o-Phenylenediamine derived carbon dots enable microfluidic-fluorometric detection of Chromium(VI) via charge transfer

A microfluidic-fluorometric platform integrated with carbon dots was developed for the real-time and sensitive detection of Chromium(VI) (Cr 6 + ). Chitosan and o-Phenylenediamine were used as precursors to synthesize nitrogen-doped carbon dots (OPD-CDs) via a solvothermal method. The OPD-CDs exhibited a monodisperse spherical nanostructure with an average diameter of 3.86 ± 0.09 nm, a quantum yield of 36.77 %, and a surface enriched with amino and carboxyl functional groups. Cr 6+ induced selective fluorescence quenching through a charge transfer mechanism, where electron donation from surface amines suppressed radiative recombination, accompanied by emission red-shift and increased Stokes shift. Furthermore, quenched OPD-CDs was able to restore fluorescence by adding H 2 O 2 . OPD-CDs demonstrated a detection limit of 0.73 μM and a linear range of 0.2–210 μM under the conventional spectrofluorometry. When incorporated into a microfluidic chip, the OPD-CDs integrated platform achieved a rapid response within 1 min, a detection limit of 1.04 μM, and high selectivity against 26 potential interfering ions. Spike-recovery tests in tap and river water samples yielded recoveries of 98.6–104.7 % (RSD ≤ 2.2 %), confirming its practicality for on-site water quality monitoring. This study innovatively merges optoelectronics features of carbon dots with the microfluidic miniaturization, enabling portable and real-time monitoring of Cr 6+ , which highlights critical needs for rapid detection of heavy metals in accidental pollution events and water quality surveillance.