## 3 Results and Discussion  
### Characterization of Prepared Two Types of ZnO-NPs  
The XRD and TEM analysis results for the \(n\)ZnO prepared by two different synthesis methods are presented in Figure 1, and the key properties are also summarized in Table 1. Overall, two \(n\)ZnO was observed to exhibit different physical properties, and the detailed information on all properties for two \(n\)ZnO is provided in the Supplementary Information. Figure 2 shows the results of the zeta potential and DLS particle size of the two types of \(n\)ZnO as a function of the change in SRHA concentration and ionic strength at pH 9. Note that zeta potential represents the electrical potential of a particle measured at slipping plane, and the value is affected by many variables (e.g., salt type, salt concentration, polymer sorption, pH) [45]. In the absence of SRHA (Figure 2a,b), the zeta potential of \(n\)ZnO-1 was less negative than that of \(n\)ZnO-2 over the whole ionic strength range (0.1-10 mM),showing no difference as a function of the change in the ionic strength (0.1-10 mM); and the zeta potential of _n_ZnO-2 was formed at approximately the isoelectric point (IEP) in the whole ionic strength range (0.1-10 mM). In the presence of SRHA, the zeta potential for two types of _n_ZnO was more negative than that in the absence of SRHA, showing a consistent tendency over the whole ionic strength range (0.1-10 mM), regardless of the SRHA concentration changes (1 and 5 mg L\({}^{-1}\)). With the presence of SRHA, no difference in the zeta potential was observed between the two types of _n_ZnO. The reason for the more negative charge in the zeta potential in the presence of SRHA was that the small amount (1 and 5 mg L\({}^{-1}\)) of SHRA added in the solution was adsorbed into the _n_ZnO to accelerate the change. This has been reported previously; SRHA was adsorbed onto metal oxide NPs to negatively increase the zeta potential [19; 46; 47; 48; 49]. In terms of the two types of _n_ZnO size (Figure 2c,d), the _n_ZnO-1 size in the absence of SRHA ranges from 190 to 220 nm over the whole ionic strength range, showing little difference as a function of change in ionic strength. This trend is consistent with the DLVO interaction energy calculation between particles, which shows the favorable interaction \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Synthesis Method**} & \multirow{2}{*}{**Sample**} & \multicolumn{2}{c}{**XRD**} & \multicolumn{2}{c}{**TEM**} & \multicolumn{2}{c}{**N\({}_{2}\) Adsorption**} \\ \cline{3-5}  & & **Crystallite Size (nm) 1  & **Crystallite Phase** & **Primary Particle Size (nm) 2  & **Surface Area (m\({}^{2}\) g\({}^{-1}\)) 3 ** \\ \hline
1 & _n_ZnO-1 & 38.1 & wurtzite & 52.4 & 10.4 \\ \hline
2 & _n_ZnO-2 & 15.8 & wurtzite & 21.5 & 58.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical properties of _n_ZnO prepared by two different synthesis methods. Figure 2: Zeta potentials (**a**,**b**) and DLS particle sizes (**c**,**d**) of _n_ZnO in the absence (w/o SHRA) and presence (w/SRHA) of SRHA at different ionic strengths in both NaCl solutions at pH 9 (adjusted with 0.1 M NaOH). condition even at the lowest ionic strength tested (0.1 mM), and the same is expected at higher ionic strengths; this will be discussed in detail in the Section 3.2. In the presence of SRHA, the size was \(\sim\)180 nm or smaller, showing little difference as a function of the change in ionic strength or SRHA concentration. In the absence of SRHA, the \(n\)ZnO-2 size ranges from 220 to 330 nm over the whole ionic strength range, showing little difference as a function of the change in ionic strength. In the presence of SRHA, the size was \(\sim\)260 nm or smaller, showing little difference as a function of the change in ionic strength or SRHA concentration. Eventually, the size of the two types of \(n\)ZnO decreased slightly in the presence of SRHA.