Polymer Optofluidic Chip for Double-Beam Photometric Determination of Nitrite in Water Samples

We present a compact polymer optofluidic lab-on-a-chip fabricated by maskless laser-direct UV lithography for double-beam photometric detection of nitrite (NO₂ ^-) in water. The chip integrates polymer optical waveguides, a 3 dB coupler, and dual microfluidic cells, implementing on-chip measurement and reference channels that compensate for light-source fluctuations and improve the robustness of the photometric measurements. The system is operated with commercial Griess-reaction-based test kits and standard RGB CMOS image sensors configured for selective RGB-channel absorbance readout. The system achieves a limit of detection of 0.08 mg L^-1 of nitrite in reagent-treated standard water samples. These results highlight the potential of low-cost polymer optofluidic chips as portable platforms for nitrite monitoring in drinking water and environmental applications.