Analyst

Microwave-enhanced electroanalytical processes: generator–collector voltammetry at paired gold electrode junctions

Liza Rassaei ^a, Robert W. French ^a, Richard G. Compton ^b and Frank Marken *^a
aDepartment of Chemistry, University of Bath, Bath, UK BA2 7AY
^bDepartment of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QZ

Generator–collector electrode systems allow redox processes and reaction intermediates from multi-step electrode reactions to be monitored. Analytically, collector electrode current responses are insightful and highly sensitive due to (i) the absence of capacitive current components and (ii) an enhanced current response due to feedback between generator and collector electrode. Here, a symmetric gold–gold junction grown by controlled electro-deposition is employed for generator–collector voltammetry in conjunction with microwave activation. Three redox systems are investigated in aqueous 0.1 M KOH: (i) the reduction of Fe(CN)₆³⁻, (ii) the reduction of chloramphenicol, and (iii) the reduction of oxygen. Microwave radiation, when focused into the electrode–solution interfacial zone, causes locally enhanced temperatures with electrode surface temperatures reaching up to typically 380 K (estimated from the shift in the Fe(CN)₆^3−/4− equilibrium potential, at both gold electrodes). The resulting increase in the rate of diffusion and the onset of convection result in non-linear Arrhenius limiting current characteristics and in an increase in collection efficiency with microwave power. The gold electrode junction geometry allows diffusion effects (which increase the feedback current within the gap) to dominate over convection effects (which suppress the feedback current).

The use of interdigitated array electrodes,¹ double-band electrodes,^2,3 ring-disc electrodes,⁴ heptode arrangements of micro-electrodes,⁵ SECM in feedback mode,⁶ or similar generator–collector electrode systems^7,8 provides powerful tools in electroanalysis, in particular for the determination of low concentration intermediates and for the suppression of capacitive background current signals.⁹ Recently, the growth of paired gold electrode junctions based on simultaneous electro-deposition and feedback current-controlled cut-off has been introduced. The resulting electrode junctions have been applied in generator–collector mode for the case of nitric oxide oxidation¹⁰ and for a range of redox systems with differing diffusion coefficients.¹¹ It was shown that an increase in diffusion coefficient generally improves the collection efficiency (defined here as the collector current divided by the generator current obtained at a symmetric junction). Quantitative analysis of current data from paired gold electrode junctions is difficult due to the complex geometry of the electro-deposited gold discs. However, these junctions are useful laboratory tools, readily grown without the need for lithography, and readily available with gap sizes approaching sub-micron dimensions. In this report the effect of microwave activation (in terms of temperature and convection effects) on paired gold electrode junctions is investigated.

Temperature in electrochemistry is an important parameter.¹² The solution temperature at the electrode surface can be changed isothermally (by heating the whole cell) or non-isothermally (by heating the electrode or the liquid close to the electrode). The role of temperature in electrochemistry has been investigated by non-isothermal techniques including radio frequency heating,¹³ at hot wire electrodes,¹⁴ for high amplitude and high frequency alternating voltage and electromagnetically-heated electrodes,¹⁵ and for laser-heated channel flow electrodes.¹⁶ In all of these processes, thermal activation occurs by heat transfer from the electrode surface to the solution, which severely limits the power supplied to the solution phase. In contrast, microwave heating occurs directly in the solution phase^17–20 and the transfer of heat is inverted in direction viz. from the solution into the metal electrode.²¹ That is, there is no limitation to how much power is applied and new phenomena like jet-boiling can be realised.²² Microwave activation (with self-focused microwave radiation at the tip of the electrode²³) is therefore an interesting alternative for the enhancement of electroanalytical processes and responses. A typical experimental cell for microwave-enhanced voltammetry is shown schematically in Fig. 1A.

	Fig. 1 (A) Schematic drawing of the electrochemical Teflon flow cell (b) placed into a multi-mode microwave cavity with port (a), reference electrode (d), counter electrode (e), and a generator–collector working electrode (c). (B) The design of the gold electrode junction with a narrow inter-diffusion zone and high temperature zones indicated.

