Th4+ tuned aggregation-induced emission: A novel strategy for sequential ultrasensitive detection and separation of Th4+ and Hg2+

Article abstract of DOI:10.1016/j.saa.2019.117926

A novel strategy, Th⁴⁺ tuned aggregation-induced emission, for sequential ultrasensitive detection and separation of Th⁴⁺ and Hg²⁺ was developed successfully. For demonstration this strategy, we designed and synthesized tw

Full text of DOI:10.1016/j.saa.2019.117926

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Th4+ tuned aggregation-induced emission: A novel strategy for
sequential ultrasensitive detection and separation of Th4+ and
Hg2+
You-Ming Zhang, Wei Zhu, Qi Zhao, Wen-Juan Qu, Hong Yao,
Tai-Bao Wei, Qi Lin
PII:
S1386-1425(19)31317-4
https://doi.org/10.1016/j.saa.2019.117926
SAA 117926
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 16 August 2019
Revised date:
21 November 2019
6 December 2019
Accepted
date:
Please cite this article as: Y.-M. Zhang, W. Zhu, Q. Zhao, et al., Th4+ tuned aggregation-
induced emission: A novel strategy for sequential ultrasensitive detection and separation
of Th4+ and Hg2+, Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy(2019), https://doi.org/10.1016/j.saa.2019.117926
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Th4+ tuned aggregation-induced emission: a novel strategy for
sequential ultrasensitive detection and separation of Th4+ and
Hg2+
You-Ming Zhanga,b,*, Wei Zhub, Qi Zhaob, Wen-Juan Qub, Hong Yaob, Tai-Bao Weib,
Qi Linb,*
a College of Chemistry and Chemical Engineering, Lanzhou City University, Lanzhou, Gansu,
730070, P. R. China. E-mail: zhangnwnu@126.com.
b Research Center of Gansu Military and Civilian Integration Advanced Structural Materials,
College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu,
730070, P. R. China. E-mail: linqi2004@126.com.
Abstract
A novel strategy, Th4+ tuned aggregation-induced emission, for sequential
ultrasensitive detection and separation of Th4+ and Hg2+ was developed successfully.
For demonstration this strategy, we designed and synthesized two tripodal gelators
TH (tri-(isoniazid-4-yl)-functionalized trimesic acylhydrazine) and TA
(tri-(pyridine-4-yl)-functionalized trimesic amide). The TH and TA could assemble
into a stable supramolecular polymer hydrogel THTA-G in DMSO/H2O (3.3:6.7, v/v)
binary-solution. The THTA-G does not show aggregation-induced emission (AIE)
effect. However, after addition of Th4+ into the THTA-G, the obtained metallogel
THTA-GTh shows strong green AIE effect, which indicated that Th4+ could tune the
gel generation of AIE effect. Interestingly, the THTA-G could ultrasensitive
fluorescently detect Th4+, and the corresponding metallogel THTA-GTh could
ultrasensitively detect Hg2+. The detection limits of THTA-G and THTA-GTh for
Th4+ and Hg2+ are 8.61 × 10-11 mol/L and 1.08 × 10-11 mol/L, respectively.
Additionally, the xerogels of THTA-G and THTA-GTh could separate Th4+ and
Hg2+ from aqueous solution with excellent ingestion capacity, and the THTA-G could
be used as a writable smart light-emitting material.
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Keywords: Ultrasensitive, Detection and separation, Aggregation-induced emission;
Supramolecular polymer hydrogel, Smart light-emitting material
1. Introduction
In recent years, ultrasensitive detection has attracted much attention due to the
development of ultrasensitive strategies for detecting specific analytes is crucial for
biological samples and environmental safety [1-3]. To date, some strategies have been
reported for detect ultralow concentrations of heavy metal ions. For example, Quan et
al. reported a ratiometric fluorescence sensor for ultrasensitive detection of Cu2+ ion
by a cation-exchange strategy [4]. Nandi et al. synthesized a nitrogen doped
PEGylated carbon dots (C-dots) for ultrasensitive detection of Hg2+ ion [5]. However,
these strategies suffer from several drawbacks that include high-cost, sophisticated
procedures, and lack of portability [6,7]. Therefore, exploring novel strategies for
efficient ultrasensitive detection of important analytes is great urgently needed.
