Fluoride detection with a redox-active naphthalene diimide metal–organic framework

Article abstract of DOI:10.1016/j.poly.2018.08.005

Fluoride is commonly added to drinking water supplies for dental health; however, exposure to elevated fluoride levels can cause significant health problems. Therefore, it is important to develop simple and robust methods for monitoring fluoride in solution. Metal–organic frameworks (MOFs) have been targeted as ideal materials for next-generation sensing technologies, due to their combination of high surface area and chemical tunability. Here we report that a mesoporous MOF with redox-active naphthalene diimide-based ligands exhibits a selective and reversible color-change response to fluoride anion. The incorporation of fluoride sensing organic molecules into a MOF allows for the development of solid-state sensor devices that offer advantages relative to solution-phase methods.

Full text of DOI:10.1016/j.poly.2018.08.005

Accepted Manuscript
Fluoride detection with a redox-active naphthalene diimide metal–organic
framework
Hanna C. Wentz, Michael G. Campbell
PII:
S0277-5387(18)30461-3
https://doi.org/10.1016/j.poly.2018.08.005
POLY 13327
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
14 June 2018
3 August 2018
4 August 2018
Please cite this article as: H.C. Wentz, M.G. Campbell, Fluoride detection with a redox-active naphthalene diimide
metal–organic framework, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.08.005
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Fluoride Detection with a Redox-Active Naphthalene Diimide Metal–
Organic Framework
Hanna C. Wentz and Michael G. Campbell*
Department of Chemistry, Barnard College, 3009 Broadway, New York, New York
1
0027, USA
*Corresponding Author (email: mcampbel@barnard.edu)
Abstract
Fluoride is commonly added to drinking water supplies for dental health; however,
exposure to elevated fluoride levels can cause significant health problems. Therefore, it
is important to develop simple and robust methods for monitoring fluoride in solution.
Metal–organic frameworks (MOFs) have been targeted as ideal materials for next-
generation sensing technologies, due to their combination of high surface area and
chemical tunability. Here we report that a mesoporous MOF with redox-active
naphthalene diimide-based ligands exhibits a selective and reversible color-change
response to fluoride anion. The incorporation of fluoride sensing organic molecules into
a MOF allows for the development of solid-state sensor devices that offer advantages
relative to solution-phase methods.
Keywords: metal–organic frameworks; fluoride; sensors
1
. Introduction
Porous metal–organic frameworks (MOFs) have recently been explored as functional
materials for next-generation chemical sensors [1–3]. MOFs are in many ways an ideal
platform for the development of “designer” sensing materials, due to a combination of
high surface area and modular synthetic design. In particular, redox-active MOFs
display great promise for sensing, including electronic sensor devices [1,3–6].
While fluoride is commonly added to drinking water supplies to promote dental health, it
is well known that exposure to elevated levels of fluoride in drinking water raises the risk
of dental fluorosis [7–11]. Therefore, effort has been devoted to developing selective
and sensitive methods of fluoride detection [12–17]. One strategy that has received
1

