A mesoporous, silica-immobilized-nanoparticle colorimetric chemosensor for the detection of nerve agents

Article abstract of DOI:10.1002/adfm.201100968

A new and an easy-to-make colorimetric azo-pyridine, 1, and its recyclable mesoporous silica-immobilized nanoparticles for nerve-agent detection are synthesized. The binding site, comprising an azo-pyridine moiety, is capable of selectively sensing diethylchlorophosphate (DCP), one of the nerve-agent mimics of chemical-warfare agents, over a series of other phosphate compounds. Compound 1 shows ratiometric changes in absorption spectroscopy to the extent of a 60 nm red-shift upon the addition of DCP, mainly due to a change in the intramolecular charge transfer (ICT) in 1 . The color change of receptor 1 from yellow to red in the concentration region = 1.0 × 10 ^-6 M is sufficient for the selective detection of the DCP nerve-agent mimic by the naked eye. With regards to solid-phase application, mesoporous silica nanoparticles using 1 ( MSIAP ) are also prepared using a sol-gel grafting reaction. The color of the MSIAP also changes from red to yellow when dipped into an aqueous DCP solution, and turns back to red when treated with NaOH solution, with nontoxic diethylphosphoric acid being given off. The absorption changes of MSIAP in the presence of DCP are consistent within the 3-9 pH range.

Full text of DOI:10.1002/adfm.201100968

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A Mesoporous, Silica-Immobilized-Nanoparticle
Colorimetric Chemosensor for the Detection of Nerve
Agents
Hyun Jung Kim, Ji Ha Lee, Hyejin Lee, Jae Hong Lee, Joung Hae Lee, Jong Hwa Jung,*
a n d Jong Seung Kim*
are fast-acting poisons that attack the
nervous system. Among chemical-warfare
agents, nerve agents are extremely dan-
gerous and their high toxicity and ease of
A new and an easy-to-make colorimetric azo-pyridine, 1, and its recyclable
mesoporous silica-immobilized nanoparticles for nerve-agent detection are
synthesized. The binding site, comprising an azo-pyridine moiety, is capable
of selectively sensing diethylchlorophosphate (DCP), one of the nerve-agent
mimics of chemical-warfare agents, over a series of other phosphate com-
pounds. Compound 1 shows ratiometric changes in absorption spectroscopy
to the extent of a 60 nm red-shift upon the addition of DCP, mainly due to a
change in the intramolecular charge transfer (ICT) in 1. The color change of
receptor 1 from yellow to red in the concentration region ≈1.0 × 10−6 M is suf-
ficient for the selective detection of the DCP nerve-agent mimic by the naked
eye. With regards to solid-phase application, mesoporous silica nanoparticles
using 1 (MSIAP) are also prepared using a sol-gel grafting reaction. The color of
the MSIAP also changes from red to yellow when dipped into an aqueous DCP
solution, and turns back to red when treated with NaOH solution, with non-
toxic diethylphosphoric acid being given off. The absorption changes of MSIAP
in the presence of DCP are consistent within the 3–9 pH range.
production underscore the need to detect
these lethal chemicals via quick and reli-
able procedures.[1,2]
Nerve agents, a class of phosphorus-
containing organic chemicals that disrupt
the mechanism by which nerves transfer
messages to organs, have been studied
extensively due to their rapid and severe
effects on human- and animal-health
systems.[3] Up to now, many detection
methods for nerve agents have been devel-
oped based on fluorogenic, colorimetric,
and enzymatic methods.[4,5] Nevertheless,
these methods suffer from some limita-
tions, such as slow response, lack of spe-
cificity, limited selectivity, low sensitivity,
difficulties in real-time monitoring, and
water solubility.[4,5] In particular, since the
1. Introduction
nerve-agent mimic is easily hydrolyzed in water within a few
minutes, non-aqueous solutions should be used in most cases
for the detection of nerve agents, in order to look at absorption
and emission spectrophotometric changes, which greatly limits
analytical application in real samples.[4,5] Therefore, simple and
efficient detection methods for nerve agents in water media
remain of great interest these days.
