Transient Electrochemical Surface-Enhanced Raman Spectroscopy: A Millisecond Time-Resolved Study of an Electrochemical Redox Process

Ionophore-Based SERS Sensing for Electrolyte Cations

We present here a surface-enhanced Raman spectroscopy (SERS)-based platform for selectively detecting electrolyte cations. This platform facilitates the reorientation of the chromoionophore I (CHI) molecule positioned on the surface of silver nanoparticles, regulated by the targeted electrolyte cation within the ionophore-based ion-selective sensing framework. When exposed to the target ions, leading to deprotonation, the aromatic plane of the CHI molecule shifts from an endwise to an edgewise configuration on the SERS substrate surface. This reorientation improves the coupling of the induced dipole within the molecule and the induced electromagnetic field normal to the surface, leading to a heightened and more discernible SERS signal. Additionally, NMR data revealed a protonation-dependent conformational change in the CHI molecules. Upon protonation, the side chain of the CHI molecule extends away from its aromatic ring, whereas upon deprotonation, the carbon chain folds back and closely approaches the edge of the aromatic plane, further confirming this conclusion. Using Ca2+ and Na+ as examples, this method shows a detection limit of 0.1 μM for Ca2+ and 1 μM for Na+ with high selectivity. The successful application of the versatile and rapidly advancing SERS technique for electrolyte cation detection shows a promising pathway for enhancing current ion detection capabilities. As a preliminary demonstration, the total Ca2+ concentration in undiluted human serum was successfully determined, with common matrix interference effectively circumvented.

Impact of Surface Enhanced Raman Spectroscopy in Catalysis

Catalysis stands as an indispensable cornerstone of modern society, underpinning the production of over 80% of manufactured goods and driving over 90% of industrial chemical processes. As the demand for more efficient and sustainable processes grows, better catalysts are needed. Understanding the working principles of catalysts is key, and over the last 50 years, surface-enhanced Raman Spectroscopy (SERS) has become essential. Discovered in 1974, SERS has evolved into a mature and powerful analytical tool, transforming the way in which we detect molecules across disciplines. In catalysis, SERS has enabled insights into dynamic surface phenomena, facilitating the monitoring of the catalyst structure, adsorbate interactions, and reaction kinetics at very high spatial and temporal resolutions. This review explores the achievements as well as the future potential of SERS in the field of catalysis and energy conversion, thereby highlighting its role in advancing these critical areas of research.

Advancements in mercury detection using surface-enhanced Raman spectroscopy (SERS) and ion-imprinted polymers (IIPs): a review

Mercury (Hg) contamination remains a major environmental concern primarily due to its presence at trace levels, making monitoring the concentration of Hg challenging. Sensitivity and selectivity are significant challenges in the development of mercury sensors. Surface-enhanced Raman spectroscopy (SERS) and ion-imprinted polymers (IIPs) are two distinct analytical methods developed and employed for mercury detection. In this review, we provide an overview of the key aspects of SERS and IIP methodologies, focusing on the recent advances in sensitivity and selectivity for mercury detection. By examining the critical parameters and challenges commonly encountered in this area of research, as reported in the literature, we present a set of recommendations. These recommendations cover solid and colloidal SERS substrates, appropriate Raman reporter/probe molecules, and customization of IIPs for mercury sensing and removal. Furthermore, we provide a perspective on the potential integration of SERS with IIPs to achieve enhanced sensitivity and selectivity in mercury detection. Our aim is to foster the establishment of a SERS-IIP hybrid method as a robust analytical tool for mercury detection across diverse fields.

