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.Ministerio de Economıá y Competitividad (CTQ2014-55583-R, CTQ2014-61914-EXP, CTQ2015- 71955-REDT) and Junta de Castilla y León (BU033-U16

Bipolar spectroelectrochemistry

Bipolar electrochemistry is receiving growing attention in the last years, not only because it is an important tool for studying electron transfer processes, but also because it is really fruitful in the development of new analytical sensors. Bipolar electrodes show promising applications as a direct analytical tool since oxidation and reduction reactions take place simultaneously on different parts of a single conductor. There are several electrochemical devices that provide information about electron transfer between two immiscible electrolyte solutions, but to the best of our knowledge, this is the first time that a bipolar device is able to record two spectroelectrochemical responses concomitantly at two different compartments. It allows deconvolving the electrochemical signal into two different optical signals related to the electron transfer processes occurring at two compartments that are electrically in contact. The combination of an electrochemical and two spectroscopic responses is indeed very useful, providing essential advantages in the study of a huge variety of systems. The study of three different electrochemical systems, such as reversible redox couples, carbon nanotubes, and conducting polymers has allowed us to validate the new cell and to demonstrate the capabilities of this technique to obtain valuable time-resolved information related to the electron transfer processes.Ministerio de Economía y Competitividad (CTQ2014-55583-R, CTQ2014-61914-EXP, CTQ2015-71955-REDT) and Junta de Castilla y León (BU033-U16

UV/Vis absorption spectroelectrochemistry of folic acid

UV/Vis absorption spectroelectrochemistry is a very promising analytical technique due to the complementary information that is simultaneously obtained from electrochemistry and spectroscopy. In this work, this technique is used in a parallel configuration to study the oxidation of folic acid in alkaline medium. Herein, UV/Vis absorption spectroelectrochemistry has been used to detect both the oxidation products and the folic acid consumed at the electrode/solution interface, allowing us to develop an analytical protocol to quantify vitamin B9 in pharmaceutical tablets. Linear ranges of three orders of magnitude have been achieved in basic medium (pH = 12.9), obtaining high repeatability and low detection limits. The spectroelectrochemical determination of folic acid in pharmaceutical tablets at alkaline pH values is particularly interesting because of the changes that occur in the optical signal during the electrochemical oxidation of FA, providing results with very good figures of merit and demonstrating the utility and versatility of this hyphenated technique, UV/Vis absorption spectroelectrochemistry.Ministerio de Economía y Competitividad (Grant CTQ2017-83935-R-AEI/FEDERUE), Junta de Castilla y León (Grant BU297P18), and Ministerio de Ciencia, Innovación y Universidades (Grant RED2018-102412-T). F.O. is grateful for the contract funded by Junta de Castilla y León, the European Social Fund, and the Youth Employment Initiative.Publicación en abierto financiada por el Consorcio de Bibliotecas Universitarias de Castilla y León (BUCLE), con cargo al Programa Operativo 2014ES16RFOP009 FEDER 2014-2020 DE CASTILLA Y LEÓN, Actuación:20007-CL - Apoyo Consorcio BUCL

Simplifying the assessment of parameters of electron-transfer reactions by using easy-to-use thin-layer spectroelectrochemistry devices

