Article pubs.acs.org/JPCC
Electrochemical System with Memimpedance Properties Kevin MacVittie and Evgeny Katz* Department of Chemistry & Biomolecular Science, Clarkson University, Potsdam, New York 13699, United States ABSTRACT: An electrochemical system based on a pH-switchable polymer-modified electrode was designed to demonstrate memory properties. The pH changes in the system were produced electrochemically in situ upon electrochemical reduction or oxidation of H2O2. Depending on the direction of the current, different pH changes were achieved and the modified electrode was switched between higher or lower resistance and capacitance. In the absence of the current the electrode preserved the last resistance and capacitance values, thus demonstrating properties of memristor and memcapacitor simultaneously (memimpedance). The switchable interface between the modified electrode and solution potentially allows functional integration between the electrochemical memimpedance device and biomolecular systems processing chemical signals.
1. INTRODUCTION Bioinspired unconventional computational systems,1,2 based on synthetic materials3−5 and biomolecular assemblies,6−8 with adaptive properties capable of both processing and storing information have received high attention and have been rapidly developed in the past decade. In addition to Boolean logic gates9,10 and various circuit elements (e.g., comparators,11 digital multiplexers/demultiplexers,12,13 encoders-decoders,14 amplifiers,15 signal-converters,15 filters,16,17 etc.), various molecular/biomolecular memory systems have attracted particular attention, being represented by different flipflop18,19 and associative memory systems.20,21 A drastic leap in the design of chemical information processing/memory systems could be achieved if the concept of memory-capable electronics, developed in physics22−24 and mostly realized in semiconductor devices,25−27 is brought to soft-matter materials and potentially to biomaterials. Presently, only a few examples of polymer-based memristors (memory-capable resistors) have been realized; however, being developed as solid state electronic devices, they are hardly compatible with biomolecular systems.28 However, switchable electrode interfaces controlled by biomolecular systems have been recently realized,29,30 keeping the promise for the development of memristive devices that include biomolecular components. It should be also noted that two kinds of memory-capable electronic systems, such as memristors and memcapacitors,31 can be easily and naturally realized in one device using switchable electrode interfaces since such interfaces change the electron transfer resistance and interfacial capacitance upon transition between different states.32 The present article is the first report on the electrochemical system demonstrating memimpedance (memory-impedance) properties (therefore uniting memristor and memcapacitor behavior) and based on modified electrodes interfaced with a solution. The present experimental realization does not include yet the biomolecular systems, being a pure electrochemical device; however, the next step, which will include the functional integration of the © 2013 American Chemical Society
switchable electrodes with enzyme-biocatalytic systems, is straightforward.30,33
2. EXPERIMENTAL SECTION 2.1. Chemicals and Supplies. Poly(4-vinyl pyridine) (P4VP, M.W. 160,000 g·mole−1, ρ = 1.101 g·cm−3, SigmaAldrich), bromomethyldimethylchlorosilane (Gelest), and other standard inorganic chemicals and organic solvents (Sigma-Aldrich) were used as supplied without any further purification. Indium−tin oxide (ITO) single-side coated conducting glass (20 ± 5 Ω/sq surface resistivity; SigmaAldrich) served as electrodes for electrochemical measurements. Ultrapure water (18.2 MΩ·cm) from NANOpure Diamond (Barnstead) source was used in all of the experiments. 2.2. Electrode Modification. The ITO-electrodes were chemically modified with P4VP-brushes using the “grafting to” method34,35 according to the following procedure. The ITOcoated glass slides were cut into 25 mm × 10 mm strips. They were cleaned with ethanol in an ultrasound bath for 15 min and dried under a stream of argon. The cleaning step was repeated using methylene chloride as a solvent. The initial cleaning steps were followed by immersing the strips into a cleaning solution (heated to 60 °C in a water bath) composed of NH4OH, H2O2, and H2O in the ratio of 1:1:1 (v/v/v) for 1 h. (Warning: This solution is highly reactive and extreme precautions must be taken upon its use.) Subsequently, the glass strips were rinsed several times with water and then dried under argon. The freshly cleaned ITO strips were reacted with bromomethyldimethylchlorosilane, 0.1% (v/v), in toluene for 20 min at 70 °C. The silanized ITO was rinsed with several aliquots of toluene and dried under argon. Then, 60 μL of the P4VP solution in nitromethane, 10 mg·mL−1, were applied to the surface of each ITO glass strip, dried to form a polymer coating, and left to Received: September 16, 2013 Published: November 6, 2013 24943
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Figure 1. (A) Schematics of the electrochemical cell. (B) Cyclic voltammogram obtained for the two-electrode device consisting of a P4VP-modified ITO electrode and a bare ITO electrode separated with a Nafion ion-exchange membrane. The background electrolyte (0.1 M NaClO4 and 5 μM acetate buffer, pH 4.0) included 0.1 M H2O2 in the compartment in direct contact with the modified electrode. Potential scan rate, 50 mV/s.
