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 (μPAD) for the determination of total ammonia (i.e., ammonia + 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 μ-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 μ-distillation chamber-based μPAD is more than double of that obtained using a gas-permeable membrane. The proposed μPAD is characterised by a limit of detection of 0.32 or 0.47 mg N L-1 and working concentration ranges of 0.5-3.0 mg N L-1 or 2.0-10 mg N L-1 when using NY or BTB indicators, respectively. This is the first μPAD 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 μPAD (3 mg N L-1) and 13 and 2.5% (8 mg N L-1) for the BTB-based μPAD. The NY-based μPAD is stable under vacuum for at least 12 days at room temperature, and 56 days if stored in a freezer (≤-20 °C).