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.

Metal-organic framework-based platforms for implantation applications: recent advances and challenges

The development of minimally invasive technology has promoted the widespread use of implant interventional materials, which play an important role in alleviating patients' pain during and after surgery. Metal-organic frameworks (MOFs) and their related hybrids formed by bridging ligands and metal nodes via covalent bonds represent one of the smart platforms in implant interventional fields due to their large surface area, adjustable compositions and structures, biodegradability, etc. Significant progresses in the implantation application of MOF-based materials have been achieved recently, but these studies are still in the initial stage. This review highlights the recent advances of MOFs and their related hybrids in orthopedic implantation, cardio-vascular implantation, neural tissue engineering, and biochemical sensing. Each correction between the structural features of MOFs and their corresponding implanted works is highlighted. Finally, the confronting challenges and future perspectives in the implant interventional field are discussed.

In situ growth of a redox-active metal-organic framework on electrospun carbon nanofibers as a free-standing electrode for flexible energy storage devices

The rational construction of free-standing and flexible electrodes for application in electrochemical energy storage devices and next-generation supercapacitors is an emerging research focus. Herein, we prepared a redox-active ferrocene dicarboxylic acid (Fc)-based nickel metal-organic framework (MOF) on electrospun carbon nanofibers (NiFc-MOF@CNFs) via an in situ approach. This in situ approach avoided the aggregation of the MOF. The NiFc-MOF@CNF flexible electrode showed a high redox-active behavior owing to the presence of ferrocene and flexible carbon nanofibers, which led to unique properties, including high flexibility and lightweight. Furthermore, the prepared electrode was utilized in a supercapacitors (SC) without the use of any binder, which achieved a specific capacity of 460 C g-1 at 1 A g-1 with an excellent cyclic retention of 82.2% after 25 000 cycles and a good rate capability. A flexible asymmetric supercapacitor device was assembled, which delivered a high energy density of 56.25 W h kg-1 and a long-lasting cycling performance. Also, the prepared electrode could be used as a freestanding electrode in flexible devices at different bending angles. The obtained cyclic voltammetry curves showed negligible changes, indicating the high stability and good flexibility of the electrode. Thus, the use of the in situ strategy can lead to the uniform growth of redox-active MOFs or other porous materials on CNFs.

Sensitive dual-mode sensing platform for Amyloid β detection: Combining dual Z-scheme heterojunction enhanced photoelectrochemistry analysis and dual-wavelength ratiometric electrochemiluminescence strategy

As a tumor biomarker, the accumulation of amyloid β oligomers (Aβo) in the brain has been suggested as a key feature in the pathogenesis and progression of Alzheimer's disease (AD). In this work, we designed a novel photoelectrochemical (PEC) and electrochemiluminescence resonance energy transfer (ECL-RET) dual-mode biosensor to achieve ultra-sensitive detection of Aβo. Specifically, the electrode surface modified Carbon Dots (C Dots) and the electrodeposited polyaniline (PANI) film formed a Z-scheme heterojunction reversing the photocurrent signal, and then the Aβo specific recognition peptide was attached to the surface via amide bonding between the amino group of PANI and carbonyl group of peptide. After that, in the presence of CdTe labeled specific recognition aptamer for Aβ (CdTe-Apt), Aβo was captured to construct a sandwich-type biosensor and exhibited a significantly enhanced cathodic photocurrent response because the formed dual Z-scheme heterojunction promoted charge separation efficiency. Interestingly, the proposed biosensor also caused a ratiometric change in the ECL intensity at 555 nm and 640 nm. Therefore, the developed biosensor achieved dual-mode detection of Aβo, where the PEC detection range of Aβo was from 10 fM to 0.1 μM (with a detection limit of 4.27 fM) and the ECL method provided a linear detection range of 10 fM to 10 nM (with a detection limit of 6.41 fM). The stability and reliability of the experimental results indicate that this has been a promising biosensing pattern and could be extended to the analysis of other biomarkers.

Therapy and diagnosis of Alzheimer's disease: from discrete metal complexes to metal-organic frameworks

Alzheimer's disease (AD) is a neurodegenerative disorder affecting 44 million people worldwide. Although many issues (pathogenesis, genetics, clinical features, and pathological aspects) are still unknown, this disease is characterized by noticeable hallmarks such as the formation of β-amyloid plaques, hyperphosphorylation of tau proteins, the overproduction of reactive oxygen species, and the reduction of acetylcholine levels. There is still no cure for AD and the current treatments are aimed at regulating the cholinesterase levels, attenuating symptoms temporarily rather than preventing the AD progression. In this context, coordination compounds are regarded as a promissing tool in AD treatment and/or diagnosis. Coordination compounds (discrete or polymeric) possess several features that make them an interesting option for developing new drugs for AD (good biocompatibility, porosity, synergetic effects of ligand-metal, fluorescence, particle size, homogeneity, monodispersity, etc.). This review discusses the recent progress in the development of novel discrete metal complexes and metal-organic frameworks (MOFs) for the treatment, diagnosis and theragnosis of AD. These advanced therapies for AD treatment are organized according to the target: Aβ peptides, hyperphosphorylated tau proteins, synaptic dysfunction, and mitochondrial failure with subsequent oxidative stress.

