Visual detection of trace lead(II) using a forward osmosis-driven device loaded with ion-responsive nanogels

A simple and portable thermometer-type device based on forward osmosis-driven liquid column rising is developed for visual detection of trace Pb²⁺. The device consists of a top indicator tube, a chamber loaded with Pb²⁺-responsive poly(N-isopropylacrylamide-co-benzo-18-crown-6-acrylamide) (PNB) smart nanogels and a bottom semipermeable membrane. Upon the recognition of Pb²⁺, PNB smart nanogels undergo a Pb²⁺-induced hydrophobic to hydrophilic transition, which simultaneously causes the increase of osmotic pressure inside the device. Driven by this osmotic pressure difference, more Pb²⁺ solution flows into the device, causing the rise of the liquid column in the indicator tube, which can be directly observed by naked eyes. The relationship between the change of liquid column height and the Pb²⁺ concentration is investigated for the quantitative detection of Pb²⁺. With the proposed forward osmosis-driven device, trace Pb²⁺ as low as 10^-10 M in aqueous solutions can be detected. This method provides a novel and simple strategy for the visual detection of trace Pb²⁺.