Since this initial report, a number of research groups have published improvements and modifications for this analytical technique to include a wider range of
analytes, experimental conditions, and theoretical considerations of the desorption/
ionization mechanism [128–133]. Effects of anodization parameters, pre-, and
post-etching treatments of porous silicon substrates upon DIOS-MS performance
were reported by Shen et al. [134], showing that a wide range of etching conditions
may be utilized without adversely affecting spectrometric activity. Little dependence of DIOS-MS upon the silicon precursor, etchant solution, or electrochemical
variables was observed. However, ‘‘double etching’’ of DIOS-MS substrates, soaking of oxidized porous silicon in aqueous HF, was generally found to improve
DIOS-MS performance despite requiring greater laser fluence, particularly for
larger analyte concentrations and for silicon substrates that had been aged in air. It
was also observed that porous silicon samples stored in ethanol for 1 day prior to
use in DIOS-MS tended to show lower intensities in light-mass background peaks,
improving the detection and assignment of analyte fragments in the lower mass
range. Research from Bohn and Sweedler employed porous silicon substrates prepared from electroless H 2 O 2 –metal–HF (HOME-HF) etching to better understand
the factors that govern analyte desorption and ionization [135]. Variation in either
photoluminescence of porous silicon or the irradiation wavelength of the laser
source was not observed to affect DIOS-MS of peptides; however, DIOS-MS substrates of large pore size typically generated mass spectra with greater reproducibility and higher quality than did specimens with smaller pores (Figure 16.20). As
reported above, it was found that storage of freshly prepared porous silicon substrates in methanol before use produced spectra with enhanced peak intensity and
little noise, reaffirming this post-etching treatment as an effective method for
improving DIOS-MS performance (Figure 16.21). The hydrophobic nature of unfunctionalized hydride-terminated porous silicon tends to restrict diffusion of hydrophilic, aqueous-derived analytes into the porous structure; considering this, it
was believed that the methanol treatment wets the pores and facilitates inclusion
of the analyte within the silicon matrix. The inferior performance of oxidized substrates was justified by acknowledging the different thermal properties of the oxide
overlayer, which may hinder the energy transfer from the laser pulse to the adsorbed analyte. Mass spectrometry of analytes deposited upon porous GaAs or
porous GaN was unsuccessful, implying that only porous silicon may have the
optimum combination of crucial properties to support direct desorption/ionization
of specimens with low fragmentation.
In mechanistic considerations of the desorption/ionization process, comparisons
of DIOS-MS to surface-assisted laser-desorption ionization mass spectrometry
(SALDI-MS) have been outlined by Alimpiev et al. [136]. DIOS-MS of adsorbed
diethylamine was reported both from anodically etched porous silicon and from
ion-etched roughened silicon substrates (Figure 16.22). These results suggest that
desorption/ionization may require not porosity, but perhaps only surface roughness. No spectra were obtained from flat polished crystalline silicon surfaces. In
contrast to the negative findings from porous GaAs and GaN [135], mass spectra
could also be obtained from ion-etched highly oriented pyrolytic graphite (HOPG).
16.4 Chemical Applications Based on Porous Silicon 543
analytes, experimental conditions, and theoretical considerations of the desorption/
ionization mechanism [128–133]. Effects of anodization parameters, pre-, and
post-etching treatments of porous silicon substrates upon DIOS-MS performance
were reported by Shen et al. [134], showing that a wide range of etching conditions
may be utilized without adversely affecting spectrometric activity. Little dependence of DIOS-MS upon the silicon precursor, etchant solution, or electrochemical
variables was observed. However, ‘‘double etching’’ of DIOS-MS substrates, soaking of oxidized porous silicon in aqueous HF, was generally found to improve
DIOS-MS performance despite requiring greater laser fluence, particularly for
larger analyte concentrations and for silicon substrates that had been aged in air. It
was also observed that porous silicon samples stored in ethanol for 1 day prior to
use in DIOS-MS tended to show lower intensities in light-mass background peaks,
improving the detection and assignment of analyte fragments in the lower mass
range. Research from Bohn and Sweedler employed porous silicon substrates prepared from electroless H 2 O 2 –metal–HF (HOME-HF) etching to better understand
the factors that govern analyte desorption and ionization [135]. Variation in either
photoluminescence of porous silicon or the irradiation wavelength of the laser
source was not observed to affect DIOS-MS of peptides; however, DIOS-MS substrates of large pore size typically generated mass spectra with greater reproducibility and higher quality than did specimens with smaller pores (Figure 16.20). As
reported above, it was found that storage of freshly prepared porous silicon substrates in methanol before use produced spectra with enhanced peak intensity and
little noise, reaffirming this post-etching treatment as an effective method for
improving DIOS-MS performance (Figure 16.21). The hydrophobic nature of unfunctionalized hydride-terminated porous silicon tends to restrict diffusion of hydrophilic, aqueous-derived analytes into the porous structure; considering this, it
was believed that the methanol treatment wets the pores and facilitates inclusion
of the analyte within the silicon matrix. The inferior performance of oxidized substrates was justified by acknowledging the different thermal properties of the oxide
overlayer, which may hinder the energy transfer from the laser pulse to the adsorbed analyte. Mass spectrometry of analytes deposited upon porous GaAs or
porous GaN was unsuccessful, implying that only porous silicon may have the
optimum combination of crucial properties to support direct desorption/ionization
of specimens with low fragmentation.
In mechanistic considerations of the desorption/ionization process, comparisons
of DIOS-MS to surface-assisted laser-desorption ionization mass spectrometry
(SALDI-MS) have been outlined by Alimpiev et al. [136]. DIOS-MS of adsorbed
diethylamine was reported both from anodically etched porous silicon and from
ion-etched roughened silicon substrates (Figure 16.22). These results suggest that
desorption/ionization may require not porosity, but perhaps only surface roughness. No spectra were obtained from flat polished crystalline silicon surfaces. In
contrast to the negative findings from porous GaAs and GaN [135], mass spectra
could also be obtained from ion-etched highly oriented pyrolytic graphite (HOPG).
16.4 Chemical Applications Based on Porous Silicon 543