Langmuir 2006, 22, 10307-10314
10307
Scanning Electrochemical Microscopy Determination of Organic Soluble MPC Electron-Transfer Rates† Rachel R. Peterson and David E. Cliffel* Department of Chemistry, Vanderbilt UniVersity, VU Station B 351822, NashVille, Tennessee 37235-1822 ReceiVed April 29, 2006. In Final Form: August 4, 2006
In this paper, we describe a novel method for measuring the forward heterogeneous electron-transfer rate constant (kf) through the thiol monolayer of gold monolayer protected clusters (MPCs) in solution using scanning electrochemical microscopy (SECM). Applying the equations for mixed mass-transfer and electron-transfer processes, we develop a new formula using only the diffusion coefficient and the tip radius and use it as part of a new method for evaluating SECM approach curves. This method is applied to determine the electron-transfer rates from a series of SECM approach curves for monodisperse hexanethiol MPCs and for polydisperse hexanethiol, octanethiol, decanethiol, dodecanethiol, and 2-phenyethylthiol gold MPCs. Our results show that as the alkanethiol length increases the rate of electron transfer decreases in a manner consistent with the previously proposed tunneling mechanism for the electron transfer in MPCs. However, the effective tunneling coefficient, β, is found to be only 0.41 Å-1 for alkanethiol passivated MPCs compared to typical values of 1.1 Å-1 for alkanethiols as self-assembled monolayers on twodimensional gold substrates. Similar SECM approach curve results for Pt and Au MPCs indicate that the electrontransfer rate is dependent mostly on the composition of the thiol layer and not on differences in the core metal.
Introduction Nanoparticle materials have seen a recent surge in interest, academically and industrially due to the wide variety of possible applications resulting from their unique optical, electronic, and electrochemical properties. Monolayer-protected nanoclusters (MPCs) are nanoparticles which consist of a metallic core protected by a passivating thiol monolayer. MPCs were first synthesized by Brust et al.,1 who combined classical gold colloid synthesis with the ability of thiols to self-assemble on a gold surface. This core-shell structure makes them very stable, inhibits particle aggregation, and allows the functionality of the MPCs to be easily manipulated.2 MPCs can also be applied to surfaces and layered to form ordered three-dimensional superlattices which exhibit characteristic optical, electronic, and electrochemical properties.3-7 The ability of MPCs to store charge as nanoscale capacitors was originally demonstrated in Murray’s group by observing quantized double layer (QDL) charging peaks for monodisperse MPCs.8-11 QDL charging results from a single electron transfer †
Part of the Electrochemistry special issue. * To whom correspondence should be addressed. E-mail: d.cliffel@ vanderbilt.edu. (1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (2) 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.; Wingnall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30. (3) Chen, S.; Murray, R. W. J. Phys. Chem. B 1999, 103, 9996-10000. (4) Zamborini, F. P.; Hicks, J. F.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 4514-4515. (5) Chen, S. Langmuir 2001, 17, 6664-6668. (6) Hicks, J. F.; Seok-Shon, Y.; Murray, R. W. Langmuir 2002, 18, 22882294. (7) Chen, S.; Deng, F. Langmuir 2002, 18, 8942-8948. (8) 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, 37033711. (9) Green, S. J.; Stokes, J. J.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W. J. Phys. Chem. B 1997, 101, 2663-2668. (10) Chen, S.; Murray, R. W.; Feldberg, S. W. J. Phys. Chem. B 1998, 102, 9898-9907. (11) Hicks, J. F.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 13322-13328.
into or out of the MPC metallic core through the thiol shell. QDL peaks can be observed for monodisperse MPCs when this singleelectron transfer causes a change in the MPC potential (∆V ) e/C) greater than the ambient thermal voltage (kBT). Improvements in techniques for obtaining monodisperse MPCs have enhanced the ability to observe sequential QDL peaks, especially for hexanethiol gold MPCs.11,12 It is essential that the electron-transfer properties of the nanoparticles be understood before they can be applied in nanotechnology. The most successful methods used to measure the kinetics of electron-transfer in MPCs involve the measurement of the “self-exchange” electron hopping rates between individual clusters within a solution-cast film. The apparent electron hopping rates for films of alkanethiol Au MPCs on an interdigitated array (IDA) were found to be on the order of 108-1011 M-1 s-1.13 This report also investigated the conductivity of MPC films protected with various thiol chain lengths and showed that the conductivity decreased exponentially as the chain length increased indicating that the electron transfer through the thiol monolayer of MPC occurred via a simple through-space tunneling mechanism. The conductivity of MPC films cast on an IDA was also employed to measure the first-order electron hopping rate constants through arenethiolate (benzylthiolate, phenylethylthiolate, phenylbutanethiolate, and cresolthiolate) gold MPC films. These rate constants were found to range from 108-1011 s-1.14 Once again the shorter thiols exhibited faster rates. It was also observed that the arenethiolate MPCs had a slightly faster electron-transfer rate than the corresponding alkanethiol MPC with a passivating monolayer of similar length. Although this technique for measuring MPC electron-transfer rates is promising, it was found that the thickness of the nanoparticle film (∼10-15 µm) was much more than the IDA finger height (0.1 µm) meaning that the MPCs were not only positioned between but also above the (12) Quinn, B. M.; Liljeroth, P.; Ruiz, V.; Laaksonen, T.; Kontturi, K. J. Am. Chem. Soc. 2003, 125, 6644-6645. (13) Wuelfing, W. P.; Green, S. J.; Pietron, J. J.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 11465-11472. (14) Wuelfing, W. P.; Murray, R. W. J. Phys. Chem. B 2002, 106, 3139-3145.
