Abstract :
[en] Singlet oxygen-based photocatalytic oxygenation reactions have emerged as an efficient technology to synthesize value-added organic molecules[1]. There are various methodologies for the production of singlet oxygen, among which the most popular involves a photoinduced electronic energy transfer from an excited state of a catalytic photosensitizer (PS) to triplet oxygen (3O2)[2].
The amino acids tyrosine, tryptophan, methionine, histidine and cysteine constitute one of the most relevant families among the photooxidizable biological substrates due to their areas of high electron density because of double bonds or sulfur moieties. Of particular interest is the photooxidation of methionine (Met) to methionine sulfoxide (MetO).
In general, sulfoxides are frequently used in organic synthesis, pharmaceutical science, biochemistry and material science. However, the classical methods to oxidize sulfides to sulfoxides present a high risk of overoxidation to sulfones. In particular, MetO is a particularly valuable synthetic intermediate with applications ranging from peptide sciences, material sciences, to organic synthesis.
PSs are commonly organic dyes bearing a (hetero)aromatic core such as Rose Bengal, Methylene Blue, Erythrosin B, porphyrins, phtalocyanines, and related tetrapyrroles. Rose Bengal (RB) is a popular photosensitizer that has been widely utilized for the production of 1O2 upon visible light aerobic irradiation[3].
The functionalization of mesoporous silica nanoparticles (MSNs) with PSs has attracted great research attention during last years to design efficient nanoplatforms to be used in nanomedicine as drug delivery systems or in targeted photodynamic cancer therapy (PDT) owing to their biocompatibility, high RB loading capacity and ease of surface functionalization, but its use as catalyst support in photooxygenation reactions is still a challenge.
Regarding the use of RB under continuous-flow photocatalytic conditions and the configuration of the flow setup, three main strategies emerge from the literature based on the use of (a) homogeneous PSs, (b) packed-bed photoreactors with heterogeneous PSs embedded on the packing material and (c) heterogeneous PSs concomitantly fed with the substrate. Although each strategy comes with assets and drawbacks, the use of free flowing heterogeneous PSs is supposedly the most interesting option, as far as, besides their efficiency, (a) they (a) are readily prepared, (b) do not accumulate within the micro/mesoreactor channels or cause clogging and (c) are easily removed downstream.
RB can be protected from photobleaching during important light exposures by a mesoporous SiO2 structure[4]. Synthesis of MSNs was made following well-known methods and the immobilization of RB at the surface of SiO2 by a covalent bond was carried out by two coupling agents: 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and (1-[Bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxide hexafluorophosphate) (HATU). A complete characterization of RB@MSNs was made by BET, Tg-MS, UV-Vis and TEM.
Finally, the influence of key parameters like RB concentration and the liquid flow were studied as a function of the coupling agent used in a microfluidic system. The efficiency of methionine photooxygenation as well as the photobleaching of the dye was followed by Proton Nuclear Magnetic Resonance (1H NMR).
This work converts MSNs in interesting support for new and clean heterogeneous continuous-flow photooxygenations. Taken together, our results show that heterogenization of PS for 1O2 production in microreactors is possible by grafting RB into MSNs by a covalent bond. This attachment is able to avoid the photobleaching of the dye and allows separating the solid support with the PS from the reactor effluent and reutilizing the PS. The potential of these NPs to be used as heterogeneous catalyst for photooxygenation of methionine has been demonstrated.
