Localized Surface Plasmon Resonance Features Research Articles

Optimizing and Quantifying Gold Nanospheres Based on LSPR Label-Free Biosensor for Dengue Diagnosis

The localized surface plasmon resonance (LSPR) due to light–particle interaction and its dependence on the surrounding medium have been widely manipulated for sensing applications. The sensing efficiency is governed by the refractive index-based sensitivity () and the full width half maximum (FWHM) of the LSPR spectra. Thereby, a sensor with high precision must possess both requisites: an effective and a narrow FWHM of plasmon spectrum. Moreover, complex nanostructures are used for molecular sensing applications due to their good values but without considering the wide-band nature of the LSPR spectrum, which decreases the detection limit of the plasmonic sensor. In this article, a novel, facile and label-free solution-based LSPR immunosensor was elaborated based upon LSPR features such as extinction spectrum and localized field enhancement. We used a 3D full-wave field analysis to evaluate the optical properties and to optimize the appropriate size of spherical-shaped gold nanoparticles (Au NPs). We found a change in Au NPs’ radius from 5 nm to 50 nm, and an increase in spectral resonance peak depicted as a red-shift from 520 nm to 552 nm. Using this fact, important parameters that can be attributed to the LSPR sensor performance, namely the molecular sensitivity, FWHM, , and figure of merit (FoM), were evaluated. Moreover, computational simulations were used to assess the optimized size (radius = 30 nm) of Au NPs with high FoM (2.3) and sharp FWHM (44 nm). On the evaluation of the platform as a label-free molecular sensor, Campbell’s model was performed, indicating an effective peak shift in the adsorption of the dielectric layer around the Au NP surface. For practical realization, we present an LSPR sensor platform for the identification of dengue NS1 antigens. The results present the system’s ability to identify dengue NS1 antigen concentrations with the limit of quantification measured to be 0.07 μg/mL (1.50 nM), evidence that the optimization approach used for the solution-based LSPR sensor provides a new paradigm for engineering immunosensor platforms.

Nanostructured materials with localized surface plasmon resonance for photocatalysis

Localized surface plasmon resonance (LSPR) enhanced photocatalysis has fascinated much interest and considerable efforts have been devoted toward the development of plasmonic photocatalysts. In the past decades, noble metal nanoparticles (Au and Ag) with LSPR feature have found wide applications in solar energy conversion. Numerous metal-based photocatalysts have been proposed including metal/semiconductor heterostructures and plasmonic bimetallic or multimetallic nanostructures. However, high cost and scarce reserve of noble metals largely limit their further practical use, which drives the focus gradually shift to low-cost and abundant nonmetallic nanostructures. Recently, various heavily doped semiconductors (such as WO3-x, MoO3-x, Cu2–xS, TiN) have emerged as potential alternatives to costly noble metals for efficient photocatalysis due to their strong LSPR property in visible-near infrared region. This review starts with a brief introduction to LSPR property and LSPR-enhanced photocatalysis, the following highlights recent advances of plasmonic photocatalysts from noble metal to semiconductor-based plasmonic nanostructures. Their synthesis methods and promising applicability in plasmon-driven photocatalytic reactions such as water splitting, CO2 reduction and pollution decomposition are also summarized in details. This review is expected to give guidelines for exploring more efficient plasmonic systems and provide a perspective on development of plasmonic photocatalysis.

Air-stable n-type Fe-doped ZnO colloidal nanocrystals.

The synthesis of Al and Fe codoped ZnO colloidal nanocrystals (NCs) using a modified etching-regrowth-doping method is presented. We show that the spectroscopic signatures associated with Fe3+ in ZnO disappear upon introduction of Al3+ donor defects into the ZnO lattice. The presence of Al3+ is confirmed by the appearance of a localized surface plasmon resonance feature indicating excess free carriers in the codoped NCs. These spectral changes suggest that Al3+ doping results in a reduction of Fe3+ dopants to the electron paramagnetic resonance-silent Fe2+ dopants that are stable under ambient conditions. These colloidal NCs provide a potential building block for manipulating magneto-optical properties and plasmon responses in colloidal NCs and higher-order nanostructures.

