Multipressure Sampling for Improving the Performance of MOF-based Electronic Noses

Metal–organic frameworks (MOFs) are a promising
class of
porous materials for the design of gas sensing arrays, which are often
called electronic noses. Due to their chemical and structural tunability,
MOFs are a highly diverse class of materials that align well with
the similarly diverse class of volatile organic compounds (VOCs) of
interest in many gas detection applications. In principle, by choosing
the right combination of cross-sensitive MOFs, layered on appropriate
signal transducers, one can design an array that yields detailed information
about the composition of a complex gas mixture. However, despite the
vast number of MOFs from which one can choose, gas sensing arrays
that rely too heavily on distinct chemistries can be impractical from
the cost and complexity perspective. On the other hand, it is difficult
for small arrays to have the desired selectivity and sensitivity for
challenging sensing applications, such as detecting weakly adsorbing
gases with weak signals, or conversely, strongly adsorbing gases that
readily saturate MOF pores. In this work, we employed gas adsorption
simulations to explore the use of a variable pressure sensing array
as a means of improving both sensitivity and selectivity as well as
increasing the information content provided by each array. We studied
nine different MOFs (HKUST-1, IRMOF-1, MgMOF-74, MOF-177, MOF-801,
NU-100, NU-125, UiO-66, and ZIF-8) and four different gas mixtures,
each containing nitrogen, oxygen, carbon dioxide, and exactly one
of the hydrogen, methane, hydrogen sulfide, or benzene. We found that
by lowering the pressure, we can limit the saturation of MOFs, and
by raising the pressure, we can concentrate weakly adsorbing gases,
in both cases, improving gas detection with the resulting arrays.
In many cases, changing the system pressure yielded a better improvement
in performance (as measured by the Kullback–Liebler divergence
of gas composition probability distributions) than including additional
MOFs. We thus demonstrated and quantified how sensing at multiple
pressures can increase information content and cross-sensitivity in
MOF-based arrays while limiting the number of unique materials needed
in the device.