Synthesis of reactive vinylsulphonyl azo dyes for application in optical pH sensing

This work describes the application of a synthon (GM1) capable of forming a variety of reactive azo dyes useful as pH indicators. GM1 possesses two functional groups, namely (a) an amino group which allows diazo coupling to form an indicator chromophore, and (b) a 2-hydroxyethyl-sulphonyl group which, after activation, allows its immobilization on cellulose. GM1 has been diazo coupled with various aromatic phenols and amines to give a variety of pH probes. After the 2-hydroxyethylsulphonyl group has been converted into a reactive vinylsulphonyl group, the dye has been covalently linked to the hydroxy groups of cellulose. We use a novel type of a transparent solid film consisting of a polyester support covered with a thin layer of cellulose acetate which, during immobilization, is converted into cellulose. Cellulose membranes coloured with such pH indicating dyes can be applied for optical pH sensing. All dyes are characterized, both in dissolved and immobilized form, in terms of optical properties in the acid and conjugate base form, pKa values and indicator properties. Semiempirical AM1 calculations have been performed in order to compute deprotonation energies of three representative dyes, and data were compared with experimental pKa's