Raman Monitoring of the Electro-Optical Synergy-Induced Enhancements in Carbon-Bromine Bond Cleavage, Reaction Rate, and Product Selectivity of p-Bromothiophenol

Electro-optical synergy has recently been targeted to improve the separation of hot carriers and thereby further improve the efficiency of plasmon-mediated chemical reactions (PMCRs). However, the electro-optical synergy in PMCRs needs to be more deeply understood, and its contribution to bond dissociation and product selectivity needs to be clarified. Herein, the electro-optical synergy in plasmon-mediated reduction of p-bromothiophenol (PBTP) was studied on a plasmonic nanostructured silver electrode using in situ Raman spectroscopy and theoretical calculations. It was found that the electro-optical synergy-induced enhancements in the cleavage of carbon-bromine bonds, reaction rate, and product selectivity (4,4'-biphenyl dithiol vs thiophenol) were largely affected by the applied bias, laser wavelength, and laser power. The theoretical simulation further clarified that the strong electro-optical synergy is attributed to the matching of energy band diagrams of the plasmonic silver with those of the adsorbed PBTP molecules. A deep understanding of the electro-optical synergy in PBTP reduction and the clarification of the mechanism will be highly beneficial for the development of other highly efficient PMCRs.

A universal method to fabricate high-valence transition metal-based HER electrocatalysts and direct Raman spectroscopic evidence for interfacial water regulation

Valence modulation of transition metal oxides represents a highly effective approach in designing high-performance catalysts, particularly for pivotal applications such as the hydrogen evolution reaction (HER) in solar/electric water splitting and the hydrogen economy. Recently, there has been a growing interest in high-valence transition metal-based electrocatalysts (HVTMs) due to their demonstrated superiority in HER performance, attributed to the fundamental dynamics of charge transfer and the evolution of intermediates. Nevertheless, the synthesis of HVTMs encounters considerable thermodynamic barriers, which presents challenges in their preparation. Moreover, the underlying mechanism responsible for the enhancement in HVTMs still needs to be discovered. Hence, the universal synthesis strategies of the HVTMs are discussed, and direct Raman spectroscopic evidence for intermediates regulation is revealed to guide the further design of the HVTM electrocatalysts. This work offers new insights for facile designing of HVTMs electrocatalysts for energy conversion and storage through adjusting the reaction pathway.

Sixty years of electrochemical optical spectroscopy: a retrospective

Sixty years ago, Reddy, Devanatan, and Bockris performed the first in situ electrochemical ellipsometry experiment, which ushered in a new era in the study of electrochemistry, using optical spectroscopy. After six decades of development, electrochemical optical spectroscopy, particularly electrochemical vibrational spectroscopy, has advanced from a phase of immaturity with few methods and limited applications to a phase of maturity with excellent substrate generality and significantly improved resolutions. Here, we divide the development of electrochemical optical spectroscopy into four phases, focusing on the proof-of-concept of different electrochemical optical spectroscopy studies, the emergence of plasmonic enhancement-based electrochemical optical spectroscopic (in particular vibrational spectroscopic) methods, the realization of electrochemical vibrational spectroscopy on well-defined surfaces, and the efforts to achieve operando spectroelectrochemical applications. Finally, we discuss the future development trend of electrochemical optical spectroscopy, as well as examples of new methodology and research paradigms for operando spectroelectrochemistry.

Time-resolved in situ vibrational spectroscopy for electrocatalysis: challenge and opportunity

Understanding the structure-activity relationship of catalysts and the reaction pathway is crucial for designing efficient, selective, and stable electrocatalytic systems. In situ vibrational spectroscopy provides a unique tool for decoding molecular-level factors involved in electrocatalytic reactions. Typically, spectra are recorded when the system reaches steady states under set potentials, known as steady-state measurements, providing static pictures of electrode properties at specific potentials. However, transient information that is crucial for understanding the dynamic of electrocatalytic reactions remains elusive. Thus, time-resolved in situ vibrational spectroscopies are developed. This mini review summarizes time-resolved in situ infrared and Raman techniques and discusses their application in electrocatalytic research. With different time resolutions, these time-resolved techniques can capture unique dynamic processes of electrocatalytic reactions, short-lived intermediates, and the surface structure revolution that would be missed in steady-state measurements alone. Therefore, they are essential for understanding complex reaction mechanisms and can help unravel important molecular-level information hidden in steady states. Additionally, improving spectral time resolution, exploring low/ultralow frequency detection, and developing operando time-resolved devices are proposed as areas for advancing time-resolved techniques and their further applications in electrocatalytic research.

