Exploring the Fate of Copper Ions in the Synthesis of Graphdiyne.

Copper is a crucial catalyst in the synthesis of graphdiyne (GDY). However, as catalysts, the final fate of the copper ions has hardly been concerned, which are usually treated as impurities. Here, it is observed that after simple washing with water and ethanol, GDY still contains a certain amount of copper ions, and demonstrated that the copper ions are adsorbed at the atomic layers of GDY. Furthermore, we transformed in situ the copper ions into ultrathin Cu nanocrystals, and the obtained Cu/GDY hybrids can be generally converted into a series of metal/GDY hybrid materials, such as Ag/GDY, Au/GDY, Pt/GDY, Pd/GDY, and Rh/GDY. The Cu/GDY hybrids exhibit extraordinary surface enhanced Raman scattering effect and can be applied in pollutant efficient enrichment and detection.

Tungsten Nitride with a Two-Dimensional Multilayer Structure for Boosting the Surface-Enhanced Raman Effect.

The development of high-performance surface-enhanced Raman scattering (SERS) substrates is an urgent and important task. Here, tungsten nitride (WN) with a two-dimensional (2D) multilayer structure has been successfully prepared through a nitriding WO2.90 precursor. In addition to the highly active "hot spots" formed on the surface of the WN sheets, a large number of gaps between the nanosheets also exhibit a strong local surface plasmon resonance effect, which greatly improves the SERS activity. Evaluated as the SERS substrate, the WN with a 2D multilayer structure exhibits good SERS characteristics and good homogeneity and stability, even after strong acid, strong alkali, or long-term light treatment. Significantly, typical environmental contaminants such as dichlorophenol and butylated hydroxyanisole also exhibit strong Raman enhancement signals. This research provides a new method for designing inexpensive, high-activity, and universal SERS substrates.

Surfactant-free interfacial growth of graphdiyne hollow microspheres and the mechanistic origin of their SERS activity.

As a two-dimensional carbon allotrope, graphdiyne possesses a direct band gap, excellent charge carrier mobility, and uniformly distributed pores. Here, a surfactant-free growth method is developed to efficiently synthesize graphdiyne hollow microspheres at liquid‒liquid interfaces with a self-supporting structure, which avoids the influence of surfactants on product properties. We demonstrate that pristine graphdiyne hollow microspheres, without any additional functionalization, show a strong surface-enhanced Raman scattering effect with an enhancement factor of 3.7 × 107 and a detection limit of 1 × 10-12 M for rhodamine 6 G, which is approximately 1000 times that of graphene. Experimental measurements and first-principles density functional theory simulations confirm the hypothesis that the surface-enhanced Raman scattering activity can be attributed to an efficiency interfacial charge transfer within the graphdiyne-molecule system.

Heteropolyacids: An Ultrasensitive Ionic Volume-Enhanced Raman Scattering Platform.

Surface-enhanced Raman scattering (SERS) is regarded as the most direct and powerful tool to identify chemical fingerprints. However, current SERS substrate materials still face some critical challenges, including low molecular utilization efficiency and low selectivity. Herein, a novel oxygen vacancy heteropolyacid─H10Fe3Mo21O51 (HFMO)─is developed as a high-performance volume-enhanced Raman scattering (VERS)-active platform. Due to its merit of water solubility, HFMO forms a special coordination bond with the probe molecule at the molecular level, which allows its enhancing ability to be comparable to that of noble metals. An enhancement factor of 1.26 × 109 and a very low detection limit of 10-13 M for rhodamine 6G were obtained. A robust O-N coordination bond was formed between the anion of HFMO and the probe molecule, resulting in a special electron transfer path (Mo-O-N) with high selectivity, which is verified using X-ray photoelectron spectroscopy analysis and density functional theory calculations. That is to say, the proposed HFMO platform has excellent VERS enhancing effect, specifically for the molecules containing the imino group (e.g., methyl blue, detection limit: 10-11 M), offering the merits of high reproducibility and uniformity, high-temperature resistance, long-time laser irradiation, and strong acid resistance. Such an initial effort on the ionic type VERS platform may enable the further development of highly sensitive, highly selective, and water-soluble VERS technology.

