The direct covalent modification of silicon nanowires with DNA oligonucleotides, and the subsequent hybridization properties of the resulting nanowire-DNA adducts, are described. X-ray photoelectron spectroscopy and fluorescence imaging techniques have been used to characterize the covalent photochemical functionalization of hydrogen-terminated silicon nanowires grown on SiO_2 substrates and the subsequent chemistry to form covalent adducts with DNA. XPS measurements show that photochemical reaction of H-terminated Si nanowires with alkenes occurs selectively on the nanowires with no significant reaction with the underlying SiO_2 substrate, and that the resulting molecular layers have a packing density identical to that of planar samples. Functionalization with a protected amine followed by deprotection and use of a bifunctional linker yields covalently linked nanowire-DNA adducts. The biomolecular recognition properties of the nanowires were tested via hybridization with fluorescently tagged complementary and non-complementary DNA oligonucleotides, showing good selectivity and reversibility, with no significant non-specific binding to the incorrect sequences or to the underlying SiO_2 substrate. Our results demonstrate that the selective nature of the photochemical functionalization chemistry permits silicon nanowires to be grown, functionalized, and characterized before being released from the underlying SiO_2 substrate. Compared with solution-phase modification, the ability to perform all chemistry and characterization while still attached to the underlying support makes this a convenient route toward fabrication of well characterized, biologically modified silicon nanowires.
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