As we all know, Thorium (typical as Th4+) is an important radioactive element in
nuclear safeguards due to its broad applications in many nuclear industries. However,
the presence of Th4+ in the environment is an important concern due to its toxicity and
health effects on human body [8,9]. In addition, Hg2+ can readily penetrate through
biological membranes and cause severe damage to the brain, heart, kidneys and
nervous system [10,11]. All of the above reasons promote urgent research on Th4+ and
Hg2+ ultrasensitive detection and separation from environmental.
In 2001, Tang’s group [12] pioneered the concept of aggregation-induced emission
(AIE), which is essentially the opposite effect to notorious aggregation-caused
quenching (ACQ) [13]. Enhanced emission is attributed to the synergetic effect of
aggregation-induced planarization and J-type aggregate formation, which help to
prevent intramolecular rotations [14,15]. Since this pioneering work, a series of
interesting molecules with AIE effect have been studied and used as chemosensors,
stimuli-responsive nanomaterials, organic light-emitting diodes, etc [16-18]. It is
worth noting that the development of AIE also provide a new platform for
ultrasensitive detection.
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Smart supramolecular polymer hydrogels are an important subclass in the area of
supramolecular polymers [19,20], which can be constructed from gelators assembling
through the driving force of multiple non-covalent interactions (H···π, H···H, O···H,
and N···H) and weak intermolecular face-to-face π-π interactions in appropriate
solvent systems [21-24]. Nowadays, supramolecular polymer hydrogels have been
widely applied in chemosensors, biomaterials, pollutant removal, drug delivery and
light-harvesting system due to their unique structures and fantastic properties [25-29].
However, smart supramolecular polymer hydrogels based on two tripodal gelators,
which can carry out sequential ultrasensitive detection and separation of Th4+ and
Hg2+ are never been reported.
Inspired by this fact and as a part of our research effort [30-32], herein, we reported
a novel strategy, Th4+ tuned aggregation-induced emission, for sequential
ultrasensitive detection and separation of Th4+ and Hg2+. First, two tripodal gelators
TH (tri-(isoniazid-4-yl)-functionalized trimesic acylhydrazine) and TA
(tri-(pyridine-4-yl)-functionalized trimesic amide) were designed and successfully
prepared. The two gelators contain multiple hydrogen-bonding sites (acylhydrazine
and amide groups), which might be great use in the supramolecular self-aggregation
process. Meanwhile, multiple coordination sites for metal ions also give the two
gelators second driving force for assembly through the strong coordination
interactions between metal ions and acylhydrazine and amide groups.
Indeed, the TH (host) and TA (guest) could assemble into a stable supramolecular
polymer hydrogel THTA-G with weak fluorescence in a mixed solvent of
DMSO/H2O (3.3:6.7, v/v). Intriguingly, after addition of Th4+ into the THTA-G, the
obtained metallogel THTA-GTh showed strong green AIE effect. These results
indicated that Th4+ could restriction of intramolecular rotation (RIR), and then tune
the gel generation of AIE effect. Additionally, Hg2+ could induce the fluorescence of
THTA-GTh to weaken again by cation-π interactions. Therefore, Th4+ tuned
aggregation-induced emission is a simple and efficient novel strategy for sequential
ultrasensitive detection of Th4+ (LOD = 8.61 × 10-11 mol/L) and Hg2+ (LOD = 1.08 ×
10-11 mol/L). Significantly, the xerogels of THTA-G and THTA-GTh could
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separation of Th4+ and Hg2+ from aqueous solution with nice adsorption property,
respectively. More importantly, the THTA-G could act as a novel smart
light-emitting material.
2. Experiment section
2.1. Materials and instruments
All starting materials were obtained from commercial supplies and used as
1
received. All cations (Table S1) were made into aqueous solution (c = 0.1 mol/L). H
NMR spectra were recorded on Mercury-600 BB spectrometer at 600 MHz and
Mercury-400 BB spectrometer at 400 MHz. 13C NMR spectra were recorded on
Mercury-600 BB spectrometer at 151 MHz. High-resolution mass spectra were
performed on Bruker Esquire 3000 plus mass spectrometer equipped with ESI
interface and ion trap analyzer. Fluorescence spectra were performed on Shimadzu
RF-5301PC fluorescence spectrophotometer. Infrared spectra were performed on
Digilab FTS-3000 FTIR spectrophotometer and X-ray diffraction (XRD) pattern was
tested using Rigaku RINT2000 diffract meter equipped (copper target; λ = 0.154073
nm). Scanning electron microscopy (SEM) images of the xerogels were investigated
using JSM-6701F instrument at an accelerating voltage of 8 kV. Transmission
electron microscope (TEM) images of the xerogels were obtained on FEI Tecna G2
F20 apparatus operating at 200 KV. Fluorescent information of the gels was
characterized using Laser Scanning Confocal Microscope (LSCM, Olympus Fluoview
FV1200).
