Functionalized g-C3N4 Quantum Dots Based Fluorescent Sensors for Detection of Toxic Metal Ions

Abstract

Exposure to toxic metal ions, such as mercury, lead, and cadmium, poses severe threats to human health. These metals have been widely investigated, and their impacts on human health are periodically assessed. The advancement of fluorescent sensors with exquisite sensitivity and selectivity for detection of toxic metal ions has attracted a great deal of recent research. Quantum dots, devoid of heavy metals and possessing excellent photoluminescence properties, are highly anticipated in the field of fluorescence sensing. This has led to the advent of quantum dots-based sensing platforms due to their distinct optical and electronic properties. Being metal-free, the graphitic carbon nitride quantum dots (g-C3N4 QDs) have attracted enormous interest in sensing applications due to their significant quantum confinement and edge effects, blue emission, high quantum yield, resistance against photobleaching and high ionic strength. The main focus of this doctoral research work is to design functionalized g-C3N4 QDs for sensing toxic metal ions in the aqueous phase. The aim was to functionalize g-C3N4 QDs either by doping with heteroatoms or by surface functionalization with organic ligands to target a specific metal ion. Here, various functionalized g-C3N4 QDs were successfully synthesized and were used as fluorescent sensors. Silver nanoparticles embedded sulfur-doped gCN quantum dots (Ag-S-gCN QDs), L-Glutathione (GSH) modified graphitic carbon nitride quantum dots (GSH@g-C3N4 QDs), L-Cysteine (L-Cys) functionalized boron doped gC3N4 QDs (L- Cys/B-gC3N4 QDs) were prepared for detection of Hg2+, Pb2+, and Cd2+ ions, respectively. The comprehensive characterization of the synthesized materials was carried out to understand their structural, morphological, and optical properties. The fluorescence sensing application of the synthesized materials was evaluated for detection of toxic metal ions. The detailed mechanistic study was conducted to understand the interaction between the probe and analyte. Eventually, the sensor system was employed in real water samples to detect the aforementioned toxic metal ions. First, silver nanoparticles embedded sulfur-doped gCN quantum dots (Ag-S-gCN QDs) were synthesized for detection of Hg2+. The as-prepared quantum dots emitted strong blue fluorescence with a relative quantum yield of 36.5%. They exhibited significant stability against photobleaching and high ionic strength. The Ag-S-gCN QDs, under optimal conditions, were employed for fast sensing of Hg2+ ions. The limit of detection (LOD) and limit of quantification (LOQ) were measured to be 0.13 μM and 0.43 μM, respectively, with a linear range of 0.1–0.6 μM. A static quenching mechanism was proposed from the average lifetime calculation accompanied by a redox reaction via electron transfer from metallic Ag to Hg2+ ions. A substantial amount (> 85%) of Hg2+ ions in the real water samples were recovered within a relative standard deviation (RSD) of ≤ 5%. Further, surface functionalization of g-C3N4 QDs was achieved by preparing L-Glutathione modified graphitic carbon nitride quantum dots (GSH@g-C3N4 QDs) for detection of Pb2+. With a relative quantum yield of 27%, the system demonstrated exceptional stability against ionic strength and photobleaching. Additionally, it was discovered that the fluorescence emission of the sensor was selectively quenched in the presence of Pb2+ ions. The limit of detection (LOD) for Pb2+ was measured to be 0.025 μM in a linear range of 0.01 μM-0.1 μM. The average lifetime calculations from the lifetime decay experiment along with UV–vis. absorption studies suggested that the quenching pattern follows a static quenching mechanism. Finally, the system was used in real water samples to detect Pb2+ with a recovery percentage greater than 90%. The functionalization of gC3N4 QDs was further extended by passivating L-Cysteine on Boron doped gC3N4 QDs (L-Cys/B-gC3N4 QDs). The blue-emitting modified quantum dots demonstrated a high quantum yield of 28% with exceptional water solubility, resistance to photobleaching, and ionic strength. Further, they were used as a fluorescent probe for monitoring Cd2+ ions at trace levels in water. They exhibited a responsive behaviour to Cd2+ ions with enhancement in the fluorescence signal. Based on time correlated single-photon counting studies (TCSPC) and UV-vis. absorption studies, the fluorescence enhancement was ascribed to the chelation-enhanced fluorescence (CHEF) mechanism. The fluorescence emission intensity of L-Cys/B-gC3N4 QDs displayed a good linear correlation within Cd2+ concentration from 0.1 to 0.7 μM, with a limit of detection (LOD) of 0.23 μM. The proposed method was effectively employed to monitor Cd2+ in tap and pond water samples, achieving exceptional recovery rates between 95-106%, with a relative standard deviation (RSD) of 2.6-3.4%.