Solvothermal synthesis yields oleic acid-coated UCLNPs with a hydrophobic surface that must be turned hydrophilic to disperse the UCLNPs in aqueous buffers – a requirement for any bioanalytical application. Furthermore, UCLNPs can only be conjugated to proteins or other biomolecules if their surface is functionalized, e.g. with thiol-reactive groups that are particularly suitable for site-specific labeling of proteins. Water-dispersible UCLNPs for bioconjugation are generated by ligand exchange of the oleic acid by a bifunctional PEG that carries a carboxylate group on one end and a thiol-reactive maleimide group on the other end. Such UCLNPs are particularly useful for any bioanalytical application that relies on low background interference. For example, in immunoassays UCLNP-antibody conjugates can be used as direct labels for the sensitive detection of analytes in raw samples. Furthermore, UCLNPs can be conjugated to small ligands that are specific for cell surface receptors. This new cell targeting approach allows for imaging thick histological samples as near-infrared light penetrates deeply into cell layers.