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Formation of Gold Nanoparticles Using Amine Reducing Agents J. D. S. Newman and G. J. Blanchard* Michigan State UniVersity, Department of Chemistry, East Lansing, Michigan 48824-1322 ReceiVed January 5, 2006. In Final Form: April 1, 2006 We report on the use of amines as reducing agents in the formation of gold nanoparticles. We can predict whether the amines will function as reducing agents in this reaction based on their redox properties. The kinetics of AuNP formation can be understood in terms of Marcus electron transfer theory, where the slower reactions proceed in the inverted region owing to the difference between the Au reduction potential and the amine oxidation potential. For a certain number of the amine reducing agents, following reduction of HAuCl4, a subsequent reaction of the amine radical cation with other reducing agent molecules in solution can form poly(amine)s. These findings point collectively to the utility of amines as reducing agents in AuNP formation and provide information on the conditions under which these reactions will proceed.
Introduction Gold nanoparticles (AuNPs) are a rapidly growing area of materials chemistry because of the chemical versatility of the AuNP surface and the relationship between their size and optical properties. AuNPs have found use in areas ranging from chemical separations1 and sensing2-11 to applications in the medical community,12-15 such as the diagnosis and treatment of certain cancers.16-19 As a result of their use in the medical arena, there is a significant effort aimed at developing stable and biocompatible AuNP preparations. AuNPs have been synthesized using a variety of methods, including citrate reduction,20 two-phase synthesis,21 and a one* To whom correspondence should be addressed. E-mail: blanchard@ chemistry.msu.edu. (1) Gross, G. M.; Nelson, D. A.; Grate, J. W.; Synovec, R. E. Anal. Chem. 2003, 75, 4558. (2) Dos Santos, D. S., Jr.; Goulet, P. J. G.; Pieczonka, N. P. W.; Oliveira, O. N., Jr.; Aroca, R. F. Langmuir 2004, 20, 10273. (3) Faulds, K.; Littleford, R. E.; Graham, D.; Dent, G.; Smith, W. E. Anal. Chem. 2004, 76, 592. (4) Frederix, F.; Friedt, J.-M.; Choi, K.-H.; Laureyn, W.; Campitelli, A.; Mondelaers, D.; Maes, G.; Borghs, G. Anal. Chem. 2003, 75, 6894. (5) Grubisha, D. S.; Lipert, R. J.; Park, H.-Y.; Driskell, J.; Porter, M. D. Anal. Chem. 2003, 75, 5936. (6) Krasteva, N.; Besnard, I.; Guse, B.; Bauer, R. E.; Muellen, K.; Yasuda, A.; Vossmeyer, T. Nano Lett. 2002, 2, 551. (7) Matsui, J.; Akamatsu, K.; Nishiguchi, S.; Miyoshi, D.; Nawafune, H.; Tamaki, K.; Sugimoto, N. Anal. Chem. 2004, 76, 1310. (8) Nath, N.; Chilkoti, A. Anal. Chem. 2004, 76, 5370. (9) Tokareva, I.; Minko, S.; Fendler, J. H.; Hutter, E. J. Am. Chem. Soc. 2004, 126, 15950. (10) Zhang, Z.-F.; Cui, H.; Lai, C.-Z.; Liu, L.-J. Anal. Chem. 2005, 77, 3324. (11) Grate, J. W.; Nelson David, A.; Skaggs, R. Anal. Chem. 2003, 75, 1868. (12) Andersson, M.; Fromell, K.; Gullberg, E.; Artursson, P.; Caldwell, K. D. Anal. Chem. 2005, 77, 5488. (13) Aslan, K.; Lakowicz, J. R.; Geddes, C. D. Anal. Chem. 2005, 77, 2007. (14) Niidome, T.; Nakashima, K.; Takahashi, H.; Niidome, Y. Chem. Commun. 2004, 1978. (15) Rojo, J.; Diaz, V.; De la Fuente, J. M.; Segura, I.; Barrientos, A. G.; Riese, H. H.; Bernad, A.; Penades, S. ChemBioChem 2004, 5, 291. (16) Hirsch, L. R.; Stafford, R. J.; Bankson, J. A.; Sershen, S. R.; Rivera, B.; Price, R. E.; Hazle, J. D.; Halas, N. J.; West, J. L. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 13549. (17) Loo, C.; Lin, A.; Hirsch, L.; Lee, M.-H.; Barton, J.; Halas, N.; West, J.; Drezek, R. Technol. Cancer Res. Treat. 2004, 3, 33. (18) Loo, C.; Lowery, A.; Halas, N.; West, J.; Drezek, R. Nano Lett. 2005, 5, 709. (19) O’Neal, D. P.; Hirsch, L. R.; Halas, N. J.; Payne, J. D.; West, J. L. Cancer Lett. 2004, 209, 171. (20) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, No. 11, 55. (21) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801.
