Light-responsive Au nanoclusters with oxidase-like activity for fluorescent detection of total antioxidant capacity

In situ fabricated gold nanostars on hydrogel beads as photo-oxidase mimics for rapid and sustainable POCT of uric acid

Synthetic enzyme mimics surpass their natural counterparts in terms of stability, efficiency, and cost-effectiveness, making them highly valuable for catalytic applications. Gold nanomaterials, particularly gold nanostars, have emerged as promising enzyme mimetic nanocatalysts due to their enhanced light interaction and superior catalytic efficiency. In this study, gold nanostars grown in situ on the surface of core-shell hydrogel beads exhibited specific oxidase-like activity when exposed to light. Photoexcitation of gold nanostars generates singlet oxygen through the interaction of positive holes and superoxide radicals, resulting in photo-oxidase-like activity. Attaching the gold nanostars to the hydrogel bead surface prevented catalytic activity loss caused by agglomeration, resulting in a marked improvement in catalytic stability. This stability is evident from the sustained catalytic activity of the hydrogel bead-embedded gold nanostars, even after 60 days of prolonged incubation in an aqueous medium, and their strong catalytic performance across multiple reaction cycles. Leveraging this photo-oxidase-like activity, a point-of-care testing (POCT) setup is developed for highly sensitive uric acid detection. The system achieved a remarkable detection limit of 0.9 μM and demonstrated excellent accuracy in blood serum and urine sample analyses. Furthermore, the integration of smartphone technology facilitated rapid and convenient on-site testing, bridging the gap between laboratory settings and real-world applications. This approach offers a practical and sustainable solution for efficient and accurate uric acid monitoring in diverse settings.

Iron nanoparticle/carbon nanotube composite as oxidase-like nanozyme for visual analysis of total antioxidant capacity

Total antioxidant capacity (TAC) is an important indicator for assessing the merit of natural plants and foods. Herein, a visual TAC assay is developed based on the oxidase-like activity of nitrogen-doped carbon nanotubes loaded with Fe nanoparticles (FeNPs@NCNT), which is prepared via high-temperature pyrolysis of metal-organic framework precursors and can catalyze the oxidation of colorless o-phenylenediamine (OPD) to colored 2,3-diaminophenazine (DAP). The addition of antioxidants (e.g., quercetin) impedes the formation of DAP, diminishing the color change, which can be analyzed via the RGB values obtained with a smartphone color-recognition APP, "Color Picker". The change of the optical signal can also be analyzed in the fluorescence mode. These two detection modes yield consistent TAC analysis of actual plant samples, in accord with results from the standard ABTS method. Results from this study highlight the unique potential of nanozymes in the development of effective TAC analysis platforms for natural plants and food.

Integrated gold nanorods-based dual-signal platform for accurate total antioxidant capacity assessment in food samples

Accurate assessment of Total Antioxidant Capacity (TAC) in food is crucial for evaluating nutritional quality and potential health benefits. This study aims to enhance the sensitivity and reliability of TAC detection through a dual-signal method, combining colorimetric and photothermal signals. Gold nanorods (AuNRs) were utilized to establish a dual-signal method duo to the colorimetric and photothermal properties. Fenton reaction can etch the AuNRs from the tips, as a result, a blue shift in the longitudinal LSPR absorption peak was obtained, leading to significant changes in color and photothermal effects, facilitating discrimination through both visual observation and thermometer measurements. In the presence of antioxidants, the Fenton reaction was suppressed or inhibited, protecting the AuNRs from etching. The colorimetric and photothermal signals were therefore positively correlated with TAC levels, enabling dual-signal detection of TAC. The linear range of AA was 4-100 μM in both colorimetry and photothermal modes, with detection limits of 1.60 μM and 1.38 μM, respectively. This dual-signal approach achieves low detection limits, enhancing precision and sensitivity. The method thus has the potential to act as a promising candidate for TAC detection in food samples, contributing to improved food quality and safety assessment.

Single-atom Fe nanozymes with excellent oxidase-like and laccase-like activity for colorimetric detection of ascorbic acid and hydroquinone

Although traditional Fe-based nanozymes have shown great potential, generally only a small proportion of the Fe atoms on the catalyst's surface are used. Herein, we synthesized single-atom Fe on N-doped graphene nanosheets (Fe-CNG) with high atom utilization efficiency and a unique coordination structure. Active oxygen species including superoxide radicals (O2•-) and singlet oxygen (1O2) were efficiently generated from the interaction of the Fe-CNG with dissolved oxygen in acidic conditions. The Fe-CNG nanozymes were found to display enhanced oxidase-like and laccase-like activity, with Vmax of 2.07 × 10-7 M∙S-1 and 4.54 × 10-8 M∙S-1 and Km of 0.324 mM and 0.082 mM, respectively, which is mainly due to Fe active centers coordinating with O and N atoms simultaneously. The oxidase-like performance of the Fe-CNG can be effectively inhibited by ascorbic acid (AA) or hydroquinone (HQ), which can directly obstruct the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). Therefore, a direct and sensitive colorimetric method for the detection of AA and HQ activity was established, which exhibited good linear detection and limit of detection (LOD) of 0.048 μM and 0.025 μM, respectively. Moreover, a colorimetric method based on the Fe-CNG catalyst was fabricated for detecting the concentration of AA in vitamin C. Therefore, this work offers a new method for preparing a single-atom catalyst (SAC) nanozyme and a promising strategy for detecting AA and HQ.

