Simultaneous electrochemical sensing of thallium, lead and mercury using a novel ionic liquid/graphene modified electrode

Critical review on toxic contaminants in surface water ecosystem: sources, monitoring, and its impact on human health

Surface water pollution is a critical and urgent global issue that demands immediate attention. Surface water plays a crucial role in supporting and sustaining life on the earth, but unfortunately, till now, we have less understanding of its spatial and temporal dynamics of discharge and storage variations at a global level. The contamination of surface water arises from various sources, classified into point and non-point sources. Point sources are specific, identifiable origins of pollution that release pollutants directly into water bodies through pipes or channels, allowing for easier identification and management, e.g., industrial discharges, sewage treatment plants, and landfills. However, non-point sources originate from widespread activities across expansive areas and present challenges due to its diffuse nature and multiple pathways of contamination, e.g., agricultural runoff, urban storm water runoff, and atmospheric deposition. Excessive accumulation of heavy metals, persistent organic pollutants, pesticides, chlorination by-products, pharmaceutical products in surface water through different pathways threatens food quality and safety. As a result, there is an urgent need for developing and designing new tools for identifying and quantifying various environmental contaminants. In this context, chemical and biological sensors emerge as fascinating devices well-suited for various environmental applications. Numerous chemical and biological sensors, encompassing electrochemical, magnetic, microfluidic, and biosensors, have recently been invented by hydrological scientists for the detection of water pollutants. Furthermore, surface water contaminants are monitored through different sensors, proving their harmful effects on human health.

Electrochemical Sensors for Heavy Metal Ion Detection in Aqueous Medium: A Systematic Review

Heavy metal ions (HMIs) are very harmful to the ecosystem when they are present in excess of the recommended limits. They are carcinogenic in nature and can cause serious health issues. So, it is important to detect the metal ions quickly and accurately. The metal ions arsenic (As3+), cadmium (Cd2+), chromium (Cr3+), lead (Pb2+), and mercury (Hg2+) are considered to be very toxic among other metal ions. Standard analytical methods like atomic absorption spectroscopy, atomic fluorescence spectroscopy, and X-ray fluorescence spectroscopy are used to detect HMIs. But these methods necessitate highly technical equipment and lengthy procedures with skilled personnel. So, electrochemical sensing methods are considered to be more advantageous because of their quick analysis with precision and simplicity to operate. They can detect a wide range of heavy metals providing real-time monitoring and are cost-effective and enable multiparametric detection. Various sensing applications necessitate severe regulation regarding the modification of electrode surfaces. Numerous nanomaterials such as graphene, carbon nanotubes, and metal nanoparticles have been extensively explored as interface materials in electrode modifiers. These nanoparticles offer excellent electrical conductivity, distinctive catalytic properties, and high surface area resulting in enhanced electrochemical performance. This review examines different HMI detection methods in an aqueous medium by an electrochemical sensing approach and studies the recent developments in interface materials for altering the electrodes.

The determination of thallium in the environment: A review of conventional and advanced techniques and applications

Thallium (Tl) is a potential toxicity element that poses significant ecological and environmental risks. Recently, a substantial amount of Tl has been released into the environment through natural and human activities, which attracts increasing attention. The determination of this hazardous and trace element is crucial for controlling its pollution. This article summarizes the advancement and progress in optimizing Tl detection techniques, including atomic absorption spectroscopy (AAS), voltammetry, inductively coupled plasma (ICP)-based methods, spectrophotometry, and X-ray-based methods. Additionally, it introduces sampling and pretreatment methods such as diffusive gradients in thin films (DGT), liquid-liquid extraction, solid phase extraction, and cloud point extraction. Among these techniques, ICP-mass spectrometry (MS) is the preferred choice for Tl detection due to its high precision in determining Tl as well as its species and isotopic composition. Meanwhile, some new materials and agents are employed in detection. The application of novel work electrode materials and chromogenic agents is discussed. Emphasis is placed on reducing solvent consumption and utilizing pretreatment techniques such as ultrasound-assisted processes and functionalized magnetic particles. Most detection is performed in aqueous matrices, while X-ray-based methods applied to solid phases are summarized which provide non-destructive analysis. This work improves the understanding of Tl determination technology while serving as a valuable resource for researchers seeking appropriate analytical techniques.

