In Situ Nanoscale Redox Mapping Using Tip-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy: a half-century historical perspective

Surface-enhanced Raman spectroscopy (SERS) has evolved significantly over fifty years into a powerful analytical technique. This review aims to achieve five main goals. (1) Providing a comprehensive history of SERS's discovery, its experimental and theoretical foundations, its connections to advances in nanoscience and plasmonics, and highlighting collective contributions of key pioneers. (2) Classifying four pivotal phases from the view of innovative methodologies in the fifty-year progression: initial development (mid-1970s to mid-1980s), downturn (mid-1980s to mid-1990s), nano-driven transformation (mid-1990s to mid-2010s), and recent boom (mid-2010s onwards). (3) Illuminating the entire journey and framework of SERS and its family members such as tip-enhanced Raman spectroscopy (TERS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and highlighting the trajectory. (4) Emphasizing the importance of innovative methods to overcome developmental bottlenecks, thereby expanding the material, morphology, and molecule generalities to leverage SERS as a versatile technique for broad applications. (5) Extracting the invaluable spirit of groundbreaking discovery and perseverant innovations from the pioneers and trailblazers. These key inspirations include proactively embracing and leveraging emerging scientific technologies, fostering interdisciplinary cooperation to transform the impossible into reality, and persistently searching to break bottlenecks even during low-tide periods, as luck is what happens when preparation meets opportunity.

High-Resolution Distance Dependence Interrogation of Scanning Ion Conductance Microscopic Tip-Enhanced Raman Spectroscopy Enabled by Two-Dimensional Molybdenum Disulfide Substrates

Scanning ion conductance microscopy (SICM) is a powerful surface imaging tool used in the electrolytic environment. Tip-enhanced Raman spectroscopy (TERS) can give more information in addition to the morphology provided by the SICM by utilizing label-free Raman spectroscopy aided by the localized plasmonic enhancement from the metal-coated probes. In this study, the integration of SICM with TERS is demonstrated through employing a silver-coated plasmonic nanopipette. Leveraging a two-dimensional (2D) molybdenum disulfide (MoS2) as a model system, the SICM-TERS enhancement factor was estimated to be ∼105, supported by finite-difference time-domain (FDTD) simulation. Moreover, the subnanometer distance dependence SICM-TERS study reveals the tensile stress and structural changes caused by the nanopipette. These findings illustrate the potential of SICM-TERS for providing comprehensive morphological and chemical insights into electrolytic environments, paving the way for future investigations of electrocatalytic and biological systems.

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.

Surface-Enhanced Raman Spectroscopy and Electrochemistry: The Ultimate Chemical Sensing and Manipulation Combination

One of the lessons we learned from the COVID-19 pandemic is that the need for ultrasensitive detection systems is now more critical than ever. While sensors' sensitivity, portability, selectivity, and low cost are crucial, new ways to couple synergistic methods enable the highest performance levels. This review article critically discusses the synergetic combinations of optical and electrochemical methods. We also discuss three key application fields-energy, biomedicine, and environment. Finally, we selected the most promising approaches and examples, the open challenges in sensing, and ways to overcome them. We expect this work to set a clear reference for developing and understanding strategies, pros and cons of different combinations of electrochemical and optical sensors integrated into a single device.

Electrochemical Tip-Enhanced Raman Spectroscopy for the Elucidation of Complex Electrochemical Reactions

Tip-enhanced Raman spectroscopy (TERS) is an emerging nanospectroscopy technique whose implementation in situ/operando, namely, in the liquid phase and under electrochemical polarization (EC-TERS), remains challenging. The investigation of electrochemical processes at the nanoscale, in real time and over wide potential windows can be of particular interest but tedious when using EC-STM-TERS. This approach was successfully applied to the investigation of a well-established but yet complex system (a thiolated nitrobenzene derivative 4-NBM) whose reduction mechanism involves various multistep reaction paths, most likely pH-dependent. In light of the EC-TERS analysis carried out under specific conditions limiting the full (6 e-/6 H+) electrochemical reduction of 4-NBM and its photocoupling, a bimolecular electrochemical reaction path, difficult to evidence from the electrochemical response only, is proposed.