Recent Advances in Nanozyme-Based Sensing Technology for Antioxidant Detection

Antioxidants are substances that have the ability to resist or delay oxidative damage. Antioxidants can be used not only for the diagnosis and prevention of vascular diseases, but also for food preservation and industrial production. However, due to the excessive use of antioxidants, it can cause environmental pollution and endanger human health. It can be seen that the development of antioxidant detection technology is important for environment/health maintenance. It is found that traditional detection methods, including high performance liquid chromatography, gas chromatography, etc., have shortcomings such as cumbersome operation and high cost. In contrast, the nanozyme-based detection method features advantages of low cost, simple operation, and rapidity, which has been widely used in the detection of various substances such as glucose and antioxidants. This article focuses on the latest research progress of nanozymes for antioxidant detection. Nanozymes for antioxidant detection are classified according to enzyme-like types. Different types of nanozyme-based sensing strategies and detection devices are summarized. Based on the summary and analysis, one can find that the development of commercial nanozyme-based devices for the practical detection of antioxidants is still challenging. Some emerging technologies (such as artificial intelligence) should be fully utilized to improve the detection sensitivity and accuracy. This article aims to emphasize the application prospects of nanozymes in antioxidant detection and to provide new ideas and inspiration for the development of detection methods.

Light-Controlled Regulation of Dual-Enzyme Properties in YbGd-Carbon Quantum Dots Nano-Hybrid for Advanced Biosensing

Compared to nanozymes with single enzyme activity, those with multiple enzyme activities possess broader application potential due to their diversified enzymatic functionalities. However, the multienzyme nanozymes currently face challenges of interference among different enzymatic activities during practical applications. In this study, we report the synthesis of a light-responsive YbGd-carbon quantum dots nano-hybrid, termed YbGd-CDs, which exhibits controllable enzyme-mimicking activities. This light-responsive behavior enables selective control of the enzymatic activities. Under visible light irradiation, YbGd-CDs demonstrate robust oxidase-like activity. Conversely, under dark conditions, they primarily exhibit peroxidase-like activity. Leveraging the dual-enzyme-mimicking capabilities of YbGd-CDs, we developed colorimetric assays for sensitive detection of total antioxidant capacity (TAC) in both normal and cancer cells as well as d-amino acids in human saliva. This study not only advances the synthesis of carbon-based nanozymes but also highlights their potential in biosensing applications.

Ultrarobust stable ABTS radical cation prepared using Spore@Cu-TMA biocomposites for antioxidant capacity assay

Herein, spore@Cu-trimesic acid (TMA) biocomposites were prepared by self-assembling Cu-based metal-organic framework on the surface of Bacillus velezensis spores. The laccase-like activity of spore@Cu-TMA biocomposites was enhanced by 14.9 times compared with that of pure spores due to the reaction of Cu2+ ions with laccase on the spore surface and the microporous structure of Cu-TMA shell promoting material transport and increasing substrate accessibility. Spore@Cu-TMA rapidly oxidized and transformed 2,2'-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) into ABTS●+ without using H2O2. Under optimum conditions, the ABTS●+ could be stored for 21 days at 4 °C and 7 days at 37 °C without the addition of any stabilizers, allowing for the large-scale preparation and long-term storage of ABTS●+. The ultrarobust stable ABTS●+ obtained with the use of Cu-TMA could effectively reduce the "back reaction" by preventing the leaching of the metabolites released by the spores. On the basis of these findings, a rapid, low-cost, and eco-friendly colorimetric platform was successfully developed for the detection of antioxidant capacity. Determination of antioxidant capacity for several antioxidants such as caffeic acid, glutathione, and Trolox revealed their corresponding limits of detection at 4.83, 8.89, and 7.39 nM, respectively, with linear ranges of 0.01-130, 0.01-140, and 0.01-180 μM, respectively. This study provides a facile way to prepare ultrarobust stable ABTS●+ and presents a potential application of spore@Cu-TMA biocomposites in food detection and bioanalysis.

A CRISPR/Cas12a-based fluorescence method for the amplified detection of total antioxidant capacity

The CRISPR/Cas12a system is a powerful signal amplification tool that has been widely used in nucleic acid detection. It has also been applied to the assay of non-nucleic acid targets, mainly relying on strategies for converting target determination into nucleic acid detection. Herein, we describe a CRISPR/Cas12a-based fluorescence method for sensitive detection of the total antioxidant capacity (TAC) by utilizing a strategy of converting TAC determination into Mn2+ detection. Specifically, the reduction of MnO2 nanosheets by antioxidants produces plenty of Mn2+, which accelerates the trans-cleavage activity of CRISPR/Cas12a. Thus, a fluorescence enhanced detection method for TAC was established, with a detection limit as low as 0.04 mg L-1 for a typical antioxidant, ascorbic acid. More importantly, this method has been proven to successfully analyze TAC in beverages. The excellent analytical performance of this method demonstrates the great potential of the CRISPR/Cas12a system in simple and sensitive TAC analysis.

