Nanoparticle Properties, Behavior, Fate in Aquatic Systems and Characterization Methods

7/23/2019 Nanoparticle Properties, Behavior, Fate in Aquatic Systems and Characterization Methods

http://slidepdf.com/reader/full/nanoparticle-properties-behavior-fate-in-aquatic-systems-and-characterization 1/29

R

E V I E W

Copyright © 2014 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal of Colloid Science and Biotechnology

Vol. 3, 1–30, 2014

Nanoparticle Properties, Behavior, Fate in AquaticSystems and Characterization Methods

Mohd Omar Fatehah1, Hamidi Abdul Aziz2, and Serge Stoll1∗

1F.-A. Forel Institute, University of Geneva, 10 route de Suisse, Versoix, 1209, Switzerland 2School of Civil Engineering, Universiti Sains Malaysia, Nibong Tebal, 14300, Pulau Pinang, Malaysia

The global demand for a wide variety of applications based on engineered nanoparticles (ENPs)

has expanded the worldwide industrial scale production and inevitably released these materials into

the environment. The increasing existence of NPs and its impact towards human health and the

environment especially the aquatic system has sparked a great concern among both the scientific

community and the public. It is therefore crucial to gain an in depth understanding of the properties

the manufactured nanoparticles possess along with the various transformations they undergo thatdetermine their behaviour and mobility. This review begins by addressing the fundamental physico-

chemical aspects of manufactured oxide nanoparticles with detailed attention given specifically to

ZnO as a representative example in a separated section. The literature collected is summarized and

focused on the essential point of view to evaluate their occurrence, fate and transport in the natural

aquatic environment as a result of their interactions with other nanoparticles or natural colloids. Key

methods and principles of nanoparticle characterization are also presented.

Keywords: Nanoparticles, ZnO, Fate, Transport, Transformation, Nanoparticle Stability, pH

Effects, Nanoparticle Characterization, Aquatic Systems.

CONTENTS1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1. Zinc Oxide Nanoparticles and Environmental Risk . . . . . . 1

1.2. Influence of Natural Organic Matter on Fate and

Transport of ZnO Nanoparticles. . . . . . . . . . . . . . . . . . . . . 31.3. Influence of pH and Zeta Potential on Stability of

ZnO Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.4. Nanoparticles and Their Removal

from the Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.5. Scope and Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2. Literature Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1. Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2. Occurrence, Fate and Transport of

Nanoparticles in Aquatic Systems . . . . . . . . . . . . . . . . . . . 11

2.3. Zinc Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.4. Methods and Principles of Nanoparticle

Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.5. Challenges of Nanoparticle Characterization

in the Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

1. INTRODUCTIONEngineered nanoparticles (ENPs) are produced by human

activities on a relatively large scale and have at least one

∗Author to whom correspondence should be addressed.

dimension in the size range of 1 to 100 nm.12 Thesenanoparticles exist in groups of carbon-based materialsand inorganic nanoparticles including metal oxides, met-

als and quantum dots.3

The increasing use of nanoparticlesin a gamut of applications comprehending industrial andhouseholds, will inadvertantly see large release of thesenanomaterials into the environment. This is supported byan increasing body of scientific evidence which suggestthat nanoparticles have been found to end up in the envi-ronment and that their fate and transformation processesare difficult to evaluate and control.45 As a result of theirnanometric dimensions and interactions with the surround-ing environment, these manufactured nanoparticles willbecome mobile due to their dissolution and disaggregationbehaviour.267 Abiotic factors that affect the mobility andtransport of nanoparticles are pH, ionic strength, particlesurface chemistry, interactions of nanoparticles with other

pollutants7

and natural organic molecules.18–10

1.1. Zinc Oxide Nanoparticles and

Environmental Risk

Some archetypes of nanoparticles are iron oxide, titaniumdioxide, fullerene, cerium oxide, carbon nanotubes andothers.1112 One of the most studied manufactured nanopar-ticles, in particular for its characteristics and behavioris zinc oxide.13–17 Nanosized ZnO has shown potential

J. Colloid Sci. Biotechnol. 2014, Vol. 3, No. 2 2 16 4- 963 4/ 20 14 /3 /0 01 /0 30 d oi :1 0. 11 66 /j cs b. 20 14 .1 09 0 1



R

E V I E W

Fatehah et al. Nanoparticle Properties, Behavior, Fate in Aquatic Systems and Characterization Methods

toxicity with its existence in the environment which hassparked a great concern from both the scientific commu-

nity and the public. In the past few years various adverse

effects of nanosized ZnO on plants, phytoplanktons, mam-mals, and even human cell lines have been reported.18–22

The mechanism of ZnO toxicity has been discussed byDjurišic et al.23 ZnO NPs in the aquatic systems have

been revealed to potentially cause harm to aquatic organ-isms, especially if dissolved Zn2+ ions are released.24 Thesolubilized ZnO NP can exert stress on cells and have

adverse impacts on different organisms.25–27 This is evi-dent in the ecotoxicity studies on ZnO NPs conducted

on bacteria such as Escherichia coli,2128 Bacillus subtilis,Streptococcus aureus,29 and marine algae.30 It is there-

fore essential to comprehend the behaviour of ZnO NPsbecause their fate, transport, behaviour and ecotoxicol-ogy are closely related to their intrinsic properties such

as particles in suspension, surface energy and colloidstabilisation.531 Additionally, understanding how exter-

nal factors such as physicochemical conditions e.g., pH,32

physicochemistry of the particles,8 and interactions with

other molecules1032 will provide a clearer view on the com-

plex system of ZnO NP and its behaviour. This is relevantbecause the NP mobility is dependent on the physicochem-ical transformation they undergo such as surface modifi-

cation, aggregation, disaggregation and dissolution.26 Thetransformation is a function of abiotic factors including pH,

ionic strength, particle surface chemistry, the interactions

of nanoparticles with other pollutants and natural organicmolecules.7–81032–34

Based on previous research on NP environmental tox-

icology, the above factors like redox conditions, light,

natural organic matter (NOM), and the presence of microorganisms may result in chemical and/or biologicaltransformations of ZnO nanomaterials and induce their

mobilization in the environment where they can poten-tially exert noxious effects on aquatic organisms and

humans.1235–37

1.2. Influence of Natural Organic Matter on Fate and

Transport of ZnO Nanoparticles

The role of NOM in the fate and transport of ZnO nanopar-ticles have been broadly studied.38–42 NOM in naturalaquatic systems mostly comprise of humic substances (HS)

and polysaccharides.43 HS are macromolecule structures44

and consists of 30–50% of dissolved organic carbon

(DOC), humic and fulvic acids. DOC is found naturallyin water with a concentration rarely exceeding 5 mg/L.

The typical DOC molecular weight ranges from 102 to106 Da.45 They carry potentially important functions in the

environment as they can control the pH balance, govern

the mobility of contaminants through absorption, aggrega-tion, and disaggregation and they can coat other surfacesto give them an overall negative charge through charge

stabilization.3946

Humic acid (HA), on the other hand, is insolubleand will precipitate out in water under acidic conditions,

especially below pH 2. It is then otherwise water soluble

at alkaline pH.45 The presence of HA is found ubiquitousin the natural environment47 and has entailed with sev-

eral investigations conducted on nanoparticles to observetheir aggregation and disaggregation behavior with HA

adsorption. The interaction of HA and negatively chargedions is mainly due to the van der Waals interactions withthe NPs in the solution. Electrostatic and steric stabi-

lizations have also been demonstrated in other indepen-dent studies involving NPs and NOM when they are in

suspensions.1945 The NOM surface coatings around theNPs indicate disaggregation through charge and steric sta-

bilization mechanisms.41–48

Polysaccharides constitute 10–30% of the NOM in nat-ural waters49 and in marine environments. Alginates are

naturally occurring polysaccharides, released by microor-ganisms such as algae, bacteria and plant roots5051 com-

monly found in the marine environments.52 Some alginatesmay also be found in nature as components of some algae

cell walls, and are likely to be excreted by the algae in the

form of extracellular organic matter.53 Alginates have thetendency to promote and enhance particle aggregation anddeposition via bridging process.54 The interaction caused

by these macromolecules will have a profound effect onthe surface chemistry and transport of nanoparticles in

aquatic systems.5556

1.3. Influence of pH and Zeta Potential on

Stability of ZnO Nanoparticles

ZnO is an amphoteric oxide and can easily dissolve inboth acids and bases.57 At acidic pH values of <63,

ZnO is hydrated to form Zn2+ cations and subsequentlyforms hydroxide layers in water at basic pH values, where

Zn(OH)2 is in equilibrium with the Zn2+, Zn(OH)−3 , andZn(OH)2−

4 species. At pH > 12, the latter two zincate ions

become the dominant species in solution.58 The majorproblem of ZnO nanoparticles arises from their poor sta-bility in water5960 which leads to the formation of aggre-

gates as it approaches the point of zero charge (PZC)or pHPZC.61 In aqueous suspensions of ZnO, certain pHregions can strongly affect the stability electrostaticallydue to the transformation of colloidal Zn(OH)2S particles

to Zn(OH)2aq as the suspension stability is highly depen-

dent on the surface charge of the constituent oxides. 6263 It

is unknown to which extent that nanoparticles will agglom-erate depending on the processing conditions and the bal-

ance between the attractive and repulsive forces among thenanoparticles as well as in between them.41

pH has a huge influence on ZnO nanoparticles and has

led researchers to further investigate its rheological andelectrophoretic properties based on measuring the viscos-ity versus the pH and amount of dispersant.58 One of the

earliest studies on zeta potential and pH was done by

J. Colloid Sci. Biotechnol. 3, 1–30, 2014 3



R E V I E

W

Nanoparticle Properties, Behavior, Fate in Aquatic Systems and Characterization Methods Fatehah et al.

Logtenberg and Stein64 who discovered that zeta potentialis distinctly influenced by changes in acidity and alka-

linity and the chemisorptions of Cl− and K+ ions. Morerecent scientific literature also addressed the zeta poten-

tial behaviour of ZnO nanoparticles.586265–69 Research on

the effects of pH and time on nanoparticle zeta potential,agglomerate size, and cellular viability have been done

by Berg et al.65 The ambient conditions surrounding the

nanoparticles have a close relationship with zeta potentialand this relationship remains a largely unexplored area.The zeta potential is affected by pH and represents the

charge of a nanoparticle with respect to that ambient sur-roundings. Zeta potential cannot be taken as the actual

measurement of the individual molecular surface charge,on the contrary, it is considered as a measurement of the

electric double layer produced by the surrounding ionsin solution (i.e., counter ions). Stability of the ZnO NPs

depend on the pH of the system, where ZnO can formaggregation or remain in colloidal form. There have been

few comprehensive reports on the aggregation behaviourof ZnO.177071 Sadowski and Polowczyk 66 reported thatwithout adjusting the pH of the suspension (pH 7.4–7.6),

and by adding cationic surfactants, will cause a positive

increase in zeta potential. In another similar study, specificadsorption of carbonate ions in ZnO solution caused a shiftof the pH at pHPZC to 8.3 as its concentration increases67

while adsorption of anionic sodium dodecyl sulfate (SDS)

and propylene glycol coating in ZnO NPs significantlyshifts the pHPZC to pH 3.70 A pH study from pH 7 topH 11 was conducted by Tang et al.58 to see the effect onthe zeta potential of ZnO nanoparticles with the addition

of cationic polyelectrolyte-polyethylenimine (PEI). Subse-

quent to that, Tang et al.69 also explored the effect of adding anionic polyelectrolyte, ammonium polyacrylate

(PAA) on the ZnO zeta potential. Other researchers exam-ined the dissolution behaviour of ZnO nanoparticles asa function of pH, ionic strength and addition of natural

organic matter, its role in the acute or chronic toxicity of

aquatic organisms and the chemical etching effect.2471–73

1.4. Nanoparticles and Their Removal

from the Environment

The exact amount of manufactured nanoparticles that

are released into the environment has yet to be deter-mined. Nonetheless, numerous studies have justified that

it does occur on a relatively large scale.74–77 This poses

a huge dilemma on environmental, health and safetyissues.1978–81 The occurrence of nanoparticles that undergophysical transformation such as disaggregation3982 willform suspended sediment particles which are known to

be important in sequestering and transporting contaminant

chemicals over significant distances. The hydrodynamicand characteristics of bodies of water and morphologyof coastal zones will largely determine the distribution

of these nanoparticles in the environment.2283 A research

by Zhang et al.84 revealed that chemical treatment and

sedimentation in water treatment is still inadequate to

remove NPs and requires microfiltration. Albeit the exten-

sive efforts in both water and wastewater treatments to

remove NPs,85–89 it is likely the unintentional release of

NPs will enter the water bodies in undetermined volumes.Therefore it is crucial to first understand the NP character-

istics such as the surface charge and how it is affected by

the aggregation and disaggregation to assess their behav-

ior, fate and transport.

Nanoparticles, also defined as colloids, are construed

by environmental processes and are largely dominated

by aggregation behavior. Aggregates forming larger than

1 m conventionally subjugates to the course of sedimen-

tation, sometimes termed as ‘colloidal pumping,’ a pro-

cess that has been well characterized to understand trace

metal behavior. The general tendency of metals to sorb

to high-specific-surface area small colloids will aggre-

gate and deposit themselves. This physical transforma-

tion transfers metals from the water matrix to sedimentsand is analogous to the behavior where water bodies

undergo ‘self-purification,’ resulting in pollutant loss from

surface waters. The ultimate fate of NP aggregation and

subsequent sedimentation is an important process in the

environment.90

1.5. Scope and Objectives

The overall objective of this review is to discuss the

physicochemical interactions that actually govern the

particle-surface and the particle–particle interactions that

represent conditions of aquatic environments.

2. LITERATURE REVIEW

2.1. Nanoparticles

Nanoparticles are generally defined as particles smaller

than 100 nm in at least one dimension 91 and have existed

for a long time in all mediums, water, air and soil.90

This definition puts them in a similar size domain as that

of ultrafine particles (air borne particulates) and places

them as a sub-set of colloidal particles.31 These materi-

als with nanoscale sized structures and components exhibit

novel and significantly improved physical, chemical and

biological properties. Nanoparticle properties are used

extensively in various fields including medicine, phar-

maceuticals, manufacturing technologies, electronics and

telecommunications.709192

In Figure 2, nanoparticles canbe divided into natural and anthropogenic origin and the

two are distinguished according to their nature. The for-

mer is naturally occuring whereby it is generated through

any number of natural processes (e.g., mineral weathering)

or it may be an unintended by-product from technologi-

cal processes while the latter results from target-oriented

manufacture.31 Both natural and manufactured NPs can be

sub-grouped into environmentally relevant carbonaceaous

4 J. Colloid Sci. Biotechnol. 3, 1–30 , 2014



R

E V I E W


Fig. 1. Schematic illustration displays the possible interactions and behavior of ENPs in the aquatic environment. (A) Chemical transformation due to

abiotic factors such as pH, light or ionic strength can lead to dissolution, redox reaction etc.; (B) Biological transformation from biological degradation

of polymer coatings on nanoparticles can affect their surface properties and lead to aggregation; (C) Physical transformation include aggregation and

dispersion which will affect the mobility of the nanoparticles.

and inorganic nanoparticles based on their chemical com-position which are later discussed.

2.1.1. Natural Nanoparticles

Natural nanoparticles have long existed on earth andcan be found in all three main mediums of earth i.e.,

atmosphere, soil and water. Sources of natural NPs inthe atmosphere include volcanic eruptions, forest fires,hydrothermal vent systems, physical and chemical weath-ering of rocks, precipitation reactions and biological

CLASSIFICATION OF

NANOPARTICLES

NATURAL ENGINEERED

• Found in atmosphere, soil and water.

• Divided into biogenic, geogenic, atmospheric and pyrogenic NPs

• Examples: organic colloids (i.e. humic acid, fulvic acid), organisms (viruses), soot, organic acids (i.e. sea salt), CNTs

• Materials purposely produced by human activities-nanoproducts.

• At least one dimension (1-100 nm)

• Classified based on chemical

composition• Examples: metals (Au, Ag, Fe), metal oxides (i.e. TiO2, ZnO, Al2O3,

CeO2), quantum dots

Fig. 2. Nanoparticles are generally categorized into two groups, nat-

ural and engineered NPs. Natural NPs are commonly formed due to

environmental processes and eventually end up as biogenic, geogenic,

atmospheric and pyrogenic products. Examples are soot, organic acid

etc. ENPs are manufactured to create nanoproducts for human use and

applications such as paint, biomedical, cosmetics etc.

processes93 including sea salt in the form of airborne

nanocrystals as a result of evaporation from sea water

sprays.20 In soil, colloids are known to constitute sil-

ica clay minerals, iron- or aluminium oxides/-hydroxides

or humic organic matter, including black carbon. There

are also forms of nanominerals i.e., ferrihydrite and nat-

ural organic-mineral aggregates. A complex matrix con-

taining particles and colloids in pore water can be found

that may adsorb and bind pollutants within the matrix

while freshwater contains very complex colloid mate-

rial which includes inorganic minerals and organic matter

such as humic substances.35 In the aquatic environment,

natural NPs comprise different forms of colloids (e.g.,

metal-sulfide nanoclusters from hydrothermal systems, and

hydrous iron and manganese oxides).93 Ocean surface

microlayer also contains colloids, sub-micron components

of phytoplankton, and carbon particles.

Carbon based natural NPs are divided into biogenic,

geogenic, atmospheric and pyrogenic NPs. Some examples

are fullerenes and CNT or geogenic or pyrogenic origin,

biogenic magnetite or atmospheric aerosols (both organic

such as organic acids and inorganic such as sea salt). 12

However, the natural background of NPs in the atmosphere

is low compared to the levels caused by the combustion

processes, diesel and gasoline-fueled vehicles and station-

ary combustion sources, which have for many years con-

tributed to the particulate material in the atmosphere.94




R E V I E

W


Here, we shall deliberate on two examples of naturalNPs. Soot, for instance, is one of the results of natu-ral combustion processes which emit a wide variety of particles from both stationary and mobile sources. Only‘ultra-fine’ particles of these natural combustion processes

correspond to the standard definition of NP. In this case,the term soot is used to represent nanosize Black Carbon(BC) combustion continuum. Re-condensation processes

during incomplete combustion of fossil and renewablefuels mainly emits soot as a product into the atmospherefrom where it distributes hemisphere-wide and is depositedonto soils and water bodies. Another example is fullerenes.

