Diffusion in face-centered cubic (fcc) opals synthesized from 250 nm-diameter silica spheres was investigated by electrochemical methods and finite-element simulations. Opal modified electrodes (OME) ((111) opal surface orientation) were prepared by thermal evaporation of Au onto ∼1 mm-thick opals. Linear sweep voltammetry of Au OMEs in aqueous solutions containing an electroactive molecule and a supporting electrolyte (0.1 M Na 2 SO 4 ) was used to determine molecular diffusion coefficients, D fcc , within the opal. D fcc is related to the diffusion coefficient of the molecule in free solution, D sol , by the relationship D fcc , ) (E/τ)D sol , where E is the interstitial volume fraction of a fcc opal (E ) 0.260 for an infinitely thick opal) and τ is the tortuosity; the tortuosity reflects the increased distance traversed by molecules as they diffuse through the curved interstitial spaces of the opal lattice, and is a function of both the direction of transport relative to the lattice and the number of layers of spheres in the opal lattice. Finite-element simulations are used to compute τ for transport orthogonal to the (111), (110), and (100) surface orientations for 1−7 layers of spheres. Values of τ ) 1.9 ± 0.7 and 3.1 ± 1.2 were obtained from experiment for transport of Ru(NH) 6 3+ and Fe(CN) 6 4-normal to the (111) surface, respectively, in reasonable agreement with a value of ∼3.0 obtained from the simulation.Introduction. The recent interest in opals comprising a closepacked face center cubic (fcc) lattice of spheres (typically SiO 2 or polystyrene of submicrometer radius) is due, in part, to their application in the synthesis of photonic crystals, 1-10 energy storage media, 11-14 novel magnetic materials, 15-17 and sensors. 18 These materials are typically prepared by infusion or diffusional transport of precursor species through the opal lattice, followed by removal of the spheres to create an inverted opal structure. 1,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] Molecular transport occurs within the tortuous interstitial spaces of the opal, Figure 1. Based on geometric factors alone, the effective diffusivity of molecules within the fcc lattice of spheres, D fcc , can be related to the diffusivitiy of molecules in free space, D sol , by eq 1:

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