Enzyme Mimetic Activity Research Articles

Au/CeO2 NR Restricted Inside Cu-MOFs: A Three-in-One Artificial Enzyme with Synergistically Enhanced Peroxidase-Like Activity for Dual-Mode Sensing of Multiple Biomarkers

Nanomaterials with inherent enzyme-mimetic properties have been extensively studied in biosensing field, but it is still challenging to overcome their natively weak enzyme-mimetic activity and further promote their stabilities. In this work, a three-in-one artificial enzyme with synergistically enhanced peroxidase-like activity and outstanding stability is constructed by restricting Au-loaded CeO2 nanorods (NR) (1% Au:CeO2 mass ratio) inside Cu-based metal–organic frameworks (Cu(PABA)), termed Au1/CeO2 NR@Cu(PABA). It has been demonstrated that the artificial enzyme possesses significantly improved performance in catalytic decomposition of H2O2, with ∼2.41 and ∼1.49 times higher activity than its individual constituents (Au1/CeO2 NR and Cu(PABA)), benefiting from the synergistic effect and the unique restricted structure; owns strong affinities to substrates, with separately ∼8.21- and ∼3.13-fold lower Michaelis constants toward H2O2 and o-phenylenediamine, respectively, than horseradish peroxidase; and exhibits desirable pH (4–12), thermal (30–80 °C), organic solvent (N,N-dimethylformamide, acetone, methanol, etc.), and long-term storage (30 days) stabilities, advantaging practical applications. Taking aforementioned superior properties of Au1/CeO2 NR@Cu(PABA), a universal colorimetric/fluorometric dual-mode sensing platform is built and utilized for detection of biomarkers (e.g., glucose, galactose, and cholesterol) in biological fluids with satisfactory recoveries (90.2–108%). This work offers new horizons in designing high-efficiency, stable, and credible biomimetic catalysts to accelerate future advanced engineering of nanozymes.

Engineering Cell Membrane-Cloaked Catalysts as Multifaceted Artificial Peroxisomes for Biomedical Applications.

Artificial peroxisomes (APEXs) or peroxisome mimics have caught a lot of attention in nanomedicine and biomaterial science in the last decade, which have great potential in clinically diagnosing and treating diseases. APEXs are typically constructed from a semipermeable membrane that encloses natural enzymes or enzyme-mimetic catalysts to perform peroxisome-/enzyme-mimetic activities. The recent rapid progress regarding their biocatalytic stability, adjustable activity, and surface functionality has significantly promoted APEXs systems in real-life applications. In addition, developing a facile and versatile system that can simulate multiple biocatalytic tasks is advantageous. Here, the recent advances in engineering cell membrane-cloaked catalysts as multifaceted APEXs for diverse biomedical applications are highlighted and commented. First, various catalysts with single or multiple enzyme activities have been introduced as cores of APEXs. Subsequently, the extraction and function of cell membranes that are used as the shell are summarized. After that, the applications of these APEXs are discussed in detail, such as cancer therapy, antioxidant, anti-inflammation, and neuron protection. Finally, the future perspectives and challenges of APEXs are proposed and outlined. This progress review is anticipated to provide new and unique insights into cell membrane-cloaked catalysts and to offer significant new inspiration for designing future artificial organelles.

Regulation of the Oxidase Mimetic Activity of Ceria Nanoparticles by Buffer Composition.

Ceria nanoparticles (CNPs) are important typical nanozymes with multiple enzyme mimetic activities, which could facilitate the oxidation of organic dyes in acidic conditions, because of the oxidase mimetic activity. Usually, the regulation of oxidase mimetic activity is focused on the adjustment of the structure, morphology, composition, surface, and other factors of nanozymes. However, the influence of the surrounding environment is not considered, which is very important during the reaction process. In this work, the oxidase mimetic activity of CNPs in buffer solutions including citric acid, acetic acid and glycine buffer solutions was investigated, with the results that carboxyl group in buffer solution could adsorb the CNPs on the surface to promote the oxidase mimetic activity. Due to the chelation with the cerium ion, the enhancement is more significant by molecules with polycarboxylic groups, and the enhancement is more efficient by carboxyl molecules in buffer solution, compared with the modification of the carboxyl groups on the surface, because of easier operation and smaller steric hindrance. From the viewpoint of increasing the oxidase mimetic activity of CNPs, the work is expected to provide references for the selection of the reaction systems to optimize the oxidase mimetic activity in bio-detection applications.

