Target triggered cleavage effect of DNAzyme: Relying on Pd-Pt alloys functionalized Fe-MOFs for amplified detection of Pb2+

Simultaneous Hg2+ and Pb2+ detection in water samples using an electrochemical aptasensor with dual signal amplification by exonuclease III and metal-organic frameworks

Heavy metal pollution in the environment has become a significant global concern due to its detrimental effects on human health and the environment. In this study, we report an electrochemical aptasensor for the simultaneous detection of Hg2+ and Pb2+. Gold nanoflower/polyethyleneimine-reduced graphene oxide (AuNFs/PEI-rGO) was introduced on the surface of a gold electrode to improve sensing performance. The aptasensor is based on the formation of a T-Hg2+-T mismatch structure and specific cleavage of the Pb2+-dependent DNAzyme, resulting in a dual signal generated by the Exo III specific digestion of methylene blue (MB) labeled at the 3' end of probe DNA-1 and the reduction of the substrate ascorbic acid (AA) catalyzed by the signal label. The decrease of MB signal and the increase of AA oxidation peak was used to indicate the content of Hg2+ and Pb2+, respectively, with detection limits of 0.11 pM (Hg2+) and 0.093 pM (Pb2+). The aptasensor was also used for detecting Hg2+ and Pb2+ in water samples with good recoveries. Overall, this electrochemical aptasensor shows promising potential for sensitive and selective detection of heavy metals in environmental samples.

Metalation of metal-organic frameworks: fundamentals and applications

Metalation of metal-organic frameworks (MOFs) has been developed as a prominent strategy for materials functionalization for pore chemistry modulation and property optimization. By introducing exotic metal ions/complexes/nanoparticles onto/into the parent framework, many metallized MOFs have exhibited significantly improved performance in a wide range of applications. In this review, we focus on the research progress in the metalation of metal-organic frameworks during the last five years, spanning the design principles, synthetic strategies, and potential applications. Based on the crystal engineering principles, a minor change in the MOF composition through metalation would lead to leveraged variation of properties. This review starts from the general strategies established for the incorporation of metal species within MOFs, followed by the design principles to graft the desired functionality while maintaining the porosity of frameworks. Facile metalation has contributed a great number of bespoke materials with excellent performance, and we summarize their applications in gas adsorption and separation, heterogeneous catalysis, detection and sensing, and energy storage and conversion. The underlying mechanisms are also investigated by state-of-the-art techniques and analyzed for gaining insight into the structure-property relationships, which would in turn facilitate the further development of design principles. Finally, the current challenges and opportunities in MOF metalation have been discussed, and the promising future directions for customizing the next-generation advanced materials have been outlined as well.

Tunable metal-organic frameworks assist in catalyzing DNAzymes with amplification platforms for biomedical applications

Various binding modes of tunable metal organic frameworks (MOFs) and functional DNAzymes (Dzs) synergistically catalyze the emergence of abundant functional nanoplatforms. Given their serial variability in formation, structural designability, and functional controllability, Dzs@MOFs tend to be excellent building blocks for the precise "intelligent" manufacture of functional materials. To present a clear outline of this new field, this review systematically summarizes the progress of Dz integration into MOFs (MOFs@Dzs) through different methods, including various surface infiltration, pore encapsulation, covalent binding, and biomimetic mineralization methods. Atomic-level and time-resolved catalytic mechanisms for biosensing and imaging are made possible by the complex interplay of the distinct molecular structure of Dzs@MOF, conformational flexibility, and dynamic regulation of metal ions. Exploiting the precision of DNAzymes, MOFs@Dzs constructed a combined nanotherapy platform to guide intracellular drug synthesis, photodynamic therapy, catalytic therapy, and immunotherapy to enhance gene therapy in different ways, solving the problems of intracellular delivery inefficiency and insufficient supply of cofactors. MOFs@Dzs nanostructures have become excellent candidates for biosensing, bioimaging, amplification delivery, and targeted cancer gene therapy while emphasizing major advancements and seminal endeavors in the fields of biosensing (nucleic acid, protein, enzyme activity, small molecules, and cancer cells), biological imaging, and targeted cancer gene delivery and gene therapy. Overall, based on the results demonstrated to date, we discuss the challenges that the emerging MOFs@Dzs might encounter in practical future applications and briefly look forward to their bright prospects in other fields.

