Oxidation Of O-phenylenediamine Research Articles

Dual-wavelength ratiometric immunosensor for Bacillus cereus: Oxidase-like MnO2-Au trigged “OFF-ON” detection strategy

As an opportunistic pathogen, Bacillus cereus (B. cereus) has caused increasing emetic and diarrheal food poisoning, even fulminant hepatic failure and death case. Herein, a mimetic oxidase-mediated ratiometric fluorescence enzyme-linked immunosorbent assay (ELISA) was developed to detect B. cereus. In this system, 3D MnO2-Au nanoflowers with layered crystalline structure can oxidize o-phenylenediamine (OPD) to generate a fluorescence peak at 570 nm (I570, response signal), while blue carbon quantum dots (bCDs) display a fluorescence peak at 455 nm (I455, reference signal). Under the effect of fluorescence resonance energy transfer (FRET), the response signal increased while the reference signal was weakened. By using MnO2-Au as the mimetic oxidase to replace the horseradish peroxidase and associate with antibody in traditional ELISA, a linear relationship was established between the ratiometric fluorescence signal (I570/I455) and the logarithm of B. cereus concentrations in the range from 1.7 × 102 to 3.3 × 106 CFU/mL. Comparing with conventional sandwich ELISA, the detection time was shortened by 43 min and the sensitivity was improved by 5-folds, indicating that the proposed ratiometric immunoassay holds great potential as a rapid and convenient tool for sensitive screening of B. cereus.

Ratiometric fluorescent hydrogel for point-of-care monitoring of organophosphorus pesticide degradation

The residues of organophosphorus pesticides have caused the potential risk in environment and human health, arousing worldwidely great concern. Herein, we fabricated a robust gold nanoclusters/MnO2 composites-based hydrogel portable kit for accurate monitoring of paraoxon residues and degradation in Chinese cabbages. With the immobilization of gold nanoclusters/MnO2 composites into a hydrogel, a ratiometric fluorescent signal is generated by catalyzing the oxidation of o-phenylenediamine, which possesses a built-in correction with low background interference. Coupling with acetylcholinesterase catalytic reactions and pesticide inhibition effect, the portable kit can sensitively detect paraoxon residues with a detection limit of 5.0 ng mL−1. For on-site quantification, the fluorescent color variations of portable kit are converted into digital information that exhibits applicative linear range toward pesticide. Notably, the hydrogel portable kit was successfully applied for precisely monitoring the residue and degradation of paraoxon in Chinese cabbage, providing a potential pathway toward practical point-of-care testing in food safety monitoring.

Integrating peroxidase-mimicking NH2-MIL-101(Fe) with molecular imprinting for high-performance ratiometric fluorescence sensing of domoic acid

Ingestion of shellfish and water containing excessive domoic acid (DA) threatens human health tremendously, and even causes death in severe cases. Herein, we have integrated the bifunctional NH2-MIL-101(Fe) with molecular imprinting to design a ratiometric fluorescent sensor for quantitative analysis of DA. In this research, NH2-MIL-101(Fe) itself can supply an intrinsic peak at 452 nm, and trigger oxidation of o-phenylenediamine (OPD) to 2,3-diaminophenazine (DAP) with peak at 556 nm owing to its excellent peroxidase-like activity. The two signal probes that influence each other owing to the inner filter effect (IFE) can form a highly sensitive composite fluorescent probe. Additionally, the introduction of molecular imprinting technology (MIT) instead of expensive antibodies can endow the ratiometric fluorescent sensor outstanding selectivity and specificity toward DA. Thus, the proposed ratiometric fluorescent sensor exhibits outstanding sensitivity, super anti-interference ability, and excellent selectivity. The experimental results demonstrate that the sensor exhibits good performance for DA sensing in the scope of 0.01–10 μM with low limit of detection (LOD) as 8.2 nM. More attractively, the successful spike and recovery experiments in real samples with the complicated matrix indicate the considerable practical application promise of our proposed ratiometric fluorescent sensor.