A colorimetric sensing platform with smartphone for organophosphorus pesticides detection based on PANI-MnO2 nanozyme

Organophosphorus pesticides (OPs) are of great concern due to its potential harms on human health and the environment. Herein, a budget-friendly, rapid and convenient colorimetric sensing platform is developed for detection of OPs in the environmental and food samples. The sensing element, PANI-MnO2 nanozyme with excellent oxidase mimetic activity is synthesized at room temperature, which is able to directly oxidize 3,3,5,5-tetramethylbenzidine (TMB) to generate blue colored oxidized TMB (OxTMB) within 2 min. Ascorbic acid (AA) can inhibit the oxidization reaction of TMB, consequently causing the blue color fading. Ascorbic acid 2-phosphate (AAP) could be hydrolyzed to produce AA by alkaline phosphatase (ALP). In the presence of OPs can effectively decrease ALP activity, resulting in the recovery of catalytic activity of PANI-MnO2. Therefore, sensitive and selective OPs detection is achieved. Under the optimal conditions, excellent detection performance in term of glyphosate as a model is achieved with a linear range from 0.50 to 50 μM, the detection limit is 0.39 μM (S/N = 3). The utility of method is further improved by combining a portable smartphone platform with a color picking application. The colorimetric platform achieves instrument-free detection of OPs and overcomes the uneven color distribution of traditional paper-based chip, providing an alternative strategy for the qualitative discernment and semi-quantitative analysis of OPs on-site.

Iron-doped carbon nitride with enhanced peroxidase-like activity for smartphone-based colorimetric assay of total antioxidant capacity

The facile detection of total antioxidant capacity (TAC) is limited by in-situ analysis, because it usually requires complex laboratory equipments. Here, a colorimetric assay for TAC detection is developed based on the peroxidase-like activity of iron-doped carbon nitride (Fe/NC) and the smartphone platform. The peroxidase-like activity of carbon nitride is greatly improved by the introduction of Fe atoms, and the active sites turn to Fe-Nx coordination groups in the Fe/NC. The inhibition mechanism of the chromogenic reaction for different kinds of antioxidants is also studied. The colorimetric assay is fabricated by the relationship of absorbance-color-antioxidant content and applied successfully to the TAC detection of several fruit juicesand commercial beverages. This work not only provides a promising approach for convenient in-situ TAC assay without the use of large instruments, but also expands the application of nanozymes in nutritional value assessment of foods.

Three-in-one covalent organic framework nanozyme: Self-reporting, self-correcting and light-responsive for fluorescence sensing 3-nitrotyrosine

Most current redox-type nanozyme-based colorimetric sensing platforms are susceptible to interference from the reductant when using chromogenic probe, and the unstable H2O2 used in the peroxidase-like nanozyme-based systems is prone to difficulty in sensing signal reproducibility, while peroxidase-like nanozyme with oxidase-mimicking activity is easy to bring background interference by O2. Since the strong structural designability of covalent organic frameworks (COFs) endows them great application value in the sensing fields, therefore, we envision the construction a COF oxidase-like nanozyme-based controllable sensing system that integrates self-reporting, self-correcting and light-responsive functions to avoid these affects. Herein, 3-nitrotyrosine (3-NT) biomarker was selected as model analyte. 1,3,5-triformylphloroglucinol (Tp) and 3,6-diaminoacridine (DA) were acted as building monomers of the multifunctional COF nanozyme (termed as TpDA). Owing to the excellent light-responsive oxidase-mimicking property of TpDA, 3-NT can be efficiently oxidized, the inner filter effect (IFE) between TpDA and the 3-NT oxidation product greatly quenches the intrinsic fluorescence of TpDA, making it a controllable self-reporting system for fluorescence turn-off sensing 3-NT. Additionally, the excessive reactive oxygen species (ROS) that generated continuously during photocatalysis can resist the interference of endogenous reductants. This study not only provides new insights to avoid the interference of H2O2, background and reductants from conventional redox-type nanozyme-based colorimetric systems but also opens avenues to rational construct versatile COF nanozyme-based sensor.

Chromogenic Mechanisms of Colorimetric Sensors Based on Gold Nanoparticles

The colorimetric signal readout method is widely used in visualized analyses for its advantages, including visualization of test results, simple and fast operations, low detection cost and fast response time. Gold nanoparticles (Au NPs), which not only exhibit enzyme-like activity but also have the advantages of tunable localized surface plasmon resonance (LSPR), high stability, good biocompatibility and easily modified properties, provide excellent platforms for the construction of colorimetric sensors. They are widely used in environmental monitoring, biomedicine, the food industry and other fields. This review focuses on the chromogenic mechanisms of colorimetric sensors based on Au NPs adopting two different sensing strategies and summarizes significant advances in Au NP-based colorimetric sensing with enzyme-like activity and tunable LSPR characteristics. In addition, the sensing strategies based on the LSPR properties of Au NPs are classified into four modulation methods: aggregation, surface modification, deposition and etching, and the current status of visual detection of various analytes is discussed. Finally, the review further discusses the limitations of current Au NP-based detection strategies and the promising prospects of Au NPs as colorimetric sensors, guiding the design of novel colorimetric sensors.

