Fluorescent proteins from marine organisms represent potential candidates for biosensor development. In this paper, we described the isolation of a native green fluorescent protein from Anemonia sulcata and the cloning and purification of its equivalent as a recombinant protein in Escherichia coli. Furthermore, the spectroscopic behaviours of the native and recombinant GFPs were investigated as a function of Cu2+, Cd2+, Pb2+ and Ni2+ concentration. Our results suggest the high selectivity of both proteins at copper than the other metals and, for the recombinant protein, a great sensitivity at a very low concentration (0.1-1 mu M). Moreover, starting from these data, using the combination of molecular biology techniques and optical setup, we developed a device for the detection of Cu2+ in water solutions. The quenching effect detected with the device showed that the relative attenuation of the signal (0.46 +/- 0.02 AU) was slightly larger than the data measured by fluorescence spectra (0.65 +/- 0.03 AU). The good sensitivity in the span of two orders of the magnitude of Cu2+ concentration, the fact that the instrument is made up of low-cost and sturdy parts and the selective quenching of rAsGFP to copper ions make this setup suited as a low cost, on-the-field, copper ion-specific biosensor.