Polymers in various forms are playing exceedingly important roles in treating various diseases. Micro- to nanostructures are used to deliver therapeutics. Their biodegradability and biocompatibility are advantageous for all bio applications. The presence of various functional groups in the polymer chain is also beneficial for achieving targeted delivery. Moreover, there are several polymers that have self-healing properties.

Meanwhile, upconversion nanoparticles are capable of converting the photons of lower energy to higher energy. Hence, they can be excited with near-infrared (NIR) radiation. NIR has high tissue penetration ability. It does not harm living cells, therefore long-duration diagnosis can be carried out without worrying about cell damage. In addition, these nanoparticles can be very useful as nanoprobes and contrast agents in bioimaging processes. The synthesis of these nanoparticles and their use in surface modification has particularly interested researchers. The use of these two materials, polymer and upconversion nanoparticles, are becoming popular for disease diagnosis and treatment.

The aim of this Special Issue is to bring together original research and review articles that discuss polymers and upconversion nanoparticles for disease diagnosis and treatment.

Potential topics include but are not limited to the following:

• Current challenges of polymers, upconversion nanoparticles, and polymer-upconversion nanoparticle composites for disease diagnosis and treatment
• Polymers as universal materials for disease treatment
• Upconversion nanoparticles for bioimaging (e.g., multimodal bioimaging and brain imaging)
• Polymer and upconversion nanoparticles for designing cellular pH sensors
• Toxicity of nanoparticles