Mesoporous Silica Containers and Programmed Catalytic Hairpin Assembly/Hybridization Chain Reaction Based Electrochemical Sensing Platform for MicroRNA Ultrasensitive Detection with Low Background

Aptamer molecular gate functionalized mesoporous SiO2@MB controlled-release system for pollutant detection using Ti(Ⅲ) self-doped TiO2 NTs as active photoanode coupled with electrostatic modulation

Atrazine (ATZ) is a widely used herbicide that can cause serious harm to organisms and ecosystems. An immobilization-free photoelectrochemical (PEC) aptasensor has been herein developed for ATZ based on aptamer molecular gate functionalized mesoporous SiO2@MB controlled release system. Compared with traditional immobilization-based sensors, immobilization-free sensors (IFSs) avoid the modification of the recognition element on the electrode surface. Mesoporous SiO2 with large surface area and good biocompatibility can be used as nanocontainers to stably encapsulate the signal shuttle molecule methylene blue (MB). The bifunctional aptamer (APT) is used not only as the recognition element for ATZ but also as the signal switch to block or release MB. In the presence of ATZ, the specific recognition between ATZ and APT will cause the detachment of APT from the surface of SiO2, thus the molecular gate will open and release MB. Due to pH modulation, the positively charged MB can reach the surface of the negatively charged Ti(III) self-doped TiO2 NTs (Ti(III)-TiO2 NTs) electrode to act as an electron donor, which increases the photocurrent. The immobilization-free aptasensor has shown ultrasensitive detection of ATZ with a wide linear range from 1.0 pM to 100.0 nM and a low detection limit of 0.1 pM. In addition, the sensor has excellent selectivity, stability and anti-interference ability, and has been used in real water sample analysis successfully. This strategy has provided a new idea for the design of advanced immobilization-free PEC sensors for environmental pollutant detection.

Nucleic acid-based electrochemical biosensors

Colorectal cancer, breast cancer, oxidative DNA damage, and viral infections are all significant and major health threats to human health, presenting substantial challenges in early diagnosis. In this regard, a wide range of nucleic acid-based electrochemical platforms have been widely employed as point-of-care diagnostics in health care and biosensing technologies. This review focuses on biosensor design strategies, underlying principles involved in the development of advanced electrochemical genosensing devices, approaches for immobilizing DNA on electrode surfaces, as well as their utility in early disease diagnosis, with a particular emphasis on cancer, leukaemia, oxidative DNA damage, and viral pathogen detection. Notably, the role of biorecognition elements and nanointerfaces employed in the design and development of advanced electrochemical genosensors for recognizing biomarkers related to colorectal cancer, breast cancer, leukaemia, oxidative DNA damage, and viral pathogens has been extensively reviewed. Finally, challenges associated with the fabrication of nucleic acid-based biosensors to achieve high sensitivity, selectivity, a wide detection range, and a low detection limit have been addressed. We believe that this review will provide valuable information for scientists and bioengineers interested in gaining a deeper understanding of the fabrication and functionality of nucleic acid-based electrochemical biosensors for biomedical diagnostic applications.

A new strategy based on a cascade amplification strategy biosensor for on-site eDNA detection and outbreak warning of crown-of-thorns starfish

Population outbreaks of the crown-of-thorns starfish (COTS) seriously threaten the sustainability of coral reef ecosystems. However, traditional ecological monitoring techniques cannot provide early warning before the outbreaks, thus preventing timely intervention. Therefore, there is an urgent need for a more accurate and faster technology to predict the outbreaks of COTS. In this work, we developed an electrochemical biosensor based on a programmed catalytic hairpin assembly (CHA) and hybridization chain reaction (HCR) cyclic amplification strategy for sensitive and selective detection of COTS environmental DNA (eDNA) in water bodies. This biosensor exhibited excellent electrochemical characteristics, including a low limit of detection (LOD = 18.4 fM), low limit of quantification (LOQ = 41.1 fM), and wide linear range (50 fM - 10 nM). The biosensing technology successfully allowed the detection of COTS eDNA in the aquarium environment, and the results also demonstrated a significant correlation between eDNA concentration and COTS number (r = 0.990; P < 0.001). The reliability and accuracy of the biosensor results have been further validated through comparison with digital droplet PCR (ddPCR). Moreover, the applicability and accuracy of the biosensor were reconfirmed in field tests at the COTS outbreak site in the South China Sea, which has shown potential application in dynamically monitoring the larvae before the COTS outbreak. Therefore, this efficient electrochemical biosensing technology offers a new solution for on-site monitoring and early warning of the COTS outbreak.

Coupled Poly(ethylenimine) Coreactant to Enhance Electrochemiluminescence of Polymer Dots for Array Imaging of Protein Biomarkers

Traditional electrochemiluminescent (ECL) bioanalysis suffers from the demand for excessive external coreactants and the damage of reaction intermediates. In this work, a poly(ethylenimine) (PEI)-coupled ECL emitter was proposed by covalently coupling tertiary amine-rich PEI to polymer dots (Pdots). The coupled PEI might act as a highly efficient coreactant to enhance the ECL emission of Pdots through intramolecular electron transfer, reducing the electron transfer distance between emitter and coreactant intermediates and avoiding the disadvantages of traditional ECL systems. Through modification of the PEI-Pdots with tDNA, a sequence partially complementary to cDNA that was complementary to the aptamer of target protein biomarker (aDNA), tDNA-PEI-Pdots were obtained. The biosensors were produced using Au/indium tin oxide (ITO) with an aDNA/cDNA hybrid, and an ECL imaging biosensor array was constructed for ultrasensitive detection of protein biomarkers. Using vascular endothelial growth factor 165 (VEGF165) as a protein model, the proposed ECL imaging method containing two simple incubations with target samples and then tDNA-PEI-Pdots showed a detectable range of 1 pg mL-1 to 100 ng mL-1 and a detection limit of 0.71 pg mL-1, as well as excellent performance such as low toxicity, high sensitivity, excellent selectivity, good accuracy, and acceptable fabrication reproducibility. The PEI-coupled Pdots provide a new avenue for the design of ECL emitters and the application of ECL imaging in disease biomarker detection.

Genetically engineered virus-like particle-armoured and multibranched DNA scaffold-corbelled ultra-sensitive hierarchical hybridization chain reaction for targeting-enhanced imaging in living biosystems under spatiotemporal light powering

Although nucleic acids-based fluorescent biosensors, exemplified by the hybridization chain reaction (HCR), have exhibited promise as an imaging tool for detecting disease-related biomolecular makers in living biosystems, they still face certain challenges. These include the need for improved sensitivity, poor bio-targeting capability, the absence of signal enrichment interface and the uncontrollable biosensing initiation. Herein, we present a range of effective solutions. First, a stacking design resembling building blocks is used to construct a special hierarchical HCR (termed H-HCR), for which a hierarchical bridge is employed to graft multiunit HCR products. Furthermore, the H-HCR components are encapsulated into a virus-like particle (VLP) endowed with a naturally peptide-mediated targeting unit through genetic engineering of plasmids, after which the biosensor can specifically identify cancer cytomembranes. By further creating a multibranched DNA scaffold to enrich the H-HCR produced detection signals, the biosensor's analyte recognition module is inserted with a photocleavage-linker, allowing that the biosensing process can be spatiotemporally initiated via a light-powered behavior. Following these innovations, this genetically engineered VLP-armoured and multibranched DNA-scaffold-corbelled H-HCR demonstrates an ultra-sensitive and specific biosensing performance to a cancer-associated microRNA marker (miRNA-155). Beyond the worthy in vitro analysis, our method is also effective in performing imaging assays for such low-abundance analyte in living cells and even bodies, thus providing a roust platform for disease diagnosis.

