Mesoporous silica is a versatile support widely applied in biomedical and catalysis field because of its desirable properties. In this work, mesoporous silica was first utilized as support for photodynamic therapy (PDT) applications. PDT is achieved by the excitation of a photosensitizer (PS) with visible light at certain wavelengths, and then the excited PS may either drive electron-transfer reactions to/from biological molecules (type I mechanism) or, alternatively, transfer its energy to molecular oxygen, thus generating singlet oxygen (1O2, type II mechanism).
Methylene blue (MB) is a known photosensitizer with desirable ability to generate singlet oxygen. The main active species is the monomeric form. However, this dye tends to self-aggregate to form dimers and higher aggregates, which are less active for 1O2 generation. Two different strategies (one-pot and post-synthesis) were designed to incorporate methylene blue into silica nanoparticles (NPs). The design of the synthesis is a key parameter to avoid both leaching of the dye and its self-aggregation. The negatively charged silica matrix allows the incorporation of the dye, which is cationic, with no leaching, whereas the presence of phenyl functions in the matrix favors the monomeric form of MB. We observe that both dimeric and monomeric forms of MB can generate 1O2 species in aqueous solution when confined inside a silica matrix, improving the activity compared to the bare dye in solution.
Matrix effect in 1O2 generation was then investigated with a series of mesoporous silica NPs of 80-100 nm diameter with various morphologies. The procedure of preparation of silica NPs was optimized considering the requirements of high colloidal stability in aqueous media for the application in PDT. Zwitterion functionalization and lipid-coating of silica NPs were attempted to improve colloidal stability but these two strategies did not meet the expectation. The results showed that the colloidal stability of all the silica NPs kept in aqueous solutions was better than that of the silica NPs previously dried after surfactant extraction and then redispersed in water. The incorporation of MB was then performed on aqueous surfactant-free samples using a fixed molar ratio between MB and silica. Water and methanol were used as solvents for 1O2 generation tests, and all the samples displayed photocatalytic activity.
Initial exploratory tests on photodynamic activity were carried out with the MB-functionalized nanoparticles, using A549 lung cancer cell lines. The killing effect is slightly improved when compared to the free methylene blue at the same concentration.
Preliminary antibacterial tests were also performed with positive Staphylococcus (S.) aureus and negative Pseudomonas (P.) aeruginosa. The results showed that silica itself and MB-containing samples had antimicrobial activity against S. aureus.
Finally, the mesoporous silica systems here prepared were also used as a support for Ru complexes, to build heterogeneous catalysts for different organic transformations. Preliminary heterogeneous catalytic tests were conducted on alcohol coupling reactions of the borrowing hydrogen type and also on photoactivated alcohol oxidation. The ruthenium functionalized nanoparticles show in some cases improved efficiency when compared to the analogous homogeneous catalysts.
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