Hyperbranched polyethylenimine-based sensor of multiple metal ions (Cu2+, Co2+ and Fe2+): colorimetric sensing via coordination or AgNP formation

Abstract

Hyperbranched polyethylenimine (HPEI), an amine rich polymer, has been used as a colorimetric probe for selective sensing of multiple metal ions (Cu²⁺, Co²⁺ and Fe²⁺) with clearly distinguishable color in aqueous solution. The selective coordination of Cu²⁺ with the chelating amines of HPEI induced a visible color change from colorless to blue across a wide pH range (pH 4.0 to 10.0). Other metal ions did not show any significant color change. Interestingly, the addition of Co²⁺ and Fe²⁺ into HPEI in the presence of silver ions (Ag⁺) leads to the formation of strong yellow and brown colors, respectively. The control studies suggest that Co²⁺ and Fe²⁺ undergo oxidation and reduce the silver ions into silver nanoparticles (AgNPs). The strong surface plasmon resonance (SPR) vibration of the AgNPs was responsible for the yellow/brown color. UV-visible studies showed a strong absorption peak at 400 nm for Co²⁺ and a broad absorption with λ_max at 420 nm for Fe²⁺. HR-TEM studies confirmed more uniform spherical AgNPs for Co²⁺ with HPEI–Ag⁺ and polydisperse AgNP aggregates for Fe²⁺. The limit of detection of the HPEI probe for Cu²⁺ is 0.25 μg L⁻¹ and those of the HPEI–Ag⁺ probe for Co²⁺ and Fe²⁺ are 40 and 30 μg L⁻¹, respectively. The practical application of the HPEI probe for selective sensing of multiple metal ions with distinguishable colors has been demonstrated on Whatman filter paper. Thus, simple commercially available HPEI has been successfully used for the selective colorimetric sensing of biologically important Cu²⁺, Co²⁺ and Fe²⁺ metal ions in aqueous medium.