Electrochemical Surface-enhanced Raman Spectroscopy Research Articles

Coupling Multidimensional Chromatography with Plasmonic Sensing: An Exploration of Electrochemical SERS as a Detection Modality for 2D-LC

Complex mixtures which may contain hundreds, if not thousands, of unique compounds pose an analytical challenge for complete and detailed analyses. For example, green tea is a well-known complex substance requiring advanced separation techniques and ultrasensitive detection methods for full characterization. Separation and identification of components in green tea help gauge tea quality, as green tea is one of the most consumed beverages in the world. In this work, the coupling of two-dimensional liquid chromatography (2D-LC) and electrochemical surface-enhanced Raman spectroscopy (EC-SERS) was explored for the first time to analyze compounds in green tea. 2D-LC offers enhanced separation compared to conventional HPLC systems due to the second dimension of separation. EC-SERS was used as an offline detection modality and qualitative tool to help identify the compounds in green tea fractions collected from the second dimension separation. 2D-LC and EC-SERS are coupled in this work for the first time and offer a pathway forward for sensitive and selective separation and identification of components in complex mixtures.

Insights into the electrochemical oxidation selectivity of hydroxymethylfurfural over humin-layered Au nanoparticles

Au nanoparticle catalysts are promising electrocatalysts for biomass upgrading. Starting from a pristine Au electrode, herein we demonstrate a simple route for the electrochemical preparation of Au NPs supported on thin layers of humin (Au/H). Utilizing an oxidized Au surface as a precursor, these electrocatalytic structures are formed upon reduction of 5-hydroxymethylfurfural (HMF) in alkaline media while performing a potential sweep. We subsequently utilize this Au/H structure for the electrochemical oxidation of ethanol and HMF in a rotating disk electrode (RDE) configuration and perform additional analysis via electrochemical surface-enhanced Raman Scattering (SERS). The RDE test reveals Au/H has different reaction kinetics towards alcohol and aldehyde functional groups, enabling a mechanistic understanding of the reaction pathway for HMF oxidation. The SERS experiment identifies the favorable reduction pathway from Au2O3 to gold, suggesting the possible active site on this catalyst for HMF oxidation.

Potential powered EC-SERS for sensitive detection of acetamiprid

Surface-enhanced Raman spectroscopy (SERS) is a powerful tool for rapid and sensitive detection of pesticide residues in food, and the applied potential regulated SERS technique can be endowed with a higher sensitivity. In this work, we developed an electrochemical-surface enhanced Raman spectroscopy (EC-SERS) sensor for the rapid detection of acetamiprid (AAP) in vegetables. AuNPs/ITO were prepared via in-situ electrodeposition of gold nanoparticles (AuNPs) on an indium tin oxide (ITO) both as a SERS substrate and working electrode. By adding AAP into an in-situ Raman electrolytic cell with an applied electrochemical potential (+0.3 V), the SERS intensity was enhanced by 4-fold and the characteristic signal was linearly related to the concentration of AAP. Under the optimal conditions, the EC-SERS method exhibited a linear detection range from 0.05 to 50 μM, with a limit of detection (LOD, 3S/M) of 0.02 μM. Simultaneously, based on density functional theory (DFT), the adsorption characteristics of AAP on AuNPs/ITO were studied in the presence of electric field. The method was finally confirmed by HPLC-MS for the detection of real samples, revealing its potential application in food safety.

Unraveling the Evolution of Dynamic Active Sites of LaNixFe1-xO3 Catalysts During OER.

Perovskites have attracted tremendous attention as potential catalysts for the oxygen evolution reaction (OER). It is well-known that the introduction of Fe into rare earth perovskites such as LaNiO3 enhances the intrinsic OER activity. Despite numerous studies on structure-property relationships, the origin of the activity and the nature of the active species are still elusive and unclear. In this work, we study a series of LaNixFe1-xO3 perovskites using in situ electrochemical surface-enhanced Raman spectroscopy and electron energy loss spectroscopy to decipher the surface evolution and formation of active species during OER. While the origin of the activity arises from NiOOH species formed from the active Ni centers in LaNiO3, our work shows that Fe serves as the active center in LaNi0.5Fe0.5O3 and forms Fe-O-Ni and FeOOH species during OER. The OER activity of LaFeO3 originates from FeOOH species, which interact with the soluble Ni species in the electrolyte forming an active electrode-electrolyte interface with high-valent stable surface iron species (Fe4+) and thereby improving the performance. Our work provides deeper insights into the synergistic effects of Ni and Fe on the catalytic activity, which in turn provides new design principles for perovskite catalysts for the OER.

