In-situ growth of iron-based metal-organic framework crystal on ordered mesoporous carbon for efficient electrocatalysis of p -nitrotoluene and hydrazine

MIL-88B(Fe) MOF modified screen-printed electrodes for non-enzymatic electrochemical sensing of malathion

An enzyme-free electrochemical approach for ultra-trace quantification of the organophosphate insecticide malathion is proposed in this study. It is premised on screen-printed carbon electrodes modified by the MIL-88B(Fe) metal-organic framework (MOF). A one-pot solvothermal method was used to synthesise MIL-88B(Fe). The surface modification of electrodes allowed for increased electroactive surface area and accelerated electron transport on the electrode. Inhibition in the redox signal of MIL-88B(Fe) was observed due to the affinity between metal centres of the MOF and the functional groups of malathion, leading to an accurate determination of malathion. The proposed sensor effectively quantified malathion in the wide concentration range of 1 × 10-12 M to 1 × 10-6 M. The limit of detection for malathion was 0.79 pM. The proposed sensor also possessed excellent stability, repeatability, and anti-interference characteristics. Furthermore, the proposed sensor demonstrated satisfactory malathion recovery in spiked vegetable samples.

Local O2 concentrating boosts the electro-Fenton process for energy-efficient water remediation

The electro-Fenton process is a state-of-the-art water treatment technology used to remove organic contaminants. However, the low O2 utilization efficiency (OUE, <1%) and high energy consumption remain the biggest obstacles to practical application. Here, we propose a local O2 concentrating (LOC) approach to increase the OUE by over 11-fold compared to the conventional simple O2 diffusion route. Due to the well-designed molecular structure, the LOC approach enables direct extraction of O2 from the bulk solution to the reaction interface; this eliminates the need to pump O2/air to overcome the sluggish O2 mass transfer and results in high Faradaic efficiencies (~50%) even under natural air diffusion conditions. Long-term operation of a flow-through pilot device indicated that the LOC approach saved more than 65% of the electric energy normally consumed in treating actual industrial wastewater, demonstrating the great potential of this system-level design to boost the electro-Fenton process for energy-efficient water remediation.

A CTAB-assisted PANI-MoS2 nanosheet flower morphology for the highly sensitive electrochemical detection of hydrazine

In this work, cetyl trimethylammonium bromide (CTAB)-assisted polyaniline-molybdenum disulfide (CPANI-MoS2) nanosheets with a flower morphology have been synthesized through in situ polymerization and a hydrothermal method. The composite was analyzed for structural modification through X-ray diffraction (XRD) to examine chemical changes and the presence of functional groups via Fourier transform infrared (FTIR) and Raman spectroscopy techniques. The surface morphology was identified by field-emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HR-TEM) techniques. The CPANI-MoS2 nanosheet glassy carbon electrode (GCE) offers a novel strategy for the electrochemical detection of carcinogenic hydrazine. The cyclic voltammetry (CV) curve demonstrated a quasi-reversible behaviour with a high-surface area. Furthermore, differential pulse voltammetry (DPV) analysis of hydrazine detection showed a wide linear range from 10 μM to 100 μM, a low limit of detection of 0.40 μM, and a high sensitivity of 7.23 μA μM cm-2. The determination of hydrazine in a water sample and the recovery percentage were found to be 100.31% and 103.73%, respectively. The CPANI-MoS2 nanosheet GCE significantly contributed to the high electroanalytical oxidation activity due to the CTAB surfactant modifying the flower-like nanosheet morphology, which enables the easy adsorption of hydrazine analyte species and exhibits a high current rate with a rapid detection response.

