Colorimetric and fluorescent sensing of a new FRET system via [5] helicene and rhodamine 6G for Hg2+ detection

A visually colorimetric'' and fluorometric sensor based on [5] helicene connected to rhodamine 6G via a hydrazide moiety, RG5H, was designed and prepared for the highly sensitive and selective detection of Hg2+. The sensor system operated through the FRET process from the [5] helicene energy donor to the ring-opened rhodamine 6G-Hg2+ complex acceptor. The sensor illustrated a very large Stokes shift up to 176 nm. In aqueous acetonitrile solutions, RG5H exhibited high selectivity and sensitivity toward Hg2+ in the presence of various competitive metal ions, such as Cu2+, Ag+, Zn2+, Pb2+, Ca2+, Cd2+, Co2+, Fe2+, Mg2+, Mn2+, Ba2+, Ni2+, Na+, Li+ and K+. The detection limit of RG5H for determination of Hg2+ was found to be 2.3 ppb. The binding of RG5H to Hg2+ was evident in both fluorescence enhancement as well as a colorimetric change in the sensor from greenish-yellow to orange, which could be easily observed by the naked eye.