Integration of GRIN lens in a digital microfluidic spectrophotometer for enhanced on-site environmental analysis

Environmental monitoring plays a crucial role in assessing and managing environmental quality, providing real-time data for decision-making, and ensuring the protection of ecosystems and public health. This study introduces a high-accuracy spectrophotometer for digital microfluidics, designed for on-site testing analysis and offering, with excellent performance in the visible spectrum range. This device features an actuator electrode with an integrated hole and a gradient index lens as a collimator, enabling real-time droplet measurement through a vertical optical path. Employing LEDs matched to the specific absorption wavelengths, the system exhibited good linearity for copper sulfate and auramine O across the visible spectrum. The applicability of the device for determining nitrite and iron in water was validated. The applicability of the device for determining ammonium concentrations in natural water using the indophenol blue method was evaluated. The device demonstrated high stability and accuracy, with results that were consistent with those from a commercial UV-2550 spectrophotometer. A detection limit of 13 μg/L was achieved, indicating the device's sensitivity for ammonium analysis. This device demonstrated excellent stability during the 24-hour on-site monitoring of ammonium concentrations in the Lijiang River. This integrated digital microfluidics platform shows potential as an innovative tool for environmental monitoring, biochemical analysis, and food safety assurance.