Rare earth metal-doped upconversion nanoparticles (UCNPs) are emerging as a new class of biomedical imaging materials due to their higher energy anti-Stokes shift, high optical penetration depth and long term repetitive imaging. In the present study, upconversion nanoparticles based on NaYF4 doped with thulium (Tm) and ytterbium (Yb) were prepared via a thermolysis method using oleic acid as a capping agent and 1-octadecene as a solvent. The X-ray diffraction pattern of the synthesized nanoparticles was found to match the standard hexagonal phase. The nanoparticles were coated with silica using tetraethyl orthosilicate (TEOS) and in order to avoid agglomeration, IGEPAL CO-520 was used as the surfactant. The coatings of SiO₂ over NaYF₄ were confirmed by the TEM image and XRD pattern. NaYF₄@SiO₂ was further functionalized by the addition of (3-aminopropyl)trimethoxysilane (APTMS) followed by either D-glucose or 2-deoxy-D-glucose (2-DG). UCNPs-D-glucose and UNCPs-2DG were examined for cell viability (MCF-7 cells) by MTT assay. The cellular uptake of UCNPs in MCF-7 cells was seen in terms of emission of a blue light. Furthermore, the uptake rate of UCNPs coated with 2-deoxy-D-glucose was found to be much faster than that of UCNPs alone under D-glucose starved conditions. The functionalization of UCNPs with 2-deoxy-D-glucose (2-DG) not only increased the uptake of nanoparticles, but also blocked the glycolysis pathway resulting in the inhibition of tumor growth as 2-deoxy-D-glucose (2-DG) is mimicking the D-glucose. The results are indicative that these upconversion nanoparticles may find applications in bioimaging, removal of tumor by precision surgery, therapy and targeted drug delivery.