Natural sources are known to have brought this nanomate-rial to earth by comets or asteroids. However, majority of fullerenes is believed to have been formed from polycylic

aromatic hydrocarbons (PAH) derived from algal matterduring metamorphosis at temperatures between 300 and500 C and in the presence of elemental sulfur or during

natural combustion processes.12 Fullerene C60 has low sol-

ubility in water,95

however they are relatively soluble ina number of organic nonpolar solvents, such as benzenes,

alkanes or naphtalenes.11

2.1.2. Manufactured Nanoparticles

Anthropogenic NPs, often referred to as engineered ormanufactured nanoparticles are materials purposely pro-duced by human activities which have at least one dimen-sion in the size range 1–100 nm and can be classified

according to their chemical composition and properties.2

Manufactured NPs, can be either inadvertently formedby a by-product, mostly during combustion, or produced

intentionally due to their particular characteristics. Theyrepresent an intermediate supramolecular state of matter

between bulk and molecular material. Manufactured NPscover a wide spectrum of substances such as fullerenes

and CNTs, both pristine and functionalized, including ele-mental metals (e.g., silver, gold and iron), metal oxides(e.g., titanium dioxide, iron oxide and aluminum oxide),

metal salts, quantum dots and fullerenes.11 The mentionedexamples are elaborated.

Fullerenes are made up of pure carbon. The simplest

fullerene, C60, is a ball made up of 60 C atoms and resem-bles a football. Fullerenes are also examples of NPs thatcan be present as a consequence of nanotechnology devel-opment. Of the large family of fullerenes, the buckmin-

sterfullerene C60 is by far the most widely investigated.12

Carbon nanotubes (CNTs) are fibrous fullerenes consist-ing of rolled up graphene sheets that may or may not be

capped at the ends by a half fullerene sphere.20 CNTs existin two main manufactured forms,(i) the single-walled or SWCNT and(ii) multi-walled or MWCNT.

Carbon nanotubes are generated by arc evaporation, laserablation, pyrolysis, and electronic methods. SWCNTs pos-sess important mechanical, thermal, photochemical and

electrical properties which are industrially useful. MWC-NTs contain several SWCNTs in their structure, andtherefore, they possibly have analogous physicochemicalproperties to those corresponding to SWCNTs.3

Metal oxides nanoparticles are known to have the

unique ability to promote faster electron transfer kineticsbetween the electrode and the active site of the desiredenzyme.96 Metal oxide NPs are widely used in a numberof applications i.e., food, material, chemical and biologicalsciences. Among of the most important commercial metaloxide NPs are elaborated below.

Nanoparticulate iron oxides (e.g., magnetite Fe3O4,maghemite Fe2O3, hematite Fe2O3 are one of the mostabundant forms of anthropogenic nanomaterials as they arefound in soil, water and the cytoplasm of living cells. Thebehavior of iron oxide NPs in aqueous media is largelygoverned by the size, shape, oxidation state and stabil-ity of the iron oxide, all varying according to the spe-cific synthesis procedure and conditions used. Iron oxide

has a positive surface in most environmentally relevant pHconditions which means that these materials will interactfavourably with the majority of negatively charged naturalcomponents in aqueous environments.11

The bulk structure of CeO2 is made of Ce(IV) atoms,eight oxygens and four ceriums. Based on the respec-tive equilibrium constants for proton adsorption, Ce3+ ionsmay be more mobile and bioavailable than nanoparticles,which raises the need to consider and quantify the con-tribution of cerium reduction and dissolution in the risk assessment of cerium nanoparticles. CeO2 dispersion ishighly stable from below pH 6, since these surfaces arestrongly positively charged in this region. Self-aggregationoccurs at above pH 6, where the CeO2 surface is less

charged as the pH approaches the IEP, which results inthe interfacial interaction between CeO2 surfaces becom-ing more attractive. This phenomenon displays kineticsthat are correlated with the distance of the pH from thePZC.11

Nano-TiO2 is produced on a large scale in the appli-cations of paints and coatings (self-cleaning, antifouling,and antimicrobial properties) and in cosmetics as a UVadsorber.77 Titanium dioxide (TiO2 is one of the mostwidely used nanomaterials in the industry.96

Metal nanoparticles such as silver NPs are nanoscaleclusters of metallic silver atoms, Ag0. Metallic silver isrelatively nonreactive. However, in aqueous environments,silver ions (Ag+ are released from the bulk metal and into

solution. Ag+

ions are used for a variety of antimicrobialapplications and sterile applications due to its antimicro-bial, antifungal and partially antiviral properties.7798 Goldnanoparticles are typically inert but become catalytic astheir size decrease to a few nanometers.39

2.1.3. Natural Aquatic Colloids

Natural aquatic colloids in surface waters are a composi-tion of environmental complex mixtures and heterogenous




R

E V I E W


phases defined as solid-phase materials having at least onedimension within the size range of 1 nm–1 m.3194 Col-

loids in the environment are formed from processes thathave taken place for several millions of years. Natural

aquatic colloids are produced (weathering, microbial pro-

cesses) and lost (aggregation and sedimentation, microbialaction) by several processes.46 Based on the size scale,

manufactured nanomaterials which has a dimension lessthan 100 nm is overlapped and thus included in the col-

loidal category.2 Colloids may originate from both naturaland anthropogenic sources. However in this case, we shall

focus more on the natural origin. Natural colloids can bedivided into two groups, organic and inorganic natural col-

loids. Organic colloids can be explained in terms of col-loidal components such as small organic macromolecules

(such as humic acid and fulvic acids, 1–2 nm in diam-

eter), fibrillar polysaccharides (1–10 nm wide and up toseveral um long), biocolloids (i.e., bacteria, viruses and

fungi) while inorganic colloids comprise of silicates (e.g.,

clays, chlorites, kaolinite), oxides, carbonates and metalsulphides.231 Natural aquatic colloids can also be classi-fied by their particle size and are generally fractal with

a 3-D network type structure. Using the small-angle neu-tron scattering (SANS), three characteristic length scales

were determined-primary particle size with ca. 3–10 nm,

small aggregates of 20–50 nm and transient networks of aggregates with a length scale of 50–200 nm.99

Besides manufactured nanoparticles, natural aquatic col-

loids too have significant effects on pollutant, nutrient

and pathogen chemistry, transport and bioavaibility. Thenature and morphology of major aquatic colloids have

been described elsewhere.49 By apprehending their chem-istry and environmental impact will provide a better under-

standing the fate and behaviour or trace elements and traceorganic pollutants as these natural NPs can cause a dele-

terious effect.3

Colloids provide a molecular milieu into and ontowhich chemicals can escape from the aqueous solution

and whose environmental fate is predominantly affected bycoagulation-breakup mechanisms, as opposed to removal

by settling.12 The physicochemical transformation of nat-

urally occurring nanoparticles in the environment include

particle aggregation, disaggregation, and surface modifica-tion; processes which usually take place in the presence of

NOM, and that respond to changes in temperature, con-centration, pH or ionic strength. Natural aquatic colloids

are of small size and large surface area per unit massmakes them important in pollutant binding as the morphol-

ogy, composition and structure of these colloids determine

their role in the environment.90 Recent studies have appliedthe colloid science principles based on the Derjaguin-

Landau-Verwey-Overbeak (DLVO) theory to gain a betterunderstanding of NP aggregation under various conditions

which will lead to further information of the fate and

behavior of trace pollutants.34 In addition, the profound

complexity and heterogeneity of the colloidal structure,and how this relates to their environmental function, is still

poorly understood.74

2.1.3.1. Humic Acid. Humic acid (HA) or its standardname Suwannee River Humic Acid (SRHA) belongs to

one of the major classes of natural organic matter otherthan fulvic acids, humins and polysaccharides.4447 Theyare naturally occurring, biogenic, heterogeneous organicmaterial, generally polydisperse and contains functionalgroups such as carboxylic acid, amine, carbonyl, alcohol

and carboxylate (–COO–). The natural pH of humic acidis pH 3.447 and its molecular weight ranges from 1000 togreater than 106 amu. HA is insoluble at low pH (<2) buteasily dissolves at alkaline pH. In aqueous solutions, HA

exist as dissolved macroligands at low concentrations, andas aggregates at higher concentrations.20

Though categorized as a supramolecular, HA has apoorly defined structure and is characterized by aromatic

and aliphatic structures in which hydrogen bonding plays a

significant role in the aqueous phase structure. Site bindingin humic acid usually involves an electrostatic interactionwhere one of the HA’s functional groups (i.e., COO–, car-

boxylate) and a cation (Ca2+, Mg2+, etc.) form an ionicbond. The main factors affecting the structure and control-ling the size are pH, the cation type and concentration,and residence time. It is also well-known that HA can

significantly modify the surface properties (e.g., electriccharge, size, chemical nature of the exposed surface sites)of natural aquatic colloids, significantly influencing theirtransport, often due to increased electrostatic repulsion.100

Under most environmental conditions, small amountsof HA can coat other surfaces to give them an overallnegative charge that results in reduced aggregation and

promote disaggregation through charge stabilization of thecomplexes under the right conditions.3446 Collectively, HAtends to aggregate as the ionic strength increases. Thisaggregation phenomenon is more important at higher pHvalues, because of the higher quantity of the dissociated

functional groups, and thus, the higher negative charge onHA. It is obvious that the behavior of HA under vari-able pH and ionic strength is tremendously complex. Thisbehavior could be completely inversed depending on(i) the pH and ionic strength,

(ii) the balance between the surface charges of HA devel-oped by the dissociation of functional groups and the con-centrations of cations present in the solution,

(iii) the preparation method: i.e., fixing the ionic strengthand varying the pH or fixing the pH and varying the ionicstrength. Consequently, different trends could be obtainedunder different conditions as stated by other researchers.

2.1.3.2. Biopolymers. Biopolymers can be dividedinto two types which are protein and polysaccharidesnanoparticles.101 Protein nanoparticles are naturally occur-ring materials such as albumin, collagen, gelatin, silk pro-

tein from sericin and fibroin nanoparticles, sericin and




R E V I E

W


keratin.102103 Polysaccharide nanoparticles are also nat-urally occurring with nanostructured surfaces which are

designed for the administration of peptides, proteins andnucleic acids.104–106 They are able to help to improvebiocompatibility of cell toxic material, which are cur-

rently being developed for novel bionanoparticle-derivedpharmaceutical formulations. Examples are alginate andchitosan.107–110

2.1.3.3. Alginate. Alginate is a natural occurring poly-

mer and represents one of the common polyelectrolytesfound in suspensions comprising of colloidal particles usedin biomedical, environmental, and industrial applications.54

It is composed of linear unbranched polysaccharidesconsisting of two types of uronic acids, -L-guluronicacid (G) and (1,4)-linked -D-mannuronic (M). Thesequence and molecular weight of the monomeric units are

grouped in three ways: blocks of alternating guluronic andmannuronic residues (MG-blocks), blocks of guluronicacids (G-blocks) and of mannuronic acids (M-blocks)

(Fig. 3). The source of alginates also varies i.e., brownseaweed,111–113 commercial sources extracted from marinealgae i.e., Laminaria hyperborea, Ascophyllum nodosum

and Macrosystis pyrifera114 or even bacteria.54 Among the

alginate characteristics that are determined by the preva-lence and sequence of the alginate block types that controlits chemistry in solution are water soluble, mucoadhesive,biocompatible and non-immunogenic.112–115

Fig. 3. Chemical structures of alginates consists of (1→ 4) linked -D-mannuronic acid (M) and -L-guluronic acid (G) residues. The structures

above depicts G-block, M-block, and alternating blocks in alginates.

Alginates are also known to undergo dissolution andbiodegradation under normal physiological conditions.113

However, they have poor mechanical properties and pro-cessing difficulties when compared with the syntheticpolymers. Alginates are negatively charged from the car-

boxyl groups located on the ring structure of both the Mand G monomers.112116 The polymer chains of alginateswith carboxyl (–COO− groups exists in the form of a

stretching conformation due to the repulsion between thedeprotonated carboxyl groups and have a high hydrophilic-ity in basic and neutral solutions. However, in an acidic

solution, the polymer chains tend to aggregate becauseof the protonation of carboxyl groups which leads to thedecreased hydrophilicity. The aggregation behavior of algi-

nate was studied by Yu et al.117 They found it difficultto precisely control the hydrophilic/hydrophobic balanceof the polymer chains of natural polymers to induce self-assembly and to form stable aggregates in aqueous media.Alginate has become a subject of academic as well as of

industrial interest because of their renewability and biode-gradibility. It has been used as a chelating agent to enhanceinteraction with Zn ion to form ZnO nanostructures.118

In another study, zinc alginate beads were prepared bydropping aqueous solution of sodium alginate into a zincsolution containing zinc nitrite or zinc acetate.109 Chenet al.54 studied enhanced aggregations rates demonstratedby nanoparticles coated with alginate. The aggregation




R

E V I E W


kinetics of alginate-coated iron oxide (hematite) increasedwith the presence of divalent ions of Ca2+, Sr2+ and Ba2+.In another study, alginate was used to control the syn-thesis of ZnO nanoparticles by microwave treatment andproduced mostly spherical in shape and hexagonal crys-

tal structure and showed strong antibacterial activity with99.9% reduction for S. aureus and 100% reduction for

E. coli after 2 hours of exposure.115

2.1.4. Nanoparticle Properties

2.1.4.1. Physicochemical Properties. With emphasis

that nanoparticles are typically engineered or formed postprocessed for specific applications, their physico-chemicalproperties and reactivity therefore vary considerably.21

Many investigators have outlined the key characteristicsof manufactured NPs that are believed to exert impor-tant controls on their environmental behavior, fate andecotoxicity,3593119 as well as uptake and distribution

within organisms, and the interactions of nanoparticles

with other pollutants.32 Due to their small size and homo-geneous composition, structure, shape or surface charac-

teristics, these manufactured NPs often exhibit a range of special physico-chemical properties and reactivities thatare expected to deviate from bulk behaviour.1235

The intrinsic properties of manufactured NP include(i) physical characteristics, particularly size and shape,surface area, electrical conductivity, state of dispersion;(ii) chemical characteristics such acid-base character of the surface charge, chemical composition, surface chem-istry and the aqueous solubility of the NPs.832

Other NP properties that could be studied are dispersibil-ity, agglomeration/aggregation, dissolution rate and reac-

tivity (e.g., catalytical activity, sorption capacity).

119

Theseproperties are very useful in toxicological studies,8 foodproduction i.e., emulsification, gelation, foaming, water-binding capacity,120 processing, packaging, additives andsafety,121 complex food and environmental samples.122123

2.1.4.2. Zeta Potential and Surface Charge. The zeta

potential represents the charge of a nanoparticle in rela-tion to the surrounding conditions. Nevertheless, the zetapotential is not an actual measurement of the individual

molecular surface charge; rather, it is a measurement of the electric double layer produced by the surrounding ionsin solution (i.e., counter ions).66 These counter ions playa role in the calculation of zeta potential measurement.

All particle systems in an aqueous media carry an elec-

tric charge which may be positive, negative, or neutral.For surface-derived nanoparticles, dissociation of an acidic

group, such as a carboxylic acid moiety on a nanoparti-cle surface will yield a negatively charged surface; whiledissociation of a basic group on a nanoparticle surfacewill yield a positively charged surface. For unmodifiednanoparticles, the individual atoms on the surface of theparticle dictate its charge. The addition of HCl affects theshift of the zeta potential of the ZnO dispersions to more

positive values, while addition of KOH results in a similarshift to more negative values.64

Surface charge is defined as an electric charge present atan interface of a NP where the NP’s propensity to interact

with charged surface and ions can be measured. Surface

charge results in the formation of an electrical double layercontaining ions attracted from the solution to the particlesurface in response to the charge. The electrical potential

at the interface of the diffuse layer and the bulk solution

can be measured, and its variation with solution chemistrycan effectively be used as a surrogate for the variation in

particle surface charge with solution chemistry.119

Surface charge of NPs may be either pH dependent,

as in oxide materials (due to protonation and deproto-nation of functional groups).124 They may also be fixed,

as in clays, where this charge results from crystal latticedefects and atomic substitution.2062 The chemistry of the

medium will influence the electrostatic surface charge of the particles, thereby affecting agglomeration/aggregation

rates and particle stability.119 In systems formed by aque-ous solutions and oxides, hydroxides, or oxide hydrox-

ides, the hydronium H3O+ and hydroxyl OH− ions are the

potential-determining species; therefore the surface charge

depends on the pH of the solution.125 Varying the solutionpH resulted in a significant change in the particle surface

and consequently, the hydrodynamic diameter.8 In anotherstudy, guar gum adsorbed on the surface enhanced the

mobility of nano zerovalent iron (NZVI) in sandy porous

media regardless of the solution chemistry for instance,pH and ionic strength.126 Surface charge is responsible

for colloidal NP properties causing particle repulsion andattractions. Polyelectrolytes are able to modify the surface

charge of NPs by giving a surface coating.69 It is closelyrelated to hydrophobicity, which is a particle’s incapacity

to interact with water. Surface charge and hydrophobicity

is presumably an important factor which determines NP

behavior in the environment.20 Particles can acquire chargewhen they adsorb ions present in the liquid.68 A study

by El Badawy et al.125 revealed that the particles canacquire charge when they adsorb ions present in the liquid.

Adjusting the surface charge can be an effective method to

modify the cytotoxicity, cellular uptake, and specificity of

targeting of NPs.128 In systems formed by aqueous solu-tions and oxides, hydroxides, or oxide hydroxides, the

hydronium, H3O+ and hydroxyl OH− are the potential-

determining species of the charges.129

2.1.4.3. Electrical Double Layer. The interface betweena metal oxide and aqueous solution is of great importance

in many fields of chemistry. The surface complexation of

the metal oxide is strongly influenced by the developmentof the surface charge which results in an electrostatic dou-

ble layer that forms around the oxide surface of each par-ticle in the electrolytic solution.129 The development of a

net charge at the particle surface affects the distribution of

ions in the surrounding interfacial region, resulting in an




R E V I E

W


Diffuse layerlons are diffusedmore freelyaround theparticle.