10.1021/la061183r CCC: $33.50 © 2006 American Chemical Society Published on Web 09/12/2006
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fingers. It was not possible to determine the effect of the MPCs above the fingers had on the measured electron-transfer rates. MPC films as electrodes in electrolyte solution have also been used to study the kinetics of electron transfer. The first-order rate constant between MPCs, composed of mixed monolayers of hexanethiol and mercaptoundecanoic acid, in a multilayer film using a metal ion carboxylate linkage was investigated with potential step chronoamperometry and found to be on the order of 106 s-1.15 The conductivity of MPC films, constructed by combining the use of metal ion carboxylate linkers and IDA electrodes, was found to range from 10-7 to 10-4 Ω-1 cm-1.16 The report demonstrated that the conductivity of the MPC film decreased as the length of the nonlinking thiols increased and that the conductivity of the MPC film was not greatly influenced by the length of the linking carboxylic acid terminated thiols, but rather by the passivating thiol monolayer. The formation and structure of the MPC films used in the previous studies was difficult to control precisely, and once again, it was impossible to determine the effect of the MPCs oriented above the gold IDA fingers on the measured rates. Recently, the rate of electron hopping of MPC films, composed of MPCs with mixed monolayers of alkanethiol and mercaptoundecanoic acid or MPCs linked with dithiols, was found to be on the order of 105 s-1, slower than previously demonstrated, using steady-state rotated disk electrode (RDE) voltammetry.17 Faster electron-transfer rates were not observed for shorter thiols, and thinner films exhibited slower kinetics. Again, the formation of the MPC films did not provide well-defined structures; therefore, a simpler method of measuring MPC kinetics would be valuable. This paper presents a method to determine the electron-transfer characteristics of MPCs in solution using the scanning electrochemical microscope (SECM).18,19 Measurement in solution allows for a simpler experimental setup which is more easily controlled and avoids any possible effects of the surface film formation. The SECM is an electrochemical approach to scanning probe microscopy that consists of a bipotentiostat and a micropositioner. In SECM, a redox mediating species is electrochemically cycled between an ultramicroelectrode (UME) tip, typically a disk electrode with a diameter less than 25 µm, and a substrate electrode. The position of the UME is controlled by a micropositioner composed of three piezoelectric motors, which allow independent manipulation in the x, y, and z directions. Additionally, in SECM, measurements are made at steady state therefore eliminating the transient currents from nonfaradaic double layer charging of the tip and substrate electrodes present in many other analytical methods such as cyclic voltammetry and chronoamperometry, making SECM a very valuable tool for measuring electron-transfer properties.18,20 The SECM has been used extensively to investigate the kinetics of heterogeneous electron-transfer characteristics of several different systems including the liquid/liquid interface of two immiscible electrolyte solutions,21,22 monolayers,23 and sur(15) Hicks, J. F.; Zamborini, F. P.; Osisek, A. J.; Murray, R. W. J. Am. Chem. Soc. 2001, 123, 7048-7053. (16) Zamborini, F. P.; Leopold, M. C.; Hicks, J. F.; Kulesza, P. J.; Malik, M. A.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 8958-8964. (17) Brennan, J. L.; Branham, M. R.; Hicks, J. F.; Osisek, A. J.; Donkers, R. L.; Georganopoulou, D. G.; Murray, R. W. Anal. Chem. 2004, 76, 5611-5619. (18) Bard, A. J.; Fan, F.-R. F.; Mirkin, M. V. In Electroanalytical Chemistry: A Series of AdVances; Bard, A. J., Ed.; Marcel Dekker: New York, 1994; Vol. 18, pp 243-373. (19) Bard, A. J.; Mirkin, M. V., Eds. Scanning Electrochemical Microscopy; Marcel Dekker: New York, 2001. (20) Liu, B.; Bard, A. J.; Mirkin, M. V.; Creager, S. E. J. Am. Chem. Soc. 2004, 126, 1485-1492. (21) Liu, B.; Mirkin, M. V. J. Am. Chem. Soc. 1999, 121, 8352-8355. (22) Zhang, Z.; Yuan, Y.; Sun, P.; Guo, J.; Shao, Y.; Girault, H. H. J. Phys. Chem. B 2002, 106, 6713-6717.