Molecular assembly of carbon nitride-based composite membranes for photocatalytic sterilization and wound healing.

Polymeric carbon nitride (pCN) has attracted increasing interest as a metal-free photocatalyst because of its high efficiency in reactive oxygen species (ROS) generation. However, due to poor solubility, compounding pCN at the molecular level into more advanced nanocomposites remains a challenge. Herein, we report the dissolution of pCN in polyphosphoric acid (PPA) for the first time and fluid-phase assembly with carbon nanotubes (CNTs) into a flexible free-standing membrane. Mechanism and generality studies disclosed that the coordination of the acidity, viscosity, and adsorption energy of the solvents led to the successful dissolution of pCN. Interestingly, the pCN/CNTs molecular composite membrane exhibited not only superior mechanical properties and cycling performance as a result of strengthened π-π interfacial interaction, but also outstanding inactivation of E. coli and S. aureus in sterilization and wound healing for laboratory mice via photogenerated oxygen radicals. It would open a new era of pCN for biomedical applications in molecular composite membranes, beyond the traditional solar fuel applications in powders.

Synthesis of Graphdiyne Hollow Spheres and Multiwalled Nanotubes and Applications in Water Purification and Raman Sensing.

Controlling the structure of graphdiyne (GDY) is crucial for the discovery of new properties and the development of new applications. Herein, the microemulsion synthesis of GDY hollow spheres (HSs) and multiwalled nanotubes composed of ultrathin nanosheets is reported for the first time. The formation of an oil-in-water (O/W) microemulsion is found to be a key factor controlling the growth of GDY. These GDY HSs have fully exposed surfaces because of the avoidance of overlapping between nanosheets, thereby showing an ultrahigh specific surface area of 1246 m2 g-1 and potential applications in the fields of water purification and Raman sensing.

Ultrathin Graphdiyne Nanowires with Diameters below 3 nm: Synthesis, Photoelectric Effect, and Enhanced Raman Sensing.

Due to the intrinsic layered structure, graphdiyne (GDY) strongly tends to form 2D materials, therefore, most of the current research are based on GDY 2D structures. Up to now, the synthesis of its ultrathin nanowires with a high aspect ratio has not been reported. Here, the ultrathin GDY nanowires with diameters below 3 nm are reported for the first time by a two-phase interface synthesis method, which has excellent crystallinity and an aspect ratio of more than 2500. Evidence shows that the GDY ultrathin nanowires are formed by the oriented-attachment mechanism of nanoparticles. The GDY ultrathin nanowires exhibit a significant quantum confinement effect, enhanced photoelectric effect, and promising applications in surface-enhanced Raman sensing.

A Metallic Niobium Nitride with Open Nanocavities for Surface-Enhanced Raman Spectroscopy.

The construction of open hot-spot structures that facilitate the entry of analytes is crucial for surface-enhanced Raman spectroscopy. Here, metallic niobium nitride (NbN) three-dimensional (3D) hierarchical networks with open nanocavity structure are first found to exhibit a strong visible-light localized surface plasmon resonance (LSPR) effect and extraordinary surface-enhanced Raman scattering (SERS) performance. The unique nanocavity structure allows easy entry of molecules, promoting the utilization of electromagnetic hot spots. The NbN substrate has a lowest detection limit of 1.0 × 10-12 M and a Raman enhancement factor (EF) of 1.4 × 108 for contaminants. Furthermore, the NbN hierarchical networks possess outstanding environmental durability, high signal reproducibility, and detection universality. The remarkable SERS sensitivity of the NbN substrate can be attributed to the joint effect of LSPR and interfacial charge transport (CT).

Directly Convert Carbonaceous Microspheres to Three-Dimensional Porous Carbon Microspheres with a Robust Self-Supporting Structure as a Metal-Free SERS Substrate for Online High-Throughput Analysis.