Colorimetric sensing for the sensitive detection of UO22+via the phosphorylation functionalized mesoporous silica-based controlled release system

In this study, we developed a simple and sensitive colorimetric sensing method for the detection of UO22+, which was built to release MB from the molybdenum disulfide with a phosphate group (MoS2-PO4) gated mesoporous silica nanoparticles functionalized phosphate group (MSN-PO4) with UO22+ chelating. In the presence of UO22+, MoS2-PO4 can be effectively adsorbed onto the surface of MSN-PO4 based on the coordination chemistry for strong affinity between the P-O bond and UO22+. The adsorbed MoS2-PO4 was then utilized as an ideal gate material to control the release of signal molecules (MB) entrapped within the pores of MSN-PO4, resulting in a detectable decrease in the absorption peak at 663 nm. This colorimetric sensing demonstrated the advantages of simplicity and easy manipulation and exhibited a linear response to the concentration of UO22+ within the range of 0.02-0.2 μM. The detection limit of UO22+ was determined to be 0.85 nM, which was lower than the limit (130 nmol L-1) set by the US Environmental Protection Agency. Furthermore, the proposed colorimetric sensing method has been utilized to determine UO22+ in samples of Xiangjiang River and tap water, and a high recovery rate was achieved. This method shows promising potential in preventing and controlling environmental pollution.

MOF-based nanocomposites as transduction matrices for optical and electrochemical sensing

Metal Organic Frameworks (MOFs), a class of crystalline microporous materials have been into research limelight lately due to their commendable physio-chemical properties and easy fabrication methods. They have enormous surface area which can be a working ground for innumerable molecule adhesions and site for potential sensor matrices. Their biocompatibility makes them valuable for in vitro detection systems but a compromised conductivity requires a lot of surface engineering of these molecules for their usage in electrochemical biosensors. However, they are not just restricted to a single type of transduction system rather can also be modified to achieve feat as optical (colorimetry, luminescence) and electro-luminescent biosensors. This review emphasizes on recent advancements in the area of MOF-based biosensors with focus on various MOF synthesis methods and their general properties along with selective attention to electrochemical, optical and opto-electrochemical hybrid biosensors. It also summarizes MOF-based biosensors for monitoring free radicals, metal ions, small molecules, macromolecules and cells in a wide range of real matrices. Extensive tables have been included for understanding recent trends in the field of MOF-composite probe fabrication. The article sums up the future scope of these materials in the field of biosensors and enlightens the reader with recent trends for future research scope.

Light-Driven Photocatalytic-Photothermal Synergetic System for Portable and Sensitive Nucleic Acid Quantification

Photosensitizers and photothermal agents have attracted increasing attention for in vitro diagnosis, but the combination remains challenging. Herein, a light-driven photocatalytic-photothermal synergetic system integrated microfluidic distance-based analytical device (PCPT-μDAD) for visual, portable, sensitive, and quantitative detection of targets was developed. Target DNA was recognized and initiated the hybridization chain reaction to form a double-stranded DNA/SYBR Green I (dsDNA/SG-I) complex. By applying the photosensitization of the dsDNA/SG-I complex and the photothermal effect of oxidized 3,3',5,5'-tetramethylbenzidine, the target concentration can effectively translate into a visual distance signal readout. Importantly, the light-driven PCPT-μDAD greatly improves the controllability of catalytic reactions and signal amplification efficiency. The light-driven PCPT-μDAD shows a low limit of detection (fM level), good stability, and high reproducibility for nucleic acid detection.

Closed Cyclic DNA Machine for Sensitive Logic Operation and APE1 Detection

DNA self-assembly has been developed as a kind of robust signal amplification strategy, but most of reported assembly pathways are programmed to amplify signal in one direction. Herein, based on mutual-activated cascade cycle of hybridization chain reaction (HCR) and catalytic hairpin assembly (CHA), a closed cycle circuit (CCC) based DNA machine is developed for sensitive logic operation and molecular recognition. Benefiting from the synergistically accelerated signal amplification, the closed cyclic DNA machine enabled the logic computing with strong and significant output signals even at weak input signals. The typical logic operations such as OR, YES, AND, INHIBIT, NOR, and NAND gate, are conveniently and clearly executed with this DNA machine through rational design of the input and computing elements. Moreover, by integrating the target recognition module with the CCC module, the proposed DNA machine is further employed in the homogeneous detection of apurinic/apyrimidinic endonuclease 1 (APE1). The precise recognition and exponential signal amplification facilitated the highly selective and sensitive detection of APE1 with limit of detection (LOD) of 7.8 × 10^-5 U mL^-1 . Besides, the normal cells and tumor cells are distinguished unambiguously by this method according to the detected concentration difference of cellular APE1, which indicates the robustness and practicability of this method.

Wheel drive-based DNA sensing system for highly specific and rapid one-step detection of MiRNAs at the attomolar level

With the use of DNA as building blocks, a variety of microRNA amplification-based sensing systems have been developed. Nevertheless, ultrasensitive, selective and rapid detection of microRNAs with a high signal-to-background ratio and point mutation discrimination ability remains a challenge. Herein, we propose a novel wheel drive-based DNA sensing system (NWDS) based on a self-assembled, self-quenched nanoprobe (SQP) to conduct highly specific and ultrasensitive one-step measurement of microRNAs. In this work, a signalling recognition DNA hairpin (DH) sequence with a self-complementary stem domain of 14 base pairs was used, which contained three functional regions, namely a recognition region for the target miRNA-21, a sticky region with 9 complementary nucleotides to the 3'terminus of a DNA wheel (DW) and a region for the hybridization with a quenching DNA primer (DP). The SQP was ingeniously self-assembled at room temperature by the DH and DP, which was capable of eliminating unwanted background signals. MiRNA-21 was employed as a target model to specifically activate the SQP, leading to specific hybridization between the HP and DW. With the assistance of a polymerase, an SQP-based wheel driving took place to induce hybridization/polymerization displacement cycles, initiating target recycling and DP displacement. As a result, a large amount of the newly formed hybrid SQP/DW accumulated to generate a substantially enhanced fluorescence signal. In this way, the newly proposed NWDS exhibits ultrasensitivity with a detection limit of 5.62 aM across a wide linear dynamic response range up to 200 nM, excellent selectivity with the capability to discriminate homologous miRNAs and one-base, two-base and three-base mismatched sequences, and an outstanding analytical performance in complex systems. In addition, the significant simultaneous advantages of one-step operation, rapid detection within 15 min and a high signal-to-background ratio of 26 offer a unique opportunity to promote the early diagnosis of cancer-related diseases and molecular biological analysis.

