Function–Topology Relationship in the Catalytic Hydrolysis of a Chemical Warfare Simulant in Two Zr-MOFs

Article abstract of DOI:10.1002/chem.202002412

Owing to their high surface area, high concentration of active metal sites, and water stability, zirconium(VI)-based metal–organic frameworks (Zr-MOFs) have shown excellent activity in the hydrolysis of organophosphorus nerve agents (OPNs). In this regard, for the first time, two topologically different Zr-MOFs (Zr-fcu-tmuc and Zr-bcu-tmuc, constructed from the same organic and inorganic building blocks; fcu=face-centered cubic, bcu=body-centered cubic) have been rationally chosen to investigate the effect of network topology on the catalytic hydrolysis of the nerve agent simulant, dimethyl 4-nitrophenyl phosphate (DMNP). A remarkable enhancement in the hydrolysis rate of DMNP was observed with Zr-bcu-tmuc, reducing the half-life more than three-fold compared with Zr-fcu-tmuc. Greater accessibility of the active Zr^VI sites in the 8-connected bcu net compared with the 12-connected fcu leads to a faster hydrolysis of DMNP on Zr-bcu-tmuc. Interestingly, the higher activity of Zr-bcu-tmuc was also confirmed by its higher fluorescence sensitivity towards DMNP (limit of detection (LOD)=0.557 μm) compared with Zr-fcu-tmuc (LOD=1.09 μm). The results show that controlling the desired topology of Zr-MOFs is a useful strategy for improving their performance in the detection and catalytic detoxification of OPNs.

Full text of DOI:10.1002/chem.202002412

Accepted Article
Accepted Article
Title: Function-Topology Relationship in Catalytic Hydrolysis of
Chemical Warfare Simulant inTwo Zr-MOFs
Authors: Ali Morsali and Hosein Ghasempour
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To be cited as: Chem. Eur. J. 10.1002/chem.202002412
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0
1/2020

Chemistry - A European Journal
10.1002/chem.202002412
Function-Topology Relationship in Catalytic Hydrolysis of
Chemical Warfare Simulant in Two Zr-MOFs
a
Hosein Ghasempour , and Ali Morsali *
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, Postal code
1
4115175, Tehran, Iran.
Abstract
Due to the grate surface area, high concentration of active metal sites and water
stability, zirconium(VI)-based metal-organic frameworks (Zr-MOFs) have shown
excellent activity for the hydrolysis of organophosphorus nerve agents (OPNs). In
this regard, for the first time, two topologically different Zr-MOFs (Zr-fcu-tmuc and
Zr-bcu-tmuc; build up from same organic and inorganic building blocks) were
rationally chosen to investigate the effect of network topology on catalytic
hydrolysis of nerve agent simulant, dimethyl-4-nitrophenyl phosphate (DMNP).
Remarkable enhancement in the DMNP hydrolysis rate was observed using Zr-bcu-
tmuc, reducing the half-life more than three times compared to Zr-fcu-tmuc. Higher
accessibility toward active Zr(VI) sites in 8-connected bcu net versus 12-connected
fcu, lead to faster hydrolysis of DMNP on Zr-bcu-tmuc. Intrestingly, the higher
activity of Zr-bcu-tmuc was also confirmed by its higher fluorescence sensitivity
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toward DMNP (LOD: 0.557 µM) compared to Zr-fcu-tmuc (LOD: 1.09 µM). The
results showed that dictating the desired topology in Zr-MOFs, is a useful strategy
to improve their performance in catalytic detoxification and detection of OPNs.
Introduction
During the last century, toxic chemical as warfare have been commonly used in as
chemical warfare agents (CWAs) and various modern chemical agents, containing
blood, blister and nerve agents were introduced [1]. Among them, organophosphorus
nerve agents (OPNs) were developed fast owing to their simple inexpensive
manufacturing process causing serious problems for nature and human kind[2].
Indeed, the point that make the OPNs distinct from other CWAs, is arisen from their
capability in rapid death of a human through shutting down the acetylcholinesterase
(AChE) enzyme activity in the acetylcholine degradation in nerve system of body[3].
