Rapid Seeded Growth of Monodisperse, Quasi-Spherical, Citrate

Publication Date (Web): September 6, 2012 .... 1,4-Benzenediboronic-Acid-Induced Aggregation of Gold Nanoparticles: Application to Hydrogen Peroxide D...
1 downloads 0 Views 408KB Size
Download PDF
Article pubs.acs.org/Langmuir

Rapid Seeded Growth of Monodisperse, Quasi-Spherical, CitrateStabilized Gold Nanoparticles via H2O2 Reduction Xiaokong Liu,† Haolan Xu,† Haibing Xia,‡ and Dayang Wang*,† †

Ian Wark Research Institute, University of South Australia, SA 5095, Australia State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China

‡

S Supporting Information *

ABSTRACT: In this report, we demonstrate a rapid and simple seeded growth method for synthesizing monodisperse, quasispherical, citrate-stabilized Au nanoparticles (Au NPs) via H2O2 reduction of HAuCl4. Au NPs with diameter ranging from 30 to 230 nm can be synthesized by simply adding 12 nm citrate stabilized Au NP seeds to an aqueous solution of H2O2 and HAuCl4 under ambient conditions. The diameter of the resulting Au NPs can be quantitatively controlled by the molar ratio of HAuCl4 to the Au seeds. The standard deviation of the Au NP sizes is less than 10%, and the ellipticity (ratio of major to minor axes) of the NPs is less than 1.1. Compared to existing ones, the present seeded growth approach is implemented within 1 min under ambient condition, and no unfavorable additives are involved because H2O2 can readily decompose into H2O during storage or via boiling.

■

obtained using hydroxylamine as a reducing agent.17 Following Natan’s work, a numbers of seeded growth methods for the synthesis of uniform, large Au NPs were successfully developed by using different kinds of reducing agents and/or stabilizing agents.20−27 Murphy and co-workers developed a step-by-step seeded growth method to synthesize Au NPs of 5−40 nm using ascorbic acid as a reducing agent and cetyltrimethylammonium bromide (CTAB) as a stabilizing agent.20 Liz-Marzán et al. extended this strategy to synthesize quasi-spherical and monodisperse Au NPs up to 180 nm by removing rod- and planar-shaped NPs after the first-step growth.22 Liu and coworkers reported a one-step seeded growth approach for the synthesis of quasi-spherical Au NPs of 30−150 nm by using 2mercaptosuccinic acid as both reducing and stabilizing agents.23 Chan and co-workers developed a simple seeded growth protocol to synthesize monodisperse and quasi-spherical Au NPs of 50−200 nm under ambient conditions by using hydroquinone as a reducing agent.24 The reducing and stabilizing agents used in the above-mentioned work greatly benefit the production of Au NPs of good quality; however, most of them are toxic.20−24 Furthermore, the stabilizing agents that strongly bind to the surface of Au (such as CTAB20−22 and 2-mercaptosuccinic acid23) hinder further functionalization of the NPs. Therefore, further careful purification is needed, which thus increases the process complexity.28 Eychmüller et al. reported the seeded growth synthesis of citrate-stabilized Au NPs of 15−300 nm with good shape and size uniformity using ascorbic acid as a reducing agent; however, meticulously

INTRODUCTION Gold nanoparticles (Au NPs) have been intensely studied for decades because of their peculiar physicochemical properties and significant applications in photonics, catalysis, electronics, biomedicine, and many others.1−4 Although anisotropic Au NPs, such as rods,5,6 prisms,7,8 and cubes,9,10 have been successfully synthesized to pursue the shape-dependent surface plasmon resonance (SPR) properties in particular, quasispherical Au NPs still remain the workhorse in many technical applications mainly because of the fact that their synthesis protocols are easy and accessible to researchers from different disciplines such as chemistry, physics, and biology. In this context, the citrate reduction of HAuCl4 in hot water, initially developed by Turkevich et al. in 195111 and further refined by others,12−15 is the most commonly used method to synthesize quasi-spherical Au NPs with controlled sizes in the range of 10 to 40 nm, depending on the ratio of citrate to HAuCl4. This strategy allows the production of Au NPs larger than 40 nm, but the sizes of the NPs thus obtained are rather polydisperse, and a considerable number of other-than-spherical NPs coexist with quasi-spherical ones.11−16 To date, seeded growth strategies have been proved to be efficient to synthesize uniform Au NPs larger than 40 nm.17−27 That is, small Au NPs, obtained via citrate reduction, for instance, are employed as seeds and HAuCl4 is selectively reduced to Au0 atop the seeds by using additional reducing agents. Natan and co-workers pioneered the seeded growth of Au NPs up to 100 nm by using citrate or hydroxylamine as a reducing agent.17−19 However, the obtained NPs were rather elongated with ellipiticities (ratio of major to minor axes) as high as 1.4 for the ∼100 nm NPs using citrate as a reducing agent, whereas a considerable number of rod-shaped NPs were © XXXX American Chemical Society

