Simultaneous enrichment, separation and detection of mercury(II) ions using cloud point extraction and colorimetric sensor based on thermoresponsive hyperbranched polymer-gold nanocomposite

A strategy for enriching, separating and detecting Hg2+ based on thermoresponsive hyperbranched polyethylenimine (HPEI) with isobutyramide (IBAm) end groups (HPEI-IBAm) capped gold nanoparticles (AuNPs) is reported. The thermoresponsive AuNPs can enrich approximately 94.5% of Hg2+ in solution, with higher enriching efficiency than that toward Ni2+, Cu2+, Mn2+, Pb2+ and Cr3+ heavy metal ions. The enriched Hg2+ can be separated by simple stratification or centrifugation. By virtue of the specific amalgam of reduced mercury(II) ions with thermoresponsive HPEI-IBAm capped gold nanoparticles (AuNPs), an unusual color change from red to fawn can be used as a colorimetric probe to detect Hg2+ with a limit of detection (LOD) of 8.8 nM by the naked eye and 1.1 nM by a UV-vis spectrophotometer. Importantly, this colorimetric sensor based on the formation of non-aggregated Au-Hg alloy stabilized by thermoresponsive HPEI-IBAm is more sensitive than that of the usual colorimetric sensors (red to blue) based on aggregation of AuNPs. After the color changed, the Au-Hg alloys did not aggregate further and could remain stable for a long observation time due to the stabilizing effects of HPEI-IBAm, which was convenient to record, detect and observe. The sensing process is very fast with a response time of less than 0.5 minutes. In addition, the proposed method showed an excellent selectivity for Hg2+ and was successfully applied in tap water and drinking water samples. Therefore, this strategy for the first time achieved simultaneous enrichment, separation and detection of Hg2+ based on the thermoresponsive HPEI-IBAm/AuNPs composite, which has the potential application for environmental renovation and groundwater analysis.