Stern layerThe particle will attracrt ions of theopposite charge. positive ions willmove closer to the surface. theseions are tightly bound immediatelyaround the surface.

Negatively chargedparticle

Charges beyond the slipping plane willnot move with the particle as an entity.ions within this boundary will move withparticle as one entity.

Hydrodynamic plane ofshear (slipping plane)

Potential enery curve

Surface potential

Stern potential

Zeta potential

Distance from particle surface

–100

mv

0

Fig. 4. The electrical double layer on the surface of a nanoparticle is based on the Gouy-Chapman-Stern model. The energy potential curve is depicted

below where the surface potential is the electrical potential surrounding the particle while the stern potential is the electrical potential at the stern layer.

The zeta potential is the electrical potential at the hydrodynamic plane of shear (slipping plane).

increased concentration of counter ions (ions of opposite

charge to that of the particle) close to the surface. The

liquid layer surrounding the particle forms two regions.

The inner region, called the Stern layer, where the ions

are strongly bound and an outer, diffuse region where they

are less firmly attached. Within the diffuse layer there is

a notional boundary inside which the ions and particlesform a stable entity. When a particle moves (e.g., due to

Brownian motion), ions within the boundary move with

it, but any ions beyond the boundary do not travel with

the particle. This boundary is called the surface hydrody-

namic shear or slipping plane. The potential that exists at

this boundary is known as the zeta potential.130 Figure 4

depicts a schematic diagram of the electrical double layer

(EDL) on the surface of a particle, with the different poten-

tials to be considered and the Debye length 1/k which is

the length where the potential has fallen to a value of 1/e

of the Stern potential.

2.1.4.4. Particle Size and Shape. The most evident

parameter to describe nanoparticles is size, as it can influ-ence a wide range of material properties such as electronic

properties, and interactions with light and other types of

electromagnetic radiation. When the particle diameter is

between 10–20 nm, the number of atoms on a particle sur-

face starts to constitute a significant fraction of the total

number of atoms in a particle, which may lead to a number

of changes related to free surface energy (lattice proper-

ties, cell parameters etc.).20

The surface charge of a nanoparticle is closely related toits size and shape. Adjusting the surface charge is gener-ally accompanied by size and shape variations, and there-fore a combined effect is achieved as both the particlesize and shape greatly affect the NP properties.128 The factthat the size of the nanoparticles itself can be a factor

in direct toxicity and pathology is extremely important,and biodegradability may be a further significant factor ingoverning harmful biological effects. Nanoparticles havea proportionately very large surface area and this surfacecan have a high affinity for metals (e.g., iron) and organicchemical combustion products such as polycyclic aromatichydrocarbons, PAHs. Apart from the particle size (one ormore dimensions of the order of 100 nm or less) whichprovides a very large surface to volume ratio, their biocom-patibility surface properties depend on the charges carriedby the particle and its chemical reactivity.22 Size also hasimportant control over other physical and chemical prop-erties such as zeta potential and metal binding. 35

2.1.4.5. Surface Area. The surface area of a nanopar-

ticle is the total area of both external and internal sur-faces available from the outside of a particle. This is animportant parameter because all interactions between NPsand other surfaces or solutes will relate to (or is quanti-tatively proportional to the particle surface). The specificsurface area instead is the ratio of the surface area to themass of a particle. Consequently, the reactivity of the NPwill strongly increase with increasing surface area or withdecreasing particle size.20




R

E V I E W


Table I. Point of zero charge (PZC) for various types of engineered nanoparticles (ENPs).

Type of ENP Provider Size (nm) PZC References

Al2O3 Sigma Aldrich, St Louis, USA <50 706 Berg et al.65

CeO2 Sigma Aldrich, St Louis, USA <25 671 Berg et al.65

Fe2O3 Synthesized via flame synthesis 23–35 424 Berg et al.65

-Fe2 O3 Synthesized by forced hydrolysis 95±

3 8 Palomino and Stoll

41

Fe2O3 Synthesized by forced hydrolysis 65 88 He et al.15

TiO2 Nanostructured and Amorphous Material Inc. ∼15 62 Loosli and Stoll48

TiO2 Evonik, Hanau-Wolfgang, Germany ∼27 519 Berg et al.65

TiO2 NanoAmor 5 5 Domingos et al.100

ZnO Sigma Aldrich, St Louis, USA <50 713 Berg et al.65

ZnO Synthesized via polyol process 200±0.5 9 Brayner et al.32

ZnO Sakai Chemical Industry Co., Ltd., Sakai, Japan 40 96 Tang et al.58

ZnO NanoAmor, Houston, USA 20 93 Mohd Omar et al.9

2.1.4.6. Point of Zero Charge and Isoelectric Point. Thepoint of zero charge (PZC) is a parameter identified at thepH at which the particle surface charge sums up to zerodue to the absence of both positive and negative charges.131

The determination of PZC on a given manufactured NPcan indicate if it is charged or uncharged (i.e., hydrophilicin an aqueous suspension). The isoelectric point (IEP) isan equally important parameter which characterizes a stateof a particle surface sorbed by an equal balance of posi-tive and negative charges (by the disassociation of H+ andOH− ions in an embedding liquid) such that the electricalpotential inside the double electric layer is equal to zero.Both the terms PZC and IEP clearly represent differentconditions eventhough the sum of charges on the colloidalparticles is nullified.20

The pHPZC is a very important value for adsorption mea-surements and surface characterization due to several rea-

sons. First, a crucial role in the sorption of protons andhydroxyl groups is played by the acid-base properties of

the surface. Second, the electrostatic sorption of electrolyteions made complicated by the chemical interaction changesat both the PZC and IEP, however, these values changein opposite directions. The pH value of PZC is an inten-

sive property, which depends on the surface chemical andphysical structure rather than on the specific surface area(SSA).125 Studies emphasizing the importance of PZC of ENPs have been vastly conducted. For example, the PZCfor iron oxides has been generally reported to be betweenpH 7.2 and pH 9.5 while it is theoretically predicted thePZC of CeO2 is at pH 7.92.11 Table II lists the values of point of zero charge for various types of ENPs at differentsizes.

2.1.4.7. Nanoparticle Structure and Morphology. The

structure of a nanoparticle can be divided into two orthree layers comprising of (i) a surface that is often func-tionalized; (ii) a shell material that may be intentionallyadded; and (iii) the core material. The layer of the sur-face is typically known to interact with a range of metalions, small molecules, surfactants or polymers. The chargeat the surface will determine which interaction will gov-ern its behavior and the type of bonding it forms withother nanoparticles or molecules. The second layer of the

nanoparticle, known as the shell, has a completely differ-ent chemical structure from the core. These layers maybe prepared intentionally in the lab for research purposes.For example, quantum dots containing a shell layer of cad-mium selenide with a core made of zinc sulfide. Generallynanomaterials containing shells do not occur through otherprocesses and are unlikely to form serendipitously. Thefinal and most essential part of the NP is the core, locatedat the centre. The centre holds the composition and prop-erties of the nanoparticle that becomes the main focus of studies by researchers.31

The morphology of a nanoparticle and its size has asignificant influence on the physical and chemical prop-erties which determines their interaction with the envi-ronment and biological systems.17113 Nanoparticles canexist in fused, aggregated or agglomerated forms with var-ious forms of morphology including spherical, tubular orirregular shaped.12132 Certain nanostructures such as ZnOcould also have novel applications in optoelectronics, sen-sors, transducers and biomedical sciences.133134 An exam-ple of primary particles would be ZnO in the hexagonalwurtzite phase where its form is spherical with sharp peaksthat indicate its high crystalline nature.66

2.2. Occurrence, Fate and Transport of

Nanoparticles in Aquatic Systems

The study on the fate and transport of manufacturednanoparticles in the environment is becoming importantdue to the current discharges to the environment.93 A fullerunderstanding of the nanomaterial domain requires anevaluation of the matrix of source materials, their trans-formation in the natural aquatic environment, and theirphysical/chemical behavior that is specific to the water

medium.135

In order to assess the risks associated withmanufactured NPs, it is necessary to first understand theirmobility, bioavailability, interactions with other materialsand toxicity.93

2.2.1. Sources and Routes of Nanoparticles

Into the Aquatic System

The expansion of nanoparticles in a diverse range of prod-ucts and applications including paints, fabrics, personal




R E V I E

W


Fig. 5. The routes of nanoparticles entering all three mediums (air, water, soil) of the environment.

health care products, electronics, biomedicine, pharmaceu-

ticals, cosmetics have increased significantly in the past

few decades.33590 The incorporation of “nano” ingredi-ents into products will increase the occurrence of man-ufactured nanoparticles in the environment as they may

be released during the life cycle of those products.4676136

The important processes and pathways of nanoparticles inthe environment are presented in Figure 5. Release of NPsmay come from point sources such as production facil-ities, production processes, landfills or wastewater treat-

ment plants or from nonpoint sources such as wear from

materials containing NP.117593 Accidental release duringproduction or transport is also possible. In addition to theunintentional release there are also NPs released inten-

tionally into the environment. NPs that are released willinevitably end up in groundwater aquifers. Their depen-

dence on the transformation and transport via a varietyof pathways will eventually bring them to their ultimate

destiny, the water/sediment interface.1277

2.2.2. Interactions of Nanoparticles in Aquatic SystemsThere is still a lot to understand about the fate and behav-ior of nanoparticles and their interactions with each other,natural colloids and pollutants. Manufactured nanoparti-

cles are considered to represent a special case, since they

may be designed to have particular surface properties and(surface) chemistry that are less likely to be found in nat-ural particles. Initial data on a number of studies suggest

that manufactured NPs interact with other contaminants

hence influencing their toxicity.81825137138 The ecotox-

icity effect imposed by NPs can be altered by disper-

sion process.7 The understanding of particle chemistry in

order to correctly interpret ecotoxicological data is nec-

essary. These include the influence of particle size, shape

and surface area and the interactions of the NPs withother material in the water or environmental matrix.35

Other factors such as microorganisms, naturally occurring

colloids, biomacromolecules (e.g., protein and polysaccha-rides), sunlight, and oxidants/reductants complicate parti-

cle behavior in the natural environments. In hard water and

seawater, nanoparticles are prone to aggregate as they are

greatly influenced by the specific type of organic matteror other natural particles (colloids) present in freshwater.

However, conclusions are normally made after taking into

account abiotic factors that may influence this, such as pH,salinity and the presence of organic matter. In the next sub-

section, we described the interactions NPs may undergo as

they enter the aquatic environment, the physicochemical

characteristics that govern their behavior and the transfor-

mations, the NPs experienced that will lead/determine theirfate and transport. Because the particles are of nanosize

(<100 nm), their interaction with solid surface, bio inter-

faces or other particles can be quite different from larger,microsized particles.7

2.2.2.1. Particle–Particle and Particle-Surface Inter-

actions. When released into the aquatic environments,

nanoparticle behaviour is dependent on particle-specific

properties (e.g., size, shape, chemical composition, surface




R

E V I E W


charge, and coating), particle state (free or matrix incorpo-rated), the surrounding solution chemistry (e.g., pH, ionicstrength, ionic composition, natural organic matter con-tent), and hydrodynamic conditions.139–141 Such factors areimportant in determining whether particles aggregate with

other particles or deposit onto various environmental sur-faces. Recognizing which interactions particles experienceunder different conditions enables the prediction of theirfate in the environment.7 Interactions leading to aggrega-tion greatly influences particle behavior under natural envi-ronment. The nanomaterial reactivity, toxicity as well astransport potential may be significantly altered due to theaggregation as a consequence of change in particle sizeand shape.142143 Nanoparticle transport through aquaticenvironment is also expected to be dominated by ran-dom Brownian diffusion where there will be an increasein particle size created by aggregation that may result ingravitational sedimentation.144 Overall it is essential to elu-cidate which physicochemical interactions govern particle-

surface and particle–particle interactions under conditionsrepresentative of aquatic environments.There are two conditions to be considered. The first

would be favourable (non-repulsive) particle-surface inter-actions where nanoparticles will be less likely to travelextensive distances. The second would be unfavourable(repulsive) deposition conditions. At any rate, particle–particle interactions and particle-surface interaction playkey-roles in controlling the aggregation and depositionbehaviour of nanoparticles in aquatic environments.145146

These interactions have traditionally been described bythe DLVO theory of colloidal stability.735 However, non-DLVO forces such as steric, magnetic and hydration forcescan also play an important role in the aggregation and

deposition of engineered nanomaterials.147

This will befurther elaborated in Subsection 2.2.3.

Iron-based nanomaterials, exhibit a magnetic dipolemoment, even in the absence of an applied magneticfield. The magnetic force may dominate the total particle–particle interaction energy, leading to aggregation. In othercases, some nanoparticles may carry hydrophilic material,functional groups, or biomolecules (e.g., proteins, polysac-charides) at their surface that can have significant amountsof bound water that may play a role in the interaction of such particles. The approach of two particles with hydratedsurfaces will generally be hindered by an additional repul-sive interaction. The range of this interaction is significantcompared to the range of electric double layer repulsion

and is expected to have an affect on nanoparticle stability,particularly at high strength.

2.2.2.2. Interaction with Natural Colloids. Colloids areusually defined as material with one dimension between1 nm and 1 m and in natural aquatic systems are a com-plex aquatic mixture including viruses and bacteria, naturalorganic matter, protein and polysaccharide exudates frominorganic matter such as oxides of iron, manganese, alu-minium and silicon.294192127148–150 Naturally occurring

organic macromolecules (e.g., NOM) in the environmentcan significantly alter the aggregation behaviour of NPs.

Even without fully defined structure, due to its macro-molecular nature, NOM is expected to prevent aggregation

and deposition presumably due to electrostatic stabiliza-

tion. Natural organic matter consists of mainly fulvic andhumic substances.Manufactured nanoparticles entering aquatic systems

will thus become components of these colloids and their

subsequent behaviour and transport will depend both onphysicochemical characteristics of the aqueous media and

interactions with other colloidal components. The stabilityof colloidal suspensions is determined by the interaction

between attractive and repulsive forces, which are gov-erned by surface charges of the colloidal material. Col-

loids carry an electrical charge, which produces a forceof mutual electrostatic repulsion between adjacent parti-

cles. If the charge is high enough, the colloids will remaindiscrete and are stabilized in suspension. Reducing or

eliminating the charge causes the colloids to agglomer-ate and settle out of suspension or form interconnected

matrices.90 Studies showed that the presence of added

humic acid altered the aggregation mechanism compared

with the nanoparticle alone as this may promote or reduceaggregation, depending on conditions.94148 The informa-

tion available suggests a complex interaction between nat-ural (organic) colloids and manufactured NPs of different

types. The observed effects include aggregation, disag-

gregation and surface film formation and are all depen-dent on conditions.3 Stabilization usually results from

NOM forming a charged stabilizing layer on the out-side of the particle. Destabilization, on the other hand,

results from particles being bridged by larger NOMmolecules, such as rigid biopolymers. This occurs for rel-

atively large-sized NOM (∼10–100 kDa) and can domi-

nate interactions between nanoparticles. Large molecular

weight biomolecules and biomacromolecules, includingpolypeptides also affect aggregation of NPs in aquatic

environments.76 Colloidal material from natural watershas been found to be coated by films of organic mate-

rial and since particle surface charges and force interac-

tions between particles are dominated by adsorbed layers.

This has important implications in understanding mecha-nisms by which colloids might bind trace elements and

pollutants.151 It has been shown that adsorption of humic

acid to various metal oxide nanoparticles (TiO2, alu-

minium oxide [Al2O3], and zinc oxide [ZnO] can result ina decrease in particle zeta potential, suggesting that HA-

coated nano-oxides could be more easily dispersed and

suspended and more stable in solution than uncoated onesbecause of their enhanced electrostatic repulsion.152

2.2.2.3. Interaction with Organisms and Pollutants.

Nanoparticles entering waterways from industrial prod-

ucts and wastes and its interactions with the aquatic biota

has been highlighted as a major concern in previous




R E V I E

W


reports.2227 Though novel properties of nanoparticles are

increasingly studied, little is known of their interations

with aquatic organisms.153 The small size of ENPs has

created opportunities for them to interact with biologi-

cal entities (i.e., cells, cellular components, bacteria and

viruses).154

At the cellular level, prokaryotes like bacteriaare more likely to be protected against the intrusion of

most ENPs as they are incapable of undergoing the mecha-

nism for the bulk transport of supramolecular and colloidal

particles throught the cell wall. In contrast, when it comes

to eukaryotes (i.e., protists and metazoans) they have a

highly developed process for the cellular internalisation of

nanoscale (<100 nm) including microscale such as endo-

cytosis and phagocytosis with a range of 100–100000 nm

particles.22 In the case of microalgae Pseudokirchneriella

subcapitata, the interaction of aggregates of TiO2 entrap-

ping algal cells played a major role in toxicity effect. 153

Other studies have proven that nanoparticles undergoing

aggregation will sediment, thus becoming less mobile and

subsequently may be ingested by organisms.7155156 Other

interactions could involve the absorption of dissolved ions

due to dissolution of metal oxides.3089157

In the case of ENPs interacting with pollutants existing

in the aquatic system, it can either amplify or alleviate the

toxicity of the compounds contained within.12 There are

many ways the pollutants can interact with ENPs. Pollu-

tants can be adsorbed on to the surfaces of ENPs, or either

adsorbed into the ENPs, co-precipitate during formation of

a natural NP or probably trapped by aggregates containing

a mixture of ENPs and adsorbed pollutants. The sorption

of pollutants onto ENPs depends on their properties such

as composition, size, purity, structure and solution condi-

tions such as pH and ionic strength.31 However, ENPs havealso proven to have an advantageous role in the environ-

ment by adsorbing toxic organic compounds as a treatment

method as elaborated elsewhere.3146158

2.2.3. Nanoparticle Stability

The classical DLVO theory of colloidal stability describes

the total interaction energy experienced by a nanoparticle

when approaching another particle (in the case of aggre-

gation) or a collector surface (in the case of deposition).159

The stability of nanoparticle suspended in an aqueous envi-

ronment can be evaluated as the sum of van der Waals and

electrical double layer interactions, likely known as the

interaction energy. The interaction energy determines theparticle stability as the two surfaces approach one another.

