Quantitative Raman spectroelectrochemistry using silver screen-printed electrodes

New screen-printed electrodes for Raman spectroelectrochemistry. Determination of p-aminosalicylic acid

BACKGROUND

The availability of new surface enhanced Raman scattering (SERS) substrates is essential to develop quantitative analytical methods. Electrochemistry is an easy, fast and reproducible methodology to prepare SERS substrates on screen-printed electrodes (SPEs).

RESULTS

This work proposes new SPEs based on a three-electrode system all made of silver. Using the same ink for the whole electrode system facilitates the fabrication process, reduces production costs, and leads to excellent analytical performance. The results showed that Raman enhancement depends strongly on the type of silver ink. To demonstrate the capabilities of the new electrodes developed, 4-aminosalicylic acid was determined in complex matrices and in the presence of strong interfering compounds such as salicylic acid and acetylsalicylic acid. The proposed analytical method is based on the electrochemical surface oxidation enhanced Raman scattering (EC-SOERS) strategy. AgCl nanocrystals are generated on the working electrode surface, which amplify the Raman signal of 4-aminosalicylic acid. Good figures of merit were obtained both in the absence and in the presence of the interfering compounds, achieving a correct estimation of a 4-aminosalicylic test sample in complex matrices.

SIGNIFICANCE

The new SPEs have been demonstrated to be very sensitive and reproducible which, together to the high specificity of the Raman signal, makes this methodology very attractive for chemical analysis.

Speciation Analysis of Metals and Metalloids by Surface Enhanced Raman Spectroscopy

The presence of metalloids and heavy metals in the environment is of critical concern due to their toxicological impacts. However, not all metallic species have the same risk level. Specifically, the physical, chemical, and isotopic speciation of the metal(loids) dictate their metabolism, toxicity, and environmental fate. As such, speciation analysis is critical for environmental monitoring and risk assessment. In the past two decades, surface-enhanced Raman spectroscopy (SERS) has seen significant developments regarding trace metal(loid) sensing due to its ultrahigh sensitivity, readiness for in situ real-time applications, and cost-effectiveness. However, the speciation of metal(loid)s has not been accounted for in the design and application of SERS sensors. In this Perspective, we examine the potential of SERS for metal(loid) speciation analysis and highlight the advantages, progress, opportunities, and challenges of this application.

Recent State and Challenges in Spectroelectrochemistry with Its Applications in Microfluidics

This review paper presents the recent developments in spectroelectrochemical (SEC) technologies. The coupling of spectroscopy and electrochemistry enables SEC to do a detailed and comprehensive study of the electron transfer kinetics and vibrational spectroscopic fingerprint of analytes during electrochemical reactions. Though SEC is a promising technique, the usage of SEC techniques is still limited. Therefore, enough publicity for SEC is required, considering the promising potential in the analysis fields. Unlike previously published review papers primarily focused on the relatively frequently used SEC techniques (ultraviolet-visible SEC and surface-enhanced Raman spectroscopy SEC), the two not-frequently used but promising techniques (nuclear magnetic resonance SEC and dark-field microscopy SEC) have also been studied in detail. This review paper not only focuses on the applications of each SEC method but also details their primary working mechanism. In short, this paper summarizes each SEC technique's working principles, current applications, challenges encountered, and future development directions. In addition, each SEC technique's applicative research directions are detailed and compared in this review work. Furthermore, integrating SEC techniques into microfluidics is becoming a trend in minimized analysis devices. Therefore, the usage of SEC techniques in microfluidics is discussed.

A Multipurpose and Multilayered Microneedle Sensor for Redox Potential Monitoring in Diverse Food Analysis

