Ionic strength mediated phase transitions of surface adsorbed DNA on single-walled carbon nanotubes

Single-stranded DNA (ssDNA) oligonucleotides have unique, and in some cases, sequence-specific molecular interactions with the surface of carbon nanotubes that remain the subject of fundamental study. In this work, we observe and analyze a generic, ionic strength mediated phase transition exhibited by over 25 distinct oligonucleotides adsorbed to single-walled carbon nanotubes (SWCNTs) in colloidal suspension. The phase transition occurs as monovalent salts are used to modify the ionic strength from 500 mM to 1 mM, causing a reversible reduction in the fluorescence quantum yield by as much as 90 percent. The phase transition is only observable by fluorescence quenching within a window of pH and in the presence of dissolved O2, but occurs independently of this optical quenching. The negatively charged phosphate backbone increases (decreases) the DNA surface coverage on an areal basis at high (low) ionic strength, and is well described by a two state equilibrium model. The resulting quantitative model is ab...