Investigating Nanoscale Electrochemistry with Surface- and Tip-Enhanced Raman Spectroscopy

Physical Adsorption and Raman Spectra of Hydrazine Hydrate on the Graphene Surface

In experimental studies, hydrazine hydrate is widely employed as a reducing agent for the conversion of graphene oxide to graphene. Herein, we conducted theoretical calculations using cluster models to investigate the adsorption behavior of hydrazine hydrate on the surface of graphene. The calculated adsorption energy reveals that hydrazine hydrate can physically bind to the graphene surface. Our findings indicate that two hydrogen bonds stabilize the hydrazine hydrate molecule, while its adsorption onto the graphene surface is primarily driven by van der Waals forces. By combining computational simulations and experimental measurements, we thoroughly examined the Raman spectra of both free and adsorbed hydrazine hydrates, which enabled us to gain detailed insights into their molecular vibrations. Notably, in the Raman spectra of free hydrazine hydrate, a strong peak at around 3300 cm-1 corresponds to the NH2 vibration. Similarly, peaks near 3300 cm-1 were observed in the Raman spectra of graphene with adsorbed hydrazine hydrate molecules. The results are expected to provide valuable references for future experimental investigations involving hydrazine hydrate.

Surface-Enhanced Raman Scattering-Based Multimodal Techniques: Advances and Perspectives

Surface-enhanced Raman scattering (SERS) spectroscopy is a versatile molecular fingerprinting technique with rapid signal readout, high aqueous compatibility, and portability. To translate SERS for real-world applications, it is pertinent to overcome inherent challenges, including high sample variability and heterogeneity, matrix effects, and nonlinear SERS signal responses of different analytes in complex (bio)chemical matrices with numerous interfering species. In this perspective, we highlight emerging SERS-based multimodal techniques to address the key roadblocks to improving the sensitivity, specificity, and reliability of (bio)chemical detection, bioimaging, theragnosis, and theragnostic. SERS-based multimodal techniques can be broadly categorized into two categories: (1) complementary methods or systems that work together to achieve a common goal where each method compensates for the weaknesses of the other to culminate in a single enhanced outcome or (2) orthogonal techniques that are independent and provide separate but corroborating results simultaneously without interfering with each other. These multimodal techniques maximize information gained from a single experiment to achieve enhanced qualitative or quantitative analysis and broaden the range of detectable analytes from small molecules to tissues. Finally, we discuss emerging directions in multimodal platform design, instrument integration, and data analytics that aim to push the analytical limits of holistic detection.

Nanoscale chemical characterization of materials and interfaces by tip-enhanced Raman spectroscopy

Materials and their interfaces are the core for the development of a large variety of fields, including catalysis, energy storage and conversion. In this case, tip-enhanced Raman spectroscopy (TERS), which combines scanning probe microscopy with plasmon-enhanced Raman spectroscopy, is a powerful technique that can simultaneously obtain the morphological information and chemical fingerprint of target samples at nanometer spatial resolution. It is an ideal tool for the nanoscale chemical characterization of materials and interfaces, correlating their structures with chemical performances. In this review, we begin with a brief introduction to the nanoscale characterization of materials and interfaces, followed by a detailed discussion on the recent theoretical understanding and technical improvements of TERS, including the origin of enhancement, TERS instruments, TERS tips and the application of algorithms in TERS. Subsequently, we list the key experimental issues that need to be addressed to conduct successful TERS measurements. Next, we focus on the recent progress of TERS in the study of various materials, especially the novel low-dimensional materials, and the progresses of TERS in studying different interfaces, including both solid-gas and solid-liquid interfaces. Finally, we provide an outlook on the future developments of TERS in the study of materials and interfaces.

In Situ Characterization Techniques for Electrochemical Nitrogen Reduction Reaction

The electrochemical reduction of nitrogen to produce ammonia is pivotal in modern society due to its environmental friendliness and the substantial influence that ammonia has on food, chemicals, and energy. However, the current electrochemical nitrogen reduction reaction (NRR) mechanism is still imperfect, which seriously impedes the development of NRR. In situ characterization techniques offer insight into the alterations taking place at the electrode/electrolyte interface throughout the NRR process, thereby helping us to explore the NRR mechanism in-depth and ultimately promote the development of efficient catalytic systems for NRR. Herein, we introduce the popular theories and mechanisms of the electrochemical NRR and provide an extensive overview on the application of various in situ characterization approaches for on-site detection of reaction intermediates and catalyst transformations during electrocatalytic NRR processes, including different optical techniques, X-ray-based techniques, electron microscopy, and scanning probe microscopy. Finally, some major challenges and future directions of these in situ techniques are proposed.