Fabrication of glass-based analytical devices by immobilizing nanomaterials on glass substrate with a fluorescent glue for the highly sensitive determination of mercury ions

A facile method is reported to develop glass-based analytical devices (GADs) based on immobilizing nanomaterials on a glass substrate with fluorescent glue. The fluorescent glue was first prepared by coupling bovine serum albumin (BSA)-protected Au nanoclusters (NCs) and sugars (i.e., ascorbic acid, AA). The glue was then used to immobilize carbon dots (C-dots) on glass substrates to fabricate the portable GADs. The liquid glue-C-dots mixture and probable GADs were developed for Hg²⁺ detection. Under 365-nm excitation wavelength, the emission at 652 nm from the glue is gradually quenched with increasing concentrations of Hg²⁺. This quenching is explained in terms of the Stern-Volmer equation and is ascribed to static quenching. The fluorescent color of the glue and GADs gradually changes from pink to blue, with increasing concentrations of Hg²⁺. The limits of detection (LODs) for Hg²⁺ determination by bare eyes are 1 nM both for the glue and GADs, suggesting an uncompromised sensing capability even after immobilization. The detection sensitivity of GADs shows a significant improvement compared with the same material-based papers (5 μM). A linear relationship is observed between the total Euclidean distances (EDs) and Hg²⁺ concentration in the range 0-100 nM, providing the potential for Hg²⁺ quantification using GADs. The LOD is estimated to be 0.84 nM. To show a potentially practical application, the GADs were used to detect Hg²⁺ in certified reference material and lake water.

Gold nanoclusters-manganese dioxide composite-based fluorescence immunoassay for sensitive monitoring of fenitrothion degradation in Chinese cabbage

Understanding the residues and degradation of organophosphorus pesticides (OPs) in crops has attracted increasing attention. Herein, we designed a sensitive fluorescence immunoassay (FIA) by employing nanobody-linked alkaline phosphatase (Nb-ALP) and gold nanoclusters anchored manganese dioxide (AuNCs-MnO₂) composite. In immunoassay protocol, Nb-ALP is used to competitively recognize the coating antigen and pesticide. After competitive immunoreaction, alkaline phosphatase catalyzes l-ascorbic acid-2-phosphate to produce ascorbic acid that can trigger the decomposition of the AuNCs-MnO₂ composite, regulating the fluorescence response. As a proof-of-concept, fenitrothion (FNT) is chosen as the target analyte. As a result, the developed FIA exhibits high detection sensitivity (IC₁₀ = 5.78 pg/mL), which is about 56-times higher than that of the conventional enzyme-linked immunosorbent assay. The developed FIA has been successfully applied for precisely monitoring the degradation of FNT in Chinese cabbage with excellent anti-interference ability and reproducibility, paving the way for the determination of pesticide residues in real food samples.

The applications of nanozymes in cancer therapy: based on regulating pyroptosis, ferroptosis and autophagy of tumor cells

Nanozymes are nanomaterials with catalytic properties similar to those of natural enzymes, and they have recently been collectively identified as a class of innovative artificial enzymes. Nanozymes are widely used in various fields, such as biomedicine, due to their high catalytic activity and stability. Nanozymes can trigger changes in reactive oxygen species (ROS) levels in cells and the activation of inflammasomes, leading to the programmed cell death (PCD), including the pyroptosis, ferroptosis, and autophagy, of tumor cells. In addition, some nanozymes consume glucose, starving cancer cells and thus accelerating tumor cell death. In addition, the electric charge of the structure and the catalytic activity of nanozymes are sensitive to external factors such as light and electric and magnetic fields. Therefore, nanozymes can be used with different therapeutic methods, such as chemodynamic therapy (CDT), photodynamic therapy (PDT) and sonodynamic therapy (SDT), to achieve highly efficient antitumor effects. Many cancer therapies induce tumor cell death via the pyroptosis, ferroptosis, and autophagy of tumor cells mediated by nanozymes. We review the mechanisms of pyroptosis, ferroptosis, and autophagy in tumor development, as well as the potential application of nanozymes to regulate pyroptosis, ferroptosis, and autophagy in tumor cells.

pH-responsive dual-enzyme mimics based on hollow metal organic framework-derivatives β-Co(OH)2 for multiple colorimetric assays

A hollow metal organic framework derivative β-Co(OH)₂ has been prepared, which possesses oxidase and peroxidase-like activities. Oxidase-like activity is derived from the generation of free radicals, and peroxidase-like activity is related to the electron transfer process. Unlike other nanozymes with dual enzyme-like activities, β-Co(OH)₂ possesses pH-responsive enzyme-like activities, among which the β-Co(OH)₂ exhibits superior oxidase and peroxidase-like activities under pH of 4 and 6, respectively, which could avoid mutual interference between multiple enzymes. Based on the phenomenon that enzyme-like activities of β-Co(OH)₂ can catalyze colorless TMB to generate blue oxidized TMB (oxTMB) with absorption peak at 652 nm, the sensors integrating total antioxidant capacity and H₂O₂ quantification were developed. The oxidase-like activity-based colorimetric system has a sensitive response to ascorbic acid, Trolox, and gallic acid, in which the limit of detection for those antioxidant substances was 0.54 μM, 1.26 μM, and 14.34 μM, respectively. The sensors based on peroxidase-like activity had low limit of detection of 1.42 μM for H₂O₂ and a linear range of 5-1000 μM. The proposed method can be well applied to the detection of the total antioxidant capacity of kiwi, Vc tables, orange and tea extract with high accuracy, and H₂O₂ determination in milk and glucose detection in beverages with satisfactory recovery (within 97-106%).

