Ultrasensitive Electrochemical Detection of Nucleic Acids Based on the Dual-Signaling Electrochemical Ratiometric Method and Exonuclease III-Assisted Target Recycling Amplification Strategy

Single-Response Duplexing of Electrochemical Label-Free Biosensor from the Same Tag

Multiplexing is a valuable strategy to boost throughput and improve clinical accuracy. Exploiting the vertical, meshed design of reproducible and low-cost ultra-dense electrochemical chips, the unprecedented single-response multiplexing of typical label-free biosensors is reported. Using a cheap, handheld one-channel workstation and a single redox probe, that is, ferro/ferricyanide, the recognition events taking place on two spatially resolved locations of the same working electrode can be tracked along a single voltammetry scan by collecting the electrochemical signatures of the probe in relation to different quasi-reference electrodes, Au (0 V) and Ag/AgCl ink (+0.2 V). This spatial isolation prevents crosstalk between the redox tags and interferences over functionalization and binding steps, representing an advantage over the existing non-spatially resolved single-response multiplex strategies. As proof of concept, peptide-tethered immunosensors are demonstrated to provide the duplex detection of COVID-19 antibodies, thereby doubling the throughput while achieving 100% accuracy in serum samples. The approach is envisioned to enable broad applications in high-throughput and multi-analyte platforms, as it can be tailored to other biosensing devices and formats.

One-pot, ultrasensitive, and multiplex detection of SARS-CoV-2 genes utilizing self-priming hairpin-mediated isothermal amplification

The global pandemic resulting from the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its emerging variants highlights the need for convenient and accurate detection protocols to facilitate timely prevention and management of the disease. Herein, we propose a new self-priming hairpin-mediated isothermal amplification (SIAM) protocol enabling one-pot and ultrasensitive identification of SARS-CoV-2 in a multiplexed way. This approach works by targeting a specific RNA sequence with a self-priming hairpin (SP) probe and promoting continuously repeated extension and nicking reactions to produce numerous trigger molecules, which could specifically bind to molecular beacons (MBs) and produce fluorescent signals. Under an isothermal condition of 37 °C, this technique allowed for the simultaneous identification of the spike (S) and nucleocapsid (N) genes of SARS-CoV-2 down to single copy/μL levels. We further validated the practical diagnostic capabilities of the SIAM method by accurately testing 20 clinical samples with 100% sensitivity and specificity. The SIAM method has a lot of potential to be a reliable nucleic acid testing protocol to identify infections caused by a wide range of pathogens.

A Dual-Enzyme Amplification Loop for the Sensitive Biosensing of Endopeptidases

A rapid and sensitive approach for the detection of endopeptidases via a new analyte-triggered mutual emancipation of linker-immobilized enzymes (AMELIE) mechanism has been developed and demonstrated using a matrix metallopeptidase, a collagenase, as the model endopeptidase analyte. AMELIE involves an autocatalytic loop created by a pair of selected enzymes immobilized on solid substrates via linkers with specific sites that can be proteolyzed by one another. These bound enzymes are spatially separated so that they cannot act upon their corresponding substrates until the introduction of the target endopeptidase analyte that can also cleave one of the linkers. This triggers the self-sustained loop of enzymatic activities to emancipate all the immobilized enzymes. In this proof of concept, signal transduction was achieved by a colorimetric horseradish peroxidase-tetramethylbenzidine (HRP-TMB-H₂O₂) reaction with HRP that are also being immobilized by one of the linkers. The pair of immobilized enzymes were collagenase and alginate lyase, and they were immobilized by an alginate linker and a short peptide chain containing the amino acid sequence of Leu-Gly-Pro-Ala for collagenase. A detection limit of 2.5 pg collagenase mL^-1 with a wide linear range up to 4 orders of magnitude was achieved. The AMELIE biosensor can detect extracellular collagenase in the supernatant of various bacteria cultures, with a sensitivity as low as 10³ cfu mL^-1 of E. coli. AMELIE can readily be adapted to provide the sensitive detection of other endopeptidases.

Self-Cleaning Recyclable Multiplexed Photoelectrochemical Sensing Strategy Based on Exonuclease III-Assisted Signal Discrimination

For discriminating the signals of multi-targets, multiplexed photoelectrochemical (PEC) detection is generally accomplished by modulating the light source or voltage, which prospect is usually limited by expensive instrumentation, tedious operational steps, and time-consuming material screening. To realize multiplexed determination on single photoelectric interface using the routine technique, a non-instrument-assisted strategy for signal discrimination needs to be explored. Herein, we propose an exonuclease III-mediated multiple PEC signals resolution strategy and construct a self-cleaning recyclable multiplexed PEC sensor using a porphyrin-bipyridine-based covalent organic framework (Por-Bpy COF) photocathode. Specifically, following the dual-target recognition event, exonuclease III cleaves the DNA strand attached to the magnetic bead so that the two signal labels can be separated. Once the signal label binds to the DNA on the electrode surface (E-DNA), exonuclease III turns to excise the DNA strand of the signal label and consequently the E-DNA can repeatedly bind different signal labels. As a result, distinguishable photocurrent signals of different targets can be generated on a single photoelectric interface. The feasibility of this multiplexed sensor is verified by detecting two coexisting mycotoxins aflatoxin B1 and zearalenone. On account of eliminating the instrumentation constraints and simplifying the experimental procedures, the proposed sensing strategy may provide a brand-new idea for the exploration of portable multiplexed PEC sensing devices.