Received: July 11, 2012 Revised: September 4, 2012

A

dx.doi.org/10.1021/la3027804 | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Table 1. Summary of the Recipes for Synthesizing Differently Sized Au NPs via Seeded Growth Based on the H2O2 Reduction of HAuCl4 batch size H2O HAuCl4 (1 wt %) citrate (1 wt %) H2O2 (30 wt %) Au seeds a

1

2

3

4

5

6

7

8

32 ± 2 nm 4.85 mL 20 μL (0.06 mM)a 25 μL (85 μM)a 5 mL (5 M)a

48 ± 4 nm 4.9 mL 33 μL (0.1 mM) 25 μL (85 μM) 5 mL (5 M)

68 ± 6 nm 4.9 mL 50 μL (0.15 mM) 25 μL (85 μM)

81 ± 7 nm 4.85 mL 100 μL (0.29 mM) 25 μL (85 μM)

120 ± 11 nm 4.85 mL 100 μL (0.29 mM) 25 μL (85 μM)

147 ± 10 nm 4.9 mL 100 μL (0.29 mM) 25 μL (85 μM)

190 ± 15 nm 4.8 mL 200 μL (0.58 mM) 25 μL (85 μM)

216 ± 19 nm 4.6 mL 300 μL (0.87 mM) 25 μL (85 μM)

5 mL (5 M)

5 mL (5 M)

5 mL (5 M)

5 mL (5 M)

5 mL (5 M)

5 mL (5 M)

100 μL (56 nM)a

50 μL (28 nM)

25 μL (14 nm) 30 μL (17 nM)

5 μL (2.8 nM)

5 μL (2.8 nM)

10 μL (5.6 nM) 5 μL (2.8 nM)

Concentration of the corresponding materials in the 10 mL reaction medium. derived from the amount of HAuCl4 used for the synthesis of Au seeds as described above), ρAu is the density of Au, d is the diameter (12 nm) of the Au seed NPs, and NA is Avogadro’s constant. Seeded Growth Synthesis of Differently Sized Au NPs. Table 1 summarizes the recipes for the seeded growth synthesis of monodisperse, quasi-spherical Au NPs with diameters from 30 to 230 nm based on the H2O2 reduction of HAuCl4 by taking 12 nm Au NPs as seeds. As listed in Table 1, the total volume of the reaction medium was set as 10 mL; 25 μL of sodium citrate (1 wt %) and 5 mL of H2O2 (30 wt %) were used for all of the NPs synthesis. The molar ratios of HAuCl4 to Au seeds were varied in order to control the sizes of the as-produced Au NPs. The whole seeded growth procedure was conducted under ambient conditions. Typically, H2O2 was added to the aqueous solution of a mixture of HAuCl4 and citrate under stirring, immediately followed by the injection of the Au seed solution, obtained as described above, under stirring at a speed of 1000 rpm. Because the fairly small volumes of the Au seed solutions (less than 100 μL) were added to the aqueous solutions of the H2O2/HAuCl4/ citrate mixtures (about 10 mL), as listed in Table 1, the reaction media were colorless or slightly yellow (derived from HAuCl4). After the addition of the Au seed solution, the color of the reaction solution changed within 1 min to pink, purple, and light orange depending on the molar ratio of HAuCl4 to Au seeds, suggesting the formation of larger Au NPs. Characterization. UV−visible spectra were recorded with a Shimadzu UV-2600 spectrophotometer. The growth kinetics of the as-prepared Au NPs was assessed by plotting the intensities of their SPR absorption maxima as a function of reaction time, which was performed with the aid of the kinetics mode of the Shimadzu UV-2600 spectrophotometer. Dynamic light scattering (DLS) studies were carried out on a Malvern Nano-ZS zetasizer at room temperature. The measurements were conducted at a scattering angle of 173° at 25 °C using a He−Ne laser with a wavelength of 633 nm. The TEM imaging of Au NPs was performed on an FEI CM100 at an acceleration voltage of 100 kV. Five microliter droplets of each sample were dropped onto a piece of ultrathin Formvar-coated 200-mesh copper grid (Ted-pella, Inc.) and left to dry in air. To determine the diameter and diameter distribution of the resulting Au NPs based on their TEM images, both the major and minor axes of more than 100 NPs were measured with the aid of the graphics editing program of Macromedia Fireworks 8. The average diameters and the standard deviations were calculated on the basis of all of the measured values of the major and minor axes. The ellipticity values of differently sized NPs were the average ratios of the major to minor axes. Calculations of the extinction spectra of the resulting Au NPs were implemented using Mie plot software by assuming spherical Au particles embedded in water at 25 °C; the mean size and standard deviation measured from the TEM images were used for calculation.