Carrier density tuning in CuS nanoparticles and thin films by Zn doping via ion exchange

Copper sulphide (covellite) nanoplatelets have recently emerged as a plasmonic platform in the near-infrared with ultrafast nonlinear optical properties. Here we demonstrate that the free-carrier density in CuS, which is an order of magnitude lower than in traditional plasmonic metals, can be further tuned by chemical doping. Using ion exchange to replace Cu with an increasing content of Zn in the nanoparticles, the free-hole density can be lowered, resulting in a long-wavelength shift of the localised plasmon resonances from 1250 nm to 1750 nm. The proposed approach provides new opportunities for tuning the plasmonic response of covellite nanocrystals as well as the carrier relaxation time which decreases for lower free-carrier densities.

Surface potential modulation as a tool for mitigating challenges in SERS-based microneedle sensors

Raman spectroscopic-based biosensing strategies are often complicated by low signal and the presence of multiple chemical species. While surface-enhanced Raman spectroscopy (SERS) nanostructured platforms are able to deliver high quality signals by focusing the electromagnetic field into a tight plasmonic hot-spot, it is not a generally applicable strategy as it often depends on the specific adsorption of the analyte of interest onto the SERS platform. This paper describes a strategy to address this challenge by using surface potential as a physical binding agent in the context of microneedle sensors. We show that the potential-dependent adsorption of different chemical species allows scrutinization of the contributions of different chemical species to the final spectrum, and that the ability to cyclically adsorb and desorb molecules from the surface enables efficient application of multivariate analysis methods. We demonstrate how the strategy can be used to mitigate potentially confounding phenomena, such as surface reactions, competitive adsorption and the presence of molecules with similar structures. In addition, this decomposition helps evaluate criteria to maximize the signal of one molecule with respect to others, offering new opportunities to enhance the measurement of analytes in the presence of interferants.

Towards time resolved characterization of electrochemical reactions: electrochemically-induced Raman spectroscopy

Structural characterization of transient electrochemical species in the sub-millisecond time scale is the all-time wish of any electrochemist. Presently, common time resolution of structural spectro-electrochemical methods is about 0.1 seconds. Herein, a transient spectro-electrochemical Raman setup of easy implementation is described which allows sub-ms time resolution. The technique studies electrochemical processes by initiating the reaction with an electric potential (or current) pulse and analyses the product with a synchronized laser pulse of the modified Raman spectrometer. The approach was validated by studying a known redox driven isomerization of a Ru-based molecular switch grafted, as monolayer, on a SERS active Au microelectrode. Density-functional-theory calculations confirmed the spectral assignments to sub-ms transient species. This study paves the way to a new generation of time-resolved spectro-electrochemical techniques which will be of fundamental help in the development of next generation electrolizers, fuel cells and batteries.

Oxygen Vacancy Engineering Synergistic with Surface Hydrophilicity Modification of Hollow Ru Doped CoNi-LDH Nanotube Arrays for Boosting Hydrogen Evolution

With the development of clean hydrogen energy, the cost effective and high-performance hydrogen evolution reaction (HER) electrocatalysts are urgently required. Herein, a green, facile, and time-efficient Ru doping synergistic with air-plasma treatment strategy is reported to boost the HER performance of CoNi-layered double hydroxide (LDH) nanotube arrays (NTAs) derived from zeolitic imidazolate framework nanorods. The Ru doping and air-plasma treatment not only regulate the oxygen vacancy to optimize the electron structure but also increase the surface roughness to improve the hydrophilicity and hydrogen spillover efficiency. Therefore, the air plasma treated Ru doped CoNi-LDH (P-Ru-CoNi-LDH) nanotube arrays display superior HER performance with an overpotential of 29 mV at a current density of 10 mA cm^-2 . Furthermore, by assembling P-Ru-CoNi-LDH as both cathode and anode for two-electrode urea-assisted water electrolysis, a small cell voltage of 1.36 V is needed at 10 mA cm^-2 and can last for 100 h without any obvious activity attenuation that showing outstanding durability. In general, the P-Ru-CoNi-LDH can improve the HER performance from intrinsic electronic structure regulation cooperated with extrinsic surface wettability modification. These findings provide an effective intrinsic and extrinsic synergistic effect avenue to develop high performance HER electrocatalysts, which is potential to be applied to other research fields.

