Capsule membranes encapsulated with smart nanogels for facile detection of trace lead(II) ions in water

Abstract

A novel method based on capsule membranes encapsulated with smart nanogels is successfully developed for facilely detecting trace lead(II) (Pb2+) ions, which are hazardous to both human health and the environment because of their toxicity. The capsule membrane system is composed of a semi-permeable calcium alginate membrane and encapsulated poly(N-isopropylacrylamide-co-acryloylamidobenzo-18-crown-6) (PNB) nanogels. The semi-permeable membrane allows Pb2+ ions and water to pass through quickly, but rejects the encapsulated nanogels and polymers totally. As soon as Pb2+ ions appear in the aqueous environment and enter into the capsule, they can be specifically recognized by encapsulated PNB nanogels via forming 18-crown-6/Pb2+ complexes that cause a Pb2+-induced phase transition of PNB nanogels from hydrophobic to hydrophilic state. As a result, the osmotic pressure inside the capsule membrane increases remarkably, and thus the elastic capsule membrane isothermally swells upon the presence of Pb2+ ions in the environmental aqueous solution. The Pb2+-induced swelling degree of the capsule membrane is dependent on the concentration of Pb2+ ions ([Pb2+]) in water. Thus, the [Pb2+] value in water is able to be easily detected by directly measuring the Pb2+-induced isothermal swelling ratio of the capsule membrane, which we demonstrate by using 15 prepared capsule membranes arranged in a line. The Pb2+-induced swelling ratios of the capsule membrane groups are easily observed with the naked eye, and the detection limit of the [Pb2+] in water is 10-9 mol L-1. Such a proposed method provides an easy and efficient strategy for facile detection of trace threat analytes in water.