Novel metal-organic framework with tunable fluorescence property: Supramolecular signaling platform for polynitrophenolics

Article abstract of DOI:10.1039/c5dt00489f

With the aid of a rotational C₃-symmetric tricarboxytriphenylamine based ligand, a new Cd-MOF was synthesized and characterized by various spectroscopic techniques as well as by single-crystal X-ray diffraction analysis. The structural investig

Full text of DOI:10.1039/c5dt00489f

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Novel metal–organic framework with tunable
fluorescence property: supramolecular signaling
platform for polynitrophenolics†
Cite this: DOI: 10.1039/c5dt00489f
Received 3rd February 2015,
Accepted 20th February 2015
M. Venkateswarulu,a Avijit Pramanikb and Rik Rani Koner*a
DOI: 10.1039/c5dt00489f
www.rsc.org/dalton
With the aid of a rotational C3-symmetric tricarboxytriphenylamine forensic investigation, mine-field and humanitarian impli-
based ligand, a new Cd-MOF was synthesized and characterized by cations.3 The nitroaromatic compounds such as 2,4,6-trinitro-
various spectroscopic techniques as well as by single-crystal X-ray toluene (TNT) and 2,4,6-trinitrophenol (TNP) are used as
diffraction analysis. The structural investigation of the crystalline principle ingredients in the explosives industry and are also
Cd-MOF complex revealed the existence of unique three symmetry found in unexploded landmines worldwide.3b,4 Thus, a selec-
independent coordination environments of Cd(II) ions with tive and sensitive sensor for nitro explosives is very much
common octahedral and pentagonal bipyramidal geometries. desirable. Various methods such as high-pressure liquid
Small cavities with dimensions 6.40 Å × 6.41 Å were present in the chromatography, electrochemical methods and surfaced-
crystal system. The tunable fluorescence property of the complex enhanced Raman spectrometry have been developed over the
was explored to detect selectively polynitrophenol based explosive years for their detection and quantification at trace levels.5
materials in the presence of other nitro explosives such as RDX, However, these techniques suffer from several drawbacks
HMX, TNT and so on. Thermogravimetric analysis and powder including their expense and poor portability; they are also
X-ray diffraction data support the high thermal stability and crystalli- highly time consuming. In order to overcome these difficulties,
nity of the complex.
recently fluorescent materials which render different optical
signals while interacting with explosive materials have being
widely used as easy, sensitive and inexpensive optical sensing
platforms. On the other hand, MOF materials have proved to
be excellent candidates for the efficient detection of explo-
sives.6 The key is the large surface area of MOFs which makes
them novel sensing platforms as they can accommodate a
large number of analytes onto their surfaces. Though literature
reports witness the existence of various MOFs for the detection
of different explosive materials, MOFs for selective detection of
aromatic polynitrophenol based explosives have rarely been
reported.6a,b,o,p,q,r Moreover, and to the best of our knowledge,
no MOF has been reported to date which is capable of dis-
tinguishing polynitrophenols and polynitroalcohols. Because
of their high negative oxygen balance as compared to poly-
nitrophenols, polynitroalcohols are inferior as explosives com-
pared with polynitrophenols.
Among the various types of solid crystalline materials being
developed for practical applications, the self-assembly of
metal–organic frameworks (MOF) with emerging engineered
architectures, has attracted considerable interest in various
scientific arenas that include gas storage, separation, sensing,
drug delivery, optical properties, catalysis, molecular magnet-
ism and too many more to mention.1 Introducing further the
luminescent properties with the help of structurally engi-
neered ligands and accessible porosity with high surface area
within MOF materials have demonstrated them to be promis-
ing candidates for chemical sensing.2 While developing
sensing materials is one of the fastest growing research fields
as these materials ensure the requirements of many areas
starting from quality control to environmental concerns,
chemical sensors for rapid and selective recognition of explo-
sive materials have attracted increasing attention owing to
their versatile application in homeland security, anti-terrorism,
The present study describes the development of
a
novel 3D luminescent supramolecular framework Cd-MOF,
[Cd5(TCA)4(H2O)2], and its application in selective and sensi-
tive detection of polynitrophenolic explosives over other
nitroaromatics.
