The Covalent Functionalization of Layered Black Phosphorus by Nucleophilic Reagents

Characterization of emerging 2D materials after chemical functionalization

The chemical modification of 2D materials has proven a powerful tool to fine tune their properties. With this motivation, the development of new reactions has moved extremely fast. The need for speed, together with the intrinsic heterogeneity of the samples, has sometimes led to permissiveness in the purification and characterization protocols. In this review, we present the main tools available for the chemical characterization of functionalized 2D materials, and the information that can be derived from each of them. We then describe examples of chemical modification of 2D materials other than graphene, focusing on the chemical description of the products. We have intentionally selected examples where an above-average characterization effort has been carried out, yet we find some cases where further information would have been welcome. Our aim is to bring together the toolbox of techniques and practical examples on how to use them, to serve as guidelines for the full characterization of covalently modified 2D materials.

Covalently Bonded Ni Sites in Black Phosphorene with Electron Redistribution for Efficient Metal-Lightweighted Water Electrolysis

The metal-lightweighted electrocatalysts for water splitting are highly desired for sustainable and economic hydrogen energy deployments, but challengeable. In this work, a low-content Ni-functionalized approach triggers the high capability of black phosphorene (BP) with hydrogen and oxygen evolution reaction (HER/OER) bifunctionality. Through a facile in situ electro-exfoliation route, the ionized Ni sites are covalently functionalized in BP nanosheets with electron redistribution and controllable metal contents. It is found that the as-fabricated Ni-BP electrocatalysts can drive the water splitting with much enhanced HER and OER activities. In 1.0 M KOH electrolyte, the optimized 1.5 wt% Ni-functionalized BP nanosheets have readily achieved low overpotentials of 136 mV for HER and 230 mV for OER at 10 mA cm-2. Moreover, the covalently bonding between Ni and P has also strengthened the catalytic stability of the Ni-functionalized BP electrocatalyst, stably delivering the overall water splitting for 50 h at 20 mA cm-2. Theoretical calculations have revealed that Ni-P covalent binding can regulate the electronic structure and optimize the reaction energy barrier to improve the catalytic activity effectively. This work confirms that Ni-functionalized BP is a suitable candidate for electrocatalytic overall water splitting, and provides effective strategies for constructing metal-lightweighted economic electrocatalysts.

Rational Design of Stimuli-Responsive Inorganic 2D Materials via Molecular Engineering: Toward Molecule-Programmable Nanoelectronics

The ability of electronic devices to act as switches makes digital information processing possible. Succeeding graphene, emerging inorganic 2D materials (i2DMs) have been identified as alternative 2D materials to harbor a variety of active molecular components to move the current silicon-based semiconductor technology forward to a post-Moore era focused on molecule-based information processing components. In this regard, i2DMs benefits are not only for their prominent physiochemical properties (e.g., the existence of bandgap), but also for their high surface-to-volume ratio rich in reactive sites. Nonetheless, since this field is still in an early stage, having knowledge of both i) the different strategies for molecularly functionalizing the current library of i2DMs, and ii) the different types of active molecular components is a sine qua non condition for a rational design of stimuli-responsive i2DMs capable of performing logical operations at the molecular level. Consequently, this Review provides a comprehensive tutorial for covalently anchoring ad hoc molecular components-as active units triggered by different external inputs-onto pivotal i2DMs to assess their role in the expanding field of molecule-programmable nanoelectronics for electrically monitoring bistable molecular switches. Limitations, challenges, and future perspectives of this emerging field which crosses materials chemistry with computation are critically discussed.

"One-stop" synergistic strategy for hepatocellular carcinoma postoperative recurrence

Residual tumor recurrence after surgical resection of hepatocellular carcinoma (HCC) remains a considerable challenge that imperils the prognosis of patients. Notably, intraoperative bleeding and postoperative infection are potential risk factors for tumor recurrence. However, the biomaterial strategy for the above problems has rarely been reported. Herein, a series of cryogels (coded as SQ-n) based on sodium alginate (SA) and quaternized chitosan (QC) were synthesized and selected for optimal ratios. The in vitro assays showed that SQ-50 possessed superior hemostasis, excellent antibacterial property, and great cytocompatibility. Subsequently, SQAP was constructed by loading black phosphorus nanosheets (BPNSs) and anlotinib hydrochloride (AL3818) based on SQ-50. Physicochemical experiments confirmed that near-infrared (NIR)-assisted SQAP could control the release of AL3818 in photothermal response, significantly inhibiting the proliferation and survival of HUVECs and H22 cells. Furthermore, in vivo studies indicated that the NIR-assisted SQAP prevented local recurrence of ectopic HCC after surgical resection, achieved through the synergistic effect of mPTT and molecular targeted therapy. Thus, the multifunctional SQAP provides a "one-stop" synergistic strategy for HCC postoperative recurrence, showing great potential for clinical application.

One-Step Electrochemical Synthesis of AlO x -Passivated Twisted-Phosphorene Nanosheets for Potentially Stable Energy Storage Devices

Black phosphorus (BP), a promising 2D material for electronics, energy storage, catalysis, and sensing, has sparked a research boom. However, exfoliated thin-layered BP is unstable and can easily be degraded under environmental conditions, severely limiting its practical applications. In this context, a simple and cost-effective method has been proposed that involves electrochemically exfoliating BP and simultaneously electrochemically depositing aluminum oxide (AlO _x ) for passivation of the exfoliated BP. The ambient stability of the exfoliated BP is studied using a time-dependent atomic force microscope (AFM). The AlO _x capping layer significantly improves the environmental stability of BP compared to uncapped BP. The thermal stability of the resulting BP is evaluated using power-dependent Raman spectroscopy. The results show that the AlO _x -passivated BP has increased thermal stability, with only a slight shift in peak position toward higher Raman power intensity. These properties can make the material suitable for stable energy storage devices. Interestingly, the electrochemical exfoliation and passivation processes resulted in the BP with a twist angle (9.86°), which is expected to exhibit unique electronic properties similar to those of graphene with a twist angle.

