Detection of TNT based on conjugated polymer encapsulated in mesoporous silica nanoparticles through FRET

Article abstract of DOI:10.1039/c2cc16115j

Amine-functionalized mesoporous silica nanoparticles containing poly(p-phenylenevinylene) provide a facile strategy to detect TNT through fluorescence resonance energy transfer (FRET). The observed linear fluorescence intensity change allows the quantitat

Full text of DOI:10.1039/c2cc16115j

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COMMUNICATION
Detection of TNT based on conjugated polymer encapsulated in
mesoporous silica nanoparticles through FRETw
Lijuan Feng, Hui Li, Ying Qu and Changli Lu*
¨
Received 1st October 2011, Accepted 6th January 2012
DOI: 10.1039/c2cc16115j
Amine-functionalized mesoporous silica nanoparticles containing
poly(p-phenylenevinylene) provide a facile strategy to detect TNT
through fluorescence resonance energy transfer (FRET). The
observed linear fluorescence intensity change allows the quantitative
detection of TNT with the detection limit of 6 Â 10À7 M.
2,4,6-Trinitrotoluene (TNT) is a worldwide concern for
environmental safety due to its highly explosivity and environ-
mental hazard. The selective and sensitive detection of TNT
has attracted much attention in recent years.1 It is well known
that the fluorescent technique has the strongpoints of high
signal output and simplicity.2 Therefore, increasingly the
fluorescence technique is used to detect TNT vapor or
solution. So far there have been several fluorescence materials
to detect TNT based on conjugated polymers, fluorescent dye
molecules, quantum dots, anti-TNT antibodies and hybrid
assemblies formed between these materials.3 Among these
materials, conjugated polymers are usually used to detect
TNT vapor through an electron transfer (ET) mechanism
because of their strongly luminescent properties and high
permeabilities to small molecular nitroaromatic analytes.4
However, given their poor solubility in solution and that
electron transfer will be hindered by the solvent, use of this
type of material for the detection of explosives in solution
environments through electron transfer is limited. Yang5 and
Trogler6 designed several conjugated polymer and silica hybrid
materials which can detect TNT in solution through ET. The
incorporation of conjugated polymers into an inorganic matrix
is an effective method to improve the environmental stability.
Silica nanoparticles are particularly suited for the realization of
the fluorescent-based chemosensors, because they are optically
transparent and photophysically inert, and their surface can
easily be modified by the coupling reactions with alkoxysilane
derivatives.7
Scheme 1 Schematic illustration of PPV@MSN-NH2 as a selective
detector for TNT through FRET.
composed of two moieties, a fluorescent moiety and TNT
binding moiety. For the fluorescent moiety, we introduced
poly(p-phenylenevinylene) (PPV) into mesoporous silica nano-
particles (MSNs) to fabricate the hybrid nanoparticles
(PPV@MSNs) by an ion-exchange and in situ polymerization
route. For selective TNT recognition and binding, 3-amino-
propyltrimethoxysilane (APTES) was used owing to its excellent
selectivity for TNT.8 The amino groups of APTES were modified
on the inner wall of PPV@MSNs through ‘‘post-grafting’’ to
form PPV@MSN-NH2. In the presence of TNT, TNT and
amino groups can form Meisenheimer complexes (TNT–amine
complexes)9 between the electron-deficient aromatic rings and
electron-rich amine ligands through a charge-transfer com-
plexing interaction (Scheme 1). The resultant amino groups of
PPV@MSN-NH2 can chemically recognize and absorb TNT
molecules by the formation of the Meisenheimer complex.
This complex absorbs the green part of visible light, and
strongly suppresses the fluorescence emission of the chosen
PPV through FRET in the mesoporous channels.
The synthetic route for the PPV@MSN-NH2 sensor is
shown in Scheme S1 (ESIw). MSNs were prepared through
hydrolysis of tetraethoxysilane by using cetyltrimethylammonium
chloride (CTACl) as template. p-Phenylenedimethylene
bis(tetramethylene sulfonium chloride) was then introduced
into the mesopores of the silica nanoparticles by ion-exchange
and in situ polymerization, and then the template was removed,
forming PPV@MSN. In the 13C NMR spectra (Fig. S1, ESIw),
the characteristic peaks of PPV (d 126.5 and 136.0 ppm)10
were observed (Fig. S1b, ESIw), indicating that the PPV was
Here, we, for the first time, designed and synthesized novel
fluorescent inorganic–organic hybrid mesoporous nanoparticles
that can selectively detect TNT through fluorescence resonance
energy transfer (FRET). As shown in Scheme 1, this probe is
Institute of Chemistry, Northeast Normal University,
Changchun 130024, P. R. China. E-mail: lucl055@nenu.edu.cn
w Electronic supplementary information (ESI) available: Experimental
details, preparation scheme, 13C and 29Si NMR, FT-IR, XRD and
TGA curves. See DOI: 10.1039/c2cc16115j
c
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Chem. Commun., 2012, 48, 4633–4635 4633