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.

Development of Meta-Chemical Surface by Dip-Pen Nanolithography for Precise Electrochemical Lead Sensing

Dip-pen nanolithography (DPN) is a powerful and unique technique for precisely depositing tiny nano-spherical cap shapes (nanoclusters) onto a desired surface. In this study, a meta-chemical surface (MCS; a pattern with advanced features) is developed by DPN and applied to electrochemical lead sensing, yielding a calibration curve in the ppb range. An ink mixture of PMMA and NTPH (which binds to Pb (II), as supported by DFT calculations) is patterned over a Pt surface. The average height of the nanoclusters is ≈13 nm with a high surface area-to-volume ratio, which depends on the ink composition and the MCS surface. This ratio affected the sensitivity of the MCS as a detecting tool. The results indicate that the sensor's features can be controlled by the ability to control the size of the nanoclusters, attributed to the unique properties of the DPN production method. These results are significant for the water-source purification industry.

A Review on Recent Trends and Future Developments in Electrochemical Sensing

Electrochemical methods and devices have ignited prodigious interest for sensing and monitoring. The greatest challenge for science is far from meeting the expectations of consumers. Electrodes made of two-dimensional (2D) materials such as graphene, metal-organic frameworks, MXene, and transition metal dichalcogenides as well as alternative electrochemical sensing methods offer potential to improve selectivity, sensitivity, detection limit, and response time. Moreover, these advancements have accelerated the development of wearable and point-of-care electrochemical sensors, opening new possibilities and pathways for their applications. This Review presents a critical discussion of the recent developments and trends in electrochemical sensing.

Anodic Stripping Voltammetric Procedure of Thallium(I) Determination by Means of a Bismuth-Plated Gold-Based Microelectrode Array

This article presents a new working electrode based on a bismuth-plated, gold-based microelectrode array, which is suitable for determining thallium(I) species using anodic stripping voltammetry (ASV). It allowed a significant increase in the sensitivity as compared to other voltammetric sensors. The main experimental conditions and the instrumental parameters were optimized. A very good proportionality between the Tl(I) peak current and its concentration was evidenced in the range from 5 × 10-10 up to 5 × 10-7 mol L-1 (R = 0.9989) for 120 s of deposition and from 2 × 10-10 up to 2 × 10-7 mol L-1 (R = 0.9988) for 180 s. A limit of detection (LOD) of 8 × 10-11 mol L-1 for a deposition time of 180 s was calculated. The effects of interfering ions on the Tl(I) analytical signal were studied. The proposed method was applied for quantitative Tl(I) detection in water certified reference material TM 25.5 as well as in spiked real water samples, for which satisfactory recovery values between 98.7 and 101.8% were determined.

Label free electrochemical sensors for Pb(II) detection based on hemicellulose extracted from Opuntia Ficus Indica cactus

We aim to develop an electrochemical sensor for a divalent metal ion (lead II), a highly toxic water contaminant. We explore a sensor formed with a hemicellulose polysaccharide extracted from the Opuntia Ficus Indica cactus associated with agarose as a sensitive layer deposited on a gold electrode. This sensor combines the functional groups of hemicellulose that could form a complex with metal ions and agarose with gelling properties to form a stable membrane. The sensor demonstrated a loading ability of Pb2+, with higher affinity compared to other metal ions such as Hg2+, Ni2+, and Cu2+, thanks to the chemical structure of hemicellulose. The detection was measured by square wave voltammetry based on a well-defined redox peak of the metal ions. The sensor shows high sensitivity towards Pb2+ with a detection limit of 1.3 fM. The application in river and sea water using the standard addition method for lead detection was studied.

