Analyte Detection with Cu-BTC Metal–Organic Framework Thin Films
by Means of Mass-Sensitive and Work-Function-Based Readout
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Abstract
Metal–organic
frameworks (MOFs) constitute a new generation
of porous crystalline materials, which have recently come into focus
as analyte-specific active elements in thin-film sensor devices. Cu-BTCalso
known as HKUST-1is one of the most theoretically and experimentally
investigated members of the MOF family. Its capability to selectively
adsorb different gas molecules renders this material a promising candidate
for applications in chemical gas and vapor sensing. Here, we explore
details of the host–guest interactions between HKUST-1 and
various analytes under different environmental conditions and study
the vapor adsorption mechanism by mass-sensitive and work-function-based
readouts. These complementary transduction mechanisms were successfully
applied for the detection of low ppm (2 to 50 ppm) concentrations
of different alcohols (methanol, ethanol, 1-propanol, and 2-propanol)
adsorbed into Cu-BTC thin films. Evaluation of the results allows
for the comparison of the amounts of adsorbed vapors and the contribution
of each vapor to the changes of the electronic properties of Cu-BTC.
The influence of the length of the alcohol chain (C1–C3) and
geometry (1-propanol, 2-propanol) as well as their polarity on the
sensing performance was investigated, revealing that in dry air, short
chain alcohols are more likely adsorbed than long chain alcohols,
whereas in humid air, this preference is changed, and the sensitivity
toward alcohols is generally decreased. The adsorption mechanism is
revealed to differ for dry and humid atmospheres, changing from a
site-specific binding of alcohols to the open metal sites under dry
conditions to weak physisorption of the analytes dissolved in surface-adsorbed
water reservoirs in humid air, with the signal strength being governed
by their relative concentration