New Parameter for Benchmarking Plasmonic Gas Sensors Demonstrated with Densely Packed Au Nanoparticle Layers.

Localized surface plasmon resonance (LSPR) gas sensitivity is introduced as a new parameter to evaluate the performance of plasmonic gas sensors. A model is proposed to consider the plasmonic sensors' surface sensitivity and plasmon decay length and correlate the LSPR response, measured upon gas exchange, with an equivalent refractive index change consistent with adsorbed gas layers. To demonstrate the applicability of this new parameter, ellipsoidal gold nanoparticles (NPs) arranged in densely packed hexagonal lattices were fabricated. The main advantages of these sensors are the small and tunable interparticle gaps (18-29 nm) between nanoparticles (diameters: 72-88 nm), with their robust and scalable fabrication technology that allows the well-ordered arrangement to be maintained on a large (cm2 range) area. The LSPR response of the sensors was tested using an LSPR sensing system by switching the gas atmosphere between inorganic gases, namely He/Ar and Ar/CO2, at constant pressure and room temperature. It was shown that this newly proposed parameter can be generally used for benchmarking plasmonic gas sensors and is independent of the type and pressure of the tested gases for a sensor structure. Furthermore, it resolves the apparent disagreement when comparing the response of plasmonic sensors tested in liquids and gases.

Aggregated gold nanoparticles rich in electromagnetic field "hotspots" for surface enhanced Raman scattering.

A simple method for one-step synthesis of aggregated gold nanoparticles (a-AuNPs) using single-layer carbon dots (s-CDs) as the capping agents has been proposed. The obtained a-AuNPs are mainly composed of several spherical AuNPs of 20-25 nm sized, which aggregate to form nanogaps of ∼1 nm. Furthermore, the obtained a-AuNPs produce a strong localized surface plasmon resonance (LSPR) absorption band centered at around 640 nm, which is quite close to the wavelength of the commonly used 633 nm laser in surface enhanced Raman scattering (SERS). Thus, under the irradiation of 633 nm laser, a lot of electromagnetic field "hot spots" are formed at around the nanogaps, and strong SERS activity is achieved. The obtained a-AuNPs are dropped on tin-foil wafers to fabricate SERS substrates, which show the advantages of high sensitivity, fast response, good repeatability and satisfactory stability. On the basis, a sensitive SERS sensor is developed to detect malachite green in aquaculture water, with a low detection limit of 1 × 10-9 mol/L.

Inkjet-Printed Localized Surface Plasmon Resonance Subpixel Gas Sensor Array for Enhanced Identification and Visualization of Gas Spatial Distributions from Multiple Odor Sources.

The visualization of the spatial distributions of gases from various sources is essential to understanding the composition, localization, and behavior of these gases. In this study, an inkjet-printed localized surface plasmon resonance (LSPR) subpixel gas sensor array was developed to visualize the spatial distributions of gases and to differentiate between acetic acid, geraniol, pentadecane, and cis-jasmone. The sensor array, which integrates gold nanoparticles (AuNPs), silver nanoparticles (AgNPs), and fluorescent pigments, was positioned 3 cm above the gas source. Hyperspectral imaging was used to capture the LSPR spectra across the sensor array, and these spectra were then used to construct gas information matrices. Principal component analysis (PCA) enabled effective classification of the gases and localization of their sources based on observed spectral differences. Heat maps that visualized the gas concentrations were generated using the mean squared error (MSE) between the sensor responses and reference spectra. The array identified and visualized the four gas sources successfully, thus demonstrating its potential for gas localization and detection applications. The study highlights a straightforward, cost-effective approach to gas sensing and visualization, and in future work, we intend to refine the sensor fabrication process and enhance the detection of complex gas mixtures.

Tunable Plasmon Resonance in Silver Nanodisk-on-Mirror Structures and Scattering Enhancement by Annealing.

