D. Gallach

Chemical stabilization of porous silicon for enhanced biofunctionalization with immunoglobulin

Abstract Porous silicon (PSi) is widely used in biological experiments, owing to its biocompatibility and well-established fabrication methods that allow tailoring its surface. Nevertheless, there are some unresolved issues such as deciding whether the stabilization of PSi is necessary for its biological applications and evaluating the effects of PSi stabilization on the surface biofunctionalization with proteins. In this work we demonstrate that non-stabilized PSi is prone to detachment owing to the stress induced upon biomolecular adsorption. Biofunctionalized non-stabilized PSi loses the interference properties characteristic of a thin film, and groove-like structures resulting from a final layer collapse were observed by scanning electron microscopy. Likewise, direct PSi derivatization with 3-aminopropyl-triethoxysilane (APTS) does not stabilize PSi against immunoglobulin biofunctionalization. To overcome this problem, we developed a simple chemical process of stabilizing PSi (CoxPSi) for biological applications, which has several advantages over thermal stabilization (ToxPSi). The process consists of chemical oxidation in H2O2, surface derivatization with APTS and a curing step at 120 °C. This process offers integral homogeneous PSi morphology, hydrophilic surface termination (contact angle θ = 26°) and highly efficient derivatized and biofunctionalized PSi surfaces (six times more efficient than ToxPSi). All these features are highly desirable for biological applications, such as biosensing, where our results can be used for the design and optimization of the biomolecular immobilization cascade on PSi surfaces.

Hybrid luminescent/magnetic nanostructured porous silicon particles for biomedical applications

This work describes a novel process for the fabrication of hybrid nanostructured particles showing intense tunable photoluminescence and a simultaneous ferromagnetic behavior. The fabrication process involves the synthesis of nanostructured porous silicon (NPSi) by chemical anodization of crystalline silicon and subsequent in pore growth of Co nanoparticles by electrochemically-assisted infiltration. Final particles are obtained by subsequent sonication of the Co-infiltrated NPSi layers and conjugation with poly(ethylene glycol) aiming at enhancing their hydrophilic character. These particles respond to magnetic fields, emit light in the visible when excited in the UV range, and internalize into human mesenchymal stem cells with no apoptosis induction. Furthermore, cytotoxicity in in-vitro systems confirms their biocompatibility and the viability of the cells after incorporation of the particles. The hybrid nanostructured particles might represent powerful research tools as cellular trackers or in cellular therapy since they allow combining two or more properties into a single particle.

Towards the Development of Electrical Biosensors Based on Nanostructured Porous Silicon

The typical large specific surface area and high reactivity of nanostructured porous silicon (nanoPS) make this material very suitable for the development of sensors. Moreover, its biocompatibility and biodegradability opens the way to the development of biosensors. As such, in this work the use of nanoPS in the field of electrical biosensing is explored. More specifically, nanoPS-based devices with Al/nanoPS/Al and Au-NiCr/nanoPS/Au-NiCr structures were fabricated for the electrical detection of glucose and Escherichia Coli bacteria at different concentrations. The experimental results show that the current-voltage characteristics of these symmetric metal/nanoPS/metal structures strongly depend on the presence/absence and concentration of species immobilized on the surface.

Calcium phosphate/porous silicon biocomposites prepared by cyclic deposition methods: spin coating vs electrochemical activation.

Porous silicon (PSi) provides an excellent platform for bioengineering applications due to its biocompatibility, biodegradability, and bioresorbability. However, to promote its application as bone engineering scaffold, deposition of calcium phosphate (CaP) ceramics in its hydroxyapatite (HAP) phase is in progress. In that sense, this work focuses on the synthesis of CaP/PSi composites by means of two different techniques for CaP deposition on PSi: Cyclic Spin Coating (CSC) and Cyclic Electrochemical Activation (CEA). Both techniques CSC and CEA consisted on alternate Ca and P deposition steps on PSi. Each technique produced specific morphologies and CaP phases using the same independent Ca and P stem-solutions at neutral pH and at room temperature. The brushite (BRU) phase was favored with the CSC technique and the hydroxyapatite (HAP) phase was better synthesized using the CEA technique. Analyses by elastic backscattering spectroscopy (EBS) on CaP/PSi structures synthesized by CEA supported that, by controlling the CEA parameters, an HAP coating with the required Ca/P atomic ratio of 1.67 can be promoted. Biocompatibility was evaluated by bone-derived progenitor cells, which grew onto CaP/PSi prepared by CSC technique with a long-shaped actin cytoskeleton. The density of adhered cells was higher on CaP/PSi prepared by CEA, where cells presented a normal morphological appearance and active mitosis. These results can be used for the design and optimization of CaP/PSi composites with enhanced biocompatibility for bone-tissue engineering.

