The development of a faradic impedimetric immunosensor for the detection of S. typhimurium in milk is described for first time. Polyclonal anti-Salmonella was cross-linked, in the presence of glutaraldehyde, on gold electrodes modified with a single 11-amino-1-undecanethiol (MUAM) self-assembled monolayer (SAM) or a mixed SAM of MUAM and 6-mercapto-1-hexanol at a constant 1 + 3 proportion, respectively. The mixed SAM was also deposited in the presence of triethylamine, which was used to prevent the formation of interplane hydrogen bonds among amine-terminated thiols. The effect of the different surface modifications on both the sensitivity and the selectivity of the immunosensors was investigated. The alteration of the interfacial features of the electrodes due to different modification or recognition steps, was measured by faradic electrochemical impedance spectroscopy in the presence of a hexacyanoferrate(II)/(III) redox couple. A substantial amplification of the measuring signal was achieved by performing the immunoreaction directly in culture samples. This resulted in immunosensors with great analytical features, as follows: (i) high sensitivity; the response of the immunosensors increases with respect to the detection time as a consequence of the simultaneous proliferation of the viable bacteria cells in the tested samples; (ii) validity; the response of the immunosensors is practically insensitive to the presence of dead cells; (iii) working simplicity; elimination of various centrifugation and washing steps, which are used for the isolation of bacteria cells from the culture. The proposed immunosensors were successfully used for the detection of S. typhimurium in experimentally inoculated milk samples. The effect of different postblocking agents on the performance of the immunosensors in real samples was also examined.

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The advantages and limitations of impedimetric sensors based on Ti/TiO 2 architectures are described. Titanium dioxide (titania) was potentiostatically formed onto titanium electrodes of 2 mm diameter, at 10 and 30 V in 1 M H 2 SO 4 . The thickness of the titania layers was ellipsometrically determined to be 30 and 86 nm respectively and they are highly insulating with charge-transfer resistances in the MW range, as they were measured with electrochemical impedance spectroscopy under specific experimental conditions. Low voltage anodization (< 10 V) results to amorphous TiO 2 , whereas at higher applied voltages (> 25 V), anatase is the predominant form. SEM images are indicative of quite smooth, compact coatings without any severe cracks. Titania films were further treated with sodium hydroxide to increase the population of hydroxyl groups onto the outer oxide layer. Various silanization procedures based on liquid and gas-phase anhydrous conditions were employed to produce amino-terminated surfaces. Using glutaraldehyde as a cross linker, avidin was immobilized by chemical bonding. All impedimetric measurements were performed using biotinylated dextran as the target analyte. The initial conductivity of titanium/titania electrodes, as it can be managed by means of the hydroxylation process (duration of chemical etching) as well as the silane layer are major factors affecting the performance of the resultant biosensors. Extended studies using electrochemical impedance spectroscopy provided important new insights into the mechanism of the signal transduction and the physical factors that control the nature of the measuring signal.

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