Deep convolutional neural network-based 3D fluorescence sensor array for sugar identification in serum based on the oxidase-mimicking property of CuO nanoparticles.

Developing sensor arrays capturing comprehensive fluorescence (FL) spectra from a single probe is crucial for understanding sugar structures with very high similarity in biofluids. Therefore, the analysis of highly similar sugar' structures in biofluids based on the entire FL of a single nanozyme probe needs more concern, which makes the development of novel alternative approaches highly wanted for biomedical and other applications. Herein, a well-designed deep learning model with intrinsic information of 3D FL of CuO nanoparticles (NPs)' oxidase-like activity was developed to classify and predict the concentration of a group of sugars with very similar chemical structures in different media. The findings presented that the overall accuracy of the developed model in classifying the nine selected sugars was (99-100 %), which prompted us to transfer the developed model to predict the concentration of the selected sugars at a concentration range of (1-100 µM). The transferred model also gave excellent results (R2 = 97-100 %). Therefore, the model was extended to other more complex applications, namely the identification of mixtures of sugars in serum and the detection of polysaccharides in different media such as serum and lake water. Notably, LOD for fructose was determined at 4.23 nM, marking a 120-fold decrease compared to previous studies. Our developed model was also compared with other deep learning-based models, and the results have demonstrated remarkable progress. Moreover, the identification of other possible coexisting interference substances in lake water samples was considered. This work marks a significant advancement, opening avenues for the widespread application of sensor arrays integrating nanozymes and deep learning techniques in biomedical and other diverse fields.

Laminarin-modulated osmium nanozymes with high substrate-affinity and selective peroxidase-like behavior engineered colorimetric assay for hydroxyl radical scavenging capacity estimation.

Hydroxyl radical (·OH) scavenging capacity (HOSC) estimation is essential for evaluating antioxidants, natural extracts, or drugs against clinical diseases. While nanozymes offer advantages in related applications, they still face limitations in activity and selectivity. In response, this work showcases the fabrication of laminarin-modulated osmium (laminarin-Os) nanoclusters (1.45 ± 0.05 nm), functioning as peroxidase-like nanozymes within a colorimetric assay tailored for rational HOSC estimation. This study validates both the characterization and remarkable stability of laminarin-Os. By leveraging the abundant surface negative charges of laminarin-Os and the surface hydroxyls of laminarin, oxidation reactions are facilitated, augmenting laminarin-Os's affinity for 3,3',5,5'-tetramethylbenzidine (TMB) (KM = 0.04 mM). This enables the laminarin-Os-based colorimetric assay to respond to ·OH more effectively than citrate-, albumin-, or other polysaccharides-based Os. In addition, experimental results also validate the selective peroxidase-like behavior of laminarin-Os under acidic conditions. Antioxidants like ascorbic acid, glutathione, tannic acid, and cysteine inhibit absorbance at 652 nm in the colorimetric platform using laminarin-Os's peroxidase-like activity. Compared with commercial kits, this assay demonstrates superior sensitivity (e.g., responds to ascorbic acid 0.01-0.075 mM, glutathione 1-15 µg/mL, tannic acid 0.5-5 µM, and monoammonium glycyrrhizinate cysteine 1.06-10.63 µM) and HOSC testing for glutathione, tannic acid, and monoammonium glycyrrhizinate cysteine. Overall, this study introduces a novel Os nanozyme with exceptional TMB affinity and ·OH selectivity, paving the way for HOSC estimation in biomedical research, pharmaceutical analysis, drug quality control, and beyond.

A colorimetric sensing strategy based on chitosan-stabilized platinum nanoparticles for quick detection of α-glucosidase activity and inhibitor screening.

