Bimetallic MOF synergy molecularly imprinted ratiometric electrochemical sensor based on MXene decorated with polythionine for ultra-sensitive sensing of catechol

Simultaneous detection of heavy metal ions in food samples using a hypersensitive electrochemical sensor based on APTES-incubated MXene-NH2@CeFe-MOF-NH2

Heavy metal ions (HMIs) pollution has become a significant food safety concern owing to rapid urbanization and industrialization. In this study, a hypersensitive electrochemical sensor based on amino-functionalized MXene and bimetallic atom MOFs nanocomposites (MXene-NH2@CeFe-MOF-NH2) was developed and applied for the detection of HMIs. MXene-NH2@CeFe-MOF-NH2 nanocomposites were obtained by incubating MXene@CeFe-MOF prepared by hydrothermal and self-assembly methods in APTES to endow them with a large amount of surface amine functional groups. The introduction of -NH2 could lead to a coordination effect between the nanocomposites and HMIs, promoting the enrichment efficiency of HMIs on the surface of the working electrode. The sensing platform demonstrated excellent detection performance, with limit of detection (LOD) values of 0.69 nM, 0.95 nM, and 0.33 nM for Cd2+, Pb2+ and Hg2+, respectively, which were far below the maximum residue levels specified by the Chinese standards. Furthermore, the sensor was employed to analyze representative real samples (fish, whole milk, rice, and corn) to simulate an accurate analysis in complex scenarios and successfully detect the three HMIs simultaneously.

Metal-organic frameworks for NH3 adsorption and separation

Ammonia (NH3) is not only an air pollutant but also a versatile and favourable chemical with widespread applications in human life. As a key component of nitrogen fertilizers, it plays a crucial role in improving crop yields. Additionally, NH3 serves as a hydrogen carrier and working fluid, contributing to the energy transition process. Given the diverse roles of NH3 and the varying requirements for adsorbents across different application scenarios, the rational design and selection of adsorbent materials are paramount. Metal-organic frameworks (MOFs) have emerged as promising adsorbent candidates due to their highly tunable structure and functionality, which can precisely match the characteristics required for NH3 adsorbents in multiple application scenarios. This review provides a comprehensive evaluation of NH3 adsorbents and delves into the stability characterization of MOFs under NH3 atmospheres and the underlying adsorption/degradation mechanisms. Additionally, we discuss the existing methods used to probe the host-guest interactions between MOFs and NH3. Finally, this study systematically summarizes the latest advancements of MOFs as NH3 adsorbents and classifies them according to the different requirements imposed by the varying roles of NH3. This review provides theoretical support for the design of more efficient NH3 adsorbents in the future.

Boron Ligands Boosting the Electrochemiluminescence Performance of Europium Metal-Organic Frameworks by Facilitating the Electronic Bridging

For optimal energy transfer in self-luminous lanthanide metal-organic frameworks (Ln-MOFs), the energy of the lowest triplet excited state must align with ideal energy levels. Failure to meet this condition can lead to reverse energy transfer, reducing luminous efficiency. In this study, we developed a mixed-ligand MOF, Eu-TCPP-BOP, which exists as an ECL self-enhancing luminophore. We used SPECM to study the role of boron ligands as a bridge for electron transport in improving the ECL performance of Eu-TCPP. The ligands H4TCPP and 5-BOP act as electron donor and shuttle, facilitating electron transport during the synthesis of Eu-TCPP-BOP and promoting energy transfer to the excited state of the acceptor Ln3+, thus enhancing overall energy transfer in Ln-MOF. The results indicate that the introduction of boron ligands enhances the ECL intensity of Eu-TCPP by a factor of 1.4 under voltage excitation. As an ECL sensing platform, it demonstrates high sensitivity and selectivity for the detection of catechol, with a concentration range of 1∼70 μM and a detection limit of 0.35 μM.

A high-sensitivity label-free electrochemical aptasensor for point-of-care measurements of low-density lipoprotein in plasma based on aptamer and MXene-CMCS-Hemin nanocomposites