Self-Reconstruction of Core–Shell Structured Electrocatalysts for Tailoring Reaction Pathways Revealed by Electrochemical Surface-Enhanced Raman Spectroscopy

Self-Reconstruction of Core–Shell Structured Electrocatalysts for Tailoring Reaction Pathways Revealed by Electrochemical Surface-Enhanced Raman Spectroscopy

Targeting antibiotic pollution: an investigation of azithromycin in wastewater by electrochemical surface-enhanced Raman spectroscopy

Azithromycin is a macrolide antibiotic that is commonly prescribed to treat infections of the skin, soft tissues, and the upper and lower respiratory tracts. Many antibiotics, including azithromycin, are overprescribed, leading to elevated concentrations of these drugs in bodies of water; this phenomenon is referred to as antibiotic pollution. Antibiotic pollution is increasingly concerning, due to its implications for the development of antibiotic resistance and its potential ecotoxicity. Rapid detection of azithromycin in wastewater could be important for addressing antibiotic pollution. This research highlights the development of a direct electrochemical surface-enhanced Raman spectroscopy method for the detection of azithromycin in synthetic wastewater. The coupling of electrochemistry and surface-enhanced Raman spectroscopy allowed enhanced detection of azithromycin and its derivatives in wastewater. To the best of our knowledge, this is the first report of direct detection of azithromycin using electrochemical surface-enhanced Raman spectroscopy. This novel detection method has promising application in future research concerning antibiotic pollution.

In Situ Spectroscopic Elucidation of the Electrochemical Potential Drop at Polyelectrolytes/Au Interfaces.

Polyelectrolytes have been widely applied in electrochemical devices. Understanding the polyelectrolyte/electrode interfaces is pivotal for polyelectrolyte-based applications. Here, we measured the electrochemical potential drop and the local activity of the mobile ion of H+ or OH- at the polyelectrolytes/Au interfaces by in situ electrochemical surface-enhanced Raman spectroscopy and voltammetry in three-electrode cells. We found that the potential dependences of the electrochemical potential drop in polyelectrolytes were smaller than that in conventional electrolyte solutions. The interfacial activity of H+ or OH- was much lower than that of bulk polyelectrolytes. The potential-dependent molecular dynamics simulations showed that the mobility of ionomers of polyelectrolytes in an electrostatic field was limited by a polymer matrix. These results suggested a characteristically thicker compact layer in the electrical double layer of a polyelectrolyte/electrode interface due to the accumulation of mobile H+ or OH- with a thicker hydration layer and immobile ionomers.

An applied potential powered screen-printed electrode for sensitive EC-SERS detection of acetamiprid

Acetamiprid (AAP) is a common neonicotinoid pesticide and their residues have emerged as a widely concerned issue on vegetables and fruits. In this work, a facile electrochemical surface-enhanced Raman spectroscopy (EC-SERS) method with one step of “mixing and detection” was proposed for detection of AAP. Wherein, screen printed electrode (SPE) was modified by dropping silver nanoparticles (AgNPs) onto the SPE surface. Compared to the normal SERS detection, the EC-SERS had a 5-fold enhancement in SERS signal with an applied potential at –0.7 V. Under the optimal applied potential, a wide linear range from 0.1 μM to 1000 μM was achieved with a low limit of detection (LOD, 0.04 μM), about 7-fold lower than that of the normal SERS method (LOD, 0.31 μM). Density functional theory (DFT) calculation revealed that the van der Waals force between AAP molecules and AgNPs was greatly enhanced. Besides, the EC-SERS method was successfully applied for the detection of AAP in Brassica chinensis L., and the results correlated well (R2 =0.9760) with those obtained from liquid chromatography-tandem mass spectrometry (HPLC-MS). The proposed method featured in easy operation, rapid detection and high sensitivity, which can act as a promising tool for monitoring pesticide residues in food samples.

