Direct identification of HMX via guest-induced fluorescence turn-on of molecular cage

Article abstract of DOI:10.1016/j.cclet.2021.05.051

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is one of the most widely used powerful explosives. The direct and selective detection of HMX, without the requirement of specialized equipment, remains a great challenge due to its extremely low volatility, unfavorable reduction potential and lack of aromatic rings. Here, we report the first chemical probe of direct identification of HMX at ppb sensitivity based on a designed metal-organic cage (MOC). The cage features two unsaturated dicopper units and four electron donating amino groups inside the cavity, providing multiple binding sites to selectively enhance host-guest events. It was found that compared to other explosive molecules the capture of HMX inside the cavity would strongly modulate the emissive behavior of the host cage, resulting in highly induced fluorescence “turn-on” (160 folds). Based on the density functional theory (DFT) simulation, the mutual fit of both size and binding sites between host and guest leads to the synergistic effects that perturb the ligand-to-metal charge-transfer (LMCT) process, which is probably the origin of such selective HMX-induced turn-on behavior.

Full text of DOI:10.1016/j.cclet.2021.05.051

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Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Direct identification of HMX via guest-induced fluorescence turn-on of
molecular cage
Chen Wang a,e,1,∗, Jin Shangb,1, Li Tiana, Hongwei Zhaoa, Peng Wang a, Kai Fenga,
Guokang Hea, Jefferson Zhe Liuc, Wei Zhud, Guangtao Lia,∗∗
a Department of Chemistry, Tsinghua University, Beijing 100084, China
b School of Energy and Environment, City University of Hong Kong, Hong Kong
c Department of Mechanical Engineering, The University of Melbourne, Parkville, VIC 3010, Australia
d School of Biology and Biological Engineering, South China University of Technology, Guangzhou 510006, China
e Institute of chemistry, Hebrew University of Jerusalem, Jerusalem 91904, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is one of the most widely used powerful explo-
sives. The direct and selective detection of HMX, without the requirement of specialized equipment, re-
mains a great challenge due to its extremely low volatility, unfavorable reduction potential and lack of
aromatic rings. Here, we report the first chemical probe of direct identification of HMX at ppb sensitivity
based on a designed metal-organic cage (MOC). The cage features two unsaturated dicopper units and
four electron donating amino groups inside the cavity, providing multiple binding sites to selectively en-
hance host-guest events. It was found that compared to other explosive molecules the capture of HMX
inside the cavity would strongly modulate the emissive behavior of the host cage, resulting in highly in-
duced fluorescence “turn-on” (160 folds). Based on the density functional theory (DFT) simulation, the
mutual fit of both size and binding sites between host and guest leads to the synergistic effects that per-
turb the ligand-to-metal charge-transfer (LMCT) process, which is probably the origin of such selective
HMX-induced turn-on behavior.
Received 24 January 2021
Revised 23 May 2021
Accepted 25 May 2021
Available online xxx
Keywords:
Chemical sensing
Explosive detection
HMX
Molecular cage
Host-guest chemistry
Fluorescence turn-on
© 2021 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia
Medica, Chinese Academy of Medical Sciences.
Efficient explosive detection has drawn intense attentions due
to increasing concerns of security [1,2]. Especially, on-site or di-
rect sensing of trace explosives is more appealing and essential
in criminal sites and forensic analysis [3]. Compared to nitroaro-
matics probes that are well-developed nowadays [4,5], detection
of nitroamine that are more powerful and widely used in ter-
rorism, e.g., RDX and HMX [6–8], are still rare due to their ul-
tralow volatility, unfavorable reduction potentials and weak elec-
tron withdrawing abilities. In particular, HMX possesses an even
lower volatility of 0.1 ppt as compared to RDX (5 ppt), making
it a great challenge to probe and identify trace HMX. One of the
commonly utilized method for direct HMX sensing is specialized
instrument analysis, including high-performance liquid chromatog-
raphy (HPLC) [9], ion mobility spectrometry [10], voltammetry
technique [11,12] and capillary electrophoresis [13]. Yet, they are
not quite suitable for on-site detection. Indirect detection through
decomposition-mediated strategies (e.g., radicals) [14] is another
choice. The existing dilemma is, however, same reactive interme-
diates (NOx) are generated from decomposition of either HMX or
RDX, thus hampering precise component identification and de-
creasing sensing selectivity. To the best of our knowledge, the di-
rect and selective detection of trace HMX using chemical recep-
tor has not been reported [1-14]. Thus, development of chemi-
cal probes for direct detection and identification of HMX with en-
hanced selectivity is still an urgent task.
Metal-organic cages (MOCs), discrete molecules assembled
through coordination interactions between metal ions and organic
ligands, have been recently considered as promising candidates in
molecular recognition owing to their well-defined cavities and tun-
able functionalities [15–17]. The rational design of individual build-
ing blocks allows fine tuning of cavity size and the integration of
chemical functionalities and multiple interactions inside the cav-
ities of MOCs, facilitating the selective binding of various guests,
∗
Corresponding author at: Department of Chemistry, Tsinghua University, Beijing
100084, China.
∗∗
Corresponding author.
E-mail addresses: chen.wang@mail.huji.ac.il (C. Wang), lgt@mail.tsinghua.edu.cn
(G. Li).
1
These two authors contributed equally to this work.
https://doi.org/10.1016/j.cclet.2021.05.051
1001-8417/© 2021 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Please cite this article as: C. Wang, J. Shang, L. Tian et al., Direct identification of HMX via guest-induced fluorescence turn-on of molec-
ular cage, Chinese Chemical Letters, https://doi.org/10.1016/j.cclet.2021.05.051

