Highly sensitive and selective fluorometric off-on K+ probe constructed via host-guest molecular recognition and aggregation-induced emission

Article abstract of DOI:10.1039/c2jm16510d

In this work, we integrated the concept of aggregation-induced emission (AIE) with the specific supramolecular recognition between K⁺ ions and crown ether moieties to develop more effective fluorometric K⁺ probes. We synthesized a novel crown ether-functionalized tetraphenylethene (TPE) derivative, TPE-(B15C5)₄, via the thiol-ene click reaction of thiol-derivatized TPE, TPE-(SH)₄, with maleimide-functionalized benzo-15-crown-5 (B15C5). In TPE-(B15C5)₄, the TPE core and four outer B15C5 moieties serve as the AIE-active motif and supramolecular K ⁺-recognizing functionalities, respectively. TPE- (B15C5)₄ molecularly dissolves in THF with negligible fluorescence emission. As we have envisaged, upon K⁺ addition, TPE-(B15C5)₄ can be effectively induced to aggregate due to K⁺-mediated cross-linking via the formation of K⁺/B15C5 (1/2 molar ratio) molecular recognition complex in a sandwiched manner. This process is concomitantly accompanied with the turn-on of fluorescence emission via the AIE mechanism. Thus, TPE-(B15C5)₄ can serve as highly sensitive and selective fluorometric off-on K⁺ probes. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm16510d

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PAPER
Highly sensitive and selective fluorometric off–on K+ probe constructed via
host–guest molecular recognition and aggregation-induced emission†
*
Xiaorui Wang, Jinming Hu, Tao Liu, Guoying Zhang and Shiyong Liu
*
Received 10th December 2011, Accepted 27th February 2012
DOI: 10.1039/c2jm16510d
In this work, we integrated the concept of aggregation-induced emission (AIE) with the specific
supramolecular recognition between K+ ions and crown ether moieties to develop more effective
fluorometric K+ probes. We synthesized a novel crown ether-functionalized tetraphenylethene (TPE)
derivative, TPE-(B15C5)4, via the thiol-ene click reaction of thiol-derivatized TPE, TPE-(SH)4, with
maleimide-functionalized benzo-15-crown-5 (B15C5). In TPE-(B15C5)4, the TPE core and four outer
B15C5 moieties serve as the AIE-active motif and supramolecular K+-recognizing functionalities,
respectively. TPE- (B15C5)4 molecularly dissolves in THF with negligible fluorescence emission. As
we have envisaged, upon K+ addition, TPE-(B15C5)4 can be effectively induced to aggregate due to
K+-mediated cross-linking via the formation of K+/B15C5 (1/2 molar ratio) molecular recognition
complex in a sandwiched manner. This process is concomitantly accompanied with the turn-on of
fluorescence emission via the AIE mechanism. Thus, TPE-(B15C5)4 can serve as highly sensitive and
selective fluorometric off–on K+ probes.
recently, the concept of aggregation-induced emission (AIE)7 or
1. Introduction
aggregation-induced enhanced emission (AIEE)8 has emerged to
be a potent and versatile strategy for fabricating novel types of
fluorescent probes. Typical molecules exhibiting AIE features,
such as tetraphenylethene (TPE), are non-emissive in molecularly
dissolved state, whereas enhanced fluorescence emission was
achieved when they are in the aggregated state. Tang et al.7,9,10
has ascribed the AIE phenomenon to the restriction of intra-
molecular rotation (RIR) of fluorogenic molecules within
aggregates, which effectively blocks the non-radiative energy
dissipation pathways and favors the radiative decay of excitons.
