## 2 Results and Discussion
### Characterization of Blm Salt-Based Nanogels and Their Thermo-Responsive Behavior  
In our previous studies [29], it has been demonstrated that the structure of IL-based monomers play a key role in the one-step synthesis of highly cross-linked nanogel. When 1-vinyl-3-(2-methoxy-2-oxyl ethyl) imidazolium chloride was copolymerized under the same conditions, the obtained particles in the submicrometer precipitated from the solvent during the polymerization process. However, when 1,4-butanediolyl-3,3'-bis-1-vinyl imidazolium bromide was copolymerized, the as-prepared nanogels  
Figure 3: \({}^{1}\)H-NMR of [C1 zVIm]Br in different deuterium solvents. ((1) D2O, 30 min later; (2) D2O, immediately; (3) DMSO-\(d\)6). * The peak is ascribed to the solvent. Figure 2: \({}^{1}\)H-NMR of [C8 VIm]Br in different deuterium solvents. ((1) D2O; (2) DMSO-\(d\)6). * The peak is ascribed to the solvent. could be well-dispersed in the solution and transform reversibly to precipitation or macrogel formation in methanol with the temperature changes [28]. Notably, novel thermo-responsive nanogel will promisingly be obtained by introducing hydrogen-bond interactions into IL-based nanogel. To investigate the relationship between the structure and thermo-response of nanogels in more detail, BIm-based monomers were copolymerized with the cross-linkers EGDMA and DVB via convention radical polymerization in selective solvent. As illustrated in Scheme 2, BIm-based monomers (\(n\) = 6, 8, and 12) can be successfully copolymerized with the cross-linkers to obtain the well-dispersed nanogels in the solution. Additionally, the nanogel solutions were very stable and could be stored without any precipitation for more than several months. The results illustrate that BIm-based monomers can stabilize the nanogels during the polymerization process. More interestingly, these BIm-based nanogels are also thermo-responsive. When the temperature decreases, the nanogels precipitate from the solvent. Inversely, the nanogels could be well-dispersed in the solvent again when the temperature increases. The performance testifies that BIm-based nanogels are of the phase behavior with UCST. The cross-linking copolymerization with different monomers and feed ratio were conducted. Then, the size and \(\zeta\)-potential of BIm-based nanogels were checked using dynamic light scattering (DLS), and the results are summarized in Table 1. It indicates that BIm-based nanogels with the size less than 160 nm can be facilely prepared via the one-step synthesis. Moreover, for the [C\({}_{6}\)VIm]Br + EGDMA series, hydrodynamic diameters decreased from 160 to 70 nm on decreasing the cross-linking. The particles would precipitate from the solution when the feed ratio of BIm-based monomer to EGDMA was less than 3:1. A similar trend can be observed when DVB is used as the cross-linker. This performance is reasonable because BIm-based monomers play the role of stabilizer in the cross-linking copolymerization process. Likewise [26, 27, 28, 29], hydrodynamic diameters of these BIm-based nanogels are in a wide distribution except Entry 5. In addition, all nanogels are positively charged, and the higher feed ratio of BIm-based monomers will enhance the \(\xi\)-potential on the surface of nanogels. As a result, positive \(\xi\)-potential can improve the stability of nanogels in the solution due to electrostatic repulsion. These results strongly demonstrate that the one-step synthesis is an effective and universal method for the preparation of imidazolium-based nanogels. The morphology of as-prepared nanogels was also examined using scanning electron microscopy (SEM). As shown in Figure 4, spherical particles with sizes of less than 200 nm can be clearly observed. Most of the particles are smaller than the corresponding hydrodynamic diameter determined by DLS (Table 1). It is because that DLS provides the data for the particles swollen in the solution, whereas SEM presents the images of dried particles.