The Calhoun lab specializes in the application, advancement, and invention of nonlinear microscopy and spectroscopy techniques for the study of a variety of systems from small molecule antibiotics to nanoparticles. The overarching goal in all of our work is to understand the role of environmental effects by probing electronic excited states with a specific focus on imaging dynamic interfaces. To this end we are employing techniques such as transient absorption microscopy, second harmonic generation spectroscopy and electronic sum frequency generation microspectroscopy to a wide array of systems including the study of drug-membrane interactions and nanoparticle surfaces.

Interactions between Biological Membranes and Small Molecules

Studying interactions between small molecules and the membranes of microbes, such as those between antibiotics and their target membranes is an active interest. Fungal and bacterial membranes are attractive drug targets as they provide a protective barrier between the cell and its environment and control the transport of ions and molecules into and out of the cell. The engineering of novel drugs is limited by an incomplete understanding of how these molecules react to different biological environments.

Through the use of transient absorption microscopy we can directly image the localization, orientation and aggregation of small molecule drugs as they interact with living microbial cells. In order to probe the dynamics of the membrane itself and monitor the transport of small molecules through it, we are simultaneously monitoring second harmonic generation and two-photon fluorescence from these systems as well. Overall, these techniques provide valuble information about the drug's mechanism of action and how the cells can resist them.

Nanoparticle Surfaces

Nanomaterials have long been studied due to their tunable size, morphology, optical properties, and high surface to volume ratio, making them ideal candidates for an extensive range of diverse applications including novel light harvesting and emitting devices. The widespread success of nanomaterials in many practical devices and applications, however, has been limited by chemical and physical processes at their surfaces. As part of an active collaboration with Oak Ridge National Laboratory, we are developing new techniques to optically interrogate surface trap states and the interfacial species that can cause them.