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

Abstract

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.