A non-enzymatic electrochemical sensor based on nitrogen-doped mesoporous carbon@hydroxyl-functionalized ionic liquid composites modified electrode for the detection of fenitrothion

The overuse of organophosphorus pesticides (OPs) results in severe environmental pollution and food safety issues. Fenitrothion (FNT) is a typical derivative of OPs, so rapid and sensitive detection of FNT plays an important role in environmental protection and public health. An FNT non-enzymatic electrochemical sensor based on nitrogen-doped mesoporous carbon@functionalized ionic liquid composites (N-CMK-3@IL) was constructed in this work. The surface topography and electrochemical properties of the sensor were investigated by scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), respectively. Because N-CMK-3@IL composites could improve the conductivity and increase the active surface area of the modified electrode, the sensor exhibited good electrocatalytic activity to FNT. Under the optimal experimental conditions, a good linear relationship for FNT was obtained in the range of 0.5-100 ng mL^-1, and the detection limit was 0.1 ng mL^-1 (S/N = 3). The sensor was successfully applied for the detection of FNT in vegetable samples.

Excellent removal performance of 4,4'-biphenyldicarboxaldehyde m-phenylenediamine Schiff base magnetic polymer towards phenanthrene and 9-phenanthrol: Experimental, modeling and DFT calculations studies

Phenanthrene (PTH) and 9-phenanthrol (9-PTH) exhibited severe health threats and ecological hazards, for this reason, exploring a high-efficient removing strategy for PTH and 9-PTH could be considered of great urgency. Herein the 4,4'-biphenyldicarboxaldehyde m-phenylenediamine Schiff base magnetic polymer (magnetic BIPH-PHEN) was successfully fabricated via Schiff base polycondensation reaction and the subsequently one-pot embedded method. The mutual aromatic nucleus of BIPH-PHEN polymer and PTH/9-PTH could form π-π interaction, thus improving the capture ability, the embedded Fe₃O₄ nanoparticles provided the possibility for rapid separation. The physical and chemical properties of the magnetic BIPH-PHEN were systematically characterized. The removal rate of magnetic BIPH-PHEN towards PTH and 9-PTH was 85.65 % and 98.52 %, respectively (PTH or 9-PTH: 8 mg/L; Adsorbent: 0.2 g/L). The DFT calculations including energy calculations and electrostatic potential distribution analyzed the different bonding modes and proposed the most possible bonding modes in the adsorbent/adsorbate system. Moreover, the LUMO and HOMO orbits combined with energy gaps analysis proved the existence and specific types of the π-π interaction. The monolayer adsorption occurred on the homogeneous magnetic BIPH-PHEN surface, simultaneously the chemisorption was dominant. This work not only proposed new sights on assembling magnetic Schiff base polymer for removing polycyclic aromatic hydrocarbons, but also provided a deeper understanding of intramolecular interactions.

Ultrasensitive detection of mercury(II) ions on a hybrid film of a graphene and gold nanoparticle-modified electrode

Aggravated by human and industrial activities, heavy metal pollution has become a severe problem, causing widespread concern in society, and cannot be ignored. Herein, a graphene/gold nanoparticle-hybrid (AuNPs/ERGO) was proposed and synthesized by electrochemical methods. Based on the AuNPs/ERGO hybrid, a novel electrochemical sensing platform was established and successfully applied for the selective, quantitative detection of Hg²⁺, taking advantage of the well-established anodic stripping voltammetry (ASV). This hybrid material not only increases the surface area and charge transfer rate but also provides more active sites for Hg deposition due to the formation of homogeneous, high density and monodispersed AuNPs on the ERGO film. The prepared AuNPs/ERGO hybrid was modified on a glassy carbon electrode (GCE) to detect Hg²⁺ with a linear range from 0.5 to 20 μg L^-1 and a low limit of detection (LOD) of 0.06 μg L^-1. The selectivity and stability of the as-prepared electrode were investigated and showed promising results. In addition, a screen-printed carbon electrode (SPCE) was also employed to verify the practical application ability of our assay with an excellent performance, which presents a bright application prospect for in situ Hg²⁺ detection.

