CF™660C picolyl azide reacts with alkyne to form 1,2,3-triazole by 1,3-dipolar Huisgen cycloaddition through the use of a much lower copper (I) concentration without sacrificing reaction efficiency. CF™660C is a cyanine-based dye engineered to be highly water-soluble without being excessively charged. As a result, CF™660C-antibody conjugates have high signal-to-noise ratio in immunostaining. CF™660C is several fold brighter and significantly more photostable than Alexa Fluor® 660. CF™660C absorbs at 667 nm but can still be sufficiently excited by the 633 or 635 nm laser with emission at 685 nm in the borderline spectral region between far-red and near-IR fluorescence. Please also see CF™660R, an exceptionally photostable 660 nm dye ideal for confocal microscopy.