UV-responsive polymeric superamphiphile based on a complex of malachite green derivative and a double hydrophilic block copolymer

Article abstract of DOI:10.1021/la203486q

We have prepared a UV-responsive polymeric superamphiphile, formed by a malachite green derivative and the double hydrophilic block copolymer methoxy-poly(ethylene glycol)₁₁₄-block-poly(l-lysine hydrochloride)₂₀₀ (PEG-b-PLKC) on the

Full text of DOI:10.1021/la203486q

ARTICLE
pubs.acs.org/Langmuir
UV-Responsive Polymeric Superamphiphile Based on a Complex of
Malachite Green Derivative and a Double Hydrophilic Block Copolymer
Peng Han,† Sichao Li,† Chao Wang,† Huaping Xu,† Zhiqiang Wang,† Xi Zhang,*,† Joice Thomas,‡ and
Mario Smet‡
†Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084,
People's Republic of China
‡Molecular Design and Synthesis, Catholic University of Leuven, Celestijnenlaan 200F, Box 2404, B-3001 Leuven, Belgium
ABSTRACT: We have prepared a UV-responsive polymeric super-
amphiphile, formed by a malachite green derivative and the double
hydrophilic block copolymer methoxy-poly(ethylene glycol)114
-
block-poly(L-lysine hydrochloride)200 (PEG-b-PLKC) on the basis
of electrostatic interactions. The malachite green derivative under-
goes photo-ionization upon UV irradiation, which makes it more
hydrophilic, resulting in changes in the self-assembly behavior of the
polymeric superamphiphile. For this reason, the polymeric super-
amphiphile originally self-assembles to form sheetlike aggregates,
which disassemble after UV irradiation because of the increased
solubility of the malachite green derivative. By use of Nile red as a
probe, the polarity of the polymeric superamphiphile solution is
confirmed to be increased after UV irradiation by fluorescence
spectra, which also explains the disassembly of the polymeric
superamphiphile.
’ INTRODUCTION
form well-defined nanostructures like vesicles and micelles and can
disassemble upon a certain stimulus because of the increased
solubility or degradation of small molecules.
Amphiphiles are able to self-assemble to form well-defined
nanostructures in aqueous solution, which can be applied in
many fields such as template synthesis1 and drug delivery.2
Compared to conventional amphiphiles, superamphiphiles refer
to amphiphiles that are formed by noncovalent interactions
instead of covalent bonds.3 Different noncovalent interactions,
such as hydrogen bonding,4 electrostatic interaction,5 charge-
transfer interaction,6 hostÀguest recognition,7 and so on have
been employed to fabricate superamphiphiles with diversified
topologies like single chain, double chain, bolaform, gemini, and
rotaxane types. When a superamphiphile based on polymers is
fabricated, polymeric superamphiphiles can be obtained. For exam-
ple, double hydrophilic block copolymers with a charged block
can attract small molecules with opposite charge, thus forming
an amphiphilic complex that can be called a polymeric
superamphiphile.8 The noncovalent preparation of the polymeric
superamphiphile avoids complicated synthesis and the use of
organic solvents during the micellization when compared with
conventional amphiphilic block copolymers. Due to the different
functionalities of the employed building blocks, superamphiphiles
allow for tuning of their amphiphilicity by external stimuli, leading to
controlled self-assembly and disassembly. For polymeric super-
amphiphiles, different responsive small molecules can be introduced
to tune the amphiphilicity of the complex by external stimuli. In our
previous work, we have introduced light-responsive, oxidation-
Among all the stimuli, light is one of the most attractive for
rapid and clean control of solution properties with specific
direction and position. Malachite green, a dye with triphenyl-
methane structure, is a well-known photochromic molecule and
has received considerable attention in both fundamental and
applied research.10 Herein, we report the fabrication of a UV-
responsive polymeric superamphiphile formed by an anionic
malachite green derivative and a cationic block copolymer, as
shown in Scheme 1. The polymeric superamphiphiles self-
assemble to form sheetlike aggregates and disassemble after
UV irradiation because of the increased solubility of the mala-
chite green derivative. It is anticipated that this work will improve
and enrich our understanding of the self-assembly and disas-
sembly of superamphiphiles.
