Ca-NIR: a ratiometric near-infrared calcium probe based on a dihydroxanthene-hemicyanine fluorophore

Fluorescent calcium probes are essential tools for studying the fluctuation of calcium ions in cells. Herein, we developed Ca-NIR, the first ratiometric calcium probe emitting in the near infrared region. This probe arose from the fusion of a BAPTA chelator and a dihydroxanthene-hemicyanine fluorophore. It is efficiently excited with common 630-640 nm lasers and displays two distinct emission bands depending on the calcium concentration (K_d = ∼8 μM). The physicochemical and spectroscopic properties of Ca-NIR allowed for ratiometric imaging of calcium distribution in live cells.