A sensitive, selective colorimetric Fe3+ detection method has been developed by using pyrophosphate functionalized gold nanoparticles (P 2O7 4--AuNPs). Gold nanoparticles were prepared by reducing HAuCl4 with sodium borohydride, in the presence of Na4P2O7. IR spectra suggested that pyrophosphates were capped on the surface of the gold nanoparticles. Aggregation of P2O7 4--AuNPs was induced immediately in the presence of Fe3+ ions, yielding a color change from pink to violet. This Fe3+-induced aggregation of P2O7 4--AuNPs was monitored using first the naked eye and then UV-vis spectroscopy with a detection limit of 5.6 μM. The P2O 7 4--AuNPs bound by Fe3+ showed excellent selectivity compared to other metal ions (Ca2+, Cd2+, Co2+, Fe2+, Hg2+, K+, Mg 2+, Mn2+, Na+, Ni2+, Pb 2+, and Zn2+). The best detection of Fe3+ was achieved in a pH range from 3 to 9. In addition, the P2O 7 4--AuNPs were also used to detect Fe3+ in lake water samples, with low interference.