Microcrystal induced emission enhancement of a small molecule probe and its use for highly efficient detection of 2,4,6-trinitrophenol in water

Article abstract of DOI:10.1007/s11426-017-9223-2

In this contribution, we reported a very simple and small molecule material, 2,5-dimethoxyterephthalaldehyde (DMA). It exhibited a relatively weak fluorescence in solution, while showed a steadily increased green fluorescence with typical aggregation- ind

Full text of DOI:10.1007/s11426-017-9223-2

SCIENCE CHINA
Chemistry
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ARTICLES• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . https://doi.org/10.1007/s11426-017-9223-2
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Microcrystal induced emission enhancement of a small molecule
probe and its use for highly efficient detection of
2,4,6-trinitrophenol in water
1
2
2
2*
2
Huan Liu , Yanyan Fu , Wei Xu , Qingguo He , Huimin Cao ,
1
1,2*
Guohua Liu & Jiangong Cheng
1
College of Life and Environmental Sciences, Shanghai Normal University, Shanghai 200234, China;
2
State Key Laboratory of Transducer Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of
Sciences, Shanghai 200050, China
Received November 23, 2017; accepted February 11, 2018; published online April 24, 2018
In this contribution, we reported a very simple and small molecule material, 2,5-dimethoxyterephthalaldehyde (DMA). It
exhibited a relatively weak fluorescence in solution, while showed a steadily increased green fluorescence with typical ag-
gregation-induced enhanced emission (AIE) effect for forming a cubic-like microcrystal structure in THF-H O mixed solvent.
2
The microcrystals presented significantly higher fluorescence than that of amorphous aggregates. The DMA microcrystals
−
7
suspension showed a good response to 2,4,6-trinitrophenol (TNP) with a LOD of 1.2×10 M, which is the best result of TNP
detection in aqueous solution. Quantum chemical calculation revealed that DMA is a donor (D)-receptor (A) type molecule with
methoxy unit as donor and carbonyl moiety as receptor. Its emission arises from an intramolecular charge transfer (ICT) from
methoxy units to carbonyl units. NMR indicated that there is a strong hydrogen bond interaction between DMA and TNP.
Hydrogen bond interaction can effectively decrease the intermolecular distance of DMA and TNP, which will increase the
efficiency of photoinduced electron transfer (PET) and fluorescence resonance energy transfer (FRET), and hence will be
advantageous for its selectivity. The microcrystal induced enhanced emission could be generally used for kinds of target
molecules analysis.
fluorescence probe, microcrystal, aggregation induced emission, intramolecular charge transfer, TNP
Citation:
Liu H, Fu Y, Xu W, He Q, Cao H, Liu G, Cheng J. Microcrystal induced emission enhancement of a small molecule probe and its use for highly
efficient detection of 2,4,6-trinitrophenol in water. Sci China Chem, 2018, 61, https://doi.org/10.1007/s11426-017-9223-2
1
Introduction
for TNP in drinking water is set to be 0.5 mg/L with an
allowable daily intake (ADI) of 1–37 μg/(kg per day) [5].
Therefore, its extensive use can contaminate soil and
groundwater, and may further result in skin irritation, ane-
mia, cancer and abnormal liver functioning, and also damage
to respiratory organs when people inhale, ingest, or touch it
[6–8]. Thus, sensitive and selective detection of TNP has
been attracted increasing attention, because of its con-
tamination to environment and the risk to human health [9].
Recently, many fluorescent probes have been developed
because of their high sensitivity, good selectivity, real-time
A great deal of challenging and considerable researches have
been devoted to detecting explosives for public security, due
to the extreme danger of the nitro aromatics (NACs) ex-
plosives such as 2,4,6-trinitrotoluene (TNT) and 2,4,6-trini-
trophenol (TNP) [1]. TNP is one of the NACs, and widely
used in the dye industry, fireworks, pharmaceuticals and
chemical laboratories, and so on [2–4]. The permissible level
*Corresponding authors (email: hqg@mail.sim.ac.cn; jgcheng@mail.sim.ac.cn)
©
Science China Press and Springer-Verlag GmbH Germany 2018 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . chem.scichina.com link.springer.com