phase synthesis in an organic solvent.22 A relatively recent development has been the use of the reducing agent as the nanoparticle capping species as well, and this technique has been demonstrated using amino acids.23,24 The vast majority of AuNP syntheses, however, are performed using citrate as the reducing and capping agent. If a capping agent other than sodium citrate is desired, an exchange reaction is required as a second synthetic step. Amines have been used in AuNP synthesis as both reducing agents and as stabilizers23-28 after AuNP formation. A variety of amines have been explored, including simple primary amines,27,28 amino acids,23-25 and multifunctional amines including polymers.26 Amines are a particularly attractive class of reducing agents because of their nearly universal presence in biological and environmental systems. However, there has been little reported about which amine properties, structural or chemical, are responsible for their utility as reducing and stabilizing agents. We are interested in understanding whether there is benefit in using amines as reducing agents, in terms of gaining control over nanoparticle growth rate, optical properties or surface passivation. We report here on our examination of a series of amines and other more widely used reducing agents20 (Chart 1) with the goal of determining whether they function as reducing agents in the formation of AuNPs. Specifically, we focus on the redox chemistry of the amines studied here and whether this property serves as a predictor of their ability to function as reducing agents for HAuCl4 from a thermodynamic standpoint. We consider the kinetics of AuNP formation and longer term stability of the resulting nanoparticles as a function of amine identity. We find that we can understand the kinetics of AuNP growth in the context of the relevant redox reaction proceeding in the Marcus inverted region for certain of the amines. We have used cyclic voltammetry to interrogate the redox chemistry of aqueous and acetonitrile solutions of HAuCl4 and (22) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655. (23) Selvakannan, P. R.; Mandal, S.; Phadtare, S.; Gole, A.; Pasricha, R.; Adyanthaya, S. D.; Sastry, M. J. Colloid Interface Sci. 2004, 269, 97. (24) Bhargava, S. K.; Booth, J. M.; Agrawal, S.; Coloe, P.; Kar, G. Langmuir 2005, 21, 5949. (25) Aslam, M.; Fu, L.; Su, M.; Vijayamohanan, K.; Dravid, V. P. J. Mater. Chem. 2004, 14, 1795. (26) Iwamoto, M.; Kuroda, K.; Zaporojtchenko, V.; Hayashi, S.; Faupel, F. Eur. Phys. J. D: Atomic, Mol., Opt. Phys. 2003, 24, 365. (27) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (28) Subramaniam, C.; Tom, R. T.; Pradeep, T. J. Nanopart. Res. 2005, 7, 209.
