Versatile Sensing Platform for Cd2+ Detection in Rice Samples and Its Applications in Logic Gate Computation

A portable solid sampling visualization nano-sensor for soil Cd based on "three-phase transforming" technique

Direct analysis of solid samples is always challenging for ionic sensors due to solidified elemental presence and matrix interference. In this work, a "three-phase transforming" technique was first established to make solid sampling elemental sensors and visual detection possible in the future. For Cd transforming from soil samples, a metal ceramic heater (MCH) electrothermal vaporizer (ETV) coupled with a dielectric barrier discharge quartz trap (DBD-QT) was first utilized to fulfill the solid sampling and preconcentration of Cd in soil; for on-site analysis, a colorimetric sensor based on the trithiocyanuric acid (TMT) functionalized gold nanoparticles (AuNPs) was chosen as a chromogenic analysis model. The portable and miniature ETV-DBD apparatus directly introduced Cd from soil and then captured Cd, consuming only <130 W and 4.5 kg weight; finally, only 200 μL water was injected as eluent to dissolve Cd for the following colorimetric detection. Herein, the Cd analyte underwent a "three-phase transforming" from solid (Cd compounds in soil), to aerosol (vaporization and transportation), to solid (Cd oxides trapped on quartz surface) and to liquid (Cd2+ in eluate). Under optimized conditions, the method limit of detection (LOD) reached 0.04 mg/kg Cd (50 mg sample), fulfilling fast monitoring of Cd contamination in soil, with <20 % relative standard deviations (RSDs). The analysis time was <10 min excluding sample digestion and acid application, as well as the interference of Pb2+ on the AuNPs sensor can be eliminated via the "three-phase transforming" process, proving an excellent anti-interference for solid analysis. This "three-phase transforming" processing technique coupled with colorimetric sensor holds a great potential for direct and on-site analysis in solid samples without complicated handling, providing a fantastic methodology for the application of ionic sensors and making solid sampling elemental sensor and visual detection possible.

A thiophene-linked terpyridine based phenanthridine chemoreceptor for Cd2+ and Cr3+ selective ratiometric fluorescence detection in environmental water and rice samples

BACKGROUND

The studied materials, Cadmium (Cd2+) and Chromium (Cr3+) are highly toxic, and it focuses on investigating various environmental sources, such as industrial processes and waste water. When quantities of Cr3+ and Cd2+ exceed the allowable limit, biological toxicity and hazardous environmental pollution are unavoidable. In order to address this problem, we introduce 5-(5-(4-([2,2':6',2″-terpyridin]-4'-yl) phenyl) thiophen-2-yl)-7,8,13,14-tetrahydrodibenzo [a,i] phenanthridine (TPTP), a dual-emission response chemosensor that employs a colorimetric and fluorescence turn-on approach for the rapid, sensitive, and discriminate detection of Cr3+ and Cd2+ ions.

RESULTS

We created a newly designed luminous TPTP sensor based on intramolecular charge transfer (ICT). TPTP sensor probe specifically determined Cr3+ and Cd2+ ions with an immediate colour shift from cyan to green and orange in CH3CN: H2O (6:4) solvent solution. The permissible level set by the Environmental Protection Agency (EPA) of the United States for the presence of Cr3+ and Cd2+ ions in drinking water was higher than the detection level of 3.5 and 9.7 nM, by this sensor respectively. NMR titrations, HRMS, and theoretical calculation methods were employed to examine the accurate sensing processes of TPTP and complexes.

SIGNIFICANCE

This is an effective method of detecting Cr3+ and Cd2+ ions in an environmental system using a ratiometric methodology. In addition, TPTP was used to determine the concentration of Cr3+ and Cd2+ ions in natural water and food samples. Fluorescent bio-imaging studies revealed that the present sensor TPTP could identify Cr3+ and Cd2+ ions inside living HeLa cells. A paper kit analysis has been done on TPTP, which has a time-to-result of less than 1 s and offers a cost-effective assay. As a result, the platform offers portability.