Exploring the Influence of Malachite Forming on Oxide-Derived Copper Electrodes on C2+ Product Selectivity

Electrocatalytic activity and C2 product selectivity of two distinguished oxide-derived Cu in electrochemical carbon dioxide reduction reaction (CO2RR) were investigated. Cu oxide layers electrodeposited at different deposition rates exhibited different morphologies, the electrode with a more compact structure was found selective to C2 products with two times higher faradaic efficiencies (40%). Both Cu+ and Cu2+ species have been identified on the surface of oxide-derived Cu electrodes by X-ray photoelectron spectroscopy (XPS). Also, oxide-derived electrodes were investigated by X-ray diffraction (XRD). Results confirmed the presence of Cu oxide phases for primary electrodes, which were then fully reduced to the metallic Cu after CO2RR. Moreover, SEM investigations helped us distinguish the morphology of the two electrodes and monitor morphological differences before and after CO2RR. To shed light on the adsorbed species, intermediate (metastable) phases, and reaction mechanisms during CO2RR, electrochemical surface-enhanced Raman spectroscopy (SERS) was utilized. This method helped us vividly observe the formation of a metastable phase (malachite) on the electrode surface, which showed lower FEs for C2 products. Moreover, the analysis of SERS indicated a strong tie between the presence of the malachite phase and strongly adsorbed CO on electrode surfaces, preventing dimerization and further reduction. This malachite phase terminating the surface can hinder the charge transport and interfere with further reductions in C2 products. Figure 1

Electrochemical surface-enhanced Raman spectroscopy analysis of malachite green on gold substrates

The pH-sensitivity and electroactivity of malachite green (MG), a cationic dye, can potentially affect the surface-enhanced Raman spectroscopy (SERS) profile during electrochemical SERS (EC-SERS) analysis. An EC-SERS investigation was performed by acquiring dynamic SERS profiles of MG on Au SERS substrates at various applied potentials to elucidate the electrochemistry of MG at the molecular level. The appropriate EC-SERS potential window of MG, mass and electron transfer modes, and the number of electrons and protons in the redox process were determined by cyclic voltammetry. The EC-SERS parameters (pH and type of supporting electrolyte) were optimized. The highest SERS signal was achieved by chronoamperometry SERS (CA-SERS) and CV-SERS at an applied potential of 0.6 V (vs. Ag/AgCl) using 0.1 M phosphate buffer at pH 4 as the supporting electrolyte. Plausible mechanisms for electrooxidation and SERS signal enhancement were proposed. The oxidized form of MG, conformational changes, and evolution of the adsorption orientation were successfully elucidated using CV-SERS. This study provides molecular-level insight into the adsorption orientation of MG and its oxidized form, which involves aromatic rings with a tilted upright orientation at positive applied potentials. The mechanism of SERS signal enhancement is expounded, which is valuable for developing EC-SERS-based MG sensing.

3D plasmonic hotspot engineering toward ultrasensitive and rapid EC-SERS recognition of plasticizers

Public attention to the health and environmental risks from plasticizer exposure in daily life has led to legal restrictions on the use of plasticizers in products. However, the lack of rapid, sensitive, and reliable analytic methods hinders the effective screening of toxic materials. In the present work, we propose a novel electrochemical surface-enhanced Raman spectroscopy (EC-SERS) technique for the detection of plasticizers such as phthalate esters (PAEs) and bisphenol A. The interior hotspots were realized through the EC deposition onto metal nanopillar (ECOMP) platforms in the presence of analytes within 2.5 min. The tiny molecules surrounded by Au produced sufficient amplification of SERS signals, leading to sub-parts-per-billion (sub-ppb) sensitivity for all the intended specimens. According to the three standard deviation equation, the limit of detection was determined to be 0.002 ppb. The ECOMP platforms with reproducibility (relative standard deviation of <10 %) and linear proportion (R2 ≥ 0.93) were highly suitable for reliable quantitative investigation. A principal component analysis method recognized their slight differences in spectral features. The variability from the first two components was achieved to be 73 %, which separated individual plasticizer groups distinctly. The plasticizer extracted from an actual polyvinyl chloride sample was successfully detected using the proposed technique. The interaction of PAEs with ethanol and the delocalization of PAEs in ECOMP platforms were also discussed. The results demonstrate the feasibility of plasticizer–ECOMP platforms being widely used for onsite testing in industrial fields such as chemical and drug safety.

