Novel planar interdigital sensors for the detection of bacterial endotoxins : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philososphy in Electronics Engineering at Massey University, Palmerston North, New Zealand
Food poisoning caused by endotoxins or Lipopolysaccharide (LPS) are associated with Gram-negative bacteria. Foodborne pathogens, Escherichia coli (E. coli) and Salmonella are examples of Gram-negative bacteria which could cause large food poisoning outbreaks. New types of planar interdigital sensors have been fabricated with different coating materials to assess their response to endotoxins. A carboxyl-funtional polymer, APTES (3-Aminopropyltriethoxysilane) and Thionine were chosen for coating the novel interdigital sensors. All coated sensors were immobilized with PmB (Polymyxin B) which has specific binding properties to LPS. The sensors were tested with different concentrations of LPS O111:B4, ranging from 0.1 μg/ml to 1000 μg/ml. Analyses of sensors’ performance were based on the Impedance Spectroscopy method. The impedance spectra were modelled using a Constant Phase-Element (CPE) equivalent circuit, and a Principal Component Analysis (PCA) was used for data classification. Sensors coated with APTES have shown better selectivity towards LPS detection. The experiments were repeated by coating APTES and immobilizing PmB to a newly improved design of silicon based interdigital sensor. Scanning electron microscope (SEM) and atomic force microscope (AFM) images were taken to analyse the APTES coating surface and PmB immobilization. The images of non-coated sensors and coated silicon sensors were studied and the thickness of a single layer coating was estimated ([approximately equal to] 268 nm). Analyses of results with LPS O111:B4 showed that these silicon sensors have higher sensitivity and selectivity to the target biomolecule LPS. The complex non-linear least squares (CNLS) fitting method was used to fit the measured impedance spectra based on chosen equivalent circuit model. PCA results were clustered, showing the parameters were related and have identified process which related to the diffusion, charge transfer and adsorption of molecules on sensors’ surface. It was also found that these sensors can detect the standard endotoxin as low as 0.01 EU/ml which is equivalent to 1 pg/ml. Selectivity, stability and sensitivity of different thickness of coated sensors were analysed. It was observed that the optimum thickness layer is 3-layers of coating which is equivalent to 800 nm. Analyses of results with food samples have shown the developed novel interdigital sensors can detect the presence of endotoxin in contaminated food samples.