Tunable localized surface plasmon resonances in tungsten oxide nanocrystals

Transition-metal oxide nanocrystals are interesting candidates for localized surface plasmon resonance hosts because they exhibit fascinating properties arising from the unique character of their outer-d valence electrons. WO(3-δ) nanoparticles are known to have intense visible and near-IR absorption, but the origin of the optical absorption has remained unclear. Here we demonstrate that metallic phases of WO(3-δ) nanoparticles exhibit a strong and tunable localized surface plasmon resonance, which opens up the possibility of rationally designing plasmonic tungsten oxide nanoparticles for light harvesting, bioimaging, and sensing.     

Recent developments and applications of hybrid surface plasmon resonance interfaces in optical sensing

Nanostructured noble metals exhibit an intense optical near field due to surface plasmon resonance, therefore promising widespread applications and being of interest to a broad spectrum of scientists, ranging from physicists, chemists, and materials scientists to biologists. A wealth of research is available discussing the synthesis, characterization, and application of noble metal nanoparticles in optical sensing. However, with respect to the sensitivity of the frequency and width of these surface plasmon resonance modes to the particle’s shape, size, and environment, in nearly every case, success strongly depends on the availability of highly stable, adhesive, and sensitive nanoparticles. This undoubtedly presents a challenging task to nanofabrication. The past decade has witnessed fascinating advances in this field, in particular, the construction of oxide-based hybrid plasmonic interfaces to overcome the problem addressed above by (1) coating the metallic nanostructures with thin overlayers to form sandwiched structures or (2) embedding metallic nanostructures in a dielectric matrix to obtain metal/dielectric matrix nanocomposite films. In this critical review, we focus on recent work related to this field, beginning with a presentation of hybrid films with enhanced structural and optical stability, readily and selectively designed using chemical and physical techniques. We then illustrate their interesting optical properties and demonstrate exciting evidence for the postulated application in surface plasmon sensing fields. Finally, we survey the work remaining to be done for that potential to be realized.     

Enhancement of Nonlinear Optical Scattering by Gold Nanoparticles through Aggregation-Induced Plasmon Coupling in the Near-Infrared

Gold nanoparticles (AuNPs) are regarded as promising building blocks in functional nanomaterials for sensing, drug delivery and catalysis. One remarkable property of these particles is the localized surface plasmon resonance (LSPR), which gives rise to augmented optical properties through local field enhancement. LSPR also influences the nonlinear optical properties of metal NPs (MNPs) making them potentially interesting candidates for fast, high resolution nonlinear optical imaging. In this work we characterize and discuss the wavelength dependence of the hyper-Rayleigh scattering (HRS) behavior of spherical gold nanoparticles (GNP) and gold nanorods (GNR) in solution, from 850 nm up to 1300 nm, covering the near-infrared (NIR) window relevant for deep tissue imaging. The high-resolution spectral data allows discriminating between HRS and two photon photoluminescence contributions. Upon particle aggregation, we measured very large enhancements (ca. 10⁴) of the HRS intensity in the NIR, which is explained by considering aggregation-induced plasmon coupling effects and local field enhancement. These results indicate that purposely designed coupled nanostructures could prove advantageous for nonlinear optical imaging and biosensing applications.     

Cyclodextrin-functionalised gold nanoparticles via streptavidin: a supramolecular approach

Biofunctionalised nanoparticles (NPs) have received increased attention both for their potential use as drug carriers and imaging agents and for their applications in medical diagnostics. Functionalised gold nanoparticles (AuNPs) bring together their unique electronic and optical properties (including strong plasmon absorption bands and enhanced light scattering) with the specific capabilities of the functionalising biological molecule. Cyclodextrins (CDs) have been used to functionalise NPs with different approaches. CDs are able to protect from physical, chemical and enzymatic degradation drugs that are included in their cavity. In this study, we report on a new supramolecular approach for the fabrication of CD-functionalised AuNPs. Particularly, we synthesised streptavidin (SA)-coated NPs modified with biotinylated β- and γ-CD, in order to exploit the interaction with SA.     

