Polyoxometalate-based iron-organic complex nanozymes with peroxidase-like activities for colorimetric detection of hydrogen peroxide and ascorbic acid

As a new type of artificial enzyme, a nanozyme is an ideal substitute for natural enzymes and has been successfully applied in many fields. However, in the application of biomolecular detection, most nanozymes have the disadvantages of long reaction times or high detection limits, prompting researchers to search for new efficient nanozymes. In this work, the enzyme-like activities of three polyoxometalate-based iron-organic complexes ([Fe(bpp)2](Mo6O19), [Fe(bpp)2]2(Mo8O26)·2CH3OH, and [Fe(bpp)2]4H[Na(Mo8O26)]3), namely, FeMo6, Fe2Mo8, and Fe4Mo8Na, were analyzed. All three polyoxometalate-based iron-organic complexes were found to be capable of catalyzing hydrogen peroxide (H2O2) to oxidize 3,3',5,5'-tetramethylbenzidine and o-phenylenediamine, resulting in visible color changes, further exhibiting peroxidase-like activity. Results showed that Fe4Mo8Na had more active sites due to its long chain structure, endowing more prominent peroxidase-like activity compared with Fe2Mo8 and FeMo6. A colorimetric sensing platform for H2O2 and ascorbic acid detection based on Fe4Mo8Na was established. The linear response range for H2O2 detection was 0.5-100 μM, and the detection limit was 0.143 μM. The linear response for ascorbic acid detection ranges from 0 to 750 μM with a detection limit of 1.07 μM. This study provides a new perspective for developing new nanozymes and expanding the sensing and detection application of nanozymes.

Synthesis of a Zn(II)-2-aminoimidazole Framework as an Efficient Carbonic Anhydrase Mimic

Carbonic anhydrase (CA) plays a crucial role in the CO2 capture processes by catalyzing the hydration of CO2. In this study, we synthesized a bioinspired carbonic anhydrase Zn-MOF (metal-organic framework) incorporating 2-aminoimidazole and Zn2+ as initial constituents. The synthesized Zn-MOF exhibited promising potential for efficiently catalyzing the CO2 hydration. Structural analyses such as SEM, XRD, and BET confirmed that the Zn-MOF crystal consisted of stacked grains with an average size of approximately 36 nm, forming a micron-sized spherical structure. Functionally, Zn-MOF exhibited effective catalytic activity toward both CO2 hydration and ester hydrolysis. The introduction of amino groups significantly enhanced the esterase activity of Zn-MOF to 0.28 U/mg at ambient temperature, which was twice that of ZIF-8. Furthermore, the introduction of amino groups resulted in remarkable hydrothermal stability, with the esterase activity reaching 0.72 U/mg after undergoing hydrothermal treatment at 80 °C for 12 h. Additionally, Zn-MOF exhibited enhanced capability in CO2 hydration at a pH value exceeding 8.5. After six repeated uses, ZIF-8 and Zn-MOF retained approximately 68 and 65% of their initial enzyme activity, respectively, underscoring the potential practical applicability of Zn-MOF in industrial CO2 capture processes. This work showcases the development of a novel Zn-MOF crystal as an efficient CA mimic, effectively emulating the active sites of natural CA using 2-aminoimidazole as a coordinating ligand for Zn2+ coordination. These findings not only advance the field of innovative enzyme mimics but also pave the way for further exploration of industrial CO2 capture catalysts.

Shape-Regulated Photothermal-Catalytic Tumor Therapy Using Polydopamine@Pt Nanozymes with the Elicitation of an Immune Response

Recently, nanozyme-based photothermal-catalytic therapy has emerged as a promising strategy for antitumor treatment. Extensive research has focused on optimizing the catalytic activity and photothermal conversion performance of nanozymes through size, morphology, and surface property regulations. However, the biological effects of nanozymes, such as cellular uptake and cytotoxicity, resulting from their physicochemical properties, remain largely unexplored. In this study, two types of polydopamine/platinum (PDA@Pt) nanozymes, flower-like (FPDA@Pt) and mesoporous spherical-like (MPDA@Pt), to comprehensively compare their enzyme-mimicking activity, photothermal conversion capacity, and antitumor efficiency are designed. These findings revealed that FPDA@Pt exhibited superior peroxidase-like activity and higher photothermal conversion efficiency compared to MPDA@Pt. This led to enhanced production of reactive oxygen species (ROS) and increased heat generation at tumor sites. Importantly, it is observed thatthe flower-like structure of FPDA@Pt facilitated enhanced cellular uptake, leading to an increased accumulation of nanozymes within tumor cells. Furthermore, the light irradiation on tumors also triggered a series of anti-tumor immune responses, further enhancing the therapeutic efficacy. This work provides a possible design orientation for nanozyme-based photothermal-catalytic tumor therapy, highlighting the importance of considering the physicochemical properties of nanozymes to optimize their therapeutic potential in antitumor strategies.