An overview on nanostructure-modified supported liquid membranes for the electromembrane extraction method

Electromembrane extraction (EME) is an extraction method on the micro scale, in which charged compounds are extracted from a donor phase (sample solution) into an acceptor phase via a supported liquid membrane (SLM) containing a water-immiscible organic solvent. To enhance the extraction efficiency and selectivity in this method, some studies have focused on the modification of the SLM, and thus many strategies have been reported for this purpose. One of these techniques is the introduction of nanomaterials in the SLM structure, which can enhance the extraction efficiency. In the current study, the different nanostructures used for SLM modification in the EME method are reviewed. Furthermore, the related analytical parameters of the developed techniques are classified and tabulated. It is hoped that this review will motivate further research in this field using other nanostructures.

A facile one-pot synthesis of magnetic iron oxide nanoparticles embed N-doped graphene modified magnetic screen printed electrode for electrochemical sensing of chloramphenicol and diethylstilbestrol

Trace determination of antibacterial agents is crucial to minimize risks of human intoxication and in the prevention of serious environmental impacts. Herein, a simple one-pot solvothermal synthesis approach for a magnetic iron oxide embed nitrogen-doped graphene (MIO@NG) nanohybrid was fabricated without the addition of any extra reductant and its application towards ultrasensitive chloramphenicol (CAP) and diethylstilbestrol (DES) electrochemical sensor is demonstrated to screen for antibiotic residue contamination in milk samples. The prepared nanohybrid was modified on a magnetic screen-printed electrode (MSPE) to make it portable for on-site detection. The determination of two additive drugs, CAP and DES, was achieved based on the reduction current response at MIO@NG modified MSPE (MIO@NG/MSPE) to eliminate interference as far as possible. Uniform dispersed MIO nanoparticles are grown in situ on the surface of nitrogen-doped graphene sheets. The morphology of MIO@NG was confirmed by transmission electron microscopy (TEM) analysis. The chemical structure of the prepared MIO@NG was characterized by x-ray diffraction (XRD), x-ray photoemission spectroscopy (XPS), Raman spectroscopy, and extended x-ray absorption fine structure (EXAFS). Moreover, the superparamagnism property was investigated by vibrating sample magnetometry (VSM). The electrochemical properties of MIO@NG were evaluated with cyclic voltammetry (CV) and square wave voltammetry (SWV). Sensor performance was evaluated by testing the electrochemical activity of CAP and DES in the presence of interferences. The MIO@NG modified electrode presented superior electrochemical performance, including high sensitivity, high catalytic activity, ultimate sensitivity, very fast detection, selectivity, and excellent performance. The MIO@NG modified electrode demonstrated a detection limit of 10 nM for the detection of CAP and 6.5 nM for DES with satisfactory recovery in real samples.

A Novel Carbon Paste Electrode Modified with N1-Hydroxy-N1,N2-Diphenylbenzamidine for the Electrochemical Determination of Cadmium(II) in Environmental Samples

The present study introduces a novel electrode for rapid, highly sensitive, and selective electrochemical sensor for cadmium(II) using 5% N¹-hydroxy-N¹,N²-diphenylbenzamidine (HDPBA) modified carbon paste electrode (CPE) (HDPBA‒CPE). Surface characterizations and structural analysis of the proposed HDPBA‒CPE were performed using several analytical techniques. The voltammetric measurements of Cd(II) were conducted by cyclic voltammetry (CV) and square wave anodic stripping voltammetry (SWASV). Several experimental conditions such as composition and pH of buffer solutions, HDPBA composition, accumulation potential and time, and other voltammetric conditions were optimized. Cd(II) was preconcentrated on the modified electrode surface for 270 s using Britton Robinson (B-R) buffer (0.1 M, pH 4) at -1.0 V versus Ag/AgCl, followed by electrochemical oxidation of the accumulated Cd(II) in the positive scan of SWASV after a quiet time of 10 s. Under optimized parameters, the proposed method showed a linear range of 0.3-100 nM Cd(II) with a detection limit of 0.032 nM. The fabricated HDPBA-modified carbon paste electrode exhibited excellent sensitivity, selectivity, stability, and reproducibility (with RSD of 3.8%). The developed HDPBA‒CPE was used for the quantification of Cd(II) in tobacco and environmental water samples, and it was found to be applicable for the determination of different types of real samples.

