A peroxidase-mimicking Zr-based MOF colorimetric sensing array to quantify and discriminate phosphorylated proteins

Dual-mode optical biocompatible-sensor enabled by enzyme-like activity of UiO-66 (Zr) for ultra-sensitive ROS detection in vitro

The simple, effective and highly sensitive detection of hydrogen peroxide (H2O2), which belongs to the reactive oxygen species (ROS), at low concentrations plays an indispensable role in the field of environmental protection, biological research and safety. In this study, a dual-mode optical biosensor, UiO-66@OPD, was developed based on the inherent peroxidase mimicking activity of UiO-66 (Zr) and the optical reaction of ortho-phenylenediamine (OPD) by extending the π-system through oxidative coupling, prototropism and elimination to form OPDox, thereby exhibiting strong orangish absorbance and greenish fluorescence. The catalase-mimicking activity of UiO-66 (Zr) was demonstrated by the catalytic oxidation of methylene blue in the presence of H2O2. Moreover, the Michaelis-Menten kinetic model confirmed the intrinsic peroxidase-like activity of UiO-66@OPD as a modified MOFzyme. The synthesized UiO-66 (Zr) facilitated the oxidation of OPD to OPDox by degrading H2O2 to the hydroxyl radicals. During the oxidation process, the absorption peak at 415 nm and the fluorescence peak at 565 nm of the synthesized probe were significantly enhanced by increasing the H2O2 concentration. Moreover, a colorimetric and fluorometric ultrasensitive sensor shows a good linear relationship between the intensity enhancement and H2O2 concentration in the range of 0-600 nM for absorption and fluorescence spectra with R2 = 0.9772, and R2 = 0.9948, respectively. To demonstrate the biological performance and biocompatibility of UiO-66@OPD as a biosensor, MTT evaluation was performed for the three cell lines MCF-10 A, HEK293 and A549, indicating high biocompatibility and good cell viability for biological applications. Ultimately, this convenient, environmentally friendly, biocompatible and cost-effective catalase-mimicking-based sensor system will open a new perspective for the development of portable kite-based biosensors In vitro.

A ratiometric fluorescence sensor for detection of organophosphorus pesticides based on enzyme-regulated multifunctional Fe-based metal-organic framework

The residues of organophosphorus pesticides (OPs) are increasing environmental pollution and public health concerns. Thus, the development of simple, convenient and sensitive method for detection of OPs is crucial. Herein, a multifunctional Fe-based MOF with fluorescence, catalytic and adsorption, is synthesized by a simple one-pot hydrothermal method. The ratiometric fluorescence sensor for detection of OPs is constructed by using only one multifunctional sensing material. The NH2-MIL-101(Fe) is able catalyze the o-phenylenediamine (OPD) into 2,3-diaminophenazine (DAP) in the presence of H2O2. The generated DAP can significantly quench the intrinsic fluorescence of NH2-MIL-101(Fe) by the fluorescence resonance energy transfer (FRET) and internal filtration effect (IFE), while producing a new measurable fluorescence. Without immobilization or molecular imprinting, pyrophosphate ion (PPi) can inhibit the peroxidase-like activity of the NH2-MIL-101(Fe) by chelating with Fe3+/Fe2+ redox couple. Moreover, PPi can also be hydrolyzed by alkaline phosphatase (ALP), the presence of OPs inhibits the activity of ALP, resulting in the increase of extra PPi preservation and signal changes of ratiometric fluorescence, the interactions of ALP with different OPs are explored by molecular docking, the OPs (e.g., glyphosate) interact with crucial amino acid residues (Asp, Ser, Ala, Lys and Arg) are indicated. The proposed sensor exhibits excellent detection performance for OPs with the detection limit of 18.7 nM, which provides a promising strategy for detection of OPs.

A bimetallic peroxidase-mimicking nanozyme with antifouling property for construction of sensor array to identify phosphoproteins and diagnose cancers

Protein phosphorylation is a significant post-translational modification that plays a decisive role in the occurrence and development of diseases. However, the rapid and accurate identification of phosphoproteins remains challenging. Herein, a high-throughput sensor array has been constructed based on a magnetic bimetallic nanozyme (Fe3O4@ZNP@UiO-66) for the identification and discrimination of phosphoproteins. Attributing to the formation of Fe-Zr bimetallic dual active centers, the as-prepared Fe3O4@ZNP@UiO-66 exhibits enhanced peroxidase-mimicking catalytic activity, which promotes the electron transfer from Zr center to Fe(II)/Fe(III). The catalytic activity of Fe3O4@ZNP@UiO-66 can be selectively inhibited by phosphoproteins due to the strong interaction between phosphate groups and Zr centers, as well as the ultra-robust antifouling capability of zwitterionic dopamine nanoparticle (ZNP). Considering the diverse binding affinities between various proteins with the nanozyme, the catalytic activity of Fe3O4@ZNP@UiO-66 can be changed to various degree, leading to the different absorption responses at 420 nm in the hydrogen peroxide (H2O2) - 2, 2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) system. By simply extracting different absorbance intensities at various time points, a sensor array based on reaction kinetics for the discrimination of phosphoproteins from other proteins is constructed through linear discriminant analysis (LDA). Besides, the quantitative determination of phosphoproteins and identification of protein mixtures have been realized. Further, based on the differential level of phosphoproteins in cells, the differentiation of cancer cells from normal cells can also be implemented by utilizing the proposed sensor array, showing great potential in disease diagnosis.

