Novel Crown Ether-Capped Cationic Gold Nanoclusters in an

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Novel Crown Ether-Capped Cationic Gold Nanoclusters in an Aqueous Medium and Their Single-Electron Charging Features P. Biji and Archita Patnaik* Department of Chemistry, Indian Institute of Technology Madras, Chennai-600036, India ReceiVed June 4, 2007. In Final Form: July 16, 2007 On the basis of the formation of a noncovalent inclusion complex through crown ether-ammonium ion interaction, a new ligand, 18-crown-6-aminoethanethiol (18C6-AET), was predesigned and synthesized with a formation constant of 4.26 × 102 M-1s-1 for the 1:1 noncovalent complex. Consequently, stable crown ether-protected gold nanoclusters were synthesized with 18C6-AET in an aqueous medium with a rate of formation of 3.688 × 10-4 min-1. The newly formed 18C6-AET-capped gold nanoclusters showed quantized double-layer charging, revealing their concentric capacitor structure, and effective cluster capacitance, CCLU was calculated to be 0.93 aF from the single-electron events. A larger CCLU in comparison to that of monolayer-protected gold clusters with longer chain lengths was attributed to a smaller ligand shell thickness as well as the cationic nature of the nanoclusters formed in an aqueous electrolyte medium.

Introduction The molecular capacitor character of monolayer-protected nanoclusters (MPCs) demonstrates the unique electron-transfer chemistry of these nanosized particles, which makes them potential building blocks for nanoelectronic circuit components such as field-effect transistors, molecular switches, and resonant tunneling diodes.1-3 Water-dispersible metal nanoclusters have promising applications as sensors and molecular markers and, in particular, in biological applications such as biolabeling and drug delivery.4-6 The controlled synthesis of metal nanoclusters by steric stabilization is a widely discussed method in the literature, though it is less well understood as compared with electrostatic stabilization of colloidal particles.7-10 However, the combined effect of both steric and electrostatic stabilization of metal nanoclusters is a less explored area in current research. Besides providing supramolecular multivalency, crown ethers are particularly useful in modeling biologically relevant iontransport processes and antibody-antigen association.11-12 Johnson et al. have synthesized new linked and threaded ruthenium cluster compounds through crown ether-ammonium ion interactions.13 It is reported that the crown ether-protected gold nanoclusters tethered to glass substrate modified by 3-mercapto* Corresponding author. E-mail: [email protected]. Phone: 009144-2257-4217. Fax: 0091-44-2257-4202. (1) Carrara, M.; Kakkassery, J. J.; Abid, J. P.; Fermin, D. J. Chem. Phys. Chem. 2004, 5, 571-575. (2) Suganuma, Y.; Dhirani, A. J. Phys. Chem. B 2005, 109, 15391-15396. (3) Ouyang, J.; Chu, C. W.; Szmanda, C. R.; Ma, L.; Yang, Y. Nat. Mater. 2004, 3, 918-922. (4) Niidome, T.; Nakashima, K.; Takahashi, H.; Niidome, Y. Chem. Commun. 2004, 1978-1979. (5) Bauer, L. A.; Birenbaum, N. S.; Mayer, G. J. J. Mater. Chem. 2004, 14, 517-526. (6) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607-609. (7) Brust, M.; Walker, M.; Bethell, D.; Shriffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (8) Paulini, R.; Frankamp, B. L.; Rotello, V. M. Langmuir 2002, 18, 23682373. (9) Bellino, M. J.; Calvo, E. J.; Gordillo, G. Phys. Chem. Chem. Phys. 2004, 6, 424-428. (10) Turkevich, J.; Stevenson, P. C.; Hiller, J. Discuss. Faraday Soc. 1951, 11, 55. (11) Mulder, A.; Huskens, J.; Reinhoudt, D. N. Org. Biomol. Chem. 2004, 2, 3409-3424. (12) Badjic, J. D.; Nelson, A.; Cantrill, S. J.; Turnbull, W. B.; Stoddart, J. F. Acc. Chem. Res. 2005, 38, 723-732.

