Silica Coated Upconversion Nanoparticles: A Versatile Platform for the Development of Efficient Theranostics

CONSPECTUS: Next generation theranostic devices will rely on the smart integration of different functional moieties into one system. These individual chemical elements will have a variety of desired chemical and physical properties and will need to behave in a multifunctional manner. Researchers have used upconversion nanoparticles (UCNPs) as a basis for superior imaging probes to locate cancerous lesions. The features of these nanoparticles, such as large anti-Stokes shifts, sharp emission bands, long-lived luminescence, and high resistance to photobleaching, have produced versatile probes. One way to improve these probes is to add a layer of dense or mesoporous silica to the outer surface of UCNPs (UCNP@SiO2). These modified UCNPs are chemically stable and much less cytotoxic than the original UCNPs. In addition, their surface can be easily modified to introduce various functional groups (e.g., -NH2, COOH, -SH) via silanization, which facilitates conjugations with various biological molecules for multimodal imaging or synergetic therapeutics. This versatility makes UCNP@SiO2 particles excellent platforms for the construction of efficient theranostics. In this Account, we provide a comprehensive summary of recent progress in the development of UCNP@SiO2 nanocomposites for theranostics in the hope of speeding their translation into the clinic. We first discuss the major design principles and protocols for engineering various nanocomposites based on UCNP@SiO2 structures including those coated with dense silica, mesoporous silica, or hollow mesoporous silica. Next we summarize several representative efforts that probe the relaxivity mechanisms of these nanostructures as a way to optimize magnetic resonance sensitivity, multimode cancer imaging, near-infrared light-triggered chemotherapy, photodynamic therapy, and synergetic therapy (the combination of radiotherapy with chemotherapy, thermotherapy, or photodynamic therapy) using UCNP@SiO2-based theranostics. By rational integration of a wide range of features that convey multiple functions (such as imaging and therapy) into the structure or onto the surfaces of UCNP@SiO2, the constructed theranostics show promise for multimodal cancer imaging, biosensing, and effective cancer therapy. Finally, we discuss the limitations of UCNP@SiO2 nanostructures, the difficulties in the design of smart theranostics, and their potential role in clinical cancer research.