Advancing stroke therapy: the potential of MOF-based nanozymes in biomedical applications

In this study, we explored the growing use of metal-organic framework (MOF)-based Nanozymes in biomedical research, with a specific emphasis on their applications in stroke therapy. We have discussed the complex nature of stroke pathophysiology, highlighting the crucial role of reactive oxygen species (ROS), and acknowledging the limitations of natural enzymes in addressing these challenges. We have also discussed the role of nanozymes, particularly those based on MOFs, their structural similarities to natural enzymes, and their potential to improve reactivity in various biomedical applications. The categorization of MOF nanozymes based on enzyme-mimicking activities is discussed, and their applications in stroke therapy are explored. We have reported the potential of MOF in treating stroke by regulating ROS levels, alleviation inflammation, and reducing neuron apoptosis. Additionally, we have addressed the challenges in developing efficient antioxidant nanozyme systems for stroke treatment. The review concludes with the promise of addressing these challenges and highlights the promising future of MOF nanozymes in diverse medical applications, particularly in the field of stroke treatment.

Colorimetric and fluorescent dual-biosensor based on zirconium and preasodium metal-organic framework (zr/pr MOF) for miRNA-191 detection

MicroRNAs (miRNAs) are associated with certain types of cancer, tumor stages, and responses to treatment, thus efficient methods are required to identify them quickly and accurately. Abnormal expression of microRNA-191 (miR-191) has been linked to particular cancers and several other health conditions, such as diabetes and Alzheimer's disease. In this study, a new dual-biosensor based on the zirconium and preasodium-based metal-organic framework (Zr/Pr MOF) was developed for the rapid, ultrasensitive, and selective detection of miRNA-191. The synthesized Zr/Pr MOF exhibited peroxidase-like activity and fluorescence properties. Our dual method involves monitoring the fluorescence and peroxidase activity of metal-organic frameworks (MOFs) in the presence of miRNAs. The Zr/Pr MOF can catalyze hydrogen peroxide (H2O2) to oxidize the chromogenic substrate 3, 3', 5, 5'-tetramethylbenzidine (TMB) to produce blue oxidized TMB (oxTMB), which exhibits ultraviolet absorption at 660 nm. However, the addition of a label-free miRNA-191 probe caused a significant change in fluorescence intensity and absorbance, indicating the binding of single-stranded miRNAs to the MOF through van der Waals interactions and π-π stacking. The presence of the target miRNA-191 caused the probe to be released from the surface of the MOF owing to hybridization, which increased the peroxidase-like activity of Zr/Pr-MOF. Both response signals showed acceptable linear relationship and low detection limits. Fluorescence and colorimetry have an LOD of 0.69 and 8.62 pM, respectively. This study demonstrates the reliability and sensitivity of miRNA identification in human serum samples.

Recent advances in metal-organic frameworks for stimuli-responsive drug delivery

After entering the human body, drugs for treating diseases, which are prone to delivery and release in an uncontrolled manner, are affected by various factors. Based on this, many researchers utilize various microenvironmental changes encountered during drug delivery to trigger drug release and have proposed stimuli-responsive drug delivery systems. In recent years, metal-organic frameworks (MOFs) have become promising stimuli-responsive agents to release the loaded therapeutic agents at the target site to achieve more precise drug delivery due to their high drug loading, excellent biocompatibility, and high stimuli-responsiveness. The MOF-based stimuli-responsive systems can respond to various stimuli under pathological conditions at the site of the lesion, releasing the loaded therapeutic agent in a controlled manner, and improving the accuracy and safety of drug delivery. Due to the changes in different physical and chemical factors in the pathological process of diseases, the construction of stimuli-responsive systems based on MOFs has become a new direction in drug delivery and controlled release. Based on the background of the rapidly increasing attention to MOFs applied in drug delivery, we aim to review various MOF-based stimuli-responsive drug delivery systems and their response mechanisms to various stimuli. In addition, the current challenges and future perspectives of MOF-based stimuli-responsive drug delivery systems are also discussed in this review.

Smartphone-assisted nanozyme sensor array constructed based on reaction kinetics for the discrimination and identification of phenolic compounds

Although the research on nanozymes has attracted widespread attention in recent years, the development of highly active and multifunctional nanozymes remains a challenge. Here, a bifunctional AMP-Cu nanozyme with laccase- and catecholase-like activities was successfully prepared at room temperature with Cu2+ as the metal ion and adenosine-5'-monophosphate (AMP) as the ligand molecule. Based on the excellent catalytic performance of AMP-Cu, a three-channel colorimetric sensor array was constructed using reaction kinetics as the sensing unit to achieve high-throughput detection and identification of six common phenolic compounds at low concentrations. This strategy simplifies the construction of sensor array and demonstrates the capacity to obtain multidimensional data from a single material. Finally, with the assistance of smartphones and homemade dark boxes, a portable on-site detection method for phenolic compounds was developed. This work would contribute to the development of portable sensors and the highly efficient identification of phenolic compounds in complex samples.

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.