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Fig. 2 Absorption spectra of APTES-TNT (a) and PPV@MSN-NH2
(b); excitation (c) and emission (d) spectra for PPV@MSN-NH2 as an
ethanol dispersion.
Fig. 1 TEM images of PPV@MSN (a), PPV@MSN-NH2 (b) and
rings leads to the formation of a Meisenheimer complex as
mentioned above, which can strongly absorb the green part of
visible light.15 As shown in Fig. 2, the absorption spectrum of
TNT-APTES solution shows an obvious absorption with lmax
at 500 nm in ethanol (Fig. 2a). However, PPV@MSN-NH2
without TNT shows only a weak band edge absorption at this
position (Fig. 2b). This shows that the interaction causing the
peak at 500 nm is due to the interaction between amine and
TNT. Comparing to the excitation spectrum (Fig. 2c), the rising
baseline at shorter absorption wavelengths of PPV@MSN-NH2
(Fig. 2b) may be due to Rayleigh scattering of the sample.13 The
fluorescence of the PPV@MSN-NH2 sensor was measured in
ethanol, as shown in Fig. 2c and d. The excitation of the sample
suspension at 373 nm leads to emission at 491 and 520 nm. These
two peaks are the characteristic emission peaks of PPV. So
PPV as an energy donor, has a spectral overlapping with the
absorption peak of TNT–amine complexes at 500 nm. This
suggests that the FRET from PPV to the TNT-amine complexes
may occur if they are spatially close to each other. The pores
of nanoparticles can absorb TNT, thereby, narrowing the
distance between TNT and PPV. So using PPV@MSN-NH2
to detect TNT fully complies with the requirements of FRET
mechanism.
relevant nitrogen adsorption-desorption isotherms (c, d).
loaded in MSNs. The amount of PPV inside the MSNs is
estimated to be about 9 wt% by thermogravimetric analysis
(Fig. S2, ESIw). APTES molecules were immobilized on the
inner walls of PPV@MSN through a ‘‘post-grafting’’ reaction,
to form PPV@MSN-NH2. Successful surface modification
with amino groups was confirmed by FT-IR (Fig. S3, ESIw).
Although N–H stretching peaks at 3380 and 3310 cmÀ1 are
overlapped with the O–H stretching peak of silanol groups in
the range of 3200–3600 cmÀ1, the characteristic asymmetric
bending peaks11 of –NH2 in the region 1382–1560 cmÀ1 can be
clearly observed (Fig. S3b, ESIw), which shows that the amino
groups have been successfully bonded on the inner surface of
mesoporous silica. The mesopore structure in the MSN can be
observed by TEM (Fig. 1a and b) and illustrates that the
average diameter of the particles is o100 nm with a wormhole
arrangement of pores, which are radially oriented from the
center to the outer surface.12 The pore structure was examined
using X-ray diffraction (Fig. S4, ESIw). All samples exhibit one
broad peak at small angle (2y = 1.51) for the first reflection,
and this peak is expected due to the structures with wormhole
arrangement and the small size of the nanoscale particles.13 In
addition, we also see that the intensity of the XRD peaks of
PPV@MSN is weaker than pure MSN because the presence of
PPV macromolecules inside the channels of MSN reduces the
scattering power.14 This result indicated that PPV was introduced
into the mesopores of MSN. The surface properties of the MSNs
were characterized with N2 adsorption–desorption isotherm
analysis (Fig. 1c and d). PPV@MSN and PPV@MSN-NH2
exhibit type-IV isotherms with a pore diameter of about
2.2 nm. The surface area and desorption cumulative volume of
PPV@MSN-NH2 are 580 m2 gÀ1 and 0.49 cm3 gÀ1 (Table S1,
ESIw), respectively. The ordered one-dimensional straight
channels as well as the large surface area and pore volume
can facilitate the accessibility of TNT to the indicator. Although
the surface area and pore volume of PPV@MSN-NH2 are
slightly reduced compared with that of non-functionalized
PPV@MSN, it has ample surface area, which can provide a
large contact space to TNT species.
Fig. 3 shows the fluorescence ratiometric responses of pure
PPV@MSN and PPV@MSN-NH2 to trace TNT analyte
in ethanol solution. The fluorescence intensities decrease
with increasing the successive concentrations of TNT in
PPV@MSN-NH2 solution (Fig. 3a). It can be clearly seen
