Electrochemical Surface-enhanced Raman Spectroscopy Research Articles

New Procedure for Detecting Pesticides By Activating SERS Properties of Screen-Printed Electrodes

The lack of sensitivity traditionally associated with Raman spectroscopy has limited the development of new analytical applications. However, enhancement of Raman intensity due to the electrochemical surface-enhanced Raman scattering (EC-SERS) effect is an interesting alternative to overcome this limitation. Although different analytical methods have been used in the detection of pesticides, the complicated instrumentation and the long procedures employed lead to the development of new and easy methodologies for quick detection.EC-SERS methodology proposed in this work for the detection of different pesticides uses metal SPEs as SERS substrates but also includes a preconcentration step as essential stage. After that, SPEs are electrochemically activated by cyclic voltammetry. Sensitivity of the methodology proposed in this work provides interesting results that allows us the detection of different pesticides. Particularly interesting are the results obtained in the detailed analysis of Raman bands of thiram and imidacloprid. Their characteristic bands allow the detection of 2.4 ppb thiram and 25 ppb imidacloprid.Figure 1. Raman spectra of thiram, imidacloprid, malathion and chlorpyrifos obtained by EC-SERS procedure using metal SPE. Figure 1

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.

Role of OH Intermediates during the Au Oxide Electro-Reduction at Low pH Elucidated by Electrochemical Surface-Enhanced Raman Spectroscopy and Implicit Solvent Density Functional Theory.

Molecular understanding of the electrochemical oxidation of metals and the electro-reduction of metal oxides is of pivotal importance for the rational design of catalyst-based devices where metal(oxide) electrodes play a crucial role. Operando monitoring and reliable identification of reacting species, however, are challenging tasks because they require surface-molecular sensitive and specific experiments under reaction conditions and sophisticated theoretical calculations. The lack of molecular insight under operating conditions is largely due to the limited availability of operando tools and to date still hinders a quick technological advancement of electrocatalytic devices. Here, we present a combination of advanced density functional theory (DFT) calculations considering implicit solvent contributions and time-resolved electrochemical surface-enhanced Raman spectroscopy (EC-SERS) to identify short-lived reaction intermediates during the showcase electro-reduction of Au oxide (AuOx) in sulfuric acid over several tens of seconds. The EC-SER spectra provide evidence for temporary Au-OH formation and for the asynchronous adsorption of (bi)sulfate ions at the surface during the reduction process. Spectral intensity fluctuations indicate an OH/(bi)sulfate turnover period of 4 s. As such, the presented EC-SERS potential jump approach combined with implicit solvent DFT simulations allows us to propose a reaction mechanism and prove that short-lived Au-OH intermediates also play an active role during the AuOx electro-reduction in acidic media, implying their potential relevance also for other electrocatalytic systems operating at low pH, like metal corrosion, the oxidation of CO, HCOOH, and other small organic molecules, and the oxygen evolution reaction.

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

Plasmonic Hot Electron-Mediated Hydrodehalogenation Kinetics on Nanostructured Ag Electrodes.

An attractive field of plasmon-mediated chemical reactions (PMCRs) is developing rapidly, but there is still incomplete understanding of how to control the kinetics of such a reaction related to hot carriers. Here, we chose 8-bromoadenine (8BrAd) as a probe molecule of hot electrons to investigate the influence of the electrode potential, laser wavelength, and power on the PMCR kinetics on silver nanoparticle-modified silver electrodes. Plasmonic hot electron-mediated cleavage of the C-Br bond in 8BrAd has been investigated by combining in situ electrochemical surface-enhanced Raman spectroscopy and density functional theory calculations. The experimental and theoretical results reveal that the energy position of plasmon relaxation-generated hot electrons can be modulated conveniently by applied potentials and laser light. This allows the proposal of a mechanism of modulating the matching energy of the hot electron of plasmon relaxation to promote the efficiency of PMCRs in electrochemical interfaces. Our work will be helpful to design surface plasmon resonance photoelectrochemical reactions on metal electrode surfaces of nanostructures with higher efficiency.

Potential-Induced Structural Alterations in the Tethered Bilayer Lipid Membrane-Anchoring Monolayers Revealed by Electrochemical Surface-Enhanced Raman Spectroscopy

Mixed self-assembled monolayers (SAMs) are widely used to anchor tethered lipid bilayer membranes (tBLMs) that mimic biological membranes and are useful platforms for fundamental studies and constr...

