Aggregation-Enhanced Emission of Fluorescent-Gemini Surfactants with High Photostability for Cell-Membrane Imaging

Article abstract of DOI:10.1002/jsde.12036

In situ studies of the aggregation behavior of traditional surfactants at the liquid interface using spectroscopic methods are often significantly affected by the large volume of fluorescent groups, such as pyrene. Fluorescent-Gemini surfactants provide an ideal solution since the fluorescent block can be designed as a spacer or a tail. In this work, we report the synthesis of a new fluorescent-Gemini surfactant with a rigid spacer (referred to as 8-TBT-8). The aggregation behavior and application in cell-membrane imaging were investigated. The unique aggregation behavior in an organic solvent and aqueous solution was studied using spectroscopy. UV–vis and photoluminescence spectra of 8-TBT-8 revealed that this new fluorescent surfactant forms H aggregates in organic solution to give blue emission, whereas it forms J aggregates in aqueous solution to give green fluorescence under UV light. In addition, the fluorescence intensity of 8-TBT-8 increases abruptly at concentrations higher than the critical micellization concentration. Good photostability and a unique structure make the synthesized Gemini surfactant very suitable for membrane imaging.

Full text of DOI:10.1002/jsde.12036

J Surfact Deterg (2018) 21: 433–440
DOI 10.1002/jsde.12036
ORIGINAL ARTICLE
Aggregation-Enhanced Emission of Fluorescent-Gemini
Surfactants with High Photostability for Cell-Membrane
Imaging
Jinwen Peng1 · Junjie Deng1 · Xuebing Huang1 · Pengfei Sun2 · Weixing Deng1
Received: 5 May 2017 / Revised: 5 October 2017 / Accepted: 20 December 2017
© 2018 AOCS
Abstract In situ studies of the aggregation behavior of
traditional surfactants at the liquid interface using spectro-
scopic methods are often significantly affected by the large
volume of fluorescent groups, such as pyrene. Fluorescent-
Gemini surfactants provide an ideal solution since the fluo-
rescent block can be designed as a spacer or a tail. In this
work, we report the synthesis of a new fluorescent-Gemini
surfactant with a rigid spacer (referred to as 8-TBT-8). The
aggregation behavior and application in cell-membrane
imaging were investigated. The unique aggregation behav-
ior in an organic solvent and aqueous solution was studied
using spectroscopy. UV–vis and photoluminescence spec-
tra of 8-TBT-8 revealed that this new fluorescent surfactant
forms H aggregates in organic solution to give blue emis-
sion, whereas it forms J aggregates in aqueous solution to
give green fluorescence under UV light. In addition, the
fluorescence intensity of 8-TBT-8 increases abruptly at
concentrations higher than the critical micellization concen-
tration. Good photostability and a unique structure make
the synthesized Gemini surfactant very suitable for mem-
brane imaging.
Keywords Gemini surfactant Á Aggregation-enhanced
emission Á Fluorescent surfactant Á Cell imaging
J Surfact Deterg (2018) 21: 433–440.
Introduction
Gemini surfactants have been attracting extensive interest
due to their unique self-assembly behavior both at inter-
faces and in solutions (Zana, 2002). Much attention has
been devoted to studies of their synthesis methods, hierar-
chal structures, and applications in various fields, including
sensors (Hong et al., 2013), drug vectors (Giuliani et al.,
2015), gene delivery (Fisicaro et al., 2014), DNA transfec-
tion (Zhou et al., 2013), membranes (Nieves et al., 2015),
and antibacterial agents (Zhang et al., 2015). So far, differ-
ent Gemini surfactants, including quaternary ammonium,
anionic, nonionic, and zwitterionic, have been designed
and synthesized. Furthermore, the solution behavior and
properties of Gemini surfactants have been well illustrated.
However, few fluorescent-Gemini surfactants have been
reported, and accordingly related phenomena are not yet
well understood (Nieves et al., 2015; Takahashi, Fukuyasu,
Horiuchi, Kondo, & Stroeve, 2013; Zhang et al., 2015).
Fluorescent blocks are functional emissive units and
have found applications in solar cells (Cheng, Yang, &
Hsu, 2009), organic light-emitting diodes (Grimsdale,
Leok, Martin, Jokisz, & Holmes, 2009), and probes
(Chen & Yin, 2014; Zhu, Liu, Yang, Lv, & Wang, 2012).
