Metal-organic framework (MOF)-derived flower-like Ni-MOF@NiV-layered double hydroxides as peroxidase mimetics for colorimetric detection of hydroquinone

BACKGROUND

Nanozymes are one of the ideal substitutes for natural enzymes because of their excellent chemical stability and simple preparation methods. However, due to the limited catalytic ability of most reported nanozymes, constructing nanomaterials with low cost and high activity is gradually becoming an exploration focus in the field of nanozymes. Heteroatom doping of metal-organic frameworks is one of potential approaches to design nanozymes with high catalytic performance. Due to their multiple valence states properties, V-doped metal-organic framework (MOF)-derived LDH is expected to be a good enzyme-like catalyst. To our knowledge, the V-doped MOF-derived LDH as nanozyme is not explored before.

RESULTS

We report the in-situ synthesis of NiV-layered double hydroxides (LDHs) on nickel-based MOF, i.e. Ni-MOF@NiV-LDHs. The MOF surface is covered by 2D nanosheets. This unique structural design increases the specific surface area of the material, enables more exposure of catalytic active sites to participate in reactions and accelerates the electron transfer rate. The Ni-MOF@NiV-LDHs have high peroxidase-like activity able to catalyze TMB oxidation by H2O2 via the generation of •OH and O2•-. Relative to Ni-MOF, the Ni-MOF@NiV-LDHs shows 47-fold peroxidase-like activity rise. It had good affinity to TMB and H2O2, with the Michaelis-Menten constants of 0.12 mM and 0.007 mM, respectively. The hydroquinone (HQ) consumed the reactive oxygen species generated in the TMB + H2O2+Ni-MOF@NiV-LDHs system to inhibit the TMB oxidation. On this basis, a sensitive and rapid assay for determining HQ was developed, with a linear range of 0.50-70 μM and a LOD of 0.37 μM.

SIGNIFICANCE

This work provided some clues for the further development of novel nanozymes with high catalytic performance via a strategy of heteroatom doping. And the constructed colorimetric analysis method was successfully utilized for the determination of HQ in actual waters, which has the potential for practical application in the analysis of environmental pollutants.

Recent advances in the colorimetric and fluorescence analysis of bioactive small-molecule compounds based on the enzyme-like activity of nanomaterials

Nanomaterials with enzyme-like activity have been widely used in the construction of colorimetric and fluorescence sensors due to their advantages of cost-effectiveness, high stability, good biocompatibility, and ease of modification. Furthermore, the colorimetric and fluorescence sensors, which are effective approaches for detecting bioactive small-molecule compounds, have been extensively explored due to their simple operation and high sensitivity. Recent significant researches have focused on designing various sensors based on nanozymes with peroxidase- and oxidase-like activity for the colorimetric and fluorescence analysis of different analytes. In this review, recent developments (from 2018 to present) in the colorimetric and fluorescent analysis of bioactive small-molecule compounds based on the enzyme-like activity of nanomaterials were summarized. In addition, the challenges and design strategies in developing colorimetric and fluorescent assays with high performance and specific sensing were discussed.

In situ green synthesis of the nanocomposites of MnO2/graphene as an oxidase mimic for sensitive colorimetric and electrochemical dual-mode biosensing

Herein we report the colorimetry and an electrochemical for the determination of dopamine (DA) by using MnO2 nanoparticles and graphene nanosheets composite (MnO2@G) that display oxidase mimicking property. MnO2@G could directly oxidize colorless 3,3',5,5'-tetramethylbenzidine (TMB) into a blue product (oxTMB) without extra oxidants such as H2O2. Nevertheless, the presence of DA will inhibit the TMB oxidation due to the presence of the competitive reaction of MnO2@G and DA, giving a product color change from blue to colorless. A colorimetric assay for detect the concentration of DA was worked out according to this finding. Response is linear in the 0.1 to 15 μM DA concentration range, and the detection limit is 0.14 μM. Wider detection range is achieved in an electrochemical method which is due to the pronounced electrocatalytic activity of MnO2@G. The MnO2@G was modified on the surface of the glassy carbon electrode in order to fabricate one type electrochemical sensor. The sensor achieves a wide detection two linear ranges from 0.4 to 70 μM, with the detection limit of 1.16 μM. The detection of DA in real serum sample proved that the nanozyme based on MnO2@G could be developed into a colorimetry and electrochemical dual-readout sensing platform.

