Gold nanoparticles with a polymerizable surfactant bilayer: synthesis, polymerization, and stability evaluation.

Gold nanoparticles were synthesized using a seed-mediated wet chemical approach with a polymerizable surfactant, 11-(acryloyloxy)undecyltrimethylammonium bromide. The polymerizable surfactant forms a bilayer on the surface of the gold nanoparticles with the quaternary positive head groups facing the bulk water, and the polymerizable groups reside in the hydrophobic region. Evidence for on-particle polymerization using a cationic free radical initiator was confirmed by NMR and FTIR analyses. Ultraviolet-visible spectroscopy, transmission electron microscopy, and dynamic light scattering measurements showed no particle aggregation after polymerization. In contrast to the nanoparticles before polymerization, the nanoparticles with a polymerized bilayer showed remarkable stability against dialysis, centrifugation-resuspension cycles, and extraction with organic solvent. This enhanced stability was linked to the decrease in surfactant desorption from the surface of the gold nanoparticles as evident from mass spectrometry analysis of supernatants. Our results indicate that the use of a polymerizable surfactant as a directing agent to prepare gold nanoparticles and subsequent "fixing" the surfactant bilayer by polymerization is an effective approach to enhance the stability of the final nanoparticles.