Portable and Low-Cost Colorimetric Sensor for Detection of Urea in Milk Samples

Presence of excess urea in milk above the maximum permissible limit can result in serious health issues, including kidney failure. While several tests exist for measuring urea concentration in milk, such tests typically require significant preprocessing of samples in a laboratory setting and result in a long turnaround time and increased cost of testing due to the use of expensive equipment operated by trained analysts. In this work, we present a point-of-use sensor for the quantification of urea concentration in milk samples, with minimal sample preprocessing and seamless readout. The assay involves the addition of p-dimethylaminobenzaldehyde (p-DMAB) to the supernatant obtained after the precipitation of milk solids, to form a yellow-green color chromogen. The intensity of the color is proportional to the amount of chromogen produced and is probed using a low-cost optical phase sensitive detection (PSD) system assembled using high-performance albeit inexpensive consumer-grade components. The data acquired by the sensor is streamed wirelessly to a smartphone for further analysis. We present preliminary results obtained using milk samples spiked with urea, with concentrations ranging from 0.1 mg/mL (safe level) to 0.7 mg/mL (maximum permissible limit), with a limit of detection (LOD) of 0.19 mg/mL.