MICROFLUIDIC TECHNOLOGICAL ADVANCEMENTS FOR IN SITU NUTRIENT ANALYSIS

Abstract

Comprehensive ocean variable mapping is an inter-disciplinary field that requires data and measurement capabilities for several areas of study, including nutrient flux dynamics in aquatic and marine environments. To this end, microfluidic technologies are uniquely suited to developing in situ sensors for remote or long-term deployment. Microfluidic devices require enabling technologies such as valves for effective fluid handling and absorbance measurement cells to spectrophotometrically determine nutrient concentrations in water samples. This thesis presents a fully automated in situ phosphate analyzer based on an inlaid microfluidic absorbance cell technology. Furthermore, embedded microvalves for integral fluid control are developed and explored for use in microfluidic devices. This thesis first presents the fabrication and characterization of a tunable microfluidic check valve for use in marine nutrient sensing. The ball-style valve makes use of a rare-earth permanent magnet, which exerts a pulling force to ensure it remains passively sealed until the prescribed cracking pressure is met. By adjusting the position of the magnet, the cracking pressure is shown to be customizable to meet design requirements. It is low cost, requires no power, and is easily implemented on microfluidic platforms. However, the microvalve displayed poor chemical resistance towards several colourimetric reagents. Alternative valve designs using elastomer membranes and compressible O-rings are explored with the intention of creating a chemically robust valve. Design parametrization and simulations using COMSOL are used to determine the viability of these valves. The microfluidic sensor developed over the course of this work employs colorimetric absorbance spectrophotometry to measure nutrient concentrations in seawater. The primary enabling technology of the device is an integrated inlaid optical absorbance cell that takes advantage of total internal reflection (TIR) to optically interrogate water samples. The sensor is field tested through its application to phosphate measurement in the Bedford Basin after undergoing in-lab validation and bench-top calibration. Finally, alternative flow configurations and optical enhancement methods are explored to improve the sampling rate and sensitivity of the in situ unit.