Top-down synthesized TiO2 nanowires as a solid matrix for surface-assisted laser desorption/ionization time-of-flight (SALDI-TOF) mass spectrometry
Graphical abstract
Introduction
Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry has been used as a powerful analytical tool for the analysis of biomolecules, such as peptides, proteins, and nucleic acids [1]. The advantages of MALDI-TOF mass spectrometry can be summarized by (1) the ease of sample preparation, (2) high sensitivity (<fM), and (3) a wide detection range (up to 300 kDa). In particular, MALDI-TOF mass spectrometry can ionize analytes without fragmentation of the analytes [2], [3]. However, this powerful tool has been restricted to the analysis of mass-to-charge ratios (m/z) higher than 500 because the matrix molecules are fragmented into irreproducible low-mass peaks. In this work, a target plate with top-down synthesized TiO2 nanowires is presented as an ideal solid matrix for the analysis of small biomolecules at a m/z of less than 500.
After Siuzdak and co-workers reported that porous silicon could be used as a solid matrix for the ionization of analytes in surface-assisted laser desorption/ionization time-of-flight (SALDI-TOF) mass spectrometry, various inorganic materials have been used as substitutes for the organic matrix for analytes at a m/z of less than 500 [4]. Nanomaterials, such as nanoparticles [5], [6], [7], carbon nanotubes [8], graphenes [9], and semiconductor nanowires [10], have been used as solid matrices in SALDI-TOF mass spectrometry. Although the exact ionization mechanism is not yet completely understood, the physical properties of nanomaterials related to energy absorption and transfer are supposed to strongly influence the ionization of analytes [11], [12]. In particular, TiO2 nanomaterials have been considered the most promising materials because of their strong absorption in the UV range, and TiO2 nanomaterials, such as thin films [13], [14], nanoparticles [15], and nanowires [10], have been reported as a solid matrix for SALDI-TOF mass spectrometry. The Ti-wire-based ionization method was also reported with probe electrospray ionization (PESI) mass spectrometry, which can be used to analyze small molecules without the influence of salts and detergents in the sample solution [16], [17], [18].
In this work, TiO2 nanowires were top-down synthesized on a solid matrix Ti plate for SALDI-TOF mass spectrometry through the hydrothermal process. The crystal and electronic structures of the top-down TiO2 nanowires were analyzed at each step of the hydrothermal process, and the optimal TiO2 nanowires were identified by checking their performance toward the ionization of analytes in SALDI-TOF mass spectrometry. Finally, the feasibility of the TiO2 nanowires in the anatase phase as a solid matrix for SALDI-TOF mass spectrometry was demonstrated using eight types of amino acids and peptides as model analytes.
Section snippets
Synthesis of TiO2 nanowire arrays
TiO2 nanowires were synthesized by the modified hydrothermal process [19], [20], [21], [22], [23], [24], [25]: (1) KOH treatment (TA), (2) water treatment (TAW) and (3) heat treatment (TAWH). Such top-down synthesized TiO2 nanowires can be prepared as arrays on a target plate using a parylene film as a protective layer against the highly corrosive etching conditions (10 M KOH) of the modified hydrothermal process, and the parylene film can be removed after the heat treatment step. As shown in
Properties of TiO2 nanowires prepared using the hydrothermal process
In this work, we focused on presenting the applicability of TiO2 nanowires prepared using the hydrothermal process to SALDI-TOF mass spectrometry. Usually, TiO2 nanowires are synthesized through the vapor–liquid–solid (VLS) mechanism using substrate materials in the gas phase under vacuum [[10], [11], [12]]. However, TiO2 nanowires from the hydrothermal process can be synthesized simply by changing aqueous solutions at ambient atmospheric conditions, as shown in Fig. 1(a). Therefore, the
Conclusions
In this work, top-down synthesized TiO2 nanowires were presented as a solid matrix for SALDI-TOF mass spectrometry. TiO2 nanowires were prepared on a Ti target plate as an array of 81 (9 × 9) nanowire zones with a diameter of 300 μm using the hydrothermal process and a protective parylene film. The feasibility of using TiO2 nanowires as a solid matrix for SALDI-TOF mass spectrometry was tested using peptides and amino acids as model analytes, and this method achieved a highly sensitive detection
Acknowledgements
This work was supported by DAPA and ADD. This research was also supported by the National Research Foundation of Korea (2009-0082188, 2013-056331 and 2013-056267).
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