Oxidation Of Tetramethylbenzidine Research Articles

Selenium Hydride-Induced Oxidase-like Activity Inhibition of Amorphous/Crystalline Manganese Dioxide: Colorimetric Assay for Selenium Detection.

Hydride generation-based optical sensors have achieved on-site visual selenium (Se) determination with high anti-interference capability, yet they rely on the change of single-color intensity with a narrow linear dynamic range. Herein, we combined selenium hydride (H2Se)-induced activity inhibition of a manganese dioxide (MnO2) nanozyme with different degrees of 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to realize sensitive multicolor visual detection of Se(IV). Due to its high oxidase-like (OXD-like) activity and sensitive response to H2Se, amorphous/crystalline manganese dioxide (ac-MnO2) was selected to form the headspace single droplet for microextraction and recognition. Via headspace redox reaction with H2Se, ac-MnO2 was reduced into low valence accompanied by in situ generation of Se nanoparticles, leading to the formation of a Se-MnOx aggregate. The experimental results and theoretical calculation indicated that, compared with MnO2, Se-MnOx had decreased active sites for adsorbing O2 to generate •O2-, resulting in the nanozyme activity inhibition that was totally dependent on Se(IV) concentration. The implementation of this strategy enabled accurate Se(IV) detection with a linear range from 10 to 600 μg L-1 and a limit of detection of 1.8 μg L-1. The portable smartphone-based detection for real sample analysis further demonstrated that this assay can be an easy, convenient, and intelligent tool for on-site selenium determination.

A colorimetric platform using highly active Prussian blue composite nanocubes for the rapid determination of ascorbic acid and acid phosphatase.

Cobalt-doped Prussian blue composite nanocubes (Co-PB NCs) were synthesized, which can quickly convert O2 to O2•- and 1O2. Due to the presence of cobalt and iron transition metal redox electron pairs, Co-PB NCs with high oxidase mimetic activity can rapidly oxidize the substrate 3,3',5,5'-tetramethylbenzidine (TMB) to produce blue products (ox-TMB) without the assistance of unstable H2O2. Using ascorbic acid-2-phosphate trisodium salt (AAP) as a substrate, it can be converted to reduced ascorbic acid (AA) under acid phosphatase (ACP) hydrolysis, resulting in suppression of TMB oxidation. Therefore, an enzyme cascade signal amplification strategy for rapid colorimetric detection of AA/ACP was developed based on the high-efficiency oxidase-like activity of Co-PB NCs combined with the hydrolysis effect of ACP. The color changes at low concentrations of AA and ACP could be observed by the naked eye, and the detection limits of AA and ACP were 1.67μM and 0.0266 U/L, respectively. The developed colorimetric method was applied to the determination of AA in beverages and ACP in human serum, and the RSDs were less than 3%, showing good reproducibility. This work provides a promising strategy for the use of metal-doped Prussian blue composite material for the construction of rapid colorimetric sensing platforms that avoid the use of unstable hydrogen peroxide.

High-loading ficin@AuNPs on polymer-UiO-66 surface with enhanced peroxidase-mimetic catalytic activity for colourimetric detection of dopamine.

Recently, MOFs@AuNPs composites-based catalysts via anchoring of AuNPs onto metal-organic-frameworks (MOFs) have attracted great attention. However, the influence of the AuNPs loading amounts on the catalytic activity of MOFs@AuNPs composites remains largely unexplored. Here, ficin (Fic) protected AuNPs (Fic@AuNPs) anchored onto the surface of UiO-66-NH2 (UiO) modified with poly(2-vinyl-4,4-dimethyl-2-oxazolidine) (PV) were designed and constructed. The UiOPVFic@AuNPs composites with longer PV chains leading to high-loading Fic@AuNPs exhibited intense peroxidase (POD)-mimetic activity in 3,3'5,5'-tetramethylbenzidine (TMB) oxidation. Further, following the colour-fading, dopamine (DA) was sensitively and selectively monitored in the composites-TMB-H2O2 system. The portable smartphone sensing platform-based colourimetric method had good linearity ranging from 3.34 to 36.7μM (R2 = 0.995), with a limit of detection of 0.3μM. This protocol explores high-loading AuNPs on polymer-MOFs composites, providing deep insights into understanding catalytic activity improvements of polymer-MOFs@AuNPs catalysts and revealing their application potential in real biological samples analysis.

