Longitudinal Plasmon Band Research Articles

Modulation of Optical Properties of Gold Nanorods on Addition of KOH

Gold nanorods are known to exhibit two distinct surface plasmon oscillations namely, transverse and longitudinal bands corresponding to oscillations of conduction electrons along width and length of gold nanorods. Considerable changes in these surface plasmon resonance peak positions occurred when KOH was added to the nanorod solution. Nanorods with initial longitudinal plasmon band at 739, 796, and 895 nm are studied with variation in KOH concentration. While the longitudinal plasmon resonance peak showed blue shift, transverse plasmon resonance peak exhibited only intensity variations. Changes could be attributed to the shape transition of gold nanorods on variation of pH in the solution. Shape transition of gold nanorods is confirmed by transmission electron microscopy images.

Nanorod‐Coated PNIPAM Microgels: Thermoresponsive Optical Properties

Nanoparticles and in particular gold nanorods have interesting optical properties arising from two well-differentiated plasmon modes. The frequency of such modes can be altered by their chemical environment and coupling with neighboring rods. This study investigates new composite materials made of gold nanorods adsorbed on thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) microgels. It is shown that the thermally induced collapse of the polymer network inside the particles leads to a red shift of the longitudinal plasmon band of the gold rods, which is found to be fully reversible.

Assembling covalently linked nanocrystals and nanotubes through click chemistry

The click reaction involving the Huisgen 1,3-cycloaddition reaction between azide and ethynyl groups has been employed to obtain assemblies of nanostructures. The click reaction between gold nanorods capped with azidoalkane- and alkyne-thiols yields chains and complex assemblies, accompanied by a large red shift of the longitudinal surface plasmon band. A similar reaction between CdSe nanocrystals results in a splitting of the photoluminescence band, possibly due to exciton splitting. Carbon nanotubes decorated by covalently linked gold nanoparticles have been obtained through click chemistry.

Preferential End Functionalization of Au Nanorods through Electrostatic Interactions

A novel methodology for the preferential end functionalization of Au nanorods with nanoparticles is reported by exploiting the electrostatic attractive interactions. The enhanced potential at the edges of Au nanorods preferentially attracts the positively charged Au nanoparticles, leading to their selective binding. Site-specific binding results in a spontaneous bathochromic shift in the longitudinal plasmon band of Au nanorods which is dependent on the size of the nanoparticles.

Optical Properties of Platinum-Coated Gold Nanorods

The optical properties of Au/Pt nanorods with different morphologies are analyzed in detail through comparison of experimental UV−vis−NIR spectra and calculations based on the boundary element method (BEM). The studied morphologies refer to two different Pt deposition modes on Au nanorods, which have been experimentally realized through the presence or absence of Ag+ ions in solution. While in the absence of Ag+, complete overcoating was obtained, the presence of these ions induced preferential growth on the tips, as recently reported by us.1 The changes in the optical spectra are different for both Pt deposition modes, and this is fully accounted for using appropriate geometrical models in the BEM calculations. The presence of Pt tips is shown to lead to red-shifts of the longitudinal plasmon band because of geometrical effects, though accompanied by damping due to increased absorption.

Plasmon Coupling in Layer-by-Layer Assembled Gold Nanorod Films

A systematic study of the optical effects derived from plasmon coupling in mono- and multilayers of gold nanorods is presented. The monolayers were prepared using the standard polyelectrolyte-assisted layer-by-layer (LbL) method and gold nanorods coated with either poly(N-vinyl pyrrolidone) or homogeneous silica shells. Such plasmon coupling leads in general to extensive red-shift and broadening of the longitudinal plasmon bands, which are discussed on the basis of recently reported theoretical modeling. Whereas for PVP-coated rods, strong interactions were observed for high-density monolayers and closely spaced multilayers, increasingly efficient screening is observed for thicker silica shells.

Spectral encoding on Gold Nanorods Doped in a Silica Sol–Gel Matrix and Its Application to High‐Density Optical Data Storage

AbstractMetallic nanorods exhibit fascinating optical properties due to surface plasmons—collective oscillation of the electron cloud within a particle. They exhibit two principle absorption bands that correspond to surface plasmon resonance (SPR) along the longitudinal and transverse directions of the nanorod. Most importantly, the longitudinal band can be tuned with the aspect ratio of the rod, making it a spectrally tuneable optical material, which can be applied to a variety of devices from bioimaging to high‐density optical storage. Here, spectral encoding for high‐density optical storage applications is demonstrated on two sizes of gold nanorods (aspect ratios of three and five) doped in a silica sol–gel matrix by femtosecond pulsed laser irradiation. It is widely known that high‐power pulsed laser irradiation causes metal nanorods to undergo shape transformations via the process of melting or fragmentation. The process is enhanced if the laser wavelength is tuned at the longitudinal surface plasmon resonance peak of the nanorods, which results in a significant reduction or shift in the surface plasmon resonance peak. As such a shape change occurs only on the subpopulation of rods that have a longitudinal plasmon band matching the laser wavelength, a size‐ or spectrum‐selective shape transition is possible in a rod mixture with varying aspect ratios. The current spectral encoding technology can be incorporated into existing optical disc technology, such as three‐dimensional bit‐by‐bit and holographic, and can increase the capacity limit by utilizing the spectral domain.