UV–Vis absorption thin-layer spectroelectrochemistry greatly contributes to the assessment of parameters of electron-transfer reactions by providing electrochemical and spectroscopic information obtained simultaneously in a confined space around the working electrode. In this work we present an ingenious modification of a spectroelectrochemistry device based on carbon nanotubes and bare optical fibers to perform UV–Vis absorption thin-layer spectroelectrochemistry measurements. The working and counter electrodes are made of carbon nanotubes filtered and press-transferred onto a flat support where a painted silver line acts as pseudo-reference electrode. To perform high sensitivity measurements, two bare optical fibers are fixed to the support in a parallel arrangement. In a novel development, a quartz plate is placed on top of the bare optical fibers in order to create a thin layer whose thickness is reproducibly controlled by the diameter of the optical fibers. This methodology is also successfully applied using commercial screen-printed electrodes. Three different electroactive compounds, ferrocenemethanol, o-tolidine and [Ru(bpy)3]2 +, are studied and good results are obtained. As demonstrated below, these long optical path UV–Vis absorption thin-layer spectroelectrochemistry cells considerably reduce the complexity associated with this type of devices, eliminating one of the major disadvantages of this technique and making it much more accessible to the scientific community.Ministerio de Economía y Competitividad (CTQ2014-55583-R, CTQ2014-61914-EXP, CTQ2015-71955-REDT) and Junta de Castilla y León (BU033-U16)Ministerio de Economía y Competitividad (CTQ2014-55583-R, CTQ2014-61914-EXP, CTQ2015-71955-REDT) and Junta de Castilla y León (BU033-U16

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.Ministerio de Economía y Competitividad (CTQ2014-55583-R; CTQ2014-61914-EXP; CTQ2015-71955-REDT; CTQ2017-83935-R AEI/FEDER, UE) and Junta de Castilla y León (BU033-U16) is gratefully acknowledged. Jesus Garoz- Ruiz thanks Ministerio de Educación, Cultura y Deporte for his FPU fellowship (FPU12/02795) and Ministerio de Economía y Competitividad for his postdoctoral contracts (CTQ2014-55583-R; CTQ2017-83935-R AEI/FEDER, UE)

Determination of uric acid in synthetic urine by using electrochemical surface oxidation enhanced Raman scattering

In this work, a new and easy methodology to determine uric acid in relevant samples using Raman spectroelectrochemistry is presented. The spectroelectrochemistry experiment is based on the in-situ formation of a suitable substrate that enables the enhancement of the Raman signal of an analyte during the oxidation stage of a silver electrode. This phenomenon is known as electrochemical surface oxidation enhanced Raman scattering (EC-SOERS) and has proved to be useful in quantitative analysis using disposable screen printed electrodes. The successful combination of EC-SOERS with PARAFAC analysis allows the determination of uric acid in a relevant complex sample avoiding the use of standard addition method and without using a baseline correction, which simplifies the application of such methodology in routine analysis.Ministerio de Economía y Competitividad (Grants CTQ2017-83935-R-AEI/FEDERUE) and Junta de Castilla y León (Grant BU297P18

Effect of chloride and pH on the electrochemical surface oxidation enhanced Raman scattering

In the present work, electrochemical surface oxidation enhanced Raman scattering (EC-SOERS) was studied using time resolved Raman spectroelectrochemistry. This multiresponse technique allows us to obtain dynamic information about the processes taking place during the electrochemical oxidation of a silver substrate. EC-SOERS is particularly found in specific electrolytic conditions, namely, HClO4 0.1 M + KCl 5·10−3 M, and has a clear dependence on chloride concentration and pH, being the optimum values between 5·10−3 M and 1·10−2 M for chloride and pH = 1. In light of the results of this study, the appearance of the phenomenon is related to the modification of the electrode surface, yielding Ag/AgCl cubes as plasmonic structures, and the stability of such structures at low pH values. The results presented in this work could shed more light into the intricate EC-SOERS phenomenon which can be summarized as the increase of the Raman signal for a Raman probe molecule exclusively during the electrochemical oxidation of silver electrodes.Ministerio de Economía y Competitividad (Grants CTQ2017-83935-R-AEI/FEDERUE) and Junta de Castilla y León (Grant BU033-U16). J.V.P-R. thanks JCyL for his postdoctoral fellowship (Grant BU033-U16

Carbon nanostructured films modified by metal nanoparticles supported on filtering membranes for electroanalysis