Reversed Regulation Effects of ssDNA on the Mimetic Oxidase and Peroxidase Activities of Covalent Organic Frameworks.

Nanomaterials with enzyme mimetic activity have attracted extensive attention, especially in the regulation of their catalytic activities by biomolecules or other polymers. Here, a covalent organic framework (Tph-BT COF) with excellent photocatalytic activity is constructed by Schiff base reaction, and its mimetic oxidase activity and peroxidase activity is inversely regulated via single-stranded DNA (ssDNA). Under light-emitting diode (LED) light irradiation, Tph-BT exhibited outstanding oxidase activity, which efficiently catalyzed oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to produce blue oxTMB, and ssDNA, especially those with poly-thymidine (T) sequences, can significantly inhibit its oxidase activity. On the contrary, Tph-BT showed weak peroxidase activity, and the presence of ssDNA, particularly poly-cytosine (C) sequences, can remarkably enhance the peroxidase activity. The influence of base type, base length, and other factors on the activities of two enzymes is also studied, and the results reveal that the adsorption of ssDNA on the surface of Tph-BT prevented intersystem crossing (ISC) and energy transfer processes to reduce 1 O2 generation, while the electrostatic interaction between ssDNA and TMB enhanced Tph-BT's affinity for TMB to facilitate the electron transfer from TMB to • OH. This study investigates multitype mimetic enzyme activities of nonmetallic D-A conjugated COFs and demonstrates their feasibility of regulation by ssDNA.

Colorimetric and fluorescence detection of circulating tumor cells based on a bimetallic-organic framework

An ultrasensitive colorimetric and fluorescence dual-readout method was developed for detecting circulating tumor cells (CTCs) using a CuFe-based bimetallic-organic framework (CuFe-NH2-MIL-101, bi-MOF). Incorporation of Cu into an Fe-based metal–organic framework (MOF) led to improved fluorescence compared with that of monometallic analogues. The synthesized bi-MOF exhibited improved fluorescence. The synthesized bi-MOF also exhibited enzyme-mimetic activity. The CTCs were bound by biotinylated antibodies and then selected and enriched by streptavidin-coated magnetic beads. With peroxidase-mimicking activity and enhanced fluorescence, the CuFe bi-MOF functioned as a signal-amplifying nanoprobe for colorimetric and fluorescence detection. The absorbance increased with increasing concentration of tumor cells, whereas the fluorescence intensity gradually decreased. Under optimized conditions, the sensor exhibited a detection range of 102–107 cells mL−1. The detection limit of this cytosensor could reach 7 cells mL−1. Moreover, this sensor showed reliable applicability in real serum samples. Overall, this dual-readout strategy holds potential for the detection of CTCs in clinical diagnostics.

Double enzyme mimetic activities of multifunctional Ag nanoparticle-decorated Co3V2O8 hollow hexagonal prismatic pencils for application in colorimetric sensors and disinfection

Since the catalytic activity of most nanozymes is still far lower than the corresponding natural enzymes, there is urgent need to discover novel highly efficient enzyme-like materials. In this work, Co3V2O8 with hollow hexagonal prismatic pencil structures were prepared as novel artificial enzyme mimics. They were then decorated by photo-depositing Ag nanoparticles (Ag NPs) on the surface to further improve its catalytic activities. The Ag NPs decorated Co3V2O8 (ACVPs) showed both excellent oxidase- and peroxidase-like catalytic activities. They can oxidize the colorless 3,3’,5,5’-tetramethylbenzidine rapidly to induce a blue change. The enhanced enzyme mimetic activities can be attributed to the surface plasma resonance (SPR) effect of Ag NPs as well as the synergistic catalytic effect between Ag NPs and Co3V2O8, accelerating electron transfer and promoting the catalytic process. ACVPs were applied in constructing a colorimetric sensor, validating the occurrence of the Fenton reaction, and disinfection, presenting favorable catalytic performance. The enzyme-like catalytic mechanism was studied, indicating the chief role of ⋅O2- radicals in the catalytic process. This work not only discovers a novel functional material with double enzyme mimetic activity but also provides a new insight into exploiting artificial enzyme mimics with highly efficient catalytic ability.

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.

DNAzyme-Derived Aptamer Reversely Regulates the Two Types of Enzymatic Activities of Covalent-Organic Frameworks for the Colorimetric Analysis of Uranium.