Covalent Functionalization of Black Phosphorus Nanosheets with Dichlorocarbenes for Enhanced Electrocatalytic Hydrogen Evolution Reaction

Two-dimensional black phosphorus (BP) has emerged as a perspective material for various micro- and opto-electronic, energy, catalytic, and biomedical applications. Chemical functionalization of black phosphorus nanosheets (BPNS) is an important pathway for the preparation of materials with improved ambient stability and enhanced physical properties. Currently, the covalent functionalization of BPNS with highly reactive intermediates, such as carbon-free radicals or nitrenes, has been widely implemented to modify the material's surface. However, it should be noted that this field requires more in-depth research and new developments. Herein, we report for the first time the covalent carbene functionalization of BPNS using dichlorocarbene as a functionalizing agent. The P-C bond formation in the obtained material (BP-CCl₂) has been confirmed by Raman, solid-state ³¹P NMR, IR, and X-ray photoelectron spectroscopy methods. The BP-CCl₂ nanosheets exhibit an enhanced electrocatalytic hydrogen evolution reaction (HER) performance with an overpotential of 442 mV at -1 mA cm^-2 and a Tafel slope of 120 mV dec^-1, outperforming the pristine BPNS.

In-Situ Electrochemical Exfoliation and Methylation of Black Phosphorus into Functionalized Phosphorene Nanosheets

Two-dimensional black phosphorus (BP) has attracted great attention as a perspective material for various applications. The chemical functionalization of BP is an important pathway for the preparation of materials with improved stability and enhanced intrinsic electronic properties. Currently, most of the methods for BP functionalization with organic substrates require either the use of low-stable precursors of highly reactive intermediates or the use of difficult-to-manufacture and flammable BP intercalates. Herein we report a facile route for simultaneous electrochemical exfoliation and methylation of BP. Conducting the cathodic exfoliation of BP in the presence of iodomethane makes it possible to generate highly active methyl radicals, which readily react with the electrode's surface yielding the functionalized material. The covalent functionalization of BP nanosheets with the P-C bond formation has been proven by various microscopic and spectroscopic methods. The functionalization degree estimated by solid-state ³¹P NMR spectroscopy analysis reached 9.7%.

Calcium Phosphate Bone Cements Incorporated with Black Phosphorus Nanosheets Enhanced Osteogenesis

For decades, calcium phosphate bone cements (CPCs) showed impressive advantages for their good biocompatibility, injectability, and osteoconductivity in the bone repair field. However, it is still difficult to prepare CPCs with outstanding antibacterial and self-curing properties, sufficient phosphorus release, and osteoinductivity for clinical application. Herein, we used partially crystallized calcium phosphate and dicalcium phosphate anhydrate particles incorporated with black phosphorous nanosheets to prepare calcium phosphate bone cements (CPCs). The curing time, compressive strength, photothermal properties, and degradation performance of BP/CPC were investigated. In addition, the cytocompatibility and osteoinductivity of BP/CPC were evaluated by cell adhesion, cytotoxicity, alkaline phosphatase detection, alizarin red staining, and western blot assay. The results indicated that BP/CPC showed adjustable curing time, good cytocompatibility, outstanding photothermal properties, and osteoinductivity, suggesting their potential application for bone regeneration.

Enhancing the anti-oxidation stability of vapor-crystallized arsenic crystals via introducing iodine

The oxidation of arsenic restricts its application in high-performance electronic devices and functional materials. Herein, a removable iodine-regulation method was proposed for the first time to enhance the anti-oxidation behavior of arsenic. In a gradient of 500-650 ℃, the introduction of 0.6-5.0 at% iodine into arsenic vapor could regulate an arsenic crystal. The oxygen content on the regulated arsenic crystal surface was lowered below 2.5 at% after exposure to ambient conditions for 96 h, reducing over 90% compared with the control group. The residual iodine barrier, which was mainly in the As-I₂ state, suppressed the long-term oxidation of arsenic. First-principles calculation suggested that the adsorbed I₂ weakened the delocalization of lone-pair electrons and inhibited charge transfer from the arsenic surface. Iodine regulation stabilized arsenic surface, which preferred (003) or (012) facets. Their surface energies were 22.4 meV and 47.6 meV, respectively. The synergistic effect of surface stabilization and I₂ passivation lowered the surface energy and continuously slowed the oxidation of arsenic. Therefore, iodine regulation comprehensively enhanced the anti-oxidation properties of arsenic. Moreover, heating at 200 ℃ left the arsenic surface iodine content below 0.1 at% with little variation in structure. The improved anti-oxidation property of arsenic preserves resources for further advanced applications.

Facile Synthesis of Black Phosphorus Nanosheet@NaReF4 Nanocomposites for Potential Bioimaging

Black phosphorus nanomaterials (BPN) have been well developed in tumor therapy. However, lack of diagnostic function limits the development of BPN in biomedicine. Lanthanide-doped nanoparticles are considered as versatile materials for fluorescence or magnetic resonance imaging. Integration of BPN with lanthanide-doped nanoparticles was rarely reported owing to the complex synthesis processes and poor modification effect. Herein, we report a simple and general method for synthesizing BPN@NaReF₄ (Re: Gd or Y, Yb, Er) nanocomposite. TEM and XRD characterization confirm efficient combination of BPN and NaGdF₄ or NaYF₄:Yb,Er (18.2 mol %) after one-step mixing. The FTIR and XPS spectra were used to prove the generation of PO₄^3--Gd and P-Gd coordination bonds and clarify ligand exchange mechanism. The anchored nanoparticles on BPN were stable and become hydrophilic. The prepared BPN@NaGdF₄ exhibit the signals of photoacoustic and magnetic resonance imaging. The obtained BPN@NaYF₄:Yb,Er (18.2 mol %) have the potential in fluorescence bioimaging. We believe that this work will expand the applications of BPN in diagnosis and therapy together.

