Fluorine-free synthesis of Ti3C2 MQDs for smartphone-based fluorescent and colorimetric determination of acetylcholinesterase and organophosphorus pesticides

Ti₃C₂ MQDs were synthesized using an effective fluorine-free method with excitation/emission maxima at 390/490 nm and a fluorescence quantum yield of 11.78%. In contrast to the traditional, hazardous, and time-consuming process of HF pretreatment, our fluorine-free method is safe and simple. Acetylcholinesterase (AChE) could catalyze the hydrolysis of acetylthiocholine (ATCh) to produce thiocholine which was further reacted with Ehrman's reagent and decomposed to form a yellow product 2-nitro-5-thiobenate anion (TNB). Due to the obvious overlap between the excitation spectrum of Ti₃C₂ MQDs and the absorption spectrum of TNB, AChE catalyzed the hydrolysis of substrate DTNB/ATCh to form TNB, which can effectively quench the fluorescence of Ti₃C₂ MQDs through the inner filter effect (IFE). However, the presence of organophosphorus (OPs) inhibited the activity of AChE, leading to a less expressed IFE and increasing recovery of fluorescence. This was used for the quantification of OPs with a detection limit of 0.20 μg·L^-1. Moreover, with the constant increase of AChE activity, the color of the reaction system changed visibly from colorless to yellow, and then from yellow to colorless with further continuous addition of OPs. A colorimetric detection with a paper-based sensor of AChE activity and OP concentration was also fabricated by analyzing changes in RGB value using a smartphone APP. In this work, we proposed an effective fluorescence/colorimetric two-mode detection method, which opened a new horizon to detect other targets.