Improved Hydrolytic Stability and Repeatability: pH sensing with APTES-coated silicon nanowire bio-FETs

This article addresses the hydrolytic instability issue of surface chemistry and the repeatability issue of silicon (Si) nanowire (NW) biosensors. Si multi-NWs or nanograting (NG) field-effect transistors (FETs) were fabricated using semiconductor lithographic processing techniques. Then, the NG surfaces were coated with 3-aminopropyltriethoxysilane (APTES) via the vapor- and solution-phase methods. Their performance, including drift, stability, sensitivity, accuracy, and linearity of pH sensing, was evaluated. Sensors treated with both APTES deposition methods exhibit linear pH response with good sensitivity. Devices treated with vapor APTES show better linearity and sensitivity, and APTES-solution-coated devices are more stable with smaller drift. A hydrolysis process was developed to significantly improve the hydrolytic stability of the APTES-coated sensor surface. As a result, an accuracy of ?0.008 pH was achieved, which is comparable to or better than commercially available ion-sensitive FETs (ISFETs), whereas the sensor drift was significantly reduced for both sensors treated with vapor and solution APTES. This hydrolysis process has greatly improved the stability and repeatability of charge sensing of our Si NG bio-FETs.