In the past few years, various AIE-based chemosensors and
biosensors for analytes ranging from heavy-metal ions, thiols,
explosive, carbohydrates, DNA, to proteins and enzymes have
been developed.9–11
Over the past decades, the exploration of novel detection strat-
egies for the K+ ion has attracted significant interest due to its
extremely important roles in various physiological activities and
biological functions.1 In comparison with other conventional
analytical approaches, colorimetric and fluorometric K+ probes
offer distinct advantages in terms of sensitivity, selectivity, fast
response, and enhanced spatial and temporal resolution.2 Until
now, a variety of elegant optical K+ chemosensors and biosen-
sors, either in an emission intensity-based or a ratiometric
manner, have been developed by utilizing K+-modulated
photoinduced electron transfer (PET),1a,3 photoinduced charge
transfer (PCT),4 or fluorescent resonance energy transfer (FRET)
mechanisms.5 The construction of small molecule-based fluoro-
metric K+ probes typically relies on the molecular recognition
between K+ ions and crown ether or cryptand hosts as they
typically exhibit weak affinity with conventional heavy metal
ion-binding ligands.2,6
To the best of our knowledge, the AIE mechanism has been
less exploited to develop fluorometric probes for K+ ions, which
are expected to offer advantages such as high detection selectivity
and sensitivity, low background interference, and enhanced
imaging contrast. In this work, we further integrated the
AIE concept with the specific supramolecular recognition
between K+ ions and crown ether moieties, aiming for the facile
development of more effective fluorometric K+ probes. We
synthesized a novel crown ether-functionalized TPE derivative,
TPE-(B15C5)4, via the thiol-ene click reaction12 of thiol-modified
TPE, TPE-(SH)4, with maleimide-functionalized benzo-15-
crown-5 (B15C5) (Scheme 1 and 2). In TPE-(B15C5)4, the TPE
core and four outer B15C5 moieties serve as the AIE-active motif
and supramolecular K+-recognizing functionalities, respectively.
TPE-(B15C5)4 molecularly dissolves in THF with negligible
In addition to the mechanism of analyte-modulated conven-
tional photophysical processes such as PET, PCT, and FRET,
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer
Science and Engineering, Hefei National Laboratory for Physical
Sciences at the Microscale, University of Science and Technology of
China, Hefei, Anhui 230026, China. E-mail: sliu@ustc.edu.cn; gyzhang@
ustc.edu.cn
† Electronic
supplementary
information
(ESI)
available:
Characterization data of 1H NMR, 13C NMR, ESI-MS, UV-Vis
absorption spectra, AFM images, and Job plot. See DOI:
10.1039/c2jm16510d
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all other reagents were purchased from Sinopharm Chemical
Reagent Co. Ltd. and used as received. Thiocyanate salts (K+,
Li+, Na+, NH4+, Ca2+, Mg2+, and Pb2+) were dissolved in CH3CN
as stock solution for all sensing experiments. 1,1,2,2-Tetrakis[4-
(2-bromoethoxy)phenyl]ethene (TBEPE, 1),14a and 2,3-(40-ami-
nobenzo)-1,4,7,10,13-pentaoxacy-clopentadeca-2-ene (40-amino-
benzo-15-crown-5, 4)14b were synthesized according to literature
procedures.
Sample synthesis
Synthetic schemes employed for the preparation of 1,1,2,2-tet-
rakis[4-(2-mercaptoethoxy)phenyl]ethane, TPE-(SH)4, (3), 2,3-
(40-maleimidebenzo)-1,4,7,10,13-pentaoxacyclopentadeca-2-ene,
B15C5-MI (6), and crown ether-functionalized tetraphenyle-
thene, TPE-(B15C5)4 (7), are shown in Scheme 2.
Scheme 1 Schematic illustration of the construction of a ‘‘light-up’’
fluorometric K+ probe based on aggregation-induced emission (AIE) of
crown ether-functionalized tetraphenylethene, TPE-(B15C5)4, induced
by supramolecular recognition between K+ ions and crown ether
moieties.
Synthesis of TPE-(SH)4 (3). General protocols for the
synthesis of precursor 2 are as follows. TBEPE (2.06 g, 2.5 mmol,
1.0 equiv.) was dissolved in DMF (20 mL). A solution of etha-
nethioic acid potassium salt (0.428 g, 3.75 mmol, 1.5 equiv.) in
DMF (10 mL) was added dropwise over ꢀ20 min. The reaction
mixture was stirred at room temperature and the progress was
monitored by TLC (SiO2, CH2Cl2). After 4 h the reaction was
complete and the solvents were removed under reduced pressure.
The residues were dissolved in CH2Cl2 (30 mL) and washed with
saturated aqueous NH4Cl three times. The organic layer was
combined and then washed with HCl (1 M), and saturated brine
successively, dried over anhydrous Na2SO4, and concentrated
under reduced pressure to yield a brown solid. Further purifi-
cation by flash column chromatography (SiO2, CH2Cl2) gave (2)
fluorescence emission. As we have envisaged, upon K+ addition,
TPE-(B15C5)4 can be effectively induced to aggregate due to
K+-mediated cross-linking via the formation of K+/B15C5 (1/2
molar ratio) molecular recognition complex in a sandwiched
manner.2,6,13 This process is concomitantly accompanied with the
turn-on of fluorescence emission via the AIE mechanism.
