Synthesis and Characterization of High Concentration Block

Mar 2, 2011 - The formation of high concentration gold nanoparticles at room temperature is reported in block copolymer-mediated synthesis where the n...
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Synthesis and Characterization of High Concentration Block Copolymer-Mediated Gold Nanoparticles Debes Ray,† Vinod K. Aswal,*,† and Joachim Kohlbrecher‡ † ‡

Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Laboratory for Neutron Scattering, Paul Scherrer Institut, CH-5232 PSI Villigen, Switzerland

bS Supporting Information ABSTRACT: The formation of high concentration gold nanoparticles at room temperature is reported in block copolymermediated synthesis where the nanoparticles have been synthesized from hydrogen tetrachloroaureate(III) hydrate (HAuCl4 3 3H2O) using block copolymer P85 (EO26PO39EO26) in aqueous solution. The formation of gold nanoparticles in these systems has been characterized using UV-visible spectroscopy and small-angle neutron scattering (SANS). We show that the presence of additional reductant (trisodium citrate) can enhance nanoparticle concentration by manyfold, which does not work in the absence of either of these (additional reductant and block copolymer). The stability of gold nanoparticles with increasing concentration has also been examined.

’ INTRODUCTION Gold nanoparticles are of a great deal of recent interest in the context of emerging nanotechnology applications. At the nanoscale, they exhibit unique quantum and surface properties, different from those of atoms as well as bulk materials.1-6 Depending on the applications they are being synthesized for, they can be synthesized in many different ways. One of the easiest and convenient ways is the chemical reduction method which involves the use of four basic materials, namely, solvent, metal salt, reducing agent, and stabilizing agent.6-10 Recently, use of block copolymers for the synthesis of gold nanoparticles is found to have many advantages; for example, block copolymer not only plays the dual role of reductant and stabilizer but also provides an economical and environmentally benign way for the synthesis of gold nanoparticles.11-14 Copolymers are a special type of polymers which have two or more different monomer units linked by covalent bonds. PEOPPO-PEO triblock copolymers are well-known nonionic surfactants (with the commercial name of Pluronics) with two dissimilar moieties, hydrophilic PEO block and hydrophobic PPO block, within the same molecule.15-18 These amphiphilic triblock copolymers possess a symmetrical structure (EO)x(PO)y(EO)x, where x and y denote the number of ethylene oxide and propylene oxide monomers per block, respectively, and are available in a range of x and y values in the form of pastes, flakes, r 2011 American Chemical Society

and liquids. In aqueous solution, the molecules self-assemble to form micelles of various forms and sizes, depending on thermodynamic parameters (entropy or enthalpy driven). The hydrophobic blocks of the block copolymers (PPO) form the core of these micellar aggregates, whereas the hydrophilic ones (PEO), with the surrounding water molecules, form the corona. The block copolymers can be used to produce metal nanoparticles because of their ability to reduce metal ions. On mixing the aqueous solution of metal (e.g., gold) salt and block copolymers, these polymeric nanostructured matrixes engulf the ionic metal precursors, which after subsequent reduction form nanoparticles. Self-assembly of block copolymer in this method is utilized to control the synthesis of gold nanoparticles.19 The formation of gold nanoparticles from AuCl4- comprises three main steps: reduction of AuCl4- ions by the block copolymers in the solution and formation of gold clusters, adsorption of block copolymers on gold clusters and reduction of AuCl4- ions on the surfaces of these gold clusters, and growth of gold particles in steps and finally its stabilization by block copolymers.20,21 The role of block copolymers in the synthesis (formation rate, yield, stability, shape, and size of nanoparticles) varies with their molecular weight, PEO/PPO block length, Received: December 7, 2010 Published: March 02, 2011 4048

