Single-molecule Forster Resonance Energy Transfer was used to observe the adsorption of fluorescentlylabeled "target" DNA oligonucleotides and their association and hybridization with complementary DNA "probes" tethered to the surface as a function of surface grafting density. Ionic strength was varied systematically to disentangle the potentially competing effects of probe accessibility and electrostatic repulsion. At high ionic strength, when the Debye length was similar to 1 nm, the adsorption of target DNA was not significantly inhibited by the presence of tethered probe DNA, even at high grafting density, and the fraction of adsorbed target strands undergoing hybridization increased systematically with grafting density, leading to a dramatic increase in the net hybridization rate at high grafting density. However, at lower ionic strength, when the Debye length was >= 3 nm, the adsorption rate of target DNA decreased and the fraction of adsorbed target strands undergoing hybridization saturated at high probe grafting density (>= 7,000 strands/mu m(2)), presumably due to electrostatic repulsion. As a result, the net rate of hybridization exhibited a maximum as a function of grafting density. This has important consequences for the design of systems that optimize surface-mediated DNA hybridization under low-salt high-stringency conditions. (C) 2020 Elsevier Inc. All rights reserved.