Metal-Mediated Polydopamine Nanoparticles-DNA Nanomachine Coupling Electrochemical Conversion of Metal-Organic Frameworks for Ultrasensitive MicroRNA Sensing

Magnetically controlled all-in-one sensing platform for triple-mode detection of organophosphorus pesticides using DNA tetrahedrons-polydopamine reporters

Accurate and on-site detection of organophosphorus pesticides (OPs) in complex matrixes is important for environmental monitoring and food safety. Herein, a colorimetric (CL), photothermal (PT) and fluorescence (FL) triple-mode sensor was constructed for OPs detection based on magnetically controlled all-in-one platform, which was featured of specific recognition unit, abundant loading of DNA tetrahedrons-Fe3+-polydopamine (TDN-Fe3+-PDA) with peroxidase-like activity, and verifiable triple-mode signal output. Based on the aptamer-target recognition and efficient separation of MBs, large amount of TDN-Fe3+-PDA reporters was released. Taking full advantages of the supernatant and precipitation, OPs was sensitively detected with the TDN-Fe3+-PDA catalyzed 3,3',5,5'-tetramethylbenzidine (TMB)-H2O2 system (CL mode), photothermal effect of TDN-Fe3+-PDA and oxidized TMB (oxTMB) (PT mode), and fluorescent signal of TDN-templated copper nanoclusters on MBs (FL mode). Using profenofos (Pro) as the model target, the triple-mode sensing platform realized sensitive detection of Pro with the detection limits 0.23 ng/mL, 0.40 ng/mL and 0.14 ng/mL for CL, PT and FL mode, respectively. The proposed strategy provided a simple and accurate method for on-site detection of OPs, holding promising application in environmental and food contamination monitoring.

A sensitive electrochemical biosensor based on Exo III cyclic amplification strategy for Phaeocystis globosa detection

Phaeocystis globosa (P. globosa), a main culprit of harmful algal blooms (HABs), is highly prone to blocking the inlet filter screens of nuclear power cold sources, thus posing a significant threat to nuclear power safety. However, existing methods for P. globosa detection fail to achieve rapid and on-site monitoring of single-cell densities prior to bloom outbreaks, limiting timely defensive measures. In this study, we developed a novel biosensor platform for efficient P. globosa detection, leveraging an Exo III-assisted signal amplification strategy to significantly enhance sensitivity and selectivity. The biosensor achieved an ultra-low limit of detection (LOD) of 268.91 fg µL-1 (3119 cells L-1), far below the benchmark concentration for P. globosa blooms (107 cells L-1), and demonstrated a wide linear detection range from 500 fg µL-1 to 10 ng µL-1. Furthermore, the biosensor's accuracy and reliability were validated through comparative analysis with droplet digital PCR (ddPCR) using actual samples from the Beibu Gulf of China, revealing a low risk of P. globosa blooms in the region at the sampling time. This study represents a significant advancement in HAB monitoring by providing a highly sensitive, rapid, and field-deployable tool for early warning of P. globosa blooms. The biosensor's innovative design and performance address critical gaps in current detection methods, offering practical implications for safeguarding coastal nuclear power facilities and protecting marine ecosystems.

Multispatially Localized DNA Walker Coupling Covalent Organic Framework for Dual-Mode Detection of Nucleocapsid Protein Using the Walking-Recycling-Conversion Strategy

DNA walkers have emerged as a powerful tool in bioanalysis; however, many existing approaches are still restricted by low reaction kinetics and inaccurate single-mode detection. Herein, a fluorescence (FL) and electrochemical (EC) dual-mode biosensor was proposed based on a multispatially localized DNA walker (m-DNA walker) coupling covalent organic framework (COF) using the walking-recycling-conversion strategy. Specifically, the functionalized COF not only served as a three-dimensional nanocarrier but also acted as an effective quencher of the walking tracks. In the presence of the target, the activated m-DNA walker moved fast along the numerous quenching tracks, leading to the cleavage of Cy3-H1 and the recovery of the FL signal. To further improve the detection sensitivity, the Cy3-H1 fragments' recycling process was implemented with the generation of a large amount of S1 and S2, which caused the assembly of DNA-Fe3+-polydopamine network amplifiers on the electrode. The rapid electrochemical conversion was introduced to convert DNA-Fe3+-polydopamine into electroactive Prussian Blue, providing a significant EC signal output. Using nucleocapsid protein (N-protein) as the model target, the designed biosensing platform produced a FL/EC dual-mode readout with the detection limits of 65.0 fg/mL for FL mode and 2.3 fg/mL for EC mode, which could eliminate the interference from different reactive pathways and improve the detection accuracy, holding potential application in early disease diagnosis and treatment.

