A highly sensitive, single selective, real-time and turn-on fluorescent sensor for Al3+ detection in aqueous media

Article abstract of DOI:10.1039/c2jm33393g

A turn-on fluorescent sensor, sodium 4,4′, 4′′-(1H-pyrrole-1,2,5-triyl)tri-benzoate (Py(PhCOO-Na)₃), has been prepared for the detection of Al³⁺ in aqueous solution. The detection mechanism, as well the mechanism specific to Al³⁺, was studied by fluorescence spectroscopy, UV-vis spectroscopy and dynamic light scattering (DLS) measurement. Py(PhCOONa)₃ exhibited an aggregation-induced emission (AIE) characteristic and was found to show a specific affinity to Al³⁺, as indicated by the enhanced and bathochromically shifted emission of the AIE fluorogen. Upon binding Al ³⁺, a significant fluorescence enhancement with a turn-on ratio of over 10-fold was triggered by the AIE process. Moreover, this sensor is highly selective for Al³⁺ over other metal ions with a detection limit of 5 μM and a quantitative detection range of 5-120 μM, and is able to be used in the determination of Al³⁺ in drinking water. The time-response investigation indicates that the emission intensity of (Py(PhCOO ^-)₃) can be quickly boosted to reach the maximum intensity in less than 10 s upon titration of Al³⁺. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2jm33393g

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PAPER
A highly sensitive, single selective, real-time and ‘‘turn-on’’ fluorescent sensor
for Al3+ detection in aqueous media†
a
a
a
a
a
Xiaoyan Shi, Huan Wang, Tianyu Han, Xiao Feng, Bin Tong, Jianbing Shi,a Junge Zhib
*
a
*
and Yuping Dong
Received 27th May 2012, Accepted 27th July 2012
DOI: 10.1039/c2jm33393g
A ‘‘turn-on’’ fluorescent sensor, sodium 4,40,400-(1H-pyrrole-1,2,5-triyl)tri-benzoate (Py(PhCOO-Na)3),
has been prepared for the detection of Al3+ in aqueous solution. The detection mechanism, as well the
mechanism specific to Al3+, was studied by fluorescence spectroscopy, UV-vis spectroscopy and
dynamic light scattering (DLS) measurement. Py(PhCOONa)3 exhibited an aggregation-induced
emission (AIE) characteristic and was found to show a specific affinity to Al3+, as indicated by the
enhanced and bathochromically shifted emission of the AIE fluorogen. Upon binding Al3+, a
significant fluorescence enhancement with a turn-on ratio of over 10-fold was triggered by the AIE
process. Moreover, this sensor is highly selective for Al3+ over other metal ions with a detection limit
of 5 mM and a quantitative detection range of 5–120 mM, and is able to be used in the determination of
Al3+ in drinking water. The time-response investigation indicates that the emission intensity of
(Py(PhCOOꢀ)3) can be quickly boosted to reach the maximum intensity in less than 10 s upon titration
of Al3+
.
selectivity and sensitivity for the rapid detection of Al3+ need to
be exploited.
Introduction
The molecular design and synthesis of chemosensors to detect
metal ions, such as Al3+, Hg2+, Pb2+ and Cu2+, has developed
rapidly due to the potential impact of their toxic effects.
Aluminium ion (Al3+) is widely used in food additives, kitchen
utensils, packing materials and some Al-based medicines, which
increases the risk of Al3+ entering into human bodies through the
digestive system. It has been found that Al3+ ions exerted several
neurotoxic effects in organisms a long time ago.1 For example,
Al3+ has an important role in Alzheimer’s disease, Parkinson’s
disease, bone softening, chronic renal failure and smoking-
related diseases.2 Since there is a close association between Al3+
and human health, the investigation of Al3+ detection attracts
more and more attention. In the last decade, several conventional
methods have been developed for detecting Al3+, including
atomic absorption spectrometry, atomic emission spectrometry,
inductively coupled plasma mass spectrometry and electro-
chemical methods.3 These methods, however, require high
instrumentation and operation costs and intricate pre-treatment
processes. Simple and accurate analytical methods with high
To the best of our knowledge, compared with other metal ions,
limited examples of fluorescent or colorimetric chemosensors for
the evaluation of Al3+ have been reported so far. Their signal
transduction is generally achieved by photoinduced electron
€
transfer, photoinduced charge-transfer, Forster resonance
energy transfer, or switching on of Au–Au interactions.4 The
majority of these methods have limitations such as weakness in
their selectivity and sensitivity over other metal cations, tedious
synthetic efforts and/or a lack of practical applicability in
aqueous solution. Thus, it is desirable to develop novel methods
for the selective detecting of Al3+ ions in aqueous environments.
