New Approach to Multivariate Standard Addition Based on Multivariate Curve Resolution by Alternating Least-Squares: Application to Voltammetric Data.

A multivariate version of the classical univariate standard addition method is proposed for the analysis of samples generating overlapping signals in the presence of notorious matrix effects. Unlike previous versions based on multivariate calibration by partial least-squares (PLS), the proposed strategy takes advantage of a self-modeling methodology: multivariate curve resolution by alternating least-squares (MCR-ALS) enhanced with signal shape constraints based on parametric functions. In this way, there is no need for the full multivariate response of a blank solution, and in multianalyte determinations, the standard additions can be made with a solution containing all of the analytes, which constitutes a clear advance as compared to PLS approach. The proposed method has been successfully tested in the voltammetric determination of hydroquinone and catechol in solutions of increasing complexity and appears to be a promising tool in the field of electroanalysis.

Electroanalysis from the past to the twenty-first century: challenges and perspectives.

A personal mini-review is presented on the history of electroanalysis and on their present achievements and future challenges. The manuscript is written from the subjective view of two generations of electroanalytical chemists that have witnessed for many years the evolution of this discipline.

Screen-Printed Electrodes for the Voltammetric Sensing of Benzotriazoles in Water.

Benzotriazoles (BZTs) are high production volume industrial chemicals that are used in various applications such as corrosion inhibitors, antifreeze agents, and UV radiation stabilizers. Given their potential ecotoxicological implications for different ecosystems and in human health, as well as their poor biodegradability, they are of increasing concern. In this study, a new voltammetric method using commercial screen-printed electrodes (SPEs) has been developed for the sensing of BZTs in water samples to help in their environmental monitoring. To this end, different types of SPEs based on carbon nanoallotropes and copper were tested under several experimental conditions to determine the two BZTs most frequently detected in the environment: 1H-benzotriazole (BZT) and 5-methyl-1H-benzotriazole (Me-BZT, tolyltriazole) as model compounds for BZTs. Carbon nanofibers electrodes exhibited the best performance, allowing detection limits as low as 0.4 mg L-1 for both BZTs, with repeatability and reproducibility of ca. 5%. The applicability of the method was tested through the determination of BZT in spiked drinking water samples, suggesting its suitability for the sensing of samples heavily polluted with BZTs.

Commercial Screen-Printed Electrodes Based on Carbon Nanomaterials for a Fast and Cost-Effective Voltammetric Determination of Paracetamol, Ibuprofen and Caffeine in Water Samples.

Carbon screen-printed electrode (SPCE), multi-walled carbon nanotubes modified screen-printed electrode (SPCNTE), carbon nanofibers modified screen-printed electrode (SPCNFE), and graphene modified screen-printed electrode (SPGPHE) were in a pioneer way tested as sensors for the simultaneous determination of the two most consumed pain-killers, paracetamol (PA) and ibuprofen (IB), and the stimulant caffeine (CF) in water by differential pulse voltammetry (DPV). Their analytical performances were compared, and the resulting sensitivities (2.50, 0.074, and 0.24 µA V mg-1 L for PA, IB, and CF, respectively), detection limits (0.03, 0.6, and 0.05 mg L-1 for PA, IB, and CF, respectively) and quantification limits (0.09, 2.2, and 0.2 mg L-1 for PA, IB, and CF, respectively) suggested that the SPCNFE was the most suitable carbon-based electrode for the voltammetric determination of the selected analytes in water at trace levels. The methodology was validated using both spiked tap water and hospital wastewater samples. The results were compared to those achieved by liquid chromatography-tandem mass spectrometry (LC-MS/MS), the technique of choice for the determination of the target analytes.

Voltammetric Electronic Tongues in Food Analysis.

A critical revision is made on recent applications of voltammetric electronic tongues in the field of food analysis. Relevant works are discussed dealing with the discrimination of food samples of different type, origin, age and quality and with the prediction of the concentration of key substances and significant indexes related to food quality.

Ag Nanoparticles Drop-Casting Modification of Screen-Printed Electrodes for the Simultaneous Voltammetric Determination of Cu(II) and Pb(II).

A new silver nanoparticle modified screen-printed electrode was developed and applied to the simultaneous determination of Pb(II) and Cu(II). Two different types of silver nanoparticles with different shapes and sizes, Ag nanoseeds and Ag nanoprisms, were microscopically characterized and three different carbon substrates, graphite, graphene and carbon nanofibers, were tested. The best analytical performance was achieved for the combination of Ag nanoseeds with a carbon nanofiber modified screen-printed electrode. The resulting sensor allowed the simultaneous determination of Pb(II) and Cu(II) at trace levels and its applicability to natural samples was successfully tested with a groundwater certified reference material, presenting high reproducibility and trueness.

