Redox Behavior of the Hexacyanoferrate Complex Immobilized into an Ionic Liquid-Modified Electrode By Using Seiras Measurement
Authors: Kitagawa Tatsuya , Inomata Tomohiko , Ozawa Tomohiro , Masuda Hideki , Motobayashi Kenta , Osawa Masatoshi 
Published: February 2020

Self-assembled monolayer (SAM) is a useful technique to modify a functional compound on a surface. However, the compounds modified on a surface often change their chemical properties. We have recently studied an ionic liquid (IL)-modified surface as a new technique to immobilize a functional compound on a substrate surface and as a material providing a novel reaction field on the surface, instead of the typical SAM method. IL is an ionic compound being in the liquid state at room temperature and a designable compound. Previously, we modified a bulky phosphonium-type IL onto a Au electrode, and succeeded in entrapping three types of metal complexes into the vacant space formed with the IL molecules without any chemical bindings, regardless of the net charges of the complexes.1) The immobilization of the complexes into the surfaces were confirmed with the electrochemical measurements, which indicated that the electrochemical properties of the complexes were similar to those in homogeneous condition. However, we did not have any structural information about the surfaces. In order to investigate the structure of the compounds modified on the surface, the surface enhanced infra-red absorption spectroscopy (SEIRAS) measurement was applied on the IL modified electrode. The SEIRAS provides an enhanced IR absorption intensity by 10-100 times stronger than that of IR-RAS measurement and simultaneous electrochemical information.2) Therefore it is expected that the redox behavior of the compound entrapped on the surface can be observed spectroscopically. In this report, we will discuss the immobilization of a hexacyanoferrate (Fe(CN)6]3−/4−) complex into the IL-modified electrode and the redox behavior by using a SEIRAS measurement.
We prepared a chemical deposited Au thin film on a Si prism as a working electrode. According to our previous report,1) the phosphonium type IL with a disulfide group has been modified on the Au electrode (IL/Au). The [Fe(CN)6]3−/4− complex was immobilized into the IL/Au by using CV measurement in the complex solution (Fe@IL/Au). The SEIRAS measurement of the IL/Au has performed during the electrochemical measurement. The IR band intensity assignable to C≡N group in the complex was increased gradually during the immobilization. After the immobilization, the redox potential of the complex immobilized on the surface has been observed at 0.1 V vs. Ag/AgCl. This redox potential is almost equal to that in an aqueous solution. The SEIRAS measurement of Fe@IL/Au was also performed during the electrochemical measurement. These results indicate that the environment of C≡N groups in Fe@IL/Au are different from the homogeneous conditions, suggesting that the coordinated C≡N groups statically interact to the cation parts of IL modified on the surface.
1) T. Kitagawa et al., Chem.
Commun., 2013, 49, 10184-10186.
2) M. Osawa, et al., Appl. Spectroscopy., 1993, 47, 1497-1502.
This study was supported by the Cooperative Research Program of Catalysis Research Center, Hokkaido University.

Figure 1