Sensing using localised surface plasmon resonance sensors

The bright colours of noble metal particles have attracted considerable interest since historical times, where they were used as decorative pigments in stained glass windows. More recently, the tuneable optical properties of metal nanoparticles and their addressability via spectroscopic techniques have brought them back into the forefront of fundamental and applied research fields. Much of the recent attention concerning metal nanoparticles such as gold and silver has been their use as small-volume, ultra-sensitive label-free optical sensors. Plasmonic nanoparticles act in this case as transducers that convert changes in the local refractive index into spectral shifts of the localized surface plasmon resonance (LSPR) band. This LSPR-shift assay is a general technique for measuring binding affinities and rates from any molecule that induces a change in the local refractive index around the metallic nanostructures. By attaching molecular recognition elements (chemical or biological ligands) on the nanostructures, specificity and selectivity to the analyte of interest are introduced into the nanosensor. In this review, we will discuss the different methods used to fabricate plasmonic nanosensors. A special emphasis will be given to techniques used to link plasmonic nanostructures to surfaces. While the difference between colorimetric and refractive index sensing approaches will be briefly described, the importance to distinguish between bulk refractive index (RI) sensing and molecular near-field refractive index sensing will be discussed. The recent progress made in the development of novel surface functionalization strategies together with the formation of optically and mechanically stable LSPR sensors will be highlighted.     

Highly sensitive optical detection of humidity on polymer/metal nanoparticle hybrid films

Porous metal films for optical humidity sensing were prepared from copper nanoparticles protected by a 2-3 nm carbon coating, a silicon tenside, and a polymeric wetting agent. Exposure to water or solvent vapor revealed an exceptional sensitivity with optical shifts in the visible light range of up to 50 nm for a change of 1% in relative humidity. These properties could be attributed to a combination of surface plasmon resonance effects at low humidity and thin film interference at higher water or solvent concentration in the surrounding air. The simple concept and use of ultra-low-cost materials suggests application of such porous metal-film-based optical humidity sensors in large-scale applications for food handling, storage, and transport.     

Plasmonic nanoprobes for intracellular sensing and imaging

Recent advances in integrating nanotechnology and optical microscopy offer great potential in intracellular applications with improved molecular information and higher resolution. Continuous efforts in designing nanoparticles with strong and tunable plasmon resonance have led to new developments in biosensing and bioimaging, using surface-enhanced Raman scattering and two-photon photoluminescence. We provide an overview of the nanoprobe design updates, such as controlling the nanoparticle shape for optimal plasmon peak position; optical sensing and imaging strategies for intracellular nanoparticle detection; and addressing practical challenges in cellular applications of nanoprobes, including the use of targeting agents and control of nanoparticle aggregation.

Gold and silver nanoparticles in sensing and imaging: sensitivity of plasmon response to size, shape, and metal composition

Plasmonic metal nanoparticles have great potential for chemical and biological sensor applications, due to their sensitive spectral response to the local environment of the nanoparticle surface and ease of monitoring the light signal due to their strong scattering or absorption. In this work, we investigated the dependence of the sensitivity of the surface plasmon resonance (frequency and bandwidth) response to changes in their surrounding environment and the relative contribution of optical scattering to the total extinction, on the size and shape of nanorods and the type of metal, that is, Au vs Ag. Theoretical consideration on the surface plasmon resonance condition revealed that the spectral sensitivity, defined as the relative shift in resonance wavelength with respect to the refractive index change of surrounding materials, has two controlling factors: first the bulk plasma wavelength, a property dependent on the metal type, and second on the aspect ratio of the nanorods which is a geometrical parameter. It is found that the sensitivity is linearly proportional to both these factors. To quantitatively examine the dependence of the spectral sensitivity on the nanorod metal composition and the aspect ratio, the discrete dipole approximation method was used for the calculation of optical spectra of Ag-Au alloy metal nanorods as a function of Ag concentration. It is observed that the sensitivity does not depend on the type of the metal but depends largely on the aspect ratio of nanorods. The direct dependence of the sensitivity on the aspect ratio becomes more prominent as the size of nanorods becomes larger. However, the use of larger nanoparticles may induce an excessive broadening of the resonance spectrum due to an increase in the contribution of multipolar excitations. This restricts the sensing resolution. The insensitivity of the plasmon response to the metal composition is attributable to the fact that the bulk plasma frequency of the metal, which determines the spectral dispersion of the real dielectric function of metals and the surface plasmon resonance condition, has a similar value for the noble metals. On the other hand, nanorods with higher Ag concentration show a great enhancement in magnitude and sharpness of the plasmon resonance band, which gives better sensing resolution despite similar plasmon response. Furthermore, Ag nanorods have an additional advantage as better scatterers compared with Au nanorods of the same size.     

