Since its origins at the late 80’s, the group of Electroanalysis of the University of Barcelona (UB) has studied the interactions between heavy metals and molecules of biological and environmental interest using electroanalytical techniques. At the beginning, these studies were performed with voltammetry at mercury electrodes and classical electrochemical modeling.
From this initial activity, the group addressed increasingly complex systems by improving not only experimental methodology, but also data processing through multivariate curve resolution by alternating least squares (MCR-ALS). This allowed us to study substances such as the metallic complexes of metallothioneins (MT), phytochelatins (PC) and other peptides synthesized by organisms in response to heavy metal contamination. In these studies, complementary non-electrochemical techniques were used like liquid chromatography with UV-vis detection, circular dichroism (CD), mass spectrometry (MS) and differential isothermal calorimetry (ITC).

Nowadays, such previous research lines have been transformed to substitute mercury electrodes by less toxic and more versatile, selective and sensitive electrochemical sensors, to carry on implementing chemometric methods for data treatment and to develop new experimental strategies to study a wider range of analytes and samples. The actual interests of the group can be classified into four general research lines. They are detailed below, as well as some relevant publications and the collaborations with other research groups which have been especially helpful:

1. Development and application of electrochemical sensors

When, at the beginning of the new millennium, mercury electrodes started to become obsolete, our research group moved to solid electrodes of carbon, gold and platinum, often coated with bismuth and antimony films to mimic some valuable properties of mercury electrodes. Later, we started to use and modify commercial screen-printed electrodes (SPE), that is, disposable devices manufactured by printing inks containing conductive materials on a ceramic support. In addition, we adapted to SPE many procedures of film deposition and immobilization of nanoparticles usually applied to conventional electrodes.
Progress has also been made in the chemical immobilization of selective substances. Thus, solid electrodes and screen-printed devices have been modified to chemically bind molecules with a great affinity for heavy metal ions, such as aminotiols or crown ethers. This chemical immobilization is done mainly by electrografting with diazonium salts and, in some cases, by electropolymerization. Now we are just working in the adaptation to screen-printed devices of molecularly imprinted polymers (MIP) originally developed for graphite-epoxi electrodes.
Recently, we have started to use as modifiers new 2D materials from the pnictogen group, such as black phosphorous, bismuthene or antimonene. Also, we have obtained promising results with a vibrating boron-doped diamond electrode (BDDE).
The development and application of all these modified electrodes made possible the determination of Cd(II), Pb(II), Zn(II), Ni(II), Cu(II), Pd(II) and Sb(III) in reference materials and natural samples. Moreover, we have applied some of these electrochemical sensors to the screening in wastewater samples of emerging pollutants such as organic UV filters present in personal care products and pharmaceutical substances.
Finally, our previous experience with flow systems has allowed us to adapt some of these SPE-based sensors to electrochemical detection in liquid chromatography and flow injection analysis, mainly through the development of a flow cell that was patented (Spanish Patent ES2549676 B1) and is now commercialised by means of a license agreement with Dropsens-Metrohm.

Relevant collaborations:

• Dr. Manel del Valle. Autonomous University of Barcelona
• Dr. Zdeněk Sofer and Dr. Rui Gusmão. University of Chemistry and Technology of Prague
• Dr. Mikko Vepsäläinen. VTT Technical Research Centre of Finland
• Dr. Antonio Florido. Polytechnical University of Catalonia
• Dr. José María Palacios. University of Cadiz
• Dr. Silvia Díaz-Cruz. Institute of Environmental Assessment and Water Research, Barcelona (IDAEA-CSIC)

Relevant publications:

Jiménez, I.; Pérez-Ràfols, C.; Serrano, N.; del Valle, M.; Díaz-Cruz, J.M.
Carbon based electrodes for the voltammetric determination of capsaicin in spicy samples.
Microchemical Journal, 191, 108757 (2023). link (Open Access)

Tapia, M.A.; Gusmão, R.; Pérez-Ràfols, C.; Subirats, X.; Serrano, N.; Sofer, Z.; Díaz-Cruz, J.M.
Enhanced voltammetric performance of sensors based on oxidized 2D layered black phosphorus.
Talanta, 238, 123036 (2022). link (Open Access)

Tapia, M.A.; Gusmão, R.; Serrano, N.; Sofer, Z.; Ariño, C.; Díaz-Cruz, J.M.; Esteban, M.
Phosphorene and other layered pnictogens as a new source of 2D materials for electrochemical sensors.
Trends in Analytical Chemistry, 139, 116249 (2021). link

