Organic-inorganic hybrid mesoporous materials as regenerable sensing systems for the recognition of nitroaromatic explosives

Article abstract of DOI:10.1002/cplu.201300140

Fluorescent organic-inorganic mesoporous hybrid materials have been prepared and characterised, and their behaviour against nitroaromatic explosives have been tested. MCM-41 silica was used as an inorganic scaffold and pyrene (P derivative containing trialkoxysilane moieties), dansyl and fluorescein (D and F derivatives also containing trialkoxysilane groups, respectively) fluorophores have been anchored on hybrid materials by a co-condensation method to obtain a homogenous distribution of dyes into the pores of the support. Six sensing materials have been prepared, of which SP, SD, SF were hydrophilic and SPh, SDh, SFh were hydrophobic. Template-free hydrophilic materials (SP, SD, SF) were obtained after repeated NH₄NO₃/ethanol extractions under temperature from as-synthesised (MP, MD and MF supports, respectively) solids. Hydrophobic materials (SPh, SDh, SFh) were prepared by using excess 1,1,1,3,3,3-hexamethyldisilazane with template-free hydrophilic (SP, SD and SF) materials. The six final materials displayed the typical emission bands of the grafted fluorophores. In particular, SP and SPh show the typical pyrene monomer (370-420 nm) and excimer (430-600 nm) emissions. SD and SDh exhibit the broad dansyl fluorescence band in the 450-600 nm range, whereas solids SF and SFh present sharp fluorescein emission centred at 525 nm. The fluorescent behaviour of the six final materials was tested in the presence of explosives (pentaerythritol tetranitrate (PETN), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), nitrobenzene (NB), 1,3,5-trinitrobenzene (TNB), 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrophenylmethylnitramine (Tetryl) and picric acid (PA)). Only nitroaromatic compounds were able to induce emission quenching. As a general trend, the quenching degree depended on the nature of the final material. The best response was obtained with explosives PA and Tetryl, which were able to significantly quench the emission of the sensing supports. The observed quenching was ascribed to the π-π stacking interactions between the electron-donor fluorophores and the electron-withdrawing nitroaromatic explosives. When using SPh for Tetryl and PA, the limits of detection were 8.5 and 1.4 ppm, respectively, whereas they were 14.4 and 1.2 ppm for SDh. Principal component analysis algorithms were applied to the fluorescence measurements taken with the six hybrid materials and the seven explosives. The obtained score plot showed well-defined clusters for the seven explosives tested. Finally, solid SDh was applied to detect trace amounts of Tetryl in soil samples with good results. Copyright

Full text of DOI:10.1002/cplu.201300140

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K. Sarkar, Y. Salinas, I. Campos,
R. Martꢀnez-MꢁÇez,* M. D. Marcos,
F. Sancenꢂn, P. Amorꢂs
&& – &&
Organic–Inorganic Hybrid Mesoporous
Materials as Regenerable Sensing
Systems for the Recognition of
Nitroaromatic Explosives
All in the family: A family of mesopo-
rous materials functionalised with fluo-
rophores (pyrene, dansyl and fluores-
cein) have been prepared, characterised
and used for the fluorogenic recogni-
tion of nitroaromatic explosives (see red
hexagons in figure). The emission of the
pyrene and dansyl functionalised mate-
rials was quenched upon the addition
of picric acid or 2,4,6-trinitrophenylme-
thylnitramine, hence easy detection was
possible.
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DOI: 10.1002/cplu.201300140
Organic–Inorganic Hybrid Mesoporous Materials as
Regenerable Sensing Systems for the Recognition of
Nitroaromatic Explosives
Krishanu Sarkar,[a] Yolanda Salinas,[a, b] Inmaculada Campos,[a, b] Ramꢁn Martꢂnez-MꢃÇez,*[a, b]
Marꢂa D. Marcos,[a, b] Fꢄlix Sancenꢁn,[a, b] and Pedro Amorꢁs[c]
Fluorescent organic–inorganic mesoporous hybrid materials
have been prepared and characterised, and their behaviour
against nitroaromatic explosives have been tested. MCM-41
silica was used as an inorganic scaffold and pyrene (P deriva-
tive containing trialkoxysilane moieties), dansyl and fluorescein
(D and F derivatives also containing trialkoxysilane groups, re-
spectively) fluorophores have been anchored on hybrid materi-
als by a co-condensation method to obtain a homogenous dis-
tribution of dyes into the pores of the support. Six sensing ma-
terials have been prepared, of which SP, SD, SF were hydro-
philic and SPh, SDh, SFh were hydrophobic. Template-free hy-
drophilic materials (SP, SD, SF) were obtained after repeated
NH4NO3/ethanol extractions under temperature from as-syn-
thesised (MP, MD and MF supports, respectively) solids. Hydro-
phobic materials (SPh, SDh, SFh) were prepared by using
excess 1,1,1,3,3,3-hexamethyldisilazane with template-free hy-
drophilic (SP, SD and SF) materials. The six final materials dis-
played the typical emission bands of the grafted fluorophores.
