Functional hydroxyazobenzene dyes for optically monitoring pH in the range from 6 to 10 were synthesised. The pK_a values of the dyes have been tailored by appropriate choice of substituents in position para to the hydroxy group, resulting in lower pK_a values for electron acceptor substituents and higher pK_a for electron donors. Furthermore, substituent effects on absorbance spectra have been evaluated, with expected shifts to longer wavelength in the case of electron donors. For immobilisation, the indicator dyes were covalently linked to cellulose films via vinylsulfonyl chemistry. Mechanical stabilization of the cellulose films was achieved by lamination onto polyethylene terephthalate foils. In order to obtain patterned sensor layers with geometrical defined sensing areas a laser-structured adhesive tape was used as protective mask in the dyeing step.