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BARC/2013/E/016BA
RC/2013/E/016
2013
GOLD NANOPARTICLES: GENERATION & CHARACTERIZATION
by
G.R. DeyRadiation & Photochemistry Division
BIBLIOGRAPHIC DESCRIPTION SHEET FOR TECHNICAL REPORT(as per IS : 9400 - 1980)
01 Security classification : Unclassified
02 Distribution : External
03 Report status : New
04 Series : BARC External
05 Report type : Technical Report
06 Report No. : BARC/2013/E/016
07 Part No. or Volume No. :
08 Contract No. :
10 Title and subtitle : Gold nanoparticles: generation and characterization
11 Collation : 26 p., 10 figs., 1 tab.
13 Project No. :
20 Personal author(s) : G..R. Dey
21 Affiliation of author(s) : Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai
22 Corporate author(s) : Bhabha Atomic Research Centre, Mumbai - 400 085
23 Originating unit : Radiation and Photochemistry Division, BARC, Mumbai
24 Sponsor(s) Name : Department of Atomic Energy
Type : Government
Contd...
BARC/2013/E/016
BARC/2013/E/016
30 Date of submission : June 2013
31 Publication/Issue date : July 2013
40 Publisher/Distributor : Head, Scientific Information Resource Division, Bhabha Atomic Research Centre, Mumbai
42 Form of distribution : Hard copy
50 Language of text : English
51 Language of summary : English, Hindi
52 No. of references : 35 refs.
53 Gives data on :
60
70 Keywords/Descriptors : NANOSTRUCTURES; GOLD; CITRIC ACID; RADIOLYSIS; PHVALUE; RADICALS; COBALT 60; STRONTIUM 90; PULSE TECHNIQUES
71 INIS Subject Category : S38
99 Supplementary elements :
Abstract : In this presentation we report the reduction of Au3+ through chemical and free radical (esolv
-)
reactions both in non-aqueous and aqueous media. In chemical reduction, the spectral nature in ascorbicacid (AA) and citric acid (CA) systems was different. The band intensity of gold nanoparticles was lower
in AA system. While in free radical reaction, the yield of nanoparticles was pure i.e. free from excessreactants. Under the study 60-200 nm size nanoparticles were generated, which are inert to oxygen. Using
pulse radiolysis technique, the initial rate for esolv
- reaction with Au3+ was determined to be 7.6 x 109 M-1
s-1.
BARC/2013/E/016BA
RC/2
013/
E/01
6
GOVERNMENT OF INDIAATOMIC ENERGY COMMISSION
BHABHA ATOMIC RESEARCH CENTREMUMBAI, INDIA
2013
GOLD NANOPARTICLES: GENERATION & CHARACTERIZATION
by
G.R. DeyRadiation & Photochemistry Division
1
svaNa^ saUxmakNa kI ]%pi<a evaM vaOSaoiYakta
saaraMSa
[sa p`itpadna mao svaNa^ Aayana ka jalaIya tqaa inaja^laIya maaGyama maMo rsaayainak evaM f`I roizkola p`itik`yaa Wara
nyaUnaIkrNa ka ivavarNa ikyaa gayaa ho . rsaayainak irD@sana maoM eskaoribak eisaz tqaa saa[iT`k eisaz p`Naailayaao ka
vaNa^k`ma saMbaMQaI svaBaava ivaiBanna qaa. eskaoribak eisaz p`NaalaI maoM svaNa^ saUxmakNa ka baOMD tIv`ata saa[iT`k eisaz
p`NaalaI kI tulanaa maoM kma qaI.jabaik,, f`I roizkola p`itik`yaa maoM saUxmakNa ka ]%pad SaUV qaa, yaaina A%yaiQak
p`itkarkao sao vaMicat. [sa AQyayana maoM, 60-200 nm Aakar ko saUxmakNa kI ]%pi<a kI ga[^,, jaao Ai@sajaona sao
inaiYk`ya qaI.plsa roiDAaolaa[isasa p`NaalaI maoM salvaoToD [lao@T/na ka svaNa^ Aayana ko saaqa p`itik`yaa ka p`aqaimak Anaupat
7.9x109 M-1 s-1 inaiScat ikyaa gayaa.
0
GOLD NANOPARTICLES: Generation & Characterization
G.R. Dey
Radiation & Photochemistry Division, Chemistry Group,
Bhabha Atomic Research Centre, Trombay, Mumbai 400085
Summary
The reduction of Au3+ ions through chemical reaction and free radical reaction under
different experimental conditions in aqueous and non-aqueous systems was studied.