VdW forces result from electrical and magnetic polar-

izations, yielding a varying electromagnetic field within

the media and in the separation distance between the two

surfaces. The dispersion interactions evaluation is pro-

posed to be based on the assumption that the potential

between two surfaces could be represented as the sum

of the interactions between pairs of atoms located within

the two surfaces (particle or collector). In aqueous envi-ronments, when particles approach each other (aggrega-

tion) or a surface (deposition), the overlap of the diffuse

electric double layers results in electrostatic double layerinteractions.160–162 Widely used equations are given for the

most commonly encountered interaction geometries (i.e.,two spherical particles or a spherical particle interacting

with a planar surface).7 The stability of a nanoparticle sus-pension can also be influenced by non-DLVO forces.163

The most significant forces encountered by engineerednanomaterials in aqueous media include steric interactions,

magnetic forces (for iron-based nanomaterials) and hydra-

tion forces.84159 Steric forces have been derived with gen-eralized expressions for particles with adsorbed layers of

polymers or surfactants that might lead to steric repul-sions. Steric interactions can be particularly important fornanoparticles in natural and engineered aquatic environ-

ments, as most particular adsorb natural organic matter

that is known to stabilize colloids.139164

The high surface area to volume ratio of the NPs resultsin high reactivity which leads to particle aggregation and

settling.127 Based on the classical Derjaguin, Landau, Ver-

wey and Overbeek (DLVO) theory, colloidal particles aresurrounded by a diffuse electrostatic double layer (EDL)(Fig. 4) and the balance between the van der Waals attrac-tion forces and the electrostatic repulsion forces deter-

mines the colloidal stability.31 However, the DLVO theory

does not take into account the effects of particle shape,surface roughness which among other factors also influ-ence the collision efficiency. The DLVO theory is only

applicable if there is no interference with such diffusive or

attractive forces.35

Aggregation may occur as homoaggregation (particles

of the same type aggregating together), or heteroaggrega-tion (particles attaching to other particle types present).Aggregation of NPs reduces the surface area to vol-

ume effects on NP reactivity. This physical transforma-

tion will cause an increase of aggregate sizes which inturn affects their transport in soil, sedimentation, reactiv-ity, uptake by organisms and toxicity. When aggregationoccurs, the number of concentration of NPs in the suspen-

sion decreases, with a concomitant increase in their effec-

tive (aggregate) size.158 This will decrease their mobilityin the environment especially in the water compartment.39

2.2.4. Nanoparticle Dissolution

Dissolution is an important process as the nanoparticlesolubility is a significant factor in determining NP prop-erties, toxicity and persistence.39136 This strongly affectsthe uptake pathway, toxicity mechanisms and the environ-

mental compartment in which NPs will have the high-

est potential impact.2573166 Dissolution is one among themany important environmental transformations that affectsthe form and concentration of NPs628159 and may be a

critical step for some NPs in determining their fate in the




R

E V I E W


environment.160 This type of environmental transformationoccurs when an ion detaches from the particle surface and

migrates through the electrical double layer into the solu-tion. One of the significant effects of dissolution on metal-

based NPs is that it can cause the release of ionic species

that are toxic towards humans and aquatic organisms.119

When nanoparticles undergo dissolution, dispersed col-

loids may enhance the mobility of environmental con-

taminants adsorbed to the colloid surfaces.167 Metal NPleaching in contaminated soil water has been reported totransport in dissolved forms.119 Some metal oxides such asZnO dissolve easily in acids.169 Anions from weak acids

form complexes with Zn2+ ions that will accelerate the

dissolution.71 The dissolution rate increases proportion-ately with the H3O+ concentration.14 Parameters such aspH and particle size have shown to have strong influence

on the dissolution rates.624267172 The rate of dissolution

is considered proportional to particle surface area137 andconsequently NPs should dissolve faster than large-sized

bulk materials, for the same mass, on surface area con-siderations alone. Studies have proven that the smaller theparticle size, the faster the dissolution rate.247182 How-

ever, denser aggregates have smaller overall surface areaand subsequently slows down the dissolution rate.119 This

impedes dissolution by reducing the average equilibriumsolubility of the particle system and by introducing kinetichindrance.19

Nanoparticles may be insoluble or have low solubility in

water but are soluble in other types of solvents. Mukher- jee et al.170 studied the dissolution of TiO2 in ascorbicacid and oxalic acid with an oxide:acid molar ratio of 1:2.

Another type of NP insoluble in water is fullerene which

is only soluble in several organic solvents.95 Nano CuOhave low solubility in water with about 12% of copperions release.18

The solubility of nanoparticles have been measured

using various methods including centrifugal ultrafilters

combined with inductively coupled plasma mass spectrom-etry, or employing the method of isothermal solution satu-ration using temperature as a variable as well as evaluation

via atomic force microscopy by monitoring the changes inparticle morphology and dissolution.6138171172

2.2.5. Nanoparticle Toxicity

Extensive usage of engineered nanomaterials in multiple

applications has brought its existence into the environ-

ment, sparking a great concern from both the scientificcommunity and the public. This has lead ecotoxicity stud-ies to be majorly performed on inorganic nanoparticlessuch as titanium dioxide, copper oxide, zinc oxide and sil-

ver nanoparticles as well as carbon based nanomaterials

(namely fullerenes and carbon nanotubes).18173–177

Ecotoxicity experiments have been demonstratedon various aquatic organisms,178 inverterbrates,79156179

bacteria,29180181 and microalgae.1930153 Several reviews

related to nanoparticle key aspects such as physico-chemistry and the transformations it undergoes includ-ing ecotoxicological impacts on the environment havebeen published within the past few years.27358297 It isknown that the mechanism of the ecotoxicity is still at its

infancy stage and requires further research. To this dateinvestigations have demonstrated that the potential toxic-ity of nanoparticles does not depend on one, but severalcharacteristics of the nanoparticle system. For example,Berg et al.65 hypothesized the cellular viability was influ-enced by both zeta potential and agglomeration state. Ina study by Hsiao and Huang174 nanorod ZnO particleswere found more toxic than the spherical shaped at afixed nanoparticle size and surface area, suggesting thatsize and shape of ZnO NPs influence their cytotoxicity.This clearly indicates that factors such as small particlesize, large surface area, shape and the ability to gener-ate reactive oxygen species play a major role in toxicityof nanoparticles. In addition, based on exposure modeling

studies on ENP effects yielded from textile product andfaçade coatings, the following criteria for the environmenthas been established—(i) indication for hazardous effects,(ii) dissolution in water increases/decreases toxic effects,(iii) fate during wastewater treatment, (iv) stability duringincineration.81

One of the prominent engineered nanoparticles, ZnO,has drawn a huge interest among researchers and beenreported on its behaviour and potential toxicity in theenvironment.9172446 Zinc oxide nanoparticles are knownto exhibit antimicrobial properties, cell selective toxicitytoward potential disease causing cells and photocatalyticability against chemical and biological species. High con-centrations of ZnO nanoparticles in biological wastewa-

ter treatment has the capability to induce the inhibitionof nitrogen and phosphorus removal and increase reactiveoxygen species (ROS) production.73182

The mechanism of ZnO nanotoxocity has been dis-cussed and is basically attributed to the release of zinc ionscaused by ZnO nanoparticle dissolution.23 The dissolutionproperties of ZnO has been associated with higher toxicityof nano ZnO which is most likely related to the physic-ochemical properties of the nanostate with respect to thebulk material.30183 However, there always exists a debatewhether the observed toxicity is caused by the effectsof Zn2+ ions, the non-dissolved ZnO particles or somecombination thereof. Solubilized ZnO nanoparticles haveshown that Zn2+ ions exerts stress on cells and can cause

adverse impacts on different microorganisms25–27

such as Escherichia coli2128 Bacillus subtilis, Streptococcusaureus

and marine algae Dunaliella tertiolecta.2930 Table II sum-marizes the impact of ZnO towards various organisms andmicroorganisms.

2.3. Zinc Oxide

Zinc oxide is categorized as a metal oxide nanoparti-cle which has a bulk, direct band gap of 3.3 eV at




R E V I E

W


Table II. Summary of ecotoxicological studies on nano ZnO on various organisms and microorganisms.

Organism/Microorganism Primary particle size Remark References

RAW 264.7 murine macrophage ∼100±20 nm Low concentration ZnO (LC-ZnO) significantly

generated ROS higher than high concentration

ZnO (HC-ZnO) and exerted cell apoptosis

yielded from the secondary size-effect, sizedependent cellular uptake and ion solubility.

Tripathy et al. (2014)184

Daphnia magna 30 nm Chronic test (of 21 days) was tested to determine

the accumulation of zinc. Toxicological effects

of ZnO is majorly attributed to the dissolved

fraction of free ZnO ions compared to

nanoparticles or aggregates.

Adam et al. (2014)157

E. coli <100 nm The interaction between ZnO NPs with E. coli

yielded ROS formation and was the key

phenomenon for antibacterial effect due to

ZnO NPs.

Dutta et al. (2012)185

E. coli 185 nm Low concentrations of ZnO induced decrease in

cell viability.

Kumar et al. (2011)186

E. coli, P. aeruginosa; S. aureus 10–30 nm The antibacterial effect towards Gram-positive

bacteria was stronger compared to the

Gram-negative.

Premanathan et al. (2011)187

Variety of Gram-positive and Gramnegative bacteria

12–307 nm Wide range of bacterial effects on bothGram-positive and Gram-negative in light UV

enhances ROS generation and antibacterial

effect.

Raghupathi et al. (2011)188

Daphnia magna <200 nm <1000 nm Acutes toxicity tests (48-h) resulted EC50 values

∼1 mg/L (within zinc toxicity range) for

Daphnia magna

Wiench et al. (2009)189

Vibrio fischeri Daphnia magna

Thamnocephalus platyurus

50–70 nm ZnO in all formations (bulk ZnO, nanoZnO and

ZnSO4) were highly toxic due to the solubilized

Zn ions.

Heinlaan et al. (2008)173

B. subtilis; E. coli 6 7 nm 820 nm Under dark conditions, bacterial growth inhibition

was observed.

Adams et al. (2006)78

room temperature with a free exciton binding energy of

60 meV.190191

The morphology of ZnO nanostructures canexist in many forms such as nanobelt and nanolasers,which have spurred an intensive research interest in studieson the ZnO nanostructure synthesis and their applications.The recent demonstrations of ZnO nanorods, nanowiresand nanotubes have shown extensive development in elec-tronic and electical applications.192193 ZnO are highlycrystalline in nature and has a wurtzite crystal structurewith an average particle size of 5.2 nm.194

ZnO has unique properties which are technologicallyimportant for semiconducting materials including opticaltransparency, electric conductivity, piezoelectricity, near-UV emission,195 photocatalytic, electronic, dermatological,photonic,17196–198 antimicrobial activity,199 direct bandgap,

large exciton binding energy, excellent chemical, mechani-cal, thermal stability, and biocompatibility.200–203 ZnO alsopossesses techno-economic attributes making it readily

available with a reasonable cost have ensured its use in anexceedingly wide range of industries.204

Among the large variety of applications include

textile materials,196 commercial suncare products,205

lasers and light-emitting diodes, field effect tran-sistors (FETs), field-emission devices, piezoelectric

nanogenerators, bio-imaging agents, biosensors, in drug

delivery vehicles, in ointments including coatings andpigments.206207 ZnO are also widely used nanomaterialsin commercial products with applications in catalysis, dye-sensitized solar cells, sensors, sunscreens, cosmetics, coat-ings, optics, electronic materials and medicine.28208

Zinc oxide has emerged as one of the most exten-sively used nanomaterials in various fields and commercialapplications such as semiconductor in microelectronics,catalysis, optoelectronic devices, sensors, photovoltaic,

main component varistors, field-effect transistors, bluelight-emitting diodes, ultraviolet laser diodes, solar cells,acousto-electrical devices and detectors, tranduscers, elec-trical and optical devices, preparing solar cells, gassensors, varistors, acoustic, electrostatic dissipative coat-

ings, luminescent devices, and could a promising com-ponent for phosphor for low-voltage luminescence in flatpanel displays. ZnO nanoparticles are used as photocat-

alysts to degrade water pollutants. With its antimicro-bial activity, it is used as an oxide in cosmetic industry.i.e., sunscreens, facial creams, and is an active material

for bioapplication as antimicrobial agents in cholesterolbiosensors, biomedicine, dietary modulators for hydro-lase’s activity, cell imagine, magnetic resonance imaging.




R

E V I E W


It is also used in products such as paints, coatings,ceramics.145178191193209210

2.3.1. Amphoteric Nature of ZnO

ZnO is an amphoteric oxide and can easily dissolve in

both acids and bases.68

Like other oxides of metal, ZnOin the presence of water undergoes hydrolysis creating ahydroxide coating on its surface (≡M–OH). The hydrox-ide surface of these hydrolysed ZnO particles may expe-rience increase in charge from the chemical and physicaladsorption of water molecules. This happens because of the amphoteric nature of the hydroxide surface that canreact with both H+ or OH− ions (Eqs. (1) and (2)).5862

≡M−OH+H+↔≡M−OH+

2 with protons (1)

≡M−OH+OH−↔ M−O−

+H2O

with hydroxides (2)

The pH increase is due to the dissolution phenomenon

that takes place at these pH values. The reactions beganimmediately after immersion of ZnO in deionized waterwith the hydrolysis of the ZnO particle surface that took place as a result of the adsorption of water moleculeswith the simultaneous formation of Zn(OH)2s layer. It isalso established that zinc hydroxide is soluble in water,and becomes more soluble as pH is lowered or increased.The predominant reactions are the dissolution of ZnOs

to Zn2+aq and O2−

aq and surface hydroxylation to Zn(OH)+swhich takes place based on the following Eqs. (3)–(4).

ZnOs ↔ Zn2+aq+O2−

aq metal oxide dissolution (3)

ZnOs+H+

aq ↔ ZnOH+s

metal oxide hydroxylation (4)

As the pH is lowered from pH 7.7 to 6.4, followed bypH 5.5 and 4.3, the following equations are known to beimportant.67

ZnOH+aq ↔ Zn2+aq+OH−

aq (5)

ZnOH2s ↔ ZnOH+aq+OH−

aq (6)

ZnOH2s ↔ ZnOH2aq (7)

ZnOH2s ↔ Zn2+aq+2OH− (8)

O2−aq+2H+

↔H2Oaq (9)

According to Degen and Kosec,62 the series of the fol-lowing reactions that take place in the region 7.7 < pH <

11.6 are:Zn2+

aq+OH−↔ ZnOH+aq (10)

ZnOH+aq+OH−↔ ZnOH2s (11)

ZnOH2s ↔ ZnOH2aq (12)

ZnOH2s+OH−↔ ZnOH−3aq (13)

ZnOH2s+2OH−↔ ZnOH2−

4aq (14)

Fig. 6. Speciation of Zn (II) species existing as Zn2+aq, Zn(OH)+aq,

Zn(OH)2aq, Zn(OH)−3aq and Zn(OH)2−4aq over a range of pH values

at 25 C. Calculations were made by using the MINTEQA2 software

(Developed by Allison Geoscience Consultants Inc. and hydroGeologic

Inc.) which applies the thermodynamic and mass balance equations tosolve geochemical equilibria and calculate the ion speciation/solubility.

In Figure 6, within the pH region of 6.6 < pH < 7.7,

the zinc species that are present in the suspension are

Zn2+aq and Zn(OH)+aq which are in equilibrium with the

surface hydroxide ≡ZnOHs or Zn(OH)2s . At 8 < pH <

8.7, Zn(OH)+aq dominates the suspension. At pH 9, there

is equilibrium between Zn(OH)+aq and Zn(OH)2aq. The

Zn(OH)2aq will be the dominating species with the con-current reduction in the Zn(OH)+aq as it is fully convertedto Zn(OH)2aq at pH 10. When the pH values are higher

than pH 10.5, more zincate ions, Zn(OH)−3aq are formedand as pH increases, Zn(OH)−3aq is gradually transformed

to Zn(OH)2−4 species especially at pH > 11.3.