The present material has the potential to distinguish nitro-
phenol from nitroalcohols.
aSchool of Basic Sciences, Indian Institute of Technology Mandi, Mandi-175001, H.P.,
India. E-mail: rik@iitmandi.ac.in; Fax: +91-1905-300027; Tel: +91-1905-237994
bDepartment of Chemistry and Biochemistry, Jackson State University, Jackson,
Mississippi 39217, USA
†Electronic supplementary information (ESI) available. CCDC 1043090. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5dt00489f
The present luminescent metal–organic framework of
pentanuclear cadmium complex, [Cd5(TCA)4(H2O)2], was
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Fig. 2 (a) C–H⋯π non-covalent interactions of aromatic centroids with
C–H moieties in Cd5 complex and other atoms are omitted for clarity.
(b) Three dimensional packing view of the Cd–TCA3− complex contains
a small porous cavity with dimensions, 6.40 Å × 6.41 Å.
Scheme 1 Synthetic methodology for TCA ligand and Cd-complex.
crystal system was removed using the SQUEEZE function of the
PLATON analysis and viewing software package. The three
dimensional space-filling view of the metal–organic framework
(Fig. 2b) illustrates a porous 1D channel with a cavity dimen-
sion, 6.405 Å × 6.405 Å along the c axis.
The photophysical properties of Cd-MOF were measured in
the solution state. The Cd-MOF exhibited strong fluorescence
at 417 nm (λem) upon excitation at 352 nm while dispersed in
DMF. The addition of TNP (600 µL) to MOF solution (1.25 mg/
3 mL, 1.98 × 10−4 M) resulted in quenching of the fluorescence
emission (Fig. 3). Similarly, fluorescence quenching was
observed when a solution of 2,4-DNP was added to the MOF
solution (Fig. S2†). But, the quenching was not found signifi-
cant while mono nitrophenols were added to MOF solution
(Fig. S3 and S4†).
Interestingly, the addition of other nitroaromatics and
nitro-explosives [2-nitrophenol (2-NP), 4-nitrophenol (4-NP),
4-nitrotoluene (4-NT), 2,4,6-trinitrotoluene (TNT), nitrobenzene
(NB), 1,4-dinitrobenzene (1,4-DNB), RDX, HMX, nitromethane
(NM) and 2,4-dinitrotoluene (DNT)] in DMF solution of Cd-
MOF caused no significant changes in fluorescence signal
which indicates the specificity of the present MOF towards
polynitrophenolic compounds (Fig. S5–S8†). While investi-
gating the fluorescence emission of the MOF in different
common organic solvents including acetonitrile, pyridine, pro-
pylamine and chloroform, we observed the highest fluo-
rescence intensity in DMF (Fig. S9†). Most importantly, this
Fig. 1 ORTEP (Oak Ridge Thermal Ellipsoid) plot of Cd–TCA complex.
Thermal ellipsoids are set at the 50% probability level; [Symmetry code:
x, y, z, −x, y + 1/2, −z + 1/2, −x, −y, −z, x, −y − 1/2, z − 1/2].
synthesized at high temperature (150 °C) upon addition of tri-
carboxytriphenylamine (TCA) into CdCl2 in DMF–ethanol
solvent mixture (Scheme 1 and see the ESI†). Structural analy-
sis revealed that the TCA3−–Cd complex crystallized in mono-
clinic P21/c space group (Fig. 1) where three symmetry
independent coordination environments of Cd(II) were present
in the crystal system.7
Two types of Cd(II) (Cd1 and Cd2) are coordinated distorted
octahedrally with four pairs of carboxylate groups of TCA3−
ligand in syn
syn, and syn
anti (O,O′) bridging modes
(Fig. 1 and Fig. S1†),8 whereas Cd3(II) is tightly held with one
water molecule and three pairs of TCA ions to show a hepta-co-
ordinated pentagonal bipyramidal geometry. The intrametallic
distances between adjacent Cd1(II)⋯Cd2(II) and Cd2(II)⋯Cd3(II)
are 3.57 Å and 3.61 Å, respectively. Within a complex, the
phenyl rings of TCA3− ligand are almost perpendicular to each
other and are stacked with other phenyl groups via C–H⋯π
interactions (C–H⋯π 3.84 Å, C–H⋯π 3.64 Å) (Fig. 2a).9 One dis-
ordered ethanol solvent molecule which was present outside of
Fig. 3 Change in fluorescence intensity of (a) Cd-MOF in the presence
of TNP and (b) upon incremental addition of TNP solution (0 to 600 µL)
the metal coordination environment but in the unit cell of the in DMF. (λex = 352,Cd-MOF = 1.25 mg/3 ml, 1.98 × 10−4 M).
