New Prospects in the Electroanalysis of Heavy Metal Ions (Cd, Pb, Zn, Cu): Development and Application of Novel Electrode Surfaces

The detection of toxic heavy metal ions, especially cadmium (Cd), lead (Pb), zinc (Zn), and copper (Cu), is a global problem due to ongoing pollution incidents and continuous anthropogenic and industrial activities. Therefore, it is important to develop effective detection techniques to determine the levels of pollution from heavy metal ions in various media. Electrochemical techniques, more specifically voltammetry, due to its properties, is a promising method for the simultaneous detection of heavy metal ions. This review examines the current trends related to electrode formation and analysis techniques used. In addition, there is a reference to advanced detection methods based on the nanoparticles that have been developed so far, as well as formation with bismuth and the emerging technique of screen-printed electrodes. Finally, the advantages of using these methods are highlighted, while a discussion is presented on the benefits arising from nanotechnology, as it gives researchers new ideas for integrating these technologies into devices that can be used anywhere at any time. Reference is also made to the speciation of metals and how it affects their toxicity, as it is an important subject of research.

Screen-Printed Technologies Combined with Flow Analysis Techniques: Moving from Benchtop to Everywhere

Screen-printed electrodes (SPEs) coupled with flow systems have been reported in recent decades for an ever-growing number of applications in modern electroanalysis, aiming for portable methodologies. The information acquired through this combination can be attractive for future users with basic knowledge, especially due to the increased measurement throughput, reduction in reagent consumption and minimal waste generation. The trends and possibilities of this set rely on the synergistic behavior that maximizes both SPE and flow analyses characteristics, allowing mass production and automation. This overview addresses an in-depth update about the scope of samples, target analytes, and analytical throughput (injections per hour, limits of detection, linear range, etc.) obtained by coupling injection techniques (FIA, SIA, and BIA) with SPE-based electrochemical detection.

Disposable and Low-Cost Electrode Based on Graphene Paper-Nafion-Bi Nanostructures for Ultra-Trace Determination of Pb(II) and Cd(II)

There is a huge demand for rapid, reliable and low-cost methods for the analysis of heavy metals in drinking water, particularly in the range of sub-part per billion (ppb). In the present work, we describe the preparation, characterization and analytical performance of the disposable sensor to be employed in Square Wave Anodic Stripping Voltammetry (SWASV) for ultra-trace simultaneous determination of cadmium and lead. The electrode consists of graphene paper-perfluorosulfonic ionomer-bismuth nano-composite material. The electrode preparation implies a key step aimed to enhance the Bi3+ adsorption into nafion film, prior to the bismuth electro-deposition. Finely dispersed bismuth nanoparticles embedded in the ionomer film are obtained. The electrode was characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX), Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS) and Electrochemical Impedance Spectroscopy (EIS). The electrode shows a linear response in the 5-100 ppb range, a time-stability tested up to almost three months, and detection limits up to 0.1 ppb for both Pb2+ and Cd2+. The electrode preparation method is simple and low in cost and the obtained analytical performance is very competitive with the state of art for the SWASV determination of Pb2+ and Cd2+ in solution.

Paper-Based Working Electrodes Coated with Mercury or Bismuth Films for Heavy Metals Determination

Paper-based carbon working electrodes were modified with mercury or bismuth films for the determination of trace metals in aqueous solutions. Both modification procedures were optimized in terms of selectivity and sensitivity for the determination of different heavy metals, aiming their simultaneous determination. Cd (II), Pb (II) and In (III) could be quantified with both films. However, Cu (II) could not be determined with bismuth films. The modification with mercury films led to the most sensitive method, with linear ranges between 0.1 and 10 µg/mL and limits of detection of 0.4, 0.1, 0.04 and 0.2 µg/mL for Cd (II), Pb (II), In (III) and Cu (II), respectively. Nevertheless, the bismuth film was a more sustainable alternative to mercury. Tap-water samples were analyzed for the determination of metals by standard addition methodology with good accuracy, by using a low-cost and easily disposable paper-based electrochemical platform. This system demonstrated its usefulness for monitoring heavy metals in water.

