Photochemical switching of the phase-transition temperatures of p-NIPAM-Pt nanoparticles thermosensitive polymer composites associated with electrodes: Functional electrodes for switchable electrocatalysis

Article abstract of DOI:10.1002/chem.201100714

The thermosensitive poly(N-isopropylacrylamide) (p-NIPAM) is electropolymerized onto Au surfaces. The incorporation of the photoisomerizable N-carboxyethyl nitrospiropyran compound into p-NIPAM allows the reversible photochemical control of the gel-to-solid phase-transition temperatures of the polymer. Whereas the gel-to-solid phase-transition temperature of the nitrospiropyran-modified p-NIPAM is 33±2 °C, the phase-transition temperature of the nitromerocyanine-functionalized p-NIPAM matrix corresponds to 38±1 °C. Upon the incorporation of Pt nanoparticles (NPs) into the photochemically controlled p-NIPAM, a hybrid photoswitchable electrocatalytic matrix is formed. At a fixed temperature corresponding to 38 °C, the effective electrocatalytic reduction of H₂O₂, or the oxidation of ascorbic acid, proceeded in the presence of the nitromerocyanine-functionalized p-NIPAM, yet these electrocatalytic transformations were inhibited in the presence of the nitrospiropyran-modified p-NIPAM. Copyright

Full text of DOI:10.1002/chem.201100714

FULL PAPER
DOI: 10.1002/chem.201100714
Photochemical Switching of the Phase-Transition Temperatures of p-
NIPAM–Pt Nanoparticles Thermosensitive Polymer Composites Associated
with Electrodes: Functional Electrodes for Switchable Electrocatalysis**
Junji Zhang,[a, b] Michael Riskin,[a] Ran Tel-Vered,[a] He Tian,[b] and Itamar Willner*[a]
Abstract: The thermosensitive poly(N-
isopropylacrylamide) (p-NIPAM) is
electropolymerized onto Au surfaces.
The incorporation of the photoisomer-
izable N-carboxyethyl nitrospiropyran
compound into p-NIPAM allows the
reversible photochemical control of the
gel-to-solid phase-transition tempera-
tures of the polymer. Whereas the gel-
to-solid phase-transition temperature
of the nitrospiropyran-modified p-
NIPAM is 33Æ28C, the phase-transi-
tion temperature of the nitromerocya-
nine-functionalized p-NIPAM matrix
corresponds to 38Æ18C. Upon the in-
corporation of Pt nanoparticles (NPs)
into the photochemically controlled p-
electrocatalytic matrix is formed. At a
fixed temperature corresponding to
388C, the effective electrocatalytic re-
duction of H2O2, or the oxidation of as-
corbic acid, proceeded in the presence
of the nitromerocyanine-functionalized
p-NIPAM, yet these electrocatalytic
transformations were inhibited in the
presence of the nitrospiropyran-modi-
fied p-NIPAM.
