## 3 Results and Discussion
### Surface Immobilization of Template Proteins  
The immobilization of template proteins was monitored using FTIR and XPS. In the spectrum depicted in Figure 4, PVC@TAA-BSA displays two new peaks at 1732 and 1713 cm\({}^{-1}\), corresponding to the C=O stretching vibration of the non-ionized and ionized carboxyl groups, respectively [39]. The new peak at 1636 cm\({}^{-1}\) corresponds to an amide I band, whereas that at 1561 cm\({}^{-1}\) is assigned to an amide II band. The bands at 1543 and 1508 cm\({}^{-1}\) are designated to belong to different in-plane ring vibrations of the tyrosine groups in BSA [40]. The XPS spectrum for the immobilization layer was composed of C 1s and N 1s peaks (Figure S2, ESI \(\dagger\)). After immobilization, two new peaks at binding energies of 288.6 and 400.1 eV were observed, which were attributed to the carboxyl (O=C-O) and amide/amino (O=C-NH/NH\({}_{2}\)) groups in BSA, respectively. More importantly, the peak  
Figure 3: SEM and TEM images of **(a)** PVC, **(b)** PVC@TAA, **(c,e)** PVC@TAA-BSA, and **(d,f)** PVC@TAA-MIP.  
of C-N+ at 286.0 eV (Figure 2e) shifted to a lower binding energy (285.7 eV, Figure S2b, ESI +), which could be attributed to the formation of ionic bonds via anion exchange affinity-based interactions between the carboxyl groups in the BSA and TAA groups at the PVC@TAA surface.  
SEM and TEM further confirmed the immobilization of the templates. As observed from the SEM images of PVC@TAA nanoparticles (Figure 3c), the morphology of PVC@TAA-BSA exhibited a distinct structure, demonstrating a rough surface. This indicated that the template proteins were successfully immobilized on the surface of the PVC@TAA nanoparticles. According to the TEM images of PVC@TAA nanoparticles (Figure 3e), the thickness of the immobilized layer of BSA was approximately 7 nm (dry state), which was close to the BSA molecular dimensions (14 \(\times\) 4 \(\times\) 4 nm) [41]. This result confirmed the near-monolayer coverage of the immobilized layer. Our synthesis procedure utilized a low BSA concentration to control the immobilized layer thickness and ensure monolayer BSA immobilization on the PVC@TAA surface.  
BSA solutions with various concentrations were used to immobilize the template proteins in PBS (pH 7.0, 10 mM). The \(\zeta\) potentials of the PVC@TAA-BSA in PBS were determined using a Zetasizer Nano Z instrument. The immobilization capacity of BSA was calculated according to Equation (1), and the results are summarized in Table S2 (ESI +). The \(\zeta\) potentials and immobilization capacities of PVC@TAA-BSA exhibited significant variation with an increase in the concentration of the BSA solution. Remarkably, the \(\zeta\) potential of PVC@TAA-BSA was reversed from positive to negative (+6.28 \(\pm\) 0.21 mV to -11.29 \(\pm\) 0.55 mV) when the concentration of the BSA solution changed from 3.6 to 4.8 \(\upmu\)M. This result confirmed that the surface of PVC@TAA was entirely covered, providing an immobilization capacity of 0.992 \(\pm\) 0.0363 \(\upmu\)mol per g of PVC@TAA. Thus, a 4.8 \(\upmu\)M BSA solution was employed to prepare the subsequent imprinted polymers. In contrast, the PVC@OH nanoparticles, as a platform to immobilize BSA via hydrogen bonding interactions, exhibited a low immobilization capacity (0.132 \(\pm\) 0.0091 \(\upmu\)mol per g of PVC@OH). This suggested that the high capacity of PVC@TAA corresponded to robust anion exchange affinity-based interactions.