Host−Guest Directed Assembly of Gold Nanoparticle Arrays

pubs.acs.org/Langmuir © 2009 American Chemical Society

Host-Guest Directed Assembly of Gold Nanoparticle Arrays Qiang Zeng,† Reena Marthi,† Andrea McNally,† Calum Dickinson,‡ Tia E. Keyes,† and Robert J. Forster*,† †

National Centre for Sensor Research, School of Chemical Sciences, Dublin City University, Dublin 9, Ireland and ‡Materials & Surface Science Institute, University of Limerick, Limerick, Ireland Received June 25, 2009. Revised Manuscript Received September 11, 2009

The formation of a three-dimensional assembly of gold nanoparticles driven by host-guest interactions is described. Assembly is achieved via host-guest interactions between cyclodextrin-modified gold nanoparticles which associate with the adamantane unit of a redox active metal complex [Os(CAIPA)3](ClO4)2, where CAIPA is 2-(4-carboxyphenyl)imidazo[4,5-f][1,10]-phenanthroline-1-adamantylamine. The electrochemical properties of thin films formed on glassy carbon electrodes have been probed using cyclic voltammetry. In aqueous LiClO4, the homogeneous charge transport diffusion coefficient, DCT, is (8.4 ( 0.4)
10-9 cm2 s-1 for both oxidation and reduction of the osmium complexes. Significantly, this charge transport rate is significantly larger than that obtained for a solid deposit of [Os(CAIPA)3](ClO4)2 alone where DCT is 2.3
10-10 cm2 s-1. The higher DCT value observed for the nanoparticle arrays suggests that the incorporated nanoparticles facilitate electron transfer between the bound osmium centers.

Introduction A key challenge in nanotechnology is to develop approaches for fabricating and organizing collections of nanoparticles into larger and more complex architectures. Such architectures have wide-ranging applications from reproducible platforms for surface-enhanced spectroscopy to sensing to nanoscale electronics.1-4 Exploiting noncovalent interactions such as directional hydrogen bonding and host-guest interactions to cooperatively link nanoparticles is attractive for several reasons. First, the strength of the interaction can be tuned over a wide range and can be thermally, photochemically, or electrochemically controlled. Second, additional functionality can be introduced by the linker, e.g., specific redox, photonic, or catalytic properties. Third, it may be possible to control the dimensionality of the assembly, for example, by controlling the number sites within the guest that are capable of binding to a host. For example, as reported previously by Kaifer and co-workers,5,6 gold colloidal particles can be capped with molecular receptors or cyclodextrins, and a nanoparticle network can be created via host-guest interactions. In addition, Reinhoudt and co-workers7 have created more complex three-dimensional structures of cyclodextrincapped gold nanoparticles by layer-by-layer techniques using the strong cyclodextrin-adamantane host-guest interactions as the supramolecular driving force. In spite of this progress, there are few reports on the use of redox or photo-active, polyvalent guests to direct the assembly of CD functionalized nanoparticles. Guests of this type are important because their distinctive electrochemical and photonic properties incorporate addressability in *To whom correspondence should be addressed. (1) Kaminska, A.; Inya-Agha, O.; Forster, R. J.; Keyes, K. E. Phys. Chem. Chem. Phys. 2008, 10, 4172. (2) Zou, S.; Schatz, G. C. Chem. Phys. Lett. 2005, 403, 62. (3) McFarland, A. D.; Van Duyne, R. P. Nano Lett. 2003, 8, 1057. (4) Kotov, N.; Tang, Z. In Nanoparticle Assemblies and Superstructure; Kotov, N. A., Ed.; CRC Press: Boca Raton, 2006. (5) Liu, J.; Mendoza, S.; Roman, E.; Lynn, M. J.; Xu, R.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 4304. (6) Liu, J.; Alvarez, J.; Ong, W.; Kaifer, A. E. Nano Lett. 2001, 1, 57. (7) Crespo-Biel, O.; Dordi, B.; Reinhoudt, D. N.; Huskens, J. J. Am. Chem. Soc. 2005, 127, 7594.