This work presents a multipurpose and multilayered stainless steel microneedle sensor for the in situ redox potential monitoring in food and drink samples, termed MN redox sensor. The MN redox sensor was fabricated by layer-by-layer (LbL) approach. The in-tube multilayer coating comprised carbon nanotubes (CNTs)/cellulose nanocrystals (CNCs) as the first layer, polyaniline (PANI) as the second layer, and the ferrocyanide redox couple as the third layer. Using cyclic voltammetry (CV) as a transduction method, the MN redox sensor showed facile electron transfer for probing both electrical capacitance and redox potential, useful for both analyte specific and bulk quantification of redox species in various food and drink samples. The bulk redox species were quantified based on the anodic/cathodic redox peak shifts (E_a/E_c) on the voltammograms resulting from the presence of redox-active species. The MN redox sensor was applied to detect selected redox species including ascorbic acid, H₂O₂, and putrescine, with capacitive limits of detection (LOD) of 49.9, 17.8, and 263 ng/mL for each species, respectively. For the bulk determination of redox species, the MN redox sensor displayed LOD of 5.27 × 10³, 55.4, and 25.8 ng/mL in ascorbic acid, H₂O₂, and putrescine equivalents, respectively. The sensor exhibited reproducibility of ~ 1.8% relative standard deviation (%RSD). The MN redox sensor was successfully employed for the detection of fish spoilage and antioxidant quantification in king mushroom and brewed coffee samples, thereby justifying its potential for food quality and food safety applications. Lastly, the portability, reusability, rapid sampling time, and capability of in situ analysis of food and drink samples makes it amenable for real-time sensing applications.

Investigating Comproportionation in Multielectron Transfers via UV-Visible Spectroelectrochemistry: The Electroreduction of Anthraquinone-2-sulfonate in Aqueous Media

UV-vis spectroelectrochemistry is assessed as a tool for the diagnosis and quantitative in situ investigation of the incidence of comproportionation in multielectron transfer processes. Thus, the sensitivity of the limiting current chronoabsorptometric signals related to the different redox states to the comproportionation kinetics is studied theoretically for different working modes (normal and parallel light beam arrangements) and mass transport regimes (from semi-infinite to thin layer diffusion). The theoretical results are applied to the spectroelectrochemical study of the two-electron reduction of the anthraquinone-2-sulfonate in alkaline aqueous solution, tuning the thermodynamic favorability of the comproportionation reaction through the electrolyte cation. The quantitative analysis of the experimental results reveals the occurrence of comproportionation in the three media examined, showing different kinetics depending on the cationic species in solution.

Review on combining surface-enhanced Raman spectroscopy and electrochemistry for analytical applications

The discovery of surface enhanced Raman scattering (SERS) from an electrochemical (EC)-SERS experiment is known as a historic breakthrough. Five decades have passed and Raman spectroelectrochemistry (SEC) has developed into a common characterization tool that provides information about the electrode-electrolyte interface. Recently, this technique has been successfully explored for analytical purposes. EC was found to highly improve the performances of SERS sensors, providing, among others, controlled adsorption of analytes and increased reproducibility. In this review, we highlight the potential of EC-SERS sensors to be implemented for point-of-need (PON) analyses as miniaturized devices, and their ability to revolutionize fields like quality control, diagnosis or environmental and food safety. Important developments have been achieved in Raman spectroelectrochemistry, which now represents a promising alternative to conventional analytical methods and interests more and more researchers. The studies included in this review open endless possibilities for real-life EC-SERS analytical applications.

What Is the Optimal Method for Cleaning Screen-Printed Electrodes?

Screen-printed electrodes-based sensors can be successfully used to determine all kinds of analytes with great precision and specificity. However, obtaining a high-quality sensor can be difficult due to factors such as lack of reproducibility, surface contamination or other manufacturing challenges. An important step in ensuring reproducible results is the cleaning step. The aim of the current work is to help researchers around the world who struggle with finding the most suitable method for cleaning screen-printed electrodes. We evaluated the cleaning efficiency of different chemical compounds and cleaning methods using cyclic voltammetry and electrochemical impedance spectroscopy. The percentage differences in polarization resistance (Rp) before and after cleaning were as follows: acetone—35.33% for gold and 49.94 for platinum; ethanol—44.50% for gold and 81.68% for platinum; H2O2—47.34% for gold and 92.78% for platinum; electrochemical method—3.70% for gold and 67.96% for platinum. Thus, we concluded that all the evaluated cleaning methods seem to improve the surface of both gold and platinum electrodes; however, the most important reduction in the polarization resistance (Rp) was obtained after treating them with a solution of H2O2 and multiple CV cycles with a low scanning speed (10 mV/s).

