Infrared and Raman chemical imaging and spectroscopy at the nanoscale

Identification of microplastics and nanoplastics in environmental water by AFM-IR

BACKGROUND

Microplastics and nanoplastics have gained worldwide attention as environmental hazards, and reliable analysis of these tiny particles is critical to accurate assessment of their impact to the environment and human health. Among the typical methods developed for analysis of microplastics, mass spectrometry-based methods are destructive and not applicable to individual nanoplastic particles, whereas vibrational spectroscopic techniques in combination with optical microscopy do not have sufficient spatial resolution needed for characterization of the much smaller nanoplastics. Therefore, a new tool is needed for the identification of nanoplastics in the environment.

RESULTS

Here we report for the first time identification of individual nanoplastics in an environmental water sample directly by atomic force microscopy-infrared (AFM-IR), a spectroscopic technique with a spatial resolution of ∼100 nm. On the basis of their spectral characteristics, four different nanoplastics, including poly(3-hydroxybutyrate) (P3HB) and a bisphenol-A based epoxy, as well as microplastics of the latter and two different polyesters were identified unambiguously, and the P3HB nanoplastic particle was found to be highly crystalline. Particles of fatty acid salt, lactic acid salt and sulfate were also identified. Amide bands were observed in the spectra of some of these particles, indicative of protein contamination. In addition, a diatom and a bacterium were identified based on their IR spectra in conjunction with the morphology and elemental composition.

SIGNIFICANCE

This work demonstrates that AFM-IR is a powerful tool for studying individual nanoplastic particles in environmental samples, capable of providing not only their identities but also detailed structure information.

Colocalized Raman and IR Spectroscopies via Vibrational-Encoded Fluorescence for Comprehensive Vibrational Analysis

Vibrational spectroscopy, including Raman scattering and infrared (IR) absorption, provides essential molecular fingerprint information, facilitating diverse applications, such as interfacial sensing, chemical analysis, and biomedical diagnostics. The complementary selection rules of Raman and IR spectroscopies offer distinct, yet mutually reinforcing, insights into molecular structure and dynamics. However, in dynamic or complex chemical environments, either technique alone is not capable of providing a complete and nuanced picture of molecular vibrations. Simultaneous detection of complementary vibrational modes within the same molecular group remains challenging due to wavelength discrepancies and sensitivity mismatches between Raman and IR spectroscopies. In this work, to address the gap between these spectroscopies, we developed an integrated approach based on vibrational-encoded fluorescence (VEF), in which the complementary vibrational information is respectively encoded into the different parts of fluorescence radiation: Stokes fluorescence carrying Raman information and anti-Stokes fluorescence reflecting IR information. This method employs a dual-resonant microsphere-on-mirror plasmonic structure to bridge the waveband gap, enabling the simultaneous detection of complete vibrational modes in the visible spectrum with ultrahigh sensitivity down to ∼100 molecules. Hyperspectral colocalization imaging demonstrates spatial correlations between the complementary vibrations. By careful calibration, the detection efficiency is improved by 8 orders of magnitude compared to unenhanced IR spectroscopy. This approach creates new opportunities for the precise identification of molecular vibrational information in complex chemical environments.

Nano-Infrared Detection and Identification of Bacteria at the Single-Cell Level

Every year, bacterial infections are responsible for over 7 million deaths globally. Timely detection and identification of these pathogens enable timely administration of antimicrobial agents, which can save thousands of lives. Most of the currently known approaches that can address these needs are time- and labor consuming. In this study, we examine the potential of innovative nano-infrared spectroscopy, also known as atomic force microscopy infrared (AFM-IR) spectroscopy, and machine learning in the identification of different bacteria. We demonstrate that a single bacteria cell is sufficient to identify Borreliella burgdorferi, Escherichia coli, Mycobacterium smegmatis, and two strains of Acinetobacter baumannii with 100% accuracy. The identification is based on the vibrational bands that originate from the components of the cell wall as well as the interior biomolecules of the bacterial cell. These results indicate that nano-IR spectroscopy can be used for the nondestructive, confirmatory, and label-free identification of pathogenic microorganisms at the single-cell level.

The role of phospholipid saturation and composition in α-synuclein aggregation and toxicity: A dual in vitro and in vivo approach

Parkinson's disease is characterized by a progressive accumulation of α-synuclein (α-syn) aggregates in Lewy bodies, extracellular deposits found in the midbrain, hypothalamus, and thalamus. The rate of α-syn aggregation, as well as the secondary structure of α-syn oligomers and fibrils, can be uniquely altered by lipids. However, the role of saturation of fatty acids (FAs) in such lipids in the aggregation properties of α-syn remains unclear. In this study, we investigated the effect of saturation of FAs in phosphatidylcholine (PC) and cardiolipin (CL), as well as a mixture of these phospholipids on the rate of α-syn aggregation. We found that although saturation plays very little if any role in the rate of protein aggregation and morphology of α-syn aggregates, it determined the secondary structure of α-syn oligomers and fibrils. Furthermore, we found that aggregates formed in the presence of both saturated and unsaturated PC and CL, as well as mixtures of these phospholipids, exert significantly higher cell toxicity compared to the protein aggregates formed in the lipid-free environment. To extend these findings, we conducted in vivo studies using C. elegans, where we assessed the effect of lipid-modified α-syn aggregates on organismal survival and neurotoxicity. Our results suggest that the saturation of FAs in phospholipids present in the plasma and mitochondrial membranes can be a key determinant of the secondary structure and, consequently, the toxicity of α-syn oligomers and fibrils. These findings provide new insights into the role of lipids in Parkinson's disease pathogenesis and highlight potential targets for therapeutic intervention.

Chemical Mapping of Supramolecular Self-Assembled Monolayers via Atomic Force Microscopy-Based Infrared with a Nanometer-Scale Lateral Resolution

Atomic force microscopy-based infrared (AFM-IR) techniques have long enabled chemical imaging of surfaces with spatial resolutions down to 10 nm. In this study, we push the boundaries of AFM-IR spatial resolution to the single-nanometer scale, achieving chemical mapping of supramolecular networks at the submolecular level. This breakthrough is made possible by implementing an innovative tapping-mode AFM-IR approach. Such unprecedented resolution opens new horizons for advancements in nanoscience.

Modulated-Illumination Intermittent-Contact Tip-Enhanced Raman Spectroscopy

This article presents a proof-of-concept for a new imaging method that combines tip-enhanced Raman spectroscopy with intermittent-contact atomic force microscopy to provide simultaneous nanometer-scale mechanical imaging with chemical contrast. The foremost difference from a standard tip-enhanced Raman microscope is the Raman illumination, which is modulated by the cantilever drive signal so that illumination is only active when the tip is close to the surface. This approach significantly reduces contact forces and thermal damage due to constant illumination while simultaneously reducing background Raman signals. Near-field optical and dynamic cantilever simulations highlight the effect of the imaging parameters on the tip-sample force and the evanescent field enhancement. The experimental images obtained with this new imaging method demonstrate a lateral resolution sufficient to identify single-walled carbon nanotube bundles with a full width at half-maximum of 20 nm.

