Chromatographic paper-based analytical devices using an oxidized paper substrate

A novel detection scheme using chromatographic retention was proposed for paper-based analytical devices (PADs). Using a wax printer and an ink-jet printer, the PADs consisted of a straight-flow channel, a circular sample zone, and a band of brilliant green (BG) printed at the connection between the sample zone and the flow channel. When a constant volume of a sample solution was applied to the sample zone, the BG band formed a colored bar along the flow channel that marked the adsorption and desorption of the paper substrate according to the principles of chromatography. If the sample solution contained an anionic complex of boric acid with chromotropic acid, the anionic complex enhanced desorption of the BG from the paper substrate via the formation of an ion-pair with the BG, which resulted in an elongated colored bar. Fundamental equations for the retention behavior of the BG on the PADs were derived using a model based on chromatographic principles and ion-pairing formation. A retardation factor, R_f, was correlated with the concentration of boric acid contained in the sample solutions. To enhance the adsorption of the BG, 2,2,6,6-tetramethylpiperidine 1-oxyl was used to oxidize the paper substrate. The oxidized paper substrate shortened the colored bar of a blank solution and formed a clear boundary for the color. When the analytical conditions were optimized for pH and the concentration of chromotropic acid, the PAD permitted the measurement of boric acid for a concentration ranging from 0.3 to 3 mM. The proposed model was validated when the fitting curve was calculated using the derived equations, which resulted in good agreement with the experimental data.