Tunable Structured 2D Nanobiocatalysts: Synthesis, Catalytic Properties and New Horizons in Biomedical Applications

2D materials have offered essential contributions to boosting biocatalytic efficiency in diverse biomedical applications due to the intrinsic enzyme-mimetic activity and massive specific surface area for loading metal catalytic centers. Since the difficulty of high-quality synthesis, the varied structure, and the tough choice of efficient surface loading sites with catalytic properties, the artificial building of 2D nanobiocatalysts still faces great challenges. Here, in this review, a timely and comprehensive summarization of the latest progress and future trends in the design and biotherapeutic applications of 2D nanobiocatalysts is provided, which is essential for their development. First, an overview of the synthesis-structure-fundamentals and structure-property relationships of 2D nanobiocatalysts, both metal-free and metal-based is provided. After that, the effective design of the active sites of nanobiocatalysts is discussed. Then, the progress of their applied research in recent years, including biomedical analysis, biomedical therapeutics, pharmacokinetics, and toxicology is systematically highlighted. Finally, future research directions of 2D nanobiocatalysts are prospected. Overall, this review to provide cutting-edge and multidisciplinary guidance for accelerating future developments and biomedical applications of 2D nanobiocatalysts is expected.

Single-Exciton Photoluminescence in a GaN Monolayer inside an AlN Nanocolumn

GaN/AlN heterostructures with thicknesses of one monolayer (ML) are currently considered to be the most promising material for creating UVC light-emitting devices. A unique functional property of these atomically thin quantum wells (QWs) is their ability to maintain stable excitons, resulting in a particularly high radiation yield at room temperature. However, the intrinsic properties of these excitons are substantially masked by the inhomogeneous broadening caused, in particular, by fluctuations in the QWs' thicknesses. In this work, to reduce this effect, we fabricated cylindrical nanocolumns of 50 to 5000 nm in diameter using GaN/AlN single QW heterostructures grown via molecular beam epitaxy while using photolithography with a combination of wet and reactive ion etching. Photoluminescence measurements in an ultrasmall QW region enclosed in a nanocolumn revealed that narrow lines of individual excitons were localized on potential fluctuations attributed to 2-3-monolayer-high GaN clusters, which appear in QWs with an average thickness of 1 ML. The kinetics of luminescence with increasing temperature is determined via the change in the population of localized exciton states. At low temperatures, spin-forbidden dark excitons with lifetimes of ~40 ns predominate, while at temperatures elevated above 120 K, the overlying bright exciton states with much faster recombination dynamics determine the emission.

Ultraviolet Photodetectors: From Photocathodes to Low-Dimensional Solids

The paper presents the long-term evolution and recent development of ultraviolet photodetectors. First, the general theory of ultraviolet (UV) photodetectors is briefly described. Then the different types of detectors are presented, starting with the older photoemission detectors through photomultipliers and image intensifiers. More attention is paid to silicon and different types of wide band gap semiconductor photodetectors such as AlGaN, SiC-based, and diamond detectors. Additionally, Ga₂O₃ is considered a promising material for solar-blind photodetectors due to its excellent electrical properties and a large bandgap energy. The last part of the paper deals with new UV photodetector concepts inspired by new device architectures based on low-dimensional solid materials. It is shown that the evolution of the architecture has shifted device performance toward higher sensitivity, higher frequency response, lower noise, and higher gain-bandwidth products.

2D Zinc Oxide - Synthesis, Methodologies, Reaction Mechanism, and Applications

Zinc oxide (ZnO) is a thermally stable n-type semiconducting material. ZnO 2D nanosheets have mainly gained substantial attention due to their unique properties, such as direct bandgap and strong excitonic binding energy at room temperature. These are widely utilized in piezotronics, energy storage, photodetectors, light-emitting diodes, solar cells, gas sensors, and photocatalysis. Notably, the chemical properties and performances of ZnO nanosheets largely depend on the nano-structuring that can be regulated and controlled through modulating synthetic strategies. Two synthetic approaches, top-down and bottom-up, are mainly employed for preparing ZnO 2D nanomaterials. However, owing to better results in producing defect-free nanostructures, homogenous chemical composition, etc., the bottom-up approach is extensively used compared to the top-down method for preparing ZnO 2D nanosheets. This review presents a comprehensive study on designing and developing 2D ZnO nanomaterials, followed by accenting its potential applications. To begin with, various synthetic strategies and attributes of ZnO 2D nanosheets are discussed, followed by focusing on methodologies and reaction mechanisms. Then, their deliberation toward batteries, supercapacitors, electronics/optoelectronics, photocatalysis, sensing, and piezoelectronic platforms are further discussed. Finally, the challenges and future opportunities are featured based on its current development.

