We report an optical fibre probe for mercury detection based on a sensitised alumina cladding. The interaction between the sensitive material and the guided light wave is effected by means of evanescent-field coupling, wherein the colorimetric changes of a Ru(II)-bis-thiocyanate complex modulate the light intensity. The improvement of the water stability measurements of the system has been achieved by treating the alumina clad in sulphuric acid before dye adsorption. A partial least squares calibration model employing spectral features from the optode response enables estimating mercury concentration at a range between 0 and 6 ppm.