In Situ Formed 3,3',5,5'-Tetramethylbenzidine-Cu(I/II) Charge-Transfer Complex Intermediates Promoting Colorimetric Assay of Cr6.

Developing a synthesis-free, multifunctional, and effective colorimetric assay system based on 3,3',5,5'-tetramethylbenzidine (TMB) oxidation is attractive yet challenging. Herein, we established a synergetic Cu2+/Cr6+-promoting strategy for TMB-based colorimetric detection of Cr6+. By introducing Cu2+, critical TMB·+···Cu(I/II)···TMB charge transfer complex (TMC) intermediates were in situ formed to reduce the activation energy of TMB oxidation, thereby accelerating Cr6+-mediated TMB oxidation. TMC intermediates also played a pivotal role in H2O2-participated TMB oxidation, clarifying the secondary responsibility of reactive oxygen species frequently caused by Fenton-like reactions. Leveraging the synergetic capacity between Cu2+ and Cr6+ for TMB oxidation, we demonstrated sensitive and specific colorimetric detections for Cr6+ with a limit of detection of 0.006 μM. With its convenient operation and rapid responsiveness, this strategy successfully enabled the practical detection of Cr6+ in real water samples. This work not only enhances the understanding of the internal acceleration mechanism in colorimetric sensing but also provides valuable opportunities to advance synthesis-free detection platforms.

Ferroelectric BaTiO3 nanoparticles as peroxidase mimics for colorimetric detection of glutathione S-transferase at physiological pH

The long-standing challenges in peroxidase-mimicking nanozymes catalysis lie in their generally lower catalytic activity in comparison to natural enzymes, as well as only exhibiting specific activities within acidic environments. Herein, BaTiO3 nanoparticles (BTO NPs) operated optimally at a physiological pH and exhibited excellent peroxidase-like activity with the catalytic constant (Kcat) up to 1.99×104 s⁻1 and 9.41×103 s⁻1 for 3,3′,5,5′-tetramethylbenzidine (TMB) and H2O2 substrate, respectively, which is fourfold and twofold that of horseradish peroxidase (4.00×103 s⁻1 and 3.48×103 s⁻1). Mechanism studies suggested that BTO NPs generated internal electric fields due to spontaneous polarization, which drove free carrier separation in different directions. The free carriers led to redox reactions with H2O2 and dissolved oxygen, which produced a variety of reactive oxygen species (ROS), such as•OH, 1O2 and O2•⁻, thus effectively oxidizing the chromogenic substrates. Furthermore, due to its strong reducing property, glutathione (GSH) can inhibit the oxidation of TMB. Conversely, glutathione S-transferase (GST) reduces the inhibition of GSH by facilitating the reaction between GSH and 1-chloro-2,4-dinitrobenzene. Ultimately, a colorimetric method was established for the detection of GST at physiological pH, with a linear range of 0.025 − 5.0 U·L⁻1. This work demonstrated the great promise of screening novel peroxidase mimics with super activities from ferroelectric materials.

Cu-MoOx-based nanozyme with enhanced peroxidase like activity for quinolone antibiotics detection

