Green and Facile Synthesis of Highly Stable Gold Nanoparticles via Hyperbranched Polymer In-Situ Reduction and Their Application in Ag+ Detection and Separation

The development of a green and facile strategy for synthesizing high stable gold nanoparticles (AuNPs) is still highly challenging. Additionally, the main problems regarding AuNPs based colorimetric sensors are their poor selectivity and low sensitivity, as well their tendency to aggregate during their synthesis and sensing process. Herein, we present an in-situ reduction strategy to synthesize thermoresponsive hyperbranched polymer (i.e., Hyperbranched polyethylenimine-terminal isobutyramide (HPEI-IBAm)) functionalized AuNPs. The HPEI-IBAm-AuNPs show excellent thermal stability up to 200 ◦C, high tolerance of a wide range of pH value (3–13), and high salt resistance. HPEI-IBAm acted as the template, the reducing agent, and the stabilizing agent for the preparation of AuNPs. The HPEI-IBAm-AuNPs can be used as colorimetric sensors for the detection of Ag+. In the detecting process, HPEI-IBAm serves as a trigger agent to cause an unusual color change from red to brown. This new non-aggregation-based colorimetric sensor showed high stability (maintaining the color lasting without fading), high selectivity, and high sensitivity with an extremely low detection limit of 7.22 nM and a good linear relationship in a wide concentration range of 0–2.0 mM (R2 = 0.9921). Significantly, based on the thermoresponsive property of the HPEI-IBAm, the AuNPs/Ag composites can be separated after sensing detection, which can avoid secondary pollutions. Therefore, the green preparation and the applications of the unusual colorimetric sensor truly embody the concepts of energy saving, environmental protection, and sustainable development.