Peterson and Cliffel
faces.24-26 The SECM has also been used to investigate the electron-transfer properties of MPCs at traditional metalelectrolyte and electrified liquid-liquid interfaces. Quinn et al. found that the electron-transfer rate, for the metal-electrolyte system, of monodisperse hexanethiol Au MPCs was very fast (k > 0.1 cm s-1) with no difference in the response for different MPC charge states using a 10 µm UME.27 In the liquid/liquid system, positively charged MPCs were expected to exhibit an increase in current upon SECM approach to a reducing electrolyte, in the aqueous phase, typical of an approach to a conductive substrate. Alternatively, the same was expected of negatively charged MPCs with an oxidizing electrolyte in the aqueous phase. The SECM approach in these systems exhibited an initial positive feedback current followed by a decrease when the tip was close to the interface and resulted in a remarkably slow liquid/liquid electron transfer is for hexanethiol MPCs (k < 10-4 cm s-1). It was rationalized that the slow heterogeneous rate is due to the large size and hydrophobicity of the MPCs, which resulted in a large separation between the MPCs and the aqueous electrolyte across the liquid/liquid interface. The apparent electron-transfer rate constant of 2-phenylethylthiol passivated MPCs across a liquid/liquid interface was found to be 76 M-1 cm s-1 using a commercial simulation package called FEMLAB.28 The lateral charge transport across a composite layer of polyelectrolyte/ MPCs has also been studied using SECM to obtain estimates of the lateral conductivity of a single MPC monolayer.29 MPCs typically have multiple oxidation states (>10) within a narrow potential range making use of analysis techniques which rely upon the use of overpotential for kinetic measurements unfeasible. As previously described,1,2 MPCs are polydisperse as prepared. The average diameter of MPCs is measured by transmission electron microscopy (TEM), whereas the ratio of metal to organic components is determined by thermogravimetric analysis (TGA). Due to difficulties in measuring the average diameter accurately, the exact concentration of even monodisperse MPCs cannot be reliably determined. Therefore, a method measuring the electron-transfer rate constants of MPCs that does not rely on knowledge of the exact concentration would be invaluable. Additionally, the development of a simple method to measure the rate constants of electron transfer for MPCs, which does not involve complicated models or multiple step analyses, would be a very useful tool to researchers. In this paper, the application of mixed mass- and kinetictransfer equations originally developed for rotating disk electrodes results in a novel equation to measure the rate of electron transfer through the thiol monolayer of MPCs using the feedback mode of SECM. This new equation is similar to conventional SECM analysis19 in that the only parameters required to fit the experimental data are the diffusion coefficient and the radius of the tip UME and results in only one fitted parameter, kf. Our SECM analysis expands on the previous study by Quinn and measures the electron-transfer rates for MPCs in solution in contrast to surface-confined methods. (23) Zhang, J.; Slevin, C. J.; Morton, C.; Scott, P.; Walton, D. J.; Unwin, P. R. J. Phys. Chem. B 2001, 105, 11120-11130. (24) Tsionsky, M.; Bard, A. J.; Dini, D.; Decker, F. Chem. Mater. 1998, 10, 2120-2126. (25) Pierce, D. T.; Unwin, P. R.; Bard, A. J. Anal. Chem. 1992, 64, 17951804. (26) Selzer, Y.; YTuryan, I.; Mandler, D. J. Phys. Chem. B 1999, 103, 15091517. (27) Quinn, B. M.; Liljeroth, P.; Kontturi, K. J. Am. Chem. Soc. 2002, 124, 12915-12921. (28) Georganopoulou, D. G.; Mirkin, M. V.; Murray, R. W. Nano Lett. 2004, 4, 1763-1767. (29) Ruiz, V.; Liljeroth, P.; Quinn, B. M.; Kontturi, K. Nano Lett. 2003, 3, 1459-1462.
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dimensionless distance parameter, L ) d/a, where d is the distance separating the electrodes, which follows
IT(L) )
il it,∞
)
Am(L) πam(L) ) 4Da 4D
(4)
when under mass-transfer control. This equation shows that IT is independent of the solution concentration, and the number of electrons transferred.18,31,33 The mass-transfer coefficient can be determined by rearranging eq 4 as follows
[4D πa ]
m(L) ) IT(L)
(5)
Substitution of eq 2 into eq 5 results in the following expression: Figure 1. Mass-transfer and kinetic transfer limiting processes. The diffusion of the species through the bulk solution to the electrode is the mass-transfer limited process while the movement of the electron from the electrode to species is the kinetically limited process.
Mass-Transfer Limited Electrochemistry. The electrochemical tip current is dependent upon many factors including mass-transfer and kinetic-transfer processes as shown for a SECM experiment in Figure 1. Mass transfer is defined as the diffusion of the species through the bulk solution to the electrode, whereas kinetic transfer is defined as the transfer between the electrode and the species at the electrode surface. If mass transfer is the limiting process, the current follows the equation
il ) nFAmC*
(1)
where il is the limiting current, F is Faraday’s constant, A is the area of the electrode, m is the mass-transfer coefficient of the redox species, and C* is the bulk concentration of the redox species.30 The mass-transfer coefficient for steady-state diffusion, mt,∞, to a disk UME far away from a surface follows
mt,∞ )
4D πa
(2)
where D is the diffusion coefficient and a is the radius of the UME. Substitution of mt,∞ for m shows that the steady-state current far away from a surface, it,∞, can be defined as
it,∞ ) 4nFDC*a
(3)
In the case of an insulating substrate, as the SECM tip approaches the surface of the substrate, the redox mediator is blocked from diffusing to the UME resulting in a decrease in the tip current, known as negative feedback. Alternatively, when a conducting substrate is approached, the redox mediator is regenerated at the substrate resulting in an increase in the tip current, known as positive feedback. The positive feedback current, il, follows eq 1 when the electrochemical current is limited by mass transfer.18,31-33 Typically, the current from a SECM approach curve is normalized through division by it,∞ to obtain the dimensionless experimental current parameter, IT. To evaluate the limiting factor in the approach curve currents, IT is plotted versus the (30) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001; pp 569-571. (31) Amemiya, S.; Ding, Z.; Zhou, J.; Bard, A. J. J. Electroanal. Chem. 2000, 483, 7-17. (32) Cliffel, D. E.; Bard, A. J. Anal. Chem. 1998, 70, 1993-1998. (33) Bard, A. J.; Fan, F.-R. F.; Kwak, J.; Lev, O. Anal. Chem. 1989, 61, 132-138.
