Osmium bipyridine-containing redox polymers based on cellulose and their reversible redox activity

Article abstract of DOI:10.1021/jp2083488

Thermo-, pH-, and electrochemical-sensitive cellulose graft copolymers, hydroxypropyl cellulose-g-poly(4-vinylpyridine)-Os(bipyridine) (HPC-g-P4VP-Os(bpy)), were synthesized and characterized. The electrochemical properties of the resulting material were investigated via cyclic voltammetry by coating the graft copolymers on the platinized carbon electrode. The results indicated that the electrochemical properties of the graft copolymer modified electrode were responsive to the pH values of the electrolyte solution. The reversible transformation between the active and inactive state originated from the changes in the architecture of the HPC-g-P4VP-Os(bpy) graft copolymer at different pH values. At high pH (e.g., above the pK_a of P4VP), the chains of P4VP collapsed, and the electrochemical activity of the electrode was reduced. With immobilization of glucose oxidase (GOx) on the graft copolymer decorated electrode, a biosensor for glucose detection was prepared. The current of the biosensor depended on the glucose concentration in the detected solution and increased with the successive addition of glucose.

Full text of DOI:10.1021/jp2083488

ARTICLE
pubs.acs.org/JPCB
Osmium Bipyridine-Containing Redox Polymers Based on Cellulose
and Their Reversible Redox Activity
||
Hongliang Kang,† Ruigang Liu,*,† Huafeng Sun,§ Jieming Zhen,§ Qinmei Li,†, and Yong Huang*,†,‡
†Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory of Molecular Sciences, Institute of Chemistry, and
‡Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
§Department of Chemistry, School of Science, Beijing Jiaotong University, Beijing 100044, China
Graduate University, Chinese Academy of Sciences, Beijing 100039, China
S Supporting Information
b
ABSTRACT: Thermo-, pH-, and electrochemical-sensitive
cellulose graft copolymers, hydroxypropyl cellulose-g-poly-
(4-vinylpyridine)-Os(bipyridine) (HPC-g-P4VP-Os(bpy)), were
synthesized and characterized. The electrochemical properties
of the resulting material were investigated via cyclic voltamme-
try by coating the graft copolymers on the platinized carbon
electrode. The results indicated that the electrochemical prop-
erties of the graft copolymer modified electrode were respon-
sive to the pH values of the electrolyte solution. The reversible
transformation between the active and inactive state originated
from the changes in the architecture of the HPC-g-P4VP-Os(bpy) graft copolymer at different pH values. At high pH (e.g., above the
pKa of P4VP), the chains of P4VP collapsed, and the electrochemical activity of the electrode was reduced. With immobilization of
glucose oxidase (GOx) on the graft copolymer decorated electrode, a biosensor for glucose detection was prepared. The current of
the biosensor depended on the glucose concentration in the detected solution and increased with the successive addition of glucose.
’ INTRODUCTION
potential applications as carriers for drug and gene delivery and in
controlled release.26,35,37 The cellulose graft copolymers gener-
ally have the reversible thermal or/and pH stimuli-responsive
properties in the literature. However, few works on the cellulose
graft copolymers with redox stimuli responsive property have
been reported.29
Stimuli-responsive polymers have attracted increasing atten-
tion in past decades. The responses of such polymers to the envi-
ronmental stimuli such as temperature,1ꢀ4 pH,5,6 ionic strength,7ꢀ9
light,10,11 biological conditions,12 or mechanical stress13,14 offer
them potential applications in many fields including oil recovery,15,16
water decontamination,17ꢀ19 analytical chemistry,20,21 purifica-
tion techniques,22 antibacterial coatings,23 and biomedicine.24,25
More recently, copolymers with well-defined architecture and multi
stimuli-responsive properties were developed on the basis of
controlled/living polymerizations.