[OPy][BF4] Selective Extraction for Trace Hg2+ Detection by Electrochemistry: Enrichment, Release and Sensing

Trace mercury ion (Hg2+) detection is important for environmental monitoring and water safety. In this work, we study the electrochemical strategy to detect trace Hg2+ based on the preconcentration of temperature-controlled N-octylpyridinium tetrafluoroborate ([OPy][BF4]) dispersive liquid-liquid microextraction. The [OPy][BF4] selectively extracted Hg2+ from aqueous solution by the adsorption of unsaturated N in [OPy][BF4], which increased the concentration of Hg2+ and decreased that of other interference ions. It was noted that the adsorption of [OPy][BF4] to Hg2+ was weakened by aqueous solution. Hence, after extraction, precipitated [OPy][BF4] was diluted by a buffer solution comprising a mixture of water and acetonitrile to release Hg2+ and the single was detected by electrochemistry. Water is proposed to release Hg2+ adsorbed by [OPy][BF4], and the acetonitrile serves as a co-solvent in buffer solution. Sensitivity and anti-inference ability of sensors were improved using the proposed method and Hg2+ releasing procedure. The detection limit (S/N = 3) of the sensor is 0.0315 μg/L with a linear range from 0.1 to 1 μg/L. And the sensor exhibits good recovery with an range from 106 % to 118%, which has great potential for trace Hg2+ determination.

Recent Advancement in Disposable Electrode Modified with Nanomaterials for Electrochemical Heavy Metal Sensors

Heavy metal pollution has gained global attention due to its high toxicity and non-biodegradability, even at a low level of exposure. Therefore, the development of a disposable electrode that is sensitive, simple, portable, rapid, and cost-effective as the sensor platform in electrochemical heavy metal detection is vital. Disposable electrodes have been modified with nanomaterials so that excellent electrochemical properties can be obtained. This review highlights the recent progress in the development of numerous types of disposable electrodes modified with nanomaterials for electrochemical heavy metal detection. The disposable electrodes made from carbon-based, glass-based, and paper-based electrodes are reviewed. In particular, the analytical performance, fabrication technique, and integration design of disposable electrodes modified with metal (such as gold, tin and bismuth), carbon (such as carbon nanotube and graphene), and metal oxide (such as iron oxide and zinc oxide) nanomaterials are summarized. In addition, the role of the nanomaterials in improving the electrochemical performance of the modified disposable electrodes is discussed. Finally, the current challenges and future prospect of the disposable electrode modified with nanomaterials are summarized.

Voltammetric sensor based on long alkyl chain tetraalkylammonium ionic liquids comprising ascorbate anion for determination of nitrite

An electrochemical sensor was fabricated utilizing ionic liquids possessing cations with long alkyl chains such as trimethyl octadecylammonium and behenyl trimethylammonium and ascorbate anion. The ionic liquids were drop-coated onto the electrode. Thin modifying layers were prepared. Cyclic voltammetric investigations revealed electrostatic interactions between the electrochemical probes and the modified surface, proving that a positive charge was established at the film surface. Hence, negatively charged species such as nitrite can be pre-concentrated on the surface of presented modified electrodes. The fabricated electrodes have been used as a voltammetric sensor for nitrite. Due to the electrostatic accumulation properties of long alkyl cation, the assay exhibits a remarkable improvement in the voltammetric response toward nitrite oxidation. The influence of pH on the electrode response was thoroughly investigated, and the mechanism of the electrode was established. The developed sensor showed a linear electrochemical response in the range 1.0–50 μM with a detection limit of 0.1 μM. The electrode revealed good storage stability, reproducibility, and anti-interference ability. The determination of nitrite performed in curing salts brought satisfactory results.