In this study, we evaluated the surface plasmon characteristics of periodic silver nanodisk structures fabricated on a dielectric thin-film spacer layer on a Ag mirror substrate (NanoDisk on Mirror: NDoM) through finite difference time domain (FDTD) simulations and experiments involving actual sample fabrication. Through FDTD simulations, it was confirmed that the NDoM structure exhibits two sharp peaks in the visible range, and by adjusting the thickness of the spacer layer and the size of the nanodisk structure, sharp peaks can be obtained across the entire visible range. Additionally, we fabricated the NDoM structure using electron beam lithography (EBL) and experimentally confirmed that the obtained peaks matched the simulation results. Furthermore, we discovered that applying annealing at an appropriate temperature to the fabricated structure enables the adjustment of the resonance peak wavelength and enhances the scattering intensity by approximately five times. This enhancement is believed to result from changes in the shape and size of the nanodisk structure, as well as a reduction in grain boundaries in the metal crystal due to annealing. These results have the potential to contribute to technological advancements in various application fields, such as optical sensing and emission enhancement.

Fabrication of Three-Dimensional Dendritic Ag Nanostructures: A SERS Substrate for Non-Invasive Detection.

This paper discusses the fabrication of three-dimensional dendritic Ag nanostructures, showcasing pronounced Localized Surface Plasmon Resonance (LSPR) effects. These nanostructures, employed in surface-enhanced Raman scattering (SERS), function as sensors for lactic acid in artificial sweat. The dendritic structures of the silver nanoparticles (AgNPs) create an effective SERS substrate, with additional hotspots at branch junctures enhancing LSPR. We achieve differential LSPR effects by varying the distribution and spacing of branches and the overall morphology. Adjustments to electrodeposition parameters, such as current and plating solution protective agents on an anodized aluminum oxide (AAO) base, allow for precise control over LSPR intensities. By pre-depositing AgNPs, the electron transmission paths during electrodeposition are modified, which leads to optimized dendritic morphology and enhanced LSPR effects. Parameter optimization produces elongated rods with main and secondary branches, covered with uniformly sized, densely packed, non-overlapping spherical AgNPs. This configuration enhances the LSPR effect by generating additional hotspots beyond the branch tips. Fine-tuning the electrodeposition parameters improved the AgNPs' morphology, achieving uniform particle distribution and optimal spacing. Compared to non-SERS substrates, our structure amplified the Raman signal for lactic acid detection by five orders of magnitude. This method can effectively tailor SERS substrates for specific analytes and laser-based detection.

Analysis and Optimization of Light Absorption and Scattering Properties of Metal Nanocages.

Metal nanocages exhibit localized surface plasmon resonance that strongly absorbs and scatters light at specific wavelengths, making them potentially valuable for photothermal therapy and biological imaging applications. However, investigations on metal nanocages are still confined to high-cost and small-scale synthesis. The comprehensive analysis of optical properties and optimal size parameters of metal nanocages is rarely reported. This paper simulates the effects of materials (Ag, Au, and Cu), size parameters, refractive index of the surrounding medium, and orientation on the light absorption and scattering characteristics of the nanocages using the finite-element method and the size-dependent refractive-index model for metal nanoparticles. The results show that the Ag nanocages have excellent light absorption and scattering characteristics and respond significantly to the size parameters, while the refractive index and orientation of the surrounding medium have less effect on them. The Au nanocages also possess superior light absorption properties at specific incident wavelengths. This study also identified the optimized sizes of three metal nanocages at incident light wavelengths commonly used in biomedicine; it was also found that, under deep therapy conditions, Ag nanocages in particular exhibit the highest volume absorption and scattering coefficients of 0.708 nm-1 and 0.583 nm-1, respectively. These findings offer theoretical insights into preparing target nanocage particles for applications in photothermal therapy and biological imaging.

Optical Properties and Applications of Diffraction Grating Using Localized Surface Plasmon Resonance with Metal Nano-Hemispheres.