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the assembly, which is essential if such architectures are to be exploited in functional systems. In most of these applications, the nanoparticles are formed as monolayer or multilayer film on a substrate, and it is essential to understand the mechanism of charge transport through thin films.8 Murray and co-workers have reported the electrochemical properties of redox-labeled monolayer-protected Au clusters (MPCs), where anthraquinone and ferrocene redox groups have been attached to the MPC core9-11 and the multilayer MPC films, in which the nanoparticles are linked together by using carboxylate/metal ion/carboxylate linking, pyridine/metal ion/pyridine linking, and dithiol linking.9,12-14 Significantly, for multiredoxlabeled MPCs, electron transfer may occur between adjacent redox centers or it can be mediated by the metallic core of the functionalized nanoparticles.9 In this contribution, we report on the formation of nanoparticle assemblies directed by the host-guest interaction of cyclodextrins bound to gold nanoparticles and a metal complex, [Os(CAIPA)3](ClO4)2, that incorporates three adamantane units, oriented at the apex of an approximately octahedral complex (Scheme 2). These units are capable of binding to the surfacebound CDs; CAIPA is 2-(4-carboxyphenyl)imidazo[4,5-f][1,10]phenanthroline-1-adamantylamine. This guest has been used because it introduces both photophysical and electrochemical functionality into the assembly. In particular, there is significant spectral overlap between the 100 nm particle plasmon centered at approximately 560 nm and the triplet MLCT transition of the guest. This overlap is a prerequisite for plasmonically induced (8) (a) Shenhar, R.; Rotello, V. M. Acc. Chem. Res. 2003, 36, 649. (b) Brust, M.; Kiely, C. Colloids Surf., A 2002, 202, 175. (c) Brennan, J. L.; Branham, M. R.; Hicks, J. F.; Osisek, A. J.; Donkers, R. L. D.; Georganopoulou, G.; Murray, R. W. Anal. Chem. 2004, 76, 5611. (9) Murray, R. W. Chem. Rev. 2008, 108, 2688. (10) Green, S. J.; Pietron, J. J.; Stokes, J. J.; Hostetler, M. J.; Vu, H.; Wuelfing, W. P.; Murray, R. W. J. Phys. Chem. B 1997, 101, 2663. (11) Ingram, R. S.; Murray, R. W. Langmuir 1998, 14, 4115. (12) Zamborini, F. P.; Hicks, J. F.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 4514. (13) Hicks, J. F.; Zamborini, F. P.; Osisek, A. J.; Murray, R. W. J. Am. Chem. Soc. 2001, 123, 7048. (14) Chen, S.; Pei, R.; Zhao, T.; Dyer, D. J. Phys. Chem. B 2002, 106, 1903.

Published on Web 11/23/2009

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Scheme 1. Chemical Structure of β-CD-(4-py)2 and Formation of β-CD-(4-py)2 Functionalized AuNPs

phenomena such as metal-enhanced fluorescence effects or surface-enhanced resonance Raman spectroscopy (SERRS). Transmission electron microscopy (TEM) reveals that the nanoparticles do not undergo Ostwald ripening but can be efficiently linked by adding the metal complex guest to the CD host decorated nanoparticles. Thin layers of these assemblies have been formed on electrodes and cyclic voltammetry used to probe the electrochemical properties and the rate of diffusion-like charge transport through the films. In this way, insights into the mechanism of electron transfer and cooperative effects between the redox-active guests and nanoparticles are obtained.

Experimental Section Materials. All reagents were of analytical grade. A colloidal solution of 100 nm diameter citrate-stabilized gold nanoparticles (AuNPs) with concentration of 5.6
109 particles per mL from BB (British Biocell) International was used as received. di-6A,6Ddeoxy-6-(4-pyridylmethyl)amino-β-cyclodextrin, β-CD-(4-py)2, and CAIPA were synthesized and fully characterized by NMR, MS, and HPLC as previously reported.15,16 Preparation of [Os(CAIPA)3](ClO4)2. 50 mg of [OsCl3] 3 3H2O (145 μmol) and 150 mg (402 μmol) of CAIP ligand were heated to reflux overnight, under nitrogen in ethylene glycol (5 mL). The solution was cooled to room temperature and a saturated aqueous solution of NaClO4 added. This solution was refrigerated at 2 °C for 3 h to promote precipitation. The greenishbrown precipitate formed was collected by filtration and washed with water and diethyl ether. It was then columned on neutral aluminum oxide, using a 90/10 (v/v) mixture of dichloromethane and methanol as eluant, and the dark green phase was collected and evaporated under reduced pressure. The pure complex was dissolved in a minimum amount of methanol and precipitated on addition of a saturated aqueous NaClO4 solution. Yield: 150 mg (75%). 1H NMR (400 MHz, d6-DMSO): 14.45 (s, 1H), 9.11 (d, 2H), 8.89 (d, 2H), 8.85 (d, 2H), 8.36 (d, 2H), 8.23 (t, 2H), 8.12 (t, 2H), 8.18 (m, 4H), 7.95 (m), 7.85 (d, 2H), 7.81 (s, 1H), 7.6 (m, 4H), 7.35 (t, 2H)’ m/z = 1390.9, M-Hþ 657.45. Preparation of β-CD-(4-py)2 Functionalized AuNPs and AuNPs Array. As shown in Scheme 1, β-CD-(4-py)2 functionalized AuNPs were prepared by mixing 2 mL of 100 nm AuNP colloidal solution with 5 mL of an aqueous solution of 0.1 mM β-CD-(4-py)2. This mixture was left standing overnight at room temperature. The number of moles of surface-active cyclodextrin used would be capable of covering an area approximately 5000 times larger than that provided by the nanoparticles. To form the nanoparticle arrays, the CD-modified AuNPs were mixed with 5 mL of 0.1 mM [Os(CAIPA)3](ClO4)2 solution (90% H2O, 5% MeOH, 5% ACN v/v), and this was left to assemble overnight. The Os complex was first dissolved in a minimum volume of ACN and MeOH mixture, and this solution was increased to 5 mL by adding the required volume of water. After each individual step, the AuNPs were collected by centrifugation, washed with Milli-Q (15) McNally, A.; Forster, R. J.; Keyes, T. E. Phys. Chem. Chem. Phys. 2009, 11, 848. (16) Pellegrin, Y.; Foster, R. J.; Keyes, T. E. Inorg. Chim. Acta 2008, 361, 2683.