According to a 2002 article in the Star-Ledger, more than 20,000 Iranian soldiers
and war medics were killed on the spot by tabun nerve gas in Iraq-Iran battle[4].
Very recently, in Japan and Syria there were other reports on using sarin gas in
terrorism attacks[5, 6]. Therefore, attempts for developing new strategies for
accelerating the detection and detoxification of these banned chemicals is one of the
most important area of academic research. The activity of OPNs arises from their
phosphonate ester-based structure containing P=O bond, a leaving group (X) and
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10.1002/chem.202002412
two alkyl/alkoxyl substituents[7]. Accordingly, common phosphonate ester
pesticides (e.g., DMNP), are known as suitable alternative for G-type OPNs
]
8[(scheme 1a).
Among different methods, the catalytic hydrolysis showed promising efficiency in
OPNs detoxification which can be carried out through polarization of the P=O bond,
Scheme 1. Phosphonate ester-based structures of G-type nerve agents and nerve agent simulant,
DMNP (a) and hydrolysis reaction of DMNP on Zr-MOFs (b).
following by selective cleavage of the labile P–X bond [9]. Although, many studies
were reported on using homogenous catalysts for the hydrolysis process of OPNs
[10-12], due to important advantages such as recyclability, stability and high
recovery, solid heterogeneous catalysts such as activated carbon or organic polymers
have received considerable interest [13, 14]. However, low porosity, low active site,
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and slow kinetics, provide significant room for introducing novel substitutes for
these materials.
Metal-organic frameworks (MOFs) are attractive heterogeneous catalysts owing to
their exceptional tunable pores, high porosity and their ability to present high
concentrations of catalytically active metal ions or clusters [15-20]. Among different
type of MOFs synthesized to date, high chemically, mechanically and thermally
stabile[21] zirconium(VI)-based MOFs (Zr-MOFs, containing Zr cluster,
6
[
Zr O (OH) ]12+}) show amazing hydrolysis rates toward OPNs [22-24] due to
6
4
4
multiple Zr–OH–Zr moieties on Zr cluster which make them very comparable to
6
the bimetallic Zn–OH–Zn active sites found in the naturally active
phosphotriesterase enzyme[25].
In addition to variety parameters such as particle size[26], pore size[27] and
functionality[28], network topology[29] can positively affect the hydrolysis
rate of OPNs. Due to different type of clusters in Zr-MOFs and simple
accessibility of reagents to their sites, systematic study of Zr-MOFs is of
interest. Many operative policies for topological design[30] of Zr-MOFs for
OPNs capture and mechanistic insights derived from in situ experimental
explanations and computational modelling, prompted the development of
promising Zr-MOF-based structures to detect and capture OPNs[31]. Agrawal
et al. modeled the uptake of OPNs and simulants including DMNP in some
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Zr-MOFs under dilute conditions and suggested that the simulant interacts
with the MOF via hydrogen bonds to the Zr−OH groups of the MOF and the
applicability of the observed interactions at very low concentrations was
studied[32]. A similar report by Florencia et al. studied the effect of Zr-MOFs
topology on the capture and detoxification of vary CWAs including OPNs.
They showed that topology depended properties of MOF, especially surface
area, pore aperture size, Zr-nodes connectivity and accessibility, play serious
role on the capture of sarin by controlling the loading, retention time and
leaching of agents[33]. In this current contribution, to highlight the direct
effect of network topology on hydrolysis rate of DMNP (scheme1b), for the
2
-
first time, two Zr-MOFs, Zr-fcu-tmuc and Zr-bcu-tmuc (tmuc : trans, trans-
muconate, fcu: face centered cubic, bcu: body centered cubic) were chosen for
2
-
comparative study. Owning the same framework backbone (tmuc ligand and
Zr cluster), these two Zr-MOFs presented to be interesting couple for a
6
comparative catalytic study.