Mechanism of MgAl-LDH-induced enhancement in the plasmonic photocatalytic performance over bismuth nanospheres

The rapid development of industry, automobiles and manufacturing has emitted an ever-growth amount of the atmospheric gas pollutants to the atmosphere, such as nitric oxide (NO x ), sulfur oxide (SO x ), volatile organic compounds (VOC) and particle matters (PM). NO is deemed as one critical air pollutant as it is the leading origins of several environmental issues, such as the acidic rain, haze, and photochemical smog. To eliminate its environmental risk, it is highly desired to develop one effective, cost-effective and environmental-friendly technology to detect, regulate and remove them. Till now, various technologies have been developed, including the selective physical/chemical adsorption, heterogeneous catalytic reduction/oxidation and photocatalysis, to show high efficiency in separation, conversion and detoxification of NO. Among them, photocatalysis is receiving an ever-increased attention by its high efficiency, low cost and green feature (this technology enables the direct utilization of solar light to trigger the generation of highly active oxidative free radicals for pollutant degradation and mineralization). In the development of photocatalytic technology, the core part is the design and synthesis of highly effective and durable photocatalyst. Plasmonic metal with a unique localized surface plasmon resonance (LSPR) feature have now emerged as one appealing class of photocatalysts with wide applications in environmental remediation and energy conversion. In the plasmonic metal-directed photocatalysis, the surface electrons of plasmonic metal oscillate resonated with the incident photon, giving rise to the generation of hot carriers for catalytic redox reactions. However, most of the current LSPR-directed photocatalysis is limited to the noble metal (Au and Ag). Very recently, the earth-abundant bismuth metal (Bi) was confirmed to show a unique LSPR feature in visible light region, and more importantly, can serve as a direct plasmonic photocatalyst in ppb-level NO removal from a gas flow under UV illumination. However, bare Bi particles do not show enough redox capability as the hot carriers produced via LSPR in Bi metal cannot travel over distances longer than tens of nanometers. In this study, we attempt to introduce the MgAl-layered double hydroxide (MgAl-LDH) to intensify photocatalytic efficiency of the Bi metal. A facile liquid-phase ultrasound-assisted assembly approach was firstly used to synthesize the Bi@MgAl-LDH nanocomposite, in which Bi-NPs are uniformly supported on MgAl-LDH nanosheets. This composite can efficiently and steadily photocatalyze the removal of ppb-level NO from a continuous air flow under ultraviolet light irradiation via the plasmonic effect Bi metal. The removal efficiency reaches 43% for Bi@MgAl-LDH, much higher than the bare Bi-NPs (43%). The combined study of the composite phase/morphology/structure, its optical property, band structure and ESR oxidation radical capture experiments reveal that MgAl-LDH cannot form a heterojunction with Bi-NPs, and the enhanced photocatalysis on Bi@MgAl-LDH originates from the abundant hydroxide ions (OH−) on the surface of MgAl-LDH. These OH− can quickly capture the photoexcited holes to generate ·OH radicals, and simultaneously promote the electron-hole separation by consuming the holes, both of which can raise the utilization efficiency of the photoexcited hot carriers and provide sufficient oxidative radicals for NO oxidation. Additionally, MgAl-LDH has a unique water molecule memory effect, and the OH− that is consumed in photocatalysis can be replenished by adsorbing water from the air, leading to a stable photocatalytic performance. The reaction pathway study of the photocatalytic NO oxidation over Bi@MgAl-LDH by in-situ DRIFTS reveals that NO is completely mineralized into nitrate. The present work should contribute to a deeper understanding of the photocatalytic mechanism and then the design of robust catalysts for air purification.

Transient localized surface plasmon induced by femtosecond interband excitation in gold nanoparticles

Localized surface plasmon resonance (LSPR) is essentially a collective oscillation of free electrons in nanostructured metals. Interband excitation may also produce conduction-band electrons above the Fermi level. However, a question here is whether these excited electrons can take part in plasmonic oscillation. To answer this question, femtosecond pump-probe measurements on gold nanoparticles were performed using interband excitation, where the pump pulse produced a large amount of electrons in the sp-conduction band and left holes in the d-band. Probing by transient absorption spectroscopy, we resolved an induced LSPR feature located at a red-shifted spectrum. This feature cannot be observed for a pumping photon energy lower than the threshold for interband transition. The commonly observed red-shift or broadening of LSPR spectrum due to electron-electron and electron-phonon scattering under strong optical excitation can be ruled out for understanding this feature by a comparison between the plasmonic dynamics at a pump above and below the interband-transition threshold. In particular, a “holding” time of about 1 ps was resolved for the interband-excitation-induced electrons to relax to the LSPR oscillation.

