Fabrication of Nanoporous Nanocomposites Entrapping Fe 3 O 4 Magnetic Nanoparticles and Oxidases for Colorimetric Biosensing

Layered Architectural Fabrication of a Novel Sulfite Nanobiosensor by Encapsulation of Sulfite Oxidase on a Polypyrrole-Multiwalled Carbon Nanotubes Composite Decorated with Platinum Nanoparticles

The fabrication of a highly selective and ultrasensitive sulfite nanobiosensor based on a layered architectural fabrication aided by the encapsulation of sulfite oxidase (SOx) in Nafion (Naf) matrix on a multiwalled carbon nanotubes-polypyrrole (MWCNTs-PPy) composite decorated with platinum nanoparticles (PtNPs) is described. The MWCNTs are deposited in the inner layer on a Pt electrode during electropolymerization of pyrrole (Py), followed by decoration with a PtNPs layer and subsequent encapsulation of SOx with Naf in the third layer capped with a fourth thin PtNPs layer. Images obtained by field emission scanning electron microscopy (FESEM) reveal that high-density PtNPs are deposited onto the 3D nanostructured inner MWCNTs-PPy layer and the electrochemical behavior is investigated. A large surface area provided by the incorporation of MWCNTs in the composite and decoration with PtNPs enables increased SOx loading, SOx retention, and substantial improvement in sensing performance. The resulting layered PtNPs/SOx-Naf/PtNPs/MWCNTs-PPy nanobiosensor exhibits a fast response time (within 3 s), a linear calibration range of 20 nmm - 6 m with an excellent sensitivity of 71 µA mm-1  cm-2 and a detection limit of 5.4 nm. The nanobiosensor  was effective in discriminating against common interferants and  was successfully applied to sulfite determination in real samples.

Recent Developments on the Catalytic and Biosensing Applications of Porous Nanomaterials

Nanoscopic materials have demonstrated a versatile role in almost every emerging field of research. Nanomaterials have come to be one of the most important fields of advanced research today due to its controllable particle size in the nanoscale range, capacity to adopt diverse forms and morphologies, high surface area, and involvement of transition and non-transition metals. With the introduction of porosity, nanomaterials have become a more promising candidate than their bulk counterparts in catalysis, biomedicine, drug delivery, and other areas. This review intends to compile a self-contained set of papers related to new synthesis methods and versatile applications of porous nanomaterials that can give a realistic picture of current state-of-the-art research, especially for catalysis and sensor area. Especially, we cover various surface functionalization strategies by improving accessibility and mass transfer limitation of catalytic applications for wide variety of materials, including organic and inorganic materials (metals/metal oxides) with covalent porous organic (COFs) and inorganic (silica/carbon) frameworks, constituting solid backgrounds on porous materials.

Immobilization of horseradish peroxidase on lysine-functionalized gum Arabic-coated Fe3O4 nanoparticles for cholesterol determination

BACKGROUND

Horseradish Peroxidase (HRP) is ranked as one of the most important industrial enzymes that is extensively used in industry. Cholesterol is routinely detected indirectly by cholesterol oxidase in the presence of O₂, liberating H₂O₂ as a by-product. The H₂O₂ content is determined through the HRP activity in the presence of a redox dye, producing a red colored quinoneimine which can be measured quantitatively. Herein, we have designed a magnetic nanoparticle for reusing and easily separating HRP as the most expensive compartment for the low-cost cholesterol assay.

METHODS

The gum Arabic coated magnetic nanoparticles were functionalized with L-lysine linker for maintaining protein flexibility on nanoparticle. Enzyme-loaded nanoparticles were characterized by TEM, FTIR, DLS, VSM and XRD analysis.

RESULTS

The immobilization efficiency was ∼65% and the immobilized HRP retained 60% of its activity after 8 times reuse. The optimum pH and thermal stability shifted from 7.0 to 8.0 and 60 to 70 °C after immobilization, respectively. Storage stability of HRP was improved by 10%, at 4 °C for 60 days. Immobilized HRP showed more catalytic activity in presence of Fe²⁺, Ca²⁺ and Na⁺. The designed system has cholesterol detection linearity range from 0.2 to 5.0 mM and detection limit of 0.08 mM and acceptable correlation coefficient of 0.9973 and 0.9982 on sample serum using both chromogens.

CONCLUSION

The HRP-loaded magnetic nanoparticles are capable of being used as a cost-effective system for cholesterol determination in laboratory due to its reusability and stability benefits.

Nanozymes in Point-of-Care Diagnosis: An Emerging Futuristic Approach for Biosensing

Nanomaterial-based artificial enzymes (or nanozymes) have attracted great attention in the past few years owing to their capability not only to mimic functionality but also to overcome the inherent drawbacks of the natural enzymes. Numerous advantages of nanozymes such as diverse enzyme-mimicking activities, low cost, high stability, robustness, unique surface chemistry, and ease of surface tunability and biocompatibility have allowed their integration in a wide range of biosensing applications. Several metal, metal oxide, metal-organic framework-based nanozymes have been exploited for the development of biosensing systems, which present the potential for point-of-care analysis. To highlight recent progress in the field, in this review, more than 260 research articles are discussed systematically with suitable recent examples, elucidating the role of nanozymes to reinforce, miniaturize, and improve the performance of point-of-care diagnostics addressing the ASSURED (affordable, sensitive, specific, user-friendly, rapid and robust, equipment-free and deliverable to the end user) criteria formulated by World Health Organization. The review reveals that many biosensing strategies such as electrochemical, colorimetric, fluorescent, and immunological sensors required to achieve the ASSURED standards can be implemented by using enzyme-mimicking activities of nanomaterials as signal producing components. However, basic system functionality is still lacking. Since the enzyme-mimicking properties of the nanomaterials are dictated by their size, shape, composition, surface charge, surface chemistry as well as external parameters such as pH or temperature, these factors play a crucial role in the design and function of nanozyme-based point-of-care diagnostics. Therefore, it requires a deliberate exertion to integrate various parameters for truly ASSURED solutions to be realized. This review also discusses possible limitations and research gaps to provide readers a brief scenario of the emerging role of nanozymes in state-of-the-art POC diagnosis system development for futuristic biosensing applications.

