Simulation of the Optical Absorption Spectra of Gold Nanorods as a Function of Their Aspect Ratio and the Effect of the Medium Dielectric Constant

Mohamed, and Mona; El‐Sayed, Mostafa A.

doi:10.1021/jp990183f

Cited by 1,500 publications

(1,373 citation statements)

References 27 publications

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“…The latter observation suggests that nanorods with the highest uniformity can be produced by quenching their growth at an early stage, within 30 min of their nucleation. where R is the nanorod aspect ratio and ε m is the effective dielectric medium of the surrounding micellar layer, which also varies as a function of R. 24,27 The application of eqs 1 and 2 is only useful to the first degree of approximation, as the nanorod shapes and micellar materials used by Link et al 24 are different from those in our study. Further modulation in the local dielectric medium by sulfide adsorption is also possible, although this effect appears to be negligible for a S:M ratio of 1:2 (cf.…”

Section: Resultsmentioning

confidence: 83%

Sulfide-Arrested Growth of Gold Nanorods

2005

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The growth of gold nanorods can be arrested at intermediate stages by treatment with Na 2 S, providing greater control over their optical resonances. Nanorods prepared by the seeded reduction of AuCl 4 in aqueous cetyltrimethylammonium bromide solutions in the presence of AgNO 3 typically exhibit a gradual blueshift in longitudinal plasmon resonance, over a period of hours to days. This "optical drift" can be greatly reduced by adding millimolar concentrations of Na 2 S to quench nanorod growth, with an optimized sulfur:metal ratio of 4:1. The sulfide-treated nanorods also experience a marked redshift as a function of Na 2 S concentration to produce stable plasmon resonances well into the nearinfrared. Sulfide treatment permitted a time-resolved analysis of nanorod growth by transmission electron microscopy, revealing two distinct periods: an initial growth burst (t < 15 min) that generates dumbbell-shaped nanorods with flared ends and a slower phase (t > 30 min) favoring growth around the midsection, leading to nanorods with the more familiar oblate geometry. The blueshift in plasmon resonance that accompanies the dumbbell-to-oblate shape transition correlates more strongly with changes in the length-to-midsection (L/D 1 ) ratio rather than the length-to-end width (L/D 2 ) ratio, based on the empirical relationship introduced by El-Sayed and co-workers.

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Section: Resultsmentioning

confidence: 83%

Sulfide-Arrested Growth of Gold Nanorods

2005

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“…[61][62][63] At the same time, well-established theoretical tools based on the Mie theory 64 and the discrete dipole approximation (DDA) 65 method have been readily exploited for a quantitative study of the nanoparticle optical properties of different size, shape, composition, and aggregation state, etc. 2,36,[66][67][68][69][70][71][72][73] In this paper, we use Mie theory and the DDA method to calculate the absorption and scattering efficiencies and optical resonance wavelengths of gold nanospheres, silica-gold nanoshells, and gold nanorods, for various nanoparticle dimensions, so as to aid the selection of nanoparticles for specific biomedical applications. The calculated optical cross-sections of all three nanoparticle classes are found to be a few orders of magnitude higher than those for conventionally used absorbing and fluorescent dyes.…”

Section: Introductionmentioning

confidence: 99%

Calculated Absorption and Scattering Properties of Gold Nanoparticles of Different Size, Shape, and Composition: Applications in Biological Imaging and Biomedicine

et al. 2006

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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 (∼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 ∼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-toshell 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 radi...

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“…For gold nanorods in an aqueous solution, the plasmon absorption splits into two bands (116,117), corresponding to the response along and perpendicular the long axis of the rods. The transverse mode (end on) corresponds to the simple case of spherical particles.…”

Section: Effects Of Nanoparticle Shapementioning

confidence: 99%

The third order nonlinear optical properties of gold nanoparticles in glasses, part II

2003

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The optical properties of gold nanoparticles have been known for a number of years and recent advances in laser power have now allowed the non-linear optical properties to be studied. In this short review paper, the various theories that have been used to describe the optical properties of gold nanoparticles are presented. Methods of preparing gold nanoparticles in glasses are explained briefly, as well as characterization techniques. The optical properties of gold nanoparticles are reviewed, as well as the effects of particle size, shape, concentration and c. Effects Of Nanoparticle SizeThe relationship between optical properties and nanoparticle radius has been studied in glasses with dispersed colloid Au nanoparticles (111,112). The peak plasmon wavelength shifts continuously to longer wavelengths with increasing average particle radius. A similar dependence was seen in silica glasses dispersed with Au colloid particles (112,113). Foss et al. (68) found significant transparency in the near-IR region and strong extinction in the visible for particles with nearsphere like shape. The wavelength of maximum extinction increased with increasing particle radius, giving rise to colour variations (red-purple for 30 nm size and blue-green for 60 nm size). This is shown in Figure 1. This was consistent with the findings of Mie (17). Hosoya et al. (73) and Muto et al. (102) found in Au:Al2O3 thin films that the spectra broadened and the peak shifted to longer wavelengths as the particle size decreased. Muto et al. (102) explained it as due to charge transfer from the metal particles to the amorphous oxide by the aluminium-metal bonds used in adhesion. The decreased electron density of the metal particles gives rise to the blue-shift. This behaviour was opposite to that of Au:SiO2, and indicates that the matrix has an effect on the blue-or red-shift of the plasmon peak with size.As the wavelength of the surface plasmon resonance changes with particle size, the χ (3) is also expected to change with size. Fukumi et al. (60), found χ (3) to be approximately proportional to the fourth power of the particle radius. They also found that χm (3) (of the metal particles on their own) had negligible dependence on the particle radius. The value of χm (3) = 2.5 x 10 -8 esu agreed with that of Hache et al. (32) and Bloemer et al. (114). Hornyak et al. (69) prepared Au nanoparticles in nanoporous alumina membranes, and absorption properties for nanoparticles of size 52 nm down to 16 nm, and with various aspect ratios. They used a modified form of MG theory, called the dynamic MG (DMG) theory (68), to model the optical properties. They found that as the nanoparticle size decreased, the wavelength of the maximum absorption intensity max approached the value predicted by DMG theory. For the smallest diameter particles, (16 nm), the values were identical with DMG theory, and they suggested that there was a quasi-static DMG limit ( max does not change from ~5 08 nm) for nanoparticles of diameter 16 nm. Alvarez et al. (115) pr...

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Simulation of the Optical Absorption Spectra of Gold Nanorods as a Function of Their Aspect Ratio and the Effect of the Medium Dielectric Constant

Cited by 1,500 publications

References 27 publications

Sulfide-Arrested Growth of Gold Nanorods

Sulfide-Arrested Growth of Gold Nanorods

Calculated Absorption and Scattering Properties of Gold Nanoparticles of Different Size, Shape, and Composition: Applications in Biological Imaging and Biomedicine

The third order nonlinear optical properties of gold nanoparticles in glasses, part II

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