Normalized ATR FT-IR spectra of virgin sericin and AgNPs. The intensity was normalized against the absorption at 1,070 cm−1. (A) Original SS shows characteristic absorptions of protein including amide I (1,700 to 1,600 cm−1, asterisk), amide II (1,560 to 1,500 cm−1, asterisk), and amide III (1,300 to 1,200 cm−1, asterisk). (B) SS-capped AgNPs show new functional groups including carboxylate (1,451, 1,404, 1,353 cm−1, asterisk) and amine salt (830 cm−1, asterisk). (C) Thermally treated SS-capped AgNPs show the same functional groups as those in (B).