Article pubs.acs.org/Langmuir
Revitalizing the Frens Method To Synthesize Uniform, QuasiSpherical Gold Nanoparticles with Deliberately Regulated Sizes from 2 to 330 nm Haibing Xia,*,† Yujiao Xiahou,† Peina Zhang,† Wenchao Ding,† and Dayang Wang*,‡ †
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China Department of Civil, Environmental and Chemical Engineering, School of Engineering, RMIT University, Melbourne, Victoria 3001, Australia
‡
S Supporting Information *
ABSTRACT: In this work, we have successfully developed a new and consistent model to describe the growth of gold nanoparticles (Au NPs) via citrate reduction of auric acid (HAuCl4) by carefully assessing the temporal evolution of the NP sizes and surface charges by means of dynamic light scattering (DLS) and zeta-potential measurements. The new model demonstrates that the nucleation and growth of the Au NPs occur exclusively in the particles of the complexes of Au+ ions and sodium acetone dicarboxylate (SAD) derived from the citrate/HAuCl4 redox reaction, which proceeds as described by the classic LaMer model. Concomitant with the Au NP growing therein, the Au+/SAD complex particles undergo reversible agglomeration with the reaction time, which may result in an abnormal color change of the reaction media but have little impact on the Au NP growth. Built on the new model, we have successfully produced monodisperse quasi-spherical Au NPs with sizes precisely regulated from 2 to 330 nm via simple citrate reduction in a one-pot manner. To date, highly uniform Au NPs with sizes spanning such a large size range could not be formed otherwise even via deliberately controlled seeded growth methods.
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NP seed sizes to the desired values.8,9 With the aid of sophisticated control of mixing and addition of HAuCl4, citrate (stabilizing agent), and ascorbic acid (reducing agent), Ziegler and Eychmüller produced uniform quasi-spherical Au NPs with sizes varying from 15 ± 2 to 30 ± 3, 69 ± 3, 121 ± 10, 151 ± 8, and 294 ± 17 nm; the Au NPs of 294 ± 17 nm in size should be the largest Au NPs produced reported in the literature thus far.10 In general, however, the success of currently available seeded growth methods is largely limited by deliberate particle separation and purification between the growth steps.8−12 Faraday reported the first two-phase (water−CS2) method for direct synthesis of Au NPs in 1857,13 whereas the modern two-phase (water−toluene) method was developed by Brust, Schiffrin, and co-workers in 1994, known as the Brust−Schiffrin method, to synthesize lipophilic Au NPs with sizes smaller than 5 nm with the aid of surfactants.14 In 1951, Turkevich et al. reported their comprehensive study of direct synthesis of monodisperse quasi-spherical Au NPs in water via citrate reduction of HAuCl4, commonly referred to as the Turkevich method.15 To date, the Turkevich method remains the
INTRODUCTION The research on colloidal gold nanoparticles (Au NPs) has been strongly inspired by the esthetic prospective of their striking color, which has been translated into many technical applications ranging from coloring glass and ceramics in ancient times to colorimetric bioassay nowadays.1,2 The color of colloidal Au NPs is the manifestation of the collective oscillation of the conduction electrons on the NP surfaces at excitation by the incoming light, coined as surface plasmon resonance (SPR), which is sensitively correlated to the NP sizes and shapes, the separation distance between the neighboring NPs, and the refractive index of the surrounding media.3−5 The modulation of the SPR properties of Au NPs relies primarily on the power of synthesis of the Au NPs with defined but varied sizes and shapes.6,7 Despite the great development in the manufacture of Au nanostructured materials, surprisingly, it is still an experimental challenge to directly synthesize uniform Au NPs with quasi-spherical shapes compared with synthesizing the NPs with other shapes, especially when the designated NP size is bigger than 40 nm. The synthesis of large quasi-spherical Au NPs requires elaborate multiple steps of deposition of gold, produced via reduction of auric acid (HAuCl4), onto preformed tiny Au NPs served as seeds, which is referred to as seeded growth method and enables stepwise enlargement of the Au © XXXX American Chemical Society
