# Innovative Antibacterial Polymer Coatings
## 2 Antibacterial Coatings
### Photoactive Antibacterial Coatings
#### 2.4.2 Photoactive Coatings Containing Photosensitizers  
The use of visible light to generate ROS could simplify the application of light-activated antibacterial coatings indoors, particularly in healthcare facilities. Photosensitizers (PS) present an interesting alternative to TiO\({}_{2}\) because their photoactive antibacterial effects can be triggered by illumination with visible light [70]. The incorporation of these compounds into disinfection coatings is inspired by photodynamic therapy (PDT), which is applied in the potential treatment of tumors, skin infections, and other diseases [71; 72; 73]. Photosensitizers used in PDT typically consist of heterocyclic ring structures, as shown in Figure 8. These compounds exhibit low toxicity in the absence of light but produce cytotoxic species upon photoactivation in the presence of oxygen [74]. When exposed to light of the appropriate wavelength, photosensitizer (PS) molecules shift from their ground state to a singlet excited state and subsequently to a longer-lived triplet excited state (Figure 9). In the triplet state, PS molecules can interact with molecular oxygen through two mechanisms [75]. The first mechanism, known as Type 1, involves the transfer of electrons or hydrogen between the excited photosensitizer (PS) and a neighboring molecule, resulting in the formation of free radicals. These free radicals then react with oxygen, leading to the production of reactive oxygen species (ROS) such as superoxide, hydroxyl radicals, and hydrogen peroxide. The second mechanism, referred to as Type 2, involves the electronic transfer of energy to oxygen, which generates the highly reactive singlet oxygen state [75]. The generation of singlet oxygen via the \"Type 2\" reaction is believed to target cancer cells, whereas the ROS produced in \"Type 1\" reactions are similarly destructive to cells [76; 77]. The development of light-activated antibacterial surfaces with included photosensizers allows for continuous and effective self-cleaning [78; 79; 80] and could play an important role in reducing nosocomial infection cases. These surfaces would inhibit the transfer cycle of microorganisms from patients to healthcare workers and subsequently to surfaces, thereby improving routine cleaning practices [65]. The use of photosensizers is advantageous because they provide a non-specific attack on nearby microorganisms, making the development of bacterial resistance unlikely [64]. Light-Activated Antibacterial Surfaces Based on Porphyrin  
Porphyrin-based compounds exhibit photobactericidal activity against pathogenic microorganisms when irradiated with visible light. Photobactericidal materials have been created by grafting porphyrin-based compounds onto nylon fibers [80], alkylated cellulose [81], poly(methyl methacrylate) [82; 83], or covalently bonding them to polyethylene terephthalate [84]. Light-activated antimicrobial materials on the base of protoporphyrin IX  
Figure 8: Commonly used photosensizer compounds in anticancer and antimicrobial PDT. Figure 9: Schematic illustration of photodynamic therapy, including the Jablonski diagram. Adapted from ref. [77] with permission from Frontiers. and zinc-protoporphyrin IX grafted onto nylon fibers have been created. They effectively photoinactivate _S. aureus_ as the sample containing zinc protoporphyrin IX exhibited greater antimicrobial efficacy [80]. These porphyrins have also been grafted onto nylon films, though the resulting photoactive antimicrobial activity was not recorded [85]. Porphyrin compounds have been covalently bonded to cellulose polymers via esterification reactions to create photoactivated bactericidal films [81]. When exposed to visible light, thus synthesized films exhibit good bactericidal properties against _S. aureus_ and _E. coli_[81]. The use of photoactivated bactericidal materials in healthcare settings could help minimize the spread of bacterial contamination, particularly from Gram-positive bacteria [80]. A recent study developed a photoactive antibacterial polymer coating using protoporphyrin.