A Sensitive Electrochemical Aptasensor for Thrombin Detection Based on Electroactive Co-Based Metal-Organic Frameworks with Target-Triggering NESA Strategy

Quenching study of Cu2S-MPA/NGODs composites in electrochemiluminescence detection by modulating resonance energy transfer and adsorption process

This study explores the principles of resonance energy transfer and adsorption modulation using composites of Cu2S-MPA/NGODs. These composites can efficiently control the quenching process of electrochemiluminescence (ECL). Mercaptopropionic acid (MPA) was added during the synthesis of Cu2S-MPA to enhance its attachment to nitrogen-doped graphene quantum dots (NGODs). The UV absorption peaks of NGODs coincided with the emission peaks of luminol ECL, enabling resonance energy transfer and enhancing the quenching capability of Cu2S-MPA. Meanwhile, there is another quenching strategy. When the readily reducible Cu+ ions underwent partial reduction to Cu when they were bound to NGODs. This weakened the electrocatalytic effect on reactive oxygen species (ROS) and had a detrimental impact on electron transfer. Under optimal conditions, the immunosensor ECL intensity decreased linearly with the logarithm of carcinoembryonic antigen (CEA) concentration in the range of 0.00001-40 ng/mL, with a detection limit of 0.269 fg/mL. The sensor was effectively utilized for the identification of CEA in actual serum samples.

Electrochemical Sensor for the Detection and Accurate Early Diagnosis of Ovarian Cancer

Ovarian cancer (OC) has the highest mortality rate among malignant tumors, primarily because it is difficult to diagnose early. Exosomes, a type of extracellular vesicle rich in parental information, have garnered significant attention in the field of cancer diagnosis and treatment. They play an important regulatory role in the occurrence, development, and metastasis of OC. Consequently, exosomes have emerged as noninvasive biomarkers for early cancer detection. Therefore, identifying cancer-derived exosomes may offer a novel biomarker for the early detection of OC. In this study, we developed a metal-organic frameworks assembled "double hook"-type aptamer electrochemical sensor, which enables accurate early diagnosis of OC. Under optimal experimental conditions, electrochemical impedance spectroscopy technology demonstrated a good linear relationship within the concentration range of 31-3.1 × 106 particles per microliter, with a detection limit as low as 12 particles per microliter. The universal exosome detection platform is constructed, and this platform can not only differentiate between high-grade serous ovarian cancer (HGSOC) patients and healthy individuals but also distinguish between HGSOC patients and nonhigh-grade serous OC (non-HGSOC). Consequently, it provides a novel strategy for the early diagnosis of OC and holds great significance in clinical differential diagnosis.

Label-free electrochemical sensing platform for sensitive detection of ampicillin by combining nucleic acid isothermal enzyme-free amplification circuits with CRISPR/Cas12a

The residue of ampicillin (AMP) in food and ecological environment poses a potential harm to human health. Therefore, a reliable system for detecting AMP is in great demand. Herein, a label-free and sensitive electrochemical sensor utilizing NH2-Co-MOF as an electrocatalytic active material for methylene blue (MB) was developed for rapid and facile AMP detection by combining hybridization chain reaction (HCR), catalytic hairpin assembly (CHA) with CRISPR/Cas12a. The surface of glassy carbon electrode modified with NH2-Co-MOF was able to undergo HCR independent of the AMP, forming long dsDNA complexes to load MB, resulting in strong original electrochemical signal. The presence of AMP could trigger upstream CHA circuit to activate the CRISPR/Cas12a system, thereby achieving rapid non-specific cleavage of the trigger ssDNA of HCR on the electrode surface, hindering the occurrence of HCR and reducing the load of MB. Significant signal change triggered by the target was ultimately obtained, thus achieving sensitive detection of the AMP with a detection limit as low as 1.60 pM (S/N = 3). The proposed sensor exhibited good stability, selectivity, and stability, and achieved reliable detection of AMP in milk and livestock wastewater samples, demonstrating its promising application prospects in food safety and environmental monitoring.