Thus, TPE-(B15C5)4 can serve as highly sensitive and selective
fluorometric off–on K+ probes (see Scheme 1).
2. Experimental
Materials
1
(1.61 g, 80%) as a white solid. H NMR (CDCl3, d, ppm, TMS,
Ethanethioic acid potassium salt (98%), hydrazine hydrate (AR),
maleic anhydride (99%), sodium acetate (anhydrous, AR), and
Fig. S1†): 6.89 (d, J ¼ 8.8 Hz, 8H), 6.62 (d, J ¼ 8.8 Hz, 8H), 4.02
(t, J ¼ 6.4 Hz, 8H), 3.23 (t, J ¼ 6.4 Hz, 8H), 2.35 (s, 12H).
Scheme 2 Synthetic routes employed for the preparation of crown ether-functionalized tetraphenylethene, TPE-(B15C5)4 (7).
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For the synthesis of TPE-(SH)4, 0.805 g of 2 (1.0 mmol, 1.0
equiv.) was dissolved in anhydrous THF. Hydrazine mono-
hydrate (250.3 mg, 5 mmol, 5.0 equiv.) in anhydrous THF
(10 mL) was added dropwise over ꢀ30 min. The solution was
then refluxed overnight. After that, all the solvents were removed
under reduced pressure, saturated brine (50 mL) was added and
the mixture was extracted with CH2Cl2 three times. The
combined organic phase was washed with saturated brine, dried
over Na2SO4 and the solvent was removed under reduced pres-
sure. Further purification by flash column chromatography
(SiO2, CH2Cl2) gave 3 as a white solid (318 mg, 50%). 1H NMR
(CDCl3, d, ppm, TMS, Fig. S2†): 6.93 (d, J ¼ 8.7 Hz, 8H), 6.65
(d, J ¼ 8.8 Hz, 8H), 4.06 (t, J ¼ 6.4 Hz, 8H), 2.87 (dd, J ¼ 14.7,
6.4 Hz, 8H).
Fluorometric sensing of K+ ions. Fluorescence spectra were
recorded using a RF-5301/PC (Shimadzu) spectrofluorometer.
The temperature of the water-jacketed cell holder was controlled
by a programmable circulation bath. The slit widths were set at
5 nm for both excitation and emission. The fluorescence experi-
ments were conducted using 4.8 mM of 7 in THF solution. The
effect of the various cations on the fluorescence emission inten-
sity was examined by adding 2.0 mL CH3CN stock solution of
thiocyanate salts into 2.0 mL THF solution of compound 7
(4.8 mM) so that the volume increase upon addition of cations
can be neglected.
Characterization
All nuclear magnetic resonance (NMR) spectra were recorded on
a Bruker AV300 NMR spectrometer (resonance frequency of
300 MHz for H and 75 MHz for 13C) operated in the Fourier
Synthesis B15C5-MI (6). 40-Aminobenzo-15-crown-5 (4,
0.340 g, 1.2 mmol, 1.0 equiv.) was dissolved in 20 mL of anhy-
drous THF and cooled to 0 ꢁC in an ice–water bath. Maleic
anhydride (1.177 g, 12 mmol, 10.0 equiv.) in 10 mL anhydrous
THF was added dropwise over 20 min. The reaction mixture was
then stirred at room temperature and the progress monitored by
TLC. After the reaction was complete, the reaction mixture was
evaporated to dryness on a rotary evaporator. The obtained
crude product (5) was used directly for the next step without any
further purification.
1
transform mode. ESI-MS experiment was performed on Agilent
6460 Triple Quadruple mass spectrometer equipped with an ESI
source, and exact masses were measured using a Thermo Scien-
tific LTQ Orbitrap Mass Spectrometer equipped with an elec-
trospray interface. UV-Vis absorption spectra were acquired on
a TU-1910 double-beam UV-Vis spectrophotometer (Puxi.
General Instrumental Company, China). Atomic force micros-
copy (AFM) measurements were performed on a Digital
Instrument Multimode Nanoscope IIID operating in the tapping
mode under ambient conditions. Silicon cantilever (RFESP) with
resonance frequency of ꢀ80 kHz and spring constant of ꢀ3 N
mꢃ1 was used. The set-point amplitude ratio was maintained at
0.7 to minimize sample deformation induced by the tip.