In-situ dynamic reaction of Ag NPs: Strategy for the construction of a sensitive electrochemical chiral sensor

A novel concept for the development of chiral sensor is proposed based on electrochemical phase transformation of silver nanoparticles (Ag NPs), which is shown for in situ photoelectric cooperative analysis mode. To demonstrate this, self-assembled Ag NPs on gold microelectrode (Ag-Au ME) serve as the sensing platform for chiral cysteine (D/L-Cys) detection. Through differences in the specific binding force, the system was validated with efficient synergistic electrochemical redox reactions and complexation of D/L-Cys with silver ions, and a novel sensing mode was developed based on the shift of oxidation peak potential of silver ions. In the proposed assay procedure, the simple principle of electrochemical phase transformation of Ag NPs, which can be monitored by electrochemical-surface enhanced Raman scattering (EC-SERS), was used as an in situ, rapid, dynamic analysis strategy for improving the detection sensitivity. Moreover, the proposed assay strategy with satisfactory sensitivity up to 74.88 μA μM−1 cm−2 was achieved, the limit of detection (LOD) is reduced to nanomolar level (8.7 nM) for D-Cys. Going beyond the conventional chiral biosensing, more importantly, the elaborate dynamic coupling of multiple mechanisms in a micro/nanoscale interface may provide a new approach for future highly sensitive biosensing.

Electrochemical SERS on 2D Mapping for Metabolites Detection.

Surface-enhanced Raman scattering (SERS) has been widely used for bioanalysis because it provides a high sensitivity for detecting analytes of ultralow concentrations. However, the clinical application of a 2D SERS-active substrate remains challenging because of the difficulty of obtaining accurate quantification, especially at low concentration. In this study, we proposed an analytical method that integrates an optimized sample mapping strategy with an electrochemical SERS (EC-SERS) technique to resolve this problem. We adopted this method to detect two metabolites of azathioprine, namely 6-thioguanine nucleotides (6-TGNs) and 6-methylmercaptopurine (6-MMP), as our proof-of-concept experiment. We first prepared a conductive SERS-active substrate by electrochemically depositing Au nanoparticles (AuNPs) on indium tin oxide glass. The two metabolites were then randomly absorbed on the surface of the AuNPs of the SERS-active substrates. When we applied a negative potential on the substrate, we observed a large enhancement of Raman intensity for both metabolites, which was attributed to both the charge transfer effect and reorientation of metabolites on the substrate surface, leading to the formation of Au-S bonds. In addition, by optimizing the mapping range, we were able to efficiently reduce the standard deviation of SERS intensity and achieve a consistent standard deviation lower than 10%. With these two features, we were able to achieve quantitative analysis of 6-TGNs and 6-MMP with a detection limit of 10 and 100 nM, respectively. The integration of EC-SERS and the mapping method provided a reliable and quantitative analytical platform for analytes, which can be electrochemically modulated, like 6-TGNs and 6-MMP.

Fast Screening of Pesticides By Raman Spectroelectrochemistry Based on EC-SERS Effect

Electrochemical Surface-enhanced Raman spectroscopy (EC-SERS) effect is an outstanding procedure that ensures the enhancement of Raman signal, a careful control of the nanostructures prepared and the possibility of detecting different analytes while the substrate preparation is taking place. Development of novel EC-SERS procedures provide these benefits in the simultaneous preparation and measurement process [1,2], and open new gates for analytical applications. Activation of metallic screen-printed electrodes (SPEs) by electrochemical routes leads to the generation of reproducible nanostructures with excellent SERS properties. In that way, gold, silver and copper SPEs circumvent the traditional reproducibility limitation and produce the enhancement of the Raman intensity to favor the detection of low concentrations.In this work, detection of different pesticides was achieved by the development of new electrochemical activation procedures using metallic SPEs as SERS substrates (Figure 1). The obtained results demonstrate that Raman spectroelectrochemistry is a powerful technique for monitoring the levels of pesticides, being this study the previous step in their detection in real samples.Figure 1. Raman spectrum of thiram obtained during a cyclic voltammetry using Ag SPE.

Fabrication of liquid–liquid self-assembled Ag arrays on disposable screen-printed electrodes and their application in the identification and analysis of the adsorption behavior of organic carboxylates through in situ electrochemical surface-enhanced Raman scattering

A disposable Ag array@screen-printed electrode (SPE) was fabricated for the identification and analysis of the adsorption behavior of organic carboxylates in in situ electrochemical surface-enhanced Raman scattering (EC-SERS).