Merging Vibrational Spectroscopy with Fluorescence Microscopy: Combining the Best of Two Worlds

Vibrational spectroscopy and fluorescence spectroscopy have historically been two established but separate fields of molecular spectroscopy. While vibrational spectroscopy provides exquisite chemical information, fluorescence spectroscopy often offers orders of magnitude higher detection sensitivity. However, they each lack the advantages of each other. In recent years, a series of novel nonlinear optical spectroscopy studies have been developed that merge both spectroscopies into a single double-resonance process. These techniques combine the chemical specificity of Raman or infrared (IR) spectroscopy with the superb detection sensitivity and spatial resolution of fluorescence microscopy. Many facets have been explored, including Raman transition versus IR transition, time domain versus frequency domain, and spectroscopy versus microscopy. Notably, single-molecule vibrational spectroscopy has been achieved at room temperature without the need for plasmonics. Even superresolution vibrational imaging beyond the diffraction limit was demonstrated. This review summarizes the growing field of vibrational-encoded fluorescence microscopy, including key technical developments, emerging applications, and future prospects.

Toward nanoscale structural and chemical analysis of microbial surfaces

Microbial surfaces play a critical role in various biological processes, including cell adhesion and biofilm formation. Understanding these surfaces at the nanoscale is essential for both fundamental and applied microbiology. This review explores recent advancements in nanoscale structural and chemical analyses of microbial surfaces, with a focus on vibrational spectroscopy, such as Raman spectroscopy, infrared spectroscopy, and atomic force microscopy. The review also discusses current challenges of these techniques, including variability in sample preparation and the reproducibility of data, and highlights future directions in nanoscale analysis that could lead to new insights in microbial physiology, antimicrobial resistance, and biofilm research.

Nano-Investigation of Mineralized Biological Samples Chemical Composition: Experimental Challenges, Constraints, and Considerations

Understanding the chemical composition of calcifications in biological tissues at the nanoscale is crucial for deciphering their formation processes and possible pathological implications. Atomic Force Microscopy Infrared Spectroscopy (AFM-IR), by allowing IR spectroscopy at the nanoscale, is thus a promising strategy to access such highly spatially resolved chemical information. However, these specimens' inherent morphological and mechanical heterogeneities pose significant challenges for standard resonance-enhanced (RE-AFM-IR) and tapping AFM-IR acquisition modes. This study introduces a dual-mode approach combining tapping and RE-AFM-IR to address these challenges. Tapping AFM-IR is first employed to acquire the topography of the soft and rough surfaces, while RE-AFM-IR provides chemical description at the submicrometric scale through hyperspectral (HS) imaging. This dual-mode methodology is validated on different mineralized biological samples, including breast microcalcifications, revealing the local chemical heterogeneous distribution within the calcium phosphate matrice. Our results outline that dual-mode AFM-IR, coupled with HS imaging, enables robust chemical characterization of highly heterogeneous biomaterials and offers a more comprehensive description compared to conventional AFM-IR single-wavenumber mapping and local spectra.

Comparison of Different Vibrational Spectroscopic Probes (ATR-FTIR, O-PTIR, Micro-Raman, and AFM-IR) of Lipids and Other Compounds Found in Environmental Samples: Case Study of Substrate-Deposited Sea Spray Aerosols

The use of vibrational spectroscopy to probe environmental samples is increasing with the development of new methods, including microspectroscopic probes. In this study, we compare different vibrational methods to interrogate lipids and other compounds found in environmental samples. In particular, we compare the vibrational spectra for different lipids that include fatty acids (protonated and deprotonated forms), fatty alcohols, and fatty esters by utilizing attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, optical photothermal infrared (O-PTIR) spectroscopy, micro-Raman spectroscopy, and atomic force microscopy infrared (AFM-IR) spectroscopy. We show the utility of infrared methods to clearly delineate the structure of the lipid, i.e., whether it is an acid, alcohol, or ester. In contrast, it is difficult to differentiate these from micro-Raman spectroscopy. Furthermore, in the case of fatty acids, the protonation state can also be determined by infrared methods. In most cases, there is a high correlation between the three different infrared techniques as seen for ATR-FTIR and O-PTIR spectroscopy; however, this is not always true with AFM-IR spectroscopy for samples with low signal-to-noise or in a liquid phase state. Additionally, substrate-deposited aerosols were collected from the Scripps Ocean-Atmosphere Research Simulator (SOARS) and examined with both the O-PTIR and micro-Raman spectroscopy to show how these two vibrational probes together can provide essential chemical insights into environmental samples that are difficult to achieve otherwise.

Atomic force microscopy imaging of plant cell walls

Plant cell walls are highly dynamic, complex structures composed of multiple biopolymers that form a scaffold surrounding the plant cell. A nanoscale understanding of their architecture, mechanical properties, and formation/degradation dynamics is crucial for revealing structure-function relationships, mechanisms of shape formation, and cell development. Although imaging techniques have been extensively used in recent decades to reveal the structural organization and chemical compositions of cell walls, observing the detailed native architecture and identifying the physicochemical properties of plant cell walls remains challenging. Atomic force microscopy (AFM) is a powerful tool for simultaneously characterizing the morphology, nanomechanical properties, single-molecule interactions, and surface potentials of living biological systems. However, studies employing AFM to investigate plant cell walls have been relatively scarce. In this review, we discuss the latest advancements in AFM for in situ imaging of the multidimensional structure of the cell wall, measuring the mechanical properties of plant tissues or single cells, specific single-molecule recognition of cell wall-related enzymes-polysaccharides, and detecting the Kelvin potential of plant cell walls. We emphasize the fundamental challenges of AFM in characterizing plant cell walls and review potential applications for state-of-the-art AFM-based infrared/Raman spectroscopy toward answering open questions in plant biology.

Determination diabetes mellitus disease markers in tear fluid by photothermal AFM-IR analysis

The tear fluids from three healthy individuals and three patients with diabetes mellitus were examined using atomic force microscopy-infrared spectroscopy (AFM-IR) and Fourier transform infrared spectroscopy (FTIR). The dried tear samples showed different surface morphologies: the control sample had a dense network of heart-shaped dendrites, while the diabetic sample had fern-shaped dendrites. By using the AFM-IR technique we identified spatial distribution of constituents, indicating how diabetes affects the structural characteristics of dried tears. FTIR showed that the dendritic structures gradually disappeared over time due to glucose-induced lysozyme damage. The tear fluid from diabetes mellitus patients has a higher concentration of glucose, which accelerates the breakdown of lysozyme and, as a result, the quick loss of the dendritic structure. Our study shows that analysis of dry tear fluid can be promising technique for the detection of glycated proteins that reveal long lasting hyperglycemia and diabetes mellitus.

Unmixing Autoencoder for Image Reconstruction from Hyperspectral Data

Due to the complexity of samples and the limitations in spatial resolution, the spectra in hyperspectral imaging (HSI) are generally contributed to by multiple components, making univariate analysis ineffective. Although feature extraction methods have been applied, the chemical meaning of the compressed variables is difficult to interpret, limiting their further applications. An unmixing autoencoder (UAE) was developed in this work for the separation of the mixed spectra in HSI. The proposed model is composed of an encoder and a fully connected (FC) layer. The former is used to compress the input spectrum into several variables, and the latter is employed to reconstruct the spectrum. Combining reconstruction loss and sparse regularization, the weights and the spectral profiles of the components will be encoded in the compressed variables and the connection weights of FC, respectively. A simulated and three experimental HSI data sets were adopted to investigate the performance of the UAE model. The spectral components were successfully obtained, from which the handwriting under papers was revealed from the image of near-infrared (NIR) diffusive reflectance spectroscopy, and the images of lipids, proteins, and nucleic acids were reconstructed from the Raman and stimulated Raman scattering (SRS) images.

