Characterization of Charge Storage in Redox ... - ACS Publications

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Characterization of Charge Storage in Redox-Active Self-Assembled Monolayers Kristian M. Roth,† Jonathan S. Lindsey,‡ David F. Bocian,*,† and Werner G. Kuhr*,† Department of Chemistry, University of California, Riverside, California 92521-0403, and Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 Received January 11, 2002. In Final Form: February 21, 2002 Self-assembled monolayers (SAMs) of thiol-derivatized alkyl ferrocenes and Zn tetraarylporphyrins on a gold surface have been characterized under open circuit conditions using fluorescence imaging microscopy and a variety of electrochemical methods. The fluorescence of the neutral form of the Zn tetraarylporphyrin SAM and the electrode cell potential are monitored simultaneously after oxidation of the SAM and subsequent establishment of an open circuit condition. These experiments capitalize on the fact that the neutral Zn tetraarylporphyrin SAM is fluorescent whereas emission from the oxidized Zn tetraarylporphyrin SAM is quenched. Accordingly, the fluorescence recovery after disconnection from an oxidizing potential provides an independent measure of the change in oxidation state of the porphyrin SAM as a function of time after the cell is open-circuited. This measurement demonstrates that the porphyrin SAM remains oxidized for an extended period (hundreds of seconds) even though the electrochemical cell potential decays rapidly to the open circuit potential (OCP). Cyclic voltammetry was used to confirm the electrochemical properties of the redox SAMs and to determine the surface coverage of the redox-active molecules. A new method, designated open circuit potential amperometry, was used to read the charge of the oxidized SAM at the OCP after charging currents have decayed away. Additionally, open circuit potential voltammetry yields a sigmoidal response with a characteristic half-wave potential identical to that observed with cyclic voltammetry, indicative of a Nernstian process. Collectively, these studies show that the electrochemical cell discharges to the OCP quickly, that the SAM remains oxidized under these conditions, and that these methods can be used to quantitatively determine the amount of stored charge in the SAM.

Introduction There is an increased interest in the utilization of selfassembled monolayers (SAMs) on electrode surfaces for many applications. In particular, arrays of SAM-coated electrodes have been proposed for uses ranging from DNA biosensors1-4 to molecular-based information storage elements.5,6 In all of these applications, selective and rapid quantitation of a redox species bound to the surface is required. For example, in recently developed chip formats for electrochemical DNA detection, large arrays of electronically active “pads” (Au microelectrodes) coated with self-assembled monolayers of specific DNA capture probes must be rapidly and efficiently interrogated.1,2 Typically, interrogation of these pads is implemented via the measurement of an amperometric signal on a rather slow time scale (tens of seconds). More recently, our groups have shown that SAMs of electroactive molecules attached to Au exhibit the requisite properties to serve as molecularbased information storage elements.5 The electroactive molecules are porphyrins; information is stored in the oxidation states of the molecule. Direct electrical com† ‡

University of California. North Carolina State University.

(1) Meade, T. J.; Kayyem, J. F. Angew. Chem., Int. Ed. Engl. 1995, 34, 352-354. (2) Creager, S.; Yu, C. J.; Bamdad, C.; O’Connor, S.; MacLean, T.; Lam, E.; Chong, Y.; Olsen, G. T.; Luo, J.; Gozin, M.; Kayyem, J. F. J. Am. Chem. Soc. 1999, 121, 1059-1064. (3) Shamansky, L. M.; Davis, C. B.; Stuart, J. K.; Kuhr, W. G. Talanta 2001, 55, 909-918. (4) Kuhr, W. G. Nat. Biotechnol. 2000, 18, 1042-1043. (5) Roth, K. M.; Dontha, N.; Dabke, R. B.; Gryko, D. T.; Clausen, C.; Lindsey, J. S.; Bocian, D. F.; Kuhr, W. G. J. Vac. Sci. Technol. B 2000, 18, 2359-2364. (6) Gryko, D.; Li, J.; Diers, J. R.; Roth, K. M.; Bocian, D. F.; Kuhr, W. G.; Lindsey, J. S. J. Mater. Chem. 2001, 11, 1162-1180.

munication with the porphyrins is accomplished by attaching the molecules to Au using a thiol linker via a self-assembly process.7,8 Upon oxidation, the porphyrin SAMs have been shown to retain charge for an extended period (tens of minutes) after disconnection of the counter electrode.5 In this case, it is extremely important to be able to measure this faradaic signal in the absence of interference from background currents. These background currents may arise either from charging the electrochemical double layer or through redox reactions with solutionbased species. In the work described herein, several methods are used to characterize the redox state of oxidized SAMs under a variety of conditions. Initially, the fluorescence characteristics of the Zn tetraarylporphyrin SAMs are used to independently assess the temporal discrimination of the faradaic process from charging current decay. These experiments capitalize on the fact that the neutral form of the Zn tetraarylporphyrin SAM is fluorescent whereas emission from the oxidized form of the Zn tetraarylporphyrin SAM is quenched (owing to the intrinsically short excited-state lifetime of a porphyrin π-cation radical (∼30 ps)9 compared with that of the neutral species (∼2 ns).10 This characteristic of the porphyrin molecule affords the possibility of independently monitoring the oxidation state of the molecule and the potential between the working (7) Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. J. Am. Chem. Soc. 1995, 117, 9529-9534. (8) Gryko, D. T.; Clausen, C.; Lindsey, J. S. J. Org. Chem. 1999, 64, 8635-8637. (9) Barley, M.; Dolphin, D.; James, B. R.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1984, 106, 3937-3943. (10) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 3, pp 1-165.

10.1021/la025525e CCC: $22.00 © 2002 American Chemical Society Published on Web 04/17/2002

Characterization of Charge Storage in SAMs

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Figure 1. Schematic diagram of a high-bandwidth current monitor. A two-electrode current monitor was constructed with a high-bandwidth current amplifier using two high-bandwidth operational amplifiers, where R1 ) 2 KΩ, R2 ) 50 Ω, and R3 ) 1 KΩ. The potential is applied though a voltage follower connected to the counter electrode via a fast-switching mercury-wetted relay. The electrochemical cell can be open-circuited at the counter electrode using this switch (rise time of 500 ns). See Experimental Section for additional details.

