Fe-N-C Single-Atom Nanozymes for the Intracellular Hydrogen Peroxide Detection

Water dispersible cobalt single-atom catalysts with efficient Chemiluminescence enhancement for sensitive bioassay

Single-atom catalysts (SACs) have been applied in various fields as they display extremely high utilization efficiency of catalytic sites. A majority of SACs prepared by high-temperature calcination suffer from poor water dispersion and lose of labelling groups. Herein cobalt SACs (CSACs) were synthesized with a solvothermal method by adopting hybridized MOFs Fe₂O₃/MIL-100(Fe) as the carriers to load cobalt atoms. Compared with original MOFs MIL-100(Fe), the carriers possess superior loading capacity, and the loading amount of cobalt element is up to 4.69 wt%. The implantation of cobalt atoms in hybridized MOFs Fe₂O₃/MIL-100(Fe) vastly improved the specific surface of the carriers for 68 times. CSACs at 1.0 μg mL^-1 can catalyze H₂O₂ to generate numerous reactive oxygen species and enormously boost the chemiluminescent emission of luminol-H₂O₂ system up to 2297 times. The CSACs also exhibit satisfactory dispersion in aqueous medium. Benefiting from these attracting features, the CSACs were applied as sensitive signal probes for detecting carbendazim in Chinese medicinal herbs with a chemiluminescent immunoassay method. The dynamic range is 10 pg mL^-1 - 50 ng mL^-1 and the limit of detection is 1.8 pg mL^-1. The proof-of-principle work paves a pathway to the exploitation of SACs as sensitive probes for tracing biological recognition events.

Single-Atom Iron Enables Strong Low-Triggering-Potential Luminol Cathodic Electrochemiluminescence

The conventional cathodic electrochemiluminescence (ECL) always requires a more negative potential to trigger strong emission, which inevitably damages the bioactivity of targets and decreases the sensitivity and specificity. In this work, iron single-atom catalysts (Fe-N-C SACs) were employed as an efficient co-reaction accelerator for the first time to achieve the impressively cathodic emission of a luminol-H₂O₂ ECL system at an ultralow potential. Benefiting from the distinct electronic structure, Fe-N-C SACs exhibit remarkable properties for the activation of H₂O₂ to produce massive reactive oxygen species (ROS) under a negative scanning potential from 0 to -0.2 V. The ROS can oxidize the luminol anions into luminol anion radicals, avoiding the tedious electrochemical oxidation process of luminol. Then, the in situ-formed luminol anion radicals will directly react with ROS for the strong ECL emission. As a proof of concept, sensitive detection of the carcinoembryonic antigen was realized by glucose oxidase-mediated ECL immunoassay, shedding light on the superiority of SACs to construct efficient cathodic ECL systems with low triggering potential.

Guided Synthesis of a Mo/Zn Dual Single-Atom Nanozyme with Synergistic Effect and Peroxidase-like Activity

We present a facile route towards a dual single-atom nanozyme composed of Zn and Mo, which utilizes the non-covalent nano-assembly of polyoxometalates, supramolecular coordination complexes as the metal-atom precursor, and a macroscopic amphiphilic aerogel as the supporting substrate. The dual single-atoms of Zn and Mo have a high content (1.5 and 7.3 wt%, respectively) and exhibit a synergistic effect and a peroxidase-like activity. The Zn/Mo site was identified as the main active center by X-ray absorption fine structure spectroscopy and density functional theory calculation. The detection of versatile analytes, including intracellular H₂ O₂ , glucose in serum, cholesterol, and ascorbic acid in commercial beverages was achieved. The nanozyme has an outstanding stability and maintained its performance after one year's storage. This study develops a new peroxidase-like nanozyme and provides a robust synthetic strategy for single-atom catalysts by utilizing an aerogel as a facile substrate that is capable of stabilizing various metal atoms.

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.

Pt–Pd Bimetallic Aerogel as High-Performance Electrocatalyst for Nonenzymatic Detection of Hydrogen Peroxide

Hydrogen peroxide (H2O2) plays an indispensable role in the biological, medical, and chemical fields. The development of an effective H2O2 detecting method is of great importance. In the present work, a series of PtxPdy bimetallic aerogels and Pt, Pd monometallic aerogels were controllably synthesized by one-step gelation method. Their morphologies and compositions were characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, and so forth. These aerogels were used as nonenzyme electrocatalysts for the detection of H2O2. The cyclic voltammetric and amperometric results demonstrated that the performance of the metal aerogels showed volcano-type behavior, with the Pt50Pd50 aerogel sitting on top. The Pt50Pd50 aerogel-based electrochemical sensor exhibited excellent comprehensive performance, with a low overpotential of −0.023 V vs. Ag/AgCl, a broad linear range from 5.1 to 3190 μM (R2 = 0.9980), and a high sensitivity of 0.19 mA mM−1 cm−2, in combination with good anti-interference ability and stability. A comprehensive study indicated that the superior sensing performance of the Pt50Pd50 aerogel is closely related to its optimized d-band center and larger cumulative pore volume. This work first applied Pt–Pd bimetallic aerogels into the detection of H2O2 and shows the promising application of noble metal aerogels in the electrochemical sensing area.

316 stainless steel wire mesh for visual detection of H2O2, glutathione and glucose based on the peroxidase-like activity

Stainless steel is a frequently used and cost-effective material. In this study, we discovered for the first time that fresh 316ss possessed an intrinsic peroxidase (POD) catalytic activity, which can catalyze the substrate of POD-like reaction 3,3',5,5'-tetramethylbenzidine (TMB) changing to a blue-colored product, oxidation of TMB, in the presence of hydrogen peroxide (H₂O₂). Subsequently, a rapid method was conducted to enable the active composites of the 316ss with reused POD activity for 25 circles at least. Based on this finding, the method exhibits a highly sensitive and selective application for H₂O₂, glutathione (GSH), and Glucose determination. The linear range of glucose detection was 5-100 μM and the detection limit was 3 μM. Finally, this method was further used for detection of glucose in human serum. This work finds a new function of 316ss and develops its novel application, which promotes the potential application of nanozyme in nanoscience and nanotechnology. Schematic representation of the enzyme mimic activities of 316ss wire mesh for the colorimetric detection of hydrogen peroxide H₂O₂ and GSH with a superior reusability for more than 25 cycles. Based on this, the colorimetric detection of glucose can be constructed combined with GOx.

