STRUCTURE-PROPERTY RELATIONSHIPS OF POLY(3,4-ETHYLENEDIOXYTHIOPHENE) (PEDOT) DERIVATIVES FOR BIOSENSING APPLICATIONS

Abstract

As one of the most well-studied conductive polymers (CPs), poly(3,4-ethylene dioxythiophene) (PEDOT) has been integrated into a wide variety of applications, including biosensors, energy storage devices, stretchable displays, and wearable electronics. PEDOT is of interest because of its outstanding electrochemical properties and chemical stability. PEDOT itself is relatively hydrophobic, limiting its utility in situations where it needs to have specific surface responses, such as in biosensors. However, it can be challenging to introduce the desired chemical functionality without sacrificing the electrochemical properties of PEDOT. In this work, functional derivatives of PEDOT were electrodeposited from monomer state into solid films, and the resulting polymers were characterized in detail. The insights obtained were used to design and construct impedimetric biosensors using immobilized recognition elements attached to the modified PEDOT. In Chapter 2, we examined the electrochemical properties of electrodeposited poly(2’-maleimideomethyl-3,4-ethylenedioxythiophene) (PEDOT-maleimide) and compared it to unmodified PEDOT. According to results of electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV), PEDOT-maleimide presented similar charge transport as well as charge storage capabilities as pristine PEDOT. Improved cyclic stability of the PEDOT-maleimide was also achieved. These results showed that PEDOT-maleimide was a promising functional PEDOT to work with for the development of applications requiring chemically active PEDOT. In Chapter 3, we examined the influence of cholesterol substitution on the properties of poly(3,4-ethylenedioxythiophene) (PEDOT-MA-cholesterol) electrodeposited from cholesterol-functionalized EDOT. Due to the insulating and bulky cholesterol side group, we found that there was a maximum charge density for electrodeposition at ~0.155 C cm-2. The inferior electrochemical properties of PEDOT-MA-cholesterol were realized through both EIS and CV. The measurement of the electrical conductivity gave a value of around 3.30 × 10-6 S cm-1 which is about six orders of magnitude lower than has been seen in PEDOT (~3 S cm−1). These inferior electronic and electrochemical properties exhibited from PEDOT-MA-cholesterol highlight damage imposed by introducing bulky and hydrophobic functional groups before the formation of conjugated backbones In Chapter 4, we focused on modifying the surface chemistry of PEDOT with cholesterol and cysteine through the post-treatment to assess their impact on the PEDOT. The post-treatment was actively monitored by EIS taken at different incubation times. It was found that the charge transfer capability, and the ion diffusion within the PEDOT film, decreased systematically with incubation time. However, the film resistance remained unchanged, indicating the relatively unaffected bulk properties. In addition, the charge storage capability also decreased, with only 23% of the original capacitance remaining after the post-treatment. In contrast, the attachment of cysteine showed almost no effect on the electrochemical properties of PEDOT-MA. Our results demonstrate that thiol-maleimide click-chemistry provides an efficient and effective route for the surface functionalization of PEDOT with biomolecules. In Chapter 5, we explored the potential of PEDOT-maleimide in the development of impedimetric biosensors. In PEDOT-maleimide, the additional chemical activity introduced by the maleimide functionality, was exploited for tethering recognition elements through carefully designed post-treatment processes. The thickness of the PEDOT-maleimide coating was identified as a critical factor dictating the optimal response of biosensors with immobilized Protein A (PrA). To achieve the most sensitive biosensor, 4 × 10-3 C cm-2 of charge density is required to electrodeposit PEDOT-maleimide, corresponding to a nominal film thickness of 100 nm. Further improvements in sensitivity were obtained by utilizing a relatively low PrA concentration (0.001 mg mL-1) in the immobilization step. A calibration curve of the relative change of charge transfer resistance (ΔRct) vs. logarithm of Immunoglobulin (IgG) concentration was constructed using the best-performing PrA-immobilized biosensor, and a positive slope (0.11 log-1(mg mL-1)) was obtained from the regression analysis of the response data, confirming that the biosensor can distinguish between different IgG concentrations. Our findings demonstrate that PEDOT-maleimide, in combination with the proposed post-treatment process, is effective for fabricating impedimetric biosensors for biosensing applications. This dissertation presents a comprehensive study of the design and development of functional PEDOT-based impedimetric biosensors, from proposing the suitable functional PEDOT exploring the desirable functionalization route validating the conceived immobilization chemistry, to finally demonstrating the sensitive and functional impedimetric biosensors. The results of this dissertation provide new insights into the design of functional PEDOT and the development of biosensing applications including more sensitive, accurate, and robust functional PEDOT-based biosensors.