and counter electrodes after initial oxidation and subsequent disconnection from the applied potential. This approach requires that (1) attachment of the porphyrin to the Au electrode does not quench all emission from the neutral species and (2) connection of the voltage-monitoring device does not inject significant charge (thereby reducing the oxidized molecules) during the course of the measurement. It will be shown that the Zn tetraarylporphyrin SAM satisfies both of these constraints and that the porphyrin SAM remains oxidized for an extended period (hundreds of seconds) even though the electrochemical cell potential decays rapidly to the open circuit potential (OCP). In information storage applications, it is important not only to affirm the presence of stored charge in the SAM but also to measure the time scale of charge retention. A variety of electrochemical methods have been used to study the redox properties of electroactive molecules immobilized on electrode surfaces.11-17 Unfortunately, none of these methods completely discriminates faradaic current from background current on a fast time scale (microseconds or less). Therefore, it was necessary to develop a new strategy to dissociate the measurement of charge associated with redox species in SAMs bound to an electrode surface from charging currents. This method, designated open circuit potential amperometry (OCPA), reads the charge of the oxidized SAM at the OCP after charging currents have decayed away. Voltammetric data can also be obtained using this technique. In this method, designated open circuit potential voltammetry (OCPV), a series of OCPA steps is performed in which the potential is successively incremented (similar to pulse voltammetric methods). The efficacies of both OCPA and OCPV are demonstrated using SAMs of two types of thiol-derivatized, redox-active molecules attached to Au, (1) an alkylferrocene and (2) a Zn tetraarylporphyrin. Collectively, these studies show that OCPA and OCPV permit the accurate measurement of the charge associated with the surface-bound redox species with very high signal quality on the microsecond time scale. (11) Creager, S. E.; Wooster, T. T. Anal. Chem. 1998, 70, 42574263. (12) Weber, K.; Creager, S. E. Anal. Chem. 1994, 66, 3164-3172. (13) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5453-5461. (14) Forster, R. J. Analyst (Cambridge, U.K.) 1996, 121, 733-741. (15) Forster, R. J.; Keyes, T. E.; Majda, M. J. Phys. Chem. B 2000, 104, 4425-4432. (16) Kertesz, V.; Chambers, J. Q.; Mullenix, A. N. Electrochim. Acta 1999, 45, 1095-1104. (17) Palecek, E.; Tomschik, M.; Stankova, V.; Havran, L. Electroanalysis 1997, 9, 990-997.

Experimental Section High-Bandwidth Current Monitor. A current monitor was constructed with a conventional high-bandwidth current amplifier using two high-bandwidth operational amplifiers (Figure 1). The first stage is a current-feedback inverting amplifier (BurrBrown OPA644) and the second stage is an inverting voltagefeedback amplifier (Analog-Devices AD744), which together provide an overall amplification of roughly 40 000. The frequency response of this amplifier is flat to 1 MHz. The potential control of the electrochemical cell is quite different from that found in a traditional potentiostat. In particular, the potential is applied through a voltage follower (Burr-Brown OPA602) that is connected to the counter electrode via a fast-switching mercurywetted relay (Aleph International Corp.). Unless otherwise noted, the counter electrode can be connected to the applied potential or open-circuited using a single-pole single-throw (SPST) switch (rise time of 500 ns). In another configuration, a double-pole single-throw (DPST) is substituted for the SPST switch and can connect the counter electrode either to the applied potential or to ground through a known resistance. The applied potentials are generated in LabVIEW (National Instruments, Austin, TX) and applied through the voltage follower. The current is monitored after amplification through the current amplifier and digitized at a rate of 5 MHz (PCI-6110E A/D//D/A, National Instruments, Austin, TX). Chemicals and Materials. Two different Zn tetraarylporphyrins each bearing three mesityl groups and one S-acetylthioderivatized linker were synthesized as previously described.18 The two porphyrins differ only in the structure of the linker: PM1, 1-(AcSCH2)-4-phenylene; PM3, 1-[AcS(CH2)3]-4-phenylene. The majority of the experiments were conducted with PM1, whereas PM3 was used only for selected studies (vide infra). An alkylferrocene with an AcS-(CH2)12 linker (C12Fc) was synthesized following the approach outlined by Chidsey et al. for an homologous compound.19 The S-acetyl protecting group has been previously shown to undergo facile cleavage upon exposure to Au.7,8 The structures of C12Fc, PM1, and PM3 are shown in Figure 2. Two different electrolytes were used for electrochemical measurements. Electrolyte A consisted of dried, distilled CH2Cl2 containing 0.1 M Bu4NPF6; electrolyte B consisted of dry ethanol containing 1 M Bu4NClO4. Poly(dimethylsiloxane) (PDMS) elastomer was purchased as Dow Corning Sylgard 184 (K. R. Anderson Co.), which consists of two components: a base and a curing agent. Gold Ball Electrode. Au ball working electrodes were prepared from 5 µm diameter Au wire sealed in soft glass.2 Initially, a ∼500 µm segment of the wire was protruding from the end of 1 mm i.d. soft glass tubing. Upon exposure to a flame, the glass forms a tight seal around the Au. The wire exposed to the flame melts into a ball that terminates at the surface of the (18) Gryko, D. T.; Clausen, C.; Roth, K. M.; Dontha, N.; Bocian, D. F.; Kuhr, W. G.; Lindsey, J. S. J. Org. Chem. 2000, 65, 7345-7355. (19) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301-4306.

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Figure 2. Structures and voltammetry of C12Fc and PM1/ PM3. The molecules were synthesized as the S-acetylthio derivatives. The S-acetyl protecting group undergoes facile cleavage upon exposure to Au (see text). The SAMs were formed on a 10 µm diameter Au ball microelectrode. The cyclic voltammograms shown are for the C12Fc SAM and the PM1 SAM. The cyclic voltammograms for PM1 are shown for the same electrode before (solid line) and after (dashed line) the OCPA experiment. The voltammogram of PM3 (not shown) is identical to that for PM1. Conditions: scan rate, 100 V/s; electrolyte A; potentials vs Ag/Ag+. glass. The electrode was then cooled in a stream of nitrogen and used immediately. The ball was found to have an average diameter of 10 µm. The SAMs were formed by placing the electrode in a 2 mg/mL solution of C12Fc, PM1, or PM3 for 20 min and sonicating for an additional 1 min. The electrode was then removed from the sample solution and rinsed with distilled CH2Cl2. The electrochemical cell consisted of the Au ball working electrode and a Ag wire counter electrode (0.5 mm diameter, 1 cm length) immersed in electrolyte A. Band Electrodes. The band electrodes consisted of a Au working electrode and a Ag counter electrode occupying the same plane on a borosilicate glass microscope slide (Fisher, Fair Lawn, NJ). The electrodes were prepared by vapor deposition of 99.99% Au (1 oz Canadian Maple Leaf, obtained locally) or 99.999% Ag (1 oz US Constitution coin, obtained locally) in a CHA SE-6004 four-pocket E-beam evaporator (CHA Industries, Menlo Park, CA). The microscope slides were first cleaned by boiling in piranha solution (30% H2O2 solution, 70% H2SO4) for 20 min, rinsed with deionized water, and dried in a vacuum oven at 100 °C. A shadow mask was used to allow deposition of 2000 Å of Au (utilizing a 100 Å Cr underlayer to promote adhesion) on either of the following: (A) four Au microband electrodes per slide, with each electrode band being 75 microns wide with a contact pad at the edge of the glass slide; or (B) a single Au macroband electrode with a single 2 mm wide band of gold. In both cases, a complementary interdigitated shadow mask was used to allow deposition of a 2 mm wide strip of Ag (2000 Å depth, utilizing a 100 Å Cr underlayer to promote adhesion) onto the microscope slide to form the counter electrode. The electrochemical cell was defined by placing a patterned 2 mm thick sheet of polymerized PDMS to frame a ∼10 mm2 area encompassing both the working and counter electrodes. PDMS adheres well to glass surfaces and prevents leakage of solution, thereby defining the area of electrode that will be exposed to electrolyte solution. This defines either (A) a 0.04 mm2 gold microband electrode or (B) a 2 mm2 gold macroband electrode and a 5-10 mm2 Ag counter electrode. In either case, the well was filled with a 2 mg/mL solution of PM1 or PM3 in dried,

