Fluorescent detection of RDX within DHPA-containing metal-organic polyhedra

Article abstract of DOI:10.1039/c3cc49355e

By incorporating the dihydropyridine amido (DHPA) group into the rationally designed ligand systems, metal-organic molecular polyhedra were obtained via self-assembly for the luminescent sensing of highly explosive RDX with a limit of detection lower than 1 ppb in solution.

Full text of DOI:10.1039/c3cc49355e

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Fluorescent detection of RDX within
DHPA-containing metal–organic polyhedra†
Cite this: Chem. Commun., 2014,
50, 3467
Liang Zhao, Yulu Chu, Cheng He and Chunying Duan*
Received 10th December 2013,
Accepted 21st January 2014
DOI: 10.1039/c3cc49355e
www.rsc.org/chemcomm
By incorporating the dihydropyridine amido (DHPA) group into the N2O chelators within the ligand backbone, a series of metal–organic
rationally designed ligand systems, metal–organic molecular polyhedra polyhedra and cyclohelicates have been constructed and used as
were obtained via self-assembly for the luminescent sensing of highly efficient chemosensors for the selective and sensitive detection of
explosive RDX with a limit of detection lower than 1 ppb in solution.
guest molecules with different sizes, shapes and interaction handles.
As the dihydropyridine amido (DHPA) group is described as the key
Detection of hidden explosive devices in minefield remediation,1 structure in NADH models16 and plays an important part in the
crime scene investigations2 and counter-terrorism applications such electron transfer process, herein, we report the assembly of CeÀZPS
as personnel or baggage screening, facility protection, and cargo and ZnÀZPT (see Scheme S1 in the ESI†) metal–organic polyhedra
screening3 is a pressing concern. The need for ultratrace detection of through combination of the DHPA group with the rationally designed
the commonly used powerful explosives having low-volatility has ligand systems. We envisioned that these capsules would be promis-
resulted in an intense interest in fluorescence methods. And ing chemosensors for RDX. The well-positioned DHPA would provide
numerous optical4,5 and electrochemical sensors6,7 for nitroaromatic appropriate matches for polar groups to address the structural
compounds were developed recently. While cyclotrimethylene diversity of RDX and that the difficulty of selectively recognizing
trinitramine (RDX) is more powerful than TNT, few high flexible conformations of RDX could be overcome. And these oxidized
sensitivity RDX probes that exhibit satisfactory properties have DHPA fragments possibly exhibited obvious emissions to transform
been reported, due to the fact that RDX is nonaromatic, it exhibits a this selective recognition information into measurable luminescent
three-dimensional flexible structure and just contains one kind of signals. Experimental results suggested that the metal–organic octa-
recognition unit (nitramine). By modifying the reduced nicotin- hedron ZnÀZPT can function as an enzyme-like pocket to encapsulate
amide adenine dinucleotide (NADH) analogue as a reducing agent, the explosive RDX molecule and prompt the photoreaction with the
metal–organic complexes8 and CdSe–ZnS QDs9 have been created DHPA group with a ca. 27 fold luminescent enhancement and the
for the fluorescent detection of RDX. It is thus hypothesized that the limit of detection was improved to 1 ppb.