Ultrafine V2O5-anchored 3D N-doped carbon nanocomposite with augmented dual-enzyme mimetic activity for evaluating total antioxidant capacity

Total antioxidant capacity (TAC) can be evaluated by detecting the content of antioxidants, such as ascorbic acid, based on the enzyme-mimetic activity of nanomaterials. Herein, we fabricated a 3D-V₂O₅/NC nanocomposite using a self-templating strategy, which achieved ultrafine particles (∼2.5 nm), a porous carbon layer, large specific surface area (152.4 m²/g), N-doping and heterogeneous structure. The strong catalytic activity of 3D-V₂O₅/NC resulted from the integrated effect between the ultrafine structure of V₂O₅ nanoparticles and the 3D porous nitrogen-doped carbon framework, effectively increasing the number of active sites. This nanozyme presented a higher catalytic activity than its components or precursors in the nanocomposite (e.g., VN/NC, NC, V₂O₅, and VO₂/g-C₃N₄). ROS scavenging experiments confirmed that the dual enzyme-like activity of 3D-V₂O₅/NC (catalase-like and oxidase-like) resulted from their co-participation of ‧O₂^-, h⁺ and ‧OH, among which ‧O₂^- played a crucial role in the catalytic color reaction. By virtue of the 3D-V₂O₅/NC nanoenzyme activity and TMB as a chromogenic substrate, the mixed system of 3D-V₂O₅/NC + TMB + H₂O₂ provided a low detection limit (0.03 μM) and suitable recovery (93.0-109.5%) for AA. Additionally, a smartphone-based colorimetric application was developed employing "Thing Identify" software to evaluate TAC in beverages. The colorimetric sensor and smartphone-detection platform provide a better or comparable analytical performance for TAC assessment in comparison to commercial ABTS test kits. The newly developed smartphone-based colorimetric platform presents several prominent advantageous, such as low cost, simple/rapid operation, and feasibility for outdoor use.

Enhanced Peroxidase-Mimic Catalytic Activity via Cerium Doping of Strontium-Based Metal-Organic Frameworks with Design of a Smartphone-Based Sensor for On-Site Salivary Total Antioxidant Capacity Detection in Lung Cancer Patients

The development of artificial nanozymes with superior catalytic performance and excellent stability has been a long-standing objective for chemists. The total antioxidant capacity (TAC) is one of the most important bioanalytical measures of oxidative stress in the body. The present work aims to develop a smartphone-assisted visual detection sensor using cerium-doped strontium-based metal-organic frameworks (Ce-SrMOFs) as peroxidase-like nanozymes for the rapid, low-cost, on-site detection of TAC. The pristine SrMOF functioned as a peroxidase nanozyme, and its enzymatic activity was enhanced after doping it with Ce(IV) ions because of the multivalent nature and synergistic impact of the heteroatoms. The Ce-SrMOFs were sensitive to the single electron transfer and hydrogen atom transfer processes, which implies that the Ce-SrMOFs can serve as an ideal nanozyme candidate for TAC analysis. The investigated mechanism revealed that •OH is the most active oxygen species for the peroxidase-like activity. The Ce-SrMOFs exhibited a strong affinity for 3,3',5,5'-tetramethylbenzidine (TMB) and H₂O₂, with K_m values of 0.082 and 0.427 mM, which are 5.29- and 8.67-fold lower than those of horseradish peroxidase (HRP), respectively. The Ce-SrMOFs were used for the detection of ascorbic acid, cysteine, and glutathione, with limits of detection of 44, 53, and 512 nM, respectively. The proposed method proved effective in measuring the TAC in saliva samples from lung cancer patients, thereby yielding results with satisfactory precision and accuracy.

Synergistic effect of silver nanoclusters and graphene oxide on visible light-driven oxidase-like activity: Construction of a sustainable nanozyme for total antioxidant capacity detection

The high cost and low reusability of natural enzymes greatly limit their application in biosensing. In this work, a sustainable nanozyme with light-driven oxidase-like activity was fabricated by integrating protein-capped silver nanoclusters (AgNCs) with graphene oxide (GO) through multiple non-covalent interactions. The prepared AgNCs/GO nanozyme could effectively catalyze the oxidation of various chromogenic substrates by activating dissolved O₂ to reactive oxygen species under visible light irradiation. Moreover, the oxidase-like activity of AgNCs/GO could be well controlled by switching on and off the visible light source. Compared with natural peroxidase and most of other oxidase-mimicking nanozymes, AgNCs/GO possessed improved catalytic activity owing to the synergistic effect between AgNCs and GO. More importantly, AgNCs/GO showed outstanding stability against precipitation, pH (2.0-8.0), temperature (10-80 °C), and storage and could be reused at least 6 cycles without obvious loss in catalytic activity. On this basis, AgNCs/GO nanozyme was used to develop a colorimetric assay for the determination of total antioxidant capacity in human serum, which had the merits of high sensitivity, low cost, and good safety. This work holds a promising prospect in developing sustainable nanozymes for biosensing and clinical diagnosis.

Rapid in-situ growth of enzyme-mimicking Pd nanoparticles on TEMPO-oxidized nanocellulose for the efficient detection of ascorbic acid

Rapid, efficient and green method of Pd nanoparticles (PdNPs) synthesis on TEMPO-oxidized cellulose nanofibril (TCNF) is demonstrated here. The nanohybrid (PdNPs/TCNF) exhibited peroxidase and oxidase-like activities evident by the oxidation of three chromogenic substrates. Enzyme kinetic studies using 3,3',5,5'-Tetramethylbenzidine (TMB) oxidation uncovered the excellent kinetic parameters (low K_m and high V_max) and good specific activities of 215 U/g and 107 U/g for peroxidase and oxidase-like activities, respectively. A colorimetric assay for ascorbic acid (AA) detection is proposed based on its ability to reduce oxidized TMB to its colorless form. However, presence of nanozyme caused re-oxidation of TMB to its blue colored form within few minutes resulting in time limitation and inaccurate detection. Thanks to the film forming nature of TCNF; this limitation was overcome by employing PdNPs/TCNF film strips that can be easily removed before AA addition. The assay allowed AA detection in the linear range of 0.25-10 μM with a detection limit of 0.039 μM. The results of AA detection in commercial beverages and vitamin C tablets are matching with the specified values. Further the nanozyme exhibited high tolerance to pH (2-10) and temperature (up to 80 °C) and good recyclability for five cycles.