Roth et al. distilled CH2Cl2. After 5 min, this solution along with the well was removed from the glass slide. The slide was copiously rinsed with dried, distilled CH2Cl2 and dried in a stream of N2. A fresh PDMS well was placed around the working and counter electrodes, filled with electrolyte B, and covered with a coverslip. Fluorescence Imaging. The Au band electrode (vide supra) was placed under the 20× objective of an epifluorescence microscope (Zeiss Axioscope; Thornwood, NY) equipped with a 100 W Hg arc lamp for epi-illumination. All images were collected and saved on a personal computer (PC) in a darkened room with a cooled Princeton Instruments MicroMax 800-PB charge-coupled device (CCD) camera system (EEV 800 × 1000 back-illuminated CCD and ST-133 controller using Winview software). Image data processing was performed with Spyglass Transform 2D software (Spyglass Software, Champaign, IL). The surface was brought into focus with light from the Hg arc lamp after 100-fold attenuation (two neutral density filters at 10% transmittance) with a CCD acquisition time of 5 ms for focusing/visible images. Fluorescence images were obtained by passing light from a Hg arc lamp through an excitation band filter (494 nm) and collecting all fluorescence at wavelengths longer than 526 nm. All fluorescence images were acquired for 3 s in the central zone of the CCD. Fluorescence Recovery of Porphyrin SAM after Oxidation. A PM1 or PM3 SAM prepared on a fresh Au band electrode was imaged under the 20× objective of a Zeiss inverted geometry microscope as described above. The working electrode was immersed in electrolyte solution in a two-electrode configuration, and the potential to the counter electrode was controlled using the home-built potentiostat. Initially, an oxidizing potential (EW) was created at the working electrode as described above (because a current follower is used to measure amperometric current, the working electrode is maintained at virtual ground). The switch was opened after 1 s to disconnect the counter electrode, thereby opening the electrochemical circuit. The initial fluorescence intensity of the neutral SAM was recorded at this time (as soon as the electrochemical cell was open-circuited). The fluorescence intensity was monitored during open circuit conditions at 60 s intervals until the initial fluorescence intensity was reobtained. The average intensity of each image was plotted versus disconnect time. These fluorescence data were fit to an exponential function, and a half-life for the fluorescence recovery time was extracted. Simultaneous Measurement of Porphyrin SAM Fluorescence and Cell Potential under Open Circuit Conditions. A high input impedance (20 GΩ) pH meter (Denver Instruments Co., Denver, CO) was used as a high impedance voltmeter and isolated from the potentiostat ground using an isolated alternating current (ac) power supply (Newark, Chicago, IL). Initially, an oxidizing potential (EW) was applied for 1 s at the working electrode via application of a negative voltage at the counter electrode. Next, an open circuit was created by throwing a DPST switch, placed between the counter electrode and the voltage input, to disconnect the counter electrode from the voltage source and simultaneously connect it to the input of the isolated voltmeter. In this arrangement, the voltmeter measures the potential drop across the electrochemical cell and not a potential referenced to ground. The potential of the cell and the average fluorescence intensity (as indicated above) were recorded simultaneously at various times after the counter electrode was open-circuited. This process was repeated until the average fluorescence intensity had recovered to that measured for the neutral SAM. Voltammetric Characterization of Redox-Active SAMs. Cyclic voltammetry was performed to establish the extent of surface coverage on the Au ball of each molecule used to create a SAM. The voltammograms for the C12Fc and PM1 SAMs are shown in Figure 2. The E1/2 values of all species in the SAM were measured at 100 V s-1 and recorded versus Ag/Ag+ in dried, distilled CH2Cl2 containing 0.1 M Bu4NPF6. The backgroundsubtracted voltammetric peaks were digitally integrated to calculate the total charge observed under each wave in each system studied. These integrated charges were used to calculate the surface coverage of each molecule in the SAM. Determination of the Open Circuit Potential.20,21 The procedure we used to determine the OCP of the electrochemical cell is schematically illustrated in Figure 3 (panels A and B).


Figure 3. Determination of the OCP. (A) The OCP is determined by incrementing the potential applied to the counter electrode, disconnecting (dashed-dotted line) the potential for a time period (τ1 ) 15 s), reconnecting the circuit (dotted line), and monitoring the resulting current. (B) Current is monitored after reconnection and not during the subsequent potential step. At any potential other than the OCP, either a positive or negative current spike is observed as the electrochemical cell is charged to the applied potential. As EW approaches the OCP, the current observed upon reconnection diminishes. The current nulls at the OCP. (C) Empirical determination of the OCP of the C12Fc SAM. Each of the current transients shown represents the charging current observed upon reconnection of the electrochemical cell. Only charging (and not faradaic) current is observed because the potential steps are in a regime that is substantially less than the E1/2 of the SAM. The OCP, as defined here, is the potential at which no current flows when no faradaic processes are occurring. Panel A shows the pulse sequence; panel B shows the current response. The OCP of each redox-active SAM on a 10 µm diameter Au ball microelectrode was determined empirically in a series of experiments performed as follows: (1) The cell potential was initially poised at an arbitrary value (EW1) where no faradaic current is expected by applying a potential to the Ag counter electrode. (2) The circuit was opened at the counter electrode for a sufficient time (τ1) to discharge the electrodes (but not the charged SAM) in the electrochemical cell. In these experiments, τ1 was at least 3 s. (3) The same potential (EW1) was then applied to the counter electrode, the circuit was closed, and the resulting current was monitored. If the applied potential was not the OCP, current flowed to create an electric field identical to that generated externally. This process was repeated at a series of different potentials (EW2, EW3, ...) to determine the potential at which no current flows. This latter potential is by definition the OCP (e.g., near EW3 in Figure 3A). The results shown in Figure 3C are described in a later section. (20) Conway, B. E.; Bai, L.; Tessier, D. F. J. Electroanal. Chem. Interfacial Electrochem. 1984, 161, 39-49. (21) Short, D. L.; Shell, G. S. G. J. Phys. E 1985, 18, 79-87.