10.1021/la060045z CCC: $33.50 © 2006 American Chemical Society Published on Web 05/26/2006
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Chart 1. Structures of Reducing Agents Used in This Work
a variety of primary, secondary, and tertiary amines including aliphatic, aromatic, and aryl systems. After determining which of the amine reducing agents would be thermodynamically capable of spontaneously reducing HAuCl4 to Au0, we have monitored the kinetics of selected reactions using time-resolved UV-visible spectroscopy. We find that the kinetics of the AuNP formation reaction depend sensitively on the identity of the reducing agent, and in certain cases, there can be competition between AuNP formation and reducing agent polymerization. The findings we report here point the way to the controlled formation of AuNP structures using amine reducing agents. We anticipate these results to be of use in the synthesis of biocompatible and sensing materials. Experimental Section Chemicals. HAuCl4‚3H2O (99.9+%), 3-aminophenol, 4-aminophenol, triethylamine, acetone, indole, 1,4-phenlyenediamine, aniline, 4-bromoaniline, 3-indolepropionic acid, 3-amino-1-propanol, 1-methylindole, pyridine, citric acid, hydroxylamine hydrochloride, sodium citrate, D,L-tryptophan, and lithium perchlorate were purchased from Aldrich Chemical Co. and used as received. Glycine was purchased from Tyron (Okemos, MI) and used as received. Electrochemistry. HAuCl4 solutions were prepared to a concentration of 0.1 M in 0.5 M LiClO4. Solutions of amines to be evaluated as reducing agents were prepared to a concentration of 0.1 M in 0.5 M LiClO4 in either acetonitrile (MeCN) or H2O, depending on solubility. Due to solubility limitations, D,L-tryptophan was prepared as a 0.005 M solution in 0.5 M LiClO4 (aq). Cyclic voltammetry was conducted using an electrochemical bench (CH Instruments model 604B, Austin, TX) in cyclic voltammetry (CV) mode with a scan rate of 50 mV/s and a sensitivity of 10-5 A full scale. A standard three electrode configuration was used with a glassy carbon working electrode, a platinum wire counter electrode,
and a reference electrode. The reference electrodes used were Ag/ AgCl with 3 M KCl (aq) for aqueous measurements and 1 mM AgNO3 (MeCN) for nonaqueous measurements. The potentials measured using the Ag/AgCl reference electrodes are reported against a standard hydrogen electrode (SHE) to facilitate comparison to reported tabular values for Au reduction potentials. Three forward and three reverse scans were recorded for each sample. Kinetics and Stability Studies. AuNPs were synthesized by combining a 0.005 M reducing agent solution (5 mL) with a 0.005 M HAuCl4 (50 µL) solution in the same solvent and shaking to agitate. The reducing agents used for the kinetics studies were 4-aminophenol, triethylamine, glycine, indole, 1,4-phenylenediamine, tryptophan, and sodium citrate. The solutions were combined and analyzed at room temperature, and UV-visible spectra were obtained at intervals for the resulting undiluted solution. The spectra were obtained at either 10 or 15 min intervals for up to 100 scans or until a depletion of the absorbance was noted. The duration of a given measurement was determined by the rate of the process being monitored. The spectra were acquired from 350 to 800 nm using a Cary model 300 UV-visible spectrometer (Varian) and acquisition time was ca. 1 min per spectrum. Initial rate information was extracted from the temporal dependence of the plasmon resonance absorbance band following the initial mixing of the reactants.
Results and Discussion The primary focus of this paper is on understanding the potential utility of amine reducing agents for the formation of AuNPs. The chief points we address are (1) achieving predictive power over which reducing agents will form AuNPs, (2) characterizing the kinetics of formation for the AuNPs, and (3) understanding whether the presence of amine containing species can give rise to other chemical reactions in solution. The issue of whether amines can be used in the formation of AuNPs is of practical
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importance because of the ubiquitous nature of amines, especially in biological systems. Despite the number of systems that can, in principle, be reacted with HAuCl4 to form AuNPs, citrate reduction is by far the most widely used reaction method. By exploring the ability of amines to reduce trivalent Au, with a range of reaction rates that depends on the amine used, we hope to bring additional chemical “tunability” to the field of nanoparticle formation. Because we are concerned with the issue of whether amines will function as reducing agents for HAuCl4 as well as the kinetic issues of AuNP formation, we have not characterized the AuNP average size and particle size distribution resulting from these reactions, and monitor the reaction product by means of the AuNP plasmon resonance band. There is a well established relationship between AuNP size and plasmon band position, with the plasmon resonance band shifting to the red and broadening with increasing particle size.29 We observed no significant degree of either PR wavelength shift or band broadening observed during AuNP growth for any of the reducing agents used in this study. This finding is consistent with the AuNP size depending on the reaction conditions for a given amine. Electrochemistry. The reduction of HAuCl4 occurs due to transfer of electrons from the amine to the metal ion, resulting in the formation of Au0, with the reaction being generically described according to Scheme 1.
