New highly fluorescent pH indicator for ratiometric RGB imaging of pCO₂

A new diketo-pyrrolo-pyrrole (DPP) indicator dye for optical sensing of carbon dioxide is prepared via a simple one step synthesis from commercially available low cost 'Pigment Orange 73'. The pigment is modified via alkylation of one of the lactam nitrogens with a tert-butylbenzyl group. The indicator dye is highly soluble in organic solvents and in polymers and shows pH-dependent absorption (λ_max 501 and 572 nm for the protonated and deprotonated forms, respectively) and emission spectra (λ_max 524 and 605 nm for the protonated and deprotonated forms, respectively). Both the protonated and the deprotonated forms show high fluorescence quantum yields (Φ_prot 0.86; Φ_deprot 0.66). Hence, colorimetric read-out and ratiometric fluorescence intensity measurements are possible. The emission of the two forms of the indicator excellently matches the response of the green and the red channels of an RGB camera. This enables imaging of carbon dioxide distribution with a simple and low cost optical set-up. The sensor based on the new DPP dye shows very high sensitivity and is particularly promising for monitoring atmospheric levels of carbon dioxide.