Ultrasensitive immunoassay of microcystins-LR using G-quadruplex DNAzyme as an electrocatalyst

<div><p>An electrochemical immunoassay for microcystin-LR (MC-LR) detection was developed using multi-labeled horseradish peroxidase-mimicking DNAzyme on carbon nanotubes (CNTs) as electrocatalyst for signal amplification. CNTs were covalently conjugated to multiple DNAzyme along with MC-LR for a competitive immunoassay. The as-prepared DNAzyme/CNTs/MC-LR biolabel was specifically captured on the electrode surface, and current responses were obtained upon the electro-catalytic reduction of hydrogen peroxide by the captured biolabels. Under optimal conditions, the electro-catalytic current decreased linearly with the increase amount of MC-LR in the range from 0.01 to 7.0 µg L⁻¹. The linear regression equation was I (µA) = 12.96 − 1.48 X _[MC–LR] (µg L⁻¹), with a correlation coefficient of 0.989. The limit of detection of MC-LR was 2.31 ng L⁻¹. Application of the immunoassay method and LC/MS/MS method for MC-LR determination on spiked reservoir water gave recovery range of 91.7–105.2% and 94.0–105.0%, respectively. The resulting versatile immunoassay exhibited high sensitivity, good precision and satisfactory reproducibility, which could have vast potential in routine water quality monitoring for various environmental toxins.</p>