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 MnO₂ chemistry, were synthesized by reacting manganese oxide, Mn₃O₄, 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 d-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 Li_0.17Mn_0.96O₂ and Na_0.16Mn_0.96O₂. 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.