Schematic of water purification with carbon nanotube ponytails. (Image: Xia Zhao and Na Lab, University of Notre Dame)To address the CNT separability issue, researchers previously did the same thing as Na's team, i.e. increase the overall size of CNTs. For example, CNTs were fixed on inactive colloidal particles. What makes the CNT ponytails special, though, is that they contain little supporting material.An interesting aspect from a material synthesis point of view is that this work demonstrates the preparation of CNT colloidal particles nearly free of support without involving exotic procedures – i.e., CNPs are made using the same standard chemical vapor deposition method for CNT growth using a slightly different catalyst material."The elimination of extensive use of support transforms CNT colloids from a composite material containing only a minor mass/volume fraction of the active component (i.e., CNTs) to a material almost exclusively made of active CNTs," says Na. "Obvious advantages include saving energy for transportation and saving space in use."In their water purification experiments, the researchers showed that their CNPs are as good as CNTs for removing contaminants from water as adsorbents, disinfectants, and catalyst supports.Unlike unbounded CNTs, as Na points out, CNPs can be separated more effectively using common techniques such as gravitational sedimentation, magnetic attraction, and membrane filtration.For water purification, processes using CNPs might be helpful in situations where commonly used water treatment infrastructures are not available, for instance, providing clean water in developing countries or remote rural areas. In addition, CNPs can be used in industrial processes – e.g., as catalyst supports – and emergency response, e.g., adsorbents for chemical spills.Going forward, the team would like to further reduce the cost of making multifunctional and recollectable hierarchical materials for water purification so that the treatments described in their paper can become more affordable."Obviously" says Na, "this would be tied to the advancement of CNT synthesis if we keep using CNTs as the building material. Alternatively, we are looking into materials that are less expensive than CNTs. Our study with CNPs has provided insights for what properties we should be looking for in the alternative materials."Given the recent advances in nanotechnology, the water community has been searching for ways to incorporate nanomaterials into treatment processes. There is, however, a tremendous dimensional disparity between the nano world and the water world. No one is interested in nanoliters of water. When people talk about water, they talk in units such as million gallons per day.The challenge is how to connect these two worlds on the dimension ladder."What we did in making CNPs has done precisely that," says Na. "By integrating nanotubes into colloidal particles, we helped CNTs climb from the nanometer rung to the micrometer rung on the dimension ladder. By incorporating magnetism into CNPs, we elevated CNTs even further to the centimeter scale.""In our future research, we will keep looking for better ways to connect nanomaterials to the world that we live in – in technical terms, we will be researching for new strategies to scale up nanoscale properties for water purification," he concludes.