Conducting polymer electrodes for electrochemical detection of amino acids and haloacetic acids

There is a growing interest in the utilisation of various kinds of conducting polymers to modify the physico-chemical properties of electrodes for use in flow injection analysis (FIA) and liquid chromatography. Conducting electroactive polymers such as polypyrrole, polythiophene and polyaniline represent a new class of organic polymers that are capable of a range of interactions enabling them to interact with the species of interest.\nThe discovery of new electrode materials has greatly expanded the range of detectable compounds using electrochemical methods. Conducting polymers have successfully been applied for the detection of electroinactive ions and proteins in flowing solutions. This work has sought to investigate the capability of conducting polymers for detection of organic compounds which includes amino acids and haloacetic acids.\nThe platinum surface was modified by depositing conducting polymers electrochemically. Two different approaches were employed. Firstly, a range of counterions were incorporated as dopant. It was observed that the polymer electroactivity was affected by the nature of the counterions incorporated. The polymer was anion, cation or both anion/cationexchanging depending upon the nature of dopant. Derivatised monomers with cation exchange groups were also synthesised, and the resultant polymer possessed both anion and cation-exchange properties. Chiral polymers were also prepared using both chiral dopant and derivatised chiral monomers. The anion exchange properties of these polymers were confirmed by mass changes monitored by use of the electrochemical quartz crystal microbalance (EQCM).\nIt has been found that conducting electroactive polymers can be used for the detection of amino acids. The responses for four amino acids belonging to each group are characteristic. Based on these observations, amino acid to polymer interaction can be characterised as neutral, anionic and cationic. The extent of interaction can be altered by careful selection of the polymers, the potential applied, current sample points and pulse width. For aspartic acid, linearity of sensor response was obtained over the concentration range of 7.5x10-6M to 6x10-5M with a correlation coefficient of 0.979. This corresponds to a 99 % confidence level. The limit of detection (LOD) for aspartic acid was 7.5x10-6M (1ppm).\nThe effect of rapid electrochemical interaction at polymer coated microelectrodes with amino acids, which can enhance the sensitivity/selectivity of the electrodes for analytes, has also been studied. With microelectrodes a background eluent of low concentration could be used which minimises interferences in FIA. Polymers were deposited with ease on microelectrodes and their FIA responses to amino acid solutions were similar to those observed at macroelectrodes. The selectivity and limit of detection for amino acids were improved at microelectrodes when compared with those at macroelectrodes. A linear calibration curve was obtained between 7.5x10-6M and 1x10-4M amino acid. The correlation coefficients were 0.992 and 0.982 for aspartic acid and glutamic acid respectively. These correspond to a 99.9% confidence level. The LOD for both aspartic acid and glutamic acid was 3x10-6M (0.4ppm).\nThe detection of haloacids at conducting polymers was carried out with pulsed integrated amperometry in suppressed eluent after separation of the mixture of organic acids by anion exchange chromatography. The selectivity factors (ratio of peak heights) can be manipulated either by changing the applied potential or polymer electrode composition. Linear calibration curves were obtained between 0.001mg/l to 30mg/l, with correlation coefficients > 0.992. This corresponds to a 99.9% confidence level. The responses were maximum at PPy/SBA. The LODs were l.0μ.g/l and 10μg/l for monochloroacetic acid and dichloroacetic acid respectively. These limits of detection were an improvement over those obtained by conductivity detection and UV method.