Morris, A.; Serrano, N.; Díaz-Cruz, J.M.; Bendavid, A.; Chen, M.; Vepsäläinen, M.
Vibrating boron-doped diamond electrode: a new, durable and highly sensitive tool for the detection of cadmium.
Analytica Chimica Acta, 1188, 339166 (2021). link (Open Access)

Torres-Rivero, K.; Pérez-Ràfols, C.; Bastos-Arrieta, J.; Florido, A.; Martí, V.; Serrano, N.
Direct As(V) determination using screen-printed electrodes modified with silver nanoparticles.
Nanomaterials, 10, 1280 (2020). link (Open Access)

Tapia, M.A.; Pérez-Ràfols, C.; Gusmão, R., Serrano, N.; Sofer, Z.; Díaz-Cruz, J.M.
Enhanced voltammetric determination of metal ions by using a bismuthene-modified screen-printed electrode.
Electrochimica Acta, 362, 137144 (2020). link

Serrano, N.; Castilla, O.; Ariño, C.; Díaz-Cruz, M.S.; Díaz-Cruz, J.M.
Commercial screen-printed electrodes based on carbon nanomaterials for a fast and cost-effective voltammetric determination of paracetamol, ibuprofen and caffeine in water samples.
Sensors, 19, 4039 (2019). link (Open Access)

Sunyer, A.; González-Navarro, A.; Serra-Roig, M.P., Serrano, N.; Díaz-Cruz, M.S.; Díaz-Cruz, J.M.
First application of carbon-based screen-printed electrodes for the voltammetric determination of the organic UV filters oxybenzone and octocrylene.
Talanta, 196, 381-388 (2019). link

Bernardo-Boongaling, V.R.R.; Serrano, N.; García-Guzmán, J.J.; Palacios-Santander, J.M; Díaz-Cruz, J.M.
Screen-printed electrodes modified with green-synthesized gold nanoparticles for the electrochemical determination of aminothiols.
Journal of Electroanalytical Chemistry, 847, 113184 (2019). link

Pérez-Ràfols, C.; Rosal, M.; Serrano, N.; Ariño, C.; Esteban, M.; Díaz-Cruz, J.M.
Expanding the possibilities of electrografting modification of voltammetric sensors through two complementary strategies.
Electrochimica Acta, 319, 878-884 (2019). link

Bastos-Arrieta, J.; Florido, A.; Pérez-Ràfols, C.; Serrano, N.; Fiol, N.; Poch, J.; Villaescusa, I.
Green synthesis of Ag nanoparticles using grape stalk waste extract for the modification of screen-printed electrodes.
Nanomaterials, 8, 946 (2018). link (Open Access)

Pérez-Ràfols, C.; Bastos-Arrieta, J.; Serrano, N.; Díaz-Cruz, J.M.; Ariño, C.; de Pablo, J.; Esteban, M.
Ag nanoparticles drop-casting modification of screen-printed electrodes for the simultaneous voltammetric determination of Cu(II) and Pb(II).
Sensors, 17(6), 1458 -1469 (2017). link (Open Access)

Ariño, C.; Serrano, N.; Díaz-Cruz, J.M.; Esteban, M.
Voltammetric determination of metal ions beyond mercury electrodes. A review.
Analytica Chimica Acta, 990, 11 - 53 (2017). link

Serrano, N.; Díaz-Cruz, J.M.; Ariño, C.; Esteban, M.
Antimony-based electrodes for analytical determinations.
Trends in Analytical Chemistry, 77, 203 - 213 (2016). link

Pérez-Ràfols, C.; Serrano, N.; Díaz-Cruz, J.M.; Ariño, C.; Esteban, M.
Glutathione modified screen-printed carbon nanofiber electrode for the voltammetric determination of metal ions in natural samples.
Talanta, 155, 8-13 (2016). link

Sosa, V.; Barceló, C.; Serrano, N.; Ariño, C.; Díaz-Cruz, J.M.; Esteban, M.
Antimony film screen-printed carbon electrode for stripping analysis of Cd(II), Pb(II) and Cu(II) in natural samples.
Analytica Chimica Acta, 855, 34 - 40 (2015). link

Serrano, N.; González-Calabuig, A.; del Valle, M.
Crown ether-modified electrodes for the simultaneous stripping voltammetric determination of Cd(II), Pb(II) and Cu(II).
Talanta, 138, 130 - 137 (2015). link