In Depth Mapping of Mesoporous Silica Nanoparticles in Malignant Glioma Cells Using Scattering-Type Scanning Near-Field Optical Microscopy

Mesoporous silica nanoparticles (MSNPs) are promising nanomedicine vehicles due to their biocompatibility and ability to carry large cargoes. It is critical in nanomedicine development to be able to map their uptake in cells, including distinguishing surface associated MSNPs from those that are embedded or internalized into cells. Conventional nanoscale imaging techniques, such as electron and fluorescence microscopies, however, generally require the use of stains and labels to image both the biological material and the nanomedicines, which can interfere with the biological processes at play. We demonstrate an alternative imaging technique for investigating the interactions between cells and nanostructures, scattering-type scanning near-field optical microscopy (s-SNOM). s-SNOM combines the chemical sensitivity of infrared spectroscopy with the nanoscale spatial resolving power of scanning probe microscopy. We use the technique to chemically map the uptake of MSNPs in whole human glioblastoma cells and show that the simultaneously acquired topographical information can provide the embedding status of the MSNPs. We focus our imaging efforts on the lamellipodia and filopodia structures at the peripheries of the cells due to their significance in cancer invasiveness.

Room-Temperature Single-Molecule Infrared Imaging and Spectroscopy through Bond-Selective Fluorescence

Infrared (IR) spectroscopy stands as a workhorse for exploring bond vibrations, offering a wealth of chemical insights across diverse frontiers. With increasing focus on the regime of single molecules, obtaining IR-sensitive information from individual molecules at room temperature would provide essential information about unknown molecular properties. Here, we leverage bond-selective fluorescence microscopy, facilitated by narrowband picosecond mid-IR and near-IR double-resonance excitation, for high-throughput mid-IR structural probing of single molecules. We robustly capture single-molecule images and analyze the combined polarization dependence, vibrational peaks, linewidths, and lifetimes of probe molecules with representative scaffolds. From bulk to single molecules, we find that vibrational lifetimes remain consistent, while linewidths are narrowed by approximately twofold and anisotropy becomes more pronounced. Additionally, unexpected peak shifts from single molecules were observed, attributed to the generation of new modes due to previously unexplored dimerization, supported by quantum chemistry calculations. These findings underscore the importance of infrared analysis on individual single molecules in ambient environments, offering molecular information crucial for functional imaging and the investigation of the fundamental properties and utilities of luminescent molecules.

Advances and challenges in dynamic photo-induced force microscopy

Photo-induced force microscopy (PiFM) represents a scanning probe technique renowned for its ability to provide high-resolution spectroscopic imaging at the nanoscale. It capitalizes on the amplification of tip motion by photo-induced forces, which are influenced by the response of the local medium, spanning from induced dipole interactions to thermal expansion. The behaviors of these force responses exhibit complexity in connection with both far-field and near-field effects, depending on their spectroscopic origins. In this review, we aim to provide a comprehensive overview of prior research endeavors, shedding light on their technical intricacies. We provide the perspectives of photo-induced dipole force and photo-induced thermal force, while exploring the dynamic PiFM modes associated with each scenario. Our article targets individuals newly venturing into this field, offering a blend of theoretical foundations and practical demonstrations covering a range from fundamental principles to advanced topics.

Understanding AFM-IR Signal Dependence on Sample Thickness and Laser Excitation: Experimental and Theoretical Insights

Photothermal induced resonance (PTIR), also known as atomic force microscopy-infrared (AFM-IR), enables nanoscale IR absorption spectroscopy by transducing the local photothermal expansion and contraction of a sample with the tip of an atomic force microscope. PTIR spectra enable material identification at the nanoscale and can measure sample composition at depths >1 μm. However, implementation of quantitative, multivariate, nanoscale IR analysis requires an improved understanding of PTIR signal transduction and of the intensity dependence on sample characteristics and measurement parameters. Here, PTIR spectra measured on three-dimensional printed conical structures up to 2.5 μm tall elucidate the signal dependence on sample thickness for different IR laser repetition rates and pulse lengths. Additionally, we develop a model linking sample thermal expansion dynamics to cantilever excitation amplitudes that includes samples that do not fully thermalize between consecutive pulses. Remarkable qualitative agreement between experiments and theory demonstrates a monotonic increase in the PTIR signal intensity with thickness, with decreasing sensitivities at higher repetition rates, while signal intensity is nearly unaffected by laser pulse length. Although we observe slight deviations from linearity over the entire 2.5 μm thickness range, the signal's approximate linearity for bands of sample thicknesses up to ≈500 nm suggests that samples with comparably low topographic variations are most amenable to quantitative analysis. Importantly, we measure absorptive undistorted profiles in PTIR spectra for strongly absorbing modes, up to ≈1650 nm, and >2500 nm for other modes. These insights are foundational toward quantitative nanoscale PTIR analyses and material identification, furthering their impact across many applications.

Nanoscale Chemical Imaging of Amyloid Fibrils in Water Using Total-Internal-Reflection Tip-Enhanced Raman Spectroscopy

Total-internal-reflection tip-enhanced Raman spectroscopy (TIR-TERS) imaging of amyloid-β (Aβ1-42-L34T) fibrils is performed with nanoscale spatial resolution in water, using TERS tips fabricated by bipolar electrodeposition. Ideal experimental parameters are corroborated by both theoretical simulations and TIR-TERS measurements. TIR-TERS imaging reveals the predominant parallel β-sheet secondary structure of Aβ1-42-L34T fibrils as well as the nanoscale spatial distribution of tyrosine, histidine, and phenylalanine aromatic amino acids. Their proportion in TERS spectra can be qualitatively explained by the combined effect of their localization in the Aβ1-42-L34T fibril structure and their molecular orientation with respect to the excitation laser light polarization. Conclusions drawn from the TERS experiments in water corroborate and significantly enrich our previous study in ambient air, thus confirming that hydration has only a marginal impact on the structure of such amyloid fibrils. This first TIR-TERS study in liquid opens fascinating perspectives for future applications in biology.

Nanoscale Structural Characterization of Amyloid β 1-42 Oligomers and Fibrils Grown in the Presence of Fatty Acids

Mono- and polyunsaturated fatty acids (FAs) are broadly used as food supplements. However, their effect on the aggregation of amyloidogenic proteins remains unclear. In this study, we investigated the effect of a large number of mono- and polyunsaturated, as well as fully saturated FAs on the aggregation of amyloid β1-42 (Aβ1-42) peptide. A progressive aggregation of this peptide is the expected molecular cause of Alzheimer's disease (AD), one of the most common neurodegenerative pathologies in the world. We found that arachidonic and stearic acids delayed the aggregation of Aβ1-42. Using Nano-Infrared spectroscopy, we found that FAs caused very little if any changes in the secondary structure of Aβ1-42 oligomers and fibrils formed at different stages of protein aggregation. However, the analyzed mono- and polyunsaturated, as well as fully saturated FAs uniquely altered the toxicity of Aβ1-42 fibrils. We found a direct relationship between the degree of FAs unsaturation and toxicity of Aβ1-42 fibrils formed in their presence. Specifically, with an increase in the degree of unsaturation, the toxicity Aβ1-42/FA fibrils increased. These results indicate that fully saturated or monounsaturated FAs could be used to decrease the toxicity of amyloid aggregates and, consequently, decelerate the development of AD.

