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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.sub.rms]), the electrode is heated up, and at the same time, a very intense electric field is created around the electrode (> [10.sup.6] V/m for electrodes 1 [micro]m 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.

An analytical evaluation of the higher ac harmonic components derived from large amplitude Fourier transformed voltammetry is provided for the reversible oxidation of ferrocenemethanol (FcMeOH) and oxidation of uric acid by an EEC mechanism in a pH 7.4 phosphate buffer at a glassy carbon (GC) electrode. The small background current in the analytically optimal fifth harmonic is predominantly attributed to faradaic current associated with the presence of electroactive functional groups on the GC electrode surface, rather than to capacitive current which dominates the background in the dc, and the initial three ac harmonics. The detection limits for the dc and the first to fifth harmonic ac components are 1.9, 5.89, 2.1, 2.5, 0.8, and 0.5/[micro]M for FcMeOH, respectively, using a sine wave modulation of 100 mV at 21.46 Hz and a dc sweep rate of 111.76 mV [s.sub.-1]. Analytical performance then progressively deteriorates in the sixth and higher harmonics. For the determination of uric acid, the capacitive background current was enhanced and the reproducibility lowered by the presence of surface active uric acid, but the rapid overall [2.sup.e-] rather than [1.sup.e-] electron transfer process gives rise to a significantly enhanced fifth harmonic faradalc current which enabled a detection limit of 0.3 [micro]M to be achieved which is similar to that reported using chemically modified electrodes. Resolution of overlapping voltammetric signals for a mixture of uric acid and dopamine is also achieved using higher fourth or fifth harmonic components, under very low background current conditions. The use of higher fourth and fifth harmonics exhibiting highly favorable faradaic to background (noise) current ratios should therefore be considered in analytical applications under circumstances where the electron transfer rate is fast.

The [V.sub.t] close-curve measurement shows that there exists a surface (X-Y) discharge contour in spite of a large sustain gap of 200/[micro]m greater than a barrier rib height of 125 [micro]m. This indicates that there are two different discharge modes, i.e., one mode is to produce the surface discharge, and the other is to produce the surface discharge including the plate gap discharge. The two discharge modes are examined and analyzed based on the [V.sub.t] close-curve movement on the cell voltage plane. The two discharge modes show the different discharge characteristics such as the sustain voltage level, [V.sub.t] close-curve behavior, luminance, and luminous efficiency. In addition, the spatial wall charge, electron density behaviors, and their current flows as a function of time are investigated in detail using the numerical analysis. Index Terms--Alternating current plasma display panel (ac-PDP), different discharge modes, large sustain gap, [V.sub.t] close curve.

We present the development of a continuous-flow, 'dielectrophoretic spectrometer' based on insulative DEP techniques and three-dimensional geometric design. Hot-embossed thermoplastic devices allow for high-throughput analysis and geometric control of electric fields via ridged microstructures patterned in a high width-to-depth aspect ratio (250:1) channel. We manipulate particles with dc-biased, ac electric fields and generate continuous-output streams of particles with a transverse outlet position specified by linear and nonlinear particle mobilities. We show, with simulation and experiment, that characteristic shape factors can be defined that capture the effects of constrictions in channel depth and that modulating the angle of these constrictions changes the resulting local DEP force. Microdevices are fabricated with an insulative constriction in channel depth, whose angle of incidence with the direction of flow varies continuously across the channel width. The resulting electric field gradients enable demonstration of a dielectrophoretic spectrometer that separates particles and controls their transverse channel position.

Alternating current mode scanning electrochemical microscopy (AC-SECM) enables local detection of electrochemical surface activity without any redox mediator present in solution. Z-approach curves toward the substrate result in a negative feedback curve of the ac signal for insulating samples. On conducting samples, however, the shape of the feedback curve was found to be dependent on the ac perturbation frequency. Approach curves over a wide range of frequencies were performed, and the results were applied to interpret laterally resolved frequency-dependent measurements obtained with combined atomic force microscopy-AC-SECM (AFM-AC-SECM). For the first time, this frequency dependence of the signal was utilized to fine-tune the electrochemical contrast in lateral imaging in AC-SECM. An array of gold microelectrodes embedded in silicon nitride displaying significant changes in electrochemical activity as well as in topography was investigated using a bifunctional AFMSECM tip with an integrated recessed ring microelectrode. Due to the unique geometrical conditions the electrochemical contrast between the conducting gold spots and the insulating [Si.sub.x][N.sub.y] is reversed, crosses zero, and inverts as a function of the applied ac frequency.