Research Area
Polymer Chemistry
Electrochemistry
Materials Science
Physical Chemistry
Current Research Projects:
The main research area of our
group is concerned with the interdisciplinary region between polymer
chemistry and electrochemistry. General
objectives of the research projects are design, preparation,
and characterization of functional polymers, which intelligently
respond to and discriminate among external stimuli, then followed
by giving appropriate "answers".
Functional polymers frequently do not work by itself, but exhibit
their performance in a particular system based on a highly organized
molecular response. The design of molecular system is also an
important target of our research projects. We focus our attention
to the systems where the responses ("answers") of functional
polymers and their systems can be electrically (electrochemically)
detected. The following are current research projects of our research
group.
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(1) Ion (Ion/Electron) Conducting Polymers and Solid State Electrochemistry
Objectives:
Develop ion conducting and ion/electron mixed conducting polymers,
explore ion transport and electron transfer properties in relation
to dynamics of the polymers, and apply these materials to solid
electrochemical devices.
Exploit these polymers as solid state solvents of electrochemical
measurements, and establish methodology of the measurements using
ultramicroelectrodes (solid state voltammetry).
Spatially structured molecular assemblies and molecular electronic
devices.
Projects:
(a) Ion conducting polymers (polymer electrolytes) based on poly(glycidyl
ether) having short oligo(ethylene oxide) side chain ---- Explore
the possibility of fast ion transport in polymers coupled with
fast relaxation of the short side chains.
(b) New polymer electrolytes based on room temperature molten
salts ---- Certain pyridinium or imidazolium salts form room temperature
molten salts (ionic liquids) with soft anions. We found that polypyridinium
or polyimidazolium salts form complexes with these ionic liquids,
and that resulting complexes have extremely high ionic conductivity,
reaching 10-3-10-2 Scm-1 at room temperature, and film forming
property. Develop new combinations having high ionic conductivity
and explore structure and dynamics of the complexes.
(c) Ion/electron mixed conducting polymers ---- Copolymers consisting
of ion conducting monomer and redox monomer; for instance poly[vinyl
ferrocene-co-w-methoxy-oligo(ethylene oxide)methacrylate], exhibit
redox activity in the solid state (in the bulk), when these copolymers
are complexed with an electrolyte salt. The redox reaction is
caused by electron hopping (electron transfer reaction) between
the adjacent redox sites. Explore the effects of the redox site
concentration, temperature, electrolyte concentration, and structure
of redox monomer (electron self-exchange rate constant) on the
electron transfer rate, in relation to the dynamics of polymers.
(d) Establishment of the methodology of solid state voltammetry
and use it as a tool to investigate the diffusion and electron
transfer dynamics of small molecules or macromolecules in the
polymer bulk ---- Solid state voltammetry, which can be enabled
by the combined use of ion conducting polymers and ultramicroelectrodes,
allows to determine diffusion coefficients of small molecules
and macromolecules in ion conducting polymers(polymer melt, network
polymer, partially crystalline polymer). In certain conditions,
electron transfer rate constant can also be determined. Try to
use several kinds of microelectrodes (micro disk, lithographically
made band array, etc.) for solid state voltammetry, establish
theory of the measurements, and exploit as a tool to explore polymer
dynamics.
(2) Design of new (macro)molecular systems to construct molecular devices
Objectives:
Preparation of functional molecules (macromolecules) having both
sensor and detector moieties. In order to allow electrochemical
detection, the detector moieties are redox active.
Design molecular systems, where responses of the sensor moieties
to external stimuli induce some structural change of functional
(macro)molecules (amplifying process), and the change can be electrochemically
detected by using the detector (redox active) moieties.
Design inverse processes, where stimuli from electrodes change
redox states of the detector moieties, and this change induces
structural change of the functional molecules.
Construct molecular devices by exploiting the above molecular
systems: chemical or physical switches and sensors, for example.
Projects:
(a) Hydrogel-modified microelectrodes ---- Ultramicroelectrodes
chemically modified with hydrogels allows electrochemical measurements
of redox molecules inside the gels, when they are immersed in
solutions containing the redox molecules. By exploiting electrochemical
methods, independent determination of concentration and diffusivity
of the redox molecules inside the gels has been made possible.
