Rational construction of a robust metal-organic framework nanozyme with dual-metal active sites for colorimetric detection of organophosphorus pesticides

A novel fluorescent and photothermal probe based on nanozyme-mediated cascade reaction for detecting organophosphorus pesticide residues

In this study, a nanozyme (ZIF-Co-Cys) with high oxidase-like catalytic activity was prepared, and a ratiometric fluorescent/photothermal dual-mode probe was constructed for organophosphorus pesticides (OPs) detection based on the competitive effect of ZIF-Co-Cys and the enzymatic reaction product of acid phosphatase (ACP) on o-phenylenediamine and the inhibition effect of OPs on ACP activity. Using dimethyl dichloroviny phosphate (DDVP) as the model, both the fluorescence intensity ratio and the temperature change of the probe solution exhibited an excellent correlation with OPs concentration. The detection limits were 1.64 ng/mL and 0.084 ng/mL, respectively. Additionally, the detection of DDVP residues in real samples verified the outstanding anti-interference and accuracy of the probe. This work not only provided a complementary dual-mode method for the accurate and rapid detection of OPs residues in complex samples, but also supplied a new insight into the design of a multi-mode sensing platform based on the cascade reaction of nanozyme.

Doughnut-shaped bimetallic Cu-Zn-MOF with peroxidase-like activity for colorimetric detection of glucose and antibacterial applications

Metal-organic frameworks (MOFs), especially bimetallic MOFs, have attracted widespread attention for simulating the structure and function of natural enzymes. In this study, different morphologies of bimetallic Cu-Zn-MOF with different peroxidase (POD)-like activities were prepared by simply controlling the molar ratio of Cu2+ and Zn2+. Among them, the doughnut-shaped Cu9-Zn1-MOF exhibited the largest POD-like activity. Cu9-Zn1-MOF was combined with glucose oxidase to construct a sensitive and selective glucose colorimetric biosensor with a linear detection range of 10-300 μM and a detection limit of 7.1 μm. Furthermore, Cu9-Zn1-MOF can efficiently convert hydrogen peroxide (H2O2) into hydroxyl radicals that effectively kill both gram-negative and gram-positive bacteria at low H2O2 level. The results of this study may promote the synthesis of bimetallic MOFs and broaden their applications in the biomedical field.

Smartphone-assisted portable swabs for blood glucose management: A point-of-use assay for dual-mode visual detection based on bifunctional carbon dots

Controlling glucose (Glu) intake is a "required course" for diabetics, thus quickly and precisely measuring the amount of Glu in food is crucial. For this purpose, a novel smartphone-assisted portable swab for the dual-mode visual detection of Glu was constructed combined the selectivity of natural enzymes with the controllable catalytic activity of nanozymes. Glu was specifically decomposed by glucose oxidase (natural enzyme) to produce H2O2, which was catalyzed by carbon dots (FeMn/N-CDs, nanozyme) to accelerate the reaction of o-phenylenediamine (OPD, colorless) to produce 2,3-diaminophenazine (DAP, yellow). As a result, the absorbance at 450 nm gradually increased with the increasing concentration of Glu, leading to a color change in the system from colorless to yellow. Meanwhile, the fluorescence of FeMn/N-CDs gradually decreased at 450 nm, while the fluorescence of DAP gradually increased at 550 nm, allowing for both ratiometric fluorescence and colorimetric dual-mode detection. Furthermore, natural enzyme and nanozyme together with OPD were co-loaded on the swabs to achieve cascade catalysis of Glu. The assembled portable swabs have detection ranges of 1-600 μM (LOD = 0.37 μM) and 4-1200 μM (LOD = 1.19 μM) for the colorimetric and fluorometric detection, respectively. The field test results on real samples demonstrated that the portable swabs have great promise for use in efficiently and accurately guiding the dietary intake of diabetics.

Vanadium-Based Metal-Organic Frameworks with Peroxidase-like Activity as a Colorimetric Sensing Platform for Direct Detection of Organophosphorus Pesticides

Colorimetry based on the bioenzyme inhibition strategy holds promising application prospects in the field of organophosphorus pesticide (OPs) detection. However, overcoming the challenges of the high cost and low stability of bioenzymes remains crucial. In this study, we successfully synthesized a peroxidase vanadium-based metal-organic framework (MOF) nanozyme named MIL-88B(V) and employed its mediated bioenzyme-free colorimetric strategy for direct OPs detection. The experimental results demonstrated that MIL-88B(V) exhibited a remarkable affinity and a remarkable catalytic rate. When the OPs target is added, it can be anchored on the MOF surface through a V-O-P bond, effectively inhibiting the MOF's activity. Subsequently, leveraging the advantages of smartphones such as convenience, speed, and sensitivity, we developed a paper sensor integrated into a smartphone for efficient OPs detection. The as-designed nanozyme-based colorimetric assay and paper sensor presented herein offer notable advantages, including affordability, speed, stability, wide adaptability, low cost, and accuracy in detecting OPs, thus providing a versatile and promising analytical approach for real sample analysis and allowing new applications of V-based MOF nanozymes.

Wide temperature adaptive oxidase-like based on mesoporous manganese based metal-organic framework for detecting total antioxidant capacity

Overcoming the intense variation of enzymatic activity among different temperatures is very critical in catalytic medicine and catalytic biology. Here, Mn-based metal-organic framework-based wide-temperature-adaptive mesoporous artificial enzymes (Mn-TMA-MOF) were designed and synthesized. The oxidase-like Mn-TMA-MOF showed excellent catalytic activity at 0-50 °C and avoided the activity loss and instability due to temperature variation that occurred. The excellent oxidase-like properties of Mn-TMA-MOF with wide temperature adaptativeness are mainly ascribed to the mixed oxidized state (Mn3+/Mn2+) and high substrate affinity (Km = 0.034 mM) of Mn. Moreover, the mesopore-micropores two-level structure of Mn-TMA-MOF provides a large space and surface area for enzyme catalysis. Based on the stability of Mn-TMA-MOF, we developed a colorimetric sensor that can detect total antioxidant capacity in fruits with a limit of detection up to 0.59 μM.

Sensitive colorimetric and fluorescence dual-mode detection of thiophanate-methyl based on spherical Fe3O4/GONRs composite nanozyme

Pesticide detection based on nanozyme is largely limited in terms of the variety of pesticides. Herein, a spherical and well-dispersed Fe3O4/graphene oxide nanoribbons (Fe3O4/GONRs) composite nanozyme was applied to firstly develop an enzyme-free and sensitive colorimetric and fluorescence dual-mode detection of thiophanate-methyl (TM). The synthesized Fe3O4/GONRs possess excellent dual enzyme-like activities (peroxidase and catalase) and can catalyze H2O2 to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into oxidized TMB (oxTMB). We found that Fe3O4/GONRs can adsorb TM through the synergistic effect of multiple forces, thereby inhibiting the catalytic activities of nanozyme. This inhibition can modulate the transformation of TMB to oxTMB, producing dual responses of absorbance decrease (oxTMB) and fluorescence enhancement (TMB). The limits of detection (LODs) of TM were 28.1 ng/mL (colorimetric) and 8.81 ng/mL (fluorescence), respectively. Moreover, the developed method with the recoveries of 94.8-100.8% also exhibited a good potential application in the detection of pesticides residues in water and food samples.

Trimetallic FeCoNi Metal-Organic Framework with Enhanced Peroxidase-like Activity for the Construction of a Colorimetric Sensor for Rapid Detection of Thiophenol in Water Samples

In recent years, nanozymes have attracted particular interest and attention as catalysts because of their high catalytic efficiency and stability compared with natural enzymes, whereas how to use simple methods to further improve the catalytic activity of nanozymes is still challenging. In this work, we report a trimetallic metal-organic framework (MOF) based on Fe, Co and Ni, which was prepared by replacing partial original Fe nodes of the Fe-MOF with Co and Ni nodes. The obtained FeCoNi-MOF shows both oxidase-like activity and peroxidase-like activity. FeCoNi-MOF can not only oxidize the chromogenic substrate 3,3,5,5-tetramethylbenzidine (TMB) to its blue oxidation product oxTMB directly, but also catalyze the activation of H2O2 to oxidize the TMB. Compared with corresponding monometallic/bimetallic MOFs, the FeCoNi-MOF with equimolar metals hereby prepared exhibited higher peroxidase-like activity, faster colorimetric reaction speed (1.26-2.57 folds), shorter reaction time (20 min) and stronger affinity with TMB (2.50-5.89 folds) and H2O2 (1.73-3.94 folds), owing to the splendid synergistic electron transfer effect between Fe, Co and Ni. Considering its outstanding advantages, a promising FeCoNi-MOF-based sensing platform has been designated for the colorimetric detection of the biomarker H2O2 and environmental pollutant TP, and lower limits of detection (LODs) (1.75 μM for H2O2 and 0.045 μM for TP) and wider linear ranges (6-800 μM for H2O2 and 0.5-80 μM for TP) were obtained. In addition, the newly constructed colorimetric platform for TP has been applied successfully for the determination of TP in real water samples with average recoveries ranging from 94.6% to 112.1%. Finally, the colorimetric sensing platform based on FeCoNi-MOF is converted to a cost-effective paper strip sensor, which renders the detection of TP more rapid and convenient.

