  The ultimate aspiration of any detection method is to achieve such a level of
sensitivity that individual quanta of a measured value can be resolved. In the
case of chemical sensors, the quantum is one atom or molecule. Such resolution
has so far been beyond the reach of any detection technique, including
solid-state gas sensors hailed for their exceptional sensitivity. The
fundamental reason limiting the resolution of such sensors is fluctuations due
to thermal motion of charges and defects which lead to intrinsic noise
exceeding the sought-after signal from individual molecules, usually by many
orders of magnitude. Here we show that micrometre-size sensors made from
graphene are capable of detecting individual events when a gas molecule
attaches to or detaches from graphenes surface. The adsorbed molecules change
the local carrier concentration in graphene one by one electron, which leads to
step-like changes in resistance. The achieved sensitivity is due to the fact
that graphene is an exceptionally low-noise material electronically, which
makes it a promising candidate not only for chemical detectors but also for
other applications where local probes sensitive to external charge, magnetic
field or mechanical strain are required.
