Metal-organic frameworks and their derivatives as signal amplification elements for electrochemical sensing

Non-enzymatic electrochemical impedance sensor for selective detection of electro-inactive organophosphate pesticides using Zr-MOF/ZrO2/MWCNT ternary composite

The existence of multiple pesticide residues in fruits and vegetables constitutes a direct peril to living organisms. Therefore, it is crucial to develop a low-cost screening method for determining organophosphate pesticides (OPPs) in food samples. This study describes the solvothermal synthesis of a ternary composite comprising multi-walled carbon nanotubes (MWCNT), zirconium oxide, and a zirconium-metal-organic framework (Zr-MOF). The ternary composite was characterised using XRD, FESEM, FTIR, and BET. The ternary composite provides a large surface area (1158 m2/g) compared with the pristine Zr-MOF (868 m2/g). The composite-modified glassy carbon electrode was used to determine nine pesticides, including organophosphate (malathion, dimethoate, chlorpyrifos, monocrotophos, and glyphosate) and non-organophosphate (thiophanate methyl, carbendazim, atrazine, and 2,4, D). In particular, various chemical combinations of OPPs were selected, such as S-P=S, P=S, P=O, and non-OPPs such as C=S (with sulphur), and without sulphur. The sensor results show that the sensor selectivity is high for OPPs containing both phosphorus and sulphur molecules. The low detection limit of the sensor was 2.02, 2.8, 2.5, 1.11, and 2.01 nM for malathion, chlorpyrifos, dimethoate, monocrotophos, and glyphosate, respectively. The electrode exhibited significant chemical stability (93%) after 100 cycles, good repeatability, and a long shelf life. The sensor is reliable for qualitative real-time applications.

A series of triphenylamine-derived fluorophores attached to a Cu-based MOF for gaseous CO optical sensing: synthesis, performance, and mechanism

A series of triphenylamine-derived fluorescent dyes were attached to a Cu2+-containing MOF (metal-organic framework), denoted as Pm@CuMOF. The molecular structures of these dyes were discussed by the single crystal structures. Their major absorption bands peaked at 410-450 nm, showing emission bands ranging from 556 to 586 nm with emission quantum yields ranging from 8.0 to 15.1%. It was found that the [-N(C2H5)2] group generally improved sensing performance, and the -OH group in the dyes helped the Cu2+ quenching effect. Pm@CuMOF was observed by SEM as nanorods with a width of ~100 nm and a length of 300 nm. Their XRD patterns and N2 adsorption/desorption isotherms were recorded to confirm their porous structure. A low probe loading level of ~4% was determined by TGA result. The CO sensing mechanism was revealed as a Cu2+/Cu+-involved sensing mechanism based on the result of NMR titration, IR, XPS, and EPR. The fluorescence of these triphenylamine-derived dyes was firstly quenched by CuMOF. In contact with CO, Cu2+ was reduced to Cu+, accompanied by the release and fluorescence recovery of the fluorescent dyes, showing emission turn-on effect towards CO gas. Pm@CuMOF showed increased emission intensity at CO level of 0.005% (versus N2), with response times ranging from 123 s to 280 s (depending on various temperatures). Good selectivity was observed over competing alkane gases, with stable emission for at least 5 days, but no linear calibration plots were observed.

Amyloid detection in neurodegenerative diseases using MOFs

Neurodegenerative diseases (amyloid diseases such as Alzheimer's and Parkinson's), stemming from protein misfolding and aggregation, encompass a spectrum of disorders with severe systemic implications. Timely detection is pivotal in managing these diseases owing to their significant impact on organ function and high mortality rates. The diverse array of amyloid disorders, spanning localized and systemic manifestations, underscores the complexity of these conditions and highlights the need for advanced detection methods. Traditional approaches have focused on identifying biomarkers using imaging techniques (PET and MRI) or invasive procedures. However, recent efforts have focused on the use of metal-organic frameworks (MOFs), a versatile class of materials known for their unique properties, in revolutionizing amyloid disease detection. The high porosity, customizable structures, and biocompatibility of MOFs enable their integration with biomolecules, laying the groundwork for highly sensitive and specific biosensors. These sensors have been employed using electrochemical and photophysical techniques that target amyloid species under neurodegenerative conditions. The adaptability of MOFs allows for the precise detection and quantification of amyloid proteins, offering potential advancements in early diagnosis and disease management. This review article delves into how MOFs contribute to detecting amyloid diseases by categorizing their uses based on different sensing methods, such as electrochemical (EC), electrochemiluminescence (ECL), fluorescence, Förster resonance energy transfer (FRET), up-conversion luminescence resonance energy transfer (ULRET), and photoelectrochemical (PEC) sensing. The drawbacks of MOF biosensors and the challenges encountered in the field are also briefly explored from our perspective.