Self-Aligned Plasmonic Lithography for Maskless Fabrication of Large-Area Long-Range Ordered 2D Nanostructures

This paper proposes a one-step maskless 2D nanopatterning approach named self-aligned plasmonic lithography (SPL) by line-shaped ultrafast laser ablation under atmospheric conditions for the first time. Through a theoretical calculation of electric field and experimental verification, we proved that homogeneous interference of laser-excited surface plasmon polaritons (SPPs) can be achieved and used to generate long-range ordered 2D nanostructures in a self-aligned way over a wafer-sized area within several minutes. Moreover, the self-aligned nanostructures can be freely transferred between embossed nanopillars and engraved nanoholes by modulating the excitation intensity of SPPs interference through altering the incident laser energy. The SPL technique exhibits further controllability in the shape, orientation, and period of achievable nanopatterns on a wide range of semiconductors and metals by tuning processing parameters. Nanopatterned films can further act as masks to transfer structures into other bulk materials, as demonstrated in silica.

Quantum confinement in chalcogenides 2D nanostructures from first principles

We investigated the impact of quantum confinement on the band gap of chalcogenides 2D nanostructures by means of density functional theory. We studied six different systems: MoS₂, WS₂, SnS₂, GaS, InSe, and HfS₂and we simulated nanosheets of increasing thickness, ranging from ultrathin films to ∼10-13 nm thick slabs, a size where the properties converge to the bulk. In some cases, the convergence of the band gap with slab thickness is rather slow, and sizeable deviations from the bulk value are still present with few nm-thick sheets. The results of the simulations were compared with the available experimental data, finding a quantitative agreement. The impact of quantum confinement can be rationalized in terms of effective masses of electrons and holes and system's size. These results show the possibility of reliably describing quantum confinement effects on systems for which experimental data are not available.

2D-Oriented Attachment of 1D Colloidal Semiconductor Nanocrystals via an Etchant

The oriented attachment (OA) of 0D semiconductor nanocrystals into 1D and 2D nanostructures with unique properties is useful for the fabrication of quantum confined nanomaterials that are otherwise difficult to produce by direct synthesis. Given that the OA of 1D nanocrystals such as nanorods generally produces linear chains, rod-couple structures, or clustered columns, linking them in a facet-specific manner to produce 2D structures is challenging. Here, we report that 1D Cu_2-xS nanorods undergo etching on exposure to hexylphosphonic acid under mild heating, which results in an increased curvature and a reduction in surface ligands at those sites. This causes the nanorods to fuse via their basal tip facets into chains and then cojoin through diametrically opposed side facets, resulting in atomically coupled, 2D raftlike structures. The stepwise OA of 1D nanocrystals into 2D nanostructures illustrated here expands the range of nanoarchitectures that can be produced via solution-processed methods.

Fluorine Engineered Self-Supported Ultrathin 2D Nickel Hydroxide Nanosheets as Highly Robust and Stable Bifunctional Electrocatalysts for Oxygen Evolution and Urea Oxidation Reactions

Developing highly efficient noble-metal-free electrocatalysts with a scalable and environmentally friendly synthesis approach remains a challenge in the field of electrocatalytic water splitting. To overcome this problem, self-supported fluorine-modified 2D ultrathin nickel hydroxide (F-Ni(OH)₂ ) nanosheets (NSs) for the oxygen evolution reaction (OER) and urea oxidation reaction (UOR) are prepared with a scalable and ascendant one-step synthesis route. The enhanced redox activity, electrical conductivity and a great number of exposed active sites of the heterogeneous catalysts improve charge migration for the electrocatalytic reactions. The density of states of the d orbitals of the Ni atoms significantly increases near the Fermi level, thereby indicating that the Ni atoms near the F-dopants promote electrical conduction in the Ni(OH)₂ monolayer. The F-Ni(OH)₂ electrocatalyst exhibits notable OER and UOR activity with onset potentials of 1.43 and 1.16 V versus RHE, respectively required to reach 10 mA cm^-2 , which are comparable to those of commercial noble-metal-based electrocatalysts. With RuCo-OH nanospheres, the settled F-Ni(OH)₂ ||RuCo-OH cell requires merely 1.55 and 1.37 V to reach 10 mA cm^-2 with superb durability for 24 h in overall water and urea electrolysis, respectively. Overall, high-quality, and efficient noble-metal-free electrocatalysts for overall water and urea electrolysis can be prepared with a simple, scalable, and reproducible preparation method.