NIPAM,
a
hybrid photoswitchable
Keywords: electrocatalysis · isom-
erization · phase transitions · Pt
AHCTUNGTRENNUNG
nanoparticles
polymers
·
thermosensitive
Introduction
the phase-transition temperature have been examined. Dif-
ferent applications of the thermosensitive p-NIPAM matri-
ces have been suggested, including controlled drug deliv-
ery,[20] controlled biocatalysis,[21] separation,[22] biological ap-
plications, such as delivery of cells,[23] culturing of cells,[24]
bacteria-induced changes of the polymer structures,[25] and
their use as nanothermometers.[26] Also, the incorporation of
coadditives into p-NIPAM has been found to affect the
phase-transition temperatures of the polymer matrices. For
example, the copolymerization of photoisomerizable units
has been found to affect the transition temperature.[27] Also,
the binding of ions, such as Cu2+, Ag+, or Hg2+, to p-
NIPAM affected differently the phase-transition tempera-
ture of the polymers, and different metal ion–p-NIPAM
thermometers have been reported.[28] Furthermore, it has
been demonstrated that the binding of metal ions to p-
NIPAM controls the charge transport through the matrices,
and upon the reduction of the ions to metal nanoclusters,
switchable electrocatalysis has been reported.[28] Herein we
report on the incorporation of nitrospiropyran photoisomer-
izable units into the p-NIPAM matrix. Nitrospiropyrans rep-
resent a broad class of photoisomerizable (photochromic)
materials.[29] Nitrospiropyran undergoes, upon UV irradia-
tion, isomerization to the zwitterionic merocyanine deriva-
tive. The irradiation of the zwitterionic merocyanine state
with visible light regenerates the nitrospiropyran state. We
found that the different photoisomer states (that are nonco-
valently associated with the polymer) control the phase-
transition temperatures of the polymer, and they enable the
photoswitching of the phases at a fixed temperature. We
then incorporated Pt nanoparticles (NPs) into the photoac-
tive, thermosensitive polymer and demonstrated photo-
Signal-triggered phase transitions of polymers have attracted
substantial research efforts aimed to develop polymer mate-
rials with new catalytic[1] and electronic[2] properties, and to
design polymers for controlled drug delivery and release,[3]
separation matrices,[4] and sensor systems.[5] Electrical,[6]
magnetic,[7] photoresponsive,[8] and thermosensitive poly-
mers[9] have been synthesized, and applied for switchable
bioelectrocatalysis,[10] controlled surface wettability,[11] sur-
face patterning,[12] and controlled release of substrates.[13]
Also, different chemical or pH stimuli have been used to
induce phase transitions in polymer matrices.[14] Among the
different signal-responsive polymers, the thermosensitive
poly(N-isopropylacrylamide) (p-NIPAM) have attracted
substantial research efforts. This polymer undergoes a gel-
to-solid transition at 32Æ18C, and this phenomenon has
been theoretically addressed.[15] Also, the effects of pH,[16]
added salt,[17] organic solvents,[18] and copolymer units[19] on
[a] J. Zhang, Dr. M. Riskin, Dr. R. Tel-Vered, Prof. I. Willner
Institute of Chemistry, The Hebrew University of Jerusalem
Jerusalem, 91904 (Israel)
Fax : (+972)2-6527715
E-mail: willnea@vms.huji.ac.il
[b] J. Zhang, Prof. H. Tian
Laboratory for Advanced Materials, Institute of Fine Chemicals
East China University of Science and Technology
Shanghai, 200237 (P.R. China)
[**] p-NIPAM=poly(N-isopropylacrylamide)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100714.
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switchable electrocatalysis with the photoisomer–p-NIPAM–
Pt NPs hybrid matrix.
the expanded polymer matrix to the molecular dopant that
diffuses into the film, followed by the thermal collapse of
the matrix to the shrunken solid state.
Although 1a is noncovalently linked to the polymer, the
crosslinking of the polymer and the entanglement of 1a in
the matrix lead to a stable composite without noticeable
leakage of the photoisomer during a period of time corre-
sponding to 24 h. The saturation of the film with 1a was fol-
lowed by Faradaic impedance spectroscopy, with
[Fe(CN)6]3À/4À as a redox couple (Figure 1A), and by quartz-
crystal microbalance (QCM) (Figure 1B). Evidently, both
the impedance and QCM measurements indicated that the
saturation of the composite with 1a occurred after five
breathing-in cycles and corresponded to a maximal loading
of 2 mgcmÀ2. Impedance spectroscopy was also proven as an
effective method to follow the thermal phase transitions of
the p-NIPAM film, and these were characterized in the pres-
ence of photoisomers 1a or 1b.
Figure 2A and Figure 2B show the Nyquist plots corre-
sponding to p-NIPAM films that include 1a or 1b, respec-
tively, at variable temperatures. Figure 2C, curve (a), depicts
the interfacial electron-transfer resistances of the p-NIPAM
film that includes the nitrospiropyran photoisomer 1a, at
variable temperatures. An increase in the interfacial elec-
tron resistance that reflects the gel-to-solid transition is ob-
served at 33Æ28C. Figure 2C, curve (b), depicts the interfa-
cial electron-transfer resistances corresponding to the p-
NIPAM film that includes the nitromerocyanine photoisom-
er state (1b) at variable temperatures. A clear shift in the
phase-transition temperature is observed, and the gel-to-
solid transition temperature is estimated to be 38Æ18C.
