Fluorescent metal-organic framework for selective sensing of nitroaromatic explosives

Article abstract of DOI:10.1039/c1cc15594f

Highly luminescent micrometre-sized fine particles of a Zn(ii) metal-organic framework (MOF) of a new π-electron rich tricarboxylate dispersed in ethanol is demonstrated as a selective sensory material for the detection of nitroaromatic explosives via a f

Full text of DOI:10.1039/c1cc15594f

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Cite this: Chem. Commun., 2011, 47, 12137–12139
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COMMUNICATION
Fluorescent metal–organic framework for selective sensing of
nitroaromatic explosivesw
Bappaditya Gole, Arun Kumar Bar and Partha Sarathi Mukherjee*
Received 9th September 2011, Accepted 28th September 2011
DOI: 10.1039/c1cc15594f
Highly luminescent micrometre-sized fine particles of a Zn(II)
metal–organic framework (MOF) of a new p-electron rich
tricarboxylate dispersed in ethanol is demonstrated as a selective
sensory material for the detection of nitroaromatic explosives
via a fluorescence quenching mechanism.
result in an exciting new class of material known as nanoscale
coordination polymers (NCPs).8 The luminescent behaviour
of a MOF is highly dependent on the organic ligand and metal
ion. Hence, enhanced performance can be achieved by careful
selection of the organic building block and the nature of
the metal.
Here, we report the synthesis of a new p-conjugated ligand
5-(4-carboxyphenylethynyl)isophthalic acid (H3L) and its
Zn(II) 3D MOF (1). Micrometre-sized particles of this Zn(II)-MOF
are strongly fluorescent when dispersed in ethanol and
show selective sensing of nitroaromatics (e.g. nitrobenzene,
DNT, TNT) through a fluorescence quenching mechanism.
The ethynyl group was introduced to make H3L p-electron
rich. Zinc(II) ions were chosen as the metal connector because
of their high complexation affinity to carboxylates and non-
detrimental nature to fluorescence. In a typical synthesis, H3L
was dissolved in N,N-dimethylacetamide (DMA) to which
Zn(OAc)2ꢀ2H2O in ethanol was added dropwise and stirred
for 2 h to obtain a very fine powder.z
In the past two decades metal–organic frameworks (MOF)
have been studied widely as potential materials for gas storage,
separation, imaging, drug delivery and heterogeneous catalysis.1
The tunable size and porosity of these materials can make
them useful as chemical sensors. The rapid detection of
explosive constituents is important for security screening,
homeland security and environmental monitoring.2 Current
detection methods rely on the use of canines3 or sophisticated
instruments.4 The present instrumental techniques need very
expensive instruments and in most of the cases they are not
easily accessible. Fluorescence quenching based sensing is a
much simpler and very sensitive technique.5 Due to the
presence of electron withdrawing nitro groups, nitroaromatics
(TNT, DNT, picric acid etc.) are strong oxidants and used as
common chemical constituents of commercial explosives.
Electron donor conjugated polymers have proven to be
excellent candidates for the detection of nitroaromatic explosives.
The delocalized p electrons of such systems increase the
electrostatic interaction between the polymer and electron
deficient compounds.6 Moreover, the polymeric backbone
facilitates efficient exciton migration to enhance sensitivity.
Recently, a very few examples of fluorescent MOFs showing
explosive vapour sensing have been reported.7 MOFs are
expected to show potential because of their high surface area,
which may allow more analyte molecules to come into contact
with the MOF surface and may reach high sensitivity. More-
over, MOFs are easy to synthesize. The vapour phase sensing
of all nitroaromatics is not always efficient because of very low
vapour pressure of the nitroaromatics. However, scaling down
of MOFs to a few micrometres to nanoscale dimensions may
X-Ray quality single crystals of the MOF were obtained by
solvothermal reaction of Zn(OAc)2ꢀ2H2O and H3L in a DMA/
ethanol mixture at 90 1C for 48 h. A single crystal X-ray
diffraction study confirmed its composition as [Zn4O(L)2ꢀ
(H2O)3]ꢀ3DMAꢀ3EtOHꢀ6H2O.
The phase purity of the bulk material was confirmed by
powder X-ray diffraction (ESIw). The X-ray single crystal
structure of the MOF (1) exhibits a three-dimensional porous
framework structure with a Zn4 cluster as a building unit.
Each Zn4 cluster possesses one octahedral, two tetrahedral and
a square-pyramidal Zn(II) ion, which are connected to each
other through a central four coordinated oxo-bridge (Fig. 1).
One-dimensional pores of 14.2 ꢁ 13.5 A2 dimensions were
observed along the b axis. These pores are filled with coordinated
water and guest solvent molecules. The void space accounts for
approximately 53.3% of the whole crystal volume (3733.2 A3
out of the 7004.1 A3 per unit cell volume) as obtained by
PLATON analysis. The sharp diffraction peaks in the powder
XRD pattern of the MOF (1) (Fig. 2) indicate that these
particles are highly crystalline in nature. The SEM image of
the MOF (Fig. 2) is also indicative of a highly crystalline
nature.
Department of Inorganic and Physical Chemistry, Indian Institute of
Science, Bangalore, India. E-mail: psm@ipc.iisc.ernet.in;
Fax: (+91) 80-2360-1552; Tel: (+91) 80-2293-3352
w Electronic supplementary information (ESI) available: Detailed
experimental procedures, powder XRD, crystallographic data in
CIF format, gas adsorption and fluorescence studies. CCDC 835355.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c1cc15594f
The photoluminescence (PL) spectrum of the micrometre-sized
particles of MOF (1) dispersed in ethanol exhibits strong
fluorescence peaks at 470 nm upon excitation at 350 nm
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Chem. Commun., 2011, 47, 12137–12139 12137