A sensitive bimetallic copper/bismuth metal-organic frameworks-based aptasensors for zearalenone detection in foodstuffs

Electrochemical aptasensors have emerged as promising platforms for effectivelydetection of various target analytes. Here, we developed a sensitive and selective electrochemical aptasensor for zearalenone (ZEN) determination based on a bimetallic organic framework (CuBi-BPDC). The results of HR-TEM, FE-SEM, XPS, etc. indicate the CuBi-BPDC possessing mixed nodes of Cu(II) and Bi(III) and multilayered nanosheets bearing nanoparticles. Due to its improved electrochemical activity and strong affinity for aptamers, the CuBi-BPDC-based aptasensor obtains a low limit of detection of 0.19 fg mL-1 (IUPAC S/N = 3) in a wide range of 1 fg mL-1-10 ng mL-1 via EIS and 0.73 fg mL-1 from 0 fg mL-1 to 1 × 107 fg mL-1 via DPV for ZEN detection, respectively. Moreover, the excellent selectivity allows this aptasensor to specifically identify ZEN from other interfering substances in raw milk and rice, indicating the potential applicability of the CuBi-BPDC-based aptasensor in sensitive and selective detection of ZEN.

CTAB-Co-MOFs@AuPt NPs as signal probes for the electrochemical detection of carcinoembryonic antigen 15-3

A sensitive electrochemical strategy for carcinoembryonic antigen 15-3 (CA15-3) detection is reported using CTAB-Co-MOFs@AuPt NPs as signal probes. The electrochemical strategy was designed as follows: First, the graphene aerogel@gold nanoparticles (GA@Au NPs) nanocomposites were employed to modify the sensing surface for promoting electron transfer rate and primary antibody (Ab1) immobilization due to GA possesses a large specific surface area, eminent conductivity, and a 3D network structure. Cobalt metal-organic frameworks (CTAB-Co-MOFs) synthesized were then used as a carrier for AuPt NPs and secondary antibody (Ab2) immobilization (notes: labelled-Ab2). With sandwich immunoreaction, the labelled-Ab2 was captured on the surface of the GA@Au NPs nanocomposites. Finally, differential pulse voltammetry (DPV) was employed to register the electrochemical signal of the immunosensor at the potential of - 0.85 V (vs SCE) in phosphate buffer saline (PBS) containing 2.5 mM H2O2. It was verified that the electrochemical reduction signal from Co3+ to Co2+ was recorded. The AuPt NPs could catalyze the reaction of H2O2 oxidizing Co2+ to Co3+, resulting in the amplification of the electrochemical signal. Under the selected conditions, the immunosensor can detect CA15-3 in the range 10 µU/mL to 250 U/mL with a low detection limit of 1.1 µU/mL. In the designed strategy, the CTAB-Co-MOFs were not only employed as carriers for AuPt NPs, but also acted as signal probes. The CTAB-Co-MOFs were investigated including SEM, TEM, XPS, and XRD. The application ability of the immunosensor was evaluated using serum sample, demonstrating the immunosensor can be applied to clinic serum analysis.

Composite of a thiacalix[4]arene-copper(I) metal-organic framework and mesoporous carbon for efficient electrochemical detection of antibiotics

Abundant use of nitrofurantoin (NFT) and metronidazole (MTZ) antibiotics has led to excessive residues in the environments and humans, resulting in serious damage to the human body and ecosystem. Therefore, effective detection of NFT and MTZ is exceedingly necessary. In this regard, metal-organic frameworks (MOFs) are promising materials as electrochemical sensors. Herein, we synthesized a new two-dimensional thiacalix [4]arene-copper (I) MOF (Cu-TC4A-M). This MOF was mixed with mesoporous carbon (MC) to a give Cu-TC4A-M@MC composite. In addition, the sensors of Cu-TC4A-M@MC(2:1)/GCE and Cu-TC4A-M@MC(1:2)/GCE were achieved (GCE = glassy carbon electrode), and then were applied for effectively detecting NFT and MTZ, respectively. Markedly, the two sensors exhibited satisfactory linear detection range, anti-interference, reproducibility and stability. When they were utilized in the real samples, such as human serum, urine, tap water and lake water, satisfactory recoveries were attained. The relative standard deviations (RSDs) were in the range of 1.16 % ∼ 1.92 % for NFT and 0.95 % ∼ 2.33 % for MTZ. This work provided a new application prospect for the thiacalix [4]arene-based MOFs as promising candidate materials for NFT and MTZ detection.

Recent Progress and Prospect of Metal-Organic Framework-Based Nanozymes in Biomedical Application

A nanozyme is a nanoscale material having enzyme-like properties. It exhibits several superior properties, including low preparation cost, robust catalytic activity, and long-term storage at ambient temperatures. Moreover, high stability enables repetitive use in multiple catalytic reactions. Hence, it is considered a potential replacement for natural enzymes. Enormous research interest in nanozymes in the past two decades has made it imperative to look for better enzyme-mimicking materials for biomedical applications. Given this, research on metal-organic frameworks (MOFs) as a potential nanozyme material has gained momentum. MOFs are advanced hybrid materials made of inorganic metal ions and organic ligands. Their distinct composition, adaptable pore size, structural diversity, and ease in the tunability of physicochemical properties enable MOFs to mimic enzyme-like activities and act as promising nanozyme candidates. This review aims to discuss recent advances in the development of MOF-based nanozymes (MOF-NZs) and highlight their applications in the field of biomedicine. Firstly, different enzyme-mimetic activities exhibited by MOFs are discussed, and insights are given into various strategies to achieve them. Modification and functionalization strategies are deliberated to obtain MOF-NZs with enhanced catalytic activity. Subsequently, applications of MOF-NZs in the biosensing and therapeutics domain are discussed. Finally, the review is concluded by giving insights into the challenges encountered with MOF-NZs and possible directions to overcome them in the future. With this review, we aim to encourage consolidated efforts across enzyme engineering, nanotechnology, materials science, and biomedicine disciplines to inspire exciting innovations in this emerging yet promising field.