Architecture of Easy-to-Synthesize and Superior Probe Based on Aminoquinoline Appended Naphthoquinone: Instant and On-Site Cu2+ Ion Quantification in Real Samples and Unusual Crystal Structure and Logic Gate Operations

Easy-to-synthesize aminoquinoline (AQ) appended naphthoquinone (NQ)-based colorimetric and ratiometric probe (AQNQ) was successfully synthesized in one step with high yield and low cost, and was utilized to supply an effective solution to critical shortcomings encountered in Cu2+ analysis. The structure of AQNQ and its interaction with Cu2+ forming an unusual AQNQ-Cu complex were enlightened with single-crystal X-ray diffraction analysis and different spectroscopic methods. AQNQ-Cu complex is the first Cu2+ containing dinuclear crystal where the octahedral coordination sphere is fulfilled through the coordination of a NQ oxygen atom. AQNQ exhibited long-term stability (more than 1 month), superior probe ability toward Cu2+ with quite fast response (30 s), high selectivity among many ions, and limit of detection of 12.13 ppb that is significantly below the highest amount of Cu2+ allowed in drinking water established by both WHO and EPA. Ratiometric determination of Cu2+ using AQNQ was performed with high recovery and low RSD values for drinking water, tap water, lake water, cherry, and watermelon samples. Colorimetric on-site determination including smartphone and paper strip applications, IMPLICATION, and INHIBIT logic gate applications were successfully carried out. The reversibility and reusability of the response to Cu2+ ions with the paper strip application were examined for the first time.

A turn-on signal biosensor for cadmium(II) based on DNAzyme and stem-loop qPCR

Cadmium is a heavy metal that is exceedingly hazardous to humans and can enter the body through tainted food or drink, causing severe harm. It is critical to develop a technology for detecting cadmium in food and water that is sensitive and accurate. One such approach, which employs nucleases, is uncommon. A cadmium(II) turn-on biosensor was successfully created in this work using repetitive cleavage of certain specific nucleases for signal conversion and sophisticated stem-loop qPCR (quantitative polymerase chain reaction) for quick signal amplification and output. The method has strong selectivity and sensitivity for precise quantification, with a detection limit of 6 nmol L-1, i.e. 0.948 g L-1, which is far lower than the 5.0 g L-1 set by the United States Environmental Protection Agency, and it also operates well in retail rice samples.

Ultralow background one-pot detection of Lead(II) using a non-enzymatic double-cycle system mediated by a hairpin-involved DNAzyme

A double-cycle system has been developed for specifically detecting trace amounts of Pb2+ by significantly decreasing the background signal. The detection involves two types of RNA cleavage reactions: one using a Pb2+-specific GR5 DNAzyme (PbDz) and the other utilizing a newly constructed 10-23 DNAzyme with two hairpins embedded in its catalytic center (hpDz). The ring-structured hpDz (c-hpDz) exhibits significantly lower activity compared to the circular 10-23 DNAzyme without hairpin structures, which plays a crucial role in reducing the background signal. When Pb2+ is present, PbDz cleaves c-hpDz to its active form, which then disconnects the molecular beacon to emit the fluorescent signal. The method allows for rapid and sensitive Pb2+ detection within 40 min for 10 fM of Pb2+ and even as short as 10 min for 100 nM of Pb2+. Additionally, visual detection is possible through the non-crosslinking assembly of Au nanoparticles. The entire process can be performed in one pot and even one step, making it highly versatile and suitable for a wide range of applications, including food safety testing and environmental monitoring.