Cotton Textile/Iron Oxide Nanozyme Composites with Peroxidase-like Activity: Preparation, Characterization, and Application

At present, both native and immobilized nanoparticles are of great importance in many areas of science and technology. In this paper, we have studied magnetic iron oxide nanoparticles and their aggregates bound on woven cotton textiles employing two simple modification procedures. One modification was based on the treatment of textiles with perchloric-acid-stabilized magnetic fluid diluted with methanol followed by drying. The second procedure was based on the microwave-assisted conversion of ferrous sulfate at high pH followed by drying. The structure and functional properties of these modified textiles were analyzed in detail. Scanning electron microscopy of native and modified textiles clearly showed the presence of iron oxide nanoparticles on the surface of the modified cotton fibers. All of the modified textile materials exhibited light to dark brown color depending on the amount of the bound iron oxide particles. Magnetic measurements showed that the saturation magnetization values reflect the amount of magnetic nanoparticles present in the modified textiles. Small-angle X-ray and neutron scattering measurements were conducted for the detailed structural characterization at the nanoscale of both the native and magnetically modified textiles, and different structural organization of nanoparticles in the two kinds of textile samples were concluded. The textile-bound iron oxide particles exhibited peroxidase-like activity when the N,N-diethyl-p-phenylenediamine sulfate salt was used as a substrate; this nanozyme activity enabled rapid decolorization of crystal violet in the presence of hydrogen peroxide. The deposition of a sufficient amount of iron oxide particles on textiles enabled their simple magnetic separation from large volumes of solutions; if necessary, the magnetic response of the modified textiles can be simply increased by incorporation of a piece of magnetic iron wire. The simplicity of the immobilized nanozyme preparation and the low cost of all the precursors enable its widespread application, such as decolorization and degradation of selected organic dyes and other important pollutants. Other types of textile-bound nanozymes can be prepared and used as low-cost catalysts for a variety of applications.

Magnetic nanomaterials with unique nanozymes-like characteristics for colorimetric sensors: A review

Colorimetric sensors for the rapid detection of numerous analytes have been widely applied in many fields such as biomedicine, food industry and environmental science due to their highly sensitive and selective response, easy operation and visual identification by naked eyes. In this review, the recent progress of the colorimetric sensors based on the magnetic nanomaterials with unique nanozymes-like catalytic activity (magnetic nanozyme) and their colorimetric sensing applications are presented. Emerging magnetic nanozyme-based colorimetric sensors, such as metal oxide/sulfides-based, metal-based, carbon-based, and aptamer-conjugated magnetic nanomaterials, offer many desirable features for target analytes detection. And due to the unique nanoscale physical-chemical properties, magnetic nanozymes have been used to mimic the catalytic activity of natural enzymes such as peroxidases, oxidases and catalases. This review also highlights the catalytic mechanisms of enzyme-like reactions, and promising colorimetric sensing system for the detection of chemical compounds like H₂O₂, pesticide, ascorbic acid, dopamine, tetracyclines, perfluorooctane sulfonate, phenolic compounds, heavy metal ion and sulfite have been deeply discussed. In addition, the remaining challenges and future directions in utilizing magnetic nanozyme for colorimetric sensors are addressed.

In vivo guiding inorganic nanozymes for biosensing and therapeutic potential in cancer, inflammation and microbial infections

Researchers have recently introduced some artificial enzymes based on nanomaterials that show significant catalytic activity relative to native enzymes called nanozyme. These nanozymes show superior performance than conventional catalysts and are considered as fascinating candidates for introducing the next generation of biomaterials in various industrial and biomedical fields. Recently, nanozymes have received a great deal of attention in biomedical applications due to their potential properties such as long-term stability, low cost, mass production capability, and controllable catalytic activity. Due to the intrinsic catalytic activity of nanoparticles (NPs) as nanozymes and their ability to be regulated in biomedical processes, this review paper focuses on the in vivo applications of nanozymes in biosensing and therapeutic activities. Despite the challenges and benefits of each approach, this paper attempts to provide an appropriate motivation for the classification of different nanozymes followed by their application in biomedical activities including in vivo biosensing and therapeutic potential in cancer, inflammation and microbial infections. Finally, some ongoing challenges and future perspective of nanozymes in biomedical application were surveyed. In conclusion, this paper may provide useful information regarding the development of nanozymes as promising platforms in biomedical settings due to expedited diagnosis, the advancement of multifactorial therapies and their pronounced stability.

