Part I Bionanomaterials - Wiley-VCH · 2014. 10. 28. · Christian Pfeiffer, Wolfgang J. Parak, and Jose Maria Montenegro 1.1 Introduction The use of silver and gold has been known

Part IBionanomaterials

Bio- and Bioinspired Nanomaterials, First Edition. Edited by Daniel Ruiz-Molina, Fernando Novio,and Claudio Roscini. 2015 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2015 by Wiley-VCH Verlag GmbH & Co. KGaA.

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1Synthesis of Colloidal Gold and Silver Nanoparticlesand their PropertiesChristian Pfeiffer, Wolfgang J. Parak, and Jose Maria Montenegro

1.1Introduction

The use of silver and gold has been known since ancient times. Silver was foundfreely in nature, but it was also highly common to find it in combination withother metals or metal compounds. A very important source was galena (lead sul-fide) and it was around 3000 BC when mankind was able to separate silver fromlead with a cupellation process, one of the milestones of metallurgy. It is alsovery common for gold and silver to come together as an alloy, called electrum,which has been also of great economic importance since ancient times. Both sil-ver and gold have been of interest as capital and widely used by almost all civili-zations as currencies and a symbol of richness. In particular, gold, for example,has been found extensively in jewelry objects and ornaments (e.g., bracelets,rings, etc.) since the Chalcolithic period (∼6000–4000 BC).The electrical and thermal conductivity of silver is the highest of all metals,

and it possesses the lowest contact resistance, which has led to its use as electri-cal contacts. Gold is the most ductile and malleable of all metals; it is possible toexpand a single gram into a sheet of 1m2 [1]. This permits us to obtain gold leafthin enough to become semi-transparent, which is useful in infrared shields invisors and in technological applications, owing to the conductivity properties ofgold. Alloys of silver and gold are widely used in jewelry. As gold is one of theleast reactive chemical elements, gold compounds are not common and their useis mainly limited to the chemical industry and research. However, silver saltsare of high interest. Silver iodide, for example, is used in seeding clouds to pro-duce rain. Silver nitrate was extensively used in photography but its importancehas been reduced by the arrival of digital photography. In former times silverwas widely used for coinage by many countries worldwide. But this ended whenthe value of the coins became greater than their exchange value.Bulk metals are not considered to be toxic but their ions show some toxicity,

in particular silver ions [2]. Already, Hippocrates of Kos knew about the bio-activity of silver [3]. Because of this biological activity, silver was used in thetreatment of wounds as wound dressing until the development of modern

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Bio- and Bioinspired Nanomaterials, First Edition. Edited by Daniel Ruiz-Molina, Fernando Novio,and Claudio Roscini. 2015 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2015 by Wiley-VCH Verlag GmbH & Co. KGaA.

antibiotics in the 1940s [4]. Since more and more bacteria are becoming resis-tant to antibiotics, the use of silver is coming back into the focus [5,6].The unconscious use of metallic nanoparticles (NPs) has been documented

since Roman times. The Lycurgus cup (Figure 1.1a) [7], a glass-metal decoratedcup, shows different color, due to the absorption or transmission of the gold andsilver NPs located in the glass matrix, depending on whether the white lightsource is located inside or outside of the cup [8,9]. This use continued over timeand can be widely seen in the stained glasses of old churches (Figure 1.1b). Thered and yellow colored glasses are a result of the presence of gold and silver NPs,respectively, in the glass matrix.Gold NPs have been detected in the golden cover of metallic objects of the

pre-Columbian era. Analysis of articles of the Sicán/Lambayeque culture showsevidence of the inclusion of NPs in the gold layer used to coat copper artifacts(Figure 1.2) [10].By the middle of the nineteenth century, science had begun to focus on

understanding the origins of these different colors. Michael Faraday, in 1857,produced the first colloidal gold suspensions by reducing an aqueous goldchloride solution with phosphorus [11]. The so-obtained “exceedingly fineparticles” formed a ruby colored solution and were synthesized using differ-ent reagents or even by simple heating of a gold chloride solution. Heobserved how changes in the solution’s pH (a concept that was not definedat the time) or the concentration of reagents induce changes in the color dueto the generation of larger NPs and precipitation, something that we knownowadays is due to the formation of agglomerates and a resulting couplingof the surface plasmons of NPs in close proximity. In a smart experimental

Figure 1.1 (a) The Lycurgus cup. Photograph by Lucas Livingston; reproduced withpermission. (b) Part of a church window in the Elisabeth Kirche, Marburg an der Lahn,Germany. Adapted from the original of José María Montenegro.

