Dense two-dimensional silver single and double nanoparticle arrays with plasmonic response in wide spectral range

We report the properties of plasmons in dense planar arrays of silver single and double nanostructures with various geometries fabricated by electron beam lithography (EBL) as a function of their size and spacing. We demonstrate a strong plasmon coupling mechanism due to near-field dipolar interactions between adjacent nanostructures, which produces a major red shift of the localized surface plasmon resonance (LSPR) in silver nanoparticles and leads to strong maximum electric field enhancements in a broad spectral range. The extinction spectra and maximum electric field enhancements are theoretically modeled by using the finite element method. Our modeling revealed that strong averaged electric field enhancements of up to 60 in visible range and up to 40 in mid-infrared result from hybridization of multipolar resonances in such dense nanostructures; these are important for applications in surface enhanced spectroscopies.     

Confined plasmons in nanofabricated single silver particle pairs: experimental observations of strong interparticle interactions

We report on the optical properties of single isolated silver nanodisks and pairs of disks fabricated by electron beam lithography. By systematically varying the disk size and surface separation and recording elastic scattering spectra in different polarization configurations, we found evidence for extremely strong interparticle interactions. The dipolar surface plasmon resonance for polarization parallel to the dimer axis exhibited a red shift as the interdimer separation was decreased; as expected from previous work, an extremely strong shift was observed. The scattering spectra of single particles and pairs separated by more than one particle radius can be well described by the coupled dipole approximation (CDA), where the particles are approximated as point dipoles using a modified dipole polarizability for oblate spheroids. For smaller particle separations (d < 20 nm), the simple dipole model severely underestimates the particle interaction, indicating the importance of multipolar fields and finite-size effects. The discrete dipole approximation (DDA), which is a finite-element method, describes the experimental results well even at d < 20 nm, including particles that have metallic bridges.     

The size dependent shift of the surface plasmon absorption band of small spherical metal particles

It has been previously established that the surface plasmon of small spherical silver particles, which are embedded in a noble gas matrix, shifts to higher energies (blue shift) as the mean diameterD of the particles decreases (100 Å>D>20 Å). This blue shift has also been found for supported silver particles, and quite recently we observed it by elastic light scattering in the gas phase. This latter experiment proves unambiguously that the blue shift in small silver particles is not induced by interactions with the environment, the presence of which is clearly recognized in less inert matrices such as O₂ or CO. From self-consistent calculations of the surface response of planar jellium surfaces one would expect a red shift, which is also directly confirmed by a few calculations for selected jellium spheres. The contradiction between the observed blue shift for small particles and the predicted red shift for jellium spheres disappears, if one accounts for thed-electrons of silver in a very simple approximation.     

Hierarchical self-assembly of silver nanocluster arrays on triblock copolymer templates

Poly(styrene-b-butadiene-b-styrene) (SBS) triblock copolymer templates which present in-plane cylinders of polystyrene (PS) aligned parallel to the plane of the substrate have been prepared by a solvent-induced order-disorder phase transition method. Silver nanoclusters have been obliquely deposited onto the SBS copolymer templates at low coverage, utilizing the directed low-energy cluster beam deposition (LECBD) method. The morphology of the samples has been characterized by a tapping-mode AFM. It is shown that the silver nanoclusters form ordered linear arrays and the intercluster distance within each individual linear array is comparable to the cluster size. Optical absorption spectra indicate that the surface plasmon resonance (SPR) of the silver nanocluster linear arrays occurs at about 444.5 nm, manifesting a red shift of approximately 21.4 nm compared to the SPR absorption of silver nanoclusters deposited on a fused quartz substrate. This is attributed mainly to the near-field electrodynamic interactions between the silver nanoclusters. This hierarchical approach to create ordered nanostructures transcends the spatial limits of lithography and provides a promising route to achieve well-ordered cluster-based nanostructures.     

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.     