Ultrathin PdAuBiTe Nanosheets as High-Performance Oxygen Reduction Catalysts for a Direct Methanol Fuel Cell Device

Ultrathin 2D metal nanostructures have sparked a lot of research interest because of their improved electrocatalytic properties for fuel cells. So far, no effective technique for preparing ultrathin 2D Pd-based metal nanostructures with more than three compositions has been published. Herein, a new visible-light-induced template technique for producing PdAuBiTe alloyed 2D ultrathin nanosheets is developed. The mass activity of the PdAuBiTe nanosheets against the oxygen reduction reaction (ORR) is 2.48 A mg_Pd ^-1 , which is 27.5/17.7 times that of industrial Pd/C/Pt/C, respectively. After 10 000 potential cyclings, there is no decrease in ORR activity. The PdAuBiTe nanosheets exhibit high methanol tolerance and in situ anti-CO poisoning properties. The PdAuBiTe nanosheets, as cathode electrocatalysts in direct methanol fuel cells, can thus give significant improvement in terms of power density and durability. In O₂ /air, the power density can be increased to 235.7/173.5 mW cm^-2 , higher than that reported in previous work, and which is 2.32/3.59 times higher than Pt/C.

Silica-Enveloped 2D-Sheet-to-Nanocrystals Conversion for Resilient Catalytic Dry Reforming of Methane

Here, lamellar confinement strategy is introduced for "sheet-to-nanocrystals (NCs)" conversion within a 2D-SiO₂ envelope, which constructs a catalytic nanocartridge holding a platoon of isolated and in-plane-aligned ultrasmall Ni-NCs, performing as a robust and coking-resistant catalytic system for dry reforming of methane. Overcoming the problem of unavoidable bulk crystal growth from multiple sheets-stack or sheet-on-open-support, silica bilayer-encasing tightly clamps the atomic-thin Ni(OH)₂ -nanosheet during thermal conversion and further hinders the migratory fusion of the resultant Ni-NCs. Upon heating-cooling cycle, the flapping silica envelope clutches the Ni-NCs like "eggs in a carton," subsequently, ensuring their thermal stability. Owing to the unique 2D-enveloped rigid architecture, Ni-NCs can circumvent sintering and coke deposition while tolerating the high temperatures (>700 °C) for long operation (>100 h), affording high conversions to syngas.

Improved Photoelectrochemical Performance of MoS2 through Morphology-Controlled Chemical Vapor Deposition Growth on Graphene

The morphology of MoS₂ nanostructures was manipulated from thin films to vertically aligned few-layer nanosheets on graphene, in a controllable and practical manner, using metalorganic chemical vapor deposition. The effects of graphene layer and MoS₂ morphology on photoelectrochemical (PEC) performance were systematically studied on the basis of electronic structure and transitions, carrier dynamic behavior, and PEC measurements. The heterojunction quality of the graphene/vertical few-layer MoS₂ nanosheets was ensured by low-temperature growth at 250−300 °C, resulting in significantly improved charge transfer properties. As a result, the PEC photocurrent density and photoconversion efficiency of the few-layer MoS₂ nanosheets significantly increased upon the insertion of a graphene layer. Among the graphene/MoS₂ samples, the few-layer MoS₂ nanosheet samples exhibited shorter carrier lifetimes and smaller charge transfer resistances than the thin film samples, suggesting that vertically aligned nanosheets provide highly conductive edges as an efficient pathway for photo-generated carriers and have better electronic contact with graphene. In addition, the height of vertical MoS₂ nanosheets on graphene should be controlled within the carrier diffusion length (~200 nm) to achieve the optimal PEC performance. These results can be utilized effectively to exploit the full potential of two-dimensional MoS₂ for various PEC applications.

Two-Dimensional Nanostructures for Electrochemical Biosensor

Current advancements in the development of functional nanomaterials and precisely designed nanostructures have created new opportunities for the fabrication of practical biosensors for field analysis. Two-dimensional (2D) and three-dimensional (3D) nanomaterials provide unique hierarchical structures, high surface area, and layered configurations with multiple length scales and porosity, and the possibility to create functionalities for targeted recognition at their surface. Such hierarchical structures offer prospects to tune the characteristics of materials-e.g., the electronic properties, performance, and mechanical flexibility-and they provide additional functions such as structural color, organized morphological features, and the ability to recognize and respond to external stimuli. Combining these unique features of the different types of nanostructures and using them as support for bimolecular assemblies can provide biosensing platforms with targeted recognition and transduction properties, and increased robustness, sensitivity, and selectivity for detection of a variety of analytes that can positively impact many fields. Herein, we first provide an overview of the recently developed 2D nanostructures focusing on the characteristics that are most relevant for the design of practical biosensors. Then, we discuss the integration of these materials with bio-elements such as bacteriophages, antibodies, nucleic acids, enzymes, and proteins, and we provide examples of applications in the environmental, food, and clinical fields. We conclude with a discussion of the manufacturing challenges of these devices and opportunities for the future development and exploration of these nanomaterials to design field-deployable biosensors.