In this study, a paired gold micro-electrode junction based on two gold discs with ca. 160 µm diameter and with approximately 5 µm average gap (estimated from scanning electron microscopy images, see Fig. 2B) is employed in order to study the mass transport and temperature conditions during microwave-enhanced voltammetry. Both generator and collector currents are shown to increase with applied microwave heating. Temperature effects are complex (non-linear Arrhenius characteristics for currents are observed) due to the combination of diffusion and convection effects, but the collection efficiency is demonstrated to systematically increase with microwave power for all redox systems investigated.

	Fig. 2 Scanning electron micrograph (SEM) images of (A) electro-deposited gold junctions grown onto 100 µm diameter platinum disc electrodes (see dashed circles). (B) Close-up view of the inter-diffusion zone between the gold deposits. The estimated average gap within the junction is approximately 5 µm.

Potassium hydroxide, K₃[Fe(CN)₆], K₄[Fe(CN)₆], KAu(CN)₂, poly(diallydimethylammonium chloride) or PDDAC were obtained from Aldrich. Chloramphenicol (2,2-dichloro-N-[(1R,2R)-1,3-dihydroxy-1-(4-nitrophenyl)propan-2-yl]acetamide, analytical grade) was obtained from Fluka. Demineralised and filtered water was taken from an Elgastat water purification system (Elga, High Wycombe, Bucks) with a resistivity of not less than 18 M cm. Argon (Pure shield, BOC) was employed for de-aeration of electrolyte solutions before and during experiments.

A PGSTAT30 bipotentiostat system (Eco Chemie, The Netherlands) was employed to control the two working electrodes during gold deposition and during microwave experiments. The reference electrode was a saturated calomel electrode (SCE, Radiometer) and the counter electrode was platinum gauze.

During microwave experiments, only the working electrodes placed into a small volume Teflon flow cell were exposed to microwave radiation (see Fig. 1). Details of the cell design and application of microwaves have been reported previously.¹⁰ The electrolyte solution flow rate was typically 0.60 cm³ min⁻¹ (applied with an external peristaltic pump to maintain the bulk temperature in the cell). A Panasonic multi-mode microwave oven (NN-3456, 2.45 GHz, electronically modified to provide a constant and smooth microwave power output) with a water energy sink and a port for the electrochemical cell was used. The microwave intensity was controlled via the anode current of the magnetron. Before and during operation, the system was tested for leaking microwave radiation with a radiation meter. Scanning electron microscopy (SEM) images were obtained with a JEOL JSM6480LV SEM system. Samples were gold sputter coated prior to taking images.

The paired gold electrode junction was prepared as described previously.¹⁰ In brief, two 100 µm diameter platinum wires were fed into a capillary glass tubes, placed into a 5 mm diameter glass tube, and then sealed at one end. By heating the glass, the two platinum wires were brought into close proximity so that after cooling down, cutting, and polishing the two platinum disc electrodes are separated by approximately 50 µm glass (see Fig. 2). The gold electro-deposition process was based on two working electrodes (WE1, WE2) attached to the two platinum disc electrodes, a gold wire counter electrode and an SCE reference electrode. The plating solution was based on an alkaline cyanide bath (KAu(CN)₂, KCN, K₂HPO₄, K₂CO₃ at 55 °C in 10 mL demineralised water²⁴) with 60 µL of 0.35 wt% poly(diallyldimethylammonium chloride) or PDDAC added to minimize dendritic crystalline gold growth. The bipotentiostatic gold growth was carried out in one step using the chronoamperometric method. Potentials applied to the platinum disc electrodes were −1.10 V and −1.11 V vs. SCE (under these conditions, the reversible potential for the gold deposition process is a ca. −0.86 V vs. SCE). The automatic cut-off function (in the GPES software) was used to automatically stop the gold deposition process when the feedback current between WE1 and WE2 exceeded ca. 90 µA. The electrode junction is then removed from the plating bath, rinsed with demineralised water, and used for experiments. Fig. 2 shows SEM images of the paired gold electrode junction.

In order to calibrate the temperature at the paired gold electrode, the temperature-dependent shift in equilibrium potential of an aqueous solution containing 5 mM Fe(CN)₆³⁻ and 5 mM Fe(CN)₆⁴⁻ in 0.1 M KOH was monitored by conventional heating and in a non-isothermal cell (dE_equilibrium/dT = −1.7 ± 0.05 mV K⁻¹, consistent with previous reports²⁵). In the presence of microwave radiation the same shift in equilibrium potential was then measured as a function of applied microwave power and a calibration graph composed (see Fig. 3). In subsequent experiments employing the same conditions (electrolyte solution, electrode, positioning) the calibration graph is used to give an estimate for the temperature at the electrode surface, T_electrode. Current responses for the limits of mass transport-controlled reduction of Fe(CN)₆³⁻ and for the mass transport-controlled oxidation of Fe(CN)₆⁴⁻ serve as current/mass transport calibration responses.