During chemical reduction of Au3+ with citric acid (CA) and ascorbic acid (AA), the
spectral nature in two different systems was different with respect to the main absorption
peak (λmax 530 nm). In the case of AA, the band at 530 nm was broader and less intense in
comparison to CA system, which is due to the different size of nanoparticles. In CA
system, the particle sizes observed were smaller (60 nm) than those measured in AA
system (100 nm). While in free radical reduction of Au3+ with esolv-, the yield of
nanoparticles is pure i.e. free from extra reactants (reducing agent), which is normally
observed in chemical reduction processes. However, in no PVA (a stabilizer) system, the
product from free radical reduction was not so impressive. Under this condition the DLS
signal obtained was poor and the size of nanoparticle was not determined, whereas in
presence of PVA the particles sizes are better with respect to polydispersity.
During pulse radiolysis, the rate constant for esolv- reaction with Au3+ was determined
following the decay of solvated electrons in methanol (kbimol = 7.6x109 M-1 s-1). In
principle, initially the reduction of Au3+ ion leads to Au(II) ion intermediate, which later
produces transients Au(I) ion, atomic gold (Auo), etc. through their subsequent reactions.
On further reaction through nucleation reactions, perhaps gold nanoparticles were
generated. Under esolv- - Au3+ ion reaction, the spectrum recorded during short pulse
irradiation exhibits three-absorption peaks with three maxima at 270, 370, and 470 nm.
The nature of all these absorption peaks follows uniformity i.e. decrease in absorbance
with time, moreover with different kinetics. Under the study, 60-200 nm-sized particles
were generated, which are stable even in presence of oxygen.
1
INTRODUCTION
Metal nanoclusters are important materials because of their role in surface-enhanced
processes. The unique properties of metal clusters, such as collective electronic and lattice
excitations depend on the size and geometry of the particle1 and have various applications
in the field of catalysis2. Study on nanometric particles of metals is also of interest to
understand their growth mechanism in photographic and catalytic sciences. Metals and
their alloys are known to act as catalysts in a wide variety of applications since ages.
Imaging as well as photographic development involves in application of clusters growth.
A number of reports on metal nanoparticles exist in the literature, which deal with the
mechanism of nucleation and growth process in photography3-5.
In the field of catalysis, metals show enhancing support for the propagation of chemical
reactions. This is mostly due to the involvement of free electrons available in metal
atoms/clusters. These electrons and their counter metal cations eventually interact with
themselves and with other reactants, producing various reaction intermediates through
different reaction channels. Metal nanoparticle provides a large surface to mass ratio as
compared to the bulk-metal, which is the primary reason for its superior catalytic activity6.
Even those materials exhibit negligible reactivity on their bulk scales are found to be
extraordinarily active in their small clusters (nanometric particles) forms. For example:
gold is an inert metal due to its non-reactive nature in the bulk state. However, its small
clusters are catalytically active due to different electronic and chemical properties,
including the size and different oxidation states of the nanoparticles. The redox potential
values for bulk gold metal and its atom are different to each other EoAu+∝/Au•
∝ = +1.68 V
and EoAu+/Au• = -1.5 V vs NHE7,8, which results in the higher reactivity of a gold atom
than the bulk gold.
The present work has been carried out to generate gold nanoparticle in water and methanol
(non-aqueous) through two different reduction techniques such as chemical and radiation
induced free radical reactions. In chemical reaction citric acid and ascorbic acids were
used as reducing agents in both aqueous and methanol systems, whereas under radiation-
induced free radical reaction studies, methanol (an alcohol) was used as reaction medium
in which two strongly reducing radicals viz. solvated electron (esolv-) and alcohol radical
(•CH2OH) are generated on radiolysis of the solvent take part in metal ion reduction.
2
EXPERIMENTAL
Gold ions as HAuCl4 (99.9% purity) from Arora-Mathey chemicals, citric acid (>99%
purity) from LOBA Chemie, India and ascorbic acid (GR grade) from E Merck and
methanol (99.8% purity) from SRL, India were used as received. The UV-vis spectra of
the clusters produced during γ-radiolysis were recorded using Hitachi U-2001
Spectrophotometer. Prior to the desire experiments, UV-vis spectra of 0.2 mM Au3+ ions in
water, methanol and water-methanol mixture solvents were recorded separately and the
spectra obtained are shown in Figure 1. It was noticed from the figure that the spectral
nature of Au3+ ion changes appreciably at 300 nm region with methanol concentrations.