Synergistic Lubrication Effect between Oxidized Black Phosphorus and Oil Molecules Triggers Superlubricity

Oxidized black phosphorus (BP) has been demonstrated as a promising oil-based nanoadditive because of its superior friction-reducing capability. However, the synergistic lubrication effect between oxidized BP and oil at the molecular level dominating the friction properties remains unclear. In this Letter, the synergistic lubrication effect between oxidized BP and two typical oil molecules (nonane and nonanoic acid) was explored with an atomic force microscope. The superlubricity of oxidized BP with an ultralow friction coefficient of 0.006 was achieved in the nonanoic acid environment, exhibiting a 96% reduction compared with that in the nonane environment. There was a confined nonanoic acid layer in the contact zone with a tilt angle of 35° because of the hydrogen bonding interaction, contributing to the superlubricity. This observation sheds light on the exploration of the lubrication mechanism of oxidized BP as a nanoadditive in oil, which reveals the considerable implications for the design of high-performance lubrication system.

Alkene-Catalyzed Rapid Layer-by-Layer Thinning of Black Phosphorus for Precise Nanomanufacturing

Black phosphorus (BP) is a promising material for electronic and optoelectronic applications. However, it is still challenging to obtain geometrically well-defined BP with desirable thickness. The method involving rapid BP surface reaction via alkene-catalyzed oxidation and easy removal of reactants by a mechanical effect was proposed to achieve the precise layer-by-layer thinning and real-time thickness monitoring of BP for nanopatterning with high spatial resolution based on mechanical scanning probe nanolithography. The enhanced electron affinity of oxygen with the assistance of a carbon-carbon double bond (C═C) in the alkene was demonstrated by density functional theory calculations, shortening the BP surface oxidation period by 99%, which provides access for the rapid thinning. The few-layer BP nanoflake with nested structure and arbitrary thickness on various substrates and the nanopatterned heterojunctions (BP/graphene and BP/hexagonal boron nitride) can be precisely fabricated by the adjustment of scanning number under a small load. This thinning technology was efficient and universal, which could be used to fabricate a BP field-effect transistor with a thinned channel to enhance the capability for current modulation, showing great potential applications for designing high-performance nanodevices.

In-situ growth of nitrogen-doped carbonized polymer dots on black phosphorus for electrochemical DNA biosensor of Escherichia coli O157: H7

Sensitive and accurate detection technology for pathogenic bacteria is of great social and economic significance in foodborne disease and food safety. In this paper, a novel portable electrochemical DNA biosensor for the detection of specific DNA sequence of Escherichia coli (E. coli) O157: H7 was constructed. To enhance the performance of the electrochemical sensor, a functionalized nitrogen-doped carbonized polymer dots in-situ grown on few-layer black phosphorus (N-CPDs@FLBP) was synthesized and used as the modifier on the surface of screen-printed electrode. Combining gold nanoparticles as immobilization matrix and methylene blue as electrochemical indicator, the analytical performance of this electrochemical DNA biosensor was evaluated using standard complementary ssDNA sequence in the linear concentration range from 1.0 × 10^-19 to 1.0 × 10^-6 mol/L with a low detection limit as 3.33 × 10^-20 mol/L (3 σ). Furthermore, the portable electrochemical DNA biosensor was proposed based on polymerase chain reaction amplification for the detection of the E. coli O157: H7 genomic DNA from chicken meat, which verified the feasibility for practical samples detection. The research has great theoretical and practical significance for the development of electrochemical biosensor of pathogenic bacteria.

Potential of Black Phosphorus in Immune-Based Therapeutic Strategies

Black phosphorus (BP) consists of phosphorus atoms, an essential element of bone and nucleic acid, which covalently bonds to three adjacent phosphorus atoms to form a puckered bilayer structure. With its anisotropy, band gap, biodegradability, and biocompatibility properties, BP is considered promising for cancer therapy. For example, BP under irradiation can convert near-infrared (NIR) light into heat and reactive oxygen species (ROS) to damage cancer cells, called photothermal therapy (PTT) and photodynamic therapy (PDT). Compared with PTT and PDT, the novel techniques of sonodynamic therapy (SDT) and photoacoustic therapy (PAT) exhibit amplified ROS generation and precise photoacoustic-shockwaves to enhance anticancer effect when BP receives ultrasound or NIR irradiation. Based on the prospective phototherapy, BP with irradiation can cause a “double-kill” to tumor cells, involving tumor-structure damage induced by heat, ROS, and shockwaves and a subsequent anticancer immune response induced by in situ vaccines construction in tumor site, which is referred to as photoimmunotherapy (PIT). In conclusion, BP shows promise in natural antitumor biological activity, biological imaging, drug delivery, PTT/PDT/SDT/PAT/PIT, nanovaccines, nanoadjuvants, and combination immunotherapy regimens.

