Smart Paper-Based Nanosensor for Simultaneous Environmental eMonitoring of Nitrate and Nitrite

Nitrate (NO₃ ^-) and nitrite (NO₂ ^-) contamination, primarily from agricultural fertilizers, food additives, and industrial effluents, poses a significant threat to the environment and public health. While current detection methods are sensitive, they often rely on complex instrumentation, trained personnel, and time-consuming procedures, limiting their applicability in field settings. To overcome these limitations, we developed a portable, IoT-integrated, paper-based fluorescent sensor that meets all WHO's REASSURED criteria for ideal sensing devices. The sensor employs phenylene-diamine-derived carbon quantum dots (CQDs) as fluorescent probes with NO₂ ^- inducing fluorescence quenching via nitrosylation and diazotization. For simultaneous detection, NO₃ ^- is converted in situ to NO₂ ^- through a simple, eco-friendly on-strip reduction step, enabling unified quantification of both analytes. The platform achieved a limit of detection (LOD) of 0.1 ppm and exhibited a linear response from 1 to 100 ppm. Its performance was validated by using real water samples, successfully determining both NO₃ ^- and NO₂ ^- concentrations. Integrated with a custom hand-held optoelectronic reader and smartphone interface, the system enables real-time data acquisition, wireless transmission, and rapid on-site decision-making. This green, low-cost, and efficient platform offers a practical solution for environmental eMonitoring by integrating nanotechnology, paper based analytical device and smart sensing in a single device.