A molecular imprinting-based selectively regulated laccase-like nanozyme strategy for onsite visual colorimetric/fluorescent assay toward microcystins in environmental waters

Given the serious harm that microcystins (MCs) cause to public health and ecosystems (e.g., freshwater systems), there is a great need for an onsite MCs environmental assay that is cost-effective, easy-to-use, reliable and precise. Regarding this point, we here proposed a molecular imprinting-based selectively regulated laccase-like copper-doped carbon dot nanozyme (MIP@Cu-CD) to engineer a smartphone-assisted portable device, as a proof of concept, yielding the onsite visual colorimetric/fluorescence imaging-driven precise and reliable detection of MC-LR (a representative MCs) in real environmental waters. The detection mechanism was derived from the combination of MC-LR with its recognizable molecularly imprinted cavity, blocking the laccase-like activity-triggered MIP@Cu-CD to oxidize 2,4-dichlorophenol (2,4-DP) into a red quinoneimine product in the presence of 4-aminophenol (4-AP). At this moment, a decrease in colorimetric absorbance at 510 nm and a recovery of intrinsic blue fluorescence at 420 nm were observed in the MIP@Cu-CD + 2,4-DP + 4-AP system, with an increase in MC-LR concentration. On this basis, the system of MIP@Cu-CD + 2,4-DP + 4-AP was integrated with a home-made smartphone-assisted portable device to perform the onsite visual colorimetric/fluorescence images-based RGB detection of MC-LR in tap water, urban sewage, and lake water, with the detection limits of 0.227 μg/L and 0.101 μg/L in colorimetric and fluorescence ways, respectively. Moreover, the reliability was also confirmed by the liquid chromatography-mass spectrometry (LC-MS) method. This work provides a feasible approach to utilize molecular imprinting-based laccase-like nanozyme strategy to integrate portable devices for realizing the on-demand onsite detection of MCs in environmental waters.