SiC Electrochemical Sensor Validation for Alzheimer Aβ42 Antigen Detection

Alzheimer's disease (AD) is a neurodegenerative disease with only late-stage detection; thus, diagnosis is made when it is no longer possible to treat the disease, only its symptoms. Consequently, this often leads to caregivers who are the patient's relatives, which adversely impacts the workforce along with severely diminishing the quality of life for all involved. It is, therefore, highly desirable to develop a fast, effective and reliable sensor to enable early-stage detection in an attempt to reverse disease progression. This research validates the detection of amyloid-beta 42 (Aβ₄₂) using a Silicon Carbide (SiC) electrode, a fact that is unprecedented in the literature. Aβ₄₂ is considered a reliable biomarker for AD detection, as reported in previous studies. To validate the detection with a SiC-based electrochemical sensor, a gold (Au) electrode-based electrochemical sensor was used as a control. The same cleaning, functionalization and Aβ_1-28 antibody immobilization steps were used on both electrodes. Sensor validation was carried out by means of Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) aiming to detect an 0.5 µg·mL^-1 Aβ₄₂ concentration in 0.1 M buffer solution as a proof of concept. A repeatable peak directly related to the presence of Aβ₄₂ was observed, indicating that a fast SiC-based electrochemical sensor was constructed and may prove to be a useful approach for the early detection of AD.

Metal-Organic Frameworks-Based Nanoplatforms for the Theranostic Applications of Neurological Diseases

Neurological diseases are the foremost cause of disability and the second leading cause of death worldwide. Owing to the special microenvironment of neural tissues and biological characteristics of neural cells, a considerable number of neurological disorders are currently incurable. In the past few years, the development of nanoplatforms based on metal-organic frameworks (MOFs) has broadened opportunities for offering sensitive diagnosis/monitoring and effective therapy of neurology-related diseases. In this article, the obstacles for neurotherapeutics, including delayed diagnosis and misdiagnosis, the existence of blood brain barrier (BBB), off-target treatment, irrepressible inflammatory storm/oxidative stress, and irreversible nerve cell death are summarized. Correspondingly, MOFs-based diagnostic/monitoring strategies such as neuroimaging and biosensors (electrochemistry, fluorometry, colorimetry, electrochemiluminescence, etc.) and MOFs-based therapeutic strategies including higher BBB permeability, targeting specific lesion sites, attenuation of neuroinflammation/oxidative stress as well as regeneration of nerve cells, are extensively highlighted for the management of neurological diseases. Finally, the challenges of the present research from perspective of clinical translation are discussed, hoping to facilitate interdisciplinary studies at the intersections between MOFs-based nanoplatforms and neurotheranostics.

Metal-Organic Frameworks as Sensors for Human Amyloid Diseases

Metal-organic frameworks (MOFs) are versatile compounds with emergent applications in the fabrication of biosensors for amyloid diseases. They hold great potential in biospecimen protection and unprecedented probing capabilities for optical and redox receptors. In this Review, we summarize the main methodologies employed in the fabrication of MOF-based sensors for amyloid diseases and collect all available data in the literature related to their performance (detection range, limit of detection, recovery, time of analysis, among other parameters). Nowadays, MOF sensors have evolved to a point where they can, in some cases, outperform technologies employed in the detection of several amyloid biomarkers (amyloid β peptide, α-synuclein, insulin, procalcitonin, and prolactin) present in biological fluids, such as cerebrospinal fluid and blood. A special emphasis has been given by researchers on Alzheimer's disease monitoring to the detriment of other amyloidosis that are underexploited despite their societal relevance (e.g., Parkinson's disease). There are still important obstacles to overcome in order to selectively detect the various peptide isoforms and soluble amyloid species associated with Alzheimer's disease. Furthermore, MOF contrast agents for imaging peptide soluble oligomers in living humans are also scarce (if not nonexistent), and action in this direction is unquestionably required to clarify the contentious link between the amyloidogenic species and the disease, guiding research toward the most promising therapeutic strategies.

Metal-organic frameworks: A promising option for the diagnosis and treatment of Alzheimer's disease

Beta-amyloid (Aβ) peptide is one of the main characteristic biomarkers of Alzheimer's disease (AD). Previous clinical investigations have proposed that unusual concentrations of this biomarker in cerebrospinal fluid, blood, and brain tissue are closely associated with the AD progression. Therefore, the critical point of early diagnosis, prevention, and treatment of AD is to monitor the levels of Aβ. In view of the potential of metal-organic frameworks (MOFs) for diagnosing and treating the AD, much attention has been focused in recent years. This review discusses the latest advances in the applications of MOFs for the early diagnosis of AD via fluorescence and electrochemiluminescence (ECL) detection of AD biomarkers, fluorescence detection of the main metal ions in the brain (Zn²⁺, Cu²⁺, Mn²⁺, Fe³⁺, and Al³⁺) in addition to magnetic resonance imaging (MRI) of the Aβ plaques. The current challenges and future strategies for translating the in vitro applications of MOFs into in vivo diagnosis of the AD are discussed.

