Langmuir 2003, 19, 8555-8559
8555
Synthesis of Mixed Monolayer-Protected Gold Clusters from Thiol Mixtures: Variation in the Tail Group, Chain Length, and Solvent Hosun Choo, Erin Cutler, and Young-Seok Shon* Department of Chemistry, Western Kentucky University, Bowling Green, Kentucky 42101 Received June 10, 2003. In Final Form: July 11, 2003 We present a systematic study on the synthesis of mixed monolayer-protected clusters (MMPCs) from mixtures of alkanethiols. Our results show that solvation-driven thermodynamic preferential adsorption of precursor ligands governs monolayer composition of MMPCs. In the mixed system HO(CH2)nSH/ CH3(CH2)mSH (n ) m or n * m) in toluene, adsorption of the polar component is largely favored because of the poorer solvation of polar tail groups in toluene compared to tetrahydrofuran (THF). When MMPCs are generated from the mixed system HO(CH2)nSH/CH3(CH2)mSH (n ) m) in THF, the tail group effect is much less. The MMPC synthesis in THF solutions containing HO(CH2)nSH/CH3(CH2)mSH (n * m) shows large thermodynamic control, which promotes a preferential adsorption of the thiols with a longer alkyl chain onto the surface of clusters. Transmission electron microscopy (TEM) and UV-vis spectroscopy data suggest that the average core dimension of MPCs generated from alkanethiols in THF is slightly smaller than that of MPCs generated from alkanethiols in toluene under the same conditions.
Introduction Due to the stabilization of a metal nanoparticle by the protection of a dense monolayer,1-4 monolayer-protected clusters (MPCs) exhibit higher stabilities in dried forms than other colloids or nanoparticles prepared by different methods, such as polyelectrolyte or polymer stabilization,5 gas-phase stabilization,6 or micelles.7 This is indicated by their facile isolation as well as dissolution in common organic solvents without causing degradation or aggregation. MPCs have been used or proposed for uses in areas such as chemical sensors,8 nanoscale electronics,9,10 catalysis,11 and optics.12 A key to bringing nanoparticle technology to the chemical and biological fields is the manipulation of the size, shape, and chemical properties of the MPCs.1 Since the precise control over the chemical and structural composition of monolayer surrounding a metal cluster can * Corresponding author. E-mail:
[email protected]. (1) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (2) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 1655. (3) Badia, A.; Demers, L.; Dickinson, L.; Morin, F. G.; Lennox, R. B.; Reven, L. J. Am. Chem. Soc. 1997, 119, 11104. (b) Badia, A.; Cuccia, L.; Demers, L.; Morin, F.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682. (4) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397. (5) Cole, D. H.; Shull, K. R.; Baldo, P.; Rehn, L. Macromolecules 1999, 32, 771. (6) Bell, R. C.; Zemski, K. A.; Castleman, A. W., Jr. J. Phys. Chem. A 1999, 103, 1585. (7) Martin, J. E.; Wilcoxon, J. P.; Odinek, J.; Provencio, P. J. Phys. Chem. B 2002, 106, 971. (8) Storhoff, J. J.; Mirkin, C. A. Chem. Rev. 1999, 99, 1849. (b) Alivastos, A. P.; Johnson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P., Jr.; Schultz, P. G. Nature 1996, 382, 609. (9) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 101. (b) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 202. (10) Sato, T.; Ahmed, H.; Brown, D.; Johnson, B. F. G. J. Appl. Phys. 1997, 82(2), 696. (11) Li, H.; Luk, Y.-Y.; Mrksich, M. Langmuir 1999, 15, 4957-4959. (12) Mulvaney, P. Langmuir 1996, 12, 788. (b) Templeton, A. C.; Pietron, J. J.; Murray, R. W.; Mulvaney, P. J. Phys. Chem. B 2000, 104, 564.