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water to remove unbound host or guest molecules, and then characterized by using UV/visible spectroscopy and surfaceenhanced resonance Raman spectroscopy. Instrumentation. UV/vis spectra were carried out on a Shimadzu UV-3100 spectrophotometer. Scanning electron microscopy (SEM) was performed using a Hitachi S-3000N system operated at the applied voltage on the cathode of 20 kV. The required volumes of sample solutions were dropped on silicon wafers, which were precleaned by rinsing with acetone and ethanol, followed by air-drying. Transmission electron microscopy (TEM) was conducted using a JEOL JEM-2011 electron microscopy operated at an accelerating voltage of 200 kV with a LaB6 filament. Images were recorded using a Gatan DualVision 600t CCD camera attached to the microscope. Samples for imaging were deposited onto a 400 mesh copper grid with a carbon film (Agar Scientific), and the liquid was allowed to dry in air at room temperature. The concentrations of samples for both unmodified and modified AuNPs are similar. The area coated by the droplet was sufficiently large to ensure that there is sufficient area available to allow the nanoparticles to form a dispersed array, i.e., the structure of the dried arrays is controlled by the host-guest interactions rather than by drying phenomena. Raman spectroscopy was obtained on a Dilor Jobinyvon Spex Labram. Argon ion and helium-neon lasers were used for excitation at 458, 514, and 632.8 nm, respectively. Before use, the wavenumber axis of the Raman was calibrated with the Rayleigh line and with silicon wafer from the phonon mode at 520 cm-1. Electrochemistry was performed in conventional threeelectrode cells using an Ag/AgCl electrode filled with 3 M KCl (aq) (CH instrument) as reference. Cyclic voltammetry (CV) was carried out using a CH Instruments model 660a electrochemical workstation. All electrolyte solutions were deoxygenated for at least 15 min using N2 gas before use and maintained under a nitrogen blanket during measurement. Glassy carbon electrodes were used as working electrodes in this experiment due to the surface stability at high potential in both neutral and acidic electrolytes. The working electrodes were polished successively with 1.0, 0.3, and 0.05 μm alumina powder before use, then modified by evaporating the required volume of a 1% [Os(CAIPA)3](ClO4)2 solution in ACN/MeOH (1:1), an aqueous solution of unmodified AuNPs, or an aqueous solution of aggregated AuNPs followed by air-drying. The maximum concentration of osmium centers within solid samples of the complex was determined as 0.71 M by measuring the density of a single crystal of the complex by flotation in nonswelling solvents, i.e., petroleum ether and dichloromethane. The maximum concentration of Os redox centers when acting as a guest within a CD monolayer modified AuNPs was calculated as 0.05 M, assuming that a close-packed monolayer of β-CD-(4-py)2 forms on the gold nanoparticle surface and every cavity is filled by [Os(CAIPA)3]2þ/3þ. This value is significantly lower than that found in the pure solid, because the volume of 100-nm-diameter gold particles has been taken into account.