Critical importance of RGB color space specificity for colorimetric bio/chemical sensing: A comprehensive study

The use of the RGB color model in colorimetric chemical sensing via imaging techniques is widely prevalent in the literature. However, the lack of specificity in the selection of RGB color space during capture and analysis presents a significant challenge in creating standardised methods for this field and possible discrepancies. In this study, we conducted a comprehensive comparison and contrast of a total of 68 RGB color spaces to evaluate their respective impacts on colorimetric bio/chemical sensing. We explore the impact of dynamic range, sensitivity, and limit of detection, and show that the lack of specificity in RGB color space selection can significantly impact colorimetric chemical sensing by 42-77%. We also explore the impact of underlying RGB comparisons and demonstrate a further 18.3% discrepancy between RGB color spaces. By emphasising the importance of proper RGB color space selection and handling, our findings contribute to a better understanding of this critical area and present valuable opportunities for future research. We further provide valuable insights for creating standardised methods in this field, which can be utilised to avoid discrepancies and ensure accurate and reliable analysis in colorimetric bio/chemical sensing.

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.

Synergistically Enhanced Oxidase-like Property of Core-Shell MOF Nanozymes by Decorating Au and Ag/AgCl Nanoparticles for l-Cysteine Colorimetric Sensing

Monitoring l-cysteine (l-Cys) is of importance for human health and food safety. Herein, we designed a novel strategy for bimetallic Au and Ag/AgCl anchoring on Ni-doped ZIF-67 to form core-shell nanocubes (Ni-ZIF-67/AuAg/AgCl) using the galvanic replacement processes. The unique properties of ZIF-67 nanocubes were conducive to generating strong synergistic catalytic effects with Au and Ag/AgCl, particularly when Ni-ZIF-67/AuAg/AgCl composites were employed as oxidase mimics for catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). The Ni-ZIF-67/AuAg/AgCl composites displayed strong affinity toward TMB, displaying a lower Michaelis constant Km value of 0.25 mM and a higher maximum initial rate Vmax of 9 × 10-8 M s-1. By virtue of the nanozyme, the colorimetric sensor was constructed for l-Cys detection with a relatively low detection limit of 0.051 μM. The superior catalytic performance of the as-prepared Ni-ZIF-67/AuAg/AgCl composites can be ascribed to the core-shell structure, large specific surface area, and strong synergistic catalytic effects, which are beneficial for exposing more active sites and enhancing the conductivity to further boost their catalytic activity.

Preparation of Co dual atomic site catalysts loaded on defect-engineered MOFs material with superb chemiluminescent enhancement effect for sensitive detection of bacteria

BACKGROUND

Dual atomic site catalysts (DASCs) have aroused extensive interest in analytical chemistry on account of the superb catalytic activity caused by the highly-exposed active centers and synergistic effect of adjacent active centers. The reported protocols for preparing DASCs usually involve harsh conditions such as acid/base etching and high-temperature calcination, leading to unfavorable water dispersity and restricted application. It is crucial to develop DASCs with satisfactory water dispersity, improved stability, and mild preparation procedures to facilitate their application as signal probes in analytical chemistry.

RESULTS

Formic acid was adopted as a modulator for preparing MOF-808 with abundant defective sites, which was used as the carrier for implanting Co atoms. Co DASCs with a special coordination structure of Co2-O10 and a high loading efficiency of 11.1 wt% were prepared with a mild solvothermal protocol. The resultant Co DASCs can significantly accelerate decay of H2O2 for forming numerous reactive oxygen radicals and boost chemiluminescent (CL) signal. Co DASCs at 1.0 μg mL-1 can enhance the CL signal of luminol-H2O2 system by about 5800 times. Thanks to their satisfactory water dispersity and excellent CL enhancement performance, they were used as ultra-sensitive CL signal probes for monitoring methicillin-resistant Staphylococcus aureus. The method shows a detection range of 102-107 CFU mL-1 and a detection limit of 47 CFU mL-1. Antibiotic susceptibility test was performed with the established CL method to prove its practicality.

SIGNIFICANCE

The water dispersible Co DASCs prepared with facile and mild solvothermal protocol exhibit prominent peroxidase-like activity and can promote the production of reactive oxygen radicals for boosting CL signal. Therefore, this study paves an avenue for implanting DASCs in defect-engineered carrier to prepare signal probes suitable for development of ultra-sensitive CL analytical methods.

Signal Amplification Strategy Design in Nanozyme-Based Biosensors for Highly Sensitive Detection of Trace Biomarkers

Nanozymes show great promise in enhancing disease biomarker sensing by leveraging their physicochemical properties and enzymatic activities. These qualities facilitate signal amplification and matrix effects reduction, thus boosting biomarker sensing performance. In this review, recent studies from the last five years, concentrating on disease biomarker detection improvement through nanozyme-based biosensing are examined. This enhancement primarily involves the modulations of the size, morphology, doping, modification, electromagnetic mechanisms, electron conduction efficiency, and surface plasmon resonance effects of nanozymes for increased sensitivity. In addition, a comprehensive description of the synthesis and tuning strategies employed for nanozymes has been provided. This includes a detailed elucidation of their catalytic mechanisms in alignment with the fundamental principles of enhanced sensing technology, accompanied by the presentation of quantitatively analyzed results. Moreover, the diverse applications of nanozymes in strip sensing, colorimetric sensing, electrochemical sensing, and surface-enhanced Raman scattering have been outlined. Additionally, the limitations, challenges, and corresponding recommendations concerning the application of nanozymes in biosensing have been summarized. Furthermore, insights have been offered into the future development and outlook of nanozymes for biosensing. This review aims to serve not only as a reference for enhancing the sensitivity of nanozyme-based biosensors but also as a catalyst for exploring nanozyme properties and their broader applications in biosensing.