Characteristic fingerprint spectrum of α-synuclein mutants on terahertz time-domain spectroscopy

α-Synuclein, a presynaptic neuronal protein encoded by the SNCA gene, is involved in the pathogenesis of Parkinson's disease. Point mutations and multiplications of α-synuclein (A30P and A53T) are correlated with early-onset Parkinson's disease characterized by rapid progression and poor prognosis. Currently, the clinical identification of SNCA variants, especially disease-related A30P and A53T mutants, remains challenging and also time consuming. This study aimed to develop a novel label-free detection method for distinguishing the SNCA mutants using transmission terahertz (THz) time-domain spectroscopy. The protein was spin-coated onto the quartz to form a thin film, which was measured using THz time-domain spectroscopy. The spectral characteristics of THz broadband pulse waves of α-synuclein protein variants (SNCA wild type, A30P, and A53T) at different frequencies were analyzed via Fourier transform. The amplitude A intensity (AWT, AA30P, and AA53T) and peak occurrence time in THz time-domain spectroscopy sensitively distinguished the three protein variants. The phase φ difference in THz frequency domain followed the trend of φWT > φA30P > φA53T. There was a significant difference in THz frequency amplitude A' corresponding to the frequency ranging from 0.4 to 0.66 THz (A'A53T > A'A30P > A'WT). At a frequency of 0.4-0.6 THz, the transmission T of THz waves distinguished three variants (TA53T > TA30P > TWT), whereas there was no difference in the transmission T at 0.66 THz. The SNCA wild-type protein and two mutant variants (A30P and A53T) had distinct characteristic fingerprint spectra on THz time-domain spectroscopy. This novel label-free detection method has great potential for the differential diagnosis of Parkinson's disease subtypes.

Controllable preparation of Ti3C2Tx/Ag composite as SERS substrate for ultrasensitive detection of 4-nitrobenzenethiol

Currently, metal carbonitride (MXene) has been identified as a hot research topic in the research area of surface-enhanced Raman scattering (SERS). In this study, Ti3C2Tx/Ag composite was fabricated as SERS substrate with different Ag contents. The fabricated Ti3C2Tx/Ag composites show good SERS behavior by detecting 4-Nitrobenzenethiol (4-NBT) probe molecules. Through calculation, the SERS enhancement factor (EF) of the Ti3C2Tx/Ag substrate was as high as 4.15 × 106. It is worth noting that the detection limit of 4-NBT probe molecules can be achieved ultralow concentration of 10-11 M. In this system, electromagnetic enhancement mechanism and chemical enhancement mechanism have synergistic effects on SERS phenomenon. Meanwhile, the Ti3C2Tx/Ag composite substrate exhibited good SERS reproducibility. In addition, the SERS detection signal hardly changed after 6 months of natural standing, and the substrate showed good stability. This work suggests that the Ti3C2Tx/Ag substrate could be used as a sensitivity SERS sensor for practical application, and could be applied in the field of environmental monitoring.

Quantifying the ultrafast and steady-state molecular reduction potential of a plasmonic photocatalyst

Plasmonic materials are promising photocatalysts as they are well suited to convert light into hot carriers and heat. Hot electron transfer is suggested as the driving force in many plasmon-driven reactions. However, to date, there are no direct molecular measures of the rate and yield of plasmon-to-molecule electron transfer or energy of these electrons on the timescale of plasmon decay. Here, we use ultrafast and spectroelectrochemical surface-enhanced Raman spectroscopy to quantify electron transfer from a plasmonic substrate to adsorbed methyl viologen molecules. We observe a reduction yield of 2.4 to 3.5% on the picosecond timescale, with plasmon-induced potentials ranging from [Formula: see text]3.1 to [Formula: see text]4.5 mV. Excitingly, some of these reduced species are stabilized and persist for tens of minutes. This work provides concrete metrics toward optimizing material-molecule interactions for efficient plasmon-driven photocatalysis.

