I have always had a bit of an obsession with monitoring things: the books I read, the movies I watch, the amount of water I drink daily, and how much food I give to my cat. Thus, it is not surprising that my PhD thesis mainly focused on the development of analytical methods for monitoring organic compounds in environmental water, particularly contaminants of emerging concern. The main challenge in this field is to find a sample preparation procedure able to extract the pollutants at trace levels, plus avoiding interferences coming from other components of the sample. I put all my efforts into developing different microextraction techniques to preconcentrate the compounds and solve the sensitivity issues. However, most of the solvents or solid materials used to extract the contaminants still failed in terms of selectivity and efficiency. That is when I started gaining more interest in porous adsorbents, including metal-organic frameworks and metal-organic polyhedra (MOPs). Thus, when I completed my PhD, I decided I needed to understand more about these materials to find a rational way of using them as extraction phases for water monitoring.
In 2021, the opportunity to do a postdoc at Kyoto University came to me as a JSPS postdoctoral fellow. I must admit that travelling to the other side of the world from the Canary Islands to another island scared me a bit; but the Furukawa group works with porous materials based on coordination bonds and both places are surrounded by water, so it made sense, right? It definitely was the perfect environment to join forces in the search for the ideal material that can be implemented in the water treatment workflow for cleaning the water and assessing its quality. I did not know them, and they did not know me, and it took some time to understand each other’s point of view, but we trusted each other to make this plan possible.
The initial idea was to use the porous gels previously developed in the Furukawa group as the extraction phase. Rhodium-based MOPs can be linked to form porous networks with extrinsic pores in the nm range that can be tuned in size depending on the linker. These networks also form colloidal nanoparticles, which are further interconnected to form colloidal gels. Thus, we already had tunable porous structures with pores big enough to fit the emerging organic pollutants. However, we still had the big question: how can we incorporate them into the water treatment workflow in a simple way without losing their properties? And the answer came unexpectedly after a group seminar where a new membrane concept was presented by Dr. Zaoming Wang. In the pore-networked membrane (PNM), the porous fillers are interconnected within the polymer matrix to form a continuous porous phase, which avoids aggregation of fillers and creates big pores accessible for the pollutants. The rhodium-based MOPs were the perfect candidates to create these PNMs.
Our first experiments compared the performance of these new PNMs and the more conventional membrane configurations, which consist of pure polymers or the dispersion of the fillers within the polymer matrix. Since the beginning, the PNMs outperformed the others in all aspects: stability, uptake capacity, and selectivity. Thus, we started investigating the uptake mechanism for specific pharmaceuticals (considered pollutants nowadays), observing the importance of the continuous porous phase in attaining high adsorption capacities and reliable results. We were so invested in this research that we started combining our expertise and the boundary between our tasks was becoming more and more blurred. Zaoming was training me to synthesize MOPs and membranes, and I was showing him how to integrate a chromatographic peak. We spent so many hours together connecting MOPs that, at some point, we even connected as individuals through a beautiful friendship (and Japanese food!).
Zaoming and I bonding over omurice. This photo was taken on one of my last days in Kyoto, when Zaoming was taking me out for dinner after work to places he likes
After one year and a half working together, we thought everything was complete. But Dr. Shuhei Furukawa made the question that led to the most interesting part of this study: “What if we make some structural changes in the MOP network by enlarging the linker or functionalizing the MOP unit?”. Considering the results so far, it was not surprising that the selectivity and uptake capacity of the PNM could be adjusted by these small modifications. Indeed, it led to the fabrication of the ultimate PNM, which resulted in the removal and detection of the pollutant diclofenac in tap and river water at concentrations corresponding to part per trillion level! If you are from the field, you might think this result is amazing.
The outcomes not only demonstrate the use of a simple membrane for both the removal and detection of these challenging compounds but also deepen our understanding of the mechanism behind the uptake process, which helps in the designing of the perfect membrane materials for specific pollutants. This research also taught us the importance of conducting interdisciplinary research, and we hope it can serve as an inspiration to open our minds, start listening to scientists from other fields, and expand our knowledge and impact in the world through collaboration and teamwork.
So, we hope you enjoy reading this paper, which required 26.5 ± 1.5 hours of meetings for the writing, fabricating dozens of membranes, using the liquid chromatograph equipment for 1289 hours, and generating 2252 chromatographic files. Yes, I could not help myself, and I had to monitor that too!