The sample was prepared by dip coating the mixture of
TPE-(B15C5)4 (4.8 mM) and K+ ions (19.2 mM, 1.0 equiv.) in
THF onto freshly cleaved mica surfaces. Dynamic light scat-
tering (DLS) experiments were conducted on a Malvern Zeta-
sizer Nano ZS Instrument.
Compound 5 obtained in the above step was dissolved in acetic
anhydride (25 mL) and heated to 60 ꢁC in an oil bath. Anhydrous
sodium acetate (0.246 g, 3.0 mmol) dissolved in anhydrous THF
ꢁ
(10 mL) was added. After stirring at 60 C for 2 h, the reaction
mixture was cooled to room temperature and then poured into
80 mL ice–water mixture. The mixture was neutralized
with saturated Na2CO3 aqueous solution and extracted with
3 ꢂ 60 mL CH2Cl2. The organic phase was combined and dried
over anhydrous MgSO4. CH2Cl2 was then removed on a rotary
evaporator and the crude product was further purified by
recrystallization from acetone. The final product was dried in
a vacuum oven at room temperature overnight to afford a yellow
crystal (0.283 g, yield: 65% over two steps). 1H NMR (CDCl3, d,
ppm, TMS; Fig. S3†): 6.96 (d, J ¼ 8.4 Hz, 1H), 6.90 (d, J ¼
2.2 Hz, 1H), 6.87 (s, 1H), 6.85 (s, 2H), 4.17 (dd, J ¼ 8.7, 5.5 Hz,
4H), 3.93 (dd, J ¼ 8.3, 3.7 Hz, 4H), 3.78 (s, 8H). 13C NMR
(CDCl3, d, ppm, TMS, Fig. S4†): 196.6, 148.9, 134.1, 131.2,
119.3, 113.8, 112.4, 70.9, 70.3, 69.3.
3. Results and discussion
Crown ether-functionalized tetraphenylethene, TPE-(B15C5)4
(7), was prepared following synthetic routes shown in Scheme 2.
Thiolated TPE, TPE-(SH)4 (3) was synthesized at first via the
reaction of 1,1,2,2-tetrakis[4-(2-bromoethoxy)phenyl]ethene
(TBEPE, 1) with ethanethioic acid potassium salt followed by
hydrazinolysis with hydrazine hydrate. The chemical structures
of precursor 2 and TPE-(SH)4 (3) were characterized by 1H NMR
(Fig. S1 and S2†). The synthesis of maleimide-functionalized
benzo-15-crown-5 (B15C5-MI, 6) proceeded via the reaction of
40-aminobenzo-15-crown-5 (4) with maleic anhydride followed
by treatment with acetic anhydride. The chemical structure of 6
was also confirmed by 1H NMR (Fig. S3†) and 13C NMR
(Fig. S4†) characterization. Taking advantage of the highly
efficient thiol-ene click reaction between TPE-(SH)4 (3) and
B15C5-MI (6), TPE derivatives containing four B15C5 moieties
at the periphery, TPE-(B15C5)4 (7) was successfully obtained. A
comparison of the 1H NMR spectrum of 7 with those of 3 and 6
further confirmed the successful synthesis of 7. Note that
considerable peak broadening in the 1H NMR spectrum of 7 was
observed and this can be ascribed to the slow relaxation of 7 due
to its high molecular weight (ꢀ2089 Da). In the 1H NMR
Synthesis of crown ether-functionalized tetraphenylethylene
(TPE-(B15C5)4, 7). Compound 6 (100 mg, 0.275 mmol, 8.0
equiv.) was dissolvedin anhydrous DMF (10 mL) anddegassed by
bubbling with N2 for 30 min. Compound 3 (22 mg, 0.034 mmol,
1.0 equiv.) was then added. After being thermostated at 35 ꢁC in
an oil bath and stirred for two days, the resulting mixture was
evaporated to dryness on a rotary evaporator and then dissolved
in a minimal amountof boiling THF. By coolingthe THF solution
ꢁ
to ꢃ20 C for several hours, the target product, TPE-(B15C5)4,
crystallizes from cold THF. After centrifugation and drying in
a vacuum oven overnight at room temperature, a white powder
was obtained (48 mg, yield: 68%). 1H NMR (DMSO-d6, d, ppm,
0
TMS; Fig. S5†): 6.60–7.20 (Ar–H, 28H); 4.00–4.12 (Hc and Hj0,
0
0
24H); 3.77 (24H, Hj0 and Hd ); 3.60 (Hk ). ESI-MS (m/z; Fig. S6†):
calcd. for [M + 2Na]2+, 1067.328; found, 1067.33.
