Bright yellow fluorescent N-doped Ti₃C₂ MXene quantum dots as an “on/off/on” nanoprobe for selective As³⁺ ion detection

Ti₃C₂ MXene quantum dots (MQDs) are considered to be an emerging nanomaterial in recent times, but the majority of MQDs exhibit limited emission properties in the blue-light region. Longer-wavelength emissive quantum dots are highly desirable in terms of various biological aspects including deep tissue penetration, superior signal-to-noise ratio, reduced radiation damage, etc. In this study, bright yellow fluorescent nitrogen-doped MQDs (N-MQDs) were successfully prepared using a one-pot hydrothermal method. The synthesized N-MQDs showed maximum emission at 570 nm upon excitation at a wavelength of 420 nm, with an optimum fluorescence quantum yield of 13.8%. Interestingly, the emission of the N-MQDs was significantly quenched upon the addition of As³⁺ ions. A mechanistic investigation revealed that static quenching was involved in the decrease in the fluorescence via the formation of a non-fluorescent complex due to the interaction of the functional groups of the N-MQDs and As³⁺. The quenched fluorescence was surprisingly recovered upon treatment of the complex with 2-amino-6-methoxybenzothiazole (MBTZ). The strong interaction of MBTZ with As³⁺ led to the detachment of the quencher from the N-MQDs, resulting in fluorescence recovery. The re-appearance of the functional groups of the N-MQDs after the addition of MBTZ was confirmed via spectroscopic study. Thus, the fluorescence “on/off/on” phenomenon of the N-MQDs nanoprobe was utilised for the instantaneous detection of As³⁺ and MBTZ. The limit of detection values were calculated to be 30 nM and 0.44 μM with a good linearity for As³⁺ and MBTZ, respectively. In addition, a solid sensor has been fabricated to recognize As³⁺ in wastewater, revealing its potential for on-site application in the near future.