development of three dimensional capsules with appropriate sizes
and NADH analogues as suitable triggers represents a powerful 1-(4-(hydrazinecarbonyl)phenyl)-4-phenyl-1,4-dihydropyridine-5-
strategy to create RDX sensors with high selectivity and sensitivity. dicarbohydrazide in an ethanol solution and characterized by
Ligand H6ZPS was synthesized from salicylaldehyde with
Metal–organic polyhedra, discrete molecular architectures that elemental analysis and spectroscopic methods. Evaporating a DMF
are constructed through the coordination of metal ions and organic solution of H6ZPS containing Ce(NO3)3Á6H2O generated CeÀZPS in a
linkers, have attracted considerable attention due to their potential high yield (57%). Single-crystal X-ray structural analysis confirmed the
for a variety of applications due to their high symmetry, stability formation of a Ce4(H2ZPS)4 tetrahedron with a crystallographic C3
and rich chemical/physical properties.10–14 The architectures that symmetry in the solid state (Fig. 1).17 The tetrahedron comprised four
generate well-defined cavities with gated pores provide specific vertical metal centers and four deprotonated H2ZPS ligands. Each
inner environments for the selective uptake and bonding of guest cerium ion was chelated by three tridentate chelating groups from
molecules and the catalysis of their reactions.15 By incorporating three different ligands to form a ternate coronary trigonal prism
coordination geometry. The four pseudo-C3 symmetric ligands posi-
tioned individually on the four triangle faces of the tetrahedron were
State Key Laboratory of Fine Chemicals, Dalian University of Technology Dalian,
defined by four metal ions. The CeÁÁÁCe separation was B13.57 Å, the
116012, China. E-mail: cyduan@dlut.edu.cn
inner volume of the tetrahedron was about 350 Å3. The rhombic
† Electronic supplementary information (ESI) available: Experimental details and
window of the tetrahedron had a size of 6.8 Â 6.5 Å2, potentially
additional spectroscopic data. CCDC 972499. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c3cc49355e
allowing the ingress and egress of small molecules i.e. RDX.
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Fig. 2 The time-dependent fluorescence growth of CeÀZPS (10 mM) in a DMF
solution upon addition of RDX (1 mM) after irradiation with 310 nm light. Spectra
were recorded at time intervals of 1 min. The inset shows the fluorescence
responses of CeÀZPS towards RDX over various explosives with the emission
intensity at 465 nm after a 5 min irradiation (excitation at 350 nm).
Fig. 1 The constitute/constructional fragments of CeÀZPS. H atoms and
solvent molecules are omitted for clarity. The metal, oxygen and nitrogen as pentaerythritol tetranitrate (PETN), 2,4,6-trinitrotoluene (TNT),
atoms are drawn in green, red and blue respectively. The carbon atoms
2,6-dinitrotoluene (DNT), 2-nitro-toluene (NT), 1,4-dinitrobenzene
(DNB), nitrobenzene (NB), 4-nitrophenol (NP) and picric acid (PA)
did not cause any significant luminescence variation. The competi-
are drawn in other colours. Selected bond distances (Å): Ce–Ophenol 2.20,
Ce–Ocarbonyl 2.44, Ce–N 2.65.
tion experiments of CeÀZPS (10 mM) in the same solution revealed
The benzene ring of DHPA was positioned outside of the tetra- that the fluorescence responses of RDX (1 mM) were unaffected in
hedron and its active site positioned in the interior of the cavity. the presence of other explosives (up to 10 mM), demonstrating the
These DHPA moieties thus provided geometric, coordinative and high selectivity of the CeÀZPS toward RDX over other nitramine
functional properties to the cage-like capsule, ensuring the size and explosives. Under optimized conditions, with the concentration of
shape selectivity for this host–guest complexation behaviour. The CeÀZPS fixed at 5 mM, the fluorescence intensity of the CeÀZPS
C–N (DHPA) distance of 1.39 Å is a formal single bond, indicating solution is nearly proportional to the RDX concentration. And the
the reduction state of the ligand. The C–O and C–N distances of addition of 10 ppb caused about 50% luminescent enhancement of the
1.26 Å and 1.32 Å, respectively, within the ligands were intermediate solution. It is postulated that the intensity enhancement contributed to
between formal single and double bonds, indicating the extensive the formation of the NAD+ analogue, whereas the selectivity and the
delocalization over the entire molecular skeleton.18 Such a conju- sensitivity were reasonably attributed to the encapsulation of the RDX
gated system is beneficial to the signal transduction within the within the cavity of the tetrahedron to enforce the proximity between
ligand backbone and it potentially enhances the fluorescence RDX and the reducing site of DHPA, giving a more quick and sensitive
response upon interaction with guests.
response towards RDX. To the best of our knowledge, CeÀZPS
ESI-MS spectra of CeÀZPS in a DMF–CH3CN solution exhibited represents the first example of the metal–organic polyhedral chemo-
intense peaks at m/z = 1710.24, 1741.73 and 1773.23 with the isotopic sensors for explosive detection in solution.