Solid-State Au Nanocone Arrays Substrate for Reliable SERS Profiling of Serum for Disease Diagnosis

Surface-enhanced Raman scattering (SERS) is a widely used rapid and noninvasive method for detecting biological substances in serum samples and is commonly employed in disease screening and diagnosis. Solid-state nanoarray SERS substrates used in serum detection may cause spectral instability due to imperfections in the detection method. For the purpose of identifying optimal detection conditions, various dilution levels of the serum were tested in this study. The study found that a complete and stable serum SERS spectrum can be obtained when the serum is diluted by a factor of 50. The study reports the successful preparation of an Au nanocone array (Au NCA) plasmonic substrate with a uniform, controllable microstructure and high activity, achieved through a combination of PS colloidal sphere template-assisted reactive ion etching (RIE) process and magnetron sputtering deposition technology. Based on this substrate, a standard detection scheme was developed to obtain highly stable and repeatable serum SERS spectra. The study verified the reliability of the optimized serum detection scheme by comparing the SERS spectra of serum samples from healthy individuals and gastric cancer patients, and confirmed the potential benefits of the scheme for disease screening and diagnosis.

Raman spectroscopy of optical-trapped single particle using bull's eye nanostructure

Surface-enhanced Raman spectroscopy (SERS) has enabled single nanoparticle Raman sensing with abundant applications in analytical chemistry, biomaterials, and environmental monitoring. Genuine single particle Raman sensing requires a cumbersome technique, such as atomic force microscopy (AFM) based tip-enhanced Raman spectroscopy; SERS-based single particle Raman sensing still collects an ensemble signal that samples, in principle, a number of particles. Here, we develop in situ Raman-coupled optical tweezers, based on a hybrid nanostructure consisting of a single bowtie aperture surrounded by bull's eye rings, to trap and excite a rhodamine-6G-dye-doped polystyrene sphere. We simulated a platform to ensure sufficient enhancement capability for both optical trapping and SERS of a single nanoparticle. Experiments with well-designed controls clearly attribute the Raman signal origin to a single 15-nm particle trapped at the center of a nanohole, and they also clarified the trapping and Raman enhancement role of the bull's eye rings. We claim Raman sensing of a smallest optically trapped particle.

Emerging Optical Microscopy Techniques for Electrochemistry

An optical microscope is probably the most intuitive, simple, and commonly used instrument to observe objects and discuss behaviors through images. Although the idea of imaging electrochemical processes operando by optical microscopy was initiated 40 years ago, it was not until significant progress was made in the last two decades in advanced optical microscopy or plasmonics that it could become a mainstream electroanalytical strategy. This review illustrates the potential of different optical microscopies to visualize and quantify local electrochemical processes with unprecedented temporal and spatial resolution (below the diffraction limit), up to the single object level with subnanoparticle or single-molecule sensitivity. Developed through optically and electrochemically active model systems, optical microscopy is now shifting to materials and configurations focused on real-world electrochemical applications.

Multi-dimensional designer catalysts for negative emissions science (NES): bridging the gap between synthesis, simulations, and analysis

Negative emissions technologies will play a critical role in limiting global warming to sustainable levels. Electrocatalytic and/or photocatalytic CO2 reduction will likely play an important role in this field moving forward, but efficient, selective catalyst materials are needed to enable the widespread adoption of these processes. The rational design of such materials is highly challenging, however, due to the complexity of the reactions involved as well as the large number of structural variables which can influence behavior at heterogeneous interfaces. Currently, there is a significant disconnect between the complexity of materials systems that can be handled experimentally and those that can be modeled theoretically with appropriate rigor and bridging these gaps would greatly accelerate advancements in the field of Negative Emissions Science (NES). Here, we present a perspective on how these gaps between materials design/synthesis, characterization, and theory can be resolved, enabling the rational development of improved materials for CO2 conversion and other NES applications.

Novel Clarification of Surface Plasmon Coupling Reactions of Aromatic Alkynamine and Nitro Compounds

This work presents a theoretical and experimental approach for the coupling of 4-ethynylaniline (4-APA) and 4-ethynylnitrobenzene (4-NPA) in the theoretical application of density functional theory (DFT) and experimental monitoring of surface-enhanced Raman spectroscopy (SERS). The results support electromagnetic enhancement to drive the conversion of aromatic alkynamine and nitro compounds and regulation by the catalytic coupling reaction conditions. In addition, this work investigates the adsorption site effect of surface plasmon coupling reactions of 4-APA and 4-NPA molecules into alkynyl azo compounds. This study presents theoretical and experimental images used to analyze the plasmon-driven surface catalytic reaction system.

