Double shielding by chitosan/maleic acid copolymer complex improves stability of gold nanoparticles in isotonic solution

Gold nanoparticles (GNPs) are currently used in various diagnostic and therapeutic applications due to their biocompatibility and unique optical properties. One of the challenges limiting their use is the sensitivity to the presence of salts, which is manifested in changes of the absorption spectra and the visual color of the sol. In this study, we investigate the physicochemical patterns of complexation at a pH of 7.4 between water-soluble chitosan and ethylene-alt-maleic acid copolymer (EMA), which is used to form a stabilizing shell around gold nanoparticles. It is found that the formation of a polyelectrolyte complex (PEC) does not cause any changes to the spectrum or size of the GNPs. Light scattering and isothermal titration calorimetry methods have shown that at pH 7.4, both carboxyl groups of the EMA unit are successively involved in the complexation process. The changes occurring in the structure of the GNPs’ shell upon the addition of chitosan have been analyzed. Comparison of the effect of salt on the spectral characteristics of GNPs stabilized by an EMA copolymer and those containing PEC in the shell has demonstrated additional stabilization of GNPs against aggregation when PEC is formed. Changes in the absorption spectrum and visual color of the sol containing PEC do not occur until a concentration of 0.15 M NaCl, which makes it promising for increasing the range of its applicability in diagnostic and therapeutic systems in environments with physiological pH and ionic strength.