In particular, SP and SPh show the typical pyrene monomer
(370–420 nm) and excimer (430–600 nm) emissions. SD and
SDh exhibit the broad dansyl fluorescence band in the 450–
600 nm range, whereas solids SF and SFh present sharp fluo-
rescein emission centred at 525 nm. The fluorescent behaviour
of the six final materials was tested in the presence of explo-
sives (pentaerythritol tetranitrate (PETN), hexahydro-1,3,5-trini-
tro-1,3,5-triazine (RDX), nitrobenzene (NB), 1,3,5-trinitrobenzene
(TNB), 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT),
2,4,6-trinitrophenylmethylnitramine (Tetryl) and picric acid
(PA)). Only nitroaromatic compounds were able to induce
emission quenching. As a general trend, the quenching degree
depended on the nature of the final material. The best re-
sponse was obtained with explosives PA and Tetryl, which
were able to significantly quench the emission of the sensing
supports. The observed quenching was ascribed to the p–p
stacking interactions between the electron-donor fluorophores
and the electron-withdrawing nitroaromatic explosives. When
using SPh for Tetryl and PA, the limits of detection were 8.5
and 1.4 ppm, respectively, whereas they were 14.4 and
1.2 ppm for SDh. Principal component analysis algorithms
were applied to the fluorescence measurements taken with
the six hybrid materials and the seven explosives. The obtained
score plot showed well-defined clusters for the seven explo-
sives tested. Finally, solid SDh was applied to detect trace
amounts of Tetryl in soil samples with good results.
Introduction
The detection of explosives and explosive-related illicit materi-
als is crucial for preventing terrorist activities, homeland securi-
ty, and environmental cleaning.[1] In this field, the development
of quick, reliable methods to detect these chemicals is of par-
ticular interest. Among the various chemical explosives avail-
able, nitroaromatic compounds are perhaps the most com-
monly used.[2] Current methods used to detect nitrated explo-
sives are based on enzymatic assays,[3] gas and liquid chroma-
tography,[4] mass spectrometry,[5] ion-mobility spectroscopy,[6]
electrochemical procedures,[7] and optodes.[8] Although each
method offers advantages, none is ideal. Principal deficiencies
include lack of portability or susceptibility to false-positive re-
sults because of environmental contaminants, or false-negative
results as a result of interfering compounds. As an alternative
to these instrumental procedures, optical probes have been
extensively researched for signalling applications given their
potential for high sensitivity, selectivity, portability and cost-ef-
fectiveness.[9] Optical detection techniques have focused on
either colorimetric or fluorescence assays.[10] In particular, fluo-
rescence is a highly sensitive technique when compared with
other light-based methods.
[a] Dr. K. Sarkar,+ Y. Salinas,+ Dr. I. Campos, Prof. R. Martꢀnez-MꢁÇez,
Dr. M. D. Marcos, Dr. F. Sancenꢂn
Centro de Reconocimiento Molecular y Desarrollo Tecnolꢂgico
Unidad Mixta Universidad Politꢃcnica de Valencia-Universidad de Valencia
Departmento de Quꢀmica. Universidad Politꢃcnica de Valencia
Camino de Vera s/n, 46022 Valencia (Spain)
Fax: (+34)96-387-93-49
E-mail: rmaez@qim.upv.es
[b] Y. Salinas,+ Dr. I. Campos, Prof. R. Martꢀnez-MꢁÇez, Dr. M. D. Marcos,
Dr. F. Sancenꢂn
CIBER de Bioingenierꢀa
Biomateriales y Naomedicina (CIBER-BBN) (Spain)
[c] Prof. P. Amorꢂs
Institut de Ciꢄncia del Materials (ICMUV)
Universitat de Valꢄncia
P.O. Box 2085, 46071 Valencia (Spain)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201300140.
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Scheme 1. Schematic representation of the sensing protocol using hydrophobic materials SPh, SDh and SFh.
For the above reasons, the preparation of optical probes to Results and Discussion
detect nitrated explosives based on fluorimetric measurements
Design and synthesis of hybrid sensing materials
has attracted much attention in recent years.[11] As a general
trend, the designed probes make full use of the electron-defi-
cient character of nitrated explosives that induces them to
form charge-transfer complexes with electron-rich molecules,
such as polycyclic aromatic hydrocarbons, dansyl derivatives or
tetrathiafulvalenes.[12] The formation of these stacked ensem-
bles usually results in the quenching of the signalling fluoro-
phore through electron-and energy-transfer processes.[13] By
making full use of this concept, molecular fluorescent recep-
tors used to detect nitrated explosives have been explored in
the last few years.[14] In spite of this, one of the most important
drawbacks of employing molecular fluorophores to sense ni-
trated explosives is lack of selectivity; that is, the stronger the
electron-withdrawing character of the nitrated explosive, the
greater the quenching of the fluorophore emission. To over-
come this lack of selectivity, researchers have incorporated mo-
lecular fluorophores into organic polymeric matrices[15] or into
inorganic supports.[16] The underlying idea relies on the fact
that the physical and chemical properties of the support can
be tuned as a way to control selectivity towards a certain ni-
trated explosive.
The proposed sensing protocol is schematically shown in
Scheme 1. As inorganic support, a mesoporous MCM-41 phase
was selected for its high load capacity (derived from the pres-
ence of a dense pore network) and its easy chemical function-
alisation.[20] The inorganic support was functionalised with
three fluorophores (pyrene, dansyl and fluorescein) by a co-
condensation procedure. This yielded hydrophilic materials SP,
SD and SF. In a second step, the hydrophilic materials were
made hydrophobic and solids SPh, SDh and SFh were ob-
tained (see Scheme 1). This synthetic procedure leads to
a family of highly emissive hybrid solids in which the polarity
of the inner pores was also controlled. As Scheme 1 explains,
the signalling protocol relies on the existence of fluorophore–
nitroaromatic explosive interactions, which are expected to
result in quenching of the emission of the grafted fluoro-
phores. Moreover, modulation of the response is expected to
occur bearing in mind both the presence of a given fluoro-
phore with a certain electron-donor character and the polarity
of the pores.