Cell Membrane and V2C MXene-Based Electrochemical Immunosensor with Enhanced Antifouling Capability for Detection of CD44

The inactive adsorption and interference of biomolecules in electrochemical biosensors is a topic of intense interest. Directly utilizing native cell membranes to endow electrochemical surfaces with antifouling and biocompatible features is a promising strategy, rather than attempting to synthetically replicate complex biological interface properties. In this study, we present a facial and sensitive sandwich-type antifouling immunoassay through platelet membrane/Au nanoparticle/delaminated V₂C nanosheet (PM/AuNPs/d-V₂C)-modified electrode as the substrate of sensing interface and methylene blue/aminated metal organic framework (MB@NH₂-Fe-MOF-Zn) as an electrochemical signal probe. The biosensor perfectly integrates the high conductivity of AuNPs-loaded V₂C MXene with the excellent loading property of NH₂-Fe-MOF-Zn to improve the electrochemical sensing performance. In addition, the excellent antifouling properties of the homogeneous cell membrane can effectively prevent the non-specific adsorption of model proteins. The obtained antifouling biosensor possesses the capability of ultrasensitive detection of CD44 and CD44-positive cancer cell in complex liquids and exhibits good analytical performance for the analysis of CD44 with a linear range from 0.5 ng/mL to 500 ng/mL. This strategy of developing cell membrane-based biosensing systems with enhanced antifouling capability can be easily expanded to the construction of other complex biosensors, and the advanced biological probes and analytical methods provide a favorable means to accurately quantify biomarkers associated with tumor progression.

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.

Electrocatalysis of Copper Sulfide Nanoparticle-Engineered Covalent Organic Frameworks for Ratiometric Electrochemical Detection of Amyloid-β Oligomer

Amyloid-β oligomer (AβO) is widely regarded as a reliable biomarker for the early diagnosis of Alzheimer's disease (AD). In this study, a signal on-off ratiometric electrochemical immunosensor has been developed for ultrasensitive detection of AβO. To achieve the dual-signal ratiometric strategy, ultrasmall copper sulfide nanoparticle-engineered covalent organic framework hybrid nanocomposites (CuS@COFs) were utilized as excellent electrocatalysts toward hydroquinone (HQ) oxidation to produce detectable signals. Meanwhile, electroactive thionine (Thi) and Aβ antibody-modified gold nanoparticles (Thi-AuNPs-Ab bioconjugates) were designed as another electrochemical indicator. Based on these two signals, an ultrasensitive sandwich-like electrochemical immunosensor was established for AβO detection. The introduction of AβO resulted in a remarkable decline in the electrochemical signal of HQ but an increase in the signal of Thi. Under optimum conditions, the ratios between the double signals (I_Thi/I_HQ) showed a proportional linear relationship with the AβO concentration (1 pM-1 μM) with a low detection limit of 0.4 pM (S/N = 3), and the biosensor was able to determine the content of AβO in real cerebrospinal fluid samples with satisfactory results. The ratiometric strategy proposed in our study offers a sensitive and efficient approach for early diagnosis of AD, and this work will promote the further applications of engineered COFs in electrochemical sensors.

Introducing a Synergistic Ligand Containing an Exotic Metal in Metal-Organic Framework Nanoarrays Enabling Superior Electrocatalytic Water Oxidation Performance

Designing and fabricating well-aligned metal-organic framework nanoarrays (MOF NAs) with high electrocatalytic activity and durability for water oxidation at large current density remain huge challenges. Here the vertical NiFc-MOF NAs constructed from agaric-like nanosheets were fabricated by introducing a ligand containing an exotic Fe atom to coordinate with Ni ion using Ni(OH)₂ NAs as a self-sacrificing template. The NiFc-MOF NAs exhibited superior water oxidation performance with a very low overpotential of 161 mV at the current density of 10 mA cm^-2. Chronoamperometry was tested at an overpotential of 250 mV, which delivered an initial industrial-grade current density of 702 mA cm^-2 and still remained at 694 mA cm^-2 after 24 h. Furthermore, it possessed fast reaction kinetics with a small Tafel slope of 29.5 mV dec^-1. The superior electrocatalytic performance can be ascribed to the structural advantage of vertically grown agaric-like NAs and the synergistic electron coupling between Ni and Fe atoms, namely, electron transfer from Ni to Fe atoms in NiFc-MOF NAs. The exposed density and valence state of active Ni sites were synchronously increased. Furthermore, the energy barrier for the adsorption/desorption of oxygenated intermediates was ultimately optimized for water oxidation. This work provides a novelty orientation to accelerate electrocatalytic performance of MOF NAs by introducing self-sacrificing templates containing one metal and synergistic ligand containing dissimilar metal.

