44993-GB5
Organic Functionalization of Porous Silicon via Hydrosilylation Pathways: Probing Monolayer Stability Through Desorption/Degradation Studies
Mesoporous and 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 of surface chemistry. 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, 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 two years, undergraduate researchers in my 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, 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 chemically demanding conditions of potential porous silicon biosensors or medicinal delivery systems in the human bloodstream or gastrointestinal track.
Degradation of the underlying porous silicon surface was monitored using both qualitative and semiquantitative infrared spectroscopy at various time intervals. The native, hydride-terminated surface of porous silicon rapidly oxidizes and degrades upon exposure to simulated acellular plasma and intestinal fluid. While unfunctionalized control samples oxidize and degrade in a window of hours, alkyl-functionalized porous silicon samples remain largely unaffected for weeks or months. In each of the three 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 and Lewis acid mediated reactions afford a higher level of protection against oxidation than the carbocation 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.
In addition to this comparative study, we report a facile, efficient, and highly-scalable reaction that results in alkyl-functionalized porous silicon by way of a multimode microwave reactor. Qualitative and semi-quantitative FTIR analysis confirmed the formation of monolayer protected porous silicon through the reaction of terminal alkenes and alkynes. Alkyl-functionalized porous silicon also demonstrated superior resistance to oxidation, as opposed to the unfunctionalized control samples under ambient, aqueous, and organic environments. High temperature, catalyst-free hydrosilylation pathways result in higher reaction yields when compared to room temperature reactions employing catalysts. Ultimately, the multimode microwave-assisted configuration successfully provided the ability to batch process numerous samples at the expense of longer reaction times. Future studies will focus on exploring microwave affects on porous silicon surface morphology as a result of surface functionalization employing microwave irradiation.
In addition to our laboratory efforts, I have managed to export the aims of our project to the undergraduate chemistry classroom and laboratory. Over the past two years, this PRF grant has provided resources and experiences that have provided the insight and ability to design and teach courses in nanoscience at both the introductory and advanced level. Initial efforts focused on spotlighting the connections between the modern research in the field and the fundamentals students learn in their introductory science and math courses. Subsequent work has focused on courses aimed at encouraging humanities and social science majors to explore topics in the physical sciences.
