Most recently, we found that the solubility of non-sulfonated Cyanine7 amine in water is much higher than for sulfo-Cyanine7 amine:
Solubility in the water of sulfo-Cyanine7 amine is 3.2 mg per mL.
Solubility in the water of Cyanine7 amine is more than 25 mg per mL.
Initially, sulfonated cyanine dyes were designed to make dyes more soluble in water. But after checking that at our lab, we realized that the solubility of non-sulfonated amine and hydrazide derivatives could be more efficient due to the average charge of the dye molecule and also the pH used to solubilize the dye.
The average charge of non-sulfonated amine-based dyes is +2; for sulfonated dyes, the average charge is 0. As a result, non-sulfonated amine-based dyes are more soluble in neutral and slightly basic water-based buffers.