Received: April 5, 2016 Revised: May 24, 2016
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DOI: 10.1021/acs.langmuir.6b01312 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. (a) Photographs of the temporal color evolution of the reaction solution after adding the aqueous solution of HAuCl4 into the boiling aqueous solution of citrate. The concentrations of citrate and HAuCl4 in the boiling reaction solution are 0.03 and 0.01 wt %, respectively. (b) Photographs of the temporal color evolution of the reaction media during incubation of the aqueous solution of HAuCl4/citrate/AgNO3 mixtures at room temperature. The volume of the HAuCl4/citrate/AgNO3 mixture solution is 2.5 mL. The concentrations of HAuCl4, citrate, and AgNO3 are 0.2, 0.6, and 1.7 × 10−3 wt %, respectively. (c) Photographs of the temporal color evolution of the reaction solution obtained by adding the aqueous solution of HAuCl4/citrate/AgNO3 mixtures (2.5 mL) into 47.5 mL of boiling Tris solution. The Tris solution is made by diluting 2 mL of 0.1 M Tris buffer solution with 45.5 mL of water. The HAuCl4/citrate/AgNO3 mixture solution (2.5 mL) is made in an identical way to that applied in (b). The concentrations of citrate and HAuCl4 in the boiling reaction solution are 0.03 and 0.01 wt %, respectively. Note that in all three reaction solutions shown in this figure, Rc/a is set as 4.1 for the benefit of comparison.
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RESULTS AND DISCUSSION Growth Mechanism of Au NPs via Citrate Reduction. Since the seminal work of Turkevich et al. in 1951, a great deal of effort has been made to study the mechanism governing the growth of Au NPs during citrate reduction of HAuCl4.21−33 The Turkevich method is very simple: rapid addition of the aqueous solution of sodium citrate into the boiling aqueous solution of HAuCl4 leads to monodisperse quasi-spherical Au NPs with sizes in the range of 12−16 nm, which exhibit a pronounced SPR band centered at 520 nm, characteristic of the ruby red color of the aqueous NP dispersions obtained. The current consensus is that citrate reduces AuCl4− ions to Au+ ions and, at the same time, it is oxidized to sodium acetone dicarboxylate (SAD). When the concentration of the Au+ ions builds up over the critical value in the reaction media,29 the selfcatalyzed disproportionation of Au+ ions to Au0 triggers the nucleation and growth of Au NPs.15 The disproportionation of Au+ ions is indicated as the key step to control the number of initial nuclei and the rate of the growth,29 and its quite slow reaction rate should be beneficial for the temporal separation of nucleation and growth and thus the growth of Au NPs with a fairly narrow size and shape distribution according to the LaMer model.34 Concomitant with the growth of Au NPs during the citrate reduction of HAuCl4, the color change of the reaction solution is hardly explicable; the solution remains light yellow for a couple of seconds and then turns gray, gray-bluish, blue, bluishpurple, and purple within 5 min or so, followed by the ruby red suddenly showing up and then remaining unchanged with time (Figure 1a). The abnormal color change was firstly described by Turkevich et al. in 195115 and then reproducibly reported by other researchers.23,29 In accord with the current understanding
