Single-Atom Catalysts in Catalytic Biomedicine

Advances in Electrostatic Spinning of Polymer Fibers Functionalized with Metal-Based Nanocrystals and Biomedical Applications

Considering the metal-based nanocrystal (NC) hierarchical structure requirements in many real applications, starting from basic synthesis principles of electrostatic spinning technology, the formation of functionalized fibrous materials with inorganic metallic and semiconductor nanocrystalline materials by electrostatic spinning synthesis technology in recent years was reviewed. Several typical electrostatic spinning synthesis methods for nanocrystalline materials in polymers are presented. Finally, the specific applications and perspectives of such electrostatic spun nanofibers in the biomedical field are reviewed in terms of antimicrobial fibers, biosensing and so on.

Single-Atom Nanozymes: Fabrication, Characterization, Surface Modification and Applications of ROS Scavenging and Antibacterial

Nanozymes are nanomaterials with intrinsic natural enzyme-like catalytic properties. They have received extensive attention and have the potential to be an alternative to natural enzymes. Increasing the atom utilization rate of active centers in nanozymes has gradually become a concern of scientists. As the limit of designing nanozymes at the atomic level, single-atom nanozymes (SAzymes) have become the research frontier of the biomedical field recently because of their high atom utilization, well-defined active centers, and good natural enzyme mimicry. In this review, we first introduce the preparation of SAzymes through pyrolysis and defect engineering with regulated activity, then the characterization and surface modification methods of SAzymes are introduced. The possible influences of surface modification on the activity of SAzymes are discussed. Furthermore, we summarize the applications of SAzymes in the biomedical fields, especially in those of reactive oxygen species (ROS) scavenging and antibacterial. Finally, the challenges and opportunities of SAzymes are summarized and prospected.

Periodic One-Dimensional Single-Atom Arrays

The orderly assembly of single atoms into highly periodic aggregates at the nanoscale is an intriguing but challenging process of high-precision atomic manufacturing. Here, we discover that an in-plane film surface shrinkage can induce molecular self-assembly to arrange single atoms with unconventional distribution, contributing them to periodic one-dimensional segregation on carbon stripes (one-dimensional single-atom arrays (SAA)). This originates from the fact that metal phthalocyanine (MPc) molecules gradually aggregate and melt to form a film under a thermal drive and the help of sodium chloride templates, accompanied by surface shrinkage, self-assembly, and deep carbonization. At the nanoscale, these periodic parallel arrays are formed due to MPc molecular interactions by π-π stacking. At the atomic scale, the single atoms are stabilized by the vertical phthalocyanine-derived multilayer graphene. This can significantly modify the electronic structure of the single-atom sites on the outermost graphene (e.g., Fe-based SAA), thus optimizing the adsorption energy of oxygen intermediates and resulting in a superior oxygen reduction reaction (ORR) performance concerning disordered single atoms. Our findings provide a general route for orderly single-atom manufacturing (e.g., Fe, Co, and Cu) and an understanding of the relationship between orderly allocation and catalytic performance.

Mn-doped single atom nanozyme composited Au for enhancing enzymatic and photothermal therapy

As an emerging technology, nanocatalytic medicine attracts much attention, especially the ones according to the enzymatic reaction by using excess H₂O₂ in the tumor. Among various candidates, single-atom catalyst (SAC) revealed unique and outstanding redox reaction performance, since the active sites consisting of single metal atoms may achieve the maximum utilization of metal atoms and emerge obviously amplified reaction rate. Here we developed an M-N_x (M = Mn, Zn) center-based SAC with a hollow structure by calcination of Mn²⁺-doped zeolitic imidazolate frameworks (ZIF-8), and PEGylation was applied to improve the hydrophilicity. According to the enzymatic reaction, the M-N_x (M = Mn, Zn) centers have an inherent peroxidase-like activity to catalyze over-expressed H₂O₂ in the weak acidic tumor microenvironment and generate a large amount of toxic reactive oxygen species (ROS) like hydroxyl radicals for therapy. To keep efficient therapeutic output, we integrated the hollow SAC with Au which could expend the glucose in tumor and supply H₂O₂ as the substrate of peroxidase-like activity. Better yet, Au may boost the photothermal effect of SAC and offer another non-invasive photothermal therapy (PTT) to promote the effect of tumor removal. This platform provided a new idea for the construction of more efficient peroxidase-like activity in tumor therapy.

Emerging single atom catalysts in gas sensors

Single atom catalysts (SACs) offer unprecedented opportunities for high-efficiency reactions taking place in many important fields of catalytic processes, electrochemistry, and photoreactions. Due to their maximized atomic utilization and unique electronic and chemical properties, SACs can provide high activity and excellent selectivity for gas adsorption and electron transport, leveraging SACs that enhance the detection sensitivity and selectivity to target gases. In the past few years, SACs including both noble (Pt, Pd, Au, etc.) and non-noble (Mn, Ni, Zn etc.) metals have been demonstrated to be very useful in optimizing sensing performances. However, a comprehensive review on this topic is still missing. Herein, we summarize the synthesis technologies of SACs that are applicable to gas sensors. The electronic and chemical interactions between SACs and host sensing materials, which are crucial to sensor functions, are discussed. Then, we highlight the application progress of various SACs in gas sensors. Prospects in the creation of new sensing materials with emerging SACs and versatile supports are also present. Finally, the challenges and prospects of SACs in the future development of sensors are analyzed.

