Electrochemical Determination of Ca2+ Based On Recycling Formation of Highly Selective DNAzyme and Gold Nanoparticle-Mediated Amplification

PAIT effect: Padlock activator inhibits the trans-cleavage activity of CRISPR/Cas12a

The CRISPR/Cas12a system is increasingly used in biosensor development. However, high background signal and low sensitivity for the non-nucleic acid targets detection is challenging. Here, a padlock activator which could inhibit the trans-cleavage activity of CRISPR/Cas12a system in the intact form by steric hindrance effect (PAIT effect) was designed for non-nucleic acid targets detection. The PAIT effect disappeared when padlock activator was separated into two split activators. To verify the feasibility of padlock activator, a Ca2+ sensor was developed based on PAIT effect with the assistance of DNAzyme, activity of which was Ca2+ dependent. In the presence of Ca2+, DNAzyme was activated to cleave its substrate, a padlock activator modified with adenine ribonucleotide, into split padlock activators which would trigger the trans-cleavage activity of Cas12a to generate fluorescence. There was a mathematical relationship between the fluorescence intensity and the logarithm of Ca2+ concentration ranging from 10 pM to 1 nM, with a limit of detection of 3.98 pM. The little interference of Mg2+, Mn2+, Cd2+, Cu2+, Na+, Al3+, K+, Fe2+, and Fe3+ indicated high selectivity. Recovery ranged from 93.32% to 103.28% with RSDs from 1.87% to 12.74% showed a good accuracy and reliability. Furthermore, the proposed sensor could be applied to detect Ca2+ in mineral water, milk powder and urine. The results were consistent with that of flame atomic absorption spectroscopy. Thus, PAIT effect is valuable for expanding the application boundary of CRISPR/Cas12a system.

Sensitive electrochemical biosensor for rapid detection of sEV-miRNA based turbo-like localized catalytic hairpin assembly

Small extracellular vesicle-associated microRNAs (sEV-miRNAs) have emerged as critical biomarkers for cancer diagnosis, yet the rapid detection of these low-abundance molecules in clinical samples remains a formidable challenge. Herein, a simple turbo-like localized catalytic hairpin assembly (TL-CHA) was proposed for sEV-miR-1246 measurement. This electrochemical sensor achieves dual localization through the ingeniously use of AuNPs and DNA nanowires, which provides rich sites for CHA cascade amplification, significantly enhancing the effective reaction and amplify the detection response. Leveraging this innovative design, this biosensor demonstrated the ability to detect sEV-miRNA at concentrations as low as 5.24 aM in a time frame of 30 min. The precision of the measurements was validated through reverse transcription quantitative polymerase chain reaction. Furthermore, the sensor was used for analyzing plasma samples from gastric cancer patients yielded AUC values of 0.973 for all stages and 0.945 for early stages. This demonstrates the sensor's robust performance in both the staging diagnosis and early screening of gastric cancer. Therefore, this platform has great potential for the clinical cancer diagnosis.

Combining plasmonic and electrochemical biosensing methods

The idea of combining electrochemical (EC) and plasmonic biosensor methods was introduced almost thirty years ago and the potential of electrochemical-plasmonic (EC-P) biosensors has been highlighted ever since. Despite that, the use of EC-P biosensors in analytics has been rather limited so far and the search for unique applications of the EC-P method continues. In this paper, we review the advances in the field of EC-P biosensors and discuss the features and benefits they can provide. In addition, we identify the main challenges for the development of EC-P biosensors and the limitations that prevent EC-P biosensors from more widespread use. Finally, we review applications of EC-P biosensors for the investigation and quantification of biomolecules, and for the study of biomolecular and cellular processes.

ABC transporter-mediated MXR mechanism in fish embryos and its potential role in the efflux of nanoparticles

ATP-binding cassette (ABC) transporters are believed to protect aquatic organisms by pumping xenobiotics out, and recent investigation has suggested their involvement in the detoxification and efflux of nanoparticles (NPs), but their roles in fish embryos are poorly understood. In this regard, this paper summarizes the recent advances in research pertaining to the development of ABC transporter-mediated multi-xenobiotic resistance (MXR) mechanism in fish embryos and the potential interaction between ABC transporters and NPs. The paper focuses on: (1) Expression, function, and modulation mechanism of ABC proteins in fish embryos; (2) Potential interaction between ABC transporters and NPs in cell models and fish embryos. ABC transporters could be maternally transferred to fish embryos and thus play an important role in the detoxification of various chemical pollutants and NPs. There is a need to understand the specific mechanism to benefit the protection of aquatic resources.

Electrochemical Biosensor Using Nitrogen-Doped Graphene/Au Nanoparticles/DNAzyme for Ca2+ Determination

An electrochemical biosensor for detecting Ca²⁺ concentration was proposed using glass carbon electrodes (GCEs) modified with nitrogen-doped graphene (NGR), gold nanoparticles (AuNPs) and DNAzyme. The resistance signal was amplified through two methods: electrochemical reduction of AuNPs on the NGR surface to increase the specific surface area of the electrode and strengthen the adsorption of DNAzyme; and increasement of the DNAzyme base sequence. The process of electrode modification was characterized by scanning electron microscopy, Raman spectroscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). Experimental parameters’ influence, such as the deposition time of gold nanoparticles and the detection time, were assessed by electrochemical methods. The linear ranges of the electrochemical biosensor were in the range from 5 × 10⁻⁶ to 5 × 10⁻⁵ and 5 × 10⁻⁵ to 4 × 10⁻⁴ M, with a detection limit of 3.8 × 10⁻⁶ M. The concentration of Ca²⁺ in the serum of dairy cows was determined by the biosensor with satisfactory results, which could be potentially used to diagnose subclinical hypocalcemia.

