Metal Centers and Organic Ligands Determine Electrochemistry of Metal-Organic Frameworks

Metal-organic frameworks-based nanozyme sensor array for the discrimination of biogenic amines and detection of histamine

Biogenic amines (BAs) are hazardous substances found in fishery products that are closely associated with fish spoilage and threaten food safety. Traditional chromatographic methods for detecting BAs are expensive, complex, and time-consuming. In this study, we developed a nanozyme-based sensor array to efficiently discriminate between four types of BAs and sensitively detect histamine. Copper-, cerium-, and manganese-based metal-organic frameworks with excellent peroxidase-like activities were employed as sensor elements. Because the catalytic activities of metal-organic frameworks could be modified by different BAs to varying degrees, the sensor array could generate a distinct colorimetric response pattern (fingerprint) for each BA. Based on this principle, the sensor array accurately discriminated BAs over a wide concentration range (10-1000 μM). Histamine could be distinguished down to 1 μM and detected with a detection limit of 4.28 μM within 20 min. In addition, mixtures of BAs, target BAs, interfering substances, and BAs in fishery product samples were well discriminated. Furthermore, our sensor array could also effectively distinguish the freshness of fish samples. This work might offer a useful strategy for the discrimination and detection of BAs and could positively contribute to food safety and public health.

Piezoelectric-mediated two-dimensional copper-based metal-organic framework for synergistic sonodynamic and cuproptosis-driven tumor therapy

Sonodynamic therapy (SDT) is a minimally invasive therapeutic approach that utilizes sonosensitizers to catalyze substrates and generate reactive oxygen species (ROS) under ultrasound stimulation, ultimately inducing tumor cell death. Enhancing the piezoelectric properties of nanomaterials and modulating the semiconductor energy band are effective strategies to improve the catalytic efficiency of sonosensitizers. In this study, we developed a two-dimensional (2D) copper-based piezoelectric metal-organic framework (MOF) sonosensitizer, denoted as CM, through the coordination of copper and dimethylimidazole. The unique 2D MOF structure imparts CM with piezoelectric characteristics, enabling it to enhance SDT efficacy by modulating the semiconductor bandgap and carrier mobility. Upon ultrasound irradiation, CM catalyzes oxygen to undergo a cascade reaction, producing highly toxic singlet oxygen. Additionally, cupric ions in CM can be reduced by glutathione, facilitating the spontaneous catalysis of hydrogen peroxide in tumors to generate hydroxyl radicals and deplete glutathione, thereby inducing oxidative damage. Moreover, cupric ions in CM can trigger tumor cell cuproptosis, which, in combination with the generated ROS, accelerates cell death. Thus, this study establishes a MOF-based system for controllably inducing multi-pathway cancer cell death and provides a foundation for enhancing ultrasound-catalyzed tumor therapy through the optimization of piezoelectric properties.

Ultrafine metal-organic framework @ graphitic carbon with MoS2-CNTs nanocomposites as carbon-based electrochemical sensor for ultrasensitive detection of catechin in beverages

GO/Co-MOF/PPy-350 (GPC-350) was synthesized by in situ growth of ultrafine Co-MOF on graphene oxide (GO), followed by encapsulation with polypyrrole (PPy) and calcination at 350.0℃. Meanwhile, MoS2-MWCNTs (MoS2-CNTs) were produced via the in situ synthesis of MoS2 within multi-walled carbon nanotubes (MWCNTs). The electrochemically superior GPC-350/MoS2-CNTs nanocomposite was then achieved by combining GPC-350 with MoS2-CNTs. The polypyrrole encapsulation serves to protect the ultrafine Co-MOF, preventing its degradation during the calcination process. The linear detection range of the GPC-350/MoS2-CNTs/GCE sensor for the determination of catechin (CA) in phosphate buffered saline (PBS) was from 5.0 to 1800.0 nM with a limit of detection of 1.78 nM. In addition, the materials were characterized using SEM, EDX, TEM, XRD, EIS, XPS, FTIR, and Raman. These results indicate that the synthesis of GPC-350/MoS2-CNTs nanocomposites is successful and CA in beverages samples can be effectively detected using electrochemical sensors. Additionally, the reaction mechanism of CA was explored through cyclic voltammetry. The application of GPC-350/MoS2-CNTs nanocomposites in sensor technology offers innovative approaches for the ultrasensitive detection of flavonoids.