The abuse of antibiotics has seriously threatened human health and living environment. Nevertheless, the detection of quinolones is currently mainly performed by high-cost and cumbersome means, such as High-Performance Liquid Chromatography (HPLC). Herein, we reported a novel method based on copper-doped MoOx nanoparticles (Cu-MoOx NPs) with peroxidase-like enhancement activity for the easy preparation, sensitive and rapid visualization of quinolone detection. Cu-MoOx NPs can make the chromogenic substrate 3,3′,5,5′-Tetramethylbenzidine (TMB) change from colorless to blue. Moreover, the addition of quantitative quinolone antibiotics can significantly accelerate the TMB oxidation reaction. Based on this phenomenon, a colorimetric method for detecting quinolone antibiotics was established with a good linear relationship ranging from 1 × 10−6 M to 1.3 × 10−4 M, and the detection limit was 0.310 μM for ciprofloxacin (CIP) and 0.520 μM for levofloxacin (LVFX). Furthermore, the mechanism was also explored, and the results showed that the peroxidase-like activity of Cu-MoOx NPs was probably derived from the generated OH, 1O2, oxygen vacancies and partially reduced Cu+, and on the other hand was derived from quinolone antibiotics and nanozymes electrostatic interaction between them.

Highly sensitive assaying study based on three-dimensional graphene/cerium oxide nanoparticle nanozyme for the detecting of organophosphate pesticides in vegetable

As an important chemical pollutant affecting the safety of agricultural products, the on-site and efficient detection of pesticide residues has become a global trend and hotspot in research. Herein, this work present a highly sensitive assaying study based on three-dimensional graphene/cerium oxide nanoparticle nanozyme for the detection of organophosphate pesticides (OPs) in vegetable. The detection mechanism is because OPs can inhibit the activity of alkaline phosphatase (ALP). Cerium oxide nanoparticles exhibited intrinsic biomimetic oxidase activity and catalyzed the oxidation of the colorless 3,3′,5,5′-tetramethylbenzidine (TMB) into blue oxidized oxTMB. Upon the introduction of ALP, L-ascorbic acid-2-phosphate can be dephosphorylated to ascorbic acid, which aroused the disintegration of cerium oxide nanoparticles into Ce3+ ions. This disintegration weakened the enzyme-mimicking activity, thus impeding the oxidation of TMB. The absorbance at 652 nm exhibited a linear relationship with the concentration of phorate in the range of 0.15–50 μg/mL, generating a limit of detection of 0.13 μg/mL, Phorate recovery in the spiked samples for the absorption detection mode was 81.0 %–96.7 %. The relative standard deviations (RSD) of the absorption were in the range of 2.18 %–4.12 %. The results in terms of detecting pesticide residues collected from food samples via this method satisfactorily agree with those obtained via gas chromatography–mass spectrometry. This simple assay is therefore suitable for sensing pesticides in complex samples, especially in combination with other portable platforms.

Constructing novel metal-free HCOF-Ph@g-C3N4 heterojunctions through molecular expansion to enhance photogenerated carrier involved molecular oxygen activation and photocatalytic hydrogen evolution

Modification of covalent organic frameworks (COFs) have exceptional stability as well as tunable structures is the key to efficient photocatalysts. In this study, hyper-crosslinked COF-Ph was constructed with g-C3N4 to obtain a novel HCOF-Ph@g-C3N4 heterojunction, and the lamellar mesoporous of g-C3N4 was successfully embedded into the pores of 3D HCOF-Ph. The synthetic strategy greatly enhanced the photocatalytic properties and molecular oxygen activation capability of the catalyst. 10 mg of HCOF-Ph@g-C3N4 heterojunction could completely degrade 30 mg/L of tetracycline (TC) within 14 min, and the rate of hydrogen generation reached 9214.69 μmol g−1 h−1. The active species in this process of photocatalysis were verified through tests with free radical trapping and ESR spectra. In addition, the molecular oxygen activation capacity of heterojunction interface and the yield of superoxide radicals were examined by 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation and degradation of nitrotetrazolium nlue chloride (NBT) experiments, respectively. We found that the molecular expansion strategy can change molecule energy band structure to improve absorption of sunlight, and the efficient segregation of the photogenerated charge were ensured by the complementary energy band structure between g-C3N4 and HCOF-Ph. This study created an efficient molecular expansion method for the synthesis of COF-based catalysts with potential uses in the energy and environmental fields.

