Improving the colloidal stability of protein@ZIF-8 nanoparticles in biologically relevant buffers

Abstract

ZIF-8 is one of the most common metal–organic frameworks used in protein encapsulation and is advantageous for the protection of protein function and delivery of protein cargo. However, protein@ZIF-8 composites typically degrade in aqueous solutions, which significantly limits their use in most biological applications that require stability in biologically relevant buffers. Here, we present a facile synthetic approach to produce reproducible and precise protein@ZIF-8 nanoparticles with bovine serum albumin (BSA) and demonstrate their colloidal stability in various buffers. We optimized the biomimetic mineralization of BSA-encapsulated ZIF-8 nanoparticles (BSA@c-ZIF-8) and demonstrated accurate synthesis of 107 ± 9 nm BSA@c-ZIF-8 nanoparticles for 36 independent batches. Dynamic light scattering and supplementary analytical techniques such as powder X-ray diffraction and scanning electron microscopy established BSA@c-ZIF-8's colloidal stability for 14 days in phosphate buffered saline, imidazole, and water. This stability enhancement is ascribed to the BSA-induced negative surface charge (−36 ± 10 mV), which repels chelating anions derived from buffers like phosphate buffered saline. To further improve the colloidal stability of BSA@c-ZIF-8 in buffers, we tested eight polymer coatings and show that poly(acrylic acid) (PAA), hexadecyltrimethylammonium bromide (CTAB), dextran and poly(vinyl alcohol) (PVA) improve colloidal stability and provide a route for further surface functionalization. This advancement can enable application of protein@ZIF-8 nanoparticles in analytical and biotechnological practices, overcoming previously described buffer instability.