In the wake of the COVID-19 pandemic and a renewed focus on infection prevention, antimicrobial materials have drawn increased attention across industries. These engineered surfaces aim to kill or repel microbes, offering passive and persistent protection against pathogens in environments where cleanliness is critical.
Antimicrobial coatings (AMCs) are surface modifications or treatments that actively reduce microbial load. They can be broadly categorized into antifouling coatings, which prevent microbial adhesion and biofilm formation, and biocidal coatings, which actively kill or inhibit microbes on contact or via released agents. They can act via various mechanisms:
- Contact-killing, where immobilized antimicrobial agents destroy microbes upon touch.
- Biocide-releasing, where agents like silver ions or copper are gradually released to maintain antimicrobial activity.
- Stimuli-responsive or triggered-release, where external signals such as pH, temperature, or quorum-sensing molecules trigger the antimicrobial response.
The ideal AMC is non-toxic, long-lasting, affordable, and effective in real-world conditions, including dry environments commonly found in hospitals.
Historically, antimicrobial coatings relied on materials with known antimicrobial effects, such as silver, copper, and quaternary ammonium compounds. However, their mechanisms of action were not well understood. Advances in materials science now enable the engineering of coatings with antimicrobial and antifouling mechanisms, moving beyond traditional approaches.
Photocatalytic Coatings
A key example of this progress is in the field of photocatalytic coatings, which harness the energy in light to produce reactive oxygen species (ROS) - highly reactive molecules like hydrogen peroxide, hydroxyl radicals, and superoxide. These ROS attack essential microbial components (lipids, proteins, and nucleic acids), rendering them nonviable.
Most photocatalytic coatings are based on metal oxide semiconductors, with titanium dioxide (TiO2) being the most common. TiO2 photocatalyst coatings typically contain TiO2 nanoparticles dispersed in a liquid or solid binder matrix. When exposed to UV or visible light, TiO₂ absorbs photons, exciting electrons from its valence band to the conduction band and leaving behind positively charged “holes”. These participate in redox reactions with ambient oxygen and moisture, generating ROS at the surface. These ROS break down microorganisms and even organic pollutants, cleaning the surface.
Importantly, photocatalytic coatings do not require the release of toxic substances, and their mechanism is non-selective, which reduces the risk of resistance development. This makes them especially appealing in the face of growing antimicrobial resistance in hospitals, such as from MRSA. Moreover, the photoinduced hydrophobicity of photocatalytic coatings imparts an innate antifouling behaviour, enabling self-cleaning of dead pathogens.
Recent Innovations
Recent years have seen significant innovations in photocatalytic AMCs.
Traditional TiO2 coatings were limited by their reliance on UV light for activation, which restricted their use indoors. Advances have aimed to mitigate this problem. For example, US9669128B2 details methods for doping TiO2 with metals such as silver, copper, or platinum, and US8551909B2 discloses doping TiO2 with non-metals such as nitrogen or fluorine to reduce the bandgap to enable activation of the photocatalytic coating by visible light.
Another challenge has been wear and photodegradation of the coating matrix. Because ROS are non-selective, extended use can lead to breakdown of the coating itself, limiting the lifetime of the coating and the surface it is protecting. Newer formulations integrate stable polymeric binders and layered structures that preserve photocatalytic activity while withstanding routine cleaning and mechanical abrasion. For instance, US5616532A describes the use of thermosetting non-oxidisable binders such as porous alumina silica, silicone, siloxane, or polymers or mixtures thereof.
Today, these coatings can now be applied to a wide range of materials (e.g. glass, metal, plastic, and textiles) using techniques like spray coating, dip coating, or plasma-assisted deposition. This has improved scalability and adoption, particularly in healthcare settings. During the COVID-19 pandemic several companies rapidly deployed TiO2-based coatings in high-risk settings. Notably, Nanoksi (a Finnish company) commercialised a nano-TiO2 coating for use in airports and commercial facilities in Europe and the UAE. These coatings were applied to high-touch surfaces like counters, screens, and handrails, showing measurable reductions in microbial load. Nanoksi’s patent application EP4157529A1 details a specialised application method for spreading their photocatalyst coating involving a stabilisation agent, compressed air and deposition at an elevated temperature for more efficient and effective application.
Hybrid and Multifunctional Approaches
Advanced systems are now exploring multi-functional coatings that integrate photocatalytic agents with nanostructured surfaces, contact-active polymers, other metal nanoparticles, or stimuli-responsive hydrogels. These hybrid coatings offer tailored performance profiles that can kill, repel, and self-clean dynamically based on the microbial burden and environmental conditions.
Conclusion
While silver, copper, and quaternary ammonium compounds have historically dominated the antimicrobial coating landscape, photocatalytic coatings offer a distinct and compelling alternative. Their light-activated, non-leaching mechanisms align with growing demands for sustainable, long-lasting, and low-toxicity solutions in healthcare and other sectors. As our understanding of surface-pathogen interactions deepens and our materials science toolkit grows more sophisticated, the next generation of antimicrobial coatings will not just be reactive barriers, but smart, responsive, and environmentally conscious technologies. In this light, photocatalytic coatings may very well be a “silver bullet” – albeit not in composition, but in impact.
