# Innovative Antibacterial Polymer Coatings
## 2 Antibacterial Coatings
### Photoactive Antibacterial Coatings
#### 2.4.2 Photoactive Coatings Containing Photosensitizers
The process consisted of several steps: (i) First, a catechol-based cationic adhesive (P(mDOPA)-co-P(DMAEMA+)) was applied to a stainless steel substrate. This adhesive serves as a universal primer, ensuring strong adhesion to the substrate; (ii) next, a nanogel made from Pox(mDOPA), which contains o-quinones in its side chain along with silver nanoparticles, was added as a middle layer. This layer promotes the covalent attachment of modified protoporphyrin, while the presence of silver nanoparticles enhances antibacterial activity; (iii) finally, an ethylene diamine derivative of protoporphyrin IX (PPIX-ED) was deposited as the top layer. The resulting photoactive coating exhibited excellent mechanical properties and demonstrated strong bactericidal effects against the Gram-positive B. subtilis and the Gram-negative _E. coli_[86]. An innovative amino-modified protoporphyrin IX, with and without incorporated silver nanoparticles (PPIX-ED/AgNPs), was developed and used as the final layer to create photoactive antibacterial coatings, following the same methodology [87]. Light-Activated Antibacterial Surfaces Based on Phenothiazze Dyes  
Photosensitizers such as methylene blue, toluidine blue O, and Bengal rose have also been used to develop photoactivated bactericidal materials [78; 79; 88; 89; 90]. Methylene blue, which has been reported to induce cell death and photodisinfection of _Pseudomonas aeruginosa_ biofilm on silicone tubing [91], has been included into polymeric resins, and the antibacterial activity of the resulting copolymer films was determined against Gram-positive and Gram-negative bacteria [92]. The composition of copolymer films, the concentration of the available photosensitizer, and the applied light dose impact antibacterial activity. The photoactivated antimicrobial activity of methylene blue-containing styrene-butadiene films was greater than that observed for corresponding polyacrylate ester films. Generally, higher concentrations of the photosensitizer and increased light energy resulted in more effective photosensitization of bacteria [92]. Additionally, the observed antibacterial activity of the photobactericidal films was more effective against the Gram-positive _S. epidermidis_ than those of Gram-negative _E. coli_. The photosensitive dye toluidine blue O (TBO) has been included in cellulose acetate, making it suitable for surface application [78; 79; 93; 94]. The cellulose acetate-based coatings containing toluidine blue O demonstrate promising light-activated antimicrobial activity against MRSA (methicillin-resistant _S. aureus_) and _Pseudomonas aeruginosa_. Further studies have explored the use of a combination of photosensitizing dyes, Bengal rose and TBO, incorporated into cellulose acetate to optimize light absorption in the visible spectrum [79; 93; 94]. This combination enhances the demonstrated lethal photobactericidal activity. Research has also demonstrated the preparation of photoactive antibacterial polymer coatings applied to a stainless steel (SS) substrate using water-based precursors. The coating is created through the layer-by-layer application of aqueous solutions, which include an anchoring layer based on a cationic polymer with pendant catechols, a nanogel decorated with ortho-quinones, and a photosensitizer of the aminoacridine type [95]. 2.4.3. Enhancing the Antibacterial Properties of Coatings Containing Photosensitizers  
The antibacterial efficacy of porphyrins and certain dyes can diminish after extended exposure to irradiation, primarily due to the photobleaching. This limitation may affect their suitability for long-term applications. Photobleaching can be prevented by coupling these dyes with metal nanoparticles or carbon materials [70; 82]. The presence of additional antibacterial agents, such as silver nanoparticles, gold nanoparticles, and others, can improve the stability and effectiveness of photosensizers (PS). Moreover, in these cases, a synergistic effect between two antibacterial agents is expected [96; 97].