This study investigates the optical properties of diffraction gratings using localized surface plasmon resonance (LSPR) with metal nano-hemispheres. We fabricated metal nano-hemisphere gratings (MNHGS) with Ga, Ag, and Au and examined their wavelength-selective diffraction properties. Our findings show that these gratings exhibit peak diffraction efficiencies at 300 nm, 500 nm, and 570 nm, respectively, corresponding to the LSPR wavelengths of each metal. The MNHGs were created through thermal nanoimprint and metal deposition, followed by annealing. The experimental and simulation results confirmed that the MNHGs selectively diffract light at their resonance wavelengths. Applying these findings to third-order nonlinear laser spectroscopy (MPT-TG method) enhances measurement sensitivity by reducing background noise through the selective diffraction of pump light while transmitting probe light. This innovation promises a highly sensitive method for observing subtle optical phenomena, enhancing the capabilities of nonlinear laser spectroscopy.

Near-Infrared Light-Driven CO2 Reduction on Cup-Stacked Carbon Nanotubes.

Carbon nanotubes feature one-dimensional nature of collective excitations, wherein strong confinement of surface plasmons severely hinders the liberation of hot electrons (HEs), posing grand challenges for their utilization in photochemistry. In this study, we prototypically achieved directed HEs flow and extraction in hybrid plasmonic CNN based on cup-stacked carbon nanotubes (CSCNTs), taking advantage of their privileged edge-plane sites. The localized pz electronic states and accessible intersubband plasmon excitations in the near-infrared (NIR) regime stands in striking contrast to the conventional concentric carbon nanotubes, as evidenced by combined photo-induced force microscopy (PiFM) and transient photocurrent response. The hybrid comprising intimately integrated CSCNTs-C3N4 effectively sustains interfacial electronic states and underlies the energy extraction out of plasmonic components. The CNN demonstrates almost near-unity NIR light-driven CO2 reduction to CO with a rate of 1.35 µmol g-1 h-1. This work sheds light on the exploitation of metal-free carbon-based plasmonic nanostructures for photocatalytic applications.

Mycotoxin Detection through Colorimetric Immunoprobing with Gold Nanoparticle Antibody Conjugates.

Driven by their exceptional optical characteristics, robust chemical stability, and facile bioconjugation, gold nanoparticles (AuNPs) have emerged as a preferred material for detection and biosensing applications in scientific research. This study involves the development of a simple, rapid, and cost-effective colorimetric immuno-sensing probe to detect aflatoxin B1 and zearalenone using AuNP antibody (AuNP-mAb) conjugates. Anti-toxin antibodies were attached to the AuNPs by using the physical adsorption method. The colorimetric immunosensor developed operates on the principle that the optical properties of the AuNP are very sensitive to aggregation, which can be induced by a critical high salt concentration. Although the presence of antibodies on the AuNP surface inhibits the aggregation, these antibodies bind to the toxin with higher affinity, which leads to exposure of the surface of AuNPs and aggregation in a salt environment. The aggregation triggers a noticeable but variable alteration in color from red to purple and blueish gray, as a result of a red shift in the surface plasmon resonance band of the AuNPs. The extent of the shift is dependent on the toxin exposure dose and can be quantified using a calibration curve through UV-Visible-NIR spectroscopy. The limit of detection using this assay was determined to be as low as 0.15 ng/mL for both zearalenone and aflatoxin B1. The specificity of the prepared immunoprobe was analyzed for a particular mycotoxin in the presence of other mycotoxins. The developed immunoprobe was evaluated for real-world applicability using artificially spiked samples. This colorimetric immunoprobe based on localized surface plasmon resonance (LSPR) has a reduced detection limit compared to other immunoassays, a rapid readout, low cost, and facile fabrication.

Recent Progress in the Synthesis of 3D Complex Plasmonic Intragap Nanostructures and Their Applications in Surface-Enhanced Raman Scattering.

Plasmonic intragap nanostructures (PINs) have garnered intensive attention in Raman-related analysis due to their exceptional ability to enhance light-matter interactions. Although diverse synthetic strategies have been employed to create these nanostructures, the emphasis has largely been on PINs with simple configurations, which often fall short in achieving effective near-field focusing. Three-dimensional (3D) complex PINs, distinguished by their intricate networks of internal gaps and voids, are emerging as superior structures for effective light trapping. These structures facilitate the generation of hot spots and hot zones that are essential for enhanced near-field focusing. Nevertheless, the synthesis techniques for these complex structures and their specific impacts on near-field focusing are not well-documented. This review discusses the recent advancements in the synthesis of 3D complex PINs and their applications in surface-enhanced Raman scattering (SERS). We begin by describing the foundational methods for fabricating simple PINs, followed by a discussion on the rational design strategies aimed at developing 3D complex PINs with superior near-field focusing capabilities. We also evaluate the SERS performance of various 3D complex PINs, emphasizing their advanced sensing capabilities. Lastly, we explore the future perspective of 3D complex PINs in SERS applications.

