Non-conductive nanomaterial enhanced electrochemical response in stripping voltammetry: The use of nanostructured magnesium silicate hollow spheres for heavy metal ions detection

Nanostructured magnesium silicate hollow spheres, one kind of non-conductive nanomaterials, were used in heavy metal ions (HMIs) detection with enhanced performance for the first time. The detailed study of the enhancing electrochemical response in stripping voltammetry for simultaneous detection of ultratrace Cd²⁺, Pb²⁺, Cu²⁺ and Hg²⁺ was described. Electrochemical properties of modified electrodes were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The operational parameters which have influence on the deposition and stripping of metal ions, such as supporting electrolytes, pH value, and deposition time were carefully studied. The anodic stripping voltammetric performance toward HMIs was evaluated using square wave anodic stripping voltammetry (SWASV) analysis. The detection limits achieved (0.186 nM, 0.247 nM, 0.169 nM and 0.375 nM for Cd²⁺, Pb²⁺, Cu²⁺ and Hg²⁺) are much lower than the guideline values in drinking water given by the World Health Organization (WHO). In addition, the interference and stability of the modified electrode were also investigated under the optimized conditions. An interesting phenomenon of mutual interference between different metal ions was observed. Most importantly, the sensitivity of Pb²⁺ increased in the presence of certain concentrations of other metal ions, such as Cd²⁺, Cu²⁺ and Hg²⁺ both individually and simultaneously. The proposed electrochemical sensing method is thus expected to open new opportunities to broaden the use of SWASV in analysis for detecting HMIs in the environment.     

Electrochemical Studies of Three Dimensional Graphene Foam as an Electrode Material

Graphene foam is one kind of network of three dimensional (3D) graphene, which inherits the properties of two dimensional graphene and overcomes the aggregation/stacking of graphene sheets. In this work, graphene foam has been characterized by scanning electron microscopy and Raman spectroscopy. A graphene foam electrode was evaluated as a new electrode material by cyclic voltammetry (CV) and used for the detection of trace level of Pb²⁺ by anodic stripping voltammetry (ASV). Under the optimized condition of deposition potential (‐1.2 V) and deposition time (2 min), the detection limit is estimated to be 40 nM for Pb²⁺ based on the 3σ method.     

Underpotential deposition of tin(II) on a gold disc electrode and determination of tin in a tin plate sample

This work describes a study of the underpotential deposition (UPD) of Sn²⁺ on a polycrystalline gold disc electrode using cyclic voltammetry (CV) and chronocoulometry (CC). Sn²⁺ ions showed well-defined peaks from UPD and UPD stripping (UPD-S) in 1 mol/L HCl solutions, while bulk deposition (BD) and BD stripping (BD-S) of the ions were also observed. The measured UPD shifts, E_UPD, between the UPD-S and the BD-S peaks were more than 200 mV. The UPD charge and the surface coverage of tin were measured by CC. A new method for determining Sn²⁺ was therefore developed, based on the excellent electrochemical properties of the Au/Sn UPD system. A plot of the UPD-DPASV (differential pulse anodic stripping voltammetry) signal versus the Sn(II) concentration was obtained for [Sn(II)] of 1.98×10^–7 to 3.64×10^–5 M. The method developed here has been applied to determine the tin in a tin plate sample.     

Anodic Stripping Voltammetry of Heavy Metals on a Metal Catalyst Free Carbon Nanotube Electrode

Anodic stripping voltammetry (ASV) determination of Pb²⁺, Cd²⁺, and Zn²⁺ was done using metal catalyst free carbon nanotube (MCFCN) electrodes. Osteryoung square wave stripping voltammetry (OSWSV) was selected for detection. The MCFCNTs are synthesized via Carbo Thermal Carbide Conversion method which leads to residual transition metal free in the CNT structure. The new material shows very good results in detecting heavy metal ions, such as Pb²⁺, Cd²⁺, and Zn²⁺. The calculated limits of detection were 13 nM, 32 nM and 50 nM for Pb²⁺, Cd²⁺ and Zn²⁺, respectively with a deposition time of 150 s.     

