Homogeneous Electrochemical Biosensor for Melamine Based on DNA Triplex Structure and Exonuclease III-Assisted Recycling Amplification

Integrating Reliable Pt-S Bond-Mediated 3D DNA Nanomachine with Magnetic Separation in a Homogeneous Electrochemical Strategy for Exosomal MicroRNA Detection with Low Background and High Sensitivity

Precise and sensitive analysis of exosomal microRNA (miRNA) is of great importance for noninvasive early disease diagnosis, but it remains a great challenge to detect exosomal miRNA in human blood samples because of their small size, high sequence homology, and low abundance. Herein, we integrated reliable Pt-S bond-mediated three-dimensional (3D) DNA nanomachine and magnetic separation in a homogeneous electrochemical strategy for the detection of exosomal miRNA with low background and high sensitivity. The 3D DNA nanomachine was easily prepared via a facile and rapid freezing method, and it was capable of resisting the influence of biothiols, thus endowing it with high stability. Notably, the as-developed magnetic 3D DNA nanomachine not only enabled the detection system to have a low background but also coupled with liposome nanocarriers to synergistically amplify the current signal. Consequently, by ingeniously combining the low background and multiple signal-amplification strategies in homogeneous electrochemical biosensing, highly sensitive detection of exosomal miRNA was successfully achieved. More significantly, with good anti-interference ability, the as-proposed method could effectively discriminate plasma samples from cancer patients and healthy subjects, thus showing a high potential for application in the nondestructive early clinical diagnosis of disease.

Homogeneous Electrochemical Immunoassay Using an Aggregation-Collision Strategy for Alpha-Fetoprotein Detection

Homogeneous immunoassays represent an attractive alternative to traditional heterogeneous assays due to their simplicity and high efficiency. Homogeneous electrochemical assays, however, are not commonly accessed due to the requirement of electrode immobilization of the recognition elements. Herein, we demonstrate a new homogeneous electrochemical immunoassay based on the aggregation-collision strategy for the quantification of tumor protein biomarker alpha-fetoprotein (AFP). The detection principle relies on the aggregation of AgNPs induced by the molecular biorecognition between AFP and AgNPs-anti-AFP probes, which leads to an increased AgNP size and decreased AgNP concentration, allowing an accurate self-validated dual-mode immunoassay by performing nanoimpact electrochemistry (NIE) of the oxidation of AgNPs. The intrinsic one-by-one analytical capability of NIE as well as the participation of all of the atoms of the AgNPs in signal transduction greatly elevates the detection sensitivity. Accordingly, the current sensor enables a limit of detection (LOD) of 5 pg/mL for AFP analysis with high specificity and efficiency. More importantly, reliable detection of AFP in diluted human sera of hepatocellular carcinoma (HCC) patients is successfully achieved, indicating that the NIE-based homogeneous immunoassay shows great potential in HCC liquid biopsy.

Application of Molecularly Imprinted Electrochemical Biomimetic Sensors for Detecting Small Molecule Food Contaminants

Environmental chemical contaminants in food seriously impact human health and food safety. Successful detection methods can effectively monitor the potential risk of emerging chemical contaminants. Among them, molecularly imprinted polymers (MIPs) based on electrochemical biomimetic sensors overcome many drawbacks of conventional detection methods and offer opportunities to detect contaminants with simple equipment in an efficient, sensitive, and low-cost manner. We searched eligible papers through the Web of Science (2000-2022) and PubMed databases. Then, we introduced the sensing mechanism of MIPs, outlined the sample preparation methods, and summarized the MIP characterization and performance. The classification of electrochemistry, as well as its advantages and disadvantages, are also discussed. Furthermore, the representative application of MIP-based electrochemical biomimetic sensors for detecting small molecular chemical contaminants, such as antibiotics, pesticides, toxins, food additives, illegal additions, organic pollutants, and heavy metal ions in food, is demonstrated. Finally, the conclusions and future perspectives are summarized and discussed.

Rational design of an allosteric G-quadruplex aptamer probe for ultra-sensitive detection of melamine in milk

Efficient and accurate detection of melamine in dairy products remains a crucial yet challenging task. Herein, an allosterically modulated G-quadruplex-integrated aptamer is rationally designed with thymine-rich recognition termini for melamine binding. The detection process is facile by simply introducing the analyte into the mixture consisting of G-quadruplex aptamer probes, exonuclease III, and thioflavin T (ThT). The detection feasibility is confirmed by the polyacrylamide gel electrophoresis and fluorescence measurement results. This exonuclease III-assisted signal amplifiable approach works well in a linear range from 0.1 nM to 0.1 μM. Moreover, a detection limit as low as 83 pM is easily achieved, which is almost five orders of magnitude smaller than the maximum allowable melamine levels (about 8 μM) defined by many countries all over the world. The whole assay time for each test is no longer than 1 h. Additionally, the scheme is highly specific and satisfactory recovery rates (from 91% to 104%) are readily obtained when challenged with melamine-spiked milk samples. Therefore, the label-free, turn-on, low-cost, and time-efficient method can be used for reliable detection of melamine in an easily manipulated and ultra-sensitive manner, which may find its utilization in the field of food safety, biomedical engineering, and clinical diagnosis.

