Mn-doped carbon dots-based fluorescent-colorimetric dual-mode probes for selective and sensitive detection of Cr(VI) ions and l-ascorbic acid via smartphone-integrated analytical platform

Background Hexavalent chromium ions (Cr(VI)), a notorious toxic heavy metal pollutant with proven carcinogenicity, endangers human health and the environment. Meanwhile, l -ascorbic acid (L-AA), a vital biological antioxidant, has abnormal levels closely tied to various diseases. Developing efficient synchronous detection methods for these two key analytes is of great value in clinical and environmental monitoring. Traditional fluorescent nanomaterials, particularly those emitting in the blue/green region, suffer from limited tissue penetration depth and autofluorescence interference, hindering their application in deep-tissue imaging and real-time monitoring. Therefore, the development of red-emitting nanomaterials with enhanced optical properties and multifunctionality is crucial for advancing biosensing and bioimaging technologies. Results Herein, we present a facile hydrothermal synthesis of manganese-doped carbon dots (Mn-CDs) that exhibit red fluorescence ( λ ex / λ em  = 520 nm/675 nm) and exceptional sensitivity for the simultaneous detection of Cr(VI) ions and L-AA. Mn-CDs were synthesized using o -phenylenediamine (OPD) and 2,5-diaminobenzenesulfonic acid (DSA) as precursors, along with manganese chloride tetrahydrate as a metal dopant source, achieving a high fluorescence quantum yield of 11 %. The Mn-CDs function as a dual-responsive "off-on-off" fluorescent sensor, demonstrating low detection limits of 0.44 μM for Cr(VI) and 1.11 μM for L-AA. Utilizing smartphone-integrated RGB analysis, the detection limits were further improved to 0.021 μM for Cr(VI) and 0.06 μM for L-AA. The Mn-CDs exhibit excellent biocompatibility and low cytotoxicity, enabling successful application in HeLa cells imaging and real-time intracellular monitoring of Cr(VI) and L-AA. Significance This study introduces a novel Mn-CDs-based fluorescent-colorimetric dual-mode probe that overcomes the limitations of traditional blue/green-emitting CDs by offering enhanced tissue penetration, minimized background interference, and multifunctional integration for biosensing and bioimaging. The integration of Mn-CDs with a smartphone-based analytical platform enhances their portability and user-friendliness for on-site detection, providing a valuable tool for heavy metal pollution monitoring and clinical biomarker detection. This work not only broadens the application scope of metal-doped carbon dots in biosensing but also provides a new strategy for environmental monitoring and clinical diagnostics, demonstrating significant application prospects in advancing biomedical research and clinical practice.