Colorimetric sensor array for discriminating and determinating phenolic pollutants basing on different ratio of ligands in Cu/MOFs

The high toxicity and low biodegradability of the phenolic pollutants destroyed the balance of the environment and influenced human health seriously. Here, we developed a three-dimensional coloremetric sensor array for discriminating and determinating phenolic pollutants basing on the distinct Cu/nucleotides MOFs. Firstly, three laccase-mimic Cu/MOFs (Cu/AMP, Cu/CMP, and Cu/GMP) were obtained by regulating the molar ratio of Cu2+ and nucleotides. Then the Cu/MOFs as the recognition elements of the sensor array catalyzed the pollutants-4-AAP-H2O2 system, obtaining the colored benzoquinone products. Subsequently, the data array obtaining from the combined training matrix (3 Cu/MOFs × 6 pollutants × 5 replicates) was projected into a new dimensional space to obtain the 3D canonical scores, and classified into individual clusters by introducing LDA method. No overlap in their respective LDA plots for the six phenolic pollutants with different concentrations suggested the prominent discriminating performance of the sensor array. Furthermore, the sensor array exhibited high selectivity compared to the "lock-and-key" sensors even other active matrices coexisting in water samples. Importantly, the most influential discrimination factor was used to monitor the levels of the six targets, evidencing the potential application in assessing water pollution and maintaining human health.

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.

A Reaction-Based Optical Fingerprinting Strategy for the Recognition of Fat-Soluble Samples: Discrimination of Motor Oils

Optical "fingerprints" are widely used for chemometrics-assisted recognition of samples of different types. An emerging trend in this area is the transition from obtaining "static" spectral data to reactions analyzed over time. Indicator reactions are usually carried out in aqueous solutions; in this study, we developed reactions that proceed in an organic solvent, thereby making it possible to recognize fat-soluble samples. In this capacity, we used 5W40, 10W40, and 5W30 motor oils from four manufacturers, with six samples in total. The procedure involved mixing a dye, sample, and reagents (HNO3, HCl, or tert-butyl hydroperoxide) in an ethanolic solution in a 96-well plate and measuring absorbance or near-infrared fluorescence intensity every several minutes for 20-55 min. The obtained photographic images were processed by linear discriminant analysis (LDA) and the k-nearest neighbors algorithm (kNN). Discrimination accuracy was evaluated by a validation procedure. A reaction of oxidation of a dye by nitric acid allowed us to recognize all six samples with 100% accuracy for LDA. Merging of data from the four reactions that did not provide complete discrimination ensured an accuracy of 93% for kNN. The newly developed indicator systems have good prospects for the discrimination of other fat-soluble samples. Overall, the results confirm the viability of the kinetics-based discrimination strategy.

Carbocyanine-Based Optical Sensor Array for the Discrimination of Proteins and Rennet Samples Using Hypochlorite Oxidation

Optical sensor arrays are widely used in obtaining fingerprints of samples, allowing for solutions of recognition and identification problems. An approach to extending the functionality of the sensor arrays is using a kinetic factor by conducting indicator reactions that proceed at measurable rates. In this study, we propose a method for the discrimination of proteins based on their oxidation by sodium hypochlorite with the formation of the products, which, in turn, feature oxidation properties. As reducing agents to visualize these products, carbocyanine dyes IR-783 and Cy5.5-COOH are added to the reaction mixture at pH 5.3, and different spectral characteristics are registered every several minutes (absorbance in the visible region and fluorescence under excitation by UV (254 and 365 nm) and red light). The intensities of the photographic images of the 96-well plate are processed by principal component analysis (PCA) and linear discriminant analysis (LDA). Six model proteins (bovine and human serum albumins, γ-globulin, lysozyme, pepsin, and proteinase K) and 10 rennet samples (mixtures of chymosin and pepsin from different manufacturers) are recognized by the proposed method. The method is rapid and simple and uses only commercially available reagents.

Recent Progress in Sensor Arrays: From Construction Principles of Sensing Elements to Applications

The traditional sensors are designed based on the "lock-and-key" strategy with high selectivity and specificity for detecting specific analytes, which however are not suitable for detecting multiple analytes simultaneously. With the help of pattern recognition technologies, the sensor arrays excel in distinguishing subtle changes caused by multitarget analytes with similar structures in a complex system. To construct a sensor array, the multiple sensing elements are undoubtedly indispensable units that will selectively interact with targets to generate the unique "fingerprints" based on the distinct responses, enabling the identification among various analytes through pattern recognition methods. This comprehensive review mainly focuses on the construction strategies and principles of sensing elements, as well as the applications of sensor array for identification and detection of target analytes in a wide range of fields. Furthermore, the present challenges and further perspectives of sensor arrays are discussed in detail.

