Multiple amplified enzyme-free electrochemical immunosensor based on G-quadruplex/hemin functionalized mesoporous silica with redox-active intercalators for microcystin-LR detection

A novel multiple amplified enzyme-free immunosensor was developed for competitive immunoassay of microcystin-LR (MC-LR). Classical electrochemical immunosensors usually employ enzymes as biocatalysts to afford amplified signals, but the proteolytic degradation and poor stability are still crucial problem. Herein, monodisperse core-shell mesoporous silica (SiO2@MSN)-functionalized DNAzyme concatamers were synthesized to load hemin and methylene blue (MB) as the mimic enzyme. Firstly, the surface of SiO2@MSN was conjugated with secondary antibody as the recognition of MC-LR antibody and with a DNA strand as the initiator. Two auxiliary DNA strands were then selected for the in-situ propagation to form a double-helix DNA through hybridization chain reaction (HCR), forming numerous DNAzymes (G-quadruplex/hemin) after the addition of hemin. Secondly, MB was inserted into the formed double-helix DNA, and also loaded in the brush-like structure of mesoporous SiO2@MSN. The molecular docking study showed that electrons can transfer more effectively with π-π stack of hemin/G-quadruplex and intercalation of MB/DNA, thus the catalytic ability of DNAzymes can be greatly improved. With the aid of MB, DNAzymes can catalyze the reduction of H2O2 to produce the electrochemical signal. This enzyme-free electrochemical immunosensor can successfully detect MC-LR in a range of 0.5 ng/L and 25 μg/L with a detection limit of 0.3 ng/L. This stable, sensitive and selective nonenzymatic electrochemical immunoassay shows promise for applications in food and environmental monitoring.