Electrochemical behavior of metal–organic framework MIL-101 modified carbon paste electrode: An excellent candidate for electroanalysis

Facet impact of CeO2@C 2D core-shell structure on electrochemical reaction kinetic factor and efficient detection of nitrite

Fine-tuning the surface structure of transition metal oxides at the atomic level is a promising way to improve the catalytic properties of materials. However, the influence of crystal surface structure on electrode reaction kinetics is still limited. In this study, we propose an in-situ synthesis strategy to obtain two-dimensional carbon/cerium oxide core-shell nanosheets by thermal decomposition of Ce-MOF nanosheets grown on the surface of carbon nanostructures, and fine-tuning the surface structure by introducing oxygen vacancies through defect engineering during the oxide nucleation process is conducted to obtain controllable exposed {111} and {110} surface CeO2@C composites. Both experiments and theoretical calculations show that the {110} -dominated nanocomplex (CeO2@C-350S) has better kinetic behavior and catalytic activity due to its abundant surface defects, which is manifested in higher active surface area, richer carrier concentration, and better promotion of diffusion and adsorption. In addition, CeO2@C-350S electrode has an extremely wide linear range and good stability in the electrochemical detection of nitrite. After 1000 times of the accelerated cycle experiments, CeO2@C-350S electrode still maintains 79.3 % of its initial current response, and recovers to 87.3 % after 10 min of stopping the test. The electrode stability is excellent, which is attributed to the clever carbon shell structure of the material. This synthesis strategy can be extended to other carbon-based oxide composite catalysts to improve the electrocatalytic performance and overall stability by adjusting the surface structure.

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

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

Design and Application of Electrochemical Sensors with Metal-Organic Frameworks as the Electrode Materials or Signal Tags

Metal-organic frameworks (MOFs) with fascinating chemical and physical properties have attracted immense interest from researchers regarding the construction of electrochemical sensors. In this work, we review the most recent advancements of MOF-based electrochemical sensors for the detection of electroactive small molecules and biological macromolecules (e.g., DNA, proteins, and enzymes). The types and functions of MOF-based nanomaterials in terms of the design of electrochemical sensors are also discussed. Furthermore, the limitations and challenges of MOF-based electrochemical sensing devices are explored. This work should be invaluable for the development of MOF-based advanced sensing platforms.

Recent advancements in metal-organic frameworks composites based electrochemical (bio)sensors

Metal-organic frameworks (MOFs) are a novel class of crystalline materials which find widespread applications in the field of microporous conductors, catalysis, separation, biomedical engineering, and electrochemical sensing. With a specific emphasis on the MOF composites for electrochemical sensor applications, this review summarizes the recent construction strategies on the development of conductive MOF composites (post-synthetic modification of MOFs, in situ synthesis of functional materials@MOFs composites, and incorporating electroactive ligands). The developed composites are revealed to have excellent electrochemical sensing activity better than their pristine forms. Notably, the applicable functionalized MOFs to electrochemical sensing/biosensing of various target species are discussed. Finally, we highlight the perspectives and challenges in the field of electrochemical sensors and biosensors for potential directions of future development.

Electrochemical Sensor for the Detection of 1-Hydroxypyrene Based on Composites of PAMAM-Regulated Chromium-Centered Metal-Organic Framework Nanoparticles and Graphene Oxide

A nanocomposite was formed by combining graphene oxide (GO) with chromium-centered metal-organic framework (Cr-MOF) nanoparticles regulated by the dendrimer polyamidoamine (PAMAM). PAMAM can successfully regulate the synthesis of Cr-MOF; in doing so, the size of Cr-MOF is reduced, its original morphology is maintained, and it has good crystallinity. A simple ultrasonication method was used to make the Cr-MOF/GO hybrid nanocomposite. Various characterization methods confirmed the successful synthesis of PAMAM/Cr-MOF/GO nanocomposites. The PAMAM/Cr-MOF/ERGO modified electrode could be used with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) to study the electrochemical behaviors of 1-hydroxypyrene (1-OHPyr). The results indicated that the constructed PAMAM/Cr-MOF/ERGO electrochemical sensor had a significantly enhanced electrocatalytic effect on the electrochemical reduction of 1-OHPyr compared with the sensors with no PAMAM and the ERGO sensor, which could be ascribed to the synergetic effect from the high porosity of Cr-MOF and the high conductivity of ERGO, as well as the further electron transport action of the nanocomposite. Under the optimal conditions, the reduction peak current and concentration of 1-OHPyr showed a good linear relationship in the range of 0.1-1.0 and 1.0-6.0 μM, and the detection limit of 1-OHPyr was calculated to be 0.075 μM. Moreover, the PAMAM/Cr-MOF/ERGO electrochemical sensor constructed in this paper can be expected to provide some instructions for the construction of electrochemical sensing platforms and wider potential applications.