4 1 Synthesis of Colloidal Gold and Silver Nanoparticles and their Properties

design, Faraday was also able to calculate the amount of gold present in asuspension via comparison with a gold ruby glass.These experiments of Faraday showed no special relevance for decades until a

new turn was given to the story. At the end of the nineteenth century, RichardAdolf Zsigmondy started to study the properties of colloids, work that wouldlead to him being awarded the Nobel Prize in Chemistry in 1925. Zsigmondywas the first scientist to study and establish the properties of colloidal gold [12]and other metallic colloids [13], developing a synthetic methodology in two stepsbased on a “nuclear method” that we know nowadays as a seed-mediated pro-cess, using formaldehyde as reducing agent. Zsigmondy was able to determinethe size and mobility of the NPs synthesized with a methodology he developedfor this purpose [14]. Further experiments carried out by Svedberg, Mie, andOstwald, and the subsequent development of a full set of analytical instrumenta-tion, techniques, and working methodologies made it possible to study the phys-ical and chemical properties of these colloidal suspensions. Adaptations of mostof these techniques are still used in the field of colloidal NPs as way we knowthem today [15–18].The definition of a NP or a nanomaterial is not as straightforward as one may

think. A common definition says that a nanomaterial must be, at least in onedimension, between 1 and 100 nm in size [19]. But this is not a general defini-tion. In other definitions, particles with at least one dimension below 1 μm(1000 nm) are considered nanomaterials [20]. Metallic NPs show special physicaland chemical properties in comparison to their bulk material. These propertiesare due to the small size and their huge surface area compared to the sameamount in bulk form. The relevant effects originating from the nm-size of NPsare reviewed for the case of gold and silver along this chapter.

Figure 1.2 Gold cup, Lambayeque culture, Perú. Reproduced with permission of Museo delOro, Perú.

1.1 Introduction 5

Nanomaterials and NPs are everywhere in our daily life. With every breath oneinhales several different NPs and they are consumed in foods and drinks [20].The two main categories of NPs are naturally occurring NPs and synthetic NPs.Natural sources of NPs are diverse, and include volcanoes, forest fires, or ero-sion; some organisms show sizes of a few nanometers and can also be includedin this division [20]. Although the bulk of NPs produced in a dust storm showsizes of a few micrometers, they can also decrease to less than 100 nm [21]. Theamount of nano- and microparticles can be so big that it can be seen by satel-lites. The ashes produced in forest fires, or the eruption of a volcano, are alsosources of nano- and microparticles. These NPs can reach the upper tropo-sphere and can be found worldwide for years. Nano- and microparticles can befound not only outdoors but, especially, also indoors [22]. Here the air can beten-times more polluted than outdoors, according to the US Environmental Pro-tecting Agency [23]. The main sources are cooking, smoking, dust, skin NPs,spores, and combustion (e.g., from candles). The World Health Organizationestimates that worldwide 1.6 million people die annually because of indoor airpollution [24].Apart from the naturally occurring NPs described so far, synthetic NPs can

also be found in huge numbers in our daily life. In this respect, most NPs can befound in cosmetics (e.g., creams, face powder, lipsticks, etc.) and personal careproducts (e.g., deodorants, soaps, toothpaste, etc.) but also in other consumerproducts [25–27]. Their use in cosmetics is due to their special properties; forexample, some NPs can penetrate deeper into the skin, show antioxidant proper-ties, or show intensive color [28]. Such products also involve silver and gold NPs.Apart from their use in consumer products, gold and silver NPs in particularalso show great promise for more advanced applications, especially for medicalapplications. In this direction, applications are based directly on the particularphysical and chemical properties of these NPs, most prominently on their sur-face plasmon absorption band. Gold and silver NPs are currently explored asdelivery vehicles [29–31], for molecular diagnostic assays [32,33], and towardstumor ablation [34–37].