propyltrimethylsiloxane formed into multilayer films through crown ether-metal ion-crown ether interactions bridging the adjacent MPCs.14 The stepwise functionalization of 2D selfassembled monolayers of macrocycles and cyclodextrins through host-guest interactions on gold/silver thin films has been reported.15-17 Rotello and co-workers have demonstrated the creation system where surface functionality can be varied through electrochemically controlled noncovalent host-guest complexation at the colloid-solution interface.18 The self-assembly of a monolayer of a fullerene crown ether derivative on a 2D gold surface by interfacial hydrogen bonding has been studied by Arias et al.19 Reports on crown ether-stabilized gold nanoclusters required the derivatization of the crown moiety, requiring complex synthetic procedures. Furthermore, such syntheses have mainly been performed in organic media.20-21 In this context, the introduction of noncovalent interactions is appropriate for the synthesis of crown ether-protected noble metal nanoclusters in aqueous medium. Murray and co-workers have proven that MPCs exhibit subattofarad double-layer capacitance (CCLU) even at room temperature, and the resolution of single-electron charging events becomes possible.22-24 Conventionally, single-electron event features of MPCs at low temperatures, commonly known as the coulomb staircase or coulomb blockade behavior, could be studied (13) Johnson, B. F. G.; Judkins, C. M. G.; Matters, J. M.; Shephard, D. S.; Parsons, S. Chem. Commun. 2000, 16, 1549-1550. (14) Pompano, R. R.; Wortley, P. G.; Moatz, L. M.; Tognarelli, D. J.; Kittredge, K. W.; Leopold, M. C. Thin Solid Films 2006, 510, 311-319. (15) Nelles, G.; Weissier, M.; Back, R.; Wohlfart, P.; Wenz, G.; Mittler Neher, S. J. Am. Chem. Soc. 1996, 118, 5039. (16) Flink, S.; Boukamp, B. A.; Van den Berg, A.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 1998, 120, 4652-4657. (17) Kitano, H.; Taira, Y.; Yamamoto, H. Anal. Chem. 2000, 72, 2976-2980. (18) Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 1999, 121, 4914-4915. (19) Arias, F.; Godinez, L. A.; Wilson, S. R.; Kaifer, A. E.; Echegoyen, L. J. Am. Chem. Soc. 1996, 118, 6086-6087. (20) Chen, C. Y.; Cheng, C. T.; Lai, C. W.; Wu, P. W.; Wu, K. C.; Chou, P. T.; Chou, Y. H.; Chiu, H. T. Chem. Commun. 2006, 263-265. (21) Lin, S. Y.; Chen, C.; Lin, M. C.; Hsu, H. F. Anal. Chem. 2005, 77, 4821-4828. (22) Lin, S. Y.; Liu, S. W.; Lin, C. M.; Chen, C. Anal. Chem. 2002, 74, 330-335. (23) Ingram, R. S.; Hostetler, M. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Whetten, R. L.; Bigioni, T. P.; Guthrie, D. K.; First, P. N. J. Am. Chem. Soc. 1997, 119, 9279-9280. (24) Song, Y.; Jimenez, V.; McKinney, C.; Donkers, R.; Murray, R. W. Anal. Chem. 2003, 75, 5088-5096.

10.1021/la701636h CCC: $37.00 © 2007 American Chemical Society Published on Web 10/06/2007

