Time-resolved Raman Spectroelectrochemistry Research Articles

Multiamperometric-SERS detection of melamine on gold screen-printed electrodes

• New multiamperometric-SERS detection method for analysis. • SERS substrates electrochemically prepared in few seconds. • Simple and reproducible methodology for SERS substrates preparation. • Good figures of merit for melamine determination. A new, simple and fast protocol to generate gold-based SERS substrates is presented in this work. Melamine is a compound widely used in the industry that can be toxic for humans if consumed even in low concentrations. EC-SERS is an excellent alternative to classical methods to detect and quantify this compound because Raman spectroscopy provides a fingerprint of the molecules, providing very good sensitivity. In this work, time-resolved Raman spectroelectrochemistry is employed to generate a SERS substrate and to detect melamine, all in a single experiment. The dynamic character of this technique provides valuable information about the interaction of the molecule with the generated substrate. An optimization of the spectroelectrochemical method based on a multi-pulse amperometric detection has been performed. The new protocol presented in this work shows significant figures of merit in a very short experiment time, just 25 s.

Electrochemical generation of surface enhanced Raman scattering substrates for the determination of folic acid

Electrochemistry is a very useful technique to generate surface enhanced Raman scattering (SERS) substrates. SERS depends on two mechanisms: the electromagnetic effect, related to the plasmonic properties of the nanostructures in the substrate, and the chemical mechanism, related to the interaction between the target molecule and the substrate. Thus, Raman enhancement for a molecule strongly depends on the nature of the nanostructures that form the substrate. In this work, time-resolved Raman spectroelectrochemistry is used to generate SERS substrates, obtaining the Raman signal during the generation of the substrate, which is very useful to obtain valuable information on the Raman performance of the substrate. Two metals, Au and Ag, have been used to determine folic acid in a drug. In this example, Au SERS substrates are highly influenced by interfering compounds present in the drug tables. On the contrary, Ag SERS substrates have been demonstrated to yield very good figures of merit in the determination of folic acid in this complex sample.

Double fingerprint characterization of uracil and 5-fluorouracil

Time Resolved Raman spectroelectrochemistry (TR-Raman-SEC) has been used for the first time to obtain two different Raman spectra of one single analyte in the same experiment. This double detection has been accomplished thanks to the use of electrochemical surface enhanced Raman scattering (EC-SERS) and electrochemical surface oxidation enhanced Raman scattering (EC-SOERS) in the same experiment. These two Raman enhancement phenomena can provide a broad insight into the interaction between analyte and substrate surface when they are combined. To prove the possibilities of this methodology, a Raman spectroelectrochemistry study of uracil (U) and 5-fluorouracil (5-FU), two analytes with relevance in medicine and biochemistry, have been performed. Density functional theory (DFT) calculations has been carried out to shed more light on the interaction of these molecules with silver substrates in acidic media.

Determination of nicotinamide in a multivitamin complex by electrochemical-surface enhanced Raman spectroscopy

Abstract Time resolved Raman spectroelectrochemistry is a powerful technique to prepare useful Surface Enhanced Raman Scattering (SERS) substrates suitable for quantitative analysis. Moreover, following the evolution of the Raman signal with the applied potential the selection of the best conditions for the analysis is quite simple. In this work, gold SERS substrates are prepared on screen-printed electrodes for the determination of nicotinamide, vitamin B3, in a multivitamin complex. The formation of the substrate has been monitored using UV/Vis absorption spectroelectrochemistry and scanning electron microscopy, which confirmed the generation of gold nanoparticles on the electrode surface. This new method has demonstrated to be suitable for the direct quantification of simple test samples using small sample volumes (50 μL). However, it failed when a complex sample, containing several interfering species, was analysed. A high interference on the preparation of the SERS substrate is observed during the analysis of a multivitamin complex. Despite that, nicotinamide was selectively determined in a multivitamin complex using the method of the standard addition, obtaining remarkable figures of merit (R2 = 0.99, %RSD

Chemical selectivity in electrochemical surface oxidation enhanced Raman scattering

Electrochemical Surface Oxidation Enhanced Raman Scattering (EC-SOERS) is an interesting and promising phenomenon capable of amplifying the Raman signal in a similar way to Surface Enhanced Raman Scattering (SERS), but EC-SOERS takes place during the oxidation of a silver substrate. This phenomenon was originally described for specific electrolytic conditions in which a small amount of chloride in acidic medium was mandatory to obtain a substantial enhancement. Herein, we demonstrate that EC-SOERS can be also observed in presence of KBr, showing a potential-dependent behavior. Moreover, in this work a novel approach to reach chemical selectivity during time resolved Raman spectroelectrochemistry (TR-Raman-SEC) experiments is proposed. This new approach is based on the effect of the electrolytic medium on the structures that are formed on the electrode surface. SEM studies were carried out to study the origin of this selectivity. Although SEM images reveal clear differences between the structures of silver halides formed on the electrode surface during the oxidation of the substrate, the absence of EC-SOERS effect at open circuit potential hinders the identification of the actual structures responsible for the phenomenon.