A Systematic Study of DFT Performance for Geometry Optimizations of Ionic Liquid Clusters

Clusters of two ion pairs of imidazolium-based ionic liquids were optimized with 43 different levels of theory, including DFT functionals and MP2-based methods combined with varying Dunning's basis sets, and added dispersion corrections. Better preforming DFT functionals were then applied to clusters consisting of four ion pairs. Excellent performance of some DFT functionals for the two ion pair clusters did not always match that of the four ion-paired clusters despite interionic distances remaining constant between the optimized two and four ion-paired clusters of the same ionic liquid. Combinations of DFT functional and basis set such as ωB97X-D/cc-pVDZ, M06-2X/aug-cc-pVDZ, B3LYP-D3/cc-pVTZ, and TPSS-D3/cc-pVTZ gave excellent results for geometry optimization of two ion-paired clusters of imidazolium ionic liquids but gave larger deviations when applied to the four ion-paired clusters of varying ionic liquids. Empirical dispersion corrections were seen to be crucial in correctly capturing correlation effects in the studied ionic liquid clusters, becoming more important in larger clusters. Dunning's double-ζ basis set, cc-pVDZ, is associated with the smallest root mean squared deviations for geometries; however, it also produces the largest deviations in total electronic energies. ωB97X-D and M06-2X produced the best performance with the augmented version of this basis set. The triple-ζ basis set, cc-pVTZ, leads to the best performance of most of the DFT functionals (especially the dispersion-corrected ones) used, whereas its augmented version, aug-cc-pVTZ, was not seen to improve results. The combinations of functional and basis set that gave the best geometry and energetics in both two and four ion-paired clusters were PBE-D3/cc-pVTZ, ωB97X-D/aug-cc-pVDZ, and BLYP-D3/cc-pVTZ. All three combinations are recommended for geometry optimizations of larger clusters of ionic liquids. PBE-D3/cc-pVTZ performed the best with an average deviation of 2.3 kJ mol^-1 and a standard deviation of 3.4 kJ mol^-1 for total electronic energy when applied to four ion-paired clusters. Geometries optimized with FMO2-SRS-MP2/cc-pVTZ produced total energy within 2.0 kJ mol^-1 off the benchmark in two ion-paired clusters, with the cc-pVDZ basis set performing unsurprisingly poorly with the same method. The error increased to 4.8 kJ mol^-1 on average in four ion-paired clusters, with the smallest RMSD deviations in geometries when compared to the benchmark ones. This study is the first report that investigated the performance of DFT functionals for two and four ion-paired clusters of a wide range of ionic liquids consisting of commonly used cations such as pyrrolidinium, imidazolium, pyridinium, and ammonium. It also identified the importance of assessing the performance of quantum chemical methods for ionic liquids on a variety of cation-anion combinations.

Pt NPs catalyzed chemiluminescence method for Hg2+ detection based on a flow injection system

Establishing a simple and accurate method for Hg²⁺ detection is of great importance for the environment and human health. In this work, platinum nanoparticles (Pt NPs) with different capped agents and morphologies were synthesized. It was found that Pt NPs exhibited peroxidase-like activity that can catalyze the chemiluminescence (CL) of the luminol system without H₂ O₂ . The most intensive CL signals were obtained by using PVP-capped Pt NPs as catalysis. Based on the fact that Hg²⁺ could further enhance the CL intensity in the Pt NPs-luminol CL system, a Pt NPs-catalyzed CL method based on a flow injection system is developed for the sensitive analysis of Hg²⁺ . When the concentration of Hg²⁺ in the system increases, the CL intensity would together increase, thereby achieving sensitive Hg²⁺ detection. The limit of detection (LOD) was calculated to be 8.6 nM. This developed method provides a simple and rapid approach for the sensitive detection of Hg²⁺ and shows great promise for applications in other complex systems.

