A novel bioluminescent detection of exonuclease I activity based on terminal deoxynucleotidyl transferase-mediated pyrophosphate release

Molecular Recognition-Triggered Aptazyme Sensor Using a Co-MOF@MCA Hybrid Nanostructure as Signal Labels for Adenosine Triphosphate Detection in Food Samples

Developing rapid detection technology for adenosine triphosphate (ATP) is crucial in quality supervision and food safety. Herein, an electrochemical aptasensor based on an aptazyme-catalyzed signal amplification strategy is constructed for ATP detection using polyethyleneimine-functionalized molybdenum disulfide (PEI-MoS₂)/Au@PtPd nanobipyramids (MoS₂/Au@PtPd NBPs) as a modification material. Additionally, a novel kind of nitrogen-rich covalent organic framework (COF) is prepared using melamine and cyanuric acid (MCA). We synthesize MCA and the Co-based metal organic framework (Co-MOF) as the signal label. Due to the fact that π-π stacking interactions of Co-MOF@MCA can expand the load efficiency and surface concentration of the signal label, the signal response is an order of magnitude higher than that of Co-MOF or MCA as the signal label. Target ATP changes the conformation of the aptazyme, and it becomes activated. With the assistance of metal ions, the signal label is circularly cleaved, causing an amplification of the signal. Among them, MoS₂/Au@PtPd NBPs have a large specific surface area and good electrical conductivity and can carry substantial DNA strands and amplify the redox signal of methylene blue (MB). Under optimal conditions, the aptasensor can detect ATP from 10 pM to 100 μM with a low limit of detection of 7.37 × 10^-10 μM. Therefore, the novel aptasensor has extensive application prospects in quality supervision and food safety.

Visual Detection of Cucumber Green Mottle Mosaic Virus Based on Terminal Deoxynucleotidyl Transferase Coupled with DNAzymes Amplification

A simple, rapid, and sensitive visual detection method for observing cucumber green mottle mosaic virus was reported based on the template-independent polymerization activity of terminal deoxynucleotidyl transferase (TdT), coupled with the cascade amplification of Mg²⁺-dependent DNAzyme and hemin/G-quadruplex DNAzyme. Briefly, the hybridized dsDNA of T1/P1 was cut into two parts at its position of 5′-AA↓CG↑TT-3′ by the restricted enzyme AcII. The longer, newborn fragment originating from P1 was tailed at its 3’-end by oligo dG, and an intact enzymatic sequence of Mg²⁺-dependent DNAzyme was generated. The substrate sequence in the loop segment of the hairpin probe (HP) hybridized with the newborn enzymatic sequence and was cleaved into two parts in the presence of Mg²⁺. The locked G-quadruplex sequence in the stem segment of the HP was released, which catalyzed the oxidation of ABTS^2- in the presence of H₂O₂, and the resulting solution turned green. A correlation between the absorbance and concentration of T1 was obtained in a range from 0.1 pM to 2 nM, with a detection limit of 0.1 pM. In addition to promoting a lower detection limit and shorter monitoring time, this method also demonstrated an excellent selectivity to single or double nucleotide changes. Therefore, the designed strategy provided a rapid and efficient platform for viral inspection and plant protection.

Exonuclease I-assisted fluorescent method for ochratoxin A detection using iron-doped porous carbon, nitrogen-doped graphene quantum dots, and double magnetic separation

In this paper, a fluorescent method was developed for ochratoxin A (OTA) detection that uses iron-doped porous carbon (MPC) and aptamer-functionalized nitrogen-doped graphene quantum dots (NGQDs-Apt) as probes. In this method, the adsorbance of the NGQDs-Apt on the MPC due to a π-π interaction between the aptamer and the MPC results in the quenching of the fluorescence of the NGQDs-Apt. However, since OTA interacts strongly with the aptamer, the presence of OTA leads to the detachment of the NGQDs-Apt from the MPC, resulting in the resumption of fluorescence from the NGQDs-Apt. When exonuclease I (Exo I) is also added to the solution, this exonuclease specifically digests the aptamer, leading to the release of the OTA back into the solution. This free OTA then interacts with another MPC-NGQDs-Apt system, inducing the release of more NGQDs into the solution, which enhances the fluorescent intensity compared to that of the system with no Exo I. Utilizing this behavior of OTA in the presence of NGQDs-Apt, it was possible to detect concentrations of OTA ranging from 10 to 5000 nM, with a limit of detection of 2.28 nM. Our method was tested by applying it to the detection of OTA in wheat and corn samples. This method has four advantages: (1) the magnetic porous carbon is easy to prepare, its porosity enhances its loading capacity for NGQDs, it highly efficiently quenches the fluorescence of the NGQDs, and its magnetic properties facilitate the separation of the MPC from other species in solution; (2) applying double magnetic separation decreases the background signal; (3) Exo I digests the free aptamer effectively, which allows the resulting free OTA to induce the release of more NGQDs-Apt, ultimately enhancing the fluorescent signal; and (4) the proposed method presented high sensitivity and a wide linear detection range. This method may prove helpful in food safety analysis and new biosensor development (achieved by using different aptamer sequences to that used in the present work). Graphical abstract Exonuclease I (Exo I)-assisted fluorescent method for ochratoxin A (OTA) detection using magnetic porous carbon (MPC), nitrogen-doped graphene quantum dots (NGQDs), and double magnetic separation.