Heterogeneous CNF/MoO3 nanofluidic membranes with tunable surface plasmon resonances for solar-osmotic energy conversion

Two-dimensional (2D) nanofluidic membranes are competitive candidates for osmotic energy harvesting and have been greatly developed. However, the use of diverse inherent characteristics of 2D nanosheets, such as electronic or optoelectronic properties, to achieve intelligent ion transport, still lacks sufficient exploration. Here, a cellulose nanofiber/molybdenum oxide (CNF/MoO3) heterogeneous nanofluidic membrane with high performance solar-osmotic energy conversion is reported, and how surface plasmon resonances (SPR) regulate selective cation transport is revealed. The SPR of amorphous MoO3 endows the heterogeneous nanofluidic membranes with tunable surface charge and good photothermal conversion. Through DFT calculations and finite element modeling, the regulation of electronic and optoelectronic properties on the surface of materials by SPR and the influence of surface charge density and temperature gradient on ion transport in nanofluidic membranes are demonstrated. By mixing 0.01/0.5 M NaCl solutions using SPR and photothermal effects, the power density can achieve a remarkable value of ≈13.24 W m-2, outperforming state-of-the-art 2D-based nanofluidic membranes. This work first reveals the regulation and mechanism of SPR on ion transport in nanofluidic membranes and systematically studies photon-electron-ion interactions in nanofluidic membranes, which could also provide a new viewpoint for promoting osmotic energy conversion.

Interfacial Bonding Induced Charge Transfer in Two-Dimensional Amorphous MoO3-x/Graphdiyne Oxide Non-Van der Waals Heterostructures for Dominant SERS Enhancement

Two-dimensional semiconductor-based nanomaterials have shown to be an effective substrate for Surface-enhanced Raman Scattering (SERS) spectroscopy. However, the enhancement factor (EF) tends to be relatively weak compared to that of noble metals and does not allow for trace detection of molecules. In this work, we report the successful preparation of two-dimensional (2D) amorphous non-van der Waals heterostructures MoO3-x/GDYO nanomaterials using supercritical CO2. Due to the synergistic effect of the localized surface plasmon resonance (LSPR) effect and the charge transfer effect, it exhibits excellent SERS performance in the detection of methylene blue (MB) molecules, with a detection limit as low as 10-14 M while the enhancement factor (EF) can reach an impressive 2.55×1011. More importantly, the chemical bond bridging at the MoO3-x/GDYO heterostructures interface can accelerate the electron transfer between the interfaces, and the large number of defective surface structures on the heterostructures surface facilitates the chemisorption of MB molecules. And the charge recombination lifetime can be proved by a ~1.7-fold increase during their interfacial electron-transfer process for MoO3-x/GDYO@MB mixture, achieving highly sensitive SERS detection.

Defect engineering to tailor structure-activity relationship in biodegradable nanozymes for tumor therapy by dual-channel death strategies

Pursuing biodegradable nanozymes capable of equipping structure-activity relationship provides new perspectives for tumor-specific therapy. A rapidly degradable nanozymes can address biosecurity concerns. However, it may also reduce the functional stability required for sustaining therapeutic activity. Herein, the defect engineering strategy is employed to fabricate Pt-doping MoOx (PMO) redox nanozymes with rapidly degradable characteristics, and then the PLGA-assembled PMO (PLGA@PMO) by microfluidics chip can settle the conflict between sustaining therapeutic activity and rapid degradability. Density functional theory describes that Pt-doping enables PMO nanozymes to exhibit an excellent multienzyme-mimicking catalytic activity originating from synergistic catalysis center construction with the interaction of Pt substitution and oxygen vacancy defects. The peroxidase- (POD), oxidase- (OXD), glutathione peroxidase- (GSH-Px), and catalase- (CAT) mimicking activities can induce robust ROS output and endogenous glutathione depletion under tumor microenvironment (TME) response, thereby causing ferroptosis in tumor cells by the accumulation of lipid peroxide and inactivation of glutathione peroxidase 4. Due to the activated surface plasmon resonance effect, the PMO nanozymes can cause hyperthermia-induced apoptosis through 1064 nm laser irradiation, and augment multienzyme-mimicking catalytic activity. This work represents a potential biological application for the development of therapeutic strategy for dual-channel death via hyperthermia-augmented enzyme-mimicking nanocatalytic therapy.

Tuning d-Orbital Electronic Structure via Au-Intercalated Two-Dimensional Fe3GeTe2 to Increase Surface Plasmon Activity

While extensive research has been dedicated to plasmon tuning within non-noble metals, prior investigations primarily concentrated on markedly augmenting the inherently low concentration of free carriers in materials with minimal consideration given to the influence of electron orbitals on surface plasmons. Here, we achieve successful intercalation of Au atoms into the layered structure of Fe3GeTe2 (FGT), thereby exerting control over the orbital electronic states or structure of FGT. This intervention not only amplifies the charge density and electron mobility but also mitigates the loss associated with interband transitions, resulting in increased two-dimensional FGT surface plasmon activity. As a consequence, Au-intercalated FGT detects crystal violet molecules as a surface-enhanced Raman scattering substrate, and the detection lines are 3 orders of magnitude higher than before Au intercalation. Our work provides insight for further studies on plasmon effects and the relation between surface plasmon resonance behavior and electronic structures.

