Red‐Emitting Carbon Dots as a Fluorescence and Chromogenic Dual Mode Probe for Rapid Determination and Smart Phone Imaging of Cu ²⁺

In this study, a kind of red-emitting carbon dots (R-CDs) was synthesized by solvothermal treatment of terephthalic acid (TPA) and 5-amino-1,10-phenanthroline (Aphen) as precursors in N, N-dimethylformamide (DMF). Comprehensive characterizations demonstrated that the R-CDs were uniform spheres with an average diameter of 2.56 nm and exhibited red fluorescence at 615 nm under 555-nm excitation. The R-CDs could form non-fluorescent ground-state aggregates with Cu²⁺, resulting in an increase in the average particle size to 3.70 nm. The aggregation also caused a significant fluorescence quenching through a static mechanism, concomitant with a change of the R-CDs solution. Consequently, a fluorescence-colorimetric dual-mode sensor was constructed for Cu²⁺ detection, with a LOD of 90 nM in the fluorescence mode and 0.73 μM in the colorimetric mode. Based on the distinct fluorescence response of R-CDs, smartphone-assisted visual detection of Cu²⁺ was also achieved in real samples. This approach demonstrates advantages of convenience, time efficiency, and low cost, providing a new strategy for in situ detection of heavy metals.