Enhancing CRISPR/Cas-mediated electrochemical detection of nucleic acid using nanoparticle-labeled covalent organic frameworks reporters

Molecular detection of nucleic acid plays an important role in early diagnosis and therapy of disease. Herein, a novel and enhanced electrochemical biosensor was exploited based on target-activated CRISPR/Cas12a system coupling with nanoparticle-labeled covalent organic frameworks (COFs) as signal reporters. Hollow spherical COFs (HCOFs) not only served as the nanocarriers of silver nanoparticles (AgNPs)-DNA conjugates for enhanced signal output but also acted as three-dimensional tracks of CRISPR/Cas12a system to improve the cleavage accessibility and efficiency. The presence of target DNA triggered the trans-cleavage activity of the CRISPR/Cas12a system, which rapidly cleaved the AgNPs-DNA conjugates on HCOFs, resulting in a remarkable decrease of the electrochemical signal. As a proof of concept, the fabricated biosensing platform realized highly sensitive and selective detection of human papillomavirus type 16 (HPV-16) DNA ranging from 100 fM to 1 nM with the detection limit of 57.2 fM. Furthermore, the proposed strategy provided a versatile and high-performance biosensor for the detection of different targets by simple modification of the crRNA protospacer, holding promising applications in disease diagnosis.

Target-driven cascade amplified assembly of covalent organic frameworks on tetrahedral DNA nanostructure with multiplex recognition domains for ultrasensitive detection of microRNA

BACKGROUND

MicroRNA (miRNA) emerges as important cancer biomarker, accurate detection of miRNA plays an essential role in clinical sample analysis and disease diagnosis. However, it remains challenging to realize highly sensitive detection of low-abundance miRNA. Traditional detection methods including northern blot and real-time PCR have realized quantitative miRNA detection. However, these detection methods are involved in sophisticated operation and expensive instruments. Therefore, the development of novel sensing platform with high sensitivity and specificity for miRNA detection is urgently demanded for disease diagnosis.

RESULTS

In this work, a novel electrochemical biosensor was constructed for miRNA detection based on target-driven cascade amplified assembly of electroactive covalent organic frameworks (COFs) on tetrahedral DNA nanostructure with multiplex recognition domains (m-TDN). COFs were employed as nanocarriers of electroactive prussian blue (PB) molecules by the "freeze-drying-reduction" method without the use of DNA as gatekeeper, which was simple, mild and efficient. The target-triggered catalytic hairpin assembly (CHA) and glutathione reduction could convert low-abundance miRNA into a large amount of Mn2+. Without the addition of exogenous Mn2+, the dynamically-generated Mn2+-powered DNAzyme cleavage process induced abundant PB-COFs probe assembled on the four recognition domains of m-TDN, resulting in significantly signal output. Using miRNA-182-5p as the model target, the proposed electrochemical biosensor achieved ultrasensitive detection of miRNA-182-5p in the range of 10 fM-100 nM with a detection limit of 2.5 fM.

SIGNIFICANCE AND NOVELTY

Taking advantages of PB-COFs probe as the enhanced signal labels, the integration of CHA, Mn2+-powered DNAzyme and m-TDN amplification strategy significantly improved the sensitivity and specificity of the biosensor. The designed sensing platform was capable of miRNA detection in complex samples, which provided a new idea for biomarker detection, holding promising potential in clinical diagnosis and disease screening.

