SECM Measurement of the Fast Electron Transfer Dynamics between

NANO LETTERS

SECM Measurement of the Fast Electron Transfer Dynamics between Au381+ Nanoparticles and Aqueous Redox Species at a Liquid/Liquid Interface

2004 Vol. 4, No. 9 1763-1767

Dimitra G. Georganopoulou,† Michael V. Mirkin,*,‡ and Royce W. Murray*,† Kenan Laboratories of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290, and Department of Chemistry and Biochemistry, Queens College, CUNY, Flushing, New York 11367 Received May 27, 2004

ABSTRACT Quantitative measurements are reported of the rate of interfacial electron transfers (ET) between gold monolayer protected clusters (Au MPCs) dissolved in 1,2-dichloroethane and aqueous redox species. Scanning electrochemical microscopy (SECM) current−distance approach curves were used to obtain the heterogeneous rate constant of ET between organic soluble Au38 clusters and an aqueous IrCl62- oxidant. The Au cluster cores, protected with a shell of phenylethylthiolate ligands, are sufficiently small (d ∼ 1.1 nm) to exhibit molecule-like redox activity. The evaluation of the ET rate constant at the nonpolarizable liquid/liquid interface was accomplished using an approach previously developed for bimolecular ET reactions between conventional redox entities. The dependences of the effective heterogeneous rate constant, kf, on the concentration of the aqueous redox mediator and reaction driving force were found to be in agreement with theoretical expectations.

Developments in the synthesis and analysis of small (e140 metal atoms) organic soluble Au nanoparticles1 have allowed researchers to study their energetics, notably the discrete quantized, single-electron charging of such clusters.2,3 The ultimate goal of these studies is to understand the nature of metal nanoparticles that have size-dependent properties, especially the stabilized monolayer protected clusters (MPCs), as well as their potential in applications such as catalysis,4 nanoscale circuits,5 machines,6 and chemical sensors.7 Among other electrochemical methods, scanning electrochemical microscopy (SECM) has been used to probe electron transfers of MPCs in Langmuir films, 8 modified slides,9 and at the liquid/liquid interface.10 Here, we present the first quantitative measurements of interfacial ET rate between MPCs confined to the waterimmiscible organic solvent (1,2-dichloroethane, DCE) and an aqueous redox species (IrCl62-). MPCs offer significant advantages in such interfacial studies. By use of either strongly hydrophobic or hydrophilic protecting monolayer ligands, the same Au MPC core size can in principle be made to be soluble in solely either an organic or aqueous phase and to resist crossing of an organic/water phase boundary. At the same time, one has the potential to systematically * Corresponding author. E-mail: [email protected] † University of North Carolina. ‡ Queens College, CUNY. 10.1021/nl049196h CCC: $27.50 Published on Web 08/06/2004

[email protected];

© 2004 American Chemical Society

vary the core size and charge and the chemistry of the protecting monolayer of the MPC. For these reasons, MPCs offer an attractive model experimental system for various aspects of ET theory.11 In the present case, we use the +1 core charge state of the organic soluble MPC Au38(PhC2S)24,2b-d where the ligand is phenylethylthiolate. Methodology for measurements of bimolecular ET rates at the water/organic solvent interface by SECM has been established previously.12 Using the feedback approach, in our experiments the ultramicroelectrode tip was positioned in the top (aqueous) phase (Figure 1). The reduced form of an aqueous redox mediator (R, IrCl63-) contained in that phase was oxidized at the electrode tip surface: R - e- f O (tip in aqueous phase)

(1)

As the tip is lowered toward the organic liquid layer (1,2dichloroethane containing the Au381+ MPCs), the mediator oxidant O (e.g., IrCl62-) reacts in an interfacial bimolecular ET with the Au381+ form of the MPCs, oxidizing it and regenerating R: k12

Au381+(o) + O (w) 98 Au382+ (o) + R (w) (liquid/liquid interface) (2) The apparent heterogeneous rate constant, kf (cm s-1), for

Figure 1. Schematic diagram of SECM approach measurement of the ET rate between an organic-soluble MPC and an aqueous redox species. Electroneutrality is maintained by transfer of perchlorate ions across the interface.