Recent and emerging trends of metal-organic frameworks (MOFs)-based sensors for detecting food contaminants: A critical and comprehensive review

Interest in the use of sensors based on metal-organic frameworks (MOFs) to detect food pollutants has been growing recently due to the desirable characteristics of MOFs, including uniform structures, large surface area, ultrahigh porosity and easy-to-functionalize surface. Fundamentally, this review offers an excellent solution using MOFs-based sensors (e.g., fluorescent, electrochemical, electrochemiluminescence, surface-enhanced Raman spectroscopy, and colorimetric sensors) to detect food contaminants such as pesticide residues, mycotoxins, antibiotics, food additives, and other hazardous candidates. More importantly, their application scenarios and advantages in food detection are also introduced in more detail. Therefore, this systematic review analyzes detection limits, linear ranges, the role of functionalities, and immobilized nanoparticles utilized in preparing MOFs-based sensors. Additionally, the main limitations of each sensing type, along with the enhancement mechanisms of MOFs in addressing efficient sensing are discussed. Finally, the limitations and potential trends of MOFs-based materials in food contaminant detection are also highlighted.

Metal-organic framework-based multienzyme cascade bioreactor for sensitive detection of methyl parathion

In this study, a cascade nanobioreactor was developed for the highly sensitive detection of methyl parathion (MP) in food samples. The simultaneous encapsulation of acetylcholinesterase (AChE) and choline oxidase (CHO) in a zeolitic imidazole ester backbone (ZIF-8) effectively improved the stability and cascade catalytic efficiency of the enzymes. In addition, glutathione-stabilized gold nanoclusters (GSH-AuNCs) were encapsulated in ZIF-8 by ligand self-assembly, conferring excellent fluorescence properties. Acetylcholine (ATCh) is catalyzed by a cascade of AChE/CHO@ZIF-8 as well as Fe(II) to generate hydroxyl radicals (·OH) with strong oxidizing properties. The ·OH radicals then oxidize Au(0) in GSH-AuNCs@ZIF-8 to Au(I), resulting in fluorescence quenching. MP, as an inhibitor of AChE, hinders the cascade reaction and thus restores the fluorescence emission, enabling its quantitative detection. The limit of detection of the constructed nanobioreactor for MP was 0.23 µg/L. This MOF-based cascade nanobioreactor has great potential for the detection of trace hazards.

Construction of Mn-decorated zeolitic imidazolate framework-90 nanostructure as superior oxidase-like mimic for colorimetric detection of glucose and choline

A Mn decorated zeolitic imidazolate framework-90 (ZIF-90) nanozyme (Mn/ZIF-90) was constructed through an effective and rapid post-synthetic strategy for the first time. The Mn in Mn/ZIF-90 exists in mixed valence states, which is doped to the ZIF-90 through the formation of Mn-O bond. The Zn-N coordination structure of ZIF-90 may change the electronic arrangement of oxygen atoms in the free carbonyl groups (-CHO), allowing the coordination of Mn with O. The prepared Mn/ZIF-90 possesses outstanding oxidase-like activity and remarkable stability. Besides, the catalytic activity of Mn/ZIF-90 can be inhibited in the presence of H2O2. Therefore, using the Mn/ZIF-90-triggered chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB) as an amplifier, a versatile enzyme cascade-based colorimetric method for the detection of glucose and choline with good sensitivity and selectivity was developed. The linear ranges for glucose and choline are 6.25-500 μM and 5-1000 μM, respectively. Furthermore, the developed method was applied in the detection of glucose and choline in rabbit plasma samples, and the recoveries are 89.5-107.3 % and 96.0-109.3 %, respectively. In short, the simple and efficient post-synthetic doping method may provide a new thought for the rational designs of enzyme mimics with improved catalytic performance. Moreover, the colorimetric method based on the excellent catalytic activity of Mn/ZIF-90 may be extended to detect other H2O2-generating or consuming molecules and evaluate the activity of bio-enzymes that can catalyze the generation of glucose or choline.

Nanozyme-induced deep learning-assisted smartphone integrated colorimetric and fluorometric dual-mode for detection of tetracycline analogs

In this work, a colorimetric and fluorescent dual-mode probe controlled by NH2-MIL-88 B (Fe, Ni) nanozymes was developed to visually detect tetracycline antibiotics (TCs) residues quantitatively, as well as accurately distinguish the four most widely used tetracycline analogs (tetracycline (TC), chrycline (CTC), oxytetracycline (OTC), and doxycycline (DC)). Colorless substrate 3,3',5,5'-tetramethylbenzidine (TMB) may be oxidized to blue oxidized TMB by the Fe Fenton reaction, which was catalyzed by the NH2-MIL-88 B (Fe, Ni) nanozyme with POD-like activity. The colorimetric detection system allows TCs to interact with NH2-MIL-88 B (Fe, Ni). This inhibits the production of ·OH, weakens the oxidation process of TMB, and ultimately lightens the blue color in the system by blocking the electron transfer between NH2-MIL-88 B (Fe, Ni) and H2O2. Furthermore, TCs can interact with NH2-MIL-88 B (Fe, Ni) as a result of the internal filtering effect, which causes the fluorescence intensity to decrease as TCs concentration increases. Additionally, a portable instrument that combines a smartphone sensing platform with colorimetric and fluorescent signals was created for the quick, visual quantitative detection of TCs. The colorimetric and fluorescent dual-mode nano platform enables color change, with detection limits (LODs) of 0.182 μM and 0.0668 μM for the spectrometer and smartphone sensor, respectively, based on the inhibition of fluorescence and enzyme-like activities by TCs. Overall, the colorimetric and fluorescence dual-mode sensor has good stability, high specificity, and an efficient way to eliminate false-positive issues associated with a single detection mode.

Graphene-based functional electrochemical sensors for the detection of chlorpyrifos in water and food samples: a review

Prolonged and excessive use of chlorpyrifos (CPS) has caused severe pollution, particularly in crops, vegetables, fruits, and water sources. As a result, CPS is detected in various food and water samples using conventional methods. However, its applications are limited due to size, portability, cost, etc. In this regard, electrochemical sensors are preferred for CPS detection due to their high sensitivity, reliability, rapid, on-site detection, and user-friendly. Notably, graphene-based electrochemical sensors have gained more attention due to their unique physiochemical and electrochemical properties. It shows high sensitivity, selectivity, and quick response because of its high surface area and high conductivity. In this review, we have discussed an overview of three graphene-based different functional electrochemical sensors such as electroanalytical sensors, bio-electrochemical sensors, and photoelectrochemical sensors used to detect CPS in food and water samples. Furthermore, the fabrication and operation of these electrochemical sensors using various materials (low band gap material, nanomaterials, enzymes, antibodies, DNA, aptamers, and so on) and electrochemical techniques (CV, DPV, EIS, SWV etc.) are discussed. The study found that the electrical signal was reduced with increasing CPS concentration. This is due to the blocking of active sites, reduced redox reaction, impedance, irreversible reactions, etc. In addition, acetylcholinesterase-coupled sensors are more sensitive and stable than others. Also, it can be further improved by fabricating with low band gap nanomaterials. Despite their advantages, these sensors have significant drawbacks, such as low reusability, repeatability, stability, and high cost. Therefore, further research is required to overcome such limitations.

