Photo-reversible and selective Cu2+ complexation of a spiropyran-carrying sulfobetaine copolymer in saline solution

Article abstract of DOI:10.1039/b921169a

A sulfobetaine copolymer (1) carrying a photochromic spiropyran residue was synthesized, which reversibly isomerized between the closed and open forms in pure water and in saline by irradiation with visible light. The thermodynamic stability of the open form of the spiropyran methacrylate (SPMA) units of compound 1 was reduced upon increasing NaCl concentration. In addition, photo-reversible and selective Cu²⁺ complexation in saline solution ([NaCl] = 1.0 wt%) was achieved using compound 1 with metal ions (Cu ²⁺, Zn²⁺, Ni²⁺, or Co²⁺). Covalently cross-linked compound 1 showed selective Cu²⁺ adsorption in pure water. In contrast, the other metal ions were increasingly adsorbed as NaCl concentration increased, resulting in lower selectivity of Cu²⁺ ion adsorption with 1, e.g., the ratios of adsorption of Cu²⁺, Zn ²⁺, Ni²⁺, and Co²⁺ with 1 in 10 wt% saline were 73, 20, 10, and 3%, respectively, while only Cu²⁺ adsorption was observed in a solution of 1.0 wt% NaCl. Because the stability of the open form of the SPMA units and the metal complexation of 1 were influenced by NaCl concentration, the electrostatically cross-linked networks of the zwitterionic sulfobetaine units of 1 may be loosened by NaCl addition, resulting in easy entry of metal ions into the network. The order of metal complexation among the four metals corresponded to the Irving-Williams series. Hydrogen-bonded networks of water molecules also may contribute to the relatively ineffective selective adsorption of Cu²⁺ ions by 1 compared to the electrically neutral spiropyran-carrying copolymer of N-isopropylacrylamide, 2.

Full text of DOI:10.1039/b921169a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Photo-reversible and selective Cu2+ complexation of a spiropyran-carrying
sulfobetaine copolymer in saline solution
*
Takayuki Suzuki, Yuki Hirahara, Kaori Bunya and Hiraku Shinozaki
Received 20th October 2009, Accepted 7th January 2010
First published as an Advance Article on the web 12th February 2010
DOI: 10.1039/b921169a
A sulfobetaine copolymer (1) carrying a photochromic spiropyran residue was synthesized, which
reversibly isomerized between the closed and open forms in pure water and in saline by irradiation with
visible light. The thermodynamic stability of the open form of the spiropyran methacrylate (SPMA)
units of compound 1 was reduced upon increasing NaCl concentration. In addition, photo-reversible
and selective Cu2+ complexation in saline solution ([NaCl] ¼ 1.0 wt%) was achieved using compound 1
with metal ions (Cu2+, Zn2+, Ni2+, or Co2+). Covalently cross-linked compound 1 showed selective Cu2+
adsorption in pure water. In contrast, the other metal ions were increasingly adsorbed as NaCl
concentration increased, resulting in lower selectivity of Cu2+ ion adsorption with 1, e.g., the ratios of
adsorption of Cu2+, Zn2+, Ni2+, and Co2+ with 1 in 10 wt% saline were 73, 20, 10, and 3%, respectively,
while only Cu2+ adsorption was observed in a solution of 1.0 wt% NaCl. Because the stability of the
open form of the SPMA units and the metal complexation of 1 were influenced by NaCl concentration,
the electrostatically cross-linked networks of the zwitterionic sulfobetaine units of 1 may be loosened by
NaCl addition, resulting in easy entry of metal ions into the network. The order of metal complexation
among the four metals corresponded to the Irving–Williams series. Hydrogen-bonded networks of
water molecules also may contribute to the relatively ineffective selective adsorption of Cu2+ ions by 1
compared to the electrically neutral spiropyran-carrying copolymer of N-isopropylacrylamide, 2.
function in an aqueous environment with no organic solvent.