dramatically affect its macroscopic properties, mixed monolayer-protected clusters (MMPCs) have found increased use in many of the aforementioned applications, including electrocatalysis,13 films,14 heavy metal detection,15 and chemical recognition.16-18 The development of a direct synthesis of MMPCs, as opposed to using two- or three-step methods known as ligand-place exchange19a,b or amide/ester coupling19b procedure, is beneficial in these areas of research. In particular, this approach will be useful for the synthesis of MMPCs with higher loading of multiple monolayer components. This method is advantageous when bulky or small incoming ligands have difficulty replacing the original ligands on the cluster surface due to either kinetic (e.g., steric hindrance) or thermodynamic effects. It has been found that ligand-place exchange at higher loading (>1:3) has been inhibited due to the presence of steric repulsion.19c Mixed two-dimensional self-assembled monolayers (SAMs) with different chain lengths and terminal groups on gold surfaces have been prepared using various methods.20-25 One approach involves the coadsorption of mixtures of alkanethiols having different functional groups and/or different chain lengths.20,21 Other ap(13) Pietron, J. J.; Murray, R. W. J. Phys. Chem. B 1999, 103, 4440. (14) Wuelfing, W. P.; Zamborini, F. P.; Templeton, A. C.; Wen, X.; Yoon, H.; Murray, R. W. Chem. Mater. 2001, 13, 87. (15) Kim, Y.; Johnson, R. C.; Hupp, J. T. Nano Lett. 2001, 1, 165. (16) Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 2000, 122, 734. (17) Fullam, S.; Rao, S. N.; Fitzmaurice, D. J. Phys. Chem. B 2000, 104, 6164. (18) Liu, J.; Alvarez, J.; Ong, W.; Roma´n, E.; Kaifer, A. E. J. Am. Chem. Soc. 2001, 123, 11148. (19) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782. (b) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081. (c) Hostetler, M. J.; Murray, R. W. Colloid Interfacial Sci. 1997, 2, 42. (20) Ostuni, E.; Yan, L.; Whitesides, G. M. Colloids Surf., B: Biointerface 1999, 15, 3. (b) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. J. Phys. Chem. 1994, 98, 563. (c) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560. (d) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665. (21) Lestelius, M.; Liedberg, B.; Tengvall, P. Langmuir 1997, 13, 5900. (b) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. Adv. Mater. 1996, 8(9), 719. (22) Zhang, M.; Anderson, M. Langmuir 1994, 10, 2807. (b) Troughton, E. B.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 365.
10.1021/la035017k CCC: $25.00 © 2003 American Chemical Society Published on Web 08/20/2003
8556
Langmuir, Vol. 19, No. 20, 2003
proaches to prepare multicomponent SAMs include the use of unsymmetrical sulfides,22 disulfides,23 and chelating alkanedithiols.24 Recently, synthesis of MPCs from unsymmetrical dialkyl disulfides26 and sulfides27 has been reported. However, these synthetic methods yield only clusters with a 1:1 composition of mixed ligands. To our knowledge, the direct one-step synthesis of MMPCs from mixtures of alkanethiols has been used, but not systematically studied.28 This paper also seeks to explore the influence that altering synthesis conditions has on the core size of the MPCs. Although recent studies have shown the effects of synthesizing MPCs under various conditions,26-29 they do not