Langmuir, Vol. 18, No. 10, 2002 4033 Open Circuit Potential Amperometry. The OCPA method is schematically illustrated in Figure 4A. Figure 4A (top panel) shows the pulse sequence; Figure 4A (bottom panel) shows the current response. Initially, an oxidizing potential (EW) is created at the working electrode via application of a negative potential at the counter electrode (because a current follower is used to measure amperometric current, the working electrode is maintained at virtual ground). A switch is then opened to disconnect the counter electrode, thereby opening the electrochemical circuit for a specified time (τ1). After the potential of the electrochemical cell decays to the OCP, the switch is closed and the circuit is reconnected to an applied potential equal to the OCP of the reduced SAM (ER, determined above). Any molecules in the SAM that have remained oxidized are immediately reduced, and the resulting faradaic current is recorded. Determination of Open Circuit Charge Retention. The magnitude of charge retention in the oxidized SAM can be determined as a function of disconnect time with the OCPA method above. This procedure is illustrated schematically in Figure 4A. Charge retention was measured by successively changing the disconnect time (τ1, τ2, ...), which is the interval between the oxidation of the SAM and the subsequent reduction of the oxidized SAM at the OCP.5 The current observed is directly proportional to the fraction of the molecules in the SAM that are retained in the oxidized state while the electrode is disconnected from the applied potential. The OCP was redetermined between successive OCPA measurements and found to remain within 5-10 mV of the originally determined value. Cyclic voltammograms were also recorded after successive OCPA measurements. The E1/2 values for the SAMs were found to be essentially identical to those initially determined (see Figure 2). Together, these experiments affirm that the potential of the counter electrode is stable during the OCPA experiments. Open Circuit Potential Voltammetry. The OCPV method is schematically illustrated in Figure 4B. Figure 4B (top panel) shows the pulse sequence; Figure 4B (bottom panel) shows the current response. OCPV was implemented by applying a series of OCPA “pulses,” where the initial step potential (EW1; in this case, an oxidizing potential) was incremented in each pulse, similar to other pulse voltammetric methods.22 To obtain voltammetric information, the potential of the initial (oxidizing) pulse (EW1, EW2, ...) was varied, but the current was always measured at the OCP (ER). The disconnect time (τ1) was kept constant, while each successive oxidation step was incremented by the same potential step (∆E ) EW2 - EW1). In these experiments, each initial oxidizing step was applied to the counter electrode for 20 ms and followed by a disconnect time (τ1) of 3 or 15 s. Once reconnected at ER, the instantaneous current was monitored at the OCP for 100 µs in each step. For a better signal to noise ratio (S/N), the current observed in each step was integrated to yield the total charge (Q, coulombs), which was normalized to the electrode area (A, cm2) to yield the charge density (σ, µC/cm2) for ease of comparison between systems at different electrodes.

Results and Discussion Previously, we have shown that charge can be “written” to a porphyrin SAM via application of a short duration voltage pulse (set at an oxidizing potential) using chronoamperometry.5 Retention of the stored charge in the porphyrin SAM was demonstrated with the approach described above by changing the time interval between the “write cycle” (oxidation of the porphyrin) and the “read cycle” (reduction of the oxidized porphyrin forming the neutral species). It was shown that the monocation radical state of the porphyrins in the SAMs persists for hundreds of seconds after the counter electrode is disconnected.5 The stability of a cation radical in the SAM during open circuit conditions can be influenced by a number of factors, including the stability of the molecule, the potential of the electrode surface reached under open circuit condi(22) Osteryoung, J.; Osteryoung, R. Anal. Chem. Symp. Ser. 1986, 25, 3-12.

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Figure 4. Procedure for OCPA and OCPV. (A) OCPA is performed by creating an oxidizing potential (EW) at the working electrode via application of a negative potential at the counter electrode (during this process, the working electrode is maintained at ground). A switch is then opened to disconnect the counter electrode (dotted line), thereby opening the electrochemical circuit for a specified time (τ1 ) 15 s). After the potential of the electrochemical cell decays to the OCP, the switch is closed and the circuit is reconnected to an externally applied potential equal to the OCP of the reduced SAM (ER, determined above), and the reductive current is monitored. The magnitude of the resulting current is directly proportional to the number of molecules that remain oxidized while the electrode is disconnected from the applied potential. In OCPA, only the disconnect time (τ) is varied, while EW and ER are constant. (B) OCPV is performed by first creating the oxidizing potential at the working electrode (EW1) and then incrementing this potential by a step (∆E). To obtain voltammetric data, the potential of the initial (oxidizing) pulse (EW1, EW2, ...) is varied, but the current is always measured at the OCP (ER). Each successive oxidation step is incremented by the same potential step (∆E ) EW2 - EW1). The current transients are only a result of reconnecting the counter electrode at ER and not from the potential step ∆E. In OCPV, only EW is varied, while the disconnect time (τ1) and ER are kept constant.

tions, and any barrier to electron transfer from the SAM to the electrode. The loss in charge does not stem from decomposition, as the redox process in the SAM can be cycled thousands of times without loss of signal (vide infra). Therefore, it seems most likely that the charge in the oxidized SAM is lost to recombination with electrons in the working electrode. A number of factors should affect charge recombination in such a system, including the distance of the cation radical from the surface, the molecular structure of the tether, and the stability of the cation radical. (Note that Zn porphyrin cations are relatively stable because the charge is delocalized over an extensive π-electron system within the molecule.) Simultaneous Measurement of Fluorescence Recovery and Potential Decay to the OCP. The neutral Zn tetraarylporphyrins are fluorescent in solution, where, in contrast, essentially all emission is quenched from the π-cation radicals.9,10 Accordingly, the oxidation state of the porphyrin can easily be assessed by monitoring its fluorescence. This characteristic of the porphyrin molecule affords the possibility of independently monitoring the oxidation state of the molecule and the potential between the working and counter electrodes after initial oxidation and subsequent disconnection from the applied potential. This approach requires that (1) attachment of the porphyrin to the Au electrode does not quench all emission from the neutral species and (2) connection of the voltagemonitoring device does not inject significant charge (thereby reducing the oxidized molecules) during the course of the measurement. We address each of these issues below.