Results and Discussion Absorption Spectroscopy. Aggregation and surface functionalization of metal nanoparticles causes the plasmon absorption band to exhibit a red-shift17-19 and to change intensity and peak shape. Figure 1A shows that the surface plasmon resonance of citrate-stabilized 100-nm-diameter AuNPs appears at 560 nm.20 (17) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2001, 123, 1471. (18) Liu, Z.; Jiang, M. J. Mater. Chem. 2007, 17, 4249. (19) Crespo-Biel, O.; Jukovic, A.; Karlsson, M.; Reinhoudt, D. N.; Huskens, J. Isr. J. Chem. 2005, 45, 353. (20) (a) Link, S.; M. El-Sayed, A. J. Phys. Chem. B 1999, 103, 4212. (b) Link, S.; El-Sayed, M. A. Int. Rev. Phys. Chem. 2000, 19, 409. (c) Jain, P. K.; Lee, K. S.; El-Sayed, I. H.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 7238.

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Article Scheme 2. Schematic Illustrations for the Preparation of the Host-Guest Inclusion Induced AuNP Assemblies

Figure 1. UV-visible spectra of (A) 100 nm citrate-stabilized AuNPs, (B) 100 nm citrate-stabilized AuNPs following exposure to β-CD, centrifugation, and resuspension, (C) 100 nm citratestabilized AuNPs following exposure to β-CD-(4-py)2, centrifugation, and resuspension, (D) 0.1 mM [Os(CAIPA)3](ClO4)2 in MeOH/ACN (1:1), and (E) β-CD-(4-py)2 modified AuNPs after addition of [Os(CAIPA)3](ClO4)2, centrifugation, and resuspension. The spectra have been displaced by approximately 0.2 A.U. for clarity.

As expected, Figure 1B shows that addition of non-surface-active β-cyclodextrin does not change the plasmon wavelength of maximum absorbance, λmax, or the peak shape. In sharp contrast, exposure of the AuNPs to β-CD-(4-py)2, centrifugation, and resuspension causes an additional broad peak to be observed centered at approximately 620 nm and λmax of the original band shifts by approximately 5 nm. This new transition is consistent with a longitudinal plasmon mode, and this, together with the shift of the original surface plasmon, suggests that β-CD-(4-py)2 binds to the surface of the gold nanoparticle in a similar way to that reported by us previously for macroscopic gold surfaces.15,21 Figure 1D shows the absorbance spectrum for [Os(CAIPA)3](ClO4)2 in MeOH/ACN (1:1) solution. The complex exhibits a singlet metal-to-ligand charge-transfer band (21) Mallon, C. T.; Campagnoli, E.; Pikramenou, Z.; Forster, R. J.; Keyes, T. E. Langmuir 2007, 23, 6997.

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(MLCT) in the visible region centered at 503 nm with a weaker, broad feature extending to approximately 680 nm that is associated with the 3MLCT charge transfer. Such spin-forbidden transitions are commonly observed in osmium polypyridyl complexes as a consequence of strong spin-orbit coupling due to the heavy osmium atom.22 The adamantyl group is a high-affinity guest for inclusion with β-CD, with an association constant of approximately 104. Therefore, the osmium complex are expected to link the individual β-CD-(4-py)2-capped gold nanoparticles together via a host-guest interaction. Figure 1E shows the absorbance spectrum of the β-CD functionalized nanoparticles following exposure to [Os(CAIPA)3](ClO4)2, centrifugation, and resuspension in solution. While the changes in the absorbance spectra are perhaps not as dramatic as those reported by Mirkin for nanoparticle aggregation driven by oligonucleotide hybridization,23 the spectrum contains additional new features as well as contributions from both the osmium complex guest and β-CD-functionalized nanoparticle host. Significantly, spectra of citrate-stabilized nanoparticles exposed to the osmium complex in solution, centrifuged, and resuspended do not contain any spectral features associated with the osmium guest; i.e., in the absence of the surface active CD on the nanoparticle, the osmium guest does not bind and link the particles. The structure of the assemblies has been further probed using Raman spectroscopy, and these data are discussed in a later section. It is perhaps important to note that, if a close-packed monolayer of β-CD-(4-py)2 is formed on the gold nanoparticle surface and every cavity filled by [Os(CAIPA)3]2þ/3þ, then each nanoparticle would contain approximately 17 500 sites for interparticle association. The nanostructure of the gold nanoparticles arrays was investigated using SEM and TEM measurements. Figure 2A shows that citrate-stabilized nanoparticles show little tendency to aggregate. Similarly, Figure 2B,C shows that β-CD-(4-py)2 functionalized AuNPs show relatively little aggregation and tend to be randomly dispersed. In addition, the TEM image shown in Figure 2E clearly demonstrates that the nanoparticles within the small number of aggregates that do form on are mostly two-dimensional and exhibit an interparticle space about 3 nm. (22) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85. (23) Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz, G. C. J. Am. Chem. Soc. 2000, 122, 4640.

