The acid-base and redox properties of the menaquinones MK-4, MK-7, and MK-9 (vitamin K 2) have been studied in DMPC monolayers on mercury electrodes. The monolayers were prepared by adhesion-spreading of menaquinone-spiked DMPC liposomes on a stationary mercury drop electrode. All three menaquinones possess pK a constants outside the experimentally accessible range, i.e., they are higher than about 12. The standard potentials of MK-4, MK-7, and MK-9 in the DMPC monolayers are very similar, i.e., 0.
The effects of the chemical environment of menaquinones (all-trans MK-4 and all-trans MK-7) incorporated in lipid monolayers on mercury electrodes have been studied with respect to the thermodynamics and kinetics of their electrochemistry. The chemical environment relates to the composition of lipid films as well as the adjacent aqueous phase. It could be shown that the addition of all-trans MK-4 to TMCL does not change the phase transition temperatures of TMCL. In case of DMPC monolayers, the presence of cholesterol has no effect on the thermodynamics (formal redox potentials) of all-trans MK-7, but the kinetics are affected. Addition of an inert electrolyte (sodium perchlorate; change of ionic strength) to the aqueous phase shifts the redox potentials of all-trans MK-7 only slightly. The formal redox potentials of all-trans MK-4 were determined in TMCL and nCL monolayers and found to be higher in nCL monolayers than in TMCL monolayers. The apparent electron transfer rate constants, transfer coefficients and activation energies of all-trans MK-4 in cardiolipins have been also determined. Most surprisingly, the apparent electron transfer rate constants of all-trans MK-4 exhibit an opposite pH dependence for TMCL and nCL films: the rate constants increase in TMCL films with increasing pH, but in nCL films they increase with decreasing pH. This study is a contribution to understand environmental effects on the redox properties of membrane bond redox systems.
Graphical abstract
In this work, we have reported glucose oxidase incorporated carbon felt bioelectrodes (GOx/CFE) as biocapacitors for energy storage. Glucose oxidase (GOx) was incorporated into a carbon felt electrode (CFE) and the electrode was characterized using X-ray diffraction, scanning electron microscope and Fourier transform infrared spectroscopy. As a result, it was found that GOx was successfully incorporated into a bare CFE and enhances the specific capacitance of the electrode and it was stable up to 500 charge-discharge cycles. Consequently, it was observed that GOx/CFE exhibits enhanced energy storage capacitance compared to that of pristine carbon felt. The capacitance of GOx/CFE is found to be 4.21 mF cm −2 (23 F g −1) while the bare CFE shows 3.68 mF cm −2 in a phosphate buffer solution (pH = 7.0). Albeit the capacitance values are small compared to conventional supercapacitors, the utility of these biocapacitors is expected to have a significant impact on glucose monitoring. Columbic efficiency obtained with the GOx/CFE matrix is 89%, and the electrode is stable up to 225 cycles with 100% retention of capacitance. After 225 cycles, the electrode loses the capacitance up to 12% retaining the capacitance of 88% up to 500 cycles. Cyclic voltammetric studies revealed that GOx/CFE is capable of energy storage with a 200 µA higher capacitive loop than the bare CFE at a scan rate of 10 mV s −1. Electrochemical impedance analysis measurements also confirmed that GOx/CFE possess minimum resistivity. Moreover, it is very eco-friendly due to which unwanted pollution can be avoided. From the proposed matrix, it is believed that a green, eco-friendly, clean, renewable material for energy storage could be realized.
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