Size sorting of citrate reduced gold nanoparticles by SdFFF

In the past few decades, gold nanoparticles (GNPs) synthesis has attracted a considerable interest in the field of nanoscience because of their remarkable physical and chemical properties. Useful properties can be incorporated into the design of GNPs for manipulation or detection of biological structures and systems. The new materials which derive from the combination of GNPs and biological molecules have great potential applications in the area of electronics, optics as well as in genomics, proteomics. GNPs are currently used in the fields of catalysis, nanophotonics, and gene and drug delivery. In the present study, the preparation of GNPs with the citrate reduction method has been investigated in the citrate/gold(III) ratio 1:1 up to 10:1 and the size of the resulting nanoparticles was measured by Sedimentation Field-Flow Fractionation (SdFFF). Experimental data, in agreement with those presented in a previous study [1], showed that the GNPs size decreases in the citrate/gold(III) ratio range 1:1 to 3:1 and then increases from 5:1 to 10:1 passing through a plateau region in between, and is almost independent of the precursor solutions concentrations. In the zone of minimum diameters the synthetic process does not produce monodispersed GNPs but often multiple distributions, very close in size, are observed as evidenced by the particle size distributions (PSD) derived from the SdFFF fractograms. It has been also proven that GNPs suspensions tend to agglomerate over the time and the agglomerates cannot be destroyed by a mechanical action. UV-Vis spectrophotometry was used throughout this work being the most common technique employed in the optical characterization of nanoparticles suspensions and SEM has been used to check data consistency. A confirmation of the nucleation-aggregation-fragmentation mechanism was inferred from the cross-correlation between UV-Vis and SdFFF results.