Hybridization Chain Reaction-Based Electrochemical Biosensors by Integrating the Advantages of Homogeneous Reaction and Heterogeneous Detection

The conventional hybridization chain reaction (HCR)-based electrochemical biosensors usually require the immobilization of probes on the electrode surface. This will limit the applications of biosensors due to the shortcomings of complex immobilization processes and low HCR efficiency. In this work, we proposed astrategy for the design of HCR-based electrochemical biosensors by integrating the advantages of homogeneous reaction and heterogeneous detection. Specifically, the targets triggered the autonomous cross-opening and hybridization oftwobiotin-labeled hairpin probes to form long-nicked dsDNA polymers. The HCR products with many biotin tags were then captured by a streptavidin-covered electrode, thus allowing for the attachment of streptavidin-conjugated signal reporters through streptavidin-biotin interactions. By employing DNA and microRNA-21 as the model targets and glucose oxidase as the signal reporter, the analytical performances of the HCR-based electrochemical biosensors were investigated. The detection limits of this method were found to be 0.6 fM and 1 fM for DNA and microRNA-21, respectively. The proposed strategy exhibited good reliability for target analysis in serum and cellular lysates. The strategy can be used to develop various HCR-based biosensors for a wide range of applications because sequence-specific oligonucleotides exhibit high binding affinity to a series of targets. In light of the high stability and commercial availability of streptavidin-modified materials, the strategy can be used for the design of different biosensors by changing the signal reporter and/or the sequence of hairpin probes.

A dual-blocker aided and dual-label-free electrochemical biosensor based on mbHCR/rGO nanocomplexes for ultrasensitive DNA detection

Heterogeneous electrochemical DNA biosensors have attracted huge attention due to their enhanced signal sensitivity, compared to homogeneous biosensors. However, the high cost of probe labeling and the reduced recognition efficiency associated with current heterogeneous electrochemical biosensors confine their potential applications. In the present work, a dual-blocker assisted and dual-label-free heterogeneous electrochemical strategy based on multi-branched hybridization chain reaction (mbHCR) and reduced graphene oxide (rGO) was fabricated for ultrasensitive detection of DNA. The target DNA could trigger the mbHCR of two DNA hairpin probes, resulting in the generation of multi-branched long chain of DNA duplexes with bidirectional arms. One direction of the multi-branched arms in the mbHCR products were then bound to the label-free capture probe on the gold electrode through multivalent hybridization with enhanced recognition efficiency. The other direction of multi-branched arms in mbHCR product could adsorb rGO via π-π stacking interactions. Two DNA blockers were ingeniously designed to block the binding of excessive H1-pAT on electrode and to prevent the adsorption of rGO by residual unbound capture probes. As a result, with the electrochemical reporter methylene blue selectively intercalated into the long chain of DNA duplex and absorbed on rGO, a remarkable electrochemical signal rise was observed. Thus, a dual-blocker aided and dual-label-free electrochemical strategy for ultrasensitive DNA detection is readily realized with the merit of cost-effective. The as-developed dual-label-free electrochemical biosensor has great potential to be employed in nucleic acid related medical diagnostics.

Engineered Escherichia coli as a Controlled-Release Biocarrier for Electrochemical Immunoassay

Micro/nanocarriers hold great potential in bioanalysis for molecular recognition and signal amplification but are frequently hampered by harsh synthesis conditions and time-consuming labeling processes. Herein, we demonstrate that Escherichia coli (Ec) can be engineered as an efficient biocarrier for electrochemical immunoassay, which can load ultrahigh amounts of redox indicators and simultaneously be decorated with detection antibodies via a facile polydopamine (PDA)-mediated coating approach. Compared with conventional carrier materials, the entire preparation of the Ec biocarrier is simple, highly sustainable, and reproducible. Moreover, immune recognition and electrochemical transduction are performed independently, which eliminates the accumulation of biological interference on the electrode and simplifies electrode fabrication. Using human epidermal growth factor receptor 2 (HER2) as the model target, the proposed immunosensor exhibits excellent analytical performance with a low detection limit of 35 pg/mL. The successful design and deployment of Ec biocarrier may provide new guidance for developing biohybrids in biosensing applications.

Superior graphdiyne self-powered biosensing platform with highly sensitivity and reliability for dual-mode detection of MicroRNA by integrating T7 Exonuclease and 3D DNA walker induced rolling circle amplification

A highly sensitivity self-powered biosensor is developed based on T7 exonuclease (T7 Exo) and 3D DNA walker induced rolling circle amplification (RCA) for electrochemical/colorimetric dual-mode detection of microRNA-21 (miRNA-21) with improved reliability. Taking its advantage of fascinating properties, such as high structure defects and good conductivity, graphdiyne is prepared and used to prepare high-performance enzyme biofuel cell. T7 Exo-assisted 3D DNA walker target recognition triggers RCA reaction to obtain a significantly amplified signal response. A capacitor is integrated to the enzyme biofuel cell to further amplify the electrochemical output signal of the self-powered biosensor. In detection system, glucose oxidase catalyzes glucose oxidation to produce hydrogen peroxide, and 3,3',5,5'-tetramethylbenzidine (TMB) is then catalyzed to generate colored products, so as to achieve the colorimetric detection of the target. Analysis signals of diverse modes are recorded independently. Consequently, detection of microRNA with improved reliability and wider signal response range are achieved by electrochemical/colorimetric dual-mode with detection limits of 0.15 and 33 fM (S/N = 3) respectively. In addition, the proposed self-powered biosensor successfully applied for the detection of miRNA-21 in human serum samples, confirming its practical applicability in clinical diagnosis. It is powerfully anticipated the proposed self-powered biosensor possesses great potential to be applied to other biomedical domains.

Nucleic acid amplification-based strategy to detect foodborne pathogens in milk: a review

Milk contaminated with trace amounts of foodborne pathogens can considerably threaten food safety and public health. Therefore, rapid and accurate detection techniques for foodborne pathogens in milk are essential. Nucleic acid amplification (NAA)-based strategies are widely used to detect foodborne pathogens in milk. This review article covers the mechanisms of the NAA-based detection of foodborne pathogens in milk, including polymerase chain reaction (PCR), loop-mediated isothermal amplification (LAMP), recombinase polymerase amplification (RPA), rolling circle amplification (RCA), and enzyme-free amplification, among others. Key factors affecting detection efficiency and the advantages and disadvantages of the above techniques are analyzed. Potential on-site detection tools based on NAA are outlined. We found that NAA-based strategies were effective in detecting foodborne pathogens in milk. Among them, PCR was the most reliable. LAMP showed high specificity, whereas RPA and RCA were most suitable for on-site and in-situ detection, respectively, and enzyme-free amplification was more economical. However, factors such as sample separation, nucleic acid target conversion, and signal transduction affected efficiency of NAA-based strategies. The lack of simple and effective sample separation methods to reduce the effect of milk matrices on detection efficiency was noteworthy. Further research should focus on simplifying, integrating, and miniaturizing microfluidic on-site detection platforms.