	Fig. 3 Plot of the variation of Telectrode (determined from the equilibrium potential in aqueous solution of 5 mM Fe(CN)63−/5 mM Fe(CN)64− in 0.1 M KOH) as a function of the applied microwave intensity (magnetron anode current).

The one-electron Fe(CN)₆^3−/4− model redox system (see eqn (1)) was employed in this study to explore the temperature and mass transport effects introduced by high intensity microwave radiation at paired gold electrode junctions.

Fe(CN)63−(aq) + e− Fe(CN)64−(aq)

(1)

Fig. 4 shows a set of typical voltammetric responses (only the forward scan is shown) recorded for the generator (see Fig. 4A) and the collector (see Fig. 4B) for a paired gold electrode junction in the absence and in the presence of microwave radiation. The effect of the feedback current on the generator current response is small and it has been explored recently.¹⁰ Therefore, in this report all voltammograms have been obtained in feedback mode. At the generator electrode, the reduction of 5 mM Fe(CN)₆³⁻ is monitored and the potential of the electrode is scanned from +0.4 V to −0.1 V vs. SCE while the potential at the collector electrode is fixed at +0.4 V vs. SCE. At room temperature, the ratio of collector current (0.8 µA) to generator current (−8 µA) corresponds to a collection efficiency of ca. 10% (see Fig. 4E). This collection efficiency is affected by both (i) the rate of diffusion and (ii) the level of convection (present even at ambient temperature due to the flow cell system). In the presence of microwave radiation, the limiting current at both generator and collector electrodes increases to −16 µA and 2.3 µA, respectively (at 371 K corresponding to a magnetron anode current of 20 mA). The collection efficiency at elevated temperatures is increased to ca. 14%, which implies that the effect of the inter-diffusion (feedback) within the junction gap (see Fig. 1B) is dominating over convection effects in bulk solution.

	Fig. 4 (A,B) Cyclic voltammograms (forward scan shown, scan rate 5 mV s−1) for the reduction of 5 mM Fe(CN)63− in aqueous 0.1 M KOH obtained at a paired gold junction electrode with (A) showing the generator current (electrode potential scanned from +0.4 to −0.1 V vs. SCE) and (B) showing the collector current (electrode potential set to +0.4 V vs. SCE) in the presence of microwave radiation (magnetron anode currents/Telectrode (i) 0 mA/293 K, (ii) 14 mA/354 K, and (iii) 20 mA/371 K). (C–E) Plot of the generator current (C), the collector current (D), and the collection efficiency (Icollector/Igenerator) versus Telectrode (E).

The activation parameter analysis based on plotting ln(current) versus 1/T shows non-linear Arrhenius behaviour with a marked increase in current at higher temperatures. A variation in the relative contributions from diffusion and convection to the total current (e.g. more convection at elevated temperatures) is likely to account for this behaviour.

Data in Fig. 4 demonstrate the effect of pulsed microwave radiation. In Fig. 4A and 4B the instant effect of pulsed microwave radiation on the generator and collector currents clearly confirms that only a localised microwave effect occurs where heating of the aqueous solution in the vicinity of the electrode surface dominates. Bulk heating of the liquid electrolyte solution remains insignificant.

Chloramphenicol is an exceedingly inexpensive antibiotic which exhibits activity against many groups of micro-organisms including bacteria^26,27 and fungi.²⁸ However, chloramphenicol has been found in rare cases to lead to serious side effects (anemia) and it has therefore been generally phased out.²⁹ The structure of chloramphenicol (see eqn (2)) consists of nitrobenzene as a well-known electrochemically active motif, which undergoes a reversible one-electron reduction in sufficiently alkaline media^30,31 (see eqn (2)). A multi-electron reduction leading to nitroso and hydroxylamine intermediates occurs more rapidly when the proton activity is increased (lower pH).³²

(2)