Gold Nanoparticles from Induced Au3+→Au0 Reaction in Polyvinyl Alcohol Molecules in Presence of Sucrose in Hot Water

In hot water (50–60°C), polyvinyl alcohol (PVA) molecules have coordination reaction with Au3+ cations, forming an Au3+-PVA polymer complex. In the proposed model reaction in small templates, the complex converts to Au0 capping in PVA molecules. Adding sucrose (5–10 times the PVA in mass) in a typical batch promotes Au3+→Au0 reaction, showing absorption coefficient α in Au0 surface plasmon band to be enhanced as much as 28 times the value in reaction with PVA. The band shifts at 547 nm from 566 nm (α = 21.4 cm−1 mol−1) in the PVA sample. Drying Au0-PVA/sucrose (2–5 wt% Au0) colloid at 60–70°C and then heating at 450°C in air burns off the organic part, leaving behind a light ash colored powder with Au0 nanoprisms or nanofibrils (∼30 nm average width). X-ray diffractogram has six reflections, (111), (200), (220), (311), (222), and (400), of Fm3m fcc Au0 of lattice parameter a = 0.4080 nm. The powder has photoluminescence in transversal and longitudinal Au0 plasmon bands of 535 and 585 nm, respectively.

Plasmon Coupling in Nanorod Assemblies: Optical Absorption, Discrete Dipole Approximation Simulation, and Exciton-Coupling Model

The shape anisotropy of nanorods gives rise to two distinct orientational modes by which nanorods can be assembled, i.e., end-to-end and side-by-side, analogous to the well-known H and J aggregation in organic chromophores. Optical absorption spectra of gold nanorods have earlier been observed to show a red-shift of the longitudinal plasmon band for the end-to-end linkage of nanorods, resulting from the plasmon coupling between neighboring nanoparticles, similar to the assembly of gold nanospheres. We observe, however, that side-by-side linkage of nanorods in solution shows a blue-shift of the longitudinal plasmon band and a red-shift of the transverse plasmon band. Optical spectra calculated using the discrete dipole approximation method were used to simulate plasmon coupling in assembled nanorod dimers. The longitudinal plasmon band is found to shift to lower energies for end-to-end assembly, but a shift to higher energies is found for the side-by-side orientation, in agreement with the optical absorption experiments. The strength of plasmon coupling was seen to increase with decreasing internanorod distance and an increase in the number of interacting nanorods. For both side-by-side and end-to-end assemblies, the strength of the longitudinal plasmon coupling increases with increasing nanorod aspect ratio as a result of the increasing dipole moment of the longitudinal plasmon. For both the side-by-side and end-to-end orientation, the simulation of a dimer of nanorods having dissimilar aspect ratios showed a longitudinal plasmon resonance with both a blue-shifted and a red-shifted component, as a result of symmetry breaking. A similar result is observed for a pair of similar aspect ratio nanorods assembled in a nonparallel orientation. The internanorod plasmon coupling scheme concluded from the experimental results and simulations is found to be qualitatively consistent with the molecular exciton coupling theory, which has been used to describe the optical spectra of H and J aggregates of organic molecules. The coupled nanorod plasmons are also suggested to be electromagnetic analogues of molecular orbitals. Investigation of the plasmon coupling in assembled nanorods is important for the characterization of optical excitations and plasmon propagation in these nanostructures. The surface plasmon resonance shift resulting from nanorod assembly also offers a promising alternative for analyte-sensing assays.

End-to-end Assemblies of Gold Nanorods Adsorbed on a Glass Substrate Modified with Polyanion Polymers

Abstract Phosphatidylcholine-passivated gold nanorods (NRs) adsorbed on polyanion-modified glass substrates. The NRs on glass substrates formed end-to-end assemblies showing red-shifted and broaden longitudinal surface plasmon (SP) bands. Addition of aminohexanethiol (AHT) induced dumping of SP bands but increased the density of NRs on glass surfaces. Thiol groups of AHT were effective to change the properties of NR assemblies on glass surfaces.