8624 | J. Mater. Chem., 2012, 22, 8622–8628
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spectrum of 7, 0 the resonance signal ascribing to TPE–
OCH2CH2S– (Hc , 4.10 ppm) still remains, whereas the reso-
nance signal at 2.87 ppm in the spectrum of 3 (Hd) ascribing to
TPE–OCH2CH2SH completely disappeared, which shifted to
0
3.77 ppm (Hd ) in the NMR spectrum of 7. In addition, peak
integral ratios between resonance signals characteristic of aryl
protons (6.6–7.2 ppm) and those aryl–OCH2– (28 : 24) further
confirmed the chemical structure of 7 (Fig. S5†). Furthermore,
ESI-MS characterization results of TPE-(B15C5)4 (7) is shown in
Fig. S6† (m/z: calcd. for [M + 2Na]2+, 1067.328; found, 1067.33)
and can further confirm its chemical structure. The detection of 7
in the dicationic form can be evidenced by intervals of the
observed isotope distributions (ca. 0.5). The above results clearly
supported that all four thiol moieties of TPE-(SH)4 (3) have
participated in the Michael addition reaction and that
TPE-(B15C5)4 (7) was successful synthesized.
In order to check whether the as-synthesized TPE-(B15C5)4 (7)
was endowed with the same AIE characteristics as those
exhibited by TPE,7,9,10 we investigated the fluorescence emission
of 7 in THF/water mixtures at varying water fractions. When
excited at 360 nm, the THF solution of TPE-(B15C5)4 exhibited
quite weak fluorescence emission (Fig. 1), indicating that 7 is
non-emissive when at a molecularly dissolved state. In contrast,
when water was added into the THF solution, the fluorescence
emission increased gradually, and
a dramatic emission
enhancement was observed when the water fraction is higher
than 70 v/v%. Thus, we confirmed that the emission behavior of
TPE-(B15C5)4 is quite comparable to those of TPE and its
fluorescence emission can be off–on switched via the AIE effect
due to aggregates formation at higher water fractions.
Considering the AIE feature of TPE core and the supramo-
lecular recognition between K+ ions and peripheral crown ether
moieties,2,6,13 we envisage that the addition of K+ will induce the
aggregation of 7 and turn on the fluorescence emission, through
which, the fluorometric detection of K+ can be achieved. As
shown in Fig. 2, upon gradual addition of K+ ions into the THF
solution of 7 (4.8 mM, [B15C5] ¼ 19.2 mM), the fluorescence
emission intensity at 460 nm increased considerably. At [K+] ¼
9.6 mM (corresponding to [K+]/[B15C5] ¼ 1/2), the solution of 7
was getting strongly emissive, with the emission intensity being
ꢀ7.5-fold higher than that in the absence of K+ ions; at [K+] ¼
19.2 mM (corresponding to [K+]/[B15C5] ¼ 1/1), the emission
intensity of the solution reached a value that is ꢀ9.5-fold higher
than the blank control. Furthermore, the enhancement of fluo-
rescence emission induced by the presence of K+ ions can also be
facilely discerned by the naked eye (Fig. 2c). If we arbitrarily
define the detection limit of K+ ions as the concentration at which
10% emission enhancement can be achieved by employing
4.8 mM of 7 in THF solution, the detection limit was then
determined to be ꢀ1.0 mM.
Fig. 1 (a) Fluorescence emission spectra (lex ¼ 360 nm; slit widths: Ex. 5
nm, Em. 5 nm; 25 ꢁC), (b) normalized fluorescence intensity changes at
lem ¼ 460 nm, and (c) optical photographs recorded under 365 nm UV
irradiation for TPE-(B15C5)4 (4.8 mM) in THF/H2O mixtures with the
volume fractions of water varying in the range of 0–90%.
has previously proposed by Tang and coworkers.7,9,10 It is worthy
of noting that before the introduction of AIE concept in 2001
by Tang et al.,7 Li et al.1j,1k reported an example of crown
ether-functionalized styryl chromophore, which also exhibits the
AIE-like fluorometric emitting properties in the presence of K+
ions or Cs+ ions due to supramolecular recognition between
alkaline metal ions and crown ether moieties. Li et al.1j termed
the system as ‘‘self-assembling fluorescence enhancement’’.
