Surface Wettability of Oxygen Plasma Treated Porous Silicon

Oxygen plasma treatment on porous silicon (p-Si) surfaces was studied as a practical and effective means to modify wetting properties of as-fabricated p-Si surfaces, that is, contact angles of the p-Si materials. P-Si samples spanning a wide range of surface nanostructures have been fabricated which were subjected to a series of oxygen plasma treatments. Reduction of the p-Si surface contact angles has been systematically observed, and the surface activation rate constant as a function of different pore geometries has been analyzed to achieve an empirical equation. The underlying diffusion mechanisms have been discussed by taking into account of different pore diameters of p-Si samples. It is envisaged that such an approach as well as the corresponding empirical equation may be used to provide relevant process guidance in order to achieve precise control of p-Si contact angles, which is essential for many p-Si applications especially in biosensor areas. 1. Introduction The past decade has seen the rapid development of porous silicon (p-Si) in many applications including microelectronics, implantation, drug delivery, and biosensor [1–3] due to its distinctive properties such as large surface-area-to-volume ratio, wide range of pore geometry and morphology, biocompatibility, There has been a particular interest in the use of p-Si for biosensors to detect biomolecular interactions and cell adhesion [4], leading to significant potentials towards applications in the fields of tissue engineering and drug delivery [5–8]. However, one of the major challenges with these applications is the precise control of surface wetting properties of p-Si. This is particularly important as both protein and living cells are very sensitive to these wetting properties, and, when not optimized, protein or cells may either lose their bioactivity on the surface or simply resist adhering to the surface [4]. For instance, it was reported that human MG63 osteosarcoma cells tend to attach and grow only on surfaces which present the contact angle of around 64° [9]. Wetting properties of p-Si surface largely rely on surface morphology and chemical bonds [10]. While morphology is usually controlled at fabrication stage [11], surface chemical bond composition can be modified during postprocessing steps which have been the focus of many research activities. As a result, approaches using molecular grafting [12, 13], UV photosensitive molecular coating [14, 15], electrochemical methods [16], and so forth have been reported. However, despite some success, these reported approaches are neither