As one of the main pollutants, heavy metal ions can accumulate in the human body and cause a cascade of damage. Electrochemical sensors provide great prospects for tracing heavy metal ions because of their properties of high sensitivity, low detection limits and fast response. Electrode surface modification materials play a key role in enhancing the performance of electrochemical sensors. Herein, we summarize in detail the recent work on electrochemical sensors modified by carbon nanomaterials (graphene and its derivatives, carbon nanofibers and carbon nanotubes), metal nanomaterials (gold, silver, bismuth and iron), complexes (MOFs, ZIFs and MXenes) and their composites for the detection of heavy metal ions (mainly include Cd(II), Hg(II), Pb(II), As(III), Cu(II) and Zn(II)) in food and water. The synthetic strategies, mechanisms, innovations, advantages, challenges and prospects of various electrode modification nanomaterials for the detection of heavy metal ions in food and water are discussed. 

Recent advance of nanomaterials modified electrochemical sensors in the detection of heavy metal ions in food and water.

A review on recent advancements in electrochemical detection of dopamine using carbonaceous nanomaterials

An effective and affordable technique for detecting a variety of substances of biological, medical, and environmental significance is electrochemical sensing (ECS). Due to their distinctive structures and abilities to offer robust electrocatalytic activity with little surface fouling, low cost, biocompatibility, and simple electron transfer kinetics, carbon materials-based electrodes like graphene (GR) and carbon nanotube (CNT) are frequently used for the development of electrochemical sensors. Since GR has a large specific surface area, it can host a large number of biomolecules and still have a decent detection sensitivity. Along with important developments in the synthesis, purification, conjugation, and biofunctionalization of GR, device integration technologies and nanofabrication of GR have evolved. Combinations of the aforementioned characteristics have accelerated the development of GR-based sensors for many critical bio-analyses. Along with important developments in the synthesis, purification, conjugation, and biofunctionalization of GR, device integration technologies and nanofabrication of GR have evolved. The rapid development of GR-based sensors for many critical bioanalyses with better detection sensitivity and selectivity has been facilitated by the combination of the aforementioned features. Direct electron transport between the enzyme and the active electrode region is made possible by the application of GR. It is possible to easily construct and execute on-site sensors for oxidases and dehydrogenases with improved selectivity thanks to the excellent electro-catalytic activities of GR on the redox reaction of hydrogen peroxide (H2O2) and nicotinamide adenine dinucleotides (NADH). Since GR's oxidation of NADH, thiols, H2O2, and other interfering species occurs at low potentials, the use of GR-based electrodes effectively suppresses these species. The two benefits of GR-based sensors for use in clinical chemistry, food quality and control, wastewater treatment, and bioprocessing are their great resistance to electrode fouling and selectivity. 

pdf/recent-developments-in-electrochemical-sensors-based-on-qu3cf04mtk.pdf

Recent developments in electrochemical sensors based on graphene for bioanalytical applications

Dopamine, a pivotal excitatory neurotransmitter, plays a crucial role in metabolic, cardiovascular, renal, central nervous, and endocrine systems. Abnormal dopamine within the human body can cause various diseases. Therefore, the precise quantification of dopamine levels, both in vivo and in vitro, holds paramount significance for clinical applications and physiological investigations. Carbon dots (CDs) exhibit a plethora of remarkable properties, including a substantial specific surface area, robust electrical conductivity, commendable biocompatibility, minimal toxicity, and high photostability. Considering these unique characteristics, CDs demonstrate substantial potential for fluorescent sensors, colorimetric sensors, and electrochemical sensors for dopamine detection. This review systematically examined the challenges and prospects for the utilization of CDs-based fluorescent sensors, electrochemical biosensors, and colorimetric sensors for monitoring dopamine levels in recent years. These findings unveil promising avenues for further advancements in the field of dopamine detection. 

Carbon dots-based dopamine sensors: Recent advances and challenges

A sulfur co-doped nitrogen-graphene quantum dot (S,N-GQD) was synthesized from polyaniline using sulfuric acid as an acid catalyst and S-doping agent. Thus, through a simple hydrothermal synthesis method, we could achieve nanosized (∼4 nm) highly crystalline and aromatic S,N-GQDs. The S,N-GQDs, for the first time, exhibited simultaneous sensing toward three of the top 10 toxic metal ions: Cd(II), Pb(II), and Hg(II) with highly sharp peaks and adequate peak-to-peak separation, while the control N-GQD (no S co-doping) exhibited current response only for Cd(II) and the current response of Cd(II) on S,N-GQD was ∼7-fold higher than that of on N-GQD. The detection limit values were lowest hitherto for Cd(II), Pb(II), and Hg(II) on S,N-GQD with 1, 10, and 1 pM, respectively, for the simultaneous sensing of the metal ions. The significantly enhanced sensitivity compared to that of N-GQD for Cd(II) and the versatile simultaneous sensing capability of S,N-GQD is assigned to the co-doping with S, which enabled the sensing of Pb(II) and Hg(II) through the M(II)–S interactions and the enhanced electronic properties, respectively. The sensing characteristics were effectively stretched to actual environmental water samples such as groundwater, seawater, and wastewater, spiked with Cd(II), Pb(II), and Hg(II), and accomplished ∼100% recovery in all the tested samples with a relative standard deviation of ≤0.5%. 

