Surface-assisted laser desorption/ionization mass spectrometry on nanostructured silicon substrates prepared by iodine-assisted etching

Abstract

Surface-assisted laser desorption/ionization (SALDI) is a matrix-free mass spectrometry (MS) approach that utilizes the unique properties of a nanostructured surface to promote desorption and ionization. However, there are still questions on what constitutes a suitable SALDI substrate for mass spectrometric application. A range of SALDI substrates prepared by anodization with an oxidizing electrolyte was investigated. The laser desorption/ionization (LDI) performance was examined on a reflectron time-of-flight (ToF) mass spectrometer. The physicochemical properties of the substrates were characterized by a number of surface analysis techniques including scanning electron microscopy (SEM), atomic force microscopy (AFM), secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS) and water contact angle measurement. Examination of surface cleaning technologies and methods for surface chemical modification were carried out. Correlation between the substrate physicochemical properties and the LDI performance was determined. It was found that only the substrate, which had a thick nanostructured layer, was effective for LDI-MS. SALDI substrate was found to have a high surface potential. However, this unique property offered no advantage for the application of LDI-MS. Surface chemistry is also an important factor in affecting the LDI performance. Plasma etching can effectively remove the surface contamination but it also increases the thickness of the oxide layer. Fluorine and hydroxyl termination is advantageous. Fluorine passivation increases the surface hydrophobicity, which confines the analyte solution droplet to a smaller area and also withdraws the electronic density from the surface, and acidifies the surface Si–OH moieties, which is believed a major proton source. The effect of laser etching was investigated by SIMS and XPS imaging and provided new insight of the SALDI ionization mechanism.