Ultra-Sensitive, Rapid and On-Site Sensing Harmful Ingredients Used in Aquaculture with Magnetic Fluid SERS

The integration of surface-enhanced Raman scattering (SERS) spectroscopy with magnetic fluid provides significant utility in point-of-care (POC) testing applications. Bifunctional magnetic-plasmonic composites have been widely employed as SERS substrates. In this study, a simple and cost-effective approach was developed to synthesize magnetic-plasmonic SERS substrates by decorating silver nanoparticles onto magnetic Fe₃O₄ nanoparticles (AgMNPs), which function both as SERS-active substrates and magnetic fluid particles. The strong magnetic responsivity from AgMNPs can isolate, concentrate, and detect target analytes from the irregular surface of fish skin rapidly. We fabricate a microfluid chip with three sample reservoirs that confine AgMNPs into ever smaller volumes under an applied magnetic field, which enhances the SERS signal and improves the detection limit by two orders of magnitude. The magnetic fluid POC sensor successfully detected malachite green from fish with excellent selectivity and high sensitivity down to the picomolar level. This work achieves a label-free, non-destructive optical sensing approach with promising potential for the detection of various harmful ingredients in food or the environment.

Heterogeneous sensing of post-translational modification enzymes by integrating the advantage of homogeneous analysis

Heterogeneous analysis has great application prospects in the detection of post-translational modification (PTM) enzymes with the advantages of signal enhancement, less sample demand, and high sensitivity and selectivity. Nevertheless, once the substrate was fixed on a solid interface, the steric hindrance might limit the approaching of catalytic center to the substrate, thus reducing the efficiency of PTM. Herein, we suggested that the avidin-modified interface could be used to develop heterogeneous sensing platforms with biotin-labeled substrates as the probes, in which the enzymatic PTM was performed in solution and the heterogeneous assay was conducted on a solid surface. The sensing strategy integrates the advantages but overcomes the defects of both homogeneous and heterogeneous assays. Protein kinase A (PKA) and histone acetyltransferase (HAT) were determined as the examples by using sequence-specific peptide substrates. The signal changes were monitored by HRP-based colorimetric assay and antibody-amplified surface plasmon resonance (SPR). The methods were used for analysis of cell lysates and evaluation of inhibition efficiency with satisfactory results. The strategy can be used for the detection of a variety of biological enzymes and provide a new idea for the design of various heterogeneous biosensors. Thus, this work should be of great significance to the popularization and practical application of biosensors.

Electrochemiluminescence Aptasensor for Charged Targets through the Direct Regulation of Charge Density in Microchannels

The change of surface charge density can cause many changes in physical or chemical properties and has been applied to design many sensitive sensors. Ochratoxin A (OTA) is a negatively charged target in neutral or alkaline solutions. In this work, a microchannel-based electrochemiluminescence (ECL) aptasensor for OTA detection based on this character had been designed. The charged target directly combined with functionalization layers of the microchannels, which caused surface charge density variation and therefore resulted in the change of ECL intensity of the (1,10-phenanthroline)ruthenium(II)/tripropylamine system. The decrease of ECL intensity is linearly dependent on OTA concentration ranging from 0.5 to 4 ng mL^-1 with a detection limit down to 0.17 ng mL^-1. This strategy has the advantages of simple interface chemistry design and universality, which offers a guiding significance for the charged target assay.

In Situ Functionalization of Silver Nanoparticles by Gallic Acid as a Colorimetric Sensor for Simple Sensitive Determination of Melamine in Milk

A simple and green colorimetric sensing assay strategy for highly efficient determination of melamine has been fabricated, which is based on the redox reaction of gallic acid with Ag+. Monodispersed Ag nanoparticles (AgNPs) were obtained using gallic acid as a reducing and stabilizing agent. However, the aggregate behavior of AgNPs was observed, while the melamine was present in the reaction medium. As a result, the color of the solution changed from vivid yellow to brown, and the density of the color was quantitatively correlated with the melamine concentration. The aggregation of AgNPs could be attributable to the formation of hydrogen bonds between melamine and gallic acid. The designed sensor exhibited a good detection limit of 0.099 μM (0.012 ppm), which was much lower than the safety limit in China (1.0 ppm) and EU (2.0 ppm). Additionally, the sensing assay displayed good selectivity toward melamine over other coexisting substances. Consequently, the proposed colorimetric sensor was successfully used for the determination of melamine detection in raw milk samples.

An ultrasensitive homogeneous electrochemical biosensor based on CRISPR-Cas12a

Taking advantage of the high-efficiency indiscriminate ssDNA cleavage activity of Cas12a in combination with the diffusivity difference of methylene blue (MB)-labeled probes (short oligonucleotides/mononucleotides) toward the negatively-charged indium tin oxide (ITO) electrode, a simple, immobilization-free, highly sensitive, and homogeneous electrochemical biosensor for the detection of human papillomavirus (HPV-16) has been fabricated. At the core of the detection process, Cas12a employed ssDNA trans-cleavage capability to achieve short-strand nucleotide cleavage, while MB-labeled probes served as high-efficiency homogeneous electrochemical emitters to achieve differential pulse voltammetric (DPV) signal. Specifically, due to strong electrostatic repulsion, MB-labeled short oligonucleotides (reporter) cannot diffuse freely to the surface of the negatively charged ITO electrode, and only weak electrochemical signals can be detected. The presence of the target HPV-16 can activate the Cas12a complex to perform indiscriminate ssDNA cleavage of the reporter to produce MB-labeled mononucleotides. The MB-labeled mononucleotides with a smaller size have almost no negative charge, so they very easily diffuse to the surface of the ITO electrode and result in an enhanced electrochemical signal response. Different electrochemical responses (DPV peak intensity) of the CRISPR-Cas12a-assisted amplification strategy can be obtained through the diffusion rate of different MB-labeled DNA on the electrode, which is also positively correlated with the input HPV-16 concentration. Given the unique combination of the CRISPR-Cas12a system with the homogeneous electrochemical solution phase, the detection limit is determined to be 3.22 pM (wide dynamic working range from 0.01 nM to 100 nM) and the two-step detection workflow could be completed within 50 min at ambient temperature, which is superior to that of the HPV-based biosensors previously reported.