As a first step to prepare the different solids, the selected
fluorophores (pyrene, dansyl and fluorescein) should be prop-
erly derivatised with trialkoxysilane moieties to anchor them
on the MCM-41 inorganic support (see Scheme 2). For this pur-
pose, 1-pyrenemethylamine (1) was treated with (3-isocyanato-
propyl)triethoxysilane (2) to yield ethoxysilane derivative P. The
1H NMR spectrum of compound P showed the typical NH pro-
tons of the urea moiety as broad triplets centred at d=5.98
and 6.47 ppm. Other significant signals were aromatic pyrene
protons, which appeared in the d=8.1–8.4 ppm interval and
the doublet centred at d=4.95 ppm, which correspond to the
methylene protons linked directly with the pyrene and urea
moieties. Moreover, the synthesis of derivatives D and F is
Bearing in mind the importance of mesoporous materials in
several scientific and technological fields,[17] our previous expe-
rience in preparing hybrid organic–inorganic materials for their
application in recognition protocols[18] and our recent interest
in designing probes for explosives,[19] we report herein the syn-
thesis, characterisation and recognition behaviour towards ni-
trated explosives of six solids functionalised with three differ-
ent fluorophores (pyrene, dansyl and fluorescein) located in hy-
drophilic or hydrophobic environments. Studies on the fluores-
cent response of these materials with selected explosives are
reported.
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solids preparation, the contents
of each fluorophore in the final
solids was somewhat different.
This may be due to the different
reactivity of the derivatives
which is apparently related with
fluorophore size; that is, the
larger the size (F>P>D), the
smaller the final amount anch-
ored onto the inorganic support.
Moreover, the polarity of the
grafted fluorophore has a clear
effect on the further functionali-
sation with hexamethyldisila-
zane. In particular, the lesser the
polarity of the fluorophore (P>
D>F), the larger the final
amount of the ÀOSi(CH3)3
groups linked to the inner area
of the pores.
Scheme 2. Synthetic procedures for the preparation of alkoxysilane derivatives P, D and F.
straightforward and has been published elsewhere.[21] In partic-
Table 1. Organic content in solids SP, SD, SF, SPh, SDh and SFh.
ular, the treatment of (3-aminopropyl)triethoxysilane (3) with
dansyl chloride (4) yielded product D, whereas F was obtained
through the treatment of 3 with fluoresceinisothiocyanate (5).
The NMR data of D and F were in full agreement with those
described in the literature.
Solid aP
aD
aF
aSiMe3
[mmolgÀ1 SiO2] [mmolgÀ1 SiO2] [mmolgÀ1 SiO2] [mmolgÀ1 SiO2]
SP
0.32
–
–
–
SPh 0.32
–
–
–
–
0.19
0.19
0.56
–
0.09
-
The functionalised hybrid MCM-41 materials were prepared
using a mixture of TEOS and the corresponding derivatised flu-
orophore (P, D and F) in a 99:1 ratio of TEOS/derivatised fluoro-
phore. The molar ratio of the reagents in the mother liquor
was fixed to 3.5:1:0.26:0.25:4.45 for TEAH3/silica source (TEOS-
derivatised fluorophore 99:1)/CTAB/NaOH/H2O. With this proce-
dure, the “as made” materials containing pyrene (MP), dansyl
(MD) and fluorescein (MF) attached in the inner porous net-
work were obtained. Then, the structure directing agent
(CTAB) was removed using ethanolic solutions of ammonium
nitrate. This extraction procedure was repeated four times to
yield the final hydrophilic solids SP, SD and SF. In another step,
these template-free hybrid solids were used to prepare the hy-
drophobic materials through a reaction with 1,1,1,3,3,3-hexam-
ethyldisilazane, which transformed the remaining silanol
groups in the solids into the corresponding trimethylsilyl moi-
eties. With this procedure, hybrid solids SPh, SDh and SFh
were obtained (see the Experimental Section for details).
SD
–
–
–
–
0.48
0.48
–
SDh
SF
SFh
–
0.16
The powder X-ray diffraction patterns (PXRD) of solids MD,
SD and SDh are shown in Figure 1, whereas the PXRD for the
remaining solids can be found in the Supporting Information.
The PXRD pattern of the siliceous MCM-41-type as-synthesised
materials (i.e., MP, MD and MF) presented four low-angle re-
flections, which are typical of the hexagonal-ordered array that
can be indexed as (100), (110), (200) and (210) Bragg peaks.
The slight shift of the (100) reflection and the broadening of
the (110) and (200) peaks in the diffraction pattern of the ex-
tracted and hydrophobic samples are clearly seen. This broad-
ening of the (110) and (200) reflections is related with a more
marked reduction in contrast (see the Supporting Information).
Nonetheless, the clear presence of the (100) reflexion in all the
PXRD patterns of the final solids suggests that the functionali-
sation with the fluorophore and the subsequent reaction with
1,1,1,3,3,3-hexamethyldisilazane did not substantially modify
the mesoporous MCM-41 scaffold.
Materials characterisation
All the prepared solids were characterised using standard pro-
cedures. For the organic–inorganic hybrid materials, one key
point associated with their characterisation was the determina-
tion of the degree of functionalisation. The contents of the
functionalised groups (i.e., fluorophore and trimethylsilane) in
the MCM-41 supports were determined from thermogravimet-
ric and elemental analyses (see Table 1).
The presence of the mesoporous structure in the functional-
ised solids was also confirmed by the TEM analysis, in which
the typical MCM-41 type matrix structure was found. By way of
example, Figure 2 shows the TEM images of the SF and SFh
materials (for the TEM images of the other materials, see the
Supporting Information).