significant attention has been the colorimetric sensing of fluoride using variously
substituted naphthalene diimide (NDI) molecules [18–23]. In some examples, the color
•–
change results from the reduction of NDIs to differently colored NDI radical anions or
2–
NDI dianions. The Saha group proposed that a selective π-anion interaction between
NDI-based receptors and fluoride can lead to an electron transfer that generates NDI•–
2–
or NDI , depending on the amount of fluoride added [24,25]. The observed response
could be further promoted through a synthetic design that preorganizes two NDI units in
an overlapping conformation. Others have pointed out, however, that direct reduction of
NDI by fluoride is unlikely on thermodynamic grounds [21,26,27], and the Gabbaï group
has put forth an alternate explanation that fluoride’s basicity leads to deprotonated
solvent side-products that ultimately serve to reduce NDI [28]. While the mechanistic
details of fluoride-induced NDI reduction are still unclear, the selective and dramatic
response of NDIs towards fluoride remains a potentially useful approach for fluoride
sensing.
We envisioned that solution-phase fluoride sensing using NDI molecules could be
translated to solid-state fluoride sensor devices, via the incorporation of NDI-based
ligands into a porous MOF. Organization of the NDI subunits in the rigid crystalline
structure of the MOF could also serve to enhance the sensing response. Solid-state
sensor materials and devices offer practical advantages as compared to solution-phase
sensor/receptor molecules. Additionally, the development of strategies for fluoride
sensing with redox-active MOFs could ultimately lead to advances such as MOF-based
fluoride sensitive electrodes [29,30].
2
. Experimental
2
.1 Materials and Methods
All manipulations were carried out under ambient atmosphere, using reagent-grade
chemicals and solvents, unless otherwise noted. Reagents and solvents were
purchased from Sigma Aldrich and used as received.
NMR spectra were recorded on a Bruker Avance spectrometer operating at 300 MHz for
1H acquisitions. UV-vis spectra were measured on a Varian Cary 5000
spectrophotometer. EPR spectra were collected on a Bruker EMX spectrometer
operating at X-band frequency; samples were measured in 2 mm quartz tubes at
2

ambient temperature. PXRD data were recorded on a PANalytical XPert3 diffractometer
with a Cu K X-ray source and PIXcel 1d detector, in a Bragg-Brentano geometry.
2
4
.2 Synthesis of H NDISA
The H NDISA ligand was synthesized according to the procedure in reference 34.
4
Under an Ar atmosphere, 1,4,5,8-napthalenetetracarboxylic dianhydride (1.0 g, 3.7
mmol, 1.0 equiv), 5-amino-2-hydroxybenzoic acid (1.2 g, 7.8 mmol, 2.1 equiv), and
anhydrous DMF (35 mL) were added to a 100 mL Schlenk flask. The reaction mixture
was stirred at 130 °C for 12 hours, then cooled to room temperature and opened to
ambient atmosphere. The resulting orange solid was isolated via vacuum filtration,
washed with DMF, diethyl ether, and methanol, and then dried under vacuum to afford
1
the product as an orange powder. H NMR analysis was consistent with the previously
reported data.
2
.3 Synthesis of Ni-NDISA
Ni-NDISA (powder or thin film) was synthesized according to the procedure in reference
4.
Powder: To a 20 mL vial was added H
104 mg, 0.36 mmol), DMF (7.5 mL), EtOH (0.5 mL), and H
3
4
NDISA (55 mg, 0.094 mmol), Ni(NO
3
)
2
•6H
2
O
(
2
O (0.5 mL). The vial was
capped, and the reaction mixture was heated at 120 °C for 12 hours. After cooling, the
solid product was isolated by vacuum filtration and washed with fresh DMF. The solids
were then stored under DMF prior to use.
Thin Films: To a 20 mL vial was added H
90 mg, 0.31 mmol), anhydrous DMF (10 mL) and anhydrous MeOH (1 mL). A glass
slide was submerged in the reaction mixture, then the vial was capped and placed at
20 °C for 20 minutes. After cooling to room temperature, the glass slide was removed
4
NDISA (55 mg, 0.094 mmol), Ni(NO
3 2 2
) •6H O
(
1
and carefully washed with fresh DMF. The MOF-coated slides were stored under DMF
prior to use.
Note: extensive drying under vacuum, or extended storage as a dry powder, results in
decomposition of Ni-NDISA as determined by PXRD analysis. We find that best results
are obtained when the MOF is stored under DMF after synthesis and purification, then
isolated via vacuum filtration immediately prior to weighing.
2
.4 Procedure for Ion Detection Experiments
3