Multiplexed electrochemical assays for clinical applications

Rapid, accurate diagnoses are central to future efficient healthcare to identify diseases at early stages, avoid unnecessary treatment, and improve outcomes. Electrochemical techniques have been applied in many ways to support clinical applications by enabling the analysis of relevant disease biomarkers in user-friendly, sensitive, low-cost assays. Electrochemistry offers a launchpad for multiplexed biomarker assays that offer more accurate and precise diagnostics compared to single biomarker assays. In this short review, we underpin the importance of multiplexed analyses and provide a universal overview of current electrochemical assay strategies for multiple biomarkers. We highlight relevant examples of electrochemical methods that successfully quantify important disease biomarkers. Finally, we offer a future outlook on possible strategies that can be employed to increase throughput, sensitivity, and specificity of multiplexed electrochemical assays.

Ionic Strength and Hybridization Position near Gold Electrodes Can Significantly Improve Kinetics in DNA-Based Electrochemical Sensors

A variety of electrochemical (EC) biosensors play critical roles in disease diagnostics. More recently, DNA-based EC sensors have been established as promising for detecting a wide range of analyte classes. Since most of these sensors rely on the high specificity of DNA hybridization for analyte binding or structural control, it is crucial to understand the kinetics of hybridization at the electrode surface. In this work, we have used methylene blue-labeled DNA strands to monitor the kinetics of DNA hybridization at the electrode surface with square-wave voltammetry. By varying the position of the double-stranded DNA segment relative to the electrode surface as well as the bulk solution's ionic strength (0.125-1.00 M), we observed significant interferences with DNA hybridization closer to the surface, with more substantial interference at lower ionic strength. As a demonstration of the effect, toehold-mediated strand displacement reactions were slowed and diminished close to the surface, while strategic placement of the DNA binding site improved reaction rates and yields. This work manifests that both the salt concentration and DNA hybridization site relative to the electrode are important factors to consider when designing DNA-based EC sensors that measure hybridization directly at the electrode surface.

Photoelectrochemical detection of microRNAs based on target-triggered self-assembly of energy band position-matched CdS QDs and C3N4 nanosheets

An ultrasensitive photochemical biosensor based on the target miRNA-triggered catalytic hairpin assembly (CHA) reaction between Au nanoparticles (AuNPs)/C₃N₄ nanosheets and CdS quantum dots (QDs) was developed for the determination of miRNAs. Firstly, AuNPs/C₃N₄ nanosheets were immobilized onto a working glassy carbon electrode. Then, the hairpin probe 1 (H1) was loaded through Au-S bonding. Afterward, the unbound sites were blocked with 6-mercaptohexanol to avoid nonspecific adsorption. In the presence of the target miRNA, the CHA reaction between the H1 and hairpin probe 2-CdS QDs (H2-CdS QDs) could be triggered. As a result, the AuNPs/C₃N₄ nanosheet and CdS QDs were linked by the double helix structure H1-H2. Unlike the other CHA reactions, H2 used in this work is longer than H1 so that the AuNPs/C₃N₄ nanosheets could touch the CdS QDs. Given the matched energy band positions between the C₃N₄ nanosheet and CdS QDs, a strong photocurrent could be obtained after the CHA reaction was triggered by the target miRNA. In addition, p-type C₃N₄ nanosheets and n-type CdS QDs presented reduction photocurrents and oxidation photocurrents, respectively. Therefore, the photocurrents were vectors in this design that can eliminate the interference of nonspecific adsorption and avoid the generation of false-positive signals. Under the optimal conditions, the limit of detection was 92 aM. The constructed photoelectrochemical biosensor showed good reproducibility and selectivity in the analysis of serum samples, which indicates its great prospects in disease diagnostics and bioanalysis.

p-Co-BDC/AuNPs-based multiple signal amplification for ultra-sensitive electrochemical determination of miRNAs

In this work, we report AuNPs-decorated pyrolyzed Co-BDC nanosheets (p-Co-BDC/AuNPs) as high-performance electrocatalyst for developing an electrochemical platform. p-Co-BDC/AuNPs as a new electrocatalyst showed superior electrocatalytic activity towards the electrochemical oxidation of methylene blue (MB). Besides, magnetic p-Co-BDC/AuNPs can be well immobilized on the magnetic glassy carbon electrode without further assistance. The oxidation of MB can be reduced by ascorbic acid. Inspired by this phenomenon, an electrochemical biosensor was constructed based on multiple signal amplification for the diagnosis of miRNAs. Firstly, p-Co-BDC/AuNPs enhanced the electrochemical oxidation of MB. Then, strand displacement amplification reaction can form lots of double helix structure DNA to embed more MB molecules. Finally, ascorbic acid in the electrolyte was utilized to reduce the oxidation of MB and improve the electrochemical signal of MB electro-oxidation. The linear detection range for the detection of miRNAs is 100 aM to 10 nM, and the limit of detection is 86 aM. Furthermore, the constructed biosensor also displayed satisfactory selectivity, good reproducibility, and excellent recovery in the detection of real samples. We are convinced that our proposed multiple signal amplification strategy will provide more promising methods for the diagnosis of cancer.

