Optical biosensor test strip for the screening and direct determination of l-glutamate in food samples

An optical glutamate biosensor test strip based on stacked membranes of nafion/sol–gel (bottom layer) and chitosan (uppermost layer) was fabricated on a piece of paper as support to form a test strip. The use of a stacked membrane system allows multiple immobilizations of sensing components directly without any covalent attachment via straight forward procedures. The uppermost membrane consisted of immobilised enzymes l-glutamate oxidase (GLOD) and horseradish peroxidase (HRP), which sensed the presence of l-glutamate and the bottom membrane contained the indicator dye, 3,3′,5,5′-tetramethylbenzidine. The test strip can be used to measure l-glutamate quantitatively by observing a colour change from light green to dark green with increasing l-glutamate concentrations. Quantitative analysis could be performed by measuring the reflectance intensity of the colour change at 550 nm. The glutamate biosensor test strip gave a linear response range of 0.01–0.30 mM to l-glutamate with a limit of detection of 5 μM. The strips were successfully applied for the estimation of l-glutamate in common food items such as sauces, soups, processed food and flavour enhancers. Results of analysis of l-glutamate in various food samples using the glutamate test strip were comparable to a standard procedure employing HPLC method.