Enhancement of limit of detection by inducing coffee-ring effect in water quality monitoring microfluidic paper-based devices

Supply of safe drinking water is a challenging task requiring not only the availability of suitable devices but also satisfactory monitoring methods to ensure that the contaminants are present well within safe limits. In this context, microfluidic devices have recently emerged as promising technologies for point-of-use screening of water samples. These devices allow for easy interpretation, rapid and cost-effective analysis. The analytical approach employed in these devices for the detection of various water contaminants have different limits of detection that are bound by the nature and stoichiometry of the underlying chemical reaction. These limitations need to be addressed to make these devices more applicable and relevant for field analysis. In this work, we report a simple and novel technique to enhance the limit of detection for determining iron content in contaminated water. We utilize the coffee-ring phenomenon induced by outward capillary flows and differential evaporation rates on paper-based microfluidic devices. On addition of coloring agent, the response is obtained as a brown ring, analogous to the coffee-ring stain. The color intensity and the width of the ring are proportional to the concentration of iron in the water sample. The proposed scheme allowed a five-fold improvement in the limit of detection compared to the unmodified detection tests. The phenomenon can be utilized to enhance the signal from other heavy metal tests as well to enhance the limit of detection and bring these devices into their relevant market for widespread adoption.