Tunable fluorescence behaviors of a supramolecular system based on a fluorene derivative and cucurbit[8]uril and its application for ATP sensing

Article abstract of DOI:10.1039/c7cp06434a

In this work, we developed a supramolecular fluorescent system based on host-guest interactions between a fluorene derivative carrying two bispyridinium units (FPy) and cucurbit[8]uril (CB[8]). In aqueous solution, the system showed outstanding tunable em

Full text of DOI:10.1039/c7cp06434a

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Tunable Fluorescence Behaviors of
Supramolecular System Based on a Fluorene
Derivative and Cucurbit[8]uril and Its
Application for ATP Sensing
†
,†
‡
†
,†
Rongqin Ye, Qianling Cui,* Chuang Yao, Ronghua Liu, and Lidong Li*
†
‡
State Key Lab for Advanced Metals and Materials, School of Materials Science and
Engineering, University of Science and Technology Beijing, Beijing 100083, China
Key Laboratory of Extraordinary Bond Engineering and Advanced Materials
Technology (EBEAM) Chongqing, Yangtze Normal University, Chongqing 408100,
China
Email: lidong@mater.ustb.edu.cn; cuiqianling@ustb.edu.cn
Abstract: In this work, we develop a supramolecular fluorescent system based on
hostꢀguest interaction between a fluorene derivative carrying two bispyridinium units
(FPy) and cucurbit[8]uril (CB[8]). In aqueous solution, the system showed
outstanding tunable emission properties. After being encapsulated into the rigid
hydrophobic cavity of CB[8] host, the fluorescence emission of fluorene had an
obvious redꢀshift with enhanced quantum yield. Interestingly, the emission behavior
of the FPy/CB[8] complex showed a twoꢀstep selfꢀassembly process when the molar
ratio of FPy to CB[8] changed from 1:1 to 1:2. Besides, the influence of several
factors on emission properties of FPy/CB[8] complex were also investigated, like pH
value, salt concentration, and temperature. Finally, the fluorescent FPy/CB[8]
complexes displayed good performance on detection of adenosineꢀ5’ꢀtriphosphate
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(ATP), which can cause aggregationꢀinduced quenching of the complexes via
electrostatic attraction.
Introduction
Luminescent molecules or systems have attracted considerable attentions because of
1
ꢀ3
their potential applications in optical electronic devices, biological applications
4
5ꢀ7
8,9
10,11
(
probes, imaging
and drug delivery ), sensors,
as well as other
12,13
applications.
Using covalent bonds to extend π systems or attach substituents to
aromatic chromophores is a general method to fabricate various luminescent
14
materials. However, it often involves elaborate design and tedious organic synthesis
with multiple steps and high cost. On the other hand, the supramolecular
selfꢀassembly of fluorophores through noncovalent force has recently emerged as a
smart strategy due to their simplicity and versatility. Several kinds of noncovalent
interaction have been used to tune the luminescent molecules’ photophysical
15
properties and selfꢀassembled nanostructures, including electrostatic attraction,
16,17
18,19
20
21
hydrogen bond,
πꢀπ stacking,
hydrophobic interaction, and coordination.
Luminescent materials based on supramolecular assemblies, especially hostꢀguest
14,22ꢀ24
interaction, are a promising alternative strategy to develop functional systems.
2
5,26
27,28
Cucurbit[n]urils (CB[n]s), along with crown ethers,
cryptands
,
2
9,30
31,32
cyclodextrins,
crucial role in supramolecular chemistry and material science in recent decades.
CB[8], a pumpkinꢀshaped macrocyclic molecule bearing a rigid hydrophobic cavity
and calixarenes,
are macrocyclic receptors that have played a
33
22
and two identical carbonyl fringed portals, has attracted great interest because of its
2

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excellent recognition property in water and wide applications in molecular
34
35
36
recognition, supramolecular organic framework, drug delivery, supramolecular
37,38
39
polymer,
molecular machine etc. CB[8] has remarkable binding abilities to form
40,41
14
1
:1, 1:2 or 1:1:1 ternary complexes with different substrates.
Ni and his
coworkers discovered that CB[8] could form stable hostꢀguest complexes with
oligo(pꢀphenylenevinylene) (OPV) dyes to form Jꢀaggregates to achieve tunable
fluorescence emission. The formation of OPV/CB[8] complex caused profound
changes in increasing fluorescent intensity, lifetime and quantum yield and tuning
fluorescent color through hostꢀguest interaction with their cavities in water.
42
Biedermann et al synthetized a kind of waterꢀsoluble perylene bis(diimide) (PDI)
dye and formed hostꢀguest complexes with CB[8] to enhance the fluorescence
emission and hinder the aggregation via π−π stacking interaction in aqueous solution.
