Measurement of mean size and evaluation of polydispersity of gold nanoparticles from spectra of optical absorption and scattering

Two methods for determination of the mean size of gold nanoparticles, based on measurement of the wavelengths of the maxima lambda(max) of side scattering and extinction in the range 400-700 nm, are compared. Four sols with mean particle diameters d of about 15, 20, 25, and 30 nm, measured using the dynamic light-scattering, technique, were studied experimentally. The slope of the size dependence lambda(max)(d) of the spectral position of the scattering peak exceeded that for the extinction peak by a factor of 2.4. This fact ensures a substantially higher accuracy of the scattering method. For simulating polydispersity, mixtures of three colloids with particle diameters of 20, 25, and 30 nm were used: sample S1, with a size distribution close to the normal one of around 25 nm, and sample S2, with equal concentrations of each of the components. The extinction spectra of mixtures S1 and S2 and the initial 25-nm sol (S0) were virtually identical, whereas their scattering spectra showed a pronounced increase in the peak amplitude in the series S0, S1, S2. These results agree with calculations based on the Mie theory. Thus, scattering spectra offer advantages over extinction spectra not only in measuring the mean size of gold particles but also in evaluating their polydispersity. (C) 2004 MAIK Nauka/Interperiodica.