Tungsten disulfide nanosheets-based colorimetric assay for glucose sensing

We have developed a glucose oxidase (GOx)-mediated strategy for glucose detection, which is based on the intrinsic peroxidase-like activity of WS₂ as a catalyst for the 3,3',5,5'-tetramethylbenzidine‑hydrogen peroxide (TMB-H₂O₂) reaction. The colorimetric assay involves two parts: generation of H₂O₂ from the oxidation of glucose catalyzed by GOx, and WS₂ nanosheets that catalyze the reaction between TMB and H₂O₂. In this colorimetric assay, the enhancement of colorimetric signals depends directly on the increased H₂O₂ concentration, which, in turn, relies on the glucose concentration. The results show that the concentrations of the glucose were directly proportional to absorbance of the TMB solutions over a range of 1 nM-500 μM with a limit of detection of 0.1445 nM. In addition, this new colorimetric assay has been utilized for glucose detection in human serum with a satisfactory result.

One pot clarification and debittering of grapefruit juice using co-immobilized enzymes@chitosanMNPs

In the present work, enzymes pectinase and naringinase were simultaneously co-immobilized on an eco-friendly chitosan coated magnetic nanoparticles (chitosanMNPs) by cross-linking using chitosan as a macro-molecular cross-linker. The maximum activity recovery of both enzymes in the co-immobilized form was obtained at chitosanMNPs to enzymes ratio of 1:3, 3% cross-linker concentration and 150 min cross-linking time. The synthesized MNPs before and after co-immobilization were characterized using different techniques. The prepared biocatalyst was found spherical with an average size below 200 nm and showed supermagnetic property with saturation magnetization of 38.28 emu/g. The optimum pH and temperature of both enzymes in co-immobilized form was found at 5.5 and 65 °C. The prepared biocatalyst exhibited an improved thermal stability with 1.8-fold increase in the half-life. The secondary structural analysis revealed that, prepared co-immobilized biocatalyst undergone changes in the conformational and structural rigidity due to macro-molecular cross-linker. The co-immobilized biocatalysts were evaluated for one pot clarification and debittering of grapefruit juice and found ~52% reduction in turbidity and ~85% reduction in the naringin content. The co-immobilized enzymes were recycled up to 7^th cycle and can be easily stored at room temperature for 30 days retaining up to 64% and 86% residual activities respectively.

N, S, and P-Co-doped Carbon Quantum Dots: Intrinsic Peroxidase Activity in a Wide pH Range and Its Antibacterial Applications

Nanozymes have drawn significant scientific interest due to their high practical importance in terms of overcoming the instability, complicated synthesis, and high cost of protein enzymes. However, their activity is generally limited to particular pHs, especially acidic ones. Herein, we report that luminescent N, S, and P-co-doped carbon quantum dots (NSP-CQDs) act as attractive peroxidase mimetics in a wide pH range, even at neutral pH, for the peroxidase substrate 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) in the presence of H₂O₂. The synergistic effects of multiple heteroatoms doping in CQDs boost the catalytic activity in a wide pH range attributed to the presence of high density of active sites for enzymatic-like catalysis and accelerated electron transfer during the peroxidase-like reactions. A possible reaction mechanism for the peroxidase-like activity of CQDs is investigated based on the radical trapping experiments. Moreover, the multifunctional activity of NSP-CQDs was further utilized for antibacterial assays for both Gram-negative and Gram-positive model species, including Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), respectively. The growths of the employed E. coli and S. aureus were found to be significantly inhibited due to the peroxidase-mediated perturbation of cell walls. The present work signifies the current advance in the rational design of N, S, and P-co-doped CQDs as highly active peroxidase mimics for novel applications in diverse fields, including catalysis, medical diagnostics, environmental chemistry, and biotechnology.

Peroxidase-like Au@Pt nanozyme as an integrated nanosensor for Ag+ detection by LSPR spectroscopy

Here we report the peroxidase-like Au@Pt nanozyme as an integrated nanosensor for selective detection of silver ions (Ag⁺), where the nanozyme plays the roles as both the signal trigger and reporter simultaneously. This method relies on two critical chemical reactions, including (1) the unique inhibitory effect of Ag⁺ on the nanozyme triggered H₂O₂ decomposition at weak acid environment and (2) H₂O₂ induced Ag⁺ reduction onto the nanozyme surface at basic environment, leading to a blueshift in the localized surface plasmonic resonance wavelength (LSPR λ_max) of the nanosensor. With this simple strategy, we demonstrated the sensitive and selective detection of Ag⁺ over a dynamic range from 0.5 to 1000 μM with a limit of detection (LOD) of 500 nM by UV-visible spectroscopy, which is below the permitted level of Ag⁺ in drinking water by U.S. Environmental Protection Agency (EPA). This method also exhibits satisfying recovery efficiency for Ag⁺ detection both in tap water and spring water from the Yuelu Mountain. With this satisfying sensing performance and excellent stability of nanoprobes, this strategy is promising for the detection of Ag⁺ in environment monitoring and food safety analysis.

Reagent-Free Colorimetric Cholesterol Test Strip Based on Self Color-Changing Property of Nanoceria

Paper-based test strip consisting of cerium oxide nanoparticles (nanoceria) as hydrogen peroxide (H₂O₂)-dependent color-changing nanozymes and cholesterol oxidase (ChOx) has been developed for convenient colorimetric determination of cholesterol without the need for chromogenic substrate. The construction of the cholesterol strip begins with physical adsorption of nanoceria on the paper surface, followed by covalent immobilization of ChOx via silanization, chitosan-mediated activation, and glutaraldehyde treatment of the nanoceria-embedded paper matrices. In the presence of cholesterol, ChOx catalyzes its oxidation to produce H₂O₂, which forms peroxide complex on the nanoceria surface and induces visual color change of the nanoceria-embedded paper from white/light yellow into intense yellow/orange, which was conveniently quantified with an image acquired by a conventional smartphone with the ImageJ software. Using this strategy, target cholesterol was specifically determined down to 40 μM with a dynamic linear concentration range of 0.1–1.5 mM under neutral pH condition, which is suitable to measure the serum cholesterol, with excellent stability during 20 days and reusability by recovering its original color-changing activity for 4 consecutive cycles. Furthermore, the practical utility of this strategy was successfully demonstrated by reliably determining cholesterol in human blood serum samples. This study demonstrates the potential of self color-changing nanozymes for developing colorimetric paper strip sensor, which is particularly useful in instrumentation-free point-of-care environments.