In the last 10 years, cellulose graft copolymers with well-
defined architecture were synthesized for the purpose of combin-
ing the properties of both the cellulose backbone and the side
chains, which could offer cellulose graft copolymer new applica-
tions in many fields. In our previous works, series stimuli-
responsive cellulose graft copolymers have been synthesized and
investigated extensively.26ꢀ34 The stimuli-responsive property
of the cellulose graft copolymers can be tailored either by the
cellulose backbone (e.g., HPC is thermal sensitive in aqueous
solution) or by the chemical structure of the side chains. The
cellulose graft copolymers can self-assemble into single-26ꢀ28,35
or multi-stimuli32ꢀ34,36 responsive micelles in the selected sol-
vents depending on the components of the graft copolymers.
Cellulose is a kind of polymer with renewable, inexpensive, and
biodegradable properties. Its stimuli-responsive micelles have
Redox polymers are usually used in the chemically modified
electrodes, which has attracted attention due to their variable
properties, particularly on the applications for bioelectroanalysis
and biosensors. The redox polymers coated onto electrode sur-
faces can mediate charge transfer between the electrode and the
target solution species. Generally, two types of redox polymers
have been reported in the literature depending on the location of
the redox groups. One is polymers with redox groups on the
backbone, such as poly(vinyl hydroquinone)s.38 The other is
those polymers contain pendent redox groups or graft chains with
redox groups, such as osmium complex or ferrocence.39,40 Osmium
complex of poly(4-vinylpyridine)41 or poly(N-vinylpyrrole)42
is the typical redox polymer of the later case and has been widely
studied due to their altering the formal potential with different
structures of ligands. These osmium complex polymers have
been applied as the mediators in the biosensors for the detection
of dopamine,43 ascorbic acid,44 lactate,45 glucose,46 H2O2,47 etc.
Received: August 30, 2011
Revised:
December 6, 2011
Published: December 07, 2011
r
2011 American Chemical Society
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In those enzyme electrodes, the structure of the polymer is one of
the key factors that affects the performance of polymer-coated
electrodes by the effect on the electron transfer rate, surface
coverage (concentration) of redox active centers, charge trans-
port and propagation, as well as diffusion and permeation of
soluble species through the polymer.48 Specially, for the purpose
of good contact with water-soluble enzymes, the redox polymers
are expected to be hydrophilic, such as poly(4-vinylpyridine)
osmium complex usually modified by amine to enhance its
solubility in water.49
In this Article, hydroxypropylcellulose-g-poly(4-vinylpyridine)-
Os(bipyridine) (HPC-g-P4VP-Os(bpy)) graft copolymers were
synthesized and characterized. The water-soluble HPC backbone
offered the solubility or swelling of the graft copolymers in water.
The redox activity of the HPC-g-P4VP-Os(bpy) was investigated
at different pH values. By immobilization of the glucose oxidase
(GOx) on the graft copolymers-coated electrode, a sensor was
fabricated for the detection of glucose.
The purified Os(bpy)2Cl2 was collected by centrifugation and
dried in a vacuum. Product yield: 68.3%.
Os(bpy)2Cl2 and HPC-g-P4VP (1/1, w/w) were dissolved in
glycol and refluxed for 24 h to obtain HPC-g-P4VP-Os(bpy)
complex. The reaction mixturewas dialyzed against deionized water
for 3 days with refreshing the outer deionized water twice a day and
freeze-dried to obtain the solid product. Product yield: 48.4%.
Modification of Electrode. Before modification, the bare
platinized carbon working electrode was carefully polished with
alumina powder (0.1 and 0.05 μm, respectively) on chamois
leather and then thoroughly cleaned ultrasonically in ethanol and
distilled water. HPC-g-P4VP-Os(bpy) was first dissolved in de-
ionic water (1.95 g/L). Next, 5 μL of the solution was coated
on the platinized carbon electrode (Φ 3 mm) and left for 1 h to
dry. For preparing enzyme and HPC-g-P4VP-Os(bpy) copolymer-
coated electrode, HPC-g-P4VP-Os(bpy) copolymer (10 g/L)
and glucose oxidase (15 g/L) aqueous solution were mixed in a
volume ratio of 1:1. 5 μL of this solution was coated on the
platinized carbon electrode and left for 1 h to dry.
