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.
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.
(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.
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.
(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.
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.
(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.