MXene quantum dots for ratiometric fluorescence detection of a Bacillus anthracis biomarker

Abstract

As a new member of the quantum dot (QD) family, MXene QDs (MQDs) have attracted increasing attention in fluorescence sensing because of their excellent photostability and resistance to photobleaching. MQD-based fluorescent sensors are still at the initial stage, especially ratiometric fluorescence methods with higher accuracy and sensitivity. Herein, blue fluorescent MQDs are prepared by acid oxidation and hydrothermal methods using Ti₃C₂ MXene as the precursor. The MQDs are rich in hydroxyl and amino groups, which can coordinate with EDTA and Eu³⁺ to form the composite probe MQDs–EDTA–Eu³⁺. The MQDs–EDTA–Eu³⁺ complex emits the blue fluorescence of MQDs since Eu³⁺ is surrounded by H₂O molecules, which are excellent quenchers to the fluorescence of Eu³⁺. 2,6-Dipicolinic acid (DPA) is a biomarker of Bacillus anthracis, and also an excellent ligand of Eu³⁺. With the addition of DPA, it can replace the H₂O molecule to form the MQDs–EDTA–Eu³⁺–DPA complex, stimulating the intense red fluorescence of Eu³⁺ at 616 nm due to the absorbance energy-transfer effect (AETE) from DPA to Eu³⁺. In the meantime, the fluorescence of MQDs at 445 nm remains stable as a reference signal. In consequence, the MQDs–EDTA–Eu³⁺ complex is used to construct a ratiometric fluorescence method for DPA detection for the first time, with a linear detection range of 0–11 μM and a detection limit of 0.26 nM. Furthermore, an MQD-based test paper is fabricated for the visual detection of DPA, with a low detection limit of 52.4 nM. This work indicates that MQDs have potential applications in the area of visual fluorescence sensing.