The Influence of Thermal Oxidation on the Surface Chemical and Adsorbent Properties of Porous Silicon
A porous silicon (pSi) based delivery system, BioSilicon™ (pSivida Ltd) is currently under development for applications including; oncology and the delivery of small molecules and peptides/proteins. To optimize the development of pSi as a delivery platform it is desirable to investigate the nano-scale porous structure and surface chemical speciation.
Porous silicon particles were oxidised in air at 200°C, 400°C, 600°C and 800°C and subsequently characterized using XPS and FTIR spectroscopy. Adsorption isotherms of a negatively charged probe molecule (orange G) were determined. The zeta potential of pSi particles were obtained with a Malvern Zetasizer Nano ZS.
Oxidation of pSi particles progresses from native Si(4-x)-Si-Hx (x=1-3) to intermediate backbonded O(4-x)-Si-Hx (x=1-3) to a completely oxidized O3-Si-OH surface. The process of pSi oxidation induces a more negatively charged surface; however varying the oxidation temperature does not appear to significantly alter surface charge. Significant orange G adsorption was observed for unoxidised and 200°C oxidised pSi, which was attributed to specific interaction with Si-Hx surface groups. In contrast, orange G adsorption onto pSi oxidised above 200°C was negligible, due to electrostatic repulsion between the negatively charged pSi surface and dye molecule.
This study has demonstrated how pSi surface speciation plays a significant role in regulating both surface charge and probe molecule adsorption, and how thermal oxidation provides significant control over these parameters.