Plasmonic and catalytic Au NBP@AgPd nanoframes for highly efficient photocatalytic reactions

Heterogeneous metal nanostructures with excellent plasmonic performance and catalytic activity are urgently needed to realize efficient light-driven catalysis. Herein, we demonstrate the preparation of hollow Au nanobipyramid (NBP)@AgPd nanostructures by employing Au NBP@Ag nanorods as templates. The products could transform from Au NBP@AgPd nanoframes to nanocages, along with the redshift and broadening of the plasmon wavelength. Particularly, the plasmon intensity of these nanostructures remained considerable among the shape evolution process. Based on the selective absorption of CTAB, the Ag atoms on the side surfaces of the Au NBP@Ag nanorods were employed as the sacrificial templates to reduce Pd atoms through galvanic replacement. The reduced Pd and Ag atoms produced through the reduction reaction were preferably co-deposited on the corners and edges at the early stage and later deposited directly on the defect sites of the side facets, as more Ag atoms were released. The discontinued distribution of the Pd atoms gives an opportunity to etch away the Ag atoms in the cores, leading to the formation of hollow Au NBP@AgPd nanostructures after the etching process. It is worth noting that the deposition of the ultrathin AgPd nanoframe had little influence on the plasmonic properties of Au NBPs, as verified by electrodynamic simulations. The Au NBP@AgPd nanoframe showed great photocatalytic activity toward Suzuki coupling reactions under laser irradiation. Taken together, these results suggest that the hot electrons successfully transfer from Au NBP to the AgPd nanoframes to participate in the photocatalytic reactions. This study affords a promising route for the synthesis of anisotropic bimetallic nanostructures with excellent plasmonic performances.

Development of Line-Detected UV-Vis Absorption Microscope and Its Application to Quantitative Evaluation of Lithium Surface Reactivity

Electrochemical reactions in practical batteries occur in confined environments where anode and cathode electrodes are separated only by a thin separator. Therefore, their electrochemical behaviors may differ from those obtained in the conventional experimental cells, where the two electrodes (working and counter electrodes) are largely separated compared to the batteries. The spatial and temporal distributions of the chemical species in the vicinity of each electrode are highly expected to be determined for quantitatively understanding the phenomena in confined environments. In the present study, we developed a line-detected UV-vis absorption microscope that simultaneously measures space-resolved UV-vis absorption spectra. This novel technique has been successfully applied to evaluate the reactivities of the highly reactive lithium (Li) surfaces in organic electrolyte solutions under in situ conditions. The quantitative evaluations of the dissolution rate of Li and the diffusion constant of the product were successfully realized by analyzing the space- and time-resolved absorption spectra based on Fick's law of diffusion. The microscopic technique is expected to open the door to understanding the fundamental electrochemistry in batteries.

Plasmonic-Thermoelectric Nanotweezers for Immersive SERS Mapping

Surface-enhanced Raman spectroscopy (SERS) technology usually uses metallic nanoparticles to enhance Raman scattering signals, thereby significantly adding to molecule-level recognition and detection. However, realization of nanometer-scaled SERS imaging in liquid environments is extremely difficult due to the requirements of both precise scanning of single metallic nanoparticle and high enhancement field and thus has never been achieved before. To overcome this obstacle, we demonstrate an immersive nanometer-scaled SERS mapping technology, based on dynamic scanning of a single metallic nanoparticle with a plasmonic-thermoelectric nanotweezers system. The technology offers greater stability in the plasmonic trapping of gold nanoparticles at relative low power, as well as generating higher electric fields in the gap region. Through its dynamics, two-dimensional nanometer-scaled SERS imaging is achieved successfully. In regard to in liquid environments, this technology provides a mapping method for label-free imaging of ultrathin materials, structures, and biological samples.

