Numerical investigation of a linear defect waveguide in a two-dimensional phononic crystal slab of water cylinders in mercury host to sense the variation of ethanol concentration in a mixture with water is carried out. Waveguide geometry is optimized by varying the waveguide core width and introducing scatterers into the core to obtain an isolated defect band within the bandgap, as close to the gap center and as narrow as possible, in order to increase sensing ability. Transmission results calculated through three-dimensional steady-state finite-element method simulations are consistent with the position and the width of the defect band obtained by band structure computations, as a function of ethanol concentration. The band center varies linearly with ethanol concentration up to 15%, where it blue-shifts with increasing concentration at a rate of approximately 480 Hz per percent variation. The bandwidth also exhibits linear variation up to significantly smaller concentrations, whereas the values calculated through the transmission spectra are considerably smaller than those obtained by band structure computations. (C) 2014 Elsevier B.V. All rights reserved.