Fluorescent/colorimetric probe for the detection of Cr(Ⅵ) based on MIL-101(Fe)–NH2 with peroxidase-like activity

In the present research, Fe-based metal-organic frameworks (MIL-101(Fe)–NH2) nanoparticles were synthesized by simple solvothermal methods and used to assay Cr(Ⅵ). The MIL-101(Fe)–NH2 performs dual functions: the 2-aminoterephthalic acid (NH2-BDC) ligand endows a strong fluorescence emission, and the Fe metal nodes are able to facilitate the oxidation of 3,3′,5,5′- tetramethylbenzidine (TMB) directly, resulting in the generation of oxidized-TMB (ox-TMB). Our research results showed that reducing agents such as ascorbic acid (AA) can collapse the structures of MIL-101(Fe)–NH2 because of the reduction of Fe3+ by AA, resulting in release of NH2-BDC. In the presence of Cr(Ⅵ), the fluorescence intensity of the MIL-101(Fe)–NH2 + AA system will be decreased due to the competitive reduction of Fe3+ and Cr(Ⅵ). Nevertheless, Cr(Ⅵ) can significantly accelerate the oxidation of TMB by MIL-101(Fe)–NH2 as it boosts the electron transfer rate between Fe3+ and Fe2+. Therefore, a fluorescent/colorimetric dual-mode platform was developed for the detection of Cr(Ⅵ) with an extensive linear range (7.5–750 μg/L and 13.3–1000 μg/L) as well as a remarkably low detection limit (0.99 μg/L and 1.98 μg/L). This MOF with the ability to release ligands not only provides inspiration for the design of new luminescent materials, but also offers a novel and reliable solution for the detection of Cr(Ⅵ).

Synthesis of Plasmonic Copper/Copper Oxide Nanostructures as (Photo)Catalysts

Plasmonic nanostructures can enhance the photocatalytic efficiency of semiconductors under visible light irradiation. While the plasmonic-enhanced photocatalysis has been widely studied, rigorous efforts focus on designing novel noble metal nanostructures by altering particle size, shape, and composition to optimize their interaction with light to influence the catalytic properties. However, the need for a more cost-effective material as an alternative for noble metals has led us toward the exploration of copper and their oxides for photocatalysis. In this work, we synthesize different morphologies of copper nanostructures (i.e., 10 nm and 50 nm nanoparticles, as well as nanowires. Copper nanostructures are prone to be oxidized over time; however, the degree of oxidation varies with the size and shape of the nanostructure, thereby creating hybrid copper/copper oxide nanostructures with unique optical and (photo)catalytic properties. The hybrid nanostructures are characterized by X-ray powder diffraction and transmission electron microscopy. Their optical properties are studied by UV-vis spectroscopy along with simulation. We then further study the use of these hybrid nanostructures as (photo)catalysts for redox reactions – the reduction of 4-nitrophenol (4-NP) and the oxidation of tetramethylbenzidine (TMB). The (photo)catalytic properties will be correlated with the size, shape, and the degree of oxidation of these hybrid nanostructures. The mechanisms of both reactions on the hybrid copper/copper oxide (photo)catalysts will be discussed.

A colorimetric platform for sensitive sensing of Hg2+ and S2− based on Se-AuNPs with Hg2+-activated peroxidase-like activity

Herein, the selenium (Se) modified gold nanoparticles (Se-AuNPs) was synthesized using cerium doped carbon dots (Ce-CDs) as a reducing agent and template. As desired, Se-AuNPs displays enhanced peroxidase (POD)-like activity in the presence of Hg2+. The mechanism for the enhanced activity was attributed to the increased affinity between Se-AuNPs-Hg2+ and the substrate, in which Se and Au elements have a strong binding capacity to Hg2+, forming Hg–Se bonds and Au–Hg amalgam to generate more ·OH. This POD-like activity of Se-AuNPs-Hg2+ correlates with the colorimetric reaction by the catalytic reaction between 3,3′,5,5′-tetramethylbenzidine (TMB) and H2O2. The oxidation of TMB was completely inhibited by the introduction of the reductive S2−. Based on the above findings, a strategy for the colorimetric detection of Hg2+ and S2− by Se-AuNPs was established with linear ranges of 0.33–66 μg/L and 0.625–75 μg/L, and low detection limits of 0.17 μg/L and 0.12 μg/L (3.3 δ/k), respectively. When the colorimetric probes for detection of Hg2+ and S2− was applied in environmental water samples, the recoveries were in the range of 90.3–108.0 %. This method will provide a new idea for the colorimetric detection strategy of Hg2+ due to the strong interaction between Hg and Se.

