High surface area of porous silicon drives desorption of intact molecules

Abstract

The surface structure of porous silicon used in desorption/ionization on porous silicon (DIOS) mass analysis is known to play a primary role in the desorption/ionization (D/I) process. In this study, mass spectrometry and scanning electron microscopy (SEM) are used to examine the correlation between intact ion generation with surface ablation and surface morphology. The DIOS process is found to be highly laser energy dependent and correlates directly with the appearance of surface ions (Si n ϩ and OSiH ϩ ). A threshold laser energy for DIOS is observed (10 mJ/cm 2 ), which supports that DIOS is driven by surface restructuring and is not a strictly thermal process. In addition, three DIOS regimes are observed that correspond to surface restructuring and melting. These results suggest that higher surface area silicon substrates may enhance DIOS performance. A recent example that fits into this mechanism is the surface of silicon nanowires, which has a high surface energy and concomitantly requires lower laser energy for analyte desorption. (J Am Soc Mass Spectrom 2007, 18, 1945-1949

Influence of Crystalline Forms of Titania on Desorption/Ionization Efficiency in Titania-Based Surface-Assisted Laser Desorption/Ionization Mass Spectrometry

“…40), 46), 54) Among these factors, the laser-induced rapid temperature increase of the SALDI substrate has been widely regarded as the thermal desorption of analyte molecules without thermal decomposition in organic matrix-free LDI-MS.…”

Surface assisted laser desorption/ionization on two-layered amorphous silicon coated hybrid nanostructures

“…AFM analysis revealed that the ␣-Si coating layer was slightly rougher on top of the hybrid, rms roughness being 0.4 nm on top of the hybrid compared with 0.1 nm on top of both silicon and SiO 2 , which might contribute toward improved ionization by several roughness related ionization mechanisms [17]. Another possible contributing factor is related to surface restructuring and melting [8,22], which has been claimed to be a key factor in the desorption process in SALDI. We observed that on the ␣-Si coated hybrid surface, clear marks were created upon exposure to the laser even at fluences that were not able to leave any visible marks on the Si and SiO 2 substrates.…”

“…Low thermal conductivity helps confine the heat [8] near the surface, but high thermal conductivity at the surface has been claimed to be conducive for transferring energy from the surface to the analytes [20]. The absorbed laser light then heats the surface, which has been identified to play a key role in the desorption process [17,21,22]. Thermal evaporation of the analyte and residual solvents has been suggested to be the desorption mechanism on porous silicon [21], but recently the role of thermally activated morphologic transitions have been emphasized by Northen et al [22], who note that the ionization threshold typically does not depend on the properties of the analyte, as would be expected for a strictly thermal desorption process.…”

“…The absorbed laser light then heats the surface, which has been identified to play a key role in the desorption process [17,21,22]. Thermal evaporation of the analyte and residual solvents has been suggested to be the desorption mechanism on porous silicon [21], but recently the role of thermally activated morphologic transitions have been emphasized by Northen et al [22], who note that the ionization threshold typically does not depend on the properties of the analyte, as would be expected for a strictly thermal desorption process. In addition to thermally driven desorption mechanisms, field desorption driven by the laser light, possibly amplified by surface nanostructures, has been proposed [6,12,17].…”

“…In addition to thermally driven desorption mechanisms, field desorption driven by the laser light, possibly amplified by surface nanostructures, has been proposed [6,12,17]. The origin of the protons (for protonated analytes) has been suggested to be either the surface itself [16,22], residual solvents, acids or adsorbed hydrocarbons [1,21], or both the surface and the solvent [23]. The role of surface topography in the desorption-ionization process remains unclear.…”