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44993-GB5
Organic Functionalization of Porous Silicon via Hydrosilylation Pathways: Probing Monolayer Stability Through Desorption/Degradation Studies
Nanostructured materials such as porous silicon continue to entice the materials research community with the promise of numerous practical applications as well as advancing fundamental understanding. Porous silicon consists of a parallel nanowire network decorated with hydride-terminated silicon nanocrystallites. Samples exhibit vast surface areas and contain nanocrystallites as small as three nanometers. For features of this size, nearly half of the silicon atoms reside at the surface. This provides an attractive substrate for the preparation and characterization of functional interfaces, owing to the enhanced ratio of surface to bulk atoms. Also, due to its easily tailored surface morphology and high surface area, porous silicon has shown great potential toward a variety of myriad of applications including microscale sensors and biomedical implants.
Unfortunately, its native hydride-termination quickly oxidizes under ambient and 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 silicon surfaces. These literature methods present a variety of methods, including thermal, microwave, carbocation, and Lewis acid mediated hydrosilylation pathways, that allow for the preparation of stable organic monolayers on porous silicon through direct, covalent silicon-carbon linkages. All of these 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.
Over the past year, undergraduate researchers in our group have reproduced the literature protocols for producing porous silicon substrates and functionalizing them with alkyl monolayers of varying chain length (C6, C12, and C18) via thermal, microwave, carbocation, and Lewis acid mediated hydrosilylation pathways. The functionalized porous silicon samples, as well as control (unfunctionalized) samples, were immersed in a variety of aqueous environments, such as simulated acellular plasma (blood), gastric, and intestinal fluids to study monolayer degradation/desorption and oxidation of the underlying silicon substrate. These environments also replicate the conditions of potential porous silicon biosensors or medicinal delivery systems in the human bloodstream or gastrointestinal tract.
Degradation of the underlying porous silicon surface was monitored using semiquantitative infrared spectroscopy at various time intervals. While unfunctionalized control samples oxidize and degrade in a window of hours, alkyl functionalized porous silicon samples remain largely unaffected for weeks in simulated acellular plasma (blood). Control samples exhibit little oxidation in simulated gastric fluid, yet oxidize quickly in the presence of the simulated intestinal fluid. In each of the four reactions pathways tested, the stability of the underlying silicon substrate was greatly improved after alkyl functionalization in comparison to the unfunctionalized control. Preliminary FTIR data suggests that the thermal, mircowave, and Lewis acid mediated reactions afford a higher level of protection against oxidation than the carboncation route. This is most likely due to some chemisorption of the trityl species observed in the infrared spectra. These trityl “defects” interfere with the alkyl monolayer packing, resulting in increased oxidation of the silicon surface. In addition, for all functionalization reactions, longer chain alkyl groups provided enhanced oxidation prevention in comparison to shorter chain alkyl monolayers.
