Ultrasensitive electrochemical detection of DNA based on Zn 2+ assistant DNA recycling followed with hybridization chain reaction dual amplification

Detection of fucosylated extracellular vesicles miR-4732-5p related to diagnosis of early lung adenocarcinoma by the electrochemical biosensor

Our preliminary investigation has identified the potential of serum fucosylated extracellular vesicles (EVs) miR-4732-5p in the early diagnosis of lung adenocarcinoma (LUAD) by a fucose-captured strategy utilizing lentil lectin (LCA)-magnetic beads and subsequent screening of high throughput sequencing and validation of real-time quantitative polymerase chain reaction (RT-qPCR). Considering the relatively complicated procedure, expensive equipment, and stringent laboratory condition, we have constructed an electrochemical biosensor assay for the detection of miR-4732-5p. miR-4732-5p is extremely low in serum, down to the fM level, so it needs to be detected by highly sensitive electrochemical methods based on the Mg2+-dependent DNAzyme splitting nucleic acid lock (NAL) cycle and hybridization chain reaction (HCR) signal amplification. In this study, signal amplification is achieved through the dual amplification reactions using NAL cycle in combination with HCR. In addition, hybridized DNA strands bind to a large number of methylene blue (MB) molecules to enhance signaling. Based on the above strategy, we further enhance our signal amplification strategies to improve detection sensitivity and accuracy. The implementation of this assay proceeded as follows: initially, miR-4732-5p was combined with NAL, and then Mg2+-dependent DNAzyme splitted NAL to release auxiliary DNA (S1) strands, which were subsequently captured by the immobilized capture probe DNA (C1) strands on the electrode surface. Following this, abundant quantities of DNA1 (H1) and DNA2 (H2) tandems were generated by HCR, and S1 strands then hybridized with the H1 and H2 tandems through base complementary pairing. Finally, MB was bonded to the H1 and H2 tandems through π-π stacking interaction, leading to the generation of a signal current upon the detection of a potential capable of inducing a redox change of MB by the electrode. Furthermore, we evaluated the performance of our developed electrochemical biosensor assay. The results demonstrated that our assay is a reliable approach, characterized by its high sensitivity (with a detection limit of 2.6 × 10-17 M), excellent specificity, good accuracy, reproducibility, and stability. Additionally, it is cost-effective, requires simple operation, and is portable, making it suitable for the detection of serum fucosylated extracellular vesicles miR-4732-5p. Ultimately, this development has the potential to enhance the diagnostic efficiency for patients with early-stage LUAD.

Proximity hybridization induced bipedal DNA walker and rolling circle amplification for label-free electrochemical detection of apolipoprotein A4

Apolipoprotein A4 (Apo-A4) is considered as a prospective molecular biomarker for diagnosis of depression due to its neurosynaptic toxicity. We develop a proximity hybridization-induced DNAzyme-driven bipedal DNA walker strategy for Apo-A4 quantification based on rolling circle amplification (RCA) triggered by poly adenine binding to Ag nanoparticles (AgNPs). With the help of DNAzyme, the free-running bipedal DNA walker can quickly and sequentially shear a molecular beacon that acts as a primer to initiate the RCA process, producing a large number of long DNA strands containing numerous adenines. The long repetitive adenine strands then absorb large amounts of AgNPs on the electrode interface, which is then electrochemically stripped of the AgNPs. The method has a linear detection range of 0.001 ∼ 100 ng mL-1 and a detection limit of 0.46 pg mL-1. The presented detection strategy is label-free, which allows high sensitivity and selectivity for detection of a wide range of protein targets by corresponding DNA-based affinity probes, which have potential applications in bioanalysis.

Proximity hybridization induced bipedal DNA walker for label-free electrochemical detection of apolipoprotein A4 based on DNA meditated Ag nanoparticles growth

Apolipoprotein A4 (Apo-A4) is considered as a prospective molecular biomarker for diagnosis of depression due to its neurosynaptic toxicity. Here, we propose a neighboring hybridization induced catalyzed hairpin assembly (CHA) driven bipedal DNA walker that mediates hybridization of Ag nanoparticles (Ag NPs) with DNA probes for highly sensitive electrochemical quantitative detection of Apo-A4. Driven by CHA, this bipedal DNA walker can spread all over the surface of the sensor, induce the HP1-HP2 double chain structure, make the surface of the sensor negatively charged, and adsorb a large number of Ag ions. After chemical reduction with hydroquinone, the Ag NPs formed provide signal tracers for electrochemical dissolution analysis of the target. The Ag NPs formed by chemical reduction of hydroquinone can provide signal traces for electrochemical stripping analysis of target thrombin. The linear range of this method is from 10 pg mL-1 to 1000 ng mL-1, and the detection limit is 5.1 pg mL-1. This enzyme-free and labeling detection method provides a new strategy for rapid clinical detection of Apo-A4 and accurate identification of depression.