Determination of Trace Lead and Cadmium in Water Samples by Anodic Stripping Voltammetry with a Nafion‐Ionic Liquid‐Coated Bismuth Film Electrode

A new chemically modified bismuth film electrode coated with an ionic liquid [(1‐ethyl‐3‐methylimidazolium tetracyanoborate (EMIM TCB)] and Nafion was developed for the simultaneous determination Pb²⁺ and Cd²⁺ by anodic stripping voltammetry. Compared with conventional bismuth film electrodes, this electrode exhibited greatly improved electrochemical activity for Pb²⁺ and Cd²⁺ detection due to the unique properties of Nafion polymer and ionic liquid. The key experimental parameters related to the fabrication of the electrode and the voltammetric measurements were optimized on the basis of the stripping signals, where the peak currents increased linearly with the metal concentrations in a range of 10–120 µg L^?1 with a detect limit of 0.2 µg L^?1 for Pb²⁺, and 0.5 µg L^?1 for Cd²⁺ for 120s deposition. High reproducibility was indicated from the relative standard deviations (1.9 and 2.5 %) for nine repetitive measurements of 20 µg L^?1 Pb²⁺ and Cd²⁺, respectively. In addition, the surface characteristics of the modified BiFE were investigated by scanning electron microscopy (SEM), and results showed that fibril‐like bismuth nanostructures were formed on the porous Nafion polymer matrix. Finally, the developed electrode was applied to determine Pb²⁺ and Cd²⁺ in water samples, indicating that this electrode was sensitive, reliable and effective for the simultaneous determination of Pb²⁺ and Cd²⁺.     

Simultaneous determination of trace levels of lead(II) and copper(II) by square wave stripping voltammetry using a glassy carbon electrode modified with hierarchical gold dendrites

A glassy carbon electrode was modified with gold hierarchical dendrites (Au HDs) by one-step electrodeposition in the presence of cytosine, which plays an important role in the formation of the Au HDs. This approach is simple, fast, feasible, controllable, without any seed, template, or surfactant. The modified electrodes were used for the simultaneous determination of Pb²⁺ and Cu²⁺ by square wave stripping voltammetry. The peak currents show good linear relationship with concentrations of Pb²⁺ and Cu²⁺ in the range of 5.0 to 15.0 μM. The recoveries of the spiked water samples are in the range of 94.0 %–107.4 % for Pb²⁺ and Cu²⁺, and their relative standard deviation are in the range of 2.7 %–4.3 % for Pb²⁺ and Cu²⁺, respectively (n?=?3).

An Electrochemical Sensing Platform for the Detection of Lead Ions Based on Dicarboxyl‐Calix[4]arene

A simple and highly sensitive electrochemical sensor COOH−C4 derived from dicarboxyl‐calix[4]arene modified on a screen printed gold electrode (Au) was developed for the determination of lead ions in water samples. A 3‐mercaptopropionic acid (MPA) monolayer was used as a template on the gold electrode for the surface modification with dicarboxyl‐calixarene. The modified electrodes were surface‐characterized using Fourier Transform infrared spectroscopy (FTIR). The data obtained proved the confirmation of each stage of the electrode modification. The electrochemical analyses of the COOH−C4 electrode showed an enhanced electrocatalytic activity and higher current towards Pb²⁺ ions as compared to the bare Au and MPA/Au electrodes. Under optimum conditions, the differential pulse voltammetry response of COOH−C4 displayed a wide linear response ranging from 280–2500 μg/L for Pb²⁺ with a detection limit of 6.2 μg/L. In addition, the fabricated electrode showed a high selectivity and stability towards the Pb²⁺ ions in presence of possible interfering species. The present method was successfully applied to determine Pb²⁺ ions in real samples with satisfactory precision, with a relative standard deviation of 3.12 % and an acceptable recovery of 92 %, which demonstrated the potential application of dicarboxyl‐calix[4]arene modified on electrodes for heavy‐metal sensing.     