Rapid detection of Pseudomonas syringae pv. actinidiae by electrochemical surface-enhanced Raman spectroscopy

Bacterial cancer caused by Pseudomonas syringae pv. actinidiae (Psa) is a major threat to kiwifruit in the world, and there is still a lack of effective control measures. The field of bacterial detection needs a fast, easy-to-use and sensitive identification platform. The current bacterial identification methods are lack of time efficiency, which brings problems to many sectors of society. Surface-enhanced Raman spectroscopy (SERS) and electrochemistry (EC) have been studied as possible candidates for bacterial detection because of their high sensitivity for the detection of biomolecules. In this work, SERS, EC and electrochemical surface-enhanced Raman spectroscopy (EC-SERS) were used for the first time to study the adsorption and EC behavior of Psa on the surface of nanostructured silver electrodes. Two different Raman spectra of a single analyte were obtained, and this dual detection was realized. Silver nanoparticles with iodide and calcium ions (Ag@ICNPs) were synthesized as SERS substrates significantly enhanced the characteristic signal peaks of Psa, and the limit of detection (LOD) is as low as 1.0 × 102 cfu/mL. Chemical imaging results show that the application of negative voltage can significantly improve the spectrum quality, showing a higher signal at −0.8 V, indicating that Psa molecules may have potential-induced reorientation on the electrode surface. Therefore, EC-SERS has the ability to greatly improve the SERS performance of bacteria in terms of peak intensity and spectral richness.

Electrochemical Surface-Enhanced Raman Spectroscopy (EC-SERS) Study of C3 Polyalcohols Oxidation Catalyzed by a Ag Roughened Electrode with Low Platinum Content

Electrochemical Surface-Enhanced Raman Spectroscopy (EC-SERS) Study of C3 Polyalcohols Oxidation Catalyzed by a Ag Roughened Electrode with Low Platinum Content

Electrochemical Surface-Enhanced Raman Spectroscopy for Aniline Adsorbed on Silver Electrodes: A DFT Study of the Anharmonic Effects of Amino Wagging Vibrational Modes

Aniline molecules exhibit a special amino wagging vibrational mode on the surface of the silver electrode. The corresponding Raman peak not only displays enhanced Raman signals in surface-enhanced Raman spectroscopy (SERS) experiments but also shows a shift toward higher wavenumbers. This work adopts a molecule-metal cluster model to investigate the normal Raman spectra of aniline molecules and their SERS spectra adsorbed on silver electrode surfaces by density functional theory (DFT) approaches. Based on the harmonic approximation, the influence of anharmonic effects on the normal Raman and SERS spectra of aniline is further considered. Our calculated results of the anharmonic Raman spectra of free aniline molecules show that their anharmonic effect mainly comes from the amino wagging vibrational mode. Then, the anharmonic Raman spectra of aniline binding to neutral and positively charged silver clusters were calculated and compared with the harmonic Raman spectra. It was found that the coupling effect between vibrational modes also greatly affects the Raman spectral characteristics. Finally, the excited state energy gap and pre-resonance Raman spectra of the aniline–silver cluster complexes are studied, and it is believed that there may be a charge-transfer enhancement mechanism.

Evaluation and classification of fentanyl-related compounds using EC-SERS and machine learning.