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detection and dual compatibility in solid and solution media
10–12]. However, it is known that conventional fluorescent
[
material usually aggregated in aqueous solution and that
fluorescence is usually weakened or quenched due to strong
intermolecular π-π interaction, which is often referred as
aggregation-induced quenching (ACQ) [13,14]. But, some
fluorescent materials based on aggregation-induced emission
or enhanced emission (AIE/AIEE) have opposite properties,
and their aggregation states have a strong emission [15,16].
Although several TNP probes based on the AIE effect have
been reported [17–19a], the relationship between the mor-
phology of aggregation state of the probes and the sensing
property is less concerned [19b]. Generally, the emission
enhancement of AIE materials (with nanoparticles, dots,
ribbon-like or hollow nanospheres morphology) is around 1
to 20 times [20–24] relative to its original state. The ag-
gregation states are rather amorphous, so the fluorescence
enhancement is rather small. As we know, a crystal state will
be highly advantageous for reducing the non-radiative tran-
sition and emitting strong fluorescence.
Figure 1 Molecular structure of DMA (color online).
Ultraviolet-visible (UV-Vis) absorption spectra were re-
corded on a model Jasco V-670 spectrophotometer (Japan).
Fluorescence spectra were performed using a model HOR-
IBA Fluoromax Spectrofluorometer (Japan). Materials Stu-
dio was used for dynamics (Forcite plus) and orbitals
(DMol3) calculation, using GGABLYP as the density func-
tional theory (DFT)-method and COMPASS II (dynamics)/
Dreilding (orbitals) as the force field. The scanning electron
microscope (SEM) by JEOL 7800F (Japan) is used to ob-
serve surface topograph. Fluorescence microscope OLYM-
PUS, IX51 (Japan). Zeta potential and Nano particle
characterization. Experimental analyts: 2,4,6-trinitrophenol
In this contribution, we endeavoured to develop a crystal
aggregation enhanced fluorescence probe for a highly effi-
cient TNP detection in aqueous phase. Herein, we developed
a small molecule 2,5-dimethoxyterephthalaldehyde (DMA,
Figure 1), whose light emission is enhanced by an increased
(TNP), nitrobenzene (NB), 1,3,5-trinitrobenzene (TNT), ni-
troglycerine (NG), methylnitrite (MN), cyclotrimethylene-
trinitramine (RDX), and pentaerithrityl tetranitrate (PETN),
1
,3-dinitrobenzene (DNB).
water ratio in a tetrahydrofuran (THF)-H O mixed solution
2
2
.2 Synthesis of DMA
via the formation of the microcrystal structure. We found that
emission maximum of DMA in the mixed solution reached
its 38 times compared with that in THF solution. By this
probe design strategy, DMA exhibited excellent response to
TNP in aqueous solution by a microcrystal aggregation
collapse upon interaction with TNP molecules. The detection
limit could be as low as 1.2×10 M, which is the best result
in TNP aqueous fluorescence detection [4,17,25,26]. NMR
spectra and quantum chemical calculation were performed to
clarify the sensing mechanism.
To a solution of 1,4-dimethoxybenzene (1) (10.0 g,
2.3 mmol) in 1,4-dioxane (30 mL), HCHO solution (38% in
water, 5 mL) and paraformaldehyde (3.0 g, 99.0 mmol) were
added in turn [27]. The resulting mixture was heated to
7
9
0 °C, concentrated HCl (2×5 mL) was added during 30 min
−
7
intervals. Heating continued for 1 h and a further 30 mL of
concentrated HCl was added. The reaction mixture was
cooled to room temperature to afford a white precipitate,
which was obtained by filtration and dried under vacuum.
The crude product was recrystallized with acetone to give
product 1 (4.5 g, 26%) as a white precipitate. A solution of
product 1 (15.0 g, 63.8 mmol) and hexamethylenetetramine
(18.0 g, 127.6 mmol) in chloroform (50 mL) was stirred at
reflux for 24 h. After cooling to r.t., the pale yellow pre-
cipitate was collected by filtration and redissolved in water
2
Experimental
2
.1 Materials and methods
,4-dimethoxybenzene, triethylamine
1
(TEA),
di-
chloromethane (DCM), dimethyl sulfoxide (DMSO), ethyl
acetate, 1,4-dioxane, methanol, toluene, tetrahydrofuran
(30 mL). The aqueous solution was acidified with
CH COOH (10 mL) and stirred at 90 °C for 24 h. The mix-
3
(
THF), acetonitrile (ACN), acetone, chloroform, acetic acid,
ethanol, methanesulfonyl chloride, anhydrous sodium sulfate
Na SO ), formaldehyde (38% in water), paraformaldehyde,
ture was cooled to r.t. and extracted with DCM (200 mL).
The organic phase was washed three times with H O
2
(
2
4
(
200 mL) and dried over anhydrous Na SO . After solvent
2 4
concentrated hydrochloric acid (37% in water, conc. HCl)
and hexamethylenetetramine were purchased from MACK-
evaporation, the residue was recrystallized from CH CH OH
3
2
1
1
to yield DMA (5.5 g, 45%) as a bright yellow solid. H NMR
LIN reagent (China). H-nuclear magnetic resonance
1
13
(300 MHz, CDCl ) δ 10.50 (s, 2H), 7.26 (s, 2H), 3.95 (s, 6H).
3
(
H NMR) and C NMR spectra were recorded on a Bruker
AVANCZ 500/600 spectrometer (Germany) at
00/600 MHz, using CDCl , d -DMSO as the solvent at
1
3
C NMR (101 MHz, CDCl ) δ 189.19, 155.73, 129.15,
3
5
2
110.92, 77.33, 77.01, 76.70, 56.23. FTMS for C10
calcd, m/z 195.1 [M+1] ; Anal. Calcd. (194.1).
H
10
O
4
:
3
6
+
98 K, and tetramethylsilane (TMS) as the internal standard.