Tumor microenvironment-activated single-atom platinum nanozyme with H2O2 self-supplement and O2-evolving for tumor-specific cascade catalysis chemodynamic and chemoradiotherapy

Nanozyme-based tumor collaborative catalytic therapy has attracted a great deal of attention in recent years. However, their cooperative outcome remains a great challenge due to the unique characteristics of tumor microenvironment (TME), such as insufficient endogenous hydrogen peroxide (H₂O₂) level, hypoxia, and overexpressed intracellular glutathione (GSH). Methods: Herein, a TME-activated atomic-level engineered PtN₄C single-atom nanozyme (PtN₄C-SAzyme) is fabricated to induce the "butterfly effect" of reactive oxygen species (ROS) through facilitating intracellular H₂O₂ cycle accumulation and GSH deprivation as well as X-ray deposition for ROS-involving CDT and O₂-dependent chemoradiotherapy. Results: In the paradigm, the SAzyme could boost substantial ∙OH generation by their admirable peroxidase-like activity as well as X-ray deposition capacity. Simultaneously, O₂ self-sufficiency, GSH elimination and elevated Pt²⁺ release can be achieved through the self-cyclic valence alteration of Pt (IV) and Pt (II) for alleviating tumor hypoxia, overwhelming the anti-oxidation defense effect and overcoming drug-resistance. More importantly, the PtN₄C-SAzyme could also convert O₂^·- into H₂O₂ by their superior superoxide dismutase-like activity and achieve the sustainable replenishment of endogenous H₂O₂, and H₂O₂ can further react with the PtN₄C-SAzyme for realizing the cyclic accumulation of ∙OH and O₂ at tumor site, thereby generating a "key" to unlock the multi enzymes-like properties of SAzymes for tumor-specific self-reinforcing CDT and chemoradiotherapy. Conclusions: This work not only provides a promising TME-activated SAzyme-based paradigm with H₂O₂ self-supplement and O₂-evolving capacity for intensive CDT and chemoradiotherapy but also opens new horizons for the construction and tumor catalytic therapy of other SAzymes.

Identification of the Stable Pt Single Sites in the Environment of Ions: From Mechanism to Design Principle

For single-atom (SA)-based catalysis, it is urgent to understand the nature and dynamic evolution of SA active sites during the reactions. In this work, an example of Pt SA-Zn_0.5 Cd_0.5 S (Pt SA-ZCS) is found to display interesting phenomena when facing the Brownian collision of ions in aqueous photocatalysis. Via synchrotron radiation, surface techniques, microscopy, and theory calculations, the results show that two kinds of Pt sites exist: Pt_Zn-sub -S₃ (Pt substituting the Zn site) and Pt_ads -S₂ (Pt adsorbing on the surface). In Na₂ S, the S^2- can coordinate with Pt atoms and peel them from the Pt_ads -S₂ sites, but leaves more stable Pt_Zn-sub -S₃ sites, bringing a low but stable catalytic activity (19.40 mmol g^-1 h^-1 ). Meanwhile, in ascorbic acid, the ascorbic acid ions show less complex ability with Pt atoms, but can decrease the migration barrier of Pt_ads -S₂ sites (67.18 down to 35.96 mmol g^-1 h^-1 , 52.03% drop after 6 h). Therefore, the Pt_ads -S₂ sites gradually aggregate into nanoclusters, bringing a high but decayed catalytic activity. Moreover, a Pt SA-ZCS-Sulfur composite is designed mainly covered by Pt_Zn-sub -S₃ sites accordingly (max: 79.09 mmol g^-1 h^-1 , 5% drop after 6 h and QE: 14.0% at 420 nm), showing a beneficial strategy "from mechanism to design principle."

Novel Strategy for Optimized Nanocatalytic Tumor Therapy: From an Updated View

Nanozyme has been experiencing rapid development in biomedical applications involving biosensors, immunoassays, and antitumor agents in recent years due to its tunable catalytic performance and desirable biocompatibility. Since the first exploration of nanozyme-based Fenton reaction for nanocatalytic therapy (NCT) against tumor, a variety of Fenton (and Fenton-like) nanozymes, such as Fe3O4, transition metal ions (Co2+, Cu2+, and Mn2+), and metal-organic frameworks (MOFs), have been proved as desirable candidates for tumor therapy, and the modulation of the tumor microenvironment (TME) is determined to be a feasible approach to improve the catalytic efficiency for in situ tumor suppression. At present, increasing studies have focused on improving the therapeutic efficiency of NCT by formulating multifunctional nanozyme-based systems to satisfy the demand for versatile and optimized applications. Herein, updated insights into the novel strategies of 1) achieving highly effective nanocatalytic reactions, including the modification of nanocatalysts and TME-modulating approaches, are provided and 2) the design and formulation of multifunctional nanozyme-based systems which achieve targeted, synergistic therapy, and theranostic applications are analyzed and concluded. Concise and concentrated comments and outlooks are illuminated at the end to outline the perspectives and the remaining challenges for the next-step explorations on further biomedical translation of NCT.

