Sub-100 nm IR spectromicroscopy of living cells

Bottom-Illuminated Photothermal Nanoscale Chemical Imaging with a Flat Silicon ATR in Air and Liquid

We demonstrate a novel approach for bottom-illuminated atomic force microscopy and infrared spectroscopy (AFM-IR). Bottom-illuminated AFM-IR for measurements in liquids makes use of an attenuated total reflection setup where the developing evanescent wave is responsible for photothermal excitation of the sample of interest. Conventional bottom-illuminated measurements are conducted using high-refractive-index prisms. We showcase the advancement of instrumentation through the introduction of flat silicon substrates as replacements for prisms. We illustrate the feasibility of this technique for bottom-illuminated AFM-IR in both air and liquid. We also show how modern rapid prototyping technologies enable commercial AFM-IR instrumentation to accept these new substrates. This new approach paves the way for a wide range of experiments since virtually any established protocol for Si surface functionalization can be applied to this sample carrier. Furthermore, the low unit cost enables the rapid iteration of experiments.

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.

AFM-IR for Nanoscale Chemical Characterization in Life Sciences: Recent Developments and Future Directions

Despite the ubiquitous absorption of mid-infrared (IR) radiation by virtually all molecules that belong to the major biomolecules groups (proteins, lipids, carbohydrates, nucleic acids), the application of conventional IR microscopy to the life sciences remained somewhat limited, due to the restrictions on spatial resolution imposed by the diffraction limit (in the order of several micrometers). This issue is addressed by AFM-IR, a scanning probe-based technique that allows for chemical analysis at the nanoscale with resolutions down to 10 nm and thus has the potential to contribute to the investigation of nano and microscale biological processes. In this perspective, in addition to a concise description of the working principles and operating modes of AFM-IR, we present and evaluate the latest key applications of AFM-IR to the life sciences, summarizing what the technique has to offer to this field. Furthermore, we discuss the most relevant current limitations and point out potential future developments and areas for further application for fruitful interdisciplinary collaboration.

Principle and applications of peak force infrared microscopy

Peak force infrared (PFIR) microscopy is an emerging atomic force microscopy (AFM)-based infrared microscopy that bypasses Abbe's diffraction limit on spatial resolution. The PFIR microscopy utilizes a nanoscopically sharp AFM tip to mechanically detect the tip-enhanced infrared photothermal response of the sample in the time domain. The time-gated mechanical signals of cantilever deflections transduce the infrared absorption of the sample, delivering infrared imaging and spectroscopy capability at sub 10 nm spatial resolution. Both the infrared absorption response and mechanical properties of the sample are obtained in parallel while preserving the surface integrity of the sample. This review describes the constructions of the PFIR microscope and several variations, including multiple-pulse excitation, total internal reflection geometry, dual-color configuration, liquid-phase operations, and integrations with simultaneous surface potential measurement. Representative applications of PFIR microscopy are also included in this review. In the outlook section, we lay out several future directions of innovations in PFIR microscopy and applications in chemical and material research.

Subdiffraction Quantum Imaging with Undetected Photons

We propose a nonlinear imaging scheme with undetected photons that overcomes the diffraction limit by transferring near-field information at one wavelength to far-field information of a correlated photon with a different wavelength generated through spontaneous photon-pair generation. At the same time, this scheme allows for retrieval of high-contrast images with zero background, making it a highly sensitive scheme for imaging of small objects at challenging spectral ranges with subdiffraction resolutions.

Liquid-Phase Peak Force Infrared Microscopy for Chemical Nanoimaging and Spectroscopy

Peak force infrared (PFIR) microscopy is an emerging atomic force microscopy that bypasses Abbe's diffraction limit in achieving chemical nanoimaging and spectroscopy. The PFIR microscopy mechanically detects the infrared photothermal responses in the dynamic tip-sample contact of peak force tapping mode and has been applied for a variety of samples, ranging from soft matters, photovoltaic heterojunctions, to polaritonic materials under the air conditions. In this article, we develop and demonstrate the PFIR microscopy in the liquid phase for soft matters and biological samples. With the capability of controlling fluid compositions on demand, the liquid-phase peak force infrared (LiPFIR) microscopy enables in situ tracking of the polymer surface reorganization in fluids and detecting the product of click chemical reaction in the aqueous phase. Both broadband spectroscopy and infrared imaging with ∼10 nm spatial resolution are benchmarked in the fluid phase, together with complementary mechanical information. We also demonstrate the LiPFIR microscopy on revealing the chemical composition of a budding site of yeast cell wall particles in water as an application on biological structures. The label-free, nondestructive chemical nanoimaging and spectroscopic capabilities of the LiPFIR microscopy will facilitate the investigations of soft matters and their transformations at the solid/liquid interface.

In Liquid Infrared Scattering Scanning Near-Field Optical Microscopy for Chemical and Biological Nanoimaging

Imaging biological systems with simultaneous intrinsic chemical specificity and nanometer spatial resolution in their typical native liquid environment has remained a long-standing challenge. Here, we demonstrate a general approach of chemical nanoimaging in liquid based on infrared scattering scanning near-field optical microscopy (IR s-SNOM). It is enabled by combining AFM operation in a fluid cell with evanescent IR illumination via total internal reflection, which provides spatially confined excitation for minimized IR water absorption, reduced far-field background, and enhanced directional signal emission and sensitivity. We demonstrate in-liquid IR s-SNOM vibrational nanoimaging and conformational identification of catalase nanocrystals and spatio-spectral analysis of biomimetic peptoid sheets with monolayer sensitivity and chemical specificity at the few zeptomole level. This work establishes the principles of in-liquid and in situ IR s-SNOM spectroscopic chemical nanoimaging and its general applicability to biomolecular, cellular, catalytic, electrochemical, or other interfaces and nanosystems in liquids or solutions.

