Functionalized Porous Silicon in a Simulated Gastrointestinal Tract: Modeling the Biocompatibility of a Monolayer Protected Nanostructured Material

Abstract

Owing to its photoluminescent properties and high surface area, porous silicon (por-Si) has shown great potential toward a myriad of applications including optoelectronics, chemical sensors, biocomposite materials, and medical implants. However, the native hydride-termination is only metastable with respect to surface oxidation under ambient conditions. Por-Si samples oxidize and degrade even more quickly when exposed to saline aqueous environments. Borrowing from solution phase synthetic methods, a selection of hydrosilylation reactions has been recently reported for functionalizing organic groups onto oxide-free, hydride-terminated porous silicon surfaces. Monolayers, bound through direct silicon-carbon bonds, are produced via thermal, microwave, Lewis acid, and carbocation mediated pathways. All of these wet, benchtop methods result in the formation of stable monolayers which protect the underlying silicon surface from ambient oxidation and chemical attack. However, no direct comparison of monolayer stability resulting from these diverse mechanisms has been reported. A variety of alkyl monolayers were prepared on porous silicon using the diverse hydrosilylation routes describe above and then immersed into a sequence of simulated gastric and intestinal fluids to replicate the conditions of potential por-Si biosensors or medicinal delivery systems in the human gastrointestinal tract. Degradation of the organic monolayers and oxidation of the underlying por-Si surfaces were monitored using both qualitative and semiquantitative transmission mode Fourier transform infrared spectroscopy (FTIR). Our initial results indicate that methods employing chemical catalysts often incorporate these species within the monolayer as defects, producing less robust surfaces compared to catalyst-free reactions. Regardless, monolayer protected por-Si samples demonstrated superior durability as opposed to the unfunctionalized controls.

Biocompatibility assessment of Si-based nano- and micro-particles

“…It was shown that bare pSi exhibited high surface oxidation after eighteen hours incubation in simulated intestinal fluid. [186] After functionalization with alkyl groups, Albrecht et al demonstrated that the porous particles had high resistance to oxidation in the gastric and intestinal fluids. [186] Surface oxidation and other chemical/physical exchanges between the particle and GI environment can produce unpredictable toxicity by reactive oxygen species (ROS).…”

“…[186] After functionalization with alkyl groups, Albrecht et al demonstrated that the porous particles had high resistance to oxidation in the gastric and intestinal fluids. [186] Surface oxidation and other chemical/physical exchanges between the particle and GI environment can produce unpredictable toxicity by reactive oxygen species (ROS). However, if the particles are already oxidized, the contribution of these processes can become negligible.…”