Dual-Mode Sensing of Fe(III) Based on Etching Induced Modulation of Localized Surface Plasmon Resonance and Surface Enhanced Raman Spectroscopy.

Convenient, rapid, highly sensitive and on-site iron determination is important for environmental safety and human health. We developed a sensing system for the detection of Fe(III) in water based on 7-mercapto-4-methylcoumarine (MMC)-stabilized silver-coated gold nanostars (GNS@Ag@MMC), exploiting a redox reaction between the Fe(III) cation and the silver shell of the nanoparticles, which causes a severe transformation of the nanomaterial structure, reverting it to pristine GNSs. This system works by simultaneously monitoring changes in the Localized Surface Plasmon Resonance (LSPR) and Surface-Enhanced Raman Spectroscopy (SERS) spectra as a function of added Fe(III). The proposed sensing system is able to detect the Fe(III) cation in the 1.0 × 10-5-1.5 × 10-4 M range, and its selectivity of the GNS@Ag@MMC sensor toward iron has been verified monitoring the LSPR and the SERS response to other cations with a clear selectivity toward Fe(III).

LSPR-susceptible metasurface platform for spectrometer-less and AI-empowered diagnostic biomolecule detection.

In response to the growing demand for biomolecular diagnostics, metasurface (MS) platforms based on high-Q resonators have demonstrated their capability to detect analytes with smart data processing and image analysis technologies. However, high-Q resonator meta-atom arrays are highly sensitive to the fabrication process and chemical surface functionalization. Thus, spectrum scanning systems are required to monitor the resonant wavelength changes at every step, from fabrication to practical sensing. In this study, we propose an innovative dielectric resonator-independent MS platform that enables spectrometer-less biomolecule detection using artificial intelligence (AI) at a visible wavelength. Functionalizing the focused vortex MS to capture gold nanoparticle (AuNP)-based sandwich immunoassays causes the resulting vortex beam profiles to be significantly affected by the localized surface plasmon resonance (LSPR) occurring between AuNPs and meta-atoms. The convolutional neural network algorithm was carefully trained to accurately classify the AuNP concentration-dependent focused vortex beam, facilitating the determination of the concentration of the targeted diagnostic biomolecule. Successful in situ identification of various biomolecule concentrations was achieved with over 99 % accuracy, indicating the potential of combining an LSPR-susceptible MS platform and AI for continuously tracking various chemical and biological compounds.

Blue Shift of Localized Surface Plasmon Resonance of Gold Ultrathin Nanorod by Forming a Single Atomic Silver Shell via Antigalvanic Process.

Gold ultrathin nanorods (Au UNRs) are anisotropic nanostructures constructed by attaching gold nanoclusters in one dimension. Au UNRs exhibit localized surface plasmon resonance (LSPR) only in the longitudinal direction because their diameter is smaller than the Fermi wavelength of an electron (<2 nm). In this study, we found that the LSPR wavelength of oleylamine-stabilized Au UNRs is blue-shifted simply by mixing with Ag(I). High-resolution elemental mapping and X-ray photoelectron spectroscopy of the resulting UNRs indicate that a Ag monatomic layer is formed on the Au UNR surface by the antigalvanic reduction of Ag(I). This process allowed us to synthesize a series of Au@Ag core-shell UNRs with LSPR wavelengths in the range of 1.2-2.0 µm.

Plasmon-Promoted Interatomic Hot Carriers Regulation Enhanced Electrocatalytic Nitrogen Reduction Reaction.

