Infrared Scanning Near-field Optical Microscopy Research Articles

An evaluation of the application of the aperture infrared SNOM technique to biomedical imaging

A single human oesophageal adenocarcinoma cell (OE33) has been imaged using aperture infrared scanning near-field optical microscopy (IR-SNOM) in transmission and reflection and also by Fourier-transform infrared (FTIR) microspectroscopy in transmission only. This work presents the first images obtained in both transmission and reflection of the same specimen using the aperture IR-SNOM technique. The results have been used to compare the two SNOM modes and also the two techniques, which have complementary capabilities. The SNOM technique necessitates a very stable source and a careful choice of wavelengths, since it is too slow to yield images at the thousands of wavelengths obtained with FTIR. However the SNOM technique is not diffraction limited and with careful fabrication of tips can yield images with high spatial resolution. There is no significant correlation between the SNOM images obtained in transmission and reflection and the correlations between images obtained at different wavelengths vary with the different imaging modes. These results are attributed to the strong dependence of the evanescent wave on both the wavelength and the distance between the tip and the source of the signal within the sample. While both transmission and reflection SNOM images show some correlation with topography this is not a dominant effect. These results indicate that with suitable calibration a combination of reflection and transmission aperture IR-SNOM measurements has the potential to reveal information on the depth distribution of the chemical structure of a specimen.

The Effect of Adjacent Materials on the Propagation of Phonon Polaritons in Hexagonal Boron Nitride

In order to apply the ability of hexagonal boron nitride (hBN) to confine energy in the form of hyperbolic phonon polariton (HPhP) modes in photonic-electronic devices, approaches to finely control and leverage the sensitivity of these propagating waves must be investigated. Here, we show that by surrounding hBN with materials of lower/higher dielectric responses, such as air and silicon, lower/higher surface momenta of HPhPs can be achieved. Furthermore, an alternative method for preparing thin hBN crystals with minimum contamination is presented, which provides opportunities to study the sensitivity of the damping mechanism of HPhPs on adsorbed materials. Infrared scanning near-field optical microscopy (IR-SNOM) results suggest that the reflections at the upper and lower hBN interfaces are primary causes of the damping of HPhPs, and that the damping coefficients of propagating waves are highly sensitive to adjacent layers, suggesting opportunities for sensor applications.

IR-SNOM Analysis of Occluding Substances in Lumina of Xylem Elements in Sapwood of Quercus serrata Attacked by Platypus quercivorus

A new microspectroscopic technique was applied to the analysis of occluding deposits in xylem elements of Quercus serrata. The production of this substance is believed to be a defense response of the sapwood against fungal infection. An occluding substance about 10 μm across was analyzed by Infrared-Scanning Near-field Optical Microscopy (IR-SNOM), which allows for the measurement of IR spectrum with high spatial resolution. The near-field IR spectrum of an occluding substance was different from those of xylem elements and featured a lack of the clear C-H absorption band that should appear at 3000-2850 cm(-1). On the other hand, the absorption band of ester bond exhibited a very strong peak. Among the near-field IR spectra of related compounds, a similar ester absorption peak was observed in the spectrum of pectin and tannic acid. The presence of a C-H absorption band as a very week peak was similar to (+)-catechin and tannic acid.

Infrared Scanning Near-Field Optical Microscopy Below the Diffraction Limit

Infrared scanning near-field optical microscopy (IR-SNOM) is an extremely powerful analytical instrument since it combines IR spectroscopy's high chemical specificity with SNOM's high spatial resolution. In order to do this in the infrared, specialty chalcogenide glass fibers were fabricated and their ends tapered to generate SNOM probes. The fiber tips were installed in a modified near-field microscope and both inorganic and biological samples illuminated with the tunable output from a free-electron laser located at Vanderbilt University. Both topographical and IR spectral images were simultaneously recorded with a resolution of ~ 50 and ~ 100 nm, respectively. Unique spectroscopic features were identified in all samples, with spectral images exhibiting resolutions of up to lambda/60, or at least 30 times better than the diffraction limited lens-based microscopes. We believe that IR-SNOM can provide a very powerful insight into some of the most important biomedical research topics.

