Carbon nanotube interfaces for single molecular level bio sensing H. Vedala * , S. Roy * , T-H. Kim * , W.B. Choi * * Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, USA, choiw@fiu.edu ABSTRACT Single molecule detection, aimed both at fundamental investigation and applications, has recently been attracting a lot of attentions. The present set of studies is focused on the design, fabrication and optimization of novel carbon nanotube (CNT) probes for recognition of biomolecules (target species) with the accuracy down to molecular level. The sensor platforms are fabricated with aligned grown CNTs on desired substrates with high level of control. We have demonstrated excellent electrochemical sensing of functionalized array of CNT for quantitative and selective detection of a range of metabolites including cholesterol, ascorbic acid and uric acid, in buffer solution as well as in human plasma and blood. In addition, a label-free detection scheme for DNA hybridization as well as environmental gas has recently been demonstrated (Nano Letters vol 8, 26 2008). In this presentation the charge transport of bio- molecular binding at the CNT-transducer will be presented and discussed. Keywords: Carbon Nanotube, DNA, Hybridization, electric conductance, bio sensors 1 INTRODUCTION Electronic detection of biomolecules at single molecular level has several advantages as compared to detection of ensemble of molecules. Single molecules studies unravel the intrinsic properties of these molecules which are essential for both fundamental studies and various technological applications [1]. Most of the current available single molecule detection techniques use spectroscopic properties of the molecule which require optical labeling. This adds more complexity to the spectroscopic detection systems. On the other hand use of electronic detection techniques based on nanomaterials provide a direct and label free alternative method. In this communication we report the development of a novel nanoelectronic platform for measuring direct electrical transport in single-molecule DNA of genomic significance. We have used single-walled carbon nanotube (SWNT) electrodes for anchoring a DNA molecule of compatible diameter (1-2 nm). Characterization of DNA using carbon nanotubes (CNT) has been pursued in the past, motivated by the prospects of CNT as a unique electrode material[2,3]. A couple of recent reports detailed the techniques of creating a nanogap in a SWNT and bridging the gap by organic molecules[4,5]. The preset study extends this concept to overcome the challenge of anchoring and electrically characterizing single-molecule DNA. We also report the influence of local environmental factors such as counterion variation, pH, temperature, ionic strength on charge transport (CT) of double-stranded (ds) DNA molecule at single molecule-level. 2 EXPERIMENTAL 2.1 Nanoelectrode fabrication SWNTs were synthesized by chemical vapor deposition technique and were suspended in isopropyl alcohol by ultrasonication. Initially a 2 µl droplet of SWNT suspension was spun on an thermally oxidized (500 nm) silicon substrate having photolithographically patterned microelectrodes and bonding pad [6,7]. Electrical contacts to individual SWNT were made by first locating them with respect to prepatterned index marks using field emission scanning electron microscope (FESEM) imaging. That was followed by making contact leads using e-beam lithography and sputtering of 50 nm of Au on 10 nm Ti adhesion layer. To fabricate a pair of nanoelectrodes, Focused Ion Beam (FIB) was used for etching near the center of an individual SWNT segment between the metal electrodes. FIB etching parameters (beam current, exposure time) were optimized to obtain a uniform gap in accordance with the length of the DNA strands. 2.2 Electrode functionalization and DNA attachment Electrical conductivity of an 80 base–pair (bp) in denatured (ssDNA) and hybridized (dsDNA) form (contour length ~27nm), encoding a portion of the H5N1 gene of avian Influenza A virus (AIV) was measured. The template strand obtained with amine modifications at the 5′ and 3′ ends was hybridized with the unmodified complementary strand at 90°C for 5 min in 10 mM NaAc buffer (pH 5.8) at equimolar concentrations. To measure the electrical conductivity of the dsDNA molecules, the SWNT nanoelectrodes were first functionalized with COOH groups to form a strong covalent bond with amine terminated DNA molecule. This was performed by chemical oxidation of SWNT as reported in [8]. In short the SWNTs were treated with HNO 3 for 1 hour followed by rinsing with DI water and vacuum drying. The sample was then incubated for 30 min in 2 mM 1-ethyl-3-(3- dimethylaminopropyl)carbodiimide hydrochloride and 5 71 NSTI-Nanotech 2008, www.nsti.org, ISBN 978-1-4200-8503-7 Vol. 1