Manipulating the optical properties of pyramidal nanoparticle arrays

This paper reports the orientation-dependent optical properties of two-dimensional arrays of anisotropic metallic nanoparticles. These studies were made possible by our simple procedure to encapsulate and manipulate aligned particles having complex three-dimensional (3D) shapes inside a uniform dielectric environment. Using dark field or scattering spectroscopy, we investigated the plasmon resonances of 250-nm Au pyramidal shells embedded in a poly(dimethylsiloxane) (PDMS) matrix. Interestingly, we discovered that the scattering spectra of these particle arrays depended sensitively on the direction and polarization of the incident white light relative to the orientation of the pyramidal shells. Theoretical calculations using the discrete dipole approximation support the experimentally observed dependence on particle orientation with respect to incident field. This work presents an approach to manipulate--by hand--ordered arrays of particles over cm(2) areas and provides new insight into the relationship between the shape of well-defined, 3D particles and their supported plasmon resonance modes.     

Anisotropic metal nanoparticles: Synthesis, assembly, and optical applications

This feature article highlights work from the authors' laboratories on the synthesis, assembly, reactivity, and optical applications of metallic nanoparticles of nonspherical shape, especially nanorods. The synthesis is a seed-mediated growth procedure, in which metal salts are reduced initially with a strong reducing agent, in water, to produce approximately 4 nm seed particles. Subsequent reduction of more metal salt with a weak reducing agent, in the presence of structure-directing additives, leads to the controlled formation of nanorods of specified aspect ratio and can also yield other shapes of nanoparticles (stars, tetrapods, blocks, cubes, etc.). Variations in reaction conditions and crystallographic analysis of gold nanorods have led to insight into the growth mechanism of these materials. Assembly of nanorods can be driven by simple evaporation from solution or by rational design with molecular-scale connectors. Short nanorods appear to be more chemically reactive than long nanorods. Finally, optical applications in sensing and imaging, which take advantage of the visible light absorption and scattering properties of the nanorods, are discussed.     

Observation of spectral anisotropy of gold nanoparticles

Metallic nanoparticles synthesized by solution-phase chemistry usually exhibit various polygonal morphologies. The shape is known to have a great impact on a nanoparticle's optical properties, for instance, the surface plasmon resonance frequency. It remains unclear, however, whether the scattering spectrum of nanoparticles is generally anisotropic in the far field as a result. This simple question turns out to be extremely challenging to address because of the particle-to-particle shape inhomogeneity in a bulk sample, and the high sensitivity of surface plasmon resonance to local environments. We report the observation of scattering angle-dependent spectra using a newly developed single-particle tracking spectroscopy (SPS). Furthermore, we show that SPS has provided a way to directly visualize the rotational random walk of individual gold nanoparticles in water for the first time.     

Calculated absorption and scattering properties of gold nanoparticles of different size, shape, and composition: applications in biological imaging and biomedicine

The selection of nanoparticles for achieving efficient contrast for biological and cell imaging applications, as well as for photothermal therapeutic applications, is based on the optical properties of the nanoparticles. We use Mie theory and discrete dipole approximation method to calculate absorption and scattering efficiencies and optical resonance wavelengths for three commonly used classes of nanoparticles: gold nanospheres, silica-gold nanoshells, and gold nanorods. The calculated spectra clearly reflect the well-known dependence of nanoparticle optical properties viz. the resonance wavelength, the extinction cross-section, and the ratio of scattering to absorption, on the nanoparticle dimensions. A systematic quantitative study of the various trends is presented. By increasing the size of gold nanospheres from 20 to 80 nm, the magnitude of extinction as well as the relative contribution of scattering to the extinction rapidly increases. Gold nanospheres in the size range commonly employed ( approximately 40 nm) show an absorption cross-section 5 orders higher than conventional absorbing dyes, while the magnitude of light scattering by 80-nm gold nanospheres is 5 orders higher than the light emission from strongly fluorescing dyes. The variation in the plasmon wavelength maximum of nanospheres, i.e., from approximately 520 to 550 nm, is however too limited to be useful for in vivo applications. Gold nanoshells are found to have optical cross-sections comparable to and even higher than the nanospheres. Additionally, their optical resonances lie favorably in the near-infrared region. The resonance wavelength can be rapidly increased by either increasing the total nanoshell size or increasing the ratio of the core-to-shell radius. The total extinction of nanoshells shows a linear dependence on their total size, however, it is independent of the core/shell radius ratio. The relative scattering contribution to the extinction can be rapidly increased by increasing the nanoshell size or decreasing the ratio of the core/shell radius. Gold nanorods show optical cross-sections comparable to nanospheres and nanoshells, however, at much smaller effective size. Their optical resonance can be linearly tuned across the near-infrared region by changing either the effective size or the aspect ratio of the nanorods. The total extinction as well as the relative scattering contribution increases rapidly with the effective size, however, they are independent of the aspect ratio. To compare the effectiveness of nanoparticles of different sizes for real biomedical applications, size-normalized optical cross-sections or per micron coefficients are calculated. Gold nanorods show per micron absorption and scattering coefficients that are an order of magnitude higher than those for nanoshells and nanospheres. While nanorods with a higher aspect ratio along with a smaller effective radius are the best photoabsorbing nanoparticles, the highest scattering contrast for imaging applications is obtained from nanorods of high aspect ratio with a larger effective radius.     

