Melanin-templated rapid synthesis of silver nanostructures

Abstract

Background: As a potent antimicrobial agent, silver nanostructures have been used in nanosensors and nanomaterial-based assays for the detection of food relevant analytes such as organic molecules, aroma, chemical contaminants, gases and food borne pathogens. In addition silver based nanocomposites act as an antimicrobial for food packaging materials. In this prospective, the food grade melanin pigment extracted from sponge associated actinobacterium Nocardiopsis alba MSA10 and melanin mediated synthesis of silver nanostructures were studied. Based on the present findings, antimicrobial nanostructures can be developed against food pathogens for food industrial applications.

Results: Briefly, the sponge associated actinobacterium N. alba MSA10 was screened and fermentation conditions were optimized for the production of melanin pigment. The Plackett-Burman design followed by a Box-Behnken design was developed to optimize the concentration of most significant factors for improved melanin yield. The antioxidant potential, reductive capabilities and physiochemical properties of Nocardiopsis melanin was characterized. The optimum production of melanin was attained with pH 7.5, temperature 35°C, salinity 2.5%, sucrose 25 g/L and tyrosine 12.5 g/L under submerged fermentation conditions. A highest melanin production of 3.4 mg/ml was reached with the optimization using Box-Behnken design. The purified melanin showed rapid reduction and stabilization of silver nanostructures. The melanin mediated process produced uniform and stable silver nanostructures with broad spectrum antimicrobial activity against food pathogens.

Conclusions: The melanin pigment produced by N. alba MSA10 can be used for environmentally benign synthesis of silver nanostructures and can be useful for food packaging materials. The characteristics of broad spectrum of activity against food pathogens of silver nanostructures gives an insight for their potential applicability in incorporation of food packaging materials and antimicrobials for stored fruits and foods.

Figure 1

Melanin production rate at different incubation time by Nocardiopsis alba MSA10.

Figure 2

The response surface plot shows the most significant variables interaction on melanin production. (A) tyrosine and sucrose, (B) temperature and sucrose, (C) salinity and sucrose, (D) temperature and tyrosine, (E) salinity and tyrosine, and (F) salinity and temperature.

Figure 3

Effects of light source on melanin production. Growth of N. alba MSA 10 on Actinomycetes isolation agar medium (A), Melanin production on tyrosine (1%) agar medium (B), Green light source on melanin production in tyrosine broth (C1) and tyrosine agar medium (C2), which produces dark brown pigment. Red light source produces brown pigment, tyrosine broth (D1) and tyrosine agar (D2). Yellow light shows light brown pigment production on tyrosine broth (E1) at 144 h incubation, but no pigment production in tyrosine agar medium (E2). The pigment production at normal light source on tyrosine broth (F1) and tyrosine agar medium (F2) produces dark brown pigment.

Figure 4

Temperature stability of melanin produced by Nocardiopsis alba MSA 10 with different incubation times measured by UV absorbance at 220 nm (mean ± S.D., n = 3).

Figure 5

Antioxidant activity in different concentration of Nocardiopsis melanin.

Figure 6

Reductive capabilities in various concentrations of Nocardiopsis melanin and a standard BHT.

Figure 7

UV absorption spectra of synthesized silver nanostructures. (A) At different temperatures. C - Control (AgNO₃ solution (1 mM), 1-40°C, 2-60°C, 3-80°C and 4-100°C (B) At different time intervals.

Figure 8

TEM images of synthesized silver nanostructures. (A) Silver nanostructures, (B) nanostructure synthesis at 100°C and (C) different sizes of synthesized silver nanostructures.

Figure 9

FTIR spectra of synthesized silver nanostructures. (A) Silver nanostructures, and (B) pure melanin.

Figure 10

Antimicrobial assay of melanin and melanin- mediated synthesis of silver nanostructures.