Optical Properties Of Gold Nanoparticles And Measurement Of Its Size And Concentration By Light Extinction Method

Gold nanoparticles exhibit excellent optical properties that bulk materials do not have. The interaction of gold nanoparticles with the incident light of a certain wavelength results in localized surface plasmon resonance phenomenon, which leads to the absorption and scattering of light by nanoparticles, as well as the local electric field around the nanoparticles, will be strongly enhanced. These special optical properties of gold nanoparticles make them have tremendous applications in many fields, including biology, medicine, chemistry, information, energy, and environment. A deep knowledge and understanding for variation and regulation mechanism of optical properties of gold nanoparticles has important guiding significance for the practical application of gold nanoparticles. Size and concentration are two important parameters of gold nanoparticle, which not only directly affects the optical properties of the particle, but also determines the properties and behavior of gold nanoparticles in the practical applications. Thus, the accurate measurement of size and concentration is critical for the preparation, characterization, and applications of gold nanoparticles. Spectral extinction method is a simple, fast, and low-cost method for measurement of particle size, it can obtain information on size and concentration of particles at the same time. Therefore, the theoretical and experimental investigations of gold nanoparticle sizing based on the spectral extinction method has actual value and significance.In this paper, the resonant optical properties and refractive index sensing of several common gold nanoparticles were systematically investigated by using Mie scattering theory and discretre dipole approximation method. For the applications of gold nanoshells in the photothermal therapy and bioimaging, we investigated the absorption and backscattering properties of gold nanoshells by using Mie scattering theory of a coated sphere, and we optimized the dimension parameters of gold nanoshells. Finally, we performed systematic investigations on the inversion problem of gold nanoparticle size and concentration based on the spectral extinction method. The main works and results are summarized as follows:(1) In the current study of the resonant optical properties of gold nanoparticles,researchers mainly investigated the extinction properties. Usually, the absorption or scattering properties of gold nanoparticles are playing important role in the many applications rather than total extinction properties, but there is few systematic study on the scattering and absorption properties. With three common gold nanoparticles(i.e., gold nanosphere, nanoshell, and nanorod) as the research object, we investigated the extinction, scattering, and absorption properties by using Mie scattering theory and discrete dipole approximation method. The results show that, gold nanoparticles absorb more light than scattering when the size is small, but the scattering become stronger with the increase of the size. Compared to the gold nanospheres, gold nanoshells and nanorods have a wider resonance wavelength range and stronger resonance peak. This investigation provides theoretical guidance for the applications of resonant optical properties of gold nanoparticles in photothermal therapy, biological imaging, biosensing etc.(2) Gold nanoellipsoid, nanocylinder, and nanobar are the three typical non-spherical gold nanoparticles. Currently, there is no systematic investigation on the refractive index sensing properties of these three nanoparticles. Thus, with gold nanoellipsoid, nanocylinder, and nanobar as the research object, we investigated the variation of resonance wavelength and refractive index sensitivity by using discrete dipole approximation method, the effects of the particle size and shape on the resonance wavelength and refractive index sensitivity were quatitatively analyzed. We also performed comparative analysis on the refractive index sensitivity of these three gold nanoparticles in order to find gold nanoparticles with high sensitivity. It was found that increase in aspect ratio or effective radius leads to the resonance wavelength redshift and the refractive index increasement. Gold nanocylinder has a greater refractive index sensitivity as compared the gold nanoellipsoid and gold nanobar of the same dimensions, thus it is more suitable for biosensing.(3) For the application of gold nanoshells in photothermal therapy and bioimaging, it is necessary to find the gold nanoshell structure with optimal absorption or backscattering properties. This paper investigated the dependence of the absorption and backscattering properties of gold nanoshells on the core radius and shell thickness by using the Mie theory for a coated sphere. We proposed the optimization idea of gold nanoshells and obtained the optimal core radius and shell thickness. The optimizedgold nanoshells can be used as ideal contrast and therapeutic agents. This straightforward idea can be directly applied to other metals with only the adjustment of the parameters in the size-dependent dielectric function. Additional particle geometries(nanorods, nanocages etc.) can also be optimized by using numerical methods such as T-matrix, discrete dipole approximation, and finite-difference time-domain.(4) The spectral extinction method is firstly used for measuring the size distribution and concentration of polydisperse gold nanospheres, it provides a simple, fast, and low-cost method to measure size and concentration of gold nanoparticles. This paper theoretically investigated the inverse problem of size and concentration of monodisperse and polydisperse spherical gold nanoparticles. It is found that, there is a one-to-one relationship between the size of monodisperse spherical gold nanoparticles and the resonance wavelength presented in extinction spectrum, thus the particle size and the particle number concentration can be determined using this relationship. For polydisperse spherical gold nanoparticles, the retrieval errors corresponding to the wavelength range 200-600 nm are smaller. When the noise added to the extinction spectra, the retrieval error becomes bigger. This paper also measured the extinction spectra of gold nanospheres with mean diameters of 80, 60, 40, and 20 nm, and retrieved the size distribution and concentration of gold nanoparticles by using the inversion algorithm. The inversion results validate the correction and reliability of the inversion algorithm.