By introducing fluorescent blocks into surfactants, it is pos-
sible to investigate the aggregation behavior of surfactants
using spectral analysis. Compared with traditional methods,
such as surface tensiometry (Zhang et al., 2014),
Electronic supplementary material The online version of this article
(doi:10.1002/jsde.12036) contains supplementary material, which is
available to authorized users.
* Weixing Deng
freewxdeng@163.com; wxdeng@glut.edu.cn
1
Guangxi Ministry-Province Jointly-Constructed Cultivation Base
for State Key Laboratory of Processing for Non-Ferrous Metal
and Featured Materials, Guilin University of Technology, Guilin,
541004, China
2
Key Laboratory for Organic Electronics & Information Displays,
Institute of Advanced Materials, Nanjing University of Posts &
Telecommunications, Nanjing, 210046, China
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conductimetry (Łudzik, Kustrzepa, & Piekarski, 2014;
fluorescent-Gemini surfactants with a conjugated unit as a
spacer have not been used in cell-membrane imaging.
In this work, we first present a fluorescent-Gemini
surfactant with aggregation-enhanced emission to label
live-cell membranes. This quaternary-ammonium probe
(8-TBT-8) comprises a thiophene–benzene–thiophene chro-
mophore and two alkane tails. 8-TBT-8 favors the forma-
tion of H aggregates in an organic solvent to give blue
emission, whereas it forms J aggregates in an aqueous
medium to give green fluorescence under UV light. Spec-
troscopy measurements reveal that the fluorescent surfac-
tant 8-TBT-8 aggregates in micelles at 1 × 10−6 mol L−1
and these micelles show high photostability, which is suit-
able for membrane imaging.
ꢀ
Łudzik, Kustrzepa, Piekarski, & Józwiak, 2016), rheology
(Lu, Huang, Li, Jia, & Fu, 2008), isothermal-titration calo-
rimetry (Li, Wettig, & Verrall, 2004), and dynamic-light
scattering (Cuenca, Falcone, Silber, & Correa, 2016), spec-
troscopy shows advantages. In fact, pyrene has been used to
study micellar systems for a long time due to its sensitivity
to microenvironments (Kalyanasundaram & Thomas, 1977).
Based on the spectral properties of pyrene, Ge et al. (2012)
studied the assembly of anionic-Gemini surfactants in the
presence of bovine serum albumin. Wang, Wettig, Foldvari,
and Verrall (2007) introduced a 1-pyrenehexyl unit as one of
the hydrophobic tails to yield a pyrene-substituted Gemini
surfactant. Keyes-Baig, Duhamel, and Wettig (2011) used
time-resolved fluorescence to characterize the behavior of
pyrene-substituted Gemini surfactants in solution. Though
plausible, the large volume of the pyrene does affect the
assembly behavior of these surfactants. In fact, except hydro-
phobic tails, spacer linkers in Gemini surfactants that can be
designed as fluorescent units also have many varieties. Con-
jugated units including fluorene, thiophene, carbazole, and
pyrrole also deserve to be exploited.
Experimental Procedures
Materials
2,5-Dibromohydroquinone,
2-chloroethyldiethylamine,
2-(tributylstannyl)thiophene, octanoyl chloride, bromo-
methane, and stannic chloride were purchased from Sigma–
Aldrich (Louis, USA) and used without further purification.
Tetrahydrofuran (THF) and CCl4 were dried over sodium
benzophenone ketyl and distilled under a dry nitrogen
atmosphere immediately prior to use. Dulbecco’s modified
Eagle’s medium (DMEM, Gibco, USA) was purchased
from Gene Tech Co. (Shanghai, China). Spectrochemical-
grade solvents were used for optical measurements.
