Tunable fluorescent amino-functionalized Ti3C2Tx MXene quantum dots for ultrasensitive Fe3+ ion sensing

The development of sensors with high sensitivity, good selectivity and reproducibility are of great importance for the detection of Fe³⁺ in contaminated water for environmental monitoring. In this work, a reflux approach has been adopted to synthesize Ti₃C₂T_x quantum dots (QDs) based on the cutting effect of tetramethylammonium hydroxide (TMAOH) on Ti₃C₂T_x at high temperature. The surface-functionalized Ti₃C₂T_x QDs contained abundant amino groups and exhibited tunable pH-dependent emission, which was attributed to the protonation and deprotonation of the surface terminations. The linearity of the radiometric fluorescence intensity versus pH indicates its great potential as a dual-emission ratiometric pH sensor. Additionally, the Ti₃C₂T_x QDs exhibited tunable excitation-dependent emission behavior, which was related to the degree of passivation by the amino groups on the surface. Furthermore, the fluorescence intensity of the Ti₃C₂T_x QDs shows a linear response toward Fe³⁺ in the nanomolar to micromolar range with a low detection limit of 2 nM, originating from the oxidation and reduction between Fe³⁺ and Ti₃C₂T_x. This ultra-sensitive and selective detection capability demonstrated the environmental application potential for Ti₃C₂T_x QDs as a nanoprobe to monitor Fe³⁺.