Temperature- and pH-Responsive Hydrogel Nanoparticles with Embedded Au Nanoparticles as Catalysts for the Reduction of Dyes

Here, we report a facile design and synthesis of Au nanoparticle (Au NP)-immobilized poly(N-isopropylacrylamide-co-methacrylic acid) hydrogel nanoparticles, also referred to as hybrid microgels (Au-MGs), by employing in situ reduction of Au(III) ions in a nanoscale polymeric network. This approach offered formation of controlled size Au NPs within the microgels with an average particle size as low as 3.3 ± 0.9 nm. The prepared hybrid microgels were shown to possess an excellent catalytic activity in the degradation of a cationic dye, methylene blue (MB). The embedded Au NPs were remarkably stable against self-aggregation thanks to the particle trapping within the polymeric network, and thus, the particles maintained their outstanding catalytic performance over a period of time. On account of the dual-responsive nature of the microgels, a tuneable catalytic activity could be achieved by adjusting the temperature and pH value of the experiments. Furthermore, other reaction parameters such as catalyst dosage, initial dye concentration, and ionic strength strongly influenced the catalytic performance of this heterostructure catalyst. The Au-MG hybrid exhibited a higher rate of MB reduction under the high Au-MG concentration (0.14 mg/mL), low MB concentration (0.05 mM), alkaline medium (pH 11), low ionic strength (no added salt), and low temperature (20 °C). The reusability of the catalyst was found to be fair enough, making it an excellent cost-effective material for dye degradation. Our strategy used herein to prepare Au-based multifunctional hybrid materials with high catalytic performance could open up new avenues for the sustainable development of materials in wastewater treatment.