Scheme 1
2.2. Synthesis and characterization of TA.
Dissolving the 1,3,5-benzenetricarbonyl trichloride (0.2655 g, 1.0 mmol) with
DMF (10 mL), then the solution was dropwise added to a mixture of 4-aminopyridine
(0.3106 g, 3.3 mmol) and TEA (1 mL) in DMF (20 mL). The reaction mixture was
stirred at room temperature for 24 h. Recrystallization of TA after the reaction was
completed, and then dried under vacuum. Yield: 90% (0.3943g). M.p.: 270～272
℃.
1H NMR (400 MHz, DMSO-d6), δ/ppm: 10.98 (s, 3H), 8.79-8.75 (t, J = 8.6 Hz, 3H),
8.55-8.52 (t, J = 5.4 Hz, 6H), 7.84-7.82 (t, J = 4.4 Hz, 6H). 13C NMR (151 MHz,
DMSO-d6), δ/ppm: 166.05, 165.66, 150.87, 150.78, 146.26, 146.13, 145.41, 135.38,
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134.92, 132.15, 131.03, 114.60, 109.24. HRMS: m/z [TA + H]+ calcd for C24H19N6O3,
439.1519; found 439.1515.
2.3. Synthesis and characterization of TH.
Dissolution of 1,3,5-benzenetricarbonyl trichloride (0.2655 g, 1.0 mmol) with
CH2Cl2 (10 mL), then the solution was dropwise added to a solution of isoniazid
(0.4526 g, 3.3 mmol) in CH2Cl2 (20 mL). After the reaction mixture was stirred at
room temperature for 24 h, the TH was filtrated and then dried under vacuum. Yield:
1
92% (0.5521g). M.p.: 167～169
℃
. H NMR (400 MHz, DMSO-d6), δ/ppm:
13
11.03-11.01 (d, J = 10.4 Hz, 6H), 8.74-8.73 (m, 9H), 7.78-7.77 (m, 6H). C NMR
(151 MHz, DMSO-d6), δ/ppm: 165.16, 164.55, 164.50, 150.64, 140.13, 138.94,
133.73, 130.37, 121.97, 121.91. HRMS: m/z [TH + Na]+ calcd for C27H22N9O6Na,
590.1512; found 590.1584.
2.4. Preparation of supramolecular polymer hydrogel THTA-G.
The THTA-G was prepared by addition of TH (0.01 g) and TA (0.0077 g) to the
DMSO/H2O (3.3:6.7, v/v) solution (300 μL) in a reagent bottle (1 mL), and then
heated the mixture until the two gelators were dissolved completely. After the solution
was cooled to room temperature, the supramolecular polymer hydrogel THTA-G
formed. Qualitatively, gelation was considered successful if no sample flow was
observed upon inversion of the container at 25 °C. Additionally, the xerogel was
obtained by drying hydrogel THTA-G under reduced pressure.
2.5. Preparation of supramolecular polymer metallogel THTA-GTh.
The gelators TH (0.01 g) and TA (0.0077 g) were put in a reagent bottle (1 mL),
and the mixture of DMSO/H2O (3.3:6.7, v/v) solution (300 μL) were also added to the
reagent bottle as solvents. Then, the aqueous solution of Th4+ (0.65 μL, 0.1 mol/L)
was added to the mixture. After that, the mixture was heated until the solid were
dissolved completely, then cooled it to room temperature, the supramolecular polymer
metallogel THTA-GTh will be obtained. Additionally, the xerogel was obtained by
drying metallogel THTA-GTh under reduced pressure.
