# Polymer-Stabilized Silver (Gold)-Zinc Oxide Nanoheterodimer Structures as Antimicrobials
## 3 Results
The degree of mutual influence of antimicrobial components--nanometals and zinc oxide (X and Y, respectively)--in their conjugates can be estimated by calculations of the fractional inhibitory concentration (FIC) and the fractional inhibitory concentration index (FICI). \[FIC(X)=\frac{MIC(X,Y)}{MIC(X)}\]  
where MIC(X,Y) is the MIC for X in combination with Y and MIC(X) is the MIC for X alone. \[FIC(Y)=\frac{MIC(Y,X)}{MIC(Y)}\]  
where MIC(Y,X) is the MIC for Y in combination with X and MIC(Y) is the MIC for Y alone. \[FICI=FIC(X)+FIC(Y)\]  
For our preparations, FICI values were calculated using the following equation:  
\[FICI=FIC(Me^{0})+FIC(ZnO)\text{ or }FICI=FIC(Me^{0})+FIC(Zn^{2+})\text{, where}\]  
\[FIC(Me^{0})=\frac{MIC(Me^{0})\text{ in }\text{preparation}}{MIC(Me^{0})}\]  
Me\({}^{0}\) is Ag\({}^{0}\) or Au\({}^{0}\)  
\[FIC(ZnO)=\frac{MIC(ZnO)\text{ in sample}}{MIC(ZnO)\text{ alone}}\]  
\[FIC(ZnO)=\frac{MIC(Zn^{2+})\text{ in sample}}{MIC(Zn^{2+})\text{ alone}}\]  
The FICI values are interpreted as follows: if FICI \(\leq\) 0.5, then the interaction of the active components of the system is defined as synergistic; if 0.5 < FICI < 1.0, then the interaction of the active components of the system is defined as additive [102]. According to other interpretation, FICI > 2.0 indicates antagonistic effects, while values from 0.5 to 2.0 characterize additive effects [103]. The revealed FICI values for all the studied objects and for all the strains used are mainly in the 0.5-2.2 range. This indicates a close-to-additive participation of bioactive components of the systems in the bactericidal process. These circumstances, apparently, can be explained by a similar mechanism of action the bactericides present in the samples on the cells of pathogens. The mechanism of antimicrobial action at the molecular level for composite Ag/ZnO was considered earlier [91]. It should be stressed here that the addition of ZnO NPs into the AgNP-containing system led, in most cases, to better results in comparison with the activity of the AgNPs preparations with added zinc cations (the FICI values were lower in the first case). This might be due to different mechanisms of action of zinc cations and zinc oxide, or the prolongation of the action of cations released from deposited in zinc oxide. Testing was carried out in the dark. It was previously shown that the inactivation of bacteria by zinc oxide follows different mechanisms in the dark and under illumination. In the dark, the action of Zn\({}^{2+}\) ions were manifested, while under illumination, mainly the process of photocatalytic injection of electrons took place [104]. For composites containing an EM copolymer, the joint action of the metal-containing components of the system was close to synergistic, and the FICI values were close to 0.5 for all microorganisms used. Apparently, this can be explained by the difference in the structure of the polymer chains of the copolymers used. With the additional introduction of zinc cations into the Ag/ZnO heterostructures, the FICI in terms of Ag/ZnO and Ag/Zn\({}^{2+}\) pairs practically did not change, which indicates that there is no expediency of introducing a third bactericidal component into the system.