In this research, we tried to prepare tunable fluorescent supramolecular system via
hostꢀguest interaction based on a fluorene derivative carrying two bispyridinium units
and CB[8] in aqueous solution. Bispyridinium group has been reported as a kind of
hydrophilic substituent and has the excellent binding abilities to interact with CB[n] to
41
form 1:1, 1:2 or 1:1:1 ternary complexes. On the other hand, fluorene unit is a
widely used luminphore because of its high emission efficiency and easy modification.
Considering the hydrophobic of polycyclic aromatic hydrocarbon and near planarity
4
3
of biphenyl, the fluorene and its derivatives are hard to dissolve in water and facile
to aggregate together via πꢀπ stacking interaction to induce the weak fluorescent
emission. Thus, the fluorescent dyes interacted with CB[8] are expected to perform
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better in aqueous solution through hostꢀguest interaction in aqueous solution.
To reach this goal, a fluorene derivative carrying two bispyridinium units was
designed and synthesized, and its selfꢀassembly behaviour with CB[8] molecules via
hostꢀguest interaction was investigated in aqueous solution. The obtained complex
showed a remarkable enhanced fluorescent intensity, increased lifetime and quantum
yield, and tuneable fluorescence emission. Additionally, its responsive behaviours
towards molar ratio of the two molecules, pH value and temperature were investigated.
Due to strong electrostatic attraction between the complex and ATP molecules which
can lead to aggregationꢀcaused quenching, this fluorescence complex can be used to
detect ATP with high sensitivity and selectivity.
Results and discussion
Figure 1 shows the designed concept of this work. Cationic aromatic guests with
amino group or nitrogen atom are suitable molecules for the stable inclusion of a
hostꢀguest interaction with the CB[8] container due to their facile entry into the
hydrophobic cavities through hydrophobic effect and electrostatic interaction in
4
4
aqueous solution. We designed and synthesized the cationic guest using fluorene as
fluorophore core and two positively charged bispyridinium end groups as the binding
sites for CB[8], which was denoted as FPy. In aqueous solution, when the molar ratio
of FPy to CB[8] is 1:1, the fluorene part would locate at the cavity of CB[8] showing
an enhanced emission, denoted as FPy/CB[8] (1:1). With further increasing the
stoichiometry to 1:2, two FPy molecules could stack together and be encapsulated by
four CB[8] host, showing a red shift emission. Considering the stability and strong
4

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emission of FPy/CB[8] (1:1), it can be used for ATP sensing via electrostatic
attraction, which caused the aggregationꢀinduced quenching. The synthetic route of
1
FPy is shown in Scheme 1, and its structure was demonstrated by H NMR and mass
spectroscopy. FPy is easily dissolved in water and polar solvents including DMSO
and methanol.
Figure 1. Schematic illustration of the formation process of FPy/CB[8] complex at
different molar ratio, and its principle on ATP sensing.
Scheme 1. Synthetic route of FPy.
Photophysical properties of FPy and FPy/CB[8] complex
The photophysical properties of FPy and FPy/CB[8] complexes with different molar
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ratios were firstly studied in water. The absorption spectra in Figure 2a shows that
FPy aqueous solution has a maximum absorption around 345 nm. As the addition of
CB[8] molecules, the absorption peak shifted toward longer wavelength, implying the
formation of complex. The maximum absorption appeared at 368 nm for FPy/CB[8]
(1:1), and shifted to 375 nm for FPy/CB[8] (1:2) showing a 30 nm redꢀshift compared
with FPy. Their corresponding fluorescence spectra under excitation at 345 nm are
shown in Figure 2b. FPy aqueous solution featured a weak greenꢀyellow emission at
5
17 nm (maximum), and its fluorescent picture is shown in Figure 2d (left). This
weak fluorescence intensity indicates the molecules are in wellꢀsolvated state due to
its strong affinity toward water. Most excitons decayed in a nonradiative channel via
intramolecular motion, leading to a weak emission. Upon addition of CB[8], the
fluorescence intensity raised up quickly as the stoichiometry increased from 1:0.25 to
1
:1, with a blue shift in maximum emission peak. For FPy/CB[8] (1:1), the maximum
fluorescence emission emerged at 490 nm with a 27 nm blue shift, while intensity of
fluorescence was remarkably enhanced 5ꢀfold compared to FPy. An intense cyan
fluorescence was observed under UV lamp illumination (Figure 2d). This
phenomenon is possibly caused by the inclusion of the fluorophore into the
hydrophobicity cavity of CB[8] avoiding some interference from water solvent.
However, further increasing the stoichiometry to 1:2 led to a remarkable reduction in
fluorescence intensity and a red shift about 20 nm compared to FPy, revealing the
formation of another kind of complex. Accordingly, its fluorescence color turned into
yellow. The twoꢀstep variation in Job’s plot in Figure 2c confirmed the 1:1 and 1:2
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stoichiometry of the complex.