Application of Fe-MOFs in Photodegradation and Removal of Air and Water Pollutants: A Review

Photocatalytic technology has received increasing attention in recent years. A pivotal facet of photocatalytic technology lies in the development of photocatalysts. Porous metal-organic framework (MOF) materials, distinguished by their unique properties and structural characteristics, have emerged as a focal point of research in the field, finding widespread application in the photo-treatment and conversion of various substances. Fe-based MOFs have attained particular prominence. This review explores recent advances in the photocatalytic degradation of aqueous and gaseous substances. Furthermore, it delves into the interaction between the active sites of Fe-MOFs and pollutants, offering deeper insights into their mechanism of action. Fe-MOFs, as photocatalysts, predominantly facilitate pollutant removal through redox processes, interaction with acid sites, the formation of complexes with composite metal elements, binding to unsaturated metal ligands (CUSs), and hydrogen bonding to modulate their respiratory behavior. This review also highlights the focal points of future research, elucidating the challenges and opportunities that lie ahead in harnessing the characteristics and advantages of Fe-MOF composite catalysts. In essence, this review provides a comprehensive summary of research progress on Fe-MOF-based catalysts, aiming to serve as a guiding reference for other catalytic processes.

An ultrasensitive electrochemical immunosensor for carcinoembryonic antigen detection based on two-dimensional PtPd/Cu-TCPP(Fe) nanocomposites

Establishing an effective signal amplification strategy is the key to achieving sensitive detection of analytes by electrochemical immunoassay. In this work, a novel sandwich-type electrochemical immunosensor with dual-signal amplification was successfully constructed using PtPd/Cu-TCPP(Fe) as the sensing platform and mesoporous silicon dioxide as the signal amplifier. Firstly, two-dimensional wrinkled Cu-TCPP(Fe) nanomaterials loaded with PtPd nanoparticles have strong affinity for the immobilization of capture antibodies and can generate excellent electrochemical signals. Meanwhile, the mesoporous silicon dioxide with large steric hindrance was used as signal label to further improve the sensitivity of the immunosensor by increasing the difference of the current response signal. Under optimal experimental conditions, the electrochemical immunosensor exhibited a wide linear detection range from 0.1 pg/mL to 1.0 μg/mL, with a detection limit as low as 0.166 fg/mL. The experimental results showed that the constructed immunosensor has a great application prospect in clinical biomarker detection.

Ultrasenstive SERS biosensor based on Zn2+ from ZnO nanoparticle assisted DNA enzyme amplification for detection of miRNA

In this work, a simple and sensitive SERS biosensor was proposed for ultrasensitive detecting miRNA 122 based on ZnO nanoparticle amplification strategy and the full utilization of DNA chain. Firstly, ZnO@S1/S2 and CoFe₂O₄@S3 complexes can flock together with the assistance of target miRNA. Accompanied with the incremental amount of miRNA, the quantity of ZnO@S1/S2 would increase. Therefore, a significant amplification capability can be obtained by converting ZnO complexes into Zn²⁺ with the assistance of HCl. In this case, the DNA chain S2 can be obtained by the ZnO dissolving. In addition, through a clever design, the obtained Zn²⁺ can be further utilized to induce DNA enzyme cycle amplification to cleave S5 into DNA chain which was similar with DNA S2. This step greatly avoided the waste of DNA chains and improved the utilization efficiency of DNA chains. The S2 and abundant S2 analogues can complement with S4 on the Raman sensing interface to imbed lots of Raman probe DOX for obtaining strong Raman signal. By this way, with the increased number of miRNA, the S2 and abundant S2 analogues would increase, so the amount of DOX would increase to produce strong Raman signal to quantitatively detect target miRNA. As a result, this SERS biosensor based on Zn⁺ amplification and high utilization efficiency of DNA chain can obtain a low detection limit of 6.82 aM and wide linear range from 10 aM to 10 pM, which shown great potential in the clinical application and medical diagnosis.

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.

