Deep Eutectic Solvent-Assisted Nitrogen, Chloride-Functionalized Carbon Quantum Dots: A Fluorescence Turn On-Off-On Approach for Detection of Mercury (II) and Glutathione in the Nanomolar Range

Abstract

In this work, we present a novel technique for the synthesis of Nitrogen & Chloride co-doped carbon quantum dots (N,Cl-CQDs) using a Choline chloride-urea based green deep eutectic solvent (DES) via a Microwave synthesis approach. Wheatgrass (Triticum aestivum) was used as a carbon source. DES played a significant role in the multi-heteroatom doping of N and Cl in respective CQDs, which XPS, FTIR, and XRD analysis confirmed. The TEM images show the spherical shape of N,Cl-CQDs with a median particle diameter of 1.75 nm, ranging from 0.5 to 4.0 nm. The N,Cl-CQDs obtained a high quantum yield (QY), i.e. 36%, compared to un-doped CQDs, which were synthesized in an aqueous medium (QY 15%). The prepared N,Cl-CQDs showed significant properties such as excellent photostability, favourable water solubility, and high optical stability. Furthermore, Hg2+ ions were detected using N,Cl-CQDs (turn-off) with a LOD value of 39 nM in a range of 0 to 0.35 μM, achieved by the dynamic quenching mechanism. Different photophysical parameters were calculated to confirm the fluorescence quenching mechanism, including average lifetime values, fluorescence quantum yield, zeta potential, radiative rate constants (kr), and non-radiative rate constants (knr). The current sensing system possesses an appealing characteristic of functioning as a “turn-on” sensor for GSH detection with a LOD value of 43 nM in a dynamic range of 0 to 0.60 μM. The N,Cl-CQDs prepared in this study exhibited a reduced detection limit and a broad linear range by an easy, environmentally friendly, and rapid method for detecting GSH and Hg2+ ions.