Infrared and Raman chemical imaging and spectroscopy at the nanoscale

The advent of nanotechnology, and the need to understand the chemical composition at the nanoscale, has stimulated the convergence of IR and Raman spectroscopy with scanning probe methods, resulting in new nanospectroscopy paradigms. Here we review two such methods, namely photothermal induced resonance (PTIR), also known as AFM-IR and tip-enhanced Raman spectroscopy (TERS). AFM-IR and TERS fundamentals will be reviewed in detail together with their recent crucial advances. The most recent applications, now spanning across materials science, nanotechnology, biology, medicine, geology, optics, catalysis, art conservation and other fields are also discussed. Even though AFM-IR and TERS have developed independently and have initially targeted different applications, rapid innovation in the last 5 years has pushed the performance of these, in principle spectroscopically complimentary, techniques well beyond initial expectations, thus opening new opportunities for their convergence. Therefore, subtle differences and complementarity will be highlighted together with emerging trends and opportunities.

Characterization of Intact Eukaryotic Cells with Subcellular Spatial Resolution by Photothermal-Induced Resonance Infrared Spectroscopy and Imaging

Photothermal-induced resonance (PTIR) spectroscopy and imaging with infrared light has seen increasing application in the molecular spectroscopy of biological samples. The appeal of the technique lies in its capability to provide information about IR light absorption at a spatial resolution better than that allowed by light diffraction, typically below 100 nm. In the present work, we tested the capability of the technique to perform measurements with subcellular resolution on intact eukaryotic cells, without drying or fixing. We demonstrate the possibility of obtaining PTIR images and spectra from the nucleus and multiple organelles with high resolution, better than that allowed by diffraction with infrared light. We obtain particularly strong signal from bands typically assigned to acyl lipids and proteins. We also show that while a stronger signal is obtained from some subcellular structures, other large subcellular components provide a weaker or undetectable PTIR response. The mechanism that underlies such variability in response is presently unclear. We propose and discuss different possibilities, addressing thermomechanical, geometrical, and electrical properties of the sample and the presence of cellular water, from which the difference in response may arise.

Nanoscale AFM-IR spectroscopic imaging of lipid heterogeneity and effect of irradiation in prostate cancer cells

The recent development of the AFM-IR technique, which combines nanoscale imaging with chemical contrast through infrared spectroscopy, opened up new fields for exploration, which were out of reach for other modalities, e.g. Raman spectroscopy. Lipid droplets (LDs) are key organelles, which are associated with stress response mechanisms in cells and their size falls into that niche. LDs composition is heterogeneous and varies depending on cancer cell type and the tumor microenvironment. Prostate cancer cells show a unique lipid metabolism manifested by an increased requirement for lipid accumulation in cytosolic LDs. In the current work, AFM-IR nanoimaging was undertaken to analyze lipids in untreated and x-ray irradiated PC-3 prostate cancer cells. Cells poor in LDs showed slightly increased lipid signal in cytoplasm close to the nucleus. On the other hand, high lipid signal coming from LDs accumulation could be found in any part of the cytoplasmic region. The observed behavior was found to be independent from irradiation and its dose. According to the band assignment of the collected AFM-IR spectra, the main components of LDs were assigned to cholesteryl esters. The size of LDs present in cells poor in lipids was found to be of less than 1 μm, whereas LDs aggregates spread out over a few microns. Analysis of AFM-IR spectra shows relative homogeneity of LDs composition in single cells and heterogeneity of LDs content within the PC-3 cell population.

Infrared nanospectroscopic mapping of a single metaphase chromosome

The integrity of the chromatin structure is essential to every process occurring within eukaryotic nuclei. However, there are no reliable tools to decipher the molecular composition of metaphase chromosomes. Here, we have applied infrared nanospectroscopy (AFM-IR) to demonstrate molecular difference between eu- and heterochromatin and generate infrared maps of single metaphase chromosomes revealing detailed information on their molecular composition, with nanometric lateral spatial resolution. AFM-IR coupled with principal component analysis has confirmed that chromosome areas containing euchromatin and heterochromatin are distinguishable based on differences in the degree of methylation. AFM-IR distribution of eu- and heterochromatin was compared to standard fluorescent staining. We demonstrate the ability of our methodology to locate spatially the presence of anticancer drug sites in metaphase chromosomes and cellular nuclei. We show that the anticancer 'rule breaker' platinum compound [Pt[N(p-HC6F4)CH2]2py2] preferentially binds to heterochromatin, forming localized discrete foci due to condensation of DNA interacting with the drug. Given the importance of DNA methylation in the development of nearly all types of cancer, there is potential for infrared nanospectroscopy to be used to detect gene expression/suppression sites in the whole genome and to become an early screening tool for malignancy.

Nanoscale chemical mapping of oxygen functional groups on graphene oxide using atomic force microscopy-coupled infrared spectroscopy

The unambiguous determination of the chemical functionality over graphene oxide (GO) is important to unleash its potential applications. However, the mapping of oxygen functionalities distribution remains to be unequivocally determined because of highly inhomogeneous non-stoichiometric structures and ultra-thin layers of GO. In this study, we report an experimental observation of the spatial distribution of oxygen functional groups on monolayer and multilayer GO using AFM-IR, atomic force microscopy coupled with infrared spectroscopy. Overcoming conventional IR diffraction limit for several micrometers, the novel AFM-IR reaches high spatial resolution ∼20 nm and could detect IR absorption on ∼1 nm thickness of monolayer GO. With nanoscale chemical mapping, the distribution of different oxygen functional groups is distinguished with AFM-IR over the GO surface. It allows us to observe that these oxygen functional groups prefer to sit on the fold areas, in discrete domains and on the edges of GO, which gave more insights into its chemical nature. The determination of the position of functional groups through precise imaging contributes to our understanding of GO structure-properties relations and paves the way for targeted tethering of polymers, biomaterials, and other nanostructures.