It is of great importance for cell-membrane imaging to
analyze the cell morphology, and the structure of cellular
membranes often folds into various shapes with different
morphologies to perform
a
variety of functions
(McMahon & Gallop, 2005; Shibata, Hu, Kozlov, & Rapo-
port, 2009). Fluorescent probes have been used to monitor
biological processes both in membrane models and in cellu-
lar experiments with high-temporal and spatial resolution
(Shim et al., 2012). Cell-membrane probes are supposed to
mimic the structure and behavior of natural lipids. Nieves
et al. (2015) developed three ceramide analogs containing
five conjugated double bonds as membrane probes to study
sphingolipids in membranes. The stability of fluorophores
in vivo is another important consideration since most tradi-
tional fluorophores suffer due to photobleaching. For high-
bright photoluminescence (PL) and photostability, Zhang
et al. (2012) prepared carboxylic-acid-modified semiconduct-
ing polymer nanoparticles using the coprecipitation method
to label human breast cancer cells. Li et al. (2014) developed
conjugated-polymer nanoparticles functionalized with plerix-
afor using a miniemulsion method for cell-membrane imag-
General Method
Nuclear magnetic resonance (NMR) spectra were recorded
on a Bruker 400 MHz spectrometer (Billerica, MA, USA) in
CDCl3 or dimethyl sulfoxide-d6 with tetramethylsilane as the
internal standard. Absorption spectra were measured usinꢀg a
Shimadzu UV-3150 spectrometer (Kyoto, Japan) at 25 C,
and PL spectra were recorded on a Shimadzu RF-530XPC
luminescence spectrometer upon excitation at the absorption
maxima. Dynamic light scattering (DLS) measurements were
carried out using a Brookhaven Instruments Corporation
90 Plus instrument (NY, USA) with λ = 532 nm and scatter-
ing angles are 90ꢀ. Hydrodynamic diameters were calculated
using the nonnegative least-squares method. Zeta potentials
were measured using a zeta potential analyzer (Zeta PALS,
Brookhaven Instruments Corp). Fluorescence images were
obtained with a confocal laser scanning microscope (TCS
SP5, Leica, Mannheim, Germany). The photostability of
fluorescent-Gemini surfactant micelles was investigated
using an Edinburgh FLSP920 fluorescence spectrophotome-
ter (Edinburgh, UK).
ing. Gemini surfactants have
a similar structure to
phospholipids making them suitable for cell imaging.
Because the emission spectra are sensitive to the aggregation
of these Gemini-surfactant molecules, fluorescent-Gemini
surfactants promise access to the imaging of not only the lat-
eral organization of biomembranes but also membrane
potentials, poorly detectable membrane tension, and so
on. However, these functions are beyond the reach of tradi-
tional dyes for imaging methods. To our knowledge,
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Cell Imaging
Synthesis of 1,4-Bis(thiophen-2-yl)-2,5-bis
(diethylaminoethoxy) Benzene (2)
The cells were first cultured in confocal microscopy dishes
at an intensity of 4 × 104 cm−2 in a complete DMEM
medium and cultured for 24 h at 37 C. Then, the cells
were washed with phosphate buffer saline (PBS) and trea-
ted with a medium containꢀing 8-TBT-8 (10−6 M). After
incubation for 10 min at 37 C, the cells were rinsed twice
with PBS and the cellular uptake of 8-TBT-8 was visual-
ized using a confocal laser scanning microscope.
The target compound 2 was obtained via Stille reactions
between 1 and 2-(tributylstannyl)thiophene in the presence
of a catalytic amount of Pd(PPh3)2Cl2. In an oven-dried
ꢀ
round-bottom
flask,
1,4-dibromo-2,5-bis(diethylami-
noethoxy) benzene (2.23 g, 4.5 mmol) and 2-(tributylstan-
nyl)thiophene (8 mL, 25 mmol) were dissolved in 40 mL
anhydrous 1,4-dione. Bis(triphenylphosphine)palladium
dichloride (0.31 g, 0.44 mmol) was added quickly.ꢀ After
degassing for 30 min, the mixture was stirred at 90 C for
48 h under a nitrogen atmosphere. The solvent was evapo-
rated under vacuum. The residue was dissolved in a mix-
ture of DCM (200 mL), and the resulting solution was
washed with statured Potassium fluoride (KF) solution
(100 mL), water, and brine, respectively, and dried over
anhydrous MgSO4. The crude product was purified by col-
umn chromatography (silica gel, EtOAc/petroleum ether
with 1% triethylamine, 1:10). Yield: 72%. 1H NMR
(400 MHz, CDCl3): δ 7.56 (2H), 7.34 (2H), 7.29 (2H),
7.11 (2H), 4.16 (4H), 2.99 (4H), 2.64 (4H), 1.08 (12H)
(Fig. S3). MALDI-TOF-MS (m/z) calcd for C26H36N2O2S2
[M]+: 472.22, found: 471.35 (Fig. S4).