Smartphone-based portable fluorescence sensor with gold nanoparticle mediation for selective detection of nitrite ions

A simple, portable device for the detection of NO2– via a fluorescence method was developed. The proposed device consisted of a dark box containing a blue LED as a low-power excitation light source and a smartphone with a mobile application for RGB analysis as a light detector. Detection was mediated by using synthesized cetyltrimethylammonium bromide-stabilized gold nanoparticles (CTAB-AuNPs). The CTAB-AuNPs were etched with NO2– to yield Au3+, which catalyzes the oxidation of o-phenylenediamine (OPD) in the presence of H2O2 to generate 2,3-diaminophenazine (DAP). Triton X-100 (TX-100) micelles were introduced to improve the DAP fluorescence emission. The fluorescence intensity of DAP was recorded by the smartphone in terms of RGB intensity, which was correlated with the NO2– concentration. This method provided a wide linear working concentration range (0.5–100 μM), a limit of detection of 0.17 μM and excellent selectivity for NO2– over other anions.

Two-Dimensional Cobalt-Doped Ti3C2 MXene Nanozyme-Mediated Homogeneous Electrochemical Strategy for Pesticides Assay Based on In Situ Generation of Electroactive Substances.

Common homogeneous electrochemical (HEC) sensors usually suffer from the drawbacks of high background signal, low signal-to-noise ratio, and even false positive results due to the preaddition of electroactive substances. Thus, it is necessary to develop novel HEC sensors based on in situ generation of electroactive substances to overcome these shortcomings, which, however, is underexplored. In this work, two-dimensional (2D) nanozymes, i.e., cobalt-doped 2D Ti3C2 MXene nanosheets (CMNSs), with excellent peroxidase-like properties were utilized to develop HEC sensors based on the in situ generation of electroactive substances for organophosphate pesticides (OPs) detection. The 2D CMNSs were synthesized via a template-directed wet chemical approach and displayed outstanding features of hydrophilia and water dispersibility, which could catalyze the oxidation of o-phenylenediamine (OPD) to generate significantly increased reduction current. Interestingly, the 2D CMNSs with peroxidase-like properties exhibited a unique response to thiol compounds and were thus employed as highly efficient catalysts to develop HEC sensors for OPs based on the hydrolysis of acetylthiocholine (ATCh) to form thiocholine catalyzed by acetylcholinesterase (AChE) and the inhibition of AChE activity by OPs. The recovery for OPs analysis of pakchoi extract solutions ranged from 97.4% to 103.3%. The as-proposed HEC sensor based on in situ generation of electroactive substances will provide a new way for the development of high-performance electrochemical sensors and demonstrate potential applicability for the determination of pesticide residues in real samples.

A universal sensing platform based on iron and nitrogen co-doped carbon dots for detecting hydrogen peroxide and related metabolites in human fluid by ratiometric fluorometry and colorimetry

A universal ratiometric fluorescence and colorimetric dual-mode sensing platform for detecting hydrogen peroxide (H2O2) and related metabolites in human fluid was constructed based on iron and nitrogen co-doped carbon dots (Fe/N-CDs). As a fluorescent nanomaterial with peroxidase-like property, Fe/N-CDs emits fluorescence at 449 nm (F449) under excitation of incident ultraviolet light, and can catalyze the oxidation of o-phenylenediamine (OPD) by H2O2 for generating 2,3-diaminophenazine (oxOPD) that exhibits obvious absorption at 420 nm (A420) and fluorescence emission at 555 nm (F555). The Förster resonance energy transfer (FRET) between Fe/N-CDs and oxOPD would result in the fluorescence quenching Fe/N-CDs and the fluorescence enhancement of oxOPD, which facilitates the quantitation of oxOPD by ratiometric fluorometry. Since the amount of generated oxOPD is determined by the amount of H2O2 consumed during the oxidation reaction, the detection of H2O2 and related metabolites can be realized by monitoring both ratiometric fluorescent (F555/F449) and colorimetric (absorption, A420) signals of oxOPD. This dual-mode sensing platform exhibits excellent selectivity and sensitivity toward with H2O2, xanthine and uric acid in both human serum and urine samples, demonstrating its good potential for monitoring H2O2 and metabolites involved in H2O2 metabolism in human body. The detection limits (LODs) of H2O2, xanthine and uric acid obtained by this sensing platform were 0.07, 0.15, and 0.14 μM for ratiometric fluorescence mode, and 0.12, 0.52, and 0.47 μM for colorimetric mode, respectively. By utilizing appropriate oxidases in this universal sensing platform, the determination of other metabolites involved with producing H2O2 can also be realized facilely.

