Tip-enhanced Raman mapping with top-illumination AFM

Electrospun Poly-L-Lactic Acid Scaffolds Surface-Modified via Reactive Magnetron Sputtering Using Different Mixing Ratios of Nitrogen and Xenon

Controlled regeneration processes involving tissue growth using the surface and structure of scaffolds, are actively used in tissue engineering. Reactive magnetron sputtering is a versatile surface modification method of both metal and polymer substrates, as the properties of the formed coatings can be modified in a wide range by changing the process parameters. In magnetron sputtering, the working gas and its composition have an influence on the chemical composition and physical characteristics of the obtained coatings. However, there are no studies addressing the influence of the nitrogen/xenon gas mixture ratio in direct current magnetron sputtering on the deposition rate, physicochemical and in vitro properties of surface-modified biocompatible poly-L-lactic acid scaffolds. In this study, the application of mixtures of nitrogen and xenon in various ratios is demonstrated to modify the surface of non-woven poly-L-lactic acid scaffolds by direct current magnetron sputtering of a titanium target. It has been found that the magnetron sputtering parameters chosen do not negatively influence the morphology of the prepared scaffolds, but increase the hydrophilicity. Moreover, quantitative spectroscopic analysis results indicate that the formed coatings are primarily composed of titanium oxide and titanium oxynitride compounds and is dependent on the gas mixture ratio only to a certain extent. Atomic force microscopy investigations of the roughness of the fibers of the electrospun scaffolds and the thickness of the coatings formed on them show that the considerable variations observed in the intrinsic fiber reliefs are due to the formation of a fine layer on the fiber surfaces. The observed decrease in roughness after plasma modification is due to temperature and radiation effects of the plasma. In vitro experiments with human osteosarcoma cells show that the scaffolds investigated here have no cytotoxic effect on these cells. The cells adhere and proliferate well on each of the surface-modified electrospun scaffolds, with stimulation of cell differentiation in the osteogenic direction.

Thin head atomic force microscope for integration with optical microscope

We present a novel thin head atomic force microscope (AFM) that can be easily integrated with an upright optical microscope (OM). The optical beam detection unit in the AFM used an obliquely incident laser beam onto the cantilever, reducing the AFM head's effective thickness to 7.3 mm. That allows an open space above the cantilever probe to accommodate the objective lens up to 0.6 numerical aperture (N.A.) without obstruction. A multi-function digital controller was developed to control the AFM and reserved interfaces to communicate with the OM. To assess the performance of the developed AFM, we first measured the noise level and bandwidths of the AFM system. Then, the imaging quality of the AFM was evaluated by both calibration grids and two-dimensional materials. Finally, the thin head AFM was integrated into a homemade white light interferometer as a demonstration of combined use with an advanced optical system. The experimental results demonstrated that our developed AFM is suitable for integration under upright OM and brings AFM high-resolution advantages to the existing OM system.

Facilitating Hotspot Alignment in Tip-Enhanced Raman Spectroscopy via the Silver Photoluminescence of the Probe

A tip-enhanced Raman spectroscopy (TERS) system based on an atomic force microscope (AFM) and radially polarized laser beam was developed. A TERS probe with plasmon resonance wavelength matching the excitation wavelength was prepared with the help of dark-field micrographs. The intrinsic photoluminescence (PL) from the silver (Ag)-coated TERS probe induced by localized surface plasmon resonance contains information about the near-field enhanced electromagnetic field intensity of the probe. Therefore, we used the intensity change of Ag PL to evaluate the stability of the Ag-coated probe during TERS experiments. Tracking the Ag PL of the TERS probe was helpful to detect probe damage and hotspot alignment. Our setup was successfully used for the TERS imaging of single-walled carbon nanotubes, which demonstrated that the Ag PL of the TERS probe is a good criterion to assist in the hotspot alignment procedure required for TERS experiments. This method lowers the risk of contamination and damage of the precious TERS probe, making it worthwhile for wide adoption in TERS experiments.