Tang’s group in 2001 first reported that the luminescence of
silole molecules is stronger in the aggregated state than that in the
solution state.5
A variety of luminogens, including dis-
tyrylbenzene, fluorene, pentacene and pyrene derivatives, were
successively proved to have the same properties.6 These mole-
cules can be generally categorized into two groups. In the first
group, the luminogenic molecules are non-emissive when dis-
solved in good solvents but become highly luminescent when
aggregated in the solid state, thus behaving exactly oppositely to
the conventional ACQ luminophores. Since the emission is
induced by aggregation, Tang coined ‘‘aggregation-induced
emission’’ (AIE) for this unusual phenomenon.5 In the second
group, the luminogens are luminescent in solution and become
more emissive in the aggregated state. Because the light emission
is enhanced by aggregate formation, the effect is referred to as
aCollege of Materials Science and Engineering, Beijing Institute of
Technology, 5 South Zhongguancun Street, Beijing, 100081, China.
E-mail: tongbin@bit.edu.cn; chdongyp@bit.edu.cn; Fax: +86 10-68948982
bCollege of Chemistry, Beijing Institute of Technology,
Zhongguancun Street, Beijing, 100081, China
5 South
† Electronic supplementary information (ESI) available: See DOI:
10.1039/c2jm33393g
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1
‘‘aggregation-induced emission enhancement’’ (AIEE).7 Such a
novel effect has enabled the AIE(E) luminogens to find potential
high-tech applications in sensors and opto-electrical devices over
the past years.5–8 Above all, the introduction of functional groups
into AIE(E) molecules will favor the new development of chemo-
or biosensors for detecting metal cations, biomolecules, organic
vapors, chiral molecules and explosives.9
All the compounds were characterized by H-NMR, infrared
spectroscopy (IR), and matrix-assisted laser desorption/ioniza-
tion time of flight mass spectrometry (MALDI-TOF MS). For
instance, two peaks at d ¼ 6.63 ppm and d ¼ 3.94 ppm attributed
to the pyrrole moiety and ester groups, respectively, are clearly
1
observed for Py(PhCOOCH3)3 in the H-NMR spectra (Fig. S1
in the ESI†). The disappearance of the peak at d ¼ 3.94 ppm
corresponded to the ester groups, as well as the appearance of a
new peak at d ¼ 13.01 ppm attributed to the carboxyl groups,
demonstrates that Py(PhCOOCH3)3 underwent a complete
hydrolysis and was entirely converted to Py(PhCOOH)3. The IR
spectra of Py(PhCOOCH3)3 and Py(PhCOOH)3 displayed
middle peaks in the 1724–1680 cmꢀ1 region that are characte-
ristics of the C]O stretching vibration of carbonyl. Compared
to the IR spectrum of Py(PhCOOCH3)3, the appearance of a
broad band in the range 3590–2680 cmꢀ1 is indicative of the
existence of carboxyl groups of Py(PhCOOH)3. Meanwhile, the
two signals around 1400 and 1600 cmꢀ1 in the IR spectrum of
Py(PhCOONa)3 are assigned to the symmetrical and anti-
symmetrical vibration of –COOꢀ, respectively. In addition, in the
1600–1430 cmꢀ1 region, the transmission bands are in accor-
dance with the skeletal vibrations of aromatic and heterocyclic
rings for all these three compounds (Fig. S2 in the ESI†). The MS
data of Py(PhCOOCH3)3 and Py(PhCOOH)3 tally with the
theoretical ones (Fig. S3 in the ESI†).