Synthesis of 1,4-Dibromo-2,5-bis(diethylaminoethoxy)
Benzene (1)
A mixture of 2,5-dibromohydroquine (27 g, 0.1 mol), 2-
diethylaminoethyl chloride hydrochloride (41 g, 0.3 mol),
potassium carbonate (82 g, 0.6 mol), and anhydrous acetone
ꢀ
300 mL was stirred at 65 C for 10 h. Then, it was cooled to
room temperature and water was added. The product was
extracted with dichloromethane (DCM). The combined
organic layers were washed with aqueous solution of
NaHCO3 and dried over MgSO4. In the next step, the solvent
was evaporated under reduced pressure. The resulting crude
products were purified by column chromatography (silica
gel, EtOAc/petroleum ether with 1% triethylamine, 1:10).
Yield: 86%. 1H NMR (400 MHz, CDCl3): δ 7.12 (2H), 4.02
(4H), 2.91 (4H), 2.64 (4H), 1.07 (12H) (Fig. S1). Matrix
assisted laser desorption/Ionization time of flight mass spec-
trometry (MALDI-TOF-MS) (m/z) calcd for C18H30Br2N2O2
[M]+: 466.07, found: 465.46 (Fig. S2).
Synthesis of P and 8-TBT-8
As shown in Scheme 1, compound P was prepared via the
Friedel–Crafts reaction, SnCl4 (1 mL, 8 mmol) was first
added to a cold solution containing 1,4-bis(thiophen-2-yl)-
a
(i)
(ii)
and
Scheme 1 Synthesis route to fluorescent-Gemini surfactants
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2,5-bis(diethylaminoethoxy) benzene (2.36 g, 5 mmol),
and then octanoyl chloride (3.4 mL, 20 mmol) in 8 mL
CCl4 was added with vigorous stirring. The reaction mix-
ture was stirred for 1 h in an ice bath and allowed for addi-
tional 2 h reaction at room temperature. Then, 3 mL 2 M
HCl was added slowly. The product was extracted with
DCM. The organic layer was washed with aqueous
NaHCO3 and water then dried with MgSO4. The solvent
was removed with a vacuum evaporator and the product
was purified by chromatography (silica gel, EtOAc/petro-
leum ether with 1% triethylamine, 1:10). Yield: 40%.
Fluorescent-Gemini surfactant (8-TBT-8) was synthesized
give 1,4-dibromo-2,5-bis(diethylaminoethoxy) benzene (1)
in 86% yield. 1,4-Dibromo-2,5-bis(diethylaminoethoxy)
benzene was converted to 1,4-bis(thiophen-2-yl)-2,5-bis
(diethylaminoethoxy) benzene (2) by the Stille coupling
reaction with 2-(tributylstannyl)thiophene for 48 h in the
presence of a catalytic amount of Pd(PPh3)2Cl2 with the
yield of 72%. Subsequently, compound P was prepared via
the Friedel–Crafts reaction with octanoyl chloride in the
yield of 40%. Finally, the cationic fluorescent-Gemini sur-
factant (8-TBT-8) was obtained by further treating com-
pound P with bromomethane in THF with 85% yield.
1
All the compounds were confirmed using H NMR. As
ꢀ
by reacting P with bromomethane in THF at 0 C for 4 h.
compared to compound P, two new peaks at 3.81 and
1
Water was then added and kept stirring for 12 h at room
temperature. 8-TBT-8 was obtained as a yellow powder by
3.06 ppm appear in the H NMR of 8-TBT-8, which were
assigned to N+ CH2 and N+ CH2 CH3, respectively,
1
freeze-drying. Detailed H NMR spectra of P and 8-TBT-8
while the peak at 3.01 ppm assigned to OCH2 CH2 N
are shown in Fig. 1.
disappears; we think that it is overlapped by the trace H2O
signal in d-DMSO. Due to the amphiphilicity of the Gemini
surfactant, it is difficult to obtain a clear 1H NMR spectrum
in other deuterated solvents.
