Ag Nanoparticles@Au Nanograting Array as a 3D Flexible and Effective Surface-Enhanced Raman Scattering Substrate.

Surface-enhanced Raman scattering (SERS) is a powerful analytical technique for chemical identification, but it remains a great challenge to realize the large-scale and well-controlled fabrication of sensitive and repeatable SERS substrates. Here, we report a facile strategy to fabricate centimeter-sized periodic Au nanograting (Au-NG) decorated with well-arranged Ag nanoparticles (Ag-NPs) (denoted as Ag-NPs@Au-NG) as a three-dimensional (3D) flexible hybrid SERS substrate with high sensitivity and good reproducibility. The Au-NG patterns with periodic ridges and grooves are fabricated through nanoimprint lithography by employing a low-cost digital versatile disc (DVD) as a master mold, and the Ag-NPs are assembled by a well-controlled interface self-assembly method without any coupling agents. Multiple coupling electromagnetic field effects are created at the nanogaps between the Ag-NPs and Au-NG patterns, leading to high-density and uniform hot spots throughout the substrate. As a result, the Ag-NPs@Au-NG arrays demonstrate an ultrahigh SERS sensitivity as low as 10-13 M for rhodamine 6G with a high average enhancement factor (EF) of 1.85 × 108 and good signal reproducibility. For practical applications, toxic organic pollutants including crystal violet, thiram, and melamine have been successfully detected with high sensitivity at a low detection limit, showing a good perspective in the rapid detection of toxic organic pollutants.

Phase-Tunable Molybdenum Boride Ceramics as an Emerging Sensitive and Reliable SERS Platform in Harsh Environments.

Traditional surface-enhanced Raman scattering (SERS) sensors rely heavily on the use of plasmonic noble metals, which have limitations due to their high cost and lack of physical and chemical stability. Hence, it is imperative to explore new materials as SERS platforms that can withstand high temperatures and harsh conditions. In this study, the SERS effect of molybdenum boride ceramic powders is presented with an enhancement factor of 5 orders, which is comparable to conventional noble metal substrates. The molybdenum boride powders synthesized through liquid-phase precursor and carbothermal reduction have ß-MoB, MoB2 , and Mo2 B5 phases. Among these phases, ß-MoB demonstrates the most significant SERS activity, with a detection limit for rhodamine 6G (R6G) molecules of 10-9  m. The impressive SERS enhancement can be attributed to strong molecule interactions and prominent charge interactions between R6G and the various phases of molybdenum boride, as supported by theoretical calculations. Additionally, Raman measurements show that the SERS activity remains intact after exposure to high temperature, strong acids, and alkalis. This research introduces a novel molybdenum boride all-ceramic SERS platform capable of functioning in harsh conditions, thereby showing the promising of boride ultrahigh-temperature ceramics for detection applications in extreme environments.

Pt Single-Atoms on Structurally-Integrated 3D N-Doped Carbon Tubes Grid for Ampere-Level Current Density Hydrogen Evolution.

To date, the excellent mass-catalytic activities of Pt single-atoms catalysts (Pt-SACs) toward hydrogen evolution reaction (HER) are categorically confirmed; however, their high current density performance remains a challenge for practical applications. Here, a binder-free approach is exemplified to fabricate self-standing superhydrophilic-superaerphobic Pt-SACs cathodes by directly anchoring Pt-SAs via Pt-NxC4-x coordination bonds to the structurally-integrated 3D nitrogen-doped carbon tubes (N-CTs) array grid (denoted as Pt@N-CTs). The 3D Pt@N-CTs cathode with optimal Pt-SACs loading is capable of operating at a high current density of 1000 mA cm-2 with an ultralow overpotential of 157.9 mV with remarkable long-term stability over 11 days at 500 mA cm-2. The 3D super-wettable free-standing Pt@N-CTs possess interconnected vertical and lateral N-CTs with hierarchical-sized open channels, which facilitates the mass transfer. The binder-free immobilization adding to the large surface area and 3D-interconnected open channels endow Pt@N-CTs cathodes with high accessible active sites, electrical conductivity, and structural stability that maximize the utilization efficiency of Pt-SAs to achieve ampere-level current density HER at low overpotentials.