simplest, most reliable, and popular one to produce Au NPs in water.15−18 The stabilizing ligand citrate is weakly bound on the resulting Au NPs and can be readily replaced by other strong ligands, leading to great flexibility and operational ease of surface functionalization. However, the classic Turkevich method allows the synthesis of uniform quasi-spherical Au NPs only with sizes in the range of 12−16 nm; the slight size variation is dependent on the experimental conditions such as the boiling state of aqueous reaction media.19 In 1973, Frens reported an extended Turkevich method, commonly referred to as Frens method, to produce Au NPs with sizes varying from 16 to 147 nm with a decrease in the citrate-to-HAuCl4 molar ratio (Rc/a), but the size distribution of the resulting NPs was rather broad and the NP shapes were rather irregular especially for the NP sizes over 40 nm.20 Herein, we meticulously assessed the temporal evolution of the sizes and colloidal stability of the Au NPs formed during citrate reduction of HAuCl4, which enabled us to reconcile the different mechanisms proposed in the literature, contradictory to each other in some aspects, into a consistent model to describe the nucleation and growth of the Au NPs. Built on this new model, we succeeded in direct synthesis of monodisperse quasi-spherical Au NPs with sizes precisely regulated from 2 ± 0.2 to 330 ± 32 nm in water by simply adjusting the Rc/a value. To date, a few methods reported in the literature allow one-pot synthesis of Au NPs with a uniform quasi-spherical shape and fairly narrow size distribution in such a broad size range. The size regulation over such a broad range was also hardly achieved by the seeded growth methods available in the literature. B
DOI: 10.1021/acs.langmuir.6b01312 Langmuir XXXX, XXX, XXX−XXX
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Figure 2. Plots of the hydrodynamic sizes (a−c) and surface charges (d−f) of the intermediate particles obtained in the aqueous solutions of HAuCl4/citrate/AgNO3 mixtures (a and d) at room temperature, after adding the mixture solutions into 47.5 mL of water at 60 °C (b and e), and in the aqueous solutions of HAuCl4/citrate/AgNO3/TB mixtures (c and f) at room temperature. The volume of the HAuCl4/citrate/AgNO3 mixture solution is 2.5 mL, and the concentrations of HAuCl4, citrate, and AgNO3 are 0.2, 0.6, and 1.7 × 10−3 wt %, respectively. The HAuCl4/citrate/ AgNO3/TB mixture solutions are made by incubating 2.5 mL of the HAuCl4/citrate/AgNO3 mixture solution for 5 min and mixing with 2.0 mL of Tris solution (0.1 M). The aqueous solutions of HAuCl4/citrate/AgNO3 mixtures (2.5 mL) and HAuCl4/citrate/AgNO3/Tris mixtures (4.5 mL) are diluted by 4 times for DLS and zeta-potential analyses. The HAuCl4/citrate/AgNO3 mixture solutions are incubated at room temperature for 5 min before the addition to water at 60 °C.
of the SPR properties of Au NPs, the appearance of the blue and purple colors of the aqueous solutions of Au NPs suggests the agglomeration of the Au NPs growing in the reaction media, which was confirmed by Chow and Zukoski with the help of dynamic light scattering (DLS) and transmission electron microscopy (TEM).23 To rationalize the abnormal color change, Turkevich et al. proposed a nonclassic growth model, in which the disproportionation of Au+ ions and the following nucleation and growth of Au NPs occurred in the multidentate Au+/SAD complexes.15,18 Turkevich et al. initially reported and others23,28,29 afterward confirmed that only a small fraction of AuCl4− ions were consumed during the initial citrate reduction, whereas the remaining AuCl4− ions tended to adsorb onto the newly formed Au NPs at an early stage of the reaction and the adsorption of citrate on the newly formed Au NPs occurred at the end of the reaction. According to surface force measurements,35 the adsorption of AuCl4− ions cannot generate sufficiently high electrostatic repulsion between the newly formed Au NPs, thus giving rise to the agglomeration of Au NPs at an early age of the reaction. In contrast, a strong electrostatic repulsion will appear when the newly formed NPs are coated dominantly by citrate, thus resulting in peptization of the NP aggregates into final NPs.29 This nonclassic model for the nucleation and growth of NPs is commonly applied to account for the abnormal color change of the reaction media. In contrast to this well-appreciated agglomeration-involved growth of Au NPs via citrate reduction, Polte et al. recently unraveled a seeded growth manner during the citrate reduction of HAuCl4 to Au NPs with the aid of synchrotron-based small-angle X-ray and X-ray absorption near-edge spectroscopy, in which the size and the number of final NPs were determined by the number of seeds with radii > 1.5 nm produced at the early stage of the reaction, no further seeds were generated at the late stages of the reaction, and no large aggregates of Au NPs were found.36
