CypHer7E NHS ester is an excellent building block for developing site-specific pH probes. It is analogous to CypHer5E NHS ester with much longer excitation and emission. It is one of the very few acidotropic pH probes that exhibit NIR fluorescence. It can be readily conjugated to amino-containing biomolecules (such as antibodies, peptides and amino-modified oligos). CypHer7E is a NIR pH-sensitive cyanine dye derivative that can be used to study most classes of cell surface receptors which internalize into the endocytic pathway when activated by a ligand. CypHer7E NHS ester is a flexible, generic fluorophore that can be coupled to a receptor or epitope tag antibody and monitors neutral to acidic intracellular events.