m(L) ) IT(L) mt,∞
(6)
As shown in Equation 5, the mass-transfer coefficient is dependent upon the radius (a) of the disk UME. The use of smaller UME diameters will increase the mass-transfer coefficient allowing faster kinetics to be ascertained. If the UME is sufficiently small, the electrochemical current will not be mass-transfer-limited but kinetically limited.18,33 The experimental approach curves are then compared to the theoretical model of mass-transfer-limited processes for positive feedback as proposed by Bard and coworkers in the following equation:
IT(L) ) 0.68 +
+ 0.3315e (0.78377 L )
(-1.0672/L)
(see refs 18, 31, 33, and 34) (7) Systems whose approach curves follow this model exhibit electrochemical currents that are mass-transfer limited. If the approach curve deviates from the model, presenting a lower feedback current, then the electron transfer is kinetically limited and its electron-transfer rate constant can be measured. The combination of eqs 5 and 7 results in
[
m(L) ) 0.68 +
+ 0.3315e (0.78377 L )
(-1.0672/L)
(8) ](4D πa )
where the mass-transfer coefficient in eq 1 can be easily calculated as function of L, if the approach curve follows the mass-transfer theory curve. SECM Determination of Mixed Mass-Transfer and Kinetic Electron-Transfer Control. As stated previously, both mass transfer and kinetic transfer can affect the observed electrochemical current. The current, iK, observed for kinetically limited processes where no back reaction occurs, follows
iK ) nFAkfC* (see ref 18)
(9)
which is similar to the mass-transfer limited current shown in eq 1 with kf in place of m. In the case of mixed mass-transfer and the kinetic-transfer control on the electrochemical currents, the limiting equation for both processes can be combined using the following equation:
1 1 1 ) + (see refs 30 and 35) iexp il iK
(10)
where iexp refers to the experimental current observed. This
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equation was originally applied to the analysis of RDE experiments30 but can be shown to apply generally to any case when eq 9 applies (i.e., when no back reaction occurs).35 Equation 10 can be rearranged into
iexp )
iliK il + i K
(11)
Substituting eq 1 into this relationship for il and eq 9 for iK results in
iexp ) nFA
( )
mkf C* m + kf
(12)
This equation will be used to evaluate the approach curves which deviate from the mass-transfer theory, eq 7, to determine the kinetic rate constant. To include any kinetic limitation to the experimental tip current far away from any surface, it,∞, it can also be rewritten using the mixed control eq 10 resulting in
it,∞ ) nFA
(
)
mt,∞kf C* mt,∞ + kf
(13)
which accounts for both mixed mass-transfer and kinetic-transfer control. As previously explained, iexp, eq 12, is divided by it,∞, eq 13, to obtain dimensionless experimental current parameter resulting from both mass transfer and kinetic transfer, IT,mix, which follows
( )
mkf m + kf iexp ) IT,mix ) it,∞ mt,∞kf mt,∞ + kf
(
)
(14)
Substitution of eq 6, m ) IT(L) mt,∞, into eq 14 and rearrangement leads to
IT,mix )
(IT(L) (mt,∞ + kf)) (IT(L) mt,∞ + kf)
(15)
Further substitution of eq 2 results in
IT,mix )
IT(L)
+k) (4D πa f
4D I (L) + kf πa T
(16)
that has only one unknown variable, kf, which is the forward heterogeneous rate constant for the MPCs. To do this, the experimental MPC approach curves are compared to IT,mix and kf will be adjusted until the best fit is achieved, resulting in the forward heterogeneous rate constant for the MPC. In the case of MPCs, the QDL charging peaks allows the number of electrons transferred to be determined. However, many cluster preparations have not revealed any QDL charging peaks because they are too polydisperse or too large, and therefore, it is impossible to determine specific standard redox potentials. (34) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L.; Cullen, W. G.; First, P. N.; Gutierrez-Wing, C.; Ascensio, J.; Jose-Yacaman, M. J. J. Phys. Chem. B 1997, 101, 7885-7891. (35) Greef, R.; Peat, R.; Peter, L. M.; Pletcher, D.; Robinson, J. Instrumental Methods in Electrochemistry; Ellis Horwood Ltd.: Chichester, U.K., 1985.