Ultrafast chemical imaging by widefield photothermal sensing of infrared absorption

Infrared (IR) imaging has become a viable tool for visualizing various chemical bonds in a specimen. The performance, however, is limited in terms of spatial resolution and imaging speed. Here, instead of measuring the loss of the IR beam, we use a pulsed visible light for high-throughput, widefield sensing of the transient photothermal effect induced by absorption of single mid-IR pulses. To extract these transient signals, we built a virtual lock-in camera synchronized to the visible probe and IR light pulses with precisely controlled delays, allowing submicrosecond temporal resolution determined by the probe pulse width. Our widefield photothermal sensing microscope enabled chemical imaging at a speed up to 1250 frames/s, with high spectral fidelity, while offering submicrometer spatial resolution. With the capability of imaging living cells and nanometer-scale polymer films, widefield photothermal microscopy opens a new way for high-throughput characterization of biological and material specimens.

Label-free detection of biotoxins via a photo-induced force infrared spectrum at the single-molecular level

Schematic illustration of photo-induced force microscopy combine principal component analysis detected and distinguish single molecule particles of biotoxins AT, RT/ETX with label-free.

Nanoscale Chemical Imaging of Individual, Chemotherapeutic Cytarabine-loaded Liposomal Nanocarriers

Dosage of chemotherapeutic drugs is a tradeoff between efficacy and side-effects. Liposomes are nanocarriers that increase therapy efficacy and minimize side-effects by delivering otherwise difficult to administer therapeutics with improved efficiency and selectivity. Still, variabilities in liposome preparation require assessing drug encapsulation efficiency at the single liposome level, an information that, for non-fluorescent therapeutic cargos, is inaccessible due to the minute drug load per liposome. Photothermal induced resonance (PTIR) provides nanoscale compositional specificity, up to now, by leveraging an atomic force microscope (AFM) tip contacting the sample to transduce the sample's photothermal expansion. However, on soft samples (e.g. liposomes) PTIR effectiveness is reduced due to the likelihood of tip-induced sample damage and inefficient AFM transduction. Here, individual liposomes loaded with the chemotherapeutic drug cytarabine are deposited intact from suspension via nES-GEMMA (nano-electrospray gas-phase electrophoretic mobility molecular analysis) collection and characterized at the nanoscale with the chemically-sensitive PTIR method. A new tapping-mode PTIR imaging paradigm based on heterodyne detection is shown to be better adapted to measure soft samples, yielding cytarabine distribution in individual liposomes and enabling classification of empty and drug-loaded liposomes. The measurements highlight PTIR capability to detect ≈ 103 cytarabine molecules (≈ 1.7 zmol) label-free and non-destructively.

Identification of Oxidative Stress in Red Blood Cells with Nanoscale Chemical Resolution by Infrared Nanospectroscopy

During their lifespan, Red blood cells (RBC), due to their inability to self-replicate, undergo an ageing degradation phenomenon. This pathway, both in vitro and in vivo, consists of a series of chemical and morphological modifications, which include deviation from the biconcave cellular shape, oxidative stress, membrane peroxidation, lipid content decrease and uncoupling of the membrane-skeleton from the lipid bilayer. Here, we use the capabilities of atomic force microscopy based infrared nanospectroscopy (AFM-IR) to study and correlate, with nanoscale resolution, the morphological and chemical modifications that occur during the natural degradation of RBCs at the subcellular level. By using the tip of an AFM to detect the photothermal expansion of RBCs, it is possible to obtain nearly two orders of magnitude higher spatial resolution IR spectra, and absorbance images than can be obtained on diffraction-limited commercial Fourier-transform Infrared (FT-IR) microscopes. Using this approach, we demonstrate that we can identify localized sites of oxidative stress and membrane peroxidation on individual RBC, before the occurrence of neat morphological changes in the cellular shape.

Determination of Polypeptide Conformation with Nanoscale Resolution in Water

The folding and acquisition of proteins native structure is central to all biological processes of life. By contrast, protein misfolding can lead to toxic amyloid aggregates formation, linked to the onset of neurodegenerative disorders. To shed light on the molecular basis of protein function and malfunction, it is crucial to access structural information on single protein assemblies and aggregates under native conditions. Yet, current conformation-sensitive spectroscopic methods lack the spatial resolution and sensitivity necessary for characterizing heterogeneous protein aggregates in solution. To overcome this limitation, here we use photothermal-induced resonance to demonstrate that it is possible to acquire nanoscale infrared spectra in water with high signal-to-noise ratio (SNR). Using this approach, we probe supramolecular aggregates of diphenylalanine, the core recognition module of the Alzheimer's β-amyloid peptide, and its derivative Boc-diphenylalanine. We achieve nanoscale resolved IR spectra and maps in air and water with comparable SNR and lateral resolution, thus enabling accurate identification of the chemical and structural state of morphologically similar networks at the single aggregate ( i. e., fibril) level.

