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

Abstract

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...

Controllable Cobalt Oxide/Au Hierarchically Nanostructured Electrode for Nonenzymatic Glucose Sensing

“…Successful down regulation was attributed to the simultaneous structural deformation of the HGN under NIR light, which was confirmed by UV-vis spectral changes in SPR, and the breakage of the Au S bond, due to heat, which resulted in the release of siRNA to the cytosol [177]. This mechanism has been described previously in photothermal transfection experiments involving DNA and is attributed to hot electron thermalization of the nanoparticle lattice [243,260,261]. Fig.…”

“…Tuning of the LSPR properties can be achieved through synthesis by exploiting differences in nanoparticle size, geometry, surface morphology, aggregation, aspect ratio, and the dielectric constant of the surrounding media [15,33,[35][36][37][38][39][40][41][42][43][44][45]. Since each application relies on a specific set of conditions for optimized efficiency, the structural parameters of the nanoparticles employed for use must be tailored accordingly.…”

“…Both the core width and shell thickness can be tuned through synthesis to produce a range of overall diameters (20-125 nm) and aspect ratios. As a two-interface system HGNs have enhanced LSPR which is the result of strong coupling in the near-field between the plasmon modes of the inner cavity surface and the outer surface [8,9,29,38]. This coupling leads to the hybridization of the two individual plasmon modes where the plasmons interact electrostatically with one another in the same manner as a coupled harmonic oscillator [8,9].…”

“…Briefly, noble metal nanoparticles, exhibiting NIR absorption in the appropriate size regime (20-100 nm) for biological applications, are modified with either an antibody or ligand specific to an antigen or receptor, on the surface of a tumor cell [38,53,190]. NIR absorbing particles are preferred since it is well known that blood and tissue are most readily penetrated by light in this wavelength region [191][192][193].…”

“…As noble metal structures, they have enhanced EM field strength because of their plasmon resonance, and surfaces can be easily functionalized to target biomolecules. They are also resistant to photobleaching and have molar extinction coefficients (ε) that are larger than those of dyes and QDS [35,38,177,367]. For instance, Lu et al recently reported that a 40 nm HGN with a 2-3 nm shell and an SPR of 800 nm has a molar absorption coefficient of 1.4 × 10 11 M −1 cm −1 [177].…”