Development of a Digital Microscope Spectrophotometric System for Determination of the Antioxidant Activity and Total Phenolic Content in Teas

This work presents a simple, low cost, and portable spectrophotometer based on a digital microscope as a detector for the determination of the antioxidant activity and total phenolic content (TPC). The antioxidant activity was expressed as ascorbic acid equivalents (AAE) using the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) assay. The TPC value was measured using the Folin-Ciocalteu method. The color of the products was captured using a digital microscope powered through the USB output of a laptop. A digital microscope, held in a 3D printing device was used to take photos of the reaction product that was illuminated with the stable light source of the digital microscope. The pictures were analyzed for quantitation. For the DPPH assay, the developed method provided a working range from 0.6 to 6 mg L⁻¹ of ascorbic acid with a limit of detection (LOD) of 0.2 mg L⁻¹ and a relative standard deviation (RSD) of 2.2%. The TPC method provided a working range from 4.6 to 100 mg L⁻¹ for gallic acid, with a LOD of 1.4 mg L⁻¹ and a RSD of 1.3%. Results of the proposed system agree well with a conventional spectrophotometric method and have been successfully applied to measure the antioxidant activity and TPC of local Thai tea samples.