Despite the presence of similar amounts of the functional-
ised D, P and F derivatives in the mother liquor during hybrid
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uptake observed at partial pressure P/P0 of 0.28–0.4 clearly in-
dicates that mesopores were uniform in size. The pore size dis-
tributions (PSD) of these samples were calculated by following
the Barret–Joyner–Halenda (BJH) method (see Table 2).[22] The
Table 2. The BET-specific surface areas, pore volumes, pore sizes (calcu-
lated from the N2 adsorption/desorption isotherms), cell parameter and
wall thickness (calculated by X-ray diffraction patterns) for the sensing
materials.
Solid
SBET
[m2 gÀ1
Pore volume
Pore size
[nm][a]
a0
[nm]
Wall thickness
[nm]
[a]
]
[cm3 gÀ1
]
SP
SPh
SD
SDh
SF
SFh
696.6
538.4
1012.6
773.2
436.5
360.4
0.63
0.44
0.92
0.62
0.37
0.22
2.51
2.11
2.84
2.53
2.76
2.57
4.98
4.98
4.62
4.62
4.88
4.88
2.47
2.87
1.78
2.09
1.86
2.31
[a] Volume (V) and diameter (D) of the mesopores.
estimated pore diameters were basically in agreement with
those found from the TEM images. In addition, the specific sur-
face areas of the sensory materials were calculated from the
adsorption data in the low-pressure range using the Brunauer–
Emmett–Teller (BET) model.[23] The specific surface areas, pore
volumes, pore sizes, cell parameter and wall thickness for the
final solids are shown in Table 2. This table clearly reveals that
the specific BET surface areas of the extracted materials (SP,
SD, SF) were larger than those corresponding to the hydro-
phobic counterparts (SPh, SDh, SFh), which is in agreement
with further functionalisation with the bulky trimethylsilane
moieties in the latter. Moreover, the pore size for SP-SPh grad-
ually decreased (from 2.51 to 2.11 nm), SD-SDh (from 2.84 to
2.53 nm) and SF-SFh (from 2.76 to 2.57 nm) also indicated the
effective functionalisation in the hydrophobic supports. Anoth-
er remarkable correlation observed was related with the areas
of the solids containing different fluorophores. Thus, despite
the degree of functionalisation (seen in Table 1), dansyl was
the smallest fluorophore and materials SD and SDh showed
the largest specific surface, whereas materials SF and SFh, con-
taining the bulky fluorescein fluorophore, presented the small-
est specific areas.
Figure 1. Small-angle powder XRD patterns of as-synthesised (MD), extract-
ed (SD) and hydrophobic (SDh) solids.
Figure 2. TEM images of solids SF and SFh showing the typical hexagonal
porosity of the MCM-41 matrix.
The N2 adsorption/desorption isotherms of the extracted
(SP) and hydrophobic material (SPh) are displayed in Figure 3
(see the Supporting Information for the N2 adsorption/desorp-
tion isotherms of SD, SF, SDh and SFh). The obtained iso-
therms can be classified as type IV, which are characteristic of
mesoporous supports. Moreover, the sharp increase in N2
Fluorescence studies
The emission responses of the six prepared materials were
tested in acetonitrile in the absence and presence of Tetryl,
which was selected in this first study as a characteristic nitrat-
ed explosive. In a typical experiment, 1 mg of the correspond-
ing hybrid material was suspended in 3 mL of acetonitrile or in
3 mL of a Tetryl solution (2.0ꢅ10À3 molLÀ1) in the same sol-
vent. In all cases, the partial quenching of emission was ob-
served in the presence of Tetryl. As an example of this behav-
iour, Figure 4 shows the emission profile of the hydrophobic
solids in acetonitrile in the presence of Tetryl.
In particular, upon excitation at 341 nm, the acetonitrile sus-
pensions of SPh showed the typical structured pyrene mono-
Figure 3. The nitrogen adsorption-desorption isotherms for solids SP and
SPh.
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emission band of fluorescein at 525 nm upon excitation at
493 nm. In this case however, addition of Tetryl induced a rela-
tively low 1.4-fold quenching of the fluorescein emission band.
Addition of Tetryl to acetonitrile suspensions of the hydrophilic
materials (SP, SD and SF) also induced quenching of the emis-
sion intensity of the three fluorophores, but this quenching
was less if compared with that observed for hydrophobic
solids. This outcome was most likely the result of the polarity
environment around the fluorophores. Tetryl is a nitroaromatic
explosive with a medium/low polarity that prefers hydrophobic
environments (in fact, its solubility in water is very low). The
presence of highly hydrophobic cavities in the SPh, SDh and
SFh materials would account for the stronger interactions of
Tetryl with the corresponding fluorophore when compared
with the hydrophilic materials, which resulted in greater
quenching efficiency. The quenching observed upon addition
of Tetryl was ascribed to p–p interactions between the elec-
tron donor-grafted fluorophores and the electron-acceptor ni-
troaromatic explosive.[12]
Bearing in mind that the larger quenching results were ob-
tained with hydrophobic solids SPh and SDh, a complete
study of fluorescent behaviour in the presence of other explo-
sives for both materials is detailed here. The experiments were
performed using 1 mg of the selected solid, which was sus-
pended in 3 mL of acetonitrile or in 3 mL of acetonitrile con-
taining explosives pentaerythritol tetranitrate (PETN), hexahy-
dro-1,3,5-trinitro-1,3,5-triazine
(RDX),
1,3,5-trinitrobenzene
(TNB), nitrobenzene (NB), 2,4,6-trinitrotoluene (TNT), 2,4-dini-
trotoluene (DNT), 2,4,6-trinitrophenylmethylnitramine (Tetryl) or
picric acid (PA) at a concentration of 2.0ꢅ10À3 molLÀ1. The
emission intensity of the suspensions (at 476 and 516 nm for
SPh and SDh, respectively) was measured after 1 minute. The
obtained results are shown in Figure 5.
