A portable optical sensor based on a one-off test strip for fast evaluation of bacterial contamination in raw tofu

Fast determination of catalase-positive bacteria was easily performed by connecting a test strip to a self-made optical sensor and adding the reaction product, which was obtained from the hydrogen peroxide dismutation catalyzed by the bacterial catalase, to the reacting area of the test strip. The new type of the one-off test strip was designed and fabricated by immobilizing peroxidase and color-developing reagents on a supporting pad for the rapid determination of hydrogen peroxide. The signal of the optical sensor, which used a light-emitting diode as the monochromatic light resource, was based on the detection of the reflecting light from the reacting area on the test strip. The sensitivity, reproducibility and stability of the test strips were investigated and detection of catalase-positive bacteria in culture media and tofu samples was carried out respectively on the optical sensor. This technique allowed the quantification of Staphylococcus aureus (S. aureus), which was used as a model of the catalase-positive bacteria, with the detection limit of 2 × 102 cfu mL−1 in culture media without bacteria enumeration. The assay time was 9.5 min, including 8 min for catalase-catalyzed reaction and 1.5 min for the detection of remaining H2O2 on the sensor, respectively. Successive analysis was conveniently accomplished by replacing the one-off test strip. The proposed methodology is a rapid, straight-forward and cost-effective approach for the determination of catalase-positive bacteria in food samples.