react in a vacuum oven at 140 °C overnight. The final cleaning steps, to remove the unbound polymer, consisted of soaking for 10 min in ethanol, followed by additional 10 min in a dilute solution of H2SO4 (pH 3). Modified electrodes were stored under ethanol. 2.3. Electrochemical Measurements. The measurements were carried out with an ECO Chemie Autolab PASTAT 10 electrochemical analyzer using the GPES 4.9 (General Purpose Electrochemical System) software package. All the measurements were performed at an ambient temperature (23 ± 2 °C) in proprietary 2-electrode sandwich cell. The working electrode was a P4VP-modified ITO-glass electrode with a geometrical area of ca. 50 mm2 (note that the typical surface roughness factor for ITO electrodes is ca. 1.6 ± 0.1).36 2.4. Electrochemical Cell Construction. All experiments were performed in a custom designed electrochemical cell. This was composed of two rubber O-rings separated by Nafion, an industry standard proton exchange membrane, held between two conductive ITO glass slides. These ITO slides were used as the electrodes in this system, with the working electrode surface modified with P4VP by the previously mentioned protocol. It should be noted that due to the microdesign of this cell no reference electrode was used. The inner volume of the O-rings was filled with the working solution, composed of acetate buffer (5 μM), sodium perchlorate (0.1 M), and potassium ferricyanide (2 mM). In addition, the chamber in contact with the working electrode contained hydrogen peroxide (0.1 M).
M). The membrane allowed for ion transport between the electrolyte solutions, but prevented the penetration of H2O2 from the compartment contacting the modified electrode to the compartment in contact with the bare ITO electrode. A voltage, controlled by a potentiostat, was applied between the two electrodes, resulting in the electrochemical process of the redox probe, [Fe(CN)6]3− and, more importantly, the electrolysis of H2O2 when appropriate potentials were applied. The cyclic voltammogram shown in Figure 1B demonstrates the reduction and oxidation of H2O2 at potentials more negative than −0.3 V and more positive than 0.4 V, respectively (note that in this experiment [Fe(CN)6]3− was excluded in order to obtain pure electrochemical response from H2O2). The processes reflected by the anodic and cathodic currents shown in Figure 1B result in pH changes concomitant to the redox transformations of H2O2, Figure 2. When the experiment was started from pH 6.0,
Figure 2. pH changes generated in situ upon application of −1.0 V (A) or 1.0 V (B) on the P4VP-modified electrode.
the oxidation of H2O2 resulted in the acidification of the electrolyte solution reaching pH ca. 4. In another experiment started from pH 4.0, the H2O2 reduction produces pH values near neutral (pH ca. 6) breaking through the buffer capacitance (note the small buffer concentration and small volume of the solution). It should be noted that the H2O2 consumption upon the electrochemically induced pH changes (measured by chronocoulometry) was less than 10% from the total amount of H2O2, thus allowing the reversible electrochemically induced pH changes several times before the H2O2 concentration decreases significantly. The P4VP-brush is known to change the electrode interface permeability for the negatively charged [Fe(CN)6]3− redox probe upon its protonation/deprotonation with the changing of the pH value of the background electrolyte solution.37 When the pH value is below 4.3, the pyridine moieties in the P4VP-
3. RESULTS AND DISCUSSION The electrochemical system specially engineered for this method (Figure 1A) included two indium−tin oxide (ITO) electrodes sandwiched with a Nafion ion-exchange membrane between them. A free volume (approximately 250 μL) for placing an electrolyte solution was obtained by organizing rubber O-rings between each electrode and the membrane. One of the ITO electrodes was modified with a poly(4-vinyl pyridine) (P4VP)-brush,37 while the second electrode was an unmodified bare ITO electrode. The inner volume of the Orings was filled with the electrolyte solution, composed of acetate buffer (5 μM; note the small buffer concentration), sodium perchlorate (0.1 M), and potassium ferricyanide (2 mM). In addition to this, the chamber in contact with the P4VP-modified electrode contained hydrogen peroxide (0.1 24944
