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.

Iron oxide-supported gold nanoparticle electrode for simultaneous detection of arsenic and sulfide on-site

The environmental behavior of arsenic (As) has garnered significant attention due to its hazardous nature. The fate of As often couples with sulfide, thus co-detecting arsenic and sulfide on-site is crucial for comprehending their geochemical interactions. While electrochemical methods are suitable for on-site chemical analysis, there currently exists no electrode capable of simultaneously detecting both arsenic and sulfide. To address this, we developed a dual-metal electrode consisting of iron oxide-encased carbon cloth loaded with gold nanoparticles (Au/FeOx/CC) using the electrochemical deposition method. This electrode enables square wave stripping voltammetry (SWASV) binary detection of As and sulfide. Comparison experiments reveal that the reaction sites for sulfide primarily reside on FeOx, while the interface synergy of iron oxide and gold nanoparticles enhances the response to arsenite (AsIII). Arsenate (AsV) is directly reduced to As0 on Fe0, obviating the need for an external reducing agent. The electrode achieves detection limits of 1.5 μg/L for AsV, 0.25 μg/L for AsIII, and 11.6 μg/L for sulfide at mild conditions (pH 7.8). Field validation was conducted in the Tengchong geothermal hot spring region, where the electrochemical method exhibited good correlation with the standard methods: Total As (r = 0.978 vs. ICP-MS), AsIII (r = 0.895 vs. HPLC-ICP-MS), and sulfide (r = 0.983 vs. colorimetric method). Principal component analysis and correlation analysis suggest that thioarsenic, could potentially be positive interferents for AsIII. However, this interference can be anticipated and mitigated by monitoring the abundance of sulfide. The study provides new insights and problems for the electrochemical detection of coexisted As and sulfide.

Electrodeposited rGO/AuNP/MnO2 Nanocomposite-Modified Screen-Printed Carbon Electrode for Sensitive Electrochemical Sensing of Arsenic(III) in Water

Herein, a simple and portable electrochemical sensor based on a reduced graphene oxide/gold nanoparticle/manganese dioxide (rGO/AuNP/MnO₂) nanocomposite-modified screen-printed carbon electrode (SPCE) was constructed by the facile stepwise electrodeposition method and used for electrochemical detection of As(III). The resultant electrode was characterized for its morphological, structural, and electrochemical properties using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). From the morphologic structure, it can be clearly observed that the AuNPs and MnO₂ alone or their hybrid were densely deposited or entrapped in thin rGO sheets on the porous carbon surface, which may favor the electro-adsorption of As(III) on the modified SPCE. It is interesting that the nanohybrid modification endows the electrode with a significant decrease in charge transfer resistance and an increase in electroactive specific surface area, which dramatically increases the electro-oxidation current of As(III). This improved sensing ability was ascribed to the synergistic effect of gold nanoparticles with excellent electrocatalytic property and reduced graphene oxide with good electrical conductivity, as well as the involvement of manganese dioxide with a strong adsorption property in the electrochemical reduction of As(III). Under optimized conditions, the sensor can detect As(III) via square wave anodic stripping voltammetry (SWASV) with a low limit of detection of 2.4 μg L^-1 and a linear range of 25-200 μg L^-1. The proposed portable sensor shows the advantages of a simple preparation procedure, low cost, good repeatability, and long-term stability. The feasibility of rGO/AuNPs/MnO₂/SPCE for detecting As(III) in real water was further verified.

Advances in Electrochemical Detection Electrodes for As(III)

Arsenic is extremely abundant in the Earth's crust and is one of the most common environmental pollutants in nature. In the natural water environment and surface soil, arsenic exists mainly in the form of trivalent arsenite (As(III)) and pentavalent arsenate (As(V)) ions, and its toxicity can be a serious threat to human health. In order to manage the increasingly serious arsenic pollution in the living environment and maintain a healthy and beautiful ecosystem for human beings, it is urgent to conduct research on an efficient sensing method suitable for the detection of As(III) ions. Electrochemical sensing has the advantages of simple instrumentation, high sensitivity, good selectivity, portability, and the ability to be analyzed on site. This paper reviews various electrode systems developed in recent years based on nanomaterials such as noble metals, bimetals, other metals and their compounds, carbon nano, and biomolecules, with a focus on electrodes modified with noble metal and metal compound nanomaterials, and evaluates their performance for the detection of arsenic. They have great potential for achieving the rapid detection of arsenic due to their excellent sensitivity and strong interference immunity. In addition, this paper discusses the relatively rare application of silicon and its compounds as well as novel polymers in achieving arsenic detection, which provides new ideas for investigating novel nanomaterial sensing. We hope that this review will further advance the research progress of high-performance arsenic sensors based on novel nanomaterials.