Nanozyme-based electrochemical biosensors for disease biomarker detection

In recent years, a new group of nanomaterials named nanozymes that exhibit enzyme-mimicking catalytic activity has emerged as a promising alternative to natural enzymes. Nanozymes can address some of the intrinsic limitations of natural enzymes such as high cost, low stability, difficulty in storage, and specific working conditions (i.e., narrow substrate, temperature and pH ranges). Thus, synthesis and applications of hybrid and stimuli-responsive advanced nanozymes could revolutionize the current practice in life sciences and biosensor applications. On the other hand, electrochemical biosensors have long been used as an efficient way for quantitative detection of analytes (biomarkers) of interest. As such, the use of nanozymes in electrochemical biosensors is particularly important to achieve low cost and stable biosensors for prognostics, diagnostics, and therapeutic monitoring of diseases. Herein, we summarize the recent advances in the synthesis and classification of common nanozymes and their application in electrochemical biosensor development. After briefly overviewing the applications of nanozymes in non-electrochemical-based biomolecular sensing systems, we thoroughly discuss the state-of-the-art advances in nanozyme-based electrochemical biosensors, including genosensors, immunosensors, cytosensors and aptasensors. The applications of nanozymes in microfluidic-based assays are also discussed separately. We also highlight the challenges of nanozyme-based electrochemical biosensors and provide some possible strategies to address these limitations. Finally, future perspectives on the development of nanozyme-based electrochemical biosensors for disease biomarker detection are presented. We envisage that standardization of nanozymes and their fabrication process may bring a paradigm shift in biomolecular sensing by fabricating highly specific, multi-enzyme mimicking nanozymes for highly sensitive, selective, and low-biofouling electrochemical biosensors.

Construction of the Nickel Oxide Nanocoral Structure on Microscope Slides for Total Self-Assembly-Oriented Probe Immobilization and Signal Enhancement

Proper orientation of probes and the binding capacity of surfaces will determine the performance of bio-applications. It has been reported that immobilizing through bio-/chemical affinity is an efficient but gentle strategy to solve the above-mentioned issue. Herein, we introduce a total self-assembly approach via the strong affinity of nickel oxide (NiO) to the polyhistidine-tag (His-tag). It allows the efficient immobilizing His-tagged proteins with orientation. Furthermore, we find that the nanocoral structure can be formed after applying rapid thermal annealing at 1100 °C, which could increase the His-tagged protein binding capacity efficiently by the enhanced surface-to-volume ratio. Lastly, we demonstrate the NiO thin film with the nanocoral structure, which has great potential for universal biosensing with a wide range of biomolecules, including DNA, protein, and bacteria. Through His-tagged monomer streptavidin (His₆-mSA) or His-tagged protein G (His₆-protein G), the biotinylated DNA or antibody could be immobilized with proper orientation on the surface consequently to complete a sensitive biomolecule detection. Moreover, the NiO nanocoral structure has the advantages of high hydrophilicity, transmittance, and pH stability that are promising to develop into several kinds of bio-applications in the near future.

Rosette-shaped graphitic carbon nitride acts as a peroxidase mimic in a wide pH range for fluorescence-based determination of glucose with glucose oxidase

Rosette-shaped graphitic carbon nitride (rosette-GCN) is described as a promising alternative to natural peroxidase for its application to fluorescence-based glucose assays. Rosette-GCN was synthesized via a rapid reaction between melamine and cyanuric acid for 10 min at 35 °C, followed by thermal calcination for 4 h. Importantly, rosette-GCN possesses a peroxidase-like activity, producing intense fluorescence from the oxidation of Amplex UltraRed in the presence of H₂O₂ over a broad pH-range of, including neutral pH; the peroxidase activity of rosette-GCN was ~ 10-fold higher than that of conventional bulk-GCN. This enhancement of peroxidase activity is presumed to occur because rosette-GCN has a significantly larger surface area and higher porosity while preserving its unique graphitic structure. Based on the high peroxidase activity of rosette-GCN along with the catalytic action of glucose oxidase (GOx), glucose was reliably determined down to 1.2 μM with a dynamic linear concentration range of 5.0 to 275.0 μM under neutral pH conditions. Practical utility of this strategy was also successfully demonstrated by determining the glucose levels in serum samples. This work highlights the advantages of GCNs synthesized via rapid methods but with unique structures for the preparation of enzyme-mimicking catalysts, thus extending their applications to the diagnostics field and other biotechnological fields. Graphical abstract.