Infrared scanning near-field optical microscopy observation of low-frequency electromagnetic field effects on functional groups in eukaryotic cells

The authors observed changes in the biochemical properties of eukaryotic cells exposed to an ac magnetic field by infrared scanning near-field optical microscopy. They specifically investigated the changes in the distribution of the inner chemical functional groups and in the cell morphology induced by a 24h exposure to a 1mT (rms), 50Hz sinusoidal magnetic field in a temperature-regulated solenoid. These results accentuate the crucial questions—raised by several recent studies—about the impact of low-frequency electromagnetic field on human cells.

Spectroscopic infrared near-field microscopy and x-ray reflectivity studies of order and clustering in lipid membranes

Lipid membranes were studied by infrared scanning near-field optical microscopy at several wavelengths and by x-ray reflectivity. Together with the x-ray data, the optical images indicate the formation of locally ordered multiple bilayers, and the topographical micrographs reveal the presence of islands at the surface, both critically important features for biotechnology and medical applications such as biosensors and gene therapy.

Spectroscopic infrared scanning near-field optical microscopy (IR-SNOM)

Scanning near-field optical microscopy (SNOM or NSOM) is the technique with the highest lateral optical resolution available today, while infrared (IR) spectroscopy has a high chemical specificity. Combining SNOM with a tunable IR source produces a unique tool, IR-SNOM, capable of imaging distributions of chemical species with a 100 nm spatial resolution. We present in this paper boron nitride (BN) thin film images, where IR-SNOM shows the distribution of hexagonal and cubic phases within the sample. Exciting potential applications in biophysics and medical sciences are illustrated with SNOM images of the distribution of different chemical species within cells. We present in this article images with resolutions of the order of λ/60 with SNOM working with infrared light. With our SNOM setup, we routinely get optical resolutions between 50 and 150 nm, regardless of the wavelength of the light used to illuminate the sample.

Very high resolution chemical imaging with Infrared Scanning Near-field Optical Microscopy (IR-SNOM)

In this paper we present chemically highly resolved images obtained with Scanning Near-field Optical Microscopy (SNOM) coupled with an Infrared (IR) Free Electron Laser (FEL) at Vanderbilt University, Nashville, USA. Main principles governing SNOM imaging as well as essential components of the experimental setup are described. Chemically resolved images showing the distribution of different phases within the boron-nitride films are presented. Universal character of the experiment and its huge potential applications in biophysics and medical sciences domain are illustrated with highly resolved SNOM images of pancreatic cells.

Chemically Resolved Imaging of Biological Cells and Thin Films by Infrared Scanning Near-Field Optical Microscopy

The infrared (IR) absorption of a biological system can potentially report on fundamentally important microchemical properties. For example, molecular IR profiles are known to change during increases in metabolic flux, protein phosphorylation, or proteolytic cleavage. However, practical implementation of intracellular IR imaging has been problematic because the diffraction limit of conventional infrared microscopy results in low spatial resolution. We have overcome this limitation by using an IR spectroscopic version of scanning near-field optical microscopy (SNOM), in conjunction with a tunable free-electron laser source. The results presented here clearly reveal different chemical constituents in thin films and biological cells. The space distribution of specific chemical species was obtained by taking SNOM images at IR wavelengths (λ) corresponding to stretch absorption bands of common biochemical bonds, such as the amide bond. In our SNOM implementation, this chemical sensitivity is combined with a lateral resolution of 0.1μm (≈λ/70), well below the diffraction limit of standard infrared microscopy. The potential applications of this approach touch virtually every aspect of the life sciences and medical research, as well as problems in materials science, chemistry, physics, and environmental research.