Label-free fluorescent and electrochemical biosensors based on defective G-quadruplexes.

Abnormal levels of guanine closely associated with plenty of diseases are usually used as a biomarker for clinical diagnosis. In order to detect guanine and its derivatives accurately, in this paper, a defective G-quadruplex (DGQ) containing a G-vacancy at one of its G-quartet layers, and two kinds of G-quadruplex specific indicators including thioflavine T (ThT) and hemin were used for constructing a fluorescent and an electrochemical biosensor, respectively. In brief, a G-rich DNA probe is designed to form either hairpin or DGQ structure. In the absence of guanine, G-rich probes prefer to maintain hairpin structure and nearly have no interaction with ThT or hemin, leading to almost negligible signals. Upon addition of guanine, the G-rich probe fold into DGQ structure and then the G-vacancy in it is filled up immediately by guanine via Hoogsteen hydrogen bonds, resulting canonical G-quadruplex formation. Accordingly, ThT or hemin can selectively combine with G-quadruplex, giving rise to distinct fluorescent or current signal changes for label-free detection of guanine. Benefiting from the perfect discriminative ability of guanine towards DGQ and ThT/hemin against standard G-quadruplex, the fluorescent and electrochemical biosensors present better sensitivity and selectivity for guanine detection with the limit of detection (LOD) as low as 18.26 and 0.36 nM, respectively. Successful attempts were also made in applying the proposed electrochemical biosensor to detect guanine in drugs and urine, obtaining satisfactory recovery rates of 99~104% and 96~106%, respectively.

A ratiometric electrochemical biosensor for the exosomal microRNAs detection based on bipedal DNA walkers propelled by locked nucleic acid modified toehold mediate strand displacement reaction.

Sensitive and selective detection of microRNAs (miRNAs) in cancer cells derived exosomes have attracted rapidly growing interest owing to their potential in diagnostic and prognostic applications. Here, we design a ratiometric electrochemical biosensor based on bipedal DNA walkers for the attomolar detection of exosomal miR-21. In the presence of miR-21, DNA walkers are activated to walk continuously along DNA tracks, resulting in conformational changes as well as considerable increases of the signal ratio produced by target-respond and target-independent reporters. With the signal cascade amplification of DNA walkers, the biosensor exhibits ultrahigh sensitivity with the limit of detection (LOD) down to 67 aM. Furthermore, owing to the background-correcting function of target-independent reporters termed as reference reporters, the biosensor is robust and stable enough to be applied in the detection of exosomal miR-21 extracted from breast cancer cell lines and serums. In addition, because locked nucleic acid (LNA) modified toehold mediate strand displacement reaction (TMSDR) has extraordinary discriminative ability, the biosensor displays excellent selectivity even against the single-base-mismatched target. It is worth mentioning that our sensor is regenerative and stable for at least 5 cycles without diminution in sensitivity. In brief, the high sensitivity, selectivity and reproducibility, together with cheap, make the proposed biosensor a promising approach for exosomal miRNAs detection, in conjunction with early point-of-care testing (POCT) of cancer.

An immobilization-free electrochemical impedance biosensor based on duplex-specific nuclease assisted target recycling for amplified detection of microRNA.

An immobilization-free electrochemical impedance biosensor for microRNA detection was developed in this work, which was based on both the duplex-specific nuclease assisted target recycling (DSNATR) and capture probes (Cps) enriched from the solution to electrode surface via magnetic beads (MBs). In the absence of miR-21, Cps cannot be hydrolyzed due to the low activity of duplex-specific nuclease (DSN) against ssDNA. Therefore, the intact Cps could be attached to the surface of magnetic glass carbon electrode (MGCE), resulting in a compact negatively charged layer as well as a large charge-transfer resistance. While in the presence of miR-21, it hybridized with Cp to form a DNA-RNA heteroduplex. Due to the considerable cleavage preference for DNA in DNA-RNA hybrids, DSN hydrolyzed the target-binding part of the Cp while liberating the intact miR-21 to hybridize with a new Cp and initiate the second cycle of hydrolysis. In this way, a single miR-21 was able to trigger the permanent hydrolysis of multiple Cps. Finally, all Cps were digested. Thus, the negatively charged layer could not be formed, resulting in a small charge-transfer resistance. By employing the above strategy, the proposed biosensor achieved ultrahigh sensitivity toward miR-21 with a detection limit of 60aM. Meanwhile, the method showed little cross-hybridization among the closely related miRNA family members even at the single-base-mismatched level. Successful attempts were made in applying the approach to detect miR-21 in human serum samples of breast cancer patients.