A sensitive and versatile electrochemical sensor based on hybridization chain reaction and CRISPR/Cas12a system for antibiotic detection

A sensitive electrochemical platform was constructed with NH2-Cu-MOF as electrochemical probe to detect antibiotics using CRISPR/Cas12a system triggered by hybridization chain reaction (HCR). The sensing system consists of two HCR systems. HCR1 occurred on the electrode surface independent of the target, generating long dsDNA to connect signal probes and producing a strong electrochemical signal. HCR2 was triggered by target, and the resulting dsDNA products activated the CRISPR/Cas12a, thereby resulting in effective and rapid cleavage of the trigger of HCR1, hindering the occurrence of HCR1, and reducing the number of NH2-Cu-MOF on the electrode surface. Eventually, significant signal change depended on the target was obtained. On this basis and with the help of the programmability of DNA, kanamycin and ampicillin were sensitively detected with detection limits of 60 fM and 10 fM (S/N = 3), respectively. Furthermore, the sensing platform showed good detection performance in milk and livestock wastewater samples, demonstrating its great application prospects in the detection of antibiotics in food and environmental water samples.

Highly efficient photoelectrochemical aptasensor based on CdS/CdTe QDs co-sensitized TiO2 nanoparticles designed for thrombin detection

A photoelectrochemical (PEC) sensor for the sensitive detection of thrombin (TB) was established. Co-sensitized combination of TiO2 nanoparticles combined with modified cadmium sulfide and cadmium telluride quantum dots (CdS/CdTe QDs) was utilized as a photoactive material. Successful growth of CdS/CdTe quantum dots on mesoporous TiO2 films occured by successive ion-layer adsorption and reaction. This interesting formation of co-sensitive structure is conducive to enhancing the photocurrent response by improving the use rate of light energy. Additionally, the step-level structure of CdS/CdTe QDs and TiO2 NPs shows a wide range of visible light absorption, facilitating the dissociation of excitons into free electrons and holes. Consequently, the photoelectric response of the PEC analysis platform is significantly enhanced. This constructed PEC aptasensor shows good detection of thrombin with a low detection limit (0.033 pM) and a wide linear range (0.0001-100 nM) in diluted actual human serum samples. In addition, this PEC aptasensor also has the characteristics of good stability and good reproducibility, which provides a novel insight for the quantitative measurement of other similar analytes.

Highly sensitive fluorescence detection of tobacco mosaic virus RNA based on polysaccharide and ARGET ATRP double signal amplification

Plant diseases caused by tobacco mosaic viruses (TMV) reduce the yield and quality of crops and cause significant losses. Early detection and prevention of TMV has important value of research and reality. Herein, a fluorescent biosensor was constructed for highly sensitive detection of TMV RNA (tRNA) based on the principle of base complementary pairing, polysaccharides and atom transfer radical polymerization by electron transfer activated regeneration catalysts (ARGET ATRP) as double signal amplification strategy. The 5'-end sulfhydrylated hairpin capture probe (hDNA) was first immobilized on amino magnetic beads (MBs) by a cross-linking agent, which specifically recognizes tRNA. Then, chitosan binds to BIBB, providing numerous active sites for fluorescent monomer polymerization, which successfully significantly amplifying the fluorescent signal. Under optimal experimental conditions, the proposed fluorescent biosensor for the detection of tRNA has a wide detection range from 0.1 pM to 10 nM (R² = 0.998) with a limit of detection (LOD) as low as 1.14 fM. In addition, the fluorescent biosensor showed satisfactory applicability for the qualitative and quantitative analysis of tRNA in real samples, thereby demonstrating the potential in the field of viral RNA detection.

Electrochemical sensor for ultrasensitive detection of paraquat based on metal-organic frameworks and para-sulfonatocalix[4]arene-AuNPs composite

The widespread occurrence of pesticides in surface water, groundwater, soil, and food has received increasing attention towards environmental safety. Paraquat (PQ) is world widely used as a rapid sterilant herbicide and is highly toxic to humans. A simple, rapid, sensitive, and on-site detection method for the water environment to detection of PQ is urgently required. Here, we prepared a zeolite imidazole skeleton-8 (ZIF-8) and para-sulfonylcalix[4]arene (pSC₄) coated gold nanoparticles composite (pSC₄-AuNPs@ZIF-8) by one-step method. An electrochemical biosensor assay for PQ was established based on pSC₄-AuNPs@ZIF-8 modified glassy carbon electrode through host-guest recognition of PQ and pSC₄. Under the optimal conditions, recoveries of targets determination results were 92.7%-103% (n = 3), respectively. The quantity PQ detection limit was found to be 0.49 pM. Therefore, the signal amplification strategy based on pSC₄-AuNPs@ZIF-8 has potential value in detecting trace pollutants in the water environment.

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.