Learning from the Heterogeneity at Electrochemical Interfaces

Understanding the structure-activity relationship at electrochemical interfaces is crucial in improving the performance of practical electrochemical devices, ranging from fuel cells, electrolyzers, and batteries to electrochemical sensors. However, functional electrochemical interfaces are often complex and contain various surface structures, creating heterogeneity in electrochemical activity. In this Perspective, we highlight the role of heterogeneity in electrochemistry, especially in the context of electrocatalysis. Current methods for revealing the heterogeneity at electrochemical interfaces, including nanoelectrochemistry tools and single-entity approaches, are discussed. Lastly, we provide perspectives on what one can learn by studying heterogeneity and how one can use heterogeneity to design more efficient electrochemical devices.

Current Methods and Prospects for Analysis and Characterization of Nanomaterials in the Environment

Analysis and characterization of naturally occurring and engineered nanomaterials in the environment are critical for understanding their environmental behaviors and defining real exposure scenarios for environmental risk assessment. However, this is challenging primarily due to the low concentration, structural heterogeneity, and dynamic transformation of nanomaterials in complex environmental matrices. In this critical review, we first summarize sample pretreatment methods developed for separation and preconcentration of nanomaterials from environmental samples, including natural waters, wastewater, soils, sediments, and biological media. Then, we review the state-of-the-art microscopic, spectroscopic, mass spectrometric, electrochemical, and size-fractionation methods for determination of mass and number abundance, as well as the morphological, compositional, and structural properties of nanomaterials, with discussion on their advantages and limitations. Despite recent advances in detecting and characterizing nanomaterials in the environment, challenges remain to improve the analytical sensitivity and resolution and to expand the method applications. It is important to develop methods for simultaneous determination of multifaceted nanomaterial properties for in situ analysis and characterization of nanomaterials under dynamic environmental conditions and for detection of nanoscale contaminants of emerging concern (e.g., nanoplastics and biological nanoparticles), which will greatly facilitate the standardization of nanomaterial analysis and characterization methods for environmental samples.

Theoretical Study on the Ultrafast Selective Excitation of Surface-Enhanced Coherent Anti-Stokes Raman Scattering Based on Fano Resonance of Disk-Ring Nanostructures by Shaped Femtosecond Laser Pulses

The enhancement and selective excitation of coherent anti-Stokes Raman scattering (CARS) and the suppression of background noise are very important problems for real-time detection at the single-molecule level. Optimizing the plasmonic substrate to ensure that all the hot spots of the pump, probe, Stokes, and anti-Stokes light are at the same position is the key to increasing the CARS signal to reach the level of single-molecule detection. The selective excitation of the target CARS peak and the suppression of the other peaks are the key to improving the signal-to-noise ratio. In this paper, we present a theoretical study to control the selective excitation and enhancement of any one of the three CARS peaks using the Fano resonance of a disk-ring structure. By optimizing the modulation of the pump, Stokes, and probe pulse, one CARS peak is maximized, while the other two are suppressed to zero. Fano resonance is applied to simultaneously enhance the four surface plasmon modes of the pump, probe, Stokes, and anti-Stokes light and to ensure that all the hot spots are located at the same position by adjusting the size of the disk-ring structure. The hot spots of the four pulses are concentrated in the disk-ring gap with a deviation distance of less than 2 nm, and the intensity of the CARS is enhanced by 1.43 × 1012 times, which is much higher than the requirement of single-molecule detection. The time, frequency, and phase distribution of the input and the response of the four pulses are studied in detail. It was found that the selective excitation and the spectra of CARS are both well preserved.

