Search

The development of nanomaterials such as two-dimensional (2D) layered materials advanced applications in many fields, including biosensors format based on field-effect transistors. The unique physical and chemical properties of 2D layered materials enable the detection limit of biomolecules as low as ∼1 pg/mL. The majority of 2D layered materials contain different structural features and defects introduced in chemical synthesis and fabrication processing. These structural features have different physicochemical properties, causing heterogeneous adsorption of bioreceptors like antibodies, enzymes, etc. Understanding the correlation between the adsorption of bioreceptors and properties of structural features is essential for building highly efficient, sensitive biosensors based on 2D layered materials. Here, we utilize a single-molecule localization-based super-resolved fluorescence imaging method to unveil the inhomogeneous adsorption of antibody fragments on 2D layered molybdenum disulfide (MoS

Optical microscopy has become an invaluable tool for investigating complex samples. Over the years, many advances to optical microscopes have been made that have allowed us to uncover new insights into the samples studied. Dynamic changes in biological and chemical systems are of utmost importance to study. To probe these samples, multidimensional approaches have been developed to acquire a fuller understanding of the system of interest. These dimensions include the spatial information, such as the three-dimensional coordinates and orientation of the optical probes, and additional chemical and physical properties through combining microscopy with various spectroscopic techniques. In this review, we survey the field of multidimensional microscopy and provide an outlook on the field and challenges that may arise.

Defects can locally tailor the electronic properties of 2D materials, including the band gap and electron density, and possess the merit for optical and electronic applications. However, it is still a great challenge to realize rational defect engineering, which requires quantitative study of the effect of defects on electronic properties under ambient conditions. In this work, we employed tip-enhanced photoluminescence (TEPL) spectroscopy to obtain the PL spectra of different defects (wrinkle and edge) in mechanically exfoliated thin-layer transition metal dichalcogenides (TMDCs) with nanometer spatial resolution. We quantitatively obtained the band gap and electron density at defects by analyzing the wavelength and intensity ratio of excitons and trions. We further visualized the strain distribution across a wrinkle and the edge-induced reconstructive regions of the band gap and electron density by TEPL line scans. The doping effect on the Fermi level and optical performance was unveiled through comparative studies of edges on TMDC monolayers of different doping types. These quantitative results are vital to guide defect engineering and design and fabrication of TMDC-based optoelectronics devices.

Dynamin has an important role in clathrin-mediated endocytosis by cutting the neck of nascent vesicles from the cell membrane. Here, using gold nanorods as cargos to image dynamin action during live clathrin-mediated endocytosis, we show that, near the peak of dynamin accumulation, the cargo-containing vesicles always exhibit abrupt, right-handed rotations that finish in a short time (~0.28 s). The large and quick twist, herein named the super twist, is the result of the coordinated dynamin helix action upon GTP hydrolysis. After the super twist, the rotational freedom of the vesicle increases substantially, accompanied by simultaneous or delayed translational movement, indicating that it detaches from the cell membrane. These observations suggest that dynamin-mediated scission involves a large torque generated by the coordinated actions of multiple dynamins in the helix, which is the main driving force for vesicle scission. Using gold nanorods and 3D-SPORT to track dynamin-induced fission in live cells, Cheng et al. show that endocytic vesicles undergo a rapid, large rotation before fission, which requires dynamin’s GTPase activity.

This review covers recent progress in using single molecule fluorescence microscopy imaging to understand the nanoconfinement in porous materials. The single molecule approach unveils the static and dynamic heterogeneities from seemingly equal molecules by removing the ensemble averaging effect. Physicochemical processes including mass transport, surface adsorption/desorption, and chemical conversions within the confined space inside porous materials have been studied at nanometer spatial resolution, at the single nanopore level, with millisecond temporal resolution, and under real chemical reaction conditions. Understanding these physicochemical processes provides the ability to quantitatively measure the inhomogeneities of nanoconfinement effects from the confining properties, including morphologies, spatial arrangement, and trapping domains. Prospects and limitations of current single molecule imaging studies on nanoconfinement are also discussed.