Molybdenum oxide nanotube caps decorated with ultrafine Ag nanoparticles: Synthesis and antimicrobial activity

In the contemporary era, microorganisms, spanning bacteria and viruses, are increasingly acknowledged as emerging contaminants in the environment, presenting significant risks to public health. Nevertheless, conventional methods for disinfecting these microorganisms are often ineffective. Additionally, they come with disadvantages such as high energy usage, negative environmental consequences, increased expenses, and the generation of harmful byproducts. The development of next-generation antifungal and antibacterial agents is dependent on newly synthesized nanomaterials with inherent antimicrobial behavior. In this study, we report an arc-discharge method to synthesize MoOx nanosheets and microbelts, followed by decorating them with ultrafine Ag nanoparticles (NPs). Scanning and transmission electron microscopies show that Ag NPs formation on the Molybdenum oxide nanostructures rolls them into nanotube caps (NTCs), revealing inner and outer diameters of approximately 19.8 nm and 105.5 nm, respectively. Additionally, the Ag NPs are ultrafine, with sizes in the range of 5-8 nm. Results show that the prepared NTCs exhibit dose-dependent sensitivity to both planktonic and biofilm cells of Escherichia coli and Candida albicans. The anti-biofilm activity in terms of biofilm inhibition ranged from 19.7 to 77.2% and 11.3-82.3%, while removal of more than 70% and 90% of preformed biofilms was achieved for E. coli and C. albicans, respectively, showing good potential for antimicrobial coating. Initial MoOx exhibits positive potential, while Ag-decorated Molybdenum oxide NTCs show dual potential effects (positive for Molybdenum oxide NTCs and negative for Ag NPs. Molybdenum oxide NTCs, with their strong positive potential, efficiently attract microbes due to their negatively charged cell surfaces, facilitating the antimicrobial effect of Ag NPs, leading to cell damage and death. These findings suggest that the synthesized NPs could serve as a suitable coating for biomedical applications.

How Amorphous Nanomaterials Enhanced Electrocatalytic, SERS, and Mechanical Properties

There is ever-growing research interest in nanomaterials because of the unique properties that emerge on the nanometer scale. While crystalline nanomaterials have received a surge of attention for exhibiting state-of-the-art properties in various fields, their amorphous counterparts have also attracted attention in recent years owing to their unique structural features that crystalline materials lack. In short, amorphous nanomaterials only have short-range order at the atomic scale, and their atomic packing lacks long-range periodic arrangement, in which the coordinatively unsaturated environment, isotropic atomic structure, and modulated electron state all contribute to their outstanding performance in various applications. Given their intriguing characteristics, we herein present a series of representative works to elaborate on the structural advantages of amorphous nanomaterials as well as their enhanced electrocatalytic, surface-enhanced Raman scattering (SERS), and mechanical properties, thereby elucidating the underlying structure-function relationship. We hope that this proposed relationship will be universally applicable, thus encouraging future work in the design of amorphous materials that show promising performance in a wide range of fields.

Oxygen-Deficient Bioceramics: Combination of Diagnosis, Therapy, and Regeneration

The journey of ceramics in medicine has been synchronized with an evolution from the first generation-alumina, zirconia, etc.-to the third -3D scaffolds. There is an up-and-coming member called oxygen-deficient or colored bioceramics, which have recently found their way through biomedical applications. The oxygen vacancy steers the light absorption toward visible and near infrared regions, making the colored bioceramics multifunctional-therapeutic, diagnostic, and regenerative. Oxygen-deficient bioceramics are capable of turning light into heat and reactive oxygen species for photothermal and photodynamic therapies, respectively, and concomitantly yield infrared and photoacoustic images. Different types of oxygen-deficient bioceramics have been recently developed through various synthesis routes. Some of them like TiO2- x , MoO3- x , and WOx have been more investigated for biomedical applications, whereas the rest have yet to be scrutinized. The most prominent advantage of these bioceramics over the other biomaterials is their multifunctionality endowed with a change in the microstructure. There are some challenges ahead of this category discussed at the end of the present review. By shedding light on this recently born bioceramics subcategory, it is believed that the field will undergo a big step further as these platforms are naturally multifunctional.

