A pH and redox dual responsive homopolypeptide: synthesis, characterization, and application in “smart” single-walled carbon nanotube dispersion

Abstract

A homopolypeptide, namely poly(S-(2-carboxylethyl)-L-cysteine) (PCELC), with pH and redox dual responsiveness was prepared by ring-opening polymerization of S-(2-(methoxycarbonyl)ethyl)-L-cysteine based N-carboxyanhydride (MCELC-NCA) and subsequent hydrolysis. The pH-induced solution phase transition of PCELC was at pH = ∼4.50 as revealed by DLS and UV-vis spectroscopy. The thioether linkages of PCELC can be readily oxidized by H₂O₂ at 40 °C yielding PCELC^ox with sulfoxide and sulfone linkages as revealed by ¹H NMR and FTIR. PCELC^ox also showed pH responsiveness with the variation of polymer aggregation sizes, but without solution phase separation. PCELC^ox can be partially reduced by 3-mercaptopropionic acid as suggested by ¹H NMR and FTIR analysis. The pH-induced solution phase transition behavior of the reduction product (i.e., PCELC^re) was different from that of the as-prepared PCELC. Moreover, PCELC can be used as a pH and oxidation dual responsive stabilizer to control the aggregation of single-walled carbon nanotubes (SWCNTs) in aqueous solution.