Nanoscale Investigation into the Cellular Response of Glioblastoma Cells Exposed to Protons

Exposure to ionizing radiation can induce cellular defense mechanisms including cell activation and rapid proliferation prior to metastasis and in extreme cases can result in cell death. Herewith we apply infrared nano- and microspectroscopy combined with multidimensional data analysis to characterize the effect of ionizing radiation on single glioblastoma nuclei isolated from cells treated with 10 Gy of X-rays or 1 and 10 Gy of protons. We observed chromatin fragmentation related to the formation of apoptotic bodies following X-ray exposure. Following proton irradiation we detected evidence of a DNA conformational change (B-DNA to A-DNA transition) related to DNA repair and accompanied by an increase in protein content related to the synthesis of peptide enzymes involved in DNA repair. We also show that proton exposure can increase cholesterol and sterol ester synthesis, which are important lipids involved in the metastatic process changing the fluidity of the cellular membrane in preparation for rapid proliferation.

Multispectral Atomic Force Microscopy-Infrared Nano-Imaging of Malaria Infected Red Blood Cells

Atomic force microscopy-infrared (AFM-IR) spectroscopy is a powerful new technique that can be applied to study molecular composition of cells and tissues at the nanoscale. AFM-IR maps are acquired using a single wavenumber value: they show either the absorbance plotted against a single wavenumber value or a ratio of two absorbance values. Here, we implement multivariate image analysis to generate multivariate AFM-IR maps and use this approach to resolve subcellular structural information in red blood cells infected with Plasmodium falciparum at different stages of development. This was achieved by converting the discrete spectral points into a multispectral line spectrum prior to multivariate image reconstruction. The approach was used to generate compositional maps of subcellular structures in the parasites, including the food vacuole, lipid inclusions, and the nucleus, on the basis of the intensity of hemozoin, hemoglobin, lipid, and DNA IR marker bands, respectively. Confocal Raman spectroscopy was used to validate the presence of hemozoin in the regions identified by the AFM-IR technique. The high spatial resolution of AFM-IR combined with hyperspectral modeling enables the direct detection of subcellular components, without the need for cell sectioning or immunological/biochemical staining. Multispectral-AFM-IR thus has the capacity to probe the phenotype of the malaria parasite during its intraerythrocytic development. This enables novel approaches to studying the mode of action of antimalarial drugs and the phenotypes of drug-resistant parasites, thus contributing to the development of diagnostic and control measures.

Mid-infrared spectroscopy and microscopy of subcellular structures in eukaryotic cells with atomic force microscopy – infrared spectroscopy

Atomic force microscopy – infrared (AFM-IR) spectroscopy allows spectroscopic studies in the mid-infrared (mid-IR) spectral region with a spatial resolution better than is allowed by the diffraction limit. We show that the high spatial resolution can be used to perform spectroscopic and imaging studies at the subcellular level in fixed eukaryotic cells. We collect AFM-IR images of subcellular structures that include lipid droplets, vesicles and cytoskeletal filaments, by relying on the intrinsic contrast from IR light absorption. We also obtain AFM-IR absorption spectra of individual subcellular structures. Most spectra show features that are recognizable in the IR absorption spectra of cells and tissue obtained with FTIR technology, including absorption bands characteristic of phospholipids and polypeptides. The quality of the spectra and of the images opens the way to structure and composition studies at the subcellular level using mid-IR absorption spectroscopy.

Atomic force microscopy – infrared (AFM-IR) spectroscopy allows spectroscopic studies in the mid-infrared (mid-IR) spectral region with a spatial resolution better than is allowed by the diffraction limit.

AFM-Based Single Molecule Techniques: Unraveling the Amyloid Pathogenic Species

Background

A wide class of human diseases and neurodegenerative disorders, such as Alzheimer’s disease, is due to the failure of a specific peptide or protein to keep its native functional conformational state and to undergo a conformational change into a misfolded state, triggering the formation of fibrillar cross-β sheet amyloid aggregates. During the fibrillization, several coexisting species are formed, giving rise to a highly heterogeneous mixture. Despite its fundamental role in biological function and malfunction, the mechanism of protein self-assembly and the fundamental origins of the connection between aggregation, cellular toxicity and the biochemistry of neurodegeneration remains challenging to elucidate in molecular detail. In particular, the nature of the specific state of proteins that is most prone to cause cytotoxicity is not established.

Methods:

In the present review, we present the latest advances obtained by Atomic Force Microscopy (AFM) based techniques to unravel the biophysical properties of amyloid aggregates at the nanoscale. Unraveling amyloid single species biophysical properties still represents a formidable experimental challenge, mainly because of their nanoscale dimensions and heterogeneous nature. Bulk techniques, such as circular dichroism or infrared spectroscopy, are not able to characterize the heterogeneity and inner properties of amyloid aggregates at the single species level, preventing a profound investigation of the correlation between the biophysical properties and toxicity of the individual species.

Conclusion:

The information delivered by AFM based techniques could be central to study the aggregation pathway of proteins and to design molecules that could interfere with amyloid aggregation delaying the onset of misfolding diseases.

Supramolecular Orientation in Anisotropic Assemblies by Infrared Nanopolarimetry

We report on the experimental characterization of anisotropic supramolecular assemblies by infrared (IR) nanopolarimetry. The presented IR absorption anisotropy imaging method simultaneously provides nanoscale-resolved insights into internal composition, intermolecular interactions, and supramolecular orientation in a label-free and noninvasive fashion. Our study of porphyrin aggregates demonstrates that their morphology can be correlated with stable J-type and metastable H-type stacking-induced anisotropic organization, revealing different oriented attachment growth mechanisms supported by theory. This analysis establishes the broad applicability of IR nanopolarimetric studies to supramolecular polymerization and biomolecular assemblies, opening up new routes in polymer science and macromolecular research.

