Near-infrared (NIR) light-activated
photosensitization represents an encouraging therapeutic method in
photodynamic therapy, especially for deep tissue penetration. In this context, two-photon activation, i.e., utilization of photons with relatively low energy but high photon flux for populating a virtual intermediate state leading to an excited state, is attractive. This concept would be highly advantageous in
photodynamic therapy due to its minimal side effects. Herein, we propose that the combination of
plasma protein serum albumin (HSA) containing several Ru complexes and NIR two-photon excitable
carbon nanodots (Cdots), termed HSA-Ru-Cdots, provides several attractive features for enhancing
singlet oxygen formation within the mitochondria of
cancer cells stimulated by two-photon excitation in the NIR region. HSA-Ru-Cdot features biocompatibility, water solubility, and photostability as well as uptake into
cancer cells with an endosomal release, which is an essential feature for subcellular targeting of mitochondria. The NIR two-photon excitation induced visible emission of the Cdots allows fluorescence resonance energy transfer (FRET) to excite the
metal-to-
ligand charge transfer of the Ru moiety, and fluorescence-lifetime imaging microscopy (FLIM) has been applied to demonstrate FRET within the cells. The NIR two-photon excitation is indirectly transferred to the Ru complexes, which leads to the production of
singlet oxygen within the mitochondria of
cancer cells. Consequently, we observe the destruction of filamentous mitochondrial structures into spheroid aggregates within various
cancer cell lines. Cell death is induced by the long-wavelength NIR light irradiation at 810 nm with a low power density (7 mW/cm2), which could be attractive for
phototherapy applications where deeper tissue penetration is crucial.