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likely due to the strong electronic coupling between the
neighbouring ligands through Zn(II) ions.
Most interestingly, different electron deficient aromatics
have different effects on the fluorescence intensity of the
dispersed solution of MOF (1) in ethanol. The fluorescence
intensity of the dispersed MOF (1) in ethanol was almost
unchanged upon addition of analytes such as chlorobenzene
(CB), bromobenzene (BB), xylene, p-cresol, benzoic acid (BA),
4-methoxybenzoic acid (4-MeO-BA) and non-aromatic nitro
compounds. But significant quenching of fluorescence intensity
was observed upon addition of nitroaromatics such as nitro-
benzene, nitrotoluene, nitrophenol, DNT, TNT. Such observations
demonstrate the potential of MOF (1) for selective sensing of
nitroaromatics. Benzoquinone has considerable quenching
efficiency due to its high reduction potential. Fig. 4 shows
the reduction in fluorescence intensity of MOF (1) upon
addition of different volumes of a 1 mM solution of TNT.
The bright blue fluorescence of the dispersed solution of
micrometre-sized MOF particles under UV-light totally
disappeared upon addition of TNT solution (inset of Fig. 4).
The quenching efficiencies (%) of different electron deficient
aromatics are plotted in Fig. 5.
Fig. 1 (a) Structure of the MOF (1) showing the binding mode of L3ꢂ
and coordination environments. (b) Highlighted view of the building
Zn4 cluster where one ZnII is in an octahedral (green), two are in a
tetrahedral (purple) and the other one is in a square-pyramidal (violet)
coordination environment. Space-filling representations of the 1D
channel viewed along (c) the a axis, (d) the b axis and (e) the c axis
(colour codes: Zn cyan, O red, C purple).
Encapsulation of nitroaromatics into the pores of the MOF
is ruled out due to the lack of porosity within this MOF. Due
to the dispersible nature of the micrometre-sized fine particles
of MOF (1), close contact with the analyte molecules is
expected and the fluorescence quenching observed can be
attributed to the photoinduced electron transfer from the
excited MOF to the electron deficient analytes adsorbed on
the surface of the MOF particles. The reason for the considerably
high efficiency in fluorescence quenching of nitroaromatics
compared to other aromatics/nitroalkanes is presumably
due to their more electron deficient nature. The detectable
response of the MOF dispersed in ethanol towards the
TNT solution is found to be at parts per billion (ppb)
concentrations (ESIw) and such a level of sensitivity falls below
the permissible level of TNT in drinking water established by
the US EPA.9 The micrometre-sized MOF could be regenerated
and reused for a significant number of cycles by centrifuging
Fig. 2 Powder XRD spectrum of MOF (1) as obtained after stirring
Zn(OAc)2ꢀ2H2O and H3L in DMA/ethanol mixture for
2 h.
Inset: SEM image.
(Fig. 3). The fluorescence of the MOF is completely different
from that of the ligand, which gives a strong fluorescence peak
at 420 nm upon excitation at 305 nm. This observed red-shift is
Fig. 4 Fluorescence titration of the dispersed MOF (1) in ethanol by
gradual addition of 1 mM solution of TNT in ethanol. (Inset: visual
colour change of 1 before and after titrating with TNT under
UV-light).
Fig. 3 Emission spectra of H3L (pink line) dissolved in ethanol
(excited at 305 nm) and of micrometre-sized particles of MOF (1)