Precision-Engineered Construction of Proton-Conducting Metal-Organic Frameworks

Proton-conducting materials have attracted considerable interest because of their extensive application in energy storage and conversion devices. Among them, metal-organic frameworks (MOFs) present tremendous development potential and possibilities for constructing novel advanced proton conductors due to their special advantages in crystallinity, designability, and porosity. In particular, several special design strategies for the structure of MOFs have opened new doors for the advancement of MOF proton conductors, such as charged network construction, ligand functionalization, metal-center manipulation, defective engineering, guest molecule incorporation, and pore-space manipulation. With the implementation of these strategies, proton-conducting MOFs have developed significantly and profoundly within the last decade. Therefore, in this review, we critically discuss and analyze the fundamental principles, design strategies, and implementation methods targeted at improving the proton conductivity of MOFs through representative examples. Besides, the structural features, the proton conduction mechanism and the behavior of MOFs are discussed thoroughly and meticulously. Future endeavors are also proposed to address the challenges of proton-conducting MOFs in practical research. We sincerely expect that this review will bring guidance and inspiration for the design of proton-conducting MOFs and further motivate the research enthusiasm for novel proton-conducting materials.

Bimetallic PdCu anchored to 3D flower-like carbon material for portable and efficient detection of glyphosate

Glyphosate (Gly), as a widely used broad-spectrum herbicide, may lead to soil and water pollution due to its persistence in the environment. Herein, the co-reduction method was employed to anchor bimetallic PdCu onto the Ni and nitrogen-doped 3D Flower-like Carbon Materials (Ni@NC), creating a composite material (PdCu/Ni@NC) with high specific surface area and good catalytic performance. This composite was used to modify screen-printed electrodes (SPE) to develop a portable and efficient Gly detection platform. In the presence of Cl⁻, the copper active sites convert to CuCl, achieving signal amplification. Upon the addition of Gly, a competitive reaction between Cu and Gly converts CuCl into a Cu-Gly complex, resulting in a sharp decrease in the electrochemical signal. This signal drop is used to detect Gly. The bimetallic PdCu nanoparticles (NPs) endowed the sensing platform with better stability and electrochemical performance due to their synergistic effect, and their stability was simply verified by Density functional theory (DFT). The sensor demonstrates a linear detection range spanning from 1 × 10⁻¹ ³ to 1 × 10⁻⁵ M, with a limit of detection (LOD) of 3.72 × 10⁻¹ ⁴ M. The sensor demonstrated a recovery rate of 95.9 % to 104.5 % in actual samples such as water and soil, indicating its potential for practical application.

Catalysis with Two-Dimensional Metal-Organic Frameworks: Synthesis, Characterization, and Modulation

The demand for the exploration of highly active and durable electro/photocatalysts for renewable energy conversion has experienced a significant surge in recent years. Metal-organic frameworks (MOFs), by virtue of their high porosity, large surface area, and modifiable metal centers and ligands, have gained tremendous attention and demonstrated promising prospects in electro/photocatalytic energy conversion. However, the small pore sizes and limited active sites of 3D bulk MOFs hinder their wide applications. Developing 2D MOFs with tailored thickness and large aspect ratio has emerged as an effective approach to meet these challenges, offering a high density of exposed active sites, better mechanical stability, better assembly flexibility, and shorter charge and photoexcited state transfer distances compared to 3D bulk MOFs. In this review, synthesis methods for the most up-to-date 2D MOFs are first overviewed, highlighting their respective advantages and disadvantages. Subsequently, a systematic analysis is conducted on the identification and electronic structure modulation of catalytic active sites in 2D MOFs and their applications in renewable energy conversion, including electrocatalysis and photocatalysis (electro/photocatalysis). Lastly, the current challenges and future development of 2D MOFs toward highly efficient and practical electro/photocatalysis are proposed.