Supercritical CO2 Directional-Assisted Synthesis of Low-Dimensional Materials for Functional Applications

Supercritical CO2 (SC CO2 ), as one of the unique fluids that possess fascinating properties of gas and liquid, holds great promise in chemical reactions and fabrication of materials. Building special nanostructures via SC CO2 for functional applications has been the focus of intense research for the past two decades, with facile regulated reaction conditions and a particular reaction field to operate compared to the more widely used solvent systems. In this review, the significance of SC CO2 on fabricating various functional materials including modification of 1D carbon nanotubes, 2D materials, and 2D heterostructures is stated. The fundamental aspects involving building special nanostructures via SC CO2 are explored: how their structure, morphology, and chemical composition be affected by the SC CO2 . Various optimization strategies are outlined to improve their performances, and recent advances are combined to present a coherent understanding of the mechanism of SC CO2 acting on these functional nanostructures. The wide applications of these special nanostructures in catalysis, biosensing, optoelectronics, microelectronics, and energy transformation are discussed. Moreover, the current status of SC CO2 research, the existing scientific issues, and application challenges, as well as the possible future directions to advance this fertile field are proposed in this review.

Strong Ferromagnetic Manipulation of SrTiO3 from CO2 -Straining Effect on Electronic Structure Modulation

2D magnetic materials are ideal to fabricate magneto-optical, magneto-electric, and data storage devices, which are proposed to be critical to the next generation of information technologies. Benefited from their labile structures, 2D perovskites are amenable for magnetic manipulation through structural optimization. In this work, 2D room-temperature ferromagnetic SrTiO₃ is achieved through straining effect induced by supercritical carbon dioxide (SC CO₂ ). According to experimental results, the cubic phase of SrTiO₃ is converted to tetragonal with exposure of (110), (200), (111), and (211) planes over the SC CO₂ treatment, leading to significant ferromagnetic enhancement. Theoretical calculations illustrate that over the conversion from cubic to tetragonal, the electronic structure of SrTiO₃ is significantly modulated. Specifically, the spin density of planes of (200), (111), and (211) is enhanced, presumably due to the stabilization of the highest occupied molecular orbital over straining by SC CO₂ , leading to magnetic optimizations. This work suggests that magnetic optimization can be achieved from SC CO₂ -induced electronic structure modulation.

Oxidation of metallic Cu by supercritical CO2 and control synthesis of amorphous nano-metal catalysts for CO2 electroreduction

Amorphous nano-metal catalysts often exhibit appealing catalytic properties, because the intrinsic linear scaling relationship can be broken. However, accurate control synthesis of amorphous nano-metal catalysts with desired size and morphology is a challenge. In this work, we discover that Cu(0) could be oxidized to amorphous Cu_xO species by supercritical CO₂. The formation process of the amorphous Cu_xO is elucidated with the aid of machine learning. Based on this finding, a method to prepare Cu nanoparticles with an amorphous shell is proposed by supercritical CO₂ treatment followed by electroreduction. The unique feature of this method is that the size of the particles with amorphous shell can be easily controlled because their size depends on that of the original crystal Cu nanoparticles. Moreover, the thickness of the amorphous shell can be easily controlled by CO₂ pressure and/or treatment time. The obtained amorphous Cu shell exhibits high selectivity for C2+ products with the Faradaic efficiency of 84% and current density of 320 mA cm^-2. Especially, the FE of C2+ oxygenates can reach up to 65.3 %, which is different obviously from the crystalline Cu catalysts.

Supercritical CO2 -induced New Chemical Bond of C-O-Si in Graphdiyne to Achieve Robust Room-Temperature Ferromagnetism

The realization of ferromagnetic ordering of two-dimensional (2D) carbon material graphdiyne (GDY) has attracted great attention due to its promising application in spin semiconductor devices. However, the absence of localized spins makes the pristine GDY intrinsically nonferromagnetic. Herein, we report the realization of robust room-temperature (RT) ferromagnetism (FM) with Curie temperature (T_C ) up to 325 K for GDY Nanosheets (GDYNs) by supercritical CO₂ (SC CO₂ ). Experimental and theoretical calculations reveal that the new chemical bond of C-O-Si can be formed because of the unique effect of SC CO₂ , which help to enhance the charge transfer and generates long-range ferromagnetic order. The RT saturation magnetization (M_S ) reaches 1.125 emu/g, which is much higher than that of carbon-based materials reported up to now. Meanwhile, by changing the conditions of SC CO₂ such as pressure, ferromagnetic responses can be manipulated, which is great for potential spintronics applications of GDY.

Plasmonic metal oxides and their biological applications

Metal oxides modified with dopants and defects are an emerging class of novel materials supporting the localized surface plasmon resonance across a wide range of optical wavelengths, which have attracted tremendous research interest particularly in biological applications in the past decade. Compared to conventional noble metal-based plasmonic materials, plasmonic metal oxides are particularly favored for their cost efficiency, flexible plasmonic properties, and improved biocompatibility, which can be important to accelerate their practical implementation. In this review, we first explicate the origin of plasmonics in dopant/defect-enabled metal oxides and their associated tunable localized surface plasmon resonance through the conventional Mie-Gans model. The research progress of dopant incorporation and defect generation in metal oxide hosts, including both in situ and ex situ approaches, is critically discussed. The implementation of plasmonic metal oxides in biological applications in terms of therapy, imaging, and sensing is summarized, in which the uniqueness of dopant/defect-driven plasmonics for inducing novel functionalities is particularly emphasized. This review may provide insightful guidance for developing next-generation plasmonic devices for human health monitoring, diagnosis and therapy.

