Silver halide sensitizing dyes are similar to the dyes used by the human eye to affect color imaging. The big difference is that the dyes adsorb to the surface of the silver halide crystal. Of course, there are dyes that form direct, light sensitive salts with silver to give a material that is light sensitive in a given region of the spectrum.

The best dyes are quite complex organic molecules and very expensive to make. Many of them are very toxic to animal life but are used at such tiny levels and decompose so quickly during processing, that they pose no problem. They also have great difficulty entering the animal body due to their very nature, but should be handled with care.

Erythrosine is an exception, being very low in toxicity.

The color of the dye is adjusted during synthesis by changing the number of CH=CH groups in the chanin between two large ring structures, the more, the further towards cyan the dye becomes until it becomes an infra red dye which is rather black. Dyes are the opposite in color to the portion of the spectrum that they sensitize the emulsion to.

I know of no azo dye that is a sensitizer for silver halide. These are dyes formed from N=N moieties.

The foremost researcher into this topic at Eastman Kodak is Paul Gilman. He 'wrote the book' so to say.

PE