Tunable Localized Surface Plasmon Resonance Research Articles

Ag–Ag2O composite structure with tunable localized surface plasmon resonance as ultrastable, sensitive and cost-effective SERS substrate

Abstract Ag–Ag2O composite structure, with fine tunability of localized surface plasmon resonance (LSPR), was fabricated by using Nd:YAG fiber pulsed laser ablation at ambient conditions. In this structure, tunable LSPR of Ag nanoparticles (NPs) with the wavelength from 384 nm to 486 nm was realized by varying the laser power to regulate both the oxidation degree and the spacing of Ag NPs. Moreover, Surface enhanced Raman scattering (SERS) was also observed due to the composite structure, which was demonstrated excellent stability and SERS sensitivity with the detectable concentration of Rhodamine B as low as 10−11 mol/L. The SERS substrates fabricated by laser ablation also exhibited favorable reproducibility and uniformity. The simulation results of finite difference time domain (FDTD) regarding the electric field distribution of all samples are in good agreement with those of above results.

Manipulating Bimetallic Nanostructures With Tunable Localized Surface Plasmon Resonance and Their Applications for Sensing.

Metal nanocrystals with well-controlled shape and unique localized surface plasmon resonance (LSPR) properties have attracted tremendous attention in both fundamental studies and applications. Compared with monometallic counterparts, bimetallic nanocrystals endow scientists with more opportunities to precisely tailor their LSPR and thus achieve excellent performances for various purposes. The aim of this mini review is to present the recent process in manipulating bimetallic nanostructures with tunable LSPR and their applications for sensing. We first highlight several significant strategies in controlling the elemental ratio and spatial arrangement of bimetallic nanocrystals, followed by discussing on the relationship between their composition/morphology and LSPR properties. We then focus on the plasmonic sensors based on the LSPR peak shift, which can be well-controlled by seed-mediated growth and selective etching. This review provides insights of understanding the “rules” involving in the formation of bimetallic nanocrystals with different structures and desired LSPR properties, and also forecasts the development directions of plasmonic sensors in the future.

Emerging charge transfer in self-coupled polymorphs for promoting charge-carrier-involved photocatalysis

Heterostructure construction has been regarded as a feasible pathway for nanomaterial fabrication towards optimized properties. Despite impressive investigations on heterostructure fabrication, assembling distinct polymorphs to form heterojunction has seldom been focused on relevant aspects. In this work, we demonstrated tunable localized surface plasmon resonance (LSPR) by Cu2S/CuS polymorphs via cation-triggered self-assembling on SnS2 substrates. During the transformation, the forming of covellite phase proceeds through cation exchange, while the simultaneous formation of chalcocite phase was achieved by the variation of the redox states of the anion framework. Furthermore, the formed polymorphs with optimized proportion on SnS2 substrates exhibited enhanced charge-carrier-involved processes by intrinsic band alignment in distinct polymorphs, resulting in excellent photocatalytic performances compared with those single-phase dominant copper sulfides on substrates. We thus hope this controllable nanostructure fabrication with distinctive properties could provide some useful hints for photo-electrochemistry applications.

Enhancing Galvanic Replacement in Plasmonic Hollow Nanoparticles: Understanding the Role of the Speciation of Metal Ion Precursors

AbstractHollow nanostructures offer great potential for plasmonic applications due to their strong and highly tunable localized surface plasmon resonance. The relationship between the plasmonic properties and geometry of hollow nanoparticles, such as core‐shell size ratio, concentricity of the cavity and porosity of the wall, is well documented. Nanoscale galvanic replacement provides a simple, versatile and powerful route for the preparation of such hollow structures. Here we demonstrate how the efficiency of reductant‐assisted galvanic replacement processes can be enhanced by controlling the degree of hydration and hydrolysis of the metal ion precursor using pH and pL as key control parameters (by analogy to pH, the letter p in the expression pL is used to indicate the decimal cologarithm associated with the concentration of the ligand L). Adjusting precursor speciation prior to the sacrificial template's hollowing process offers a new strategy to tune the morphology and optical properties of plasmonic hollow nanostructures.

Ultrathin Plasmonic Tungsten Oxide Quantum Wells with Controllable Free Carrier Densities.

