Nanoscale characterization of plasma functionalized graphitic flakes using tip-enhanced Raman spectroscopy

High-precision Helicobacter pylori infection diagnosis using a dual-element multimodal gas sensor array

Helicobacter pylori (H. pylori) is a globally widespread bacterial infection. Early diagnosis of this infection is vital for public and individual health. Prevalent diagnosis methods like the isotope 13C or 14C labelled urea breath test (UBT) are not convenient and may do harm to the human body. The use of cross-response gas sensor arrays (GSAs) is an alternative way for label-free detection of metabolite changes in exhaled breath (EB). However, conventional GSAs are complex to prepare, lack reliability, and fail to discriminate subtle changes in EB due to the use of numerous sensing elements and single dimensional signal. This work presents a dual-element multimodal GSA empowered with multimodal sensing signals including conductance (G), capacitance (C), and dissipation factor (DF) to improve the ability for gas recognition and H. pylori-infection diagnosis. Sensitized by poly(diallyldimethylammonium chloride) (PDDA) and the metal-organic framework material NH2-UiO66, the dual-element graphene oxide (GO)-composite GSAs exhibited a high specific surface area and abundant adsorption sites, resulting in high sensitivity, repeatability, and fast response/recovery speed in all three signals. The multimodal sensing signals with rich sensing features allowed the GSA to detect various physicochemical properties of gas analytes, such as charge transfer and polarization ability, enhancing the sensing capabilities for gas discrimination. The dual-element GSA could differentiate different typical standard gases and non-dehumidified EB samples, demonstrating the advantages in EB analysis. In a case-control clinical study on 52 clinical EB samples, the diagnosis model based on the multimodal GSA achieved an accuracy of 94.1%, a sensitivity of 100%, and a specificity of 90.9% for diagnosing H. pylori infection, offering a promising strategy for developing an accurate, non-invasive and label-free method for disease diagnosis.

Regioselective Tip-Enhanced Raman Spectroscopy of Lipid Membranes with Sub-Nanometer Axial Resolution

Noninvasive and label-free analysis of cell membranes at the nanoscale is essential to comprehend vital cellular processes. However, conventional analytical tools generally fail to meet this challenge due to the lack of required sensitivity and/or spatial resolution. Herein, we demonstrate that tip-enhanced Raman spectroscopy (TERS) is a powerful nanoanalytical tool to analyze dipalmitoylphosphatidylcholine (DPPC) bilayers and human cell membranes with submolecular resolution in the vertical direction. Unlike the far-field Raman measurements, TERS spectra of the DPPC bilayers reproducibly exhibited a uniquely shaped C-H band. These unique spectral features were also reproducibly observed in the TERS spectrum of human pancreatic cancer cells. Spectral deconvolution and DFT simulations confirmed that the TERS signal primarily originated from vibrations of the CH₃ groups in the choline headgroup of the lipids. The reproducible TERS results obtained in this study unequivocally demonstrate the ultrahigh sensitivity of TERS for nanoanalysis of lipid membranes under ambient conditions.

Nanoscale chemical analysis of 2D molecular materials using tip-enhanced Raman spectroscopy

Two-dimensional (2D) molecular materials have attracted immense attention due to their unique properties, promising a wide range of exciting applications. To understand the structure-property relationship of these low-dimensional materials, sensitive analytical tools capable of providing structural and chemical characterisation at the nanoscale are required. However, most conventional analytical techniques fail to meet this challenge, especially in a label-free and non-destructive manner under ambient conditions. In the last two decades, tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful analytical technique for nanoscale chemical characterisation by combining the high spatial resolution of scanning probe microscopy and the chemical sensitivity and specificity of surface-enhanced Raman spectroscopy. In this review article, we provide an overview of the application of TERS for nanoscale chemical analysis of 2D molecular materials, including 2D polymers, biomimetic lipid membranes, biological cell membranes, and 2D reactive systems. The progress in the structural and chemical characterisation of these 2D materials is demonstrated with key examples from our as well as other laboratories. We highlight the unique information that TERS can provide as well as point out the common pitfalls in experimental work and data interpretation and the possible ways of averting them.

Tip-enhanced nanoscopy of two-dimensional transition metal dichalcogenides: progress and perspectives

The optoelectronic properties of two-dimensional (2D) transition metal dichalcogenide (TMD) thin layers prepared by exfoliation or chemical vapour deposition are strongly modulated by defects at the nanoscale. The mediated electronic and optical properties are expected to be spatially localised in a nanoscale width neighbouring the defects. Characterising such localised properties requires an analytical tool with nanoscale spatial resolution and high optical sensitivity. In recent years, tip-enhanced nanoscopy, represented by tip-enhanced Raman spectroscopy (TERS) and tip-enhanced photoluminescence (TEPL), has emerged as a powerful tool to characterise the localised phonon and exciton behaviours of 2D TMDs and heterojunctions (HJs) at the nanoscale. Herein, we first summarise the recent progress of TERS and TEPL in the characterisation of several typical defects in TMDs, such as edges, wrinkles, grain boundaries and other defects generated in transfer and growth processes. Then the local strain and its dynamic control of phonon and exciton behaviours characterised by TERS and TEPL will be reviewed. The recent progress in characterising TMD HJs using TERS and TEPL will be subsequently summarised. Finally, the progress of TERS and TEPL combined with optoelectronic sensitive electronic scanning probe microscopy (SPM) in the applications of TMDs will be reviewed.