this reaction can be evaluated from theoretical fits of the tip current vs tip-to-interface distance (iT vs d) curves.12 The apparent rate constant, kf can then be related to the bimolecular rate constant k12 (M-1cm s-1) for reaction 2 via the equation13 kf ) k12CMPC where CMPC is the concentration of MPCs in the organic phase. SECM has been a powerful tool for probing the dependence of bimolecular rate constants on the driving force of heterogeneous reactions.12 The driving force for the ET reaction is determined by the difference of standard potentials of the organic and aqueous redox mediators (∆E° ) E°acceptor - E°donor, where IrCl62- is the acceptor and MPC1+ is the donor) and the Galvani potential drop across the interface: ∆G° ) -F(∆E° + ∆°w φ)

(3)

The Galvani potential difference, ∆°w φ, in SECM experiments is established by the partitioning of a common ion in both phases and is described by a Nernst-Donnan equation, ∆°w φ, that depends on the ratio of concentrations of this common ion in water, [ClO4-]w, and in organic phase, [ClO4-]o

∆°w φ ) const - 0.059 log

[ClO4-]w [ClO4-]o

(4)

ClO4- ions also maintain electroneutrality (Figure 1) by crossing the interface in company to the heterogeneous electron transfer described in reaction 2, and as indicated in Figure 1. 1764

Figure 2. Differential pulse (lower) and steady state (upper) voltammetry of 0.4 mM Au38(PhC2S)24 clusters in DCE solvent obtained at 3-mm-diameter Pt disk at 10 mV/s (lower) and 12.5 µm radius microdisk electrode (upper). The solution contained 0.01 M Hx4NClO4. Oxidation states of MPCs corresponding to a given DPV peak or CV wave are identified in the figure. Potentials are corrected vs SHE scale.

The voltammetry of organic soluble Au38(PhC2S)24 nanoparticles stabilized with phenylethylthiolate (PhC2S-) ligands14,15 is summarized in Figure 2. The lower panel shows differential pulse voltammetry in DCE solvent; the MPC core charge states indicated have been assigned in previous work.2c,d The Au38 nanoparticle core is small enough that a molecule-like HOMO-LUMO gap has developed as reflected in the large potential interval between the first oneelectron oxidation and the first one-electron reduction. The voltammetric peaks labeled 1+/0 and 2+/1+ correspond to successive loses of electrons from the HOMO molecular orbital. Figure 2, upper, shows a typical steady-state microelectrode voltammogram of the 0.4 mM MPC in DCE solution; the two waves by reference to the lower panel in Figure 2 represent the Au38+1/0 and Au38+2/+1 charge state changes. The current plateau for Au381+ lies nearly at zero current; i.e., more positive potentials oxidize Au381+ (to Au382+) and more negative potentials reduce Au381+ (to Au380). The prepared MPC solution is thus Au381+ (with a small proportion, ∼10%, of Au380). Using eq 5 for the steadystate diffusion limiting current at a microdisk electrode, iT,∞ ) 4nFaDMPCCMPC

(5)

where n ) 1 is the number of transferred electrons, F is the Faraday constant, a ) 12.5 µm is the disk radius, DMPC and CMPC are the diffusion coefficient and concentration of MPCs, respectively, one can extract DMPC ) 2.1 × 10 -6 cm2 s-1 from the voltammogram in Figure 2, upper. This result is in good agreement with other studies.16 The SECM approach curves obtained with different concentrations of IrCl63- redox mediator in the aqueous phase and Au38 MPCs in DCE are shown in Figure 3. In each case, the tip currents are normalized to iT,∞ and distance to tip Nano Lett., Vol. 4, No. 9, 2004

instead the commercial simulation package FEMLAB to solve the steady-state SECM diffusion problem, which is very similar to that formulated in ref 18. Our computational results agree very well with those presented by Barker et al.18 However, it turned out that the two dimensionless parameters used in ref 18 can be combined into one, Dow )

Figure 3. SECM current-distance curves for a 12.5 µm radius Pt tip in aqueous solution approaching the water/DCE interface. Currents are normalized to iT,∞ and distance to tip electrode radius. The aqueous solution contained 0.1 M NaClO4 and (1) 0.5, (2) 0.32, (3) 0.145, or (4) 0.057 mM Na3IrCl6. DCE contained 0.01 M Hx4NClO4 and 0.4 mM MPC (curves 2-4). The tip potential was held at 0.8 V vs Ag/AgCl, corresponding to the plateau current for oxidation of IrCl63-. The tip was approached at 1 µm/s. Solid lines are: theory for pure negative feedback (curve 1),20b simulated curves for finite heterogeneous kinetics (curves 2-4), and theory for a diffusion-controlled process (curve 5).20b