Silica-based nanoparticles are of great interest for biomedical-
and environmental-research applications such as bioseparation,
drug targeting, cell isolation, enzyme immobilization, and pro-
tein purification because of their biocompatibility and stability
against degradation.[6–10] It is clear that receptor-immobilized
nanoparticles have some important advantages as solid chemo-
sensors. Firstly, such nanoparticles are readily synthesized by
sol-gel condensation, a versatile technique that permits the
presence of chemical functionalities. Secondly, immobilized
receptors on an inorganic support can remove guest molecules
(toxic metal ions and anions) from the pollutant solution.
Thirdly, the nanoparticles can be easily isolated from pollutants
by filtration and repeatedly utilized with suitable treatment. In
this aspect, the homogeneous porosity and large surface area of
mesoporous silicas (SBA-15 and MCM-41) make them prom-
ising as inorganic supports.[11–14] If hybrid nanomaterials can
A chemical warfare agent is a substance “intended for use in
military operations to kill, seriously injure, or incapacitate man
because of its physiological effects.” In the modern era, chemical-
warfare agents have been divided into five categories: nerve
agents, vesicants, choking agents, blood agents, and incapaci-
tants.[1,2] The development of chemical-warfare agents during
the Second World War led to the so-called “nerve agents”, which
H. J. Kim, J. H. Lee, J. S. Kim
Department of Chemistry
Korea University
Seoul 136-701, Korea
E-mail: jongskim@korea.ac.kr
J. H. Lee, H. Lee, J. H. Jung
Department of Chemistry and Research Institute of Natural Science
Gyeongsang National University
Jinju 660-701, Korea
E-mail: jonghwa@gnu.ac.kr
J. H. Lee
Korea Research Institute of Standards and Science
Daejon 305-600, Korea
DOI: 10.1002/adfm.201100968
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As a first step, the reactivity of 1 with DCP,
PMP, DMMP, TEP, and TBP in acetonitrile
was tested. These organophosphates have
been widely used as simulants, as they dis-
play reactivities similar to those of nerve
agents such as Tabun, Sarin and Soman, yet
they lack their toxicity.[3]
The absorption spectra of the stable trans
isomers of the azo-pyridine chemosensor,
1, exhibit an intense peak at 345 nm (ε =
2.3 × 104 −1 cm−1 ), which corresponds to the
M
π–π* transition.[16] Upon addition of 10 equiv
of DCP to a solution of 1, the absorption
band was red-shifted to 405 nm (ε = 2.5 ×
−1
104
M
cm−1 ), which resulted in a modula-
tion from yellow to red in organic media
(Figure 1a). This spectroscopic-change event
is obviously due to enhanced intramolecular
charge transfer (ICT). The enhancing ICT,
Scheme 1. Chemical structures of 1–4 and MSIAP.
0.6
0.4
0.2
0.0
a
be provided as portable systems for chemosensors,[15] silica-
based hybrid nanoamaterials will be easily able to detect spe-
cific guest molecules in biological and environmental fields,
and thus could be utilized as recyclable sensors.
1, PMP, DMMP, TEP, TBP
DCP
We have investigated a method of detecting nerve agents
by using a nerve-agent mimic, diethylchlorophosphate (DCP),
which functions as an acetylcholinesterase inhibitor, but
which is rather less harmful to the human body than real
nerve agants.[5d] In real nerve agents, this inhibition has been
known to lead to paralysis and death.[3] The desired probe for
DCP would therefore be selectively unresponsive to much-
less-harmful agents that lack reactive acyl or phosphoryl halide
groups, such as pinacolylmethylphosphonate (PMP), dimethyl-
methyl phosphonate (DMMP), triethylphosphate (TEP) and
tributylphosphate (TBP).
200
300
400
500
600
Wavelength (nm)
Herein we present a synthesis of azo-pyridine (1), which
contains a colorimetric mesoporous silica-immobilized chem-
osensor, and its highly selective UV–vis spectrophotometric
changes, along with changes detectable by the naked-eye for DCP.
