Photothermal Excitation of Neurons Using MXene: Cellular Stress and Phototoxicity Evaluation

Understanding the communication of individual neurons necessitates precise control of neural activity. Photothermal modulation is a remote and non-genetic technique to control neural activity with high spatiotemporal resolution. The local heat release by photothermally active nanomaterial will change the membrane properties of the interfaced neurons during light illumination. Recently, it is demonstrated that the two-dimensional Ti3 C2 Tx MXene is an outstanding candidate to photothermally excite neurons with low incident energy. However, the safety of using Ti3 C2 Tx for neural modulation is unknown. Here, the biosafety of Ti3 C2 Tx -based photothermal modulation is thoroughly investigated, including assessments of plasma membrane integrity, mitochondrial stress, and oxidative stress. It is demonstrated that culturing neurons on 25 µg cm-2 Ti3 C2 Tx films and illuminating them with laser pulses (635 nm) with different incident energies (2-10 µJ per pulse) and different pulse frequencies (1 pulse, 1 Hz, and 10 Hz) neither damage the cell membrane, induce cellular stress, nor generate oxidative stress. The threshold energy to cause damage (i.e., 14 µJ per pulse) exceeded the incident energy for neural excitation (<10 µJ per pulse). This multi-assay safety evaluation provides crucial insights for guiding the establishment of light conditions and protocols in the clinical translation of photothermal modulation.

Ti3C2Tx MXene Flakes for Optical Control of Neuronal Electrical Activity

Understanding cellular electrical communications in both health and disease necessitates precise subcellular electrophysiological modulation. Nanomaterial-assisted photothermal stimulation was demonstrated to modulate cellular activity with high spatiotemporal resolution. Ideal candidates for such an application are expected to have high absorbance at the near-infrared window, high photothermal conversion efficiency, and straightforward scale-up of production to allow future translation. Here, we demonstrate two-dimensional Ti₃C₂T_x (MXene) as an outstanding candidate for remote, nongenetic, optical modulation of neuronal electrical activity with high spatiotemporal resolution. Ti₃C₂T_x's photothermal response measured at the single-flake level resulted in local temperature rises of 2.31 ± 0.03 and 3.30 ± 0.02 K for 635 and 808 nm laser pulses (1 ms, 10 mW), respectively. Dorsal root ganglion (DRG) neurons incubated with Ti₃C₂T_x film (25 μg/cm²) or Ti₃C₂T_x flake dispersion (100 μg/mL) for 6 days did not show a detectable influence on cellular viability, indicating that Ti₃C₂T_x is noncytotoxic. DRG neurons were photothermally stimulated using Ti₃C₂T_x films and flakes with as low as tens of microjoules per pulse incident energy (635 nm, 2 μJ for film, 18 μJ for flake) with subcellular targeting resolution. Ti₃C₂T_x's straightforward and large-scale synthesis allows translation of the reported photothermal stimulation approach in multiple scales, thus presenting a powerful tool for modulating electrophysiology from single-cell to additive manufacturing of engineered tissues.

Modified MAX Phase Synthesis for Environmentally Stable and Highly Conductive Ti3C2 MXene

One of the primary factors limiting further research and commercial use of the two-dimensional (2D) titanium carbide MXene Ti₃C₂, as well as MXenes in general, is the rate at which freshly made samples oxidize and degrade when stored as aqueous suspensions. Here, we show that including excess aluminum during synthesis of the Ti₃AlC₂ MAX phase precursor leads to Ti₃AlC₂ grains with improved crystallinity and carbon stoichiometry (termed Al-Ti₃AlC₂). MXene nanosheets (Al-Ti₃C₂) produced from this precursor are of higher quality, as evidenced by their increased resistance to oxidation and an increase in their electronic conductivity up to 20 000 S/cm. Aqueous suspensions of stoichiometric single- to few-layer Al-Ti₃C₂ flakes produced from the modified Al-Ti₃AlC₂ have a shelf life of over ten months, compared to 1 to 2 weeks for previously published Ti₃C₂, even when stored in ambient conditions. Freestanding films made from Al-Ti₃C₂ suspensions stored for ten months show minimal decreases in electrical conductivity and negligible oxidation. Furthermore, oxidation of the improved Al-Ti₃C₂ in air initiates at temperatures that are 100-150 °C higher than that of conventional Ti₃C₂. The observed improvements in both the shelf life and properties of Al-Ti₃C₂ will facilitate the widespread use of this material.

Modified MAX Phase Synthesis for Environmentally Stable and Highly Conductive Ti3C2 MXene

One of the primary factors limiting further research and commercial use of the two-dimensional (2D) titanium carbide MXene Ti₃C₂, as well as MXenes in general, is the rate at which freshly made samples oxidize and degrade when stored as aqueous suspensions. Here, we show that including excess aluminum during synthesis of the Ti₃AlC₂ MAX phase precursor leads to Ti₃AlC₂ grains with improved crystallinity and carbon stoichiometry (termed Al-Ti₃AlC₂). MXene nanosheets (Al-Ti₃C₂) produced from this precursor are of higher quality, as evidenced by their increased resistance to oxidation and an increase in their electronic conductivity up to 20 000 S/cm. Aqueous suspensions of stoichiometric single- to few-layer Al-Ti₃C₂ flakes produced from the modified Al-Ti₃AlC₂ have a shelf life of over ten months, compared to 1 to 2 weeks for previously published Ti₃C₂, even when stored in ambient conditions. Freestanding films made from Al-Ti₃C₂ suspensions stored for ten months show minimal decreases in electrical conductivity and negligible oxidation. Furthermore, oxidation of the improved Al-Ti₃C₂ in air initiates at temperatures that are 100-150 °C higher than that of conventional Ti₃C₂. The observed improvements in both the shelf life and properties of Al-Ti₃C₂ will facilitate the widespread use of this material.