In situ synthesis of Co-doped MoS2 nanosheet for enhanced mimicking peroxidase activity

To enhance the catalytic activity of two-dimensional layered materials as versatile materials, the modification of transition metal dichalcogenide nanosheets such as MoS₂ by doping with heteroatoms has drawn great interests. However, few reports are available on the study of the enzyme-like activity of doped MoS₂. In this study, a facile in situ hydrothermal method for the preparation of various ultrathin transition metals (Fe, Cu, Co, Mn, and Ni) doped MoS₂ nanosheets has been reported. Through the density functional theory (DFT) and steady-state kinetic analysis, the Co-doped MoS₂ nanosheets exhibited the highest peroxidase-like catalytic activity among them. Furthermore, a typical colorimetric assay for H₂O₂ was presented based on the catalytic oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to a blue product (oxTMB) by Co-MoS₂. The proposed colorimetric method showed excellent tolerance under extreme conditions and a broad linear range from 0.0005 to 25 mM for H₂O₂ determination. Concerning the practical application, in situ detection of H₂O₂ generated from SiHa cells was also fulfilled, fully confirming the great practicability of the proposed method in biosensing fields.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10853-022-07201-z.

Reversible inhibition of the oxidase-like activity of Fe single-atom nanozymes for drug detection

Mechanism research of nanozymes has always been of great interest since their emergence as outstanding mimics of friable natural enzymes. An important but rarely mentioned issue in mechanism research of nanozymology is the inhibitory effect of nanozymes. And conventional nanozymes with various active sites hinder the mechanism research, while single-atom Fe-N-C nanozymes with similar active sites to natural enzymes exhibit structural advantages. Herein, we synthesized Fe single-atom nanozymes (Fe-SANs) with ultrahigh oxidase-like activity and found that a common analgesic-antipyretic drug 4-acetamidophenol (AMP) had inhibitory effects for the oxidase-like activity of Fe-SANs. We investigated the inhibitory effects in detail and demonstrated that the inhibition type was reversible mixed-inhibition with inhibition constants (K i and ) of 0.431 mM and 0.279 mM, respectively. Furthermore, we put forward a colorimetric method for AMP detection based on nanozyme inhibition. The research on the inhibitory effects of small molecules on nanozymes expands the scope of analysis based on nanozymes and the inhibition mechanism study may offer some insight into investigating the interaction between nanozymes and inhibitors.

Engineering single-atom catalysts toward biomedical applications

Due to inherent structural defects, common nanocatalysts always display limited catalytic activity and selectivity, making it practically difficult for them to replace natural enzymes in a broad scope of biologically important applications. By decreasing the size of the nanocatalysts, their catalytic activity and selectivity will be substantially improved. Guided by this concept, the advances of nanocatalysts now enter an era of atomic-level precise control. Single-atom catalysts (denoted as SACs), characterized by atomically dispersed active sites, strikingly show utmost atomic utilization, precisely located metal centers, unique metal-support interactions and identical coordination environments. Such advantages of SACs drastically boost the specific activity per metal atom, and thus provide great potential for achieving superior catalytic activity and selectivity to functionally mimic or even outperform natural enzymes of interest. Although the size of the catalysts does matter, it is not clear whether the guideline of "the smaller, the better" is still correct for developing catalysts at the single-atom scale. Thus, it is clearly a new, urgent issue to address before further extending SACs into biomedical applications, representing an important branch of nanomedicine. This review begins by providing an overview of recent advances of synthesis strategies of SACs, which serve as a basis for the discussion of emerging achievements in improving the enzyme-like catalytic properties at an atomic level. Then, we carefully compare the structures and functions of catalysts at various scales from nanoparticles, nanoclusters, and few-atom clusters to single atoms. Contrary to conventional wisdom, SACs are not the most catalytically active catalysts in specific reactions, especially those requiring multi-site auxiliary activities. After that, we highlight the unique roles of SACs toward biomedical applications. To appreciate these advances, the challenges and prospects in rapidly growing studies of SACs-related catalytic nanomedicine are also discussed in this review.

Peroxidase-like Active Nanomedicine with Dual Glutathione Depletion Property to Restore Oxaliplatin Chemosensitivity and Promote Programmed Cell Death

The nanocatalytic activity of nanozymes provides a vision for tumor treatment. However, the glutathione (GSH)-related antioxidant defense system (ADS) formed on the basis of excessive GSH in the tumor microenvironment limits its catalytic activity. Here, dendritic mesoporous silica nanoparticles (DMSNs) were employed as nanocarrier; ultrasmall Fe₃O₄ nanoparticles, Mn²⁺ ions, and glutaminase inhibitor Telaglenastat (CB-839) were subsequently integrated into large mesopores of DMSNs, forming DMSN/Fe₃O₄-Mn@CB-839 (DFMC) nanomedicine. This nanomedicine exhibits peroxidase mimicking activities under acidic conditions, which catalyzes the decomposition of hydrogen peroxide (H₂O₂) into hydroxyl radical (^•OH). This also promotes the formation of lipid peroxides, which is required for ferroptosis. Furthermore, this nanomedicine can effectively deplete the existing GSH, thereby enhancing reactive oxygen species (ROS)-mediated tumor catalytic therapy. Moreover, the introduced CB-839 blocks the endogenous synthesis of GSH, further enhancing GSH depletion performance, which reduces the excretion of oxaliplatin (GSH-related resistance) from tumor cells, thereby restoring the chemical sensitivity of oxaliplatin. The dual GSH depletion property significantly weakens the GSH-related ADS and restores the chemical sensitivity of oxaliplatin, leading to the high DFMC-induced apoptosis and ferroptosis of tumor cells. Our developed nanomedicine based on integrated nanotechnology and clinical drug may aid the development of tumor treatment.