For better understanding, the absorbances at 310 nm were plotted against methanol
percentage in water and the plot is shown as inset of figure 1. It is noticed that the
absorption at λmax = 310 nm is minimum when there is no methanol, however, it increases
with methanol concentration and reaches maximum around 40% of methanol and remains
steady (see inset figure 1). Hence in the present work, >90% methanol was used for
carrying out all desire experiments assuming at this concentration, the nature of the parent
Au3+ ions remains identical.
RESULTS & DISCUSSION
Chemical Reaction
Generation of gold nanoparticles through chemical reduction
A chemical reaction is a process that always results in the inter conversion of chemical
substances. The substance(s) initially involved in a chemical reaction are called
reactant(s). Chemical reactions are usually characterized by a chemical change, and they
yield to one or more products, which are in general different from the reactant(s). The
performance of these reactions mostly depends on four parameters viz. temperature,
concentration, surface area and catalyst. Under this direction, the chemical reduction of
Au3+ ions (one reactant) to gold nanoparticles (product) was studied using ascorbic acid
and citric acid as reducing agent (another reactant) at room temperature (25oC) separately.
Observations during the course of reaction are discussed below.
3
Reduction with CA
HAuCl4 (0.1 mM) solution in water was mixed with 0.1 mM CA at its natural pH (pH ≈ 3)
under aerated condition in which oxygen concentration present in the solution was 0.2
mM. With time, the initial light yellow colour solution changed to wine red colour
indicating the formation of gold nanoparticles possibly due to chemical reaction between
Au3+ ions with CA. The UV-vis spectrum recorded shown in Figure 2A exhibits absorption
maximum at 530 nm, which is similar to the reported spectrum for gold nanoparticles9-12.
Different concentration ratios of Au3+ ions to CA were employed to observe the different
behavior with respect to color intensity, kinetics, etc. exists if any during the study. The
peak at 530 nm did not change appreciably when the concentration of CA was tripled,
however, when the concentration of CA increased to 1 mM (10 times as compared to 0.1
mM), the absorption intensity at 530 nm was increased to three fold with a red shift to 550
nm. As mentioned above, the chemical reactions depend on concentration (one of the four
parameters) of at least one of the reactant, which is also supported by the experimental
results obtained at higher concentration of CA, a reactant in the Au3+ ions system. It is
interesting to note here that instantaneous development of color occurred as soon as the
concentration of CA increased to 10 time reveals that the kinetics of the reaction became
faster on increase in reactant (CA) concentration. Furthermore, lower concentration of CA
(0.1 and 0.3 mM) was not sufficient to reduce entire Au3+ ions to gold nanoparticles,
which is the possible reason to observe low yields (less absorbance at 530 nm).
Similar experiments were carried out in presence of 1 mM PVA, a stabilizer and the UV-
vis spectra obtained are shown in Figure 2B. The difference between the spectra observed
in with and without PVA systems was the band intensity. In presence of PVA, 1:1 and 1:3
ratios of Au3+ ions to CA, the peak at 530 nm was remain almost steady whereas at 1:10
Au3+ ions to CA ratio the absorbance value was doubled (0.45 to 1.1). The sizes of the
nanoparticles were measured using DLS (Dynamic Light Scattering) technique and listed
in Table 1. Particle size and the spectral properties (peak sharpness & intensity) together
reveal that CA under the study acts a stabilizer of gold nanoparticles, as there is no
influence of PVA. Both in presence and absence of PVA the particle size (~60 nm) and
peak intensity (Abs546 nm ~ 1.1) remain identical.