We first investigated the fluorescence characteristics of the PM1 SAM. The neutral SAM did exhibit sufficient emission such that the fluorescence signal could be imaged using fluorescence microscopy with a cooled CCD camera. This is illustrated in Figure 5 (panel A) which shows the fluorescence intensity of a neutral PM1 SAM on a Au microband electrode immersed in electrolyte B. Using an integration time of 3 s, this signal is >500 counts above background (obtained from a bare Au electrode immersed in electrolyte B). The working electrode was then poised at 850 mV for 1 s to oxidize the PM1 SAM and then opencircuited. Upon oxidation of the PM1 SAM, the observed fluorescence signal immediately decreased to essentially that of the background. A series of fluorescence images were taken as a function of time after opening the circuit. As time progressed, the fluorescence intensity of the SAM recovered due to reduction of the porphyrin cation to the neutral porphyrin (Figure 5, panels C-F). Images were taken at 60 s intervals until the average fluorescence intensity reached the original value of the neutral SAM. The fluorescence recovery followed first-order kinetics with t1/2 ) 138 ( 7 s. An OCPA experiment was then performed on this same electrode, and the t1/2 value was measured to be 111 ( 8 s. These experiments provide independent evidence that the fluorescence and OCPA measurements are both monitoring the oxidation state of the PM1 SAM. Also, the charge-retention t1/2 value determined using the Au microband electrode and electrolyte B is very similar to that measured using the Au ball electrode and electrolyte A (t1/2 ∼167 s, see Table 1).5


Langmuir, Vol. 18, No. 10, 2002 4035

Table 1. Charge-Retention Characteristics of the C12Fc SAM and the PM1 SAM on an Au Ball Electrode redox process

σCVa µC cm-2

σ0b (τ1 ) 0 s) µC cm-2

σ10 (τ1 ) 10 s) µC cm-2

σ100 (τ1 ) 100 s) µC cm-2

t1/2b s

σDLc (CA) µC cm-2

σBKGDd (OCPA) µC cm-2

C12Fc+ f C12Fc PM1+ f PM1 PM12+ f PM1

6.5 3.7 7.1

6.7 4.2 6.8

5.6 4.1 6.7

0.6 2.8 4.1

31 167 254

28.6 19.5 e

∼0 0.1 e

a σ , the integrated charge density obtained from cyclic voltammetry, was obtained by digitally integrating the current under each peak CV obtained with cyclic voltammetry at 100 V s-1 and normalizing to electrode area (see Figure 2). b σ0, the charge density at t ) 0 s, and t1/2, the charge-retention half-life, were determined by fitting the decay in the observed charge density (σ(τ), 25 µs integration time) vs disconnect time (τ1) to a first-order rate law (ref 5). c σDL (CA), the integrated charging current obtained from chronoamperometry, was obtained by digitally integrating the current under each transient from 0 to 300 mV and then normalizing to electrode area (160 µs integration time; see dotted lines, Figure 7A,B). d σBKGD (OCPA), the integrated background current obtained in OCPA, was obtained by digitally integrating the current under each transient when each SAM was in the reduced state prior to the OCPA measurement and then normalizing to electrode area (25 µs integration time; see dotted lines, Figure 7C,D). e Not measured.

Figure 5. Fluorescence images of the PM1 SAM during OPCA. The neutral Zn tetraarylporphyrin is fluorescent, where, in contrast, essentially all emission is quenched from the π-cation radical (refs 9 and 10). Accordingly, the oxidation state of the porphyrin can easily be assessed by monitoring the fluorescence. Fluorescence images were taken at a microband Au electrode covered with a PM1 SAM (A) before application of a 1 s, 850 mV oxidizing potential and (B) 2, (C) 390, (D) 510, (E) 630, and (F) 870 s after disconnection of the oxidizing potential and the creation of an open circuit. The fluorescence recovery followed first-order kinetics with t1/2 ) 138 ( 7 s. An OCPA experiment (see Figure 8) was then performed on this same PM1 SAM, and the t1/2 value was measured (111 ( 8 s).

The experiment used above was modified to monitor both the charge retention and the cell potential independently. The cell potential (the potential drop across the working and counter electrodes) was monitored with a high input impedance voltmeter during the disconnect phase. Initial studies performed using PM1 on the Au microband electrode showed a significant bias current through the voltmeter (roughly 50 pA, large enough to discharge a microelectrode in seconds). This compromises the charge-retention measurement because the bias current produced by the voltmeter reduces the oxidized SAM. Therefore, a much larger Au macroband electrode was used, with roughly 1000 times larger surface area, to dramatically increase the amount of charge stored on the surface, which therefore minimizes the effect of this bias current. The Au macroband electrode also had a proportionately larger double-layer capacitance, increasing the RC time constant of the cell and lengthening the time to equilibrate to the OCP after the potential is disconnected. To minimize this type of perturbation, the measurements were made on the PM3 SAM rather than the PM1 SAM. We have previously shown that the PM3 SAM has a significantly longer charge-retention time than the PM1 SAM (885 vs 167 s, respectively).5 Initially, the fluorescence characteristics of the PM3 SAM were investigated on the Au macroband electrode without the voltmeter connected. The charge-retention

Figure 6. Measurement of PM3 SAM fluorescence intensity and open circuit potential at the same electrode during OCPA. A PM3 SAM on a macroband Au electrode was oxidized at 1 V vs Ag/Ag+ for 1 s, and then a high impedance voltmeter (20 GΩ input impedance) was connected across the electrochemical cell. The OCP of the cell was measured as a function of disconnect time, shown here as the difference from the OCP of 500 mV. The time course of the decay of the cell voltage (triangles) is shown (left axis), which fits an exponential decay (as expected for a RC phenomenon) as plotted (solid line, t1/2 ) 7 ( 0.7 s). At the same time, fluorescence images were taken (see Figure 5) at various times immediately following the disconnection of the applied potential. The fluorescence intensity of each image was averaged, and the average fluorescence intensity (squares, right axis) is plotted as a function of time after creation of the open circuit. The t1/2 value for fluorescence recovery is 424 ( 62 s (solid line). As shown, the fluorescence recovery (indicative of the reduction of the π-cation radical) is almost 2 orders of magnitude slower than the relaxation of the cell potential.