Nanozymes are nanomaterials with enzyme-mimetic activity. It is known that DNA can interact with various nanozymes in different ways, enhancing or inhibiting the activity of nanozymes, which can be used to develop various biosensors. In this work, we synthesized a photosensitive covalent-organic framework (Tph-BT) as a nanozyme, and its oxidase and peroxidase activities could be reversely regulated by surface modification of single-stranded DNA (ssDNA) for the colorimetric detection of UO22+. Tph-BT exhibits excellent oxidase activity and weak peroxidase activity, and it is surprising to find that the UO22+-specific DNA aptamer can significantly inhibit the oxidase activity while greatly enhancing the peroxidase activity. The present UO22+ interacts with the DNA aptamer to form secondary structures and detaches from the surface of Tph-BT, thereby restoring the enzymatic activity of Tph-BT. Based on the reversed regulation effects of the DNA aptamer on the two types of enzymatic activities of Tph-BT, a novel "off-on" and "on-off" sensing platform can be constructed for the colorimetric analysis of UO22+. This research demonstrates that ssDNA can effectively regulate the different types of enzymatic activities of single COFs and achieve the sensitive and selective colorimetric analysis of radionuclides by the naked eye.

Cerium oxide nanozymes confer a cytoprotective and bio-friendly surface micro-environment to methacrylate based oro-facial prostheses

Poly-(methyl methacrylate) (PMMA) is the preferred biomaterial for orofacial prostheses used for the rehabilitation of naso-palatal defects. However, conventional PMMA has limitations determined by the complexity of the local microbiota and the friability of oral mucosa adjacent to these defects. Our purpose was to develop a new type of PMMA, i-PMMA, with good biocompatibility and better biological effects such as higher resistance to microbial adhesion of multiple species and enhanced antioxidant effect. The addition of cerium oxide nanoparticles to PMMA using a mesoporous nano-silica carrier and polybetaine conditioning, resulted in an increased release of cerium ions and enzyme mimetic activity, without tangible loss of mechanical properties. Ex vivo experiments confirmed these observations. In stressed human gingival fibroblasts, i-PMMA reduced the levels of reactive oxygen species and increased the expression of homeostasis-related proteins (PPARg, ATG5, LCI/III). Furthermore, i-PMMA increased the levels of expression of superoxide dismutase and mitogen-activated protein kinases (ERK and Akt), and cellular migration. Lastly, we demonstrated the biosafety of i-PMMA using two in vivo models: skin sensitization assay and oral mucosa irritation test, respectively. Therefore, i-PMMA offers a cytoprotective interface that prevents microbial adhesion and attenuates oxidative stress, thus supporting physiological recovery of the oral mucosa.

Different Dimensional Copper-Based Metal–Organic Frameworks with Enzyme-Mimetic Activity for Antibacterial Therapy

Fighting against bacterial infection and accelerating wound healing remain important and challenging in infected wound care. Metal-organic frameworks (MOFs) have received much attention for their optimized and enhanced catalytic performance in different dimensions of these challenges. The size and morphology of nanomaterials are important in their physiochemical properties and thereby their biological functions. Enzyme-mimicking catalysts, based on MOFs of different dimensions, display varying degrees of peroxidase (POD)-like activity toward hydrogen peroxide (H2O2) decomposition into toxic hydroxyl radicals (•OH) for bacterial inhibition and accelerating wound healing. In this study, we investigated the two most studied representatives of copper-based MOFs (Cu-MOFs), three-dimensional (3D) HKUST-1 and two-dimensional (2D) Cu-TCPP, for antibacterial therapy. HKUST-1, with a uniform and octahedral 3D structure, showed higher POD-like activity, resulting in H2O2 decomposition for •OH generation rather than Cu-TCPP. Because of the efficient generation of toxic •OH, both Gram-negative Escherichia coli and Gram-positive methicillin-resistant Staphylococcus aureus could be eliminated under a lower concentration of H2O2. Animal experiments indicated that the as-prepared HKUST-1 effectively accelerated wound healing with good biocompatibility. These results reveal the multivariate dimensions of Cu-MOFs with high POD-like activity, providing good potential for further stimulation of specific bacterial binding therapies in the future.

