Electrochemical sensing of mercury ions in electrolyte solutions by nitrogen-doped graphene quantum dot electrodes at ultralow concentrations

Parthenium hysterophorus derived nanostructures as an efficient carbocatalyst for the electrochemical sensing of mercury(II) ions

The sustainable utilization of resources motivate us to create eco-friendly processes for synthesizing novel carbon nanomaterials from waste biomass by minimizing chemical usage and reducing energy demands. By keeping sustainability as a prime focus in the present work, we have made the effective management of Parthenium weeds by converting them into carbon-based nanomaterial through hydrothermal treatment followed by heating in a tube furnace under the nitrogen atmosphere. The XPS studies confirm the natural presence of nitrogen and oxygen-containing functional groups in the biomass-derived carbon. The nanostructure has adopted a layered two-dimensional structure, clearly indicated through HRTEM images. Further, the nanomaterials are analyzed for their ability towards the electrochemical detection of mercury, with a detection limit of 6.17 μM, while the limit of quantification and sensitivity was found to be 18.7 μM and 0.4723 μM μA-1 cm-2, respectively. The obtained two-dimensional architecture has increased the surface area, while the nitrogen and oxygen functional groups act as an active site for sensing the mercury ions. This study will open a new door for developing metal-free catalysts through a green and sustainable approach by recycling and utilization of waste biomass.

Microwave synthesized N-doped carbon dots for dual mode detection of Hg(II) ion and degradation of malachite green dye

One of the most intriguing materials today is carbon dots, which offer a variety of possible uses owing to their distinct photophysical and chemical characteristics. The current study examines the electrochemical and photochemical aspects of carbon dots produced in a single pot for environmental sustainability. Domestic microwave-assisted pyrolysis of urea and glucose yielded chemically synthesized nitrogen-doped carbon dots (microwave synthesized N-doped carbon dots (M-NCDs)) with blue fluorescence and a quantum yield of 14.9 %. High water dispersibility, stability, and biocompatibility were the significant attributes of synthesized M-NCDs. Customarily fluorescent carbon dots were initially used for sensing studies. Fluorescent and electrochemical studies manifest the excellent stability, sensitivity, and selectivity of M-NCDs for mercuric ions. Both methods' Hg (II) procure detection limits of 3.5 nM and 6.1 nM. In addition to sensing traits, the subsequent section deals with the potential of M-NDCs to bring about the exhaustive degradation of malachite green (MG) dye. Within 60 min, 98 % of the dye was catalytically degraded by M-NCD by first-order kinetics based on the Langmuir-Hinshelwood model. This is the first time reporting the catalytic degradation of malachite green dye utilizing carbon dot in its natural form rather than being doped with any metal atom or converted to any composite form.

Novel Amperometric Mercury-Selective Sensor Based on Organic Chelator Ionophore

A novel amperometric sensor for the direct determination of toxic mercury ions, Hg²⁺, based on the organic chelator ionophore N, N di (2-hydroxy-5-[(4-nitrophenyl)diazenyl]benzaldehyde) benzene-1,2-diamine (NDBD), and multiwalled carbon nanotubes (MWCNT) immobilized on a glassy carbon electrode surface was developed. The parameters influencing sensor performance including the ionophore concentration, the applied potential, and electrolyte pH were optimized. The sensor response to Hg²⁺ was linear between 1-25 µM with a limit of detection of 60 nM. Interferences from other heavy metal ions were evaluated and the sensor showed excellent selectivity towards Hg²⁺. The method was successfully applied to the determination of mercury ions in milk and water samples.

Metal ion sensing with graphene quantum dots: detection of harmful contaminants and biorelevant species

Graphene quantum dots (GQDs) are attractive materials for use as highly selective and sensitive chemical sensors, owing to their simple preparation and affordability. GQDs have been successfully deployed as sensors for toxic metal ions, which is a significant issue due to the ever-increasing environmental contamination from agricultural and industrial activities. Despite the success of GQDs in this area, the mechanisms which underpin GQD-metal ion specificity are rarely explored. This lack of information can result in difficulties when attempting to replicate published procedures and can limit the judicious design of new highly selective GQD sensors. Furthermore, there is a dearth of GQD examples which selectively detect biologically relevant alkali and alkaline earth metals. This review will present the current state of GQDs as metal ion sensors for harmful contaminants, highlighting and discussing the discrepancies that exist in the proposed mechanisms regarding metal ion selectivity. The emerging field of GQD sensors for biorelevant metal ion species will also be reviewed, with a perspective to the future of this highly versatile material.