Applications of metal-organic framework-based bioelectrodes

Metal-organic frameworks (MOFs) are an emerging class of porous nanomaterials that have opened new research possibilities. The inherent characteristics of MOFs such as their large surface area, high porosity, tunable pore size, stability, facile synthetic strategies and catalytic nature have made them promising materials for enormous number of applications, including fuel storage, energy conversion, separation, and gas purification. Recently, their high potential as ideal platforms for biomolecule immobilization has been discovered. MOF-enzyme-based materials have attracted the attention of researchers from all fields with the expansion of MOFs development, paving way for the fabrication of bioelectrochemical devices with unique characteristics. MOFs-based bioelectrodes have steadily gained interest, wherein MOFs can be utilized for improved biomolecule immobilization, electrolyte membranes, fuel storage, biocatalysis and biosensing. Likewise, applications of MOFs in point-of-care diagnostics, including self-powered biosensors, are exponentially increasing. This paper reviews the current trends in the fabrication of MOFs-based bioelectrodes with emphasis on their applications in biosensors and biofuel cells.

Triple-Helix Molecular Switch Triggered Cleavage Effect of DNAzyme for Ultrasensitive Electrochemical Detection of Chloramphenicol

The abuse of chloramphenicol (CAP) in animal-derived products leads to serious food safety problems, so the sensitive and accurate determination of CAP residues has great noteworthiness for public health. Herein, we present a novel electrochemical aptasensor that incorporates a poly(diallyldimethylammonium chloride) functionalized graphene/Ag@Au nanosheets (PDDA-Gr/Ag@Au NSs) composite modified electrode and a DNAzyme signal amplification effect triggered by a triple-helix molecular switch (THMS) for detecting CAP. The PDDA-Gr/Ag@Au NSs composite has the advantages of high surface area, great conductivity, and dispersibility and has successfully improved the electrochemical performance of the electrode. Specific interaction with CAP will cause the signal transduction probe (STP) to be released from the THMS. After that, the DNAzyme will be activated with the help of Pb²⁺ and remove the immobilized signal probe on the electrode surface. The signal change was recorded by square wave voltammetry (SWV) and led to an accurate quantification of CAP. With all these features, the proposed sensing strategy yielded a satisfactory analytical performance with linearity between 1 pM and 1 μM and a limit of detection of 18.6 fM. Furthermore, the aptasensor shows excellent specificity for CAP in the presence of other antibiotics and resists interference with other common metal ions. Importantly, the performance is not diminished when the constructed aptasensor is applied to measuring CAP in milk powder. This THMS-based method is easy to design, and alteration to different targets can be achieved by simply replacing the aptamer sequence in the THMS. Therefore, this method shows significant prospects as a flexible platform for accurate monitoring of antibiotic residues in foodstuffs.

DNAzyme-based cascade signal amplification strategy for highly sensitive detection of lead ions in the environment

Lead ions are one of many common environmental pollutants, that can cause posing a serious threat to people's health, thus, their efficient and sensitive detection is important. We propose a cascade signal amplification sensor using a DNAzyme-based strand displacement amplification (SDA) and hybridization chain reaction (HCR) for the high-sensitivity detection of Pb²⁺. In the demonstrated sensor system, the target metal ion can activate DNAzyme to cause a strand displacement reaction. Under the synergistic action of polymerase and nickase, large numbers of DNA strands are generated that can initiate HCR. The subsequent HCR can restore the fluorescence intensity of the FAM quenched for the fluorescence resonance energy transfer effect, which exhibits a strong fluorescence signal. The DNAzyme-based sensor allowed the detection of Pb²⁺ down to 16.8 pM and resulted in a good dynamic line relationship ranging from 50 pM to 500 nM, and the biosensor also showed good selectivity. Furthermore, we confirmed that the proposed sensor can still detect lead ions in complex environments such as lake water, milk, and serum. We believe these findings will provide new ideas for the detection of toxic elements ions in the environment and food.

Direct Electrodeposition of Bimetallic Nanostructures on Co-Based MOFs for Electrochemical Sensing of Hydrogen Peroxide

