Size and shape of gold nanoparticles in toluene via SAXS
A solution of colloidal gold nanoparticles was characterized for size and shape by the SAXSpace system.
The demand for colloidal gold nanoparticles (AuNP), famous for their vibrant colors, has increased in the past decades due to their remarkable optoelectronic properties. Their diverse applications range from organic photovoltaics, therapeutic agents, drug delivery, functional and medical surfaces, sensors and catalysis to name just a few. There are several routes known to synthesize AuNP with different stabilizing shells. The chemical nature of those shells determines the solvent in which the colloidal gold can be electrostatically or sterically stabilized.
Experimental and Results
The AuNPs were sterically stabilized to form a colloid using a dodecanthiol shell and dispersed in toluene. The concentration was 2.0 wt%. The geometric radius according to TEM measurements was found to be in between 3 and 5 nm.
SAXS measurements were done using the SAXSpace instrument, offering a line-collimated X-ray beam ideal for isotropic scatterers. The sample was filled into the TubeCell, which is resistant against many organic solvents and shows very low parasitic background scattering. The SAXS curve after toluene subtraction and transmission normalization is displayed in Figure 1 (black curve). For the particle size evaluation by indirect Fourier transformation the GIFT software package (developed by Glatter et al.) was used. It allows for smearing a model, thus no desmearing routines had to be used. The resulting approximated (red) and desmeared (green) scattering curves obtained from GIFT are plotted in Fig. 1. The slope of the scattering curve at small values of q follows the power law of q0, indicating a spherical shape of the AuNP. The pronounced form factor further proves the narrow size distribution.
Fig. 1 Experimental, approximated and desmeared SAXS curve of AuNP after toluene subtraction
The pair distance distribution function (PDDF) obtained after indirect Fourier transformation (IFT) is shown in Figure 2. It can be seen that the most prominent pair distance (maximum of the PDDF) of the colloid is 2.4 nm (corresponding to the AuNP radius), with a maximum dimension of ~ 5 nm. These results are in excellent agreement with the geometric radius known from TEM measurement.
Fig. 2 The p(r) function (pair distance distribution function) exhibits a Gaussian shaped curve, indicating the monodisperse and nearly perfect shaped spheres.