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.

Electrochemical sensing of Hg(ii) in chicken liver and snail shell extract samples using novel modified SDA/MWCNT electrodes

Heavy metal ions (Hg(ii)) were detected in fresh chicken liver and snail shell extract samples using novel synthesised SDA/MWCNT-modified electrodes. The synthesized N,N'-bis(salicylaldehyde)-1,2-diaminobenzene (SDA) ligand was characterized via FT-IR, 1H-NMR, and 13C-NMR spectroscopy. The hydroxyl and imine functional groups present in SDA act as active sites and bind to the MWCNT surface. The surface morphology of the modified SDA/MWCNT electrode exhibited a star-like crystal structure and the preconcentration of Hg(ii)-SDA/MWCNTs lead to a crystal cloud structure, as characterized by SEM with EDX. The enhancement of current and conductance of the SDA/MWCNT- and MWCNT-modified electrode was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The conductance (σ) values for the MWCNT- and SDA/MWCNT-modified electrodes are 234.1 × 10-5 S cm-1 and 358.4 × 10-5 S cm-1, respectively, as determined by electrochemical impedance spectroscopy. Consequently, an electrochemical sensor with outstanding performance in terms of reproducibility, stability and anti-interference ability was fabricated. The stripping analysis of Hg(ii) was performed using square wave anodic stripping voltammetry (SWASV) and cyclic voltammetry (CV). Using SWASV, a linear range of Hg(ii) response was found to be 1.3 to 158 μg L-1, and the limit of detection (LOD) was 0.24 μg L-1. Finally, the results of the recovered value of Hg(ii) in freshly prepared chicken liver and snail shell extract samples by SWASV were compared with the atomic absorption spectroscopy (AAS) results.

Perovskite chemical gardens: highly fluorescent microtubes from self-assembly and ion exchange

We report the shape-preserving conversion of self-assembled CaCO3 microtubes to PbCO3 and MAPbBr3 perovskite.

Electrochemical detection of mercuric(ii) ions in aqueous media using glassy carbon electrode modified with synthesized tribenzamides and silver nanoparticles

This study reports the synthesis, characterization, and mercuric ion detection ability of novel tribenzamides having flexible and rigid moieties.

Electrospun polyacrylonitrile/hydroxyapatite composite nanofibrous membranes for the removal of lead ions from aqueous solutions

A novel Polyacrylonitrile/Hydroxyapatite (PAN/HAP) composite nanofibrous membranes were successfully prepared via the electrospinning approach. The scanning electron microscope, Fourier transforms infrared, X-ray diffraction were selected to serve as characterization techniques...

Formation and phase evolution of calcium phosphates modulated by ion exchange ionomer Nafion

The phase transition of calcium phosphates regulated by Nafion with the inherent acidity and ion exchange features.

The synthesis of super-small nano hydroxyapatite and its high adsorptions to mixed heavy metallic ions

A "molecular cage" was possibly built in glucose alkaline solution to limit the chemical reaction in a tiny space to control the size and morphology of HA nanoparticles. A possible reaction mechanism of HA nanoparticles confined space synthesis was proposed in this work. The super-small HA nanoparticles were successfully obtained in the first time in mild and green condition which had a 7 nm diameter. The adsorptions to Pb²⁺, Zn²⁺, Ni²⁺, Cd²⁺ ions in their mixture solution by the synthesized HA nanoparticles were studied. The adsorbent showed huge adsorption capacities for the targets (Pb²⁺ = 3289 mg/g, Zn²⁺ = 3150 mg/g, Ni²⁺ = 3061 mg/g and Cd²⁺ = 2784 mg/g), which indicated an excellent performance of the synthesized nano material in water treatment. Freundlich equation analysis and SEM images of the adsorption products suggested multilayer adsorptions to the targets, and a new recrystalline type adsorption on the surface of the host was found. Some Ca atoms in HA nanoparticles were also found to be substituted by the heavy metallic elements in the initial stage of the adsorptions. The synthesized nano material exhibits better application prospect in wastewater treatment field for its easy synthesis, low-cost and environmental friendly properties.

Investigating structure-property relationships of biomineralized calcium phosphate compounds as fluorescent quenching-recovery platform

The structure-property relationship between biomineralized calcium phosphate compounds upon a fluorescent quenching-recovery platform and their distinct crystalline structure and surficial functional groups are investigated. A fluorescence-based sensing platform is shown to be viable for the sensing of 8-hydroxy-2-deoxy-guanosine in simulated systems.

Dual detection of nitrate and mercury in water using disposable electrochemical sensors

Here we report a disposable, cost effective electrochemical paper-based sensor for the detection of both nitrate and mercury ions in lake water and contaminated agricultural runoff. Disposable carbon paper electrodes were functionalized with selenium particles (SePs) and gold nanoparticles (AuNPs). The AuNPs served as a catalyst for the reduction of nitrate ions using differential pulse voltammetry techniques. The AuNPs also served as a nucleation sites for mercury ions. The SePs further reinforced this mercury ion nucleation due to their high binding affinity to mercury. Differential pulse stripping voltammetry techniques were used to further enhance mercury ion accumulation on the modified electrode. The fabricated electrode was characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and electrochemistry techniques. The obtained results show that the PEG-SH/SePs/AuNPs modified carbon paper electrode has a dual functionality in that it can detect both nitrate and mercury ions without any interference. The modified carbon paper electrode has improved the analytical sensitivity of nitrate and mercury ions with limits of detection of 8.6µM and 1.0ppb, respectively. Finally, the modified electrode was used to measure nitrate and mercury in lake water samples.

Mesoporous carbonated hydroxyapatite/chitosan porous materials for removal of Pb(ii) ions under flow conditions

Mesoporous carbonated hydroxyapatite/chitosan porous materials are fabricated for the first time, which show good adsorption properties for Pb(ii) ions even under flow conditions.