Dago, A.; Navarro, J.; Ariño, C.; Díaz-Cruz, J.M.; Esteban, M.
Carbon nanotubes and graphene modified screen-printed carbon electrodes as sensitive sensors for the determination of phytochelatins in plants using liquid chromatography with amperometric detection.
Journal of Chromatography A, 1409 (2015) 210–217. link

2. Design of sensor arrays: electronic tongues and hybrid sensing systems

Regarding multichannel voltamperometric measurements, combinations of SPE (modified or not) have been studied in ensembles of sensors with cross-response which constitute voltammetric electronic tongues. The purpose of the tongues is to improve the selectivity of the set by combining sensors of different selectivity towards a series of analytes. In the last years, the group has been working in the development of tongues for the analysis of complex mixtures of heavy metal ions and mixtures of aminotiols without prior separation. In this sense, the various modifications of electrodes mentioned above provide a wide range of sensors susceptible to being successfully integrated into voltammetric electronic tongues.
Very recently, the group has acquired a SPELEC instrument by Metrohm-Dropsens and has started to investigate the combination of simultaneous voltammetric and spectrophotometric measurements to design hybrid sensing systems.

Relevant collaborations:

• Dr. Manel del Valle. Autonomous University of Barcelona
• Dr. Álvaro Colina and Dr. Aránzazu Heras. University of Burgos

Relevant publications:

Ustabasi, G.S.; Bastos-Arrieta, J.; Pérez-Ràfols, C.; Serrano, N.; Díaz-Cruz, J.M.
Considerations on the use of spectroelectrochemistry in reflection mode for quantitative analysis: Study of the Fe(III)/Fe(II) – orthophenanthroline system.
Microchemical Journal, 181, 107678 (2022). link (Open Access)

Pérez-Ràfols, C.; Serrano, N.; Díaz-Cruz, J.M.
A hybrid sensing system combining simultaneous optical and electrochemical measurements: application to beer discrimination.
Talanta,123273 (2022). link (Open Access)

Díaz-Cruz, J.M.; Pérez-Ràfols, C.; Cetó, X.; Serrano, N.; Ariño, C.; Esteban, M.
Voltammetric Electronic Tongues. (Book chapter)
in Recent Advances in Analytical Techniques, Vol. 4, 68-98, Bentham Science, 2020. link

Pérez-Ràfols, C.; Serrano, N.; Ariño, C.; Esteban, M.; Díaz-Cruz, J.M.
Voltammetric electronic tongues in food analysis.
Sensors, 19, 4261 (2019). link (Open Access)

Pérez-Ràfols, C.; Serrano, N.; Díaz-Cruz, J.M.; Ariño, C.; Esteban, M.
Simultaneous determination of Tl(I) and In(III) using a voltammetric sensor array.
Sensors and Actuators: B. Chemical, 245, 18-24 (2017). link

Pérez-Ràfols, C.; Gómez, A.; Serrano, N.; Díaz-Cruz, J.M.; Ariño, C.; Esteban, M.
A voltammetric electronic tongue based on commercial screen–printed electrodes for the analysis of aminothiols by differential pulse voltammetry.
Electroanalysis, 29, 1559 - 1565 (2017). link

Pérez-Ràfols, C.; Serrano, N.; Díaz-Cruz, J.M.; Ariño, C.; Esteban, M.
A screen-printed voltammetric electronic tongue for the analysis of complex mixtures of metal ions.
Sensors and Actuators B: Chemical, 250, 393 - 401(2017). link

Pedroza, R.H.P.; Serrano, N.; Díaz-Cruz, J.M.; Ariño, C.; Esteban, M.
Integration of commercial screen-printed electrodes into a voltammetric electronic tongue for the analysis of aminothiols.
Electroanalysis, 28(7), 1570 - 1577 (2016). link

Serrano, N.; Prieto-Simón, B.; Cetó, X.; del Valle, M.
Array of peptide-modified electrodes for the simultaneous determination of Pb(II), Cd(II) and Zn(II).
Talanta, 125, 159 - 166 (2014). link

