Switchable selectivity for gating ion transport with mixed polyelectrolyte brushes:approaching ‘smart’ drug delivery systems

Abstract

A pH-responsive mixed polyelectrolyte brush from tethered polyacrylic acid (PAA) and poly(2-vinylpyridine)(P2VP) (PAA:P2VP = 69:31 by weight) was prepared and used for selective gating transport of anions and cationsacross the thin film. An ITO glass electrode was modified with the polymer brush and usedto study the switchable permeability of the mixed brush triggered by changesin pH of the aqueous environment in the presence of two soluble redox probes:[Fe(CN)6]4− and[Ru(NH3)6]3+. The responsive behavior of the brush was also investigated using the in situellipsometric measurements of the brush swelling, examination of the brushmorphology with atomic force microscopy (AFM), and contact angle measurementsof the brush samples extracted from aqueous solutions at different pHvalues. The mixed brush demonstrated a bipolar permselective behavior. AtpH<3 thepositively charged P2VP chains enabled the electrochemical process for the negatively charged redoxprobe, [Fe(CN)6]4−,while the redox process for the positively charged redox probe was effectively inhibited. On the contrary,at pH>6 a reversible redox process for the positively charged redox probe,[Ru(NH3)6]3+, was observed, while the redox process for the negatively charged redox species,[Fe(CN)6]4−, was fully inhibited. Stepwise changing the pH value and recording cyclic voltammogramsfor the intermediate states of the polymer brush allowed electrochemical observation of thebrush transition from the positively charged state, permeable for the negatively chargedspecies, to the negatively charged state, permeable for the positively charged species. Thedata of ellipsometric, AFM and contact angle measurements are in accord with theelectrochemical study. The discovered properties of the brush could be used for thedevelopment of ‘smart’ sensors and drug delivery systems, for example, a smart drugdelivery capsule which could release negatively charged molecules of drugs in acidicconditions, while positively charged molecules of drugs will be released in neutralconditions.