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Langmuir polymer concentration, and temperature.22-26 The synthesis is enhanced with an increase in molecular weight and PEO block length. The reduction of bound AuCl4- ions proceeds via oxidation of the oxyethylene and oxypropylene segments by the metal center. PEO is known to form a conformation similar to pseudo-crown-ether structure that is able to bind with metal ions. The induced cyclization is caused by ion-dipole interactions between the templating ion and the electron lone pairs of the ethylene oxide linkages. The interaction between AuCl4ions and PEO becomes important for longer PEO (>EO7) chains. Several oxygen atoms in the PEO chain interact with one ion, and therefore, the strength of the attraction depends on the length of the PEO chain.21 PPO facilitates growth of gold clusters to nanoparticles by block copolymer adsorption on gold clusters and reduction of gold ions on the surface of these clusters. On the other hand, the increase in the block copolymer concentration and temperature increases the reaction activity and hence the nanoparticle formation is enhanced. The limitation of this method is that it has quite low yield.21,22 The concentration of nanoparticles simply does not increase with the increase in the gold salt concentration. In this paper, we have used UV-visible spectroscopy and small-angle neutron scattering (SANS) for the understanding of the reasons for the limited yield and how it can be improved by manyfold. The measurement of surface plasmon resonance (SPR) of the gold nanoparticles by UV-visible spectroscopy provides information on the formation of gold nanoparticles, the synthesized concentration, and the stability of gold nanoparticles.27 SANS is used to examine the role of different components in the synthesis and characterize the nanoparticles.28 In particular, contrast variation SANS is an ideal technique to study such multicomponent systems.29

’ EXPERIMENTAL DETAILS Materials. Pluronic block copolymer P85 was obtained from BASF Corp., Mount Olive, New Jersey. The gold salt of hydrogen tetrachloroaureate(III) hydrate (HAuCl4 3 3H2O) and trisodium citrate (Na3C6H5O7 3 5.5H2O) were purchased from Sigma-Aldrich. All products were used as received. Synthesis of Gold Nanoparticles. The synthesis of the gold nanoparticles was carried out by varying the concentration of gold salt (HAuCl4 3 3H2O) at a fixed concentration of block copolymer in H2O. The block copolymer concentration used was 1 wt % (2.2 mM), and the concentration of gold salt varied in the range from 0 to 1 wt % (025.4 mM). The synthesis has been compared without and with the presence of additional reductant trisodium citrate (Na3Ct). After mixing all the components for synthesis, samples were kept at room temperature without any disturbances. Characterization of Gold Nanoparticles. The formation of gold nanoparticles is confirmed using UV-visible spectroscopy. This technique measures absorbance or transmittance to give qualitative and quantitative information at the molecular level. SPR band determination is one of the most familiar applications of this technique, which arises due to the resonance between the incident radiation and collective oscillation of conducting electrons of metal nanostructures.27,30 The measurements were carried out using a 6505 Jenway UV-visible spectrophotometer. This instrument is suitable for measurements in the scanning wavelength range between 190 and 1100 nm (with an accuracy of (1 nm). It makes use of a tungsten halogen lamp as the visible light source and a deuterium discharge lamp for the UV light source. The instrument was operated in spectrum mode with a wavelength interval of 1 nm, and the samples were held in quartz cuvettes of path length 1 cm.

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SANS is used to study the role of block copolymer in the synthesis of gold nanoparticles. In SANS, one measures the coherent differential scattering cross section (dΣ/dΩ) per unit volume as a function of wave vector transfer Q (= 4π sin(θ/2)/λ, where λ is the wavelength of the incident neutrons and θ is the scattering angle). It provides information about the shape and size of the scattering particles in the length scale of 10-1000 Å.28,29 The measurements were carried out at SANS-I facility, Swiss Spallation Neutron Source SINQ, Paul Scherrer Institut, Switzerland.31 The wavelength of the neutron beam used was 6 Å. The experiments were performed at sample-to-detector distances of 2 and 8 m to cover a Q range of 0.007-0.32 Å-1. The sample solutions were kept in a 2 mm thick quartz cell with Teflon stoppers. The scattered neutrons were detected using two-dimensional 96 cm  96 cm detector. All the measured data were corrected and normalized to absolute scale using BerSANS-PC data processing software.32

’ SANS ANALYSIS In SANS, dΣ/dΩ for a system of particles dispersed in a medium can be written as33 dΣ ðQ Þ ¼ np Vp 2 ðFp - Fs Þ2 PðQ Þ SðQ Þ þ B dΩ