Raman and fluorescence spectroelectrochemical monitoring of resazurin-resorufin fluorogenic system

Raman and fluorescence spectroelectrochemistry techniques have been used for the in-situ study of the reaction mechanism of resazurin/resorufin/dihydroresorufin system. Resazurin (RZ) is one of the most highly referenced substances used in studies on cell viability and cytotoxicity for a wide range of biological and environmental systems. Electrochemical behaviour of this tracer is based on the irreversible reduction of RZ, a slightly fluorescent compound, to resorufin (RS), a high fluorescent compound, while a subsequent and reversible reduction of RS generates dihydroresorufin (DHRS), a non-fluorescent compound. Time-resolved Raman spectroelectrochemistry has been used, for the first time, to perform dynamic studies of this system. The continuous optical acquisition provides a large number of spectra during the whole electrochemical reaction and not only at discrete potentials, offering a novel contribution to the literature related to this tracer. The characteristic Raman and fluorescence bands associated with each molecule as well as their evolution with potential allow us to obtain valuable information about the interconversion between RZ, RS and DHRS. In addition, spectroelectrochemistry demonstrates that experimental parameters play an essential role in the fluorescent response of this coloured system. In that way, operando multiresponse techniques shed more light for the complete understanding of RZ, RS and DHRS system, and they yield novel information to explain the electrochemical conversion of these compounds.

Versatility of Time-Resolved Raman Spectroelectrochemistry: From the Resolution of Mixture of Vitamins to the Detection Pesticides

Electrochemical Surface-enhanced Raman scattering (EC-SERS) is a powerful analytical tool for the sensitive and selective detection of different molecules adsorbed on a nanostructured metal. This methodology consists of the electrochemical generation of metal nanostructures with excellent SERS properties and the analytical detection of different molecules in a unique experiment. Detection of pesticides in food is an essential step in regulating and monitoring the levels of pesticides. However, there is currently a clear demand in the development of new and easy methodologies that allow a quick detection. In order to demonstrate the utility of EC-SERS in this field, two pesticides have been selected. Imidacloprid and permethrin are widely used for controlling different insects in agricultura and their detection is completely required. EC-SERS allows the fast and simple detection of these pesticides, opening new gates for future application in this field. However, the versaility of EC-SERS has been demonstrated in this work since this technique can be also used in a different, way, and for example is really useful in the resolution of mixtures. As proof of concept, mixtures of different vitamins (B1, B2, B3, B6 and B12) have been studied by time-resolved Raman spectroelectrochemistry. Raman spectra show the characteristic bands of each vitamin, allowing us to differeciate between them. However, apart from differenciate the vitamins present in the mixture, this methodology allows the calculation of the chemical composition of different samples.

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.

Electrochemical surface oxidation enhanced Raman scattering

In this work, an unexpected enhancement of the Raman signal for uric acid during the electrochemical oxidation of a silver electrode is presented. This behavior cannot be easily explained using classical models of Surface Enhanced Raman Scattering (SERS). Time resolved Raman spectroelectrochemistry is used to study this interesting process strongly dependent on the experimental conditions. The new phenomenon was observed in different molecules and was found to be reproducible and robust, allowing us to use this methodology for the determination of citric acid. The enhancement of the Raman signal only takes place when a potential is applied to the electrode and therefore, this new phenomenon can be denoted as Electrochemical Surface Oxidation Enhanced Raman Scattering (EC-SOERS). In this work, EC-SOERS is presented not only as an alternative to SERS for detection of molecules but also as a reproducible process that can be used for quantitative analysis.

Quantitative Raman spectroelectrochemistry using silver screen-printed electrodes

Surface enhanced Raman scattering (SERS) is a powerful technique based on the intensification of the Raman signal because of the interaction of a molecule with a nanostructured metal surface. Electrochemically roughened silver has been widely used as SERS substrate in the qualitative detection of analytes at the ultra-trace level. However, its potential for quantitative analysis has not been widely exploited yet. In this work, the combination of time-resolved Raman spectroelectrochemistry with silver screen-printed electrodes (SPE) is proposed as a novel methodology for the preparation of SERS substrates. The in situ activation of a SERS substrate is performed simultaneously with the analytical detection of a probe molecule, controlling the process related to the preparation of the substrate and performing the analytical measurement in real time. The results show the good performance of silver SPE as electrochemically-induced surface-enhanced Raman scattering substrates. Raman spectra were recorded at fairly low integration times (250 ms), obtaining useful spectroelectrochemical information of the processes occurring at the SPE surface with excellent time-resolution. By recording the microscopic surface images at different times during the experiment, we correlated the different data obtained: structural, optical and electrochemical. Finally, the in situ activation process was used to obtain a suitable in situ SERS signal for ferricyanide and tris(bipyridine)ruthenium (II) quantification. The detection of the analytes at concentrations of a few tens of nM was possible with a low integration time (2 s) and good precision, demonstrating the exceptional performance of the Raman spectroelectrochemical method and the possibility to use cost-effective screen-printed electrodes for applications where a high sensitivity is needed.

