Ester Pastor

Electrochemical Oscillation of Open Circuit Potential during Immersion Plating of Copper on Silicon

A new type of oscillation emerging during copper immersion plating on silicon in solutions containing HF was observed. It is the spontaneous oscillation of open-circuit potential rather than the oscillations which have been observed on silicon under high polarization. The oscillation appears at less-noble potential and in a limited HF concentration range, and the amplitude is small. The behavior is affected by the type of dopant and dopant concentration and by the surface condition. The phenomenon is likely to be explained based on reactions occurring during immersion plating in the presence of fluoride ions: the oxide formation-breakdown-repair model. However, some results cannot be simply explained by the model.

Pore size is a critical parameter for obtaining sustained protein release from electrochemically synthesized mesoporous silicon microparticles

Mesoporous silicon has become a material of high interest for drug delivery due to its outstanding internal surface area and inherent biodegradability. We have previously reported the preparation of mesoporous silicon microparticles (MS-MPs) synthesized by an advantageous electrochemical method, and showed that due to their inner structure they can adsorb proteins in amounts exceeding the mass of the carrier itself. Protein release from these MS-MPs showed low burst effect and fast delivery kinetics with complete release in a few hours. In this work, we explored if tailoring the size of the inner pores of the particles would retard the protein release process. To address this hypothesis, three new MS-MPs prototypes were prepared by electrochemical synthesis, and the resulting carriers were characterized for morphology, particle size, and pore structure. All MS-MP prototypes had 90 µm mean particle size, but depending on the current density applied for synthesis, pore size changed between 5 and 13 nm. The model protein α-chymotrypsinogen was loaded into MS-MPs by adsorption and solvent evaporation. In the subsequent release experiments, no burst release of the protein was detected for any prototype. However, prototypes with larger pores (>10 nm) reached 100% release in 24–48 h, whereas prototypes with small mesopores (<6 nm) still retained most of their cargo after 96 h. MS-MPs with ∼6 nm pores were loaded with the osteogenic factor BMP7, and sustained release of this protein for up to two weeks was achieved. In conclusion, our results confirm that tailoring pore size can modify protein release from MS-MPs, and that prototypes with potential therapeutic utility for regional delivery of osteogenic factors can be prepared by convenient techniques.

Electrochemical Oxidation of Mesoporous Silicon: Structural and Morphology Properties of the Obtained ox-porSi Material

Morphological and structural changes in porous silicon por-Si grown on differently doped Si wafers under anodic oxidation in phosphoric acid were studied. The two degrees of oxidation were confirmed: slighter (pore walls) and stronger (bulky) oxidation. Slighter oxidized por-Si does not experience any structural or morphological changes but chemical composition of the pore walls is gradually changed to Si-OH terminations. Stronger oxidation is accompanied by progressive morphological and structural transformations. Conductivity of the porous layer plays a significant role in the observed behavior. In less doped samples only a stronger oxidation started at the bottom of the porous layer is found. Porous layer becomes electrically separated from the Si wafer (electrode) and surface Si-Hx groups are preserved. Thus, doping level and conductivity of the Si wafer determines a spatial distribution of the porous electrode/ electrolyte interface and possibility to conserve native chemical composition of por-Si after electrochemical oxidation.

Nano Suspension of Porous Silicon in Water: Two Methods of Material Preparation and Modification by Surfactants

Luminescent silicon nanocrystals (Si-nc) are promising material for biomedical purposes. However, as-prepared Si-nc are Hterminated and not miscible with biological (aqueous) liquids. We used physical and chemical adsorption of undecylenic acid to make porous silicon (porSi, material containing Si-nc assemblies) miscible with water solutions. PorSi powders prepared by two different methods, electrochemical anodization and stain etching were modified and suspended in water. Luminescent properties of aqueous suspensions have been then monitored for long time. The manyfold buildup of initial luminescence intensity during the first 3-6 days has been observed for electrochemically prepared porSi material. This phenomenon was explained as disaggregating of the primary micelles formed from surfactantcovered porSi particles into smaller ones. Contrary to this, the stain etched porSi powder exhibited a continuous decrease of the luminescence intensity with time and, moreover, was much less luminescent. Thus, electrochemically prepared porSi is found more appropriate for photosensitizing biomedical applications.

A Three Pulse Electrochemical Regime as a Fabrication Platform for Nano-structured Multifunctional Silicon Based Materials and Their Primary Prototypes

Nano-Silicon is an extremely versatile material that can be structured at the level of its porosity, multi-film thicknesses or size of particles. Sciences keen interest in these new nanomaterials is motivated by new fascinating properties and their tunability. Different forms of nano-Si primary prototypes can be made using a novel cyclic three-pulse electrochemical regime presented in this work. It consists of three pulses of different intensity: a working pulse when layer with determined thickness and porosity is formed; a cut pulse that introduces an embedded pre-partition between the layers; and an etch stop pulse when electrolyte is refreshed. An easy control by electrical means of overall morphology opens the possibility for large variations of nano-structuring in the obtained material. In-situ deposition of some metals simultaneously with layers formation through variation of the electrolyte composition and some parameters of the electrical regime is attainable as well.