Modulating Visible-Light Driven NIR Lanthanide Polymer Photocatalysis for Amplification Detection of Exosomal Proteins and Cancer Diagnosis.

Near-infrared (NIR) luminescent lanthanide materials hold great promise for bioanalysis, as they have anti-interference properties. The approach of efficient luminescence is sensitization through a reasonable chromophore to overcome the obstacle of the aqueous phase. The involvement of the surfactant motif is an innovative strategy to arrange the amphiphilic groups to be regularly distributed near the polymer to form a closed sensitized space. Herein, a lanthanide polymer (TCPP-PEI70K-FITC@Yb/SDBS) is designed in which the meso-tetra(4-carboxyphenyl)porphine (TCPP) ligand serves as both a sensitizer and photocatalytic switch. The surfactant sodium dodecyl benzenesulfonate (SDBS) wraps the photosensitive polymers to form a hydrophobic layer, which augments the light-harvesting ability and expedites its photocatalysis. TCPP-PEI70K-FITC@Yb/SDBS is subsequently applied as an amplified photocatalysis toolbox for universally regulating the generation of reactive oxygen species (ROS). Boosting 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to produce blue products, a dual-mode biosensor is fabricated for improving the diagnosis of programmed death ligand-1-positive (PDL1) cancer exosomes. Exosomes were captured by Fe3O4 modified by the PDL1 aptamer, enabling replacement of alkaline phosphatase (ALP)-labeled multiple hybridized chains; then, the isolated ALP triggered a hydrolysis reaction to block the generation of oxTMB. Detection sensitivity improves by 1 order of magnitude through SDBS modulation, down to 104 particles/mL. The sensor performed well clinically in distinguishing cancer patients from healthy individuals, expanding physiological applications of near-infrared lanthanide luminescence.

Facile synthesis of copper carbonate analog with peroxidase-like activity for colorimetric detection of isoniazid

In this article, copper carbonate analog with good peroxidase-like activity was successfully synthesized for the first time via a simple co-precipitation of CuSO4▪5H2O and Na2CO3. The obtained copper carbonate analog exhibited excellent intrinsic peroxidase-like activity towards a classical peroxidase substrate of 3, 3′, 5, 5′ -tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H2O2) under an acidic environment. The study of the catalytic mechanism confirmed that the hydroxyl radical produced from the decomposition of H2O2 is the main reactive oxygen species responsible for the catalytic oxidation of TMB to oxTMB. Moreover, results from kinetic parameter analysis indicated that H2O2 was more easily and/or likely to attach to the copper carbonate analog than TMB. Subsequently, the effects of experimental conditions (buffer pH, temperature, and incubation time) on the catalytic activity of the copper carbonate analog were also optimized. Finally, a copper carbonate analog-based colorimetric sensor was developed to determine isoniazid. Under the optimal conditions, the linear range for isoniazid was as broad as 0–178.6 μM, and the detection limit was as low as 8.47 μM. The spiked recoveries of isoniazid in normal human serum has been observed in the range of 94.8%–105.5 %. This strategy focuses on the development of a green, cost-efficient peroxidase mimic with high activity, good biocompatibility, and a simple synthesis process.

