Interactions of Ti3C2 MXene with Aqueous Zwitterionic Biological Buffers: Implications for Applications in Biological Systems

MXenes, a family of ultrathin layered two-dimen-sional (2D) transition-metal carbides, nitrides, and carbonitrides, are steadily advancing as novel inorganic nanosystems for a broad range of applications. While high conductivity, solution process-ability, hydrophilic nature, and the presence of various surface functional groups enable the use of 2D MXenes in aqueous systems, applications that involve the use of MXenes in aqueous systems and biological solutions, including but not limited to water treatment, water desalination, and biological assays, are only beginning to emerge. For the successful application of MXene in aqueous systems, their interactions with common aqueous solution constituents must be better understood. This paper describes the structural and functional properties of Ti3C2Tx (Tx = O-, OH-, F-) MXenes in N-substituted commonly used biological buffers like N-(2-hydroxyethyl)-piperazine-N '-ethanesulfonic acid (HEPES) and 3-(N-morpholino)-propane sulfonic acid (MOPS) and with a series of piperazine-based molecules dissolved in phosphate buffer solutions. Dissolving MXenes in these buffers significantly impacts their optical, structural, and electrical properties. The interactions of Ti3C2Tx MXenes with N-substituted zwitterionic buffer molecules are structure-dependent and mostly but not fully reversible. X-ray photoelectron spectroscopy measurements reveal small but measurable surface oxidation through the formation of Ti-O bonds. Our results suggest a complex interaction pattern where some interactions are driven by hydrogen bonding and electrostatic forces, while others involve chemisorption, which results in a permanent impact on the MXene nanosheets. This study is an important step toward understanding the stability of MXenes in complex aqueous media and the impact of interactions of MXenes with nitrogen-containing molecules on MXene properties, which are especially important in applications of MXenes in biological systems. The nature of interactions between MXenes and biological buffer molecules, revealed in our study, suggests that surface chemistry modifications of MXenes are required to preserve their chemical stability and enable their applications in complex biological solutions.