AuNP/Cu-TCPP(Fe) metal-organic framework nanofilm: a paper-based electrochemical sensor for non-invasive detection of lactate in sweat

Effective and real-time detection of lactate (LA) content in human sweat has attracted considerable attention from researchers. In this work, a novel electrochemical paper-based analysis device (ePAD) was developed for the non-invasive detection of LA in sweat. The electrocatalytic properties of AuNP/Cu-TCPP(Fe) hybrid nanosheets, which were prepared by an optimised synthetic method, were studied by CV and EIS electrochemical methods for the first time and the working electrode can be fabricated using a drip coating method. The lactate sensor was optimised and validated for usability, adoptability and interpretability. To the best of our knowledge, this was the fastest, lowest detection line and widest linear range method reported to date for the detection of lactate. It achieved the detection limit of 0.91 pM and a linear range from 0.013 nM to 100 mM. The dual catalytic effects of the hybrid NSs shortened the detection time by nearly two times and enhanced the sensitivity approximately two times, an accuracy unmatched until now. Furthermore, this sensor was employed for LA analysis and validated by high performance liquid chromatography (HPLC). The ePAD shows superior biocompatibility, accuracy, and high sensitivity and can be easily manufactured. Hence, it is applicable for the long-term monitoring of sweat LA concentrations in point-of-care testing, athletic testing of athletes and military personnel and other subjects in different extreme environments.

An electrochemical biosensor for the detection of aflatoxin B1 based on the specific aptamer and HCR biological magnification

Aflatoxin B1 (AFB1) is a highly toxic mycotoxin, which causes severe acute or cumulative poisoning. Therefore, it is important to develop sensitive and selective detection methods for AFB1 for the safety of food and medicinal herbs. Herein, we have developed a "signal-on" electrochemical aptasensor based on the high specificity of the aptamer and hybridization chain reaction (HCR) biological amplification for AFB1 detection. In this work, thiol-modified complementary DNA (cDNA) immobilized on the surface of a gold electrode (GE) served as an initiator DNA. When AFB1 was present, it competed with the cDNA for binding to the aptamers, which resulted in the detaching of aptamers from the cDNA-aptamer duplexes. Then, the single-stranded cDNA acted as an initiator to trigger the HCR signal amplification. Therefore, long double-stranded DNA (dsDNA) products were produced, which could load large amounts of methylene blue (MB) molecules to generate a distinct electrochemical signal. Under the optimized conditions, the proposed electrochemical aptasensor achieved the ultrasensitive detection of AFB1 with a linear detection range of 0.01-100 pg mL^-1, and a limit of detection (LOD) down to 2.84 fg mL^-1. Furthermore, the electrochemical aptasensor was successfully applied for detecting AFB1 in corn and two kinds of traditional Chinese medicine samples, indicating the potential value for AFB1 detection in practical samples.

Simultaneous Detection of Bladder Cancer Exosomal MicroRNAs Based on Inorganic Nanoflare and DNAzyme Walker

Bladder cancer (BC) is one of the most common cancers in the world, with high morbidity and mortality. It is essential to develop a non-invasive, highly accurate, and simple method for BC diagnosis. This work proposed a fluorescent biosensor based on inorganic nanoflares combined with a DNAzyme walker for the simultaneous detection of BC exosomal microRNAs (miRNAs). This biosensor was constructed on the Au nanoparticle (AuNP) modified with the carbon dot (CD)-labeled substrates and DNAzyme strands (AuNP@CDs inorganic nanoflares-DNAzyme, APCD). In the presence of target miRNAs, DNAzyme was activated and then cleaved the CD-labeled substrates and automatically walked along the AuNP, allowing fluorescence recovery. Due to the structure and functional composition, the APCD biosensors demonstrated high sensitivity and specificity, with the reached limit of detection for a single miRNA at the femtomolar level and wide linear range from 50 fM to 10 nM. Furthermore, the simultaneous analysis of BC-related exosomal miR-133b and miR-135b in clinical serum specimens was achieved and consistent with qRT-PCR, suggesting it is a potential method for the diagnosis of BC and other cancers.

Proximity hybridization induced DNA assembly for label-free surface-enhanced Raman spectroscopic detection of carcinoembryonic antigen

In our research, label-free and surface-enhanced Raman dyes-free Raman spectroscopy which was used to detect carcinoembryonic antigen (CEA) according to poly adenine (Poly A)-regulated self-assembly methods was developed and studied. CEA induced partial hybridization of Ab-H2 and Ab-H1, and Ab-H1-CEA-Ab-H2 (a sandwich proximity CEA-DNA complex) was formed, which unfolded molecular beacon 1 (MB1) and modified the substrate. Subsequently, MB2-AuNPs were hybridized with MB1, and Ab-H1-CEA-Ab-H2 was released via toehold regulated displacements of DNA strands. Therefore, hybridization processes of MB2 and MB1 were induced and promoted by CEA-DNA complexes which worked as catalysts. The misplaced target then induced a next round of strand exchange, and the signals for determination of CEA were amplified by AuNPs absorbed on the substrate. It was indicated that the spectral characteristics of adenine at 736 cm^-1 were consistent with the SERS spectrum of DNA. Adenine acted as an internal marker for label-free SERS detection of CEA. Moreover, satisfactory stability and reproducibility were found. Meanwhile, the antibody could specifically recognize the corresponding antigen. Since adenine was dominant in SERS spectra, which was also proximal to Au surface, the sensitivity of the novel method was high without modifications. The analytical performance of this method in determining serum CEA was satisfactory.