Sensitive and Simple Electrochemical Detection of Lead(II) with Carbon Ionic Liquid Electrode

In this paper a carbon ionic liquid electrode (CILE) was fabricated by using ionic liquid 1‐ethyl‐3‐methylimidazolium ethylsulphate ([EMIM]EtOSO₃) as the modifier and further used as the working electrode for the sensitive anodic stripping voltammetric detection of Pb²⁺. The characteristics of the CILE were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). In pH 4.5 NaAc‐HAc buffer Pb²⁺ was accumulated on the surface of CILE due to the extraction effect of IL and reduced at a negative potential (‐1.20 V). Then the reduced Pb was oxidized by differential pulse anodic stripping voltammetry with an obvious stripping peak appeared at ?0.67 V. Under the optimal conditions Pb²⁺ could be detected in the concentration range from 1.0 × 10^?8 mol/L to 1.0 × 10^?6 mol/L with the linear regression equation as I_p(μA) = ?0.103 C (μmol/L) + 0.0376 (γ = 0.999) and the detection limit as 3.0 × l0^?9 mol/L (3σ). Interferences from other metal ions were investigated and Cd²⁺ could be simultaneously detected in the mixture solution. The proposed method was further applied to the trace levels of Pb²⁺ detection in water samples with satisfactory results.     

Electrochemical Determination of Cd2+ and Pb2+ Using NSAID‐mefenamic Acid Functionalized Mesoporous Carbon Microspheres Modified Glassy Carbon Electrode

This paper describes the electrochemical behaviors of Cd²⁺ and Pb²⁺ on the proposed mesoporous carbon microspheres/mefenamic acid/nafion modified glassy carbon electrode (MC/MA/Nafion/GC) studied by square wave anodic stripping voltammetry (SWASV). The prepared material is characterized by XRD, SEM, FTIR, RAMAN and BET analysis. Experimental parameters, such as the deposition potential and time, the pH value of buffer solution were optimized. Under the optimized conditions, the electrode responded linearly to Cd²⁺ and Pb²⁺ in the concentration range from 50 to 300 nM, and the detection limits were 24.2 and 11.26 nM respectively. The sensitivity determined was 0.0623 μA/nM (Cd²⁺) and 0.192 μA/nM (Pb²⁺). Multiple metal ion detection with clear demarcation of peaks was produced by the electrode. Moreover, the modified electrode has possessed good selectivity and reproducibility of Cd²⁺ and Pb²⁺ detection. We also investigated the interference of various anions and surfactants for the detection of Cd²⁺ and Pb²⁺ ions. Finally the modified electrode was used to detect the presence of metal ions in practical samples and the results obtained are comparatively good with respect to AAS.     

Electroanalytical Determination of Cadmium(II) and Lead(II) Using an Antimony Nanoparticle Modified Boron‐Doped Diamond Electrode

We report the simultaneous electroanalytical determination of Pb²⁺ and Cd²⁺ by linear sweep anodic stripping voltammetry (LSASV) using an antimony nanoparticle modified boron doped diamond (Sb‐BDD) electrode. Sb deposition was performed in situ with the analytes, from a solution of 1 mg L^?1 SbCl₃ in 0.1 M HCl (pH 1). Pb²⁺ inhibited the detection of Cd²⁺ during simultaneous additions at the bare BDD electrode, whereas in the presence of antimony, both peaks were readily discernable and quantifiable over the linear range 50–500 μg L^?1.     

Selective Determination of Copper (II) Based on Aluminum Silicon Carbide Nanoparticles Modified Glassy Carbon Electrode by Square Wave Stripping Voltammetry