1.2Physical and Chemical Properties of Gold and Silver Nanoparticles

Nowadays, NPs with controlled chemical and physical properties can beobtained by precise tuning of their sizes, shapes, and surface coatings [38–42].The physical and chemical properties of these NPs are determined by the modesof excitation of their electrons [43]. Excitation depends on the space to whichthe electrons are confined. The lower limit is at the atomic level, which is thestrongest type of electronic confinement, because the electron is closely boundto the atom within a length scale of ∼50 pm [43]. Ideal bulk metals are the upperlevel, in which, to a first approximation, free electrons (from the conduction


band) are free to move with all kinetic, but no potential, energy. For metal NPs,the size reduction transforms the initially conductive metal into a semiconductor(∼3 nm diameter) or even an insulator (at diameters below 1 nm) [44]. This isdue to the increasing separation between the valence and conduction bands.While the mean free path in bulk gold and silver is ∼50 nm, NPs smaller thanthis are expected to have no scattering from the bulk and the interactions withlight will instead be produced on the surface [45].

1.2.1

Optical Properties of Gold and Silver Nanoparticles

Gold and silver NPs show an intense absorption in the visible–near UV. Thisabsorption is due to a coherent oscillation of the free surface electrons, calledsurface plasmon resonance [46]. The physical explanation of this phenomenonwas given by Mie, solving Maxwell’s equations for the interaction of electromag-netic radiation and spherical metallic NPs in one of the earliest studies in thefield of nanotechnology [16].The interaction of light with colloidal dispersions of metallic gold and silver

NPs involves scattering and absorption. Both these optical properties arestrongly dependent on the size and shape of the NPs, as well as the dielectricconstant of the surrounding medium [47] and the electronic interactionsbetween the NP metallic core and the protective surfactant [48]. This is anadvantage because it is possible to tune these parameters to obtain NPs withdefined and controlled optical properties. Size effects come into play in particu-lar for NPs of sufficiently small size (<50 nm), comparable or lower than themean free path of the electrons [48,49]. Spherical gold NP dispersions showa characteristic red color with a maximum absorbance in the green (e.g.,λabs≈ 520 nm for Au NPs with a diameter up to ∼40 nm) [45,50,51]. Sphericalsilver NP dispersions are yellow, with an absorbance in the blue (e.g., λabs≈ 420nm for Ag NPs with a diameter up to 30 nm) [45,52]. At sizes below 50 nm, theplasmon resonance corresponds to the excitation of dipolar modes. The increasein size above 100 nm leads to the appearance of multipole plasmon modes [53].Not only the size, but also the introduction of anisotropy to NPs leads to

large changes in their optical properties [54]. Anisotropic NPs have absorbancesstronger than isotropic NPs and are composed of two or more absorptionbands. This is due to the oscillation of excited electrons along and perpendicularto the long axis in the case of rods as well as multipole phenomena indistinct shapes like nanoprisms, nanorods, nanoshells, or nanorectangles(Figure 1.3) [55–63].However, if nanorods are embedded in a matrix, organized in an array

arrangement, only one of the two bands is visible and it is blue-shifted [64,65].The strong absorption of NPs is also responsible for enhancing other electro-

magnetic radiation interacting processes like fluorescence [66–68], surface-enhanced Raman scattering [69–73], and generation of harmonics [74–76].

1.2 Physical and Chemical Properties of Gold and Silver Nanoparticles 7

1.2.2Electronic Properties of Gold and Silver Nanoparticles

The electronic properties of Au and Ag NPs are a result of the size-dependentelectronic band structure. The electronic band structure changes upon the tran-sition from individual atoms to bulk metal. NPs have intermediate propertiesbetween atoms and bulk metal. Figure 1.4 shows a scheme illustrating the transi-tion of electron binding energies between atoms and metal [77,78]. The metallicproperties appear at ∼1 nm diameter and a band structure similar to that of bulkmetal at ∼3 nm diameter [44,79].Electron dynamics in metallic NPs can be used to determine the dynamics

of chemical reactions on their surfaces. This can be realized, for example, upon

Figure 1.3 (a) Absorption spectra of goldNPs of various sizes and shapes. Adapted fromEustis and El-Sayed [45] 2006. The RoyalSociety of Chemistry. (b) Shape-dependence

of surface plasmon resonance in gold NPs.Adapted from Tréguer-Delapierre et al. [54], 2008 Springer Verlag.

