Characterization and application of nanocolloidal Mn(IV) in a chemiluminescence system for estimating the total phenolic content in pomegranate juices using a nanodroplet microfluidics platform

An extensive investigation has been carried out to test the nanocolloidal manganese (IV) chemiluminescence (CL) system and to utilize it for the determination of total phenolic content (TPC) in fruit juices. Initially, a detailed characterization of the produced nanocolloidal Mn(IV) was conducted including oxidation state, elemental analysis, size, shape and morphology. The electrochemical studies confirmed the oxidation state of Mn(IV), and the elemental analysis confirmed the formation of MnO₂. The size of the spherical nanocolloidal Mn(IV) was found to be approximately 5 ± 1 nm. Using these nanocolloidals, a new nanodroplet microfluidics chip was used to develop a CL method for the determination of the TPC in fruit juices. In comparison to the published work on the determination of TPC in food samples using nanocolloidal Mn(IV), the reduction in the flow rates of the proposed setup is 650- fold due to the novel chip design. The novel chip consists of three main zones; a serpentine mixing zone, a CL nanodroplet reaction zone and a serpentine detection zone. These three zones were kept very close to each other such that the total area is less than 1.8 cm². This small area enabled fast and robust analysis despite the low flow rates utilized here. The study was then extended to investigate 18 of the phenolic compounds that are commonly found in food samples. The limits of detection (LOD) obtained were between 1.3 ng mL⁻¹ for gallic acid and 16.5 ng mL⁻¹ for rutin. The proposed method is characterized by its excellent repeatability and reproducibility (0.4% and 4% respectively using gallic acid solution of 1000 ng mL⁻¹). The sensitivity and the selectivity of the nanocolloidal Mn(IV) CL toward phenolic compounds were compared with those obtained by the acidic KMnO₄ - formaldehyde CL system. Finally, the method was applied successfully to the determination of the TPC in pomegranate juice.