A tetraphenylethene and maltoheptaose conjugate with aggregation-induced emission (AIE) characteristic for temperature sensors

Article abstract of DOI:10.1039/c8nj03338b

An amphiphilic maltoheptaose-tetraphenylethylene (Mal-TPE) conjugate can assemble in an aqueous medium and the aggregates exhibit an aggregation-induced emission (AIE) effect. The sizes of Mal-TPE aggregates decrease with the increase in temperature, resulting in temperature-dependent fluorescence properties.

Full text of DOI:10.1039/c8nj03338b

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Tetraphenylethene and Maltoheptaose Conjugate with
Aggregation-Induced Emission (AIE) Characteristic for
Temperature Sensor
Received 00th January 20xx,
Accepted 00th January 20xx
Jie Hao,a Wen Yi Lv, b Chang Ming Li,a Bin Wanga and Li Qun Xu*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
The amphiphilic maltoheptaose-tetraphenylethylene prepared for applications in sensors,19,20 disease
(Mal-TPE) conjugate can assemble in aqueous medium theranostics,21 bioimaging22-25 and opto-electronic
and the aggregrates exhibit aggregation-induced systems.26 Tetraphenylethylene (TPE), a propeller-like
emission (AIE) effect. The sizes of Mal-TPE aggregrates molecule bearing four peripheral phenyl rings around a
decrease with the increase in temperature, resulting in central C═C bond, is a classical AIEgens.20 Free rotation
temperature-dependent fluorescence property.
of the four phenyl rings in the soluble state could serve
as a nonradiative relaxation pathway for the excited
electrons, resulting in no or weak fluorescence emission.
However, the restricted intramolecular rotation of these
phenyl rings in the aggregated state could prevent the
nonradiative relaxation, leading to strong fluorescence
enhancement.20,27,28 The solubility, phase and self-
assembly behaviors, intermicellar interactions, and
surface forces of non-ionic surfactants display
temperature-dependent effects.29 Since TPE is
hydrophobic and insoluble in aqueous media, its
conjugation with a hydrophilic component would lead to
the formation of amphiphilic surfactant with thermo-
responsive properties. The light-up characteristic of TPE
derivative could be modulated by lowering temperature
and used to monitor the change of temperature. As far
as we know, this is the first report on the construction of
fluorescent thermometer based on the amphiphilic AIE
luminogen.
Fluorescent thermometers have attracted increasing
attention in clinical, biomedical, industrial and
environmental applications.1-4 They allow “remote”
sensing of temperature based on various fluorescent
parameters, including fluorescence intensity, lifetime
and anisotropy, quantum yield, and peak wavelength.5-7
Among these characteristics, monitoring of changes in
the relative fluorescence intensity has been commonly
used due to its visually readable signal and simple
instrumental setup.5 To date, a wide variety of
luminophores, including metal-ligand complexes,8
organic and inorganic fluorophores,9-11 conjugated
polymers,12 metal organic frameworks,13 nanoclusters,14
quantum dots,15 and upconversion nanoparticles,16 have
been used to construct the fluorescent thermometers.
Aggregation-Induced Emission (AIE) is an exotic
photophysical phenomenon and have an exactly
opposite property of earlier aggregation-caused
quenching (ACQ) effect. The compound with AIE effect
is non-emissive in the soluble state but emits strong
fluorescence in the aggregated state.17 Since Tang et al.
first reported the AIE phenomenon of silole compound
in 2001,18 various AIE luminogens (AIEgens) have been
In this work, the azide-containing TPE was conjugated
with hydrophilic alkyne-terminated maltoheptaose via
copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC)
reaction (Figure 1). Maltoheptaose was obtained by a
ring-opening reaction of β-cyclodextrin, followed by
functionalization at the distal end with alkyne group.
Hydroxyl group in TPE-OH was methanesulfonated to
convert into azide moiety via azidation. The
intermediate products, Mal-alkyne and TPE-N3 were
reacted in the presence of copper catalyst to form the
Mal-TPE conjugate. The final products were successfully
a.Institute for Clean Energy and Advanced Materials, Faculty of Materials and
Energy, Southwest University, Chongqing 400715, P. R. China. E-mail:
xulq@swu.edu.cn
b.Key Laboratory of Luminescent and Real-Time Analytical Chemistry, Southwest
University, Chongqing 400715, P. R. China.
Electronic Supplementary Information (ESI) available: Experimental details, 1H NMR synthesized, as demonstrated by 1H NMR (Figure S1, ESI)
and mass spectra of Mal-TPE, time-dependent fluorescence spectra of Mal-TPE (20
and mass (Figure S2, ESI) spectroscopy.
μM). See DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C8NJ03338B
Figure 1. Schematic illustration for the synthesis of Mal-TPE conjugate.
The UV-visible absorption spectra of Mal-TPE in arising from the trigger of AIE characteristic upon the
phosphate-buffered saline (PBS) solution and N,N- aggregation of Mal-TPE in aqueous medium. The
dimethylformamide (DMF) (Figure 2a) exhibit aggregation of Mal-TPE in aqueous medium was verified
absorption peaks at about 300 and 305ꢀnm, respectively. by 1H NMR spectroscopy. As shown in Figure S3 (ESI), the
The absorption band of Mal-TPE in DMF is red-shifted by 1H NMR spectrum of Mal-TPE in D2O only shows partial
ca. 5 nm, which may be attributed to the change in proton signals of Mal segment. The shield of proton
solvation or interactions between Mal-TPE and solvents. signals of TPE segment suggests it has been assembled
Due to the good solubility of Mal-TPE in DMF, its solution within the interior of aggregates. The aggregation of
emits almost negligible fluorescence under UV light Mel-TPE in aqueous medium was further confirmed by
irradiation. However, a strong blue fluorescence is transmission electron microscopy (TEM) image. As
