Tailoring the surface chemistry of SiO2-based monoliths to enhance the selectivity of SALDI-MS analysis of small molecules

Mohamed O Amin; Entesar Al-Hetlani

doi:10.1016/j.talanta.2019.03.078

Tailoring the surface chemistry of SiO₂-based monoliths to enhance the selectivity of SALDI-MS analysis of small molecules

Talanta. 2019 Aug 1:200:458-467. doi: 10.1016/j.talanta.2019.03.078. Epub 2019 Mar 22.

Authors

Mohamed O Amin¹, Entesar Al-Hetlani²

Affiliations

¹ Chemistry Department, Faculty of Science, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait. Electronic address: mohamed.amin@ku.edu.kw.
² Chemistry Department, Faculty of Science, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait. Electronic address: entesar.alhetlani@ku.edu.kw.

PMID: 31036209
DOI: 10.1016/j.talanta.2019.03.078

Abstract

Due to their 3D geometry, surface chemistry, high porosity and large surface area, SiO₂-based monoliths were explored as novel surface-assisted laser desorption/ionization-mass spectrometry (SALDI-MS) substrates for the sensitive and selective detection of small molecules of varying polarities. For this purpose, the surface of a hydrophilic SiO₂ monolith was functionalized with moderately hydrophobic C-8 and more hydrophobic C-18 groups to form C8-SiO₂ and C18-SiO₂ monoliths, respectively. These monoliths were characterized using N₂ adsorption-desorption, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). All of the monoliths displayed V isotherms and H1-type hysteresis loops, and they had surface areas of no less than 400 m² g^-1. The presence of bands at 2853 and 2927 cm^-1 in the FTIR spectra of the respective C8-SiO₂ and C18-SiO₂ monoliths demonstrated that they contained alkyl chains on their surfaces. A proof-of-concept study was conducted to assess the ability of the SiO₂-based monoliths to serve as substrates for SALDI-MS analysis of small molecules, including hydrophilic vitamin C, moderately hydrophobic caffeine and hydrophobic carbazole. When the hydrophilic SiO₂ monolith was used as the SALDI substrate, the spectrum of vitamin C showed a large molecular ion signal, while that of carbazole exhibited only an insignificant ion signal. By contrast, the use of the hydrophobic monolith C18-SiO₂ led to a high intensity molecular ion signal in the SALDI spectrum of nonpolar carbazole, while the spectrum of vitamin C showed a minimal ion signal. The observed selectivities appear to be a consequence of strong interactions occurring between the substrates and analytes of matching polarities that prevent early laser irradiation-induced analyte desorption. Furthermore, the studies showed that the more hydrophobic monolith C18-SiO₂ displayed higher selectivity toward enhancing the SALDI signal of the longer chain fatty acid arachidic acid (C20), while the hydrophilic SiO₂ monolith was highly selective in promoting shorter chain myristic acid (C14). In addition, the feasibility of using the C18-SiO₂ monolith for SALDI detection of the hydrophobic antidepressant drugs desipramine and trimipramine in carbonated malt beverages was also evaluated. The results showed that this monolith enabled the detection of the protonated forms of these drugs, with respective limits of detection (LODs) of 10 µg mL^-1 and 1 ng mL^-1. This proof-of-principle study demonstrates that SiO₂-based monoliths are potentially important SALDI substrates for the selective enhancement of SALDI signals of a vast range of small molecule analytes.

Keywords: 3D structure SALDI; Antidepressant drugs; Hydrophilic/hydrophobic substrate; MADLI; Polar/nonpolar molecules; SiO(2).