α-Glucosidase (α-Glu) is implicated in the progression and pathogenesis of type II diabetes (T2D). In this study, we developed a rapid colorimetric technique using platinum nanoparticles stabilized by chitosan (Ch-PtNPs) to detect α-Glu activity and its inhibitor. The Ch-PtNPs facilitate the conversion of 3,3',5,5'-tetramethylbenzidine (TMB) into oxidized TMB (oxTMB) in the presence of dissolved O2. The catalytic hydrolysis of 2-O-α-D-glucopyranosyl-L-ascorbic acid (AA-2G) by α-Glu produces ascorbic acid (AA), which reduces oxTMB to TMB, leading to the fading of the blue color. However, the presence of α-Glu inhibitors (AGIs) hinders the generation of AA, allowing Ch-PtNPs to re-oxidize colorless TMB back to blue oxTMB. This unique phenomenon enables the colorimetric detection of α-Glu activity and AGIs. The linear range for α-Glu was found to be 0.1-1.0 U mL-1 and the detection limit was 0.026 U mL-1. Additionally, the half-maximal inhibition value (IC50) for acarbose, an α-Glu inhibitor, was calculated to be 0.4769 mM. Excitingly, this sensing platform successfully detected α-Glu activity in human serum samples and effectively screened AGIs. These promising findings highlight the potential application of the proposed strategy in clinical diabetes diagnosis and drug discovery.

Advances and perspectives of nanozymes in respiratory diseases.

Respiratory diseases, some of the most common human diseases, have become a prominent public health and medical problem. Feasible treatment and prevention strategies are still required to prepare for respiratory emergencies. Nanotechnology has provided new technological conceptions in respiratory disease-related applications and inspired the exploration of various multifunctional nanomaterials. Among them, "nanozymes" with enzyme-like activities and nanomaterials' physicochemical properties may propel the development in this field. Over the past few decades, nanozymes have distinguished themselves in the fields of biosensing, biomedicine, imaging, and environmental protection due to their outstanding enzymatic properties, reactive oxygen species-regulating mechanism, high stability, modifiability, mass production, and others. Herein, this article reviews the research progress of nanozymes in diagnosing, treating, and preventing respiratory diseases, hoping to bring new ideas for promoting nanozymes and their beneficial applications in respiratory diseases.

Machine learning-based sensor array: full and reduced fluorescence data for versatile analyte detection based on gold nanocluster as a single probe.

Different acquisition data approaches have been used to fetch the fluorescence spectra. However, the comparison between them is rare. Also, the extendability of a sensor array, which can work with heavy metal ions and other types of analytes, is scarce. In this study, we used first- and second-order fluorescent data generated by 6-Aza-2-thiothymine-gold nanocluster (ATT-AuNCs) as a single probe along with machine learning to distinguish between a group of heavy metal ions. Moreover, the dimensionality reduction was carried out for the different acquisition data approaches. In our case, the accuracy of different machine learning algorithms using first-order data outperforms the second-order data before and after the dimensionality reduction. For proving the extendibility of this approach, four anions were used as an example. As expected, the same finding has been found. Furthermore, random forest (RF) showed more stable and accurate results than other models. Also, linear discriminant analysis (LDA) gave acceptable accuracy in the analysis of the high-dimensionality data. Accordingly, using LDA in high-dimensionality data (the first- and second-order data) analysis was highlighted for discrimination between the selected heavy metal ions in different concentrations and in different molar ratios, as well as in real samples. Also, the same method was applied for the anion's discrimination, and LDA gave an excellent separation ability. Moreover, LDA was able to differentiate between all the selected analytes with excellent separation ability. Additionally, the quantitative detection was considered using a wide concentration range of Cd2+, and the LOD was 60.40 nM. Therefore, we believe that our approach opens new avenues for linking analytical chemistry, especially sensor array chemistry, with machine learning.

Carboxylated chitosan enabled platinum nanozyme with improved stability and ascorbate oxidase-like activity for a fluorometric acid phosphatase sensor.