Stable Isotope Probing-nanoFTIR for Quantitation of Cellular Metabolism and Observation of Growth-Dependent Spectral Features

This study utilizes nanoscale Fourier transform infrared spectroscopy (nanoFTIR) to perform stable isotope probing (SIP) on individual bacteria cells cultured in the presence of 13C-labelled glucose. SIP-nanoFTIR simultaneously quantifies single-cell metabolism through infrared spectroscopy and acquires cellular morphological information via atomic force microscopy. The redshift of the amide I peak corresponds to the isotopic enrichment of newly synthesized proteins. These observations of single-cell translational activity are comparable to those of conventional methods, examining bulk cell numbers. Observing cells cultured under conditions of limited carbon, SIP- nanoFTIR is used to identify environmentally-induced changes in metabolic heterogeneity and cellular morphology. Individuals outcompeting their neighboring cells will likely play a disproportionately large role in shaping population dynamics during adverse conditions or environmental fluctuations. Additionally, SIP-nanoFTIR enables the spectroscopic differentiation of specific cellular growth phases. During cellular replication, subcellular isotope distribution becomes more homogenous, which is reflected in the spectroscopic features dependent on the extent of 13C-13C mode coupling or to specific isotopic symmetries within protein secondary structures. As SIP-nanoFTIR captures single-cell metabolism, environmentally-induced cellular processes, and subcellular isotope localization, this technique offers widespread applications across a variety of disciplines including microbial ecology, biophysics, biopharmaceuticals, medicinal science, and cancer research.

Mechanochemistry: Fundamental Principles and Applications

Mechanochemistry is an emerging research field at the interface of physics, mechanics, materials science, and chemistry. Complementary to traditional activation methods in chemistry, such as heat, electricity, and light, mechanochemistry focuses on the activation of chemical reactions by directly or indirectly applying mechanical forces. It has evolved as a powerful tool for controlling chemical reactions in solid state systems, sensing and responding to stresses in polymer materials, regulating interfacial adhesions, and stimulating biological processes. By combining theoretical approaches, simulations and experimental techniques, researchers have gained intricate insights into the mechanisms underlying mechanochemistry. In this review, the physical chemistry principles underpinning mechanochemistry are elucidated and a comprehensive overview of recent significant achievements in the discovery of mechanically responsive chemical processes is provided, with a particular emphasis on their applications in materials science. Additionally, The perspectives and insights into potential future directions for this exciting research field are offered.

Under Heparin-Free Conditions Unsaturated Phospholipids Inhibit the Aggregation of 1N4R and 2N4R Tau

A progressive aggregation of Tau proteins in the brain is linked to both Alzheimer's disease (AD) and various Tauopathies. This pathological process can be enhanced by several substances, including heparin. However, very little if anything is known about molecules that can inhibit the aggregation of Tau isoforms. In this study, we examined the effect of phosphatidylserines (PSs) with various lengths and saturations of fatty acids (FAs) on the aggregation properties of Tau isoforms with one (1N4R) and two (2N4R) N-terminal inserts that enhance binding of Tau to tubulin. We found that PS with unsaturated and short-length FAs inhibited Tau aggregation and drastically lowered the toxicity of Tau oligomers that were formed in the presence of such phospholipids. Such an effect was not observed for PS with fully saturated long-chain FAs. These results suggest that a short-chain irreversible disbalance between saturated and unsaturated lipids in the brain could be the trigger of Tau aggregation.

Taking Photoacoustic Force into Account in Liquid-Phase Peak Force Infrared Microscopy

The microscopic structure of the material's solid-liquid interface significantly influences its physicochemical properties. Peak force infrared microscopy (PFIR) is a powerful technique for analyzing these interfaces at the nanoscale, revealing crucial structure-activity relationships. PFIR is recognized for its explicit photothermal signal generation mechanism but tends to overlook other photoinduced forces, which can disturb the obtained infrared spectra, thereby reducing spectral signal-to-noise ratio (SNR) and sensitivity. We have developed a multiphysics-coupled theoretical model to assess the magnitudes of various photoinduced forces in PFIR experiments and have found that the magnitude of the photoacoustic force is comparable to that of the photothermal expansion force in a liquid environment. Our calculations show that through simple modulation of the pulse waveform it is possible to effectively suppress the photoacoustic interference, thereby improving the SNR and sensitivity of PFIR. This work aims to alert researchers to the potential for strong photoacoustic interference in liquid-phase PFIR measurements and enhance the performance of PFIR by clarifying the photoinduced forces entangled in the signals.

Fabrication of plasmonic probes for reproducible nanospectroscopic investigation of lipid monolayers - The electrochemical etching with DC-pulsed voltage

Tip-enhanced Raman spectroscopy (TERS) is a label-free analytical technique that characterizes molecular systems, potentially even with a nanometric resolution. In principle, the metallic plasmonic probe is illuminated with a laser beam generating the localized surface plasmons, which induce a strong local electric field enhancement in close proximity to the probe. Such field enhancement improves the Raman scattering cross-section from the sample volume localized near the probe apex. TERS provides a high spatial resolution and a great sensitivity, however, it is rather rarely used due to technical limitations causing unstable enhancement and the relative lack of data reproducibility. Despite many scientific efforts for the fabrication of effective TER probes providing robust TER enhancement still requires further investigations. In this work, we explore new possibilities based on preparation of scanning tunnelling microscopy (STM) plasmonic probes, since by nature of the tunnelling effect, they potentially could offer a very high spatial resolution in STM guided TERS experiments. Here we compare two methods of STM-TERS probe preparation for effective spectra acquisition. Our results strongly indicate that an application of square pulse voltage upon the etching procedure significantly improves the quality of the TER data over those obtained with a constant voltage one. To demonstrate the efficiency of our probes we present the results of hyperspectral TER mapping of the 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) monolayer deposited on an ultra-pure and atomically flat gold substrate.

Elucidating the mechanisms of α-Synuclein-lipid interactions using site-directed mutagenesis

α-Synuclein (α-syn) is a small protein that is involved in cell vesicle trafficking in neuronal synapses. A progressive aggregation of this protein is the expected molecular cause of Parkinson's disease, a disease that affects millions of people around the world. A growing body of evidence indicates that phospholipids can strongly accelerate α-syn aggregation and alter the toxicity of α-syn oligomers and fibrils formed in the presence of lipid vesicles. This effect is attributed to the presence of high copies of lysines in the N-terminus of the protein. In this study, we performed site-directed mutagenesis and replaced one out of two lysines at each of the five sites located in the α-syn N-terminus. Using several biophysical and cellular approaches, we investigated the extent to which six negatively charged fatty acids (FAs) could alter the aggregation properties of K10A, K23A, K32A, K43A, and K58A α-syn. We found that FAs uniquely modified the aggregation properties of K43A, K58A, and WT α-syn, as well as changed morphology of amyloid fibrils formed by these mutants. At the same time, FAs failed to cause substantial changes in the aggregation rates of K10A, K23A, and K32A α-syn, as well as alter the morphology and toxicity of the corresponding amyloid fibrils. Based on these results, we can conclude that K10, K23, and K32 amino acid residues play a critical role in protein-lipid interactions since their replacement on non-polar alanines strongly suppressed α-syn-lipid interactions.

