Assembly of citrate gold nanoparticles on hydrophilic monolayers

Self-assembled monolayers (SAMs) as model surfaces were linked onto planar gold films thoroughlipoic acid or disulfide groups. The molecules used were polyethylene glycol (EG-S-S), N-[tris-(hydroxymethyl)methyl]acrylamide polymers with and without lipoic acid (Lipa-pTHMMAA andpTHMMAA) and a lipoic acid triazine derivative (Lipa-MF). All the layers, but Lipa-MF with a primaryamino group were hydroxyl terminated. The layers were characterized by contact angle measurementsand atomic force microscopy, AFM. Citrate stabilized nanoparticles, AuNPs in water and phosphate bufferwere allowed to assemble on the layers for 10 min and the binding was followed in real-time with surfaceplasmon resonance, SPR. The SPR resonance curves were observed to shift to higher angles and becomeincreasingly damped, while also the peaks strongly broaden when large nanoparticles assembled on thesurface. Both the angular shift and the damping of the curve was largest for nanoparticles assembling onthe EG-S-S monolayer. High amounts of particles were also assembled on the pTHMMAA layer withoutthe lipoic acid group, but the damping of the curve was considerably lower with a more even distributionof the particles. Topographical images confirmed that the highest number of particles were assembled onthe polyethylene glycol monolayer. By increasing the interaction time more particles could be assembledon the surface.