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brush are protonated and the positively charged polymer is swollen, hydrophilic, and permeable for the negatively charged redox probe. In the case of pH values above 5.5, the pyridine units are deprotonated and the neutral polymer is collapsed to the hydrophobic state, which is not permeable for the ionic redox probe, thus inhibiting the electron transfer process. In the presently designed system the anodic current corresponding to the H2O2 oxidation resulted in the pH value of ca. 4, thus producing the protonated, swollen and open state of the P4VPmodified electrode, while the cathodic current, corresponding to the H2O2 reduction, resulted in a pH of ca. 6, thus yielding the deprotonated hydrophobic and closed state of the modified interface. The impedance spectra for the open and closed electrode states recorded in the two-electrode device are shown in Figure 3. It should be noted that the reversible transition
Figure 4. Switchable Ret (A) and Cdl (B) of the memimpedance device: (1) initial open state of the modified electrode, pH 4.0, (2) closed state of the modified electrode, pH ca. 6, produced by passing the cathodic current at −1.0 V applied to the modified electrode for 20 min, (3) repeat of the measurement after 20 min without passing any current, (4) re-open of the electrode, pH ca. 4, produced by passing the anodic current of 1.0 V applied to the modified electrode for 20 min, (5) repeat of the measurement after 20 min without passing any current, (6) reset of the system to the closed state by adding solution with pH 6.0 to the compartment facing the modified electrode, (7) reset of the system to the open state by adding solution with pH 4.0 to the compartment facing the modified electrode.
the closing of the electrode interface, thus resulting in the increased electron transfer resistance, Ret ca. 320 kΩ, and decreased double layer capacitance, Cdl ca. 13 μF, derived from the impedance spectrum, Figure 4A,B (step 2). After the resistance/capacitance change was completed, we disconnected the electrodes from the external source of voltage and remeasured the impedance spectrum after 20 min. The impedance spectrum has demonstrated almost no changes keeping the resistance and capacitance values essentially the same as when obtained after passing the cathodic current through the modified interface, Figure 4A,B (step 3), thus demonstrating the memory properties of the system. Then the potential of +1.0 V (measured vs the unmodified counter electrode) was applied on the modified electrode for 20 min yielding the anodic current (note the current direction opposite to the previous electrochemical step) and resulting in the pH corresponding to the opening of the electrode interface, thus producing the low electron transfer resistance, Ret ca. 8 kΩ, and high double layer capacitance, Cdl ca. 16.5 μF, Figure 4A,B (step 4). It should be noted that while the closed state of the modified electrode surface inhibited the redox process of the anionic redox probe [Fe(CN)6]3−, it was still permeable for the small molecule H2O2, thus allowing its oxidation and the concomitant pH changes. After disconnecting the electrodes from the potentiostat and letting the system sit for 20 min, the impedance spectrum was remeasured showing almost unchanged resistance and capacitance, thus confirming the system memory in this state, Figure 4A,B (step 5). In the final steps of the experiments the system was also reset to the closed and then open states by injections of the electrolyte solutions with the pH values of 6.0 and 4.0 corresponding to the deprotonated and protonated forms of the P4VP-brush, respectively, Figure 4A,B (steps 6 and 7). One of the features of the classical solid-state memristor devices is the hysteresis loop in the current−voltage function, Figure 5, inset.24,31 This feature was also nicely confirmed for the studied electrochemical device, Figure 5. The cyclic voltammogram was started from the potential of 0.6 V applied to the modified switchable electrode facing the solution with pH 4.0 corresponding to the open state of the interface (Figure 5, point a). While proceeding to the negative potentials, the
Figure 3. Impedance spectra obtained on the closed, pH 6.0, (a) and open, pH 4.0, (b) states of the P4VP-modified electrode. Bias potential −450 mV (vs the counter electrode); frequency range 100 mHz−10 kHz.