A Sensitive Thrombin Aptasensor Based on Target Circulation Strategy

A convenient homogeneous electrochemical thrombin sensor based on potential-assisted Au-S deposition and a dual signal amplification strategy was established in this study. Potential-assisted Au-S deposition does not require the modification of the gold electrode, thus eliminating the tedious pre-modification of the electrode. To better amplify the output signal, both ends of the signal hairpin probes were modified with a new electroactive substance, tetraferrocene, which was synthesized by the authors. Thrombin was immediately hybridized with a thiol-modified probe to open the stem-loop structure. After chain hybridization, thrombin was replaced and participated in the next round of the reaction; thus, the cascade amplification of the signal was realized. The hybrid chain formed an Au-S deposition under potential assistance, and the electrochemical signal of tetraferrocene could then be measured through differential pulse voltammetry (DPV) and consequently used for the quantitative detection of target thrombin. In addition, the detection limit of thrombin was as low as 0.06 pmol/L, and the detection of common interfering proteins was highly specific.

A label-free electrochemical platform based on a thionine functionalized magnetic Fe-N-C electrocatalyst for the detection of microRNA-21

Taking a composite of a nanomaterial and a signal molecule as a substrate material can provide a label-free electrochemical platform. Besides, the nanomaterial with a high catalytic activity towards the signal molecule can improve the sensitivity of the platform. Herein, a thionine functionalized Fe-N-C nanocomposite was employed as the substrate. Firstly, the electrocatalytic activity of Fe-N-C towards the electroreduction of thionine was explored. Then, an immobilization-free and label-free electrochemical platform for the determination of microRNA-21 based on Fe-N-C-thionine/Fe₃O₄@AuNPs was constructed. A magnetic glassy carbon electrode (MGCE) was used to keep the magnetic Fe-N-C-thionine/Fe₃O₄@AuNPs modified onto the surface of the MGCE. Fe-N-C and Fe₃O₄ nanoparticles can co-catalyze the electroreduction of thionine and a strong electrochemical reduction signal of thionine could be realized in the differential pulse voltammetry (DPV) test. Also, a catalytic hairpin assembly (CHA) reaction was utilized to enhance the sensitivity of the developed electrochemical biosensor. Besides, the developed biosensor shows excellent specificity and reproducibility in the test of human serum samples, indicating its wide application prospects in the early-stage diagnosis of tumors.

Amplified electrochemical biosensing based on bienzymatic cascade catalysis confined in a functional DNA structure

Herein, an amplified and renewable electrochemical biosensor was developed via bienzymatic cascade catalysis of glucose oxidase (GOx) and horseradish peroxidase (HRP), which were confined in a functional Y-shaped DNA nanostructure oriented by a dual-thiol-ended hairpin probe (dSH-HP) with a paired stem as a rigid scaffold and unpaired loop as enclosed binding platform. For proof-of-concept assay of sequence-specific biomarker DNA related to Alzheimer's disease (aDNA), GOx and redox ferrocene-modified HRP (Fc@HRP) were chemically conjugated in two enzyme strands (GOx-ES1 and Fc@HRP-ES2), respectively. The repeated recycling of aDNA was powered by the displacement of GOx-ES1 by aDNA and exonuclease III (ExoIII)-assisted cleavage reaction for amplified output of numerous GOx-ES1 as dependent transducers, together with Fc@HRP-ES2 which was simultaneously hybridized with dSH-HP to assemble this DNA structure. Rationally, the bienzymatic cascade catalysis was motivated through GOx-catalyzed glucose oxidization to in situ generate hydrogen peroxide (H₂O₂) and overlapped HRP-catalyzed H₂O₂ decomposition to promote the electron transfer, producing significantly enhanced electrochemical signal of Fc with an ultrahigh sensitivity down to 0.22 fM of aDNA. Benefited from the unique design of dSH-HP-oriented bienzymatic cascades, this one-step strategy without non-specific blockers passivation was simple and renewable, and would pave a promising avenue for sensitive electrochemical assay of biomolecules.