As depicted in Figure 5, addition of PA and Tetryl induced
stronger quenching among all the nitrated explosives tested.
Moderate quenching was observed upon the addition of TNT,
TNB, DNT and NB, whereas negligible changes were measured
for RDX and PETN. Almost the same selectivity trend was
found for both the SPh and SDh sensing materials. The negli-
gible quenching found upon the addition of RDX and PETN is
clearly the result of the chemical nature of these explosives.
RDX is a nitramine, whereas PETN is a nitrate ester, and both
are non-aromatic. This non-aromatic character precludes the
interaction with grafted fluorophores and a poor fluorescent
response was therefore observed. The other nitroaromatic
compounds tested induced several quenching degrees, which
clearly relate with their electron-withdrawing character. As
a general trend, the more nitro groups in their structure, the
greater the quenching measured. Moreover, the selectivity and
sensitivity observed for SPh and SDh could also originate from
a highly cooperative association of PA and Tetryl with the
grafted electron-rich fluorophores. The incorporation of these
fluorophores into the mesoporous matrix induced the forma-
tion of hydrophobic preorganised cavities with sizes that
matches well with those of PA and Tetryl. These cooperative
adsorption processes have been recently used for the selective
sensing and separation of certain species.[25]
Figure 4. The fluorescence spectra of hydrophobic solids SPh, SDh and SFh
alone and after the reaction with Tetryl (2.0ꢅ10À3 molLÀ1).
mer emission band in the 370–420 nm interval together with
a broad unstructured band at 476 nm, which was ascribed to
the excimer emission band of pyrene.[24] The presence of the
latter red-shifted emission indicates that the pyrene groups
come close enough in the SPh solid to yield the excimer com-
plex. Moreover, the hydrophobic environment in the inner of
the pores also favours the interaction between excited pyrenes
and their counterparts in the ground state. Addition of Tetryl
induced a 3.5-fold quenching of both monomer and excimer
emissions. The acetonitrile suspensions of SDh show the typi-
cal broad emission band of the dansyl fluorophore centred at
516 nm upon excitation at 347 nm. In this particular case, addi-
tion of Tetryl also induced a 6-fold quenching of the emission
band. Finally, the suspensions of SFh exhibited the narrow
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Figure 6. Calibration curve of acetonitrile suspensions of solid SDh upon the
addition of increasing quantities of PA.
Figure 5. Fluorescent emission representation of the solid suspensions of
SPh (lex =341 nm, lem =476 nm) and SDh (lex =347 nm, lem =516 nm) in
the presence of different nitroaromatic explosives (2.0ꢅ10À3 molLÀ1). The
inset presents photographs of the suspensions of solids alone (left) and in
the presence of Tetryl (middle) and PA (right).
One interesting feature of both the SPh and SDh solids was
the greater quenching observed with PA, when compared with
that observed for TNT. When comparing this result with the lit-
erature, it is apparent that most of the described chromogenic
and fluorogenic methods were unable to distinguish between
PA and TNT because of their similar structure and chemical
properties.[26] In our case, both hybrid materials SPh and SDh
displayed a differentiable response towards these nitroaromat-
ic explosives and may allow the selective detection of PA. In
fact the sensing systems for PA are greatly demanded for
homeland security needs (PA has strong explosion ability when
compared with TNT, and is capable of exploding with no trig-
ger).[27]
Figure 7. Fluorescence emission recovery for the SPh material upon addition
of Tetryl (2.0ꢅ10À3 molLÀ1) and subsequent nitroaromatic explosive removal
upon washing with THF and NaOH (0.08 molLÀ1).
Complementary studies on explosive desorption were car-
ried out from the SPh material and its potential reusability was
investigated. The SPh solid, which was in contact with an ace-
tonitrile solution of PA (2.0ꢅ10À3 molLÀ1), was filtered and the
solid material was washed with THF (150 mL), NaOH (50 mL,
0.08 molLÀ1), acetone (10 mL) and dried.[28] The solid was then
suspended again and the PA adsorption/desorption cycle was
repeated five times. The recovery results are provided in
Figure 7, which indicates that the SPh material can be reused
at least for five cycles with no significant loss of its recognition
ability. The unique observed effect was the reduced fluores-
cence of the free material after several cycles (ca. 70% of the
initial emission), which is most likely due to a partial hydrolysis
of the grafted pyrene moiety upon washing with NaOH.
Having established the response of SPh and SDh with sever-
al explosives, the limit of detection (LOD) was calculated for
explosives PA and Tetryl, which displayed a more marked re-
sponse. For these studies, 1 mg of the corresponding solid was
suspended in 3 mL of acetonitrile solutions containing differ-
ent concentrations of PA or Tetryl and, after 1 minute, the in-
tensity of the emission band was measured (at 476 and
516 nm for SPh and SDh, respectively). As a representative
result, the fluorescence calibration curve for the SDh-PA
system is shown in Figure 6 (see the Supporting information
for the other calibration curves). The LODs for PA and Tetryl
were calculated to be 1.4 ppm and 8.5 ppm using solid SPh,
whereas they were 1.2 ppm and 14.4 ppm for SDh.
