Figure 4.

(a) Energy diagram of ZnS:Mn nanoparticles excited by multiphoton absorption. Two-photon absorption of ZnS (600 nm) or Mn²⁺ (NIR-II region) induces orange emission. Three-photon absorption of ZnS also induces orange emission. (b) Two-photon image of FITC and three-photon image of ZnS:Mn nanoparticles. Three-photon imaging showed better spatial resolution. (c) Transmission spectra of tumor tissue and blood. The NIR-II region exhibits improved transmission. (d) PL intensity of ZnS:Mn nanoparticles at different excitation wavelengths. For comparison, Rhodamine 6G is measured at 900 nm. S is the slope of the linear fitting. Between 900 and 1000 nm, the excitation mechanism is a three-photon absorption, and between 1100 and 1180 nm, the excitation mechanism is switched to two-photon absorption. The efficiency of PL is highest when excited at 1180 nm. Reprinted with permission from refs (42), (97), and (101). Copyright 2013 Macmillan Publishers Limited and 2013 American Chemical Society.