Nanoscale Chemical Imaging of Human Cell Membranes Using Tip-Enhanced Raman Spectroscopy

Lack of appropriate tools for visualizing cell membrane molecules at the nanoscale in a non-invasive and label-free fashion limits our understanding of many vital cellular processes. Here, we use tip-enhanced Raman spectroscopy (TERS) to visualize the molecular distribution in pancreatic cancer cell (BxPC-3) membranes in ambient conditions without labelling, with a spatial resolution down to ca. 2.5 nm. TERS imaging reveals segregation of phenylalanine-, histidine-, phosphatidylcholine-, protein-, and cholesterol-rich BxPC-3 cell membrane domains at the nm length-scale. TERS imaging also showed a cell membrane region where cholesterol is mixed with protein. Interestingly, the higher resolution TERS imaging revealed that the molecular domains observed on the BxPC-3 cell membrane are not chemically "pure" but also contain other biomolecules. These results demonstrate the potential of TERS for non-destructive and label-free imaging of cell membranes with nanoscale resolution.

Visualizing Surface Phase Separation in PS-PMMA Polymer Blends at the Nanoscale

Phase-separated polymer blend films are an important class of functional materials with numerous technological applications in solar cells, catalysis, and biotechnology. These technologies are underpinned by the precise control of phase separation at the nanometer length-scales, which is highly challenging to visualize using conventional analytical tools. Herein, we introduce tip-enhanced Raman spectroscopy (TERS), in combination with atomic force microscopy (AFM), confocal Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), as a sensitive nanoanalytical method to determine lateral and vertical phase-separation in polystyrene (PS)-poly(methyl methacrylate) (PMMA) polymer blend films. Correlative topographical, molecular, and elemental information reveals a vertical phase separation of the polymers within the top ca. 20 nm of the blend surface in addition to the lateral phase separation in the bulk. Furthermore, complementary TERS and XPS measurements reveal the presence of PMMA within 9.2 nm of the surface and PS at the subsurface of the polymer blend. This fundamental work establishes TERS as a powerful analytical tool for surface characterization of this important class of polymers at nanometer length scales.

Probing Nanoscale Exciton Funneling at Wrinkles of Twisted Bilayer MoS2 Using Tip-Enhanced Photoluminescence Microscopy

In twisted bilayer (t2L) two-dimensional (2D) transition metal dichalcogenides, local strain at wrinkles strongly modulates the local exciton density and PL energy resulting in an exciton funneling effect. Probing such exciton behaviors especially at nanometer length scales is beyond the limit of conventional analytical tools due to the limited spatial resolution and low sensitivity. To address this challenge, herein we applied high-resolution tip-enhanced photoluminescence (TEPL) microscopy to investigate exciton funneling at a wrinkle in a t2L MoS₂ sample with a small twist angle of 0.5°. Owing to a spatial resolution of <10 nm, excitonic behavior at nanoscale sized wrinkles could be visualized using TEPL imaging. Detailed investigation of nanoscale exciton funneling at the wrinkles revealed a deformation potential of -54 meV/%. The obtained results provide novel insights into the inhomogeneities of excitonic behaviors at nanoscale and would be helpful in facilitating the rational design of 2D material-based twistronic devices.

Molecular Perturbation Effects in AFM-Based Tip-Enhanced Raman Spectroscopy: Contact versus Tapping Mode

Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for nondestructive and label-free surface chemical characterization at nanometer length scales. However, despite being considered nondestructive, the interaction of the TERS probe used in the analysis can alter the molecular organization of the sample. In this study, we investigate the role of the atomic force microscopy (AFM) feedback (contact mode and tapping mode) on molecular perturbation in TERS analysis of soft samples using a self-assembled monolayer (SAM) of 2-chloro-4-nitrobenzene-1-thiol (Cl-NBT) as a test sample. Surprisingly, the tapping mode shows a consistently higher TERS signal resulting from a minimal perturbation of the Cl-NBT SAM compared to the contact mode. This study provides novel insights into the choice of the correct AFM-TERS operation mode for nanoscale chemical analysis of soft and delicate samples and is expected to expedite the growing application of TERS in this area.

Visualising structural modification of patterned graphene nanoribbons using tip-enhanced Raman spectroscopy

Graphene nanoribbons (GNRs) fabricated using electron beam lithography are investigated using tip-enhanced Raman spectroscopy (TERS) with a spatial resolution of 5 nm under ambient conditions. High-resolution TERS imaging reveals a structurally modified 5-10 nm strip of disordered graphene at the edge of the GNRs. Furthermore, hyperspectral TERS imaging discovers the presence of nanoscale organic contaminants on the GNRs. These results pave the way for nanoscale chemical and structural characterisation of graphene-based devices using TERS.