Experimental
Materials and Instruments. Chemical reagents were obtained from commercial
sources and used without further purification. Zr-MOFs and DMNP were
synthesized according to the literature methods[34-36]. Powder X-ray diffraction
(PXRD) patterns were recorded on a Panalytical X’pert diffractometer with
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monochromatic Cu-Kα radiation (λCu = 1.5406 Å). Volumetric N sorption
2
measurements of samples were collected using a TriStar II 3020 (Micromeritics) at
7
7 K. The (Vt) was calculated at P/P = 0.3. UV−Vis absorbance spectra were
0
measured on a Varian Cary 50 UV−Vis spectrophotometer equipped with a single-
beam facility with a spectral resolution of 0.2 nm. The Fluorescence properties of
samples were measured in different solvent emulsions containing MOFs using a
PerkinElmer-LS55 Fluorescence Spectrometer at room temperature. NMR spectra
were collected on a Bruker Avance 250 NMR instrument (static magnetic field: 11.7
T) working at Larmor frequencies of 250 MHz. FE-SEM and EDS analysis were
performed by TESCAN MIRA instrument. Leached metal ion concentrations were
determined on a Varian Vista-PRO ICP-OES instrument (Springvale, Australia)
using a charge-coupled detector.
Hydrolysis processes. In a vial, 13 mg of Zr-fcu-tmuc or 7 mg of Zr-bcu-tmuc (5
mol% based on Zr) was added followed by 10 mL of aqueous solution (D O/H O
2
2
1
:9) containing 0.45 ml N-ethylmorpholine (as a buffer at pH 10) and about 25 mg
0.1mmol) DMNP. The solution was stirred at room temperature for two hours.
Periodic monitoring was carried out by removing a 0.5 mL aliquot from the reaction
(
3
1
mixture and diluting it with the buffer (1 mL) prior to PNMR measurements.
3
1
Progress of the reaction was monitored by PNMR spectra of the reaction mixture
and conversion was tracked by integrating the peaks associated with DMNP (δ=-5.1
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ppm) versus the dimethyl phosphate (δ=2.2 ppm) product obtained from cleavage of
the phosphorus-nitrophenol bond. Kinetic plots were created by plotting natural log
of the conversion versus time (Figure S12). Initial half-life (t ) was calculated from
1/2
following equation:
t =ln 2/k
1
/2
where k is the slope of kinetic plots[36].
Fluorescence Measurements. In a typical procedure, 0.5 mg of an activated MOF
was grinded down, and then immersed in methanol, ethanol and water solvents (2
ml), separately and were scanned. To investigate the fluorescence quenching effect
of DMNP in detail, the Stern-Volmer (SV) plot is created according to SV equation:
(
I /I)= K [A] + 1 , where I is the initial fluorescence intensity of MOF in methanol,
0
sv
0
I is the fluorescence intensity in the presence of analyte, K is the quenching
sv
-1
constant (M ) and [A] is the molar concentration of analyte). The Detection Limits
(LOD) were calculated on the basis of fluorescence titration. The PL spectra of Zr-
MOFs were scanned 10 times, and the fluorescence intensities were plotted versus
the concentration of DMNP. LOD were calculated from following equation: LOD
=
3∂/m, where m is the slope of the calibration curve and ∂ is the standard deviation
of the blank measurement.
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Results and discussion
Zr-fcu-tmuc and Zr-bcu-tmuc were synthesized according to the published
methods [34, 35]. Comparing PXRD pattern of synthesized samples with
reported data confirmed the purity of products (Figures S3). Zr-fcu-tmuc is
isostructural with the UiO-66 structure forming well-known 12-connected fcu
topology with two type tetrahedral and octahedral cages (Figure 1c). In the
8
+
case of Zr-bcu-tmuc, each Zr cluster ([Zr (µ -O) (µ -OH) (OH) (H O) ] )
6
6
3
4
3
4
4
2
4
was bridged by 8 organic linkers to coordinate to 8 other clusters forming 8-
connected bcu topology with one-dimensional channels (Figure 1d). In fact,
bcu topology derives from the fcu topology through changing geometry of
ligand from linear to zigzag which one-third of bridged ligands were removed
to form overall an 8-connected bcu topology. As shown in the figure 1, the
2
-
tmuc linker in zigzag geometry, lead to bcu net with 8-connected clusters
instead of common 12-connected fcu net. N sorption isotherms measured at
2
7
7
K
indicated
that
Zr-fcu-tmuc
and
Zr-bcu-tmuc
have
2
−1
Brunauer−Emmett−Teller (BET) surface area around 1480 and 758m .g ,
respectively which are comparable with reported values [35, 37] (Figure S4).