Figure 2. (a) UVꢀvis absorption and (b) fluorescence emission spectra of FPy/CB[8]
in water with different molar ratios. (c) Job’s plot obtained by recording the emission
at 517 nm for the solution of FPy/CB[8] complex in aqueous solution at 25 °C
([FPy]+[CB[8]]=2 ꢁM, confirming the 1:1 and 1:2 stoichiometry of the complex. (d)
Fluorescent photos of FPy/CB[8] complex in water with different molar ratio under
irradiation of UV lamp (365 nm). The concentration of FPy is 10 ꢁΜ.
To further understand the relationship between optical properties and selfꢀassembly
behavior, the fluorescence decay experiments and absolute quantum yields
measurements were carried out and shown in Figure 3 and Table 1. FPy shows a
biꢀexponential decay curve with lifetimes of 1.61 ns and 7.82 ns, indicating the
existence of two species in aqueous solution. When the FPy/CB[8] (1:1) complex
formed, it showed a linear decay curve with only one lifetime of 4.31 ns. On the other
hand, the estimated absolute fluorescence quantum yield (Φf(abs)) value increased from
2
4.31% to 63.63% upon encapsulation by the CB[8] host (Table 1). These findings
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proved that all the FPy fluorophores entered into the cavity of CB[8] and had a less
influence from solvent molecules. Thus, some of these relaxation processes are
severely hindered by the hydrophobic cavity, which prolonged the excitedꢀstate
lifetime and enhanced the emission efficiency. In the case of FPy/CB[8] (1:2), it also
displayed a biꢀexponential decay curve with lifetimes of 4.72 ns (19.95%) and 24.2 ns
(80.05%), and the quantum yields had a slight decrease to 60.58%. Combined with
the redꢀshift in absorption and fluorescence spectra, we can conclude that in the
FPy/CB[8] (1:2) complex system, the fluorophores not only existed in the cavity of
CB[8] but also formed some kind of complicated complexes, which have a longer
lifetime but a decreased emission efficiency. The dominant longer lifetime was likely
to result of the interaction of fluorene and positivelyꢀcharged bispyridinium in the
cavity of CB[8] as shown in Figure 1, which might lead to a charge transfer from
fluorene to bispyridinium.
Figure 3. Decay curves of FPy aqueous solution (black curve), complex of FPy/CB[8]
(1:1) (pink curve) and FPy/CB[8] (1:2) (blue curve) at emission of 537 nm excited by
3
45 nm.
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Table 1. Absolute quantum yields and fluorescent lifetime of FPy, FPy/CB[8] (1:1),
FPy/CB[8] (1:2)
a
Sample
Absolute quantum
Lifetime
a
yield /%
τ/ns
Percent/%
51.62/48.38
100
FPy
24.31
63.63
60.58
1.61/7.82
4.31
4.72/24.2
FPy/CB[8] (1:1)
FPy/CB[8] (1:2)
19.95/80.05
a
The fluorescent lifetime and absolute quantum yield were measured at their
maximum emission peak.
Self-assembly behaviors
1
H NMR spectroscopy is a powerful technique to investigate the hostꢀguest
interaction, since the slight change in environment where the protons locate will cause
an obvious shift in their chemical shift. Here, the binding behaviour between guest
1
molecule FPy and host CB[8] was investigated by the H NMR in D O (Figure 4).
2
Generally, the proton signals of guest molecules shift to higher field when it is
incorporated into the cavity of CB[8], whereas the ones outside the cavity exhibit
4
5
2
downfield shifts. In our case, free FPy displayed clear and sharp signals in D O:
peaks marked with a, b, c, d were ascribed to aromatic protons of bispyridinium units,
while peaks of e, f, g were attributed to aromatic protons of fluorene. After
incorporation of 1 equiv CB[8], FPy/CB[8] (1:1) showed broad and weak proton
signals, indicating the formation of a single type of complex in the solution. The
a b c d
proton signals (H , H , H , H ) of the bispyridinium moiety underwent an obvious
complexationꢀinduced downfield shift of 0.18, 0.4, 0.19. 0.3 ppm, respectively,
indicating they were outside of the CB[8] cavity. On the other hand, the fluorene
moiety showed an obvious upfield shift, attributed to the encapsulation of CB[8] and
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the shielding effect of fluorene moiety to each other. As a contrast, we also
1
investigated the H NMR spectra of FPy/CB[7] (1:1) to comprehend the complexes’
formation deeply, which showed a similar variation of fluorescence spectra (Figure S1,
ESI†). The electrospray ionization mass spectrometry (ESIꢀMS) of the FPy/CB[8]
(1:1) complex proved the presence of a [1+1] complex (m/z, found 902.3, calcd value
ꢀ
2+
9
02.3 for [Mꢀ2Cl ] , Figure S3, ESI†). In addition, the correlation spectroscopy
(COSY) spectra also revealed the supramolecular assembled structure of the complex
in the solution (Figure S4, ESI†). After validation of the 1:1 binding stoichiometry,
4
ꢀ1
the lower limit of the complex formation constant (K ) was estimated as 2.05
×
10 M
S
1
5
ꢀ1
from H NMR integration and the K
S
of FPy/CB[7] complex was 1.49
×
10 M ,
indicating that the higher affinity of the fluorene moiety and CB[7] than FPy/CB[8]
complex (Figure S5ꢀS6, ESI†).
