People with faulty kidney function often require dialysis to rid their blood of harmful compounds. Conventional treatments are not perfect. They do a poor job removing a number of toxic compounds, such as p-cresyl sulfate and indoxyl sulfate. These molecules are stripped away by healthy kidneys, but in patients undergoing dialysis they may bind to human serum albumin and remain in the blood. Activated carbon and other adsorbents can trap them, but these materials are not selective—they also capture pharmaceuticals and other molecules that are better left in the body. This lack of selectivity led a Northwestern University team headed by Satoshi Kato and Omar K. Farha to wonder whether metal-organic framework compounds (MOFs), which are highly selective porous crystalline materials, could help trap the toxins. Early results look promising. The researchers chose a series of MOFs on the basis of expected interactions between their binding sites and the toxic compounds’ functional groups and evaluated their trapping efficiency. The top performer, NU-1000, a zirconium-based MOF with pyrene linkers, removed 94% of p-cresyl sulfate from aqueous solutions that simulate the blood of patients with chronic kidney disease (J. Am. Chem. Soc. 2019, DOI: 10.1021/jacs.8b12525). Analyses show that the MOF’s high uptake is due to highly hydrophobic adsorption sites sandwiched between two pyrene linkers and to the presence of Zr6 units, which bind guest molecules’ polar functional groups.