A colorimetric and SERS dual-mode aptasensor for the detection of Shiga toxin type II based on Mn/Fe-MIL(53)@AuNSs

Shiga toxin type II (Stx2), the major virulence component of enterohemorrhagic Escherichia coli, is strongly associated with the life-threatening hemolytic uremic syndrome thus posing a substantial risk to food safety and human health. In this work, a dual-mode aptasensor with colorimetric and surface-enhanced Raman scattering was developed for Stx2 specific detection based on noble metal nanoparticles and Raman reporter loaded metal-organic framework (Mn/Fe-MIL(53)@AuNSs-MBA). The Mn/Fe-MIL(53)@AuNSs could catalyze the H2O2-mediated oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), thereby enabling visual detection. Meanwhile, the SERS signal from MBA can be enhanced by the decorated AuNSs. Under optimal conditions, a linear range of 0.05-500 ng/mL with limit of detection (LOD) of 26 pg/mL was achieved in colorimetric mode and a linear range of 5-1000 ng/mL with LOD of 0.82 ng/mL in SERS mode, in which the dual-mode results complement each other, widening the linear range, increasing the accuracy and reliability of the detection. The method was further applied to the detection of Stx2 in milk with average recovery of 101.1 %, demonstrating its superior potential for bacterial toxin monitoring.

Fe-Co-Based Metal-Organic Frameworks as Peroxidase Mimics for Sensitive Colorimetric Detection and Efficient Degradation of Aflatoxin B1

Building multifunctional platforms for integrating the detection and control of hazards has great significance in food safety and environment protection. Herein, bimetallic Fe-Co-based metal-organic frameworks (Fe-Co-MOFs) peroxidase mimics are prepared and applied to develop a bifunctional platform for the synergetic sensitive detection and controllable degradation of aflatoxin B1 (AFB1). On the one hand, Fe-Co-MOFs with excellent peroxidase-like activity are combined with target-induced catalyzed hairpin assembly (CHA) to construct a colorimetric aptasensor for the detection of AFB1. Specifically, the binding of aptamer with AFB1 releases the prelocked Trigger to initiate the CHA cycle between hairpin H2-modified Fe-Co-MOFs and hairpin H1-tethered magnetic nanoparticles to form complexes. After magnetic separation, the colorimetric signal of the supernatant in the presence of TMB and H2O2 is inversely proportional to the target contents. Under optimal conditions, this biosensor enables the analysis of AFB1 with a limit of detection of 6.44 pg/mL, and high selectivity and satisfactory recovery in real samples are obtained. On the other hand, Fe-Co-MOFs with remarkable Fenton-like catalytic degradation performance for organic contaminants are further used for the detoxification of AFB1 after colorimetric detection. The AFB1 is almost completely removed within 120 min. Overall, the introduction of CHA improves the sensing sensitivity; efficient postcolorimetric-detection degradation of AFB1 reduces the secondary contamination and risk to the experimental environment and operators. This strategy is expected to provide ideas for designing other multifunctional platforms to integrate the detection and degradation of various hazards.

A portable colorimetric sensing platform for rapid and sensitive quantification of dichlorvos pesticide based on Fe-Mn bimetallic oxide nanozyme-participated highly efficient chromogenic catalysis

BACKGROUND

Dichlorvos (DDVP), as a highly effective insecticide, is widely used in agricultural production. However, DDVP residue in foodstuffs adversely affects human health. Conventional instrumental analysis can provide highly sensitive and accurate detection of DDVP, while the need of bulky and expensive equipment limits their application in resource-poor areas and on-site detection. Therefore, the development of easily portable sensing platforms for convenient, rapid and sensitive quantification of DDVP is very essential for ensuring food safety.

RESULT

A portable colorimetric sensing platform for rapid and sensitive quantification of DDVP is developed based on nanozyme-participated highly efficient chromogenic catalysis. The Fe-Mn bimetallic oxide (FeMnOx) nanozyme possesses excellently oxidase-like activity and can efficiently catalyze oxidation of 3, 3', 5, 5'-tetramethylbenzidine (TMB) into a blue oxide with a very low Michaelis constant (Km) of 0.0522 mM. The nanozyme-catalyzed chromogenic reaction can be mediated by DDVP via inhibiting the acetylcholinesterase (AChE) activity. Thus, trace DDVP concentration-dependent color evolution is achieved and DDVP can be sensitively detected by spectrophotometry. Furthermore, a smartphone-integrated 3D-printed miniature lightbox is fabricated as the colorimetric signal acquisition and processing device. Based on the FeMnOx nanozyme and smartphone-integrated lightbox system, the portable colorimetric sensing platform of DDVP is obtained and it has a wide linear range from 1 to 3000 ng mL-1 with a low limit of detection (LOD) of 0.267 ng mL-1 for DDVP quantification.

SIGNIFICANCE

This represents a new portable colorimetric sensing platform that can perform detection of DDVP in foodstuffs with simplicity, sensitivity, and low cost. The work not only offers an alternative to rapid and sensitive detection of DDVP, but also provides a new insight for the development of advanced sensors by the combination of nanozyme, 3D-printing and information technologies.

Enhanced oxidase-mimic constructed by luminescent carbon dots loaded on MIL-53(Fe)-NO2 for dual-mode detection of gallic acid and biothiols in food and humans

Monitoring of gallic acid (GA) in food and biothiols in humans is crucial for body health. Nanozyme-mediated colorimetric strategy for evaluating them has been widely applied nowadays, however, the inferior efficient and susceptible single-signal recognition limit its further application. Herein, a sensitive biosensor was first constructed for bimodal detection of GA and biothiols based on CDs@MIL-53(Fe)-NO2, prepared through a facile and time-saving microwave treatment. Benefiting from the excellent fluorescent and electron transfer properties of CDs, CDs@MIL-53(Fe)-NO2 exhibited significant enhanced blue fluorescence and oxidase-like activity, which could oxide colorless 3,3',5,5'-tetramethylbenzidine without H2O2, and the blue product could quench the fluorescence of composite. The dual-mode assay based on such bifunctional nanozyme showed an extremely sensitivity towards GA/l-cysteine/homocysteine with the detection limit of 62/65/124 nM and 17/16/27 nM in colorimetric/fluorescent modes, respectively. The practicability in real samples and portability based on a smartphone of the analysis has been investigated with reliable results.

A dual-mode sensing platform based on metal-organic framework for colorimetric and ratiometric fluorescent detection of organophosphorus pesticide

Pesticide residues have raised considerable concern about environmental health and food safety. Despite a great advance in enzymatic sensors for pesticide detection, the intrinsic fragility of native enzyme and possible fake results due to single mode signal have hindered its wide application. Here, a novel dual-mode sensor is reported for organophosphorus pesticide detection by using metal-organic framework (MOF) nanozyme NH2-CuBDC as sensing element. The intrinsic peroxidase-mimicking activity and fluorescence property of NH2-CuBDC enable both colorimetric and fluorescent detection of chlorpyrifos. Compared with previously reported chlorpyrifos sensors, our sensor exhibits outstanding sensitivity, and the limits of detection (LOD, S/N = 3) in colorimetric and fluorescent modes are 1.57 ng/mL and 2.33 ng/mL, respectively. No obvious interferences from other substances were measured and chlorpyrifos analysis in real samples presented good reliability, showing practical potential. This work is anticipated to provide new insights to develop multifunctional nanozymes and integrated multi-mode sensing platforms.

Thermal-Driven Curcumin Release Film with Dual-Mode Synergistic Antibacterial Behavior for Efficient Tangerine Preservation

Antimicrobial packing showed great potential in extending the shelf life of food. However, developing a new biocomposite film with an intelligent and efficient antimicrobial performance is still desirable. Herein, a Fe-MoOx encapsulated with curcumin (Cur) filled chitosan-based composite film (CCF films) was prepared by solvent casting method. The total color differences of the CCF films were less than 30%, and satisfactory surface color, transparency, hydrophobicity, and thermal stability were also obtained. Besides, the UV-light/water/oxygen barrier capability and mechanical properties were enhanced with the incorporation of Cur@Fe-MoOx. Moreover, CCF films showed photothermal performance and thermal-controlled curcumin release ability, which endowed the CCF0.15 film with excellent antibacterial capability toward E. coli (≥99.95%) and S. aureus (≥99.96%) due to the synergistic antibacterial effect. Fe-MoOx exhibited high cell viability and less than 5% hemolysis even under the concentration of 500 μg mL-1. Based on those unique characteristics, the CCF0.15 film was chosen for tangerine preservation. The CCF0.15 film could prolong the shelf life of tangerine by at least 9 days compared with the unpacking group, and the tangerines could maintain the freshness characteristics over a 24 day storage period. Such thermal-mediated antibacterial film proposed by our work showed promising potential in food packaging.