Zr4+-mediated hybrid chain reaction and its application for highly sensitive electrochemical detection of protein kinase A

An electrochemical platform has been developed to detect protein kinase activity through the combined actions of Zr⁴⁺ mediated signal transition and hybridization chain reaction (HCR)-stimulated DNAzymes nanowires. First of all, protein kinase A (PKA) phosphorylates substrate peptides immobilized on gold electrode surface. Thereafter, the DNA1 containing 5'-phosphoryl ends is linked to the phosphorylated substrate peptide via the robust phosphate-Zr⁴⁺-phosphate linkages. By the introduction of molecular beacons (MBs), the DNA1 can open the hairpin structures of MBs through toehold mediated strand displacement (TMSDR), leading to an autonomous stem-opening process and subsequent assembly of G-quadruplex-containing DNA chains by HCR. After the addition of hemin, the formed HRP-mimicking DNAzymes can catalyze the hydroquinone-H₂O₂ system to generate amplified electrochemical signals. As expected, this method can achieve ultrahigh analytical performance with a low detection limit of 0.02U/mL and exhibit high cost-savings potential without the need for antibody, protease and labeling. Therefore, this method can serve as a new tool for the assay of protein kinase A and its inhibitor screening in the future.

Sensitive and Programmable "Signal-Off" Electrochemiluminescence Sensing Platform Based on Cascade Amplification and Multiple Quenching Mechanisms

A versatile and sensitive quantum dot (QD)-based "signal-off" electrochemiluminescence (ECL) sensing system was constructed using target-initiated dual Mg²⁺-dependent DNAzyme (MNAzyme) recycling and catalytic hairpin assembly (CHA) amplification strategies. After the cascade amplification, numerous ferrocene-labeled Y-shaped DNA complexes generated on the QD-modified electrode surface. In the presence of hemin, moreover, the terminal sequence of the formed complex could assemble into hemin/G-quadruplex. Therefore, the highly efficient ECL quenching was achieved due to the multiple quenching mechanisms, including electron/energy transfer between ferrocene and QDs, the steric hindrance effects, and the horseradish peroxidase-mimicking activity of hemin/G-quadruplex. Furthermore, owing to the flexibility in regulating the recognition sequences of MNAzyme, the assaying targets can be programmed. Based on the cascade amplification and multiple ECL quenching mechanisms, the developed programmable "signal-off" ECL sensing platform demonstrates excellent sensitivity and the detection limits of 35.00 aM, 3.71 fM, and 0.28 pM (S/N = 3) for target DNA, aptamer substrate (ATP as a model), and ion (Ag⁺ as a model), respectively.

Zn2+ -Dependent DNAzymes: From Solution Chemistry to Analytical, Materials and Therapeutic Applications

Since 1994, deoxyribozymes or DNAzymes have been in vitro selected to catalyze various types of reactions. Metal ions play a critical role in DNAzyme catalysis, and Zn²⁺ is a very important one among them. Zn²⁺ has good biocompatibility and can be used for intracellular applications. Chemically, Zn²⁺ is a Lewis acid and it can bind to both the phosphate backbone and the nucleobases of DNA. Zn²⁺ undergoes hydrolysis even at neutral pH, and the partially hydrolyzed polynuclear complexes can affect the interactions with DNA. These features have made Zn²⁺ a unique cofactor for DNAzyme reactions. This review summarizes Zn²⁺ -dependent DNAzymes with an emphasis on RNA-/DNA-cleaving reactions. A key feature is the sharp Zn²⁺ concentration and pH-dependent activity for many of the DNAzymes. The applications of these DNAzymes as biosensors for Zn²⁺ , as therapeutic agents to cleave intracellular RNA, and as chemical biology tools to manipulate DNA are discussed. Future studies can focus on the selection of new DNAzymes with improved performance and detailed biochemical characterizations to understand the role of Zn²⁺ , which can facilitate practical applications of Zn²⁺ -dependent DNAzymes.