Utilizing hot carriers for efficient plasmon-mediated chemical reactions (PMCRs) to convert solar energy into secondary energy is one of the most feasible solutions to the global environmental and energy crisis. Finding a plasmonic heterogeneous nanostructure with a more efficient and reasonable hot carrier transport path without affecting the intrinsic plasmonic properties is still a major challenge that urgently needs to be solved in this field. Herein, the mechanism by which plasmon-promoted interatomic hot electron redistribution on the surface of Au3Cu alloy nanoparticles promotes the electrocatalytic nitrogen reduction reaction (ENRR) is successfully clarified. The localized surface plasmon resonance (LSPR) effect can boost the transfer of plasmon hot electrons from Au atoms to Cu atoms, trigger the interatomic electron regulation of Au3Cu alloy nanoparticles, enhance the desorption of ammonia molecules, and increase the ammonia yield by approximately 93.9 %. This work provides an important reference for rationally designing and utilizing the LSPR effect to efficiently regulate the distribution and mechanism of plasmon hot carriers on the surface of heterogeneous alloy nanostructures.

Controlling Plasmonic Catalysis via Strong Coupling with Electromagnetic Resonators.

Plasmonic excitations decay within femtoseconds, leaving nonthermal (often referred to as "hot") charge carriers behind that can be injected into molecular structures to trigger chemical reactions that are otherwise out of reach─a process known as plasmonic catalysis. In this Letter, we demonstrate that strong coupling between resonator structures and plasmonic nanoparticles can be used to control the spectral overlap between the plasmonic excitation energy and the charge injection energy into nearby molecules. Our atomistic description couples real-time density-functional theory self-consistently to an electromagnetic resonator structure via the radiation-reaction potential. Control over the resonator provides then an additional knob for nonintrusively enhancing plasmonic catalysis, here more than 6-fold, and dynamically reacting to deterioration of the catalyst─a new facet of modern catalysis.

Development of a localized surface plasmon-enhanced electron beam-pumped nanoscale light source for electron beam excitation-assisted optical microscopy.

We have demonstrated localized surface plasmon (LSP)-enhanced cathodoluminescence (CL) from an atomic layer deposition (ALD)-grown Al2O3/ZnO/Al2O3 heterostructure to develop a bright nanometer-scale light source for an electron beam excitation-assisted (EXA) optical microscope. Three types of metals, Ag, Al, and Au, were compared, and an 181-fold enhancement of CL emission was achieved with Ag nanoparticles (NPs), with the plasmon resonance wavelength close to the emission wavelength energy of ZnO. The enhanced emission is plausibly attributed to LSP/exciton coupling. However, it is also attributed to an increase in coupling efficiency with penetration depth and also to an increase in light extraction efficiency by grading the refractive indices at the heterostructure.

Unveiling the Mechanism of Plasmon Photocatalysis via Multiquantum Vibrational Excitation.

Plasmon photocatalysis reactions are thought to occur through vibrationally activated reactants, driven by nonthermal energy transfer from plasmon-induced hot carriers. However, a detailed quantum-state-level understanding and quantification of the activation have been lacking. Using anti-Stokes surface-enhanced Raman scattering (SERS) spectroscopy, we mapped the vibrational population distributions of reactants on plasmon-excited nanostructures. Our results reveal a highly nonthermal distribution with an anomalously enhanced population of multiquantum excited states (v ≥ 2). The shape of the distribution and its dependence on local field intensity and excitation wavelength cannot be explained by photothermal heating or vibronic optical transitions of the metal-molecule complex. Instead, it can be modeled by hot electron-molecule energy transfer mediated by the transient negative ions, establishing direct links among nonthermal reactant activation, plasmon-induced hot electrons, and negative ion resonances. Moreover, the presence of multiquantum excited reactants, which are far more reactive than those in the ground state or first excited state, presents opportunities for vibrationally controlling chemical selectivities.

Nanoflower Microreactor Based Versatile Enhancer for Recognition Cofactor-Dependent Enzyme Biocatalysis toward Saxitoxin Detection.

