Observation of the neutrinoless double beta decay is the only practical way to establish that neutrinos are their own antiparticles(1). Because of the small masses of neutrinos, the lifetime of neutrinoless double beta decay is expected to be at least ten orders of magnitude greater than the typical lifetimes of natural radioactive chains, which can mimic the experimental signature of neutrinoless double beta decay(2). The most robust identification of neutrinoless double beta decay requires the definition of a signature signal-such as the observation of the daughter atom in the decay-that cannot be generated by radioactive backgrounds, as well as excellent energy resolution. In particular, the neutrinoless double beta decay of(136)Xe could be established by detecting the daughter atom,Ba-136(2+), in its doubly ionized state(3-8). Here we demonstrate an important step towards a 'barium-tagging' experiment, which identifies double beta decay through the detection of a single Ba(2+)ion. We propose a fluorescent bicolour indicator as the core of a sensor that can detect single Ba(2+)ions in a high-pressure xenon gas detector. In a sensor made of a monolayer of such indicators, the Ba(2+)dication would be captured by one of the molecules and generate a Ba2+-coordinated species with distinct photophysical properties. The presence of such a single Ba2+-coordinated indicator would be revealed by its response to repeated interrogation with a laser system, enabling the development of a sensor able to detect single Ba(2+)ions in high-pressure xenon gas detectors for barium-tagging experiments. A fluorescent bicolour sensor is proposed as the basis of a barium-tagging technique for the detection of neutrinoless double beta decay in xenon gas experiments.