Molecular vibrational imaging at nanoscale

The demand to visualize the spatial distribution of chemical species based on vibrational spectra is rapidly increasing. Driven by such a need, various Raman and infrared spectro-microscopies with a nanometric spatial resolution have been developed over the last two decades. Despite rapid progress, a large gap still exists between the general needs and what these techniques can achieve. This Perspective highlights the key challenges and recent breakthroughs of the two vibrational nano-imaging techniques, scattering-type scanning near-field optical microscopy and tip-enhanced Raman scattering.

Progress of tip-enhanced Raman scattering for the last two decades and its challenges in very recent years

Tip-enhanced Raman scattering (TERS) has recently attracted remarkable attention as a novel nano-spectroscopy technique. TERS, which provides site-specific information, can be performed on any material surface regardless of morphology. Moreover, it can be applied in various environments, such as ambient air, ultrahigh vacuum (UHV), solutions, and electrochemical environments. This review reports on one hand progress of TERS for the last two decades, and on the other hand, its challenges in very recent years. Part of the progress of TERS starts with the prehistory and history of TERS, and then, the characteristics and advantages of TERS are described. Significant emphasis is put on the development of TERS instrumentation and equipment such as ultrahigh vacuum TERS, liquid TERS, electrochemical-TERS, and tip-preparations. Applications of TERS, particularly those with nanocarbons, biological materials, and surface and interface analysis, are mentioned in some detail. In the part on challenges, we focus on the very recent advances in TERS; progress in spatial resolution to the angstrom scale is the hottest topic. Recent TERS studies performed under UHV, for example chemical imaging at the angstrom scale and Raman detection of bond breaking and making of a chemisorbed up-standing single molecules at single-bond level, are reviewed. Of course, there is no clear border between the two parts. In the last part the perspective of TERS is discussed.

Quantification of doping state of redox sensitive nanoparticles for probing the invasiveness of cancer cells using surface enhanced Raman scattering

Redox activity is known to regulate migration, invasion, metastasis, proliferation, and vascularization of cancer. Because cancer is heterogeneous, the role of redox activity in different cancers and cancer-related processes vary widely. In this study, water soluble, Tween 80-coated polyaniline (TPAni) nanoparticles were synthesized and used as nano-agents for sensing the redox activities of various cancer cells. To identify the relationship between the redox activity and the aggressiveness of cancer cells, two different cancer cell lines, derived from the same tissue but different with regards to aggressiveness, were selected for study. First, the cancer cell lines were incubated with TPAni nanoparticles, and an absorbance ratio obtained from the cell culture media was used as a colorimetric indicator of the redox activities of the cells. Simultaneously, hydrophobically modified filter papers coated with silver nanosnowflakes (SNSF) were used as sensing substrates for surface enhanced Raman scattering (SERS). SERS spectra obtained from varying concentrations of rhodamine 6G were used to confirm the detection limit of the SNSF-based SERS substrate. Cell culture media containing TPAni nanoparticles were treated with the SNSF-containing SERS substrates to examine the redox activities of the various cancer cell lines.The redox activities of cancer cell lines were confirmed by absorbance spectral analysis, and these redox activities were better identified via an SERS analysis method. A SNSF-containing SERS substrate, fabricated from SNSF and filter paper, was used to sense redox activity in cancer cell lines and to further identify correlations between redox activity and cancer cell line aggressiveness, as indicated by the use of EpCAM as a biomarker. Finally, potential of ​in vivo ​redox activity sensing was also confirmed.

In Situ/Operando Insights into the Stability and Degradation Mechanisms of Heterogeneous Electrocatalysts

The further commercialization of renewable energy conversion and storage technologies requires heterogeneous electrocatalysts that meet the exacting durability target. Studies of the stability and degradation mechanisms of electrocatalysts are expected to provide important breakthroughs in stability issues. Accessible in situ/operando techniques performed under realistic reaction conditions are therefore urgently needed to reveal the nature of active center structures and establish links between the structural motifs in a catalyst and its stability properties. This review highlights recent research advances regarding in situ/operando techniques and improves the understanding of the stabilities of advanced heterogeneous electrocatalysts used in a diverse range of electrochemical reactions; it also proposes some degradation mechanisms. The review concludes by offering suggestions for future research.

