Upconversion nanoparticles (UCN) that are excited in the near infrared (NIR) region were synthesized and modified to enable their application to biological systems for imaging. The UCN obtained are oleic acid capped and hence hydrophobic in nature. Since the particles were to be used for imaging cells, a surface modification to make them hydrophilic and biocompatible was performed. Silica coating was chosen for the modification due to the possibility to further functionalize the surface and conjugate biomolecules. Cardiac cells which are capable of forming gap junctions were selected to be labeled. Gap junction specific antibodies were conjugated to the silica-coated UCN. The fluorescence emission spectrum of the particles was obtained with a continuous wave 980 nm laser and size of the particles before and after coating was determined to be 30 and 50 nm, respectively, by TEM. A covalent coupling method was used to bind the gap junction specific antibody to the nanoparticles. The fluorescence imaging experiments were carried out on cardiomyoblast cells and co-culture of bone marrow stem cells/cardiomyoblast cells after incubation with the antibody-modified UCN. Images of the particles after incubation with cardiac cells obtained over days demonstrated the potentials of the UCN for fluorescence imaging.