show the effects of surfactant tail groups and solvent in controlling the core size of MPCs. We anticipate that this study will enable us to have a better understanding of the intrinsic reaction mechanism of MPC formation. Experimental Section Materials. The following materials were purchased from the indicated suppliers and used as received: hydrogen tetrachloroaurate (HAuCl4‚3H2O), sodium borohydride (NaBH4), 1-dodecanethiol, 1-hexanethiol, acetonitrile (Acros), 11-mercapto-1undecanol, 6-mercapto-1-hexanol, tetraoctylammonium bromide (Aldrich), ethyl alcohol, toluene, tetrahydrofuran (THF), iodine crystals, acetone (Fisher), and dichloromethane (EM Science). Water was purified by a Millipore Simplicity Nanopure Ultrapure water system. Synthesis of MPCs in THF/H2O. The procedure given is a standard one for the synthesis of MPCs from mixtures of alkanethiols. The following specific reaction conditions were systematically varied: (i) the particular organic moiety and (ii) the mole ratio of organic moiety. The total mole ratio of mixtures of alkanethiols to AuCl4- was a fixed 2-to-1 ratio for all experiments. A mixture of 0.12 g (0.6 mmol) of 1-dodecanethiol and 0.12 g (0.6 mmol) of 11-mercapto-1-undecanol, dissolved in 20 mL of THF, was added under vigorous stirring to a solution of 0.24 g (0.6 mmol) of HAuCl4‚3H2O in 50 mL of THF. The reaction mixture was stirred for ca. 10 min at room temperature, before 0.23 g (6.0 mmol) of NaBH4 in 10 mL of Nanopure water was added over a period of ca. 5 s. The color of the solution changed immediately from pale yellow to black upon the reductant addition. The reaction mixture was allowed to stir for 2 h. The solvent was removed using a rotary evaporator. The black precipitate was suspended in 100 mL of acetonitrile, briefly sonicated to promote dissolution of impurities, and placed on a glass filtration frit. The product was exhaustively washed with Nanopure water, acetonitrile, and acetone. Synthesis of MPCs in Toluene/H2O. The synthesis of gold MPCs in toluene/H2O from alkanethiol was analogous to the Schiffrin reaction.7a A 0.24 g (0.6 mmol) sample of HAuCl4‚3H2O in 20 mL of water was placed in the reaction flask. AuCl4- was phase-transferred into toluene (50 mL) using 1.25 g (2.3 mmol) (23) Chen, S.; Li, L.; Boozer, C. L.; Jiang, S. J. Phys. Chem. B 2001, 105, 2975. (b) Noh, J.; Hara, M. Langmuir 2000, 16, 2045. (c) Heister, K.; Allara, D. L.; Bahnck, K.; Frey, S.; Zharnikov, M.; Grunze, M. Langmuir 1999, 15, 5440. (24) Shon, Y.-S.; Lee, S.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 1278. (b) Shon, Y.-S.; Lee, S.; Colorado, R., Jr.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 7556. (25) Allara, D. L.; Dunbar, T. D.; Weiss, P. S.; Bumm, L. A.; Cygan, M. T.; Tour, J. M.; Reinerth, W. A.; Yao, Y.-T.; Kozaki, M.; Jones, L., II Ann. N. Y. Acad. Sci. 1998, 852, 349. (26) Shon, Y.-S.; Mazzitelli, C.; Murray, R. W. Langmuir 2001, 17, 7735. (27) Shelley, E. J.; Ryan, D.; Johnson, S. R.; Couillard, M.; Fitzmaurice, D.; Nellist, P. D.; Chen, Y.; Palmer, R. E.; Preece, J. A. Langmuir 2002, 18, 1791. (28) Examples of the synthesis of mixed MPCs from thiol mixtures: (a) Chen, S.; Murray, R. W. J. Phys. Chem. B 1999, 103, 9996. (b) Yang, W.; Chen, M.; Knoll, W.; Deng, H. Langmuir 2002, 18, 4124. (29) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17.