Magnetic and Hydrophobic Composite Polyurethane Sponge for Oil–Water Separation

Crude oil spills from offshore oil fields will cause serious pollution to the marine ecological environment. Many 3D porous materials have been used for oil–water separation, but they cannot be widely used due to complex preparation processes and expensive preparation costs. Here, a facile and cheap approach to disperse expanded graphite (EG), stearic acid, and Fe3O4 magnetic nanoparticles on the skeleton surface of polyurethane (PU) sponge to prepare the magnetic and hydrophobic composite polyurethane sponge for oil–water separation. The results show that the composite PU sponge had a strong oil absorption capacity for various oils, the oil adsorption capacities has reached 32–40 g/g, and it has become more hydrophobic. The addition of Fe3O4 magnetic nanoparticles endowed the sponge with magnetic responsivity, and the composite PU sponge still had a strong oil adsorption capacity after several adsorbing-squeezing cycles. The magnetic and hydrophobic composite polyurethane sponge is a very promising material for practical oil adsorption and oil–water separation.

Nanozymes for medical biotechnology and its potential applications in biosensing and nanotherapeutics

Recent past years have witnessed the development of several artificial enzymes, using different materials to mimic natural enzymes with respect to their structure and functions. The nanozymes are nanomaterials possessing similar characteristics to the natural enzymes and have emerged recently as an innovative class of artificial enzymes. The nanozymes have got remarkable attention from the researchers and notable developments have been achieved owing to their unique properties compared with natural enzymes and classic artificial enzymes. In this regard, several nanomaterials have been scrutinized so far to mimic different natural enzymes for wider applications ranging from imaging, sensing, water treatment, pollutant removal, and therapeutics. The applications of nanozymes in biomedicine research are fast-growing and various nanozymes have been implicated in diagnostic medicine, targeted cancer therapy. Such abilities make them an appropriate alternative for the development of affordable, sustainable and safe diagnostic as well as therapeutic agents.

Recent Advances in Nanozyme Research

As a new generation of artificial enzymes, nanozymes have the advantages of high catalytic activity, good stability, low cost, and other unique properties of nanomaterials. Due to their wide range of potential applications, they have become an emerging field bridging nanotechnology and biology, attracting researchers in various fields to design and synthesize highly catalytically active nanozymes. However, the thorough understanding of experimental phenomena and the mechanisms beneath practical applications of nanozymes limits their rapid development. Herein, the progress of experimental and computational research of nanozymes on two issues over the past decade is briefly reviewed: (1) experimental development of new nanozymes mimicking different types of enzymes. This covers their structures and applications ranging from biosensing and bioimaging to therapeutics and environmental protection. (2) The catalytic mechanism proposed by experimental and theoretical study. The challenges and future directions of computational research in this field are also discussed.

The Intrinsic Enzyme Activities of the Classic Polyoxometalates

The mimicking enzyme activities of eighteen classic POMs with different structures, Keggin (H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀, H₄GeW₁₂O₄₀, K₄GeW₁₂O₄₀, H₃PMo₁₂O₄₀, H₄SiMo₁₂O₄₀ and Eu₃PMo₁₂O₄₀), Wells-Dawson (H₆P₂Mo₁₈O₆₂, α-(NH₄)₆P₂W₁₈O₆₂ and α-K₆P₂W₁₈O₆₂·14H₂O), lacunary-Keggin (Na₈H[α-PW₉O₃₄], Na₁₀[α-SiW₉O₃₄], Na₁₀[α-GeW₉O₃₄] and K₈[γ-SiW₁₀O₃₆]), the transition-metal substituted-type (α-1,2,3-K₆H[SiW₉V₃O₃₄] and H₅PMo₁₀V₂O₄₀), sandwich-type (K₁₀P₂W₁₈Fe₄(H₂O)₂O₆₈) and an isopolyoxotungstate (Na₁₀H₂W₁₂O₄₂) were screened and compared. The mechanisms and reaction conditions of POMs with mimicking enzyme-like activities were also analyzed. The results shown that the structures, the hybrid atoms, the coordination atoms, the substituted metal atoms, pH and substrate are the effect factors for the enzyme mimic activities of POM. Among the eighteen POMs, H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀, H₄GeW₁₂O₄₀, α-(NH₄)₆P₂W₁₈O₆₂, α-K₆P₂W₁₈O₆₂·14H₂O, Na₈H[α-PW₉O₃₄], Na₁₀[α-SiW₉O₃₄], Na₁₀[α-GeW₉O₃₄], K₈[γ-SiW₁₀O₃₆], K₁₀P₂W₁₈Fe₄(H₂O)₂O₆₈ and Na₁₀H₂W₁₂O₄₂ had the peroxidase activities. Eu₃PMo₁₂O₄₀, H₃PMo₁₂O₄₀, H₄SiMo₁₂O₄₀, α-1,2,3-K₆H [SiW₉V₃O₃₄], H₆P₂Mo₁₈O₆₂ and H₅PMo₁₀V₂O₄₀ showed the oxidase-like activities. K₄GeW₁₂O₄₀ did not show the peroxidase and oxidase activities. The Na₈H[α-PW₉O₃₄], Na₁₀[α-SiW₉O₃₄] and Na₁₀[α-GeW₉O₃₄] showed intrinsic enzyme activities at alkaline conditions, which were different from other type of POMs. The sandwich-type K₁₀P₂W₁₈Fe₄(H₂O)₂O₆₈ displayed the strongest peroxidase activity, which is similar to natural horseradish peroxidase.