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Cd-MOF were available for these nitroaromatics, whereas the
addition of TNP to the same solution resulted in a drastic
decrease in the fluorescence intensity (Fig. S12†). These results
suggested that the Cd-MOF could be used as efficient sensing
platforms for polynitrophenols even in complicated systems.
To investigate its practical applicability in more benign sol-
vents such as water, fluorescence titrations were performed in
DMF–H2O (80 : 20) medium. Interestingly, a much higher and
faster quenching efficiency of TNP was observed in the pres-
ence of mixed aqueous media (Fig. S13 and S14†). Further,
competitive experiments were also performed to examine the
selectivity of Cd-MOF for TNP in the presence of other
nitroaromatics in mixed aqueous media. In these experiments
an excess amount of nitroaromatics were added to the
aqueous medium of Cd-MOF followed by addition of an
aqueous solution of TNP to the same solution. Surprisingly,
Fig. 4 The change in fluorescence intensity of Cd-MOF in DMF upon
addition of 2,4-DNP and 2,4-DNBA solutions (λex = 352 nm, λem
=
415 nm).
Cd-MOF showed
a higher sensitivity in mixed aqueous
medium. Thus, Cd-MOF showed unprecedented selectivity and
sensitivity for TNP in both DMF and in 20% aqueous medium.
The reusability of the Cd-MOF has been checked and it has
been observed that the Cd-MOF can be reused many times as
∼89% quenching efficiency was observed even after five cycles
(Fig. S15†). The limit of detection was calculated using (3σ/
slope) method and found to be in the nanomolar range.10 The
MOF could detect as low as 2.3 and 1.7 nM of 2,4 DNP and
TNP, respectively (Fig. S16 and S17†).
supramolecular platform can efficiently differentiate polynitro-
phenols from polynitroalcohols through their optical signal.
For example, the addition of 2,4-DNP resulted in a drastic
quenching of the fluorescence signal of the MOF while the
addition of 2,4-dinitrobenzyl alcohol (2,4-DNBA) did not cause
any effect on the fluorescence property of the MOF (Fig. 4). To
check the sensitivity of the MOF towards TNP, fluorescence
titrations were performed. The gradual decrease in fluo-
rescence intensity of the MOF was observed upon incremental
addition of TNP (600 µl) to MOF solution (Fig. 3b).
The quenching efficiency of all the explosive materials was
calculated and plotted (Fig. 5). It was observed that the pres-
ence of TNP and 2,4-DNP caused maximum quenching
(91.78% and 82%) within 1 minute of addition (Fig. S10 and
11†). On the other hand, only 1–10% quenching was observed
upon addition of much higher amounts (1200 µl) of other
nitroaromatic compounds (Fig. S12†).
