Application of Red Onion Peel Extract for Green Synthesis of Silver Nanoparticles in Hydrogels Exhibiting Antimicrobial Properties

Abstract

UV-initiated green synthesis of metal nanoparticles by using plant extracts as photoreducing agents is of particular interest since it is an environmentally friendly, easy-to-maintain, and cost-effective method. Plant molecules that act as reducing agents are assembled in a highly controlled way which makes them suitable for metal nanoparticle synthesis. Depending on the plant species, their application for green synthesis of metal nanoparticles for diverse applications may contribute to the mediation/reduction in organic waste amounts, thus enabling the implementation of the circular economy concept. In this work, UV-initiated green synthesis of Ag nanoparticles in hydrogels and hydrogel's thin films containing gelatin (matrix), red onion peel extract of different concentrations, water, and a small amount of 1 M AgNO₃ have been investigated and characterized using UV-Vis spectroscopy, SEM and EDS analysis, XRD technique, performing swelling experiments and antimicrobial tests using bacteria (Staphylococcus aureus, Acinetobacter baumannii, Pseudomonas aeruginosa), yeasts (Candida parapsilosis, Candida albicans) and microscopic fungi (Aspergillus flavus, Aspergillus fumigatus). It was found that the antimicrobial effectiveness of the silver-enriched red onion peel extract-gelatin films was higher at lower AgNO₃ concentrations as compared to those usually used in the commercially available antimicrobial products. The enhancement of the antimicrobial effectiveness was analyzed and discussed, assuming the synergy between photoreducing agent (red onion peel extract) and silver nitrate (AgNO₃) in the initial gel solutions leading to the intensification of Ag nanoparticles production.

Keywords: Ag NPs; UV-Vis spectrophotometry; antimicrobial activity; gels; green synthesis; red onion peel.

Figure 1

UV-Vis absorbance spectra of diluted red onion peel extract solutions.

Figure 2

UV-Vis spectra of red onion extract-gel-silver nitrate hydrogels containing 25 wt.% of onion peel extract and its substitutes.

Figure 3

Photographs of differently exposed cuvettes filled with onion peel extract-gelatin-silver nitrate hydrogels and gelatin.

Figure 4

UV-Vis spectra of the UV-exposed red onion peel-gelatin-silver nitrate hydrogels indicating the creation of Ag NPs: 1—not exposed gel; 2—5 min exposed gel; 3—10 min exposed gel; 4—20 min exposed gel.

Figure 5

A photograph of red onion peel extract–gelatin-silver nitrate gel films in a Petri dish (a) and read onion bulbs (b).

Figure 6

UV-Vis spectra of the exposed red onion peel–gelatin-AgNO₃ films indicating formation of Ag (a) and color changes of hydrogel films after UV exposure (b). UV-Vis Spectrum of red onion-gelatin gel is provided for comparison.

Figure 7

Exposure-duration-related intensity variations of the LSPR absorbance in red-onion-peel–gelatin films continuing photosynthesized Ag nanoparticles.

Figure 8

XRD pattern of red-onion-extract-gelatin-AgNO₃ film after 20 min UV exposure.

Figure 9

SEM images and sample composition (EDX measurement): not irradiated hydrogel films (R); UV-exposed (20 min) hydrogel films (L).

Figure 10

Swelling capacity of red onion-gelatin solution with silver nanoparticles (t = 20 °C).

Figure 11

Antimicrobial activity of red onion peel extract hydrogels containing silver nanoparticles against Pseudomonas aeruginosa bacteria ATCC 15442: (1) control; (2) 5 min exposed gel; (3) 10 min exposed gel; (4) 20 min exposed gel; (5) 60 min exposed gel.

Figure 12

Antimicrobial activity of red onions peel extract hydrogel films containing silver nanoparticles against Pseudomonas aeruginosa bacteria ATCC 15442: (1) control; (2) 5 min exposed film; (3) 10 min exposed film; (4) 20 min exposed film; (5) 60 min exposed film.