Disclosed are water soluble azo dyes, represented by Formula I or II, and their synthesis and use. The water soluble azo dyes are synthesized by diazotizing 4-aminopyridine and coupling the resulting diazo intermediate with alkylated metanilic or orthanilic acid. To be suitable for measuring blood glucose levels, the dyes are combined with m-nitrophenyl boronic acid in an aqueous solvent. A combined ao form of the dyes with glucose shows a peak at 540 nm. At this wavelength, the absorptivity has a tendency to increase with the increasing of glucose level, which allows the measurement of blood glucose levels. ##STR1## wherein R has a 1-4 carbon atom length and is an alkyl, a hydroxy alkyl, a carboxyl-containing alkyl, or a sulfonyl-containing alkyl.