controlled mixing procedures of the reactants and further boiling-mediated recrystallization are needed to lead to the quasi-spherical Au NPs.25 Just recently, Bastús et al. reported a multistep seeded growth method to synthesize Au NPs up to 200 nm by solely using citrate as both reducing and stabilizing agents, but the deliberate control of experimental parameters at each step is crucial to maintaining the narrow size and shape distribution of the resulting quasi-spherical NPs.26 Thus, it is still a big challenge to develop a simple protocol to synthesize monodisperse, quasi-spherical Au NPs with sizes larger than 40 nm without the involvement of unfavorable additives. Herein we have demonstrated that large, monodisperse, quasi-spherical Au NPs can be produced by simply adding 12 nm citrate-stabilized Au NPs to a mixture of H2O2 and HAuCl4 in water under ambient conditions. The whole seeded growth process is completed within 1 min. The NPs size can be quantitatively tuned from 30 to 230 nm by the molar ratio of HAuCl4 to Au NP seeds. H2O2 is used as a reducing agent and can be easily removed via decomposition to H2O during storage or via boiling.

■

EXPERIMENTAL SECTION

Materials. Hydrogen tetrachloroaurate(III) (99.9%, metals basis, Au > 49% min, item no. 12325) was purchased from Alfa Aesar, and an aqueous HAuCl4 (1 wt %) solution was centrifuged at 18 000g for 2 h before use to remove any possible aggregates. Silver nitrate (99.9%) and trisodium citrate dihydrate (99%) were purchased from Aldrich and used as received. Hydrogen peroxide (30 wt %) was purchased from Chem-Supply Australia. Glassware and stirring bars were cleaned with aqua regia (3:1 v/v HCl (37%)/HNO3 (65%)) solutions and then rinsed thoroughly with H2O before use. (Caution! Aqua regia solutions are dangerous and should be used with extreme care; never store these solutions in closed containers.) The water in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18.2 MΩ cm−1. Synthesis of 12 nm Citrate-Stabilized Au Nanoparticle (NP) Seeds. Twelve nanometer gold seeds were synthesized via the modified Turkevich method reported elsewhere.13 A HAuCl4 aqueous solution (0.5 mL, 1 wt %) and a AgNO3 aqueous solution (42.5 μL, 0.1 wt %) were added to 1.5 mL of a citrate aqueous solution (1 wt %) under stirring, followed by water addition to bring the volume of the solution to 2.5 mL. After 5 min of incubation, this mixture was quickly poured into boiling water (47.5 mL) in a 100 mL flask. The reaction solution was further refluxed for 1 h under stirring to warrant the formation of uniform quasi-spherical Au NPs. After the as-prepared Au NP dispersion was cooled to room temperature, water was added to bring the volume of the dispersion to 50 mL. By assuming that the amount of HAuCl4 used is completely consumed for the production of Au seeds during synthesis and the obtained Au seeds are spherical, the molar amount of Au seeds in the as-prepared dispersion can be calculated by 6mAu/(ρAuπd3NA), where mAu is the mass of Au (which is

■

RESULTS AND DISCUSSION H2O2 is generally acknowledged to be an oxidative agent.29 However, it has been reported to be able to reduce HAuCl4 to synthesize Au NPs in a broad pH range, although the B

dx.doi.org/10.1021/la3027804 | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

