Immobilization of papain on nitrogen-doped graphene quantum dots improves the enzymatic properties and makes it a biosensor for cystatin C

In recent years enzymes had many applications, such as catalysts in organic synthesis and producing biosensors. In this study, nitrogen-doped graphene quantum dots (NGQDs) were functionalized with glutaraldehyde (NGQD-Glu) and used as a support for the immobilization of papain (NGQD-Glu-Papain). Immobilization of papain on NGQDs has considerably improved papain stability. So, with incubation of the enzymes at 70 °C, immobilized papain maintained 80% of its activity for 150 min while the free enzyme's residual activity was 55% after 50 min. The stability results in the presence of organic solvents showed that the immobilization increased papain stability, especially in polar solvents. The activity results showed that the Km value decreases from 2.02 g L−1 to 1.20 g L−1 after immobilization, and the Vmax value decreases from 932.1 μmol mg−1 min−1 to 62.5 μmol mg−1 min−1. The fluorimetric studies showed that the NGQD-Glu-Papain is specific sensitive to cystatin C at concentrations ranging from 0.05 ng L−1 to 50 ng L−1 with a 0.05 ng L−1 of limit for detection. In conclusion, NGQDs can be suitable for enzyme stabilization and biosensor production. [Display omitted] •Immobilization leads to enhanced enzymatic properties.•The stability of the immobilized enzyme has been significantly improved.•Immobilized papain is high stable in organic solvents and can be used as a catalyst.•Immobilized papain on nitrogen-doped graphene quantum dots has biosensor properties.•This biosensor is specific for cystatin C with 0.05 ng L−1 of limit detection.