Fluorescence biosensing of the leukemia gene by combining Target-Programmed controllable signal inspiring engineering

Clinical diagnosis urgently requires ultrasensitive, accurate and rapid monitoring of low-abundance biomarkers. A biosensing strategy capable of detecting target genes at the femtomolar scale was designed in this work. In the biosensing strategy, the target can induce the specially designed hairpin probe H1 to self-fold and form a 3' blunt-ended structure. When there are the hybrid double-stranded P1-T1, ligase, polymerase and nickase, the target gene was recycled, and at the same time the system produces a lot of T1 and T2. T1 and T2 can simultaneously trigger HCR, causing the modified fluorophore FAM on the DNA strand to move away from the quencher group BHQ. The amplified fluorescent signal can be captured by a fluorescence instrument. It is exciting for us that three signal amplifications are involved to achieve femtomolar detection of target genes, namely target recycling, dual-triggered HCR of T1 and T2, and HCR. In addition, it still has good detection ability in actual samples simulated by serum. We expect that the sensing strategy proposed in this paper offers great potential for biomarker detection of leukemia for early clinical diagnosis.

An Ultrasensitive Electrochemical Biosensor Integrated by Nicking Endonuclease-Assisted Primer Exchange Reaction Cascade Amplification and DNA Nanosphere-Mediated Electrochemical Signal-Enhanced System for MicroRNA Detection

Specific and sensitive microRNAs (miRNAs) detection is essential to early cancer diagnosis. The development of these technologies including functional nuclease-mediated target amplification and DNA nanotechnology possesses tremendous potential for the high-performance detection of miRNAs in the accurate diagnosis of disease. In this study, we have established an ultrasensitive electrochemical biosensor by combining nicking endonuclease-assisted primer exchange reaction (PER) cascade amplification with a DNA nanosphere (DNS)-mediated electrochemical signal-enhanced system for the detection of miRNA-21 (miR-21). The cascade amplification is initiated by a nicking endonuclease that can cleave specific DNA substrates and highly amplify translation of the target to single-stranded DNA fragments (sDNA). Then, the PER cascade is powered by strand-displacing polymerase and generates a large amount of nascent single-stranded connector DNA (cDNA) via sDNA triggering of the dumbbell probe (DP), thus achieving the cascade amplification of the target. Finally, the DNS loaded with plenty of electroactive substances can be captured on the electrode via cDNA for further enhancing the electrochemical signal and highly sensitive detection of miR-21. The proposed electrochemical biosensor exhibits a wide detection range of 1 aM to 0.1 nM and a low detection limit of 0.58 aM. The excellent selectivity allows the biosensor to discriminate miR-21 from other miRNAs, even the one base-mismatched sequence. Moreover, the practicability of the biosensor is investigated by analyzing miR-21 in human serum and cancer cell lysate. Therefore, our proposed nicking endonuclease-assisted PER cascade amplification strategy provides a powerful platform for the early detection of miRNA-related disease and molecular diagnosis.

Cascade signal enhancement by integrating DNA walking and RCA reaction-assisted "silver-link" crossing electrode for ultrasensitive electrochemical detection of Staphylococcus aureus

The key factor to control the incidence rate of diseases caused by bacteria is rapid detection and early diagnosis. Herein, we proposed a new electrochemical bacterial sensor by coupling DNA walking and rolling circle amplification (RCA) reaction-assisted "silver-link" crossing electrode. Staphylococcus aureus (S. aureus) was detected using this proof-of concept strategy. Aptamer/DNA walker and auxiliary sequence (AS)/RCA reaction probe (RP) duplexes were modified on the electrode surface. The binding of S. aureus with its aptamer caused the disintegration of aptamer/DNA walker and released DNA walker. With the help of Exo III, DNA walker moved along the electrode surface and AS in AS/RP duplex was continuously digested to release RP. By introducing phi29 DNA polymerase, RCA reaction was performed using RP as the reaction primer to form long single-strand RCA extension products between the electrodes. The "silver-link" crossing electrode was formed by metallization of "gene-link", significant conductivity was thus acquired for bacteria detection. The limit of detection (LOD) was 10 CFU/mL and detection time was 2 h. The proposed sensor has high efficiency, good stability and low background signal, human serum and milk samples were successfully detected, which emerged a promising potential in the food monitoring and clinical diagnosis.

Novel integrating polymethylene blue nanoparticles with dumbbell hybridization chain reaction for electrochemical detection of pathogenic bacteria

Pathogenic bacteria infections pose a major threat to human health which can be found in contaminated food and infected humans. Herein, an electrochemical sensor was developed for pathogenic bacteria assay using a dual amplification strategy of polymethylene blue nanoparticles (pMB NPs) and dumbbell hybridization chain reaction (DHCR). The strong binding ability of aptamer to targets endowed outstanding performance in identifying Staphylococcus aureus (S. aureus) among other typical bacteria. The released T strands were hybridized with capture DNA on electrode surface which triggered DHCR in the presence of two dumbbell-shaped helper DNA, leading to the formation of extended and tight dsDNA polymers. In combination with pMB NPs (redox indicators), S. aureus was quantitatively detected in a range of 10-10⁸ CFU/mL and the detection limit reached 1 CFU/mL. Moreover, this sensor was successfully applied for S. aureus detection in human serum and foods, demonstrating the reliability in practical applications.

A waste-free entropy-driven DNA nanomachine for smartly designed photoelectrochemical biosensing of MicroRNA-155

DNA nanotechnology has been booming in many fields such as biosensors, logic gates, and material science. Typically, as a kind of powerful isothermal and enzyme-free DNA amplifier in biosensors, entropy-driven DNA nanomachines are superior to hairpin-based ones in speed, specificity, stability, and simplicity. However, the atomic economy of non-covalent molecular reactions in these machines is not high, and DNAs waste is typically generated during operation. Herein, in order to further save costs and improve the performance, we report a novel design for a smart photoelectrochemical (PEC) biosensor of microRNA-155 by engineering waste-free entropy-driven DNA amplifiers conjugated to superparamagnetic Fe₃O₄@SiO₂ particles. This elegant design efficiently avoids leaving redundant DNA strands and waste complex in the amplification system, and all the displaced DNA strands can be regenerated into double-stranded structures, making the reaction irreversible. Thanks to superparamagnetic Fe₃O₄@SiO₂ particles, this strategy is achieved by effectively enriching, extracting, and cleaning target analogs to prevent co-existing species from remaining on the modified electrode surface, enabling a highly specific and sensitive PEC biosensor. This innovative study will be a new perspective on microRNAs detection in complex biological systems, paving the way for the design of waste-free DNA molecular machines and promoting the development of DNA nanotechnology.

A novel electrochemical biosensor based on MIL-101-NH2 (Cr) combining target-responsive releasing and self-catalysis strategy for p53 detection

A novel electrochemical biosensor was constructed to detect p53 gene based on MIL-101-NH₂ (Cr) by combining target-responsive releasing and self-catalysis strategy. MIL-101-NH₂ (Cr) with suitable pore structure was used to encapsulate methylene blue (MB) as signal probe. The hairpin DNA (HP) containing rich-G sequences was used as gatekeeper to seal up the pores and avoid MB leakage through covalent immobilization. The p53 gene could hybridize with the loop portion of HP for the formation of dsDNA, which had the specific nicking site of the nicking endonuclease (Nt.BstNBI). Then Nt.BstNBI recognized the specific recognition site and cleaved HP to open the pore for releasing of MB. Meanwhile, the cleavage of HP released the target DNA to trigger the target recycling for signal amplification. More importantly, the plentiful rich-G sequences were exposed to form Hemin/G-quadruplex DNAzymes, which could unite MIL-101-NH₂ (Cr) to catalyze redox reaction of MB released by itself for signal amplification. The biosensor for p53 had wide linear range from 1 × 10^-14 to 1 × 10^-7 M and a low detection limit of 1.4 × 10^-15 M. The combination of target-responsive releasing and self-catalysis strategy provided a promising way for constructing ultrasensitive and simple biosensor.

