Attractive Interactions between DNA-Carbon Nanotube Hybrids in Monovalent Salts

DNA carbon nanotube (DNA-CNT) hybrids are nanometer-sized, highly charged, rodlike molecules with complex surface chemistry, and their behaviors in aqueous solutions are governed by multifactorial interactions with both solvent and cosolutes. We have previously measured the force between DNA-CNTs as a function of their interaxial distance in low monovalent salts where interhybrid electrostatic repulsion dominates. The characteristics of DNA-CNT forces were further shown to closely resemble that of double-stranded DNA (dsDNA) in low salts. However, contrasting behaviors emerge at elevated monovalent salts: DNA-CNT condenses spontaneously, whereas dsDNA remains soluble. Here we report force distance dependencies of DNA-CNTs across wide-ranging monovalent salt concentrations. DNA-CNT force curves are observed to deviate from dsDNA curves above 300 mmol/L NaCl, and the deviation grows with increasing salts. Most notably, DNA-CNT forces become net attractive above 1 mol/L NaCl, whereas dsDNA forces are repulsive at all salt concentrations. We further discuss possible physical origins for the observed DNA-CNT attraction in monovalent salts, in consideration of the complex surface chemistry and unique polyelectrolyte properties of DNA-CNT hybrids.