Crown Ether-Capped Gold Nanoclusters

by scanning tunneling microscopy/spectroscopy (STM/STS).25-26 Recently, a significant literature base has emerged in the area of multivalent redox properties of MPCs substantiating singleelectron charging events.27-29 Voltammetric measurements of smaller MPCs in solution or self-assembled on an electrode surface are known to show single electron transfer behavior at room temperature because of their “quantized double-layer charging” (QDL).30 Most of the electrochemical studies of MPCs have mainly been confined to organic media,31-33 with very few experiments conducted in aqueous environments.34-35 Herein, we report the synthesis of crown ether-ammonium ion complexcapped gold nanoclusters from 18-crown-6 and 2-aminoethanethiol. These crown ether-capped gold nanoclusters exhibited single-electron charging features in an aqueous medium. Experimental Section Materials and Methods. Reagents used for the synthesis of the crown ether complex ligand were 18-crown-6 (99.0%, Merck), 2-aminoethanethiol hydrochloride, [AET] (98.0%, Merck), and methanol (Extrapure AR, SRL). Sodium borohydride (98.0%, Merck) was used for the reduction of HAuCl4·3H2O (98.0%, Aldrich) in ultrapure water, (Millipore-Academic) of resistivity 18.2 MΩ cm. All chemicals were used as received without purification unless otherwise stated. All glassware was scrupulously cleaned with chromic acid solution and rinsed with Millipore water. A 1:1 complex of 18-crown-6/2-aminoethanethiol was synthesized in methanol from 1 mmol of each reactant upon vigorous stirring for 30 min at room temperature. Needle-shaped, colorless crystals of the new 18C6-AET complex resulted upon slow evaporation of the solvent at room temperature. Synthesis of Crown Ether-Capped Gold Nanoclusters. The synthesis of crown ether-functionalized, monolayer-protected gold clusters was accomplished in a single step. Twenty milliliters of a 1 mM HAuCl4 aqueous solution in ultrapure water was stirred at room temperature with 5 mL of a 1 mM ligand solution, followed by the reduction of Au3+ to (Au0)n by the dropwise addition of an ice-cold solution of 1 mL of freshly prepared 0.1 M NaBH4 in water. The solution was vigorously stirred for 3 h. The resulting stable gold hydrosol was transparent brown, indicating the small particle size of the gold clusters as further confirmed by TEM analysis. Characterization Methods. The needle-shaped, colorless single crystals grown from methanol were used for single-crystal X-ray diffraction analysis. The X-ray data were collected on a Nonius Kappa CAD-4 diffractometer with graphite-monochromated Cu KR radiation (λ ) 1.54180 Å) operating in ω/2θ scanning mode at 293(2) K. Unit cell parameters were determined by full matrix leastsquares refinement. The ranges of h, k, and l used were 0 e h e 26, 0 e k e 28, and 0 e l e 10. The absorption spectra of the aqueous solution of crown ether-capped gold nanoparticles were recorded with a UV-vis spectrophotometer (Shimadzu UV-2100). Conductometry experiments were done with a digital conductivity meter (Digisun Electronics, DI-909) with a cell constant of 1.01. The size and morphology of the gold nanoparticles were determined (25) Lee, D.; Donkers, R. L.; Wang, G.; Harper, A. S.; Murray, R. W. J. Am. Chem. Soc. 2004, 126, 6193-6199. (26) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098-2101. (27) Quinn, B. M.; Liljeroth, P.; Ruiz, V.; Laaksonen, T.; Kontturi, K. J. Am. Chem. Soc. 2003, 125, 6644-6645. (28) Dorogi, M.; Gomez, J.; Osifchin, R.; Andres, R. P.; Reifenberger, R. Phys. ReV. B 1995, 52, 9071-9077. (29) Su, B.; Girault, H. H. J. Phys. Chem. B 2005, 109, 11427-11431. (30) Wuelfing, W. P.; Green, S. J.; Pietron, J. J.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 11465-11472. (31) Chen, S.; Murray, R. W. J. Phys. Chem. B 1998, 102, 9898-9907. (32) Su, B.; Zhang, M.; Shao, Y.; Girault, H. H. J. Phys. Chem. B 2006, 110, 21460-21466. (33) Wolfe, R. L.; Murray, R. W. Anal. Chem. 2006, 78, 1167-1173. (34) Chen, S. J. Am. Chem. Soc. 2000, 122, 7420-7421. (35) Deng, F.; Chen, S. Phys. Chem. Chem. Phys. 2005, 7, 3375-3381.