seeds under identical experimental conditions, even when the concentration of H2O2 was as high as 5 M (Figure 3). Figure 2e shows the growth kinetics of the Au NPs prepared by the reduction of HAuCl4 using different concentrations of H2O2 in the presence and absence of Au NP seeds by plotting the intensities of their corresponding SPR absorption maxima as a function of reaction time. H2O2 reduction of HAuCl4 can occur in the absence of Au NP seeds, but it becomes exceedingly fast in the presence of Au NP seeds. The seeded growth rate of Au NPs is drastically increased with H2O2 concentration; the growth of Au NPs with sizes of about 80 nm from 12 nm NP seeds is shortened to 18 s from 2000 s by increasing the H2O2 concentration to 5 M from 1 × 10−3 M (Figure 2e). The highly accelerated growth of Au NPs in the presence of Au seeds clearly underlines the important catalytic role of Au seeds during the H2O2 reduction of HAuCl4. In this scenario, the fast, selective deposition of Au0 on the Au seeds can be envisioned, which would significantly suppress the formation of new Au NPs via H2O2 reduction of HAuCl4 without the aid of the Au seeds as catalysts, thus keeping the size distribution of the growing NPs similar to that of the seeds. As shown in Figure 2, the exceedingly fast seeded growth is necessitated to produce quasi-spherical Au NPs; they can be obtained using only 5 M H2O2 whereas elongated NPs noticeably appear when the H2O2 concentration is 1 M or below. Encouraged by the aforementioned success, we harnessed H2O2 as a reducing agent for the seeded growth of 12 nm citrate-stabilized Au seeds to differently sized NPs by simply adding 12 nm citrate-stabilized Au NPs to a mixture of H2O2 and HAuCl4 in water under ambient conditions (Experimental Section). Figures 4 and S2 show TEM images of the resulting Au NPs produced from eight different molar ratios of HAuCl4 to Au seeds listed in Table 1. The resulting Au NPs are uniform and quasi-spherical, and their sizes increase from 32 ± 2 to 48 ± 4, 68 ± 6, 81 ± 7, 120 ± 11, 147 ± 10, 190 ± 15, and 216 ± 19 nm with the molar ratio of HAuCl4 to the Au seeds (listed in Table 1). All of the NP sizes exhibit standard deviations below 10% (Table 2), indicating a fairly narrow size distribution. The ellipticities of all of the resulting NPs shown in Figure 4 are below 1.1 (Table 2), underlining the quasi-spherical shape of the NPs. Histograms (Figures S3 and S4) of the diameter and ellipticity (measured from TEM images) distributions for the differently sized Au NPs are further evidence of their narrow size distribution and quasi-sphericity. The monodispersity of the resulting Au NPs in size and shape can also be confirmed by the monomodal distribution of the hydrodynamic NP diameters with a small polydispersity index, obtained via DLS measurement (Figure S5). By assuming that both NP seeds and large NPs obtained thereof are spherical and the amount of HAuCl4 used is exclusively consumed for the enlargement of the seeds to the large NPs, we can estimate the theoretical sizes of Au spherical NPs obtained via seeded growth by πD3ρN/6 = m + Δm, where D is the theoretical diameter of the resulting Au NPs, ρ is the Au density, N is the number of the resulting Au NPs and is assumed to equal the number of the Au seeds used, m is the mass of the ensemble of the Au seeds used, and Δm is the mass of Au derived from HAuCl4. As suggested in Table 2, the calculated diameters of the Au NPs obtained via the present seeded growth approach are fairly comparable to the experimentally measured ones with a discrepancy of less than 5 nm, underlining the quantitative control of the resulting Au NP sizes by the molar ratio of HAuCl4 to the Au seeds. This also confirms no secondary nucleation occurring during the

morphology and size of the Au NPs thus generated are rather nonuniform.30−34 The proposed reaction equation is32 yields 3 3 AuCl−4 + H 2O2 ⎯⎯⎯⎯→ Au 0 + 4Cl− + 3H+ + O2 2 2 Recently, it has been increasingly evidenced that H2O2, produced during the biocatalyzed oxidation of substances such as glucoses, can readily reduce HAuCl4 to Au0 catalyzed by Au NPs, thus leading to the NPs' enlargement and in turn a noticeable red shift of the NP SPR absorption band, which has been used for the fabrication of biosensors.35−37 Willner and co-workers reported the first study of using H2O2 to reduce HAuCl4 (2 × 10−4 M) in the presence of 12 nm citratestabilized Au NP seeds (3 × 10−10 M) as catalysts and cetyltrimethylammonium chloride (CTAC, 2 × 10−3 M) as an additional stabilizer.35 They observed that 2.5−7 nm Au clusters were dominantly formed on the surfaces of the Au NP seeds at a lower H2O2 concentration (5 × 10−5 M, comparable to the H2O2 concentration produced from biocatalytic reactions) whereas in the reaction solutions in coexistence with the enlarged Au NPs at a higher H2O2 concentration (2.4 × 10−4 M) the former caused a red shift of ca. 15 nm for the SPR absorption band and the latter caused a blue shift of ca. 10 nm. In contrast to the work of Willner et al., when we conducted the H2O2 reduction of HAuCl4 (2.9 × 10−4 M) in an aqueous dispersion of 12 ± 1 nm citrate-stabilized Au NPs (1.7 × 10−8 M, shown in Figure S1) in the absence of CTAC, we observed that the SPR absorption band of the Au NPs significantly red shifted from 515 nm to ca. 550 nm at high H2O2 concentrations (e.g., 1 × 10−3 M) (Figure 1).

