Methylene blue (MB) was strongly retained inside the cavity of host layered barium phosphate, without leaching, and the intercalated compound was characterized through X-ray power diffraction (XRD), IR spectra and electrochemical measurements. The intercalated dye compound was incorporated into a carbon paste electrode and, by means of cyclic voltammetry and amperometry, its electrochemical properties were investigated. In various electrolyte solutions, changing the pH between 3 and 9, the midpoint potential remained practically constant at -0.15 V. This is not the usual behavior for MB, since it is known that in the solution phase the midpoint potential changes considerably with pH. These data suggest that methylene blue behaves as a guest molecule in the lamellar structure of the barium phosphate. The electrode formed by this host/guest composition was used to investigate NADH electrochemical oxidation. (C) 2004 Elsevier B.V. All rights reserved.52318995