Triplet-Triplet Annihilation Upconversion Based on Silica Nanoparticles

Photon upconversion based on triplet-triplet annihilation (TTA) composed of organic photosensitizer and emitter, has attracted widespread attention due to its unique photophysical properties and enormous applications in photovoltaic cells, photocatalysis, bio-imaging, and photodynamic therapy.Particularly, in biological systems, long-wavelength excitation light can efficiently reduce the interference of background fluorescence and increase the penetration depth of biological tissue, thereby avoiding the use of high-energy excitation light and reducing the damage to biological samples.However, most of the upconversion dyes based on TTA mechanism are water-insoluble organic compounds, which greatly limits their application in the biological field.Herein we synthesized a TTA upconversion system based on silica nanoparticles, which can achieve upconversion emission in water.Specifically, the photosensitizer (fluorinated tetraphenylporphyrin platinum) and the emitter (siloxane derivatized 9,10-diphenylanthracene) for photon upconversion were designed and synthesized, whose upconversion performance in dichloromethane solution was firstly studied by UV-Vis spectrophotometer and fluorescence spectrometer.Clear blue upconversion emission from emitter could be observed when the photosensitizer was excited by 532 nm laser.The triplet energy transfer efficiency between photosensitizer and emitter is 60%.The optimal ratio of photosensitizer to emitter was 1∶40.Based on this ratio, the stable upconversion silica nanoparticles with uniform size in water were constructed by micellar template method.The average diameter characterized by transmission electron microscopy (TEM) is 15.5 nm and the hydration diameter characterized by dynamic light scattering (DLS) is 22.5 nm.When the 532 nm laser is used as the excitation source, the upconversion emission in water was achieved.Their upconversion luminescence lifetime and quantum yield are 12 μs and 0.8%, respectively.Finally, the upconversion mechanism in silica nanoparticles was studied.The upconversion intensities in silica nanoparticles show quadratic and first-order dependences on the incident intensity in the low and high excitation intensity ranges respectively, proving a triplet-triplet annihilation mechanism.