Development of Functional Interfaces for Sensing Applications

Abstract

An electrochemical device has been developed for the detection of gaseous tricresyl phosphate (TCP). Monitoring of neurotoxic TCP is important since it has been widely used as an additive in commercial jet engine oils, acting as a flame retardant and plasticizer. The process of air recirculation in most aircrafts could allow TCP to enter the cabin, if oil leakage occurs, and potentially harm the health and safety of the crew and passengers. Although a few of the new airplanes have begun to use a different air recirculation system which can reduce the possibility of TCP contamination, it is still a big issue for most airplanes that are currently in service throughout the world. Since TCP has low saturated vapor pressure, a gaseous sample is not readily available and a special procedure was developed in our laboratory for conducting experiments. A TCP methanol solution was heated while N2 was bubbled though a flow system to vaporize the TCP. Since TCP is not electro-active and cannot be detected with electrochemical approaches, it was hydrolyzed to cresol using a special hydrolysis column in the flow system. Both of these operations were also performed with an automatic sampling device that was built in our lab. The presence of TCP in the hydrolysate samples of TCP, as well as real TCP-contained engine oils, was successfully detected by the electrochemical device within the linear range of 30-300 ppb of TCP in gas phase. However, the electrochemical detection procedure results in oxidative polymerization of cresol on the electrode surface and this significantly distorts the results of measurements. A functional interface of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) composites has been introduced on the electrode surface to prevent electrode fouling. With this modified electrode, a similar linear range was obtained. More importantly, the modified electrochemical device was able to continuously detect gaseous TCP and has reliable responses over a longer period of time than with unmodified electrodes. Electrode fouling is a common problem during the electrochemical analysis of phenolic compounds. A potential-drop-based model has been created in order to better understand the mechanism of electrode fouling, and it was able to quantitatively predict the electrode fouling in terms of time, applied potential, and the concentration of cresol. In order to obtain this model, the current-potential relationship was studied, and the amount of potential drop across the fouling layer was measured with a copper deposition method. This scientific model was comparable with experimental results, and would be helpful for the development of anti-fouling strategies.