Degradation Kinetic Studies of BSA@ZIF-8 Nanoparticles with Various Zinc Precursors, Metal-to-Ligand Ratios, and pH Conditions

Nanosized zeolitic imidazolate framework particles (ZIF-8 nanoparticles [NPs]) have strong potential as effective carriers for both in vivo and in vitro protein drug delivery. Synthesis of ZIF-8 and stability of protein encapsulation within ZIF-8 are affected by several factors, notably the metal ion source and molar ratio. To systematically investigate these factors, we investigated such effects using BSA as a model test protein to synthesize BSA@ZIF-8 NPs at various metal-to-ligand (M:L) ratios. SEM, FTIR, XRD, and DLS were applied to characterize the morphology and structure of BSA@ZIF-8 NPs and their effects on protein loading capacity. Degradation kinetics and protein release behavior of BSA@ZIF-8 NPs were evaluated at pH 5.0 (to simulate the tumor environment) and pH 7.4 (to mimic the blood environment). Our objective was to define optimal combinations of the high protein loading rate and rapid release under varying pH conditions, and we found that (i) the yield of BSA@ZIF-8 NPs decreased as the M:L ratio increased, but the protein content increased. This highlights the need to strike a balance between cost-effectiveness and practicality when selecting ZIF-8 NPs with different molar ratios for protein-based drug formulation. (ii) BSA@ZIF-8 NPs exhibited cruciate flower-like shapes when synthesized using Zn(NO3)2 as the zinc precursor at M:L ratios of 1:16 or 1:20. In all other cases, the NPs displayed a regular rhombic dodecahedral structure. Notably, the release behavior of the NPs did not differ significantly between these morphologies. (iii) When Zn(OAc)2 was used as the zinc precursor, the synthesized ZIF-8 NPs exhibited a smaller size compared to the Zn(NO3)2-derived ZIF-8 NPs. (iv) The release rate and amount of BSA protein were higher at pH 5.0 compared to pH 7.4. (v) Among the different formulations, BSA@ZIF-8 with an M:L ratio of 1:16 at pH 5.0 was observed to have a shorter time to reach a plateau (0.5 h) and higher protein release, making it suitable for locally rapid administration in a tumor environment. BSA@ZIF-8 prepared from Zn(OAc)2 at an M:L ratio of 1:140 showed the slower release of BSA protein over a 24-h period, indicating its suitability for sustained release delivery. In conclusion, our findings provide a useful basis for the practical application of ZIF-8 NPs in protein-based drug delivery systems.