The morphology and structure of as‐prepared aluminum silicon carbide (Al₄SiC₄) were characterized using X‐ray diffraction (XRD) patterns, scanning electron microscope (SEM), transmission electron microscopy (TEM) and UV‐vis spectra. The Al₄SiC₄ nanoparticles modified glassy carbon electrode (GCE) was further investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Based on this, this kind of new electrode was used for the detection of trace Cu²⁺ by square wave anodic stripping voltammetry (SWASV) for the first time. The electrochemical parameters influencing on deposition and stripping of metal ions, such as supporting electrolytes, pH value, deposition potential and deposition time, were also optimized. The results showed that the Al₄SiC₄ modified GCE exhibited excellent stripping response of Cu²⁺ and the stripping peaks response increased linearly with increasing concentration of Cu²⁺ in the ranges of 400 to 2200 nM. Under the optimized conditions the favorable sensitivity of the Al₄SiC₄ modified GCE toward trace Cu²⁺ was 1.49 μA μM⁻¹ and the limit of detection (S/N=3) was estimated to be 2.76 nM. More importantly, Al₄SiC₄ modified GCE had an excellent stability and negligible interference from other coexisting metal ions in the electrochemical determination of Cu²⁺.     

The Fabrication and Characterization of a Bismuth Nanoparticle Modified Boron Doped Diamond Electrode and Its Application to the Simultaneous Determination of Cadmium(II) And Lead(II)

We report the simultaneous electroanalytical determination of Pb²⁺ and Cd²⁺ by square‐wave anodic stripping voltammetry (SWASV) using a bismuth nanoparticle modified boron doped diamond (Bi‐BDD) electrode. Bi deposition was performed in situ with the analytes, from a solution of 0.1 mM Bi(NO₃)₃ in 0.1 M HClO₄ (pH 1.2), and gave detection limits of 1.9 μg L^?1 and 2.3 μg L^?1 for Pb(II) and Cd(II) respectively. Pb²⁺ and Cd²⁺ could not be detected simultaneously at a bare BDD electrode, whilst on a bulk Bi macro electrode (BiBE) the limits of detection for the simultaneous determination of Pb²⁺ and Cd²⁺ were ca. ten times higher.     

The influence of organic adsorbates on the UPD process. Oxidation of formic acid at UPD lead-modified platinum electrodes

The oxidation of formic acid at Pt electrodes in the presence of underpotentially deposited (UPD) Pb has been studied using an electrochemical quartz crystal microbalance (EQCM). Although the current associated with the UPD process is largely obscured by current from the oxidation of formic acid, the mass response is dominated by the changes in UPD coverage. Thus examination of mass responses accompanying cyclic voltammetric and constant-potential experiments reveals both variations in UPD coverage and the manner in which the underpotential deposits are affected by adsorbates derived from formic acid. At low concentrations of formic acid there is some suppression of the underpotential deposit and data suggest that strongly adsorbing intermediates form most rapidly in the hydrogen adsorption region of potential. Mass responses also indicate slight increases in UPD coverage upon removal of strongly adsorbed species by oxidation. Oxidation of high concentrations (0.1 M) of formic acid induces a significant positive shift in the potential for removal of the UPD deposit on the positive scan, and on the subsequent negative scan the rapid reaction between the oxidized Pt surface and formic acid removes the oxide at a higher potential than normal and consequently allows the UPD process to begin at a more positive potential. Adsorption of Pb²⁺ at oxidized Pt surfaces is also inhibited by the presence of formic acid.     

Characteristics of subtractive anodic stripping voltammetry of Pb and Cd at silver and gold electrodes

Silver and gold electrodes are useful for the quantitative determination of lead and cadmium with subtractive anodic stripping voltammetry (SASV). The use of SASV is essential for achieving good separation between the two peaks, to eliminate the interference of nitrates when cadmium is present and to allow analysis at very low concentrations without the removal of oxygen. The deposition and dissolution of Pb²⁺ and Cd²⁺ proceed at underpotential (UPD) on both electrodes. The UPD properties of the deposits are the main factor determining the analytical characteristics of the ASV method and are strongly affected by the type and concentration of the electrolyte. The effects of anions (Cl⁻, Br⁻, SO₄²⁻, NO₃⁻) and acids (HNO₃, HClO₄, H₂SO₄, HCl) are shown. The two electrodes complement each other and, in addition, enable the qualitative identification of Pb²⁺ and Cd²⁺, since the peaks appear in opposite order on the two electrodes. Analysis of mixtures of the two analytes is restricted on gold but not on silver. At gold the two peaks overlap: (i) at concentrations of cadmium higher than 250 nM at deposition times greater than 30 s, (ii) in the presence of copper at concentrations higher than 1 μM, and (iii) in the presence of Triton X-100 at concentrations above 10 mg/l. The repeatability at 10 nM analyte is better than 2.5%. The detection limits for Pb²⁺ and Cd²⁺ at 120 s deposition time and 3500 rpm rotation rate are: dl_Pb/Ag=0.04 nM; dl_Cd/Ag=0.7 nM; dl_Pb/Au=0.1 nM; dl_Cd/Au=0.3 nM. The analysis of lead and cadmium in natural waters has been performed.     

