pH sensitivity of Si-C linked organic monolayers on crystalline silicon surfaces

The electrochemical behaviour of Si-C linked organic monolayers is studied in electrolyte-insulator-Si devices, under conditions normally encountered in potentiomeric biosensors, to gain fundamental knowledge on the behaviour of such Si electrodes under practical conditions. This is done via titration experiments, Mott-Schottky data analysis, and data fitting using a site-binding model. The results are compared with those of native SiO2 layers and native SiO2 layers modified with hexamethyldisilazone. All samples display pH sensitivity. The number of Si-OH groups on the alkylated samples is calculated to be less than 0.7% of that of a pure SiO2 insulator, which still causes a pH sensitivity of approximate to 25 mV per pH unit in the pH range: 4-7. The alkylated samples hardly suffer from response changes during up and down-going titrations, which indicates that very little oxide is additionally formed during the measurements. The pK(a) values of all samples with monolayers (4.0-4.4) are lower than that of native SiO2 (6.0). The long-term drift (of approximately 1 mV h(-1)) is moderate. The results indicate that biosensors composed of alkylated Si substrates are feasible if a cross-sensitivity towards pH in the sensor signal is taken into account.