Swamidoss Abraham John

This research introduces a novel fluorescence sensor ‘on-off-on’ employing nitrogen-doped carbon dots (N-CDs) with an ‘on-off-on’ mechanism for the selective and sensitive detection of Hg(II) and L-cysteine (L-Cys). N-CDs was synthesized using citric acid as the carbon precursor and urea as the nitrogen source in dimethylformamide (DMF) solvent, resulting in red emissive characteristics under UV light. Comprehensive spectroscopic analyses, including UV-Vis, fluorescence, FT-IR, XRD, XPS, Raman, and Zeta potential techniques, validated the structural and optical characteristics of the synthesized N-CDs. The maximum excitation and emission of N-CDs were observed at 548 and 622 nm, respectively. The quantum yield of N-CDs was calculated to be 16.1%. The fluorescence of N-CDs effectively quenches upon the addition of Hg(II) due to the strong coordination between Hg(II) and the surface functionalities of N-CDs. Conversely, upon the subsequent addition of L-Cys, the fluorescence of N-CDs was restored. This restoration can be attributed to the stronger affinity of the -SH group in L-Cys towards Hg(II) relative to the surface functionalities of N-CDs. This dual-mode response enabled the detection of Hg(II) and L-Cys with impressive detection limits of 15.1 nM and 8.0 nM, respectively. This sensor methodology effectively detects Hg(II) in lake water samples and L-Cys levels in human urine, with a recovery range between 99 and 101%. Furthermore, the N-CDs demonstrated excellent stability, high sensitivity, and selectivity, making them a promising fluorescence on-off-on probe for both environmental monitoring of Hg(II) and clinical diagnostics of L-Cys.

The development of efficient H₂O₂ sensors is crucial because of their multiple functions inside and outside the biological system and the adverse effects that a higher concentration can cause. This work reports a highly sensitive and selective non-enzymatic electrochemical H₂O₂ sensor achieved through the hybridization of Co₃S₄ and graphitic carbon nitride nanosheets (GCNNS). The Co₃S₄ is synthesized via a hydrothermal method, and the bulk g-C₃N₄ (b-GCN) is prepared by the thermal polycondensation of melamine. The as-prepared b-GCN is exfoliated into nanosheets using solvent exfoliation, and the composite with Co₃S₄ is formed during nanosheet formation. Compared to the performances of pure components, the hybrid structure demonstrates excellent electroreduction towards H₂O₂. We investigate the H₂O₂-sensing performance of the composite by cyclic voltammetry, differential pulse voltammetry, and amperometry. As an amperometric sensor, the Co₃S₄/GCNNS exhibits high sensitivity over a broad linear range from 10 nM to 1.5 mM H₂O₂ with a high detection limit of 70 nM and fast response of 3 s. The excellent electrocatalytic properties of the composite strengthen its potential application as a sensor to monitor H₂O₂ in real samples. The remarkable enhancement of the electrocatalytic activity of the composite for H₂O₂ reduction is attributed to the synergistic effect between Co₃S₄ and GCNNS.

We report a comprehensive study of the interaction of transition and heavy metal ions with graphene quantum dots-capped silver nanoparticles (AgGQDs) using different spectroscopic and microscopic techniques. High-resolution transmission electron microscopy studies show that the interaction of metal ions with AgGQDs leads to the formation of metal oxides, the formation of zerovalent metals, and the aggregation of Ag nanoparticles (AgNPs). The metal ions may interact with AgGQDs through selective coordination with -OH and -COOH functionalities, adsorption on the graphene moiety, and directly to AgNPs. For instance, the interaction of Cd²⁺ with AgGQDs altered the spherical shape of AgNPs into a chain-like structure. On the contrary, the formation of PbO is observed after the addition of Pb²⁺ to AgGQDs. Interestingly, the interaction of AgGQDs with Hg²⁺ results in the complete dissolution of Ag⁰ from the surface of GQDs and subsequent deposition of Hg⁰ on the graphene moiety of GQDs. Unlike transition metal ions, Cd²⁺, Pb²⁺, and Hg²⁺ can adsorb strongly on the graphene surface at the bridge, hollow, and top sites, respectively. This special interaction of heavy metal ions with the graphene surface would decide the mechanistic pathway in which the reaction proceeds. The transition metal ions Cu²⁺, Zn²⁺, Co³⁺, Mn²⁺, Ni²⁺, and Fe³⁺ induced the aggregation of AgNPs.

The electrocatalytic activity of graphitic carbon nitride nanosheets prepared via thermal and solvent exfoliation is compared towards nitrobenzene reduction.

Several researchers have synthesized graphitic carbon nitride (GCN) from various precursors and attached it to electrode substrates after exfoliation under different conditions and have reported inconsistent data on electrochemical impedance, electroactive surface areas, and electrocatalytic activity. Thus, the present study aims to study the same systematically in addition to morphology after modifying GCN on the GC substrate from different exfoliation times in water assisted by sonication. The GCN was prepared from urea by bulk condensation pyrolysis and then attached to the GC substrate by drop casting to study its morphology, electrochemical impedance, and electrocatalytic activity with respect to exfoliation. The SEM image of a GCN-modified GC plate after 15 and 30 min of exfoliation shows bulky structure whereas thin sheets of GCN were noticed after 120 min of exfoliation. On the other hand, broken sheets were observed when GCN was coated from beyond 120 min of exfoliation. The electrochemical impedance studies show that the charge transfer resistance (R_CT) of GCN modified from 15 and 30 min of exfoliation was higher than that for the bare GC electrode. However, it started to decrease while increasing the exfoliation time, and 1.8 kΩ was obtained after 120 min of exfoliation. The R_CT value was again increased to 3.2 and 5.0 kΩ for GCN coated after 150 and 180 min of exfoliation, respectively. The electroactive surface area (EASA) of GCN modified by 15 and 30 min of exfoliation was less than that of the bare GC electrode, whereas it was 3.8-fold higher for GCN coated from 120 min of exfoliation. The electrocatalytic activity of the GC electrodes modified with GCN was then tested by studying ascorbic acid (AA) and dopamine (DA) oxidation and reduction of hydrogen peroxide (HP). Among the different exfoliation times, GCN modified from 120 min of exfoliation displayed the highest electrocatalytic activity toward AA, DA, and HP. This was attributed to its higher EASA and lower charge-transfer resistance.