The organic-inorganic hybrid mesoporous silica-immobilized
azo-pyridine based on 1 (MSIAP) that we have developed is the
first recyclable colorimetric sensor that can be used in 100%
aqueous solution for DCP sensing.
0.6
0.4
0.2
0.0
b
0 eq~ 20 eq
2. Results and Discussion
Azo-compound 1[16] was prepared by the reaction of 2-hydrazi-
nopyridine with 1,4-benzoquinone in acidic media. Reference
compounds 2[17] and 3[18] were also prepared using literature pro-
cedures reported previously. Compound 4 was also prepared by
the reaction of 1 with 3-triethoxysilylpropylisocyanate in CH3CN.
Subsequently, MSIAP was synthesized by coupling 4 with mes-
oporous silica in toluene. All of the chemical structures of 1–4
300
400
500
600
700
Wavelength (nm)
Figure 1. a) UV–vis spectra of 1 (20.0 × 10−6 M) upon addition of DCP,
PMP, DMMP, TEP, and TBP (10 equiv) in CH3CN. b) UV–vis titration
spectra of 1 (20.0 × 10−6 M) upon addition of various amounts of DCP
in CH3CN.
1
and MSIAP (Scheme 1) were confirmed by H-NMR spectros-
copy, 13C-NMR spectroscopy, and fast-atom-bombardment mass
spectrometry (FAB-MS) (see Supporting Information).
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Weak ICT
Strong ICT
Scheme 2. Proposed mechanism of ICT change of 1 in DCP sensing.
promoted by the strong push-pull effect between the electron-
donating oxygen (OH) and the electron-accepting pyridinium
group,[19] exerts a 60 nm bathochromic shift of the maximum
wavelength in the UV–vis spectrum. Figure 1b gives the
detailed absorption changes of 1 upon gradual titration of DCP
(0–20 equiv) (Scheme 2) .
In acidic solution, 1 showed a strong absorption band cen-
tered at 405 nm, as seen in Figure S1, Supporting Information,
which supports the notion that the formation of the pyridinium
ion of 1 enhances the ICT when it reacts with DCP. The new
band at 405 nm is also similar to that of a 2-pyridine nitrogen,
methylated azo compound that was previously reported by
Velasco et al.[16]
To elucidate the role of the pyridine nitrogen atom in the
nucleophilic interaction with 1 on the ICT development, we
synthesized two structural analogues, 2 and 3. We found how-
ever that 2 did not show any spectral changes on DCP addition,
implying the DCP binding to 1 is dominated by the presence
of the pyridine nitrogen atom not by the phenolic OH group.
Compound 3, which does not have an electron-donating group
such as OH, showed smaller spectral changes (20 nm) in the
presence of DCP than 1 (Figure S2, Supporting Information);
this firmly demonstrates that the push-pull-mediated ICT from
the OH to the pyridine N atom is of critical importance in DCP
detection.
The 1:1 binding was evidenced by a peak at m/z = 337.2
(calcd 337.12), corresponding to [1 + DCP + H]2+ in the
electrospray-ionization mass-spectrometry (ESI-MS) spectrum
of a mixture of 1 and 20 × 10−6 M DCP (Figure S3, Supporting
Information).
420
400
380
360
340
0
1
2
3
4
5
6
7
Figure 2. Plot for the absorbance of MSIAP by alternated dipping in a
1.0 × 10−6 M aqueous solution of DCP and 10.0 × 10−6 M NaOH treatment.
The cyclic index is the number of alternating dipping/rinsing cycles, with
the vertical axis showing the absorbance for the MSIAP at 405 nm.
In consideration of extending its usefulness, MSIAP was
fabricated by a sol-gel reaction of 4 with mesoporous silica.