Progress on two-dimensional binary oxide materials

Two-dimensional van der Waals (2D vdW) materials have attracted much attention because of their unique electronic and optical properties. Since the successful isolation of graphene in 2004, many interesting 2D materials have emerged, including elemental olefins (silicene, germanene, etc.), transition metal chalcogenides, transition metal carbides (nitrides), hexagonal boron, etc. On the other hand, 2D binary oxide materials are an important group in the 2D family owing to their high structural diversity, low cost, high stability, and strong adjustability. This review systematically summarizes the research progress on 2D binary oxide materials. We discuss their composition and structure in terms of vdW and non-vdW categories in detail, followed by a discussion of their synthesis methods. In particular, we focus on strategies to tailor the properties of 2D oxides and their emerging applications in different fields. Finally, the challenges and future developments of 2D binary oxides are provided.

Recent advances in the fabrication of 2D metal oxides

Atomically thin two-dimensional (2D) metal oxides exhibit unique optical, electrical, magnetic, and chemical properties, rendering them a bright application prospect in high-performance smart devices. Given the large variety of both layered and non-layered 2D metal oxides, the controllable synthesis is the critical prerequisite for enabling the exploration of their great potentials. In this review, recent progress in the synthesis of 2D metal oxides is summarized and categorized. Particularly, a brief overview of categories and crystal structures of 2D metal oxides is firstly introduced, followed by a critical discussion of various synthesis methods regarding the growth mechanisms, advantages, and limitations. Finally, the existing challenges are presented to provide possible future research directions regarding the synthesis of 2D metal oxides. This work can provide useful guidance on developing innovative approaches for producing both 2D layered and non-layered nanostructures and assist with the acceleration of the research of 2D metal oxides.

A molybdenum oxide-based degradable nanosheet for combined chemo-photothermal therapy to improve tumor immunosuppression and suppress distant tumors and lung metastases

Molybdenum oxide (MoOx) nanosheets have drawn increasing attention for minimally invasive cancer treatments but still face great challenges, including complex modifications and the lack of efficient accumulation in tumor. In this work, a novel multifunctional degradable FA-BSA-PEG/MoOx nanosheet was fabricated (LA-PEG and FA-BSA dual modified MoOx): the synergistic effect of PEG and BSA endows the nanosheet with excellent stability and compatibility; the FA, a targeting ligand, facilitates the accumulation of nanosheets in the tumor. In addition, DTX, a model drug for breast cancer treatment, was loaded (76.49%, 1.5 times the carrier weight) in the nanosheets for in vitro and in vivo antitumor evaluation. The results revealed that the FA-BSA-PEG/MoOx@DTX nanosheets combined photothermal and chemotherapy could not only inhibit the primary tumor growth but also suppress the distant tumor growth (inhibition rate: 51.7%) and lung metastasis (inhibition rate: 93.6%), which is far more effective compared to the commercial Taxotere®. Exploration of the molecular mechanism showed that in vivo immune response induced an increase in positive immune responders, suppressed negative immune suppressors, and established an inflammatory tumor immune environment, which co-contributes towards effective suppression of tumor and lung metastasis. Our experiments demonstrated that this novel multifunctional nanosheet is a promising platform for combined chemo-photothermal therapy.

Plasmonic Oxygen Defects in MO3- x (M = W or Mo) Nanomaterials: Synthesis, Modifications, and Biomedical Applications

Nanomedicine is a promising technology with many advantages and provides exciting opportunities for cancer diagnosis and therapy. During recent years, the newly developed oxygen-deficiency transition metal oxides MO_{3- x} (M = W or Mo) have received significant attention due to the unique optical properties, such as strong localized surface plasmon resonance (LSPR) , tunable and broad near-IR absorption, high photothermal conversion efficiency, and large X-ray attenuation coefficient. This review presents an overview of recent advances in the development of MO_{3- x} nanomaterials for biomedical applications. First, the fundamentals of the LSPR effect are introduced. Then, the preparation and modification methods of MO_{3- x} nanomaterials are summarized. In addition, the biological effects of MO_{3- x} nanomaterials are highlighted and their applications in the biomedical field are outlined. This includes imaging modalities, cancer treatment, and antibacterial capability. Finally, the prospects and challenges of MO_{3- x} and MO_{3- x} -based nanomaterial for fundamental studies and clinical applications are also discussed.

Manipulation on Two-Dimensional Amorphous Nanomaterials for Enhanced Electrochemical Energy Storage and Conversion

Low-carbon society is calling for advanced electrochemical energy storage and conversion systems and techniques, in which functional electrode materials are a core factor. As a new member of the material family, two-dimensional amorphous nanomaterials (2D ANMs) are booming gradually and show promising application prospects in electrochemical fields for extended specific surface area, abundant active sites, tunable electron states, and faster ion transport capacity. Specifically, their flexible structures provide significant adjustment room that allows readily and desirable modification. Recent advances have witnessed omnifarious manipulation means on 2D ANMs for enhanced electrochemical performance. Here, this review is devoted to collecting and summarizing the manipulation strategies of 2D ANMs in terms of component interaction and geometric configuration design, expecting to promote the controllable development of such a new class of nanomaterial. Our view covers the 2D ANMs applied in electrochemical fields, including battery, supercapacitor, and electrocatalysis, meanwhile we also clarify the relationship between manipulation manner and beneficial effect on electrochemical properties. Finally, we conclude the review with our personal insights and provide an outlook for more effective manipulation ways on functional and practical 2D ANMs.