These results suggest that at 368C, the 1a-containing p-
NIPAM matrix exists mostly in the solid form, whereas the
1b-containing p-NIPAM is mostly in the gel form. These re-
sults indicate that the gel-to-solid phase transitions of the p-
NIPAM matrix can be photomodulated by the photoisomer
state of the additive. The slightly higher gel-to-solid phase
transition of the 1b–p-NIPAM matrix as compared to the
1a–p-NIPAM system may be attributed to H bonds or
dipole interactions between 1b and the acrylamide groups
of p-NIPAM that perturb the temperature-induced collapse
of the amide groups. That is, the normal collapse of the p-
NIPAM gel to the solid state involves the elimination of
water from the matrix and the formation of inter H bonds
among the acrylamide units.
Results and Discussion
p-NIPAM was electropolymerized onto Au electrodes under
crosslinking conditions in the presence of N-isopropylacryla-
mide, with Na2S2O8 as an initiator. The mechanism of the
electrochemically induced polymerization of the N-isopropy-
lacrylamide under these conditions has been previously dis-
cussed.[30] The thickness of the p-NIPAM film was estimated
by AFM measurements to be 10Æ1 nm. N-carboxyethyl ni-
trospiropyran (1a) was incorporated into the p-NIPAM film
by five “breathing-in” cycles, in which the photoisomeriza-
ble substrate was taken up by soaking the film in the 1a so-
lution at 228C (at this stage the polymer is in the gel state)
and elevating the temperature to 408C to form the solid
polymer state. By the repeated uptake of 1a by p-NIPAM,
the film was saturated with the photoisomer, Figure 1. The
breathing-in mechanism involves the primary exposure of
In the presence of the hydrophilic merocyanine dopant,
the confinement of the water in the matrix is enhanced, thus
a higher temperature to induce the collapse of the film is re-
quired. Indeed, Figure 2D shows that the interfacial elec-
tron-transfer resistance of p-NIPAM can be cycled, at 368C,
between high and low values upon the reversible photoiso-
merization of the additive between the states 1a and 1b, re-
spectively. The increase in the interfacial electron-transfer
resistances of the p-NIPAM-modified electrode upon the
gel-to-solid phase transition may be explained by the rela-
tive permeabilities of the [Fe(CN)6]3À/4À redox couple
through the polymer film. Whereas the redox couple pene-
Figure 1. A) Faradaic impedance spectra (at 208C, in the form of Nyquist
plots) corresponding to [Fe(CN)6]3À/4À (2 mm) and following the breath-
ing-in process of nitrospiropyran photoisomerizable units into the p-
NIPAM matrix. Number of breathing-in cycles: a) 0, b) 1, c) 2, d) 3, e) 4,
f) 5. Inset: The dependence of the interfacial electron-transfer resistances
on the number of breathing-in cycles. Data were recorded in an aqueous
NaNO3 solution (0.2m, pH 9.5) in the frequency range corresponding to
10 kHz to 100 mHz, at a potential of E=0.185 V versus SCE and by
using an alternate perturbation potential of DE=10 mV. B) Mass
changes, detected by QCM, corresponding to the breathing-in of nitro-
spiropyran photoisomerizable units into the p-NIPAM matrix.
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Functional Electrodes for Switchable Electrocatalysis
FULL PAPER
Figure S1). After five cycles,
the film was saturated with the
Pt NPs, and the loading corre-
sponded to about 2.5 mgcmÀ2.
The gel-to-solid transition of
the p-NIPAM containing the
photoisomerizable units and the
Pt NPs is depicted in Fig-
ure 3A. It should be noted that
the phase-transition tempera-
ture of the p-NIPAM matrix
that included the Pt NPs, but
lacked the photoisomerizable
component, was not affected
upon illumination at l=365 nm
or l>475 nm. The photoactive
p-NIPAM–Pt NPs matrix was
then applied to photostimulate
the electrocatalytic reduction of
H2O2 or the electrocatalytic oxi-
dation
of
ascorbic
acid
(Scheme 1).
