Electro-osmotic flow system with integrated planar optical waveguide sensing

A combined optical waveguide sensor and electro-osmotic flow system is described. The waveguide sensor is based on the resonant mirror (RM), which uses frustrated total internal reflection to couple light into and out of the waveguide layer. This sensing technique can be used to monitor refractive index, optical absorption and fluorescence within a thin surface layer. The surface of the sensor has been made compatible with electro-osmotic flow. The dielectric layers used to form the waveguide sensor are deposited by chemical vapour deposition (CVD). They consist of a 550 nm thick layer of silica as a spacer with an 80 nm layer of silicon nitride as the waveguide. These are deposited on a Schott SF10 substrate, which had been etched with fluorosilicic acid to define the flow cell. To maintain compatibility with electro-osmotic flow, a drilled glass cover plate is bonded to the waveguide to close the channel, leaving ports at either end to act as electrode reservoirs. Electro-osmotic flow is observed using voltages between 100 and 200 V. To detect the flow of solution, the refractive index of the solution at a point midway along the channel is monitored using the waveguide sensor. A refractive-index boundary created by adding glycerol-containing buffer to one end of the flow cell could be moved into and out of the channel by applying the appropriate polarity across the electrodes. Control experiments have been performed without a refractive-index change to eliminate the possibility of thermal or electro-optical effects giving the observed signal changes.