Localized surface plasmon resonances (LSPR) of nanostructures can be tuned by controlling their morphology, local dielectric environment, and free carrier concentration. We report the colloidal synthesis of an ∼3 tungsten-oxygen (W-O) layer thick (∼1 nm), two-dimensional (2D) WO3-x nanoplatelets (NPLs) (x ≈ 0.55-1.03), which display tunable near-infrared LSPR properties and additionally high free electron density (Ne) that arises predominantly from the large shape factor of 2D NPLs. Importantly, the W to O composition ratios inferred from their LSPR measurements show much higher percentage of oxygen vacancies than those determined by X-ray diffraction analysis, suggesting that the aspect ratio of ultrathin WO3-x NPLs is the key to producing an unprecedentedly large Ne, although synthesis temperature is also an independent factor. We find that NPL formation is kinetically controlled, whereas thermodynamic parameter manipulation leads to Ne values as high as 4.13 × 1022 cm-3, which is close to that of plasmonic noble metals, and thus our oxide-based nanostructures can be considered as quasi-metallic. The unique structural properties of 2D nanomaterials along with the high Ne of WO3-x NPLs provide an attractive alternative to plasmonic noble metal nanostructures for various plasmon-driven energy conversions and design of photochromic nanodevices.

Tunable localized surface plasmon resonances in MoO3−x-TiO2 nanocomposites with enhanced catalytic activity for CO2 photoreduction under visible light

The photocatalytic reduction of CO2 with H2O to fuels and chemicals using solar energy is one of the most attractive but highly difficult routes. Thus far, only a very limited number of photocatalysts has been reported to be capable of catalyzing the photocatalytic reduction of CO2 under visible light. The utilization of the localized surface plasmon resonance (LSPR) phenomenon is an attractive strategy for developing visible-light photocatalysts. Herein, we have succeeded in synthesizing plasmonic MoO3−x-TiO2 nanocomposites with tunable LSPR by a simple solvothermal method. The well-structured nanocomposite containing two-dimensional (2D) molybdenum oxide (MoO3−x) nanosheets and one-dimensional (1D) titanium oxide nanotubes (TiO2-NT) showed LSPR absorption band in the visible-light region, and the incorporation of TiO2-NT significantly enhanced the LSPR absorption band. The MoO3−x-TiO2-NT nanocomposite is promising for application in the photocatalytic reduction of CO2 with H2O under visible light irradiation.

Cu5FeS4 Nanoparticles With Tunable Plasmon Resonances for Efficient Photothermal Therapy of Cancers.

Localized surface plasmon resonances (LSPRs) in heavily doped copper chalcogenides are unique because LSPR energy can be adjusted by adjusting doping or stoichiometry. However, there are few investigations on the LSPRs of bimetal copper-based chalcogenides. Herein, bimetal Cu5FeS4 (CFS) nanoparticles were synthesized by a facile hot injection of a molecular precursor. The tunable plasmon resonance absorption of CFS nanoparticles is achieved by the decrease of the ratio of copper to iron and the treatment of n-dodecylphosphoric acid (DDPA). After surface modification with polyethylene glycol (PEG), the CFS nanoparticles with a plasmon resonance absorption peak at 764 nm can serve as promising photothermal agents, showing good biocompatibility and excellent photothermal performance with a photothermal conversion efficiency of up to 50.5%, and are thus used for photothermal therapy of cancers under the irradiation of an 808-nm laser. Our work provides insight into bimetal copper-based chalcogenides to achieve tunable LSPRs, which opens up the possibility of rationally designing plasmonic bimetal copper-based chalcogenides.

Spectrally tunable infrared plasmonic F,Sn:In2O3 nanocrystal cubes.