A sandwich-type electrochemical immunosensor for CYFRA 21-1 based on probe-confined in PtPd/polydopamine/hollow carbon spheres coupled with dendritic Au@Rh nanocrystals

A signal-on sandwich-like electrochemical immunosensor was built for determination of cytokeratin 19 fragments 21-1 (CYFRA 21-1) in non-small cell lung cancer (NSCLC) by confining electroactive dye (e.g., methylene blue, MB) as a probe for amplifying signals. Specifically, core-shell gold@rhodium dendritic nanocrystals (Au@Rh DNCs) behaved as a substrate for primary antibody and accelerate interfacial electron transfer. Besides, hollow carbon spheres (HCSs) were subsequently modified with polydopamine (PDA) and PtPd nanoparticles for sequential integration of the secondary antibody and confinement of MB as a label, termed as MB/PtPd/PDA/HCSs for clarity. The built sensors showed a broad linear range (100 fg mL^-1 ~ 100 ng mL^-1) for detection of CYFRA 21-1 with an ultra-low detection limit (31.72 fg mL^-1, S/N = 3), coupled with satisfactory performance in human serum samples. This work can be explored for assays of other proteins and provides some constructive insights for early and accurate diagnosis of NSCLC.

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.

Ratiometric Electrochemical Immunosensor Based on L-cysteine Grafted Ferrocene for Detection of Neuron Specific Enolase

In order to realize the ultra sensitive detection of Neuron specific enolase (NSE) in human serum, we chose electrochemical immunosensor as a simple analytical method. During the experiment, we found that the peak value signals of Cu-MOFs-Au and Fc-_L-Cys were significantly changed at -0.20 V and 0.20 V potentials by DPV. So a ratiometric electrochemical immunosensor for quantitative analysis of NSE was prepared for Cu-MOFs-Au as the electrode sensing surface and Fc-_L-Cys as the label of Ab₂. The data and performances of the immunosensor were tested and analyzed by DPV. Cu-MOFs not only provide the required signal for the immunosensor, but also have a large specific surface area, which can provide more sites for the placement of Au nanoparticles. _L-cysteine (_L-Cys) can prevent a large amount of Fc-COOH leakage, so that Fc⁺ can stably provide another required signal. With the beefing up of NSE concentration, redox peak of Cu-MOFs-Au decreased and that of Fc-_L-Cys raised. The ratio (ΔI=ΔI_Cu/ΔI_Fc) of two different signals was linear with the logarithm of NSE concentration in a certain value range. In brief, with the optimized experimental conditions, the immunosensor showed excellent performance in the concentration range of 1 pg/mL to 1 μg/mL, and the detection limit was 0.011 pg/mL. Compared with other immunosensors, it showed an unexpected high sensitivity. This method also provided a new idea for the ultra sensitive quantitative detection of other biomarkers.

Concluding remarks: current and next generation MOFs

This paper describes the content of my "Concluding remarks" talk at the Faraday Discussion meeting on "MOFs for energy and the environment" (online, 23-25 June 2021). The panel consisted of sessions on the design of MOFs and MOF hybrids (synthetic chemistry), their applications (e.g., capture, storage, separation, electrical devices, photocatalysis), advanced characterization (e.g., transmission electron microscopy, solid-state nuclear magnetic resonance), theory and modeling, and commercialization. MOF chemistry is undergoing a significant evolution from simply network chemistry to the chemistry of synergistic integration with heterogeneous materials involving other disciplines (we call this the fourth generation type). As reflected in the papers of the invited speakers and discussions with the participants, the present and future of this field will be described in detail.

Fluorescent enzyme-linked immunosorbent assay based on alkaline phosphatase-responsive coordination polymer composite

The fabrication of alkaline phosphatase (ALP)-responsive coordination polymer (CP) composite is demonstrated for establishing a fluorescent immunoassay. The CP composite (ThT@GMP/Eu) was synthesized by encapsulating thioflavin T (ThT) into the CP host that was composed of europium ion (Eu³⁺) and guanine monophosphate (GMP). The ThT@GMP/Eu composite shows a strong fluorescence in aqueous solution due to the confinement effect of GMP/Eu CPs, which restricts the conformational rotation of ThT. However, upon the addition of ALP, the structure of GMP/Eu CPs was disrupted to release ThT into solution. This results in the quenching of the fluorescence of ThT@GMP/Eu. The fluorescence of ThT@GMP/Eu has a linear response that covers 0.8 to 120 mU/mL ALP with a detection limit of 0.26 mU/mL and exhibits excellent specificity towards ALP against other enzymes. On this basis, inspired by the wide application of ALP as an enzyme label in enzyme-linked immunosorbent assay (ELISA), an ALP-based fluorescent immunoassay was further developed for the detection of mouse immunoglobulin G (mIgG). The developed immunoassay displays a linear fluorescent response towards mIgG from 0.8 to 100 ng/mL, and the detection limit is 0.16 ng/mL. The fluorescent immunoassay was successfully applied to the determination of mIgG in serum samples. Schematic of the responsivity of ThT@GMP/Eu to ALP that hydrolyzes GMP to release ThT, which leads to fluorescent quenching, and its application in the construction of a fluorescent immunoassay for mIgG determination.