Chitosan modification has attracted considerable interest in the nanozyme field last decade. As a chitosan derivative, carboxylated chitosan (CC) has been less explored. Herein, PtNPs with an average size of approximately 3.3 nm and zeta potential of -44.8 ± 0.3 mV (n = 3) have been prepared by using CC as the surface modification (CC-PtNPs). We have carried out an in-depth investigation of CC-PtNPs, including the characterization, colloidal stability, and ascorbate oxidase-like activity. Due to the contribution of carboxylated chitosan, CC-PtNPs present improved colloidal stability and ascorbate oxidase-like activity compared to chitosan-modified Pt nanozyme. Inspired by these results, a fluorometric acid phosphatase sensor was proposed based on the improved performance of CC-PtNPs. This sensor exhibits excellent sensitivity and selectivity towards acid phosphatase in the linear range of 0.25-18 U/L with a low limit of detection (1.31 × 10-3 U/L). The concentration of acid phosphatase in human semen samples has been successfully measured.

Fructose oxidase-like activity of CuO nanoparticles supported by phosphate for a tandem catalysis-based fructose sensor.

A surge of nanozymes with oxidase-like activities is emerging in various fields, whereas nanozymes with the ability to catalyze the oxidation of saccharides have less been explored. Herein, CuO nanoparticles (NPs) with phosphate-supported fructose oxidase-like activity have been reported. Notably, reactive oxygen species (ROS) have been confirmed as the products during the process. By coupling the fructose oxidase-like activity with the peroxidase-like activity of CuO NPs, a tandem catalysis-based fructose sensor can be fabricated. In detail, CuO NPs can catalyze the fructose oxidation under O2 to yield ROS (e.g., H2O2, •OH, and O2·-) and effectively decompose H2O2 into ·OH. After that, terephthalic acid can be oxidized by •OH produced from the tandem catalysis to generate a fluorescent product. This sensor shows a linear range toward fructose (0.625-275 µÐœ) with a low limit of detection (0.5 µÐœ), which can be successfully conducted to detect fructose from real samples. Overall, this work aims to expand the catalytic types of nanozymes and provide a desirable fructose sensor.

Immunofluorescent-aggregation assay based on anti-Salmonella typhimurium IgG-AuNCs, for rapid detection of Salmonella typhimurium.

Sensitive and rapid detection of pathogenic bacteria plays an important role in avoiding food poisoning. However, the practical application value of conventional assays for detection of foodborne bacteria, are limited by major drawbacks; these include the laboriousness of pure culture preparation, complexity of DNA extraction for polymerase chain reaction, and low sensitivity of enzyme-linked immunosorbent assay. Herein, we designed a non-complex strategy for the sensitive, quantitative, and rapid detection of Salmonella typhimurium with high specificity, using an anti-Salmonella typhimurium IgG-AuNC-based immunofluorescent-aggregation assay. Salmonella typhimurium was agglutinated with fluorescent anti-Salmonella typhimurium IgG-AuNC on a glass slide, and observed using a fluorescence microscope with photoexcitation and photoemission at 560 nm and 620 nm, respectively. Under optimized reaction conditions, the AuNC-based immunofluorescent-aggregation assay had a determination range between 7.0 × 103 and 3.0 × 108 CFU/mL, a limit of detection of 1.0 × 103 CFU/mL and an assay response time of 3 min. The technique delivered good results in assessing real samples.

A peroxidase-like activity-based colorimetric sensor array of noble metal nanozymes to discriminate heavy metal ions.

Heavy metal ions (HMIs), including Cu2+, Ag+, Cd2+, Hg2+, and Pb2+ from the environment pose a threat to human beings and can cause a series of life-threatening diseases. Therefore, colorimetric sensors with convenience and flexibility for HMI discrimination are still required. To provide a solution, a peroxidase-like activity-based colorimetric sensor array of citrate-capped noble metal nanozymes (osmium, platinum, and gold) has been fabricated. Some studies reported that some HMIs could interact with the noble metal nanozymes leading to a change in their peroxidase-like activity. This phenomenon was confirmed in our work. Based on this principle, different concentrations of HMIs (Cu2+, Ag+, Cd2+, Hg2+, and Pb2+) were discriminated. Moreover, their practical application has been tested by discriminating HMIs in tap water and SiYu lake water. What is more, as an example of the validity of our method to quantify HMIs at nanomolar concentrations, the LOD of Hg2+ was presented. To sum up, our study not only demonstrates the differentiation ability of this nanozyme sensor array but also gives hints for using nanozyme sensor arrays for further applications.

