Current applications of colourimetric microfluidic devices (smart phone based) for soil nutrient determination

Soil health monitoring is crucial to maintain a sustainable agricultural yield and food security. Therefore, the efficiency and accuracy of monitoring methods are of paramount importance. Colourimetric detection relies on a color change given by interaction or reaction between the analyte and a chemical reagent. The colourimetric method must be robust, low-cost, and straightforward and is ideally the best option for the in situ test. Commercial colourimetric test kits often rely on visual inspection with the naked eye to estimate the target level, but the interferences, such as background light intensity and soil color, limit their accuracy and therefore their applicability. As such, these commercial test kits can only provide an estimation of a concentration range instead of quantified results for each analyte. With the use a smartphone, some of these issues can be overcome through normalization and digitization of colourimetric test results. The smartphone is a good starting point to link new technologies, such as artificial intelligence (AI), unmanned devices, and human-machine interface (HMI), to traditional analysis in both agricultural and environmental protection. Furthermore, if combined with the implementation of the chemistry in a microfluidic device, it could provide a cost-effective “lab-on-chip” system for in-field colourimetric application. In this chapter, we focus on the selection of chemical reagents for colourimetric detection, and discuss the possible interferences in both the colourimetric reaction and the digital data processing using smartphones, and then link to the recent progress of microfluidic devices used for colourimetric soil nutrients testing.