This is a schematic illustration of the assembly of DNA-functionalized nanoparticles (NPs) at positively charged interfaces. (a) In the absence of salt, interactions are dominated by the electrostatic repulsion between DNA chains. (b) The 2D assemblies can be altered by programming the interactions between the NPs. By introducing monovalent salt, an attractive interaction between the NPs is switched ON, due to DNA hybridization. Change in interaction between NPs provides the path to tune the structure of the 2D assemblies at the interface.