Nanoscale Infrared, Thermal, and Mechanical Characterization of Telaprevir-Polymer Miscibility in Amorphous Solid Dispersions Prepared by Solvent Evaporation

Miscibility is of great interest for pharmaceutical systems, in particular, for amorphous solid dispersions, as phase separation can lead to a higher tendency to crystallize, resulting in a loss in solubility, decreased dissolution rate, and compromised bioavailability. The purpose of this study was to investigate the miscibility behavior of a model poorly water-soluble drug, telaprevir (TPV), with three different polymers using atomic force microscopy-based infrared, thermal, and mechanical analysis. Standard atomic force microscopy (AFM) imaging together with nanoscale infrared spectroscopy (AFM-IR), nanoscale thermal analysis (nanoTA), and Lorentz contact resonance (LCR) measurements were used to evaluate the miscibility behavior of TPV with three polymers, hydroxypropyl methylcellulose (HPMC), HPMC acetate succinate (HPMCAS), and poly(vinylpyrrolidone-co-vinyl acetate) (PVPVA), at different drug to polymer ratios. Phase separation was observed with HPMC and PVPVA at drug loadings above 10%. For HPMCAS, a smaller miscibility gap was observed, with phase separation being observed at drug loadings higher than ∼30-40%. The domain size of phase-separated regions varied from below 50 nm to a few hundred nanometers. Localized infrared spectra, nano-TA measurements, images from AFM-based IR, and LCR measurements showed clear contrast between the continuous and discrete domains for these phase-separated systems, whereby the discrete domains were drug-rich. Fluorescence microscopy provided additional evidence for phase separation. These methods appear to be promising to evaluate miscibility in drug-polymer systems with similar Tgs and submicron domain sizes. Furthermore, such findings are of obvious importance in the context of contributing to a mechanistic understanding of amorphous solid dispersion phase behavior.

Broadband near-field infrared spectromicroscopy using photothermal probes and synchrotron radiation

In this paper, we experimentally demonstrate the use of infrared synchrotron radiation (IR-SR) as a broadband source for photothermal near-field infrared spectroscopy. We assess two methods of signal transduction; cantilever resonant thermal expansion and scanning thermal microscopy. By means of rapid mechanical chopping (50-150 kHz), we modulate the IR-SR at rates matching the contact resonance frequencies of atomic force microscope (AFM) cantilevers, allowing us to record interferograms yielding Fourier transform infrared (FT-IR) photothermal absorption spectra of polystyrene and cyanoacrylate films. Complementary offline measurements using a mechanically chopped CW IR laser confirmed that the resonant thermal expansion IR-SR measurements were below the diffraction limit, with a spatial resolution better than 500 nm achieved at a wavelength of 6 μm, i.e. λ/12 for the samples studied. Despite achieving the highest signal to noise so far for a scanning thermal microscopy measurement under conditions approaching near-field (dictated by thermal diffusion), the IR-SR resonant photothermal expansion FT-IR spectra measured were significantly higher in signal to noise in comparison with the scanning thermal data.

The importance of correcting for variable probe–sample interactions in AFM-IR spectroscopy: AFM-IR of dried bacteria on a polyurethane film

Interplay between AFM-IR probe – sample interactions and signal transduction for bacteria – PU bilayer.

Miscibility of Itraconazole-Hydroxypropyl Methylcellulose Blends: Insights with High Resolution Analytical Methodologies

Drug-polymer miscibility is considered to be a prerequisite to achieve an optimally performing amorphous solid dispersion (ASD). Unfortunately, it can be challenging to evaluate drug-polymer miscibility experimentally. The aim of this study was to investigate the miscibility of ASDs of itraconazole (ITZ) and hydroxypropyl methylcellulose (HPMC) using a variety of analytical approaches. The phase behavior of ITZ-HPMC films prepared by solvent evaporation was studied before and after heating. Conventional methodology for miscibility determination, that is, differential scanning calorimetry (DSC), was used in conjunction with emerging analytical techniques, such as fluorescence spectroscopy, fluorescence imaging, and atomic force microscopy coupled with nanoscale infrared spectroscopy and nanothermal analysis (AFM-nanoIR-nanoTA). DSC results showed a single glass transition event for systems with 10% to 50% drug loading, suggesting that the ASDs were miscible, whereas phase separation was observed for all of the films based on the other techniques. The AFM-coupled techniques indicated that the phase separation occurred at the submicron scale. When the films were heated, it was observed that the ASD components underwent mixing. The results provide new insights into the phase behavior of itraconazole-HPMC dispersions and suggest that the emerging analytical techniques discussed herein are promising for the characterization of miscibility and microstructure in drug-polymer systems. The observed differences in the phase behavior in films prepared by solvent evaporation before and after heating also have implications for processing routes and suggest that spray drying/solvent evaporation and hot melt extrusion/melt mixing can result in ASDs with varying extent of miscibility between the drug and the polymer.

Metal-dielectric-metal resonators with deep subwavelength dielectric layers increase the near-field SEIRA enhancement

Plasmonic nanostructures presenting either structural asymmetry or metal-dielectric-metal (M-D-M) architecture are commonly used structures to increase the quality factor and the near-field confinement in plasmonic materials. This characteristic can be leveraged for example to increase the sensitivity of IR spectroscopy, via the surface enhanced IR absorption (SEIRA) effect. In this work, we combine structural asymmetry with the M-D-M architecture to realize Ag-Ag(2)O-Ag asymmetric ring resonators where two Ag layers sandwich a native silver oxide (Ag(2)O) layer. Their IR response is compared with the one of fully metallic (Ag) resonators of the same size and shape. The photothermal induced resonance technique (PTIR) is used to obtain near-field SEIRA absorption maps and spectra with nanoscale resolution. Although the native Ag(2)O layer is only 1 nm to 2 nm thick, it increases the quality factor of the resonators' dark-mode by ≈27% and the SEIRA enhancement by ≈44% with respect to entirely Ag structures.