1
The H NMR spectra of the FPy/CB[8] (1:2) displayed that the proton signals (a”,
b”, c”) of bispyridinium moieties underwent an obvious complexationꢀinduced upfield
shift compared to FPy/CB[8] (1:1), showing a large difference compared to free FPy,
indicating they were also encapsulated in the cavity of CB[8]. As the contrast, the
proton signals of the FPy/CB[7] (1:2) mixture had a comparable shift change
compared to FPy/CB[7] (1:1) and free FPy, while the proton signals of FPy:CB[8]
(1:2) exhibited a significant shift after adding 2 equiv CB[8] (Figure S2, ESI†). This
large difference in proton shifts after adding CB[7] and CB[8] indicated that the
structure of FPy/CB[7] (1:2) and FPy:CB[8] (1:2) complex was not the same. The
complexationꢀinduced upfield shift of the FPy/CB[7] counteracted the shielding effect
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of bispyridinium unit. CB[8] has bigger cavity than CB[7] which is capable of
3
3,45
simultaneously encapsulating two guest molecules to form a stable complex.
Therefore, FPy/CB[7] (1:2) complex has one bispyridinium unit in the cavity, while
FPy/CB[8] (1:2) complex has two guest units in the cavity. Based on these evidences
we inferred that the bipyridinium moiety and fluorene unit were concurrently included
in the cavity of CB[8] through chargeꢀtransfer interaction and the end unit
(bispyridinium moiety) was also entrapped in the cavity of CB[8]. In particular, a new
set of proton of the CB[8] host appeared in the spectrum, which can be attributed to
the CB[8] host interaction with different moiety of FPy (fluorene and bispyridinium,
respectively). This result clearly suggested that new complex was formed by the two
different CB[8]ꢀbased hostꢀguest interaction, which was confirmed by COSY result
1
1
(
Figure S8, ESI†). The Hꢀ H ROESY spectra is useful for determining which signals
arise from protons that close to each other in space even if they are not bonded. The
ROESY spectra revealed the spatial proximity of proton Hd’’ꢀHe’’ and Hb’’ꢀHf’’ as
the stacking of fluorene moiety and bispyridinium unit in the FPy/CB[8] (1:2)
complex (Figure S9, ESI†). Furthermore, the ESIꢀMS results of FPy/CB[8] (1:2)
clearly indicated the formation of a [2+4] complex instead (m/z 1052.3, calcd value
ꢀ
+ 3+
1
052.3 for [Mꢀ2Cl +Na ] , Figure S7, ESI†). The reduced fluorescence intensity and
quantum yield of FPy/CB[8] (1:2) compared to FPy/CB[8] (1:1) can be explained by
the chargeꢀtransfer interaction between bispyridinium moiety and fluorene unit.
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1
Figure 4. H NMR spectra of (a) FPy in D
1:2). The signals of CB[8] and solvent are denoted as symbol ● and ○, respectively.
Molecular modelling
2
O, (b) FPy/CB[8] (1:1), (c) FPy/CB[8]
(
We used theoretical calculations and molecular dynamic (MD) simulations to
investigate the interaction and stable structure of ionized FPy and CB[8] in water
solution. The electrostatic potential surface and MD simulation results were showed
in Figure 5. We calculated the electrostatic potential surfaces of ionized FPy and CB[8]
and simulated a system comprising ionized FPy and CB[8] in water solution. The
geometric structure and electrostatic potential surface of ionized FPy and CB[8] were
calculated by density functionally theory (DFT) calculations based on B3LYP/6ꢀ31G*
46
level using ORCA program. The related electrostatic potential surface of ionized
FPy and CB[8] were shown in Figure 5a and 5b, which indicated that the ionized FPy
showed a positive electrostatic potential around the whole molecular surface. The end
of the cavity of CB[8] showed a negative electrostatic potential, which was mainly
caused by the lone pair electrons on oxygen atoms. The molecular dynamics
47
simulations were carried out by the Gromacs 5.0.4 package. The GROMOS 54A7
force field model was used as an atomistic force field for the simulation system. A
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48
stochastic velocityꢀrescaling thermostat was used to maintain a constant temperature
with a relaxation time of 0.1 ps. The pressure coupling was controlled by the
49
Parrinello–Rahman approach with a relaxation time of 5.0 ps. The cutoff distances
for van der Waals and Coulombic interactions were 1.5 nm, and the long range
electrostatic interactions were computed by the ParticleꢀMesh Ewald (PME)
50
method. The equations of motion were integrated with a time step of 0.001 ps. The
ꢀ
initial system contained one ionized FPy, a CB[8], 2014 water and 2 Cl . As the MD
simulation time went, ionized FPy was sucked in the cavity of CB[8] by the coulomb
interaction which was consistent with the electrostatic potential surface analysis. At
the last 10 ns, the ionized FPy and CB[8] kept a stable structure as shown in Figure 5c.
