A hydrogel containing lanthanide-based nanoparticles releases proteins inside the gel when hit with near-infrared light (J. Am. Chem. Soc., DOI: 10.1021/ja308876j). The nanoparticle-studded gel could enable doctors to cue the release of protein drugs deep inside a patient’s tissues, the developers say. Yue Zhao of the University of Sherbrooke, in Quebec, and Neil R. Branda of Simon Fraser University, in British Columbia, wanted to make hydrogels responsive to near-IR light, which is safer and can penetrate deeper into tissue than ultraviolet light. They impregnated a hydrogel with nanoparticles composed of a NaYF4 core doped with thulium and ytterbium and wrapped with a NaYF4 shell. The hydrogel is a cross-linked web of polyacrylamide and polyethylene glycol, held together by photoresponsive o-nitrobenzyl groups. When exposed to near-IR light, the nanoparticles emit UV light through a process called upconversion. The UV light then triggers the nitrobenzyl groups to cleave the polymer chains. The team tested their gel by filling it with a fluorescently labeled protein. After about 50 minutes of irradiation with near-IR light, almost 70% of the protein escaped from the gel.