Dynamic Light Scattering (DLS) Measurements: This technique is also called as ‘quasi
elastic light scattering technique’, which provides rapid detection of the precise size and
4
size distribution of materials with nano to micro range particles. When light (from Laser
source) is made incident on the solution, light scatters in all direction. Thus an analyzer
(fast photon counter) kept at a specific angle detects the analyzer beam and hence these
time dependent fluctuations in the scattering intensity are seen. These fluctuations are due
to the small molecules in the solution undergoing Brownian movement and are directly
related to the rate of diffusion of the molecule through the solvent. Hence, these
fluctuations are analyzed to determine the hydrodynamic radius of the particles present in
the solution. A typical plot from DLS measurement is shown in Figure 3A wherein a
representative change of the electric field correlation function g1(τ) for the gold
nanoparticle in aqueous phase is recorded at a scattering angle of 90˚. The solid line is a fit
of the experimental data using the method of cumulants13. The particles were found to be
highly poly-disperse in nature and the polydispersity index (ratio of variance and square of
mean decay rate13) obtained from cumulant analysis. Figure 3B represents the particles
distributions in the solution and the peak position corresponds to the dominating size of
the nanoparticles.
Similarly, TEM and SEM analyses are used for size determination and image
monitoring14,15 of nanometric particles.
Reduction with AA
In addition to the generation of gold nanoparticles using CA, the chemical reduction of
Au3+ ions with AA (another reducing agent) has also been carried out under identical
conditions. The UV-vis spectra recorded for without and with PVA are shown in Figure 4.
In absence of PVA, the peak around 550 nm because of gold nanoparticles was shifted
towards red with respect to AA concentration, which is due to increase in particle size.
However, in presence of PVA the peak was relatively sharper when the concentration of
AA increased to 1 mM (10 times) (Figure 4B). The particle sizes measured by DLS
method are listed in Table 1.
Particle size and the spectral properties (peak sharpness & intensity) together reveal that
AA under the study does not act as a stabilizer of gold nanoparticles, since there is
significant change in peak intensity (Abs535 nm ~ 0.93 from 0.6 for 1:10 ratio) and the
particle size (~140 to 100 nm) in presence of PVA. Hence AA under the study is not a
potential stabilizer as CA does.
5
Similar experiments on chemical reduction of Au3+ ions were also carried out in methanol
medium using both CA and AA separately. It was observed that even after 72 hr of mixing
of two reactants, the solution color did not change to wine red, an indication of chemical
reaction of Au3+ ions with CA/AA in aqueous systems or vis-à-vis gold nanoparticle
formation, which designates negligible or a very slow reaction (if any) in methanol solvent
with these reactants. This dissimilar reaction behavior of gold nanoparticle formation in
methanol and water systems could be explained with the difference in solvent properties
viz. a low dielectric constant of the solvent methanol (32.63) as compared to water
(78.25)16.
The reactions taking place during the formation of gold nanoparticles through chemical
reduction of Au3+ ions with CA and AA are:
2HAuCl4 + Citrate ion gold particles + CO2 + HCOOH + etc. (1)
2HAuCl4 + 3C6H8O6 2Auo + C6H6O6 + 8HCl (2)
Where C6H8O6 and C6H6O6 represent ascorbic acid and dehydroascorbic acid respectively.
Soon after Auo takes part in nanoparticle formation.
Similarly, using other reducing agents such as sodium borohydride (NaBH4)17, aldehydes
(RCHO)18, and Carbon monoxide (CO)19, metal nanoparticleds are also generated through
chemical reduction method.
Radiolytic Reaction
Generation of gold nanoparticles through radiation induced free radical reaction
In radiation chemical research, high-energy ionizing radiation is generally used as
the energy source, whereas a light source is used in photochemical studies. The high
ionizing radiation emitting sources generally used for such work are radioactive nuclides
viz. 60Co, 90Sr, 3H and 137Cs in addition neutron source from nuclear reactors is also used.
In laboratory, machine sources such as X-ray tubes, Van de graaff generators, cyclotron,
synchrotrons betatron, febetron and other electron accelerators are used as high-energy
ionizing radiation sources. Therefore, in addition to an electronic excitation routinely
occur in the later type studies, ionization and super excitation of molecules are caused by
high-energy ionizing radiation20,21. Temporally, starting at femto-second time scale,
6
interaction of radiations with matter produces electrons in a high and wide energy range.