Figure 1. UV−vis spectra of 12 nm Au seeds and Au NPs obtained via seeded growth based on the H2O2 reduction of HAuCl4. The Au seeds are 12 nm in diameter. The concentrations of the Au seeds and HAuCl4 are 1.7 × 10−8 and 2.9 × 10−4 M, respectively. The H2O2 concentrations are 0.001, 0.1, 1, and 5 M.

Transmission electron microscopy (TEM) imaging of the resulting Au NPs indicates the coexistence of differently shaped NPs. As shown in Figure 2a−d, the population of the rodlike and flakelike NPs declines with increasing H2O2 concentration, and monodisperse, quasi-spherical 81 ± 7 nm NPs are exclusively obtained at a H2O2 concentration of 5 M (Figure 2a). This significant improvement in the shape uniformity has been evidenced by the significant reduction of the SPR absorption in the wavelength range above 700 nm (Figure 1). As in previous reports,32 fairly nonuniform Au NPs were obtained via H2O2 reduction of HAuCl4 in the absence of Au C

dx.doi.org/10.1021/la3027804 | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Figure 2. (a−d) TEM images of Au NPs obtained via seeded growth based on the reduction of HAuCl4 by H2O2. The Au seeds are 12 nm in diameter. The concentrations of the Au seeds and HAuCl4 are 1.7 × 10−8 and 2.9 × 10−4 M, respectively. The H2O2 concentrations are (a) 5, (b) 1, (c) 0.1, and (d) 0.001 M. The scale bars are all 200 nm. (e) Plots of the intensities of SPR absorption maxima of the resulting Au NPs vs reaction time during seeded growth via the reduction of HAuCl4 (2.9 × 10−4 M) by H2O2 at different concentrations (given in the legend) in the presence (−) or absence (---) of 12 nm Au seeds (1.7 × 10−8 M). The SPR absorption maxima of the Au NPs prepared in the presence of Au seeds using 5, 1, 0.1, and 0.001 M H2O2 are at 553, 552, 552, and 554 nm, respectively. The SPR absorption maxima of the Au NPs prepared in the absence of Au seeds using 5 and 0.001 M H2O2 are at 556 and 570 nm, respectively. The maximum intensities of the SPR absorption maxima of the different Au NPs obtained are normalized for the benefit of easy comparison.

shown in Figure 4. When the NPs are smaller than 81 ± 7 nm, the dipolar SPR bands of the resulting NPs are noticeably symmetric and have a nearly zero absorption baseline, identifying the excellent sphericity of the NP shape.11−16 The dipolar SPR band of the NPs red shifts from 515 nm for the 12 nm seeds to 600 nm for 120 ± 11 nm NPs (Table 2, exp); it continuously red shifts while becoming broad and extends to the near-infrared wavelength regime with further NP size increases. A noticeable quadrupolar SPR band is visible when the resulting Au NPs are larger than 120 ± 7 nm; it red shifts from 546 nm for 147 ± 10 nm NPs to 578 nm for 216 ± 19 nm NPs. The SPR band red shift of the resulting NPs with the NP sizes is clearly reflected in the color of the aqueous dispersions of the NPs (inset in Figure 5a). According to the geometry measured in Figure 4, we calculated the extinction spectra of the differently sized Au NPs on the basis of standard Mie theory of spherical Au particles (Figure 5b).38 The calculated SPR bands match well with the experimentally observed ones in terms of both absorption profile and maxima (Table 2 and Figure 5), which is a further indication of the quasi-spherical shape and narrow size distribution of the Au NPs obtained via the present seeded growth. In the current work, the growth kinetics of Au NPs via H2O2 reduction of HAuCl4 was noticeably slowed down with the increase in the molar ratio of HAuCl4 to Au seeds, but the growth of Au NPs derived from the eight molar ratios of HAuCl4 to Au seeds, listed in Table 1, were implemented within 1 min. When the largest molar ratio of HAuCl4 to Au seeds was used to produce the largest NPs in the current work, the NP growth took just 45 s (Figure S6). Note that a small amount of sodium citrate (8.5 × 10−5 M) was involved in the present seeded growth process but the citrate was utilized dominantly to enhance the colloidal stability of the resulting Au NPs rather than to aid the reduction of HAuCl4. As shown in Figure 6, the UV−vis spectra and the growth kinetics of the Au NPs synthesized in the presence and absence of citrate are almost identical, suggesting that the citrate addition has no effect on the shape and sizes of the Au NPs synthesized by the currently proposed seeded growth method.

Figure 3. (a) TEM images and (b) UV−vis spectra of Au NPs obtained by the reduction of HAuCl4 (2.9 × 10−4 M) with H2O2 (5 M) in the absence of Au seeds.