We also found the choice of solvents also plays key roles in
observing the K+-induced AIE. In preliminary experiments, we
have also tried CHCl3, DMSO, and acetone as the solvents.
However, for 7 dissolved in CHCl3, the addition of K+ typically
leads to the formation of quite large aggregates, which prevents
the continuous monitoring of K+ concentration-dependent
emission enhancement. This might be ascribed to the fact that in
CHCl3, the molecular recognition events between B15C5 moie-
ties and K+ is more favored compared to that in THF.2,6,13 When
using DMSO as the solvent, the K+-induced emission enhance-
ment was not that prominent as compared to those in THF or
CHCl3. In acetone and acetonitrile, negligible fluorescence turn-
on can be observed. Thus, it turned out that THF is the best
choice in terms of the compromise between emission turn-on
The fluorescence emission enhancement behavior observed for
7 in THF in the presence of K+ ions can be attributed to the
supramolecular recognition between peripheral crown ether
moieties of 7 and K+ ions, which leads to the formation of
K+/B15C5 sandwich complex in a 1 : 2 molar ratio and the
concomitant formation of cross-linked network with TPE-
(B15C5)4 (7) as the building block (see Scheme 1). The network
formation leads to the turn-on of RIR process, simultaneously
boosting the fluorescence emission of TPE cores, this mechanism
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diameter, <Dh>, of 7 in THF is ꢀ1 nm, which indicated that 7
can be molecularly dissolved in THF. However, nanoparticle
aggregates of ꢀ200 nm clearly formed upon addition of 1.0
equiv. K+ ions.
With an attempt to further demonstrate that K+-induced
aggregation and the emission turn-on via AIE was caused by the
formation of intermolecular K+/crown ether sandwich
complexes, further competition experiments were then con-
ducted. As shown in Fig. 4, upon gradual addition of free benzo-
15-crown-5 (B15C5) to the mixture of 7 (4.8 mM) and K+ ions
(19.2 mM) in THF, the fluorescence emission intensity at 460 nm
gradually decreased. In the presence of a large excess of free
B15C5 (ꢀ350 equiv. relative to K+), the emission spectrum
almost restored to resemble that of 7 in the absence of K+ ions.
The inset in Fig. 4b shows that the intense cyan emission of 7/K+
mixture was greatly weakened upon addition of free B15C5. The
above results confirmed that the supramolecular recognition
between 7 and K+ ions is responsible for K+-induced aggregation
and emission enhancement.
Fig. 2 (a) Fluorescence emission spectra (lex ¼ 360 nm; slit widths: Ex.
5 nm, Em. 5 nm; 25 ꢁC), (b) relative emission intensity changes at lem
¼
460 nm, and (c) optical photographs recorded under 365 nm UV light
irradiation for TPE-(B15C5)4 solution in THF upon gradual addition of
K+ ions (0–1.0 equiv. relative to crown ether moieties); TPE-(B15C5)4
concentrations were fixed to be 4.8 mM in all cases.
efficiency and sufficient colloidal stability of the cross-linked
network, although the underlying mechanism interpreting the
observed phenomenon remains to be solved.
Further experiments were conducted to probe K+-induced
aggregation of 7 in THF. Fig. S7 shows the UV-Vis absorption
spectra of 7 upon addition of varying amount of K+ ions.† The
absorption band at around 250 nm decreased and the absorption
intensity at around 330 nm gradually increased with the addition
of K+ ions, and an isosbestic point was observed at 290 nm.
Furthermore, absorbance baselines at >400 nm also gradually
increased, suggesting the formation of colloidal aggregates.
Furthermore, AFM measurements for the 7/K+ mixture in THF
directly revealed the formation of supramolecular cross-linked
spherical nanoparticles with the average dimension being
ꢀ230 nm (Fig. 3a and Fig. S8†). Also note that the nanoparticles
are quite monodisperse in terms of size distributions. The DLS
results are shown in Fig. 3b, the intensity-average hydrodynamic
Fig. 3 (a) AFM height image (10 ꢂ 10 mm) recorded for supra-
molecularly cross-linked nanoparticles formed in the mixture of
TPE-(B15C5)4 (4.8 mM) and K+ ions (19.2 mM) in THF, (b) DLS profiles
of 4.8 mM of 7 in THF solution in the absence and presence of 1.0 equiv.