Thrombin Determination Using Graphene Oxide Sensors with Co-Assisted Amplification

Graphene oxide (GO) is widely used in sensors. The detection of proteins based on bare GO has been developed; however, the detection sensitivity needs to be improved. In this paper, a novel GO-DNA sensor for thrombin detection was developed using an aptamer linked to the surface of GO. Polyethylene glycol (PEG) was further used to prevent thrombin from nonspecific adsorption and to improve the sensitivity of the sensor for detection of thrombin. In order to improve the limit of detection for thrombin, we developed a GO and RecJf exonuclease co-assisted signal amplification strategy, and a detection limit of 24.35 fM for thrombin was achieved using this strategy. The results show that it is a promising method in analytical applications.

Recent Progress on Highly Selective and Sensitive Electrochemical Aptamer-based Sensors

Highly selective, sensitive, and stable biosensors are essential for the molecular level understanding of many physiological activities and diseases. Electrochemical aptamer-based (E-AB) sensor is an appealing platform for measurement in biological system, attributing to the combined advantages of high selectivity of the aptamer and high sensitivity of electrochemical analysis. This review summarizes the latest development of E-AB sensors, focuses on the modification strategies used in the fabrication of sensors and the sensing strategies for analytes of different sizes in biological system, and then looks forward to the challenges and prospects of the future development of electrochemical aptamer-based sensors.

Persulfate activation by ferrocene-based metal-organic framework microspheres for efficient oxidation of orange acid 7

Ferrocene-based metal-organic framework with different transition metals (M-Fc-MOFs, M = Fe, Mn, Co) was synthesized by a simple hydrothermal method and used as a heterogeneous catalyst for persulfate activation. The samples were characterized by X-ray diffraction, transmission electron microscopy, X-ray electron spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. Meanwhile, the influences of factors such as catalyst dosage, persulfate concentration, and pH on the degradation of acid orange 7 (AO7) were studied in detail. The results showed that hollow cobalt-based ferrocenyl metal-organic framework microspheres (Co-Fc-MOFs) exhibited the best catalytic performance, which is closely related to the synergy of Fc/Fc⁺ and Co(II)/Co(III) cycles in persulfate activation. Free radical quenching studies indicated that both sulfate and hydroxyl appeared to contribute to the degradation of AO7.

MicroRNA-21 electrochemiluminescence biosensor based on Co-MOF-N-(4-aminobutyl)-N-ethylisoluminol/Ti3C2Tx composite and duplex-specific nuclease-assisted signal amplification

A novel electrochemiluminescence (ECL) biosensor for the determination of microRNA-21 (miRNA-21) was developed, based on a hybrid luminescent Co-MOF-ABEI/Ti₃C₂T_x composite as an ECL luminophore combined with a duplex-specific nuclease (DSN)-assisted signal amplification strategy. The synthesized Co-MOF-ABEI/Ti₃C₂T_x composite carrying N-(4-aminobutyl)-N-ethylisoluminol (ABEI) exhibited strong and stable ECL in the presence of reactive oxygen species (ROS). The ECL biosensor was fabricated by adsorbing Co-MOF-ABEI/Ti₃C₂T_x onto a glassy carbon electrode and covalently coupling the probe DNA onto the surface of the Co-MOF-ABEI/Ti₃C₂T_x-modified electrode. In the presence of the target miRNA-21, the DSN selectively cleaved the complementary DNA section (S1) to miRNA-21, resulting in the release of the transduction section (S2) and the reuse of miRNA-21 in the subsequent amplification cycle. The interaction of the stem-loop structure of the probe DNA with the Co-MOF-ABEI/Ti₃C₂T_x-modified glassy carbon electrode with S2 strands led to the opening of the annular part of the probe DNA. Then, the opened guanine (G)-rich sequences of probe DNA were exposed and folded into a hemin/G-quadruplex DNAzyme in the presence of hemin. The catalysis of H₂O₂ to ROS by the hemin/G-quadruplex DNAzyme significantly enhanced ECL intensity, and this intensity was logarithmically proportional to the concentration of target miRNA-21 between 0.00001 and 10 nM, having a limit of detection of 3.7 fM. The designed ECL biosensor can  detect miRNA-21 extracted from HeLa cells, indicating its promising application in clinical diagnosis and disease prognosis analysis.