Detection of Fe3+ and Hg2+ ions through photoluminescence quenching of carbon dots derived from urea and bitter tea oil residue

In this study, we prepared nitrogen-doped carbon dots (xNCDs) using hydrothermally-treated bitter tea oil residue with urea for the detection of metal ions by monitoring the photoluminescence quenching. The quantum yields of the xNCDs increased from approximately 3.85% (CDs) to 5.5% (3NCDs) and 7.2% (1NCDs), revealing that nitrogen doping effectively increases the fluorescence emission. The increased emission of the xNCDs can be attributed to radiative recombination resulting from the π-π* transition of the C=C or the n-π* transition between the C=O or N=O of sp³ units. Moreover, the CDs have abundant surface-attached phenolic and hydroxyl groups that coordinate with Fe³⁺ ions and quench the fluorescence. Conversely, Hg²⁺ ions preferentially adsorb on nitrogen-containing groups, such as amide-carbonyl groups (O=C-NH₂) and pyridinic and pyrrolic functionalities, on the surface of the NCDs owing to their strong affinity, quenching the substantial photoluminescence emissions. Our results suggest that bitter tea oil residue-derived carbon dots can be used to selectively detect metal ions, such as Fe³⁺ and Hg²⁺, by doping with nitrogen using urea as a nitrogen precursor.

Measurement of mercury with highly selective fluorescent chemoprobe by carbon dots and silver nanoparticles

This work describes a novel fluorescent chemoprobe that uses carbon dots and silver nanoparticles (AgNPs) to monitor mercury ions in aqueous samples attributed to the principle of inner filter effect. The fluorescent response signal of the carbon dots is diminished by AgNPs, attributed to inner filter effect, and is restored with the addition of Hg²⁺. The fluorescent chemoprobe was specific over the range from 0.01 to 2.5 μM and a high sensitivity of 3.6 nM. The chemoprobe was validated using real local aqueous samples, and the spike recoveries of 97.4%-103% were excellent and satisfied. The data indicated that the developed fluorescent chemoprobe was sensitive, selective, stable and reliable. This fluorescent chemoprobe provides a sensitive tool with broad prospects for mercury detection in aqueous samples and the work will offer ideas for designing and constructing novel fluorescent probes.

Facile and ultra-sensitive voltammetric electrodetection of Hg2+ in aqueous media using electrodeposited AuPtNPs/ITO

In this study, we have investigated the use of electrodeposited Au-Pt nanoparticles (AuPtNPs) on indium tin oxide (ITO) for the detection of Hg²⁺ heavy ions in water samples. The mechanism of AuPtNP electrocrystallization on ITO glass in an aqueous solution containing 0.5 mM HAuCl₄ + 0.5 mM H₂PtCl₆ is described for the first time. The nucleation mechanism of monometallic AuNPs on ITO was found to be progressive; however, a transition from progressive to instantaneous was observed for bimetallic AuPtNPs at elevated overpotentials. The modified ITOs were then assessed for the electrodetection of Hg²⁺ in aqueous media. It was shown by differential pulse voltammetry (DPV) that the sensitivity of the constructed AuPtNPs/ITO electrode toward Hg²⁺ was about 2.08 μA nM^-1. An approximate detection limit of 4.03 nM Hg²⁺ was achieved, which is below the permissible level of 30.00 nM Hg²⁺ in drinking water, according to the World Health Organization (WHO). Characterization of AuPt nanostructures was carried out by X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM), and different electrochemical techniques (cyclic voltammetry (CV), chronoamperometry, and electrochemical impedance spectroscopy (EIS)). Our results indicate a good potential of a facile and robust electrochemical assembly for on-site detection of heavy metals in water samples.

