Development of highly sensitive and selective microfluidic paper-based analytical devices (µPADs) for environmental monitoring

© 2014 Dr. Badra Manori Jayawardane KaththotaralalageThe increase in world population and the level of industrialisation of many developed and developing countries have affected the quality of the world’s environment significantly since last century. High demand for raw materials and agricultural products has resulted in the release of increasing amounts of mining toxins, fertilisers and biocides. Electronic wastes contain large quantities of hazardous materials, including heavy metals, and their disposal presents huge environmental challenges to all developed and most developing countries. These problems highlight the necessity for low-cost, portable and accurate chemical analysers as indispensable monitoring and analysis tools for environmental protection activities. The objective of the work described in this thesis was to develop microfluidic paper-based analytical devices (µPADs) for the detection of reactive phosphate, ammonia, nitrate/nitrite and copper to meet the increasing need for rapid, sensitive and selective analysis of nutrients in drinking, natural, and industrial and domestic wastewaters. Interest in the determination of reactive phosphate in aquatic systems stems from the realisation in the late-1960s to mid-1970s that phosphorus was a critical nutrient that promoting eutrophication in aquatic systems. A simple µPAD was prepared and evaluated for the determination of reactive phosphate based on the phosphomolybdenum reaction. Hydrophilic reagent zones were defined by printing filter paper with a hydrophobic paper-sizing agent using an inkjet printer. Under optimal conditions, the µPAD is characterised by a working range of 0.1–10 mg L-1 P, and limits of detection (LOD) and quantitation (LOQ) of 0.05 and 0.16 mg L-1 P, respectively. There was no significant difference between the results obtained using the µPAD and reference methods. A simple and effective µPAD for the determination of ammonia in wastewaters was based on gas-diffusion separation combined with colourimetric detection. The sample was spotted onto the µPAD and ammonium ion present in the sample reacted with sodium hydroxide to form molecular ammonia, which diffused through a hydrophobic microporous membrane into an adjacent paper zone containing an acid-base indicator (3-nitrophenol or bromothymol blue). The resultant change in indicator colour was monitored using a desktop scanner and the Red, Green and Blue (RGB) signal was used for quantification. Under optimal conditions, the µPAD gave LOD of 0.8 and 1.8 mg L-1 N and repeatability, expressed as relative standard deviation (RSD) (n ≥ 10), of 3.1% and 3.7% at 20 mg L-1 N for 3-nitrophenol and bromothymol blue, respectively. The small dimensions, minimal reagent consumption, low cost and simplicity of application make the proposed µPAD an attractive option for on-site wastewater process and discharge monitoring. A low-cost disposable colourimetric µPAD was developed for the determination of nitrite and nitrate by the Griess reaction. First, nitrate is reduced to nitrite in the µPAD’s hydrophilic channel using immobilised zinc microparticles. Under optimal conditions, the LOD and LOQ for nitrite are 1.0 and 7.8 µM respectively, while the corresponding values for nitrate are 19 and 48 µM respectively. This µPAD is suitable for on-site measurement of environmental and drinking waters. A µPAD based on a polymer inclusion membrane (PIM) was developed for the selective measurement of Cu(II) in drinking and mine-tailing waters. The PIM consists of di(2-ethlyhexyl) phosphoric acid as the carrier, dioctyl phthalate as a plasticiser, poly(vinyl chloride) as the base polymer and 1-(2’-pyridylazo)-2-naphthol as the colourimetric reagent. Under optimal conditions, the device is characterised by LOD and LOQ of 0.06 and 0.21 mg L-1 Cu(II), respectively. This µPAD was stable for more than 5 months under ambient conditions