Molecular-Spectroscopic Techniques for Label-Free Imaging

Abstract

Molecular spectroscopy, including Raman spectroscopy and infrared (IR) spectroscopy, is used to study the interactions between electromagnetic fields and molecules. The measured spectrum can provide information on molecular vibrations in the sample. Vibrational spectroscopy serves as a powerful tool in both research and industry, enabling molecule identification, gas sensing, and temperature measurements. Also, both Raman and IR spectroscopy can be adapted to a microscope configuration and used for label-free imaging. In this dissertation, we will start with brief introductions to both the Raman spectroscopy and IR spectroscopy, then discuss several improved spectroscopic techniques. Five experiments will be presented to demonstrate their applications. Spontaneous Raman spectroscopy and microscopy have several drawbacks, like low sensitivity, strong fluorescence background, and limited spatial resolution. To overcome these issues, people developed surface-enhanced Raman scattering (SERS), tip-enhanced scattering (TERS), and coherent anti-Stokes Raman scattering (CARS) techniques. We exhibit a SERS experiment by using semiconductor nanoparticles, which can enhance the CARS signal over 109 times. The second experiment uses TERS to image RNA strands and achieves 90% averaged accuracy of the sequencing, which shows the TERS technique provides sub-nanometer resolution. CARS can have a higher signal than spontaneous Raman and avoid fluorescence background. We build a CARS microscope to demonstrate its ability for noninvasive label-free imaging.

Traditional IR micro-spectroscopy has several drawbacks; it cannot be used for samples with water and is limited in spatial resolution because of the long wavelength used. This dissertation will present a novel IR technique called infrared-resonant third-order sum-frequency (ITS) spectroscopy which can overcome these issues. Spectroscopic measurements are used to demonstrate and explain the ITS mechanism. Then I will show a wide-field microscopy setup based on ITS, providing label-free imaging with 1 μm resolution that breaks the diffraction limit of the mid-IR input light.