Heavy metals are well-known environmental contaminants and are released through a variety of industrial processes with the main risk of human exposure stemming from the contamination of natural water sources. Small-molecule fluorescent sensors make it possible to detect such metal ions in aqueous solutions, and they can then be removed using a plethora of adsorption materials, including hydrogels and porous silicas. A significant challenge is combining these processes to allow for the simultaneous detection and removal of heavy metal ions. Metal–organic frameworks (MOFs) exhibit the same ion-capturing capability as hydrogels and silicas, and recently their metal-free cousins, covalent organic frameworks (COFs), have come to light as potential candidates for ion capture because they share the well-exploited porosity of MOFs.

Now, Wei Wang and a team of researchers from Lanzhou and Sun Yat-Sen universities in China have designed a system that combines the sensing capabilities of thiol groups with the robust stabilizing network provided by COFs, creating a material capable of detecting and removing mercury ions from solution (J. Am. Chem. Soc. 138, 3031–3037; 2016). By the co-condensation of a thioether monomer under mild conditions, the researchers synthesized a COF with functional thioether side-chain branches embedded within the framework that act as cation receptors (pictured). The material fluoresces under normal conditions but exhibits strong fluorescence quenching in response to the addition of mercury ions — attributed to their interactions with the thioether groups — demonstrating its detecting ability.


When Wang and colleagues tested the fluorescence response of the material to a number of different metal ions, they found that significant quenching was exhibited only for mercury, showing the COF to be selective for this ion. Furthermore, they found that when suspended in a dilute aqueous solution of Hg(ClO4)2, the COF could take up over 98 percent of the mercury ions through their absorption into its porous channels, where the ions interact with the thiol branches. This system showed better performance for mercury ion removal than its MOF counterpart.

Wang and colleagues also showed that their COF was robust to recycling. Experiments were carried out to exchange the mercury ions out of the framework using Na2S and the crystalline structure and fluorescence sensitivity were preserved during a number of cycles. With its combined sensing and capture properties, this material shows promise in expanding the role of COFs towards environmental applications.