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3
Results and discussion
demonstrated an aggregation-induced emission (AIE) char-
acter. To monitor the morphology change with f , SEM was
w
3
.1 Optical properties and AIE characterization
used to measure the DMA samples with different f . As
w
The absorption and fluorescence spectra of DMA were
shown in Figure 2(a). It indicates that there are mainly two
absorption bands located at 272 and 396 nm, respectively,
and emission maximum is at 461 nm. We tested the fluor-
escent spectra in different polarity solvents. As shown in
Figure 2(b), the emission peak moved from 450 nm in to-
luene to 480 nm in DMSO as the solvent polarity increases,
which indicates the intramolecular charge transfer (ICT)
character of DMA. To further confirm the ICT character, the
energy levels of the frontier orbitals were calculated with
DMol3 while the molecule stacking of DMA was calculated
with Forcite Plus by Materials Studio 8 software. As shown
in Figure 3(a), the lowest unoccupied molecular orbital
shown in Figure 2(e), DMA tended to grow up to cube-like
microstructure by a steady addition of water from f =10% to
w
90%. A clean morphology with only cubic microstructure
was formed until f =60%.
w
To know the character of the microstructure, the DMA
particles in THF solution were characterized by dynamic
light scattering (DLS) experiment as shown in Figure S1(a)
(Supporting Information online), which showed that they are
well dispersed and uniform in size, and their sizes are about
0.455−0.485 μm with an average diameter of 0.47 nm. But,
when f =70%, the size of the microcrystal grew into particles
w
about 1.59−1.65 μm with an average diameter of 1.62 μm
(Figure S1(b)). This shows that DMA forms larger particles
in the mixed solvent than in the THF solution. And the
(
LUMO) mainly was localized on the carbonyl groups and
phenyl unit, while the highest occupied molecular orbital
HOMO) was localized on methoxy groups and phenyl unit,
indicating an intramolecular charge transfer from methyoxy
donor) to carbonyl units (acceptor). It can be seen from
fluorescence image of DMA in f =70% was collected by a
w
(
direct observation of its suspension on the glass sustrate,
indicating a very clear microcrystal structure (Figure S2). In
order to understand crystal more clearly, we did the XRD
analysis, as shown in Figure S3, the sample prepared from
fw=0 exhibited a very weak and sharp reflections at 2θ=11.2°,
(
Figure 2(c, d), the fluorescence intensity changed gradually
with the ratio of H O/THF. When f was increased to 10%, a
2
w
weak fluorescence of DMA can be observed, due to the di-
pole-dipole interaction between the DMA in its excited state
with the increase of f , the diffraction peak steadily in-
w
creased, at f =70%, the diffraction signal reached its max-
w
and the surrounding solvent molecules [28]. As f further
imum. It indicated a transformation from poor crystalline
state to good crystalline state. In other words, self-assembly
cubic microcrystal plays a role in the highly efficient pho-
toluminescence.
w
increases, the emission intensity enhanced with it. At
fw=70%, the emission intensity reached its maximum, which
is as 38 times its original intensity. Obviously, the DMA
Figure 2 (a) Absorption and emission spectra of DMA in DCM; (b) emission spectra of DMA in toluene, DCM, TCM, THF, CH
PL spectra of DMA recorded in water/THF mixed solvent with different f ; (d) the change of emission with different f ; (e) the SEM image of DMA in water/
THF, from f =0% to 90% (color online).
3
OH and DMSO; (c) the
w
w
w