Heterogeneous synthesis and electrochemical performance of LiMnPO4/C composites as cathode materials of lithium ion batteries

In this study, a facile yet efficient interfacial hydrothermal process was successfully developed to fabricate LiMnPO₄/C composites. In this strategy, the walls of carbon nanotubes were employed as heterogeneous nucleation interfaces and biomass of phytic acid (PA) as an eco-friendly phosphorus source. By comparing the experimental results, a reasonable nucleation-growth mechanism was proposed, suggesting the advantages of interfacial effects. Meanwhile, the as-synthesized LiMnPO₄/C samples exhibited superior rate performances with discharge capacities reaching 161 mA h g⁻¹ at C/20, 134 mA h g⁻¹ at 1C, and 100 mA h g⁻¹ at 5C. The composites also displayed excellent cycling stabilities by maintaining 95% of the initial capacity over 100 continuous cycles at 1C. Electrochemical impedance spectroscopy showed that the superior electrochemical performances were attributed to the low charge-transfer resistance and elevated diffusion coefficient of lithium ions. In sum, the proposed approach for the preparation of LiMnPO₄/C composites looks promising for future production of composite electrode materials for high-performance lithium-ion batteries.

A heterogeneous nucleation technique was used to prepare LiMnPO₄/C. The walls of carbon nanotubes were employed as nucleation interfaces and phytic acid as an eco-friendly phosphorus source. The product exhibits superior electrochemical performance.

Rapid and colorimetric detection of nucleic acids based on entropy-driven circuit and DNAzyme-mediated autocatalytic reactions

In this work, a novel, rapid and enzyme-free colorimetric biosensor for the detection of nucleic acids has been developed based on entropy-driven (EDC) circuit and DNAzyme-mediated autocatalytic reactions. On sensing the target DNA, the EDC reaction could be initiated and the intact Mg2+-dependent DNAzyme was formed in the reaction product; then, a "mimic target" DNA was generated during the cleavage process of DNAzyme, which in turn catalyzed the EDC reaction corresponding to an autocatalytic process. Meanwhile, numerous G-quadruplex structures were released and further interacted with hemin to form peroxidase-mimicking DNAzyme, inducing a remarkably amplified colorimetric signal. This autocatalytic EDC (AEDC) sensing system exhibited a linear relationship in the range from 20 pM to 10 nM with a detection limit of 10.2 pM. More importantly, benefitting from the Mg2+-dependent DNAzyme-mediated autocatalytic reaction, the detection time (20 min) was significantly reduced compared to that for the reported EDC strategies. In addition, this sensing system has been applied for the detection of target DNA in human serum samples, indicating that it is promising for the on-site and real-time detection of nucleic acids in biomedical research and disease diagnosis.

UV Lithography-Assisted Fabrication of Low-Cost Copper Electrodes Modified with Gold Nanostructures for Improved Analyte Detection

An in-house UV lithography setup has been optimized to fabricate low-cost disposable electrochemical sensing Cu electrodes using a copper clad board. In view of the high oxidation probability of copper, the low-cost electrodes were modified using different gold nanostructures and a conducing polymer PEDOT:PSS to attain maximal signal output and improved shelf-life. Zero-dimensional (0D) gold nanoparticles (∼40 nm) and three-dimensional (3D) gold nanoflowers (∼38 nm) mixed with PEDOT:PSS were used as signal-enhancing conductors for the ultrasensitive detection of our model contaminant, methylene blue dye (MB). The bare copper electrode was sensitive to MB, linearly within the range of 4-100 μM, with a limit of detection of 3.49 μM. While for gold nanoparticle-PEDOT:PSS-modified electrode, the sensitivity of the electrode was found to increase linearly in the range of 0.01-0.1 μM, and for gold nanoflowers-PEDOT:PSS, the sensitivity achieved was 0.01-0.1 μM with the LOD as 0.0022 μM. For a PEDOT:PSS-modified Cu electrode, used as a comparative to study the contributing role of gold nanostructures towards improved sensitivity, the linearity was found to be in the range of 0.1-1.9 μM with the LOD as 0.0228 μM. A 6 times improvement in signal sensitivity for the nanoflower-PEDOT:PSS electrode compared to the nanoparticle-PEDOT:PSS-modified electrode indicates the influence of nanoparticle shape on the electrode efficiency. 3D gold nanoflowers with a large surface area-to-volume ratio and a high catalytic activity prove to be a superior choice for electrode modification.

Titania/Electro-Reduced Graphene Oxide Nanohybrid as an Efficient Electrochemical Sensor for the Determination of Allura Red

