Mesoporous aluminium organophosphonates: A reusable chemsensor for the detection of explosives

Article abstract of DOI:10.1016/j.jssc.2016.04.004

Rapid and sensitive detection of explosives is in high demand for homeland security and public safety. In this work, electron-rich of anthracene functionalized mesoporous aluminium organophosphonates (En-AlPs) were synthesized by a one-pot condensation process. The mesoporous structure and strong blue emission of En-AlPs were confirmed by the N₂ adsorption-desorption isotherms, transmission electron microscopy images and fluorescence spectra. The materials En-AlPs can serve as sensitive chemosensors for various electron deficient nitroderivatives, with the quenching constant and the detection limit up to 1.5×10⁶ M^-1 and 0.3 ppm in water solution. More importantly, the materials can be recycled for many times by simply washed with ethanol, showing potential applications in explosives detection.

Full text of DOI:10.1016/j.jssc.2016.04.004

Author’s Accepted Manuscript
Mesoporous aluminium organophosphonates: a
reusable chemsensor for the detection of
explosives
Dongdong Li, Xiang Yu
www.elsevier.com/locate/yjssc
PII:
S0022-4596(16)30126-8
http://dx.doi.org/10.1016/j.jssc.2016.04.004
YJSSC19350
DOI:
Reference:
To appear in:
Journal of Solid State Chemistry
Received date: 25 January 2016
Revised date: 22 March 2016
Accepted date: 3 April 2016
Cite this article as: Dongdong Li and Xiang Yu, Mesoporous aluminium
organophosphonates: a reusable chemsensor for the detection of explosives
Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j.jssc.2016.04.004
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Mesoporous aluminium organophosphonates: a reusable chemsensor for the
detection of explosives
Dongdong Li*, Xiang Yu
Department of Materials Science, Key Laboratory of Automobile Materials of MOE,
Jilin University, Changchun, 130012, P. R. China
*Corresponding author. Tel: +86 431 85166879. E-mail: lidongdong@jlu.edu.cn
Abstract
Rapid and sensitive detection of explosives is in high demand for homeland
security and public safety. In this work, electron-rich of anthracene functionalized
mesoporous aluminium organophosphonates (En-AlPs) were synthesized by a one-pot
condensation process. The mesoporous structure and strong blue emission of En-AlPs
were confirmed by the N2 adsorption-desorption isotherms, transmission electron
microscopy images and fluorescence spectra. The materials En-AlPs can serve as
sensitive chemosensors for various electron deficient nitroderivatives, with the
quenching constant and the detection limit up to 1.5×106 M–1 and 0.3 ppm in water
solution. More importantly, the materials can be recycled for many times by simply
washed with ethanol, showing potential applications in explosives detection.
Keywords: Explosives detection; Mesoporous; Aluminium phosphate; Recyclable
1. Introduction
1

The reliable and accurate detection of explosives has become increasingly
important and urgent issue in modern society [1,2]. So far, many methods have been
developed for the detection of explosives, such as trained canine teams, gas
chromatography, ion mobility spectrometry (IMS), and surface-enhanced Raman
spectroscopy, and so on [3-7]. However, none of them is ideal for the detection of
explosives due to certain features such as more complicated, lack of selectivity, high
cost, time-consuming. Fluorescence-based detection of nitroderivatives such as
2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT) and picric acid (PA) by
harnessing organic dyes have drawn much more attention owing to their high
sensitivity, easy visualization and real-time monitoring with fast response time [8-12].
It is worth noting that nitroderivatives contain electron withdrawing nitro groups (–
NO2), which often act as good electron acceptors [13]. Thus using π-electron rich of
fluorophores as sensors would significantly improve the sensitivity of explosives
detection [14-16]. The fluorescence of electron-rich aromatic rings of anthracene is
known to be rapidly quenched upon exposure to nitroderivatives based on
electron-transfer and/or energy transfer mechanism [17,18]. However, few works are
reported on the incorporation of anthracene into porous materials for the detection of
explosives.
Porous materials have been widely used in various fields such as separation,
catalysis, and sensors due to their structural stability, high surface areas, large pore
volumes and controllable pore sizes [19-22]. As an important part of porous materials,
mesoporous metal phosphates possess abundant potential applications in optics,
2

