Voltron2 is a chemigenetic voltage sensor. Voltron2 consists of a rhodopsin from Acetabularia acetabulum, mutated to carry a D81N amino acid substitution which inactivates the proton pump, fused to a Halo-tag which can covalently bind a fluorophore dye ligand such as JF[[525]]. Voltron2 also contains a A122D amino acid change in the rhodopsin sequence (compared to its progenitor, Voltron), which results in an approximately 3-fold increase in sensitivity with lower baseline fluorescence. Changes in membrane potential result in a change in the level of fluorescence resonance energy transfer (FRET) quenching of the fluorophore dye by the rhodopsin; depolarization results in decreased fluorescence (PMID:37015225).