Reduction of tip–sample interaction forces for scanning near-field optical microscopy in a liquid environment

Live endothelial cells imaged by Scanning Near-field Optical Microscopy (SNOM): capabilities and challenges

The scanning near-field optical microscopy (SNOM) shows a potential to study details of biological samples, since it provides the optical images of objects with nanometric spatial resolution (50-200 nm) and the topographic information at the same time. The goal of this work is to demonstrate the capabilities of SNOM in transmission configuration to study human endothelial cells and their morphological changes, sometimes very subtle, upon inflammation. Various sample preparations were tested for SNOM measurements and promising results are collected to show: 1) the influence of α tumor necrosis factor (TNF-α) on EA.hy 926 cells (measurements of the fixed cells); 2) high resolution images of various endothelial cell lines, i.e. EA.hy 926 and HLMVEC (investigations of the fixed cells in buffer environment); 3) imaging of live endothelial cells in physiological buffers. The study demonstrate complementarity of the SNOM measurements performed in air and in liquid environments, on fixed as well as on living cells. Furthermore, it is proved that the SNOM is a very useful method for analysis of cellular morphology and topography. Changes in the cell shape and nucleus size, which are the symptoms of inflammatory reaction, were noticed in TNF-α activated EA.hy 926 cells. The cellular structures of submicron size were observed in high resolution optical images of cells from EA.hy 926 and HLMVEC lines.

Recent progress of nano-technology with NSOM

Recent progress of nano-technology with near-field scanning optical microscope (NSOM) is surveyed in this article. We focus mainly on NSOM, nano-scale spectroscopy with NSOM, probe technology of NSOM, and study of nano-structured metallic surface with NSOM. First, we follow developments of aperture NSOM and apertureless NSOM, and then address progress of NSOM-combined spectroscopy which is so sufficiently advanced with apertureless NSOM technology to provide chemical information on length scales of a few nanometers. Recent achievement of nano-scale Raman and IR spectroscopy will be introduced. Finally, research on nano-optic elements using surface plasmon polariton with NSOM is introduced as an example of NSOM applications to nano-structured metallic surfaces.

Near-field scanning optical microscopy in cell biology and cytogenetics

Light microscopy has proven to be one of the most versatile analytical tools in cell biology and cytogenetics. The growing spectrum of scientific knowledge demands a continuous improvement of the optical resolution of the instruments. In far-field light microscopy, the attainable resolution is dictated by the limit of diffraction, which, in practice, is about 250 nm for high-numerical-aperture objective lenses. Near-field scanning optical microscopy (NSOM) was the first technique that has overcome this limit up to about one order of magnitude. Typically, the resolution range below 100 nm is accessed for biological applications. Using appropriately designed scanning probes allows for obtaining an extremely small near-field light excitation volume (some tens of nanometers in diameter). Because of the reduction of background illumination, high contrast imaging becomes feasible for light transmission and fluorescence microscopy. The height of the scanning probe is controlled by atomic force interactions between the specimen surface and the probe tip. The control signal can be used for the production of a topographic (nonoptical) image that can be acquired simultaneously. In this chapter, the principle of NSOM is described with respect to biological applications. A brief overview of some requirements in biology and applications described in the literature are given. Practical advice is focused on instruments with aperture-type illumination probes. Preparation protocols focussing on NSOM of cell surfaces and chromosomes are presented.

High-resolution near-field optical imaging of single nuclear pore complexes under physiological conditions

Scanning near-field optical microscopy (SNOM) circumvents the diffraction limit of conventional light microscopy and is able to achieve optical resolutions substantially below 100 nm. However, in the field of cell biology SNOM has been rarely applied, probably because previous techniques for sample-distance control are less sensitive in liquid than in air. Recently we developed a distance control based on a tuning fork in tapping mode, which is also well-suited for imaging in solution. Here we show that this approach can be used to visualize single membrane protein complexes kept in physiological media throughout. Nuclear envelopes were isolated from Xenopus laevis oocytes at conditions shown recently to conserve the transport functions of the nuclear pore complex (NPC). Isolated nuclear envelopes were fluorescently labeled by antibodies against specific proteins of the NPC (NUP153 and p62) and imaged at a resolution of approximately 60 nm. The lateral distribution of epitopes within the supramolecular NPC could be inferred from an analysis of the intensity distribution of the fluorescence spots. The different number densities of p62- and NUP153-labeled NPCs are determined and discussed. Thus we show that SNOM opens up new possibilities for directly visualizing the transport of single particles through single NPCs and other transporters.

