Single plasmonic nanoparticle tracking studies of solid supported bilayers with ganglioside lipids

Single-particle tracking experiments were carried out with gold nanoparticle-labeled solid supported lipid bilayers (SLBs) containing increasing concentrations of ganglioside (GM₁). The negatively charged nanoparticles electrostatically associate with a small percentage of positively charged lipids (ethyl phosphatidylcholine) in the bilayers. The samples containing no GM₁ show random diffusion in 92% of the particles examined with a diffusion constant of 4.3(±4.5) × 10^-9 cm²/s. In contrast, samples containing 14% GM₁ showed a mixture of particles displaying both random and confined diffusion, with the majority of particles, 62%, showing confined diffusion. Control experiments support the notion that the nanoparticles are not associating with the GM ₁ moieties but instead most likely confined to regions in between the GM₁ clusters. Analysis of the root-mean-squared displacement plots for all of the data reveals decreasing trends in the confined diffusion constant and diameter of the confining region versus increasing GM₁ concentration. In addition, a linearly decreasing trend is observed for the percentage of randomly diffusing particles versus GM₁ concentration, which offers a simple, direct way to measure the percolation threshold for this system, which has not previously been measured. The percolation threshold is found to be 22% GM₁ and the confining diameter at the percolation threshold only ∼50 nm.