A novel magneto-mediated electrochemical biosensor integrated DNAzyme motor and hollow nanobox-like Pt@Ni-Co electrocatalyst as dual signal amplifiers for vanilla detection

Herein, a novel magneto-mediated electrochemical aptasensor using the signal amplification technologies of DNAzyme motor and electrocatalyst for vanilla (VAN) detection was fabricated. The D/B duplex, formed by the DNAzyme motor that was each silenced by a blocker, and hairpin DNA1 (H1) containing adenosine ribonucleotide (rA) site were tethered on the sites of the gold nanoparticles@hollow porphyrinic-Metal-organic framework/polyethyleneimine-reduced graphene oxide (AuHPCN-222/PEI-rGO)-modified gold electrode (AuE). Then, after homogeneous and specific recognition in the presence of the VAN, trigger DNA was released and enriched by magnetic separation technique and introduced to the sensing platform to activate the DNAzyme motor, which efficiently improved target recognition capability and avoided the obstacle of multiple DNA strands tangling. More interestingly, the activated DNAzyme motor could repeatedly bind to and cleave H1 in the presence of Mg2+, leading to the exposure of a plethora of capture probes. The thionine (Thi) functionalized hairpin DNA2 (H2)-Pt@Ni-Co as signal probes could hybridize with capture probes. Additionally, the Pt@Ni-Co electrocatalysts presented catalytic activity towards Thi to obtain stronger electrochemical signals. VAN with concentrations ranging from 1 × 10-6 to 10 μM was determined and a detection limit was down to 0.15 pM. The designed electrochemical sensor was highly selective with specificity, stability, reproducibility, and reliable capability for monitoring the VAN in real samples.

Insight into the selection of oxidant in persulfate activation system: The effect of the target pollutant properties

As a rising branch of advanced oxidation processes, persulfate activation has attracted growing attention. Unlike catalysts that have been widely studied, the selection of persulfate is previously overlooked. In this study, the affecting factors of persulfates were studied. The effect of target pollutant properties on superior persulfate species (the species with a higher degradation efficiency) was investigated by multiwalled carbon nanotube (MWCNT)/persulfate catalytic systems. Innovatively, the EHOMO (or vertical ionization potential (VIP)) value of the target pollutant was proposed to be an index to judge the superior persulfate species, and the threshold is VIP= 6.397-6.674 eV, EHOMO= -8.035∼- 7.810 eV, respectively. To be specific, when the VIP of phenolic compounds is higher (or EHOMO of phenolic compounds is lower) than the threshold, the catalytic performance of peroxymonosulfate would be higher than that of peroxydisulfate. Moreover, the effects of coexisting cations on peroxydisulfate superior species were further investigated. It was illustrated that the hydrated cation radius of coexisting cations would influence the pollutant degradation efficiency under some circumstances. This study provides a new approach to improve the cost of persulfate activation systems and promotes the underlying downstream application of persulfate activation systems.

Thiacalix[4]arene-based metal-organic framework/reduced graphene oxide composite for electrochemical detection of chlorogenic acid

A novel metal-organic framework [Co2LCl4]·2DMF (Co-L) based on thiacalix[4]arene derivative was synthesized using the solvothermal method. Then Co-L was respectively mixed with reduced graphene oxide (RGO), multi-walled carbon nanotubes (MWCNT) and mesoporous carbon (MC) to prepare corresponding composite materials. PXRD, SEM and N2 adsorption-desorption illustrated that composite materials have been successfully prepared. After optimizing experimental conditions for detecting chlorogenic acid (CGA), the Co-L@RGO(1:1) composite material showed the optimal electrocatalytic activity for CGA, which may be because RGO possessed large specific surface area and hydroxyl and carboxyl groups that could form hydrogen-bonding with the oxide of CGA. Benefiting from the synergetic effect of Co-L and RGO, the glassy carbon electrode modified with Co-L@RGO(1:1) (Co-L@RGO(1:1)/GCE) exhibited a low limit of detection (LOD) of 7.24 nM for CGA within the concentration of 0.1-2 μM and 2-20 μM. Co-L@RGO(1:1)/GCE also showed excellent selectivity, stability, and reproducibility for the CGA detection. Co-L@RGO(1:1)/GCE could detect the CGA in honeysuckle with satisfactory results. This work provided a great example for the thiacalix[4]arene-based MOF in the application of electrochemical sensors.