Manipulating Coupled Field Enhancement in Slot-under-Groove Nanoarrays for Universal Surface-Enhanced Raman Scattering.

Surface-enhanced Raman scattering (SERS) is an ultrasensitive spectroscopic technique that can identify materials and chemicals based on their inelastic light-scattering properties. In general, SERS relies on sub-10 nm nanogaps to amplify the Raman signals and achieve ultralow-concentration identification of analytes. However, large-sized analytes, such as proteins and viruses, usually cannot enter these tiny nanogaps, limiting the practical applications of SERS. Herein, we demonstrate a universal SERS platform for the reliable and sensitive identification of a wide range of analytes. The key to this success is the prepared "slot-under-groove" nanoarchitecture arrays, which could realize a strongly coupled field enhancement with a large spatial mode distribution via the hybridization of gap-surface plasmons in the upper V-groove and localized surface plasmon resonance in the lower slot. Therefore, our slot-under-groove platform can simultaneously deliver high sensitivity for small-sized analytes and the identification of large-sized analytes with a large Raman gain.

SERS spectral evolution of azo-reactions mediated by plasmonic Au@Ag core-shell nanorods.

The mechanism and application of localized surface plasmon resonance induced photocatalytic reactions remain an issue of interest. In this work, we used Au@Ag core-shell nanorods as a platform for plasmon-driven photocatalysis, which was in situ investigated by surface-enhanced Raman scattering (SERS) spectroscopy. The para-aminothiophenol (PATP) and para-nitrothiophenol (PNTP) adsorbed on the nanorods were irradiated with different excitation wavelengths (633 nm, 785 nm) and transformed into 4,4'-dimercaptoazobenzene (DMAB) as evidenced by the emerging Raman peaks at 1142 cm-1, 1390 cm-1, 1440 cm-1, and 1477 cm-1, corresponding to hot carrier dominated oxidation of PATP and reduction of PNTP. Preliminary azo-reaction kinetics and in situ SERS measurements were conducted by comparing the relative intensity ratio of SERS peaks at 1440 cm-1 (DMAB stretching of N[double bond, length as m-dash]N) and 1080 cm-1 (C-S stretching of PATP and PNTP). These results indicate that the catalytic efficiency was dominated by the excitation wavelength as well as the resonance condition between the plasmon band of the nanorods and the excitation line. As a proof of concept, the Au@Ag core-shell nanorods were used to catalyze 4-nitrophenol molecules, and 4-hydroxyazobenzene molecules as the product were confirmed by in situ SERS spectra as well theoretical predictions, showing potential in plasmon driven catalysis and degradation of organic molecules.

Structurally integrated 3D carbon tube grid-based high-performance filter capacitor.

Filter capacitors play a critical role in ensuring the quality and reliability of electrical and electronic equipment. Aluminum electrolytic capacitors are the most commonly used but are the largest filtering components, limiting device miniaturization. The high areal and volumetric capacitance of electric double-layer capacitors should make them ideal miniaturized filter capacitors, but they are hindered by their slow frequency responses. We report the development of interconnected and structurally integrated carbon tube grid-based electric double-layer capacitors with high areal capacitance and rapid frequency response. These capacitors exhibit excellent line filtering of 120-hertz voltage signal and volumetric advantages under low-voltage operations for digital circuits, portable electronics, and electrical appliances. These findings provide a sound technological basis for developing electric double-layer capacitors for miniaturizing filter and power devices.

In-Situ Monitoring the SERS Spectra of para-Aminothiophenol Adsorbed on Plasmon-Tunable Au@Ag Core-Shell Nanostars.