Fabrication of Carbon Nanofiber Incorporated with CuWO4 for Sensitive Electrochemical Detection of 4-Nitrotoluene in Water Samples

In the current work, copper tungsten oxide (CuWO₄) nanoparticles are incorporated with carbon nanofiber (CNF) to form CNF/CuWO₄ nanocomposite through a facile hydrothermal method. The prepared CNF/CuWO₄ composite was applied to the electrochemical detection of hazardous organic pollutants of 4-nitrotoluene (4-NT). The well-defined CNF/CuWO₄ nanocomposite is used as a modifier of glassy carbon electrode (GCE) to form CuWO₄/CNF/GCE electrode for the detection of 4-NT. The physicochemical properties of CNF, CuWO₄, and CNF/CuWO₄ nanocomposite were examined by various characterization techniques, such as X-ray diffraction studies, field emission scanning electron microscopy, EDX-energy dispersive X-ray microanalysis, and high-resolution transmission electron microscopy. The electrochemical detection of 4-NT was evaluated using cyclic voltammetry (CV) the differential pulse voltammetry detection technique (DPV). The aforementioned CNF, CuWO₄, and CNF/CuWO₄ materials have better crystallinity with porous nature. The prepared CNF/CuWO₄ nanocomposite has better electrocatalytic ability compared to other materials such as CNF, and CuWO₄. The CuWO₄/CNF/GCE electrode exhibited remarkable sensitivity of 7.258 μA μM^-1 cm^-2, a low limit of detection of 86.16 nM, and a long linear range of 0.2-100 μM. The CuWO₄/CNF/GCE electrode exhibited distinguished selectivity, acceptable stability of about 90%, and well reproducibility. Meanwhile, the GCE/CNF/CuWO₄ electrode has been applied to real sample analysis with better recovery results of 91.51 to 97.10%.

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.

Spherical silver oxide nanoparticles for fabrication of electrochemical sensor for efficient 4-Nitrotoluene detection and assessment of their antimicrobial activity

This research article reports an economic and affordable microwave-assisted synthesis of spherical silver oxide nanoparticles (Ag₂O NPs) (80-90 nm) for an efficient electrochemical sensing of a hazardous organic pollutant 4-nitrotoluene (4-NT). Such well-characterized Ag₂O NPs were utilized to modify gold (Au) electrode in order to fabricate Ag₂O-NPs/Au sensor to detect 4-NT using cyclic voltammetry (CV) and linear sweep voltammetry (LSV) techniques. Ag₂O-NPs/Au sensor exhibited a distinguished electrical response as a function of varying 4-NT concentration in neutral medium samples. Ag₂O-NPs/Au sensor demonstrated an ultrahigh sensitivity as 15.33 μA (μM)^-1 cm^-2, a low detection limit of 62.3 nM, and linear response in detection ranges of 0.6-5.9 μM and 37-175 μM. The sensing performance of fabricated Ag₂O-NPs/Au sensor is reproducible, reusable, selective in presence of other chemical species, and validated using real samples. The Ag₂O/Au sensor had much rapid and easy fabrication process and offered much lower LOD for 4-NT detection than many previously reported sensors. Along with efficient electrochemical activity, the spherical Ag₂O NPs also exhibit remarkable antimicrobial activity against harmful gram negative Escherichia coli (E. coli) and gram positive Staphylococcus aureus (S. aureus) bacteria. Herein projected efficient Ag₂O-NPs/Au electrochemical sensor for 4-NT is affordable with the capability of scaling up to perform point-of-care 4-NT testing needed for successful environmental monitoring and water quality assurance.

Glucose oxidase decorated fluorescent metal-organic frameworks as biomimetic cascade nanozymes for glucose detection through the inner filter effect