Evaluation of mercury stress in plants from the Almadén mining district by analysis of phytochelatins and their Hg complexes.

To evaluate plant response to Hg stress, glutathione, phytochelatins, and their Hg complexes were analyzed using HPLC with amperometric detection in samples of Asparagus acutifolius grown in the Almadén mining district (Ciudad Real, Spain), one of the most Hg-contaminated sites in the world. Soils of the Almadén mining district, and specifically from the Almadenejos zone, are highly contaminated, with some zones having values above 4,000 µg Hg g(-1) soil. Although soils have an extremely high concentration of mercury, generally less than 2% is available for plants, as is shown by various soil extractions simulating bioavailability. In plants, Hg concentration increases depending on the content of Hg in soils. In addition, Hg levels in roots are higher than in aerial parts, which is a strategy of plants for protecting their more sensitive aerial parts from the deleterious effects of metal stress. The total content of phytochelatins (PCs) and their complexes are directly related with the amount of mercury in soils. These findings highlight the important role of thiol compounds and their metal complexes in capturing and fixing Hg from soils, giving plants the capacity to deal with the heavy metal toxicity of polluted soils.

Parametric Signal Fitting by Gaussian Peak Adjustment: implementation of 2D transversal constraints and its application for the determination of pKa and complexation constants by differential pulse voltammetry.

A new method, GPA2D, is presented as a significant improvement of the previously described Gaussian Peak Adjustment (GPA) which includes, for the first time, transversal constraints to increase the consistency of the resolution along the different signals of a voltammetric dataset. The constraints deal with the evolution of peak potentials versus pH and with the implementation of chemical equilibrium constants. Moreover, pkavolt, a new tool for the pKa determination, is proposed as an example of the great versatility of the PSF-GPA (Parametric Signal Fitting by Gaussian Peak Adjustment) methodology. GPA2D and pkavolt have been successfully tested on the systems Cd(ii):phytochelatin, Cu(ii):6-benzylaminopurine, Cd(ii):1,10-phenanthroline, Zn(ii):oxalate and captopril drug by determining formation constants (metal complex systems) or pKa values (captopril).

Combination of chemometrically assisted voltammetry, calorimetry, and circular dichroism as a new method for the study of bioinorganic substances: application to selenocystine metal complexes.

Selenium-containing compounds play an important role in antioxidant defense systems, binding to toxic metals, preventing their uptake into cells, and thus protecting cells from metal-induced formation of reactive oxygen species. Here, we present a proposal for a relatively new method as a complement to the more usual methods used in selenium studies. A systematic study of the metal-binding properties of selenocystine (SeCyst) in the presence of divalent metal cations (Cd, Co, Hg, Ni, and Zn) is reported. Isothermal titration calorimetry provides thermodynamic parameters of the systems. Titrations produced curves that could be fit reasonably well to the one set of sites model. The data clearly demonstrate that one M(2+) binds one SeCyst molecule, and the stable M(SeCyst) complex is formed under these conditions. The order of the SeCyst binding constant for the metal ions is Hg(2+) > Cd(2+) ~ Zn(2+) > Ni(2+)> Co(2+). Cadmium ion was selected as a modulator for the behavior of SeCyst in the presence of a nonessential metal, and zinc was selected for the case of an essential element. These interactions of SeCyst with Cd(2+) and Zn(2+), either individually or combined, were studied in aqueous buffered solutions at physiological pH by differential pulse polarography and circular dichroism spectroscopy. Furthermore, recently developed chemometric tools were applied to differential pulse polarography data obtained in mixtures of SeCyst and glutathione in the presence of Cd(2+) at physiological pH.

Application of different chemometric strategies to voltammetric and UV-vis spectroscopic data to obtain a complexation model: study of the Cu(II) binding with the phytohormone 6-benzylaminopurine.

The complexation processes of Cu(II) by the phytohormone and the possible antitumoral agent 6-benzylaminopurine (BAP) were studied by Differential Pulse Polarography (DPP) and ultraviolet-visible spectroscopy (UV-vis) in combination with chemometric programs such as Multivariate Curve Resolution by Alternating Least-Squares (MCR-ALS) and Gaussian Peak Adjustment (GPA). All data confirm the formation of the predominant 1 : 1 and 1 : 2 Cu(2+) : BAP complexes. The corroboration of the presented stoichiometries was performed by ElectroSpray Ionization-Mass Spectrometric (ESI-MS) experiments of the Cu(II)-BAP system at different ratios.

