Functionalization of a gold surface with DNA is often complicated by kinetic traps from unintended DNA base adsorption. Herein, we communicate that BC serves as a robust backfilling agent displacing selected DNA bases on gold. Traditional thiol backfillers are too strong, while even 300 mM BC is well tolerated. Conjugates prepared with BC hybridize 10-fold faster and resist DNA release with better colloidal stability yielding highly sensitive probes. From colorimetric and Raman assays, adsorption affinity ranks as F- < T approximate to Cl- < C < G approximate to BR- < A < I-, allowing Br- to displace nonpoly-A sequences from gold. This well-controlled biointerface will impact biosensing, drug delivery, and directed assembly of nanomaterials.