One-step synthesis of graphene-Au nanoparticle hybrid materials from metal salt-loaded micelles

In this study, we present a facile one-step method to synthesize graphene-Au nanoparticle (NP) hybrid materials by using HAuCl4-loaded poly(styrene)-block-poly(2-vinylpyridine) (PS-P2VP) micelles as solid carbon sources. N-doped graphene with controllable thickness can be grown from PS-P2VP micelles covered by a Ni capping layer by an annealing process; simultaneously, the HAuCl4 in the micelles were reduced into Au NPs under a reductive atmosphere to form Au NPs on graphene. The decoration of Au NPs leads to an obviously enhanced electrical conductivity and a slightly increased work function of graphene due to the electron transfer effect. The graphene-Au NP hybrid materials also exhibit a localized surface plasmon resonance feature of Au NPs. This work provides a novel and accessible route for the one-step synthesis of graphene-Au NP hybrid materials with high quality, which might be useful for future applications in optoelectronic devices.

Photocatalytic H2 Production from Ethanol–Water Mixtures Over Pt/TiO2 and Au/TiO2 Photocatalysts: A Comparative Study

Pt/TiO2 (Pt loadings 0-4 wt%) and Au/TiO2 (Au loadings 0-4 wt%) photocatalysts were synthesized, char- acterized and tested for H2 production from ethanol-water mixtures (80 vol% ethanol, 20 vol% H2O) under UV exci- tation. Average metal nanoparticle sizes determined by TEM were 1-3 nm for Pt in the Pt/TiO2 photocatalysts and 5-7 nm for Au in the Au/TiO2 photocatalysts. Au/TiO2 showed an intense localized surface plasmon resonance feature at *570 nm, typical for metallic Au nanoparticles of diameter *5 nm supported on TiO2. X-ray photoelectron spectros- copy and X-ray diffraction analyses established that Pt and Au were present in metallic form on the TiO2 support. X-ray fluorescence revealed close accord between nominal and actual Pt and Au loadings. The Au/TiO2 and Pt/TiO2 phot- ocatalysts both displayed very high activities for H2 pro- duction under UV irradiation, with the Au/TiO2 samples affording slightly superior rates of H2 production at most metal loadings. The 2 wt% Au/TiO2 and 1 wt% Pt/TiO2 photocatalysts showed the highest H2 production rates (32-34 mmol g -1 h -1 ). Photoluminescence studies confirmed that Pt and Au nanoparticles positively enhance the photocatalytic properties of P25 TiO2 for H2 production by acting as electron acceptors and thereby suppressing electron-hole pair recombination in TiO2.

A displacement principle for mercury detection by optical waveguide and surface enhanced Raman spectroscopy

A novel displacement principle of metal nanoparticles for target analysis, differing from the usual target-induced aggregation principle, was proved feasible by the use of para-aminothiophenol coupled Au nanoparticles (PATP-Au) multilayer as probes to detect Hg2+. The PATP-Au multilayer was fabricated through layer-by-layer assembly of Au nanoparticles on optical waveguide (OWG) chip surface using para-aminothiophenol (PATP) as coupling molecules. The localized surface plasmon resonance (LSPR) extinction from Au nanoparticles and the PATP as a Raman reporter enable to easily capture changes in PATP-Au multilayer by OWG and of surface enhanced Raman spectroscopy, respectively. The introduction of the Hg2+, which has a higher binding affinity to the thiol group of PATP, greatly destroyed the multilayer structure, and produced a large change, several folds higher than the noise, in LSPR features and Raman signals of PATP-Au multilayer probes, and resulted in an excellent selectivity for Hg2+ detection at a low level of 1nM. This investigation provides us more ideas on the future development of surface analysis techniques for the detection of various target analytes.

Phenolic acid induced growth of gold nanoshells precursor composites and their application in antioxidant capacity assay

In the present work, the gold nanoshells (GNSs) precursor composites were preadsorbed onto the surface of ITO substrates. With the treatment of modified electrodes immersed in the gold nanoparticles (GNPs) growth solution containing different phenolic acids, the GNSs precursor composites were enlarged to varying degrees. Phenolic acids with one or more phenolic hydroxyl groups served as reductants for the growth of GNPs. The enlargement conditions varied with the different reducing capacity of phenolic acids, exhibiting specific morphologies differ from the complete GNSs. Consequently, the UV–vis–NIR spectra and cyclic voltammetry curves for the phenolic acid-treated ITO electrode were gradually changed. Results showed that the higher reducing capacity for phenolic acid to reduce AuCl 4 − to Au 0 resulted in the intensified localized surface plasmon resonance features and reduced cathodic currents. The spectral wavelength peaks red shifted hundreds of nanometers across the visible region. Moreover, the antioxidant capacity of phenolic acids correlates well with their reducing activity, both of which reflect their tendency to donate electrons. Thus, the optical and electrochemical results could be used to evaluate the antioxidant capacity of phenolic acids by utilizing GNSs precursor composites as nanoprobes. The method is simple, rapid and could be used in visual analysis to a certain extent.