Direct detection of DNA with an electrocatalytic threading intercalator.

Herein we report the synthesis, intercalating properties, and analytical applications of an imidazole-substituted naphthalene diimide, N,N'-bis(3-propylimidazole)-1,4,5,8-naphthalene diimide (PIND), functionalized with electrocatalytic redox moieties. PIND was prepared in a single-step reaction from the corresponding dianhydride. Attachment of the redox moieties to PIND relied upon ligand exchange with one of the liable chloride ligands of an Os(bpy)2Cl2 (bpy = 2,2'-bipyridine) complex. The Os(bpy)2Cl2 complex was grafted onto PIND through coordinative bonds with the two imidazole groups at its termini, forming a PIND-[Os(bpy)2Cl]+ compound (PIND-Os). Gel electrophoretic studies revealed that PIND-Os binds more strongly to double-stranded DNA (ds-DNA) than its parent compound 1,4,5,8-naphthalene diimide. The naphthalene diimide group binds to ds-DNA in a "classical" threading intercalation mode, while the two Os(bpy)2Cl+ pendants interact with DNA via electrostatic interaction, reinforcing the intercalation by "locking up" the naphthalene diimide group in place. An electrochemical biosensor was fabricated using the redox-active and catalytic PIND-Os intercalator. An increase in sensitivity of 2500-fold over direct voltammetry was obtained in electrocatalytic amperometry, making this an interesting system for amperometric DNA sensing. Under optimized experimental conditions, the biosensor allowed the detection of a 50-mer target DNA in the range of 1.0-300 pM with a detection limit of 600 fM (1.5 amol, 23 fg).