Fluoride-capped nanoceria as a highly efficient oxidase-mimicking nanozyme: inhibiting product adsorption and increasing oxygen vacancies

Nanozymes aim to mimic enzyme activities using nanomaterials. Nanoceria (CeO2 nanoparticles) is an important model nanozyme for its rich redox chemistry. In particular, its oxidase-like activity allows oxidation reactions without the need of unstable and toxic H2O2. Fluoride can significantly improve its oxidase-like activity, and this work aims to understand the mechanism of fluoride-promoted catalysis. First, fluoride can adsorb on CeO2 tighter than other halides, but not as strong as phosphate as characterized by isothermal titration calorimetry (ITC). FT-IR spectroscopy indicates adsorption of fluoride likely via exchange with surface hydroxide groups. Fluoride capping inverses the surface charge of CeO2, facilitating desorption of the ABTS oxidation product, significantly increasing the turnover number. The Raman, EPR and XPS spectroscopy results demonstrate that the concentration of Ce3+ and the accompanying oxygen vacancy significantly increased upon adding F-, which can explain the enhanced catalytic activity. Finally, the electron transfer properties of fluoride-capped CeO2 were more efficient than that of the bare CeO2 as determined by a direct electrochemical measurement on a glass carbon electrode. This study has provided new insight into nanoceria, and can also further confirm the role of nanoceria as a model for engineering the surface of nanozymes.

Direct glucose detection in whole blood by colorimetric assay based on glucose oxidase-conjugated graphene oxide/MnO2 nanozymes

We herein report a facile approach for the preparation of horseradish peroxidase (HRP)-mimic glucose oxidase-conjugated graphene oxide/MnO2 (GOD-GO/MnO2) as new nanozyme to detect the glucose concentration in whole blood. The nano-sized of MnO2 nanoparticles embedded in bovine serum albumin (BSA)-coated GO by in situ growth were evaluated focusing on the principle of HRP-mimic activity catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide. Furthermore, we constructed dual sensing platforms based on the combination of a plasma separation pad and GOD-GO/MnO2 for direct detection of glucose concentration in whole blood by colorimetric assay without blood sample pretreatment. As a proof-of-concept, a limit of detection of 3.1 mg dL-1 for glucose was obtained with a wide linear quantification range from 25 mg dL-1 to 300 mg dL-1 through visual observation and quantitative analysis, suggesting potential clinical applications in blood glucose monitoring for diabetic patients.

Magnetic Nanoparticles-Embedded Enzyme-Inorganic Hybrid Nanoflowers with Enhanced Peroxidase-Like Activity and Substrate Channeling for Glucose Biosensing

It is reported that glucose oxidase (GOx)-copper hybrid nanoflowers embedded with Fe₃ O₄ magnetic nanoparticles (MNPs) exhibit superior peroxidase-mimicking activity as well as substrate channeling for glucose detection. This is due to the synergistic integration of GOx, crystalline copper phosphates and MNPs being in close proximity within the nanoflowers. The preparation of MNP-embedded GOx-copper hybrid nanoflowers (MNPs-GOx NFs) begins with the facile conjugation of amine-functionalized MNPs with GOx molecules via electrostatic attraction, followed by the addition of copper sulfate that leads to full blooming of the hybrid nanoflowers. In the presence of glucose, the catalytic action of GOx entrapped in the nanoflowers generates H₂ O₂ , which is subsequently used by peroxidase-mimicking MNPs and copper phosphate crystals, located close to GOx molecules, to convert Amplex UltraRed substrate into a highly fluorescent product. Using this strategy, the target glucose is successfully determined with excellent selectivity, stability, and magnetic reusability. This biosensor based on hybrid nanoflowers also exhibits a high degree of precision and reproducibility when applied to real human blood samples. Such novel MNP-embedded enzyme-inorganic hybrid nanoflowers have a great potential to be expanded to any oxidases, which will be highly beneficial for the detection of various other clinically important target molecules.

High-Performance Integrated Enzyme Cascade Bioplatform Based on Protein-BiPt Nanochain@Graphene Oxide Hybrid Guided One-Pot Self-Assembly Strategy

Nanozymes provide new opportunities for facilitating next generation artificial enzyme cascade platforms. However, the fabrication of high-performance integrated artificial enzyme cascade (IAEC) bioplatforms based on nanozymes remains a great challenge. A facile and effective self-assembly strategy for constructing an IAEC system based on an inorganic/protein hybrid nanozyme, β-casein-BiPt nanochain@GO (CA-BiPtNC@GO) nanohybrid with unique physicochemical surface properties and hierarchical structures, is introduced here. Due to the synergetic effect of the protein, GO, and Bi³⁺ , the hybrid acts as highly adaptable building blocks to immobilize natural enzymes directly and noncovalently without the loss of enzyme activity. Simultaneously, the CA-BiPtNC@GO nanohybrid exhibits outstanding peroxidase-mimicking activity and works well with natural oxidases, resulting in prominent activity in catalyzing cascade reactions. As a result, the proposed IAEC bioplatform exhibits excellent sensitivity with a wide linear range of 0.5 × 10^-6 to 100 × 10^-6 m and a detection limit of 0.05 × 10^-6 m for glucose. Meticulous design of ingenious hierarchically nanostructured nanozymes with unique physicochemical surface properties can provide a facile and efficient way to immobilize and stabilize nature enzymes using self-assembly instead of chemical processes, and fill the gap in developing robust nanozyme-triggered IAEC systems with applications in the environment, sensing, and synthetic biology.