Calculations of Stern–Volmer constants
To quantify the interaction of explosives PA and Tetryl with the
grafted pyrene and dansyl fluorophores in solids SPh and SDh,
the corresponding Stern–Volmer quenching constants were
determined. Figures 8 and 9 depict the emission quenching of
solids SPh and SDh upon the addition of increasing quantities
of PA and Tetryl. As seen in the inset, the changes in emission
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Figure 8. Emission changes of the hybrid SPh solid suspended in acetonitrile
upon the addition of increasing quantities of PA and Tetryl. The inset pro-
vides a plot of F0/F versus CQ, a Stern–Volmer quenching representation.
Figure 9. Emission changes of the hybrid SDh solid suspended in acetoni-
trile upon the addition of increasing quantities of PA and Tetryl. The inset
shows a plot of F0/F versus CQ, a Stern–Volmer quenching representation.
intensity upon the addition of Tetryl are linear, whereas they
are exponential for PA. In the latter case, the Stern–Volmer
constants were calculated within the linear range.
Table 3. Quenching constants for solids SPh and SDh with Tetryl and PA.
[a]
Solid
Explosive
Kd [MÀ1
]
b[b]
R2[c]
SPh
SPh
SDh
SDh
Tetryl
PA
Tetryl
PA
1028
14189
1425
1.02
1.16
0.99
0.92
0.94
0.95
0.95
0.98
Stern and Volmer considered the quenching process to be
a bimolecular reaction that competes with the radiative pro-
cess and all other molecular processes. Accordingly, an equa-
tion which can be expressed as F0/F=1+Kd[Q] [Equation (1)]
can be derived.[29] In the equation, F0 and F are the fluores-
cence intensities in the absence and the presence of a quench-
er at concentration Q, and Kd is the so-called quenching con-
stant. Following this procedure, the Stern–Volmer quenching
constants for SPh and SDh with Tetryl and PA were calculated
and they are included in Table 3.
14110
[a] Kd, the Stern–Volmer quenching constant from [Eq. (1)]. [b] b, the inter-
cept from [Eq. (1)]. [c] R2, the correlation coefficient from [Eq. (1)].
Principal component analysis studies
The values of the obtained Stern–Volmer quenching con-
stants indicate that PA was able to provide the strongest inter-
action with the pyrene or dansyl fluorophores grafted in solids
SPh and SDh. This finding is in agreement with the more elec-
tron-deficient character of PA when compared with Tetryl. It is
also noteworthy that the quenching constants measured for
both solids were of the same order of magnitude as those re-
ported for polymeric materials or molecular-based fluorescence
probes in the presence of nitroaromatic explosives.[30]
The response of the six sensory materials (SP, SD, SF, SPh, SDh
and SFh) towards the seven nitrated explosives tested was un-
selective and different quenching degrees were obtained de-
pending on the solid and explosive used. The response of
a given solid in the presence of a certain explosive therefore
depends on the intrinsic chemical nature of the binding site
(the fluorophore), the shape, size and electron-acceptor charac-
ter of the explosive as the presence, or not, of trimethylsilyl
groups.
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Signalling applications: sensing
explosives in soil samples
After studying the response of
the different solids to nitrated
explosives in solution, we ex-
tended our interest to the possi-
ble use of these materials to
detect explosives in soil samples.
It is known that the removal of
obsolete explosives and muni-
tions has not always been per-
formed in an environmentally
satisfactory manner, and even
today, the analysis of trace levels
of explosive contaminants in
complex environmental matrices
such as soils still requires the de-
velopment of new easy-to-use
sensing protocols.[9] For this
study, solid SDh as support and
Tetryl as a representative explo-
sive were selected. Based on the
research described above, the
Figure 10. Principal component analysis (PCA) score plot using the data set obtained from the response of solids
SP, SD, SF, SPh, SDh and SFh and diverse nitrated explosives (2.0ꢅ10À5 molLÀ1). Data shown from three different
trials.
These subtle differences were reflected in the specific final
quenching observed. This differential response of the solids
was a well-suited prerequisite for pattern recognition. In this
approach the response of solids SP, SD, SF, SPh, SDh and SFh
with the set of explosives can be combined to search for selec-
tive pattern recognition fingerprints. This can be carried out,
for example, when the data matrix is analysed by principal
component analysis (PCA) algorithms.[31] The PCA score plot of
the results for solids SP, SD, SF, SPh, SDh and SFh in the pres-
ence of the nitrated explosives studied was shown in Figure 10
for three different trials.
detection of an explosive can be achieved by simply extracting
the explosive from a contaminated soil sample with acetoni-
trile, by the addition of the SDh sensing support and by taking
additional emission intensity measurements. In one particular
experiment, 10 g of soil were spiked with a known amount of
Tetryl (28.71 ppm) and the dopant was extracted simply with
acetonitrile (3ꢅ15 mL). Then the obtained extract was dried in
a rotary evaporator, the crude was dissolved in 10 mL of aceto-
nitrile and the SDh sensing material was added. After
1 minute, emission was measured upon exciting at 347 nm.
