Cancer theragnostics, the integration of diagnostic and therapeutic modalities, has emerged as a promising strategy for personalized medicine. In this study, we present the design and characterization of upconversion core-shell nanoconstructs tailored for advanced cancer theragnostics. The nanoconstructs consist of a core of upconversion nanoparticles, which efficiently convert near-infrared (NIR) light into higher energy emission, surrounded by a versatile shell with multifunctional components. The upconversion core facilitates deep tissue penetration and minimizes background signal, making it an ideal candidate for imaging in the NIR window. Additionally, the core-shell architecture allows for the incorporation of various functional components, such as targeting ligands for specific cancer cell recognition, therapeutic agents for localized drug delivery, and imaging probes for real-time monitoring. The synthesis and optimization of these nanoconstructs were carried out systematically, ensuring their biocompatibility and stability under physiological conditions. In vitro and in vivo studies demonstrate the efficient targeting of cancer cells, triggered drug release, and simultaneous imaging, providing a comprehensive platform for theragnostics. The versatility of the nanoconstructs enables customization for different cancer types, optimizing the therapeutic efficacy while minimizing off-target effects. Furthermore, the presented upconversion core-shell nanoconstructs exhibit enhanced photothermal and photodynamic therapy capabilities, offering a synergistic approach to cancer treatment. The combination of imaging, targeted drug delivery, and therapeutic interventions within a single nanoconstruct underscores the potential of these multifunctional platforms in advancing precision medicine for cancer patients. Overall, our study showcases the potential of upconversion core-shell nanoconstructs as a versatile and effective tool for cancer theragnostics, paving the way for the development of tailored and personalized approaches in the diagnosis and treatment of various malignancies.