Recent Progress and Prospect of Metal-Organic Framework-Based Nanozymes in Biomedical Application

A nanozyme is a nanoscale material having enzyme-like properties. It exhibits several superior properties, including low preparation cost, robust catalytic activity, and long-term storage at ambient temperatures. Moreover, high stability enables repetitive use in multiple catalytic reactions. Hence, it is considered a potential replacement for natural enzymes. Enormous research interest in nanozymes in the past two decades has made it imperative to look for better enzyme-mimicking materials for biomedical applications. Given this, research on metal-organic frameworks (MOFs) as a potential nanozyme material has gained momentum. MOFs are advanced hybrid materials made of inorganic metal ions and organic ligands. Their distinct composition, adaptable pore size, structural diversity, and ease in the tunability of physicochemical properties enable MOFs to mimic enzyme-like activities and act as promising nanozyme candidates. This review aims to discuss recent advances in the development of MOF-based nanozymes (MOF-NZs) and highlight their applications in the field of biomedicine. Firstly, different enzyme-mimetic activities exhibited by MOFs are discussed, and insights are given into various strategies to achieve them. Modification and functionalization strategies are deliberated to obtain MOF-NZs with enhanced catalytic activity. Subsequently, applications of MOF-NZs in the biosensing and therapeutics domain are discussed. Finally, the review is concluded by giving insights into the challenges encountered with MOF-NZs and possible directions to overcome them in the future. With this review, we aim to encourage consolidated efforts across enzyme engineering, nanotechnology, materials science, and biomedicine disciplines to inspire exciting innovations in this emerging yet promising field.

A MOF nanozyme-mediated acetylcholinesterase-free colorimetric strategy for direct detection of organophosphorus pesticides

Although some simple and rapid colorimetric methods have been developed to detect organophosphorus pesticides (OPs), the difficult extraction and easy denaturation of acetylcholinesterase (AChE) are still drawbacks needing to be overcome. Here, we propose a MOF nanozyme-mediated AChE-free colorimetric strategy for the direct detection of OPs. In the presence of OPs (pirimiphos-methyl as a model), the intense blue of oxidized 3,3',5,5'-tetramethylbenzidine (TMB) becomes light due to the quenching effect of OPs towards hydroxyl radicals (˙OH) that are generated by the decomposition of H2O2 catalyzed by the Cu4Co6 ZIF nanozyme with excellent peroxidase (POD)-like activity. The developed colorimetric sensor exhibits assay performance and offers a universal and promising analysis strategy for detecting OPs in practical samples.

Multifunctional Ni-NPC Single-Atom Nanozyme for Removal and Smartphone-Assisted Visualization Monitoring of Carbamate Pesticides

A multifunctional single-atom nanozyme, denoted as 3D Ni,N-codoped porous carbon (Ni-NPC), was devised that exhibits remarkable adsorption capabilities and a repertoire of enzyme mimetic functions (oxidase- and peroxidase-like). These attributes stem from the distinctive mesoporous thin-shell structure and well-dispersed Ni sites. The efficient adsorption capacity of Ni-NPC was assessed with respect to three carbamate pesticides (CMPs): metolcarb, carbaryl, and isoprocarb. Moreover, a colorimetric detection method for CMP was established based on its robust peroxidase-like catalytic activity and sequential catalytic interactions with acetylcholinesterase. Furthermore, a portable colorimetric sensor based on a hydrogel sphere integrated with a smartphone platform was devised. This sensor enables rapid, on-site, and quantitative assessment of CMP, boasting an extraordinarily low detection limit of 1.5 ng mL-1. Notably, this sensor was successfully applied to the analysis of CMP levels in lake water and vegetable samples (pakchoi and rape), propelling the progress of real-time detection technologies in food and environment monitoring.

A label-free ratiometric fluorescent aptasensor based on a peroxidase-mimetic multifunctional ZrFe-MOF for the determination of tetrodotoxin

A Fe/Zr bimetal-organic framework (ZrFe-MOF) is utilized to establish a ratiometric fluorescent aptasensor for the determination of tetrodotoxin (TTX). The multifunctional ZrFe-MOF possesses inherent fluorescence at 445 nm wavelength, peroxidase-mimetic activity, and specific recognition and adsorption capabilities for aptamers, owing to its organic ligand, and Fe and Zr nodes. The peroxidation of o-phenylenediamine (OPD) substrate generates fluorescent 2,3-diaminophenazine (OPDox) at 555 nm wavelength, thus quenching the inherent fluorescence of ZrFe-MOF because of the fluorescence resonance energy transfer (FRET) effect. TTX aptamers, which are absorbed on the material surface without immobilization or fluorescent labeling, inhibit the peroxidase-mimetic activity of ZrFe-MOF. It causes the decreased OPDox fluorescence at 555 nm wavelength and the inverse restoration of ZrFe-MOF fluorescence at 445 nm wavelength. With TTX, the aptamers specifically bind to TTX, triggering rigid complex release from ZrFe-MOF surface and reactivating its peroxidase-mimetic activity. Consequently, the two fluorescence signals exhibit opposite changes. Employing this ratiometric strategy, the determination of TTX is achieved with a detection limit of 0.027 ng/mL and a linear range of 0.05-500 ng/mL. This aptasensor also successfully determines TTX concentrations in puffer fish and clam samples, demonstrating its promising application for monitoring trace TTX in food safety.

Bimetallic nanozyme triple-emission fluorescence intelligent sensing platform-integrated molecular imprinting for ultrasensitive visual detection of triclosan

Triclosan (TCS) is an endocrine disruptor, which has been widely used in daily chemicals, resulting in the potential risk to the ecosystem and human health. Herein, a smartphone-integrated bimetallic nanozyme triple-emission fluorescence capillary imprinted sensing system was developed for ultrasensitive and intelligent visual microanalysis of TCS. Carbon dots (CDs) and bimetallic organic framework (MOF-(Fe/Co)-NH2) were used as fluorescence sources to synthesize nanozyme fluorescence molecularly imprinted polymer (MOF-(Fe/Co)-NH2@CDs@NMIP), which oxidized o-phenylenediamine to 2,3-diaminophenazine (OPDox), resulting in the derivation of a new fluorescence peak at 556 nm. In the existence of TCS, the fluorescence of MOF-(Fe/Co)-NH2 at 450 nm was restored, the fluorescence of OPDox at 556 nm was suppressed, and the CDs fluorescence of at 686 nm remained constant. The color of triple-emission fluorescence imprinted sensor varied from yellow to pink to purple to blue. The response efficiency (F450/F556/F686) of this sensing platform based on the capillary waveguide effect demonstrated a significant linear relationship toward the concentration of TCS ranged from 1.0 × 10-12 to 1.5 × 10-10 M with the LOD of 8.0 × 10-13 M. Compared with dual-emission capillary fluorescence sensor, this sensing system has higher sensitivity and richer visual color. Combined with the smartphone-integrated portable sensing platform, the color of fluorescence was transformed into an RGB value to calculate TCS concentration with the LOD of 9.6 × 10-13 M, providing a novel method for intelligent visual microanalysis (18 μL/time) of environmental pollutants.

A colorimetric sensor array based on nanoceria crosslinked and heteroatom-doped graphene oxide nanoribbons for the detection and discrimination of multiple pesticides

Nanozymes have demonstrated high potential in constructing colorimetric sensor array for pesticides. However, rarely array for pesticides constructed without bio-enzyme were reported. Herein, nanoceria crosslinked graphene oxide nanoribbons (Ce-GONRs) and heteroatom-doped graphene oxide nanoribbons (Ce-BGONRs and Ce-NGONRs) were prepared, demonstrating excellent peroxidase-like activities. A colorimetric sensor array was developed based on directly inhibiting the peroxidase-like activities of the above three nanozymes, which realized the discrimination and quantitative analysis of six pesticides. In the presence of pesticides including carbaryl (Car), fluroxypyr-mepthyl (Flu), thiophanate-methyl (Thio), thiram (Thir), diafenthiuron (Dia) and fomesafen (Fom), the peroxidase-like activities of three nanozymes were inhibited to different degrees, resulting in different fingerprint responses. The six pesticides in the concentration range of 0.1-50 μg/mL and two pesticides mixtures at varied ratios could be detected and discriminated, and minimum detection limit for pesticides was 0.022 μg/mL. In addition, this sensor array has been successfully applied for pesticides discrimination in lake water and apple samples. This work provided a new strategy of constructing simple and sensitive colorimetric sensor array for pesticides based on directly inhibiting the catalytic activities of nanozymes.