A synthetic challenge in faceted metal oxide nanocrystals (NCs) is realizing tunable localized surface plasmon resonance (LSPR) near-field response in the infrared (IR). Cube-shaped nanoparticles of noble metals exhibit LSPR spectral tunability limited to visible spectral range. Here, we describe the colloidal synthesis of fluorine, tin codoped indium oxide (F,Sn:In2O3) NC cubes with tunable IR range LSPR for around 10 nm particle sizes. Free carrier concentration is tuned through controlled Sn dopant incorporation, where Sn is an aliovalent n-type dopant in the In2O3 lattice. F shapes the NC morphology into cubes by functioning as a surfactant on the {100} crystallographic facets. Cube shaped F,Sn:In2O3 NCs exhibit narrow, shape-dependent multimodal LSPR due to corner, edge, and face centered modes. Monolayer NC arrays are fabricated through a liquid-air interface assembly, further demonstrating tunable LSPR response as NC film nanocavities that can heighten near-field enhancement (NFE). The tunable F,Sn:In2O3 NC near-field is coupled with PbS quantum dots, via the Purcell effect. The detuning frequency between the nanocavity and exciton is varied, resulting in IR near-field dependent enhanced exciton lifetime decay. LSPR near-field tunability is directly visualized through IR range scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS). STEM-EELS mapping of the spatially confined near-field in the F,Sn:In2O3 NC array interparticle gap demonstrates elevated NFE tunability in the arrays.

Ultrasmall aqueous starch-capped CuS quantum dots with tunable localized surface plasmon resonance and composition for the selective and sensitive detection of mercury(ii) ions.

Ultrasmall starch-capped CuS quantum dots (QDs) with controllable size were chemically fabricated in an aqueous medium. The phase of the CuS QDs was confirmed via X-ray diffraction (XRD), whereas the characteristic localized surface plasmon resonance (LSPR) peak in the near-infrared (NIR) region was measured using UV-Vis spectroscopy. Transmission electron microscopy and high bandgap analysis confirmed the formation of ultrasmall CuS QDs in the size range of 4–8 nm. CuS QDs have been used for the selective and sensitive detection of Hg2+ ions through colorimetric and spectroscopic techniques. The selective sensing of Hg2+ ions from various metal ions was detected via a remarkable change in color, damping in LSPR intensity, significant change in the Fourier-transform infrared spectra and X-ray photoelectron spectroscopic measurements. The mechanism of interaction between the CuS QDs and Hg2+ ions has been deeply explored in terms of the role played by the starch and the reorganization of sulfide and disulfide bonds to facilitate the access of Hg2+ ions into the CuS lattice. Finally, an intermediate Cu2−xHgxS nanostructure resulted in the leaching of Cu+ ions into the solution, which were further recovered and reused for the formation of fluorescent Cu2S nanoparticles. Thus, the entire process of synthesis, sensing and reuse paves the way for sustainable nanotechnology.

Galvanic replacement synthesis of multi-branched gold nanocrystals for photothermal cancer therapy.

We present a facile organic phase synthesis method for producing multi-branched gold nanocrystals (nanostars) with a broad localized surface plasmon resonance (LSPR) across near-infrared (NIR) to short-wave infrared (SWIR) wavelengths. In this approach, galvanic replacement of copper by gold, in seed particles produced in situ, initiates growth of multi-branched structures. The method enables broad tuning of the LSPR energy by manipulating the branch length, with peak LSPR absorbance tuned from 850 to 1880 nm, reaching SWIR wavelengths covering the second and third optical transparency windows in biological media, rarely achieved with noble metal plasmonic nanostructures. After a ligand-exchange process, the gold nanostars readily disperse in water while retaining their LSPR absorbance. The multi-branched Au nanocrystals (NCs) exhibit exceptionally high photothermal transduction efficiency, exceeding that of Au nanorods and nanoparticles, to which we make direct comparisons here. At the same time, their synthesis is much more straightforward than hollow structures like nanocages, nanoshells, and nanomatryoshkas that can also exhibit high photothermal efficiency at NIR wavelengths. In vitro photothermal heating of multi-branched Au NCs in the presence of human cervical cancer cells causes effective cell ablation after 10 min laser irradiation. Cell viability assays demonstrate that the NCs are biocompatible at doses required for photothermal therapy. These results suggest that the multi-branched Au NCs can serve as a new type of photothermal therapy agent and in other applications in which strong NIR to SWIR absorbers are needed.

Tailoring Fluorescence and Singlet Oxygen Generation of a Chlorophyll Derivative and Gold Nanorods via a Silica Shell

Gold nanorods deserve special attention as they exhibit tunable longitudinal localized surface plasmon resonances (LSPRs). In our study, gold nanorods of the aspect ratio of 2.25 (maximum of LSPR b...

