pH is a widely used indicator of soil and water quality and controls reaction conditions and the structure of biomolecules. Because of this, the ability to accurately measure pH is extremely important. The most prevalent methods for measuring pH to date include glass electrodes and pH indicator strips. However, the former is limited due to its fragility and frequent maintenance needs whereas the latter, while quick, lacks precision and is single use. A promising alternative sensor type is a resistive sensor which requires highly conductive materials of construction with surface functional groups. One such material that meets these requirements is the recently discovered class of two-dimensional nanomaterials known as MXenes, specifically Ti₃C₂T_z. Herein, we demonstrate the ability to fabricate polycation/Ti₃C₂T_z resistive sensors with high pH sensitivity. Due to their highly negative ζ-potential, MXenes are an ideal constituent material for layer-by-layer assembly. Further, the MXene nanosheets’ hydroxyl surface groups render them pH-responsive. By changing the constituent polycation in the multilayer from the pH-insensitive poly (diallyldimethylammonium) to the pH-sensitive branched polyethylenimine, we demonstrate the ability to significantly increase the pH sensitivity of the sensor by up to 460%. We also compare the response of our multilayers to those assembled with rGO, another common two-dimensional nanomaterial, to further demonstrate the high pH-responsivity of Ti₃C₂T_z. Our results indicate the ability to tune these polycation/Ti₃C₂T_z multilayers to specific stimuli by changing the polycation, opening the door for future sensing applications of these MXene based multilayers.

Hyosung An

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