Metal-organic framework (MOF)-based biosensors for miRNA detection

MicroRNAs (miRNAs) play several crucial roles in the physiological and pathological processes of the human body. They are considered as important biomarkers for the diagnosis of various disorders. Thus, rapid, sensitive, selective, and affordable detection of miRNAs is of great importance. However, the small size, low abundance, and highly similar sequences of miRNAs impose major challenges to their accurate detection in biological samples. In recent years, metal-organic frameworks (MOFs) have been applied as promising sensing materials for the fabrication of different biosensors due to their distinctive characteristics, such as high porosity and surface area, tunable pores, outstanding adsorption affinities, and ease of functionalization. In this review, the applications of MOFs and MOF-derived materials in the fabrication of fluorescence, electrochemical, chemiluminescence, electrochemiluminescent, and photoelectrochemical biosensors for the detection of miRNAs and their detection principle and analytical performance are discussed. This paper attempts to provide readers with a comprehensive knowledge of the fabrication and sensing mechanisms of miRNA detection platforms.

Fe-MOF/AuNP-based ratiometric electrochemical immunosensor for the detection of deoxynivalenol in grain products

A ratiometric assay was designed to improve the sensitivity and reliability of electrochemical immunosensors for deoxynivalenol (DON) detection. The indicator signal caused by the Fe-based metal-organic framework nanocomposites loaded with gold nanoparticles and the internal reference signal from the [Fe(CN)6]3-/4- in the electrolyte came together at the immunosensor. When immunoreactivity occurred, the indicator signals decreased as the concentration of DON increased, while the internal reference signals increased slightly. The ratio of the indicator signal to the internal reference signal was available for reproducible and sensitive monitoring of DON. The prepared immunosensor showed excellent performance in the range from 0.5 to 5000 pg mL-1, and the detection limit was 0.0166 pg mL-1. The immunosensor achieved satisfactory detection toward DON in spiked and actual samples and has a promising application in the control of DON in grain products.

Nanomaterials in electrochemical nanobiosensors of miRNAs

Nanomaterial-based biosensors have received significant attention owing to their unique properties, especially enhanced sensitivity. Recent advancements in biomedical diagnosis have highlighted the role of microRNAs (miRNAs) as sensitive prognostic and diagnostic biomarkers for various diseases. Current diagnostics methods, however, need further improvements with regards to their sensitivity, mainly due to the low concentration levels of miRNAs in the body. The low limit of detection of nanomaterial-based biosensors has turned them into powerful tools for detecting and quantifying these biomarkers. Herein, we assemble an overview of recent developments in the application of different nanomaterials and nanostructures as miRNA electrochemical biosensing platforms, along with their pros and cons. The techniques are categorized based on the nanomaterial used.

Constructing DNAzyme-driven three-dimensional DNA nanomachine-mediated paper-based photoelectrochemical device for ultrasensitive detection of miR-486-5p

As a unique class of dynamic nanostructures, biomimetic DNA walking machines that exhibit geometrical complexity and nanometre precision have gained great success in photoelectrochemical (PEC) bioanalysis. Despite certain achievements, the slow reaction kinetics and low processivity severely restrict the amplification efficiency of the DNA walker-mediated biosensors. Herein, by taking advantage of efficient DNA rolling machines, a three-dimensional (3D) DNA nanomachine-mediated paper-based PEC device for speedy ultrasensitive detection of miR-486-5p was successfully constructed. To achieve it, a novel In2S3/SnS2 sensitized heterojunction was firstly in-situ grown on the Au-modified paper fibers and implemented as the photoanode with effective separation of photogenerated carriers to achieve an enhanced initial photocurrent. Subsequently, the copper hexacyanoferrate(II)-modified CuO nanosphere was introduced as a multifunctional signal regulator via the competitive capture of electron donors and photon energy with the photoelectric layer for efficiently quenching the PEC signal. With the introduction of targets, the DNAzyme-driven DNA nanomachine with editable motion modes was gradually activated and it could continuously cleave the tracks DNA labeled quenching probes, finally achieving the recovery of PEC signal. As a proof of concept, the elaborated paper-based PEC device presented a wide linear range from 0.1 fM to 100 pM and a detection limit of 35 aM for miR-486-5p bioassay. This work provides an innovative insight to the exploitation of DNA nanobiotechnology and nucleic acid signal amplification strategy.