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Figure 1. View of the linker conformations (a-b), structure along c axe (c-d) and Zr cluster
6
connectivity (e-f) in Zr-fcu-tmuc (up) and Zr-bcu-tmuc (down), (The structure of UiO-66 was used
to show Zr-fcu-tmuc structure). Comparison of fcu-a and bcu-a networks (g-h). C; t urquoise blue,
-
O; red, Zr; purple and Zr
clarity.
6
clusters; purple polyhedron, terminal OH and H
2
O were eliminated for
Since Zr-MOFs have attracted extensive interest as heterogenous catalyst for
detoxification of OPNs [38, 39], the catalytic hydrolysis of DMNP (as an OPNs
simulant) was assessed on these two Zr-MOFs. Possessing the same organic
2
-
and inorganic building blocks (tmuc ligand and Zr cluster), similar particle
6
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and pore aperture size, but different network topology, these two Zr-MOFs
presented to be interesting couple for a comparative catalytic study. First, we
followed the catalytic process in the test reaction in an aqueous buffer at
pH=10 (0.45 M N-ethylmorpholine) by UV-Vis spectroscopy. As an example,
in figure S5 it is clear that by passing time UV-Vis absorbing band of DMNP
at 270 nm was disappearing while new band at 407 nm (p-nitrophenoxide) was
growing up attributing to hydrolysis of DMNP on Zr-MOFs. Further, the
3
1
31
PNMR spectroscopy was used to evaluate the catalytic process. PNMR
spectra of the mixtures were collected during desired time and conversions
were determined by integrating the peaks related with DMNP versus the
dimethyl phosphate which is produced from hydrolysis of the phosphorus-
3
1
nitrophenol bond (Figures 2a, S6 and S7). The PNMR spectrum exhibits a
resonance at δ=2.2 ppm related to the desired hydrolysis product (dimethyl
phosphate) and the peak at δ=-5.1 ppm is ascribed to unreacted DMNP.
Initially, to study the hydrolytic degradation of DMNP on Zr-MOFs, different
reaction conditions were performed and the results was summarized in table
S1. Hydrolysis experiments in distilled water at room temperature resulted in
no hydrolysis after 24 hours even by increasing temperature up to 50 °C. Also,
when hydrolysis of DMNP was tried in the buffer in the absence of any
catalyst (blank reaction), trace amount of conversion was attained within 24
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hours (Table S1, entries 1-4). It has been observed that using H tmuc ligand
2
and ZrCl salt as catalyst in buffer solution, resulted in negligible conversion
4
(Table S1, entries 6 and 8). While applying 2.5 mol % (based on Zr) of Zr-
MOFs in buffer solution led to a considerable amplified in hydrolysis
conversion within only 2 hours, 31 % and 40.6 % conversion for Zr-bcu-tmuc
and Zr-fcu-tmuc, respectively which means higher activity of Zr-bcu-tmuc.
By increasing the amount of catalysts (5 mol %) in buffer solution, we
observed 92% and 60.9% conversions after 1 hour for Zr-bcu-tmuc and Zr-
fcu-tmuc, respectively (Table S1, Figure S8). Notably, no considerable
product was observed for the blank reaction (hydrolysis reaction in buffer
without catalyst) over the same time course (<3 % conversion after 60 min;
Figures. S9 and 2b, purple cycles). According to the results the optimum
condition was carried out at ambient temperature by stirring 5 mol % catalyst
in buffer within desired period of time to calculate the hydrolysis rate of
DMNP. Figure 2b shows the conversion percentage versus time profiles for
both Zr-fcu-tmuc and Zr-bcu-tmuc in comparison with blank reaction. Kinetic
plots were created by plotting natural log of the conversion versus time (Figure
S10). Initial half-life (t ) was calculated from related equation, noted in
1
/2
experimental section. Interestingly, initial rate calculation showed that using
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Zr-bcu-tmuc as catalyst significantly enhanced the DMNP hydrolysis by
reducing the half-life from 25.4 to 7.3 minutes (Table 1). This relatively high
Figure 2. 31PNMR of DMNP after hydrolysis catalysis in the presence of Zr-bcu-tmuc (a).
