Influence of conditioning and surface modification of PEDOT used as solid-contact in K+-selective electrodes
Tediashvili, Davit (2019)
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Åbo Akademi University
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In the last few decades, there has been a rapid development of chemical sensors. The main driving force for this development is attractive properties of these devices, such as their price, simplicity of operation and fast response. However, one of the main limitations in potentiometric chemical sensors is an internal filling solution. This element serves as a signal transducer in a device, converting an ionic signal into an electronic one, which is recorded afterward. Even though this transduction is well studied and efficient, the presence of liquid is undesirable. Water is prone to evaporation, leading to unstable signals unless calibrated frequently and is sensitive towards temperature and pressure. All of these limit a sensor’s application, especially in remote sensing. Another emerging trend is towards developing miniaturized devices, and the presence of the water layer is causing some problems in this area as well. Numerous attempts have been made to eliminate liquid from ion-selective electrodes. However, results obtained with this device led to the conclusion that some intermediate part with mixed ionic and electronic conductivity is essential for the proper functioning of the sensor. The most promising approach is the usage of conductive polymers. It was shown that conductive polymers with enough redox capacity can serve as effective signal transductors. One of the most common material for this purpose is doped poly(3,4-ethylenedioxythiophene) (PEDOT). However, signal stability is the main issue for solid contact electrodes (SCEs). One of the reasons could be hydrophilic properties of solid contact (SC) itself, which leads to the formation of a small reservoir between the ion-selective membrane and the SC. One of the ways to solve this problem is to make more hydrophobic contacts. This problem is addressed in this work. Electropolymerized PEDOT(PSS) and PEDOT(Cl) were immersed into alkylthiol solutions, which could lead to thiolation of the PEDOT surface and increased hydrophobicity due to bound alkyl chains. Water contact angle and energy dispersive X-ray (EDX) measurements showed increased sulfur in films, indicating successful thiolation. Electrodes with thiolated SCs performed worse than unmodified ones. One of the reasons could be the damaging effect of the organic solvent. The effect of organic solvent on solid contacts was minimized by using fast potential scans to speed up the thiolation process. The effect of conditioning solution for PEDOT(PSS) on the potential stability of K+-selective electrodes was studied as well. It was shown that dilution of conditioning solution for PEDOT(PSS) can greatly enhance piece-to-piece reproducibility of electrode potentials, reducing the deviation of measured E0s from 19.74 mV (conditioned in 10-1M KCl) to 0.49mV (in 10-3M KCl).