  Integration of nanoporous materials such as metal organic frameworks (MOFs)
with sensitive transducers can result robust sensing platforms for monitoring
gases and chemical vapors for a range of applications. Here, we report on an
integration of the zeolitic imidazolate framework-8 (ZIF-8) MOF with surface
acoustic wave (SAW) and thickness shear mode quartz crystal microbalance (QCM)
devices to monitor carbon dioxide(CO2)and methane (CH4) at ambient conditions.
The MOF was directly coated on the custom fabricated Y-Z LiNbO3 SAW delay lines
(operating frequency,f0 = 436 MHz)and AT-cut Quartz TSM resonators (resonant
frequency, f0 = 9 MHz) and the devices were tested for various gases in N2 at
ambient condition. The devices were able to detect the changes in CO2 or CH4
concentrations with relatively higher sensitivity to CO2, which was due to its
higher adsorption potential and heavier molecular weight. The sensors showed
full reversibility and repeatability which were attributed to the physisorption
of the gases into the MOF and high stability of the devices. Both types of the
sensors showed linear responses relative to changes in the binary gas
compositions thereby allowing to construct calibration curves which correlated
well with the expected mass changes in the sorbent layer based on mixed-gas
gravimetric adsorption isotherms measured on bulk samples.For 200 nm thick
films, the SAW sensitivity to CO2 and CH4 were 1.44x10^-6/vol-%, respectivel
yagainst the QCM sensitivities 0.24x10^-6/vol-% and 1x10^-8/vol-%,respectively
which were evaluated as the fractional change in the signal. The SAW sensors
were also evaluated for 100 nm - 300 nm thick films, the sensitivities of which
were found to increase with the thickness due to the increased number of pores
for adsorption of larger amount of gases. Also, the SAW devices had a good
wireless response for detecting gases remotely.