Introduction
Some approaches have been reported to overcome the former
problem. Winkler and coworkers detected target metal ions in
ethanol or benzene by analysis of the metal-complexed spi-
ropyran, based on intensity of fluorescent emission for metal
complexes with different metal species.4 Another well-known
approach is modification of spiropyrans with additional ligands
for metal chelation at the position ortho to the oxygen nucleus of
the pyran ring,5,6 or attachment of crown ethers7–10 to the
molecule to recognize metal size to some extent. Yang and
coworkers reported selective Cu2+ ion complexation in ethanol–
water by synthetic spiropyrans modified with a morpholino-
methyl group acting as the chelating functional group.11
Interest in the design and synthesis of new materials for metal ion
sensors, resulting in ‘‘one-time’’ use sensors, has been motivated
by non-reversible interactions between a metal ion and its sensing
receptor. An important feature for the next generation of metal
ion sensing will be reversibility in the recognition process because
it contributes to ‘‘real-time’’ metal ion monitoring. The materials
showing non-reversible metal adsorption cannot detect a low
concentration of the metal ions continuously in the situation
after adsorbing high quantities of metal ions. The attractive
features of photochromic spiropyrans have prompted a number
of studies of derivatives of these compounds and their metal-
binding characteristics.1,2
However, no accounts of spiropyrans showing specific binding
to a target metal ion in a purely aqueous environment have been
reported because of the difficulty of satisfying the requirements
for (a) and (b) with one molecule. The molecular structure of the
spiropyrans, especially modification of the pyran ring, is likely to
influence the isomerization characteristics of the closed and open
forms and their metal complexation characteristics.
Upon irradiation with UV or visible light, spiropyrans isom-
erize between open and closed forms, respectively.3 Metal ions
can influence this isomerization by association with the open
form through electron-rich oxygen atoms. In contrast, visible
light produces high concentrations of the closed form, and thus
hinders metal binding. Spiropyrans, therefore, have potential for
providing photo-reversible metal complexation.
The application of spiropyran-carrying copolymers has been
reported for use in aqueous solution.12,13 The spiropyran units in
the copolymers can perform limited functions, i.e., target metal
complexation, because water solubility of the copolymers is
achieved with the other co-monomer unit. Here, photo-reversible
and selective Cu2+ complexation of spiropyran-carrying sulfo-
betaine copolymer, 1 (Scheme 1), is described in water and in
saline solution without an organic solvent, where photo-revers-
ible and Cu2+ selective complexation occurs at the spiropyran
unit, and the copolymer exhibits high solubility in saline solution
However, some problems of spiropyrans remain as a material
for repeated-use metal ion sensing, i.e. ‘‘real-time’’ monitoring of
metal ions, for biological and environmental applications: (a)
high sensitivity to a target metal ion, and (b) photo-reversible
Department of Green & Sustainable Chemistry, Faculty of Engineering,
Tokyo Denki University, 2-2 Nishiki-cho Kanda, Chiyoda-ku, Tokyo,
Japan. E-mail: suzutaka@cck.dendai.ac.jp; Fax: +81-3-5280-3758; Tel:
+81-3-5280-3758
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Scheme 1 The chemical structure of 1.
Fig. 1 Absorption spectra of saline solutions ([NaCl] ¼ 1.0 wt%) of 1.
Solutions contained: no metal ions (black), or 1.0 mM Cu2+ (red), Zn2+
(blue), Ni2+ (green), or Co2+ (yellow) ions at 25 ꢀC. [SPMA unit] ¼
0.1 mM at equilibrium (—); after visible light irradiation (---).
owing to the sulfobetaine unit. McCormick, and coworkers
reported that zwitterions in sulfobetaine polymers remain in their
diionic form over a broader pH range than the carboxy betaine
polymers.14 The spiropyran units in the copolymers favor
protonation over metal complexation while in the open form.
Thus, having the sulfobetaine unit as the other co-monomer
allows the spiropyran unit to form a metal complex efficiently.
The sulfobetaine copolymers with photo-reversible and selective
Cu2+ complexation show promise for useful applications in cell
biology, molecular biology, and biochemistry due to the ability
to function in saline solution.
suppressed the thermodynamic stability of the open form of the
SPMA units, as indicated by a trend in decrease in intensity of
the 530 nm band at 3.0 and 10 wt% aqueous NaCl (Fig. 2).
Interfering with the electrostatic interaction, which constructed
the cross-linked network with oppositely charged groups in the
sulfobetaine units, causes hydration of 1 in response to NaCl
addition, leading to an unfavorable environment for thermody-
namic stability of the open form of SPMA units. Bergbreiter and
coworkers reported that sodium salts, whether kosmotropes or
chaotropes, could increase the surface tension of the interface of
water and the hydrophobic portions of poly(N-isopropy-
lacrylamide), P(NIPAAm).20 Compound 1 also contains both
hydrophobic and hydrophilic portions similar to P(NIPAAm).
The subsequent high surface tension of the 1/water interface
upon NaCl addition results in hydrophobic hydration of 1, which
can explain the decrease in the number of SPMA units in the
open form.