2
0 mg of Ni-NDISA was added to a 20 mL vial, and then suspended in 3 mL of DMF.
Tetra-n-butylammonium halide (TBAX; X = F, Cl, Br, I) was then added (3 equiv, relative
to equivalents of NDI in the MOF), and the suspension was briefly swirled. No changes
were observed for X = Cl, Br, I, whereas a rapid color change to black was observed for
X = F (see Figure 3).
Note: All ion detection experiments were conducted under ambient atmosphere, using
reagent-grade solvents that were neither dried nor deoxygenated.
2
.5 Fabrication of Paper Fluoride Test Strips
Paper fluoride test strips were made by cutting filter paper (VWR Grade 413) into strips
1 × 6 cm). Each strip was first saturated with DMF. A suspension of Ni-NDISA (20 mg)
(
in DMF (5 mL) was immersed in an ultrasonic bath for 5 minutes to disperse the solids.
The end of a paper strip was then held in the MOF suspension, and sonication was
continued for 1–2 minutes to embed the MOF solids into the paper. The MOF-infused
paper strips were kept wet with DMF until use.
3
. Results and Discussion
3
.1 Choice of Suitable MOF for Fluoride Sensing
In targeting a suitable MOF for fluoride detection in solution, we considered several
design criteria: high porosity to allow for efficient ion transport; accessibility of the NDI
subunits to fluoride ions in the MOF pores; and stability of the MOF towards fluoride.
We also chose not to use the large number of NDI-based MOFs that exhibit
photochromism [31–33], in order to avoid potentially competing responses between
fluoride and ambient light. From surveying the literature, we identified the mesoporous
MOF-74 analogues M-NDISA (M = Mg, Ni) as promising candidates [34]. Synthesis of
M-NDISA, as either polycrystalline bulk samples or as thin films, is accomplished via
standard solvothermal methods from M(NO
)
3 2
and the free ligand H NDISA (Figure 1,
4
top). The H NDISA ligand is synthesized in one step from commercially available
4
starting materials. Thin films of M-NDISA have previously been shown to exhibit fast
and reversible electrochromic switching from transparent to dark states, based on redox
activity of the NDI ligands. We anticipated that intercalation of fluoride via the ~3.3 nm
mesopores of the MOF should allow for a selective response via reduction of the NDI
subunits of the ligands (Figure 1, bottom).
4

Figure 1. Synthesis and structure of the mesoporous MOFs M-NDISA (top); conceptual
schematic of fluoride-induced reduction to produce a selective color change response
(bottom).
3
.2 Evaluation of M-NDISA Stability Towards Fluoride
Initial tests were conducted using bulk polycrystalline samples of the MOF. The as-
synthesized M-NDISA MOFs were suspended in DMF, after which tetra-n-
butylammonium fluoride (TBAF) was added to the suspension. In accordance with
results from previous studies using molecular NDI-based fluoride sensors [24], fluoride
amounts in the range of ≤5 equivalents were used (equivalents of fluoride were
calculated with respect to the NDI subunits in the MOF, using the reported molecular
formulae for M-NDISA). We found that Mg-NDISA was unstable towards fluoride:
addition of TBAF to suspensions of Mg-NDISA quickly resulted in dissolution of the
framework. Gratifyingly, however, Ni-NDISA displayed stability towards fluoride
5

solutions over the course of several hours, as established by powder X-ray diffraction
PXRD). Diffraction patterns measured for Ni-NDISA matched well with the simulated
(
pattern, and no change was observed after fluoride exposure (Figure 2). Longer
exposure times (e.g. several days) did lead to eventual degradation of Ni-NDISA.
Figure 2. PXRD data for Ni-NDISA: simulated diffraction pattern (bottom); as-
synthesized Ni-NDISA (middle); Ni-NDISA after exposure to a solution of TBAF in DMF
(top).
3
.3 Response of Ni-NDISA to Anion Solutions
In order to evaluate the response of Ni-NDISA towards fluoride and other halide anions,
suspensions of Ni-NDISA were prepared in DMF, and then 3 equivalents of halide
(fluoride, chloride, bromide, and iodide) were added as their tetra-n-butylammonium
salts. Addition of TBAF produced a rapid and dramatic color change in the MOF solids
from red/orange to black. Addition of other halide salts did not produce an observable
color change as compared to the as-synthesized MOF (Figure 3, left). These results are
consistent with previous studies on molecular NDI-based fluoride sensors, and show
that selective fluoride-induced reduction of NDIs can also take place in a solid-state
framework. The rapid response also suggests facile ion transport through the
mesopores of the MOF. Leaching of the colored NDI ligand into solution was not seen
on the timescale of our experiments (minutes to hours), in line with the observed
stability of Ni-NDISA towards fluoride. Interestingly, when the MOF solids were isolated
6

