  We report the fabrication and characterization of a simple gate-free graphene
device as a pH sensor. The graphene sheets are made by mechanical exfoliation.
Platinum contact electrodes are fabricated with a mask-free process using
focused ion beam, and then expanded by silver paint. Annealing is used to
improve the electrical contact. The experiment on the fabricated graphene
device shows that the resistance of the device decreases linearly with
increasing pH values (in the range of 4-10)in the surrounding liquid
environment. The resolution achieved in our experiments is approximately 0.3 pH
in alkali environment. The sensitivity of the device is calculated as
approximately 2 k\Omega /pH. The simple configuration, miniaturized size and
the integration ability make graphene-based sensors promising candidates for
future micro/nano applications.