Scheme 1 HAuCl4 + 3 NR3 f Au0 + 3 NR3+• + H+ + 4ClThe resulting metallic gold then undergoes nucleation and growth to form AuNPs. The fate of the oxidized amine clearly depends on the amine used. In some cases, we expect a monomeric entity, and for other systems, for which the formation of a radical cation is the first step in a polymerization reaction, we expect the formation of oligomeric and polymeric species.28 We have performed NMR measurements for some systems, and these data point to the formation of short chain oligomers of the amines resulting from the redox reaction. See the Supporting Information for representative data for indole. The initial step in this work is to understand whether the reaction shown in Scheme 1 is thermodynamically allowed. We have evaluated the oxidation and reduction potentials of HAuCl4 in both aqueous and MeCN solutions. In both solvents, we observe stable oxidation and reduction potentials for Au. In aqueous solution, the reduction potential of HAuCl4 to Au0 was found to be 0.853 V (vs SHE) and the oxidation potential of Au0 to Au1+ was 1.425 V (vs SHE), in 0.5 M LiClO4 solution. In MeCN solution, the reduction potential of the HAuCl4 was 0.888 V (vs SHE), whereas the oxidation potential of Au0 was 1.555 V (vs SHE), in 0.5 M LiClO4 (Figure 1, panels a and b). These values, compared to the reduction potentials of the various amines used in this work (Chart 1), will determine from a thermodynamic standpoint which of the amines will be capable of reducing HAuCl4. For our systems, the HAuCl4 reduction to Au0 is irreversible, and the corresponding amine oxidation reaction, NR3 oxidizing to its radical cation, is also irreversible. Oxidation of Au0 or reduction of the oxidized amine will result in the formation of species that are different than the initial reactants. If the oxidation potential of the amine falls within the range of potentials between the reduction and oxidation potentials of the gold system, HAuCl4 reduction should be thermodynamically allowed. The essential issue is that for the reaction to proceed, (29) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036.
Figure 1. (a) Comparison between the CV of HAuCl4 (solid line) and tryptophan (dashed line) in aqueous 0.5 M LiClO4 electrolyte, (b) CV of HAuCl4 0.5 M LiClO4 electrolyte (MeCN), and (c) CV of pyridine in 0.5 M LiClO4 (MeCN).
the reduction potential of HAuCl4 is lower than the oxidation potential of the amine under study (Figure 1a,b). When an amine reducing agent has an oxidation potential lower than the reduction potential of HAuCl4, AuNPs will not form because HAuCl4 will not be reduced spontaneously. For amine oxidation potentials above that for the oxidation of Au0 to Au1+, there will be no reaction, because Au0 would oxidize before the amine would. This is the case for pyridine (Figure 1c). In this situation, the reduction reaction would be HAuCl4 to Au0 and the oxidation reaction would be Au0 to Au1+, precluding AuNP formation. If the reduction potential of HAuCl4 is greater than the potential at which the amine is oxidized, AuNP formation will not proceed spontaneously on thermodynamic grounds. For non-amine systems, all of those with literature precedent for use in AuNP formation (e.g., acetone, citric acid, hydroxylamine HCl, and sodium citrate)20 are characterized by reduction potentials that lie between the HAuCl4 reduction and Au0 oxidation potentials. Experimentally, we find that those amines predicted by electrochemistry not to allow for AuNP formation, in fact, do not yield AuNPs. Of the amines we consider here, only 3-amino1-propanol, 1-methylindole, and pyridine are disallowed thermodynamically (Table 1). There are several amine reducing agents, however, that are predicted on thermodynamic grounds to allow for AuNP formation, but we do not observe facile nanoparticle growth. These include 3-aminophenol (oxidation potential 1.012 V vs SHE), aniline (oxidation potential 1.183 V vs SHE), and 4-bromoaniline (oxidation potential 1.112 V vs SHE). It is clear from these experimental data that the potentials of the relevant redox reactions for the different amine reducing
