Biosensor Based on Malic Dehydrogenase Immobilized in a CdS-Graphene-Chitosan Nanocomposite for Root-Exuded Malic Acid Determination

A new measurement method for root-exuded malic acid based on CdS-graphene-chitosan (CdS-G-CHIT) nanocomposite was reported. The preparation of CdS-G-CHIT was performed by dispersing CdS-G nanocomposite in chitosan solution. The amperometric biosensor was constructed by coating CdS-graphene-chitosan (CdS-G-CHIT) nanocomposite on glassy carbon electrode (GCE), and then adsorbing malic dehydrogenase (MDH). The pH in a pyrophosphate buffer solution, the concentration of oxidized nicotinamide adenine dinucleotide (NAD(+)), and the potential applied in the measurement system on the current response of the fabricated biosensor was investigated. Under optimum conditions, the biosensor showed an acceptable linear response from 1.8 mu M to 120 mu M, with a detection limit of 0.6 mu M. Moreover, the biosensor exhibited high specificity, good stability, and reproducibility. This approach was suitable for the direct measurement of malic acid in root exudates.