Plasmon catalytic PATP coupling reaction on Ag-NPs/graphite studied via in situ electrochemical surface-enhanced Raman spectroscopy.

The p-aminothiophenol (PATP) coupling reaction on plasmon substrates such as Ag and Au nanoparticles has received extensive attention since the catalytic effect of the surface plasmon was found. Currently, in situ kinetic studies of this reaction are rare, especially those focusing on the specific role of the hot electron-hole carriers. Here, in situ electrochemical surface-enhanced Raman spectroscopy (SERS) is developed to study the plasmon catalytic reaction of PATP in a controlled aqueous environment involving the factors of O2, electron and hole carriers, and solution pH. Ag nanoparticles supported on graphite serve as a SERS substrate, which could separate hot electron-hole pairs effectively and is beneficial to study the effects of hot carriers on plasmon-driven reactions. In situ electrochemical SERS measurements reveal two reaction paths for the PATP coupling reaction. One is that plasmon-induced hot holes activate the dehydrogenation of PATP and then the radical coupling reaction to form p,p'-dimercaptoazobenzene (DMAB) under O2-free conditions. Another is likely to be that the surface Ag2O/AgOH, which is generated from Ag and 1O2/O2-, catalyzes the oxidation of PATP and then the coupling process under O2-rich conditions. Benefitting from the potential/atmosphere controlled measurements in situ, the intermediate species of PATP(NH)/PATP(N) are observed with vibrational bands at around 1056, 1202, 1253, 1395, 1514 and 1540 cm-1.

Electrochemical SERS and SOERS in a single experiment: A new methodology for quantitative analysis

In the present work, a new methodology which combines two different phenomena to enhance the Raman signal is used to resolve a mixture of two compounds with similar molecular structures. The use of Raman spectroelectrochemistry (Raman-SEC) allows us to collect simultaneously, with high time-resolution, the enhancement of the Raman signal of the compounds present in a sample during the electrochemical oxidation-reduction cycle (ORC) of a silver screen-printed electrode. During such ORC two different phenomena appears depending on the stage of the electrochemical modification of the silver substrate, which are known as electrochemical surface enhanced Raman scattering (EC-SERS) and electrochemical surface oxidation enhanced Raman scattering (EC-SOERS). This work is a proof of concept that demonstrates the advantage of using EC-SOERS and EC-SERS in a single experiment to resolve mixtures of similar molecules such as vitamin B3, which components are nicotinic acid and nicotinamide. Although the interaction between analytes and substrates influence a univariate calibration, the trilinear character of Raman-SEC makes possible to deconvolve such interactions and provide a good calibration curve for both, nicotinic acid and nicotinamide.

Single Functionalized pRNA/Gold Nanoparticle for Ultrasensitive MicroRNA Detection Using Electrochemical Surface-Enhanced Raman Spectroscopy.

Controlling the selective one‐to‐one conjugation of RNA with nanoparticles is vital for future applications of RNA nanotechnology. Here, the monofunctionalization of a gold nanoparticle (AuNP) with a single copy of RNA is developed for ultrasensitive microRNA‐155 quantification using electrochemical surface‐enhanced Raman spectroscopy (EC‐SERS). A single AuNP is conjugated with one copy of the packaging RNA (pRNA) three‐way junction (RNA 3WJ). pRNA 3WJ containing one strand of the 3WJ is connected to a Sephadex G100 aptamer and a biotin group at each arm (SEPapt/3WJ/Bio) which is then immobilized to the Sephadex G100 resin. The resulting complex is connected to streptavidin‐coated AuNP (STV/AuNP). Next, the STV/AuNP–Bio/3WJa is purified and reassembled with another 3WJ to form a single‐labeled 3WJ/AuNP. Later, the monoconjugate is immobilized onto the AuNP‐electrodeposited indium tin oxide coated substrate for detecting microRNA‐155 based on EC‐SERS. Application of an optimum potential of +0.2 V results in extraordinary amplification (≈7 times) of methylene blue (reporter) SERS signal compared to the normal SERS signal. As a result, a highly sensitive detection of 60 × 10−18 m microRNA‐155 in 1 h in serum based on monoconjugated AuNP/RNA is achieved. Thus, the monofunctionalization of RNA onto nanoparticle can provide a new methodology for biosensor construction and diverse RNA nanotechnology development.

Nanoscale Surface Redox Chemistry Triggered by Plasmon-Generated Hot Carriers.