Selective Shortening of Single-Crystalline Gold Nanorods by Mild Oxidation

Gold nanorods (NRs) have received much attention due to their size-dependent surface plasmon-related optical properties. A seed-mediated approach has recently been developed for the synthesis of Au NRs with varying length-to-diameter aspect ratios. With the introduction of silver ions in the growth solution, Au NRs of narrow size distributions can be produced in high yields. Herein we describe an approach for the continuous and selective shortening of Au NRs synthesized by the silver ion-assisted seed-mediated method through oxidation with environmentally benign oxygen at slightly elevated temperatures. UV-visible extinction measurements indicate that the longitudinal surface plasmon band of Au NRs decreases in intensity and blue-shifts as a function of the oxidation time. Transmission electron microscopy (TEM) imaging shows that the length of Au NRs decreases with oxidation and their diameter stays almost constant, which suggests that oxidation starts at the ends of Au NRs. The size distributions of shortened Au NRs are similar to those of starting NRs. Further oxidation transforms Au NRs into nanospheres, which become smaller in diameter and finally completely disappear. It has been found that the oxidation rate of Au NRs can be controlled by temperature and acid concentration. Furthermore, high-resolution TEM studies reveal that Au NRs synthesized by the silver ion-assisted seed-mediated method are single crystalline and they stay single crystalline during oxidation. It is expected that Au NRs of any aspect ratio with narrow size distributions within the limit of that possessed by starting NRs can be produced by this mild oxidation approach.

Quantitation of Metal Content in the Silver-Assisted Growth of Gold Nanorods

The seed-mediated approach to making gold nanorods in aqueous surfactant solutions has become tremendously popular in recent years. Unlike the use of strong chemical reductants to make spherical gold nanoparticles, the growth of gold nanorods requires weak reducing conditions, leading to an unknown degree of gold reduction. The metal content of gold nanorods, made in high yield in the presence of silver ion, is determined by inductively coupled plasma atomic emission spectroscopy. Through the use of the known gold concentration in nanorods, molar extinction coefficients are calculated for nanorods of varying aspect ratios from 2.0 to 4.5. The extinction coefficients at the longitudinal plasmon band peak maxima for these nanorods vary from 2.5x10(9) to 5.5x10(9) M-1 cm-1, respectively, on a per-particle basis. Many of the gold ions present in the growth solution remain unreacted; insights into the growth mechanism of gold nanorods are discussed.

Contributions from radiation damping and surface scattering to the linewidth of the longitudinal plasmon band of gold nanorods: a single particle study

The scattering spectra of single gold nanorods with aspect ratios between 2 and 4 have been examined by dark field microscopy. The results show that the longitudinal plasmon resonance (electron oscillation along the long axis of the rod) broadens as the width of the rods decreases from 14 to 8 nm. This is attributed to electron surface scattering. Analysis of the data using gamma = gamma(bulk) + Anu(F)/L(eff), where L(eff) is the effective path length of the electrons and nu(F) is the Fermi velocity, allows us to determine a value for the surface scattering parameter of A = 0.3. Larger rods with widths of 19 and 30 nm were also examined. These samples also show spectral broadening, which is attributed to radiation damping. The relative strengths of the surface scattering and radiation damping effects are in excellent agreement with recent work on spherical gold nanoparticles by Sönnichsen et al., Phys. Rev. Lett., 2002, 88, 077402; and by Berciaud et al., Nano Lett., 2005, 5, 515.

Aspect ratio dependence on surface enhanced Raman scattering using silver and gold nanorod substrates

Silver and gold nanorods with aspect ratios from 1 to 16 have been used as substrates for surface enhanced Raman spectroscopy (SERS) in colloidal solution. The nanorod aspect ratio is varied to give different degrees of overlap between the nanorod longitudinal plasmon band and excitation source in order to determine its effect on overall surface enhancement. Results suggest that enhancement factors are a factor of 10-10(2) greater for substrates that have plasmon band overlap with the excitation source than for substrates whose plasmon bands do not.

In vitro and in vivo two-photon luminescence imaging of single gold nanorods

Gold nanorods excited at 830 nm on a far-field laser-scanning microscope produced strong two-photon luminescence (TPL) intensities, with a cos(4) dependence on the incident polarization. The TPL excitation spectrum can be superimposed onto the longitudinal plasmon band, indicating a plasmon-enhanced two-photon absorption cross section. The TPL signal from a single nanorod is 58 times that of the two-photon fluorescence signal from a single rhodamine molecule. The application of gold nanorods as TPL imaging agents is demonstrated by in vivo imaging of single nanorods flowing in mouse ear blood vessels.