PH-regulated reversible photoluminescence and localized surface plasmon resonances arising from molybdenum oxide quantum dot

Localized surface plasmon resonances (LSPR) and photoluminescence are important for applications of transition-metal oxides. However, simultaneous control of these optical properties is challenging. Here, a facile strategy is presented that simultaneously tunes photoluminescence and visible LSPR of molybdenum oxide quantum dots (MoOx QDs) by controlling lattice vacancies. Specifically, N-doped MoOx QDs were prepared with a one-pot protocol. The introduction of N in MoOx QDs surfaces via ammonia (NH3) not only trapped oxygen molecules in the process of forming MoOx QDs, but also provided enough free electrons to enable tunable optical properties. Thus, a MoOx QD-based dual-modal fluorescence and LSPR assay was demonstrated via lattice vacancy concentration tuning. Upon introduction of H+ or OH−, pH-reversible tunability of the fluorescence and plasmonic resonance was observed. The dual-mode probe was used to detect extreme acidity in bacterial cells. Overall, tunable LSPR and photoluminescence within one nanostructure via pH-regulation should enable multi-modal signal-outputs for sensing platforms and photoelectric nanodevices.

Design of Aluminum Bowtie Nanoantenna Array with Geometrical Control to Tune LSPR from UV to Near-IR for Optical Sensing

Plasmonic nanoantennas have earned strong recognition for their unique capability to confine light from free space into sub-wavelength dimensions with strong electric field (E-field) enhancement factor due to localized surface plasmon resonance (LSPR). Broad spectral tuning of LSPR from ultraviolet (UV) to near-infrared (NIR) is required for incident light wavelength and material sensitive plasmonic applications in different spectral regions. In this article, we introduced and designed a novel aluminum plasmonic platform consisting of a bowtie nanoantenna (BNA) array with metal-insulator-metal (MIM) configuration where LSPR peak position was broadband tunable from UV to NIR through geometric control of antenna parameters. Furthermore, we designed and numerically analyzed a plasmonic biosensor platform that detected concentration of glycerol in de-ionized (DI) water with a concentration in the range of 0 to 40 wt% (refractive index = 1.333 to 1.368) with a sensitivity of 497 nm/RIU (refractive index units). The designed plasmonic platform can also be used as a surface-enhanced Raman scatting (SERS) substrate with enhancement factor as high as 4.82 × 109 for 1042 nm excitation wavelength. The reported hybrid dielectric-metallic plasmonic nanostructured system is a universal plasmonic platform for a wide range of applications including single-molecule SERS, biosensing, fluorescence microscopy, plasmonic nanocavity, nanolasers, and solid-state lighting.

Plasmon-Enhanced Oxidase-Like Activity and Cellular Effect of Pd-Coated Gold Nanorods.

Local surface plasmon resonance (LSPR)-enhanced catalysis has attracted much attention recently. Palladium nanoparticles have been reported to have various nanozyme activities and exhibit promising potentials for biomedical applications. However, as Pd is a poor plasmonic metal, little attention has been paid to its LSPR-regulated nanozyme activity. Herein, by using Au nanorods (AuNRs) as a strong plasmonic core, we coated a thin layer Pd to form a rod-shaped core-shell structure. The obtained Au@PdNRs showed tunable LSPR bands in the near-infrared (NIR) spectral range inheriting from the Au core and yet an exposed Pd surface for catalysis. The oxidase-like activity was investigated in the dark and upon SPR excitation. The plasmon-enhanced activity was observed and was mainly ascribed to the local photothermal effect. Finally, to enhance biocompatibility, mesoporous silica-coated nanorods were used to detect the oxidase-like activity in cells. After being endocytosed by cells, upon plasmon excitation, the oxidase activity of Au@PdNRs could be manifested and lead to higher cytotoxicity and depolarization of mitochondrial membrane potential. Our studies highlight the feasibility of regulating the nanozyme activity of plasmonic nanostructures using their unique NIR plasmonic features with spatiotemporal control.