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3
.2 Sensing performances to TNP
ently toward different explosives. For a clear comparison, we
made a histogram to illustrate the response of explosives.
Figure 3(f) indicated that the quenching efficiency of TNP is
2.2 times that of TNT, 4.7 times that of DNB and 11 times
that of PETN. While for RDX, MN and NG, there is even a
0.1 times fluorescence enhancement. This means that as a
sensing probe DMA showed a good selectivity to TNP.
We investigated the fluorescence quenching ratio of DMA
fw from 0 to 90%) upon the addition of TNP (Figure 3(b)).
(
Clearly, a ratio of 90% gives the best quenching efficiency,
but as mentioned above, the emission intensity of f =70% is
the largest, and the quenching efficiency of 70% is com-
parable with that of 90%. So f =70% was chosen for the
w
1
w
In order to better understand the sensing mechanism, H
following research. Figure 3(c) showed the fluorescent ti-
tration of DMA upon a gradual addition of TNP aqueous
solution. The emission was quenched with it, and the largest
quenching efficiency is 97% with a TNP concentration of
NMR spectra was used to explore of the intermolecular in-
teraction of TNP and DMA. With the addition of TNP to
DMA in d -DMSO, the proton signals of position a (–CHO),
6
b (Ph–H) and c (–CH ) were upfield shifted (Δ=0.03, 0.04
3
−
5
4
.3×10 M. As shown in Figure 3(c), when the increasing
concentrations of TNP were introduced into mixed solution
and 0.02 ppm) as shown in Figure 4(A). It indicates that there
is a strong hydrogen bonding interaction between DMA and
TNP arising from –CHO unit and –OH unit [17]. Further-
more, we also studied the morphology change of DMA be-
fore and after the addition of TNP by SEM as shown in
Figure S4(a, b).
Clearly, the cubic microcrystals were destroyed into very
small particles arising from the strong H bonding interaction
of DMA with TNP. To know the origin of the fluorescence
quenching, the energy level of DMA was compared with
those of TNP and TNT. The calculated LUMO (−3.47 eV) of
DMA is higher than those of TNP (−3.93 eV) and TNT
(−3.80 eV) (Figure 4(B)) [30,31], suggesting that a photo-
induced electron transfer (PET) process is responsible for the
fluorescence quenching for both TNP and TNT. Considering
that TNP showed a more efficient fluorescence quenching
than TNT, other effect may also be involved. We tested the
absorption spectra of TNP and emission spectrum DMA
−
4
of DMA (1.1×10 M), the emission peak at 500 nm was
decreased rapidly. The linear regression equation was de-
termined to be F=−12187.4[PA]+244227.8 (n=10, R =0.97).
2
Accordingly, the limit of detection (LOD) could be calcu-
lated according to the linear relationship of the emission
intensity and TNP concentration (Figure 3 (d)). Here
LOD=3S /ρ [29], where S is the standard deviation of the
d
d
blank measurements and ρ is the slope of intensity versus
sample concentration. The LOD is calculated to be
−
7
1
.2×10 M (0.0275 mg/L), which is the best result for TNP
detection in water. It is far lower than the permissible level
(0.5 mg/L) for TNP in drinking water.
Selectivity of DMA to TNP was tested against other ex-
plosives such as nitroarence (NB, DNB and TNT), nitroester
PETN, NG and MN), nitramine (RDX). As shown in Figure
(e), the fluorescence of DMA was quenched quite differ-
(
3
Figure 3 (a) The distribution of HOMO and LUMO of DMA calculated by TD-DFT; (b) the fluorescence intensity different f
w
in 1 equiv. TNP; (c) PL
=70%); (d) plot of fluorescence
intensity versus the concentration of TNP; (e) the emission of DMA added into explosives such as TNP, TNT and DNT, and others; (f) quenching efficiency
bar diagram for different nitro analytes in the case of DMA) (color online).
−
4
−4
spectra of DMA (1.1×10 M) upon gradual addition of TNP with 6.6×10 M (from 5 to 250 μL) in mixture solvent (f
w
(

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1
Figure 4 (A) The H NMR of DMA, +0.5 equiv. TNP, 1.0 equiv. TNP, 1.5 equiv. TNP; (B) electron transfer process between DMA and PA, TNT (color
online).
and Technology (2016YFA0200800), the National Natural Science Foun-
under solution condition. It demonstrated that there is partial
overlap in their spectra (Figure S5), confirming the fluores-
cence resonance energy transfer (FRET) between DMA and
dation of China (51473182, 61731016, 61771460), and the Youth Innova-
tion Promotion Association of Chinese Academy of Sciences (2015190).
TNP. As comparison, there is no spectral overlap between
TNT and DMA, implying no FRET between them. So the
additional quenching effect will result in the selectivity of
DMA to these two explosives. In addition, as mentioned
above, there is H-bonding interaction between DMA and PA,
so DMA could come much closer with PA than that of TNT.
As we know, both PET and FRET are related to the distance
of the donor and acceptor molecules, so the PETwill be more
efficient between DMA and TNP than that of TNT, needless
to say there is also FRET process between DMA and TNP.
This explained why DMA can selectively detect TNP while
avoiding the interference of analog TNT.
Conflict of interest The authors declare that they have no conflict of
interest.
Supporting information The supporting information is available online at
chem.scichina.com and link.springer.com/journal/11426. The supporting
materials are published as submitted, without typesetting or editing. The
responsibility for scientific accuracy and content remains entirely with the
authors.
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