The development of efficient luminescent materials in the aggregated or solid state is a topic of current interest on account of their potential applications in the fields of electronics, optics, biological sciences, and as simple sensors for the presence of toxic heavy-metal ions, such as Hg²⁺. However, although fluorescent materials are highly emissive in the solution state, they become weakly emissive or even non-emissive when aggregated in the condensed phase. This aggregation-caused quenching (ACQ) effect has significantly limited the high-tech application of conventional fluorophores.

In 2001, Ben Zhong Tang, Hong Kong University of Science & Technology, and colleagues observed the phenomenon of aggregation-induced emission (AIE) in some propeller-shaped molecules that is the exact opposite of the ACQ effect [1].

Tang and colleagues now report a simple strategy to tune the emission behavior of a tetraphenylethene (TPE)-functionalized benzothiazolium salt (TPEBe–X). All the TPEBe–X luminogens showed faint red emission in solution, but by varying the counteranion from I^– to ClO₄^– or PF₆^–, TPEBe–X changed its emission behavior from ACQ to AIE.

Addition of Hg²⁺ into the aqueous solution of TPEBe–I enhanced the light emission 80-fold; this result suggests that this salt could make an excellent fluorescent Hg²⁺ sensor with high sensitivity and selectivity.

The sensing process can also be performed in a solid film with a detection limit of 1 μM, thereby demonstrating the use of AIE compounds for Hg²⁺ detection in real-world conditions and opening a new way to detect Hg²⁺ in aqueous solution based on the AIE mechanism.

• Effect of the Counterion on Light Emission: A Displacement Strategy to Change the Emission Behaviour from Aggregation-Caused Quenching to Aggregation-Induced Emission and to Construct Sensitive Fluorescent Sensors for Hg²⁺ Detection,
  Na Zhao, Jacky W. Y. Lam, Herman H. Y. Sung, Hui Min Su, Ian D. Williams, Kam Sing Wong, Ben Zhong Tang,
  Chem. Eur. J. 2013.
  DOI: 10.1002/chem.201303251

[1] J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok, X. Zhan, Y. Liu, D. Zhu, B. Z. Tang, Aggregation-induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole, Chem. Commun. 2001, 1740–1741. DOI: 10.1039/B105159H