Switchable Nanozyme Activity of Porphyrins Intercalated in Layered Gadolinium Hydroxide

In this study, organo-inorganic nanohybrids LHGd-MTSPP with enzyme-like activity were prepared by in situ intercalation of anionic 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin and its complexes with Zn(II) and Pd(II) (MTSPP, M = 2H, Zn(II) and Pd(II)) into gadolinium layered hydroxide (LHGd). The combination of powder XRD, CHNS analysis, FT-IR, EDX, and TG confirmed the layered structure of the reaction products. The basal interplanar distances in LHGd-MTSPP samples were 22.3-22.6 Å, corresponding to the size of an intercalated tetrapyrrole molecule. According to SEM data, LHGd-MTSPP hybrids consisted of individual lamellar nanoparticles 20-50 nm in thickness. The enzyme-like activity of individual constituents, LHGd-Cl and sulfoporphyrins TSPP, ZnTSPP and PdTSPP, and hybrid LHGd-MTSPP materials, was studied by chemiluminescence analysis using the ABAP/luminol system in phosphate buffer solution. All the individual porphyrins exhibited dose-dependent antioxidant properties with respect to alkylperoxyl radicals at pH 7.4. The intercalation of free base TSPP porphyrin into the LHGd preserved the radical scavenging properties of the product. Conversely, in LHGd-MTSPP samples containing Zn(II) and Pd(II) complexes, the antioxidant properties of the porphyrins changed to dose-dependent prooxidant activity. Thus, an efficient approach to the design and synthesis of advanced LHGd-MTSPP materials with switchable enzyme-like activity was developed.

Recent Progress in Aptamer-Functionalized Metal-Organic Frameworks-Based Optical and Electrochemical Sensors for Detection of Mycotoxins

Mycotoxin contamination in foodstuffs and agricultural products has posed a serious hazard to human health and raised international concern. The progress of cost-effective, facile, rapid and reliable analytical tools for mycotoxin determination is in urgent need. In this regard, the potential utility of metal-organic frameworks (MOFs) as a class of crystalline porous materials has sparked immense attention due to their large specific surface area, adjustable pore size, nanoscale framework structure and good chemical stability. The amalgamation of MOFs with high-affinity aptamers has resulted in the progress of advanced aptasensing methods for clinical and food/water safety diagnosis. Aptamers have many advantages over classical approaches as exceptional molecular recognition constituents for versatile bioassays tools. The excellent sensitivity and selectivity of the MOF-aptamer biocomposite nominate them as efficient lab-on-chip tools for portable, label-free, cost-effective and real-time screening of mycotoxins. Current breakthroughs in the concept, progress and biosensing applications of aptamer functionalized MOFs-derived electrochemical and optical sensors for mycotoxins have been discussed in this study. We first highlighted an overview part, which provides some insights into the functionalization mechanisms of MOFs with aptamers, offering a foundation to create MOFs-based aptasensors. Then, we discuss various strategies to design high-performance MOFs-based aptamer scaffolds, which serve as either signal nanoprobe carriers or signal nanoprobes and their applications. We perceived that applications of optical aptamers are in their infancy in comparison with electrochemical MOFs-derived aptasensors. Finally, current challenges and prospective trends of MOFs-aptamer sensors are discussed.

Colorimetric determination of biothiols based on peroxidase-mimicking Ag nanoparticles decorated Ti3C2 nanosheets

Ag nanoparticle-decorated Ti₃C₂ nanosheets (AgNPs@Ti₃C₂ NSs) were facilely synthesized via a self-reduction approach, in which Ti₃C₂ NSs acted as both reductant and supporter. The AgNPs@Ti₃C₂ NS nanocomposite exhibited excellent peroxidase-like activity with o-phenylenediamine (OPD) and H₂O₂ as substrates. The catalytic behavior followed the typical Michaelis-Menten kinetics; Michaelis constant (K_m) and maximum initial velocity (V_max) for OPD were 0.263 mM and 43.2 × 10^-8 M^-1 s, indicating high affinity and high catalytic efficiency towards OPD. The catalytic mechanism was revealed to be an accelerated electron transfer process. Based on the inhibition effect on the peroxidase-like activity of AgNPs@Ti₃C₂ NSs, a simple, fast, and sensitive colorimetric method for detection of low-weight biothiols (cysteine (Cys), homocysteine (Hcy), and glutathione (GSH)) was developed by measuring the absorbance at 425 nm. The colorimetric method displayed wide linear range (50 nM to 50 μM for Cys, 10 nM to 250 μM for Hcy, 10 nM to 50 μM for GSH), low limit of detection (48.5 nM for Cys, 5.5 nM for Hcy, 7.0 nM for GSH), and good selectivity and short assay time (3 min). Moreover, the feasibility of this colorimetric sensor was demonstrated by accurately determining Cys in diluted human serum samples; good recovery (95.9-101.0%) and low relative standard deviations (2.8-4.9%) were obtained, showing great promise for point-of-care test in clinical samples.