Hydrogen peroxide (H₂O₂) is the most significant reactive oxygen species in biological systems. Here, we reported an electrochemical sensor for the detection of H₂O₂ on the basis of bimetallic gold-platinum nanoparticles (Au₃Pt₇ NPs) supported by Co-based metal organic frameworks (Co-MOFs). First, Au₃Pt₇ NPs, with optimal electrocatalytic activity and accessible active surface, can be deposited on the surface of the Co-MOF–modified glassy carbon electrodes (Au₃Pt₇/Co-MOFs/GCE) by one-step electrodeposition method. Then, the electrochemical results demonstrated that the two-dimensional (2D) Co-MOF nanosheets as the supporting material displayed better electrocatalytic properties than the 3D Co-MOF crystals for reduction of H₂O₂. The fabricated Au₃Pt₇/2D Co-MOF exhibited high electrocatalytic activity, and the catalytic current was linear with H₂O₂ concentration from 0.1 μM to 5 mM, and 5–60 mM with a low detection limit of 0.02 μM (S/N = 3). The remarkable electroanalytical performance of Au₃Pt₇/2D Co-MOF can be attributed to the synergistic effect of the high dispersion of the Au₃Pt₇ NPs with the marvelous electrochemical properties and the 2D Co-MOF with high-specific surface areas. Furthermore, this sensor has been utilized to detect H₂O₂ concentrations released from the human Hela cells. This work provides a new method for improving the performance of electrochemical sensors by choosing the proper support materials from diverse crystal morphology materials.

DNAzyme-Functionalized Nanomaterials: Recent Preparation, Current Applications, and Future Challenges

DNAzyme-nanomaterial bioconjugates are a popular hybrid and have received major attention for diverse biomedical applications, such as bioimaging, biosensor development, cancer therapy, and drug delivery. Therefore, significant efforts are made to develop different strategies for the preparation of inorganic and organic nanoparticles (NPs) with specific morphologies and properties. DNAzymes functionalized with metal-organic frameworks (MOFs), gold nanoparticles (AuNPs), graphene oxide (GO), and molybdenum disulfide (MoS₂ ) are introduced and summarized in detail in this review. Moreover, the focus is on representative examples of applications of DNAzyme-nanomaterials over recent years, especially in bioimaging, biosensing, phototherapy, and stimulation response delivery in living systems, with their several advantages and drawbacks. Finally, the perspective regarding the future directions of research addressing these challenges is also discussed and highlighted.

An overview of Structured Biosensors for Metal Ions Determination

The determination of metal ions is important for nutritional and toxicological assessment. Atomic spectrometric techniques are highly efficient for the determination of these species, but the high costs of acquisition and maintenance hinder the application of these techniques. Inexpensive alternatives for metallic element determination are based on dedicated biosensors. These devices mimic biological systems and convert biochemical processes into physical outputs and can be used for the sensitive and selective determination of chemical species such as cations. In this work, an overview of the proposed biosensors for metal ions determination was carried out considering the last 15 years of publications. Statistical data on the applications, response mechanisms, instrumentation designs, applications of nanomaterials, and multielement analysis are herein discussed.

Biosensing with DNAzymes

This article provides a comprehensive review of biosensing with DNAzymes, providing an overview of different sensing applications while highlighting major progress and seminal contributions to the field of portable biosensor devices and point-of-care diagnostics. Specifically, the field of functional nucleic acids is introduced, with a specific focus on DNAzymes. The incorporation of DNAzymes into bioassays is then described, followed by a detailed overview of recent advances in the development of in vivo sensing platforms and portable sensors incorporating DNAzymes for molecular recognition. Finally, a critical perspective on the field, and a summary of where DNAzyme-based devices may make the biggest impact are provided.

Exonuclease III-Driven Dual-Amplified Electrochemical Aptasensor Based on PDDA-Gr/PtPd@Ni-Co Hollow Nanoboxes for Chloramphenicol Detection

Herein, a hierarchically porous Zr-MOF-labeled electrochemical aptasensor based on the composite of PtPd@Ni-Co hollow nanoboxes (PtPd@Ni-Co HNBs) and poly (diallyldimethylammonium chloride)-functionalized graphene (PDDA-Gr) was developed for ultrasensitive detection of chloramphenicol (CAP). PtPd@Ni-Co HNBs have excellent conductivity and provide binding sites for aptamers; the functionalized PDDA-Gr improves its dispersibility and conductivity as a substrate material, which can be successfully used to increase the electrode surface area and support more PtPd@Ni-CoHNBs. Besides, hierarchically porous Zr-MOFs (HP-UiO-66) were utilized as signal probes and showed a stronger load capacity for signal molecules than conventional UiO-66. In the presence of CAP, two ingeniously designed Exo III-assisted cyclic amplification strategies further improved the sensitivity of the aptasensor: CAP causes cycle I to release a large amount of trigger DNA (Tr DNA), and then, Tr DNA initiated cycle II, which causes the exposed capture DNA to further bind the signal probes. With these advantages, the constructed aptasensors performed with satisfactory sensitivity in a wide linear range (10 fM-10 nM) and a detection limit of 0.985 fM. Several signal amplification strategies adopted in this study have effectively improved the performance of the sensor, providing a new avenue for the development of ultrasensitive sensors in the food analysis field.

