Development of a micro-distillation microfluidic paper-based analytical device as a screening tool for total ammonia monitoring in freshwaters

An easy-to-use, portable 3D microfluidic paper-based analytical device (mPAD) for the determination of total ammonia (i.e., ammonia thorn ammonium) in freshwaters is described. It consists of two layers of paper patterned with hydrophilic circular zones, one impregnated with sodium hydroxide (sample zone) and another (detection zone) with an acid-base indicator (nitrazine yellow (NY) or bromothymol blue (BTB)), separated by a m-distillation chamber. Ammonium ions present in the water sample are converted into ammonia gas by reaction with sodium hydroxide in the sample zone. Ammonia then diffuses through a headspace and reacts with an acid-base indicator in the detection zone, the reflectance of which can be related to the total ammonia concentration. The analytical signal at 7.8 mg L-1 offered by the m-distillation chamber-based mPAD is more than double of that obtained using a gas-permeable membrane. The proposed mPAD is characterised by a limit of detection of 0.32 or 0.47mg N L-1 and working concentration ranges of 0.5-3.0 mg N L-1 or 2.0-10mg N L-1 when using NY or BTB indicators, respectively. This is the first mPAD whose working range covers almost the entire trigger value range (0.32 -2.3 mg N L-1) for ammonia nitrogen in freshwater systems which makes it suitable as a field screening tool for ammonia in freshwaters. An inter-and intra-device repeatability of 7.6% and 9.0%, respectively, has been achieved for the NY-based mPAD (3mg N L-1) and 13 and 2.5% (8 mg N L-1) for the BTB-based mPAD. The NY-based mPAD is stable under vacuum for at least 12 days at room temperature, and 56 days if stored in a freezer (<=-20 degrees C). (C) 2019 Elsevier B.V. All rights reserved.