between the open and closed electrode states changes not only the resistance through the switchable interface reflected by the real component of the impedance spectra, but also the capacitance value reflected by the imaginary part of the impedance. The simplest definition of the memristor’s properties states that the resistance of the device (also capacitance for a memcapacitor) depends on the direction of the current passing through the device, while in the absence of the current the device remembers its last resistance (or capacitance).23,24,31 In order to demonstrate the memimpedance features of the system we performed the following set of experiments analyzing the system changes by impedance spectroscopy. The experiments were started with the working solution at a pH of 4.0, in the cell compartment facing the P4VP-modified electrode. At this pH value the modified electrode is open for the [Fe(CN)6]3− redox probe, thus resulting in a low value of the electron transfer resistance, Ret ca. 8 kΩ, Figure 4A (step 1). The swollen hydrophilic polymer brush also yields a high interfacial double-layer capacitance on the electrode surface, Cdl ca. 17 μF, Figure 4B (step 1). When the potential of −1.0 V (measured vs the unmodified counter electrode) was applied on the modified electrode for 20 min yielding the cathodic current, the electrochemically induced pH changes resulted in 24945
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bias potential, the device can be switched between the memristor behavior, with the variable Ret and Cdl, and the classical resistor behavior, with the constant high resistance.
4. CONCLUSIONS The article reports on the first experimental realization of the electrochemical system with memimpedance properties, thus demonstrating the memristor and memcapacitance in one device. It should be noted that the slow processes resulting in the transition of the electrochemical device from the open to the closed states and back originate from the bulk electrolysis of H2O2 resulting in the pH changes of the solution. Further miniaturization of the device, reducing the electrolyte volume, will decrease the waiting time for the switching processes. However, this switching process will never be as fast as in semiconductor solid-state devices because of the time required for the restructuring of the polymer brush on the electrode surface. The present system does not pretend to be a competitor for solid-state devices but opens future options for combining electrochemical memory devices with unconventional molecular/biomolecular information processing systems. A vast array of future applications in (bio)sensing, and other medicinal subfields, rather than in electronic systems, becomes increasingly feasible.
Figure 5. Cyclic voltammogram demonstrating the hysteresis loop in the current−voltage function: (a) open electrode at initial pH 4.0, (b) the potential of −1.0 V was applied for 20 min to allow switching from the open to closed state, (c) closed state of the electrode at pH ca. 6 produced electrochemically, (d) the potential of 1.0 V was applied for 20 min to allow switching from the closed to open state. Potential scan rate between points a−b and c−d was 50 mV/s. Inset: Schematic hysteresis loop in the current−voltage function characteristic of a memristor device.
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cyclic voltammogram demonstrated the cathodic peak corresponding to the reduction of the [Fe(CN)6]3− redox probe. When the final potential of −0.4 V was reached (Figure 5, point b) the potential sweep was stopped, and the potential of −1.0 V was applied for 20 min to allow the electrochemically induced pH changes resulting in pH ca. 6. Then the potential was returned to −0.4 V, and the potential sweep was continued in the positive direction (Figure 5, from point c to point d). During this potential sweep, the electrode was already in the closed state and the redox process of [Fe(CN)6]3− was inhibited, thus the anodic peak on the cyclic voltammogram was not observed. Note that the final point d in the cyclic voltammogram does not coincide with the starting point a, thus resulting in a hysteresis loop similar to the one expected for memristor devices. Lastly, the potential of 1.0 V was applied for 20 min resulting in the pH changes back to the acidic values (pH ca. 4) and opening the modified electrode interface. Therefore, the next cycle starting again from point a was possible. It should be noted that the potential sweep was not performed in the entire potential range of ±1.0 V only to show the cathodic peak of [Fe(CN)6]3− redox probe on the cyclic voltammogram, which would be screened by the large currents of the reduction/oxidation of H2O2 had the whole potential range been applied. Another interesting feature of the electrochemical memimpedance system is its dependence on the bias potential applied for the impedance measurements. Since the [Fe(CN)6]3− redox probe species were applied in their oxidized state, the bias potential, Eb = −450 mV, was more negative than the redox potential of the probe, E1/2 = 100 mV (vs the counter electrode), thus inducing the reductive process. This process was activated and inhibited by the open and closed states of the modified electrode, respectively, resulting in the switchable Ret and Cdl. When the bias potential was more positive than the redox potential of the probe used, the electrochemical process was not possible (note that the oxidized probe cannot be further oxidized), and the high Ret resistance was observed regardless of the electrode state. Therefore, by changing the
AUTHOR INFORMATION
Corresponding Author
*(E.K.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding of our research by the NSF, via awards CCF 1015983 and CBET-1066397, is gratefully acknowledged. REFERENCES
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