It is of great significance for practical applications to directly convert readily available biomass carbon into three-dimensional (3D) porous carbon microspheres with a self-supporting structure. Herein, we report the convenient conversion of biomass carbon microspheres to hierarchical porous carbon microspheres (HP-CMSs) with a robust self-supporting framework structure. A general SiO2-induced etching mechanism is proposed for the formation of the HP-CMSs. Benefiting from this robust 3D self-supporting frame structure, these HP-CMSs have outstanding mechanical, chemical, and thermal stability. As a metal-free surface-enhanced Raman scattering (SERS) substrate with an ultrahigh specific surface area (4216 m2 g-1) and a high density of active sites, the HP-CMSs exhibit high sensitivity with a detection limit of 10-10 M and a Raman enhancement factor of 3.5 × 106. By integrating the enrichment and sensing functions of the HP-CMSs in a microfluidic channel, online high-throughput SERS detection of 20 samples within 5 min is achieved in a single channel, and the relative standard deviation of the signals between samples is only 5.1%. The current work develops a convenient preparation method that converts sustainable biomass carbon to 3D hierarchical porous carbon and provides a potential material for sensing, energy, catalysis, and other fields.

Photoelectron Storages in Functionalized Carbon Nitrides for Colorimetric Sensing of Oxygen.

Colorimetric sensors have been widely used for centuries across diverse fields, thanks to their easy operation and uncompromisingly high sensitivity with no need for electricity. However, it is still a great challenge for conventional chromogenic systems to perform multiple measurements meanwhile maintaining high robustness. Here, we reported that carbon nitrides (CNs), the raw materials that are abundant, structure-tunable, and stable semiconductors with photoelectron storage capability, can be developed as a chromogenic system for colorimetric sensors. Beyond conventional metal oxides that only demonstrated a single blue-color switch after photoelectron storage, CN exhibited a multicolor switch under identical conditions owing to the unusual multiple photoelectron storage pathways. Mechanism studies revealed cyano and carbonyl groups in CN crucially elongated the centroid distance of electrons/holes, which exclusively stabilized the specific excited states that have different light absorption; meanwhile, the counter cations strengthened these processes. As a result, O2, a proof-of-concept analyte, was quantitatively detected by the CN-derived colorimetric sensor, showing high reversibility in hundreds of cycles and adaptable sensitivity/detection range, outperforming most reported and commercial oxygen sensors. These intriguing features of CN are highly envisioned for the next generation of colorimetric sensors, especially in developing countries or fieldworks, to improve the detection reliability and lower the sensing cost.

In Situ Surface Restraint-Induced Synthesis of Transition-Metal Nitride Ultrathin Nanocrystals as Ultrasensitive SERS Substrate with Ultrahigh Durability.

It is a major challenge to synthesize crystalline transition-metal nitride (TMN) ultrathin nanocrystals due to their harsh reaction conditions. Herein, we report that highly crystalline tungsten nitride (W2N, WN, W3N4, W2N3) nanocrystals with small size and excellent dispersibility are prepared by a mild and general in situ surface restraint-induced growth method. These ultrafine tungsten nitride nanocrystals are immobilized in ultrathin carbon layers, forming an interesting hybrid nanobelt structure. The hybrid WN/C nanobelts exhibit a strong localized surface plasmon resonance (LSPR) effect and surface-enhanced Raman scattering (SERS) effect, including a lowest detection limit of 1 × 10-12 M and a Raman enhancement factor of 6.5 × 108 comparable to noble metals, which may be one of the best records for non-noble metal SERS substrates. Moreover, they even can maintain the SERS performance in a variety of harsh environments, showing outstanding corrosion resistance, radiation resistance, and oxidation resistance, which is not available on traditional noble metal and semiconductor SERS substrates. A synergistic Raman enhancement mechanism of LSPR and interface charge transfer is found in the carbon-coated tungsten nitride substrate. A microfluidic SERS channel integrating the enrichment and detection of trace substances is constructed with the WN/C nanobelt, which realizes high-throughput dynamic SERS analysis.

Molybdenum Nitride Porous Prisms with a Strong Plasmon Resonance Effect in the Visible Region for Surface-Enhanced Raman Spectroscopy.

