Jeffery L. Coffer

Evaluation of mesoporous silicon/polycaprolactone composites as ophthalmic implants.

The suitability of porous silicon (pSi) encapsulated in microfibers of the biodegradable polymer polycaprolactone (PCL) for ophthalmic applications was evaluated, using both a cell attachment assay with epithelial cells and an in vivo assessment of biocompatibility in rats. Microfibers of PCL containing encapsulated pSi particles at two different concentrations (6 and 20 wt.%) were fabricated as non-woven fabrics. Given the dependence of Si particle dissolution kinetics on pSi surface chemistry, two different types of pSi particles (hydride-terminated and surface-oxidized) were evaluated for each of the two particle concentrations. Significant attachment of a human lens epithelial cell line (SRA 01/04) to all four types of scaffolds within a 24h period was observed. Implantation of Si fabric samples beneath the conjunctiva of rat eyes for 8 weeks demonstrated that the composite materials did not cause visible infection or inflammation, and did not erode the ocular surface. We suggest that these novel composite materials hold considerable promise as scaffolds in tissue engineering with controlled release applications.

High-porosity poly(ε-caprolactone)/mesoporous silicon scaffolds: Calcium phosphate deposition and biological response to bone precursor cells

In this study, the fabrication and characterization of highly porous composites composed of poly(epsilon-caprolactone) and bioactive mesoporous silicon (BioSilicon) prepared using salt-leaching and microemulsion/freeze-drying methods are described. The role of silicon, along with porosity, in the scaffolds on calcium phosphate deposition was assessed using acellular in vitro calcification analyses. The presence of bioactive silicon in these scaffolds is essential for the deposition of calcium phosphate while the samples are immersed in simulated body fluid (SBF). Silicon-containing scaffolds produced using salt-leaching methods are more likely to calcify as a consequence of SBF exposure than those produced using microemulsion methods. In vitro proliferation and cell viability assays of these porous composites using human embryonic kidney fibroblast cells indicate that no cytotoxic effects are present in the scaffolds under the conditions used. Preliminary analyses of bone sialoprotein and alkaline phosphatase expression using orthopedically relevant mesenchymal cells derived from bone marrow suggest that such scaffolds are capable of mediating osteoblast differentiation. Overall, the results show that these porous silicon-containing polymer scaffolds enhance calcification, can be considered nontoxic to cells, and support the proliferation, viability, attachment, and differentiation of bone precursor cells.

Surface reactivity of luminescent porous silicon

The effects of addition of a series of organoamine molecules on the luminescence of porous silicon has been examined by steady‐state photoluminescence (PL) and Fourier transform infrared spectroscopies. These samples, prepared nonanodically via stain etching techniques and characterized by atomic force microscopy, show dramatic quenching of visible PL upon addition of dilute solutions of the above Lewis base adsorbates. The fractional changes in integrated PL intensity as a function of quencher concentration obey a simple equilibrium model, demonstrating Langmuir‐type behavior from which equilibrium constants can be calculated. An observation concomitant with this loss of PL is a diminution of the silicon hydride stretching frequencies near 2100 cm−1.

Porosified Silicon Wafer Structures Impregnated With Platinum Anti-Tumor Compounds: Fabrication, Characterization, and Diffusion Studies

In this work, the incorporation and characterization of cis-platin (cis-diammine dichloroplatinum(II)), carbo-platin [cis-diammine(cyclobutane-1,1-dicarboxylato] platinum(II)), and Pt(en)Cl2 (ethylenediamminedichloro platinum(II)) within layers of calcium phosphate on porous Si/Si substrates are described. These materials have been characterized by scanning electron microscopy, secondary ion mass spectrometry, and X-ray energy dispersive spectroscopy. The diffusion of platinum species from the doped calcium phosphate layers has also been investigated by UV-visible absorption spectrometry and inductively-coupled plasma spectroscopy. The influence of initial platinum concentration, the impact of thermal annealing of the calcium phosphate/porous Si/Si matrix, as well as the effect of varying the ligand coordination sphere of the Pt complex on its ability to be delivered to the surroundings have also been analyzed. For the case of cis-platin, it is found that increasing the concentration of platinum complex in the electrolyte during cathodic growth of calcium phosphate results in a relatively greater concentration of Pt incorporated into the calcium phosphate layers and a larger amount of Pt which subsequently can be delivered to the surrounding medium upon exposure to solvent.

Porous Silicon Nanotube Arrays as Anode Material for Li-Ion Batteries

We report the electrochemical performance of Si nanotube vertical arrays possessing thin porous sidewalls for Li-ion batteries. Porous Si nanotubes were fabricated on stainless steel substrates using a sacrificial ZnO nanowire template method. These porous Si nanotubes are stable at multiple C-rates. A second discharge capacity of 3095 mAh g(-1) with a Coulombic efficiency of 63% is attained at a rate of C/20 and a stable gravimetric capacity of 1670 mAh g(-1) obtained after 30 cycles. The high capacity values are attributed to the large surface area offered by the porosity of the 3D nanostructures, thereby promoting lithium-ion storage according to a pseudocapacitive mechanism.