Research on the Migration and Adsorption Mechanism Applied to Microplastics in Porous Media: A Review

In recent years, microplastics (MPs) have emerged as a significant environmental pollutant, garnering substantial attention for their migration and transformation behaviors in natural environments. MPs frequently infiltrate natural porous media such as soil, sediment, and rock through various pathways, posing potential threats to ecological systems and human health. Consequently, the migration and adsorption mechanisms applied to MPs in porous media have been extensively studied. This paper aims to elucidate the migration mechanisms of MPs in porous media and their influencing factors through a systematic review. The review encompasses the characteristics of MPs, the physical properties of porous media, and hydrodynamic factors. Additionally, the paper further clarifies the adsorption mechanisms of MPs in porous media to provide theoretical support for understanding their environmental behavior and fate. Furthermore, the current mainstream detection techniques for MPs are reviewed, with an analysis of the advantages, disadvantages, and applications of each technique. Finally, the paper identifies the limitations and shortcomings of current research and envisions future research directions.

Fast Nano-IR Hyperspectral Imaging Empowered by Large-Dataset-Free Miniaturized Spatial-Spectral Network

The emerging field of nanoscale infrared (nano-IR) offers label-free molecular contrast, yet its imaging speed is limited by point-by-point traverse acquisition of a three-dimensional (3D) data cube. Here, we develop a spatial-spectral network (SS-Net), a miniaturized deep-learning model, together with compressive sampling to accelerate the nano-IR imaging. The compressive sampling is performed in both the spatial and spectral domains to accelerate the imaging process. The SS-Net is trained to learn the mapping from small nano-IR image patches to the corresponding spectra. With this elaborated mapping strategy, the training can be finished quickly within several minutes using the subsampled data, eliminating the need for a large-labeled dataset of common deep learning methods. We also designed an efficient loss function, which incorporates the image and spectral similarity to enhance the training. We first validate the SS-Net on an open stimulated Raman-scattering dataset; the results exhibit the potential of 10-fold imaging speed improvement with state-of-the-art performance. We then demonstrate the versatility of this approach on atomic force microscopy infrared (AFM-IR) microscopy with 7-fold imaging speed improvement, even on nanoscale Fourier transform infrared (nano-FTIR) microscopy with up to 261.6 folds faster imaging speed. We further showcase the generalization of this method on AFM-force volume-based multiparametric nanoimaging. This method establishes a paradigm for rapid nano-IR imaging, opening new possibilities for cutting-edge research in materials, photonics, and beyond.

AFM-IR investigation of thin PECVD SiO x films on a polypropylene substrate in the surface-sensitive mode

Thin silicon oxide films deposited on a polypropylene substrate by plasma-enhanced chemical vapor deposition were investigated using atomic force microscopy-based infrared (AFM-IR) nanospectroscopy in contact and surface-sensitive mode. The focus of this work is the comparison of the different measurement methods (i.e., contact mode and surface-sensitive mode) with respect to the chemical surface sensitivity. The use of the surface-sensitive mode in AFM-IR shows an enormous improvement for the analysis of thin films on the IR-active substrate. As a result, in this mode, the signal of the substrate material could be significantly reduced. Even layers that are so thin that they could hardly be measured in the contact mode can be analyzed with the surface-sensitive mode.

Surface-phonon-polariton-enhanced photoinduced dipole force for nanoscale infrared imaging

The photoinduced dipole force (PiDF) is an attractive force arising from the Coulombic interaction between the light-induced dipoles on the illuminated tip and the sample. It shows extreme sample-tip distance and refractive index dependence, which is promising for nanoscale infrared (IR) imaging of ultrathin samples. However, the existence of PiDF in the mid-IR region has not been experimentally demonstrated due to the coexistence of photoinduced thermal force (PiTF), typically one to two orders of magnitude higher than PiDF. In this study, we demonstrate that, with the assistance of surface phonon polaritons, the PiDF of c-quartz can be enhanced to surpass its PiTF, enabling a clear observation of PiDF spectra reflecting the properties of the real part of permittivity. Leveraging the detection of the PiDF of phonon polaritonic substrate, we propose a strategy to enhance the sensitivity and contrast of photoinduced force responses in transmission images, facilitating the precise differentiation of the heterogeneous distribution of ultrathin samples.

Signal2signal: Pushing the Spatiotemporal Resolution to the Limit by Single Chemical Hyperspectral Imaging

There is growing interest in developing a high-performance self-supervised denoising algorithm for real-time chemical hyperspectral imaging. With a good understanding of the working function of the zero-shot Noise2Noise-based denoising algorithm, we developed a self-supervised Signal2Signal (S2S) algorithm for real-time denoising with a single chemical hyperspectral image. Owing to the accurate distinction and capture of the weak signal from the random fluctuating noise, S2S displays excellent denoising performance, even for the hyperspectral image with a spectral signal-to-noise ratio (SNR) as low as 1.12. Under this condition, both the image clarity and the spatial resolution could be significantly improved and present an almost identical pattern with a spectral SNR of 7.87. The feasibility of real-time denoising during imaging was well demonstrated, and S2S was applied to monitor the photoinduced exfoliation of transition metal dichalcogenide, which is hard to accomplish by confocal Raman spectroscopy. In general, the real-time denoising capability of S2S offers an easy way toward in situ/in vivo/operando research with much improved spatial and temporal resolution. S2S is open-source at https://github.com/3331822w/Signal2signal and will be accessible online at https://ramancloud.xmu.edu.cn/tutorial.

Structural characterization of amyloid aggregates with spatially resolved infrared spectroscopy

The self-assembly of proteins and peptides into ordered structures called amyloid fibrils is a hallmark of numerous diseases, impacting the brain, heart, and other organs. The structure of amyloid aggregates is central to their function and thus has been extensively studied. However, the structural heterogeneities between aggregates as they evolve throughout the aggregation pathway are still not well understood. Conventional biophysical spectroscopic methods are bulk techniques and only report on the average structural parameters. Understanding the structure of individual aggregate species in a heterogeneous ensemble necessitates spatial resolution on the length scale of the aggregates. Recent technological advances have led to augmentation of infrared (IR) spectroscopy with imaging modalities, wherein the photothermal response of the sample upon vibrational excitation is leveraged to provide spatial resolution beyond the diffraction limit. These combined approaches are ideally suited to map out the structural heterogeneity of amyloid ensembles. AFM-IR, which integrates IR spectroscopy with atomic force microscopy enables identification of the structural facets the oligomers and fibrils at individual aggregate level with nanoscale resolution. These capabilities can be extended to chemical mapping in diseased tissue specimens with submicron resolution using optical photothermal microscopy, which combines IR spectroscopy with optical imaging. This book chapter provides the basic premise of these novel techniques and provides the typical methodology for using these approaches for amyloid structure determination. Detailed procedures pertaining to sample preparation and data acquisition and analysis are discussed and the aggregation of the amyloid β peptide is provided as a case study to provide the reader the experimental parameters necessary to use these techniques to complement their research efforts.

