Chemichromic azodye from 2,4-dinitrobenzenediazonium o-benzenedisulfonimide and γ-acid for monitoring blood parameters: structural study and synthesis optimisation.

Azo coupling of 6-amino-4-hydroxy-2-naphthalenesulfonic acid (γ-acid) with 2,4-dinitrobenzenediazonium ion in alkaline solution gave a mixture of aminoazo and hydroxyazo dyes. The use of dry diazonium salt, stabilised as o-benzenedisulfonimide derivative, afforded a higher quantity of desired hydroxyazo dye compared to use of diazonium salt prepared via nitrosylsulfuric acid. The addition of a highly acidic diazonium salt solution determines a local pH-value decreasing. In the mixing zones the acid-base equilibrium corresponds to the “microscopic”, lower pH-values and the aminoazo dye is formed. Only hydroxyazo dye was obtained when N-acetyl derivative of γ-acid was used as a coupling agent. In any case, the use of 2,4-dinitrobenzenediazonium o-benzenedisulfonimide allowed for an easier experimental procedure. 1H NMR analysis, in particular 1H–1H COSY and 1H–1H NOESY techniques, elucidated the structure of dyes evidencing that coupling took place at the para-position to the hydroxy group in hydroxyazo dye and at usual ortho-position in aminoazo dye. Preliminary spectroscopic study showed that p-hydroxyazo dye gave change in colour in the physiological range of pH, an important requisite for its use in a sensor applicable for the monitoring of blood parameters. [Copyright &y& Elsevier]