(blue line) dispersed in ethanol (excited at 350 nm).
c
12138 Chem. Commun., 2011, 47, 12137–12139
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nitroaromatics such as DNT and TNT over other electron
deficient aromatics. Excited state electron transfer from the
electron rich fluorophore to electron deficient nitroaromatics is
a probable mechanism of such quenching. Bulky macroscopic
MOFs need to be processed in several steps like activation and
incorporation of the sensed species before realizing as a sensor
material. The dispersible nature of the MOF (1) particles of a
few micrometre dimension makes this material an attractive
candidate for straightforward use. This methodology of
designing p-electron rich fluorescent MOFs for selective and
efficient sensing of electron deficient oxidising explosives may
enable the future development of much improved sensors for
infield explosives sensing.
The authors are grateful to the Department of Science and
Technology (DST), India for financial support and P.S.M.
thanks the Johnson Matthey Pvt. Ltd. U.K. for providing
Pd(II) salt on loan.
Fig. 5 Reduction in fluorescence intensity (plotted as quenching
efficiency) observed upon the addition of several quenchers.
CB = chlorobenzene, BB = bromobenzene, BA = benzoic acid,
4-MeO-BA = 4-methoxybenzoic acid, BQ = benzoquinone, NT =
nitrotoluene, NB = nitrobenzene, DNT = dinitrotoluene, NP =
nitrophenol and TNT = 2,4,6-trinitrotoluene.
Notes and references
z In a typical synthesis procedure H3L (15 mg) was dissolved in DMA
(2 mL) to which 1.5 equivalents of Zn(OAc)2ꢀ2H2O dissolved in
ethanol (1 mL) was added dropwise. Upon addition of Zn(OAc)2ꢀ
2H2O the solution turned turbid, which indicated the formation of
fine particles of MOF (1). The mixture was kept under gentle stirring
for a further 2 h at room temperature. The mixture was centrifuged to
isolate the micrometre-sized fine powder of MOF (1). Single crystals of
1 were grown upon solvothermal treatment at 90 1C followed by slow
cooling.
the dispersed solution after use and washing several times with
ethanol. It is noteworthy that almost regaining the initial
fluorescence intensity over repeated cycles implies a high
photostability of the material for its long time infield explosive
detection application (Fig. 6).
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In conclusion, a luminescent Zn(II) MOF (1) has been
synthesized using a new p-electron rich tricarboxylic acid
incorporating ethynyl functionality. The dispersed solution
of the micrometre-sized particles of 1 in ethanol exhibits strong
fluorescence emission and its initial fluorescence intensity
was quenched efficiently upon addition of small amounts of
nitroaromatic explosives. The choice of the highly conjugated
p-electron rich organic ligand makes this material fluorescent
and an efficient sensory material for explosive sensing. Our
study revealed that 1 is quite selective towards explosive
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Fig. 6 Reproducibility of the quenching ability of 1 dispersed in
ethanol to TNT solution. The material was recovered by centrifuging
after each experiment and washed several times with ethanol. The blue
bars represent the initial fluorescence intensity and the purple bars
represent the intensity upon addition of 200 mL (1 mM) of a solution
of TNT.
c
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Chem. Commun., 2011, 47, 12137–12139 12139