Immobilization of the receptor 4 was conducted under reflux
conditions for 24 h in toluene to give the MSIAP. In this process,
the triethoxylsilyl group of 4 undergoes hydrolysis and covalently
attaches to the surface of the mesoporous silica. After cooling
to room temperature, the red solid product was filtered, washed
with tetrahydrofuran (THF), and then dried overnight. The
MSIAP was characterized by transmission electron microscopy
(TEM), Fourier transform IR (FTIR) spectroscopy, Brunauer–
Emmett–Teller (BET) isotherms, time-of-flight secondary-ion
mass spectrometry (TOF-SIMS), and X-ray photoelectron spec-
troscopy (XPS).
It is well known that strong bases such as NaOH can react
with phosphates (DCP or DFP) to make a less-toxic hydrolysis
product (phosphoric acid).[20] From the absorbance changes and
color revival from red to yellow, we also confirmed that the dis-
sociation of DCP from the 1-DCP complex occurs easily with
NaOH treatment, to return to the original 1, making the com-
pound recyclable (Figure 2) .
In Figure 3, the TEM picture of MSIAP clearly shows the for-
mation of a well-ordered hexagonal arrangement of mesoporous
channels after attaching 4. To investigate the porosity changes
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water was initially red in color. In the pres-
ence of DCP, however, the suspension of
MSIAP showed a color change from red to
yellow. On the other hand, no significant
changes in absorption were observed in par-
allel experiments with PMP, DMMP, TEP,
and TBP (Figure 4). From these spectro-
scopic changes, we noticed that the MSIAP
reveals a high selectivity for DCP, showing
a similar spectroscopic response to that of
1, mentioned eariler. The results imply that
the MSIAP is considerably applicable to
the environmental field as a new organic-
inorganic hybrid sensor for the detection of
the nerve agent, DCP. An evaluation of the
Figure 3. a) Photograph of the MSIAP. b) TEM image of the MSIAP.
time course for the absorption change of
of the mesoporous silica induced by the introduction of 4, we
measured the surface areas, pore volumes, and pore diameters
of both the mesoporous silica and the MSIAP by way of their
nitrogen adsorption-desorption isotherms (Figure S4, Sup-
porting Information). The mesoporous silica had a BET surface
area of 1050.8 m2 g−1 and a pore volume of 0.47 cm3 g−1 . On
the other hand, we observed that the MSIAP had a BET surface
area of 850.35 m2 g−1 and a pore volume of 0.32 cm3 g−1 . The
mesoporous silica and the MSIAP had Barrett–Joyner–Halenda
(BJH) pore diameters of 8.53 and 7.20 nm, respectively. The
decreased surface area and pore diameter in MSIAP are attribut-
able to the attachment of 4 to the mesoporous silica (Figure S4,
Supporting Information).
MSIAP at 405 nm (Figure S9, Supporting Information) indi-
cates that immediately after the addition of DCP, the absorp-
tion intensity of MSIAP starts to increase and that, by 30 s,
the absorption intensity is almost saturated. That is, this fast
response time makes it a rapid and convenient method for the
quantification of DCP in aqueous solution.
In order to extend the above performance to a portable
chemosensor kit, a disk-type pellet was prepared from MSIAP
(Figure 5). The red color of the disk-type pellet of MSIAP
changed to yellow when dipped in DCP (10 × 10−6 M) aqueous
solution. On the other hand, no significant changes in absorp-
tion were observed in parallel experiments in the cases of PMP,
Moreover, for further structural proof of the MSIAP, IR spec-
troscopy for both mesoporous silica and MSIAP was performed.
For the mesoporous-silica nanoparticles, IR peaks appeared at
3450, 1658, and 1084 cm−1 , whereas peaks appeared at 3010,
2976, 2933, 1646, 1633, 1540, 1471, 1446, 1428, and 1382 cm−1
for the MSIAP (Figure S5, Supporting Information), with the
new peaks originating from the azo-pyridine, 4, giving solid
evidence that 4 was attached to the surface of the mesoporous
silica.