Results and Discussion
Synthesis and Molecular Characterization
Optical Properties
The target compound was prepared starting from 2,5-dibromo-
hydroquinone, which reacts with 2-chloroethyldiethylamine to
Figure 2 shows the UV–vis spectra and PL spectra of 8-
TBT-8 (10−5 mol L−1 in units of rod block) in DCM and
Fig. 1 1H NMR spectra of compound P in CDCl3 (up) and 8-TBT-8 in DMSO-d6 (down)
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Fig. 2 UV–vis spectra (a) and PL spectra (b) of 8-TBT-8 in different solvents. The inset shows fluorescent colors of 8-TBT-8 in DCM and aque-
ous solution
aqueous solution. 8-TBT-8 has maximum absorption peaks
at 391 nm in DCM and 402 nm in aqueous solution, which
are attributed to π–π* transition intramolecular charge-
transfer between benzene rings and thiophene rings. In
addition, 8-TBT-8 shows a shoulder absorption peak at
334 nm in aqueous solution, which is assigned to the effect
of carbonyl groups in the 8-TBT-8 unit. A similar
bathochromic shift takes place in the emission spectra of
the Gemini surfactant (8-TBT-8) and reveals solvent-
dependent fluorescence emission. 8-TBT-8 shows two
emission peaks at 445 and 459 nm in DCM and a single
band at 505 nm in aqueous solution; this solvent-dependent
change can be distinguished by the naked eye under UV
light (Fig. 2, inset). It is obvious that the optical properties
of 8-TBT-8 in DCM are different from the optical proper-
ties of compound P in DCM (Figs. S5 and S6). Compounds
P and 8-TBT-8 share the same conjugated block but show
different optical properties in the same solvent indicating
that the conjugated blocks take different aggregation. The
optical properties illustrate the aggregation behavior of 8-
TBT-8 in DCM and aqueous solution (Figs. S7–S11 and
5a). In aqueous solution, the variation of the aggregation of
8-TBT-8 molecules was further improved due to the amphi-
philicity of 8-TBT-8.
Compared with the emission spectra of 8-TBT-8 in
DCM, the blue-shifted emission in aqueous solution is
attributed to the assembly of a H-aggregate structure, in
which dyes tend to form a head-to-tail or head-to-head
aggregates of the rod units (Fig. 3a). On the other hand, the
J-aggregated structure is attributed to parallel packing of
rod units exhibiting a red-shift in the emission spectra
(Fig. 3b). The long rigid spacer and two alkyl tails make 8-
TBT-8 soluble in an organic solvent and these rigid spacers
favor H-type aggregates through π–π stacking interactions.
In DCM solution, the rod unit has good solubility and
undergoes head-to-head aggregation. However, in aqueous
solution, alkane chains with charges have better solubility
than rod units and rod units form J-type aggregates through
the hydrophobic interactions and electrostatic repulsion.
This red-shifted emission of 8-TBT-8 in aqueous solution
is beneficial for cell imaging.
The self-assembly behavior of 8-TBT-8 in aqueous solu-
tion was investigated by DLS. Figure 4 shows the hydrody-
namic radius distribution of 8-TBT-8 in aqueous solution
with the mean diameter of 182 nm at a concentration of
10−5 mol L−1. Zeta potential of the 8-TBT-8 Gemini sur-
factant is 45 mV in aqueous solution. The particle size of
8-TBT-8 molecules in aqueous solution far outweighs the
size of a single molecule of 8-TBT-8 indicating the aggre-
gation of 8-TBT-8 molecules in aqueous solution. The high
value of zeta potential enables a highly positive charge den-
sity on the surface of aggregated particles.
To study the aggregation behavior of the Gemini surfac-
tant (8-TBT-8) in aqueous solution, the critical micelle con-
centration (CMC) was determined by the fluorescence
intensity change versus the 8-TBT-8 concentration curve.