Evaluation of electrochemical, UV/VIS and Raman spectroelectrochemical detection of Naratriptan with screen-printed electrodes

Naratriptan, active pharmaceutical ingredient with antimigraine activity was electrochemically detected in untreated screen-printed carbon electrodes (SPCEs). Cyclic voltammetry and differential pulse voltammetry were used to carry out quantitative analysis of this molecule (in a Britton-Robinson buffer solution at pH 3.0) through its irreversible oxidation (diffusion controlled) at a potential of +0.75V (vs. Ag pseudoreference electrode). Naratriptan oxidation product is an indole based dimer with a yellowish colour (maximum absorption at 320nm) so UV–VIS spectroelectrochemistry technique was used for the very first time as an in situ characterization and quantification technique for this molecule. A reflection configuration approach allowed its measurement over the untreated carbon based electrode. Finally, time resolved Raman Spectroelectrochemistry is used as a powerful technique to carry out qualitative and quantitative analysis of Naratriptan. Electrochemically treated silver screen-printed electrodes are shown as easy to use and cost-effective SERS substrates for the analysis of Naratriptan.

SERS spectroelectrochemical study of electrode processes at copper hexacyanoferrate modified electrode

Electrochemical redox processes taking place at copper hexacyanoferrate (CuHCF) modified electrode have been investigated by near-infrared laser (785nm) induced surface-enhanced Raman spectroscopy (SERS). Raman bands observed within the spectral ranges of 2200–2000cm−1 and 500–100cm−1, as well as their dependence on electrode potential, have been analysed. The most characteristic Raman bands, related to triple CN bond vibrations and centered at 2187 and 2127cm−1 were assigned to the oxidised and the reduced forms of CuHCF, respectively. Time-resolved Raman spectroelectrochemical study shows that the electrochemical redox interconversions between these two forms proceed relatively slow, thus resembling the behaviour of structurally related cobalt hexacyanoferrate, and differing essentially from that of Prussian blue layer studied previously. It has been shown by the time-resolved Raman spectroelectrochemistry that the rate of some redox processes of solute species like the anodic oxidation of ascorbate or cathodic reduction of hydrogen peroxide at CuHCF modified electrode appears to be limited by the slow electrochemical redox transformations within the modifier layer itself rather than by the redox interactions of a modifier with the solute species.

Monitoring charge transfer at polarisable liquid/liquid interfaces employing time-resolved Raman spectroelectrochemistry

Abstract In-situ Raman spectroscopy is implemented for the first time to monitor dynamic charge transfer processes at polarisable interfaces between two immiscible electrolyte solutions (ITIES) in real time. A custom-designed new electrochemical cell is described which allows probing the Raman signals of ferroin ions as a function of the potential applied across the water|1,2-dichlorobenzene (DCB) interface. This approach is also used for investigating the heterogeneous electron transfer reaction involving dimethylferrocene in DCB and potassium hexacyanoferrate (II/III) in the aqueous phase. The evolution of the Raman signals during potentiodynamic measurements is recorded in real-time with a resolution of a few seconds.

Dynamic Raman Spectroelectrochemistry of Single Walled Carbon Nanotubes modified electrodes using a Langmuir-Schaefer method

Raman spectroelectrochemistry is a fundamental technique to characterize single walled carbon nanotube (SWCNT) films. In this work, we have performed the study of SWCNT films transferred to a glassy carbon electrode using a Langmuir-Schaefer method. Langmuir balance has allowed us to control the characteristics of the film that can be easily transferred to the electrode support. Time-resolved Raman spectroelectrochemistry experiments at scan rates between 20 and 400mV s−1 were done in two different solvents, water and acetonitrile. Spectroscopic results indicate that electron transfer of carbon nanotubes is a very fast process. The electrochemical process is reversible in acetonitrile and, on the contrary, nanotubes are degraded when the characterization is performed in water being the degradation independent on the scan rate.

Electrochemical redox processes at cobalt hexacyanoferrate modified electrodes: An in situ Raman spectroelectrochemical study

Cobalt hexacyanoferrate (CoHCF) modified electrode has been prepared by electrodeposition, and studied by cyclic voltammetry and Raman spectroscopy using near-infrared laser excitation (785nm). Raman features observed within the spectral range of 2200–2000cm−1, related to tripple C–N bond vibrations, have been analysed. Based on in situ spectroelectrochemical study performed in potassium or sodium chloride or nitrate electrolytes, Raman bands located around 2130 and 2080cm−1 have been assigned to reduced, and the band around 2185cm−1 – to oxidised form of CoHCF. With the use of time-resolved Raman spectroelectrochemistry, it has been shown that the electrochemically driven redox interconversion between the two redox forms of CoHCF proceeds relatively slow, in contrast to fast transformations occuring within Prussian blue layers studied previously. It was also shown that the efficiency of some electrode processes taking place at CoHCF modified electrode, like the cathodic reduction of hydrogen peroxide, appears to be limited by slow redox transformations within the modifier layer.