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.

Ultrathin quasi-hexagonal gold nanostructures for sensing arsenic in tap water

Monodispersed colloidal gold nanoparticles (AuNPs) were synthesized by an easy, cost-effective, and eco-friendly method. The AuNPs were mostly quasi-hexagonal in shape with sizes ranging from 15 to 18 nm. A screen-printed electrode modified with AuNPs (AuNPs/SPE) was used as an electrochemical sensor for the detection of As(iii) in water samples. The mechanistic details for the detection of As(iii) were investigated and an electrochemical reaction mechanism was proposed. Under the optimal experimental conditions, the sensor was highly sensitive to As(iii), with a limit of detection of 0.11 μg L-1 (1.51 nM), which is well below the regulatory limit of 10 μg L-1 established by the United States Environmental Protection Agency and the World Health Organization. The sensor responses were highly stable, reproducible, and linear over the As(iii) concentration range of 0.075 to 30 μg L-1. The presence of co-existing heavy metal cations such as lead, copper, and mercury did not interfere with the sensor response to As(iii). Furthermore, the voltammogram peaks for As(iii), lead, copper, and mercury were sufficiently separate for their potential simultaneous measurement, and at very harsh acidic pH it may be possible to detect As(v). The AuNPs/SPE could detect As(iii) in tap water samples at near-neutral pH, presenting potential possibilities for real-time, practical applications.

Regulating Surface Facets of Metallic Aerogel Electrocatalysts by Size-Dependent Localized Ostwald Ripening

It is well known that the activity and stability of electrocatalysts are largely dependent on their surface facets. In this work, we have successfully regulated surface facets of three-dimensional (3D) metallic Au ^{m- n} aerogels by salt-induced assembly of citrate-stabilized gold nanoparticles (Au NPs) of two different sizes and further size-dependent localized Ostwald ripening at controlled particle number ratios, where m and n represent the size of Au NPs. In addition, 3D Au ^{m- n}-Pd aerogels were further synthesized on the basis of Au ^{m- n} aerogels and also bear controlled surface facets because of the formation of ultrathin Pd layers on Au ^{m- n} aerogels. Taking the electrooxidation of small organic molecules (such as methanol and ethanol) by the resulting Au ^{m- n} and Au ^{m- n}-Pd aerogels as examples, it is found that surface facets of metallic aerogels with excellent performance can be regulated to realize preferential surface facets for methanol oxidation and ethanol oxidation, respectively. Moreover, they also indeed simultaneously bear high activity and excellent stability. Furthermore, their activities and stability are also highly dependent on the area ratio of active facets and inactive facets on their surfaces, respectively, and these ratios are varied via the mismatch of sizes of adjacent NPs. Thus, this work not only demonstrates the realization of the regulation of the surface facets of metallic aerogels by size-dependent localized Ostwald ripening but also will open up a new way to improve electrocatalytic performance of 3D metallic aerogels by surface regulation.

Thermally stable hybrid polyarylidene(azomethine-ether)s polymers (PAAP): an ultrasensitive arsenic(III) sensor approach

A new category of thermally stable hybrid polyarylidene(azomethine-ether)s and copolyarylidene(azomethine-ether)s (PAAP) based on diarylidenecycloalkanones has been synthesized using solution polycondensation method. For potential cationic sensor development, a thin layer of PAAP onto a flat glassy carbon electrode (GCE, surface area: 0.0316 cm²) was prepared with conducting nafion (5%) coating agent to fabricate a sensitive and selective arsenic (III) [As³⁺] ion in short response time in neutral buffer system. The fabricated cationic sensor was measured the analytical performances such as higher sensitivity, linear dynamic range, detection limit, reproducibility, and long-term stability towards As³⁺ ions. The sensitivity and detection limit were calculated as 2.714 μAμM^-1cm^-2 and 6.8 ± 0.1 nM (SNR of 3; 3N/S) respectively from the calibration curve. This novel approach can be initiated a well-organized route of an efficient development of heavy selective arsenic sensor for hazardous pollutants in biological, environmental, and health care fields. Real sample analysis for various environmental sample was performed with PAAP-modified-GCE.

Tuning DNA adsorption affinity and density on metal oxide and phosphate for improved arsenate detection

Arsenic (As) contamination in groundwater presents a major health and environmental concern in developing countries. Typically, As is found in two oxidation states. Most chemical tests for inorganic arsenic are focused on As(III), and few have been developed for As(V). We are interested in developing biosensors for As(V) based on its similarity with phosphate. Building upon previous work involving DNA-capped Fe₃O₄ nanoparticles for As(V) detection, we investigated two other nanomaterials: CeO₂ and CePO₄ in terms of DNA adsorption and As(V) induced DNA desorption. Fluorescently labeled DNA is physically adsorbed to the surface sites on the nanoparticle surface via its phosphate backbone. In the cases of CeO₂ and Fe₃O₄, the fluorescence was quenched due to electron transfer, whereas for the insulating CePO₄, no quenching was observed. Arsenate, being similar to phosphate, can also bind to the surface of the nanoparticles and displace the DNA, increasing the fluorescence signal. The length and sequence of DNA were systematically studied. Using this method, CeO₂ performed significantly better than Fe₃O₄, lowering the detection limit by almost 10-fold. In addition, for CeO₂ and CePO₄, using shorter DNA was more effective for As(V) detection than using the longer DNA since they both adsorb DNA more tightly than Fe₃O₄ does. Overall, CeO₂ has the best performance since it has an intermediate adsorption affinity of DNA, while CePO₄ adsorbs DNA too strongly.