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Figure 1. (a) Electron density cross section of the 18C6-AET ligand along the macrocycle plane, as estimated from DFT calculations. (b) Graphical illustration of the ligand-stabilized gold nanocluster for the estimation of the number of ligands per cluster with R ) 1 nm and L ) 4.59 Å. by transmission electron microscopy using a JEOL 3010 instrument. Electrochemical measurements were carried out using a CHI-660B electrochemical workstation (CH Instruments). Measurements were performed in a three-electrode cell with a Pt microelectrode of 25 µm diameter as the working electrode and a Pt wire counter electrode. All potentials were recorded with respect to a Ag/AgCl (1 M KCl) reference electrode and with 0.1 M KCl as the supporting electrolyte in which 1 mM 18C6-AET-capped gold nanoclusters acted as the multivalent redox species. The solution was purged with N2 before each experiment.

Results and Discussion Design of the 18C6-AET Ligand. In the present investigation, an inclusion complex of crown ether 18-crown-6 with protonated 2-aminoethanethiol (AET) was used as a stabilizing agent for gold nanoclusters. The 18-crown-6/AET complex was predesigned and geometry optimized using DFT calculations at the B3LYP/3-21g level. The gold nanoclusters were stabilized by the crown ether moiety of the ligand through steric interactions along with electrostatic stabilization from the ammonium ion part of the AET. The electron density contour cross section along the crown ether plane of the 18C6-AET complex is shown in Figure 1a, which depicts the electrostatic interaction between the maximum density region around the oxygens of the ethylene oxide linkages of 18C6 and the ammonium part. The free thiol group of the ligand forms a Au-S bond with the nanogold clusters. Heath et al. have demonstrated a model to estimate the magnitude of excess volume, which gives an indication of the amount that a ligand shell associated with one particle may interpenetrate into the ligand shell of a second particle.36 However, such ligand shell interpenetrations may lead to particle-particle aggregation, especially when ω-functionalized ligands are used to cap the nanoparticles. Such aggregation could be avoided for ligands functionalized with sterically bulky end functionalities such as crown ethers. In such cases, the approximate number of ligands associated with a nanocluster can be estimated by comparing the footprint area occupied by a single ligand to the total surface area of the cluster. This is illustrated graphically in Figure 1b. Because the area of the crown ether present on the ligand is very large compared to the alkyl chain associated with it, the total area occupied by a single ligand will be equivalent to that of the crown moiety. Also, the possibility of interpenetration of the neighboring ligand shell is much less for such a nanocluster because of the steric hindrance of the crown ether. Such a steric restriction prevents a second ligand from being attached on the (36) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189-197.

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Table 1. Hydrogen Bonding and Geometry (Å) from the Crystal Structure of the Complex D-H‚‚‚A

d(D-H)

d(H‚‚‚A)

〈DHA〉

N(1)-H(1D)‚‚‚O(2) N(1)-H(1C)‚‚‚O(4) N(1)-H(1E)‚‚‚O(6)

