Spectroelectrochemical Determination of Isoprenaline in a Pharmaceutical Sample.

UV/Vis absorption spectroelectrochemistry (SEC) is a multi-response technique that has been commonly used for the characterization of materials and the study of reaction mechanisms. However, it has been scarcely used for quantitative purposes. SEC allows us to obtain two analytical signals simultaneously, yielding a dual sensor in just one experiment. In the last years, our group has developed new devices useful for analysis. In this work, a SEC device in parallel configuration, based on optical fibers fixed on screen-printed electrodes, was used to determine isoprenaline in a commercial drug, using both, the electrochemical and the spectroscopic signals. In this commercial drug, isoprenaline is accompanied in solution by other compounds. Among them is sodium metabisulfite, an antioxidant that strongly interferes in the isoprenaline determination. A simple pretreatment of the drug sample by bubbling wet-air allows us to avoid the interference of metabisulfite. Here, we demonstrate again the capabilities of UV/Vis absorption SEC as double sensor for analysis and we propose a simple pretreatment to remove interfering compounds.

Derivative UV/Vis spectroelectrochemistry in a thin-layer regime: deconvolution and simultaneous quantification of ascorbic acid, dopamine and uric acid.

In this work, UV/Vis spectroelectrochemistry (SEC), in a thin-layer regime and parallel configuration, is selected to solve a complex mixture that contains dopamine (DA), ascorbic acid (AA) and uric acid (UA). These molecules, like many other biological compounds, are assuming a highly important place in analytical and biomedical fields due to the fundamental role that they play in human metabolism. In addition, low or high levels of these compounds are associated with diseases such as Parkinson's disease. For this reason, the quantification of these biomolecules is becoming increasingly critical. However, some drawbacks must be overcome, because the three molecules coexist in the human body, and the species are subject to mutual interference. In fact, they are all oxidized at similar potentials, and their UV/Vis absorption bands overlap, greatly complicating their quantification. For this reason, derivative SEC together with suitable chemometric tools such as PARAFAC are proposed to solve this complex matrix. This technique allows us to separate the contribution of each of these molecules present in a sample and to quantify all of them, achieving high resolution and reproducibility. Besides, detection limits at the micromolar level are achieved for DA, AA and UA in mixture solutions. This work thus demonstrates the great potential for derivative potentiodynamic SEC combined with the appropriate chemometric tools in solving complex mixtures, a field where SEC is still taking the first steps. Graphical abstract.

A Flexible Platform of Electrochemically Functionalized Carbon Nanotubes for NADH Sensors.

A flexible electrode system entirely constituted by single-walled carbon nanotubes (SWCNTs) has been proposed as the sensor platform for ß-nicotinamide adenine dinucleotide (NADH) detection. The performance of the device, in terms of potential at which the electrochemical process takes place, significantly improves by electrochemical functionalization of the carbon-based material with a molecule possessing an o-hydroquinone residue, namely caffeic acid. Both the processes of SWCNT functionalization and NADH detection have been studied by combining electrochemical and spectroelectrochemical experiments, in order to achieve direct evidence of the electrode modification by the organic residues and to study the electrocatalytic activity of the resulting material in respect to functional groups present at the electrode/solution interface. Electrochemical measurements performed at the fixed potential of +0.30 V let us envision the possible use of the device as an amperometric sensor for NADH detection. Spectroelectrochemistry also demonstrates the effectiveness of the device in acting as a voltabsorptometric sensor for the detection of this same analyte by exploiting this different transduction mechanism, potentially less prone to the possible presence of interfering species.

Application of spectroelectroanalysis for the quantitative determination of mixtures of compounds with highly overlapping signals.

The amount of qualitative and quantitative information provided by a UV-vis absorption spectroelectrochemistry (SEC) experiment is sometimes wasted. However, almost all electrochemical and spectroscopic data can provide valuable information. In this spirit, the main objective proposed in this work is the quantitative resolution of catechol/dopamine (CAT/DA) and dopamine/epinephrine (DA/EP) mixtures, using spectroelectrochemical sensors in long optical path length arrangement based on bare optical fibers in parallel configuration with respect to carbon nanotubes or screen-printed electrodes. These compounds show extremely similar electrochemical and spectroscopic responses at high acidic pH, being impossible to determine their concentrations in the mixtures just using univariate regression models. To our knowledge, the SEC ability to resolve complex mixtures has never been demonstrated before with signals with this degree of overlapping. The quantitative analysis of these mixtures is possible using multivariate regression analysis of a set of time-resolved spectroelectrochemical data with a powerful statistical tool such as parallel factor analysis (PARAFAC). PARAFAC enables us to extract all the information from the experiments, allowing us to quantify the different analytes in mixtures of varying concentrations with excellent results. This milestone for spectroelectroanalysis illustrates the expected capabilities of SEC and demonstrates experimentally the potential of this technique for sensing of biomolecules.

Direct Determination of Ascorbic Acid in a Grapefruit: Paving the Way for In Vivo Spectroelectrochemistry.

