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Unformatted text preview: Importance of the Debye Screening Length on Nanowire Field Effect Transistor Sensors Eric Stern, Robin Wagner, Fred J. Sigworth, Ronald Breaker, Tarek M. Fahmy,* , and Mark A. Reed* ,, | Departments of Biomedical Engineering, Electrical Engineering, Molecular, Cellular, and De V elopmental Biology and Howard Hughes Medical Institute, Applied Physics, Yale Uni V ersity, P.O. Box 208284, New Ha V en, Connecticut 06520 Received July 23, 2007; Revised Manuscript Received September 10, 2007 ABSTRACT Nanowire field effect transistors (NW-FETs) can serve as ultrasensitive detectors for label-free reagents. The NW-FET sensing mechanism assumes a controlled modification in the local channel electric field created by the binding of charged molecules to the nanowire surface. Careful control of the solution Debye length is critical for unambiguous selective detection of macromolecules. Here we show the appropriate conditions under which the selective binding of macromolecules is accurately sensed with NW-FET sensors. The ability to rapidly sense minute concentrations of specific macromolecules such as DNA sequences is critical for clinical diagnostics, 1,2 genomics, 3,4 and drug discovery 3,4 and useful for applications in defense and homeland security. 5 Most current systems for macromolecular sensing rely on labels, such as radiolabeled tags or fluorophores. 6- 8 Tech- niques that could distinguish these without the need for labels, i.e., label-free sensing, are of great interest because they would not only significantly decrease the cost and time needed for sample preparation but would also eliminate issues related to modification of target molecules. 9,10 One of the most promising platforms for unlabeled sensing is the nanowire field effect transistor (NW-FET). 9- 11 These devices operate similarly to conventional chemical FETs, sensing the presence of bound species by their intrinsic charge, with the advantage of enhanced sensitivity due to the nanoscale channel confinement. 11,12 By binding a receptor protein or a single-stranded DNA (ssDNA) oligomer to the NW-FET surface, the binding of the specific ligand or complementary ssDNA modifies the electric field surround- ing the device, enabling direct electronic detection. 13- 16 The integration issues faced by traditional, as-grown NWs have been overcome with the advent of NW-like devices patterned by top-down microlithography. 14- 18 Although early devices suffered from low signal-to-noise ratios, a top- down method producing high-quality nanosensors capable of detecting specific antibodies at , 10 fM concentrations have recently been demonstrated. 17 In this Letter, we demonstrate the effect of molecular charge screening by dissolved solution counterions, Debye screening, 19 on sensor response. This is a critical consideration for designing optimal protocols for label-free sensing using NW-FETs....
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- Spring '09