Integrating Pt nanoparticles with carbon nanodots to achieve robust cascade superoxide dismutase-catalase nanozyme for antioxidant therapy

Nanozymes, nanomaterials with intrinsic enzyme mimetic activity, have emerged and been used in a broad range of applications yet are constrained with limited catalytic efficiency. Herein, by integrating carbon nanodots (CNDs) with platinum nanoparticles (PtNPs), a novel Pt@CNDs nanocomposite was engineered as an efficient nanozyme with superoxide dismutase (SOD)- and catalase (CAT)-like specific activities of 12,605 U/mg and 3172 U/mg, respectively. The PtNPs were deposited on the surface of CNDs by in situ chemical decomposition of Pt4+ to produce Pt@CNDs, in which CNDs served as a SOD mimic while PtNPs could mimic CAT, thereby forming a cascade antioxidant nanozyme system. Of note, the Pt@CNDs could significantly scavenge hydroxyl radicals as well. The carbonyl and hydroxyl groups of CNDs could bind with PtNPs, which promoted the electron transfer between PtNPs and CNDs that endowed the Pt@CNDs with excellent catalytic performance. Furthermore, Pt@CNDs could enter living cells and target mitochondria, thereby reducing the upregulated reactive oxygen species (ROS) level. Moreover, in vivo experimental results indicated that Pt@CNDs could effectively relieve ROS-induced inflammation in living mice. This work provides a promising strategy to design nanozymes with desired catalysis activity evidence by integrating CNDs and nanometals.

Antioxidative 0-dimensional nanodrugs overcome obstacles in AKI antioxidant therapy.

Acute kidney injury (AKI) is a commonly encountered syndrome associated with various aetiologies and pathophysiological processes leading to enormous health risks and economic losses. In the absence of specific drugs to treat AKI, hemodialysis remains the primary clinical treatment for AKI patients. The revelation of the pathology opens new horizons for antioxidant therapy in the treatment of AKI. However, small molecule antioxidant drugs and common nanozymes have failed to challenge AKI due to their unsatisfactory drug properties and renal physiological barriers. 0-Dimensional (0D) antioxidant nanodrugs stand out at this time thanks to their small size and high performance. Recently, a number of research studies have been carried out around 0D nanodrugs for alleviating AKI, and their multi-antioxidant enzyme mimetic activities, smooth glomerular filtration barrier permeability and excellent biocompatibility have been investigated. Here, we comprehensively summarize recent advances in 0D nanodrugs for AKI antioxidant therapy. We classify these representative studies into three categories according to the characteristics of 0D nanomaterials, namely ultra-small metal nanodots, inorganic non-metallic quantum dots and polymer nanodots. We focus on the antioxidant mechanisms and their distribution in vivo in each inspiring work, and the purpose and ingenuity of each design are rigorously captured and described. Finally, we provide our reflections and prospects for 0D antioxidant nanodrugs in AKI treatment. This mini review provides unique insights and valuable clues in the design of 0D nanodrugs and other kidney absorbable drugs.

Development of nanozyme based sensors as diagnostic tools in clinic applications: a review.

Since 1970, many artificial enzymes that imitate the activity and structure of natural enzymes have been discovered. Nanozymes are a group of nanomaterials with enzyme-mimetic properties capable of catalyzing natural enzyme processes. Nanozymes have attracted great interest in biomedicine due to their excellent stability, rapid reactivity, and affordable cost. The enzyme-mimetic activities of nanozymes may be modulated by numerous parameters, including the oxidative state of metal ions, pH, hydrogen peroxide (H2O2) level, and glutathione (GSH) concentration, indicating the tremendous potential for biological applications. This article delivers a comprehensive overview of the advances in the knowledge of nanozymes and the creation of unique and multifunctional nanozymes, and their biological applications. In addition, a future perspective of employing the as-designed nanozymes in biomedical and diagnostic applications is provided, and we also discuss the barriers and constraints for their further therapeutic use.

Light-Mediated Modulation of Enzyme-Mimetic Activity of CuMnO2 Nanosheets.

As typical copper-based-manganese delafossites, CuMnO2 nanoflakes (CuMnO2 NFs) display excellent light absorption capacity, showing great promise for facile regulation of catalysis reactions by light stimulation. Here, we report that the as-prepared CuMnO2 NFs possess intrinsic peroxidase- and oxidase-like activities, which are capable of catalyzing the oxidation of the enzymatic substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2 or O2, respectively. Additionally, CuMnO2 NFs exhibit significant light-enhanced dual enzyme-like performance (peroxidase and oxidase) for accelerating the oxidation of TMB, owing to a much greater light-induced reactive oxygen species (ROS) yield. The light-induced hydroxyl radicals (·OH) and superoxide radicals (O2•-) hold the key to rapidly catalyzing the oxidation of TMB into the two-electron charge transfer complex diamine, subsequently improving the oxidation capacity and enzyme-like activity of CuMnO2 NFs as peroxidase and oxidase mimics. These findings confirm an efficient way to improve the enzyme-like performance of nanozymes and regulate the reactive oxygen species (ROS) level by simply employing irradiated light. This work also provides a general method for constructing external field-modulated smart nanozymes and an insightful approach to the corresponding enzyme-mimetic reaction mechanism.