half-life of the PM3 SAM measured using the fluorescence data was 595 ( 87 s. This value is consistent with the t1/2 value of ∼885 s that we previously determined using OCPA for the PM3 SAM with a different electrode/electrolyte system. The fluorescence recovery was then measured with the voltmeter in the circuit. In this experiment, the oxidizing potential (1 V) was applied to the cell for 1 s; then the counter electrode was switched to the isolated input of the voltmeter. Initially, the measured cell voltage was near 1 V and this voltage quickly decayed to the OCP, which was determined to be ∼500 mV under these conditions. The time course of the decay of the cell voltage is shown in Figure 6 (squares). The decay is exponential (as expected for an RC phenomenon) with t1/2 ) 7 ( 0.7 s. The recovery of fluorescence intensity is also a firstorder process with t1/2 ) 424 ( 62 s (Figure 6, triangles). This decay process is almost 2 orders of magnitude slower than the relaxation of the cell potential. These data independently demonstrate that the SAM remains oxi-

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dized long after the potential across the cell has decayed to the OCP. The charge-retention t1/2 value for PM3 measured with the voltmeter in the circuit is somewhat shorter than that measured in the absence of the meter (∼424 versus ∼595 s). This is due to the fact that the pH meter has finite input impedance; therefore, it injects charge into the electrode/SAM. Using the input impedance of the pH meter (Rin ) 20 GΩ), the total amount of charge injected over the time course of decay to the OCP is calculated to be 2.65 × 10-8 C (which represents the integrated bias current, i ) V/Rin, where V is the measured voltage and Rin is defined above). Using the electrochemically measured area of a molecule in the PM3 SAM (300 Å2) and the geometric area of the electrode (2 mm2) and assuming 100% oxidation of the SAM, Q0 of the PM3 SAM is calculated to be 1.06 × 10-7 C. These values indicate that the voltmeter will inject sufficient charge to reduce ∼25% of the oxidized molecules during the time course of the experiment. This amount of charge, while of minimal impact in this measurement, would be sufficient to completely discharge a microelectrode SAM in a matter of seconds. Therefore, this effect could only be observed at a large electrode area. Amperometric Measurement of Charge Storage. Rapid, efficient electrochemical measurement of charge associated with redox species in SAMs requires the minimization of background currents for optimal signal quality. When large, dense arrays of electrochemical elements must be interrogated, two compounding problems are manifested. (1) Higher densities of elements require that each element becomes smaller (ultimately, to micron dimensions); hence, the signal amplitude (proportional to area) decreases. (2) As the number of elements increases, the time per measurement must decrease proportionately, which necessitates that the bandwidth of the measurement increases. The combination of these two effects leads to a situation where less signal must be measured with a wider bandwidth, leading to severe S/N problems. A variety of approaches have been previously utilized to overcome S/N problems associated with the detection of dense arrays of electrochemical elements. These approaches include modulation techniques and chronoamperometric methods. We first discuss the merits and shortcomings of these methods and then describe the properties of the new OCP methods reported herein and discuss the application of these methods to interrogate the SAMs of C12Fc, PM1, and PM3. Electrochemical Methods for the Measurement of the Redox Properties of SAMs. Modulation Techniques. Modulation techniques have been used very effectively in a number of circumstances to improve signal quality from surface-immobilized redox species.2 For example, Creager et al. have utilized ac voltammetry to study the kinetics of monolayers of redox-active molecules tethered to electrode surfaces via alkane thiols11 or molecular-scale wires.2 In ac voltammetry, a potential ramp is applied to the electrode (typically 10-50 mV/s), a small amplitude sine wave (always less than 50 mV, usually 10 mV)23 is superimposed onto the linear ramp, and the signal is measured at the fundamental or second harmonic frequency using a lock-in amplifier. Small amplitude modulations are exclusively used to ensure that the faradaic response remains essentially linear.23 Because the potential is modulated at 100-1000 times the fundamental frequency of the ramp, modulation frequencies (23) Eccles, G. N. Crit. Rev. Anal. Chem. 1991, 22, 345-380.

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are typically in the tens to hundreds of hertz. Thus, the slope of the linear ramp determines the scan rate; the time of analysis typically varies between 2 and 200 s. While ac voltammetry is extremely useful in the determination of the kinetic parameters for redox processes of surface-immobilized species, there are practical limitations to the analytical implementation of the approach. In particular, this technique does not completely discriminate against background currents under conditions of low surface coverage or when fast measurements are required (e.g., fast scan rates).2 Chronoamperometry. Chronoamperometry has also been utilized to rapidly interrogate the redox properties of surface-bound molecules.14-16,24 Pulse methods are extremely effective in discriminating the faradaic current arising from redox processes of diffusing species from the charging current in the time domain because charging currents decay much more rapidly than the diffusionlimited faradaic current (exp(-t/RC) vs t-1/2, respectively). However, it is much more difficult to dissociate the current that arises from redox processes in a SAM from the charging current at the electrode interface. This is especially problematic when the redox kinetics are faster than the RC time constant of the electrochemical cell, because both faradaic and charging currents will decay with the same time constant. The above-noted problems in the chronoamperometry of the SAMs of C12Fc and PM1 are illustrated in Figure 7, which shows the transient current observed when a 300 mV potential step is applied to the C12Fc SAM (Figure 7A) and PM1 SAM (Figure 7B). The observed signal contains both the faradaic current (solid line) and a charging current (dotted line). Integration of only the charging current under each chronoamperogram indicates that a great deal of charge is produced whenever a potential step (300 mV as shown) is applied to the electrode (Table 1). This would create a tremendous problem in the coulometric measurement of the charge in the SAM, in that this measured charge contains a significant charging current. As can be seen in both cases, the faradaic current resulting from oxidation of the SAM has the same time course as the charging current. This result indicates that the rate of electron transfer for C12Fc and PM1 is much faster than the RC time constant of the electrochemical cell.15 In such cases, the instantaneous faradaic current arising from the redox process cannot be easily discriminated from the instantaneous charging current that arises from the capacitive properties of the electrode. This situation becomes particularly problematic when the electrode has less than 100% monolayer coverage or when the redox-active species is diluted with a non-redox-active material. Quantitation of Surface-Immobilized Species. Coulometry is perhaps one of the oldest and most straightforward techniques available for the quantitation of redox species. Faraday’s law {Q ) nFN, where Q is charge (coulombs), n is the number of electrons in each reaction, F is Faraday’s constant (96 485 C/mol), and N is the number of redox molecules (moles)} is the fundamental relationship between the measured faradaic charge (corresponding to the oxidation or reduction of the redox species) and the number of molecules consumed in the redox process. The only assumption in this relationship is that the measured charge does not contain a significant background charge. Thus, it is essential to isolate the faradaic current from any background current to get the (24) Sevilla, J. M.; Pineda, T.; Madueno, R.; Roman, A. J.; Blazquez, M. J. Electroanal. Chem. 1998, 442, 107-112.


