Antimicrobial Photodynamic Inactivation Using Topical and Superhydrophobic Sensitizer Techniques: A Perspective from Diffusion in Biofilms

Abstract

This review describes nanoparticle and dye diffusion in bacterial biofilms in the context of antimicrobial photodynamic inactivation (aPDI). aPDI requires the diffusion of a photosensitizer (Sens) into the biofilm and subsequent photoactivation of oxygen for the generation of reactive oxygen species (ROS) that inactivate microbes. Molecular diffusion in biofilms has been long investigated, whereas this review is intended to draw a logical link between diffusion in biofilms and ROS, a combination that leads to the current state of aPDI and superhydrophobic aPDI (SH-aPDI). This review should be of interest to photochemists, photobiologists and researchers in material and antimicrobial sciences as is ties together conventional aPDI with the emerging subject of SH-aPDI.

Figure 1.

Schematic comparison of aPDI and SH-PDI. Antimicrobial photodynamic inactivation (aPDI) with the administration of photosensitizer to the biofilm is followed by irradiation in the presence of oxygen. This approach requires the diffusion of photosensitizer into the biofilm as a precursor to the formation of reactive oxygen species. In contrast, superhydrophobic antimicrobial photodynamic inactivation (SH-aPDI) has airborne singlet ¹O₂ delivered to the biofilm with the Sens isolated from the biofilm. The tips of the SH surface do not contain photosensitizer, and either can touch the biofilm or be used at a short ~0.5 mm distance away from it.

Figure 2.

Photodynamic inactivation of biofilm bacteria starts with a photosensitizer (sens) in the ground state (S₀) that is excited by light to reach its first excited singlet state (S₁, e.g., porphyrin or phthalocyanine Q-band) or second excited singlet state (S₂, e.g., Soret band, not shown). These singlet excited states can decay by fluorescence or undergo intersystem crossing to reach the triplet state (T₁). The Sens in its triplet state can undergo a type I reaction (formation of oxygen radicals or radical ions) or transfer energy to ³O₂ thereby producing ¹O₂ in a type II reaction. Both type I and type II processes contribute to aPDI, although SH-aPDI is exclusively type II due to gas phase delivery which radicals cannot access.

Figure 3.

Superhydrophobic surface partially embedded with Si-phthalocyanine (Si-Pc) infused solgel particles A) schematic of a water droplet poised on the surface with inset showing formation of singlet oxygen and diffusion into the droplet; B) photograph of a water droplet poised on the superhydrophobic surface; C) scanning electron micrograph of the polydimethylsiloxane posts partially embedded with Si-Pc particles. “Adapted with permission from Aebisher, D.; Bartusik, D.; Liu, Y.; Zhao, Y.; Barahman, M.; Xu, Q.; Lyons, A. M.; Greer, A. Superhydrophobic Photosensitizers. Mechanistic Studies of ¹O₂ Generation in the Plastron and Solid/Liquid Droplet Interface, J. Am. Chem. Soc., 2013, 135 (50), pp 18990–1899. Copyright (2013) American Chemical Society."

Figure 4.

Schematic representation of a nanoparticle that can be used for multiple applications. 1) Targeting agent attached to the nanoparticle, 2) light activation done by a suitable wavelength, 3) fluorescent molecules for imaging, 4) photodynamic therapy by generation of reactive oxygen species (ROS), and 5) controlled released of synergistic agents for delivery of a high therapeutic payload.