Fig. 10 Two-photon excited quantum dots FRET system enhanced PDT. Large two-photon absorption cross section and deep tissue penetration

Chou and co-workers coupled aluminum sulfonated phthalocyanine (AlPcS) to two-photon excited (TPE) quantum dots (QDs) to form QD-AlPcS couples for PDT(Chou et al. 2013). Because the quantum dots have a high two-photon absorption cross section (TPACS), the quantum dots can be excited by a low-power-density 800 nm unfocused femtosecond laser and then transfer the energy to the conjugated AlPcS via FRET. The QD-AlPcS coupling has a FRET efficiency of up to 90% in water. The FRET process of cellular QD-AlPcS was also observed in KB and HeLa cells under the action of two-photon excitation by 800 nm femtosecond laser, and effectively produced ¹O₂ which finally succeeded in killing cancer cells. Fowley and co-workers designed a carbon quantum dot (CQD)-protoporphyrin (IX) sensitizer coupling(Fowley et al. 2013). Under 800 nm laser irradiation, the large two-photon absorption cross section of CQDs was used to indirectly excite protoporphyrin (IX) by FRET to produce large amounts of singlet oxygen, which resulted in an 82% decrease in HeLa cell viability. In addition, mice treated with intratumoral injection of CQD-protoporphyrin (IX) conjugate showed 60% tumor suppression after 800 nm laser irradiation in a fibrosarcoma tumor-bearing mice model.