observed in the PBS solution of Mal-TPE at the same shown in Figure S4 (ESI), the Mal-TPE aggregates possess
condition (Inset of Figure 2b). This phenomenon may be irregular spherical shape and broad size distribution.
Figure 2. (a) UV-visible adsorption spectra of Mal-TPE (40 μM) in PBS solution and DMF; (b)
fluorescence spectra of Mal-TPE (40 μM) in PBS-DMF mixtures (0-100% for DMF fraction) with an
excitation wavelength of 350 nm; (c) fluorescence spectra of Mal-TPE in water at different pH; (d)
pH-dependent maximum fluorescence intensity of Mal-TPE in water.
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DOI: 10.1039/C8NJ03338B
800
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0
750
(a)
(b)
20 oC
25 oC
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40 oC
45 oC
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65 oC
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20 25 30 35 40 45 50 55 60 65
Temperature (oC)
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Figure 3. (a) Fluorescence spectra of Mal-TPE (40 μM) in PBS solution at different temperatures with an
excitation wavelength of 350 nm; (b) temperature-dependent maximum fluorescent intensity of Mal-TPE
at different concentrations (20 and 40 μM); (c) changes in the maximum fluorescence intensity of Mal-
TPE (40 μM) during repeated heating and cooling processes.
Figure 2b shows the fluorescence spectra of Mal-TPE in
Figure 3a and Figure S6 (ESI) represent the
PBS-DMF mixtures. The fluorescence intensity centered temperature-dependent fluorescence spectra of Mal-
at about 466 nm increases with the increase in the TPE in PBS solution at concentrations of 40 and 20 μM,
aqueous fraction, clearly indicating the AIE respectively, at different temperatures ranging from 20
o
characteristic of Mal-TPE. The absolute quantum yields - 65 C. It is evident that the maximum fluorescence
(Φ) of Mal-TPE in PBS solution and DMF are determined intensity of Mal-TPE decreases with the increase in
to be 0.354 and 0.007, respectively. The lifetime of Mal- temperature. This tread is more pronounced for Mal-
TPE in PBS solution is evaluated to be 4.5 ns (Figure S5, TPE at a high concentration (Figure 3b). Upon heating
ESI). The effect of pH value on the fluorescence from 20 to 65 oC, the corresponding maximum
characteristic of Mal-TPE in water was then investigated. fluorescence intensity is decreased by 90% and 86%,
As shown in Figure 2c and 2d, the fluorescence intensity respectively, for Mal-TPE at concentrations of 20 and 40
of Mal-TPE aqueous solution increases with the increase μM. Figure 3c shows the changes of maximum
in pH value in the range from 1 to 7, and decreases with fluorescence intensity of Mal-TPE when temperature
the further increase. A maximum fluorescence intensity changes repeatedly between 20 and 55 °C. The result
is achieved at neutral pH. The changes in fluorescence clearly shows the PBS solution of Mal-TPE exhibits
intensity of Mal-TPE at alkaline (or acidic) pH may be reversible fluorescence intensity changes upon
attributed to the dissociation (or protonation) of alternative heating and cooling processes. This indicates
hydroxyl groups in Mal-TPE, increasing the solubility of that the aggregates of Mal-TPE in aqueous medium
Mal-TPE and weakening its AIE characteristic. To undergo smooth morphological change in response to
eliminate the influence of pH, the temperature- temperature, thereby allowing the “turn-on/turn-off”
dependent fluorescence behavior of Mal-TPE was model of AIE characteristic.
studied in buffer solution.
Dynamic light scattering (DLS) analysis was used to
examine the sizes of Mal-TPE aggregates in PBS solution
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at different temperature. As shown in Figure 4a, the size the Central Universities (SWU115V0ie0w5Article Oannlinde
distribution of Mal-TPE is relatively broad at 20 oC. When XDJK2018B006).
DOI: 10.1039/C8NJ03338B
o
the temperatures increase to 40 and 60 C, their size
distributions become narrow and particle sizes become
Conflicts of interest
small. A schematic illustration of the change in particle
sizes during the heating and cooling process is provided
in Figure 4b. This phenomenon may be due to the
hydrogen bonding interaction within oligosaccharide in
Mal-TPE is sensitive to temperature, leading to the
increased solubility of amphiphilic Mal-TPE conjugate at
high temperature. The Mal-TPE aggregates could be
partially dissociated during the heating process.
Therefore, the reduced particle sizes of Mal-TPE
aggregates weaken their AIE effect, resulting in the
decreased fluorescence intensity at high temperature.
There are no conflicts to declare.
Notes and references
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In conclusion, a simple amphiphilic AIEgen with
thermo-responsive property was presented. The
resulting Mal-TPE conjugate emits weak
fluorescence in good solvent, and emits strong blue
fluorescence in aqueous media, due to its
aggregation. The aggregates of amphiphilic Mal-TPE
exhibit thermo-responsive behavior, and their sizes
decrease with the increase in temperature. The
decreased sizes of Mal-TPE aggregates can influence
their AIE effect, resulting in temperature-dependent
fluorescence intensities. This study provides a new
perspective for the construction of fluorescent
thermometers based on amphiphilic AIEgens.
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This research was supported by National Natural
Scientific Foundation of China (21504072 and
51741304), and Fundamental Research Funds for
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Amphiphilic maltoheptaose-tetraphenylethene conjugate exhibits AIE effect in aqueous medium and its
fluorescence property shows temperature-dependent behavior.
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