Pinhole-Free Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy for Interference-Free Probing of Electrochemical Reactions

Investigating the behavior of analytes at the electrode surface is crucial in understanding the electrochemical and electrocatalytic reactions. Although Surface Enhanced Raman Scattering (SERS) is sensitive to minor chemical changes in the analyte, it is not widely used to study the reaction mechanisms on nonplasmonic surfaces because of the interference from plasmonic SERS substrates. In this study, we have investigated the redox reaction of Nile Blue A on a glassy carbon surface using pinhole-free silica-coated silver nanoparticles for Raman signal enhancement. The silver nanostructures were synthesized by a chemical reduction method, and the quality of the silica layer was confirmed using microscopic and electrochemical method. The in situ spectroelectrochemical data reveals the catalytic interference from silver which considerably alters the native reaction mechanism. The pinhole-free silica layer prevents the hot electron transfer and yields an interference-free enhancement to the Raman signals.

DNA/Nano based advanced genetic detection tools for authentication of species: Strategies, prospects and limitations

Authentication, detection and quantification of ingredients, and adulterants in food, meat, and meat products are of high importance these days. The conventional techniques for the detection of meat species based on lipid, protein and DNA biomarkers are facing challenges due to the poor selectivity, sensitivity and unsuitability for processed food products or complex food matrices. On the other hand, DNA based molecular techniques and nanoparticle based DNA biosensing strategies are gathering huge attention from the scientific communities, researchers and are considered as one of the best alternatives to the conventional strategies. Though nucleic acid based molecular techniques such as PCR and DNA sequencing are getting greater successes in species detection, they are still facing problems from its point-of-care applications. In this context, nanoparticle based DNA biosensors have gathered successes in some extent but not to a satisfactory stage to mark with. In recent years, many articles have been published in the area of progressive nucleic acid-based technologies, however there are very few review articles on DNA nanobiosensors in food science and technology. In this review, we present the fundamentals of DNA based molecular techniques such as PCR, DNA sequencing and their applications in food science. Moreover, the in-depth discussions of different DNA biosensing strategies or more specifically electrochemical and optical DNA nanobiosensors are presented. In addition, the significance of DNA nanobiosensors over other advanced detection technologies is discussed, focusing on the deficiencies, advantages as well as current challenges to ameliorate with the direction for future development.

Toward a mechanistic understanding of electrocatalytic nanocarbon

Electrocatalytic nanocarbon (EN) is a class of material receiving intense interest as a potential replacement for expensive, metal-based electrocatalysts for energy conversion and chemical production applications. The further development of EN will require an intricate knowledge of its catalytic behaviors, however, the true nature of their electrocatalytic activity remains elusive. This review highlights work that contributed valuable knowledge in the elucidation of EN catalytic mechanisms. Experimental evidence from spectroscopic studies and well-defined molecular models, along with the survey of computational studies, is summarized to document our current mechanistic understanding of EN-catalyzed oxygen, carbon dioxide and nitrogen electrochemistry. We hope this review will inspire future development of synthetic methods and in situ spectroscopic tools to make and study well-defined EN structures.

Electrochemical Tip-Enhanced Raman Spectroscopy: An In Situ Nanospectroscopy for Electrochemistry

Revealing the intrinsic relationships between the structure, properties, and performance of the electrochemical interface is a long-term goal in the electrochemistry and surface science communities because it could facilitate the rational design of electrochemical devices. Achieving this goal requires in situ characterization techniques that provide rich chemical information and high spatial resolution. Electrochemical tip-enhanced Raman spectroscopy (EC-TERS), which provides molecular fingerprint information with nanometer-scale spatial resolution, is a promising technique for achieving this goal. Since the first demonstration of this technique in 2015, EC-TERS has been developed for characterizing various electrochemical processes at the nanoscale and molecular level. Here, we review the development of EC-TERS over the past 5 years. We discuss progress in addressing the technical challenges, including optimizing the EC-TERS setup and solving tip-related issues, and provide experimental guidelines. We also survey the important applications of EC-TERS for probing molecular protonation, molecular adsorption, electrochemical reactions, and photoelectrochemical reactions. Finally, we discuss the opportunities and challenges in the future development of this young technique.

