Single-Cell Lipidomics: Characterizing and Imaging Lipids on the Surface of Individual Aplysia californica Neurons with Cluster Secondary Ion Mass Spectrometry

Neurons isolated from Aplysia californica, an organism with a well-defined neural network, were imaged with secondary ion mass spectrometry, C₆₀-SIMS. A major lipid component of the neuronal membrane was identified as 1-hexadecyl-2-octadecenoyl-<i>sn</i>-glycero-3-phosphocholine [PC­(16:0e/18:1)] using tandem mass spectrometry (MS/MS). The assignment was made directly off the sample surface using a C₆₀-QSTAR instrument, a prototype instrument that combines an ion source with a commercial electrospray ionization/matrix-assisted laser desorption ionization (ESI/MALDI) mass spectrometer. Normal phase liquid chromatography mass spectrometry (NP-LC–MS) was used to confirm the assignment. Cholesterol and vitamin E were also identified with in situ tandem MS analyses that were compared to reference spectra obtained from purified compounds. In order to improve sensitivity on the single-cell level, the tandem MS spectrum of vitamin E reference material was used to extract and compile all the vitamin E related peaks from the cell image. The mass spectrometry images reveal heterogeneous distributions of intact lipid species, PC­(16:0e/18:1), vitamin E, and cholesterol on the surface of a single neuron. The ability to detect these molecules and determine their relative distribution on the single-cell level shows that the C₆₀-QSTAR is a potential platform for studying important biochemical processes, such as neuron degeneration

Dibromocarbene Functionalization of Boron Nitride Nanosheets: Toward Band Gap Manipulation and Nanocomposite Applications

We report the covalent functionalization of exfoliated boron nitride nanosheets (BNNSs) using dibromocarbene (DBC) species. The functionalization of BNNSs is enabled as the nanosheets are utilized as two-dimensional phase-transfer catalysts for the migration of carbenes across the organic–aqueous phase boundary. We postulate that BNNSs stabilize carbenes by forming B-CBr₂ ylides and in turn act as the reaction substrate. DBC functionalization of BNNSs results in the formation of B–C and B–N bonds to the sp²-hybridized BNNS lattice via the formation of dibromo-bridged bicyclo BCN systems. The covalent functionalization was characterized using HR-TEM, AFM, EELS, XRD, EDX, ToF-SIMS, TGA, Raman, XPS, FTIR, and UV–vis techniques. Utilization of CBr₂ groups as a means by which BNNSs may be integrated and interfaced with solvents, molecular species, and condensed-phase materials was demonstrated by grafting alkyl chains from the functional groups via alkyl/halogen exchange. Alkyl-functionalized BNNSs were integrated within polyethylene (LDPE) and extruded to form BNNS-nanocomposite fibers. Implications of the covalent functionalization of h-BN are considered in the context of band gap manipulation and the versatility of the CBr₂ functional groups to enable subsequent chemical derivatization