A novel methodology to prepare sensors based on carbon nanostructures electrodes modified by metal nanoparticles is proposed. As a proof of concept, a novel bismuth nanoparticle/carbon nanofiber (Bi-NPs/CNF) electrode and a carbon nanotube (CNT)/gold nanoparticle (Au-NPs) have been developed. Bi-NPs/CNF films were prepared by 1) filtering a dispersion of CNFs on a polytetrafluorethylene (PTFE) filter, and 2) filtering a dispersion of Bi-NPs chemically synthesized through this CNF/PTFE film. Next the electrode is prepared by sticking the Bi-NPs/CNF/PTFE film on a PET substrate. In this work, Bi-NPs/CNF ratio was optimized using a Cd2+ solution as a probe sample. The Cd anodic stripping peak intensity, registered by differential pulse anodic stripping voltammetry (DPASV), is selected as target signal. The voltammograms registered for Cd stripping with this Bi-NPs/CNF/PTFE electrode showed well-defined and highly reproducible electrochemical. The optimized Bi-NPs/CNF electrode exhibits a Cd2+ detection limit of 53.57 ppb. To demonstrate the utility and versatility of this methodology, single walled carbon nanotubes (SWCNTs) and gold nanoparticles (Au-NPs) were selected to prepare a completely different electrode. Thus, the new Au-NPs/SWCNT/PTFE electrode was tested with a multiresponse technique. In this case, UV/Vis absorption spectroelectrochemistry experiments were carried out for studying dopamine, demonstrating the good performance of the Au-NPs/SWCNT electrode developed.Ministerio de Economía y Competitividad (CTQ2014-61914-EXP, CTQ2014-55583-R, TEC2014-51940-C2-2R, CTQ2015-71955-REDT) and Junta de Castilla y León (BU033-U16)Ministerio de Economía y Competitividad (CTQ2014-61914-EXP, CTQ2014-55583-R, TEC2014-51940-C2-2R, CTQ2015-71955-REDT) and Junta de Castilla y León (BU033-U16

UV/Vis spectroelectrochemistry of o-vanillin: Study of the antioxidant properties

UV/Vis absorption spectroelectrochemistry is a very useful technique to study complex electrochemical mechanisms. In this work, we have studied the electrochemistry of o-vanillin in presence and absence of oxygen. This compound exhibits good antioxidant properties. The products of the reaction of o-vanillin with the superoxide anion, electrogenerated when oxygen is reduced in an aprotic medium, have been detected using UV/Vis absorption spectroelectrochemistry. This technique allows us to shed more light on the antioxidant properties of o-vanillin, providing valuable information not only on the antioxidant properties of this compound but also about other compounds that follow a similar mechanism. In this work we have deconvolved the electrochemical signal in the different components that are related to the processes taking place at the electrode and in the solution adjacent to it.Ministerio de Economía y Competitividad (CTQ2017-83935-R-AEI/FEDERUE), Junta de Castilla y León (BU297P18), Ministerio de Ciencia, Innovación y Universidades (RED2018- 102412-T)

Antimony tin oxide (ATO) screen-printed electrodes and their application to spectroelectrochemistry

Spectroelectrochemistry studies spectral changes as a function of applied potential or current. While there is no standard experimental setup, transparent electrodes are most typically used in transmission mode. Working in reflection mode forces light across the sample twice, resulting in higher sensitivities, but in turn requires the use of highly reflective electrodes. Here we present the production and characterization of screen-printed electrodes made from different antimony tin oxide (ATO) conducting particles. The resulting electrodes display excellent spectroelectrochemical properties, such as reflectivities up to 20 times higher than conventional graphite screen-printed electrodes, but with comparable electron transfer rates. These electrodes represent an attractive alternative to conventional materials and widen the choice of suitable electrode materials for electrochemistry in general and spectroelectrochemistry in particularFEDER funds managed by the Catalan Secretary of Universities and Research through project PROD-0000114 (Enterprise and Knowledge, Industry Department, Generalitat de Catalunya)