Suppression of DNA coffee-ring by compacting agents via adsorption at water/substrate and water/air interfaces.

The drying of sessile drops of DNA solutions commonly leads to inhomogeneous deposition caused by the so-called coffee-ring effect, which hampers uniform surface immobilization in biosensing and microarray applications. Here, we investigate how DNA-compacting agents influence DNA patterns formed on negatively and positively charged solid substrates upon drop drying. Using poly-L-lysine and spermine as model compacting agents, we demonstrate that the suppression of the coffee-ring effect can arise from two distinct phenomena depending on the compacting agent concentration regime. At low concentrations, compacting agents mainly induce electrostatic adsorption of DNA at the water/substrate interface. Conversely, at higher concentrations, DNA compacts and aggregates, leading to trapping of aggregates at the water/air interface during evaporation and preventing their transport to, and accumulation at, the drop contact line. These findings constitute a new way to suppress the coffee-ring effect through DNA neutralization and compaction, and establish adsorption at interfaces as a general strategy for achieving homogeneous DNA coatings.