Nanoclusterzyme for Dual Colorimetric Sensings: A Case Study on [Au14 (Dppp)5 I4 ]2

The enzymatic activity of atomically precise metal nanoclusters has recently been recognized; however, the number of nanoclusterzymes is very small. Besides, the applications of nanoclusterzyme wait to be explored. Herein, a novel nanoclusterzyme is synthesized and its structure is majorly resolved by single-crystal X-ray diffraction and mass spectrometry, which reveal that the nanocluster consists of an Au₁₃ icosahedron capped by an exterior shell including four I, three Dppp (1,3-bis(diphenylphosphino) propane) ligands, and a rarely reported Dppp-Au-Dppp handle staple, which contributes a lot to the enzyme activity of [Au₁₄ (Dppp)₅ I₄ ]²⁺ nanocluster. The as-obtained nanocluster can catalyze oxygen to O₂ ^•- under visible light irradiation with a specific activity up to 0.182 U·mg^-1 and lead to the blue color of 3,3',5,5'-tetramethylbenzidine (TMB) in both solution and solid states. With the addition of acetylcholinesterase (AChE), the blue color of (Au₁₄  + TMB) solution system disappears due to the nanoclusterzyme activity inhibition, but the further addition of organophosphorus pesticides (OPs) into the above mixture can restore the nanoclusterzyme and recover the blue color. Based on the color turn-off and on, the various nanoclusterzyme-containing systems are used to colorimetrically sense AChE and OPs with the detection limits reaching 0.04 mU·mL^-1 and 0.02 ng·mL^-1 , respectively.

Photo-responsive oxidase-like nanozyme based on a vanadium-docked porphyrinic covalent organic framework for colorimetric L-Arginine sensing

This study reports the development of a vanadium-docked porphyrinic covalent organic framework as a novel class of highly polar photoactive materials. Thanks to its extended π-electron conjugation and high chemical stabilities, this framework can serve as an oxidase-Like photo-nanozyme for photocatalytic oxidation of o-phenylenediamine (o-PDA) and a colorimetric substrate for the production of the yellow-colored oxidized o-PDA (o-PDAox). The physicochemical properties of the as-prepared photo-nanozyme were characterized by several analytical techniques. Its enhanced light harvesting and charge separation and transfer were also verified by electrochemical and spectroscopic analysis. This photo-nonenzymatic colorimetric assay was applied for the sensitive L-Arginine (L-Arg) detection as a typical amino acid in the linear range of 8.1 nM-330 μM with a limit of detection (LOD) of 3.5 nM. The findings of this research confirmed the safety and feasibility of the proposed photo-nonenzymatic colorimetric sensing strategy for the detection of L-Arg and other similar biomolecules in food samples. Kinetic investigation revealed that the photo-responsive oxidase mimic exhibits satisfactory K_m (0.47 mM) and V_max (42.0 μM/s) values. This work broadened our insight into the development of modified porphyrinic-COF-based visible light-responsive oxidase-like photo-nanozyme for environmentally friendly colorimetric biosensing.

BSA-stabilized silver nanoclusters for efficient photoresponsive colorimetric detection of chromium(VI)

Hexavalent chromium is a highly toxic substance, which will pose a serious threat to human life and health and the entire ecosystem. Therefore, it is crucial to establish a simple and rapid detection method for hexavalent chromium. In this work, we fabricated bovine serum albumin-stabilized silver nanocluster (BSA-Ag₁₃ NC) which exhibited photoresponsive oxidase-like activity, catalyzing the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to the blue oxidized state TMB (oxTMB) in a short time. Interestingly, 8-hydroxyquinoline (8-HQ) can significantly inhibit the color reaction of TMB oxidation while Cr(VI) can interact specifically with 8-HQ to restore this chromogenic reaction. Based on the above facts, a colorimetric sensing system for detecting Cr(VI) was developed. The sensing system shows a wide linear range, and good selectivity, with a low detection limit of 2.32 nM. Moreover, this sensing system could be successfully applied to the detection of Cr(VI) in lake water, tap water, and sewage with satisfactory results.

A redox reaction-induced ratiometric fluorescence platform for the specific detection of ascorbic acid based on Ag2S quantum dots and multifunctional CoOOH nanoflakes

