Effect of Nanoparticle Surface Chemistry on Adsorption and Fluid Phase Partitioning in Aqueous/Toluene and Cellular Systems

Copolymers of di(ethylene glycol) methyl ether methacrylate (x = MeO₂MA) and oligo(ethylene glycol) methyl ether methacrylate (y = OEGMA) display lower critical solution phenomena in aqueous systems that are tunable by the copolymer ratio (x:y), ionic strength, and temperature. These properties enable tuning the hydrophobicity of macromolecular systems by variation of (x:y). For nanoparticles stabilized with these macromolecules, this provides a systematic approach to understanding the impact of surface chemistry, specifically hydrophobicity, on the equilibrium and transport properties of nanomaterials in biphasic systems. We synthesized a homologous series of gold nanoparticles capped by these copolymers, Au@(MeO₂MA _x -co-OEGMA _y ). By varying the copolymer 95:5 <(x:y) < 80:20 ratio, we studied the effect of surface hydrophobicity on the nanoparticle equilibrium adsorption isotherm and phase transfer at the aqueous-toluene interface. The increase in hydrophobicity from (x:y)= 80:20 to (x:y)= 95:5 is accompanied by an increase in the fractional coverage of the aqueous-toluene interface from f = 0.3 to f >1, or multilayer adsorption and an increase in the characteristic adsorption timescale from τ_D = 31 to τ_D = 450 seconds. The equilibrium partition coefficient for the aqueous/toluene systems, K_{T /W} is also a strong function of (x:y), increasing from K_{T /W} (80:20) = 0.7 to K_{T /W} (95:5) = 9.8. We also observed an increase in cellular uptake for increasing (x:y) suggesting that surface chemistry alone plays a significant role in intercellular transport processes.