A core-shell structured AuNPs@ZnCo-MOF SERS substrate for sensitive and selective detection of thiram

Surface-enhanced Raman scattering (SERS) enables pesticide residue monitoring to become facile and efficient. In this study, a core-shell structured gold nanoparticles@ZnCo metal-organic framework (AuNPs@ZnCo-MOF) SERS substrate was designed and successfully synthesized for efficient and selective detection of thiram. The bimetallic ZnCo-MOF shell can not only enrich the targeted molecules in the electromagnetic field because of its excellent absorptive capacity, but also act as a stabilized matrix for protecting the AuNPs from aggregation. The AuNPs@ZnCo-MOFs exhibited a high enhancement factor (EF) of 3.51 × 106 and a low detection limit of 1 × 10-7 mol L-1. Besides, the substrate material showed exceptional stability for up to 28 days at room temperature. The AuNPs@ZnCo-MOFs were used to detect thiram which displayed wide linearity (1 × 10-7 to 1 × 10-4 mol L-1) and high recoveries (83.45-99.61%). Moreover, the AuNPs@ZnCo-MOF SERS substrate exhibited excellent anti-interference ability and size selectivity for the target molecules. These indicate that the AuNPs@ZnCo-MOF substrate has great potential for the detection of thiram residues in practical applications.

Tuning microscopic structure of La-MOFs via ligand engineering effect towards enhancing phosphate adsorption

The selection of different organic ligands when synthesizing metal organic framework (MOFs) can change their effects on the adsorption performance. Here, four La-MOFs adsorbents (La-SA, La-FA, La-TA and La-OA) with different organic ligands and structures were synthesized by solvothermal method for phosphate adsorption, and the relationship between their adsorption properties and structures was established. Among four La-MOFs, their phosphate adsorption capacities and adsorption rates followed La-SA > La-FA > La-TA > La-OA. The results indicated that average pore diameter played a key role in phosphate adsorption and there was a positive correlation between average pore diameter and adsorption capacity (R2 = 0.86). Coexisting ion experiments showed that phosphate adsorptions on three La-MOFs (La-SA, La-FA and La-TA) were inhibited in the presence of CO32- and HCO3-. The inhibition of CO32- was the most pronounced and the results of redundancy analysis pointed out that it was mainly due to the change of pH value. In contrast, La-OA showed enhanced phosphate adsorption in the presence of CO32- and HCO3-, and the combination of pH experiments showed that phosphate adsorption by La-OA was increased under alkaline conditions. Further combined with FT-IR, XRD, high resolution energy spectra of XPS (La 3d, P 2p and O 1s) and XANES, the adsorption mechanisms were derived electrostatic attraction, chemical precipitation and inner sphere complexation, and the last two were identified as the main mechanisms. Moreover, it can be identified from XPS 2p that the phosphate adsorption on La-FA and La-OA were mainly in the LaPO4 state, while La-SA and La-TA mainly existed in the form of LaPO4·xH2O crystals and inner sphere complexes. From the perspective of material morphology, this work provides a thought for the rational design of MOFs with adjustable properties for phosphate adsorption.

Enhanced peroxidase-like activity of MOF nanozymes by co-catalysis for colorimetric detection of cholesterol

Metal-organic frameworks (MOFs) have been widely used as nanozymes with a great development prospect due to their unique advantages. It is known that the current Fe-based or Cu-based MOF, etc., exhibits the catalytic activity of nanozymes through the Fenton catalytic reaction. And the conversion efficiency of the Fe3+/Fe2+ or Cu2+/Cu+ cycle is key to the catalytic activity. Therefore, we proposed a novel co-catalytic method to promote the reaction rate of the rate-limiting step of Cu2+/Cu+ conversion in the Fenton reaction of Cu2+/H2O2 to enhance the catalytic activity of the nanozymes. As a proof of concept, the MoCu-2MI nanozyme with high catalytic activity was successfully synthesized using Mo-doped Cu-2MI (2-methylimidazole). By using 3,3',5,5'-tetramethylbenzidine (TMB) as the chromogenic substrate, MoCu-2MI exhibited higher peroxidase-like activity than pure Cu-2MI. Then, it was confirmed that the newly introduced Mo played a crucial co-catalytic role by characterizing the possible catalytic mechanism. Specifically, Mo acted as a co-catalyst to accelerate the electron transfer in the system, and then promote the Cu2+/Cu+ cycle in the Cu-Fenton reaction, which was conducive to accelerating the production of a large number of reactive oxygen species (ROS) from H2O2, and finally improve the activity. Ultimately, a biosensor platform combined with MoCu-2MI and cholesterol oxidase realized the one-step colorimetric detection of cholesterol in the range of 2-140 μM with the detection limit as low as 1.2 μM. This study provides a new strategy for regulating the activity of MOF nanozymes.