Enhanced photocatalytic activity of S-doped graphitic carbon nitride hollow microspheres: Synergistic effect, high-concentration antibiotic elimination and antibacterial behavior

For the past few years, graphitic carbon nitride (g-C₃N₄) has been widely used to eliminate environmental pollutants, but limited active site on surface and low separation/migration ability suppress its practical uses. Herein, we adopted a supramolecular self-assembly route followed with S doping to synthesize S-doped g-C₃N₄ with a hollow microsphere composition (SCNHM), where the shell was demonstrated to compose of ultrathin nanosheets. The unique structural characteristics endow the SCNHM with high specific surface area (∼81 m² g^-1) to provide abundant reaction sites and enhanced light-harvesting due to the light-scattering effect of hollow structure. Moreover, the S dopant meliorated the electronic structure to narrow the bandgap and promoted the charge separation/transfer capability. With this synergistic effect, the SCNHM presented greatly improved photocatalytic activity for degrading tetracycline hydrochloride (TC) compared to the CN, SCN and CNHM samples. This photocatalyst could eliminate high-concentration TC (50 mg L^-1) in 18 min, and the 30 min removal efficiencies of 100 mg L^-1 and 200 mg L^-1 reached 92 % and 60 %, which is much better than the reported photocatalysts in literatures (usually ≤ 20 mg L^-1). Additionally, the good photocatalytic durability was confirmed and the degradation pathway of TC was proposed. Furthermore, the SCNHM was proved to meanwhile possess superior performance for inactivating the typical Gram-positive bacterium of Staphylococcus aureus (S. aureus) and the typical Gram-negative bacterium of Escherichia coli (E. coli). Finally, based on determination of band alignment and detection of active species, a plausible photocatalytic mechanism was proposed.

Review: Synthesis of metal organic framework-based composites for application as immunosensors in food safety

Ensuring food safety continues to be one of the major global challenges. For effective food safety monitoring, fast, sensitive, portable, and efficient food safety detection strategies must be devised. Metal organic frameworks (MOFs) are porous crystalline materials that have attracted attention for use in high-performance sensors for food safety detection owing to their advantages such as high porosity, large specific surface area, adjustable structure, and easy surface functional modification. Immunoassay strategies based on antigen-antibody specific binding are one of the important means for accurate and rapid detection of trace contaminants in food. Emerging MOFs and their composites with excellent properties are being synthesized, providing new ideas for immunoassays. This article summarizes the synthesis strategies of MOFs and MOF-based composites and their applications in the immunoassays of food contaminants. The challenges and prospects of the preparation and immunoassay applications of MOF-based composites are also presented. The findings of this study will contribute to the development and application of novel MOF-based composites with excellent properties and provide insights into advanced and efficient strategies for developing immunoassays.

Efficient detection of metronidazole by a glassy carbon electrode modified with a composite of a cyclotriveratrylene-based metal-organic framework and multi-walled carbon nanotubes

Constructing a sensitive and efficient sensor for determination of metronidazole (MNZ) is crucial in food field. Herein, a new cyclotriveratrylene-based metal-organic framework (MOF), namely, [Cd₆L₂(cyclen)₂(H₂O)₂] (1), was constructed by self-assembly of functionalized 5,6,12,13,19,20-hexacarboxy-propoxy-cyclotriveratrylene (H₆L), 1,4,7,10-tetraazacyclododecane (cyclen) and Cd(II) cation under solvothermal condition. In 1, adjacent Cd(II) cations are linked by L^6- to produce a 2D polymeric structure with carboxylate and phenolic oxygen atoms. To enhance conductivity of 1, it was combined with conducting carbon materials, including mesoporous carbon (MC), reduced graphene oxide (RGO) and multi-walled carbon nanotubes (MWCNT), respectively, producing a series of composite materials. Remarkably, electrochemical tests showed that 1@MWCNT(1:1) featured a much better electrochemical detection performance for metronidazole (MNZ) than 1@MC and 1@RGO. The linear range for the detection of MNZ is up to 0.4-500 μM and the limit of detection (LOD) for MNZ reached 0.25 μM. Importantly, the fabricated sensor 1@MWCNT(1:1) was employed for the detection of MNZ in honey and egg with satisfactory result. High-performance liquid chromatography (HPLC) validated the high accuracy of the electrochemical method for the determination of honey and egg.

Biomolecular sensors for advanced physiological monitoring

Body-based biomolecular sensing systems, including wearable, implantable and consumable sensors allow comprehensive health-related monitoring. Glucose sensors have long dominated wearable bioanalysis applications owing to their robust continuous detection of glucose, which has not yet been achieved for other biomarkers. However, access to diverse biological fluids and the development of reagentless sensing approaches may enable the design of body-based sensing systems for various analytes. Importantly, enhancing the selectivity and sensitivity of biomolecular sensors is essential for biomarker detection in complex physiological conditions. In this Review, we discuss approaches for the signal amplification of biomolecular sensors, including techniques to overcome Debye and mass transport limitations, and selectivity improvement, such as the integration of artificial affinity recognition elements. We highlight reagentless sensing approaches that can enable sequential real-time measurements, for example, the implementation of thin-film transistors in wearable devices. In addition to sensor construction, careful consideration of physical, psychological and security concerns related to body-based sensor integration is required to ensure that the transition from the laboratory to the human body is as seamless as possible.