Protein-Assisted Osmium Nanoclusters with Intrinsic Peroxidase-like Activity and Extrinsic Antifouling Behavior.

Extensive studies have laid the groundwork for understanding peroxidase-like nanozymes. However, improvements are still required before their practical applications. On one hand, it is significant to explore highly reactive nanozymes. On the other hand, it is necessary to avoid fouling formed on the surface of nanozymes, which will affect their activity and the results of H2O2 sensors or H2O2-related applications. Herein, a strategy is reported to design osmium nanoclusters (Os NCs) with the existence of bovine serum albumin (BSA) through biomineralization. BSA-Os NCs were found to possess intrinsic peroxidase-like activity with a high specific activity (6120 U/g). Studies also found that the catalytic activity of BSA-Os NCs was better than those of reported protein-assisted metal nanozymes (e.g., BSA-Pt NPs and BSA-Au NCs). More significantly, BSA has been confirmed as a protective shell to give Os NCs extrinsic antifouling property in some typical ions (e.g., Hg2+, Ag+, Pb2+, I-, Cr6+, Cu2+, Ce3+, S2-, etc.), saline (0-2 M), or protein (0-100 mg/mL) conditions. Under optimal conditions, a colorimetric sensor was established to realize a linear range of H2O2 from 1.25 to 200 µM with a low detection limit of 300 nM. On this basis, remarkable features enable a BSA-Os NCs-based colorimetric sensor to detect H2O2 from complex systems with clear color gradients. Together, this work highlights the advantages of protein-assisted Os nanozymes and provides a paragon for peroxidase-like nanozymes in H2O2-related applications.

Platinum group element-based nanozymes for biomedical applications: an overview.

With a rapid advancement of nanotechnology and the close integration of disciplines, research on nanozymes (nanomaterials with enzyme-like activities), is becoming an expeditiously developing field. In recent years, platinum group element (PGE)-based (Pt, Pd, Ru, Rh, Ir, and Os) nanozymes developed successively, have not only promoted the research of nanozymes but also expanded the biomedical applications of nanomaterials. Generally speaking, PGE-based nanozymes process high catalytic efficiency, specific surface area, stability, and other physical/chemical properties, which benefit for their applications in biosensing, biological medicine, biomedical imaging, and environmental protection. This paper will introduce the research progress of PGE-based nanozymes including their synthesis, characterization, enzyme-like activities, stability, biocompatibility, toxicity, and applications for biological detection and clinical relevance. Our emphasis is put on unfolding the roles of PGE-based nanozymes in biomedical applications and how they overcome the limitations. Last but not least, trends and future perspectives of PGE-based nanozymes in biomedical applications are also provided.

Single gold nanocluster probe-based fluorescent sensor array for heavy metal ion discrimination.

There is a continuing high demand to design effective sensors for the determination of heavy metal ions (HMIs) since they are hazardous to both human health and the environment. In this study, we reported a facile fluorescent sensor array for rapid discrimination of HMIs based on a single gold nanocluster (AuNC) probe. This AuNC probe was prepared by using 2-mercapto-1-methylimidazole (MMI) as a ligand and polyvinypyrrolidone (PVP) as a dispersing agent. The fluorescence emission of PVP/MMI-AuNC was observed to be closely related to the pH value of the aqueous solution, which displays yellow (λmax = 512 nm) and red (λmax = 700 nm) fluorescence at pH 12.0 and 6.0, respectively. Further experiments indicated that different HMIs can produce differential effects on the photoluminescence of PVP/MMI-AuNC and thus generate distinct fluorescent responses at 512 and 700 nm. On the basis of this phenomenon, a fluorescent sensor array based on the PVP/MMI-AuNC was then built by simply changing pH value in the sensor element. A total of seven HMIs had their unique response patterns and were successfully distinguished by hierarchical cluster analysis and linear discriminant analysis both in buffer solution and spiked water samples, achieving 100% identification accuracy. This study provides a simple and powerful fingerprinting sensing platform for multiple HMIs, showing broad application prospects in the field of environmental monitoring.