In this work, a ratiometric fluorescent nanoplatform for the detection of ascorbic acid (AA) was constructed based on the Ag₂S quantum dots (QDs) and multifunctional hydroxyl cobalt oxide nanoflakes (CoOOH NFs). Ag₂S QDs can be assembled on the surface of CoOOH NFs by electrostatic adsorption, resulting in the quenching of the NIR fluorescence emission of Ag₂S QDs at 680 nm effectively through the inner filter effect (IFE). o-Phenylenediamine (OPD), a common substrate of oxidase-like (OXD) mimic, is rapidly oxidized into the fluorescent product of 2,3-diaminophenazine (DAP) with the appearance of an emission peak at 575 nm under the catalysis of CoOOH NFs. After AA was added, the fluorescence emission of DAP declined because of the decline in the OXD-like activity of CoOOH NFs due to the transformation of Co²⁺. Simultaneously, Ag₂S QDs were released, accompanied by the recovery of red fluorescence. These two fluorescent signals can be excited at the same excitation wavelength, simplifying the detection procedure. Using F₅₇₅/F₆₈₀ as the readout, the quantification of AA can be realized with the linear range and detection limit of 0.2 μM-20 mM and 0.014 μM, respectively. The ratiometric fluorescence sensor can be effectively used to determine the content of AA in real samples such as juice and serum. This work integrates the in-situ formation of the fluorescent species via the catalysis of the nanozyme and the redox reaction to destroy the CoOOH NFs nanozyme as well as the two dimensional nanoflake induced turn-off-on strategy for Ag₂S QDs, which provides a specific strategy for the selective detection of AA and may offer a reliable approach for the construction of other biosensing platforms.

Zeolitic imidazolate frameworks as light-responsive oxidase-like mimics for the determination of adenosine triphosphate and discrimination of phenolic pollutants

In this study, 3,3',5,5'-tetramethylbenzidine (TMB) was selected as a chromogenic substrate to evaluate the light-responsive oxidase-like activity of different zeolitic imidazolate frameworks (ZIFs). The synthesized ZIFs were systematically characterized by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction analysis. Several main operational parameters, including ZIFs and TMB concentrations, pH value, radiation time, and working current, in the reaction process were optimized. The kinetic measurement results show that ZIF-90 exhibits higher affinity to the substrate than horseradish peroxidase. Furthermore, given that adenosine triphosphate (ATP) can specifically combine with Zn²⁺ binding site and destroy the structure of ZIF-90, a specific and sensitive colorimetric method was established for the quantitative detection of ATP within the range 10 - 240 μM. In addition, on the basis that phenolic pollutants can impact the reaction kinetics diversely on different ZIFs, a sensor array was constructed and successfully applied to differentiate five phenolic pollutants in lake water samples. This work is expected to shed light on the establishment of ZIF-based light-responsive oxidase-like nanozymes for the highly selective colorimetric detection and sensor array.

Nanozyme-encoded luminescent detection for food safety analysis: An overview of mechanisms and recent applications

With the rapid growth in global food production, delivery, and consumption, reformative food analytical techniques are required to satisfy the monitoring requirements of speed and high sensitivity. Nanozyme-encoded luminescent detections (NLDs) integrating nanozyme-based rapid detections with luminescent output signals have emerged as powerful methods for food safety monitoring, not only because of their preeminent performance in analysis, such as rapid, facile, low background signal, and ultrasensitive, but also due to their strong attractiveness for future sensing research. However, the lack of a full understanding of the fundamentals of NLDs for food safety detection technologies limits their further application. In this review, a systematic overview of the mechanisms of NLDs and their applications in the food industry is summarized, which covers the nanozyme-mimicking types and their luminescent signal generation mechanisms, as well as their applications in monitoring common foodborne contaminants. As demonstrated by previous studies, NLDs are bridging the gap to practical-oriented food analytical technologies and various opportunities to improve their food analytical performance to be considered in the future are proposed.

Construction of Pd Single Site Anchored on Nitrogen-Doped Porous Carbon and Its Application for Total Antioxidant Level Detection

Natural enzymes have excellent catalytic activity. However, due to their unstable nature and high cost, current research has turned to the synthesis and development of enzyme-like nanomaterials and single-atomic nanozymes. In this study, a single-atomic palladium-loaded nitrogen-doped porous carbon catalyst (SA-Pd/NPC) was prepared and used as a mimetic peroxidase to catalyze the substrates oxidation. The catalytic capability of the SA-Pd/NPC was tested by the TMB-H₂O₂ system, and it expressed a superior catalytic capability owing to the plentiful catalytic centers of the single-atom Pd, its high porosity, the large specific surface area, and the strong electron transfer capability of the NPC. For the color reaction of TMB, thiol antioxidants (e.g., glutathione, GSH) and non-thiol antioxidants (e.g., ascorbic acid, AA) are suitable for different inhibition mechanisms. GSH and AA are typical substances of these two main antioxidant types, respectively. Here, we demonstrate that this prepared catalyst could be used to simultaneously determine a variety of major known physiologically relevant thiol-containing and thiol-free antioxidants, accompanied by a blue color gradient change with UV-Vis spectra at 652 nm through the SA-Pd/NPC-catalyzed TMB-H₂O₂ system. Linear responses to GSH and AA could be obtained in the concentration ranges of 0.01-0.10 mM and 1-13 μM (both R² values were greater than 0.970), respectively, while the limits of detection were 3 μM and 0.3 μM, respectively. The ability of the nanozyme to detect overall antioxidant levels (TAL) was also confirmed in subsequent tests on artificial saliva and biological samples.