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

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

Constructing functional metal-organic frameworks by ligand design for environmental applications

Metal-organic frameworks (MOFs) with unique physical and chemical properties are composed of metal ions/clusters and organic ligands, including high porosity, large specific surface area, tunable structure and functionality, which have been widely used in chemical sensing, environmental remediation, and other fields. Organic ligands have a significant impact on the performance of MOFs. Selecting appropriate types, quantities and properties of ligands can well improve the overall performance of MOFs, which is one of the critical issues in the synthesis of MOFs. This article provides a comprehensive review of ligand design strategies for functional MOFs from the number of different types of organic ligands. Single-, dual- and multi-ligand design strategies are systematically presented. The latest advances of these functional MOFs in environmental applications, including pollutant sensing, pollutant separation, and pollutant degradation are further expounded. Furthermore, an outlook section of providing some insights on the future research problems and prospects of functional MOFs is highlighted with the purpose of conquering current restrictions by exploring more innovative approaches.

Quasi-Cu-MOFs: highly improved water stability and electrocatalytic activity toward H2O2 reduction among pristine 3D MOFs

QHKUST-1 calcined at 250 °C for 1 h maintains the perfect octahedral morphology of HKUST-1 and exhibits superior moisture stability and enhanced electrocatalytic activity compared to the original water-sensitive HKUST-1.

Nanosphere Structures Using Various Materials: A Strategy for Signal Amplification for Virus Sensing

Nanomaterials have been explored in the sensing research field in the last decades. Mainly, 3D nanomaterials have played a vital role in advancing biomedical applications, and less attention was given to their application in the field of biosensors for pathogenic virus detection. The versatility and tunability of a wide range of nanomaterials contributed to the development of a rapid, portable biosensor platform. In this review, we discuss 3D nanospheres, one of the classes of nanostructured materials with a homogeneous and dense matrix wherein a guest substance is carried within the matrix or on its surface. This review is segmented based on the type of nanosphere and their elaborative application in various sensing techniques. We emphasize the concept of signal amplification strategies using different nanosphere structures constructed from a polymer, carbon, silica, and metal-organic framework (MOF) for rendering high-level sensitivity of virus detection. We also briefly elaborate on some challenges related to the further development of nanosphere-based biosensors, including the toxicity issue of the used nanomaterial and the commercialization hurdle.

MXene-Derived Metal-Organic Framework@MXene Heterostructures toward Electrochemical NO Sensing

The electrochemical sensing of nitric oxide (NO) molecules by metal-organic framework (MOF) catalysts has been impeded, to a large extent, owing to their poor electrical conductivity and weak NO adsorption. In this work, incomplete in situ conversion of V₂ CT_x (T = terminal atoms) MXene to MOF is adopted, forming MOF@MXene heterostructures, which outperform MXene and MOF monocomponents toward electrochemical NO sensing. Density functional theory (DFT) calculation results indicate metal-like electronic characters for the heterostructure benefiting from the dominating contribution of the V 3d orbitals of the metallic MXene. Moreover, plane-averaged charge density difference shows substantial charge redistribution occurs at the heterointerfaces, producing a built-in field, which facilitates charge transfer. Besides, molecular mechanics-based simulated annealing calculation reveals greatly enhanced adsorption energies of NO molecules on the heterointerfaces than that on separate MOFs and MXenes. Hence, the facilitated charge transfer and preferential NO adsorption are responsible for the dramatically promoted performance toward NO sensing. The prudent design of MOF@MXene heterostructure may spur advanced electrocatalysts for electrochemical sensing.

Recent Advances in Metal Nanocomposite-Based Electrochemical (Bio)Sensors for Pharmaceutical Analysis

Rising rates of drug abuse and pharmaceutical pollution throughout the world as a consequence of increased drug production and utilization pose a serious risk to public health and to environmental integrity. It is thus critical that reliable analytical approaches to detecting drugs and their metabolites in a range of sample matrices be developed. Recent advances in the design of nanomaterial-based electrochemical sensors and biosensors have enabled promising new approaches to pharmaceutical analysis. In particular, the development of a range of novel metal nanocomposites with enhanced catalytic properties has provided a wealth of opportunities for the design of rapid and reliable platforms for the detection of specific pharmaceutical compounds. The present review provides a comprehensive overview of representative metal nanocomposites with synergistic properties and their recent (2017-2022) application in the context of electrochemical sensing as a means of detecting specific antibiotic, tuberculostatic, analgesic, antineoplastic, antipsychotic, and antihypertensive drugs. In discussing these applications, we further explore a variety of testing-related principles, fabrication approaches, characterization techniques, and parameters associated with the sensitivity and selectivity of these sensor platforms before surveying the future outlook regarding the fabrication of next-generation (bio)sensor platforms for use in pharmaceutical analysis.

