Tip-Enhanced Raman Spectroscopy

Emerging SERS and TERS MoS2 platforms for the characterization of plasma-derived extracellular vesicles

Extracellular vesicles (EVs) play a crucial role in intercellular communication processes. In addition, their biomolecular cargoes such as lipids, proteins, and nucleic acids are useful for identifying potential biomarkers related to different stages of cancer disease. However, the small size and heterogenicity of tumor-related EVs represent a major challenge in properly identifying the content of EVs' cargoes with common characterization protocols. To address these issues, surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) are powerful alternatives to assign the vibrational fingerprints to the biomolecules contained in cancer EVs, providing high specificity and spatial resolution. Transition metal dichalcogenides are particularly interesting as SERS and TERS substrates due to the high sensitivity of their 2D surface through coulombic and van der Waals interactions when in contact with an analyte or small object such as the charged membranes of EVs. These interactions induce subtle changes in the work function of the flakes which can be measured through drastic changes of optical processes. We investigate the use of MoS2 flakes synthesized by atmospheric pressure chemical vapor deposition as a potential label-free SERS and TERS platform for the identification of plasma EVs. To exemplify this technology, we isolated plasma EV samples from donors with early-stage [FIGO (I/II)] with high-grade serous carcinoma (HGSC) by size exclusion chromatography (SEC). Both surface- and tip-enhanced measurements were conducted individually, enabling the identification of a series of markers from ovarian cancer donors, highlighting the complementarity of SERS and TERS measurements.

Nanoscale resolved mapping of the dipole emission of hBN color centers with a scattering-type scanning near-field optical microscope

Color centers in hexagonal boron nitride (hBN) are promising candidates as quantum light sources for future technologies. In this work, we utilize a scattering-type near-field optical microscope (s-SNOM) to study the photoluminescence (PL) emission characteristics of such quantum emitters in metalorganic vapor phase epitaxy grown hBN. On the one hand, we demonstrate direct near-field optical excitation and emission through interaction with the nanofocus of the tip resulting in a subdiffraction limited tip-enhanced PL hotspot. On the other hand, we show that indirect excitation and emission via scattering from the tip significantly increases the recorded PL intensity. This demonstrates that the tip-assisted PL (TAPL) process efficiently guides the generated light to the detector. We apply the TAPL method to map the in-plane dipole orientations of the hBN color centers on the nanoscale. This work promotes the widely available s-SNOM approach to applications in the quantum domain including characterization and optical control.

Polarization-Dependent Plasmon-Induced Doping and Strain Effects in MoS2 Monolayers on Gold Nanostructures

Monolayers of transition-metal dichalcogenides, such as MoS2, have attracted significant attention for their exceptional electronic and optical properties, positioning them as ideal candidates for advanced optoelectronic applications. Despite their strong excitonic effects, the atomic-scale thickness of these materials limits their light absorption efficiency, necessitating innovative strategies to enhance light-matter interactions. Plasmonic nanostructures offer a promising solution to overcome those challenges by amplifying the electromagnetic field and also introducing other mechanisms, such as hot electron injection. In this study, we investigate the vibrational and optical properties of MoS2 monolayer deposited on gold substrates and gratings, emphasizing the role of strain and plasmonic effects using conventional spectroscopic techniques. Our results reveal significant biaxial strain in the supported regions and a uniaxial strain gradient in the suspended ones, showing a strain-induced exciton and carrier funneling effect toward the center of the nanogaps. Moreover, we observed an additional polarization-dependent doping mechanism in the suspended regions. This effect was attributed to localized surface plasmons generated within the slits, as confirmed by numerical simulations, which may decay nonradiatively into hot electrons and be injected into the monolayer. Photoluminescence measurements further demonstrated a polarization-dependent trion-to-A exciton intensity ratio, supporting the hypothesis of additional plasmon-induced doping. These findings provide a comprehensive understanding of the strain-mediated funneling effects and plasmonic interactions in hybrid MoS2/Au nanostructures, offering valuable insights for developing high-efficiency photonic devices and quantum technologies, including polarization-sensitive detectors and excitonic circuits.

Contactless manufacturing of TERS-active AFM tips by bipolar electrodeposition

Tip-enhanced Raman spectroscopy (TERS) is a powerful technique for nanoscale chemical imaging. However, its worldwide expansion is still limited by the challenging fabrication of cheap, robust and efficient TERS tips as optical nanosources to amplify the Raman signal. An original method based on bipolar electrodeposition is described here to prepare gold-coated AFM cantilevers used as TERS tips. This wireless method is simple to implement, cost-effective, and allows for the parallel fabrication of several TERS tips with good reproducibility of the metal thickness and a relatively long lifetime. The TERS activity was confirmed by imaging graphene oxide flakes with high spatial resolution (below 10 nm). A promising yield of 64% was achieved for the fabrication of active TERS tips. Therefore, this method could pave the way for the development of new chemical routes for the preparation of TERS tips and other plasmonic nanostructures.

Strained Silicon Technology: Non-Destructive High-Lateral-Resolution Characterization Through Tip-Enhanced Raman Spectroscopy

The semiconductor industry is undergoing a transformative phase, marked by the relentless drive for miniaturization and a constant demand for higher performance and energy efficiency. However, the reduction of metal-oxide-semiconductor field-effect transistor sizes for advanced technology nodes below 10 nm presents several challenges. In response, strained silicon technology has emerged as a key player, exploiting strain induction in the silicon crystal lattice to improve device performance. At the same time, there has been a growing need for characterization techniques that allow in-line monitoring of sample conditions during semiconductor manufacturing, as an alternative to traditional methods such as transmission electron microscopy or high-resolution X-ray diffraction, which have several limitations in terms of measurement time and sample destructiveness. This paper explores the application of advanced spectroscopic characterization techniques, in particular µ-Raman spectroscopy and tip-enhanced Raman spectroscopy (TERS), to meet the evolving needs of the semiconductor industry for quality control and failure analysis, increasingly requiring faster and non-destructive characterization techniques. µ-Raman provides insight into strain values and distributions of strained layers with different thicknesses and germanium concentrations, but its lateral resolution is constrained by the Abbe diffraction limit. TERS, on the other hand, emerges as a powerful non-destructive technique capable of overcoming diffraction limits by exploiting the combination of an atomic force microscope with a Raman spectrometer. This breakthrough makes it possible to estimate the chemical composition and induced strain in the lattice by evaluating the Raman peak position shifts in strained and unstrained silicon layers, providing crucial insights for nanoscale strain control. In particular, this paper focuses on the TERS characterization of Si0.7Ge0.3 epitaxial layers grown on a silicon-on-insulator device, demonstrating the effectiveness of this technique and the high lateral resolution that can be achieved.

