Metal-Organic Frameworks as Sensors: A ZIF-8 Based Fabry-Pe´rot Device
as a Selective Sensor for Chemical Vapors and Gases
Guang Lu and Joseph T. Hupp*
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208
Received February 17, 2010; E-mail: j-hupp@northwestern.edu
Permanently microporous metal-organic framework (MOF)
materials
1
have attracted considerable attention on account of their
large internal surface areas, uniform channels, (sub)nanometer-sized
cavities, thermal stability, and chemical tailorability.
2
Among the
potential applications for which proof-of-concept reports have
proliferated are gas storage,
3
chemical catalysis,
4
and small-
molecule separations.
5
At first glance MOFs would also appear to
be attractive for chemical sensing. Nevertheless, reports of MOF-
based sensing are comparatively few.
6
The challenge is signal
transduction: the cavities of MOFs are generally too small to tailor
with reporter molecules, i.e. moieties that can readily signal analyte-
binding events via changes in color, redox potential, or other
properties. In the handful of cases where sensing has been described,
advantage has generally been taken of framework luminescence,
7
with signal transduction consisting of luminescence quenching.
6,8
Here we describe an alternative approach that circumvents the
need for molecular-level reporters and instead relies upon a readout
of changes in a macroscopic property of the sensing material, the
refractive index. Specifically, we have configured the material as a
transparent thin film on an appropriate support material (glass or
silicon) and optically monitored the energies of Fabry-Pe´rot
interference peaks as a function of analyte exposure.
9
These peaks
are observable when the thickness of a supported material is
comparable to the wavelength of light (λ). Importantly, their
energies also depend on the film’s refractive index.
As the sensing material we chose a zeolitic imidazolic frame-
work, ZIF-8. This chemically robust and thermally stable material
is characterized by the sodalite (SOD) zeolite-type structure with
large cavities (11.6 Å) and small pore apertures (3.4 Å).
10
Diverse
protocols
10
are available to produce ZIF-8 crystals with sizes from
micrometers to nanometers.
10d
While dense ZIF-8 membranes have
recently been synthesized on porous supports using solvothermal
conditions for the purpose of gas separation,
11
it is not easy to
precisely control thicknesses of films on the submicrometer scale
using these conditions. Several strategies have been developed to
prepare MOF thin films,
12
but they either produce poor morphol-
ogies
13
or are time-consuming
14
to grow with thicknesses over 100
nm. In our experiments, ZIF-8 was obtained in thin-film form
(Figure 1A) simply by immersing glass or silicon slides in a freshly
prepared methanolic solution of 2-methylimidazole and Zn(NO
3
)
2
at room temperature (see Supporting Information (SI)). Quartz
crystal microgravimetry experiments (gold surfaces; see SI) show
initially rapid film growth that subsequently slows and is largely
complete after 30 min. Thicker films can be obtained by repeating
the process with fresh solutions (Figure 1B and 1C). Cross-sectional
scanning electron microscopy (SEM) images demonstrate that the
films thicken by ca. 100 nm per growth cycle (Figure 1D). (The
initial cycle, however, yields a ca. 50 nm thick film.) X-ray
diffraction measurements (see SI) establish that the films comprise
ZIF-8.
Figure 2 shows a series of ZIF-8 films of various thicknesses, l,
grown on silicon substrates. While ZIF-8 itself is colorless, the
silicon-supported thin films are not. The variable coloration is a
manifestation of film-thickness-dependent optical interference in
the visible region. At normal incidence the wavelengths of the
(reflected) interference peaks are given by
where m is an integer and n is the thin film’s refractive index.
The key to chemical sensing is the tunability of n. For a
microporous MOF, n is a volume-weighted average of the indices
for the cavities (vacuum, n
vac
) 1) and the framework (n
fram
> 1).
Inserting into the cavity any polarizable molecule will displace
vacuum and increase the overall refractive index. From eq 1, this
will result in red shifts of the interference peaks.
Fabry-Pe´rot fringes for transmission of visible light through a
ca. 1000 nm thick film (10-cycle growth) of ZIF-8 on glass were
monitored to demonstrate the sensing properties.
15
Exposure to
propane shifts the visible-region fringes by up to 49 nm (Figure
3A). The extents of the shifts are sensitive to the propane partial
pressure (Figure 3B). The analyte-induced shift occurs within 1
Figure 1. Cross-sectional SEM images of ZIF-8 films grown on silicon
substrates with cycles of (A) 1, (B) 10, and (C) 40; (D) thickness of ZIF-8
film versus numbers of growth cycles.
Figure 2. Photograph of a series of ZIF-8 films of various thicknesses
grown on silicon substrates.
mλ ) 2nl (1)
10.1021/ja101415b XXXX American Chemical Society J. AM. CHEM. SOC. XXXX, xxx, 000
9
A
min and is reversed by replacing analyte with pure nitrogen gas
(see SI). From the magnitudes of the shifts in the interference
fringes, we calculated that the volume fraction of analyte in the
framework is ∼0.13 for pure propane at 1 atm.
While responsive to a range of vapors and gases, the ZIF-8 sensor
does display some chemical selectivity. For example, linear
n-hexane is readily sensed, but the sterically more demanding
cyclohexane is not, consistent with the small portal size for ZIF-8
cavities. Consistent with the known hydrophobicity of ZIF-8,
10a
the sensor is unresponsive to water vapor. In contrast, ethanol is
readily detected (Figure 4A). Exposure to the vapor above ethanol/
water mixtures of various ethanol contents gives rise to ethanol-
concentration-dependent responses, with the sensor response satu-
rating at ca. 40% ethanol (Figure 4B). We have taken advantage
of this observation to calculate the volume fraction of analyte in
the framework: 0.25 for pure ethanol, which agrees reasonably well
with the void volume fraction (0.20) indicated by single-crystal
X-ray structural measurements.
16
The detection limit depends, in part, on the resolution of the
spectrophotometer. For 1 nm resolution, the estimated detection
limit for ethanol in water is ca. 0.3 vol %, corresponding to an
ethanol vapor concentration of ca. 100 ppm.
In summary, we have constructed MOF-based Fabry-Pe´rot
devices that function as selective sensors for chemical vapors and
gases. To obtain suitable devices we rely upon a simple, but
controllable and effective, method for fabricating ZIF-8 films.
Virtues of this method include gentle reaction conditions (room
temperature), rapid growth rate (∼100 nm/30 min), good control
over thickness (∼ 100 nm/cycle), no special requirements for
surface modification of substrates, and ease of removal of solvent
(methanol).
Acknowledgment. We thank DTRA, AFOSR, and the North-
western Nanoscale Science and Engineering Center for support of
our work.
Supporting Information Available: Materials and methods, QCM,
XRD, EDXS, FT-IR, TGA, N
2
gas sorption, gas- and vapor-testing
data, and effective refractive index and volume fraction calculation.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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JA101415B
Figure 3. (A) UV-vis transmission spectra of 10-cycle ZIF-8 film grown
on glass substrate after exposure to propane of various concentrations (blue
curve for 0% and red curve for 100%) and (B) corresponding interference
peak (originally at 612 nm) shift versus propane concentration. The propane
concentration is expressed as a percentage of the total gas flow where
nitrogen is used as diluent.
Figure 4. (A) UV-vis transmission spectra of 10-cycle ZIF-8 film grown
on glass substrate on exposure to vapors of ethanol and water. (B)
Interference peak (originally at 612 nm) shift versus ethanol concentration
in ethanol/water solutions. The concentration is expressed as a volume
percentage.
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