Determination of glutamate using paper-based microfluidic devices with colorimetric detection for food samples

A paper-based device (PAD) capable of colorimetric detection was developed to determine the presence of glutamate in various food samples. The PAD employs an enzymatic reaction with glutamate followed by an oxidation reaction with N-benzoyl leucomethylene blue (BLMB) in the presence of horseradish peroxidase. The designed PAD consists of a sample introduction zone connected to a channel that transports a sample solution to three detection zones. The detection zones contain pre-deposited reagents: glutamate oxidase, horseradish peroxidase, BLMB, a phosphate buffer, and poly(acrylic acid). The PAD is perpendicularly immersed into a sample solution and bent at a right angle using a 3D-printed holder to allow the sample to simultaneously flow into three different detection zones. When the PAD is immersed into a sample containing glutamate, glutamate oxidase produces hydrogen peroxide, which changes the pale blue color of BLMB to a deep blue color in the presence of horseradish peroxidase. Under the optimum conditions, the calibration curve between the logarithm of the glutamate concentrations and the color intensity was linear within a range of from 5 × 10-6 mol L-1 to 10-2 and with a correlation coefficient of 0.994. Using this system, the PAD successfully determined glutamate in soup stocks, sauces, snacks, and tomato juice without the need of complicated sample pretreatment. These results agreed with those of a commercially available glutamate assay kit, which was employed as a certification method (tstat = 1.95, tcrit = 2.57). The developed PAD is simple, easy to fabricate, portable, and could be used outside of equipped laboratories to determine the presence of glutamate in food samples.