A sensitive molecularly imprinted electrochemical aptasensor for highly specific determination of melamine

An electrochemical aptamer sensor based on gold nanoparticles (AuNPs) was developed for highly specific sensing of melamine (MEL), which combines molecularly imprinted polymers (MIPs) and aptamers. AuNPs were synthesized by simple reduction of sodium citrate and characterized by transmission electron microscopy. The MIP membranes with particular recognition sites were formed by electropolymerization of dopamine (DA) with polythymine (poly T) aptamers as functional monomers and melamine as template molecules. Under optimal experimental conditions, this molecularly imprinted electrochemical aptamer sensor (MIEAS) exhibits a linear relationship between 10^-12 M and 10^-4 M for detecting MEL with the detection limit of 6.7 × 10^-13 M. Moreover, this sensor displays excellent selectivity, reproducibility and stability. The milk samples analysis has confirmed the potential application of this MIEAS to quantitative detection of melamine.

Aptamer Recognition-Driven Homogeneous Electrochemical Strategy for Simultaneous Analysis of Multiple Pesticides without Interference of Color and Fluorescence

Credible and simultaneous determination of multiple pesticides is highly desirable for guaranteeing food safety. However, the current methods are limited to significant interference of color and fluorescence or electrode's modification and mainly focus on the analysis of a single pesticide. Herein, we proposed a novel aptamer-based homogeneous electrochemical system for highly sensitive and simultaneous analysis of multiple pesticides based on target pesticide-switched exonuclease III (Exo III)-assisted signal amplification. The recognition of hairpin probes by target pesticides impels the production of pesticide-DNA complexes, which hybridize with electroactive dye-labeled DNA to form double-stranded DNA, subsequently initiating an Exo III-assisted digestion reaction to generate abundant electroactive dye-tagged mononucleotides. In comparison with pesticide deficiency, two higher differential pulse voltammetry (DPV) currents are measured, which rely on the amount of target pesticides. Therefore, simultaneous analysis of two pesticides is realized with limits of detection of 0.0048 and 0.0089 nM, respectively, comparable or superior to those of known methods that focused on a single pesticide. Moreover, the proposed system is successfully employed to simultaneously evaluate the residual level of acetamiprid and profenofos in Brassica chinensis and thus will find more useful applications for pesticide-related food safety.

"Aptamer-locker" DNA coupling with CRISPR/Cas12a-guided biosensing for high-efficiency melamine analysis

Herein, we report a method that combined "aptamer-locker" DNA with CRISPR/Cas12a-based biosensing for sensitive and rapid melamine analysis. Three strategies were harnessed for designing the DNA sensors that were well characterized by circular dichroism (CD) spectroscopy and isothermal titration calorimetry (ITC) in the absence and presence of melamine. The detection parameters were optimized to achieve good analytic performance. As a result, a limit of detection (LOD) as low as 38 nM was achieved, which is below the threshold (1.0 mg/kg) of allowable melamine in infant milk products. In addition, the sensors show high selectivity for melamine against other analogues such as cyanuric acid, ammeline and ammelide. Moreover, our method was effective for rapid melamine analysis in whole milk samples, with or without sample pretreatment, in less than 20 min. Adopting a commercially available portable fluorimeter, on-site analysis of melamine in milk was accomplished. The strategies demonstrated here can expand to detect other non-nucleic-acid targets by simply replacing the aptamers.

SYBR Green I promotes melamine binding to poly-thymine DNA and FRET-based ratiometric sensing

Using SYBR Green I for DNA melting experiments, polythymine DNA binding to melamine was found to be an intramolecular reaction, allowing the design of a FRET-based biosensor and its sensitivity was enhanced by SYBR Green I.

Homogeneous electrochemical biosensor for microRNA based on enzyme-driven cascaded signal amplification strategy

Infectious diseases are a long-standing and severe global public health problem. The rapid diagnosis of infectious diseases is an urgent need to solve this problem. MicroRNA (miRNA) plays an important role in the intervention of some infectious diseases and is expected to become a potential biomarker for the diagnosis and prognosis of infectious diseases. It is of great significance to develop rapid and sensitive methods for detecting miRNA for effective control of infectious diseases. In this study, a simple and highly sensitive homogeneous electrochemical method for microRNAs using enzyme-driven cascaded signal amplification has been developed. In the presence of target miRNA, the reaction system produced plenty of MB-labeled single-nucleotide fragments (MB-MF) containing a few negative charges, which can diffuse to the negative surface of the ITO electrode easily, so an obvious electrochemical signal enhancement was obtained. Without the target, MB-HP contains a relatively large amount of negative charges due to the phosphates on the DNA chain, which cannot be digested by the enzyme and cannot diffuse freely to the negatively charged ITO electrode, so only a small signal was detected. The enhanced electrochemical response has a linear relationship with the logarithm of miRNA concentration in the range of 10 fM to 10 nM and the limit of detection as low as 3.0 fM. Furthermore, the proposed strategy showed the capability of discriminating single-base mismatch and performed eligibly in the analysis of miRNA in cell lysates, exhibiting great potential for disease diagnosis and biomedical research. Graphical abstract.

A Fluorescence Resonance Energy Transfer Biosensor Based on Graphene Quantum Dots and Protoporphyrin IX for the Detection of Melamine

Graphene quantum dots (GQDs), which have high photostability, anti-photobleaching and scintillation, good biocompatibility and low toxicity, are important member of the fluorescent material family, and have attracted extensive research interest. In this paper, a fluorescence resonance energy transfer (FRET) biosensor based on protoporphyrin IX (PpIX) and GQDs was developed for melamine detection. PpIX was bound to the surface of GQDs to produce self-assembled nanosensors, and a FRET process occurred between GQDs and PpIX. However, due to the combination of melamine and PpIX, the FRET process was shut down in the presence of melamine. The FRET system could quickly and accurately detect melamine with a detection range of 1.0 × 10^-8 to 2.0 × 10^-6 mol/L based on the fluorescence intensity ratio of PpIX and GQDs, and the detection limit was 3.6 × 10^-9 mol/L. This method obtained satisfactory results when it was employed to the determination of melamine in milk samples.

