Evaluation of a carbon ink chemically modified electrode incorporating a copper-neocuproine complex for the quantification of antioxidants
Graphical abstract
Introduction
Electrochemical detection using chemically modified surfaces (known as chemically modified electrodes or CMEs) is a useful tool to quantify compounds in a variety of samples, because these interfaces enhanced accuracy during the analysis [[1], [2], [3], [4], [5]]. Additionally, CMEs are analytical alternatives where the chemical requirements should be diminished because they are already integrated, reducing the time of each analysis and facilitating the quantification procedures [6,7]. One modification strategy for preparing CMEs is based on the preparation of carbon inks, in which a given compound acting as the chemical probe of the analytical system, is incorporated within a mixture of conductive carbon (either Vulcan ®, graphite, etc.) and an agglomerant (mostly Nafion ®) on the electrode surface, which is later dried and used [8]. Carbon inks are attractive for sensing because they are inexpensive, have low background currents and broad potential windows [9]; also, they allow for the production of inexpensive and rapid detection systems in several matrices, providing robustness, good detection limits and easy miniaturization in various areas of analytical chemistry [1]. In the literature there are several works, presenting the development of CME for the quantification of antioxidants, some of them supporting the radical 2,2-diphenyl-1-picrylhydrazyl (DPPH͘˚͘) immovilized with cetylpyridinium bromide [10,11], carbon paste electrode with MgO nanoparticles for the quantification of the antioxidant tert-butylhydroxyanisole using [12] and vitamins quantification with ZrO2 nanoparticles [13].
Furthermore, antioxidant content has become an important analytical determination, as antioxidants are widely distributed in samples including biological systems, natural extracts, foods, drugs or cosmetic products. Antioxidant content is conventionally quantified by the indirect oxidation of those compounds, employing a chemical reagent, followed by the spectroscopic detection of the reduced species [[14], [15], [16], [17]]; the determined value is known as the total antioxidant capacity (TAC). Even though these methodologies are commonly used, the required time for the analysis is long, and the results are strongly determined by both the methodology employed for each specific analysis or the antioxidants being analysed [18]. Additionally, many methodologies require the preparation of several specific and fresh solutions to achieve the proper analytical response (i.e., to generate specific complexes in solution), thus prolonging the time of analysis.
Our workgroup have developed an alternative for these spectroscopic determinations using an electrochemical detection following the CUPRAC method (originally developed by Apak and co-workers) [19,20]. CUPRAC method is based on the oxidation of antioxidants by the formation of a complex between Cu(II) and neocuproine (Nc), following the next reaction:
TAC is proportional to the amount of chemically generated , which is in turn oxidized electrochemically. Electrochemical CUPRAC has a good analytical performance and the results are statistically comparable with the conventional CUPRAC method [[20], [21], [22]]. These results led to the proposal of the preparation of a carbon ink CME, by incorporating a previously prepared Cu(II)-Nc salt, thus overcoming the limitations concerning solution preparation and the time required for analysis, this study consists of a feasibility analysis of the development of a CUPRAC-modified surface and its possible use in the quantification of antioxidants.
Section snippets
Synthesis and characterization of [Cu(Nc)2](NO3)2
The Cu(II)-Nc salt was prepared using a 0.6 mmol of Cu(NO3)5H2O (0.14 g methanol solution in 5 mL) stirred solution, in which 1.2 mmol of Nc (0.25 g in methanol) was added. Later, the solvent was evaporated at room temperature as suggested by Hall [23,24]. The solid obtained was filtered and washed with ethyl ether. Obtained powder was characterized by IR spectroscopy and gas chromatography.
CME preparation and electrochemical measurements
Electrochemical experiments and determinations were carried out with a potentiostat/galvanostat PSTAT
Complex of copper II
The salt prepared employing Cu(NO3) and Nc at room temperature was characterized by Infrared spectroscopy (IR) and Gas chromatography/Mass Spectroscopy. The IR spectrum of the complex (see supplementary information (SI)), showed main absorption bands at 815, 840 and 1384 cm−1, all related to the NO3 ion. The observed band at 1621 cm−1 is typically attributed to the alkenyl stretching (CN), which confirms the presence of the neocuproine ligand.
In the mass spectrum (see SI) two major peaks
Conclusions
In this work, a Chemically Modified Electrode was prepared, integrating prepared [Cu(Nc)2](NO3)2, as the analytical probe within a carbon ink surface, which presents a similar electrochemical response to the one observed in CUPRAC solutions. This electrode allowed obtaining reproducible results in the evaluation of the Total Antioxidant Content of model antioxidants, such as Trolox, gallic acid and ascorbic acid, where the anodic peak current of the cyclic voltammograms of the immobilized
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
A. Cárdenas gratefully acknowledge the financial support of CONACYT-Mexico through the scholarship: 274103 and the financial support for her postdoctoral studies. C. Frontana thanks CONACYT-Mexico for support through project 256943(Fondo de Investigación Cientifica Basica 2015 SEP-CONACyT). We would like to thank Enrique Rodríguez Nuñez for their help with the analysis of tea samples.
Carlos Frontana obtained his BSc. Degree from Universidad Nacional Autonoma de Mexico in 2001 and his PhD at Universidad Autonoma Metropolitana-Iztapalapa, in Mexico in 2006. He is a full researcher at Centro de Investigacion y Desarrollo Tecnologico en Electroquimica (CIDETEQ, SC) in Queretaro, Mexico, where his main research interests are molecular electrochemistry, electron transfer dynamics, proton coupled electron transfer –mainly electrochemically controlled hydrogen bonding-,
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Carlos Frontana obtained his BSc. Degree from Universidad Nacional Autonoma de Mexico in 2001 and his PhD at Universidad Autonoma Metropolitana-Iztapalapa, in Mexico in 2006. He is a full researcher at Centro de Investigacion y Desarrollo Tecnologico en Electroquimica (CIDETEQ, SC) in Queretaro, Mexico, where his main research interests are molecular electrochemistry, electron transfer dynamics, proton coupled electron transfer –mainly electrochemically controlled hydrogen bonding-, spectroelectrochemistry, electroanalytical chemistry and design and processing in electrochemically stimulated microbial reactors for waste treatment. He is currently Head of Research in Health, as part of the Science Department.
Arely Cardenas received her Ph.D. Degree from the Center of Research and Technological Development in Electrochemistry, Mexico, in 2015. She is currently a researcher at CONACYT assigned to the Autonomous University of Queretaro. Her research is focused on electroanalytical chemistry, electrochemical sensors, bioelectrochemistry, environmental engineering and wastewater treatment.