Optimisation of UV Enhanced Core–Shell Lanthanide-Doped Upconversion Nanoparticles for Integration with UV-Responsive Polymers to Achieve Optimal Drug Release Under NIR Excitation

In recent years, drug delivery systems have been suggested as an alternative to conventional cancer treatments. Among the various materials employed in controlled drug delivery systems, photo responsive polymeric nanoparticles have been paid much attention because they have several attractive features, such as their high tumor-specific targeting with low side effects and higher loading capacity. One of the few limitations of most photo responsive polymers is that they can only be triggered by ultraviolet (UV) light. However, the UV region is not only harmful to tissues and cells, but also has a disadvantage of low tissue penetration depth. In contrast, near infrared (NIR) light penetrates harmlessly through the body and can be easily directed and focused on specific sites, making it a more desirable option as an external stimulus in clinical applications.<br/>Lanthanide-ion-doped upconversion nanoparticles (UCNPs) have emerged as a new class of luminescent materials that offer excellent chemical stability, good biocompatibility, narrow bandwidth, long luminescence times, good resistance to photobleaching and photo blinking. Most importantly, UCNPs can absorb at NIR and emit in the UV-visible region. Therefore, UCNPs represent an excellent candidate as intermediate activators for UV responsive polymers.<br/>The aim of this study is to optimise the optical properties of UCNPs for their use in combination with UV responsive polymers to trigger the release of drug molecules under NIR light. Unfortunately, UCNPs are still restricted by low upconversion (UC) efficiency. In order to enhance the UV UC quantum efficiency, two strategies have been combined. Coating an inert shell has been considered as the primary approach, in which the shell can protect the optically active core from surface quenching effects. The second step was to follow a thermal annealing to the core @ shell structures in the presence of excess oleic acid (OA) to improve crystallinity and reconstruct the crystal edges of the nanoparticles.<br/>In this study, several core–shell structures were synthesized using NaYbF₄: Tm/Fe as the core template to regulate the shell growth of different NaLnF_4.By designing a core @ shell structure followed by post annealing treatment; markedly improved luminescence has been achieved at the 350 nm wavelength. A library of phenacyl monomers were synthesised and employed to form polymers via a reversible addition–fragmentation chain transfer (RAFT) polymerisation. In early-stage studies, these polymers were added to the surface of UCNPs, which resulted in a quenching of the UCNP UV emission following NIR irradiation. The future aims of the study are to incorporate drug mimics into the surface bound polymers, which will release following polymeric structural changes upon UV absorption following UCNP NIR irradiation.