2.6. The technical details for the measurement of fluorescence spectra of gel sample.
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First, in order to investigate the fluorescence response properties of THTA-G to
metal ions, various metal ions aqueous solution (Hg2+, Fe3+, Ag+, Zn2+, Ca2+, Ni2+,
Co2+, Pb2+, Cr3+, Cu2+, Mg2+, Ba2+, Cd2+, Al3+, Eu3+, Tb3+, La3+ and Th4+) were added
into THTA-G (3.3 wt%, 10 mg/ml = 1.0%), respectively. Any fluorescence changes
of THTA-G after addition of different metal ions were recorded on a Shimadzu
RF-5301PC fluorescence spectrometer. As a result, only upon the addition of Th4+
into THTA-G, the fluorescence emission of THTA-G could be enhanced and make
the fluorescence become strong green. The obtained Th4+ coordinated metallogel is
named as THTA-GTh.
Next, for THTA-GTh, in order to study the successive fluorescence response
properties of THTA-GTh to metal ions, a series of metal ions aqueous solution such
as Hg2+, Fe3+, Ag+, Zn2+, Ca2+, Ni2+, Co2+, Pb2+, Cr3+, Cu2+, Mg2+, Ba2+, Cd2+, Al3+,
Eu3+, Tb3+ and La3+ were added into THTA-GTh, respectively. Then, we also use
Shimadzu RF-5301PC spectrometer to record the changes in fluorescence of the
metallogels. As a result, only Hg2+ could induce the fluorescence of THTA-GTh to
weaken again.
Fig. 1.
3. Results and discussion
The gelators TH and TA were prepared via two simple routes (Scheme 1) and well
characterized using 1H NMR, 13C NMR and HRMS techniques (Fig. S1-S6). The TH
and TA could form a stable supramolecular polymer hydrogel THTA-G in a mixed
solvent system of DMSO/H2O (3.3:6.7, v/v) (Fig. 1). The gelation concentration of
THTA-G is about 3.3 wt%, and gel-sol transition temperature (Tgel) is approximately
115
℃. The fluorescence intensity of THTA-G is very strong when the temperature
is higher than Tgel. However, after the THTA-G formed, the fluorescence intensity
decreased approximately by 2.7-fold and the emission maxima shifted from 470 to
485 nm (Fig. 2a). The laser scanning confocal microscopy (LSCM) experiments also
demonstrated the THTA-G emitting weak fluorescence under ultraviolet light (Fig.
3a). This phenomenon of decreased fluorescence could be satisfactorily explained by
a typical ACQ (aggregation-caused quenching) mechanism [33,34]. In this case,
intermolecular hydrogen bond interactions or other non-radiative channels to form
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excimer or excinlex consume the energy of excited state and cause a decrease in
radiative emission.
Fig. 2.
Fig. 3.
The possible assembly mechanism of TH and TA was investigated via
high-resolution mass spectrometry (HRMS), proton nuclear magnetic resonance
spectroscopy (1H NMR), two-dimensional NOESY nuclear magnetic resonance
spectroscopy (2D NOESY NMR), fourier-transform infrared spectroscopy (FT-IR),
X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) analyses. In the HRMS of xerogel THTA-G (Fig. S7), a
peak exhibited at m/z = 1006.3138 [TH+TA+H]+, which confirmed the stoichiometry
of TH and TA was 1:1. As shown in Fig. 4a, with the addition of TA to TH, the
signals of protons Ha and Hb on TH shifted downfield, and Hc, Hd, He on TH shifted
upfield. Meanwhile, the signals of protons H1, H2, H3 and H4 on TA shifted downfield.
As the concentration of TH and TA increased (Fig. 4b), the signals of protons Ha and
Hb on TH shifted downfield, while Hc, Hd and He shifted upfield. At the same time,
the signals of protons H1, H2, H3 and H4 on TA shifted downfield. These results
confirmed the existence of intermolecular interactions in the assemble process of TH
and TA, and stable intermolecular hydrogen bonds also formed between N-H and
C=O groups. Moreover, all the signal peaks became broad at high concentration,
which illustrated the formation of supramolecular polymer driven by intermolecular
interactions. The 2D NOESY NMR (Fig. S8) experiments displayed the correlation
between signals of protons Ha, Hb, Hc, Hd, He and H1, H2, H3, H4, which also
demonstrated the existence of intermolecular interactions between TH and TA.
Fig. 4.
