**4. Conclusions**

In conclusion, we have presented the results of the synthesis, the size and morphology characteristics, spectral properties and APDT activity of the aqueous solution of ZnTPP/D-g-PNIPAM/AuNPs triple hybrid nanosystem containing zinc tetraphenylporphyrin (ZnTPP) photosensitizer, thermoresponsive dextran poly (N-isopropylacrylamide) polymer (D-g-PNIPAM) and Au nanoparticles. Spectroscopic manifestations of binding of ZnTPP molecules to D-g-PNIPAM/AuNPs macromolecules have been obtained. The ZnTPP/D-g-PNIPAM/AuNPs nanohybrid has demonstrated high morphological stability (absence of the aggregation) up to 7 days after preparation. Optimal concentrations of the components of hybrid nanosystem providing the weak aggregation and the high plasmonic enhancement of electronic processes in photosensitizer molecules have been determined as following: ZnTPP—0.001 g/L, D-g-PNIPAM—0.078 g/L, Au—0.077 g/L. The shrinking of D-g-PNIPAM macromolecule at a thermally induced LCST phase transition has been revealed to lead to Au NPs and ZnTPP molecules release from the ZnTPP/D-g-PNIPAM/AuNPs macromolecule, as well as to strengthening of plasmon enhancement of the optical processes in ZnTPP molecules bound with the polymer macromolecule. The 2.7-fold plasmon enhancement of the photogeneration of singlet oxygen under excitation resonant with LSPR in Au NPs has been demonstrated for ZnTPP/D-g-PNIPAM/AuNPs system, which indicates its potential for PDT applications. The data obtained in vitro on *Staphylococcus aureus* wild strains have proved the potential of such nanosystem for rapid inactivation of microorganisms. In particular, the significant increase of PDT based bactericidal efficiency of ZnTPP/D-g-PNIPAM/AuNPs nanohybrid at the temperatures higher than LCST transition point has been observed at irradiation doses lower than 8 J/mL. This indicates that aqueous solution of ZnTPP/D-g-PNIPAM/AuNPs nanohybrid has a potential for thermally driven and controlled PDT applications at low light irradiation doses, in particular for rapid antibacterial PDT.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/nano12152655/s1, Aging effect on ZnTPP/D-g-PNIPAM/AuNPs nanosystem in aqueous solution from DLS measurements (Figure S1), Transformation of absorption spectrum of ZnTPP in water and aqueous solutions of hybrid nanosystems (Figure S2 and analysis), Transformation of FL spectrum of ZnTPP in water and aqueous solutions of hybrid nanosystems (Figure S3 and analysis), Remarks on the reabsorption effect on concentration dependence of FL, Additional information on photodynamic antibacterial activity of nanosystems in vitro (Figures S4–S8).

**Author Contributions:** Conceptualization, O.A.Y. and N.V.K.; methodology, O.A.Y., N.V.K., P.A.V. and V.A.C.; writing—original draft, O.A.Y.; writing—review & editing, O.A.Y., N.V.K., L.C. and G.N.; funding acquisition, O.A.Y., N.V.K. and G.N.; investigation, A.V.T., P.S.K., P.A.V., V.A.C., Y.I.K., A.I.M. and L.C.; formal analysis, A.V.T., P.S.K., P.A.V. and V.A.C. All authors have read and agreed to the published version of manuscript.

**Funding:** This work was financially supported by National Research Foundation of Ukraine Project (No. 2020.02/0022), Ministry of Education and Science of Ukraine Project (No. 0122U001818), Guangxi Innovation Driven Development Major Project (No. Guike AA20302013), Nanning Scientific Research and Technology Development Plan Project (No. RC20200001), "Yongjiang Plan" Project of Leading Talents of Innovation and Entrepreneurship in Nanning City (No. 2020024), Yulin City Science and Technology Transformation Project (No. 19040003), French PAUSE program for emergency welcome of Ukrainian scientists.

**Data Availability Statement:** Data are within the article and Supplementary Materials.

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