Graphene-derived nanomaterials as recognition elements for electrochemical determination of heavy metal ions: a review

This review (with 155 refs.) summarizes the progress made in the past few years in the field of electrochemical sensors based on graphene-derived materials for the determination of heavy metal ions. Following an introduction of this field and a discussion of the various kinds of modified graphenes including graphene oxide and reduced graphene oxide, the review covers graphene based electrodes modified (or doped) with (a) heteroatoms, (b) metal nanoparticles, (c) metal oxides, (d) small organic molecules, (e) polymers, and (f) ternary nanocomposites. Tables are provided that afford an overview of representative methods and materials for fabricating electrochemical sensors. Furthermore, sensing mechanisms are discussed. A concluding section presents new perspectives, opportunities and current challenges. Graphical Abstract Schematic illustration of electrochemical sensor for heavy metal ion sensing based on heteroatom-doped graphene, metal-modified graphene, metal-oxide-modified graphene, organically modified graphene, polymer-modified graphene, and ternary graphene based nanocomposites.

A review on graphene-based nanocomposites for electrochemical and fluorescent biosensors

Biosensors with high sensitivity, selectivity and a low limit of detection, reaching nano/picomolar concentrations of biomolecules, are important to the medical sciences and healthcare industry for evaluating physiological and metabolic parameters.

Dopamine crosslinked graphene oxide membrane for simultaneous removal of organic pollutants and trace heavy metals from aqueous solution

A graphene oxide-polydopamine-(β-cyclodextrin) (GPC) ultrafiltration membrane was fabricated by using the method of drop-coating combined with vacuum filtration. The prepared GPC membrane was characterized with FTIR and XPS spectrophotometry and evaluated for its performances for the rejection of organic molecules (methylene blue) and adsorption of trace heavy metals (Pb²⁺) from aqueous solutions. The membrane exhibited an excellent rejection coefficient of 99.2% for methylene blue and the permeation flux was 12 L m^-2 h^-1 at 0.1 bar. The membrane also exhibited fast adsorption kinetics for Pb²⁺ and the adsorption capacity was 101.6 mg g^-1 at a solution pH of 6. The performance of the membrane could almost be completely recovered after a simple clean and regeneration process. These results indicate that the GPC membrane has potential applications in treatment of complex industrial wastewater streams.

New portable smartphone-based PDMS microfluidic kit for the simultaneous colorimetric detection of arsenic and mercury

A smartphone-based microfluidic platform was developed for point-of-care (POC) detection using surface plasmon resonance (SPR) of gold nanoparticles (GNPs). The simultaneous colorimetric detection of trace arsenic and mercury ions (As3+ and Hg2+) was performed using a new image processing application (app). To achieve this goal, a microfluidic kit was fabricated using a polydimethylsiloxane (PDMS) substrate with the configuration of two separated sensing regions for the quantitative measurement of the color changes in GNPs to blue/gray. To fabricate the microfluidic kit, a Plexiglas mold was cut using a laser based on the model obtained from AutoCAD and Comsol outputs. The colorimetric signals originated from the formation of nanoparticle aggregates through the interaction of GNPs with dithiothreitol - 10,12-pentacosadiynoic acid (DTT-PCDA) and lysine (Lys) in the presence of As3+ and Hg2+ ions. This assembly exhibited the advantages of simplicity, low cost, and high portability along with a low volume of reagents and multiplex detection. Heavy Metals Detector (HMD), as a new app for the RGB reader, was programmed for an Android smartphone to quantify colorimetric analyses. Compared with traditional image processing, this app provided significant improvements in sensitivity, time of analysis, and simplicity because the color intensity is measured through a new normalization equation by converting RGB to an Integer system. As a simple, real-time, and portable analytical kit, the fabricated sensor could detect low concentrations of As3+ (710 to 1278 μg L-1) and Hg2+ (10.77 to 53.86 μg L-1) ions in water samples at ambient conditions.