These electrons lose energy through various physical processes and lead to the formation
of low energy (eV) electrons. Interaction of matter with such low-energy electrons
produce ionic and/or excited species, which disappear through different processes such as
dissociation, recombination, luminescence, internal conversion, intersystem crossing, ion
molecule reactions and solvation, generating various end-products. In water/aqueous
medium, the primary transient species produced due to the interaction of high-energy
radiation in picosecond time scale and diffused homogeneously throughout the medium
within 0.1 μs time are eaq-, H•, •OH, H2, H2O2, H3O
+. Amongst these, eaq- and H• are
reducing in nature and their reduction potentials are: eaq- + H+/½H2 = –2.9 V and H• + e- +
H+/H2 = -2.3 V vs. NHE respectively whereas •OH is an oxidizing radical, which
possesses reduction potential •OH + H+ + e-/H2O = 2.72 V in acidic solution and •OH + e-
/OH- = 1.89 V vs. NHE in basic solutions (pKa •OH O- + H+ is 11.9)20. The yields of
primary species in water radiolysis vary with pH of the solution. The yield in radiation
chemistry is generally expressed as G-value (G), which is the number of species generated
or destroyed per 100 eV of absorbed dose. In SI unit, G is expressed in mol J-1. At near
neutral pH, the primary species of water radiolysis with G-values (in μmol J-1) are given
as:
eaq- (0.28), H• (0.062), •OH (0.29), H2O2 (0.07), H2 (0.05), H3O
+ H2O
(0.33)20, thus allowing quantitative measurement of the ensuing chemical change.
Similarly, the radiolysis of non-aqueous solvents generates a number of species
through different reactions. The generation of two strong reducing species such as solvated
electrons (esolv-) and alcohol radical along with other species during the radiolysis of
alcohol such as methanol, ethanol, 2-propanol is well known22-27. The different species
generated in radiolysis of methanol are presented below:
esolv-, CH3OH2
+, CH3O-, H•, H2, CH3O
•, •CH2OH (3) CH3OH
In the absence of any added solute, the transient absorption spectrum obtained on
radiolysis has an absorption maximum at 620 nm caused by esolv- species [21].
In this system, H• or H-atom produced on radiolysis reacts with methanol producing •CH2OH radicals (reaction rate constant = 2.6 x106 M-1 s-1 in aqueous system)28. H• +
7
CH3OH •CH2OH + H2 and CH3O• + H2. Hence, the existence of H• is negligible in
methanol system. Moreover, H• does not react directly with low concentration of the
solute present in methanol. •CH2OH, a reducing radical, generated during radiolysis as
well as through H• scavenging reaction with methanol has a reduction potential of –1.18 V
vs. NHE at neutral pH29.
Steady state radiolysis
Generation of Au nanoparticles in methanol
In our laboratory, for the generation of gold nanoparticles, 60Co γ-source was used for
steady state radiolysis. Typical Fricke’s dosimetry is employed to determine the dose
absorbed by the solution under identical experiments conditions20. Deoxygenated solution
of 0.2 mM Au3+ in methanol was γ-irradiated using the available 60Co γ-source at
Radiation and Photochemistry division, BARC (dose rate ~ 29 Gy/min in methanol) for
different time intervals. It was found that up to 3.5 kGy the intensity of yellow color
experimental solution decreases. On further irradiation (beyond 3.5 kGy dose), a purple
color solution was observed which is quite similar to the color reported for gold
nanoparticles. The absorption spectra of all samples before and after irradiation were
recorded, and shown in Figure 5. From figure 5, the absorption at 220 and 320 nm was
found to be decreasing with irradiation time, whereas after irradiation to 3.5 kGy dose, a
new broad band with absorbance maximum around 525 – 550 nm was appeared. This band
is due to the gold nanoparticles, and their size was >400 nm with high polydispersity.
Generation of Au-nanoparticles in methanol in presence of stabilizer
The effect of stabilizer in the generation of Au-nanoparticles through free radical induced
reaction was also studied. For which a solution of 0.2 mM Au3+ in methanol containing 1
mM PVA (a stabilizer) was irradiated using 60Co γ-source under Ar-purged conditions.
Figure 6 represents the spectra recorded for irradiated samples along with non-irradiated
one. It was observed that after γ-irradiation to 300 Gy dose, the solution color starts
changing to purple, this became more intense after 3.5 kGy dose. The peak at 530 nm in
PVA system is quite different than that observed in without PVA system (see figure 5).
Under DLS measurements the size of Au-nanoparticles was found to be ~100 nm, which is
more stable in contrast to without PVA under identical conditions. The reaction for the
8
formation of nanoparticle including coagulation is fairly controlled in PVA containing
system due to the increase in solution density; as a result smaller-sized nanoparticles were
produced.
Later the gold nanoparticles in both solutions (with and without PVA) were exposed to air
(oxygen) and the observed spectra were found to be identical to their respective spectra
recorded before the exposure to air. This study suggests that the gold nanoparticle
generated in methanol during radiolysis is stable and inactive to oxygen in contrast to
silver nanoparticle in methanol30.
