Production of silica/zeolite nanoparticle from incinerated cow dung and antimicrobial potential

Abstract

Green synthesis of silica nanoparticle from cow dung ash is a promising prospect due to the low-cost and environmental friendliness. The burning of dried cow dung results in the production of fly-ash that is grey in colour. Silica nanoparticles yield of around 76% was obtained via acid/base treatments of cow dung ash using NaOH and H2SO4 respectively. The silica nanoparticles are characterized based on their structure and morphology using various techniques. FESEM analysis shows that the silica nanoparticles have even distribution of shapes and sizes, amorphous in nature with agglomerated particles and near-spherical shapes while the fly-ash has a less buniform size distribution with irregular shapes. The synthesized silica nanoparticles have dan average size of around 35 nm. It was found through EDX analysis that silicon and oxygen are two of the major components and make-up 48.01 wt% and 61.34 wt% of fly-ash and silica nanoparticles for SEM-EDX and 80.96 wt% and 79.72 wt% for TEM-EDX respectively. This confirms the purity silica nanoparticles in fly-ash at 79.72% with the presence of other impurities. In FTIR analysis of silica nanoparticles and fly-ash, peaks can be seen at 3436.1 cm-1 and 3459.5 cm-1, at 1631.1 cm-1 and 1641.1 cm-1, at 1037.9 cm-1 and 1102.37 cm-1, at 783.65 cm-1 and 797 cm-1, and at 476.7 cm-1 and 473.36 cm-1, which correspond to silanol group, traces of water molecule, siloxane stretching, Si-O bending vibrations and out of plane Si-O bending vibrations respectively. Silica nanoparticles produced an absorption spectrum at 240 nm with a band gap energy of 5.1 eV in UV-Visible Spectroscopy analysis. Through Dynamic Light Scattering (DLS) analysis, the mean size of the produced silica nanoparticles is found to be 361.68 nm with a polydispersity index of 0.171. TGA analysis confirms the thermal stability of the synthesized silica nanoparticles with only 4% of weight loss at elevated temperatures. Silicon and oxygen are found to be prominent in silica nanoparticles based XPS analysis which coincides with the results of EDX analysis despite the mirror presence of impurity such as sodium in silica nanoparticles. Sharp and intensive peaks are apparent at 2θ = 26.58° for fly-ash and at 2θ = 19.09° for silica nanoparticles which show the presence of crystalline quartz that confirms the crystal shape of silica nanoparticles based on XRD analysis. Disc diffusion assay was utilized in determining the minimum inhibitory concentration (MIC) of Ampicillin as a positive control to assess the antimicrobial potential of the silica nanoparticles against Bacillus subtilis and Escherichia coli. The MIC of Ampicillin of B. subtilis and E. coli were found to be 0.2 mg/ml and 0.6 mg/ml respectively. Unmodified silica nanoparticles did not show any antimicrobial activity against B. subtilis and E. coli. Silica nanoparticles that have been modified with APTES and Glutaraldehyde and attached with Ampicillin on the surface showed antimicrobial activity although the minimal inhibitory constant of Ampicillin with similar concentrations as the silica nanoparticles suspension produced larger clear zones.