Tip-enhanced Raman spectroscopy reveals the structural rearrangements of tau protein aggregates at the growth phase

Tau protein aggregates inside neurons in the course of Alzheimer's disease (AD). Because of the enormous number of people suffering from AD, this disease has become one of the world's major health and social problems. The presence of tau lesions clearly correlates with cognitive impairments in AD patients, thus, tau is the target of potential treatments for AD, next to amyloid-β. The exact mechanism of tau aggregation has not been understood in detail so far; especially little is known about the structural rearrangements of tau aggregates at the growth phase. The research into tau conformation at each step of the aggregation pathway will contribute to the design of effective therapeutic approaches. To follow the secondary structure of individual tau aggregates at the growth phase, we applied tip-enhanced Raman spectroscopy (TERS). The nanospectroscopic approach enabled us to follow the structure of individual aggregates occurring in the subsequent phases of tau aggregation. We applied multivariate data analysis to extract the spectral differences for tau aggregates at different aggregation phases. Moreover, atomic force microscopy (AFM) allowed the tracking of the morphological alterations for species occurring with the progression of tau aggregation.

The influence of zwitterionic and anionic phospholipids on protein aggregation

The progressive aggregation of misfolded proteins is the underlying molecular cause of numerous pathologies including Parkinson's disease and injection and transthyretin amyloidosis. A growing body of evidence indicates that protein deposits detected in organs and tissues of patients diagnosed with such pathologies contain fragments of lipid membranes. In vitro experiments also showed that lipid membranes could strongly change the aggregation rate of amyloidogenic proteins, as well as alter the secondary structure and toxicity of oligomers and fibrils formed in their presence. In this review, the effect of large unilamellar vesicles (LUVs) composed of zwitterionic and anionic phospholipids on the aggregation rate of insulin, lysozyme, transthyretin (TTR) and α- synuclein (α-syn) will be discussed. The manuscript will also critically review the most recent findings on the lipid-induced changes in the secondary structure of protein oligomers and fibrils, as well as reveal the extent to which lipids could alter the toxicity of protein aggregates formed in their presence.

Spectral insights: Navigating the frontiers of biomedical and microbiological exploration with Raman spectroscopy

Raman spectroscopy (RS), a powerful analytical technique, has gained increasing recognition and utility in the fields of biomedical and biological research. Raman spectroscopic analyses find extensive application in the field of medicine and are employed for intricate research endeavors and diagnostic purposes. Consequently, it enjoys broad utilization within the realm of biological research, facilitating the identification of cellular classifications, metabolite profiling within the cellular milieu, and the assessment of pigment constituents within microalgae. This article also explores the multifaceted role of RS in these domains, highlighting its distinct advantages, acknowledging its limitations, and proposing strategies for enhancement.

Hot-Melt Extrusion: from Theory to Application in Pharmaceutical Formulation-Where Are We Now?

Hot-melt extrusion (HME) is a globally recognized, robust, effective technology that enhances the bioavailability of poorly soluble active pharmaceutical ingredients and offers an efficient continuous manufacturing process. The twin-screw extruder (TSE) offers an extremely resourceful customizable mixer that is used for continuous compounding and granulation by using different combinations of conveying elements, kneading elements (forward and reverse configuration), and distributive mixing elements. TSE is thus efficiently utilized for dry, wet, or melt granulation not only to manufacture dosage forms such as tablets, capsules, or granule-filled sachets, but also for designing novel formulations such as dry powder inhalers, drying units for granules, nanoextrusion, 3D printing, complexation, and amorphous solid dispersions. Over the past decades, combined academic and pharmaceutical industry collaborations have driven novel innovations for HME technology, which has resulted in a substantial increase in published articles and patents. This article summarizes the challenges and models for executing HME scale-up. Additionally, it covers the benefits of continuous manufacturing, process analytical technology (PAT) considerations, and regulatory requirements. In summary, this well-designed review builds upon our earlier publication, probing deeper into the potential of twin-screw extruders (TSE) for various new applications.

Controlling the Fluctuating Tip-Enhanced Raman Spectra of Chloramben on Silver Nanocubes

Tip-enhanced Raman (TER) scattering from molecules residing at plasmonic junctions can be used to detect, identify, and image single molecules. This is most evident for flat molecules interrogated under conditions of extreme temperatures and pressure. It is also the case for (bio)molecular systems that feature preferred orientations/conformations under ambient laboratory conditions. More complex molecules that can adopt multiple conformations and/or feature different protonation or charge states give rise to complex TER spectra. We illustrate how the latter can be controlled in the case of chloramben molecules coated onto plasmonic silver nanocubes. We show that characteristic molecular Raman spectra cannot be obtained when tunneling plasmons are operative, i.e., when the tip is in direct contact with the chemically functionalized plasmonic nanoparticles. We rationalize these observations and propose an approach to less invasive and hence more analytical TER spectral imaging.

Nanoscale Chemical Diversity of Coke Deposits on Nanoprinted Metal Catalysts Visualized by Tip-Enhanced Raman Spectroscopy

Coke formation is the prime cause of catalyst deactivation, where undesired carbon wastes block the catalyst surface and hinder further reaction in a broad gamut of industrial chemical processes. Yet, the origins of coke formation and their distribution across the catalyst remain elusive, obstructing the design of coke-resistant catalysts. Here, the first-time application of tip-enhanced Raman spectroscopy (TERS) is demonstrated as a nanoscale chemical probe to localize and identify coke deposits on a post-mortem metal nanocatalyst. Monitoring coke at the nanoscale circumvents bulk averaging and reveals the local nature of coke with unmatched detail. The nature of coke is chemically diverse and ranges from nanocrystalline graphite to disordered and polymeric coke, even on a single nanoscale location of a top-down nanoprinted SiO2 -supported Pt catalyst. Surprisingly, not all Pt is an equal producer of coke, where clear isolated coke "hotspots" are present non-homogeneously on Pt which generate large amounts of disordered coke. After their formation, coke shifts to the support and undergoes long-range transport on the surrounding SiO2 surface, where it becomes more graphitic. The presented results provide novel guidelines to selectively free-up the coked metal surface at more mild rejuvenation conditions, thus securing the long-term catalyst stability.

Designing Surface and Interface Structures of Copper-Based Catalysts for Enhanced Electrochemical Reduction of CO2 to Alcohols

Electrochemical CO2 reduction (ECR) has emerged as a promising solution to address both the greenhouse effect caused by CO2 emissions and the energy shortage resulting from the depletion of nonrenewable fossil fuels. The production of multicarbon (C2+) products via ECR, especially high-energy-density alcohols, is highly desirable for industrial applications. Copper (Cu) is the only metal that produces alcohols with appreciable efficiency and kinetic viability in aqueous solutions. However, poor product selectivity is the main technical problem for applying the ECR technology in alcohol production. Extensive research has resulted in the rational design of electrocatalyst architectures using various strategies. This design significantly affects the adsorption energetics of intermediates and the reaction pathways for alcohol production. In this review, we focus on the design of effective catalysts for ECR to alcohols, discussing fundamental principles, innovative strategies, and mechanism understanding. Furthermore, the challenges and prospects in utilizing Cu-based materials for alcohol production via ECR are discussed.