Integrating CRISPR-Cas12a with catalytic hairpin assembly as a logic gate biosensing platform for the detection of polychlorinated biphenyls in water samples

Polychlorinated biphenyls (PCBs) are ubiquitous persistent organic pollutants that cause harmful effects on environmental safety and human health. There is an urgent need to develop an intelligent method for PCBs sensing. In this work, we proposed a logic gate biosensing platform for simultaneous detection of multiple PCBs. 2,3',5,5'-tetrachlorobiphenyl (PCB72) and 3,3',4,4'-tetrachlorobiphenyl (PCB77) were used as the two inputs to construct biocomputing logic gates. We used 0 and 1 to encode the inputs and outputs. The aptamer was used to recognize the inputs and release the trigger DNA. A catalytic hairpin assembly (CHA) module is designed to convert and amplify each trigger DNA into multiple programmable DNA duplexes, which initiate the trans-cleavage activity of CRISPR/Cas12a for the signal output. The activated Cas12 cleaves the BHQ-Cy5 modified single-stranded DNA (ssDNA) to yield the fluorescence reporting signals. In the YES logic gate, PCB72 was used as the only input to carry out the logic operation. In the OR, AND, and INHIBIT logic gates, PCB72 and PCB77 were used as the two inputs. The output signals can be visualized by the naked eye under UV light transilluminators or quantified by a microplate reader. Our constructed biosensing platform possesses the merits of multiple combinations of inputs, intuitive digital output, and high flexibility and scalability, which holds great promise for the intelligent detection of different PCBs.

Recent Aptamer-Based Biosensors for Cd2+ Detection

Cd²⁺, a major environmental pollutant, is heavily toxic to human health. Many traditional techniques are high-cost and complicated; thus, developing a simple, sensitive, convenient, and cheap monitoring approach is necessary. The aptamer can be obtained from a novel method called SELEX, which is widely used as a DNA biosensor for its easy acquisition and high affinity of the target, especially for heavy metal ions detection, such as Cd²⁺. In recent years, highly stable Cd²⁺ aptamer oligonucleotides (CAOs) were observed, and electrochemical, fluorescent, and colorimetric biosensors based on aptamers have been designed to monitor Cd²⁺. In addition, the monitoring sensitivity of aptamer-based biosensors is improved with signal amplification mechanisms such as hybridization chain reactions and enzyme-free methods. This paper reviews approaches to building biosensors for inspecting Cd²⁺ by electrochemical, fluorescent, and colorimetric methods. Finally, many practical applications of sensors and their implications for humans and the environment are discussed.

Multifunctional Photoelectrochemical Biosensor Based on ZnIn2S4/ZnS QDs@Au-Ag-Reversed Photocurrent of Cu-Metal-Organic Framework Coupled with CRISPR/Cas-12a-Shearing for Assay of Dual Targets

False positives and negatives in bioanalytical assays remain a persistent problem. Herein, a multifunctional photoelectrochemical (PEC) biosensor based on ZnIn₂S₄ (ZIS)/ZnS quantum dots (QDs)@Au-Ag-reversed photocurrent of Cu-metal-organic framework (MOF) coupled with CRISPR/Cas-12a-shearing was innovatively developed for assay of dual targets. First, Cu-MOF as a good PEC material shows cathodic photocurrent. Then, numerous ZIS/ZnS QDs were assembled to the Au-Ag nanoparticles (NPs) to prepare a stable and highly amplified signal probe, which can just match the energy level of Cu-MOFs and realized the polarity-reversed photocurrent of Cu-MOF for the first time. As the empty-core nanostructure of Au-Ag NPs has a high specific surface area and low material density, the bimetallic nanocrystal can much increase the reaction rate and improve the redox efficiency. When target CEA-produced cDNA opened the hairpin DNA (HP1 DNA) on the electrode, the ZIS/ZnS QDs@Au-Ag signal probe was conjugated to the electrode via DNA hybridization, achieving a significantly reversed PEC current for CEA detection. Moreover, the specific binding of kanamycin/aptamer generated the acDNA (activator), which can activate the trans-cleavage activity of the CRISPR-CAS12a system on ssDNA, so the signal probe was sheared and caused the obvious decrease of PEC signal for kanamycin detection. The newly developed ZIS/ZnS QDs@Au-Ag NPs displayed excellent PEC properties and reversed photocurrent to MOF and were combined with the unique CRISPR-Cas12a system to achieve sensitive detection of dual targets, which can open a new polarity-reversed PEC sensing platform for rapid and accurate analysis of multiple targets and can effectively avoid false positives results in clinical testing.

