Redox-responsive thermal sensitivity based on a selenium-containing small molecule

Article abstract of DOI:10.1039/c3cc49753d

Thermoresponsive spherical assemblies formed by a selenium-containing small molecule exhibit lower critical solution temperature (LCST) behavior in aqueous solution, which can be reversibly switched by mild redox treatment. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc49753d

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Redox-responsive thermal sensitivity based
on a selenium-containing small molecule†
Cite this: Chem. Commun., 2014,
50, 2585
Yan Ding, Yu Yi, Huaping Xu, Zhiqiang Wang and Xi Zhang*
Received 24th December 2013,
Accepted 16th January 2014
DOI: 10.1039/c3cc49753d
www.rsc.org/chemcomm
Thermoresponsive spherical assemblies formed by a selenium- of thermoresponsive polymers with redox-responsive LCST
containing small molecule exhibit lower critical solution temperature behavior have been reported.5 However, the only ones that could
(LCST) behavior in aqueous solution, which can be reversibly switched be used in a physiological environment are still limited to disulfide
by mild redox treatment.
containing polymers. In addition, reversibility remains a problem in
those systems. Thermosensitive materials with sensitive and reversible
Thermoresponsive polymers that exhibit LCST behavior are of responsiveness to redox triggers are highly desirable.
great interest for the fabrication of thermosensitive materials, We have demonstrated that selenium-containing copolymers are
which may find application in sensing, drug delivery, tissue promising biomedical materials with unique redox properties.6
engineering and separation.1 Compared to the extensive study on Compared to sulfide compounds, selenium-containing compounds
thermoresponsive polymers, thermal sensitivity based on small are more sensitive to redox stimuli and can respond to mild redox
molecules has gained attention only recently.2 Among the few treatment.7 What’s more, for selenide-containing molecules, the
small-molecule systems reported, especially interesting are those oxidation of the selenide group to selenoxide can be fully reversed.8
where molecules are non-covalently bound together through self-
Herein, we report a selenide-containing small molecule SeG
assembly to give thermoresponsive assemblies that display similar (Scheme 1), which upon self-assembly forms spherical assemblies
LCST properties to thermoresponsive polymers.2a,3 The process can exhibiting LCST behavior in aqueous solution. As mentioned above,
be considered as ‘‘non-covalent synthesis’’ of thermoresponsive the selenide group can be oxidized to selenoxide under the
aggregates, providing a complementary route to realize thermal conditions of 0.1% v/v hydrogen peroxide. The oxidation can
sensitivity. The supramolecular strategy can save the labor of drastically increase the hydrophilicity of the SeG molecule,
polymerization. Moreover, it also takes the advantage of the well- leading to the switch of the LCST properties accompanied by the
defined composition of small molecules over polydispersed disassembly of the spherical assemblies. Moreover, the oxidized
polymers, which will facilitate the application of the thermo- SeG molecule can be fully reduced by ascorbic acid, which restores
responsive assemblies in the field of biomedicine.
thermoresponsiveness of the system. The spherical assemblies
For practical purpose, it is often required to tune the LCST of
a thermoresponsive system. Different external stimuli have been
applied to alter the hydrophilic–hydrophobic balance of thermo-
responsive polymers and thus adjust their LCST, including pH,
light, ions, etc.4 In contrast, control on the thermal properties of a
small-molecule system has been rarely addressed.2d,3b Among
various external stimuli, redox is particularly intriguing because
the local redox potential of inflammatory or tumor cells is different
from that of normal cells, which means that the change in LCST
may be selectively triggered within specific cells. A few examples
Key Lab of Organic Optoelectronics and Molecular Engineering,
Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China.
E-mail: xi@mail.tsinghua.edu.cn; Fax: +86-10-6277-1149; Tel: +86-10-6279-6283
† Electronic supplementary information (ESI) available: Materials and instru-
ments. See DOI: 10.1039/c3cc49753d
Scheme 1 Redox-responsive thermal sensitivity of the spherical assem-
blies formed by SeG.
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transparent again. The reversibility is very good for a supramole-
cular system with almost no shift of the transition temperature.
DLS was also employed to follow the size of the aggregates in the
solution of SeG. As shown in Fig. 2b, no big change in the hydro-
dynamic diameter of the aggregates can be observed at low tempera-
tures while a rapid increase occurs as the temperature goes above the
LCST, which is consistent with the transmittance variation.