Further addition of Tetryl allowed the detection of Tetryl in the
soil sample following a standard addition method. The results
obtained are shown in Figure 11, which depicts the fluores-
Separate to the response of the explosives a blank sample
(in the absence of analite) was also included. A total of 144
measurements (i.e. 8 sampling dataꢅ3 replicatesꢅ6 sensory
materials) were used to generate the PCA score plot. The first
principal component contained only 60.92% of the variance of
the data, whereas the first three components used in the rep-
resentation (see Figure 10) accounted for 80.12% of total var-
iance. Recognition patterns can be identified clearly for all the
explosives studied, even for explosives that gave a fairly small
response. Moreover, a detailed look at the PCA plot suggests
that there is some correlation between the position of the ex-
plosive clusters on the PC1 axis and their electron-acceptor
ability. Thus, TNB, PETN, RDX, DNT and TNT appear at positive
values of PC1, whereas the Tetryl and PA are placed at negative
PC1 values. In addition, when PC2 and PC3 were also taken
into account a remarkable clustering of all the samples can be
observed. From all the explosives studies PA was the one that
showed a larger distance in the score plot from the blank
sample.
Figure 11. Fluorescence intensity at 516 nm (lexc =347 nm) in acetonitrile
suspensions containing SDh and Tetryl for a solution obtained from a soil
sample. Increasing concentrations of Tetryl were added to the former solu-
tion in order to detect the Tetryl concentration in soil following an addition
standard procedure.
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croscope. The 1H NMR spectra were acquired with a Varian 300
spectrometer (Sunnyvale, CA, USA).
cence response of the SDh-Tetryl system in terms of Tetryl con-
centration after extraction with acetonitrile. This simple proce-
dure determined a concentration of Tetryl of 23.58 ppm (82%
of recovery).
Chemicals
Chemicals 1-pyrenemethylamine hydrochloride (1·HCl), (3-isocyana-
topropyl)triethoxysilane (2), (3-aminopropyl)triethoxysilane (3),
dansyl chloride (4), fluorescein isothiocyanate (5), tetraethylorthosi-
licate (TEOS), n-cetyltrimethylammonium bromide (CTAB), sodium
hydroxide, triethanolamine (TEAH3), triethylamine, sodium sul-
phate, ammonium nitrate, 1,1,1,3,3,3-hexamethyldisilazane (HMDS),
2,4-dinitrotoluene (DNT), nitrobenzene (NB) and picric acid (PA)
were provided by Sigma–Aldrich and used as received. The 3% so-
lution in acetonitrile of the subsequently nitrated explosives 2,4,6-
trinitrophenylmethylnitramine (Tetryl), 2,4,6-trinitrotoluene (TNT),
hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), pentaerythritol tetrani-
trate (PETN) and 1,3,5-trinitrobenzene (TNB) were purchased from
SelectLab Chemicals. Caution! Explosives should be used with ex-
treme caution and handled only in small quantities. Analytical-
grade solvents were acquired from Scharlab (Barcelona, Spain). All
the reagents were used as received.
Conclusion
Six new fluorescent organic–inorganic hybrid MCM-41-type
materials were prepared, characterised and used for the detec-
tion of nitroaromatic explosives. The grafted fluorophores em-
ployed were pyrene, dansyl and fluorescein. Moreover, the po-
larity of inner pores was modulated through the formation of
silyl ether moieties. Of the six materials, three were hydrophilic
(SP, SD and SF) and the rest were hydrophobic (SPh, SDh and
SFh). Addition of nitroaromatic explosives induced several de-
grees of quenching, and the most remarkable were those ob-
tained with the hydrophobic SPh and SDh materials. The
quenching observed was clearly related to the electron-accept-
or character of the nitroaromatic explosives, with the most im-
portant emission change obtained upon addition of PA and
Tetryl. These emission modulations were ascribed to the p–p
stacking interaction between the electron-rich fluorophores
and the electron-deficient nitroaromatic explosives. Materials
SPh and SDh presented fairly good sensitivities to PA and
Tetryl with limits of detection of 1.4 ppm and 8.5 ppm (for
SPh) and of 1.2 ppm and 14.4 ppm (for SDh), respectively. It is
demonstrated that solid SPh can be reused for at least five
cycles with no significant loss of its recognition ability. The
sensing performances (selectivity, limits of detection and
quenching constants) of the hydrophobic SPh and SDh solids
are similar to those measured for other fluorescent materials
(see comparative table in the Supporting Information). More-
over, solid SDh was used to detect trace amounts of Tetryl ex-
plosives from contaminated soil, which might open up the
way for promising applications in detection and remediation in
real samples.
Synthetic methods
Extraction of 1-pyrenemethylamine (1): 1-pyrenemethylamine hy-
drochloride (1·HCl, 500 mg, 1.86 mmol) was dissolved in basic
water (pH>10) and then extracted with diethyl ether. The ether
layer was then dried with Na2SO4 and filtered. The solvent was re-
moved by rotary evaporation and finally dried under vacuum. The
final compound was obtained as a yellow solid.
Synthesis of P: 1 (500 mg, 2.16 mmol) and 2 (536 mL, 2.16 mmol)
were allowed to react in anhydrous acetonitrile (60 mL) at RT
under an inert atmosphere for 24 h. Triethylamine (3 drops) was
used to catalyse the reaction. The final yellow solid (P, 806 mg,
1.68 mmol, 77%) was obtained by evaporating the solvent and
then drying with a vacuum pump for 1 h. 1H NMR (400 Mhz,
DMSO): d=0.53 (t, 2H, J=7.0 Hz, Si-CH2-CH2), 1.09 (t, 9H, J=7.0 Hz,
CH3-CH2-O), 1.44 (q, 2H, J=7.0 Hz, Si-CH2-CH2), 3.03 (m, 2H, CH2-
NH-CO), 3.72 (q, 6H, J=7.0 Hz, CH3-CH2-O), 4.95 (d, 2H, J=5.5 Hz,
Pyr-CH2-N), 5.98 (t, 1H, J=5.5 Hz, NH-CO-NH), 6.47 (t, 1H, J=5.5 Hz,
NH-CO-NH), 8.10-8.40 ppm (m, 9H, Pyr); HRMS (ESI): m/z calcd for
C27H34N2O4Si: 478.2288; found: 479.2343 [M+H+].