Chemometric analysis of voltammetric data on metal ion binding by selenocystine.

The behavior of selenocystine (SeCyst) alone or in the presence of various metal ions (Bi(3+), Cd(2+), Co(2+), Cu(2+), Cr(3+), Ni(2+), Pb(2+), and Zn(2+)) was studied using differential pulse voltammetry (DPV) over a wide pH range. Voltammetric data matrices were analyzed using chemometric tools recently developed for nonlinear data: pHfit and Gaussian Peak Adjustment (GPA). Under the experimental conditions tested, no evidence was found for the formation of metal complexes with Bi(3+), Cu(2+), Cr(3+), and Pb(2+). In contrast, SeCyst formed electroinactive complexes with Co(2+) and Ni(2+) and kinetically inert but electroactive complexes with Cd(2+) and Zn(2+). Titrations with Cd(2+), Co(2+), Ni(2+), and Zn(2+) produced data that were reasonably consistent with the formation of stable 1:1 M(SeCyst) complexes.

Asymmetric logistic peak as a suitable function for the resolution of highly asymmetric voltammograms in non-bilinear systems.

The asymmetric logistic peak is tested as a new function for the parametric signal fitting (PSF) of highly asymmetric electrochemical signals in non-bilinear datasets, such as those obtained in linear sweep voltammetry (LSV) or in the presence of irreversible electrochemical processes. This new multivariate curve resolution strategy (PSF-ALPA) is successfully applied to LS voltammograms measured for the Cd(II)-glutathione system with a hanging mercury drop electrode, where Cd(II) is reversibly reduced, and to differential pulse voltammograms (DPV) measured at a glassy carbon electrode, where Cd(II) reduction becomes irreversible. Matrix augmentation by using LS voltammograms measured at different scan rates provides good results and encourages the development of ALPA methodology for third order data.

Circular dichroism and voltammetry, assisted by multivariate curve resolution, and mass spectrometry of the competitive metal binding by phytochelatin PC5.

The competitive binding of Cd(2+) and Pb(2+) by the phytochelatin (γGlu-Cys)5-Gly (PC5) has been examined by several analytical techniques. Positive-mode electrospray ionization mass spectrometry (ESI-MS) is used to determine the stoichiometries of the complexes, while voltammetric and spectroscopic data are analyzed by multivariate curve resolution with alternating least-squares (MCR-ALS) and some recently developed chemometric tools, which allows one to follow the displacement induced by Cd(2+) or Pb(2+) in the binding of Pb(2+) or Cd(2+), respectively, by PC5, and to obtain a complete overview of the processes involved. Differential pulse polarography (DPP) signals of these systems present, besides overlapping peaks due to free metal ion and metal complexes, interference of mercury anodic signals. Circular dichroism (CD) spectroscopy is a useful complementary technique due to the absence of these types of signals and its selective response to different species. A tentative complexation/electrochemical model is proposed for Cd(2+) and Pb(2+) competition toward PC5 complexation. The formation of the previously unreported ternary CdPb(PC5) complex was suggested by DPP and CD spectroscopy experiments and confirmed by ESI-MS.

Electrochemical survey of the chain length influence in phytochelatins competitive binding by cadmium.

Multivariate curve resolution with alternating least squares (MCR-ALS) was applied to voltammetric data obtained in the analysis of the competitive binding of glutathione (GSH) and phytochelatins [(gammaGlu-Cys)(n)-Gly, PC(n), n=2-5] by Cd(2+). The displacements between ligands and chain length influence on the competitive binding of PC(n) toward Cd(2+) were investigated. The analysis of the resulting pure voltammograms and concentration profiles of the resolved components suggests that ligands containing more thiol groups are able to displace the shortest chain ligands from their metal complexes, whereas the opposite does not happen. However, when the length of the chain surpasses that of PC(3), the binding capacity of the molecule still increases (i.e., it can bind more metal ions), but the position and shape of the voltammetric signals practically rest unchanged. This suggests that at this level, the stability of metal binding could depend more on the nature of the binding sites separately than on the quantity of the sites (i.e., the chain length).

Cadmium binding in mixtures of phytochelatins and their fragments: a voltammetric study assisted by multivariate curve resolution and mass spectrometry.