Choo et al. of tetraoctylammonium bromide, and the aqueous layer was discarded. A mixture of 0.12 g (0.6 mmol) of 1-dodecanethiol and 0.12 g (0.6 mmol) of 11-mercapto-1-undecanol in 20 mL of toluene was added to the reaction mixture. The reaction mixture was stirred for ca. 10 min at room temperature, before addition of 0.23 g (6.0 mmol) of NaBH4 in 10 mL of Nanopure water over a period of ca. 5 s. The solution quickly darkened during borohydride addition (however, more slowly than the above synthesis in THF/H2O). After being stirred for 2 h, the water phase was discarded and the toluene was removed under vacuum, leaving a black solid. The black precipitate was suspended in 50 mL of acetonitrile and placed on a glass filtration frit. The product was exhaustively washed with acetonitrile and acetone. Some MPCs generated from mixtures of alkanethiols were found to be insoluble in toluene as they precipitated out during the reaction process. The black precipitate was collected on a glass filtration frit and washed with Nanopure water, toluene, and acetonitrile. Measurements. Proton NMR spectra were recorded on a JEOL CPX FT-NMR spectrometer operating at 270 MHz in CDCl3 solutions and internally referenced to δ 7.26 ppm. Infrared spectra were obtained, using a Perkin-Elmer 1600 FT-IR spectrometer, of films of MPCs pressed into a KBr plate. The spectra were recorded from 4500 to 450 cm-1. UV-vis spectra of dichloromethane solutions in quartz cells were acquired on a Shimadzu UV-2101 PC spectrophotometer. Transmission electron microscopic (TEM) images of nanoparticles were obtained with a JEOL 120CX scanning/transmission electron microscope operating at 120 keV. Samples were prepared for TEM by casting a single drop of ∼1 mg/mL solution (either hexane or ethanol) onto standard carbon-coated (80-100 Å) Formvar film on copper grids (600 mesh) and drying in air for at least 30 min. Several regions were imaged at 100000×. Size distributions of the gold cores were obtained from digitized photographic enlargements with Scion Image Beta Release 2.
Results and Discussion In general, the synthesis of MPCs begins with AuCl4and Oct4N+ forming a complex in toluene. In this two-step Schiffrin reaction,2a a 3-fold excess of alkanethiols can completely reduce Au(III) to Au(I), which is indicated by the solution color change from dark purple (AuCl4-Oct4N+) to clear (a polymeric Au(I)-SR complex).26,30 The addition of BH4- to the reaction mixture initially causes darkening of solution, which indicates the reduction of Au(I) to Au(0) and cluster formation. This Schiffrin protocol was systematically altered to understand the coadsorption pattern of mixtures of alkanethiols relative to the solvents, tail groups, and alkyl chain length. In THF, AuCl4- can be dissolved without the assistance of phase transfer reagents (Oct4N+). The yellow AuCl4-/THF solution slightly fades after adding the 2 molar ratio of alkanethiols to AuCl4-, indicating the partial formation of Au(I)-thiolate polymers.30 The formation of MPC in THF seems faster than that in toluene, which is indicated by the faster darkening of solution. Variation in the Tail Group. To investigate the effect of the nature of the tail group on the synthesis of MMPCs, we studied a two-component system, composed of various mole fractions of HO(CH2)11SH and CH3(CH2)11SH. In both toluene and THF, the variation in the mole ratio of alkanethiols resulted the distinct solubility of MMPCs. MMPCs generated from the mixture of alkanethiols having 50% or higher concentration of HO(CH2)11SH in toluene were precipitated out during the reaction. The isolated MPCs were found to be only soluble in polar protic solvents such as ethanol and methanol, indicating an excessive (30) Bachman, R. E.; Bodolosky-Bettis, S. A.; Glennol, S. C.; Sirchio, S. A. J. Am. Chem. Soc. 2000, 122, 7146. (b) Al-Sa’ady, A. K. H.; Moss, K.; McAuliffe, C. A.; Parish, R. V. J. Chem. Soc., Dalton Trans. 1984, 1609.