Manipulating Hot-Electron Injection in Metal Oxide Heterojunction Array for Ultrasensitive Surface-Enhanced Raman Scattering

Efficient photoinduced charge transfer (PICT) resonance is crucial to the surface-enhanced Raman scattering (SERS) performance of metal oxide substrates. Herein, we venture into the hot-electron injection strategy to achieve unprecedented enhanced PICT efficiency between substrates and molecules. A heterojunction array composed of plasmonic MoO₂ and semiconducting WO_3-x is designed to prove the concept. The plasmonic MoO₂ generates intense localized surface plasmon resonance under illumination, which can generate near-field Raman enhancement as well as accompanied plasmon-induced hot-electrons. The hot-electron injection in direct interfacial charge transfer and plasmon-induced charge transfer process can effectively promote the PICT efficiency between substrates and molecules, achieving a record Raman enhancement factor among metal oxide substrates (2.12 × 10⁸) and the ultrasensitive detection of target molecule down to 10^-11 M. This work demonstrates the possibility of hot-electron manipulation to realize unprecedented Raman enhancement in metal oxides, offering a cutting-edge strategy to design high-performance SERS substrates.

Electrocatalytic Glycerol Oxidation with Concurrent Hydrogen Evolution Utilizing an Efficient MoOx /Pt Catalyst

Glycerol electrolysis affords a green and energetically favorable route for the production of value-added chemicals at the anode and H₂ production in parallel at the cathode. Here, a facile method for trapping Pt nanoparticles at oxygen vacancies of molybdenum oxide (MoO_x ) nanosheets, yielding a high-performance MoO_x /Pt composite electrocatalyst for both the glycerol oxidation reaction (GOR) and the hydrogen evolution reaction (HER) in alkaline electrolytes, is reported. Combined electrochemical experiments and theoretical calculations reveal the important role of MoO_x nanosheets for the adsorption of glycerol molecules in GOR and the dissociation of water molecules in HER, as well as the strong electronic interaction with Pt. The MoO_x /Pt composite thus significantly enhances the specific mass activity of Pt and the kinetics for both reactions. With MoO_x /Pt electrodes serving as both cathode and anode, two-electrode glycerol electrolysis is achieved at a cell voltage of 0.70 V to reach a current density of 10 mA cm^-2 , which is 0.90 V less than that required for water electrolysis.

Role of supercritical carbon dioxide (scCO2) in fabrication of inorganic-based materials: a green and unique route

In recent times, the supercritical carbon dioxide (scCO₂) process has attracted increasing attention in fabricating diverse materials due to the attractive features of environmentally benign nature and economically promising character. Owing to these unique characteristics and high-penetrability, as well as diffusivity conditions of scCO₂, this high-pressure technology, with mild operation conditions, cost-effective, and non-toxic, among others, is often applied to fabricate various organic and inorganic-based materials, resulting in the unique crystal architectures (amorphous, crystalline, and heterojunction), tunable architectures (nanoparticles, nanosheets, and aerogels) for diverse applications. In this review, we give an emphasis on the fabrication of various inorganic-based materials, highlighting the recent research on the driving factors for improving the quality of fabrication in scCO₂, procedures for production and dispersion in scCO₂, as well as common indicators utilized to assess quality and processing ability of materials. Next, we highlight the effects of specific properties of scCO₂ towards synthesizing the highly functional inorganic-based nanomaterials. Finally, we summarize this compilation with interesting perspectives, aiming to arouse a more comprehensive utilization of scCO₂ to broaden the horizon in exploring the green/eco-friendly processing of such versatile inorganic-based materials. Together, we firmly believe that this compilation endeavors to disclose the latent capability and universal prevalence of scCO₂ in the synthesis and processing of inorganic-based materials.

Ultra-thin g-C3N4/MFM-300(Fe) heterojunctions for photocatalytic aerobic oxidation of benzylic carbon centers

In situ growth of the metal-organic framework material MFM-300(Fe) on an ultra-thin sheet of graphitic carbon nitride (g-C3N4) has been achieved via exfoliation of bulk carbon nitride using supercritical CO2. The resultant hybrid structure, CNNS/MFM-300(Fe), comprising carbon nitride nanosheets (CNNS) and MFM-300(Fe), shows excellent performance towards photocatalytic aerobic oxidation of benzylic C-H groups at room temperature under visible light. The catalytic activity is significantly improved compared to the parent g-C3N4, MFM-300(Fe) or physical mixtures of both. This facile strategy for preparing heterojunction photocatalysts demonstrates a green pathway for the efficient and economic oxidation of benzylic carbons to produce fine chemicals.

