An anion sensing photonic gel by hydrogen bonding of anions to the N-allyl-N′-ethyl urea receptor

Abstract

In this paper, a new concept for an anion sensor is demonstrated. We synthesized anion-responsive N-allyl-N′-ethylurea (AEU) that can form hydrogen-bonded complexes with various anions. An allyl functional group enables AEU to participate in copolymerization with hydroxyethyl methacrylate (HEMA) and a crosslinker to form a crosslinked gel. Photopolymerization of the mixed monomer was conducted in the presence of a polymeric opal template, and the subsequent removal of the template resulted in a blue-colored photonic gel in organic solvents owing to light reflection by an inverse opal (IO) structure. After immersing the photonic gel into an acetonitrile (MeCN) solution containing an anion sample with tetrabutylammonium counterion, the reflective color of the gel red-shifted from swelling. For anions such as acetate and dihydrogen phosphate, the photonic gel exhibited color changes within the entire visible spectrum for anion concentration ranges of 10⁻⁵ to 10⁻² M. Assuming that the charge increase by anion binding is mainly responsible for the longitudinal swelling of a photonic gel sensor, we proposed a mathematical relationship between the swelling factor α and the equilibrium constant K for hydrogen bonding (H-bonding) of an anion with AEU. For five different anion samples, variations in the normalized reflectance peak were plotted as a function of anion concentration, and the equilibrium constant K was calculated by fitting the experimental data to a mathematical function. The sequence of anion sensitivity for five samples was found to be in good accordance with previous investigations on urea–anion complexation, while the lower K values obtained in our system were presumably from the weak electron donating substituent in AEU and the hindered diffusion of the analyte within the gel. However, a photonic gel anion sensor has advantages from the standpoint of simplicity. For instance, a detection apparatus is not required, and the sensor can be used multiple times. With further modification of the urea receptor, the photonic gel will find useful applicability, such as in reusable probe sensors and for the in situ monitoring of ionic species in a continuous process.