A supramolecular fluorescent vesicle based on a coordinating aggregation induced emission amphiphile: Insight into the role of electrical charge in cancer cell division

Article abstract of DOI:10.1039/c6cc06432a

Binding of Zn²⁺ to the coordinating supramolecular vesicle based on an aggregation induced emission amphiphile TPE-BPA immediately triggers the formation of charged vesicles. This induces vesicle fission and fluorescence reduction, suggesting a

Full text of DOI:10.1039/c6cc06432a

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DOI: 10.1039/C6CC06432A
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Supramolecular Fluorescent Vesicle Based on Coordinating
Aggregation Induced Emission Amphiphile: Inspiration for the
Role of Electrical Charges on Cancer Cell Division
Received 00th January 20xx,
Accepted 00th January 20xx
Jie Lia, Kangjie Shi,a Markus Drechslerb, Ben Zhong Tangc,*，Jianbin Huanga,* and Yun Yana,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Binding of Zn2+ to the coordinating supramolecular vesicle based ionic supramolecular self-assembly of an aggregation induced
on aggregation induced emission amphiphile TPE-BPA emission (AIE)29 amphiphile. This vesicle divides into many
immediately triggers formation of charged vesicles. This induces smaller ones when triggered by electrical charges, which is
vesicle fission and fluorescence reduction, suggesting a looser accompanied by the reduction of fluorescence, suggesting a
molecular packing in the charged vesicle membrane. Since cancer looser molecular packing in the charged vesicles. Drug
cells are highly charged, this inspires that the quick fission of releasing test indicates that the releasing rate of the charged
cancer cells may have electrial charge origin.
vesicle is 7 times of the uncharged one. This model system,
taking the advantage of the coordinating aggregation induced
emission (AIE) molecule, for the first time revealed that
electrical charges not only promote the membrane division of
cancer cells, but also leads to looser molecular packing in the
cancer cells which facilitates much quicker mass exchange
through the membrane.
The structure of TPE-BPA is given in Scheme 1a. Since each
ligand carries 2 negative charges, TPE-BPA is highly soluble in
water and emits weakly. However, upon addition of cationic
surfactant CTAB (cetyltrimethylammonium bromide), intense
Cancer cells are notorious for their quick reproduction and
easy metastasis which raise serious threats to human lives.1-6
Up to date, the code that governs the quick fission and
metastasis of cancer hasn’t been cracked, yet it is well-known
that the membrane of cancer cells have much higher electrical
charges7-10 which is generally considered caused by the change
of the membrane contents.11 Recent study shows that
alternating the electrical field may inhibit the metastatic
spread of cancer 12-15. However, so far the role of electrical
charge on the fission of the membrane of cancer cells hasn’t
been clearly revealed, which is to a large extent due to the lack
of proper modelling system.
Vesicles have long been considered as model systems for
biological cells, yet so far studies in this regard are focused on
the membrane behaviour of normal cells.16-22 Corresponding
studies on cancer cells are scarce, because most vesicles self-
assembled from small molecules disassemble if the electrical
charge of the component molecules increases,23-26 whereas
polymeric vesicles often undergo “breathing” behaviour,
namely, the size of the same vesicle swells upon increasing
electrical charges but contract on discharging.27, 28 It is highly
desired to design robust vesicles that keeps vesicular structure
at high electrical charges for the modelling of cancer cells.
Herein we report a case of fluorescent vesicle built with the
emission can be induced, indicating it is a typical AIE
30
molecule.29,
The Zeta potential of TPE-BPA in aqueous
solution is -17 mV, but it increases sharply upon addition of
cationic CTAB. A zero potential is obtained at the molar ratio of
CTAB: TPE-BPA=8:1(Figure 1a), suggesting the formation of
neutral electrostatic complex of TPE-BPA@8CTAB. In line with
this, the fluorescence show maximum emission (Fig.1 b,c), and
UV spectra display the largest red-shift (Fig. 1d), confirming
the occurrence of the optimal interaction between TPE-BPA
and CTAB at the molar ratio of 1:8.