2D Co3O4 modified by IrO2 nanozyme for convenient detection of aqueous Fe2+ and intercellular H2O2.

A portable sensor for visual monitoring of Fe2+ and H2O2, two-dimensional Co3O4 modified by nano-IrO2 (IrO2@2D Co3O4) was prepared in this work, for the first time, with the help of microwave radiation at 140°C, which was further stabilized onto common test strips. The present IrO2@2D Co3O4 possessed superior dual-function enzyme-like activity with low toxicity and excellent biocompatibility. Especially, trace Fe2+ and H2O2 could exclusively alter their enzyme-like catalytic activity with discriminating hyperchromic or hypochromic effect, i.e., from blue to colorless or to dark blue for both IrO2@2D Co3O4 dispersion and its functionalized test strips. The linear regression equations were A652 = 0.5940 - 0.00041 cFe2+ (10-8M, R2 = 0.9927) for Fe2+ and ∆A652 = 0.0023 cH2O2 + 0.00025 (10-7M, R2 = 0.9982) for H2O2, respectively. When applied to visual monitoring of aqueous Fe2+ and intercellular H2O2, the recoveries were 101.2 ~ 102.5% and 95.8 ~ 103.7% with detection limits of 1.25 × 10-8mol/L and 1.02 × 10-7mol/L, respectively, far below the permitted values in drinking water set by the World Health Organization. The mechanisms for the enhancing enzyme-mimetic activity of IrO2@2D Co3O4 and its selective responses to Fe2+ and H2O2 were investigated in detail.

Noninvasive Delivery of Self-Regenerating Cerium Oxide Nanoparticles to Modulate Oxidative Stress in the Retina.

Diseases affecting the retina, such as age-related macular degeneration (AMD), diabetic retinopathy, macular edema, and retinal vein occlusions, are currently treated by the intravitreal injection of drug formulations. These disease pathologies are driven by oxidative damage due to chronic high concentrations of reactive oxygen species (ROS) in the retina. Intravitreal injections often induce retinal detachment, intraocular hemorrhage, and endophthalmitis. Furthermore, the severe eye pain associated with these injections lead to patient noncompliance and treatment discontinuation. Hence, there is a critical need for the development of a noninvasive therapy that is effective for a prolonged period for treating retinal diseases. In this study, we developed a noninvasive cerium oxide nanoparticle (CNP) delivery wafer (Cerawafer) for the modulation of ROS in the retina. We fabricated Cerawafer loaded with CNP and determined its SOD-like enzyme-mimetic activity and ability to neutralize ROS generated in vitro. We demonstrated Cerawafer's ability to deliver CNP in a noninvasive fashion to the retina in healthy mouse eyes and the CNP retention in the retina for more than a week. Our studies have demonstrated the in vivo efficacy of the Cerawafer to modulate ROS and associated down-regulation of VEGF expression in the retinas of very-low-density lipoprotein receptor knockout (vldlr-/-) mouse model. The development of a Cerawafer nanotherapeutic will fulfill a hitherto unmet need. Currently, there is no such therapeutic available, and the development of a Cerawafer nanotherapeutic will be a major advancement in the treatment of retinal diseases.

MIL-88B(Fe)-reduced graphene oxide as an artificial enzyme for gold nanorod etching and its application to develop the prostate-specific antigen immunosensor.

