Effect of the phenyl substituent’s position on the encapsulation of porphyrins inside lignin nanoparticles: Photophysical and antibacterial properties

Abstract

Porphyrins are well studied molecules that are widely used in Photodynamic Therapy, aiming for the eradication of tumours, but also for the eradication of bacteria. The formulation of porphyrins is a crucial factor when developing photodynamic applications, due to the limited stability and low solubility of porphyrins in aqueous media. Our research group has previously demonstrated that porphyrins, and especially 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin are easily encapsulated inside acetylated lignin spherical nanoparticles, while conserving their main photophysical features. In the present study we encapsulated 5,10,15,20-tetrakis(3-hydroxyphenyl)porphyrin, the precursor of the well-known photosensitizer Temoporfin, in acetylated lignin nanoparticles, yielding a stable formulation while keeping the photophysical properties of the encapsulated porphyrin. This formulation was tested as a photodynamic antibacterial agent against Staphylococcus aureus CIP 76.25 and Escherichia coli CIP 53.126, where it efficiently eradicated S. aureus, although also showing non-neglectable dark toxicity against the bacterial cells. In conclusion, the 3-hydroxyphenyl substituted porphyrin yielded nanoparticles physically and photophysically similar to those prepared with its isomer. However, the change in the position of the phenyl substituent seems to increase the dark toxicity of the nanoparticles, a phenomenon that still needs to be explored.