magnetics, electronics, mechanics, and electron transfer [23-26]. So far, many
organically functionalized mesoporous metal organophosphonates have been
synthesized [27]. The materials can rapidly associate analytes and drugs inside the
large pores via physical diffusion and/or chemical interaction, which can efficiently
interact with the functional groups in/on the pore wall. In addition, the pore size of
mesoporous materials generally lies in the working distances of photo-induced
electron transfer and/or energy transfer. In this regard, organic fluorescent group
functionalized mesoporous metal organophosphonates maybe served as a new
generation of chemosensor for the detection of explosives.
The present work will contribute to the one-pot synthesis of mesoporous
aluminium organophosphonates functionalized with anthracene groups (En-AlPs) and
systematically investigated their application in explosives detection. The electron-rich
of fluorophore anthracene will have donor-acceptor (D–A) interactions with the
electron-deficient of nitroderivatives, resulting in a favorable photo-induced electron
transfer, which accounts for the most of the fluorescence-based explosive detection.
Furthermore, the large porous of En-AlPs would allow the explosives to enter and
interact with anthracene groups, enhancing the fluorescence quenching process.
Indeed, the En-AlPs show sensitive fluorescence quenching to PA in water solution,
with the quenching constant up to 1.5×106 M–1. The materials can also be recycled for
many times by simply washed with ethanol, showing potential application in
explosives detection.
2. Experimental
3

2.1. Materials
Aluminium isopropoxide (Al(i-PrO)4, Beijing Beihua Chemical Co., Ltd.), Cetyl
trimethyl ammonium chloride (CTAC, Beijing Beihua Chemical Co., Ltd.), Picric
acid (PA, Xilong Chemical Co., Ltd.), Phosphoric acid (H3PO4, Tianjin Tiantai Fine
Chemical Co., Ltd.), Tetramethylammonium hydroxide (TMAH, 25 wt%,
Sigma-Aldrich). All chemicals used in the experiments were purchased and used as
received
without
further
purification.
Organophosphonates
9,10-bis(dihydroxyphosphinylmethyl)anthracene (PAEn) was prepared by using a
modified literature method [28,29].
2.2. Synthesis of mesoporous aluminium organophosphonates
Mesoporous aluminium organophosphonates was prepared as described
previously with a modify [30], the process is as follows: 1.19 g CTAC and 0.13 g
PAEn were added into 2.2 mL water containing 2.7 g TMAH (25 wt%), and 0.48 mL
H3PO4. Then the solution was stirred at room temperature until completely dissolved.
Subsequently, 1.53 g Al(i-PrO)4 was added slowly, yielding a milky suspension after
stirred for 24 h. The mixture was transferred to a Teflon-lined autoclave and
crystallized statically at 130 °C under autogenous pressure for about 5 days. After
cooling to room temperature, the obtained materials was certified and washed with
deionized water and DMSO several times, and then refluxed in ethanol for 24 h to
remove the template. After dried in the freezer dryer overnight, the materials
functionalized with anthracene groups were obtained, marked as En-AlP2. Other
loading levels were obtained with the same process just by varied the amount of
4

PAEn to 0.065g, 0.55g for En-AlP1, En-AlP3, respectively. The contents of
anthracene loaded into the materials determined by CHN elemental analysis were 0.15,
0.31, and 0.92 mmol g–1.
2.3. The detection of explosives with En-AlPs
3 mg En-AlPs were well dispersed into 3 mL water by ultrasonic. Certain amounts
of explosives solution (10–4 M) was added into the system each time. Meanwhile, the
fluorescence was recorded with the addition of different amounts of explosives in
THF solution by spectrofluorometer under UV light illumination (365 nm).
2.4. Characterizations
The N2 adsorption-desorption isotherms at 77 K were measured on a
Micromeritics Gemini V analyzer. Transmission electron microscopy (TEM) images
were recorded with a JEM-3010 electron microscope. UV-Vis adsorption spectra
were obtained on a Shimadzu UV-2550 spectrophotometer. The UV/Vis excitation
and emission spectra were obtained on a Shimadzu RF-5301PC spectrofluorometer.
CHN elemental analyses were performed on a varioMICRO elemental analyzer.
3. Results and Discussion
The organophosphonates 9,10-bis(dihydroxyphosphinylmethyl)anthracene (PAEn)
was prepared according to the synthetic route (Fig. 1). Then it was introduced into
mesoporous aluminium organophosphonates through P-O-Al covalent bonds by a
one-pot condensation process with the assistance of surfactant CTAC. After removing
the surfactant by refluxed in ethanol, the materials functionalized with anthracene
groups were obtained, marked as En-AlPs. The mesostructure of the En-AlPs were
5