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stretches in the membrane of vesicle, the smallesVtiehweAigrtihclte Ofonrlinae
dry vesicle should be equal to the sum DoOfI:tw10o.10c3o9l/lCa6pCseCd06T4P32EA-
BPA layers, namely, about 5.0 nm. Therefore, the height of
21.0 and 39.8 nm corresponds to multilamellar vesicles with 4
and 8 shells, respectively. Obviously, the vesicle membrane is
formulated by the monolayer of TPE-BPA, and CTAB, with an
extending length of 1.5 nm, has to embed into the voids
between the coordinating heads of TPE-BPA (Scheme 1b).
Since TPE-BPA on its own cannot self-assemble, we expect that
the increased hydrophobic effect in the TPE-BPA@8CTAB
supramolecular building block is the main driving force for the
formation of vesicles.
1.0
a)
b)
0.8
0.6
0.4
0.2
0.0
1
10
100
1000
Radius/nm
Scheme 1. a) Molecular structures of TPE-BPA and CTAB. b) Illustration of
self-assembly of TPE-BPA/CTAB vesicles and electrical charge triggered
fission of the TPE-BPA/CTAB vesicle.
Figure 2. Morphology and size information for the TPE-BPA@8CTAB
vesicles: a) cryo-TEM image; b) size distribution obtained from DLS
measurement. Inset in b) is the CLSM image of the TPE-BPA@8CTAB
vesicle.
25
a)
b)
20
15
10
5
CTAB/TPE-BPA
As the hydrophilic heads of TPE-BPA molecules are capable
of coordinating with metal ions, such as Zn2+,31 Ni2+,32
Ca2+,32etc., Zn2+ was added to the TPE-BPA@8CTAB vesicular
system. (Zn2+ has no quenching effect to the luminescence of
TPE group.32) Fig. 3a shows that fluorescence keeps decreasing
upon addition of Zn2+ and reaches a plateau at the ratio of
Zn2+/TPE-BPA=2. Meanwhile, the UV absorption at 255 nm
keeps increasing and levels off at this ratio (Fig. 3b). Since each
TPE-BPA contains 4 coordinating heads (pyridine dicarboxylate
0
0
2
4
6
8
10 12 14 16 18
-5
CTAB/TPE-BPA
-10
-15
-20
2.0
2000
1500
1000
500
0
CTAB/TPE-BPA
CTAB/TPE-BPA
d)
c)
0
2
4
6
0
4
8
12
16
1.5
1.0
0.5
0.0
ligand), and in principle each Zn2+ can coordinate with two
31, 33
8
pyridine dicarboxylate ligands,
this implies every two
10
12
14
16
20
coordinating heads share one Zn2+to satisfy the space of a
octahedral field,34,35as illustrated in Fig. S3. Therefore, we
propose that the neighbouring two pyridine dicarboxylate
ligands coordinate to the same Zn2+, as illustrated in Scheme
1b.
400
450
500
550
600
280 320 360 400 440
Wavelength/nm
Wavelength/nm
Figure 1. a) Variation of the zeta potential with the molar ratio of
CTAB/TPE-BPA; b) photo of the CTAB/TPE-BPA samples under 365 nm UV
lamp at various molar ratios; c) fluorescence spectra for the CTAB/TPE-BPA
DLS measurements (Fig. S4a) suggest that smaller particles
with the hydrodynamic diameter about 35 nm are formed
samples of different molar ratio; d) UV spectra for the CTAB/TPE-BPA upon addition of Zn2+ to the TPE-BPA@8CTAB vesicular system.
samples. [TPE-BPA]=50 μM, T=298K. λex=365nm.