Construction of a Tl(I) voltammetric sensor based on ZIF-67 nanocrystals: optimization of operational conditions via response surface design

An electroanalytical sensor was constructed constituted on a carbon paste electrode (CPE) with a ZIF-67 modifier and devoted to the quantification of Tl(I). Several characterization tests including XRD, BET, FT-IR, SEM/EDS/mapping, TEM, impedance spectroscopy (EIS), and cyclic voltammetry (CV) were performed on the synthesized ZIF-67 nanocrystals and CPE matrix. Central composite design (CCD) was used to assess the impact of variables affecting the sensor response, including the weight percent of ZIF-67 (14%), the pH of the thallium accumulation solution (6.4), and accumulation time (315 s) as well as the accumulation potential (-1.2 V). The direct linear relationship between the sensor response and the concentration of Tl(I) is in the interval of 1.0×10^-10 to 5.0×10^-7 M (coefficient of determination = 0.9994). The detection limit is approximately 1.0 × 10^-11 M. The right selection of the MOF makes this sensor highly resistant to the interference of other ions. High selectivity against common interferences in the measurement of thallium (such as Pb(II) and Cd(II)) is an important feature of this sensor. To confirm the performance of the prepared sensor, the amount of thallium in the real sample was determined.

Operando spectroscopy of nanoscopic metal/covalent organic framework electrocatalysts

Metal and covalent organic frameworks (MOFs and COFs) are increasingly finding exceptional utility in electrocatalytic systems. Their chemically defined porous nature grants them key functions that may enhance their electrocatalytic performance relative to conventional molecular or heterogeneous materials. In order to obtain insights into their function, mechanism, and dynamics under electrocatalytic conditions, operando spectroscopy, that which is performed as the catalyst is functioning, has been increasingly applied. This mini review highlights several key works emerging in recent years that have used various operando spectroscopic techniques, namely UV-vis absorption, Raman, Infrared, and X-ray absorption spectroscopy, to investigate electrocatalytic MOFs and COFs. A brief introduction to each technique and how it was applied to investigate MOF/COF-based electrolytic systems is detailed. The unique set of data obtained, interpretations made, and progress attained all point to the power of operando spectroscopy in truly opening the functionality of MOFs and COFs across many aspects of catalysis.

Employing Conductive Metal-Organic Frameworks for Voltammetric Detection of Neurochemicals

This paper describes the first implementation of an array of two-dimensional (2D) layered conductive metal-organic frameworks (MOFs) as drop-casted film electrodes that facilitate voltammetric detection of redox active neurochemicals in a multianalyte solution. The device configuration comprises a glassy carbon electrode modified with a film of conductive MOF (M₃HXTP₂; M = Ni, Cu; and X = NH, 2,3,6,7,10,11-hexaiminotriphenylene (HITP) or O, 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP)). The utility of 2D MOFs in voltammetric sensing is measured by the detection of ascorbic acid (AA), dopamine (DA), uric acid (UA), and serotonin (5-HT) in 0.1 M PBS (pH = 7.4). In particular, Ni₃HHTP₂ MOFs demonstrated nanomolar detection limits of 63 ± 11 nM for DA and 40 ± 17 nM for 5-HT through a wide concentration range (40 nM-200 μM). The applicability in biologically relevant detection was further demonstrated in simulated urine using Ni₃HHTP₂ MOFs for the detection of 5-HT with a nanomolar detection limit of 63 ± 11 nM for 5-HT through a wide concentration range (63 nM-200 μM) in the presence of a constant background of DA. The implementation of conductive MOFs in voltammetric detection holds promise for further development of highly modular, sensitive, selective, and stable electroanalytical devices.