Multiple analytical techniques for the screening of fentanyl-related compounds exist. High discriminatory methods such as GC-MS and LC-MS are expensive, time-consuming, and less amenable to onsite analysis. Raman spectroscopy provides a rapid, inexpensive alternative. Raman variants such as electrochemical surface-enhanced Raman scattering (EC-SERS) can provide signal enhancements with 1010 magnitudes, allowing for the detection of low-concentration analytes, otherwise undetected using conventional Raman. Library search algorithms embedded in instruments utilizing SERS may suffer from accuracy when multicomponent mixtures involving fentanyl derivatives are analyzed. The complexing of machine learning techniques to Raman spectra demonstrates an improvement in the discrimination of drugs even when present in multicomponent mixtures of various ratios. Additionally, these algorithms are capable of identifying spectral features difficult to detect by manual comparisons. Therefore, the goal of this study was to evaluate fentanyl-related compounds and other drugs of abuse using EC-SERS and to process the acquired data using machine learning-convolutional neural networks (CNN). The CNN was created using Keras v 2.4.0 with Tensorflow v 2.9.1 backend. In-house binary mixtures and authentic adjudicated case samples were used to evaluate the created machine-learning models. The overall accuracy of the model was 98.4 ± 0.1% after 10-fold cross-validation. The correct identification for the in-house binary mixtures was 92%, while the authentic case samples were 85%. The high accuracies achieved in this study demonstrate the advantage of using machine learning to process spectral data when screening seized drug materials comprised of multiple components.

In-situ surface enhanced Raman spectroscopy investigations on surface transformations of oxide derived copper electrodes during CO2RR

We investigated the catalytic activity and C2 selectivity in electrochemical carbon dioxide reduction reaction (CO2RR) on two distinguished electrodeposited Cu oxides with distinct morphologies and structures. The electrode with a compact structure exhibited two times higher faradaic efficiencies of C2 products (40%). Through utilizing electrochemical surface-enhanced Raman spectroscopy (SERS), it was realized that the formation of a metastable phase (malachite) on electrode surfaces by consumption of HCO3- could cause a shift in local pH. The analysis of SERS indicated a strong correlation between the presence of the malachite phase and strongly-adsorbed CO on electrode surfaces, preventing dimerization and further reduction. This malachite phase terminating the surface can hinder the charge exchange and interfere with further reductions in C2 products.

Voltage Modulation of Nanoplasmonic Metal Luminescence from Nano-Optoelectrodes in Electrolytes.

Metallic nanostructures supporting surface plasmon modes can concentrate optical fields and enhance luminescence processes from the metal surface at plasmonic hotspots. Such nanoplasmonic metal luminescence contributes to the spectral background in surface-enhanced Raman spectroscopy (SERS) measurements and is helpful in bioimaging, nanothermometry and chemical reaction monitoring applications. Although there is growing interest in nanoplasmonic metal luminescence, its dependence on voltage modulation has received limited attention in research investigations. Also, the hyphenated electrochemical surface-enhanced Raman spectroscopy (EC-SERS) technique typically ignores voltage-dependent spectral background information associated with nanoplasmonic metal luminescence due to limited mechanistic understanding and poor measurement reproducibility. Here, we report a combined experiment and theory study on dynamic voltage-modulated nanoplasmonic metal luminescence from hotspots at the electrode-electrolyte interface using multiresonant nanolaminate nano-optoelectrode arrays. Our EC-SERS measurements under 785 nm continuous wavelength laser excitation demonstrate that short-wavenumber nanoplasmonic metal luminescence associated with plasmon-enhanced electronic Raman scattering (PE-ERS) exhibits a negative voltage modulation slope (up to ≈30% V-1) in physiological ionic solutions. Furthermore, we have developed a phenomenological model to intuitively capture the plasmonic, electronic, and ionic characteristics at the metal-electrolyte interface to understand the observed dependence of the PE-ERS voltage modulation slope on voltage polarization and ionic strength. The current work represents a critical step toward the general application of nanoplasmonic metal luminescence signals in optical voltage biosensing, hybrid optical-electrical signal transduction, and interfacial electrochemical monitoring.