Simple and green synthesis of piperazine-grafted reduced graphene oxide and its application for the detection of Hg(II)

In this paper, piperazine-grafted reduced graphene oxide (NH-rGO) was synthesized via a simple and green two-step procedure: (i) opening of the resulting epoxides of graphene oxide (GO) with piperazine (NH) through nucleophilic substitution; (ii) reduction of GO with ascorbic acid. Its structure and morphology were characterized by scanning electron microscopy and x-ray photoelectron spectroscopy. The NH-rGO modified glassy carbon electrode was explored as an electrochemical sensor for the determination of Hg(II) using a differential pulse anodic stripping voltammetry technique. Hg(II) can be efficiently accumulated and deposited on the surface of a modified electrode by strong coordination chemical bonds formed between Hg(II) and NH. And then the anodic stripping current can be significantly enhanced by rGO with the merits of large specific surface area and high conductivity, which served as a signal amplifier, finally realizing the highly sensitive determination of Hg(II). The experimental parameters including the pH value of the acetate buffer, deposition potential and deposition time were optimized. Under optimal conditions, the developed sensor exhibited a wide linear range from 0.4-12 000 nM with a low limit of detection of 0.2 nM, which is well below the guideline value in drinking water set by the WHO. Moreover, the practical application of this method was confirmed by an assay of Hg(II) in tap water samples with acceptable results.

A colorimetric assay for vanillin detection by determination of the luminescence of o-toluidine condensates

Vanillin (4-hydroxy-3-methoxybenzaldehyde), a food additive with rich milk flavor, is commonly used in the food, beverage and cosmetic industries. However, excessive consumption of vanillin may cause liver and kidney damage. Therefore, methods for detecting and controlling the level of vanillin in food, especially in infant powder, have important practical significance. In this study, we established a colorimetric assay for vanillin detection. The detection was performed under high-temperature and acidic conditions, which can induce the reaction of the aldehyde group of vanillin with the amino group of o-toluidine. The resulting product had a maximum absorption at 363 nm, which was quantified by a UV spectrophotometer. This assay had a limit of detection (LOD) of 1 pg mL⁻¹ and a linear range between 1 μg mL⁻¹ and 100 μg mL⁻¹. The average recoveries at three spiked levels were in the range from 91.1% to 101.6% with a relative standard deviation (RSD) of 4.62% ~ 7.27%.

Electrode Modified by Reduced Graphene Oxide for Monitoring of Total Thallium in Grain Products

Grain products and the associated industry have a notable economic and social impact all over the world. The toxicological safety of grain products is a nutritional prerogative. This study focused on the determination of thallium content in grain product samples collected from a commercial brand commonly available in Poland. The samples were analyzed with the use of differential pulse anodic stripping voltammetry (DPASV) with graphene oxide based on glassy carbon. The stripping anodic peak current of thallium was linear over its concentration range from 9.78 × 10⁻⁹ to 97.8 × 10⁻⁹ M. The limit of detection (LOD) was calculated according to the formula LOD = (κ × SDa)/b, where κ is 3.3, SDa is the standard deviation of the intercept, and b is the slope. The determined value of LOD was 1.229 µg L⁻¹ (6.01 × 10⁻⁹ M). The proposed method was successfully applied for the determination of thallium ions in samples of actual grain products. The obtained results confirmed that thallium was present in the studied cereal samples (average content at 0.0268 ± 0.0798 mg/kg). Thallium has a half-life of 60 days; therefore, the consumption of foods with thallium content of approximately 0.08 mg/kg has the potential for harmful bioaccumulation in the body. Thallium contamination in cereal products should be a critical parameter for health environmental regulations.