Since the charging peaks cannot be observed, it is impossible to determine the number of electrons transferred at any applied potential. Additionally, although it is possible to determine an approximate MPC concentration, it is impossible to determine the exact MPC concentration. A few very large particles can skew the estimated concentration and therefore introduce error into the measurement of the electron-transfer rate. The use of eq 16 allows the forward heterogeneous rate constant to be determined for MPCs that exhibit no clearly defined oxidation and reduction QDL peaks. This eliminates the need for the standard redox potential, number of electrons transferred, and concentration making it a very easy method of measuring electrontransfer rates for MPCs. Experimental Procedure Reagents. Dodecanethiol (C12S-H, 98.5%+) and hexanethiol (C6S-H, 96%) were purchased from Acros; octanethiol (C8S-H, 98.5+%), decanethiol (C10S-H, 96%), and 2-phenylethyl thiol (Ph(CH2)2S-H, 98%) from Sigma; acetonitrile (MeCN, 99%), methylene chloride (CH2Cl2, ACS grade), sodium nitrate (NaNO3), and sodium borohydride (NaBH4, 98+%) from Fisher; and tetrabutylammonium hexafluorophosphate (TBAPF6, g99%) and tetraoctylammonium bromide (TOABr, g98%) were purchased from Fluka. Potassium hexafluorophosphate (KPF6, 99%) was purchased from Aldrich, hexane (HPLC grade) from Burdick and Jackson, ferrocenylmethyltrimethylammonium iodide (FcTMA+I-) from Strem Chemicals, and sulfuric acid (H2SO4, 95.0-98.0%) from EM Science. Tetrachloroauric acid (HAuCl4‚3H2O) was prepared according to the literature.36 Ferrocenylmethyltrimethylammonium hexafluorophosphate (FcTMA+PF6-) was prepared from (FcTMA+I-) according to the procedure given by Mirkin and co-workers using KPF6.37 Water was deionized using a Solution 2000 Water Purification system (g18 MΩ cm). All chemicals were used as purchased unless otherwise specified. Synthesis of Alkanethiol MPCs. All MPCs were synthesized following the procedure first given by Brust et al.1 Briefly, a 3:1 ratio of thiol to HAuCl4 and a 10:1 ratio of NaBH4 to HAuCl4 were used in the synthesis of the polydisperse MPCs. The MPC solution was held at 0 °C during the addition of NaBH4. Each prepared MPC reaction was stirred overnight before isolation. Acetone, ethanol, acetonitrile, and hexane were used to wash the MPCs in order to remove impurities. The hexanethiol MPCs were separated into more monodisperse samples via solvent fractionation. These nanoparticles were placed in acetone and allowed to sit for ∼1 h. The MPCs were then vacuum filtered using a glass frit. The MPCs which remained on the frit were completely insoluble in acetone indicating the nanoparticles were larger. This fraction was named cut C. MPC solution was allowed to evaporate under the vacuum after the initial filtration. The acetone-MPC solution was then slowly evaporated, and the particles that crashed out of solution first as partially insoluble in acetone were named cut B. The particles which remained in solution were named cut A and were highly soluble in acetone. Cut B, the most monodisperse fraction as determined by proton NMR, MALDI mass spectrometry, and square wave voltammetry (Figure 2), was used in these studies as the monodisperse hexanethiol MPC. SECM Probe Approach Curves. An SECM (CHI900, CH Instruments) was used to acquire the approach curves. The electrochemical cell consisted of a 5 or 10 µm UME Pt (99.9%, Goodfellow, Huntingdon, England) working electrode (WE) fabricated according to Bard et al.,18 a 2 mm Pt substrate electrode (CHI303), a Ag/AgCl, 3M KCl aqueous (CHI111) or Ag/Ag+ nonaqueous reference electrode (RE) (CHI112), and a Pt wire (VWR) counter electrode (CE), unless otherwise specified. The UME and (36) Brauer, G. Handbook of PreparatiVe Inorganic Chemistry, 2nd ed.; Academic Press: New York, 1963. (37) Forouzan, F.; Bard, A. J.; Mirkin, M. V. Israel J. Chem. 1997, 37, 155163.
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Figure 2. Square wave voltammogram of hexanethiol Cut B MPCs which exhibits both positive and negative charging of the nanoparticle. the substrate electrode were polished by using successively 240, 400, 600, and 1200 grit Carbimet Paper Disks and a solution of 0.1 and 0.05 µm Micropolish II with 8′′ Microcloth on a Metaserv 2000 Grinder/Polisher (Buehler). Each electrode was examined for defects using an Olympus BX41 optical microscope. The electrodes were electrochemically cleaned using 0.5 M sulfuric acid before they were employed in the SECM studies. Test approach curves were conducted on each of the constructed and polished UMEs using a solution of 1 mM FcTMA+PF6- in aqueous 0.1 M KCl. Each of the electrodes achieved at least 800% positive feedback current that fit mass-transfer theory before they were used to analyze the MPC solutions. MPC solutions were prepared by dissolving 20 mg of the MPC sample in 5.0 mL of 0.10 M TBAPF6 in CH2Cl2. During the approach curves of the MPC solutions, the UME was biased between -0.9 and +1.0 V vs Ag/Ag+ while the substrate electrode was left at the open potential of the solution or biased from 0 to +1.0 V vs Ag/Ag+. The tip electrode approached the substrate electrode at a step size of at most 0.5 µm with a withdrawal distance of at least 20 µm and a quiet time of 100 s or more. The absolute zero distance position was established during the approach curve by the slow and gentle touching of the tip into the substrate; however, not all tipsubstrate touches resulted in high currents from direct contact, as some contacts were made by the glass sheath around the UME tip. Biased vs Unbiased Substrate. In this research, SECM approach curves were employed to investigate the electron-transfer characteristics of alkanethiol MPCs. In order obtain the simplified form on the right side of eq 4, it is essential that the number of electrons, n, either be known for the tip during approach, n1, and far away, n2, or be equal and cancel. To illustrate this, n1 and n2 can be substituted into the left side of eq 4 as follows:
Figure 3. SECM approach curves of hexanethiol MPCs with a 10 µm UME at various potentials with the substrate electrode held at 0 V. The samples consisted of 20 mg of MPC in 5 mL of 0.1 M TBAPF6 in CH2Cl2. chloride depending on size and composition.38 The diffusion layer thickness was compared to 20 µm because this was the minimum distance the UME was backed away from the substrate between approach curves. Before each approach curve a quiet time of at least 100 s was employed in which the UME and substrate electrode are both held at the potentials used during the approach curve to allow the redox states of the MPCs to equilibrate within the diffusion layer thickness of 245 µm according to eq 18. The concentration profile within the diffusion layer can be found using the complementary error function, erfc[x/2(Dt)1/2], and for x ) 20 µm, most (∼95%) of the MPCs within the tip travel distance will be equilibrated to the substrate potential. Therefore, the quiet time was sufficient to allow the MPCs to be biased by the substrate electrode within the tip travel distance, and the number of electrons transferred is the same whether the UME is far from the substrate or the UME is close to the substrate (n1 ) n2). Consequently, the approach curves completed with the substrate electrode biased and unbiased are equally acceptable in canceling n out of eq 17.
