Microfluidic paper-based analytical devices for antioxidant vitamins C and E in foods

In this study, we developed microfluidic paper-based analytical devices (μPADs) for the determination of antioxidant vitamins. The proposed μPADs utilize the reduction of metal ions by hydrophilic and hydrophobic antioxidant vitamins, which is followed by colorimetric reactions with chelating reagents. Hydrophilic vitamin C reduces Fe(III) to Fe(II) and forms a stable Fe(II)-bathophenanthroline complex in an aqueous solution. By contrast, this complex is unstable in organic solvents, and hydrophobic vitamin E requires Fe(III) and bathophenanthroline to be replaced with Cu(II) and bathocuproine. In these results, the relationship between the logarithm of a vitamin's concentration and its color intensity was linear and ranged from 4.4 to 35 mg L^-1 for ascorbic acid and 50-200 mg L^-1 for α-tocopherol. The limits of detection, estimated from the standard deviation of blank samples, were 3.1 mg L^-1 for ascorbic acid and either 27 mg L^-1 (in hexane) or 48 mg L^-1 (in ethanol) for α-tocopherol. The proposed method was used to quantify vitamin C in bell peppers, mandarin oranges, kiwifruit, and lemons, as well as vitamin E in almonds, almond milk, and dietary supplements. The results demonstrate the effectiveness of these μPADs for the practical analysis of antioxidant vitamins in food samples.