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Table 1. Oxidation Potentials of Amines in a 0.5 M LiClO4 Electrolyte Solution amine
solvent
conc. (M)
oxidation potential (V)
3-aminophenol 4-aminophenol triethylamine glycine 4-bromoaniline indole 1,4-phenylenediamine aniline D,L-tryptophan 3-indolepropionic acid 3-amino-1-propanol 1-methylindole pyridine
MeCN MeCN MeCN H2O MeCN MeCN MeCN MeCN H2O MeCN MeCN MeCN MeCN
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.005 0.1 0.1 0.1 0.1
1.021 1.062 1.079 1.112 1.122 1.129 1.160 1.183 1.283 1.308 1.589 1.641 1.980
agents can have a significant effect on the formation of AuNPs in a given system as well as their formation kinetics. We believe that our experimental data on which amines readily form AuNPs and those which exhibit slow growth kinetics are consistent with the electrochemical data and we understand these findings within the framework of Marcus electron transfer theory.30-33 Although thermodynamic considerations dictate whether the reaction can proceed, there are important kinetic issues that can play a deterministic role in the formation of AuNPs. It is clear that we can achieve some control over these systems by controlling the reaction temperature. The work we report here was carried out at room temperature, and it is useful to keep this in mind in our discussion of the kinetic behavior of these systems. AuNP Growth Kinetics and Stability. For the studies we present below, the amine reducing agent and HAuCl4 solutions were each 0.005 M, with either water or MeCN as the solvent, depending on the system. For these measurements, we used the time-evolution of the AuNP plasmon resonance band to monitor the reaction kinetics. A word is in order regarding the details of the AuNP plasmon resonance band used in our kinetics studies. As noted above, the plasmon band maximum depends on particle size as well as the solvent used and the degree of aggregation in solution.29 For a given nanoparticle-forming system, we see little time-dependent variation in the plasmon resonance band profile, suggesting it is the concentration rather than the size of the nanoparticles that we are monitoring. Although the band position can vary with the use of different reducing agents and solvents, the wavelengths used for our kinetics studies are either 530 or 560 nm, depending on the system. We note that there are two contributions to these kinetic results: Au3+ reduction and AuNP formation. The functional form of the data we present below suggests that the electrochemical step plays the mediating role in AuNP formation. Within the group of amine reducing agents with a single nitrogen per molecule, the rate of AuNP formation was found to depend exponentially on the amine reduction potential up to 1.425 V (aq) or 1.555 V (MeCN), the oxidation potential of Au0 to Au1+ (Figure 2 and Table 2). For reducing agents that possess two nitrogens per molecule, we are tempted to note a decrease in the rate of AuNP formation with increasing oxidation potential. We caution, however, that we examined only two amines possessing two nitrogens per molecule, limiting our ability to identify a trend with any degree of certainty. The operating premise of Marcus electron transfer theory is that the actual electron-transfer event for a reaction occurs on (30) Marcus, (31) Marcus, (32) Marcus, (33) Marcus,
R. R. R. R.
A. A. A. A.
J. J. J. J.
Chem. Chem. Chem. Chem.
Phys. Phys. Phys. Phys.
1956, 1956, 1957, 1957,
24, 24, 26, 26,
966. 979. 867. 872.
Figure 2. Relationship between oxidation potential and initialgrowth rate when one nitrogen is present per mole reducing agent.