Although it may not elucidate the abnormal color change during the citrate reduction of HAuCl4 to Au NPs, the seededgrowth mechanism, proposed by Polte et al., can well rationalize the size monodispersity and the shape uniformity of the resulting Au NPs, which was de facto little discussed in the nonclassic agglomeration-involved growth model proposed by other research teams. It was also able to explain early TEM observation,22 in which Au NPs were found to be polycrystalline and composed of primary monocrystalline domains with sizes of 1−2 nm. Although a great deal of effort was devoted to unraveling the growth mechanism of Au NPs via citrate reduction, most of the studies focused on the synthesis of Au NPs with sizes of 12−16 nm via the classic Turkevich method. To date, the Frens method is commonly applied for the direct production of large Au NPs via citrate reduction, but the size polydispersity and shape irregularity of the resulting Au NPs remains an unsolved challenge especially when the NP sizes are above 40 nm (Table S1 and Figure S1). To circumvent this technical challenge, the present work was aimed at establishing a consistent model to explicitly describe the NP growth and, more importantly, to enable direct synthesis of uniform Au NPs with the sizes precisely regulated in a broad range. Citrate is known to simultaneously act as a reducing agent, a stabilizing agent, and a pH buffer during the growth of Au NPs.28 To minimize the pH buffer role of citrate,28 we developed a premixing method to produce Au NPs with an exceedingly narrow size distribution and high shape sphericity, in which citrate, AgNO3, and HAuCl4 were first mixed in water and then the mixture solutions were added into boiling water.31 Note that the Ag+ ions were used not only to catalyze the oxidation of citrate to SAD to speed up the formation of Ag+/SAD complex particles but also to selectively deposit onto the facets of the newly growing Au NPs to reshape the NPs into a quasi-spherical shape.31 Here, this C
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early stage of citrate reduction of HAuCl4 occurs in coincidence with the sharp fluctuation in the zeta-potential value of the particle surfaces, which is usually expected to cause agglomeration of charged particles.35 This inspired us to postulate that polydisperse large particles, obtained either after a 13-min incubation of HAuCl4/citrate/AgNO3 mixtures in water at room temperature (Figure 2a) or immediately after adding the HAuCl4/citrate/AgNO3 mixtures into water at 60 °C (Figure 2b), are the agglomerates of uniform 75 nm particles formed at the early stage of the reaction (Figure 2a). These particle agglomerate clumps are readily broken into uniform small particles at the end of the reaction (Figure 2b). According to the nonclassic nucleation model,15,23 the uniform particles with sizes of 75 nm, observed at the very beginning of the reaction, should be made of Au+/SAD complexes because the formation of Au+/SAD complexes is well-accepted as the first step occurring soon after citrate reduction of HAuCl4. Taking into account the slow reduction rate of the citrate reduction of HAuCl4 (Figure S3), the concentration of newly formed Au+ ions and SAD ions should remain sufficiently low to produce secondary Au+/SAD complex particles, which is evidenced by the fairly narrow size distribution of the resulting complex particles observed in Figure 2. Thus, the Au+ ions and SAD ions newly formed in the reaction solution should dominantly diffuse into the existing Au+/SAD complex particles, which causes dense crosslinking of the complex particles and in turn leads to uniform particles with sizes of ca. 75 nm (Figure 2a). At the same time, the nucleation and growth of Au NPs are expected to occur within the complex particles when the concentration of Au+ ions increases above the critical value (ca. 10 nM).29 In this scenario, the agglomeration of the Au+/SAD complex particles may facilitate the