Response of Local Evanescent Array-Coupled Biosensors to Organic Nanofilms

A label-free planar optical waveguide immunosensor that operates on the novel principle of local evanescent field shift is demonstrated in this paper. Increased local refractive index at the waveguide's upper surface due to the formation of an organic adlayer shifts the evanescent field distribution up, and hence, changes the light intensity both above and below the waveguide structure. Beam propagation simulations show increased modulation ratio sensitivity to adlayer thickness with increasing detection distance below the waveguide. The local nature of detection allows sensors to be implemented in array formats on a single waveguide for multiple-analyte sensing. Both near-field scanning optical microscopy and integrated buried detector arrays are employed to study the response to patterned organic nanofilms including immunocomplexes, photoresist, and adsorbed bovine serum albumin (BSA) layers. Buried polysilicon detector arrays integrated with silicon nitride waveguides in a commercial CMOS process exhibit a 15% photocurrent modulation ratio response to an approximately 1-nm-thick adsorbed film of BSA. CMOS compatibility enables a low-cost sensor system on a chip. Temperature dependence measurements show that sensor has a 0.3%/degC change in modulation ratio, which is thousands of times less than traditional resonant biosensors.