Atomic Rearrangement and Amorphization Induced by Carbon Dioxide in Two-Dimensional MoO3-x Nanomaterials

Supercritical carbon dioxide (SC CO₂) has shown great potential in fabrication of two-dimensional (2D) amorphous nanomaterials with excellent electric and optical properties, while the amorphization mechanism led by SC CO₂ is still unclear. In this work, by investigating the amorphization kinetics of MoO_3-x nanomaterials in SC CO₂, we find two amorphization mechanisms dependent on the SC CO₂ pressure. At lower pressure, forming oxygen vacancies is the dominant effect, while at higher pressure, atomic rearrangement is the controlling factor. Furthermore, we demonstrate that amorphization directly affects the optical performance of MoO_3-x nanosheets because of the change in coordination, which further indicates the atomic rearrangement during the amorphization process. Therefore, this work reveals the amorphization mechanism led by SC CO₂ and builds a link between amorphization and optical performance; it also provides new inspiration for fabrication of amorphous nanomaterials with tunable optical and photocatalytic performance.

CO2-Assisted Synthesis of 2D Amorphous MoO3-x Nanosheets: From Top-Down to Bottom-Up

Supercritical CO₂ has shown great potential in the top-down fabrication of two-dimensional (2D) amorphous nanomaterials. However, a few works focus on the SC CO₂-assisted synthesis of 2D amorphous nanomaterials by a bottom-up approach. Here we report the facile bottom-up synthesis of 2D amorphous MoO_3-x nanosheets, using SC CO₂ as a surface confining agent. Moreover, the morphology of the MoO_3-x can be tailored by simply adjusting the pressure of the SC CO₂. The as-prepared 2D amorphous MoO_3-x nanosheets exhibit enhanced surface plasma resonance in the visible and near-infrared regions, showing outstanding photothermal conversion performance. This work constructs a new approach for the preparation of 2D amorphous nanosheets, throwing light on the amorphization mechanism of 2D materials.

Multimode Imaging-Guided Photothermal/Chemodynamic Synergistic Therapy Nanoagent with a Tumor Microenvironment Responded Effect

The development of near-infrared (NIR) laser triggered phototheranostics for multimodal imaging-guided combination therapy is highly desirable. However, multiple laser sources, as well as inadequate therapeutic efficacy, impede the application of phototheranostics. Here, we develop an all-in-one theranostic nanoagent PEGylated DCNP@DMSN-MoO_x NPs (DCDMs) with a flower-like structure fabricated by coating uniformly sized down-conversion nanoparticles (DCNPs) with dendritic mesoporous silica (DMSN) and then loading the ultrasmall oxygen-deficient molybdenum oxide nanoparticles (MoO_x NPs) inside through an electrostatic interaction. Owing to the doping of Nd ions, when excited by an 808 nm laser, DCNPs emit bright NIR-II emissions (1060 and 1300 nm), which have characteristic high spatial resolution and deep tissue penetration. In terms of treatment, MoO_x NPs could be specifically activated by excessive hydrogen peroxide (H₂O₂) in the tumor microenvironment, thus generating ¹O₂ via the Russell mechanism. In addition, the excessive glutathione (GSH) in the tumor cells could be depleted through the Mo-mediated redox reaction, thus effectively decreasing the antioxidant capacity of tumor cells. Importantly, the excellent photothermal properties (photothermal conversion efficiency of 51.5% under an 808 nm laser) synergistically accelerate the generation of ¹O₂. This cyclic redox reaction of molybdenum indeed ensured the high efficacy of tumor-specific therapy, leaving the normal tissues unharmed. MoO_x NPs could also efficiently catalyze tumor endogenous H₂O₂ into a considerable amount of O₂ in an acidic tumor microenvironment, thus relieving hypoxia in tumor tissues. Moreover, the computed tomography (CT) and T₁-weighted magnetic resonance imaging (MRI) effect from Gd³⁺ and Y³⁺ ions make DCNPs act as a hybrid imaging agent, allowing comprehensive analysis of tumor lesions. Both in vitro and in vivo experiments validate that such an "all-in-one" nanoplatform possesses desirable anticancer abilities under single laser source irradiation, benefiting from the NIR-II fluorescence/CT/MR multimodal imaging-guided photothermal/chemodynamic synergistic therapy. Overall, our strategy paves the way to explore other noninvasive cancer phototheranostics.

