Fluorescein ver. 2.0: A new, highly fluorescent, pH-sensitive trihydroxytrioxatriangulenium dye (H-TOTA) has been synthesised and characterised. The dye is closely related to fluorescein and may be considered to be a two-dimensional extended version. This new dye can exist in four different protonation states (see graphic) depending on the pH, and its use as a sensitive fluorescent ratiometric pH probe in a physiological buffer is demonstrated.