Conversion versus time plots of the DMNP hydrolysis by Zr-fcu-tmuc (blue) and Zr-bcu-tmuc
(
red) in comparison with blank reaction (purple) and post filter (green) reactions(b). Photograph
images of reaction mixture in NMR tubes after 12 hours (c). FE-SEM images of used Zr-fcu-tmuc
d) and Zr-bcu-tmuc (e) crystals.
(
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hydrolysis rate in Zr-bcu-tmuc was also evidenced by appearing crystalized
p-nitrophenol in NMR tubes containing test reaction mixture in natural water,
which are observed by naked eye. As shown in figure 2c, the amount of the
crystalized product in Zr-bcu-tmuc tube (con. %=27.3, entry 13 table 1) is
significantly more than Zr-fcu-tmuc tube (con. %=11.3, entry 9 table 1) which
is another reason for great catalytic activity of Zr-bcu-tmuc. The high
accessibility of Zr cluster in 8-connected Zr-bcu-tmuc might is the main
6
reason of higher hydrolysis activity.
It was found that [36, 40], DMNP is too large (ca. 11×5 ꢀ) to access the interior
pores of UiO-66 type MOFs. The pore aperture size of Zr-fcu-tmuc is in the similar
range of UiO-66 (about 6 ꢀ). Similarly, Zr-bcu-tmuc has 1D channels along c axis
with estimated aperture size around 7 ꢀ which may allow partial diffusion of DMNP
inside the framework channels. Accordingly, most of the reaction occurs on the
exterior surface of these Zr-MOF crystals. Although scanning electron microscopy
(
FE-SEM) images of Zr-MOFs revealed that both Zr-bcu-tmuc and Zr-fcu-tmuc
have crystals with almost same size (Figures 2d and 2e), the hydrolysis results
showed that surface of Zr-bcu-tmuc is relatively more active than Zr-fcu-tmuc
which can attribute to its more accessible Zr(VI) sites in less connected bcu
topology.
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Figures S11-16 illustrate the FE-SEM images, corresponding elemental map images
and EDS analysis of as-synthesized, used Zr-MOFs and after washing with acidic
solution (0.02 M HCl). It is clear that after hydrolysis reaction both of catalyst
surfaces are covered with phosphorus component which was confirmed with
elemental map images and EDS spectra by appearing the phosphorus (Figures S12
and S15). Also, these observations indicated that washing the catalysts is needed for
regeneration. So, catalysts were washed with distilled water several times and were
analysed with FE-SEM, elemental map and EDS. The map image and EDS analysis
showed that catalysts were not regenerated successfully through applying water.
While using 0.02 M HCl solution for washing resulted in the regenerated Zr-MOF
catalysts which can be used again for hydrolysis of DMNP (Figures. S13 and S16).
Very recently, Chen et al. suggested detailed hydrolysis mechanism of DMNP on
Nu-1000 through computational investigation[41]. Considering this report and our
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experimental results, the schematically hydrolysis mechanism was proposed in
scheme 2, involving binding mode between DMNP and Zr(IV) nodes in the reactant
state (RS) and transition state (TS). First, the non-bridging oxygen in DMNP (O=P)
make an interaction with active Lewis-acidic sites of Zr(IV) nodes resulting in
cluster-DMNP complex as shown in the RS in scheme 2. This interaction causes the
polarizing of P=O bond, followed by weakening of the phosphorus−nitrophenol
(leaving group) bond which make it very prone to accept a nucleophilic attack. Like
typical S substitution, the TS mainly involves attack of the nucleophile but not the
N2
-
departure of the leaving group. OH anion from buffer solution attacks the
phosphorus atom in a proposed mechanism, where only one TS is involved.
Noticeable point in the TS on the Zr(IV) nodes is the presence of a hydrogen bond
-
between the leaving group oxygen and an OH group on the node (calculated 2.01 Å
in ref 39) which effectively increases the leaving group ability. Finally, washing with
acidic solution dissociates nontoxic product from the cluster-phosphate complex
toward regenerated Zr-MOFs (Scheme 2).