Pulse radiolysis
Pulse radiolysis technique is a well-accepted method for studying kinetics and the spectral
properties of the short-lived transient and/or reaction intermediate species (≥ ns life-
time)31. Such studies have been carried out using 7 MeV electron pulses of 50, 100, 200 ns
and 2 μs durations from a linear electron accelerator (Forward Industries, UK). The
experimental set up is currently available at Radiation and Photochemistry Division,
BARC for pulse radiolysis studies and its detail has been reported earlier32. The schematic
diagram of pulse radiolysis set-up is shown in Figure 7. In brief, an electron pulse beam
irradiates the sample contained in a 10 mm X 10 mm ‘‘Suprasil’’ square cuvette kept at a
distance of ~120 mm from the exit window, where the electron beam diameter is ~10 mm.
The transient changes in the absorbance of the solution caused by the electron beam are
monitored with the help of a collimated light beam from a pulsed 450 W Xenon arc lamp.
The analyzing light has been detected in PMT (Hamamatsu, R-955) placed at the exit of
monochromator (Kratos, model GMA 252). The output signal from PMT is fed to 100
MHz 1 GS/s Tektronix TDS 220 Storagescope. Spectral and kinetic data acquisition and
analysis has been carried out with the help of an IBM compatible PC33. Aerated 10 mM
potassium thiocyanate solution is used for determining the absorbed dose using a value of
2.59x10-4 m2 J-1 for Gε at 475 nm34. Pulse doses are 10, 20, 40 and 80 Gy respectively for
50, 100, 200 ns and 2 μs pulses. The pulse doses are variable and the desired dose can be
achieved by changing instrumental parameters. Care need to be taken while preparing the
methanolic solutions so that the water content always remains less than 1%. The water
contamination in alcohols plays an important role in radiolysis25 and the possible effects
are: i) change in matrix properties; ii) change in reaction rate constants. Anhydrous CuSO4
was used to test the water content in alcohols27. If the anhydrous CuSO4 (white) failed to
9
transform into blue color in alcohols then the presence of negligible or very low water
contamination (within the limit permissible for such studies) was indicated. Presently, to
reconfirm its’ purity, the yield and kinetics profiles of transient are additionally checked
by adding 1% water to alcohol. Unchanged kinetic behavior confirms the suitability of the
methanol solvent and permissible for such experiments. High concentrated (24 mM) Au3+
ion stock solution in water prepared previously is mixed with neat methanol to formulate
desired experimental solutions of <1% water as impurity.
Reduction of Au3+ ions in methanol The time resolved spectra due to the transient species generated during electron pulse
irradiation on deoxygenated methanol are shown in Figure 8. The spectra at 0.4 and 0.8 μs
time after electron pulse are due to solvated electron and the spectra are matching well
with the reported one21. The decay of esolv- was faster in the presence of Au3+ ions in
methanol than measured in neat (without Au3+ ions) solvent, which authenticate the
reaction between Au3+ ions and esolv-. The decay time profiles obtained in different conc.
of Au3+ ions are shown in Figure 9. The reaction rate constant (kbimol) was determined by
following the change in pseudo first order decay (kϕ) of esolv- absorption at 620 nm21 with
respect to Au3+ ions concentrations. From the slope of the plot kϕ vs. [HAuCl4], (shown as
inset of Figure 9), the bimolecular reaction rate constant was evaluated to be 7.6×109 dm3
mol-1 s-1.
The time resolved spectra recorded during the course of esolv- reaction with Au3+ ions (0.2
mM HAuCl4) are shown in Figure 10, which exhibit three absorption maxima at 270, 370
and 470 nm. The nature of the absorbing species at these three wavelengths is quite
similar; at each peak position the absorption intensity decreases with time. The inset of
Figure 10 represents the time profiles obtained at 470 and 270 nm in electron pulse
irradiated deoxygenated 0.2 mM Au3+ ions in methanol. The 470 nm time profile shows
the faster decay of esolv- initially which is due to solvated electron reaction with Au3+ ions
followed by the formation of reduced Au3+ ions intermediate species. The decay of esolv-
attains its minimum value around 1.5 μs whereas the formation of reduced Au3+
intermediate species is completed within 10 μs after the electron pulse. This observation
concludes that the formation of the species absorbing at 470 nm is not only due to simple
first reduction species but also consisting different intermediate species originating from
10
the solvated electron reaction. At longer time (up to 5 ms) also no intermediate species
absorbing within the measurement region were generated after the subsequent decay of
reduced Au3+ ion species absorbing at 470 nm. Similar observation in 2-propanol system
has been reported recently35. Moreover, it is difficult to interpret that these three peaks are
due to one species. Comparing the reducing radical yields in methyl viologen (MV2+)
systems containing with and without Au3+ ions and the transient spectrum observed in 2-
propanol system, the spectrum of very first reduced Au3+ species (i.e. Au2+•) has been
derived and discussed earlier35.