Biomimetic Metal-Pyrimidine Nanoflowers: Enzyme Immobilization Platforms with Boosted Activity

For the enzyme immobilization platform, enhancing enzyme activity retention while improving enzyme stability remains a challenge for sensitive sensing analysis. Herein, an in situ biomimetic immobilized enzyme carrier (metal-pyrimidine nanoflowers, MPNFs) synthesized by the coordination of DNA base derivative (2-aminopyrimidine) with Zn2+ in the aqueous phase at room temperature is developed. The biocompatibility of 2-aminopyrimidine and the hydrophilicity and green synthetic conditions of MPNFs allows the immobilized enzymes to retain above 91.2% catalytic activity. On this basis, a cascade catalytic platform is constructed by simultaneously immobilizing acetylcholinesterase (AChE), choline oxidase (CHO), and horseradish peroxidase (HRP) in MPNFs (AChE/CHO/HRP@MPNFs) for organophosphorus pesticides (OPs) colorimetric biosensing detection. The assay could specifically detect parathion-methyl within 13 min with a wider linear range (0.1-1000.0 nm) and a lower limit of detection (LOD) (0.032 nm). The remarkable stability of the immobilized enzymes is also achieved under harsh environments, room temperature storage, and recycling. Furthermore, a portable and cost-effective biosensing platform is developed by integrating AChE/CHO/HRP@MPNFs with a smartphone-assisted paper device for the on-site detection of OPs. Overall, the high catalytic activity retention and the enhanced detection performance demonstrate that MPNF is a robust carrier in enzyme immobilization and holds great promise in biosensing and other field applications.

Sugeno fuzzy inference system modeling and DFT calculations for the treatment of pesticide-laden water by newly developed arginine functionalized magnetic Mn-based metal organic framework

The uncontrolled utilization of pesticides poses a significant risk to the environment and human health, making its management essential. In this regard, a new arginine functionalized magnetic Mn-based metal-organic framework (Arg@m-Mn-MOF) was fabricated and assessed for the removal of cypermethrin (CYP) and chlorpyrifos (CHL) from aqueous system. The Arg@m-Mn-MOF was characterized by scanning electron microscopy, energy dispersive X-ray, Fourier transform infrared spectroscopy, X-ray diffraction, and Brunauer-Emmett-Teller analysis. Various parameters were optimized in a series of batch experiments and the following conditions were found optimal: pH: 4 and 5, contact time: 20 min, adsorbent dosage: 0.6 and 0.8 g L-1 with initial concentration: 10 mg L-1 and temperature: 298 K for CYP and CHL, respectively. The composite attained a maximum removal capacity of 44.84 and 71.42 mg g-1 for CYP and CHL, respectively. The elucidated data was strongly fitted to the pseudo-second-order model of kinetics (R2 > 0.98) and Langmuir isotherm (R2 > 0.98). Based upon 350 experimental datasets obtained from batch studies and interpolated data, the adsorption capacity of the adsorbent was elucidated with R2 > 0.97 (CHL) and > 0.91 (CYP). The adsorption energy (- 11.67 kcal mol-1) calculated by Gaussian software suggests a good interaction between arginine and CHL through H-bonding. The present study's findings suggested the prepared Arg@m-Mn-MOF as a promising adsorbent for the efficient removal of pesticides from agriculture runoff.

Introducing Mn into ZIF-8 nanozyme for enhancing its catalytic activities and adding specific recognizer for detection of organophosphorus pesticides

In order to design and establish a highly efficient and selective nanozyme-based sensing platform for the UV-vis detection of organophosphorus pesticides (OPs), Mn was introduced into ZIF-8 nanozyme for enhancing its catalytic activities and adding specific recognizer. The Mn-doped ZIF-8 (Mn-ZIF-8) nanocomposites were synthesized with a very facile one-pot method by heating the mixture of ZnO, 2-methylimidazole (Hmin) and Mn(CH3COO)2·4H2O in a solvent-free system at 180 °C for 8 h. The Mn-ZIF-8 nanocomposite showed a higher peroxidase activity and an additional thiocholine (TCh)-degradable property compared to the pristine ZIF-8. OPs could inhibit acetylcholinesterase (AChE) to catalyze the hydrolysis of acetylthiocholine (ATCh) to produce TCh, thus blocking the degradation of Mn-ZIF-8 and protecting the catalysis of the oxidation of colorless 3,3',5,5'-tetramethylbenzydine (TMB) to blue oxidized TMB (ox-TMB). Accordingly, a detection method for OPs with high sensitivity and selectivity was designed and established on the basis of the Mn-ZIF-8 nanozyme with a linear range of 0.1-20 nM and a limit of detection (LOD) as low as 54 pM.

Rapid Trace Detection of Sulfite Residue in White Wine Using a Multichannel Colorimetric Nanozyme Sensor

As a commonly used food additive, sulfite (SO32-) is popular with food manufacturers due to the functions of bleaching, sterilizing, and oxidation resistance. However, excess sulfites can pose a threat to human health. Therefore, it is particularly important to achieve rapid and sensitive detection of SO32-. Herein, a colorimetric sensor was invented for visual, meticulous, and rapid detection of SO32- based on MIL-53(Fe/Mn). Bimetallic nanozyme MIL-53(Fe/Mn) was prepared by a one-pot hydrothermal reaction. The prepared MIL-53(Fe/Mn) can effectively catalyze the oxidation of colorless TMB to a blue oxidation product (oxTMB). The introduction of SO32- causes significant discoloration of the reaction system, gradually transitioning from a visible blue color to colorless. Hence, a sensitive colorimetric sensor for SO32- detection was developed based on the decolorization degree of the detection system. Further, the discoloration was ascribed to the inactivation of nanozyme and the strong reducing ability of SO32-. Under the optimal experimental conditions, there was a good linear relationship between the absorbance at 652 nm and SO32- concentration in the linear range of 0.5-6 μg mL-1 with a limit of detection (LOD) of 0.05 μg mL-1. The developed method was successfully applied to the detection of actual samples of white wine with good accuracy and recovery. Compared to traditional methods, this colorimetric sensor produces similar detection results but significantly reduces the detection time. Compared to traditional methods, this colorimetric sensor can not only reduce the detection costs effectively but also help the food industry maintain quality standards. Strong anti-interference capability, simple operation, and low detection limits ensure the excellent performance of the colorimetric sensor in detecting SO32- in white wine. The combination of a smartphone and a colorimetric analysis application has also greatly facilitated the semi-quantitative, visual on-site detection of SO32-, which has opened up an application prospect of an MIL-53(Fe/Mn)-based detection platform. Our work has indicated a new direction for the detection of SO32- and provided important assurance for food safety.

Diazo-reaction based dual-mode colorimetric-electrochemical sensing of nitrite in pickled food

Development of an effective and convenient sensor for sensitive detection of nitrites is of great concern since excessive amounts of nitrites can be harmful to both human health and the environment. In this work, Cu-MOF modified exfoliated graphite paper (EGP) was employed as a signal reporter to enable the visual and electrochemical dual-mode sensing of nitrites. Cu-MOFs were in situ synthesized on EGP, which exhibited an excellent oxidase enzyme-like activity to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into its oxidation product (oxTMB). The multi-layer structure and the superior electrical conductivity of EGP not only facilitated the loading of the Cu-MOF nanozyme for colorimetric sensing but also enabled its use as an underlying backbone to support electroanalysis. Based on the recognition of nitrite through a highly specific diazo reaction between nitrite and oxTMB, the addition of nitrite caused the colorimetric sensing solution to change color from blue to green, which allowed for the colorimetric sensing of nitrite with a limit of detection (LOD) of 8.5 × 10-6 mol L-1. Meanwhile, the Cu-MOF/EGP electrochemical platform was employed for ratiometric detection of nitrite based on the electrochemical oxidation of nitrite and TMB. Compared with the colorimetric mode, the electrochemical mode possessed higher sensitivity with a LOD of 5.4 × 10-7 mol L-1, indicating the high sensitivity and accuracy of the proposed dual-mode sensing strategy. Furthermore, the determination of nitrite in different pickled food samples is demonstrated.

