A K⁺-sensitive photonic crystal hydrogel sensor for efficient visual monitoring of hyperkalemia/hypokalemia

Potassium ions (K⁺) play crucial roles in many biological processes. Abnormal K⁺ levels in the body are usually associated with physiological disorders or diseases, and thus, developing K⁺-sensitive sensors/devices is of great importance for disease diagnosis and health monitoring. Herein, we report a K⁺-sensitive photonic crystal hydrogel (PCH) sensor with bright structural colors for efficient monitoring of serum potassium. This PCH sensor consists of a poly(acrylamide-co-N-isopropylacrylamide-co-benzo-15-crown-5-acrylamide) (PANBC) smart hydrogel with embedded Fe₃O₄ colloidal photonic crystals (CPCs), which could strongly diffract visible light and endow the hydrogel with brilliant structural colors. The rich 15-crown-5 (15C5) units appended on the polymer backbone could selectively bind K⁺ ions to form stable 2 : 1 [15C5]₂/K⁺ supramolecular complexes. These bis-bidentate complexes served as physical crosslinkers to crosslink the hydrogel and contracted its volume, and thus reduced the lattice spacing of Fe₃O₄ CPCs and blue-shifted the light diffraction, and finally reported on the K⁺ concentrations by a color change of the PCH. Our fabricated PCH sensor possessed high K⁺ selectivity and pH- and thermo-sensitive response performances to K⁺. Most interestingly, the K⁺-responding PANBC PCH sensor could be conveniently regenerated via simple alternate flushing with hot/cold water due to the excellent thermosensitivity of the introduced PNIPAM moieties into the hydrogel. Such a PCH sensor provides a simple, low-cost and efficient strategy for visualized monitoring of hyperkalemia/hypokalemia, which will significantly promote the development of biosensors.