Cellular interface supported toehold strand displacement cascade for amplified dual-electrochemical signal and its application for tumor cell analysis

Label-free dual-mode sensing platform based on target-regulated CRISPR-Cas12a activity for ochratoxin A in Morinda officinalis

Many traditional Chinese herbs are susceptible to ochratoxin A (OTA), a potent mycotoxin, which causes serious effects on the quality of the herb and on people's health. The development of methods to detect OTA is extremely important. Most methods for detecting OTA are based on a single-signal output mode, which might be easily influenced by complex environmental conditions. In this research, by taking advantage of the cleavage of DNA by target-induced CRISPR-Cas12a activity and the difference in electrostatic force of DNA to different charge electrochemiluminescent (ECL) and electrochemical (EC) probes, a biosensor is developed for the detection of OTA. First, the CRISPR-Cas12a system consists of a well-designed crRNA, its complementary strand (also as an aptamer for OTA), and Cas12a. Without the target, this CRISPR-Cas12a system is in the "activated stage", which digests hairpin DNA on the electrode, resulting in a weak ECL signal and strong current response. With the introduction of OTA bound with the aptamer, CRISPR-Cas12a activity is inhibited ("locked stage"). Thus, hairpin DNA remained intact on the electrode, resulting in recovery of the ECL signal and attenuation of the current intensity. As a result, this label-free dual-mode sensing platform realizes an assay for OTA in Morinda officinalis. This target-regulated CRISPR-Cas12a activity-sensing platform with dual-mode output not only provides high sensitivity (due to the CRISPR-Cas12a system), but also has good anti-interference ability against complex substrates (due to dual-mode output), and exhibits a broad range of prospects for application.

Electrochemical biosensing based on folic acid-triazine-grafted reduced graphene oxide: a highly selective breast cancer cell sensor

Based on the results of this research, a new electrochemical sensor has been developed to detect human breast cancer cells (MCF-7). A folic acid (FA)-functionalized triazine-grafted reduced graphene oxide (RGOTrz) was used as the modifier of a glassy carbon electrode (GCE) for application as the sensing element. The composition of the resulting FA-RGOTrz/GCE was investigated using XRD (X-ray diffraction), FT-IR (Fourier-transform infrared) spectroscopy, SEM (scanning electron microscopy) and UV-vis spectroscopy studies. CV (cyclic voltammetry) and EIS (electrochemical impedance spectroscopy) techniques were also used to characterize the electrochemical proficiency of the new electrode. Further, MCF-7 cancer cells were examined in solutions of phosphate buffer and [Fe(CN)6]3-/4- as a suitable supporting electrolyte and a useful probe, respectively. The FA-RGOTrz/GCE provides a suitable substrate to reversible redox reactions and provides good electrochemical signals after binding to cancer cells. DPV (differential pulse voltammetry) results indicated that the binding of folate receptor (FR) in the MCF-7 cell to the RGOTrz-modified electrode, in the presence of [Fe(CN)6]3-/4-, reduced folic acid, diminished electron transfer and collapsed the current signal. During the measured flow, a detection limit of 50 human breast cancer cells per milliliter was obtained. The FA-RGOTrz/GCE, with its unique structural design, significantly increases the electron transfer and electrochemical activity towards the detection of MCF-7 cells. This FA-RGOTrz/GCE sensor, due to its special structure, shows high sensitivity to FR in MCF-7 cells and excellent, reliable and satisfactory performance and a great promise for use in industries and medical field.

An immobilization-free electrochemical aptamer-based assay for zearalenone based on target-triggered dissociation of DNA from polydopamine nanospheres with strand displacement amplification

Zearalenone (ZEN), a widespread mycotoxin, can cause great harm to people's health. In order to assay ZEN, an immobilization-free electrochemical sensor has been developed. A multifunctional hairpin DNA has been carefully designed, including three functions: the aptamer for zearalenone (ZEN), primer, and template sequence. This hairpin DNA can anchor on polydopamine nanospheres (PDANSs), which can protect DNA against the digestion of enzymes and prevent the occurrence of strand displacement amplification (SDA). In the presence of ZEN, the hairpin DNA is dissociated from PDANSs due to the interaction between ZEN and the aptamer, and the SDA reaction is initiated with the help of endonuclease and polymerase. During the SDA process, substantial amounts of negatively charged dsDNA are generated. The MB molecules are embedded into the dsDNA grooves to obtain the complex with a negative charge. The confined MB is repelled on the surface of the negatively charged ITO electrode, leading to the decline of the current. This immobilization-free method possesses high sensitivity (LOD of 0.18 pg mL^-1) and good selectivity and can be applied to assay ZEN in corn flour.

