Colloidal Stability, Sedimentation, and Aggregation of Crystalline Two-Dimensional Crumpled Birnessite Flakes, Their Dye Adsorption and Immune Cell Response.

Highly crystalline two-dimensional (2D) flakes of birnessite, a polymorph of manganese oxide with a MnO chemistry, were synthesized by reacting manganese oxide, MnO, at 80 °C with aqueous solutions of tetramethylammonium hydroxide (TMAH) for tens of hours. Their colloidal stability, aggregation, and sedimentation were studied as a function of ionic strengths of Na and Li cations. After reaction, a water-based stable colloidal suspension (ζ-potential ∼ -31 ± 1 mV) was obtained. Mixing the colloidal suspension with a LiCl or NaCl aqueous solution resulted in the sedimentation of crumpled flakes, as evidenced by electron microscopy (transmission and scanning). Concomitant with the sedimentation, the TMA cations present after synthesis are exchanged by the alkali ions, as evidenced by a decrease in the -spacings between the 2D sheets illustrated by X-ray diffraction (XRD). Both Na and Li uptakes were quantified by elemental analysis via inductively coupled plasma tandem mass spectrometry, giving LiMnO and NaMnO. Rhodamine 6G dye was also studied as a sedimentation agent, resulting in a maximum uptake of 550 mg (1.15 mmol) of dye per g of birnessite. To explore the immune response of the Li-intercalated crumpled flakes, the activation of antigen-presenting cells by the flakes was investigated. It was found that the immune cells were slightly activated in a dose-dependent manner, indicating that the materials may have good biocompatibility and thus possibe applications in healthcare.