Trace Voltammetric Determination of Lead at a Recycled Battery Carbon Rod Electrode

Carbon rod electrodes (CREs) were obtained from recycled zinc–carbon batteries and were used without further modification for the measurement of trace concentrations of lead (Pb). The electrochemical behavior of Pb at these electrodes in a variety of supporting electrolytes was investigated by cyclic voltammetry. The anodic peaks obtained on the reverse scans were indicative of Pb being deposited as a thin layer on the electrode surface. The greatest signal–to–noise ratios were obtained in organic acids compared to mineral acids, and acetic acid was selected as the supporting electrolyte for further studies. Conditions were optimized, and it was possible to determine trace concentrations of Pb by differential pulse anodic stripping voltammetry. A supporting electrolyte of 4% v/v acetic acid, with a deposition potential of −1.5 V (vs. SCE) and a deposition time of 1100 s, was found to be optimum. A linear range of 2.8 µg/L to 110 µg/L was obtained, with an associated detection limit (3σ) of 2.8 µg/L. A mean recovery of 95.6% (CV=3.9%) was obtained for a tap water sample fortified with 21.3 µg/L.

A new multivariate standard addition strategy for stripping voltammetric electronic tongues: Application to the determination of Tl(I) and In(III) in samples with complex matrices

A new multivariate standard addition strategy applicable to stripping methods was proposed as an extention of the classical univariate standard addition method for the resolution of complex samples involving overlapped peaks and complex matrices. The proposed strategy consists in alternate additions of the considered analytes and the further extrapolation to a simulated blank solution measured by skipping the preconcentration step (deposition time = 0). This calibration approach was successfully tested in tonic water samples spiked with Tl(I) and In(III) using a sensor array based on a SeCyst-SPCNFE and an ex-situ-BiSPCE, providing good concordance between replicates and much better accuracy than the usual multivariate external calibration method.

Application of screen-printed carbon electrode modified with lead in stripping analysis of Cd(II)

In the work presented, a lead film electrode was prepared in situ on a screen-printed carbon support using a reversibly deposited mediator (Zn) and applied to the determination of Cd(II) by anodic stripping voltammetry. The electrochemical method for lead film formation is based on a co-deposition of a metal of interest (Pb), with a reversibly deposited zinc mediator, followed by oxidation of zinc, with additional deposition of lead at the appropriate potential. It serves to increase the density of lead particles, promoting lead film growth, and consequently helps to improve the electrochemical properties of the electrode. This was confirmed by microscopic and voltammetric studies. The obtained detection limit of Cd(II) is equal to 6.6 × 10−9 mol L−1 (−1.6 V for 180 s and then −0.95 V for 5 s). The presented procedure was successfully applied to cadmium determination in Bystrzyca River water samples.

Recent Advances in the Fabrication and Application of Screen-Printed Electrochemical (Bio)Sensors Based on Carbon Materials for Biomedical, Agri-Food and Environmental Analyses

This review describes recent advances in the fabrication of electrochemical (bio)sensors based on screen-printing technology involving carbon materials and their application in biomedical, agri-food and environmental analyses. It will focus on the various strategies employed in the fabrication of screen-printed (bio)sensors, together with their performance characteristics; the application of these devices for the measurement of selected naturally occurring biomolecules, environmental pollutants and toxins will be discussed.

Penicillamine-modified sensor for the voltammetric determination of Cd(II) and Pb(II) ions in natural samples

A new penicillamine-GCE was developed based on the immobilization of d-penicillamine on aryl diazonium salt monolayers anchored to the glassy carbon electrode (GCE) surface and it was applied for the first time to the simultaneous determination of Cd(II) and Pb(II) ions by stripping voltammetric techniques. The detection and quantification limits at levels of µg L(-1) suggest that the penicillamine-GCE could be fully suitable for the determination of the considered ions in natural samples.