Plasmonic nanoparticle sensors: current progress, challenges, and future prospects

Plasmonic nanoparticles (NPs) have played a significant role in the evolution of modern nanoscience and nanotechnology in terms of colloidal synthesis, general understanding of nanocrystal growth mechanisms, and their impact in a wide range of applications. They exhibit strong visible colors due to localized surface plasmon resonance (LSPR) that depends on their size, shape, composition, and the surrounding dielectric environment. Under resonant excitation, the LSPR of plasmonic NPs leads to a strong field enhancement near their surfaces and thus enhances various light-matter interactions. These unique optical properties of plasmonic NPs have been used to design chemical and biological sensors. Over the last few decades, colloidal plasmonic NPs have been greatly exploited in sensing applications through LSPR shifts (colorimetry), surface-enhanced Raman scattering, surface-enhanced fluorescence, and chiroptical activity. Although colloidal plasmonic NPs have emerged at the forefront of nanobiosensors, there are still several important challenges to be addressed for the realization of plasmonic NP-based sensor kits for routine use in daily life. In this comprehensive review, researchers of different disciplines (colloidal and analytical chemistry, biology, physics, and medicine) have joined together to summarize the past, present, and future of plasmonic NP-based sensors in terms of different sensing platforms, understanding of the sensing mechanisms, different chemical and biological analytes, and the expected future technologies. This review is expected to guide the researchers currently working in this field and inspire future generations of scientists to join this compelling research field and its branches.

Fundamentals and Applications of Raman-Based Techniques for the Design and Development of Active Biomedical Materials

Raman spectroscopy is an analytical method based on light-matter interactions that can interrogate the vibrational modes of matter and provide representative molecular fingerprints. Mediated by its label-free, non-invasive nature, and high molecular specificity, Raman-based techniques have become ubiquitous tools for in situ characterization of materials. This review comprehensively describes the theoretical and practical background of Raman spectroscopy and its advanced variants. The numerous facets of material characterization that Raman scattering can reveal, including biomolecular identification, solid-to-solid phase transitions, and spatial mapping of biomolecular species in bioactive materials, are highlighted. The review illustrates the potential of these techniques in the context of active biomedical material design and development by highlighting representative studies from the literature. These studies cover the use of Raman spectroscopy for the characterization of both natural and synthetic biomaterials, including engineered tissue constructs, biopolymer systems, ceramics, and nanoparticle formulations, among others. To increase the accessibility and adoption of these techniques, the present review also provides the reader with practical recommendations on the integration of Raman techniques into the experimental laboratory toolbox. Finally, perspectives on how recent developments in plasmon- and coherently-enhanced Raman spectroscopy can propel Raman from underutilized to critical for biomaterial development are provided.

Toward Modeling the Complexity of the Chemical Mechanism in SERS

Surface-enhanced Raman scattering (SERS) provides detailed information about the binding of molecules at interfaces and their interactions with the local environment due to the large enhancement of Raman scattering. This enhancement arises from a combination of the electromagnetic mechanism (EM) and chemical mechanism (CM). While it is commonly accepted that EM gives rise to most of the enhancement, large spectral changes originate from CM. To elucidate the rich information contained in SERS spectra about molecules at interfaces, a comprehensive understanding of the enhancement mechanisms is necessary. In this Perspective, we discuss the current understanding of the enhancement mechanisms and highlight their interplay in complex local environments. We will also discuss emerging areas where the development of computational and theoretical models is needed with specific attention given to how the CM contributes to the spectral changes. Future efforts in modeling should focus on overcoming the challenges presented in this review in order to capture the complexity of CM in SERS.

Label-Free Optical Technologies to Enhance Noninvasive Endoscopic Imaging of Early-Stage Cancers

White light endoscopic imaging allows for the examination of internal human organs and is essential in the detection and treatment of early-stage cancers. To facilitate diagnosis of precancerous changes and early-stage cancers, label-free optical technologies that provide enhanced malignancy-specific contrast and depth information have been extensively researched. The rapid development of technology in the past two decades has enabled integration of these optical technologies into clinical endoscopy. In recent years, the significant advantages of using these adjunct optical devices have been shown, suggesting readiness for clinical translation. In this review, we provide an overview of the working principles and miniaturization considerations and summarize the clinical and preclinical demonstrations of several such techniques for early-stage cancer detection. We also offer an outlook for the integration of multiple technologies and the use of computer-aided diagnosis in clinical endoscopy.

Unveiling brain disorders using liquid biopsy and Raman spectroscopy

Brain disorders, including neurodegenerative diseases (NDs) and traumatic brain injury (TBI), present significant challenges in early diagnosis and intervention. Conventional imaging modalities, while valuable, lack the molecular specificity necessary for precise disease characterization. Compared to the study of conventional brain tissues, liquid biopsy, which focuses on blood, tear, saliva, and cerebrospinal fluid (CSF), also unveils a myriad of underlying molecular processes, providing abundant predictive clinical information. In addition, liquid biopsy is minimally- to non-invasive, and highly repeatable, offering the potential for continuous monitoring. Raman spectroscopy (RS), with its ability to provide rich molecular information and cost-effectiveness, holds great potential for transformative advancements in early detection and understanding the biochemical changes associated with NDs and TBI. Recent developments in Raman enhancement technologies and advanced data analysis methods have enhanced the applicability of RS in probing the intricate molecular signatures within biological fluids, offering new insights into disease pathology. This review explores the growing role of RS as a promising and emerging tool for disease diagnosis in brain disorders, particularly through the analysis of liquid biopsy. It discusses the current landscape and future prospects of RS in the diagnosis of brain disorders, highlighting its potential as a non-invasive and molecularly specific diagnostic tool.