Sensitive Dual-Mode Biosensors for CYFRA21-1 Assay Based on the Dual-Signaling Electrochemical Ratiometric Strategy and "On-Off-On" PEC Method

Herein, an electrochemical (EC)-photoelectrochemical (PEC) dual-mode biosensor was constructed for cytokeratin 19 fragment 21-1 (CYFRA21-1) assay based on the dual-signaling electrochemical ratiometric strategy and "on-off-on" PEC method. The indium tin oxide (ITO) electrode was modified by 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)@C₆₀ and gold nanoparticles (Au NPs), and the double-stranded DNA composed of thiol/methylene blue (MB)-labeled single-stranded DNA (ssDNA) (S0-MB) and antibody/ferrocene (Fc)-labeled ssDNA (Ab1-S1-Fc) was immobilized on the Au NPs/PTCDA@C₆₀/ITO electrode via the Au-S bond between Au NPs and thiol of S0-MB. With the help of another antibody-labeled ssDNA (Ab2-S2), the presence of CYFRA21-1 triggered a typical antigen-antibody sandwich immune reaction (Ab1, CYFRA21-1, and Ab2) and proximity hybridization between Ab1-S1-Fc and Ab2-S2. This caused the release of Ab1-S1-Fc from the modified electrode and the change of S0-MB to a hairpin structure, resulting in a decrease (an increase) of the oxidation peak current of Fc (MB) and an increase of the photocurrent due to the enhancing (inhibiting) effect of MB (Fc) on the photoelectric performance of the Au NPs/PTCDA@C₆₀/ITO electrode. Thus, CYFRA21-1 was detected by the developed EC-PEC dual-mode sensing platform sensitively, and the linear response ranges of 0.001-40 ng/mL with a detection limit of 0.3 pg/mL for the EC technique and 0.0001-4 ng/mL with a detection limit of 0.03 pg/mL for the PEC method were obtained. Furthermore, by changing the specific antibodies of disease-related biomarkers, the developed dual-mode biosensing platform could be readily extended to detect other antigens, implying its great potential applications in biological analysis and early disease diagnosis.

Hairpin DNA-Mediated isothermal amplification (HDMIA) techniques for nucleic acid testing

Nucleic acid detection is of great importance in a variety of areas, from life science and clinical diagnosis to environmental monitoring and food safety. Unfortunately, nucleic acid targets are always found in trace amounts and their response signals are difficult to be detected. Amplification mechanisms are then practically needed to either duplicate nucleic acid targets or enhance the detection signals. Polymerase chain reaction (PCR) is one of the most popular and powerful techniques for nucleic acid analysis. But the requirement of costly devices for precise thermo-cycling procedures in PCR has severely hampered the wide applications of PCR. Fortunately, isothermal molecular reactions have emerged as promising alternatives. The past decade has witnessed significant progress in the research of isothermal molecular reactions utilizing hairpin DNA probes (HDPs). Based on the nucleic acid strand interaction mechanisms, the hairpin DNA-mediated isothermal amplification (HDMIA) techniques can be mainly divided into three categories: strand assembly reactions, strand decomposition reactions, and strand creation reactions. In this review, we introduce the basics of HDMIA methods, including the sensing principles, the basic and advanced designs, and their wide applications, especially those benefiting from the utilization of G-quadruplexes and nanomaterials during the past decade. We also discuss the current challenges encountered, highlight the potential solutions, and point out the possible future directions in this prosperous research area.

Ratiometric Electrochemistry: Improving the Robustness, Reproducibility and Reliability of Biosensors

Electrochemical biosensors are an increasingly attractive option for the development of a novel analyte detection method, especially when integration within a point-of-use device is the overall objective. In this context, accuracy and sensitivity are not compromised when working with opaque samples as the electrical readout signal can be directly read by a device without the need for any signal transduction. However, electrochemical detection can be susceptible to substantial signal drift and increased signal error. This is most apparent when analysing complex mixtures and when using small, single-use, screen-printed electrodes. Over recent years, analytical scientists have taken inspiration from self-referencing ratiometric fluorescence methods to counteract these problems and have begun to develop ratiometric electrochemical protocols to improve sensor accuracy and reliability. This review will provide coverage of key developments in ratiometric electrochemical (bio)sensors, highlighting innovative assay design, and the experiments performed that challenge assay robustness and reliability.

Interaction between the functionalized probes: The depressed efficiency of dual-amplification strategy on ratiometric electrochemical aptasensor for aflatoxin B1

Signal amplification is one of the most effective ways to develop the high-performance electrochemical sensors. However, it can be more complicated for ratiometric detections. Herein, a ratiometric electrochemical aptasensor for aflatoxin B1 (AFB1) was proposed by taking advantage of a dual-amplification strategy by coupling of DNA walker (DW) with hybridization chain reaction (HCR). The special binding of AFB1 with ferrocene (Fc)-labelled aptamer triggers DW on hairpin DNA (hDNA) tracks to produce abundant double-stranded DNA (dsDNA). HCR-based strand amplification occurs on these dsDNA to absorb more methylene blue (MB). Then current ratio of MB (I_MB) and Fc (I_Fc) is designed as a yardstick to detect AFB1. Our experiments reveal that the interaction between Fc and MB (i.e., steric hindrance, electron mediator) varies. In addition to steric hindrance, the presence of MB also acts as electron mediator, thereby facilitating the electron transfer between Fc and electrode. Such combined effect consequently depresses the efficiency of dual-amplification strategy to improve the detection. The developed ratiometric electrochemical aptasensor allows the accurate detection of AFB1 in the 0.003-3 pg mL^-1 range. Our work has shed light on the amplification strategy for ratiometric sensing, and provided a new route in integrating different amplification strategies.