Recent advances in plasmon-enhanced Raman spectroscopy for catalytic reactions on bifunctional metallic nanostructures

Metallic nanostructures exhibit superior catalytic performance for diverse chemical reactions and the in-depth understanding of reaction mechanisms requires versatile characterization methods. Plasmon-enhanced Raman spectroscopy (PERS), including surface-enhanced Raman spectroscopy (SERS), shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), and tip-enhanced Raman spectroscopy (TERS), appears as a powerful technique to characterize the Raman fingerprint information of surface species with high chemical sensitivity and spatial resolution. To expand the range of catalytic reactions studied by PERS, catalytically active metals are integrated with plasmonic metals to produce bifunctional metallic nanostructures. In this minireview, we discuss the recent advances in PERS techniques to probe the chemical reactions catalysed by bifunctional metallic nanostructures. First, we introduce different architectures of these dual-functionality nanostructures. We then highlight the recent works using PERS to investigate important catalytic reactions as well as the electronic and catalytic properties of these nanostructures. Finally, we provide some perspectives for future PERS studies in this field.

A discrete interaction model/quantum mechanical method for simulating surface-enhanced Raman spectroscopy in solution

Since surface-enhanced Raman scattering (SERS) is of considerable interest for sensing applications in aqueous solution, the role that solvent plays in the spectroscopy must be understood. However, these efforts are hindered due to a lack of simulation approaches for modeling solvent effects in SERS. In this work, we present an atomistic electrodynamics-quantum mechanical method to simulate SERS in aqueous solution based on the discrete interaction model/quantum mechanical method. This method combines an atomistic electrodynamics model of the nanoparticle with a time-dependent density functional theory description of the molecule and a polarizable embedding method for the solvent. The explicit treatment of solvent molecules and nanoparticles results in a large number of polarizable dipoles that need to be considered. To reduce the computational cost, a simple cut-off based approach has been implemented to limit the number of dipoles that need to be treated without sacrificing accuracy. As a test of this method, we have studied how solvent affects the SERS of pyridine in the junction between two nanoparticles in aqueous solution. We find that the solvent leads to an enhanced SERS due to an increased local field at the position of the pyridine. We further demonstrate the importance of both image field and local field effects in determining the enhancements and the spectral signatures. Our results show the importance of describing the local environment due to the solvent molecules when modeling SERS.

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.

Tip-enhanced Raman spectroscopy for nanoscale probing of dynamic chemical systems

Dynamics are fundamental to all aspects of chemistry and play a central role in the mechanism and product distribution of a chemical reaction. All dynamic processes are influenced by the local environment, so it is of fundamental and practical value to understand the structure of the environment and the dynamics with nanoscale resolution. Most techniques for measuring dynamic processes have microscopic spatial resolution and can only measure the average behavior of a large ensemble of sites within their sampling volumes. Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for overcoming this limitation due to its combination of high chemical specificity and spatial resolution that is on the nanometer scale. Adapting it for the study of dynamic systems remains a work in progress, but the increasing sophistication of TERS is making such studies more routine, and there are now growing efforts to use TERS to examine more complex processes. This Perspective aims to promote development in this area of research by highlighting recent progress in using TERS to understand reacting and dynamic systems, ranging from simple model reactions to complex processes with practical applications. We discuss the unique challenges and opportunities that TERS presents for future studies.