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.

Metal-organic framework-based materials for the abatement of air pollution and decontamination of wastewater

Developing new and efficient technologies for environmental remediation is becoming significant due to the increase in global concerns such as climate change, severe epidemics, and energy crises. Air pollution, primarily due to increased levels of H₂S, SO_x, NH₃, NO_x, CO, volatile organic compounds (VOC), and particulate matter (PM) in the atmosphere, has a significant impact on public health, and exhaust gases harm the natural sulfur, nitrogen, and carbon cycles. Similarly, wastewater discharged to the environment with metal ions, herbicides, pharmaceuticals, personal care products, dyes, and aromatics/organic compounds is a risk for health since it may lead to an outbreak of waterborne pathogens and increase the exposure to endocrine-disrupting agents. Therefore, developing new and efficient air and water quality management systems is critical. Metal-organic frameworks (MOFs) are novel materials for which the main application areas include gas storage and separation, water harvesting from the atmosphere, chemical sensing, power storage, drug delivery, and food preservation. Due to their versatile structural motifs that can be modified during synthesis, MOFs also have a great promise for green applications including air and water pollution remediation. The motivation to use MOFs for environmental applications prompted the modification of their structures via the addition of metal and functional groups, as well as the creation of heterostructures by mixing MOFs with other nanomaterials, to effectively remove hazardous contaminants from wastewater and the atmosphere. In this review, we focus on the state-of-the-art environmental applications of MOFs, particularly for water treatment and air pollution, by highlighting the groundbreaking studies in which MOFs have been used as adsorbents, membranes, and photocatalysts for the abatement of air and water pollution. We finally address the opportunities and challenges for the environmental applications of MOFs.

State-of-the-art developments in carbon quantum dots (CQDs): Photo-catalysis, bio-imaging, and bio-sensing applications

Carbon quantum dots (CQDs), the intensifying nanostructured form of carbon material, have exhibited incredible impetus in several research fields such as bio-imaging, bio-sensing, drug delivery systems, optoelectronics, photovoltaics, and photocatalysis, thanks to their exceptional properties. The CQDs show extensive photonic and electronic properties, as well as their light-collecting, tunable photoluminescence, remarkable up-converted photoluminescence, and photo-induced transfer of electrons were widely studied. These properties have great advantages in a variety of visible-light-induced catalytic applications for the purpose of fully utilizing the energy from the solar spectrum. The major purpose of this review is to validate current improvements in the fabrication of CQDs, characteristics, and visible-light-induced catalytic applications, with a focus on CQDs multiple functions in photo-redox processes. We also examine the problems and future directions of CQD-based nanostructured materials in this growing research field, with an eye toward establishing a decisive role for CQDs in photocatalysis, bio-imaging, and bio-sensing applications that are enormously effective and stable over time. In the end, a look forward to future developments is presented, with a view to overcoming challenges and encouraging further research into this promising field.

Metal organic frameworks as advanced functional materials for aptasensor design

BACKGROUND

Advancement in coordination chemistry has achieved an impressive development of metal organic frameworks (MOFs) as the supramolecular hybrid materials, comprising harmonized metal nodes with organic ligands. Scope and approach: MOFs offer the unique properties of easy synthesis, nanoscale structure, adjustable size and morphology, high porosity, large surface area, supreme chemical tunability and stability, and biocompatibility. The features provide an exceptional opportunity for the widely usage of MOFs in the different scientific fields, e.g. biomedicine, electrocatalysis, food safety, energy storage, environmental surveillance, and biosensing platforms. The synergistic incorporation of the aptamer advantages and the superiorities of MOFs attains the novel MOF-based aptasensors. The excellent selectivity and sensitivity of the MOF-based aptasensors nominate them as efficient lab-on-chip tools for cost-effective, label-free, portable, and real-time monitoring of diverse targets.

KEY FINDINGS AND CONCLUSIONS

Here, we review the achievements in the sensor design by cooperation of MOF motifs and aptamers with the conspicuous potential of determining the targets. Finally, some results are expressed that provide a valuable viewpoint for developing the novel MOF-based test strips in the future.

A Versatile Electrochemical Biosensor for the Detection of Circulating MicroRNA toward Non-Small Cell Lung Cancer Diagnosis

Circulating microRNAs (miRNAs) can be used as noninvasive biomarkers and are also found circulating in body fluids such as blood. Dysregulated miRNA expression is associated with many diseases, including non-small cell lung cancer (NSCLC), and the miRNA assay is helpful in cancer diagnosis, prognosis, and monitoring. In this work, a versatile electrochemical biosensing system is developed for miRNA detection by DNAzyme-cleavage cycling amplification and hybridization chain reaction (HCR) amplification. With cleavage by Mn²⁺ targeted DNAzyme, DNA-walker can move along the predesigned DNA tracks and contribute to the transduction and enhancement of signals. For the electrochemical process, the formation of multiple G-quadruplex-incorporated long double-stranded DNA (dsDNA/G-quadruplex) structures is triggered through HCR amplification. The introduction of G-quadruplex allows sensitive measurement of miRNA down to 5.68 fM with good specificity. Furthermore, by profiling miRNA in the NSCLC cohort, this designed strategy shows high efficiency (area under the curve (AUC) of 0.879 using receiver operating characteristic (ROC) analysis) with the sensitivity of 80.0% for NSCLC early diagnosis (stage I). For the discrimination of NSCLC and benign disease, the assay displays an AUC of 0.907, superior to six clinically-acceptable protein tumor markers. Therefore, this platform holds promise in clinical application toward NSCLC diagnosis and prognosis.