Ce/Zr-MOF with Dual Cycle Synergistic Catalysis Pathway Enabling Enhanced Peroxidase-like Performance for Wearable Hydrogel Patch Visualization Sensing Platform

Engineering the activity of enzyme-like catalysts should be a top priority to make them superior substitutes for natural enzymes. Herein, a Ce/Zr bimetal-organic framework (Ce/Zr-MOF) was designed and synthesized by a one-pot hydrothermal method, which has enhanced performance in mimicking peroxidase (POD) than its single-metal counterparts. To further comprehend the mechanism of activity enhancement, the role of the bimetallic synergistic catalysis process in H2O2 decomposition and reactive oxygen species formation was elucidated, and the possible dual cycle synergistic catalysis pathway of bimetallic catalysis is proposed for the first time. The enhanced POD-like activity mainly depends on the introduction of Ce, which improved the conductivity and electron-transfer capability of Ce/Zr-MOF and promoted the generation of •OH. Integrated with a hydrogel substrate, a wearable all-solid-state H2O2 sensor for early diagnosis of plant health was produced. The detection limit can be as low as 3.3 μM, which is lower than that of some instrument-based colorimetric methods and has great potential in the development of visualized sensing applications. The concept of dual cycle synergistic catalysis pathway we proposed not only deepens the comprehension regarding sensing and catalytic mechanisms but also provides novel perspectives into the design of enzyme-like catalysts for extensive usage.

Integrated Design of a Dual-Mode Colorimetric Sensor Driven by Enzyme-like Activity Regulation Strategy for Ultratrace and Portable Detection of Hg2

Colorimetric analysis for mercury detection has great application potential in the prevention of health damage caused by mercury in the environment. Sensitivity, selectivity, and portability are core competencies of sensors, and concentrating these properties in a single sensor for efficient mercury detection remains a great challenge. Herein, a hollow structure CuS@CuSe@PVP (CCP) was prepared in which the enzyme-like activities could be activated by Hg2+ due to the antagonism between Hg and Se, inspiring the establishment of a colorimetric method for Hg2+ detection. As for Hg2+ detection performance, the linear range (LR) and limit of detection (LOD) were 1-900 and 0.81 nM in the POD-like activity system, respectively. Also, 5-550 nM of LR and 2.34 nM of LOD were achieved in the OD-like activity system. Further, a smartphone-mediated portable RGB nanosensor was fabricated, with a LOD down to 6.65 nM in the POD-like system and 7.97 nM in the OD-like system. Moreover, the excellent self-calibration and satisfactory recovery of 94.77%-106.16% were shown in the application of real water samples analysis. This study represented advanced progress toward emerging applications of nanozymes with multiple enzyme-like activities in heavy metal detection and will accelerate the development of efficient and portable heavy metal sensors.

High Peroxidase-Mimicking Metal-Organic Frameworks Decorated with Platinum Nanozymes for the Colorimetric Detection of Acetylcholine Chloride and Organophosphorus Pesticides via Enzyme Cascade Reaction

The sensitive detection of acetylcholinesterase (AChE) and organophosphorus pesticides (OPs) is very important for the protection of human health. Herein, a hybrid material, Pt NPs/Fe-MOF, consisting of a metal-organic framework (MIL-88B-NH2, Fe-MOF) decorated with platinum nanoparticles (Pt NPs), was prepared first and exhibited remarkably improved and excellent peroxidase-mimicking activity compared to the Fe-MOF material resulting from the synergistic catalysis effect between Fe-MOF and Pt NPs, which can effectively catalyze 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to generate a blue product (oxidized TMB, oxTMB). Interestingly, in the presence of AChE and acetylcholinesterase, the peroxidase-mimicking activity from Pt NPs/Fe-MOF was inhibited obviously, and thus, a colorimetric sensing platform for AChE can be constructed; more importantly, after the addition of OPs, this nanozyme activity can be recovered, inducing the further successful construction of a sensitive colorimetric sensing platform for OPs. The related sensing mechanism and condition optimization were studied, and the as-prepared Pt NPs/Fe-MOF nanozyme-based colorimetric method for AChE and OP detection displayed superior analytical performances with wide linearities and low detection limits. Furthermore, the designed method offers satisfactory real application ability. We expect the as-proposed Pt NPs/Fe-MOF nanozyme-based colorimetric sensing platform for AChE and OPs via the enzyme cascade reaction to show great potential application.

Nanozyme Rich in Oxygen Vacancies Derived from Mn-Based Metal-Organic Gel for the Determination of Alkaline Phosphatase

Vacancy engineering as an effective strategy has been widely employed to regulate the enzyme-mimic activity of nanomaterials by adjusting the surface, electronic structure, and creating more active sites. Herein, we purposed a facile and simple method to acquire transition metal manganese oxide rich in oxygen vacancies (OVs-Mn2O3-400) by pyrolyzing the precursor of the Mn(II)-based metal-organic gel directly. The as-prepared OVs-Mn2O3-400 exhibited superior oxidase-like activity as oxygen vacancies participated in the generation of O2•-. Besides, steady state kinetic constant (Km) and catalytic kinetic constant (Ea) suggested that OVs-Mn2O3-400 had a stronger affinity toward 3,3',5,5'-tetramethylbenzidine and possessed prominent catalytic performance. By taking 2-phospho-l-ascorbic acid as the substrate, which can be converted into reducing substance ascorbic acid in the presence of alkaline phosphatase (ALP), OVs-Mn2O3-400 can be applied as an efficient nanozyme for ALP colorimetric analysis without the help of destructive H2O2. The colorimetric sensor established by OVs-Mn2O3-400 for ALP detection showed a good linearity from 0.1 to 12 U/L and a lower limit of detection of 0.054 U/L. Our work paves the way for designing enhanced enzyme-like activity nanozymes, which is of significance in biosensing.

Rational design of three-in-one Pt/MnO2/GO hybrid nanozyme with enhanced catalytic performance for colorimetric detection of ascorbic acid and cysteine

In this work, a novel three-in-one Pt/MnO₂/GO hybrid nanozyme with wide pH and temperature working range was rational prepared using simple hydrothermal and reduction strategy. The prepared Pt/MnO₂/GO displayed enhanced catalytic activity than single active component due to the excellent conductivity of GO, the increased active sites, the increased electron transfer capacity, the synergistic effect between each component and the decreased binding energy for adsorbed intermediates. Combing chemical characterization and theoretical simulation calculations, the O₂ reduction process on the Pt/MnO₂/GO nanozymes and the generated reactive oxygen species in the nanozyme-TMB system were thoroughly illustrated. Based on the excellent catalytic activity of Pt/MnO₂/GO nanozymes, a colorimetric strategy was proposed to detect the ascorbic acid (AA) and cysteine (Cys), and the experimental results indicated that detection range of AA was 0.35-56 μM with a LOD of 0.075 μM and detection range of Cys was 0.5-32 μM with a LOD of 0.12 μM. Good recoveries were achieved in the human serum and fresh fruit juice detection procedures, demonstrating the potential applications of Pt/MnO₂/GO-based colorimetric strategy in complex biological and food samples.

Sn/MoC@NC hollow nanospheres as Schottky catalyst for highly sensitive electrochemical detection of methyl parathion

Developing electrode materials with excellent electrocatalytic properties for detecting pesticide residues plays a vital role in the safety of agricultural products and environmental applications. Herein, we designed a new electrochemical sensor on the basis of N-doped carbon hollow nanospheres modified with Sn/MoC Schottky junction (Sn/MoC@NC) for methyl parathion (MP) detection. The Sn/MoC@NC was prepared by self-assembled polymerization-anchoring strategy and high-temperature carbonization design. Sn/MoC Schottky junction and hollow nanosphere structure endow Sn/MoC@NC with a larger surface area, more active sites, and faster electron transfer, which is beneficial to enhancing its electrocatalytic performance. The structural characterizations and physicochemical properties of Sn/MoC@NC were explored through various microscopy, spectroscopic and electrochemical techniques. The experimental results confirmed that the calibration curve for current and MP concentration (0.05-10 μg/mL) was available under optimized conditions, and the sensitivity and detection limit were respectively determined to be 9.02 μA μM¹ cm² and 8.9 ng/mL. Furthermore, the constructed sensor displayed excellent selectivity, repeatability, and stability, which qualified it for use in detecting MP in grapes and tap water with satisfactory recovery. This work may provide some interesting prospects for constructing high-performance electrocatalysts for MP detection.