Multifunctional SERS chip mediated by black phosphorus@gold-silver nanocomposites inserted in bilayer membrane for in-situ detection and degradation of hazardous materials

Self-cleaning surface-enhanced Raman scattering (SERS) substrates dependent on versatile two-dimensional semiconductors offer an efficient channel for the sensitive monitoring and timely degradation of hazardous molecules. Herein, a kind of sophisticated SERS-active nanocomposites was developed by incorporating Au-Ag nanoparticles onto black phosphorus (BP) nanosheets via photo-induced self-reduction. Combining the substantial electromagnetic "hot spots" triggered by bimetallic plasma coupling effect and the efficient charge transfer from BP to probe molecules, the proposed nanocomposites featured attractive SERS enhancement, facilitating a limit of detection down to 4.5 × 10^-10 M. Attributed to the remarkable restriction of electron-hole recombination stemming from "Schottky contact", the photocatalytic activity of BP was prominently boosted, demonstrating a complete degradation time as short as 65 min. Furthermore, the disgusting instability of BP was considerably hindered by inserting the nanocomposites into various bilayer matrices with diverse hardness and viscosity inspired by cling film principle. Moreover, a significantly elevated collection rate high to 93.1% for in-situ detection was also achieved by the as-manufactured flexible SERS chips based on tape. This study illustrates a clear perspective for the development of versatile BP-based SERS chips which might facilitate sensitive analysis and treatment of perilous contaminants in complicated real-life scenarios.

Engineered 2D materials for optical bioimaging and path toward therapy and tissue engineering

Two-dimensional (2D) layered materials as a new class of nanomaterial are characterized by a list of exotic properties. These layered materials are investigated widely in several biomedical applications. A comprehensive understanding of the state-of-the-art developments of 2D materials designed for multiple nanoplatforms will aid researchers in various fields to broaden the scope of biomedical applications. Here, we review the advances in 2D material-based biomedical applications. First, we introduce the classification and properties of 2D materials. Next, we summarize surface and structural engineering methods of 2D materials where we discuss surface functionalization, defect, and strain engineering, and creating heterostructures based on layered materials for biomedical applications. After that, we discuss different biomedical applications. Then, we briefly introduced the emerging role of machine learning (ML) as a technological advancement to boost biomedical platforms. Finally, the current challenges, opportunities, and prospects on 2D materials in biomedical applications are discussed.

Graphical abstract

A Perfect Pair: Stabilized Black Phosphorous Nanosheets Engineering with Antimicrobial Peptides for Robust Multidrug Resistant Bacteria Eradication

Black phosphorus (BP) nanosheets emerged as promising 2D nanomaterial that have been applied to eradicate antibiotic-resistant bacteria. However, their applications are limited by intrinsic ambient instability. Here, the ε-poly-l-lysine (ε-PL)-engineered BP nanosheets are constructed via simple electrostatic interaction to cater the demand for passivating BP with amplified antibacterial activity. The dual drug-delivery complex named BP@ε-PL can closely anchor onto the surface of bacteria, leading to membrane disintegration. Subsequently, in situ hyperthermia generated by BP under near-infrared (NIR) irradiation can precisely eradicate pathogenic bacteria. In vitro antibacterial studies verify the rapid disinfection ability of BP@ε-PL against Methicillin-resistant Staphylococcus aureus (MRSA) within 15 min. Moreover, ε-PL can serve as an effective protector to avoid chemical degradation of bare BP. The in vivo antibacterial study shows that a 99.4% antibacterial rate in a MRSA skin infection model is achieved, which is accompanied by negligible toxicity. In conclusion, this work not merely provides a new conjecture for protecting the BP, but also opens a novel window for synergistic antibiotic-resistant bacteria therapy based on antimicrobial peptides and 2D photothermal nanomaterial.

Electrochemical Detection of Alpha-Fetoprotein Based on Black Phosphorus Nanosheets Modification with Iron Ions

Black phosphorus nanosheets (BPNSs) were synthesized with liquid exfoliation combined with the ultrasonic method and loaded with Fe³⁺ by simply mixing. The morphology, structure and electrochemical properties of the synthesized Fe³⁺/BPNSs were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV), etc. The load of Fe³⁺ can improve the electrochemical performance of BPNSs and enhance the sensitivity of the detection. Additionally, Fe³⁺/BPNSs display good biocompatibility. In this study, immunosensors based on Fe³⁺/BPNSs were constructed to detect alpha-fetoprotein (AFP). The detection is due to the specific binding between the AFP antigen and antibody on the surface of the immunosensors, which can reduce the current response of Fe³⁺/BPNSs. The immunosensors have a good linear relationship in the range of 0.005 ng·mL⁻¹ to 50 ng·mL⁻¹, and the detection limit is 1.2 pg·mL⁻¹. The results show that surface modification with metal ions is a simple and effective way to improve the electrochemical properties of BPNSs, which will broaden the prospects for the future application of BPNSs in the electrochemical field.

Unveiling the Degradation Chemistry of Fibrous Red Phosphorus under Ambient Conditions

The practical applications of fibrous red phosphorus (FRP), an emerging quasi-one-dimensional material, might be hindered by its environmental instability. Although other phosphorus allotropes such as white phosphorus, violet phosphorus, and black phosphorus are reported unstable under ambient conditions, the chemical stability of FRP remains unexplored. Herein, we investigate the degradation chemistry of FRP by combining experimental study and density functional theory calculations. The results reveal that both oxygen and water can react with FRP, while light illumination may accelerate these reactions. Furthermore, the degradation behavior of FRP shows a pseudo-first-order reaction in oxygenated water, while it follows a pseudo-zero-order reaction in deoxygenated water. Such different reaction kinetics originates from the preferable dissociative adsorption behaviors of O₂ molecular and H₂O molecular on a FRP surface or at a FRP edge. A covalent modification approach using an aryl diazonium salt was adopted to passivate the surface of FRP flakes and significantly enhance their stability in air.