Suppressing Sulfite Dimerization at a Polarized Gold Electrode/Water Solution Interface for High-Quality Gold Electrodeposition

Solid/liquid interfacial structure occupies great importance in chemistry, biology, and materials. In this paper, by combining EC-SERS study and DFT calculation, we reveal the adsorption and dimerization of sulfite (SO₃^2-) at a gold electrode/water solution interface, and establish an adsorption displacement strategy to suppress the dimerization of sulfite. At the gold electrode/sodium sulfite solution interface, at least two layers of SO₃^2- anions are adsorbed on the electrode surface. As the applied potential shifts negatively, the adsorption strength of the first SO₃^2- layer is weakened gradually and then is dimerized with the second orientated SO₃^2- layer to form S₂O₅^2-, and S₂O₅^2- is further reduced to S₂O₃^2-. After hydroxyethylene disphosphonic acid (HEDP) is introduced to the gold electrode/sodium sulfite solution interface, the second oriented SO₃^2- layer is replaced by a HEDP coadsorption layer. This results in the first layer of SO₃^2- being desorbed directly without any structural transformation or chemical reaction as the potential shifts negatively. The suppression of sulfite dimerization by HEDP is more clear at the gold electrode/gold sulfite solution interface owing to the electroreduction of gold ions. Furthermore, the electrochemical studies and electrodeposition experiments show that as the sulfite dimerization reaction is suppressed, the electroreduction of gold ions is accelerated, and the deposited gold coating is bright and dense with finer grains.

Single-nanoparticle spectroelectrochemistry studies enabled by localized surface plasmon resonance

This review describes recent progress of spectroelectrochemistry (SEC) analysis of single metallic nanoparticles (NPs) which have strong surface plasmon resonance properties. Dark-field scattering (DFS), photoluminescence (PL), and electrogenerated chemiluminescence (ECL) are three commonly used optical methods to detect individual NPs and investigate their local redox activities in an electrochemical cell. These SEC methods are highly dependent on a strong light-scattering cross-section of plasmonic metals and their electrocatalytic characteristics. The surface chemistry and the catalyzed reaction mechanism of single NPs and their chemical transformations can be studied using these SEC methods. Recent progress in the experimental design and fundamental understanding of single-NP electrochemistry and catalyzed reactions using DFS, PL, and ECL is described along with selected examples from recent publications in this field. Perspectives on the challenges and possible solutions for these SEC methods and potential new directions are discussed.

Surface-Enhanced Raman Spectroscopy-Scanning Electrochemical Microscopy: Observation of Real-Time Surface pH Perturbations

Understanding and controlling chemical dynamics at electrode interfaces is key to electrochemical applications in sensing, electrocatalysis, and energy storage. Here, we introduce colocalized surface-enhanced Raman scattering-scanning electrochemical microscopy (SERS-SECM) as a multimodal tool able to simultaneously probe and affect electrochemical interfaces in real time. As a model system to demonstrate SERS-SECM, we used a self-assembled monolayer of 4-mercaptopyridine (4MPy), a pH sensitive Raman indicator, anchored to silver nanoparticles as a substrate. We modulated the local pH at the surface with chronoamperometry, inducing the hydrogen evolution reaction (HER) at the SECM tip and observed subsequent Raman peak height changes in the 4MPy. We then performed cyclic voltammetry of HER at the SECM tip while measuring SERS spectra every 200 ms to highlight the technique's real-time capabilities. Our results show the capability to sensitively interrogate and trigger chemical/electrochemical dynamic surface phenomena. We hope SERS-SECM will provide insight on the link between heterogeneous and homogeneous reactivity at electrochemical interfaces.

In Situ Surface-Enhanced Raman Spectroscopy Characterization of Electrocatalysis with Different Nanostructures

As energy demands increase, electrocatalysis serves as a vital tool in energy conversion. Elucidating electrocatalytic mechanisms using in situ spectroscopic characterization techniques can provide experimental guidance for preparing high-efficiency electrocatalysts. Surface-enhanced Raman spectroscopy (SERS) can provide rich spectral information for ultratrace surface species and is extremely well suited to studying their activity. To improve the material and morphological universalities, researchers have employed different kinds of nanostructures that have played important roles in the development of SERS technologies. Different strategies, such as so-called borrowing enhancement from shell-isolated modes and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS)-satellite structures, have been proposed to obtain highly effective Raman enhancement, and these methods make it possible to apply SERS to various electrocatalytic systems. Here, we discuss the development of SERS technology, focusing on its applications in different electrocatalytic reactions (such as oxygen reduction reactions) and at different nanostructure surfaces, and give a brief outlook on its development.