In surface-enhanced Raman spectroscopy (SERS) detection, the structure of the Raman-scattering substrate is critical to the sensitivity and stability of the detector. Herein, molybdenum nitride (MoN) porous structures with a well-defined hexagonal prism shape were synthesized via a precursor nitriding route. As a typical metallic transition-metal nitride (TMN), these molybdenum nitride porous hexagonal prisms exhibit a rare strong SPR effect in the visible region, with a resonance peak centered at 534 nm. Benefiting from the strong SPR effect and their huge surface area and porosity, these MoN porous hexagonal prisms exhibit surface-enhanced Raman scattering effects comparable to those of noble metals, with a Raman enhancement factor of 5.5 × 106. More importantly, these MoN SERS substrates exhibit ultrahigh chemical stabilities that noble metal and semiconductor substrates do not possess, which can prevent corrosion by strong acids, alkalis, and high-temperature oxidation.

Moving MoO2/C Nanospheres with the Functions of Enrichment and Sensing for Online-High-Throughput SERS Detection.

The development of online surface-enhanced Raman spectroscopy (SERS) detection methods is crucial to achieving high-throughput efficiency. Herein, a non-noble-metal moving substrate that integrates the functions of enrichment and sensing is developed for the microfluidic online-high-throughput detection of pollutants. The lowest limit of detection of 1 × 10-12 M and a Raman enhancement factor of 6.3 × 108 are obtained on the nanospheres. In a single detection channel, the analysis of 20 samples is achieved within 5 min, and the relative standard deviation of the signals is less than 6.8%. Compared with static SERS detection of fixed substrates, this dynamic SERS detection method greatly reduces the contamination memory effect of the analyte residue, enabling it to perform the sequential quantitative detection of samples with large concentration differences. Moreover, the current online SERS platform realizes the rapid quantitative detection of multicomponent samples.

Plasmonic Rare-Earth Nanosheets as Surface Enhanced Raman Scattering Substrates with High Sensitivity and Stability for Multicomponent Analysis.

Looking for high-performance substrates is an important goal of current surface enhanced Raman scattering (SERS) research. Herein, ultrathin multilayer rhenium (Re) nanosheets as a rare-earth metal substrate are found to have extraordinary SERS performance. These Re nanosheets are prepared through a convenient low-temperature molten salt strategy, and their total thickness is ∼5 nm, including 3-4 layers of ultrathin nanosheets with a thickness of only ∼1 nm. The viscosity of molten salt plays a key role in the formation of these ultrathin layered nanosheets. These nanosheets exhibit a strong and well-defined localized surface plasmon resonance (SPR) effect in the visible light region. The plasmonic Re nanosheets show excellent SERS performance with high sensitivity, chemical stability, and signal repeatability. The lowest detection limit for toxic compounds is 10-12 mol, and the corresponding Raman enhancement factor is 9.1 × 108. A composite enhancement mechanism caused by localized-SPR and charge transport has played an important role in the rare-earth-SERS. High-throughput multiassay analysis is performed on the flexible membrane assembled from the Re nanosheets, which highlights that our system is capable of rapid separation and identification of the samples containing various analytes.

Nitrogen-Doped Titanium Monoxide Flexible Membrane for a Low-Cost, Biocompatible, and Durable Raman Scattering Substrate.

The development of low-cost, biocompatible, and durable high-performance substrates is an urgent issue in the field of surface-enhanced Raman scattering (SERS). Herein, by reducing and exfoliating the TiO2-layered nanoplates in the gas phase, nitrogen-doped titanium monoxide (N-TiO) ultrathin nanosheets composed of 2-3 single layers with a thickness of only ∼1.2 nm are synthesized. Compared with pure TiO, the oxidation resistance of N-TiO is greatly improved, in which the oxidation threshold is significantly increased from 187.5 to 415.6 °C. The N-TiO ultrathin nanosheets are found to have strong surface plasmon resonance in the visible region. These ultrathin N-TiO nanosheets can be easily assembled into a large-scale flexible membrane and exhibit remarkable SERS effects. Moreover, this low-cost flexible SERS substrate combines the high durability of noble-metal substrates and the high biocompatibility of semiconductor substrates.

δ-MoN Yolk Microspheres with Ultrathin Nanosheets for a Wide-Spectrum, Sensitive, and Durable Surface-Enhanced Raman Scattering Substrate.