T7 Endonuclease I-mediated voltammetric detection of KRAS mutation coupled with horseradish peroxidase for signal amplification

Rapid and selective sensing of KRAS gene mutation which plays a crucial role in the development of colorectal, pancreatic, and lung cancers is of great significance in the early diagnosis of cancers. In the current study, we developed a simple electrochemical biosensor by differential pulse voltammetry technique for the specific detection of KRAS mutation that uses the mismatch-specific cleavage activity of T7-Endonuclease I (T7EI) coupled with horseradish peroxidase (HRP) to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) substrate in the presence of hydrogen peroxide (H₂O₂). In addition, we synthesized the nanocomposite composed of multi-walled carbon nanotube/chitosan-ionic liquid/gold nanoparticles (MWCNT/Chit-IL/AuNPs) on screen-printed carbon electrode surface to increase the electrode surface area and electrochemical signal. In principle, T7E1 enzyme recognized and cleaved the mismatched site formed by the presence of KRAS gene mutation, removing 5'-biotin of capture probes and subsequently reducing the differential pulse voltammetry signal compared to wild-type KRAS gene. With this proposed strategy, a limit of detection of 11.89 fM was achieved with a broad linear relationship from 100 fM to 1 µM and discriminated 0.1% of mutant genes from the wild-type target genes. This confirms that the developed biosensor is a potential platform for the detection of mutations in early disease diagnosis.

MOF-derivated MnO@C nanocomposite with bidirectional electrocatalytic ability as signal amplification for dual-signal electrochemical sensing of cancer biomarker

Dual-signal strategy has great potential in improving the accuracy and sensitivity of cancer biomarker determination. However, most sensors based on nanomaterials as signal amplification usually output single detectable signal. It is still a challenge to achieve dual-signal sensing of biomarkers with nanomaterials as signal amplification. Herein, MnO@C nanocomposite was prepared with Mn-MOF-74 as precursor by pyrolysis. It possesses bidirectional electrocatalytic ability toward both oxidation and reduction of H₂O₂ for its fully exposed crystal facets. After loading AuNPs, MnO@C@AuNPs can connect aptamer (Apt) via Au-S and then as a signal amplification for the construction of sandwich-type aptasensor for dual-signal electrochemical sensing of cancer biomarker. Thus, taking mucin 1 (MUC1) as a model system. The aptasensor has the parallel output of differential pulse voltammetry (DPV) and chronoamperometry responses based on oxidation and reduction of H₂O₂, respectively, which implemented sensitive and accurate measurements to avoid false results. The linear response ranges of 0.001 nM-100 nM (detection limit of 0.31 pM) for DPV technique and 0.001 nM-10 nM (detection limit of 0.25 pM) for chronoamperometry technique were obtained. It opens up a new way to design elegant dual-signal aptasensors with potential applications in early disease diagnosis.

Metal-organic frameworks based hybrid nanocomposites as state-of-the-art analytical tools for electrochemical sensing applications

Metal-organic frameworks (MOFs) are remarkably porous materials that have sparked a lot of interest in recent years because of their fascinating architectures and variety of potential applications. This paper systematically summarizes recent breakthroughs in MOFs and their derivatives with different materials such as, carbon nanotubes, graphene oxides, carbon fibers, enzymes, antibodies and aptamers etc. for enhanced electrochemical sensing applications. Furthermore, an overview part is highlighted, which provides some insights into the future prospects and directions of MOFs and their derivatives in electrochemical sensing, with the goal of overcoming present limitations by pursuing more inventive ways. This overview can perhaps provide some creative ideas for future research on MOF-based materials in this rapidly expanding field.

Femtomolar Detection of Thrombin in Serum and Cerebrospinal Fluid via Direct Electrocatalysis of Oxygen Reduction by the Covalent G4-Hemin-Aptamer Complex

Thrombin, a serine protease playing a central role in the coagulation cascade reactions and a potent neurotoxin produced by injured brain endothelial cells, is a recognized cardiac biomarker and a critical biomarker for Alzheimer's disease development. Both in vivo and in vitro, its low physiological concentrations and nonspecific binding of other components of physiological fluids complicate electroanalysis of thrombin. Here, femtomolar levels of thrombin in serum and an artificial cerebrospinal fluid (CSF) were detected by the indicator-free electrochemical methodology exploiting the O₂ reduction reaction directly, with no electron transfer mediators, electrocatalyzed by the covalent G4-hemin DNAzyme complex naturally self-assembling upon thrombin binding to the hemin-modified 29-mer DNA aptamer sequence tethered to gold via an alkanethiol linker. Coadsorbed PEG inhibited nonspecific protein binding and allowed the sought signal resolution. The proposed assay exploiting the "oxidase" activity of G4-hemin DNAzyme does not require any coreactants necessary for the traditional peroxidase activity-based assays with this DNAzyme, such as H₂O₂ and redox mediators, or solution deaeration and allows fast, overall 30 min analysis of thrombin in aerated buffer, CSF, and 1% human serum solutions. This pioneer approach exploiting the oxidase activity G4-hemin DNAzyme is simple, sensitive, and selective and represents a new tool for ultrasensitive electrocatalytic assays based on simple and efficient O₂-dependent DNAzyme labels.