Results and Discussion
(18)
Electron-Transfer Studies for Monodisperse Hexanethiol Au MPCs. A typical set of approach curves, IT vs L, obtained using hexanethiol MPCs with a 10 µm Pt UME is shown in Figure 3. The substrate potential was held at 0.0 V while the potential applied to the Pt tip was varied in different approaches from 200 to 600 mV. These approach curves follow the masstransfer limited model, IT(L), which indicates that the electrochemical current is mass-transfer limited at the applied tip potentials.18 The mass-transfer coefficient was determined to be 0.083 cm/s using eq 6 and the observed UME maximum current, IT, of 9. Because the hexanethiol MPCs positive feedback current was shown to be mass-transfer limited, the kinetic rate of the electron transfer was at least as fast as the mass-transfer coefficient (kf g 0.083 cm/s). This observation is in complete agreement with earlier hexanethiol Au MPC SECM observations by Quinn (kf > 0.1 cm/s).27 As shown in eq 2, a reduction in the UME size will create a faster mass-transfer coefficient; consequently, a smaller UME with an increased mass-transfer coefficient is required to further investigate the forward heterogeneous rate constant, kf, of the hexanethiol MPCs. SECM approach curves of hexanethiol MPCs with a 5 µm Pt UME shown in Figure 4 exhibited a deviation from mass-transfer theory. Although the current reached an IT as high as 8 before
where D is approximately 3 × 10-6 cm2/s. The diffusion coefficients for alkanethiol MPCs range from 2 to 4 x 10-6 cm2/s in methylene
(38) Wuelfing, W. P.; Templeton, A. C.; Hicks, J. F.; Murray, R. W. Anal. Chem. 1999, 71, 4069-4974.
IT(L) )
il n1Am(L) n1πam(L) ) ) it,∞ 4n2Da 4n2D
(17)
The MPCs in solution have a resting charge state before the experiment begins. The use of a biased substrate will cause local charging of the MPCs to the substrate potential resulting in a biased state rather than their resting state. During an approach curve, the MPC is redox-cycled between the tip and substrate, which causes the nanoparticle to be charged to the substrate potential transferring n1 electrons. Conversely, when the tip is far from the substrate, the MPC can then diffuse through the solution and return to the native charge state of the MPC solution transferring n2 electrons. The use of an unbiased substrate electrode guarantees that the number of electrons transferred at the substrate is the same as the number of electrons transferred far away, therefore ensuring that the two variables can be canceled (n1 ) n2). For biased substrates, the diffusion layer thickness, δ, which follows δ ) x2Dt
10312 Langmuir, Vol. 22, No. 25, 2006
Figure 4. SECM approach curves of hexanethiol MPCs with a 5 µm UME at various positive potentials with an unbiased substrate electrode, Ag/Ag+ nonaqueous reference electrode, and a Pt wire counter electrode. The samples comprised of 20 mg of MPC in 5 mL of 0.1 M TBAPF6/CH2Cl2.