Improved photocatalytic activity and stability of black phosphorus/multi-walled carbon nanotube hybrid for RhB degradation

Black phosphorus (BP) is a two-dimensional (2D) semiconductor that has recently attracted much interest due to its unique characteristics. However, BP is susceptible to oxidization under ambient conditions. In this work, a facile one-step route is presented, in which stable P-C bonds were formed by ball milling bulk BP and multi-walled carbon nanotubes (MWCNTs) mixture without any additives. The BP-MWCNTs hybrid and the milled BP (m-BP) were both dispersed in water under ambient conditions, and their optical absorbances were monitored. The resulting data showed that the absorbance value of the BP-MWCNTs hybrid decreased by 10.87% after 5 d, whereas the m-BP decreased by 59.21%. Surprisingly, the BP-MWCNTs hybrid also exhibited ultrahigh photocatalytic activity in the visible light range. Within 60 min of irradiation, the removal efficiency of rhodamine B (RhB) by the BP-MWCNTs hybrid reached 88.42%, which is four times higher than that of the bare m-BP. This improvement can be attributed to the formation of the P-C bond and the enhanced surface adsorption capacity resulting from the introduction of the MWCNTs, indicating that the utilization of the charges on the surface of the photocatalyst is further improved. In short, this study not only provides an easy method to synthesize the stable BP-based material for practical applications but also represents a new approach to enhance the photocatalytic activity of BP.

Self-supporting Electrocatalyst Film based on Self-assembly of Heterogeneous Bottlebrush and Polyoxometalate for Efficient Hydrogen Evolution Reaction

Developing efficient electrocatalysts to promote the hydrogen evolution reaction (HER) is essential for green and sustainable future energy supply. For practical applications, it is a challenge to achieve self-assembly of electrocatalyst from microscopic to macroscopic scales. Herein, we propose a facile strategy to fabricate a self-supporting electrocatalyst film (CNT-g-PSSCo/PW₁₂ ) for HER by electrostatic interaction induced self-assembly of cobalt polystyrene sulfonate-grafted carbon nanotube heterogeneous bottlebrush (CNT-g-PSSCo) and polyoxometalates (PW₁₂ ). Co²⁺ ions of CNT-g-PSSCo can function as junctions for interconnecting neighbouring bottlebrushes to form the 3D nanonetwork structure and enable electrostatic capture of negatively-charged PW₁₂ nanodots. Moreover, CNT backbones can provide highly conductive pathways to CNT-g-PSSCo/PW₁₂ . Such a self-assembled CNT-g-PSSCo/PW₁₂ displays a low overpotential of 31 mV at a current density of 10 mA cm^-2 and a small Tafel slope of 25 mV dec^-1 , showing high efficiency toward HER. Furthermore, CNT-g-PSSCo/PW₁₂ with a stable self-supporting film morphology exhibits long-term electrocatalytic stability over 1000 CV cycles without noticeable overpotential change in acidic media. Our findings may provide a new avenue for constructing self-assembled functional nanonetwork materials with well-orchestrated structural hierarchy for many applications in energy, environment, catalysis, medicine, and others. This article is protected by copyright. All rights reserved.

Recent trends in covalent functionalization of 2D materials

Covalent functionalization of the surface is more crucial in 2D materials than in conventional bulk materials because of their atomic thinness, large surface-to-volume ratio, and uniform surface chemical potential. Because...

Electron Matters: Recent Advances in Passivation and Applications of Black Phosphorus

2D materials have experienced rapid and explosive development in the past decades. Among them, black phosphorus (BP) is one of the most promising materials on account of its thickness-dependent bandgap, high charge-carrier mobility, in-plane anisotropic structure, and excellent biocompatibility, as well as the broad applications brought by the properties. In view of the electron configuration, the most unique feature of BP is the lone-pair electrons on each P atom. The lone-pair electrons inevitably cause high reactivity of BP, particularly toward water/oxygen, which greatly limits the practical application of BP under ambient conditions. The other side of the coin is that BP can serve as an electron donor to promote the construction of BP-based hybrid materials and/or to boost the performance of BP or BP-based hybrid materials in applications. Here, recent advances in passivation and application of BP by addressing the interaction between the lone-pair electrons of BP and the other materials are discussed, and prospects for future research on BP are also proposed.

Molecularly Engineered Black Phosphorus Heterostructures with Improved Ambient Stability and Enhanced Charge Carrier Mobility

Overcoming the intrinsic instability and preserving unique electronic properties are key challenges for the practical applications of black phosphorus (BP) under ambient conditions. Here, it is demonstrated that molecular heterostructures of BP and hexaazatriphenylene derivatives (BP/HATs) enable improved environmental stability and charge transport properties. The strong interfacial coupling and charge transfer between the HATs and the BP lattice decrease the surface electron density and protect BP sheets from oxidation, resulting in an excellent ambient lifetime of up to 21 d. Importantly, HATs increase the charge scattering time of BP, contributing to an improved carrier mobility of 97 cm² V^-1 s^-1 , almost three times of the pristine BP films, based on noninvasive THz spectroscopic studies. The film mobility is an order of magnitude larger than previously reported values in exfoliated 2D materials. The strategy opens up new avenues for versatile applications of BP sheets and provides an effective method for tuning the physicochemical properties of other air-sensitive 2D semiconductors.