Investigating organic carriers' utilization efficiency and bioactivity within organic-inorganic hybrid nanoflowers is critical to constructing sensitive immunosensors. Nevertheless, the sensitivity of immunosensors is interactively regulated by different classes of biomolecules such as antibodies and enzymes. In this work, we introduced a new alkaline phosphatase-antibody-CaHPO4 hybrid nanoflowers (AAHNFs) microreactor based colorimetric immunoprobe. This system integrates a biometric unit (antibody) with a signal amplification element (enzyme) through the biomineralization process. Specifically, the critical factors affecting antibody recognition activity in the formation mechanism of AAHNFs are investigated. The designed AAHNFs retain antibody recognition ability with enhanced protection for encapsulated proteins against high temperature, organic solvents, and long-term storage, facilitating the selective construction of lock structures against antigens. Additionally, a colorimetric immunosensor based on AAHNFs was developed. After ascorbic acid 2-phosphate hydrolysis by alkaline phosphatase (ALP), the generated ascorbic acid decomposes I2 to I-, inducing the localized surface plasmon resonance in the silver nanoplate, which is effectively tuned through shape conversion to develop the sensor. Further, a 3D-printed portable device is fabricated, integrated with a smartphone sensing platform, and applied to the data of collection and analysis. Notably, the immunosensor exhibits improved analytical performance with a 0.1-6.25 ng·mL-1 detection range and a 0.06 ng·mL-1 detection limit for quantitative saxitoxin (STX) analysis. The average recoveries of STX in real samples ranged from 85.9% to 105.9%. This study presents a more in-depth investigation of the recognition element performance, providing insights for improved antibody performance in practical applications.

Dual Plasmons with Bioinspired 3D Network Structure Enabling Ultrahigh Efficient Solar Steam Generation.

Plasmonic nanomaterials such as Au, Ag, and Cu are widely recognized for their strong light-matter interactions, making them promising photothermal materials for solar steam generation. However, their practical use in water evaporation is significantly limited by the trade-off between high costs and poor stability. In this regard, we introduce a novel, nonmetallic dual plasmonic TiN/MoO3-x composite. This composite features a three-dimensional, urchin-like biomimetic structure, with plasmonic TiN nanoparticles embedded within a network of plasmonic MoO3-x nanorods. As a solar absorber, the TiN/MoO3-x composite achieves a high evaporation rate of ∼2.05 kg m-2 h-1 with an energy efficiency up to 106.7% under 1 sun illumination, outperforming the state-of-the-art plasmonic systems. The high photothermal stability and unique dual plasmonic nanostructure of the TiN/MoO3-x composite are demonstrated by advanced in situ laser-heating transmission electron microscopy and photon-induced near-field electron microscopy/electron energy-loss spectroscopy, respectively. This work provides new inspiration for the design of plasmonic materials.

Aqueous Synthesis of Au10Pt1 Nanorods Decorated with MnO2 Nanosheets for the Enhanced Electrocatalytic Oxidation of Methanol.

Developing novel catalysts with high activity and high stability for the methanol oxidation reaction (MOR) is of great importance for the ever-broader applications of methanol fuel cells. Herein, we present a facile technique for synthesizing Au10Pt1@MnO2 catalysts using a wet chemical method and investigate their catalytic performance for the MOR. Notably, the Au10Pt1@MnO2-M composite demonstrated a significantly high peak mass activity of 15.52 A mg(Pt)-1, which is 35.3, 57.5, and 21.9 times greater than those of the Pt/C (0.44 A mg(Pt)-1), Pd/C (0.27 A mg(Pt)-1), and Au10Pt1 (0.71 A mg(Pt)-1) catalysts, respectively. Comparative analysis with commercial Pt/C and Pd/C catalysts, as well as Au10Pt1 HSNRs, revealed that the Au10Pt1@MnO2-M composite exhibited the lowest initial potential, the highest peak current density, and superior CO anti-poisoning capability. The results demonstrate that the introduction of MnO2 nanosheets, with excellent oxidation capability, not only significantly increases the reactive sites, but also promotes the reaction kinetics of the catalyst. Furthermore, the high surface area of the MnO2 nanosheets facilitates charge transfer and induces modifications in the electronic structure of the composite. This research provides a straightforward and effective strategy for the design of efficient electrocatalytic nanostructures for MOR applications.