Rapid Determination of the 'Legal Highs' 4-MMC and 4-MEC by Spectroelectrochemistry: Simultaneous Cyclic Voltammetry and In Situ Surface-Enhanced Raman Spectroscopy

The synthetic cathinones mephedrone (4-MMC) and 4-methylethcathinone (4-MEC) are two designer drugs that represent the rise and fall effect of this drug category within the stimulants market and are still available in several countries around the world. As a result, the qualitative and quantitative determination of 'legal highs', and their mixtures, are of great interest. This work explores for the first time the spectroelectrochemical response of these substances by coupling cyclic voltammetry (CV) with Raman spectroscopy in a portable instrument. It was found that the stimulants exhibit a voltammetric response on a gold screen-printed electrode while the surface is simultaneously electro-activated to achieve a periodic surface-enhanced Raman spectroscopy (SERS) substrate with high reproducibility. The proposed method enables a rapid and reliable determination in which both substances can be selectively analyzed through the oxidation waves of the molecules and the characteristic bands of the electrochemical SERS (EC-SERS) spectra. The feasibility and applicability of the method were assessed in simulated seized drug samples and spiked synthetic urine. This time-resolved spectroelectrochemical technique provides a cost-effective and user-friendly tool for onsite screening of synthetic stimulants in matrices with low concentration analytes for forensic applications.

Detection of dithiocarbamate, chloronicotinyl and organophosphate pesticides by electrochemical activation of SERS features of screen-printed electrodes

Enhancement of Raman intensity due to the electrochemical surface-enhanced Raman scattering (EC-SERS) effect is an interesting alternative to overcome the lack of sensitivity traditionally associated with Raman spectroscopy. Furthermore, activation of metallic screen-printed electrodes (SPEs) by electrochemical route leads to the reproducible generation of nanostructures with excellent SERS properties. EC-SERS procedure proposed in this work for the detection of several pesticides (thiram, imidacloprid and chlorpyrifos) with different nature, uses gold SPEs as SERS substrates, but also includes a preconcentration step as the initial and essential stage. Taking into account the small volume of solution employed, only 60 µL, the preconcentration cannot be performed for more than 15 min in order to ensure the proper contact of the solution with WE, RE and CE. Furthermore, selected temperature, 34 °C, is not very high to allow the exhaustive control of the drop volume. Optimization of preconcentration parameters (time and temperature) displays a crucial step, particularly in the detection of low concentrations of pesticides, because it will provide higher Raman intensity in EC-SERS experiments. After the initial step, gold SPEs were electrochemically activated by cyclic voltammetry, allowing the detection of very low concentration (µg·L^-1) of pesticides due to the generation of fresh nanostructures with SERS effect.

Screen-Printed Electrodes Modified with Metal Phthalocyanines: Characterization and Electrocatalysis in Chlorinated Media

Metal phthalocyanines are well-known sensing phases with applications in different scientific fields due to their interesting properties. Detailed characterization by Raman spectroscopy was performed in order to study the shifting of the vibrational bands related to the coordination sphere of each metal phthalocyanine. In this work, a study involving the use of screen-printed electrodes (SPEs) with various metal phthalocyanines to electrochemically detect and quantify chlorine (Cl₂) gas is presented. The Cl₂ gas was generated in-situ via oxidation of the chloride present in form of aqueous salt solutions. The developed method offers not only the possibility to quantify chlorine, but also to discriminate among several chlorinated species due to the changes observed in the voltammetric profiles associated with the interaction between the specie assayed and the phthalocyanine metallic center. Optimization of detecting parameters was also performed to apply this procedure for the quantification of chlorine generated from commercial chlorine tablets. The development of this proof of concept shows interesting possibilities and easy-to-use applications with novel on metal phthalocyanines based SPE sensors.

Investigation into the electrochemical behaviour of silver in alkaline solution and the influence of Au-decoration using operando Raman spectroscopy

To explore the basic chemistry in the electrochemical environment, the electrochemical behavior of Ag and the influence of Au decoration is investigated with cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and operando Raman measurements in a 1 M KOH solution. During the anodic CV sweep, Ag is oxidized to Ag₂O in the first step through a one-electron process, and then, AgO in the second step through another one-electron process. Meanwhile, some AgO is formed at a relatively low potential under the irradiation of visible lights (photoelectrochemical oxidation). In the GCD mode, it is found that apart from the two one-electron processes, part of the Ag is oxidized to AgO directly through a two-electron process in the second oxidation step, implying slightly different activities of these reactions in the CV and GCD mode. During cathodic CV sweep and galvanostatic discharge, opposite reactions take place respectively. The coulombic efficiency is calculated to be only ∼82% from the CV cycle at 5 mV s⁻¹ due to the formation of silver hydroxyl species (oxidation state) in a low potential range. For the Au decorated Ag, Raman signals from these species disappeared and the coulombic efficiency is enhanced to 95%, indicating an obvious improvement in reversibility.