Reversible Ratiometric Electrochemiluminescence Biosensor Based on DNAzyme Regulated Resonance Energy Transfer for Myocardial miRNA Detection

Myocardial miRNAs in peripheral blood are closely related to the pathogenic process of myocardial infarction. Rapid identification and accurate quantification of myocardial miRNAs are of great significance to clinical interventions for treating cardiovascular lesions. Therefore, a ratiometric electrochemiluminescence (ECL) biosensor integrating DNAzyme with a resonance energy transfer (RET) system was designed to detect myocardial miRNA. The dual-signal system was composed of rA marked substrate strand functionalized CdTe quantum dots (QDs) as reductive-oxidative (R-O) emitters and Cy5-labeled strand-functionalized Ru(bpy)₃²⁺-filled silica nanoparticles (RuSi NPs) as oxidative-reductive (O-R) emitters. In the presence of target miRNA, DNAzyme was activated to cut substrate strands on the CdTe QDs and release triggers for opening hairpin probes. Then, the Cy5 molecule-labeled hairpin DNA on the RuSi NPs was opened to introduce Cy5 molecules and RuSi NPs into the system. The R-O ECL was quenched by ECL-RET between CdTe QDs and Cy5 molecules and the O-R ECL was introduced by the RuSi NPs. In this way, based on the simultaneous changing of the ECL signal, the dual-potential dynamic signal ratiometric ECL sensing platform was developed. By measuring the ratio of O-R ECL signal to R-O ECL signal, the concentration of miRNA-499 was accurately quantified in the range of 10 fM to 10 nM, and the detection limit was as low as 2.44 fM (S/N = 3). This DNAzyme guided dual-potential ratiometric ECL method provides a sensitive and reliable method for myocardial miRNA detection, and it has great potential in clinical diagnosis and treatment.

Nanoarchitectured prototypes of mesoporous silica nanoparticles for innovative biomedical applications

Despite exceptional morphological and physicochemical attributes, mesoporous silica nanoparticles (MSNs) are often employed as carriers or vectors. Moreover, these conventional MSNs often suffer from various limitations in biomedicine, such as reduced drug encapsulation efficacy, deprived compatibility, and poor degradability, resulting in poor therapeutic outcomes. To address these limitations, several modifications have been corroborated to fabricating hierarchically-engineered MSNs in terms of tuning the pore sizes, modifying the surfaces, and engineering of siliceous networks. Interestingly, the further advancements of engineered MSNs lead to the generation of highly complex and nature-mimicking structures, such as Janus-type, multi-podal, and flower-like architectures, as well as streamlined tadpole-like nanomotors. In this review, we present explicit discussions relevant to these advanced hierarchical architectures in different fields of biomedicine, including drug delivery, bioimaging, tissue engineering, and miscellaneous applications, such as photoluminescence, artificial enzymes, peptide enrichment, DNA detection, and biosensing, among others. Initially, we give a brief overview of diverse, innovative stimuli-responsive (pH, light, ultrasound, and thermos)- and targeted drug delivery strategies, along with discussions on recent advancements in cancer immune therapy and applicability of advanced MSNs in other ailments related to cardiac, vascular, and nervous systems, as well as diabetes. Then, we provide initiatives taken so far in clinical translation of various silica-based materials and their scope towards clinical translation. Finally, we summarize the review with interesting perspectives on lessons learned in exploring the biomedical applications of advanced MSNs and further requirements to be explored.

Dual-mode sensor based on the synergy of magnetic separation and functionalized probes for the ultrasensitive detection of Clostridium perfringens

Clostridium perfringens is an important foodborne pathogen, which has caused serious public health problems worldwide. So, there is an urgent need for rapid and ultrasensitive detection of C. perfringens. In this paper, a dual-mode sensing platform using the synergy between fluorescent and electrochemical signals for Clostridium perfringens detection was proposed. An electrochemical aptasensor was constructed by a dual-amplification technology based on a DNA walker and hybridization chain reaction (HCR). When the C. perfringens genomic DNA was present, it specifically bonded with FAM-labeled aptamer which triggered the DNA walker on hairpin DNA (hDNA) tracks to start the synthesis of double-stranded DNA. HCR occurred subsequently and produced long-chain DNA to absorb more methylene blue (MB). In this cycle, the fluorescent signals of released FAM-labeled aptamer could also be detected. The synergistic effects of MB and FAM significantly improved the sensitivity and accuracy of the dual-mode sensor. As a result, the biosensor displayed an excellent analytical performance for C. perfringens at a concentration of 1 to 10⁸ CFU g⁻¹. A minimum concentration of 1 CFU g⁻¹ and good accuracy were detected in real samples. The proposed ultrasensitive detection method for detecting C. perfringens in food showed great potential in controlling foodborne diseases.

Clostridium perfringens is an important foodborne pathogen, which has caused serious public health problems worldwide.

Advances in the DNA Nanotechnology for the Cancer Biomarkers Analysis: Attributes and Applications

The most commonly used clinical methods are enzyme-linked immunosorbent assay (ELISA) and quantitative PCR (qPCR) in which ELISA was applied for the detection of protein biomarkers and qPCR was especially applied for nucleic acid biomarker analysis. Although these constructed methods have been applied in wide range, they also showed some inherent shortcomings such as low sensitivity, large sample volume and complex operations. At present, many methods have been successfully constructed on the basis of DNA nanotechnology with the merits of high accuracy, rapid and simple operation for cancer biomarkers assay. In this review, we summarized the bioassay strategies based on DNA nanotechnology from the perspective of the analytical attributes for the first time and discussed and the feasibility of the reported strategies for clinical application in the future.

Recent advances in catalytic hairpin assembly signal amplification-based sensing strategies for microRNA detection

Accumulative evidences have indicated that abnormal expression of microRNAs (miRNAs) is closely associated with many health disorders, making them be regarded as potentialbiomarkers for early clinical diagnosis. Therefore, it is extremely necessary to develop a highly sensitive, specific and reliable approach for miRNA analysis. Catalytic hairpin assembly (CHA) signal amplification is an enzyme-free toehold-mediated strand displacement method, exhibiting significant potential in improving the sensitivity of miRNA detection strategies. In this review, we first describe the potential of miRNAs as disease biomarkers and therapeutics, and summarize the latest advances in CHA signal amplification-based sensing strategies for miRNA monitoring. We describe the characteristics and mechanism of CHA signal amplification and classify the CHA-based miRNA sensing strategies into several categories based on the "signal conversion substance", including fluorophores, enzymes, nanomaterials, and nucleotide sequences. Sensing performance, limit of detection, merits and disadvantages of these miRNA sensing strategies are discussed. Moreover, the current challenges and prospects are also presented.

