Patterned threads as solid-state reagent storage and delivery medium for automated periodic colorimetric monitoring of the environment

Environmental monitoring requires periodic measurement of parameters such as pH, heavy metals, nitrates and phosphates in field settings, autonomously. Microfluidic devices have been considered for environmental sensing due to its inherent advantages of lower volume, faster sensing and low-cost fabrication. Various sensing methods including colorimetric, fluorescence and electrochemistry have been studied. Among them, colorimetric sensing is attractive due to its simple instrumentation and wide range of selective reagents. Nevertheless, it is limited by the use of liquid reagents which necessitates the use of energy-demanding valves and pumps that limit the lifetime of colorimetric systems in the field. Here, we demonstrate that threads can be used for solid state reagent storage in the dried form to facilitate autonomous measurement. When integrated together with a microfluidic device, the system was shown to control the volume of sample exposed to the stored reagent on a thread for a controlled sample-reagent interaction and demonstrated a measurable color change using a simple, low-cost system that minimizes energy cost by reducing the number of pumps and valves necessary. We develop a new approach to pattern and separate the dried reagents using paraffin wax to allow the thread to slide smoothly into the microfluidic device while preventing the sample from wetting the remaining thread allowing experiments to be performed at different timepoints. We demonstrate this platform for periodic measurement using pH and nitrites as examples of environmental monitoring which is extendable to other colorimetric analytes for periodic autonomous monitoring. We show that the device operates in a range of 1–5 mg/mL of nitrite and pH range of 6–7.4 which covers the regulatory range in many countries.