touching, the slope did not follow the mass-transfer theory,18 and therefore, the electrochemical current was kinetically limited. The approach curve conducted with the UME at 100 mV resulted in no positive feedback because there was not enough of a potential difference between the tip and substrate to result in the significant charging of the nanoparticle. At all other tip potentials (0.2-0.7 V), positive feedback was observed, and the amount of positive feedback was remarkably similar for each approach. The similar positive feedback independent of tip potential results from a combination of the lack of significant overpotentials because of the closely spaced standard potentials for the MPCs and the normalization of the number of electrons transferred in the dimensionless current. The forward heterogeneous rate constant, kf, was found by fitting of the IT,mix curve. The rate of the electron transfer through the hexanethiol monolayer was found to be 0.11 ( 0.03 cm/s by averaging the values for tip potentials from Figure 4. It was observed that the hexanethiol MPC showed a positive feedback in current for both positive and negative potentials (not shown) indicating that the hexanethiol MPC can be both positively and negatively charged which is consistent with the previously observed charging peaks from square wave voltammetry for the hexanethiol MPCs (Figure 2). Larger hexanethiol Au MPCs (cut C) showed similar positive feedback approach currents as measured by IT despite the fact they did not show QDL charging peaks. SECM Approach Curves for Polydisperse Alkanethiol MPCs. A 10 µm Pt UME was used to acquire SECM approach curves of octanethiol, decanethiol, and dodecanethiol MPCs with polydisperse sizes. Typical approach curves, IT vs L, for each of the alkanethiol MPCs are presented in Figure 5 along with the mass-transfer limited theory curve. The approach curves of the alkanethiol MPCs exhibited currents that were substantially lower than the mass-transfer limited model; therefore, the electrochemical current was kinetically limited even at a wide range of applied tip potentials similar to the hexanethiol Au MPCs. Each experimental approach curve was then compared to a IT,mix curve from eq 16 to determine the kinetic rate constant, kf, for each approach. The calculated curves closely match the actual data until the tip approached the substrate electrode to a distance of ∼100 nm ensuring a good approximation of kf (presented in Figure 5). It was found that the kf decreased as the alkanethiol chain length increased; that is, the octanethiol MPCs with a chain length of 1.02 nm had a kf of 0.024 ( 0.003 cm/s, the decanethiol MPCs with a chain length of 1.27 nm had a kf of 0.011 ( 0.002 cm/s, and the dodecanethiol MPCs with a chain
Peterson and Cliffel
Figure 5. Typical SECM approach curves of octanethiol, decanethiol, dodecanethiol, and 2-phenylethylthiol Au MPCs. 10 µm Pt UME at 1 V, a 2 mm Pt substrate electrode at 0 V, Ag/Ag+ nonaqueous reference electrode, and a Pt wire counter electrode. The samples consisted of 20 mg of sample in 5 mL of 0.1 M TBAPF6 in CH2Cl2. Table 1. Comparison of Thiol Ligand Length and Electron-Transfer Rate kf for the Organic Solule MPCs Showing the Decrease in Electorn-Transfer Rate with an Increase in Thiol Lengtha MPC
ligand length (nm)
kf rate (cm/s)
hexanethiol 2-phenylethylthiol octanethiol decanethiol dodecanethiol
0.77 0.68 1.02 1.27 1.52
0.11 ( 0.026 0.035 ( 0.001 0.024 ( 0.004 0.011 ( 0.002 0.0048 ( 0.0008
a n > 7 measurements averaged. Ligand lengths obtained from Hyperchem and were previously reported in ref 8 for the alkanethiols and in ref 14 for the 2-phenylethylthiol.
length of 1.52 nm had a kf of 0.0048 ( 0.0008 cm/s (Table 1). This indicates that the longer thiol chains insulated the metallic core better than the shorter thiol chains, therefore retarding the electron transfer. This correlates with previous studies that demonstrated a decrease in the electron-transfer rate with increasing thiol lengths indicating a through-space tunneling mechanism for the electron transfer.13,14 The decrease in electrontransfer rate for longer protecting thiol molecules correlates well with a recent SECM study which demonstrated that the rate of electron transfer through a monolayer of pentadecanethiol on a two-dimensional surface was 3.7 × 10-4 and 1.0 × 10-3 cm/s using two different electrochemical mediators.20 Although twodimensional SAMs typically have better packed monolayers (more rigid packing leading to less pinholes), they are comparable to three-dimensional monolayers on MPCs. The correlation of the magnitudes of kinetic rate constants between the two- and threedimensional surfaces suggests that the values found in this study are reasonable. Additionally, the use of different mediators had a large impact on the electron-transfer rate observed for the twodimensional surface. The use of the SECM to determine the electron-transfer properties of MPCs with the nanoparticles acting as the redox mediators in this study eliminates the any possible errors resulting from an external mediator. An SECM approach curve of 2-phenylethylthiol MPCs is shown in Figure 5. As shown previously, it was expected that conjugated thiol MPC would exhibit a faster electron-transfer rate than the alkanethiol MPCs because the probability of electron tunneling is proportional to exp(-βx).30 Creager et al. have shown that β is smaller for conjugated molecules than for aliphatic molecules.39 The hexanethiol molecule is similar in length to that of the 2-phenylethylthiol, a partially conjugated thiol;
Organic Soluble MPC Electron-Transfer Rates
Langmuir, Vol. 22, No. 25, 2006 10313
Figure 6. Alkanethiol Pt MPC approach curves under identical conditions as Figure 5.
