Label-free electrochemical homogeneous detection of the depression marker human apolipoprotein A4 based on proximity hybridization triggered rolling circle amplification.

In this paper, we developed a label-free homogeneous electrochemical sensor for detection of apolipoprotein A4 based on proximity hybridization triggered rolling circle amplification induced G-quadruplex formation. The presence of apolipoprotein A4 promoted the formation of a proximate complex via the proximity hybridization of the aptamer DNA strands, which unfolded the molecular beacon, the stem part of molecular beacon as a primer to initiate the RCA process. Thus, with the electrochemical indicator hemin selectively intercalated into the multiple G-quadruplexes, a significant electrochemical signal drop is observed, which is dependent on the concentration of the target apolipoprotein A4. Thus, using this "signal-off" mode, label-free homogeneous electrochemical strategy for sensitive apolipoprotein A4 assay with a wide range from 1 pg mL-1 to 100 ng mL-1, with a low detection limit of 0.51 pg mL-1. And it rendered satisfactory analytical performance for the determination of apolipoprotein A4 in serum samples. Furthermore, this method also exhibits additional advantages of simplicity and low cost, since both expensive labeling and sophisticated probe immobilization processes are avoided. The satisfactory results indicated that the proposed sensor had promising potential in the clinical diagnosis of depression.