Results and discussion
Solubility and photochromism of 1 in saline solution
One feature of polymeric betaines is the tendency of the zwit-
terionic fragments to form a cyclic structure involving the
cationic and anionic groups of neighboring monomer units
(intragroup), or a head-to-tail stacking (interchain) structure
within single macromolecules and interchain ion contacts
between neighboring macromolecules. The consequent prox-
imity of the anionic and cationic groups causes the appearance of
electrostatically cross-linked networks.15–17 Therefore, polymeric
betaines are usually sparingly soluble in pure water.18
A homopolymer of sulfobetaine, one of the co-monomer units
of 1, is insoluble in pure water. The copolymer of 1 containing 1.0
mol% of spiropyran methacrylate (SPMA) units was also insol-
uble because the hydrophobic closed form of SPMA is dominant
in the equilibrium between the closed and open forms in ordinary
solvents, involving water.
The addition of electrolytes, such as NaCl and KCl is often
used to solubilize polymeric betaines because of the interference
of the dissociated electrolytes with the electrostatically cross-
linked networks constructed by the zwitterionic fragments. A
transparent aqueous solution of compound 1 was prepared by
adding 1 to saline solution ([NaCl] ¼ 1.0 wt%, pH z 7.0) at
25 ꢀC. The absorption spectrum of the saline solution of 1
showed a strong band at 530 nm (Fig. 1) attributed to the open
form of the SPMA units.19 This band was photo-bleached by
irradiation with visible light (>420 nm), but reappeared upon
standing in the dark. Further addition of NaCl to the solution
Fig. 2 Absorption spectra of saline solutions of 1 with different NaCl
concentrations: 1% (black), 3% (red), and 10 wt% NaCl (blue) at 25 ꢀC.
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Table 1 Concentration of metal ions in saline solution of 1 before and
after irradiation with visible light, measured by ICP-AESa
Cu2+
Zn2+
Ni2+
Co2+
In the dark
After visible light irradiation
1.3
1.7
2.9
3.0
2.9
2.9
3.0
3.0
a
Unit: mg mLꢁ1, initial concentration of metal chlorides was 3.0 mg mLꢁ1
,
which was one tenth of the concentration of SPMA units of 1. [NaCl] ¼
1.0 wt%, at 25 ꢀC.
to that of the solutions of the soluble non-cross-linked 1, i.e.,
while there was no change in absorption spectra of the cross-
linked copolymer after addition of Zn2+, Ni2+, or Co2+; only the
solution containing Cu2+ showed the appearance of an absorp-
tion band near 420 nm and a decrease in intensity of the 530 nm
band (Fig. 1). Metal ion measurement of the solution (initial
metal chloride concentration: 3.0 mg mLꢁ1, [NaCl] ¼ 1.0 wt%)
was performed using inductively coupled plasma atomic emis-
sion spectroscopy, ICP-AES, before and after irradiation with
visible light (Table 1). Only the solution containing Cu2+ showed
a change in metal ion concentration before and after visible light
irradiation. Inefficient desorption of complexed Cu2+ ions from
the hydrogel due to irradiation with visible light is due to slow
diffusion from inside the gel, indicated by the colorless solution
under visible light irradiation.
Fig. 3 A hydrogel of covalently cross-linked 1 in aqueous NaCl solu-
tion: (a) 0 wt% NaCl (pure water); (b) 1.0 wt% NaCl; and (c) 5.0 wt%
NaCl at 25 ꢀC. The plate printed with the grid (1 cm ꢂ 1 cm squares) was
behind the gel.
A covalently cross-linked copolymer of 1 was also prepared
with methylenebisacrylamide as a cross-linker by radical copo-
lymerization in trifluoroethanol. The transparent hydrogel
obtained from this copolymer in pure water produced
a maximum absorption band at a wavelength of 530 nm (not
shown), which was identical to the saline solution of 1. The non-
turbid nature of the hydrogel without addition of NaCl (Fig. 3a)
indicates that the polymer chains of the covalently cross-linked
copolymer were distributed efficiently in a purely aqueous
medium compared to the covalently non-cross-linked copoly-
mer, because the cross-linked structure of the copolymer
prevents the zwitterionic sulfobetaine units from entering their
intra- and intertether areas, or from forming stacked layers
within single macromolecules or interchain ion contacts between
neighboring macromolecules. The polymerization process
accompanied by cross-linking may place sulfobetaine co-mono-
mers in electrostatically unfavorable conformations as they
became a part of the propagating polymer chains in the gel
structure. In addition, reversible swelling/shrinking of the
hydrogel by changing the concentration of NaCl was observed,
as shown in Fig. 3. The size of the hydrogel immersed in 5.0 wt%
NaCl solution was 2.2-fold larger than that in pure water. The
reversible behavior also supports the presence of electrostatic
interactions between the oppositely charged groups of the zwit-
terionic sulfobetaine units of 1 in aqueous solution.
