Evaluation of Photoelectrochemical Glutathione Sensors Using of Electrochemical Impedance Spectroscopy
Authors: Katagishi Kensuke , Kuragano Asako , Terashima Chiaki , Suzuki Norihiro , Nakata Kazuya , Katsumata K. , Hoshi Yoshinao , Shitanda Isao , Kondo T. , Yuasa M. , Itagaki Masayuki , Fujishima Akira 
Published: February 2020

Human blood contains a high concentration of glutathione (GSH), which is an antioxidant component. The higher the concentration of the in-vivo GSH, the higher a risk of cancer. Therefore, it is important to detect the GSH concentration of the high density region. Photoelectrochemical sensors using photocatalyst can be kept low applied potential from becoming a source of energy from light and electricity. Therefore, high sensitivity sensing can be expected. According to the previous report, it was confirmed that a hybrid Au-TiO2 photocatalyst material can detect GSH. In this study, deeper studies were carried out in order to design a smarter and more sensitive GSH sensor. This is the method for manufacturing the electrode of the present study. Au-TiO2 thin film was prepared by two steps method. First, a mixture of titanium (IV) bis (ethyl acetoacetato) diisopropoxide and 1-butanol was spin-coated on FTO glass. Second, the coated FTO glass was calcined at 450 °C for 1 h. After cooling to room temperature, a solution of gold chloride dissolved in PVP, ethanol, and acetic acid was deposited on the TiO2-modified FTO glass using electrospinning (ES) method, and heated at 500 °C for 2 h.
The physical properties of prepared film were evaluated by X-ray diffraction (XRD) and diffuse reflectance spectrum analysis. XRD was let found to be the titanium oxide of anatase type. In addition, the diffuse reflection showed an increase in the peak of gold due to the increased amount of gold (about 570 - 580 nm).
The morphology of TiO2 is found to be fiber structure in our study. To reproducibly deposite titanium oxide film of the fiber is required to have humidity control, and it was possible to produce fiber membranes by increasing the humidity. This is because the hydrolysis tends to occur by high humidity.
This study was performed on the impedance measurement using an electrolyte prepared by adding 0.5 mM of GSH in phosphate buffered saline (PBS). In this research, reference electrode is Ag/AgCl, counter electrode is platinum and the light source is a solar simulator (1 SUN).
By changing the ES method, different samples of Au-TiO2 on FTO was prepared and electrochemical properties were measured. Equivalent circuit was designed in this experimental system. Charge transfer resistance and space charge layer capacitance between electrolyte interface with Au-TiO2 electrode were studied and fitted to the experimental results. The measurement results, loading of Au nanoparticles was decreased charge transfer resistance and increased space charge layer capacity (Fig.1). This indicates that carrier density in TiO2 would increase, which was further confirmed by Mott-Schottky plot. Therefore, recombination of exited electrons of TiO2 can be suppressed by modifying with Au nanoparticles.
Furethermore, we measured chronoamperometry in order to know the concentration range that can be put to practical use as a sensor. GSH was then detected by the electrode (Au 0.1 g) and detection level is found to be in the range of 1 - 5 mM (Fig.2).
From the above result, the fiber morphology as well as the loading of Au nanoparticles enable high-sensitivity sensing by using an electrode of Au 0.1 g, leading to the determination at a high concentration range.
Reference
1) A. Devadoss et al., J. Mater. Chem B, 2016, 4, 220-228.

Figure 1