The authors would like to acknowledge the assistance of Dr. Mughees Khan (Wyss Institute for Bio-inspired Engineering) in the fabrication of the arrays structures. The work was partially supported by the ARPA-E under award #: DE-AR0000326. The work was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under Award ECS-0335765. L.M. thanks the US Department of Homeland Security (DHS) for the fellowship. The DHS Scholarship and Fellowship Program is administered by the Oak Ridge Institute for Science and Education (ORISE) through an interagency agreement between the US Department of Energy (DOE) and DHS. ORISE is managed by Oak Ridge Associated Universities (ORAU) under DOE Contract DE-AC05-06OR23100.