Voltammetry using an electrode surface continuously renewed by laser ablation and its demonstration on electro-oxidation of l-ascorbic acid

Laser heated boron doped diamond electrodes: effect of temperature on outer sphere electron transfer processes

Thermoelectrochemical experiments can reveal significant information about electrochemical processes compared to ambient only measurements. Typical thermoelectrochemistry is performed using resistively heated wires or laser heated electrodes, both of which can suffer drawbacks associated with the electrode material employed. Boron doped diamond (BDD) is ideal for thermoelectrochemical investigations due to its extremely high thermal conductivity and diffusivity, extreme resistance to thermal ablation (can withstand laser power densities, Pd, of GW cm(-2) for nanosecond pulses) and excellent electrochemical properties (low background currents and wide potential window). In this paper we describe the use of a pulsed laser technique to heat the rear of a 1 mm diameter conducting BDD disc electrode, which drives electrochemical solution reactions at the front face. Maximum electrode temperatures of 90.0 °C were recorded experimentally and confirmed by finite element modelling (FEM). The effect of laser pulsed heating (maximum 3.8 kW cm(-2); 10 ms on and 90 ms off) on the cyclic voltammetric response of two fast (reversible) outer sphere electron transfer redox mediators (Ru(NH3)6(3+/2+) and IrCl6(2-/3-)) are investigated. In particular, we observe pulsed increases in the current, which increase with increasing Pd. The potential of the peak current is shifted positively for the Ru(NH3)6(3+/2+) couple (in accordance with a positive temperature coefficient, β, +0.68 mV K(-1)) and negatively for the IrCl6(3-/2-) couple (β = -0.48 mV K(-1)). Scanning backwards, in contrast to that observed for a macrodisc electrode in ambient solution, a cathodic peak is again observed for Ru(NH3)6(3+/2+) and an anodic peak for IrCl6(3-/2-) couple. We attribute this response to the entropy of the redox reaction and the time-dependant change in mass transport due to the induced thermal gradients at the electrode/electrolyte interface. The observed responses are in qualitative agreement with FEM simulations.

Sodium dodecyl sulfate-modified electrochemical paper-based analytical device for determination of dopamine levels in biological samples

We report the development of an electrochemical paper-based analytical device (ePAD) for the selective determination of dopamine (DA) in model serum sample. The ePAD device consists of three layers. In the top layer, SU-8 photoresist defines a hydrophilic sample application spot on the filter paper. The middle layer was made from transparency film and contained two holes, one for sample preconcentration and the other for the surfactant to allow transfer to the third layer. A screen-printed carbon electrode formed the bottom layer and was used for electrochemical measurements. In the absence of the anionic surfactant, sodium dodecyl sulfate (SDS), the oxidation peaks of DA, ascorbic acid (AA) and uric acid (UA) overlapped. With the addition of SDS, the DA oxidation peak shifted to more negative values and was clearly distinguishable from AA and UA. The oxidation potential shift was presumably due to preferential electrostatic interactions between the cationic DA and the anionic SDS. Indeed, whilst the SDS-modified paper improved the DA current five-fold, the non-ionic Tween-20 and cationic tetradecyltrimethylammonium bromide surfactants had no effect or reduced the current, respectively. Furthermore, only the SDS-modified paper showed the selective shift in oxidation potential for DA. DA determination was carried out using square-wave voltammetry between -0.2 and 0.8 V vs. Ag/AgCl, and this ePAD was able to detect DA over a linear range of 1-100 μM with a detection limit (S/N=3) of 0.37 μM. The ePAD seems suitable as a low cost, easy-to-use, portable device for the selective quantitation of DA in human serum samples.

