Towards the development of paper analytical devices for testing alkaline phosphatase, starch, and urea in milk

We developed novel assays on paper platform for alkaline phosphatase (ALP), starch, and urea by employing 5-bromo-4-chloro-3-indolyl phosphate/nitro blue tetrazolium, iodometric test, and p-dimethylaminobenzaaldehyde (DMAB) reagent, respectively. Under optimum conditions, the working ranges of the assays were 2000 to 3.82 × 10−3 U mL−1, 10 to 1.5 × 10−1%, and 5 to 1.9 × 10−2% for ALP, starch, and urea, respectively. The linear range, limit of detection (LOD) and limit of quantification (LOQ) were 3.0 × 10−2 to 3.8 × 10−3 U mL−1, 6.4 × 10−3 U mL−1, and 21.1 × 10−3 U mL−1, respectively, for ALP assay. The linear range, LOD and LOQ of paper-based starch assay were 12.5 × 10−1 to 1.5 × 10−1%, 0.3%, and 1.0%, respectively. Similarly, linear range, LOD and LOQ of urea assay were 2.1 × 10−1 to 1.0 × 10−2%, 5.0 × 10−2%, and 1.7 × 10−1%, respectively. We used the newly developed assay methods to test pasteurisation and adulteration of 85 milk samples collected locally.