First passage time study of DNA strand displacement

DNA strand displacement, in which a single-stranded nucleic acid invades a DNA duplex, is pervasive in genomic processes and DNA engineering applications. The kinetics of strand displacement have been studied in bulk; however, the kinetics of the underlying strand exchange were obfuscated by a slow bimolecular association step. Here, we use a novel single-molecule fluorescence resonance energy transfer approach termed the "fission" assay to obtain the full distribution of first passage times of unimolecular strand displacement. At a frame time of 4.4 ms, the first passage time distribution for a 14-nucleotide displacement domain exhibited a nearly monotonic decay with little delay. Among the eight different sequences we tested, the mean displacement time was on average 35 ms and varied by up to a factor of 13. The measured displacement kinetics also varied between complementary invaders and between RNA and DNA invaders of the same base sequence, except for T → U substitution. However, displacement times were largely insensitive to the monovalent salt concentration in the range of 0.25-1 M. Using a one-dimensional random walk model, we infer that the single-step displacement time is in the range of ∼30-300 μs, depending on the base identity. The framework presented here is broadly applicable to the kinetic analysis of multistep processes investigated at the single-molecule level.

Ultrasensitive electrochemical biosensor for attomolar level detection of let 7a based on toehold mediated strand displacement reaction circuits and molecular beacon mediated circular strand displacement polymerization

In this study, an ultrasensitive electrochemical miRNA biosensor based on toehold mediated strand displacement reaction circuits (SDRCs) and molecular beacon mediated isothermal circular strand displacement polymerization reaction (ICSDPR) has been proposed. During the SDRCs module, the cascade strand displacement reaction induces the recycling of the target let 7a and generation of a large amount of strand A (SA). The SA recognition opens the hairpin capture probe immobilized on the gold electrode, thus, varying the distance between the redox molecules and electrode surface. The primer mediated ICSDPR is observed to further generate a large amount of SA, thus, leading to a reduction in the signal. Considering these merits, the proposed method is observed to exhibit a log-linear linearity from 10 aM to 100 pM and ultrahigh sensitivity towards let 7a down to 6.2 aM, with a capability of distinguishing the let 7a family members, thereby, providing a new electrochemical route for early cancer screening.

Facile incorporation of DNA-templated quantum dots for sensitive electrochemical detection of the oral cancer biomarker interleukin-8

Recent studies reveal a great value of interleukin-8 (IL-8), a pro-inflammatory cytokine, as a potent biomarker for early diagnosis of oral cancer. Herein, a new electrochemical method is proposed to detect IL-8 by facilely incorporating DNA-templated quantum dots (QDs). In principle, target IL-8 is first treated with the reducing agent tris(2-carboxyethyl)phosphine (TCEP) to yield active thiols and then captured by antibody-functionalized magnetic beads (MBs). Thereafter, via the Michael addition reaction between the active thiol and maleimide group, a maleimide-modified DNA probe is linked to the surface of MBs, which can initiate a process of rolling circle amplification. In this way, long-range DNA strands are generated on the MB surface, subsequently recruiting DNA-templated CdTe/CdS QDs (DNA-QDs) to act as electrochemical reporters. By tracing the responses of DNA-QDs, the method allows IL-8 detection in a linear range from 5 to 5000 fg/mL with a detection limit of 3.36 fg/mL. The selectivity, reproducibility, and applicability in complex serum samples are also demonstrated to be favorable, indicating that the method may have a great potential in the future. More importantly, the use of TCEP treatment in the method not only provides a facile way to incorporate DNA-QDs, avoiding the complicated and time-consuming preparation process of antibody-DNA conjugates or functional nanomaterials; but also makes the method capable of being extended to detect other protein biomarkers in view of widespread presence of disulfides, which may hold a broad potential to facilitate efficient biosensing designs.