Polymeric Electrochemical Sensor for Calcium Based on DNA

Plastic membranes containing deoxyribonucleic acid (DNA) as an electroactive material were acting as Ca²⁺ selective sensors. Diethyl phthalate (DEP), dioctyl Phthalate (DOP), or nitrophenyl octyl ether (NPOE) were used as plasticizers and polyvinyl chloride (PVC) was the membrane matrix. A sensor with a membrane composition of 120 mg PVC, 60 mg DOP plasticizer, and 2 mg DNA ionophore (DNA: DOP: PVC, 1.0:29.2:0.1 mole) was found to have the best performance. The slope of the calibration graph was 30 mV decade⁻¹. The optimum pH range was 5.7–9.5 for 0.01 M Ca²⁺. The sensor response time was fast (2–3 s) with a long working period (up to 3 weeks). Excellent selectivity for Ca²⁺ was indicated by the values of selectivity coefficients for different selected interference. The sensor was used effectively for the estimation of calcium in real samples (fruits, calcium syrup, milk, and dairy products).

DNAzyme-based sensing probe protected by DNA tetrahedron from nuclease degradation for the detection of lead ions

Lead poisoning endangers soil, plants and human health due to its toxic effect. It is urgent to develop ideal tool for the in vivo detection of Pb²⁺.In this study, tetrahedron-based Pb²⁺-sensitive DNAzyme sensor (TPS) is constructed by taking advantages of a classic Pb²⁺-dependent GR-5 DNAzyme and DNA tetrahedral structure, where the cleavage substrate and DNAzyme are modified with fluorophore FAM and quencher BHQ-1, respectively. DNA tetrahedron is arranged at the terminus of substrate/DNAzyme duplex to offer the protective shield against the nuclease attack. In the absence of Pb²⁺, FAM and BHQ-1 are kept close and FAM fluorescence is efficiently quenched. However, in the presence of Pb²⁺ cofactor, the DNAzyme exhibits the catalytic activity and cleaves the substrate strands, spatially separating the FAM away from BHQ-1 and releasing fluorescence. Utilizing the sensing probe, the Pb²⁺ can be quantitatively detected down to 1 nM without the interference from nontarget metal ions. Even if incubating in the human serum solution for 12 h, no substantial nuclease degradation is detected. In different complex biological milieu, the TPS can preserve the 85% of fluorescence signal, indicating that the developed TPS is a promising tool for the future application in the in vivo detection of Pb²⁺.

DNAzyme Sensor for the Detection of Ca2+ Using Resistive Pulse Sensing

DNAzymes are DNA oligonucleotides that can undergo a specific chemical reaction in the presence of a cofactor. Ribonucleases are a specific form of DNAzymes where a tertiary structure undergoes cleavage at a single ribonuclease site. The cleavage is highly specificity to co-factors, which makes them excellent sensor recognition elements. Monitoring the change in structure upon cleavage has given rise to many sensing strategies; here we present a simple and rapid method of following the reaction using resistive pulse sensors, RPS. To demonstrate this methodology, we present a sensor for Ca²⁺ ions in solution. A nanoparticle was functionalised with a Ca²⁺ DNAzyme, and it was possible to follow the cleavage and rearrangement of the DNA as the particles translocate the RPS. The binding of Ca²⁺ caused a conformation change in the DNAzyme, which was monitored as a change in translocation speed. A 30 min assay produced a linear response for Ca²⁺ between 1–9 μm, and extending the incubation time to 60 min allowed for a concentration as low as 0.3 μm. We demonstrate that the signal is specific to Ca²⁺ in the presence of other metal ions, and we can quantify Ca²⁺ in tap and pond water samples.

Two-Step Hydrothermal Preparation of Carbon Dots for Calcium Ion Detection

It is well known that the calcium ion is essential for maintaining life activities in living organisms, and it is of great significance to detect the intracellular calcium concentration. For the detection of calcium ions, we developed a new type of fluorescent carbon dots (CDs), whose surface was modified by ethylenebis(oxyethylenenitrilo)tetraacetic acid (EGTA) through a secondary hydrothermal method. This is a simple and convenient chemical preparation method because all reactions are carried out in the same autoclave, and the final product is directly the EGTA-modified CDs. The CDs exhibit bright blue fluorescence, and as the calcium concentration increases, the fluorescence intensity drops sharply. The fluorescence quenching correlates with the concentration of calcium ions and has a good linearity in the range of 15-300 μM with a detection limit of 0.38 μM. The experimental results confirmed that the detection of calcium ions by CDs is a static fluorescence quenching process. Also, cytotoxicity test and cellular imaging experiments have shown that the CDs are nontoxic and biocompatible.

Ultrasensitive electrochemical detection of miRNA based on DNA strand displacement polymerization and Ca2+-dependent DNAzyme cleavage

An ultrasensitive electrochemical sensing strategy for the detection of miRNA is developed combining strand displacement polymerization and a DNAzyme-catalyzed cleavage reaction.