Covalent organic frameworks (COFs) have great potential as materials for batteries, sensors, and “smart” membranes that selectively admit or reject other molecules. But current manufacturing methods find COFs awkward to work with. Now, a team of researchers has developed a method of preparing COFs as inks that can be sprayed onto many different surfaces. They proved the concept by building an acoustic sensor to detect meat spoilage. They used the U.S. Department of Energy's (DOE's) Advanced Photon Source (APS) to characterize the device and show its nanoscale structure was indeed what they had hoped. Their work, published in the journal Advanced Materials, will open the way to effective manufacturing techniques to make COFs commercially viable materials.

Covalent organic frameworks are two-dimensional (2-D) polymers with a regular structure, a large ratio of surface area to weight, and strong covalent bonds. The large surface area makes them ideal for storing other molecules (this could allow them to act as batteries, for example), filtering substances by size at the molecular level, or as ultra-sensitive chemical detectors. But conventionally processed crystalline COF powders are insoluble and difficult to process, making it hard to manufacture them into thin-film devices using the typical methods.

Researchers from Northwestern University, DuPont Electronics and Imaging, and the DOE's Oak Ridge National Laboratory took a different approach. They mixed hexahydroxytriphenylene (HHTP), a polycyclic aromatic hydrocarbon, with a boronate ester, and various COF units. HHTP is a common building block for two-dimensional polymers and is often used to make self-assembling metal organic frameworks, both two- and three-dimensional. The boronate ester acted as a solvent to make the mixture into a colloidal liquid, with the COF building blocks suspended and easily sprayable. It also acted as a linker, connecting the COF building blocks. (The researchers note that other linking solvents would also be likely to work with this method; boronate esters were just the type chosen for this study.)

They then used a commercial Iwata airbrush to spray the COF colloid over a stencil on various substrates. When they removed the stencil, they found they had a finely patterned COF film with details as small as 50 micrometers (Fig. 1).

The team then used the technique to make acoustic sensors out of 2-D COF thin films. Acoustic sensors work by detecting shifts in frequency as a function of mass. The COF thin-film sensor increased in mass when it bound with the targeted molecule, creating a detectible signal. The sensors were able to detect trimethylamine, the scent of rotting meat, at a concentration of only 10 parts per billion. That matches the most sensitive detectors on the market. And the COF devices were fabricated in just two minutes, using less than 100 micrograms of COF.

The researchers were not surprised at the effectiveness of the sensor: 2-D COFs have molecularly well-defined pores with high affinity for targeted molecules. This molecular precision of the pores gives 2-D COF sensors an advantage in sensitivity and selectivity compared to current state-of-the-art sensors. By changing the specific COF used, such acoustic sensors should be able to detect a diverse range of molecules for many different purposes. The group has already found that the spray technique works for mixtures of different COFs, which would enable a single device to detect multiple target molecules.

The team used the X-ray Science Division (XSD) Chemical and Material Science Group’s beamline 12-ID-D and DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) beamline 5-ID-D at the APS to perform small- and wide-angle x-ray scattering investigations. These techniques can reveal structural features at the nanoscale, allowing the researchers to be sure the spray ink technique really had created the COF devices they intended. The DND-CAT beamline instrumentation, in particular, is excellent for characterizing thin-film materials and was the ideal beamline for this experiment.

They also used XSD Dynamics & Structure Group’s beamline 8-ID-E at the APS to perform grazing Incidence x-ray diffraction to confirm the surface features of the COF devices. (The APS is a DOE Office of Science user facility at Argonne National Laboratory.)

All the analysis done using the APS high-brightness x-rays (as well as other analytical methods) showed the spray coating method was effective. The team’s results show they have a scalable additive manufacturing technique for 2-D COFs that could make it possible for these materials to be used commercially.  ― Kim Krieger

See: Austin M. Evans¹, Nathan P. Bradshaw¹, Brian Litchfield², Michael J. Strauss¹, Bethany Seckman², Matthew R. Ryder³, Ioannina Castano¹, Christopher Gilmore², Nathan C. Gianneschi¹, Catherine R. Mulzer²***, Mark C. Hersam¹*, and William R. Dichtel¹**, “High-Sensitivity Acoustic Molecular Sensors Based on Large-Area, Spray-Coated 2D Covalent Organic Frameworks,” Adv. Mater. 32, 2004205 (2020). DOI: 10.1002/adma.202004205

Author affiliations: ¹Northwestern University, ²DuPont Electronics and Imaging, ³Oak Ridge National Laboratory

Correspondence: * m-hersam@northwestern.edu, ** wdichtel@northwestern.edu, *** catherine.mulzer@dupont.com

The authors acknowledge the Army Research Office for a Multidisciplinary University Research Initiatives (MURI) award under grant number W911NF-15- 1-0447. N.P.B. and A.M.E. also acknowledge the Department of Energy (DOE) (Grant DE-SC0019356) for support of the 2D COF spray coating work. A.M.E. (DGE-1324585), M.J.S. (DGE-1842165), I.C. (DGE-1842165), and N.P.B. were supported by National Science Foundation Graduate Research Fellowships. A.M.E., M.J.S., and I.C were supported by Ryan Fellowships provided by the International Institute of Nanotechnology. N.P.B. and M.C.H. acknowledge the DOE (Grant DE-SC0019356) and the National Science Foundation (Grant DMR-1720139) for support of the 2D COF spray coating work. M.R.R. acknowledges the U.S. DOE Office of Science-Basic Energy Sciences for research funding and the National Energy Research Scientific Computing Center (NERSC), a U.S. DOE Office of Science User Facility, operated under Contract No. DE-AC02-05CH11231, for access to supercomputing resources. DND-CAT is supported by Northwestern University, E.I. DuPont de Nemours & Co., and the Dow Chemical Company. This work has also made use of the IMSERC, EPIC, Keck II, and NUANCE facilities at Northwestern University, which have received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the MRSEC program (NSF DMR-1720139) at the Materials Research Center, the Keck Foundation, the State of Illinois, and International Institute for Nanotechnology (IIN). This research used resources of the Advanced Photon Source and Center for Nanoscale Materials, both U.S. DOE Office of Science User Facilities operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE- AC0206CH11357.

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