Silicon-based hybrid luminescent/magnetic porous nanoparticles for biomedical applications

Silicon-based porous nanoparticles showing at the same time intense visible luminescence and magnetic response were fabricated. The hybrid luminescent/magnetic nanoparticles (hLMNPs) were fabricated by the electrodeposition of cobalt and iron into nanostructured porous silicon. These nanoparticles were subsequently functionalized and internalized into cells. The hybrid behavior of the hLMNPs is a relevant feature for the development of research tools as nontoxic cellular tracker for progenitor cells and consequently able to be used in many strategies of cellular therapy. Additionally, the hLMNPs can be functionalized with various biomolecules that will endow them with new functionalities.

Characterization of hybrid cobalt-porous silicon systems: protective effect of the Matrix in the metal oxidation

In the present work, the characterization of cobalt-porous silicon (Co-PSi) hybrid systems is performed by a combination of magnetic, spectroscopic, and structural techniques. The Co-PSi structures are composed by a columnar matrix of PSi with Co nanoparticles embedded inside, as determined by Transmission Electron Microscopy (TEM). The oxidation state, crystalline structure, and magnetic behavior are determined by X-Ray Absorption Spectroscopy (XAS) and Alternating Gradient Field Magnetometry (AGFM). Additionally, the Co concentration profile inside the matrix has been studied by Rutherford Backscattering Spectroscopy (RBS). It is concluded that the PSi matrix can be tailored to provide the Co nanoparticles with extra protection against oxidation.

Luminescence and fine structure correlation in ZnO permeated porous silicon nanocomposites

Nanocomposites formed by porous silicon (PS) and zinc oxide (ZnO) have potential for applications in optoelectronic devices. However, understanding the distribution of both materials in the nanocomposite, and especially the fine structure of the synthesized ZnO crystals, is key for future device fabrication. This study focuses on the advanced characterization of a range of PS-ZnO nanocomposites by using photon- and ion-based techniques, such as X-ray absorption spectroscopy (XAS) and elastic backscattering spectroscopy (EBS), respectively. PS substrates formed by the electrochemical etching of p(+)-type Si are used as host material for the sol-gel nucleation of ZnO nanoparticles. Different properties are induced by annealing in air at temperatures ranging from 200 °C to 800 °C. Results show that wurtzite ZnO nanoparticles form only at temperatures above 200 °C, coexisting with Si quantum dots (QDs) inside a PS matrix. Increasing the annealing temperature leads to structural and distribution changes that affect the electronic and local structure of the samples changing their luminescence. Temperatures around 800 °C activate the formation of a new zinc silicate phase and transform PS into an amorphous silicon oxide (SiOx, x≈ 2) matrix with a noticeably reduced presence of Si QDs. Thus, these changes affect dramatically the emission from these nanocomposites and their potential applications.

Microstructure based optical modeling of ZnO- porous silicon permeated nanocomposites

ZnO composites with porous silicon (PSi) are increasingly used in advanced optical and electronic structures. ZnO/PSi nanocomposites have been prepared by permeating anodized PSi with ZnO sols based on zinc acetate. Upon thermal annealing the ZnO sols form surface wurzite nanocrystals, as indicated by XRD from annealing temperatures of 400 °C. By increasing the annealing up to 800 °C, electron microscopies evidence that ZnO diffuses through the columnar PSi, while void ZnO crystallites decorate the surface. Angular dependent x-ray photoelectron spectra agree with the partial coverage of the PSi surface by disperse ZnO nanocrystals. In depth composition, analyzed using C-resonant backscattering spectroscopy confirms an activation of ZnO diffusion and PSi oxidation at high temperatures. This microstructural information was used to analyse the optical properties through models adapted to critical processing temperatures. A uniaxial anisotropic layer, included to consider columnar PSi, and an evolution of optical coefficients in agreement with thermally induced effects (namely PSi oxidation and ZnO diffusion-transformation) allows to satisfactorily simulate ellipsometric spectra. The results are relevant for the optimization of bifunctional electronic-antireflective ZnO/PSi structures.

Development of electrical biosensors based on nanostructured porous silicon

Nanostructured porous silicon (nanoPS) can be described as a network of silicon crystals with sizes in the range of a few nanometers. The typical large specific surface area and high reactivity of nanoPS make this material very suitable for many different applications in the field of sensing. Moreover, its biocompatibility and biodegradability opens the way to the development of biosensors. Within this context, in the present work the use of nanoPS in the field of electrical biosensing is explored. More specifically, nanoPS-based devices with the structure Al/nanoPS/silicon/Al and AuNiCr/nanoPS/silicon/Al were fabricated for the electrical detection of glucose and the bacteria Escherichia Coli. The experimental results show that the current-voltage characteristics of the metal/nanoPS/silicon/metal structures show a strong dependence on the presence/absence and surface concentration of glucose and the bacteria Escherichia Coli. The present work describes our findings in the correlation between surface concentration of glucose and bacteria E. Coli and current for a given voltage.