ꢀ
The initial system contained two ionized FPy, four CB[8], 2001 water and 4 Cl . As
the MD simulation time went, the two ionized FPy were approached closer and the
fluorene moiety of the first molecule was close in parallel to the bispyridinium moiety
of another molecule with CB[8] by the coulomb interaction (Figure 5d). And the other
two bispyridinium moieties also were encapsulated in the cavity of CB[8] respectively.
1
The result is consistent with the structure that we inferred from H NMR and prove
the structure is stable in this condition.
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Figure 5. The electrostatic potential surfaces calculation of (a) ionized FPy and (b)
CB[8], MD simulation system structure for (c) FPy/CB[8] (1:1).and (d) FPy/CB[8]
(1:2).
Characterization of FPy/CB[8] complex
The size and morphology of FPy/CB[8] complexes were characterized by dynamic
light scattering (DLS), zeta potentials and scanning electron microscopy (SEM). DLS
result in Figure 6a showed the FPy molecules dissolved well in water and no
aggregates formed. After addition of 1 equiv of CB[8], a size distribution around 44.8
nm emerged, indicating the formation of complex and aggregates. As further adding
CB[8], a larger size distribution around 224.2 nm appeared for FPy/CB[8] (1:2). On
the other hand, the zeta potential of the complex in Figure 6b displayed a decreasing
trend with increasing CB[8] molecules. FPy/CB[8] (1:2) showed a weak positive
charge of +0.91 mV because the positivelyꢀcharged bispyridinium moieties entered
into the cavity of CB[8] which screened the charges. SEM image in Figure 6c reveals
the FPy/CB[8] (1:1) dispersed well approximately nanorod structures, while the
FPy/CB[8] (1:2) selfꢀassemblies transformed into nanocubes. This kind of regular
morphologies was also observed for other CBꢀbased hostꢀguest selfꢀassemblies
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51ꢀ53
reported before.
In the case of FPy:CB[8] (1:1) complex, the interaction of
bispyridinium units on two adjacent complex induced the linear selfꢀassembly of the
complex through hydrophobic interaction and πꢀπ interactions, giving rise a rodꢀlike
morphology. For the FPy:CB[8] (1:2) complex, its rigid molecular conformation
facilitated the formation of cubic structures.
Figure 6. (a) DLS distribution and (b) zeta potential of free FPy, FPy/CB[8] (1:1) and
FPy/CB[8] (1:2). (c) SEM images of complexes of FPy/CB[8] (1:1) and (d) FPy/CB[8]
(1:2).
The response of selfꢀassembly system toward external stimuli, such as temperature,
concentration, pH or salts, has a great influence on the practical application. Firstly,
concentration factor on the selfꢀassembly behaviour of FPy and CB[8] was studied.
As shown in Figure S10 (ESI†), when the concentration of FPy was below 5 ꢁM, the
fluorescence maximum peak of FPy/CB[8] (1:2) located at 493 nm, similar to the
FPy/CB[8] (1:1). If the FPy concentration raised above 5 ꢁM, the fluorescence
emission peak had a slight red shift, which further increased when the concentration
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came up to 10 ꢁM. It reveals that the concentration was a key factor to induce the
formation of FPy/CB[8] (1:2). This is understandable because sufficient concentration
was required for CB[8] to efficiently meet with the FPy/CB[8] (1:1) complex and
formed FPy/CB[8] (1:2).
Next, we investigated the fluorescence response behaviour of FPy/CB[8] (1:1)
complex towards variation in pH value and salt concentration (Figure 7). The
fluorescence intensity of FPy/CB (1:1) complex only showed a slight fluctuation over
pH=5ꢀ12 range, implying the selfꢀassembly was less affected by pH change (Figure
7
a). Electrostatic attraction plays an important role in the hostꢀgust interaction
between positively charged bispyridinium units and CB[8] cavity. The presence of salt
ions possibly screen the charges and have an influence on the selfꢀassembly behavior.
The experiment turned out that the fluorescence spectra recorded for the FPy/CB[8]
(1:1) complex do not change over the variation of NaCl concentration which suggest
that FPy/CB[8] complex was of high thermodynamic stability (Figure 7b).
Figure 7. (a) Variation in fluorescence intensity of FPy/CB[8] (1:1) at different pH
value. (b) Fluorescence spectra of FPy/CB[8] (1:1) in the presence of NaCl with
concentration from 60 nM to 6 mM.
In the following, we investigated the response behaviour of FPy, FPy/CB[8] (1:1)
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and FPy/CB[8] (1:2) complex toward changes in temperature. When the free FPy
solution was heated from 20 ºC to 60 ºC, the intensity of fluorescence slightly
decreased (Figure 8a). This is because at higher temperature the excitons decay more
likely in a nonradiative way via intramolecular motion, leading to a reduced emission.