SPR coupling of the Au NPs growing therein, which accounts for the appearance of the dark blue color of the reaction media at room temperature (Figure 1b) or purple color at high temperatures (Figure 1a). In terms of colloidal stabilization, one can expect that the Au+/SAD complex particles may be initially stabilized by citrate ions as well as by AuCl4− ions, whereas the Au NPs newly formed inside of the complex particles may be stabilized mainly by AuCl4− ions. As the reaction progresses, more AuCl4− ions are reduced to Au+ ions, and the citrate ions may be the dominant stabilizers of the Au+/SAD complex particles. This can account for the reduction of the particle zeta potential from −27.5 to −42.7 mV at the early stage of the reaction (Figure 2d). As a result of AuCl 4 − to Au + ions and the disproportionation of the latter, meanwhile, more HCl is released especially from the Au+/SAD complex particles, which may cause a sudden reduction of the local pH close to the complex particle surfaces and thus protonation of the citrate molecules adsorbed on the complex particles. This can account for the increase of the zeta potential from −42.7 to −18.4 mV at the middle stage of the reaction at room temperature (Figure 2d) and for the colloidal instability and agglomeration of the complex particles (Figure 1b). At the end of the reaction when most AuCl4− ions are consumed (at high temperatures), excess citrate in the surrounding environment can buffer the surface pH of the Au+/SAD complex particles, which causes deprotonation of the citrate molecules adsorbed on the complete particle surfaces to make the particle surfaces more negatively charged. This should be the rationale for the reversible agglomeration of the complex particles (Figure 2b,e).
premixing method was adopted for the study of the temporal evolution of the size and zeta potential of the particles growing in the reaction media over the course of citrate reduction of HAuCl4 at room temperature. The Rc/a of the reaction solution was set mainly as 4.1, the value commonly used in the classic Turkevich method to produce Au NPs with sizes of 12−16 nm for the study of the NP growth mechanism, which enabled us to compare our results with those reported in the literature. The quite low reaction rate of citrate reduction of HAuCl4 at room temperature was also beneficial for the kinetic study. Figure 1b shows that when HAuCl4 and citrate are mixed in water at room temperature (Rc/a = 4.1), the yellow color of the mixture solution gradually fades and the dark blue color becomes very intense after 10 min and remains little changed with time, which is similar to that observed at the early stage of citrate reduction of HAuCl4 to Au NPs in boiling water (Figure 1a). Note that black precipitates might become visible with increasing reaction time, but adding the HAuCl4/citrate mixture solutions into boiling water led to a ruby red solution, indicative of the formation of monodisperse quasi-spherical Au NPs with sizes in the range of 12−16 nm. A series of aliquots were taken out from the aqueous solutions of HAuCl4/citrate/AgNO3 mixtures at different reaction times to assess the temporal evolution of the sizes and surface charges of the particles formed in the reaction media by means of DLS and zeta-potential measurements. Figure 2a shows that ca. 170 nm particles with a fairly narrow size distribution were observed in ca. 3 min after the mixing of HAuCl4, citrate, and AgNO3 in water at room temperature, whereas the particle sizes gradually decreased to 75 nm in ca. 9 min with little change in the size distribution. This size change is accompanied by the fading-out of the yellow color of the reaction media and by the gray hue becoming noticeable (Figure 1b). Afterward, the particle sizes rapidly increase to ca. 1600 nm with a fairly broad size distribution within 2 min, which occurs in concurrence with the dark blue color of the reaction media showing up intensively (Figure 1b). Coincident with the temporal evolution of the particle size and the reaction solution color, the zeta potential of the particles decreases from −27.5 to −42.7 mV in ca. 7 min and then