Recent progress in SERS monitoring of photocatalytic reactions

Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical technique renowned for its ultra-high sensitivity. Extensive research in SERS has led to the development of a wide range of SERS substrates, including plasmonic metals, semiconductors, metal organic frameworks, and their assemblies. Some of these materials are also excellent photocatalysts, and by taking advantage of their bifunctional characteristics, the photocatalytic processes that occur on their surface can be monitored in situ via SERS. This provides us with unique opportunities to gain valuable insights into the intricate details of the photocatalytic processes that are challenging to access using other techniques. In this review, we highlight key development in in situ and/or real-time SERS-tracking of photocatalytic reactions. We begin by providing a brief account of recent developments in SERS substrates, followed by discussions on how SERS can be used to elucidate crucial aspects of photocatalytic processes, including: (1) the influence of the surrounding media on charge carrier extraction; (2) the direction of charge carrier transfer; (3) the pathway of photocatalytic activation; and (4) differentiation between the effects of photo-thermal and energetic electrons. Additionally, we discuss the benefits of tip-enhanced Raman spectroscopy (TERS) due to the ability to achieve high-spatial-resolution measurements. Finally, we address major challenges and propose potential directions for the future of SERS monitoring of photocatalytic reactions. By leveraging the capabilities of SERS, we can uncover new insights into photocatalytic processes, paving the way for advancements in sustainable energy and environmental remediation.

Rapid single-particle chemical imaging of nanoplastics by SRS microscopy

Plastics are now omnipresent in our daily lives. The existence of microplastics (1 µm to 5 mm in length) and possibly even nanoplastics (<1 μm) has recently raised health concerns. In particular, nanoplastics are believed to be more toxic since their smaller size renders them much more amenable, compared to microplastics, to enter the human body. However, detecting nanoplastics imposes tremendous analytical challenges on both the nano-level sensitivity and the plastic-identifying specificity, leading to a knowledge gap in this mysterious nanoworld surrounding us. To address these challenges, we developed a hyperspectral stimulated Raman scattering (SRS) imaging platform with an automated plastic identification algorithm that allows micro-nano plastic analysis at the single-particle level with high chemical specificity and throughput. We first validated the sensitivity enhancement of the narrow band of SRS to enable high-speed single nanoplastic detection below 100 nm. We then devised a data-driven spectral matching algorithm to address spectral identification challenges imposed by sensitive narrow-band hyperspectral imaging and achieve robust determination of common plastic polymers. With the established technique, we studied the micro-nano plastics from bottled water as a model system. We successfully detected and identified nanoplastics from major plastic types. Micro-nano plastics concentrations were estimated to be about 2.4 ± 1.3 × 105 particles per liter of bottled water, about 90% of which are nanoplastics. This is orders of magnitude more than the microplastic abundance reported previously in bottled water. High-throughput single-particle counting revealed extraordinary particle heterogeneity and nonorthogonality between plastic composition and morphologies; the resulting multidimensional profiling sheds light on the science of nanoplastics.

Isotopic effects on in-plane hyperbolic phonon polaritons in MoO3

Hyperbolic phonon polaritons (HPhPs), hybrids of light and lattice vibrations in polar dielectric crystals, empower nanophotonic applications by enabling the confinement and manipulation of light at the nanoscale. Molybdenum trioxide (α-MoO3) is a naturally hyperbolic material, meaning that its dielectric function deterministically controls the directional propagation of in-plane HPhPs within its reststrahlen bands. Strategies such as substrate engineering, nano- and heterostructuring, and isotopic enrichment are being developed to alter the intrinsic die ectric functions of natural hyperbolic materials and to control the confinement and propagation of HPhPs. Since isotopic disorder can limit phonon-based processes such as HPhPs, here we synthesize isotopically enriched 92MoO3 (92Mo: 99.93 %) and 100MoO3 (100Mo: 99.01 %) crystals to tune the properties and dispersion of HPhPs with respect to natural α-MoO3, which is composed of seven stable Mo isotopes. Real-space, near-field maps measured with the photothermal induced resonance (PTIR) technique enable comparisons of inplane HPhPs in α-MoO3 and isotopically enriched analogues within a reststrahlen band (≈820 cm-1 to ≈ 972 cm-1). Results show that isotopic enrichment (e.g., 92MoO3 and 100MoO3) alters the dielectric function, shifting the HPhP dispersion (HPhP angular wavenumber × thickness vs IR frequency) by ≈-7% and ≈ +9 %, respectively, and changes the HPhP group velocities by ≈ ±12 %, while the lifetimes (≈ 3 ps) in 92MoO3 were found to be slightly improved (≈ 20 %). The latter improvement is attributed to a decrease in isotopic disorder. Altogether, isotopic enrichment was found to offer fine control over the properties that determine the anisotropic in-plane propagation of HPhPs in α-MoO3, which is essential to its implementation in nanophotonic applications.

Secondary structure and toxicity of transthyretin fibrils can be altered by unsaturated fatty acids

Transthyretin amyloidosis is a severe pathology characterized by the progressive accumulation of transthyretin (TTR) in various organs and tissues. This highly conserved through vertebrate evolution protein transports thyroid hormone thyroxine. In our bodies, TTR can interact with a large number of molecules, including ω-3 and ω-6 polyunsaturated fatty acids (PUFAs) that are broadly used as food supplies. In this study, we investigated the effect of ω-3 and ω-6 PUFAs, as well as their fully saturated analog, on TTR aggregation. Our results showed that both ω-3 and ω-6 PUFAs strongly decreased the rate of TTR aggregation. We also found that in the presence of PUFAs, TTR formed morphologically different fibrils compared to the lipid-free environment. Nano-Infrared imaging revealed that these fibrils had drastically different secondary structures compared to the secondary structure of TTR aggregates formed in the PUFAs-free environment. Furthermore, TTR fibrils formed in the presence of ω-3 and ω-6 PUFAs exerted significantly lower cell toxicity compared to the fibrils formed in the absence of fatty acids.

Cholesterol and Sphingomyelin Uniquely Alter the Rate of Transthyretin Aggregation and Decrease the Toxicity of Amyloid Fibrils

Transthyretin (TTR) is a small tetrameric protein that aggregates, forming highly toxic oligomers and fibrils. In the blood and cerebrospinal fluid, TTR can interact with various biomolecules, phospho- and sphingolipids, and cholesterol on the red blood cell plasma membrane. However, the role of these molecules in TTR aggregation remains unclear. In this study, we investigated the extent to which phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol (Cho), important components of plasma membranes, could alter the rate of TTR aggregation. We found that PC and SM inhibited TTR aggregation whereas Cho strongly accelerated it. The presence of these lipids during the stage of protein aggregation uniquely altered the morphology and secondary structure of the TTR fibrils, which changed the toxicity of these protein aggregates. These results suggest that interactions of TTR with red blood cells, whose membranes are rich with these lipids, can trigger irreversible aggregation of TTR and cause transthyretin amyloidosis.