For the FPy/CB[8] (1:1) complex, the intensity of fluorescence did not show any
changes with the temperature increased (Figure 8b). It indicated that FPy encapsulated
in the hydrophobic cavity of CB[8] could hinder FPy molecules to interact with
solvent molecules which can decrease the excitons decay via nonradiative channel.
Interestingly, when the temperature of the solution of the FPy/CB[8] (1:2) was heated
from 20 ºC to 60 ºC, a relative increase in intensity as well as a remarkable blueꢀshift
of the emission peak were detected from 537 nm to 490 nm. Correspondingly, the
fluorescence color of the solution under UV light was changed from yellow to cyan
(
Figure 8c). The reversible process can be modulated repeatedly completely in the
heating and cooling cycle (Figure 8d). These changes indicated that the FPy/CB[8]
1:2) was not as stable as FPy/CB[8] (1:1) complex, which would break apart and
(
inclined to form the FPy/CB[8] (1:1) complex when heated. This process was also
1
demonstrated by H NMR result under different temperature (Figure S11, ESI†).
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Figure 8. Fluorescence emission spectra from 20 °C to 60 °C of (a) FPy, (b)
FPy/CB[8] (1:1), (c) FPy/CB[8] (1:2). (d) Reversibility of the heating and cooling
process for FPy/CB[8] (1:2) between 20 ºC and 60 ºC.
Application for ATP sensing
The asꢀprepared FPy/CB[8] (1:1) complex provides abundant coordination sites for
anion, attributed to the presence of bispyridinium units. ATP molecules contain three
negatively charged phosphate groups, and strong electrostatic attraction will occur
between ATP and positively charged species, like FPy. The ATP with negatively
charge can make the FPy/CB[8] (1:1) complex aggregate together due to the
electrostatic attraction between the bispyridinium and ATP. Upon binding with ATP,
the aggregation of FPy/CB[8] complex lead to the increased nonradiative decay of the
excitons. Accordingly, the fluorescence intensity of the FPy was decreased in this
aggregated state, showing typical aggregationꢀcaused quenching, as shown in Figure
9
a.
Here, we chose the FPy/CB[8] (1:1) with a FPy concentration of 1 ꢁM for ATP
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sensing. After addition of ATP, the absorption curve of FPy/CB[8] (1:1) in Figure 9b
showed an obvious decrease, possibly attributed to the aggregation of the complex.
The fluorescence response of the FPy/CB[8] (1:1) complex in aqueous solution in the
presence of ATP with different concentration were recorded (Figure 9c). It clearly
showed that the fluorescence of the complex remarkably quenched upon addition of
ATP. Figure 9c showed that fluorescence quenching was observed at a very low level
of ATP (1 n
fluorescence intensity (1ꢀI/I
ꢀ9 nM (Figure 9d), which was favorable to quantitative ATP assay at nM level. And
the linear relationship was about 1ꢀI/I =0.0297[ATP]+0.0255. The fluorescence
M
) in the beginning. Nearly linear relationship between the variation in
0
) and ATP concentrations was observed in the range of
1
0
intensity further reduced rapidly as ATP concentration increased, and about 50% of
the emission intensity was quenched in the presence of 10 ꢁM of ATP. The completed
quenching would not occur when 1000 equiv of ATP were added. This phenomenon
could be explained by the encapsulation of FPy into the cavity of CB[8] and the
quenching mechanism. Compared to the free FPy molecule, the FPy/CB[8] (1:1)
complex has a shield to prevent aggregating together due to electrostatic attraction
and πꢀπ stacking interaction. Consequently, these complexes can attract the ATP
molecules easily and tightly at a low concentration.
On the other hand, we checked the fluorescent spectra about FPy/CB[8] (1:2)
complex in the presence of ATP with different concentration (Figure S12, ESI†). The
addition of ATP did not cause any obvious change of the fluorescence spectra of
FPy/CB[8] (1:2), indicating almost no ATPꢀinduced aggregation took place. This
19

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phenomenon was reasonable since the positively charged bispyridinium moieties
located in the cavity of CB[8] which screened the charges. Thus, ATP molecules
cannot interact with the bispyridinium units anymore, and no aggregationꢀinduced
fluorescence quenching was observed. This observation further demonstrate the
quenching mechanism for FPy/CB[8] (1:1) complex after adding ATP, as well as the
assumption on the assembly structure of FPy/CB[8] (1:2).