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.

Vertical Diffusion of Ions within Single Particles during Electrochemical Charging

Determining the trajectory of ionic transport and diffusion within single electroactive nanomaterials is critical for understanding the charging kinetics and capacity fading associated with ion batteries, with implications for rational design of excellent-performance electrode materials. While the horizontal pathway of mass transport has been feasibly investigated by optical superlocalization methods and electron microscopes, determination on the vertical trajectory has proven a more challenging task. Herein, we developed dual-angle total internal reflection microscopy by simultaneously introducing different angle-dependent illumination depths to trace the optical centroid shifts of nano-objects in the vertical dimension. We first demonstrated the proof of concept by resolving the vertical moving trails of a nanosphere doing Brownian motion and subsequently explored the picture of mass transport in the interior of single Prussian blue (PB) particles during electrochemical cycling. The results indicated that the vertical centroids of single PB particles remained unchanged when ions were inserted or extracted, suggesting an outside-in ionic transport pathway instead of bottom-up trajectory that one would intuitively expect.

Recent progress in mycotoxins detection based on surface-enhanced Raman spectroscopy

Mycotoxins are toxic compounds naturally produced by certain types of fungi. The contamination of mycotoxins can occur on numerous foodstuffs, including cereals, nuts, fruits, and spices, and pose a major threat to humans and animals by causing acute and chronic toxic effects. In this regard, reliable techniques for accurate and sensitive detection of mycotoxins in agricultural products and food samples are urgently needed. As an advanced analytical tool, surface-enhanced Raman spectroscopy (SERS), presents several major advantages, such as ultrahigh sensitivity, rapid detection, fingerprint-type information, and miniaturized equipment. Benefiting from these merits, rapid growth has been observed under the topic of SERS-based mycotoxin detection. This review provides a comprehensive overview of the recent achievements in this area. The progress of SERS-based label-free detection, aptasensor, and immunosensor, as well as SERS combined with other techniques, has been summarized, and in-depth discussion of the remaining challenges has been provided, in order to inspire future development of translating the techniques invented in scientific laboratories into easy-to-operate analytic platforms for rapid detection of mycotoxins.

Nanoelectrochemistry in the study of single-cell signaling

Label-free biosensing has been the dream of scientists and biotechnologists as reported by Vollmer and Arnold (Nat Methods 5:591-596, 2008). The ability of examining living cells is crucial to cell biology as noted by Fang (Int J Electrochem 2011:460850, 2011). Chemical measurement with electrodes is label-free and has demonstrated capability of studying living cells. In recent years, nanoelectrodes of different functionality have been developed. These nanometer-sized electrodes, coupled with scanning electrochemical microscopy (SECM), have further enabled nanometer spatial resolution study in aqueous environments. Developments in the field of nanoelectrochemistry have allowed measurement of signaling species at single cells, contributing to better understanding of cell biology. Leading studies using nanoelectrochemistry of a variety of cellular signaling molecules, including redox-active neurotransmitter (e.g., dopamine), non-redox-active neurotransmitter (e.g., acetylcholine), reactive oxygen species (ROS), and reactive nitrogen species (RNS), are reviewed here.

In Situ Spectroscopic Methods for Electrocatalytic CO2 Reduction

Electrochemical reduction of CO2 to value-added chemicals and fuels is a promising approach to store renewable energy while closing the anthropogenic carbon cycle. Despite significant advances in developing new electrocatalysts, this system still lacks enough energy conversion efficiency to become a viable technology for industrial applications. To develop an active and selective electrocatalyst and engineer the reaction environment to achieve high energy conversion efficiency, we need to improve our knowledge of the reaction mechanism and material structure under reaction conditions. In situ spectroscopies are among the most powerful tools which enable measurements of the system under real conditions. These methods provide information about reaction intermediates and possible reaction pathways, electrocatalyst structure and active sites, as well as the effect of the reaction environment on products distribution. This review aims to highlight the utilization of in situ spectroscopic methods that enhance our understanding of the CO2 reduction reaction. Infrared, Raman, X-ray absorption, X-ray photoelectron, and mass spectroscopies are discussed here. The critical challenges associated with current state-of-the-art systems are identified and insights on emerging prospects are discussed.

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.

Spatially resolved electrochemistry enabled by thin-film optical interference

Electrochemical reactions occurring on the local surface can be spatially resolved by successive interferometric imaging of the nanochannel membrane coated electrode.