Figure 2. A) Faradaic impedance spectra corresponding to [Fe(CN)6]3À/4À (2 mm) on the 1a–p-NIPAM-modified
electrode at variable temperatures: a) 208C, b) 268C, c) 288C, d) 308C, e) 328C, f) 348C, g) 368C, and h) 408C.
B) Faradaic impedance spectra under the same conditions as in (A) for the 1b–p-NIPAM-modified electrode,
at: a) 208C, b) 228C, c) 248C, d) 268C, e) 288C, f) 348C, g) 368C, h) 388C, i) 40 8C, j) 448C. C) Interfacial elec-
tron-transfer resistances measured at different temperatures for: a) the 1a–p-NIPAM-modified electrode and
b) the 1b–p-NIPAM-modified electrode. D) Cyclic changes in the interfacial electron-transfer resistances upon
the photochemically induced transition between: a) the 1a–p-NIPAM-modified electrode, and b) the 1b–p-
NIPAM-modified electrode at T=368C. Data were recorded in an aqueous NaNO3 solution (0.2m, pH 9.5)
containing [Fe(CN)6]3À/4À (2 mm).
Figure 3B depicts the linear-
sweep voltammograms corre-
sponding to the electrocatalytic
reduction of H2O2 by the 1a–p-
NIPAM–Pt NPs matrix, curve
(a), and by the 1b–p-NIPAM–
Pt NPs composite, curve (b).
trates the gel form of the film, the solid form of the p-
NIPAM film blocks the penetration of the labels, thus in-
creasing the interfacial electron-transfer resistances. Also,
control experiments revealed that irradiation of
a p-
NIPAM-modified electrode at either l=365 nm or at l>
475 nm, in the absence of 1a, did not lead to any differences
in the gel-to-solid phase transitions (phase-transition tem-
perature, 32Æ18C). These results indicate that the photosti-
mulated control of the phase-transition temperatures of p-
NIPAM, indeed, originates from the photoisomerizable ad-
ditive.
Realizing that the photoisomerizable component embed-
ded in the p-NIPAM matrix photostimulates a change in the
transition temperature of the polymer, we implemented the
system to develop photoswitchable electrocatalytic transfor-
mations. In contrast to previous studies that photoswitched
an electrocatalytic process by photostimulated binding or
dissociation of metallic nanoparticles to, or from, the elec-
trode surface,[31] the present approach attempts to use the
different phases of the polymer film to switch the electroca-
talytic functions of the modified electrode. Toward this goal,
we used the breathing-in process to incorporate Pt NPs into
the p-NIPAM film. Accordingly, we have applied five
breathing-in cycles to saturate the film with the Pt NPs,
while monitoring the gravimetric loading of the film with
the NPs by using QCM (see the Supporting Information,
Scheme 1. Photostimulated electrocatalytic reduction of H2O2 or oxida-
tion of ascorbic acid on a photoisomer–p-NIPAM–Pt NPs matrix.
Evidently, the electrocatalytic current is up to 70% higher
in the presence of the 1b–p-NIPAM–Pt NPs. Because the
1b–p-NIPAM–Pt NPs exists (at 388C) mainly in the gel
state, whereas at the same temperature, the 1a–p-NIPAM–
Pt NPs composite exists mainly in the solid phase, the
higher currents in the 1b–p-NIPAM–Pt NPs composite are
attributed to the enhanced permeability of the H2O2 sub-
strate to the Pt NPs sites in the gel phase of the polymer. By
the cyclic photoisomerization (at 388C) between the 1a–p-
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11239

