Ratiometric Hg2+ /Ag+ Probes with Orange Red-White-Blue Fluorescence Response Constructed by Integrating Vibration-Induced Emission with an Aggregation-Induced Emission Motif

Abstract

Two novel ratiometric fluorometric probes 6 a and 6 b have been designed and synthesized for the detection of Hg²⁺ and Ag⁺ , respectively. Each of these probes is comprised of a 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC) skeleton, a tetraphenylethylene (TPE) unit, and two thymine/adenine groups. In this proof-of-concept work, the DPAC unit featuring vibration-induced emission (VIE) characteristics serves as a dual-emission chromogen, whereas the TPE moiety, which is noted for its aggregation-induced emission (AIE) properties, functions mainly as a signal amplifier. The coordination between the thymine/adenine groups and Hg²⁺ /Ag⁺ leads to the formation of aggregates, which restricts the intramolecular vibrations of DPAC and the intramolecular rotations of TPE, activating the AIE effect of TPE and resulting in the bent form of DPAC. Accordingly, the overall fluorescence changed from moderate orange-red (I_orange-red ) to intense blue light (I_blue ) passing through the white-light region, concurrently with an enhancement of fluorescence at 475 nm and a dramatic increase in the ratio of I_blue /I_orange-red . Furthermore, the superb sensitivity of 6 a/6 b towards Hg²⁺ /Ag⁺ has been demonstrated by the detection limit, which is as low as 6.1 nm and 0.1 μm, respectively. Both of these values are below the maximum concentration in drinking water permitted by the U.S. Environmental Protection Agency, demonstrating their great potential to be applied for real-world metal-ion sensing.

Keywords: aggregation-induced emission (AIE); mercury ion; ratiometric fluorometric probes; silver ion; vibration-induced emission (VIE).