Graphene/fluorescein dye-based sensor for detecting As(III) in drinking water

A complex of reduced graphene oxide (rGO) and fluorescein (FL) dye nanoparticles of size between 50 and 100 nm has been prepared and its sensing performance for detection of As(III) in drinking water has been reported. When As(III) binds to the rGO-FL nanoparticles the relative quenching of fluorescence was increased with increase in As(III) concentration thus provide two linear calibration ranges (0-4.0 mmol L-1 and 4.0-10 mmol L-1). The fluorescence quenching mechanism was investigated by using time-resolved fluorescence spectroscopy and molecular modeling. The detection limit of this sensor has been determined as equal to 0.96 µg L-1 which is about 10 times lower than the WHO stipulated standard for As(III) in drinking water (10 µg L-1). The analytical performance and potential application of the nanosensor was compared to commercial field kits used in arsenic monitoring. The sensor proposed in this study is fast, sensitive and accurate for detection of As(III) in drinking water and environmental samples.

A novel and sensitive electrochemical sensor based on nanoporous gold for determination of As(III)

Three-dimensional porous gold nanoparticles (NPG) were synthesized in situ on indium-doped tin oxide (ITO) substrates by a green and convenient one-step electrodeposition method to achieve super-sensitive As(III) detection. The introduction of NPG method not only greatly improves the electron transfer capacity and surface area of sensor interface but provides more active sites for As(III) enrichment, thus boosting sensitivity and selectivity. The sensor was characterized by scanning electron microscopy, energy dispersion spectroscopy, differential pulse anode stripping voltammetry (DPASV), and electrochemical impedance to evaluate its morphology, composition, and electrochemical performance. The wall thickness of NPG was customized by optimizing the concentration of electroplating solution, dissolved electrolyte, deposition potential, and reaction time. Under optimal conditions, the electrochemical sensor showed a wide linear range from 0.1 to 50 μg/L As(III), with a detection limit (LOD) of 0.054 μg/L (S/N = 3). The LOD is far below 10 μg/L, the recommended maximum value by the world health organization for drinking water. Stability, reproducibility, and repeatability of NGP/ITO were determined to be 2.77%, 4.9%, and 4.1%, respectively. Additionally, the constructed sensor has been successfully applied to determine As(III) in three actual samples, and the results are in good agreement with that of hydride generation atomic fluorescence spectrometry (AFS). Graphical abstract.

Automated Determination of As(III) in Waters with an Electrochemical Sensor Integrated into a Modular Microfluidic System

The presence of high levels of arsenic in waters poses a threat to the human health in many countries all over the world. Effective surveillance programs of water quality require the implementation of in-field tests to assess early the presence of this metal ion and other contaminants. To date, there exist few market-available analytical approaches that suffer from important limitations related to cost, in addition to complex reactions, very long analysis times, and/or high limits of detection. This work describes a robust electrochemical sensor integrated into a modular microfluidic system that shows a clear potential to be deployed for the on-site monitoring of inorganic As(III) species. Flexible and transparent microfluidic modules are fabricated by rapid prototyping techniques and include different microfluidic components among them, flow cells where electrochemical sensors can be easily and reversibly inserted. The electrochemical sensor comprises a gold nanoparticle (AuNP)-modified gold thin-film electrode that is readily applied to the sensitive detection of As(III) by anodic stripping linear sweep voltammetry. The microfluidic system enables the automatic sensor calibration, sample uptake, and preconditioning as well as As(III) detection. The system response to As(III) is linear in a concentration range of 1-150 μg L^-1, with a detection limit of 0.42 μg L^-1, which is well below the threshold value of 10 μg L^-1 set by the World Health Organization. Analysis of tap water and two water samples from two Argentinean aquifers, spiked with different As(III) concentrations, demonstrates the excellent performance of the system.

Dual anion colorimetric and fluorometric sensing of arsenite and cyanide ions

A naphthalene appended probe, 2-((2-hydroxynaphthalen-1-yl)methylene)hydrazine carbothioamide, was synthesized and found to recognize AsO₂⁻ and CN⁻ ions with turn-on emission fluorescence over different anions in DMF–H₂O (HEPES buffer, 7.2 pH) (9 : 1, v/v solution) medium.