H2O2 reduction of HAuCl4 and at the same time further underlines that the resulting Au NPs are monodisperse and quasi-spherical. Figure 5a shows the SPR absorption spectra of differently sized Au NPs obtained by seeded growth via H2O2 reduction D

dx.doi.org/10.1021/la3027804 | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Figure 4. TEM images of Au NPs with sizes of (a) 32, (b) 48, (c) 68, (d) 81, (e) 120, (f) 147, (g) 190, and (h) 216 nm obtained by seeded growth via H2O2 reduction of HAuCl4. The high-magnification TEM images of the Au NPs are shown in the insets. The H2O2 concentration is 5 M. The Au seeds are 12 nm in diameter. The concentrations of HAuCl4 and the Au seeds are listed in Table 1.

Table 2. Summary of the Sizes and Ellipticities of the Au NPs Shown in Figure 4 and the Measured and Calculated Positions of Their SPR Bands batch

1

2

3

4

5

diameter (D) deviationa ellipticityb calculated D SPR mode Miee expf

32 6.2% 1.05 33.1 dipo.c 523 525

48 8.3% 1.06 48.8 dipo. 530 530

68 8.8% 1.06 70.6 dipo. 542 542

81 8.6% 1.09 83.2 dipo. 550 553

120 9.2% 1.12 120.6 dipo. 587 600

6

dipo. 635 655

147 6.8% 1.09 152.0 quadru.d 545 546

7

8

190 7.9% 1.10 191.4 dipo. quadru. 740 555 770 560

216 8.8% 1.11 220.0 dipo. quadru. 810 573 835 578

a

Standard deviation of the Au NP diameter. bEllipticity is estimated as the ratio of the major to minor axes. cAbbreviation of dipolar mode (nm). Abbreviation of quadrupolar mode (nm). eSPR band positions calculated by using Mie theory. fSPR band positions measured by UV−vis spectroscopy. d

Figure 5. (a) UV−vis spectra of 12 nm Au seeds (curve 0) and differently sized Au NPs (curves 1−8) obtained by seeded growth via H2O2 reduction of HAuCl4. The H2O2 concentration is 5 M. The concentrations of HAuCl4 and the Au seeds are listed in Table 1. All spectra are normalized to 400 nm. The inset shows photographs of the aqueous dispersions of the corresponding Au NPs. (b) Extinction spectra of Au spheres calculated on the basis of Mie theory with the mean size and size distribution as obtained experimentally. (a, b) Curves 1−8 represent the UV−vis spectra of the Au NPs with average diameters of 32, 48, 68, 81, 120, 147, 190, and 216 nm, respectively.

■

CONCLUSIONS We have demonstrated a rapid, simple seeded growth method for synthesizing monodisperse, quasi-spherical Au NPs via the H2O2 reduction of HAuCl4. The NP sizes can be quantitatively tuned from 30 to 230 nm by increasing the molar ratio of HAuCl4 to Au seeds. The standard deviation of the resulting NP sizes is smaller than 10%, and the ellipticity of the NPs is less than 1.1. As compared to existing methods, the present seeded growth method is implemented in less than 1 min under

ambient conditions. H2O2 used as reducing agent can be readily decomposed to H2O during storage or via boiling, which leaves behind no unfavorable additives in the aqueous dispersions of the resulting Au NPs. Thus, the present seeded growth protocol should be more easily popularized and adopted by researchers from a diversity of scientific disciplines thanks to its operation simplicity, short processing time, no unfavorable contamination, and high quality of the Au NPs obtained. E

dx.doi.org/10.1021/la3027804 | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