Catalytic hairpin assembly-based electrochemical biosensor with tandem signal amplification for sensitive microRNA assay

We demonstrate for the first time the construction of a low background electrochemical biosensor with tandem signal amplification for sensitive microRNA assay based on target-activated catalytic hairpin assembly (CHA) of heteroduplex-templated copper nanoparticles. This electrochemical biosensor exhibits high sensitivity, good specificity, single-base mismatch discrimination capability, excellent stability and reproducibility, and it can sensitively detect microRNA in cancer cells.

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.

Proximity hybridization triggered hybridization chain reaction for label-free electrochemical homogeneous aptasensors

A label-free homogeneous electrochemical aptasensor was developed for detection of thrombin based on proximity hybridization triggered hybridization chain reaction induced G-quadruplex formation. Thrombin promoted the formation of a complex via the proximity hybridization of the aptamer DNA strands, which unfolded the molecular beacon, the stem part of molecular beacon as a primer to initiate the hybridization chain reaction process. Thus, with the electrochemical indicator hemin selectively intercalated into the multiple G-quadruplexes, a significant electrochemical signal drop is observed, which is dependent on the concentration of the target thrombin. Thus, using this"signal-off" mode, label-free homogeneous electrochemical strategy for sensitive thrombin assay with a detection limit of 44 fM is realized. Furthermore, this method also exhibits additional advantages of simplicity and low cost, since both expensive labeling and sophisticated probe immobilization processes are avoided. Its high sensitivity, acceptable accuracy, and satisfactory versatility of analytes led to various applications in bioanalysis.

Cytochrome c-multiwalled carbon nanotube and cobalt metal organic framework/gold nanoparticle immobilized electrochemical biosensor for nitrite detection

Based on cytochrome c-multiwalled carbon nanotubes (Cyt c-MWCNTs) and cobalt metal–organic frameworks/gold nanoparticles (Co-MOFs/AuNPs), an electrochemical biosensor was proposed for the detection of nitrite. Herein, Co-MOFs and AuNPs were immobilized on gold electrodes via surface layer assembly. Their advantages including large surface area and high conductivity provided an excellent platform for the immobilization of Cyt c-MWCNTs. Cyt c-MWCNTs were prepared via electrostatic adsorption and possessed good biocompatibility and superior electrocatalytic activity towards nitrite. Notably, MWCNTs and AuNPs could provide a good microenvironment for the electron transfer of Cyt c, which further significantly promoted the dispersion of MWCNTs. All of the above features led to outstanding electrochemical performance and achieved signal amplification for nitrite detection. Therefore, the biosensor displayed a linear range from 0.005 μmol L⁻¹ to 1000 μmol L⁻¹ with a detection limit of 0.0044 μmol L⁻¹ for nitrite detection. In addition, the designed biosensor exhibited excellent selectivity and could be applied in real samples.

Based on cytochrome c-multiwalled carbon nanotubes (Cyt c-MWCNTs) and cobalt metal organic frameworks/gold nanoparticles (Co-MOFs/AuNPs), an electrochemical biosensor was proposed for the detection of nitrite.

Electrochemical Aptasensors Based on Hybrid Metal-Organic Frameworks

Metal-organic frameworks (MOFs) offer a unique variety of properties and morphology of the structure that make it possible to extend the performance of existing and design new electrochemical biosensors. High porosity, variable size and morphology, compatibility with common components of electrochemical sensors, and easy combination with bioreceptors make MOFs very attractive for application in the assembly of electrochemical aptasensors. In this review, the progress in the synthesis and application of the MOFs in electrochemical aptasensors are considered with an emphasis on the role of the MOF materials in aptamer immobilization and signal generation. The literature information of the use of MOFs in electrochemical aptasensors is classified in accordance with the nature and role of MOFs and a signal mode. In conclusion, future trends in the application of MOFs in electrochemical aptasensors are briefly discussed.

