Kelvin Probe Force Microscopy Analysis of the Covalent Functionalization and DNA Modification of Gallium Phosphide Nanorods

The growth, covalent functionalization, and subsequent DNA modification of gallium phosphide (GaP) nanorods is presented. Analysis of the nanorods by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) revealed important information regarding their physical properties such as the presence of twinning defects. The nanorods were deposited onto glass substrates for further functionalization and biomolecule immobilization. Plasma cleaning was employed to remove the surfactant present on the nanorods’ surfaces. Kelvin probe force microscopy (KPFM) was used to analyze the extent of plasma cleaning and how it affected the functionalization that employed thiol chemistry. KFPM analysis of the subsequent modification of functionalized nanorods with single-stranded DNA (ssDNA) revealed that immobilization was dependent on the amount of plasma cleaning to which the nanorods had been exposed. Nanorods were then exposed to the cDNA strand and KPFM was again used to detect successful hybridization. ■ INTRODUCTION The functionalization of semiconductor surfaces with organic molecules has become an increasingly popular topic because of its potential applications in the area of devices, especially biosensors. A large focus lies on silicon due to its wellcharacterized properties and variety of functionalization schemes. However, with the advancement of technology comes the advent of continuously smaller devices. At these smaller scales, silicon’s normally advantageous properties begin to break down. Furthermore, silicon’s band-gap energy is not suited for efficient biosensor devices. Therefore, the next generation of electronic devices will rely on a different variety of semiconductors, namely, III−V semiconductors. Gallium phosphide (GaP) is a III−V semiconductor with excellent inherent properties such as carrier mobility and has demonstrated favorable biocompatibility in the past. The main challenge with making III−V semiconductors, such as GaP, valuable for future biosensing devices is developing schemes for biomolecule immobilization. The covalent functionalization of a planar GaP surface with both thiol and terminal alkene organic linker molecules has been demonstrated by our group in the past. In another study, we used Kelvin probe force microscopy (KPFM), a derivative of atomic force microscopy that is capable of mapping the surface potential, to demonstrate the functionalization of planar GaP with a terminal alkene cross-linker using microcontact printing followed by the immobilization of single-stranded DNA and its complement. It was concluded that the concentration of the linker molecule solution that reacted with the GaP surface strongly affected the orientation of the DNA. Nanorods are more advantageous to future biosensing devices compared to planar surfaces due to their advantageous electrical properties, their size, and their large surface area to volume (S/V) ratio, which allows for a maximum amount of biomolecule immobilization. Greater coverage of biomolecules on a semiconducting surface allows for greater sensitivity of a field-effect biosensing device, for example. Indeed, DNA immobilization on silicon nanorods has been successfully demonstrated by use of X-ray photoelectron spectroscopy (XPS) and fluorescence microscopy. Biomolecule immobilization on III−V semiconductor nanorods is also feasible, and devices that incorporate them have demonstrated excellent sensitivity. Recent studies have demonstrated that KPFM on nanorods can reveal important information such as the location of electrical trapping centers and dopant distribution. In this study, we evaluate the progressive functionalization and immobilization of DNA on GaP nanorods via KPFM. The KPFM technique is once again appropriate for this study since thiol molecules have been shown to elicit a pronounced KPFM response. It also proved useful for investigating the effects of plasma cleaning on the removal of surfactant from the nanorods. We used 11-aminoundecanethiol as the cross-linker whose terminal amine group can provide for biomolecule immobilization. DNA was chosen as a representative biomolecule because it is a negatively charged molecule and therefore provides good KPFM contrast. In a recent study, Received: March 9, 2012 Revised: May 18, 2012 Published: May 22, 2012 Article