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Figure 2. SEM images obtained from (A) citrate-stabilized AuNPs, (B,C) β-CD-(4-py)2 functionalized AuNPs, and (D) AuNPs arrays following host-guest formation between CD functionalized NPs and [Os(CAIPA)3](ClO4)2. TEM images for β-CD-(4-py)2 functionalized AuNPs and the AuNPs arrays following host-guest formation are shown as (E) and (F). 1328 DOI: 10.1021/la902258s

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Figure 3. Resonance Raman spectra (from top to bottom) of dry β-CD-(4-py)2 film on a roughened gold surface, β-CD-(4-py)2 modified AuNPs on a silicon wafer, citrate-stabilized AuNPs on a silicon wafer, citrate-stabilized AuNPs following addition of [Os(CAIPA)3](ClO4)2, and β-CD-(4-py)2 excited at 632.8 nm.

In contrast, Figure 2D,F shows that CD-functionalized nanoparticles aggregate into rough spherical three-dimensional assemblies when exposed to [Os(CAIPA)3](ClO4)2 that is capable of forming a host-guest complex with the surface-bound CDs. Raman Spectroscopy. An important property of closely spaced metal nanoparticles is their capacity to significantly enhance the Raman response of surface-bound species. This was exploited here to characterize the assemblies, to confirm the presence of β-CD-(4-py)2 and [Os(CAIPA)3](ClO4)2 within the aggregates, and to measure the surface enhancement factors. Thin films of citrate-stabilized AuNPs, β-CD-(4-py)2 functionalized AuNPs, citrate-stabilized AuNPs following addition of [Os(CAIPA)3](ClO4)2, and β-CD-(4-py)2 stabilized AuNPs following addition of [Os(CAIPA)3](ClO4)2 were formed on silicon wafers and Raman spectra recorded using 632.8 nm excitation. As a comparison, an additional Raman experiment was carried out on β-CD-(4-py)2 monolayer on gold electrode under the same conditions. Excitation at 632.8 nm is associated with the plasmon absorption of the host-guest linked AuNPs giving rise to the possibility of surface-enhanced resonance Raman spectroscopy (SERRS). As shown in Figure 3, the features at 1590, 1363, 949, and 303 cm-1 are attributed to the citrate-stabilized AuNPs, and no features are obtained in Raman spectra of bulk solid β-CD-(4py)2, because the excitation wavelength is not resonant with the absorption bands and CD is a weak Raman scatterer. However, the Raman spectrum of CD-functionalized AuNPs shows enhanced features of β-CD-(4-py)2 at 1614, 1541, 1422, 1468, 1278, 1214, and 1021 cm-1 due to surface enhancement. This observation is supported by the Raman spectrum of a β-CD-(4-py)2 monolayer on a roughened gold electrode shown in Figure 3. Also, Raman spectra of citrate-stabilized AuNPs collected following exposure to [Os(CAIPA)3](ClO4)2 followed by centrifugation show features only at 1590, 949, and 305 cm-1, which are associated with the AuNPs alone; i.e., there are no modes associated with the Os complex suggesting that citrate-stabilized AuNPs and the Os complex do not associate. In all cases, the feature marked with an asterisk at 520 cm-1 arises from the underlying silicon wafer. Thin films of the nanoparticle assemblies were coated on a glassy carbon electrode and Raman spectra recorded using excitation at 458, 514, and 632.8 nm. These excitation wavelengths are capable of exciting both the nanoparticle plasmon and Langmuir 2010, 26(2), 1325–1333