Advancements in Neurosurgical Intraoperative Histology

Despite their relatively low incidence globally, central nervous system (CNS) tumors remain amongst the most lethal cancers, with only a few other malignancies surpassing them in 5-year mortality rates. Treatment decisions for brain tumors heavily rely on histopathological analysis, particularly intraoperatively, to guide surgical interventions and optimize patient outcomes. Frozen sectioning has emerged as a vital intraoperative technique, allowing for highly accurate, rapid analysis of tissue samples, although it poses challenges regarding interpretive errors and tissue distortion. Raman histology, based on Raman spectroscopy, has shown great promise in providing label-free, molecular information for accurate intraoperative diagnosis, aiding in tumor resection and the identification of neurodegenerative disease. Techniques including Stimulated Raman Scattering (SRS), Coherent Anti-Stokes Raman Scattering (CARS), Surface-Enhanced Raman Scattering (SERS), and Tip-Enhanced Raman Scattering (TERS) have profoundly enhanced the speed and resolution of Raman imaging. Similarly, Confocal Laser Endomicroscopy (CLE) allows for real-time imaging and the rapid intraoperative histologic evaluation of specimens. While CLE is primarily utilized in gastrointestinal procedures, its application in neurosurgery is promising, particularly in the context of gliomas and meningiomas. This review focuses on discussing the immense progress in intraoperative histology within neurosurgery and provides insight into the impact of these advancements on enhancing patient outcomes.

Toward the nanoscale chemical and physical probing of milk-derived extracellular vesicles using Raman and tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy (TERS) is an advanced technique to perform local chemical analysis of the surface of a sample through the improvement of the sensitivity and the spatial resolution of Raman spectroscopy by plasmonic enhancement of the electromagnetic signal in correspondence with the nanometer-sized tip of an atomic force microscope (AFM). In this work, TERS is demonstrated to represent an innovative and powerful approach for studying extracellular vesicles, in particular bovine milk-derived extracellular vesicles (mEVs), which are nanostructures with considerable potential in drug delivery and therapeutic applications. Raman spectroscopy has been used to analyze mEVs at the micrometric and sub-micrometric scales to obtain a detailed Raman spectrum in order to identify the 'signature' of mEVs in terms of their characteristic molecular vibrations and, therefore, their chemical compositions. With the ability to improve lateral resolution, TERS has been used to study individual mEVs, demonstrating the possibility of investigating a single mEV selected on the surface of the sample and, moreover, analyzing specific locations on the selected mEV with nanometer lateral resolution. TERS potentially allows one to reveal local differences in the composition of mEVs providing new insights into their structure. Also, thanks to the intrinsic properties of TERS to acquire the signal from only the first few nanometers of the surface, chemical investigation of the lipid membrane in correspondence with the various locations of the selected mEV could be performed by analyzing the peaks of the Raman shift in the relevant range of the spectrum (2800-3000 cm-1). Despite being limited to mEVs, this work demonstrates the potential of TERS in the analysis of extracellular vesicles.

Proposed Quantum Twisting Scanning Probe Microscope over Twisted Bilayer Graphene

Twisted bilayer graphene (TBG) has the natural merits of tunable flat bands and localized states distributed as a triangular lattice. However, the application of this state remains obscure. By density functional theory (DFT) and pz orbital tight-binding model calculations, we investigate the tip-shaped electrostatic potential of top valence electrons of TBG at half filling. Adsorption energy scanning of molecules above the TBG reveals that this tip efficiently attracts molecules selectively to AA-stacked or AB-stacked regions. Tip shapes can be controlled by their underlying electronic structure, with electrons of low bandwidth exhibiting a more localized feature. Our results indicate that TBG tips offer applications in noninvasive and nonpolluting measurements in scanning probe microscopy and theoretical guidance for 2D material-based probes.

Resonant Raman scattering on graphene: SERS and gap-mode TERS

Nanoscale deformations and corrugations occur in graphene-like two-dimensional materials during their incorporation into hybrid structures and real devices, such as sensors based on surface-enhanced Raman scattering (SERS-based sensors). The structural features mentioned above are known to affect the electronic properties of graphene, thus highly sensitive and high-resolution techniques are required to reveal and characterize arising local defects, mechanical deformations, and phase transformations. In this study, we demonstrate that gap-mode tip-enhanced Raman Scattering (gm-TERS), which offers the benefits of structural and chemical analytical methods, allows variations in the structure and mechanical state of a two-dimensional material to be probed with nanoscale spatial resolution. In this work, we demonstrate locally enhanced gm-TERS on a monolayer graphene film placed on a plasmonic substrate with specific diameter gold nanodisks. SERS measurements are employed to determine the optimal disk diameter and excitation wavelength for further realization of gm-TERS. A significant local plasmonic enhancement of the main vibrational modes in graphene by a factor of 100 and a high spatial resolution of 10 nm are achieved in the gm-TERS experiment, making gm-TERS chemical mapping possible. By analyzing the gm-TERS spectra of the graphene film in the local area of a nanodisk, the local tensile mechanical strain in graphene was detected, resulting in a split of the G mode into two components, G+ and G-. Using the frequency split in the positions of G+ and G- modes in the TERS spectra, the stress was estimated to be up to 1.5%. The results demonstrate that gap-mode TERS mapping allows rapid and precise characterization of local structural defects in two-dimensional materials on the nanoscale.

