Fig. 1. (A) Schematic of photochemically triggered fusion of liposomes L₁ modified with o-nitrophenyl phosphate locked hairpin units (1) with liposomes L₂ modified with nucleic acid (2). The liposome L₁ is loaded with upconversion nanoparticles, UCNPs, and with Tb³⁺-ions. Liposomes L₂ are loaded with the ligand DPA. NIR irradiation of the system, λ = 980 nm, leads to the UCNP-stimulated generation of the 365 nm internal light source that deprotects the o-nitrophenyl phosphate units and fragments the hairpin structure (1′/1′′). The (2)-functionalized liposome displaces the strand (1′′) resulting in the (1′/2) crosslinked liposome that leads to fusion. The fusion yields the fluorescent Tb³⁺–DPA complex that quantifies the full fusion efficiency. (B) Time-dependent size-changes of the liposome mixture L₁/L₂, evaluated by dynamic light-scattering upon: (a) the NIR-irradiation of the mixture of liposomes L₁ and L₂. (b) Non-irradiated mixture of L₁ and L₂ (error bars derived from N = 4 experiments). (C) Time-dependent percentage contents of the fused liposomes (or liposome fusion efficiency) corresponding to: (a) the NIR-irradiated mixture of liposomes L₁ and L₂ (liposome fusion efficiency was calculated according to the fluorescence increase corresponding to the Tb³⁺–DPA complex generated upon the full fusion of the two liposomes, see details in the Experimental section in the ESI†). (b) Non-irradiated mixture of the liposomes L₁ and L₂ (error bars derived from N = 3 experiments).