Facile and rapid synthesis of emission color-tunable molybdenum oxide quantum dots as a versatile probe for fluorescence imaging and environmental monitoring

Recent years have seen molybdenum oxide quantum dots (MoOx QDs) as a booming material due to their attractive physical and chemical properties. However, there is still a large demand for MoOx QDs with long-wavelength emission by a facile strategy but these are more challenging to obtain. Herein, we rationally designed and successfully prepared nitrogen and phosphorus co-doped green emitting MoOx QDs (N,P-MoOx QDs) through a microwave-assisted rapid method. They exhibit a maximum emission at 500 nm under a 430 nm excitation. Moreover, by controlling their sizes in the process, we find that such a strategy enables the tuning of the emission color of N,P-MoOx QDs from green to blue. N,P-MoOx QDs show a significant fluorescence response to pH changes, and also display pH-sensitive near-infrared localized surface plasmon resonance (LSPR) at 866 nm. An effective and simple pH probe with a dual-signal response is achieved using N,P-MoOx QDs. As environmental sensors, N,P-MoOx QDs can be applied for sensitive detection of the concentrations of permanganate and captopril, offering the linear range from 0.08 to 25 μM and 0.1 to 31 μM, respectively. Benefitting from the effect of doping nitrogen and phosphorus, the probe could detect a wide range of pH changes (2-9) and is endowed with superior biocompatibility. Further, it is successfully used to "see" the intracellular pH variation by fluorescence confocal imaging. These findings not only demonstrate the achievement of a promising multifunctional probe for biosensing and environmental detection, but also pave the way for the fabrication of transition metal oxide QDs with tunable optical properties.

Mo-Based Layered Nanostructures for the Electrochemical Sensing of Biomolecules

Mo-based layered nanostructures are two-dimensional (2D) nanomaterials with outstanding characteristics and very promising electrochemical properties. These materials comprise nanosheets of molybdenum (Mo) oxides (MoO2 and MoO3), dichalcogenides (MoS2, MoSe2, MoTe2), and carbides (MoC2), which find application in electrochemical devices for energy storage and generation. In this feature paper, we present the most relevant characteristics of such Mo-based layered compounds and their use as electrode materials in electrochemical sensors. In particular, the aspects related to synthesis methods, structural and electronic characteristics, and the relevant electrochemical properties, together with applications in the specific field of electrochemical biomolecule sensing, are reviewed. The main features, along with the current status, trends, and potentialities for biomedical sensing applications, are described, highlighting the peculiar properties of Mo-based 2D-nanomaterials in this field.

Supercritical CO2 synthesis of Co-doped MoO3-x nanocrystals for multifunctional light utilization

Here, we demonstrate, for the first time, that Co-MoO_3-x nanocrystals (NCs) have been synthesized with the assistance of supercritical CO₂. Their unique structural features of transition-metal doping and high oxygen vacancy concentrations, lead to synchronous outstanding surface enhanced Raman scattering (SERS) detection and photothermal conversion performances.

Hybrid Plasmonic Fiber-Optic Sensors

With the increasing demand of achieving comprehensive perception in every aspect of life, optical fibers have shown great potential in various applications due to their highly-sensitive, highly-integrated, flexible and real-time sensing capabilities. Among various sensing mechanisms, plasmonics based fiber-optic sensors provide remarkable sensitivity benefiting from their outstanding plasmon-matter interaction. Therefore, surface plasmon resonance (SPR) and localized SPR (LSPR)-based hybrid fiber-optic sensors have captured intensive research attention. Conventionally, SPR- or LSPR-based hybrid fiber-optic sensors rely on the resonant electron oscillations of thin metallic films or metallic nanoparticles functionalized on fiber surfaces. Coupled with the new advances in functional nanomaterials as well as fiber structure design and fabrication in recent years, new solutions continue to emerge to further improve the fiber-optic plasmonic sensors' performances in terms of sensitivity, specificity and biocompatibility. For instance, 2D materials like graphene can enhance the surface plasmon intensity at the metallic film surface due to the plasmon-matter interaction. Two-dimensional (2D) morphology of transition metal oxides can be doped with abundant free electrons to facilitate intrinsic plasmonics in visible or near-infrared frequencies, realizing exceptional field confinement and high sensitivity detection of analyte molecules. Gold nanoparticles capped with macrocyclic supramolecules show excellent selectivity to target biomolecules and ultralow limits of detection. Moreover, specially designed microstructured optical fibers are able to achieve high birefringence that can suppress the output inaccuracy induced by polarization crosstalk and meanwhile deliver promising sensitivity. This review aims to reveal and explore the frontiers of such hybrid plasmonic fiber-optic platforms in various sensing applications.

Cascade Chromogenic System with Exponential Signal Amplification for Visual Colorimetric Detection of Acetone

The signal of the traditional chromogenic systems is directly proportional to analyte concentration, leading to an unsatisfactory sensitivity. Herein, we report a cascade chromogenic system to realize exponential amplification of colorimetric signal through coupling chemical oxidation with photoinduced radical chain reaction. The chemical oxidation of o-phenylenediamine (OPD) by Fe³⁺ generates Fe²⁺ and photoactive 2,3-diaminophenazine (DAP). Under blue-light irradiation, DAP initiates the formation of holes and H₂O₂ that reacts with Fe²⁺ to hydroxyl radicals (·OH) and Fe³⁺ via an intersystem crossing (ISC) process. Moreover, the holes oxidize water to yield ·OH as well. The resulting ·OH and regenerated Fe³⁺ in turn oxidize OPD to yield more DAP, leading to a self-propagating reaction cycle that continues to proceed until all the OPD molecules are consumed, along with a distinct color change from colorless to yellow. Through the generation of the complex between DAP and acetone that limits the ISC process, and therefore quenches the colorimetric signal, the highly sensitive and selective naked-eye detection of acetone is achieved from 50 μM to 3 mM, with a limit of detection of 35 μM. Additionally, the feasibility of this colorimetric assay to detect acetone in real water samples is also demonstrated.