Fabrication of an electrochemical sensor based on metal-organic framework ZIF-8 for quantitation of silver ions: optimizing experimental conditions using central composite design (CCD)

ZIF-8 was synthesized and carbon paste electrodes (CPEs) modified with this metal-organic framework were utilized for quantitation of silver(i) by the differential pulse anodic stripping voltammetry (DPASV) technique.Prepared ZIF-8 and the matrix of the electrodes were distinguished by impedance spectroscopy (EIS), XRD, FT-IR spectroscopy, cyclic voltammetry (CV), TEM and SEM/EDX methods. To obtain the strongest stripping peak currents, several significant variables were optimized with response surface methodology (RSM), including the ligand amount (near 11% w/w), applied potential for preconcentration (approximately -1.36 V), pH of the preconcentration solution (about 8.5) and preconcentration time (about 275 s). A calibration curve was acquired in the limits from 1.0 × 10-10 to 5.0 × 10-7 M with the Pearson correlation coefficient R = 0.9993. The limiting detectable concentration (LDC) was determined to be 1.0 × 10-11 M. The developed sensor has high selectivity for mercury(ii). The excellent pH, potential and especially size-exclusion based selectivity of the prepared sensor are unique characteristics that are very important in the determination of silver ions. The developed method was effectively employed for the quantitation of silver(i) ions in environmental and industrial samples.

MOF-Based Supramolecule Helical Nanomaterials: Toward Highly Enantioselective Electrochemical Recognition of Penicillamine

For the first time, we report the formation of a chiral MOF-based helical nanomaterial (h-HDGA@ZIF-67) through arranging zeolitic imidazolate framework (ZIF-67) nanocrystals on helical l-glutamic acid terminated bolaamphiphile (h-HDGA) via a facile process at room temperature. The self-assembly leads to the chiral function of the ZIF-67 from an achiral ligand. The h-HDGA@ZIF-67 served as a new type of electrochemical sensing interface for recognizing and quantifying Pen enantiomers that realize significant enantioselectivity, satisfactory stability, and reproducibility. The synergetic effect from ZIF-67 nanocrystals on h-HDGA and stereoselectivity of h-HDGA@ZIF-67 lead to the excellent enantioselectivity. The present strategy showed the first example of a chiral MOF-based supramolecule helical nanomaterial, presenting high enantioselectivity for electrochemical enantiomeric determination.

A new electrochemical sensor for the detection of fentanyl lethal drug by a screen-printed carbon electrode modified with the open-ended channels of Zn(ii)-MOF

An electrochemical fentanyl sensor based on modified screen-printed carbon electrode by Zn(ii)-MOF.

Sensing organic analytes by metal–organic frameworks: a new way of considering the topic

In this review, our goal is comparison of advantageous and disadvantageous of MOFs about signal-transduction in different instrumental methods for detection of different categories of organic analytes.

Graphite paste electrodes modified with a sulfo-functionalized metal-organic framework (type MIL-101) for voltammetric sensing of dopamine

The metal-organic frameworks MIL-101 and sulfo-MIL-101 were used to modify graphite paste electrodes (GPEs) to obtain sensors for determination of dopamine (DA). Taking advantage of the catalytic activity of metal-organic frameworks (MOFs) and of the electrical conductivity of graphite, the modified GPEs show enhanced voltammetric responses, and the GPE modified with the sulfo-MOF displays superior sensitivity when operated at a working potential of -0.4 to 0.8 V (vs. Ag/AgCl). The sensor works in the 0.07 to100 μM DA concentration range and has a 43 nM detection limit. It is concluded that the sulfo group provides open sites for efficient electrostatic and hydrogen bonding interactions, which facilitates electron transfer. Graphical abstractSchematic representation of the structure of the sulfo-functionalized MOF (sulfo-MIL-101) and the different voltammetric signals of dopamine at the graphite paste electrodes (GPEs) modified with sulfo-MIL-101 and the parent MOF (MIL-101).