electrode radius. As expected, when the DCE contains no MPC (curve 1), pure negative feedback is observed when the 12.5 µm radius Pt tip approaches the DCE/water interface, which shields the diffusion field to the microelectrode tip. The approach curves obtained with 0.4 mM MPCs in the organic phase (curves 2 to 4) were fitted to the theory for finite heterogeneous kinetics (solid lines); these curves lie higher than the negative feedback curve 1 but lower than the theoretical curve for an entirely diffusion-controlled (i.e., very fast) ET reaction (curve 5). The normalized tip currents in Figure 3 decrease markedly with increasing concentration of IrCl63- in water (from curve 4 to 2). The decrease is due to increasing diffusion limitations in the bottom (organic) phase. It was shown previously17 that the transport limitations diminish the SECM feedback current unless the ratio of concentrations of redox species in the two phases (i.e., CMPC/CIrCl63-) is sufficiently high. Barker et al.18 carried out extensive simulations and described the effect of the diffusion in the bottom phase on the shape of the approach curve as a function of two parameters, i.e., the ratios of concentrations and diffusion coefficients of the redox reactants in the two liquid phases. In our experiments, the diffusion coefficient of MPC (2.1 × 10-6 cm2 s-1) is substantially lower than that of IrCl63- (7.6 × 10-6 cm2 s-1), as found from steady-state voltammetry. At the same time, the limiting solubility of MPCs in DCE forced us to keep CMPC at e0.4 mM. Therefore, at any feasible concentration of the aqueous redox mediator (i.e., CIrCl63- g 30 µM), the diffusion limitations in the bottom organic phase had to be taken into account. Because the simulated data in ref 18 depend on several kinetic parameters and cannot be reduced to an analytical expression or a universal working curve, it was not possible to use them to fit our experimental approach curves. We used Nano Lett., Vol. 4, No. 9, 2004

DMPCCMPC DaqCaq

where Daq and Caq represent the diffusion coefficient and concentration of aqueous redox species. Dow is a product of two parameters used in ref 18. The shape of an SECM approach curve is fully determined by the values of Dow and a dimensionless rate constant, K, K)

aCMPCk12 Daq

(6)

where k12 is the apparent bimolecular rate constant (mol-1 cm4 s-1) for reaction 2. When Dow J 20, the diffusion in the bottom phase is fast and the shape of the approach curve is essentially independent of the value of Dow. In Figure 3, 0.35 e Dow e 1.9 and the feedback current strongly depends on concentration of the aqueous redox mediator. The dimensionless rate constant K ) 5 ( 1 was obtained by fitting experimental approach curves to the theory at different concentrations of IrCl63- in the aqueous phase (Figure 3, curves 2-4). As ideally expected, K was found to be essentially constant (within experimental error) and independent of the Dow value. From eq 6, the corresponding value of the apparent bimolecular rate constant is k12 ) 7.6 × 104 mol-1 cm4 s-1 (or 76 M-1 cm s-1). To compare this measured rate constant to those reported previously for ET reactions between conventional molecules at the liquid/liquid interface, one needs to know the driving force for the interfacial ET reaction. It was shown previously17b that the driving force can be evaluated as the difference between half-wave potentials of the aqueous and organic redox species (∆E1/2) measured with respect to a common (e.g., aqueous) reference electrode. Using this approach, ∆E1/2 ) 315 mV for the ET between Au381+ and IrCl62- when the concentrations of ClO4- in water and DCE are equal to 0.1 and 0.01 M, respectively. The measured k12 value for the Au381+/IrCl62- reaction is significantly larger than those reported previously for ET reactions between conventional molecules at the liquid/liquid interface. For example, the bimolecular rate constant measured for the ET between aqueous Mo(CN)84- and the oxidized form of zinc porphyrin (ZnPor+) in benzene at a similar driving force was about 5 M-1 cm s-1. 17b At the water/DCE interface, k12 ) 18 M-1 cm s-1 was measured19 for the reaction between decamethylferrocene and Ru(CN)63smaller than the MPC1+/IrCl62- result, even though the driving force for the former reaction (0.54 V) was much larger than ∆E1/2 for MPC/IrCl62-. For the reaction between decamethylferrocene and IrCl62-, where the driving force was even higher (0.68 V), the rate constant value was only ca. 1765