Optical properties of nanoengineered gold blocks

We report on control over the extinction spectrum of tetragonal gold nanoblocks by changing their lateral aspect ratio. Nanoblocks were patterned on a glass substrate by electron-beam lithography and were 40-400 nm in lateral dimensions and spanned from 1 to 9 in aspect ratio. This allowed us to tune the localized surface plasmon bands from 700 nm to 1.5 microm (longitudinal mode) and from 700 to 550 nm (transverse mode). Unprecedented polarization selectivity of the transverse and longitudinal plasmon bands was achieved via alignment and 3D control of the dimensions of the nanoblocks.

A DDA calculation on the optical properties of silver nanorods

The optical properties of metal nanoparticles are quite different from those of the bulk materials mainly due to the collective oscillations of their conduction electrons known as the surface plasmon resonance（SPR），which is strongly dependent on the particle shape and size，and the dielectric properties of the local environment where the nanoparticles are embedded in. Based on the discrete dipole approximation（DDA）method，we studied the optical properties of silver nanorods with different aspect ratios in some special dielectric environment including air，water，acetone，methylene chloride and pyridine. The DDA simulation of the ultraviolet-visible（UV-Vis）extinction spectra of silver nanorods with varying aspect ratios shows the plasmons absorption splits into two bands corresponding to the oscillation of the free electrons along and perpendicular to the long axis of the rods. The transverse mode shows almost a fixed resonance at about 350 nm while the resonance of the longitudinal mode is red-shifted and strongly depends on the aspect ratio of the nanorods. An empirical formula was given to predict the peak position of the longitudinal palsmon band of the silver nanorods with different aspect ratios in the air. The calculation result also shows the maximum of the longitudinal plasmon band of a silver nanorod with a fixed aspect ratio depends on the medium dielectric constant in a linear way. The TEM image and corresponding UV-Vis extinction spectrum of silver nanosphere and nanorods synthesized by our lab are in good agreement with the DDA simulation results.

Starch-Mediated Shape-Selective Synthesis of Au Nanoparticles with Tunable Longitudinal Plasmon Resonance

We report the synthesis ofAu nanoparticles, with tunable longitudinal plasmon band and shape selectivity, mediated by starch in the presence of ultrasonic waves. The synthesis was carried out by reduction of HAuCl4, at various concentrations, using H2O2 as the reducing agent. When the reactions were carried out in the absence of ultrasonic waves, there was no occurrence of the longitudinal resonance band, while the transverse plasmon resonance band shifted toward a higher wavelength. Transmission electron microscopic measurements revealed an increase in particle sizes with increasing higher initial HAuCl4 concentration. On the other hand, in the presence of ultrasonic waves, as the initial concentration of HAuCl4 was increased, while the transverse plasmon resonance band remained the same, the longitudinal plasmon resonance band increasingly shifted toward a higher wavelength. Transmission electron microscopic measurements revealed the change in shape from spherical to triangular to hexagonal particles with increasing initial HAuC14 concentration. We also report that the starch-stabilized nanoparticles could be precipitated from the solution by a starch digesting enzyme which also binds with the particles resulting in its precipitation.

Immobilization of Gold Nanorods on the Glass Substrate by the Electrostatic Interactions for Localized Plasmon Sensing

Abstract Gold nanorods (NRs) have two localized surface plasmon bands in the visible–near infrared region. We have succeeded in well-dispersed deposition and immobilization of the gold nanorods on the surface of a glass substrate by the electrostatic interactions. The outermost PDDA layer was effective to prevent the lateral diffusion of NRs on the PSS layer. The longitudinal plasmon band showed clear shifts with solvents in the near-infrared region.

Vibrational response of nanorods to ultrafast laser induced heating: theoretical and experimental analysis.

In this paper, we elucidate the vibrational response of cylindrical nanorods to ultrafast laser-induced heating. A theoretical analysis of the expected behavior is first presented. This analysis predicts that both extensional and breathing vibrational modes of the rods should be excited by laser-induced heating. Analytical formulas are derived assuming that the heating/expansion process is instantaneous, and that the lengths of the rods are much greater than their radii. These results show that the breathing mode dominates the mechanical deformation of the rod. However, because the frequency of the extensional mode is much lower than that of the breathing mode, the extensional mode will dominate the response for a real experiment (a finite-time heating/expansion process). The results of this model are compared to data from transient absorption experiments performed on gold nanorods with average lengths between 30 and 110 nm. The transient absorption traces show pronounced modulations with periods between 40 and 120 ps, which are only observed when the probe laser is tuned to the longitudinal plasmon band. The measured periods are in good agreement with the expected values for the extensional modes of the rods. For rods wider than 20 nm, the breathing mode can also be observed and, again, the measured periods are in good agreement with the theoretical calculations. The breathing mode is not observed for thinner rods (<20 nm width) because, in this case, the period is comparable to the time scale for lattice heating.