The thermal sensitivity of the spherical assemblies formed by
SeG results from the reversible disruption of the hydrogen bonds
between the OEG groups and water molecules, which resembles the
mechanism for the LCST transition of thermoresponsive polymers
such as poly(oligo(ethylene glycol) methacrylates) (POEGMA).11
Below the LCST, hydrogen bonds are formed between the oxygen
of the OEG groups on the spherical assemblies and the hydrogen
of H2O molecules, which facilitates the solubilization of the
spherical assemblies. However, when the temperature goes
beyond the LCST, the hydrogen bonds break. Thus the OEG groups
on the surface of the spherical assemblies get dehydrated and
collapse, leading to the aggregation and phase separation of the
spherical assemblies. It should be noted that upon aggregation
the small spherical assemblies will also merge with each other
to form large ill-defined aggregates.
Taking advantage of the redox chemistry of selenium, the
LCST properties of the SeG assemblies can be reversibly switched
by mild redox stimuli. Herein, diluted hydrogen peroxide was
adopted as the oxidant. A certain amount of H2O2 was added into
an aqueous solution of SeG so that the final concentration of
H2O2 was 0.1% v/v. After oxidation, the solution of SeG displays
no change in transmittance in the whole range of 25 to 75 1C
(Fig. 2a). Thus the LCST transition has been totally switched
off. Meanwhile, no aggregates can be observed by TEM, which
indicates the disassembly of the spherical assemblies upon
oxidation. In reverse, it is feasible to restore the thermorespon-
siveness of the system using reductants such as ascorbic acid.
28.2 mg ascorbic acid was added to a 2 mL solution of SeG that
had been treated with H2O2, and the resultant solution was allowed
to stand for 1 hour at room temperature before it was used for
transmittance measurement. As shown in Fig. 2a, the reduced
sample exhibits similar thermal behavior to the original solution of
SeG, although the LCST has shifted to 41.4 1C. Thus, reversible
control on the thermal behavior of the spherical assemblies of SeG
has been achieved. In addition, the shift of the LCST may result
from the existence of the oxidized ascorbic acid and the surplus
ascorbic acid in the solution.
Fig. 1 (a) TEM image of the spherical assemblies formed by SeG at room
temperature. (b) Size distribution of the spherical assemblies of SeG by DLS
measurement at 25 1C.
formed by SeG may find application in temperature-mediated
cell targeting9 and redox-controlled drug delivery.
SeG was synthesized according to our previous work.10 The SeG
molecule is amphiphilic, bearing a hydrophobic core of tris(selanyl
methyl) benzene and a hydrophilic periphery of oligo(ethylene
glycol) (OEG). As shown in the TEM image (Fig. 1a), SeG can self-
assemble into spherical assemblies in aqueous solution at room
temperature. The spherical assemblies are not uniform in size. The
diameter can vary in the range of 50 to 150 nm. Based on the TEM
image, the average diameter of the assemblies is estimated to be
110 nm. DLS measurements give a mean diameter of 95 nm at
25 1C (Fig. 1b), which is close to the result of the TEM observation.
The critical aggregation concentration (CAC) of SeG at 20 1C was
measured to be 1.2 mM by fluorescence with pyrene as the probe of
polarity.10 The concentration of SeG in all the solutions used in this
work was fixed at 2.0 mM unless otherwise mentioned.
The spherical assemblies of SeG exhibit LCST behavior in
aqueous solution. The transmittance at 700 nm of the solution of
SeG at varied temperatures was measured using UV-vis spectro-
scopy. As shown in Fig. 2a, upon heating, the transmittance starts
to drop gradually at 30 1C, followed by an abrupt decrease above
40 1C. Meanwhile, the initially clear solution becomes a turbid
suspension. The LCST of the system is determined to be 38.0 1C,
which is defined as the temperature at which the transmittance
drops to 90% of the initial value. Reversely, when the temperature
is lowered, the transmittance increases nearly following the same
path with the heating process and the turbid suspension turns
The thermal properties of the spherical assemblies of SeG are
switched due to the disturbance of the hydrophilic–hydrophobic
balance of the SeG molecule. When exposed to H2O2, the
selenide group in the SeG molecule is oxidized to selenoxide,
1
which can be confirmed by H-NMR and ESI-MS. As shown in
Fig. 3a and b, after addition of H2O2, the signals of the protons
near the selenium atom (Ha,b,c) shift downfield, indicating the
oxidation of the selenium atom. In particular, the peak for Hb
splits into two doublets, which is characteristic of the formation of
the selenoxide group in PhCH2SeQO. The split occurs because
the addition of one oxygen atom onto the selenium atom hinders
the free rotation of the Ph–C and C–Se bonds so that the two
Fig. 2 (a) Temperature dependence of the transmittance of the solution
of SeG (black for heating, red for cooling), of the solution treated with
0.1% v/v H2O2 (purple) and of the solution further reduced with ascorbic
acid after the H2O2 treatment. Experiments were conducted at a heating–
cooling rate of 1 1C minÀ1. (b) Size variation of the aggregates in the
solution of SeG with temperature.