Figure 4. SECM approach curve for 0.4 mM MPC solution in DCE, when the aqueous phase contains 0.03 mM Fe(CN)64-. For other parameters see Figure 3. Solid line is the theory for pure negative feedback.20b

2-fold larger than ours (180 M-1 cm s-1).19 The finding that the ET reactions involving MPCs are faster than those between conventional redox species is qualitatively in line with predictions of Marcus theory.11 The standard bimolecular rate constant for ET across the liquid/liquid interface can then be evaluated as

( )

k0 ) k12 exp

2π(a1 + a2)∆R2kel1 kel2 ∆G) RT κν

(7)

where a1 and a2 are the radii of the two reactants, ∆R is an ET parameter ∼1 Å, k1el and k2el are the standard heterogeneous ET rate constants for the two redox couples at a metal electrode in water and organic solvent respectively, and κν is the product of the adiabaticity factor and the nuclear motion frequency. Thus, k12 should be proportional to the sum of two radii, a1 + a2 (depending on a specific model of the liquid/liquid interface, k12 may also be proportional to (a1 + a2)2; see eqs 9 and 12 in ref 11c). One can assume that heterogeneous rate constants of MPCs at a metal electrode are not much smaller than those of normal redox species (for example, in ref 10, such a rate constant was found to be >0.1 cm/s). Since the radius of an MPC is larger than that of a typical molecule, one can qualitatively expect higher k12 values for ET reactions involving MPCs. To investigate the effect of an unfavorable driving force on the rate of ET involving MPCs, we obtained SECM approach curves with Fe(CN)64-/3- as an aqueous redox mediator (Figure 4). The standard potential of hexacyanoferrate is much more negative than that of the IrCl63/2couple, and ∆E1/2 ) -0.15 V was measured for the Fe(CN)63-/MPC+ reaction. The rate of this ET, which is an uphill process, should be very slow. In agreement with this expectation, the approach curves obtained with different concentrations of Fe(CN)64- in the aqueous phase fit the theory for a pure negative feedback (Figure 4). The above findings for the MPC1+/IrCl62- reaction rate are at variance with a recent publication,10 in which the rates of ET reactions between organic-soluble MPCs and various 1766

aqueous redox species appeared to be too slow to measure by SECM. The main source of this discrepancy seems to be the lack of preequilibration of Au clusters with aqueous redox species, which resulted in the formation of a thick diffusion layer at the interface and precluded fitting of the experimental approach curves to the theory. We also note that the MPCs used in that study were less monodisperse and much larger in core size (and a correspondingly different pattern of ET charge state changes) than those used here. In summary, the bimolecular rate constant of the ET reaction between organic-soluble Au38 clusters and an aqueous redox species has been measured for the first time. In qualitative agreement with the prediction of Marcus theory, this process was found to be faster than similar ET reactions between conventional aqueous and organic redox species at the liquid/liquid interface. Further studies aimed at probing the driving force dependence of the ET rate and checking other aspects of the ET theory are underway in our laboratories. Acknowledgment. M.V.M. acknowledges support from the National Science Foundation (CHE-0315558) and PSCCUNY; R.W.M. acknowledges support by the Office of Naval Research, the National Science Foundation, and NASA (equipment). We thank Drs. Dongil Lee and Robert L. Donkers for supplying the pure Au38 sample. References (1) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (c) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (2) (a) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (b) Donkers R. L.; Lee, D.; Murray R. W. Langmuir 2004, 20, 1945. (c) Lee, D.; Donkers R. L.; DeSimone J. M.; Murray R. W. J. Am. Chem. Soc. 2003, 125, 1182. (d) Lee, D.; Donkers, R. L.; Wang, G.; Harper, A. S.; Murray, R. W. J. Am. Chem. Soc. 2004, 126, 6193. (e) Jimenez, V. L.; Georganopoulou, D. G.; White, R. J.; Harper A. M.; Mills, A.; Lee, D.; Murray, R. W. Anal. Chem. 2004, ASAP Article. (3) (a) Hicks, J. F.; Templeton, A. C.; Chen, S.; Sheran, K. M.; Jasti, R.; Murray, R. W.; Debord, J.; Schaaff, T. G.; Whetten, R. L. Anal. Chem. 1999, 71, 3703. (b) Hicks, J. F.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 13322. (4) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (5) (a) Hadley, P.; Mooij, J. E. Electronic Materials Series 2000, 6, 1. (b) Service, R. F. Science 2001, 293, 782. (c) Langford, S. J.; Raymo, F. M.; Stoddart, J. F. Mol. Electron. 1997, 325. (6) (a) Gomez-Lopez, M.; Stoddart, J. F. Handbook of Nanostructured Materials and Nanotechnology 2000, 5, 225. (b) Molecular Machines Special Issue, Acc. Chem. Res. 2001, 34. (7) (a) Vo-Dinh, T.; Cullum, B. M.; Stokes, D. L. Sens. Actuators B 2001, B74, 2. (b) Zamborini, F. P.; Leopold, M. C.; Hicks, J. F.; Kulesza, P. J.; Malik, M. A.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 8958. (c) Han, L.; Daniel, D. R.; Maye, M. M.; Zhong, C.-J. Anal. Chem. 2001, 73, 4441. (8) Quinn, B. M.; Prieto, I.; Haram, S. K.; Bard, A. J. J. Phys. Chem. 2001, 105, 7474. (9) (a) Zhang, J.; Lahtinen, R. M.; Kontturi, K.; Unwin, P. R.; Schiffrin, D. J. Chem. Commun. 2001, 1818. (b) Chen, S. J. Am. Chem. Soc. 2000, 122, 7420. (10) Quinn, B. M.; Liljeroth, P.; Ruiz, V.; Laaksonen, T.; Kontturi, K. J. Am. Chem. Soc. 2003, 125, 6644. (11) (a) Marcus, R. A. J. Phys. Chem. 1990, 94, 1050. (b) Marcus, R. A. J. Phys. Chem. 1990, 94, 4152. (c) Marcus, R. A. J. Phys. Chem. 1991, 95, 2010. Nano Lett., Vol. 4, No. 9, 2004

