Ratiometric Multimode Detection of pH and Fe³⁺ by Dual-Emissive Heteroatom-Doped Carbon Dots for Living Cell Applications

Dual-emissive carbon dots (D-CDs) are in high demand for multipurpose applications. Herein, nitrogen-doped carbon dots possessing inherent dual emissions at green (490 nm) and yellow (570 nm) are synthesized hydrothermally from 3,4-diaminobenzoic acid and hydrazine hydrate. The relative intensity of the two emission bands is dependent markedly on the pH of medium, making the D-CDs a robust pH sensor within the pH range of 2.0–7.6. The ratiometric emission of D-CDs is sensitive to trace amounts of Fe3+; the 490 nm emission intensity diminishes with simultaneous amplification of the 570 nm emission intensity. A discernible color change from light green to bright yellow is evident under an ultraviolet (UV) lamp with increasing Fe3+ concentrations, and the CIE color coordinate shifts from (0.33, 0.43) to (0.47, 0.50). Primarily, the complexation between D-CDs and Fe3+ and the accompanying aggregation play a dominant role in the ratiometric change of the emission intensities. For the on-spot detection of Fe3+, smartphone-based and solid-phase sensing is accomplished. The D-CDs also display decent anticancer properties against the breast cancer cell line MCF-7 and effectively detect the intracellular pH and exogenous Fe3+ inside the MCF-7 cells.