Liquid exfoliation of V8C7 nanodots as peroxidase-like nanozymes for photothermal-catalytic synergistic antibacterial treatment

Nanozymes are effective antibiotics that use reactive oxygen species (ROS) produced by Fenton/Fenton-like reactions to kill bacteria. However, its activity is still not satisfactory and requires large amounts of hydrogen peroxide (H₂O₂) with side effects on normal tissues. Herein, ultrasmall V₈C₇ nanodots (NDs) are successfully constructed by the liquid-phase exfoliation method for photothermal-catalytic synergistic antibacterial treatment. The prepared V₈C₇ NDs are horseradish peroxidase (HRP)-like nanozymes that can efficiently catalyze H₂O₂ to produce a large amount of ROS. Unlike traditional HRP-like nanozymes, V₈C₇ NDs can have a good catalytic effect under slightly acidic conditions (pH=5.5). Moreover, V₈C₇ NDs have good near-infrared (NIR) absorption and high photothermal conversion efficiency (PTCE, 50.39%), which can be used for photothermal treatment (PTT) of bacteria. In addition, the mild photothermal effect can further enhance the HRP-like catalytic activity of V₈C₇ NDs, thereby further enhancing the antibacterial performance of V₈C₇ NDs. In vitro results show that V₈C₇ NDs can effectively eradicate Escherichia coli (E. coli, gram-negative) and methicillin-resistant Staphylococcus aureus (MRSA, gram-positive) under laser irradiation and the presence of H₂O₂, possessing spectral antibacterial properties. More importantly, V₈C₇ NDs display satisfactory therapeutic effects on wounds infected by MRSA in vivo, and their toxicity is negligible, suggesting that they may have great potential for application as powerful and safe antibacterial agents. This work presents a practical antibacterial strategy by combining PTT and catalytic therapy to achieve efficient treatment of bacteria-infected wounds. STATEMENT OF SIGNIFICANCE: (1) Ultrasmall V₈C₇ NDs were prepared by the liquid-phase exfoliation method. (2) V₈C₇ NDs showed good photothermal, catalytic properties. (3) V₈C₇ NDs achieved satisfactory photothermal-catalytic synergistic antibacterial treatment.

Achieving a highly efficient oxygen reduction reaction via a molecular Fe single atom catalyst

Molecular Fe phthalocyanine (FePc) is successfully anchored on a defective mesoporous carbon framework for the highly efficient oxygen reduction reaction (ORR) with a half-wave potential of 0.86 V (vs. RHE) and a limited current density of 5.40 mA cm^-2. DFT calculations further suggest that the non-planar structure incorporating FePc can promote charge polarization and decrease the energy barrier.

Emerging Prospects of Nanozymes for Antibacterial and Anticancer Applications

The ability of some nanoparticles to mimic the activity of certain enzymes paves the way for several attractive biomedical applications which bolster the already impressive arsenal of nanomaterials to combat deadly diseases. A key feature of such 'nanozymes' is the duplication of activities of enzymes or classes of enzymes, such as catalase, superoxide dismutase, oxidase, and peroxidase which are known to modulate the oxidative balance of treated cells for facilitating a particular biological process such as cellular apoptosis. Several nanoparticles that include those of metals, metal oxides/sulfides, metal-organic frameworks, carbon-based materials, etc., have shown the ability to behave as one or more of such enzymes. As compared to natural enzymes, these artificial nanozymes are safer, less expensive, and more stable. Moreover, their catalytic activity can be tuned by changing their size, shape, surface properties, etc. In addition, they can also be engineered to demonstrate additional features, such as photoactivated hyperthermia, or be loaded with active agents for multimodal action. Several researchers have explored the nanozyme-mediated oxidative modulation for therapeutic purposes, often in combination with other diagnostic and/or therapeutic modalities, using a single probe. It has been observed that such synergistic action can effectively by-pass the various defense mechanisms adapted by rogue cells such as hypoxia, evasion of immuno-recognition, drug-rejection, etc. The emerging prospects of using several such nanoparticle platforms for the treatment of bacterial infections/diseases and cancer, along with various related challenges and opportunities, are discussed in this review.

Nanomaterials alleviating redox stress in neurological diseases: mechanisms and applications

Overproduced reactive oxygen and reactive nitrogen species (RONS) in the brain are involved in the pathogenesis of several neurological diseases, such as Alzheimer's disease, Parkinson's disease, traumatic brain injury, and stroke, as they attack neurons and glial cells, triggering cellular redox stress. Neutralizing RONS, and, thus, alleviating redox stress, can slow down or stop the progression of neurological diseases. Currently, an increasing number of studies are applying nanomaterials (NMs) with anti-redox activity and exploring the potential mechanisms involved in redox stress-related neurological diseases. In this review, we summarize the anti-redox mechanisms of NMs, including mimicking natural oxidoreductase activity and inhibiting RONS generation at the source. In addition, we propose several strategies to enhance the anti-redox ability of NMs and highlight the challenges that need to be resolved in their application. In-depth knowledge of the mechanisms and potential application of NMs in alleviating redox stress will help in the exploration of the therapeutic potential of anti-redox stress NMs in neurological diseases.

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.

Strategies for enhancing cancer chemodynamic therapy performance

Chemodynamic therapy (CDT) has emerged to be a frontrunner amongst reactive oxygen species-based cancer treatment modalities. CDT utilizes endogenous H2O2 in tumor microenvironment (TME) to produce cytotoxic hydroxyl radicals (•OH) via Fenton or Fenton-like reactions. While possessing advantages such as tumor specificity, no need of external stimuli, and low side effects, practical applications of CDT are still impeded owing to the heterogeneity, complexity, and reductive environment of TME. Over the past couple of years, strategies to enhance CDT for efficient tumor regression are in rapid development in synergy with the growth of nanomedicine. In this review, we initially outline the fundamental understanding of Fenton and Fenton-like reactions and their relationship with CDT. Subsequently, the development in the design of nanosystems for CDT is highlighted in a general manner. Furthermore, recent advancement of the strategies to augment Fenton reactions in TME for enhanced CDT is discussed in detail. Finally, perspectives toward the future development of CDT for better therapeutic outcome are presented. This review is expected to draw attention for collaborative research on CDT in the best interest of its future clinical applications.

