Construction of turn-on mode fluorescent sensor for Pb²⁺ and Al³⁺ with structure of phthalic acid and tetraphenylethylene core

Lead is one of the most hazardous metals to humans. It accumulates in liver, kidney and central nervous system, and interferes with a variety of physiological processes such as biosynthesis of hemoglobin. Lead is especially dangerous for children, causing mental retardation. Aluminum has an important role in Alzheimer's disease, Parkinson's disease, bone softening, chronic renal failure. However, both lead and aluminum are widely used in industrial applications and daily life. The limits of Pb2+ and Al3+ for drinking water according to the World Health Organization (WHO) standard are 0.5 and 7.41 mu mol/L respectively. Thus, it is necessary to develop real-time and rapid detection of Pb2+ and Al3+. Fluorescent probes are favored by researchers because of their high sensitivity and selectivity, rapid real-time monitoring and potential application in biological systems. The emission of the traditional panel-like chromophore is weakened or even quenched at high concentration or aggregation, while at low concentration, it exhibits low sensitivity and is easy to photobleaching. Tang and coworkers noticed that some luminogens with twisted conformations are nearly nonemssive in solution but exhibit boosted emission in aggregate states, thus, Tang coined this phenomenon as aggregation induced emission (AIE). With the disclosure of the AIE mechanism of restriction of motion (RIM), many AIE active luminogens have been developed. Some turn-on mode fluorescent sensors for Pb2+ and Al3+ have also been reported based on AIE mechanism, but few of them worked in pure water. Two water-soluble fluorescent sensors are constructed using phthalic acid structure as electron acceptor and tetraphenylethylene as core: 4,4'-(2,2-diphenylethene-1,1-diyl)diphthalic acid (1) and 4,4'-(2,2-bis(4(dimethylamino)phenyl)ethene-1,1-diyl)diphthalic acid (2). Both 1 and 2 are nearly nonemissive in solution, but their solids are highly luminescent with quantum yield of 17.1% and 7.3%. Emission of 1 and 2 in water in the presence of KOH is turned on upon addition of Pb2+ or Al3+, while exhibit nearly no response to the addition of other metal ions, indicating the high selectivity for Pb2+ or Al3+. The detection limit of 1 and 2 towards Al3+ was estimated as 2 and 0.17 mu mol/L, which meets the limit for drinking water according to the WHO standard (7.41 mu mol/L). Fluorescent intensities of 1 and 2 enhanced by Pb2+ and Al3+ are not much affected by the existence of the interfering ions. The exception is that the emission intensity of 1 can be greatly enhanced by Zn2+ when detecting Pb2+, which may afford the the construction for highly sensitive sensors for Pb2+ through combination of 1 and Zn2+. The average particles size of 1 and 2 in water solution are 36 and 98 nm determined by dynamic light scattering (DLS) analysis, respectively. The average particles size of 1 increased to 707 and 292 nm upon addition of Pb2+ and Al3+, respectively. And average particles size of 2 increased to 986 and 906 nm upon addition of Pb2+ and Al3+. Thus the emission enhancement of 1 and 2 upon addition of Pb2+ and Al3+ should be caused by the aggregate formation. The emissions of the water solutions of 1 or 2 in the presence of Pb2+ or Al3+ were quenched upon addition of ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na) due to the competing coordination of EDTA-2Na with metal ions.