Electrochemical Biosensors for Detection of MicroRNA as a Cancer Biomarker: Pros and Cons

Cancer is the second most fatal disease in the world and an early diagnosis is important for a successful treatment. Thus, it is necessary to develop fast, sensitive, simple, and inexpensive analytical tools for cancer biomarker detection. MicroRNA (miRNA) is an RNA cancer biomarker where the expression level in body fluid is strongly correlated to cancer. Various biosensors involving the detection of miRNA for cancer diagnosis were developed. The present review offers a comprehensive overview of the recent developments in electrochemical biosensor for miRNA cancer marker detection from 2015 to 2020. The review focuses on the approaches to direct miRNA detection based on the electrochemical signal. It includes a RedOx-labeled probe with different designs, RedOx DNA-intercalating agents, various kinds of RedOx catalysts used to produce a signal response, and finally a free RedOx indicator. Furthermore, the advantages and drawbacks of these approaches are highlighted.

Electrochemical Analysis of Target-Induced Hairpin-Mediated Aptamer Sensors

The state of probe DNA at the biosensing interface greatly affects the detection performance of electrochemical DNA biosensors. Herein, we constructed a target-induced hairpin-mediated biosensing interface to study the effect of probe DNA on the analytical performance of adenosine triphosphate aptamer (ATPA) and adenosine triphosphate (ATP) detection. Moreover, we also explored the electrochemical contribution of the coexisting hairpin and double-stranded DNA (dsDNA) to this sensing interface. Experimental results suggested that the molecular recognition ability and detection performance of the biosensing interface were majorly dependent on the surface density of methylene blue (MB)-labeled probe hairpin DNA and partly affected by the spatial state of the formed dsDNA. When the surface density of hairpin DNA was moderate (5.72 pmol cm^-2), this sensing interface determined as low as 0.74 fM ATPA and 5.04 pM ATP with high selectivity and excellent regeneration, respectively. Furthermore, we calculated that the formed dsDNA had a 31.87% contribution in the total electrochemical signal for 10 pM ATPA detection. Based on the above results, we designed an XOR logic gate based on the biosensing interface for ATPA and ATP detection.

Bifunctional MOFs-Based Ratiometric Electrochemical Sensor for Multiplex Heavy Metal Ions

In this study, a ratiometric electrochemical sensor based on metal-organic frameworks (MOFs) was developed for sensing of multiplex metal ions. The bifunctional MOFs were prepared in a way to integrate two signal tags and a detection probe. In the presence of target metal ions, the target metal ions can replace the framework metal-ion center in the original MOFs through an ion-exchange reaction, leading to ratiometric electrochemical signals under different applied potentials. One consisted of the Cu²⁺ signal generated from electroactive MOFs selected as internal reference signals. The other consisted of the signal induced by other target metal ions. Using the I_metal ions/I_Cu²⁺ signal as the output, the prepared ratiometric probe was able to eliminate disturbance caused by the sensing environment. Moreover, the large surface area and abundant active sites in MOFs produced a multiplex ratiometric electrochemical sensor with improved characteristics in terms of reproducibility, stability, and sensitivity. The sensor was also simple without sophisticated instrumentation, amplification processes, or an acid dissolution/preconcentration procedure, hence promising for practical applications.

Metal-Free Photoinduced Atom Transfer Radical Polymerization for Highly Sensitive Detection of Lung Cancer DNA

Convenient and sensitive detection of biomolecules is of great significance to disease diagnosis. In this work, a metal-free photoinduced atom transfer radical polymerization (photoATRP) by a reductive quenching pathway as a novel strategy is applied to achieve lung cancer DNA detection. Thiolated PNA is exploited to specifically recognize target DNA, and the initiator of photoATRP is linked to the electrode surface via phosphate-Zr⁴⁺ -carboxylate. Under the excitation of blue light, the reductive quenching pathway is activated with eosin Y (EY) as photoredox catalyst and N,N,N',N'',N'-pentamethyldiethylenetriamine (PMDETA) as electron donor, and numerous polymeric chains are formed. Under optimal conditions, the linear range of this strategy is from 0.1 pm to 10 nm (R² =0.989) with a limit of detection (LOD) of 1.4 fm (14 zmol in 10 μL). The variety of possible light sources for photoATRP and simple operation endow this biosensor with great potential for practical applications.

Double-signal mode based on metal–organic framework coupled cascaded nucleic acid circuits for accurate and sensitive detection of serum circulating miRNAs

A double-signal mode based on metal–organic framework coupled cascaded nucleic acid circuits was developed for the accurate and sensitive detection of serum circulating miRNAs.

