Probing the surface of organic and bioconjugated nanocrystals by using mass spectrometric imaging

The time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) imaging technique was utilized to characterize surface ligands conjugated onto nanocrystals in sizes of 6-12 nm. Nanocrystals were dropped onto a Si(100) substrate precleaned in a piranha solution and dried under vacuum, for TOF-SIMS analysis. The appearance of [Zn+MPA-H]^- in the mass spectra and the absence of an S-H stretching band in the IR spectra demonstrate the zinc-thiolate linkage between MPA and CdSe/ZnS nanocrystals. The overlapping images of [sterate]^- and [Si+ODA]^- occupying a nanocrystal-free space and the uncoordinated COO^- stretching bands in the IR spectra, imply the coexistence of sterate and ODA on the substrate. The results also show that the images of [MPO-H]^- and [AET-H]^- ions overlap with the [ZnS]^- image representing nanocrystals.