Characterizations. 1H NMR measurements were carried
out with a Bruker DMX 400 NMR instrument using dimethyl
sulfoxide-d6 as the solvent. FT-IR spectra were recorded on a
Bruker-Equinox 55 FT-TR spectrometer by KBr method. The
molecular weight and the molecular weight distribution of HPC
and the graft copolymers were measured by gel permeation chro-
matography (GPC) (Waters 515), on a system equipped with
a Waters 515 pump, three columns, and a 2410 differential re-
fractometer detector. DMF was used as the eluant at the flow rate
of 0.5 mL/min. Monodisperse polystyrene was used as the standard
to generate the calibration curve. Elemental analysis (CHN) was
performed on a Flash EA 1112 elemental analyzer. Os content in
graft copolymer was determined by inductively coupled plasma
(ICP) atomic emission spectroscopy (Varian 710-ES, U.S.).
Dynamic light scattering (DLS) experiments were carried out
on a commercial spectrometer (ALV/SP-150) equipped with
an ALV-5000 multi-τ digital time correlator and a solid-state
laser (ADLS DPY 425II, output power about 400 MW at λ =
632.8 nm) as the light source. All copolymer solutions were
1 mg/mL and filtered through the Milipore Millex-FH nylon
filter (0.45 μm) before measurements. The hydrodynamic radius
(Rh) was obtained by fitting the correlation function with the
CONTIN program. The samples were equilibrated for 30 min at
the desired temperature that was controlled by a thermostatic bath.
All DLS experiments were carried out at the scattering angle of 90ꢀ.
The transmittance of the copolymer solution was determined
at the fixed wavelength of 500 nm on a Shimadzu 1601PC UVꢀ
vis spectrophotometer. The concentration of the polymer was
kept at 1 mg/mL in aqueous solution.
’ EXPERIMENTS
Materials. 4-Vinylpyridine (4VP) (95%, Acros) was distilled
from calcium hydride under reduced pressure before use. Hydro-
xypropyl cellulose (HPC) (Mn = 10 000 g/mol, Mw = 80 000 g/mol,
Aldrich) was dried for 48 h under a vacuum at 50 ꢀC before use.
poly(vinyl pyridine) (P4VP) (Mw = 60 000 g/mol, Aldrich),
ammonium ceric nitrate (CAN) (A.R. grade, Aladdin), K2OsCl6
(99%, Alfa Aesar), and 2,20-bipyridine (98%, Aldrich) were used
as received. Other chemicals (A.R. grade) were supplied by local
chemical suppliers and used as received. The water was Milli-Q
water. Dialyser had the cutoff molecular weight of 14 kDa.
Glucose oxidase (>100 U/mg, Aladdin) was used as received.
Synthesis of Hydroxypropylcellulose-g-poly(4-vinylpyridine)
(HPC-g-P4VP). Hydroxypropylcellulose-g-poly(4-vinylpyridine)
(HPC-g-P4VP) copolymers were synthesized by free radical graft
copolymerization initiated by CAN. In detail, HPC was dissolved
in water under continuous stirring and bubbled with N2 to
remove oxygen in the solution. The solution was then transferred
to an oil bath settled at 35 ꢀC, and nitric acid was added until the
pH value of the solution was 1.0. Next, degassed 4VP was added
and dissolved for 15 min, after which the degassed initiator of
CAN aqueous solution was added. The reaction vessel was then
sealed, and the reaction was stopped by adding hydroquinone at
a desired period of reaction time. The reaction mixture was
dialyzed against water to remove unreacted monomers and its
low molecular weight homopolymers and other impurities. The
graft copolymers were obtained by freeze-drying after dialysis in
water for 5 days.
Electrochemical experiments were carried out on a CHI660D
electrochemical workstation (CH instruments, Chenhua Instru-
ments Corp., Shanghai, China). The three-electrode system
consisted of a platinized carbon electrode as working electrode,
a Pt wire as secondary electrode, and Ag|AgCl as the reference
electrode. Cyclic voltammeters were carried in a 0.1 M phosphate
buffer solution.