The electrochemical chloramphenicol determination has been reported based on processes at mercury electrodes,^33,34 at boron-doped diamond electrodes,³⁵ at screen-printed carbon electrodes,³⁶ at activated glassy carbon fibre electrodes,³⁷ and aided by molecularly imprinted polymer films.³⁸

In aqueous 0.1 M KOH the reversible one-electron reduction of chloramphenicol associated with the formation of the chloramphenicol radical anion occurs at ca. −0.73 V vs. SCE (see Fig. 5A). This cathodic process is chosen here for the investigation of the generator–collector process under microwave radiation. However, a second anodic and much weaker voltammetric response occurs at −0.2 V vs. SCE (see Fig. 5A) possibly associated with the gold-catalysed oxidation of the benzylic hydroxide (this process is chemically irreversible and does not result in feedback currents, vide infra).

	Fig. 5 (A–C) Cyclic voltammograms (forward scan shown, scan rate 5 mV s−1) for the reduction of 1 mM chloramphenicol in aqueous 0.1 M KOH obtained at a paired gold junction electrode with (A) showing the generator current (electrode potential scanned from −0.2 to −1.0 V vs. SCE) and (B,C) showing the collector current (electrode potential set to −0.2 and −0.5 V vs. SCE, respectively) in the presence of microwave radiation (magnetron anode currents/Telectrode (i) 0 mA/293 K, (ii) 12 mA/363 K, and (iii) 20 mA/371 K). (D–G) Plot of the generator current and collector current for −0.2 V vs. SCE (D), the collection efficiency versus Telectrode (E), the generator and collector current for −0.5 V vs. SCE (F), and the collection efficiency (Icollector/Igenerator) versus Telectrode (G).

Both the generator current and the collector current are affected by microwave radiation and a dramatic increase in current is observed in particular at higher applied temperatures. The collector current obtained at −0.2 V vs. SCE clearly shows the presence of the one-electron reduced chloramphenicol radical anion. The trend in collection efficiency for this process (see Fig. 5E, ca. 10% at ambient temperature and increasing with microwave power) is very similar to that observed for the reduction of Fe(CN)₆³⁻, confirming the reversible one-electron character of the process. A positive offset current in the collector response (see Fig. 5B) is indicative of an additional anodic process occurring at −0.2 V vs. SCE at sufficiently high temperatures and, indeed, this process can be seen also in the generator response at high microwave power (see Fig. 5A). This process appears to be chemically irreversible and not leading to significant feedback current responses (see Fig. 5B). Next, the collector potential was shifted negative to −0.5 V vs. SCE in order to achieve a more stable baseline. Fig. 5C and 5F show the corresponding voltammetric responses and limiting current data. Interestingly, the collector response under these conditions appears peak-shaped, which indicates a reduced yield of chloramphenicol radical anion at more negative generator potentials. Collection efficiencies of typically 8% (ambient, increasing with microwave power) are observed. The collector current baseline at −0.5 V vs. SCE is strongly affected by traces of remaining oxygen in the electrolyte solution and this effect is also enhanced at increased temperatures in the presence of microwaves (see Fig. 5C).

Analysis of the current responses based on plotting ln(current) versus 1/T again suggests non-linear Arrhenius characteristics with convection effects becoming more important at elevated temperature.

The electrochemical reduction of oxygen is an important process in fuel cells,³⁹ for gas sensors,⁴⁰ and in the electro-synthesis of hydrogen peroxide.⁴¹ The reaction pathway is highly dependent on the electrode material⁴² as well as the solution pH.⁴³ The electrochemical reduction of oxygen at hot wire platinum electrodes has been reported by Zerihun and Gründler.⁴⁴ Microwave-enhanced electrochemistry allows highly non-isothermal electrochemical processes to be performed and this approach is particularly advantageous in the study of dissolved gases. The solubility of gases such as oxygen in aqueous media decreases dramatically upon heating^45,46 and bulk heating would therefore lead to the loss of a considerable amount of oxygen. Here, the effect of localised microwave heating (without loss of oxygen) on the electro-reduction of oxygen at paired gold electrode junctions is investigated.

Fig. 6A and 6B show representative current–potential curves for reduction of ca. 0.2 mM oxygen (saturated⁴⁷) in aqueous 0.1 M KOH for the generator and the collector electrode in the presence and in the absence of microwave radiation. As expected for gold electrodes,^48,49 there are two distinct processes: (i) P1 at ca. −0.6 V vs. SCE (consistent with the 2-electron reduction of oxygen to hydrogen peroxide, see eqn (3)) and (ii) P2 at ca. −1.0 V vs. SCE (consistent with the 4-electron reduction of oxygen to water, see eqn (4)).