According to the FT-IR spectra of TH, TA and THTA-G (Fig. S9a), for THTA-G,
compared with TH (N-H: 3470 cm-1, C=O: 1664 cm-1) and TA (N-H: 3475 cm-1,
C=O: 1615 cm-1), the stretching vibration peaks of N-H and C=O all became a broad
peak and shifted to 3465 cm-1 and 1610 cm-1, respectively, which also indicated that
hydrogen bond interactions involved in the assemble process of TH and TA. In
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addition, the assembly process of TH and TA was also investigated by the
small-angle XRD pattern. As shown in Fig. S10a, there were many sharp peaks at 2θ
from 20 to 30 degrees, indicating that TH and TA have long-range ordered structure
in the xerogel state. However, there were only two sharp peaks at 2θ from 10 to 15
degrees in THTA-G, suggesting that π-π stacking interaction were not involved in the
gelation process, and hydrogen bond interactions involved in the assemble process of
TH and TA [35,36]. Importantly, the SEM images (Fig. S11) also supported above
proposed assemble mechanism. The TA shows rod-like structure and TH shows
amorphous structure. However, the xerogel of THTA-G shows tire-shaped structure
with tightly stacking. Moreover, in TEM image of THTA-G (Fig. S12a), the distance
between layer and layer of tire is about 72 nm. These results indicated that the TA and
TH could assemble into supramolecular polymer hydrogel THTA-G in aqueous
media by intermolecular interactions (Scheme 2a).
Scheme 2.
Fig. 5.
The metal ions response properties of THTA-G were investigated carefully by the
addition of various metal ions aqueous solution including Hg2+, Fe3+, Ag+, Zn2+, Ca2+,
Ni2+, Co2+, Pb2+, Cr3+, Cu2+, Mg2+, Ba2+, Cd2+, Al3+, Eu3+, Tb3+, La3+ and Th4+ into the
THTA-G, respectively. As a result, only Th4+ could enhance the fluorescence of
THTA-G and make the fluorescence become strong green (Fig. 5a and 6a). The
results suggested that THTA-G could selectively detect Th4+. According to
fluorescent titrations of THTA-G for Th4+ (Fig. 6c) and 3δ/S method [37], the
detection limit of THTA-G for Th4+ is 8.61 × 10-11 mol/L (Fig. 7a and S13).
Moreover, the THTA-G shows good binding affinity with Th4+ with an association
constant (Ka) of 7.14 × 104 mol/L (Fig. S14). Importantly, the LOD (limit of detection)
of THTA-G for Th4+ is lower than some other reported fluorescent sensors for Th4+
(Table S2), which indicated that the THTA-G could carry out ultrasensitive and
selective detection of Th4+ from aqueous solution. Meanwhile, in order to investigate
the selectivity of THTA-G for Th4+, the control experiments were carried out under
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the competition conditions. The coexist competition cations could not induce any
interfere for the Th4+ sensing process for THTA-G (Fig. S15).
Fig. 6.
Fig. 7.
One of the values mentioned is that the THTA-G could be tuned by Th4+ to form
metallogel THTA-GTh. The fluorescence intensity of THTA-GTh is negligible
when the temperature is higher than 95 ℃. However, when the THTA-GTh formed,
the fluorescence intensity increased approximately by 5.6-fold and the emission
maxima shifted from 465 to 470 nm (Fig. 2b). The LSCM experiments also
demonstrated the THTA-GTh emitting strong green fluorescence under ultraviolet
light (Fig. 3b). This phenomenon of enhanced fluorescence could be explained by an
amazing AIE mechanism [38,39]. In this case, Th4+ could restriction of intramolecular
motions in the aggregation state, and then blocks the nonradiative decay of excited
state species and opening up a radiative channel. Moreover, being influenced by the
aggregation effect, the emission maximum of metallogel was found to be red shifted
by 5 nm. Therefore, the strong green fluorescence is an aggregation-induced emission
(AIE).
Then, the successive metal ions response properties of THTA-GTh were
investigated by adding a series of metal ions aqueous solution such as Hg2+, Fe3+, Ag+,
Zn2+, Ca2+, Ni2+, Co2+, Pb2+, Cr3+, Cu2+, Mg2+, Ba2+, Cd2+, Al3+, Eu3+, Tb3+ and La3+
into THTA-GTh, respectively. As a result, the THTA-GTh could selectively
recognize Hg2+ obviously (Fig. 5b). The LSCM experiment demonstrated the complex
of THTA-GTh and Hg2+ also emitting weak fluorescence under ultraviolet light (Fig.