Near-field asymmetries in plasmonic resonators

Surface-enhanced infrared absorption (SEIRA) spectroscopy exploits the locally enhanced field surrounding plasmonic metamaterials to increase the sensitivity of infrared spectroscopy. The light polarization and incidence angle are important factors for exciting plasmonic nanostructures; however, such angle dependence is often ignored in SEIRA experiments, typically carried out with Cassegrain objectives. Here, the photothermal induced resonance technique and numerical simulations are used to map the distribution and intensity of SEIRA hot-spots surrounding gold asymmetric split ring resonators (ASRRs) as a function of light polarization and incidence angle. The results show asymmetric near-field SEIRA enhancements as a function of the incident illumination direction which, in analogy with the symmetry-breaking occurring in asymmetric transmission, we refer to as symmetry-breaking absorption. Numerical calculations reveal that the symmetry-breaking absorption in ASRRs originates in the angle-dependent interference between the electric and magnetic excitation channels of the resonators' dark-mode. Consequently, to maximize the SEIRA intensity, ASRRs should be illuminated from the dielectric side at an angle that maximizes the constructive interference of the two excitation channels, (35° for the structures studied here), in place of the Cassegrain objectives. These results can be generalized to all structures characterized by plasmonic excitations that give rise to a surface-normal magnetic moment and that possess an electric dipole.

Infrared Imaging and Spectroscopy Beyond the Diffraction Limit

Progress in nanotechnology is enabled by and dependent on the availability of measurement methods with spatial resolution commensurate with nanomaterials' length scales. Chemical imaging techniques, such as scattering scanning near-field optical microscopy (s-SNOM) and photothermal-induced resonance (PTIR), have provided scientists with means of extracting rich chemical and structural information with nanoscale resolution. This review presents some basics of infrared spectroscopy and microscopy, followed by detailed descriptions of s-SNOM and PTIR working principles. Nanoscale spectra are compared with far-field macroscale spectra, which are widely used for chemical identification. Selected examples illustrate either technical aspects of the measurements or applications in materials science. Central to this review is the ability to record nanoscale infrared spectra because, although chemical maps enable immediate visualization, the spectra provide information to interpret the images and characterize the sample. The growing breadth of nanomaterials and biological applications suggest rapid growth for this field.

Quantifying nanoscale biochemical heterogeneity in human epithelial cancer cells using combined AFM and PTIR absorption nanoimaging

Subcellular chemical heterogeneity plays a key role in cell organization and function. However the biomechanics underlying the structure-function relationship is governed by cell substructures which are poorly resolved using conventional chemical imaging methods. To date, advances in sub-diffraction limited infrared (IR) nanoscopy have permitted intracellular chemical mapping. In this work we report how image analysis applied to a combination of IR absorption nanoimaging and topographic data permits quantification of chemical complexity at the nanoscale, enabling the analysis of biochemical heterogeneity in mammalian cancer cells on the scale of subcellular features.

Metal-carbonyl units for vibrational and luminescence imaging: towards multimodality

Metal-carbonyl complexes are attractive structures for bio-imaging. In addition to unique vibrational properties due to the CO moieties enabling IR and Raman cell imaging, the appropriate choice of ancillary ligands opens up the opportunity for luminescence detection. Through a classification by techniques, past and recent developments in the application of metal-carbonyl complexes for vibrational and luminescence bio-imaging are reviewed. Finally, their potential as bimodal IR and luminescent probes is addressed.

Prospects for vibrational-mode EELS with high spatial resolution

Taking advantage of previous measurements by Geiger and co-workers, we discuss the possibilities and problems of measuring vibrational modes of energy loss in a transmission electron microscope fitted with a monochromator and a high-resolution energy-loss spectrometer. The tail of the zero-loss peak is seen to be a major limitation, rather than its full-width at half-maximum. Because of the low oscillator strengths and small cross-sections involved, radiation damage will limit the spatial resolution if this technique is applied to organic specimens. Delocalization of the inelastic scattering may also be a limitation, if a dipole description of the scattering process is valid.

Monitoring TriAcylGlycerols Accumulation by Atomic Force Microscopy Based Infrared Spectroscopy in Streptomyces Species for Biodiesel Applications

An atomic force microscope coupled with a tunable infrared laser source (AFM-IR) was used to measure the size and map the distribution of oil inclusions inside of microorganism without staining or other special sample preparation. The microorganism under study is Streptomyces, a soil bacterium that possesses the capability, under some specific nutritional conditions, to store its carbon source into TriAcylGlycerols, a potential direct source of biodiesel.

Atomic force microscope infrared spectroscopy of griseofulvin nanocrystals

The goal of this work was to evaluate the ability of photothermal-induced resonance (PTIR) to measure the local infrared absorption spectra of crystalline organic drug nanoparticles embedded within solid matrices. Herein, the first reports of the chemical characterization of sub-100 nm organic crystals are described; infrared spectra of 90 nm griseofulvin particles were obtained, confirming the chemical resolution of PTIR beyond the diffraction limit. Additionally, particle size distributions via dynamic light scattering and PTIR image analysis were found to be similar, suggesting that the PTIR measurements are not significantly affected by inhomogeneous infrared absorptivity of this system. Thus as medical applications increasingly emphasize localized drug delivery via micro/nanoengineered structures, PTIR can be used to unambiguously chemically characterize drug formulations at these length scales.