I. Willner et al.
NIPAM–Pt NPs and the 1b–p-NIPAM–Pt NPs composites,
the electrocatalytic cathodic currents are reversibly switched
between low and high values, respectively (Figure 3B, inset,
curve (I)). For comparison, the electrocatalytic cathodic cur-
rents corresponding to the reduction of H2O2 by the p-
NIPAM–Pt NPs composite that lacks the photoisomerizable
component, under illumination at l=365 nm or at l>
475 nm, are depicted in Figure 3B, inset, curve (II). Under
these conditions, lower cathodic currents of a comparably
constant value of about 2.5 mA are observed, irrespective of
the wavelength of the irradiation. The nonswitchable current
value implies that the photoswitchable electrocatalysis origi-
nates from the photoisomerization of the coadditive in the
1a and 1b states. In a further control experiment, we
probed the possibility that the photoisomers 1a/1b affect
the catalytic functions of the Pt NPs. Accordingly, Pt NPs
were functionalized with a monolayer of 1-mercaptobutyl ni-
trospiropyran. The resulting 2a-capped Pt NPs were immo-
bilized on a benzene-dithiol-monolayer-functionalized Au
electrode. The electrocatalytic reduction of H2O2 by the ni-
trospiropyran- and the nitromerocyanine-functionalized Pt
NPs-modified electrode was then examined (Figure S2 in
the Supporting Information). We found that the electrocata-
lytic activities of the Pt NPs are identical in the two photo-
isomer states, indicating that the photoisomer states do not
affect the catalytic functions of the Pt NPs. These control
experiments imply that the photoswitchable electrocatalytic
activities of the 1a/1b–p-NIPAM–Pt NPs composite origi-
nate from the effect of the photoisomerizable components
on the polymer phase, rather than an effect on the catalytic
functions of the Pt NPs.
Similar photoswitchable electrocatalytic functions of the
photoisomerizable p-NIPAM–Pt NPs were observed upon
the electrochemical oxidation of ascorbic acid, Figure 3C.
The linear-sweep voltammogram corresponding to the oxi-
dation of ascorbate by the 1a–p-NIPAM–Pt NPs (at 388) is
shown in Figure 3C, curve (a), whereas the voltammogram
corresponding to the oxidation of the substrate by the 1b–p-
NIPAM–Pt NPs composite (at 388) is shown in Figure 3C,
curve (b). Clearly, the electrocatalyzed oxidation of ascorbic
acid by the 1b–p-NIPAM–Pt NPs composite yields about
2.5-fold higher electrocatalytic anodic currents, indicating a
superior electrocatalytic function of the 1b–p-NIPAM–Pt
NPs composite. Also, the electrocatalytic functions of the
photoisomerizable composite are photoswitchable, and upon
the photochemical cycling of the polymer between the 1a
and 1b–p-NIPAM–Pt NPs states, the electrocatalytic func-
tions of the system are switched between low and high am-
perometric responses, respectively (Figure 3C, inset, curve
(I)). In a control experiment, the photochemical irradiation,
at l=365 nm or at l>475 nm, of the p-NIPAM–Pt NPs
hybrid that lacked the photoisomerizable units, did not lead
to photoswitchable electrocatalytic functions, and resulted
(at 388C) only in low anodic currents (Figure 3C, inset,
curve (II)). These low current responses are attributed to
the fact that the p-NIPAM–Pt NPs composite reveals, in the
absence of the photoisomerizable units, a phase-transition
Figure 3. A) Interfacial electron-transfer resistances measured at different
temperatures for a) the 1a–p-NIPAM–Pt NPs-modified electrode and
b) the 1b–p-NIPAM–Pt NPs-modified electrode. Data were recorded in
an aqueous NaNO3 solution (0.2m, pH 9.5) containing [Fe(CN)6]3À/4À
(2 mm). B) Linear sweep voltammograms corresponding to the electroca-
talytic H2O2 reduction on a) the 1a–p-NIPAM–Pt NPs-modified elec-
trode and b) the 1b–p-NIPAM–Pt NPs-modified electrode. Inset: Switch-
able electrocatalytic reduction of H2O2 on I) the 1a–p-NIPAM–Pt NPs-
modified electrode and II) a p-NIPAM–Pt NPs-modified electrode lack-
ing the photoisomerizable units. C) Similar measurements as in (B) for
the electrocatalytic oxidation of ascorbic acid. Measurements were re-
corded in an aqueous NaNO3 solution (0.2m, pH 9.5) that included H2O2
(36 mm) or ascorbic acid (2.5 mm) at T=388C. Scan rate: 10 mVsÀ1
.