In our previous research, we found that 1,2,5-triphenylpyrrole
(TPP) derivatives exhibited good AIEE properties. Especially,
sodium
4-(2,5-diphenyl-1H-pyrrol-1-yl)benzoate
(TriPP-
COONa), with one carboxylate group in TPP, can form highly
fluorescent aggregates with aluminium and has been used for
selective and sensitive detecting Al3+ in THF–water mixtures
(25 : 75, v/v) based on the AIE mechanism.10 Herein, to further
improve the water solubility of TPP derivatives and thus to
realize the detection of metal ions in aqueous media, we
synthesized a new TPP derivative, sodium 4,40,400-(1H-pyrrole-
1,2,5-triyl)tribenzoate (Py(PhCOONa)3), and explored their
utility as metal ion sensors on the basis of its AIE characteristic.
(Py(PhCOONa)3) was proved to be a highly sensitive, singly
selective, real-time and ‘‘turn-on’’ fluorescence sensor for Al3+ in
aqueous media.
Results and discussion
Aggregation-induced emission
Synthesis and characterization of Py(PhCOONa)3
Py(PhCOONa)3 exhibits good solubility in water. The dilute
aqueous solution of Py(PhCOONa)3 hardly shows any fluores-
cence (fluorescence quantum yield, 0.003%) as shown in Fig. 1
and 2. Because THF is a poor solvent for it, Py(PhCOOꢀ)3 must
have aggregated in the H2O–THF mixture. To investigate the
AIE property of Py(PhCOOꢀ)3, the fluorescence behaviour of
the H2O–THF mixtures in different ratios was examined at the
same Py(PhCOOꢀ)3 concentration under identical measurement
conditions. The fluorescence spectrum of Py(PhCOOꢀ)3 (10 mM)
exhibits an emission band at 413 nm in H2O–THF (5 : 95, v/v)
mixture when excited at 328 nm. As THF content increased from
The general synthetic procedure is shown in Scheme 1. Dimethyl
4,40-(buta-1,3-diyne-1,4-diyl)dibenzoate (DMBDB) was synthe-
sized from starting material methyl 4-ethynylbenzoate (MEA) by
the Glaser coupling reaction. The Schulte–Reisch reaction of
DMBDB with methyl 4-aminobenzoate gave the corresponding
trimethyl 4,40,400-(1H-pyrrole-1,2,5-triyl)tribenzoate (Py(Ph-
COOCH3)3). Hydrolysis of Py(PhCOOCH3)3 to produce 4,40,400-
(1H-pyrrole-1,2,5-triyl)tribenzoic acid (Py(PhCOOH)3) followed
by neutralization with sodium hydroxide afforded sodium
4,40,400-(1H-pyrrole-1,2,5-triyl)tribenzoate (Py(PhCOONa)3) in a
good yield.
Fig. 1 Emission spectra of Py(PhCOOꢀ)3 (10 mM) in H2O–THF mixture
Scheme 1 Synthesis route to Py(PhCOONa)3.
with different THF contents. Excitation wavelength: 340 nm.
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Fig. 2 Fluorescent quantum yield of Py(PhCOOꢀ)3 vs. composition of
H2O–THF mixture. Quinine sulfate in 0.1 M H2SO4 was used as
reference.
Fig. 3 Emission spectra of Py(PhCOOꢀ)3 (100 mM) in H2O–THF (96/4,
v/v) mixture upon addition of Al3+. The Al3+ concentrations are 0, 10, 20,
30.160 mM, from bottom to top.
0% to 95%, the photoluminescence (PL) intensities and fluores-
cent quantum yield were dramatically enhanced (Fig. 1 and 2,
and Fig. S4 in the ESI†). Even in the H2O–THF (90/10, v/v)
mixture, the fluorescence was still much higher than that in pure
water. The photoluminescence (PL) emission, however, reached
the maximum value at 75% THF content. The decrement of
fluorescent quantum yield and PL intensity along with the THF
content from 75% to 95% was probably due to the quick
agglomeration of the Py(PhCOOꢀ)3 molecules in a random way,
which forms less emissive, ‘‘redder’’, amorphous powders.6b–d
It’s wholly different from TriPPCOOꢀ whose emission inten-
sities almost remained in nullity at ‘‘low’’ THF fraction content
(<40%) and only showed an uprush at much higher THF
contents (>40%).10b The enhancement in luminescence, after the
addition of nonsolvent, resulted from the changing of the existing
forms (the relative distance between the molecules and geometry
of compounds in the solution state) from a solution state in water
to the aggregated particles in the H2O–THF mixture. That is, the
emission of Py(PhCOOꢀ)3 is spectacularly boosted by aggrega-
tion, in other words, Py(PhCOOꢀ)3 is AIE active. Since the
solubility of Py(PhCOONa)3 with three charged carboxylate
groups in THF is much lower than that of TriPPCOONa, that is,
the formation of Py(PhCOOꢀ)3 aggregates is easier than that of
TriPPCOOꢀ in the presence of THF, it will provide more
possibilities of detecting metal ions in aqueous solution with
lower THF content.
