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

Abstract

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 <i>m/z</i> ranges. We developed a functionalized TiO<sub>2</sub> nanowire as a solid substrate for IMS of low-molecular-weight species in biological specimens. We prepared TiO<sub>2</sub> 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 TiO<sub>2</sub> 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 <i>Catharanthus roseus</i> and (2) microbial co-culture of <i>Burkholderia</i> <i>cenocepacia </i>869T2 vs <i>Phellinus noxius</i>. We observed that secondary metabolites are distributed heterogeneously in a petal, which is consistent with previous results reported for the <i>C. roseus</i> 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 <i>B</i>. <i>cenocepacia</i> 869T2 in presence of <i>P. noxius</i>, which were unable to be detected by MALDI-MS approach, were revealed by our newly developed approach. This suggested that the functionalized TiO<sub>2</sub> nanowire substrates-based SALDI is a powerful technique complementary to MALDI-MS.

Surface‐assisted laser desorption/ionization mass spectrometry imaging: A review

“…Metal targets can also be directly etched to form Ag etched substrates, for example (Figure 11B) (Schnapp et al, 2016). A titanium target has also already been etched through an inexpensive modified hydrothermal process to form TiO 2 nanowires (Figure 11C) (Dutkiewicz et al, 2019). Nanowires also constitute the silicon nanostructure of the NALDI™ plate used in various SALDI-MSI studies (Krasny et al, 2015;Skriba & Havlicek, 2018;Tata et al, 2012Tata et al, , 2014Vidova et al, 2010).…”

“…This method is generally employed in the SALDI variant referred as NIMS (i.e., nanostructure-initiator MS). Cells (e.g., cell cultures [Stopka & Vertes 2020], bacteria [Dutkiewicz et al, 2019], embryos [Ferreira et al, 2014], and cancerous cells [O'Brien et al, 2013]) are also imaged following their deposition on the substrate. The disadvantage of this method in the case of tissue imaging is that it requires very thin tissue sections (<5µm), which are difficult to prepare and usually require the embedding of the sample in a protective material.…”

“…Moreover, aptamers offer a wide range of targets, including DNA or RNA sequences, large proteins, and even biological cells (Ocsoy et al, 2013). In an imaging context, Dutkiewicz et al improved the selectivity toward Catharanthus roseus secondary metabolites called vinca alkaloids by functionalizing TiO 2 nanowires with perfluorooctyl chain (Dutkiewicz et al, 2019). The specificity toward specific analytes can also be enhanced by modifying the physico-chemical properties of the nanosubstrate surface through functionalization (Iakab et al, 2020;Wu et al, 2017).…”

“…P. noxius was also studied in the work of Dutkiewicz et al in which trichloro (1H, 1H, 2H, 2H‐perfluorooctyl)silane functionalized TiO 2 nanowires were used as solid nanosubstrate to image the distribution of the secondary metabolites in the microbial coculture involving bacteria ( Burkholderia cenocepacia 869T2 ) and fungi ( P. noxius ) (Dutkiewicz et al, 2019). B. cenocepacia 869T2 is a bacterium of particular interest as it is capable of inhibiting several phytopathogens such as P. noxius .…”

“…AgNPET (Niziol & Ruman, 2013a;Niziol et al, 2019Niziol et al, , 2020 Etched Ag foils (Schnapp et al, 2016), TiO 2 nanowire surface (Dutkiewicz et al, 2019),…”