Figure 7. Chronoamperometry and OCPA of the C12Fc SAM and PM1 SAM. All experiments were performed on a 10 µm diameter Au ball in electrolyte A, using a two-electrode 1 MHz bandwidth current monitor. Chronoamperometry: Standard chronoamperometry was performed at (A) the C12Fc SAM from 0 to -300 mV (dashed line) and from 300 to 0 mV vs Ag/Ag+ (solid line) and (B) the PM1 SAM from 300 to 0 mV vs Ag/Ag+ (dashed line) and from 800 to 500 mV vs Ag/Ag+ (solid line). The dashed traces in the two panels show the current observed when the potential is stepped in a range in which the SAM is not redox active (i.e., where no faradaic current flows); the solid line represents a step which contains both faradaic and charging currents. OCPA: The same SAMs on the same electrodes were examined with OCPA. (C) The C12Fc SAM, oxidized at EW ) +300 mV vs Ag/Ag+ for 20 ms prior to disconnection. The reductive current was measured after reconnection of the counter electrode at the OCP (ER ) -400 mV vs Ag/Ag+, determined as shown in Figure 3) after a disconnect time (τ1) of 100 s. (D) The PM1 SAM, oxidized at EW ) +800 mV vs Ag/Ag+ for 20 ms prior to disconnection. The reductive current was measured after reconnection of the counter electrode at the OCP (ER ) +125 mV vs Ag/Ag+) with a disconnect time (τ1) of 100 s. In panels C and D, the null traces show the current observed when the SAM is in a neutral state (i.e., no faradaic current flows); the intense traces represent steps that contain primarily faradaic current.

best quantitative information. One way to accomplish this goal is to discharge the charging current prior to the measurement of the faradaic charge associated with a redox-active SAM. Measurement of the stored charge in a SAM in such an electrochemical cell requires that (1) the OCP is welldefined, can be readily determined, and is at an analytically useful (i.e., in this case, reducing) potential; (2) the molecules in the SAM remain oxidized under open circuit conditions; and (3) the molecules that remain oxidized in the SAM are readily reduced upon reconnection of the circuit. As will be shown below, the C12Fc SAM and the PM1 SAM satisfy these conditions. We have already demonstrated that oxidized porphyrin SAMs formed on Au microelectrodes retain charge for an extended period (tens of minutes) after disconnection of the counter electrode in a two-electrode cell.5 Once the circuit is opened, any charging current arising from the application of an oxidizing potential dissipates and the cell potential discharges to an OCP dictated by the composition of the electrochemical cell. If the cell is reconnected to an external potential that corresponds to the OCP of the reduced SAM, any current that is measured upon reconnection should

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reflect only that current which is derived from a redox process that occurred within the cell. Determination of the OCP. As previously noted, the OCP is the potential at which no current flows through the external circuit of an electrochemical cell.20,21 The empirical determination of the OCP of a C12Fc SAM is shown in Figure 3 (panel C). Each of the current transients shown represents the current observed upon reconnection of the electrochemical cell after a τ1 of 15 s. This measurement is possible due to the “imperfect” nature of the disconnected circuit; inevitably, there is an electrical path for current to flow to ground even if the circuit is open. No faradaic current is observed in this case because the potential steps were performed in a potential regime that is substantially more negative than the E1/2 of the SAM. Therefore, the SAM is in a neutral state throughout the OCP determination. The OCP for the C12Fc SAM was determined to be -400 mV versus Ag/Ag+, because the current transient at that potential is virtually flat (Figure 3, panel C). The PM1 SAM determined under identical solution conditions gave an OCP of +125 mV versus Ag/Ag+, which is quite different from that of the C12Fc SAM. The value of the OCP for each monolayer is reproducible (within 10-20 mV) between electrodes as long as the preparation of the SAM and solution conditions remain the same. These measurements indicate that (1) the OCP is dependent upon the composition of the SAM and (2) the electrochemical cell can be connected and disconnected at the OCP after this initial discharge at any time without additional flow of current. OCPA of the C12Fc SAM and PM1 SAM. The OCPA method is qualitatively similar to conventional chronoamperometry; however, these methods differ in that the measurement of the charge associated with the oxidized SAM is temporally dissociated from the charging current. Temporal dissociation of these currents is accomplished by allowing the electrochemical cell to discharge to the OCP prior to the amperometric measurement. This occurs due to the imperfect nature of the disconnected circuit. As will be shown below, even an “open” circuit, with very high impedance (>100 GΩ), passes enough current (picoamperes) to discharge an electrochemical cell containing a microelectrode to its OCP within seconds. Once the OCP has been reached, current is measured upon reconnection of the circuit to a potential equal to that of the OCP. This current arises primarily from the reduction of any molecules that are still oxidized in the SAM (Figure 4A). In other words, after oxidation of the SAM, the circuit is left open for a sufficient time (disconnect time, τ1) to allow the charging current associated with the oxidation step to dissipate. Because the circuit is open, an electrode/ electrolyte interface is produced in which the oxidized SAM is stable under open circuit conditions. Reconnection of the circuit with an applied potential equal to the OCP (ER) reestablishes the electric field at the OCP without the need to inject any charging current into the cell (Figure 3A). Upon reconnection of the circuit, the oxidized species in the SAM are immediately reduced, producing a faradaic current (Figure 3B). We evaluated the effect of the magnitude of the open circuit resistance on the dissipation of charging current by modifying this experiment slightly. Rather than disconnecting the counter electrode from the applied potential completely, a DPDT switch was used to connect the counter electrode to ground through a known resistance. This resistor provided a well-defined current path to ground during τ1. When a 2 GΩ resistor was placed between ground and the counter electrode, the time required to reach OCP decreased by 73% compared with