Improved atomic force microscope infrared spectroscopy for rapid nanometer-scale chemical identification

Atomic force microscope infrared spectroscopy (AFM-IR) can perform IR spectroscopic chemical identification with sub-100 nm spatial resolution, but is relatively slow due to its low signal-to-noise ratio (SNR). In AFM-IR, tunable IR laser light is incident upon a sample, which results in a rise in temperature and thermomechanical expansion of the sample. An AFM tip in contact with the sample senses this nanometer-scale photothermal expansion. The tip motion induces cantilever vibrations, which are measured either in terms of the peak-to-peak amplitude of time-domain data or the integrated magnitude of frequency-domain data. Using a continuous Morlet wavelet transform to the cantilever dynamic response, we show that the cantilever dynamics during AFM-IR vary as a function of both time and frequency. Based on the observed cantilever response, we tailor a time-frequency-domain filter to identify the region of highest vibrational energy. This approach can increase the SNR of the AFM cantilever signal, such that the throughput is increased 32-fold compared to state-of-the art procedures. We further demonstrate significant increases in AFM-IR imaging speed and chemical identification of nanometer-scale domains in polymer films.

Super-resolved FT-IR spectroscopy: Strategies, challenges, and opportunities for membrane biophysics

Direct correlation of molecular conformation with local structure is critical to studies of protein- and peptide-membrane interactions, particularly in the context of membrane-facilitated aggregation, and disruption or disordering. Infrared spectroscopy has long been a mainstay for determining molecular conformation, following folding dynamics, and characterizing reactions. While tremendous advances have been made in improving the spectral and temporal resolution of infrared spectroscopy, it has only been with the introduction of scanned-probe techniques that exploit the raster-scanning tip as either a source, scattering tool, or measurement probe that researchers have been able to obtain sub-diffraction limit IR spectra. This review will examine the history of correlated scanned-probe IR spectroscopies, from their inception to their use in studies of molecular aggregates, membrane domains, and cellular structures. The challenges and opportunities that these platforms present for examining dynamic phenomena will be discussed. This article is part of a Special Issue entitled: FTIR in membrane proteins and peptide studies.

Chemical imaging beyond the diffraction limit: experimental validation of the PTIR technique

Photothermal induced resonance (PTIR) has recently attracted great interest for enabling chemical identification and imaging with nanoscale resolution. In this work, electron beam nanopatterned polymer samples are fabricated directly on 3D zinc selenide prisms and used to experimentally evaluate the PTIR lateral resolution, sensitivity and linearity. It is shown that PTIR lateral resolution for chemical imaging is comparable to the lateral resolution obtained in the atomic force microscopy height images, up to the smallest feature measured (100 nm). Spectra and chemical maps are produced from the thinnest sample analyzed (40 nm). More importantly, experiments show for the first time that the PTIR signal increases linearly with thickness for samples up to ≈ 1 μm (linearity limit); a necessary requirement towards the use of the PTIR technique for quantitative chemical analysis at the nanoscale. Finally, the analysis of thicker samples provides the first evidence that the previously developed PTIR signal generation theory is correct. It is believed that the findings of this work will foster nanotechnology development in disparate applications by proving the basis for quantitative chemical analysis with nanoscale resolution.

Nanoscale infrared spectroscopy: improving the spectral range of the photothermal induced resonance technique

Photothermal induced resonance (PTIR) is a new technique which combines the chemical specificity of infrared (IR) spectroscopy with the lateral resolution of atomic force microscopy (AFM). PTIR requires a pulsed tunable laser for sample excitation and an AFM tip to measure the sample expansion induced by light absorption. The limited tunability of commonly available laser sources constrains the application of the PTIR technique to a portion of the IR spectrum. In this work, a broadly tunable pulsed laser relying on a difference frequency generation scheme in a GaSe crystal to emit light tunable from 1.55 μm to 16 μm (from 6450 cm(-1) to 625 cm(-1)) was interfaced with a commercial PTIR instrument. The result is a materials characterization platform capable of chemical imaging, in registry with atomic force images, with a spatial resolution that notably surpasses the light diffraction limit throughout the entire mid-IR spectral range. PTIR nanoscale spectra and images allow the identification of compositionally and optically similar yet distinct materials; organic, inorganic, and composite samples can be studied with this nanoscale analog of infrared spectroscopy, suggesting broad applicability. Additionally, we compare the results obtained with the two tunable lasers, which have different pulse lengths, to experimentally assess the recently developed theory of PTIR signal generation.

Atomic force microscope infrared spectroscopy on 15 nm scale polymer nanostructures

We measure the infrared spectra of polyethylene nanostructures of height 15 nm using atomic force microscope infrared spectroscopy (AFM-IR), which is about an order of magnitude improvement over state of the art. In AFM-IR, infrared light incident upon a sample induces photothermal expansion, which is measured by an AFM tip. The thermomechanical response of the sample-tip-cantilever system results in cantilever vibrations that vary in time and frequency. A time-frequency domain analysis of the cantilever vibration signal reveals how sample thermomechanical response and cantilever dynamics affect the AFM-IR signal. By appropriately filtering the cantilever vibration signal in both the time domain and the frequency domain, it is possible to measure infrared absorption spectra on polyethylene nanostructures as small as 15 nm.