Synthesis of HPC-g-P4VP-Os(bpy) Complex. HPC-g-P4VP-
Os(bpy) complex was synthesized by the reaction of HPC-
g-P4VP graft copolymers with Os(bpy)2Cl2. Os(bpy)2Cl2 was
synthesized as follows. K2OsCl6 and 2,20-bipyridine (1:2, mole
ratio) were dissolved in 5 mL of DMF and refluxed at 140 ꢀC in
an oil bath for 3 h. The reaction mixture was cooled to room
temperature, and KCl crystals were formed in the reaction mix-
ture. KCl crystals were filtered, and the left solution was diluted
by adding 3 mL of ethanol and precipitated in ether. The crude
’ RESULTS AND DISCUSSION
Os(bpy)2Cl3 2H2O was collected by centrifugation and dried in
Synthesis of HPC-g-P4VP-Os(bpy). The synthesis route of
HPC-g-P4VP-Os(bpy) is shown in Scheme 1. First, HPC-
g-P4VP is synthesized by using free radical polymerization with
Ce4+ as the initiator in aqueous solution. Because HPC is a
3
air overnight, and was then dissolved in the methanol/DMF/
H2O (volume ratio of 1:2:20) mixture solvent. The Na2S2O4
solution was added dropwise to precipitate Os(bpy)2Cl2.
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Scheme 1. Preparation Process of HPC-g-P4VP-Os(bpy)
Figure 2. FTIR spectra of (a) HPC-g-P4VP, (b) HPC, and (c) P4VP.
of pyridine groups of P4VP. These results confirm the success of
the synthesis of HPC-g-P4VP.
The success of the complexing of ꢀOs(bpy)2 onto the HPC-
g-P4VP graft copolymers was characterized by UVꢀvis absorption
spectra. The content of Os in the complex graft copolymers was
determined by inductively coupled plasma (ICP) atomic emis-
sion spectroscopy, and the results are listed in Table 1. Figure 3
shows the UVꢀvis absorption spectra of HPC-g-P4VP-Os(bpy),
HPC-g-P4VP, and Os(bpy)2Cl2. They exhibit the characteristic
metal-to-ligand charge-transfer (MLCT) bands in the visible
region.50 After [Os(bpy)2]2+ complexes with the pyridine groups
of P4VP side chains of the cellulose graft copolymer, the bands
move to lower wavenumber. Two kinds of HPC-g-P4VP copoly-
mers with different branch length were used to function to
osmium. As shown in Table 1, samples A1 and A2 have a similar
content of P4VP (6.76 repeat units of 4VP for every glucose
ring in cellulose) and different contents of Os. The different
amounts of the Os complex bound to the branch chains of P4VP
were obtained by changing the mole ratio of HPC-g-P4VP and
Os(bpy)2Cl2. Figure 4 shows the hydrodynamic radius (Rh)
distribution of HPC-g-P4VP-Os(bpy) and HPC-g-P4VP in aque-
ous solution. The result indicates that the ÆRhæ of the HPC-
g-P4VP graft copolymer is about 50 nm. After HPC-g-P4VP
copolymer is functionalized with ꢀOs(bpy)2 redox units, the
ÆRhæ of HPC-g-P4VP-Os(bpy) is about 92 nm. This is because
the pendant redox unit ꢀOs(bpy)2 is hydrophobic, which may
cause the association of the graft copolymers.
Temperature and pH Response. HPC is a thermo-sensitive
polymer with the LCST at around 45 ꢀC. It can form metastable
nanoparticle aggregates in aqueous solution via chain association
at only a couple of degrees higher than LCST.51 Figure 5 repre-
sents the transmittance as a function of temperature of HPC,
HPC-g-P4VP, and HPC-g-P4VP-Os(bpy) in water. The results
show that for the graft copolymer HPC-g-P4VP6.76, which has
high content of P4VP, the transmittance of the solution is
independent of the temperature. This is because the properties
of the HPC backbones have been changed by the high graft density
and P4VP content. For the graft copolymer with relatively lower
P4VP content, HPC-g-P4VP2.07, the transmittance of the solu-
tion starts to decrease at about 50 ꢀC, and the solution becomes
opaque during the process. The reason is that when the temper-
ature of the solution is above its LCST, the HPC backbones
Figure 1. 1H NMR spectra of HPC-g-P4VP and HPC in DMSO-d6
solvent.
thermal-sensitive polymer with the LCST of 45 ꢀC, the graft
copolymerization is carried out at 35 ꢀC to keep the good solu-
bility of HPC in aqueous solution. The content of the P4VP in
the HPC-g-P4VP graft copolymers was adjusted by varying the
concentration of the initiator CAN and monomer 4VP in the
reaction system as well as the reaction time (details are available
in the Supporting Information). The graft copolymers are puri-
fied by dialysis against water to remove the free P4VP and un-
reacted monomers. Next, the HPC-g-P4VP-Os(bpy) is synthe-
sized by complexation between the nitrogen in the pyridine ring
of the P4VP side chains and osmium-bipyridine.
