Ultraviolet (UV) light can trigger a wide range of useful photochemical reactions for incredibly diverse applications, such as for photocatalysis, photopolymerization, or drug delivery. These applications typically require penetration of high energy photons deep into materials, yet delivering these photons beyond the surface is extremely challenging due to absorption and scattering effects. Triplet-triplet annihilation upconversion (TTA-UC) shows great promise to circumvent this issue by generating one high energy photon from two lower energy photons. However, the small organic molecules that facilitate TTA-UC usually have poor water solubility, restricting their applications in aqueous-based environments. To address this issue, we engineer a nanoencapsulation method to fabricate water-compatible UC micelles, enabling on-demand UV photon generation deep into materials. We demonstrate a range of TTA-UC sensitizers with excellent solubilities that facilitate the formation of highly emissive aqueous UV micelles. This encapsulation method is generalizable to 19 UV UC systems. As a proof-of-principle demonstration, we use upconversion micelles to successfully uncage a fluorescent dye at a focal point nearly a centimeter beyond the surface of the dye solution. This motivates further deployment of blue-to-UV upconversion systems to trigger photochemical and biological applications and reveals opportunities for spatially controlled manipulation of UV-responsive materials.

Spatially controlled UV light generation at depth by upconversion micelles

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