Effective Electrochemical Modulation of SERS Intensity Assisted by Core-Shell Nanoparticles

An effective and reversible tuning of the intensity of surface-enhanced Raman scattering (SERS) of nonelectroactive molecules at nonresonance conditions by electrochemical means has been developed on plasmonic molecular nanojunctions formed between Au@Ag core-shell nanoparticles (NPs) and a gold nanoelectrode (AuNE) modified with a self-assembled monolayer. The Au@Ag nanoparticle on nanoelectrode (NPoNE) structures are formed in situ by the electrochemical deposition of Ag on AuNPs adsorbed on the AuNE and can be monitored by both the electrochemical current and SERS signals. Instead of introducing molecular changes by the applied electrode potential, the highly effective SERS intensity tuning was achieved by the chemical composition transformation of the ultrathin Ag shell from metallic Ag to insulating AgCl. The electrode potential-induced electromagnetic enhancement (EME) tuning in the Au@Ag NPoNE structure has been confirmed by finite-difference time-domain simulations. Moreover, the specific Raman band associated with Ag-molecule interaction can also be tuned by the electrode potential. Therefore, we demonstrated that the electrode potential could effectively and reversibly modulate both EME and chemical enhancement in Au@Ag NPoNE structures.

Ultrasensitive SERS detection of antitumor drug methotrexate based on modified Ag substrate

Methotrexate (MTX) is a drug with broad-spectrum antitumor activity that is of great importance in therapeutic drug monitoring applications. In this essay, the two-step modified concentrated Ag colloid with the assistance of KF and MgSO₄ was used as the SERS active substrate for the ultrasensitive detection of MTX and its commercial formulations (tablets). It can be found that the two-step modification of the samples is a crucial procedure to remove the by-products in the synthesis of Ag colloid and further concentrate the Ag colloid. Under the optimal detection conditions, the minimum detection concentration of MTX is 1 × 10^-16 mol/L. And, there is a good linear relationship over a wide concentration range of 1 × 10^-16-1 × 10^-6 mol/L. The labelled amounts of the two manufacturers of MTX commercial tablets are in the range of 96.4-104.3% with the RSDs between 1.8% and 3.5% by this method, which are in accordance with the methodological requirements. These results prove that the proposed SERS method exhibits a good reproducibility and ultra-high sensitivity for the detection of the antitumor drug MTX.

Cascaded nanooptics to probe microsecond atomic-scale phenomena

Plasmonic nanostructures can focus light far below the diffraction limit, and the nearly thousandfold field enhancements obtained routinely enable few- and single-molecule detection. However, for processes happening on the molecular scale to be tracked with any relevant time resolution, the emission strengths need to be well beyond what current plasmonic devices provide. Here, we develop hybrid nanostructures incorporating both refractive and plasmonic optics, by creating SiO₂ nanospheres fused to plasmonic nanojunctions. Drastic improvements in Raman efficiencies are consistently achieved, with (single-wavelength) emissions reaching 10⁷ counts⋅mW^-1⋅s^-1 and 5 × 10⁵ counts∙mW^-1∙s^-1∙molecule^-1, for enhancement factors >10¹¹ We demonstrate that such high efficiencies indeed enable tracking of single gold atoms and molecules with 17-µs time resolution, more than a thousandfold improvement over conventional high-performance plasmonic devices. Moreover, the obtained (integrated) megahertz count rates rival (even exceed) those of luminescent sources such as single-dye molecules and quantum dots, without bleaching or blinking.

Surface-enhanced Raman spectroscopy: benefits, trade-offs and future developments

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique with sensitivity down to the single molecule level that provides fine molecular fingerprints, allowing for direct identification of target analytes. Extensive theoretical and experimental research, together with continuous development of nanotechnology, has significantly broadened the scope of SERS and made it a hot research field in chemistry, physics, materials, biomedicine, and so on. However, SERS has not been developed into a routine analytical technique, and continuous efforts have been made to address the problems preventing its real-world application. The present minireview focuses on analyzing current and potential strategies to tackle problems and realize the SERS performance necessary for translation to practical applications.

