Hg²⁺ crosslinked two-dimensional photonic crystal hydrogel sensor for the “off–on” detection of iodine ion

Iodide is an essential microelement for human, playing a crucial role in thyroid hormone synthesis, which is vital for mental development, metabolism and cell growth. Current iodide ion (I⁻) detection methods typically require expensive instruments, are complex to operate, and lack portability, making them impractical for home applications. Therefore, there is a pressing need for a novel, portable and point-of-care testing (POCT) method for the detection of I⁻. In this study, we developed poly(acrylamide-allylthiourea) hydrogels embedded with two-dimensional polystyrene colloidal arrays. The functional groups on the hydrogel chain, containing N, O, and S atoms, can selectively coordinate with Hg²⁺. This coordination increases the cross-linking density of the hydrogel, leading to smaller microsphere spacing, which triggers the “off” state. The two-dimensional photonic crystal hydrogels (2DPCH) sensor developed for I⁻ detection operates on this principle. Upon interaction with I⁻, stable HgI₂ is formed, causing the release of the coordinated Hg²⁺ ions. This results in a reduction in the cross-linking density, causing the hydrogel to swell. Hence, the spacing between the 2D colloidal microspheres increases, shifting the system to the “on” state. We observed a linear relationship between the change of microsphere spacing and the concentration of I⁻ within 0.4 − 200 μM, with an LOD of 0.04 μM. The 2DPCH sensor was successfully detected I⁻ in kelp and Yanhe River samples. Moreover, the sensor demonstrated its potential for use in clinical sample analysis. This method offers a novel strategy for clinical disease diagnosis and holds great promise for POCT applications.