Synthesis of MMPCs from Thiol Mixtures
Langmuir, Vol. 19, No. 20, 2003 8557
Figure 1. Mole fraction of OH-terminated alkanethiolate in MMPCs generated from mixtures of HO(CH2)11SH and CH3(CH2)11SH. Composition of monolayers was determined from the integration of 1H NMR spectra of I2-decomposed MMPCs.
presence of hydrophilic terminal groups in the monolayer surrounding a cluster. This was indicative of the preferential adsorption of HO(CH2)11SH over CH3(CH2)11SH in toluene. MPCs synthesized from the mixtures of HO(CH2)11SH and CH3(CH2)11SH with less than 50% molar concentration of HO(CH2)11SH remained soluble in toluene, suggesting the presence of more hydrophobic tail groups in the monolayer. MMPCs synthesized from mixtures of HO(CH2)11SH and CH3(CH2)11SH in THF do not precipitate out during the reaction. These results suggest that the solubility of MMPCs roughly reflects their ligand compositions on the surface of clusters. The composition of monolayers was determined from the integration of 1H NMR spectra, which provided the stoichiometry of monolayers on clusters.26,31 The MMPCs were I2-decomposed prior to taking an NMR spectrum. This provides better integration results as the MPCs quantitatively liberate the thiolate (RS-) ligands from the clusters as disulfides. A typical analytical procedure could be found in the literature.26,31 The NMR results for the MMPCs generated from mixtures of HO(CH2)11SH and CH3(CH2)11SH are shown in Figures 1 and 2. Note the presence of CH3 resonance at 0.9 ppm and CH2OH resonance at 3.6 ppm in Figure 2. We obtained Figure 1 from the integration of these two resonances. These results showed that MMPCs prepared from mixtures of HO(CH2)11SH and CH3(CH2)11SH having 50% or higher molar concentration of HO(CH2)11SH in toluene contain mostly HO(CH2)11S- rather than CH3(CH2)11S-. When the concentration of CH3(CH2)11SH (75% and 90% molar concentration) was increased in toluene, an increased adsorption of CH3(CH2)11S- (∼46% and ∼82%, respectively) ligands on the surface of clusters could be observed. This result clearly supports a preferential adsorption of HO(CH2)11SH over CH3(CH2)11SH in toluene. The NMR results for MMPCs synthesized from mixtures of alkanethiols in THF indicate only a slight preferential adsorption of CH3(CH2)11S- on the surface of clusters throughout the different molar concentration of alkanethiols. The concentration of HO(CH2)11S- on the surface of gold clusters increases almost linearly as the concentration of HO(CH2)11SH in solution (THF) increases. Infrared spectroscopy provides structural and conformational information regarding monolayers on metal clusters.9,32 Figure 3 shows IR spectra of MPCs generated from mixtures of HO(CH2)11SH and CH3(CH2)11SH.32 The (31) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845.
Figure 2. 1H NMR spectra (CDCl3) of I2-decomposed MMPCs generated from mixtures of HO(CH2)11SH and CH3(CH2)11SH in THF. The sharp resonance at 1.54 ppm is due to H2O impurity in the CDCl3 solution.
Figure 3. IR spectra of MMPCs generated from mixtures of HO(CH2)11SH and CH3(CH2)11SH in THF: (a) χ(OH)THF ) 0.75 and (b) χ(OH)THF ) 0.25.
presence of O-H stretching bands can be observed at 3300 cm-1 and weak CH3 antisymmetric stretching bands at 2955 cm-1 for the MMPCs containing higher concentration of HO(CH2)11S- on the surface of clusters. The antisymmetric C-H stretching region (νa(CH2) ) ∼2920 cm-1) is indicative of methylene chain ordering.14,32 The results suggest that MMPCs have monolayers that exhibit a degree of packing and chain crystallinity similar to that of single-component MPCs. The relationship between the composition of alkanethiols in solution and the composition of MMPC monolayers can be addressed by predominantly thermodynamic control. The assumption of thermodynamic equilibrium requires reversible ligand displacement during and after (32) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212.
8558
Langmuir, Vol. 19, No. 20, 2003
Choo et al.