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the MLCT state of the osmium complex, opening up the possibility of SERS and SERRS. For comparison, Raman spectroscopy of solid deposits of [Os(CAIPA)3](ClO4)2 were also conducted under the same conditions. Figure 4A(1) shows the Raman spectrum for solid [Os(CAIPA)3](ClO4)2 excited at 458 nm, which is resonant with the MLCT absorption for this complex. The features at 1616, 1566, 1503, 1457, 1422, 1368, 1313, 1251, 1195, 1145, 1079, 1048, 733, and 443 cm-1 are all characteristic of CAIPA ligand vibration.16,24,25 Figure 4A(2) shows that the same features are observed for nanoparticle assemblies created by the host-guest interaction of nanoparticle bound β-CD-(4-py)2 and [Os(CAIPA)3](ClO4)2, confirming the presence of the metal complex in the assembly. The intensity of AuNPs arrays is lower than that obtained in solid deposits probably due to the lower concentration of Os in the AuNPs arrays. This observation is confirmed by the surface coverage in both solid deposits and AuNPs arrays obtained from electrochemistry measurement in the following section. Excitation at 514 nm is resonant with both the MLCT of the osmium complex and the plasmon absorption of AuNPs. As shown in Figure 4B, the peak positions and intensities of solid [Os(CAIPA)3](ClO4)2 are insensitive to the excitation wavelength. For the host-guest associated AuNPs film, the intensity of the CAIPA features are approximately an order of magnitude more intense than those observed for the solid complex, despite the lower concentration of [Os(CAIPA)3](ClO4)2 within the nanoparticle assembly. The overall enhancement factor is approximately 103suggesting that the Raman spectrum is surface enhanced due to the close proximity of the guest to the AuNPs surface when included within the CD. This conclusion is further confirmed, since although all of the features present at 458 nm remain at 514 nm for the aggregated AuNPs film, additional new features are enhanced at 776, 617, 536, 495, and 468 cm-1, which are attributed to the cyclodextrin pyridine unit vibration25 and a 354 cm-1 mode associated with the Os-N stretch.16 These modes are enhanced at this wavelength because the exciting laser moves into resonance with the nanoparticle surface plasmon. This result agrees with our previous work on β-CD-(py) layers prepared on electrochemically roughened Au that give significant enhancement factors.26 Figure 4C shows the Raman spectra for solid [Os(CAIPA)3](ClO4)2 and nanoparticle assembly excited at 632.8 nm. For the thin film of [Os(CAIPA)3](ClO4)2, the Raman features are lost under the tail of the emission arising from the complex, which emits at 740 nm. For the nanoparticle aggregate, 632.8 nm is not resonant with the MLCT absorbance, but there is strong resonance with the metal plasmon of the aggregate. The observed spectrum is a complicated mix of modes associated with the β-CD-(4-py)2 unit. Modes from the pyridine anchors at 1609, 1460, 1425, 1288, 1024, 778, and 619 cm-1,27 respectively, are particularly enhanced. Similarly, carbohydrate modes associated with the β-CD are observed which are analogous to those reported previously for γ-CD-(4py)2 adsorbed on roughened gold15 and Figure 3. These studies confirm that the β-CD-(4-py)2 and the [Os(CAIPA)3](ClO4)2 units are simultaneously present in the film. The observation that the SERS spectrum is dominated by the β-CD-(4-py)2 unit confirms that this is the surface-bound moiety. Because of the (24) Abraham, B.; Sastri, C. V.; Maiya, B. G.; Umapathy, S. J. Raman Spectrosc. 2004, 35, 13. (25) Clarke, R. J. H.; Turtle, P. C.; Strommen, D. P.; Streusand, B.; Kincaid, J.; Nakamoto, R. Inorg. Chem. 1977, 84. (26) Mcnally, A. Ph.D. Thesis, Dublin Institute of Technology. 2005. (27) Cai, W.-B. J. Chem. Soc., Faraday Trans. 1998, 94, 3127.

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Figure 4. Resonance Raman spectra of dry [Os(CAIPA)3](ClO4)2 film (1) and the AuNPs assemblies (2) coated on 3 mm glassy carbon electrodes: (A) excited at 458 nm, (B) excited at 514 nm, (C) excited at 632.8 nm.

luminescence from the [Os(CAIPA)3](ClO4)2 thin film at 632.8 nm, it was not possible to estimate the enhancement factor. Nonetheless, given the relative magnitude of the 632.8 nm Raman spectrum and the fact that the laser intensity is about 50% less for this line than the argon ion lines, it is clear that there is very significant SERS enhancement from the aggregates for the assembled β-CD-(4-py)2 and [Os(CAIPA)3](ClO4)2 units. It is interesting to note that the background on the SERS spectrum of the assembly is small compared with the [Os(CAIPA)3](ClO4)2 solid, suggesting that the osmium emission is quenched by the nanoparticles. Redox Properties. Supramolecular assemblies of this kind may exhibit useful electrochemical properties because of cooperative effects between the redox-active osmium linkers and the metal nanoparticles. For example, the overall conductivity of the assemblies may be controlled by electron transfer between the nanoparticles or by electron self-exchange between adjacent osmium guests on the surface of the nanoparticles. Cyclic voltammetry was used to investigate the charge transport mechanism through thin films of the host-guest assemblies. Solid Deposits of the Osmium Complex. To compare the voltammetric properties of the osmium complex in the solid state and as a guest within the nanoparticle assemblies, voltammograms of solid-state deposits of the osmium complex were collected. As illustrated in Figure 5, the formal potential, E°0 , for the Os2þ/3þ couple within a solid deposit is 631 mV when in contact with 0.1 M LiClO4 aqueous electrolyte. This value is significantly more positive than that found for the complexes dissolved in DMF/H2O (4:1) containing 0.1 M LiClO4 where E°0 is 581 mV. This shift suggests that there are significant differences in the solvation shell or a relatively high Gibbs energy barrier for ion transport into/within the solid film 1330 DOI: 10.1021/la902258s

