Fast Electron Transfer Reaction of Azurin Fixed on the Self-Assembled Monolayer Gold Nanoparticle Electrode in Hydrated Ionic Liquids
Authors: Fujita Kyoko , Kuwahara Jun , Nakamura Nobuhumi , Ohno Hiroyuki 
Published: February 2020

Ionic liquids (ILs), which are composed of ions and have melting points below 100 °C, have attracted much attention as solvents because of their fascinating features, such as negligible vapor pressure and high thermal, chemical, and electrochemical activity. ILs have been widely studied, not only in organic synthesis and electrochemistry, but also in bio-applications, such as biocatalytic reactions, biosensors, and protein stabilization. We have been reported the biocatalytic processes of enzymes in a hydrated IL, hydrated cholinium dihydrogen phosphate (hydrated [ch][dhp]). [1] Proteins dissolved in hydrated [ch][dhp] were shown to have long term stability and improved thermal stability compared with that in buffer solution. However, when the biocatalytic reaction was detected as an electrochemical response, the observed redox current was smaller than that in buffer solution. [2] In this work, the redox reaction rate between a fixed protein and electrode was investigated to clarify the reason for the depression of the redox response of biocatalytic reactions in hydrated [ch][dhp]. [3]
Azurin was selected as a target protein that realizes direct electron transfer to electrodes. The redox reaction rate between the electrode and the protein, which was fixed on the self-assembled monolayer (SAM)-modified electrode, was analyzed in hydrated [ch][dhp]. Mixed SAM-modified gold nanoparticles (AuNP) electrodes were prepared by immersing the AuNP electrodes into mixed ethanol solutions of thiols with head group H3C–:HO– of 1:1 mole ratio. Azurin was fixed on the SAM-modified AuNP electrodes by soaking.
The cyclic voltammograms obtained for azurin fixed on the SAM-AuNP electrode in ammonium acetate buffer and hydrated [ch][dhp] showed reversible responses. When the redox response was evaluated in other hydrated ILs, 1-butyl-3-methylimidazolium tetrafluoroborate, and 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, weak and late responses were observed only for short times. A liner relationship between the redox current and the scan rate was observed in ammonium acetate buffer and in hydrated [ch][dhp]. When the peak separation was analyzed at different scan rates, it was larger in ammonium acetate buffer than in hydrated [ch][dhp], especially at high scan rates. The electron transfer rate constant between azurin and the electrode in ammonium acetate buffer and hydrated [ch][dhp] were k
s = 44 s-1 and 202 s-1, respectively. It is revealed that the electron transfer rate between azurin and electrode was fast in hydrated [ch][dhp] as same that in buffer, though a slight shrinkage of azurin fixed on the SAM-AuNP electrode was suggested as one of the factors for the small differences in behavior at the electrode surface in buffer and in hydrated [ch][dhp].
Reference
[1] K. Fujita, H. Ohno, Biopolymers, 93, 1093, 2010
[2] K. Fujita et al., Electrochimica Acta, 56, 7224, 2011
[3] K. Fujita et al., Journal of Electrochemical Society, 163, G79, 2016