Photodynamic Inactivation of Staphylococcus aureus Biofilms Using a Hexanuclear Molybdenum Complex Embedded in Transparent polyHEMA Hydrogels

Abstract

Three new photoactive polymeric materials embedding a hexanuclear molybdenum cluster (Bu4N)2[Mo6I8(CH3COO)6] (1) have been synthesized and characterized by means of Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and emission spectroscopy. The materials are obtained in the format of transparent and thin sheets, and the formulations used to synthesize them are comprised of 2-hydroxyethyl methacrylate (HEMA), as a polymerizable monomer, and ethylene glycol dimethacrylate (EGDMA) or poly­(ethylene glycol)­dimethacrylate (PEGDMA), as cross-linkers. All the polymeric hydrogels generate singlet oxygen (1O2) upon irradiation with visible light (400–700 nm), as demonstrated by the reactivity toward two chemical traps of this reactive species (9,10-dimethylanthracene and 1,5-dihydroxynaphthalene). Some differences have been detected between the photoactive materials, probably attributable to variations in the permeability to solvent and oxygen. Notably, one of the materials resisted up to 10 cycles of photocatalytic oxygenation reactions of 1,5-dihydroxynaphthalene. All three of the polyHEMA hydrogels doped with 1 are efficient against S. aureus biofilms when irradiated with blue light (460 nm). The material made with the composition of 90% HEMA and 10% PEGDMA (Mo6@polymer-III) is especially easy to handle, because of its flexibility, and it achieves a notable level of bacterial population reduction (3.0 log10 CFU/cm2). The embedding of 1 in cross-linked polyHEMA sheets affords a protective environment to the photosensitizer against aqueous degradation while preserving the photochemical and photobactericidal activity.

Photodynamic Inactivation of Pseudomonas aeruginosa by PHEMA Films Loaded with Rose Bengal: Potentiation Effect of Potassium Iodide

“…Synthesis of PHEMA films was carried out by thermally initiated radical polymerisation ( AIBN as initiator) as previously reported [ 51 ]. The corresponding amount of RB disodium salt (from 0.005 to 0.1% wt.)…”

“…The polymeric hydrogels were synthesized according to the following procedure [ 51 ]: Rose Bengal disodium salt (from 0.005 to 0.1% wt., relative to the total mass of monomers and crosslinker) was dissolved in a mixture of HEMA : ATAC : PEGDMA (85:5:10 wt.%, respectively). Then, AIBN was added and dissolved (1 wt.% relative to the total mass of monomers and crosslinker).…”

“…The evolution of the photoreactions was monitored over time by means of UV−vis absorption spectrophotometry (decrease of absorbance at 377 nm). The kinetic traces were fitted to the following pseudo-first order model [ 51 ]: ln (A/A 0 ) = −k obs · t, where A 0 is the initial absorbance of DMA , A is the absorbance of DMA at time t, and k obs is the observed kinetic constant.…”

“…However, for the method described here, the final concentration of the photosensitizer inside the matrix is exactly the initial concentration before polymerization. As mentioned in the Introduction, we have successfully used this strategy for entrapping a variety of optically active molecules, from aromatic ketones [46,47,50] and xanthenic dyes [49], to flavylium cations [44,45] and molybdenum clusters [51], all retaining their properties after the thermal polymer formation. The chosen internal proportion of the acrylic polymerizable molecules HEMA:ATAC: PEGDMA was 85:5:10 (in wt.%).…”

“…Since 2005 we have designed and synthesized tailored coloured or emissive PHEMA hydrogels for various sensing applications [ 44 , 45 , 46 , 47 , 48 , 49 , 50 ], and more recently we have encapsulated a photoactive hexanuclear molybdenum complex in a PHEMA matrix. It has been found to be active against Staphylococcus aureus after irradiation with visible light [ 51 ]. As a continuation of such work, we are now reporting the encapsulation of the photosensitizer Rose Bengal ( RB ) into a matrix of cross-linked cationic PHEMA ( RB @PHEMA) displaying photo-antibacterial properties against Gram-negative bacteria P. aeruginosa in planktonic suspension.…”