3. Application of Chemometrics to Electroanalysis

The study of increasingly complex systems, with the simultaneous presence of different analytes, the overlapping of its signals, matrix effects, signal drifts and a large amount of data from different sources, has made even more necessary the use of Chemometrics for data processing.
In systems behaving linearly, MCR-ALS has been the first choice for the study of stoichiometries and relative stabilities of metal complexes. Moreover, the methodology of calibration by standard addition has been successfully extended to multivariate analysis on the basis of MCR-ALS. For non-linear systems, it has been necessary to develop new strategies based on the use of functions with adjustable parameters capable of reproducing displacements of the voltammetric peaks in the potential axis as well as variations in width and symmetry.
As for the multivariate calibration required for electronic tongues, partial least squares (PLS) has been used under linear conditions, whereas, in the absence of linearity, artificial neural networks (ANN) has been applied.
In the characterisation and discrimination of food samples by chromatographic and/or electrochemical means, techniques like principal component analysis (PCA), linear discriminant analysis (LDA) and partial least squares discriminant analysis (PLS-DA) have been used. In more involved situations, support vector machine discriminant analysis (SVM-DA) has provided promising results.

Relevant publications:

Tapia, M.A.; Pérez-Ràfols, C.; Ariño, C.; Serrano, N.; Díaz-Cruz, J.M.
New approach to multivariate standard addition based on multivariate curve resolution by alternating least squares: Application to voltammetric data.
Analytical Chemistry, 92, 3396-3402 (2020). link

Serrano, N.; Pérez-Ràfols, C.; Ariño, C.; Esteban, M.; Díaz-Cruz, J.M.
MCR-ALS of voltammetric data for the study of environmentally relevant substances.
Microchemical Journal, 158, 105177 (2020). link

Marín, J.; Serrano, N.; Ariño, C.; Díaz-Cruz, J.M.
A chemometric survey about the ability of voltammetry to discriminate pharmaceutical products from the evolution of signals as a function of pH.
Chemosensors, 8, 46 (2020). link (Open Access)

Esteban, M.; Ariño, C.; Díaz-Cruz, J.M.
Chemometrics in Electrochemistry. (Book chapter)
in Comprehensive Chemometrics: Chemical and Biochemical Data Analysis.  Brown S.; Tauler, R.; Walczak B. Eds., 2nd Edition. Elsevier, Oxford, 2020. link

Pérez-Ràfols, C.; Puy-Llovera, J.; Serrano, N.; Ariño, C.; Esteban, M.; Díaz-Cruz, J.M.
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.
Talanta, 192, 147-153 (2019). link

Díaz-Cruz, J.M.; Esteban, M.; Ariño, C.
Chemometrics in Electroanalysis. (book)
Inside 'Monographs in Electrochemistry', Edited by F. Scholz, Springer Nature, Cham, Switzerland, 2019. link

Martínez, K.; Ariño, C.; Díaz-Cruz, J.M.; Serrano, N.; Esteban, M.
Multivariate standard addition for the analysis of overlapping voltammetric signals in the presence of matrix effects: Application to the simultaneous determination of hydroquinone and catechol.
Chemometrics and Intelligent Laboratory Systems, 178, 32-38 (2018). link

Aragó, M.; Ariño, C.; Dago A.; Díaz-Cruz, J.M.; Esteban, M.
Simultaneous determination of hydroquinone, catechol and resorcinol by voltammetry using graphene screen-printed electrodes and partial least squares calibration.
Talanta, 160, 138-143 (2016). link

Dago, A.; González, I; Ariño, C.; Díaz-Cruz, J.M.; Esteban, M.
Chemometrics applied to the analysis of induced phytochelatins in Hordeum vulgare plants stressed with various tòxic non-essential metals and metalloids.
Talanta, 118, 201-209 (2014). link

Cavanillas, S.; Serrano, N.; Díaz-Cruz, J.M.; Ariño, C.; Esteban, M.
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.
Analyst, 138(7), 2171 - 2180 (2013). link

Cavanillas, S.; Díaz-Cruz, J. M.; Ariño, C.; Esteban, M.
Parametric Signal Fitting by Gaussian Peak Adjustment: a New Multivariate Curve Resolution Method for Non-bilinear Voltammetric Measurements.
Analytica Chimica Acta, 689(2), 198 - 205 (2011). link

Gusmão, R.; Cavanillas, S.; Ariño, C.; Díaz-Cruz, J. M.; Esteban, M.
Circular Dichroism and Voltammetry, assisted by Multivariate Curve Resolution, and Mass Spectrometry of the competitive metal binding by phytochelatin PC5.
Analytical Chemistry , 82(21), 9006 - 9013 (2010). link