Smartphone assisted paper strip-based colorimetric sensing of phosphate and copper ions utilizing bi-ligand intercalated cobalt-MOF as a dual functional nanozyme

Monitoring phosphate and copper ions is vital for environmental and food safety due to associated health risks. To address this need, a new cobalt and bi-organic ligand based metal organic framework (BDC-Co-BIm MOF) has been developed. This MOF acts as a dual-function nanozyme, displaying peroxidase-like activity and selective detection abilities for PO43- ion. The Co(II) nodes and sp2 carbon within the framework exhibit excellent Fenton-like reaction, efficiently catalyzing the conversion of 3,3′,5,5′-tetramethylbenzidine (TMB) into oxidised TMB (oxTMB). Also, steady-state kinetic analysis indicated that BDC-Co-BIm MOF exhibited high affinity toward both TMB and H2O2. The Co(II) nodes and positive charge of the BDC-Co-BIm MOF capture PO43- ion through Co-O-P bonding and electrostatic interaction, limiting the electron transfer between BDC-Co-BIm MOF and H2O2. This reduces TMB oxidation and changes the color from blue to colorless. When ionic Cu2+ ion are added to the system, they react with PO43- ion to form a stable Cu3(PO4)2 complex. This reactivates the electron transfer process, allowing the H2O2 to decompose into •OH. As a result, TMB oxidation is restored and producing the blue color product again. Based on these principles, a colorimetric assay has been developed for the rapid and selective detection of PO43- and Cu2+ ions. A strong linear correlation has been established between their concentrations and the UV absorbance at 652 nm, with limit of detection (LOD) of PO43- and Cu2+ was 0.05 and 0.13 µM, respectively. These limits are notably lower than the thresholds established by the Environmental Protection Agency and World Health Organization (EPA & WHO) at 0.18 µM for PO43- and 30 µM for Cu2+. A Paper-based smartphone-assisted sensing of PO43- and Cu2+ ions was successfully achieved using intelligent Red, Green, and Blue (RGB) analysis. The platform showed high selectivity and performance with environmental water samples. This research presents a novel method for visually detecting PO43- and Cu2+ ions using versatile nanozyme, advancing portable on-site environmental safety analysis.

Nanozyme-Inhibited SERS Multichannel Paper-Based Sensor Array for the Quantification and Identification of Biothiols and Cancer Cells Based on Three Ag-Based Nanomaterials.

Biothiols play essential roles in maintaining normal physiological functions, resisting oxidative stress, and protecting cell health. Establishing an effective and reliable sensor array for the accurate quantification and discrimination of diverse biothiols is extremely meaningful. In this work, Ag/Mn3O4, Ag3PO4, and Ag3Cit with excellent oxidase-mimetic activity and surface-enhanced Raman scattering (SERS)-enhanced features have been prepared and loaded onto Whatman filter paper (WFP) to build SERS paper chips as three sensing channels, which can induce 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to SERS-active reporters (TMBox) and concurrently generate prominent SERS signals. Nevertheless, the addition of biothiols can suppress conversion from TMB to TMBox, which can cause the reduction of the SERS signal from TMBox. Interestingly, each SERS sensing channel can generate different TMBox signals' variations due to differences in the oxidative inhibition abilities of diverse biothiols and exclusive properties of each paper chip, which can be plotted as specific fingerprint patterns of each biothiol and further translated into intuitive two-dimensional (2D) clustering profiles through linear discriminant analysis (LDA) and hierarchical cluster analysis (HCA) techniques for precise identification of these six biothiols with the minimum concentration of 1 μM. More importantly, this SERS sensor array is exploited for the precise quantification of intracellular glutathione (GSH), and can differentiate between normal and cancer cells based on different intracellular GSH contents and even identify different types of tumor cells, demonstrating its powerful application prospects in disease diagnosis.

Electrochemical Lateral Flow Immunoassay with Built-In Electrodes for Ultrasensitive and Wireless Detection of Inflammatory Biomarkers.