2.3.2. ZnO Behavior as a Nanoparticle

In aquatic systems, the intrinsic properties of the ZnOnanoparticles in suspension, in particular, the surfaceenergy and nanoparticle stabilization will determine their

fate, transport, behaviour and ecotoxicology.531 Abiotic

factors that affect the mobility and transport of nanoparti-cle are pH, ionic strength, particle surface chemistry, andthe interactions of nanoparticles with other pollutants.7

Factors that influence aggregation or disaggregation are

dissolved NOM, ionic strength, type of electrolyte, aggre-gation state, and pH which can all affect the stability of

nanoparticles in water. Under most environmental con-ditions, small quantities of humic substances can coatother surfaces to give them an overall negative charge

depending on the NPs point of zero charge and solu-

tion pH that result in reduced aggregation through chargestabilization.146 Clearly, dissolved NOM enhances the sta-

bility of Zn NPs in solution as NOM adsorption signifi-cantly increases the electric double layer repulsive energy

and produces a net energy barrier between NPs. However,




R E V I E

W


the stabilizing effect of NOM on NP disaggregation willnot only cause the particle size to decrease and resus-pend but will also enhance the NPs toxicity on aquaticlife as they become more mobile and dispersed in naturalwaters.32178

The pH has a huge influence on ZnO nanoparticlesthat has lead researchers to further investigate its rheolog-ical and electrophoretic properties and dispersion in thepresence of surfactants.58 Pradhan et al.193 reported thatpH influences the ZnO structures between a pH range 5to 10 i.e., disk, rod, spindle or flower-like. Research hasalso been conducted to understand the transport proper-ties under saturated flow conditions in the presence andabsence of stabilizing agents36167211–213 as well as thesynthesis methods that affects the morphology of ZnOstructures.133204 One of the earliest studies on zeta poten-tial and pH was by Logtenberg and Stein64 who discoveredthat zeta potentials are distinctly influenced by changes inacidity and alkalinity of the suspension and the chemisorp-

tion of Cl

−

and K

+

ions. More recent scientific litera-ture also addressed the zeta potential behaviour of ZnOnanoparticles.586265–69 On the other hand there have beena few comprehensive reports on the aggregation behaviourof ZnO.1726 71 Sadowski and Polowczyk 66 reported thatwithout adjusting the pH of the suspension (pH 7.4–7.6),adding cationic surfactants caused a positive increase inzeta potential. In another similar study, specific adsorptionof carbonate ions onto the ZnO NPs caused a shift of thepHPZC to 8.3 as its concentration increases.67 Adsorptionof anionic SDS and propylene glycol coating on ZnO NPswas found to significantly shift the pHPZC to pH 3.26 pHeffect was studied from pH 7 to 11 by Tang et al.58 onthe zeta potential of ZnO nanoparticles with the addition

of cationic polyelectrolyte-polyethylenimine (PEI). Subse-quent to that, Tang et al.69 also explored the effect of adding anionic polyelectrolyte, ammonium polyacrylate(PAA) on the ZnO zeta potential. Other researchers exam-ined the dissolution behaviour of ZnO nanoparticles asa function of pH, ionic strength and addition of naturalorganic matter, its role in the acute or chronic toxicity of aquatic organisms and the chemical etching effect.24 71–73

2.4. Methods and Principles of Nanoparticle

Characterization

A large number of nanoparticle parameters such as size,size distribution, specific surface area, surface charge,degree of agglomeration, shape and chemistry have

been used by previous researchers involving investi-gations such as ecotoxicological,8 32 synthesis of ZnOnanoparticles,196 198 nanocomposites,194 215–217 nanospher-icals and nanobundles,134 nanofilms, nanostructures,218

nanoparticles in the environment,1 100 particlestability38497 and biopolymers.113

Characterization is one of the main challenges toquantify the concentration and changes to the physico-chemical form of these nanoparticles especially fate,

transport and toxicity studies.96219 Several state-of-the-art analytical techniques are able to determine the par-ticle morphology (i.e., transmission electron microscopy,TEM; scanning electron microscopy, SEM; atomic forcemicroscopy, AFM), particle size and particle size distri-

bution (i.e., dynamic light scattering, DLS, nanoparticletracking analysis, NTA), structure (i.e., X-ray diffraction)as well as spectrometry and chromatography techniques(i.e., flow field flow fractionation, FIFFF; size exclusionchromatography, SEC; fluorescence correlation spectrom-etry, FCS).8 123 135 220–223

Identifying different types of ENPs may vary in ana-lytical methods and techniques. In the case of char-acterizing engineered nanoparticles in the environment,no single ‘ideal’ technique is able to perform a com-plete characterization analysis to get an insight of its‘true’ nature in the form of natural colloidal suspen-sion. An overall look can be obtained by combin-ing different instrumental methods.94120121 For example,

small angle X-ray scattering (SAXS) in combina-tion with TEM to provide superior information withregard to shape and size of nanoparticles in disper-sions or powders.224 Techniques for the characteriza-tion of colloid structure and colloid-pollutant interactionshave been developed and applied to environmentalcolloids including FIFFF-ICP-MS, FIFFF-LIBD andCE-MS.46

Many research studies involving laboratory work on the aggregation or dissolution of ENPs simulatingenvironmental aqueous systems utilizes a combination of analytical techniques to evaluate the behaviour of thenanomaterials. This is summarized in a table by Petosaet al.7 enlisting various types of nanoparticles character-

ized with different techniques. It must be noted that thetechniques available for the detection and characteriza-tion may overlap to a great extent as most are based ondifferent fundamental principles.100225 This section aimsto introduce the basic principles, descriptions of meth-ods and some examples for the characterization of ENPsincluding natural colloids. At the end of this section, theprinciples, advantages and limitations of the techniquesdiscussed here are summarized.

2.4.1. Particle Morphology

The size and morphology of nanoparticles have a signif-icant influence on the physical and chemical propertieswhich determines their interaction with the environment

and biological systems.113 Electron microscopy (EM) tech-niques such as TEM, SEM and AFM are among the tech-niques which enable visualization and characterization of nanosized objects. The great resolving power of EM is aresult of the application of an electron beam with a wave-length well below the nm range. Optical microscopy, oper-ating at wavelengths in the range 400–800 nm givesa resolution of ∼200 nm. Sample types such as emul-sions, turbid and clear liquids can be freeze-dried prior




R

E V I E W


to applying electron-microscopy techniques.121 Other elec-tron imaging techniques are atmospheric scanning electronmicroscope (ASEM), WetSEM, FESEM and others.226227

2.4.1.1. Scanning Electron Microscopy. Scanning elec-tron microscopy (SEM) is one of the most popular electronmicroscopic techniques. SEMs today achieve resolutionsof 1 nm by using high-resolution imaging instrumentswhereas ∼3 nm by using conventional instruments.3121

Before an SEM analysis, the nanoparticle solution isfreeze-dried to convert them into dry powder. An alterna-tive method, is when the SEM samples are prepared bydrying several drops of 10 mg/L nanoparticle suspensions(prepared in nanopure water) on a thin slab.84 The sam-ple is then placed on a sample holder followed by coatingwith a conductive metal (gold) using a sputter coater. Thesample is then scanned with a focused fine beam of elec-trons. An image of the sample scanned from the samplesurface with a low-energy beam of electrons (1–30 keV)is created by detecting the electrons scattered off by the

sample. SEM also allows us to estimate the sample compo-sition roughly with energy-dispersive X-ray spectroscopy(EDS).93

2.4.1.2. Transmission Electron Microscopy. TEM isused as an approach to understand the aggregation andbehaviour of ENPs, where the images can provide a lotof information. The breadth of the NP clustering can becalculated using the radial distribution function (RDF), aprobability of measuring a particle at radial distances fromthe centre of the representative sample.228

TEM operates on a different principle as it offers greaterresolution. Four protocols are initially employed to preparethe TEM grids:(1) air-drying a small portion (a droplet) of suspension

directly onto a grid which had previously been covered byan electron-transparent film:(2) fast-freezing a droplet, then subliming away the water;(3) fast-freezing a droplet, then sectioning it and placingthe sections on a grid;(4) adding a droplet of sample to a tiny portion of Nanoplast (a hydrophilic melamine embedding resin),polymerizing the Nanoplast, then sectioning it and placingthen ultrathin sections on a grid.

TEM, with appropriate detectors and software, gives arange of other data such as fractal dimensions, elementalcomposition and chemistry bonding and redox activity.100

Figure 7 displays an example of a TEM image.2.4.1.3. Atomic Force Microscopy. Atomic force

microscopy (AFM) is among the powerful tool for deter-mining surface topography. It is a very high-resolutiontype of scanning probe microscopy that obtains 3D imageswith a resolution of 10 nm.93 The high resolution of AFMis able to discern particles of ∼0.1 nm and has been uti-lized to view directly single atoms, molecules and hardspheres that have dimensions of a few nm (soft objectsare difficult to analyze). AFM images provide informa-tion on the surface structure of biomolecular systems

Fig. 7. TEM image of zinc oxide aggregates formed at pH 7.7 (ini-

tial pH) with dark zones representing dense aggregates and lighter areas

which are aggregate branches. The ZnO individual nanoparticles reveal

both hexagonal-like and circular shapes. Samples were prepared by dry-

ing a drop of ZnO nanoparticle suspension (100 mg/L) on a copper grid at

room temperature for 3 minutes. TEM image was captured using CM12

equipped with analysis Docu Version 3.2 image analysis [Philips Electron

Optics, Eindhoven, Netherlands] [Soft Imaging System GmbH, Munster,Germany].

that is complementary to other established techniques (e.g.,light microscopy and EM, nuclear magnetic resonance andX-ray crystallography).120 AFM is also able to directlyobserve NP aggregation behaviours.6

AFM has the advantage of imaging almost any type of surface, including polymers, ceramics, composites, glassand biological samples, and produces a high-resolution,three-dimensional profile of the surface under study. AFMmeasurements may not always be in accordance with otherelectron microscopy techniques. For example, the same sil-icon nanoparticles were determined at 100 nm by SEM

and 94.2 nm by TEM. However, this was not achieved byAFM which measured it at 50.9 nm.26

In a study by Baalousha,2 AFM was proven useful toconfirm the role of HA in the disaggregation process bydemonstrating the sorption and the formation of surfacecoating of HA at the iron oxide NP aggregates.

2.4.1.4. Environmental Scanning Electron Microscopy.

Environmental scanning electron microscopy (ESEM) is atechnique capable of imaging micron and submicron par-ticles. Generally ESEM has been used to study flocs in

wastewater treatment. Redwood et al.45 applied it to imageand quantify natural organic matter (standard SuwaneeRiver humic acid) to observe humic aggregate structuresas a function of humidity and pH, the first study ESEM

application to natural environmental particles and colloids.Fractal analysis of ESEM images was performed to probethe change in aggregate structure as a function of pH.

ESEM has also served as an important complementarytechnique to other analytical methods. Though ESEM can-not be used to image nonperturbal natural samples, it canevaluate ENPs in foodstuff which does not require anysample preparation enabling its image to be captured inan unperturbed state.121 However, the method is ideal for




R E V I E

W


probing the changes in colloid structure as function of hydration state and has the potential to perform fully quan-titative and nonperturbing analysis of colloidal structure.Poorly and electron dense material such as natural organicsdo not require staining because of the amplication of sig-

nal due to the interactions of the electron beam and vaporwithin the ESEM sample chamber. The resolution is sim-ilar to, but slightly reduced compared with atomic forcemicroscopy (AFM), conventional SEM, and transmissionelectron microscopy (TEM). Ideally, resolution limits areca. 5–10 nm but will likely be lower for nonideal envi-ronmental materials. ESEM also has the unique capabilityof allowing the hydration state to be altered under con-ditions between 0 and 100% humidity through the use of controlled pressure and temperature conditions, allowingthe impacts of dehydration and rehydration processes onorganic structure to be probed.

Theoretically, ESEM is able to overcome the precludingof imaging wet samples in their native state as required

in SEM.93227

A short working distance of a few mm atthe sample stage is at a pressure of 1–10 Torr. Using awater-cooled Peletier stage to choose the correct temper-ature (ca. 2–10 C), the sample can be maintained underliquid water and the water content varied by altering pres-sure or temperature. There is no requirement for stainingpoorly electron dense material such as humic substance.The interactions of the secondary electrons and the watervapor around the sample produces a cascade of ionizedgas atoms, which amplify the signal allowing even naturalorganic material to be imaged.

2.4.2. Nanoparticle Stability

Assessing the stability of nanoparticles in the environ-

ment requires evaluating their ability to aggregate andto interact with other particles.122 Electrophoretic mobil-ity, related to the formation of the electric double layeraround the nanoparticle, is a function of the zeta poten-tial of the nanoparticle. The zeta potential can provideinformation regarding the nature of material encapsulatedwithin the nanocapsule or coated onto the surface.113 Thezeta potential of the nanoparticles can be determinedfrom their electrophoretic mobility according to Smolu-chowski’s equation.84 Since zeta potential is affected bythe nanoparticle surface charge, use of modifying agentsin the mobile-phase buffer can alter the rate of migrationtowards the electrode.135 Laser Doppler velocymetry is theusual technique used to measure the zeta potential. It is

based on the evaluation of the velocity of particles by theshift caused in the interference fringe, which is producedby the intersection of two laser beams. The electrophoreticmobility is then transformed into zeta potential. Most nat-ural colloidal particles have negative zeta potential valuesranging from about −100 to −5 mV. Surface charges pre-vent the agglomeration of nanoparticles in the dispersionsdue to the strong electrostatic repulsions, thereby enhanc-ing the stability of the nanoparticles.

2.4.3. Nanoparticle Structure

2.4.3.1. X-Ray Diffraction. X-ray diffraction (XRD) isa primary tool for investigating the structure of crystallinematerials, from atomic arrangement to crystallite size andimperfections.63184190 XRD also analyzes phase compo-

sition, crystallite size and shape, lattice distortions andfaulting, composition variations, orientation and in situ

structure development of the nanoparticles.196229230 Thebroadening of the peaks in the XRD patterns of solids isattributed to nanoparticle size effects. Usually, the XRDpattern is obtained by illuminating the sample with anX-ray source (Copper K line) with wavelength of 1.54 Åand scanning the diffraction within a certain range at anangle of 2.113196

2.4.3.2. Fourier Transform Infrared Spectroscopy.

Another technique to supplement XRD is Fourier trans-form infrared spectroscopy (FTIR). FTIR is capable to pro-vide information about the structural details of substancesin solution with greater spatial and temporal resolution.

Samples commonly characterized are usually lyophilizednanoparticles in minute quantities. The basic principle thatgoverns is that the bonds and groups of bonds vibrate atcharacteristic frequencies. A molecule that is exposed toinfrared rays absorbs infrared energy at frequencies whichare characteristic to that molecule.

Principally, FTIR analysis is carried out by illuminatingthe sample with a modulated IR beam. The sample trans-mittance and reflectance of the infrared rays at differentfrequencies is translated into an IR absorption plot, whichis then analyzed and matched with known signatures of identified materials in the FTIR library.113 In another studyby Bitenc and Orel,198 FTIR was used to characterize anddetermine the shapes of ZnO at wavebands in a regionfrom 680 up to 300 cm−1. It shows spherical particles wereobtained at 458 cm−1. On the other hand, when ZnO wasmodified with polymethacrylic acid (PMAA), the carboxylgroups, –COO interacted with the surface of ZnO, caus-ing a shift on the absorption bands of the FTIR indicat-ing the formation of poly(zinc methacrylate) complex onthe surface of nano-ZnO. This was due to polymer chainsof PMAA creating covalent bonds onto the nanoparticlesurface.231

2.4.3.3. ATR-FTIR Spectroscopy. Attenuated totalreflectance Fourier transform infrared (ATR-FTIR) spec-troscopy is generally conducted by using a horizontalliquid cell. The sample of NPs is prepared by drying a

suspension in methanol (1 mL) on the ATR element forquantification. The cells are crimp sealed, covered in foilto inhibit photoinduced chemistry. It is then mixed end-over-end on a circular rotater (Cole-Palmer) for at leasttwo hours. Aqueous samples are then extracted so that thedissolved concentration of the sample can be quantifiedwith a HPLC instrument. Wavelengths of the instrumentare adjusted accordingly to the type of samples analyzedand the concentration is then measured.24150




R

E V I E W


The principles of spectrum collection is based on the IR

light which is directed into the crystal element at an angle

that it is totally reflected at each point of incidence inside

the crystal. The evanescent waves, known as the reflec-

tion part of the light, pass beyond the crystal interface and

interacts with the sample, which is placed in close contactwith the crystal. The light ultimately exits the crystal and

is directed to the IR detector, from which a signal is then

directed to the data processing system and then measured

by the analytical instrument.232

Studies applying ATR-FTIR were used to compare the

adsorption of adipic and oxalic acid on the two different

sizes of anatase nanoparticles. It showed significant dif-

ferences in the adsorption bands present for oxalic acid

adsorbed on both nanparticle sizes, 5 nm and 32 nm. This

clearly indicates that different adsorption sites or different

distribution of adsorption sites exists for oxalic acid.150 In

another study, the adsorption of humic acid on ZnO NPs

were investigated and revealed a spectrum of ZnO NPs

with adsorption bands at 1400 and 1586 cm−1 due to car-

boxylate which are assigned to symmetric and asymmetric

stretching modes. In correspondence to CH2 rocking mode

and C–C stretching mode, the adsorption bands were at

1333 and 1022 cm−1 the surface adsorbed acetate.24

2.4.4. Separation Methods

2.4.4.1. Flow Field-Flow Fractionation. Flow field-

flow fractionation (FIFFF) is chromatography-like, flow-

assisted hydrodynamic separation technique that permits

physical separation of small quantities (injected mass in

the ng-ug range) of macromolecules or nanoparticles.1220

Separation is achieved by retention of particles in the FlFF

channel compared to the average flow velocity of the car-rier. Retention is caused by balancing the diffusion move-

ment of the particles by an externally generated field force,

which acts on the particles perpendicular to the carrier-

driven flow through the FFF channel.123 The particles are

accumulated by being pushed towards the back wall, from

which they diffuse back into the channel because of the

concentration gradient. The more diffusive (smaller) par-

ticles migrate farther into the channel at higher flow rates

and thus, elute first. When fractograms are coupled with

absorbance detection, the diffusion coefficients (mode val-

ues) are determined from the fractograms. When coupled

with absorbance detection, the diffusion coefficients (and

their derived sizes) corresponded most closely to a massbased diffusion coefficient, Dw (or diameter dw.100

FIFFF using an F1000 model Universal Fractionator,

Postnova Analytics Europe, Landsberg, Germany, was per-

formed to examine the aggregation process between iron

oxide nanoparticles and standard Suwannee River Humic

Acid (SHRA) under a restricted set of conditions (pH 2–6)

relevant to environmental conditions.1 In another study

using the same model, natural particles from freshwater

sites were determined with size ranges <4.2 nm, 4.2–15.8 nm and 15.8–32.4 nm as accordance to FIFFF theory

(<25 nm).74

2.4.4.2. Size Exclusion Chromatography. Size exclu-

sion chromatography (SEC) is applicable for particles

smaller than 100 nm. A porous packing-material col-umn is required, and unspecific adsorption can cause

unwanted interactions. So it is necessary to add addi-tives to block active sites. SEC combined with detec-

tion techniques (e.g., voltammery, inductively coupled

plasma mass spectrometry (ICP-MS), DLS, multi-angle

laser light scattering (MALLS) can be successfully appliedto characterize nanoparticles.4693 To determine the effec-

tiveness of SEC for isolating NPs, the pore size of thestationary phase must be able to reject those particles

with diameters exceeding the targeted ENPs. However, as

applied ENPs may be presented in functionalized forms,

targeting unmodified ENPs could possibly miss a signif-icant fraction of a more generic ENP group in aquatic

systems. In the case for aquatic matrices analysis, theapplication of SEC requires a pre-treatment step in order

to prevent pore blockage.135

2.4.4.3. Fluorescence Correlation Spectroscopy. Fluo-

rescence Correlation Spectroscopy (FCS) is a technique

that is based on diffusion coefficients where samples such

as fluorescent molecules and particles need to pass througha defined optically, laser illuminated volume. The analysis

process involves an autocorrelation curve that is derivedfrom measured fluorescence intensity due to temporal fluc-

tuations. The translational diffusion of the fluorophore

can be determined from the autocorrelation curve in the

absence of chemical reactions that may affect sample fluo-rescence over the time scale of the measurement. A num-

ber average diffusion coefficient, Dn, is provided for a nearsingle molecule detection. In the case of singly labelled

particles or a weight average diffusion coefficient, Dw, is

provided for conditions of several fluorescent labels are

bound to each nanoparticle.100

In a study by Domingos et al.100 FCS measurements

were performed on a Leica TCS SP5 laser scanning micro-scope using an argon ion laser fluorescence excitation at

488 or 514 nm. Fluorescence emission due to adsorbed

humic substances (TiO2, ZnO) was followed at 500–

530 nm while quantum dots (QD) particle fluorescencewas measured in the range 607–682 nm. Another exam-

ple of an FCS analysis was conducted by Lead et al. 233

to determine the diffusion coefficients of humic and ful-vic acid in order to measure the effects of various envi-

ronmental factors. The advantages of FCS is that it can

be used to measure diffusion coefficients of fluorescentmacromolecules under a wider range of solution condi-

tions, including low concentration (≥0.5 mg/L). The diffu-sion times in solution were measured directly to eliminate

the possibility of interactions with a solid phase. Due to

the fact that only a fraction of humic macromolecules are




R E V I E

W


Table III(a). Summary of advantages and disadvantages of microscopy techniques on nanoparticles.