Target-Controlled Redox Reaction and Ru(II) Release of a Smart Metal-Organic Framework Nanomaterial for Highly Sensitive Ratiometric Homogeneous Electroanalysis of Cadmium(II)

In this work, a highly sensitive ratiometric homogeneous electroanalysis (HEA) strategy of cadmium(II) (Cd2+) was proposed via a Cd2+-controlled redox reaction and Ru(bpy)32+ (Ru(II)) release from a smart metal-organic framework (MOF) nanomaterial. For achieving this purpose, Ru(II) was entrapped ingeniously into the pores of an MOF material (UiO-66-NH2) and subsequently gated by the double-strand hybrids of a Cd2+-aptamer (Apt) and its complementary sequences (CP) to form a novel smart nanomaterial (denoted as Ru@UiO-66-NH2); meanwhile, Fe(III) was selected as an additional probe present in electrolyte to facilitate the Ru(II) redox reaction: Fe(III) + Ru(II) → Fe(II) + Ru(III). Owing to the strong binding effect of the Cd2+ target to the specific Apt, the Apt-CP hybridization at Ru@UiO-66-NH2 would be destroyed in the presence of Cd2+, and the related Apt was further induced away from the smart nanomaterial, leading to the opening of the gate and release of Ru(II). Meanwhile, the released Ru(II) was quickly oxidized chemically by Fe(III) to Ru(III). On the basis of the generated Ru(III) and consumed Fe(III), the ratio of the reduction currents between Ru(III) and Fe(III) exhibits an enhancement and it is dependent on the level of Cd2+; thus, a novel HEA strategy of Cd2+ was then designed. Under the optimal conditions, the HEA sensor shows a wide linearity ranging from 10.0 pM to 500.0 nM, and the achieved detection limit of Cd2+ is 3.3 pM. The as-designed ratiometric HEA strategy not only offers a unique idea to realize a simple and sensitive assay for Cd2+ but also possesses significant potential as an effective tool to be introduced for other target analysis just via altering the specific Apt.

Nanoporous Crystalline Materials for the Recognition and Applications of Nucleic Acids

Nucleic acid plays a crucial role in countless biological processes. Hence, there is great interest in its detection and analysis in various fields from chemistry, biology, to medicine. Nanoporous crystalline materials exhibit enormous potential as an effective platform for nucleic acid recognition and application. These materials have highly ordered and uniform pore structures, as well as adjustable surface chemistry and pore size, making them good carriers for nucleic acid extraction, detection, and delivery. In this review, the latest developments in nanoporous crystalline materials, including metal organic frameworks (MOFs), covalent organic frameworks (COFs), and supramolecular organic frameworks (SOFs) for nucleic acid recognition and applications are discussed. Different strategies for functionalizing these materials are explored to specifically identify nucleic acid targets. Their applications in selective separation and detection of nucleic acids are highlighted. They can also be used as DNA/RNA sensors, gene delivery agents, host DNAzymes, and in DNA-based computing. Other applications include catalysis, data storage, and biomimetics. The development of novel nanoporous crystalline materials with enhanced biocompatibility has opened up new avenues in the fields of nucleic acid analysis and therapy, paving the way for the development of sensitive, selective, and cost-effective diagnostic and therapeutic tools with widespread applications.

Engineering of catalytic hairpin-rigidified Y-shaped DNA-functionalized nanomachine to rapidly detect mRNA

The catalytic hairpin-rigidified Y-shaped DNA through layer-by-layer assembly has been fixed on the surface of copper sulfide nanoparticles for the detection of survivin mRNA. The distance between the CHA probes fixed on the Y-shaped DNA is significantly shortened. The results show that the fluorescence of this nanomachine reached the maximum value in 50 min (excitation wavelength at 488 nm and emission wavelength 526 nm), and its reaction rate is more than 5-fold faster than that of the free-CHA control system. In addition, the nanomachine showed high sensitivity (LOD of 3.5 pM) and high specificity for the survivin mRNA detection. Given its fast response time and excellent detection performance, we envision that the catalytic hairpin-rigidified Y-shaped DNA-functionalized nanomachine will offer potential applications in disease diagnostics and clinical applications.

