A novel phenanthridine and terpyridine based D-π-A fluorescent probe for the ratiometric detection of Cd2+ in environmental water samples and living cells

A "turn-on" Donor-π-Acceptor (D-π-A) containing phenanthridine-functionalized extended π-conjugate terpyridine, 5-(4'-([2,2':6',2''-terpyridin]-4'-yl)-[1,1'-biphenyl]4-yl)7,8,13,14-tetrahydrodibenzo [a, i] phenanthridine (TBTP) was synthesised. It shows strong selectivity for the detection of toxic Cd2+ without interference from other metal ions. In the presence of Cd2+, the absorption of the TBTP changes dramatically along with the fluorescent emission with the large Stokes shift of 6300 cm-1. When the compound TBTP is exposed to UV light, its colour changes from blue to orange over the addition of Cd2+. Adding other transition metal ions has no effect. This is based on the mechanism of intramolecular charge transfer. The detection limit for Cd2+ was found to be around 1.181 × 10-8 M. An investigation of the sensing mechanism includes job plot, NMR titration, DFT calculation, and HRMS analyses. Excitingly, the recognition of Cd2+ in CH3CN: H2O (8:2, v/v) medium is quantitative without interference from Zn2+, which is a common interferent for Cd2+. Furthermore, the probe was used for detecting Cd2+ in real water samples and cell imaging in living cells was also performed.