Activation Effects of a Platinum Electrode by Laser Pulse Irradiation on the Electro-Oxidation of Glucose in Alkaline Solution

In order to demonstrate the activation effects of a Pt electrode by laser pulse irradiation, the electro-oxidation of glucose was tested at an activated Pt electrode by cyclic voltammetry. A fixed potential was applied to the electrode, and then the electrode was irradiated with laser pulses from a Nd:YAG laser at 20 Hz for 20 s. Activation by the laser pulse irradiation gave two remarkable effects on cyclic voltammograms from the electro-oxidation of glucose in a 0.1 mol dm(-3) NaOH solution, i.e., surface modulation and cleaning effects. Significant differences were found in the cyclic voltammograms at the activated and at the simply polished electrodes. Such differences in the oxidation waves are attributed to a crystallographic change of the electrode surface induced by a laser ablation, accompanied by laser pulse irradiation. Due to the cleaning effect, the activated Pt electrode gave a sharp oxidation wave at -0.3 V even in real samples containing various organic compounds that could foul the electrode, though the activated Pt electrode lacked selectivity to the electro-oxidation of glucose.

Laser activation voltammetry: selective removal of reduced forms of methyl viologen deposited on glassy carbon and boron-doped diamond electrodes

The effect of high-intensity laser pulses on the reduction of methyl viologen at glassy carbon electrodes in aqueous solution is investigated using laser activation voltammetry (LAV) under both channel flow and no-flow conditions and compared with the effect of conventional variable-temperature voltammetry. The reduction proceeds in two consecutive one-electron steps, and the neutral two-electron-reduction product of methyl viologen is shown by voltammetry and in situ optical microscopy to form two types of deposits, amorphous and crystalline, on the electrode surface. Laser activation voltammetry using a 10 Hz pulsed Nd-YAG 532 nm laser is shown to remove the deposits from the electrode surface at different laser intensities: the amorphous material is more easily ablated than the crystalline deposit. By conventional variable-temperature voltammetry, it is shown that the two stripping peaks disappear as the temperature is increased. However, with conventional heating, the opposite ease of removal is detected compared to the case of laser activation voltammetry: the stripping response associated with the crystalline material disappears at lower temperatures compared to that for the amorphous material. In the presence of high-intensity laser pulses (>0.17 W cm(-2)), glassy carbon surfaces are damaged and the voltammetric characteristics become poor. It is shown that, by the employment of a thin-film boron-doped diamond electrode grown using a chemical vapor deposition procedure on a tungsten substrate, much higher laser intensities can be applied and well-defined LAV signals can be obtained without deactivation of the electrode.

Laser-activated voltammetry: measurement of the diffusion coefficients of electropassivating species. Application to pyrrole and phenol in aqueous solution

Clean electrode surfaces can be achieved during the electrolysis of otherwise passivating species in aqueous and other solutions by means of surface ablation using a 10-Hz pulsed Nd:YAG 532-nm laser. This ability to remove passivating electrolytically generated layers on glassy carbon and platinum electrodes is shown first by an investigation of the stripping peaks formed from the electrogeneration of the neutral forms of methyl viologen and heptyl viologen during reduction in aqueous solution of dications. Next, laser ablation was conducted under well-defined hydrodynamic conditions using a channel flow cell to identify laser power thresholds below which transport-limited currents could be seen, which were in quantitative agreement with those expected in the absence of irradiation. Levich plots recorded in the channel flow cell for K4Fe(CN)6 at glassy carbon and platinum electrodes showed such agreement for laser intensities lower than 0.17 and 0.65 W cm(-2), respectively. Working at intensities below these thresholds, steady-state voltammetry was observed for the oxidation of both phenol and pyrrole at glassy carbon and platinum electrodes, respectively, in aqueous solution. The diffusion coefficients of these two species were then measured under hydrodynamic conditions using laser ablation voltammetry to continuously clean the surface. Diffusion coefficients were inferred using the Levich equation. The result for phenol at a pH of 12 in aqueous solution was 0.9 (+/-0.1) x 10(-5) cm2 s(-1), which is in good agreement with an independent nonelectrochemical method. The diffusion coefficient of pyrrole in aqueous solution was similarly evaluated as 1.25 (+/-0.1) x 10(-5) cm2 s(-1).