3c). According to fluorescent titrations of THTA-GTh for Hg2+ (Fig. 6d) and 3δ/S
method, the LOD of THTA-GTh for Hg2+ is 1.08 × 10-11 mol/L (Fig. 7b and Fig.
S16). Importantly, the LOD of THTA-GTh for Hg2+ is also lower than some other
reported fluorescent sensors for Hg2+ (Table S3). Thus, the THTA-GTh is capable of
ultrasensitive and selective detection of Hg2+ from aqueous solution. Meanwhile, in
order to investigate the selectivity of THTA-GTh for Hg2+, the control experiments
were carried out under the competition conditions. The coexist competition cations
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could not induce any interfere for the Hg2+ sensing process for THTA-GTh (Fig.
S17).
The possible sequential ultrasensitive detection mechanism of THTA-G for Th4+
and Hg2+ was investigated carefully by HRMS, FT-IR, XRD, SEM and TEM analysis.
In HRMS (Fig. S18), a positive ion peak showed at m/z = 1234.3651
[TH+TA+Th-3H]+, which verified a 1:1 stoichiometric complex formed between
THTA-G and Th4+. In the FT-IR spectra of THTA-G (Fig. S9b), the stretching
vibration peaks of N-H and C=O appeared at 3465 cm-1 and 1610 cm-1, respectively.
After addition of Th4+ into the THTA-G and the formation of THTA-GTh, the
stretching vibration peaks of N-H and C=O shifted to 3435 cm-1 and 1635 cm-1,
respectively, which was attributed to the coordination of Th4+ with N-H and C=O
groups in THTA-GTh. In addition, the THTA-GTh didn’t have sharp peaks at 2θ
from 15 to 30 degrees, and only one sharp peak appeared at 2θ from 10 to 15 degrees
(Fig. S10b). These results indicated that the THTA-G could combine with Th4+ by
metal coordination interactions [40-42], and π-π stacking interactions were not
1
involved in THTA-GTh. As shown in Fig. 8a, in the concentration-dependent H
NMR spectra of TH, with increasing concentration of TH, the signals of protons Ha,
Hb, Hc, Hd and He on TH all showed distinct shifted downfield and these signal peaks
became broad at high concentration, which confirmed that gelator TH could
assemble to high-molecular-weight aggregates via hydrogen bond interactions. After
addition of Hg2+ to THTA, the signals of protons Ha and Hd almost no shifted,
meanwhile, the signals of protons Hc and He on TH, and H3 and H4 on TA all shifted
downfield (Fig. 8b). To our surprise, for the complex of THTA-GTh and Hg2+, the
stretching vibration peaks of N-H and C=O shifted to 3459 cm-1 and 1617 cm-1
respectively (Fig. S9b), which indicated the hydrogen bonds were formed again.
Meanwhile, the d-spacings of 4.13 Å and 3.17 Å at 2θ = 21.49° and 28.17° appeared
(Fig. S10b). These results confirmed that Hg2+ didn’t interact with N-H and C=O
groups, but formed cation-π interactions with the THTA-GTh [43,44]. Moreover, the
1:3 stoichiometric complex between THTA-G and Hg2+ was confirmed by HRMS
(Fig. S19). However, after Hg2+ was added into the THTA-GTh, we didn’t find Th4+
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participate in the reaction. Therefore, after addition of Hg2+ into the THTA-GTh,
Th4+ no longer interacts with the gel.
Fig. 8.
In SEM (Fig. S11), the THTA-GTh shows tire-shaped structure, but compared
with the THTA-G, the distance between layer and layer of tire is wider. After
addition of Hg2+ into the THTA-GTh, the micromorphology also shows tire-shaped
tightly stacking structure. In TEM, for THTA-GTh, the distance between layer and
layer of tire structure is about 150 nm (Fig. S12b). After adding Hg2+ to the
THTA-GTh, the distance between layer and layer of tire is about 57 nm (Fig. S12c).