Quinone-modified metal-organic frameworks MIL-101(Fe) as heterogeneous catalysts of persulfate activation for degradation of aqueous organic pollutants

In this work, quinone-modified metal-organic framework MIL-101(Fe)(Q-MIL-101(Fe)), as a novel heterogeneous Fenton-like catalyst, was synthesized for the activation of persulfate (PS) to remove bisphenol A (BPA). The synthetic Q-MIL-101(Fe) was characterized via X-ray diffraction, scanning electron microscope, Fourier transform infrared, electrochemical impedance spectroscopy, cyclic voltammetry and X-ray photoelectron spectroscopy. As compared to the pure MIL-101(Fe), Q-MIL-101(Fe) displayed better catalytic activity and reusability. The results manifested that the Q-MIL-101(Fe) kept quinone units, which successfully promoted the redox cycling of Fe³⁺/Fe²⁺ and enhanced the removal efficiency. In addition, the reaction factors of Q-MIL-101(Fe) were studied (e.g. pH, catalyst dosage, PS concentration and temperature), showing that the optimum conditions were [catalyst] = 0.2 g/L, [BPA] = 60 mg/L, [PS] = 4 mmol/L, pH = 6.79, temperature = 25 °C. On the basis of these findings, the probable mechanism on the heterogeneous activation of PS by Q-MIL-101(Fe) was proposed.

Cu2+-BTC based metal–organic framework: a redox accessible and redox stable MOF for selective and sensitive electrochemical sensing of acetaminophen and dopamine

A Cu²⁺ plus benzenetricarboxylate (BTC) based ‘3D’ metal–organic framework HKUST-1 was synthesized via a facile microwave assisted route.

Highly sensitive and selective determination of malathion in vegetable extracts by an electrochemical sensor based on Cu-metal organic framework

This article demonstrates the first application of a copper-based porous coordination polymer (BTCA-P-Cu-CP) as a carbon paste electrode (CPE) modifier for the detection of malathion. The electrochemical behavior of BTCA-P-Cu-CP/CPE was explored using cyclic voltammetry (CV) while chrono-amperometry methods were applied for the analytical evaluation of the sensor performance. Under optimized conditions, the developed sensor exhibited high reproducibility, stability, and wide dynamic range (0.6-24 nM) with the limits of detection and sensitivity equal to 0.17 nM and 5.7 µAnMcm^-1, respectively, based on inhibition signal measurement. Furthermore, the presence of common coexisting interfering species showed a minor change in signals (<4.4%). The developed sensor has been applied in the determination of malathion in spiked vegetable extracts. It exhibited promising results in term of fast and sensitive determination of malathion in real samples at trace level with recoveries of 91.0 to 104.4%. (RSDs < 5%, n = 3). A comparison of the two studied techniques showed that the HPLC technique is unable to detect malathion when the concentration is lower than 1.8 µM while 0.006 µM is detected with appropriate RSDs 0.2-5.2% (n = 3) by amperometric method. Due to the high sensitivity and selectivity, this new electrochemical sensor will be useful for monitoring trace malathion in real samples.

Efficient detection of hazardous catechol and hydroquinone with MOF-rGO modified carbon paste electrode

Reduced graphite oxide (rGO) was incorporated into a metal organic framework (MOF) MIL-101(Cr) for the modification of carbon paste electrode. Taking advantages of the large surface area of MOF and the electrical conductivity of rGO, the resulted electrodes exhibited high sensitivity and reliability in the simultaneous electrochemical identification and quantification of catechol (CC) and hydroquinone (HQ). Specifically, in the mixture solution of catechol and hydroquinone (constant concentration of an analyte), the linear response ranges for catechol and hydroquinone were 10-1400 μM and 4-1000 μM, and detection limits were 4 μM and 0.66 μM (S/N = 3) for individual catechol and hydroquinone, respectively. Therefore, the relatively easy fabrication of modified CPE and its fascinating reliability towards HQ and CC detection may simulate more research interest in the applications of MIL-101(Cr)-rGO composites for electrochemical sensors.