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the time required under “open” circuit conditions. Use of a 1 GΩ resistance yielded a still shorter time to reach OCP (decreased by 83%). In both cases, the OCPA method still measured a substantial amount of stored charge at long times, albeit diminished in magnitude. If the resistance was less than 10 MΩ, the decay to OCP was too fast to measure with this experimental arrangement and no current was measured. These data indicate that charging current is leaking from the counter electrode to ground once the cell is open-circuited. A large faradaic current is measured in OCPA of oxidized SAMs of C12Fc and PM1 on Au microelectrodes, as shown in Figure 7 (solid lines, panels C and D, respectively). As shown in the dotted lines in Figure 7, only a very small current is observed if either the C12Fc SAM or PM1 SAM is in the reduced (i.e., neutral) form prior to the OCP measurement (under identical conditions at the same electrode), indicating that reconnection of the circuit at the OCP of the reduced moiety does not induce a significant background current (including charging current). As we have previously shown, the nature of the redox species and the linker both influence the stability of the oxidized species under open circuit conditions.5 Clearly, for molecules that have relatively short charge-retention halflives (t1/2), the sensitivity of the OCPA measurement can be enhanced by decreasing the disconnect time (τ1). The shortest disconnect time can be determined by how rapidly the charging current discharges while the cell is under open circuit conditions. Under the conditions employed in the present studies, the charging current (for the electrode discharging from EW1 to ER (the OCP)) decayed exponentially, with t1/2 ) 0.58 s. To ensure that at least 95% of this current has dissipated, one would have to wait at least three t1/2 values, or ∼1.8 s, prior to reconnection. This discharge time is dictated by the electrical characteristics of the electrochemical cell and potentiostat and not by the nature of the molecules used to form the SAM. Integration of OCPA to Monitor Charge-Retention Behavior. The charge-retention half-life represents the kinetics of charge recombination of the oxidized SAM with electrons in the working electrode surface (see below). Charge-retention information can be obtained through integration of OCPA current transients (Figure 8).5 The instantaneous current measured during OCPA was digitally integrated as a function of time and recorded as the instantaneous charge, normalized to electrode area. The disconnect time (τ1) was varied from 1 to 100 s for the C12Fc SAM (Figure 8, panel A) and from 15 to 420 s for the PM1 SAM (panel B). As shown, the magnitude of the charge measured in each series was found to decrease as a function of disconnect time. The charge measured after 25 µs in each trace was used to represent the total retained charge of the SAM, termed Q(t). Q(t) was normalized to the electrode area to yield the charge density (σ, µC/cm2) for ease of comparison between systems at different electrodes. σ0 (the charge density extrapolated to t ) 0 s) and t1/2 (the charge-retention half-life) were determined by fitting the decay in the observed charge density versus disconnect time to a first-order rate law.5 The OCPA charge density at t ) 0 s (σ0), representing the charge density stored in the SAM prior to any decay processes, is essentially the same as that obtained through integration of a voltammetric peak at the same electrode (Table 1). Because these values are the same within experimental error, this indicates that the charge density observed in OCPA is well correlated to that measured with cyclic voltammetry. Integration of the OCPA background current under these conditions indicates that only a small amount of residual charge is observed (Table 1). Under these

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Figure 8. Integration of OCPA current transients as a function of disconnect time (τ1). OCPA was performed for the C12Fc SAM and the PM1 SAM as described in Figure 7. The instantaneous current was digitally integrated as a function of time and recorded as the instantaneous charge, normalized to electrode area. The disconnect time (τ1) was varied from 1 to 100 s for the C12Fc SAM (electrode area ) 2500 µm2, panel A) and from 15 to 420 s for the PM1 SAM (electrode area ) 790 µm2, panel B). The charge measured at 25 µs in each trace was used to represent the instantaneous retained charge of the SAM, σ(t). The magnitude of the charge measured was found to decrease exponentially as a function of τ1, indicating a firstorder process for charge recombination (see text for details).

conditions, this corresponds to roughly a 200-600-fold diminution of background charge versus that observed with chronoamperometry. Increasing τ1 can further minimize this background charge. The charge-retention characteristics of the C12Fc and PM1 SAMs are summarized in Table 1. Previously, we have shown that charge retention for the PM1 (and PM3) SAM fits a first-order rate law with extremely high fidelity, allowing the calculation of the initial charge density and a charge-retention half-life for the process.5,25,26 The data obtained for C12Fc indicate that these charge-retention characteristics are also well modeled by first-order kinetics. The kinetics of the charge-recombination process observed here correspond to that found in the absence of an electric field and, therefore, is analogous to that of (25) Clausen, C.; Gryko, D. T.; Dabke, R. B.; Dontha, N.; Bocian, D. F.; Kuhr, W. G.; Lindsey, J. S. J. Org. Chem. 2000, 65, 7363-7370. (26) Clausen, C.; Gryko, D. T.; Yasseri, A. A.; Diers, J. R.; Bocian, D. F.; Kuhr, W. G.; Lindsey, J. S. J. Org. Chem. 2000, 65, 7371-7378.


spontaneous electron-transfer reactions in homogeneous solution (i.e., when no external potential is applied). While the electrochemical cell is open-circuited, the microscopic distribution of the electric field must be such that the oxidized redox molecules sit at a potential that would not cause spontaneous reduction, even though the macroscopic potential would indicate that this should take place. The microscopic distribution of the electric field has been modeled for a SAM under potential control,27 but it has not yet been evaluated for a system under open circuit conditions. This is very different from what is observed when the circuit is reconnected and the kinetics of the field-driven electron transfer is similar to that observed with a controlled-potential amperometric experiment (i.e., when reading charge in the OCPA experiment). The charge-retention behavior of the C12Fc is qualitatively similar to that observed for PM1, though there are some differences. Inspection of Table 1 reveals that the total amount of charge initially stored by the C12Fc SAM (6.7 µC cm-2) is larger than for the singly oxidized PM1 SAM (4.2 µC cm-2). The total charge stored depends on the total number of molecules in the SAM. This number is in turn dictated by the effective size of the molecule (which depends on its actual size and its orientation with respect to the surface) and the packing density of the molecules in the SAM. The larger σ0 value for the C12Fc SAM (6.7 µC cm-2) versus the PM1 SAM (4.2 µC cm-2) most likely reflects the intrinsically smaller surface area of the former molecule. On the other hand, the chargeretention t1/2 value for the singly oxidized PM1 SAM (167 s) is much longer than that for the C12Fc SAM (31 s). As a consequence, ∼66% of the molecules in the PM1 SAM that are initially oxidized remain oxidized after a 100 s disconnect time. In contrast, only ∼9% of the molecules in the C12Fc SAM remain oxidized after 100 s. The large differences in the charge-retention t1/2 values illustrate the fact that the stability of the cation radical in each SAM varies dramatically for the two types of surfaceimmobilized species. The stability of stored charge within the molecule will significantly impact the utility of OCPA. The distance dependence of charge-storage stability has been demonstrated with four Zn tetraarylporphyrins bearing Sacetylthio-derivatized linkers, which differ only in the number of methylene spacers (0-3) between the aryl substituent of the porphyrin and the sulfur atom.18,25,26 These molecules formed SAMs in which the chargeretention lifetime increased dramatically with increasing tether length,5 indicating a strong dependence of charge retention on the distance of the redox center from the electrode surface. In contrast, a ferrocene linked to the surface with a short tether (C6) produced a SAM with no discernible charge retention (

Characterization of Charge Storage in Redox ... - ACS Publications

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