pH Sensitivity of Si-C Linked Organic Monolayers on Crystalline Silicon Surfaces

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Abstract
The electrochemical behavior of SiC linked organic monolayers is studied in electrolyte–insulator–Si devices, under conditions normally encountered in potentiometric biosensors, to gain fundamental knowledge on the behavior 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 hexamethyldisilazane. 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 ≈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 pKa 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.