Construction of molecular logic gates using heavy metal ions as inputs based on catalytic hairpin assembly and CRISPR-Cas12a

We successfully constructed several molecular logic gates using heavy metal ions as inputs based on catalytic hairpin assembly (CHA) and CRISPR-Cas12a. The corresponding DNAzymes were used to recognize heavy metal ions (Hg²⁺, Cd²⁺, Pb²⁺, and Mn²⁺). The specific cleavage between heavy metal ions and DNAzymes leads to the release of the trigger DNA, which can be used to activate CHA through logic computation. The CHA-generated DNA duplexes contain the protospacer adjacent motifs (PAM) sequence, which can be distinguished by CRISPR-Cas12a. The hybridization interactions between the duplexes and gRNA will activate the trans-cleavage capability of Cas12a, which can cleave the single-stranded DNA (ssDNA) reporter. The separation of the fluorescence group and quench group in ssDNA will generate a high fluorescence signal for readout. Using Hg²⁺ and Cd²⁺ as the two inputs, several basic logic gates were constructed, including OR, AND, and INHIBT. Using Hg²⁺, Cd²⁺, Pb²⁺, and Mn²⁺ as the four inputs, cascaded logic gates were further fabricated. With the advantages of scalability, versatility, and logic computing capability, our proposed molecular logic gates can provide an intelligent sensing system for heavy metal ions monitoring.

Integrating CRISPR/Cas12a with strand displacement amplification for the ultrasensitive aptasensing of cadmium(II)

Cadmium ion (Cd(II)) is a pernicious environmental pollutant that has been shown to contaminate agricultural lands, accumulate through the food chain, and seriously threaten human health. At present, Cd(II) monitoring is dependent on centralized instruments, necessitating the development of rapid and on-site detection platforms. Against this backdrop, the present study reports on the development of a fluorometric aptasensor designed to target Cd(II), which is achieved through the integration of strand displacement amplification (SDA) and CRISPR/Cas12a. In the absence of Cd(II), the aptamer initiates SDA, resulting in the generation of a profusion of ssDNA that activates Cas12a, leading to a substantial increase in fluorescence output. Conversely, the presence of Cd(II) curtails the SDA efficiency, culminating in a significant reduction in fluorescence output. The proposed approach has been demonstrated to enable the selective detection of Cd(II) at concentrations of 60 pM, with the performance of the aptasensor validated in real water and rice samples. The proposed platform based on aptamer-target interaction holds immense promise as a signal-amplified and precise method for the detection of Cd(II) and has the potential to transform current hazard detection practices in food samples.

A fluorescent biosensor for Cd2+ detection in water samples based on Cd2+-fueled wheel DNAzyme walker and its logic gate applications

A fluorescent biosensor was developed for Cd²⁺ detection based on a Cd²⁺-fueled wheel DNAzyme walker. Cd²⁺ can activate the wheel to roll along the DNA walking tracks through DNAzyme cleavage and toehold-mediated strand displacement. The substrate strand was modified with BHQ and Cy5. Through continuous cleavage reactions toward the substrate strands, a high fluorescence signal can be obtained. The biosensor is ultrasensitive, and the detection limit is 0.2 pM (S/N = 3). The fluorescent assay is robust and has been applied to the determination of Cd²⁺ in real water samples with good accuracy and reliability. Using Cd²⁺, Pb²⁺, and Hg²⁺ as the three inputs, we also construct a concatenated AND logic gate. The input combination of (111) can produce an output of 1. Other input combinations produce an output of 0. Our proposed detection platform and logic system hold great promise for the ultrasensitive and intelligent sensing of different heavy metal ions in water samples.

Construction and Application of DNAzyme-based Nanodevices

The development of stimuli-responsive nanodevices with high efficiency and specificity is very important in biosensing, drug delivery, and so on. DNAzymes are a class of DNA molecules with the specific catalytic activity. Owing to their unique catalytic activity and easy design and synthesis, the construction and application of DNAzymes-based nanodevices have attracted much attention in recent years. In this review, the classification and properties of DNAzyme are first introduced. The construction of several common kinds of DNAzyme-based nanodevices, such as DNA motors, signal amplifiers, and logic gates, is then systematically summarized. We also introduce the application of DNAzyme-based nanodevices in sensing and therapeutic fields. In addition, current limitations and future directions are discussed.