’ EXPERIMENTAL SECTION
Materials. The block copolymer methoxy-poly(ethylene glycol)114
-
block-poly(L-lysine hydrochloride)200 (PEG-b-PLKC) was purchased
from Alamanda Polymers. Nile red was purchased from SigmaÀAldrich.
3-Bromopropane-1-sulfonic acid was also obtained from SigmaÀAldrich.
Received: September 5, 2011
responsive, and enzyme-responsive molecules into polymeric
Revised:
Published: November 08, 2011
October 21, 2011
superamphiphiles.9 These superamphiphiles can self-assemble to
r
2011 American Chemical Society
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Synthesis of Malachite Green Derivative (MG, 2). Alcohol
6.74 (d, J = 8.1 Hz, 2H), 6.55 (d, J = 8.5 Hz, 4H), 3.91 (br s, 2H), 3.57 (br s,
2H), 3.48 (br s, 2H), 2.95 (br s, 2H), 2.84 (s, 12H), 1.97 (br s, 2H); 13C
NMR (75 MHz, CDCl3, 25 °C, TMS) δ= 158.4, 150.0, 134.0, 130.2, 130.0,
129.4, 121.8, 114.7, 112.4, 69.9, 69.0, 67.4, 55.6, 48.1, 40.6, 25.0; MS CI
[MH]+ calcd 538, found 538.
1
10d (100 mg, 0.24 mmol) and 3-bromopropane-1-sulfonic acid (97 mg,
0.48 mmol) were dissolved in dry N,N-dimethylformamide (DMF;
8 mL) under argon atmosphere. NaH (60 mg of a 60% dispersion in
mineral oil, 2.4 mmol) was added and the resulting suspension was
stirred for 14 h at 80 °C. After this, the mixture was evaporated to
dryness. The reaction mixture was quenched with ice water (20 mL) and
extracted with CH2Cl2 (5 Â 30 mL), dried over MgSO4, filtered, and
evaporated to dryness to afford the crude product . Purification by
column chromatography (silica, eluent CH2Cl2ÀMeOH 9:1) afforded 2
(88 mg, 68%) as a slightly greenish amorphous solid. 1H NMR (300 MHz,
CDCl3, 25 °C, TMS) δ = 7.05 (d, J = 8.3 Hz, 2H), 6.96 (d, J = 8.8 Hz, 4H),
1
Instruments. H NMR spectra were recorded on a JEOL JNM-
ECA600 spectrometer. UV/vis and fluorescence spectra were obtained
on Hitachi U-3010 and Hitachi F-7000 spectrophotometers, respec-
tively. UV light was provided by a high-pressure mercury lamp with an
optical fiber and intensity of 900 mW/cm2. Transmission electron
microscopy (TEM) was performed on a JEMO 2010 electron micro-
scope at an acceleration voltage of 110 kV. The samples for TEM
observation were prepared by dropping the aqueous solution on the
carbon-coated copper grid and staining with 0.2% phosphotungstic acid
hydrate solution. Dynamic light scattering (DLS) was measured by a
Nano ZS90 instrument, Malvern Instruments Ltd.
Scheme 1. Self-Assembly and Disassembly of the
UV-Responsive Polymeric Superamphiphile
Preparation of Polymeric Superamphiphile. First the MG
derivative was dissolved in water and neutralized with 1 equiv of sodium
hydroxide at a concentration of 5 Â 10À4 mol/L. PEG-b-PLKC was
dissolved in pure water at a concentration of 4 Â 10À6 mol/L, in which
the concentration of the ammonium cation is 8 Â 10À4 mol/L. Then
1.6 mL of the MG solution (the total amount of negative charge was 8 Â
10À7 mol) was slowly added into 1 mL of the PEG-b-PLKC solution
(the total amount of positive charge was 8 Â 10À7 mol) under stirring,
and the mixture was diluted with 2.4 mL of water for further experi-
ments. The charge ratio of anionic MG and cationic PEG-b-PLKC in the
polymeric superamphiphile (MGÀPEG-b-PLKC for short) was 1:1. pH
of the mixed solution is about 7, and the two aromatic tertiary amine
groups are only slightly protonated, which should have little effect on the
electrostatic interactions between the MG molecule and PEG-b-PLKC.