compared to solution. The peak shape is independent of scan rate up to 100 mV s-1, and the signal is consistent with a surfaceconfined response. Moreover, as shown in the inset of Figure 5, the peak height scales linearly with increasing scan rate, and plots of log peak current vs log scan rate have slopes of 1.01 ( 0.05. The peak to peak separation (ΔEp) is 26 ( 3 mV, and the full width at half-maximum (fwhm) is 184 ( 6 mV. An fwhm of 90.6 mV is expected for a one-electron transfer process under finite diffusion control, and the broadened peaks suggest that there are repulsive interactions between adjacent adsorbates. Using the geometric area of the electrode, the charge passed in the background corrected voltammograms recorded at 5 mV s-1 yields a surface coverage of 8.0
10-9 mol cm-2. The bulk concentration of redox centers within the solid as determined from the flotation measurement is 0.71 M. Therefore, assuming that the solid deposit does not swell when in contact with water, this surface coverage corresponds to a layer thickness of 110 nm or approximately 35 ML equiv. The rate of charge transport through thin films of the osmium complex alone can be estimated by measuring the homogeneous charge transfer diffusion coefficient, DCT. In fast scan cyclic voltammetry, only a small fraction of the total amount of material immobilized is electrolyzed and the depletion zone remains well within the film. Under these conditions, linear diffusion predominates and the peak current varies as υ1/2. Figure 6 illustrates voltammograms for a thin film of the osmium complex at scan rates between 200 and 500 mV s-1. For this range of scan rates, the ΔEp value is 79 ( 4 mV and voltammetric peak current increases linearly with υ1/2. This observation is in reasonable agreement with that expected for a reversible redox reaction under semi-infinite linear diffusion control, and the dependence of the peak current, Langmuir 2010, 26(2), 1325–1333

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Figure 5. Cyclic voltammograms for a [Os(CAIPA)3](ClO4)2 film coated on 3 mm glassy carbon electrode in 0.1 M aqueous LiClO4

electrolyte. Scan rates are (from bottom to top) 10, 20, 30, 40, 50, and 100 mV s-1. Γ = 8.0
10-9 mol cm-2. The inset shows a plot of peak current vs scan rate.

Figure 6. Scan rate dependence of the voltammetric response of [Os(CAIPA)3](ClO4)2 film on 3 mm glassy carbon electrode in 0.1 M LiClO4. υ = 500, 400, 300, and 200 mV s-1 (from top to bottom). Γ = 8.0
10-9 mol cm-2. Inset: ip vs υ1/2 under same conditions.

ip, on the scan rate can be expressed in terms of the Randles-Sevcik equation ip ¼ 2:69
105 n3=2 ADCT 1=2 Ceff v1=2

ð1Þ

where n is the number of electrons transferred, A is the area of the working electrode, DCT is the homogeneous charge transport diffusion coefficient, and Ceff is the effective fixed site concentration of the redox center, which has been measured as 0.71 M. This equation allows the DCT value of the osmium film to be determined from the slope of the plot of ip vs υ1/2. The DCT estimated for both oxidation and reduction processes is (28) Keane, L.; Hogan, C.; Forster, R. J. Langmuir. 2002, 18, 4826. (29) Walsh, D. A.; Keyes, T. E.; Forster, R. J. J. Electroanal. Chem. 2002, 75, 538.

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(2.3 ( 0.2)
10-10 cm2 s-1, which is similar to that reported previously for structurally related systems.28,29 Nanoparticle Thin Films. The situation with regards to the nanoparticle assemblies is rather different. As the osmium centers are guests within the nanoparticle bound CDs, their concentration within the assembly is significantly lower (0.05 M) than that of the solid deposit (0.71 M). Moreover, switching the redox state of the osmium may be facilitated by electron hopping between the gold nanoparticles whose concentration is above the percolation limit. Figure 7 shows cyclic voltammograms of the AuNPs assemblies coated on 3 mm glassy carbon electrode in contact with 0.1 M aqueous LiClO4 at scan rates between 10 and 100 mV s-1. For this range of scan rates, E°0 is 620 mV and does not depend on the scan rate. As shown in the inset of Figure 7, the peak current scales linearly with scan rates. The ΔEp is 43 ( 8 mV and fwhm is 173 ( 10 mV, which are both higher than the values DOI: 10.1021/la902258s

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Figure 7. Voltammetric response of the thin film of AuNPs array coated on 3 mm glassy carbon electrode in 0.1 M aqueous LiClO4 at scan rates between 10 and 100 mV s-1 (from bottom to top). Γ = 1.1
10-9 mol cm-2. Inset is the plot of the peak current vs scan rate.