Ultrasensitive Detection of Exosomes by Target-Triggered Three-Dimensional DNA Walking Machine and Exonuclease III-Assisted Electrochemical Ratiometric Biosensing

Exosomes are membrane-enclosed phospholipid extracellular vesicles. In spite of their great promise as noninvasive biomarkers for cancer diagnosis, sensitive detection of exosomes is still challenging. Herein, the detection of exosomes was changed to the detection of DNA after recognition of exosomes with its aptamers. CD63 aptamer and EpCAM aptamer were used for the detection of MCF-7 cell-secreted exosome. The recognition process was amplified through the movements of a three-dimensional DNA walker. And then, Exonuclease III- assisted electrochemical ratiometric sensor was applied for further signal amplification. Under optimal conditions, the detection limit of 1.3 × 10⁴ particles/mL was obtained with excellent selectivity. Furthermore, clinical application test for the detection of exosomes in human serum was also verified.

Signal-on electrochemical detection of DNA methylation based on the target-induced conformational change of a DNA probe and exonuclease III-assisted target recycling

A promising electrochemical system was explored for DNA methylation detection according to the construction of a signal-on biosensor. Based on the ingenious design of probe DNA and auxiliary DNA, methylated target DNA triggered the exonuclease III (Exo III) digestion of auxiliary DNA from 3'-terminus, resulting in the conformational change of probe DNA with an electroactive methylene blue (MB) tag at 5'-terminus. Consequently, the MB tag in the probe DNA was close to the electrode surface for electron transfer, generating an increased current signal. Because of the target recycling of methylated DNA, significant signal amplification was obtained. Moreover, bisulfite conversion conferred an efficient approach for the universal analysis of any CpG sites without the restriction of specific DNA sequence. As a result, the target DNA with different methylation statuses were clearly recognized, and the fully methylated DNA was quantified in a wide range from 10 fM to 100 pM, with a detection limit of 4 fM. The present work realized the assay of methylated target DNA in serum samples with satisfactory results, illustrating the application performance of the system in complex sample matrix.

Design and Biosensing of a Ratiometric Electrochemiluminescence Resonance Energy Transfer Aptasensor between a g-C3N4 Nanosheet and Ru@MOF for Amyloid-β Protein

A dual-wavelength ratiometric electrochemiluminescence resonance energy transfer (ECL-RET) aptasensor based on the carbon nitride nanosheet (g-C₃N₄ NS) and metal-organic frameworks (Ru@MOFs) as energy donor-receptor pairs is first designed for the detection of the amyloid-β (Aβ) protein. The cathode ECL of g-C₃N₄ NS gradually decreased, whereas the anode ECL from Ru@MOF pyramidally enhanced along with the increasing concentration of Aβ in a 0.1 M phosphate-buffered saline solution containing 0.1 M S₂O₈^2-. Additionally, it is worth noting that 2-amino terephthalic acid from MOF not only can load abundant amounts of luminophor Ru(bpy)₃²⁺ but also promote the conversion of more amounts of S₂O₈^2- that served as a coreactant accelerator into SO₄^•-, further enhancing the ECL signal of Ru@MOF. Besides, the ECL intensity from the g-C₃N₄ NS had a tremendous spectrum overlap with the UV-vis spectrum of Ru@MOF, demonstrating the high-efficiency ECL-RET from g-C₃N₄ NS to Ru@MOF. According to the ratio of ECL_460nm/ECL_620nm, the constructed aptasensor for the detection of Aβ showed a wide linear range from 10^-5 to 500 ng/mL and a low detection limit of 3.9 fg/mL (S/N = 3) with a correction coefficient of 0.9965. The obtained results certified that the dual-wavelength ratiometric ECL sensor could provide a reliable direction and have the potential for application in biosensing and clinical diagnosis fields.

Novel 3D Printed Device for Dual-Signaling Ratiometric Photoelectrochemical Readout of Biomarker Using λ-Exonuclease-Assisted Recycling Amplification

A ratiometric photoelectrochemical (PEC) sensing strategy was proposed for monitoring of carcinoembryonic antigen (CEA) based on a homemade 3D printing device with dual-working photoelectrodes (PE₁ and PE₂), coupling λ-exonuclease (λ-Exo)-assisted recycling amplification with CdS quantum dots. Gold nanoparticles-functionalized ZnO nanorods were utilized as PEC substrate for generating initial photocurrent and immobilizing DNA probe. Upon incubation of target with DNA trigger/CEA aptamer-modified magnetic bead (tri/apt-MB), DNA trigger dissociated from magnetic bead and then hybridized with capture probe (cp) on PE₁ or opened hairpin probe (hp) on PE₂ to form double-stranded DNA (dsDNA). The exonuclease could recognize and cleave two newly generated dsDNA, leading to the release of trigger. The free trigger strand continued to hybridize with the remaining cp/hp, which were cleaved by λ-Exo, and then trigger was released again and restarted next recycle with the λ-Exo. After digestion of λ-Exo, the number of capture probes on PE₁ was reduced, and many short DNA fragments were produced on PE₂, thereby resulting in the decreasing CdS QDs on PE₁ and the increasing CdS QDs on PE₂. As a result, it was observed that the ratio value of photocurrents (PE₁/PE₂) significantly decreased with the increasing CEA. Under optimum conditions, the sensing method showed a good linear relationship toward CEA within the dynamic range of 0.02-10 ng mL^-1 and a detection limit of 6.0 pg mL^-1. Moreover, the ratiometric PEC sensor exhibited good reproducibility, satisfying stability, and remarkable anti-interference performance, which suggests its promising application prospect to detect target CEA.