The intensity of the 420 nm band of the solution of 1 con-
taining Cu2+ increased with NaCl concentration, while the
530 nm band decreased in intensity (Fig. 4). The change in the
intensity of the bands at 420 nm and 530 nm accompanied by
Cu2+ complexation was evaluated in terms of the equilibria of the
isomerization reaction of SPMA units and Cu2+ complexation
with the open form of the SPMA units. Since the concentration
of the SPMA units remained constant (0.1 mM), the sum of the
concentrations of the closed and the non-complexed open forms
decreases as Cu2+ complexation increases. The reversible isom-
erization of SPMA units is an equilibrium between the closed
Metal complexation of the copolymer in saline solution
Four metal chlorides (CuCl2, ZnCl2, NiCl2, and CoCl2), all salts
of first-row transition metals, were added separately into a saline
solution ([NaCl] ¼ 1.0 wt%) of 1. No change was seen in the
absorption spectra of any of the solutions, except for the CuCl2
solution as shown in Fig. 1, indicating that no metal complexes of
1 with Zn2+, Ni2+, or Co2+ are formed. Only the solution con-
taining Cu2+ ions contained an absorption band near 420 nm
along with a decrease in the intensity of the 530 nm band.
To confirm Cu2+-selective adsorption and desorption by 1 in
saline solution via light irradiation, the metal ion concentration
was measured upon photo-reversible isomerization of 1. As
shown in Fig. 3, the color of covalently cross-linked 1 was similar
Fig. 4 Absorption spectra of a saline solution of 1 containing Cu2+ (1.0
mM) at different concentrations of NaCl at 25 ꢀC. Concentration of
NaCl: 1.0 wt% (black); 5.0 wt% (red); and 10 wt% (blue). [SPMA unit] ¼
0.1 mM.
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form (CF) and the open form (OF) (eqn (1)), and concentration
of the non-complexed open form is determined by its equilibrium
constant in saline of i% NaCl, Ki. Thus, concentration of the
closed form of SPMA units also decreases in proportion to the
concentration of the non-complexed open form with Ki.
[CF]b + [OF]b ¼ [CF]a + [OF]a + [MF] (¼ [SPMA])
(3)
where [CF]b and [CF]a, or [OF]b and [OF]a are the concentrations
of CF or OF before and after the addition of metal ions,
respectively. Eqn (3) can be rewritten as:
Ki
*
[MF] ¼ ([CF]b + [OF]b) ꢁ ([CF]a + [OF]a)
(4)
CF
OF
(1)
)
In contrast, similar but small changes in the intensity of the 420
and 530 nm bands for all solutions of 1 containing the Zn2+, Ni2+,
or Co2+ were also observed at 5.0 and 10 wt% of NaCl at 25 ꢀC. A
high NaCl concentration contributes to metal complexation of
the SPMA units with inefficient metal ion selectivity.
where [MF] is the concentration of the total generated metal
complexes of SPMA units after metal ion addition.
Isomerization of the SPMA units of 1 between CF and OF is
a reversible reaction with an equilibrium constant (Ki) in i%
NaCl solution as follows:
As mentioned, the thermodynamic stability of the open form
of SPMA units of 1 decreased in the more concentrated NaCl
solutions, showing a decrease in the intensity of the 530 nm band.
Since both the stability of the open form of SPMA units and the
metal complexation of 1 were closely connected to the NaCl
concentration, the NaCl may loosen the electrostatically cross-
linked networks of the zwitterionic sulfobetaine units of 1,
allowing the metal ions easy entry into the network. The order of
metal complexation among the four metals followed the Irving–
Williams series, which refers to the relative stabilities of
complexes formed by a metal ion.21
Ki ¼ [OF]/[CF]
(5)
This leads to eqn (6):
À
Á
ꢁ1
½MFꢃ ¼ ðKi þ 1Þ ½OFꢃ ꢁ½OFꢃ
b
a
ꢁ1
¼ ðKi þ 1Þð1 ꢁ xÞ½OFꢃ
b
À
Á
¼ ð1 ꢁ xÞ ½CFꢃ þ½OFꢃ
¼ ð1 ꢁ xÞ½SPMAꢃ
b
b
(6)
Calculation of amount of metal ion adsorption with 1
SPMA-coordinating Cu2+ complexes exist with different SPMA
coordination numbers, such as OF–Cu2+ and (OF)2–Cu2+. The
amounts are dependent upon the equilibria among the Cu2+
complexes (OF + Cu2+ % OF–Cu2+ and OF–Cu2+ + OF %
(OF)2–Cu2+). The dominant metal complexes can be determined
by Job’s method.22 However, the quantity of Cu2+ ions adsorbed
on 1 cannot be easily calculated by spectroscopic analysis of the
420 nm band. Filley and coworkers reported that the molar
absorption coefficients of the metal complexes of spiropyrans
were not proportional to the number of spiropyran ligands, i.e.,
the molar absorption coefficient of (OF)2–Metal is not twice that
of OF–Metal.23 In contrast, the 530 nm band is due to the non-
complexed open form of the SPMA units of 1. The quantity of
the non-complexed open form of the SPMA units is related to the
concentration of the closed form of the SPMA units through the
equilibrium constant (Ki) for isomerization between closed and
open forms in saline (i% of NaCl). Therefore, the 530 nm band
absorbance is appropriate for estimating the total amount of all
Cu2+ complexes of 1 because the quantity of the Cu2+ complexes
involving (OF)2–Cu2+ and OF–Cu2+ can be calculated by
subtraction of the sum of the closed and the non-complexed open
forms from the initial concentration of SPMA units.