Cardiovascular disease (CVD) remains a significant worldwide health challenge, with mortality rates rising rapidly. Low-density lipoprotein (LDL) is a crucial serum biomarker for the early diagnosis of CVD, which can significantly improve outcomes and reduce mortality. Herein, a label-free electrochemical aptasensor for rapid detection of LDL was developed based on the titanium carbide-carboxymethyl chitosan-hemin (MXene-CMCS-Hemin) nanocomposites as the electrochemical signal probe. Firstly, gold nanoparticles (Au NPs) were electrodeposited onto a screen-printed carbon electrode (SPCE) to form a conductive substrate. Subsequently, the MXene-CMCS-Hemin nanocomposites were anchored onto the Au NPs/SPCE surface. Then LDLApt was immobilized on the surface of MXene-CMCS-Hemin/Au NPs/SPCE to construct the electrochemical aptasensor. When LDL specifically bound with the LDLApt to form LDL-LDLApt complexes, hindering the electron transfer and reducing the Hemin oxidation current, LDL detection can be achieved via differential pulse voltammetry (DPV). Under optimal circumstances, the changes of Hemin's oxidation current showed a good linear response with LDL concentration in the range of 0.1-4.0 μmol/L with a detection limit of 0.095 μmol/L (S/N = 3). The aptasensor demonstrated good performance with the relative errors of 0.60 % to 6.58 % for the direct detection of LDL in human serum samples, which offers a novel tool for the clinical diagnosis of CVD.

Enhanced Electrochemical Detection of Valganciclovir Using a Hierarchically Structured Lisianthus Flower-Inspired Bimetallic Ni-Ce Organic Framework

This study reports the development of an innovative electrochemical sensor based on organometallic framework nanostructures for detecting valganciclovir (VLCV). VLCV is employed in the treatment of cytomegalovirus retinitis in AIDS patients. Rational design of nanoarchitectures for electroactive materials is a crucial approach for boosting their electrocatalytic performance. Herein, Lisianthus flower-inspired Ni-Ce-metal-organic framework (MOF), Ni-MOF, and rod-inspired Ce-MOF were synthesized by the solvothermal method. An electrochemical sensor for VLCV was developed by employing a multilayer approach using Lisianthus flower-inspired Ni-Ce metal-organic framework/multiwall carbon nanotubes (Ni-Ce-MOF/MWCNTs) modification on a glassy carbon electrode (GCE). Incorporating a bimetallic Ni-Ce-MOF into a conventional conductive material, such as MWCNTs, significantly increases the specific surface area and improves the conductivity and catalytic properties of the MWCNTs. Relative to the rod-inspired Ce-MOF and Ni-MOF, the electrocatalytic performance of the Lisianthus flower-inspired Ni-Ce-MOF coated on MWCNTs surpasses that of the rod-inspired Ce-MOF, showcasing enhanced performance in VLCV oxidation. This superiority arises from their enhanced electrical conductivity and enlarged surface area. The Lisianthus flower-inspired Ni-Ce-MOF/MWCNTs/GCE demonstrated extensive linear ranges (ranging from 4.0 to 3800.0 nM), a lower detection limit (1.4 nM), remarkable selectivity, and sustained stability over an extended period in the context of VLCV sensing. The real samples underwent analysis through using both electrochemical and UV-vis spectrophotometry methods, and the findings from both methods exhibited no substantial difference, validating the sensor's remarkable practical performance. These results suggest that Lisianthus flower-inspired Ni-Ce-MOF/MWCNTs/GCE electrocatalysts provide a promising sensing platform for analyzing biological samples.

Progress in MXenes and Their Composites as Electrode Materials for Electrochemical Sensing and Dye-Sensitized Solar Cells

In the present mini-review article, we have compiled the previously reported literature on the fabrication of MXenes and their hybrid composite materials based electrochemical sensors for the determination of phenolic compounds and counter electrodes for platinum (Pt)-free dye-sensitized solar cells (DSSCs). MXenes are two-dimensional (2D) materials with excellent optoelectronic and physicochemical properties. MXenes and their composite materials have been extensively used in the construction of electrochemical sensors and solar cell applications. In this paper, we have reviewed and compiled the progress in the construction of phenolic sensors based on MXenes and their composite materials. In addition, co1.unter electrodes based on MXenes and their composites have been reviewed for the development of Pt-free DSSCs. We believe that the present review article will be beneficial for the researchers working towards the development of phenolic sensors and DSSCs using MXenes and their composites as electrode materials.