Sodium Alginate Modified Platinum Nanozymes With Highly Efficient and Robust Oxidase-Like Activity for Antioxidant Capacity and Analysis of Proanthocyanidins.

Platinum nanozymes exhibiting highly efficient and robust oxidase-like activity are successfully synthesized and modified using sodium alginate (SA-PtNPs). According to a steady-state dynamic assay, Michaelis-Menton constant (K m ) is calculated as 11.6 µM, indicating that the affinity of SA-PtNPs toward the substrate, 3, 3', 5, 5'-tetramethylbenzidine (TMB), is high. It shows in the paper that SA-PtNPs exhibit a significant oxidant effect on substrate-O2 to produce O 2 • - as an oxidase mimic. Moreover, the oxidase-like activity fluctuated slightly under changes in environmental pH and incubation time, implying that SA can increase the dispersibility and stability of PtNPs. A colorimetric assay for oligomeric proanthocyanidins (OPC) was realized given how few explorations of the former there are. We found that the significant inhibitory effect of OPC on the oxidase-like activity is due to the competitive effect between OPC and TMB for binding to the active site of SA-PtNPs, resulting in a color change. Under optimal conditions, the logarithmic value of the chromogenic difference (ΔA450nm) to OPC concentration was linear (4-32.5 µM, r = 0.999) with a limit of detection (LOD) of 2.0 µM. The antioxidant capacity of OPC obtained by the Soxhlet extraction method from grape seeds was 2.85 U/mg. The recovery from the experiment in which OPC was added to grape seeds ranged from 97.0 to 98.6% (RSDs of 0.5-3.4%), suggesting a high accuracy in OPC detection. These findings are important because OPC is an internationally recognized antioxidant that eliminates free radicals in the human body and, therefore, may prevent a variety of diseases. Thus, we envisage that this Pt nanozyme-based assay may be prevalent for antioxidant capacity evaluation and analytical applications.

Decisive role of pH in synthesis of high purity fluorescent BSA-Au20 nanoclusters.

Various types of bovine serum albumin (BSA)-protected fluorescent gold nanoclusters (BSA-AuNCs) have been fabricated and applied in various fields. However, the conventional synthesis methods for BSA-AuNCs usually yield a low photoluminescence quantum yield (PLQY) in solution. In this study, we systematically examined the influences of incubation time, temperature, and pH on the formation process of BSA-AuNCs and then developed a novel strategy to synthesize BSA-AuNCs with PLQY (26%), far exceeding that of existing counterparts. Of the three important factors, pH, temperature, and time, pH plays a key role in the formation of BSA-AuNCs with different compositions and fluorescence properties. The matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) results showed that BSA-Au20NCs with high purity can be produced at a pH value of 10 and the correct combination of incubation temperature and reaction time. The advantages of the obtained BSA-Au20NCs, including small size, high PLQY, long lifetime, high purity, as well as facile modification, make them ideal candidates for luminescent probes in imaging and sensing applications.

Protein-Supported RuO2 Nanoparticles with Improved Catalytic Activity, In Vitro Salt Resistance, and Biocompatibility: Colorimetric and Electrochemical Biosensing of Cellular H2O2.

Protein-supported nanoparticles have a great significance in scientific and nanotechnology research because of their "green" process, low cost-in-use, good biocompatibility, and some interesting properties. Ruthenium oxide nanoparticles (RuO2NPs) have been considered to be an important member in nanotechnology research. However, the biosynthetic approach of RuO2NPs is relatively few compared to those of other nanoparticles. To address this challenge, this work presented a new way for RuO2NP synthesis (BSA-RuO2NPs) supported by bovine serum albumin (BSA). BSA-RuO2NPs are confirmed to exert peroxidase-like activity, electrocatalytic activity, in vitro salt resistance (2 M NaCl), and biocompatibility. Results indicate that BSA-RuO2NPs have higher affinity binding for 3,3',5,5'-tetramethylbenzidine or H2O2 than bare RuO2NPs. Moreover, BSA turns out to be a crucial factor in promoting the stability of RuO2NPs. Taking the advantages of these improved properties, we established colorimetric (linear range from 2 to 800 µM, a limit of detection of 1.8 µM) and electrochemical (linear range from 0.4 to 3850 µM, a limit of detection of 0.18 µM) biosensors for monitoring in situ H2O2 secretion from living MCF-7 cells. Herein, this work offers a new biosynthesis strategy to obtain BSA-RuO2NPs and sheds light on the sensitive biosensors to monitor the H2O2 secreted from living cells for promising applications in the fields of nanotechnology, biology, biosensors, and medicine.