Number averaged analysis indicates that the small particles are
the dominant species (Fig. S4b). Cryo-TEM observation reveals
Cryo-TEM observation revealed that unilamellar and
that the smaller particles still keep vesicular structure (Fig. 3b,
multilamellar vesicles (Fig. 2a and Fig. S1) with average size of
inset), which is further confirmed in the AFM measurement
~145 nm have been formed, which is in consistent with the
(Fig. 3c). The height profiles indicate that the collapsed
result obtained from DLS measurement (Fig. 2b). The Confocal
particles are about 10.5 nm and 5.1 nm (Fig. 3c). These values
Laser Scanning Microscopy (CLSM) image indicates that these
are much smaller than 35 nm, suggesting these particles are
vesicles are fluorescent (Inset in Fig. 2b), confirming the
hollow vesicles. It is worth noting that the scattering intensity
presence of TPE-BPA components in the vesicle membrane.
(Fig. 3d) for the small vesicles is about 25% higher than that of
AFM image in Figure S2 shows that the thickness of the
the larger ones. Since the scattering intensity is proportional to
R6, where R is the radius of the scattering particle, the
collapsed vesicles can be 21.0 nm, 39.8 nm, etc. Since the
extending length of TPE-BPA is about 2.5 nm and it always
observation of increased scattered light intensity strongly
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indicates that the number of the small particles is tremendous. the self-assembled structure develops from micelVleiesw(ArPtic=le0O~n1li/n3e)
Therefore, it is convinced that Zn2+ ions have triggered the to vesicles(P=1/2~1). Generally speaking, in the reasonable P
DOI: 10.1039/C6CC06432A
fission of the TPE-BPA@8CTAB vesicles (Scheme 1b).
range, larger P will lead to larger particles. As the surface
charge of the vesicles is increased, repulsive forces between
molecules in the membrane are generated, which leads to
larger occupied area per molecule. As a result, the value of P is
decreased, so that smaller vesicles are formed.
2.0
Zn2+/TPE-BPA
Zn2+/TPE-BPA
b)
a)
4000
3000
2000
1000
0
1
2
3
4
0
0
1
2
4
1.5
1.0
0.5
0.0
It is worth noting that the electrical charge triggered fission
of vesicles is not the same as the ‘breathing’ vesicles reported
28
in literature,27,
where increasing electrical charges don’t
change the number of vesicles but generate a larger one as a
result of repulsion inside the same vesicle. In contrast, in the
present study, the increased charge has resulted in increased
number of vesicles but decreased size. Fig. 5a clearly shows
that after addition of Zn2+ ions for 1 min, the vesicles become
ellipsoid, and divide into several smaller ones.
Since the membrane of cancer cells are highly charged
compared to those normal ones,7-10 the present results
indicate that the high electrical charges may accelerate the cell
fission and generate a looser molecular packing. Because TPE-
BPA is a typical AIE molecule which displays emission
proportional to the extent of aggregation,39 the decrease of
250
300
350
400
400
450
500
550
600
Wavelength/nm
Wavelength/nm
80
60
40
20
0
d)
c)
2+
vesicle
2Zn
@vesicle
Figure 3. a) Variation of the fluorescence, b) UV/Vis absorption with the the emission is a reflection of the looser molecular packing. To
addition of Zn2+ to the TPE-BPA@8CTAB vesicular system; c) AFM image
confirm this argument further, the releasing rates displayed by
vesicles with different charges were compared. Figure 5b
and the height profile of the 2Zn2+@TPE-BPA@8CTAB vesicles; d) Scattered
intensity of the TPE-BPA@8CTAB vesicular system before and after addition
shows when a hydrophilic drug DOX is entrapped in the native
charge neutral vesicle of TPE-BPA@8CTAB, only 5% of the drug
is released within 60 min. In contrast, as the charges were
increased by increasing the composition of CTAB to form TPE-
BPA@12CTAB vesicle (ζ = 6,4 mV), the releasing rate was
enhanced to 15% within 60 min. Not surprisingly, nearly 35%
of DOX is released from the 2Zn2+@vesicle within the same
time scale since it has a much higher potential of ζ = 16,5 mV.