Nanoscale chemical imaging with structured X-ray illumination

High-resolution imaging with compositional and chemical sensitivity is crucial for a wide range of scientific and engineering disciplines. Although synchrotron X-ray imaging through spectromicroscopy has been tremendously successful and broadly applied, it encounters challenges in achieving enhanced detection sensitivity, satisfactory spatial resolution, and high experimental throughput simultaneously. In this work, based on structured illumination, we develop a single-pixel X-ray imaging approach coupled with a generative image reconstruction model for mapping the compositional heterogeneity with nanoscale resolvability. This method integrates a full-field transmission X-ray microscope with an X-ray fluorescence detector and eliminates the need for nanoscale X-ray focusing and raster scanning. We experimentally demonstrate the effectiveness of our approach by imaging a battery sample composed of mixed cathode materials and successfully retrieving the compositional variations of the imaged cathode particles. Bridging the gap between structural and chemical characterizations using X-rays, this technique opens up vast opportunities in the fields of biology, environmental, and materials science, especially for radiation-sensitive samples.

Role of Plasmonic Antenna in Hot Carrier-Driven Reactions on Bimetallic Nanostructures

Noble metal nanostructures can efficiently harvest electromagnetic radiation, which, in turn, is used to generate localized surface plasmon resonances. Surface plasmons decay, producing hot carriers, that is, short-lived species that can trigger chemical reactions on metallic surfaces. However, noble metal nanostructures catalyze only a very small number of chemical reactions. This limitation can be overcome by coupling such nanostructures with catalytic-active metals. Although the role of such catalytically active metals in plasmon-driven catalysis is well-understood, the mechanistics of a noble metal antenna in such chemistry remains unclear. In this study, we utilize tip-enhanced Raman spectroscopy, an innovative nanoscale imaging technique, to investigate the rates and yields of plasmon-driven reactions on mono- and bimetallic gold- and silver-based nanostructures. We found that silver nanoplates (AgNPs) demonstrate a significantly higher yield of 4-nitrobenzenehtiol to p,p'-dimercaptoazobisbenzene (DMAB) reduction than gold nanoplates (AuNPs). We also observed substantially greater yields of DMAB on silver-platinum and silver-palladium nanoplates (Ag@PtNPs and Ag@PdNPs) compared to their gold analogues, Au@PtNPs and Au@PdNPs. Furthermore, Ag@PtNPs exhibited enhanced reactivity in 4-mercatophenylmethanol to 4-mercaptobenzoic acid oxidation compared to Au@PtNPs. These results showed that silver-based bimetallic nanostructures feature much greater reactivity compared to their gold-based analogues.

Toward conformational identification of molecules in 2D and 3D self-assemblies on surfaces

The design of supramolecular networks based on organic molecules deposited on surfaces, is highly attractive for various applications. One of the remaining challenges is the expansion of monolayers to well-ordered multilayers in order to enhance the functionality and complexity of self-assemblies. In this study, we present an assessment of molecular conformation from 2D to 3D supramolecular networks adsorbed onto a HOPG surface under ambient conditions utilizing a combination of scanning probe microscopies and atomic force microscopy- infrared (AFM-IR). We have observed that the infrared (IR) spectra of the designed molecules vary from layer to layer due to the modifications in the dihedral angle between the C=O group and the neighboring phenyl ring, especially in the case of a 3D supramolecular network consisting of multiple layers of molecules.

Elucidating the Role of Lipids in the Aggregation of Amyloidogenic Proteins

The abrupt aggregation of misfolded proteins is linked to the onset and spread of amyloidogenic diseases, including diabetes type 2, systemic amyloidosis, and Alzheimer's (AD) and Parkinson's diseases (PD). Although the exact cause of these pathological processes is unknown, a growing body of evidence suggests that amyloid diseases are triggered by misfolded or unfolded proteins, forming highly toxic oligomers. These transient species exhibit high structural and morphological heterogeneity. Protein oligomers can also propagate into β-sheet-rich filaments that braid and coil with other filaments to form amyloid fibrils and supramolecular structures with both flat and twisted morphologies. Microscopic examination of protein deposits formed in the brains of both AD and PD patients revealed the presence of fragments of lipid membranes. Furthermore, nanoscale infrared analysis of ex vivo extracted fibrils revealed the presence of lipids in their structure (Zhaliazka, K.; Kurouski, D. Protein Sci. 2023, 32, e4598). These findings demonstrated that lipid bilayers could play an important role in the aggregation of misfolded proteins.Experimental findings summarized in this Account show that (i) lipids uniquely change the aggregation rate of amyloidogenic proteins. In this case, the observed changes in the rates directly depend on the net charge of the lipid and the length and saturation of lipid fatty acids (FAs). For instance, zwitterionic phosphatidylcholine (PC) with 14:0 FAs inhibited the aggregation of insulin, lysozyme, and α-synuclein (α-Syn), whereas anionic phosphatidylserine with the same FAs dramatically accelerated the aggregation rate of these proteins (Dou, T., et al. J. Phys. Chem. Lett. 2021, 12, 4407. Matveyenka, M., et al. FASEB J. 2022, 36, e22543. Rizevsky, S., et al. J. Phys. Chem. Lett. 2022, 13, 2467). Furthermore, (ii) lipids uniquely alter the secondary structure and morphology of protein oligomers and fibrils formed in their presence. Utilization of nano-infrared spectroscopy revealed that such aggregates, as well as ex vivo extracted fibrils, possessed lipids in their structure. These findings are significant because (iii) lipids uniquely alter the toxicity of amyloid oligomers and fibrils formed in their presence. Specifically, PC lowered the toxicity of insulin and lysozyme oligomers, whereas α-Syn oligomers formed in the presence of this phospholipid were found to be significantly more toxic to rat dopaminergic cells compared to α-Syn oligomers grown in the lipid-free environment. Thus, the toxicity of protein oligomers and fibrils is directly determined by the chemical structure of the lipid and the secondary structure of amyloidogenic proteins (Dou, T., et al. J. Phys. Chem. Lett. 2021, 12, 4407. Matveyenka, M., et al. FASEB J. 2022, 36, e22543. Rizevsky, S., et al. J. Phys. Chem. Lett. 2022, 13, 2467). Experimental results discussed in this Account also suggest that amyloidogenic diseases could be caused by pathological changes in the lipid composition of both plasma and organelle membranes, which, in turn, may trigger protein aggregation that results in the formation of highly toxic oligomers and fibrils. Finally, the Account discusses the effects of polyunsaturated FAs on the aggregation properties of amyloidogenic proteins. Experimental findings reported by the author's laboratory revealed that polyunsaturated FAs drastically accelerated the aggregation rate of both insulin and α-Syn as well as strongly changed the secondary structure of amyloid fibrils formed in their presence.

Saturation of fatty acids in phosphatidic acid uniquely alters transthyretin stability changing morphology and toxicity of amyloid fibrils

Transthyretin (TTR) is a small, β-sheet-rich tetrameric protein that transports thyroid hormone thyroxine and retinol. Phospholipids, including phosphatidic acid (PA), can uniquely alter the stability of amyloidogenic proteins. However, the role of PA in TTR aggregation remains unclear. In this study, we investigated the effect of saturation of fatty acids (FAs) in PA on the rate of TTR aggregation. We also reveal the extent to which PAs with different length and saturation of FAs altered the morphology and secondary structure of TTR aggregates. Our results showed that TTR aggregation in the equimolar presence of PAs with different length and saturation of FAs yielded structurally and morphologically different fibrils compared to those formed in the lipid-free environment. We also found that PAs drastically lowered the toxicity of TTR aggregates formed in the presence of this phospholipid. These results shed light on the role of PA in the stability of TTR and transthyretin amyloidosis.