Novel UV-activated biofunctionalization of up-converting nanocrystals for detection of proteins

Lanthanide doped nanocrystals capable to emit higher energy photons under excitation with lower energy radiation are promising for a broad range of applications including biodetection, biosensing, and bioimaging. However, the adaptation of these nanoparticles to the biological environment that requires good water-solubility, stability and ease of further functionalization still remains a challenge. The application of nanoparticles for biodetection or in various assays encountered many difficulties arising mainly from the strong tendency of nanoparticles for aggregation or nonspecific binding. Here we present a new method to obtain soluble and stable in water-based buffers NaYF4:Yb3+Er3+ nanocrystals with modified surface ready for further conjugation with biomolecules. In the presented approach polyvinylpyrrolidone/vinyl alcohol copolymer (PVP/VA) with photo-activatable linker (N-5-Azido-2-nitrobenzoyl group—ANB-NOS) was used for initial coating due to its high non-covalent affinity to nanoparticles surface. Subsequent coating with aminated dextran by ultraviolet light activation of ANB-NOS was carried out. This step has a significant impact on nanocrystals stability in the physiological buffer as well as on protein conjugation. Conjugation of biomolecules was possible by employing another photo-linker—sulfosuccinimidyl 4,4′-azipentanoate (sulfo-SDA). Bacterial Protein G has been selected to create a universal immune-imaging agent due to its ability to bind Fc fragment of most mammalian immunoglobulins. Moreover, the direct attachment of antibodies to nanoparticles was also examined. The activity of biofunctionalized nanocrystals was tested in immunoblot test, that confirmed preserved activity of attached molecules and lack of undesired nonspecific binding or precipitation on the assay membrane. Graphic abstract