In Vitro and Ex Vivo Investigation of the Effects of Polydopamine Nanoparticle Size on Their Antioxidant and Photothermal Properties: Implications for Biomedical Applications

Polydopamine (PDA) is a polymer that derives from the self-polymerization of the biomolecule dopamine. It can be easily synthesized to obtain spherical nanoparticles (PDNPs), tunable in terms of size, loaded cargo, and surface functionalization. PDNPs have been increasingly attracting the attention of the research community due to their elevated versatility in the biomedicine field, for their excellent ability to encapsulate drugs, to convert near-infrared (NIR) radiation into heat, and to act as an antioxidant agent. Size is an important aspect to be considered, especially concerning the specific intended field of application. This work aims at investigating how changes in the size of PDNPs affect the nanoparticle properties relevant for biomedical applications, especially focusing on cancer nanomedicine. A library of differently sized PDNPs (from 145 to 957 nm) has been obtained by varying the ammonia/dopamine molar ratio during the synthesis procedure, and detailed characterization in terms of biocompatibility, cell internalization, antioxidant capacity, and photothermal conversion has been carried out. Experiments showed that nanoparticles with a larger diameter display higher NIR absorbance, superior resistance to degradation, and higher photothermal conversion capacity (the latter confirmed by a mathematical model). On the other hand, a reduction in diameter size induces both improved antioxidant properties and enhanced cellular uptake. Herein, we provide a useful tool, allowing one to choose the proper size of PDNPs tailored for specific biomedical applications.