P1:O2 + H+ + 2e− HO2−

(3)

P2:O2 + 4H+ + 4e− 2H2O

(4)

	Fig. 6 (A,B) Cyclic voltammograms (forward scan shown, scan rate 5 mV s−1) for the reduction of ca. 0.2 mM oxygen in aqueous 0.1 M KOH obtained at a paired gold junction electrode with (A) showing the generator current (electrode potential scanned from 0 to −1.2 V vs. SCE) and (B) showing the collector current (electrode potential set to 0 V vs. SCE) in the presence of microwave radiation (magnetron anode current/Telectrode (i) 0 mA/293 K (ii) 14 mA/354 K, and (iii) 20 mA/371 K). (C–E) Plot of the generator currents for processes P1 and P2 (C), the collector current (D), and the collection efficiency (Icollector/Igenerator) versus Telectrode (E).

Both the generator and collector currents increase with microwave power. However, the onset potentials for the oxygen reduction to hydrogen peroxide (ca. −0.14 V vs. SCE) and for reduction of oxygen to water (ca. −0.71 V vs. SCE) as well as the halfwave potentials appear to change only insignificantly with temperature. The collector current is detected only for process P1, consistent with hydrogen peroxide causing the feedback current. The collection efficiency is ca. 4% at ambient temperature and increasing to ca. 7% at elevated temperatures, which is lower compared to collection efficiencies observed for Fe(CN)₆³⁻ and for chloramphenicol. Hydrogen peroxide is known to be unstable and undergoing disproportionation in solution and at surfaces⁵⁰ and this can explain the reduced collection efficiency. Indeed, the magnitude for the current for process P1 is always higher compared to that for process P2, in particular at elevated temperatures, and the reason for this is again the disproportionation of hydrogen peroxide (which causes the limiting current for P1 to increase and the limiting current for P2 to decrease).

The presence of sharp, well-defined current pulses when pulsed microwave radiation is applied is consistent with localised thermal effects. In Fig. 6A it can be seen that the pulsed generator currents appear to reach a lower current compared to the ambient limiting current, which may be attributed to local oxygen concentration gradient effects during rapid temperature pulsing. Analysis of currents based on Arrhenius plots again suggests non-linear behaviour and probably a combination of diffusion and convection effects. The beneficial effect of microwave radiation on the collection efficiency and therefore on the sensitivity is most clearly demonstrated in Fig. 6B where a characteristic current peak is obtained. This peak is considerably enhanced in the presence of microwave radiation. The formation of hydrogen peroxide occurs with an onset of ca. −0.14 V vs. SCE and hydrogen peroxide as an intermediate is consumed at the generator electrode at potential negative of −0.6 V vs. SCE.

Fig. 7 summarises the combined effect of convection and diffusion in generator–collector experiments at paired gold electrode junctions in the presence of microwave radiation. Due to the small and relatively protected inter-diffusion zone between the two gold electrodes, the thermal increase in diffusion coefficient is dominating the feedback response and the collection efficiency is increased. Only for conditions of more forceful convection (e.g. achieved when degassed solutions are employed in microwave experiments⁵¹) a decrease in collection efficiency is anticipated.

	Fig. 7 Schematic plot of the effects of diffusion coefficient and convection on the collection efficiency in generator–collector voltammetry experiments conducted at junction electrodes in the presence of microwave activation.

It has been shown that paired gold electrode junctions can be used under microwave heating conditions. Both electrodes experience the same temperature increase and generator–collector current responses are obtained up to high electrode temperatures, T_electrode 371 K. The effects of the elevated temperature on the rate of diffusion and on the convection processes at the electrodes cause both generator and collector currents to increase. However, the collection efficiency (defined here as the collector current divided by the generator current) is improved at elevated temperatures due to the geometry of the electrode junction. Within the gap, diffusion processes dominate and these result in improved collection efficiencies whereas convection effects reduce the collection efficiency. The non-isothermal junction electrode system allows reaction intermediates to be detected and investigated and it allows high temperature redox processes involving gases to be studied without solubility limitations.