The reason for the maximum quenching in fluorescence
emission of Cd-MOF solution in the presence of polynitrophe-
nols may be ascribed to the energy transfer from fluorophore
to the analyte.6a,f The energy transfer depends on the extent of
overlap between the absorption band of the analyte and the
emission band of the fluorophore. It has been observed that
the absorption spectrum of polynitrophenol shows 41.2%
overlap with the emission spectrum of Cd-MOF, while no
overlap was observed with other explosive materials
(Fig. S18†). The fluorescence quenching efficiency was also
investigated by the Stern–Volmer equation: (I0/I = Ksv[A] + 1,
where I0 and I are the fluorescence intensities before and after
addition of the analyte respectively, Ksv is the quenching con-
stant (M−1), [A] is the molar concentration of analyte. As
shown in (Fig. S19–S21†), the SV plot for MOF with TNP
appeared linear at low concentration and lost its linearity at
higher concentration. This non-linear nature of the plot may
be attributed to self-absorption or energy transfer.11 The
quenching constants of all explosive materials were calculated
and the quenching constant of TNP was found to be 5.9 × 104
which is ca. 11 times greater than for RDX, NB and TNT. It was
also observed that the quenching constant value for TNP is
comparable with the reported MOF and common organic
polymers.12
Next we investigated the potential of Cd-MOF as a selective
sensor for polynitrophenolics, particularly TNP. The addition
of an excess amount of a large number of various nitro-
aromatics to Cd-MOF dispersed in DMF did not show any fluo-
rescence quenching although high affinity binding sites of
In general, nitroaromatic based explosive detection by
MOFs largely depends on the electronic structure of the ligand
and the electron-donor/electron-acceptor orbital overlap where
the MOF acts as an electron donor and the nitroaromatic acts
as an electron acceptor. In the case of the present MOF, the
ligand contains basic nitrogen which possibly induces strong
Fig. 5 The quenching percentage of Cd-MOF in DMF upon incremental
addition of TNP, 2,4-DNP,4-NP, 2,4-NP, 1,4-DNB, 2,4-DNT, 4-NT, NB,
RDX, HMX, NM and 2,4-DNBA solutions. (λex = 352 nm, λem = 415 nm).
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electrostatic interaction with the highly acidic OH functionality
of TNP. This effect helps to bring TNP closer to the MOF
resulting in a better confinement of TNP into the MOF which
in turn helps in effective energy transfer from MOF to TNP
(Förster type strut-to-strut energy transfer).1k,6a
(c) J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang and C. Y. Su,
Chem. Soc. Rev., 2014, 43, 6011–6061; (d) G. Ferey and
C. Serre, Chem. Soc. Rev., 2009, 38, 1380–1399; (e) Q.-L. Zhu
and Q. Xu, Chem. Soc. Rev., 2014, 43, 5468–5512;
(f) K. M. Choi, H. M. Jeong, J. H. Park, U. B. Zhang,
J. K. Kang and O. M. Yaghi, ACS Nano, 2014, 8, 7451–7457;
(g) R. Custelcean and B. A. Moyer, Eur. J. Inorg. Chem.,
2007, 1321–1340; (h) R. Custelcean, V. Sellin and
B. A. Moyer, Chem. Commun., 2007, 1541–1543;
(i) S. C. Sahoo, T. Kundu and R. Banerjee, J. Am. Chem.
Soc., 2011, 133, 17950–17958; ( j) M. Zhao, S. Ou and
C. D. Wu, Acc. Chem. Res., 2014, 47, 1199–1207;
(k) L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf,
R. P. V. Duyne and J. T. Hupp, Chem. Rev., 2012, 112, 1105–
1125.
2 (a) M. Zhang, G. Feng, Z. Song, Y. P. Zhou, H. Y. Chao,
D. Yuan, T. T. Y. Tan, Z. Guo, Z. Hu, B. Z. Tang, B. Liu and
D. Zhao, J. Am. Chem. Soc., 2014, 136, 7241–7244;
(b) C. Wang and W. Lin, J. Am. Chem. Soc., 2011, 133, 4232–
4235; (c) K. C. Stylianou, R. Heck, S. Y. Chong, J. Bacsa,
J. T. A. Jones, Y. Z. Khimyak, D. Bradshaw and
M. J. Rosseinsky, J. Am. Chem. Soc., 2010, 132, 4119–
4130.