concentration from 0 to 170 mM, the emission band at 428 nm
showed a steady and smooth enhancement and red-shift when
excited at 328 nm. A ‘‘turn-on’’ ratio over 10-fold was triggered
with the addition of 1.6 equiv. of Al3+. Fig. 4 shows the depen-
dence of the changes in emission intensity on the Al3+ concen-
tration. It exhibited a three-phase increment upon the increasing
of Al3+ concentration, a slow growth before 50 mM, a rapid
growth in the range of 50–130 mM, and a suspended manner
afterward. Notably, there exists a good linear relationship
between the changes in PL intensity and the square of Al3+
concentration (R2 ¼ 0.9979, 0–130 mM). It’s quantitative detec-
tion range is much more broader than that of TriPPCOOꢀ,10b
Fluorogenic Al3+ sensing
Given the fact that Py(PhCOOꢀ)3 has already existed in an
aggregated form in H2O–THF (96/4, v/v) mixtures, one can
expect that the introduction of Al3+ into Py(PhCOOꢀ)3 solution
would lead to the formation of Py(PhCOOꢀ)3-nAl3+ (n ¼ 1, 2 or
3) salts with lower solubility. As a result of increasing the
aggregation degree, a ‘‘turn-on’’ fluorescent signal could be
detected based on the AIE mechanism. The emission spectra of
100 mM Py(PhCOOꢀ)3 in H2O–THF (96/4, v/v) mixture upon
titration with Al3+ are shown in Fig. 3. With increasing Al3+
Fig. 4 Emission intensities of Py(PhCOOꢀ)3 (100 mM) in H2O–THF
mixture (96/4, v/v) as a function of [Al3+] or [Al3+ 2
] (0–170 mM). Exci-
tation wavelength: 340 nm. Inset: Linear relationship between emission
growth ratio and Al3+ concentration from 0 mM to 130 mM. Curve-fit
equation: (I ꢀ I0)/I0 ¼ 0.27809 + 5.67207*10ꢀ4[Al3+]2, R2 ¼ 0.99786.
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indicating that this approach is more applicable for quantitative
detection of Al3+
In addition, the detection limit of
.
Py(PhCOOꢀ)3 towards Al3+ was estimated as 5.3 mM, which
meets the limit for drinking water according to the WHO stan-
dard (7.41 mM).
To further evaluate the specificity of Py(PhCOONa)3 towards
Al3+, we carried out fluorescence titration with other cations,
including Al3+, N(CH3)+, Au3+, Cr2+, Ni2+, Cd2+, Hg2+, Fe3+
,
Pb2+, Ag+, Ce2+, Cu2+, Zn2+, Ba2+, Mg2+, Ca2+ and K+, at
concentrations of 500 mM to make sure of its maximum fluo-
rescent response. Although Py(PhCOOꢀ)3 has
a positive
response towards Au3+, Cr2+, Pb2+ and Cu2+, as well as a
negative response toward Hg2+, Fe3+ and Ag+, it is clearly seen
that the most striking effects are observed for Al3+, confirming
the probe is selectively responsive to Al3+ (Fig. 5). Compared
with the detection result in pure water, the ratio of H2O and THF
(96/4, v/v) used in the sensory experiments was chosen according
to its relatively higher selectivity and stronger amplification
ability owing to the formation of Py(PhCOOꢀ)3 AIE aggregates
induced by THF (Fig. 5). Compared with TriPPCOOꢀ,
Py(PhCOOꢀ)3 exhibits improved selectivity due to its relative
higher solubility and COOꢀ density. To further testify single
selectivity of Py(PhCOOꢀ)3 for Al3+ in practical application, we
chose K+, Mg2+, Ca2+, and Ba2+ ions as the interfering ions,
some of which have a relatively high concentration in biological
tissue and drinking water. As shown in Fig. 5, the experimental
results indicated that fluorescent intensities of Py(PhCOOꢀ)3 at
460 nm enhanced by Al3+ aren’t much affected by the existence of
the interfering ions, thus providing a potential application for
biological detection and water quality monitoring.