Review: Cantilever-Based Sensors for High Speed Atomic Force Microscopy

This review critically summarizes the recent advances of the microcantilever-based force sensors for atomic force microscope (AFM) applications. They are one the most common mechanical spring-mass systems and are extremely sensitive to changes in the resonant frequency, thus finding numerous applications especially for molecular sensing. Specifically, we comment on the latest progress in research on the deflection detection systems, fabrication, coating and functionalization of the microcantilevers and their application as bio- and chemical sensors. A trend on the recent breakthroughs on the study of biological samples using high-speed atomic force microscope is also reported in this review.

Local structural changes in graphene oxide layers induced by silver nanoparticles

Structural changes of graphene oxide (GO) in silver/graphene oxide (AGO) nanocomposites are investigated using tip-enhanced Raman scattering (TERS). Because of markedly high spatial resolution of the TERS technique, the measurements of molecular information at specific nano-scaled positions can be achieved by constructing line-profile TERS spectra straight from the center of silver nanoparticles (AgNPs) on GO layers. The results show evidences that AgNPs cause shortening of C-C bonds beneath AgNPs, flattening of GO layers, and critical bending on GO layers. Additionally, a connection of carbon atoms via a C-C network subsequently expands structural changes with a distance of 200-250 nm from the center of AgNPs, even though this distance is larger than the size of AgNPs. The proposed model of GO structural changes unveils new understanding about changes in properties from GO to AGO nanocomposites, which will contribute to a development of advanced nanostructures/nanocomposites in the near future.

Polarization Control with Plasmonic Antenna Tips: A Universal Approach to Optical Nanocrystallography and Vector-Field Imaging

Controlling the propagation and polarization vectors in linear and nonlinear optical spectroscopy enables us to probe the anisotropy of optical responses providing structural symmetry selective contrast in optical imaging. Here, we present a novel tilted antenna-tip approach to control the optical vector-field by breaking the axial symmetry of the nanoprobe in tip-enhanced near-field microscopy. This gives rise to a localized plasmonic antenna effect with significantly enhanced optical field vectors with control of both in-plane and out-of-plane components. We use the resulting vector-field specificity in the symmetry selective nonlinear optical response of second-harmonic generation (SHG) for a generalized approach to optical nanocrystallography and imaging. In tip-enhanced SHG imaging of monolayer MoS₂ films and single-crystalline ferroelectric YMnO₃, we reveal nanocrystallographic details of domain boundaries and domain topology with enhanced sensitivity and nanoscale spatial resolution. The approach is applicable to any anisotropic linear and nonlinear optical response and enables the optical nanocrystallographic imaging of molecular or quantum materials.

Tip-enhanced Raman spectroscopy - from early developments to recent advances

An analytical technique operating at the nanoscale must be flexible regarding variable experimental conditions while ideally also being highly specific, extremely sensitive, and spatially confined. In this respect, tip-enhanced Raman scattering (TERS) has been demonstrated to be ideally suited to, e.g., elucidating chemical reaction mechanisms, determining the distribution of components and identifying and localizing specific molecular structures at the nanometre scale. TERS combines the specificity of Raman spectroscopy with the high spatial resolution of scanning probe microscopies by utilizing plasmonic nanostructures to confine the incident electromagnetic field and increase it by many orders of magnitude. Consequently, molecular structure information in the optical near field that is inaccessible to other optical microscopy methods can be obtained. In this general review, the development of this still-young technique, from early experiments to recent achievements concerning inorganic, organic, and biological materials, is addressed. Accordingly, the technical developments necessary for stable and reliable AFM- and STM-based TERS experiments, together with the specific properties of the instruments under different conditions, are reviewed. The review also highlights selected experiments illustrating the capabilities of this emerging technique, the number of users of which has steadily increased since its inception in 2000. Finally, an assessment of the frontiers and new concepts of TERS, which aim towards rendering it a general and widely applicable technique that combines the highest possible lateral resolution and extreme sensitivity, is provided.

