Effect of surface Zeta potential on the deposition of inorganic nanoparticles on polydopamine (PDA) grafting layers

Polydopamine (PDA) is a versatile, bio-inspired material that enables the controlled grafting of inorganic nanoparticles onto different substrates. In this paper, we investigate how the surface charge of a PDA layer—tuned by changing the working pH—dictates the assembly efficiency and surface density of three distinct nanoparticle types: spherical silver nanoparticles (AgNPs), non-spherical gold nanostars (AuNS), and cubic Prussian Blue nanoparticles (PBNPs). AgNPs and AuNS bear a negative surface charge, while PBNPs were prepared with both negative and positive surface charge. By changing the pH during the nanoparticle grafting on a PDA functionalized glass, we modulate the surface Zeta potential and thus the extent of electrostatic attraction or repulsion with each nanoparticle suspension. In all cases, optimal surface coverage of a nanoparticle monolayer correlates with complementary charges between PDA and the particles: negatively charge PDA favors grafting of positively charged particles, while grafting of negative nanoparticles is optimal when PDA is positively charged. All the functionalized surfaces were characterized by means of optical techniques (UV-Vis spectroscopy) and electron microscopy (SEM), to determine the surface coverage on each sample. Our results demonstrate that precise control of PDA's surface charge provides a universal, facile strategy to tailor nanoparticle loading and surface density, with implications for sensors, catalysis, and biointerfaces.