Next, we demonstrated the utility of an encapsulated blue-to-UV system to trigger a UV-sensitive photochemical reaction in aqueous solution. Specifically, we selected a caged fluorophore with a photocleavable protecting group as a UV upconversion reporter, such that removal of the protecting group using UV photolysis results in visible color changes and measurable absorbance and fluorescence changes. The UV upconversion reporter was fluorescein bis-(5-carboxymethoxy-2-nitrobenzyl) ether, hereafter referred to as caged fluorescein (Fig. ). We hypothesized that blue incident light would photolyze caged fluorescein only when UV upconversion micelles were integrated into the solution (Fig. ). For this demonstration, we selected the ppy2/PPO sensitizer/annihilator pair due to its upconversion emission profile extending deepest into the UV spectrum (Fig. ), where caged fluorescein has high absorptivity. The fabrication of ppy2/PPO micelles was optimized to maximize upconversion counts (Fig. and SI Methods).