Technique and parameters Advantages Limitations References

Scanning electronmicroscopy

• Imaging

• Particle morphology• Determine aggregation,dispersion, size, structureand shape of NPs.

Image magnification up to 10 nm.Provides a better overall visual image

of the sample: detailed 3D and

topographical images, versatileinformation collected from differentdetectors.

Easy to operate and more flexible invariety of samples. Sample does notneed to be nearly as thin as withTEM.

Can analyze samples such as largerwear debris particles and distressedmachine surfaces.

Preparation of samples may result inartifacts.

Samples types are limited to solid,

inorganic with small size sufficient tofit inside the vacuum chamber andcan handle moderate vacuumpressure.

Lower resolution than TEM.Samples condition: Dry or will disturb

the efficiency of SEM measurement.Incurs huge cost, large in size, requires

free interference of any possibleelectric, magnetic or vibration.

Farré et al. (2011)93

Peters et al. (2011)120

Gontijo et al., (2004)248

Transmission electronmicroscopy

• Imaging• Determine aggregation,

dispersion, size, structureand shape of NPs.

Powerful in assessing NP structure.Delivers direct images and localinformation on size and shape of nanoparticles.

Greater resolution than SEM.Magnification approximately 10

times that of an SEM (objects assmall as three to 10 Å for TEM.Easy to operate.Allows also physical characterization of

shape, size, structure, fractaldimensions, elemental compositions,chemistry, aggregation and internalstructure (when they are sufficient insize and electron density).

Data accuracy depends on suitablesample preparation rather than correctuse and choice of instrumentation.

Ju-Nam and Lead(2008)3


Domingos et al. (2009)100


Weinberg et al. (2011)135

Pabisch et al. (2012)224

Atomic Force Microscopy• Imaging• Determine aggregation,

dispersion, size, structureand shape of NPs.

Obtains 3D images from the tipmovement with a resolution of 10 nm.

High resolution (∼0.1 nm) is able toview directly single atoms ormolecules with few nm dimensionsand hard spheres.

Wide range of NPs characterization,different media (i.e., ambient air,controlled environments, liquidconditions), any type of surface (i.e.,polymers, ceramics, composites, glassand biological samples).

Elemental composition of a sample canbe gained if AFM is paired withanalytical electron microscopy(AEM).

Structural characterization of proteins,polysaccharides and liposomes.

Can measure lateral distances.

Soft objects are difficult to analyze.Measurements are only possible for

NPs that are sufficiently attached tothe substrate.

Bias could occur due to the preferentialcollection of smaller material(<50 nm).

Alteration of the AFM derived signaldue to the uptake of nanoparticles tothe AFM cantilever.

Lead and Wilkinson(2006)46

Farré et al., (2011)93




Environmental scanningelectron microscopy

• Imaging

Captures NP images in more naturalconditions.

Yields best results for imaging naturalaquatic colloids.

Allows imaging samples under ambientconditions at lower relative humidity.

Does not require any samplepreparation, enabling the specimen tobe visualized in an undisturbed state.

Sample chamber and detector cannotachieve atmospheric pressure (max.10–50 torr).

Has difficulties analyzing smallest NPs.Does not work at ‘real’ atmospheric

pressures which is very important forenvironmental studies.

Ju-Nam and Lead(2008)3

Farré et al., (2011)93


Dudkiewicz et al.(2011)121

Luo et al. (2013)227




R

E V I E W


Table III(b). Summary of advantages and disadvantages of techniques measuring nanoparticle morphology and structure.


X-Ray Diffraction

• Crystalline structureThe XRD technique has a

non-destructive nature.It is more accurate for measuring large

crystalline structures compared to small

ones.

Gontijo et al., (2004)248

Fourier transform infraredspectroscopy

• Surface particle

• Structure

Offers continuous real-timeoperation.

Able to provide information about

structural details of substances in

solution with greater spatial and

temporal resolution compared to

other crystallographic techniques.

Spatial resolution with a synchrotron sourceusing FTIR imaging in transmission is

inherently worse than ATR-FTIR.

Lower sensitivity and lower signal-to-noise

ratio compared to ATR-FTIR.

Need to apply complex mathematical

procedures i.e., Kramers-Kronig

transformation

Sundar et al. (2010)113

Li et al. (2007)232

Doyle (1991)249

Kazarian and Chan (2006)250

Attenuated total

reflectance-fourier

transform infrared

spectroscopy

• Adsorption studies

Requires minimal or no sample

preparation prior to spectral

measurements.

Procedures: easy and time efficient

to obtain ‘pure’ samples.

A powerful tool for analyzing

biomedical samples or substances

with strong infrared absorption

i.e., water. Measurements onlyrequire small sample areas.

Good contact and corresponding

reliable FTIR images by simply

placing the sample directly on the

diamond of ATR crystal.

Spatial resolution achievable with a

synchrotron source by micro

ATR-FTIR with a Ge crystal.

Better signal-to-noise ratio and

increased sensitivity, no

significant spectral distortion.

Lower spatial resolution of about 100 m,

compared with about 20 m achievable

by transmission or reflectance methods.

Sample contact is necessary and this may

damage the condition of the sample’s

surface.

Bian et al., (2011)24

Pettibone et al. (2008)150

Li et al. (2007)232

Kazarian and Chan (2006)250

fluorescent, thus FCS measurements cannot fully represent

humic acid during the brief time (<1 ms) the experiments

were conducted.233

2.4.5. Dynamic Light Scattering

Dynamic light scattering (DLS), also known as photon-

correlation spectroscopy (PCS), is currently the fastest and

most popular method of determining particle size and size

distribution of ENPs in aqueous suspensions.113120 The

major advantage of this instrument is that it can moni-

tor the nanoparticle aggregation behavior in situ over a

wide size range (10 nm–6 m). The zeta potential of

ZnO nanoparticles can be calculated concomitantly from

the electrophoretic mobility measurement according to the

Smoluchowski equation.165 In a capillary cell, the elec-

trophoretic mobility of the sample is measured directlywhen an electric field ( E) is applied. The suspended

charged nanoparticles in the sample are attracted towards

the electrode of opposite charge. As these nanoparticles

move, their velocity (v) is measured and expressed as a

unit field strength ( as their mobility. The electrophoretic

mobility (e is related to the velocity through e = v/E.

The zeta potential ( is then calculated via the Smolu-

chowski equation e = r0/, where r is the static

dielectric constant of the medium (water), 0 is the per-mittivity of free space, and is the dynamic viscosity of

the medium.Depending on the type and model of the analytical

instrument, particle sizes and size distributions are nor-mally operated with a He-Ne laser at a wavelength of 633 nm using back scattered light.1100234 In DLS, theautocorrelation of the temporal fluctuations in scatteredlight intensity due to Brownian motion of particles in adilute suspension is evaluated in order to determine theintensity-weighted average translational diffusion coeffi-cient D of the particles. From the diffusion coefficient, theaverage hydrodynamic radius Rh can then be calculatedusing the Stokes-Einstein equation: Rh = kBT /6D ,where kB is the Boltzmann constant, T is the absolutetemperature, and is the viscosity of the medium. This

method yields a hydrodynamic diameter that is a calcu-lated particle diameter of a sphere that has the same mea-sured motion in the solute as the actual particle.167

DLS has been widely used to estimate the par-ticle size of different nanoparticle suspensions anddetermine the aggregation behavior at different ionicstrengths,2426434770126149235 interactions betweennatural organic matter2414895236 and in colloidal aqueousenvironments.228234237




R E V I E

W


Table III(c). Summary of advantages and disadvantages of separation techniques on nanoparticle.


Flow field-flow fractionation

• Separation and

characterization of

environmental colloids.• Size

High size resolution. Does not

require a stationary phase for

ENP separation.

Wide application in ENP analysis(soil suspensions and colloids in

freshwater and marine water

samples).

Enables further characterization of

NPs after fractionation.

Useful for separating the dissolved

phase metal from the NP metals

(i.e., inorganic NPs) with high

size resolution fractionation

technique (<100 nm).

The possibility of aggregation when sample

preconcentration is required which

contradicts that sample pre-concentration

can promote disaggregation.Requires coupling with other

characterization techniques to quantify

ENPs in natural environment.

Combination with other analytical

instrument will only measure strong (i.e.,

non-labile) complexes due to a

continuous re-equilibration in the

channel.

Ju-Nam and Lead (2008)3

Lead and Wilkinson (2006)46



Size exclusion

chromatography

• Isolation of NPs.

• Size

Applicable for particles <100 nm.

Suitable for NP separation.

Can be a stand alone technique to

characterize nanosize particles

ranging from 3–20 nm.

Requires combination detection methods to

further characterize NPs.

Efficiency depends on pore size of

stationary phase to reject other particles

with bigger diameters than the targeted

NPs.Requires pre-treatment step to prevent

blockage of pores for analysis of aquatic

matrices.

Unspecific adsorption can cause unwated

interactions.

Irreversible adsorption of NPs to stationary

phase is still a challenge.



Weinberg et al., (2011)135

Wu et al., (2011)223

Fluorescence correlation

spectroscopy

• Diffusion coefficients

• Hydrodynamic diameter

Measures diffusion coefficients of

fluorescent macromolecules under

a wide range of solution

conditions i.e., low concentration

(≥0.5 mg/L) and sample volumes

(<10 L).

Directly measures particle number

and signal amplication not

required.

Analyzed by far standards of humic

substances and polysaccharides.

Single-molecule sensitivity.Lead and Wilkinson (2006)46


Multiangle laser light scattering (MALLS) is another

light scattering technique worth mentioning, which is able

to measure root mean square radius and average molecu-

lar weight of both polymers and biopolymers238 including

aggregation kinetics experiments.239

2.4.6. Nanoparticle Tracking Analysis

Nanoparticle tracking analysis (NTA) is the latest tech-

nique developed by NanoSight Ltd. to specifically charac-

terize particles at nanoscale by providing size, count and

concentration measurements. It has a unique capability to

directly size and visualize nano-scale particles in liquid

suspension with high resolution, in real-time with mini-

mal sample preparation for systems that are even com-

plex and polydisperse.240 Individual particle trajectories are

tracked and the mean squared distances that are travelled

by the particles in two dimensions are recorded in order

to determine the number based on diffusion coefficients.The technique is used in conjunction with an ultramicro-scope which permits small particles in liquid suspensionto be observed moving under Brownian motion based on aslight modification to photon-correlation spectroscopy.241

Since the scattering of small particles varies strongly withparticle radius, larger particles can still mask the signalof smaller nanoparticles. However, the effect is less sig-nificant than in DLS. Using a specific computer software,

mean square displacements of single particles are ableto be tracked and recorded while concurrently estimatingtheir hydrodynamic radius, using the Stokes-Einstein equa-tion. The mean size corresponds to the arithmetic averageof all particles sizes while the modal size corresponds tothe most frequently observed particle diameter.98113

The NTA technique has been applied in diversifiedresearch areas related to characterizing protein aggre-gations and drug delivery nanoparticles, metal oxide




R

E V I E W


Table III(d). Summary of advantages and disadvantages of light scattering techniques on nanoparticle.


Dynamic light

scattering/photon-correlation

sprectroscopy

• Particle size distributions• Hydrodynamic diameter

Analyze polydisperse samples.

Provides a more sensitve means of

detecting large aggregates.

Nanoparticles aggregation behaviourcan be monitored in situ over a

wide size range (10 nm–6 m).

Accurate and simple to perform.

Frequently used for in situ NP

sizing and physical

characterization of aggregated

NPs.

The signal is dominated by the

larger, more intensely scattering

particles, and the result is an

intensity weighted mean size.

Results may be influenced by the presence

of dust or agglomerated fractions in a

sample.

Capital and running costs.Analysis turnaround time.

Limited ability to resolve particle size

distribution profiles.

Large ensemble of particles are analyzed

and only a z-average particle mean is

obtained.

In some cases, relatively small number of

highly scattering larger particles (e.g.,

aggregates or contaminants) masks the

presence of the bulk of smaller particles,

leading to a limitation of intensity

weighted, z-average.



Pabisch et al. (2012)224

Mohd Omar et al. (2014)243

Gillespie et al. (2011)246

Carr and Malloy (2007)251

Nanoparticle Tracking Analysis

• Particle size distributions

• Number of particles

Requires minimal sample

preparation.

Applicable for complex and

polydisperse systems.Delivers insight of realtime, true

size distributions and state of

aggregation.

High resolution particle sizing of

individual particle.

Measurement is based on

concentration and particle count.

Minimum size to 10 nm, and

material dependent.

Each particle size class is given a

number versus intensity.

Unaffected from occasional

presence of an anomalous dust of

aggregate.

Possible to generate particle size

distribution profiles that reflect

the actual number of particlesseen on camera.

Concentration of particles present

and their size distribution can be

estimated based on the tracking

of individual particles.

Sensitive to ambient vibrations.

Sample concentrations should be within

105–1010 particles/ml for effective

measurement.Larger size particles become increasing

difficult to measure accurately.

Particles approaching or exceeding 600 nm

will restrict the Brownian motion and

hence reduce the measurement accuracy.

Malloy (2011)240

Walker (2012)241

Filipe et al. (2010)242

Mohd Omar et al. (2014)243

Gillespie et al. (2011)246

James and Driskell (2013)247

Carr and Malloy (2007)251

nanoparticle size distribution and aggregation behaviourof catalyst nanoparticles in proton exchange membrane

fuel cells including virus particle determination.242–244 TheNTA technique has also demonstrated as a new, feasi-ble analytical instrument by furnishing additional infor-

mation not offered by DLS in other studies related to

growth of particles of a poorly water soluble drug andbio-conjugation of gold nanoparticles withal characteriz-ing protein-protein interactions.240245–247 In respect to theNTA-based methods that have been developed, NTA pro-vides a more accurate determination in many novel aspectsfor nanoparticle analysis.

All the techniques mentioned above are summarized inthe Tables III(a)–(d) including principle mechanism, meth-ods, limits and advantages.

2.5. Challenges of Nanoparticle Characterization inthe Environment

Much is known about the diversity of engineered nanopar-

ticles n the environment but their characterization and

detection in the natural aquatic environment are incom-

pletely understood. We currently lack methods that

are able to directly collect or obtain data on occur-

rence levels, fate and transport of ENPs in aquatic

systems.4694120135227 Sequential steps need to be taken to

gain a better understanding of the nanomaterial domain.

Prior to analyzing the ENPs as it is, an evaluation of

(i) the matrix of source materials;

(ii) their transformation in the natural aquatic environment

and




R E V I E

W


(iii) their physical/chemical behavior that is specific to thewater medium are required beforehand.

ENP exclusive properties are easily affected by the pro-cess of sampling, extraction and quantification.177 Oneobstacle to overcome is the ability to detect and to quan-

tify NPs in the environment, to distinguish natural fromengineered products contained in the same sample andto differentiate between the toxicity of these materials.35

This remains as one of the most critical priorities for theadvancement of the nanomaterial industry as there is no

simple method to characterize the exact molecular struc-ture of an unknown NP pollutant in a complex environ-mental matrix (e.g., water).226

Although efforts have been exhausted to analyze the

ENPs without perturbing their natural existence, for exam-ple by Tiede et al.252 who applied WetSEM on environ-mental samples for the first time by retaining the aqueoussamples in its liquid form, there were still limitations.

Another example of a first time application was attempted

by Jarvie and King99 who applied small-angle neutronscattering (SANS) to quantitatively analyze the nano-structure and length scales in natural freshwater aquatic

colloidal dispersion from a number of different samples.SANS was found to be an appropriate assessment tech-nique of natural aquatic colloidal samples with minimalperturbation.

The reasonable step to conduct analysis would beinvolving multiple instruments in quantifying the nanopar-ticles in aqueous environmental samples accurately. Some

samples containing NPs are not initially separated from

larger particles, and depend on the analytical methods.The requirement of individual component analysis in thesample may need to be size fractionated which couldbe destructive towards the original state of the sample.

The inability to determine the presence and/or chemicalstructure of ENPs accurately severely hinders efforts toevolve treatment technologies that use them or to investi-gate the impact of these materials once they are released

in an uncontrolled environment.135 For example, in astudy on the fate of Al2O3 in a fresh water microcosm,replicating a natural lake water ecosystem, a combina-tion of analytical techniques was applied. X-ray diffrac-

tion analysis to analyze the crystalline properties of Al 2O3

nanoparticles, TEM analysis to evaluate the size and shape,DLS to measure the hydrodynamic size distribution andmean hydrodynamic diameter while aggregation of par-

ticles were tested using the DLS method.178237

HoweverMajedi et al.228 and his group attested another time andcost effective approach by employing orthogonal arraydesign (OAD). OAD is a multiparametric technique to

evaluate optimal conditions to investigate environmentalinteractions among water chemistries (i.e., organic acidtype, organic acid concentration, NP concentration, pH,salt content, and electrolyte type) and their effects on ZnO

aggregation behaviour.