A Review of Chinese Raman Spectroscopy Research Over the Past Twenty Years

This paper introduces the major Chinese research groups in the fields of biomedicine, food safety, environmental testing, material research, archaeological and cultural relics, gem identification, forensic science, and other research areas of Raman spectroscopy and combined methods spanning the two decades from 1997 to 2017. Briefly summarized are the research directions and contents of the major Chinese Raman spectroscopy research groups, giving researchers engaged in Raman spectroscopy research a more comprehensive understanding of the state of Chinese Raman spectroscopy research and future development trends to further develop Raman spectroscopy and its applications.

Present and Future of Surface-Enhanced Raman Scattering

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

Shell isolated nanoparticle enhanced Raman spectroscopy for renewable energy electrocatalysis

This review covers the use of shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) to investigate heterogeneous electrocatalytic processes.

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.

An In situ Raman study of intermediate adsorption engineering by high-index facet control during the hydrogen evolution reaction

An in situ Raman study of the mechanism of HER catalytic performance enhanced by high-index facets on Ti@TiO₂ nanosheets.

Multibranch Gold Nanoparticles as Surface-Enhanced Raman Spectroscopy Substrates for Rapid and Sensitive Analysis of Fipronil in Eggs

A sensitive strategy to rapidly detect fipronil residues in eggs using multibranch gold nanoparticles (AuNPs) as the substrate of surface-enhanced Raman spectroscopy (SERS) was investigated in this study. Under optimized conditions, fipronil molecules preferentially deposited on the multibranch gold nanoparticles with preferential (111) facet-oriented growth due to its low surface energy. This anisotropic growth promoted the increase of SERS "hot spots", inducing a huge enhancement of Raman signals of the fipronil. An external standard calibration method was employed for quantitative analysis, and the method was validated for linearity, sensitivity, repeatability and recovery. Good linearity were found in the concentration range of 10 ng/L~10 mg/L in fipronil acetone solution (R2 = 0.9916) and 8 × 10-5 mg/m2 to 0.8 mg/m2 on eggshells (R2 = 0.9906), respectively. The recovery rate based on acetone recovered fipronil on eggshells and in egg liquids was 80.13%~87.87%, and 81.34%~88.89%, respectively. The SERS assay was successfully used to monitor fipronil in eggs.

Observing Transient Bipolar Electrochemical Coupling on Single Nanoparticles Translocating through a Nanopore

We report the observation of transient bipolar electrochemical coupling on freely moving 40 nm silver nanoparticles. The use of an asymmetric nanoelectrochemical environment at the nanopore orifice, for example, an acid inside the pipette and halide ions in the bulk, enabled us to observe unusually large current blockages of single Ag nanoparticles. We attribute these current blockages to the formation of H₂ nanobubbles on the surface of Ag nanoparticles due to the coupled faradaic reactions, in which the reduction of protons and water is coupled to the oxidation of Ag and water under potentials higher than 1 V. The appearance of large current blockages was strongly dependent on the applied voltage and the choice of anions in the bulk solution. The correlation between large current blockages with the oxidation of Ag nanoparticles and their nanopore translocation was further supported by simultaneous fluorescence and electric recordings. This study demonstrates that transient bipolar electrochemistry can take place on small metal nanoparticles below 50 nm when they pass through nanopores where the electric field is highly localized. The use of a nanopore and the resistive-pulse sensing method to study transient bipolar electrochemistry of nanoparticles may be extended to future studies in ultrafast electrochemistry, nanocatalyst screening, and gas nucleation on nanoparticles.

