In-channel modifications for improved optical detection of phosphate in a microfluidic chip design

Recent developments in the area of low cost optical analysers has enabled rapid, reliable and robust analysis of water nutrient levels, such as phosphate, in water systems. Herein, describes an enhancement study of a previously demonstrated lab- on-a-disc (LOAD) centrifugal microfluidic device for the detection of phosphate in freshwater. The LOAD device utilizes a microfluidic sample processing to enable high precision metering and reagent mixing, followed by colorimetric analysis (at 880 nm) of the resultant complex. A customisable and complementary, in-house analysis system was also developed to enhance user interaction and enable rapid analysis. This analysis system delivers both disc centrifugation and automated colourimetric detection of the LOAD device, with recording of data transmitted via PC interface. The aim of this study is to maintain the same level of sensitivity of the current[1] system with a reduced pathlength. The limit of detection (LOD) and limit of quantification (LOQ) for this new revised system are as follows: The blackened chip obtained the best sensitivity with an LOD and LOQ of 6 and 19 μg L-1 respectively, followed by the P80 roughened chip which achieved 13 and 38 μg L-1, these results will be discussed further in this paper.The previously demonstrated microfluidic platform demonstrated an optical path length detection of 75 mm for optimal detectability, resulting in low quantity of sample testing per disc. This work details the optimisation of the disc design through a reduction of path length, therefore increasing the number of test replications on-disc by two-fold and a reduction in reagent consumption, whilst maintaining the same sensitivity using photo-enhancement techniques. The photo-enhancement techniques described in this paper utilize detection channel surface modifications, e.g. coatings and refractive index modification. These assist in significantly improving the signal-to-noise (S/N) with increased transmittance, hence increasing the overall sensitivity that can be achieved using the system. This piece of work focuses on the optical improvement using surface modification strategies in detection enhancement.