“Molecular Cage” Could Help Remove Drugs and Chemical Pollutants From Water

Researchers have developed a new material that could help to remove unwanted pollutants — including leftover medicines and synthetic chemicals — from our waterways.

The metal-organic cage (MOC) molecules are designed to absorb harmful pollutants and trap them inside their cage-like structures. While MOCs have been used previously for gas and chemical capture in organic solvents, this latest research introduces a rare water-soluble MOC made with an easily adaptable synthesis technique. The research is published in Cell Reports Physical Sciences.

Removing harmful contaminants from water

Unmetabolized medications and chemicals left over from hygiene and personal care products have become of increasing concern in recent years. Studies suggest that these compounds could pose a danger to animal health and welfare when present as environmental contaminants.

“While domestic wastewater passes through wastewater treatment plants, these plants frequently do not meet the capacity or the level of chemical purification required to completely prevent medicinal and healthcare products reaching our ecosystems,” senior study author Dr. Imogen Riddell, a Royal Society University Research Fellow at the University of Manchester, told Technology Networks.

“Once introduced into natural waterways, these chemicals bioaccumulate in the fatty tissues of aquatic organisms resulting in toxicity and negative effects on the physiology, development and reproduction of the organisms that consume them.”

In search of better treatment options that can selectively bind to and remove these contaminants, researchers have turned to MOCs. These cage-like molecules have previously been used to encapsulate, store and release pollutants — such as the potent greenhouse gas SF₆ — but have rarely been applied to water-based systems.

“A longstanding challenge in the MOC community has been to design useful MOCs which are also water-soluble. This has been difficult because MOCs often include organic linkers which have poor solubility in water,” explained first author Jack Wright, a PhD student in Riddell’s research group at the University of Manchester. “Strategies that allow the introduction of water-solubilizing groups, which might address this challenge, often negatively impact the self-assembly of the MOCs themselves.”

To make their MOC water-soluble, the researchers incorporated sulfonates into their MOC’s structure. Sulfonate-based ligands have good solubility in water, however their poor commercial availability has previously limited their use in water-soluble MOCs. In their new paper, the researchers demonstrate an alternative synthesis technique, based on a simple ring-opening addition reaction, that more easily introduces sulfonate moieties into ligands for water-soluble MOCs.

How do metal-organic cages work?

MOCs are made up of metal ions linked together by organic molecules, leaving a hollow core where other compounds can be trapped. This trapping occurs through an effect known as “hydrophobic binding”.

“Effectively what is happening upon binding in our cage is that the oil-like, water-hating (hydrophobic) pollutants are preferentially located inside of our MOC, where they are protected from the repulsion of the water molecules. At the same time, water molecules prefer to be outside the internal binding space than trapped inside it and they are therefore readily displaced by the incoming pollutants,” Riddell explained.

To demonstrate the effectiveness of their new MOC at trapping environmental contaminants, the researchers conducted a series of host binding studies using a number of potential water contaminants, including hormone medications and hygiene product chemicals.

“Ethinylestradiol is the synthetic derivative of the naturally occurring molecule estradiol, which is commonly used in female contraceptives,” Riddell explained. “Ethinylestradiol has been shown to make its way into our waterways through the excretion of prescribed supplements into domestic wastewater. Once there, it has been implicated in the feminization of fish, which is when male fish change their gender due to external chemical stimuli.”

“Similarly, tonalide – a ubiquitous synthetic chemical found in fragrances, laundry products and soaps – has been shown to enter into waterways through everyday human behavior,” Riddell said. “Tonalide is highly toxic and is implicated in physiological changes as well as reproductive and developmental problems.”

Analysis by NMR spectroscopy indicated that the new MOC was able to capture estradiol, ethinylestradiol, testosterone, progesterone, tonalide and cholesterol in its internal cavity.

“Future efforts will focus on the development of robust and efficient recycling protocols for this MOC system,” Riddell said. “We believe that a promising approach would involve the development of an immobilized version of the MOC, which would allow extraction of pollutants from a stream of wastewater which is flown over the MOC. The MOC could then be emptied/regenerated by flowing an organic solvent over it – which would bind the pollutants preferentially.”

“In addition to developing a robust recycling protocol enabling reuse of the MOC, we are interested to make new MOCs with different sized binding pockets which would allow for capture of a wider variety of guest molecules. We also want to further probe the fundamental design rules of these MOCs which would allow us to design new MOCs with specific functions, maximizing their utility and potentially opening up new application areas for this exciting class of molecule.”

Reference: Wright JD, Whitehead GFS, Pyzer-Knapp EO, Riddell IA. Encapsulation of hydrophobic pollutants within a large water-soluble [Fe₄L₆]⁴⁻ cage. Cell Rep Phys Sci. 2025:102404. doi: 10.1016/j.xcrp.2025.102404

Dr. Imogen Riddell and Jack Wright were speaking to Alexander Beadle, Science Writer for Technology Networks.

About the interviewees:

Dr. Imogen Riddell completed her PhD at Cambridge University working for Prof. Nitschke, where she explored new strategies for the self-assembly of metal-organic container molecules. She then undertook her postdoctoral training with Prof. Lippard at the Massachusetts Institute of Technology (MIT), where her research was directed at understanding the mechanisms of non-classical inorganic anticancer complexes. In 2017, she was awarded a University of Manchester Dame Kathleen Ollerenshaw Research Fellowship, and a Royal Society URF in 2018, which enabled her to start her own research program looking at the design and discovery of metal-organic materials for biomolecule encapsulation.

Jack Wright is a PhD student and researcher at the University of Manchester. He holds a Master’s degree from the University of Birmingham. His research is focused on the development of water-soluble cages for the binding of biomolecules.