ð1Þ

where np denotes the number density of particles, Fp and Fs are, respectively, the scattering length densities of particle and solvent, and Vp is the volume of the particle. P(Q) is the intraparticle structure factor, and S(Q) is the interparticle structure factor. B is a constant term representing incoherent background, which is mainly due to hydrogen present in the sample. P(Q) depends on the shape and size of the particle and is the square of single particle form factor F(Q) expressed as   ð2Þ PðQ Þ ¼ jFðQ Þj2 For a sphere of radius R, F(Q) is given by   3 sinðQRÞ - QR cosðQRÞ FðQ , RÞ ¼ ðQRÞ3

ð3Þ

S(Q) correlates the particles present in the system, and it is the Fourier transform of the radial distribution function g(r) for the mass centers of the particles. For diluted samples, S(Q) ∼ 1. The scattering from individual block copolymer unimers is given by the Gaussian chain conformation as34     dΣ ðQ Þ ∼ ðQRg Þ-4 Q 2 Rg 2 - 1 þ expð - Q 2 Rg 2 Þ ð4Þ dΩ u where Rg is the radius of gyration of the block copolymer. The scattering from block copolymer micelles is calculated using core-shell particles with different scattering length densities for core and shell. The structure of these micelles is described by a model consisting of noninteracting Gaussian PEO chains attached to the surface of the PPO core. The scattering cross section of the micelle is the combination of four different terms: the self-correlation of the core, the self-correlation of the chains, the cross term between core and chain, and the cross term between different chains. It is given by35 I m ðQ Þ ¼ Ns 2 bs 2 Fs ðQ Þ þ 2Ns bc 2 Fc ðQ Þ þ 2Ns ð2Ns - 1Þbc 2 Scc ðQ Þ þ 4Ns 2 bs bc Ssc ðQ Þ

ð5Þ

where bs and bc are excess scattering length of the core and chain, respectively, and Ns is the aggregation number of the micelles. 4049

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Figure 1. Photograph of gold nanoparticles in aqueous solutions of 1 wt % P85 with varying HAuCl4 3 3H2O concentration. The labels show the concentration of gold salt in wt %.

The scattering from gold nanoparticles stabilized by block copolymers can be expressed as36   dΣ ðQ Þ ¼ dΩ np h i2 nnp ðFnp - Fshell ÞVnp FðQ , RÞ þ ðFshell - Fs ÞVt FðQ , R þ tÞ þB

ð6Þ

where Fnp, Fshell, and Fs are, respectively, the scattering length densities of the nanoparticle, block copolymer shell, and solvent. R is the radius of the nanoparticle, and t is the thickness of the block copolymer shell on the nanoparticle. Vnp (= 4πR3/3) and Vt [= 4π(R þ t)3/3] are the volumes of the nanoparticle and total volume of nanoparticle along with block copolymer shell, respectively. In the case when both gold nanoparticles and free block copolymers (unimers and/or micelles) coexist, the total scattering is determined by the sum of the individual components as       dΣ dΣ dΣ ðQ Þ ¼ ðQ Þ þ ðQ Þ þ B ð7Þ dΩ t dΩ np dΩ bcp where the subscripts np and bcp correspond to nanoparticles and block copolymer, respectively. On contrast matching the block copolymers to the solvent, the above equation for a dilute system reduces to only one term, that of scattering from the gold nanoparticles as given by     dΣ dΣ ðQ Þ ¼ ðQ Þ dΩ t dΩ np ¼ nnp Vnp 2 ðFnp - Fs Þ2 Pnp ðQ Þ ð8Þ The polydispersity is incorporated as Z dΣ dΣ ðQ Þ ¼ ðQ , RÞ f ðRÞ dR þ B ð9Þ dΩ dΩ The polydispersity has been calculated by a log-normal distribution as given by "  # 1 1 R 2 pffiffiffiffiffiffi exp - 2 log f ðRÞ ¼ ð10Þ 2σ Rmed Rσ 2π where Rmed is the median value of the distribution and σ is the polydispersity of the distribution. The mean radius (Rm) is given by " # σ2 Rm ¼ Rmed exp ð11Þ 2

Figure 2. SPR peak in UV-visible absorption spectra of 1 wt % P85 with varying HAuCl4 3 3H2O concentration.