Fabrication of hydroxyapatite/chitosan porous materials for Pb(ii) removal from aqueous solution

Hydroxyapatite/chitosan porous materials are fabricated by a freeze-drying method for Pb(ii) removal from aqueous solution.

Room temperature gas sensing properties of tubular hydroxyapatite

Tubular hydroxyapatite was synthesized and showed high response, selectivity, good reproducibility and short response and recovery time to ammonia.

Bio-inspired formation of nanostructured arrays on flexible substrates with superoleophobicity

A bio-inspired synthesis of nanoarrays on a flexible substrate using Mn₃(PO₄)₂·3H₂O as a model is demonstrated and achieves superoleophobicity.

Synthesis and characterisation of 3-dimensional hydroxyapatite nanostructures using a thermoplastic polyurethane nanofiber sacrificial template

A facile synthesis of shape controlled three dimensional hydroxyapatite nanostructures (HAp) using a sacrificial thermoplastic polyurethane (TPU) nanofiber template.

Multi-dimensional hydroxyapatite (HAp) nanocluster architectures fabricated via Nafion-assisted biomineralization

The Nafion-assisted biomineralization strategy utilized for the fabrication of multi-dimensional hydroxyapatite (HAp) will be of great significance not only for constructing other functional materials with complex structure but also for their further extensive applications.

A fluorescence-quenching platform based on biomineralized hydroxyapatite from natural seashell and applied to cancer cell detection

As a typical biomineral, hydroxyapatite (HAp) is widely applied in bone implants and other related fields. However, the inherent nature of HAp can potentially be altered through restricting its fabrication conditions. Here, HAp fabricated by a hydrothermal treatment of pieces of natural seashell is demonstrated to have the capability of fluorescence quenching. To the best of the author's knowledge, this is the first time that this new property of HAp has been reported. Consequently, we assembled a fluorescence-quenching platform based on the biomineralized HAp substrate following a hydrothermal treatment and associated with a DNA molecular beacon and applied to cancer cell detection by the transformation from “OFF state” (fluorescence quenching) to “ON state” (fluorescence recovery). Herein, we found that the outer surface of HAp material after hydrothermal biomineralization for 5 days has considerable capability for both fluorescence quenching and recovery. These results may also have implications in the further detection of various targets such as cancer cells with other special surface antigens, significant biological small molecules or disease related microRNA, just by changing the sequence of the nucleic acid beacon according to the corresponding aptamer.

[Properties of modified amperometric biosensors based on methanol dehydrogenase and Methylobacterium nodulans cells]

The properties of amperometric biosensors based on methanol dehydrogenase (MDH), Methylobacterium nodulans cells, and the ferrocene-modified carbon paste electrode were investigated. It was shown that the addition ofhydroxyapatite (HA) to a carbon paste increased the sensitivity and operating stability of MDH biosensors. The linear range of the electrode was 0.0135-0.5 and 0.032-1.5 mM for methanol and formaldehyde, respectively. The detection limit of methanol and formaldehyde was 4.5 and 11.0 microM, respectively. The loss of activity of the electrode within 10 days of storage in the presence of 2.0 mM KCN did not exceed 12%. Cyanide (10 mM) completely inhibited the sensor responses to formaldehyde (1.0 mM), which allowed for the selective determination of methanol in the presence of formaldehyde. The biosensor based on cells exhibited lower stability and sensitivity toward methanol and formaldehyde; the sensitivity coefficients were 980 and 21 nA/mM, respectively.

Advances in synthesis of calcium phosphate crystals with controlled size and shape

Calcium phosphate (CaP) materials have a wide range of applications, including biomaterials, adsorbents, chemical engineering materials, catalysts and catalyst supports and mechanical reinforcements. The size and shape of CaP crystals and aggregates play critical roles in their applications. The main inorganic building blocks of human bones and teeth are nanocrystalline CaPs; recently, much progress has been made in the application of CaP nanocrystals and their composites for clinical repair of damaged bone and tooth. For example, CaPs with special micro- and nanostructures can better imitate the biomimetic features of human bone and tooth, and this offers significantly enhanced biological performances. Therefore, the design of CaP nano-/microcrystals, and the shape and hierarchical structures of CaPs, have great potential to revolutionize the field of hard tissue engineering, starting from bone/tooth repair and augmentation to controlled drug delivery devices. Previously, a number of reviews have reported the synthesis and properties of CaP materials, especially for hydroxyapatite (HAp). However, most of them mainly focused on the characterizations and physicochemical and biological properties of HAp particles. There are few reviews about the control of particle size and size distribution of CaPs, and in particular the control of nano-/microstructures on bulk CaP ceramic surfaces, which is a big challenge technically and may have great potential in tissue engineering applications. This review summarizes the current state of the art for the synthesis of CaP crystals with controlled sizes from the nano- to the macroscale, and the diverse shapes including the zero-dimensional shapes of particles and spheres, the one-dimensional shapes of rods, fibers, wires and whiskers, the two-dimensional shapes of sheets, disks, plates, belts, ribbons and flakes and the three-dimensional (3-D) shapes of porous, hollow, and biomimetic structures similar to biological bone and tooth. In addition, this review will also summarize studies on the controlled formation of nano-/microstructures on the surface of bulk ceramics, and the preparation of macroscopical bone grafts with 3-D architecture nano-/microstructured surfaces. Moreover, the possible directions of future research and development in this field, such as the detailed mechanisms behind the size and shape control in various strategies, the importance of theoretical simulation, self-assembly, biomineralization and sacrificial precursor strategies in the fabrication of biomimetic bone-like and enamel-like CaP materials are proposed.