Optical detection of analytes through evanescent waves in lab-on-chip devices

We present an optoelectronic system-on-glass for on-chip quantitative detection of a specific analyte in a biological solution. The system includes an amorphous silicon photodetector and an optical waveguide, integrated on a single glass substrate. The solution under investigation is placed on a waveguide's region where a monochromatic light is travelling. Light coupling depends on the sample's optical properties. Two different waveguiding structures have been investigated: glass-made waveguides, obtained by the double ion-exchange method on the glass substrate, and SU-8 polymer waveguides, obtained by photolithography. Optical losses around 0.5 dB/cm at 532 nm were achieved. The analyte detection is performed by a thin-film amorphous silicon p/i/n junction, fabricated with microelectronic technologies. The tested photodiodes present shot noise currents of about 2 fA/sqrt(Hz) and responsivity around 500 mA/W at 532 nm. As proof of concept, hemoglobin was studied as analyte for a demonstration scenario, involving optical simulations interpolated with experimental data, resulting in a detection limit around 100 ppm, in line with well-established colorimetric methods currently on the market.