Three dimensional graphene transistor for ultra-sensitive pH sensing directly in biological media

•A three-dimensional graphene transistor for pH sensing is presented.•It shows sensitivity of 71 ± 7 mV/pH even in high ionic strength media.•High sensitivity attributed to 3D foam structure and all-around liquid gating.•Enables real-time pH sensing in biological media without need of desaltation.

In this work, pH sensing directly in biological media using three dimensional liquid gated graphene transistors is presented. The sensor is made of suspended network of graphene coated all around with thin layer of hafnium oxide (HfO2), showing high sensitivity and sensing beyond the Debye-screening limit. The performance of the pH sensor is validated by measuring the pH of isotonic buffered, Dulbecco's phosphate buffered saline (DPBS) solution, and of blood serum derived from Sprague-Dawley rat. The pH sensor shows high sensitivity of 71 ± 7 mV/pH even in high ionic strength media with molarities as high as 289 ± 1 mM. High sensitivity of this device is owing to suspension of three dimensional graphene in electrolyte which provides all around liquid gating of graphene, leading to higher electrostatic coupling efficiency of electrolyte to the channel and higher gating control of transistor channel by ions in the electrolyte. Coating graphene with hafnium oxide film (HfO2) provides binding sites for hydrogen ions, which results in higher sensitivity and sensing beyond the Debye-screening limit. The 3D graphene transistor offers the possibility of real-time pH measurement in biological media without the need for desaltation or sample preparation.

Graphical abstract(a) Test setup – Direct rat blood serum pH measurements (b) Measured transfer characteristics of the transistor for blood serum at different pH values, and (c) Zoomed in version around direct point.Figure optionsDownload full-size imageDownload as PowerPoint slide