Antimicrobial photodynamic inactivation in nanomedicine: small light strides against bad bugs

Abstract

The relentless advance of drug-resistance among pathogenic microbes, mandates a search for alternative approaches that will not cause resistance. Photodynamic inactivation (PDI) involves the combination of nontoxic dyes with harmless visible light to produce reactive oxygen species that can selectively kill microbial cells. PDI can be broad-spectrum in nature and can also destroy microbial cells in biofilms. Many different kinds of nanoparticles have been studied to potentiate antimicrobial PDI by improving photosensitizer solubility, photochemistry, photophysics and targeting. This review will cover photocatalytic disinfection with titania nanoparticles, carbon nanomaterials (fullerenes, carbon nanotubes and graphene), liposomes and polymeric nanoparticles. Natural polymers (chitosan and cellulose), gold and silver plasmonic nanoparticles, mesoporous silica, magnetic and upconverting nanoparticles have all been used for PDI.

Keywords: bacteria; carbon nanomaterials; fungi; liposomes; photodynamic therapy; photoinactivation; photosensitizer; titania photocatalysis; upconversion.

Figure 1. Jablonski diagram

Ground state photosensitizer (⁰PS) absorbs light to form first excited singlet state (¹PS) that (in addition to losing energy by fluorescence or conversion to heat) undergoes intersystem crossing to form the long lived first excited triplet state (³PS). The triplet state can undergo type 1 (electron transfer) photochemical reaction to form superoxide and hydroxyl radical, and/or type 2 (energy transfer) photochemical reaction to form singlet oxygen. These ROS can oxidatively damage and kill all known forms of microorganism. hν_abs: Absorbed light; hv_em: Emitted light; IC: Internal conversion; ISC: Intersyetem crossing; ROS: Reactive oxygen species; PS: Photosensitizer.

Figure 2. Gram-positive and Gram-negative cell walls

(A) Gram-negative bacteria have a double lipid bilayer (inner and outer membrane) separated by periplasm and peptidoglycan. The outer membrane contains porins and lipoproteins and is decorated with lipopolysaccharide chains with a negative charge. (B) Gram-positive bacteria have a single lipid bilayer surrounded by a thick but porous layer of peptidoglycan, with teichuronic and lipoteichoic acids providing a negative charge.

Figure 3. Nanoparticles that have been covalently modified with photosensitizers

(A) Dendrimers conjugated to PS. (B) Macromolecules with biotargeting properties such as antibodies. (C) Lipid-conjugated PS self-assemble into liposomes. (D) Solid nanoparticles can be conjugated to PS. PS: Photosensitizer.

Figure 4. Noncovalent encapsulation of photosensitizers

Polymeric nanoparticles are sub-µm colloidal particles designed to solubilize hydrophobic photosensitizer. They include: (A) nanomicelles in which amphiphilic copolymers with hydrophobic and hydrophilic blocks self-assemble to entrap the cargo; (B) nanocapsules, in which the cargo is in solution and surrounded by a shell-like wall; (C) nanospheres, in which the cargo is dissolved, adsorbed or dispersed throughout the matrix, attached to the surface or attached to the polymer matrix; and (D) liposomes in which an amphiphlic polymer self-assembles into a lipid bilayer that forms a unilamellar vesicle that encapsulates the cargo [50].

Figure 5. Titania photocatalysis

Schematic illustration of main processes in the photocatalytic reaction of TiO₂. Nanoparticles have a sufficiently large surface area to allow this process to be efficient. Electrons are excited by UVA light from the semiconductor valence band to the conductance band. The electrons in the conductance band undergo electron transfer to oxygen to form superoxide, and the holes in the valence band react with water to form hydroxyl radicals. The ROS produced (O2^·− and HO^·) can kill microorganisms. C_B: Conduction band; E_g: Energy gap; UVA: Ultraviolet A; V_B: Valence band.

Figure 6. Fullerenes used in antimicrobial photodynamic therapy

(A) The tri-cationic C60 fullerene BF6. (B) The deca-cationic C60 fullerene, C60[>M(C₃N₆⁺C₃)₂]. (C) The deca-cationic C70 fullerene, C70[>M(C₃N₆⁺C₃)₂].

Figure 7. Carbon nanotubes

(A) Multiwalled carbon nanotubes. (B) Single-walled arbon nanotubes.

Figure 8. Titanium dioxide coated carbon nanotubes

The vertical array of multiwalled carbon nanotubes acts as a filter for bacterial retention while TiO₂ acts as a bactericidal photocatalyst.

Figure 9. Graphene-loaded photosensitizers

(A) Hydrophobic photosensitizer can be ‘sandwiched’ between graphene sheets by π–π stacking. Examples of photosensitizer that have been loaded on to graphene are: (B) HPPH; (C) hypocrellin a; (D) methylene blue. HPPH: 2-(1-Hexyloxyethyl)-2-devinyl pyropheophorbide-a.

Figure 10. Chemical structure of antimicrobial photosensitizers

(A) 5-[4-(1-dodecanoylpyridinium)]-10,15,20-triphenyl-porphyrin; (B) m-tetrahydroxyphenyl-chlorin; (C) chlorin(e6); (D) hypericin; (E) methylene blue; (F) Rose Bengal; (G) CNC-Por(1); (H) toluidine blue O; (I) 5,15-bisphenyl-10,20-bis(4-methoxycarbonylphenyl)- porphyrin] platinum (t-PtCP); (J) Zn-phthalocyanine.

Figure 11. Naturally occurring biopolymers used to form nanoparticles

(A) Chitosan. (B) Cellullose.

Figure 12. Gold nanoparticle-conjugated photosensitizer

(A) Gold nanoshell encapsulating a PS. (B) Plasmonic gold nanoparticle. The local electric field caused by conductance electrons potentiates the optical field close to the surface and increases the fluorescence or photoactivity of an attached PS. PS: Photosensitizer.

Figure 13. Magnetic nanoparticle-conjugated photosensitizer

Consists of a magnetite core (Fe₃O₄) coated by a biologically compatible layer such as PEG, and the PS are covalently attached by flexible linkers. PS: Photosensitizer.

Figure 14. Upconverting nanoparticle-mediated photodynamic therapy

Nanoparticles made of rare earth salts such as NaYF4 absorb continuous wavelength 980-nm light and emit 490-nm light that can excite a conjugated PS. PS: Photosensitizer.