Ratiometric fluorescence and colorimetric dual-mode sensing platform based on carbon dots for detecting copper(II) ions and D-penicillamine.

A sensing platform with both ratiometric fluorescence and colorimetric responses towards copper(II) ions (Cu2+) andD-penicillamine (D-pen) was constructed based on carbon dots (CDs). o-Phenylenediamine (OPD) was employed as a chromogenic development reagent for reaction with Cu2+to generate the oxidation product 2,3-diaminophenazine (oxOPD), which not only emits green fluorescence at 555nm, but also quenches the blue fluorescence of CDs at 443nm via the inner filter effect (IFE) and Förster resonance energy transfer (FRET). Additionally, oxOPD exhibits obvious absorption at 420nm. Since the intense chelation affinity of D-pen to Cu2+ greatly inhibits the oxidation of OPD, the intensity ratio of fluorescence at 443nm to that at 555nm (F443/F555) and the absorbance at 420nm (A420) were conveniently employed as spectral response signals to represent the amount of D-pen introduced into the testing system. This dual-signal sensing platform exhibits excellent selectivity and sensitivity towards both Cu2+ and D-pen, with low detection limits of 0.019μM and 0.092μM, respectively. In addition, the low cytotoxicity of the testing reagents involved in the proposed sensing platform facilitates its application for live cell imaging.

A FRET ratiometric fluorescence biosensor for the selective determination of pyrophosphate ion and pyrophosphatase activity based on difunctional Cu-MOF nanozyme

A ratiometric fluorescent method based on the difunctional copper (II)-based metal-organic framework (Cu-MOF) for label-free monitoring pyrophosphatase (PPase) activity and pyrophosphate ion (PPi) was developed. Difunctional Cu-MOF possessed peroxidase mimetic activity and fluorescent emission property. The catalytic activity of Cu-MOF would be inhibited by the coordination reaction between Cu2+ and PPi because of the good affinity between Cu2+ and PPi. However, the PPi can be hydrolyzed by PPase catalyzation to Pi which can hardly affect the Cu-MOF nanozyme activity. Once the PPase participated, the catalytic activity of Cu-MOF was recovered and catalyzed the decomposition of hydrogen peroxide to oxidize o-phenylenediamine (OPD) to form a 2,3-diaminophenazine (DAP) solution and turns on the Förster resonance energy transfer (FRET) from Cu-MOF as a donor to DAP as an acceptor. The LODs of the fluorescence sensing platform were 0.30 mU mL−1 for PPase and 0.53 μmol L−1 for PPi, respectively. This method will broaden the applications of nanozyme and shows a great potential application in the diagnosis of PPi or PPase-related diseases.

A hydrated 2,3-diaminophenazinium chloride as a promising building block against SARS-CoV-2

Phenazine scaffolds are the versatile secondary metabolites of bacterial origin. It functions in the biological control of plant pathogens and contributes to the producing strains ecological fitness and pathogenicity. In the light of the excellent therapeutic properties of phenazine, we have synthesized a hydrated 2,3-diaminophenazinium chloride (DAPH+Cl−·3H2O) through direct catalytic oxidation of o-phenylenediamine with an iron(III) complex, [Fe(1,10-phenanthroline)2Cl2]NO3 in ethanol under aerobic condition. The crystal structure, molecular complexity and supramolecular aspects of DAPH+Cl− were confirmed and elucidated with different spectroscopic methods and single crystal X-ray structural analysis. Crystal engineering study on DAPH+Cl− exhibits a fascinating formation of (H2O)2…Cl−…(H2O) cluster and energy framework analysis of defines the role of chloride ions in the stabilization of DAPH+Cl−. The bactericidal efficiency of the compound has been testified against few clinical bacteria like Streptococcus pneumoniae, Escherichia coli, K. pneumoniae using the disc diffusion method and the results of high inhibition zone suggest its excellent antibacterial properties. The phenazinium chloride exhibits a significant percentage of cell viability and a considerable inhibition property against SARS-CoV-2 at non-cytotoxic concentration compared to remdesivir. Molecular docking studies estimate a good binding propensity of DAPH+Cl− with non-structural proteins (nsp2 and nsp7-nsp-8) and the main protease (Mpro) of SARS-CoV-2. The molecular dynamics simulation studies attribute the conformationally stable structures of the DAPH+Cl− bound Mpro and nsp2, nsp7-nsp8 complexes as evident from the considerable binding energy values, − 19.2 ± 0.3, − 25.7 ± 0.1, and − 24.5 ± 0.7 kcal/mol, respectively.