Plasmon-induced photocatalysis on noble metal surfaces has attracted broad attention due to its application in sunlight energy conversion, while the selectivity of plasmonic platforms remains unclear. Herein, we present the controlled plasmon-mediated oxidation of para-aminothiophenol (p-ATP) by employing Au@Ag core-shell nanostars with tunable tip plasmons in visible-near-infrared range as reactors. In-situ Raman measurements indicate that Au@Ag core-shell nanostars essentially promote the conversion of p-ATP to 4,4'-dimercaptoazobenzene (DMAB) due to hot carriers excited by localized surface plasmon resonance. Au@Ag nanostars with plasmon modes under resonant light excitation suggested higher catalytic efficiency, as evidenced by the larger intensity ratios between 1440 cm-1 (N=N stretching of DMAB) and 1080 cm-1 shifts (C-S stretching of p-ATP). Importantly, the time-dependent surface-enhanced Raman scattering spectra showed that the conversion efficiency of p-ATP was mainly dictated by the resonance condition between the tip plasmon mode of Au@Ag core-shell nanostars and the excitation light, as well as the choice of excitation wavelength. These results show that plasmon bands of metal nanostructures play an important role in the efficiency of plasmon-driven photocatalysis.

Nanocomposite-Decorated Filter Paper as a Twistable and Water-Tolerant Sensor for Selective Detection of 5 ppb-60 v/v% Ammonia.

Ammonia (NH3) sensors proposed for the simultaneous exhalation diagnosis, environmental pollution monitoring, and industrial leakage alarm require high flexibility, selectivity, stability, humidity tolerance, and wide-concentration-range detection; however, technical challenges still remain. Herein, twistable and water-tolerant paper-based sensors integrated over surgical masks have been developed for NH3 detection at room temperature, via decorating specially designed ternary nanocomposites (ternary-NCs) on the commercial filter paper. The NCs consist of a multiwalled carbon nanotube framework with a polypyrrole nanolayer and are further loaded with Pt nanodots. Benefiting from the synergy effect of ternary components, the ternary-NCs exhibit an ultrasensitive response to 5 ppb-60 v/v% NH3 and present high selectivity confirmed by the theory calculations. Remarkably, the filter-paper-based sensors possess outstanding stability against twisting 0-1080°, along with excellent cuttability and foldability. Critically, such paper-based sensors can be integrated over surgical masks for simulated exhaled diagnosis and display superior water tolerance even being immersed in water for 24 h. Practically, the detecting accuracy of the filter-paper-based sensor toward the simulated exhaled NH3, environmental NH3 pollution, and industrial NH3 leakage is validated using ion chromatography.

Efficient electrocatalytic reduction of nitrate to nitrogen gas by a cubic Cu2O film with predominant (111) orientation.

A (111) predominant cubic Cu2O film terminated with nanopyramids was electrodeposited on copper foam as the cathode for electrocatalytic reduction of nitrate. The nitrate removal efficiency reached 94.3% and the selectivity for nontoxic nitrogen gas was 49.2%, 99% and 64.2% in neutral solution, alkaline solution and spiked actual lake water, respectively.

Template-assisted fabrication of Ag-nanoparticles@ZnO-nanorods array as recyclable 3D surface enhanced Raman scattering substrate for rapid detection of trace pesticides.

We present a template-assisted fabrication method for a large-scale ordered arrays of ZnO nanorods (ZnO-NRs) modified with Ag nanoparticles (Ag-NPs), which possess high-density three-dimensional (3D) hot spots uniformly dispersed all over the substrate, being beneficial to ultrahigh sensitivity of surface enhanced Raman scattering (SERS) detection. These achieved Ag-NPs@ZnO-NRs arrays show high sensitivity, good spectral uniformity and reproducibility as substrates for SERS detection. Using the arrays, both dye molecules (rhodamine 6G, R6G) and organic pollutants like toxic pesticides (thiram and methyl parathion) are detected, with the detection limits of thiram and methyl parathion being 0.79 × 10-9 M and 1.51 × 10-8 M, respectively. In addition, the Ag-NPs@ZnO-NRs arrays have a self-cleaning function because the analyte molecules can be photocatalytic degraded using ultraviolet irradiation, showing that the 3D recyclable arrays have promising opportunities to be applied in rapid SERS-based detection of toxic organic pesticides.