0.89 0.89 0.89

2.06 2.11 2.12

176.0 160.7 153.3

surface of the cluster, within the occupied area of one ligand. Because the diameter of 18C6 is 9.32 Å from the optimized structure, the occupied footprint area of each ligand on the surface of a gold nanocluster can be considered to be equivalent to the area of 18C6. Accordingly, ligand area ALIG was calculated to be ALIG ) πr2 ) 68.25 Å2, with r being the radius of the 18C6 ring. Transmission electron microscopy (TEM) investigations revealed the gold nanoclusters to be spherical with an average diameter of 2 nm. The total surface area of the cluster ACLU ) 4πR2, with R as the radius of the gold nanocluster, was calculated to be 12.56 nm2, which, when compared with the occupied area of the ligand, yielded ∼18 ligands (ACLU/ALIG) present on the surface of the particle. The monolayer-protected cluster is a concentric capacitor in an electrolyte medium with the metal cluster surrounded by an insulating dielectric shell of ligands. In view of a concentric capacitor model for the 18C6-AETcapped Au nanoparticle, the thickness of the dielectric ligand shell (L) was estimated to be 4.59 Å from the optimized structure. Single-Crystal X-ray Diffraction Structure Analysis. The molecular structure and solid-state conformation of the noncovalent inclusion complex were investigated by single-crystal X-ray diffraction analysis, which showed two-center, three-point hydrogen bonding between protonated AET and a set of three planar oxygens of 18C6. Needle-shaped single crystals of the complex crystallized in the Pbcn orthorhombic space group. The hydrogen bonds formed between oxygen atoms O(2), O(4), and O(6) with the three hydrogen atoms on N(1) are responsible for the formation of the complex. The corresponding hydrogen bond distances for the same are shown in Table 1. The distance between nitrogen atom N(1) of the AET and the mean plane of 18C6 was found to be 0.729 Å. The molecular structure derived from the crystallographic data in Figure 4 shows the protonated amine group of the AET as being slightly tilted away from the mean plane of the crown ether moiety, as evident from the hydrogen bond distances as well as the dihedral angles listed in Table 1. This conformation leads to O‚‚‚O separations of 4.953 Å between O(1) and O(3) and 4.874 Å between O(2) and O(4). The average O‚‚‚O distance between adjacent oxygen atoms is found to be 2.879 Å. X-ray crystallographic analysis of single crystals showed the formation of a disulfide linkage in the ligand, as evident from the ORTEP diagram in Figure 2. The two 18C6-AET moieties exist trans to each other with respect to the S-S linkage as a result of the oxidation of the thiol group to disulfide during the slow evaporation of the solvent for crystallization.37-38 The Au3+ ions are reduced by the addition of reducing agent NaBH4 in the presence of the 18C6-AET complex to small gold nanoclusters. The disulfide linkage present in the ligand in the process was reduced to a free thiol group in the presence of the reducing agent and was then bound to the gold nanocluster surface, forming Au-S bonds. Determination of the Formation Constant of the Ligand. As reported previously, the tetrahedral ammonium ion can bind (37) Brodbelt, J. S.; Lion, C.-C. Pure. Appl. Chem. 1993, 65, 409-414. (38) Biaglow, J. E.; Issels, R. W.; Gerweck, L. E.; Varnes, M. E.; Jacobson, B.; Mitchell, J. B.; Russo, A. Radiat. Res. 1984, 100, 298-312.

Figure 2. ORTEP drawing of the complex normal to the mean plane. The displacement ellipsoids are drawn at the 50% probability level. Dashed lines indicate H bonding.

to three of the six available oxygen atoms in the 18C6 ring to form a stable inclusion complex.39 In this case, alkyl chain R of the protonated AET, R-NH3+, presumably protrudes upward from the center of and perpendicular to the plane of the oxygens. The formation of the inclusion complex of 18C6 with AET in methanol was investigated by conductometry. Titration was conducted in a jacketed cell, and the temperature was strictly maintained at 25 °C using a thermostat. A known amount of AET was prepared in methanol, to which was then added 18C6 of known concentration in 0.5 mL aliquots, and the conductance of the solution was measured after each addition. The addition of 18C6 was continued until the molar conductance became nearly constant. Upon addition of 18C6, the molar conductance of the solution decreased, and the results are shown in Table 2. Binding of the protonated AET (R-NH3+) ion with the 18C6 crown ether ligand (L) can be expressed by the following equilibrium: Kf

+ R-NH+ 3 + L {\} R-NH3L

(1)

Accordingly, the complex formation constant in terms of the molar conductances can be expressed as40

Kf )

(Λm - Λobs) (Λobs - Λc)[L]

(2)

where

[L] ) CL -

CAET(Λm - Λobs) (Λm - Λc)

(3)