A Library of ROS-Catalytic Metalloenzyme Mimics with Atomic Metal Centers

MetalN-coordinated centers supported by carbonaceous substrates have emerged as promising artificial metalloenzymes (AMEs) to mimic the biocatalytic effects of their natural counterparts. However, the synthesis of well-defined AMEs that contain different atomic metalN centers but present similar physicochemical and coordination structures remains a substantial challenge. Here, 20 different types of AMEs with similar geometries and well-defined atomic metalN-coordinated centers are synthesized to compare and disclose the catalytic activities, substrate selectivities, kinetics, and reactive oxygen species (ROS) products. Their oxidase (OXD)-, peroxidase (POD)-, and halogen peroxidase (HPO)-mimetic catalytic behaviors are systematically explored. The Fe-AME shows the highest OXD- and HPO-mimetic activities compared to the other AMEs due to its high v_max (0.927 × 10^-6 m s^-1 ) and low K_m (1.070 × 10^-3 m), while the Cu-AME displays the best POD-like performance. Furthermore, theoretical calculation reveals that the ROS-catalytic paths and activities are highly related to the electronic structures of the metal centers. Benefiting from its facile adsorption of H₂ O₂ molecule and lower energy barrier to generating •O₂ ^- , the Fe-AME displays higher ROS-catalytic performances than the Mn-AME. The engineered AMEs show not only remarkably high ROS-catalytic performances but also provide new guidance toward developing metalN-coordinated biocatalysts for broad application fields.

A Bioinspired Five-Coordinated Single-Atom Iron Nanozyme for Tumor Catalytic Therapy

Single-atom nanozymes (SAzymes) represent a new research frontier in the biomedical fields. The rational design and controllable synthesis of SAzymes with well-defined electronic and geometric structures are essential for maximizing their enzyme-like catalytic activity and therapeutic efficacy but remain challenging. Here, a melamine-mediated pyrolysis activation strategy is reported for the controllable fabrication of iron-based SAzyme containing five-coordinated structure (FeN₅ ), identified by transmission electron microscopy imaging and X-ray absorption fine structure analyses. The FeN₅ SAzyme exhibits superior peroxidase-like activity owing to the optimized coordination structure, and the corresponding catalytic efficiency of Fe-species in the FeN₅ SAzyme is 7.64 and 3.45 × 10⁵ times higher than those in traditional FeN₄ SAzyme and Fe₃ O₄ nanozyme, respectively, demonstrated by steady-state kinetic assay. In addition, the catalytic mechanism is jointly disclosed by experimental results and density functional theory studies. The as-synthesized FeN₅ SAzyme demonstrates significantly enhanced antitumor effect in vitro and in vivo due to the excellent peroxidase-like activity under tumor microenvironment.

Construction of a colorimetric sensor array based on the coupling reaction to identify phenols

Phenols are harmful to the human body and the environment. Since there are a variety of phenols in actual samples, this requires a sensor which possesses the ability to simultaneously distinguish them. Herein, we report a colorimetric sensor array, which uses two nanozymes (Fe-N-C nanozymes and Cu-N-C nanozymes) as electronic tongues for fingerprint identification of six phenols (2,4,6-trichlorophenol (2,4,6-Tri), 4-nitrophenol (P-np), phenol (Phe), 3-chlorophenol (3-CP), 4-chlorophenol (4-CP), and o-nitrophenol (O-np)) in the environment. Nanozymes catalyzed the reaction of hydrogen peroxide, different phenols and 4-aminoantipyrine (4-AAP) to produce different color variations. These signal changes as fingerprints encouraged us to develop a pattern recognition method for the identification of phenols by linear discriminant analysis (LDA). The six phenols at 50 nM have their own response patterns, respectively. Surprisingly, this sensor array had distinguished the six phenols in actual samples successfully.

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.

pH-programmed responsive nanoplatform for synergistic cancer therapy based on single atom catalysts

The development of stimuli-responsive nanoplatform provides powerful tool for simultaneously enhancing the efficiency and accuracy of cancer therapies. Herein, we develop a pH-programmed responsive and synergistically theranostic nanoplatform based on CaCO₃ mineralized single atom iron nanoparticles (SAF NPs). Basically, the highly active site on SAF NPs nanoagent can trigger in-situ produce toxic •OH in tumor microenvironment (TME) that kill cancer cells for Fenton-reaction-based chemodynamic therapy (CDT). The porous structure of SAF NPs can serve as delivery platforms to package and programmed release chemotherapeutic drug doxorubicin (DOX) to enhance chemotherapy (CT) efficiency. The nanoplatform was simultaneously in-situ mineralized with CaCO₃ and A549 cell membrane (CM) which could avoid DOX leakage during transport in bloodstream and target homologous cancer cells. In addition, overload Ca²⁺ decomposed from CaCO₃ triggers mitochondrial dysfunction, induces cytoskeleton collapse and oxidative stress to formulate calcium ions interference therapy (CIT). With the combination of CDT, CT and CIT, the designed multi-synergetic nanoplatform exhibits excellent biocompatibility, specificity and tunable drug release behavior, which has a broad application prospect in tumor therapy.

