Enhanced selectivity and sensitivity for colorimetric determination of glyphosate using Mn–ZnS quantum dot embedded molecularly imprinted polymers combined with a 3D-microfluidic paper-based analytical device

We report a novel three-dimensional microfluidic paper-based analytical device (3D-μPAD) with colorimetric detection, using Mn–ZnS quantum dot embedded molecularly imprinted polymer (Mn–ZnS QD-MIP), for selective glyphosate determination in whole grain samples. Detection is based on the catalytic activity of Mn–ZnS QD-MIP in the H2O2 oxidation of ABTS. Glyphosate imprinted polymer is successfully synthesized on the Mn–ZnS QD surface using a poly (N-isopropylacrylamide) (NIPAM) and N, N′-Methylenebisacrylamide (MBA) as the functional monomers. The catalytic activity depends on binding or non-binding of glyphosate molecules on the synthetic recognition sites of the Mn–ZnS QD-MIP. Glyphosate selectively binds to the cavities embedded on the Mn–ZnS QD surface, and subsequently turns-off or inhibits the ABTS oxidation and color change to light green. The change of reaction color from dark green to light green depends on the concentration of glyphosate. We report, for the first time, using the relatively new penguard enamel colour to create a hydrophobic barrier. The foldable 3D-μPAD comprises three layers (top/center/bottom), named as the detection zone, immobilized Mn–ZnS QD-MIP disc, and sample loading. Assay on the 3D-μPAD can determine glyphosate by ImageJ detection, over an operating range of 0.005–50 μg mL−1 and with a detection limit of 0.002 μg mL−1. Our 3D-μPAD exhibits high accuracy, with a 0.4% (intra-day) and 0.7% (inter-day) relative difference from the certified CRM value. Moreover, the fabricated 3D-μPAD provides good reproducibility (1.7% RSD for ten devices). The developed 3D-μPAD was successfully applied to determine the glyphosate concentration in whole grain samples and shows great promise as an alternative highly selective and sensitive colorimetric method. The 3D-μPAD is well suited to food-quality control and onsite environmental-monitoring applications, without sophisticated instrumentation.