Divalent metal ion-sensitive holographic sensors

A holographic sensor for real-time detection of divalent metal ions (Ca²⁺, Mg²⁺, Ni²⁺, Co²⁺ and Zn²⁺) has been fabricated by incorporating a chelating monomer into a hydrogel matrix. A methacrylated analogue of iminodiacetic acid (IDA) was prepared and co-polymerised with hydroxyethyl methacrylate (HEMA) and ethylene glycol dimethacrylate (EDMA) as a cross-linker to form polymer films. A silver-based reflection hologram was incorporated into the hydrogel by diffusion followed by holographic recording using a frequency-doubled Nd/YAG laser. Changes in the replay wavelength of the hologram were used to characterise the swelling behaviour of the matrix as a function of its chemical composition and concentration of analyte in the media. The effects of active monomer, cross-linker, pH and ionic strength on the swelling of the matrix and on metal detection sensitivity have been studied. Polymers containing >10 mol% of chelating monomer and 6 mol% of cross-linker showed significant responses (46.3 nm) within 30 s at an ion concentration of 0-40 nm. The selectivity of the holograms towards the different ions tested was Ni²⁺>Zn²⁺>Co²⁺>Ca²⁺>Mg²⁺. The sensor showed fully reversible responses, permitting real-time monitoring of calcium ion efflux during the germination of Bacillus megaterium spores.