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Fig. 4 Transmittance variation of the SeG system with alternate addition
of H2O2 and ascorbic acid at 60 1C. Into a turbid suspension of SeG at x =
0, H2O2 was added at each odd number on the x axis while ascorbic acid
was added at each even number.
The turbid–clear–turbid cycle can be repeated more than 5 times.
In addition, the transitions proceed fast. It takes less than 5 min for
the transmittance of the system to reach equilibrium upon each
addition of the oxidant or the reductant. These features indicate the
thermal stability of the SeG molecule, and also demonstrate the
great reversibility of the oxidation of selenide to selenoxide, which
is a big advantage of selenide-containing compounds over disulfide
compounds. Therefore, in addition to the thermally induced LCST
transition, isothermal phase transition triggered by redox stimuli
has been achieved with the solution of SeG.
In summary, thermoresponsive spherical assemblies have
been prepared by the self-assembly of a selenium-containing small
molecule. Making use of the sensitivity of selenium to mild oxida-
tion, the LCST properties have been successfully switched by H2O2
as diluted as 0.1% v/v. Furthermore, the good reversibility of
the oxidation of the selenide group allows the system to regain
its thermoresponsiveness upon reduction by ascorbic acid. The
spherical assemblies of SeG may be promising carriers for thermally
targeted drug delivery because the hydrophilic-to-hydrophobic
transition above the LCST can enhance the cellular uptake of the
assemblies while the oxidation-induced disassembly enables the
intracellular release of the drug molecules.
Fig. 3 1H-NMR for SeG before oxidation (a), after oxidation with 0.1% v/v
H2O2 (b), and after further reduction with ascorbic acid (c). All the samples
were prepared with D2O as the solvent.
protons of PhCH2SeQO are no longer chemically equivalent,
leading to the geminal 2J coupling of the two protons. Similarly,
2
the peak for Hc also splits due to the geminal J coupling. ESI-MS
results show that after oxidation the molecular ion peak for SeG
([M + Na+]) at m/z 819.6 disappears while peaks for [M + 3O + H+]
and [M + 3O + Na+] arise at m/z 845.5 and m/z 867.4, respectively,
which also strongly supports the formation of the selenoxide group.
The oxidation of selenide to selenoxide greatly increases the hydro-
philicity of the SeG molecule, which makes the oxidized SeG totally
soluble in water as individual molecules, thus leading to the switch
of the LCST as well as the disassembly of the spherical assemblies.
1
Upon reduction, the peaks of the oxidized SeG in the H-NMR
spectrum (Fig. 3b) shift back to the initial chemical shifts of
SeG (Fig. 3c). What’s more, the area ratios between the different
peaks for the reduced molecule are unchanged compared to the
original SeG molecule, which indicates that all the selenoxide
groups have been reduced to selenide. In addition, ESI-MS gives
only the molecular ion peak for SeG at m/z 819.8. Therefore, the
oxidation of SeG can be fully reversed with ascorbic acid to
restore the LCST of the system.
As shown in Fig. 2, the difference in transmittance between
the on and off states of the thermoresponsiveness lies above the
LCST. If the temperature is fixed at a value higher than the LCST,
it is possible to reversibly trigger the phase transition of the
suspension of SeG with redox stimuli. To test this, a solution of
SeG was heated to 60 1C to give a turbid suspension, which was
then kept at the same temperature. Then H2O2 and ascorbic acid
were added alternately into the system and the transmittance of
the system was recorded. As shown in Fig. 4, the transmittance
goes close to 100% upon each addition of H2O2, which means the
turbid suspension becomes a transparent solution. On the other
hand, the resultant solution gets turbid again each time ascorbic
acid is added, indicating that the phase separation occurs again.
This work was financially supported by the National Basic
Research Program of China (2013CB834502), the Foundation
for Innovative Research Groups of NSFC (21121004) and the
NSFC-DFG joint grant (TRR 61).
