Abstract: Iron oxide nanoparticles (IONPs) modified with functional proteins hold great promise in the biomedical field. However, conventional protein modification strategies, such as adsorption and covalent coupling, are either unstable or nonspecific, or may result in the changes of protein structure and ultimately the loss of protein activity. Modification of active proteins on small-sized IONPs with a particle size of less than 30 nm is especially difficult due to their high surface energy. Herein, we developed a universal modification method based on Spy chemistry for rapid and stable protein immobilization on small-sized IONPs, which only requires the presence of active groups on the surface of nanoparticles that can couple with SpyCatcher. In short, the SpyCatcher peptides were first coated on the surface of IONPs by cross-linking with activated groups, and then the SpyTag peptide fused with a model protein (enhanced green fluorescent protein, EGFP) was engineered (SpyTag-EGFP) and directly coupled to SpyCatcher-modified IONPs by self-assembly, which is spontaneous and robust while avoiding the effect of chemical reactions on functional protein activity. The obtained EGFP-functionalized IONPs exhibited enhanced and stable green fluorescence and improved magnetic properties. In addition, the cell internalization efficiency of EGFP-functionalized IONPs was significantly increased as compared to unmodified IONPs, providing an ideal solution for efficient cell labeling and tracking. In conclusion, here we report a rapid and easy strategy for EGFP immobilization on IONPs based on Spy chemistry, which could be further adapted to other functional proteins in the future. SpyCatcher-modified IONPs and SpyTag-X (arbitrary functional fusion proteins) hold great potential to be applied as a versatile platform for protein immobilization on IONPs and enable its multifunctional application in the future.

Key words: Iron oxide nanoparticles, Spy chemistry, Protein immobilization, Magnetic resonance imaging, Cell labeling