Effective medium theory to the description of plasmonic resonances: Role of Au and Ti nanoparticles embedded in MoO3 thin films

The growing interest in functional transition metal oxides for efficient energy consumption or in the bio-sensing process; indicates that is necessary to develop a new theoretical method that describes experiments. This article presents a new theoretical methodology to characterize molybdenum trioxide (MoO₃) thin films doped with resonant gold – nanoparticles (Au – NPs) and non-resonant titanium – nanoparticles (Ti – NPs). The modulation of surface plasmon resonance (SPR) and the implications in the MoO₃ transmittance spectrum is described by applying an effective medium theory. The transmittance modulation was modified by variating three parameters, the radius of the NPs, the concentration of the NPs as well as the variation of the MoO₃ thin films thickness. It was found that the nanoparticles concentration is the most important parameter in the transmittance modulation. Additionally, the orthorhombic and monoclinic structure of MoO₃ was studied, from which it was obtained that the monoclinic structure of the MoO₃ doped with Au – NPs favors the reduction in the transmittance values in the visible region which is associated with the increase of the SPR signal. Similar analyses are performed for non-resonant nanoparticles such as Ti, where it was found that optical modulation is not as marked as the case of gold nanoparticles.

CO2-assisted fabrication of two-dimensional amorphous transition metal oxides

This Frontier presents the recent developments and applications of two-dimensional (2D) amorphous transition metal oxides (TMOs) obtained by using supercritical CO₂. CO₂ molecules can affect the crystal transformation of layered materials and allow diffusive atomic disordering during the exfoliation process. If amorphous structures are introduced into TMOs, the strong localization tail states that exist in the band gap of TMOs can effectively promote chemical reactions. We also discuss the future challenges to be overcome for this class of supercritical CO₂ and 2D amorphous materials.

Accurate Control of VS2 Nanosheets for Coexisting High Photoluminescence and Photothermal Conversion Efficiency

In two-dimensional (2D) amorphous nanosheets, the electron-phonon coupling triggered by localization of the electronic state as well as multiple-scattering feature make it exhibit excellent performance in optical science. VS₂ nanosheets, especially single-layer nanosheets with controllable electronic structure and intrinsic optical properties, have rarely been reported owing to the limited preparation methods. Now, a controllable and feasible switching method is used to fabricate 2D amorphous VS₂ and partial crystallized 2D VO₂ (D) nanosheets by altering the pressure and temperature of supercritical CO₂ precisely. Thanks to the strong carrier localization and the quantum confinement, the unique 2D amorphous structures exhibit full band absorption, strong photoluminescence, and outstanding photothermal conversion efficiency.

Photo-induced synthesis of molybdenum oxide quantum dots for surface-enhanced Raman scattering and photothermal therapy

By means of a simple and photo-induced method, four colors of molybdenum oxide quantum dots (MoO_x QDs) have been synthesized, using Mo(CO)₆ as the structural guiding agent and molybdenum source. The as-prepared MoO_x QDs display diverse optical properties due to the different configurations of oxygen vacancies in various nanostructures. Among them, crystalline molybdenum dioxide (MoO₂) with a deep blue color shows the most intense localized surface plasmon resonance effect in the near-infrared (NIR) region. The strong NIR absorption endows MoO₂ QDs with a high photothermal conversion efficiency of 66.3%, enabling broad prospects as a photo-responsive nanoagent for photothermal therapy of cancer. Moreover, MoO₂ QDs can also serve as a novel semiconductor substrate for ultrasensitive surface-enhanced Raman scattering (SERS) analysis of aromatic molecules, amino acids and antibiotics, with SERS performance comparable to that of noble metal-based substrates. The therapeutic applications of MoO₂ QDs open up a new avenue for tumor nanomedicine.

Supercritical CO2-assisted amorphization of WO2.72 and its high-efficiency photothermal conversion

Amorphization of WO_2.72 was successfully achieved with the assistance of supercritical carbon dioxide (SC CO₂). Amorphous SC CO₂-treated sample has strong optical absorbance and excellent photothermal conversion efficiency of 52.5% indicates they can be a promising photothermal agent.

Amorphous MoO3-x nanosheets prepared by the reduction of crystalline MoO3 by Mo metal for LSPR and photothermal conversion

Amorphous MoO3-x with enhanced LSPR has been fabricated successfully by introducing Mo atoms into the interlayers of MoO3 nanosheets via a hydrothermal method. The inserted Mo atom could bond with inherent Mo atoms and further form a distorted atomic configuration structure. Thus, the amorphous MoO3-x possesses a relatively excellent photothermal conversion efficiency of 61.79%.