Raman spectroscopy and its plasmon-enhanced counterparts: A toolbox to probe protein dynamics and aggregation

Protein unfolding and aggregation are often correlated with numerous diseases such as Alzheimer's, Parkinson's, Huntington's, and other debilitating neurological disorders. Such adverse events consist of a plethora of competing mechanisms, particularly interactions that control the stability and cooperativity of the process. However, it remains challenging to probe the molecular mechanism of protein dynamics such as aggregation, and monitor them in real-time under physiological conditions. Recently, Raman spectroscopy and its plasmon-enhanced counterparts, such as surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS), have emerged as sensitive analytical tools that have the potential to perform molecular studies of functional groups and are showing significant promise in probing events related to protein aggregation. We summarize the fundamental working principles of Raman, SERS, and TERS as nondestructive, easy-to-perform, and fast tools for probing protein dynamics and aggregation. Finally, we highlight the utility of these techniques for the analysis of vibrational spectra of aggregation of proteins from various sources such as tissues, pathogens, food, biopharmaceuticals, and lastly, biological fouling to retrieve precise chemical information, which can be potentially translated to practical applications and point-of-care (PoC) devices. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Diagnostic Tools > Diagnostic Nanodevices Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Statistical Study of Low-Intensity Single-Molecule Recognition Events Using DeepTipTM Probes: Application to the Pru p 3-Phytosphingosine System

The interaction between the plant lipid transfer protein Pru p 3 and phytosphingosine was assessed using an atomic force microscope. Phytosphingosine was covalently immobilized on DeepTipTM probes and Pru p 3 on MicroDeckTM functionalized substrates. Single-molecular interaction events between both molecules were retrieved and classified and the distribution for each one of the identified types was calculated. A success rate of over 70% was found by comparing the number of specific Pru p 3-phytosphingosine interaction events with the total number of recorded curves. The analysis of the distribution established among the various types of curves was further pursued to distinguish between those curves that can mainly be used for assessing the recognition between phytosphingosine (sensor molecule) and Pru p 3 (target molecule) in the context of affinity atomic force microscopy, and those that entail details of the interaction and might be employed in the context of force spectroscopy. The successful application of these functionalized probes and substrates to the characterization of the low-intensity hydrophobic interaction characteristic of this system is a clear indication of the potential of exploiting this approach with an extremely wide range of different biological molecules of interest. The possibility of characterizing molecular assembly events with single-molecule resolution offers an advantageous procedure to plough into the field of molecular biomimetics.

Biophotonics as a new application in optical technology: A bibliometric analysis

Biophotonics procures wide practicability in life sciences and medicines. The contribution of biophotonics is well recognized in various Nobel Prizes. Therefore, this paper aims to conduct a bibliometric analysis of biophotonics publications. The scientific database used is the Web of Science database. Harzing's Publish or Perish and VOSviewer are the bibliometric tools used in this analysis. This study found an increasing trend in the number of publications in recent years as the number of publications peaked at 347 publications in 2020. Most of the documents are articles (3361 publications) and proceeding papers (1632 publications). The top three subject areas are Optics (3206 publications), Engineering (1706 publications) and Radiology, Nuclear Medicine, and Medical Imaging (1346 publications). The United States has the highest number of publications (2041 publications) and citation impact (38.07 citations per publication; h-index: 125). The top three publication titles are Proceedings of SPIE (920 publications), Journal of Biomedical Optics (599 publications), and Proceedings of the Society of Photo Optical Instrumentation Engineers SPIE (245 publications). The potential areas for future research include to overcome the optical penetration depth issue and to develop publicly available biosensors for the detection of common diseases.

Cell wall dynamics: novel tools and research questions

Years ago, a classic textbook would define plant cell walls based on passive features. For instance, a sort of plant exoskeleton of invariable polysaccharide composition, and probably painted in green. However, currently, this view has been expanded to consider plant cell walls as active, heterogeneous, and dynamic structures with a high degree of complexity. However, what do we mean when we refer to a cell wall as a dynamic structure? How can we investigate the different implications of this dynamism? While the first question has been the subject of several recent publications, defining the ideal strategies and tools needed to address the second question has proven to be challenging due to the myriad of techniques available. In this review, we will describe the capacities of several methodologies to study cell wall composition, structure, and other aspects developed or optimized in recent years. Keeping in mind cell wall dynamism and plasticity, the advantages of performing long-term non-invasive live-imaging methods will be emphasized. We specifically focus on techniques developed for Arabidopsis thaliana primary cell walls, but the techniques could be applied to both secondary cell walls and other plant species. We believe this toolset will help researchers in expanding knowledge of these dynamic/evolving structures.

Superhydrophobic Surface Modification of Polymer Microneedles Enables Fabrication of Multimodal Surface-Enhanced Raman Spectroscopy and Mass Spectrometry Substrates for Synthetic Drug Detection in Blood Plasma

Microneedles are widely used substrates for various chemical and biological sensing applications utilizing surface-enhanced Raman spectroscopy (SERS), which is indeed a highly sensitive and specific analytical approach. This article reports the fabrication of a nanoparticle (NP)-decorated microneedle substrate that is both a SERS substrate and a substrate-supported electrospray ionization (ssESI) mass spectrometry (MS) sample ionization platform. Polymeric ligand-functionalized gold nanorods (Au NRs) are adsorbed onto superhydrophobic surface-modified polydimethylsiloxane (PDMS) microneedles through the control of various interfacial interactions. We show that the chain length of the polymer ligands dictates the NR adsorption process. Importantly, assembling Au NRs onto the micrometer-diameter needle tips allows the formation of highly concentrated electromagnetic hot spots, which provide the SERS enhancement factor as high as 1.0 × 106. The micrometer-sized area of the microneedle top and high electromagnetic field enhancement of our system can be loosely compared with tip-enhanced Raman spectroscopy, where the apex of a plasmonic NP-functionalized sharp probe produces high-intensity plasmonic hot spots. Utilizing our NR-decorated microneedle substrates, the synthetic drugs fentanyl and alprazolam are analyzed with a subpicomolar limit of detection. Further analysis of drug-molecule interactions on the NR surface utilizing the Langmuir adsorption model suggests that the higher polarizability of fentanyl allows for a stronger interaction with hydrophilic polymer layers on the NR surface. We further demonstrate the translational aspect of the microneedle substrate for both SERS- and ssESI-MS-based detection of these two potent drugs in 10 drug-of-abuse (DOA) patient plasma samples with minimal preanalysis sample preparation steps. Chemometric analysis for the SERS-based detection shows a very good classification between fentanyl, alprazolam, or a mixture thereof in our selected 10 samples. Most importantly, ssESI-MS analysis also successfully identifies fentanyl or alprazolam in these same 10 DOA plasma samples. We believe that our multimodal detection approach presented herein is a highly versatile detection technology that can be applicable to the detection of any analyte type without performing any complicated sample preparation.