One-Step Synthesis of Methylene Blue-Encapsulated Zeolitic Imidazolate Framework for Dual-Signal Fluorescent and Homogeneous Electrochemical Biosensing

In vitro diagnosis requires target biomarkers to be reliably detected at an ultralow level. A dual-signal strategy permits self-calibration to overcome the interferences of experimental and environmental factors, and thus is regarded as a promising approach. However, currently reported works mainly concentrated on the same forms of energy of output signals. Herein, we propose a one-step strategy for synthesis of methylene blue-encapsulated zeolitic imidazolate framework-90 (MB@ZIF-90) with high loading, unique dual-signal property, exceptional recognition capability, and good stability, and we further pioneer MB@ZIF-90 as a dual-signal biosensor for label-free, enzyme-free, and ultrasensitive detection of adenosine triphosphate (ATP) by integration of fluorescence and homogeneous electrochemical techniques. The recognition of MB@ZIF-90 by target ATP spurs the decomposition of ZIF-90, subsequently permitting MB to be released into a supernatant. As compared to the case where ATP does not exist, obviously increased intensities in fluorescence and differential pulse voltammetry current are observed and both signals are directly proportional to ATP concentrations. Thus, the MB@ZIF-90-based biosensor achieved dual-signal detection of ATP in an ultrasensitive manner and displayed a more reliable diagnosis result than previously reported ATP biosensors. This dual-signal strategy provides a new opportunity to develop high-performance biosensors for in vitro diagnosis and demonstrates great potential for future applications in bioinformatics and clinical medicine.

A homogeneous electrochemical DNA sensor on the basis of a self-assembled thiol layer on a gold support and by using tetraferrocene for signal amplification

An unmodified electrochemical biosensor has been constructed, which can directly detect DNA in homogeneous solution. The synthesized new compound tetraferrocene was used for signal amplification. The dual-hairpin probe DNA was tagged with a tetraferrocene at the 3' terminal and a thiol at the 5' terminal. Without being hybridized with target DNA, the loop of probe prevented the thiol from contacting the exposed gold electrode surface with an applied potential. After hybridization with the target DNA, the loop-stem structure of the probe was opened, which led to the formation of the hairpin DNA structure. Afterwards, the thiol easily contacted the electrode and accomplished potential-assisted Au-S self-assembly. Its current signal depends on the concentration of target DNA in the 1.8 × 10^-13 to 1.8 × 10^-9 M concentration range, and the detection limit is 0.14 pM. The technique is a meaningful study because of its high selectivity and sensitivity. Graphical abstract Schematic diagram of the electrochemical DNA sensor operation. Target DNA and probe DNA hybridization, resulting in the disappearance of the steric hindrance of the probe stem ring. A higher signal was generated when tetraferrocene reached the electrode. The electrochemical signals were determined by differential voltammetric pulses (DPV).

Electrostatic attraction-induced aggregation of polymer dots for the facile detection of melamine migration

Many polymer dot (Pdot)-based assays involve complicated modifications for target recognition and detection. In this work, the fluorescence quenching of Pdots based on electrostatic attraction-induced aggregation was proposed for the first time. It was demonstrated that the prepared Pdots were negatively charged and electron-rich (e-Pdots), while protonated melamine was positively charged and electron-withdrawing. Therefore, the melamine was likely to electrostatically attract the e-Pdots, resulting in the aggregation of a melamine–e-Pdot complex. Meanwhile, the electron-transfer from the e-Pdots to the protonated melamine resulted in a remarkable fluorescence quenching. Accordingly, an e-Pdot-based assay was developed for the facile detection of melamine in the range of 0.1–100 nM and the limit of detection was as low as 0.03 nM. Furthermore, this method was applied for monitoring the melamine migration from a resin bowl, and the satisfactory results prove the promising applications of these e-Pdots.

The electrostatic attraction between electron-rich Pdots and electron-withdrawing melamine was investigated, and the e-Pdots was used for the facile detection of melamine migration.

Homogeneous Electrochemiluminescence Biosensor for the Detection of RNase A Activity and Its Inhibitor

Ribonuclease A (RNase A) is increasingly considered as a biomarker for tumor diagnosis, and it is of great significance to develop an ultrasensitive, cost-effective assay for RNase A detection. Electrochemiluminescence (ECL) technology has distinctive advantages in the development of biosensors for diverse targets. However, most of the ECL biosensors require the complex process of electrode modification, which is laborious and time consuming. In this work, an immobilization-free homogeneous ECL assay was developed for the highly sensitive detection of RNase A activity for the first time. On the basis of the fact that RNase A can specifically hydrolyze RNA at the site of ribonucleotide uracil (rU), a rU-containing chimeric DNA probe is designed and labeled with Ru(bpy)₃²⁺ (act as ECL indicator). The chimeric DNA probe hardly diffuses to the surface of negatively charged indium tin oxide (ITO) electrode due to the strong electrostatic repulsion between the negatively charged DNA and ITO electrode, resulting in a weak ECL signal detected. When the RNase A is present, the chimeric DNA probe is hydrolyzed into small fragments, which contains little negative charge and can diffuse easily to the ITO electrode surface due to the decreased electrostatic repulsion. In this case, an enhanced ECL signal can be detected. Under the optimal conditions, there is a linear relationship between the ECL signal and the concentration of RNase A in the range of 0.001-0.10 ng/mL, and the detection limit is 0.2 pg/mL. In addition, the proposed ECL sensing system is also applied to detect the RNase A inhibitor, taking As³⁺ as an example. The proposed homogeneous ECL sensing system provides a new approach for the highly sensitive and convenient detection of RNase A as well as other ribonucleases only by redesigning a responding chimeric DNA probe.