by filtration after fluoride exposure, the color quickly reverted back to orange. Removal
from the fluoride-containing solution likely allows for re-oxidation of the NDI ligands by
air.
Figure 3. Optical images of Ni-NDISA suspensions before and after exposure to halide
anions in DMF solutions (left); paper test strips fabricated from Ni-NDISA, showing a
color change after exposure to fluoride in solution (right).
For practical applications, fluoride sensing in aqueous solutions is important. We
therefore explored the effect of water on the response of Ni-NDISA towards fluoride.
Exposure of Ni-NDISA to fluoride in pure water was not successful due to the limited
stability of Ni-NDISA in water, so we tested mixed solvent systems. We found that
addition of water to DMF solutions completely shut down reactivity of the MOF with
fluoride: if Ni-NDISA was suspended in a 85/15 mixture of DMF/H O, addition of TBAF
2
produced no response; similarly, after TBAF was added to a suspension of Ni-NDISA in
DMF, subsequent addition of water reversed the observed color change. If DMSO was
used in place of DMF, the color change response upon addition of TBAF was nearly
identical (Figure 4, left). Unlike DMF, however, a mixed solvent system of 85/15
DMSO/H O still led to a color change response upon addition of TBAF, albeit less
2
pronounced (Figure 4, right). This solvent-dependent behavior is consistent with the
hypothesis that acid/base reactivity with solvent mediates NDI reduction in the presence
of fluoride [28].
7

Figure 4. Optical images of Ni-NDISA suspensions before and after exposure to TBAF
in DMSO solutions, showing the effect of added water.
3
.4 Paper Test Strips for Fluoride Detection
Previously reported examples of molecular NDI-based fluoride sensors have primarily
focused on solution-phase reactivity, which limits practical applications [18–25]. We
envision that a significant advantage of incorporating the NDI moiety into a porous MOF
is the ability to develop solid-state fluoride sensor devices. As a proof-of-concept
demonstration, we have fabricated simple “dip strip” papers using Ni-NDISA. Strips of
filter paper were infused with solid Ni-NDISA by immersing the ends of the paper in a
DMF suspension of Ni-NDISA, in an ultrasonic bath. The MOF-infused paper strips
displayed a pale orange color, which selectively turned dark brown after dipping into a
DMF solution of TBAF (Figure 3, right). We found that the color change was less
dramatic for the paper strips, as compared to suspensions of Ni-NDISA. This is primarily
due to the reversibility of the color change: as with the bulk solids, the “dip strips” rapidly
lost their dark color after removal from the fluoride solution.
3
.5 Spectroscopic Characterization
The selective color change of the MOF in the presence of fluoride anion was probed
using electronic absorption spectroscopy. Thin films of Ni-NDISA were grown on the
surface of glass slides. Transmission UV-vis spectra were then acquired before and
after exposure to a TBAF solution in DMF (Figure 5). As with the bulk solids, Ni-NDISA
thin films displayed a selective transparent-to-dark color change response to fluoride
(Figure 5, inset). The spectrum before fluoride exposure is consistent with previous data
for Ni-NDISA and displays characteristic peaks for ππ* transitions of the neutral NDI
8