The construction of Mycobacterium tuberculosis 16S rDNA MSPQC sensor based on Exonuclease III-assisted cyclic signal amplification

Tuberculosis caused by Mycobacterium tuberculosis (M. tuberculosis) remains one of the most serious infectious diseases all over the world. The key to reduce the spread and mortality rate of tuberculosis is to develop faster and more sensitive approaches for detection of M. tuberculosis. However, current detection methods can not meet the requirements of rapid clinical M. tuberculosis detection in terms of detection time. Herein, a new 16S rDNA multichannel series piezoelectric quartz crystal (MSPQC) sensor based on Exonuclease III (Exo III)-aided target recycling has been developed for rapid detection of M. tuberculosis. The specific 16S rDNA fragment of M. tuberculosis was used as biomarker, DNA capture probes complementary to the biomarker were designed and modified on the surface of AuNPs. The Exo III which could recognise hybrid duplexes and selectively digest DNA capture probe was used to assist digestion cycle by digesting DNA capture probe and releasing the intact target fragment. After all DNA probes loading on the surface of AuNPs were removed, the surface of AuNPs was exposed and conductive connection was formed between the nanogap network electrode by self-catalytic growth of exposed AuNPs in the glucose and HAuCl₄ solution. This resulted in sensitive response of M. tuberculosis sensor and M. tuberculosis was detected by recording this response. The limit of detection (LOD) of the method was 20 CFU/mL and the detection time was less than 3 h. It was expected to be widely used in detection methods of M. tuberculosis.

Aptamer based ratiometric electrochemical sensing of 17β-estradiol using an electrode modified with gold nanoparticles, thionine, and multiwalled carbon nanotubes

Gold nanoparticles (AuNPs) with a size of ~3 nm were placed on a thionine-multiwalled carbon nanotube (Thi-CNTs) conjugate to form a novel AuNP-Thi-CNTs nanocomposite. Its morphology and composition were characterized by transmission electron microscopy, atomic force microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The nanocomposite was placed on an electrode and used as a redox-active signaling interface to fabricate a ratiometric electrochemical aptasensor for 17β-estradiol (E2). The potentiostatic insertion method was applied to insert an aptamer against E2 into a thin alkane monolayer to warrant an adequate distance between aptamers. The aptamer against E2 acts as both a collector and separator to specifically bind E2. The electrode displays two peak signals (at +0.50 V vs. SCE for E2; and at -0.32 V for Thi) which increase and decrease, respectively, in the 12 pM to 60 nM E2 concentration range. Therefore, the current ratio can be used to reliably, reproducibly, and sensitively quantify the concentration of E2. Graphical abstract Schematic presentation of a novel AuNP-Thi-CNTs nanocomposite. AuNP-Thi-CNTs showed good electrocatalytic oxidation to E2. AuNP-Thi-CNTs was used as self-redox signal interface to fabricate aptasensor. Dual signals of extrinsic E2 and inner Thi was applied to monitor the concentration of E2.

Ratiometric Electrochemical Sensors Associated with Self-Cleaning Electrodes for Simultaneous Detection of Adrenaline, Serotonin, and Tryptophan

Electrochemical sensors have long suffered from issues such as nonspecific adsorption, poor anti-interference ability, and internal and external disturbances. To address these challenges, we developed a facile electrochemical method, which integrated a ratiometric strategy with self-cleaning electrodes. In the novel sensing system, the self-cleaning electrode was realized via forming a hydrophobic layer on carbonized ZIF-67@ZIF-8 (cZIF) by polydimethylsiloxane (PDMS) precursor vaporization. As for ratiometry, it is worth to mention that the measurements were conducted by adding an interior reference (methylene blue) directly into electrolyte solution, which is more facile and flexible to operate compared with conventional ones. Sensing performance of the self-cleaning electrode as well as the newly established ratiometric strategy was explored fully, and it turned out that PDMS@cZIF nanocomposites provided decent electrocatalytic ability, superhydrophobic property, and stability. Furthermore, the ratiometric strategy significantly elevated the robustness and reproducibility of electrochemical sensing. Simultaneous detection of Adr, 5-HT, and Trp was performed under the optimum experimental conditions with wide linear ranges and low detection limits. Finally, the original ratiometric electrochemical sensor was successfully applied for monitoring the three target molecules in biological samples.

