Jesús López-Palacios

Spectroelectrochemistry at Screen-Printed Electrodes: Determination of Dopamine

A new device to perform spectroelectrochemical measurements in the UV/visible spectral region using screen-printed electrodes has been developed. Neurotransmitter dopamine has been selected as a proof of concept of the capabilities of the new device. The results obtained have allowed us both to study the oxidation mechanism of dopamine and to carry out the spectroelectrochemical detection of this neurotransmitter. Differences in dopamine oxidation mechanism have been observed depending on the initial concentration. Thus, dopamine concentrations lower than 10(-3) M led to a higher generation of dopaminochrome and its derivatives with a band centered at 305 nm, which was the best wavelength to determine dopamine spectrophotometrically at these concentrations. However, if dopamine concentration is higher than 10(-3) M, dopaminoquinone is stable enough to use its maximum of absorbance, 395 nm, to detect this neurotransmitter. Dopamine concentration can also be calculated from the electrochemical data in spectroelectrochemistry, the results being comparable to that obtained from spectroscopic data. Comparison between spectrophotometric and electrochemical determinations demonstrates that the two methods measure this analyte indistinctively, proving that spectroelectrochemistry represents an autovalidated technique. Partial least-squares regression has also been used, obtaining good results in the full dopamine concentration range. Finally, as spectroelectrochemistry is an intrinsically trilinear technique, PARAFAC has been used to study the effect of probable interfering species.

Optimization of the experimental parameters in the determination of copper(II) by differential-pulse anodic stripping voltammetry and evaluation of the characteristic detection curves

Differential-pulse anodic stripping voltammetry with a mercury microelectrode was used for the determination of copper with ethylenediaminetetraacetic acid as complexing agent. Interactions between analytical factors and their optimal levels were investigated using two factorial designs and the steepest ascent method. The calibration graph, calculated by least median squares, guarantees the linearity of the proposed calibration. Characteristic curves for the operating detection limit were established.

Spatial scanning spectroelectrochemistry. Study of the electrodeposition of Pd nanoparticles at the liquid/liquid interface

Spatial scanning spectroelectrochemistry is a new analytical technique that provides spectral information at different distances from an electrified liquid/liquid interface where an electrochemical process takes place. As a proof of concept, we have studied two different electrochemical processes at the electrified liquid/liquid interface: (1) Ru(bpy)(3)(2+) transfer through the water/1,2-dichloroethane interface and (2) electrodeposition of Pd nanoparticles at the water/1,2-dichloroethane interface. The instrumental setup developed consists of a movable slit for the light beam to sample at well-defined positions on both sides of the interface, providing important information about the chemical process occurring. If the slit is scanned at different distances from the interface during an electrochemical experiment, a complete picture of the reactions and equilibria in the diffusion layer can be obtained. For example, in the case of the Ru(bpy)(3)(2+), the experiments show clearly how the complex is transferred from one phase to the other. In the case of electrosynthesis of Pd nanoparticles, it is demonstrated that nanoparticles are not only deposited at the interface but diffuse to the aqueous bulk solution. These in situ observations were confirmed by ex situ experiments using transmission electron microscopy.

Bidimensional chronoabsorptometric study of electropolymerisation of 4,4′-bis(2-methylbutylthio)-2,2′-bithiophene

Abstract Bidimensional chronoabsorptometry is a novel spectroelectrochemical technique that monitors simultaneously three different signals: current and absorbance both normal to the electrode plane and parallel to this plane during a time in which a fixed potential is imposed. This technique is applied in the visible range to the study of the electropolymerisation of 4,4 ′ -bis(2-methylbutylthio)- 2,2 ′ -bithiophene (MBTBT). Experiments are performed in a spectroelectrochemical cell under finite diffusion conditions (thin layer cell) with the aim of interpreting the processes taking place both at the electrode surface and in the adjacent solution during the potentiostatic electrogeneration and deposition of the polymer. Correlations are drawn out among the trends of the oligomers concentration in solution, the polymer electrodeposition and charging, and the current flow, on the time scales of the different steps of the process.

A spectroelectrochemical approach to the electrodeposition of bismuth film electrodes and their use in stripping analysis

UV-vis reflection spectroelectrochemistry has proven to be a very useful multiresponse technique to evaluate the quality of bismuth films obtained by electrochemical deposition on glassy carbon electrodes (GCEs). Bismuth films have recently emerged as a promising and environmental friendly alternative to mercury electrodes for stripping analysis. Spectroelectrochemical measurements, carried out in a flow cell, allowed us to follow in situ the electrodeposition and stripping of bismuth and cadmium. Electrochemical and spectroscopic responses individually have led to successfully quantify the amount of cadmium in test solutions.

Multiple standard addition with latent variables (MSALV): Application to the determination of copper in wine by using differential-pulse anodic stripping voltammetry

Abstract MSALV, multiple standard addition with latent variables, is a new method designed to overcome the difficulties presented by the determination of an analyte in a complex sample in which equilibria exist whose displacement may alter the response of the sensors when the analyte is added in a multiple addition. MSALV has been used to quantify the Cu content in samples of wine using differential-pulse anodic stripping voltammetry (DPASV) with a mercury electrode and ethylenediaminetetraacetic acid (EDTA) and KCl as complexing agents. The existence of the EDTA-Cu(II) complex and Cu(I)-chloride complex intermediate leads to a change in the sensibility of the sensors when Cu(II) is added, which must be born in mind during construction of the multiple addition model. The use of latent variables calculated by partial least squares (PLS) provides a solution which is both quantitatively adequate and chemically interpretable. The results of four repeated determinations of Cu in samples of red wine are shown. In each case the latent variables were calculated from the intensities recorded at fifteen potentials in eight voltammograms corresponding to the successive additions of wine and/or copper.

Digital simulation model for bidimensional spectroelectrochemistry

A digital simulation model has been developed taking into account the simultaneous acquisition of one electrical and two spectroscopic signals generated in a bidimensional spectroelectrochemistry experiment. In this type of measurements, light absorption is simultaneously considered in both the normal and parallel directions to a plane electrode surface. The difficulty in resolving the equation giving the absorbance in parallel configuration as a function of the time and the distance to the electrode makes advisable the use of a digital simulation model. The model developed is valid for any potential controlled experiment, either in the equilibrium state or under finite/semi-infinite diffusion, that is to say, from thin-layer to bulk cells.