Aquamarine blue emitting silver nanoparticles as fluorescent sensor for melamine detection

Designing an exceptional probe to detect minute quantities of melamine is of huge importance for the safety and health of the human race. In this study, an aquamarine blue and rapid fluorescent sensor was devised by chemical reduction synthesis method for highly selective and sensitive detection of melamine in milk using polyethyleneimine protected silver nanoparticles (PEI-AgNPs). The optical properties of PEI-AgNPs were studied as a function of reactive material concentration, incubation time, temperature and pH. The formation of PEI-AgNPs was verified by using transmission electron microscopy (TEM) and found that the PEI-AgNPs were spherical in shape with an average diameter of 15 nm. The prepared PEI-AgNPs were aquamarine blue luminescent under UV light and show a maximum emission at 470 nm, when excited at 348 nm. The fluorescence of the PEI-AgNPs can be effectively quenched by the addition of melamine due to its ability to interact with citric acid (CA) through hydrogen bonding and form silver-melamine complexes with Ag⁺ via coordination bonds, while the addition of other nitro compounds induced feeble quenching of PEI-AgNPs, suggesting the high selectivity of PEI-AgNPs. The present sensor system exhibits a good linear response to melamine ranging from 0.16 to 56 μM, and the limit of detection was found to be 132 nM (Sb/K = 3). Significantly, the fluorescence sensor exhibited good selectivity and acted as a potential probe for detection of melamine in milk samples.

Immobilization-free photoelectrochemical aptasensor for environmental pollutants: Design, fabrication and mechanism

Atrazine (ATZ) is one of the most widely used and highly toxic triazine herbicides in the world. Photoelectrochemical (PEC) method is an attractive and sensitive alternate for ATZ. However, for conventional PEC sensors, recognition elements usually need to immobilize on electrode surface, where a complex procedure is unavoidable and the reproducibility of sensors fabrication is usually poor. Therefore, we herein proposed a new and feasible strategy for developing a signal-on immobilization-free PEC aptasensor to ATZ. Aptamer for ATZ is combined with graphene to obtain APT-GN complex, serving as the recognition element in solution. TiO₂ nanotubes (NTs) electrode deposited with Au nanoparticles (NPs) is used as the substrate electrode. After further self-assembled with 1-Mercaptooctane (MCT), the photo-generated carriers transfer between the resultant electrode and the electrolyte will be blocked, leading to a signal-off of the photocurrent. But when sensing ATZ, aptamers on APT-GN will be grasped by ATZ, leaving free graphene to assemble onto MCT/Au NPs/TiO₂ NTs, which will largely "turn on" the photocurrent response of the substrate electrode due to the efficient carrier transport efficiency of graphene. Meanwhile, simultaneous addition of deoxyribonuclease I (DNase I) can bring about further cycling amplification of the signal enhancement. The as-designed PEC aptasensor exhibits a linear range from 50.0 fM to 0.3 nM with detection limit of 12.0 fM for ATZ. Since the reaction of recognition elements and targets ATZ occurs in homogeneous solution rather than on the photoelectrode surface, this PEC aptasensor exhibits advantages of high stability, anti-interference ability, reproducibility, and wide pH and ion strength feasibility range. A promising immobilization-free aptasensing platform has thus been provided not only for ATZ but also for other kinds of environmental pollutants.

Ultrasensitive and simple fluorescence biosensor for detection of the mecA gene of Staphylococcus aureus by using an exonuclease III-assisted cascade signal amplification strategy

In this work, a label-free fluorescence biosensor for ultrasensitive and simple detection of the mecA gene of Staphylococcus aureus was proposed by using an exonuclease III (Exo III)-assisted cascade signal amplification strategy. The 3' end-extruding hairpin probe (HP) acted as the target recognition element and the caged G-quadruplex was used as the signal reporter. Without the mecA gene, the HP probe cannot be digested by Exo III, as the G-rich sequences are blocked in the stem of the HP probe. In the presence of the mecA gene, the hybridization of the mecA gene with the 3' end-extruding HP probe triggers the digestion reaction of Exo III, liberating the mecA gene and the mecA gene analogue. Both the released mecA gene and the mecA gene analogue can hybridize with other HP probes and activate another round of the cleavage reaction. Consequently, the released free G-quadruplex is "lit up" by N-methylmesoporphyrin IX (NMM), displaying a dramatically enhanced fluorescence intensity. This sensing platform showed a high sensitivity towards the mecA gene with a detection limit as low as 2.4 fM without any labelling, immobilization, or washing steps. The designed sensing system also exhibits excellent selectivity for the mecA gene in the presence of other interfering DNA sequences. Furthermore, the presented biosensor is robust and has been successfully applied for the detection of the mecA gene in a real food sample with satisfactory results. Owing to its simplicity, cost-effectiveness and ultrasensitivity, our proposed sensing strategy provides a promising platform for the detection of other genes by substituting the target-recognition element.