Integration of Hybrid Plasmonic Au-BaTiO3 Metamaterial on Silicon Substrates.

Silicon integration of nanoscale metamaterials is a crucial step toward low-cost and scalable optical-based integrated circuits. Here, a self-assembled epitaxial Au-BaTiO3 (Au-BTO) hybrid metamaterial with highly anisotropic optical properties has been integrated on Si substrates. A thin buffer layer stack (<20 nm) of TiN and SrTiO3 (STO) was applied on Si substrates to ensure the epitaxial growth of the Au-BTO hybrid films. Detailed phase composition and microstructural analyses show excellent crystallinity and epitaxial quality of the Au-BTO films. By varying the film growth conditions, the density and dimension of the Au nanopillars can be tuned effectively, leading to highly tailorable optical properties including tunable localized surface plasmon resonance (LSPR) peak and hyperbolic dispersion shift in the visible and near-infrared regimes. The work highlights the feasibility of integrating epitaxial hybrid oxide-metal plasmonic metamaterials on Si toward future complex Si-based integrated photonics.

Tunable localized surface plasmon resonance by self-assembly of trimetallic and bimetallic alloy nanoparticles via Ag sublimation from Ag/Au/Pt tri-layers

In this work, various configurations, size, density and composition of AgAuPt and AuPt alloy NPs are demonstrated via the solid-state dewetting (SSD) of Ag/Au/Pt tri-layers on the transparent c-plane sapphire (0001) and the corresponding LSPR characteristics are thoroughly investigated along with the FDTD simulation. The SSD is adapted to convert the sputtered Ag/Au/Pt tri-layers into the definite alloy NPs based on the diffusion, interdiffusion and energy minimization process. The resulting AgAuPt and AuPt NPs demonstrate much stronger plasmonic characteristics as compared to their counterparts with the tunable LSPR bands in the UV and VIS regions along with the various plasmon resonance modes such as dipolar (DR), quadrupolar (QR), multipolar (MR) and higher order (HO). Furthermore, the Ag atom sublimation demonstrates a significant role in the dewetting process, which significantly alters the size, shape and composition of alloy NPs, giving a rise to the development of AuPt NPs. In specific, the LSPR response attenuates with the sublimation, however, as the AuPt NPs in this study are significantly improved in terms of composition and configuration, i.e. shape, size and spacing, the LSPR responses are much stronger and dynamic as compared to the pure Pt NPs on sapphire in the previous studies.

Tailoring thermo-optical properties of eosin B dye using surfactant-free gold-silver alloy nanoparticles.

Surfactant free gold, silver and gold-silver alloy nanoparticles were synthesized using a laser mediated method for localized surface plasmon resonance tuning. The effect of these nanoparticles on the thermo-optical properties of eosin B dye was investigated. Dual beam mode matched thermal lens method was implemented to evaluate the thermal diffusivity of the eosin B with gold, silver and gold-silver alloy nanoparticles. Concentration and composition dependant changes in thermo-optical properties of the eosin B-nanoparticle systems were quantified. As the concentration of nanoparticles incorporated into the dye solution increased, the thermal diffusivity and fluorescence emission intensity of the samples were found to be decreased. At the same time an enhancement of the thermal lens signal was observed with the introduction of nanoparticles into the system. Further enhancement in signal and reduction in thermal diffusivity and fluorescence intensity can be obtained with fine tuning of surface plasmon resonance wavelength by gold, silver and gold-silver alloy nanoparticles.

Improved localized surface plasmon resonance responses of multi-metallic Ag/Pt/Au/Pd nanostructures: systematic study on the fabrication mechanism and localized surface plasmon resonance properties by solid-state dewetting