Throughout the data analysis, corrections were made for instrumental smearing. The calculated scattering profiles were smeared by the appropriate resolution function to compare with the measured data. The parameters in the analysis were optimized by means of a nonlinear least-squares fitting program.37

’ RESULTS AND DISCUSSION The synthesis of gold nanoparticles involves mixing of aqueous solutions of gold salt and block copolymer together. Figure 1 shows the photograph of gold nanoparticles prepared from aqueous solutions of 1 wt % P85 with varying HAuCl4 3 3H2O concentration. After some time of mixing (few minutes), the transparent block copolymer solution starts to turn purple. The color intensity of the equilibrated solution strongly depends on the salt concentration. The maximum color intensity exists at 0.008 wt % HAuCl4 3 3H2O and concentrations below and above which it falls. The purple color in the solution is an indication of the formation of gold nanoparticles, and their SPR is known to show such a color.30 Figure 2 shows UV-visible absorption spectra of these samples. The peak centered at 540 nm originates from the SPR of the gold nanoparticles, and the magnitude of this peak depends on the nanoparticle concentration and structure of gold nanoparticles. It is observed that as the concentration of gold salt increases the SPR peak height rises up to some maximum value and after that decreases and broadens. The maximum 4050

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Table 1. Neutron Scattering Length Densities (G) of Different Components in the Block-Copolymer-Mediated Gold Nanoparticle System component F (1010 cm-2)

Figure 3. SANS data of 1 wt % P85 with varying HAuCl4 3 3H2O concentration at (a) 30 °C and (b) 15 °C. The critical micelle temperature (CMT) of 1 wt % P85 is 26 °C. The solid curves are theoretical fits to the experimental data (using eqs 4 and 7). Inset in (a) shows the variation of calculated micellar fraction with increase in gold salt concentration.

absorbance at SPR (proportional to concentration of gold nanoparticles) has been found at 0.008 wt % HAuCl4 3 3H2O and is consistent with color intensity variation as in Figure 1. These results clearly show that the concentration of the gold nanoparticles simply does not increase with gold salt concentration. The block-copolymer-mediated synthesis is believed to be a quite fast method, as the synthesis reaches equilibrium within about 150 min (see Figure S1 in the Supporting Information). The role of block copolymer in the synthesis of gold nanoparticles has been examined by SANS. Figure 3 shows the SANS data of 1 wt % P85 with varying gold salt concentration corresponding to the samples in Figures 1 and 2 prepared in D2O. The use of D2O in place of H2O provides better contrast particularly for hydrogenous systems (block copolymer) as well as gives rise to low incoherent background. SANS data do not show any observable changes up to the gold salt concentration corresponding to the maximum nanoparticle concentration (0.008 wt %). This suggests that the scattering is mostly dominated by block copolymers and no significant scattering occurs from gold nanoparticles (eq 7) which could be because of