Efficient immobilization of aptamers on the layered double hydroxide nanohybrids for the electrochemical proteins detection

Despite the use of layered double hydroxides (LDH) in different electrochemical (bio)sensors, the construction of aptasensors using LDH-based surfaces was not reported to the best of our knowledge. This may be due to the lack of a suitable linker to attach aptamers to the LDH-modified surface. LDH-based aptasensors are established here as very sensitive and reliable devices in serum and cerebrospinal fluid (CSF) analysis. 5'-NH₂ DNA aptamer probes were immobilized on the LDH-based surfaces in a vertical conformation without any linker materials. Due to the low electron conductivity of the LDH, carbon nanotubes (CNT) with high electronic conductivity and high surface area were combined with LDH. Thrombin was used as a model protein for aptasensing. The sensor shows a linear range of 0.005-12,000 pmol L^-1 and a limit of detection of 0.1 fmol L^-1. Moreover, the aptasensor was used for the sensing of thrombin in CSF and serum samples acquired from both healthy and patients with different disease.

Highly sensitive host-guest mode homogenous electrochemical thrombin signal amplification aptasensor based on tetraferrocene label

The development of sensitive and convenient detection methods to monitor thrombin without the use of enzymes or complex nanomaterials is highly desirable for the diagnosis of cardiovascular diseases. In this article, tetraferrocene was first synthesized and then a sensitive and homogeneous electrochemical aptasensor was developed for thrombin detection based on host-guest recognition between tetraferrocene and β-cyclodextrin (β-CD). In the absence of thrombin, the double stem-loop of thrombin aptamer (TBA) prevented tetraferrocenes labeled at both ends from entering the cavity of β-CD deposited on gold electrode surface. After binding with thrombin, the stem-loop structure of TBA opened and transformed into special G-quarter structure, forcing tetraferrocene into the cavity of β-CD. As a result, thrombin allowed eight ferrocene molecules to reach the gold electrode surface, greatly amplifying the response signal. The obtained aptasensors showed dynamic detection range from 4 pM to 12.5 nM with detection limit around 1.2 pM. Overall, the results indicate that the proposed aptasensors are promising for future rapid clinical detection of thrombin and development of signal amplification strategies for detection of various proteins.

Hemin-Bridged MOF Interface with Double Amplification of G-Quadruplex Payload and DNAzyme Catalysis: Ultrasensitive Lasting Chemiluminescence MicroRNA Imaging

Here, we report a double-amplified sensing platform for ultrasensitive chemiluminescence (CL) miRNA detection in real patients' blood in which a hemin-bridged metal-organic framework (MOF) is employed as a functional interface to boost the payload and catalysis of G-quadruplex (G4) DNAzymes. Hemin is here used as the organic ligand for the MOF synthesis, which endows the MOF with an intrinsic peroxidase-like catalytic activity. Most importantly, the MOF surface provides a large amount of binding sites for polymeric G4 DNAzymes that are produced by miRNA-triggered rolling circle amplification reactions, and meanwhile, the interfaced G4 DNAzymes on MOFs (G4/MOFzymes) display an about 100-fold higher catalytic activity than those in solution. By using the G4/MOFzyme catalysts in the luminol/H₂O₂ CL system, the amplification detection of two acute myocardial infarction (AMI)-related miRNAs (low to 1 fM seen with naked eyes) is achieved in human serum with a smartphone as a portable imaging detector, which provides a facile methodology for point-of-care (POC) diagnosis of AMI. Compared with previous smartphone-based counterparts not requiring sophisticated equipment, this new facile methodology shows both 6 orders of magnitude higher sensitivity and an ∼50-fold longer duration for CL miRNA imaging. These unique features allow our developed G4/MOFzymes to be further employed as a novel luminescent ink for printing commonly used patterns.

Ultra-Stable UiO-66 Involved Molecularly Imprinted Polymers for Specific and Sensitive Determination of Tyramine Based on Quartz Crystal Microbalance Technology

A rapid method was developed to determine the content of tyramine in food on the basis of the combination of molecular imprinting technique and the metal-organic frameworks. We developed the new molecular imprinted polymers based on metal-organic frameworks UiO-66 (named UiO-66@MIPs) as the sensing recognition element, the non-molecular imprinted polymers based on UiO-66 (named UiO-66@NIPs) was synthesized according the same steps without tyramine for comparison. The characterization of obtained UiO-66@MIPs was investigated through a series of characterization experiments. The results indicated that the octahedral shaped UiO-66 was encapsulated in the sol-gel polymer film, with a desirable thermal stability and possessed a specific surface area (SSA) of 994.3 m²·g⁻¹. The imprinting factor of the UiO-66@MIPs for tyramine was 1.956 in static experiment. This indicates the synthesized UiO-66@MIPs have outstanding performance compered to UiO-66@NIPs on the static adsorption quantity and selective adsorption affinity. It’s to make use of advantages of the synthetic materials to develop a quartz crystal microbalance (QCM) sensor for the sensitive detection of tyramine. The detection limit of the system was 61.65 μg·L⁻¹ within measurable concentration range from 80 to 500 μg·L⁻¹. The prepared QCM sensor was verified in selectivity and application. The UiO-66@MIPs possess good behavior on selectivity, absorptivity, and chemical stability, so the UiO-66@MIPs achieve accurate and rapid trace detection of biogenic amines in food combining with the quartz crystal microbalance.