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Fig. 1. ESI-MS spectra of host-guest complexes. TNT@cage A (a), RDX@cage A (b),
Scheme 1. Schematic illustration of the direct identification of HMX within the
cage A and structures of ligand and metal (S in copper unit represents the addi-
tionally coordinated solvent molecules H2O and CH3OH), and the target molecules
include nitroaromatic TNT and nitroamines RDX, HMX and PETN.
HMX@cage A (c) and PETN@cage A (d).
days. Single-crystal X-ray analysis reveals the formation of a M2L4
lantern-type structure with two paddlewheel dicopper motifs
bridged by four ligands L-NH2 (Fig. S2 and Table S1 in Supporting
information). The distance between two Cu2+ is 9.394 A, and the
˚
even at single molecule level. More importantly, guest molecules
binding in molecular cages could in turn induce unusual phenom-
ena that are hardly observed in guest-free state due to the am-
plified host-guest events in confined environment, such as guest-
induced emission amplification [18], guest-induced rearrangement
[19], stabilization of reactant transition state [20], enhancement of
reaction selectivity [21]. Taking advantages of these distinctive fea-
tures of metal-organic cages, the rational design of ideal receptors
for selective binding given target molecules should be possible.
Herein we developed a non-fluorescent M2L4 molecular cage
(cage A), acting as a remarkable receptor for direct and selec-
tive sensing of HMX with exceptionally high fluorescence turn-
on behavior (Scheme 1). The cage A was decorated with unsatu-
rated Cu2+ and amino groups to amplify the guest binding events
and induce detectable outputs. After binding HMX, the emission
of HMX@cage A complex is enhanced remarkably up to 160-folds
with a detection limit of 3.5 ppb. No response to RDX and PETN,
and only minute response to TNT were detected, demonstrating
the unique sensing selectivity towards HMX. DFT simulation was
carried out to understand the mechanism behind. Compared with
the cases of TNT and RDX, DFT simulation showed that HMX ex-
hibits the largest charge transfer to Cu2+, indicating the strongest
modulation of the Cu2+single occupied molecular orbitals (SOMO),
correlating well with experimental results. Therefore, the origin
of such fluorescence enhancement probably is the mutual fit of
both size and binding sites between host and guest, thus leading
to the guest-induced perturbation of the ligand-to-metal charge-
transfer (LMCT) process. To the best of our knowledge, this is the
first chemical receptor for direct and selective detection of HMX
based on a fluorescence turn-on approach. It should be noted that,
compared to traditional turn-off MOCs explosive sensors [22–24],
the turn-on approach would significantly improve the sensitivity
as well as the selectivity of the probe, making it a preference for
˚
distance between two opposing aniline rings is 11.973 A. The cage
crystals are soluble and stable in DEF/dichloromethane solution.
The intense peak at 1962.20 in electrospray ionization mass (ESI-
MS) spectrum further confirmed the composition of [Cu4L4]+Na+
(Fig. S3 and discussion in Supporting information).
In our work, we selected two kinds of explosives as tar-
gets, including nitroaromatic TNT and nitroamines RDX, HMX
and PETN (Scheme 1). The host-guest interaction between the
cage
A and explosive molecules was first studied by ESI-MS
technique. In Figs. 1a-c, the intense peaks at m/z 2295.73,
2262.50, and 2357.62 in ESI-MS spectra were observed, which are
assigned correspondingly to [Cu4L4@C7H5N3O6]·4CH3OH+H+
species,
[Cu4L4@C3H6N6O6]·2H2O·2CH3OH+H+
and
[Cu4L4@C4H8N8O8]·H2O·2CH3OH+K+ species, proving the 1:1
stoichiometric host-guest complexation. However, the intense peak
at m/z 1962.20 in Fig. 1d shows that PETN was not encapsulated
in the cage A, probably due to the steric hindrance between PETN
and the cage A. However, due to the paramagnetic property of
copper(II), NMR analysis could not be performed.
Thus, the host-guest interaction was carefully investigated and
confirmed by UV–vis titration. As shown in Fig. 2a, the cage A solu-
tion exhibits obvious ligand-based charge-transfer bands at 378 nm
in DEF solution. Upon addition of RDX and HMX, the intensity of
absorption at visible area decreases while TNT increases the ab-
sorption at visible area, which could be attributed to the charge-
transfer interaction between the amino group of cage A and TNT
(Figs. 2a and b). After the addition of TNT into the solution of
the cage A, the color changes from colorless to deep red, indicat-
ing the guest-binding behavior inside the cavity (Fig. S4 in Sup-
porting information). In the case of RDX and HMX, two isosbestic
points are observed at 355 nm and 413 nm, together with the
decrease of the absorption peak at 378 nm (Figs. 2c and d), in-
dicative of the prospective guest binding within the cage. The de-
crease of the absorption band at 378 nm might be due to the
partial blockage of ligand-to-metal charge transfer (LMCT) upon
interaction between HMX or RDX and metal ion center of the
cage, thus leading to the strong modulation of the single occu-
pied molecular orbitals (SOMO) of Cu2+ and further modulation
of the fluorescence. The detailed mechanism would be discussed
in the following context. However, the UV–vis spectra of the cage
A titrated by PETN didn’t change due to the size-selectivity of
HMX detection.
ꢀ
The
ligand
3,3 -((2-amino-5-isopropyl-1,3-phenylene)
bis(ethyne-2,1-diyl))dibenzoic acid (L-NH2) was synthesized from
2,6-dibromo-4-isopropylaniline (for the synthetic route, see Fig. S1
in Supporting information) to create the M2L4 cage A (Scheme 1).
Through slowly layering methanol onto N,N-diethylformamide
(DEF) solution that contains Cu2(OAc)4 (1 equiv.) and ligand
l-NH2 (2 equiv.) at room temperature [25,26], the molecular cage
[Cu4(L-NH2)4(S)4]·xS with a yield of 80% was harvested after 5
2