Modulating the Charge Transfer Plasmon in Bridged Au Core-Satellite Homometallic Nanostructures

The localized surface plasmon resonance (LSPR) is one of the important properties for noble metal nanoparticles. Tuning the LSPR on demand thus has attracted tremendous interest. Beyond the size and shape control, manipulating intraparticle coupling is an effective way to tailor their LSPR. The charge transfer plasmon (CTP) is the most important mode of conductive coupling between subunits linked by conductive bridges that are well studied for structures prepared on substrates by lithography method. However, the colloidal synthesis of CTP structure remains a great challenge. This work reports the colloidal synthesis of extraordinary bridged Au core-satellite structures by exploiting the buffer effect of polydopamine shell on Au core for Au atom diffusion, in which the Au bridge is well controlled in terms of width and length. Benefiting from the tunable Au bridges, the resonance energy of the CTP can be readily controlled. As a result, the LSPR absorptions of the core-satellite structures are continuously tuned within the NIR spectral range (from 900 to >1300 nm), demonstrating their great potentials for ultrafast nano-optics and biomedical applications.

Uncovering the cytotoxic effects of air pollution with multi-modal imaging of in vitro respiratory models

Annually, an estimated seven million deaths are linked to exposure to airborne pollutants. Despite extensive epidemiological evidence supporting clear associations between poor air quality and a range of short- and long-term health effects, there are considerable gaps in our understanding of the specific mechanisms by which pollutant exposure induces adverse biological responses at the cellular and tissue levels. The development of more complex, predictive, in vitro respiratory models, including two- and three-dimensional cell cultures, spheroids, organoids and tissue cultures, along with more realistic aerosol exposure systems, offers new opportunities to investigate the cytotoxic effects of airborne particulates under controlled laboratory conditions. Parallel advances in high-resolution microscopy have resulted in a range of in vitro imaging tools capable of visualizing and analysing biological systems across unprecedented scales of length, time and complexity. This article considers state-of-the-art in vitro respiratory models and aerosol exposure systems and how they can be interrogated using high-resolution microscopy techniques to investigate cell-pollutant interactions, from the uptake and trafficking of particles to structural and functional modification of subcellular organelles and cells. These data can provide a mechanistic basis from which to advance our understanding of the health effects of airborne particulate pollution and develop improved mitigation measures.

Raman Spectroscopy on Brain Disorders: Transition from Fundamental Research to Clinical Applications

Brain disorders such as brain tumors and neurodegenerative diseases (NDs) are accompanied by chemical alterations in the tissues. Early diagnosis of these diseases will provide key benefits for patients and opportunities for preventive treatments. To detect these sophisticated diseases, various imaging modalities have been developed such as computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET). However, they provide inadequate molecule-specific information. In comparison, Raman spectroscopy (RS) is an analytical tool that provides rich information about molecular fingerprints. It is also inexpensive and rapid compared to CT, MRI, and PET. While intrinsic RS suffers from low yield, in recent years, through the adoption of Raman enhancement technologies and advanced data analysis approaches, RS has undergone significant advancements in its ability to probe biological tissues, including the brain. This review discusses recent clinical and biomedical applications of RS and related techniques applicable to brain tumors and NDs.

Current Status and Challenges of Analytical Methods for Evaluation of Size and Surface Modification of Nanoparticle-Based Drug Formulations

The present review discusses the current status and difficulties of the analytical methods used to evaluate size and surface modifications of nanoparticle-based pharmaceutical products (NPs) such as liposomal drugs and new SARS-CoV-2 vaccines. We identified the challenges in the development of methods for (1) measurement of a wide range of solid-state NPs, (2) evaluation of the sizes of polydisperse NPs, and (3) measurement of non-spherical NPs. Although a few methods have been established to analyze surface modifications of NPs, the feasibility of their application to NPs is unknown. The present review also examined the trends in standardization required to validate the size and surface measurements of NPs. It was determined that there is a lack of available reference materials and it is difficult to select appropriate ones for modified NP surface characterization. Research and development are in progress on innovative surface-modified NP-based cancer and gene therapies targeting cells, tissues, and organs. Next-generation nanomedicine should compile studies on the practice and standardization of the measurement methods for NPs to design surface modifications and ensure the quality of NPs.

Evidence for a Local Field Effect in Surface Plasmon-Enhanced Sum Frequency Generation Vibrational Spectra

Surface plasmon-enhanced vibrational spectroscopy has been demonstrated to be an important highly sensitive diagnostic technique, but its enhanced mechanism is yet to be explored. In this study, we couple femtosecond sum frequency generation vibrational spectroscopy (SFG-VS) with surface plasmon generated by the excitation of localized gold nanorods/nanoparticles and investigate the plasmonically enhanced factors (EFs) of SFG signals from poly(methyl methacrylate) films. Through monitoring the SFG intensity of carbonyl and ester methyl groups, we have established a correlation between EFs and the coupling of localized surface plasmon resonance with SFG and visible beams. It is found that the total enhanced factor is approximately proportional to the square of an enhanced factor of the SFG electromagnetic field and the fourth power of the enhanced factor of the visible electromagnetic field. The local field effect is roughly expressed to be the square of an enhanced factor of the visible electromagnetic field. This finding will help to guide the experimental design of plasmon-enhanced SFG to drastically improve the detection sensitivity and thus provide greater insight into the ultrafast dynamics near plasmonic surfaces.