distribution patterns separated by 0.50 Da, assignable to [Ce4(H2ZPS)2-
As the N2O chelator and the DHPA reductive triggers were
(HZPS)2]2À, [Ce4(H2ZPS)2(HZPS)2Á2CH3OH]2À and [Ce4(H2ZPS)2- robust enough, our fabrication strategy could be extended to other
(HZPS)2Á4CH3OH]2À, respectively, indicating the successful assembly Werner-type capsules composed of transition metal ions. Ligand
of a Ce-based M4L4 tetrahedron. Upon the addition of RDX, ESI-MS H3ZPT was obtained by a similar Schiff based formation reaction
spectra exhibited a new peak at m/z = 1821.26. An exact comparison of using 2-pyridyl aldehyde and malonohydrazide in an ethanol
this peak with the simulation results obtained on the basis of natural solution. The metal–organic octahedron was constructed using
isotopic abundances revealed the assignment of the peak to the zinc ions with a d10 electron configuration and ligand H3ZPT
species [Ce4(H2ZPS)2(HZPS)2 * RDX]2À, demonstrating a 1 : 1 stoi- through the same strategy.20 EA and 1H NMR suggested the
chiometric inclusion behavior. These results suggested that the cavity formation of a new compound in the solution. ESI-MS spectra of
of the CeÀZPS tetrahedron could encapsulate RDX directly, in which the assembly species exhibited an intense peak at m/z = 1027.51
the encapsulated RDX reacted with DHPA efficiently to form highly with the isotopic distribution patterns separated by 0.33 Da. The
luminescent active species for the experimental detection.
peak was assigned to [Zn6(HZPT)3(ZPT)]3+ species, indicating the
Compound CeÀZPS (10 mM) exhibited an absorption band successful assembly of a Zn6L4 molecular octahedron in solution
centered at about 390 nm in DMF solvent (log e = 5.22), assignable (Fig. 3). Upon the addition of RDX, ESI-MS spectra exhibited a new
to the absorptions endemic to the deprotonated phenol groups.19 peak at m/z = 1101.52 with the isotopic distribution patterns separated
When excited at 350 nm, the solution of CeÀZPS (10 mM) gave an by 0.33 Da. The peak is assignable to [Zn6(HZPT)3(ZPT) * RDX]3+
initially weak emission. The addition of the RDX (1 mM) caused a species by the comparison of this peak with the simulation results
ca. 8-fold luminescent enhancement after a 5 min irradiation with obtained on the basis of natural isotopic abundances. These results
310 nm light at 298 K (Fig. 2). The addition of other explosives such also demonstrated the formation of a 1 : 1 host–guest complex,
3468 | Chem. Commun., 2014, 50, 3467--3469
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nearly proportional to the RDX concentration, and the addition of
1 ppb RDX causes about 20% fluorescent enhancement within
5 min. The detection limit of 1 ppb represents the most sensitive
chemosensor for RDX in solution. This more sensitive detection
compared to the mass spectrometric detection21,22 suggests the
opportunity for its application in the field of rapid detection of trace
amounts of explosives.
In order to detect explosive RDX in real world applications,
the visual detection study of RDX was performed by preparing
substrates spotted with ZnÀZPT and explosive RDX solution
using DMF–acetonitrile as the solvent. The mixed solutions of
ZnÀZPT (5 mM) and the desired RDX concentration were spotted
onto a filter paper with low background fluorescence using a
glass microsyringe. A solvent blank was spotted next to each
explosive. After a 1 min irradiation with a 310 nm light, while
RDX shows turn-on fluorescence detection as low as 1 ng, other
explosives do not show any turn-on fluorescence even for spot
loadings as high as 50 ng.
Fig. 3 ESI-MS spectra of (a) ZnÀZPT (0.1 mM) and (b) ZnÀZPT (0.1 mM)
upon addition of 1 equiv. of RDX without illumination in a DMF–CH3CN
solution. Insets display the measured and simulated isotopic patterns at
1027.51 (top picture) and 1101.52 (bottom picture), respectively.
This work was supported by NSFC 21171029 and 21025102
and the Program for Changjiang Scholars and Innovative
Research Team in University (IRT1213).
Notes and references
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`
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