The influence of the peptide molar ratios on the functionalization of gold nanoparticles

Au nanoparticles (NPs) functionalized with L-cysteine (Cys) and cysteine-glycine (Cys- Gly) were synthetized. The AuNPs were prepared using sodium citrate as reducing agent. The influence of the molar concentrations of Cys and Cys-Gly, as well as the sodium citrate is studied on particle size and particle size distribution. TEM measurements revealed the formation of AuNPs with diameter in the range 5-35 nm which corresponds to nontoxic sizes [we should add a reference here, perhaps number one]. The optimal particle size for biomedical application along with narrow particle size distribution was observed for samples prepared with molar ratio of C_Au:C_citrate = 1:10. The results of UV-Vis spectroscopy revealed the interaction of the AuNPs with Cys and Gly-Cys demonstrated by a visible change in the absorption intensities of the plasmon peak located at 520 nm after AuNP functionalization and a slight shifting of this gold nanoparticles plasmon peak. Thus, any dielectric shell on surface of particles with more refraction index (and, correspondingly, dielectric function) can produce the particles with the red shift. Such effect of the surface shell with red-shift in the range of few nanometers observed for the AuNPs functionalized with Cys and Cys- Gly (Fig. 4) can be interpreted as thin or discontinuous layer of aminoacid molecules according to the data of optical spectra simulation. The optimum concentration of precursors was found for the suitable nontoxic AuNPs according to two main criteria. Firstly, particle size suitable to overcome blood brain barrier without being toxic (~20 nm) and nontoxic spherical shape. AuNPs obtained using 10 μM of citrate demonstrated the optimum narrow particle size distribution with 20 nm colloids preferentially formed. After particles functionalization with the aminoacid and peptide the particle size distribution did not change significantly. However, the number of particles with the size of 20 nm increases more than (add %) compare to the citrate method.