Self-triggered fluorescent metal-organic framework mimic enzyme for competitive immunoassay of hypoglycemic drug in functional tea

Colorimetric or fluorescent biosensors based on mimic enzymes have come into the spotlight in virtue of their visual detection. In traditional visual sensors, fluorescent-changing or color-changing substances should be introduced for the catalytic reaction with mimic enzymes. Herein, a mimic enzyme (Au@Fe-MIL-88B) with self-triggered fluorescent property was prepared. By incorporating Au nanoparticles (Au NPs) in Fe-MIL-88B, a higher peroxidase activity of Au@Fe-MIL-88B was monitored due to the synergistic effect between Au NPs and Fe-MIL-88B. Besides, Au NPs can change the valence of Fe ion in metal organic framework (MOF), thus lower background fluorescence was discovered, but the addition of H₂O₂ can trigger the self-fluorescence of Au@Fe-MIL-88B. By using Au@Fe-MIL-88B as a label to anchor secondary antibody, a competitive immunosensor based on fluorescence and photoelectrochemistry was constructed for the immunoassay of rosiglitazone (RSG), a kind of hypoglycemic drug. Finally, a portable instrument was homemade for the on-site and convenient detection of RSG in functional tea. This self-triggered fluorescent MOF may provide a possible route to design biosensors for the detection of hazardous materials.

An efficient differential sensing strategy for phenolic pollutants based on the nanozyme with polyphenol oxidase activity

To realize the efficient differential sensing of phenolic pollutants in sewage, a novel sensing strategy was successfully developed based on one nanozyme (GMP-Cu) with polyphenol oxidase activity. Phenolic pollutants can be oxidized by GMP-Cu, and the oxidation products reacts subsequently with 4-aminoantipyrine to produce a quinone-imine compound. The absorption spectra of final quinone-imine products resulted from different phenolic pollutants showed obvious differences, which were due to the interaction difference between GMP-Cu and phenolic pollutants, as well as the different molecular structures of the quinone-imine products from different phenolic pollutants. Based on the difference of absorption spectra, a novel differential sensing strategy was developed. The genetic algorithm was used to select the characteristic wavelengths at different enzymatic reaction times, HCA and PLS-DA algorithms were utilized for the discriminant sensing of seven representative phenolic pollutants, including hydroquinone, resorcinol, catechol, resorcinol, phenol, p-chlorophenol, and 2,4-dichlorophenol. Scientific wavelength selection algorithm and recognition algorithm resulted in the successful identification of phenolic pollutants in sewage with a discriminant accuracy of 100%, and differentiation of the phenolic pollutants regardless of their concentration. These results indicate that sensing strategy can be used as an effective tool for the efficient identification and differentiation of phenolic pollutants in sewage.

Interactions of proteins with metal-based nanoparticles from a point of view of analytical chemistry - Challenges and opportunities

Interactions of proteins with nanomaterials draw attention of many research groups interested in fundamental phenomena. However, alongside with valuable information regarding physicochemical aspects of such processes and their mechanisms, they more and more often prove to be useful from a point of view of bioanalytics. Deliberate use of processes based on adsorption of proteins on nanoparticles (or vice versa) allows for a development of new analytical methods and improvement of the existing ones. It also leads to obtaining of nanoparticles of desired properties and functionalities, which can be used as elements of analytical tools for various applications. Due to interactions with nanoparticles, proteins can also gain new functionalities or lose their interfering potential, which from perspective of bioanalytics seems to be very inviting and attractive. In the framework of this article we will discuss the bioanalytical potential of interactions of proteins with a chosen group of nanoparticles, and implementation of so driven processes for biosensing. Moreover, we will show both positive and negative (opportunities and challenges) aspects resulting from the presence of proteins in media/samples containing metal-based nanoparticles or their precursors.

Fabrication of a polyoxotungstate/metal-organic framework/phosphorus-doped reduced graphene oxide nanohybrid modified glassy carbon electrode by electrochemical reduction and its electrochemical properties

Hybrid nanocomposites based on polyoxometalates (POMs), metal-organic frameworks (MOFs), and graphene oxide (GO) have a unique set of properties. They have specific properties such as high acidity, oxygen-rich surface, and good redox capability from POMs. In contrast, they do not have weaknesses of POMs such as a low surface area, and high solubility in aqueous media. Herein, a novel organic-inorganic nanohybrid compound based on H3PW12O40 (PW12), a Co-based MOF, and GO was prepared. The prepared hybrid nanocomposite (PW12/MOF/GO) was characterized using different techniques. Then, a PW12/MOF/GO nanocomposite modified glassy carbon electrode (GCE) was fabricated by the drop-casting method and next was dried at room temperature. Then, the PW12/MOF/GO/GCE was subjected to electrochemical reduction at a constant potential of -1.5 V, in 0.1 M H3PO4 solution containing 0.10% w/v PW12/MOF/GO additive. The morphology, electrochemical activity, and stability of the modified electrode (PW12/MOF/P@ERGO/GCE) were studied with FE-SEM coupled with EDS, CV, and amperometry. The obtained results confirmed that the PW12/MOF/P@ERGO/GCE could be effective in hydrogen evolution reaction (HER). The electrochemical activity of the PW12/MOF/P@ERGO/GCE due to the desirable microstructure of the electrocatalyst (e.g. high active surface area and homogeneous distribution of the PW12/MOF/P@ERGO), and also the synergistic effect of the blocks, is more than those of PW12/GCE, MOF/GCE, PW12/MOF/GCE, and P@ERGO/GCE. Moreover, the PW12/MOF/P@ERGO/GCE showed an excellent long-term stability under the air atmosphere.

