Polymer Brush-Coated Magnetic Nanoparticles: Diels–Alder/Retro-Diels–Alder Chemistry-Mediated Dual Functionalization for Targeted Drug Delivery

Polymer-coated magnetic nanoparticles play a vital role in many biomedical technologies ranging from disease diagnostics to therapeutic delivery. The effective postpolymerization functionalization of polymer brushes is important for enabling their utilization for intended applications. In this regard, the thiol-maleimide conjugation reaction provides an efficient tool for the attachment of bioactive molecules and ligands. In this study, maleimide-containing polymer brushes are fabricated on nanoparticle surfaces by using a graft-from approach. A furan-protected maleimide monomer is utilized during the polymerization, followed by deprotection of the thiol-reactive maleimide group using the retro-Diels–Alder cycloreversion. Using a thiol-containing fluorescent hydrophobic dye, we demonstrate effective functionalization of the maleimide-containing polymer brush-coated nanoparticles. Interestingly, the partial deprotection of the maleimide groups furnishes dual-functionalizable polymer brush-coated nanoparticles. While the maleimide groups provide a conjugation handle for thiol-containing molecules, the furan-protected bicyclic adduct undergoes cycloaddition with tetrazine-containing molecules through the inverse-electron-demand Diels–Alder reaction. Dual functionalization of the polymeric coating is demonstrated through the conjugation of two different fluorescent dyes in an orthogonal fashion. To highlight the utility of such dual-functionalizable platforms, a thiol-containing anticancer drug, doxorubicin, and a tetrazine-containing cyclic-peptide-based targeting group are conjugated. The targeted drug-conjugated polymer brush-coated nanoparticles show enhanced cytotoxicity and internalization into the MDA-MB-231 breast cancer cell lines. We envision that the dual-functionalizable polymer brush-coated nanoparticles reported here are amenable to facile functionalization and thus can be tailored for a variety of biomedical applications.