Part I, Fabrication and surface modification of composite biomaterials based on silicon and calcium disilicide; Part II, Synthesis and characterization of erbium doped silicon nanocrystals encapsulated by aluminum and zinc oxides

Abstract

A dry-etch spark ablation method was used to produce porous silica (SiO2/Si) and calcium disilicide (CaSi2/Si) layers on silicon (Si) surfaces for the electrochemical growth of apatitic phosphates (CaP). Both SiO2/Si and CaSi2/Si composite electrodes readily calcify in vitro under the application of a small electric potential, and with proper treatment the electrodeposition of CaP is localized to the sparked areas. Porous SiO2 films can also be fabricated via a guided ablation technique and subsequently produce patterns of CaP on Si. In addition to increasing the local concentration of Ca2?, interfacial layers of CaSi2 on Si exhibit exceptional site-selectivity towards CaP formation under bias due to the difference in conductivity between Si and CaSi2.^The proposed mechanism for bias-assisted biomineralization of CaSi2/Si layers on spark-processed Si accounts for the physicochemical properties of deposited CaP films.This work also describes routes to surface modification of calcified composite electrodes with medicinally relevant compounds such as alendronate and norfloxacin. To assess the suitability of this material as an antibiotic delivery platform, release of the latter compound was also monitored as a function of time. Next, biomineralization of CaSi2/Si layers on Si surfaces under zero bias was followed by means of Scanning Electron Microscopy (SEM), X-Ray Energy Dispersive Analysis (EDX), and Raman spectroscopy. CaSi2/Si wafers are bioinert at 25?C and bioactive at 37?C. Mechanistic insights regarding biomineralization were derived from an analysis of film growth morphology and chemical composition after various soaking periods in standard SBF.^Changes in CaSi2 calcification behavior as a function of reaction temperature and pH, SBF concentration, and various surface modification processes were also employed for this purpose.During CaSi2/Si calcification under zero bias, CaP growth is significantly dependent on the structural degradation of CaSi2 grains. Surface silanol groups, initially present on the as-prepared material, cannot induce CaP nucleation which begins only upon delamination of CaSi2 layers. The CaP phases, which are present during various growth stages, possibly include a combination of Mg-substituted whitlockite, monetite, and tricalcium phosphate. The incorporation of CaSi2 grains within a polycaprolactone (PCL) framework results in bioactive and biodegradable scaffolds which may be used in bone tissue regeneration. Porous PCL scaffolds were prepared via a combination of salt-leaching/microemulsion methods.^To provide markedly different structural environments for the inorganic phase, calcium disilicide powder was either added to a mixed-composition porogen during a given scaffold's preparation, or alternatively added to pre-formed scaffolds.Selective fluorescent labeling, SEM, and EDX were employed to assess scaffold calcification in vitro. The process of CaSi2/PCL scaffold calcification under zero bias, during which CaP growth is significantly dependent on the structural degradation of CaSi2 grains, has a similar mechanism to CaP growth on bioactive glasses/ceramics. The biomineralization of these scaffolds is initiated solely by the silicide phase and can be accelerated by the degradation of the polymer matrix. A separate part of this work deals with rare earth-doped Si nanocrystals. Photoexcited erbium-doped silicon nanocrystals (Er/Si-NCs) emit at 1.54 um, the wavelength of light which is the most compatible with existing silica-based fiber optics.^Several selective surface modification reactions with inorganic capping layers comprised of either aluminum or zinc oxide were analyzed in an attempt to improve the photoluminescence (PL) efficiency of these nanocrystals by reducing interfacial defect density. It is shown that coating Er/Si-NCs with aluminum oxide via kinetically controlled chemical reaction doubles the PL efficiency. Zinc oxide, deposited under thermodynamic control, improves the PL by a factor of four. Such process demonstrate the impact of surface modification on dopant photophysics.