Practical and Inexpensive Evanescent-Wave Sensing Platform for Methylene Blue Detection in Water

This paper presents a performance assessment of a fabricated optoelectronic Lab-on-Chip device for the detection of dyes in polluted water. It monolithically integrates a polymeric waveguide and a hydrogenated amorphous silicon (a-Si:H) photodetector on a compact BK7 glass chip measuring a few square centimeters in footprint. The device senses the complex refractive index of a liquid sample, leveraging the interaction between the evanescent field of the guided light and a solution droplet, monitoring the optical losses at the end of the light path, and linking them to the analyte concentration in the solution. The device's performance was assessed for methylene blue (MB) solutions in water through numerical simulations conducted at a working wavelength of 613 nm. The sample-chip interaction was examined, and the results were combined with the experimentally measured photoresponse of the fabricated a-Si:H detector. The sensor demonstrated a sensitivity of 2.43 pA/ppm, along with a limit of detection of 78 ppb. As the device does not include bulky detectors, chemical treatments, or complex fabrication steps, the reported performance of this highly compact, cost-effective, and easy-to-use sensor appears very promising for health and environmental monitoring applications, particularly for in-situ detection of pollutants in water.