Metal-organic frameworks (MOFs) as a peroxidase mimic have been integrated with glucose oxidase (GOx) to achieve one-step glucose detection. However, limited by the loading amount of GOx, the performances of the developed glucose sensing assays still remain to be further improved to meet sensing requirements in diverse biological samples. Herein, with Fe3+ as the metal ion and 2-amino-benzenedicarboxylic acid as a ligand, a fluorescent Fe-based organic framework (NH2-MIL-101) with peroxidase-like activity was synthesized. Due to the large specific surface area (791.75 m2 g-1), 68 μg mg-1 GOx could be immobilized through the amidation coupling reaction, and the product was designated GOx@NH2-MIL-101. With OPD as the substrate, Gox@NH2-MIL-101 achieved highly efficient biomimetic cascade catalysis for one-step glucose detection through an inner filter effect: upon reacting with glucose, GOx@NH2-MIL-101 catalytically oxidized glucose using dissolved O2, and the produced H2O2 concurrently oxidized o-phenylenediamine (OPD) to oxidized OPD (oxOPD), accompanied by the fluorescence of GOx@NH2-MIL-101 at 456 nm being quenched and that of oxOPD at 565 nm being enhanced. With the fluorescent ratio F565/F456 used as a readout signal, a wide linear range of 0.1-600 μM was obtained, and the detection limit was 0.0428 μM. Based on the excellent selectivity and high stability of GOx@NH2-MIL-101, the developed assay was successfully applied to glucose detection in human serum and saliva, presenting potential applications in diverse biological samples and even medical diagnosis.

Pt Nanoparticles Anchored on NH2-MIL-101 with Efficient Peroxidase-Like Activity for Colorimetric Detection of Dopamine

Dopamine (DA) is an important catecholamine neurotransmitter that plays a highly relevant role in regulating the central nervous system, and abnormal DA content can cause many immune-related diseases. Hence, it is of significance to sensitively and specifically identify DA for clinical medicine. In this work, Pt/NH2-MIL-101 hybrid nanozymes with bimetallic catalytic centers were fabricated by forming coordinate bonds between Pt nanoparticles (Pt NPs) and –NH2 on metal–organic frameworks (MOF). The catalytic activity of Pt/NH2-MIL-101 was increased by 1.5 times via enlarging the exposure of more active sites and improving the activity of the active sites through the strategy of forming bimetallic catalytic centers. In the presence of DA, competing with 3, 3′, 5, 5′-tetramethylbenzidine (TMB) for the generated hydroxyl radicals (•OH), the blue oxidation state TMB (Ox-TMB) is reduced to colorless TMB, showing dramatic color changes. The Pt/NH2-MIL-101-based colorimetric assay enables the sensitive and robust detection of DA molecules with a detection limit of only 0.42 μM and has an observable potential in clinical applications.

Low-temperature (NO + O2) adsorption performance of alkaline earth metal-doped C-FDU-15

To improve the removal capacity of NO + O₂ effectively, the alkaline earth metal-doped order mesoporous carbon (A-C-FDU-15(0.001) (A = Mg, Ca, Sr and Ba)) and Mg-C-FDU-15(x) (x = 0.001-0.003) samples were prepared, and their physicochemical and NO + O₂ adsorption properties were determined by means of various techniques. The results show that the sequence in (NO + O₂) adsorption performance was as follows: Mg-C-FDU-15(0.001) (93.2 mg/g) > Ca-C-FDU-15(0.001) (82.2 mg/g) > Sr-C-FDU-15(0.001) (76.1 mg/g) > Ba-C-FDU-15(0.001) (72.9 mg/g) > C-FDU-15 (67.1 mg/g). Among all of the A-C-FDU-15(0.001) samples, Mg-C-FDU-15(0.001) possessed the highest (NO + O₂) adsorption capacity (106.2 mg/g). The species of alkaline earth metals and basic sites were important factors determining the adsorption of NO + O₂ on the A-C-FDU-15(x) samples, and (NO + O₂) adsorption on the samples was mainly chemical adsorption. Combined with the results of (NO + O₂)-temperature-programmed desorption ((NO + O₂)-TPD) and in situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) characterization, we deduced that there were two main pathways of (NO + O₂) adsorption: one was first the conversion of NO and O₂ to NO₂ and then part of NO₂ was converted to NO₂^- and NO₃^-; and the other was the direct oxidation of NO to NO₂^- and NO₃^-.