Carbon-on-metal films for surface plasmon resonance detection of DNA arrays

Surface plasmon resonance (SPR) imaging affords label-free monitoring of biomolecule interactions in an array format. A surface plasmon conducting metal thin film is required for SPR measurements. Gold thin films are traditionally used in SPR experiments as they are readily functionalized with thiol-containing molecules through formation of a gold-sulfur bond. The lability of this gold-thiol linkage upon exposure to oxidizing conditions and ultraviolet light renders these surfaces incompatible with light-directed synthetic methods for fabricating DNA arrays. It is shown here that applying a thin carbon overlayer to the gold surface yields a chemically robust substrate that permits light-directed synthesis and also supports surface plasmons. DNA arrays fabricated on these carbon-metal substrates are used to analyze two classes of biomolecular interactions: DNA-DNA and DNA-protein. This new strategy allows the combinatorial study of binding interactions directly from native, unmodified biomolecules of interest and offers the possibility of discovering new ligands in complex mixtures such as cell lysates.     

The surface enhanced Raman scattering effects of composite nanocrystals of Ag-TiO2

The composite particles of Ag-TiO2 both in a nanocrystal form was prepared by photoreduction of AgNO3 in the rutile colloidal solution. The surface plasmon absorption (SPA) band of the silver clusters supported on the TiO2 nanocrystal was red shift, and a strong surface enhanced Raman scattering (SERS) effect for the composite particles of Ag-nanocrystalline TiO2 was observed. The SERS effect is relating to the red shift phenomenon of SPA band, which could be explained by induced-resonance effect.     

Two-dimensional self-organization of polystyrene-capped gold nanoparticles

Thiol end-functionalized polystyrene chains have been introduced onto the surface of gold nanoparticles via a two-step grafting-to method. This simple grafting procedure is demonstrated to be efficient for gold nanoparticles of different sizes and for particles initially dispersed in either aqueous or organic media. The method has been applied successfully for a relatively large range of polystyrene chain lengths. Grafting densities, as determined by thermogravimetric analysis, are found to decrease with increasing chain length. In all cases, the grafting density indicates a dense brush conformation for the tethered chains. The resulting functionalized nanoparticles self-organize into hexagonally ordered monolayers when cast onto solid substrates from chloroform solution. Furthermore, the distance between the gold cores in the dried monolayer is controlled by the molecular weight of the grafted polystyrene. Optical absorption spectra recorded for the organized monolayers show the characteristic plasmon absorption of the gold particles. Importantly, the plasmon resonance frequency exhibits a distinct dependence on interparticle separation that can be attributed to plasmon coupling between neighboring gold cores.     

Gold particle interaction in regular arrays probed by surface enhanced Raman scattering

Lithographically designed two-dimensional arrays consisting of gold nanoparticles deposited on a smooth gold film are used as substrate to examine the SERS effect of the trans-1,2-bis (4-pyridyl) ethylene molecule. These arrays display two plasmon bands instead of the single one observed for the same arrays of particles but deposited on indium tin oxide coated glass. Laser excitation within the short wavelength band does not bring about any SERS spectrum, while excitation within the long wavelength band yields SERS spectra with a gain per molecule rising up to 10(8). The simultaneous investigation of extinction and Raman spectra of arrays exhibiting various topography parameters enables us to suggest an interpretation for both the occurrence of the two plasmon resonances and for the high Raman enhancement. We suggest to assign the short wavelength band to a plasmon wave propagating at the gold glass interface and the long wavelength one to an air/gold surface plasmon mode modified by particle-particle interaction.     

Self-assembly of poly(N-isopropylacrylamide)-carrying microspheres into two-dimensional colloidal arrays

Droplets containing polymer particles were deposited on a substrate. Poly(N-isopropylacrylamide) (PNIPAM) hydrogel and particles with PNIPAM graft chains on the surface self-assembled into a two-dimensional (2-D) superlattice when their dilute dispersions were dried on substrates. The capillary force between the particles induced ordered array formation during water evaporation. The presence of a PNIPAM layer on the particle surface gave the particles steric stability during ordered array formation. By grafting PNIPAM chains on particle surfaces by living radical polymerization, we successfully controlled the structural patterns of the colloidal arrays. These, controllable, 2-D colloidal arrays were generated on various substrates upon air-drying.     

