Abstract: The present invention concerns an improved optical method and optical sensing device for determining the levels of polyhydroxyl-substituted organic molecules in vitro in aqueous or organic media. Specifically, a dye is combined with an conjugated nitrogen-containing aromatic heterocyclic aromatic boronic acid-substituted bis-onium compound in the presence of a sugar, such as fructose or glucose. The viologens are preferred as the aromatic conjugated nitrogen containing boronic acid substituted compounds. The method is useful to determine sugar levels in fermentation and process streams.