In-situ Generation of Gold, Platinum and Palladium Nanoparticles in N-isopropylacrylamide-surfmer Copolymer Hydrogels and Catalytic Activity of the Gels

Preparation and catalytic activity of N-isopropylacrylamide (NIPAM)/surfmer copolymer hydrogels containing gold, palladium and platinum nanoparticles are described. The functional hydrogels were obtained in three steps. First a thermoresponsive copolymer gel was prepared upon ⁶⁰Co-gamma irradiation of a micellar aqueous solution of NIPAM and (2-methacryloyloxyethyl)dodecyldimethylammonium bromide (MEDDAB) or (11-methacryloyloxyundecyl)trimethylammonium bromide (AUTMAB). Secondly tetrachlorometalate anions were introduced in the gel upon ion exchange of the bromide counterions of the positively charged surfmer head groups. For this purpose, the gel was de-swollen at 50 °C and re-swollen in aqueous solution of tetrachloroauric acid, sodium tetrachloropalladate, or sodium tetrachloroplatinate at 20 °C. Thirdly, the metal nanoparticles were in-situ generated by exposing the de-swollen gel to an aqueous solution of sodium borohydride, which led to re-swelling and reduction of the complex anions. The formation of nanoparticles caused an immediate colour change of the gel from yellow or brown to golden black or black. TEM studies indicate aggregates of nanoparticles of approximately 3.5–5.3 nm in diameter. The metal-containing gels catalyze the reduction of 4-nitrophenolate (4-NP) into 4-aminophenolate below the lower critical solution temperature (LCST) and at high osmotic pressure, where the gels are highly swollen. NIPAM/MEDDAB gels are active at 20 °C and 30 °C, whereas at 40 °C (readily above the LCST of 29 °C) the activity is low and comparable with the metal-free gel. NIPAM/AUTMAB gels with large counterion dissociation are still active at 40 °C although the LCST is only 33.8 °C. Further studies indicate that the gold-containing gels exhibit the highest catalytic activity, and that the gels can be repeatedly used for catalysis because the metal nanoparticles are only slowly washed out.