Nanomaterial-based fluorescent sensors for the detection of lead ions

Lead (Pb) poisoning has been a scourge to the human to pose sighnificant health risks (e.g., organ disorders, carcinogenicity, and genotoxicity) as observed from many different parts of the world, especially in developing countries. The demand for accurate sensors for its detection, especially in environmental media (soil, water, food, etc.) has hence been growing steadily over the years. The potential utility of fluorescent nanosensors as an important analytical tool is recognized due to their astonishing characteristics (e.g., high sensitivity/selectivity, enhanced detection performance, low cost, portability, and rapid on-site detection ability). This review is organized to offer insight into the recent developments in fluorescent nanosensing technology for the detection of lead ions (Pb²⁺). To this end, different types of nanomaterials explored for such applications have been classified and evaluated with respect to performance, especially in terms of sensitivity. This review will help researchers gain a better knowledge on the status and importance of optical nanosensors so as to remediate the contamination of lead and associated problems. The technical challenges and prospects in the development of nanosensing systems for Pb²⁺ are also discussed.

Cobalt metal-organic framework modified carbon cloth/paper hybrid electrochemical button-sensor for nonenzymatic glucose diagnostics

In the growing pandemic, family healthcare is widely concerned with the increase of medical self-diagnosis away from the hospital. A cobalt metal-organic framework modified carbon cloth/paper (Co-MOF/CC/Paper) hybrid button-sensor was developed as a portable, robust, and user-friendly electrochemical analytical chip for nonenzymatic quantitative detection of glucose. Highly integrated electrochemical analytical chip was successfully fabricated with a flexible Co-MOF/CC sensing interface, effectively increasing the specific area and catalytic sites than the traditional plane electrode. Based on the button-sensor, rapid quantitative detection of glucose was achieved in multiple complex bio-matrixes, such as serum, urine, and saliva, with desired selectivity, stability, and durability. With the advantages of low cost, high environment tolerance, ease of production, our nanozyme-based electrochemical analytical chip achieved reliable nonenzymatic electrocatalysis, has great potential for the application of rapid on-site analysis in personalized diagnostic and disease prevention.

Review of recent progress on DNA-based biosensors for Pb2+ detection

Lead (Pb) is a highly toxic heavy metal of great environmental and health concerns, and interestingly Pb²⁺ has played important roles in nucleic acids chemistry. Since 2000, using DNA for selective detection of Pb²⁺ has become a rapidly growing topic in the analytical community. Pb²⁺ can serve as the most active cofactor for RNA-cleaving DNAzymes including the GR5, 17E and 8-17 DNAzymes. Recently, Pb²⁺ was found to promote a porphyrin metalation DNAzyme named T30695. In addition, Pb²⁺ can tightly bind to various G-quadruplex sequences inducing their unique folding and binding to other molecules such as dyes and hemin. The peroxidase-like activity of G-quadruplex/hemin complexes was also used for Pb²⁺ sensing. In this article, these Pb²⁺ recognition mechanisms are reviewed from fundamental chemistry to the design of fluorescent, colorimetric, and electrochemical biosensors. In addition, various signal amplification mechanisms such as rolling circle amplification, hairpin hybridization chain reaction and nuclease-assisted methods are coupled to these sensing methods to drive up sensitivity. We mainly cover recent examples published since 2015. In the end, some practical aspects of these sensors and future research opportunities are discussed.

