Binary superlattices formed by charged grafted gold nano-particles*

Taking advantage of the aqueous biphasic behavior of polyethylene glycol (PEG), it has been shown that gold nanoparticles (AuNPs) grafted with PEG (PEG-AuNPs) form two- and three-dimensional crystalline structures (including binary structures) by varying salinity and temperature of the suspensions. To extend the control over binary superlattices, we introduce charged end groups to PEG, in this case, carboxylic (-COOH) and amine (-NH2) groups. We show that the effective surface charge per grafted nanoparticle can be tuned by pH leading to assembly into complex super-structures. Using small-angle X-ray scattering (SAXS) technique, we show that by mixing oppositely charged PEG-AuNPs, i.e., COOH-PEG-AuNPs and NH2-PEG-AuNPs, a few binary cubic superlattices can be formed at room temperature by manipulating mixing ratios and pH level.