ZEUS (ZIF-based electrochemical ultrasensitive screening) device for isopentane analytics with focus on lung cancer diagnosis

Breath analytics is currently being explored for the development of point-of-care devices in non-invasive disease detection. It is based on the measurement of volatile organic compounds (VOCs) and gases that are produced by the body because of the metabolic pathways. The levels of these metabolites vary due to alteration in the endogenous oxidative stress-related metabolic pathways and can be correlated to understand the underlying disease condition. The levels of exhaled hydrocarbons in human breath can be used to design a rapid, easy to use method for lung cancer detection. This work outlines the development of an electrochemical sensing platform that can be used for the non-invasive diagnosis of lung cancer by monitoring isopentane levels in breath. This electrochemical sensor platform involves the use of [BMIM]BF4@ZIF-8 for sensing the target analyte. This synthesized nanocomposite offers advantages for gas sensing applications as it possesses unique properties such as an electrochemically active Room Temperature Ionic Liquid (RTIL) and a crosslinking Metal Organic Framework (MOF) that provides increased surface area for gas absorption. This is the first report of a hydrocarbon-based sensor platform developed for lung cancer diagnosis. The developed sensor platform displays sensitivity and specificity for the detection of isopentane up to 600 parts-per-billion. We performed structural and morphological characterization of the synthesized nanocomposite using various analytical techniques such as PXRD, FESEM, FTIR, and DLS. We further analyzed the electrochemical activity of the synthesized nanocomposite using a standard glassy carbon electrode. The application of the nanocomposite for isopentane sensing was done using a commercially available carbon screen printed electrode. The results so obtained helped in strengthening our hypothesis and serve as a proof-of-concept for the development of a breathomics-enabled electrochemical strategy. We illustrated the specificity of the developed nanocomposite by cross-reactivity studies. We envision that the detection platform will allow sensitive and specific sensing of isopentane levels such that it can used for point of care applications in noninvasive and early diagnosis of lung cancer, thereby leading to its early treatment and decrease in mortality rate.

Fenton reaction-mediated dual-attenuation of signal for ultrasensitive amperometric immunoassay

In order to alter the complexion of immunoprobe with large impedance as negative factor in sensitivity of amperometric immunosensor, a strategy of Fenton reaction-mediated dual-attenuation of signal was proposed. Herein, metal-polydopamine-Fe³⁺ composite with the ability of Fenton reaction was initially prepared as immunoprobe for an ultrasensitive immunoassay. The polymerization of dopamine occurred on the surface of ZIF-67 to gain the metal-polydopamine shell, which possessed rich functional groups, negative charge and high specific surface. Then the prepared functional shell was further used to absorb Fe³⁺ and immobilize labeling antibody as immunoprobe, which was used to construct a sandwich type immunosensor. With addition of H₂O₂ and aniline, Fenton reaction was triggered to produce hydroxyl radicals, which can not only decrease the current value by degrading methylene blue molecules, but also further initiate aniline to polymerize into non-conductive polyaniline for successive abatement of signal intensity. Therefore, the dual-attenuation of signal model rendered the immunoprobe into a favorable factor and synchronously enhance sensitivity. Expectedly, the detection performance with a linear range from 1.0 × 10^-4-100 ng mL^-1 and ultralow detection limit of 9.07 × 10^-5 ng mL^-1 toward neuron-specific enolase was obtained under optimal conditions. This work offered a novel tactic for enhancing sensitivity of immunosensor through the preparation of functional immunoprobe and its rational utilization as signal enhancer.

ZENose (ZIF-Based Electrochemical Nose) Platform for Noninvasive Ammonia Detection

Breathomics is a widely emerging tool for noninvasive disease diagnosis and focuses on the detection of various levels of volatile organic compounds and inorganic gases present in human breath. One of the rapid, easy-to-use, and noninvasive detection methods being investigated is a system that can measure exhaled breath ammonia levels and can be correlated to the functional state of protein metabolic pathways and the renal functioning system. In this work, we have demonstrated the development of an electrochemical nose system using ferrocene encapsulated into zeolitic imidazole framework, Fc@ZIF-8, which can be successfully used for the detection of ammonia levels in breath. This is the first report of an electrochemical gas sensor platform that uses a faradaic probe (that is ferrocene) encapsulated into a metal-organic framework cavity used for disease diagnosis by monitoring the levels of the target gas and can be used for breathomics applications. This work demonstrates that low levels of ammonia gas (up to 400 ppb) can be detected with high sensitivity and specificity. The morphological and structural characterization of the novel, synthesized Fc@ZIF-8 nanocomposite has been performed using powder X-ray diffraction, field emission scanning electron microscopy, Fourier transform infrared, ultraviolet-visible spectroscopy, and dynamic light scattering. Electrochemical characterization of the material has been performed using a standard glassy carbon electrode, and further application of the material has been shown using the in-house designed and reported spiral electrochemical notification coupled electrode, used for ammonia gas sensing. Cross-reactivity studies have also been performed to demonstrate sensor specificity toward the target gas. We demonstrate the first of its kind electrochemical bifunctional probe platform that can be used for sensing ammonia levels in breath, with high sensitivity and specificity, due to the hybrid material system-zinc-imidazole framework 8 (having excellent physisorption properties) and ferrocene (acting as a redox mediator). We envision that such a sensing system will allow noninvasive and early diagnosis of chronic kidney disease, thus leading to early treatment and a decrease in the mortality rate.