Construction of Donor-Acceptor Heteroporous Covalent Organic Frameworks as Photoregulated Oxidase-like Nanozymes for Sensing Signal Amplification

Nanomaterials with enzyme-like characteristics (called nanozymes) show their extreme potentials as alternatives to natural enzymes. Covalent organic frameworks (COFs) as metal-free nanozymes have attracted huge attention for catalytic applications due to their flexible molecular design and synthetic strategies and conjugated, porous, and chemically stable architectures. Designing high-performance two-dimensional (2D) porous COF materials embedded with functional building units for modulating nanozymes' catalytic activity is of immense importance in contemporary research. The proper combination of donor-acceptor (D-A) fragments within a porous COF skeleton is an effective strategy to decrease the band gap and provide a strong charge-transfer pathway for highly effective charge separation. Herein, two donor-acceptor heteroporous COFs using an electron-deficient 4,4'-(thiazolo[5,4-d]thiazole-2,5-diyl)dibenzaldehyde (Tz) unit or 4,4'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzaldehyde (Td) unit and electron-rich tetrakis(4-aminophenyl)ethane (ETTA) linkers were presented. The resulting crystalline and heteroporous COFs showed outstanding oxidase-like activity under light irradiation, which can catalyze the oxidation of typical substrates and corresponding evolution in color and absorption. The light-activatable ETTA-Tz COF with prominent oxidase-like activity can serve as a colorimetric probe for quantitative detection of sulfide ions with a linear range of 1-50 μM and a detection limit of 0.27 μM within 3 min. The colorimetric approach could also be used for sulfide ion detection in human serum samples. The research demonstrated the future potential of D-A motifs within fully conjugated COFs to obtain excellent mimic enzyme activity.

Copper ions assisted fluorescent detection of some dithiocarbamates based on nickel nanocluster with aggregation-induced emission enhancement behavior

Pesticide residue contamination has become an urgent issue since it threatens both the natural environment and public health. In this study, a fluorescent method for detecting dithiocarbamate (DTC) compounds was constructed based on novel nickel nanoclusters (Ni NCs) and copper ions (Cu²⁺). The water-soluble fluorescent Ni NCs were synthesized for the first time through a one-pot method using glutathione as stabilizer and ascorbic acid as reducing agent. The as-prepared Ni NCs exhibited a maximum fluorescence emission at 445 nm when excited by 380 nm. And they displayed aggregation-induced emission enhancement when ethylene glycol was introduced into the nanocluster aqueous solution. Based on the Ni NCs, a label-free fluorescence quenching sensor was established for sensitive and selective detection of DTC compounds with the assistance of Cu²⁺. The complex formed by DTC and Cu²⁺ led to fluorescence quenching of Ni NCs through inner filter effect. The sensing method was successfully applied to two typical DTC compounds, thiram and disulfiram, with good linearity over a wide linear range and a low detection limit. Moreover, the proposed approach was capable of thiram detection in real samples, which confirms the potential of this sensing method as a platform for DTC compound detection.

Nanozyme-enabled sensing strategies for determining the total antioxidant capacity of food samples

Gold doped copper hexacyanoferrate (Au@Cu-HCF) nanozyme-based colorimetric sensing strategies were exploited to measure total antioxidant capacity (TAC) of some plant-derived food samples. The new developed Au@Cu-HCF nanozyme replaces natural enzymes to catalyze a redox reaction, and antioxidants can compete with substrates in the interaction with OH, leading to an antioxidant concentration-dependent color change. Depending on the Au@Cu-HCF-based ABTS colorimetric strategy, a smartphone-based sensor was devised using smartphone's camera as a "smart detector". The proposed sensor was successfully utilized to measure the TAC of lotus root (4.61 mM), citrus juice (6.35 mM), and lemon beverage (1.00 mM) with standard deviations of 0.16 mM, 0.16 mM, and 0.06 mM, respectively. These results all agree well with the commercial kit (4.55 mM for lotus root, 6.27 mM for citrus juice and 1.11 mM for lemon beverage), indicating this sensor has good practical applications in the TAC measurement of food samples.

Construction of metal (Mn, Ce, Eu)-containing species in CN nanocomposites with photo-responsive oxidase-mimicking activity for multi-antioxidant discrimination

On the basis of three M-CN nanocomposites with photo-oxidase activity, a colorimetric sensor is proposed for the pattern recognition of antioxidants.

Nanoscale Covalent Organic Frameworks with Donor-Acceptor Structures as Highly Efficient Light-Responsive Oxidase-like Mimics for Colorimetric Detection of Glutathione

Although organic artificial enzymes have been reported as biomimetic oxidation catalysts and are widely used for colorimetric biosensors, developing organic artificial enzymes with high enzymatic activity is still a challenge. Two-dimensional (2D) covalent organic frameworks (COFs) have shown superior potential in biocatalysts because of their periodic π-π arrays, tunable pore size and structure, large surface area, and thermal stability. The interconnection of electron acceptor and donor building blocks in the 2D conjugated COF skeleton can lead to narrower band gaps and efficient charge separation and transportation and thus is helpful to improve catalytic activity. Herein, a donor-acceptor 2D COF was synthesized using tetrakis(4-aminophenyl)pyrene (Py) as an electron donor and thieno[3,2-b]thiophene-2,5-dicarbaldehyde (TT) as an electron acceptor. Under visible light irradiation, the donor-acceptor 2D COF exhibited superior enzymatic catalytic activity, which could catalyze the oxidation of chromogenic substrates such as 3,3',5,5'-tetramethylbenzidine (TMB) by the formation of superoxide radicals and holes. Based on the above property, the photoactivated donor-acceptor 2D COF with enzyme-like catalytic properties was designed as a robust colorimetric probe for cheap, highly sensitive, and rapid colorimetric detection of glutathione (GSH); the corresponding linear range of GSH was 0.4-60 μM, and the limit of detection was 0.225 μM. This study not only presents the construction of COF-based light-activated nanozymes for environmentally friendly colorimetric detection of GSH but also provides a smart strategy for improving nanozyme activity.