Functionalized Titanium Oxide Nanowire Substrate for Surface-Assisted Laser Desorption/ionization Imaging Mass Spectrometry

Imaging mass spectrometry (IMS) is a powerful technique that enables analysis of various molecular species at a high spatial resolution with low detection limits. In contrast to the standard matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) approach, surface-assisted laser desorption/ionization (SALDI) is more effective in the detection of small molecules due to the absence of interfering background signals in low m/z ranges. We developed a functionalized TiO₂ nanowire as a solid substrate for IMS of low-molecular-weight species in biological specimens. We prepared TiO₂ nanowires using the inexpensive modified hydrothermal process and subsequently functionalized it chemically with various silane analogs to overcome the problem of superhydrophilicity of the substrate. Chemical modification changed the selectivity of imprinting of samples deposited on the surface of the plate and thus improved the detection limits. Due to the enhanced performance, the functionalized TiO₂ nanowire substrate could be successfully used for imaging of complex native samples. We applied our new substrate to image distribution of the secondary metabolites in (1) petal of the medicinal plant Catharanthus roseus and (2) microbial co-culture of Burkholderia cenocepacia 869T2 vs Phellinus noxius. We observed that secondary metabolites are distributed heterogeneously in a petal, which is consistent with previous results reported for the C. roseus plant leaf and stem. We verified the semi-quantitative capabilities of the imprinting/imaging approach by comparing results using standard LC-MS analysis of the plant extracts. Several bacteria-related metabolites produced by B. cenocepacia 869T2 in presence of P. noxius, which were unable to be detected by MALDI-MS approach, were revealed by our newly developed approach. This suggested that the functionalized TiO₂ nanowire substrates-based SALDI is a powerful technique complementary to MALDI-MS.