High resolution size determination of 20 nm colloidal gold particles by SedFFF

Purpose. Assessment of lower size limit of Sedimentation Field-Flow Fractio nation (SedFFF), specifically to evaluate if the method is suitable to dete rmine the size and size distribution of 20 nm colloidal gold particles with high resolution. Methods. Sedimentation Field-Flow Fractionation was used to determine the s ize of the colloidal particles. Due to the high density of gold it was poss ible to extend the lower size limit of SedFFF well below 20 nm. The size di stribution of a gold colloid was obtained from the peak broadening caused b y the polydispersity of the sample. The peak broadening due to instrumental imperfections was determined. For comparison purpose the particles were al so sized using SEM and PCS. Results. The mean diameter of the particles was determined to be (20.87 +/- 0.05) nm, the standard deviation in size being 1.04 nm (about 5%). SEM cou ld confirm that the particles are about 20 nm in diameter. A sizing with PC S was not possible. The particles have a strong tendency to aggregate and P CS yields a diameter that is much too large. Conclusions. At optimized analytical parameters Sedimentation Field-Flow Fr actionation is an effective method to measure the size of gold particles as small as 15 nm with an accuracy of about 0.1 nm. The polydispersity of the sample can easily be determined.