Biosensing applications of single-walled carbon nanotubes have been demonstrated in solid-state device structures(1-3). Bioanalyte sensing schemes based on coupling of reversible nanotube fluorescence quenching to redox reactions paired to enzymatic peroxide generation have also been pursued(4,5). Here we show a new approach to highly sensitive nanotube-based optical sensing. Single-walled carbon nanotubes interacting with dye ligand conjugates-a redox-active dye molecule that is covalently bound to a biological receptor ligand ( such as biotin in this case)-showed fluorescence quenching. Further interaction between the receptor ligand on the conjugates and target analytes ( avidin in this case) induced the recovery of the quenched fluorescence, forming the basis of the sensing scheme. Nanomolar sensitivity was attained with high specificity for the target analyte. This is a versatile approach because a wide range of conjugation possibilities exists between the potential receptors and redox quenchers.