Self-limiting gold nanoparticle surface assemblies through modulation of pH and ionic strength

Techniques to assemble monolayers of nanoparticles on surfaces are crucial for manufacturing devices for applications ranging from bio-sensing to tribology. Electrostatic-mediated assembly has numerous potential attributes, including self-limiting deposition and the ability to tune nanoparticle density and order through solution conditions. Herein, we establish the synergistic role of pH, ionic strength (I), and particle functionalization to identify the conditions for electrostatic assembly that yield maximum process stability and particle coverage. When the particle and surface are oppositely charged, the density of adsorbed 11.4-nm gold nanoparticles (AuNPs) could be tuned with both pH and ionic strength. The resulting monolayer arrays were disordered, in agreement with random sequential adsorption (RSA) theory. Finally, AuNPs stabilized by associated citrate molecules provided a larger processing window (pH 3–9, I = 1–10 mM) than AuNPs capped with a covalently bound mercaptopropanesulfonate (MPS) ligand shell (pH 3–9, I = 0.1–5 mM). These processing regimes provide a standard for predicting structural formations at reduced particle-surface interactions.