Molybdenum Selenide/Porous Carbon Nanomaterial Heterostructures with Remarkably Enhanced Light-Boosting Peroxidase-like Activities.

Nanozymes have emerged as a fascinating nanomaterial with enzyme-like characteristics for addressing the limitations of natural enzymes. Nevertheless, how to improve the relatively low catalytic activity still remains challenging. Herein, a facile recrystallizing salt template-assisted chemical vapor deposition method was utilized to synthesize MoSe2/PCN heterostructures. This heterostructure displays remarkably enhanced light boosting peroxidase-like activities. Notably, the maximal reaction velocity of this heterostructure attains 17.81 and 86.89 μM min-1 [for o-phenylenediamine (OPD) and 3,3'5,5'-tetramethylbenzidine (TMB), respectively]. Moreover, various characterization means were performed to explore the mechanism deeply. It is worth mentioning that the photoinduced electrons generated by the heterostructure directly react with H2O2 to yield plentiful •OH for the effective oxidation of OPD and TMB. Therefore, this work offers a promising approach for improving peroxidase-like activity by light stimulation and actuating the development of enzyme-based applications.

Quantitative analysis of galactose using LDI-TOF MS based on a TiO2 nanowire chip

A novel method for quantifying galactose was developed to serve as a newborn screening test for galactosemia using laser desorption/ionization time-of-flight (LDI-TOF) mass spectrometry (MS) with a TiO2 nanowire chip. Herein, phosphate citrate buffer, serum, and dried blood spot (DBS) were employed for the quantitative analysis of galactose. To quantitatively analyze galactose, its reduction potential was used to oxidize o-phenylene diamine (OPD) into 2,3-diaminophenazine (DA), which were both detected using LDI-TOF MS with a TiO2 nanowire chip according to the concentration of galactose. The reproducibility and the interference of glucose were determined to demonstrate the applicability of this method. Moreover, mixtures of galactose, phenylalanine, and 17 α-OHP were analyzed to determine the interference induced by other biomarkers of metabolic disorders. The OPD oxidation of galactose was found to be selectively achieved under high-glucose conditions, similar to human blood, thereby showing good reproducibility. The intensities of the mass peaks of OPD and DA based on LDI-TOF MS with a TiO2 nanowire chip were linearly correlated in the galactose concentration range of 57.2–220.0 μg/mL (r2 = 0.999 and 0.950, respectively) for serum samples and 52.5–220.0 μg/mL (r2 = 0.993 and 0.985, respectively) for DBS after methanol precipitation/extraction. The enzyme immunoassay and LDI-TOF MS analysis results were statistically analyzed, and a mixture of phenylalanine, 17 α-OHP, and galactose was simultaneously investigated quantitatively at the cutoff level.