Homogeneous voltammetric sensing strategy for lead ions based on aptamer gated methylthionine chloride@UiO-66-NH2 framework as smart target-stimulated responsive nanomaterial

Herein an innovative electrochemical method is proposed for the determination of lead ions (Pb²⁺) based on a homogeneous voltammetric (HVC) sensing strategy using an aptamer gated methylthionine chloride@UiO-66-NH₂ framework as a smart target-stimulated responsive material. The proposed HVC sensor exhibits excellent sensing performance: ultralow detection limit (0.166 pM) and wide linearity (5.0 pM-500.0 nM), simultaneously, it avoids electrodeposition processes and it is simple to modify the electrode compared to previous electrochemical methods for Pb²⁺ detection. Thus our method shows great potential in the highly efficient detection of Pb²⁺ and other heavy metal ions by simply altering the related specific aptamer.

Ratiometrically homogeneous electrochemical biosensor based on the signal amplified strategy of dual DNA nanomachines for microRNA analysis

Precise and sensitive microRNA (miRNA) analysis is very significant for early disease diagnosis. In this work, a dual DNA nanomachines-based homogeneous electrochemical biosensor was constructed for the sensitively ratiometric detection of miRNA by a nicking enzyme (Nt.AlwI)-assisted cycling signal amplification strategy. The Co-based metal organic frameworks (Co-MOFs) and toluidine blue (TB) were employed as signal probes and internal reference probes, respectively. The introduction of internal reference probes can actually calibrate the interferent factors of the analytical system to improve the stability in detection procedure. In addition, with the help of the magnetic separation technique, the homogeneous electrochemical biosensor provides a more simpler way for the development of immobilization-free electrochemical miRNA biosensors, avoiding the complex modification procedure of traditional electrochemical biosensing interfaces. Consequently, taking advantages of this proposed dual DNA nanomachines-based homogeneous electrochemical biosensor, the highly sensitive and selective detection of miRNA-141 as model could be accomplished in ranging from 1 fM to 10 nM with detection limit of 0.46 fM. This strategy exhited good sensitivity and stability to integrate the nicking enzyme-powered dual DNA nanomachines with the ratiometric electrochemical output modes, which open new opportunities for the sensitive and reliable diagnosis of miRNA-related diseases.

Target recognition initiated self-dissociation based DNA nanomachine for sensitive and accurate MicroRNA (miRNA) detection

As a valuable biomarker for various tumor, sensitive and reliable quantitative determination of microRNA (miRNA) is crucial for both disease diagnosis and cancer treatment. Herein, we depict a novel simple and sensitive miRNA detection approach by exploiting an elegantly designed target recognition initiated self-dissociation based DNA nanomachine. In this nanomachine, target recognition assists the formation of active DNAzyme secondary conformation, and the active DNAzyme generates a nicking site in H probe, initiating the self-assembly of H probe. With the reflexed sequences as primer, dual signal recycles are formed under the cooperation of DNA polymerase and Nb.BbvCI. Eventually, the method exhibits a high sensitivity with the limit of detection as low as 12 fM. In addition, the method is also demonstrated with a high selectivity that can distinguish one mismatched base pair. We believe the established approach can be a robust tool for the early-diagnosis of a variety of cancers and also in anticancer drug development.

Wedged DNA Walker Coupled with a Bimetallic Metal-Organic Framework Electrocatalyst for Rapid and Sensitive Monitoring of DNA Methylation

The dissociation of the walking strand from the track gives rise to decreased efficiency and long reaction time of DNA walkers. In this work, we constructed a DNA walker combining the introduction of a wedge segment with a bimetallic metal-organic framework (MOF) electrocatalyst to solve this problem. The target methylated DNA acted as a single-legged walker, and the immobilization probe assembled on the track contained a wedge segment that was complementary to the target methylated DNA persistently, inhibiting its dissociation from the track. The fuel strand modified with a bimetallic MOF would drive the target strand to conduct branch migration and move processively along the track. The stepwise movement of the target strand resulted in the loading of numerous bimetallic MOF catalysts to reduce H₂O₂ at the electrode interface, thereby a significantly increased current response would be obtained for the detection of methylated DNA. This DNA walker achieved a detection limit of 200 aM within 20 min and effectively distinguished DNA with different methylation statuses, which would pave a way for rapid and sensitive monitoring of DNA methylation.

