Biosensing in the optical switch configuration on strong plasmonic gratings enabling differential referenced detection

A deep gold-coated sinusoidal grating is proposed as a transducer for label-free real-time biosensing, operating in a new configuration based on the optical switch effect, which produces complementary optical outputs enabling differential and referenced detection. Biosensing experiments are reported for the first time on this platform, using immunoassays involving biospecific pairs consisting of bovine serum albumin and its antibody, and human serum albumin and its antibody. Direct and sandwich immunoassays are demonstrated along with negative controls. A limit of detection of 6 pg/mm² was obtained. A theoretical model correlating the variation in the differential referenced output optical signal with adlayer growth is presented and supports the experimental results. The proposed detection device operating in the optical switch configuration makes a promising case for point-of-care detection applications because the differential detection of two diffracted orders enables common noise suppression and robust interrogation.