Three types of SPMA units of 1 in saline solution containing
metal ions exist: the closed form (CF), the non-complexed open
form (OF), and the metal-complexed open form (MF), which
involves metal complexes with different binding ratios, e.g., OF–
Metal and (OF)2–Metal. The total concentration of the SPMA
units ([SPMA]) of 1 is constant, as follows:
[SPMA] ¼ [CF] + [OF] + [MF]
(2)
Fig. 5 The ratios of metal ion complexes to spiropyran units of copol-
ymers 1 (a) and 2 (b) in solutions with different NaCl concentrations. The
graphs show the adsorption of: Cu2+ (red), Zn2+ (blue), Ni2+ (green), and
Co2+ (yellow).
Concentrations of the SPMA units of 1 before and after
addition of metal ions are:
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where x is the ratio of the 530 nm band intensity after metal ion
addition to that before the addition, i.e., [OF]a ¼ x[OF]b. The
value of x, measured from the absorption spectra, varies between
0 and 1. The absorbance at 550 nm was used as the initial
absorbance for the aqueous solution containing 1 before addi-
tion of metal ions to estimate the amount of metal adsorption by
1 in saline.24
aqueous solution (LCST of 32 ꢀC in pure water and 10 ꢀC in 10
wt% NaCl).25 The effect of dissociated NaCl ions on the phase
transition of aqueous polymer solutions is indirect and can be
explained by changes in the structure and properties of water.
The LCST of 2, comprising 2 mol% spiropyran acrylate (SPAA)
ꢀ
units and 98 mol% NIPAAm units, was 28 C, which is lower
than that for P(NIPAAm) due to the hydrophobic SPAA units in
the copolymer.12
Fig. 5a shows ratios oꢀf the amount of metal complex to total
SPMA units of 1 at 25 C. A case with extremely low or high
metal complexation is difficult to analyze because of the
accuracy of x. Therefore, the initial Cu2+ solution was prepared
with three-fold greater concentration than the concentration of
SPMA units of 1 due to the relatively high ability of Cu2+ to
complex with 1, whereas the concentration of the other three
metal ions was ten-fold greater than that of the SPMA units.
Nevertheless, selective and photo-reversible Cu2+ adsorption
was complete in saline ([NaCl] ¼ 1.0 wt%). Increasing NaCl
concentration in the solutions increased the amount of metal
complexation for all of the metal ions, as shown in Fig. 5a.
This resulted in a lower effective selectivity for Cu2+ ion
adsorption with 1 at 10 wt% NaCl.
This report focused on the influence of NaCl concentration on
the thermodynamic stability of the open form of the SPAA units
of 2. The absorbance of the 530 nm band attributed to the open
form decreased as the NaCl concentration increased, whereas the
intensity of the 295 nm band increases with the concentration of
the closed form of the SPAA units. The change in the absorption
spectra of a saline solution of 2 was the same as that for 1.