Pathogen Identification: Ultrasensitive Nucleic Acid Detection via a Dynamic DNA Nanosystem-Integrated Ratiometric Electrochemical Sensing Strategy

Sensitively determining trace nucleic acid is of great significance for pathogen identification. Herein, a dynamic DNA nanosystem-integrated ratiometric electrochemical biosensor was proposed to determine human immunodeficiency virus-associated DNA fragment (HIV-DNA) with high sensitivity and selectivity. The dynamic DNA nanosystem was composed of a target recycling unit and a multipedal DNA walker unit. Both of them could be driven by a toehold-mediated strand displacement reaction, enabling an enzyme-free and isothermal amplification strategy for nucleic acid determination. The target recycling unit could selectively recognize HIV-DNA and activate the multipedal DNA walker unit to roll on the electrode surface, which would lead to bidirectional signal variation for ratiometric readout with cascade signal amplification. Benefiting from the synergistic effect of the dynamic DNA nanosystem and the ratiometric output mode, the ultrasensitive detection of HIV-DNA was achieved in a wide linear range of 6 orders of magnitude with a limit of detection of 36.71 aM. The actual usability of the proposed sensor was also verified in complex biological samples with acceptable performance. This dynamic DNA nanosystem-integrated ratiometric sensing strategy might be promising in the development of reliable point-of-care diagnostic devices for highly sensitive and selective pathogen identification.

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.

Zinc-Air Battery-Based Self-Powered Sensor with High Output Power for Ultrasensitive MicroRNA let-7a Detection in Cancer Cells

Self-powered sensors do not require a power supply and are easy to miniaturize, which have potential for constructing wearable, portable, and real-time detection devices. However, it is challenging for the detection of low abundant targets due to the low output power density of fuel cells and much interference of complex biological environment. Herein, a new kind of photocatalytic zinc-air battery-based self-powered electrochemical sensor (ZAB-SPES) was constructed for the detection of microRNA let-7a (miRNA let-7a) by combining magnetic nanobeads (MBs) with a metal-organic framework loaded with glucose oxidase (MOFs@GOX). Poly(1,4-di(2-thienyl))benzene (PDTB) was used as the photocathode material, and the proposed ZAB-SPES had a high power density of 22.8 μW/cm², which was 2-3-fold of commonly used photofuel cells. MBs can capture and separate miRNA from complex samples quickly with a high separation efficiency of 99% within 60 s. The competitive reaction of oxygen reduction reaction between PDTB and MOFs@GOX would change the output power density of the ZAB-SPES. Based on the relationship between output power density and target concentration, the ZAB-SPES realized ultrasensitive detection of miRNA let-7a with a detection limit down to 1.38 fM. Furthermore, the successful detection of miRNA let-7a in A549 cancer cells indicated the great prospects of ZAB-SPES in clinical analysis and early diagnosis of cancers.

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.

Dual signal magnification for ultrasensitive biosensing based on well-regulated SERS of AuNTs@AuHg and DSN-assisted amplification

AuNTs@AuHg alloy with well-regulated SERS properties was proposed, which displayed wonderful SERS intensity and effective salt resistance. Using miRNA-21 as a model analyte and combining with DSN-assisted amplification, a dual signal amplification strategy for ultrasensitive miRNA biosensing with a low detection limit (0.53 fM) and satisfactory selectivity was designed.

Design and Application of Electrochemical Sensors with Metal-Organic Frameworks as the Electrode Materials or Signal Tags

Metal-organic frameworks (MOFs) with fascinating chemical and physical properties have attracted immense interest from researchers regarding the construction of electrochemical sensors. In this work, we review the most recent advancements of MOF-based electrochemical sensors for the detection of electroactive small molecules and biological macromolecules (e.g., DNA, proteins, and enzymes). The types and functions of MOF-based nanomaterials in terms of the design of electrochemical sensors are also discussed. Furthermore, the limitations and challenges of MOF-based electrochemical sensing devices are explored. This work should be invaluable for the development of MOF-based advanced sensing platforms.

