Instant synthesis of nitrogen-doped Ti3C2 MXene quantum dots for fluorescence and electrochemical dual-mode detection of norepinephrine with a portable smartphone assay†

Next-generation 2D materials, such as transition metal carbides and nitrides (MXenes), have received increasing attention owing to their physicochemical properties. In this study, we synthesized highly intense fluorescent materials, nitrogen-doped MXene quantum dots (N-MQDs) using an easy and less time-consuming microwave-assisted method. These N-MQDs are spherical, fluorescent, and highly sensitive materials, as confirmed by high-resolution transmission electron microscopy, atomic force microscopy, UV-visible, fluorescence, Fourier transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, zeta potential, and contact angle measurements. The N-MQDs were used as dual probes for the fluorescence and electrochemical sensing of neurotransmitter norepinephrine (NE-0.1 to 500 μM). The sensing strategy is based on the Förster resonance energy transfer acquired by the N-MQDs, leading to fluorescence quenching at 400 nm. A new emission peak at 500 nm with color changes and NE-to-NE quinone conversion in an electrochemical reaction. Fluorescence and electrochemical analyses were revealed using the human serum sample limit of detection (LOD) values of 40 and 33 nM, respectively. For point-of-care analysis, we developed a smartphone-integrated sensor array to calculate intensity changes, and the relative red/green/blue (RGB) values were measured at different concentrations of NE. The synthesized fluorescent probe is a promising candidate for detecting NE in biofluids. It is highly selective toward NE and is suitable for the early diagnosis of neurological diseases.