Silicon Nanowires for Sensing

CMOS compatible silicon nanowires (SiNWs) operated as ion-sensitive field-effect transistors (ISFETs) are investigated for the use as chemical and biochemical sensors. ISFETs are sensing devices based on metal oxide semiconductor field-effect transistors. Thereby the gate metal is replaced by the solution carrying the analyte species. The electrical potential of the solution affects the output of the ISFET. Reactions of charged analytes with corresponding ligand groups at the sensor surface cause a surface charge which leads to an additional surface potential. This change in surface potential is monitored and can be related to the number of adsorbed analytes.

Using different surfaces and surface functionalizations the sensitivity to specific target analytes can be tuned. We achieved ideal pH sensitivity and specific alkaline ion detection using arrays of silicon nanowires in a differential setup.

References:

• Investigation of the dominant 1/f Noise Source in Silicon Nanowire Sensors
  Bedner, K., V. A. Guzenko, A. Tarasov, M. Wipf, R. Stoop, S. Rigante, J. Brunner, W. Fu, and C. David<em>, et al</em>.

  Sensors and Actuators B 191 , 270-275 (2014) .

• pH- Response of Silicon Nanowire Sensors: Impact of Nanowire Width and Gate Oxide
  Bedner, K., V. A. Guzenko, A. Tarasov, M. Wipf, R. L. Stoop, D. Just, S. Rigante, W. Fu, and R. A. and Minamisawa<em>, et al</em>.

  Sensors and Materials 25 (8) , 567-576 (2013) .

• Selective Sodium Sensing with Gold-Coated Silicon Nanowire Field-Effect Transistors in a Differential Setup
  Wipf, Mathias, Ralph L. Stoop, Alexey Tarasov, Kristine Bedner, Wangyang Fu, Iain A. Wright, Colin J. Martin, Edwin C. Constable, and Michel Calame<em>, et al</em>.

  ACS Nano 7 (7) , 5978-5983 (2013) .

• Understanding the Electrolyte Background for Biochemical Sensing with Ion-Sensitive Field-Effect Transistors
  Tarasov, A., M. Wipf, K. Bedner, J. Kurtz, Wangyang Fu, Vitaliy A. Guzenko, Oren Knopfmacher, Ralph L. Stoop, and Michel Calame<em>, et al</em>.

  ACS Nano 6 (10) , 9291-9298 (2012) .

• A True Reference Nanosensor Realized with Silicon Nanowires
  Tarasov, A., M. Wipf, K. Bedner, J. Kurtz, Wangyang Fu, Vitaliy A. Guzenko, Oren Knopfmacher, Ralph L. Stoop, and Michel Calame<em>, et al</em>.

  Langmuir 28 (25) , 9899-9905 (2012) .
  [Abstract]

  Conventional gate oxide layers (e.g., SiO2, Al2O3, or HfO2) in silicon field-effect transistors (FETs) provide highly active surfaces, which can be exploited for electronic pH sensing. Recently, great progress has been achieved in pH sensing using compact integrateable nanowire FETs. However, it has turned out to be much harder to realize a true reference electrode, which – while sensing the electrostatic potential – does not respond to the proton concentration. In this work, we demonstrate a highly effective reference sensor, a so-called reference FET, whose proton sensitivity is suppressed by as much as 2 orders of magnitude. To do so, the Al2O3 surface of a nanowire FET was passivated with a self-assembled monolayer of silanes with a long alkyl chain. We have found that a full passivation can be achieved only after an extended period of self-assembling lasting several days at 80 °C. We use this slow process to measure the number of active proton binding sites as a function of time by a quantitative comparison of the measured nonlinear pH-sensitivities to a theoretical model (site-binding model). Furthermore, we have found that a partially passivated surface can sense small changes in the number of active binding sites reaching a detection limit of ?Ns ? 170 ?m–2Hz–1/2 at 10 Hz and pH 3.

• Silicon-based ISFET shows negligible dependence on salt concentration at constant pH
  Knopfmacher, Oren, Alexey Tarasov, Mathias Wipf, Wangyang Fu, Michel Calame, and Christian Schoenenberger.

  ChemPhysChem 13 (5) , 1157-1160 (2012) .
  [Abstract]

  The response of (miniaturized) ion-sensitive field-effect transistors (ISFETs) with oxide interfaces to changes in the electrolyte concentration is discussed. It is shown that FETs covered with a thin alumina layer are almost insensitive to changes in the ionic strength of the electrolyte, while being extremely pH-sensitive.

• Signal-to-noise ratio in dual-gated silicon nanoribbon field-effect sensors
  Tarasov, A., W. Fu, O. Knopfmacher, J. Brunner, M. Calame, and C. Schoenenberger.

  Appl. Phys. Lett. 98 , 12114 (2011) .
  [Abstract]

  Recent studies on nanoscale field-effect sensors reveal the crucial importance of the low-frequency noise for determining the ultimate detection limit. In this letter, the 1/f-type noise of Si nanoribbon field-effect sensors is investigated. We demonstrate that the signal-to-noise ratio can be increased by almost two orders of magnitude if the nanoribbon is operated in an optimal gate voltage range. In this case, the additional noise contribution from the contact regions is minimized, and an accuracy of 0.5‰ of a pH shift in 1 Hz bandwidth can be reached.

• Nernst Limit in Dual-Gated Si-Nanowire FET Sensors
  Knopfmacher, O., A. Tarasov, W. Fu, M. Wipf, B. Niesen, M. Calame, and C. Schönenberger.

  Nano Letters 10 , 2268-2274 (2010) .

• Dual Gated Silicon Nanowire Field Effect Transistors
  Knopfmacher, O., D. Keller, M. Calame, and C. Schönenberger.

  Procedia Chemistry 1 (1) , 678-681 (2009) .