Quadrilateral Nucleic Acid Frame-Accelerating DNAzyme Walker Kinetics for Biosensing Based on Host-Guest Recognition-Enhanced Electrochemiluminescence

Depending on the reaction between walkers and tracks, DNA walker is able to output signals continuously, which has attracted great attention from the bioanalytical community. Therefore, how to improve its reaction kinetics for efficient signal readout is of great significance. Herein, a quadrilateral DNAzyme walker was fabricated by colocalizing one walker and three DNA tracks in the quadrilateral nucleic acid frame to form a reaction unit (abbreviated as qDNA walker). Impressively, in contrast to the common free DNAzyme walker, the reaction kinetics of the qDNA walker was 2.3 times faster, which could achieve microRNA detection within 30 min. Meanwhile, an electrochemiluminescence (ECL) emitter of anthracene-cucurbituril supramolecular nanocrystals (Ant-CB SNCs) was obtained based on the self-assembly of cucurbituril (CB, host molecule) and anthracene (Ant, guest molecule). Benefiting from the host-guest recognition effect, the prepared Ant-CB SNCs exhibited enhanced ECL efficiency due to the supramolecular interaction between CB and Ant, which could inhibit vibration and rotation of the Ant molecules. We defined this new enhanced ECL phenomenon as "host-guest recognition-enhanced ECL." As a proof of concept, an ECL biosensor for microRNA-21 (miRNA-21) was constructed by combining the high-efficiency DNAzyme walker and the advanced ECL emitter of Ant-CB SNCs, which showed a linear range from 50 aM to 50 pM with a low limit of detection (11 aM), highlighting the great potential in clinical diagnosis.

Highly Sensitive Detection of miR-21 through Target-Activated Catalytic Hairpin Assembly of X-Shaped DNA Nanostructures

MicroRNAs (miRNAs) are found in extremely low concentrations in cells, so highly sensitive quantitation is a great challenge. Herein, a simple dual-amplification strategy involving target-activated catalytic hairpin assembly (CHA) coupled with multiple fluorophores concentrated on one X-shaped DNA is reported. In this strategy, four hairpin probes (H1, H2, H3, and H4) are modified with FAM and BHQ1 at both sticky ends, while a circulating hairpin probe (H0) is used to activate CHA circuits once it binds to complementary sequences in the target miR-21 (T). The powerful dual-amplification cascades in Förster resonance energy transfer (FRET)-based nonenzymatic nucleic acid circuits are triggered by T-H0-activated formation of the X-shaped DNA nanostructure, freeing T-H0 for the next CHA reaction cycle. CHA circuits increase the fluorescence due to the wide distance between FAM and BHQ1 in the formed X-shaped DNA nanostructure, resulting in signal amplification and highly sensitive detection of miR-21, with a limit of detection (LOD, 3σ) of 0.025 nM, which is 25.6 or 57.6 times lower than that obtained through a single-amplification strategy without multiple fluorophores on one X-shaped DNA or CHA circuit. Furthermore, this cascade reaction was completed in 45 min, effectively avoiding target degradation. This new enzyme-free signal amplification strategy holds promising potential for sensitively detecting different DNA or RNA sequences by simply adapting the fragment of the H0 sequence complementary to the target.

Dual 3D DNA Nanomachine-Mediated Catalytic Hairpin Assembly for Ultrasensitive Detection of MicroRNA

Herein, we designed a dual 3D DNA nanomachine (DDNM)-mediated catalytic hairpin assembly (DDNM-CHA) to construct an electrochemical biosensor for ultrasensitive detection of miRNA, which possesses quite a faster reaction rate and much higher amplification efficiency than those of traditional catalytic hairpin assembly (CHA). Impressively, since the DDNM skillfully increases the local concentration of reactants and decreases the steric hindrance of substrates simultaneously, the DDNM-CHA could be endowed with higher collision efficiency and more effective reaction compared with traditional CHA, resulting in a hyper conversion efficiency up to 2.78 × 10⁷ only in 25 min. This way, the developed DDNM-CHA could easily conquer the main predicaments: long reaction time and low efficiency. As a proof of the concept, we adopt the gold nanoparticles (AuNPs) and the magnetic nanoparticle (Fe₃O₄) as the kernel of DNM-A and DNM-B, respectively, and harness the magnetic electrode to directly adsorb the products H1-H2/Fe₃O₄ for constructing an immobilization-free biosensor for high-speed and ultrasensitive detection of miRNA with a detection limit of 0.14 fM. As a result, the DDNM-CHA we developed carves out a new insight to design a functional DNA nanomachine and evolve the analysis method for practical amplification in the sensing area and promotes the deeper exploration of the nucleic acid signal amplification strategy and DNA nanobiotechnology.

Rapid heavy metal sensing platform: A case of triple signal amplification strategy for the sensitive detection of serum copper

Heavy metals are considered as hazardous substances to human because of their toxicity, persistence and bioaccumulation, and the level in serum is an important factor to evaluate the caused health risk, which depends on efficient and sensitive analytical methods. Here, a triple signal-amplified electrochemical sensing platform based on metal-dependent DNAzymes was fabricated for sensitive determination of heavy metals in serum (copper as a model target). Under the optimized conditions, the proposed method showed good sensitivity (limit of detection, 0.33 fM for Cu²⁺) with excellent selectivity and stability, which is ascribed to: (i) tetrahedral DNA nanostructures (TDNs) that was used as a promising scaffold to adjust the selective transformation between heterogeneous and homogeneous reactions, preventing the nonspecific binding of electrodes surface and DNA probes; (ii) the magnetic beads (MBs) used which led to signal amplification and decreased background owing to its excellent properties of extracting equivalent targets from the complex samples; (iii) two signal amplification strategy of catalytic hairpin assembly (CHA) and hybridization chain reaction (HCR). In addition, the proposed sensing platform displayed satisfactory accuracy through the validation with inductively coupled plasma-mass spectrometry (ICP-MS) and a spike-recovery analysis (recoveries, 87.92-111.61%; RSD, 4.89-8.85%), indicating the great potential for rapid and sensitive detection of Cu²⁺ or other metal ions.

Recent Progress in DNA Hybridization Chain Reaction Strategies for Amplified Biosensing

With the continuous development of DNA nanotechnology, various spatial DNA structures and assembly techniques emerge. Hybridization chain reaction (HCR) is a typical example with exciting features and bright prospects in biosensing, which has been intensively investigated in the past decade. In this Spotlight on Applications, we summarize the assembly principles of conventional HCR and some novel forms of linear/nonlinear HCR. With advantages like great assembly kinetics, facile operation, and an enzyme-free and isothermal reaction, these strategies can be integrated with most mainstream reporters (e.g., fluorescence, electrochemistry, and colorimetry) for the ultrasensitive detection of abundant targets. Particularly, we select several representative studies to better illustrate the novel ideas and performances of HCR strategies. Theoretical and practical utilities are confirmed for a range of biosensing applications. In the end, a deep discussion is provided about the challenges and future tasks of this field.