An electrochemical aptasensor based on AuPt alloy nanoparticles for ultrasensitive detection of amyloid-β oligomers

Amyloid-β oligomer is an important biomarker and a potential therapeutic target of Alzheimer's disease in its early stage. Here, we combined superhydrophobic carbon fiber paper (CFP) with AuPt alloy nanoparticles to prepare a CFP/AuPt nanocomposite with larger specific surface area and hydrophobic surface. On this basis, we constructed an electrochemical aptasensor based on CFP/AuPt for the ultrasensitive detection of amyloid-β oligomers. The surface-coated AuPt nanoparticles greatly enhanced the electroactive area, and the hydrophobic surface increased the resisting nonspecific adsorption performance of sensor. A combination of these two features significantly improved the sensitivity and specificity of the sensor. This electrochemical aptasensor based on CFP/AuPt displayed a low detection limit of 0.16 pg/mL. This work shows a promising future in clinical diagnosis of Alzheimer's disease and provides a possible solution to electrochemical biosensors that are susceptible to interference in biological fluids.

A three-dimensional CoNi-MOF nanosheet array-based immunosensor for sensitive monitoring of human chorionic gonadotropin with core-shell ZnNi-MOF@Nile Blue nanotags

A CoNi-based metal-organic framework (CoNi-MOF) nanosheet array is synthesized by the treatment of a CoNi layered double hydroxide nanosheet array on Ni foam with 3,5-diaminobenzoic acid. The CoNi-MOF nanosheet array with amino and carboxyl groups can be used to capture the human chorionic gonadotropin (HCG) primary antibody (HCG Ab1). Nile Blue decorated ZnNi-MOF (NB@ZnNi-MOF) spheres immobilized with HCG secondary antibodies (HCG Ab2) are used for signal amplification. When HCG exists in an analytical sample, a sandwich structure is formed and an electrochemical signal is produced. The analytical signal generated during the detection is caused by the conversion of Co(ii) and Co(iii) in the CoNi-MOF nanosheet array. The Nile Blue of the NB@ZnNi-MOF sphere, as a kind of redox-active species, is responsible for the electrochemical signal amplification in the immunosensor. On the basis of the above advantages, the HCG immunosensor exhibits a lower limit of detection (1.85 × 10-3 mIU mL-1) and a wide linear range from 0.005 mIU mL-1 to 250 mIU mL-1. Additionally, this immunosensor is used to quantitatively detect HCG in human blood serum and shows good correlations with the standard enzyme-linked immunosorbent assay (ELISA), providing a high value on clinical diagnosis.

Adsorption of Amyloid Beta Peptide by Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are capable of adsorbing a wide range of molecules. In addition to the more commonly investigated small molecules, researchers have demonstrated that MOFs adsorb much larger molecules, such as proteins and peptides. We have investigated whether MOFs are capable of adsorbing amyloid beta peptide. Amyloid beta plays a pivotal role in the progression of Alzheimer's disease because individual copies of the peptides can aggregate, forming neurotoxic oligomers and the amyloid plaques found in brains of Alzheimer's patients. After synthesizing a number of commonly studied MOFs, their adsorption capabilities were tested. We found that the MOFs tested readily adsorbed small amounts of amyloid beta (as determined by gel electrophoresis). It was determined that in most cases, adsorption occurs rapidly, with complete adsorption within minutes of incubation. Overall adsorption capacity was found to vary between different MOFs as well. Once adsorbed, amyloid beta peptide can subsequently be eluted from some MOFs by treatment with acetonitrile/water solutions, though retention strength varied between different MOFs. In some cases, MOFs that showed complete adsorption also saw high levels of peptide elution, but others showed little to no elution of the peptide. Together these data can help us begin to understand the interactions between amyloid beta and MOFs.

Electrochemical Applications of Ferrocene-Based Coordination Polymers

Ferrocene and its derivatives, especially ferrocene-based coordination polymers (Fc-CPs), offer the benefits of high thermal stability, two stable redox states, fast electron transfer, and excellent charge/discharge efficiency, thus holding great promise for electrochemical applications. Herein, we describe the synthesis and electrochemical applications of Fc-CPs and reveal how the incorporation of ferrocene units into coordination polymers containing other metals results in unprecedented properties. Moreover, we discuss the usage of Fc-CPs in supercapacitors, batteries, and sensors as well as further applications of these polymers, for example in electrocatalysts, water purification systems, adsorption/storage systems.