A Co-based MOF as nanozyme with enhanced oxidase-like activity for highly sensitive and selective colorimetric differentiation of aminophenol isomers

Nanozyme with the merit of excellent and adjustable catalytic activity, outstanding stability and low cost is a promising alternative for natural enzymes widely applied in a variety of fields. In the present study, a new two-dimensional cobalt-based MOF nanocomposite designated as MVCM@β-CD was synthesized. Combined with the strategies of increasing the ratio of Co(Ⅲ)/Co(Ⅱ) and modifying with small molecule β-cyclodextrin (β-CD), MVCM@β-CD displayed remarkably enhanced oxidase-mimicking activity, which was attributed to synergistic effect from large surface area of two dimensional Co-MOF nanosheet, numerous exposed active sites, high-proportioned trivalence of cobalt and regulating action of β-cyclodextrin. The addition of aminophenol isomers inhibited the catalytic oxidation process, resulting in different color change of the solution and UV-Vis absorption behaviors, based on which a sensitive ratiometric colorimetry for m-aminophenol (m-Ap) and a simple colorimetric p-aminophenol (p-Ap) detection method were developed with the detection limit of 0.16 μM and 1.01 μM, respectively. This method realized the colorimetric differentiation of aminophenol isomers, which provided a simple, accurate and low-cost approach for visual discrimination without complicated instrument and procedure, especially appropriate for on-site detection.

Rational design of a novel MOF-based ternary nanocomposite for effectively monitoring harmful organophosphates in foods and the environment

Methyl parathion (MP) is a widely used organophosphate insecticide that is extremely toxic due to its ability to irreversibly inhibit acetylcholinesterase in the body and persistently accumulate in the environment. Timely detection of MP can prevent harmful residue exposure to humans. Therefore, the development of fast, efficient electrochemical methods to detect trace MP has been highly beneficial for monitoring harmful residues in foods and environment to ensure food safety and ecological conservation. Herein, a novel hybrid metal-organic framework (MOF) nanocomposite composed of Pt nanoparticles (PtNPs), multi-walled carbon nanotubes (MWCNTs), and UiO-66-NH₂ (PtNPs/UiO-66-NH₂/MWCNTs) was rationally designed and prepared by a facile two-step strategy for the sensitive determination of MP. The synergistic effects are illustrated in detail using XRD, XPS, FTIR, TEM, and SEM studies as well as electrochemical technologies such as CV, EIS, and DPV. In addition, the performance of the ternary nanocomposite for detecting MP was investigated by comparing it with the binary-component one. The results showed that the PtNPs/UiO-66-NH₂/MWCNT-based electrochemical sensor exhibited outstanding sensitivity of 21.9 μA μM^-1 cm^-2, satisfactory low detection limit of 0.026 μM and wide linear range of 0.11-227.95 μM for MP analysis. Furthermore, the fabricated sensor delivered distinguished freedom from interferences, outstanding regeneration ability, and adequate recoveries for fresh foods and river water samples. In conclusion, the proposed PtNPs/UiO-66-NH₂/MWCNT-based sensor provides a potentially useful analytical tool for determining hazardous residues of OPs in foods and the environment.

Advanced visual sensing techniques for on-site detection of pesticide residue in water environments

Pesticide usage has increased to fulfil agricultural demand. Pesticides such as organophosphorus pesticides (OPPs) are ubiquitous in world food production. Their widespread usage has unavoidable detrimental consequences for humans, wildlife, water, and soil environments. Hence, the development of more convenient and efficient pesticide residue (PR) detection methods is of paramount importance. Visual detecting approaches have acquired a lot of interest among different sensing systems due to inherent advantages in terms of simplicity, speed, sensitivity, and eco-friendliness. Furthermore, various detections have been proven to enable real-life PR surveillance in environment water. Fluorometric (FL), colourimetric (CL), and enzyme-inhibition (EI) techniques have emerged as viable options. These sensing technologies do not need complex operating processes or specialist equipment, and the simple colour change allows for visual monitoring of the sensing result. Visual sensing techniques for on-site detection of PR in water environments are discussed in this paper. This paper further reviews prior research on the integration of CL, FL, and EI-based techniques with nanoparticles (NPs), quantum dots (QDs), and metal-organic frameworks (MOFs). Smartphone detection technologies for PRs are also reviewed. Finally, conventional methods and nanoparticle (NPs) based strategies for the detection of PRs are compared.

A versatile platform for colorimetric, fluorescence and photothermal multi-mode glyphosate sensing by carbon dots anchoring ferrocene metal-organic framework nanosheet

Concerns regarding pesticide residues have driven attempts to exploit accurate, prompt and straightforward approaches for food safety pre-warning. Herein, a nanozyme-mediated versatile platform with multiplex signal response (colorimetric, fluorescence and temperature) was proposed for visual, sensitive and portable detection of glyphosate (GLP). The platform was constructed based on a N-CDs/FMOF-Zr nanosensor that prepared by in situ anchoring nitrogen-doped carbon dots onto zirconium-based ferrocene metal-organic framework nanosheets. The N-CDs/FMOF-Zr possessed excellent peroxidase (POD)-like activity and thus could oxide colorless 3, 3', 5, 5'-tetramethylbenzidine (TMB) into a blue oxidized TMB (oxTMB) in presence of H₂O₂. Intriguingly, owing to the blocking effect triggered by multiple interaction between GLP and N-CDs/FMOF-Zr, its POD-like activity of the latter was remarkably suppressed, which can modulate the transformation of TMB into oxTMB, generating tri-signal responses of fluorescence enhancement, absorbance and temperature decrease. More significantly, the temperature mode can be facilely realized by a portable home-made mini-photothermal device and handheld thermometers. The proposed multimodal sensing was capable of providing sensitive results by fluorescence mode and simultaneously realized visual/portable testing by colorimetric and photothermal channels. Consequently, it exhibited more adaptability for practical applications, which can satisfy different testing requirements according to sensitivity and available instruments/meters, presenting a new horizon for exploiting multifunctional sensors.

Colorimetric aptasensor for fumonisin B1 detection based on the DNA tetrahedra-functionalized magnetic beads and DNA hydrogel-coated bimetallic MOFzyme

The toxicity and incidence of fumonisin B₁ (FB₁) pose a major challenge to public health and the environment, prompting the development of alternative quantitative strategies for FB₁. Herein, a colorimetric aptasensor was constructed based on DNA tetrahedra-functionalized magnetic beads (MBs) and DNA hydrogel-coated Mn-Zr bimetallic metal-organic frameworks-based nanozyme (MOFzyme). Initially, MBs functionalized by DNA tetrahedra demonstrated excellent capturing capability for FB₁. Along with the capture of FB₁, catalyst DNA (C) was released into the supernatant. Aided by fuel DNA (F), C can trigger continuous cleavage of the main chains and cross-linking points of the DNA hydrogel through an entropy-driven DNA circuit integrated into the hydrogel coating. Subsequently, the bimetallic MOFzyme encapsulated inside the DNA hydrogel was exposed and exerted its superb peroxidase-like activity, producing a colorimetric signal whose intensity was positively dependent on the amount of FB₁. The developed aptasensor exhibited good linearity in the range of 5 × 10^-4 to 50 ng mL^-1 with a limit of detection (LOD) of 0.38 pg mL^-1, and reasonable specificity in different matrices. Furthermore, the aptasensor was successfully applied to quantify FB₁ in actual samples with recoveries fell within 92.25 %- 108.00 %, showing its great potential in environmental monitoring and food safety.

