Synthesis and characterization of thermo-responsive and photo-cleavable block copolymers as nanocarriers

In this study, we synthesized thermo-responsive and photo-cleavable amphiphilic block copolymers containing photodegradable linkers as junction points between hydrophilic and hydrophobic chains. We synthesized PNiPAAm-ONB-PXCL block copolymers by using a combination of ring-opening polymerization and nucleophilic substitution reactions using 5-hydroxy-2-nitrobenzyl alcohol as a difunctional photo-responsive initiator. These PNiPAAm-ONB-PXCL copolymers consisting of soft domains of amorphous PNiPAAm and PXCL exhibited amorphous T_g. The polymer micelles exhibited dual responsiveness to heat and light. The lower critical solution temperature of PNiPAAm₂₀-ONB-PMCL₄₉ was 39.3°C, which is approximately the temperature of tumor tissue. When we exposed the polymer solutions to ultraviolet (UV) irradiation, we observed major changes in the structure and morphology of the particles. Fluorescence emission measurements indicated the release of Nile red (NR), a hydrophobic dye, encapsulated by the PNiPAAm-ONB-PXCL micelles, in response to irradiation because of disruption of the micelles. Release of indomethacin (IMC) was rapid under UV irradiation at 42°C and approximately 90% of the encapsulate IMC was released in a sustained manner during the first 7 h. The nanoparticles were associated with nonsignificant toxicity at concentrations <100 μg mL^-1. Doxorubicin (DOX)-loaded micelles facilitated the uptake of DOX by HeLa cells within 30 min of treatment, and were predominantly retained in the cytoplasm. The DOX-loaded micelles were associated with low cytotoxicity against HeLa cells.