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Functional Electrodes for Switchable Electrocatalysis
FULL PAPER
sulting mixture was heated at 608C for 10 min and allowed to react at
temperature corresponding to 32Æ18C. Thus, at 388C, the
polymer exists mostly in the solid phase; this prohibits the
accessibility of ascorbic acid to the Pt NPs catalytic sites.
Furthermore, the photoswitchable catalytic functions of the
1a/1b–p-NIPAM–Pt NPs composite are attributed to the
effect of the photoisomers on the transition temperatures of
the p-NIPAM hybrids. The nitrospiropyran–p-NIPAM–Pt
NPs composite exists (at 388C) mostly in the solid phase,
leading to an inefficient electrocatalytic oxidation of ascor-
bic acid. On the contrary, the merocyanine–p-NIPAM–Pt
NPs hybrid exists (at 388C) mostly in the gel phase, allowing
the easy permeation of the ascorbic acid molecules into the
matrix and effective oxidation at the catalytic sites. As
before, the electrocatalytic oxidation of ascorbic acid by a
monolayer of Pt NPs functionalized with the nitrospiropyran
or nitromerocyanine components, yet lacking the polymer
matrix, revealed no noticeable differences. These results in-
dicate that the electrocatalyzed oxidation of ascorbic acid
by the Pt NPs is not affected by the different photoisomer
states, and demonstrate that the photoswitchable electroca-
talytic functions of the hybrid polymer matrices originate
from the effect of the photoisomerizable units on the phase-
transition temperatures of the polymer.
room temperature overnight. The resulting precipitate was filtered and
washed with butanone to yield
a
yellow powder (1.2 g). 1H NMR
(CDCl3): d=1.14 (s, 3H), 1.28 (s, 3H), 2.71 (t, 2H), 3.65 (t, 2H), 5.85 (d,
1H), 5.91 (d, 1H), 6.62–7.18 (m, 5H), 8.05 (m, 2H).
Pt NPs synthesis: Citrate-capped Pt NPs were prepared by heating a
À
PtCl6 solution (100 mL, 1 mm) to reflux, followed by the addition of an
aqueous sodium citrate solution (10 mL, 38.8 mm). After 10 min of boil-
ing, the solution turned from clear to black colored, after which heating
was turned off and the solution was stirred for additionally 10 min. Final-
ly, the solution was allowed to cool to room temperature, filtered through
a 0.2 mm cellulose acetate filter (Schleicher and Schuell, Keene, NH), and
rinsed two times through a 100000 MW cutoff Centricon tube (Millipore
Inc., Billerica, MA) with water. For the preparation of mercaptobutyl ni-
trospiropyran (2a)-capped Pt NPs, N-4-mercaptobutyl nitrospiropyran
(1 mm, 1 mL, dissolved in a 1:4 mixture of DMSO:H2O) was added into
the citrate-capped Pt-NPs solution (9 mL). The mixture was kept in the
dark for 4 h, and the resulting 2a-capped Pt NPs precipitated. Following
a centrifugation at 5000 rpm for 15 min, the precipitated Pt NPs were pu-
rified by repeated resuspension and centrifugation in a NaNO3 solution
(0.2m).