High sensitive and broad-range detection of cortisol in human saliva using a trap lateral flow immunoassay (trapLFI) sensor

Cortisol, a steroid hormone, is a main biomarker of psychological stress. Early diagnosis and proper treatment of such stress is crucial to prevent the excessive secretion of cortisol. However, cortisol has a low molecular weight and cannot provide sufficient recognition sites for sandwich immunoreaction; it has previously been measured using a competitive immunoassay instead of a general sandwich immunoassay. The disadvantage of this approach is that quantitative measurements are limited because of the narrow measurable range that is key for biosensors. To overcome this limitation, we propose a new detection platform that enables small molecules such as cortisol to be quantified with high detection sensitivity. A trap lateral flow immunoassay (trapLFI) sensor has deletion and detection zones instead of the test and control zones in general lateral flow immunoassay (LFI) sensors. The conjugates used to minimize possible detection targets at low concentration are gold nanoparticles that include an antibody against cortisol and an enzyme for signal generation. Target-bound conjugates are captured in the detection zone, whereas conjugates not binding with targets are trapped in the deletion zone. Using this platform, enzyme-catalyzed color signals increase in the detection zone and decrease in the deletion zone with the concentration of cortisol. The ratio of signal from deletion zone and detection zone supplied a wide analytical range (0.01-100 ng mL-1) with high detection sensitivity (9.9 pg mL-1). Analysis of 15 human saliva samples showed a good correlation with conventional ELISA results (R2 = 0.9432).

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

In this work, toehold strand displacement cascade (TSDC) has been delicately designed and carried out on the cellular interface for the amplification and output of dual-electrochemical signal. Specifically, antibody cross-linked T strand can recognize cell which is linked with immune-magnetic bead. Subsequently, T strand on the cellular interface can mediate the occurrence of TSDC, resulting the change of SN₃/S1-MB to SN₃/S2-Fc ratio in the supernatant after magnetic separation. The resultant SN₃/S1-MB and SN₃/S2-Fc can be immobilized on the electrode interface through click chemistry and give the amplified double electrochemical signal. So the tumor cell amount can closely correlate with the change of the double signal. Except for output of the double signal for improvement of analytical accuracy, the double magnetic separation not only eliminate the interference of the complicated substances in serum, but also remove the influence of cell on click reaction on the electrode interface. So based on cellular interface supported TSDC for amplified dual-electrochemical signal, the established method has been successfully applied to analyze the tumor cells in serum accurately and sensitively.

An electrochemical dual-signaling aptasensor for the ultrasensitive detection of insulin

Insulin plays a central role in physiological glycolmetabolism and is associated with diabetes and related diseases. In this work, a dual-signaling electrochemical aptasensor for insulin detection with high sensitivity and specificity has been reported. Methylene blue (MB)-modified insulin-binding aptamer (IBA) as "signal-off" probe, and (DNA2)/Ferrocene (Fc) co-modified gold nanoparticles (DNA2Fc@GNPs) as the "signal-on" probe were integrated with linker mDNA to fabricate the DNA2Fc@GNPs/mDNA/MB-IBA modified Au electrode as the sensing interface, and the current responses of MB and Fc were used as signal indicators. As expected, the incubation of insulin with DNA2Fc@GNPs/mDNA/MB-IBA/Au electrode resulted in the current responses of MB and Fc decreased and increased, respectively. Based on this strategy, the detection of insulin was successfully achieved, and a linear range from 10 pM to 10 nM with the detectable lowest concentration of 0.1 pM was obtained. By measuring the insulin concentrations in serum samples, this proposed aptasensor has been proven to be of high specificity and accuracy. Moreover, the dual-signaling is useful for the more accurate and reproducible detection through their self-referencing capability. This aptasensor possesses such advantages as simplicity, rapid responses, high sensitivity, specificity and accuracy, which is significant for improving the diagnosis of insulin-related diseases.

Recent advances in molybdenum disulfide-based electrode materials for electroanalytical applications

The primary objective of this review article is to summarize the development and structural diversity of 2D/3D molybdenum disulfide (MoS₂) based modified electrodes for electrochemical sensors and biosensor applications. Hydrothermal, mechanical, and ultrasonic techniques and solution-based exfoliation have been used to synthesize graphene-like 2D MoS₂ layers. The unique physicochemical properties of MoS₂ and its nanocomposites, including high mechanical strength, high carrier transport, large surface area, excellent electrical conductivity, and rapid electron transport rate, render them useful as efficient transducers in various electrochemical applications. The present review summarizes 2D/3D MoS₂-based nanomaterials as an electrochemical platform for the detection and analysis of various biomolecules (e.g., neurotransmitters, NADH, glucose, antibiotics, DNA, proteins, and bacteria) and hazardous chemicals (e.g., heavy metal ions, organic compounds, and pesticides). The substantial improvements that have been achieved in the performance of enzyme-based amperometry, chemiluminescence, and nucleic acid sensors incorporating MoS₂-based chemically modified electrodes are also addressed. We also summarize key sensor parameters such as limits of detection (LODs), sensitivity, selectivity, response time, and durability, as well as real applications of the sensing systems in the environmental, pharmaceutical, chemical, industrial, and food analysis fields. Finally, the remaining challenges in designing MoS₂ nanostructures suitable for electroanalytical applications are outlined. Graphical abstract • MoS₂ based materials exhibit high conductivity and improved electrochemical performance with great potential as a sensing electrode. • The role of MoS₂ nanocomposite films and their detection strategies were reviewed. • Biomarkers detection for disease identification and respective clinical treatments were discussed. • Future Challenges, as well as possible research development for "MoS₂ nanocomposites", are suggested.