Alkali metal-modified C-FDU-15: Highly efficient adsorbents for adsorption of NO and O2 at low temperatures

The alkali metal (M = Na, K, Rb, and Cs)-modified C-FDU-15 (M-C-FDU-15(x); x was the M/C-FDU-15 M ratio, and equal to 0.01-0.03) samples were prepared through an in situ process, and characterized by means of the TG, XRD, TEM, EDS, N₂ adsorption-desorption, O₂-TPD, and CO₂-TPD techniques. The (NO + O₂) adsorption mechanism was investigated using the (NO + O₂)-TPD and DRIFTS techniques. The results show that the sequence of (NO + O₂) adsorption performance was Na-C-FDU-15(0.01) (104.1 mg/g) > K-C-FDU-15(0.01) (92.4 mg/g) > C-FDU-15 (76.2 mg/g) > Rb-C-FDU-15(0.01) (65.1 mg/g) > Cs-C-FDU-15(0.01) (62.3 mg/g). The alkali metal was uniformly distributed in C-FDU-15 and its doping enhanced the amount of the basic sites in the sample. Moreover, the optimal Na/C-FDU-15 M ratio was 0.02. (NO + O₂) were chemically adsorbed mainly in the forms of nitrite (NO₂^-) and nitrate (NO₃^-) on M-C-FDU-15(x). A more amount of NO was converted to nitrate than to nitrite. There were three key factors of enhancing the (NO + O₂) adsorption capacity of C-FDU-15 due to alkali metal doping: the first factor was the increasing of surface area and pore volume of the sample, the second one was the enhancement in amount of the active sites in the sample, and the third one was the smaller alkali metal ionic radius in the sample.

BTEX Vapor Detection with a Flexible MOF and Functional Polymer by Means of a Composite Photonic Crystal

Owing to superior sorption properties, structural variability, and versatility, metal-organic frameworks (MOFs) are used as sensing materials with both high selectivity and sensitivity. Herein, integrating a MOF with a polymer, a multilayered photonic crystal (PC) sensor, which is composed of NH₂-MIL-88B nanocrystals and poly(styrene-acrylic acid) nanoparticles, is fabricated. Synthetically, by taking advantage of the sensitive breathing effect of the MOF and excellent stimuli-response of the copolymer, the sensor outputs significant optical signals that can be visually recognized and captured with the assistance of the spectrum with the detection limits of 3.70, 0.87, 0.42, and 0.22 g/m³ when exposed to benzene, toluene, ethylbenzene, and xylene (BTEX), respectively. Thanks to the porous construction and ultrathin feature, the PC sensor reaches a sensing balance within 3 s in BTEX streams and restores its initial state immediately after the rapid volatilization of the vapors. The function of the MOF material is confirmed by comparing the sensing properties of MOF/polymer PC with those of the SiO₂/polymer one. In addition, as the designed MOF/polymer-based PC sensor shows different spectrum characteristics compared with those of other reported MOF-based ones, finite element simulation technology is adopted to help explain the relationship between optical property and material structure feature of the multilayered PC structure.

Core-Shell Metal-Organic Frameworks as the Mixed-Mode Stationary Phase for Hydrophilic Interaction/Reversed-Phase Chromatography

Stationary phases with mixed-mode mechanisms have emerged as a hot research topic. In the present research, monodisperse core-shell UiO-67@SiO₂ materials were prepared and further served as the packed column for mixed-mode hydrophilic interaction liquid chromatography/reversed-phase liquid chromatography. The developed UiO-67@SiO₂ materials were characterized via thermogravimetric analysis, scanning electron microscopy, X-ray Powder diffraction, and Fourier transform infrared techniques. The developed UiO-67@SiO₂ column shows flexible selectivity for separation of both hydrophobic (anilines, alkylbenzenes, and polycyclic aromatic hydrocarbons) and hydrophilic (thioureas) compounds. Furthermore, the UiO-67@SiO₂ column was also utilized to characterize potential pollutants in lake water samples. In summary, the UiO-67@SiO₂ column provided flexible selectivity and wide-range retention behaviors for both hydrophilic and hydrophobic analytes.