AFM-IR: combining atomic force microscopy and infrared spectroscopy for nanoscale chemical characterization

Polymer and life science applications of a technique that combines atomic force microscopy (AFM) and infrared (IR) spectroscopy to obtain nanoscale IR spectra and images are reviewed. The AFM-IR spectra generated from this technique contain the same information with respect to molecular structure as conventional IR spectroscopy measurements, allowing significant leverage of existing expertise in IR spectroscopy. The AFM-IR technique can be used to acquire IR absorption spectra and absorption images with spatial resolution on the 50 to 100 nm scale, versus the scale of many micrometers or more for conventional IR spectroscopy. In the life sciences, experiments have demonstrated the capacity to perform chemical spectroscopy at the sub-cellular level. Specifically, the AFM-IR technique provides a label-free method for mapping IR-absorbing species in biological materials. On the polymer side, AFM-IR was used to map the IR absorption properties of polymer blends, multilayer films, thin films for active devices such as organic photovoltaics, microdomains in a semicrystalline polyhydroxyalkanoate copolymer, as well as model pharmaceutical blend systems. The ability to obtain spatially resolved IR spectra as well as high-resolution chemical images collected at specific IR wavenumbers was demonstrated. Complementary measurements mapping variations in sample stiffness were also obtained by tracking changes in the cantilever contact resonance frequency. Finally, it was shown that by taking advantage of the ability to arbitrarily control the polarization direction of the IR excitation laser, it is possible to obtain important information regarding molecular orientation in electrospun nanofibers.

Nanometer-scale infrared spectroscopy of heterogeneous polymer nanostructures fabricated by tip-based nanofabrication

There is a significant need for chemical identification and chemical imaging of nanofabricated structures and devices, especially for multiple materials integrated at the nanometer scale. Here we present nanofabrication, chemical identification, and nanometer-scale chemical imaging of polymer nanostructures with better than 100 nm spatial resolution. Polymer nanostructures of polyethylene, polystyrene, and poly(3-dodecylthiophene-2,5-diyl) were fabricated by tip-based nanofabrication. Nanometer-scale infrared measurements using atomic force microscopy infrared spectroscopy (AFM-IR) obtained quantitative chemical spectra of these nanostructures. We show chemical imaging of intersecting patterns of nanometer-scale polymer lines of different chemical compositions. The results indicate that for closely packed heterogeneous nanostructures, the spatial resolution of AFM-IR is not limited by nanometer-scale thermal diffusion, but is instead limited by the cantilever sensitivity and the signal-to-noise ratio of the AFM-IR system.

Analysis of bacterial polyhydroxybutyrate production by multimodal nanoimaging

In this paper, we will employ two microscopy techniques, transmission electron microscopy and infrared nanospectromicroscopy, to study the production of polyhydroxybutyrate in Rhodobacter capsulatus and to evaluate the influence of glucose and acetone on the production yield. The results overlap which leads us to a consistent conclusion, highlighting that each technique brings specific and complementary information. By using electron microscopy and infrared nanospectromicroscopy we have proved that both glucose and acetone had a positive effect on the biopolymer production, although the first study done by Fourier transform infrared spectroscopy only identified the effect of acetone. In conclusion, we have now established a method to be able to perform fast diagnostic for PHB production.

Infrared microspectroscopy combined with conventional atomic force microscopy

This paper reports nanotopography and mid infrared (IR) microspectroscopic imaging coupled within the same atomic force microscope (AFM). The reported advances are enabled by using a bimaterial microcantilever, conventionally used for standard AFM imaging, as a detector of monochromatic IR light. IR light intensity is recorded as thermomechanical bending of the cantilever measured upon illumination with intensity-modulated, narrowband radiation. The cantilever bending is then correlated with the sample's IR absorption. Spatial resolution was characterized by imaging a USAF 1951 optical resolution target made of SU-8 photoresist. The spatial resolution of the AFM topography measurement was a few nanometers as expected, while the spatial resolution of the IR measurement was 24.4 μm using relatively coarse spectral resolution (25-125 cm(-1)). In addition to well-controlled samples demonstrating the spatial and spectral properties of the setup, we used the method to map engineered skin and three-dimensional cell culture samples. This research combines modest IR imaging capabilities with the exceptional topographical imaging of conventional AFM to provide advantages of both in a facile manner.

Biological applications of synchrotron radiation infrared spectromicroscopy

Extremely brilliant infrared (IR) beams provided by synchrotron radiation sources are now routinely used in many facilities with available commercial spectrometers coupled to IR microscopes. Using these intense non-thermal sources, a brilliance two or three order of magnitude higher than a conventional source is achievable through small pinholes (<10 μm) with a high signal to-noise ratio. IR spectroscopy is a powerful technique to investigate biological systems and offers many new imaging opportunities. The field of infrared biological imaging covers a wide range of fundamental issues and applied researches such as cell imaging or tissue imaging. Molecular maps with a spatial resolution down to the diffraction limit may be now obtained with a synchrotron radiation IR source also on thick samples. Moreover, changes of the protein structure are detectable in an IR spectrum and cellular molecular markers can be identified and used to recognize a pathological status of a tissue. Molecular structure and functions are strongly correlated and this aspect is particularly relevant for imaging. We will show that the brilliance of synchrotron radiation IR sources may enhance the sensitivity of a molecular signal obtained from small biosamples, e.g., a single cell, containing extremely small amounts of organic matter. We will also show that SR IR sources allow to study chemical composition and to identify the distribution of organic molecules in cells at submicron resolution is possible with a high signal-to-noise ratio. Moreover, the recent availability of two-dimensional IR detectors promises to push forward imaging capabilities in the time domain. Indeed, with a high current synchrotron radiation facility and a Focal Plane Array the chemical imaging of individual cells can be obtained in a few minutes. Within this framework important results are expected in the next years using synchrotron radiation and Free Electron Laser (FEL) sources for spectro-microscopy and spectral-imaging, alone or in combination with Scanning Near-field Optical Microscopy methods to study the molecular composition and dynamic changes in samples of biomedical interest at micrometric and submicrometric scales, respectively.