Fluorescent Detection of Two Pesticides Based on CRISPR-Cas12a and Its Application for the Construction of Four Molecular Logic Gates

An intelligent detection platform was developed through molecular logic gate operation based on CRISPR-Cas12a and signal amplification circuits using two kinds of pesticides [acetamiprid (ACE) and atrazine (ATR)] as inputs. The pesticide-aptamer bindings activate the signal amplification process to produce numerous double-stranded DNA, which can be identified by CRISPR-Cas12a. Under the optimal assay conditions, the sensor exhibits excellent analytical performance, with the detection limits for ACE and ATR of 2.5 and 0.2 pM, respectively. The practicality of the platform was verified by testing pesticide concentrations in food samples. Several molecular logic gates (OR, AND, XOR, and INHIBIT) were constructed using "0" and "1" to encode the target pesticides and the fluorescence readout. The logic detection platform with simple operation, high sensitivity, and multiple logic functions is promising to become a powerful sensing system for the intelligent assay of different pesticides in food samples.

Molecular logic operations from complex coacervation with aggregation-induced emission characteristics

Leveraging complex coacervation of a polycation and a bivalent anion with aggregation-induced emission characteristics, we accomplish eight basic logic operations with environmental stimuli as inputs, producing Boolean-like fluorescence intensity or turbidity 'outputs' with contrast higher than one order of magnitude. Storage of information of a fluorescent pattern and thermo-sensor applications are also demonstrated.

A novel signal amplification biosensor for detection of Cd2+ based on asymmetric PCR

In this work, a novel signal amplification biosensor was utilized to detect Cd²⁺ based on asymmetric PCR. In the presence of Cd²⁺, it can bind with Cd²⁺-aptamer C1 which caused the complementary strand C2 to be released from double-stranded DNA C1-C2. Because the single-stranded C1 cannot be hydrolyzed by Exo III, it can be used as a template to take part in asymmetric PCR reaction. In the absence of Cd²⁺, the C1-C2 was digested by Exo III and no PCR template was left. During the experiment, an interesting phenomenon was found that the asymmetric PCR can obtain higher level of fluorescent signal than that of symmetric PCR. To the best of our knowledge, this is the first report of using asymmetric PCR to detect Cd²⁺. Through the asymmetric PCR amplification strategy, this biosensor had a low detection limit (19.93 nM) and a wide linear range (0-500 nM). Meanwhile, this biosensor showed a satisfactory selectivity and recovery rate.

Highly sensitive biosensor based on aptamer and hybridization chain reaction for detection of cadmium ions

In this work, a highly sensitive biosensor for detecting cadmium ions (Cd²⁺ ) was developed based on Cd²⁺ -specific DNA aptamer and hybridization chain reaction (HCR). The Cd²⁺ -aptamer (named S0) was used to recognize Cd²⁺ and trigger HCR reaction. Without Cd²⁺ , S0 initiated the HCR to form long nicked dsDNA structures to quench the fluorescence. Then, Cd²⁺ can bind with S0 to block HCR to recover fluorescence. This biosensor had high sensitivity with the detection limit of 0.36 nM and a linear range from 0 to 10 nM. Moreover, it showed a satisfactory selectivity and recovery rates.

A bifunctional fluorescent probe based on "AND logic" for the simultaneous recognition of H2S/HNO and its bioimaging applications

The reaction of NO and H₂S to form HNO is a classical pathway in physiological conditions. The reported single recognition-type fluorescent probes are difficult to track precisely the relationships of H₂S and HNO. It is necessary to develop a bifunctional fluorescence probe (NJA) for monitoring simultaneously the production of endogenous HNO and H₂S. Using 7-Nitrobenzofurazan (NBD) and 2-(diphenylphosphine) benzoate as recognition sites, the obatined NJA can detect specifically HS^- and HNO. The detection limit of HS^- and HNO are 0.46 μM and 1.42 μM, respectively. Based on the dual recognition sites and input signals of the probe, a molecular "AND" logic gate was established to detect successfully H₂S and HNO in MCF-7 cells. NJA based on "AND logic" provided a simple and robust tool for monitoring the production of endogenous HNO correlative with H₂S and NO in living cells.

