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
).
1
H 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 (
K
a
= 3.77 × 10
6
mol
–1
L) compared
to Zn(Cur)O (
K
a
= 0.645 × 10
6
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.
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