Animmunosensor based on gold nanorods (AuNRs) etchant activity of a metal-organic framework (MOF): MIL-88B(Fe)-reduced graphene oxide (rGMOF) was developed for the determination of prostate-specific antigen (PSA). Several techniques, including FTIR, UV-Vis spectrophotometry, XRD, and electron microscopy, were employed to characterize the MOFs containing iron-oxygen clusters on the surface of reduced graphene oxide. Enzyme mimetic activity of rGMOF before and after bioconjugation with antibodies was calculated as 8.4 and 2.5 Umg-1, respectively. The primary anti-PSA was conjugated to a magnetic bead and used as PSA-specific capturing. Then, the secondary anti-PSA was grafted to the rGMOF. In the presence of antigen, an immuno-sandwich was formed between the conjugations mentioned above. Afterward, AuNRs were etched by rGMOF, and the related spectrum was recordedin the wavelength range 350 to 900nm.By progressing the etching procedure, the longitudinal LSPR peak of AuNRs was gradually blue-shifted with a linear correlation with the PSA concentration from 0.1pgmL-1 to 100ngmL-1. The detection limit was 0.09pgmL-1. Theproposed immunosensor was successfully employed to determine PSA levels in real samples. Since the obtained results showed an excellent correlation with those acquired by the chemiluminescence gold standard method, it has the potential for PSA determination in clinical assays.

Single-atom nanozymes: From bench to bedside.

Single-atom nanozymes (SANs) are the new emerging catalytic nanomaterials with enzyme-mimetic activities, which have many extraordinary merits, such as low-cost preparation, maximum atom utilization, ideal catalytic activity, and optimized selectivity. With these advantages, SANs have received extensive research attention in the fields of chemistry, energy conversion, and environmental purification. Recently, a growing number of studies have shown the great promise of SANs in biological applications. In this article, we present the most recent developments of SANs in anti-infective treatment, cancer diagnosis and therapy, biosensing, and antioxidative therapy. This text is expected to better guide the readers to understand the current state and future clinical possibilities of SANs in medical applications.

In situ controllable growth of Ag particles on paper for smartphone optical sensing of Hg2+ based on nanozyme activity stimulation

Given the huge threat of Hg2+ to human health, it is always significant to develop Hg2+ monitoring tools. Utilizing its impacts on nanozyme catalysis is a promising strategy to realize Hg2+ detection. However, nearly all the assays fabricated based on the principle should be performed in solution and in laboratory and require bulky signal reading instruments, hindering their wider applications. It is highly desired to develop portable, efficient, and practical Hg2+ sensors. Herein, we proposed the in situ controllable growth of Ag nanoparticles (AgNPs) on paper to fabricate a smartphone optical nanosensor for highly sensitive Hg2+ detection based on enzyme-mimetic activity stimulation. The ultrasmall AgNPs were directly grown on paper wafers to obtain test strips, which further integrated a 3D-printed holder and an APP built-in smartphone to fabricate the sample-to-answer sensor. The AgNPs-based test strips exhibited a target-stimulated peroxidase-like activity to catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine, and the chromogenic information was captured and processed by the smartphone to rapidly report the level of Hg2+. With the portable nanosensor, highly sensitive and specific detection of Hg2+ was achieved, with a detection limit down to 0.44 nM. The sensor gathers the features of easy construction, simple operation, excellent capability, and robust performance together, promising great potential for rapid on-site analysis.

Sensitive colorimetric assay of hydrogen peroxide and glucose in humoral samples based on the enhanced peroxidase-mimetic activity of NH2-MIL-88-derived FeS2@CN nanocomposites compared to its precursors.

By employing NH2-MIL-88 as a template, we synthesized the intermediate Fe@CN under high-temperature calcination and further fabricated the FeS2@CN nanocomposites in the presence of sulfur powder. Under varying temperatures (300-600 °C) and Fe@CN-to-S ratios (1:3-6), FeS2@CN500-5 nanocomposites had the highest peroxidase-mimetic activity. Under optimized conditions (incubation temperature 40 °C; solution pH 4.0 and nanocomposite concentration 10 μg/mL; 652-nm absorption), the Michaelis-Menten constant (Km) of FeS2@CN was much lower than that of horseradish peroxidase (HRP), therefore demonstrating that it had a higher affinity for both chromogenic substrates than conventional HRP. The limits of detection for H2O2 and glucose were 0.15 and 0.30 μmol/L, respectively, and the recoveries for glucose were 91.8-103% with RSDs <5.2%. The novelty of this study lies in (1) the FeS2@CN was confirmed to possess stronger enzyme-mimetic activity than its precursors (NH2-MIL-88 and Fe@CN); (2) the enhanced activity resulted from the unsaturated sites of N and S doping and the plentiful defects on the porous carbon surface; and (3) free radical trapping experiments evidenced that •OH played a major role in the catalytic reaction, while h+ and •O2- simultaneously participated in the catalytic process. These convincing performance metrics lead us to postulate that the FeS2@CN-based colorimetric biosensor provides a promising approach for several real-world applications, such as point-of-care diagnosis and workplace health evaluations.