Biomimics of phenazine oxidase activity of a cobalt (III)‐dipyridylamine complex: Spectroscopic, structural, and computational studies†

AbstractThis manuscript demonstrates the synthesis, structural characterization, computational studies, and biomimics of the phenazine oxidase activity of a newly designed cobalt (III) complex, [Co (dpa)(dpa‐H+)(N3)2]Cl2 (1) [dpa = 2,2′‐dipyridylamine] under an aerobic condition. The crystal structure analysis reveals that the cobalt (III) center adopts an octahedral geometry, and the complex forms a beautiful supramolecular framework through noncovalent interactions. The cobalt (III) catalyst turns out to be a promising catalyst for the oxidative coupling of o‐phenylenediamine (OPD) in oxygen‐saturated methanol with an excellent turnover number, kcat = 7.85 × 103 h−1. Spectrophotometric, electrochemical, mass spectrometry, and computational analysis ensure that the course of catalysis undergoes through a catalyst‐substrate complexation, facilitating the development of cobalt‐iminobenzoquinone species in the solution. The computational calculations employing the density functional theory (DFT) throw a light to the mechanistic insights of the phenazine oxidase mimics. ETS‐NOCV (extended transition state‐natural orbitals for chemical valence) plots of the reactive intermediates portray the coordination‐driven depletion of electron density from the nitrogens of OPD to cobalt center leading to the enhancement of electrophilic character on para‐positioned C‐atoms with respect to N‐atoms of OPD, therby catalyzing the nucleophilic attack by second OPD to produce the oxidation product, 2,3‐diaminophenazine (DAP). Interestingly, we are able to isolate the oxidation product of the OPD oxidation reaction as a hydrated chloride salt, DAPH+Cl− ·3H2O (2). The crystal engineering perspectives of 2 attribute the intriguing fate of the secondary chlorides for the stabilization of the oxidation product in crystalline phase.

Unveiling the Actual Catalytic Sites in Nanozyme-Catalyzed Oxidation of o-Phenylenediamine.

Nanozymes have offered remarkable advantages over natural enzymes and found widespread applications including biosensors, immunoassays, nanomedicines, and environmental remediation. Oxidation of o-phenylenediamine (OPD) by nanozymes has been listed as a standard protocol for determining nanozyme activities. Given the complexity of OPD oxidation processes, however, the mechanism of nanozyme-catalyzed oxidation of OPD remains elusive. In this report, mechanistic studies of nanozyme-catalyzed oxidation of OPD are performed and a distinguishably different mechanism from that of natural enzymes is found. A combination of Fourier transform infrared spectroscopy, nuclear magnetic resonance, electrospray ionization mass spectrometry, and electron microscopic studies provides compelling evidence that polymerization of OPD occurs on the surface of several different nanozymes. The unexpected polymerization causes a dense coating layer of poly(o-phenylenediamine) (POPD) on nanozymes renders the intrinsic properties of nanozymes. Therefore, this fundamental discovery raise serious concerns using OPD-based colorimetric method for determining nanozyme activities. Without examining the surface change of nanozymes after catalytic reactions, the use of OPD-based colorimetric method for determining nanozyme activities is strongly discouraged. Furthermore, POPD is discovered as a new oxidase mimic, and this new mechanism also provides a general and robust method to coat nanomaterials with POPD polymers of enzyme-mimicking properties.

Thermocatalytic Activity of Gold Truncated Nanopyramids on Strontium Titanate Nanocube

Gold truncated nanopyramids smaller than 10 nm with clean surfaces were formed on SrTiO3 naocubes (Au TNPy//SrTiO3 NCs, // denotes heteroepitaxial junction) by the deposition precipitation method without surface modifier, while the shape of Au nanoparticles (NPs) on irregular shaped SrTiO3 NPs was hemisphere-like (Au HS/SrTiO3 NPs). As a probe reaction, the oxidation of o-phenylenediamine (OPD) to 2,3-diaminophenazine (DAP) by H2O2 was carried out with the catalysts in the dark at 25 °C. Au TNPy//SrTiO3 NCs exhibit significantly higher catalytic activity for the oxidation reaction than Au HS/SrTiO3 NPs. Kinetic analyses indicated that H2O2 can be more effectively activated to be utilized for the OPD oxidation in the former system than the latter system.