Therefore, NaCl addition to a solution of 2 also suppressed
thermodynamic stability of the open form of the SPAA units,
indicating that dissociated NaCl electrolytes could increase the
cost of hydrophobic hydration, i.e., increase the surface tension
of the polymer/water interface.20
Three aqueous solutions were prepared with varying NaCl
concentrations: 0, 1.0, and 3.0 wt%. Absorption spectra of the
saline solutions of 2 upon addition of a metal ion (Cu2+, Zn2+,
Cu2+ ion adsorption with electrically neutral copolymer of acryl
amide
Ni2+, or Co2+) were measured at 10 C, a temperature far from
ꢀ
the LCST to avoid generation of a precipitate of 2. Quantita-
tive analysis to determine metal adsorption by 2 in saline
solution was conducted as shown in Fig. 5b. Even under
conditions disadvantageous for the adsorption of Cu2+ ions
([Cu2+] : [SPAA] ¼ 3 : 1) for comparison with the other metal
ions ([Metal2+] : [SPAA] ¼ 10 : 1), absorption of Cu2+ ions was
greatest among the four metal ions. Upon increasing the NaCl
concentration, adsorption of all of the metal ions by 2
improved, while the selectivity of metal ion adsorption
decreased.
An electrically neutral water-soluble copolymer, poly(N-iso-
propylacrylamide-co-spiropyran acrylate), 2 (Scheme 2), was
compared to the selective Cu2+ ion adsorption with 1. Zwitter-
ionic-type polyeletrolytes have a small positive N value. The N
value is the number of H-bonds disrupted because of the presence
of one monomer residue in the polymer. The N value of zwit-
terionic polyeletrolytes is similar to that for electrically neutral
polymers.10 Thus, the difference in the metal complexation
between 1 and 2 could reveal the contribution of electrostatically
cross-linked networks of sulfobetaine units to selective Cu2+ ion
adsorption by 1.
The change in metal adsorption as a function of NaCl
concentration for 2 was similar to that for 1. These results
indicate that the open form of the spiropyran units of 1 and 2
is less thermodynamically stable in the hydrophobic hydrated
environment resulting from the addition of NaCl electrolytes.
The consequent hydration results in lower levels of H-bonded
arrangements of water molecules, leading to greater oppor-
tunities for the spiropyran units to come into contact with
metal ions, in contrast to increased H-bonding in dilute NaCl
solution. This could improve metal adsorption by the spi-
ropyran units of 1 and 2, and decrease metal ion selectivity.
Breaking the H-bonded structure by addition of NaCl prob-
ably also occurs for 1 to the same extent as 2 because zwit-
terionic polybetaines have a N value similar to that for
electrically neutral polymers. However, increasing the NaCl
concentration to 10 wt% did not significantly improve the
amount of metal complexes of 1, except for the level of Cu2+
complexation in comparison to 2, as shown in Fig. 5. This
indicates that electrostatically cross-linked networks of sulfo-
betaine units are better at protecting against metal ion entry
than the H-bonding networks of water molecules. Some
reports described the interaction between spiropyrans and
Cu2+ ions, leading to selective Cu2+ complexation.11,26 There
may be a similar interaction between the sulfobetaine units of
1 and Cu2+ ions.
Poly(N-isopropylacrylamide), P(NIPAAm), is a thermosensi-
tive polymer, showing a phase transition at the lowest critical
solution temperature (LCST). Such a polymer is sensitive to the
surrounding conditions, e.g., temperature, pH, and ion strength
of aqueous solutions, accompanied by a large structural change
in polymer chains. According to Maeda and co-workers, the
LCST of P(NIPAAm) varies with NaCl concentration in
Scheme 2 Chemical structure of 2.
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Preparation of a cross-linked copolymer of 1
Experimental section
The radical copolymerization of [2-(methacryloyloxy)ethyl]-
dimethyl-(3-sulfopropyl)-ammonium hydroxide (1340 mg),
SPMA (36 mg) and methylenebisacrylamide (15 mg) in 2,2,2-
trifluoroethanol (2 mL) was performed under a nitrogen atmo-
sphere at 60 ꢀC for 5 h with AIBN on a glass plate to afford
a cross-linked copolymer membrane of 1. The obtained copol-
ymer membrane was immersed in water several times to remove
the trifluoroethanol.
Materials
[2-(Methacryloyloxy)ethyl]-dimethyl-(3-sulfopropyl)-ammoniu-
m hydroxide (reagent grade) was purchased from Sigma-Aldrich,
Inc. N-Isopropylacrylamide was purchased from Wako Pure
Chemical Industries, Ltd. and used after recrystallization.
10,30,30-Trimethyl-6-hydroxyspiro(2H-1-benzopyran-2,20-indo-
line) was purchased from Acros Organics. N-Iso-
propylacrylamido (NIPAAm, Wako Pure Chemical Industries,
Ltd.) was purified by recrystallization from benzene–n-hexane.