Highly sensitive colorimetric detection of allergies based on an immunoassay using peroxidase-mimicking nanozymes

Nanomaterials that exhibit enzyme-like characteristics, which are called nanozymes, have recently attracted significant attention due to their potential to overcome the intrinsic limitations of natural enzymes, such as low stability and relatively high cost for preparation and purification. In this study, we report a highly efficient colorimetric allergy detection system based on an immunoassay utilizing the peroxidase-mimicking activity of hierarchically structured platinum nanoparticles (H-Pt NPs). The H-Pt NPs had a diameter of 30 nm, and were synthesized by a seed-mediated growth method, which led to a significant amount of peroxidase-like activity. This activity mainly occurs because of the high catalytic power of the Pt element, and the fact that the H-Pt NPs have a large surface area available for catalytic events. The H-Pt NPs were conjugated to an antibody for the detection of immunoglobulin E (IgE) in the analytes; IgE is a representative marker for the diagnosis of allergies. They were then successfully integrated into a conventionally used allergy diagnostic test, the ImmunoCAP diagnostic test, as a replacement for natural signaling enzymes. Using this strategy, total and specific IgE levels were detected within 5 min at room temperature, with high specificity and sensitivity. The practical utility of the immunoassay was also successfully verified by correctly determining the levels of both total and specific IgE in real human serum samples with high precision and reproducibility. The present H-Pt NP-based immunoassay system would serve as a platform for rapid, robust, and convenient analysis of IgE, and can be extended to the construction of diagnostic systems for a variety of clinically important target molecules.

Functional nanomaterials with unique enzyme-like characteristics for sensing applications

We highlight the recent developments in functional nanomaterials with unique enzyme-like characteristics for sensing applications.

A one-pot and modular self-assembly strategy for high-performance organized enzyme cascade bioplatforms based on dual-functionalized protein–PtNP@mesoporous iron oxide hybrid

A one-pot and modular self-assembly strategy for high-performance enzyme cascade bioplatform based on dual-functionalized protein/inorganic hybrid.

Development of colorimetric cholesterol detection kit using TPU nanofibre/cellulose acetate membrane

In this study, the authors report a simple fabrication of thermoplastic polyurethane (TPU) nanofibres-based kit for cholesterol detection. TPU is a polymer that is highly elastic, resistant to microorganisms, abrasion and compatible with blood; thus, making it a natural selection as an immobilisation matrix for cholesterol oxidase (ChOx) enzyme. The nanofibre was fabricated by electrospinning process and was characterised using scanning electron microscopy and Fourier transform-infrared spectroscopy. ChOx was covalently immobilised on TPU nanofibre and cholesterol level/concentration was visually found using 4-aminoantipyrine, a dye that reacts with H₂O₂ produced from the oxidation of cholesterol by ChOx and changes colour from yellow to red. The efficacy of the nanofibre to act as a detecting substrate was compared with cellulose acetate (CA) membrane, a well-documented enzyme immobilisation matrix. The optimisation of enzyme concentration and dye quantity were performed using standard ChOx spectrophotometric assay and the same was used in CA membrane and TPU nanofibre. The ChOx immobilised nanofibre showed good linear range from 2 to 10 mM with a lower detection limit of 2 mM and was highly stable compared to that of CA membrane. The enzyme immobilised nanofibre was further validated in serum samples.

Integrated nanozymes: facile preparation and biomedical applications

Nanozymes have been viewed as the next generation of artificial enzymes due to their low cost, large specific surface area, and good robustness under extreme conditions. However, the moderate activity and limited selectivity of nanozymes have impeded their usage. To overcome these shortcomings, integrated nanozymes (INAzymes) have been developed by encapsulating two or more different biocatalysts (e.g., natural oxidases and peroxidase mimics) together within confined frameworks. On the one hand, with the assistance of natural enzymes, INAzymes are capable of specifically recognizing targets. On the other hand, nanoscale confinement brought about by integration significantly enhances the cascade reaction efficiency. In this Feature Article, we highlight the newly developed INAzymes, covering from synthetic strategies to versatile applications in biodetection and therapeutics. Moreover, it is predicted that INAzymes with superior activities, specificity, and stability will enrich the research of nanozymes and pave new ways in designing multifunctional nanozymes.

Multifunctional nanozymes: enzyme-like catalytic activity combined with magnetism and surface plasmon resonance

Over decades, as alternatives to natural enzymes, highly-stable and low-cost artificial enzymes have been widely explored for various applications. In the field of artificial enzymes, functional nanomaterials with enzyme-like characteristics, termed as nanozymes, are currently attracting immense attention. Significant progress has been made in nanozyme research due to the exquisite control and impressive development of nanomaterials. Since nanozymes are endowed with unique properties from nanomaterials, an interesting investigation is multifunctionality, which opens up new potential applications for biomedical sensing and sustainable chemistry due to the combination of two or more distinct functions of high-performance nanozymes. To highlight the progress, in this review, we discuss two representative types of multifunctional nanozymes, including iron oxide nanomaterials with magnetic properties and metal nanomaterials with surface plasmon resonance. The applications are also covered to show the great promise of such multifunctional nanozymes. Future challenges and prospects are discussed at the end of this review.

Phage capsid protein-directed MnO2 nanosheets with peroxidase-like activity for spectrometric biosensing and evaluation of antioxidant behaviour

Small molecular weight proteins (5.21 kDa) were used as bio-templates to synthesize MnO₂ nanosheets (NSs). This work will open up a protein-directed avenue to synthesize 2D morphology. Further, the as-prepared MnO₂ NSs showed intrinsic peroxidase-like activity and were then applied for glucose detection and evaluation of antioxidant behaviours of typical antioxidants.