Defects can induce drastic changes of the electronic properties of two-dimensional transition metal dichalcogenides and influence their applications. It is still a great challenge to characterize small defects and correlate their structures with properties. Here, we show that tip-enhanced Raman spectroscopy (TERS) can obtain distinctly different Raman features of edge defects in atomically thin MoS2, which allows us to probe their unique electronic properties and identify defect types (e.g., armchair and zigzag edges) in ambient. We observed an edge-induced Raman peak (396 cm−1) activated by the double resonance Raman scattering (DRRS) process and revealed electron–phonon interaction in edges. We further visualize the edge-induced band bending region by using this DRRS peak and electronic transition region using the electron density-sensitive Raman peak at 406 cm−1. The power of TERS demonstrated in MoS2 can also be extended to other 2D materials, which may guide the defect engineering for desired properties. Probing inevitable defects in two- dimensional materials is challenging. Here, the authors tackle this issue by using tip-enhanced Raman spectroscopy (TERS) to obtain distinctly different Raman features of edge defects in atomically thin MoS2, and further probe their unique electronic properties as well as identify the armchair and zigzag edges.

The insightful comprehension of in situ catalytic dynamics at individual structural defects of two-dimensional (2D) layered material, which is crucial for the design of high-performance catalysts via defect engineering, is still missing. Here, we resolved single-molecule trajectories resulted from photocatalytic activities at individual structural features (i.e., basal plane, edge, wrinkle, and vacancy) in 2D layered indium selenide (InSe) in situ to quantitatively reveal heterogeneous photocatalytic dynamics and surface diffusion behaviors. The highest catalytic activity was found at vacancy in a four-layer InSe, up to ~30× higher than that on the basal plane. Moreover, lower adsorption strength of reactant and slower dissociation/diffusion rates of product were found at more photocatalytic active defects. These distinct dynamic properties are determined by lattice structures/electronic energy levels of defects and layer thickness of supported InSe. Our findings shed light on the fundamental understanding of photocatalysis at defects and guide the rational defect engineering.

Electrochemical tip-enhanced Raman spectroscopy (EC-TERS), which provides molecular fingerprint information with nanometer-scale spatial resolution, is a promising technique to study the structure-activity relationships of the electrochemical interface. In this work, we developed the electrochemical tip-enhanced Raman spectroscopy (EC-TERS) that possesses high sensitivity and nanoscale spatial resolution, as well as methods to fabricate TERS tips with a high enhancement. Based on the developed systems, we in-situ monitor the plasmon driven decarboxylation reaction. The spatial distribution of the effective hot carriers was visualized by TERS imaging of the nanoscale reaction region, which provides mechanistic insights into plasmon driven reactions.

In the field of surface plasmon-mediated photocatalysis, the coupling reactions of p-aminothiophenol (PATP) and p-nitrothiophenol (PNTP) to produce p, p'-dimercaptoazobenzene (DMAB) are the most widely investigated systems. However, a clear understanding of the structure-function relationship is still required. Here, we used tip-enhanced Raman spectroscopy (TERS) to study the coupling reactions of PATP and PNTP on well-defined Ag(111) and Au(111) surfaces using 632.8 and 532 nm lasers. On Au(111), the oxidative coupling of PATP can proceed under irradiation by a 632.8 nm laser, and the reductive coupling of PNTP can only occur under irradiation by a 532 nm laser. Neither wavelength of laser light can induce the coupling reactions of these two molecules on Ag(111). Density functional theory (DFT) was used to calculate the stable adsorption configurations of PATP and PNTP on Ag(111) and Au(111). Both the adsorption configurations of the two molecules on the surfaces and laser energies were, experimentally and theoretically, found to determine whether the coupling reactions can occur on different substrates. These results may help the rational design of photocatalysts with enhanced reactivity.