Due to the hydrophobicity of the thiophene–benzene–
thiophene unit, 8-TBT-8 yields very weak emission signals
when the concentration is lower than the CMC. However,
the fluorescence intensity increases significantly when 8-
TBT-8 aggregates into micelles, which provide a relatively
hydrophobic microenvironment for the thiophene–ben-
zene–thiophene units (Fig. S11). The CMC of the Gemini
surfactant (8-TBT-8) is 1 × 10−6 mol L−1 determined from
the breakpoint of the fluorescence intensity as shown in
Fig. 5. Such a low CMC value is mainly because the dimer
structure of 8-TBT-8 has two hydrophobic alkyl chains
rather than one. Figure 5 also shows the photostability of
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Fig. 3 H aggregates and J aggregates of 8-TBT-8 in DCM and aqueous solution
8-TBT-8 micelles both in DCM and in aqueous solution.
The PL intensity remains almost unchanged after 10 min of
continuous UV irradiation. These results reveal that 8-
TBT-8 micelles are highly resistant to photobleaching and
exhibit excellent photostability.
5-diphenyltetrazolium bromide (MTT) cell viability assay
against NIH-3T3 and it shows low cytotoxicity at low
concentrations (Fig. S12). The cell imaging assay of 8-
TBT-8 was performed for HeLa and HepG2 cells
(Fig. 6). As expected, confocal laser scanning microscopy
imaging results show that 8-TBT-8 is located on the
membrane after 10 min incubation, which was verified by
colocalization with a cell-membrane dye Dil. These cells
show green channel and red channel fluorescence, respec-
tively. The merged images indicate that the staining of 8-
TBT-8 fits well with the cell-membrane dye Dil. The inten-
sity channel also varies in close synchrony. Moreover, the
intensity scatter plot of the green channel and the red
Cell Viability and Cell Uptake
Gemini surfactants have a similar structure to phospho-
lipids with two hydrophobic alkane tails and hydrophilic
cationic head groups, so they may aggregate on cell
membranes just like phospholipids. Cytotoxicity of 8-
TBT-8 was evaluated in a 3-(4, 5-dimethylthiazol-2-yl)-2,
Fig. 4 Hydrodynamic size distribution (a) and zeta potential (b) of 8-TBT-8 in water
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Fig. 5 Plot of fluorescence intensity for 8-TBT-8 (λex = 402 nm) versus log[8-TBT-8] (a) and luminescence time traces of 8-TBT-8 under UV
light illumination for 10 min (b), indicating the high photostability of 8-TBT-8 in both DCM and aqueous solution
Fig. 6 Confocal luminescence images and intensity profiles across cells costained by 8-TBT-8 and Dil. Scale bar represents 30 μm
channel is highly correlated with a high-overlap Pearson’s
coefficient (0.86 for HeLa cells and 0.92 for HepG2 cells).
The results further confirm that fluorescent-Gemini surfac-
tant (8-TBT-8) mainly locates in the cell membrane.
J aggregates to give a green fluorescence upon UV irradiation.
Spectrometry showed that the fluorescent surfactant 8-TBT-8
aggregates into micelles at 1 × 10−6 mol L−1, the micelles
formed have high photostability, which is suitable for mem-
brane imaging. Water soluble 8-TBT-8 showed low cytotox-
icity at low concentrations and located on cell membranes
effectively.
Conclusions
A fluorescent-Gemini surfactant (8-TBT-8) with a thiophene–
benzene–thiophene-conjugated spacer was synthesized and
its spectral properties were investigated. It was found that 8-
TBT-8 forms H aggregates to give a blue emission and forms
Acknowledgements This work was financially supported by the
National Natural Science Foundation of China (No. 51363004), Natu-
ral Science Foundation of Guangxi (2014GXNSFBA118244), Guangxi
Universities Science Research Project (YB2014154), and Key
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Laboratory of New Processing Technology for Nonferrous Metals and
Materials (13KF-2).
(dimethyloctylammoniumbromide)
(dimethyloctylammonium bromide) by electric conductance mea-
and
dodecylene-1,12-bis
surements. Journal of Chemical
4165–4172.
& Engineering Data, 59:
ꢀ
Łudzik, K., Kustrzepa, K., Piekarski, H., & Józwiak, M. (2016)
Application of conductance study to analyze micellization behavior
of cationic Gemini surfactants in water and water–ethanol solvent
mixed media. Journal of Chemical & Engineering Data, 61:
1047–1053.
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