# Cotton Terry Textiles with Photo- and Bio-Activity in a Model Study and Real Conditions
## 4 Discussion

Table 6: _Cont._declared by the manufacturers and obtained from the DLS analysis may be related to the different measurement techniques used. The analysis of SEM micrographs of modified fabric has shown that the described padding-squeezing method of Terry fabric modification allows the obtainment of NPs randomly distributed on the fibers surface, mainly in nanoscale size with a contribution of submicro-particles. Energy dispersive X-ray spectroscopy (EDX) was used to identify the elements present on the fabric surface. The determined amount of Zn and Ti atoms on the surface of the modified cotton Terry towels corresponds well with the composition of the finishing bath, as was confirmed by EDX analysis. It was observed that the modification process affects the fabric surface mass and time of absorption. Increases in surface mass and decreases in absorption time are probably connected with the deposition of NPs into fiber surfaces. The photodegradation of blackcurrant juice staining on the surface of diagonally woven fabric modified with ZnO and N-dopped TiO\({}_{2}\) NPs under UV/VIS irradiation was confirmed. Most semi-conductors, such as ZnO and TiO\({}_{2}\), are activated by light only in the UV region, which limits their use in textiles for outdoor application. For indoor applications, where artificial light is predominantly in use, only visible light responsive photocatalysts can be used. For instance, N-doping of TiO\({}_{2}\) NPs extends the spectral response of photocatalysts to visible light [12; 19; 20]. ZnO and TiO\({}_{2}\) deposited in this study onto fiber surfaces show synergic photocatalytic effects. The authors are aware that this type of modification can cause the random distribution of both types of semiconductors on the fibers surface. However, preliminary research results indicate the synergic activity of both semiconductors, but this topic is beyond the scope of this publication. In model conditions of the photocatalytically modified textiles, low and medium biostatic activity (A = 0.4-1.62) was observed towards the five tested microorganisms. Their sensitivity was ranked in the following order: _B. subtilis, S. aureus, E. coli, C. albicans_ and _A. niger_. Hence, higher sensitivity was demonstrated for Gram-positive bacteria than for Gram-negative ones, which complies with the studies by [PERSON] et al. [47] and [PERSON] et al. [48]. Other authors state that Gram-negative bacteria are more sensitive to photocatalysis than Gram-positive ones, which results from the much thinner structure of the cell membrane (fewer layers of peptidoglycan) in Gram-negative bacteria [4; 6; 49]. In the majority of papers, as confirmed by the present model studies, yeast and mold are indicated as organisms that are less sensitive to photocatalysis than bacteria, which can be attributed to the different structure of the cell envelopes and the formation of resistant spores by mold [7]. The discrepancies between the literature and the present data concerning bacterial sensitivity to photocatalysis may result from different model conditions (different photocatalysts, lighting and microclimatic conditions used in the tests). Consequently, various processes can take place, whose mechanisms are responsible for the microorganism growth inhibition effect. In the present model studies, the direct impact of biocides in the form of TiO\({}_{2}\) and ZnO on the microorganisms was the main biological mechanism on the material surface. The present model studies were carried out under high relative humidity conditions (RH = 80%), with no access to light (incubation in a climate chamber), which determines the biological effect by metal ion penetration into the microorganism cells, multi-dimensional action through binding Ti\({}^{2+}\) and Zn\({}^{+}\) cations to proteins and DNA, inhibition of protein biosynthesis and, consequently, inhibition of metabolism. The mechanism of such activity upon direct contact of the microorganisms with Ag, Zn and Ti metal nanoparticles has been described in the literature [50; 51].