Sustained Antibacterial Activity from Triclosan-Loaded Nanostructured Mesoporous Silicon

In this work, nanostructured particles of porous silicon are demonstrated to act as an effective carrier for the sustained delivery of antibacterial agents with an enhanced inhibitory activity. Methods are described for the incorporation of significant amounts of the established antibacterial compound triclosan (Irgasan) into mesoporous silicon of varying porosities. Such materials were characterized by a combination of scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), and antimicrobial assays. Assessment of antibacterial activity was carried out versus the bacterium Staphylococcus aureus as a function of time with concomitant assessment of triclosan release; significant, sustained inhibition of bacterial growth is demonstrated in the triclosan-containing porous Si for time intervals greater than 100 days. Significantly, enhanced dissolution (relative to room temperature equilibrium solubility) of the triclosan was observed for the initial 15 days of drug release, inferring some amorphization or nanostructuring by the porous Si matrix.

Medicinal Surface Modification of Silicon Nanowires: Impact on Calcification and Stromal Cell Proliferation

Medicinal surface modification of silicon nanowires (SiNWs) with selected bisphosphonates, such as the known antiosteoporotic drug alendronate, is described. In terms of specific assays relevant to orthopedic applications, the impact of selected bisphosphonate attachment on acellular calcification in simulated plasma is reported. To further investigate biocompatibility, proliferation assays of these modified nanowires were carried out using an orthopedically relevant cell line: mesenchymal stem cells derived from mouse stroma. It is found that the identity of the bisphosphonate ligand strongly and sensitively impacts its resultant cytotoxicity.

Size control of erbium-doped silicon nanocrystals

This work describes the effects of pyrolysis oven length and erbium precursor on the preparation of discrete erbium-doped silicon nanoparticles. These doped nanoparticles were prepared by the co-pyrolysis of disilane and the volatile complex Er(tmhd)3 (tmhd=2,2,6,6-tetramethyl-3,5-heptanedionato). The particle sizes and size distributions were determined using high resolution and conventional transmission electron microscopy. Erbium-doped silicon nanoparticles exhibit a selected area electron diffraction pattern consistent with the diamond cubic phase and a distinctive dark contrast in the transmission electron microscope. The presence of erbium is confirmed by x-ray energy dispersive spectroscopy. In general, the mean diameter of the individual nanoparticles increases as the length of the pyrolysis oven used during their preparation is increased.

The influence of adenine content on the properties of Q-CdS clusters stabilized by polynucleotides

Abstract In this work the nature of the interaction between quantum-confined cadmium sulfide (Q-CdS) clusters and either single or double-stranded polynucleotide stabilizers of varying nucleic acid base composition is investigated. We specifically focus here on the influence of adenine content on cluster size, photoluminescence behavior, and the relative ability of these small particles to increase in average diameter upon conditions of mild heating. Thermolysis of Q-CdS/polynucleotide samples affects the interfacial interaction between cluster and stabilizer giving rise to a shift in the emission maximum, a change in luminescence quantum yield, and in some cases an increase in average size. For a fixed set of reactant concentrations, Q-CdS stabilized by polyadenylic acid (Poly[A]) and other adenine-rich polynucleotides are unique in stabilizing the formation of smaller clusters with an average diameter near 38 A under ambient conditions, but Poly[A]-stabilized Q-CdS clusters readily fuse to larger (about 60 A) clusters upon mild heating. In contrast, Q-CdS stabilized by other types of single-stranded hompolymers (Poly[U], [G], and [C]) as well as double-stranded calf thymus and Escherichia coli deoxyribonucleic acids form somewhat larger clusters during intitial synthesis and typically exhibit greater protection against radical changes in size upon thermolysis.

Porous silica glasses doped with quantum-confined cadmium selenide

The formation of stable cadmium selenide-doped silica glasses, via semiconductor diffusion into a porous glass obtained using a sol-gel process, is described. Exposure of quantum-confined (‘Q-state’) CdSe clusters, of average diameter 30 A and dissolved in an ethylpyridine solution, to a colorless sol-gel disk possessing an average pore diameter of 35 A yields an intense red-green glass within hours. Subsequent drying and heat treatment up to 200°C does not degrade the samples. These CdSe-doped glasses are characterized by both absorption and emission (photoluminescence) spectroscopy as well as surface area measurements. The effect of different functional groups present in the gel pores, Cl − versus OH − , on the properties of the diffused CdSe is also reported.