3D hierarchical LDHs-based Janus micro-actuator for detection and degradation of catechol

Micro/nanomotors that combine the miniaturization and autonomous motion have attracted much research interest for environmental monitoring and water remediation. However, it is still challenging to develop a facile route to produce bifunctional micromotors that can simultaneously detect and remove organic pollutants from water. Herein, we developed a novel Janus micromotor with robust peroxide-like activity for simultaneously colorimetric detection and removal of catechol from water. Such laccase (Lac) functionalized Janus micromotor consisted of calcined MgAl-layered double hydroxides (MgAl-CLDHs) nanosheets and Co₃O₄-C nanoparticles (Lac-MgAl-CLDHs/Co₃O₄-C), revealing unique 3D hierarchical microstructure with highly exposed active sites. The obtained Janus micromotors exhibited autonomous motion with a maximum velocity of 171.83 ± 4.07 µm/s in the presence of 7 wt% H₂O₂ via a chemical propulsion mechanism based on the decomposition of H₂O₂ by Co₃O₄-C layer on the hemisphere surface of Janus micromotors. Owing to the combination of autonomous motion and high peroxide-like activity, Lac-MgAl-CLDHs/Co₃O₄-C Janus micromotors could sensitively detect catechol with the limit of detection of 0.24 μM. In addition, such Janus micromotors also could quickly degrade catechol by •OH generated from a Fenton-like reaction. It is a first step towards using autonomous micromotors for highly selective, sensitive, and facile detection and quick removal of catechol from water.

Novel Ce-based coordination polymer nanoparticles with excellent oxidase mimic activity applied for colorimetric assay to organophosphorus pesticides

Cerium, as a lanthanide, has attracted considerable interest because of its excellent catalytic activity. Here, we propose a novel cerium-based coordination polymer nanoparticles named DPA-Ce-GMP, which have excellent oxidase-mimicking properties. Furthermore, a colorimetric probe that can act as an inhibitor to suppress the activity of acetylcholinesterase (AChE) was developed for detecting organophosphorus pesticides (OPs). DPA-Ce-GMP catalyzes colorless 3,3',5,5'-tetramethylbenzidine (TMB) to produce a blue color, and AChE catalyzes acetylthiocholine to produce thiocholine (TCh), which can weaken DPA-Ce-GMP-catalyzed TMB. After the addition of OPs, the enzymatic activity of AChE was inhibited to produce less amount of TCh, resulting in more DPA-Ce-GMP-catalyst oxidized TMB to show an increasing blue color. Dichlorvos, as the samples, with the limit of 0.024 μg/L. Overall, we believe that the colorimetric probe can be used for the rapid, low-cost, and large-scale field detection of OPs in food samples.

Hollow-Channel Paper Analytical Devices Supported Biofuel Cell-Based Self-Powered Molecularly Imprinted Polymer Sensor for Pesticide Detection

Herein, a paper-based glucose/air biofuel cell (BFC) was constructed and implemented for self-powered pesticide detection. Our developed paper-based chip relies on a hollow-channel to transport fluids rather than capillarity, which reduces analysis times as well as physical absorption. The gold nanoparticles (Au NPs) and carbon nanotubes (CNTs) were adapted to modify the paper fibers to fabricate the flexible conductive paper anode/cathode electrode (Au-PAE/CNT-PCE). Molecularly imprinted polymers (MIPs) using 2,4-dichlorophenoxyacetic acid (2,4-D) as a template were synthesized on Au-PAE for signal control. In the cathode, bilirubin oxidase (BOD) was used for the oxygen reduction reaction. Based on a competitive reaction between 2,4-D and glucose-oxidase-labeled 2,4-D (GOx-2,4-D), the amount of GOx immobilized on the bioanode can be simply tailored, thus a signal-off self-powered sensing platform was achieved for 2,4-D determination. Meanwhile, the coupling of the paper supercapacitor (PS) with the paper-based chip provides a simple route for signal amplification. Combined with a portable digital multi-meter detector, the amplified signal can be sensitively readout. Through rational design of the paper analytical device, the combination of BFC and PS provides a new prototype for constructing a low-cost, simple, portable, and sensitive self-powered biosensor lab-on-paper, which could be easily expanded in the field of clinical analysis and drug delivery.

Polydopamine-based nanozyme with dual-recognition strategy-driven fluorescence-colorimetric dual-mode platform for Listeria monocytogenes detection

Development of a simple and efficient dual-mode analytical technique with the built-in cross reference correction feature is benefit to achieve the highly accurate detection of the target pollutants and avoid the false-positive outputs in environmental media. Here, we synthesized a Fe-doped polydopamine (Fe@PDA)-based nanozyme with prominent peroxide-mimetic enzyme activity and high fluorescence emission ability. On this basis, we designed a dual-recognition strategy-driven fluorescence-colorimetric dual-mode detection platform, consisting of Listeria monocytogenes (L. monocytogenes) recognition aptamer-modified Fe@PDA (apt/Fe@PDA) and vancomycin-functionalized Fe₃O₄ (van/Fe₃O₄), for L. monocytogenes. Owing to van/Fe₃O₄-powered magnetic separation, there was a L. monocytogenes concentration-dependent fluorescence enhancement of apt/Fe@PDA for performing fluorescence assay in the precipitate. In this case, the prominent peroxide-mimetic enzyme activity of the residual apt/Fe@PDA in the precipitation could catalyze H₂O₂ to further oxidate colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue oxTMB, which displayed a L. monocytogenes concentration-dependent absorbance enhancement for carrying out colorimetric assay as well. As a result, a fluorescence-colorimetric dual-mode analytical platform was proposed to successfully detect the residual L. monocytogenes in real environmental media with acceptable results. This work showed the great prospects by integrating dual-recognition strategy into fluorescence nanozyme to develop efficient and reliable dual-mode analytical platforms for safeguarding environmental health.

Optical sensing techniques for rapid detection of agrochemicals: Strategies, challenges, and perspectives

In recent years, the irrational use of agrochemicals has caused great harm to the environment and public health. Along with the rapid development of optical technology and nanotechnology, the research of optical sensing methods in agrochemical detection has been developed rapidly owing to its advantages of simplicity, fast response, and cost-effectiveness. In this review, the strategies of employing optical systems based on colorimetric sensor, fluorescence, chemiluminescence, terahertz spectroscopy, surface plasmon resonance, and surface-enhanced Raman spectroscopy for sensing agrochemicals were summarized. In addition, the challenges in the practical application of optical sensing technologies for agrochemical detection were discussed in-depth, and potential future trends and prospects of these techniques were addressed. A variety of nanomaterials have been developed for enhancing the sensitivity of optical sensing systems. The optical properties of nanomaterials are governed by their size, shape, and chemical structure. Although each optical sensing system holds its advantages, there are still many challenges that need to be overcome in practical applications. With the continuous developments in novel functional nanomaterials, sample preparation methods, and spectral processing algorithms, optical sensors are expected to have powerful potential for rapid testing of agrochemicals in the environment and foods.

Potentially tunable ratiometric electrochemiluminescence sensing based on conjugated polymer nanoparticle for organophosphorus pesticides detection

In general, suitable double luminophores and their coreactants are necessary for constructing electrochemiluminescence (ECL) ratio strategy. However, the complexity of matching double luminophores and the stability and repeatability problem suffered by introducing exogenous coreactant would greatly limit the application of ratio detection. An original single-luminophore-based ECL ratio sensing was developed excluding any exogenous coreactants in this work. The poly [9,9-bis(3'-(N,N-dimethylamino)propyl)- 2,7-fluorene]-alt-2,7-(9,9-dioctylfluorene)] nanoparticles (PFN NPs) were explored to emit two anodic ECL signals. One centered at + 1.25 V (ECL-1) with the scanning potential of 0 ~ + 1.25 V and the other at + 1.95 V (ECL-2) with the scanning potential of 0 ~ + 1.95 V. ECL-1 showed a very strong emission without any exogenous coreactant. Importantly, hydrogen peroxide (H₂O₂) was able to efficiently weaken ECL-1 but strengthen ECL-2. When organophosphorus pesticides (OPs) were absent, the immobilized acetylcholinesterase-choline oxidase (AChE-ChOx) would catalyze the substrate acetylthiocholine chloride (ATCl) to produce H₂O₂, resulting in a quenched ECL-1 and an enhanced ECL-2. With the introduction of OPs, ECL-1 increased while ECL-2 accordingly decreased as OPs prohibited production of H₂O₂ by inhibiting activity of AChE. Highly sensitive ECL ratio detection for OPs was realized based on the change of the ratio of two signals. The dual anode emission properties of PFN NPs coupled with the opposite regulation of H₂O₂ on the two signals paved a new avenue for potentially tunable ECL ratio sensing strategy, and showed enormous potential applications for OPs analysis.