Zein film functionalized with gold nanoparticles and the factors affecting its mechanical properties

Abstract

In this article, we report a simple method to synthesize biodegradable zein films functionalized with gold nanoparticles (AuNPs) with significantly improved mechanical properties, as an environmentally benign substitute to biologically hazardous polymers. Zein-coated AuNPs were synthesized using the zein protein as a reducing agent and characterized with IR, UV, CD, ζ-potential, and TEM measurements. The zein protein interaction with the negatively charged surface of AuNPs provides excellent strength to the zein thin film. For the first time, FT-IR spectral studies suggested the strong interaction between AuNPs and zein protein, which was further supported by the higher binding constant (K _b) value. The films were characterized for mechanical properties with spectroscopic and physical experimental investigations. The surface morphology of AuNP-doped zein film was explored by AFM and SEM, which suggested that the AuNPs prevent the buckling of zein film and increase the strength as well as flexibility of the film.

Fig. 1. (a) UV-visible spectral changes of zein protein (0.20%; black line) upon addition of HAuCl₄ (0.20 mM, red line) in 10 mM SDS aqueous solution at 65 °C. (b) UV-visible spectral titration monitored at 550 nm for the formation of AuNPs as a function of temperature (from 20 °C to 90 °C). (c) CD spectra of zein (blue line) and AuNPs (red line) recorded in aqueous medium at 25 °C (d) FT-IR spectra of zein (blue line) and AuNPs (red line) recorded in KBr at 25 °C.

Scheme 1

Fig. 2. (a) Fluorescence spectral changes of zein protein (0.25 μM; blue line) upon addition of AuNPs (0.14 nM, red line) in 10 mM SDS aqueous solution at 70 °C. (b) CD spectra of AuNP doped zein film (magenta line) and zein (red line) recorded in aqueous medium at 25 °C (c) FT-IR spectra of film (magenta line) and AuNPs (red line) recorded in KBr at 25 °C. (d) TGA graph of the AuNP doped film recorded from room temperature to 500 °C.

Fig. 3. (a) Optical microscope image of buckled zein protein film. The average wavelength was found to be 150 μm (b) Optical microscope image of AuNPs doped zein protein film. The addition of AuNPs alleviates the compressive stress and prevents buckling.

Fig. 4. (a) Tensile strength and (b) strain at failure of various films made from different amounts of AuNPs vs. zein (15% v/v) (c) AFM image of zein protein film doped with AuNPs. (d) SEM image of AuNPs doped zein film showing the random distribution of NPs (yellow circles) (e) images of the AuNP functionalized film showing the bending at different angles.