Fe-N-C single-atom nanozymes based sensor array for dual signal selective determination of antioxidants

Machine learning algorithms as a powerful tool can efficiently utilize and process large quantities of data generated by high-throughput experiments in various fields. In this work, we used a general ionic salt-assisted synthesis method to prepare oxidase-like Fe-N-C SANs. The possible reason for the excellent enzyme-mimicking activity and affinity of Fe-N-C SANs was further verified by density functional theory calculations. Due to the remarkable oxidase-mimicking activity, the prepared Fe-N-C SANs were used to detect ascorbic acid (AA) with a detection limit of 0.5 μM. Based on the machine learning algorithms, we successfully distinguished six antioxidants (ascorbic acid, glutathione, L-cysteine, dithiothreitol, uric acid, and dopamine) with the same concentration by either one kind of Fe-N-C SANs or three kinds of different Fe-N-C SANs. The usefulness of the Fe-N-C SANs sensor arrays was further validated by the hierarchal cluster analysis, where they also can be correctly identified. More importantly, a SANs-based digital-image colorimetric sensor array has also been successfully constructed and thereby achieved visual and informative colorimetric analysis for practical samples out of the lab. This work not only provides a design synthesis method to prepare SANs but also combines machine learning algorithms with SANs sensors to identify analytes with similar properties, which can further expand to the detection of proteins and cells related to diseases in the future.

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.

Highly Sensitive Chemiluminescent Immunoassay of Mycotoxins Using ZIF-8-Derived Yolk-Shell Co Single-Atom Site Catalysts as Superior Fenton-like Probes

Superior to traditional nanoscale catalysts, single-atom site catalysts (SASCs) show such merits as maximal catalysis efficiency and outstanding catalytic activity for the construction of analytical methodological platforms. Hereby, an in situ etching strategy was designed to prepare yolk-shell Co SASCs derived from ZIF-8@SiO₂ nanoparticles. On the basis of direct chemical interactions between precursors and supports, the Co element with isolated atomic dispersion was anchored on ZIF-8@SiO₂ nanoparticles. The Co SASCs possess high Fenton-like activity and thus can catalyze the decomposition of H₂O₂ to produce massive superoxide radical anions instead of singlet oxygen and hydroxyl radicals. With the activity for producing superoxide radical anion, Co SASCs can greatly improve the chemiluminescent (CL) response of a luminol system by 3133.7 times. Furthermore, the SASCs with active sites of Co-O₅ moieties were utilized as the CL probes for establishment of an immunoassay method for sensitive detection of mycotoxins by adopting aflatoxin B1 as a mode analyte. The quantitation range is 10-1000 pg/mL, and the limit of detection is 0.44 pg/mL (3σ) for aflatoxin B1. The proof-of-principle work elucidates the practicability of direct chemical interactions between precursors and supports for forming SASCs with ultrahigh CL response, which can be extended to the exploitation of more sorts of SASCs for tracing biological binding events.

Reference material of Prussian blue nanozymes for their peroxidase-like activity

The development process of a Prussian blue nanozyme certified reference material for peroxidase-like activity.

Nanocatalyst-Mediated Chemodynamic Tumor Therapy

Traditional tumor treatments, including chemotherapy, radiotherapy, photodynamic therapy, and photothermal therapy, are developed and used to treat different types of cancer. Recently, chemodynamic therapy (CDT) has been emerged as a novel cancer therapeutic strategy. CDT utilizes Fenton or Fenton-like reaction to generate highly cytotoxic hydroxyl radicals (•OH) from endogenous hydrogen peroxide (H₂ O₂ ) to kill cancer cells, which displays promising therapeutic potentials for tumor treatment. However, the low catalytic efficiency and off-target side effects of Fenton reaction limit the biomedical application of CDT. In this regard, various strategies are implemented to potentiate CDT against tumor, including retrofitting the tumor microenvironment (e.g., increasing H₂ O₂ level, decreasing reductive substances, and reducing pH), enhancing the catalytic efficiency of nanocatalysts, and other strategies. This review aims to summarize the development of CDT and summarize these recent progresses of nanocatalyst-mediated CDT for antitumor application. The future development trend and challenges of CDT are also discussed.

Metal-Support Interactions of Single-Atom Catalysts for Biomedical Applications

The development of single-atom catalysts (SACs) has become a rapidly growing research field. It is a critical challenge to understand the interactions between the single-atom metal active sites and the support materials. Recently, original research reports of SACs in biomedical applications have emerged in the literature, yet this topic has seldom been reviewed. Here, this review focuses on the latest advances in single-atom catalysis for biomedical applications and highlights the keys for the design of SACs, such as understanding the interactions between metals and supports and classifying various enzyme-like activities. This review helps bridge the knowledge of multiple disciplines and provides prospects regarding the development of SACs for biomedicine.