Paper-based lateral flow immunoassays (LFIAs) are cost-effective, portable, and simple methods for detection of diverse analytes, which however only provide qualitative or semiquantitative results and lack sufficient sensitivity. A combination of LFIA and electrochemical detection, namely, electrochemical lateral flow immunoassay (eLFIA), enables quantitative detection of analytes with high sensitivity, but the integration of external electrodes makes the system relatively expensive and unstable. Herein, the working, counter, and reference electrodes were prepared directly on the nitrocellulose membrane using screen printing, which remarkably simplified the structure of eLFIA and decreased the cost. Moreover, a horseradish peroxidase (HRP)-based electrochemical signal amplification strategy was used for further increasing the analytical sensitivity. HRP captured on the working electrode can catalyze the oxidation of tetramethylbenzidine (TMB) to form the TMB-TMBox precipitate on the electrode surface, which as an electrochemically active product can output an amplified current for quantification. We demonstrated that the eLFIA could detect low-abundant inflammatory biomarkers in human plasma samples with limits of detection of 0.17 and 0.54 pg mL-1 for interleukin-6 and C-reactive protein, respectively. Finally, a fully portable system was fabricated by integrating eLFIA with a flexible and wireless electrochemical workstation, realizing the point-of-care detection of interleukin-6.

Colorimetric Immunoassays with Boronic Acid-Decorated, Peroxidase-like Metal-Organic Frameworks as the Carriers of Antibodies and Enzymes.

The sensitivity of immunoassays is generally limited by the low signal reporter/recognition element ratio. Nanomaterials serving as the carriers can enhance the loading number of signal reporters, thus improving the detection sensitivity. However, the general immobilization strategies, including direct physical adsorption and covalent coupling, may cause the random orientation and conformational change in proteins, partially or completely suppressing the enzymatic activity and the molecular recognition ability. In this work, we proposed a strategy to load recognition elements of antibodies and enzyme labels using boronic acid-modified metal-organic frameworks (MOFs) as the nanocarriers for signal amplification. The conjugation strategy was proposed based on the boronate ester interactions between the carbohydrate moieties in antibodies and enzymes and the boronic acid moieties on MOFs. Both enzymes and MOFs could catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H2O2, therefore achieving dual signal amplification. To indicate the feasibility and sensitivity of the strategy, colorimetric immunoassays of prostate specific antigen (PSA) were performed with boronic acid-modified Cu-MOFs as peroxidase mimics to catalyze TMB oxidation and nanocarriers to load antibody and enzyme (horseradish peroxidase, HRP). According to the change in the absorbance intensity of the oxidized TMB (oxTMB), PSA at the concentration range of 1~250 pg/mL could be readily determined. In addition, this work presented a site-specific and oriented conjugation strategy for the modification of nanolabels with recognition elements and signal reporters, which should be valuable for the design of novel biosensors with high sensitivity and selectivity.

Multi-mode sensing platform for gallic acid determination based on the peroxidase-like activity of magnetic Fe3O4@Co/Ni/Al-LDH nanospheres

The determination of gallic acid (GA), one of the most prevalent antioxidants in various plants, provides information for food nutrition evaluation. A novel colorimetric, fluorescent, and smartphone multi-mode GA sensing platform based on Fe3O4@Co/Ni/Al-layered double hydroxide (LDH) nanospheres was designed in this study. The core-shell magnetic Fe3O4@Co/Ni/Al-LDH nanospheres were synthesized through a simple co-precipitation approach. They showed dominate peroxidase-like activities capable of oxidizing 3,3′,5,5′-tetramethylbenzidine (TMB) into a blue colored oxidation oxTMB. Meanwhile, the generated oxTMB could quench the fluorescence of neutral red via the inner filter effect. GA acted as an inhibitor to prevent the oxidation of TMB to oxTMB and restoring the fluorescence of neutral red. Under optimized conditions, the linear ranges were 0.5–120 μmol/L with detection limits of 0.137–0.415 μmol/L. Besides, the sensor exhibited good selectivity toward GA over various interferents. Moreover, it was successfully utilized to analyze GA in tea beverages with ideal recoveries of 92.8–105%, demonstrating good reliability and stability in practical applications. This study provided Fe3O4@Co/Ni/Al-LDH nanospheres as candidate nanozymes for the construction of biosensors. The multi-mode self-checking function and the “turn on” fluorescent sensing strategy offered this sensor superior anti-interference ability, and opens up a new strategy for the determination of antioxidants in foods.