Full-color tuning of surface plasmon resonance by compositional variation of Au@Ag core-shell nanocubes with sulfides

In the present study, we demonstrate the precise tuning of surface plasmon resonance over the full visible range by compositional variation of the nanoparticles. The addition of sulfide ions into the Au@Ag core-shell nanocubes generates stable Au@Ag/Ag(2)S core-shell nanoparticles at room temperature, and the plasmon extinction maximum shifts to the longer wavelength covering the entire visible range of 500-750 nm. Based on the optical property, the Au@Ag core-shell nanocubes are employed as a colorimetric sensing framework for sulfide detection in water. The detection limit is measured to be 10 ppb by UV-vis spectroscopy and 200 ppb by naked eyes. Such nanoparticles would be useful for decoration and sensing purposes, due to their precise color tunability and high stability.     

Electromagnetic interactions with rough metal surfaces

When surfaces are structured on the scale of the wavelength, we can expect incident light to be strongly modified by the surface. This is especially the case when the surface is metallic. We have developed a formalism for computing these modifications, closely analogous to electron scattering theory, which we briefly review and present some results for optical properties of, and electron energy loss in, colloids. Our main theme is another effect associate with rough or structured metallic surfaces: Surface Enhanced Raman Scattering, or SERS. We model the rough surface by a periodic array of spheres and obtain the correct magnitude for the enhancement and for the frequency shifts observed.     

AuNPs/PNIPAM复合颗粒的制备及其温敏性质

将金纳米颗粒(AuNPs)组装到聚N-异丙基丙烯酰胺(PNIPAM)水凝胶微球表面制备出AuNPs/PNIPAM复合颗粒. 将PNIPAM 凝胶的温敏特性与AuNPs的光学性质结合, 通过改变温度调节AuNPs的局部表面等离子共振(LSPR)吸收峰位置. 研究结果表明, 温度升高使AuNPs的LSPR吸收峰发生红移, 并且这种效应是可逆的. 同时发现, AuNPs的光学性质还可以作为表征PNIPAM水凝胶微球温敏行为的一种手段. 利用透射电镜、紫外-可见光谱仪及动态光散射仪对AuNPs/PNIPAM复合颗粒的形貌、光学性质、粒径变化等进行了分析.     

Aerosol assisted chemical vapor deposition using nanoparticle precursors: a route to nanocomposite thin films

Gold nanoparticle and gold/semiconductor nanocomposite thin films have been deposited using aerosol assisted chemical vapor deposition (CVD). A preformed gold colloid in toluene was used as a precursor to deposit gold films onto silica glass. These nanoparticle films showed the characteristic plasmon absorption of Au nanoparticles at 537 nm, and scanning electron microscopic (SEM) imaging confirmed the presence of individual gold particles. Nanocomposite films were deposited from the colloid concurrently with conventional CVD precursors. A film of gold particles in a host tungsten oxide matrix resulted from co-deposition with [W(OPh)(6)], while gold particles in a host titania matrix resulted from co-deposition with [Ti(O(i)Pr)(4)]. The density of Au nanoparticles within the film could be varied by changing the Au colloid concentration in the original precursor solution. Titania/gold composite films were intensely colored and showed dichromism: blue in transmitted light and red in reflected light. They showed metal-like reflection spectra and plasmon absorption. X-ray photoelectron spectroscopy and energy-dispersive X-ray analysis confirmed the presence of metallic gold, and SEM imaging showed individual Au nanoparticles embedded in the films. X-ray diffraction detected crystalline gold in the composite films. This CVD technique can be readily extended to produce other nanocomposite films by varying the colloids and precursors used, and it offers a rapid, convenient route to nanoparticle and nanocomposite thin films.     