2D Monomolecular Nanosheets Based on Thiacalixarene Derivatives: Synthesis, Solid State Self-Assembly and Crystal Polymorphism

Synthetic organic 2D materials are attracting careful attention of researchers due to their excellent functionality in various applications, including storage batteries, catalysis, thermoelectricity, advanced electronics, superconductors, optoelectronics, etc. In this work, thiacalix[4]arene derivatives functionalized by geranyl fragments at the lower rim in cone and 1,3-alternate conformations, that are capable of controlled self-assembly in a 2D nanostructures were synthesized. X-ray diffraction analysis showed the formation of 2D monomolecular-layer nanosheets from synthesized thiacalix[4]arenes, the distance between which depends on the stereoisomer used. It was established by DSC, FSC, and PXRD methods that the obtained macrocycles are capable of forming different crystalline polymorphs, moreover dimethyl sulphoxide (DMSO) is contributing to the formation of a more stable polymorph for cone stereoisomer. The obtained crystalline 2D materials based on synthesized thiacalix[4]arenes can find application in material science and medicine for the development of modern pharmaceuticals and new generation materials.

Workers' Exposure Assessment during the Production of Graphene Nanoplatelets in R&D Laboratory

Widespread production and use of engineered nanomaterials in industrial and research settings raise concerns about their health impact in the workplace. In the last years, graphene-based nanomaterials have gained particular interest in many application fields. Among them, graphene nanoplatelets (GNPs) showed superior electrical, optical and thermal properties, low-cost and availability. Few and conflicting results have been reported about toxicity and potential effects on workers’ health, during the production and handling of these nanostructures. Due to this lack of knowledge, systematic approaches are needed to assess risks and quantify workers’ exposure to GNPs. This work applies a multi-metric approach to assess workers’ exposure during the production of GNPs, based on the Organization for Economic Cooperation and Development (OECD) methodology by integrating real-time measurements and personal sampling. In particular, we analyzed the particle number concentration, the average diameter and the lung deposited surface area of airborne nanoparticles during the production process conducted by thermal exfoliation in two different ways, compared to the background. These results have been integrated by electron microscopic and spectroscopic analysis on the filters sampled by personal impactors. The study identifies the process phases potentially at risk for workers and reports quantitative information about the parameters that may influence the exposure in order to propose recommendations for a safer design of GNPs production process.

Strongly Confined Excitons in GaN/AlN Nanostructures with Atomically Thin GaN Layers for Efficient Light Emission in Deep-Ultraviolet

Fascinating optical properties governed by extremely confined excitons have been so far observed in 2D crystals like monolayers of transition metal dichalcogenides. These materials, however, are limited for production by epitaxial methods. Besides, they are not suitable for the development of optoelectronics for the challenging deep-ultraviolet spectral range. Here, we present a single monolayer of GaN in AlN as a heterostructure fabricated by molecular beam epitaxy, which provides extreme 2D confinement of excitons, being ideally suited for light generation in the deep-ultraviolet. Optical studies in the samples, supplemented by a group-theory analysis and first-principle calculations, make evident a giant enhancement of the splitting between the dark and bright excitons due to short-range electron-hole exchange interaction that is a fingerprint of the strongly confined excitons. The practical significance of our results is in the observation of the internal quantum yield of the room-temperature excitonic emission as high as ∼75% at 235 nm.

Kinetic Control over Morphology of Nanoporous Graphene on Surface

On-surface synthesis of high-quality nanoporous graphene (NPG) for application in nanotechnology and nanodevices remains challenging. Rational design of molecular precursors and proper kinetic control over the reaction process are the two key factors to tune the synthesis. Herein, we report a detailed study of the coupling reactions of a planar halogen-substituted nanographene molecular precursor, hexaiodo-peri-hexabenzocoronene (I₆ -HBC), on the Au(111) surface in the synthesis of NPG. The influence of three basic kinetic processes - molecular adsorption, migration, and coupling - on the synthesis was investigated. The results show that the HBC molecules deposited at low temperature predominantly desorb from the Au(111) surface during the annealing process, whereas depositing the precursor molecules onto a hot surface (700 K) can lead to the formation of NPG. However, at such a high surface temperature, simultaneous intermolecular dehydrogenative coupling between HBC monomers can hinder the ordered growth of NPG through Ullmann coupling. Moreover, the deposition rate of the precursors greatly influences the growth morphology of the NPG nanostructures.