Ferrocene-functionalized nanocomposites as signal amplification probes for electrochemical immunoassay of Salmonella typhimurium

An electrochemical immunosensor based on ferrocene (Fc)-functionalized nanocomposites was fabricated as an efficient electroactive signal probe to amplify electrochemical signals for Salmonella typhimurium detection. The electrochemical signal amplification probe was constructed by encapsulating ferrocene into S. typhimurium-specific antimicrobial peptides Magainin I (MI)-Cu₃(PO₄)₂ organic-inorganic nanocomposites (Fc@MI) through a one-step process. Magnetic beads (MBs) coupled with antibody were used as capture ingredient for target magnetic separation, and Fc@MI nanoparticles were used as signal labels in the immunoassays. The sandwich of MBs-target-Fc@MI assay was performed using a screen-printed carbon electrode as transducer surface. The immunosensor platform presents a low limit of detection (LOD) of 3 CFU·mL^-1 and a linear range from 10 to 10⁷ CFU·mL^-1, with good specificity and precision, and was successfully applied for S. typhimurium detection in milk. Graphical abstract One-pot process antimicrobial peptides Magainin I-Cu₃(PO₄)₂ organic-inorganic nanocomposites (Fc@MI) were used as ideal electrochemical signal label, integrating both essential functions of biological recognition and signal amplification. Screen-printed carbon electrode (SPCE) was used as the electrochemical system for Salmonella typhimurium detection.

Gold nanoparticles decorated bimetallic CuNi-based hollow nanoarchitecture for the enhancement of electrochemical sensing performance of nitrite

Gold nanoparticles (AuNPs) decorated bimetallic CuNi-based hollow nanoarchitecture (CNHN) are reported for the first time as a nonenzymatic sensor for the quantification of nitrite in neutral solution . The CNHN was prepared via a convenient calcining routine using the bimetallic CuNi-MOFs as a coprecursor. The unique chemical structure of hollow CNHN with high specific surface area and abundant terminal amino groups effectively avoid the aggregation of AuNPs and facilitate the subsequent adsorption of nitrite. The Au/CNHN exhibited high electrocatalytic activity towards nitrite oxidation due to the synergetic catalytic effect of AuNPs and CNHN. Chronoamperometric detection of nitrite at the Au/CNHN/GCE achieved a lower linear calibration range of 0.05 to 1.15 mM, with an LOD of 0.017 μM compared with previous reports. The proposed method obtained satisfactory recoveries for nitrite determination in practical applications, which was verified by UV-Vis spectrophotometry. The prepared sensor based on Au/CNHN featured favorable selectivity and stability, which provides a promising approach for real sample analysis. Graphical abstract.

Electrochemical Biosensors Based on Nanomaterials for Early Detection of Alzheimer's Disease

Alzheimer's disease (AD) is an untreatable neurodegenerative disease that initially manifests as difficulty to remember recent events and gradually progresses to cognitive impairment. The incidence of AD is growing yearly as life expectancy increases, thus early detection is essential to ensure a better quality of life for diagnosed patients. To reach that purpose, electrochemical biosensing has emerged as a cost-effective alternative to traditional diagnostic techniques, due to its high sensitivity and selectivity. Of special relevance is the incorporation of nanomaterials in biosensors, as they contribute to enhance electron transfer while promoting the immobilization of biological recognition elements. Moreover, nanomaterials have also been employed as labels, due to their unique electroactive and electrocatalytic properties. The aim of this review is to add value in the advances achieved in the detection of AD biomarkers, the strategies followed for the incorporation of nanomaterials and its effect in biosensors performance.

Design principles and fundamental understanding of biosensors for amyloid-β detection

Aβ as biomarker in Alzheimer’s disease (AD) drives the significant research efforts for developing different biosensors with different sensing strategies, materials, and mechanisms for Aβ detection.

Ferrocene-Encapsulated Zn Zeolitic Imidazole Framework (ZIF-8) for Optical and Electrochemical Sensing of Amyloid-β Oligomers and for the Early Diagnosis of Alzheimer's Disease

In this work, the ferrocene-encapsulated Zn zeolitic imidazole framework (ZIF-8) was prepared by the self-assembly of Zn ions and 2-methylimidazole and used for the dual detection of amyloid-beta oligomers (AβO), which is the main neuropathological hallmark of Alzheimer's disease. Ferrocene is an optically and electrochemically active signal which was successfully encapsulated inside of the ZIF-8 and released by the competitive coordination between Zn ions and AβO after being treated with AβO. The released ferrocene content was monitored by ultraviolet/visible spectrophotometry and cyclic voltammetry. The dual determination of AβO played a synergetic role in the quick qualitative and precise quantitative analyses in a wide detection range of 10^-5 to 10² μM and good feasibility in artificial cerebrospinal fluid.

