Competitive redox reaction of Au-NCs/MnO2 nanocomposite: Toward colorimetric and fluorometric detection of acid phosphatase as an indicator of soil cadmium contamination.

In this study, by rational regulating the competitive redox reaction of Au-NCs/MnO2 nanocomposite between the dye indigo carmine (IC) and the enzymatic product L-ascorbic acid (AA), we have established a colorimetric and fluorometric double-channel responsive assay for acid phosphatase (ACP), which could serve as an indicator of soil cadmium (Cd) contamination. Initially, the gold nanoclusters (Au-NCs) were added to the suspension of MnO2 nanosheets to form Au-NCs/MnO2 nanocomposite with enhanced oxidative degradation ability. When IC was subsequently added, the blue color of IC faded due to oxidative degradation, and the mixture showed the yellow color of Au-NCs/MnO2 nanocomposite. Meanwhile, based on the inner filter effect (IFE), the fluorescence of Au-NCs was suppressed by MnO2 nanosheets during this process. However, with the presence of AA, hydrolyzed from L-ascorbic-2-phosphate (AAP) by ACP, the MnO2 nanosheets in Au-NCs/MnO2 nanocomposite were reduced to Mn2+ immediately. As a consequence, IC remained its blue color, in the meantime, the fluorescence of Au-NCs recovered, which essentially constituted a new mechanism for ACP detection with colorimetric and fluorometric double-channel response. With the method we developed, soil ACP activity can either be directly visualized by bare eyes or detected reliably through double channels. Furthermore, the dynamic changes of ACP activity during soil Cd contamination could also be monitored; the sharp increase of ACP activity at an appropriate time point could serve as a unique alarm for cadmium (Cd) contamination in soil, which is of great importance for soil quality evaluation and ecological risk assessment.