The overall reactions taking place during the formation of gold nanoparticles through free
radical induced Au3+ ions reduction are:
In deoxygenated (Ar/N2-purged) system:
The initial species Au•++, Au(I) and Auo possessing optical absorption in 250-720 nm
wavelength region generated during the course of reaction of esolv- with Au3+ ions as
discussed above are shown below through reactions 4-7.
Au3+ + esolv- Au•++ (4)
Au•++ + Au•++ (Au•++)2 Au3+ + Au•+ (5)
Au•+ + Au•++ Au3+ + Auo (6)
Au•+ + Au•+ Au•++ + Auo (7)
The absorption of (Au•++)2 and Au•++ species as shown earlier in 2-propanol systems30 are
formed during the reduction of Au3+ ions, undergoes decay successively. These reactions
continued to the formation of gold nanoparticles. Hence, the absorption shown in figure 10
is quite similar to 2-propanol system, which is due to the contribution of Au•++, Au•+ and
Auo intermediate species. These intermediate species are stabilized with solvent molecules
or the species generated from alcohol radiolysis. For a single pulse (~30 Gy) irradiation,
the formation of the gold nanoparticle was not observed whereas purple coloration due to
gold nanoparticle appeared after either long time radiolysis or repetitive pulse irradiation
up to dose ≥ 400 Gy.
In conclusion, two different systems (water and methanol) were used for the generation of
gold nanoparticles using chemical as well as free radical induced reduction processes. In
11
chemical reduction reaction, AA & CA both generate nanoparticles in aqueous systems
whereas their chemical reactions in methanol are quite slow, highlighting the importance
of solvent properties. In free radical induced Au3+ ions reduction, there are many reports in
aqueous systems available in the literature; hence it is not discussed in this presentation.
Moreover, as compared to radical (esolv- and •CH2OH) reaction with Au3+ in water,
reaction rates in methanol are diffusion controlled.
Acknowledgement: The author would like to thank Ms. D.G. Deepanjali (M.Sc student from Pune University) and Dr. R. Ganguli of Chemistry Division, BARC for their assistance during the course of studies/experiments. He also thanks the LINAC maintenance team for their help during pulse radiolysis measurements. REFERENCES
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13
Table 1
Sizes of the gold nanoparticles generated under different conditions:
Sr. no. Systems Size (nm)
1 0.1 mM Au3+ ions with 0.1 mM CA in water 60
2 0.1 mM Au3+ ions with 0.1 mM CA in water with PVA 57
3 0.1 mM Au3+ ions with 0.3 mM CA in water 72
4 0.1 mM Au3+ ions with 0.3 mM CA in water with PVA 75
5 0.1 mM Au3+ ions with 1 mM CA in water 147
6 0.1 mM Au3+ with 1 mM CA in water PVA 150
7 0.1 mM Au3+ ions with 0.1 mM AA in water 167
8 0.1 mM Au3+ with 0.1 mM AA in water with PVA 104
9 0.1 mM Au3+ ions with 0.3 mM AA in water 111
10 0.1 mM Au3+ ions with 0.3 mM AA in water with PVA 120
11 0.1 mM Au3+ ions with 1 mM AA in water 190
12 0.1 mM Au3+ with 1 mM AA in water PVA 125
13 0.1 mM Au3+ ions on γ-radiolysis in methanol (deaerated) -
14 0.1 mM Au3+ ions on γ-radiolysis in methanol with PVA (deaerated)
100
14
240 320 400 480 5600.0
0.3
0.6
0.9
1.2
1.5
0 25 50 75 1000.2
0.3
0.4
0.5
a
d c
Abs
λ (nm)
b
Abs 31
0
% of Methanol
Figure 1. UV-vis spectra of 0.2 mM HAuCl4 recorded in different concentrations of methanol in water. (a) no methanol, (b) 20% methanol, (c) 30% methanol, (d) 40% methanol. Inset: the plot of absorbance at 310 nm vs. % of methanol in the solutions containing 0.2 mM HAuCl4.