(12) Mirkin, M. V.; Tsionsky, M. In Scanning Electrochemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.; Marcel Dekker: New York, 2001; p 299. (13) Amemiya, S.; Ding, Z.; Zhou, J.; Bard, A. J. J. Electroanal. Chem. 2000, 483, 7. (14) The synthesis of Au38(PhC2S)24 has been reported elsewhere.2b Briefly, the modified Brust method involves a phase transfer of hydrochloroaurate prepared from 99.999% pure gold with tetra-noctylammonium bromide (Aldrich, 98%), into toluene (Fisher, reagent grade), followed by the treatment with a 3-fold molar excess (relative to Au) of phenylethanethiol (Aldrich, 99%). The gold(I)-thiol polymer formed is subsequently reduced with an excess of sodium borohydride (Aldrich, 99%). Successive fractionation and washing allowed the isolation and purification of Au38(PhC2S)24. The Au38(PhC2S)24 clusters thus prepared were further purified with silica column chromatography, leaving the average core charge in a +1 state. (15) Cyclic voltammetry and SECM measurements were carried out with a commercially available SECM instrument (CHI-900, CH Instruments). A two-electrode configuration was used with Ag wire and Ag/AgCl reference electrodes in the organic and aqueous phase,

Nano Lett., Vol. 4, No. 9, 2004

(16) (17)

(18) (19) (20)

respectively. The working electrode was a 12.5-µm-radius Pt disk prepared as described previously.20a Aqueous solutions were prepared using Barnstead NANOpure purified H2O (18.2 MΩ) and the organic solvent was dichloroethane (Aldrich, 99.9%). LiClO4 (Fisher, reagent grade) and Hx4NClO4 (Fisher, reagent grade) were used as received. The SECM procedures were described previously.17 All measurements were carried out at ambient room temperatures. Guo, R.; Donkers, R. L.; Brennan, J. L.; Murray, R. W. unpublished results, UNC-CH, 2003. (a) Tsionsky, M.; Bard, A. J.; Mirkin, M. V. J. Phys. Chem. 1996, 100, 17881. (b) Tsionsky, M.; Bard, A. J.; Mirkin, M. V. J. Am. Chem. Soc. 1997, 119, 10785. Barker, A. L.; Unwin, P. R.; Amemiya, S.; Zhou, J.; Bard, A. J. J. Phys. Chem. B 1999, 103, 7260. Zhang, J.; Barker, A. L.; Unwin, P. R. J. Electroanal. Chem. 2000, 483, 95. (a) Kwak, J.; Bard, A. J. Anal. Chem. 1989, 61, 1794. (b) Kwak, J.; Bard, A. J. Anal. Chem. 1989, 61, 1221.

NL049196H

1767