Resolving the Difference in Electric Potential within a Charged Macromolecule

Abstract

The difference of the electric potential between the middle and end of polystyrenesulfonate (PSS–) chain is discovered experimentally. Using a pH-responsive fluorophore attached to these two locations on the PSS– chain, the local pH value was determined by single molecule fluorescence technique: photon counting histogram (PCH). By the observation of a very high accumulation of proton (2–3 orders of magnitude in concentration) at the vicinity of the PSS– as a result of the electrostatic attraction between the charged chain and protons, the electric potential of the PSS– chain is determined. A higher extent of counterion adsorption is discovered at the middle of the PSS– chain than the chain end. The entropy effect of the counterion adsorption is also discovered—upon the dilution of protons, previously adsorbed counterions are detached from the chain.