Figure 4. Mole fraction of OH-terminated alkanethiolate in MMPCs generated from thiol mixtures with different alkyl chain lengths. Composition of monolayers was determined from the integration of 1H NMR spectra of I2-decomposed MMPCs.
the formation of MMPCs. This leads to the incorporation of thermodynamically more stable components on the surface of clusters. However, the rate of ligand displacement in a preformed monolayer on a cluster by thiols or disulfides in solution at room temperature is too slow to account for the results considering the rapid initial adsorption of monolayer precursors.20e Therefore, an equilibrium cannot be established by desorption and readsorption of the monolayer components in MPCs. The monolayer composition is more likely controlled by a fast initial equilibration, which is influenced by the relative solvation and diffusion rate of precursor ligands. The reactivity of the thiol headgroup should be the same for all the adsorbates. The component with lower solvation will have a higher reactivity in solution and hence will be preferentially adsorbed onto the surface of clusters. The nature of the solvent influences both the relative solvation and reactivity of the thiols. Therefore, the preferential adsorption should vary with the choice of solvent. Our results showed that the adsorption of HO(CH2)11SH in toluene was preferred, reflecting better solvation of CH3(CH2)11SH and higher reactivity (diffusion) of HO(CH2)11SH. This preference was not observed in THF. When MMPCs contain mostly a single component in the monolayers, they show no sign of flocculation or aggregation over time. However, MMPCs with ∼50% loading of OH ligands tend to slowly aggregate over time and become insoluble in several organic solvents. As described in an earlier report,26 the aggregation of MMPCs containing large amounts of both CH3- and OH- functional groups in several solvents is probably due to the presence of two or more different types of inter-MPC noncovalent bonding including van der Waals and hydrogen bonding interactions. These aggregated MMPCs can be reversibly dissolved in more hydrophobic alcohols such as 1-pentanol. Variation in Chain Length. MMPCs were also synthesized from the mixed system HO(CH2)nSH/ CH3(CH2)mSH (n * m) to observe the effects of ligand chain length. MMPCs generated from 1:1 mixture of both HO(CH2)11SH/CH3(CH2)5SH and HO(CH2)6SH/ CH3(CH2)11SH in toluene exhibited high solubility only in polar-protic solvents. They were insoluble in other solvents such as toluene, hexane, and dichloromethane. MPCs generated from 1:1 mixture of HO(CH2)11SH/ CH3(CH2)5SH in THF exhibited similar solubility. However, MPCs generated from 1:1 mixture of HO(CH2)6SH/ CH3(CH2)11SH in THF were only soluble in nonpolar or polar-aprotic solvents, indicating the presence of hydrophobic functional groups in the monolayer. 1H NMR results shown in Figure 4 confirm that MMPCs generated from thiol mixtures in toluene contain an excess of HO(CH2)11S-
Figure 5. Transmission electron micrograph and core size histogram of MPCs generated from mixtures of HO(CH2)11SH and CH3(CH2)11SH (χ(OH)solution ) 0.25) (a) in toluene and (b) in THF.
or HO(CH2)6S- in the monolayer due to solvation-driven thermodynamic preference. In contrast, MPCs generated from thiol mixtures in THF show a preferential adsorption of long-chain alkanethiolate species (HO(CH2)11S- or CH3(CH2)11S-) on the surface of the clusters. Since alkanethiols with a short alkyl chain move faster, the kinetic controls favor adsorption of the shorter chain thiols. Therefore, this preferential adsorption of long chains supports thermodynamic control over the adsorption process. This is because intermolecular interactions between hydrocarbon chains favor adsorption of the longer chains. In toluene, solvation of CH3(CH2)11SH and a faster diffusion of HO(CH2)6SH overcome these intermolecular interactions. Cluster Size. It has been found that the cluster formation behaves as a nucleation-growth-passivation process.14,33,34 Therefore, the average core size of MPCs gets smaller when a larger thiol:gold mole ratio is used,14 the reductant is added faster (