Ag-Nanoparticles@Bacterial Nanocellulose as a 3D Flexible and Robust Surface-Enhanced Raman Scattering Substrate.

We present a well-designed, low-cost, and simple synthetic approach to realizing the hybrid composites of Ag nanoparticle-decorated bacterial nanocellulose (denoted as Ag-NPs@BNC) as a three-dimensional (3D) flexible surface-enhanced Raman scattering (SERS) substrate with ultrahigh SERS sensitivity, excellent signal reproducibility, and stability. The homogeneous Ag-NPs with high density were in situ grown on the networked BNC fibers by the controlled silver mirror reaction and volume shrinkage treatment, which created uniformly distributed SERS "hot spots" in the 3D networked hybrid substrate. Attributed to these unique 3D hot spots, the as-presented Ag-NPs@BNC substrates exhibited ultrahigh sensitivity and good spectral reproducibility. Moreover, the hydrophilic BNC exhibits good permeability and adsorption performances, which could capture the target molecules in the highly active hot spot areas to further improve the SERS sensitivity. As a result, not only dye molecules (rhodamine 6G) but also toxic organic pollutants such as 2-naphthalenethiol and thiram have been detected using the hybrid substrates as SERS substrates, with sensitivities of 1.6 × 10-8 and 3.8 × 10-9 M, respectively. The good linear response of the intensity and the logarithmic concentration revealed promising applications in the rapid and quantitative detection of toxic organic pollutants. Besides, this self-supported Ag-NPs@BNC substrate demonstrated good stability and flexibility for varied detection conditions. Therefore, the 3D networked, flexible, ultrasensitive, and stable Ag-NPs@BNC substrate shows potential as a versatile SERS substrate in the rapid identification of various organic molecules.

Plasmonic hot electrons for sensing, photodetection, and solar energy applications: A perspective.

In plasmonic metals, surface plasmon resonance decays and generates hot electrons and hot holes through non-radiative Landau damping. These hot carriers are highly energetic, which can be modulated by the plasmonic material, size, shape, and surrounding dielectric medium. A plasmonic metal nanostructure, which can absorb incident light in an extended spectral range and transfer the absorbed light energy to adjacent molecules or semiconductors, functions as a "plasmonic photosensitizer." This article deals with the generation, emission, transfer, and energetics of plasmonic hot carriers. It also describes the mechanisms of hot electron transfer from the plasmonic metal to the surface adsorbates or to the adjacent semiconductors. In addition, this article highlights the applications of plasmonic hot electrons in photodetectors, photocatalysts, photoelectrochemical cells, photovoltaics, biosensors, and chemical sensors. It discusses the applications and the design principles of plasmonic materials and devices.

Silver nanoparticle-assembled micro-bowl arrays for sensitive SERS detection of pesticide residue.

It is of great significance to develop a simple and effective method for constructing large-scale high-quality surface-enhanced Raman scattering (SERS) substrate. Here, an Ag nanoparticle-assembled micro-bowl array was prepared by a close-packed polystyrene (PS) sphere monolayer templated electrodeposition approach. The fabricated Ag nanoparticle-assembled micro-bowl array shows high SERS sensitivity to rhodamine 6G (R6G) under an ultra-low concentration of 1 fM, and exhibits excellent SERS spectral uniformity with a small relative standard deviation (RSD) of 7.6% and good reproducibility (a RSD ∼8.2% for the average peak intensities from different batches of SERS substrates). The fabricated micro-bowl array SERS substrate was employed to detect pesticide residue (thiram and methyl parathion) on vegetables. The limit of detections (LODs) for the two pesticides are lower than the maximum residue limits (MRLs) set by the European Union respectively, showing promising application in rapid inspection of food safety.