Figure 3. CV of aniline in 0.5 M LiClO4 (MeCN). Table 2. Summary of Kinetic Data from UV-Visible Spectroscopy reducing agent
oxidation potential (V)
rate (AU/min)
4-aminophenol triethylamine (TEA) glycine indole 1,4-phenylenediamine tryptophan sodium citrate
1.062 1.079 1.112 1.129 1.160 1.283 1.271
3.4 × 10-3 6.6 × 10-4 2.5 × 10-5 5.9 × 10-6 2.8 × 10-3 8.2 × 10-4 6.1 × 10-5
a time scale faster than the inertial processes that mediate any structural rearrangement associated with the reaction. It is thus useful to consider the reaction coordinate for an electron-transfer process in the context of the Franck-Condon principle. Within the framework of this model, as the difference between the potential energy minima of the reactant and the product increases, there is an increase in the rate of the electron transfer reaction. This relationship holds true until the optimum reaction rate is achieved at the point where the product energy curve passes through the minimum of the reactant energy curve. Here the electron transfer proceeds along an essentially barrierless surface. If viewed in the context of the solvent environment of the reactants and products, the electron transfer reaction will proceed at a rate related to the driving force up to the point where the product and reactant species become sufficiently structurally different that a large change in either the molecular structure or the solvation environment are required to stabilize the product. Such inertially mediated considerations can serve to slow the kinetics of the
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Figure 4. 1H NMR in CDCl3 for (a) aniline after completion of electrochemistry and (b) prior to electrochemical oxidation.
electron-transfer process. In this corollary, the reactant energy is represented by the oxidation potential of the amine and the product energy is represented by the reduction potential of the HAuCl4. From Marcus theory, there should be an increase in the rate of the electron-transfer reaction, and therefore an increase in the AuNP plasmon resonance growth rate, as the amine reduction potential increases beyond the HAuCl4 reduction potential. At some combination of HAuCl4 reduction potential and amine oxidation potential the optimum conditions are achieved and beyond this point the electron transfer reaction rate, and consequently the nanoparticle formation and growth rate, should decrease. The experimentally observed decrease in plasmon resonance growth rate with difference between the reduction potential of HAuCl4 and oxidation potential of the amine reducing agent is consistent with Marcus electron transfer theory and suggests that, for these systems, we are operating in the Marcus inverted region. There are several amine systems that, while thermodynamically allowed, do not lead to the formation of AuNPs. The solutions of aniline, 3-aminophenol and 4-bromoaniline do not react to form AuNPs when combined with HAuCl4. We believe that aniline and 3-aminophenol do not exhibit measurable AuNP growth because they participate in a competitive polymerization reaction of the reducing agent.34,35 This finding does not mean that Au0 is not formed during the reaction, but rather it is not detected due to the competitive reaction of the amine reducing agent to form its corresponding polymer. For these systems, either AuNPs that are formed initially by monomer oxidation and are later oxidized during the polymerization reaction, if the reaction rates of HAuCl4 reduction and polymer formation are (34) Mallick, K.; Witcomb, M. J.; Dinsmore, A.; Scurrell, M. S. Macromol. Rapid Commun. 2005, 26, 232. (35) Shan, J.; Han, L.; Bai, F.; Cao, S. Polym. AdV. Technol. 2003, 14, 330.
sufficiently different, or the steady-state concentration of Au0 is below that required for AuNP formation if the Au3+ reduction rate and reducing agent polymerization reaction rate are similar. Aniline and 3-aminophenol show similar time-resolved spectral profiles. For illustrative purposes, we consider the data for aniline (the data for 3-aminophenol may be found in the Supporting Information). The CV for aniline shows an oxidation peak at 1.183 V (vs SHE) with a broad peak after the initial oxidation (Figure 3). The broad peak is indicative of a radical initiated polymerization resulting from the oxidation of aniline. This oxidative polymerization has literature precedent34 in the presence of an oxidizing agent. The 1H NMR of the aniline sample after the CV was conducted shows evidence of a slight downfield shift accompanied by a peak broadening (Figure 4), a typical result for polymer formation. Additionally, the time-resolved UV-visible spectrum shows the development of a peak in the 410 nm range which is consistent with a previously reported absorption feature associated with the polymerization of aniline (Figure 5).36 The inability of the 4-bromoaniline to produce AuNPs may be explained either by the strong electron withdrawing nature of the bromide substituent or by the competitive rates of polymer formation and Au3+ reduction. 4-Aminophenol represents a case where both AuNP formation and polymerization can be observed. We believe this is the case because for 4-aminophenol the kinetics of HAuCl4 reduction and polymerization are not similar. 4-Aminophenol exhibits the fastest AuNP plasmon resonance growth rate for any of the amines we have studied. AuNP solutions formed with this reducing agent are, however, the least stable. In the presence of HAuCl4 or Au0, 4-aminophenol exhibits spectral growth characteristic of polymerization (Figure 6, panels a and b), with the band at 375 nm (36) Kogan, Y. L.; Davidova, G. I.; Knerelman, E. I.; Gedrovich, G. V.; Fokeeva, L. S.; Emelina, L. V.; Savchenko, V. I. Synth. Met. 1991, 41, 887.
