Polypeptide core–shell silica nanoparticles with high grafting density by N-carboxyanhydride (NCA) ring opening polymerization as responsive materials and for bioconjugation

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Abstract

Silica nanoparticles were furnished with a functional polypeptide shell to create a pH-responsive inorganic–organic hybrid material. Free amine groups present on silica nanoparticles initiated the N-carboxyanhydride (NCA) polymerization of particular amino acid NCAs directly onto the inorganic support, offering a convenient method to functionalize silica core nanoparticles with a uniformly dense polypeptide shell. Poly(γ-benzyl-L-glutamate) (PBLG), poly(ε-carbobenzyoxy-L-lysine) (PZLL), and S-tert-butyl protected polycysteine (PtBLC) were grafted from the silica core both independently as homopolypeptides and simultaneously to form a copolypeptide shell, highlighting the versatility that the grafting mechanism possesses. The grafting of PBLG was investigated in detail at 0 °C and 20 °C to determine any differences in the size and uniformity of the polypeptide shell formed. Dynamic light scattering (DLS) analysis revealed a correlation between the thickness of the uniform organic shell and the amount of amino acid in the monomer feed, with higher linearity at the lower polymerization temperature. Size Exclusion Chromatography (SEC) analyses of degrafted PBLG confirmed the DLS results. A high grafting density of around 0.4 PBLG chains per nm2 was calculated highlighting the control afforded in this approach to polypeptide grafting. Subsequent deprotection of the PBLG homopolypeptide shell afforded pH-sensitive poly(glutamic acid) (PGA) coupled silica nanoparticles. The selective release of a model rhodamine B dye was demonstrated to emphasize the potential that these hybrid nanomaterials have for the on-demand release of payload molecules in response to a targeted pH trigger. Moreover, covalent bioconjugation was successfully shown by attachment of green fluorescent protein.

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