3. CONCLUSION

This review discusses the nanoparticle physicochemicalproperties by adopting ZnO as an example, to obtain anindepth sight on some possible uncertainties that lack fun-damental knowledge for a deeper understanding of ENPs

environmental fate and behaviour. It is evident the drivingforce that governs ENP behaviour is generally controlledby their surface properties and formation of corona in pres-ence of aquagenic compounds. However, there is still awhole new domain to explore each nanoparticle type, invarious conditions with many factors to consider.

Individual analytical methods on ENPs elaborated ear-lier were more distinct on their principle mechanism,advantages and limitations in respect to their physico-chemical properties. It should be noted as well that theanalytical methods of evaluating ENP entities in com-

plex environmental media particularly in aqueous systems,which we currently are lacking, is gaining a huge inter-est among researchers. The consensus of reviews conclude

that due to the complexity of ENPs and their occurrence,fate and behaviour in the environment, it is unlikely toapply a single suitable technique in order to determinecertain physico-chemical characteristics. There are a num-ber of analytical parameters which require equal atten-tion in order to represent the in-situ state of the ENPsin the aqueous environment. Among them include surfacearea, surface charge, size distribution, particle morphologyand/or the mineral phase of the ENPs. Surrounding exter-nal factors that exist in aquatic systems may dictate theinteractions between the ENPs and natural water compo-nents. Though efforts have been made to analyze the ENPswith minimal perturbance, there are still limitations whichrequire detailed attention for future work. For example,

to address the need for reproductible and accurate exper-iments, a large amount of work and effort is still neededon the development of relevant dispersion protocols forcomplex powdered samples.

ACRONYMS/ABBREVIATIONS

AFM Atomic force microscopyATR-FTIR Attenuated total reflectance Fourier trans-

form infraredASEM Atmospheric scanning electron microscopy

DLS Dynamic light scattering

DLVO Derjaguin-Landau-Verwey-Overbeek EDL Electric double layer

EM Electron microscopyENP Engineered nanoparticle

ESEM Environmental scanning electronmicroscopy

FCS Fluorescence correlation spectroscopyFET Field effect transistors

FIFFF Flow field-flow fractionationFTIR Fourier transform infrared red

HA Humic acid




R

E V I E W


HS Humic substanceIEP Isoelectric point

MALLS Multi-angle laser light scatteringNOM Natural organic matter

NP Nanoparticle

NTA Nanoparticle tracking analysisOAD Orthogonal array design

PAH Polycyclic aromatic hydrocarbons

PCS Photo-correlation spectroscopyPEMFC Proton exchange membrane fuel cell

PZC Point of zero charge

RDF Radial distribution functionSC Surface charge

SEC Size-exclusion chromatographySEM Scanning electron microscopy

SRHA Suwannee River Humic Acid

SSA Specific surface areaTEM Transmission electron microscopy

vdW Van der Waals

WetSEM Wet scanning electron microscopyXRD X-ray diffraction.

Acknowledgments: The authors gratefully acknowl-

edges Arnaud Clavier for his assistance and contribu-

tion to the content of this publication. The authors alsoacknowledge the research support received from the Inter-

national Foundation of Science (Sweden) in part of theIFS programme Individual Research Approach (Grant no.W/5334) for the School of Civil Engineering, Universiti

Sains Malaysia. This work is additionally supported by theSwiss National Foundation (200021_135240).

References and Notes1. M. Baalousha, A. Manciulea, S. Cumberland, K. Kendall, and J. R.

Lead, Environ. Toxicol. Chem. 27 (2008).

2. M. Baalousha, Sci. Total Environ. 407 (2009).

3. Y. Ju-Nam and J. R. Lead, Sci. Total Environ. 400 (2008).

4. T. P. Dasari and H. M. Hwang, Sci. Total Environ. 408 (2010).

5. A. Weir, P. Westerhoff, L. Fabricius, K. Hristovski, and N. von

Goetz, Environ. Sci. Technol. 46 (2012).

6. R. D. Kent and P. J. Vikesland, Environ. Sci. Technol. 46 (2012).

7. A. R. Petosa, D. P. Jaisi, I. R. Quevedo, M. Elimelech, and

N. Tufenkji, Environ. Sci. Technol. 44 (2010).

8. J. Jiang, G. Oberdörster, and P. Biswas, J. Nanopart. Res. 11

(2009).

9. F. M. Omar, H. A. Aziz, and S. Stoll, Sci. Total Environ. 468

(2014).

10. R. Molina, Y. Al-Salama, K. Jurkschat, P. J. Dobson, and I. P.

Thompson, Chemosphere 83 (2011).

11. J. Labille and J. Brant, Nanomedicine 5 (2010).

12. B. Nowack and T. D. Bucheli, Environ. Pollut. 150 (2007).

13. S. S. Alias, A. B. Ismail, and A. A. Mohamad, J. Alloy Compd.

499 (2010).

14. H. Gerischer and N. Sorg, Electrochim. Acta 37 (1992).

15. C. He, T. Sasaki, H. Usui, Y. Shimizu, and N. Koshizaki, J. Pho-

toch. Photobio. A 191 (2007).

16. M. Jitianu and D. V. Goia, J. Colloid Interf. Sci. 309 (2007).

17. D. Zhou and A. A. Keller, Water Res. 44 (2010).

18. I. Blinova, A. Ivask, M. Heinlaan, M. Mortimer, and A. Kahru,

Environ. Pollut. 158 (2010).

19. N. M. Franklin, N. J. Rogers, S. C. Apte, G. E. Batley, G. E. Gadd,

and P. S. Casey, Environ. Sci. Technol. 41 (2007).

20. E. J. Joner, T. Hartnik, and C. E. Amundsen, Norwegian Pollution

Control Authority Report. TA-2304/2007 (2007).

21. H. Ma, P. L. Williams, and S. A. Diamond, Environ. Pollut. 172(2013).

22. M. N. Moore, Environ. Inter. 32 (2006).

23. A. B. Djurišic, X. Chen, Y. H. Leung, and A. M. C. Ng, J. Mater.

Chem. 22 (2012).

24. S. W. Bian, I. A. Mudunkotuwa, T. Rupasinghe, and V. H. Grassian,

Langmuir 27 (2011).

25. W. Bai, Z. Zhang, W. Tian, X. He, Y. Ma, Y. Zhao, and Z. Chai,

J. Nanopart. Res. 12 (2010).

26. G. Liu, D. Wang, J. Wang, and C. Mendoza, Sci. Total Environ.

409 (2011).

27. N. R. von Moos and V. I. Slaveykova, Nanotoxicology 8 (2014).

28. M. Li, D. Lin, and L. Zhu, Environ. Pollut. 173 (2013).

29. Y. W. Baek and Y. J. An, Sci. Total Environ. 409 (2011).

30. S. Manzo, M. L. Miglietta, G. Rametta, S. Buono, and G. D.

Francia, Sci. Total Environ. 445 (2013).

31. P. Christian, F. von der Kammer, M. Baalousha, and Th. Hofmann,

Ecotoxicology 17 (2008).32. R. Brayner, S. A. Dahaumane, C. Yéprémian, C. Djediat, M. Meyer,

A. Couté, and F. Fiévet, Langmuir 26 (2010).

33. M. Baalousha, P. Le Coustumer, I. J ones, and J. R. Lead, Environ.

Chem. 7 (2010).

34. S. M. King and H. P. Jarvie, Environ. Sci. Technol. 46 (2012).

35. R. D. Handy, F. von der Kammer, J. R. Lead, M. Hassellöv,

R. Owen, and M. Crane, Exotoxicology 17 (2008).

36. S. R. Kanel and S. R. Al-Abed, J. Nanopart. Res. 13 (2011).

37. B. C. Lee, S. Kim, H. K. Shon, S. Vigneswaran, S. D. Kim, J. Cho,

I. S. Kim, K. H. Choi, J. B. Kim, H. J. Park, and J. H. Kim,


38. F. Claret, T. Schäfer, A. Bauer, and G. Buckau, Sci. Total Environ.

317 (2003).

39. G. V. Lowry, K. B. Gregory, S. C. Apte, and J. R. Lead, Environ.

Sci. Technol. 46 (2012).

40. T. J. Manning, T. Bennett, and D. Milton, Sci. Total Environ. 257(2000).

41. D. Palomino and S. Stoll, J. Nanopart. Res. 15 (2013).

42. K. Schmeide, S. Sachs, and G. Bernhard, Sci. Total Environ. 419

(2012).

43. K. L. Chen, S. E. Mylon, and M. Elimelech, Environ. Sci. Technol.

40 (2006).

44. M. Baalousha, M. Motelica-Heino, and P. Le Coustumer, Colloids

and Surfaces A 272 (2006).

45. P. S. Redwood, J. R. Lead, R. M. Harrison, I. P. Jones, and S. Stoll,

Environ. Sci. Technol. 39 (2005).

46. J. R. Lead and K. J. Wilkinson, Environ. Chem. 3 (2006).

47. K. L. Chen and M. Elimelech, J. Colloid Interf. Sci. 309 (2007).

48. F. Loosli and S. Stoll, J. Colloid Sci. Biotechnol. 1 (2012).

49. J. Buffle, K. J. Wilkinson, S. Stoll, M. Filella, and J. Zhang, Envi-

ron. Sci. Technol. 32 (1998).

50. S. Stoll and J. Buffle, J. Colloid Interf. Sci. 180 (1996).

51. A. J. de Kerchove and M. Elimelech, Macromolecules 39 (2006).

52. W. R. Gombotz and S. F. Wee, Adv. Drug Deliv. Rev. 31 (1998).

53. M. George and T. E. Abraham, J. Controlled Release 114 (2006).

54. K. L. Chen, S. E. Mylon, and M. Elimelech, Langmuir 23 (2007).

55. N. Bhattarai and M. Zhang, Nanotechnology 18 (2007).

56. B. Espinasse, E. M. Hotze, and M. R. Wiesner, Environ. Sci. Tech-

nol. 41 (2007).

57. J. Zhai, X. Tao, Y. Pu, X.-F. Zeng, and J.-F. Chen, Appl. Surf. Sci.

257 (2010).

58. F. Tang, T. Uchikoshi, and Y. Sakka, J. Am. Ceram. Soc. 85 (2002).




R E V I E

W


59. D. M. Yebra, S. Kiil, C. E. Weinell, and K. Dam-Johansen, Prog.

Org. Coat. 56 (2006).

60. S. Fardad, R. Massudi, A. Manteghi, and M. A. Amini, Int. Conf.

Nanotech. (2007).

61. M. Nakade and M. Ogawa, J. Mater. Sci. 42 (2007).

62. A. Degen and M. Kosec, J. Eur. Ceram. Soc. 20 (2000).

63. R. Y. Hong, J. Z. Qian, and J. X. Cao, Powder Technol. 163 (2006).

64. E. H. P. Logtenberg and H. N. Stein, Colloids Surf. 17 (1986).

65. J. M. Berg, A. Romoser, N. Banerjee, R. Zebda, and C. M. Sayes,

Nanotoxicology 3 (2009).

66. Z. Sadowski and I. Polowczyk, Int. J. Miner. Process. 74 (2004).

67. A. Sedlak and W. Janusz, Phys. Prob. Miner. Process. 42 (2008).

68. K. S. Suganthi and K. S. Rajan, Int. J. Heat Mass Transfer 55

(2012).

69. F. Tang, Y. Sakka, and T. Uchikoshi, Mater. Res. Bull. 38 (2003).

70. W. S. Liu, Y. H. Peng, C. E. Shiung and Y. H. Shih, J. Nanopart.

Res. 14 (2012).

71. R. A. T. P. Rupasinghe, Theses and Disertations, University of Iowa

(2011).

72. E. A. Meulenkamp, J. Phys. Chem. B 102 (1998).

73. T. Xia, M. Kovochich, M. Liong, L. Mädler, B. Gilbert, H. Shi,

J. I. Yeh, J. I. Zink, and A. E. Nel, Am. Chem. Soc. 2 (2008).

74. M. Baalousha and J. R. Lead, Sci. Total Environ. 386 (2007).

75. T. M. Benn and P. Westerhoff, Environ. Sci. Technol. 42 (2008).76. E. M. Hotze, T. Phenrat, and G. V. Lowry, J. Environ. Qual. 39

(2010).

77. N. C. Mueller and B. Nowack, Environ. Sci. Technol. 42 (2008).

78. L. K. Adams, D. Y. Lyon, and P. J. J. Alvarez, Water Res. 40

(2006).

79. J. Cheng, E. Flahaut, and S. H. Cheng, Environ. Toxicol. Chem. 26

(2007).

80. A. Kahru, H. C. Dubourguier, I. Blinova, A. Ivask, and K.

Kasemets, Sensors 8 (2008).

81. C. Som, P. Wick, H. Krug, and B. Nowack, Environ. Inter. 37

(2011).

82. C. Levard, E. M. Hotze, G. V. Lowry, and G. E. Brown Jr., Environ.

Sci. Technol. 46 (2012).

83. C. Priadi, S. Ayrault, S. Pacini, and P. Bonte, Int. J. Environ. Sci.

Technol. 8 (2011).

84. Y. Zhang, Y. Chen, P. Westerhoff, K. Hristovski, and J. C.

Crittenden, Water Res. 42 (2008).85. C. M. Chou and H. L. Lien, J. Nanopart. Res. 13 (2011).

86. J. Hu, G. Chen, and I. M. C. Lo, J. Environ. Eng. 132 (2006).

87. L. K. Limbach, R. Bereiter, E. Muller, R. Krebs, R. Galli, and W. J.

Stark, Environ. Sci. Technol. 42 (2008).

88. P. Westerhoff, G. Song, K. Hristovski, and M. A. Kiser, J. Environ.

Monit. 13 (2011).

89. Q. Zhang, B. Pan, S. Zhang, J. Wang, W. Zhang, and L. Lv,


90. S. J. Klaine, P. J. J. Alvarez, G. E. Batley, T. F. Fernandes, R. D.

Handy, D. Y. Lyon, S. Mahendra, M. J. McLaughlin, and J. R.

Lead, Environ. Toxicol. Chem. 27 (2008).

91. K. Schmid and M. Riediker, Environ. Sci. Technol. 42 (2008).

92. N. Akaighe, R. I. Maccuspie, D. A. Navarro, D. Saga, S. Banerjee,

M. Sohn, and V. K. Sharma, Sci. Total Environ. 441 (2012).

93. M. Farré, J. Sachis, and D. Barcelo, Anal. Chem. 30 (2011).

94. N. S. Wigginton, K. L. Haus, and M. F. Hochella Jr., J. Environ. Monit. 9 (2007).

95. K. L. Chen and M. Elimelech, Environ. Sci. Technol. 42 (2008).

96. J. Xu, C. Liu, and Z. Wu, J. Solid State Electrochem. 16 (2012).

97. I. Chowdhury, Y. Hong, and S. L. Walker, Colloids Surf. A. 368

(2010).

98. C. Marambio-Jones and E. M. V. Hoek, J. Nanopart. Res. 12

(2010).

99. H. P. Jarvie and S. M. King, Environ. Sci. Technol. 41 (2007).

100. R. F. Domingos, N. Tufenkji, and K. J. Wilkinson, Environ. Sci.

Technol. 43 (2009).

101. P. Kumar, K. P. Sandeep, S. Alavi, V. D. Truong, and R. E. Gorga,

J. Food Eng. 100 (2010).

102. D. A. M. Abd-El-Haleem, M. A. AlMa’adeed, and N. Al-Thani,

Global J. Environ. Res. 1 (2007).

103. B. B. Mandal, A. S. Priya, and S. C. Kundu, Acta Biomater. 5

(2009).

104. R. B. Garcia, J. L. M. S. Ganter, and R. R. Carvalho, Eur. Poly. J.

36 (2000).

105. R. A. de Freitas, M. F. Drenski, A. M. Alb, and W. F. Reed, Mater.

Sci. Eng., C. 30 (2010).

106. E. E. Oliveira, A. E. Silva, T. N. Junior, M. C. S. Gomes, L. M.

Aguiar, H. R. Marcelino, I. B. Ajaujo, M. P. Bayer, N. M. P. S.

Ricardo, A. G. Oliveira, and E. S. T. Egito, Bioresour. Technol. 101

(2010).

107. J. R. Pan, C. Huang, S. Chen, and Y.-C. Chung, Colloids Surf. A

147 (1999).

108. F. Renault, B. Sancey, P.-M. Badot, and G. Crini, Eur. Polym. J. 45

(2009).

109. P. K. Singh, B. Bhattacharya, R. K. Nagarale, K.-W. Kim, and

H.-W. Rhee, Eur. Polym. J. 45 (2010).

110. X. Wang, B. Liu, J. Ren, C. Liu, X. Wang, J. Wu, and R. Sun,

Compos. Sci. Technol. 70 (2010).

111. S. Baskoutas, P. Giabouranis, S. N. Yannapoulos, V. Dracopoulos,

L. Toth, A. Chrissanthopoulos, and N. Bouropoulos, Thin Solid Films 515 (2007).

112. K. T. Shalumon, K. H. Anulekha, S. V. Nair, S. V. Nair, K. P.

Chennazhi, and R. Jayakumar, Int. J. Biol. Macromol. 49 (2011).

113. S. Sundar, J. Kundu, and S. C. Kundu, Sci. Technol. Adv. Mater. 11

(2010).

114. C. V. Liew, L. W. Chan, A. L. Ching, and P. W. S. Heng, Int. J.

Pharm. 309 (2006).

115. L. V. Trandafilovic, D. K. Bozanic, S. Dimitrijevic-Brankovic, A. S.

Luyt, and V. Djokovic, Carbohydr. Polym. 88 (2012).

116. K. Y. Lee and D. J. Mooney, Prog. Polym. Sci. 37 (2012).

117. C. Y. Yu, H. Wei, Q. Zhang, X. Z. Zhang, S. X. Cheng, and R. X.

Zhuo, J. Phys. Chem. Lett. B 113 (2009).

118. M. Thirumavalavan, K.-L. Huang, and J.-F. Lee, Colloids Surf. A

41 (2013).