confirmed by N2 sorption analysis and TEM observations. The N2 adsorption–
desorption isotherms of En-AlP1 and En-AlP2 showed type IV isotherms with distinct
hysteresis loops in the 0.4–1.0 P/P0 range, revealing the typical character of
mesoporous materials (Fig. 2). One narrow peak around 2.4 nm were observed in the
pore size distribution curve, and the respective Brunauer–Emmett–Teller (BET)
surface area, pore volume are 132 m2 g–1, 0.24 cm3 g–1, and 172 m2 g–1, 0.28 cm3 g–1,
respectively. When more PAEn was added during the preparation process, the N2
adsorption–desorption isotherm of En-AlP3 becomes higher and shows a distinct
hysteresis loop in the 0.7–0.95 P/P0 range. Note that the pore size distribution of
En-AlP3 contains a new broader pore range indicating the presence of newly formed
larger mesopores. The reason can be contributed to the penetration of more bulky
bridging groups PAEn into the mesopore walls. Furthermore, some PAEn molecules
may also act as swelling agents incorporated in the formed CTAC micelles, leads to
the formation of much larger diameter pores. All the textural parameters of the
corresponding materials are summarized in Table 1. The wormlike pore structure of
the En-AlPs could be clearly seen from Fig. 3, consistent with the result of N2
sorption data.
Fig. 4 shows the emission spectra of En-AlPs in water solution. Upon UV
irradiation (365 nm), all of them exhibit strong blue emission, and their
photoluminescence (PL) intensity is enhanced with the increased loading amounts of
PAEn within a certain range. When the PAEn was 0.92 mmol g–1, the PL intensity
was not further increased, which maybe caused by the aggregation-caused quenching
6

(ACQ) effect. As anthracene is a well-known π-electron rich of fluorophores, the
En-AlPs may show excellent sensoring performance to electron deficient
nitroderivatives. We take commercially available picric acid (PA) as a model for the
explosive detection in water solution.
The PL intensity of En-AlPs decreases significantly with the increasing loading
amount of PA. Fig. 5 shows the fluorescence emission spectra of En-AlP2 with the
addition of different amounts of PA in water solution. The PL quenching can be
clearly discerned at a [PA] level as low as 1.4 µM or 0.3 ppm. When the PA
concentration is increased to 33 µM, the quenching of fluorescence can clearly visible
to the naked eye under the illumination of UV light at 365 nm. It is noting that the
materials En-AlP1, En-AlP3 show similar quenching process, but slower than that of
En-AlP2. The Stern–Volmer plots of relative PL intensity (I0/I) of En-AlPs versus PA
concentration are shown in Fig. 5B to further quantify the quenching efficiency,
where I0 and I are the PL intensities of the En-AlPs before and after adding PA,
respectively. The I0/I versus [PA] plots bend upwards quickly with quenching
constants of about 0.4×106, 1.5×106, 1.3×106 M–1 for En-AlP1, En-AlP2, and
En-AlP3, respectively. The sensitivity of them shows the sequence of En-AlP2 >
En-AlP3 > En-AlP1, consistent with their fluorescence intensity.
The sensitive fluorescence quenching behaviors of En-AlPs to PA can be
explained by the photoinduced electron transfer and/or energy transfer mechanism.
From Fig. 6, it can be seen that the absorption of PA overlaps with the emission of
En-AlPs from 380–500 nm in the water solution, which allows the energy transfer
7