3D V2O5-MoS2/rGO nanocomposites with enhanced peroxidase mimicking activity for sensitive colorimetric determination of H2O2 and glucose

In this work, we reported a novel nanozyme (3D V₂O₅-MoS₂/rGO) by decorating MoS₂ nano-flowers and V₂O₅ nanoparticles on reduced graphene oxide (rGO). The 3D V₂O₅-MoS₂/rGO nanocomposites exhibited intrinsic peroxidase mimicking activity and catalyzed the oxidation of 3, 3', 5, 5'-tetramethylbenzidine (TMB) to produce a blue colored product in the presence of H₂O₂. Compared with horseradish peroxidase (HRP), 3D V₂O₅-MoS₂/rGO nanocomposites displayed high catalytic velocity (V_max) and affinity (K_m) for substrates (H₂O₂ and TMB). The study of the catalytic mechanism showed that the reduction of V⁵⁺ and the oxidation of S^2- in the 3D V₂O₅-MoS₂/rGO nanocomposites accelerate electron transfer between H₂O₂ and TMB, which enhanced the peroxidase mimicking activity of 3D V₂O₅-MoS₂/rGO nanocomposites. The as-synthetized 3D V₂O₅-MoS₂/rGO could be used for the colorimetric detection of H₂O₂ in the range of 20.00-800.00 μM with the LOD of 12.40 μM (3σ/S). Moreover, the 3D V₂O₅-MoS₂/rGO could also be used for the detection of glucose in the range of 4.00-300.00 μM with the LOD of 3.99 μM (3σ/S). In addition, the as-synthetized novel peroxidase mimics has good applicability for sensitive colorimetric determination of glucose in human blood samples and artificial urine samples, and has broad application prospects as a multi-functional sensing platform in clinical diagnosis.

Emerging Single-Atom Catalysts/Nanozymes for Catalytic Biomedical Applications

Single-atom catalysts (SACs) are a type of atomically dispersed nanozymes with the highest atom utilization, which employ low-coordinated single atoms as the catalytically active sites. SACs not only inherit the merits of traditional nanozymes, but also hold high catalytic activity and superb catalytic selectivity, which ensure their tremendous application potential in environmental remediation, energy storage and conversion, chemical industry, nanomedicine, etc. Nevertheless, undesired aggregation effect of single atoms during preactivation and reaction processes is significantly enhanced owing to the high surface free energy of single atoms. In this case, appropriate substrates are requisite to prevent the aggregation event through the powerful interactions between the single atoms and the substrates, thereby stabilizing the high catalytic activity of the catalysts. In this review, the synthetic methods and characterization approaches of SACs are first described. Then the application cases of SACs in nanomedicine are summarized. Finally, the current challenges and future opportunities of the SACs in nanomedicine are outlined. It is hoped that this review may have implications for furthering the development of new SACs with improved biophysicochemical properties and broadened biomedical applications.

Multienzyme Cascades Based on Highly Efficient Metal-Nitrogen-Carbon Nanozymes for Construction of Versatile Bioassays

Distinguished by the coupled catalysis-facilitated high turnover and admirable specificity, enzyme cascades have sparked tremendous attention in bioanalysis. However, three-enzyme cascade-based versatile platforms have rarely been explored without resorting to tedious immobilization procedures. Herein, we have demonstrated that formamide-converted transition metal-nitrogen-carbon (f-MNC, M = Fe, Cu, Mn, Co, Zn) with a high loading of atomically dispersed active sites possesses intrinsic peroxidase-mimetic activity following the activity order of f-FeNC > f-CuNC > f-MnNC > f-CoNC > f-ZnNC. Ulteriorly, benefitting from the greatest catalytic performance and explicit catalytic mechanism of f-FeNC, versatile enzyme cascade-based colorimetric bioassays for ultrasensitive detection of diabetes-related glucose and α-glucosidase (α-Glu) have been unprecedentedly devised using f-FeNC-triggered chromogenic reaction of 3,3',5,5'-tetramethylbenzidine as an amplifier. Notably, several types of α-Glu substrates can be effectively utilized in this three-enzyme cascade-based α-Glu assay, and it can be further employed for screening α-Glu inhibitors that are used as antidiabetic and antiviral drugs. These versatile assays can also be extended to detect other H₂O₂-generating or -consuming biomolecules and other bioenzymes that are capable of catalyzing glucose generation procedures. These nanozyme-involved multienzyme cascades without intricate enzyme-engineering techniques may provide a concept to facilitate the deployment of nanozymes in celestial versatile bioassay fabrication, disease diagnosis, and biomedicine.

Single-atom Nanozymes for Biomedical Applications： Recent Advances and Challenges

Nanozymes have received extensive attention in the fields of sensing and detection, medical therapy, industry, and agriculture thanks to the combination of the catalytic properties of natural enzymes and the physicochemical properties of nanomaterials, coupled with superior stability and ease of preparation. Despite the promise of nanozymes, conventional nanozymes are constrained by their oversized size and low catalytic capacity in sophisticated practical application environments. single-atom nanozymes (SAzymes) were characterized as nanozymes with high catalytic efficiency by uniformly distributed single atoms as catalysis sites, thus effectively addressing the defects of conventional nanozymes. This paper reviews the activity improvement scheme and catalytic mechanism of SAzymes and highlights the latest research progress of SAzymes in the fields of biomedical sensing and therapy. Eventually, the challenges and future directions of SAzymes are discussed in this paper.

Cluster Nanozymes with Optimized Reactivity and Utilization of Active Sites for Effective Peroxidase (and Oxidase) Mimicking

Single-atom catalysts have attracted attention in the past decade since they maximize the utilization of active sites and facilitate the understanding of product distribution in some catalytic reactions. Recently, this idea has been extended to single-atom nanozymes (SAzymes) for the mimicking of natural enzymes such as horseradish peroxidase (HRP) often used in bioanalytical applications. Herein, it is demonstrated that those SAzymes without constructing the reaction pocket of HRP still undergo the OH radical-mediated pathway like most of the reported nanozymes. Their positively charged single-atom centers resulting from support electronegative oxygen/nitrogen hinder the reductive conversion of H₂ O₂ to OH radicals and hence display low activity per site. In contrast, it is found that this step can be facilitated over their metallic counterparts on cluster nanozymes with much higher site activity and atom efficiency (cf. SAzymes with 100% atom utilization). Besides the mimicking of HRP in glucose detection, cluster nanozymes are also demonstrated as a better oxidase mimetic for glutathione detection.