(2) Grzelczak, M.; Pérez-Juste, J.; Mulvaney, P.; Liz-Marzán, L. M. Shape Control in Gold Nanoparticle Synthesis. Chem. Soc. Rev. 2008, 37, 1783−1791. (3) Sardar, R.; Funston, A. M.; Mulvaney, P.; Murray, R. W. Gold Nanoparticles: Past, Present and Future. Langmuir 2009, 25, 13840− 13851. (4) Sun, Y.; Xia, Y. Shape-Controlled Synthesis of Gold and Silver Nanoparticles. Science 2002, 298, 2176−2179. (5) Gole, A.; Murphy, C. J. Seed-Mediated Synthesis of Gold Nanorods: Role of the Size and Nature of the Seed. Chem. Mater. 2004, 16, 3633−3640. (6) Huang, X.; Neretina, S.; EI-Sayed, M. A. Gold Nanorods: From Synthesis and Properities to Biological and Biomedical Applications. Adv. Mater. 2009, 21, 4880−4910. (7) Ha, T. H.; Koo, H.-J.; Chung, B. H. Shape-Controlled Syntheses of Gold Nanoprisms and Nanorods Influenced by Specific Adsorption of Halide Ions. J. Phys. Chem. C 2007, 111, 1123−1130. (8) Millstone, J. E.; Métraux, G. S.; Mirkin, C. A. Controlling the Edge Length of Gold Nanoprisms via a Seed-Mediated Approach. Adv. Funct. Mater. 2006, 16, 1209−1214. (9) Sohn, K.; Kim, F.; Pradel, K. C.; Wu, J.; Peng, Y.; Zhou, F.; Huang, J. Construction of Evolutionary Tree for Morphological Engineering of Nanoparticles. ACS Nano 2009, 3, 2191−2198. (10) Chen, H.; Sun, Z.; Ni, W.; Woo, K. C.; Lin, H.-Q.; Sun, L.; Yan, C.; Wang, J. Plasmon Coupling in Clusters Composed of TwoDimensionally Ordered Gold Nanocubes. Small 2009, 5, 2111−2119. (11) Turkevich, J.; Stevenson, P. C.; Hillier, J. A Study of the Nucleation and Growth Processes in the Synthesis of Colloidal Gold. Discuss. Faraday Soc. 1951, 11, 55−75. (12) Frens, G. Controlled Nucleation for the Regulation of the Particle Size in Monodisperse Gold Suspensions. Nat. Phys. Sci. 1973, 241, 20−22. (13) Xia, H.; Bai, S.; Hartmann, J.; Wang, D. Synthesis of Monodisperse Quasi-Spherical Gold Nanoparticles in Water via Silver(I)-Assisted Citrate Reduction. Langmuir 2010, 26, 3585−3589. (14) Ji, X.; Song, X.; Li, J.; Bai, Y.; Yang, W.; Peng, X. Size Control of Gold Nanocrystals in Citrate Reduction: The Third Role of Citrate. J. Am. Chem. Soc. 2007, 129, 13939−13948. (15) Ojea-Jimenez, I.; Romero, F. M.; Bastús, N. G.; Puntes, V. Small Gold Nanoparticles Synthesized with Sodium Citrate and Heavy Water: Insights into the Reaction Mechanism. J. Phys. Chem. C 2010, 114, 1800−1804. (16) Kimling, J.; Maier, M.; Okenve, B.; Kotaidis, V.; Ballot, H.; Plech, A. Turkevich Method for Gold Nanoparticle Synthesis Revisited. J. Phys. Chem. B 2006, 110, 15700−15707. (17) Brown, K. R.; Walter, D. G.; Natan, M. J. Seeding of Colloidal Au Nanoparticle Solutions. 2. Improved Control of Particle Size and Shape. Chem. Mater. 2000, 12, 306−313. (18) Brown, K. R.; Lyon, L. A.; Fox, A. P.; Reiss, B. D.; Natan, M. J. Hydroxylamine Seeding of Colloidal Au Nanoparticles. 3. Controlled Formation of Conductive Au Films. Chem. Mater. 1999, 12, 314−323. (19) Brown, K. R.; Natan, M. J. Hydroxylamine Seeding of Colloidal Au Nanoparticles in Solution and on Surfaces. Langmuir 1998, 14, 726−728. (20) Jana, N. R.; Gearheart, L.; Murphy, C. J. Seeding Growth for Size Control of 5−40 nm Diameter Gold Nanoparticles. Langmuir 2001, 17, 6782−6786. (21) Jana, N. R.; Gearheart, L.; Murphy, C. J. Evidence for SeedMediated Nucleation in the Chemical Reduction of Gold Salts to Gold Nanoparticles. Chem. Mater. 2001, 13, 2313−2322. (22) Rodriguez-Fernandez, J.; Perez-Juste, J.; Garcia de Abajo, F. J.; Liz-Marzán, L. M. Seeded Growth of Submicron Au Colloids with Quadrupole Plasmon Resonance Modes. Langmuir 2006, 22, 7007− 7010. (23) Niu, J.; Zhu, T.; Liu, Z. One-Step Seed-Mediated Growth of 30−150 nm Quasispherical Gold Nanoparticles with 2-Mercaptosuccinic Acid as a New Reducing Agent. Nanotechnology 2007, 18, 325607.

Figure 6. (a) UV−vis spectra of the Au NPs with average diameters of 48 and 190 nm synthesized via seeded growth based on the H2O2 reduction of HAuCl4 in the presence and absence of sodium citrate. The concentrations of H2O2, HAuCl4, Au seeds, and sodium citrate are given in Table 1. (b) Plots of the intensities of the SPR absorption maxima (at 553 nm) of Au NPs with an average diameter of 81 nm synthesized in the presence and absence of citrate vs the reaction time.