Single-molecule biosensors: Recent advances and applications

Single-molecule biosensors serve the unmet need for real time detection of individual biological molecules in the molecular crowd with high specificity and accuracy, uncovering unique properties of individual molecules which are hidden when measured using ensemble averaging methods. Measuring a signal generated by an individual molecule or its interaction with biological partners is not only crucial for early diagnosis of various diseases such as cancer and to follow medical treatments but also offers a great potential for future point-of-care devices and personalized medicine. This review summarizes and discusses recent advances in nanosensors for both in vitro and in vivo detection of biological molecules offering single-molecule sensitivity. In the first part, we focus on label-free platforms, including electrochemical, plasmonic, SERS-based and spectroelectrochemical biosensors. We review fluorescent single-molecule biosensors in the second part, highlighting nanoparticle-amplified assays, digital platforms and the utilization of CRISPR technology. We finally discuss recent advances in the emerging nanosensor technology of important biological species as well as future perspectives of these sensors.

Nanoscale electrochemical kinetics & dynamics: the challenges and opportunities of single-entity measurements

The development of nanoscale electrochemistry since the mid-1980s has been predominately coupled with steady-state voltammetric (i-E) methods. This research has been driven by the desire to understand the mechanisms of very fast electrochemical reactions, by electroanalytical measurements in small volumes and unusual media, including in vivo measurements, and by research on correlating electrocatalytic activity, e.g., O2 reduction reaction, with nanoparticle size and structure. Exploration of the behavior of nanoelectrochemical structures (nanoelectrodes, nanoparticles, nanogap cells, etc.) of a characteristic dimension λ using steady-state i-E methods generally relies on the well-known relationship, λ2 ∼ Dt, which relates diffusional lengths to time, t, through the coefficient, D. Decreasing λ, by performing measurements at a nanometric length scales, results in a decrease in the effective timescale of the measurement, and provides a direct means to probe the kinetics of steps associated with very rapid electrochemical reactions. For instance, steady-state voltammetry using a nanogap twin-electrode cell of characteristic width, λ ∼ 10 nm, allows investigations of events occurring at timescales on the order of ∼100 ns. Among many other advantages, decreasing λ also increases spatial resolution in electrochemical imaging, e.g., in scanning electrochemical microscopy, and allows probing of the electric double layer. This Introductory Lecture traces the evolution and driving forces behind the "λ2 ∼ Dt" steady-state approach to nanoscale electrochemistry, beginning in the late 1950s with the introduction of the rotating ring-disk electrode and twin-electrode thin-layer cells, and evolving to current-day investigations using nanoelectrodes, scanning nanocells for imaging, nanopores, and nanoparticles. The recent focus on so-called "single-entity" electrochemistry, in which individual and very short redox events are probed, is a significant departure from the steady-state approach, but provides new opportunities to probe reaction dynamics. The stochastic nature of very fast single-entity events challenges current electrochemical methods and modern electronics, as illustrated using recent experiments from the authors' laboratory.

Raman Techniques: Fundamentals and Frontiers

Driven by applications in chemical sensing, biological imaging and material characterisation, Raman spectroscopies are attracting growing interest from a variety of scientific disciplines. The Raman effect originates from the inelastic scattering of light, and it can directly probe vibration/rotational-vibration states in molecules and materials. Despite numerous advantages over infrared spectroscopy, spontaneous Raman scattering is very weak, and consequently, a variety of enhanced Raman spectroscopic techniques have emerged. These techniques include stimulated Raman scattering and coherent anti-Stokes Raman scattering, as well as surface- and tip-enhanced Raman scattering spectroscopies. The present review provides the reader with an understanding of the fundamental physics that govern the Raman effect and its advantages, limitations and applications. The review also highlights the key experimental considerations for implementing the main experimental Raman spectroscopic techniques. The relevant data analysis methods and some of the most recent advances related to the Raman effect are finally presented. This review constitutes a practical introduction to the science of Raman spectroscopy; it also highlights recent and promising directions of future research developments.