Here, Λm is the molar conductance of protonated AET before the addition of the 18C6 ligand (L); Λc, the molar conductance of the complexed AET; Λobs, the molar conductance of the solution during titration; CL, the concentration of crown ligand added; and CAET, the concentration of protonated AET. The formation constant, Kf, and the molar conductance of the complex, Λc, were obtained by fitting eqs 2 and 3 to the molar conductancemole ratio data using a nonlinear least-squares curve-fitting program in the Origin 7.0 software. (39) Gunduz, C.; Salan, U.; Ozkul, N.; Basaran, I.; Cakir, U.; Bulut, M. Dyes Pigm. 2006, 71, 161-167. (40) Hasani, M.; Shamsipur, M. J. Inclusion Phenom. Mol. Recognit. Chem. 1993, 16, 123-137.

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Scheme 1. Illustration of the Formation of the 18C6-AET Complex in Methanol and the Disulfide Linkage Formed, Reducing to -SH in the Presence of the NaBH4 Reducing Agent

Table 2. Variation of Λobs upon the Addition of 18C6 to AET volume of 18C6 Λobs volume of 18C6 Λobs added (mL) added (mL) (S cm-2 mol-1) (S cm-2 mol-1) 0 0.5 1 1.5 2 2.5 3 4.0 5 6 7 8

39.3 38.6 37.9 37.2 36.5 35.7 35.0 33.6 32.4 31.3 30.3 29.4

9 10 11 12 13 14 15 16 17 18 19

28.5 27.7 27.4 27.0 26.7 26.4 26.3 25.9 25.8 25.7 25.6

It can be inferred from Figure 3 that the addition of crown ether to AET causes a continuous decrease in the molar conductance of the solution, indicating the lower mobility of the complexed R-NH3+ ion compared to that of the solvated cation. The slope of the conductance-mole ratio plot changes sharply at the point where the ligand-to-cation mole ratio equals 1, indicating the formation of a fairly stable 1:1 complex. The formation constant, Kf, of the resulting 1:1 inclusion complex was found to be 4.26 × 102 M-1 s-1. The standard free energy of complex formation, ∆G0 ) -RT ln K, was estimated to be -14.99 kJ mol-1, indicating the formation of a stable 18C6AET complex.

Electron Microscopy Analysis of Gold Nanoclusters. The core size and size distribution of the crown ether-capped Au nanoclusters were examined by transmission electron microscopy. Figure 4a shows a representative transmission electron microscopy image of the aqueous Au colloidal suspension on a carboncoated copper grid. The histogram of the particle diameter plotted in Figure 4b indicates a nearly monodisperse size with an average core diameter of 2 ( 0.2 nm. The smaller core size of the gold clusters indicates the effective capping of the cluster with the crown ether complex ligand. Kinetics of 18C6-AET-Capped Gold Nanocluster Formation. The stable hydrosol of 18C6-AET-capped gold nanoclusters were transparent brown, indicating the small particle size of the gold cluster as further confirmed by TEM analysis. Noble metal nanoparticles exhibit a strong UV-visible surface plasmon absorption band when the incident photon frequency is resonant with the collective excitation of the conduction band electrons.41 The formation of Au nanoclusters capped by the 18C6-AET complex was verified by UV-vis absorption spectroscopy, wherein the characteristic plasmon bands were observed at around 526 nm, as depicted in Figure 5A. The stability of the nanoclusters was checked by monitoring the red shift of the surface plasmon band. The minor red shift of the plasmon band as well as the color variation with respect to time indicated the slow growth of particle size.42 Figure 5B represents the kinetics of particle growth with respect to time. The plot of Amax at 526 nm with time increased exponentially and reached a maximum absorbance. The rate constant, k, was determined by fitting the experimental data to the first-order rate equation43

ln(A∞ - At) ) ln(A∞ - A0) - kt

(4)

where t is the time and At, A0, and A∞ are the absorbances at time t, zero time, and infinite time, respectively. The rate constant for the growth of Au nanoclusters was calculated from the slope of the ln[A∞ - At] versus time plot as 3.688 × 10-4 min-1 and was found to be first order. The stability of the 18C6-AET-capped nanoclusters was monitored spectrophotometrically by varying the [Au3+]/[ligand] ratio (Supporting Information), and 1:3 [Au3+]/[ligand] formed the most stable gold nanoclusters. Quantized Double-Layer Charging of the Nanoclusters. Monolayer-protected clusters (MPCs) behave as multivalent redox

Figure 3. Variation of molar conductance as a function of the molar ratio of 18C6/AET in methanol.

