Intense pH Sensitivity Modulation in Carbon Nanotube-Based Field-Effect Transistor by Non-Covalent Polyfluorene...

Area of Science:

• Materials Science
• Nanotechnology
• Chemical Sensors

Background:

• Carbon nanotube field-effect transistors (CNTFETs) are promising for sensing applications.
• Functionalization of CNTFETs is explored to enhance their performance, particularly for pH sensing.
• Existing CNTFET pH sensors have limitations in sensitivity and operational range.

Purpose of the Study:

• To compare the pH sensing performance of non-functionalized CNTFETs (p-CNTFET) and CNTFETs functionalized with a polyfluorene polymer bearing urea moieties (f-CNTFET).
• To evaluate the sensitivity and response of these devices in different buffer solutions.
• To understand the mechanism behind the enhanced pH sensitivity in functionalized devices.

Main Methods:

• Fabrication of electrolyte-gated, PMMA-passivated, 5 µm-channel FETs using inkjet-printed single-walled CNT.
• Functionalization of CNTFETs with a conjugated polyfluorene polymer (FF-UR) containing urea-based moieties.
• Measurement of drain current sensitivity to pH variations in phosphate-buffered saline (PBS) and borate buffer solutions (BBS) at a gate and drain voltage of -0.8 V.

Main Results:

• Non-functionalized CNTFETs (p-CNTFETs) showed a linear pH response in PBS (pH 3-9) with 26%/pH unit sensitivity.
• Functionalized CNTFETs (f-CNTFETs) exhibited a significantly higher sensitivity of 373%/pH unit in PBS (pH 7-9) and 96%/pH unit in BBS (pH 5-9).
• The enhanced sensitivity in f-CNTFETs is attributed to interactions between the FF-UR functionalization and buffer ions, affecting electrolyte gating and surface charge.

Conclusions:

• Functionalization with FF-UR polymer dramatically improves CNTFET pH sensing performance, exceeding the current state-of-the-art by over 10 times in BBS.
• This study demonstrates the first significant improvement in CNTFET pH sensing using functionalization beyond carboxylate moieties.
• Challenges remain in translating this high performance to real-world water matrices due to potential interference from competing species.