Photosensitizer-loaded nanoparticles: characterization and encapsulation efficiency

Amphiphilic diblock copolymers, methoxy poly(ethylene glycol)/poly(ε-caprolactone) (PECL), were synthesized by ring-opening polymerization of ε-caprolactone initiated by the hydroxyl groups of methoxy poly(ethylene glycol). The prepared PECL diblock copolymers were characterized using Fourier transform infrared spectroscopy, proton (¹H) nuclear magnetic resonance spectroscopy, gel permeation chromatography, differential scanning calorimetry and wide-angle X-ray diffraction. Nanoparticles of these copolymers were prepared by the double-emulsification solvent evaporation method and were used to load methylene blue (MB) as a model drug photosensitizer. Scanning electron microscopy micrographs revealed that the unloaded nanoparticles were nearly spherical and had particle sizes ranging from 263 to 346 nm with relatively uniform size distribution. It was found that the hydrophobic block length of PECL nanoparticles was a crucial factor in controlling the size of MB-loaded nanoparticles and the encapsulation efficiency. The MB-loaded nanoparticles were irregular and not uniform and had a larger size that ranged from 304 to 367 nm. Moreover, MB encapsulation efficiency increased by about 30% with reducing content of the hydrophobic poly(ε-caprolactone) in PECL copolymer nanoparticles. The amphiphilic PECL nanoparticles in this study offer potential as alternative nanomaterial vehicles for MB aimed at photodynamic therapy applications.