According to aforementioned, Th4+ could induce the THTA-G to carry out
supramolecular aggregates through metal-ligand coordination, leading to the
THTA-GTh showed distinct green AIE effect. Clearly, this fluorescent enhancement
process attributing to Th4+ tuned aggregation induced emission. In addition, Hg2+ has
high ionic strength [45], which is easy to induce the π electrons on benzene ring
groups of TH and TA transfer to Hg2+ and act as cation-π interactions, so that Hg2+
could be identified through cation-π interactions. Therefore, Hg2+ could form stable
cation-π interactions with the THTA-GTh [46], and then the fluorescence of
THTA-GTh was induced to weaken again. These results cause the selectivity
response of THTA-GTh for Hg2+ over other metal ions.
As shown in Fig. S20, there arise five broad absorption bands around 275 nm in
the UV-vis spectra of TH, TA, THTA-G, THTA-GTh and THTA-GTh with analyte
ions. Compared with the absorbance of TH and TA, the absorbance of THTA-G was
obviously decreased, which indicated that the TH and TA could assemble into
aggregates through intermolecular hydrogen bonding [47]. Meanwhile, due to
aggregation, the emission of THTA-G in the gel state was weak significantly.
However, after addition of Th4+ into the THTA-G, the Th4+ could restriction of
intramolecular motions (RIM) in the aggregation state, and makes the absorbance of
THTA-GTh increase again. In addition, the characteristic absorption presented a
slight red shift and broadening with increasing Hg2+ to the THTA-GTh, as well as the
absorbance of complex was also decreased, suggesting the Th4+ could combine with
the aromatic moieties of TH and TA molecules via cation-π interactions [48].
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Therefore, the THTA-G could carry out sequential ultrasensitive detection of Th4+
and Hg2+ through a novel mechanism (Scheme 2b). Based on sequential ultrasensitive
detection of THTA-G to Th4+ and Hg2+, a cations-controlled fluorescence response
switch was prepared, and the switching performance could be repeated at least four
successive cycles with little fluorescent efficiency loss (Fig. S21).
In order to investigate the adsorption rates of THTA-G for Th4+ and THTA-GTh
for Hg2+, the xerogels sample of THTA-G (0.001 g) and THTA-GTh (0.001 g) were
placed in dilute aqueous solution of Th4+ (5 mL, c = 1 × 10-4 mol/L) and Hg2+ (5 mL,
c = 1 × 10-4 mol/L), respectively. After the two mixtures were stirred at room
temperature for 20 h, the precipitates were separated by centrifugation (1000r/min, 10
min). Then, the supernatant was analysed by inductively coupled plasma (ICP). As a
result, the adsorption rates of THTA-G for Th4+ and THTA-GTh for Hg2+ are up to
99.82% and 99.95%, respectively (Table 1). These results indicated that the xerogels
of THTA-G and THTA-GTh have nice separation properties for Th4+ and Hg2+ in
aqueous solution, respectively.
Table 2.
Fig. 9.
A gel film based on the THTA-G was prepared by loading THTA-G on a clean
glass surface. Upon addition of Th4+, the fluorescence of the gel film was changed
from weak to strong green. However, after Hg2+ was added, the fluorescence of the
gel film weakens again (Fig. 9). Therefore, the gel film based on THTA-G is suitable
for immediate visualization of Th4+ and Hg2+ by a very simple practical method that
should be applicable for rapid on-site testing.
4. Conclusion
In conclusion, a novel strategy for sequential ultrasensitive detection of Th4+ and
Hg2+ based on “Th4+ tuned aggregation-induced emission” mechanism was proposed.
Two easy-to-make tripodal gelators TH and TA could assemble into a stable
supramolecular polymer hydrogel THTA-G by intermolecular interactions in aqueous
media. Interestingly, Th4+ could tune the THTA-G shows strong green AIE effect
with ultrasensitive response to Th4+. Meanwhile, the obtained metallogel THTA-GTh
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shows ultrasensitive stimuli-responsive to Hg2+. The detection limits of THTA-G and
THTA-GTh for Th4+ and Hg2+ are 8.61 × 10-11 mol/L and 1.08 × 10-11 mol/L,
respectively. Moreover, the separate rates of THTA-G and THTA-GTh for Th4+ and
Hg2+ reached up to 99.82% and 99.95%, respectively. Therefore, the THTA-G and
THTA-GTh have good potential for practical application in ions detection and
separation, and smart light-emitting materials. This work enriches the AIE effect and
provides new thinking for the smart materials design.