Fluorescence Measurements. Twenty microliters of a 5 Â 10À4
mol/L Nile red solution in acetone was added to 1 mL of the polymeric
superamphiphile solution described above or ro the polymeric super-
amphiphile solution irradiated with UV light for a certain time: 30, 60,
100, 300, or 900 s. Then the fluorescence spectra of the mixtures were
recorded with excitation at 500 nm. The polarity of the solution can be
estimated by the red shift and the quenching of fluorescence emission of
Nile red.
Scheme 2. Synthetic Route to MG Molecule 2
’ RESULTS AND DISCUSSION
Photo-ionization of MG. To fabricate the UV-responsive
polymeric superamphiphile, we first designed and synthesized
the compound MG bearing a malachite green group, a well-
known UV-sensitive chromophore. The chemical structure of
MG is shown in Scheme 2. It is well-known that, upon UV
irradiation, the malachite green group can exclude a CNÀ,
Figure 1. Photo-ionization of MG (a) without and (b) with PEG-b-PLKC. The concentration of MG in both solutions is 1.6 Â 10À5 mol/L.
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changing from a neutral hydrophobic moiety to a cationic
hydrophilic moiety. Photo-ionization of the MG molecule was
monitored by UV/vis spectrometry, as shown in Figure 1a. After
UV irradiation, the colorless aqueous solution turns green and
the absorbance at 606 and 453 nm increases quickly, while the
absorbance at 259 nm drops dramatically. All these results show
that the malachite green group undergoes an ionization process;
that is, a CNÀ is excluded from the molecule and a cationic state
is formed.10e The photo-ionization process is considered as a
first-order reaction, according to the formula ln [(A0 À Aeq)/(At À
further demonstrated by NMR. As shown in Figure 2A, the
protons Ha, Hb, Hc, and Hd on the aromatic rings of MG give very
clear and sharp peaks between 6.9 and 7.2 ppm. After UV
irradiation, the peaks of Hc and Hd move to lower field while
the resonances of Ha and Hb are approximately the same. For the
mixture of MG and PEG-b-PLKC, the protons on the aromatic
rings of MG were all screened (Figure 2C), but they appeared
after UV irradiation (Figure 2D). This is probably due to
incorporation of MG into the superamphiphile and aggregation
of the formed superamphiphile. The full incorporation resulted
in the absence of MG proton signals. The results also indicate
that the electrostatic interaction between MG and PEG-b-PLKC
is able to induce formation of the polymeric superamphiphile and
that the superamphiphile is disassembled after UV irradiation.
Self-Assembly of Polymeric Superamphiphile. We were
curious about the self-assembled structure of the MGÀPEG-b-
PLKC superamphiphile. The critical micelle concentration
(cmc) of MG is about 2 Â 10À4 M, determined by measuring
the conductivity as a function of increasing MG concentration.
To avoid its own aggregation, the concentration of MG in the
polymeric superamphiphiles is kept below its cmc. TEM is
employed to observe the aggregated structure. As shown in
Figure 3a,b, the polymeric superamphiphile self-assembles to
form sheetlike aggregates in aqueous solution. Moreover, the
sheetlike structure shows little change when the charge ratio of
anion and cation is altered. Interestingly, upon UV irradiation,
the sheetlike structure disappears, as shown in Figure 3c. The
self-assembled sheetlike structure of the polymeric superamphi-
phile may be a result of πÀπ interactions between the aromatic
rings on the MG molecules. Upon UV irradiation, the MG
molecules undergo an ionization process, switching from the
anionic to the zwitterionic state, which strongly increases the
solubility of the molecule and breaks the πÀπ interaction
between the aromatic rings due to charge repulsion, thus result-
ing in disassembly of the polymeric superamphiphile. It should
Aeq)] = kt, where the rate constant k is calculated as 0.078 sÀ1
.