Polyvinylpyrrolidone-stabilized Pt nanoclusters as robust oxidase mimics for selective detection of ascorbic acid

In this work, the Pt nanoclusters with an average size of 1.7 ± 0.3 nm were prepared by a facile and green method using polyvinylpyrrolidone (PVP) as both stabilizer and reductant. The oxidase-like activity of PVP-stabilized Pt nanoclusters (PVP-Pt NCs) was investigated by the catalytic oxidation of o-phenylenediamine (OPD). The mechanism studies indicated that PVP-Pt NCs performed as oxidase enzyme mimics by catalyzing the dissolved oxygen to generate the reactive oxygen species (singlet oxygen and superoxide anion) intermediates. Ascorbic acid (AA) could also be oxidized to dehydroascorbic acid (DHA) which could react with OPD to generate a fluorescent compound. On the basis of the robust oxidase-like activity of PVP-Pt NCs, both colorimetric assay and fluorescence method for AA determination were established. Compared with colorimetric assay, the fluorescence method showed higher sensitivity and selectivity for AA detection. The proposed fluorescence method with the LOD of 1.17 μM could overcome the interference of most reducing substances and showed promising applications in biosensor. The as-prepared PVP-Pt NCs possessed intrinsic oxidase-like property by catalyzing dissolved oxygen to ROS (superoxide anions (·O 2 - ) and singlet oxygen ( 1 O 2 )) which could oxide the OPD to OPD ox . The inhibition of AA to the catalytic oxidation of OPD was employed to establish the colorimetric assay for AA determination. AA could also be oxidized to DHA by·O 2 - and 1 O 2 intermediates, and the DHA could react with OPD to form QD with a fluorescence emission peak at 434 nm. The fluorescence method for AA detection was carried out when the solution changed to yellow color which was the signal of the reaction terminal. Comparing with colorimetric assay, the fluorescence method exhibited enhanced sensitivity and selectivity.

A highly sensitive dual-read assay using nitrogen-doped carbon dots for the quantitation of uric acid in human serum and urine samples.

A simple dual-read assay for uric acid (UA) was developed based on a combined ratiometric fluorescent and colorimetric strategy using nitrogen-doped carbon dots (N-CDs). The biosensor relies on the oxidation of UA by uricase to produce H2O2, which was then converted to •OH radicals by I-, resulting in the oxidation of o-phenylenediamine (OPD) to 2,3-diaminophenazine (DAP). In the presence of UA, the colorless biosensor system changed to yellow. Furthermore, the presence of DAP quenched the fluorescence emission of the N-CDs at 427 nm based on the inner filter effect (IFE). With increasing UA concentrations, the fluorescence intensity of the biosensor at 427 nm decreased but increased at 580 nm, demonstrating the ratiometric response. A strong linearity was observed between the fluorescence intensity ratio of DAP to N-CDs (I580/I427) and the corresponding UA concentration over the range 0.5−150 μM, and a limit of detection (S/N ratio of 3) of 0.06 μM was calculated. The dual-read assay was successfully employed in the quantitation of UA in human serum and urine samples, revealing its potential for measuring UA in clinical samples.Graphical abstract Supplementary InformationThe online version contains supplementary material available at 10.1007/s00604-021-04971-2.

Fluorescent MOF-based nanozymes for discrimination of phenylenediamine isomers and ratiometric sensing of o-phenylenediamine

Although peroxidase-like nanozymes have made great progress in bioanalysis, few current nanozyme-based biosensors are constructed for discriminating isomers of organic compounds. Herein, fluorescent metal-organic framework (MOF)-based nanozyme is utilized for phenylenediamine isomers discrimination and detection. NH2-MIL-101(Fe), as a member of Fe-based MOFs, functions as not only fluorescent indicator but also peroxidase mimics. In the presence of H2O2, NH2-MIL-101(Fe) can catalyze the oxidation of o-phenylenediamine (OPD) and p-phenylenediamine (PPD) into their corresponding oxidation products (OPDox and PPDox), which in turn quench its intrinsic fluorescence at 445 nm via inner filter effect (IFE). Differently, a new fluorescence peak at 574 nm is observed for OPDox. Thus, a ratiometric fluorescence method for the detection of OPD can be designed with the fluorescence intensity ratio F574/F445 as readout. This proposed strategy displays excellent discrimination ability for three phenylenediamines and may open new applications of MOFs in environmental science.