Dual-Mode Lateral Flow Immunoassay Based on "Pompon Mum"-Like Fe3O4@MoS2@Pt Nanotags for Sensitive Detection of Viral Pathogens

Lateral flow immunoassay (LFIA) has been widely used for the early diagnosis of diseases. However, conventional colorimetric LFIA possesses limited sensitivity, and the single-mode readout signal is easily affected by the external environment, leading to insufficient accuracy. Herein, multifunctional Fe3O4@MoS2@Pt nanotags with a unique "pompon mum"-like structure were triumphantly prepared, exhibiting excellent peroxidase (POD)-like activity, photothermal properties, and magnetic separation capability. Furthermore, the Fe3O4@MoS2@Pt nanotags were used to establish dual-mode LFIA (dLFIA) for the first time, enabling the catalytic colorimetric and photothermal dual-mode detection of severe acute respiratory syndrome coronavirus 2 nucleocapsid protein (SARS-CoV-2 NP) and influenza A (H1N1). The calculated limits of detection (cLODs) of SARS-CoV-2 NP and H1N1 were 80 and 20 ng/mL in catalytic colorimetric mode and 10 and 8 ng/mL in photothermal mode, respectively, demonstrating about 100 times more sensitive than the commercial colloidal Au-LFIA strips (1 ng/mL for SARS-CoV-2 NP; 1 μg/mL for H1N1). The recovery rates of dLFIA in simulated nose swab samples were 95.2-103.8% with a coefficient of variance of 2.3-10.1%. These results indicated that the proposed dLFIA platform showed great potential for the rapid diagnosis of respiratory viruses.

NIR-induced improvement of catalytic activity and antibacterial performance over AuAg nanorods in Rambutan-like Fe3O4@AgAu@PDA magnetic nanospheres

Pathogenic bacteria and difficult-to-degrade pollutants in water have been serious problems that always plague people. Therefore, finding a "one stone-two birds" method that can quickly catalyze the degradation of pollutants and show effective antibacterial behavior become an urgent requirement. This work reports a facile one-step strategy for fabricating a Rambutan-like Fe₃O₄@AgAu@PDA (Fe₃O₄@AgAu@Polydopamine) core/shell nanosphere with both catalytic and antibacterial activities which can be critically improved by externally applying an NIR laser irradiation (NIR, 808 nm) and a rotating magnetic field. Typically, the Rambutan-like Fe₃O₄@AgAu@PDA nanosphere have a rather rough surface due to the AuAg bimetallic nanorods sandwiched between the Fe₃O₄ core and the PDA shell. Owing to the penetrated PDA shell, AgAu nanorods show high and magnetically recyclable photothermal-enhanced catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol and they can also be applied to initiate TMB oxidation under the help of NIR heating condition. Moreover, Fe₃O₄@AgAu@PDA shows a moderate antibacterial activity due to the weak release of Ag⁺. Under applying a rotating external magnetic field, the rough-surface Fe₃O₄@AgAu@PDA nanospheres produce a controllable magnetolytic force on the bacterial due to their good affinity. As a result, the Fe₃O₄@AgAu@PDA nanospheres show a "magnetolytic-photothermal-Ag⁺" synergistic antibacterial behavior against E. coli and S. aureus.