Phosphorylated Protein Levels in Animal-Sourced Food Muscles Based on Fe3+ and UV/Vis Spectrometry

Protein phosphorylation, a post-translational modification of proteins, is important in biological regulation. The quantity of phosphorylated proteins is a key requirement for the quality change of animal muscle foods. In the present study, a new approach to quantify phosphorylated proteins and/or peptides was developed based on ferric ions (Fe³⁺) and UV/vis spectrometry. This method is proved to be ultra-effective in discriminating phosphopeptides and non-phosphopeptides with the assistance of Fe³⁺. The protocol of extracting proteins with 0.1% trifluoroacetic acid (TFA) solution from animal muscle samples coupled with Fe³⁺ was verified by using an artificial mixture of peptides with different phosphorylation sites and was successfully used to characterize the phosphorylation quantity in the samples via UV/vis spectrometry. A peptide with one phosphorylated site was taken as a reference standard and successfully utilized for the absolute quantification of phosphorylated proteins in caprine muscles during frozen storage and in fish muscle food samples. This present study paves a new way for the evaluation of phosphorylated protein quantitative levels in bio-samples.

Co-N/C-900 metal-organic framework-derived nanozyme as a H2O2-free oxidase mimic for the colorimetric sensing of L-cysteine

Nanozymes have attracted considerable attention as a new type of promising artificial enzyme in recent years. Here, an oxidase-like cobalt-nitrogen-carbon (Co-N/C-900) nanozyme with well-regulated metal atom spatial distribution has been derived from Co-Zn bimetal zeolitic imidazolate framework precursors and used to develop a facile colorimetric sensing method for L-cysteine. With the aid of Co-N/C-900, the colorless 3,3',5,5'-tetramethylbenzidine (TMB) was oxidized to blue oxidized TMB in the absence of H₂O₂. However, the oxidation was inhibited after the addition of L-cysteine, and the blue color faded to colorless. Thus, Co-N/C-900 exhibited quite good oxidase-like activity with high catalytic efficiency. Therefore, a facile and efficient colorimetric method to sensitively determine L-cysteine with a low detection limit of 33 nM (S/N = 3) has been developed. Furthermore, favorable selectivity and anti-interference ability towards the determination of L-cysteine based on this approach have also been achieved. It is believed that this colorimetric method for the detection of L-cysteine based on Co-N/C-900 will show potential applications in bioscience and bioengineering.

Colorimetric quantification of sodium benzoate in food by using d-amino acid oxidase and 2D metal organic framework nanosheets mediated cascade enzyme reactions

A rapid colorimetric method for detecting sodium benzoate in food products was established based on the d-amino acid oxidase (DAAO) and 2D metal organic framework (2D MOF) nanosheets mediated cascade enzyme reactions. Firstly, the synthesized 2D MOF nanosheets served as high efficient nanozyme with outstanding peroxidase-like catalytic activity and catalyzed the color reaction between H₂O₂ and 3, 3', 5, 5'- tetramethylbenzidine. Secondly, sodium benzoate as a competitive inhibitor of DAAO, could influence the production of H₂O₂ in DAAO mediated oxidation reaction. After a combination of those two reactions, this colorimetric quantitative method was constructed and validated for sodium benzoate determination with wide linear range (2.0-200.0 μM), low limit of detection (2.0 μM), high accuracy (recovery rate in 95.80-108.00%) and satisfied selectivity. Lastly, this method was utilized to analyze sodium benzoate concentration in juice, wine and vinegar by naked eyes.

Simulation design of natural enzyme binding pocket structure in MOFs for enhanced catalytic activity

This study confirms the activity gap between MOFs (Fe) and horseradish peroxidase can be bridged by simulating the binding pocket structure and adding active center. The customized structure promoted the...

Highly efficient peroxidase-like activity of a metal–oxide-incorporated CeO2–MIL(Fe) metal–organic framework and its application in the colorimetric detection of melamine and mercury ions via induced hydrogen and covalent bonds

The illegal addition of melamine to dairy products and the contamination of water with Hg2+ are serious threats to human health. This necessitates the search for new and efficient probe for detection of melamine in foods and Hg2+ in water samples.

An imine-linked covalent organic framework for renewable and sensitive determination of antibiotic

Lanthanide-functionalized porous organic materials have been the promising candidates in the chemical and biological sensing. Considering the superior thermal and solvent stability of covalent organic frameworks (COFs), the development of lanthanide ions-functionalized COFs based sensing platform is meaningful, while remains to be a challenge. In this work, a new imine-linked COF which provides suitable coordination sites for Tb³⁺ was constructed via the Schiff base reaction between P-phenylenediamine (Pda) and 2,6-Diformylpyridine (Dfp). Benefiting from its high signal-to-noise, the COF@Tb shows excellent ability to determinate ciprofloxacin (CIP) with a detection limit of 3.01 nM. The measurement can maintain good stability in the presence of potential interference or in actual sample. Being washed with ethanol after each measurement, COF@Tb can be recycled for five times. This work provides a novel alternative strategy for efficient construction of lanthanide-grafted COFs and may promote the development of porous organic materials based chemical sensing.