Fig. 6 Absorption response of Py(PhCOOꢀ)3 (100 mM) to the titration
of Al3+ in H2O–THF (96/4, v/v) mixture. The [Al3+] total increases from
0 to 60 mM along the direction of the arrow. Inset: Linear relationship
between absorbance and Al3+ concentration at 329 and 391 nm.
(Fig. 6). Upon addition of Al3+ (0–60 mM), the absorbance at
329 nm decreased gradually with slight red-shift while the
absorbance at 346 nm remained constant along with the increase
of the Al3+ concentration. In the meanwhile, although a new
absorption band was not obviously observed, a low energy (LE)
shoulder developed at 391 nm (Fig. 6). The presence of a well-
defined isosbestic point at 346 nm indicated that interconversion
between the uncomplexed and complexed species had occurred.
Besides, the transmittance of the solution decreased upon titra-
tion with Al3+ and reached its minimum value when the Al3+
concentration was over 200 mM (Fig. S5 in the ESI†), which
implies the amount of aggregates became larger and/or the size of
aggregates became bigger with increasing the Al3+ content. These
assumptions were further confirmed by dynamic light scattering
(DLS) measurement. The average particle size of Py(PhCOOꢀ)3
Moreover, ion recognition properties of Py(PhCOOꢀ)3 have
also been studied by using an absorption technique. The UV-vis
spectra of Py(PhCOOꢀ)3 in water–THF (96/4, v/v) mixtures
showed only one p–p* transition band centered at 329 nm,
which can be assigned to the aryl-substituted pyrrole moiety
Fig. 5 The maximum fluorescent response of Py(PhCOOꢀ)3 (100 mM)
upon addition of different metal ions (500 mM) in H2O–THF (96/4, v/v)
mixtures (black columns) or pure water (red columns), listed from left to
right: (A) Al3+, (B) N(CH3)4+, (C) Au3+, (D) Cr2+, (E) Ni2+, (F) Cd2+
,
Fig. 7 Particle size distribution in Py(PhCOOꢀ)3 (100 mM, H2O–THF
96/4, v/v) solution with (b) and without (a) the addition of Al3+ (500 mM)
determined by DLS.
(G) Hg2+, (H) Fe3+, (I) Pb2+, (J) Ag+, (K) Ce2+, (L) Cu2+, (L) Zn2+, (N)
Ba2+, (O) Mg2+, (P) Ca2+, (Q) K+, and (R) Al3++Ba2++Mg2++Ca2++K+.
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in H2O–THF (96/4, v/v) solution increased from 50 to 130 nm
upon addition of Al3+ (500 mM) (Fig. 7). Thus, an AIE mecha-
nism is contributed to the ‘turn-on’ detection of Py(PhCOOꢀ)3
for Al3+ herein.
In order to elucidate the effect of Al3+ on aggregate formation,
the morphology of Py(PhCOOꢀ)3 aggregates in H2O–THF (96/4,
v/v) mixtures without and with Al3+ was examined by trans-
mission electron microscopy (TEM). The TEM image of the
sample without Al3+ showed the amorphous and incompact
aggregates because of the electrostatic repulsion between
carboxylate ions (Fig. 8a). The diameters of the approximative
spheres ranged from 15 to 100 nm with a 45.3 nm average
diameter, which is consistent with that from DLS analysis
(Fig. 7, line a). Part of the nanospheres further clumped together
to form nanoclusters with random morphologies. In the Al3+
-
containing sample, the TEM image, however, revealed that the
introduction of Al3+ leads to the morphological transition of
aggregates in H2O–THF (96/4, v/v) mixtures. Aggregates
composed of spheres, capsules and ribbons were observed
(Fig. 8). Especially, the morphology of spheres and capsules is
more regular and more compact than that of Py(PhCOOꢀ)3
aggregates in H2O–THF (96/4, v/v) mixtures without Al3+, which
indicates that the introduction of Al3+ promotes Py(PhCOOꢀ)3
to form regular aggregates. The diameters of the spheres and
capsules ranges from 23 to 71 nm with a 47 nm average diameter.
