title: 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.
authors.affiliation: Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg, D-93040 Regensburg, Germany.
authors.forename: Raphaela B
authors.initials: RB
authors.lastname: Liebherr
authors.forename: Tero
authors.initials: T
authors.lastname: Soukka
authors.forename: Otto S
authors.initials: OS
authors.lastname: Wolfbeis
authors.forename: Hans H
authors.initials: HH
authors.lastname: Gorris
mesh_terms: D000818:Animals; D002417:Cattle; D003102:Colloids / Q000737:chemistry; D049408:Luminescent Agents / Q000737:chemistry*; D053758:Nanoparticles / Q000737:chemistry* / Q000648:ultrastructure; D036103:Nanotechnology; D009829:Oleic Acids / Q000737:chemistry*; D017785:Photons; D011092:Polyethylene Glycols / Q000737:chemistry; D013438:Sulfhydryl Compounds / Q000737:chemistry; D005719:gamma-Globulins / Q000737:chemistry
chemical_list: D003102:Colloids; D049408:Luminescent Agents; D009829:Oleic Acids; D013438:Sulfhydryl Compounds; D005719:gamma-Globulins; D011092:Polyethylene Glycols