core (max = 342, 363, and 385 nm) [34,35]. Exposure to fluoride produces new
absorption maxima throughout the visible region of the spectrum (max = 474, 607, 700,
and 780 nm). The spectral features that appear after fluoride exposure are nearly
identical to spectroelectrochemical data for films of Ni-NIDSA in which the ligands have
•–
been reduced to the NDI radical anion form [34]. In contrast to solution-phase fluoride
2–
sensing with NDI-based molecules, peaks corresponding to the NDI dianion were not
observed, even after extended exposure times to a larger excess of TBAF [24]. After
removing the Ni-NDISA film from the TBAF solution, the color faded within seconds, and
the UV-vis spectrum was identical to the as-synthesized material. This could be
repeated for several cycles of TBAF exposure, although after a few cycles the film
began to delaminate from the glass substrate. The observed reversibility is promising
for the development of reusable MOF-based fluoride sensors.
Figure 5. Normalized transmission UV-vis spectra of Ni-NDISA thin films before and
after exposure to a solution of TBAF in DMF; the inset shows optical images of the films
on glass slides inside of quartz cuvettes.
Consistent with the UV-vis data, electron paramagnetic resonance (EPR) spectroscopy
clearly indicates the formation of radical anion ligands after fluoride exposure. While the
as-synthesized Ni-NDISA MOF is EPR silent, exposure to fluoride results in the
appearance of a signal centered at approximately g = 2.006 (Figure 6). Typical of NDI•–
radical anions, a 13-line pattern is observed, due to coupling with two equivalent
9

nitrogen nuclei and four equivalent protons in the NDI core [35]. The combined data
show that fluoride exposure results in reduction of the MOF ligands to their radical anion
form, without destruction of the MOF framework.
Figure 6. X-band EPR spectrum of Ni-NDISA after exposure to a solution of TBAF in
DMF (100 kHz modulation frequency, 0.2 G modulation amplitude); no signal is
observed prior to fluoride exposure.
Because fluoride exposure generates ligand-based radicals, we were also curious about
the electrical response of Ni-NDISA in the presence of fluoride. We performed
preliminary electrical conductivity studies on Ni-NDISA before and after fluoride
exposure and observed slight increases in conductivity. However, these efforts were
hampered by the fact that the color change response does not persist if the MOF is
isolated from the fluoride-containing suspension, which complicates device fabrication.
Additionally, under our measurement conditions, the change in conductivity was not
anion-specific. Exposure to TBACl also produced a comparable increase in conductivity.
Future studies in our lab will aim to further explore the electrical response of NDI-based
MOFs to fluoride exposure – the development of new fluoride-sensitive electrodes
based on redox-active MOFs would be a significant advance.
4
. Conclusions
We have reported the first example of using a redox-active MOF for the detection of
fluoride anions in solution. Incorporation of naphthalene diimide-based ligands into a
1
0

mesoporous MOF allows for rapid and selective color-change response in the presence
of fluoride, which represents a practical advantage over previously reported solution-
phase systems. We anticipate that future work will result in improved MOF-based
technologies for monitoring fluoride levels in solution. In particular, the development of
water-stable, redox-active MOFs that exhibit a selective electrical response towards
fluoride in aqueous solutions would provide a platform for next-generation fluoride
sensors.
Acknowledgements
We thank Khalid Al Kaabi for helpful discussions and for simulation of the Ni-NDISA
structure, and Steffen Jockusch for assistance with EPR data collection. We thank the
Department of Chemistry and the Office of the Provost at Barnard College for financial
support. Powder X-ray diffraction was performed at the Shared Materials
Characterization Laboratory (SMCL) at Columbia University. Use of the SMCL was
made possible by funding from Columbia University.
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Graphical abstract
Synopsis: We report the first example of using a redox-active metal–organic framework (MOF)
for the detection of fluoride anions in solution. These results provide a platform for the
development of improved MOF-based fluoride sensor devices.
1
3