Paper-based microfluidic device for serum zinc assay by colorimetry

Zinc is an essential micronutrient playing several crucial roles in human pathophysiology and its deficiency leads to micronutrient malnutrition. Therefore, a rapid, inexpensive, and accurate protocol for serum zinc concentration measurement becomes essential in community healthcare. This study demonstrates the design, fabrication, and characterization of a low-cost, paper-based microfluidic device (μPAD) to detect serum zinc concentration by colorimetric techniques. The μPAD comprises circular spotting zones doped with diphenylthiocarbazone, commonly known as dithizone, that produces pink-colored chelates upon reacting with zinc and the color intensity monotonically changes with concentration even across the physiological range (i.e., 5-25 μM). The design and the doping protocol were optimized to generate a linear correlation (in water, R² = 0.94; in artificial plasma, R² = 0.98) between a suitable optical measure (i.e., the normalized Euclidean shift) evaluated by image analysis of photographs captured by the camera of a standard smartphone and zinc concentration. The calibration curve for artificial plasma was further used to evaluate the zinc concentrations in real blood serum samples, resulting in a high parity with the respective gold standard method. The device is expected to significantly contribute in diagnosis of micronutrient malnutrition with a particular emphasis on community healthcare and to reach resource-limited settings.