Activation of persulfates by ferrocene–MIL-101(Fe) heterogeneous catalyst for degradation of bisphenol A

In this study, a novel and high-performance catalyst was prepared and used as the heterogeneous catalyst to activate persulfate for bisphenol A (BPA) degradation. Ferrocene was anchored to NH₂-MIL-101(Fe) post-synthetically by the condensation of amine group from NH₂-MIL-101(Fe) with the carbonyl group of ferrocenecarboxaldehyde. The synthesized ferrocene tethered MIL-101(Fe)–ferrocene was characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photo-electron spectra, cyclic voltammetry and electrochemical impedance spectroscopy. The ferrocene acts as a redox mediator, which makes the ferrocene functionalized NH₂-MIL-101(Fe) highly active in the degradation of BPA by accelerating the rate of the charge-transfer processes in aqueous solution. MIL-101(Fe)–Fc was proved to be the most effective catalyst, removing more than 99.9% of BPA. In addition, the catalyst can be reused without significant loss in activity.

In this study, a novel and high-performance catalyst ferrocene-MIL-101(Fe) was prepared and used as the heterogeneous catalyst to activate persulfate for bisphenol A degradation.

Quinone-modified NH2-MIL-101(Fe) composite as a redox mediator for improved degradation of bisphenol A

A novel quinone-modified metal-organic frameworks NH₂-MIL-101(Fe) was synthesized using a simple chemical method under mild condition. The introduced 2-anthraquinone sulfonate (AQS) can be covalently modified with NH₂-MIL-101(Fe) and acts as a redox mediator to enhance the degradation of bisphenol A (BPA) via persulfate activation. The obtained AQS-NH-MIL-101(Fe) was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectra, cyclic voltammetry and electrochemical impedance spectroscopy. AQS-NH-MIL-101(Fe) exhibited better catalytic performance compared with NH₂-MIL-101(Fe) and NH₂-MIL-101(Fe) with free AQS (NH₂-MIL-101(Fe)/AQS). That is, AQS-NH-MIL-101(Fe) was proved to be the most effective in that more than 97.7% of BPA was removed. The degradation rate constants (k) of AQS-NH-MIL-101(Fe) was 9-fold higher than that of NH₂-MIL-101(Fe) and 7-fold higher than NH₂-MIL-101(Fe)/AQS, indicating that AQS is a great electron-transfer mediator when modified with NH₂-MIL-101(Fe). Based on the above results, the possible mechanism of catalytic reaction has been investigated in view of the trapping experiments. In addition, the AQS-NH-MIL-101(Fe) catalyst exhibited excellent stability and can be used several times without significant deterioration in performance.

An updated roadmap for the integration of metal–organic frameworks with electronic devices and chemical sensors

This review highlights the steps needed to bring the properties of MOFs from the chemical lab to the microelectronics fab.

Perspectives on metal–organic frameworks with intrinsic electrocatalytic activity

This highlight article focuses on the rapidly emerging area of electrocatalytic metal–organic frameworks (MOFs) with a particular emphasis on those systems displaying intrinsic activity.

Novel UIO-66-NO2@XC-72 nanohybrid as an electrode material for simultaneous detection of ascorbic acid, dopamine and uric acid

The UIO-66-NO₂@XC-72 sensor exhibited excellent linear responses to AA, DA and UA due to the synergistic effect of UIO-66-NO₂ and XC-72, as well as the hydrogen bond effect. The detection ranges were 0.2–3.5 μM for AA, 0.03–2.0 μM for DA and 0.75–22 μM for UA.

A novel electrochemical sensor of tryptophan based on silver nanoparticles/metal–organic framework composite modified glassy carbon electrode

A familiar metal–organic framework MIL-101(Fe) and silver nanoparticles composite acted as a novel electrode modified material for the detection of tryptophan.

Recent Electroanalytical Studies of Metal-Organic Frameworks: A Mini-Review

Metal-organic frameworks (MOFs) are attracting considerable attention because of their unique structural properties, such as high surface areas, tunable pore sizes, and open metal sites, which enable them to have potential applications in gas storage, catalysis, sensors, drug release, and separation. Also, MOFs can be fabricated and functionalized as electrochemically functional frameworks with perfect electrochemical properties and electrocatalytic activities. This review focuses on the electroanalytical applications of MOFs between 2010 and 2014 years.

One-step synthesis of a copper-based metal–organic framework–graphene nanocomposite with enhanced electrocatalytic activity

A facile one-step method to prepare a Cu-MOF–graphene composite and its electrochemical application.