Transferrin guided quasi-nanocuboid as tetra-enzymic mimics and biosensing applications

Given the promising prospect of nanozymes system with multi-enzyme mimetic activities, it is also a challenge for designing a controllable nanostructure as multi-enzymes mimics by protein-guided strategy. Here, transferrin (Trf)-directed manganese oxide with 3D nanomorphology was developed. Trf-Mn₃O₄ quasi-nanocuboids (NCs) was obtained by improved conditions for biomineralization, which employing Trf as biotemplate and Mn²⁺ as metal source in alkaline solution. Fortunately, not only was the controllable structure discovered, but Trf-Mn₃O₄ NCs also showed tetra-enzyme mimic activities involving peroxidase-, ascorbic acid oxidase-, catalase- and superoxide dismutase-mimic activities. Further, the catalytic properties and steady-state kinetic of Trf-Mn₃O₄ NCs was explored, systematically. Moreover, we develop simple colorimetric sensor based on the peroxidase-mimic activity of Trf-Mn₃O₄ NCs for the detection of gallic acid (GA) with the linear range within 0.1-40 μΜ, and the limit of detection was 41.2 nM (S/N = 3). Besides, a novel fluorimetric sensor for the detection of AA based on the ascorbic acid oxidase-mimic activity was proposed with the linear range of 0.5 μM-10 μM (R² = 0.9906) and 10 μM-5 mM (R² = 0.9927), and LOD of 0.24 μM (S/N = 3) was obtained. Both the proposed sensors showed outstanding detection performance, accuracy and repeatability, which realized the detection of GA and AA in real sample, respectively. The as-synthesized tetra-enzymic mimics not only enriched the species of nanozymes, particularly for the nanozymes with multi-enzyme mimic activities, the proposed sensors showed promising potential applications for biosensor in complex samples.

Dual enzyme-mimic nanozyme based on single-atom construction strategy for photothermal-augmented nanocatalytic therapy in the second near-infrared biowindow

Nanozyme-based catalytic therapy, an emerging therapeutic pattern, has significantly incorporated in the advancement of tumor therapy by generating lethal reactive oxygen species. Nevertheless, most of the nanozymes have mono catalytic performances with H₂O₂ in the tumor microenvironment (TME), which lowers their therapeutic efficiency. Herein, we design a newly-developed single-atom Fe dispersed N-doped mesoporous carbon nanospheres (SAFe-NMCNs) nanozyme with high H₂O₂ affinity for photothermal-augmented nanocatalytic therapy. The SAFe-NMCNs nanozyme possesses dual enzyme-mimic catalytic activity which not only acts as a catalase-mimic role to achieve ultrasonic imaging in tumor site by O₂ generation, but also exhibits the superior peroxidase-mimic catalytic performance to generate •OH for nanocatalytic therapy. Besides, the SAFe-NMCNs nanozyme with strong optical absorption in the second near-infrared (NIR-II) region shows excellent photothermal conversion performance. The peroxidase-mimic catalytic process of SAFe-NMCNs nanozyme is realized using density functional theory (DFT). Both in vitro and in vivo results indicate that the SAFe-NMCNs nanozyme can efficiently suppress tumor cells growth by a synergistic therapy effect with photothermal-augmented nanocatalytic therapy. The work developed a single-atom-coordinated nanozyme with dual-enzyme catalytic performance and achieve hyperthermia-augmented nanocatalytic therapy effect, can open a window for potential biological applications.

Thermally Stable Single-Atom Heterogeneous Catalysts

Single-atom catalysts (SACs) have attracted extensive attention in fields related to energy, environment, and material sciences because of the high atom efficiency and the unique properties of these materials. Many approaches have hitherto been successfully established to prepare SACs, including impregnation, pyrolysis-involved processes, atom trapping, and coprecipitation. However, under typical reaction conditions, single atoms on catalysts tend to migrate or agglomerate, forming nanoclusters or nanoparticles, which lowers their surface free energy. Efforts are required to develop strategies for improving the thermal stability of SACs while achieving excellent catalytic performance. In this Progress Report, recent advances in the development of thermally durable single-atom heterogeneous catalysts are discussed. Several important preparation approaches for thermally stable SACs are described in this article. Fundamental understanding of the coordination structures of thermally stable single atom prepared by these methods is discussed. Furthermore, the catalytic performances of these thermally stable SACs are reviewed, including their activity and stability. Finally, a perspective of this important and rapidly evolving research field is provided.

Amplified Peroxidase-like Activity of Co2+ Using 8-Hydroxyquinoline and Its Application for Ultrasensitive Colorimetric Detection of Clioquinol

8-Hydroxyquinoline (8HQ) and its derivatives display diverse bioactivities and therapeutic potentials. In this study, we unveiled that 8HQ can boost the peroxidase-like activity of Co²⁺ in the presence of bicarbonate (HCO₃ ^- ) in neutral pH at room temperature. With 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulphonate) (ABTS) as the substrate, the formed Co²⁺ /8HQ/HCO₃ ^- complex shows robust catalytic activity with the turnover number (k_cat ) tens to hundreds of times higher than that of Co₃ O₄ and other Co²⁺ complexes in terms of per cobalt ion. This system was used to design colorimetric sensors for ultrasensitive detection of 8HQ-based drugs by activating the activity of Co²⁺ . Take detecting clioquinol as an example, a detection limit of 2.4 nM clioquinol with a linear range from 0.01 to 0.2 μM was obtained. This work not only revealed a new kind of ligand that activated the activity of Co²⁺ , but also provided a facile, low-cost, ultrasensitive, easy-to-use, and universal strategy for sensing various 8HQ-based drugs. Further development of this catalytic system might be beneficial to overcome drug resistance by combined medication.