Facing the complex environment of on-site detection, the development of active substrates with wide-spectrum surface-enhanced Raman scattering (SERS) activity is essential. Herein, we report on the low temperature and reproducible synthesis of plasmonic δ-MoN yolk microspheres by in situ-nitriding amorphous MoO2 microspheres at 500 °C and 1 atm. The yolk-structured δ-MoN exhibits strong and wide-spectrum surface plasmon resonance and SERS effects and can perform highly selective detection for probes with different absorption wavelengths under excitation of 532, 633, and 785 nm lasers, with a limitation of 10-11 M and an enhanced factor of 3.6 × 107. Moreover, the plasmonic δ-MoN yolk microspheres have high environmental durability, which can maintain high sensitivity in strong acid and alkaline solutions.

General Microwave Route to Single-Crystal Porous Transition Metal Nitrides for Highly Sensitive and Stable Raman Scattering Substrates.

The synthesis of metallic transition metal nitrides (TMNs) has traditionally been performed under harsh conditions, which makes it difficult to prepare TMNs with high surface area and porosity due to the grain sintering. Herein, we report a general and rapid (30 s) microwave synthesis method for preparing TMNs with high specific surface area (122.6-141.7 m2 g-1) and porosity (0.29-0.34 cm3 g-1). Novel single-crystal porous WN, Mo2N, and V2N are first prepared by this method, which exhibits strong surface plasmon resonance, photothermal conversion, and surface-enhanced Raman scattering effects. Different from the conventional low-temperature microwave absorbing media such as water and polymers, as new concept absorbing media, hydrated metal oxides and metallic metal oxides are found to have a remarkable high-temperature microwave heating effect and play key roles in the formation of TMNs. The current research results provide a new-concept microwave method for preparing high lattice energy compounds with high specific surface.

Selective Preparation of Mo2N and MoN with High Surface Area for Flexible SERS Sensing.

γ-Mo2N and δ-MoN are the two most important molybdenum nitrides, but controllable preparation of them with high surface area has not been achieved. Herein, we achieved selective preparation of γ-Mo2N and δ-MoN. The key factor for the selective preparation of γ-Mo2N and δ-MoN is to control the crystal phase of the precursor MoO3. In H2O and NH3 mixed gas, the α-MoO3 nanoribbons are nitridated to obtain γ-Mo2N single-crystal porous nanobelts, while the h-MoO3 prisms are nitrided to obtain δ-MoN hierarchical porous columns. The corrosion effect of H2O plays a key role in the formation of single-crystal porous structure. The γ-Mo2N flexible membrane composed of the single-crystal porous nanobelts exhibits strong localized surface plasmon resonance and surface enhanced Raman scattering effect, which show highly sensitive response to polychlorinated phenol.

Vanadium dioxide nanostructures with remarkable surface-enhanced Raman scattering activity.

Structure and size control are always considered to be effective routes to enhance the sensitivity of materials. Herein, rough VO2 (D) nanostars and nanospheres with highly dense and rough surfaces were synthesized. Accompanied by the properties inherited from the rough VO2 nanospheres, i.e., high adsorption and strong plasma resonance, these VO2 (D) nanospheres exhibit highly sensitive activity as a surface enhanced Raman spectrum substrate. The detection limit of Rhodamine 6G on this semiconductor SERS substrate is as low as 10-9 M.

General molten-salt route to three-dimensional porous transition metal nitrides as sensitive and stable Raman substrates.

Transition metal nitrides have been widely studied due to their high electrical conductivity and excellent chemical stability. However, their preparation traditionally requires harsh conditions because of the ultrahigh activation energy barrier they need to cross in nucleation. Herein, we report three-dimensional porous VN, MoN, WN, and TiN with high surface area and porosity that are prepared by a general and mild molten-salt route. Trace water is found to be a key factor for the formation of these porous transition metal nitrides. The porous transition metal nitrides show hydrophobic surface and can adsorb a series of organic compounds with high capacity. Among them, the porous VN shows strong surface plasmon resonance, high conductivity, and a remarkable photothermal conversion efficiency. As a new type of corrosion- and radiation-resistant surface-enhanced Raman scattering substrate, the porous VN exhibits an ultrasensitive detection limit of 10-11 M for polychlorophenol.