Au 4.66

P85 0.47

H2O -0.56

D2O 6.40

the low concentration and low contrast of these particles. The calculated scattering length densities of the different constituents present in the system are given in Table 1. There is an increase in the scattering intensity beyond the gold salt concentration corresponding to the maximum concentration of nanoparticles and can be explained as a result of the enhanced micellization at higher salt concentrations. This is further confirmed by the SANS data taken at lower temperature (15 °C; which is much lower than the CMT of P85, i.e., 26 °C) where the micellization of block copolymers is suppressed (Figure 3b). All the SANS data at 15 °C overlap to one curve which represents the scattering from block copolymer unimers (eq 4). These observations thus suggest that while the ratio of block copolymer-to-gold salt ion concentration controls the synthesis, most of the block copolymer remains unassociated to gold nanoparticles after the synthesis. The concentration of synthesized gold nanoparticles is low, and hence, the fraction of block copolymers that would remain associated with particles to stabilize them will also be low. The analysis from the SANS data at 15 °C using eq 4 gives the radius of gyration of block copolymer unimers around 23 Å. The higher scattering intensity at 30 °C arises because of the temperatureinduced micellization of block copolymer. Both the block copolymer unimers and micelles coexist in these systems. The fraction of block copolymer micelles remains the same up to the gold salt concentration of 0.008 wt %, and thereafter it increases as a result of enhanced micellization (salting out effect) with the salt concentration. The micellar size is found to be almost similar having a core radius of 40 Å surrounded by a PEO chain of radius of gyration of 12 Å and aggregation number around 70. We have seen that the concentration of gold nanoparticles for 1 wt % block copolymer is the maximum at 0.008 wt % gold salt, which in turn suggests that it cannot be simply enhanced by increasing salt concentration. This gives rise to the minimum molar ratio (rmin) of block copolymer-to-gold salt ion if about 11 for the formation of gold nanoparticles from gold ions. This can be understood in terms of the following mechanism of the synthesis of gold nanoparticles:21 (i) Reduction of AuCl4- ions by block copolymers and formation of gold clusters. AuCl4 - þ nðPEO-PPO-PEOÞ f ðAuCl4 - Þ-ðPEO-PPO-PEOÞn m½ðAuCl4 - Þ-ðPEO-PPO-PEOÞn  f Aum þ 4mCl- þ oxidation products (ii) Adsorption of block copolymers on the gold clusters and further reduction of AuCl4- ions on the surfaces of these gold clusters. Aum þ l½ðAuCl4 - Þ-ðPEO-PPO-PEOÞn  f Aup -ðPEO-PPO-PEOÞq þ 4lClþ oxidation products (iii) Continued growth of gold nanoparticles using step (ii) and finally the stabilization by block copolymers. 4051

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Figure 4. Photograph of gold nanoparticles in aqueous solutions of 1 wt % P85 þ 0.02 wt % HAuCl4 3 3H2O with varying trisodium citrate (Na3Ct) concentration. The labels show the concentration of Na3Ct in wt %.

Figure 5. UV-visible absorption spectra of 1 wt % P85 with varying (HAuCl4 3 3H2O þ Na3Ct) concentration.

One of the important steps [step (i)] in the synthesis is the reduction of gold salt ions, and it requires n to be greater than rmin. The fact that rmin is quite large gives rise to the limited yield for a given concentration of block copolymer. It has been seen in the UV-visible absorption spectra that if n is less than rmin (for concentration greater than 0.008 wt %), most of the gold salt ions do not undergo reduction (see Figure S2 in the Supporting Information). We also calculate the fraction of block copolymers associated with the coating of the gold nanoparticles, which is found to be very small (less than 1%).38,39 Therefore, it is expected that most of the block copolymers remain free, and this is consistent with the fact that SANS data do not show any significant change at the gold salt concentration corresponding to the maximum nanoparticle concentration than that of from the pure block copolymer solution (Figure 3). This also implies that the limited yield is primarily because of the limited reduction as high value of n (>rmin) is required. On the other hand, steps (ii) and (iii) are expected to favor the synthesis as long as reduction can take place. Based on this understanding, we have made use of additional reductant by which the nanoparticle concentration can be improved by manyfold. Na3Ct, one of the most widely used reducing agents, is used for additional reduction. The optimization on Na3Ct was carried out by varying its concentration for a fixed concentration of gold salt. Figure 4 shows the photograph of the concentration improvement of the

Figure 6. Calculated concentration of gold nanoparticles in 1 wt % P85 with varying (HAuCl4 3 3H2O þ Na3Ct) concentration. The comparison of nanoparticle concentration without the use of block copolymer is also shown.

gold nanoparticles in 1 wt % P85 with 0.02 wt % HAuCl4 3 3H2O by varying the Na3Ct concentration. It is seen that the nanoparticle concentration increases with the increase in Na3Ct concentration and has a maximum around 0.02 wt %. The molecular weights of HAuCl4 3 3H2O and Na3Ct are similar which suggests typically 1:1 involvement of these two components in the reduction. The nanoparticle concentration has been examined up to 1 wt % of gold salt concentration, and it has been found that it now increases with the concentration (see Figure S3 in the Supporting Information). This is confirmed by the UV-visible spectroscopy data as shown in Figure 5. The SPR peak increases with the gold salt concentration. It may be mentioned that to avoid the saturation effect in absorption at high concentrations, these data were measured on the samples diluted to 0.005 wt %, and those shown in the Figure 5 are the measured data multiplied by the dilution factor. The calculated nanoparticle concentration from these samples as a function of gold salt concentration and the comparison of nanoparticle concentration in presence of Na3Ct without the block copolymer are shown in Figure 6. The nanoparticle concentration (proportional to the integrated absorbance) of the synthesis has been calculated from the area under the SPR peak covering the wavelength range 450-700 nm. It is clear that the addition of Na3Ct has improved the reduction and subsequently the concentration of the gold nanoparticles, 4052