Apart from the traditional charge storage mechanism demonstrated in CV, a two-electron process of Ag to AgO is revealed in GCD.

Spectroelectrochemical and computational studies of tetrahydrocannabinol (THC) and carboxy-tetrahydrocannabinol (THC-COOH)

Electrochemical SERS allows for the rapid detection of both THC and THC-COOH in bodily fluid matrices without interferences from matrix species, paving the way to a point-of-need tool for cannabinoid detection.

Spectroelectrochemistry, the future of visualizing electrode processes by hyphenating electrochemistry with spectroscopic techniques

Reviewing the future of electrochemistry combined with infrared, Raman, and nuclear magnetic resonance spectroscopy as well as mass spectrometry.

3D-Printed Low-Cost Spectroelectrochemical Cell for In Situ Raman Measurements

Raman spectroelectrochemistry is a powerful technique for characterizing structural changes of materials during electrochemical reactions and investigating the mechanism of film deposition and adsorption processes on the surfaces of electrodes. Moreover, in situ measurements enable identification of catalytic sites and reaction intermediates, which facilitates the comprehension of reaction mechanisms. The limitations of this technique include the high-cost and the complexity of the experimental arrangement required by commercial spectroelectrochemical cells (SEC). Thus, 3D-printing technology emerges as an excellent alternative for the production of SEC, with desirable shape, low-cost, and robustness in a short period of time. In this work, an SEC and a 3D-printed working electrode were fabricated from acrylonitrile-butadiene-styrene (ABS) and conductive graphene polylactic acid (PLA) filaments, respectively. The proposed SEC and the 3D-printed electrode were printed within 3.5 h with an estimated cost of materials of less than US $2. Then, the 3D-printed SEC and the electrode were used in a study of structural changes of Prussian blue according to different voltage bias.

Spectroelectrochemical elucidation of B vitamins present in multivitamin complexes by EC-SERS

Raman spectroelectrochemistry based on electrochemical surface-enhanced Raman scattering (EC-SERS) effect is an interesting alternative to overcome the lack of sensitivity of normal Raman spectroscopy. Electrochemical activation of metallic screen-printed electrodes (SPEs) leads to the reproducible generation of nanostructures with excellent SERS properties. In that way, gold SPEs circumvent the traditional reproducibility limitation and produce the enhancement of the Raman intensity to favor the detection of low concentrations. Furthermore, fingerprint features of Raman spectroscopy make possible the dynamic spectroelectrochemical analysis of B vitamins. The accuracy assignments of Raman bands associated with B1, B2, B3, B6 and B12 vitamins present in multivitamin complexes provides valuable information, allowing us not only the detection of B vitamin present in mixtures, but also to understand the interaction between vitamins and metallic SERS surfaces.

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.

Resolution of mixed dyes by in situ near infrared (NIR) spectroelectrochemistry

NIR spectroelectrochemistry has scarcely been used for deconvolving aqueous mixtures due to the water restriction in this spectral range. However, this work offers an interesting approach for the study of mixtures of molecules with similar electrochemical and spectroscopic behaviour by overcoming the limitations of this hybrid technique. As a proof of concept, the resolution of mixtures of two dyes with similar chemical structures demonstrates the usefulness of NIR spectroelectrochemistry.

Advanced Nanoscale Approaches to Single-(Bio)entity Sensing and Imaging

Individual (bio)chemical entities could show a very heterogeneous behaviour under the same conditions that could be relevant in many biological processes of significance in the life sciences. Conventional detection approaches are only able to detect the average response of an ensemble of entities and assume that all entities are identical. From this perspective, important information about the heterogeneities or rare (stochastic) events happening in individual entities would remain unseen. Some nanoscale tools present interesting physicochemical properties that enable the possibility to detect systems at the single-entity level, acquiring richer information than conventional methods. In this review, we introduce the foundations and the latest advances of several nanoscale approaches to sensing and imaging individual (bio)entities using nanoprobes, nanopores, nanoimpacts, nanoplasmonics and nanomachines. Several (bio)entities such as cells, proteins, nucleic acids, vesicles and viruses are specifically considered. These nanoscale approaches provide a wide and complete toolbox for the study of many biological systems at the single-entity level.

Towards single-molecule in situ electrochemical SERS detection with disposable substrates

Nanostructure and surface charge can be tuned during the electrochemical activation of SERS substrates achieving highly active surfaces with feasible detection of a few molecules.