Target-switched triplex nanotweezer and synergic fluorophore translocation for highly selective melamine assay

This paper describes a triplex DNA nanotweezer to specifically capture melamine (MEL). The triplex-forming oligonucleotide (TFO) arm can be switched from the open state to the closed state once MEL binds to the abasic site (AP site) in duplex via the bifacial hydrogen bonding with thymines. Following this nanotweezer operation, the AP site-bound fluorophore is translocated to the terminal triplet to subsequently light up the nanotweezer. The TFO arm is found to be pivotal for permitting the AP site binding. The synergic processes of target competition and fluorophore translocation support a high selectivity for the MEL assay even against the inherent adenosine and the MEL hydrolysis products. Chelerythrine is employed as the fluorescent probe. The detection limit of MEL was estimated to be about 140 nM assuming a signal-to-noise ratio of 3. It was applied to the determination of MEL in spiked milk samples without any separation procedure. Conceivably, this method opens a new avenue towards highly selective triplex-based sensors by making use of other commercially available DNA modifications for recognizing other analytes. Graphical abstract Schematic presentation of a triplex nanotweezer with an open-to-close conversion upon the abasic site binding of melamine. The assay is based on a synergic fluorophore translocation. The corresponding duplex otherwise shows no binding with melamine. Chelerythrine (CHE) with a yellow-green emission peaking at 544 nm is employed as the fluorescent probe.

Target-induced diffusivity enhancement for rapid and highly sensitive homogeneous electrochemical detection of BLM in human serum

A simple, rapid, and sensitive homogeneous electrochemical bleomycin (BLM) bioassay has been successfully developed through the target-induced specific/efficient cleavage reaction. The designed probe, denoted as MB-DNA, contains both methylene blue (MB) and target recognizable sequences, and presents relatively low electrochemical signal. Upon the addition of BLM, the recognition/cleavage reaction occurs and leads to the in-situ generation of MB tag (MB-DNA-1), leading to the reduced electrostatic repulsive force. As a result, an obvious enhancement in differential pulse voltammetry (DPV) current is determined, which is relied on the amount of BLM. Thus, a turn on homogeneous electrochemical method for BLM is really achieved, and exhibits high sensitivity of 33 pM, and the shortest response time of 20 min. Furthermore, this electrochemical bioassay presents excellent sensing performance in the analysis of BLM in real samples. Comparing with other sensing strategies for BLM, this proposed electrochemical platform is just consisted of one DNA probe alone, and affords a really rapid and sensitive strategy for BLM analysis.

Fluorescence detection of melamine based on inhibiting Cu2+-induced disaggregation of red-emitting silver nanoclusters

Herein, we report a facile method to synthesize red-emitting, water-soluble Ag nanoclusters (Ag NCs) employing lipoic acid as a stabilizing agent. The Ag NCs show aggregation-induced emission property and have good stability and optical properties. Cu²⁺ can disperse the aggregated Ag NCs, accompanied by the quenching of fluorescence. However, the formed Cu²⁺-melamine complex by the coordination chemistry between free copper ion and melamine is able to effectively avoid the quenching process of Cu²⁺ to the Ag NCs. Hence, the Ag NCs can be applied to design a novel fluorescent probe based on this property to detect melamine. In the determination of melamine, the as-prepared fluorescent Ag NCs exhibit favorable sensitivity and high selectivity. The limit of detection (LOD) down to 0.022 mg/L and good recoveries of real sample experiments were obtained. This fluorescent probe is proved to be convenient and rapid for detecting melamine, with potential application to trace melamine analysis in complicated samples.

A homogeneous electrochemical sensor for Hg2+ determination in environmental water based on the T-Hg2+-T structure and exonuclease III-assisted recycling amplification

A simple, fast, sensitive, and homogeneous electrochemical sensor based on the T-Hg2+-T structure and exonuclease III-assisted recycling amplification has been constructed for mercury ion (Hg2+) detection. The cT and methylene blue-labeled DNA probes (MB-TDNA) were designed to contain poly T sequences, which were repulsed from the negatively charged indium tin oxide (ITO) electrode due to their abundant negative charges. Hg2+ could trigger the formation of double-stranded DNA (dsDNA) between two DNA probes owing to the stable T-Hg2+-T structure. Then, Exo III specifically recognizes the cleavage of the double-stranded structure to release a methylene blue-labeled mononucleotide fragment (MB-MF). Moreover, the release of the target Hg2+ induces new hybridization and produces a large number of MB-MFs; MB-MFs are not repulsed by the negatively charged ITO electrode surface, thus producing a significant current signal. Under optimal conditions, the differential pulse voltammetric (DPV) response had a linear relationship with the logarithm of Hg2+ concentration in the range of 1.0 nM-0.5 μM, and the proposed method displayed great applicability for detecting Hg2+ in tap-water samples.

Homogeneous electrochemical aptasensor for mucin 1 detection based on exonuclease I-assisted target recycling amplification strategy

A sensitive and homogeneous electrochemical aptasensor was fabricated for the detection of mucin 1 (MUC1) by combining a well-designed DNA bulge-loop (L-DNA) structure with high-efficient exonuclease I (Exo I)-assisted target recycling amplification strategy. The L-DNA probe was constructed via the hybridization of the MUC1 aptamer and methylene blue (MB) labeled complementary DNA (cDNA) (cDNA-MB) and hence could not diffuse freely to the negatively charged ITO electrode surface due to the strong electrostatic repulsion, so small electrochemical signal was detected. The addition of MUC1 caused the dissociation of L-DNA structure due to the specificity between aptamer and MUC1. Then Exo I was implemented to digest the released cDNA-MB into mononucleotides and then produced short MB-labeled mononucleotides fragments (MB-MFs). As the MB-MFs contained few negative charges, it diffused easily to the negatively charged ITO electrode surface and resulted in the enhanced electrochemical signal. Meanwhile, the MUC1-aptamer complex was also specifically digested by Exo I, resulting in the liberation of MUC1 and hence realized the target recycling and then caused the amplification of the electrochemical signal. The enhanced electrochemical signal has a good linear relationship with logarithm of MUC1 concentration in the range of 1.0 pg mL^-1-50 ng mL^-1 with a limit of detection of 0.40 pg mL^-1 (S/N = 3). Additionally, the fabricated aptasensor has been successfully applied to detect MUC1 in serum samples with satisfactory results and thereby it exhibits great potential in the practical application of clinical diagnosis.

