A coumarin-naphthalimide-based fluorescent probe for the ratiometric detection of Hg2+ utilizing an ICT-FRET mechanism

Abstract

Mercury, as one of the most toxic elements in environmental and biological samples, can poses a grave threat to the integrity of the nervous, reproductive, immune, and digestive systems. Hence, it is significant to exploit convenient and rapid fluorescent probes featuring high sensitivity and selectivity for monitoring the concentration level of mercury ions (Hg²⁺). In this paper, a ratiometric fluorescent probe for Hg²⁺ based on intramolecular charge transfer (ICT) and fluorescence resonance energy transfer (FRET) mechanisms was developed by using coumarin ad energy donor and naphthalimide as energy acceptor, and thionocarbonate as the recognizing group for Hg²⁺. When the probe alone was present in the system, the thionocarbonate group of the probe prevented the electron transfer and the ICT process was off, thus preventing the FRET process of the probe as well. The probe alone emitted the intrinsic blue fluorescence of coumarin upon excitation at 397 nm. After providing Hg² the thionocarbonate unit reacted with Hg²⁺, which restored the ICT process of the naphthalimide donor, and the FRET process in turn took place. Upon adding Hg²⁺, the probe emitted yellow fluorescence. In addition, the fluorescence intensity ratio at 548 nm and 476 nm (I_{548 nm}/I_{476 nm}) of the probe linearly corrected with the concentration of Hg²⁺ ions in the range of 0.1–12 μM. A limit of detection of 0.055 μM was obtained. Meanwhile, the probe exceptional recognized Hg²⁺ across a broad pH range (pH=4.00–10.00), including physiological pH. Cellular studies further confirmed the probe's negligible cytotoxicity and its potential for ratiometric fluorescence imaging of intracellular Hg²⁺ ions in A549 cells. In addition, the probe was loaded on filter paper to make test strips and combined with a smartphone to achieve rapid visual quantitative detection of Hg²⁺.