15
300 400 500 600 700 8000.0
0.3
0.6
0.9
1.2
1.5
300 400 500 600 700 8000.0
0.3
0.6
0.9
1.2
1.5
1:11:3
1:10
Abs
orba
nce
λ / nm
B
1:3
1:1
1:10
A
A
bsor
banc
e
λ / nm
Figure 2. UV-vis spectra of the gold nanoparticles generated during the reaction of CA with Au3+. Systems used are: 1:10, 1:3 & 1:1 ratios of Au3+ ions to CA without (A) & with (B) 1 mM PVA in aerated water.
16
10-1 100 101 102 103 104
0.0
0.2
0.4
0.6
0.8
10 100 10000
3
6
9
12
15
A
Cor
rela
tion
Fun
ctio
n (g
1 (τ))
Time (μSec)
B
Num
ber
Size
Figure 3. (A): Plot of correlation function vs. time for gold nanoparticles. (B): The distribution of particles of the same sample for figure 3 A.
17
300 400 500 600 700 8000.0
0.3
0.6
0.9
1.2
1.5
1.8
300 400 500 600 700 8000.0
0.3
0.6
0.9
1.2
1.5
B
1:1
1:3
1:10
Abs
orba
nce
λ / nm
A
1:1
1:3
1:10
Abs
orba
nce
λ / nm
Figure 4. UV-vis spectra of the gold nanoparticles generated during the reaction of AA with Au3+. Systems used are: 1:10, 1:3 & 1:1 ratios of Au3+ ions to AA without (A) & with (B) PVA in aerated water.
18
300 400 500 600 700 8000.0
0.3
0.6
0.9
1.2
1.5
b
cd
e
a
Ab
s
λ (nm)
Figure 5. UV-vis spectra of gold nanoparticles generated during γ-radiolysis of deaerated 0.2 mM Au3+ ion solution in methanol before (a) and after (b) 150 Gy; (c) 600 Gy; (d) 3.5 kGy and (e) 5 kGy irradiation.
Figure 6. UV-vis spectra of gold nanoparticles generated during γ-radiolysis of deaerated 0.2 mM Au3+ ion solution in methanol containing 1 mM PVA before a) and after (b) 450 Gy; (c) 1.7 kGy; (d) 3.5 kGy and (e) 5 kGy irradiation.
200 300 400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
ed
ba
Ab
sorb
an
ce
λ / nm
c
19
400 500 600 700 800
0.000
0.006
0.012
0.018
0.024
7 μs
0.8 μs
ΔA
λ / nm
0.4 μs
Figure 7. Typical pulse radiolysis setup.
Figure 8. The time resolved spectra obtained during electron beam irradiation of deoxygenated methanol.
XeLamp L L M PM T
PC
7M ev electro n pulse beam
O scilloscope
Sam pleShutter
M – MonochromatorL - LensX e – X enon arc la m pPC – C om puterPMT – Photomult ip lier tube
20
0 2 4 6 8-0.005
0.000
0.005
0.010
0.015
0.020
0.0 0.1 0.2 0.3 0.40
10
20
30
40
k φ /105 s
-1
[HAuCl4] / 10-3 M
d
c
b
a
ΔA
Time / μs
0 5 10 15 20
-0.02
0.00
0.02
0.04
0.06
0 5 10 15 20-0.002
0.000
0.002
0.004
0.006
300 400 500 600 700
-0.02
0.00
0.02
0.04
0.06
Time (μs)
ΔA
270 nm
Time (μs)
470 nm
4
20 μs
2
0.8
ΔA
λ (nm)
Figure 9. The time profiles at 620 nm obtained during electron beam irradiation of deoxygenated methanol in absence (a) and in presence of different concentrations of Au(III) ions (b: 0.01 mM; c: 0.025 mM and d: 0.4 mM). Inset: plot of kφ vs. concentration of HAuCl4
Figure 10. The time resolved spectra obtained in electron pulse irradiated deoxygenated 0.2 mM HAuCl4 solutions in methanol after 0.8 to 20 μs (Dose/pulse 16 Gy). Inset: time profiles at 270 and 470 nm.