1
Figure 1 shows the H NMR spectra of HPC and HPC-
g-P4VP. The multiple peaks around δ = 3.0ꢀ5.0 ppm and the
single peak at δ = 1.1 ppm are attributed to the protons of glucose
rings and methyl protons of HPC, respectively. The new charac-
teristic chemical shifts at about 8.3, 6.6, and 1.5 ppm correspond
to the protons of P4VP. On the FT-IR spectra (Figure 2), the
bands at 3439 and 1083 cmꢀ1 are attributed to the stretching
of ꢀOH and CꢀOꢀC in glucose rings of HPC. The prominent
additional bands at 1597 and 1556 cmꢀ1 are due to the stretching
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become hydrophobic and collapse to form a core that stabilized
with hydrophilic P4VP side chains at the surface of the HPC
aggregation. The opaque solution is stable and turns transparent
again when the temperature of the solution decreases to a tem-
perature lower than its cloud point, for example, room tempera-
ture. From the curve of the transmittance versus the temperature,
the cloud point of the HPC-g-P4VP2.07 can be estimated at about
55 ꢀC, which is higher than that of HPC. The increase of cloud
point of the graft copolymer may be due to the higher hydrophilic
content in the graft copolymer. The thermal sensitivity of the
HPC-g-P4VP graft copolymers has been discussed in detail in
previous work, which is because the HPC backbone collapses to
form the core of the micelles that stabilized with P4VP in the shell
of the micelles.32 The graft copolymer HPC-g-P4VP2.07 func-
tionalized with ꢀOs(bpy)2 redox units also shows thermal sensi-
tivity with a cloud point at about 54 ꢀC, as that indicated the
transmittance as a function of temperature (Figure 5). Above the
LCST, the graft copolymer can assemble into micelles with a
diameter around 100 nm (Figure 6).
separation ΔEp= 59/n mV (n = 1, the number of electrons in a
single electro transfer acts characteristic of the Os-complex
electrochemistry55) at potential scan rate v = 100 mV/s. More-
over, the peaks on the cyclic voltammograms are stable for many
potential cycles and the washing of the electrode. However,
Ip is proportional to the square root of the potential scan rate
(Figure 8b), different from the theoretical prediction that Ip
P4VP is a weak polyelectrolyte (pKa ≈ 5) and becomes hydro-
phobic and precipitates from aqueous solution at pH > 5 due
to deprotonation of nitrogen atoms in the pyridine rings.52,53
In case of HPC-g-P4VP graft copolymers, the hydrophilic HPC
backbone is expected to stabilize the P4VP side chains to aggre-
gate, preventing precipitation from the aqueous solution at pH > 5.
HPC-g-P4VP6.76-Os(bpy) also shows a pH-sensitive property.
Figure 7 is the transmittance of the HPC-g-P4VP6.76-Os(bpy)
solution in water at different pH values. It can be seen that the
transmittance of solution decreases constantly with the increase
in the pH values until the graft copolymer precipitates at about
pH 13. It cannot assemble into stable micelles after increasing the
pH values, forming aggregations. The result is different from
previous work, in which the HPC-g-P4VP graft copolymers syn-
thesized by ATRP can form stable micelles at higher pH values.32
This may be attributed to that the pKa of the HPC-g-P4VP graft
copolymers depends on the architecture (block or graft) and the
length of P4VP chains.54 The deprotonation of pyridine groups
in HPC-g-P4VP6.76-Os(bpy) with the pH value increasing makes
the macromolecules more hydrophobic and aggregate.
Figure 3. UVꢀvis absorption spectra of HPC-g-P4VP-Os(bpy), HPC-
g-P4VP, and Os(bpy)2Cl2 in methanol.
