Optical metal ion sensor based on diffusion followed by an immobilizing reaction. Quantitative analysis by a...

A new optical metal ion sensor based on diffusion followed by an immobilizing reaction has been developed. The current sensor is based on a model that unifies two fundamental processes which a metal analyte undergoes when it is exposed to a porous, ligand-grafted monolith: (a) diffusion of metal ions to the binding sites and (b) metal-ligand (ML(n)) complexation. A slow diffusion of the metal ions is followed by their fast immobilizing reaction with the ligands in the monolith to give a complex. Inside the region where the ligands have been saturated, the diffusion of the metal ions reaches a steady state with a constant external metal ion concentration (C(0)). If the complex ML(n) could be observed spectroscopically, the absorbance of the product A(p) follows: A(p) = Kt(1/2), K = 2epsilon(p)(L(0)C(0)D)(1/2). D = diffusion constant of the metal ions inside the porous solid; L(0) = concentration of the ligands grafted in the monolith; and t = time. This equation is straightforward to use, and the K vs C(0)(1/2) plot provides the correlations with the concentrations (C(0)) of the metal ions. This is a rare optical sensor for quantitative metal ion analysis. The use of the model in a mesoporous sol-gel monolith containing grafted amine ligands for quantitative Cu(2+) sensing is demonstrated. This model may also be used in other chemical sensors that depend on diffusion of analytes followed by immobilizing reactions in porous sensors containing grafted/encapsulated functional groups/molecules.