Sun Kil Kang

In vivo calibration of the subcutaneous amperometric glucose sensors using a non-enzyme electrode.

A new two-point calibration method for the subcutaneous amperometric continuous glucose sensor is reported. The proposed method is based on direct measurement of the background current (I(o)) using a non-enzyme electrode. For in vivo test, three electrodes were implanted in rabbits. Two of the three were identical needle-type enzyme electrodes with perfluorinated polymer outer layers (Pt/enzyme layer/Kel-F/PTFE/Kel-F/Nafion) that were placed in subcutaneous tissue and in a vessel (ear artery), respectively. And one non-enzyme electrode with exactly the same membrane composition as those of other two was in the subcutaneous layer to measure the background current. Implantation in the subcutaneous layer generated many crevices on the protecting layers of the electrodes. The signals from enzyme electrodes were effectively corrected by the measured background current from the non-enzyme electrode. In addition, a telemetric monitoring system was developed and evaluated for in vivo continuous glucose monitoring in order to alleviate the problems of motion artifact.

Electrochemical behavior of calix[4]arenediquinones and their cation binding properties

Abstract The redox and cation-binding properties of three quinone-functionalized calix[4]arenes have been studied in acetonitrile. Cation-bound calix[4]arenediquinone produces a new set of redox peaks at positively shifted potentials. Thus, the association constant between cation and calix[4]arenediquinone is found to be enhanced by the electrochemical reduction of the receptor molecule. The enhancement is dependent on the charge and size of cations. The magnitude of the potential shift can be explained on the basis of stabilization due to the transition of binding characteristics from ion-dipole to ion-ion interaction by an electron transfer reaction. Substituents in the lower rim exert considerable influence on the redox behavior of the complex. NH + 4 produces extraordinary voltammetric behavior due to the fact that only NH + 4 can form hydrogen bondings with calix[4]arenes, in contrast to other metal ions.

Reproducible fabrication of miniaturized glucose sensors: preparation of sensing membranes for continuous monitoring.

The immobilization process of glucose oxidase(GOx) in the poly(1,3-diaminobenzene) (poly(1,3-DAB)) network was closely investigated in situ using an electrochemical quartz crystal microbalance(EQCM). GOx captured in approximately 50 nm thick poly-1,3-DAB layer causes a 514 Hz frequency increase, corresponding to 541 ng, and distributes mostly in the outer part of the polymer film. The presence of poly-L-lysine and glutaraldehyde during electropolymerization of poly(1,3-DAB) improves sensitivity by raising the amount of GOx immobilized. Adding a protective membrane on to the enzyme layer from poly(tetrafluoroethylene) (PTFE) dispersed in aqueous media lets the entire fabrication procedure finish perfectly without nonaqueous solvent. The finalized needle-type glucose sensors show competent functions in sensitivity, stability, biocompatibility, lifetime, interference and reproducibility.

Electrochemical recognition of Ca2+ ion in basic aqueous media using quinone-derivatized calix[4]arene

The voltammetric study on a water-soluble calix[4]arene (calix[4]arene-triacid-monoquinone, CTA) in basic aqueous solution in the presence of Ca 2 ion provided important information about the unique electrochemical behavior of Ca 2 ‐CTA complex. The redox behavior of CTA and voltammetric responses to Ca 2 ion are reminiscent of those of quinone-derivatized calix[4]arenes in aprotic media. Using CTA, Ca 2 ion in aqueous solution could be recognized quantitatively by voltammetric techniques.

Interaction between various alkylammonium ions and quinone-derivatized calix[4]arenes in aprotic media☆

The complexation and electrochemical behavior of redox-dependent ionophores in the presence of various alkylammonium ions were examined with calix[4]arenediquinone compounds. In spite of the small ligating pore, semi-empirical calculation and 1 H NMR spectra showed that calix[4]arenediquinones were able to form complexes with protonated amine-type guests by virtue of three or four hydrogen bonds between the guest and carbonyl oxygen atoms in the host molecules. As a result of encapsulation, the structure of the calixarenes underwent a large conformational change, especially in the lower rim, which was rearranged to provide a symmetrically tetragonal binding environment. In addition, the reduction potential of the quinone moiety shifted to the positive direction. The magnitude of the potential shift depended on the strength of the hydrogen bond with the quinone. Therefore, the acidity and capability of forming multiple hydrogen bonds were predominant factors among other properties of the guest. The cooperative shortening effect of neighboring hydrogen bonds between the ammonium ion and carbonyl oxygen atoms produced an extra enhancement by making the redox center easily reducible and fast proton transfer possible.

Selective electrochemical recognition of ions in solution and at self-assembled monolayers

Quinone-functionalized calix[4]arenes having carboxylic acid groups or thiol groups were prepared and their spontaneous adsorption on silver and gold surfaces, respectively, was studied. Since the cavity-like structure of calixarenes was immobilized on the noble metal electrodes, they exhibited a selective affinity towards specific hard metal ions in aqueous media. Voltammetric and spectroscopic studies showed the well-ordered deposition of organic receptors and entrapment of metal ions. It also was found that the repeated capture and removal of metal ions reversibly with chelating agents such as ethylenediaminetetraacetic acid (EDTA) was possible. This is the first example, to our knowledge, of voltammetric detection of hard metal ions in aqueous media using a chemically modified electrode with redox-active macrocyclic receptors.

Modulation of Quinone PCET Reaction by Ca2+ Ion Captured by Calix[4]quinone in Water

Calix[4]arene-triacid-monoquinone (CTAQ), a quinone-containing water-soluble ionophore, was utilized to investigate how proton-coupled electron transfer (PCET) reactions of quinones were influenced by redox-inactive metal ions in aqueous environment. This ionophoric quinone derivative captured a Ca(2+) ion that drastically altered the voltammetric behavior of quinone, showing a characteristic response to pH and unique redox wave separation. Spectroelectrochemistry verified significant stabilization of the semiquinone, and electrocatalytic currents were observed in the presence of Ca(2+)-free CTAQ. Using digital simulation of cyclic voltammograms to clarify how the thermodynamic properties of quinones were altered, a simple scheme was proposed that successfully accounted for all the observations. The change induced by Ca(2+) complexation was explained on the basis of the combined effects of the electrostatic influence of the captured metal ion and hydrogen bonding of water molecules with the support of DFT calculation.