Hydrogen-Bonded Biohybrid Framework-Derived Highly Specific Nanozymes for Biomarker Sensing

Nanozymes are of particular interest due to their enzyme-mimicking activity and high stability that are favorable in biomedical sensing and immunoassays. In this work, we report a highly specific N-doped nanozyme through pyrolysis of framework-confined bovine serum albumin (BSA). This strategy allows one to translate the low-cost and featureless BSA into a highly active enzyme mimic. The obtained carbon nanozyme (denoted as HBF-1-C800) displays 3- to 7-fold enhancement on peroxidase (POD) activity compared with the conventional carbon nanozymes and also shows ca. 5-fold activity enhancement compared to the reported N-doping graphene. Such excellent POD activity originates from high N-doping efficiency, protein-induced defective sites, and the intrinsic porous structure of HBF-1-C800, which provides abundantly accessible active sites and accelerates substrate diffusion simultaneously. Importantly, the HBF-1-C800 nanozyme has highly specific POD activity and also enables resistance to several harsh conditions that should denature natural enzymes. These features allow it with high accuracy, stability, and sensitivity for biosensing applications. Moreover, HBF-1-C800 has been designed as a promising platform for colorimetric biosensing of several biomarkers including H₂O₂, glutathione, and glucose, with wide linear ranges and low limits of detection that are satisfied with the disease diagnosis.

Facile in situ microwave synthesis of Fe3O4@MIL-100(Fe) exhibiting enhanced dual enzyme mimetic activities for colorimetric glutathione sensing

In recent decades, artificial nanozymes with excellent stability, low cost and availability have been gradually explored to avoid the limits of natural enzymes such as poor stability, high cost and difficult preparation. Herein, for the first time, we investigated the capability of nanoscale Fe₃O₄@MIL-100(Fe) as a nanozyme, which was quickly synthesized in situ by a microwave-assisted method within 20 min using Fe₃O₄ as the metal precursor. The obtained Fe₃O₄@MIL-100(Fe) showed satisfactory intrinsic dual enzyme mimetic activities, including peroxidase (POD)- and catalase (CAT)-like activities. Moreover, a simple and effective colorimetric biosensor was fabricated to detect glutathione (GSH) based on its POD-like activity. The proposed measurement had a linear range of 1-45 μM and a limit of detection (LOD) of 0.26 μM (3.3 δ/S). It was proved that the established colorimetric sensing system could be successfully applied to detect GSH in actual biological samples. Importantly, the outstanding reusability and stability made it extremely valuable as a catalyst. The present work implied that Fe₃O₄@MIL-100(Fe) synthesized in situ by the microwave-assisted method was a very promising candidate for biocatalyst and biosensing.

Designed multifunctional ratiometric fluorescent probe for directly detecting fluoride ion/ dichromate and indirectly monitoring urea

UiO-66-NH₂@eosin Y composite was obtained by confining eosin Y (EY) into the cavities of Zr-MOF and could emit two fluorescence peaks at 453 and 543 nm at an excitation wavelength of 355 nm. This multi-responsive and multifunctional ratiometric fluorescent nanoprobe not only enable directly distinct detection of F^-/Cr₂O₇^2- with ultra-high selectivity and sensitivity, but also could indirectly monitor the concentration of urea based on unique enzymatic hydrolysis reaction. The multifunctional probe was utilized for fluorescence labeling F-/Cr₂O₇^2- in sweat latent fingerprint through an environmentally friendly powder strategy and exhibited obvious luminescence visualization changes. Notably, the corresponding portable on-line test strips of probe for detection of F^- and Cr₂O₇^2- were made for monitoring the levels of F^- and Cr₂O₇^2-. Furthermore, the probe was applied to evaluate the degrees of F^-/Cr₂O₇^2- in HepG-2 cell and urea in serum with superior results,which indicate the potential application of the as-synthesized UiO-66-NH₂@EY as multifunctional probe for the detection of F^-, Cr₂O₇^2- and urea in biological samples. Finally, in order to extend the device-based applications of probe, an AND-OR-coupled molecular logic gate was put on agenda.

Development of enzyme-free single-step immunoassays for glycocholic acid based on palladium nanoparticle-mediated signal generation

Palladium nanoparticles (PdNPs) are composed mainly of inert noble metals, and their outstanding properties have attracted wide attention. PdNPs are not only capable of mimicking the oxidase-like characteristics of natural bio-enzymes, but they also present a clear black band in the test zone. In this work, the synthesized PdNPs promoted a transformation of colorless tetramethylbenzidine (TMB) to a blue oxidation product of TMB, providing a K_m value of 0.09 mM for TMB, and revealing the good catalytic performance of the synthesized PdNPs. For both signal generation and amplification, PdNPs effectively replaced natural bio-enzymes as a new labeling tag. Thus, the PdNP-based enzyme-free single-step immunoassays were successfully developed for efficient and sensitive detection of glycocholic acid (GCA). Under optimal conditions, a noticeable linear relationship was identified by the enzyme-linked immunosorbent assay (ELISA) over a range of 8-2390 ng/mL, while the visual limit of detection (vLOD) in the constructed lateral flow immunoassay (LFA) was 10 ng/mL for GCA. The recovery rate in spiked urine samples obtained by the ELISA ranged from 84.2 to 117.9%, which was consistent with the results in LFA. The present work demonstrates the potential of PdNPs as labeling matrices in enzyme-free single-step immunoassays.