Unique laser-scanning optical microscope for low-temperature imaging and spectroscopy

Development of a confocal scanning microscope for fluorescence imaging and spectroscopy at variable temperatures

We developed and tested a confocal scanning optical microscope that fits into a thermally controlled, commercial research cryostat designed for operation from ambient temperature down to below 4 K. The home-built microscope is a fiber-coupled, self-contained instrument based on readily available mechanical and optical components. Its sample module is sealed in a protective stainless steel tube that minimizes vibrations caused by the flow of cryogenic gas. A high numerical aperture microscope objective specifically designed for cryogenic and high-vacuum applications focuses the excitation light onto the sample, while the core of an optical fiber attached to an avalanche photodiode acts as the confocal detection pinhole. The sample is displaced using a piezotube scanner mounted on top of a three-axis, low-temperature nanopositioner assembly for coarse sample positioning. A broadband polarizing cube beam splitter in the emission path allows for polarization-resolved imaging and spectroscopy. Fluorescence excitation scans are acquired with custom-written software that correlates fluorescence photon counts with the output from a high precision wavelength meter, which is part of a narrow-band, tunable dye laser setup. The imaging and spectral data acquisition capabilities of the microscope were confirmed using a variety of samples and excitation wavelengths at temperatures ranging from 5 K to room temperature.

A confocal optical microscope for detection of single impurities in a bulk crystal at cryogenic temperatures

A compact sample-scanning confocal optical microscope for detection of single impurities below the surface of a bulk crystal at cryogenic temperatures is described. The sample, lens, and scanners are mounted inside a helium bath cryostat and have a footprint of only 19 × 19 mm. Wide field imaging and confocal imaging using a Blu-ray lens immersed in liquid helium are demonstrated with excitation at 370 nm. A spatial resolution of 300 nm and a detection efficiency of 1.6% were achieved.

Imaging with Raman spectroscopy

Raman spectroscopy, based on the inelastic scattering of a photon, has been widely used as an analytical tool in many research fields. Recently, Raman spectroscopy has also been explored for biomedical applications (e.g. cancer diagnosis) because it can provide detailed information on the chemical composition of cells and tissues. For imaging applications, several variations of Raman spectroscopy have been developed to enhance its sensitivity. This review article will provide a brief summary of Raman spectroscopy-based imaging, which includes the use of coherent anti-Stokes Raman spectroscopy (CARS, primarily used for imaging the C-H bond in lipids), surface-enhanced Raman spectroscopy (SERS, for which a variety of nanoparticles can be used as contrast agents), and single-walled carbon nanotubes (SWNTs, with its intrinsic Raman signal). The superb multiplexing capability of SERS-based Raman imaging can be extremely powerful in future research where different agents can be attached to different Raman tags to enable the interrogation of multiple biological events simultaneously in living subjects. The primary limitations of Raman imaging in humans are those also faced by other optical techniques, in particular limited tissue penetration. Over the last several years, Raman spectroscopy imaging has advanced significantly and many critical proof-of-principle experiments have been successfully carried out. It is expected that imaging with Raman Spectroscopy will continue to be a dynamic research field over the next decade.

A sensitive and versatile laser scanning confocal optical microscope for single-molecule fluorescence at 77 K

We developed a cryostat suitable for a laser scanning confocal microscope which allows for a short working distance and thus the usage of an objective with a high numerical aperture ensuring high collection efficiency. The in situ preparation of a thin layer of amorphous water is realized in a part of the cryostat, a Dewar vessel, which is put onto a custom-made, liquid-nitrogen immersed spin-coater. First tests on the setup are performed on a perylenemonoimide/polymethyl methacrylate model system using a standard oil objective and a dry objective at ambient temperature as well as a dry objective at liquid nitrogen temperature (77 K). Fluorescence resonance energy transfer (FRET) measurements on doubly labeled, freeze-quenched polyproline chains show the applicability of the new method on biomolecules. The alternating laser excitation (ALEX) is modified to a line-scanning process (slow ALEX) to optimize the sorting of the labeled molecules. Photophysics and photochemistry at liquid nitrogen temperature are investigated.

Confocal single molecule fluorescence spectroscopy in ultrahigh vacuum

We have constructed an ultrahigh vacuum confocal fluorescence microscope with the purpose of performing single molecule spectroscopy under highly defined conditions. The microscope is designed for high stability while affording the capability of sample preparation, sample transfer, and optical detection in ultrahigh vacuum. It achieves near-diffraction-limited performance and allows the observation of single molecule fluorescence dynamics over extended periods of time. The design of the microscope is discussed in detail and the performance is demonstrated with single molecule fluorescence images and trajectories of N,N'-dibutylperylene-3,4,9,10-dicarboxyimide deposited onto several different surfaces. This instrument further enhances the array of available surface science techniques, permitting spectroscopic investigations of molecule-surface interactions at the single molecule level and on insulating surfaces.

Molecular Imaging with Single-Walled Carbon Nanotubes

Nanoparticle-based molecular imaging has emerged as an interdisciplinary field which involves physics, chemistry, engineering, biology, and medicine. Single-walled carbon nanotubes (SWCNTs) have unique properties which make them suitable for applications in a variety of imaging modalities, such as magnetic resonance, near-infrared fluorescence, Raman spectroscopy, photoacoustic tomography, and radionuclide-based imaging. In this review, we will summarize the current state-of-the-art of SWCNTs in molecular imaging applications. Multifunctionality is the key advantage of nanoparticles over traditional approaches. Targeting ligands, imaging labels, therapeutic drugs, and many other agents can all be integrated into the nanoparticle to allow for targeted molecular imaging and molecular therapy by encompassing many biological and biophysical barriers. A multifunctional, SWCNT-based nanoplatform holds great potential for clinical applications in the future.