Near-infrared light-triggered drug release from UV-responsive diblock copolymer-coated upconversion nanoparticles with high monodispersity.

The preparation of a new near-infrared (NIR) light-responsive nanocarrier for controlled drug release is demonstrated. Upconversion nanoparticles (UCNPs) were coated with an amphiphilic diblock copolymer through surface-initiated atom transfer radical polymerization, in which the inner block is hydrophobic, ultraviolet (UV)-sensitive poly(4,5-dimethoxy-2-nitrobenzyl methacrylate) (PNB), and the outer block is hydrophilic poly(methoxy polyethylene glycol monomethacrylate) (POEG). The resulting polymer/UCNP nanocarrier is thermally stable in water over a wide temperature range (5-70 °C) and is uniform in size (120 nm hydrodynamic diameter, polydispersity index <0.1). The diblock copolymer self-assembly on the surface of each UCNP occurs in aqueous solution, which allows encapsulation of antitumor drugs like doxorubicin (DOX) by the hydrophobic "micelle-like" core of PNB surrounding the NIR-sensitive UCNP. Under 980 nm laser exposure, the UV light emitted by the single UCNP is absorbed by the PNB inner layer, which results in cleavage of o-nitrobenzyl groups and formation of carboxylic acid groups. The increasing hydrophilicity of the diblock copolymer resulting from the NIR light-triggered photochemical reaction can thus disrupt the nanocarrier and leads to the release of DOX molecules. This diblock copolymer self-assembly-based approach to constructing NIR light-responsive nanocarriers of well-defined structures is general and offers possibilities for photocontrolled drug delivery.