Electrochemical Response. The electrochemical properties
of the redox-functionalized cellulose graft copolymer modified
electrode were studied by cyclic voltammetry. The results indi-
cate that samples A1 and A2 have electrochemical responsive
properties. However, sample B1 does not show electrochemical
responsive properties. Figure 8a shows cyclic voltammograms of
the electrode coated with sample A1 at different potential scan
rates at pH 7.4. The results indicate that the cyclic voltammo-
grams are the reversible electrochemical process at low potential
scanrates. The standard potentialEꢀ = 0.333 V, with peak-to-peak
Figure 4. Size distributions of HPC-g-P4VP and HPC-g-P4VP-Os-
(bpy) in aqueous solution at 25 ꢀC. The concentration is 1 mg/mL.
Table 1. Summary of Characteristic Data of the Graft Copolymers HPC-g-P4VP-Os(bpy)
repeat unit of P4VP
HPC-g-P4VP-Os(bpy)
Mn (g/mol)a
PDI
EAb
NMRc
content of Os (μmol/mg)d
4VP/Os (mole ratio)
A1
A2
B1
7.45 ꢁ 105
7.45 ꢁ 105
9.47 ꢁ 104
1.30
1.30
3.34
3.24
3.24
2.52
6.76
6.76
2.07
0.54e
1.40f
0.37
11.85
4.55
1.02
a GPC analysis of HPC-g-P4VP. b The calculated results from CHN elemental analysis, representing the repeat units of branch P4VP as compared to
every glucose ring. c The calculated results from 1H NMR analysis, representing the repeat units of branch P4VP as compared to every glucose ring. d The
content of Os element by ICP characterized. e The reaction ratio of HPC-g-P4VP and Os(bpy)2Cl2 is 2/1 (w/w). f The reaction ratio of HPC-g-P4VP
and Os(bpy)2Cl2 is 1/1 (w/w).
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Figure 5. The transmittance against temperature curves for HPC-
g-P4VP-Os(bpy), HPC-g-P4VP copolymers, and HPC.
Figure 8. (a) Cyclic voltammograms of the HPC-g-P4VP-Os(bpy)
modified electrode obtained upon application of different potential scan
rates;. (b) The peak current and peak-to-peak separation dependence on
the square-root of the potential scan rate. The measurements were
performed in 0.1 M phosphate buffer, pH 7.4.
Figure 6. TEM images of micelles of HPC-g-P4VP2.07-Os(bpy).
complex with P4VP bound to an electrode.58 The surface
coverage of the electrode with the redox species, Γ = 2.2 ꢁ
10ꢀ10 mol/cm2 (n = 1), by integrating the peak on the cyclic
voltammogram was recorded at a low scan rate, v = 100 mV/s.
The redox property of the graft copolymer HPC-g-P4VP-
Os(bpy) (sample A1) is also sensitive to pH. Figure 9a shows the
cyclic voltammograms of the HPC-g-P4VP-Os(bpy) modified
electrode with the background solution at different pH values.
The cyclic voltammograms indicate that the peak drops at pH 7.0
and becomes weaker with the further increase in pH value until it
almost disappears at pH 14.0 (Figure 9b). The ꢀOs(bpy)2
pendant redox groups are subjected to the reductionꢀoxidation
process without the participation of protons in the reaction, which
suggests that the pH value has not affected the redox potential
theoretically. It should be noted that the mole ratio of P4VP to
Os is 11.85, which means that large amounts of the pyridine ring
left without ligand with Os in the HPC-g-P4VP-Os(bpy) copo-
lymer. Therefore, the dependence of the redox properties of the
HPC-g-P4VP-Os(bpy) copolymer on pH is attributed to the de-
protonations of the pyridine groups, which leads to the collapse
of the graft polymer at higher pH values. As a result, the graft
copolymer loses the quasi-diffusion flexibility, and the electrochemical
Figure 7. The transmittance against pH curves for HPC-g-P4VP6.76
Os(bpy).