Titania/electro-reduced graphene oxide nanohybrids (TiO2/ErGO) were synthesized by the hydrolysis of titanium sulfate in graphene oxide suspension and in situ electrochemical reduction. It provides a facile and efficient method to obtain nanohybrids with TiO2 nanoparticles (TiO2 NPs) uniformly coated by graphene nanoflakes. TiO2/ErGO nanohybrids were characterized by transmission electron microscopy, X-ray diffraction, cyclic voltammogram, and electrochemical impedance spectroscopy in detail. Compared with pure ErGO and TiO2 NPs, TiO2/ErGO nanohybrids greatly enhanced the electrocatalytic activity and voltammetric response of Allura Red. In the concentration range of 0.5-5.0 μM, the anodic peak currents of Allura Red were linearly correlated to their concentrations. However, the linear relationship was changed to the semi-logarithmic relationship at a higher concentration region (5.0-800 μM). The detection limit (LOD) was 0.05 μM at a signal-to-noise ratio of 3. The superior sensing performances of the proposed sensor can be ascribed to the synergistic effect between TiO2 NPs and ErGO, which provides a favorable microenvironment for the electrochemical oxidation of Allura Red. The proposed TiO2/ErGO/GCE showed good reproducibility and stability both in determination and in storage, and it can accurately detect the concentration of Allura Red in milk drinks, providing an efficient platform for the sensitive determination of Allura Red with high reliability, simplicity, and rapidness.

Ultrasensitive DNA electrochemical biosensor based on MnTBAP biomimetic catalyzed AGET ATRP signal amplification reaction

In this paper, an ultrasensitive, highly selective and green electrochemical biosensor for quantifying DNA sequences (aM DNA) based on a MnTBAP catalyst for AGET ATRP reaction is proposed.

Morphology-dependent MnO2/nitrogen-doped graphene nanocomposites for simultaneous detection of trace dopamine and uric acid

Four nanostructured MnO₂ with various controllable morphologies, including nanowires, nanorods, nanotubes and nanoflowers were synthesized, and then further composited with nitrogen-doped graphene (NG) with the assistance of ultrasonication. The surface morphologies, phase structures, and electrochemical performances of the proposed MnO₂/NG nanohybrids were investigated by various techniques, and their catalytic activities on the electrooxidation of dopamine (DA) and uric acid (UA) were compared systematically. The sensing performances were found to be highly correlated with their morphologies. Among these morphologies, the nanoflower-like MnO₂, composited with NG, displayed the most sensitive response signals for DA and UA. The boosted electrocatalytic activity was ascribed to the unique porous structure, large electroactive area, and low charge transfer resistance (R_ct), which facilitated the electron transfer between electrode and analytes. Two linear response ranges (0.1 μM-10 μM and 10 μM-100 μM) were accompanied with very low detection limits of 34 nM and 39 nM for DA and UA, respectively. Moreover, the successful application of the MnO₂NFs/NG composites for the simultaneous detection of DA and UA in human serum was realized using second-derivative linear sweep voltammetry (SDLSV). These findings give valuable insights for understanding the morphology-dependent sensing properties of MnO₂ based nanomaterials, which is conducive to the rapid development of ubiquitous MnO₂-based electrochemical sensors.

A general scheme for fluorometric detection of multiple oligonucleotides by using RNA-cleaving DNAzymes: application to the determination of microRNA-141 and H5N1 virus DNA

A widely applicably method is described for fluorometric determination of targets such as microRNA and viral DNA. It is making use of a Mg(II)-dependent DNAzyme and a G-quadruplex. In the absence of analyte, an inactive DNAzyme is formed by the hybridization of split DNAzymes and substrate. On addition of target analyte, the end of each strand of the split DNAzymes bind the analyte. This leads to the generation of an active DNAzyme. In the presence of Mg(II), the activated DNAzyme is formed and can cleave the substrate strand. Hence, the caged G-quadruplex sequences will be released. These released G-quadruplexes combine with thioflavin T to generate a G-quadruplex/thioflavin T complex and thereby cause amplified fluorescence. The method shows a 70 pM detection limit for H5N1 and works over a wide linear range 1 nM to 400 nM. Conceivably, this detection scheme has a wide scope in that it may be applied to other assays for microRNAs and DNAs by variation of the type of DNAzyme. Graphical abstract Schematic presentation of target detection: the DNAzyme cannot cleave the substrate strand when target is absent. Once the target is added, the active DNAzyme can cleave the substrate strand in the presence of Mg²⁺, resulting in significant fluorescence enhancement when the release of the caged G-quadruplex sequences binding with 2-[4-(dimethylamino)phenyl]-3,6-dimethylbenzothiazolium chloride (ThT).

Ultrasensitive Detection of DNA via SI-eRAFT and in Situ Metalization Dual-Signal Amplification

In this work, we report a new amplification strategy based on electrochemically mediated reversible addition-fragmentation chain transfer (eRAFT) and in situ metalization for electrochemical detection of DNA. First, peptide nucleic acid (PNA) probes were immobilized on the surface of the gold electrode, and when they hybridized with the target DNA, the chain transfer agent (CTA), 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPAD), of RAFT was connected to the PNA/DNA heteroduplex formed by the coordination bonding of Zr⁴⁺. Then glycosyloxyethyl methacrylates (GEMA) were assembled on the surface of the electrode by electrochemically mediated surface-initiated reversible addition-fragmentation chain transfer (SI-eRAFT) to form a polymer-containing sugar glucose. Next, the o-hydroxyl groups on the polysaccharide molecular skeleton were oxidized to aldehyde groups by sodium periodate (NaIO₄). The aldehyde groups generated then reduce silver ions to silver particles deposited on the electrode surface in situ, and this system was then subjected to differential pulse voltammetry (DPV). Under optimal conditions, the intensity of the stripping current and the logarithm of the target DNA (tDNA) concentration has a good linear relationship in the range of 10 aM to 1 pM (R² = 0.996), and the detection limit can go down to 5.4 aM (S/N = 3). Moreover, the method is suitable for single-nucleotide polymorphism (SNP) analysis and has strong anti-interference ability for the analysis of target ssDNA in serum samples.

