The potentiality and limitations of electrochemical impedance spectroscopic methods for molecular film applications

Author(s)

Beatriz Lucas Garrote

• Document type: Master's Dissertation
• Press: Araraquara. 2018-04-16.
• Institution: Universidade Estadual Paulista (Unesp). Instituto de Química. Araraquara
• Defense date: 2018-03-27
• Advisor: Flávio Cesar Bedatty; Paulo Robero Bueno

Abstract

Biomedicine research is directing its effort to achieve fast, simple, point–of–care analytical devices for disease diagnostic. Label-free electrochemical biosensors based on electrochemical impedance spectroscopy (EIS) are an interesting tool for this purpose. For analytical objective, EIS is more sensitive than other electrochemical techniques, such as amperometric, potentiometric or voltammetry. EIS measures the complex resistance of the system. In faradaic EIS system, variations on the electrode surface are monitored by the charge transfer resistance (Rct), so correlations between the variation in Rct value and the concentration of a specific target could be made. Thus, EIS is a widely used technique for biosensing. Here, an EIS based system for the detection of the protein interleukin-6 (IL-6) was proposed. The working electrode was modified with a self-assembled monolayer (SAM) of alkanethiols, in which the biological receptor (Ab IL-6) was covalently immobilized. During the development of the system experimental problems compromised the analytical response. It was analysed every step of the biosensor construction. It was concluded that defects or pinholes on the SAM were the responsible for the absence of response, since EIS is a technique based on charge transfer and the presence of pinholes on the self-assembled monolayer could allow to free ionic and/or electronic migration. Because of this, it was proposed a second system for IL-6 detection based on electrochemical capacitance spectroscopy (ECS) and using a redox peptide SAM. Again, the sensitive of the system was not high enough to the protein detection. According to previous work published by our group, it was possible to correlate the sensitivity of the system with the molecular weight of the biological receptor and the specific target. It was shown that systems with bigger specific target and similar receptors resulted in more sensitive devices. Moreover, it was observed that system with small receptors and bigger targets showed higher sensitivity. IL-6 is a 26 kDa protein and is the smaller protein tested in an ECS based system by our group, and the biological receptor of the system was an antibody (150 kDa), 5.8 times higher than the biological receptor. Thus, the absence of response of the ECS system was attributed to the small molecular weight of the IL-6 protein and the high difference in weight between the biological receptor and the target. At the same time, it was performed a preliminary electrochemical study of a system with a redox probe confined on the electrode surface and a second one in solution. The objective was study the signal amplifier behaviour observed when two redox probes are involved on the system and understand the electrochemical process. It was observed the variation from a capacitive-based system, with the confined redox probe, to an impedance one, when the redox probe in solution was present. The presence of two redox probes decreased the redox capacitance of the electroactive film and the resonance resistance of the electrons between the electrode surface and the film; moreover, increased the energy of the system and the frequency of relaxation, due to the increase of the electron diffusion with the solution. (AU)

• FAPESP's process: 15/17332-1 - Label Free Electrochemical Multiplexing Prostate Cancer Diagnostics
• Grantee: Beatriz Lucas Garrote
• Support Opportunities: Scholarships in Brazil - Master