We propose programed potential modulation strategies to balance the ion intercalation/deintercalation, surface tailoring and bubbling dispersion processes in the electrochemical exfoliation of graphite, resulting in high-quality graphene with high crystallinity, low oxidation degree, uniform size distribution and few layers.

Dynamin plays an important role in clathrin-mediated endocytosis by cutting the neck of nascent vesicles from the cell membrane. Gold nanorods were used as imaging probes to observe dynamin action on cargo vesicles during live endocytosis events. Invariant is that at the peak of dynamin accumulation, the cargo-containing vesicle always gives abrupt, right-handed rotations that finishes in a short time (~ 0.28 s). The large and quick twist, herein named the super twist, is the result of the coordinated dynamin helix action upon GTP hydrolysis. After the super twist, the rotational freedom of the vesicle drastically increases, accompanied with simultaneous or delayed translational movement, indicating that it detaches from the cell membrane. These observations suggest that dynamin-mediated scission at the final stage involves a large torque generated by coordinated actions of multiple dynamins in the helix, which is the main driving force for scission. The super twist presumably results in membrane tube hemi-fission and partial destruction of the dynamin helix, followed by vesicle fission.

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.

A multimodality spectromicroscopy imaging system has been developed to offer the essential capability of in situ characterization of functional materials at multiple length scales during the morphology evolution and phase development under external stimuli. The photoactive layer of bulk heterojunction solar cell, whose performance is strongly correlated to the structural features over a wide range of length scales, was characterized under thermal stress. Three stages of thermotropic evolution were monitored continuously by the spectromicroscopy imaging system to reveal the critical information from the molecular level to meso- and microscale. The optimized thermal annealing temperature window and preferred temperature dropping operation were identified to promote the performance of the photoactive layer.

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.

Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive technique with molecular specificity, making it an ideal analytical tool in various fields. However, the breadth of practical applications of SERS has been severely limited because it is still a great challenge to achieve simultaneously a high sensitivity and a high reproducibility. Herein, we report a highly controllable method to fabricate periodic bowtie SERS substrates with a narrow nanogap, high SERS enhancement, and good uniformity over a large area. The periodic bowtie template is first fabricated over a gold film by holographic lithography (HL), followed by Au deposition to obtain a conductive plasmonic bowtie array. The gap size is then narrowed down by pulsed electrodeposition of Ag simultaneously monitored in situ by electrochemical dark field spectroscopy. Thus, we are able to observe the most sensitive change in the scattering spectra when the gap is just about to merge and obtain uniform SERS substrates with a gap size down to around 5 nm. The average enhancement factor of 5 × 107 to 1 × 108 is obtained, which is 50 times larger than that from Au nanoparticle-assembled substrates and 140 times larger than that from commercial Klarite chips. This substrate offers a promising opportunity for SERS practical applications.

Nanoconfinement imposes physical constraints and chemical effects on reactivity in nanoporous catalyst systems. In the present study, we lay the groundwork for quantitative single-molecule measurements of the effects of chemical environment on heterogeneous catalysis in nanoconfinement. Choosing hydrophobicity as an exemplary chemical environmental factor, we compared a range of essential parameters for an oxidation reaction on platinum nanoparticles (NPs) confined in hydrophilic and hydrophobic nanopores. Single-molecule experimental measurements at the single particle level showed higher catalytic activity, stronger adsorption strength, and higher activation energy in hydrophobic nanopores than those in hydrophilic nanopores. Interestingly, different dissociation kinetic behaviors of the product molecules in the two types of nanopores were deduced from the single-molecule imaging data.

