Nanomolar Level Detection of Metal Ions by Improving the Monodispersity and Stability of Nitrogen-Doped Graphene Quantum Dots

Heteroatom doping of graphene quantum dots (GQDs) leads to the modification of their intrinsic properties, and is used as a fluorescent sensor for metal ions sensing. Here, nitrogen-doped GQDs (N-GQDs) were synthesized using graphene oxide as a precursor in the presence of N,N′-dimethylformamide through a hydrothermal method. The particle size of N-GQDs was examined by a high-resolution transmission electron microscope (HRTEM) image. The surface functional groups and composition of N-GQDs were determined by X-ray photoelectron spectroscopy (XPS) and FTIR spectroscopic techniques. The introduction of nitrogen atoms in the framework of GQDs enhances its quantum yield, water solubility, and stability. The as-prepared nanoprobe has high sensitivity and selectivity for the label-free determination of Fe3+ ions. The values of the quantum yield, average lifetime, different photophysical parameters (such as radiative (kr) and non-radiative (knr) constants), rate of electron transfer (kET), and electron transfer efficiency (ΦEET) have been evaluated to confirm the dynamic quenching between the N-GQDs and Fe3+ ions. The prepared sensor showed high sensitivity with a very good limit of detection at the nanomolar range, i.e., 17 nM for Fe3+ ions, as compared to the other sensors. The prepared sensing probe was also used for the sensing of Fe3+ ions in iron supplementation, and it can be further used for different real sample detection. The synthesized fluorescent probe is very fast, inexpensive, and environmentally friendly.