Laser Spectroscopy of Biological and Material Interfaces
Our research focuses on molecular structure and dynamics of surfaces and interfaces. Two complementary aspects of our research program are (1) development of new and more powerful surface spectroscopy techniques with better sensitivity and detection limits, ultrafast time resolution, and improved molecular-level information content; and (2) applications of these techniques for studies of molecular structure, organization, reactivity, and dynamics in these interesting and complex environments.
Currently, we are focusing on vibrational spectroscopy (mid-infrared spectral range) because it provides a wealth of information on the molecular structure and organization at surfaces and interfaces. To achieve surface selectivity, we utilize even-order nonlinear optical processes, such as Sum Frequency Generation (SFG) and Second Harmonic Generation (SHG), which are symmetry- forbidden in isotropic bulk media in the electric dipole approximation.
In addition to the standard frequency-domain spectroscopic measurements that characterize ensemble averaged structures, we are working on an array of ultrafast (femtosecond) time-domain techniques, such as SFG-FID (Free Induction Decay) for complementary studies of the molecular dynamics in real time. Our recent developments include a mixed Spectrally- and Time-Resolved SFG (STiR-SFG), a novel technique capable of measuring spectral evolution of vibrational coherences at surfaces, and heterodyne-detected SFG (HD- SFG), a broad-band spectral interferometry technique capable of ultrasensitive detection on a few percent of a monolayer level while simultaneously providing phase information on the molecular vibrations.
We are interested in surfaces and interfaces important in life sciences, nano- and bio- technology, and material science. Our current projects fall into three categories:
Structure and dynamics of the hydrogen bonding network at aqueous interfaces, with the long-range goal of better understanding biomembrane surfaces. H-bonding underlies most of the important and unique properties of water in the interfacial region, in particular 'biological water', a thin layer adjacent to biointerfaces. Using surface-selective spectroscopy, we are systematically investigating how the ultrafast dynamics of the aqueous H-bond network is affected by the chemical functionalities and electrostatics of interfaces. To this end, we investigate the ultrafast vibrational and rotational dynamics of the water molecules themselves as well as carefully chosen small probe molecules at aqueous interfaces.
Molecular organization of monolayer and surface materials. Molecularly ordered surfaces and thin films are emerging as the base materials for nanoscale devices, molecular electronics, and biotechnologies. The 'tailored' surface materials under investigation include: Langmuir-Blodgett monolayers, Self-Assembled Monolayers, polymer surfaces, and bio-functionalized surfaces used, e.g., for cell adhesion. We study details of molecular orientation, conformation, packing, dynamics and relaxation, and surface functionalization chemistry at these surfaces.
Surfaces and interfaces of nanostructures. A common motif in many emerging nanotechnology applications involves nanostructures covered with chemi- or physisorbed organic molecules which perform the desired physical, chemical, or biological function. We apply nonlinear spectroscopy to study organization, conformation, dynamics, and reactivity of molecules on surfaces of metal and semiconductor nanostructures, e.g. nanoparticles of different size and shape, and nanoparticle aggregates. In particular, we focus on the new effects arising when the characteristic size of the nanostructure approaches the molecular scale. Another aspect of this project is the exploration of the enhancement of nonlinear spectroscopic signals due to local nanoplasmon effects, with potential application in sensors with ultra-low detection limits and high molecular information content.