2.1.2. Determination of Molar Ratio of Na2SeO3 to Vc

According to the reaction equation, sodium selenite (Na2SeO3) reacts with ascorbic acid (Vc) at the coefficient of 1:2. However, in fact, an excess of VC needs to be added to the system to prevent SeNPs from being oxidized. Therefore, in this study, fixing the CS concentration of 0.1 mg/mL, the reaction time of 3 h and the reaction temperature of 55 ◦C, the molar ratio of Na2SeO3 to Vc varied from 1:2 to 1:10. The particle sizes of SeCS prepared with different molar ratios of Na2SeO3 to Vc were shown in Figure 1B. The results showed that the average particle diameters of SeCS obtained with the molar ratio of Na2SeO3 to Vc of 1:2 were 187.3 ± 12.48 nm. As the molar ratio of Na2SeO3 to Vc increased to 1:8, the particle size reached the smallest. It was speculated that excessive Vc could improve the growth of the crystal nucleus to stabilize SeCS [20]. However, further increasing Vc concentration meant the particle size became larger due to the unstable reaction system. So, it might be the reason why the diameter of the SeCS prepared with a molar ratio of Na2SeO3 to Vc of 1:10 was larger than that obtained with a molar ratio of Na2SeO3 to Vc of 1:8. Therefore, the optimal molar ratio of Na2SeO3 to Vc was selected as 1:8.

**Figure 1.** Effects of different template dosage (**A**), reactants ratio (**B**), reaction time (**C**), and reaction temperatures (**D**) on particle size of SeCS. Bars with a1, b1 and c1 represented a statistical difference (*p* < 0.05) among different template dosages. Bars with a2 and b2 represented a statistical difference (*p* < 0.05) among different reactants ratios. Bars with a3, b3 and c3 represented a statistical difference (*p* < 0.05) among different times. Bars with a4 and b4 represented a statistical difference (*p* < 0.05) among different reaction temperatures.
