1.1 Preface
A bio-detection system is involved with one or several detection techniques such as such as optical, electrochemical and mechanical detection methods and the correspondent equipment and material. Electrochemical techniques are advantageous compared with others within bio-detection methods due to its low-cost and easy-to-integrate features. There is also no need for complex signal transduction processes because the subsequent data acquisition unit after the sensing event can usually directly attain an electrical signal (A.J. Bard and Faulkner, 2000).
Electrochemical impedance spectroscopy (EIS) is a powerful electrochemical method for bio-analyte detection in affinity-based biosensors and can be integrated into miniaturized devices due to its label-free and sensitive characteristics (Daniels and Pourmand, 2007). This trait is attractive because electrodes can easily be fabricated into a miniaturized device which the sample usage can be lowered down to a few micro-liters.
Among various electrode configurations, interdigitated array (IDA) electrodes are symmetric electrodes and have the advantages of enhanced sensitivity, increased active area and miniaturized sizes (Mazlan et al., 2017). They have been widely used in various applications (Yan et al., 2011). Their steady-state current and concentration behaviour
have been extensively researched using analytical and numerical approaches (Aoki et al., 1988).
Aptamers are artificially synthesized DNA or RNA that can be selected in vitro. This is another growing field of interest for the affinitive and selective tendency towards their target (Hianik and Wang, 2009). Their robustness allows them to survive harsh conditions and remain functional over an extended period of time (Y. C. Lim et al., 2010). This important feature can contribute to the bio-detection system, permitting them to be immobilized as sensing elements on a biosensor chip (aptasensors), packed up in miniaturized chips and delivered to remote regions. Researches on aptasensors using IDA electrode chips along with EIS bio-detection has recently gained high popularity due to several combined advantages (Arya et al., 2018; Ding et al., 2017; Zhurauski et al., 2018).
These chips have a large potential in the development of portable, real-time, multiplex and smart device integrated sensing applications.
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1.2 Research Motivation
Novel sensing architectures are presented frequently without having a strong and powerful enough model in accordance to their detection methods. This may lead to valuable resources being eventually abandoned due to the lack of ability for its interpretation.
Taking EIS detection of IDA electrodes for example, the periodic-steady-state diffusion impedance of IDA electrodes has never been tackled using an analytical approach. A Warburg element is still often used in equivalent circuit modeling of EIS data.
However, it fails at low frequencies due to the arising steady-state current (Allen J. Bard et al., 1986; Niwa et al., 1990). In order to obtain a general method for interpreting EIS data using an IDA electrode, a model must be constructed to better evaluate the diffusion phenomenon exhibited. If that is achieved, then parameters such as wg (gap width) or we
(electrode bandwidth) can be accurately determined.
For miniaturization of electrochemical systems, symmetric electrode setups are often used due to its fabrication simplicity and high yield rate. The shape of EIS plots using these setups are known to be identical to a typical three electrode setup, and thus a single Randles circuit can be used for equivalent circuit fitting. However, to the best of the author’s knowledge, there hasn’t been any study that derives the relationship between
the parameters of the fitted data and the real parameters if a symmetric Randles circuit in series is used. In order to find the real parameters of a symmetric electrode setup using a single Randles circuit, a relationship must be established.
To sum up, the thesis is divided into two parts. The former part aims to develop a theory for finding a solution to model the diffusion impedance of IDA electrodes. The solution can be degraded for calculating the limiting current, and an equivalent circuit element is constructed for fitting the EIS data obtained. The latter part focuses on impedimetric aptasensing using symmetric electrodes. The parameter relationship between a single and symmetric Randles circuit is derived and verified, and is used in application for detection of thrombin and the tumor marker mucin-1 (MUC1).
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1.3 Research Aims
This research is dedicated to modeling IDA and other symmetric electrodes which can for be applied for impedimetric aptasensing of proteins including tumor markers. For derivation of the diffusion impedance of IDA electrodes, not only can the theory help researchers fit their data and obtain parameters more accurately, but also assist them understand the underlying phenomenon more correctly. For relationship establishment of parameters between a single Randles circuit and symmetric electrode setups, it can simplify the fitting model used for characterization. For fabrication of impedimetric aptasensor, it has label-free and simple advantages, and can be monitored during each stage. The individual objectives for this thesis are listed as below:
1. Solution derivation for the diffusion impedance of IDA electrodes.
2. Equivalent circuit fitting using the IDA diffusion element.
3. Relationship establishment of Randles circuit parameters between three and symmetric electrode setups.
4. Impedimetric aptasensor fabrication using symmetric SGEs.
5. Fabrication of a regenerable and target specific impedimetric aptasensor using IDA electrodes.
1.4 Research Framework
This thesis consists of theoretical and applied stages. Due to incomplete fundamentals for impedimetric biosensors using symmetric electrodes, the derivations for the IDA diffusion impedance and the symmetric equivalent circuit analyzing method is carried out. Moreover, the IDA diffusion impedance is firsthand derived, and is used to make complete the theory of symmetric equivalent circuits, which derives the relationship of Randles circuit parameters between three and two symmetric electrode setups. The theory thereafter is applied for MUC1 detection using an impedimetric aptasensor made from symmetric electrodes. Parameters of two and three electrode setups for MUC1 detection are compared to verify the derived relationship, and specificity between the aptamer and target is also confirmed using a two electrode setup. At last, the two theories are used for analysis of an impedimetric aptasensor fabricated on IDA chips for thrombin detection. The whole framework is architected as in Figure 1-1.
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Figure 1-1 The research framework for this thesis.