Atomic Force Microscopy Based Top-Illumination Electrochemical Tip-Enhanced Raman Spectroscopy

Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) is a powerful technique for the in situ study of the physiochemical properties of the electrochemical solid/liquid interface at the nanoscale and molecular level. To further broaden the potential window of EC-TERS while extending its application to opaque samples, here, we develop a top-illumination atomic force microscopy (AFM) based EC-TERStechnique by using a water-immersion objective of a high numerical aperture to introduce the excitation laser and collect the signal. This technique not only extends the application of EC-TERS but also has a high detection sensitivity and experimental efficiency. We coat a SiO₂ protection layer over the AFM-TERS tip to improve both the mechanical and chemical stability of the tip in a liquid TERS experiment. We investigate the influence of liquid on the tip-sample distance to obtain the highest TERS enhancement. We further evaluate the reliability of the as-developed EC-AFM-TERS technique by studying the electrochemical redox reaction of polyaniline. The top-illumination EC-AFM-TERS is promising for broadening the application of EC-TERS to more practical systems, including energy storage and (photo)electrocatalysis.

One-side metal-coated pyramidal cantilever tips for highly reproducible tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy (TERS) has been recognized as a useful tool for nanoscale chemical analysis, and it can further reach down to the sub-nanometer scale in the gap-mode configuration. Using an atomic force microscopy (AFM) in gap-mode TERS for position control of a metallic tip, a unique and correlative analysis can be even realized at the single molecule level. However, one of crucial issues in AFM-based gap-mode TERS is the fabrication of reliable and reproducible cantilver metallic tips. Here, we propose a simple, cost-effective fabrication method of metal-coated tips for AFM-based gap-mode TERS by means of the physical vapor deposition technique in a reproducible way. Our plamonic tips have extremely smooth silver layers on one side of the pyramidal tip, which is totally different from the regular metallic tips that hold granular metallic structures randomly arranged on their bodies. Importantly, all fabricated tips exhibited a reasonably high enhancement factor of more than 10⁴, which indicates that the reproducibility of our plasmonic tip is virtually 100% in the gap-mode configuration. The excellent reproducibility of gap-mode TERS measurement holds great promise for rendering AFM-based TERS as a powerful analytical technique in a broad range of fields.

Infrared and Raman chemical imaging and spectroscopy at the nanoscale

The advent of nanotechnology, and the need to understand the chemical composition at the nanoscale, has stimulated the convergence of IR and Raman spectroscopy with scanning probe methods, resulting in new nanospectroscopy paradigms. Here we review two such methods, namely photothermal induced resonance (PTIR), also known as AFM-IR and tip-enhanced Raman spectroscopy (TERS). AFM-IR and TERS fundamentals will be reviewed in detail together with their recent crucial advances. The most recent applications, now spanning across materials science, nanotechnology, biology, medicine, geology, optics, catalysis, art conservation and other fields are also discussed. Even though AFM-IR and TERS have developed independently and have initially targeted different applications, rapid innovation in the last 5 years has pushed the performance of these, in principle spectroscopically complimentary, techniques well beyond initial expectations, thus opening new opportunities for their convergence. Therefore, subtle differences and complementarity will be highlighted together with emerging trends and opportunities.

A WS2-gold nanoparticle heterostructure-based novel SERS platform for the rapid identification of antibiotic-resistant pathogens

The emergence of antibiotic-resistant bacteria is the biggest threat to our society. The rapid discovery of drug resistant bacteria is very urgently needed to guide antibiotic treatment development. The current manuscript reports the design of a 2D-0D heterostructure-based surface enhanced Raman spectroscopy (SERS) platform, which has the capability for the rapid identification of the multidrug resistant strain of Salmonella DT104. Details of the synthesis and characterization of the heterostructure SERS platform using a two dimensional (2D) WS₂ transition metal dichalcogenide (TMD) and zero dimensional (0D) plasmonic gold nanoparticles (GNPs) have been reported. The current manuscript reveals that the 2D-0D heterostructure-based SERS platform exhibits extremely high Raman enhancement capabilities. Using Rh-6G and 4-ATP probe molecules, we determined that the SERS sensitivity is in the range of ∼10^-10 to 10^-11 M, several orders of magnitude higher than 2D-TMD on its own (10^-3 M) or 0D-GNPs on their own (∼10^-6 to 10^-7 M). Experimental and theoretical finite-difference time-domain (FDTD) simulation data indicate that the synergistic effect of an electromagnetic mechanism (EM) and a chemical mechanism (CM) on the heterostructure is responsible for the excellent SERS enhancement observed. Notably, the experimental data reported here show that the heterostructure-based SERS has the ability to separate a multidrug resistance strain from a normal strain of Salmonella by monitoring the antibiotic-pathogen interaction within 90 minutes, even at a concentration of 100 CFU mL^-1.

