## 3 Results  
### Preparation and Characterization of PSBMA Nanogels  
Compared with emulsion polymerization, reflux-precipitation polymerization is a simple, one-step and surfactant-free method to obtain spherical nanogels with uniform size. The obtained nanogels can be facilely organized and utilized as functional nanobi-materials for delivery application [36; 37; 38]. SBMA, which contains a quaternary ammonium group with a positive charge and a sulfo group with a negative charge, was selected as the zwitterionic monomer in this work. Meanwhile, DVB was used as the crosslinker, considering its high reaction activity and strong hydrophobic character. The chemical structure for the prepared PSBMA nanogels was characterized by FTIR spectroscopy. As shown in Figure 1a, two characteristic peaks at 1043 cm\({}^{-1}\) and 1484 cm\({}^{-1}\) can be attributed to the symmetric and asymmetric stretching vibrations of the sulfonate groups (-SO\({}_{3}\)) in the structure of SBMA, and the strong peaks at 1719 cm\({}^{-1}\) and 1179 cm\({}^{-1}\) are attributed to the stretching vibration of -C=O and -C-O-C-, respectively. In order to study the effect of the monomer/crosslinker feeding ratio on the physicochemical properties of the nanogels, a series of PSBMA nanogels with different cross-linking degrees (the total weight of monomer and crosslinker was 200 mg) were prepared. The colloidal data are shown in Table 1. As the DVB content increased from 10 wt% to 40 wt% (PSBMA-1-PSBMA-4), the diameter of PSBMA nanogels decreased from 294.7 nm to 219.9 nm, and all nanogels displayed narrow size distribution with polydispersity index (PDI) lower than 0.2 (Figure 2). This result was expected because the higher degree of cross-linking led to more compacted nanogels and smaller hydrodynamic sizes [39]. Furthermore, the zeta potential decreased from \(-\)4 mV to \(-\)14 mV when the cross-linking degree increased from 10 wt% to 40 wt%. The high surface potential was beneficial for colloidal stability. The SEM images of PSBMA-4 nanogels displayed a uniform and spherical morphology, with a calculated diameter of 120-150 nm (Figure 3a,b). It is worth noting that the hydrodynamic size calculated from DLS was larger than that observed by SEM, indicating that the nanogels were highly swollen in aqueous solution and greatly shrunk under dry conditions.  
Figure 1: FTIR spectra of PSBMA nanogels (**a**) and pHEMA contact lens (**b**).  
Figure 2: Size distributions for the series of PSBMA nanogels determined by DLS.  
Nanogels used in biomedicine as drug carriers should have good stability in the physiological environment. As shown in Figure 4, the PSBMA-4 nanogels displayed good colloidal stability, as the size of particles remained nearly unchanged in water over 30 days, with the change of particle sizes from initial size after storage in the range of \(\pm\)10 nm, indicating good storage stability. Meanwhile, it was essential to measure the stability of nanogels against salt conditions, as salt exists in humans. In Figure 4, the nanogels exhibited excellent narrow size distribution, even when the concentration of NaCl increased to 1 M, and the hydrodynamic diameters of nanogels increased only slightly (within 15 nm). The high stability of nanogels can be attributed to their cross-linked structure and the shielding effect of PSBMA zwitterionic chains against the outside environment.