The study of real samples is more complicated than the study of other systems. However, the inherent advantages of UV-vis absorption spectroelectrochemistry should overcome some difficulties related to direct measurements in complex matrices. For this reason, a singular spectroelectrochemistry device has been fabricated and validated. The novel cell is based on single-walled carbon nanotubes, which are filtered and subsequently press-transferred on a polyethylene terephthalate support using a stencil with a custom design. With this new methodology, working, counter, and reference electrodes are completely flat on the surface, where two optical fibers are fixed in a long optical path length configuration. To demonstrate the usefulness of this device and the power of spectroelectrochemistry techniques to solve problems of the current world, this device is used to quantitatively detect the concentration of ascorbic acid in a complex matrix such as a fruit, directly, without any previous sample pretreatment. The ease to fabricate the device, the advantages related to its use, and the excellent results obtained not only with univariate but also with multivariate analysis, shed more light on the analysis of samples as they occur in nature. According to the particular features of this cell, to the best of our knowledge this is the first spectroelectrochemical sensor that can be inserted directly in a biological matrix, laying the groundwork to perform in vivo measurements in a near future.

Simultaneous UV-Visible Absorption and Raman Spectroelectrochemistry.

The development of a new device based on the use of UV-vis bare optical fibers in a long optical path length configuration and the measurement of the Raman response in normal arrangement allows us to perform UV-vis and Raman spectroelectrochemistry simultaneously in a single experiment. To the best of our knowledge, this is the first time that a spectroelectrochemistry device is able to record both spectroscopic responses at the same time, which further expands the versatility of spectroelectrochemistry techniques and enables us to obtain much more high-quality information in a single experiment. Three different electrochemical systems, such as ferrocyanide, dopamine, and 3,4-ethylenedioxythiophene, have been studied to validate the cell and to demonstrate the performance of the device. Processes that take place in solution can be properly distinguished from processes that occur on the electrode surface during the electrochemical experiment, providing a whole picture of the reactions taking place at the electrode/solution interface. Therefore, this device allows us to study a larger number of complex electrochemical processes from different points of view taking into account not only the UV-vis spectral changes in the solution adjacent to the electrode but also the Raman signal at any location. Furthermore, complementary information, which could not be unambiguously extracted without considering together the two spectroscopic signals and the electrochemical response, is obtained in a novel way.

Electrodeposition and Screening of Photoelectrochemical Activity in Conjugated Polymers Using Scanning Electrochemical Cell Microscopy.

A number of renewable energy systems require an understanding and correlation of material properties and photoelectrochemical activity on the micro to nanoscale. Among these, conducting polymer electrodes continue to be important materials. In this contribution, an ultrasensitive scanning electrochemical cell microscopy (SECCM) platform is used to electrodeposit microscale thin films of poly(3-hexylthiophene) (P3HT) on an optically transparent gold electrode and to correlate the morphology (film thickness and structural order) with photoactivity. The electrochemical growth of P3HT begins with a thin ordered film up to 10 nm thick, after which a second more disordered film is deposited, as revealed by micro-Raman spectroscopy. A decrease in photoactivity for the thicker films, measured in situ immediately following film deposition, is attributed to an increase in bulk film disorder that limits charge transport. Higher resolution ex situ SECCM phototransient measurements, using a smaller diameter probe, show local variations in photoactivity within a given deposit. Even after aging, thinner, more ordered regions within a deposit exhibit sustained enhanced photocurrent densities compared to areas where the film is thicker and more disordered. The platform opens up new possibilities for high-throughput combinatorial correlation studies, by allowing materials fabrication and high spatial resolution probing of processes in photoelectrochemical materials.

Development of a Novel Bidimensional Spectroelectrochemistry Cell Using Transfer Single-Walled Carbon Nanotubes Films as Optically Transparent Electrodes.

A really easy method to transfer commercial single-walled carbon nanotubes (SWCNTs) to different substrates is proposed. In this paper, a homogeneous transference of SWCNTs films to nonconductor and transparent supports, such as polyethylene terephthalate, glass, and quartz, and to conductor supports, such as indium tin oxide, aluminum, highly ordered pyrolytic graphite, and glassy carbon, was achieved using a very fast, reproducible, and clean methodology. In order to test these transferences, SWCNTs films transferred on quartz were used as working optically UV-vis transparent electrodes due to their optimal electrical and optical properties. A new easy-to-use, homemade optical fiber based cell for bidimensional spectroelectrochemistry was developed, offering the possibility to measure in normal and parallel configuration. The cell was tested with ferrocenemethanol, a compound widely used in electrochemistry but scarcely studied by spectroelectrochemistry, covering the UV-vis spectral region.

Optical fiber spectroelectrochemical device for detection of catechol at press-transferred single-walled carbon nanotubes electrodes.

A new long-optical-pathway spectroelectrochemical cell for absorptometric measurements in the UV-Vis region was developed. This cell consists of two optical fibers brought face to face and fixed on the working electrode support. As a proof of concept, the spectroelectrochemical cell was applied to the determination of catechol using a press-transferred single-walled carbon nanotube film as the working electrode. Voltabsorptometry was demonstrated to be very helpful in understanding the mechanism of catechol oxidation. The experiments showed that the main oxidation product is o-benzoquinone, but other soluble side products are also observed. Multivariate calibration explains the selection of 390 nm as the best wavelength for the univariate absorptometric determination of catechol, avoiding the interference of oxidation side products. Catechol was quantified using both the electrochemical and the spectroscopic signal, demonstrating that this hybrid technique is an autovalidated analytical method. Dual detection of catechol was also carried out using amperometric spectroelectrochemistry. Finally, spectroelectrochemistry was used to quantify catechol in the presence of hydroquinone.