from the excited state of En-AlPs to the ground state of PA, leading to rapid
fluorescence quenching response to the explosive PA. In addition, the large porous are
convenient for electron acceptor PA to enter and interact with the electron-rich
anthracene, improving the capacity of photo-induced electron transfer. Other electron
deficient nitroderivatives such as 3-nitrophenol (NP), 2,4-dinitrotoluene (DNT),
4-nitrotoluene (NT), and 4-nitrobenzaldehyde (NBD) were also investigated for
En-AlP2. As illustrated in Fig. 7, the En-AlP2 show lower sensing ability towards NP,
DNT, NT and NBD in the systems compared to PA under the same conditions. The
strong spectral overlap of PA with En-AlP2 is responsible for the high sensitivity.
Furthermore, substitution of more electron-withdrawing nitro groups on the aromatic
ring will also favor the fluorescence quenching of the materials. NP, NT and NBD
only have one nitro group, but the NP behaves as a relatively stronger acid because of
the –OH group affixed on benzene ring, which makes it can form hydrogen bond with
the hydroxyl group on the surface of En-AlP2. While NT and NBD are hydrophobic
in nature and the En-AlP2 are hydrophilic materials. It can be expected that the
compatibility of the En-AlP2 to NT and NBD will be poor, resulting in lower
quenching efficiencies.
The reusability of the anthracene functionalized mesoporous aluminium
organophosphonates was further investigated by taking En-AlP2 as example. After
the emission was quenched, the materials was certified, and washed with ethanol
several times. Then the experimental procedure was repeated. It can be clearly seen
from Fig. 8 that En-AlP2 could be regenerated and reused for at least five cycles
8

without significant change of PL intensity, indicating that the materials can be used as
environmentally friendly detectors for explosives.
4. Conclusion
In summary, electron-rich of anthracene functionalized mesoporous aluminium
organophosphonates was prepared by a one-pot condensation process, which can be
used for the detection of electron-deficient nitroderivatives in water solution based on
the photo-induced electron and/or energy transfer mechanism. The large porous
enable explosives to enter and interact with the anthracene chromophores, leading to
rapid fluorescence quenching response to PA, with the quenching constant and the
detection limit up to 1.5×106 M–1 and 0.3 ppm, respectively. More importantly, the
materials can be recycled for many times by simply washed with ethanol, proving that
they can be used as an excellent fluorescence probe for future practical applications in
explosives detection.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (Grant
No.: 21501063).
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Captions:
Fig. 1 The synthetic route of PAEn.
Fig. 2 (A) N2 adsorption/desorption isotherms of (a) En-AlP1, (b) En-AlP2, (c)
En-AlP3, respectively. (b) Corresponding pore size distributions based on the
adsorption branch of the isotherm with the Barrett-Joyner-Halenda (BJH) method.
Fig. 3 TEM images of (a) En-AlP1, (b) En-AlP2, (c) En-AlP3.
Fig. 4 Fluorescence spectra of (a) En-AlP1, (b) En-AlP2, (c) En-AlP3 in water.
Excitation wavelength: 365 nm.
Fig. 5 (A) Fluorescence emission spectra of En-AlP2 with the addition of different
amounts of PA in water solution. (B) Stern–Volmer plots of I0/I versus [PA] in water
(a) En-AlP2; (b) En-AlP3; (c) En-AlP1.
Fig. 6 Absorption spectrum of (a) PA, (b) NT, (c) DNT, (d) NP, (e) NBD; the PL
spectra of (f) En-AlP1, (g) En-AlP2, and (h) En-AlP3 in water.
Fig. 7 Fluorescence quenching efficiencies of En-AlP2 toward different analytes. The
samples are ordered as PA (1), NP (2), DNT (3), NT (4), NBD (5).
Fig. 8 The quenching and recovery test of En-AlP2 in water solution. The inset shows
fluorescence images in the water solution with [PA] of 0 and 33 µM, which are taken
under UV light illumination (365 nm).
13

Table 1 Textural parameters of En-AlPs.
Pore Volume
(cm3/g)
0.24
Anthracene loading
Sample
SBET (m2/g)
DBJH (nm)
(mmol/g)
0.15
En-AlP1
En-AlP2
En-AlP3
132
172
104
2.4
2.4
0.28
0.31
15.0
0.34
0.92
Fig. 1
14

Fig. 2
Fig. 3
15

Fig. 4
Fig. 5
16

Fig. 6
Fig. 7
17

Fig. 8
Highlights
 Anthracene functionalized mesoporous aluminium organophosphonates were
synthesized.
 The materials serve as sensitive chemosensors for nitroderivatives.
 The materials can be recycled for many times by simply washed with ethanol.
 The materials show potential applications in explosives detection.
18

Graphical abstract
Electron-rich
of
anthracene
functionalized
mesoporous
aluminium
organophosphonates can serve as sensitive and recycled chemosensors for
nitroderivatives with the quenching constant up to 1.5×106 M–1 in water solution.
19