Surface Chemistry, Reactivity, and Pore Structure of Porous Silicon Oxidized by Various Methods

Abstract

Oxidation is the most commonly used method of passivating porous silicon (PSi) surfaces against unwanted reactions with guest molecules and temporal changes during storage or use. In the present study, several oxidation methods were compared in order to find optimal methods able to generate inert surfaces free of reactive hydrides but would cause minimal changes in the pore structure of PSi. The studied methods included thermal oxidations, liquid-phase oxidations, annealings, and their combinations. The surface-oxidized samples were studied by Fourier transform infrared spectroscopy, isothermal titration microcalorimetry, nitrogen sorption, ellipsometry, X-ray diffraction, electron paramagnetic resonance spectroscopy, and scanning electron microscopy imaging. Treatment at high temperature was found to have two advantages. First, it enables the generation of surfaces free of hydrides, which is not possible at low temperatures in a liquid or a gas phase. Second, it allows the silicon framework to partially accommodate a volume expansion because of oxidation, whereas at low temperature the volume expansion significantly consumes the free pore volume. The most promising methods were further optimized to minimize the negative effects on the pore structure. Simple thermal oxidation at 700 °C was found to be an effective oxidation method although it causes a large decrease in the pore volume. A novel combination of thermal oxidation, annealing, and liquid-phase oxidation was also effective and caused a smaller decrease in the pore volume with no significant change in the pore diameter but was more complicated to perform. Both methods produced surfaces that were not found to react with a model drug cinnarizine in isothermal titration microcalorimetry experiments. The study enables a reasonable choice of oxidation method for PSi applications.

Quantification and Reduction of the Residual Chemical Reactivity of Passivated Biodegradable Porous Silicon for Drug Delivery Applications

“…Chemical reactivity of porous silicon has been ascribed to hydrides associated with the surfaces of the particles (12,21). These hydride surfaces reduce Resazurin (blue), a marginally fluorescent dye, to pink Resorufin which is highly fluorescent.…”

“…Biodegradable semiconducting silicon degrades into orthosilicic acid in vivo, which is naturally present in the human body [19], but can also release trace levels of silane [20] and can contain residual silicon-hydride bonds after thermal passivation (oxidation) treatments (21). If insufficient post-etch rinsing and drying is carried out the material can also contain sufficient HF-based electrolyte, solvent residues and etch by-products to cause cytotoxicity and reactivity [22].…”

“…As etched (as-anodized) porous silicon contains a high concentration of silicon hydride bonds at its surface; which act as a reducing agent. Therefore to make the material non-reactive with APIs, the Si-H bonds are oxidized to yield a passivated product, which is less reactive due to lower amounts of hydrides [20,21]. Derivatization of Si-H bonds with aminosilane compounds like APTES or APDMES has also been employed to make the pore wall surface more stable and more compatible with biomolecules such as oligonucleotides [24].…”

Oxidized Porous Silicon Nanostructures Enabling Electrokinetic Transport for Enhanced DNA Detection

“…This decrease is attributed to the volume expansion of the material during oxidation. [ 55 ] Conventional thermal oxidization of PSi is carried out at 500-900 °C for durations of several minutes to an hour, [ 11,52,[56][57][58][59] primarily to improve the nanostructure stability in aqueous media. Oxidation at temperatures higher than 1000 °C leads to structural deterioration of the porous layer, accompanied by drastic decrease in the surface area and transition of the elongated columns into closed sphere-like pores; [60][61][62] thus unsuitable for biosensing applications.…”

Characterization of Porous Silicon by Calorimetry

“…Salonen (Salonen et al 1999b) Reactivity in solution IMC Salonen (Salonen et al 1997a(Salonen et al , 2001), Riikonen (Riikonen et al 2012 Plummer (Plummer et al 2008), Becker (Becker et al 2010) fraction of the drug in the sample can be calculated relatively accurate (Salonen et al 2005a). This makes the combination of TG and DSC a fast and simple method to estimate the amount and physical state of the drug loaded.…”