A Cascaded DNA Circuit in Bead Arrays for Quantitative Single-Cell MicroRNA Analysis

Single-cell microRNA (miRNA) analysis helps people understand the causes of diseases and formulate new disease treatment strategies. However, miRNA from a single cell is usually very rare and requires signal amplification for accurate quantification. Here, to amplify the signal, we constructed the cascaded DNA circuits consisting of catalytic hairpin assembly and hybrid chain reaction into the bead array platform, on which the uniformly distributed beads were adopted for miRNA quantification. After exponential signal amplification, a consistent linear correlation between the percentage of fluorescent beads and the copy number of miRNA was detected. The proposed bead array can achieve ultrahigh sensitivity as low as 60 copies of miR-155 and high specificity for distinguishing single nucleotide differences. This method has been successfully applied to the quantitative detection of miRNA in a single cancer cell. The high sensitivity, programmability, and simple workflow of the bead array chip will give a huge advantage in basic and clinical research.

Ultrasensitive detection of microRNA-21 by using specific interaction of antimonene with RNA as electrochemical biosensor

MicroRNA exhibits different levels of expression in cancer and can affect the transformation, metastasis, and carcinogenesis of the cancer cell. Herein, we developed a novel kind of electrochemical microRNA biosensor based on two-dimensional nanomaterial of antimonene nano-flakes (AMNFs) and carbon quantum dots (CQDs) which were used as substrating to cadmium ion (Cd²⁺) for specific detection of breast cancer-relevant biomarker-microRNA-21. Compared to graphene, the first principle energetic calculation shows that the AMNFs have completely a stronger force interaction with single strand (ssRNA), due to the antimonene has a more delocalized 5 s/5p orbital. After the addition of complementary microRNA, due to the low adsorption affinity of double-stranded RNA (dsRNA) to antimonene, the hybridized target is easy to desorb from the antimonene interface, and the oxidation peak of metal ions is significantly reduced. Results showed the microRNA-21 concentration can be detected from 100 aM to 1 nM, the limit of detection as low as 21 aM toward microRNA-21, which is 3 times lower than those of the established microRNA biosensors. The unique combination of not be attempted before existing sensing material which has special adsorption properties represents an approach to the detection of breast cancer. And it provides a promising method for early diagnosis, monitoring, and staging of breast cancer.

Hybridization chain reaction and its applications in biosensing

To pursue the sensitive and efficient detection of informative biomolecules for bioanalysis and disease diagnosis, a series of signal amplification techniques have been put forward. Among them, hybridization chain reaction (HCR) is an isothermal and enzyme-free process where the cascade reaction of hybridization events is initiated by a target analyte, yielding a long nicked dsDNA molecule analogous to alternating copolymers. Compared with conventional polymerase chain reaction (PCR) that can proceed only with the aid of polymerases and complicated thermal cycling, HCR has attracted increasing attention because it can occur under mild conditions without using enzymes. As a powerful signal amplification tool, HCR has been employed to construct various simple, sensitive and economic biosensors for detecting nucleic acids, small molecules, cells, and proteins. Moreover, HCR has also been applied to assemble complex nanostructures, some of which even act as the carriers to execute the targeted delivery of anticancer drugs. Recently, HCR has engendered tremendous progress in RNA imaging applications, which can not only achieve endogenous RNA imaging in living cells or even living animals but also implement imaging-guided photodynamic therapy, paving a promising path to promote the development of theranostics. In this review, we begin with the fundamentals of HCR and then focus on summarizing the recent advances in HCR-based biosensors for biosensing and RNA imaging strategies. Further, the challenges and future perspective of HCR-based signal amplification in biosensing and theranostic application are discussed.

Direct detection of intracellular miRNA in living circulating tumor cells by tumor targeting nanoprobe in peripheral blood

Molecular analysis of circulating tumor cells (CTCs) is of critical significance for the non-invasive early detection of tumors. However, in situ detection of intracellular nucleic acids of CTCs in whole blood still remains challenge. By using a highly efficient tumor targeting nanoprobe, we realize in situ detection of microRNA-21 (miR-21) of living CTCs in unprocessed whole blood. In the nanoprobe, a catalytic hairpin assembly (CHA) system is complexed with protamine sulfate (PS), and then decorated by SYL3C conjugated hyaluronic acid (SHA) and hyaluronic acid (HA). The CHA system can be specifically delivered into living CTCs in whole blood, followed by hybridization between the CHA system and intracellular miR-21 in CTCs to induce strong fluorescence emission. After isolation of CTCs by membrane filtration, CTCs of cancer patients can be directly visualized by a fluorescence microscope for miR-21 detection at a single-cell level. Our study provides an efficient strategy to realize in situ genomic analysis of living CTCs in whole blood.

The mechanism and improvements to the isothermal amplification of nucleic acids, at a glance

A comparative review of the most common isothermal methods is provided. In the last two decades, the challenge of using isothermal amplification systems as an alternate to the most extensive and long-standing nucleic acids-amplifying method-the polymerase chain reaction-has arisen. The main advantage of isothermal amplification is no requirement for expensive laboratory equipment for thermal cycling. Considerable efforts have been made to improve the current techniques of nucleic acid amplification and the development of new approaches based on the main drawbacks of each method. The most important and challenging goal was to achieve a low-cost, straightforward system that is rapid, specific, accurate, and sensitive.

A dual-model "on-super off" photoelectrochemical/ratiometric electrochemical biosensor for ultrasensitive and accurate detection of microRNA-224

A dual-model "on-super off" photoelectrochemical (PEC)/ratiometric electrochemical (EC) biosensor based on signal enhancing and quenching combining three-dimensional (3D) DNA walker strategy was designed for the ultrasensitive and accurate detection of microRNA-224 (miRNA-224). The "signal on" PEC state was achieved by methylene blue labeled hairpin DNA (MB-DNA) for sensitizing CdS QDs. Then numerous transformational ferrocene labeled DNAs (Fc-DNAs) converted by target-induced 3D DNA walker amplification with the help of Ag nanocubes (NCs) label DNA (Ag-DNA) were introduced to open hairpin MB-DNA. Such configuration change would relocate the sensitizer MB and the quencher Fc, whereas energy transfer placed between Ag NCs and CdS QDs, thereby significantly quenching the PEC signal to obtain "super off" state. Meanwhile, these changes resulted in a decreased oxidation peak current of MB (I_MB) and an increased that of Fc (I_Fc). MiRNA-224 was also detected on basis of the dual-signaling EC ratiometric method for complementary PEC detection. Benefiting from different mechanisms and relatively independent signal transduction, this approach not only avoided interference from difficult assembly but also outstandingly increased sensitivity by distance-controllable signal enhancing and quenching strategies. As a result, the detection ranges of 0.1-1000 fM with a low detection limit of 0.019 fM for PEC, and 0.52 to 500 fM with a low detection limit of 0.061 fM for EC, were obtained for miRNA-224, which opens a new avenue for designing numerous elegant biosensors with potential utility in bioanalysis and early disease diagnosis.

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.

Aptamer-functionalized metal-organic frameworks (MOFs) for biosensing

As a class of crystalline porous materials, metal-organic frameworks (MOFs) have attracted increasing attention. Due to the nanoscale framework structure, adjustable pore size, large specific surface area, and good chemical stability, MOFs have been applied widely in many fields such as biosensors, biomedicine, electrocatalysis, energy storage and conversions. Especially when they are combined with aptamer functionalization, MOFs can be utilized to construct high-performance biosensors for numerous applications ranging from medical diagnostics and food safety inspection, to environmental surveillance. Herein, this article reviews recent innovations of aptamer-functionalized MOFs-based biosensors and their bio-applications. We first briefly introduce different functionalization methods of MOFs with aptamers, which provide a foundation for the construction of MOFs-based aptasensors. Then, we comprehensively summarize different types of MOFs-based aptasensors and their applications, in which MOFs serve as either signal probes or signal probe carriers for optical, electrochemical, and photoelectrochemical detection, with an emphasis on the former. Given recent substantial research interests in stimuli-responsive materials and the microfluidic lab-on-a-chip technology, we also present the stimuli-responsive aptamer-functionalized MOFs for sensing, followed by a brief overview on the integration of MOFs on microfluidic devices. Current limitations and prospective trends of MOFs-based biosensors are discussed at the end.