Graphene Quantum Dots Doped with Sulfur and Nitrogen as Versatile Electrochemical Sensors for Heavy Metal Ions Cd(II), Pb(II), and Hg(II)

A highly stable copper (Cu) nanocluster (NC), which exhibited stability for more than one year, was synthesized using nitrogen-doped graphene quantum dots (N-GQDs) as reducing and capping agents and smaller glutathione molecules as additional capping agents. The synthesized NC, CuNC@N-GQDs, successfully sensed dopamine (DA), serotonin (SER), and nicotine (NIC) simultaneously with well-defined peaks and good peak-to-peak separation, whereas none of the controls including CuNCs and N-GQDs exhibited the simultaneous sensing properties. In addition, they exhibited enhanced sensitivity with current responses ∼4, ∼4, and ∼2 times those of the corresponding control for DA, SER, and NIC. The limits of detection obtained were 0.001, 1.00, and 0.01 nM for DA, SER, and NIC, respectively. The higher sensitivity and the simultaneous sensing are indicative of the synergistic effect of CuNCs and N-GQDs in the CuNC@N-GQDs. The sensing performance was successfully extended to real blood and urine samples spiked with DA, SER, and NIC.

A highly stable copper nano cluster on nitrogen-doped graphene quantum dots for the simultaneous electrochemical sensing of dopamine, serotonin, and nicotine: a possible addiction scrutinizing strategy.

Herein, a graphene-nano-molybdenum disulphide (pGr-MoS2), synthesized from pulverized graphite and using precursors of MoS2, was investigated for the electrochemical sensing of dihydroxy benzene isomers (DHBI): hydroquinone (HQ), catechol (CA), and resorcinol (RE). Interestingly, the material could sense the three isomers simultaneously, with well-defined peaks and an adequate potential difference between each peak. The detection limits (3σ method) of HQ, CA, and RE on the glassy carbon electrode (GCE) modified with pGr-MoS2 are 10-13, 10-12, and 10-8 M (i.e., 0.1 pM, 1 pM, and 10 nM), respectively, and are the lowest reported so far for the isomers. The pGr-MoS2/GCE exhibited selectivity towards DHBI, in the presence of other toxic contaminants and metal ions such as phenol, dinitrophenol, trinitrophenol, urea and glucose, Hg(II), Ca(II), Ni(II), Zn(II), Cu(II), Na(I) and K(I). A possible mechanism for this superior selectivity of pGr-MoS2 towards DHBI is discussed based on the structural properties of pGr-MoS2 with evidence. Further, the pGr-MoS2 sensor exhibited reproducibility (with six different electrodes), stability (≥90 days), and repeatability properties. The sensing performance was successfully demonstrated in real water samples such as ground-, tap-, and river- water spiked with HQ, CA, and RE.

Picomolar level electrochemical detection of hydroquinone, catechol and resorcinol simultaneously using a MoS2 nano-flower decorated graphene.

In this work, we present an ultrastable gold–copper nanocluster on nitrogen-doped graphene quantum dot (AuCuNC@N-GQD), with a stability of ≥1 year, achieved through a galvanic exchange process, which displayed intense solid- and solution-state near-infrared emission. Furthermore, the NC exhibited selective and dual sensing of glycine (GLY) through (nonenzymatic) electrochemical (EC) and fluorescence methods, which is assigned to the unique combination of the strong emission and conducting properties of Au–CuNC and N-GQD, respectively, in the same material and is a first-time report of all the above-mentioned. The limits of detection (LODs) obtained for GLY by EC and fluorescence sensing were 10 nM and 1 μM, respectively and are the lowest LOD values reported hitherto in the respective categories. Compared to the controls, AuCuNC@N-GQDs exhibited a ∼20-fold enhancement with a sensitivity of 0.377 μA μM–1 cm–2. The preferential Au–GLY interaction is considered to be the reason for the selective sensing of GLY by AuCuNC@N-GQD and the improvement in the electronic and conductivity characteristics due to N-GQD to the enhanced sensitivity. The sensing performance was successfully extended to real blood and urine samples spiked with GLY without interference. 

Ultrastable Gold–Copper Nanoclusters on Nitrogen-Doped Graphene Quantum Dots for Selective Electrochemical and Fluorescence Sensing of Glycine

This work explores and demonstrates the electrochemical sensing ability of a highly stable (≥1 year) bimetallic Cu–Au nanocluster, AuCuNC@N-GQD, toward Pb(II) with high selectivity. Pb(II) is one among the top 10 hazardous substances of foremost communal health concern in the list of the World Health Organization, and its prevalent usage in batteries and other industries has made the detection and one-to-one care of Pb(II) in environmental samples mandatory. The selective sensing is assigned to the selective interaction between Au(I) in AuCuNC@N-GQD and Pb(II), and the heightened sensing properties were accredited to the electron-rich N-GQD, which facilitated the electron transport and reduction of Pb(II) to Pb(0). The LOD value was the lowest reported hitherto for Pb(II) with 1 pM (4.8 × 10–15 g/L); the previously reported lowest was 90 pM. The detecting capacity of AuCuNC@N-GQD/GCE was verified in samples collected from the environment, such as seawater, groundwater, and wastewater, which are Pb(II) spiked, and was found to perform efficiently with ≤100% recovery, with a corresponding relative standard deviation of <0.5%. 

Picomolar Selective Electrochemical Sensing of Lead Ions by a Gold–Copper Nanocluster-Nitrogen-Doped Graphene Quantum Dot Combination

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