Here we propose a strategy of radical oxidation reaction for the high-efficiency production of graphene oxide (GO). GO plays important roles in the sustainable development of energy and the environment, taking advantages of oxygen-containing functional groups for good dispersibility and assembly. Compared with Hummers' method, electrochemical exfoliation of graphite is considered facile and green, although the oxidation is fairly low. To synthesize GO with better crystallinity and higher oxidation degree, we present a photosynergetic electrochemical method. By using oxalate anions as the intercalation ions and co-reactant, the interfacial concentration of hydroxyl radicals generated during electrochemical exfoliation was promoted, and the oxidation degree was comparable with that of GO prepared by Hummers' method. In addition, the crystallinity was improved with fewer layers and larger size. Moreover, the aniline coassembled GO membrane was selectively permeable to water molecules by the hydrogen-bond interaction, but it was impermeable to Na+, K+, and Mg2+, due to the electrostatic interactions. Thus, it has a prospective application to water desalination and purification. This work opens a novel approach to the direct functionalization of graphene during the electroexfoliation processes and to the subsequent assembly of the functionalized graphene.

Active oxygen species (AOS) play key roles in many important catalytic reactions relevant to clean energy and environment. However, it remains challenging to characterize the active sites for producing AOS and to image the surface properties of AOS, especially on multicomponent metallic catalyst surfaces. Herein, we utilize tip-enhanced Raman spectroscopy (TERS) to probe the local generation and diffusion of OH radicals on a Pd/Au(111) bimetallic catalyst surface. The reactive OH radicals can be catalytically generated from hydrogen peroxide (H2O2) at the metal surface, which then oxidizes the surface adsorbed thiolate, a reactant that is used as the TERS probe. By TERS imaging of the spatial distribution of unreacted thiolate molecules, we demonstrate that the Pd surface is active for generation of OH radicals and the Pd step edge shows much higher activity than the Pd terrace, whereas the Au surface is inactive. Furthermore, we find that the locally generated OH radicals at the Pd step edge could diffuse to both the Au and the Pd surface sites to induce oxidative reactions, with a diffusion length estimated to be about 5.4 nm. Our TERS imaging with few-nanometer spatial resolution not only unravels the active sites but also characterizes in real space the diffusion behavior of OH radicals. The results are highly valuable to understand AOS-triggered catalytic reactions. The strategy of using reactants with large Raman cross sections as TERS probes may broaden the application of TERS for studying catalysis with reactive small molecules.

Tip-enhanced Raman spectroscopy (TERS), known as nanospectroscopy, has received increasing interest as it can provide nanometer spatial resolution and chemical fingerprint information of samples simultaneously. Since Ag tips are well accepted to show a higher TERS enhancement than that of gold tips, there is an urgent quest for Ag TERS tips with a high enhancement, long lifetime, and high reproducibility, especially for atomic force microscopy (AFM)-based TERS. Herein, we developed an electrodeposition method to fabricate Ag-coated AFM TERS tips in a highly controllable and reproducible way. We investigated the influence of the electrodeposition potential and time on the morphology and radius of the tip. The radii of Ag-coated AFM tips can be rationally controlled at a few to hundreds nanometers, which allows us to systematically study the dependence of the TERS enhancement on the tip radius. The Ag-coated AFM tips show the highest TERS enhancement under 632.8 nm laser excitation and a broad localized surface plasmon resonance (LSPR) response when coupled to a Au substrate. The tips exhibit a lifetime of 13 days, which is particularly important for applications that need a long measuring time.

Surface and interfaces play key roles in heterogeneous catalysis, electrochemistry and photo(electro)chemistry. Tip-enhanced Raman spectroscopy (TERS) combines plasmon-enhanced Raman spectroscopy with scanning probe microscopy to simultaneously provide a chemical fingerprint and morphological information for the sample at the nanometer spatial resolution. It is an ideal tool for achieving an in-depth understanding of the surface and interfacial processes, so that the relationship between structure and chemical performance can be established. We begin with the background of surfaces and interfaces and TERS, followed by a detailed discussion on some issues in experimental TERS, including tip preparation and TERS instrument configuration. We then focus on the progress of TERS for studying the surfaces and interfaces under different conditions, from ambient, to UHV, solid-liquid and electrochemical environments, followed by a brief introduction to the current understanding of the unprecedented high spatial resolution and surface selection rules. We conclude by discussing the future challenges for TERS practical applications in surfaces and interfaces.

