Microscale pH gradient generation by electrolysis on a light-addressable planar electrode

This paper describes a light-addressable electrolytic system to produce microscale pH gradients. Light illumination to a photoconductive planar electrode generates a conducting point serving as a photo-anode or cathode, where applied positive or negative voltage electrolytically produces protons or hydroxide ions leading to an increased or decreased pH gradient. A digital micromirror device (DMD) was installed in this system to provide a patterned light illumination. This flexible spatial addressability is a great advantage over previous on-chip pH gradient generation systems with metal thin film microelectrodes. The fluorescence pH imaging showed a proof of principle that electrolysis at the addressed photo-anodes and cathodes could generate localized pH gradients. The spatio-temporal patternability of photo-electrodes allowed characterization of various gradient profiles without any re-fabrication of electrode patterns, a prerequisite in conventional electrolytic generation with metal thin film microelectrodes. A spatio-temporal switching between the photo-anode and cathode allowing use of a combination of bimodal electrolysis enabled the fine-tuning of gradient profiles and enriched the variety of available gradient patterns.