■

ASSOCIATED CONTENT

S Supporting Information *

TEM image of 12 nm Au seeds. Low-magnification TEM images of as-prepared Au NPs. Histograms of the size and ellipticity distribution of the as-prepared Au NPs. DLS measurement results of the as-prepared Au NPs. Growth kinetics of the as-prepared largest Au NPs. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS We acknowledge the Australian Research Council (DP110104179 and DP120102959) for financial support. REFERENCES

(1) Daniel, M. C.; Astruc, D. Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related Properties, and Applications toward Biology, Catalysis, and Nanotechnology. Chem. Rev. 2004, 104, 293−346. F

dx.doi.org/10.1021/la3027804 | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

(24) Perrault, S. D.; Chan, W. C. W. Synthesis and Surface Modification of Highly Monodispersed, Spherical Gold Nanoparticles of 50−200 nm. J. Am. Chem. Soc. 2009, 131, 17042−17043. (25) Ziegler, C.; Eychmüller, A. Seeded Growth Synthesis of Uniform Gold Nanoparticles with Diameters of 15−300 nm. J. Phys. Chem. C 2011, 115, 4502−4506. (26) Bastús, N. G.; Comenge, J.; Puntes, V. Kinetically Controlled Seeded Growth Synthesis of Citrate-Stabilized Gold Nanoparticles of up to 200 nm: Size Focusing versus Ostwald Ripening. Langmuir 2011, 27, 11098−11105. (27) Bakshi, M. S. A Simple Method of Superlattice Formation: Stepby-Step Evaluation of Crystal Growth of Gold Nanoparticles through Seed-Growth Method. Langmuir 2009, 25, 12697−12705. (28) Leonov, A. P.; Zheng, J.; Clogston, J. D.; Stern, S. T.; Patri, A. K.; Wei, A. Detoxification of Gold Nanorods by Treatment with Polystyrenesulfonate. ACS Nano 2008, 2, 2481−2488. (29) Zhang, Q.; Li, N.; Goebl, J.; Lu, Z.; Yin, Y. A Systematic Study of the Synthesis of Silver Nanoplates: Is Citrate a “Magic” Reagent? J. Am. Chem. Soc. 2011, 133, 18931−18939. (30) Sarma, T. K.; Chowdhury, D.; Paul, A.; Chattopadhyay, A. Synthesis of Au Nanoparticle−Conductive Polyaniline Composite Using H2O2 as Oxidising as well as Reducing Agent. Chem. Commun. 2002, 1048−1049. (31) Sarma, T. K.; Chattopadhyay, A. Starch-Mediated ShapeSelective Synthesis of Au Nanoparticles with Tunable Longitudinal Plasmon Resonance. Langmuir 2004, 20, 3520−3524. (32) Panda, B. R.; Chattopadhyay, A. Synthesis of Au Nanoparticles at “All” pH by H2O2 Reduction of HAuCl4. J. Nanosci. Nanotechnol. 2007, 7, 1911−1915. (33) Pacławski, K.; Fitzner, K. Kinetics of Reduction of Gold(III) Complexes Using H2O2. Metall. Mater. Trans. B 2006, 37, 703−714. (34) Li, Q.; Lu, B.; Zhang, L.; Lu, C. Synthesis and Stability Evaluation of Size-Controlled Gold Nanoparticles via Nonionic Fluorosurfactant-Assisted Hydrogen Peroxide Reduction. J. Mater. Chem. 2012, 22, 13564−13570. (35) Zayats, M.; Baron, R.; Popov, I.; Willner, I. Biocatalytic Growth of Au Nanoparticles: From Mechanistic Aspects to Biosensors Design. Nano Lett. 2005, 5, 21−25. (36) Willner, I.; Baron, R.; Willner, B. Growing Metal Nanoparticles by Enzymes. Adv. Mater. 2006, 18, 1109−1120. (37) Zheng, X.; Liu, Q.; Jing, C.; Li, Y.; Li, D.; Luo, W.; Wen, Y.; He, Y.; Huang, Q.; Long, Y.-T.; Fan, C. Catalytic Gold Nanoparticles for Nanoplasmonic Detection of DNA Hybridization. Angew. Chem., Int. Ed. 2011, 50, 11994−11998. (38) Mie, G. Beiträge zur Optik Trüber Medien, Speziell Kolloidaler Metallösungen. Ann. Phys. 1908, 330, 377−445.

G

dx.doi.org/10.1021/la3027804 | Langmuir XXXX, XXX, XXX−XXX