In order to characterize the thermal stability of the
complex, thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) were performed. As shown in
Fig. S22,† the Cd-MOF exhibited weight loss at the temperature
∼180 °C and ∼300 °C. TGA and DSC analyses also show that
the major weight loss occurred at ∼380 °C and ∼475 °C, which
can be attributed to the decomposition and combustion of the
complex. The formation of Cd-MOF complex as well as its
stability during explosive sensing was investigated with the
help of powder X-ray diffraction (PXRD) studies. A comparative
study on PXRD patterns of the Cd-MOF powder, reusable Cd-
MOF powder (obtained after one round of the sensing experi-
ment) and simulated Cd-MOF complex has been shown in
Fig. S23.† The PXRD pattern of Cd-MOF powder was found to
be similar to that of the simulated pattern. These results
support our conclusion that the present MOF maintains its
crystalline property even in the bulk phase. Moreover, the simi-
larity in PXRD patterns between unused MOF and reused MOF
powder indicated that the MOF is stable in the dispersion.
These results were further supported by FT-IR studies (Fig. S24†).
In conclusion, the development and importance of a new
Cd-MOF have been demonstrated. The results suggest that the
present MOF can act as a novel sensing platform for the highly
sensitive and selective detection of polynitrophenol-based
explosive materials. Besides, the Cd-MOF was found to be an
interesting candidate for selective detection of TNP in the pres-
ence of other nitroaromatic materials in environmental con-
ditions which may offer a broad range of applications in the
field of detection of explosive materials. The selective detec-
tion of polynitrophenols over polynitroalcohols is quite inter-
esting and a useful application of this newly developed MOF.
3 (a) Y. Salinas, R. M. Manez, M. D. Marcos, F. Sancenon,
A. M. Costero, M. Parra and S. Gil, Chem. Soc. Rev., 2012,
41, 1261–1296; (b) J. I. Steinfeld and J. Wormhoudt, Annu.
Rev. Phys. Chem., 1998, 49, 203–232.
4 H. Sohn, M. J. Sailor, D. Magde and W. C. Trogler, J. Am.
Chem. Soc., 2003, 125, 3821–3830.
5 (a) A. W. Czarnik, Nature, 1998, 394, 417–418;
(b) D. S. Moore, Rev. Sci. Instrum., 2004, 75, 2499–
2512.
6 (a) S. S. Nagarkar, B. Joarder, A. K. Chaudhari,
S. Mukherjee and S. K. Ghosh, Angew. Chem., Int. Ed., 2013,
52, 2881–2885; (b) S. S. Nagarkar, A. V. Desai and
S. K. Ghosh, Chem. Commun., 2014, 50, 8915–8918;
(c) A.-J. Lan, K.-H. Li, H.-h. Wu, D. H. Olson, T. J. Emge,
W. Ki, M.-C. Hong and J. Li, Angew. Chem., Int. Ed., 2009,
48, 2334–2338; (d) D.-X. Ma, B.-Y. Li, X.-j. Zhu, Q. Zhou,
K. Liu, G. Zeng, G.-H. Li, Z. Shi and S.-H. Feng, Chem.
Commun., 2013, 49, 8964–8966; (e) S. R. Zhang, D.-Y. Du,
J.-S. Qin, S.-J. Bao, S.-L. Li, W.-W. He, Y.-Q. Lan, P. Shen
and Z.-M. Su, Chem. – Eur. J., 2014, 20, 3589–3594;
(f) J. H. Lee, S. Kang, J. Y. Lee, J. Jaworski and J. H. Jung,
Chem. – Eur. J., 2013, 19, 16665–16671; (g) H. Xu, F. Liu,
Y. Cui, B. Chen and G. Qian, Chem. Commun., 2011, 47,
3153–3155; (h) B. Gole, A. K. Bar and P. S. Mukherjee,
Chem. Commun., 2011, 47, 12137–12139; (i) Y.-N. Gong,
L. Jiang and T.-B. Lu, Chem. Commun., 2013, 49, 11113–
11115; ( j) D. Banerjee, Z. Hu, S. Pramanik, X. Zhang,
H. Wang and J. Li, CrystEngComm, 2013, 15, 9745–9750;
(k) Y.-S. Xue, Y. He, L. Zhou, F.-J. Chen, Y. Xu, H.-B. Du,
X.-Z. You and B. Chen, J. Mater. Chem. A, 2013, 1, 4525–
4530; (l) D. Tian, Y. Li, R.-Y. Chen, Z. Chang, G.-Y. Wang
and X.-H. Bu, J. Mater. Chem. A, 2014, 2, 1465–1470;
(m) T. K. Kim, J. H. Lee, D. Moon and H. R. Moon, Inorg.