Ultrathin Single-Crystalline CdTe Nanosheets Realized via Van der Waals Epitaxy

Due to the novel physical properties, high flexibility, and strong compatibility with Si-based electronic techniques, 2D nonlayered structures have become one of the hottest topics. However, the realization of 2D structures from nonlayered crystals is still a critical challenge, which requires breaking the bulk crystal symmetry and guaranteeing the highly anisotropic crystal growth. CdTe owns a typical wurtzite crystal structure, which hinders the 2D anisotropic growth of hexagonal-symmetry CdTe. Here, for the first time, the 2D anisotropic growth of ultrathin nonlayered CdTe as thin as 4.8 nm via an effective van der Waals epitaxy method is demonstrated. The anisotropic ratio exceeds 10³ . Highly crystalline nanosheets with uniform thickness and large lateral dimensions are obtained. The in situ fabricated ultrathin 2D CdTe photodetector shows ultralow dark current (≈100 fA), as well as high detectivity, stable photoswitching, and fast photoresponse speed (τ_rising = 18.4 ms, τ_decay = 14.7 ms). Besides, benefitting from its 2D planar geometry, CdTe nanosheet exhibits high compatibility with flexible substrates and traditional microfabrication techniques, indicating its significant potential in the applications of flexible electronic and optoelectronic devices.

Tuning the Structural Color of a 2D Photonic Crystal Using a Bowl-like Nanostructure

Structural colors of the ordered photonic nanostructures are widely used as an effective platform for manipulating the propagation of light. Although several approaches have been explored in attempts to mimic the structural colors, improving the reproducibility, mechanical stability, and the economic feasibility of sophisticated photonic crystals prepared by complicated processes continues to pose a challenge. In this study, we report on an alternative, simple method for fabricating a tunable photonic crystal at room temperature. A bowl-like nanostructure of TiO2 was periodically arranged on a thin Ti sheet through a two-step anodization process where its diameters were systemically controlled by changing the applied voltage. Consequently, they displayed a broad color distribution, ranging from red to indigo, and the principal reason for color generation followed the Bragg diffraction theory. This noncolorant method was capable of reproducing a Mondrian painting on a centimeter scale without the need to employ complex architectures, where the generated structural colors were highly stable under mechanical or chemical influence. Such a color printing technique represents a potentially promising platform for practical applications for anticounterfeit trademarks, wearable sensors, and displays.

Single-Crystalline Rhodium Nanosheets with Atomic Thickness

CO confinement strategy for ultrathin Rh nanosheets: CO is introduced as a confining agent to regulate the anisotropic growth of unique 2D structure. The single-crystalline Rh nanosheets have a thickness of three to five atomic layers and tunable edge length ranging from 500 to 1300 nm. By understanding the formation mechanism, surface-clean Rh nanosheets are also prepared and display better catalytic performance that their surfactant-capped nanosheets.

van der Waals epitaxial ultrathin two-dimensional nonlayered semiconductor for highly efficient flexible optoelectronic devices

Despite great progress in synthesis and application of graphene-like materials, it remains a considerable challenge to prepare two-dimensional (2D) nanostructures of nonlayered materials that may bring us surprising physical and chemical properties. Here, we propose a general strategy for the growth of 2D nonlayered materials by van der Waals epitaxy (vdWE) growth with two conditions: (1) the nonlayered materials satisfy 2D anisotropic growth and (2) the growth is implemented on the van der Waals substrates. Large-scale ultrathin 2D Pb(1-x)Sn(x)Se nanoplates (∼15-45 nm) have been produced on mica sheets by applying this strategy. Benefiting from the 2D geometry of Pb(1-x)Sn(x)Se nanoplates and the flexibility of mica sheet, flexible photodetectors that exhibit fast, reversible, and stable photoresponse and broad spectra detection ranging from UV to infrared light (375, 473, 632, 800, and 980 nm) are in situ fabricated based on Pb(1-x)Sn(x)Se nanoplates. We anticipate that more nonlayered materials will be developed into 2D nanostructures through vdWE, enabling the exploitation of novel electronic and optoelectronic devices.