Fig. 3. Kinetic analysis of removal of surface-bound particles shows spatial preference for cell periphery.
a Interference reflection microscopy images of a HeLa cell spreading and tearing off biotin-neutravidin surface immobilized virus-sized gold nanoparticles. Where the cell formed close contacts with the substrate, light reflection and interference with the incident light creates the dark contrast. The nanoparticles scattered the incident light and thus appear bright. In the maximum projection with temporal color-code stable particles are displayed in white, while particles that were removed within the first 30 min are blue and particles that were moved between 30 and 60 min appear red and the cell outline at t0 + 30 min is printed in blue. Scale bar, 10 µm. b Schematic of the regions of interest (ROIs) for single ROI analysis and analysis using multiple ROIs. In the single ROI analysis, we considered particles, which fall within the blue ROI defined as the projected cell area (black circles) at tx = t0 + 30 min Data were normalized to the particles in this ROI at t0. In the analysis using multiple ROIs we grouped particles (similar gray areas), that stayed underneath the projected cell area for at least the indicated interaction time t*. Data were normalized to the available number of particles per interaction time and corrected for the already torn off particles at all shorter times. c Normalized number of surface-bound nanoparticles without ligands under HeLa cells over time either analyzed in a single ROI or multiple ROIs. Data are represented as mean ± s.e.m. and fitted with two-phase decay functions. d For the particles being actively removed by the HeLa cells, the fraction a and the off rate koff are shown as obtained by fitting from the two-phase-decay of particles (n = 16 cells, 3 technical replicates, box-plots represent median ± 95% CI with whiskers to the min and max values, one-way ANOVA with post hoc Tukey test). Source data are provided as a source data file.