Platinum Ultramicroelectrodes Modified with Electrogenerated Surfactant‐Templated Mesoporous Organosilica Films: Effect of Film Formation Conditions on Its Performance in Preconcentration Electroanalysis

Pt µdisc electrodes have been modified by mesoporous organosilica thin films by electrochemically assisted self‐assembly (EASA) of mercaptopropyltrimethoxysilane (MPTMS), tetraethoxysilane (TEOS), and the surfactant cetyltrimethylammonium bromide (CTAB). The EASA process involves the generation of hydroxide ions at the electrode/solution interface, upon the application of a cathodic current density, leading to TEOS and MPTMS polycondensation around the CTAB template and concomitant growing of a thiol‐functionalized mesoporous film onto the electrode surface. The experimental conditions (current density, deposition time, silane concentration and molar ratio between surfactant template and silane) were optimised to form a thin and permeable film likely to be used in preconcentration electroanalysis. The morphology of the film electrodes were characterised by scanning electron microscopy. The permeability properties of the modified Pt µdisc electrodes have been evidenced by cyclic voltammetry using Ru(NH₃)₆³⁺ as a redox probe. The best parameters identified for the film preparation are a current density of ? 8 mA cm^?2 applied for 15 s in a solution containing 110 mM of hydrolysed silane precursors and 70.4 mM of CTAB. Pt µdisc electrodes modified in these conditions were used for the open‐circuit preconcentration of Hg(II) species prior to their detection by anodic stripping voltammetry in a mercury‐free solution. In the optimized conditions, a sensitivity of 14.3 mA cm^?2 µM^?1 was obtained for the 0.02–0.08 µM concentration range. The analytical performance of such organosilica films could decay by up to two orders of magnitude for the materials prepared in conditions other than the optimized ones, highlighting the need for a fine control of the deposition parameters to elaborate sensors based on such modified ultramicroelectrodes.     

A Simple and Highly Sensitive Electrochemically Reduced p‐Nitrobenzoic Acid Film Modified Sensor for Determination of Mercury

Herein, a simple electrochemical sensor was fabricated for sensing Hg²⁺ ions by using electrochemically reduced p‐nitrobenzoic acid molecules modified (ERpNBA) glassy carbon electrode (GCE). The modified electrode was applied for the determination of Hg²⁺ ions by using differential pulse anodic stripping voltammetry (DPASV). Experimental parameters such as concentration of p‐nitrobenzoic acid used for electrode modification, pH, accumulation time and deposition potential used for the determination of Hg²⁺ ions were optimized. The strong interaction between the Hg²⁺ ions and the lone pair of electrons on the nitrogen atoms of ERpNBA molecules leads to highly selective adsorption of Hg²⁺ ions on the modified electrode. Under the optimum experimental conditions, the sensor showed higher sensitivity and very low detection limit for Hg²⁺ ions than other metal ions such as Cd²⁺, Pb²⁺ and Zn²⁺ ions. The LOD for Hg²⁺ ions was 240 pM which is below the guideline value given by the World Health Organization and the earlier reports.     