(41) Link, S.; EI-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212-4217. (42) Eustis, S.; EI. Sayed, M. A. Chem. Soc. ReV. 2006, 35, 209-217. (43) Mokhtar, I.; Takakazu, Y.; Patigul, I. J. Zhejiang UniV., Sci. 2005, 6, 722-724.

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Figure 5. (A) UV-vis absorption spectra of the ligand-capped aqueous gold nanoclusters with respect to time. The inset corresponds to photographs of the gold hydrosol after the reduction of 10-3 M HAuCl4 with a 1:3 [Au3+]/[ligand] molar ratio for (a) 5 min, (b) 5 days, (c) 8 days, and (d) 2 months. (B) Evolution of surface plasmon absorbance with time. Inset shows the time dependent changes of the absorbance at 526 nm.

Figure 4. (a) Representative transmission electron micrograph, (b) histogram for particle size distribution, and (c) high-resolution image of the aqueous crown ether-capped colloidal gold suspensions.

species as charge injection into the core is quantized. The resulting consecutive single-electron double-layer charging events of MPCs are analogous to the current peaks seen in traditional redox reactions44 (i.e., the quantized double-layer charging currents are diffusion-controlled). The mass transport of MPCs with adjacent states of core charge (Z) is a mixed-valence solution, which follow the Nernst equation with regard to the average core potential. The very small (sub-attofarad) double-layer capacitance (CMPC) associated with MPCs in electrolyte solutions is an important quantized, size-dependent property and is an electrochemical analogue to coulomb staircase charging known as quantized double-layer charging, QDL. The consequence of the (44) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153-1175.

small capacitance is that voltammetric current peaks for serial single-electron transfer (SET) charging of the MPC double layers are separated by readily measurable electrode potential intervals (∆V ) e/CCLU). The small MPC capacitance originates from the combination of a tiny core radius and the small dielectric of the surrounding insulating organic monolayer. The length of the ligand monolayer of a certain thickness (d) is related to the MPC double-layer capacitance (CCLU) according to the concentric spherical capacitor equation

CCLU ) 4π0

(dr)(r + d)

(5)

where  is the static dielectric constant of the monolayer, 0 is the permittivity of free space (8.854 × 10-12 F m-1), and r is the core radius.45 From this equation, one would expect MPCs with a longer chain length to have a smaller double-layer capacitance. More specifically, CCLU is associated with the ionic space charge formed around an MPC dissolved in the electrolyte (45) Chaki, N. K.; Singh, P.; Dharmadhikari, C. V.; Vijayamohanan, K. P. Langmuir 2004, 20, 10208-10217.

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Figure 6. (A) Cyclic voltammogram and (B) differential pulse voltammogram of the 18C6-AET-capped gold nanoclusters in an aqueous solution with a scan rate of 100 mV/s. Each star represents a single-electron event in the DPV profile. (C) Z plot of the MPCs and CCLU as determined from the slope of the straight line.

solution upon electronic charging of the core, and the charging is controlled by electrostatic (double-layer) principles. It has been shown that, assuming fast electron transfer, the SET charging reactions of MPCs follow the Nernst relation and each SET can be assigned a “formal potential”. The formal potentials of these charging events are described by 0 EZ,Z-1 ) EPZC +

(Z - (1/2))e CCLU

(6)