Complementary Powerful Techniques for Investigating the Interactions of Proteins with Porous TiO2 and Its Hybrid Materials: A Tutorial Review

Understanding the adsorption and interaction between porous materials and protein is of great importance in biomedical and interface sciences. Among the studied porous materials, TiO₂ and its hybrid materials, featuring distinct, well-defined pore sizes, structural stability and excellent biocompatibility, are widely used. In this review, the use of four powerful, synergetic and complementary techniques to study protein-TiO_2-based porous materials interactions at different scales is summarized, including high-performance liquid chromatography (HPLC), atomic force microscopy (AFM), surface-enhanced Raman scattering (SERS), and Molecular Dynamics (MD) simulations. We expect that this review could be helpful in optimizing the commonly used techniques to characterize the interfacial behavior of protein on porous TiO₂ materials in different applications.

Tip-Enhanced Photoluminescence of Freestanding Lateral Heterobubbles

Two-dimensional (2D) semiconducting materials have promising applications in flexible optoelectronics, nanophotonics, and sensing based on the broad tunability of their optical and electronic properties. 2D nanobubbles form exciton funnels due to localized strain that can be used as local emitters for information processing. Their nanoscale optical characterization requires the use of near-field scanning probe microscopy (SPM). However, previous near-field studies of 2D materials were performed on SiO₂/Si and metallic substrates using the plasmonic gap mode to increase the signal-to-noise ratio. Another challenge is the deterministic control of bubble size and location. We addressed these challenges by investigating the photoluminescence (PL) signals of freestanding monolayer lateral WSe₂-MoSe₂ heterostructures under the influence of strain exerted by a plasmonic SPM tip. For first time, we performed tip-enhanced PL imaging of freestanding 2D materials and studied the competition between the PL enhancement mechanisms by nanoindentation as a function of the tip-sample distance. We observed the tunability of PL as a function of bubble size, which opens new possibilities to design optoelectronic nanodevices.

Molecular and plasmonic resonances on tip-enhanced Raman spectroscopy

Plasmon has been widely investigated and applied, because it can greatly enhance molecular Raman spectral intensity. In this study, the resonance Raman effect of the tetra-tert-butylnaphthalocyanine (TTBN) is analyzed, including the Raman wave number shift and enhancement factor, resulting from light of different incident wavelengths. Furthermore, the optical properties of TTBN are obtained, such as charge transfer, the electronic circular dichroism (ECD) spectrum, etc. Lastly, we study the tip-enhanced Raman spectroscopy (TERS) by adjusting the parameters of the metal tip to achieve the highest electromagnetic enhancement at different incident wavelengths. Combining the resonance excitation effect and the tip enhanced Raman effect, the enhancement factor of TERS can reach up to 10⁸-10⁹. This study provides significant help for a profound understanding of the TERS mechanism.

Applied surface enhanced Raman Spectroscopy in plant hormones detection, annexation of advanced technologies: A review

Plant hormones are the molecules that control the vigorous development of plants and help to cope with the stress conditions efficiently due to vital and mechanized physiochemical regulations. Biologists and analytical chemists, both endorsed the extreme problems to quantify plant hormones due to their low level existence in plants and the technological support is devastatingly required to established reliable and efficient detection methods of plant hormones. Surface Enhanced Raman Spectroscopy (SERS) technology is becoming vigorously favored and can be used to accurately and specifically identify biological and chemical molecules. Subsistence molecular properties with varying excitation wavelength require the pertinent substrate to detect SERS signals from plant hormones. Three typical mechanisms of Raman signal enhancement have been discovered, electromagnetic, chemical and Tip-enhanced Raman spectroscopy (TERS). Though, complex detection samples hinder in consistent and reproducible results of SERS-based technology. However, different algorithmic models applied on preprocessed data enhanced the prediction performances of Raman spectra by many folds and decreased the fluorescence value. By incorporating SERS measurements into the microfluidic platform, further highly repeatable SERS results can be obtained. This review paper tends to study the fundamental working principles, methods, applications of SERS systems and their execution in experiments of rapid determination of plant hormones as well as several ways of integrated SERS substrates. The challenges to develop an SERS-microfluidic framework with reproducible and accurate results for plant hormone detection are discussed comprehensively and highlighted the key areas for future investigation briefly.

Two-dimensional materials for electrochemical CO2 reduction: materials, in situ/operando characterizations, and perspective

Electrochemical CO₂ reduction (CO₂ ECR) is an efficient approach to achieving eco-friendly energy generation and environmental sustainability. This approach is capable of lowering the CO₂ greenhouse gas concentration in the atmosphere while producing various valuable fuels and products. For catalytic CO₂ ECR, two-dimensional (2D) materials stand as promising catalyst candidates due to their superior electrical conductivity, abundant dangling bonds, and tremendous amounts of surface active sites. On the other hand, the investigations on fundamental reaction mechanisms in CO₂ ECR are highly demanded but usually require advanced in situ and operando multimodal characterizations. This review summarizes recent advances in the development, engineering, and structure-activity relationships of 2D materials for CO₂ ECR. Furthermore, we overview state-of-the-art in situ and operando characterization techniques, which are used to investigate the catalytic reaction mechanisms with the spatial resolution from the micron-scale to the atomic scale, and with the temporal resolution from femtoseconds to seconds. Finally, we conclude this review by outlining challenges and opportunities for future development in this field.