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Fig. 2. (a) The UV–vis spectra of the cage A and the formed explosive@cage A com-
plexes in DEF solvent; (b) UV–vis spectra as obtained during the titration of the
cage A (10−5 L/mol) with 10−3 L/mol TNT in DEF/CH2Cl2 solution; (c) UV–vis spec-
tra as obtained during the titration of the cage A (10−5 L/mol) with 10−3 L/mol
RDX in DEF/CH2Cl2 solution, inset shows zoomed in spectrum; (d) UV–vis spectra
as obtained during the titration of the cage A (10−5 L/mol) with 10−3 L/mol HMX
in DEF/CH2Cl2 solution, inset shows zoomed in spectrum.
Fig. 3. (a) Emission spectra, as obtained during the titration of cage (10−7 L/mol)
with 10−5 L/mol HMX in DEF solution when excited at 390 nm; inset shows the
titration isotherm; (b) Fluorescence enhancement of HMX@cage A, TNT@cage A,
RDX@cage A and PETN@cage A as compared to pure cage in DEF solution, inset
shows the fluorescence response of the cage toward HMX, TNT, RDX and PETN af-
ter fluorescence titrations reach saturation; (c) Fluorescent images of HMX@cage A,
TNT@cage A, RDX@cage A under UV light (365 nm).
the cage, which is consistent with the ESI-MS results. The bind-
ing affinity of TNT@cage A was calculated to be 1.43 × 104 L/mol
by assuming
a 1:1 binding model. For RDX and HMX, the
cage A shows a moderate binding affinity of 1.03 × 104 L/mol
and 1.60 × 103 L/mol, respectively (Fig. S5 in Supporting informa-
tion). It should be noted that the substrate with higher binding
constant does not necessarily indicate the better sensitivity. Both
size- and shape-dependence determine the selectivity and sensing
ability with our cage, which is an efficient sensing strategy towards
selective explosive molecules trapping.
Table 1
Charge transfer and binding energy between the cage A and dif-
ferent guests. The values of charge transfer are given in electron
charge.
Cu2+
-NH2
Binding energy (eV)
To further study the sensing ability and selectivity of the de-
signed cage A, we investigated the fluorescence behavior of the
host-guest complexes through fluorescence titration. As expected,
irradiation of the cage A in pure DEF solution gave no emission
due to fluorescence quenching by Cu2+ ions, while the ligand L-
NH2 exhibited high fluorescence at 445 nm (Fig. S6 in Support-
ing information). Interestingly, high fluorescence enhancement at
445 nm could be observed upon addition of HMX into the solu-
tion of the cage, together with a blue-shift of emission (Fig. 3a).
The limit of detection (LOD) was calculated to be 3.5 ppb from 3
σ/S. In contrast, the titration of the cage with TNT in DEF solu-
tion led to a little increase of the emission band at 445 nm, while
RDX and PETN caused almost no change in fluorescence spectra
(Fig. S7 in Supporting information). The emission band at 445 nm
of HMX@cage A in DEF solution showed a significant fluorescence
enhancement up to 160-folds, whereas the cage A exhibited no flu-
orescence response to RDX and PETN, and only minute response to
TNT (Figs. 3b and c). Controlled fluorescence and absorbance ex-
periments were also carried out that pure ligand L-NH2 was mixed
with explosives (Figs. S8 and S9 in Supporting information). No flu-
orescence and absorbance changes were observed, indicating the