Direct photoexcitation of charges at a plasmonic metal hotspot produces energetic carriers that are capable of performing photocatalysis in the visible spectrum. However, the mechanisms of generation and transport of hot carriers are still not fully understood and under intense investigation because of their potential technological importance. Here, spectroscopic evidence proves that the reduction of dye molecules tethered to a Au(111) surface can be triggered by plasmonic carriers via a tunneling mechanism, which results in anomalous Raman intensity fluctuations. Tip-enhanced Raman spectroscopy (TERS) helps to correlate Raman intensity fluctuations with temperature and with properties of the molecular spacer. In combination with electrochemical surface-enhanced Raman spectroscopy, TERS results show that plasmon-induced energetic carriers can directly tunnel to the dye through the spacer. This organic spacer chemically isolates the adsorbate from the metal but does not block photo-induced redox reactions, which offers new possibilities for optimizing plasmon-induced photocatalytic systems.

Determination of Molecular Orientation in Bi-analyte Mono-molecule Layer through Electrochemical Surface-enhanced Raman Scattering Measurements

Surface-enhanced Raman scattering measurements have been conducted using plasmonic dimer structures under electrochemical potential control. We have confirmed the potential dependence of the adsorp...

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.

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.

On the origin of electrochemical surface-enhanced Raman spectroscopy (EC-SERS) signals for bacterial samples: the importance of filtered control studies in the development of new bacterial screening platforms

The importance of filtered control studies for ascertaining the origin of bacterial SERS signals is highlighted.

Photovoltaic cells as a highly efficient system for biomedical and electrochemical surface-enhanced Raman spectroscopy analysis.

Surface-enhanced Raman scattering (SERS) has been intensively used recently as a highly sensitive, non-destructive, chemical specific, and label-free technique for a variety of studies. Here, we present a novel SERS substrate for: (i) the standard ultra-trace analysis, (ii) detection of whole microorganisms, and (iii) spectroelectrochemical measurements. The integration of electrochemistry and SERS spectroscopy is a powerful approach for in situ investigation of the structural changes of adsorbed molecules, their redox properties, and for studying the intermediates of the reactions. We have developed a conductive SERS platform based on photovoltaic materials (PV) covered with a thin layer of silver, especially useful in electrochemical SERS analysis. These substrates named Ag/PV presented in this study combine crucial spectroscopic features such as high sensitivity, reproducibility, specificity, and chemical/physical stability. The designed substrates permit the label-free identification and differentiation of cancer cells (renal carcinoma) and pathogens (Escherichia coli and Bacillus subtilis). In addition, the developed SERS platform was adopted as the working electrode in an electrochemical SERS approach for p-aminothiophenol (p-ATP) studies. The capability to monitor in real-time the electrochemical changes spectro-electro-chemically has great potential for broadening the application of SERS.

Electrochemical SERS for in Situ Monitoring the Redox States of PEDOT and Its Potential Application in Oxidant Detection

In response to recent developments for applying conducting polymers on various biomedical applications, the development of characterization techniques for evaluating the states of conducting polymers in liquids is beneficial to the applications of these materials. In this study, we propose a platform using electrochemical surface-enhanced Raman scattering (EC-SERS) technology, which allows a direct measurement of the redox states of conducing polymers in liquids. A thiophene-based conducting polymer, hydroxymethyl poly(3,4-ethylenedioxythiophene) or poly(EDOT-OH), was used to demonstrate this concept. Poly(EDOT-OH) films were coated on Au nanoparticle-coated ITO glass as SERS-active substrates. Taking the advantage of Raman enhancement, we can in situ and clearly monitor the redox behavior of poly(EDOT-OH) in aqueous solutions. The Raman peak intensity decreases as the poly(EDOT-OH) film is oxidized. Furthermore, we demonstrated our idea to utilize this phenomenon as the sensing mechanism for oxidant detection. The Raman intensity of conducting polymers reduces faster when oxidants exist, and we obtain a quantitative analysis for the detection of oxidants. Moreover, the oxidized poly(EDOT-OH) films can be reused for detection of oxidants simply by applying a reduction potential to activate the poly(EDOT-OH) films. The film stability was also confirmed, and the detection of two other oxidants, namely ammonium persulfate and iron chloride, were also demonstrated. The results show different SERS spectra of poly(EDOT-OH) films oxidized by using different oxidants. Besides, the oxidized films can be easily recovered simply by applying a cathodic potential, which allows repeating usage and makes it possible for continuous monitoring applications. To the best of our knowledge, this is the first time to apply PEDOT's Raman feature for detection purposes.