Cell membrane-coated nanoparticles as peroxidase mimetics for cancer cell targeted detection and therapy

The development of novel and efficient recognition molecules that can be easily modified by nanomaterials to achieve ultra-sensitive and specific cancer cell analysis is of great significance for its early diagnosis and timely prognosis. Herin, a new nanostructured hybrid based on cell membrane-coated Au cores- ultrathin Pt skins composite nanoparticles (Au@Pt@CM NPs) were developed for in vitro detection and treatment of cancer cells. In this strategy, the Au@Pt NPs acted as the signal transducer, and the cell membrane were used as the cancer-cell recognition tool. The synthesized Au@Pt@CM NPs could catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of the hydrogen peroxide and were demonstrated to have excellent peroxidase-like activity. Coated with the source cancer cell membrane, the nanoparticles achieved highly specific self-recognition to the source cell. Therefore, the colorimetric method based on Au@Pt@CM NPs could detect the cancer cells in the linear range from 50 to 100000 cells/mL with a limit of detection of 5 cells/mL, which is much lower than other colorimetric detection methods. Afterwards, the nanoparticles as a mimetic enzyme were used for therapeutics of cancer cells through the ROS-mediated oxidative damage. Due to the change of the redox state in the cells by the Au@Pt@CM NPs, the hybrid can achieve the growth inhibitory effect and the selective killing effect on cancer cells. It can be expected that this novel hybrid membrane coating method will bring new insight into developing targeted nanomaterials for tumor treatment and detection.

Peroxidase-like activity of Ru-N-C nanozymes in colorimetric assay of acetylcholinesterase activity

Herein, the Ru-N-C nanozymes with abundant active Ru-N_x sites have been successfully prepared by pyrolyzing Ru(acac)₃ trapped zeolitic-imidazolate-frameworks (Ru(acac)₃@ZIF-8). Taking advantages of the remarkable peroxidase-mimicking activity, outstanding stability and reusability of Ru-N-C nanozymes, a novel biosensing system with explicit mechanism is strategically fabricated for sensitively determining acetylcholinesterase (AChE) and tacrine. The limit of detection for AChE activity can achieve as low as 0.0433 mU mL^-1, and the IC₅₀ value of tacrine for AChE is about 0.190 μmol L^-1. The robust analytical performance in serums test verifies the great application potential of this assay in real matrix. Furthermore, "INH" and "IMPLICATION-AND" logic gates are rationally constructed based on the proposed colorimetric sensor. This work not only provides one sustainable and effective avenue to fabricate Ru-N-C-based peroxidase mimic with high catalytic performance, and also gives new impetuses for developing novel biosensors by applying Ru-N-C-based enzyme mimics as substitutes for the natural enzyme.

Co-N-C single-atom nanozymes with oxidase-like activity for highly sensitive detection of biothiols

Biothiol detection is of great importance for clinical disease diagnosis. Previous nanozyme-based colorimetric sensors for biothiol detection showed unsatisfactory catalytic activity, which led to a high detection limit. Therefore, developing new nanozymes with the high catalytic activity for biothiol detection is extremely necessary. Recently, single-atom nanozymes (SAzymes) have attracted much attention in biosensing due to their 100% atom utilization and excellent catalytic activity. Most previous works focus on the peroxidase-like activity of Fe-based SAzymes by using unstable and destructive H₂O₂ as the oxidant. It is essential to develop new SAzymes with high oxidase-like activity for biosensing to break through the limitation. Herein, Co-N-C SAzymes with high oxidase-like activity are explored. Furthermore, Co-N-C SAzymes are used as a biosensor for colorimetric detection of biothiols (GSH/Cys) based on the inhibition of thiols toward the oxidase-like activity of Co-N-C SAzymes, which showed high sensitivity with a low detection limit of 0.07 µM for GSH and 0.06 µM for Cys. Besides, the method showed good reproducibility and high selectivity against other amino acids. This work offers new insights using Co-N-C SAzymes in the biosensing field.

Perspective for Single Atom Nanozymes Based Sensors: Advanced Materials, Sensing Mechanism, Selectivity Regulation, and Applications

Nanozymes are a kind of nanomaterial mimicking enzyme catalytic activity, which has aroused extensive interest in the fields of biosensors, biomedicine, and climate and ecosystems management. However, due to the complexity of structures and composition of nanozymes, atomic scale active centers have been extensively investigated, which helps with in-depth understanding of the nature of the biocatalysis. Single atom nanozymes (SANs) cannot only significantly enhance the activity of nanozymes but also effectively improve the selectivity of nanozymes owing to the characteristics of simple and adjustable coordination environment and have been becoming the brightest star in the nanozyme spectrum. The SANs based sensors have also been widely investigated due to their definite structural features, which can be helpful to study the catalytic mechanism and provide ways to improve catalytic activity. This perspective presents a comprehensive understanding on the advances and challenges on SANs based sensors. The catalytic mechanisms of SANs and then the sensing application from the perspectives of sensing technology and sensor construction are thoroughly analyzed. Finally, the major challenges, potential future research directions, and prospects for further research on SANs based sensors are also proposed.

Cascaded Nanozyme System with High Reaction Selectivity by Substrate Screening and Channeling in a Microfluidic Device

Surpassing natural enzymes in cost, stability and mass production, nanozymes have attracted wide attention in fields from disease diagnosis to tumor therapy. However, nanozymes intrinsically have low reaction selectivity, which significantly restricts their applications. A general method is reported to address this challenge by following a biomimetic operation principle of substrates channeling and screening. Two oxidase- and peroxidase-like nanozymes (i.e., emerging N-doped carbon nanocages and Prussian blue nanoparticles), were cascaded as a proof of concept to improve the reaction selectivity in transforming the substrate into the targeted product by more than 2000 times. The cascaded nanozymes were also adopted to a spatially confined microfluidic device, leading to more than 100-fold enhancement of the reaction efficiency due to signal amplification.

Hollow spheres of iron oxide as an “enzyme-mimic”: preparation, characterization and application as biosensors

Nanostructured hollow spheres of iron oxide are demonstrated as “nanozymes” for the dual mode (spectrophotometric and electrochemical) detection of hydrogen peroxide & cholesterol biomarkers and a novel electrochemical sensing mechanism is proposed.

Tuning the Ratio of Pt(0)/Pt(II) in Well-Defined Pt Clusters Enables Enhanced Electrocatalytic Reduction/Oxidation of Hydrogen Peroxide for Sensitive Biosensing

Rational design and construction of advanced sensing platforms for sensitive detection of H₂O₂ released from living cells is one of the challenges in the field of physiology and pathology. Noble metal clusters are a kind of nanomaterials with well-defined chemical composition and special atomic structures, which have been widely explored in catalysis, biosensing, and therapy. Compared with noble metal nanoparticles, noble metal clusters exhibit great potential in electrochemical biosensing due to their high atom utilization efficiency and abundant reactive active sites. Herein, Pt nanoclusters anchored on hollow carbon spheres (Pt_NCS/HCS) were successfully prepared for sensitive detection of H₂O₂. By tuning the ratio of Pt(0)/Pt(II) at different annealing temperatures, the optimized Pt_NCS/HCS-550 showed higher H₂O₂ reduction and oxidation catalytic activities than other control samples. Density functional theory calculations revealed that H₂O₂^*can be better activated and dissociated in the Pt_0II model featured with the co-existence of Pt(0)/Pt(II) and the key intermediates OOH*/OH* have a stronger interaction with the Pt_0II model. As a concept application, the electrochemical biosensing platform was successfully applied to sensitive detection of H₂O₂ released from the cells.