Targeted fentanyl screening utilizing electrochemical surface-enhanced Raman spectroscopy (EC-SERS) applied to authentic seized drug casework samples

Effective and rapid screening methods for seized drug analysis are crucial to ensure the safety of first responders and laboratory personnel, while reducing overall analysis time and improving reliability. The drug landscape has been overwhelmed by fentanyl and fentanyl analogs that are extremely potent and generally present in low concentrations with other drugs and diluents. We have previously reported the use of electrochemical surface-enhanced Raman spectroscopy (EC-SERS) as a novel screening method for detecting fentanyl and fentanyl analogs in the presence of commonly encountered analytes. Herein, we present the application of this targeted method to authentic seized drug casework samples to assess the performance and fit-for-purpose of the developed method to accurately identify fentanyl and fentanyl-like substances. Authentic sample sets contained a wide range of analytes, and a varying number of compounds present in each sample, representing both true positive and true negative samples. EC-SERS results were compared to the ground-truth as established by gas chromatography-mass spectrometry (GC–MS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS), as well as the results of chemical color tests. Application to authentic samples allowed for identification of fentanyl and fentanyl analogs. The targeted approach was shown to provide preferential enhancement of the fentanyl signal. The overall accuracy for the targeted screening method for the presence of a fentanyl/fentanyl-like substance was 87.5 % and the fentanyl samples averaged between 6 wt% to 9 wt% fentanyl or fentanyl analog. EC-SERS provided an alternative fentanyl screening approach demonstrating results within minutes and the absence of false positives.

Ultraminiaturized Microfluidic Electrochemical Surface‐Enhanced Raman Scattering Chip for Analysis of Neurotransmitters Fabricated by Ship‐in‐a‐Bottle Integration

Electrochemical surface‐enhanced Raman scattering (EC‐SERS) is a promising technique for the diagnosis of trace amounts of neurotransmitters, because it can elucidate neurotransmitters’ behavior on electrodes to deduce their functions in the human body. However, the current EC‐SERS devices need several tens of milliliters of analyte solution to collect enough signal for analysis. Miniaturization of EC‐SERS devices is crucial for the early diagnosis of disease and point‐of‐care testing. Herein, a new type of EC‐SERS sensor based on 3D microfluidic chips for the analysis of neurotransmitters in ultrasmall volumes is proposed. The microfluidic EC‐SERS chip is fabricated by a ship‐in‐a‐bottle technique based on hybrid laser processing. The working electrode is modified using silver/zinc oxide materials, enabling the formation of a unique “candy apple” structure. To assess the fabricated microfluidic EC‐SERS chips, ascorbic acid is analyzed using the ingenious microfluidic EC‐SERS chips to elucidate its redox reaction by EC‐SERS spectroscopy. Significantly, a sub‐10 μL volume of analyte solution is sufficient for EC‐SERS analysis, which is several orders smaller in volume than the requirements of current EC‐SERS devices. The unprecedented microfluidic EC‐SERS chips fabricated by the ship‐in‐a‐bottle integration technique can be used in portable and smart analyzers for next‐generation biomedicines and catalysts.

Potential controlled redox cycling of 4-aminothiophenol by coupling plasmon mediated chemical reaction with electrochemical reaction

The plasmon-mediated chemical reaction (PMCR) is a promising new direction for photocatalysis. By harvesting light through photon-electron interactions using nanostructures of plasmonic metals, chemical reactions of immobilized molecules on the metal surface can be activated. As demonstrated in recent years, the electrode potential (E) applied to the metal surface also provides a unique means for electrosynthesis by changing the local electrostatic field and controlling the electron transfer and redistribution in the molecules immobilized on the electrode surface. Here, as a case study, we studied the E-modulated PMCR of 4-aminothiophenol (4-ATP) confined in the nano junction formed between the gold nanoparticle (AuNP) and gold nanoelectrode (AuNE) by using the time-resolved electrochemical-surface-enhanced Raman spectroscopy (EC-SERS), a powerful tool to track the chemical reaction in-situ on the plasmonic metal surfaces in solution. We demonstrated that the negative E can effectively suppress the plasmon-mediated photo-oxidation of 4-ATP to 4,4-dimercaptoazobenzene (DMAB) and meanwhile activate the EC reduction of DMAB to 4-ATP. We are able to use E to regulate the competition between photo-oxidation and EC reduction and achieve the redox cycling between 4-ATP and DMAB in continuous E cycles. By analyzing the E-modulated dynamic SERS signal changes, we also found the applied E could affect the SERS changes through two intertwined nonreaction and reaction mechanisms. This approach can improve our understanding of the photon-electron-molecule interactions and the interplay between PMCR and EC reactions in plasmonic molecular junctions.