Triplex DNA formation-mediated strand displacement reaction for highly sensitive fluorescent detection of melamine

Since melamine is a strong hazard to human health, the development of new methods for highly sensitive detection of melamine is highly desirable. Herein, a novel fluorescent biosensing strategy was designed for sensitive and selective melamine assay based on the recognition ability of abasic (AP) site in triplex towards melamine and signal amplification by Mg²⁺-dependent DNAzyme. In this strategy, the melamine-induced formation of triplex DNA was employed to trigger the strand displacement reaction (SDR). The SDR process converted the specific target recognition into the release and activation of Mg²⁺-dependent DNAzyme, which could catalyze the cleavage of fluorophore/quencher labeled DNA substrate (FQ), resulting in a significantly increased fluorescent signal. Under the optimal conditions, the fluorescent signal has a linear relationship with the logarithm of the melamine concentration in a wide range of 0.005-50 μM. The detection limit was estimated to be 0.9 nM (0.1ppb), which is sufficiently sensitive for practical application. Furthermore, this strategy exhibits high selectivity against other potential interfering substances, and the practical application of this strategy for milk samples reveals that the proposed strategy works well for melamine assay in real samples. Therefore, this strategy presents a new method for the sensitive melamine assay and holds great promise for sensing applications in the environment and the food safety field.

Homogeneous and label-free electrochemiluminescence aptasensor based on the difference of electrostatic interaction and exonuclease-assisted target recycling amplification

The difference of electrostatic interaction between free Ru(phen)₃²⁺ and Ru(phen)₃²⁺ embedded in double strand DNA (dsDNA) to the negatively charged indium tin oxide (ITO) electrode has been applied to develop a homogeneous and label-free electrochemiluminescence (ECL) aptasensor for the first time. Ochratoxin A (OTA) has been chosen as the model target. The OTA aptamer is first hybridized with its complementary single strand DNA (ssDNA) to form dsDNA and then interacted with Ru(phen)₃²⁺ via the grooves binding mode to form dsDNA-Ru(phen)₃²⁺ complex, which remains negatively charged feature as well as low diffusion capacity to the negatively charged ITO electrode surface owing to the electrostatic repulsion. Meanwhile, the intercalated Ru(phen)₃²⁺ in the grooves of dsDNA works as an ECL signal reporter instead of the labor-intensive labeling steps and can generate much more ECL signal than that from the labeling probe. In the presence of target, the aptamer prefers to form an aptamer-target complex in lieu of dsDNA, which induces the releasing of Ru(phen)₃²⁺ from the dsDNA-Ru(phen)₃²⁺ complex into the solution. With the assistance of RecJ_f exonuclease (a ssDNA specific exonuclease), the released ssDNA and the aptamer in the target-complex were digested into mononucleotides. In the meantime, the target can be also liberated from OTA-aptamer complex and induce target cycling and large amount of free Ru(phen)₃²⁺ present in the solution. Since Ru(phen)₃²⁺ contains positive charges, which can diffuses easily to the ITO electrode surface because of electrostatic attraction, causing an obviously enhanced ECL signal detected. Under the optimal conditions, the enhanced ECL of the system has a linear relationship with the OTA concentration in the range of 0.01-1.0 ng/mL with a detection limit of 2 pg/mL. This innovative system not only expands the immobilization-free sensors in the electrochemiluminescent fields, but also can be developed for the detection of different targets easily with the same strategy by changing the aptamer used.

Origin of Both Right- and Left-Handed Helicities in a Supramolecular Gel with and without Ni2+ at the Supramolecular Level

We demonstrate the different origins of helical directions in polymeric gels derived from a hydrazone reaction in the absence and presence of Ni²⁺. The right-handed helicity of polymeric gels without Ni²⁺ originates from the enantiomeric d-form alanine moiety embedded in the building block. However, the right-handed helicity is inverted to a left-handed helicity upon the addition of Ni²⁺, indicating that added Ni²⁺ greatly affects the conformation of the polymeric gel by overcoming the influence of the enantiomer embedded in the building block on the helicity at the supramolecular level. More interestingly, the ratio of the right-toleft-handed helical fibers varies with the concentration of Ni²⁺, which converts from 100% right-handed helical fiber to 90% left-handed helical fiber. In the presence of Ni²⁺, both right- and left-handed helical fibers coexist at the supramolecular level. Some fibers also exhibit both right- and left-handed helicities in a single fiber.

Convenient and sensitive colorimetric detection of melamine in dairy products based on Cu(ii)-H2O2-3,3′,5,5′-tetramethylbenzidine system

The illegal adulteration of melamine in dairy products for false protein content increase is a strong hazard to human health. Herein, a simple and sensitive colorimetric method was developed for the quantification of melamine in dairy products based on a Cu²⁺-hydrogen peroxide (H₂O₂)-3,3′,5,5′-tetramethylbenzidine (TMB) system. In this strategy, Cu²⁺ exhibits peroxidase-like activity and can catalyze the oxidation of TMB to oxidized TMB (oxTMB) in the presence of H₂O₂ with a blue colour change of the solution. However, the presence of melamine quickly interacts with H₂O₂ leading to the consumption of H₂O₂ and thus strongly hinders the oxidation of TMB. Under the optimal conditions, the absorbance change of oxTMB has a linear response to the concentration of melamine from 1 to 100 μM with a detection limit of 0.5 μM for melamine. The proposed method has many merits including more simplicity, good selectivity, and more cost-effectiveness without using any nanomaterials. The method was further successfully applied to detect melamine in dairy products including milk and infant formula powder.

