Green synthesis of silk sericin-capped silver nanoparticles and their potent anti-bacterial activity

Abstract

In this study, a 'green chemistry' approach was introduced to synthesize silk sericin (SS)-capped silver nanoparticles (AgNPs) under an alkaline condition (pH 11) using SS as a reducing and stabilizing agent instead of toxic chemicals. The SS-capped AgNPs were successfully synthesized at various concentrations of SS and AgNO3, but the yields were different. A higher yield of SS-capped AgNPs was obtained when the concentrations of SS and AgNO3 were increased. The SS-capped AgNPs showed a round shape and uniform size with diameter at around 48 to 117 nm. The Fourier transform infrared (FT-IR) spectroscopy result proved that the carboxylate groups obtained from alkaline degradation of SS would be a reducing agent for the generation of AgNPs while COO- and NH2 + groups stabilized the AgNPs and prevented their precipitation or aggregation. Furthermore, the SS-capped AgNPs showed potent anti-bacterial activity against various gram-positive bacteria (minimal inhibitory concentration (MIC) 0.008 mM) and gram-negative bacteria (MIC ranging from 0.001 to 0.004 mM). Therefore, the SS-capped AgNPs would be a safe candidate for anti-bacterial applications.

Figure 1

Formation of SS-capped AgNPs at pH 9 and 11. (A) 5 mg/mL SS + 1, 5, 10 mM AgNO₃ at pH 9, (B) 10 mg/mL SS + 1, 5, 10 mM AgNO₃ at pH 9, (C) 5 mg/mL SS + 1, 5, 10 mM AgNO₃ at pH 11, (D) 10 mg/mL SS + 1, 5, 10 mM AgNO₃ at pH 11.

Figure 2

Formation of SS-capped AgNPs synthesized from SS and AgNO ₃ at pH 11.

Figure 3

Morphology of SS-capped AgNPs synthesized from SS and AgNO₃ at pH 11, observed on TEM. Scale bar = 1 μm.

Figure 4

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).

Figure 5

Stability of SS-capped AgNPs synthesized at pH 11 when stored at different temperatures. (empty square) 5 mg/mL SS + 5 mM AgNO₃, 37°C, (filled square) 10 mg/mL SS + 5 mM AgNO₃, 37°C, (empty triangle) 5 mg/mL SS + 5 mM AgNO₃, 25°C, (filled triangle) 10 mg/mL SS + 5 mM AgNO₃, 25°C, (empty circle) 5 mg/mL SS + 5 mM AgNO₃, 4°C, (filled circle) 10 mg/mL SS + 5 mM AgNO₃, 4°C.