Construct a Magnetic Pt/Ru Alloy Peroxidase Mimic As a Reusable and Cost-Effective "Signal-Off" Sensing Platform for Sensitive and Wide-Linear-Range Assay.

"Signal-off" nanozyme sensing platforms are usually employed to detect analytes (e.g., ascorbic acid (AA) and alkaline phosphatase (ALP)), which are mostly based on oxidase (OXD) nanozymes. However, their drawbacks, like dissolved oxygen-dependent catalysis capability, relatively low enzyme activity, limited amount, and kind, may not favor sensing platforms' optimization. Meanwhile, with the need for sustainable development, a reusable "signal-off" sensing platform is essential for cutting down the cost of the assay, but it is rarely developed in previous studies. Magnetic peroxidase (POD) nanozymes potentially make up the deficiencies and become reusable and better "signal-off" sensing platforms. As a proof of concept, we first construct Fe3O4@polydopamine-supported Pt/Ru alloy nanoparticles (IOP@Pt/Ru) without stabilizers. IOP@Pt/Ru shows high POD activity with Vmax of 83.24 × 10-8 M·s-1 for 3,3',5,5'-Tetramethylbenzidine (TMB) oxidation. Meanwhile, its oxidation rate for TMB is slower than the reduction of oxidized TMB by reducers, favorable for a more significant detection signal. On the other hand, IOP@Pt/Ru possesses great magnet-responsive capability, making itself be recycled and reused for at least 15-round catalysis. When applying IOP@Pt/Ru for AA (ALP) detection, it performs better detectable adaptability, with a linear range of 0.01-0.2 mM (0.1-100 U/L) and a limit of detection of 0.01 mM (0.05 U/L), superior to most of OXD nanozyme-based ALP sensing platform. Finally, IOP@Pt/Ru's reusable assay was demonstrated in real blood samples for ALP assay, which has never been explored in previous studies. Overall, this study develops a reusable "signal-off" nanozyme sensing platform with superior assay capabilities than traditional OXD nanozymes, paves a new way to optimize nanozyme-based "signal-off" sensing platforms, and provides an idea for constructing inexpensive and sustainable sensing platforms.

Engineering nickel-supported osmium bimetallic nanozymes with specifically improved peroxidase-like activity for immunoassay

Researchers have shown significant interest in modulating the peroxidase-like activity of nanozymes. Among these, bimetallic nanozymes have shown superior peroxidase-like activity over monometallic counterparts, offering enhanced performance and cost-efficiency in nanozyme designs. Herein, bimetallic nanozymes comprising nickel (Ni) and osmium (Os) incorporated into hyaluronate (HA) have been developed, resulting in HA-Nin/Os nanoclusters. Subsequently, comprehensive characterizations have been conducted. Further investigation has revealed that HA-Nin/Os efficiently catalyzed 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation with hydrogen peroxide (H2O2), confirming its peroxidase-like behavior and role as a nanozyme. Impressively, HA-Ni2/Os (Ni/Os = 2:1) displays heightened substrate affinity, accelerated reaction rates, enhanced hydroxyl radical production in acidic conditions, and exhibits activity unit of 1224 U/mg, representing more than two-fold increase compared to non-Ni-supported Os nanozyme. Theoretical calculations indicate that Ni support enhances the peroxidase-like process of Os nanozyme by improving H2O2 adsorption and TMB oxidation. Crucially, the support of Ni does not significantly alter the other enzyme-like activities of Os nanozymes, thereby enabling Ni to selectively enhance their peroxidase-like activity. In terms of application, the peroxidase-like ability of HA-Ni2/Os, facilitated by HA's carboxyl groups enabling crosslinking, proves effective in a squamous carcinoma antigen immunoassay. Moreover, HA-Ni2/Os exhibit reliable stability, promising as a peroxidase substitute. This work underscores the advantages of incorporating Ni into Os, specifically enhancing peroxidase-like activity, highlighting the potential of Os bimetallic nanozymes for peroxidase-based applications.