DIFFUSIONAL ELECTROTITRATION - GENERATION OF PH GRADIENTS OVER ARRAYSOF ULTRAMICROELECTRODES DETECTED BY FLUORESCENCE

Electrode reactions near the surface of planar platinum and gold ultra microelectrodes in aqueous electrolyte solutions were visualized by fl uorescence microscopy. Millisecond pulses of direct current (0.5-5 V d c) caused pH alterations in the vicinity of the electrodes and in the interelectrode gaps. These changes caused differences in the fluoresce nce intensity of fluorescein or of its dextran conjugate (MW 2 x 10(6) ), allowing visualization. Redox reactions at the electrodes created p H gradient patterns on a micrometer and submicrometer scale. It is pos sible to delay the diffusional dissipation of the electrogenerated ''p rotonation clouds'' by adding up to 30% (w/v) poly-(vinylpyrrolidone) (MW 7.5 x 10(5)). We used 50, 20, 10, and 3 mu m wide electrode strips with either dc, pulsed dc, or dc-shifted ac (kHz-MHz) excitation. Ste ep pH gradients could be moved perpendicular to the electrode strip ar ray. Traveling pH waves with speeds of about similar to 1 mm/s could b e generated. We have developed a numerical procedure for modeling the time-dependent concentration of electroactive species. The numerical p H patterns are in good agreement with the experimentally obtained ones . Electrotitration and its numerical modeling open new perspectives. M icro- and ultramicroelectrodes should find applications in analytical chemistry and biomedical research. Titration techniques within microco mpartments may now be practicable. The visualization of cathodically d riven electrodes is also useful for assessing the working state of mic roelectrode arrays.