Enhancing potassium-ion battery performance by defect and interlayer engineering.

Defect and interlayer engineering is applied to exploit the large van der Waals gaps of transition metal dichalcogenides for potassium-ion batteries (KIBs). As a demonstrator, MoS2 nanoflowers with expanded interlayer spacing and defects in the basal planes are used as KIB anodes in the voltage range of 0.5-2.5 V, where an intercalation reaction rather than a conversion reaction takes place to store K-ions in the van der Waals gaps. The nanoflowers show enhanced K-storage performance compared to the defect-free counterpart that has a pristine interlayer spacing. Kinetic analysis verifies that the K-ion diffusion coefficient and surface charge storage are both enhanced in the applied voltage range of the intercalation reaction. The collective effects of expanded interlayer spacing and additionally exposed edges induced by the in-plane defects enable facile K-ion intercalation, rapid K-ion transport and promoted surface K-ion adsorption simultaneously.

Detection of Dithiocarbamate Pesticides with a Spongelike Surface-Enhanced Raman Scattering Substrate Made of Reduced Graphene Oxide-Wrapped Silver Nanocubes.

Dithiocarbamate (DTC) pesticides are widely used for fruits, vegetables, and mature crops to control fungal diseases. Their residues in food could pose a threat to human health. Therefore, a surface-enhanced Raman scattering-based (SERS-based) sensor is developed to detect DTC pesticides because SERS can provide the characteristic spectrum of pesticides and avoid the use of a molecular recognition probe in the sensor. For the acquisition of high sensitivity, good anti-interference ability, and robustness of the SERS sensor, a silver nanocube-reduced graphene oxide (AgNC-rGO) sponge is devised. In the AgNC-rGO sponge, the rGO sheets form a porous scaffold that physically holds the AgNCs, which create narrow gaps between the neighboring AgNCs, leading to the formation of "hot spots" for SERS-signal amplification. When DTC pesticides coexist with aromatic pesticides in a sample matrix, the AgNC-rGO sponge can selectively detect DTC pesticides because of the preferential adsorption of DTC pesticides on the Ag surface and aromatic pesticides on the rGO surface, which can effectively eliminate the interference of the SERS signals of aromatic pesticides, and facilitate the qualitative and quantitative analysis of DTC pesticides. The AgNC-rGO sponge shows great potential as a SERS substrate for selective detection of DTC pesticides.

Long-range surface plasmon resonance and surface-enhanced Raman scattering on X-shaped gold plasmonic nanohole arrays.

A multilayered architecture including a thin Au film supporting an X-shaped nanohole array and a thick continuous Au film separated by a Cytop dielectric layer is reported in this work. Long-range surface plasmon resonance (LR-SPR) was generated at the top Au/water interface, which also resulted in a long-range surface-enhanced Raman scattering (LR-SERS) effect. LR-SPR originates from the coupling of surface plasmons (SPs) propagating along the opposite sides of the thin Au film embedded in a symmetric refractive index environment with Cytop (n = 1.34) and water (n = 1.33). The finite-difference time-domain (FDTD) simulation method was used to investigate the optimal dimensions of the substrate by studying the reflectance spectra and electric field profiles. The calculated optimal structure was then fabricated via electron beam lithography, and its LR-SERS performance was demonstrated by detecting rhodamine 6G and 4-mercaptobenzoic acid in the refractive index-matched environment. We believe that this structure as a LR-SPR or LR-SERS substrate can have broad applications in biosensing.

A flexible transparent Ag-NC@PE film as a cut-and-paste SERS substrate for rapid in situ detection of organic pollutants.

