Gold Nanoparticle-Polyelectrolyte Complexes with Tunable Structure Probed by Synchrotron Small-Angle X-ray Scattering: Implications for the Production of Colloidal Crystals-Based Nanophotonic Materials

Close-packed colloidal crystals of gold nanoparticles (AuNPs) are of great interest in the field of nanophotonics due to their lattice-pattern-dependent optical properties. One challenge resides in producing such crystalline assemblies by a scalable approach involving polydisperse AuNPs obtained by standard synthesis routes and structuring natural molecules in water at room temperature. Electrostatic complexation between functionalized AuNPs and oppositely charged polyelectrolytes (PELs) is a very simple way to create AuNP self-assemblies of different morphologies and compactness. Our work investigates, using synchrotron small-angle X-ray scattering, their structure as a function of concentration, PEL persistence length and ionic strength. By decreasing the radius, R, of the positively charged gold nanoparticles to a few nm (R <= 3 nm) and increasing the polyelectrolyte persistence length, LT, substituting flexible sodium polystyrenesulfonate for natural semiflexible hyaluronan, we tuned the characteristic ratio LT/R up to values of >= 1.85. Such ratios allow the successful formation of a new AuNPs arrangement with a high degree of short-range order at low ionic strength and of crystalline order when interactions are screened by addition of salt. This approach involving commercially available natural water-soluble semiflexible PELs, opens a low-cost and promising way for the one-step production of gold colloidal crystals, which can be of ubiquitous utility in nanophotonics.