Nanoscale metal-organic frameworks in detecting cancer biomarkers

Following the efficient performance of metal-organic frameworks (MOFs) as recognition elements in gas sensors, biosensors based on MOFs are now being investigated to capture and quantify potential cancer biomarkers, such as circulating tumor cells (CTCs), nucleic acids and proteins. The current status of MOF-based biosensors in the detection of early stages of cancer is in its infancy, although it has significantly emerged since the beginning of this decade. That said, salient research has been conducted in the past five years to utilize the distinctive porous crystalline structure of MOFs for highly sensitive and selective detection of cancer biomarkers. In this pursual, MOFs designed with bimetallic assembly, doped with magnetic nanoparticles, coated with polymers, and even conjugated with peptides or oligonucleotides have shown promising outcomes in detecting CTCs, nucleic acids and proteins. In particular, aptamer-conjugated MOFs are able to perform at a lower limit of detection down to the femtomolar, implying their efficacy for the point of care testing in clinical trials. In this way, aptasensors based on aptamer-conjugated MOFs present a newer sub-branch, to be coined as a MOFTA sensor in the current review. Considering the emerging progress and promising outcomes of MOFTA sensors as well as a variety of MOF-based techniques of detecting cancer biomarkers, this review will highlight their significant advances and related aspects in the recent five years on the context of detecting CTCs, nucleic acids and proteins for the early-stage detection of cancer.

Measurement of aflatoxin M1 in powder and pasteurized milk samples by using a label-free electrochemical aptasensor based on platinum nanoparticles loaded on Fe-based metal-organic frameworks

In the present study, a sensitive label-free electrochemical aptasensor is introduced to measure aflatoxin M1 (AFM1) by using platinum nanoparticles (PtNPs) decorated on a glassy carbon electrode (GCE) modified with Fe-based metal-organic frameworks, MIL-101(Fe). The MIL-101(Fe) and the PtNP/MIL-101(Fe) are synthesized and characterized by Fourier transform infrared spectroscopy, X-ray diffraction, UV-Visible spectroscopy, and field-emission scanning electron microscopy. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) are done to monitor the fabrication processes of the aptasensor. In optimum conditions, the linear calibration range of 1.0 × 10^-2 to 80.0 ng mL^-1 and the detection limit of 2.0 × 10^-3 ng mL^-1 are obtained to measure AFM1 concentration using the EIS method. Finally, the fabricated aptasensor is successfully applied to measure AFM1 concentration in powder and pasteurized milk samples.

Catalytic hairpin assembly-based double-end DNAzyme cascade-feedback amplification for sensitive fluorescence detection of HIV-1 DNA

In this work, a simple all-nucleic acid cascade-feedback amplification strategy for homogeneous and protein enzyme-free fluorescence detection of HIV-1 related DNA (HIV-1 DNA) has been proposed by integrating catalytic hairpin assembly (CHA) circuit with double-end Mg²⁺-dependent DNAzyme autocatalytic feedback amplification. Here, the active double-end DNAzyme assemblies were derived from target-catalyzed CHA circuit, which further circularly cleaved the ribonucleotide-containing quenched fluorogenic hairpin substrates to generate distinctly amplified fluorescence signal. Meanwhile, the released quencher-labeled fragments as target DNA analogues were also able to autocatalyze CHA-DNAzyme reaction process, thus improving the determination sensitivity of HIV-1 DNA. The result demonstrated that the fluorescence intensity increment of double-end DNAzyme was over 3 times higher than that of single-end DNAzyme. The sensing method displayed a good linear range from 1 pM to 2 nM with a detectable minimum concentration of 1 pM and high specificity towards different mismatched target DNAs. Moreover, the practical application potential of the proposed method for target DNA detection in complex biological matrices was also assessed. Considering the appealing feature of programmable nucleic acids in CHA-DNAzyme sensing platform, the current strategy may provide a prospective design for detection of broad-spectrum nucleic acid biomarkers.