Facet-Dependent Reduction Reaction of Diruthenium Metal-String Complexes by Face-to-Face Linked Gold Nanocrystals

The facet-dependent redox reactions of diruthenium metal-string complexes by gold nanoparticles (AuNPs) are explored by using the surface-enhanced Raman scattering (SERS) technique. Gold nano-rhombic dodecahedrons (AuRDs), gold nanocubes (AuNCs), and gold octahedrons (AuOhs) with exclusive facets {110}, {100}, and {111}, respectively, were synthesized. These AuNPs linked face-to-face by metal-string complexes Ru₂M(dpa)₄Cl₂ (dpa = dipyridyl amino, M = Ni, Cu) with chloride axial ligands serve as both SERS substrates and reducing agents in the reactions. We employ the diruthenium core in these complexes with multiple redox states to study the reduction ability of varied AuNP facets upon plasmonic excitation. In Ru₂Ni(dpa)₄Cl₂, the Ru-Ru stretching mode ν_Ru-Ru str. lies at 327 cm^-1 on the SERS substrate AuOh, but this band shifts to 313 cm^-1 on the AuRD and AuNC. The diruthenium moiety was reduced to [Ru₂]⁴⁺ by the AuRD facet {110} and the AuNC {100}. The gold nanorods in the solution prepared with metal-string complexes bridging head-to-head on {111} facets were used for the SERS substrate. The SERS curves of the complexes in these self-assembled head-to-head rods display ν_Ru-Ru str. at 327 cm^-1, which is assigned to having an [Ru₂]⁵⁺ core. Hence, facets {110} and {100} have a reduction reactivity greater than that of {111}. In Ru₂Cu(dpa)₄Cl₂, the ν_Ru-Ru str. is observed to lie at 312 cm^-1 on AuRD, but shifts to 320 cm^-1 on the AuNC and AuOh. In the latter cases, the diruthenium moiety was reduced to having a charge of 4+ with electronic configuration π*²δ*², whereas the former case band at 312 cm^-1 with a weaker Ru-Ru bonding is also attributed to [Ru₂]⁴⁺ but with electron configuration π*⁴. π*⁴ lies at an energy greater than π*²δ*². The electrochemical SERS spectra of diruthenium complexes were recorded to verify their oxidation states. Conclusively, these results yield the reduction reactivity of the following facet: {110} > {100} > {111}. According to the results of the redox reactions, the valence states of the diruthenium metal-string complexes are verified. In the [Ru₂] ⁿ⁺ core, n = 4 π*⁴, 4 π*²δ*², 5 π*²δ*, and 6 π*δ*, and the ν_Ru-Ru str. is 312, 320, 327, and 337 cm^-1, respectively.

Electrochemical Reflective Absorption Microscopy for Probing the Local Diffusion Behavior in the Electrochemical Interface

Electrochemical interfaces determine the performance of electrochemical devices, including energy-related systems. An in-depth understanding of the heterogeneous interfaces requires in situ techniques with high sensitivity and high temporal and spatial resolution. We develop here an electrochemical reflective absorption microscope (EC-RAM) by using the absorption signals of reacting species with a reasonably good spatial resolution and high sensitivity. We systematically study the response of absorbance ( A) and its derivative, i.e. d A/d t, at different positions of the electrode surface and at electrodes with different sizes (50 μm, 500 μm, and 2 mm) both experimentally and theoretically. We find that the derivative cyclic voltabsorptometry (DCVA) frequently used to obtain the local current response in conventional electrochemical optical microscopy techniques is only applicable to reactions of surface species or solution species under linear diffusion control. For processes when the radial diffusion cannot be ignored, as in the case of a microelectrode or the edge of a large electrode, the DCVA curves show distinct diffusion behaviors for the electroactive species in different regions of the electrode, which cannot be directly related to the CV curves. When the radial diffusion dominates the reaction, CVA curves follow the same shape as the CV curves. The developed EC-RAM technique can be applied to extract in situ the local response of an electrochemical system during the dynamic reaction processes.

Hot Holes Assist Plasmonic Nanoelectrode Dissolution

Strong light-absorbing properties allow plasmonic metal nanoparticles to serve as antennas for other catalysts to function as photocatalysts. To achieve plasmonic photocatalysis, the hot charge carriers created when light is absorbed must be harnessed before they decay through internal relaxation pathways. We demonstrate the role of photogenerated hot holes in the oxidative dissolution of individual gold nanorods with millisecond time resolution while tuning charge-carrier density and photon energy using snapshot hyperspectral imaging. We show that light-induced hot charge carriers enhance the rate of gold oxidation and subsequent electrodissolution. Importantly, we distinguish how hot holes generated from interband transitions versus hot holes around the Fermi level contribute to photooxidative dissolution. The results provide new insights into hot-hole-driven processes with relevance to photocatalysis while emphasizing the need for statistical descriptions of nonequilibrium processes on innately heterogeneous nanoparticle supports.