An efficient electrochemical assay for miR-3675-3p in human serum based on the nanohybrid of functionalized fullerene and metal-organic framework

Idiopathic pulmonary fibrosis (IPF) is an interstitial lung disease with unclear pathogenesis, for which diagnosis has been a great challenge. Recent researches have revealed that miR-3675-3p is a promising biomarker for IPF diagnosis. Herein, the present work describes a novel electrochemical microRNA biosensor for rapid and sensitive detection of miR-3675-3p based on multiple signal amplification strategies. First of all, fullerene (C₆₀) is doped with poly(amidoamine) (PAMAM)-functionalized metal-organic framework (MOF) to form a new nanohybrid of C₆₀@PAMAM-MOF, which exhibits more remarkable redox activity compared with the other two synthesized C₆₀-based nanohybrids when triggered by tetraoctylammonium bromide (TOAB). C₆₀@PAMAM-MOF also possesses a large specific surface area and abundant amino groups to anchor Au nanoparticles (AuNPs) for the immobilization of signal probe (SP) to form tracer label and enhance the electrochemical response signal. In addition, core@shell Au-Pt nanoparticles (Au@PtNPs) are absorbed on chitosan-acetylene black (CS-AB) to act as sensing platform, which can promote electron transfer and increase the loading of capture probe (CP). Under optimum conditions, the proposed biosensor displays a wide linear range for miR-3675-3p from 10 fM to 10 nM, with a limit of detection (LOD) as low as 2.99 fM. More significantly, this biosensor shows a lower LOD and wider linear range than that of qRT-PCR, and its trial application in human serum shows favorable results, which exhibits a promising prospect for IPF diagnosis.

Electrochemical Affinity Biosensors Based on Selected Nanostructures for Food and Environmental Monitoring

The excellent capabilities demonstrated over the last few years by electrochemical affinity biosensors should be largely attributed to their coupling with particular nanostructures including dendrimers, DNA-based nanoskeletons, molecular imprinted polymers, metal-organic frameworks, nanozymes and magnetic and mesoporous silica nanoparticles. This review article aims to give, by highlighting representative methods reported in the last 5 years, an updated and general overview of the main improvements that the use of such well-ordered nanomaterials as electrode modifiers or advanced labels confer to electrochemical affinity biosensors in terms of sensitivity, selectivity, stability, conductivity and biocompatibility focused on food and environmental applications, less covered in the literature than clinics. A wide variety of bioreceptors (antibodies, DNAs, aptamers, lectins, mast cells, DNAzymes), affinity reactions (single, sandwich, competitive and displacement) and detection strategies (label-free or label-based using mainly natural but also artificial enzymes), whose performance is substantially improved when used in conjunction with nanostructured systems, are critically discussed together with the great diversity of molecular targets that nanostructured affinity biosensors are able to quantify using quite simple protocols in a wide variety of matrices and with the sensitivity required by legislation. The large number of possibilities and the versatility of these approaches, the main challenges to face in order to achieve other pursued capabilities (development of antifouling, continuous operation, wash-, calibration- and reagents-free devices, regulatory or Association of Official Analytical Chemists, AOAC, approval) and decisive future actions to achieve the commercialization and acceptance of these devices in our daily routine are also noted at the end.

A novel Fe-hemin-metal organic frameworks supported on chitosan-reduced graphene oxide for real-time monitoring of H2O2 released from living cells

Herein, a kind of novel hemin-based metal organic frameworks (Fe-hemin-MOFs) with unique peroxidase-like bioactivity was developed for the first time. The synthesized Fe-hemin-MOFs exhibited satisfactory catalytic activity toward hydrogen peroxide (H₂O₂). When it was further supported on Chitosan-reduced graphene oxide (CS-rGO), amplified electrochemical signal could be obtained. The Fe-hemin-MOFs/CS-rGO composite was used to construct a novel H₂O₂ electrochemical sensor. The electrocatalytic reduction of H₂O₂ displayed two segments linearity range from 1 to 61 μM and 61-1311 μM, as well as a low detection limit of 0.57 μM. Furthermore, the proposed sensor was successfully used for real-time monitoring of H₂O₂ released from living cells, which extended the practical application of MOFs-based sensors in monitoring the pathological process in living cells.