银纳米粒子自组装结构的光谱性质研究

采用自组装方法在玻璃基底表面上构筑了银纳米粒子二维亚单层结构, 进而以对巯基苯胺为耦联分子进行银粒子的二次组装, 构成具有分子尺寸间隙的银粒子簇. 银粒子表面等离子体共振依赖于粒子间距、表面吸附分子和粒子组装方式. 同层内的银粒子相互间电磁偶合可导致银粒子偶极子等离子体共振蓝移; 对巯基苯胺的吸附则使得表面等离子体共振红移. 表面增强拉曼光谱结果表明, 具有分子尺寸间隙的银粒子簇对耦联于粒子间的对巯基苯胺分子的拉曼散射具有极大的增强效应, 同时也使耦联的对巯基苯胺与银粒子间产生更大程度的电荷转移.     

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.     

Light-induced coherent interactions between silver nanoparticles in two-dimensional arrays

Silver nanoparticles arranged in two-dimensional arrays experience quadrupolar coupling of plasmon resonances when irradiated with visible light. This coupling leads to the formation of the coherent plasmon mode characterized by an intense narrow resonance in the blue spectral range in the extinction spectrum. The coupling and the intensity of this mode can be effectively controlled by varying the distance between particles. The interparticle distance was varied by biaxial stretching of the arrays prepared in transparent elastomeric film of poly(dimethylsiloxane). The observed phenomenon exemplifies a generic approach in which new optical properties of materials can be engineered by organizing metal nanoparticles in various one-, two-, and three-dimensional structures. Further development of this approach will result in the discovery of novel principles of both fundamental and practical importance.     

Reversible Modulation of Surface Plasmons in Gold Nanoparticles Enabled by Surface Redox Chemistry

Switchable surface redox chemistry is demonstrated in gold@iron/iron oxide core–shell nanoparticles with ambient oxidation and plasmon‐mediated reduction to modulate the oxidation state of shell layers. The iron shell can be oxidized to iron oxide through ambient oxidation, leading to an enhancement and red‐shift of the gold surface plasmon resonance (SPR). This enhanced gold SPR can drive reduction of the iron oxide shell under broadband illumination to reversibly blue‐shift and significantly dampen gold SPR absorption. The observed phenomena provide a unique mechanism for controlling the plasmonic properties and surface chemistry of small metal nanoparticles.     

Multiple Fano resonances in monolayer hexagonal non-close-packed metallic shells

In this study, we first numerically investigate the appearance and properties of multiple Fano resonances in two-dimensional hexagonal non-close-packed arrays of symmetric metallic shells. The coexistence of broad sphere-like plasmon modes formed from the near-field interaction between the individual sphere plasmons and substantially narrower void plasmon modes supported by the inner surface of the individual shell resonant over the same range of energies can produce such Fano resonances. In particular, void and sphere-like plasmon modes of different angular momentum could directly interact without the need of symmetry breaking in the structure. A cost-effective colloidal crystal templating method is utilized to prepare the arrays of the metallic shells with small openings. The effect of the symmetry breaking on the Fano resonances in metallic cup arrays is experimentally and numerically investigated. Further tunability on the Fano resonances is gained by changing the size of the inner dielectric core, hence changing the moment of the void plasmon modes and consequently the resonance frequency. By adopting the polymer dielectric core with gain materials, our study may offer realizable experimental opportunities towards subwavelength low threshold plasmonic lasing.     