that the fluorescence quenching of PPV@MSN-NH2 is much
larger than that of pure PPV@MSN at the same concentration
of TNT. Also the PL intensity of PPV@MSN shows almost
no change with increase of TNT concentration (Fig. 3b). This
indicates that the amine ligands within the mesoporous silica
particles play an important role as recognition receptors of
TNT molecules by the strong interactions mentioned above.
The TNT binding will lead to fluorescence quenching of PPV
by FRET. FRET is the predominant quenching process rather
than electron transfer (ET) in the presence of the TNT–amine
complex. To further confirm that the PPV@MSN-NH2 can
enhance the sensitivity to TNT, we investigated 2,4-dinitrotoluene
(DNT) which is a structural analogue of TNT. The fluorescence
intensities also decrease with increasing DNT concentration
(Fig. 3c) but the quenching level is much lower than for TNT at
the same concentration of PPV@MSN-NH2. It is well known
TNT is colorless in solution and does not absorb any visible
light. However, with addition of organic amines such as
APTES, the electron transfer from amino groups to aromatic
c
4634 Chem. Commun., 2012, 48, 4633–4635
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complexes and PPV. Compared with the electron transfer
mechanism previously proposed, the FRET mechanism can
improve the sensitivity of TNT detection. In addition, we
expect this study can open up a new perspective in the design
of ratiometric sensors for various analytes based on FRET
technology.
This work was supported by the National Natural Science
Foundation of China (21074019) and Natural Science Foundation
of Jilin Province (20101539).
Notes and references
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Fig. 3 Evolution of fluorescence spectra of PPV@MSN-NH2 (a) and
of pure PPV@MSN (b) with concentration increase of TNT and for
PPV@MSN-NH2 with DNT (c). (d) Stern–Volmer plots corresponding
to the above graphs. The concentrations of TNT and DNT from top
to bottom are 0, 1, 2, 5, 10, 15 and 25 mM, respectively. The amount of
silica nanoparticles is 3.4 mg mLÀ1 in ethanol.
that the electron accepting ability of DNT with two nitro
groups is much weaker than TNT molecules with three
nitro groups, and the consequent low affinity towards the
amino groups and weak electron-accepting ability lead to the
low quenching efficiencies of DNT.16 The quenching response
was analyzed by fitting the data to the Stern–Volmer equation
((I0/I) À 1 = KSVCA)17 (Fig. 3d), where I0 and I are the
fluorescence intensity in the absence and presence of analyte
(quencher, A), respectively, CA is the molar concentration of
the analyte, and KSV is the corresponding Stern–Volmer
quenching constant. The TNT quenching fluorescence toward
PPV@MSN-NH2 had a distinct linear response to this equa-
tion in the concentration range of 0–25 mM with a corre-
lation coefficient of 0.9987, and a linear regression equation of
(I0/I) À 1 = À0.03 + [7.42 Â 104]CTNT (the DNT data and
that for PPV@MSN are given in Table S2, ESIw). The
KSV value of PPV@MSN-NH2 to TNT is about 4.8-fold
relative to that for DNT and 20-fold that of pure PPV@MSN.
This result shows that the PPV@MSN-NH2 greatly amplifies
the quenching response to TNT through highly efficient
FRET. The detection limit of TNT using PPV@MSN-NH2
is 6 Â 10À7 M (CL = kSB/m, where k is a constant, usually
k = 3, SB is the standard deviation of the blank signal (F0),
and m is the quenching constant of quencher, which is equal
to the KSV of the Stern–Volmer equation), while for pure
PPV@MSN the detection limit is only 1 Â 10À5 M. Therefore
the amino-functionalized PPV@MSN nanosensor can improve
the sensitivity of TNT detection nearly 16 times as compared to
pure PPV@MSN.
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In summary, we have demonstrated that the PPV@MSN-NH2
can be used as a nanosensor for TNT sensing. It shows good
fluorescence quenching sensitivity towards TNT through FRET
because of the good energy matching between TNT–amine
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c
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Chem. Commun., 2012, 48, 4633–4635 4635