This report presents a simple and inexpensive fabrication approach to a flexible transparent composite film as a "cut-and-paste" surface-enhanced Raman scattering (SERS) substrate for in situ detection of organic pollutants. First, a self-assembled monolayer of Ag-nanocubes (Ag-NCs) is obtained at the air/water interface. Then, the Ag-NC monolayer is retrieved onto a flexible transparent polyethylene (PE) film to achieve an Ag-NC@PE composite film as a flexible SERS substrate. As the Ag-NCs in the monolayer are closely and uniformly packed on the PE film, the Ag-NC@PE composite film shows high SERS-activity with good signal homogeneity and reproducibility. Furthermore, the flexible transparent Ag-NC@PE composite film is "cut into" small pieces and directly "pasted" onto contaminated fruits for in situ SERS detection, as a result 10 nM thiram, 1 µM 4-polychlorinated biphenyl and 10 nM methyl parathion contaminants on oranges are detected, respectively. Therefore the Ag-NC@PE composite film is an inexpensive and effective SERS substrate for rapid in situ detection of organic pollutants in aqueous solutions, on fruits and other solid objects.

An ordered array of hierarchical spheres for surface-enhanced Raman scattering detection of traces of pesticide.

An ordered array of hierarchically-structured core-nanosphere@space-layer@shell-nanoparticles has been fabricated for surface-enhanced Raman scattering (SERS) detection. To fabricate this hierarchically-structured chip, a long-range ordered array of Au/Ag-nanospheres is first patterned in the nano-bowls on the planar surface of ordered nanoporous anodic titanium oxide template. A ultra-thin alumina middle space-layer is then conformally coated on the Au/Ag-nanospheres, and Ag-nanoparticles are finally deposited on the surface of the alumina space-layer to form an ordered array of Au/Ag-nanosphere@Al2O3-layer@Ag-nanoparticles. Finite-difference time-domain simulation shows that SERS hot spots are created between the neighboring Ag-nanoparticles. The ordered array of hierarchical nanostructures is used as the SERS-substrate for a trial detection of methyl parathion (a pesticide) in water and a limit of detection of 1 nM is reached, indicating its promising potential in rapid monitoring of organic pollutants in aquatic environment.

Fabrication of hexagonally patterned flower-like silver particle arrays as surface-enhanced Raman scattering substrates.

Hierarchical assembly of plasmonic nanostructures can induce high surface-enhanced Raman scattering (SERS) activity. However, it is a challenge to uniformly disperse the hierarchical nanostructures onto a planar substrate to achieve SERS signal reproducibility. This report presents a facile route to fabricate a hexagonally patterned flower-like silver particle array as the SERS substrate. First, hexagonally ordered silver hemisphere arrays with smooth surface are molded in the pores of an anodic aluminum oxide template. Ag-nanosheets are then electrodeposited onto the surface of individual silver hemispheres. The numerous nano-edges and nano-gaps between adjacent nanosheets render a large number of hot spots, leading to high SERS activity over a larger area of chip. The silver flower-like array is employed as the SERS substrate, which is able to detect 0.1 nM rhodamine 6 G and 1 µM 3,3',4,4'-tetrachlorobiphenyl (PCB-77, a persistent organic pollutant).

Surface-Enhanced Raman Scattering from Au-Nanorod Arrays with Sub-5-nm Gaps Stuck Out of an AAO Template.

Ordered arrays of Au-nanorod-tips protruding from an anodic aluminum oxide (AAO) template are reported as reproducible and active surface-enhanced Raman scattering (SERS) substrates. The Au-nanorods were grown in the nanochannels of the AAO template by use of alternative current electrodeposition, then the template was strengthened using a polymer, and finally the template bottom side was selectively etched to expose the Au-nanorod tips. By controlling the thinning of the AAO-porewalls, the inter-nanorod-gaps were tuned to ~5 nm, forming dense and uniform nano-gap induced "hot spots" among the adjacent Au-nanorod tips. As a result, the electromagnetic field of the Au-nanorod-tip arrays was uniformly enhanced, and demonstrated high SERS sensitivity with good signal reproducibility. The Au-nanorod-tips have the potential to be used in SERS-based applications in order to rapidly detect trace pollutants in the environment.