Metal Ion-Sensitive Holographic Sensors

Abstract

Holographic sensors for Na⁺ and K⁺ have been fabricated from crown ethers incorporated into polymeric hydrogels. The methacrylate esters of a homologous series of hydroxyether crown ethers were synthesized and copolymerized with hydroxyethyl methacrylate and the cross-linker ethylene dimethacrylate (3 mol %) to form stable hydrogel films (∼10 μm thick) containing covalently bound (0−97 mol %) 12-crown-4, 15-crown-5, and 18-crown-6 pendant functionalities. The films were transformed into silver-based volume holograms using a diffusion method coupled with a holographic recording using a frequency-doubled Nd:YAG laser. The resulting holographic reflection spectrum was used to characterize the shrinkage and swelling behavior of the holograms as a function of polymer composition and the nature and concentration of alkali, alkaline earth, and NH₄⁺ ions in the test media. Optimized film compositions containing 50 mol % crown ether showed substantial responses (≤200 nm) within 30 s at ion concentrations of ≤30 mM, which could be rationalized on the basis of the known complexation behavior of the crown ethers. An 18-crown-6 holographic film was shown to be able to quantitate K⁺ concentrations over the physiologically relevant range. It was virtually unaffected by variations in the Na⁺ background concentration within the normal physiological variation (∼0.13−0.15 M) and shows promise for developing simple, low-cost K⁺ sensors for medical applications.