Recent Progress on Luminescent Metal-Organic Framework-Involved Hybrid Materials for Rapid Determination of Contaminants in Environment and Food

The high speed of contaminants growth needs the burgeoning of new analytical techniques to keep up with the continuous demand for monitoring and legislation on food safety and environmental pollution control. Metal-organic frameworks (MOFs) are a kind of advanced crystal porous materials with controllable apertures, which are self-assembled by organic ligands and inorganic metal nodes. They have the merits of large specific surface areas, high porosity and the diversity of structures and functions. Latterly, the utilization of metal-organic frameworks has attracted much attention in environmental protection and the food industry. MOFs have exhibited great value as sensing materials for many targets. Among many sensing methods, fluorometric sensing is one of the widely studied methods in the detection of harmful substances in food and environmental samples. Fluorometric detection based on MOFs and its functional materials is currently one of the most key research subjects in the food and environmental fields. It has gradually become a hot research direction to construct the highly sensitive rapid sensors to detect harmful substances in the food matrix based on metal-organic frameworks. In this paper, we introduced the synthesis and detection application characteristics (absorption, fluorescence, etc.) of metal-organic frameworks. We summarized their applications in the MOFs-based fluorometric detection of harmful substances in food and water over the past few years. The harmful substances mainly include heavy metals, organic pollutants and other small molecules, etc. On this basis, the future development and possible application of the MOFs have prospected in this review paper.

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.

A dual-model SERS and RRS analytical platform for Pb(II) based on Ag-doped carbon dot catalytic amplification and aptamer regulation

Several carbon dots doping with diferent elements (Ca, Ag, Au) were fabricated and their catalytic properties had been investigated in this paper. It was found that the Ag-doped carbon dots (CDAg) had played a role of mimic enzyme on the reaction of HAuCl4-H2O2 and generated nanogold particles with surface enhanced Raman scattering (SERS) and resonance Rayleigh scattering (RRS) effects. The aptamer (Apt) can be adsorbed on the CDAg surface and cause the catalysis weakening. When the target Pb(II) was added, it would combine with the Apt to produce firm complexes Pb-Apt and desorb CDAg, which caused its catalytic effect restore. The formed nanogold had a strong RRS peak (at 375 nm) and a high SERS peak (at 1615 cm-1) in the presence of molecular probe (Victoria blue B, VBB). The dual-model signals of SERS and RRS increased linearly with Pb(II) concentration increase within the scope of 0.006-0.46 μmol/L and 0.01-0.46 μmol/L. And their detection limits respectively were 0.0032 μmol/L and 0.0048 μmol/L Pb(II).

Ultrasensitive electrochemical detection of Mycobacterium tuberculosis IS6110 fragment using gold nanoparticles decorated fullerene nanoparticles/nitrogen-doped graphene nanosheet as signal tags

Tuberculosis (TB), caused by Mycobacterium tuberculosis (MTB), remains the top fatal infection continuing to threat public health, and the present detection method for MTB is facing great challenges with the global TB burden. In response to this issue, a novel electrochemical DNA biosensor was developed for detecting the IS6110 fragment within MTB. For the first time, the nanohybrid of gold nanoparticles decorated fullerene nanoparticles/nitrogen-doped graphene nanosheet (Au-nano-C₆₀/NGS) directly served as a new signal tag to generate signal response without additional redox molecules and subsequently labeled with signal probes (SPs) to form tracer label to achieve signal amplification. Additionally, a biotin-avidin system was introduced to immobilize abundant capture probes (CPs), further improving the sensitivity of the proposed biosensor. After a typical sandwich hybridization, the proposed electrochemical DNA biosensor was incubated with tetraoctylammonium bromide (TOAB), which was used as a booster to induce the intrinsic redox activity of the tracer label, resulting in a discriminating current response. The proposed electrochemical DNA biosensor shows a broad linear range for MTB determination from 10 fM to 10 nM with a low limit of detection (LOD) of 3 fM. In addition, this proposed biosensor not only distinguishes mismatched DNA sequence, but also differentiates MTB from other pathogenic agents. More importantly, it has been preliminarily applied in clinical detection and displayed excellent ability to identify the PCR products of clinical samples. There is great potential for this developed method to be used in early diagnosis and monitor of TB.