Copolymerization of NIPAAm and SPAA (2)
The radical copolymerization of NIPAAm and SPAA in ethanol
ꢀ
was performed under a nitrogen atmosphere at 60 C with 2,20-
10,30,30-Trimethyl-6-methacryloylspiro[2H-1-benzopyran-2,20-
indoline] (SPMA)
azobis(isobutyronitrile) (AIBN) as an initiator to afford
P(NIPAAm-SPAA). The molecular weight for the copolymer
was determined (Mw ¼ 1.3 ꢂ 105, Mw/Mn ¼ 1.4) by GPC
(Shodex KF-804L and KF-803L) using tetrahydrofuran as the
eluent and polystyrenes as the molecular weight standard. The
ratio of SPAA in P(NIPAAm-SPAA) was 2 mol%, as determined
10,30,30-Trimethyl-6-hydroxyspiro(2H-1-benzopyran-2,20-indo-
line) (6.8 g) was reacted with methacryloyl chloride (1.3 mL) at
ꢀ
0 C in chloroform (10 mL) with triethylamine (1.9 mL) for an
hour. The solvent was removed in vacuo, followed by extraction
(chloroform–water). The residue was purified by column chro-
matography on silica gel with chloroform as the eluent (Rf ¼
0.67, chloroform) to give SPMA in 51% yield (4.3 g); MS m/z 361
(M+, 100%); 1H NMR (500 MHz, CDCl3) d 1.15 (3H, s, CCH3),
1.30 (3H, s, CCH3), 2.03 (3H, s, methacryloyl-CH3), 2.72 (3H, s,
NCH3), 5.70 (1H, d, J ¼ 10.3 Hz), 5.71 (1H, dd, J ¼ 0.6, 1.5 Hz,
methacryloyl-H), 6.30 (1H, dd, J ¼ 0.7 Hz, methacryloyl-H),
6.5–7.1 (8H, m, Ar–H).
1
by H NMR.
General methods
1H NMR spectra were recorded at 500 MHz on a JEOL JNM-
ECP500 instrument. UV/visible absorption spectra were
acquired on an Agilent 8453 diode array spectrophotometer.
Visible light (>420 nm) for photo-irradiation was obtained by
passing light from a 500 W Xe lamp (USHIO) through a color
filter, SCF-50S-42L (SIGMA KOKI).
10,30,30-Trimethyl-6-acryloylspiro[2H-1-benzopyran-2,20-indoline]
(SPAA)
Conclusions
The zwitterionic copolymer of 1 has electrostatically cross-linked
networks of zwitterionic sulfobetaine units, which improve Cu2+
ion selectivity as well as the amount of Cu2+ ion adsorption. In
contrast, H-bonded networks of cross-linked water molecules in
the hydration shell of polymer chains of 1 and 2 do not seem to
contribute effectively to metal ion selectivity in metal complex-
ation.
10,30,30-Trimethyl-6-hydroxyspiro(2H-1-benzopyran-2,20-indo-
ꢀ
line) (4.7 g) was reacted with acryloyl chloride (1.6 g) at 0 C in
toluene (43 mL) with triethylamine (1.8 g) for 15 h. The solvent
was removed in vacuo, followed by extraction (toluene–water).
The residue was purified by column chromatography on silica gel
with chloroform as the eluent (Rf ¼ 0.82, chloroform) to give
SPAA in 66% yield (3.10 g); mp 88–90 ꢀC; MS m/z 347 (M+,
100%); 1H NMR (500 MHz, CDCl3) d 1.14 (3H, s, CCH3), 1.29
(3H, s, CCH3), 2.71 (3H, s, NCH3), 5.70 (1H, d, J ¼ 10.3 Hz),
5.97 (1H, dd, J ¼ 1.1, 10.6 Hz, acryloyl-H), 6.28 (1H, dd, J ¼
10.5, 16.5 Hz, acryloyl-H), 6.56 (1H, dd, J ¼ 0.7, 17.2 Hz,
acryloyl-H), 6.5–7.1 (8H, m, Ar–H).
Acknowledgements
We thank the Research Institute for Technology (No. Q09M-02)
of Tokyo Denki University, Japan.
Notes and references
1 R. C. Bertelson, in Photochromism, ed. G. H. Brown, Wiley, New
York, 1971.
Copolymerization of sulfobetaine methacrylate and SPMA (1)
2 R. J. Guglielmetti, in Photochromism, ed. H. Durr and
H. Bouas-Laurent, Elsevier, Amsterdam, 1990.
3 R. C. Bertelson, in Organic Photochromic and Thermochromic
Compounds, ed. J. C. Crano and R. J. Guglielmetti, Plenum Press,
New York, 1999.
4 J. D. Winkler, C. M. Bowen and V. Michelet, J. Am. Chem. Soc.,
1998, 120, 3237–3242.