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

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

Engineering DNA/Fe-N-C single-atom nanozymes interface for colorimetric biosensing of cancer cells

Single atom nanozymes (SAzymes) represent the state-of-the-art technology in nanomaterial-based catalysis, which have attracted attentions in catalysis, cancer treatment, disinfection and biosensing fields. However, numerous SAzymes suffered from low aqueous dispersion and without recognition capacity, which impeded their applications in bioanalysis. Herein, we engineered DNA onto SAzymes to obtain the DNA/SAzymes conjugates, which significantly improved the aqueous dispersion and recognition ability of SAzymes. We synthesized iron SAzymes (Fe-N-C SAzymes) as the catalytic nanomaterials, and investigated the interactions between Fe-N-C SAzymes and DNA. We compared A₁₅, T₁₅ and C₁₅ adsorption of Fe-N-C SAzymes in HEPES containing 2 mM MgCl₂. We found that 50 μg mL^-1 Fe-N-C SAzymes produced nearly 100% A₁₅ adsorption, 90% T₁₅ adsorption and only 69% C₁₅ adsorption, indicating that adenine and thymine had higher adsorption affinity on Fe-N-C SAzymes. More importantly, DNA modification did not affect the peroxidase-like activity of Fe-N-C SAzymes and the bioactivity of the adsorbed DNA. Taking the advantage of the diblock DNA with one DNA sequence (adenine) binding to Fe-N-C SAzymes and the other DNA sequence (i.e., aptamer) binding to cancer cells, we designed Apt/Fe-N-C SAzymes for colorimetric detection of cancer cells, which offered new insights for the use of SAzymes in biomedicine.

Fe-N-C Single-Atom Catalyst Coupling with Pt Clusters Boosts Peroxidase-like Activity for Cascade-Amplified Colorimetric Immunoassay

Although single-atom catalysts with high enzyme-like activities have been found, the rational design of highly active peroxidase (POD)-like nanozymes is still a formidable challenge. Herein, highly active POD-like nanozymes were synthesized through loading Pt clusters on the Fe single-atom (Fe_SA-Pt_C) nanozymes. The POD-like activity of Fe_SA-Pt_C nanozymes is enhanced 4.5-fold and 7-fold, in comparison to that of Fe_SA and Pt_C nanozymes, respectively, which is attributed to the unexpected synergistic effect between Fe single atoms and Pt clusters. Based on the outstanding POD-like activity of Fe_SA-Pt_C nanozymes, a cascade signal amplification strategy was constructed by combining glucose oxidase for the colorimetric biosensing of prostate-specific antigens, exhibiting satisfactory sensitivity, high selectivity, a low detection limit of 1.8 pg/mL, and practical feasibility in serum sample detection. This work may serve as a tough foundation to guide the design of superior POD-like nanozymes and expand the application in biosensing.

Antioxidant identification using a colorimetric sensor array based on Co-N-C nanozyme

Here we develop a simple and effective nose/tongue sensor array based on Co-N-C single-atom nanozymes-3,3',5,5'-tetramethylbenzidine (TMB)-H₂O₂ for colorimetric discrimination of antioxidants, which makes use of the color reaction of TMB oxidation by H₂O₂ in two different pH (3.8 and 4.6) environments under the catalysis of Co-N-C nanoenzyme with peroxidase-like activity. Different antioxidants have varying reducing ability to the oxidation products of TMB (oxTMB), thus resulting in differential absorbance and color changes. Linear discriminant analysis (LDA) results indicate that the sensor array successfully identified 7 antioxidants, i.e., glutathione (GSH), ascorbic acid (AA), cysteine (Cys), tannin (TA), Catechin (C), dopamine (DA), and uric acid (UA) in both buffer and even serum samples. Additionally, the performance of the sensor array was validated with antioxidant mixtures, individual antioxidants with different concentrations, and target antioxidants and interfering substances. In general, the versatile sensor array based on Co-N-C single-atom nanozymes provides an excellent strategy for identifying a variety of antioxidants, which exhibits a broad application prospect in medical diagnosis.

Colorimetric determination of phenolic compounds using peroxidase mimics based on biomolecule-free hybrid nanoflowers consisting of graphitic carbon nitride and copper

Hybrid nanoflowers consisting of graphitic carbon nitride (GCN) and copper were successfully constructed without the involvement of any biomolecule, by simply mixing them at room temperature to induce proper self-assembly to achieve a flower-like morphology. The resulting biomolecule-free GCN-copper hybrid nanoflowers (GCN-Cu NFs) exhibited an apparent peroxidase-mimicking activity, possibly owing to the synergistic effect from the coordination of GCN and copper, as well as their large surface area, which increased the number of catalytic reaction sites. The peroxidase-mimicking GCN-Cu NFs were then employed in the colorimetric determination of selected phenolic compounds hydroquinone (HQ), methylhydroquinone (MHQ), and catechol (CC). For samples without phenolic compounds, GCN-Cu NFs catalyzed the oxidation of the peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂, producing an intense blue color signal. Conversely, in the presence of phenolic compounds, the oxidation of TMB was inhibited, resulting in a significant reduction of the color signal. Using this strategy, HQ, MHQ, and CC were selectively and sensitively determined in a linear range up to 100 μM with detection limits down to 0.82, 0.27, and 0.36 μM, respectively. The practical utility of this assay system was also validated by using it to detect phenolic compounds spiked in tap water, yielding a good recovery of 97.1-108.9% and coefficient of variation below 3.0%, demonstrating the excellent reliability and reproducibility of this strategy. Colorimetric determination of phenolic compounds using peroxidase mimics based on biomolecule-free hybrid nanoflowers consisting of graphitic carbon nitride and copper.