Ratiometric electrochemical immunoassay based on 2D Co/Fe MOF decorated with toluidine blue and Fc-labeled Schiff base for accurate assay of alpha-fetoprotein in clinical serum

The high level of alpha-fetoprotein (AFP) expression is closely related to hepatocellular carcinoma (HCC). Herein, a dual signal ratiometric electrochemical immunosensor based on chitosan-ferrocenecarboxaldehyde-spindle gold (Chit-Fc-SAu) and Co/Fe metal-organic framework-toluidine blue/polydopamine (Co/Fe MOF-TB/PDA) was proposed for quantitative analysis of AFP. Specifically, Chit-Fc-SAu worked as a substrate to trap more primary antibodies (Ab1) generating the first electrochemical signal from Fc. Thanks to the large specific surface area, the synergistic and electronic effects of Co/Fe MOF nanosheets, and the rich functional groups of PDA, Co/Fe MOF-TB/PDA could load more secondary antibodies (Ab2) and signal molecules (TB) providing another amplified electrochemical signal. In the presence of AFP, Ab1-AFP-Ab2 formed a sandwich structure, and as the AFP concentration increased, the peak current ratio of TB to Fc (ITB/IFc) also increased. The dual signal ratiometric strategy can avoid environmental signal interference and achieve signal self-calibration, thereby improving the accuracy and reproducibility of detection. After a series of exploration, this self-calibrated ratiometric immunosensor exhibited a wide linear range (0.001-200 ng mL-1), a low detection limit (0.34 pg mL-1), and good repeatability. When applied to the assay of clinical serum samples, the detection results of ratiometric sensor were consistent with that of commercial electrochemiluminescence (ECL) immunoassay, significantly superior to that of non-ratiometric sensor. The self-calibrated strategy based on ratiometric sensor helps to improve the accuracy of AFP in clinical diagnosis.

Recent advancements in modifications of metal-organic frameworks-based materials for enhanced water purification and contaminant detection

Metal-organic frameworks (MOFs) have emerged as a key focus in water treatment and monitoring due to their unique structural features, including extensive surface area, customizable porosity, reversible adsorption, and high catalytic efficiency. While numerous reviews have discussed MOFs in environmental remediation, this review specifically addresses recent advancements in modifying MOFs to enhance their effectiveness in water purification and monitoring. It underscores their roles as adsorbents, photocatalysts, and in luminescent and electrochemical sensing. Advancements such as pore modification, defect engineering, and functionalization, combined synergistically with advanced materials, have led to the development of recyclable MOF-based nano-adsorbents, Z-scheme photocatalytic systems, nanocomposites, and hybrid materials. These innovations have broadened the spectrum of removable contaminants and improved material recyclability. Additionally, this review delves into the creation of multifunctional MOF materials, the development of robust MOF variants, and the simplification of synthesis methods, marking significant progress in MOF sensor technology. Furthermore, the review addresses current challenges in this field and proposes potential future research directions and practical applications. The growing research interest in MOFs underscores the need for an updated synthesis of knowledge in this area, focusing on both current challenges and future opportunities in water remediation.

Electrochemical sensing performance of two CuO nanomaterial-modified dual-working electrodes

Two CuO nanostructures, namely, nanospheres (CuONSs) and nanochains (CuONCs) with different shapes but similar diameters, were synthesized and characterized. With these two nanomaterials as electrode modifiers, a systematic comparative study was conducted to examine their electrochemical sensing of catechol (CT) using a dual-working electrode system. The results suggest that for CuONS- and CuONC-modified glassy carbon electrodes, the electrode process for the CT redox is diffusion-controlled, and the modification amount and electrolyte pH have a similar effect on the response. However, the CuONCs showed a higher peak current and lower peak potential, as well as a lower detection limit for the electrochemical oxidation of CT. This is explained by the lower charge transfer impedance and higher electroactive surface area of the CuONCs. Notably, an unexpected peak appeared in the cyclic voltammograms when the pH was <4 for the CuONCs and <3 for the CuONSs. For this phenomenon, UV-Vis spectra, zeta potential, and size distribution experiments demonstrated changes in the two CuO nanostructures at lower pH, illustrating that CuONSs can tolerate a higher pH as compared to CuONCs. The multiple comparisons between the two nanomaterials presented here can provide references for the selection of electrochemical sensing materials.

Molecularly imprinted electrochemical sensor based on CoNi-MOF/RGO nanocomposites for sensitive detection of the hippuric acid

Toluene, a volatile organic compound, may have adverse effects on the nervous and digestive system when inhaled over an extended period. The assessment of environmental toluene exposure can be effectively conducted by detecting hippuric acid (HA), a toluene metabolite. In this investigation, a molecularly imprinted electrochemical sensor was developed for HA detection, utilizing the synergistic effects of reduced graphene oxide (RGO) and a bimetallic organic skeleton known as CoNi-MOF. Initially, graphene oxide (GO) was synthesized using a modified Hummers' method, and RGO with better conductivity was achieved through reduction with ascorbic acid (AA). Subsequently, CoNi-MOF was introduced to enhance the material's electron transport capabilities further. The molecularly imprinted membrane was then prepared via electropolymerization to enable selective HA recognition. Under optimal conditions, the synthesized sensor exhibited accurate HA detection within a concentration range of 2-800 nM, with a detection limit of 0.97 nM. The sensor's selectivity was assessed using a selectivity coefficient, yielding an imprinting factor of 6.53. The method was successfully applied to the quantification of HA in urine, demonstrating a favorable recovery rate of 93.4%-103.9%. In conclusion, this study presents a practical platform for the detection of human metabolite detection.