K+ ions, respectively.
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Fig. 5 Relative emission intensity at lem ¼ 460 nm recorded for TPE-
(B15C5)4 (4.8 mM) in THF solution upon addition of 1.0 equiv. different
interfering ions (Li+, Na+, NH4+, Ca2+, Mg2+ and Pb2+) and upon addition
of 1.0 equiv. K+ ions in the absence or presence of 1.0 equiv. different
interfering ions, (lex ¼ 360 nm; slit widths: Ex. 5 nm, Em. 5 nm; 25 ꢁC).
4. Conclusions
In summary, a novel ‘‘light up’’ fluorometric K+ probe based on
the AIE feature of crown ether-functionalized tetraphenylethene,
TPE-(B15C5)4 (7), was successfully constructed. The fluoro-
metric K+ detection limit can be down to ꢀ1.0 mM and it also
exhibits excellent selectivity over other common cations such as
Li+, Na+, NH4+, Ca2+, Mg2+ and Pb2+. It works on the basis of
K+-induced aggregation of 7 in THF via the formation of
supramolecular K+/crown ether 1 : 2 sandwich complex. In
addition, quite uniform spherical supramolecular nanoparticles
possessing internally cross-linked networks formed in solution.
Further experiments are currently underway to demonstrate the
practical application of the reported AIE-based fluorometric K+
sensing system, for example, the design of novel analogues of 7
with good water solubility for the in situ detection and imaging of
K+ ions under physiological conditions.
Fig. 4 (a) Normalized fluorescence emission spectra (lex ¼ 360 nm; slit
widths: Ex. 5 nm, Em. 5 nm; 25 ꢁC) and (b) emission intensity changes at
lem ¼ 460 nm recorded for TPE-(B15C5)4 (4.8 mM) in THF in the
presence of 1.0 equiv. of K+ ions (relative to crown ether moieties in
TPE-(B15C5)4) upon gradual addition of free benzo-15-crown-5 (B15C5,
0–350 equiv. relative to K+ ions). The inset in (b) shows optical photo-
graphs recorded under 365 nm UV irradiation for TPE-(B15C5)4/K+
mixture in THF before and after the addition of 350 equiv. B15C5.
It is well known that crown ether hosts exhibit strong binding
affinity for guest molecules due to their macrocyclic structure
and the multidentate binding cooperativity. The binding selec-
tivity largely depends on the host cavity sizes. For B15C5-K+
host–guest recognition system, the formation of 2 : 1 sandwich
complex is thermodynamically favored. Fig. S9 shows Job plot of
crown ether moieties in TPE-(B15C5)4 with K+ ions.† The total
concentration of the hosts (crown ether moieties in TPE-
(B15C5)4) and guests (K+ ions) in THF was fixed at 28.8 mM, and
the molar fraction of K+ was varied. It is clearly evident that
when the molar ratio between K+ and crown ether moieties is at
1 : 2, the fluorescence intensity (I ꢃ I0) reaches the maximum
value (Fig. S9†). This demonstrates that crown ether moieties in
TPE-(B15C5)4 form a 2 : 1 stoichiometric complex with K+ ions.
Finally, we checked the fluorometric K+-sensing selectivity of 7
in THF over other common cations. As shown in Fig. 5, after
addition of 1.0 equiv. of competitive ions (Li+, Na+, NH4+, Ca2+,
Mg2+ and Pb2+), the fluorescence emission of 7 exhibited negli-
gible changes at around 460 nm; however, when 1.0 equiv. of K+
ions was added, the fluorescence emission intensity was greatly
enhanced either in the presence or absence of competitive ions
(Li+, Na+, NH4+, Ca2+, Mg2+ and Pb2+). These results clearly
indicated that 7 can act as a highly selective fluorometric K+
chemosensor.
Acknowledgements
The financial support from National Natural Scientific Foun-
dation of China (NNSFC) Projects (20874092, 91027026, and
51033005) and Fundamental Research Funds for the Central
Universities is gratefully acknowledged.
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