Correlation of surface-enhanced Raman scattering (SERS) with the surface density of gold nanoparticles: evaluation of the critical number of SERS tags for a detectable signal

The use of plasmonic nanotags based on the surface-enhanced Raman scattering (SERS) effect is highly promising for several applications in analytical chemistry, biotechnological assays and nanomedicine. To this end, a crucial parameter is the minimum number of SERS tags that allows for the collection of intense Raman signals under real operating conditions. Here, SERS Au nanotags (AuNTs) based on clustered gold nanoparticles are deposited on a substrate and analyzed in the same region using Raman spectroscopy and transmission electron microscopy. In this way, the Raman spectra and the surface density of the SERS tags are correlated directly, showing that 1 tag/µm² is enough to generate an intense signal above the noise level at 633 nm with an excitation power of only 0.65 mW and an acquisition time of just 1 s with a 50× objective. The AuNT density can be even lower than 1 tag/µm² when the acquisition time is extended to 10 s, but must be increased to 3 tags/µm² when a 20× objective is employed under the same excitation conditions. In addition, in order to observe a linear response, it was found that 10 SERS AuNTs inside the probed area are required. These findings indicate that a better signal-to-noise ratio requires high-magnification optics, while linearity versus tag number can be improved by using low-magnification optics or a high tag density. In general the suitability of plasmonic SERS labels for ultrasensitive analytical and biomedical applications is evident.

In Situ Nanoscale Redox Mapping Using Tip-Enhanced Raman Spectroscopy

Electrochemical atomic force microscopy tip-enhanced Raman spectroscopy (EC-AFM-TERS) was used for the first time to spatially resolve local heterogeneity in redox behavior on an electrode surface in situ and at the nanoscale. A structurally well-defined Au(111) nanoplate located on a polycrystalline ITO substrate was studied to examine nanoscale redox contrast across the two electrode materials. By monitoring the TERS intensity of adsorbed Nile Blue (NB) molecules on the electrode surface, TERS maps were acquired with different applied potentials. The EC-TERS maps showed a spatial contrast in TERS intensity between Au and ITO. TERS line scans near the edge of a 20 nm-thick Au nanoplate demonstrated a spatial resolution of 81 nm under an applied potential of -0.1 V vs Ag/AgCl. The intensities from the TERS maps at various applied potentials followed Nernstian behavior, and a formal potential ( E^0') map was constructed by fitting the TERS intensity at each pixel to the Nernst equation. Clear nanoscale spatial contrast between the Au and ITO regions was observed in the E^0' map. In addition, statistical analysis of the E^0' map identified a statistically significant 4 mV difference in E^0' on Au vs ITO. Electrochemical heterogeneity was also evident in the E^0' distribution, as a bimodal distribution was observed in E^0' on polycrystalline ITO, but not on gold. A direct comparison between an AFM friction image and the E^0' map resolved the electrochemical behavior of individual ITO grains with a spatial resolution of ∼40 nm. The variation in E^0' was attributed to different local surface charges on the ITO grains. Such site-specific electrochemical information with nanoscale spatial and few mV voltage resolutions is not available using ensemble spectroelectrochemical methods. We expect that in situ redox mapping at the nanoscale using EC-AFM-TERS will have a crucial impact on understanding the role of nanoscale surface features in applications such as electrocatalysis.

Spiers Memorial Lecture. Surface-enhanced Raman spectroscopy: from single particle/molecule spectroscopy to ångstrom-scale spatial resolution and femtosecond time resolution

Four decades on, surface-enhanced Raman spectroscopy (SERS) continues to be a vibrant field of research that is growing (approximately) exponentially in scope and applicability while pushing at the ultimate limits of sensitivity, spatial resolution, and time resolution. This introductory paper discusses some aspects related to all four of the themes for this Faraday Discussion. First, the wavelength-scanned SERS excitation spectroscopy (WS-SERES) of single nanosphere oligomers (viz., dimers, trimers, etc.), the distance dependence of SERS, the magnitude of the chemical enhancement mechanism, and the progress toward developing surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS) are discussed. Second, our efforts to develop a continuous, minimally invasive, in vivo glucose sensor based on SERS are highlighted. Third, some aspects of our recent work in single molecule SERS and the translation of that effort to ångstrom-scale spatial resolution in ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) and single molecule electrochemistry using electrochemical (EC)-TERS will be presented. Finally, we provide an overview of analytical SERS with our viewpoints on SERS substrates, approaches to address the analyte generality problem (i.e. target molecules that do not spontaneously adsorb and/or have Raman cross sections <10^-29 cm² sr^-1), SERS for catalysis, and deep UV-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.

