Figure 1.

Preparation of the charged nanoparticles and the experimental system used for observations of the particle-MT interaction. (A) Negatively-stained image of polyacrylamide particles after fractionation via sucrose density gradient centrifugation. Bar = 500 nm. (Inset) Magnified view (bar = 100 nm). The aspect ratio of the particle (ratio of major to minor axis) was 1.13 ± 0.13 (mean ± SD, n = 865), indicating that particles can be regarded as spheres rather than ellipsoids. (B) A side view of the particles showing their flattening on the charged surface of the carbon film (bar = 100 nm). Such side views were obtained from a few accidental occasions where the carbon film was torn and curved. Based on such images, a ratio of equatorial radius to height, a measure of shape flattening, was calculated to be 1.27 ± 0.16 (n = 11). (C) Distribution of the particle diameters for a fraction of particles with an amine density 0.96 nm⁻³, after correction for shape flattening. The average of the diameters was 57 ± 13 nm (mean ± SD, n = 865). For details of the diameter measurement, see Fig. S1 and Methods in the Supporting Material. (D) Interactions between charged nanoparticles and MTs were observed by DFM. To eliminate the electrostatic interactions of particles or MTs with the glass surface, MTs were attached to a hydrophobically modified glass surface (see Materials and Methods for details). (E) A representative DFM image of nanoparticles interacting with MTs. Bar = 3 μm.