Controllable synthesis of three-dimensional nitrogen-doped hierarchical porous carbon and its application in the detection of lead

In this study, gelatin-based microcapsules were first proposed as a carbon source for the synthesis of nitrogen-doped hierarchical porous carbon (N-HPC) via a facile one-pot high-temperature treatment. The morphologies of the microcapsules could be well controlled by adjusting the synthesis parameters; this ensured the repeatability of the calcined products. The as-prepared N-HPC possesses a favorable three-dimensional network structure and hierarchical porous structure. As a promising modified electrode, N-HPC displayed remarkably improved stability and sensitivity for lead ion (Pb²⁺) detection. Moreover, two factors are responsible for the good analytical performance: (i) the morphologies of the microcapsules are controllable and reproducible; this improves the detection stability; and (ii) the nitrogen atoms in the shells of the microcapsules can efficiently interact with Pb²⁺; this enhances the detection sensitivity. The influences of various experimental parameters, including the pH value of the supporting electrolyte, deposition potential and deposition time, on the stripping signal of Pb²⁺ were investigated. The method displayed a wide linear range of the Pb²⁺ concentration from 7 nM to 7000 nM with the detection limit of 1.44 nM under the optimized conditions. The modified electrode possessed high selectivity, which might be due to the high binding affinity of the NH₂ ^- groups to Pb²⁺. The developed method has been successfully applied to the detection of Pb²⁺ in actual water samples; this demonstrates that the N-HPC-based electrochemical sensors have prospective applications in the environmental monitoring of Pb²⁺.

Highly sensitive and specific electrochemical biosensor for microRNA-21 detection by coupling catalytic hairpin assembly with rolling circle amplification

BACKGROUND

MicroRNA plays a significant role in gene regulation and is usually regarded as an important biological marker. Electrochemical biosensors are excellent tools for microRNA detection.

METHODS

In this experiment, we take miRNA-21 as a target, combining catalytic hairpin assembly (CHA) and rolling circle amplification (RCA) as a dual signal amplification strategy for the detection of microRNA in an electrochemical biosensor.

RESULTS

This strategy has a good linear range of 0.5-12 500 pmol of microRNA. The limit of detection (LOD) for miRNA is as low as 290 fmol, showing excellent performance. Finally, this method has been successfully applied to the detection of miRNA-21 from HeLa cells.

CONCLUSION

This method can be applied not only for microRNA detection with high sensitivity and speed, but can also detect small molecules and proteins combined with aptamers.

A hybridization-triggered DNAzyme cascade assay for enzyme-free amplified fluorescence detection of nucleic acids

An enzyme-free and ultrasensitive fluorescence assay for the detection of nucleic acids was successfully established by a hybridization-triggered DNAzyme cascade (HTDC).

A DNA nanostructured aptasensor for the sensitive electrochemical detection of HepG2 cells based on multibranched hybridization chain reaction amplification strategy

Sensitive detection of cancer cells is beneficial to the early diagnosis of cancer and individual treatment. In the present study, a DNA nanostructured aptasensor was used for the sensitive electrochemical detection of human liver hepatocellular carcinoma cells (HepG2) based on multibranched hybridization chain reaction amplification strategy. We established a well-designed platform by immobilizing DNA tetrahedron, a three-dimensional DNA nanostructure, on the gold electrode to capture HepG2 cells more specifically and efficiently. Meanwhile, functional hybrid nanoprobes consisted of MIL-101@AuNPs (Au nanoparticles), numerous hemin/G-quadruplex DNAzyme from multibranched hybridization chain reaction, and natural horseradish peroxidase (HRP) was designed. The hybrid nanoprobes possessed the functions of specific discernment and enzymatic signal amplification simultaneously. With the help of nanoprobes, HepG2 cells were recognized and captured to form a DNA tetrahedron-cell-nanoprobe sandwich-like structure on the electrode surface. The lower detection limit of this established cytosensor is 5 cells per ml. Moreover, it delivered a broad detection range from 10² to 10⁷ cells per ml. The results revealed that the as-proposed cytosensor may be utilized as a powerful tool for early diagnosis of cancer in the future.

