Purification of Nano‐Porous Silicon for Biomedical Applications

Abstract

Recently, various bio‐medical applications of nanoporous silicon (np‐Si) have been suggested. This work investigates the biocompatibility of np‐Si particles taking into account hazardous residua confined in the pores after preparation. The emphasis is on the potential application of such particles as oxygen photosensitizer for photodynamic therapy of cancer, which requires both negligible toxicity of np‐Si particles in darkness and a high photo‐cyto‐toxic effect due to generation of singlet oxygen under illumination. Considerable amounts of water soluble toxic impurities are found to be present in the nanoporous shell of micrometer‐sized np‐Si particles immediately after their preparation by chemical etching of bulk silicon powder. The effects of several ordinary cleaning treatments are investigated by using thermal effusion mass‐spectroscopy and FTIR spectroscopy. A particular purification procedure is developed, capable to reduce the concentration of residual impurities to levels acceptable for bio‐medical applications while preserving the required photo‐activity of the np‐Si particles.

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

“…Trace etch residues, such as silicon tetrafluoride and hexafluorosilicic acid, could potentially introduce chemical reactivity [22]; however, rigorous water and solvent rinsing with multiple exchanges was employed in sample preparation. Fluoride levels in selected batches were undetectable by XRF and XPS [Loni A. unpublished data 45] but the sensitivity of these techniques for this element is not particularly high.…”

“…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]. Pharmacopeia requirements for drug formulations include very low levels of "Active Pharmaceutical Ingredient" (API) modification during long term storage (e.g.…”

Porous Silicon Formation by Anodization

“…The secondary reaction of H 2 SiF 6 with water in the aqueous environment of the pores may produce metasilicic acid (H 2 SiO 3 ) -this can polymerize to a less-soluble (poly-silicate) gel (and ultimately a solid), which may entrap the residual toxic species mentioned above (Koynov et al 2011). The complete removal of all toxic species is a prerequisite for most applications and is particularly important for the biomedical field.…”

“…The presence of residual toxic chemical species, such as the electrolyte (HF) itself and the etch by-products silicon tetrafluoride (SiF 4 ) and hexafluorosilicic acid (H 2 SiF 6 ), all water soluble, must be considered after anodization (Koynov et al 2011) and, likewise, any residual electrolyte additives. The secondary reaction of H 2 SiF 6 with water in the aqueous environment of the pores may produce metasilicic acid (H 2 SiO 3 ) -this can polymerize to a less-soluble (poly-silicate) gel (and ultimately a solid), which may entrap the residual toxic species mentioned above (Koynov et al 2011).…”

Bi‐Modal Nonlinear Optical Contrast from Si Nanoparticles for Cancer Theranostics

“…Second, in contrast to porous silicon‐based NPs, large Si NPs produced by laser ablation or laser pyrolysis of silanes are ideally spherical in shape and low‐size‐dispersed, while their large size should contribute to their prolonged circulation in the blood stream and not prevent their subsequent complete excretion via renal clearance due to the biodegradability option . Third, being prepared by wet chemistry‐free methods they are free of any contaminants and should not provide any toxic effects, as it was proved in our early studies in vitro and in vivo (in contrast, porous silicon‐based NPs cannot be completely cleaned from sub‐products of wet chemical synthesis).…”