pH-Response of polycation/Ti3C2Tx MXene layer-by-layer assemblies for use as resistive sensors

The importance and widespread need for accurate pH monitoring necessitates the fabrication of new pH sensors with high sensitivity that can be used in a variety of environments. However, typical pH sensors have certain limitations (e.g., glass electrodes are fragile and require consistent upkeep, colorimetric pH strips are single use and inaccurate). Herein, we examine the pH-response of multilayers consisting of Ti3C2Tx nanosheets and polycations fabricated using layer-by-layer (LbL) assembly. The MXene sheets themselves are pH-responsive due to their hydroxyl surface groups, and this effect may be amplified with the choice of an appropriate polycation. Specifically, the performance of multilayers assembled with the strong electrolyte poly (diallyldimethylammonium) (PDADMA) or pH-sensitive branched polyethylenimine (BPEI) is compared. As expected, the use of a pH-sensitive constituent leads to a 464% increase in pH sensitivity (116 k omega pH(-1) unit vs. 25 k omega pH(-1) unit) as compared to PDADMA. This is due to the conformational changes that BPEI undergoes with (de)protonation as pH changes. Further comparisons with reduced graphene oxide (rGO), which is far less pH responsive, confirm the unique pH responsivity of MXene nanosheets themselves. The ability to enhance response to particular stimuli by changing the constituent polycation demonstrates promise for future use of MXenes in resistive sensors for a variety of stimuli.