DNA Molecules on GaP (100) Surfaces: Spectroscopic Characterization and Biospecificity Assessment

Abstract

DNA-modified surfaces have been the subject of considerable research activity in the field of bionanotechnology. Such surfaces can be incorporated into novel diagnostic devices that utilize sequencing and gene mapping technologies. 3] For example, the use of DNAbased sensors shows promise for rapid, economical and accurate detection of genetic diseases. In such sensor devices it is very important to have a robust and reproducible packing of DNA molecules. Different research groups have explored the absorption of biomolecules and the integration of biological systems on inorganic materials such as gold and silicon. Recently, there has been an interest in extending such studies to III–V semiconductor substrates. Gallium phosphide (GaP) is an attractive semiconductor material due to its usage in charge storage devices and low-noise detection photodiodes. This material offers promise for the fabrication of novel biosensors. Our previous work demonstrated that GaP ACHTUNGTRENNUNG(100) remains stable after surface functionalization with well-parked adlayers and high molecular coverages. Herein, we describe the covalent functionalization and characterization of H-doped GaP ACHTUNGTRENNUNG(100) with modified DNA strands. We demonstrate that photochemical functionalization with undecylenic acid (UDA) can modify GaP substrates and that the terminal carboxylic acid groups can be used for the successful immobilzation of biomolecules (Scheme 1). In our approach, we control the orientation of modified DNAs by reacting the carboxylic acid-terminated GaP ACHTUNGTRENNUNG(100) surfaces with a mixture of amine-terminated ssDNA and a spacer, hexylamine (HA). We also confirm the bioactivity of biotin-modified DNA by the use of streptavidin-modified nanoparticles and Cy3-labeled streptavidin. The following techniques are used to complete the physical characterization of the surfaces: water contact angle (WCA), atomic force microscopy (AFM), Fourier transform infrared reflectance absorbance spectroscopy (FT–IRRAS). Prior to any spectroscopic analysis each modified surface is evaluated using WCA and AFM. The data, summarized in the Supporting Information, follows the expected hydrophicilicity trend due to the nature of the end groups. In addition, no major changes in roughness are observed after each treatment. Initial spectroscopic analysis of the modified surfaces by FT–IRRAS spectroscopy provides us with information about the adsorbates’ orientation on the GaP surface. The FT–IRRAS vibrations for H-doped, Br-modified DNA/HA, biotin-modified DNA/HA and biotin-modified DNA/HA/streptavidin nanoparticles GaP ACHTUNGTRENNUNG(100) steps are shown in Figure 1. We place a Br-label at the 5’ end of the DNA so that we can use it as a way to prove that molecules are on the surface by X-ray photoelectron spectroscopy (XPS). The FT–IRRAS spectrum of H-doped GaP ACHTUNGTRENNUNG(100) does not show any peaks in the low and high frequency regions, as one would expect to be the case immedi[a] Dr. R. Flores-Perez, Prof. A. Ivanisevic Department of Chemistry Weldon School of Biomedical Engineering, Purdue University 206 S. Martin Jischke Drive, West Lafayette, IN 47907 (USA) Fax: (+1)7654961459 E-mail : albena@purdue.edu [b] Dr. D. Y. Zemlyanov Birck Nanotechnology Center Purdue University, West Lafayette, IN 47907 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200800166. Scheme 1. Surface coupling chemistry used to modify GaP with DNA molecules.