crucial role of mutual fit of HMX in MOC’s confined cavity to in-
duce amplified detectable sensing signals. In Fig. 3c, the fluorescent
image of HMX@cage A in DEF solution shows a significant fluo-
rescence enhancement as compared to TNT@cage A and RDX@cage
A under irradiation. The quantum yields of the cage A in the ab-
sence and presence of HMX were measured to be 0.004 and 0.062,
respectively. Luminescence lifetimes were also measured. The life-
time of cage A is 4.02 ns. In the presence of HMX, the lifetime is
TNT@cage A
RDX@cage A
HMX@cage A
−0.035
−0.091
−0.150
−0.023
−0.009
−0.006
−0.84
−1.20
−1.25
2.73 ns, therefore the type of luminescence is fluorescence (Figs.
S10 and S11 in Supporting information). In fact, due to the des-
ignable features of metal-organic cages, cages can be facilely mod-
ified to be hydrophilic through the appropriate selection or mod-
ification of ligands or metal ions for further investigation of the
sensing performance and improvement of practical applications.
In our work, numerous attempts were made to obtain the crys-
tal structure of host-guest complex under different conditions (e.g.,
solvent, temperature) to clarify host–guest interactions at molecu-
lar level, but unfortunately failed. Thus, DFT simulations were car-
ried out to further study the inclusion of the explosives in the
cage A and the possible fluorescence turn-on mechanism. The sim-
ulated geometries of the cage and host-guest complexes provide
the 1:1 binary complexation (Fig. 4). In all three explosives, nitro
groups are found to point toward amino groups of the cage. Partic-
ularly, in the case of HMX, two nitro groups point to amino groups
and the other two nitro groups point to the Cu2+center of the
cage A (Fig. 4d). Compared with RDX and TNT, HMX induces much
stronger charge transfer within the cage, and the charge transfer
to Cu2+ center was much larger than that to amino groups, indi-
cating a stronger modulation to the Cu2+single occupied molecu-
lar orbitals (SOMO) (Table 1). In fact, the electronic structures of
Cu2+ in MOC could be easily modulated through the coordination
with solvent or some special molecules at the apical position in d9
3

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Declaration of competing interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
Acknowledgments
This research was made possible as a result of a generous grant
from the National Natural Science Foundation of China (NSFC,
Nos. 21773135, 22032003, 21821001), the Ministry of Science
and Technology (MOST, No. 2017YFA0204501), and the Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation, No.
TRR61). The authors wish to acknowledge Professor Wei Zhu for
his helpful discussions regarding the experimental design.
Supplementary materials
Fig. 4. DFT simulation of the molecular cage A and difference-electron density of
different host-guest complexes. Simulated molecular M2L4 (a), difference-electron
density of HMX@cage A (b), TNT@cage A (c) and RDX@cage A (d) complexes with
electron accumulation represented by red and depletion by dark blue. The isosur-
faces are plotted at 0.001207 e−/bohr3 for difference-election density results.
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.cclet.2021.05.051.
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