rapidly increases to −18.4 mV in ca. 3 min, followed by a gradual decrease to −40.0 mV in ca. 15 min (Figure 2d). A similar size evolution was also observed when the HAuCl4/ citrate/AgNO3 mixtures with different Rc/a values, for example 6.9 and 0.8, were incubated in water at room temperature (Figure S2). When the aqueous solutions of HAuCl4/citrate/ AgNO3 mixtures are added into water at 60 °C, however, large particles with sizes of ca. 675 nm are immediately formed and rapidly shrink to ca. 50 nm within ca. 7 min and then gradually to ca. 20 nm in ca. 30 min (Figure 2b), which is accompanied by the quick reduction in the zeta potential from −10 to −50 mV in ca 7 min and by little change thereafter (Figure 2e). The present data underline the temporal separation of nucleation and growth of Au NPs during citrate reduction; the former readily occurs at room temperature, whereas the latter is significantly activated at high temperatures. This is in line with ́ the electrochemical model proposed by Rodriguez-Gonzá lez et 29 al. It is worth noting that the particle sizes (ca. 50 nm), observed at the rapid size reduction stage at 60 °C (Figure 2b), are comparable to those (ca. 75 nm) observed at the gradual size reduction stage at room temperature (Figure 2a). Figure 2d highlights that the formation of uniform 75 nm particles at the D
DOI: 10.1021/acs.langmuir.6b01312 Langmuir XXXX, XXX, XXX−XXX
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Scheme 1. (a) Schematic Depiction of the Deprotonation and Hydrolysis of HAuCl4, the Protonation of Citrate Ions, the Redox Reaction between AuCl4− and Citrate Ions, Au+/SAD Complexation, and the Disproportionation of Au+ Ions to Au Atoms and (b) Schematic Description of the Formation and Reversible Agglomeration of the Au+/SAD Complex Particles and the Growth of Au NPs Therein Occurring at the Early, Middle, and Final Stages during Citrate Reduction of HAuCl4a
a
The size and zeta potential of the particles obtained in the reaction solutions at the corresponding stages and the reaction solution color are highlighted according to the results shown in Figures 1 and 2.
Here, we found that the fluctuation in the zeta potential of Au+/SAD complex particles and their agglomeration occurring thereafter could be suppressed by introducing an additional stabilizer, tris(hydroxymethyl) aminomethane (Tris), into the reaction media. After incubation of HAuCl4/citrate/AgNO3/ Tris mixtures (Rc/a = 4.1) in water at room temperature for 20 min, particles with sizes of ca.75 nm and a zeta potential of ca. −22.5 mV were obtained, which hardly changed as the reaction progressed (Figure 2c,f). The initial zeta potential of the
resulting Au+/SAD complex particles was about −33 mV, indicating that the particles were stabilized by citrate ions at the initial stage of the reaction. However, the amino groups of Tris were able to interact with Au+ ions and Au atoms more strongly than the carboxyl and/or hydroxyl groups of citrate ions, so one can expect that the surfaces of the resulting Au+/SAD complex particles were gradually stabilized by both Tris and citrate together, which can be the rationale for the increase in the zeta E
DOI: 10.1021/acs.langmuir.6b01312 Langmuir XXXX, XXX, XXX−XXX
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Table 1. Summary of the Experimental Details for the Synthesis of Uniform Au NPs with Sizes Ranging from 2 to 330 nm and the Morphological and SPR Features of the Resulting Au NPs SPR band position (nm) synthesis methods GSH-assisted premixing method premixing method
Tris-assisted premixing method
Tris-assisted Turkevich method
size (nm)
PDI
ellipticity
expa
Mie
SPR band FWHM (nm) expa
Mie
[citrate] (mM)
[AuCl4−] (mM)
Rc/a
[Ag+] (μM)
[GSH] (μM)
[Tris] (mM)
Tp‑m (min)
1.03
0.25
4.1
5.05
300
0
6
2
9.5%
1.26
4 6 9 12 15 18 21 24 27 30 33 36 39 40
9.6% 8.3% 5.5% 8.3% 6.7% 5.5% 9.5% 8.3% 9.6% 9.6% 9.0% 8.3% 7.7% 7.5%
1.13 1.09 1.06 1.06 1.07 1.07 1.09 1.11 1.08 1.06 1.07 1.07 1.08 1.05
515 515 516 516 519 519 520 521 522 521 522 523 521
522 522 522 522 522 524 524 524 526 526 526 528 528
82 76 72 73 67 64 64 60 62 61 60 62 62
58 58 57 58 58 58 58 58 58 58 58 58 58