For the ribbons, their average width and length are about 30 and
120 nm, respectively. The results also explain the corresponding
attributions of the two peaks showed in Fig. 7, line b. According
to previous studies, reducing the anionic repulsion between Al3+
and amphiphilic Py(PhCOOꢀ)3 headgroups enables closer
packing to fabricate spheres and can provide an additional
impetus for capsule formation.11 Because the concentration of
Al3+ was much higher than that of Py(PhCOOꢀ)3 herein, the
decrease in charge of partial unilamellar vesicle bilayers could, in
turn, induce the stacks of lamellae to form the ribbons by
decreasing the repulsive force between anionic bilayers.12
Fig. 9 Fluorescence response of Al3+ with different concentrations
(5–500 mM) at different times.
maximum after the addition of Al3+ in a few seconds and stays
invariable over an extended period of time. This performance is
much faster than that of TriPPCOOꢀ reported by our group,
which increased to the maximum in 300 s.10b We compared this
phenomenon to that attributed with the TriPPCOOꢀ aggregates,
and there are more carboxylate groups that could bind with Al3+
through electrostatic attractions existing on the surface of
Py(PhCOOꢀ)3 aggregates. This provides a potential ‘‘zero-wait’’
detecting method for Al3+
.
Response mechanism of Py(PhCOOꢀ)3 to Al3+
We supposed that the fluorescence enhancement after addition
of Al3+ is assigned to a synergetic effect based on the AIE
mechanism. On one hand, according to literature, both the
experimental results and the theoretical calculations have
revealed that the bidentate complex formed between carboxy-
late groups and Al3+ is more stable than the mono-dentate
complex.13 When one carboxylate group in Py(PhCOOꢀ)3 is
used to form a bidentate complex with Al3+ ion, the pyrrole ring
easily constructs the coplanar conformation with the benzyl
ring, which further favours the aggregation and suppresses
intramolecular-rotation causes non-radiative decay, that is, an
AIE mechanism (Scheme 2). Thus, the PL intensity of
Py(PhCOOꢀ)3 is enhanced with the increasing of Al3+ concen-
tration. On the other hand, little and loose aggregates are
considered to be formed in the initial Py(PhCOOꢀ)3 (100 mM,
THF/water, 96/4, v/v) solution, which can be proved by the
DLS data and TEM images (Fig. 7 and 8). Owing to its small
size, high charge density and oxophilicity, Al3+ will immediately
get close to carboxylate groups existing on the surface of these
aggregates and then form a chelation complex with lower
solubility through electrostatic attractions. This can not only
increase the amount and the size of the aggregates but also
compact them. All these impacts can suppress intramolecular-
rotation, cause non-radiative decay and boost the fluorescence
emission based on the AIE mechanism (Scheme 2).
Time response of Py(PhCOOꢀ)3 to Al3+
The time-dependent fluorescence intensity of H2O–THF (96/4,
v/v) solution of Py(PhCOOꢀ)3 to Al3+ was evaluated. Because
the minimum response time that can be measured is limited in the
manipulation time, all the measurements with different Al3+
contents were started at 10 s. As shown in Fig. 9 and S6 in the
ESI,† the emission intensity was immediately enhanced up to the
Fig. 8 TEM images of the Py(PhCOOꢀ)3 (100 mM) in H2O–THF (96/4,
v/v) mixture (a) without and (b) with 500 mM Al3+
.
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70 ꢁC for 15.5 h and poured into 500 mL dilute hydrochloric acid
solution, the white precipitate was washed by H2O, ethanol and
dichloromethane several times, yield: 87.6%. 1H NMR (400
MHz, DMSO) d (ppm): 13.01 (s, 3H), 7.92–7.90 (d, 2H), 7.76 (s,
4H), 7.22–7.11 (m, 6H), 6.70 (s, 2H). MS (MALDI-TOF): calcd
for C25H17NO6, 427.11; found, 426.9.
Py(PhCOONa)3: Py(PhCOOH)3 (0.05 mmol) was neutralized
with aqueous sodium hydroxidesolution (0.15 mmol), stirred
overnight and then Py(PhCOONa)3 was afforded after the
1
drying process. H NMR (400 MHz, D2O) d (ppm): 7.72–7.57
(m, 6H), 7.07–7.03 (m, 6H), 6.58 (s, 2H).