Figure 1.4 Diagram of the evolution ofelectronic states from atoms to metal. BE isthe binding energy; EFermi is the Fermi energylevel; Evacuum is the vacuum energy level; eϕ is

the metal work function. Reproduced withpermission from Bäumer and Freund [77], 1999 Elsevier Limited.


photo-excitation of electrons for subsequent triggering of the chemicalprocesses [79].

1.3Synthesis of Gold and Silver Core Nanoparticles

Metallic NPs can be synthesized via top-down and bottom-up approaches. NPsof many metals such as copper [80], platinum [81], cobalt [82], or nickel [83]have been synthesized. In this chapter, we focus on the two dominant metals inthe field, gold and silver. Top-down synthesis of metallic NPs starts from thebulk material and uses physical microfabrication methods such as laser ablationto obtain NPs dispersions [84–94]. In a bottom-up synthesis the starting point isa chemical precursor, usually a salt, which is chemically reacted (typicallyreduced) to obtain the final NPs via crystal growth from a supersaturated solu-tion. Most current synthetic methodologies, in particular wet-chemical methods,are based on this approach [95–99].Wet-chemical methods usually yield NPs with a good control of size, shape, and

coating. Turkevich et al., in 1951, pioneered a scientifically practical synthesis ofgold NPs [100]. This synthesis was based on the reduction of Au(III) ions in aque-ous solutions, using several reagents, including citric acid, hydroxylamine, acety-lene, or oxalic acid, leading to different sizes and shapes depending on the reagentused. The synthesis in aqueous media usually uses phosphanes [101] or thiols [102]as stabilizers, owing to the high stability of Au-P and Au-S bond. Silver NPs canbe synthesized in aqueous solutions in a similar way upon reduction with NaBH4

using cetyltrimethylammonium bromide (CTAB) [103] or sugars [104]. The dis-advantage of this method is that sizes smaller than 10nm could not be achievedin a straightforward way on the basis of the original protocol. A good methodologyto solve this problem, based on the reduction of Au(III) ions in organic solvent,reaching smaller sizes, was published in 1994 by Brust et al. [105]. The Brust–Schiffrin method is based on a two-phase system using NaBH4 as reducing agent.It allows for the generation of stable monodisperse gold NPs using alkylthiolchains or arylthiols to stabilize the NPs and thus preventing agglomera-tion [42,105]. The synthesis of NPs in organic solvents yields stable NPs of differ-ent sizes and shapes. In this way, nanocubes [39,106,107], spherical NPs [50,108],or nanoclusters [38,109–111] can be obtained. Although the properties of iso-tropic plasmonic NPs (such as spherical Au and Ag NPs) allow their use for manyapplications, the anisotropy (e.g., of Au and Ag nanorods) introduces additionalproperties (i.e., polarized absorption, scattering) that merit the effort needed todevelop efficient synthetic methodologies that permit control of these properties.There are several strategies by which to synthesize anisotropic NPs, which can beclassified into several basic concepts, as outlined in the following.Seed-mediated methods are the most common methodologies used to produce

Au and Ag NPs in a large variety of shapes. These methods are based onthe early synthesis strategies of gold NPs as described by Zsigmondy [13] and