Modification of the electrodes: Electropolymerization of the p-NIPAM
film was performed in an aqueous solution, purged by nitrogen for
30 min, that contained N-isopropylacrylamide (1.0m), N,N’-methylenebi-
sacrylamide (40 mm), NaNO3 (0.2m), and Na2S2O8 (0.01m), by using 60
repetitive cyclic voltammetry scans, ranging between À0.35 and À1.35 V
versus SCE, at a scan rate of 100 mVsÀ1. Following the electropolymeri-
zation process, the electrodes were washed with distilled water to remove
residues of the monomers. The breathing-in of N-carboxyethyl nitrospiro-
pyran 1a was performed by soaking the p-NIPAM-modified electrodes in
an aqueous solution that contained 1a (2 mm) for 2 h. The breathing-in
of Pt NPs was similarly performed. In a control experiment, a Au slide
was immersed for 4 h in an ethanolic solution of benzene dithiol (2 mm)
and the modified electrode was reacted with the 2a-capped Pt NPs to
form a monolayer of the photoisomerizable particles on the Au surface.
Conclusion
In conclusion, the present study has demonstrated that the
incorporation of a photoisomerizable unit into the thermo-
sensitive p-NIPAM polymer matrix allows photochemical
control of the gel/solid phase transitions of the polymer by
means of the photoisomer states of the additive substrate.
This allowed the selection of a specific temperature (368C),
at which the p-NIPAM existed in the gel phase (in the pres-
ence of the nitromerocyanine (1b) isomer), which could be
photochemically transformed into the solid phase (the nitro-
spiropyran (1a) isomer). This photoswitchable control of the
phase of the thermosensitive p-NIPAM matrices allowed the
design of polymer–NPs composites with inherent photo-
switchable electrocatalytic properties. The incorporation of
Pt NPs into the photoisomerizable p-NIPAM matrix allowed
the photoswitchable electrocatalyzed reduction of H2O2 or
the electrocatalyzed oxidation of ascorbic acid by the light-
induced controlled permeability of the substrates to the Pt
NP catalytic sites, and through the transformations between
gel and solid phases of the polymer.
Instrumentation: Nanopure (Barnstead) ultrapure water was used in the
preparation of the different solutions. Au-coated glass plates (Evaporated
Coatings, PA, USA) were used as working electrodes. Prior to modifica-
tion, the Au surface was flame-annealed for 5 min in an n-butane flame
and allowed to cool down for 10 min under a stream of N2. Linear-sweep
voltammetry experiments were carried out with a PC-controlled (Autolab
GPES software) electrochemical analyzer potentiostat/galvanostat (mAu-
tolab, type III). A graphite rod (d=5 mm) was used as a counter elec-
trode, and the reference was a saturated calomel electrode (SCE). A
HETO HMT 200 thermostated bath (Æ0.28C), in which the electrochem-
ical cell was installed, was used throughout the experiments. Faradaic im-
pedance measurements were recorded at 0.185 V versus SCE in the fre-
quency range of 10 kHz to 100 mHz with an alternating voltage of
Æ10 mV. Prior to each measurement, the cell temperature was equilibrat-
ed for 15 min. QCM measurements were performed with a home-built in-
strument linked to a frequency analyzer (Fluke) by using Au-quartz crys-
tals (AT-cut 10 MHz). Photochemical transformations were carried out
with a UV lamp (Upland, USA, P=8 W) at l=365 nm for 25 min and a
Xe lamp (Oriel Instruments, USA, model 6255OF, 150 W) in an Oriel
Research Housing (model 66002 with a power supply Oriel 68700) at l>
475 nm, for 25 min.
Experimental Section
Acknowledgements
Synthesis of 1a: The material was prepared by a slight modification of a
previously reported procedure.[32] 2,3,3-trimethylindoline (6.4 mL,
40 mmol) and iodopropionic acid (7.8 g, 39 mmol) were heated in a flask
for 3 h. The resulting red solid was washed with butanone to yield 1-(b-
carboxyethyl)-2,3,3-trimethylindolynium iodide (9 g). The product (2 g,
5.7 mmol) was dissolved in boiling butanone (5 mL) in the presence of pi-
peridine (0.5 mL). The solution was then reacted with a solution of 2-hy-
droxyl-5-nitrobenzaldehyde (1 g, 6 mmol) in butanone (2 mL). The re-
This research is supported by the FP7 EU ECCell. J.Z. Acknowledges
the Chinese Scholarship Council (CSC).
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11241

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Received: March 8, 2011
Published online: August 25, 2011
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