Gold nanoparticles as spectroscopic enhancers for in vitro studies on single viruses

Gold particles with diameters between 2.5 and 4.5 nm have been introduced in the inner cavity of an icosahedral brome mosaic virus. The optical properties of single gold-marked virions have been tested in-vitro with respect to the characteristic plasmon polariton resonance. The shift in the plasmon polariton resonance of a single Au particle encapsulated in a virus with respect to a free particle in solution indicates a close interaction between the basic residues on the inner wall of the capsid and the negative surface charge of the particle. Incorporation of a pair of Au particles, approximately 4 nm diameter, is shown to be a frequent event. In this case, the dependence of the two-particle surface plasmon spectrum on the interparticle distance and the strong particle/capsid wall interaction suggest that, in the future, it will be possible to use encapsulated Au particles to track changes in the viral capsid volume in real-time and in a physiological environment.     

Grating-induced plasmon mode in gold nanoparticle arrays

We study the dipolar coupling of gold nanoparticles arranged in regular two-dimensional arrays by extinction micro-spectroscopy. When the interparticle spacing approaches the plasmon resonance wavelength of the individual particles, an additional band of very narrow width emerges in the extinction spectrum. By systematically changing the particles dielectric environment, the particles shape, the grating constant and angle of incidence, we show how this band associated to a grating induced-resonance can be influenced in strength and spectral position. The spectral position can be qualitatively understood by considering the conditions for grazing grating orders whereas the strength can be related to the strength of dipolar scattering from the individual particles.     

Interaction of DNA bases with silver nanoparticles: assembly quantified through SPRS and SERS

Colloidal silver nanoparticles were prepared by reducing silver nitrate with sodium borohydride. The synthesized silver particles show an intense surface plasmon band in the visible region. The work reported here describes the interaction between nanoscale silver particles and various DNA bases (adenine, guanine, cytosine, and thymine), which are used as molecular linkers because of their biological significance. In colloidal solutions, the color of silver nanoparticles may range from red to purple to orange to blue, depending on the degree of aggregation as well as the orientation of the individual particles within the aggregates. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and absorption spectroscopy were used to characterize the assemblies. DNA base-induced differential silver nanoparticle aggregation was quantified from the peak separation (relates to color) of surface plasmon resonance spectroscopy (SPRS) and the signal intensity of surface-enhanced Raman scattering (SERS), which rationalize the extent of silver-nucleobase interactions.     

Dielectric medium effects on collective surface plasmon coupling interactions in oligothiophene-linked gold nanoparticles

The near-field coupling interactions between surface plasmon modes of neighboring metal nanoparticles (NPs) are investigated in thin films of oligothiophene-linked Au NPs. The oligothiophene linker facilitates near-field coupling between adjacent NPs, and disruption of the conjugation in the oligothiophene by chemical oxidation leads to a decrease in surface plasmon resonance (SPR) coupling between neighboring particles. The SPR coupling between NPs was found to be highly dependent on the dielectric constant of the medium that the films are exposed to, where a higher dielectric medium leads to weaker coupling. The dependence of the SPR coupling on the dielectric constant of the medium is explained using electrodynamic theory.     

Iminodiacetic acid-functionalized gold nanoparticles for optical sensing of myoglobin via Cu2+ coordination

A novel gold nanoparticle (AuNP)-based optical sensing system has been developed for the detection of myoglobin (Mb), which is of significant importance for early disease diagnosis. Two thiol molecules containing an iminodiacetic acid moiety (IDA) were synthesized. This detection is based on the Mb-induced aggregation of IDA-functionalized AuNPs resulting from the structures of Mb sandwiched between the functionalized AuNPs via Cu(2+) bridges in the coordination interactions of IDA-Cu(2+)-histidine residues available on the Mb surface, which was confirmed by UV-vis spectroscopy, transmission electron microscopy, dynamic light scattering, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The induction aggregation resulted in a red shift in plasmon resonance band of the AuNPs concomitant with a change in solution color from red to purple. The qualitative and quantitative detections of Mb can be achieved by colorimetric observations and UV-vis spectral measurements, respectively. The selectivity of protein assay with the functionalized AuNPs was further investigated, and it is found that the optical sensing of histidine-rich proteins is closely related to number and distribution of surface histidine residues as well as size of proteins.