An electrochemical sensor based on Ce-MOF-derived Ce-doped poly(3,4-ethylenedioxythiophene) composite for efficient determination of rutin in food

As a common antioxidant and nutritional fortifier in food chemistry, rutin has positive therapeutic effects against novel coronaviruses. Here, Ce-doped poly(3,4-ethylenedioxythiophene) (Ce-PEDOT) nanocomposites derived through cerium-based metal-organic framework (Ce-MOF) as a sacrificial template have been synthesized and successfully applied to electrochemical sensors. Due to the outstanding electrical conductivity of PEDOT and the high catalytic activity of Ce, the nanocomposites were used for the detection of rutin. The Ce-PEDOT/GCE sensor detects rutin over a linear range of 0.02-9 μM with the limit of detection of 14.7 nM (S/N = 3). Satisfactory results were obtained in the determination of rutin in natural food samples (buckwheat tea and orange). Moreover, the redox mechanism and electrochemical reaction sites of rutin were investigated by the CV curves of scan rate and density functional theory. This work is the first to demonstrate the combined PEDOT and Ce-MOF-derived materials as an electrochemical sensor to detect rutin, thus opening a new window for the application of the material in detection.

Organic metal matrix Mil-88a nano-enzyme for joint repair in the osteoarthritis mouse model

Introduction: In this paper we tried to conduct a novel nanomaterial strategy to overcome osteoarthritis (OA) in a mouse model. Methods: In this regard, after synthesizing the Mil-88a nanozyme, as a certain Fe-MOF, its toxic effects were detected by CCK-8 method and live-dead staining. The OA model of mouse was constructed, and paraffin sections of joints were taken for histological evaluation. In addition, immunofluorescence and immunohistochemistry were used to identify the OA progression and OARSI was used to evaluate the OA grades. We observed that Mil-88a could be easily synthesized and has high biocompatibility. Results: We observed that Mil-88a could significantly promote the expression of OA anabolism-related genes such as Col2 and also significantly inhibit the expression of OA catabolism-related genes MMP13. Besides, we observed better OARSI score in animals treated with Mil-88a nano-enzyme loading on organic metal matrix. Discussion: Overall, Mil-88a nano-enzyme could be used as a novel strategy to treat OA.

Tuning the structure of cerium-based metal-organic frameworks for efficient removal of arsenic species: The role of organic ligands

The ability of organic ligands to change the structure of metal-organic frameworks (MOFs) in nature and influence their adsorption efficiency for arsenic species is enormous. The current work was designed to investigate the adsorption performance of cerium-based MOFs with tunable structures through the use of organic ligands (Ce-MOF-66 and Ce-MOF-808) towards arsenic species from water. The structural features of Ce-MOF-66 and Ce-MOF-808 with varying crystallinity, morphology, particle size, and surface area are considerably altered by organic ligands tuning, resulting in clearly distinct arsenate (As (V)) and arsenite (As (III)) adsorption capabilities. The experimental results showed that the Langmuir adsorption capacities of As (V) by Ce-MOF-66 and Ce-MOF-808 reached 355.67 and 217.80 mg/g, respectively, while for As (III) were 5.52 and 402.10 mg/g for Ce-MOF-66 and Ce-MOF-808, respectively. Except for the impact of PO₄^3- on As (V), co-existing ions had no significant influence on adsorption, illustrating the high selectivity. Furthermore, to understand the structure and adsorption mechanism, two adsorbents were characterized by powder X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, specific surface area, Fourier transform infrared and X-ray photoelectron spectroscopy, in which identified that unsaturated sites and ligand exchange were the main adsorption mechanisms of As (V) and As (III). Overall, this research presents a novel approach for developing high-performance Ce-derived MOFs adsorbents to capture arsenic species.