  Wireless and passive surface acoustic wave (SAW) devices with nanoporous
metal-organic framework (MOF) sensing layers are attractive gas sensors for
applications in many fields such as energy industries and air pollution
control. Here, we report on enhancing the sensitivity and detection limit of
zeolitic imidazolate framework-8 (ZIF-8) MOF-coated SAW reflective delay line
mass sensors by increasing the operating frequency for sensitive detection of
carbon dioxide (CO2) and methane (CH4) at ambient conditions. In particular, we
show at least four times higher sensitivity of an 860 MHz (4-{\mu}m
periodicity) SAW reflective delay line coated with a 240 nm thick ZIF-8
compared to the sensitivity of a 430 MHz (8-{\mu}m periodicity) otherwise
identical sensor device to the targeted gases. The detection limits of the
higher frequency wireless devices for CO2 and CH4 were estimated to be 0.91
vol-% and 7.01 vol-%, respectively. The enhanced sensitivity for higher
frequency devices is explained in terms of the frequency dependent acoustic
wave energy confinement.
