The interfacial behaviour of singlepoly(N,N-dimethylacrylamide) chains as a function of pH

Abstract

We have studied the pH-dependent conformational behaviour ofpoly(N,N-dimethylacrylamide) (PDMAc) at silicon and gold surfaces using single-moleculeforce spectroscopy and a quartz crystal microbalance (QCM). Despite the pHdependence, nuclear magnetic resonance and titration experiments demonstrate thatPDMAc is not a weak polybase. The interaction between single chains and asilicon surface (with native oxide layer intact) in aqueous solution was investigatedusing force spectroscopy. Single-molecule force measurements were performedusing thiol-functionalized PDMAc and gold-coated AFM cantilevers. The forces ofinteraction between PDMAc and the native oxide-coated silicon surface varywith the pH. The shape of the retraction curve for low pH solution includes agreater number of ‘train’ conformations, which suggests a stronger interactionwith the surface relative to the surrounding media at low pH. The adsorptionbehaviour of PDMAc, from liquid onto silicon surfaces, was monitored using aQCM, which shows greater PDMAc adsorption onto silicon at low pH. The forcespectroscopy and QCM investigations confirm that the PDMAc chain is moreextended and stiffer in low pH solution. We attribute the pH-dependent behaviourto an increased number of hydrogen bonding sites on the silicon surface at lowpH.