Maleimide activation of photon upconverting nanoparticles for bioconjugation

Abstract

Photon upconverting nanoparticles (UCNPs) have become an important new class of optical labels. Their unique property of emitting visible light after photo-excitation with near-infrared radiation enables biological imaging without background interference or cell damage. Biological applications require UCNPs that are dispersible in water and allow the attachment of biomolecules. Oleic acid-coated UCNPs obtained by solvothermal synthesis were functionalized with both hydrophilic PEG and thiol-reactive maleimides, either by ligand exchange or by silanization. Three different types of maleimide-functionalized UCNPs were prepared and characterized by transmission electron microscopy, dynamic light scattering and Raman spectroscopy. Ligand exchange of oleic acid by maleimide-PEG-COOH yielded UCNPs that did not aggregate, were colloidally stable and reacted readily with proteins. Such luminescent labels are required for background-free imaging and many other bioanalytical applications.