Facile preparation of Fe3O4@Pt nanoparticles as peroxidase mimics for sensitive glucose detection by a paper-based colorimetric assay

A simple strategy to rapidly detect glucose was developed by utilizing core (Fe₃O₄)-shell (Pt) magnetic nanoparticles (Fe₃O₄@Pt NPs) as a nanoenzyme and a paper-based colorimetric sensor. In the presence of H₂O₂, Fe₃O₄@Pt NPs catalyze the redox reaction of 3,3',5,5'-tetramethylbenzidine (TMB) and generate a colour change from colourless to blue. On this basis, a colorimetric glucose sensing method assisted by glucose oxidase (GOx) was developed. Under the optimal conditions, the detection limits of the proposed assay for H₂O₂ and glucose were 0.36 µM and 1.27 µM, respectively. Furthermore, the fabricated colorimetric method was successfully applied to analyze glucose concentrations by using a paper device as a measuring platform without a spectrometer. In addition, this method exhibited satisfactory recovery for glucose detection in human serum samples and urine samples, which satisfied the requirements for normal detection of real samples. This study provides a good candidate for health monitoring of glucose and also expands the applications of nanoenzymes and paper-based colorimetric assays in point-of-care testing.

Application of Au or Ag nanomaterials for colorimetric detection of glucose

In recent years, Au and Ag nanomaterials have been widely used in the determination of glucose owing to their specific properties such as large specific surface area, high extinction coefficient, strong localized surface plasmon resonance effect and enzyme-mimicking activity. Compared with other methods, colorimetric determination of glucose with Au or Ag nanomaterials features the advantages of simple operation, low cost and easy observation. In this review, several typical synthesis methods of Au and Ag nanomaterials are introduced. Strategies for the colorimetric determination of glucose by Au or Ag nanomaterials are elaborated. The challenges and prospects of the application of Au or Ag nanomaterials for colorimetric detection of glucose are also discussed.

Three-dimensional MoS2 nanoflowers supported Prussian blue and Au nanoparticles: A peroxidase-mimicking catalyst for the colorimetric detection of hydrogen peroxide and glucose

Well-dispersed Prussian blue (PB) and Au nanoparticles (Au NPs) loaded three dimensional MoS₂ nanoflowers (PB-Au@MoS₂ NFs) was synthesized by a simple and economical method. The structure, morphology and composition of the hybrid were characterized by XRD, SEM and EDS. Similar to the reported literature, MoS₂ nanoflowers showed peroxidase-like activity in catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). This peroxidase-mimicking activity could be enhanced with the introduction of PB and Au NPs. Herein, PB-Au@MoS₂ NFs could be used to establish a new platform for the determination of H₂O₂ and glucose by the chromogenic reaction. Wide linear ranges with 0-15 μM and 0-120 μM for H₂O₂ and glucose detection were finally obtained. The detection limits were as low as 0.25 μM and 3 μM (with signal to noise ratio of 3), respectively. The established platform was also used successfully for the determination of glucose in human serum and fruit juice samples with excellent sensitivity and stability.

Layer-by-Layer Electrodeposition of FTO/TiO2 /Cux O/CeO2 (1 < x < 2) Photocatalysts with High Peroxidase-Like Activity by Greatly Enhanced Singlet Oxygen Generation

Inorganic nanomaterials have attracted much attention as enzyme mimics because of simple and stable spatial conformation of those artificially synthesized nanocatalysts. Cu₂ O, as an important kind of narrow band gap semiconductor, is identified as effective as visible-light-driven photocatalysts, which can catalyze decomposition of H₂ O₂ into reactive oxygen species. Moreover, after forming Cu_x O/CeO₂ hybrids, the strongly coupled interface between the two components will further improve their catalytic performance. In this paper, the authors try to construct FTO/TiO₂ /Cu_x O/CeO₂ (1 < x < 2) nanohybrids with such a kind of active interface via a layer-by-layer electrodeposition strategy by aid of the following surface etching process. It is found that FTO/TiO₂ /Cu_x O/CeO₂ exhibits good peroxidase mimic activity in the dark but much better performance under visible light irradiation (λ ≥ 420 nm) during catalytic oxidation of 3,3',5,5'-tetramethylbenzidine substrates in the presence of H₂ O₂ . Detailed characterizations disclose that the construction of TiO₂ /Cu₂ O pn-heterojunctions do effectively accelerate separation of photogenerated carriers, and the formation of a highly active Cu_x O/CeO₂ interface is synergistically favorable for selectively generating singlet oxygen to boost the catalytic performance of FTO/TiO₂ /Cu_x O/CeO₂ .

