Phospholipid-Tailored Titanium Carbide Nanosheets as a Novel Fluorescent Nanoprobe for Activity Assay and Imaging of Phospholipase D

As one of the emerging inorganic graphene analogues, two-dimensional titanium carbide (Ti₃C₂) nanosheets have attracted extensive attention in recent years because of their remarkable structural and electronic properties. Herein, a sensitive and selective nanoprobe to fluorescently probe Phospholipase D activity was developed on the basis of an ultrathin Ti₃C₂ nanosheets-mediated fluorescence quenching effect. Ultrathin Ti₃C₂ nanosheets with ∼1.3 nm in thickness were synthesized from bulk Ti₃AlC₂ powder by a two-step exfoliation procedure and further modified by a natural phospholipid that is doped with rhodamine B-labeled phospholipid (RhB-PL-Ti₃C₂). The close proximity between RhB and Ti₃C₂ leads to efficient fluorescence quenching (>95%) of RhB by energy transfer. Phospholipase D-catalyzed lipolysis of the phosphodiester bond in RhB-PL results in RhB moving away from the surface of Ti₃C₂ nanosheets and subsequent fluorescence recovery of RhB, providing a fluorescent "switch-on" assay for the Phospholipase D activity. The proposed nanoprobe was successfully applied to quantitatively determine Phospholipase D activity with a low limit of detection (0.10 U L^-1) and to measure its inhibition. Moreover, in situ monitoring and imaging the activity of Phospholipase D in living cells were achieved using this biocompatible nanoprobe. These results reveal that Ti₃C₂ nanosheets-based probes exhibit great potential in fluorometric assay and clinical diagnostic applications.