Dielectric constant, electrical conductivity and relaxation time measurements of different gold nanoparticle sizes

Abstract

Gold nanoparticles (GNPs) offer a great possibility for biomedical application, not only to pharmaceutics approaches, but also as novel diagnostic and therapeutic approaches. One of the important concerns is about their safety in clinical applications. Nanoparticle size has been shown to be an extremely important parameter affecting the nanoparticle uptake and cellular internalization. The aim of the present study was to investigate the dielectric constant, electrical conductivity and relaxation time of different GNP sizes. The electrical parameters were measured in the frequency range of 20 Hz up to 1 MHz using a WAYNE KERR precision component analyzer. The sample cell has two squared platinum black electrodes each having an area of 1 × 1 cm2 with an inner electrode distance of 1 cm. For a dielectric material placed between two parallel plate capacitor, the measured value of capacitance (C) and resistance (R) were used to calculate the real () and imaginary part () of the complex permittivity , in addition to calculating the conductivity (s) and the relaxation time (t). The sizes of GNPs were calculated from the images taken by the transmission electron microscope (TEM). It became evident that relatively simple methods can be used to obtain the populations of different GNP sizes, which allow simultaneous detection of several targets. The presented dielectric data indicates that GNPs have strong dielectric dispersion corresponding to the alpha relaxation region in the frequency range of 20 Hz to 100 kHz which was identified as anomalous frequency dispersion. The measured conductivity values decreased with increasing GNPs size. Moreover, at high frequencies, the conductivity rapidly increased for all the examined GNPs size. The GNPs show a relaxation process. The relaxation time decreased with increasing GNPs size and was found to be 2.5, 3.5 and 4 ms for 10, 20 and 50 nm GNP size, respectively. A rapid decrease in the dielectric constant may be attributed to the tendency of dipoles in the GNPs to orient themselves in the direction of the applied field in the low-frequency range. However, in the high-frequency range, the dipoles will hardly be able to orient themselves in the direction of the applied field and hence, the value of the dielectric constant is nearly constant. The relaxation time may be attributed to increase in the localized charges distribution within the medium which was confirmed by the conductivity data. This study demonstrates that the dielectric, electrical conductivity and relaxation time values decreased with increasing the GNPs size, e.g. these changes are particle-size dependent. This study suggests that the increase in dielectric constant, electrical conductivity and relaxation time observed with the smaller 10 and 20 nm GNPs compared with 50 nm GNPs may be used as important risk factors for bioaccumulation and toxicity of the smaller GNPs. Thus additional histological and histochemical experiments are needed to confirm this hypothesis. Key words: Gold nanoparticles, rheological parameters, size, temperature, dielectric, conductivity.