119. P. S. Tourinho, C. A. M. van Gestel, S. Lofts, C. Svendsen, A. M.

V. M. Soares, and S. Loureiro, Environ. Toxicol. Chem. 31 (2012).

120. R. Peters, G. Ten Dam, H. Bouwmeester, H. Helsper, G. Allmaier,

F. vd Kammer, R. Ramsch, C. Solans, M. Tomaviova, J. Hajslova,

and S. Weigel, Trends Anal. Chem. 30 (2011).

121. A. Dudkiewicz, K. Tiede, K. Loeschner, L. H. S. J. Jensen,

E. Jensen, R. Wierzbicki, A. B. A. Boxall, and K. Molhave, Trends

Anal. Chem. 30 (2011).

122. B. F. da Silva, S. Perez, P. Gardinalli, R. K. Singhal, A. A. Mozeto,

and D. Barcelo, Trends Anal. Chem. 30 (2011).

123. F. von der Kammer, S. Legros, E. H. Larsen, K. Loeschner, and

T. Hofmann, Trends Anal. Chem. 30 (2011).

124. K. L. Chen and M. Elimelech, Environ. Sci. Technol. 43 (2009).

125. S. I. Pechanyuk, Russ. Chem. Bull. 48 (1999).

126. A. Tiraferri, K. L. Chen, R. Sethi, and M. Elimelech, J. Colloid

Interf. Sci. 324 (2008).

127. A. M. El Badawy, T. P. Luxton, B. G. Silva, K. G. Scheckel, M. T.

Suidan, and T. M. Tolaymat, Environ. Sci. Technol. 44 (2010).

128. J. Zhang, G. Dong, A. Thurber, Y. Hou, M. Gu, D. A. Tenne, C. B.Hanna, and A. Punnoose, Adv. Mater. 24 (2012).

129. K. Bourikas, T. Hiemstra, and W. H. V. Riemsdijk, Langmuir 17

(2001).

130. Malvern Instruments Ltd., Zetasizer Nano Series Technical Note,

MRK654-01, Worcesteshire, UK.

131. K. Bourikas, C. Kordulis, and A. Lycourghiotis, Environ. Sci. Tech-

nol. 39 (2005).

132. A. Testino, I. R. Bellobono, V. Buscaglia, C. Canevali,

M. D’Arienzo, S. Polizzi, R. Scotti, and F. Morazanni, J. Am. Soc.

129 (2007).




R

E V I E W


133. J. W. Shim, J. W. Kim, S. H. Han, I. S. Chang, H. K. Kim, H.-H.Kang, O. S. Lee, and K. D. Suh, Colloids Surf. A 207 (2002).

134. Z. L. Wang, J. Phys.: Condens. Matter. 16 (2004).135. H. Weinberg, A. Galyean, and M. Leopold, Trends Anal. Chem. 30

(2011).136. T. M. Scown, R. van Aerle, and C. R Tyler, Crit. Rev. Toxicol. 40

(2010).137. A-.J. Miao, X.-Y. Zhang, Z. Luo, C.-S. Chen, W-.C. Chin, P. H.Santschi, and A. Quigg, Environ. Toxicol. Chem. 29 (2010).

138. J. M. Unrine, B. P. Colman, A. J. Bone, A. P. Gondikas, and C. W.Matson, Environ. Sci. Technol. 46 (2012).

139. R. A. French, A. R. Jacobson, B. Kim, S. L. Isley, R. L. Penn, andP. C. Baveye, Environ. Sci. Technol. 43 (2009).

140. E. Illès and E. Tombácz, J. Colloid Surf. Sci. 295 (2006).141. L. C. Kai and M. Elimelech, J. Colloid Interf. Sci. 309 (2007).142. D. Jassby, J. F. Budarz, and M. Wiesner, Environ. Sci. Technol. 46

(2012).143. X. Jiang, M. Tong, H. Li, and K. Yang, 350 (2010).144. A. Jasper, H. H. Salih, G. A. Sorial, R. Sinha, R. Krishnan, and

C. L. Patterson, Environ. Eng. Sci. 27 (2010).145. A. Nelson, S. W. Depner, S. Banerjee, V. K. Sharma, and M. Sohn,

Sci. Total Environ. 441 (2012).146. M. Seijo, S. Ulrich, M. Filella, J. Buffle, and S. Stoll, Environ. Sci.

Technol. 43 (2009).147. S. Ma, L. Changli, K. Yang, and D. Lin, Sci. Total Environ. 439

(2012).148. E. B. Ö Güngör, and M. Bekbölet, Geoderma. 159 (2010).149. J. D. Hu, Y. Zevi, X. M. Kou, J. Xiao, X. J. Wang, and Y. Jin, Sci.

Total Environ. 408 (2010).150. J. M. Pettibone, D. M. Cwiertny, M. Scherer, and V. H. Grassian,

Langmuir 24 (2008).151. J. R. Lead, D. Muirhead, and C. T. Gibson, Envriron. Sci. Technol.

39 (2005).152. K. L. Chen, Thesis and dissertation, Yale University (2008).153. V. Aruoja, H.-C. Dubourguier, K. Kasemets, and A. Kahru, Sci.

Total Environ. 407 (2009).154. Y. Teow, P. V. Asharani, M. P. Hande, and S. Valiyaveettil, Chem.

Commun. 47 (2011).155. M. Ates, J. Daniels, Z. Arslan, and I. O. Farah, Environ. Monit.

Assess. 18 (2013).156. A. Baun, N. B. Hartmann, K. Grieger, and K. O. Kusk, Ecotoxi-

cology 17 (2008).157. N. Adam, C. Schmitt, J. Galceran, E. Companys, A. Vakurov,

R. Wallace, D. Knapen, and R. Blust, Nanotoxicology. 8 (2014).158. J. R. Peralta-Videa, L. Zhao, M. L. Lopez-Moreno, G. de la Rosa,

J. Hong, and J. L. Gardea-Torresdey, J. Hazard. Mater. 186 (2011).159. J. Brant, H. Lecoanet, and M. R. Wiesner, J. Nanopart. Res. 7

(2005).160. J. Gao, S. Youn, A. Hovsepyan, V. L. Llaneza, Y. Wang, G. Bitton,

and J.-C. J. Bonzongo, Environ. Sci. Technol. 43 (2009).161. N. Maximova and O. Dahl, Curr. Opin. Colloid Interface Sci. 11

(2006).162. X. Liu, M. Wazne, T. Chou, R. Xiao, and S. Xu, Water Res. 45

(2011).163. J. Zhang and J. Buffle, J. Colloid Interf. Sci. 174 (1995).164. N. Dutta and D. Green, Langmuir 24 (2008).165. C. P. Tso, C. M. Zhung, Y. H. Shih, Y. M. Tseng, S. C. Wu, and

R. A. Doong, Water Sci. Technol. 61 (2010).166. S. K. Misra, A. Dybowska, D. Berhanu, S. N. Luoma, and

E. Valsami-Jones, Sci. Total Environ. 438 (2012).167. L. Denaix, R. M. Semlali, and F. Douay, Environ. Pollut. 113

(2001).168. P. Borm, F. C. Klaessig, T. D. Landry, B. Moudgil, J. Pauluhn,

K. Thomas, R. Trottier, and S. Wood, Toxicol. Sci. 90 (2006).169. O. Fruhwirth, G. W. Herzog, and J. Poulios, Surf. Technol. 24

(1985).

170. A. Mukherjee, A. M. Raichur, and J. M. Modak, Chemosphere 61(2005).

171. A. I. Chen, D. Xu, X. Y. Chen, W. Y. Zhang, and X. H. Liu, Trans.

Nonferrous Metals Soc. China 22 (2012).172. R. B. Reed, D. A. Ladner, C. P. Higgins, P. Westerhoff, and J. F.

Ranville, Environ. Toxicol. Chem. 31 (2012).173. M. Heinlaan, A. Ivask, I. Blinova, H. C. Dubourguier, and

A. Kahru, Chemosphere 71 (2008).174. I. L. Hsiao and Y. J. Huang, Sci. Total Environ. 409 (2011).175. H. Wang, R. L. Wick, and B. Xing, Environ. Pollut. 157 (2009).176. D. Xiong, T. Fang, L. Yu, X. Sima, and W. Zhu, Sci. Total Environ.

409 (2011).177. S. Perez, M. L. Farré, and D. Barceló, Trends Anal. Chem. 28

(2009).178. K. Van Hoecke, K. A. C. De Schamphelaere, P. V. der Meeren,

G. Smagghe, and C. R. Janssen, Environ. Pollut. 159 (2011).179. Z. Pipan-Tkalec, D. Drobne, A. Jemec, T. Romih, P. Zidar, and

M. Bele, Toxicology 269 (2010).180. X. Fang, R. Yu, B. Li, P. Somasundaran, and K. Chandran, J. Col-

loid Interf. Sci. 348 (2010).181. K. Kasemets, A. Ivask, H.-C. Dubourguier, and A. Kahru, Toxicol.

Vitro 23 (2009).182. D. M. Berube, J. Nanopart. Res. 10 (2008).

183. X. Zhu, J. Wang, X. Zhang, Y. Chang, and Y. Chen, Nanotechnol-ogy 20 (2009).

184. N. Tripathy, T. K. Hong, H. S. Jeong, and Y. B. Hahn, J. Hazard.

Mater. 270 (2014).185. R. K. Dutta, B. P. Nenavathu, M. K. Gangishetty, and A. V. Reddy,

Colloid Sur. B. 1 (2012).186. A. Kumar, A. K. Pandey, S. S. Singh, R. Shanker, and A. Dhawan,

Free Radic. Biol. Med. 51 (2011).187. M. Premanathan, K. Karthikeyan, K. Jeyasubramanian, and

G. Manivannan, Nanomed.: Nanotech. Biol. Med. 7 (2011).188. K. R. Raghupathi, R. T. Koodali, and A. C. Manna, Langmuir 27

(2011).189. K. Wiench, W. Wohlleben, V. Hisgen, K. Radke, E. Salinas, S. Zok,

and R. Landsiedel, Chemosphere 76 (2009).190. R. Hong, T. Pan, J. Qian, and H. Li, Chem. Eng. J. 119 (2006).191. O. Lupan, L. Chow, G. Chai, and B. Roldan, Mater. Sci. Eng. B

179 (2007).

192. S. K. Arya, S. Saha, J. E. Ramirez-Vick, V. Gupta, S. Bhansali,and S. P. Sing, Anal. Chim. Acta 737 (2012).

193. A. K. Pradhan, K. Zhang, G. B. Loutts, U. N. Roy, Y. Cui, andA. Burger, J. Phys. Condens. Matter 16 (2004).

194. Y. X. Kong and Z. L. Wang, Nano Lett. 3 (2003).195. Z. Yang, C. Xie, X. Xia, and S. Cai, J. Mater. Sci.: Mater. Med. 19

(2008).196. A. Becheri, M. Dürr, P. Lo Nostro, and P. Baglioni, J. Nanopart.

Res. 10 (2008).197. M. Salvati-Niasari, F. Davar, and A. Khansari, J. Alloys Compd.

509 (2011).198. M. Bitenc and Z. C. Orel, Mater. Res. Bull. 44 (2009).199. S. Dhobale, T. Thite, S. L. Laware, C. V. Rode, S. J. Koppikar,

R. K. Ghanekar, and S. N. Kale, J. App. Phys. 104 (2008).200. S. Thomas, N. Birbilir, M. S. Venkatraman, and I. S. Cole, Corros.

Sci. 69 (2013).

201. S. Music, S. Popovic, M. Maljkovic, and D. Dragcevic, J. AlloysCompd. 347 (2002).202. P. A. Rodnyi and I. V. Khodyuk, Opt. Spectrosc. 111 (2011).203. M. Mouanga, P. Berçot, and J. Y. Rauch, Corros. Sci. 52 (2010).204. A. Moezzi, A. M. McDonagh, and M. B. Cortie, Chem. Eng. J. 185

(2012).205. Z. A. Lewicka, A. F. Benedetto, D. N. Benoit, W. W. Yu, J. D.

Fortner, and V. L. Colvin, J. Nanopart. Res. 13 (2011).206. Y. B. Hahn, J. Chem. Eng. 28 (2011).207. R. Zamiri, A. Zakaria, H. A. Ahangar, M. Darroudi, and A. K. Zak,

J. Alloys Compd. 516 (2012).




R E V I E

W


208. T. M. Benn, P. Westerhoff, and P. Herckes, Environ. Pollut. 159(2011).

209. A. P. Hynes, R. H. Doremus, and R. W. Siegel, J. Am. Ceram. Soc.

85 (2002).210. M. Ristic, S. Music, M. Ivanda, and S. Popovic, J. Alloys Compd.

397 (2005).

211. T. Ben-Moshe, I. Dror, and B. Berkowitz, Chemosphere 81 (2010).212. A. A. Keller, H. Wang, D. Zhou, H. S. Lenihan, G. Cherr, B. J.Cardinale, R. Miller, and R. Ji, Environ. Sci. Technol. 44 (2010).

213. S. Liufu, H. Xiao, and Y. Li, Powder Technol. 145 (2004).214. S. Baruah and J. Dutta, J. Cryst. Growth 311 (2009).215. B. K. Sharma, A. K. Gupta, N. Khare, S. K. Dhawan, and H. C.

Gupta, Synth. Met. 159 (2009).216. P. X. Gao and Z. L. Wang, J. App. Phys. 97 (2005).217. Z. L. Wang, X. Y. Kong, Y. Ding, P. Gao, W. L. Hughes, R. Yang,

and Y. Zhang, Adv. Funct. Mater. 14 (2004).218. K.-S. Choi, H. C. Lichtenegger, and G. D. Stucky, J. Am. Chem.

Soc. 124 (2002).219. A. S. Dukhin, P. J. Goetz, X. Fang, and P. Somasundaran, J. Colloid

Interf. Sci. 342 (2010).220. M. Baalousha, F. V. D. Kammer, M. Motelica-Heino, H. S. Hilal,

and P. Le Coustumer, J. Chromatogr. 1104 (2006).221. R. D. Boyd, S. K. Pichaimuthu, and A. Cuenet, Colloids Surf. A

387 (2011).222. J. K. Lim, S. P. Yeap, H. X. Che, and S. C. Low, Nanoscale Res.

Lett. 8 (2013).223. C. S. Wu, F. K. Liu, and F. H. Ko, Anal. Bioanal. Chem. 399

(2011).224. S. Pabisch, B. Feichtenschlager, G. Kickelbick, and H. Peterlik,

Chem. Phy. Lett. 521 (2012).225. M. S. Diallo, C. J. Glinka, W. A. Goddard III, and J. H. Johnson

Jr., J. Nanopart. Res. 7 (2005).226. K. Tiede, M. Hassellöv, E. Breitbarth, Q. Chaudhry, and A. B. A.

Boxall, J. Chromatogr. A. 1216 (2009).227. P. Luo, I. Morrison, A. Dudkiewicz, K. Tiede, E. Boyes, P. O’Toole,

S. Park, and A. B. Boxall, J. Microsc. 250 (2013).228. S. M. Majedi, B. C. Kelly, and H. K. Lee, J. Hazard. Mater. 264

(2014).229. S. Tachikawa, A. Noguchi, T. Tsuge, M. Hara, O. Odawara, and

H. Wada, Materials 4 (2011).230. C. A. Lanzl, J. Baltrusaitis, and D. M. Cwiertny, Langmuir 28(2012).

231. E. Tang, G. Cheng, X. Ma, X. Pang, and Q. Zhao, App. Surf. Sci.

252 (2006).232. Z. Li, P. M. Fredericks, L. Rintoul, and C. R. Ward, J. Coal Geo.

70 (2007).233. J. R. Lead, K. J. Wilkinson, K. Starchev, S. Canonica, and J. Buffle,

Environ. Sci. Technol. 34 (2000).

234. D. Zhou, A. I. Abdel-Fattah, and A. A. Keller, Environ. Sci. Tech-nol. 46 (2012).235. Y. Zhang, Z. Chen, P. Westerhoff, and J. Crittenden, Water. Res. 43

(2009).236. X. Jiang, M. Tong, H. Li, and K. Yang, J. Colloid Interface Sci.

350 (2010).237. S. Pakrashi, S. Dalai, R. B. Sneha, N. Chandrasekaran, and

A. Mukherjee, Ecotoxicol. Environ. Saf. 84 (2012).238. T. J. Manning, T. Bennett, and D. Milton, Sci. Total Environ. 257

(2000).239. N. D. Saleh, L. D. Prefferle, and M. Elimelech, Environ. Sci. Tech-

nol. 42 (2008).240. A. Malloy, Mater. Today 14 (2011).241. J. G. Walker, Measurement Sci. Technol. 23 (2012).242. V. Filipe, A. Hawe, and W. Jiskoot, Pharma. Res. 27 (2010).243. F. M. Omar, H. A. Aziz, and S. Stoll, J. Colloid Interface Sci.

(accepted for publication) (2014).

244. P. Kramberger, M. C. Ciringer, A. Štrancar, and M. Peterka, Virol. J. 9 (2012).

245. P. Hole, K. Sillence, C. Hannell, C. M. Maguire, M. Roesslein,G. Suarez, S. Capracotta, Z. Magdolenova, L. Horev-Azaria,A. Dybowska, L. Cooke, A. Haase, S. Contal, S. Mano, A. Ven-nemann, J. J. Sauvain, K. C. Staunton, S. Anguissola, A. Luch,M. Dusinska, R. Korenstein, A. C. Gutleb, M. Wiemann, A. Prina-Mello, M. Riediker, and P. Wick, J. Nanopart. Res. 15 (2013).

246. C. Gillespie, P. Halling, and D. Edwards, Colloids Surf. A 384(2011).

247. A. E. James and J. D. Driskell, Analyst 138 (2013).248. L. C. Gontijo, R. Machado, E. J. Miola, L. C. Casteletti, and P. A.

P. Nascente, Surf. Coat. Technol. 183 (2004).249. W. M. Doyle, Process Control Qual. 2 (1992).250. S. G. Kazarian and K. L. A. Chan, Lap Chip 6 (2006).251. B. Carr and A. Malloy, Nanosight Ltd., Salisbury, Wiltshire

(2007).252. K. Tiede, S. P. Tear, H. David, and A. B. A. Boxall, Water. Res. 43(2009).

Received: 25 June 2014. Accepted: 24 September 2014.


Documents

Nanoparticle Properties, Behavior, Fate in Aquatic Systems and Characterization Methods