The Chemistry and Applications of Metal-Organic Frameworks (MOFs) as Industrial Enzyme Immobilization Systems

Enzymatic biocatalysis is a sustainable technology. Enzymes are versatile and highly efficient biocatalysts, and have been widely employed due to their biodegradable nature. However, because the three-dimensional structure of these enzymes is predominantly maintained by weaker non-covalent interactions, external conditions, such as temperature and pH variations, as well as the presence of chemical compounds, can modify or even neutralize their biological activity. The enablement of this category of processes is the result of the several advances in the areas of molecular biology and biotechnology achieved over the past two decades. In this scenario, metal–organic frameworks (MOFs) are highlighted as efficient supports for enzyme immobilization. They can be used to ‘house’ a specific enzyme, providing it with protection from environmental influences. This review discusses MOFs as structures; emphasizes their synthesis strategies, properties, and applications; explores the existing methods of using immobilization processes of various enzymes; and lists their possible chemical modifications and combinations with other compounds to formulate the ideal supports for a given application.

Target amplification-free detection of urinary microRNA for diabetic nephropathy diagnosis with electrocatalytic reaction

Diabetic nephropathy (DN) is a serious diabetic complication, usually developed from type II diabetes mellitus (T2DM) and known as type II DN (T2DN). New emerging biomarkers for T2DN are microRNAs (miRNAs) which have been studied for the noninvasive early-stage detection of the disease. In this work, a nucleic acid amplification-free miRNA-124 sensor based on target-induced strand displacement on magnetic beads, and by using methylene blue-loaded silica particles as a label was developed. Measurement methods can be either visual observation, spectrophotometry, or electrochemistry. After incubation and separation of the magnetic particles, a blue-violet solution (564 nm) appeared, depending on the concentration of miRNA displaced. For electrochemical detection, methylene blue on the silica served as a redox mediator for the coupled reaction with ferricyanide in the solution phase. At the electrode surface, ferricyanide was re-reduced to ferrocyanide, and was thus available for further reaction with methylene blue, forming an amplification cycle. After optimization, the total assay time was 60 min, and limits of detection were 1 pM, 6 fM, and 0.65 fM, by the naked eye, spectrophotometry and electrochemistry, respectively. The miRNAs in 42 suspected urine samples from patients suffering from either diabetic nephropathy, diabetes mellitus, or chronic kidney disease were validated by comparing with the droplet digital polymerase chain reaction (ddPCR).

Rapid 16S rDNA electrochemical sensor for detection of bacteria based on the integration of target-triggered hairpin self-assembly and tripedal DNA walker amplification

Diseases caused by bacteria pose great challenges to human health. The key to reduce disease transmission and mortality is to develop accurate and rapid methods for the detection and identification of bacteria. Herein, a rapid bacteria 16S rDNA electrochemical sensor based on target-triggered hairpin self-assembly and tripedal DNA walker (TD walker) amplification strategy was constructed. Specific variable region of 16S rDNA fragment of bacteria was used as biomarker. The target-triggered hairpin self-assembly strategy was used to prepare a TD walker. The hairpin DNA probes labeled with ferrocene (F_c) were designed and modified on surface of electrode. The "legs" of TD walker hybridized with three hairpin probes and opened their hairpin structures. Exo III enzyme recognised hybrid duplexes and selectively digest hairpin probes. The "legs" of TD walker was released and hybridized with the other three hairpin probes. In this way, the enzyme drived the walkers to walk along electrode interface, until hairpin DNA probes were all removed from the electrode, the F_c was far away from electrode interface. A significantly current reduction signal was obtained and bacteria were detected by recording this response. This strategy was low-cost and scalable, it could continuously recycle low-concentration targets, thus enhanced the detection sensitivity. As the proof-of-concept work, the electrochemical sensor was utilized as detector. The limit of detection (LOD) of detecting Staphylococcus aureus (S. aureus) was 20 CFU mL^-1 and detection time was less than 3 h. It was expected to be widely used in clinical early diagnosis.

A porphyrin-triazatruxene dyad for ratiometric two-photon fluorescent sensing of intracellular viscosity

By combining an electron-rich triazatruxene unit (TAT) to an electron-deficient zinc porphyrin fluorophore (ZnPor) via an ethynyl bridge, a new two-photon fluorescent viscosity rotor (TAT-ZnPor) with typical donor-π-acceptor (D-π-A) electronic...