  Searching for neutrinoless double beta decays ($\beta\beta0\nu$) is the only
practical way to establish if the neutrinos are their own antiparticles. Due to
the smallness of neutrino masses, the lifetime of $\beta\beta0\nu$ is expected
to be at least ten orders of magnitude smaller than the noise associated with
the natural radioactive chains. A positive identification of $\beta\beta0\nu$
decays requires, ultimately, finding a signal that cannot be mimicked by
radioactive backgrounds. This signal could be the observation of the daughter
atom in the decay, since no known background processes induce a Z+2
transformation. In particular, the $\beta\beta0\nu$ decay of Xe-136 could be
established by detecting the doubly ionised daughter atom, Ba$^{2+}$. Such a
detection could be achieved via a sensor made of a monolayer of molecular
indicators. The Ba$^{2+}$ would be captured by one of the molecules in the
sensor, and the presence of the single chelated indicator would be subsequently
revealed by a strong fluorescent response from repeated interrogation with a
laser system. Here we describe a fluorescent bicolor indicator that binds
strongly to Ba$^{2+}$ and shines very brightly, shifting its emission colour
from green to blue when chelated in dry medium, thus allowing the unambiguous
identification of single barium atoms in the sensor, and permitting a positive
identification of the $\beta\beta0\nu$ decay of Xe-136 in a gas chamber, that
could led to a background-free experiment.