Tip in-light on: Advantages, challenges, and applications of combining AFM and Raman microscopy on biological samples

Scanning probe microscopies and spectroscopies, especially AFM and Confocal Raman microscopy are powerful tools to characterize biological materials. They are both non-destructive methods and reveal mechanical and chemical properties on the micro and nano-scale. In the last years the interest for increasing the lateral resolution of optical and spectral images has driven the development of new technologies that overcome the diffraction limit of light. The combination of AFM and Raman reaches resolutions of about 50-150 nm in near-field Raman and 1.7-50 nm in tip enhanced Raman spectroscopy (TERS) and both give a molecular information of the sample and the topography of the scanned surface. In this review, the mentioned approaches are introduced, the main advantages and problems for application on biological samples discussed and some examples for successful experiments given. Finally the potential of colocated AFM and Raman measurements is shown on a case study of cellulose-lignin films: the topography structures revealed by AFM can be related to a certain chemistry by the colocated Raman scan and additionally the mechanical properties be revealed by using the digital pulsed force mode. Microsc. Res. Tech. 80:30-40, 2017. © 2016 Wiley Periodicals, Inc.

Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS), a combination of Raman spectroscopy and apertureless near-field scanning optical microscopy using a metallic tip which resonates with the local mode of the surface plasmon, can provide a high-sensitive and high-spatial-resolution optical analytical approach. The basic principle of TERS, common experimental setups, various SPM technologies, and excitation/collection configurations are introduced as well as recent research progress with respect to TERS.

Laser Scanning-Assisted Tip-Enhanced Optical Microscopy for Robust Optical Nanospectroscopy

Laser-scanning-assisted tip-enhanced optical microscopy was developed for robust optical nanospectroscopy. The laser-scanning system was utilized to automatically set and keep the center of a tight laser-focusing spot in the proximity of a metallic tip with around 10 nm precision. This enabled us to efficiently and stably induce plasmon-coupled field enhancement at the apex of the metallic probe tip. The laser-scanning technique was also applied to tracking and compensating the thermal drift of the metallic tip in the spot. This technique is usable for long-term tip-enhanced optical spectroscopy without any optical degradation.

Exploring the Nanoscale: Fifteen Years of Tip-Enhanced Raman Spectroscopy

Spectroscopic methods with high spatial resolution are essential to understand the physical and chemical properties of nanoscale materials including biological and chemical materials. Tip-enhanced Raman spectroscopy (TERS) is a combination of surface-enhanced Raman spectroscopy (SERS) and scanning probe microscopy (SPM), which can provide high-resolution topographic and spectral information simultaneously below the diffraction limit of light. Even examples of sub-nanometer resolution have been demonstrated. This review intends to give an introduction to TERS, focusing on its basic principle and the experimental setup, the strengths followed by recent applications, developments, and perspectives in this field.

Tip-enhanced Raman scattering--Targeting structure-specific surface characterization for biomedical samples

Tip-enhanced Raman scattering (TERS) has become a powerful tool for nanoscale structural analysis for several branches of organic, inorganic, and biological chemistry. This highly sensitive technique enables molecular characterization with a lateral resolution far beyond Abbe's diffraction limit and correlates structural and topographic information on a nanometer scale. In this review, the current experimental concepts with respect to their strengths and obstacles are introduced and discussed. A further focus was set to biochemistry comprising applications like nucleic acids, proteins, and microorganisms, thus demonstrating the potential use towards the pharmaceutically relevant challenges where nanometer resolution is required.

Gold-coated AFM tips for tip-enhanced Raman spectroscopy: theoretical calculation and experimental demonstration

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.

