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

Understanding the influence of the pH value towards microenvironments of bioanalytical systems, especially inside 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 this limitation, the present work reports on light-addressable electrodes (LAEs) as a tool to create pH gradients at the micro scale. Light-addressable electrodes are based on semiconductor materials in which electron-hole pairs are generated 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 on glass/fluorine-doped tin oxide/titanium dioxide heterostructures. This work studies the influence of the applied external potential, illumination brightness and illumination area on the maximum pH change and width of the pH gradient using a pH-sensitive fluorescent dye. Furthermore, we evaluate the correlation between the pH change and electrical charge transfer. Finally, we provide an outlook towards tailoring complex pH gradients inside microchannels.