Highly emitting red-CDs for extreme acidic pH monitoring, and multi-mode ratiometric detection of Ag+

Background Red-emitting carbon dots (R-CDs), characterized by their long-wavelength fluorescence in the red spectral region, offer significant advantages as fluorescent probes due to their capacity to effectively mitigate light scattering and suppress autofluorescence interference from biological matrices. This inherent optical property positions them as one of the most promising candidates for high-contrast bioimaging and sensitive sensing applications. Herein, a novel type of highly luminescent R-CDs was synthesized via a tailored precursor design and doping strategy. Their potential as advanced fluorescent probes was systematically evaluated, focusing on their exceptional optical properties, photostability, and responsiveness to specific substances. Results The emitting of R-CDs exhibits strong pH sensitivity in the range of 1.5-4.0, enabling monitoring of extremely acidic environments. Additionally, R-CDs were employed as probes to develop a multi-mode sensing platform integrating ratiometric fluorescence, ratiometric colorimetry, and smartphone-based imaging for Ag + detection. Fluorescence mode: Ag + selectively quenches the red emission of R-CDs, while yellow emission of Cu-CDs remains stable. Quantification detection relies on the I 624 /I 550 ratio, with a linear range of 0–185.3 μM and LOD of 0.266 μM. Colorimetry mode: Ag + -induced aggregation of the R-CDs weakened the original absorption band and generates a new peak at 680 nm, enabling detection via the A 680 /A 610 ratio with a linear range of 0–208.0 μM and LOD of 0.398 μM. Moreover, the R-CDs solution undergoes a distinct color change under exposure to Ag + ; by leveraging smartphone-based color recognition, a portable visual sensing platform has been successfully constructed. Significance In this work, the synthesis of R-CDs provides a strategic reference for developing high-performance, multicolor-tunable, and function-oriented doped CDs. The multi-mode sensing platform achieves functional complementarity and data cross-validation, thereby improving the reliability, anti-interference capability and on-site applicability of detection. Analytical robustness is further enhanced by the ratiometric strategy's built-in reference signal, which effectively compensates for instrumental, environmental, and operational variables. This design strategy features universal principles and demonstrates strong potential across a wide range of applications.