Notes and references
1 (a) D. Roy, W. L. A. Brooks and B. S. Sumerlin, Chem. Soc. Rev., 2013,
42, 7214; (b) C. de Las Heras Alarcon, S. Pennadam and
C. Alexander, Chem. Soc. Rev., 2005, 34, 276; (c) S. Dai, P. Ravi and
K. C. Tam, Soft Matter, 2009, 5, 2513; (d) E. S. Gil and S. M. Hudson,
Prog. Polym. Sci., 2004, 29, 1173.
2 (a) J. E. Betancourt and J. M. Rivera, J. Am. Chem. Soc., 2009,
131, 16666; (b) S. Dong, B. Zheng, Y. Yao, C. Han, J. Yuan,
M. Antonietti and F. Huang, Adv. Mater., 2013, 25, 6864; (c) S. Lee,
J. S. Lee, C. H. Lee, Y. S. Jung and J. M. Kim, Langmuir, 2011,
27, 1560; (d) H. Hayashi, K. Ohkubo, S. Karasawa and N. Koga,
Langmuir, 2011, 27, 12709.
´
´
´
3 (a) L. M. Negron, Y. Melendez-Contes and J. M. Rivera, J. Am. Chem.
Soc., 2013, 135, 3815; (b) T. Hirose, M. Irie and K. Matsuda, Adv. Mater.,
2008, 20, 2137; (c) Y. Ding, Z. Wang and X. Zhang, Chem. Commun.,
2013, 49, 5580; (d) H. Chen, Y. Yang, Y. Wang and L. Wu, Chem.–Eur. J.,
2013, 19, 11051; (e) G. J. Richards, J. Labuta, J. P. Hill, T. Mori and
K. Ariga, J. Phys. Chem. Lett., 2010, 1, 1336; ( f ) K. Kono, E. Murakami,
Y. Hiranaka, E. Yuba, C. Kojima, A. Harada and K. Sakurai, Angew.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 2585--2588 | 2587

View Article Online
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ChemComm
Chem., Int. Ed., 2011, 50, 6332; (g) F. Wang, A. Klaikherd and
S. Thayumanavan, J. Am. Chem. Soc., 2011, 133, 13496.
6 H. Xu, W. Cao and X. Zhang, Acc. Chem. Res., 2013, 46, 1647.
7 (a) N. Ma, Y. Li, H. Xu, Z. Wang and X. Zhang, J. Am. Chem. Soc.,
2009, 132, 442; (b) W. Cao, X. Zhang, X. Miao, Z. Yang and H. Xu,
Angew. Chem., Int. Ed., 2013, 52, 6233.
8 H. Ren, Y. Wu, N. Ma, H. Xu and X. Zhang, Soft Matter, 2012, 8, 1460.
9 (a) S. R. Abulateefeh, S. G. Spain, K. J. Thurecht, J. W. Aylott, W. C. Chan,
M. C. Garnett and C. Alexander, Biomater. Sci., 2013, 1, 434; (b) S. Salmaso,
P. Caliceti, V. Amendola, M. Meneghetti, J. P. Magnusson, G. Pasparakis
and C. Alexander, J. Mater. Chem., 2009, 19, 1608.
`
4 (a) D. Han, X. Tong, O. Boissiere and Y. Zhao, ACS Macro Lett., 2011,
1, 57; (b) F. D. Jochum and P. Theato, Chem. Soc. Rev., 2012, 42, 7468;
(c) T. Ito, T. Hioki, T. Yamaguchi, T. Shinbo, S.-i. Nakao and
S. Kimura, J. Am. Chem. Soc., 2002, 124, 7840.
5 (a) K. Rahimian-Bajgiran, N. Chan, Q. Zhang, S. M. Noh, H.-i. Lee
and J. K. Oh, Chem. Commun., 2013, 49, 807; (b) M. J. Summers,
D. J. Phillips and M. I. Gibson, Chem. Commun., 2013, 49, 4223;
(c) D. J. Phillips and M. I. Gibson, Chem. Commun., 2012, 48, 1054; 10 Y. Yi, S. Fa, W. Cao, L. Zeng, M. Wang, H. Xu and X. Zhang, Chem.
(d) H. Fu, D. M. Policarpio, J. D. Batteas and D. E. Bergbreiter, Polym. Commun., 2012, 48, 7495.
Chem., 2010, 1, 631; (e) N. Kuramoto and Y. Shishido, Polymer, 1998, 11 (a) J.-F. Lutz, K. Weichenhan, O. Akdemir and A. Hoth, Macromole-
¨
39, 669.
cules, 2007, 40, 2503; (b) Y. Maeda, Langmuir, 2001, 17, 1737.
2588 | Chem. Commun., 2014, 50, 2585--2588
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