Upconversion nanoparticles (UCNPs) are a class of inorganic fluorophores that follow the anti-Stokes mechanism, to which the wavelength of emission is shorter than absorption. This unique optical behavior generates relatively long-lived intermediate energy levels of lanthanides that stabilize the excitation state in the fluorescence process. Longer-wavelength light sources, e.g., near-infrared (NIR), penetrate deeper into biological materials such as tissue and cells that provide a larger working space for cell biology applications and imaging, whereby UCNPs have recently gained increasing interest in medicine. In this report, the emission intensity of a gadolinium-based UCNP was screened by changing the concentrations of the constituents. The optimized condition was utilized as a luminescent nanoprobe for targeting the mitochondria as a distinguished subcellular organelle within differentiated neuroblastoma cells. The main goal of this study is to illustrate the targeting process within the cells in a native state using modified UCNPs. Confocal microscopy on the cells treated with the functionalized UCNPs indicated a selective accumulation of UCNPs after immunolabeling. To tackle the insolubility of as-synthesized particles in water-based media, the optimized UCNPs were surface-coated with polyamidoamine (PAMAM) dendrimers that due to peripheral amino groups are suitable for functionalizing with peptides and antibodies. Ulti-mately, we concluded that UCNPs are potentially versatile and ideal tools for NIR bioimaging and capable of making adequate contrast against biomaterials to be detectable in electron microscopy (EM) imaging.