*2.1. Determination of Preparation Conditions of SeCS*

#### 2.1.1. Determination of CS Concentration

In order to evaluate the effect of CS concentration on the formation of SeCS, the particle sizes of SeCS prepared with different concentrations of CS were measured by nanoparticle size analyzer. As shown in Figure 1A, the particle sizes of SeNPs with 0.05 mg/mL CS surface decoration were 278.70 ± 9.47 nm. The average particle diameters of SeNPs were significantly decreased to 113.7 ± 2.89 nm in the presence of 0.1 mg/mL CS, indicating that CS between 0.05 mg/mL and 0.1 mg/mL could inhibit the aggregation of SeNPs with the increase of CS concentration. The result was in accordance with the previous study, which demonstrated that the particle diameters of SeNPs decorated with 1.0 mg/mL chitosan were smaller than that synthesized with 0.6 mg/mL chitosan [19]. However, when CS concentration is greater than 0.1 mg/mL, the particle size of SeCS in the system became larger. This might be because CS molecules aggregated to reduce its hydrophilic groups' function, resulting in the particle size become larger. Therefore, the optimal concentration of CS was selected as 0.1 mg/mL.