1.3 Synthesis of Gold and Silver Core Nanoparticles 9

involve the fabrication of metallic NPs that will act as seed. On top of the seedsthere is further growth upon reduction of the metallic precursor with mildreducing agents. In this way, nanorods [95,96,112], nanoprisms [113], nanohexa-pods [114], nanoribbons [115], or nanobranches [116] can be grown. Thermaldecomposition based approaches were stimulated by work of Fievet in 1989[117]. They make use of polyols like poly(vinyl pyrrolidone) (PVP), ethylene gly-col, or 1,2-propylene glycol as solvents and also as reducing agents. Polymers canbe used as stabilizers, which offer several advantages like enhancement of NPstability, adjustment of the solubility, increased amphiphilicity, tuning of the sur-face density, and so on. There are two synthetic routes to NPs stabilized withpolymers: “grafting to” and “grafting from.” Although both yield NPs coatedwith polymers, only the “grafting to” methodology involves direct NP synthe-sis [95]. The “grafting to” methods can be divided into two strategies. The firstuses ligand-functionalized polymers to stabilize the gold NPs. In some cases, thesame polymer can act as reducer of the gold salts, but typically the addition of anadditional reducing agent (e.g., NaBH4) is required. Polymers functionalized withthiols [118–126], nitrogen [127], arsenic [128], or ionic polymers [129] havebeen used for this purpose. The second strategy implies the use of functionalizedpolymers as templates. Poly(acrylic acid) copolymers [130] or N-isopropylacryla-mide [131] are some of the polymers that have been used. In the same way, den-drimers and dendrons can be used for the synthesis and stabilization of gold andsilver NPs. In this way, PAMAM dendrimers of different generations have beenused with good results [132,133]. Pulse radiolysis involves γ-ray irradiation forthe reduction of AuIII. Using this method, Au nanorods have been obtained [59].With these methodologies, a wide variety of sizes and shapes can be obtained.

Anisotropic NPs are of great interest due to their distinctive properties. Nano-rods, nanoprisms, nanoshells, nanocages, and branched nanostructures havebeen designed and some of the concepts leading to these structures are outlinedin the following. Nanorods can be synthesized from many different materials,involving also Au and Ag. Wet-chemical syntheses of Au and Ag nanorods typi-cally involve reduction of chemical Au or Ag precursors in aqueous solution.Electrochemical [134], radiolytic [59], and chemical seed-mediated methods arethe most commonly used routes to synthesize tunable nanorods with controlledsize and high monodispersity [96,97,112,135]. Most frequently, cetyltrimethy-lammonium bromide (CTAB) is used as surfactant and the synthesis is carriedout in two steps. First, small metallic NPs (around 4-nm diameter) are formed inaqueous solution by the addition of sodium borohydride to a mixture of AuIII

and CTAB solution. These spherical NPs are then grown further upon the addi-tion of more metal precursor and reducing agent (Figure 1.5). For gold nano-rods, addition of AgNO3 has been reported to improve the final shape of thenanorods [99]. This behavior seems to be related to the formation of AgBr thatis adsorbed on the Au NP surface, thereby stabilizing the rod instead of thespherical shape.Nanoplates are planar NPs, which are usually synthesized using the same

methodologies as for nanorods, but by changing some parameters such as the


concentration of reagents or pH. There are several types of nanoplates. Trigonalnanoprisms [58,113,136,137] and hexagonal nanoplates [138] are some of thestructures commonly synthesized. Nanoshell NPs are nanocomposites of adielectric core (silica, polystyrene, etc.) covered with a thin layer of gold. Theoptical properties of these structures are characterized by intense absorbanceand scattering [139]. In nanoshells, the surface plasmon resonance absorptioncan be tuned via modification of the gold thickness [140]. Silica core sphericalgold nanoshells [60,139,140] and gold sulfide gold nanoshells [141,142] are twoexamples of these nanomaterials. Nanocages and other hollow nanostructuresare very attractive due to the presence of pores and their high surface/volumeratio. The main synthetic strategy is based on galvanic replacement methods.Hence, nanocages, nanorings, and nanotubes have been obtained [143].Branched nanostructures have also been widely investigated, as the anisotropy ofsuch NPs produces very interesting optical features. The presence of tips oredges produces “hot spots” that are useful in surface enhanced Raman scattering(SERS) and like light confiners [144–146].

1.4Transfer to Aqueous Media of Gold and Silver Nanoparticles from Organic Solvents

Although aqueous-based synthetic methods for Au and Ag NPs are widely avail-able, organic solvent based processes are also frequently used due to improvedcontrol of sizes and shapes. When NPs synthesized in organic solvents are to be

Figure 1.5 Seed-mediated synthesis of gold nanorods. Adapted with permission from Murphyet al. [112], 2005 American Chemical Society.

