Naphthalimide-Porphyrin Hybrid Based Ratiometric Bioimaging Probe for Hg2+: Well-Resolved Emission Spectra and Unique Specificity

In this paper, we unveil a novel naphthalimide-porphyrin hybrid based fluorescence probe (1) for ratiometric detection of Hg2+ in aqueous solution and living cells. The ratiometric signal change of the probe is based on a carefully predesigned molecule containing two independent Hg2+-sensitive fluorophores with their maximal excitation wavelengths located at the same range, which shows reversibly specific ratiometric fluorescence responses induced by Hg2+. In the new developed sensing system, the emissions of the two fluorophores are well-resolved with a 125 nm difference between two emission maxima, which can avoid the emission spectra overlap problem generally met by spectra-shift type probes and is especially favorable for ratiometric imaging intracellular Hg2+. It also benefits from a large range of emission ratios and thereby a high sensitivity for Hg2+ detection. Under optimized experimental conditions, the probe exhibits a stable response for Hg2+ over a concentration range from 1.0 x 10(-7) to 5.0 x 10(-5) M, with a detection limit of 2.0 x 10(-8) M. The response of the probe toward Hg2+ is reversible and fast (response time less than 2 min). Most importantly, the ratiometric fluorescence changes of the probe are remarkably specific for Hg2+ in the presence of other abundant cellular metal ions (i.e., Na+, K+, Mg2+, and Ca2+), essential transition metal ions in celsl (such as Zn2+, Fe3+, Fe2+, Cu2+, Mn2+, Co2+, and Ni2+) and environmentally relevant heavy metal ions (Ag+, Pb2+, Cr3+, and Cd2+), which meets the selective requirements for biomedical and environmental monitoring application. The recovery test of Hg2+ in real water samples demonstrates the feasibility of the designed sensing system for Hg2+ assay in practical samples. It has also been used for ratiometric imaging of Hg2+ in living cells with satisfying resolution, which indicates that our novel designed probe has effectively avoided the general emission spectra overlap problem of other ratiometric probes.