Phytochelatins (PC(n), (gamma-Glu-Cys)(n)Gly) are cysteine-rich peptides synthesized by plants which are involved in metal bioregulation and phytoremediation. Multivariate Curve Resolution by Alternating Least Squares (MCR-ALS) is applied to voltammetric data obtained from the analysis of the competitive binding of Cys or Cys-Gly with PC(2) or PC(3) by Cd(2+). The displacements between ligands, the chain length dependence on the competitive binding to PC(n) and the possible existence of mixed ligand metal-complexes are investigated. The shape analysis of the resulting pure voltammograms and concentration profiles of the components resolved by MCR-ALS suggests that ligands containing more thiol groups are able to displace the shorter chain ligands from their metal complexes, whereas the opposite does not happen. Electrochemical results are compared with ESI-MS measurements.

Non-linear multivariate curve resolution analysis of voltammetric pH titrations.

A new chemometric approach is put forward, dealing with the non-linear behaviour observed in the multivariate curve resolution (MCR) analysis of certain overlapping voltammetric signals obtained in titrations of metal complexes where pH is progressively changed. In such cases, non-reversible reduction signals move along the potential axis as a consequence of the involvement of H(+)-ions in the electrochemical process and cause a dramatic loss of linearity, which hinders accurate MCR analysis. The method proposed is based on the least-squares fitting of peak potential vs. pH datasets to parametric linear and sigmoid functions through the decomposition of the data matrix into both a concentration profile matrix and a unit signal matrix, in a similar way as in the alternating least-squares algorithm of MCR (ALS). Such calculations are carried out through several home-made Matlab programs which are freely available as Supplementary Material of the present work. The fitted parameters, along with the evolution of resolved concentrations and potential shifts with pH, provide valuable information on the complexation/reduction processes. The method is tested first on the relatively simple Cd(II)-NTA system and then applied to the study of the binding of Cd(II)-ions by glutathione (gamma-Glu-Cys-Gly, GSH) and the phytochelatin PC(2) ((gamma-Glu-Cys)(2)-Gly).

Stripping analysis of heavy metals in tap water using the bismuth film electrode.

A commercially available screen-printed carbon electrode coated with an ex situ deposited bismuth film (BiSPCE) has been applied to the determination of Pb(II) and Zn(II) ions in tap water (Barcelona water distribution network) by means of stripping voltammetry (SV) and stripping chronopotentiometry (SCP). A good reproducibility of the measurements and a satisfactory agreement between SV and SCP data were observed for both heavy metal ions. Although, in principle, the procedure could be also suited to the determination of Cd(II), this species was not detected. The results were also consistent with the routine ICP-OES measurements of the water distribution company, thus confirming the potential usefulness of such BiSPCE disposable devices for the analysis of heavy metals in natural waters.

Competitive binding of cadmium by plant thiols: an electrochemical study assisted by multivariate curve resolution.

Multivariate curve resolution with alternating least squares (MCR-ALS) has been applied to voltammetric data obtained from analysis of the competitive binding of cysteine (Cys) and cysteine-glycine (Cys-Gly) by Cd(II) as a first approach towards mixtures of phytochelatins and related compounds in natural media. From different starting points, the possibilities of formation of mixed complexes and/or displacements between ligands are investigated. Analysis of the resulting unitary voltammograms and concentration profiles of the resolved components by MCR-ALS suggests that the strongest ligand (Cys-Gly) is able to displace the weakest (Cys) from its metal complexes, whereas this does not happen in the opposite direction. On the other hand, no evidence of Cd mixed-ligand complexes was found.

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.

Combined use of the potential shift correction and the simultaneous treatment of spectroscopic and electrochemical data by multivariate curve resolution: analysis of a Pb(II)-phytochelatin system.

The combined use of differential pulse polarography (DPP), circular dichroism (CD) and multivariate curve resolution by alternating least squares (MCR-ALS), with potential shift correction for data analysis, provides valuable information on the complexation of Pb(ii) by the phytochelatin (gamma-Glu-Cys)(3)-Gly (PC(3)). All data confirm the formation of the predominant 1 : 1 Pb-PC(3) complex. However, in the presence of an excess of Pb(ii), the same complex seems to be present but with some modification in its structure. This is suggested by the shift towards more negative potentials of the DPP signal, and by a dramatic change in CD spectra. This evolution seems to be more related to a conformational change of the complex than with a modification of the electrochemical process kinetics.