Mid-infrared photothermal hyperspectral imaging of biomolecular systems
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The development of novel techniques in spectroscopy and microscopy that are label-free, contactless and accessible is useful among many scientific disciplines, ranging from Materials Science to Biomedical Engineering. Hyperspectral photothermal imaging using vibrational spectroscopy promises to be a new tool in the arsenal for analysis and characterization of materials. This technique can be used for understanding structural composition of a material that is advantageous to the materials scientist. A combination of microscopy and spectroscopy is also beneficial to the biologist or pathologist that analyzes a complex sample with rich morphology. Photothermal hyperspectral microscopy is a label-free nondestructive method that utilizes specific vibrational bands of a molecule giving spectral information to an image. The method is based on changes in the thermal state, and the associated change in the refractive index of the sample as it is irradiated with mid-infrared light. Photothermal microscopy has rapidly emerged as one of the most sensitive label-free optical spectroscopic methods, rivaling current well-established methods based on fluorescence. The method has been used to image single non-fluorescent molecules in room temperature and to directly characterize biological features such as mitochondria and red blood cells. Despite great breakthroughs in the visible regime, the method has not been explored in the mid-infrared regime where most of the biological molecules have characteristic vibrational modes that constitute an intrinsic molecular "fingerprint" . This thesis presents the development of a new technique to measure the linear and nonlinear mid-infrared photothermal response induced by tunable high power lasers such as Quantum Cascade Lasers (QCLs). Photothermal response can be measured in pump-probe heterodyne detection, using short wavelength visible lasers and compact fiber lasers as a probe. This allows for direct detection of the fingerprint mid-infrared vibrational modes through ultrasensitive photodetectors. Integrated into a mid-infrared microscope, the system facilitates the acquisition of spectra and images on condensed phase samples. Photothermal heterodyne mid-infrared hyperspectral vibrational technique is used to image biological samples such as bird brain and other biomolecules First photothermal images on specially designed plasmonic metamaterials, designed to either enhance or suppress a selected mid-infrared vibrational normal mode, are demonstrated. Plasmonic metamaterials can be engineered using electron beam lithography for functional studies on biomolecules enhancing selected vibrational infrared resonances. This study takes advantage of the strong interaction between light and matter and investigates properties of the material that are difficult to detect through conventional spectroscopic methods. The new technique has the ability to advance studies in many fields, as it is applicable to different types of materials, non-destructive, accessible and inexpensive.
Thesis (Ph.D.)--Boston University