0 where EZ,Z-1 is the formal potential of the Z/(Z - 1) charge state couple and corresponds experimentally to a DPV peak potential. Values for Z > 0 and Z < 0 correspond to core “oxidation” and “reduction”, respectively. The formal potentials of the charging peaks are referenced to the potential of zero charge (EPZC) of the MPC that lies midway between the formal potentials of the MPC+1/0 and MPC0/-1 charge state couples. A Z plot of E0z versus the charge state is linear provided CCLU is constant and gives an average value of CCLU from the slope CCLU ) e/slope, where e is the charge of an electron.46 The cyclic voltammogram (CV) and the DPV profile of the 18C6-AET-capped gold nanoclusters are shown in Figure 6. The CV of the nanoclusters in an aqueous medium showed weak charging peaks as indicated by arrows in Figure 6A. Small QDL peaks are more evident in the DPV (Figure 6B) of the gold nanoclusters. The single-electron charging events are indicated

(46) Chen, S.; Sommers, J. M. J. Phys. Chem. B 2001, 105, 8816-8820.

by a star. Because the gold nanostructures formed are very small, ∼2 nm, the point of zero charge (PZC) lies in the middle of +1/0 and 0/-1 peaks at 0.21 V, and is the HOMO-LUMO gap because the electronic structure of small gold nanoclusters can resemble moleculelike behavior, with a peak separation between adjacent single-electron charging events of ∆V ) 0.22V. Figure 6C depicts the Z plot for 18C6-AET-capped gold nanoclusters, where linear behavior in the potential range of -0.1 to +0.8 V can be seen, as expected for QDL behavior. From the linear regression, the effective MPC capacitance, CCLU, was evaluated from the slope of the Z plot as 0.93 aF. The weak QDL peaks represent the monodisperse proportion of the MPC sample, whereas the underlying continuum arises from the polydispersity of the nanoparticles. The phenomenon indicates the single-electron charging behavior of the 18C6-AET-capped gold nanoclusters and hence points toward its capacitance properties in an aqueous medium. The relatively large value of the MPC capacitance is attributed to the presence of a highly effective dielectric environment created by the aqueous electrolyte medium and the smaller ligand-shell thickness, as per eq 5. In related reports,47-48 the single-electron charging events of alkanethiolate-protected gold nanoparticles have exhibited cluster capacitances of ∼0.5-0.6 aF in an organic medium. Because the 18C6-AET-capped gold nanoclusters are cationic in nature as a result of the presence of positive charge present on the nitrogen (47) Hicks, J. F.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 13322-13328. (48) Chen, S. J. Electroanal. Chem. 2004, 574, 153-165.

12054 Langmuir, Vol. 23, No. 24, 2007

Biji and Patnaik

atom of the protonated amine part of AET, the surface concentration of anions present in the electrolyte medium will be comparatively large in the vicinity of the gold nanocluster surface, which may further enhance the capacitance value.

toward their capacitance behavior in the aqueous medium, as evidenced by the electrochemical studies. The cationic nature and the reduced ligand shell thickness contributed to the increased capacitance of these tiny nanocapacitors in the aqueous medium.

Conclusions

Acknowledgment. We gratefully acknowledge Dr. Babu Varghese, SAIF, IIT Madras, India, for single-crystal XRD measurements and useful discussions. We also acknowledge the DST unit of Nanoscience (DSTUNS), IIT Madras, for HRTEM measurements. P.B. acknowledges IIT Madras for a research fellowship.

Because aminothiols are an essential part of biological molecules, an introduction of multivalency onto aminothiols for the protection of gold nanostructures in the aqueous phase promises biologically relevant applications. A new ligand, 18C6aminoethanethiol in which the crown ether-ammonium ion interaction was exploited, was designed and synthesized. Multivalent interactions have thereby been introduced for the stabilization of gold nanoclusters, resulting in smaller cationic gold clusters in the aqueous medium. The single-electron charging behavior of these 18C6-AET-capped gold nanoparticles pointed

Supporting Information Available: Single-crystal X-ray diffraction data and UV-visible absorption spectra for crown ethercapped gold nanoclusters at various ligand concentrations. This material is available free of charge via the Internet at http://pubs.acs.org. LA701636H