Heparin-platinum nanozymes with enhanced oxidase-like activity for the colorimetric sensing of isoniazid.

In this study, a colorimetric sensing assay of isoniazid based on excellent oxidase-like activity of heparin sodium stabilized platinum nanoparticles (HS-PtNPs) has been demonstrated. The newly prepared HS-PtNPs exhibit a great dispersion with an average size distribution of 4.8 ± 0.6 nm, and maintain more than 90% catalytic activity under strong acid and alkali or long-term storage conditions, indicating a robust nanomaterial with attractive potential. The HS-PtNPs show distinct oxidase-like activity with an ultrahigh affinity (Km = 0.01012 mM) for 3, 3', 5, 5'-tetramethylbenzidine (TMB). More significantly, we found that the pyridine ring of isoniazid has a strong reductive hydrazyl substitution, which can compete with TMB for the catalytic site of HS-PtNPs resulting in a colorless solution. Accordingly, a colorimetric sensing of isoniazid was fabricated. A linear relationship for isoniazid was achieved in 2.5 × 10-6 to 2.5 × 10-4 M (R2 = 0.998) with a low limit of detection 1.7 × 10-6 M (S/N = 3). Recovery experiments in drug tablets show that the standard recovery rates were 95%-103%. The quantitative detection data for isoniazid in drug tablets calculated respectively from the standard method and this method exhibited a high correlation coefficient (a slope of 0.9995), suggesting that high accuracy in isoniazid detection.

Highly sensitive colorimetric sensor for detection of iodine ions using carboxylated chitosan-coated palladium nanozyme.

Although a massive research has been devoted on the exploration of noble metal-based nanozyme, less progress has been made in the investigation of palladium (Pd) nanozyme and the interaction between ions and Pd nanozyme. In this study, a new type of Pd nanozyme was prepared by a facile one-pot approach by using carboxylated chitosan as the stabilizer. Owing to the synergistic effect of carboxylated chitosan stabilized Pd nanoparticles (CC-PdNPs) can effectively catalyze the H2O2-mediated oxidation of 3,3',5,5'-tetramethylbenzidine sulfate (TMB) accompanied by a blue color change (oxidized TMB), indicating the peroxidase-like activity of CC-PdNPs. Furthermore, the Michaelis-Menten constants and catalytic stability of CC-PdNPs render them suitable for environmental analysis and bio-detection. Here, we found that while introducing the iodine ions (I-) into the reaction medium, the peroxidase-like activity of CC-PdNPs has been rapidly and effectively inhibited through the formation of Pd-I bond; thus, the active sites of PdNPs can be blocked by I-. Based on this specific inhibition by I-, a facile colorimetric assay has been performed for the detection of I- with an extremely low limit of detection (0.19 nM). Furthermore, the practicality of the proposed sensor also has been demonstrated in tap water, and the satisfactory recoveries were obtained. Our study not only demonstrated a novel Pd-based nanozyme but also provided guidance for I- sensing for environmental analysis, food inspection, and bio-detection. Graphical abstract.

Rational Design of High-Performance Donor-Linker-Acceptor Hybrids Using a Schiff Base for Enabling Photoinduced Electron Transfer.