Voltammetric behavior and trace determination of Pb at a mercury-free screen-printed silver electrode

Screen-printed silver electrodes (AgSPEs), without chemical modification, has been investigated as disposable sensors for the measurement of trace levels of Pb²⁺. Potential segment analysis indicates that the formation of underpotential and bulk depositions of Pb is not strongly coupled on the AgSPE. The possibility of determining Pb²⁺ at trace levels using the reversible underpotential deposition peak was examined by square-wave anodic stripping voltammetry without removal of oxygen. Under the optimized analytical conditions, the obtained sensitivity, linearity, and detection limit are 0.355 μA/ppb, 5-80 ppb (r=0.9992), and 0.46 ppb (S/N=3), respectively. The electrode is quite stable for repetitive measurements. The interference effect was thoroughly studied with various metals and no significant change in current was found in the determination of 5 ppb Pb²⁺. The practical applications were demonstrated to measure trace Pb²⁺ in natural waters.     

Detection of Trace Heavy Metal Ions Using Carbon Nanotube‐ Modified Electrodes

A sensitive voltammetric method for detection of trace heavy metal ions using chemically modified carbon nanotubes (CNTs) electrode surfaces is described. The CNTs were covalently modified with cysteine prior to casting on electrode surfaces. Cysteine is an amino acid with high affinities towards some heavy metals. In this assay, heavy metals ions accumulated on the cysteine‐modified CNT electrode surfaces prior to being subjected to differential pulse anodic stripping voltammetry analysis. The resulting peak currents were linearly related to the concentrations of the metal ions. The method was optimized with respect to accumulation time, reduction time and reduction potential. The detection limits were found to be 1 ppb and 15 ppb for Pb²⁺ and Cu²⁺ respectively. The technique was used for the detection of Pb²⁺ and Cu²⁺ in spiked lake water. The average recoveries of Pb²⁺ and Cu²⁺ were 96.2% and 94.5% with relative standard deviations of 8.43% and 7.53% respectively. The potential for simultaneous detection of heavy metal ions by the modified CNTs was also demonstrated.     

Remote Stripping Analysis of Lead and Copper by a Mercury‐Coated Platinum Microelectrode

The performance of a remote stripping sensor based on mercury microelectrodes (MM‐RS) for the in situ detection of trace metals in aquatic systems, was investigated. The submersible device employed here consists of a single mercury‐coated platinum disk microelectrode assembled in a two‐electrode cell configuration, and connected remotely by a 30 m long shielded cable. First, the MM‐RS device is characterized in Ru(NH₃) and Pb²⁺ synthetic aqueous solutions by applying cyclic voltammetry and anodic stripping voltammetry (ASV), respectively. The results obtained show that the small electrode dimensions and the related low currents involved, the long remote connection cable or the use of a two‐electrode system do not cause noise effects or uncompensated resistance problems in the measurements. Using square‐wave voltammetry in the stripping step, linear calibration graphs for Pb²⁺ ions over the concentration range 1×10^?9?5×10^?7 M were obtained, and a detection limit, DL, of 0.15 nM was found. The relative standard deviation (RSD), at 5×10^?8 M Pb²⁺ level, was within 5%. The effect of humic acid and of sodium dodecylsulfate surfactants on the stripping responses was also investigated. The performance of the submersible MM‐RS system was tested for the in situ monitoring of the labile fraction of lead and copper on a site of the Lagoon of Venice. In situ Pb²⁺ and Cu²⁺ concentrations were monitored for about 8 hours, by leaving the sensor immersed in the lagoon waters (2 m depth) and recording the response every hour. Under these field conditions, reliable in situ data for the labile fraction of these metal ions with a satisfactory precision, the RSD being within 7 and 9 % for lead and copper, respectively, were obtained.     

Zinc oxide nanostructured platform for electrochemical detection of heavy metals

ZnO nanoparticles (ZnO-NP) were prepared by a facile precipitation technique using di-isopropyl amine as precipitating agent. The morpho-structure and porosity of the as-prepared nano-powder were investigated by FT-IR analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), and BET analysis. By drop-casting, a composite film was deposited to obtain ZnO-NP-Nafion/GCE modified electrode. The modified electrode was investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and square wave anodic stripping voltammetry (SWASV) for the detection of Pb²⁺, Cd²⁺, Cu²⁺, and Fe³⁺, and it was successfully applied for the detection of Pb²⁺ and Cu²⁺ in real water samples.