Heterogeneity of the catalyst system was also studied using ICP analysis of the
filtrate reaction mixture after 90 minutes (in the case of Zr-bcu-tmuc with
conversion > 99%) in comparison with the digested Zr-MOFs (with same amount
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Scheme 2. Proposed catalytic hydrolysis mechanism of DMNP on the Zr
6
cluster based Zr-MOF
catalyst.
which used for hydrolysis reaction) in 5% HF solution. Our analyses reveal that less
than 0.005% of zirconium ion was detected in the cases of whole MOFs. In addition,
1
the HNMR spectrum of the same sample (conversion > 99%) exhibited just product
peaks (Figure S17), and there is no detectable organic linker in reaction solution
which confirms no leaching of the Zr(VI) ions or the linker during the catalytic
procedure. Furthermore, in a typical filtration test, the reaction was stopped after 8
minutes and the catalyst was filtered off and the conversion was determined by
31PNMR. Consequently, the reaction was allowed to continue for 24 hours. As
shown in figures 2b(green) and S18 no reaction was progressed after catalyst
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removal during the test reactions of both Zr-MOFs. The BET surface area of used
Zr-fcu-tmuc and Zr-bcu-tmuc decreased from 1480 and 758 to 989 and 425 m2g-1
after hydrolysis, respectively (Figure. S4). We can attribute this to clogging of the
pores or apertures of MOFs by the various reagents and products. The MOFs become
yellow after catalysis due to existence of various reagents and products (Figure S19).
According to the EDS and elemental map analysis data, it has been found that the
used catalysts can be recycle and reused without significant change in their catalytic
activity (Figure S8). This hypothesis is supported by PXRD analysis since no notable
losses in crystallinity of catalysts after third run reaction is observed (Figure 3).
Figure 3. PXRD data for Zr-fcu-tmuc (a) and Zr-bcu-tmuc as-made samples (b) compared to
simulated and after third hydrolysis run pattern. No significant change in peak position or intensity
is observed.
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Catalytic hydrolysis results encouraged us to study the fluorescence sensing ability
of these frameworks toward DMNP. We found enhanced hydrolysis rate for Zr-bcu-
tmuc (t =7.3 min) versus Zr-fcu-tmuc (t =25.4 min). Further, we postulated that
1
/2
1/2
Zr-bcu-tmuc would therefore show higher sensitivity toward DMNP compared to
Zr-fcu-tmuc given that Zr-bcu-tmuc shows superior hydrolysis activity to Zr-fcu-
tmuc. The fluorescence properties of Zr-fcu-tmuc and Zr-bcu-tmuc were
preliminary evaluated by recording the PL spectra on 0.5 mg activated MOFs
dispersed in methanol (due to complete separation between excitation and emission
bands with highest intensity, the scans were performed in methanol solvent). These
frameworks exhibit strong fluorescence emissions at 318 nm and 323 nm upon
excitation wavelength (λ ) of 251 and 263 nm for Zr-fcu-tmuc and Zr-bcu-tmuc,
ex
respectively (Figure S20). These observed PL activities may be arise from the charge
transfer transitions between the organic ligand and metal cluster, which is previously
described in the many case of MOFs[42]. Notably, these two MOFs exhibit rapid
and high fluorescence quenching upon incremental addition of a DMNP solution
(200 µM) with the quenching efficiency of 86% and 95% for Zr-fcu-tmuc and Zr-
bcu-tmuc, respectively (Figures S21 and S22). To investigate the fluorescence
quenching effect of DMNP in detail, the Stern-Volmer (SV) plot is created through
monitoring the PL intensity of both Zr-fcu-tmuc and Zr-bcu-tmuc in the presence of
different concentration (4-200 µM) of DMNP, (See Supplementary information,
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Figures 4a and 4c). The SV plots show a linear manner with an SV quenching
4
-1
4
-1
constant (K ) of 3.25×10 M and 2.29×10 M for Zr-fcu-tmuc and Zr-bcu-tmuc,
sv
respectively, which means higher sensitivity of Zr-bcu-tmuc (Figure 4b). The
Detection Limits (LOD) of Zr-fcu-tmuc and Zr-bcu-tmuc were calculated on the
basis of fluorescence titration (See Supplementary information, Figure 4d). LOD for
Zr-fcu-tmuc was obtained 1.09 µM which is signifyingly higher than Zr-bcu-tmuc
(0.557 µM). Results clearly indicate that the fluorescence response of Zr-bcu-tmuc
toward DMNP is more effective and faster than Zr-fcu-tmuc. Photo-induced electron
transfer (PET) mechanism have been widely used in the evolution of fluorescent
detection of analytes which is confirmed by observing the: i) linear SV behavior and
ii) No overlap between the UV-Vis absorption band of analyte and the PL emission
band of sensor [43, 44]. Since no significant overlap was observed between the UV-
Vis absorption region of DMNP and the emission bands of Zr-MOFs (Figure S23),
thus, in this case, probably PET is the main quenching mechanism. Several reports
were previously done by our group focusing on the florescence sensing ability of
MOFs with extended aromatic system toward nitro aromatics such as
nitrobenzene[45], trinitrophenol[46] and etc, based on PET mechanism. Similar to
reported results, here the electron withdrawing nitro phenol substituent of DMNP
molecule, made it very suitable fluorescence quenching agent due to electron
transfer from electron rich Zr-MOFs backbone to electron deficient DMNP.
