Progress in organic fluorescent probes for detecting metal ions in environment

The massive emission of various pollutants leads to severe environmental problems, and the air and water issues attract enormous attention. Heavy metal ions highly affect our life and may cause many diseases, which is a critical issue to be solved. As reported, copper ion is highly related to some neurodegenerative diseases, such as Menkes and Wilson's diseases. Children are susceptible to mercury ion during the early stages of development, leading to a variety of diseases relating to the brain, kidney and central nervous system. Thus, it is of vital importance to develop facile and fast detection methods for life-threatening metal ions. Organic dyes with various functional groups have shown great potential for detecting various metal ion pollutants through apparent color and fluorescence changes, exhibiting excellent selectivity and sensitivity. In terms of photophysical properties and chemical structures of organic dyes, we summarize recent research progress of the commonly seen detection mechanisms for metal ions. The detection mechanisms include photoinduced electron transfer (PET), intramolecular charge transfer (ICT), fluorescence resonance energy transfer (FRET), through-bond energy transfer (TBET), aggregation-disaggregation effect and rearrangement of molecular structures. As one of the mostly studied mechanisms, PET has been widely used in the construction of metal ion probes. These metal ion probes generally consist of three parts including fluorophores, spacers, and recognition sites. Most probes based on the PET mechanism are easily designed with high sensitivity and selectivity. However, most of these probes have no built-in correction resulting in severe external environment interference. Compared with the PET mechanism, ICT mechanism has great superiority for the construction of ratiometric probes with built-in correction which can minimize external interference and make quantitative detection more reliable. In addition. ICT mechanism provides another way for detecting some electron-donating and electron-accepting metal ions. such as Fe2+, CU2+ and Zn2+. To achieve FRET within the probe. fluorophores are usually linked by a nonconjugated spacer, and the energy transfers from the donor to acceptor. In this scenario, spectral overlap is required. On the contrary. in TBET-based fluorescent chemosensors, a rigid conjugated linker is needed to chemically connect fluorophores and thus spectral overlap is not required. However, most of the reported FRET and TBET chemosenmsors are based on rhodamine dyes. It is highly necessary to develop novel energy transfer systems for expanding the variousness of metal ions probes. Naked-eye detection is the one of the most convenient methods for environmental metal ion sensing. Modulating the pi-conjugated system of probes result in huge absorption spectrum change and lead to apparent color change of the probe, which is a promising strategy for developing naked-eye metal ions probes with high contrast. Moreover, mechanisms with aggregation-induced signal change including aggregation-caused quenching (ACQ) and aggregation-induced emission (ALE) are studied for environmental pollutant metal ion detection. Above all, these sensing strategies guarantee the successful development of metal ion probes. In this review, the basic design principles and responding mechanisms of the metal ion probes are discussed. Most of the probes can chelate with metal ions and some special reactions between probes and metal ions are also introduced including desulfurization reaction, redox reaction and Heck reaction etc. At last, the developing trends and the prospects of metal ion probes are presented.