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and reduced forms,35 so the disappearance of the AuNP plasmon resonance peak with time is likely due to an equilibrium between the Au0 in the form of AuNPs and Au1+ in solution. In this instance, the AuNPs are seen as an intermediate in the polymerization reaction. The postulated equilibration between the oxidized and reduced forms of poly(4-aminophenol)35 necessitates also an equilibrium between the Au0 and one of the oxidized forms. Because Au1+ is the most stable Au ion, the equilibrium likely exists between Au0 and Au1+ accounting for the disappearance of the plasmon resonance peak for the AuNPs as the polymer peak increases instead of a equilibration of this peak with time.
Conclusions Figure 5. Time-resolved US-visible spectrum of 100:1 0.005 M aniline/0.005 M HAuCl4 conversion to poly(aniline).
Figure 6. UV-visible spectrum of (a) 100:1 0.005 M 4-aminophenol/0.005 M HAuCl4 and (b) excess Au0 and 0.005 M 4-aminophenol conversion to poly(4-aminophenol).
being associated with the growth of the polymer. Electrochemical and enzymatic polymerization of 4-aminophenol has literature precedent35,37,38 and we postulate that the redox reaction taking place between the 4-aminophenol and HAuCl4 not only creates Au0 in the form of AuNPs but also oxidizes the 4-aminophenol to a radical cation, which subsequently reacts to form poly(4aminophenol). Poly(4-aminophenol) can exist in both the oxidized (37) Salavagione, H. J.; Arias-Pardilla, J.; Perez, J. M.; Vazquez, J. L.; Morallon, E.; Miras, M. C.; Barbero, C. J. Electroanal. Chem. 2005, 576, 139. (38) Taj, S.; Ahmed, M. F.; Sankarapapavinasam, S. J. Electroanal. Chem. 1992, 338, 347.
The oxidation potential of amines relative to that of the reduction potential of HAuCl4 and oxidation potential of Au0 provides a first line of prediction on the ability of a given amine to reduce HAuCl4 to Au0, with the subsequent formation of AuNPs. Amines that have a reduction potential between that of the oxidation of Au0 to Au1+ and the reduction of HAuCl4 to Au0 are candidate reducing agents. Those with oxidation potentials outside this range will not react with HAuCl4 to form AuNPs. In some cases, as in aniline and 3-aminophenol, competitive polymerization of the reducing agent can result in an inability to detect an AuNP plasmon resonance band in the visible spectrum, due either to a large polymer absorption in same spectral region or the rapid consumption of Au0 by the amine polymerization process. For some amines, such as 4-bromoaniline, the presence of strong electron withdrawing groups removes electron density from the amine moiety which renders the amine unable to reduce the HAuCl4. The kinetics of the reaction also play a role in the ability of a given reducing agent to produce AuNPs. At room temperature, compounds with the same number of nitrogens show a decrease in plasmon resonance growth rate with increasing oxidation potential. For reducing agents with one amine nitrogen per molecule, increasing the oxidation potential results in an exponential decay in the plasmon resonance growth rate. These results provide the basis for the prediction of which amines will function as reducing agents in AuNP production, and the variation in kinetics for AuNP formation can be understood either in the context of Marcus theory or in terms of competition between the reduction of Au3+ and the formation of polymeric amines. We anticipate these results will be of use in the formation of AuNPs in matrixes that are either water-processible or are bio-compatible. Acknowledgment. We are grateful to the U. S. Department of Energy for support of this work through Grant DEFG0299ER15001. Supporting Information Available: NMR, CV, and UVvisible spectra. This is available free of charge at http://pubs.acs.org LA060045Z