Conclusions
In summary, a singly selective and highly sensitive water-soluble
probe for the real-time and ‘‘turn-on’’ detection of Al3+ was
synthesized by the introduction of three carboxylate functional
groups into the aryl-substituted pyrrole derivative. The
complexation of Al3+ with Py(PhCOOꢀ)3 triggered a significant
enhancement of fluorescence intensity in aqueous solution based
on the AIE mechanism. We suppose that the selectivity is
probably due to the difference of the electrostatic binding ability
between metal ions and –COOꢀ as well as the solubility of the
complex. Such a real-time detection method in aqueous solution
based on AIE mechanism has a huge prospect of application in
chemosensing and biosensing fields. Further studies on the
design of novel fluorescent probes for metal ions with this prin-
ciple are underway.
Scheme 2 The sensing process based on the AIE mechanism.
Experimental section
All chemicals were purchased from Alfa Aesar and Beijing
Chemical Reagent Company without further purification.
Solvents were purified according to standard procedures. The
structures of the products synthesized in this paper were identi-
1
fied by H-NMR spectra, elemental analysis and mass spectra
through Bruker ARX-400 spectrometer, Elementar Vario EL
and Autoflex III MALDI-TOF, respectively. UV-visible
absorption spectra were measured by TU-1901 spectrophoto-
meter. The emission spectra were carried out on a Perkin-Elmer
VARIAN 55 spectrophotometer. Particle size distribution anal-
ysis was determined by an ALV-5000 dynamic laser light scat-
tering. Samples were examined in a Tecnai Spirit 120 kV
transmission electron microscope. The TEM was operated at
80 kV accelerating potential. The samples were prepared as
following steps: the solutions of Py(PhCOOꢀ)3 (100 mM) without
(as blank) or with 500 mM Al3+ in H2O–THF(96/4 v/v) mixture
were absorbed onto copper mesh for 1 minute and dried at
ambient temperature, then they were washed with pure water. In
addition, the blank was re-dyed with uranyl acetate for 30
seconds.
Acknowledgements
The authors are grateful to the National Natural Scientific
Foundation of China (Grant nos. 20634020, 51073026,
51061160500, 20944004, 21074011), the Specialized Research
Fund for the Doctoral Program of Higher Education of China
(Grant nos. 20091101110031 and 20101101120029), Basic
Research Foundation of Beijing Institute of Technology,
Excellent Young Scholars Research Fund of Beijing Institute of
Technology (Grant no. 2009Y0914), Technology Innovation
Programme for Graduate Students of Beijing Institute of Tech-
nology (Grant no. 2011cx03032) for financial support of this
work.
Py(PhCOOCH3)3: compound 1,4-di(4-(methoxycarbonyl)
phenyl)buta-1,3-diyne was synthesized according to our previous
work.10b A mixture of 1,4-di(4-(Methoxycarbonyl)phenyl)buta-
1,3-diyne (20.0 mmol), copper(I) chloride (2.0 mmol), and methyl
4-aminobenzoate (30.0 mmol) was stirred under argon for 12 h at
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170 C. The crude product was purified by column chromatog-
raphy (ethyl acetate/petroleum ether (the boiling range: 30–
60 ꢁC) 7 : 1), and then recrystallized from chloroform and n-
hexane, yield: 32.8%. 1H NMR (400 MHz, CDCl3) d(ppm): 7.98–
7.96 (d, 2H), 7.88–7.86 (d, 4H), 7.11–7.09 (d, 6H), 6.63 (s, 2H),
3.94–3.91 (d, 9H). Anal. calcd for C28H23NO6: C 71.63, H 4.94,
N 2.98; found: C 71.29, H 4.96, N 2.93%. MS (MALDI-TOF):
calcd for C28H23NO6, 469.15; found, 469.0.
Py(PhCOOH)3: Py(PhCOOCH3)3 (0.8 mmol) in 100 mL and
50 mL of H2O in which dissolved NaOH (5 mmol) was added
into a 250 mL round bottom flask, then the mixture was stirred at
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J. Mater. Chem., 2012, 22, 19296–19302 | 19301

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