Laser Desorption/Ionization Mass Spectrometry Using TiO2 Nanopillar Array Substrates with Tunable Surface Roughness and Wettability

The ability to sensitively detect trace quantities of substances is important in many practical applications. Herein, the use of TiO2 nanopillar arrays, formed using oblique electron-beam deposition and thermal treatment at 800 °C, as substrates for laser desorption/ionization mass spectrometry (LDI-MS) is reported. The TiO2 nanopillar arrays effectively assist ionization of various analytes, including small amino acids, a sugar, pesticides, peptides, and proteins with molecular weights of up to 24,000. The fabricated surface nanostructures were analyzed by measuring contact angles and applying the Cassie–Baxter and Wenzel wetting models. The TiO2 nanopillar surface density was shown to significantly affect the thermal properties of the substrate and the wettability of analyte-containing droplets. A TiO2 nanopillared substrate with a lower surface density exhibited more intense signals for the detection of small (∼1.2 kDa) analytes. The use of the benzyl pyridinium ion ([BP]+), as a thermometer chemical, revealed that TiO2 substrates with the lower pillar density exhibited effective heat confinement due to the pillar morphology, which plays a significant role in increasing the signal intensities of small molecules. The wetting behavior of the droplet on the substrate revealed that analyte enrichment, facilitated by surface hydrophobization, is another factor that contributes to enhancing small-molecule LDI performance. In contrast, the homogeneous adsorption of target molecules onto the nanopillared surface is likely to be a dominant factor for the detection of proteins. In this work, it is demonstrated that the appearance and disappearance of “coffee rings” during the evaporation of droplets on the surface are closely related to analyte distribution and, consequently, the intensities of the detected LDI-MS signals. The heat confinement effect of the TiO2 nanopillars, along with suitable sample wettability, delivers significant LDI performance.