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Figure 7. SANS data of 1 wt % P85 with varying (HAuCl4 3 3H2O þ Na3Ct) concentration at 15 °C.

Figure 8. SANS data of 1 wt % P85 with varying (HAuCl4 3 3H2O þ Na3Ct) concentration at 15 °C. The block copolymers are contrastmatched to the solvent. The solid curves are theoretical fits to the experimental data (using eq 8).

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which only works in the presence of block copolymer. The nanoparticle concentration is now shown to increase with gold salt concentration in the presence of additional reductant. The high concentration nanoparticles have been characterized using SANS. The scattering at room temperature is still dominated by the micelles and increases as expected with salt concentration (see Figure S4 and Table T1 in the Supporting Information). From such data, it is difficult to separate the relatively weak scattering from gold nanoparticles. Therefore, the SANS data (Figure 7) were also measured at low temperature (15 °C) to avoid the strong scattering from block copolymer micelles. At low temperature, block copolymers remain as unimers and therefore they give rise to much smaller scattering as compared to micelles at room temperature (eq 4). A distinct scattering buildup in the low Q region is seen because of the scattering from gold nanoparticles (eq 7). This buildup is visible for gold salt concentration beyond 0.1 wt %. The overall increase in the scattering intensity at 1 wt % is again because of salt-induced micellization of block copolymers. The scattering in Figure 7 has contributions from gold nanoparticles coated with block copolymer as well as from block copolymers as unimers and their micelles, if any (eq 7). These data show that the scattering is mostly dominated by the individual block copolymers and the fact that it is difficult to fit the data of that adsorbed block copolymer on gold nanoparticle with core-shell structure. To simplify the analysis, SANS data (Figure 8) were taken on block copolymers contrast-matched to the solvent (85% H2O in H2O and D2O mixture). In this case, no scattering from block copolymers is seen and whatever is measured is the scattering from gold nanoparticles (eq 8). The scattering in Figure 8 increases as the number density of the gold nanoparticles increases with increasing gold salt concentration. The gold nanoparticles are fitted with polydispersed spherical particles using eq 9. The schematic of gold nanoparticle is shown in Figure 9. Although we do not directly measure the adsorbed block copolymers on the nanoparticle surface, their stability particularly observed at higher temperature suggests that the hydrophobic PPO should be covering the surface of the gold nanoparticles surrounded by the solvated PEO. Na3Ct remains uniformly dissolved in the solution, as it is highly soluble in aqueous solution (solubility in water ∼42.5 g/100 mL at 25 °C). The particle size is found to increase with a decrease in polydispersity

Figure 9. Schematic of structure of block-copolymer-stabilized gold nanoparticles. The samples in cells show that the concentration of gold nanoparticles in the presence of additional reductant can be enhanced. 4053

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Figure 10. Calculated size distributions of gold nanoparticles in 1 wt % P85 at varying gold nanoparticle concentration.

