Harnessing the Materials Chemistry of Mesoporous Silicon Nanoparticles to Prepare "Armor-Clad" Enzymes

There is a growing interest in nanomaterials that can encapsulate enzymes while retaining their ability to function within the confines of a nanocage. Here porous silicon nanoparticles (pSiNPs) are evaluated as an enzyme cage, utilizing the aqueous chemistry of silicon to dynamically restructure the mesopore structure, immobilizing, and confining the enzyme. The common bioluminescent reporter enzyme nanoluciferase (Nluc) is used to evaluate two different trapping chemistries, and impacts on the stability and catalytic performance of the enzyme are compared with controls involving free enzyme and enzyme electrostatically adsorbed to a pSiNP host without the use of trapping chemistry. The two chemistries exploited in this study are (1) oxidative trapping, where mild aqueous oxidation of the elemental silicon skeleton in the mesoporous silicon host swells and restructures the pore walls, physically trapping the Nluc payload in a porous SiO2 matrix, and (2) calcium ion-induced condensation, where localized precipitation of calcium silicate entraps the Nluc protein in a porous silicate matrix. The two trapping chemistries form robust nanoscale cages with substantially smaller pores (9.8 +/- 0.4 and 8.8 +/- 0.3 nm, respectively) compared to the pSiNP starting material (15.3 +/- 1.8 nm), such that the enzyme does not leach from the pSiNPs in aqueous buffer or under assay conditions. Enzyme stability is substantially improved using the two trapping chemistries; the caged materials retain 30-45% activity after heating to 80 degrees C for 30 min or when exposed to organic solvents; either of these denaturing conditions result in complete or near-complete loss of activity for the free enzyme or for enzyme that is electrostatically adsorbed to pSiNPs. Finally, we explore the potential for the use of the Nluc-encapsulated nanocomposite as a cellular probe by demonstrating the luminescent reporting function of the nanoparticles in HeLa human cell cultures.