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Mid-infrared spectroscopy is a widely used tool for material identification and secondary structure analysis in chemistry, biology and biochemistry. However, the diffraction limit prevents nanoscale protein studies. Here we introduce mapping of protein structure with 30 nm lateral resolution and sensitivity to individual protein complexes by Fourier transform infrared nanospectroscopy (nano-FTIR). We present local broadband spectra of one virus, ferritin complexes, purple membranes and insulin aggregates, which can be interpreted in terms of their α-helical and/or β-sheet structure. Applying nano-FTIR for studying insulin fibrils—a model system widely used in neurodegenerative disease research—we find clear evidence that 3-nm-thin amyloid-like fibrils contain a large amount of α-helical structure. This reveals the surprisingly high level of protein organization in the fibril’s periphery, which might explain why fibrils associate. We envision a wide application potential of nano-FTIR, including cellular receptor in vitro mapping and analysis of proteins within quaternary structures.

Scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared nanospectroscopy (nano-FTIR) are emerging tools for nanoscale chemical material identification. Here, we push s-SNOM and nano-FTIR one important step further by enabling them to quantitatively measure local dielectric constants and infrared absorption. Our technique is based on an analytical model, which allows for a simple inversion of the near-field scattering problem. It yields the dielectric permittivity and absorption of samples with 2 orders of magnitude improved spatial resolution compared to far-field measurements and is applicable to a large class of samples including polymers and biological matter. We verify the capabilities by determining the local dielectric permittivity of a PMMA film from nano-FTIR measurements, which is in excellent agreement with far-field ellipsometric data. We further obtain local infrared absorption spectra with unprecedented accuracy in peak position and shape, which is the key to quantitative chemometrics on the nanometer scale. SECTION: Spectroscopy, Photochemistry, and Excited States R adiation in the infrared (IR) and terahertz (THz) spectral regions is sensitive to molecular and electronic properties of matter, which makes it an excellent instrument for nondestructive identification and characterization of materials. Fourier transform infrared spectroscopy (FTIR) is thus a powerful and widely used technique for chemical materials identification based on IR fingerprint absorption spectroscopy. 1 Likewise, IR ellipsometry 2 or impedance spectroscopy 3 can be utilized to measure the dielectric permittivity of a sample. Together, these techniques provide a complete identification and characterization of optical properties of a sample. However, the spatial resolution in both techniques is limited by diffraction to about half of the illumination wavelength λ, limiting their application potential in chemistry, biology, nanoscience, and the semiconductor industry, where the examination of single nano-objects is often desired.The diffraction limit can be circumvented by scattering-type near-field optical microscopy (s-SNOM), which provides wavelength-independent nanoscale resolution even at IR and THz frequencies. 4 s-SNOM is a scanning probe technique often based on atomic force microscopy (AFM). In addition to the sample topography, s-SNOM yields optical amplitude and phase images, obtained by interferometric detection of the radiation scattered from the illuminated probe (typically a sharp metalized AFM tip). The scattered radiation depends on the near-field interaction between the tip and the sample, thus providing access to the optical properties of the sample at the nanoscale (we further refer to such properties as local). 5−15 By employing broad-band illumination and performing Fourier transform spectroscopy of the tip-scattered light, s-SNOM enables local spectroscopy (nano-FTIR) of plasmons and phonons, 16−19 (bio)minerals, 20 and polymers 21,22 with less than 20 n...

Infrared nanospectroscopy enables novel possibilities for chemical and structural analysis of nanocomposites, biomaterials or optoelectronic devices. Here we introduce hyperspectral infrared nanoimaging based on Fourier transform infrared nanospectroscopy with a tunable bandwidth-limited laser continuum. We describe the technical implementations and present hyperspectral infrared near-field images of about 5,000 pixel, each one covering the spectral range from 1,000 to 1,900 cm−1. To verify the technique and to demonstrate its application potential, we imaged a three-component polymer blend and a melanin granule in a human hair cross-section, and demonstrate that multivariate data analysis can be applied for extracting spatially resolved chemical information. Particularly, we demonstrate that distribution and chemical interaction between the polymer components can be mapped with a spatial resolution of about 30 nm. We foresee wide application potential of hyperspectral infrared nanoimaging for valuable chemical materials characterization and quality control in various fields ranging from materials sciences to biomedicine.

We report the development of infrared-resonant antenna probes for tip-enhanced optical microscopy. We employ focused-ion-beam machining to fabricate high-aspect ratio gold cones, which replace the standard tip of a commercial Si-based atomic force microscopy cantilever. Calculations show large field enhancements at the tip apex due to geometrical antenna resonances in the cones, which can be precisely tuned throughout a broad spectral range from visible to terahertz frequencies by adjusting the cone length. Spectroscopic analysis of these probes by electron energy loss spectroscopy, Fourier transform infrared spectroscopy, and Fourier transform infrared near-field spectroscopy corroborates their functionality as resonant antennas and verifies the broad tunability. By employing the novel probes in a scattering-type near-field microscope and imaging a single tobacco mosaic virus (TMV), we experimentally demonstrate high-performance mid-infrared nanoimaging of molecular absorption. Our probes offer excellent perspectives for optical nanoimaging and nanospectroscopy, pushing the detection and resolution limits in many applications, including nanoscale infrared mapping of organic, molecular, and biological materials, nanocomposites, or nanodevices.

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We establish a solid basis for the interpretation of infrared near-field spectra of thin organic films on highly reflective substrates and provide guidelines for their straightforward comparison to standard far-field Fourier transform infrared (FTIR) spectra. Particularly, we study the spectral behavior of near-field absorption and near-field phase, both quantities signifying the presence of a molecular resonance. We demonstrate that the near-field phase spectra only weakly depend on the film thickness and can be used for an approximate comparison with grazing incidence FTIR (GI-FTIR) spectra. In contrast, the near-field absorption spectra can be compared more precisely with far-field spectra: for ultrathin films they match well GI-FTIR spectra, while for thick films a good agreement with standard transmission FTIR spectra is found. Our results are based on experimental data obtained by nanoscale FTIR (nano-FTIR) spectroscopy and supported by a comprehensive theoretical analysis. V

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