Selective sodium sensing with gold-coated silicon nanowire field-effect transistors in a differential setup.

Ion-sensitive field-effect transistors based on silicon nanowires with high dielectric constant gate oxide layers (e.g., Al2O3 or HfO2) display hydroxyl groups which are known to be sensitive to pH variations but also to other ions present in the electrolyte at high concentration. This intrinsically nonselective sensitivity of the oxide surface greatly complicates the selective sensing of ionic species other than protons. Here, we modify individual nanowires with thin gold films as a novel approach to surface functionalization for the detection of specific analytes. We demonstrate sodium ion (Na(+)) sensing by a self-assembled monolayer (SAM) of thiol-modified crown ethers in a differential measurement setup. A selective Na(+) response of ≈-44 mV per decade in a NaCl solution is achieved and tested in the presence of protons (H(+)), potassium (K(+)), and chloride (Cl(-)) ions, by measuring the difference between a nanowire with a gold surface functionalized by the SAM (active) and a nanowire with a bare gold surface (control). We find that the functional SAM does not affect the unspecific response of gold to pH and background ionic species. This represents a clear advantage of gold compared to oxide surfaces and makes it an ideal candidate for differential measurements.