## 4 Discussion
The antimicrobial potential of TiO\({}_{2}\) nanotube arrays' surfaces has been determined for the different microorganisms, as shown in \[\text{TiO}_{2}+\text{h}\
u\rightarrow\text{e}\text{ }_{\text{CB}}{}^{-}+\text{h}_{\text{VB}}{}^{+} \tag{8}\]3 To determine whether the observed activity is related to the thickness and chemical composition of the cell wall, the microorganisms were grouped into groups reflecting the cell wall characteristics, as shown in \[\text{TiO}_{2}+\text{h}\
u\rightarrow\text{e}\text{ }_{\text{CB}}{}^{-}+\text{h}_{\text{VB}}{}^{+} \tag{8}\]4 Statistical analysis showed that the data did not deviate from normality, as a \(p\)-value of 0.075 was observed for the Shapiro-Wilk normality test. The application of one-way ANOVA using the Tuckey post hoc test showed that TiO\({}_{2}\)NT and TiO\({}_{2}\)NT-Ag surfaces were equally effective (\(p\)-value 0.999) in the removal of microorganisms, while also being significantly different for TiO\({}_{2}\)NT-S (\(p\)-value 0.003) and TiO\({}_{2}\)NT-P (\(p\)-value \(<\) 0.001). Furthermore, there were no significant differences in the observed survivability of microorganisms between TiO\({}_{2}\)NT-S and TiO\({}_{2}\)NT-P surfaces (\(p\)-value 0.336). These observations further indicate that the activity of the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**pH**} \\ \hline  & **pH 2** & **pH 3** & **pH 6** & **pH 7** & **pH 9** \\ \hline TiO\({}_{2}\)NT Surface & +ve & +ve & Neutral & -ve & -ve \\ Tannic acid & +ve & +ve & Neutral & -ve & -ve \\ Interaction & Repulsion & Repulsion & Equal & Repulsion & Repulsion \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of the charges and interactions under different pH conditions between the TiO\({}_{2}\)NT surface and tannic acid. thinner layers is inferior to that of their thicker counterparts. The difference in antimicrobial activity between TiO\({}_{2}\)NT-S and TiO\({}_{2}\)NT-P is not immediately evident, possibly indicating that the interaction of the microorganisms with the surfaces might also influence the reaction rate. The formation of organic surface layers that may form after the lysis of the cell membrane might interfere with the interaction of the other bacteria with the oxidising species. The shape of the tubes might provide preferential sites for attachment of the bacteria resulting in a fouled surface. The addition of silver was also ineffective, indicating that it was present in a sub-biocidal dose. Studies carried out by [PERSON] et al. and [PERSON] et al. [46; 47] show that biofilm formation and bacterial inactivation on titanium nanotubes are dependent on the pore diameter, surface roughness and defects. The smaller the pore diameter, the larger the amount of biofilm formed whilst the bacterial inactivation was increased with a decrease in surface defects. Furthermore, [PERSON] et al. [48] showed that the addition of 20 ppm of silver nanoparticles to titanium nanotubes eliminated 99.99% of _Staphylococcus aureus_ after 60 min under sunlight irradiation. In this study, the TiO\({}_{2}\)-NT-Ag only resulted in the removal of 58.2% of _Staphylococcus aureus_, indicating that the impregnation process used was not suitable. This could be due to the incomplete adhesion of the particles to the surface, which did not provide a suitable contact area for the formation of a Schottky barrier. To determine the difference in the susceptibility of a microorganism to TiO\({}_{2}\)-NT surfaces, these were grouped into four groups depending on the characteristic of the cell walls of different microorganisms, as shown in Figure 9, corresponding to Gram-negative bacteria, Gram-positive bacteria, yeast, and fungi. Statistical analysis showed that there was a significant difference in % survivability between Gram-positive and Gram-negative bacteria (\(p\)-value 0.015) and between fungi and yeast (\(p\)-value \(<\) 0.001). On the other hand, no significant difference was observed in the susceptibility between yeast and Gram-negative bacteria (\(p\)-value 0.998).