A simple paper-based analytical device using UV resin screen-printing for the determination of ammonium in soil

This study reports a convenient and low cost paper-based analytical device (PAD) for measuring ammonium in soil. This PAD has been developed using an inexpensive UV resin solution, as a new hydrophobic material, with a screen-printing method to create hydrophobic areas. The paper-based colorimetric device was developed using a modified Berthelot reaction in which salicylate and dichloroisocyanurate are used in order to produce a green compound of 2-2 dicarboxyindophenol in the presence of ammonium. The concentration of ammonium is proportional to the intensity of the resulting green color, as analysed by ImageJ software. A variety of tests was carried out to optimize and evaluate various aspects of the PAD's fabrication and utilization. A linear range was obtained in the range of 10-100 mg L-1 with a limit of detection and limit of quantitation of 0.5 mg L-1 and 1.7 mg L-1, respectively. The relative standard deviation of intra-day measurements was 3.0% and the inter-day precision was 3.2% with good reproducibility. When recovery of ammonium added to samples was evaluated it ranged from 95.5 to 107.5%. The PAD was applied to detect ammonium in a variety of actual soil samples, and the results were validated against spectrophotometric results using a paired t-test, showing good accuracy. A convenient color comparison chart was also created on paper to enable the user to interpret the color results without the need for a color scanner. This developed PAD is fast, easy, inexpensive, and provides an alternative to existing methods for the determination of ammonium in soil.