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Figure 4. Comparison of fluorescence emission spectra of Zr-bcu-tmuc (a) and Zr-fcu-tmuc (c) in
the presence of DMNP with different concentrations. Comparison of SV plots in the presence of
0
.5 mg of Zr-bcu-tmuc (red) and Zr-fcu-tmuc (blue) in different DMNP concentrations (b).
Intensity versus concentration of DMNP of Zr-bcu-tmuc (red) and Zr-fcu-tmuc (blue) (d).
As described above, results indicated that Zr-bcu-tmuc can hydrolysis DMNP
around three times faster than fcu analogous which is related to more accessible
active Zr(VI) sites in 8 connected bcu topology. These observations also were
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confirmed by fluorescence sensing data showing that Zr-bcu-tmuc has more
sensitivity toward DMNP compared to Zr-fcu-tmuc. Considering both sensing and
hydrolysis results, we concluded that in Zr-bcu-tmuc with less connected Zr cluster,
6
the P=O binding to Zr cluster (first step of hydrolysis process, scheme 2) can occur
6
faster, while same binding step proceeded slower in Zr-fcu-tmuc through possible
defects on the nodes.
Conclusions
In conclusion, we highlighted the topology controlling of Zr-MOFs as a useful
strategy to improve the hydrolysis rate of nerve agent simulant, DMNP. A
comparative study was done on two topologically distinct Zr-MOFs, Zr-fcu-tmuc
and Zr-bcu-tmuc, on catalytic hydrolysis of DMNP. We demonstrated that
hydrolysis of DMNP to be faster with the Zr-bcu-tmuc (t =7.3 min) than Zr-fcu-
1
/2
tmuc (t =25.4 min). The interaction of P=O bond with Lewis-acidic Zr(IV) nods
1
/2
on Zr cluster cause the polarizing this bond and make it very prone to accept a
6
nucleophilic attack of hydroxide ions. The more accessible active Zr(IV) nods in 8-
connected Zr-bcu-tmuc in comparison with 12-connected Zr-fcu-tmuc leads to an
amplified response and fast hydrolysis. Further the fluorescence detection results
confirmed the hydrolysis results which showed that fluorescence response of Zr-
4
-1
bcu-tmuc toward DMNP (LOD: 0.557 µM, K : 3.25×10 M ) is significantly higher
sv
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Chemistry - A European Journal
10.1002/chem.202002412
4
-1
than Zr-fcu-tmuc (LOD: 1.09 µM, K : 2.29×10 M ). We believe that this work can
sv
open new horizons for scientists to further design promising MOFs for detoxification
of toxic chemicals through controlling the network topology.
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Chemistry - A European Journal
10.1002/chem.202002412
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was funded by Tarbiat Modares University and Catalan Institute of
Nanoscience and Nanotechnology (ICN2).
Supporting Information
1
The Supporting Information is including of synthesis methods, FT-IR, UV-Vis, PL, H-
3
1
NMR, PNMR, EDS, PXRD diagrams, elemental maps, FE-SEM images and N adsorption
2
Isotherms.
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