Development of the DNA-based biosensors for high performance in detection of molecular biomarkers: More rapid, sensitive, and universal

The molecular biomarkers are molecules that are closely related to specific physiological states. Numerous molecular biomarkers have been identified as targets for disease diagnosis and biological research. To date, developing highly efficient probes for the precise detection of biomarkers has become an attractive research field which is very important for biological and biochemical studies. During the past decades, not only the small chemical probe molecules but also the biomacromolecules such as enzymes, antibodies, and nucleic acids have been introduced to construct of biosensor platform to achieve the detection of biomarkers in a highly specific and highly efficient way. Nevertheless, improving the performance of the biosensors, especially in clinical applications, is still in urgent demand in this field. A noteworthy example is the Corona Virus Disease 2019 (COVID-19) that breaks out globally in a short time in 2020. The COVID-19 was caused by the virus called SARS-CoV-2. Early diagnosis is very important to block the infection of the virus. Therefore, during these months scientists have developed dozens of methods to achieve rapid and sensitive detection of the virus. Nowadays some of these new methods have been applied for producing the commercial detection kit and help people against the disease worldwide. DNA-based biosensors are useful tools that have been widely applied in the detection of molecular biomarkers. The good stability, high specificity, and excellent biocompatibility make the DNA-based biosensors versatile in application both in vitro and in vivo. In this paper, we will review the major methods that emerged in recent years on the design of DNA-based biosensors and their applications. Moreover, we will also briefly discuss the possible future direction of DNA-based biosensors design. We believe this is helpful for people interested in not only the biosensor field but also in the field of analytical chemistry, DNA nanotechnology, biology, and disease diagnosis.

A nanozyme-catalysis-based ratiometric electrochemical sensor for general detection of Cd2+

AuPt–rGO showed good peroxidase-like activity for the oxidation of OPD to DAP (a novel internal reference) and achieved sensitive and reliable detection of Cd2+ based on a ratiometric strategy.

Ratiometric fluorescence detection of pathogenic bacteria based on dual-recognition nanoprobes with controllable G-quadruplex release

A dual-recognition carbon dot-based nanoprobe with controllable G-quadruplex release is developed for ratiometric fluorescence detection of pathogenic bacteria in a fast and precise way, which opens a promising avenue for efficient detection and early warning of pathogenic bacteria in food matrices.

Visual Test Paper for on-Site Polychlorinated Biphenyls Detection and Its Logic Gate Applications

A visual detection method was proposed for polychlorinated biphenyls (PCBs) detection using lateral flow test paper as the sensing platform. The aptamer sequence was used to recognize the target 3,3',4,4'-tetrachlorobiphenyl (PCB77). The integration of Zn²⁺-dependent DNAzyme with toehold-mediated strand displacement reaction significantly improved the response signals. Gold nanoparticles were utilized as the signal tracers in the test paper, making the results visible directly by the naked eye. Under optimal conditions, the paper enables the visual detection of PCB77 as low as 10 pM without additional instrumentation. The assay displays a high selectivity for PCB77 against potential interfering molecules. The visual test paper is robust and has been applied to the detection of PCB77 in milk samples with good recovery and satisfactory accuracy. Using two different PCBs (PCB77 and PCB72) as inputs, we further fabricated OR and AND logic gates, which is conducive to the development of an intelligent detection strategy for PCBs monitoring. Given the attractive characteristics of disposability, low cost, logic operation, and intuitive output, the test paper shows great promise for on-site screening of PCBs in resource-limited areas.