Selective and Sensitive Electrochemical Sensor for Aflatoxin M1 with a Molybdenum Disulfide Quantum Dot/Metal-Organic Framework Nanocomposite

Aflatoxins are the hepatotoxic secondary metabolites which are highly carcinogenic and known to cause several adverse effects on human health. The present study reports a simple, sensitive, and novel electrochemical sensor for aflatoxin M1 (AFM1). The sensor has been fabricated by modifying the screen-printed carbon electrodes with a functional nanocomposite of molybdenum disulfide (MoS₂) quantum dots (QDs) and a zirconium-based metal-organic framework (MOF), that is, UiO-66-NH₂. The MoS₂/UiO-66-modified electrodes were decorated with the AFM1-specific monoclonal antibodies and then investigated for the electrochemical detection of AFM1. Based on the electrochemical impedance spectroscopy analysis, it was possible to detect AFM1 in the concentration range of 0.2-10 ng mL^-1 with a limit of detection of 0.06 ng mL^-1. The realization of an excellent sensing performance can be attributed to the electroactivity of MoS₂ QDs and the large surface to volume area achieved by the addition of the MOF. The presence of UiO-66-NH₂ is also useful to attain readily available amine functionality for the robust interfacing of antibodies. The performance of the developed sensor has also been validated by detecting AFM1 in the spiked milk samples.

Fabrication of Au/Fe 3 O 4 /RGO based aptasensor for measurement of miRNA‐128, a biomarker for acute lymphoblastic leukemia (ALL)

Due to their high sensitivity, simplicity, portability, self‐contained, and low cost, the development of electrochemical biosensors is a beneficial way to diagnose and anticipate many types of cancers. An electrochemical nanocomposite‐based aptasensor is fabricated for the determination of miRNA‐128 concentration as the acute lymphoblastic leukemia (ALL) biomarker for the first time. The aptamer chains were immobilized on the surface of the glassy carbon electrode (GCE) through gold nanoparticles/magnetite/reduced graphene oxide (AuNPs/Fe₃O₄/RGO). Fast Fourier transform infrared (FTIR), X‐ray diffraction (XRD), vibrating sample magnetometer (VSM), and transmission electron microscopy (TEM) were used to characterize synthesized nanomaterials. Cyclic voltammetry (CV), square wave voltammetry (SWV), and electrochemical impedance spectroscopy (EIS) were used to characterize the modified GCE in both label‐free and labeled methods. The results indicate that the modified working electrode has high selectivity and for miRNA‐128 over other biomolecules. The hexacyanoferrate redox system typically operated at around 0.3 V (vs. Ag/AgCl), and the methylene blue redox system ran at about 0 V, were used as an electrochemical probe. The detection limit and linear detection range for hexacyanoferrate and methylene blue are 0.05346 fM, 0.1–0.9 fM, and 0.005483 fM, 0.01–0.09 fM, respectively. The stability and diffusion control analyses were performed as well. In both label‐free and labeled methods, the modified electron showed high selectivity for miRNA‐128. The use of methylene blue as a safer redox mediator caused miRNA‐128 to be detected with greater accuracy at low potentials in PBS media. The findings also show the substantial improvement in detection limit and linearity by using reduced graphene oxide‐magnetite‐gold nanoparticles that can be verified by comparing with previous studies on the detection of other miRNAs.

Label-Free ZnIn2S4/UiO-66-NH2 Modified Glassy Carbon Electrode for Electrochemically Assessing Fish Freshness by Monitoring Xanthine and Hypoxanthine

Considering that simultaneous detection of xanthine (XA) and hypoxanthine (HXA) has been proved to be a reliable and feasible method for assessing fish freshness, a novel electrochemical sensing platform based on the ZnIn2S4/UiO-66-NH2 modified glassy carbon electrode (GCE) was constructed in this study for XA and HXA determination. X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FT-IR) were performed to exhibit the morphology and structural characteristics of ZnIn2S4/UiO-66-NH2. The Brunauer–Emmett–Teller (BET) displayed that the introduction of UiO-66-NH2 can improve the specific surface area of the hybrid. Besides, the electrochemical sensing performance of ZnIn2S4/UiO-66-NH2 was evaluated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). For simultaneously detecting XA and HXA, the fabricated electrochemical sensor shows wide linear ranges (0.025–40 µM and 0.3–40 µM) with low detection limits (0.0083 µM and 0.1 µM). This sensor also has 96–103% recovery in detecting XA and HXA content in large yellow croaker meat samples, demonstrating a promising application in the marine food industry.

