*3.6. Antioxidant Activity Evaluation of SeCS*

#### 3.6.1. DPPH Radical Scavenging Assay

The antioxidant activity of SeCS was evaluated using 2,2-diphenyl-1-picrylhydrazyl (DPPH) reagent according to a previously described method with some modifications [27]. Different concentrations (0.1–0.5 mg/mL) of CS, SeNPs and SeCS solutions were respectively added into 0.2 mM DPPH solution in methanol. After 30 min incubation in a dark place, the color change of the reaction mixture was read at 517 nm using a TU-1901 double beam UV-visible spectrophotometry (Persee, Beijing, China). Positive control in this assay was ascorbic acid. The formula of the DPPH scavenging rate was listed below:

DPPH scavenging rate (%) = [A0 − (A1 − A2)]/A0 × 100%

In the equation, A0, A1 and A2 separately represented the absorbance values of control, the samples with DPPH solution and the samples without DPPH solution.
