Selective Biofunctionalization all-(111) Surface Silicon Nanowires
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1236-SS08-22
Selective Biofunctionalization of All-(111) Surface Silicon Nanowires M. N. Masood, S. Chen, E. T. Carlen and A. van den Berg BIOS Lab on a Chip Group, MESA+ Institute for Nanotechnology, University of Twente, Enschede, 7500 AE, The Netherlands. ABSTRACT Selective biomolecular functionalization of our all-(111) surface silicon nanowire (SiNW) biosensors using covalently linked alkyl- monolayers is demonstrated. Monolayers were made using a commercially available six member carbon precursor N-(5-Hexynyl) phthalimide and UV based hydrosylilation reaction. Contact angle and x-ray photoelectron spectroscopy (XPS) measurements were used to characterize the monolayer at different stages on planar Si (111) samples. Terminal amine groups on the monolayer surface were used for further conjugation with (+)-Biotin N-hydroxysuccinimide ester after deprotection of the phthalimide group with a methylamine solution. Selective biofunctionalization was demonstrated by reacting the SiNWmonolayer-biotin surface with 5 nm gold nanoparticles conjugated with streptavidin and subsequent high resolution scanning electron microscopy imaging. INTRODUCTION Silicon nanowire (SiNW) field-effect biosensors are an emerging technology for the highly sensitive, label free and real time detection of biomolecular binding and have been reported for the ultra-trace detection of proteins, single stranded DNA, and viruses [1-4]. The high detection sensitivity has been attributed to the sensors nanoscale size, large surface-to-volume ratio, and three-dimensional multi-gate structures, which all contribute to the improved sensitivity, compared the conventional planar devices [5]. The transformation of a SiNW into a biosensor requires surface modification, such that biologically active probe molecules are conjugated to the sensor surface [6]. Conventionally, the conjugation of the probe molecule is done directly to the native, or grown, silicon dioxide layer on the silicon sensor surface. However, conjugating to the oxide layer reduces the selectivity of the sensor. The alkylation of hydrogen-terminated silicon surfaces precludes the need for biomolecular conjugation to oxide passivated silicon surfaces [7]. This direct conjugation to the silicon surface has several advantages over oxide surfaces for biosensing, which includes improved sensitivity, as the probe molecule is very close (~1 nm) to the sensing surface, and improved selectivity since only the SiNW surfaces are functionalized with the probe molecules. Additionally, the electrically imperfect oxide layer, which typically suffers from large trapped (fixed) charge densities, is replaced with a direct silicon-carbon (Si-C) covalent bond between the organic and inorganic phases. It is well known that the (111) silicon surface is preferred for the chemical attachment of organic monolayers due to the atomic arrangement, which leads to densely packed layers [8]. However, existing reports of alkyl monolayer formation on SiNWs have been conducted on a mixture of surface orientations due to li
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