Electroactive metal-organic framework composites: Design and biosensing application

Metal-organic frameworks (MOFs) as molecular crystalline materials have been extensively applied in various fields such as catalysis, separation, and biomedical engineering. However, the applications of MOFs materials are limited in electrochemical biosensing due to the poor conductivity, less selectivity, and lack of modification sites. By incorporating the functionalized nanoparticles into MOF structures, MOF-based composites are endowed with high electronic conductivity and strong catalytic activity, which process the advantages over single-component MOFs. With a particular focus on the electrochemical applications of MOF composites, this review summarizes the comprehensive guidelines on design of electroactive MOF composites: dopant modification of electroactive ligands, in situ synthesis of nanoparticle@MOF composites and post-modification of MOF structure. The illustrative examples of electroactive MOF composites in the last five years are highlighted in electrochemical, electrochemiluminescent, and photoelectrochemical biosensing. The prospects and challenges for future work are also included. Understanding the structure-function relationship of electroactive MOF composites benefits the design of next-generation electrochemical biosensors.

Facile, Rapid, and Low-Cost Electrophoresis Titration of Thrombin by Aptamer-Linked Magnetic Nanoparticles and a Redox Boundary Chip

An aptamer-linked assay of a target biomarker (e.g., thrombin) is facing the challenges of long-term run, complex performance, and expensive instrument, unfitting clinical diagnosis in resource-limited areas. Herein, a facile chip electrophoresis titration (ET) model was proposed for rapid, portable, and low-cost assay of thrombin via aptamer-linked magnetic nanoparticles (MNPs), redox boundary (RB), and horseradish peroxidase (HRP). In the electrophoresis titration-redox boundary (ET-RB) model, thrombin was chosen as a model biomarker, which could be captured within 15 min by MNP-aptamer 1 and HRP-aptamer 2, forming a sandwich complex of (MNP-aptamer 1)-thrombin-(HRP-aptamer 2). After MNP separation and chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB) within 10 min, an ET-RB run could be completed within 5 min based on the reaction between a 3,3',5,5'-tetramethylbenzidine radical cation (TMB^•+) and l-ascorbic acid in the ET channel. The systemic experiments based on the ET-RB method revealed that the sandwich complex could be formed and the thrombin content could be assayed via an ET-RB chip, demonstrating the developed model and method. In particular, the ET-RB method had the evident merits of simplicity, rapidity (less than 30 min), and low cost as well as portability and visuality, in contrast to the currently used thrombin assay. In addition, the developed method had high selectivity, sensitivity (limit of detection of 0.04 nM), and stability (intraday: 3.26%, interday: 6.07%) as well as good recovery (urine: 97-102%, serum: 94-103%). The developed model and method have potential to the development of a point-of-care testing assay in resource-constrained conditions.

Quenched sandwich-type photoelectrochemical aptasensor for protein detection based on exciton energy transfer

This work proposes a quenched photoelectrochemical sensing method for highly selective and sensitive detection of protein via Energy Transfer (ET) effect between the AuNPs and CdS:Mn quantum dots. This detection was performed on a sandwich-type aptamer sensing interface. Chitosan modified CdS:Mn/TiO₂/ITO electrode was used to immobilize capture DNA (S1) via -CONH- bond. In the presence of target protein, AuNPs labeled DNA (AuNPs-S2) was further bonded to the protein to fabricate sandwich sensing platform, which forced the AuNPs away from the electrode surface. In this state, the photocurrent was greatly depressed, mainly due to two factors: (a) the ET effect produced by interparticle distance between CdS:Mn and AuNPs; (b) the steric hindrance of AuNPs-S2 partly obstructs the diffusion of the electron donor. The photocurrent decreased with the increasing concentration of the target protein. Using thrombin as a target, this sensitized method showed a detectable range of 0.1 pM to 8 nM and a detection limit of 30 fM. It possessed high selectivity and good stability for detection of thrombin. This method is extremely flexible and can be extended to varieties of protein targets.

Substrate-free and label-free electrocatalysis-assisted biosensor for sensitive detection of microRNA in lung cancer cells

We develop a substrate-free and label-free electrocatalysis-assisted biosensor for sensitive detection of microRNA using the iron-embedded nitrogen-rich carbon nanotubes (FeCN) as the catalytic elements. This biosensor exhibits excellent selectivity and high sensitivity with a detection limit of 8.53 × 10^-16 M and a large dynamic range of 6 orders of magnitude. It can be further applied for accurate quantification of microRNA in lung cancer cells.