Photocharging of Carbon Nitride Thin Films for Controllable Manipulation of Droplet Force Gradient Sensors

Intentional generation, amplification, and discharging of chemical gradients is central to many nano- and micro-manipulative technologies. We describe a straightforward strategy to direct chemical gradients inside a solution via local photo-electric surface charging of organic semiconducting thin films. We observed that the irradiation of carbon nitride thin films with ultra-violet light generates local and sustained surface charges in illuminated regions, inducing chemical gradients in adjacent solutions via charge-selective immobilization of surfactants onto the substrate. We studied these gradients using droplet force gradient sensors, complex emulsions with simultaneous and independent responsive modalities to transduce information on transient gradients in temperature, chemistry, and concentration via tilting, morphological reconfiguration, and chemo-taxis. Fine control over the interaction between local, photo-electrically patterned, semiconducting carbon nitride thin films and their environment yields a new method to design chemo-mechanically responsive materials, potentially applicable to micro-manipulative technologies including micro-fluidics, lab-on-a-chip devices, soft robotics, biochemical assays, and the sorting of colloids and cells.