To bind two entities together, an attractive interaction is needed. In biological systems, such interactions are often between ligands and receptors. But this interaction constantly breaks and forms because it is (too) weak. To ensure a lasting bond, the system can form multiple weak bonds that form an overall strong bond – similar to velcro. An interesting feature of aweak multivalent systemis the sharp discrimination between surfaces based on receptor density. That means that when multivalent particles encounter surfaces with the specific receptor but different densities, they will most likely bind to the surface with the highest density, because it has the highest binding probability. This phenomenon is called superselectivity and emerges from the large entropic contribution in amultivalent system: The more ligands and receptors are involved in the binding, the more possibilities the system has to form a bond and hence a large entropy. In this thesis we investigate how the interaction strength and entropy influences superselective binding. In doing so, we study superselective binding of microparticleswith hundreds of interactions and, additionally, particles with only few interactions of the size of nanometers. @en

Reliably distinguishing between cells based on minute differences in receptor density is crucial for cell-cell or virus-cell recognition, the initiation of signal transduction, and selective targeting in directed drug delivery. Such sharp differentiation between different surfaces based on their receptor density can only be achieved by multivalent interactions. Several theoretical and experimental works have contributed to our understanding of this "superselectivity." However, a versatile, controlled experimental model system that allows quantitative measurements on the ligand-receptor level is still missing. Here, we present a multivalent model system based on colloidal particles equipped with surface-mobile DNA linkers that can superselectively target a surface functionalized with the complementary mobile DNA-linkers. Using a combined approach of light microscopy and Foerster resonance energy transfer (FRET), we can directly observe the binding and recruitment of the ligand-receptor pairs in the contact area. We find a nonlinear transition in colloid-surface binding probability with increasing ligand or receptor concentration. In addition, we observe an increased sensitivity with weaker ligand-receptor interactions, and we confirm that the timescale of binding reversibility of individual linkers has a strong influence on superselectivity. These unprecedented insights on the ligand-receptor level provide dynamic information into the multivalent interaction between two fluidic membranes mediated by both mobile receptors and ligands and will enable future work on the role of spatial-temporal ligand-receptor dynamics on colloid-surface binding.@en