Probing the Influence of the Particle in Single Particle Tracking Measurements on Lipids

Abstract

We present a systematic study of the influence of the nanoparticle used as a probe in Single Particle Tracking experiments with a supported lipid bilayer as a model system. Quantum Dots, 40 nm and 200 nm diameter fluorescent latex spheres, and 40 nm diameter gold colloids were attached to headgroup-labeled lipid molecules in the bilayer by means of an antibody. The percentage of the particle's surface covered with antibody was also varied. We argue that the best estimate of the diffusion coefficient is derived from the first two points of the particle's mean square displacement. The accuracy of this estimate is equivalent to a more rigorous analysis based on the cumulated probability function. We show that, under optimal conditions, Quantum Dots and also 40 nm latex beads have a negligible influence on the lipid diffusion. The diffusion coefficient of gold colloids and 200 nm diameter latex particles was systematically reduced 2- to 3-fold compared to the reference value obtained by Fluorescence Recovery After Photobleaching. Unexpectedly, the diffusion coefficient of Quantum Dots and 40 nm latex beads depended on the percentage of the nanoparticle's surface covered with antibody: at very low percentage the perturbation to the lipid diffusion due to the particle increased. We speculate that in this regime subtle details of antibody binding to the nanoparticle become important.In an alternative system using particles tethered with 30 or 90 bp DNA strands, particles attached with the longer tether showed the highest diffusion coefficient. We propose that the proximity of the nanoparticle to the membrane may modify lipid dynamics.ReferenceP. Mascalchi et al. 2012. Probing the influence of the particle in Single Particle Tracking measurements of lipid diffusion. Soft Matter, 2012,8, 4462-4470.