Single-atom nanozymes and environmental catalysis: A perspective

The nanomaterials intrinsic enzyme-like activity has gained enormous traction since its discovery in 2007 by Gao and colleagues. The wide range of applications with nanozymes made it more attractive among the scientific community across the world. The area of artificial-enzymes is still evolving, with the development of Single-Atom Nanozymes (SANs), and there is a lot of opportunity in the design and development of SANs that has plenty of real-time applications. The irregular active site distribution or truncated densities of active sites present on the surface of nanozymes can be result in the reduced activity and specificity of nanozymes. Individually spreading these active sites evenly on a solid support will help to curtail the uneven distribution of active sites, resulting in the formation of SANs. SANs, like homogeneous catalysts, are very effective and active due to the nearly uniform distribution of active sites on solid support, and their recovery and recyclability, like heterogeneous catalysts, make them green and sustainable. This review provides a brief overview of architecture, synthesis, and implementations of SANs in various fields. Also, the possibility of SANs in environmental catalysis is discussed along with the key challenges and prospects lying ahead in this field.

A critical comparison of natural enzymes and nanozymes in biosensing and bioassays

Nanozymes (NZs) are nanomaterials that mimic enzyme-like catalytic activity. They have attracted substantial attention due to their inherent physicochemical properties for use as promising alternatives to natural enzymes (NEs) in a variety of research fields. Particularly, in biosensing and bioassays, NZs have opened a new horizon to eliminate the intrinsic limitations of NEs, including their denaturation at extreme pH values and temperatures, poor reusability and recyclability, and high production costs. Moreover, the catalytic activity of NZs can be modulated in the preparation step by following an appropriate synthesis strategy. This review aims to gain insight into the potential substitution of NEs by NZs in biosensing and bioassays while considering both the pros and cons.

Proton-Regulated Catalytic Activity of Nanozymes for Dual-Modal Bioassay of Urease Activity

Benefiting from the merits of high stability and superior activity, nanozymes are recognized as promising alternatives to natural enzymes. Despite the great leaps in the field of therapy and colorimetric sensing, the development of highly sensitive nanozyme-involved photoelectrochemical (PEC) biosensors is still in its infancy. Specifically, the investigation of multifunctional nanozymes facilitating different catalytic reactions remains largely unexplored due to the difficulty in synergistically amplifying the PEC signals. In this work, mesoporous trimetallic AuPtPd nanospheres were synthesized with both efficient oxidase and peroxidase-like activities, which can synergistically catalyze the oxidation of 4-chloro-1-naphthol to produce benzo-4-chlorohexadienone precipitation on the surface of photoactive materials, and thus lead to the decreased photocurrent as well as increased charge-transfer resistance. Inspired by the proton-dependent catalytic activity of nanozymes, a self-regulated dual-modal PEC and electrochemical bioassay of urease activity was innovatively established by in situ regulating the activity of AuPtPd nanozymes through urease-mediated proton-consuming enzymatic reactions, which can remarkably improve the accuracy of the assay. Meanwhile, the determination of urease activity in spiked human saliva samples was successfully realized, indicating the reliability of the biosensor and its application prospects in clinical diagnosis.

A Review on Metal- and Metal Oxide-Based Nanozymes: Properties, Mechanisms, and Applications

Since the ferromagnetic (Fe3O4) nanoparticles were firstly reported to exert enzyme-like activity in 2007, extensive research progress in nanozymes has been made with deep investigation of diverse nanozymes and rapid development of related nanotechnologies. As promising alternatives for natural enzymes, nanozymes have broadened the way toward clinical medicine, food safety, environmental monitoring, and chemical production. The past decade has witnessed the rapid development of metal- and metal oxide-based nanozymes owing to their remarkable physicochemical properties in parallel with low cost, high stability, and easy storage. It is widely known that the deep study of catalytic activities and mechanism sheds significant influence on the applications of nanozymes. This review digs into the characteristics and intrinsic properties of metal- and metal oxide-based nanozymes, especially emphasizing their catalytic mechanism and recent applications in biological analysis, relieving inflammation, antibacterial, and cancer therapy. We also conclude the present challenges and provide insights into the future research of nanozymes constituted of metal and metal oxide nanomaterials.

Two-Dimensional-Plasmon-Boosted Iron Single-Atom Electrochemiluminescence for the Ultrasensitive Detection of Dopamine, Hemin, and Mercury

Single-atom catalysts (SACs) have recently been exploited for luminol-dissolved oxygen electrochemiluminescence (ECL); however, they still suffer from low sensitivity and narrow detection range for a real sample assay. In this work, we boost markedly the ECL response of the iron SAC (Fe-SAC)-based system, for the first time, by the excitation of two-dimensional plasmons derived from the Au@SiO₂ nanomembrane. The plausible mechanism of plasmon enhancement in the Fe-SAC ECL system has been discussed. The constructed Fe-SAC ECL system has been applied for the ECL detection of dopamine, hemin, and mercury (Hg²⁺), with pretty low limits of detection of 0.1, 0.7, and 0.13 nM and wider linear ranges of 0.001-1.0, 0.001-10, and 0.01-0.5 nM, respectively, under optimal conditions.

Bound oxygen-atom transfer endows peroxidase-mimic M-N-C with high substrate selectivity

Advances in nanoscience have stimulated the wide exploration of nanozymes as alternatives to enzymes. Nonetheless, nanozymes often catalyze multiple reactions and are not specialized to a specific substrate, restricting their broad application. Here, we report that the substrate selectivity of the peroxidase-mimic M-N-C can be significantly altered via forming bound intermediates with variable interactions with substrates according to the type of metal. Taking two essential reactions in chemical sensing as an example, Fe-N-C and Co-N-C showed opposite catalytic selectivity for the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) and 3-aminophthalhydrazide (luminol), respectively, by factors of up to 200-fold. It was revealed that specific transition metal-N coordination was the origin of the selective activation of H2O2 forming critically bound oxygen intermediates (M[double bond, length as m-dash]O) for oxygen-atom transfer and the consequent oxidization of substrates. Notably, owing to the embedded ligands in the rigid graphitic framework, surprisingly, the selectivity of M-N-C was even superior to that of commonly used horseradish peroxidase (HRP).