A "turn-off" SERS aptasensor based DNAzyme-gold nanorod for ultrasensitive lead ion detection

It is great significance to precisely monitor lead (II) ions (Pb²⁺) for environment protection and human health monitoring. We designed a sensitive detection strategy for sensitive and selective determination of Pb²⁺, based on a Pb²⁺-specific DNAzyme as the catalytic unit, Cy3-labeled DNA modified gold nanorods (AuNRs) as SERS reporter. Firstly, AuNRs surface were employed as a platform for the immobilization of thiolated probe DNA, and then hybridized with DNAzyme catalytic beacons. By taking advantage of DNAzyme digest, a molecular beacon, causes a "turn-off" SERS signal by disrupting the labeled probes. Under the optical conditions, the DNAzyme-AuNRs sensor system exhibited high sensitivity, acceptable stability and reproducibility with a wide linear range from 0.5 to 100 nM (R² = 0.9973), and an ultra-low detection limit of 0.01 nM. The proposed strategy has additional advantages of being less time-consuming, low-cost and remote query, and avoids the interference of some metals such as Fe³⁺, Cd²⁺, Ba²⁺, Cu²⁺, Zn²⁺. The SERS biosensor system has been successfully applied for detecting Pb²⁺ in real samples with a satisfactory result. The result indicated that the proposed sensing strategy not only enriches SERS platform of monitoring Pb²⁺ but also exhibits potential for the point-of-care diagnostic application of the clinical screening in complicated biological samples.

Application of hairpin DNA-based biosensors with various signal amplification strategies in clinical diagnosis

Biosensors have been commonly used in biomedical diagnostic tools in recent years, because of a wide range of application, such as point-of-care monitoring of treatment and disease progression, drug discovery, commonly use food control, environmental monitoring and biomedical research. Additionally, development of DNA biosensors has been increased enormously over the past few years as confirmed by the large number of scientific publications in this field. A wide range of techniques can be used for the development of DNA biosensors, such as DNA nano-machines and various signal amplification strategies. This article selectively reviews the recent advances in DNA base biosensors with various signal amplification strategies for detection of cancer DNA and microRNA, infectious microorganisms, and toxic metal ions.

Multifunctional drug carrier on the basis of 3d–4f Fe/La-MOFs for drug delivery and dual-mode imaging

Multifunctional drug carriers for simultaneous imaging and drug delivery have emerged as an important new direction for the treatment of cancer.

A label-free immunosensor for the detection of nuclear matrix protein-22 based on a chrysanthemum-like Co-MOFs/CuAu NWs nanocomposite

In this study, a new, simple, and label-free electrochemical immunosensor was presented for the detection of nuclear matrix protein-22 (NMP-22). In order to accurately monitor very small amounts of NMP-22, it was advantageous to use highly efficient nanomaterials as signals. For this reason, we synthesized a chrysanthemum-like nanocomposite (Co-MOFs/CuAu NWs), using Co-based metal-organic frameworks (Co-MOFs) as carriers and copper gold nanowires (CuAu NWs) wrapped around their surface, which was applied for modifying a glassy carbon electrode (GCE). The Co-MOFs/CuAu NWs possessed outstanding catalytic capabilities, which served as signal materials and simultaneously carried the anti-NMP-22 antibody (Ab). When different concentrations of the NMP-22 antigen (Ag) were specifically attached to the immunosensor, the current responses decreased by varying degrees. The designed biosensor used the principle to establish a linear regression equation and achieve an accurate quantification of NMP-22. After optimization, the NMP-22 sensor exhibited a good linear response over a concentration range from 0.1 pg mL^-1 to 1 ng mL^-1, with a lower detection limit of 33 fg mL^-1 (based on S/N = 3). The proposed biosensor demonstrated the advantages of ultra-sensitivity, high specificity and acceptable reproducibility, suggesting that the proposed strategy has the potential for the quantification of NMP-22 in human urine samples. Moreover, the novel nanocomposite Co-MOFs/CuAu NWs are promising materials for electrochemical sensors to detect other biomolecules.

Metal-Organic Frameworks for the Development of Biosensors: A Current Overview

This review focuses on the fabrication of biosensors using metal-organic frameworks (MOFs) as recognition and/or transducer elements. A brief introduction discussing the importance of the development of new biosensor schemes is presented, describing these coordination polymers, their properties, applications, and the main advantages and drawbacks for the final goal. The increasing number of publications regarding the characteristics of these materials and the new micro- and nanofabrication techniques allowing the preparation of more accurate, robust, and sensitive biosensors are also discussed. This work aims to offer a new perspective from the point of view of materials science compared to other reviews focusing on the transduction mechanism or the nature of the analyte. A few examples are discussed depending on the starting materials, the integration of the MOF as a part of the biosensor and, in a deep detail, the fabrication procedure.