Optimised self-calibrating microfluidic systems towards design optimisation

Quane, Nile and Cheneler, David (2022) Optimised self-calibrating microfluidic systems towards design optimisation. PhD thesis, Lancaster University.

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Abstract

Clean water is a finite resource, and the quality of such is best monitored by colorimetric in-situ sensors, which allow frequent, non-labour intensive sampling, and are low-cost and simple to manufacture. There are multiple types of sensors that exist in the literature, however, many are cost-prohibitive for wide deployment, or the literature does not not fully elaborate on their operation. The aim of this research was to extend the lifetime and improve the performance of a colorimetric in-situ sensor, Aquamonitrix colorimetric sensor, that was produced by T.E. Laboratories, in addition to characterising sensor behaviour. Its operation was focused on the Griess reaction, in which a vivid azo dye is produced in the presence of nitrite, that can be linearly calibrated to the absorbance by the dye from a photodectector placed at the opposite end of a microfluidic detector channel to a monochromatic light source. Using multiobjective optimisation on a numerical model of a Y-junction micromixer, it was found that both sensitivity could be increased and reagent could be conserved, by limiting the proportion of reagent used during testing to 5% to 7.5% of testing solution, as opposed to the 50% originally used by the system. The conservation of the reagent allowed for an increased sensor deployment lifetime of up to tenfold. To better understand how the parameters of analyte concentration, reagent proportion of test solution, and mean flow velocity of the solution affect sensor output, both mechanistic and data-based modelling of the continuous and stopped flow stages of the sensor were undertaken. Third-order and second-order models were identified for the continuous and stopped flow data respectively. The second-order model is analagous to the two-step Griess reaction, of which there is a first, faster step. Further characterisation of the zeroes, poles and transfer function coefficients of the third order models showed that parameterisation was possible and, using principal component analysis, reduction of parameters. Other testing on the effects of order of cycles, turbidity and heavy metals was also conducted to measure their impact on sensor output. Carryover between sensor cycles was found to be the most interfering factor on sensor output, due to the microfluidic connector components, which was eliminated after eight cycles. Overall, the performance and efficiency of the existing sensor was improved, iii and the methodologies in this dissertation can be used for other continuous-flow colorimetric sensors and reactions, or even other microreactor applications, such as in green chemistry.