Advances in Applications of Molecular Logic Gates

Logic gates are devices that can perform Boolean logic operations and are the basic components of integrated circuits for information processing and storage. In recent years, molecular logic gates are gradually replacing traditional silicon-based electronic computers with their significant advantages and are used in research in water quality monitoring, heavy metal ion detection, disease diagnosis and treatment, food safety detection, and biological sensors. Logic gates at the molecular level have broad development prospects and huge development potential. In this review, the development and application of logic gates in various fields are used as the entry point to discuss the research progress of logic gates and logic circuits. At the same time, the application of logic gates in quite a few emerging fields is briefly summarized and predicted.

Development of a DNAzyme-based colorimetric biosensor assay for dual detection of Cd2+ and Hg2

A colorimetric biosensor assay has been developed for Cd²⁺ and Hg²⁺ detection based on Cd²⁺-dependent DNAzyme cleavage and Hg²⁺-binding-induced conformational switching of the G-quadruplex fragment. Two types of multifunctional magnetic beads (Cd-MBs and Hg-MBs) were synthesized by immobilizing two functionalized DNA sequences on magnetic beads via avidin-biotin chemistry. For Cd²⁺ detection, Cd-MBs are used as recognition probes, which are modified with a single phosphorothioate ribonucleobase (rA) substrate (PS substrate) and a Cd²⁺-specific DNAzyme (Cdzyme). In the presence of Cd²⁺, the PS substrate is cleaved by Cdzyme, and single-stranded DNA is released as the signal transduction sequence. After molecular assembly with the other two oligonucleotides, duplex DNA is produced, and it can be recognized and cleaved by FokI endonuclease. Thus, a signal output component consisting of a G-quadruplex fragment is released, which catalyzes the oxidation of ABTS with the addition of hemin and H₂O₂, inducing a remarkably amplified colorimetric signal. To rule out false-positive results and reduce interference signals, Hg-MBs modified with poly-T fragments were used as Hg²⁺ accumulation probes during the course of Cd²⁺ detection. On the other hand, Hg-MBs can perform their second function in Hg²⁺ detection by changing the catalytic activity of the G-quadruplex/hemin DNAzyme. In the presence of Hg²⁺, the G-quadruplex structure in Hg-MBs is disrupted upon Hg²⁺ binding. In the absence of Hg²⁺, an intensified color change can be observed by the naked eye for the formation of intact G-quadruplex/hemin DNAzymes. The biosensor assay exhibits excellent selectivity and high sensitivity. The detection limits for Cd²⁺ and Hg²⁺ are 1.9 nM and 19.5 nM, respectively. Moreover, the constructed sensors were used to detect environmental water samples, and the results indicate that the detection system is reliable and could be further used in environmental monitoring. The design strategy reported in this study could broadly extend the application of metal ion-specific DNAzyme-based biosensors.

Dual recognition element-controlled logic DNA circuit for COVID-19 detection based on exonuclease III and DNAzyme

Two fragments of the COVID-19 genome (specific and homologous) were used as two inputs to construct an AND logic gate for COVID-19 detection based on exonuclease III and DNAzyme. The detection sensitivity of the assay can reach fM levels. Satisfactory recovery values were obtained in real sample analysis.

An amplifying DNA circuit coupled with Mg2+-dependent DNAzyme for bisphenol A detection in milk samples

As bisphenol A (BPA) is harmful to human health, it is of great significance to develop a new method for BPA detection. Herein, we designed a BPA biosensor by integrating an amplifying DNA circuit with Mg²⁺-dependent DNAzyme into the sensing system. The BPA-aptamer binding activated a DNA circuit for signal amplification based on toehold-mediated strand displacement. A catalytic Mg²⁺-dependent DNAzyme was formed through synergistically DNA hybridization, which can cleave the dual-labeled substrate DNA into two segments. The separation of the fluorophore and quencher produces a high fluorescence response for BPA detection. This biosensor exhibited a superior sensitivity with a detection limit of 50 fM. The method is selective and robust, which can work even in milk samples with satisfactory accuracy. The biosensor analytical results were also verified by liquid chromatography coupled with mass spectrometry (LC-MS) and no obvious difference existed between the two methods.