A ratiometric electrochemical biosensor for the exosomal microRNAs detection based on bipedal DNA walkers propelled by locked nucleic acid modified toehold mediate strand displacement reaction

Sensitive and selective detection of microRNAs (miRNAs) in cancer cells derived exosomes have attracted rapidly growing interest owing to their potential in diagnostic and prognostic applications. Here, we design a ratiometric electrochemical biosensor based on bipedal DNA walkers for the attomolar detection of exosomal miR-21. In the presence of miR-21, DNA walkers are activated to walk continuously along DNA tracks, resulting in conformational changes as well as considerable increases of the signal ratio produced by target-respond and target-independent reporters. With the signal cascade amplification of DNA walkers, the biosensor exhibits ultrahigh sensitivity with the limit of detection (LOD) down to 67 aM. Furthermore, owing to the background-correcting function of target-independent reporters termed as reference reporters, the biosensor is robust and stable enough to be applied in the detection of exosomal miR-21 extracted from breast cancer cell lines and serums. In addition, because locked nucleic acid (LNA) modified toehold mediate strand displacement reaction (TMSDR) has extraordinary discriminative ability, the biosensor displays excellent selectivity even against the single-base-mismatched target. It is worth mentioning that our sensor is regenerative and stable for at least 5 cycles without diminution in sensitivity. In brief, the high sensitivity, selectivity and reproducibility, together with cheap, make the proposed biosensor a promising approach for exosomal miRNAs detection, in conjunction with early point-of-care testing (POCT) of cancer.

Synthesis of a multi-functional DNA nanosphere barcode system for direct cell detection

Nucleic acid-based technologies have been applied to numerous biomedical applications. As a novel material for target detection, DNA has been used to construct a barcode system with a range of structures. This paper reports multi-functionalized DNA nanospheres (DNANSs) by rolling circle amplification (RCA) with several functionalized nucleotides. DNANSs with a barcode system were designed to exhibit fluorescence for coding enhanced signals and contain biotin for more functionalities, including targeting through the biotin-streptavidin (biotin-STA) interaction. Functionalized deoxynucleotide triphosphates (dNTPs) were mixed in the RCA process and functional moieties can be expressed on the DNANSs. The anti-epidermal growth factor receptor antibodies (anti-EGFR Abs) can be conjugated on DNANSs for targeting cancer cells specifically. As a proof of concept, the potential of the multi-functional DNANS barcode was demonstrated by direct cell detection as a simple detection method. The DNANS barcode provides a new route for the simple and rapid selective recognition of cancer cells.

Enzyme-free, signal-amplified nucleic acid circuits for biosensing and bioimaging analysis

Enzyme-free, signal-amplified nucleic acid circuits utilize programmed assembly reactions between nucleic acid substrates to transduce a chemical input into an amplified detection signal.

Impedimetric genosensor for ultratrace detection of hepatitis B virus DNA in patient samples assisted by zeolites and MWCNT nano-composites

Nanocrystals of zeolites (Nanocrys Zeo) and Multi-walled carbon nanotubes (MWCNT) based diagnostic genosensor was employed for detection of polymerase chain (PCR) amplified HBVDNA in blood of hepatitis B patients. The ssDNA-nanocomposite modified electrode was characterized by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The hybridization between ss DNA probe and target ss DNA was detected by reduction in current, generated by interaction of methylene blue (MB) with free guanine (3'G) of ssDNA. Nanocrys zeo were deposited on the Fluorine doped tin oxide glass electrode (FTO) by drop-casting method for better immobilization of ss DNA while MWCNTs are incorporated into the zeolite-assembly to enhance the electro-conductivity of the present genosensor. The ssDNA-nanocomposite modified FTO electrode exhibited optimum current within 5s, at pH 5.6, and incubation temperature of 45°C. The value of charge transfer resistance (Rct) was linear with the number of copies of target DNA between 150 and 10(6) copies/ml. The limit of detection (LOD) of the sensor was 50 copies/ml. Within and between batches coefficients of variation (CV) were 2.5% and 3.2% respectively. Results obtained with our genosensor were also correlated with those by RT-PCR and r(2) value found with good accuracy of 97%. The electrode was reused by dipping it into 0.1M NaOH for 3min and lost 50% of its initial activity in 4 weeks. Furthermore the technique employed for detection of HBV is EIS, which is convenient and required less analysis time.

Label-free electrochemical genosensor based on mesoporous silica thin film

A novel label-free electrochemical strategy for nucleic acid detection was developed by using gold electrodes coated with mesoporous silica thin films as sensing interface. The biosensing approach relies on the covalent attachment of a capture DNA probe on the surface of the silica nanopores and further hybridization with its complementary target oligonucleotide sequence, causing a diffusion hindering of an Fe(CN)6 (3-/4-) electrochemical probe through the nanochannels of the mesoporous film. This DNA-mesoporous silica thin film-modified electrodes allowed sensitive (91.7 A/M) and rapid (45 min) detection of low nanomolar levels of synthetic target DNA (25 fmol) and were successfully employed to quantify the endogenous content of Escherichia coli 16S ribosomal RNA (rRNA) directly in raw bacterial lysate samples without isolation or purification steps. Moreover, the 1-month stability demonstrated by these biosensing devices enables their advanced preparation and storage, as desired for practical real-life applications. Graphical abstract Mesoporous silica thin films as scaffolds for the development of novel label-free electrochemical genosensors to perform selective, sensitive and rapid detection of target oligonucleotide sequences. Application towards E. coli determination.