Ratiometric fluorescence and smartphone dual-mode detection of glutathione using carbon dots coupled with Ag+-triggered oxidation of o-phenylenediamine

Developing ratiometric fluorescence and smartphone dual-mode bioanalysis methods is important but challenging. A ratiometric fluorescence method for determining glutathione (GSH) using carbon dots (CDs) and Ag+-triggered o-phenylenediamine (OPD) oxidation is described here. Ag+ oxidizes OPD to give 2,3-diaminophenazine (oxOPD), which effectively quenches CD fluorescence at 436 nm through the inner filter effect and causes a new emission peak at 561 nm. GSH chelates with Ag+ and prevents the Ag+ oxidizing OPD and therefore effectively preserves CD emission at 436 nm (blue) and allows only weak oxOPD fluorescence at 561 nm (orange) to occur. The oxOPD to CD fluorescence intensity ratio decreased linearly as the GSH concentration increased in the range 0–150 nM, and the detection limit was 15 nM. The ratiometric fluorescence probe lit with an ultraviolet lamp clearly changed color from orange to blue as the GSH concentration increased. An image was acquired using a smartphone camera and converted into digital values. The blue and red channel ratio was calculated and used to quantify GSH. The method therefore allows dual-mode detection of GSH.

Enhanced oxidase-like activity of g-C3N4 nanosheets supported Pd nanosheets for ratiometric fluorescence detection of acetylcholinesterase activity and its inhibitor

The undesirable enzymatic activity of nanozymes under near neutral pH condition and the traditional single signal output always restrict the analytical application of nanozyme-based biosensors. Herein, graphitic carbon nitride nanosheets supported palladium nanosheets composite (Pd/g-C3N4) with both oxidase-like activity and fluorescent property is synthesized. Notably, Pd/g-C3N4 exhibits enhanced oxidase-like activity compared to Pd NSs under pH 7.4. By combining Pd/g-C3N4 with o-phenylenediamine (OPD), a ratiometric fluorescence assay for acetylcholinesterase (AChE) activity detection is developed. Pd/g-C3N4 can catalyze oxidation of nonfluorescent OPD to fluorescent oxidized OPD (oxOPD, Em = 565 nm), which can quench fluorescence of g-C3N4 supporter (Em = 441 nm) through fluorescence resonance energy transfer (FRET). However, in presence of AChE, acetylthiocholine can be hydrolyzed into thiocholine, which will block the oxidase-like activity of Pd/g-C3N4 and then hamper the FRET process. This ratiometric fluorescence assay is also viable to screen AChE inhibitor. This work will guide design of ratiometric fluorescence assay based on nanozymes with improved enzymatic activity.

Ratiometric fluorescent sensing of mercury (II) ion based on the Pt nanozyme-triggered fluorescence resonance energy transfer between Si quantum dots and 2,3-diaminophenazine

The demand for efficient methods to determine mercury ion (Hg2+) is increasing. Herein, we developed a ratiometric fluorescence assay by the coupling of nanozyme and fluorescence resonance energy transfer (FRET) to determine Hg2+. Platinum nanoparticles (PtNPs) was synthesized as effective nanozyme to catalyze the oxidation of o-phenylenediamine (OPD) at the presence of hydrogen peroxide (H2O2) to produce 2,3-diaminophenazine (DAP). The fluorescence emission of Si quantum dots (QDs) at 414 nm (I414) was observed to be quenched by DAP (emission at 576 nm (I576)) via FRET mechanism, whereas Hg2+ can depress the catalytic activity of PtNPs to generate DAP. Based on this principle, SiQDs/PtNPs/OPD/H2O2-based ratiometric fluorescence assay was developed that the emission ratio of DAP and SiQDs (I576/I414) decreased with a higher concentration of Hg2+. Noteworthy, the sensitivity of such system was found to be related with the value of I576/I414. The developed assay offered a detection range of 20–3000 nM and a detection limit of 10.5 nM. This assay was applied to analyze Yangtze river water, and its reliability has been validated by the standard ICP-MS method. Our work provided an effective and convenient method for Hg2+ detection.