## 3 Results and Discussion
### Adsorption Behavior of GO-MIPs for 4-MEC
#### 3.2.2 Adsorption Isotherms  
The 4-MEC recognition ability of GO-MIPs was further investigated by the static absorption experiments. As shown in Figure 5a, the equilibrium adsorption capacity (\(Q_{e}\)) of 4-MEC increased quickly with the increase of the initial concentration at first, and then increased slowly until saturation. The maximum adsorption capacity of the 4-MEC onto the GO-MIPs was measured as 23.22 mg g\({}^{-1}\), which was about 3 times higher than that of the control non-imprinted GO-NIPs (7.53 mg g\({}^{-1}\)). On the basis of the specific recognition  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**Samples**} & \multirow{2}{*}{\(Q_{e,\text{exp}}\) (mg g\({}^{-1}\))} & \multicolumn{3}{c}{**Pseudo-First-Order**} & \multicolumn{3}{c}{**Pseudo-Second-Order**} \\ \cline{3-8}  & & \(k_{1}\) & \(Q_{e}\) (mg g\({}^{-1}\)) & \(\mathbf{R^{2}}\) & \(k_{2}\) & \(Q_{e}\) (mg g\({}^{-1}\)) & \(\mathbf{R^{2}}\) \\ \hline GO-MIPs & 22.36 & 0.0285 & 18.690 & 0.9393 & 0.00159 & 26.455 & 0.9927 \\ \hline GO-NIPs & 7.39 & 0.0261 & 4.759 & 0.9805 & 0.00789 & 8.157 & 0.9934 \\ \hline \end{tabular}
\end{table}
Table 1: Kinetic constants of the pseudo-first-order equation and pseudo-second-order equation for 4-MEC on GO-MIPs and GO-NIPs.  
Figure 4: Dynamic adsorption (**a**), pseudo-first-order kinetic for adsorption (**b**), and pseudo-second-order kinetic for adsorption (**c**) of GO-MIPs and GO-NIPs.  
of the imprinting cavities, GO-MIPs possessed a better adsorption ability to 4-MEC. The isotherm models of [PERSON] and [PERSON] were applied to describe the interaction between the target 4-MEC and GO-MIPs, and the applied linear model could be calculated using these equations [40]:  
\[\frac{C_{e}}{Q_{e}} =\frac{1}{Q_{m}K_{L}}+\frac{C_{e}}{Q_{m}} \tag{4}\] \[\ln Q_{e} =\frac{1}{n}\ln C_{e}+\ln K_{F} \tag{5}\]  
In Equations (4) and (5), \(C_{e}\) (mg L\({}^{-1}\)) is the equilibrium concentration of 4-MEC in solution, \(Q_{e}\) (mg g\({}^{-1}\)) is the adsorption capacity of 4-MEC at equilibrium, \(Q_{m}\) (mg g\({}^{-1}\)) is the maximum adsorption capacity, \(K_{L}\) (L mg\({}^{-1}\)) is the Langmuir constant, and \(K_{F}\) and \(n\) are the Freundlich constants. As described in Figure 5b, plotting \(C_{e}/Q_{e}\) against \(C_{e}\) gives a straight line, and the values of \(Q_{m}\) and \(K_{L}\) can be obtained from the slope and intercept. In addition, the values of \(K_{F}\) and \(n\) also can be obtained from the intercept and slope of the linearized plot of ln\(Q_{e}\) vs. ln\(C_{e}\) (Figure 5c). The calculated coefficients and constants were summarized in Table 2. As we know, the Langmuir isotherm model presumes an ideal adsorption process in which all the absorbing sites are homogenous and the adsorption is restricted only up to the mono-layer, while the Freundlich isotherm model describes the multi-layer adsorption of the adsorbate onto a heterogeneous surface [40]. The Freundlich model (R\({}^{2}\) = 0.9842) is more adequate in describing the adsorption process than the Langmuir model (R\({}^{2}\) = 0.9772) for 4-MEC adsorption onto GO-MIPs, suggesting that the adsorption in this study can be considered as multi-layer sorption. When the value of \(n\) is higher than 1, this indicates a favorable adsorption process [41]. In contrast, Langmuir isotherm (R\({}^{2}\) = 0.9768) is concluded to be a better fit to describe the adsorption activity of GO-NIPs compared to Freundlich isotherm (R\({}^{2}\) = 0.9489), which is probably because mono-layer adsorption mainly relies on the non-selective action of the surface and the fact that there is no selective binding point inside the GO-NIPs.