Figure 9. (a) Photograph of FPy/CB[8] (1:1) solutions in the absence of ATP and in
the presence of ATP (1 mM) under a handꢀheld UV lamp illumination. (b) Absorption
spectra of FPy/CB[8] (1:1) before and after addition of 1 mM ATP. (c) Fluorescence
spectra of FPy/CB [8] (1:1) upon continuous addition of ATP with concentration from
1
nM to 1 mM. (d) Linear relationship between the variation in fluorescence intensity
(
1ꢀI/I ) and ATP concentrations in the range of 1ꢀ9 nM. The concentration of FPy
0
solution was 1 ꢁM.
In living cells, adenosine 5’ꢀdiphosphate (ADP) and adenosine 5’ꢀdiphosphate
(AMP) are always present, which have similar structures compared to ATP but
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contain only two or one phosphate groups respectively. They also could interact with
FPy/CB[8] (1:1) complex via electrostatic attraction to induce the aggregationꢀcaused
quenching. To test this possibility, the fluorescence spectra of the complexes was
tested (Figure 10a). After adding 1 mM AMP or ADP under the same experimental
conditions as ATP, the fluorescence intensity of FPy/CB[8] (1:1) complexes exhibited
a decrease, but much fewer changes compared with ATP. The variation of
fluorescence spectra of FPy/CB[8] (1:1) complex as a function of the addition of
AMP and ADP with different concentration was also examined (Figure S13, ESI†).
The fluorescence intensity of FPy/CB[8] (1:1) also showed a decrease trending with
the continuous addition of ADP and AMP, but the change of fluorescence intensity
was much less compared to ATP at same concentration. This difference binding
performance might be caused by the difference in the number of phosphate groups on
the analytes. The lower charge density on ADP and AMP leads to a weaker
interaction with the positive molecule, which cannot efficiently prevent the complexes
stacking and cause the fluorescence quenching. Considering the fact that the
concentration of ADP and AMP is much lower than ATP in the living cells, their
interferences to ATP sensing can be ignored. Besides, other negative species also
could induce the aggregation of FPy/CB[8] and possible interference on ATP sensing.
In order to test the possibility, the fluorescence changes of FPy/CB[8] complexes
ꢀ
ꢀ
ꢀ
2ꢀ
were recorded in the presence of various anions, including Cl , F , H
2
PO
COO . As Figure 10b shown, only ATP
induced a large quenching in the fluorescence intensity of the complex, where the
4 4
, HPO ,
ꢀ
ꢀ
2ꢀ
ꢀ
ꢀ
ꢀ
3 2 3 3 3 3
NO ,NO , CO , HCO , HSO and CH
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anions showed slight effects on the fluorescence response. It is worthy to notice that
2
ꢀ
3
carbonate ion (CO ) has two negative charges, which displayed much more
quenching than other anions with one charge. But the presence of carbonate ion only
caused a slight quenching on the fluorescence of FPy/CB[8] (1:1) complexes
compared to ATP. Thus, the interference of anions could be ignored.
Figure 10. (a) Fluorescence emission spectra of FPy/CB[8] (1:1) (1 ꢁM) upon
addition of sodium salt of ATP (1 mM), ADP (1 mM), and AMP (1 mM) in aqueous
solution. (b) Variation in fluorescence intensity (I/I
addition of various anions (1 mM).
Conclusions
0
) of FPy/CB[8] (1:1) after
In summary, a facile CB[8]ꢀbased supramolecular approach to fabricate
dynamically smart fluorescent material in aqueous solution is described. It greatly
decreased the number of required synthetic steps and produced a system with tunable
and reversible photophysical properties. The hostꢀguest interaction based on
macrocyclic container was produced by simple addition of CB[8] to a solution of FPy
which caused fluorescent emission shift. In addition, the encapsulation of FPy into the
cavity also induced long lifetime, high quantum yield and regular topological
aggregates. Furthermore, the temperature triggers fluorescent responses between
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green and yellow emission. Upon binding with ATP, the fluorescent intensity of the
FPy/CB[8] (1:1) complexes remarkably decreased due to aggregationꢀcaused
quenching, which exhibited specific ATP sensing with high sensitivity and selectivity.
Experimental section
2
,4ꢀdinitrochlorobenzene, 4,4'ꢀbipyridine, 2,7ꢀdiaminofluorene, cucurbit[7]uril
CB[7]), cucurbit[8]uril (CB[8]) were purchased from J&K Chemical Co. Adenosine
′ꢀtriphosphate (ATP) sodium salt, adenosine 5′ꢀdiphosphate (ADP) sodium salt, and
(
5
adenosine 5′ꢀmonophosphate (AMP) sodium salt were bought from SigmaꢀAldrich.
Other inorganic salts and organic solvents were purchased from Beijing Chemical
Works and used without further purification. Ultrapure Millipore water (18.6 MΩ·cm)
was used throughout the experiments.