An integrated experimental and theoretical study on the optical properties of uniform hairy noble metal nanoparticles

We report a viable route to plasmonic nanoparticles with well-controlled sizes, shapes, and compositions. A series of monodisperse Ag and Au nanoparticles capped with polystyrene chains (i.e., "hairy" nanoparticles) are crafted by capitalizing on star-like diblock copolymers as nanoreactors. Such monodisperse nanoparticles render an accurate absorption spectrum, providing a strong basis for theoretical investigation into their optical properties. By combining the experimental study with the three-dimensional finite element calculation of electromagnetic field distributions, the contributions of both intra-band and inter-band transitions to plasmonic absorption are revealed. The calculated absorption spectra perfectly reproduce the experimental observations, including the peak positions, shapes, and trends of peak shifting or broadening as a function of nanoparticle sizes. The influences of nanoparticle dimensions and surface ligands on plasmonic absorption of metallic nanoparticles are also systematically explored.

Preparation of surfaces of composite samples for tip based micro-analyses using ion beam milling

Ion beam milling, as a method of surface design for tip analytical techniques, was explored. A sample of clay, embedded in a resin, was treated by the ion beam and allowed AFM (a typical tip technique) to be successfully applied. The method is suitable for advanced tip analyses based on AFM, like TERS or SNOM, and for samples that are not possible to prepare by standard mechanical methods. The approach can be useful for characterisation of the surfaces of many different types of materials in versatile applications such as catalysis, corrosion science or advanced material characterisation.

A Rotaxane Scaffold Bearing Multiple Redox Centers: Synthesis, Surface Modification and Electrochemical Properties

A rotaxane scaffold incorporating two dithiolane anchoring units for the modification of gold surfaces has been functionalized with multiple copies of a redox unit, namely ferrocene. Surface modification has been first assessed at the single molecule level by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) imaging, while tip enhanced Raman spectroscopy (TERS) provided the local vibrational signature of the ferrocenyl subunits of the rotaxanes grafted onto the gold surface. Finally, oxidation of the redox moieties within a rotaxane scaffold grafted onto gold microelectrodes has been investigated by ultrafast cyclic voltammetry. Intramolecular electron hopping is indeed extremely fast in this system. Moreover, the kinetics of charge injection depends on the molecular coverage due to the influence of intermolecular contacts on molecular motions.

Tannic Acid/Fe3+/Ag Nanofilm Exhibiting Superior Photodynamic and Physical Antibacterial Activity

Silver nanoparticles (AgNPs) enwrapped in the biologically safe tannic acid (TA)/Fe³⁺ nanofilm are synthesized by an ultrafast, green, simple, and universal method. The physical antibacterial activity and photodynamic antibacterial therapy (PAT) efficacy of the TA/Fe³⁺/AgNPs nanofilm were investigated for the first time, which exhibited a strong physical antibacterial activity as well as great biocompatibility, through in vitro and in vivo studies. The results disclosed that this hybrid coating could possess high PAT capabilities upon irradiation under a visible light of 660 nm, which is longer than those of previously reported green and blue sensitization light, thus allowing deeper light penetration into biological tissues. Electron spin resonance (ESR) spectra proved that the PAT efficacy of the TA/Fe³⁺/AgNPs nanofilm was associated with the yields of singlet oxygen (¹O₂) under the irradiation of visible light (660 nm). A higher PAT efficiency of 100 and 94% against Escherichia coli and Staphylococcus aureus could be achieved within 20 min of illumination under 660 nm visible light, whereas the innate physical antibacterial activity of AgNPs could endow the implants with long-term prevention of bacterial infection. The mechanism of PAT may be associated with the formation of oxidative stress and oxidative damage to key biomolecules (proteins and lipids) in bacteria. Our results reveal that the synergistic action of both PAT and physical action of AgNPs in this hybrid nanofilm is an effective way to inactivate bacteria, with minimal side effects.

Plasmonic tunnel junctions for single-molecule redox chemistry

Nanoparticles attached just above a flat metallic surface can trap optical fields in the nanoscale gap. This enables local spectroscopy of a few molecules within each coupled plasmonic hotspot, with near thousand-fold enhancement of the incident fields. As a result of non-radiative relaxation pathways, the plasmons in such sub-nanometre cavities generate hot charge carriers, which can catalyse chemical reactions or induce redox processes in molecules located within the plasmonic hotspots. Here, surface-enhanced Raman spectroscopy allows us to track these hot-electron-induced chemical reduction processes in a series of different aromatic molecules. We demonstrate that by increasing the tunnelling barrier height and the dephasing strength, a transition from coherent to hopping electron transport occurs, enabling observation of redox processes in real time at the single-molecule level.