## 3 Results and Discussion
### Synthesis of BSA-Imprinted Thin Films  
In the proposed molecular imprinting method, an external imprinted film was synthesized over PVC@TAA-BSA during the second-stage SI-PIMP with AAm and MBAA as the functional monomer and crosslinker, respectively. As shown in Figure S1 (ESI +), two significant bands at 1731 and 1638 cm\({}^{-1}\), corresponding to the C=O stretching modes of MBAA and AAm, respectively, indicated the successful growth of covalently bonded polymers (MBAA) on the surface [42]. Further, the peak at 1208 cm\({}^{-1}\), corresponding to the C=S stretching vibration of the dithiocarbamate group, is also observed in Figure S1  
Figure 4: FT\(-\)IR spectra of PVC@TAA and PVC@TAA\(-\)BSA nanoparticles. (ESI +). In addition, the XPS spectrum analysis showed a new binding energy of 288.8 eV (Figure S2e, ESI +), which could have been derived from the O=C-NH bonds, indicating the deposition of a poly(AAm-co-MBAA) thin film on the surface of the substrate. The S 2p core-level spectrum (Figure S2g, ESI +) consisted of the S 2p\({}_{3/2}\) and S 2p\({}_{1/2}\) doublet at approximately 161.6, 162.8, 163.5, and 164.9 eV [43], which could be attributed to the dithiocarbamte groups at the polymer surface, revealing that the imprinted thin films were successfully polymerized over the substrate surface by SI-PIMP. In addition, peaks corresponding to C-N\({}^{+}\) (Figure S2e, ESI +), and C-N\({}^{+}\) (Figure S2f, ESI +) showed no signal in the corresponding core-level spectrum, confirming that the absence of residual TAA groups on the surface could endow the imprinted films with a weak non-specific binding property. The size and surface morphology of the PVC@TAA-MIP nanoparticles were examined using SEM and TEM. As can be seen in Figure 3d, the PVC@TAA-MIP had a rough surface as a result of the cross-linking reaction and removal of the template proteins by the elution solution. In addition, the TEM image of PVC@TAA-MIP (Figure 3f) shows that the unified ultrathin polymer film had a thickness of approximately 15 nm. This suggests that the imprinting effect may be further enhanced by controlling the shell layer thickness via the SI-PIMP reaction. The specific surface areas (Table S1, ESI +) of PVC@TAA-MIP and PVC were determined to be 104.59 and 83.41 m\({}^{2}\cdot\)g\({}^{-1}\), and the average pore diameters were 9.57 and 14.63 nm, respectively. PVC@TAA-MIP had a significantly large specific surface area, which might have been due to the large number of imprinted cavities located on the outside surface. Furthermore, the average pore diameter of the PVC@TAA-MIP satisfied the release and rebinding transfer requirements of the template. Based on the above discussion, the successful preparation of a PVC@TAA-MIP core-shell structure using an immobilized template technique and surface-initiated polymerization could be confirmed by the complete characterization provided through SEM, TEM, BET, FT-IR, and XPS analyses. Additionally, after polymerization, the supernatant had no clear signal for BSA, as measured by UV after centrifugation, suggesting that the BSA was entirely immobilized during the polymerization process without significant leakage. These observations further proved that applying an immobilization template via the anion exchange affinity interaction approach could effectively construct binding sites within the imprinted films. Controlling the imprinted film thickness is critical for forming properly imprinted cavities with high specific recognition ability. A prolonged irradiation time results in a thick imprinted film, which in part prevents the washing of the template protein from the imprinted film and inhibits the formation of binding cavities after polymerization [44]. The thickness of the imprinted film estimated by TEM increased linearly with the irradiation time (R\({}^{2}\) = 0.9897, slope = 0.1552 \(\pm\) 0.0112 nm min\({}^{-1}\), Figure 5). The results indicated that SI-PIMP could proceed from the dithiocarbamte groups on the substrate surface.