Topography-induced variations of localized surface plasmon resonance in tip-enhanced Raman configuration

We report on topography-induced changes of the localized surface plasmon resonance (LSPR) enhanced luminescence of gold tip on SrTiO₃ nanostructures with apertureless scanning near-field optical microscopy (aSNOM) in tip-enhanced Raman spectroscopy (TERS) configuration. Our experimental and simulated results indicate that the averaged refractive index of the dielectric environment of the tip apex containing both air and SrTiO₃ in variable volume ratios, is dependent on the topography of the sample. This reveals that the local topography has to be taken into consideration as an additional contribution to the position of the LSPR.

Multimodal Characterization of Resin Embedded and Sliced Polymer Nanoparticles by Means of Tip-Enhanced Raman Spectroscopy and Force-Distance Curve Based Atomic Force Microscopy

Understanding the property-function relation of nanoparticles in various application fields involves determining their physicochemical properties, which is still a remaining challenge to date. While a multitude of different characterization tools can be applied, these methods by themselves can only provide an incomplete picture. Therefore, novel analytical techniques are required, which can address both chemical functionality and provide structural information at the same time with high spatial resolution. This is possible by using tip-enhanced Raman spectroscopy (TERS), but due to its limited depth information, TERS is usually restricted to investigations of the nanoparticle surface. Here, TERS experiments are established on polystyrene nanoparticles (PS NPs) after resin embedding and microtome slicing. With that, unique access to their internal morphological features is gained, and thus, enables differentiation between information obtained for core- and shell-regions. Complementary information is obtained by means of transmission electron microscopy (TEM) and from force-distance curve based atomic force microscopy (FD-AFM). This multimodal approach achieves a high degree of discrimination between the resin and the polymers used for nanoparticle formulation. The high potential of TERS combined with advanced AFM spectroscopy tools to probe the mechanical properties is applied for quality control of the resin embedding procedure.

Enhanced subwavelength coupling and nano-focusing with optical fiber-plasmonic hybrid probe

Metallic nanowires supporting surface plasmon polaritons can localize optical fields at nanoscale tapered ends for near-field imaging. Radially polarized light through the optical fiber or free space efficiently excites the converging radial plasmons at the apex of a sharp tip. However, such radial vector mode excitation through optical fiber requires precise polarization control and strongly polarization maintaining state in optical fiber, thus inducing a complexity for optical applications. In this paper, we propose a photonic-plasmonic probe that uses the linearly polarized source to excite the nanoscale plasmonic hotspot. The linearly polarized fiber mode is converted to radial surface plasmon polaritons (SPPs) through asymmetric coupling at the base of the metallic nano-tip. The radial SPP's then propagate along the half ellipsoid tip and are focused at the tip apex giving rise to the nanoscale concentration of optical energy. The probe can be implemented with near-field imaging techniques such as tip-enhanced Raman microscopy to obtain topographic and chemical spectroscopic information in atomic resolution for studying light-matter interaction at the nanoscale.