Two-dimensional graphitic carbon nitride nanosheets for biosensing applications

Two-dimensional graphitic carbon nitride nanosheets (CNNSs) with planar graphene-like structure have stimulated increasingly research interest in recent years due to their unique physicochemical properties. CNNSs possess superior stability, high fluorescence quantum yield, low-toxicity, excellent biocompatibility, unique electroluminescent and photoelectrochemical properties, which make them appropriate candidates for biosensing. In this review, we first introduce the preparation and unique properties of CNNSs, with emphasis on their superior properties for biosensing. Then, recent advances of CNNSs in photoelectrochemical biosensing, electrochemiluminescence biosensing and fluorescence biosensing are highlighted. An additional attention is paid to the marriage of CNNSs and nucleic acids, which exhibits great potentials in both biosensing and intracellular imaging. Finally, current challenges and opportunities of this 2D material are outlined. Inspired by the unique properties of CNNSs and their advantages in biological applications, we expect that more attention will be drawn to this promising 2D material and extensive applications can be found in bioanalysis and diseases diagnosis.

A layered tungsten disulfide/acetylene black composite based DNA biosensing platform coupled with hybridization chain reaction for signal amplification

A 2-dimensional tungsten disulfide-acetylene black (WS₂-AB) composite is synthesized by a simple hydrothermal method to achieve excellent electrochemical properties for applications as a DNA biosensor. The biosensor is fabricated based on the Au nanoparticles (AuNPs) and WS₂-AB composite modified electrode, which subsequently is used to couple with a capture probe by an Au-S bond, then modified with target DNA, auxiliary DNA and bio-H1-bio-H2 (H1-H2) to perform hybridization chain reaction for signal amplification. Herein, two DNA hairpins H1 and H2 are opened by the recognition probe. The nicked double helices from hybridization chain reaction are used to immobilize horseradish peroxidase enzymes via biotin-avidin reaction, which produces signal-amplification detection of target DNA through the catalytic reaction of the hydrogenperoxide + hydroquinone system. Under optimum conditions, the as-prepared biosensor shows a good linear relationship between the current value and logarithm of the target DNA concentration ranging from 0.001 pM to 100 pM and a detection limit as low as 0.12 fM. Moreover, the fabricated biosensor exhibits good selectivity to differentiate the one-base mismatched DNA sequence. This work will open a pathway for ultrasensitive detection of other biorecognition events and gene-related diseases based on layered WS₂-AB and hybridization chain reaction.

Ultrasensitive electrochemical detection of UO22+ based on DNAzyme and isothermal enzyme-free amplification

A novel enzyme-free amplification biosensor for uranyl detection was developed based on UO₂²⁺-specific DNAzyme and a hybridization chain reaction.

In situ amplified electrochemical aptasensing for sensitive detection of adenosine triphosphate by coupling target-induced hybridization chain reaction with the assembly of silver nanotags

Biomolecular immobilization and construction of the sensing platform are usually crucial for the successful development of a high-efficiency detection system. Herein we report on a novel and label-free signal-amplified aptasensing for sensitive electrochemical detection of small molecules (adenosine triphosphate, ATP, used in this case) by coupling with target-induced hybridization chain reaction (HCR) and the assembly of electroactive silver nanotags. The system mainly consisted of two alternating hairpin probes, a partial-pairing trigger-aptamer duplex DNA and a capture probe immobilized on the electrode. Upon target ATP introduction, the analyte attacked the aptamer and released the trigger DNA, which was captured by capture DNA immobilized on the electrode to form a newly partial-pairing double-stranded DNA. Thereafter, the exposed domain at trigger DNA could be utilized as the initator strand to open the hairpin probes in sequence, and propagated a chain reaction of hybridization events between two alternating hairpins to form a long nicked double-helix. The electrochemical signal derived from the assembled silver nanotags on the nicked double-helix. Under optimal conditions, the electrochemical aptasensor could exhibit a high sensitivity and a low detection limit, and allowed the detection of ATP at a concentration as low as 0.03 pM. Our design showed a high selectivity for target ATP against its analogs because of the high-specificity ATP-aptamer reaction, and its applicable for monitoring ATP in the spiking serum samples. Improtantly, the distinct advantages of the developed aptasensor make it hold a great potential for the development of simple and robust sensing strategies for the detection of other small molecules by controlling the apatmer sequence.