A dual-signal readout enzyme-free immunosensor based on hybridization chain reaction-assisted formation of copper nanoparticles for the detection of microcystin-LR. 
Enzyme-based electrochemical biosensors are widely used in immunoassays, but the intrinsic disadvantages of enzymes including instability or sensitivity to temperature and pH should be considered.  Herein, an enzyme-free and dual-signal readout immunoassay was established to detect microcystin-LR (MC-LR) sensitively and selectively.  Firstly, the microplate was modified with gold nanoparticles-decorated-carbon nanotubes (AuNP-CNT) to immobilize sufficient antigens by the high surface area of CNT and high affinity of AuNP.  Then, silver nanoparticles were decorated on gold nanorods to form corn-like AgNP/AuNR composite and then capture secondary antibody and initiator DNA strand.  After hybridization chain reaction, long double helix DNA strands can be formed on AgNP/AuNR to germinate copper nanoparticles.  A dual-signal readout from the current responses of both silver and copper ions was obtained by using differential pulse stripping voltammetry with the aid of acid-treatment.  By using a competitive immunoreaction, MC-LR can be detected in a linear range from 0.005 μg/L to 20 μg/L with a lower detection limit of 2.8 ng/L. The reproducibility, stability and specificity were all acceptable, indicating its promising application in environment monitoring and sensitive electrochemical detection for other analytes.