Assembly of gold nanoparticles mediated by multifunctional fullerenes

The understanding of the interparticle interactions of nanocomposite structures assembled using molecularly capped metal nanoparticles and macromolecular mediators as building blocks is essential for exploring the fine-tunable interparticle spatial and macromolecular properties. This paper reports the results of an investigation of the chemically tunable multifunctional interactions between fullerenes (1-(4-methyl)-piperazinyl fullerene, MPF) and gold nanoparticles. The interparticle spatial properties are defined by the macromolecular and multifunctional electrostatic interactions between the negatively charged nanoparticles and the positively charged fullerenes. In addition to characterization of the morphological properties, the surface plasmon resonance band, dynamic light scattering, and surface-enhanced Raman scattering (SERS) properties of the MPF-mediated assembly and disassembly processes have been determined. The change of the optical properties depends on the pH and electrolyte concentrations. The detection of the Raman-active vibration modes (Ag(2) and Hg(8)) of C60 and the determination of their particle size dependence have demonstrated that the adsorption of MPF on the nanoparticle surface in the MPF-Au nm assembly is responsible for the SERS effect. These findings provide new insights into the delineation between the interparticle interactions and the nanostructural properties for potential applications of the nanocomposite materials in spectroscopic and optical sensors and in controlled releases.     

Light scattering investigations on mercury ion induced amalgamation of gold nanoparticles in aqueous medium

With the aids of SEM,XPS measurements,localized plasmon resonance light scattering(PRLS) spectrometry and light scattering imaging,investigations on the amalgamation process of both cetyltrimethylammonium bromide(CTAB) and citrate-coated gold nanoparticles(AuNPs) in the presence of Hg2+ showed that the Au-Hg amalgam process of gold nanoparticles is surface coating dependent in aqueous medium,and the scattering light color change of AuNPs under a dark-field microscope is blue-shifted from red-orange into yellow-orange or even yellow.The former one involves the reduction of Hg2+ to Hg0 species and adsorption of Hg0 on the surfaces of AuNPs,while the later one indicates the shape-evolution of AuNPs.     

Surface-enhanced resonance Raman scattering and background light emission coupled with plasmon of single Ag nanoaggregates

We investigated the optical properties of isolated single aggregates of Ag nanoparticles (Ag nanoaggregates) on which rhodamine 6G molecules were adsorbed to reveal experimentally a correlation among plasmon resonance Rayleigh scattering, surface-enhanced resonance Raman scattering (SERRS), and its background light emission. From the lack of excitation-laser energy dependence of background emission maxima we concluded that the background emission is luminescence, not Raman scattering. The polarization dependence of both SERRS and background emission was the same as that of the lowest-energy plasmon resonance maxima, which is associated with a longitudinal plasmon. From the common polarization dependence, we identified that the lowest-energy plasmon is coupled with both SERRS and background emission. In addition, we revealed that the lowest-energy plasmon with a higher quality factor (Q factor) yields larger SERRS and background emission intensity. Also, we identified that the Q factor dependence of the SERRS intensity was similar to that of the background emission intensity. This similarity directly supported us to demonstrate an enhancement of both SERRS and background emission by coupling with a common plasmon radiative mode.     

单个等离子体纳米颗粒在生化分析和生物成像中的应用

等离子体纳米颗粒（PNPs）因其独特的物理、化学、光学和生物学特性而被广泛地应用于材料科学、生物学和医药学等研究领域。PNPs的光学性质是可以通过改变其组成、形状和大小来进行调控的,所以利用可控合成的方式能够筛选出适合的光散射探针。在单分子水平上实时研究PNPs的动态行为对于理解细胞及活体组织的生命活动机制、制备功能型纳米材料和开发新型化学生物传感器等有着重要的意义。基于传统的暗场显微镜（DFM）,通过对光源、检测器及其它光学元件的择优组装和调试,我们开发出了一系列具有高灵敏度、高时空分辨率和高通量的等离子体光散射成像技术,并将其应用于单分子检测、多颗粒传感、单细胞成像以及生物过程示踪等领域。基于具有光学各向异性的PNPs,我们还研制出了活细胞三维扫描成像系统和超连续激光光片成像与高速毛细管电泳联用系统,推进了单分子光谱方面的研究。本文将总结近十年来本课题组在PNP单颗粒分析及成像中的工作,并为该领域未来的发展提出一些新的思路。