Radical Surface Chemistry: The Surprising Reactivity of Silica and Partially Oxidized Porous Silicon Surfaces

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Silica and amorphous silicon oxides are generally considered to be inert substrates but have recently been shown to exhibit surprising catalytic activity as a base.1,2 While their weakly acidic nature has long been recognized, recent work has shown that during the formation of microdroplets contact electrification at the water/SiO2 interface is related to formation of radical species such as •OH and H•.3,4 Nano- and micro-structuring, asperities, and anisotropic charge distributions within the double layer may all play roles in the generation of electric fields that facilitate electrochemical processes such as the stripping of an electron from OH– generated by water autoionization that leads to •OH radical formation.5,6 Such radicals may participate in aqueous phase chemistry or produce surface bound radicals such as SiO• that act as active sites. We present evidence for such radical-mediated chemistry at amorphous silica and partially oxidized porous silicon surfaces. Examples include the formation of disulfide linkages from amino acids including cysteine, penicillamine, and glutathione that are mediated by a surface radical site on amorphous silica surfaces.7 Partially oxidized porous silicon is found to be a prodigious producer of H2O2 formed by the reactions of •OH radicals (or other reactive oxygen species) in solution. The reactive oxygen species formed couple synergistically with reduced pH to effectively kill salmonella. Experimental studies are supported by density functional theory (DFT) calculations, which are used to screen possible reactions based on their thermodynamics as well as to explore reaction pathways. 1Y. Li, T. F. Mehari, Z. Wei, Y. Liu, and R. G. Cooks, Angew. Chem., Int. Ed. Engl. 60, 2929 (2021). 2Y. Li, K.-H. Huang, N. Morato, and R. G. Cooks, Chem. Sci. 12, 9816 (2021). 3X. Chen, Y. Xia, Z. Zhang, L. Hua, X. Jia, F. Wang, and R. N. Zare, J. Am. Chem. Soc. 145, 21538 (2023). 4M. A. Mehrgardi, M. Mofidfar, and R. N. Zare, J. Am. Chem. Soc. 144, 7606 (2022). 5C. F. Chamberlayne and R. N. Zare, J. Chem. Phys. 152, 184702 (2020). 6H. Hao, I. Leven, and T. Head-Gordon, Nature Communications 13, 280 (2022). 7Y. Li, K. W. Kolasinski, and R. N. Zare, Proc. Natl. Acad. Sci. U. S. A. 120, e2304735120 (2023).

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