We wondered whether the photo-ionization could happen in the
polymeric superamphiphile. UV/vis results for the polymeric
superamphiphile are shown in Figure 1b. As can be seen, the
malachite green group underwent the same photo-ionization
process, but it needed a much longer time to reach photo-
equilibrium. By the same formula mentioned above, the rate
constant of the ionization of MG in the polymeric superamphi-
phile is calculated as 0.004 sÀ1, which is much smaller than that of
MG itself. This is reasonable because, in the polymeric super-
amphiphile, the interaction between the block copolymer and
MG restricts the free movement of MG, and the block copolymer
may absorb some UV irradiation, leaving less energy for the
photo-ionization of MG.
Interaction between MG and PEG-b-PLKC. The interaction
between MG and PEG-b-PLKC in aqueous environment was
Figure 2. 1H NMR of MG (A) before UV and (B) after UV for 600 s,
and 1H NMR of MGÀPEG-b-PLKC (C) before UV and (D) after UV
for 600 s. The concentration of MG is 1.6 Â 10À4 mol/L in all solutions.
The charge ratio of MG to PEG-b-PLKC is 1:1.
Figure 4. Fluorescence spectra of Nile red in 1:1 MGÀPEG-b-PLKC
solution with different UV irradiation times.
Figure 3. TEM images of MGÀPEG-b-PLKC at charge ratios of (a) 1:2, (b) 1:1, and (c) 1:1 after UV for 300 s.
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be noted that PEG-b-PLKC is doubly hydrophilic and cannot
self-aggregate in the given concentration, as indicated by the DLS
results that the count rate of the PEG-b-PLKC solution is
approximately that of pure water.
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self-assembled structures in solution, the ionized MG has a strong
absorption at 633 nm, which is a commonly used laser wavelength in
DLS, making DLS not appropriate for monitoring the structure
change of the polymeric superamphiphile during UV irradiation.
Thus fluorescence spectroscopy was selected to monitor the
structural change of the polymeric superamphiphile upon UV
irradiation. Nile red was chosen as the probe. Normally, the
emission of Nile red is red-shifted and quenched when the
environment becomes more polar. From the results shown in
Figure 4, it can be seen that the fluorescence emission of Nile red
shifts to the red by about 7 nm after 300 s of UV irradiation and the
intensity drops with increasing irradiation time. These results clearly
show that the polarity of the solution increased after UV irradiation.
In aqueous solution, the hydrophobic Nile red is usually incorpo-
rated in the hydrophobic part of the aggregates. Thus the results
indicate an environment with higher polarity has been obtained after
UV irradiation, which confirms the UV-induced disassembly of the
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’ CONCLUSION
In summary, we have fabricated a UV-sensitive polymeric
superamphiphile consisting of an anionic malachite green deri-
vative and a cationic block copolymer on the basis of electrostatic
interaction. The polymeric superamphiphile self-assembles into
sheetlike aggregates in aqueous solution and disassembles under
UV irradiation due to the increased solubility of MG. It is
anticipated that a similar concept can be extended to fabricate
stimuli-responsive polymeric superamphiphiles for controlled
self-assembly and disassembly.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail xi@mail.tsinghua.edu.cn.
’ ACKNOWLEDGMENT
This work was financially supported by the NSFC (50973051,
20974059), NSFC-DFG Joint Grant (TRR61), the Foundation
for Innovative Research Groups of the National Natural Science
Foundation of China (21121004), Tsinghua University Initiative
Scientific Research Program (2009THZ02230), National Basic
Research Program (2007CB808000), and the Bilateral Grant
BIL 09/08 between Belgium and China.
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