Self-Assembly of Polyacrylate-Capped Platinum Nanoparticles on a Polyelectrolyte Surface: Kinetics of Adsorption and Effect of Ionic Strength and Deposition Protocol

Polyacrylate-capped Pt nanocrystallites of 2.5 ± 0.6 nm diameter were synthesized by reduction of Pt(IV) with citrate in the presence of polyacrylate and were self-assembled layer-by-layer in poly(diallyldimethylammonium chloride) by virtue of the Coulombic attraction between the negatively charged capping agent and the cationic polyelectrolyte. The self-assembly protocol resulted in a reproducible consecutive surface-charge buildup, leading to a linear incorporation of nano-Pt per layer. TEM imaging revealed a chainlike distribution of the nanoparticles on the polymer surface at submonolayer coverage. The density of assembled nanoparticles per layer can be controlled by varying the dipping time, the Pt solution ionic strength, and its composition or by resorting to an interrupted-deposition protocol involving multiple rinsing/drying cycles per layer. The dynamics of nano-Pt assembly on PDDA was followed ex-situ by UV−visible spectroscopy. The adsorption was found to take place via two kinetics regimes at different time scales. Initially, the adsorption rate is believed to be limited by the nanoparticles availability at the solution/surface interface. At high surface coverage, the slower polyelectrolyte−nanoparticle surface rearrangement necessary for further attachment becomes the rate-determining step, leading to a pseudosaturation within ∼1 h.