1. Department of Physics and Astronomy, University of Arkansas at Little Rock, 2801 South University Avenue, Little Rock, AR 72204, USA
2. Division of Microbiology, National Center for Toxicological Research, US Food and Drug Administration, Jefferson, AR 72079, USA
3. NCTR-ORA Nanotechnology Core Facility, National Center for Toxicological Research, US Food and Drug Administration, Jefferson, AR 72079, USA