Sensitive Precise pH Measurement with Large-Area Graphene Field-Effect Transistors at the Quantum-Capacitance Limit

Ibrahim Fakih, Farzaneh Mahvash, Mohamed Siaj, and Thomas Szkopek
Phys. Rev. Applied 8, 044022 – Published 30 October 2017

Abstract

A challenge for pH sensing is decreasing the minimum measurable pH per unit bandwidth in an economical fashion. Minimizing noise to reach the inherent limit imposed by charge fluctuation remains an obstacle. We demonstrate here graphene-based ion-sensing field-effect transistors that saturate the physical limit of sensitivity, defined here as the change in electrical response with respect to pH, and achieve a precision limited by charge-fluctuation noise at the sensing layer. We present a model outlining the necessity for maximizing the device carrier mobility, active sensing area, and capacitive coupling in order to minimize noise. We encapsulate large-area graphene with an ultrathin layer of parylene, a hydrophobic polymer, and deposit an ultrathin, stoichiometric pH-sensing layer of either aluminum oxide or tantalum pentoxide. With these structures, we achieve gate capacitances 0.6μF/cm2, approaching the quantum-capacitance limit inherent to graphene, along with a near-Nernstian pH response of 55±2mV/pH. We observe field-effect mobilities as high as 7000cm2V1s1 with minimal hysteresis as a result of the parylene encapsulation. A detection limit of 0.1mpH in a 60-Hz electrical bandwidth is observed in optimized graphene transistors.

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  • Received 13 June 2017

DOI:https://doi.org/10.1103/PhysRevApplied.8.044022

© 2017 American Physical Society

Physics Subject Headings (PhySH)

Interdisciplinary PhysicsCondensed Matter, Materials & Applied Physics

Authors & Affiliations

Ibrahim Fakih1,*, Farzaneh Mahvash1,2, Mohamed Siaj2, and Thomas Szkopek1,†

  • 1Department of Electrical and Computer Engineering, McGill University, Montreal, Quebec H3A 2A7, Canada
  • 2Département de Chimie et Biochimie, Universite du Québec à Montréal, Montreal, Quebec H3C 3P8, Canada

  • *ibrahim.fakih@mail.mcgill.ca
  • thomas.szkopek@mcgill.ca

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Vol. 8, Iss. 4 — October 2017

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