Amino-functionalization of carbon nanotubes by using a factorial design: human cardiac troponin T immunosensing application

A simple amino-functionalization method for carbon nanotubes and its application in an electrochemical immunosensor for detection of the human cardiac troponin T are described. Amino-functionalized carbon nanotubes allow oriented antibodies immobilization via their Fc regions, improving the performance of an immunosensor. Herein multiwalled carbon nanotubes were amino-functionalized by using the ethylenediamine reagent and assays were designed by fractional factorial study associated with Doehlert matrix. Structural modifications in the carbon nanotubes were confirmed by Fourier transform infrared spectroscopy. After amino-functionalization the carbon nanotubes were attached to screen-printed carbon electrode and a sandwich-type immunoassay was performed for measuring the cardiac troponin T. The electrochemical measurements were obtained through hydrogen peroxide reaction with peroxidase conjugated to the secondary antibody. Under optimal conditions, troponin T immunosensor was evaluated in serum samples, which showed a broad linear range (0.02 to 0.32 ng mL⁻¹) and a low limit of detection, 0.016 ng mL⁻¹. This amino platform can be properly used as clinical tool for cardiac troponin T detection in the acute myocardial infarction diagnosis.

Development of an electrochemically reduced graphene oxide modified disposable bismuth film electrode and its application for stripping analysis of heavy metals in milk

A novel electrochemical sensing platform based on electrochemically reduced graphene oxide film modified screen-printed electrode was developed. This disposable electrode shows excellent conductivity and fast electron transfer kinetics. By in situ plating bismuth film, the developed electrode exhibited well-defined and separate stripping peaks for cadmium and lead. Several parameters, including electrolytes environment and electrodeposition conditions, were carefully optimized to achieve best stripping performance. The linear range for both metal ions at the disposable bismuth film electrode was from 1.0 μg L(-1) to 60.0 μg L(-1). The detection limit was 0.5 μg L(-1) for cadmium ion and 0.8 μg L(-1) for lead ion. Milk sample analysis demonstrates that the developed electrode could be effectively used to detect low levels (μg L(-1)) of cadmium ion and lead ion. Graphene based disposable bismuth film electrode is a sensitive, stable, and reliable sensing platform for heavy metals determination.

Simultaneous Stripping Detection of Pb(II), Cd(II) and Zn(II) Using a Bimetallic Hg-Bi/Single-Walled Carbon Nanotubes Composite Electrode

A new, sensitive platform for the simultaneous electrochemical assay of Zn(II), Cd(II) and Pb(II) in aqueous solution has been developed. The platform is based on a new bimetallic Hg-Bi/single-walled carbon nanotubes (SWNTs) composite modified glassy carbon electrode (GCE), demonstrating remarkably improved performance for the anodic stripping assay of Zn(II), Cd(II) and Pb(II). The synergistic effect of Hg and Bi as well as the enlarged, activated surface and good electrical conductivity of SWNTs on GCE contribute to the enhanced activity of the proposed electrode. The analytical curves for Zn(II), Cd(II) an Pb(II) cover two linear ranges varying from 0.5 to 11 μg L(-1) and 10 to 130 μg L(-1) with correlation coefficients higher than 0.992. The limits of detection for Zn(II), Cd(II) are lower than 2 μg L(-1) (S/N = 3). For Pb(II), moreover, there is another lower, linear range from 5 to 1100 ng L(-1) with a coefficient of 0.987 and a detection limit of 0.12 ng L(-1). By using the standard addition method, Zn(II), Cd(II) and Pb(II) ions in river samples were successfully determined. These results suggest that the proposed method can be applied as a simple, efficient alternative for the simultaneous monitoring of heavy metals in water samples. In addition, this method demonstrates the powerful application of carbon nanotubes in electrochemical analysis of heavy metals.