A modified transmission tip-enhanced Raman scattering (TERS) setup provides access to opaque samples

The combination of scanning probe microscopy and Raman spectroscopy enables chemical characterization of surfaces at highest spatial resolution. This so-called tip-enhanced Raman scattering (TERS) can be employed for a variety of samples where a label-free characterization or identification of constituents on the nanometer scale is pursued. Present TERS setup geometries are always a compromise for specific dedicated applications and show different advantages and disadvantages: Transmission back-reflection setups, when using immersion objectives with a high numerical aperture, intrinsically provide the highest collection efficiency but cannot be applied for opaque samples. Those samples demand upright setups, at the cost of lower collection efficiency, even though very efficient systems using a parabolic mirror for illumination and collection have been demonstrated. In this contribution it is demonstrated that the incorporation of a dichroic mirror to a transmission TERS setup provides easy access to opaque samples without further modification of the setup.

Reflection-mode TERS on Insulin Amyloid Fibrils with Top-Visual AFM Probes

Tip-enhanced Raman spectroscopy provides chemical information while raster scanning samples with topographical detail. The coupling of atomic force microscopy and Raman spectroscopy in top illumination optical setup is a powerful configuration to resolve nanometer structures while collecting reflection mode backscattered signal. Here, we theoretically calculate the field enhancement generated by TER spectroscopy with top illumination geometry and we apply the technique to the characterization of insulin amyloid fibrils. We experimentally confirm that this technique is able to enhance the Raman signal of the polypeptide chain by a factor of 10⁵, thus revealing details down to few molecules resolution.

Invited review article: combining scanning probe microscopy with optical spectroscopy for applications in biology and materials science

This is a comprehensive review of the combination of scanning probe microscopy (SPM) with various optical spectroscopies, with a particular focus on Raman spectroscopy. Efforts to combine SPM with optical spectroscopy will be described, and the technical difficulties encountered will be examined. These efforts have so far focused mainly on the development of tip-enhanced Raman spectroscopy, a powerful technique to detect and image chemical signatures with single molecule sensitivity, which will be reviewed. Beyond tip-enhanced Raman spectroscopy and/or topography measurements, combinations of SPM with optical spectroscopy have a great potential in the characterization of structure and quantitative measurements of physical properties, such as mechanical, optical, or electrical properties, in delicate biological samples and nanomaterials. The different approaches to improve the spatial resolution, the chemical sensitivity, and the accuracy of physical properties measurements will be discussed. Applications of such combinations for the characterization of structure, defects, and physical properties in biology and materials science will be reviewed. Due to the versatility of SPM probes for the manipulation and characterization of small and/or delicate samples, this review will mainly focus on the apertureless techniques based on SPM probes.

Note: tip enhanced Raman spectroscopy with objective scanner on opaque samples

We report on 14 nm lateral resolution in tip-enhanced Raman spectroscopy mapping of carbon nanotubes with an experimental setup that has been designed for the analysis of opaque samples in confocal side-access through a novel piezo-driven objective scanner. The objective scanner allows for fast and stable laser-to-tip alignment and for the adjustment of the focus position with sub-wavelength precision to optimize the excitation of surface plasmons. It also offers the additional benefit of imaging the near-field generated Raman scattering at the gap between tip and sample as direct control of the tip enhancement.

Developments in and practical guidelines for tip-enhanced Raman spectroscopy

This feature review provides an overview of the state-of the art and recent developments in tip-enhanced Raman spectroscopy (TERS), in-depth information about the different available types of instruments including their (dis-)advantages and capabilities as well as a short glance at a number of samples that have recently been investigated using TERS. Issues concerning the progression of TERS from point spectroscopy to an imaging technique are discussed, as well as problems arising from background and contamination signals. This review is concluded with a short TERS 'user guideline', trying to aid researchers new in the field to properly align and test their own TERS setups. Finally, a short outlook is given and some critical issues are raised that need to be solved by the community sooner or later, in order to promote TERS towards a 'push-button' operation.