with increasing gold salt concentration. The calculated size distributions of the nanoparticles with increasing concentration for some of the systems are shown in Figure 10. We also observed similar results using small-angle X-ray scattering (see Figure S5 in the Supporting Information) and transmission electron microscopy (see TEM micrographs in Figure S6 and calculated size distributions in Figure S7). For these two techniques, only gold nanoparticles are seen as the contrast for block copolymer is negligible. The increase in nanoparticle concentration is not found to be linear with gold salt concentration as shown in Figure 6 and can be explained in terms of increase in the size of the particle as well as decrease in stability if any. The increase in the size has already been observed, and this will decrease the number density and hence the nanoparticle concentration. Figure 11 shows the time evolution of synthesis of gold nanoparticles for different gold salt concentrations. Time-dependent UV-visible absorption spectra were collected over a period of time. The data have been taken up to 4 h after the mixing of the components. For all the samples, nanoparticle concentration increases with the synthesis time and is found to be reaching the maximum after about 1 h. The rate of synthesis is higher when the concentration is high. The nanoparticle concentration rate increases about 10 times when the gold salt concentration increased from 0.05 to 1 wt %. There is saturation at the maximum concentration for the samples of gold concentrations up to 0.2 wt %. However, it is observed that at 0.5 and 1 wt % gold salt concentration the nanoparticle concentration decreases and saturates after achieving the maximum value. The decrease in nanoparticle concentration is possible, because, unlike block copolymer, a too large amount of citrate increases the ionic strength of the solution, resulting in the instability of gold particles in the solution.40,41 It may be mentioned that we have not made any attempt by SANS to examine the structural evolution of particles in Figure 11 since the large measuring time required for

Figure 11. Time-dependent variation of the integrated absorbance (400-800 nm) of SPR peak in 1 wt % P85 at different nanoparticle concentrations.

these samples (about 1 h) makes it difficult to study the growth of the particles which are associated with smaller time scale (about few minutes). However, recently SAXS using synchrotron has been used to study structural evolution of particles in similar systems as there exists a high flux of X-rays as well as high scattering contrast of gold nanoparticles for X-rays.42 We have seen that the nanoparticle concentration of block copolymer-mediated synthesis of gold nanoparticles is limited by the capability of the reduction of gold ions by block copolymer (Figure 2). If the reduction is enhanced by the presence of strong reducing agent such as Na3Ct, the nanoparticle concentration is increased (Figure 5). There have been numerous studies reported where Na3Ct alone has been used for the synthesis of gold 4054

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Langmuir nanoparticles.41,43,44 This requires Na3Ct solution to be used at high temperature (>80 °C) so that there are sufficient dissociated citrate ions that are needed around the nanoparticles to stabilize them.45-50 The rate of reduction is also enhanced with the increase in temperature which many times leads to the oversized nanoparticles (>100 nm) and instability of the nanoparticles in solution. Therefore additional stabilizers such as CTAB, thiolated and phosphine surfactants are used to control the stability of gold nanoparticles synthesized using Na3Ct.50,51 With the addition of Na3Ct, citrate molecules in the presence of block copolymers adsorb on the surface of the growing gold clusters, enhancing the reduction of AuCl4- ions in step (i). The subsequent growth of the clusters [step (ii)], the formation of nanoparticles, and their stabilization using block copolymers [step (iii)] completes the synthesis, thus improving the nanoparticle yield by manyfold as long as the reduction can be maintained. This method can be viewed as the additive effect of the enhanced reduction by Na3Ct with control and stabilization by the block copolymers. Unlike the block copolymer, a too large amount of citrate increases the ionic strength of the solution, resulting in the instability of nanoparticles in the solution. To the best of our knowledge, we present for the first time the room temperature synthesis of Na3Ct-assisted blockcopolymer-mediated high concentration stable gold nanoparticles.

’ CONCLUSIONS A high concentration synthesis of gold nanoparticles using block copolymer P85 at room temperature has been studied. This is achieved in the presence of additional reductant (Na3Ct) to enhance the reduction and hence nanoparticle concentration, which does not work in the absence of block copolymer and/or Na3Ct. The nanoparticle concentration is found to increase drastically with gold salt concentration in the presence of additional reductant. The size of the nanoparticles also increases with the nanoparticle concentration. The time-dependent decrease in the concentration of nanoparticles observed at higher gold salt concentrations is related to the increased surface coating of gold nanoparticles. ’ ASSOCIATED CONTENT

bS

Supporting Information. Time-dependent UV-visible spectra of synthesis, concentration-dependent UV-visible spectra of gold chloride ions, photograph of high concentration gold nanoparticle systems, fitted SANS data of block copolymer micellization, fitted SAXS data, TEM micrographs of gold nanoparticles, and their size distribution histograms in high yield systems. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Telephone: þ91 22 25594606. Fax: þ91 22 25505151.

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