Figure 8. Cyclic voltammograms for solid deposits of Os complex (top) and thin film of AuNPs array (bottom) on 3 mm glassy carbon electrode in 0.1 M LiClO4. The scan rate is 300 mV s-1. The solid lines represent experimental data, and theoretical fits generated according to the Butler-Volmer formalism of electrode kinetics are denoted by the open circles.

expected for a reversible one-electron transfer reaction involving a surface-confined species. The peak-to-peak separations and the associated error bars are somewhat larger for the nanoparticle array than those observed for the solid deposits of the complex alone. However, the voltammetric peaks are significantly sharper and the formal potential is shifted in a negative potential direction compared to the solid deposits. These results indicate that oxidation of the osmium centers is more facile in the nanoparticle arrays. As the scan rate is increased from 100 to 500 mV s-1 ΔEp increases to 136 mV. This observation may suggest that the rate of heterogeneous electron transfer across the electrode/deposit interface influences the voltammetric response of the thin film of host-guest linked AuNPs. Significantly, the total cell resistance of the thin film is approximately 320 Ω, leading to an ohmic drop of approximately 11 mV, which is negligible compared with the ΔEp observed at high scan rates. Figure 8 illustrates theoretical 1332 DOI: 10.1021/la902258s

fits to the experimental cyclic voltammograms for both Os complex solid deposits and AuNPs array thin film, which are generated according to the Butler-Volmer formalism of electrode kinetics under the assumption of semi-infinite linear diffusion. Clearly, while the Butler-Volmer model is capable of accurately reproducing the peak potentials and currents using two parameters, i.e., DCT and k°, it does not reproduce the peak shapes accurately. This behavior is consistent with the peak broadening presented in Figure 7 and suggests either a range of effective formal potentials or heterogeneous electron transfer rates. Despite these effects, the best fit DCT and k° values are expected to provide a useful insight into the effect of incorporating metal nanoparticles on the rate of homogeneous charge transport. Fits of the CVs obtained for thin films of the Os complex alone yield a diffusion coefficient, DCT, of (2.0 ( 0.4)
10-10 cm2 s-1 and a standard heterogeneous electron transfer rate constant, k°, of (1.3 ( 0.5)
10-4 cm s-1. The diffusion coefficient obtained by Langmuir 2010, 26(2), 1325–1333

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fitting the complete voltammograms is indistinguishable from that found using the Randles-Sevcik equation, (2.3 ( 0.2)
10-10 cm2 s-1. For thin films of the AuNPs arrays, DCT is (8.4 ( 0.4)
10-9 cm2 s-1 and k° is (3.1. ( 0.3)
10-3 cm s-1. Significantly, the apparent rates of homogeneous charge transport and heterogeneous electron transfer are more than an order of magnitude higher for the nanoparticle arrays than for thin films of the osmium complex alone. This result suggests that the metal nanoparticles participate in the charge transport mechanism most likely through electron hopping between the metallic cores.

Conclusions A new three-dimensional gold nanoparticle array, driven by the host-guest interactions of cyclodextrin-decorated gold nanoparticles and the adamantane host of an Os metal complex, has been formed and characterized. The structure of a film of this material has been probed using UV-visible, resonance, and

Langmuir 2010, 26(2), 1325–1333

surface-enhanced Raman spectroscopies. The redox properties and the diffusion-like electron transport dynamics of this AuNPs multilayer have been investigated using cyclic voltammetry. The homogeneous charge transport diffusion coefficient, DCT, associated with oxidizing or reducing the osmium redox labels is on the order of 10-9 cm2 s-1. Significantly, this rate of homogeneous charge transport is approximately 40-fold higher than that found for a solid deposit of the complex alone, i.e., the metal nanoparticles appear to promote charge propagation through the assembly. Acknowledgment. We appreciate the financial support of the Higher Education Authority under the Program for Research in Third Level Institutions under funding of the National Biophotonics and Imaging Platform. This work was in part supported by Science Foundation Ireland under the Research Frontiers Programme Award No. CHE0085. We are very grateful for the useful insights provided by the referees of this manuscript.

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