Iron-Imprinted Single-Atomic Site Catalyst-Based Nanoprobe for Detection of Hydrogen Peroxide in Living Cells

Fe-based single-atomic site catalysts (SASCs), with the natural metalloproteases-like active site structure, have attracted widespread attention in biocatalysis and biosensing. Precisely, controlling the isolated single-atom Fe-N-C active site structure is crucial to improve the SASCs' performance. In this work, we use a facile ion-imprinting method (IIM) to synthesize isolated Fe-N-C single-atomic site catalysts (IIM-Fe-SASC). With this method, the ion-imprinting process can precisely control ion at the atomic level and form numerous well-defined single-atomic Fe-N-C sites. The IIM-Fe-SASC shows better peroxidase-like activities than that of non-imprinted references. Due to its excellent properties, IIM-Fe-SASC is an ideal nanoprobe used in the colorimetric biosensing of hydrogen peroxide (H₂O₂). Using IIM-Fe-SASC as the nanoprobe, in situ detection of H₂O₂ generated from MDA-MB-231 cells has been successfully demonstrated with satisfactory sensitivity and specificity. This work opens a novel and easy route in designing advanced SASC and provides a sensitive tool for intracellular H₂O₂ detection.

Mesoporous TiO2-based architectures as promising sensing materials towards next-generation biosensing applications

In the past two decades, mesoporous TiO2 has emerged as a promising material for biosensing applications. In particular, mesoporous TiO2 materials with uniform, well-organized pores and high surface areas typically exhibit superior biosensing performance, which includes high sensitivity, broad linear response, low detection limit, good reproducibility, and high specificity. Therefore, the development of biosensors based on mesoporous TiO2 has significantly intensified in recent years. In this review, the expansion and advancement of mesoporous TiO2-based biosensors for glucose detection, hydrogen peroxide detection, alpha-fetoprotein detection, immobilization of enzymes, proteins, and bacteria, cholesterol detection, pancreatic cancer detection, detection of DNA damage, kanamycin detection, hypoxanthine detection, and dichlorvos detection are summarized. Finally, the future perspective and research outlook on the utilization of mesoporous TiO2-based biosensors for the practical diagnosis of diseases and detection of hazardous substances are also given.

2D bimetallic Ni/Fe MOF nanosheet composites as a peroxidase-like nanozyme for colorimetric assay of multiple targets

In this contribution, 2D Ni/Fe MOF nanosheets were synthesized by a simple two-step ultrasound strategy at room temperature, i.e. the 2D Ni-MOF with a lamellar structure was first synthesized by the top-down ultrasonic assisted stripping route, followed by introducing Fe3+ ions as a metal node and terephthalic acid as an organic ligand to form 2D Ni/Fe MOF nanosheets that exhibited weak oxidase-like and strong peroxidase-like properties. Relative to that of the single metal Ni-MOF and Fe-MOF, the peroxidase-mimicking capability of the 2D Ni/Fe MOF nanosheets increased by over 14-fold and 3-fold, respectively. Reactive oxygen trials indicated that the 2D Ni/Fe MOF nanosheets can efficiently catalyze the decomposition of H2O2 to generate the ˙OH and O2˙- radicals, which can oxidize TMB to oxTMB from colorless to blue. The kinetic trial demonstrated the high affinity of the 2D Ni/Fe MOF nanosheet to H2O2 with a Km of 0.037 mM, which was 100 times lower than that of HRP. These impressive characteristics are likely related to the good dispersion of the in situ formed Fe MOF in the 2D Ni-MOF nanosheet structure with coordinatively unsaturated metal sites. This allows the 2D Ni/Fe MOF nanosheets to expose more active metal sites and to enhance the intrinsic catalytic activity of each site due to the synergistic interaction between the two metals. Interestingly, glutathione can obviously restrict the peroxidase-like activity of the 2D Ni/Fe MOF nanosheet, while the inhibited TMB oxidation can be restored upon further introducing Hg2+ ions due to the high and specific affinity of Hg2+ to thiol groups in glutathione. Based on the above facts, the 2D Ni/Fe MOF nanozyme was used to construct a nanoplatform to determine multiple targets, i.e. H2O2, glutathione and Hg2+. The 2D Ni/Fe MOF nanozyme-based colorimetric assay exhibits a linear response to H2O2, glutathione and Hg2+ ions over the 0.01-100 μM, 0.02-100 μM, and 100 nM to 200 μM ranges, respectively. The limits of detection (3σ) for the determination of H2O2, glutathione and Hg2+ are 10 nM, 10 nM, and 100 nM, respectively. This method was used to determinate the content of Hg2+ ions in real water samples.

Single-atom Fe catalytic amplification-gold nanosol SERS/RRS aptamer as platform for the quantification of trace pollutants

Bisphenol A (BPA), as a typical endocrine disruptor, poses a serious threat to human health. Therefore, it is urgent to establish a rapid, sensitive, and simple method for the determination of BPA. In this paper, based on the aptamer-mediated single-atom Fe carbon dot catalyst (SA_Fe) catalyzing the HAuCl₄-ethylene glycol (EG) nanoreaction, a new SERS/RRS di-mode detection method for BPA was established. The results show that SA_Fe exhibits a strong catalytic effect on the HAuCl₄-EG nanoreaction, which could generate purple gold nanoparticles (AuNPs) with resonance Rayleigh scattering (RRS) signals and surface-enhanced Raman scattering (SERS) effects. After the addition of BPA aptamer (Apt), it could encapsulate SA_Fe through intermolecular interaction, thus inhibiting its catalytic action, resulting in the reduction of AuNPs generated and the decrease of RRS and SERS signals of the system. With the addition of BPA, Apt was specifically combined with BPA, and SA_Fe was re-released to restore the catalytic ability; the generated AuNPs increased. As a result of this RRS and SERS signals of the system recovered, and their increment was linear with the concentration of BPA. Thus, the quantification of 0.1-4.0 nM (RRS) and 0.1-12.0 nM (SERS) BPA was realized, and the detection limits were 0.08 nM and 0.03 nM, respectively. At the same time, we used molecular spectroscopy and electron microscopy to study the SA_Fe-HAuCl₄-ethylene glycol indicator reaction, and proposed a reasonable SA_Fe catalytic reaction mechanism. Based on Apt-mediated SA_Fe catalysis gold nanoreaction amplification, a SERS/RRS di-mode analytical platform was established for targets such as BPA.

