Abstract
The work reports on zinc oxide bentonite nanocomposite (ZnOBt) chemical route synthesis, characterization, and investigation of curcumin (Cur) functionalization for a label-free colorimetric detection of total aflatoxins (AFs) in foods. XRD of ZnO nanoparticles (NPs) confirmed the wurtzite structure (2θ = 36.2°) and that of ZnOBt showed the intercalated interlayer composite phase. The Debye–Scherrer relation calculated the crystallite size as 20 nm (ZnO) and 24.4 nm (ZnOBt). Surface morphology by SEM exhibited flower-like hexagonal, rod-shaped ZnO NPs on the bentonite surface. Colorimetric reaction involved two-stage redox reactions between ZnOBt and dye Cur followed by AFs phenolic group and Zn(Cur)OBt. Cur gets oxidized at its diketone moiety in the presence of ZnOBt to form a red colored complex Zn(Cur)OBt, which further scavenge protons from AFs phenolic group, and gets oxidized to AFs-Zn(Cur)OBt (yellow). Binding of AFs-Zn(Cur)OBt is characterized by FT-IR ascribed to C–H bending (1966.615 cm–1), O–H stretching (3256.974 cm–1), and C=O stretching (1647.362 cm–1). 1H NMR chemical shifts (δ) (ppm) showed an increase in proton at the aliphatic region (0 to 4.4) while removal of proton in ether at 4.4 to 6 regions. Job plot calculation using UV–Vis data resulted in a higher total AF binding coefficient of Zn(Cur)OBt (Ka = 3.77 × 106 mol–1 L) compared to Zn(Cur)O (Ka = 0.645 × 106 mol–1 L) as well as a molar ratio of 1:1 by the Benesi–Hildebrand plot equation. Corn and almond food samples showed the total AFs LOD of 2.74 and 4.34 ppb, respectively. The results are validated with standard LC/MS-MS in compliance with MRL value as per the regulatory standard (EU).The NP-based method is facile and rapid and hence can be utilized for onsite detection of total AFs in foods.
Highlights
Aflatoxins have major subtypes as B1, G1, B2, and G2 and are carcinogenic, heterocyclic, oxygenated, stable difuranocoumarins metabolites of Aspergillus flavus and Aspergillus parasiticus, prevalent mostly in cereals, nuts, and tree nuts.[1]
The results showed that the ochratoxin A (OTA), ZEN, and DON standard solutions showed a red color with no observed peak at 430 nm, implying that they do not bind with zinc oxide bentonite nanocomposite (ZnOBt)
A simple rapid label-free method using the ZnOBt nanocomposite is developed for total AFs detection in corn and almond
Summary
Aflatoxins have major subtypes as B1, G1, B2, and G2 and are carcinogenic, heterocyclic, oxygenated, stable difuranocoumarins metabolites of Aspergillus flavus and Aspergillus parasiticus, prevalent mostly in cereals, nuts, and tree nuts.[1]. Among other metal oxide nanomaterials, ZnO has several advantages due its wide band gap (3.35 eV), low cost, high aspect ratio, and improved optoelectronic properties It shows unique structural properties like it has a second coordination sphere, which entails itself in hydrogen binding likely with ligands, water molecules, and hydrophobic cores, for improved binding results.[24] it was reported that catalytic activity of these oxide materials can be enhanced when they are linked or immobilized on a natural clay material such as bentonite, which has a higher surface area.[25] introducing nanoparticles with other substrates/fillers would result in composites with novel and enhanced properties that cannot be achieved by the individual components. Functionalizing ZnOBt with Cur may enhance the absorption and luminescence properties of ZnO and overall adsorption efficiency of ZnOBt and can be utilized for colorimetric total AFs detection in our colorimetric sensor studies
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