**4. Conclusions**

We have designed, synthesized and characterized a novel "three bullet" nanoconstruct based on mesoporous silica nanoparticles for potential combined cancer photo-chemotherapy. Such a multifunctional nanoplatform is able to generate 1O2 and NO under selective excitation with green and blue light, respectively, and release the noncovalently entrapped anticancer DOX under physiological conditions. A remarkable advantage of this system is the excellent preservation of the photochemical properties of the active components once simultaneously integrated in the same sca ffold and despite the copresence of the DOX. This is the result of the strategy adopted which permitted to integrate the PS and the NOPD in the outer and the inner sites of the nanosca ffold, avoiding potential and undesired intermolecular photoprocesses (i.e., energy/electron transfer) which would have otherwise precluded the final goal. This allows the individual photoactivable components to be operated either singularly or in tandem upon the appropriate choice of the excitation wavelengths. At this regard, preliminary biological results performed on A375 cells expressing ABC transporters show a potentiated activity of DOX incorporated in our "three bullet" nanoconstruct upon irradiation with blue light, due to the e ffect of the NO photoreleased. Note that the reduction of the cell viability observed under light excitation (ca. 50%) is considered e ffective in overcoming DOX-resistance in cells expressing ABC transporters [47,48], as those used in the present work. Besides its e fficacy, a plus of our approach is the absence of toxicity exerted by PS-MSNs/NOPD/DOX in the absence of irradiation. Since light can be easily controlled and directed towards tumor tissue, in a translational perspective, our approach can be considered a tumor-selective strategy that avoids undesired toxicity on nontransformed tissues and overcomes a severe limitation of currently used chemotherapy. Detailed photobiological investigations addressed to elucidate the single and the combined e ffects of all the active species produced by this novel multifunctional drug delivery system are currently under investigation in our laboratories.

**Author Contributions:** Conceptualization, A.L.T. and S.S.; Investigation, A.L.T., A.F., A.C.P.d.S., E.G., and C.R.; Methodology, C.R.; Supervision, S.S.; Writing—Original Draft, S.S.

**Funding:** This research was funded by Italian Association for Cancer Research (AIRC) IG-19859.

**Acknowledgments:** We thank S. Petralia (STMicroelectronics, Catania) for the help in TEM analysis. A.L.T. thanks UTFPR and CAPES for the financial support process no. 88881.119200/2016-01.

**Conflicts of Interest:** The authors declare no conflicts of interest.