Two-dimensional biomaterials: material science, biological effect and biomedical engineering applications

To date, nanotechnology has increasingly been identified as a promising and efficient means to address a number of challenges associated with public health. In the past decade, two-dimensional (2D) biomaterials, as a unique nanoplatform with planar topology, have attracted explosive interest in various fields such as biomedicine due to their unique morphology, physicochemical properties and biological effect. Motivated by the progress of graphene in biomedicine, dozens of types of ultrathin 2D biomaterials have found versatile bio-applications, including biosensing, biomedical imaging, delivery of therapeutic agents, cancer theranostics, tissue engineering, as well as others. The effective utilization of 2D biomaterials stems from the in-depth knowledge of structure-property-bioactivity-biosafety-application-performance relationships. A comprehensive summary of 2D biomaterials for biomedicine is still lacking. In this comprehensive review, we aim to concentrate on the state-of-the-art 2D biomaterials with a particular focus on their versatile biomedical applications. In particular, we discuss the design, fabrication and functionalization of 2D biomaterials used for diverse biomedical applications based on the up-to-date progress. Furthermore, the interactions between 2D biomaterials and biological systems on the spatial-temporal scale are highlighted, which will deepen the understanding of the underlying action mechanism of 2D biomaterials aiding their design with improved functionalities. Finally, taking the bench-to-bedside as a focus, we conclude this review by proposing the current crucial issues/challenges and presenting the future development directions to advance the clinical translation of these emerging 2D biomaterials.

Covalent modification of black phosphorus with alkoxy groups to improve the solubility and ambient stability

Black phosphorus (BP), a new 2D material as a layered allotrope of phosphorus, has regained attention due to its outstanding semiconductor characteristics. However, the major hurdles of using few-layer BP for applications are its poor solution processability and low ambient stability. Here, we report a covalent modification of BP nanosheets by a chemical reaction with sodium alkoxide. Fourier transform infrared spectra, Raman spectra, X-ray photoemission spectra and thermogravimetric analyses all confirmed the successful introduction of alkoxy groups on the BP surface with P-O-C bonds, which increased the solubility and ambient stability of BP. The introduced alkoxy groups as soluble side chains on the BP surface not only increase the solubility of BP nanosheets by almost 3 times, but also decrease the degradation ratio of the modified BP by about 2 times because of the encapsulation. In this work we developed a facile synthetic strategy to covalently modify BP by introducing soluble side chains, suggesting an effective way to realize its full potential application in electronics.

Polydopamine-decorated black phosphorous to enhance stability in polymer scaffold

Black phosphorous (BP) is recognized as an effective reinforcement for polymer scaffold because of its excellent mechanical property and biocompatibility. Nevertheless, its poor stability in physiological environment limits its application in bone repair. In this work, BP was modified with dopamine by self-polymerization approach (donated as BP@PDA) to improve its stability, and then was introduced into poly-L-lactic acid (PLLA) scaffold fabricated by selective laser sintering technology. Results showed the compressive and tensile strength of PLLA/BP@PDA scaffold were improved by 105% and 50%, respectively. The enhanced strength was ascribed to the increased stability of BP and the improved compatibility of BP@PDA with PLLA matrix after modifying with polydopamine. Simultaneously, the bioactivity of PLLA scaffold was significantly improved. It was attributed to that BP@PDA provided the sustained source ofPO43-ions which could capture Ca²⁺ions from physiological medium to facilitatein situbiomineralization, thereby promoting cell adhesion, proliferation and differentiation. This study demonstrated the great potential of BP@PDA in bone repair.

Covalent and non-covalent chemistry of 2D black phosphorus

The post-graphene era is undoubtedly marked by two-dimensional (2D) sheet polymers, such as black phosphorus (BP). This emerging material has a fascinating structure and outstanding electronic properties and has been postulated for a plethora of applications. The need to circumvent the pronounced oxophilicity of P atoms has dominated the research on this material in recent years, with the objective of finding the most effective method to improve its environmental stability. When it comes to chemical functionalization, the few approaches reported so far involve some drawbacks such as low degree of addition and low production ability. This review presents the concepts and strategies of our studies on the chemical functionalization of BP, both non-covalent and covalent, emphazising the current synthetic challenges. Moreover, we also provide some effective pathways for the chemical activation of the unreactive basal plane, the identification of the effective binding strategies, and the concept to overcome hurdles associated with characterization tools. This work will provide fundamental insights into the controlled chemical functionalization and characterization of BP, fostering the research on this appealing 2D material.

Two-Dimensional Functionalized Germananes as Photoelectrocatalysts

Succeeding graphene, monoelemental two-dimensional (2D) materials such as germanene and silicene, coined as "Xenes", have attracted vast scientific and technological interests. Adding covalently bonded hydrogen on both sides of germanene leads to germanane (i.e., hydrogen-terminated germanene, GeH). Further, the covalent functionalization of germanane allows the tuning of its physical and chemical properties. Diverse variants of germananes have been synthesized, but current research is primarily focused on their fundamental properties. As a case in point, their applications as photo- and electrocatalysts in the field of modern energy conversion have not been explored. Here, we prepare 2D germanene-based materials, specifically germanane and germananes functionalized by various alkyl chains with different terminal groups-germanane with methyl, propyl, hydroxypropyl, and 2-(methoxycarbonyl)ethyl-and investigate their structural, morphological, optical, electronic, and electrochemical properties. The bond geometries of the functionalized structures, their formation energies, and band gap values are investigated by density functional theory calculations. The functionalized germananes are tested as photoelectrocatalysts in the hydrogen evolution reaction (HER) and photo-oxidation of water. The performance of the germananes is influenced by the functionalized groups, where the germanane with -CH₂CH₂CH₂OH termination records the lowest HER overpotentials and with -H termination reaches the highest photocurrent densities for water oxidation over the entire visible spectral region. These positive findings serve as an overview of organic functionalization of 2D germananes that can be expanded to other "Xanes" for targeted tuning of the optical and electronic properties for photo- and electrochemical energy conversion applications.

Does black phosphorus hold potential to overcome graphene oxide? A comparative review of their promising application for cancer therapy

Although graphene oxide (GO) is leading the way in the biomedical field of 2D materials, nanosized black phosphorus (NBP) has recently come to attention for use in this challenging field. A direct comparison between these two materials, in this context, has never been described. Therefore, in this mini-review, we will critically compare the applications of NBP and GO in cancer therapy. Material functionalisation, biodegradation by design, phototherapy and immunotherapy will be summarised. This work aims to inspire researchers in designing the next generation of safe NBP platforms for cancer treatment, taking advantage of the vast experience gained with GO.

