Fig. 2. Absorption (a) and luminescence emission (b) spectra of L123 during red-light irradiation (630 nm) of liposomes functionalized with 3²⁺. Dashed line: spectrum at t = 0; black solid line: spectrum at t = 180 minutes; grey lines: spectra measured every 15 minutes. (c) Difference in absorbance at 490 nm, after baseline correction, during red-light irradiation (630 nm) of L123 (black filled circles), a 1 : 1 volume mixture of L12 and L3 (dark-grey filled squares), and L23 (light-grey filled diamonds). Error bars represent standard deviation of three independent experiments. (d) Integrated upconversion emission (400–600 nm, black filled circles) and integrated sensitizer emission (750–900 nm, empty circles) during red-light irradiation of L123. Error bars represent standard deviation of three independent experiments. Irradiation conditions: power 120 mW, beam diameter 4 mm, intensity 0.95 W cm^–2, T = 310 K, sample volume 1.5 ml, 8% of sample volume simultaneously irradiated. Liposome dispersions used in these experiments were prepared as in Table 1, and then diluted with PBS buffer prior to measurement so that every time [1] = 0.25 μM, and [2] = 2.5 μM. The bulk concentration of 3²⁺ was experimentally determined with ICP-OES and was 19 ± 1 μM, 21 ± 3 μM, and 20 ± 2 μM for L123, L23, and L3, respectively.