Experimental Section
Synthesis of D: 3 (575 mL, 2.45 mmol), 4 (1 g, 3.71 mmol), and trie-
thylamine (516 mL, 3.69 mmol) were heated at reflux for 2 h in dry
dichloromethane under an inert atmosphere. After the reaction
was complete the solvent was evaporated by rotary evaporation
and then dried with a vacuum pump. The final product (D, 1.25 g,
Methods
Powder XRD, thermogravimetric analyses (TGA), TEM images and
N2 adsorption/desorption studies were employed to characterise
synthesised materials. Fluorescence spectroscopy was used for ki-
netic and quenching studies of the materials after interactions with
different explosives. Powder X-ray diffraction measurements were
carried out on a Bruker D8 Advance diffractometer using CuKa radi-
ation. The thermogravimetric analyses were carried out on a TGA/
SDTA 851e Mettler Toledo balance using an oxidant atmosphere
(air, 80 mLminÀ1) with a heating programme consisting of a ramp
of 108CminÀ1 from 208C to 10008C, and an isothermal heating
step at this temperature for 30 minutes. The N2 adsorption/desorp-
tion isotherms were recorded with a Micromeritics ASAP2010 auto-
mated sorption analyser. Samples were degassed at 1208C in
vacuum overnight. Specific surface areas were calculated from the
adsorption data in the low pressures range using the BET model.
Pore size was determined following the BJH method. Fluorescence
spectroscopy was carried out with a JASCO FP-8500 spectrofluor-
ometer. TEM images were obtained with a 100 kV Philips CM10 mi-
1
2.83 mmol, 77%) was obtained as an orange oil. H and 13C NMR
data are fully coincident with those reported in the literature.[21a]
Synthesis of F: 5 (500 mg, 1.28 mmol) and 3 (299 mL, 1.28 mmol)
were allowed to react in anhydrous ethanol in the dark for 4 h. An
orange yellow product (F, 801 mg, 1.31 mmol, 98%) was obtained
after evaporating the solvent and subsequent drying under
vacuum. 1H and 13C NMR data are fully coincident with those re-
ported in the literature.[21b]
Synthesis of SP: The synthetic strategy for preparing hybrid MCM-
41-type materials was an application of the “atrane” route with
a minor experimental modification. Here TEOS and P were used
(TEOS/P 99:1) instead of only TEOS. The molar ratio of the reagents
in the mother liquour was fixed at 3.5:1:0.26:0.25:4.45 for TEAH3/
silica source (TEOS-P 99:1)/CTAB/NaOH/H2O. In a typical synthesis,
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CTAB (4.67 g, 12.8 1 mmol) was added at 1188C to a solution of
TEAH3 (25.76 g, 173 mmol) containing NaOH (0.48 g, 12 mmol) and
then a silatrane derivative (10.89 mL, 48.77 mmol of TEOS and
0.24 g, 0.5 mmol of P) was added to TEAH3 at 708C; for example,
in the form of Si(TEA)(TEAH2), where TEA was the fully deprotonat-
ed ligand. Then deionised water (80 mL) was added with vigorous
stirring at 708C. After some minutes, a yellowish suspension was
obtained. This mixture was aged at RT for 2 h and was finally auto-
claved at 1008C for 36 h. The resulting as-made powder (MP) was
collected by filtration and repeatedly washed with water until the
filtrate reached pH 6–7, and the solid was subsequently dried at
708C. Then the structure-directing agent (CTAB) was removed
using ethanolic solutions of ammonium nitrate. Typically, MP (1 g)
was exchanged by stirring with NH4NO3 (0.3 g) in ethanol (150 mL)
at 608C for 15 min. Then it was filtered and repeatedly washed
with cold ethanol and finally dried in an oven at 408C for 12 h. Fi-
nally, solid SP was obtained by repeating this exchange procedure
four times.
10À4 molLÀ1) with acetonitrile were added in five different samples,
and the final volume of each sample was adjusted to 10 mL by
adding acetonitrile. Finally, 3 mL of each sample solution were
added to 1 mg of SDh, stirred for 1 min, and the fluorescent emis-
sion was measured by exciting at 347 nm.
Acknowledgements
Financial support from the Spanish Government (Project
MAT2012–38429-C04–01) and the Generalitat Valencia (Project
PROMETEO/2009/016) is gratefully acknowledged. Y.S. is grateful
to the Spanish Ministry of Science and Innovation for her grant.
I.C is also grateful to the Universidad Politꢃcnica de Valencia for
her grant.
Keywords: fluorogenic recognition · fluorophores · hybrid
materials · mesoporous materials · nitroaromatic explosives
Synthesis of SD: The synthetic procedure was the same used for
the preparation of solid SP. The only difference lies with the use of
D as the silica source. SD was obtained after the removal of surfac-
tant from the as-synthesised material (MD) following the same pro-
cedure used for SP.
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added to 3 mL of acetonitrile. The solution was stirred for 1 min
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Soil extraction experiment
Ten grams of finely ground homogeneous soil sample was doped
with a known concentration of Tetryl (10 mL of concentration 1.0ꢅ
10À4 molLÀ1). After 3 h of stirring, this dopant was extracted with
acetonitrile (3ꢅ15 mL). Then the obtained extract was dried in
a rotary evaporator until the solvent was eliminated. The crude ex-
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Received: April 19, 2013
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