therefore, it was expected that the 2-phenylethylthiol MPC would have a rate of electron transfer at least as fast as the hexanethiol MPC. The electron-transfer rate for the 2-phenylethylthiol MPCs was found to be 0.035 ( 0.001 cm/s which is slower than the rate of 0.11 ( 0.026 cm/s for the hexanethiol MPCs (Table 1) and only slightly faster than the electron-transfer rate through octanethiol MPCs which is 0.024 ( 0.004 cm/s. This indicates that the phenyl ring did not enhance the electron transfer but in fact slowed the rate of electron transfer. The slower electrontransfer rate of the 2-phenylethylthiol may result from the difference in the orientation of this thiol on the MPC surface. It has been shown by Laibinis et al. that alkanethiols typically orient with a tilt angle of 26-28° from the surface normal of a gold substrate on a two-dimensional SAM.40 It follows that the alkanethiols would orient similarly on the three-dimensional surface of the MPC. Although there have been no reports as to the orientation of 2-phenylethyl thiol on a two- or threedimensional surface, the role of the molecular backbone was studied by comparing the orientation of 4-mercaptobenzoic acid and 4-methyl-4-mercaptobiphenyl which are similar in length. It was found that the phenyl thiol tilt angle had an upper limit of 19° which is smaller than the tilt angle of alkanethiols. The smaller tilt angle resulted because the phenyl molecules do not need to tilt to the same degree as alkanethiols to maximize their van der Waals interactions.41 A smaller tilt angle would result in a slower electron-transfer rate for the 2-phenylethylthiol MPCs than for the hexanethiol MPCs of a similar length because of the longer effective distance required for electron transfer. The effect of aromatic groups on electron transfer warrants more investigation via additional phenyl terminated MPCs to include completely conjugated thiols. Our recent results42 on the synthesis of platinum nanoclusters have shown that hexanethiol (-SC6) and dodecanethiol (-SC12) functionalized platinum particles can be made by a one-phase synthesis, in which the platinum is reduced by lithium triethethylborohydride (LiTEBH, “super hydride”) in dry tetrahydrofuran (THF). All of these platinum clusters have been characterized by NMR, TGA, and UV spectroscopy and are polydisperse as made. In general, these Pt clusters show remarkably similar electron-transfer results to their gold MPC equivalents in the SECM as shown in Figure 6, which strongly suggests that the composition of the organothiolate ligand is the controlling factor in the electron-transfer kinetics. While QDL charging has not yet been shown for alkanethiol Pt MPCs, the (39) Creager, S.; Yu, C. J.; Bamdad, C.; O’Connor, S.; MacLean, T.; Lam, E.; Cong, Y.; Olsen, G. T.; Luo, J.; Cozin, M.; Kayyem, J. F. J. Am. Chem. Soc. 1999, 121, 1059-1064. (40) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (41) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151-256. (42) Eklund, S. E.; Cliffel, D. E. Langmuir 2004, 20, 6012-6018.
Figure 7. Plot of ln kf versus estimated chain length of the organic soluble MPCs (Table 1) to determine the probability of tunneling, β.
SECM approach curve in Figure 6 does confirm that these particles can store charge in the same manner as the Au MPCs. Tunneling Charge Transfer. As the kinetic rate constants seem to be governed by the length of the alkanethiol and are relatively independent of the tip overpotential applied for reasons stated above, we considered a simple model of electron throughspace tunneling through thiols between two metal surfaces, albeit where one of the metal surfaces is the freely diffusing MPC core. The probability of an electron tunneling follows
probability of tunneling ∝ e(-βx)
(19)
where x is the distance of the electron transfer and β (Å-1) is a factor which depends on the height of the energy barrier and the nature of the medium between the states (a larger β results in a slower electron-transfer rate).30 It has also been shown that the β typically ranges from 1 to 1.2 Å-1 for saturated chains and 0.4 to 0.6 Å-1 for π-conjugated molecules.30 The electron is transferred via tunneling if the conductivity decreases exponentially with an increase in distance, which is typically the chain length.13,14,16 Therefore, the heterogeneous rate constant can be used to evaluate β as shown in Figure 7. For the organic soluble MPCs, β was found to be 0.41 ( 0.05 Å-1 which is a considerably smaller value than that shown previously for SAMs of saturated molecules. This suggests that the calculated chain length, x, may not reflect an accurate electron-transfer distance for the monolayer on the MPCs. The MPC core is a threedimensional surface allowing the thiols to be more flexible and possibly less dense than on two-dimensional SAMs. This could bring about a higher tilt angle from the surface normal of the substrate resulting in a smaller distance between the metal surfaces than for two-dimensional experiments. This smaller actual distance gives rise to a higher tunneling probability for the electron transfer. Thus, the smaller tunneling probability, β, for MPCs likely results from the inaccurate use of chain length as the electron-transfer distance.
Conclusions A novel method for measuring the electron-transfer rates through the thiol monolayer of MPCs was developed. The alkanethiol MPCs showed positive feedback curves allowing their electron-transfer properties to be investigated. The hexanethiol, octanethiol, decanethiol, dodecanethiol, and 2-phenylethylthiol MPCs showed kinetically limited approach curves, and consequently their forward heterogeneous rate constant, kf, was determined. The partially conjugated 2-phenylethylthiol
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MPCs did not result in an expected increase in the electrontransfer rate compared to hexanethiol MPCs, possibly due to an increase in the actual tunneling distance of the electron. It was shown that as the alkanethiol length increases the rate of electron transfer decreases which is consistent with the tunneling mechanism for the electron transfer proposed for MPCs previously. The electron-transfer rate did not increase with an increased electrode potential as might be expected from the Butler-Volmer model of kinetics because even monodisperse large MPCs are known to have multiple redox states closely spaced in potential.
Peterson and Cliffel
The exponent of tunneling, β, for the alkanethiol MPCs was found to be 0.41 ( 0.05 Å-1 which is less than shown previously for alkanethiol molecules on SAMs. This difference suggests that the thiol molecules may not present the same electron-transfer distance for MPCs as for SAMs. Acknowledgment. We thank the Vanderbilt Institute for Nanoscale Science and Engineering and a GAANN fellowship (R.R.P.) for the partial financial support of this work. We also thank Dr. Sven Eklund for providing the Pt MPC samples. LA061183R