GOX-hemin nanogels with enhanced cascade activity for sensitive one-step glucose detection

Early glucose detection is important in both healthy people and diabetic patients. The glucose colorimetric detection techniques usually consist of multiple steps and their preparation processes are time consuming. In this work, we fabricate a GOX-hemin nanogel (GHN) that could be used for one-step colorimetry detection of glucose. The GHN was prepared by carrying out polymerization on the surface of GOX. Each GOX-hemin nanogel consists of a single GOX encapsulated with a thin polymer network containing hemin. The proximity of hemin to GOX facilitates two reactions, i.e. the oxidation of glucose catalysed by GOX to yield H₂O₂, and the subsequent 3,3',5,5'-tetramethylbenzidine (TMB) oxidation reaction catalysed by hemin to yield the blue colored product. These processes work in tandem, which greatly enhances the efficacy, sensitivity and stability of the detection system. The limit of detection in our system was determined to be as low as 4 μM. Furthermore, the glucose detection activity still maintained more than 70% even after being incubated at 55 °C for 30 minutes, or in 20% (v/v) aqueous solution of DMF, CH₃CN or THF for 25 minutes at room temperature. It is anticipated that this work can provide a method for developing diverse functional materials based on proteins.

A non-enzymatic electrochemical biosensor based on Au@PBA(Ni-Fe):MoS2 nanocubes for stable and sensitive detection of hydrogen peroxide released from living cells

Hydrogen peroxide (H2O2) is the main product of enzymatic reactions and plays an important role in biological processes. The detection of H2O2 inside organisms or cells is critical. Here, we report a nickel-iron Prussian blue analogue nanocube doped with molybdenum disulfide and Au nanoparticles (Au@PBA(Ni-Fe):MoS2) as an electrochemical sensing material for the stable detection of H2O2 in neutral solutions for a long time. First, the Prussian blue analogue (PBA(Ni-Fe)) is synthesized by a simple charge-assembly technology, and then etched into PBA(Ni-Fe):MoS2 hollow nanocubes by a high-temperature hydrothermal reaction. Finally, Au nanoparticles are reduced inside the PBA(Ni-Fe):MoS2in situ to generate Au@PBA(Ni-Fe):MoS2 nanocubes. Ni-doping enhances the nanocube's stability in neutral solutions; as a result, the sensor can maintain a stable current response towards H2O2 reduction for more than 1 h. The sensing material can meet the needs of a long-time test. The introduction of Au enhances the electron transfer efficiency, which endows the sensor with good reduction ability for H2O2 at 0 V over a wide linear range (0.5-200 μM and 210-3000 μM) and with a low detection limit (0.23 μM (S/N = 3)), which fulfills the requirements for the detection of H2O2 in a biological system. The sensor can sense H2O2 released from cells stimulated by ascorbic acid. Au@PBA(Ni-Fe):MoS2 provides good guidance for the future development of efficient biosensors to be applied in cell biology.

Zero-Dimensional/Two-Dimensional AuxPd100-x Nanocomposites with Enhanced Nanozyme Catalysis for Sensitive Glucose Detection

Here, we report facile fabrication of two-dimensional (2D) Pd nanosheet (NS)-supported zero-dimensional (0D) Au nanoparticles via galvanic replacement. In the synthesis, the surface-clean Pd NSs premade not only acted as a sacrifice template for replacing Pd atoms by Au³⁺ ions, but served as a support substrate to support Au nanoparticles. The morphology, structure, and composition of products relied on the Au/Pd feed atomic ratio. Interestingly, the as-obtained 0D/2D Au_xPd_100-x (x = 4.5, 9.8, and 21) nanocomposites showed remarkably enhanced peroxidase-mimic catalysis in the model oxidation reaction, which followed the typical Michaelis-Menten theory. Compared to Pd NSs, the enhanced catalysis of Au_xPd_100-x was closely related to both the increased specific surface area and the modified electronic structure of Pd NSs, which resulted in a change in the catalytic pathway, that is, from hydroxyl radical generation to rapid electron transfer. The work provides a simple yet efficient avenue to build highly efficient heterogeneous catalysts based on metallic NSs, as exemplified by the superior nanozyme activity of 0D/2D bimetallic nanostructures for glucose detection.

The design and characterization of a hypersensitive glucose sensor: two enzymes co-fixed on a copper phosphate skeleton

A new glucose sensor GOx&DhHP-6–Cu₃(PO₄)₂ showed the best catalytic ability at a neutral temperature and pH.

Accelerating the peroxidase-like activity of MoSe2 nanosheets at physiological pH by dextran modification

One-pot synthesis of dextran-modified MoSe₂ nanosheets with peroxidase-like activity at physiological pH for bio-sensing.