Copper single-atom catalysts with photothermal performance and enhanced nanozyme activity for bacteria-infected wound therapy

Nanozymes have become a new generation of antibiotics with exciting broad-spectrum antibacterial properties and negligible biological toxicity. However, their inherent low catalytic activity limits their antibacterial properties. Herein, Cu single-atom sites/N doped porous carbon (Cu SASs/NPC) is successfully constructed for photothermal-catalytic antibacterial treatment by a pyrolysis-etching-adsorption-pyrolysis (PEAP) strategy. Cu SASs/NPC have stronger peroxidase-like catalytic activity, glutathione (GSH)-depleting function, and photothermal property compared with non-Cu-doped NPC, indicating that Cu doping significantly improves the catalytic performance of nanozymes. Cu SASs/NPC can effectively induce peroxidase-like activity in the presence of H2O2, thereby generating a large amount of hydroxyl radicals (•OH), which have a certain killing effect on bacteria and make bacteria more susceptible to temperature. The introduction of near-infrared (NIR) light can generate hyperthermia to fight bacteria, and enhance the peroxidase-like catalytic activity, thereby generating additional •OH to destroy bacteria. Interestingly, Cu SASs/NPC can act as GSH peroxidase (GSH-Px)-like nanozymes, which can deplete GSH in bacteria, thereby significantly improving the sterilization effect. PTT-catalytic synergistic antibacterial strategy produces almost 100% antibacterial efficiency against Escherichia coli (E. coli) and methicillin-resistant Staphylococcus aureus (MRSA). In vivo experiments show a better PTT-catalytic synergistic therapeutic performance on MRSA-infected mouse wounds. Overall, our work highlights the wide antibacterial and anti-infective bio-applications of Cu single-atom-containing catalysts.

Multifunctional Composite Nanosystems for Precise/Enhanced Sonodynamic Oxidative Tumor Treatment

Ultrasound-activated therapies have been regarded as the efficient strategy for tumor treatment, among which sonosensitizer-enabled sonodynamic oxidative tumor therapy features intrinsic advantages as compared to other exogenous trigger-activated dynamic therapies. Nanomedicine-based nanosonosensitizer design has been extensively explored for improving the therapeutic efficacy of sonodynamic therapy (SDT) of tumor. This review focuses on solving two specific issues, i.e., precise and enhanced sonodynamic oxidative tumor treatment, by rationally designing and engineering multifunctional composite nanosonosensitizers. This multifunctional design can augment the therapeutic efficacy of SDT against tumor by either improving the production of reactive oxygen species or inducing the synergistic effect of SDT-based combinatorial therapies. Especially, this multifunctional design is also capable of endowing the nanosonosensitizer with bioimaging functionality, which can effectively guide and monitor the therapeutic procedure of the introduced sonodynamic oxidative tumor treatment. The design principles, underlying material chemistry for constructing multifunctional composite nanosonosensitizers, intrinsic synergistic mechanism, and bioimaging guided/monitored precise SDT are summarized and discussed in detail with the most representative paradigms. Finally, the existing critical issues, available challenges, and potential future developments of this research area are also discussed for promoting the further clinical translations of these multifunctional composite nanosonosensitizers in SDT-based tumor treatment.

Single-Atom (Iron-Based) Catalysts: Synthesis and Applications

Supported single-metal atom catalysts (SACs) are constituted of isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and metal oxides. Their thermal stability, electronic properties, and catalytic activities can be controlled via interactions between the single-metal atom center and neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomic dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased, thus offering new possibilities to control the selectivity of a given transformation as well as to improve catalyst turnover frequencies and turnover numbers. This review aims to comprehensively summarize the synthesis of Fe-SACs with a focus on anchoring single atoms (SA) on carbon/graphene supports. The characterization of these advanced materials using various spectroscopic techniques and their applications in diverse research areas are described. When applicable, mechanistic investigations conducted to understand the specific behavior of Fe-SACs-based catalysts are highlighted, including the use of theoretical models.

Thermal Atomization of Platinum Nanoparticles into Single Atoms: An Effective Strategy for Engineering High-Performance Nanozymes

Although great progress has been made in artificial enzyme engineering, their catalytic performance is far from satisfactory as alternatives of natural enzymes. Here, we report a novel and efficient strategy to access high-performance nanozymes via direct atomization of platinum nanoparticles (Pt NPs) into single atoms by reversing the thermal sintering process. Atomization of Pt NPs into single atoms makes metal catalytic sites fully exposed and results in engineerable structural and electronic properties, thereby leading to dramatically enhanced enzymatic performance. As expected, the as-prepared thermally stable Pt single-atom nanozyme (Pt_TS-SAzyme) exhibited remarkable peroxidase-like catalytic activity and kinetics, far exceeding the Pt nanoparticle nanozyme. The following density functional theory calculations revealed that the engineered P and S atoms not only promote the atomization process from Pt NPs into Pt_TS-SAzyme but also endow single-atom Pt catalytic sites with a unique electronic structure owing to the electron donation of P atoms, as well as the electron acceptance of N and S atoms, which simultaneously contribute to the substantial enhancement of the enzyme-like catalytic performance of Pt_TS-SAzyme. This work demonstrates that thermal atomization of the metal nanoparticle-based nanozymes into single-atom nanozymes is an effective strategy for engineering high-performance nanozymes, which opens up a new way to rationally design and optimize artificial enzymes to mimic natural enzymes.

Organic/Inorganic Hybrid Fibers: Controllable Architectures for Electrochemical Energy Applications

Organic/inorganic hybrid fibers (OIHFs) are intriguing materials, possessing an intrinsic high specific surface area and flexibility coupled to unique anisotropic properties, diverse chemical compositions, and controllable hybrid architectures. During the last decade, advanced OIHFs with exceptional properties for electrochemical energy applications, including possessing interconnected networks, abundant active sites, and short ion diffusion length have emerged. Here, a comprehensive overview of the controllable architectures and electrochemical energy applications of OIHFs is presented. After a brief introduction, the controllable construction of OIHFs is described in detail through precise tailoring of the overall, interior, and interface structures. Additionally, several important electrochemical energy applications including rechargeable batteries (lithium-ion batteries, sodium-ion batteries, and lithium-sulfur batteries), supercapacitors (sandwich-shaped supercapacitors and fiber-shaped supercapacitors), and electrocatalysts (oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction) are presented. The current state of the field and challenges are discussed, and a vision of the future directions to exploit OIHFs for electrochemical energy devices is provided.