Effective Peroxidase-Like Activity of Co-Aminoclay [CoAC] and Its Application for Glucose Detection

In this study, we describe a novel peroxidase-like activity of Co-aminoclay [CoAC] present at pH ~5.0 and its application to fluorescent biosensor for the determination of H₂O₂ and glucose. It is synthesized with aminoclays (ACs) entrapping cationic metals such as Fe, Cu, Al, Co., Ce, Ni, Mn, and Zn to find enzyme mimicking ACs by sol-gel ambient conditions. Through the screening of catalytic activities by the typical colorimetric reaction employing 2,2'-azino-bis(3-ethylbenzo-thiazoline-6-sulfonic acid)diammonium salt (ABTS) as a substrate with or without H₂O₂, Fe, Cu, and CoACs are found to exhibit peroxidase-like activity, as well as oxidase-like activity was observed from Ce and MnACs. Among them, CoAC shows exceptionally high peroxidase-like activity, presumably due to its ability to induce electron transfer between substrates and H₂O₂. CoAC is then used to catalyze the oxidation of Amplex® UltraRed (AUR) into a fluorescent end product, which enables a sensitive fluorescent detection of H₂O₂. Moreover, a highly sensitive and selective glucose biosensing strategy is developed, based on enzyme cascade reaction between glucose oxidase (GOx) and CoAC. Using this strategy, a highly linear fluorescence enhancement is verified when the concentration of glucose is increased in a wide range from 10 μM to 1 mM with a lower detection limit of 5 μM. The practical diagnostic capability of the assay system is also verified by its use to detect glucose in human blood serum. Based on these results, it is anticipated that CoAC can serve as potent peroxidase mimetics for the detection of clinically important target molecules.

A colorimetric sensor of H2O2 based on Co3O4–montmorillonite nanocomposites with peroxidase activity

Schematic of the colorimetric detection of H₂O₂ catalyzed using Co₃O₄–MMT NPs.

Enzymatic activity of Fe-grafted mesoporous silica nanoparticles: an insight into H2O2and glucose detection

Iron-grafted MSNs were synthesized by post-synthesis and applied as a biosensor for detection of glucose and H₂O₂.

Highly active fluorogenic oxidase-mimicking NiO nanozymes

NiO nanoparticles can quickly catalyze oxidation of Amplex red to produce fluorescent products for intracellular imaging, much more efficiently than other types of tested nanozymes.

Influence of VO₂ Nanoparticle Morphology on the Colorimetric Assay of H₂O₂ and Glucose

Nanozyme-based colorimetric sensors have received considerable attention due to their unique properties. The size, shape, and surface chemistry of these nanozymes could dramatically influence their sensing behaviors. Herein, a comparative study of VO₂ nanoparticles with different morphologies (nanofibers, nanosheets, and nanorods) was conducted and applied to the sensitive colorimetric detection of H₂O₂ and glucose. The peroxidase-like activities and mechanisms of VO₂ nanoparticles were analyzed. Among the VO₂ nanoparticles, VO₂ nanofibers exhibited the best peroxidase-like activity. Finally, a comparative quantitative detections of H₂O₂ and glucose were done on fiber, sheet, and rod nanoparticles. Under the optimal reaction conditions, the lower limit of detection (LOD) of the VO₂ nanofibers, nanosheets, and nanorods for H₂O₂ are found to be 0.018, 0.266, and 0.41 mM, respectively. The VO₂ nanofibers, nanosheets, and nanorods show the linear response for H₂O₂ from 0.025-10, 0.488-62.5, and 0.488-15.625 mM, respectively. The lower limit of detection (LOD) of the VO₂ nanofibers, nanosheets, and nanorods for glucose are found to be 0.009, 0.348, and 0.437 mM, respectively. The VO₂ nanofibers, nanosheets, and nanorods show the linear response for glucose from 0.01-10, 0.625-15, and 0.625-10 mM, respectively. The proposed work will contribute to the nanozyme-based colorimetric assay.

WSe2 few layers with enzyme mimic activity for high-sensitive and high-selective visual detection of glucose

As one of the transition metal dichalcogenide materials, WSe₂ in the form of a few layers (nanosheets) have received much attention due to their unique physical, optical and electrical properties. Herein, we demonstrate that WSe₂ nanosheets possess intrinsic enzyme mimic activity. Under acidic conditions, they exhibit pronounced peroxidase-like property. The Michaelis-Menten kinetics indicate that the catalytic activity of WSe₂ nanosheets is comparable to that of horseradish peroxidase. Based on the color reaction, a platform of WSe₂ nanosheets was constructed to detect glucose concentration and it showed high sensitivity and high selectivity, which means that WSe₂ nanosheets with peroxidase-like property can be used to develop a highly sensitive and selective colorimetric method for glucose detection. Moreover, due to the electron-transferring and antioxidant properties of WSe₂ few layers, the peroxidase-like catalysis is proposed. These findings pave the way for further application of WSe₂ nanosheets in nano-biomedicine.

Iron Oxide Nanozyme: A Multifunctional Enzyme Mimetic for Biomedical Applications

Iron oxide nanoparticles have been widely used in many important fields due to their excellent nanoscale physical properties, such as magnetism/superparamagnetism. They are usually assumed to be biologically inert in biomedical applications. However, iron oxide nanoparticles were recently found to also possess intrinsic enzyme-like activities, and are now regarded as novel enzyme mimetics. A special term, "Nanozyme", has thus been coined to highlight the intrinsic enzymatic properties of such nanomaterials. Since then, iron oxide nanoparticles have been used as nanozymes to facilitate biomedical applications. In this review, we will introduce the enzymatic features of iron oxide nanozyme (IONzyme), and summarize its novel applications in biomedicine.