Influence of nonlocal dielectric response on the Au tip-enhanced fluorescence effect

Tip-enhanced fluorescence (TEF) with ultra-high detection sensitivity and spatial resolution has been a powerful characterization technique in the study of surface science and life science. Herein, a systematically theoretical investigation in the visible range had been performed to study TEF properties of a single molecule located inside a nanogap formed by Au tip and substrate. In the strong localized surface plasmon coupling effect, the contribution of nonlocal dielectric response to the fluorescence quantum yield as well as radiative and energy dissipated decay rates were calculated. It is found that the nonlocal dielectric effects become comparable to the radiative and energy dissipated decay rates with the increasing of the tip-molecule distance, as a result, the nonlocal dielectric effect significantly suppresses the fluorescence process. The huge excitation enhancement at the shorter tip-molecule distance can efficiently compensate the low quantum yield, leading to the great fluorescence enhancement. The results show that the maximum enhancement obtained from the calculations can reach as high as four orders of magnitude by optimizing the tip-molecule distance. These results are not only helpful to our understanding of the TEF mechanism but also valuable for its further applications.

Probing Bias-Induced Electron Density Shifts in Metal-Molecule Interfaces via Tip-Enhanced Raman Scattering

Surface charging effects at metal-molecule interfaces, for example, charge transfer, charge transport, charge injection, and so on, have a strong impact on the performance of organic electronics. Only having molecules bound or adsorbed on different metals results in a doping-like behavior at the interface by the different work functions of the metals and creates hybrid surface states, which strongly affect the efficiencies. With the ongoing downsizing and thinning of the organic components, the impact of the interface will even further increase. However, most of the investigations only monitor the interface without the additional charging effects from applying a voltage to the interface. In this work we present a spectroscopic approach based on tip-enhanced Raman spectroscopy (TERS) to study metal-molecule interfaces with an applied voltage simulating the electric field strength in real devices. We monitor how an intrinsic inductive effect of partial functional groups in molecules can shift the molecular electron density (ED) distribution when a bias voltage is applied. Therefore, we choose two molecules as model systems, which are similar in size and binding condition to a smooth gold surface, but with different electronic structure. By placing the tip 1 nm over the molecular surface at a fixed position and changing the applied bias voltage, we record electric-field-dependent tip-enhanced Raman spectra. Specific vibrational bands exhibit voltage-dependent intensity changes related to the shift of the local ED inside the molecules. We believe this experiment is valuable to gain deeper insights into charged metal-molecule interfaces.

Coral-like plasmonic probes for tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy is a powerful tool for the analysis of system interfaces, enabling access to chemical information with nanometric spatial resolution and sensitivity up to the single molecule level. Such features are due to the presence of proper metallic nanostructures at the TERS probe apex, which, via the excitation of a plasmonic field, confine light to a nanometric region. The nano-sized characteristic of such metallic structures intrinsically renders the fabrication of high performing and reproducible TERS probes still a challenge. In this paper, we present a facile, rapid and effective approach to prepare Ag-based TERS probes. The fabrication process proposed herein is based on spinodal dewetting of Ag-coated AFM-probes through a RF plasma treatment. The obtained probes appear covered with a coral-like silver nanotexture, endowed with an excellent plasmonic activity. Intriguingly, such a texture can be easily tuned by changing some process parameters, such as Ag film thickness and exposure time to the plasma. The as-prepared TERS probes show a high TERS enhancement, reaching 107, and allow a good spatial resolution, down to 10 nm. Finally, we suggest an easy and effective procedure to restore oxidized TERS tips following exposure to ambient air, which can be applied to all types of Ag-based TERS tips.

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.

Tuning temperature gradients in subwavelength plasmonic nanocones with tilted illumination

Inducing and controlling temperature gradients in illuminated subwavelength plasmonic structures is a challenging task. Here, we present a strategy to remotely induce and tune temperature gradients in a subwavelength metallic nanocone by adjusting the angle of incidence of linearly polarized continuous-wave illumination. We demonstrate, through rigorous three-dimensional numerical simulations, that properly tilting the incident illumination angle can increase or decrease the photoinduced temperature gradients within the nanostructure. We analyze the apex-base photoinduced temperature gradient for different illumination directions, resembling typical illumination schemes utilized in surface or tip-enhanced Raman spectroscopy.

Sample induced intensity variations of localized surface plasmon resonance in tip-enhanced Raman spectroscopy

Tip-enhanced spectroscopy techniques, in particular tip-enhanced Raman spectroscopy (TERS), rely on a localized surface plasmon resonance (LSPR). This LSPR depends on the near field antenna, its material and shape, and the surrounding medium with respect to its relative permittivity and the volume fraction of the optical near field occupied by the sample. Here, we investigate the effects of the surface composition and topography on the change of the LSPR intensity in tip-enhanced spectroscopy on SrTiO₃ nanoislands by monitoring the LSPR enhanced luminescence of gold tips. Our experimental results and analytical estimates indicate that by affecting the effective permittivity of the dielectric environment at the tip apex, the material composition as well as topography of the studied sample induce a change in LSPR intensity. This result significantly helps the understanding of the evolution or origin of the LSPR intensity during a typical TERS measurement, which in turn leads to a more accurate assessment of the relative intensity of different Raman modes in TERS.

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.

Raman spectroscopy and neuroscience: from fundamental understanding to disease diagnostics and imaging

Neuroscience would directly benefit from more effective detection techniques, leading to earlier diagnosis of disease. The specificity of Raman spectroscopy is unparalleled, given that a molecular fingerprint is attained for each species. It also allows for label-free detection with relatively inexpensive instrumentation, minimal sample preparation, and rapid sample analysis. This review summarizes Raman spectroscopy-based techniques that have been used to advance the field of neuroscience in recent years.

Excited-State Imaging of Single Particles on the Subnanometer Scale

At the intersection of spectroscopy and microscopy lie techniques that are capable of providing subnanometer imaging of excited states of individual molecules or nanoparticles. Such approaches are particularly important for imaging macromolecules or nanoparticles large enough to have a high probability of containing a defect. These inevitable defects often control properties and function despite an otherwise ideal structure. We discuss real-space imaging techniques such as using scanning tunneling microscopy tips to enhance optical measurements and electron energy-loss spectroscopy in a scanning transmission electron microscope, which is based on focused electron beams to obtain high-resolution spatial information on excited states. The outlook for these methods is bright, as they will provide critical information for the characterization and improvement of energy-switching, electron-switching, and energy-harvesting materials.