A Biofilm Microenvironment-Activated Single-Atom Iron Nanozyme with NIR-Controllable Nanocatalytic Activities for Synergetic Bacteria-Infected Wound Therapy

Biofilm microenvironment (BME)-activated antimicrobial agents display great potential for improved biofilm-related infection therapy because of their superior specificities and sensitivities, effective eliminations, and minimal side effects. Herein, BME-activated Fe-doped polydiaminopyridine nanofusiform-mediated single-atom nanozyme (FePN SAzyme) is presented for photothermal/chemodynamic synergetic bacteria-infected wound therapy. The photothermal therapy (PTT) function of SAzyme can be specifically initiated by the high level of H₂ O₂ and further accelerated through mild acid within the inflammatory environment through "two-step rocket launching-like" process. Additionally, the enhanced chemodynamic therapy (CDT) for the FePN SAzyme can also be endowed by producing hydroxyl radicals through reacting with H₂ O₂ and consuming glutathione (GSH) of the BME, thereby contributing to more efficient synergistic therapeutic effect. Meanwhile, FePN SAzyme could catalyze biofilm-overexpressed H₂ O₂ decomposing into O₂ and overcome the hypoxia of biofilm, which significantly enhances the susceptibility of biofilm and increases the synergistic efficacy. Most importantly, the synergistic therapy of bacterial-induced infection diseases can be switched on by the internal and external stimuli simultaneously, resulting in minimal nonspecific damage to healthy tissue. These remarkable characteristics of FePN SAzyme not only develop an innovative strategy for the BME-activated combination therapy but also open a new avenue to explore other nanozyme-involved nanoplatforms for bacterial biofilm infections.

Tailoring the Electronic Structure of an Atomically Dispersed Zinc Electrocatalyst: Coordination Environment Regulation for High Selectivity Oxygen Reduction

Accurately regulating the selectivity of the oxygen reduction reaction (ORR) is crucial to renewable energy storage and utilization, but challenging. A flexible alteration of ORR pathways on atomically dispersed Zn sites towards high selectivity ORR can be achieved by tailoring the coordination environment of the catalytic centers. The atomically dispersed Zn catalysts with unique O- and C-coordination structure (ZnO₃ C) or N-coordination structure (ZnN₄ ) can be prepared by varying the functional groups of corresponding MOF precursors. The coordination environment of as-prepared atomically dispersed Zn catalysts was confirmed by X-ray absorption fine structure (XAFs). Notably, the ZnN₄ catalyst processes a 4 e^- ORR pathway to generate H₂ O. However, controllably tailoring the coordination environment of atomically dispersed Zn sites, ZnO₃ C catalyst processes a 2 e^- ORR pathway to generate H₂ O₂ with a near zero overpotential and high selectivity in 0.1 M KOH. Calculations reveal that decreased electron density around Zn in ZnO₃ C lowers the d-band center of Zn, thus changing the intermediate adsorption and contributing to the high selectivity towards 2 e^- ORR.

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.

TiO2 supported single Ag atoms nanozyme for elimination of SARS-CoV2

The outbreak of SARS-coronavirus 2 (SARS-CoV2) has become a global health emergency. Although enormous efforts have been made, there is still no effective treatment against the new virus. Herein, a TiO₂ supported single-atom nanozyme containing atomically dispersed Ag atoms (Ag-TiO₂ SAN) is designed to serve as a highly efficient antiviral nanomaterial. Compared with traditional nano-TiO₂ and Ag, Ag-TiO₂ SAN exhibits higher adsorption (99.65%) of SARS-CoV2 pseudovirus. This adsorption ability is due to the interaction between SAN and receptor binding domain (RBD) of spike 1 protein of SARS-CoV2. Theoretical calculation and experimental evidences indicate that the Ag atoms of SAN strongly bind to cysteine and asparagine, which are the most abundant amino acids on the surface of spike 1 RBD. After binding to the virus, the SAN/virus complex is typically phagocytosed by macrophages and colocalized with lysosomes. Interestingly, Ag-TiO₂ SAN possesses high peroxidase-like activity responsible for reactive oxygen species production under acid conditions. The highly acidic microenvironment of lysosomes could favor oxygen reduction reaction process to eliminate the virus. With hACE2 transgenic mice, Ag-TiO₂ SAN showed efficient anti-SARS-CoV2 pseudovirus activity. In conclusion, Ag-TiO₂ SAN is a promising nanomaterial to achieve effective antiviral effects for SARS-CoV2.

Synergistic Multimodal Cancer Therapy Using Glucose Oxidase@CuS Nanocomposites

Multimodal nanotherapeutic cancer treatments are widely studied but are often limited by their costly and complex syntheses that are not easily scaled up. Herein, a simple formulation of glucose-oxidase-coated CuS nanoparticles was demonstrated to be highly effective for melanoma treatment, acting through a synergistic combination of glucose starvation, photothermal therapy, and synergistic advanced chemodynamic therapy enabled by near-infrared irradiation coupled with Fenton-like reactions that were enhanced by endogenous chloride.