0.76 1.72 1.03 1.03 0.76 0.48 0.58 0.52 0.48 0.45 0.24 0.21 0.17 0.34
0.25 0.25 0.25 0.25 0.25 0.25 0.38 0.38 0.38 0.38 0.25 0.25 0.25 0.25
3 6.9 4.1 4.1 3.0 2.0 1.6 1.4 1.3 1.2 1.0 0.8 0.69 1.4
5.05 5.05 5.05 5.05 5.05 5.05 5.05 5.05 5.05 5.05 5.05 5.05 5.05 5.05
300 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 4
8 16 8 5 4.5 4 5 5.5 5 4.5 2.5 2.5 2 5
60 82 95 105
5.0% 8.9% 6.3% 9.5%
1.08 1.09 1.09 1.09
524 539 541 565
535 551 564 573
62 73 72 110
60 79 91 109
0.27 0.24 0.21 0.21
0.25 0.25 0.25 0.25
1.1 1.0 0.8 0.8
5.05 5.05 5.05 0
0 0 0 0
4 4 4 4
4 4 3 0
593 551 657 584 855 575 701
151
158
0.10 0.07
0.25 0.25
0.4 0.27
0 0
0 0
4 4
0 0
0.03
0.25
0.14
0
0
4
0
0.03
0.25
0.14
0
0
5
0
125 166
10% 10%
1.15 1.14
240
10%
1.16
580 551 634 596
330
10%
1.14
592
a
The SPR bands of Au NPs in Tris solution are slightly blue-shifted, compared with those in citrate solution because of the refractive index change, so the positions of SPR bands of Au NPs in Tris solution are smaller than those in pure water by Mie theory.
potential of the Au+/SAD complex particles with the reaction time. When the HAuCl4/citrate/AgNO3 mixtures (Rc/a = 4.1) are added into boiling water with Tris, the color of the reaction solution changes from colorless to light pink, pink, red, and wine red (Figure 1c), which is in line with the color changes with the particle size normally anticipated based on Mie theory.37 We also observed a similar color change when a huge excess of citrate was used, for example Rc/a > 6.9 (Figure S4). The success in turning the abnormal color change to the normal one in the reaction solution confirms that the aforementioned rationalization of the growth of Au NPs via citrate reduction is plausible, which is summarized in Scheme 1. Overall, the new model underlines that the formation of Au+/ SAD complex particles at the early stage of citrate reduction of HAuCl4 is the key step to regulate the growth of Au NPs. These complex particles serve as uniform microscopic reactors for the nucleation and growth of Au NPs, whereas they undergo reversible agglomeration at the middle stage of the reaction as a result of sharp fluctuation in their surface zeta potential. On the one hand, our model endorses the reversible agglomeration of the Au+/SAD complex particles, which is in line with the nonclassic growth mechanism.15,23 On the other hand, it stresses the exclusive growth of single Au NPs within
individual Au+/SAD complex particles, which proceeds in the seeded growth fashion proposed by Polte and co-workers.36 According to our model, the reversible agglomeration of Au+/ SAD complex particles and the growth of Au NPs therein are coincident but are independent of each other. The abnormal color change of the reaction solution is a result of the SPR coupling of single Au NPs formed in neighboring Au+/SAD complex particles during the reversible agglomeration of the complex rather than that of the neighboring Au NPs formed in individual Au+/SAD complex particles. As a result, the new model enables us to reconcile different mechanisms currently available in the literature into one consistent framework to describe the growth of Au NPs via citrate reduction of HAuCl4. One-Pot Synthesis of Monodisperse Quasi-Spherical Au NPs with Sizes from 2 to 330 nm. According to our new model, the growth of Au NPs and especially the sizes of the final NPs should be controlled by the formation of Au+/SAD complex particles at the early stage of citrate reduction of HAuCl4. As suggested in the literature,29 the reduction of AuCl4− to Au+ ions is the rate-determining step for the formation of Au+/ SAD complex particles. The successful use of Tris to realize the normal color change of the reaction solution should inspire another possible way to regulate the sizes of final Au NPs by F
DOI: 10.1021/acs.langmuir.6b01312 Langmuir XXXX, XXX, XXX−XXX
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Figure 3. TEM images of Au NPs synthesized by the GSH-assisted premixing method at Rc/a of 4.1 (a) and 3 (b). The insets are high-resolution TEM (HRTEM) images of corresponding Au NPs. The experimental details are listed in Tables 1 and S2.
