Photocatalytic Antimicrobials

We’re always on the lookout for new ways to use visible light. This month, we recommend a fascinating in-depth review on Photocatalytic Antimicrobial Materials. The work in question entitled “Photocatalytic Antimicrobials: Principles, Design Strategies, and Applications” appears in Chem Reviews by Xiaojun Peng and coworkers from Shenzhen University (Ref 1). The topic itself is vast and the review is wonderfully in depth. The authors discuss applications in areas of “Local infection therapy, personal protective equipment, water purification, antimicrobial coatings, wound dressings, food safety, textiles, [and] air purification”. If you have the same preconceptions as we do that this topic is limited to shining UV light on some surfaces as a disinfectant, then you’ll be surprised at how much visible light photocatalysis has to offer this field.

Leaving aside the inequalities of cost and availability, we live in a world where small molecule antibiotics are abundant. So much so that antibiotic overuse has led to significant problems with drug-resistant bacteria. Drug resistant microbes such as methicillin- resistant (MRSA) are a significant public health crisis. Add in the challenges and differences needed to selectively treat bacteria, viruses, fungi, protozoa, algae, and researchers are in a constant search for new drugs to treat new threats and stay ahead of drug-resistance. An alternate approach expressed in the present review is photocatalytic antimicrobial therapy (PCAT). Idealistically, treating infections with a low-cost administration of a photocatalyst and mild selective light source in a localized area portends a green low-cost precision treatment avoiding damage to healthy tissues. A key feature of this approach? A mechanism of action where photocatalysts are locally producing reactive oxygen species to efficiently kill microorganisms is not believed to lead to resistance in pathogens.

Photocatalysis has found use in many, many diverse areas. We’ve written about more than a few of them. Most rely on the simple idea of finding compounds capable of absorbing light and then finding a way to harness that energy for some purpose. And it’s no surprise that PCAT is no different. Activate a nontoxic photocatalyst with a harmless wavelength of light suitable for the catalyst’s band gap to simultaneously generate strong oxidants and reductants. In this case, we’re looking to oxidize a water molecule to form a hydroxyl radical while an oxygen molecule gets reduced to form superoxide radicals and then we’re off to the races with these and additional reactive oxygen species (ROS). Generate these active oxidants in the presence of bacteria cell walls, organelles and other functional biomolecules and you initiate a process that ultimately leads to cell death.

How visible-light photocatalysts actually kill microorganisms; however, is often a mixed bag. We’ll spare the details discussing the differences between treating different types of microorganisms and instead focus on a few general trends. The authors define 4 ways that a photocatalyst can lead to cell/virus deactivation that need to be understood when determining a mechanism of action.

Physical damage: The direct interaction between the sharp edge of the catalysts and a bacterial cell membrane can lead to damage. Positive or negative interactions influence the type of catalyst that should be chosen for the microbe of interest. The authors discuss the effect of catalysts based on flat graphene and graphene oxide and their interactions on cell walls.

Metal releasing ions: Release of metal from the photocatalyst materials into the can pass through the cell wall and interact directly with biomolecules. Examples with Ag+, Zn2+ and Cu2+.

Chemical oxidation: The traditional idea. Generation of ROS destroy to cell function.

Other species: One additional topic discussed are materials that are designed to selectively release CO release upon initiation by light.

An extensive discussion of the types of photocatalytic antimicrobials (PCAM) that are often used in PCAT are categorized and described in detail. The materials are broken down into metal oxides, metal sulfides, carbon materials (graphene, carbon nanotubes), metal nanoparticles, polymeric semiconductors and the always necessary “other”. We thought we would focus on a few examples that caught our eye with visible light.

The first example we want to highlight is a catalyst based on perylene diimide(PDI), a catalyst commonly used for photoredox catalysis and Sn3O4 heterostructures (Ref 2). The PDI self-assembles onto Sn3O4 nanosheets forming what the authors describe as a “hook-and-loop” sticky surface that can attract bacteria through π- π interactions. The complex structure can absorb light over the full visible spectrum and generates ROS to kill MRSA in wounds of mice.

Our second highlight are copper nanoparticles that have been applied as a coating to surgical masks (Ref 3). Clearly a project inspired by our global COVID nightmare, the coating both increased the hydrophobicity of the masks, exhibited photocatalytic antimicrobial activity and allowed the masks to be reusable. When exposed to solar illumination, the temperature of the photoactive antiviral mask (PAM) rapidly increased to >70 °C while also generating free radicals to disrupt the membrane nanosized virus particles.

Finally, since we are contractually required to highlight porphyrins whenever present in a paper. Our final highlight is a porphyrin doped MOF (metal organic framework) (Ref 4). The material called PB@MOF combines a porphyrin as a photosensitizer into a Prussian Blue metal organic framework. The catalyst promotes singlet oxidation upon illumination at 660 nm. The compound exhibited excellent antibacterial effects on E. Coli and S. aureus.

There are truly many more interesting examples that we could highlight but we’ll leave that up to you. If you want an in-depth discussion on materials and their use as antimicrobials we suggest that you check out the review in November’s Chem Reviews.

As we wind down 2023, we’ve started to compile our list of favorite photochemistry papers from the year.

Send us your favorites at [email protected]

 

 

References:

(1) Ran, B.; Ran, L.; Wang, Z.; Liao, J.; Li, D.; Chen, K.; Cai, W.; Hou, J.; Peng, X. Photocatalytic Antimicrobials: Principles, Design Strategies, and Applications. Chem. Rev. 2023. https://doi.org/10.1021/acs.chemrev.3c00326.

(2) Yang, R.; Song, G.; Wang, L.; Yang, Z.; Zhang, J.; Zhang, X.; Wang, S.; Ding, L.; Ren, N.; Wang, A.; et al. Full solar-spectrum- driven antibacterial therapy over hierarchical Sn3O4/PDINH with enhanced photocatalytic activity. Small 2021, 17, 2102744. https://onlinelibrary.wiley.com/doi/abs/10.1002/smll.202102744

(3) Kumar, S.; Karmacharya, M.; Joshi, S. R.; Gulenko, O.; Park, J.; Kim, G. H.; Cho, Y. K. Photoactive antiviral face mask with self- sterilization and reusability. Nano Lett. 2021, 21, 337-343. https://pubs.acs.org/doi/pdf/10.1021/acs.nanolett.0c03725

(4) Luo, Y.; Li, J.; Liu, X.; Tan, L.; Cui, Z.; Feng, X.; Yang, X.; Liang, Y.; Li, Z.; Zhu, S.; et al. Dual metal-organic framework heterointerface. ACS Central Sci. 2019, 5, 1591-1601. https://pubs.acs.org/doi/10.1021/acscentsci.9b00639

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