Combined scanning electrochemical/optical microscopy with shear force and current feedback

A technique that combines scanning electrochemical microscopy (SECM) and scanning optical microscopy (OM) was developed. Simultaneous scanning electrochemical/optical microscopy (SECM/OM) was performed by a special probe tip, which consists of an optical fiber core for light passage, surrounded by a gold ring electrode, and an outermost electrophoretic insulating sheath, with the tip attached to a tuning fork. To regulate the tip-substrate distance, either the shear force or the SECM tip current was employed as the feedback signal. The application of a quartz crystal tuning fork (32.768 kHz) for sensing shear force allowed simultaneous topographic, along with SECM and optical imaging in a constant-force mode. The capability of this technique was confirmed by obtaining simultaneously, for the first time, topographic, electrochemical, and optical images of an interdigitated array electrode. Current feedback from SECM also provided simultaneous electrochemical and optical images of relatively soft samples, such as a polycarbonate membrane filter and living diatoms in a constant-current mode. This mode should be useful in mapping the biochemical activity of a living cell.

Fluorescence anisotropy near-field scanning optical microscopy (FANSOM): a new technique for nanoscale microviscometry

A near-field scanning optical microscope system was implemented and adapted for nanoscale steady-state fluorescence anisotropy measurement. The system as implemented can resolve approximately 0.1 cP microviscosity variations with a resolution of 250 nm laterally in the near field, or approximately 10 microm when employed in a vertical scanning mode. The system was initially used to investigate the extent of microviscous vicinal water over surfaces of varying hydrophilicity. Water above a cleaved mica surface was found to have a decreased microviscosity, while water above a hydrophobic surface showed no change (detection limit approximately 0.1 cP at approximately 30 + nm from the surface).

High spatial resolution imaging with near-field scanning optical microscopy in liquids

The mechanism of tuning fork-based shear-force near-field scanning optical microscopy is investigated to determine optimal experimental conditions for imaging soft samples immersed in liquid. High feedback sensitivity and stability are obtained when only the fiber probe, i.e., excluding the tuning fork prongs, is immersed in solution, which also avoids electrical shorting in conductive (i.e., buffer) solutions. Images of MEH-PPV were obtained with comparable spatial resolution in both air and water. High optical resolution (approximately160 nm fwhm) was observed.

Control of the vibration damping through surface functionalization of a shear force probe

Shear force mapping on thiolipid Langmuir-Blodgett monolayers probed with Al-coated tips leads to a contrast highly dependent on the monolayer molecular organization, which does not correspond to the topographical relief of the sample. The use of functionalized surface probes offers the possibility to better control the probe-to-sample interaction. In addition, hydrophilic surface probes are totally insensitive to alkyl chain arrangements in the monolayer. Hydrophobic probes, instead, can be chosen to map shear force on soft samples in liquid environment, since their mechanical properties are not influenced by the surrounding liquid.

Continuous wave two-photon scanning near-field optical microscopy

We have implemented continuous-wave two-photon excitation of near-UV absorbing fluorophores in a scanning near-field optical microscope (SNOM). The 647-nm emission of an Ar-Kr mixed gas laser was used to excite the UV-absorbing DNA dyes DAPI, the bisbenzimidazole Hoechst 33342, and ethidium bromide in a shared aperture SNOM with uncoated fiber tips. Polytene chromosomes of Drosophila melanogaster and the nuclei of 3T3 Balb/c cells labeled with these dyes were readily imaged. The fluorescence intensity showed the expected nonlinear (second order) dependence on the excitation power in the range of 8-180 mW. We measured the fluorescence intensity as a function of the tip-sample displacement in the direction normal to the sample surface in the single- and two-photon excitation modes (SPE, TPE). The fluorescence intensity decayed faster in TPE than in SPE.