Surface- and Tip-Enhanced Raman Scattering in Tribology and Lubricant Detection—A Prospective

Surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS) are fast, convenient, and non-destructive molecular detection techniques, which provide a practical method for studying interfacial reactions with high resolution and accuracy. Both techniques are able to provide quantitative and qualitative information on the chemical properties, conformational changes, order state, and molecular orientation of various surfaces. This paper aims at summarizing the research efforts in the field of SERS and TERS related to tribological systems with a special emphasis on thin film and nanoparticles. This overview starts with a brief introduction for both techniques. Afterwards, it summarizes pros and cons of both techniques related to the advanced characterization of tribologically induced reactions layers. Moreover, the feasibility of both techniques to evaluate the friction and wear performance of new lubricant additives including solid lubricants is discussed. At the end of this review article, the main challenges and future directions in this field are prospected to emphasize the development direction of SERS and TERS in tribology and lubricants.

Controllable plasmon-induced catalytic reaction by surface-enhanced and tip-enhanced Raman spectroscopy

The controllable catalytic reaction plays a pivotal role in heterogeneous catalysis. Surface-enhanced Raman scattering (SERS) and tip enhanced Raman spectroscopy (TERS) are considered promising techniques for the study of catalytic reactions due to the highly localized sensitivity of SERS and the nanoscale spatial resolution of TERS. Herein, Ag/Au composite films were employed as catalyst for in situ monitoring of the catalytic reaction of 4‑nitrobenzenethiol (4NBT) to p, p'‑dimercaptoazobenzene (DMAB). The catalytic reaction of 4NBT adsorbed on Au film can be manipulated at the nanoscale using TERS by controlling the height between the tip-apex and the sample surface in Ag tip-Au substrate geometry. According to finite difference time domain (FDTD) simulations, the 'hot electron' induced by the localized surface plasmon is sufficient for promoting the catalytic reaction. These findings provide a novel way for controllable graph drawing of molecules at the nanoscale level.

Numerical investigations on the electromagnetic enhancement effect to tip-enhanced Raman scattering and fluorescence processes

In the present work, we theoretically study the electromagnetic (EM) enhancement of the Raman and fluorescence signals for a molecule placed in a nanocavity formed by a metallic tip and substrate that mimics a tip-enhanced Raman scattering (TERS) setup using three-dimensional finite element method calculations. The influence of tip size and tip-molecule distance on the EM enhancements of the incident field as well as the radiative and non-radiative decay rates of the molecule are systematically investigated. Simulation results show that the maximum EM enhancement to the incident light as provided by the localized surface plasmon resonance in the nanocavity can reach  ∼285 for the configuration considered in the present work. Meanwhile, it was found that, at the classical limit, decreasing the apex radius or the tip-molecule distance can both reduce the spatial distribution (as characterized by the full width at half maximum) of the Raman enhancement in a linear fashion. Moreover, simulation results show that the nonlocal dielectric response of the tip and the substrate plays a key role to the fluorescence quantum yield of the molecule. However, it was found that the strong EM excitation enhancement is the dominating factor for the tip enhanced fluorescence (TEF) effect and stronger fluorescence enhancement has been found when increasing the apex radius or reducing the tip-molecule distance with an incident wavelength of 532 nm. The best TERS and TEF enhancements were found to be  ∼[Formula: see text] and  ∼[Formula: see text], respectively, with the tip-molecule distance around 1 nm.

The role of a plasmonic substrate on the enhancement and spatial resolution of tip-enhanced Raman scattering

Since the first report in the early 2000s, there have been several experimental configurations that have demonstrated enhancement and spatial resolution of tip-enhanced Raman spectroscopy (TERS). The combination of a plasmonic substrate and a metallic tip is one suitable approach to achieve even higher enhancement and lateral resolution. In this contribution, we demonstrate TERS on a monolayer of MoS₂ on an array of Au nanodisks. The Au nanodisks were prepared by electron beam writing. Thereafter, MoS₂ was transferred onto the plasmonic substrate via the exfoliation technique. We witness an unprecedented enhancement and spatial resolution in the experiments. In the TERS image a ring-like shape is observed that matches the edges of the nanodisks. TERS enhancement at the edges is about 170 times stronger than at the center of the nanodisks. For a better understanding of the experimental results, finite element method (FEM) simulations were employed to simulate the TERS image of the MoS₂/plasmonic heterostructure. Our calculations show a higher electric field concentration at the edges that exponentially decays to the center. Therefore, it reproduces the ring-like shape of the experimental image. Moreover, the calculations suggest a TERS enhancement of 135 at the edges compared to the center, which is in very good agreement with the experimental data. According to our calculations, the spatial resolution is also increased at the edges. For comparison, FEM simulations of a tip-flat metal substrate system (conventional gap-mode TERS) were carried out. The calculations confirmed a 110 times stronger enhancement at the edges of the nanodisks than that of conventional gap-mode TERS and explained the experimental maps. Our results provide not only a deeper understanding of the TERS mechanism of this heterostructure, but can also help in realizing highly efficient TERS experiments using similar systems.

Photocatalytic Reversible Reactions Driven by Localized Surface Plasmon Resonance

In this study, we review photocatalytic reversible surface catalytic reactions driven by localized surface plasmon resonance. Firstly, we briefly introduce the synthesis of 4,4′-dimercaptoazobenzene (DMAB) from 4-nitrobenzenethiol (4NBT) using surface-enhanced Raman scattering (SERS) technology. Furthermore, we study the photosynthetic and degradation processes of 4NBT to DMAB reduction, as well as factors associated with them, such as laser wavelength, reaction time, substrate, and pH. Last but not least, we reveal the competitive relationship between photosynthetic and degradation pathways for this reduction reaction by SERS technology on the substrate of Au film over a nanosphere.