Effect of Solvent on Fluorescence Emission from Polyethylene Glycol-Coated Graphene Quantum Dots under Blue Light Illumination

To explore aggregate-induced emission (AIE) properties, this study adopts a one-pot hydrothermal route for synthesizing polyethylene glycol (PEG)-coated graphene quantum dot (GQD) clusters, enabling the emission of highly intense photoluminescence under blue light illumination. The hydrothermal synthesis was performed at 300 °C using o-phenylenediamine as the nitrogen and carbon sources in the presence of PEG. Three different solvents, propylene glycol methyl ether acetate (PGMEA), ethanol, and water, were used for dispersing the PEG-coated GQDs, where extremely high fluorescent emission was achieved at 530-550 nm. It was shown that the quantum yield (QY) of PEG-coated GQD suspensions is strongly dependent on the solvent type. The pristine GQD suspension tends to be quenched (i.e., QY: ~1%) when dispersed in PGMEA (aggregation-caused quenching). However, coating GQD nanoparticles with polyethylene glycol results in substantial enhancement of the quantum yield. When investigating the photoluminescence emission from PEG-coated GQD clusters, the surface tension of the solvents was within the range of from 26.9 to 46.0 mN/m. This critical index can be tuned for assessing the transition point needed to activate the AIE mechanism which ultimately boosts the fluorescence intensity. The one-pot hydrothermal route established in this study can be adopted to engineer PEG-coated GQD clusters with solid-state PL emission capabilities, which are needed for next-generation optical, bio-sensing, and energy storage/conversion devices.

A novel N,S-rich COF and its derived hollow N,S-doped carbon@Pd nanorods for electrochemical detection of Hg2+ and paracetamol

Covalent-organic frameworks (COFs) are conjugate crystalline polymers with high porosity, controllable pores and structure as well as large specific surface area, showing great potential for electrochemical sensors. Here, a new N,S-rich COF_BTT-TZT is proposed by direct amine-aldehyde dehydration condensation between 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline (TZT) and benzo [1,2-b:3,4-b':5,6-b'']trithiophene-2,5,8-tricarbaldehyde (BTT). The COF_BTT-TZT has a hexagonal hcb structure with theoretical pore of 2.2 nm and presents rod-like morphology with many small flakes on its surface. Particularly, there are lots of S and N atoms in COF_BTT-TZT, which provides abundant adsorption sites for Hg²⁺ so that it can be used to detect Hg²⁺. The proposed Hg²⁺ sensor has a linear range of 0.54 nM-5.0 μM and a detection limit of 0.18 nM. Besides, using COF_BTT-TZT as precursor and template, the hollow N,S-doped C@Pd nanorods which possesses many tiny Pd nanoparticles embedded in rods-like hollow structure are obtained. An electrochemical paracetamol sensor is also proposed based on the N,S-doped C@Pd nanorods, showing low detection limit of 11 nM and wide linear range of 33 nM-120 μM. The good results provide an important guidance for the application of COF in electrochemical sensors.

Graphene Quantum Dots by Eco-Friendly Green Synthesis for Electrochemical Sensing: Recent Advances and Future Perspectives

The continuous decrease in the availability of fossil resources, along with an evident energy crisis, and the growing environmental impact due to their use, has pushed scientific research towards the development of innovative strategies and green routes for the use of renewable resources, not only in the field of energy production but also for the production of novel advanced materials and platform molecules for the modern chemical industry. A new class of promising carbon nanomaterials, especially graphene quantum dots (GQDs), due to their exceptional chemical-physical features, have been studied in many applications, such as biosensors, solar cells, electrochemical devices, optical sensors, and rechargeable batteries. Therefore, this review focuses on recent results in GQDs synthesis by green, easy, and low-cost synthetic processes from eco-friendly raw materials and biomass-waste. Significant advances in recent years on promising recent applications in the field of electrochemical sensors, have also been discussed. Finally, challenges and future perspectives with possible research directions in the topic are briefly summarized.

Computational studies and biosensory applications of graphene-based nanomaterials: a state-of-the-art review

Graphene, graphene oxide (GO) and graphene quantum dots (GQDs) are expected to play a vital role in the diagnosis of severe ailments. Computer-based simulation approaches are helpful for understanding theoretical tools prior to experimental investigation. These theoretical tools still have a high computational requirement. Thus, more efficient algorithms are required to perform studies on even larger systems. The present review highlights the recent advancement in structural confinement using computer simulation approaches along with biosensory applications of graphene-based materials. The computer simulation approaches help to identify the interaction between interacting molecules and sensing elements like graphene sheets. The simulation approach reduces the wet-lab experiment time and helps to predict the interaction and interacting environment. The experimental investigation can be tuned at a molecular level easily to predict small changes in structural configuration. Here, the molecular simulation study could be useful as an alternative to actual wet experimental approaches. The sensing ability of graphene-based materials is a result of interactions like hydrogen bonding, base-base interaction, and base-to-pi interaction to name a few. These interactions help in designing and engineering a substrate for sensing of various biomolecules.