Carboxylated Carbon Nanotubes in Hydrogels

Carboxyl-functionalized multi-walled carbon nanotubes (MWCNT-COOH) with average outer diameter 30 nm and length 1–5 µm (PD30L1-5-COOH, purity > 95%) were obtained from NanoLab (Waltham, MA, USA). These nanotubes were prepared by reflux in concentrated sulfuric/nitric acid and are highly functionalized (2–7 wt% of COOH groups by titration as reported by the manufacturer). The following products were supplied by Sigma-Aldrich (Schnelldorf, Germany) and used without further purification: N-Isopropylacrylamide (NIPAM, 97%, 415324), N,N′-methylenebisacrylamide (MBA, 99%, 146072), and 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959, 98%, 410896). Ultrapure deionized water (resistivity 18.2 MΩ·cm) was produced by a Direct-Q3 UV water purification system (Millipore, Molsheim, France) and used in all preparations.