Synthesis of 1-(2,4-dinitrophenyl)-4,4'-bipyridinium (Compound 1)
Compound 1 was synthesized by Zincke reaction according to previously reported
54
procedure. In brief, a solution of 2,4ꢀdinitrochlorobenzene (3.25 g, 16 mmol) and
4
,4'ꢀbipyridine (2.50 g, 16 mmol) in 25 mL of ethanol was heated under reflux for 16
h. The mixture was cooled to room temperature and added to diethyl ether (250 mL)
under stirring. The goldenꢀbrown precipitate was filtered off and washed with diethyl
1
ether followed by drying in vacuum (4.5 g, 78.4%). H NMR (Figure S14, 400 MHz,
D
2
O): δ 9.43 (s, 1H, ArꢀH), 9.28 (d,
ArꢀH), 8.88 (d, = 4.6 Hz, 2H, PyꢀH), 8.72 (d,
0.2 Hz, 1H, ArꢀH), 8.08 (d, = 4.6 Hz, 2H, PyꢀH) ppm.
J
= 5.2 Hz, 2H, PyꢀH), 8.97 (d,
J = 8.8 Hz, 1H,
J
J
= 5.3 Hz, 2H, PyꢀH), 8.31 (d, J =
1
J
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Synthesis of FPy
Compound 1 (2.87 g, 8 mmol) was added to a ethanol solution (50 mL) of
,7ꢀdiaminofluorene (0.39 g, 2 mmol) and the mixture was refluxed under intense
2
stirring for 48 h. The hot solution was filtered and the solvent was removed under
reduced pressure. The solid was washed using dichloromethane. The resulting
brownꢀblack solid was redissolved in a minimum amount of methanol with heating
and reprecipitated by addition of diethyl ether. The precipitate was collected and then
1
dried under vacuum to give compound FPy (0.78 g, 71.5%). H NMR (Figure S15,
4
00 MHz, D
.63 (d, = 6.5 Hz, 4H, H
), 7.89 (d,
MHz, CD OD, 25
2
O, 25
°
C): δ 9.30 (d,
J
= 6.5 Hz, 4H, H
d
), 8.87 (d,
J
= 6.1 Hz, 4H, H
= 6.3 Hz, 6H, H
)ppm; C NMR (Figure S16, 100
a
),
8
J
c
), 8.31 (d,
J
= 8.3 Hz, 2H, H ), 8.11 (d,
e
J
b
,
13
H
g
J
= 9.8 Hz, 2H, H
f
), 4.31 (s, 2H, H
h
°
C): δ=152.61, 148.12, 145.03, 143.77, 141.61, 141.28, 140.76,
3
1
24.42, 122.22, 121.30, 121.05, 119.99, 35.50 ppm; MALDIꢀTOFꢀMS: m/z calcd for
2
4
+
C H Cl N : 476.3; found: 476.4.
33
24
2
1
Characterization. The H NMR, correlation spectroscopy (COSY), rotating frame
13
nuclear overhauser effect spectroscopy (ROESY) and C NMR spectra were recorded
on a 400 MHz and 100 MHz AC Bruker spectrometer in the indicated solvents at
room temperature (298 K), respectively. The molecular weight of FPy was measured
on a Bruker Daltonics BIFLEX III MALDIꢀTOF analyzer in matrixꢀassisted laser
desorption ionization timeꢀofꢀflight (MALDI) mode. The molecular weight of
FPy/CB[8] complex was measured on a Bruker 9.4T Solarix Fourier transform ion
cyclotron resonance mass spectrometer using electrospray ionization (ESI) mode.
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Ultravioletꢀvisible (UVꢀvis) absorption and photoluminescence (PL) emission spectra
were measured on a Hitachi Uꢀ3900H spectrophotometer and a Hitachi Fꢀ7000
fluorescence spectrophotometer, respectively. The hydrodynamic diameters and zeta
potentials of complexes in aqueous solution were determined by dynamic light
scattering (DLS) with a Malvern Zetasizer Nano ZS90 at 25 ºC. The size and
morphology of the complexes were measured with scanning electron microscope
(SEM, JEOL JSMꢀ7401F). The SEM samples were prepared by dropping a portion of
the complexes onto silicon wafers, which were then lyophilized under reduced
pressure to keep the initial shapes of the formed aggregates. The absolute fluorescence
quantum yields were determined using
a
spectrofluorometer (NanologR
FluoroLogꢀ3ꢀ2ꢀiHR320, Horiba Jobin Yvon) equipped with an integrating sphere. The
excitation wavelength was set at 345 nm. The scattering spectral range was set from
3
40 nm to 350 nm, and the emission spectral range was from 490 to 700 nm. The
fluorescent lifetime was determined using a spectrofluorometer (UltraFast lifetime
spectrofluorometer Delta flex, Horiba Jobin Yvon).
Acknowledgments
This work was supported by the National Natural Science Foundation of China
(
51503015, 51673022), the Fundamental Research Funds for the Central Universities
FRFꢀBRꢀ16ꢀ021A).
(
Conflict of interest
The authors declare no conflict of interest.
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Keywords:
fluorescence,
fluorene,
supramolecular
system,
host-guest,
cucurbit[8]uril, ATP sensing
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