centered cubic Au (0.234 nm).42,43 The quality of the resulting small Au NPs is comparable to that produced via the two-phase Brust−Schiffrin method.14 The recipe used for the production of uniform 2 nm Au NPs was almost identical to that used for the production of uniform 12 nm Au NPs; Rc/a was set as 4.1. The incubation of HAuCl4/ citrate/AgNO3/GSH mixtures in water at room temperature caused immediate formation of uniform ca. 30 nm particles with a zeta potential of −30 mV, which remained little changed with the reaction time. This observation was in stark contrast to that during the incubation of HAuCl4/citrate/AgNO3 mixtures in water at room temperature, in which uniform ca. 75 nm particles were formed and reversibly agglomerated (Figure 2a) in accompaniment with the sharp fluctuation in the particle zeta potential (Figure 2d). When the HAuCl4/citrate/AgNO3/GSH mixture solutions were added into boiling water, large particles with sizes of ca. 250 nm were initially observed but rapidly shrunk to uniform small NPs; the NPs were reduced to 2 nm after 30 min of reaction (Figure S6). The zeta potential of the resulting particles in the reaction solutions remained fairly negative in the range from −30 to −50 mV despite the sharp fluctuation at the beginning of the reaction (Figure S6). These findings confirm that the presence of GSH in the reaction media not only can facilitate the reduction of HAuCl4 and thus nucleation and growth of tiny Au NPs but also provide the tiny NPs with effective stabilization and thus prevent them from agglomeration. Synthesis of Monodisperse, Quasi-Spherical Au NPs with Sizes in the Range of 6−39 nm in Size Increments of 3 nm by the Premixing Method. The sizes of the aggregates of tiny charged particles or molecules are expected to depend on the surface charge density.44 Because the Au+/ SAD complex particles formed at Rc/a of 4.1 (Figure 2d) and 0.8 (Figure S7) had comparable surface potential values, the complex particle sizes might be hardly affected by the Rc/a. This is evidenced by the comparable sizes of Au+/SAD complex particles obtained at different Rc/a values (ca. 75 nm) (Figures 2a and S2). According to the kinetics study, on the other hand, the citrate reduction of AuCl4− ions to Au+ ions is enhanced with increasing Rc/a (Figure S3), so more Au+/SAD complexes are expected to be formed at a larger Rc/a at the early stage of the reaction. However, this will increase the number of Au+/ SAD complex particles in the reaction media with little change in the particle sizes by taking into account little dependence of the Au+/SAD complex particle size on Rc/a discussed above.
introducing secondary stabilizers into the reaction system. Built on our new model, here we succeeded in the direct synthesis of monodisperse quasi-spherical Au NPs with sizes ranging from 2 to 330 nm via citrate reduction. The experimental details of the synthesis of differently sized Au NPs are summarized in Table 1. Synthesis of Uniform Au NPs with Sizes of 2 and 4 nm via the Glutathione-Assisted Premixing Method. To date, the synthesis of Au NPs with sizes of