Declaration of Competing Interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(Nos. 21661028; 21662031; 21574104), the Program for Changjiang Scholars and
Innovative Research Team in University of Ministry of Education of China (IRT
15R56).
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Author statement
You-Ming Zhang: Conceptualization, Funding acquisition, Validation.
Wei Zhu: Software, Formal analysis, Data Curation, Writing - Original Draft.
Qi Zhao: Data Curation.
Wen-Juan Qu: Data Curation.
Hong Yao: Supervision, Project administration.
Tai-Bao Wei: Funding acquisition, Methodology, Visualization.
Qi Lin: Funding acquisition, Investigation, Writing - Review & Editing.
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
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Figure Captions
Scheme 1. Synthesis of compounds TA and TH
.
Fig. 1. Gelation property of supramolecular polymer hydrogel THTA-G (3.3 wt%, 10
mg/ml = 1.0%) in DMSO/H2O (3.3:6.7, v/v) binary-solution.
Fig. 2. Temperature-dependent fluorescence spectra of (a) THTA-G and (b)
THTA-GTh during the gelation process (λex = 370 nm).
Fig. 3. The laser scanning confocal microscopy (LSCM) images of (a) THTA-G; (b)
THTA-GTh; (c) THTA-GTh+Hg2+.
Fig. 4. (a) Partial 1H NMR (400 MHz, DMSO-d6) spectra of THTA, from bottom to
top: 2.82 × 10-2 mol/L TH; 3.65 × 10-2 mol/L TA; 2.82 × 10-2 mol/L TH and 5.64 ×
10-2 mol/L TA. (b) Partial concentration-dependent 1H NMR spectra (400 MHz,
DMSO-d6) of THTA, from bottom to top: 2.82 × 10-2 mol/L TH; 2.74 × 10-2 mol/L
TA; 1.05 × 10-1 mol/L TH and 8.22 × 10-1 mol/L TA.
Scheme 2. Schematic illustration of the sequential ultrasensitive detection and
separation of Th4+ and Hg2+ from aqueous solution based on the Th4+ tuned
aggregation induced emission mechanism.
Fig. 5. (a) Fluorescence response properties of THTA-G to different metal ions. (b)
Fluorescence response properties of THTA-GTh to different metal ions.
Fig. 6. Fluorescence spectra of (a) THTA-G and THTA-GTh; (b) THTA-GTh and
THTA-GTh+Hg2+. Fluorescence spectra of (c) THTA-G with increasing amounts of
Th4+; (d) THTA-GTh with increasing amounts of Hg2+ (λex = 370 nm).
Fig. 7. The detection limits of (a) THTA-G for Th4+; and (b) THTA-GTh for Hg2+.
Fig. 8. (a) Partial concentration-dependent 1H NMR (400 MHz, DMSO-d6) spectra of
TH, from bottom to top: 2.47 × 10-2 mol/L TH; 1.76 × 10-1 mol/L TH. (b) Partial 1H
NMR spectra (400 MHz, DMSO-d6) of THTA with various equivalents of Hg2+, from
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bottom to top: 2.82 × 10-2 mol/L TH; 2.74 × 10-2 mol/L TA; 1.0 equivalent of Hg2+;
2.0 equivalents of Hg2+.
Fig. 9. Photographs of THTA-G-based films fluorescently (365 nm UV-lamp) sense
Th4+ and Hg2+.
Table 1. Adsorption rates of THTA-G for Th4+ and THTA-GTh for Hg2+.
Initial concentration
(mol/L)
Residual concentration
(mol/L)
Ion
Adsorption rate%
99.82%
1.0 × 10-4
1.80 × 10-7
Th4+
Hg2+
1.0 × 10-4
5.0 × 10-8
99.95%
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Graphical abstract
A novel strategy, Th4+ tuned aggregation-induced emission was developed
successfully. Based on this strategy, sequential ultrasensitive detection and separation
of Th4+ and Hg2+were achieved.
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Highlights
1. A novel strategy for sequential ultrasensitive detection of Th4+ and Hg2+.
2. Stable supramolecular polymer hydrogel THTA-G was constructed.
3. Th4+ could tune the hydrogel THTA-G generation of AIE effect.
4. The xerogels of THTA-G and THTA-GTh could separation of Th4+ and Hg2+.
5. THTA-G could act as a novel smart light-emitting material.
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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5