Ultrahigh frequency voltammetry: effect of electrode material and frequency of alternating potential modulation on mass transport at hot-disk microelectrodes.

Ultrahigh frequency voltammetry involves low scan rate voltammetric measurements with microelectrodes polarized by high-frequency large-amplitude alternating potential. The method provides a simple means for studying electrothermal and dielectrophoretic effects, which are important in micro and nanofluidic systems. The method also allows for indirect measurements of electrode impedance at gigahertz frequencies. This increases the upper frequency limit in impedance measurements about 1000 times. In this work we demonstrated, for the first time, that the effect of dielectric relaxation of water can be observed in a simple voltammetric experiment. The paper focuses on the description of electrothermal convection at ac heated disk microelectrodes as a function of frequency and provides a comparison of numerical simulations with experimental results.

Effect of large-amplitude alternating current modulation on apparent reversibility of electrode processes.

We examined the effect of a large-amplitude high-frequency alternating potential modulation on direct currents associated with irreversible, quasi-reversible, and reversible electron-transfer processes occurring at microelectrodes under voltammetric conditions. All irreversible processes appear to be accelerated by the superimposed ac modulation, and under certain conditions this may even lead to an electrochemical etching of noble metal electrodes. In the case of electrode processes which are reversible on the time scale of a dc polarization, but quasi-reversible on the time scale of the ac modulation, the distortion of voltammograms caused by the ac modulation can provide useful information about the kinetics of fast electron-transfer processes. For completely reversible electrode processes the effect of the large-amplitude ac modulation is essentially trivial; the distortion of voltammetric curves causes broadening of analytical signals without providing any useful information.

Dielectrophoretic and electrothermal effects at alternating current heated disk microelectrodes.

When a disk microelectrode is polarized with an alternating potential of very high frequency (0.1-2 GHz) and a high amplitude (up to 2.8 V rms), the electrode is heated up, and at the same time, a very intense electric field is created around the electrode (>10(6) V/m for electrodes 1 microm in radius). This strong electric field gives rise to positive or negative dielectrophoretic effects. Positive dielectrophoretic effects can be used to assemble nanowires from nanoparticles at the electrode edge. On the other hand, a negative dielectrophoretic effect is probably responsible for "jet boiling" observed at overheated microelectrodes. In addition, a combination of a high temperature gradient and a high potential gradient generates an intense electrothermal flow of solution which very strongly enhances the mass transport and is responsible for intense convection in such systems. The electrothermal flow and dielectrophoretic forces can be generated directly on a microelectrode employed in electrochemical detection because the high frequency ac polarization of the electrode does not interfere with the acquisition of analytical signals.

Cyclic chronopotentiometric determination of sugars at Au and Pt microelectrodes in flowing solutions.

The main advantage of the application of cyclic chronopotentiometry (CCP) in end-column CE detection arises from the fact that the detection parameters and the magnitude of the analytical signal are (in contrast with other electrochemical detection methods) independent of the ohmic polarization of the solution caused by the separation current at the detection end of the capillary. CCP was used to determine sugars on platinum and gold microelectrodes after separation by CE. The results obtained with a gold microelectrode were better. Subsequently this detection method was used for quantitative determination of sugars in honeys and for their authentication.

Cyclic chronopotentiometry as a detection tool for flowing solution systems.

Cyclic chronopotentiometry provides a very simple detection method, which may be particularly useful in capillary electrophoresis (CE) and microseparation systems. It has been shown that for disk microelectrodes it is possible to define safe reduction and oxidation currents that would never lead to the formation of H2 or O2 gas bubbles, even if they are applied for an indefinitely long time period. During end-column CE detection, currents passing through the working microelectrode can be completely controlled by the external electronic circuit and they are not affected by the separation current. Consequently, problems created by the offset potential in CE can be completely eliminated. The detection can be accomplished through a variety of different mechanisms; however, generation of the electrode response as a result of analyte adsorption seems to be most common. The method is applicable to many analytes, which do not have to be electroactive. The analytical signal is obtained by monitoring the change in the average electrode potential (calculated for either a cathodic or an anodic half-cycle) caused by an analyte interacting with the electrode. The analytical signal is proportional to the analyte concentration, within a concentration range extending over approximately 2 orders of magnitude.

High frequency faradaic rectification voltammetry at microelectrodes.

An experimental setup for carrying out faradaic rectification measurements at micrometer-sized electrodes under potential control is described. A new method of data analysis is proposed that allows the determination of the standard rate constant and the electron-transfer coefficient of a fast charge transfer process without knowing the impedance of the microelectrode. This method is based on the frequency dependence of the shape of the faradaic rectification voltammograms (i.e., the average width of the peaks and the ratio of the peak heights) rather than on the magnitude of the faradaic rectification signal. The method was tested in the determination of heterogeneous electron transfer kinetics of Fe(CN)6(3-/4-) and Ru(NH3)6(2+/3+) in aqueous solutions on a platinum microelectrode (12.5 microm in radius) and ferrocene/ferrocinum redox couple in a dimethylformamide solution on a gold microelectrode (12.5 microm in radius).

Hot microelectrodes.

Heat generation at disk microelectrodes by a high-amplitude (few volt) and high-frequency (0.1-2 GHz) alternating voltage is described. This method allows changing electrode temperature very rapidly and maintaining it well above the boiling point of solution for a very long time without any indication of boiling. The size of the hot zone in solution is determined by the radius of the electrode. There is no obvious limit in regard to the electrode size, so theoretically, by this method, it should be possible to create hot spots that are much smaller than those created with laser beams. That could lead to potential applications in medicine and biology. The heat-generating waveform does not electrically interfere with normal electroanalytical measurements. The noise level at hot microelectrodes is only slightly higher, as compared to normal microelectodes, but diffusion-controlled currents at hot microelectrodes may be up to 7 times higher, and an enhancement of kinetically controlled currents may be even larger. Hot microelectrodes can be used for end-column detection in capillary electrophoresis and for in-line or in vivo analyses. Temperature gradients at hot microelectrodes may exceed 1.5 x 10(5) K/cm, which makes them useful in studies of Soret diffusion and thermoelectric phenomena.