Donor-linker-acceptor (D-L-A)-based photoinduced electron transfer (PET) has been frequently used for the construction of versatile fluorescent chemo/biosensors. However, sophisticated and tedious processes are generally required for the synthesis of these probes, which leads to poor design flexibility. In this work, by exploiting a Schiff base as a linker unit, a covalently bound D-L-A system was established and subsequently utilized for the development of a PET sensor. Cysteamine (Cys) and N-acetyl-l-cysteine (NAC) costabilized gold nanoclusters (Cys/NAC-AuNCs) were synthesized and adopted as an electron acceptor, and pyridoxal phosphate (PLP) was selected as an electron donor. PLP can form a Schiff base (an aldimine) with the primary amino group of Cys/NAC-AuNC through its aldehyde group and thereby suppresses the fluorescence of Cys/NAC-AuNC. The Rehm-Weller formula results and a HOMO-LUMO orbital study revealed that a reductive PET mechanism is responsible for the observed fluorescence quenching. Since the pyridoxal (PL) produced by the acid phosphatase (ACP)-catalyzed cleavage of PLP has a weak interaction with Cys/NAC-AuNC, a novel turn-on fluorescent method for selective detection of ACP was successfully realized. To the best of our knowledge, this is the first example of the development of a covalently bound D-L-A system for fluorescent PET sensing of enzyme activity based on AuNC nanoprobes using a Schiff base.

A Heparinase Sensor Based on a Ternary System of Hg2+-Heparin-Osmium Nanoparticles.

A visual assay for the detection of heparinase was developed on the basis of a ternary system of Hg2+-heparin-osmium nanoparticles (OsNPs). First, heparin-capped OsNPs (heparin-OsNPs) were synthesized by a facile reduction method using heparin as the protecting/stabilizing agent. The oxidase-like activity of heparin-OsNPs, however, turned out to be low, which somewhat limits their application. We discovered that Hg2+ can significantly/specifically boost the oxidase-like activity of heparin-OsNPs via electrostatic interaction. The oxidase-like activity of heparin-OsNPs toward the oxidation of the substrate, 3,3',5,5'-tetramethylbenzidine, by dissolved O2 was found to increase by 76-fold in the presence of Hg2+. More significantly, heparin in heparin-OsNPs could be specifically hydrolyzed into small fragments in the presence of heparinase, which resulted in the weakening of the oxidase-like activity of Hg2+/heparin-OsNPs. On the basis of these findings, a linear response of the sensor for heparinase was obtained in the range 20-1000 µg/L with a low detection limit (15 µg/L), which is comparable to those of other reported sensors. Further, the colorimetric sensor was employed for the detection of heparinase in human serum samples with satisfactory results. We speculate that combining such surface modification of the osmium nanozyme with a sensing element could be an interesting direction for promoting nanozyme research in medical diagnosis.

Ascorbate Oxidase Mimetic Activity of Copper(II) Oxide Nanoparticles.

Although oxidase mimetic nanozymes have been widely investigated, specific biological molecules have rarely been explored as substrates, particularly in the case of ascorbate oxidase (AAO) mimetic nanozymes. Herein, we demonstrate for the first time that copper(II) oxide nanoparticles (CuO NPs) catalyze the oxidation of ascorbic acid (AA) by dissolved O2 (as a green oxidant) to form dehydroascorbic acid (DHAA), thus functioning as a new kind of AAO mimic. Under neutral conditions, the Michaelis-Menten constant of CuO NPs (0.1302 mm) is similar to that of AAO (0.0840 mm). Furthermore, the robustness of CuO NPs is greater than that of AAO, thus making them suitable for applications under various conditions. As a demonstration, a fluorescence AA sensor based on the AAO mimetic activity of CuO NPs was developed. To obtain a fluorescent product, o-phenylenediamine (OPDA) was used to react with the DHAA produced by the oxidation of AA catalyzed by CuO NPs. The developed sensor was cost-effective and easy to fabricate and exhibited high selectivity/sensitivity with a wide linear range (1.25×10-6 to 1.125×10-4 m) and a low detection limit (3.2×10-8 m). The results are expected to aid in expanding the applicability of oxidase mimetic nanozymes in a variety of fields such as biology, medicine, and detection science.