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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 (MIn) 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 (Co). If the complex MLn, could be observed spectroscopically, the absorbance of the product Ap follows: Ap = Kt1/2, K = 2√2ϵp(LoCoD)1/2. D = diffusion constant of the metal ions inside the porous solid; Lo = concentration of the ligands grafted in the monolith; and t time. This equation is straightforward to use, and the Kvs Co1/2 plot provides the correlations with the concentrations (Co) 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 Cu2+ 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. [ABSTRACT FROM AUTHOR]