Advanced Hybrid materials in electrochemical sensors: Combining MOFs and conducting polymers for environmental monitoring

The integration of conducting polymers (CPs) with metal-organic frameworks (MOFs) has arisen as a dynamic and innovative approach to overcome some intrinsic limitations of both materials, representing a transformative method to address the pressing need for high-performance environmental monitoring tools. MOFs, with their intricate structures and versatile functional groups, provide tuneable porosity and an extensive surface area, facilitating the selective adsorption of target analytes. Conversely, CPs, characterized by their exceptional electrical conductivity and redox properties, serve as proficient signal transducers. By combining these two materials, a novel class of hybrid materials emerges, capitalizing on the unique attributes of both components. These MOF/CP hybrids exhibit heightened sensitivity, selectivity, and adaptability, making them primordial in detecting and quantifying environmental contaminants. This review examines the synergy between MOFs and CPs, highlighting recent advancements, challenges, and prospects, thus offering a promising solution for developing advanced functional materials with tailored properties and multifunctionality to be applied in electrochemical sensors for environmental monitoring.

Preparation of surface molecular imprinting fluorescent sensor based on magnetic porous silica for sensitive and selective determination of catechol

A magnetic fluorescent molecularly imprinted sensor was successfully prepared and implemented to determine catechol (CT). Fe3O4 nanoparticles were synthesized by the solvothermal technique and mesoporous Fe3O4@SiO2@mSiO2 imprinted carriers were prepared by coating nonporous and mesoporous SiO2 shells on the surface of the Fe3O4 subsequently. The magnetic surface molecularly imprinted fluorescent sensor was created after the magnetic mesoporous carriers were modified with γ-methacryloxyl propyl trimethoxy silane to introduce double bonds on the surface of the carries and the polymerization was carried out in the presence of CT and fluorescent monomers. The magnetic mesoporous carriers were modified with γ-methacryloxyl propyl trimethoxy silane and double bonds were introduced on the surface of the carriers. After CT binding with the molecularly imprinted polymers (MIPs), the fluorescent intensity of the molecularly imprinted polymers (Ex = 400 nm, Em = 523 nm) increased significantly. The fluorescent intensity ratio (F/F0) of the sensor demonstrated a favorable linear correlation with the concentration of CT between 5 and 50 μM with a detection limit of 0.025 μM. Furthermore, the sensor was successfully applied to determine CT in actual samples with recoveries of 96.4-105% and relative standard deviations were lower than 3.5%. The results indicated that the research of our present work provided an efficient approach for swiftly and accurately determining organic pollutant in water.

Recent advances using MXenes in biomedical applications

An MXene is a novel two-dimensional transition metal carbide or nitride, with a typical formula of Mn+1XnTx (M = transition metals, X = carbon or nitrogen, and T = functional groups). MXenes have found wide application in biomedicine and biosensing, owing to their high biocompatibility, abundant reactive surface groups, good conductivity, and photothermal properties. Applications include photo- and electrochemical sensors, energy storage, and electronics. This review will highlight recent applications of MXene and MXene-derived materials in drug delivery, tissue engineering, antimicrobial activity, and biosensors (optical and electrochemical). We further elaborate on recent developments in utilizing MXenes for photothermal cancer therapy, and we explore multimodal treatments, including the integration of chemotherapeutic agents or magnetic nanoparticles for enhanced therapeutic efficacy. The high surface area and reactivity of MXenes provide an interface to respond to the changes in the environment, allowing MXene-based drug carriers to respond to changes in pH, reactive oxygen species (ROS), and electrical signals for controlled release applications. Furthermore, the conductivity of MXene enables it to provide electrical stimulation for cultured cells and endows it with photocatalytic capabilities that can be used in antibiotic applications. Wearable and in situ sensors incorporating MXenes are also included. Major challenges and future development directions of MXenes in biomedical applications are also discussed. The remarkable properties of MXenes will undoubtedly lead to their increasing use in the applications discussed here, as well as many others.