We investigated tip-enhanced Raman spectra excited by high-order fiber vector beams. Theoretical analysis shows that the high-order fiber vector beams have stronger longitudinal electric field components than linearly polarized light under tight focusing conditions. By introducing the high-order fiber vector beams and the linearly polarized beam from a fiber vector beam generator based on an electrically-controlled acoustically-induced fiber grating into a top-illumination tip-enhanced Raman spectroscopy (TERS) setup, the tip-enhanced Raman signal produced by the high-order fiber vector beams was 1.6 times as strong as that produced by the linearly polarized light. This result suggests a new type of efficient excitation light beams for TERS.

Reproducible fabrication of sharp gold- or silver-coated tips has become the bottleneck issue in tip-enhanced Raman spectroscopy, especially for atomic force microscopy (AFM)-based TERS. Herein, we developed a novel method based on pulsed electrodeposition to coat a thin gold layer over atomic force microscopy (AFM) tips to produce plasmonic TERS tips with high reproducibility. We systematically investigated the influence of the deposition potential and step time on the surface roughness and sharpness. This method allows the rational control of the radii of gold-coated TERS tips from a few to hundreds of nanometers, which allows us to systematically study the dependence of the TERS enhancement on the radius of the gold-coated AFM tip. The maximum TERS enhancement was achieved for the tip radius in the range of 60-75 nm in the gap mode. The coated gold layer has a strong adhesion with the silicon tip surface, which is highly stable in water, showing the great potential for application in the aqueous environment.

Wide applications of surface plasmon resonance rely on the in-depth understanding of the near-field distribution over a metallic nanostructure. However, precisely locating the strongest electric field in a metallic nanostructure still remains a great challenge in experiments because the field strength decays exponentially from the surface. Here, we demonstrate that the hot spot position for gold nanoparticles over a metal film can be precisely located using surface-enhanced Raman spectroscopy (SERS) by rationally choosing the probe molecules and excitation wavelengths. The finite difference time domain simulation verifies the experimental results and further reveals that the enhancement for the above system is sensitive to the distance between nanoparticles and the metal film but insensitive to the distance of nanoparticles. On the basis of this finding, we propose and demonstrate an approach of using a nanoparticles-on-metal film substrate as a uniform SERS substrate. This work provides a convenient way to probe the location of strong near-field enhancement with SERS and will have important implications in both surface analysis and surface plasmonics.

Surface plasmon polaritons (SPPs) are extremely sensitive to the surrounding refractive index and have found important applications in ultrasensitive label-free sensing. Reducing the linewidth of an SPP mode is an effective way to improve the figure of merit (FOM) and hence the sensitivity of the plasmonic mode. Many efforts have been devoted to achieving a narrow linewidth by mode coupling, which inevitably results in an asymmetrical lineshape compromising the performance. Instead, the SPP modes are directly narrowed by elaborately engineering periodic plasmonic structures with minimized feature sizes to effectively reduce the radiative losses. A narrow linewidth smaller than 8 nm is achieved over a wide wavelength ranging from 600 to 960 nm and a minimum full width at half maximum of 3 nm at 960 nm. Benefiting from the almost perfect Lorentzian lineshape and the extremely narrow linewidth, a record FOM value of 730 is obtained. The sensor is capable of detecting bovine serum albumin with an ultralow concentration of 10-10 m. The sensor has great potential for practical application for its ultrahigh FOM, broad working wavelength, and ease of high-throughput fabrication.

