Fig. 1: Concept of the Crown Ether/SiP platform for Pb²⁺ ion detection.

a 3-D illustration of the photonic Pb²⁺ ion sensor based on the crown ether-decorated SiP platform. The functionalization performed in the sensing region is indicated. For the sake of clarity, the 20 nm SiO₂ deposited on top of the waveguides in the sensing region is not indicated. Information is provided in Fig. 3a. b The zoomed-in illustration of the sensing region, where the binding between the Pb²⁺ ions and crown ether functional layer is depicted. The technology sub-layers in the sensing region are labeled: SiP, Fischer esterification, and the crown ether functional layer. c Micrograph image of the Pb²⁺ ion sensor, where the sensing arm and scale bar (500 μm) are indicated d The Pb²⁺ photonic sensor assembly, consisting of the photonic chip and a microfluidic chamber. e Elucidated operating principle of the photonic Pb²⁺ ion sensor. The inset shows the exemplary applications that the ion detection platform can be extended to.