The charge dependent releasing rate demonstrates that the
molecular exchange through the membrane has been
significantly promoted in the charged vesicle. We expect that
this may facilitate the metastatic spread of cancer factors.
of Zn2+. [TPE-BPA]= 50 μM, [CTAB]= 400 μM, [Zn2+]= 100 μM. Inset in b) is
the cryo-TEM image of the 2Zn2+@TPE-BPA@8CTAB vesicles.
The Zn2+ triggered vesicle fission is attributed to the
increased surface charge. Fig. 4a demonstrates that after
addition of Zn2+ ions, the zeta potential of the vesicles keeps
increasing and reaches a plateau at the optimal coordinating
ratio of Zn2+/TPE-BPA= 2. In a controlled experiment where the
Zn2+ is replaced with Ca2+, a cation displaying much weaker
binding affinity to the head of TPE-BPA,36 the zeta potential of
the vesicles only slightly increases (Fig.4a). In line with this, the
size of the vesicles is hardly influenced (Fig. 4b). Accordingly,
the fluorescence intensity and UV absorbance (Figure S5) are
not influenced, too.
50
8CTAB@TPE-BPA
a)
b)
2Zn2+@Vesicle
40
30
20
10
0
12CTAB@TPE-BPA
20
a)
Ca2+/TPE-BPA
1.0
b)
16
12
8
0
1
2
3
4
0.8
0.6
0.4
0.2
0.0
Ca2+
Zn2+
0
20
40
60
80
4
Time (min)
0
Figure 5 a) TEM image of the fission of TPE-BPA@8CTAB vesicle triggered by Zn2+.
The image was made after addition of Zn2+ within 1 minute. [TPE-BPA]= 50 μM,
[CTAB]= 400 μM, [Zn2+]= 100 μM. b)Comparison of DOX releasing rate in the
0
1
2
3
4
1
10
100
Radius/nm
1000 10000
Metal/TPE-BPA
Finger 4. a) Comparison of the zeta potential of the TPE-BPA@8CTAB vesicle native TPE-BPA@8CTAB vesicle and the charged vesicle of TPE-
triggered by Zn2+ and Ca2+. b) DLS size distribution of the TPE-BPA@8CTAB vesicle BPA@12CTAB, 2Zn2+@vesicle. [TPE-BPA]=50 μM, [CTAB]=400 μM, [Zn2+]=
in the presence of various amount of Ca2+.
100 μM, [DOX]=100 μM, T=25°C.
The electrical charge triggered vesicle fission can be
qualitatively explained with the theory of molecular packing37,
38, where the packing parameter P is expressed as P=v/al, with
v, a, l being the volume of the hydrophobic part, the average
occupied area of the component molecule, and the extending
length of the hydrophobic chains, respectively. P increases as
In summary, we for the first time report the triggered fission
of vesicles with increasing electrical charge. The vesicle is built
with the supramolecular vesicle of ionic self-assembly of a
coordinating aggregation induced emission amphiphile. The
original low charged strong luminescent vesicle immediately
undergo fission upon coordination with Zn2+, which is
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25. D.-S. Guo, K. Wang, Y.-X. Wang and Y. Liu, J. Am. Chem. Soc., 2012,
accompanied by the drastic increase of the surface electrical
potential and the decrease of the fluorescence, indicating a
much looser molecular packing in the highly charged vesicles.
Drug releasing experiment reveals a faster molecular exchange
through the charged vesicular membrane, which confirms the
occurrence of looser molecular packing. Because a prominent
feature of cancer cells is their abnormally high surface charge
potential, this model system clearly indicates that the
electrical charges are very relevant to the fission and
molecular arrangement in the membrane of cancer cells,
which may help to reveal the mystery behind the easy
metastasis of the cancer cells and inspire novel strategy for
cancer therapy.
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