Acknowledgements
Financial support from the Department of Science and Tech-
nology (DST), India [grant no. SR/FT/CS 57/2010(G)] is thank-
fully acknowledged. R. R. K. is thankful to IIT-Mandi for her
fellowship. M. V. thanks CSIR, India for research fellowship.
We thankfully acknowledge the Director, IIT-Mandi, for
research facilities. We are grateful to the Advance Materials
Research Centre, IIT-Mandi, for sophisticated instrument
facilities. We are thankful to Dr Anirban Karmakar for his sug-
gestions during manuscript preparation. We thankfully
acknowledge the reviewers for their valuable comments.
Notes and references
1 (a) O. M. Yaghi, H. Li, C. Davis, D. Richardson and
T. L. Groy, Acc. Chem. Res., 1998, 31, 474–484; (b) Z. Hu,
B. J. Deibert and J. Li, Chem. Soc. Rev., 2014, 43, 5815–5840;
Dalton Trans.
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Communication
Chem., 2013, 52, 589–595; (n) G.-Y. Wang, C. Song,
D.-M. Kong, W.-J. Ruan, Z. Chang and Y. Li, J. Mater. Chem.
A, 2014, 2, 2213–2220; (o) X.-H. Zhou, L. Li, H.-H. Li, A. Li,
T. Yanga and W. Huang, Dalton Trans., 2013, 42, 12403–
8 W. C. Voegtli, N. Khidekel, J. Baldwin, B. A. Ley,
J. M. Bollinger and A. C. Rosenzweig, J. Am. Chem. Soc.,
2000, 122, 3255–3261.
9 M. Nishio, CrystEngComm, 2004, 6, 130–158.
12409; (p) B. Joarder, A. V. Desai, P. Samanta, S. Mukherjee 10 (a) G. L. Long and J. D. Winefordner, Anal. Chem., 1983, 55,
and S. K. Ghosh, Chem. – Eur. J., 2015, 21, 965–969;
(q) C. Zhang, L. Sun, Y. Yan, J. Li, X. Song, Y. Liu and
Z. Liang, Dalton Trans., 2015, 44, 230–236; (r) J.-D. Xiao,
712A–724A; (b) G. L. Long, E. G. Voigtman, M. A. Kosinski
and J. D. Winefordner, Anal. Chem., 1983, 55(8), 1432–
1434.
L.-G. Qiu, F. Ke, Y.-P. Yuan, G.-S. Xu, Y.-M. Wang and 11 (a) H. Sohn, M. J. Sailor, D. Magde and W. C. Trogler, J. Am.
X. Jiang, J. Mater. Chem. A, 2013, 1, 8745–8752.
Chem. Soc., 2003, 125, 3821–3830; (b) W. Wu, S. Ye, G. Yu,
Y. Liu, J. Qin and Z. Li, Macromol. Rapid Commun., 2012,
33, 164–171; (c) D. Zhao and T. M. Swager, Macromolecules,
2005, 38, 9377–9384.
7 Crystal data for C84H52Cd5N4O26, M = 2095.35, CCD =
736670, T = 298(2) K, monoclinic, space group P2(1)/c,
a = 14.6163(3) Å, b = 23.1639(4) Å, c = 15.7113(4) Å, β =
108.061(3)°, V = 5057.28 Å3, Z = 2, μ = 1.100 mm−1, 10 323 12 Y. Salinas, R. Martinez-Manez, M. D. Marcos, F. Sancenon,
reflections, 8303 unique, (Rint = 0.0447), R(F) = 0.0574
(I > 2σ(I), wR(F2) = 0.1358 (all data).
A. M. Castero, M. Parra and S. Gil, Chem. Soc. Rev., 2012,
41, 1261–1296.
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