In Situ Electrochemical Tip-Enhanced Raman Spectroscopy with a Chemically Modified Tip

Chemically modified tips in scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been used to improve the imaging resolution or provide richer chemical information, mostly in ultrahigh vacuum (UHV) environments. Tip-enhanced Raman spectroscopy (TERS) is a nanoscale spectroscopic technique that already provides chemical information and can provide subnanometer spatial resolution. Chemical modification of TERS tips has mainly been focused on increasing their lifetimes for ambient and in situ experiments. Under UHV conditions, chemical functionalization has recently been carried out to increase the amount of chemical information provided by TERS. However, this strategy has not yet been extended to in situ electrochemical (EC)-TERS studies. The independent control of the tip and sample potentials offered by EC-STM allows us to prove the in situ functionalization of a tip in EC-STM-TERS. Additionally, the Raman response of chemically modified TERS tips can be switched on and off at will, which makes EC-STM-TERS an ideal platform for the development of in situ chemical probes on the nanoscale.

Facile fabrication of microfluidic surface-enhanced Raman scattering devices via lift-up lithography

We describe a facile and low-cost approach for a flexibly integrated surface-enhanced Raman scattering (SERS) substrate in microfluidic chips. Briefly, a SERS substrate was fabricated by the electrostatic assembling of gold nanoparticles, and shaped into designed patterns by subsequent lift-up soft lithography. The SERS micro-pattern could be further integrated within microfluidic channels conveniently. The resulting microfluidic SERS chip allowed ultrasensitive in situ SERS monitoring from the transparent glass window. With its advantages in simplicity, functionality and cost-effectiveness, this method could be readily expanded into optical microfluidic fabrication for biochemical applications.

Silicon nanohybrid-based SERS chips armed with an internal standard for broad-range, sensitive and reproducible simultaneous quantification of lead(ii) and mercury(ii) in real systems

Lead ions (Pb²⁺) and mercury ions (Hg²⁺), the two commonly coexisting heavy metal ions, pose severe risks to environment and human health. To date, no surface-enhanced Raman scattering (SERS) sensor has been reported for the simultaneous quantification of Pb²⁺ and Hg²⁺ in real systems. Herein, the first demonstration of SERS chips for simultaneous quantification of Pb²⁺ and Hg²⁺ in real systems is presented based on the combination of reproducible silicon nanohybrid substrates and a corrective internal standard (IS) sensing strategy. This chip was made of a silver nanoparticle-decorated silicon wafer via modification of the IS, i.e. 4-aminothiophenol, molecules. The as-prepared chip was further functionalized with Pb²⁺- and Hg²⁺- specific DNA strands capable of simultaneously detecting Pb²⁺ and Hg²⁺. Quantitatively, upon correction by the IS Raman signals, the broad dynamic ranges from 100 pM to 10 μM for Pb²⁺ and from 1 nM to 10 μM for Hg²⁺ were achieved, with the detection limit down to 19.8 ppt for Pb²⁺ and 168 ppt for Hg²⁺. For real applications, we further demonstrated that Pb²⁺ and Hg²⁺ spiked into industrial wastewater could be readily distinguished via the presented chip, and the relative standard deviation (RSD) value was less than ∼15%. More significantly, the resulting SERS chip can be well coupled with a hand-held Raman instrument and can then be used for the qualitative analysis of both Pb²⁺ and Hg²⁺ in real systems in a portable manner. Our results suggest that this high-quality SERS chip is a powerful tool for on-site detection of various heavy metal ions in real samples in the field of food safety and environment protection.

Surface-enhanced Raman spectroscopy: bottlenecks and future directions

This feature article discusses developmental bottleneck issues in surface Raman spectroscopy in its early stages and surface-enhanced Raman spectroscopy (SERS) in the past four decades and future perspectives.