Boosted peroxidase-like activity of metal-organic framework nanoparticles with single atom Fe(Ⅲ) sites at low substrate concentration

Single atom nanomaterials possess catalytic activity like natural enzymes are termed as SAzymes which have gained great attention during last two years because of the maximal utilization of atoms and the benefit of understanding structure-property relationship. However, most of SAzymes are fabricated based on hydrophobic carbon, which disperse poorly in water and exhibit inferior affinity towards substrates, which may limit their biomedical applications. Here, we report a peroxidase-like SAzyme through the post-modification route based on hydrophilic defective metal-organic frameworks. Hydrochloric acid (HCl) is employed as ligand modulator to fabricate defective NH₂-UiO-66 nanoparticles (HCl-NH₂-UiO-66 NPs). Compared with the NPs fabricated through acetic acid modulation method (Ac-NH₂-UiO-66 NPs), HCl-NH₂-UiO-66 NPs have more missing linkers. Hence, more Fe(Ⅲ) ions can be successfully doped onto Zr₆ clusters in HCl-NH₂-UiO-66 NPs in a single atom state via formation of Fe-O-Zr bridge. The HCl-NH₂-UiO-66 NPs doped with Fe(Ⅲ) ions (Fe-HCl-NH₂-UiO-66 NPs) possess higher peroxidase-like activity than Fe-Ac-NH₂-UiO-66 NPs due to the higher loading amount of Fe. Besides, both Fe-HCl-NH₂-UiO-66 NPs and Fe-Ac-NH₂-UiO-66 NPs exhibit lower Michaelis-Menten constants (K_m) for hydrogen peroxide (H₂O₂) than most reported nanomaterials, indicating their higher affinity to H₂O₂. Due to their excellent catalytic activity to low concentration of substrates, Fe-HCl-NH₂-UiO-66 NPs can detect H₂O₂ with a limit of detection (LOD) of 1.0 μM. Thus, our system can be used to detect the low cellular H₂O₂ concentration. With high peroxidase-like activity induced by plenty of single atom Fe(Ⅲ) sites, Fe-HCl-NH₂-UiO-66 NPs can also find wide applications in other fields including nanomedicine, pollution degradation and catalysis.

Directly Probing the Local Coordination, Charge State, and Stability of Single Atom Catalysts by Advanced Electron Microscopy: A Review

The drive for atom efficient catalysts with carefully controlled properties has motivated the development of single atom catalysts (SACs), aided by a variety of synthetic methods, characterization techniques, and computational modeling. The distinct capabilities of SACs for oxidation, hydrogenation, and electrocatalytic reactions have led to the optimization of activity and selectivity through composition variation. However, characterization methods such as infrared and X-ray spectroscopy are incapable of direct observations at atomic scale. Advances in transmission electron microscopy (TEM) including aberration correction, monochromators, environmental TEM, and micro-electro-mechanical system based in situ holders have improved catalysis study, allowing researchers to peer into regimes previously unavailable, observing critical structural and chemical information at atomic scale. This review presents recent development and applications of TEM techniques to garner information about the location, bonding characteristics, homogeneity, and stability of SACs. Aberration corrected TEM imaging routinely achieves sub-Ångstrom resolution to reveal the atomic structure of materials. TEM spectroscopy provides complementary information about local composition, chemical bonding, electronic properties, and atomic/molecular vibration with superior spatial resolution. In situ/operando TEM directly observe the evolution of SACs under reaction conditions. This review concludes with remarks on the challenges and opportunities for further development of TEM to study SACs.

Catalytic Nanomaterials toward Atomic Levels for Biomedical Applications: From Metal Clusters to Single-Atom Catalysts

Single-atom catalysts (SACs) featuring the complete atomic utilization of metal, high-efficient catalytic activity, superior selectivity, and excellent stability have been emerged as a frontier in the catalytic field. Recently, increasing interests have been drawn to apply SACs in biomedical fields for enzyme-mimic catalysis and disease therapy. To fulfill the demand of precision and personalized medicine, precisely engineering the structure and active site toward atomic levels is a trend for nanomedicines, promoting the evolution of metal-based biomedical nanomaterials, particularly biocatalytic nanomaterials, from nanoparticles to clusters and now to SACs. This review outlines the syntheses, characterizations, and catalytic mechanisms of metal clusters and SACs, with a focus on their biomedical applications including biosensing, antibacterial therapy, and cancer therapy, as well as an emphasis on their in vivo biological safeties. Challenges and future perspectives are ultimately prospected for SACs in diverse biomedical applications.

Fabrication of Bioresource-Derived Porous Carbon-Supported Iron as an Efficient Oxidase Mimic for Dual-Channel Biosensing

Herein, we designed a new strategy for fabricating a renewable bioresource-derived N-doped hierarchical porous carbon-supported iron (Fe/NPC)-based oxidase mimic. The obtained results suggested that Fe/NPC possessed a large specific surface area (1144 m²/g) and pore volume (0.62 cm³/g) to afford extensive Fe-Nx active sites. Taking advantages of the remarkable oxidase-mimicking activity, outstanding stability, and reusability of Fe/NPC, a novel dual-channel biosensing system was strategically fabricated for sensitively determining acetylcholinesterase (AChE) through the integration of Fe/NPC and fluorescent silver nanoclusters (AgNCs) for the first time. The limits of detection for AChE can achieve as low as 0.0032 and 0.0073 U/L by the outputting fluorometric and colorimetric dual signals, respectively. Additionally, this dual-signal system was applied to analyze human erythrocyte AChE and its inhibitor with robust analytical performance. This work provides one sustainable and effective avenue to apply a bioresource for fabricating an Fe/NPC-based oxidase mimic with high catalytic performance and also gives new impetuses for developing novel biosensors by applying Fe/NPC-based enzyme mimics as substitutes for the natural enzyme.

A ferrocene-linked metal-covalent organic polymer as a peroxidase-enzyme mimic for dual channel detection of hydrogen peroxide

A novel ferrocene-linked metal-covalent organic polymer (MCOP-NFC) was synthesized through the Claisen-Schmidt condensation reaction of 1,1'-diacetyl ferrocene and tris(4-formylphenyl)amine. MCOP-NFC acts as a highly efficient artificial enzyme for mimicking peroxidase, and shows good stability in harsh chemical environments including strong bases and acids, and boiling water. Based on the peroxidase-like activity of MCOP-NFC, a highly sensitive dual channel detection method for hydrogen peroxide was developed. For the colorimetric detection strategy, the limit of detection (LOD) reached 2.1 μM, while the limit of detection was found to be as low as 0.08 μM based on the electrochemical detection channel. This study offers a new strategy for the development of an enzyme mimetic on the basis of the covalent assembly of nanostructures, and the proposed electrochemical-colorimetric sensor for H₂O₂ detection has great potential for applications in biology and biomedicine.