Electrochemical biosensors for measurement of colorectal cancer biomarkers

Colorectal cancer (CRC) is associated with one of the highest rates of mortality among cancers worldwide. The early detection and management of CRC is imperative. Biomarkers play an important role in CRC screening tests, CRC treatment, and prognosis and clinical management; thus rapid and sensitive detection of biomarkers is helpful for early detection of CRC. In recent years, electrochemical biosensors for detecting CRC biomarkers have been widely investigated. In this review, different electrochemical detection methods for CRC biomarkers including immunosensors, aptasensors, and genosensors are summarized. Further, representative examples are provided that demonstrate the advantages of electrochemical sensors modified by various nanomaterials. Finally, the limitations and prospects of biomarkers and electrochemical sensors in detection are also discussed. Graphical abstract.

Hybridizing clinical translatability with enzyme-free DNA signal amplifiers: recent advances in nucleic acid detection and imaging

Nucleic acids have become viable prognostic and diagnostic biomarkers for a diverse class of diseases, particularly cancer. However, the low femtomolar to attomolar concentration of nucleic acids in human samples require sensors with excellent detection capabilities; many past and current platforms fall short or are economically difficult. Strand-mediated signal amplifiers such as hybridization chain reaction (HCR) and catalytic hairpin assembly (CHA) are superior methods for detecting trace amounts of biomolecules because one target molecule triggers the continuous production of synthetic double-helical DNA. This cascade event is highly discriminatory to the target via sequence specificity, and it can be coupled with fluorescence, electrochemistry, magnetic moment, and electrochemiluminescence for signal reporting. Here, we review recent advances in enhancing the sensing abilities in HCR and CHA for improved live-cell imaging efficiency, lowered limit of detection, and optimized multiplexity. We further outline the potential for clinical translatability of HCR and CHA by summarizing progress in employing these two tools for in vivo imaging, human sample testing, and sensing-treating dualities. We finally discuss their future prospects and suggest clinically-relevant experiments to supplement further related research.

The Recent Development of Hybridization Chain Reaction Strategies in Biosensors

With the continuous development of biosensors, researchers have focused increasing attention on various signal amplification strategies to pursue superior performance for more applications. In comparison with other signal amplification strategies, hybridization chain reaction (HCR) as a powerful signal amplification technique shows its certain charm owing to nonenzymatic and isothermal features. Recently, on the basis of conventional HCR, this technique has been developed and improved rapidly, and a variety of HCR-based biosensors with excellent performance have been reported. Herein, we present a systematic and critical review on the research progress of HCR in biosensors in the last five years, including the newly developed HCR strategies such as multibranched HCR, migration HCR, localized HCR, in situ HCR, netlike HCR, and so on, as well as the combination strategies of HCR with isothermal signal amplification techniques, nanomaterials, and functional DNA molecules. By illustrating some representative works, we also summarize the advantage and challenge of HCR in biosensors, and offer a deep discussion of the latest progress and future development trends of HCR in biosensors.

Fluorescence sensing strategy based on aptamer recognition and mismatched catalytic hairpin assembly for highly sensitive detection of alpha-fetoprotein

At present, alpha fetoprotein (AFP) is mainly used as a serum marker of primary Hepatocellular carcinoma. A simple, enzyme-free sensing strategy is introduced for highly sensitive fluorescence detection of AFP. This detection strategy is based on aptamer recognition and mismatched catalytic hairpin assembly (MCHA). At first, Trigger is locked by aptamer before the introduction of AFP in this aptamer-MCHA system. The aptamer preferentially combines with AFP via powerful attraction in the presence of AFP. This results in the release of trigger and initiation of MCHA cycle, thus forming the H1 and H2 double chain complexes (denoted as H1@H2). Finally, H1@H2 and double chain structure containing fluorophore and its quenched group- BHQ1 (denoted as F@Q) initiated displacement reaction, which caused double chain separation and fluorescence recovery. This assay produces a wide detection range, which is from 0.1 ng mL^-1 to 10 μg mL^-1 and the limit of detection as 0.033 ng mL^-1. The whole detection process was performed at 37 °C for 60 min. In addition, this assay had high anti-interference ability and could be used to detect AFP in clinical serum. This novel AFP detection strategy is able to screen of Hepatocellular carcinoma.

An enzyme-free electrochemical biosensor based on target-catalytic hairpin assembly and Pd@UiO-66 for the ultrasensitive detection of microRNA-21

MicroRNA-21 (miR-21) has been widely investigated as important biomarkers for cancer diagnosis and treatment. Herein, a highly sensitive nonenzymatic electrochemical biosensor based on Pd@metal-organic frameworks (Pd@UiO-66) and target-catalytic hairpin assembly (CHA) with target recycling approach has been proposed for the detection of miR-21. The proposed biosensor integrates the efficient CHA strategy and excellent electrocatalytic performance of Pd@UiO-66 nanocomposites. The concentration of miRNA-21 is related to the amount of the adsorbed electrocatalyst, leading to the different electrochemical signals for readout towards paracetamol (AP). This biosensor shows a low limit of detection of 0.713 fM with the dynamic range of 20 fM -600 pM under the optimal experimental conditions, providing a powerful platform for detecting miR-21. Furthermore, the designed biochemical self-assembly strategy of this electrochemical biosensor is promising candidate for potential applications in the analysis of other important genetic biomarkers for early diagnosis of cancers.

Target-Catalyzed Assembly of Pyrene-Labeled Hairpins for Exponentially Amplified Biosensing

Rapid and sensitive detection of nucleic acids is vital for disease diagnosis. This work designed an enzyme-free isothermal strategy for rapid exponential signal amplification through target-triggered catalytic hairpin assembly (CHA) to induce the spatially sensitive fluorescent signal of the pyrene excimer. Functionally, this system consisted of three pyrene labelled hairpins (H1, H2, and H3) and one catalyst DNA C. In the presence of C, the CHA was activated to generate intermediate I, which contained a single-stranded region identical to the C sequence for initiating the second cycle of CHA to obtain 2I and thus achieved the exponential formation of I along with the switching of pyrene excimer. The fluorescent signal of the pyrene excimer could be further enhanced via the inclusion of γ-cyclodextrin and showed a linear increase upon increasing logarithm of C concentration. Through the introduction of a helping hairpin H4-containing C sequence and a region specific to the target, this strategy could be extended to realize the quick and sensitive detection of different analytes. Using dengue virus RNA as an analyte model, the proposed fluorescent method showed a linear range from 0.1 to 50 nM with a limit of detection of 0.048 nM at 3σ and good selectivity. The excellent performance and convenient operation demonstrated its promising application in clinical disease diagnosis.