Three dimensional sea-urchin-like PdAuCu nanocrystals/ferrocene-grafted-polylysine as an efficient probe to amplify the electrochemical signals for ultrasensitive immunoassay of carcinoembryonic antigen

A novel sandwich-like immunosensor was efficiently fabricated for detection of carcinoembryonic antigen (CEA) with three dimensional sea-urchin-like PdAuCu nanocrystals (PdAuCu NCs)/ferrocene-grafted-polylysine (Fc-g-PLL) as the label of secondary antibodies (Ab₂) and Au nanoparticles (Au NPs) as the substrate material. Herein, PdAuCu NCs were directly synthesized with polyethylene oxide (PEO) as a growth-directing agent by a facile one-step aqueous method without any organic solvent. Meanwhile, Fc-g-PLL was obtained by covalent linkage of Fc with PLL via Schiff-base reaction. The well-dispersed PdAuCu NCs by Fc-g-PLL have the enlarged surface area, enhanced catalytic properties and superior biocompatibility to amplify the current signals. The resultant immunosensor shows linear relationship of the electrochemical responses with the CEA concentrations within a broader linear range (0.001-100.0 ng mL^-1) and a lower detection limit (0.23 pg mL^-1, S/N = 3). Furthermore, the immunosensor was explored for practical assay of CEA in human serum samples with accredited results. The novel immunoassay provides a feasible platform for early medical diagnosis.

Zn and Co redox active coordination polymers as efficient electrocatalysts

New redox active 1D helical coordination polymers M(fcdHp) (M(ii) = Zn(1), Co(2)) have been obtained.

Highly stable Ni-MOF comprising triphenylamine moieties as a high-performance redox indicator for sensitive aptasensor construction

Electroactive metal-organic frameworks (MOFs) with large surface area and manipulatable structural properties show promise as a new type of signal probe for electrochemical biosensing application. In this work, an electroactive Ni-MOF, assembled by the redox-active ligands 4,4',4″-Tricarboxytriphenylamine (H₃TCA), a triphenylamine derivatives, as the electroactive source and magnetic ordered Ni₄O₄ clusters as electronic transport nodes, is first designed and applied for electrochemical aptasensing of thrombin (Tb). The designed Ni-MOF probe realizes a stable and sensitive electrochemical signal output based on simple sandwich-type aptasensing because the high-content TCA active sites and good magnetic ordered intermediate of Ni₄O₄ clusters are periodically arranged in well-defined porous structure of the MOF. The Ni-MOF probe assembled by redox-active ligand presents the high stability and can be directly applied in electrochemical aptasensor, avoiding any post-modification and the addition of redox mediators. As a result, the constructed electrochemical aptasensor shows a wide linear relationship for Tb from 0.05 pM to 50 nM and a detection limit of 0.016 pM (S/N = 3). Furthermore, the proposed aptasensor is successfully applied to analysis of target Tb in real serum sample with satisfactory results. The present work indicates that fabricating a redox-active organic molecule in functionalized MOFs offer a feasible strategy to design high-stable electroactive MOFs for construction of electrochemical biosensors with simplicity, high selectivity and sensitivity.

Integrated Antibody with Catalytic Metal-Organic Framework for Colorimetric Immunoassay

Enzyme-linked immunosorbent assay has been widely used as a gold standard in biomedical field, but some inevitable drawbacks still exist in its practical applications, especially the laborious preparation of enzyme-antibody conjugates by a covalent linkage. In this work, we proposed a new strategy to prepare enzyme-antibody conjugate by integrating antibody with catalytic metal-organic framework (MOF) to form dual-functional MOF/antibody composite. As models, rabbit antimouse immunoglobulin G antibody (RIgG) and Cu-MOF with peroxidase-like activity were used to fabricate RIgG@Cu-MOF composite for colorimetric immunoassay. It was found that Cu-MOF as a host not only has no influence on the original capture ability of RIgG to its corresponding antigen (mIgG), but also can shield RIgG against long-term storage, high temperature, and biological degradation. More importantly, upon the formation of sandwiched immunocomplex between RIgG@Cu-MOF and capture antibody, Cu-MOF can serve as a signal amplification unit to perform colorimetric immunoassay. The detection limit of RIgG@Cu-MOF toward mIgG was obtained at 0.34 ng/mL, which is 3-fold lower than that of horseradish peroxidase labeled RIgG. Furthermore, the successful determination of mIgG in serum sample demonstrates the applicability of RIgG@Cu-MOF in detecting real sample. Therefore, it is highly anticipated that this study can offer a new way to prepare enzyme-antibody conjugates, facilitating the exploration of MOF composites in biomedical field.

Role of the cell membrane interface in modulating production and uptake of Alzheimer's beta amyloid protein

The beta amyloid protein (Aβ) plays a central role in Alzheimer's disease (AD) pathogenesis and its interaction with cell membranes in known to promote mutually disruptive structural perturbations that contribute to amyloid deposition and neurodegeneration in the brain. In addition to protein aggregation at the membrane interface and disruption of membrane integrity, growing reports demonstrate an important role for the membrane in modulating Aβ production and uptake into cells. The aim of this review is to highlight and summarize recent literature that have contributed insight into the implications of altered membrane composition on amyloid precursor protein (APP) proteolysis, production of Aβ, its internalization in to cells via permeabilization and receptor mediated uptake. Here, we also review the various membrane model systems and experimental tools used for probing Aβ-membrane interactions to investigate the key mechanistic aspects underlying the accumulation and toxicity of Aβ in AD.