The TOF-SIMS spectrum of the MSIAP displayed fragments
of 4-3OEt + 3H (m/z = 316), thereby providing a proof that 4-
3OEt + 3H was anchored onto the surface of the silica particles
(Figure S6, Supporting Information). The mesoporous-silica
nanoparticles before and after functionalization of 4 were fur-
ther confirmed by XPS (Figure S7, Supporting Information).
The XPS spectrum of the mesoporous-silica nanopaticles
before immobilization of 4 showed the Si2P and O1S binding
energies (Figure S7a, Supporting Information), whereas the C1S
and N1S binding energies for carbon and nitrogen, respectively,
appeared for MSIAP (Figure S7b, Supporting Information). We
further measured the TGA thermogram of MSIAP. The TGA
results (Figure S8, Supporting Information) showed that the
MSIAP consisted of only 35.5 wt% of receptor 4.
Nerve-agent-sensor development under aqueous conditions
is hampered by the instability of suitable chloride-containing
electrophiles (DCP is hydrolyzed in water buffered to a pH of 7
within a few minutes). An important advantage of the MSIAP
in our study is that it is more practically accessable: it is a
100%-water-soluble and recyclable nerve-agent detector. In the
absence of nerve-agent simulants, a suspension of MSIAP in
Figure 4. UV–vis spectra of MSIAP (20.0 × 10−6 M) upon addition of DCP,
PMP, DMMP, TEP, and TBP (10 equiv) in H2O.
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to less-toxic molecules after treatment with NaOH, and then
turns back to the original, recyclable MSIAP. The azo-pyridine
motif, as an ICT framework for DCP sensing, is a new field and
other nerve-agent sensors can be easily designed based on the
present article as well.
4. Experimental Section
Instruments and Reagents: The UV–vis absorption spectra were
recorded using a Sinco S-3100 spectrophotometer. The NMR and mass
spectra were recorded using a Varian (400 MHz) NMR spectrometer and
a JMS-700 MStation mass spectrometer, respectively. The TEM images
were captured using a JEOL JEM-2100 F microscope. The nitrogen-
adsorption isotherms were measured at 78 K using a Micromeritics
ASAP 2010 analyzer. TOF-SIMS was performed using a Model PHI
7200 instrument equipped with Cs- and Ga-ion guns for positive- and
negative-ion mass detection. The XPS spectra were recorded using a
Thermo VG Scientific spectrometer.
Figure 5. Photogram of MSIAP (10 mg) (a), and the MSIAP in the pres-
ence of DCP (b), DMP (c), DMMP (d), TEP (e), and TBP (f) (10 equiv).
The reactants, 2-hydrazinopyridine, 1,4-benzoquinone, phenol,
sodium nitrite, aniline, and the organophorphorus compounds of DCP,
PMP, DMMP, TEP, and TBP were purchased from Aldrich and used as
received. All of the solvents were analytical-reagent-grade chemicals
from Duksan Pure Chemical Co., Ltd. For the absorbance spectra, the
MeCN used was HPLC-reagent grade.
Preparation of 4: Under nitrogen, a solution of 1 (0.1 g, 0.5 mmol)
and 3-triethoxysilylpropylisocyanate (0.125 g, 0.5 mmol) was suspended
in dry CHCl3 (10 mL). Et3N (10 mL) was then added and the mixture
was refluxed under N2 for 48 h. The mixture was evaporated by
rotary evaporation and the crude product was purified by column
chromatography (silica, ethyl acetate/hexane 1:4) to give 0.2 g of 4 (89%
yield).
DMMP, TEP, and TBP solutions (10 × 10−6 M). In addition, the
solid fluorescence spectrum of the disk-type pellet of MSIAP
with DCP was the same as that obtained from MSIAP dispersed
in aqueous solution. The fluorescence change was found to be
fully reversible when the disk-type pellet prepared from MSIAP
was rinsed using NaOH (10 × 10−6 M) solution. This result
implies that the disk-type pellet prepared from MSIAP is appli-
cable as a portable chemosensor for the detection of DCP in the
biological and environmental fields.