1.4 Transfer to Aqueous Media of Gold and Silver Nanoparticles from Organic Solvents 11

used for biological applications that require water-soluble NPs, the hydrophobicNPs have to be made hydrophilic in a post-synthesis step. The two main proto-cols to transfer metallic NPs from organic solvents to water are given below.Ligand exchange approaches (Figure 1.6) are based on exchange of the

hydrophobic surfactant coating with ligand molecules that on one end arereactive to the NP surface and on the other end possess a hydrophilicgroup [52,147–150]. Note that it is necessary to find appropriate ligand mol-ecules for each NP type.Coating approaches, on the other hand, overcoat the original hydrophobic lig-

and shell of the NPs with an additional amphiphilic shell, which most ofteninvolves amphiphilic polymers, although lipids are also employed for this pur-pose. The use of amphiphilic polymers (Figure 1.7) to transfer hydrophobic NPsto aqueous media is an easy way to obtain highly stable dispersions of NPs andincrease the possibilities of post-modification due to the reactive organic groupspresent on the surface.Many different polymers have been reported, such as polymers bearing

carboxylic (Scheme 1.1) [151,152], phosphonic, or tetraalkylammonium groups[153]. An extensive review has been given by Zhang et al. [154].

Figure 1.6 Ligand exchange of a silver NP coated with dodecanethiol with mercaptoundeca-noic acid. Adapted from Caballero-Díaz et al. [52], 2013 John Wiley & Sons.

Scheme 1.1 Synthesis of dodecylamino-poly(isobutylene-alt-maleic anhydride) amphiphilicpolymer. Adapted from Lin et al. [151], 2008 John Wiley & Sons.


1.5Some Applications of Gold and Silver Nanoparticles

Since the development of nanotechnology, plasmonic Au and Ag NPs have beensuggested as very promising materials in diverse fields of research:

1) Lithography: Originally, lithography referred to a process of printing inwhich a non-polar ink is applied to a hydrophilic surface patterned with ahydrophobic image. However, nowadays the term is applied to variousmethods for replicating a predetermined pattern on a substrate. NPs canhelp in lithographic techniques because it is possible to work in the sub-100 nm scale [155,156].

2) SERS: Surface enhanced Raman spectroscopy (or surface enhanced Ramanscattering) is an analytical technique that uses the electronic properties ofsurfaces to enhance Raman scattering of molecules adsorbed on them.Raman spectroscopy provides rich structural information, is non-invasive,and does not require specific sample treatment [48,157,158]. Metallic NPscan be used as SERS probes. The plasmon resonance of metallic NPs canresult in a huge magnification of the local field at the surface. Theenhancement is so strong that it is possible to reach the single-moleculelevel [70,71,159]. Detection of ions [160], DNA [161], narcotics [162], andexplosives [163] has been achieved.

3) Fluorescence enhancement: the use of metal NPs for enhancing the fluores-cence of chromophores located close to the surface shows a distancedependent enhancement of the signal at very close distances and quench-ing of fluorescence at higher distances [164], which also depends on theNP shape of the NPs [66].

4) Catalysis: The possibility to use less material (due to the high surface-to-volume ratio) and the different properties associated with the

Figure 1.7 Scheme of polymer coating. Surface carboxylic groups point out into the aqueoussolution. Adapted from Lin et al. [151], 2008 John Wiley & Sons.

1.5 Some Applications of Gold and Silver Nanoparticles 13

different sizes and shapes make NPs a very promising tool for cataly-sis [45]. Both homogeneous and heterogeneous catalysis have beeninvestigated [165].

The appropriate functionalization of NPs makes them potentially relevantin a large number of biological applications, including biodiagnosis, labeling,molecular delivery, use as hyperthermia agents, and sensing [51,166,167].Owing to the higher toxicity of silver NPs in comparison withgold NPs [52,168–170], more biological studies have been carried out withgold NPs [51,166]. In the following, some of the applications mentioned arehighlighted:

