Formation of controllable pH gradients inside microchannels by using light-addressable electrodes

Understanding the influence of the pH value towards microenvironments of bioanalytical systems, especiallyinside lab-on-a-chip or micro total analysis systems, is crucial for the success of experiments. Different ap­proaches are known to control the pH value inside those microchannels and to tailor pH gradients. Nevertheless,the existing concepts often lack the possibility for a flexible adaption of these gradients. To overcome thislimitation, the present work reports on light-addressable electrodes (LAEs) as a tool to create pH gradients at themicro scale. Light-addressable electrodes are based on semiconductor materials in which electron-hole pairs aregenerated by illumination. These free charge carriers can trigger chemical reactions at the semiconductor-electrolyte interface, including the change of the pH value. For this purpose, we have designed LAEs based onglass/fluorine-doped tin oxide/titanium dioxide heterostructures. This work studies the influence of the appliedexternal potential, illumination brightness and illumination area on the maximum pH change and width of thepH gradient using a pH-sensitive fluorescent dye. Furthermore, we evaluate the correlation between the pHchange and electrical charge transfer. Finally, we provide an outlook towards tailoring complex pH gradientsinside microchannels