Are silicone-supported [C60]-fullerenes an alternative to Ru(ii) polypyridyls for photodynamic solar water disinfection?

Abstract

Different photosensitizing materials manufactured by immobilizing (0.5–3.0 g m⁻²) tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) (RDP²⁺), [C₆₀]-fullerene, or 1-(4-methyl)-piperazinylfullerene (MPF) on porous neutral (pSil) or surface-modified anionic (pSil⁻) poly(dimethylsiloxane) are compared on the grounds of their singlet molecular oxygen (¹O₂) production and photodynamic solar water disinfection capability. The C₆₀-based sensitizers display a broad weak absorption in the visible and strong absorption in the UV, while absorption of light by RDP²⁺ supported on pSil is strong in both the UV and blue regions. The ¹O₂ emission lifetimes (τ_Δ) determined for RDP²⁺ and MPF on porous silicone materials under air are similar (40–50 μs) and correspond to the decay of ¹O₂ generated by sensitizers dissolved in the polymer support. In contrast, τ_Δ measured for C₆₀ in pSil is similar to that observed for MPF or RDP²⁺ when immobilized at low loading on pSil, but dramatically increases up to 5 ms if C₆₀ aggregates are formed in the porous material as evidenced by microscopy evaluation. The photosensitizing properties of the dyes, together with their electrical charge and the overall charge of the porous silicone-based materials, lead to highly different sunlight-driven bacteria inactivation efficiencies, as tested with waterborne E. faecalis. RDP/pSil provides efficient disinfection by photosensitization unlike MPF/pSil, which leads to reduced bacteria inactivation rates due to poorer ¹O₂ production. C₆₀/pSil and MPF/pSil⁻ materials, despite their ¹O₂ photogeneration, show unsuccessful waterborne bacteria inactivation due to the negative surface charge of fullerene aggregates in contact with water, and to the net negative charge of the pSil⁻, respectively.