4. Discrimination of foods and beverages

Recently we have started to investigate the possibilities of liquid chromatography with UV-vis and/or amperometric detection with SPE for the discrimination of foods and beverages by using either the concentration of relevant substances (e.g., selected aminothiols in the case of white wine) or the fingerprint derived from groups of analytes (e.g., phenolic substances in the case of paprika). Most analyses have been made with typical reversed-phase C18 columns, but also hydrophilic interaction liquid chromatography (HILIC) columns have been used.
As for the use of sensors and sensor arrays, commercially available SPE are yielding promising results in the discrimination of beer samples, especially when combined with simultaneous UV-vis measurements.
Concerning to the data treatment, it has been carried out using the techniques of pattern recognition cited above: PCA, LDA, PLS-DA and SVM-DA.
Using this strategy, we have been able to discriminate among different varieties of paprika, white wine and beer, and now we are just working in the discrimination of different types of species and herbs by using both liquid chromatography and electronic tongues.

Relevant collaborations:

• Dr. Óscar Núñez and Dr. Xavier Subirats. University of Barcelona
• Dr. Manel del Valle. Autonomous University of Barcelona

Relevant publications:

Pages-Rebull, J.; Pérez-Ràfols, C.; Serrano, N.; Díaz-Cruz, J.M.
Analytical methods for cinnamon authentication.
Trends in Food Science & Technology, in press, 104388 (2024). link (Open Access)

Pages-Rebull, J.; Sagristà, G.; Pérez-Ràfols, C.; Serrano, N.; Díaz-Cruz, J.M.
Application of HPLC-UV combined with chemometrics for the detection and quantification of ‘true cinnamon’ adulteration.
Talanta, 271, 125676 (2024). link (Open Access)

Pérez-Ràfols, C.; Serrano, N.; Díaz-Cruz, J.M.
Authentication of soothing herbs by UV–vis spectroscopic and chromatographic data fusion strategy.
Chemometrics and Intelligent Laboratory Systems, 235, 104783 (2023). link (Open Access)

Pages-Rebull, J.; Pérez-Ràfols, C.; Serrano, N.; del Valle, M.; Díaz-Cruz, J.M.
Classification and authentication of spices and aromatic herbs by means of HPLC-UV and chemometrics.
Food Bioscience, 52, 102401 (2023). link (Open Access)

Pérez-Ràfols, C.; Serrano, N.; Díaz-Cruz, J.M.
A hybrid sensing system combining simultaneous optical and electrochemical measurements: application to beer discrimination.
Talanta, 123273 (2022). link (Open Access)

Roselló, A.; Serrano, N.; Díaz-Cruz, J.M.; Ariño, C.
Discrimination of beers by cyclic voltammetry using a single carbon screen-printed electrode.
Electroanalysis, 33, 864-872 (2021). link

Serrano, N.; Díaz-Cruz, J.M.
Authentication of spices and herbs by chromatographic techniques. (Book chapter)
in Chromatographic and Related Separation Techniques in Food Integrity and Authenticity, Vol. B: Relevant Applications, 157-185, O. Núñez and G. Campmajó Eds., World Scientific Publishing Company, Singapore, 2021. link

Cetó, X.; Sánchez, C.; Serrano, N.; Díaz-Cruz, J.M.; Núñez, O.
Authentication of paprika using HPLC-UV fingerprints.
LWT - Food Science and Technology, 124, 109153 (2020). link

Pérez-Ràfols, C.; Subirats, X.; Serrano, N.; Díaz-Cruz, J.M.
New discrimination tools for harvest year and varieties of white wines based on hydrophilic interaction liquid chromatography with amperometric detection.
Talanta, 201, 104-110 (2019). link

Serrano, N.; Cetó, X.; Núñez, O.; Aragó, M.; Gámez, A.; Ariño, C.; Díaz-Cruz, J.M.
Characterization and classification of Spanish paprika (Capsicum annuum L.) by liquid chromatography coupled to electrochemical detection with screen-printed carbon-based nanomaterials electrodes.
Talanta, 189, 296-301 (2018). link

Cetó, X.; Serrano, N.; Aragó M.; Gámez, A.; Esteban, M.; Díaz-Cruz, J.M.; Núñez, O.
Determination of HPLC-UV fingerprints of Spanish paprika (Capsicum annuum L.) for its classification by linear discriminant analysis.
Sensors,18, 4479 (2018). link (Open Access)