-
linearly increases with the elevated potential scan rate for
the surface-confined electrochemical process. Such relation-
ship suggests the quasi-diffusional translocation of the redox
units bound to the flexible polymer chains on the electrode
surface. This phenomenon has also been observed for ferro-
cene groups linked to the flexible polymer56,57 or osmium
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Figure 9. (a) Cyclic voltammograms of the HPC-g-P4VP-Os(bpy)
modified electrode at different pH values of the background solution.
(b) Peak current dependence on the pH value of the background
solution. The measurements were performed in 0.1 M phosphate buffer
with the potential scan rate, 100 mV/s.
Figure 11. Cyclic voltammograms corresponding to the oxidation of
glucose catalyzed by glucose oxidase (GOX), 0.014 mM. The measure-
ments were performed in 0.1 M phosphate buffer (pH 7.0) with the
potential scan rate, 50 mV/s.
Figure 10. (a) Cyclic voltammograms of the HPC-g-P4VP-Os(bpy)
modified electrode obtained upon stepwise measurements performed in
pH 5.56 and pH 10 solution. (b) Reversible switching of the HPC-
g-P4VP-Os(bpy) modified electrode activity. The measurements were
performed in 0.1 M phosphate buffer with the potential scan rate,
100 mV/s.
Figure 12. Amperometric response of the GOX/HPC-g-P4VP-Os-
(bpy) biosensor to different concentrations of glucose in PBS (0.1 M,
pH 7.0) at an applied potential of 0.4 V. Inset: Calibrated curve of the
biosensor with successive addition of glucose.
activity of the redox groups is reduced. It has been reported that
the electrode modified with P4VP-Os-complex redox units shows
pH dependence property, and the redox peak disappears on the
cyclic voltammogram curve at pH > 6. Specifically, our system
can extend to high pH values, such as 11 and 13, at which the
cyclic voltammograms of the HPC-g-P4VP-Os(bpy) modified
electrode can still be measured (Figure 9). This is due to the
specific architecture of the HPC-g-P4VP graft copolymers. With
the increase in the pH value even above the pKa of P4VP, the
HPC backbone is still hydrophilic, which provides the channels
for the electrochemical reaction. Our system extends the applica-
tions of the P4VP-based redox system.
The above-mentioned pH dependence of the redox property
ofHPC-g-P4VP-Os(bpy)graftcopolymersisreversible. Figure10
shows the electrochemical experiments performed in the solu-
tion at pH values of 5.6 and 10 at intervals. The cyclic voltam-
mograms of the HPC-g-P4VP-Os(bpy) modified electrode
show switchable property and are reproducible for several circles.
At low pH (e.g., 5.6), the electrochemical activity of the HPC-
g-P4VP-Os(bpy) modified electrode is better than that at higher
pH (e.g., 10), which is due to that the P4VP side chains collapse
and are “frozen” and the electrochemical activity of the electrode
is reduced as a result. However, the hydrophiphlic property of the
backbone still keeps the electrochemical process.
Application for Glucose Detection. Figure 11 shows the
cyclic voltammograms of HPC-g-P4VP-Os(bpy) modified elec-
trode in the solutions with and without glucose. The oxidation
of glucose is catalyzed by glucose oxidase (GOx). It is shown that
the anodic electrocatalytic current corresponding to the oxida-
tion of glucose has been biocatalyzed by GOx and mediated by HPC-
g-P4VP-Os(bpy) graft copolymer. Figure 12 shows the ampero-
metric response of the GOX/HPC-g-P4VP-Os(bpy) biosensor to
different concentrations of glucose in PBS (0.1 M, pH 7.0) at an
applied potential of 0.4 V. The results indicate that the biosensor
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is sensitive to the concentration of glucose and the current obvi-
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The corresponding calibration curve of the biosensor is shown in
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S
Supporting Information. Details of the experimental
b
parameters and their effect on the content of the graft P4VP
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lable free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Tel.: +8610-82618573. Fax: +8610-62554670. E-mail: rgliu@
iccas.ac.cn (R.L.); yhuang@iccas.ac.cn (Y.H.).
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’ ACKNOWLEDGMENT
The financial support of the National Natural Science Foun-
dation of China (Grant nos. 50821062, 20974114, 21174156) is
greatly appreciated. We also wish to thank Ping Yu for electro-
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