Tuning nanozyme property of Co@NC via V doping to construct colorimetric sensor array for quantifying and discriminating antioxidant phenolic compounds

Through V₂O₅ etching of ZIF-67 and subsequent pyrolysis in an argon flow, the V doped Co@NC (V/Co@NC) with mixed-valence Co(II)/Co(III) and V(III)/V(IV) was successfully obtained. V doping plays an important role in regulating the enzyme-like activity of Co@NC. Specifically, the Co@NC has both oxidase-like activity and peroxidase-mimic activity, while the V/Co@NC possesses the specific oxidase-like activity. Benefiting from the elevated Co²⁺ level due to electrons transfer from the reduced V(III) to Co³⁺ and recyclable redox reactions between the Co(III)/Co(II) and V(IV)/V(III) couples, the V/Co@NC displays 4-fold increase in the oxidase-like activity, smaller K_m (0.18 mM) and larger V_max (4.01 × 10^-8 M s^-1) toward TMB relative to Co@NC. The origin of V/Co@NC as oxidase mimic is likely attributed to the generation of ¹O₂ and •OH. Different phenolic compounds (PC), like gallic acid, kaempferol, caffeic acid, quercetin, and catechin, have distinct antioxidant capacity, showing a differential inhibiting effect on the V/Co@NC-TMB system. The different PC antioxidants in the V/Co@NC-TMB system lead to unique decrease in the absorbance at 652 nm (A₆₅₂), resulting in a unique absorbance signal response mode. By choosing different combinations of A₆₅₂ signals at various time points, multichannel information can be extracted from a single nanozyme for pattern recognition. Based on this, a colorimetric array sensing platform for the identification of PC is established successfully. Furthermore, the constructed sensor array can be used for quantifying and discriminating multiple PC antioxidants.

Self-template synthesis of mesoporous vanadium oxide nanospheres with intrinsic peroxidase-like activity and high antibacterial performance

Mesoporous vanadium oxide nanospheres are a very promising nanozyme for antibacterial and chemical sensing. However, controllable synthesis of mesoporous vanadium oxide nanospheres with uniform structure and small diameter (<200 nm) remains challenging. Herein, mesoporous vanadium oxide nanospheres (MVONs) with a small, uniform and adjustable particle size (52-105 nm), large mesopore size (5.1-5.8 nm), and high specific surface area (up to 63.7 m² g^-1) are constructed via a self-template strategy using tannic acid, formaldehyde and vanadium compounds as a polymerizable ligand, cross-linking agent and metal source, respectively. The relationships between synthesis conditions and material nanostructure are systematically investigated. The particle size and peroxidase-like activity of MVONs can be easily changed by adding different amounts of Pluronic block copolymer F127. Owing to the mesoporous structure, high specific surface area and small particle size, MVONs can effectively convert H₂O₂ into extremely toxic reactive oxygen species, and further kill Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). This research establishes a universal, reliable method for synthesizing mesoporous vanadium oxide nanospheres, which might be used in catalysis, biosensors, and antibacterial treatment.

Multifaceted Therapy of Nanocatalysts in Neurological Diseases

With the development of enzymes immobilization technology and the discover of nanozymes, catalytic therapy exhibited tremendous potential for neurological diseases therapy. In especial, since the discovery of Fe₃O₄ nanoparticles possessing intrinsic peroxidase-like activity, various nanozymes have been developed and recently started to explore for neurological diseases therapy, such as Alzheimer's disease, Parkinson's disease and stroke. By combining the catalytic activities with other properties (such as optical, thermal, electrical, and magnetic properties) of nanomaterials, the multifunctional nanozymes would not only alleviate oxidative and nitrosative stress on the basis of multienzymes-mimicking activity, but also exert positive effects on immunization, inflammation, autophagy, protein aggregation, which provides the foundation for multifaceted treatments. This review will summarize various types of nanocatalysts and further provides a valuable discussion on multifaceted treatment by nanozymes for neurological diseases, which is anticipated to provide an easily accessible guide to the key opportunities and current challenges of the nanozymes-mediated treatments for neurological diseases.

Ag-doped Fe-metal–organic framework nanozymes for efficient antibacterial application

Illustration of an Fe-MOF-Ag nanozyme for antibacterial application.

A NiAg-graphene quantum dot-graphene hybrid with high oxidase-like catalytic activity for sensitive colorimetric detection of malathion

This paper reports the synthesis of a nickel-silver-graphene quantum dot-graphene hybrid.