Convenient and sensitive colorimetric detection of melamine in dairy products based on a Cu(ii)-H₂O₂-3,3′,5,5′-tetramethylbenzidine system was reported.

Electro-Grafted Electrode with Graphene-Oxide-Like DNA Affinity for Ratiometric Homogeneous Electrochemical Biosensing of MicroRNA

This work demonstrated for the first time a simple and rapid approach to endow the electrode with the excellent discrimination ability over single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) through the robust electrochemical grafting of in situ generated 1-naphthalenesulfonate (NS^-) diazonium salt onto the surface of indium tin oxide (ITO) electrode. On the basis of understanding the influence of sequence and length on the binding affinity of ssDNA and dsDNA toward NS^- grafted ITO (NS^--ITO) electrode, these interesting findings were successfully employed to rationally develop a ratiometric homogeneous electrochemical biosensing platform for microRNA based on the affinity-mediated signal transduction. The achievement of ultrasensitive detection of microRNA lies in a compatibly designed T7 exonuclease-assisted isothermal amplification strategy, in which the presence of target microRNA initiated the continual and opposite affinity inversion of two rationally engineered electrochemical signal reporters, methylene blue (MB) labeled hairpin reporter and ferrocene (Fc) labeled dsDNA reporter, toward NS^--ITO electrode, thereby providing the ratiometric transduction and amplification of the homogeneous electrochemical output signal. By measuring the distinct variation in the peak current intensity ratios of Fc and MB tags, this ratiometric homogeneous electrochemical microRNA biosensing platform showed a detection limit of 25 aM, which is much lower than that of the reported homogeneous electrochemical biosensors. Therefore, we envision that the proposed approach will find useful applications in disease molecular diagnoses and biomedicine.

Enzymatic Fuel Cell-Based Self-Powered Homogeneous Immunosensing Platform via Target-Induced Glucose Release: An Appealing Alternative Strategy for Turn-On Melamine Assay

Enzymatic fuel cell (EFC)-based self-powered biosensors have attracted considerable attention because of their unique feature of no need for extra power sources during the entire detection process, which endows them with the merits of simplicity, rapidness, low cost, anti-interference, and ease of use. Herein, we proposed, for the first time, an EFC-based self-powered homogeneous immunosensing platform by integrating the target-induced biofuel release and bioconjugate immunoassay for ultrasensitive melamine (ME) detection. In this design, the biofuel, i.e., glucose molecules, was entrapped in the pores of positively charged mesoporous silica nanoparticles and capped by the biogate AuNPs-labeled anti-ME antibody (AuNPs-Ab). The presence of the target ME triggered the entrapped glucose release due to the removal of the biogate via immunoreaction, which resulted in the transfer of electrons produced by glucose oxidation at the bioanode to the biocathode, and thus, the open-circuit voltage of the EFC-based self-powered immunosensor dramatically increased, realizing the ultrasensitive turn-on assay for ME. The limit of detection for ME assay was down to 2.1 pM (S/N = 3), superior to those previously reported in the literature. Notably, real milk samples need no special sample pretreatment for the detection of ME because of the good anti-interference ability of EFC-based self-powered biosensors and the excellent selectivity of the homogeneous immunoassay. Therefore, this appealing self-powered homogeneous immunosensing platform holds great promise as a successful prototype of portable and on-site bioassay in the field of food safety.

Triplex DNA-based Bioanalytical Platform for Highly Sensitive Homogeneous Electrochemical Detection of Melamine

Melamine detection has attracted much attention since the discovery of the damage of melamine to human health. Herein, we have developed a sensitive homogeneous electroanalytical platform for melamine detection, which is relied on the formation of triplex molecular beacon integrated with exonuclease III (Exo III)-mediated signal amplification. The formation of triplex molecular beacon was triggered by the recognition and incorporation of melamine to the abasic (AP) site contained in the triplex stem. The stem of the triplex molecular beacon was designed to have a protruding double-strand DNA, which can be recognized and hydrolyzed by Exo III for releasing methylene blue (MB)-labeled mononucleotide. These released MB molecules exhibit high diffusivity toward indium tin oxide electrode with negative charge, thus producing a significantly increased electrochemical response. Taking advantages of the high binding affinity of the DNA triplex structure containing AP sites towards melamine and the unique features of Exo III, this sensing platform is capable for sensitive and selective melamine assay with a detection limit as low as 8.7 nM. Furthermore, this strategy shows good applicability for melamine assay in real samples. Therefore, this strategy broadens the application of triplex DNA and presents a new method for sensitive detection of melamine.

Exonuclease-Catalyzed Methylene Blue Releasing and Enriching onto a Dodecanethiol Monolayer for an Immobilization-Free and Highly Sensitive Electrochemical Nucleic Acid Biosensor

Herein, a unique and versatile immobilization-free electrochemical nucleic acid biosensor architecture is proposed for the first time based on the catalyzed release of a methylene blue (MB)-tagged mononucleotide by exonuclease III (Exo III) and the successive enrichment onto a dodecanethiol monolayer, which can be attributed to the hydrophobic force between the alkyl chain of the dodecanethiol monolayer and the hydrophobic part of the MB-tagged mononucleotide. The fabricated biosensor demonstrates considerable advantages including assay simplicity, rapidness, and high sensitivity owing to its immobilization-free and homogenous operation for the biorecognition and amplification process. A low detection limit of approximately 1 pM toward the target DNA could be achieved with an excellent selectivity. The proposed immobilization-free electrochemical biosensing strategy was also extended for the assay of Exo I and III activity. Furthermore, it might be easily extended for the detection of a wide spectrum of targets and thus provide a promising avenue for the development of immobilization-free and sensitive electrochemical biosensors.