Interfacial properties are highly important to the performance of some energy-related systems. The in-depth understanding of the interface requires highly sensitive in situ techniques that can provide fingerprint molecular information at nanometer resolution. We developed an electrochemical tip-enhanced Raman spectroscopy (EC-TERS) by introduction of the light horizontally to the EC-STM cell to minimize the optical distortion and to keep the TERS measurement under a well-controlled condition. We obtained potential-dependent EC-TERS from the adsorbed aromatic molecule on a Au(111) surface and observed a substantial change in the molecule configuration with potential as a result of the protonation and deprotonation of the molecule. Such a change was not observable in EC-SERS (surface-enhanced), indicating EC-TERS can more faithfully reflect the fine interfacial structure than EC-SERS. This work will open a new era for using EC-TERS as an important nanospectroscopy tool for the molecular level and nanoscale analysis of some important electrochemical systems including solar cells, lithium ion batteries, fuel cells, and corrosion.

Tip-enhanced Raman spectroscopy (TERS) can not only provide very high sensitivity but also high spatial resolution, and has found applications in various fields, including surface science, materials, and biology. Most of previous TERS studies were performed in air or in the ultrahigh vacuum. If TERS study can be performed in the electrochemical environment, the electronic properties of the surface can be well controlled so that the interaction of the molecules with the substrate and the configuration of the molecules on the surface can also be well controlled. However, the EC-TERS is not just a simple combination of electrochemistry with TERS, or the combination of EC-STM with Raman. It is a merge of STM, electrochemistry and Raman spectroscopy, and the mutual interference among these techniques makes the EC-TERS particularly challenge: the light distortion in EC system, the sensitivity, the tip coating to work under EC-STM and retain the TERS activity and cleanliness. We designed a special spectroelectrochemical cell to eliminate the distortion of the liquid layer to the optical path and obtain TER spectra of reasonably good signal to noise ratio for surface adsorbed molecules under electrochemical potential control. For example, potential dependent TERS signal have been obtained for adsorbed aromatic thiol molecule, and much obvious signal change compared with SERS has been found, manifesting the importance of EC-TERS to reveal the interfacial structure of an electrochemical system. We further extended EC-TERS to electrochemical redox system, and clear dependence of the species during redox reaction can be identified.

After over 15 years of development, tip-enhanced Raman spectroscopy (TERS) is now facing a very important stage in its history. TERS offers high detection sensitivity down to single molecules and a high spatial resolution down to sub-nanometers, which make it an unprecedented nanoscale analytical technique offering molecular fingerprint information. The tip is the core element in TERS, as it is the only source through which to support the enhancement effect and provide the high spatial resolution. However, TERS suffers and will continue to suffer from the limited availability of TERS tips with a high enhancement, good stability, and high reproducibility. This review focuses on the tip-related issues in TERS. We first discuss the parameters that influence the enhancement and spatial resolution of TERS and the possibility to optimize the performance of a TERS system via an in-depth understanding of the enhancement mechanism. We then analyze the methods that have been developed for producing TERS tips, including vacuum-based deposition, electrochemical etching, electrodeposition, electroless deposition, and microfabrication, with discussion on the advantages and weaknesses of some important methods. We also tackle the issue of lifetime and protection protocols of TERS tips which are very important for the stability of a tip. Last, some fundamental problems and challenges are proposed, which should be addressed before this promising nanoscale characterization tool can exert its full potential. Graphical Abstract ᅟ.

The optimal gold-coated atomic force microscopy (AFM) tip-substrate system for tip-enhanced Raman spectroscopy (TERS) was designed theoretically and demonstrated experimentally. By optimizing the tip, excitation laser, and the substrate, the TERS enhancement factor can be tuned to as high as 9 orders of magnitude, and the spatial resolution could be down to 5 nm. Preliminary experimental results for AFM tips coated with gold layer of different thicknesses reveal that the maximum enhancement can be achieved when the thickness is about 60-80 nm, which is in good agreement with the theoretical prediction. Our results not only provide a deep understanding of the underlying physical mechanism of AFM tip-based TERS, but also guide the rational construction of a working AFM-TERS system with a high efficiency.

not Available.