A slight change of the external solutions, such as temperature,
polarity, pH, ionic strength, electric field, and concentration
of specific molecules, induces volume phase transition of hydrogels.
This transition greatly affects the electrochemical response,
via the changes in concentration and diffusivity of the redox
molecules. This methodology is used for the fundamental understanding
of static and dynamic behaviors of hydrogels, and further, provides
a way to convert the phase transition to electric signal.
(b) Redox molecule end-capped poly(ethylene oxide) ---- KOH catalyzed
anionic ring-opening polymerization of ethylene oxide in the presence
protic redox molecules, like ferrocenylmethanol and phenothiazine,
gives redox molecule end-capped poly(ethylene oxide)s. Poly(ethylene
oxide) can be a sensor moiety to external stimuli, and the end-capped
redox molecule can be a detector moiety. Poly(ethylene oxide)
chain has specific interactions to alkali metal ions, proton-donating
compounds such as ureas and thiophenols, and proton-donating polymers
such as poly(acrylic acid) and poly(methacrylic acid). When these
compounds are added to solutions of these modified poly(ethylene
oxide)s, the redox response is greatly affected by their addition.
Time-course of the complex formation can also be explored by monitoring
time-dependence of the redox response. These modified poly(ethylene
oxide)s can also be utilized for the study of polymer diffusion
in the melts.
(c) Photochromic/redox conjugated molecules ----- Functional molecules
having both photochromic and redox moieties are prepared to construct
molecular photo-switching systems. Electronic state change of
the redox moiety induced by the photochromic reaction causes the
change in the redox potential. If the electrode potential is kept
at the bi-stable potential, the electrolytic current is switched
on or off by the photochromism induced by photo-irradiation. This
molecular photo-switching system may be first realized in solutions,
and then in ion-conducting polymers.
(3) Biomedical Materials and Systems
Objectives:
Develop electrochemically controlled drug-delivery-system using
redox active micelles.
Explore antigen-antibody reaction electrochemically, and eventually
construct electrochemical immuno-assay system.
Develop enzymatic sensors using polymeric mediators.
Projects:
(a) Electrochemical control of drug release from micelles consisting
of redox active surfactants ---- A nonionic surfactant, ferrocenylalkyl
poly(ethylene oxide), forms micelles above the critical micellar
concentration. In this surfactant, ferrocenylalkyl group functions
as hydrophobic group, and poly(ethylene oxide) functions as hydrophilic
group. Hydrophobic drugs can be solubilized in the micelles. When
ferrocenyl group in the surfactant is oxidized to ferrocenium
ion, the micelles are broken up to monomers. Simultaneously, the
drugs dissolved in the micelles are released to the outside. The
effects of the balance and molecular weight of hydrophobic/hydrophilic
groups, and structures of incorporated drugs and redox active
group on the amount and position of the solubilized drugs, in
relation to their chemical structure, and the performance of controlled
release are explored.
(b) Electrochemical detection of antigen-antibody reaction ----
One end of the redox molecule end-capped poly(ethylene oxide),
described in (2)-(b), is -OH group. This functionality is used
for the chemical modification of the end-capped poly(ethylene
oxide) to antibody (IgG). In this bio-conjugate, IgG function
as a sensor to specific chemicals. When immuno-complexes are formed
by antigen-antibody reaction, molecular weight of the immuno-complexes
greatly increases, compared with that of the bio-conjugate itself.
The antigen-antibody reaction can, thus, be electrochemically
detected by measuring electrolytic current of the end-capped redox
molecule. Monochronal antibody will be used, and the construction
of electrochemical immuno-assay system is the final goal of this
project.
(c) Enzymatic sensors using polymeric mediators ---- After fully
understanding of electron transfer properties of some ion/electron
mixed conductors, described in (1)-(c), and redox molecule end-capped
poly(ethylene oxide), described in (2)-(b), these polymers are
used as polymeric mediators in enzymatic sensors, typically glucose
sensors. These polymers are mixed with or chemically attached
to enzymes, and the chemical reaction between the substrate and
enzyme is electrochemically detected.