A Single Microfluidic Paper-Based Analytical Device for the Determination of Ammonia and pH Using Mulberry Fruit Extract as a Natural Indicator

This work presents a microfluidic paper-based analytical device (µPAD) designed for the screening and measurement of ammonia and pH, utilizing mulberry fruit extract as a natural indicator. For ammonia determination, the method is based on a membraneless gas-separation microfluidic device. The µPAD co-nsists of three layers: the upper layer serves as the detection area, featuring two circles with a diameter of 8 mm, where mulberry fruit extract at pH 3.07 and pH 4.93 is applied for the measurement of ammonia and pH, respectively. Ammonia gas is generated in the lower donor layer and diffuses through the gap in the middle layer to the acceptor layer. The concentration of ammonia is determined by analyzing the color change of the mulberry fruit extract indicator. Images of the colored detection zones are captured using a smartphone camera, and their color intensity is analyzed with ImageJ software. For pH measurement, the color change is compared to a standard pH scale ranging from 1 to 14. The proposed method offers a linear calibration range from 0 to 150 mg/L N, with a limit of detection of 4.89 mg/L N. The relative standard deviation was 3.43% (at 50 mg/L N, n = 10). This method was successfully applied to measure ammonia and pH in natural water, pond water, wastewater, and fertilizer samples, with recovery rates ranging from 95 to 109%. The results obtained from the µPAD correlated well with those from the spectrophotometric method for ammonia and a pH meter for pH values.