Constructed MXene matrix composites as sensing material and applications thereof: A review

Most studies on MXene matrix composites for sensor development have primarily focused on synthesis and application. Nevertheless, there is currently a lack of research on how the introduction of different materials affects the sensing properties of these composites. The rapid development of MXene has raised intriguing questions about improving sensor performance by combining MXene with other materials such as polymers, metals and inorganic non-metals. This review will concentrate on the construction of MXene-based composites and explore ways to enhance their sensor applications. Specifically, this review describes why the introduction of materials to the system brings the advantage of low concentration and high sensitivity assays, as well as the MXene-based frameworks that have been recently investigated. Lastly, in order to capture the current trend of MXene-based composites in sensor applications and identify promising research directions, this review will critically evaluate the potential applications of newly developed MXene systems.

Molecularly Imprinted Heterostructure-Based Electrochemosensor for Ultratrace and Precise Detection of 2-Methylisoborneol in Water

Ultratrace 2-methylisoborneol (2-MIB, ∼ng/L) in source water is the main odorant in the algae-derived odor episodes, whose accurate on-site detection will have a promising application potential. Due to the chemical inertness of 2-MIB, sensitive and selective detection of 2-MIB remains much challenging. Herein, molecularly imprinted polymer cavities were polymerized on the heterostructure Ti3C2Tx@CuFc-metal-organic framework to selectively capture 2-MIB, where the heterostructure could catalyze the probe redox reaction of [Fe(CN)63-/4-] and amplify the corresponding current signals. The prepared electrochemical sensor showed higher sensitivity on 2-MIB detection than the reported ones. Excellent stability, reusability, and selectivity for 2-MIB detection were also verified. The linear range and limit of detection of our sensor for 2-MIB were optimized to 0.0001-100 μg/L and 30 pg/L, respectively, performing much better than the reported sensors. Comparable performance to gas chromatography-mass spectrometry was achieved when the sensor was applied to real water samples with or without 2-MIB standards. Overall, our research has made great progress in the application of an on-site sensor in 2-MIB detection and well advances the development of molecularly imprinted polymer-based electrochemical sensors.

Molecularly Imprinted Polymer-Based Sensors Integrated with Transition Metal Dichalcogenides (TMDs) and MXenes: A Review

Molecularly imprinted polymer (MIP)-based electrochemical sensors have been extensively researched due to their higher sensitivity, quick response, and operational ease. To develop more advanced sensing devices with enhanced properties, MIPs have been integrated with two-dimensional (2D) layered materials such as transition metal dichalcogenides (TMDs) and MXenes. These 2D materials have unique electronic properties and an extended surface area, making them promising sensing materials that can improve the performance of MIPs. In this review article, we describe the methods used for the synthesis of TMDs and MXenes integrated MIP-based electrochemical sensors. Furthermore, we have provided a critical review of a wide range of analytes determined through the application of these electrochemical sensors. We also go over the influence of TMDs and MXenes on the binding kinetics and adsorption capacity which has enhanced binding recognition and sensing abilities. The combination of TMDs and MXenes with MIPs shows promising synergy in the development of highly efficient recognition materials. In the future, these sensors could be explored for a wider range of applications in environmental remediation, drug delivery, energy storage, and more. Finally, we address the challenges and future perspectives of using TMDs and MXenes integrated MIPs. We conclude with a focus on future development and the scope of integrating these materials in sensing technology.

Molecularly Imprinted Polymer-Based Biomimetic Systems for Sensing Environmental Contaminants, Biomarkers, and Bioimaging Applications

Molecularly imprinted polymers (MIPs), a biomimetic artificial receptor system inspired by the human body's antibody-antigen reactions, have gained significant attraction in the area of sensor development applications, especially in the areas of medical, pharmaceutical, food quality control, and the environment. MIPs are found to enhance the sensitivity and specificity of typical optical and electrochemical sensors severalfold with their precise binding to the analytes of choice. In this review, different polymerization chemistries, strategies used in the synthesis of MIPs, and various factors influencing the imprinting parameters to achieve high-performing MIPs are explained in depth. This review also highlights the recent developments in the field, such as MIP-based nanocomposites through nanoscale imprinting, MIP-based thin layers through surface imprinting, and other latest advancements in the sensor field. Furthermore, the role of MIPs in enhancing the sensitivity and specificity of sensors, especially optical and electrochemical sensors, is elaborated. In the later part of the review, applications of MIP-based optical and electrochemical sensors for the detection of biomarkers, enzymes, bacteria, viruses, and various emerging micropollutants like pharmaceutical drugs, pesticides, and heavy metal ions are discussed in detail. Finally, MIP's role in bioimaging applications is elucidated with a critical assessment of the future research directions for MIP-based biomimetic systems.