Turning Waste into Wealth: Remotely NIR Light-Controlled Precious Metal Recovery by Covalently Functionalized Black Phosphorus

It is a great challenge to refine precious metals from e-wastes under mild conditions without hazardous reagents. Herein, black phosphorus (BP) was covalently functionalized with poly(N-isopropylacrylamide) (PNIPAM) to obtain thermo/near-infrared (NIR)-responsive BP-P for precious metal recovery. Precious metals (Au, Ag, and Pd) with higher redox potentials than BP-P could be efficiently recovered by reduction-driven enrichment. Taking Au as an example, the recovery process presented fast kinetics (<15 min), excellent selectivity, and high efficiency (≈98 %). Remote operation with NIR light could generate heat by BP, which induced the hydrophilic-to-hydrophobic transition of PNIPAM, allowing the spontaneous gathering, facile collection, and practical recycle of BP-P following Au extraction. Thanks to the unique features of BP-P, not only could high-quality Au nanoparticles (20-30 nm) be economically extracted (cost: $0.731-1.222 g^-1 Au nanoparticles; 5-6 orders of magnitude lower than the market price), but also the formed BP-P-Au nanocomposites have potential application in hydrogen evolution reaction.

Interfacial Covalent Bonds Regulated Electron-Deficient 2D Black Phosphorus for Electrocatalytic Oxygen Reactions

Developing resource-abundant and sustainable metal-free bifunctional oxygen electrocatalysts is essential for the practical application of zinc-air batteries (ZABs). 2D black phosphorus (BP) with fully exposed atoms and active lone pair electrons can be promising for oxygen electrocatalysts, which, however, suffers from low catalytic activity and poor electrochemical stability. Herein, guided by density functional theory (DFT) calculations, an efficient metal-free electrocatalyst is demonstrated via covalently bonding BP nanosheets with graphitic carbon nitride (denoted BP-CN-c). The polarized PN covalent bonds in BP-CN-c can efficiently regulate the electron transfer from BP to graphitic carbon nitride and significantly promote the OOH* adsorption on phosphorus atoms. Impressively, the oxygen evolution reaction performance of BP-CN-c (overpotential of 350 mV at 10 mA cm^-2 , 90% retention after 10 h operation) represents the state-of-the-art among the reported BP-based metal-free catalysts. Additionally, BP-CN-c exhibits a small half-wave overpotential of 390 mV for oxygen reduction reaction, representing the first bifunctional BP-based metal-free oxygen catalyst. Moreover, ZABs are assembled incorporating BP-CN-c cathodes, delivering a substantially higher peak power density (168.3 mW cm^-2 ) than the Pt/C+RuO₂ -based ZABs (101.3 mW cm^-2 ). The acquired insights into interfacial covalent bonds pave the way for the rational design of new and affordable metal-free catalysts.

Progress in the therapeutic applications of polymer-decorated black phosphorus and black phosphorus analog nanomaterials in biomedicine

Wonderful black phosphorus (BP) and some BP analogs (BPAs) have been increasingly studied for their biomedical applications owing to their fascinating properties and biodegradability, but opportunities and challenges have always coexisted in their study. Poor stability upon exposure to the natural environment is the major obstacle hampering their in vivo applications. BP/polymer and BPAs/polymer nanocomposites can not only efficiently prevent their oxidation and aggregation but also exhibit "biological activity" due to synergistic effects. In this review, we briefly describe the synthesis methods and stability strategies of BP/polymer and BPAs/polymer. Then, advances pertaining to their exciting therapeutic applications in various fields are systematically introduced, such as cancer therapy (phototherapy, drug delivery, and synergistic immunotherapy), bone regeneration, and neurogenesis. Some challenges for future clinical trials and possible directions for further study are finally discussed.

Recent advances in black phosphorus-based electrochemical sensors: A review

Since the discovery of liquid-phase-exfoliated black phosphorus (BP) as a field-effect transistor in 2014, BP, with its 2D layered structure, has attracted significant attention, owing to its anisotropic electroconductivity, tunable direct bandgap, extraordinary surface activity, moderate switching ratio, high hole mobility, good biocompatibility, and biodegradability. Several pioneering research efforts have explored the application of BP in different types of electrochemical sensors. This review summarizes the latest synthesis methods, protection strategies, and electrochemical sensing applications of BP and its derivatives. The typical synthesis methods for BP-based crystals, nanosheets, and quantum dots are discussed in detail; the degradation of BP under ambient conditions is introduced; and state-of-the-art protection methodologies for enhancing BP stability are explored. Various electrochemical sensing applications, including chemically modified electrodes, electrochemiluminescence sensors, enzyme electrodes, electrochemical aptasensors, electrochemical immunosensors, and ion-selective electrodes are discussed in detail, along with the mechanisms of BP functionalization, sensing strategies, and sensing properties. Finally, the major challenges in this field are outlined and future research avenues for BP-based electrochemical sensors are highlighted.

Surface Modification of Black Phosphorus with Group 13 Lewis Acids for Ambient Protection and Electronic Tuning

Herein we introduce a facile, solution-phase protocol to modify the Lewis basic surface of few-layer black phosphorus (bP) and demonstrate its effectiveness at providing ambient stability and tuning of electronic properties. Commercially available group 13 Lewis acids that range in electrophilicity, steric bulk, and Pearson hard/soft-ness are evaluated. The nature of the interaction between the Lewis acids and the bP lattice is investigated using a range of microscopic (optical, atomic force, scanning electron) and spectroscopic (energy dispersive, X-ray photoelectron) methods. Al and Ga halides are most effective at preventing ambient degradation of bP (>84 h for AlBr₃ ), and the resulting field-effect transistors show excellent IV characteristics, photocurrent, and current stability, and are significantly p-doped. This protocol, chemically matched to bP and compatible with device fabrication, opens a path for deterministic and persistent tuning of the electronic properties in bP.

