Synthesis of gold nanoparticle necklaces using linear-dendritic copolymers

Linear-dendritic copolymers containing hyperbranched poly(citric acid) and linear poly(ethylene glycol) blocks (PCA-PEG-PCA) were used as reducing and capping agents to synthesize and support gold nanoparticles (AuNPs). PCA-PEG-PCA copolymers with 1758, 1889 and 3446 molecular weights, called A1, A2 and A3 through this work, respectively, were synthesized using 2, 5, and 10 citric acid/PEG molar ratios. The diameter of A1, A2 and A3 in a fresh water solution was investigated using dynamic light scattering (DLS) and it was between 1.8 and 2.8 nm. AuNPs were simply synthesized and supported by addition a boiling aqueous solution of HAuCl₄ to aqueous solutions of A1, A2 and A3. Supported AuNPs were stable in water for several months and agglomeration was not occurred. The loading capacity of A1, A2 and A3 and the size of synthesized AuNPs were investigated using UV spectroscopy and transmission electron microscopy (TEM). It was found that the loading capacity of PCA-PEG-PCA copolymers depend on the concentration of copolymers and the size of their poly(citric acid) parts directly. For example average loading capacities for 400 μM concentration of A1, A2 and A3 were 32.24, 37.4 and 41.52 μM, respectively, and average loading capacities for 400, 200 and 100 μM concentration of A1 were 32.24, 20.28 and 9.1 μM, respectively. Interestingly there was a reverse relation between the size of synthesized AuNPs and size of poly(citric acid) parts of PCA-PEG-PCA copolymers.