Facile synthesis of dual-emission carbon dots and fabrication of Poly(vinyl alcohol)/Alginate/carbon dots films for selective fluorescence detection and smartphone-assisted monitoring of NO2- and Fe3+ in water

Carbon dots (CDs) have demonstrated high selectivity as fluorescent probes for pollutant detection. However, most reported systems rely on single-emission intensity responses, which are susceptible to signal fluctuation, environmental interference, and limited reliability in complex matrices. In addition, translating solution-phase CD sensors into stable solid-state platforms remains challenging due to background fluorescence and the potential leaching of nanomaterials. Herein, dual-emission CDs with characteristic peaks at 420 and 520 nm were synthesized, enabling selective discrimination of and Fe3+ via distinct quenching responses. The sensor exhibited excellent linearity with low detection limits of 0.12 ppm for and 0.17 ppm for Fe3+. Crucially, a flexible PVA/Alg/CD fluorescent film was fabricated by immobilizing the CDs within a co-polymer PVA/alginate matrix. The composite film displayed strong and uniform blue fluorescence under 365 nm UV excitation, excellent optical stability, and effective confinement of CDs without observable leaching in aqueous environments. Upon exposure to NO ₂ -, the film exhibited rapid and visually distinguishable fluorescence quenching, with intensity decreasing progressively over the 0 - 50 ppm range. Notably, smartphone-assisted RGB image analysis enabled quantitative evaluation of fluorescence changes, generating linear calibration curves with coefficients of determination exceeding 0.93, consistent with spectroscopic measurements. These findings highlight the strong potential of the PVA/Alg/CDs film as a portable, solid-state platform for on-site water quality monitoring.