Multi-metallic nanoparticles (NPs) can offer dynamic and tunable localized surface plasmon resonance (LSPR) properties that are suitable for various catalysis, sensing and energy harvesting applications due to the wide range of tunability and applicability. In this work, the systematic fabrication and improved LSPR characteristics of multi-metallic alloy NP arrays are demonstrated based on the solid-state dewetting (SSD) of multi-layers of Ag/Pt/Au/Pd on sapphire (0001). The evolution of surface NPs in terms of configurational and elemental specifications yields vary strong and dynamic LSPR bands in the UV and VIS wavelengths based on the excitation of various plasmonic modes, i.e. dipolar (DR), quadrupolar (QR), multipolar (MR) and higher order (HO) bands, which is further exploited by the finite difference time domain simulations. Through the systematic control of multi-layer thickness, layer ratio and growth conditions, various nanostructures such as voided nanoclusters, network-like NPs and isolated semispherical NPs are obtained, which are unique in terms of morphology and elemental composition at each stage of dewetting process. The growth mechanism of multi-metallic alloy NP arrays is proposed based on the temperature driven thermal diffusion, alloying, Rayleigh-like instability and energy minimization mechanisms. Due to the subsequent sublimation of Ag atoms at above 650 °C, a sharp alteration in the elemental and morphological characteristics is demonstrated. In specific, the high percentage of Ag alloy NPs exhibits strong LSPR bands and gradually weakened along with the Ag sublimation. At the same time, however, the alloy or mono-metallic NPs without Ag still demonstrate much stronger LSPR bands as compared to the monometallic NPs by the SSD of pure films.

Tailored Engineering of Bimetallic Plasmonic Au@Ag Core@Shell Nanoparticles.

A distinctive synthetic method for the efficient synthesis of multifunctional bimetallic plasmonic Au@Ag core@shell nanoparticles (NPs) with tunable size, morphology, and localized surface plasmon resonance (LSPR) using Triton X-100/hexanol-1/deionized water/cyclohexane-based water-in-oil (W/O) microemulsion (ME) is described. The W/O ME acted as a “true nanoreactor” for the synthesis of Au@Ag core@shell NPs by providing a confined and controlled environment and suppressing the nucleation, growth, agglomeration, and aggregation of the NPs. High-resolution transmission electron microscopic analysis of the synthesized Au@Ag core@shell NPs revealed an “unusual core@shell” contrast, and the selected area electron diffraction and Moiré patterns showed that Au layers are paralleled to Ag layers, thus indicating the formation of Au@Ag core@shell NPs. Interestingly, the UV–visible spectrum of the Au@Ag core@shell NPs exhibited enthralling plasmonic properties by introducing a high-frequency quadrupolar LSPR mode originated from the isolated Au@Ag NPs along with a low-frequency dipolar LSPR mode originated from the coupled Au@Ag NPs. The effective plasmonic enhancement of the Au@Ag core@shell NPs is attributed to the extreme enhancement of the localized electromagnetic field by coupling of the localized surface plasmons of the Au core and Ag shell. The mechanisms for the nucleation and growth of Au@Ag core@shell NPs in W/O ME have been proposed. A unique electron transfer phenomenon between the Au core and Ag shell is elucidated for better understanding and manipulation of the electronic properties, which evinced the development of Au@Ag core@shell NPs through suppression of the galvanic replacement reaction.

Systematic investigation on quad-metallic AgAuPdPt and tri-metallic AuPdPt NPs through the solid-state dewetting of quad-layer Ag/Au/Pd/Pt thin films on c-plane sapphire.

Multi-metallic alloy nanoparticles (MNPs) can offer valuable opportunities to meet the various demands of applications. MNPs consist of various noble metallic elements can combine diverse electronic, optical and catalytic properties in a single NP configuration, thus taking the advantage of each element. In this paper, the fabrication of tri- and quad- metallic alloy NPs with noble elements (Ag, Au, Pd and Pt) and the corresponding localized surface plasmon resonance (LPSR) properties are systematically demonstrated. Tri- and quad-metallic alloy NPs come in various size and configurations by the solid-state dewetting of Ag/Au/Pd/Pt quad-layers on sapphire (0001). Tri-metallic AuPdPt NPs are demonstrated by the systematic control of growth temperature along with the significant Ag atom sublimation. Strongly enhanced and tunable LPSR is exerted in the UV-VIS regions depending upon the size, configuration, spacing and elemental composition of the MNPs. The size dependent LSPR response of MNPs is discussed based on the absorption and scattering along with the excitation of dipolar, quadrupolar, high order and multipolar resonance modes. The MNPs exhibit much stronger and dynamic LSPR bands as compared with the monometallic Pt and Pd NPs with the comparable size and configurations.