Facile synthesis of Mn, Ce co-doped g-C3N4 composite for peroxymonosulfate activation towards organic contaminant degradation

Peroxymonosulfate (PMS)-based advanced oxidation processes for wastewater treatment have received extensive attention in the past years. Here, a novel Mn, Ce co-modified g-C₃N₄ (MnCe-CN) composite was successfully synthesized by one-step pyrolysis for activating PMS. The physical and chemical characterization of MnCe-CN/PMS was conducted, indicating that Mn and Ce were evenly distributed on g-C₃N₄ and existed in the form of Mn-N structure and CeO₂, respectively. The MnCe-CN/PMS system could effectively degrade pollutants such as acetaminophen (ACT), methylparaben (MeP), p-nitrophenol (PNP), and 2,4-dichlorophenol (2,4-DCP). Among them, 2,4-DCP could be rapidly degraded, reaching 100% within 30 min. The masking experiments and electrochemical testing results revealed that 2,4-DCP was degraded via superoxide radicals (O₂˙-), singlet oxygen (¹O₂), and electron transfer path. The cyclic experiments and real water treatment experiments testified that the oxidative system had excellent stability and applicability. This study provides a facile synthetic method to fabricate bimetallic co-modified g-C₃N₄ for the enhancement of PMS activation.

Nitrogen-doping coupled with cerium oxide loading co-modified graphitic carbon nitride for highly enhanced photocatalytic degradation of tetracycline under visible light

The increasingly serious pollution of antibiotics brings an enormous threat to the ecological environment and human health. Graphite phase carbon nitride (g-C₃N₄), as a popular photocatalytic material, is widely used in photocatalytic degradation of antibiotics in water. In order to make up for the shortage of g-C₃N₄ monomer, CeO₂/N-doped g-C₃N₄ (CeNCN) composite photocatalysts co-modified with nitrogen doping and CeO₂ loading were designed and synthesized with the idea of expanding visible light absorption and promoting photogenerated carrier separation. CeNCN exhibits excellent photodegradation performance, the removal rate of tetracycline reached 80.09% within 60 min, which is much higher than that of g-C₃N₄ (CN) and N-doped g-C₃N₄ (NCN); and the quasi-first-order degradation rate constant is 0.0291, which is 7.86 and 2.29 times higher than CN and NCN. Electron spin resonance and free radical trapping experiments confirmed that h⁺, O₂^- and OH are the active substances in the photocatalytic system. After 5 cycles, the degradation efficiency of tetracycline still exceeds 75%, which indicates that CeNCN has good stability. This work proves that N-doping and CeO₂ loading can effectively broaden the photoresponse range of g-C₃N₄, facilitate the separation of photogenerated electron-hole pairs, and provide a reference for the construction of g-C₃N₄-based photocatalyst with high-efficiency photodegradation activity.

Advanced Metal–Organic Frameworks-Based Catalysts in Electrochemical Sensors

Developing efficient catalysts is vital for the application of electrochemical sensors. Metal–organic frameworks (MOFs), with high porosity, large specific surface area, good conductivity, and biocompatibility, have been widely used in catalysis, adsorption, separation, and energy storage applications. In this invited review, the recent advances of a novel MOF-based catalysts in electrochemical sensors are summarized. Based on the structure–activity–performance relationship of MOF-based catalysts, their mechanism as electrochemical sensor, including metal cations, synthetic ligands, and structure, are introduced. Then, the MOF-based composites are successively divided into metal-based, carbon-based, and other MOF-based composites. Furthermore, their application in environmental monitoring, food safety control, and clinical diagnosis is discussed. The perspective and challenges for advanced MOF-based composites are proposed at the end of this contribution.

A sensitive fluorescence "turn on" nanosensor for glutathione detection based on Ce-MOF and gold nanoparticles

Glutathione (GSH) as an essential biothiol maintains redox homeostasis in human body, the aberrant level of it has been related to various diseases. In this work, we constructed a facile and environment-friendly strategy by using Ce based metal-organic frameworks and gold nanoparticles (AuNPs) for detection of GSH. The fluorescence intensity of the Ce-MOF was quenched by AuNPs, which is ascribed to the existence of fluorescence resonance energy transfer (FRET) and electrostatic interaction between Ce-MOFs and AuNPS. Because of the formation of Au-SH between AuNPs and GSH, the addition GSH induced the Ce-MOF/AuNPs and prevented the occurrence of FRET and electrostatic interaction between Ce-MOFs and AuNPS, which futher recovered the fluorescence of Ce-MOF. Under the optimized conditions, this "turn-on" sensing process revealed a high selectivity toward GSH and displayed good linearity in range of 0.2-32.5 μM with low detection limit of 58 nM. In addition, the practicability of the strategy was testified through analyzing GSH in real human serum samples.

A two-dimensional thin Co-MOF nanosheet as a nanozyme with high oxidase-like activity for GSH detection

A two-dimensional thin Co-MOF (ZIF-67) nanosheet with high oxidase-like activity was applied for sensitive visual GSH detection.

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.