Interface Amorphization of Two-Dimensional Black Phosphorus upon Treatment with Diazonium Salts

Two-dimensional (2D) black phosphorus (BP) represents one of the most appealing 2D materials due to its electronic, optical, and chemical properties. Many strategies have been pursued to face its environmental instability, covalent functionalization being one of the most promising. However, the extremely low functionalization degrees and the limitations in proving the nature of the covalent functionalization still represent challenges in many of these sheet architectures reported to date. Here we shine light on the structural evolution of 2D-BP upon the addition of electrophilic diazonium salts. We demonstrated the absence of covalent functionalization in both the neutral and the reductive routes, observing in the latter case an unexpected interface conversion of BP to red phosphorus (RP), as characterized by Raman, 31 P-MAS NMR, and X-ray photoelectron spectroscopies (XPS). Furthermore, thermogravimetric analysis coupled to gas chromatography and mass spectrometry (TG-GC-MS), as well as electron paramagnetic resonance (EPR) gave insights into the potential underlying radical mechanism, suggesting a Sandmeyer-like reaction.

Basic Pharmacological Characterization of EV-34, a New H2S-Releasing Ibuprofen Derivative

BACKGROUND

Cardioprotective effects of H2S are being suggested by numerous studies. Furthermore, H2S plays a role in relaxation of vascular smooth muscle, protects against oxidative stress, and modulates inflammation. Long-term high-dose use of NSAIDs, such as ibuprofen, have been associated with enhanced cardiovascular risk. The goal of the present work is the synthesis and basic pharmacological characterization of a newly designed H2S-releasing ibuprofen derivative.

METHODS

Following the synthesis of EV-34, a new H2S-releasing derivative of ibuprofen, oxidative stability assays were performed (Fenton and porphyrin assays). Furthermore, stability of the molecule was studied in rat serum and liver lysates. H2S-releasing ability of the EC-34 was studied with a hydrogen sulfide sensor. MTT (3-(4,5-dimethylthiazol 2-yl)-2,5-(diphenyltetrazolium bromide)) assay was carried out to monitor the possible cytotoxic effect of the compound. Cyclooxygenase (COX) inhibitory property of EV-34 was also evaluated. Carrageenan assay was carried out to compare the anti-inflammatory effect of EV-34 to ibuprofen in rat paws.

RESULTS

The results revealed that the molecule is stable under oxidative condition of Fenton reaction. However, EV-34 undergoes biodegradation in rat serum and liver lysates. In cell culture medium H2S is being released from EV-34. No cytotoxic effect was observed at concentrations of 10, 100, 500 µM. The COX-1 and COX-2 inhibitory effects of the molecule are comparable to those of ibuprofen. Furthermore, based on the carrageenan assay, EV-34 exhibits the same anti-inflammatory effect to that of equimolar amount of ibuprofen (100 mg/bwkg).

CONCLUSION

The results indicate that EV-34 is a safe H2S releasing ibuprofen derivative bearing anti-inflammatory properties.

Self-floating black phosphorous nanosheets as a carry-on solar vapor generator

Efficient solar vapor generation has provided a feasible way to solve the water shortage in undeveloped and arid areas. However, some photo-thermal materials need additional large-volume support materials being harmful to carry around to obtain float capacity, while suppressing the solar steam generation. Herein, the decrease approach of surface tension was presented for solving the moist-environment degradation behavior of black phosphorus (BP) nanosheets and obtaining the self-floating capacity, thus applying BP nanosheets in solar vapor generation field. With the in situ cross-linking polymerization, trichloro(1H,1H,2H,2H-perfluorooctyl) silane successfully modified BP nanosheets (F-BP). Due to the significantly decreased surface tension, F-BP nanosheets are capable of self-floating easily on water surface and spreading out spontaneously. With assistance of desirable photo-thermal conversion capacity, self-floating BP nanosheets convert the incident photon into local heat, showing excellent vapor generation performance. With simulated sun illumination (1 kW/m²), 238.7 g/m² BP nanosheets present the evaporation rate of ~0.9437 kg/(m²·h) and efficiency of ~64.63 ± 2.3%. Meanwhile, the super-hydrophobicity successfully imparts BP nanosheets with resistance to the deterioration caused by salt precipitation. The carriable F-BP nanosheets are an ideal photo-thermal convertor to produce drinking water, successfully providing a feasible way to solve water shortage in limited condition.

The Art of Constructing Black Phosphorus Nanosheet Based Heterostructures: From 2D to 3D

Assembling different kinds of 2D nanosheets into heterostructures presents a promising way of designing novel artificial materials with new and improved functionalities by combining the unique properties of each component. In the past few years, black phosphorus nanosheets (BPNSs) have been recognized as a highly feasible 2D material with outstanding electronic properties, a tunable bandgap, and strong in-plane anisotropy, highlighting their suitability as a material for constructing heterostructures. In this study, recent progress in the construction of BPNS-based heterostructures ranging from 2D hybrid structures to 3D networks is discussed, emphasizing the different types of interactions (covalent or noncovalent) between individual layers. The preparation methods, optical and electronic properties, and various applications of these heterostructures-including electronic and optoelectronic devices, energy storage devices, photocatalysis and electrocatalysis, and biological applications-are discussed. Finally, critical challenges and prospective research aspects in BPNS-based heterostructures are also highlighted.