The highly selective DCP recognition of MSIAP demon-
strates that the approach employed in the present study is
capable of co-operatively enhancing and controlling the selec-
tivity towards DCP. More importantly, quantitative measure-
ments of the absorption maximum of DCP-bound MSIAP
indicate that the absorption change correlates linearly with the
[DCP] over the 17–84 ppb range investigated (Figure S10, Sup-
porting Information). We determined that the detection limit of
MSIAP for DCP is 0.53 ppb.
1H NMR (400 MHz, CDCl3, δ): 8.70 (d, J = 4.8 Hz, 1H), 8.04 (d, J =
8.9 Hz, 2H), 7.86 (t, J = 8.0 Hz, 1H), 7.78 (d, J = 8.0 Hz, 1H), 7.37 (t,
J = 4.8 Hz, 1H), 7.28 (d, J = 8.8 Hz, 2H), 5.65 (t, J = 5.8 Hz, 1H), 3.76
(q, 6H), 3.23 (q, 2H), 1.67 (q, 2H), 1.22 (t, J = 8.2 Hz, 9H), 0.68 (t, J =
8.2 Hz, 2H); 13C NMR (100 MHz, CDCl3, δ): 162.9, 154.4, 154.1, 149.6,
138.6, 125.1, 125.0, 122.3, 115.7, 58.7, 43.8, 31.8, 23.2, 18.5, 14.3, 7.9;
FAB-MS (m/z): [M + H]+ calcd for C21H31N4O5Si, 447.58; found 447.3.
Preparation of Mesoporous Silica: Mesoporous silica was synthesized
starting from the preparation of
a hydrochloric acid solution of
poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene
oxide) triblock copolymer (P-123). Tetraethyl orthosilicate (TEOS) was
then added and the mixture was stirred at 40 °C for 20 h. The molar
composition was 1:5.9:193:0.017 TEOS:HCl:H2O:P-123. The solid
was aged at 65 °C for 1 d and was then filtered, washed and dried at
90 °C. To cleave the template to generate the mesopores, 1.0 g of the
as-synthesized SBA-15 silica was mixed with 100 mL of 60 wt% H2SO4
solution and refluxed at 95 °C for 1 d. The product was recovered by
washing with water and was dried at 90 °C. To generate the mesopores,
the acid-treated sample was heated to 200 °C in air. To remove any
cationic surfactants from the resulting dried, fiber-like flocculates and
particles, the sample was calcined in a box furnace in air at 500 °C for
We have also investigated the effect of pH on the spectro-
photometric behaviour of MSIAP in both the absence and the
presence of DCP, because, ideally for biological and environ-
mental applications, sensing should be practical over a range
of pH values. Over the pH range from 3 to 9, MSIAP showed
no absorption in the absence of DCP, whereas upon addition
of DCP, MSIAP displayed almost equal absorption intensities
at all pH values investigated within this range. These results
clearly confirm the suitability of MSIAP for use in physiological
environments spanning the 3–9 pH range (Figure S11, Sup-
porting Information).
5 h, at a ramp rate of 1 °C min−1
.
Immobilization of Receptor 4 onto Mesoporous Silica: Compound
4 (100 mg) was dissolved in toluene (10 mL). The mesoporous silica
(100 mg) was added as a solid. The suspension of silica was stirred
under reflux conditions for 24 h in toluene. Then, the collected solid was
washed copiously with toluene (50 mL) to rinse away any surplus 4 and
dried under vacuum.
3. Conclusions
We synthesized chromogenic azo-pyridine (1) and its recyclable
MSIAP for the detection of the nerve-agent DCP. The MSIAP
shows a high sensitivity and selectivity for DCP over other orga-
nophosphorus compounds, with respect to absorption changes
in 100% aqueous solution, and the corresponding color changes
are easily recognizable by the naked-eye. Moreover, MSIAP not
only detects the toxins, but can simultaneously decompose them
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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Acknowledgements
This work was supported by a grant from the CRI program (No. 2011-
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