5 J.-W. Zhou, Y.-T. Li and X.-Q. Song, J. Photochem. Photobiol., A,
1995, 87, 37–42.
6 T. Tamaki and K. Ichimura, J. Chem. Soc., Chem. Commun., 1989,
1477–1479.
7 M. V. Alfimov, O. A. Fedorova and S. P. Gromov, J. Photochem.
Photobiol., A, 2003, 158, 183–198.
The radical copolymerization of [2-(methacryloyloxy)ethyl]-
dimethyl-(3-sulfopropyl)-ammonium hydroxide (830 mg) and
SPMA (10.8 mg) in 2,2,2-trifluoroethanol (2 mL) was performed
under a nitrogen atmosphere at 60 ꢀC for 4 h with 2,20-azobi-
s(isobutyronitrile) (AIBN) as an initiator to afford 1. The
molecular weight of the copolymer (Mw ¼ 7.5 ꢂ 105, Mw/Mn ¼
2.9) was determined by GPC (OHpak SB-806MHQ) using saline
([NaCl] ¼ 0.4 mol mLꢁ1) as the eluent and Shodex Standard P-82
as the molecular weight standard. The ratio of SPMA in 1 was 1
1
mol%, as determined by H NMR.
2778 | J. Mater. Chem., 2010, 20, 2773–2779
This journal is ª The Royal Society of Chemistry 2010

View Online
8 K. Kimura, H. Sakamoto and M. Nakayama, Bull. Chem. Soc. Jpn.,
2003, 76, 225–245.
18 H. Kitano, M. Imai, K. Sudo and M. Ide, J. Phys. Chem. B., 2002,
106, 11391.
9 M. Inouye, M. Ueno, K. Tsuchiya, N. Nakayama, T. Konishi and
T. Kitao, J. Org. Chem., 1992, 57, 5377.
10 O. A. Fedorova, S. P. Gromov, Y. V. Pershina, S. S. Sergeev,
Y. P. Strokach, V. A. Barachevsky, M. V. Alfimov, G. Pepe,
A. Samat and R. Guglielmetti, J. Chem. Soc., Perkin Trans. 2, 2000,
563–570.
19 We reported the 530 nm band attributed to the open form of SPMA
and spiropyran acrylate (SPAA) as one of the comonomers of
copolymers in aqueous solution. Metal complexation and
protonation of the open form of SPMA and SPAA units of the
copolymers generates another band around 420 nm. Thus, 1 has no
ability to form a Na+ complex in aqueous solution.
11 N. Shao, J. Y. Jin, H. Wang, Y. Zhang, R. H. Yang and W. H. Chan,
Anal. Chem., 2008, 80(9), 3466–3475.
20 Y. J. Zhang, S. Furyk, D. E. Bergbreiter and P. S. Cremer, J. Am.
Chem. Soc., 2005, 127, 14505–14510.
12 T. Suzuki, T. Kato andH. Shinozaki, Chem. Commun., 2004, 2036–2037.
13 T. Suzuki, Y. Kawata, S. Kahata and T. Kato, Chem. Commun., 2003,
2004–2005.
14 E. E. L. Kathmann, L. A. White and C. L. McCormick,
Macromolecules, 1997, 30, 5297.
21 H. M. N. H. Irving and R. J. P. Williams, J. Chem. Soc., 1953, 3192–
3210.
22 M. T. Stauffer and S. G. Weber, Anal. Chem., 1999, 71, 1146–1151.
23 J. Filley, M. A. Ibrahim, M. R. Nimlos, A. S. Watt and D. M. Blake,
J. Photochem. Photobiol., A, 1998, 117, 193–198.
15 S. Kudaibergenov, W. Jaeger and A. Laschewsky, Adv. Polym. Sci.,
2006, 201, 157–224.
16 J. C. Salamone, W. C. Rice, Polyampholytes, in Encyclopedia of
Polymer Science and Engineering, ed. H. F. Mark, Wiley, New
York, 1998, Vol. 11, p 514.
24 Because there is still a small overlap of the 420 nm and the 530 nm
bands at 530 nm, we used absorption at 550 nm for analysis to
estimate the amount of metal adsorption.
25 Y. Maeda, T. Higuchi and I. Ikeda, Langmuir, 2001, 17, 7535.
26 N. Shao, J. Y. Jin, S. M. Cheung, R. H. Yang, W. H. Chan and
T. Mo, Angew. Chem., Int. Ed., 2006, 45, 4944–4948.
17 A. B. Lowe and C. L. McCormick, Chem. Rev., 2002, 102, 4177.
This journal is ª The Royal Society of Chemistry 2010
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