Characterization of immobilized DNA on sulfur-passivated InAs surfaces
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Characterization of immobilized DNA on sulfur-passivated InAs surfaces EunKyung Cho,1 Pae Wu,2 Minhaz Ahmed,2 April Brown,2 and T. F. Kuech1 1 Department of Chemical and Biological Engineering, University of Wisconsin-Madison, 1415 Engineering Dr., Madison, WI 53706, U.S.A. 2 Department of Electrical and Computer Engineering, Duke University ABSTRACT The immobilization of DNA on passivated n-type InAs (100) surfaces has been studied using X-ray and ultraviolet photoelectron spectroscopy. The benefits of sulfur passivation using ammonium sulfide solution ((NH4)2S) for DNA immobilization were examined. The XPS/UPS data carried out on non-functionalized and functionalized surfaces demonstrate that the DNA probes reacted with the sulfur-passivated InAs surface. The XPS data in combination with fluorescently-tagged DNA indicate that the sulfur passivation process leads to a higher and more uniform attachment of DNA over the surface compared to non-sulfur-passivated InAs surfaces. The XPS data obtained immediately after sulfur passivation clearly observes In-S bonding, with little or no As-S. In addition, the XPS spectra of As 3d core-levels immediately after sulfur passivation shows that there is a negligible amount of As-Ox, but the peak become considerable after exposure to the aqueous DNA probe solution. The increase in As-Ox is likely due to the presence of non-sulfur bonded As atoms present on the surface. The presence of sulfur on the surface does lead to the high areal density of attached ssDNA. This system forms the basis of a DNA sensing system. While chemically passivating the surface against oxidation and facilitating probe attachment, the changes in Fermi level position were also monitored by UPS. UPS spectra show that the Fermi level of a clean InAs surface is located ~0.6 eV above the valence band maximum. The changes in electronic states induced by sulfur passivation and the pinning of EF are discussed. INTRODUCTION Single-stranded DNA (ssDNA) immobilized on solid surfaces has became important in biotechnology applications such as DNA microarrays [1,2] and biosensors [1,3]. The immobilization of DNA on InAs is interesting because InAs is one of the few semiconductors that possess a surface Fermi level typically positioned above the conduction band edge [4,5]. This leads to the presence of a quasi-two-dimensional electron gas at the surface that can be modulated by adsorbed species and other binding events. Thus, this material is an excellent candidate for a variety of sensing applications. Recently, surface passivation of InAs has been investigated using ammonium sulfide ((NH4)2Sx) [6], thioacetamide [7] and methyl-terminated alkane thiols [8]. Such passivation techniques can remove surface oxides and other contaminants by creating a covalently bonded sulfur layer with good short-term stability in ambient air and aqueous solutions. Surface functionalization with ssDNA has been largely unexamined on InAs surfaces and III-V semiconductors in general. In this paper, we present a study of the immobilization of
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