1) Biodiagnosis and biosensing: The use of NPs for biodetection has been ofgreat interest in recent years [171]. The goal of sensing a single molecule in aliving cell seems possible with the use of NPs [172]. The analytes of interestcan be detected by observing changes in the optical properties of the NPsupon their interaction with the analyte. A change in plasmon resonanceupon the formation of NP agglomerates is one of the methods reported mostfrequently. This principle has led, for example, to the development of DNAsensors. They are based on gold NPs functionalized with oligonucleotidestrands complementary to the sequence of interest [173,174].In addition, the quenching of fluorescence emission of fluorophores

located close to the NPs surface of NPs can be used for sensing purposes.Competitive strategies where fluorescent residues are released upon linkageof the analyte to the NP [175] and the use of DNA hybridization to increase/decrease the separation between fluorophores and NPs [176–179] are thetwo strategies used most often.Analyte molecules located close to the surface of NPs can be detected with

surface-enhanced Raman scattering (SERS). The functionalization of goldand silver NPs with analyte-specific SERS-active ligands permitted the detec-tion of DNA [161,180] and proteins [181–183].

2) Labeling: The role of NPs used for labeling can be considered as “passive,”in contrast to the “active” role they have in sensing and diagnosis. Thismeans that there is no need for NP properties changes upon binding tothe target region. There are several labeling processes for which NPs havepresented promising properties. For example, in immunostaining, the NPsare modified with antibodies (Ab), specific for certain molecular or biologi-cal targets. This specificity directs the Ab-NPs conjugates to their target.At the target, the NPs provide high contrast for imaging, for example, fluo-rescence microscopy or transmission electron microscopy [184–186]. Sin-gle particle tracking experiments also use Ab functionalized NPs, but usinga low NP concentration. This avoids NP saturation on the NP target andallows the tracing of individual NPs [187–190]. Finally, besides fluores-cence microscopy and TEM, Au and Ag NPs can be used as contrastagents for X-rays [191,192].


3) Molecular delivery: The use of NPs for molecular delivery in cells can bepursued by two approaches, via forced or natural NP–cell interactions. Anexample of a forced approach is the use of so-called gene guns, a ballisticsimile where the NP plays the role of a bullet. Gene guns are frequentlyused to insert plasmids inside cells, whereby the genes are adsorbed ontothe NP “bullets.” Shooting can be carried out with gas pressure or electricdischarges (Figure 1.8a) [193,194].The uptake of NPs by cells also occurs naturally, typically via endocy-

totic pathways. Incorporation of NPs by cells can be specific (via a recep-tor–ligand interaction) or nonspecific. Specific incorporation requires thefunctionalization of NPs with molecules that interact with membranereceptors (Figure 1.8b) [195–197]. This specificity permits, for example,the design of systems where the NPs are incorporated preferably by cancercells that present specific surface receptors. After incorporation, NPs aretypically located inside endosomes and/or lysosomes [197,198]. For manydelivery applications, the NPs must be released from these organelles topermit delivery to the cytosol. In this direction, NPs can be functionalizedwith cell-penetrating peptides that permit translocation to the cyto-sol [198–200]. Once inside cells, NPs can release different moleculesdepending on their functionalization. NPs with DNA residues as well asanticancer drugs have been reported [201–203].

4) Local heating: Human cells typically initiate apoptosis at temperaturesabove 42 °C. The preferential incorporation of NPs in tumor cells uponmodification with specific targets together with the enhanced permeationand retention (EPR) effect [204], and the increase in temperature of plas-monic NPs produced upon light excitation at their resonance frequency,permits the design of strategies in which tissues enriched with NPs are

Figure 1.8 (a) Gene gun injection of functionalized Au NPs into a cell; (b) natural uptake of AuNPs by a cell. Reproduced with permission from Sperling et al. [51], 2008 The Royal Societyof Chemistry.

1.5 Some Applications of Gold and Silver Nanoparticles 15

irradiated and apoptosis is induced upon local illumination and thesubsequent generation of high temperatures. The low penetration of lightin tissues limits somewhat this application to tissues close to theskin [205–208].While most of the applications reported here are still in an exploration

phase, it can be foreseen that in the next decade some of them will be putinto industrial and clinical practice.

Acknowledgments

Parts of this work were supported by the European Commission (projectNamdiatream to W.J.P.).

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Part I Bionanomaterials - Wiley-VCH · 2014. 10. 28. · Christian Pfeiffer, Wolfgang J. Parak, and Jose Maria Montenegro 1.1 Introduction The use of silver and gold has been known