Surface modification: how nanoparticles assemble to molecular imaging probes

Nanomaterials have attracted widespread attention due to their unique chemical and physical properties, such as size-dependent optical, magnetic, or catalytic properties, thus have the great potential application, especially in the fields of new materials and devices. The emergence of nanoparticle-based probe has led to important innovations in molecular imaging field. Several types of nanoparticles have been employed for molecular imaging application, including Au/Ag nanoparticles, upconversion nanoparticles (UCNPs), quantum dots, dye-doped nanoparticles, magnetic nanoparticles (MNPs), etc. The preparation of nanoparticle-based probe for molecular imaging routinely includes three steps: synthesis, surface modification, and bioconjugation, among which surface modification plays an important role for the whole procedure. Surface modification usually possesses the safety, biocompatibility, stability, hydrophilicity, and terminal functional groups for further conjugation. This review aims to outline the surface modification of how nanoparticles assemble to probes, focusing on the developments of two widely used nanoparticles, UCNPs and MNPs. Recent advances of different types of linkers, a core component for surface modification, are summarized. It shows the intimate relationship between chemistry and nanoscience. Finally, perspectives and challenges of nanoparticle-based probe in the field of molecular imaging are expected.