Reactivity mapping of nanoscale defect chemistry under electrochemical reaction conditions

Electrocatalysts often show increased conversion at nanoscale chemical or topographic surface inhomogeneities, resulting in spatially heterogeneous reactivity. Identifying reacting species locally with nanometer precision during chemical conversion is one of the biggest quests in electrochemical surface science to advance (electro)catalysis and related fields. Here, we demonstrate that electrochemical tip-enhanced Raman spectroscopy can be used for combined topography and reactivity imaging of electro-active surface sites under reaction conditions. We map the electrochemical oxidation of Au nanodefects, a showcase energy conversion and corrosion reaction, with a chemical spatial sensitivity of about 10 nm. The results indicate the reversible, concurrent formation of spatially separated Au₂O₃ and Au₂O species at defect-terrace and protrusion sites on the defect, respectively. Active-site chemical nano-imaging under realistic working conditions is expected to be pivotal in a broad range of disciplines where quasi-atomistic reactivity understanding could enable strategic engineering of active sites to rationally tune (electro)chemical device properties.

Identifying reacting species locally with nanometer precision is a major challenge in electrochemical surface science. Using operando Raman nanoscopy, authors image the reversible, concurrent formation of nanometer-spatially separated Au₂O₃ and Au₂O species during Au nanodefect oxidation.

Tip-Enhanced Raman Excitation Spectroscopy (TERES): Direct Spectral Characterization of the Gap-Mode Plasmon

The plasmonic properties of tip-substrate composite systems are of vital importance to near-field optical spectroscopy, in particular tip-enhanced Raman spectroscopy (TERS), which enables operando studies of nanoscale chemistry at a single molecule level. The nanocavities formed in the tip-substrate junction also offer a highly tunable platform for studying field-matter interactions at the nanoscale. While the coupled nanoparticle dimer model offers a correct qualitative description of gap-mode plasmon effects, it ignores the full spectrum of multipolar tip plasmon modes and their interaction with surface plasmon polariton (SPP) excitation in the substrate. Herein, we perform the first tip-enhanced Raman excitation spectroscopy (TERES) experiment and use the results, both in ambient and aqueous media, in combination with electrodynamics simulations, to explore the plasmonic response of a Au tip-Au substrate composite system. The gap-mode plasmon features a wide spectral window corresponding to a host of tip plasmon modes interacting with the plasmonic substrate. Simulations of the electric field confinement demonstrate that optimal spatial resolution is achieved when a hybrid plasmon mode that combines a multipolar tip plasmon and a substrate SPP is excited. Nevertheless, a wide spectral window over 1000 nm is available for exciting the tip plasmon with high spatial resolution, which enables the simultaneous resonant detection of different molecular species. This window is robust as a function of tip-substrate distance and tip radius of curvature, indicating that many choices of tips will work, but it is restricted to wavelengths longer than ∼600 nm for the Au tip-Au substrate combination. Other combinations, such as Ag tip-Ag substrate, can access wavelengths as low as 350 nm.

Electrochemical Tip-Enhanced Raman Spectroscopy with Improved Sensitivity Enabled by a Water Immersion Objective

Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) appears as a promising in situ nanospectroscopic tool for characterization and understanding of the electrochemical interfacial processes at the nanometer scale and molecular level. However, the wide application of EC-TERS is hampered by its low sensitivity as a result of the optical path distortion due to the refractive index mismatch of the multilayer media (air, glass, and electrolyte). Here, we propose a new side-illumination EC-TERS setup by coupling a water immersion objective with a high numerical aperture to a scanning tunneling microscope scanning head customized with a large open space and a compact spectroelectrochemical cell. It not only effectively eliminates the optical distortion but also increases the sensitivity remarkably, which allows sensitive monitoring of the electrochemical redox processes of anthraquinone molecules. More importantly, EC-TERS is able to independently control the tip position and laser illumination position. By utilizing this feature, we reveal that the irreversible reduction reaction of anthraquinone observed in EC-TERS is induced by the synergistic effect of the negative potential and laser illumination rather than the localized surface plasmon. The highly improved sensitivity and the flexibility to control the tip and laser illumination position on the nanometer scale endows EC-TERS as an important tool for the fundamental understanding of the photo- or plasmon electrochemistry and the interfacial structure-activity relationship of important electrochemical systems.