controlled, in part, by variation of the etching conditions and the nitrate treatment. Explosion of porous silicon by condensation of dioxygen within the pores
has also been reported [118]. Hydride-passivated porous silicon cooled below 90 K
was spontaneously exploded upon saturation with dioxygen gas. Detonation of
dioxygen-saturated, oxide-terminated substrates could also be induced through illumination with UV light or by weak impact. Such an explosion was calculated to
occur within 1 ms, rivaling many conventional strong munitions, and to expend
nearly 12 kJ g
À1 , exceeding the release of TNT (4.2 kJ g
À1 ). The intensity of the
explosion was sufficient to annihilate the underlying bulk silicon to micron-sized
powder. It has been proposed that the explosion is ‘‘sparked’’ when condensed dioxygen reacts with dangling silicon bonds to yield highly reactive, oxidizing species, specifically hydroxyl, oxygen, and hydrogen radicals. The rapid exothermic
chain reactions of these radicals with silicon surface atoms result in the uncontrolled oxidation and consequent detonation of the porous silicon. Besides atomic
emission, applications that could use this explosive property include microchip arrays for elemental analysis, propulsive devices for microelectromechanical systems
(MEMS), power sources for electronic systems, and self-destructing computer circuits for the preservation of commercial and national securities.
16.4.5
Desorption/Ionization on Silicon Mass Spectrometry (DIOS-MS)
Matrix-assisted laser/desorption ionization mass spectrometry (MALDI-MS), introduced in the 1980s, is widely employed in mass measurements of organic and
biological macromolecules such as proteins, polymers, and drug candidates [119,
120]. In this technique, an analyte is dispersed within an organic matrix exhibiting
high UV absorptivity and this is then irradiated with a UV laser source. Energy
accumulated from the laser pulse by the matrix is transferred to the incorporated
analyte; this exchange induces vaporization and ionization of the dispersed molecules which are then detected and measured by time-of-flight (TOF) mass spectrometry. In contrast to direct desorption/ionization methods from matrix-less
surfaces that may yield a plethora of fragmented ion products from a single
analyte, MALDI-MS is considered a ‘‘soft’’ ablation technique in that the energy
transfer is indirect and relatively gentle, substantially reducing the extent of ion
degradation and simplifying the interpretation of the mass spectra [121–126].
Furthermore, its sensitivity to femtomolar and attomolar levels of analytes and its
tolerance for mixtures and biological contaminants afford the characterization of
minimal sample sizes of only modest purity. Characterization of masses to nearly
200,000 Da in size is possible; however, spectrometry of low-mass analytes is generally not feasible with this method owing to concurrent ablation of the organic
matrix with the cocrystallized analyte. The resultant mass spectrum may exhibit
substantial complexity in the low-mass region due to interference of matrix fragment signals with those of the ionized analyte. To fully optimize this spectrometric
method for specimens of masses less than 1000 Da, the implementation of an
inert and nonablative matrix is imperative.
16 Recent Developments in the Chemistry and Chemical Applications of Porous Silicon
540
has also been reported [118]. Hydride-passivated porous silicon cooled below 90 K
was spontaneously exploded upon saturation with dioxygen gas. Detonation of
dioxygen-saturated, oxide-terminated substrates could also be induced through illumination with UV light or by weak impact. Such an explosion was calculated to
occur within 1 ms, rivaling many conventional strong munitions, and to expend
nearly 12 kJ g
À1 , exceeding the release of TNT (4.2 kJ g
À1 ). The intensity of the
explosion was sufficient to annihilate the underlying bulk silicon to micron-sized
powder. It has been proposed that the explosion is ‘‘sparked’’ when condensed dioxygen reacts with dangling silicon bonds to yield highly reactive, oxidizing species, specifically hydroxyl, oxygen, and hydrogen radicals. The rapid exothermic
chain reactions of these radicals with silicon surface atoms result in the uncontrolled oxidation and consequent detonation of the porous silicon. Besides atomic
emission, applications that could use this explosive property include microchip arrays for elemental analysis, propulsive devices for microelectromechanical systems
(MEMS), power sources for electronic systems, and self-destructing computer circuits for the preservation of commercial and national securities.
16.4.5
Desorption/Ionization on Silicon Mass Spectrometry (DIOS-MS)
Matrix-assisted laser/desorption ionization mass spectrometry (MALDI-MS), introduced in the 1980s, is widely employed in mass measurements of organic and
biological macromolecules such as proteins, polymers, and drug candidates [119,
120]. In this technique, an analyte is dispersed within an organic matrix exhibiting
high UV absorptivity and this is then irradiated with a UV laser source. Energy
accumulated from the laser pulse by the matrix is transferred to the incorporated
analyte; this exchange induces vaporization and ionization of the dispersed molecules which are then detected and measured by time-of-flight (TOF) mass spectrometry. In contrast to direct desorption/ionization methods from matrix-less
surfaces that may yield a plethora of fragmented ion products from a single
analyte, MALDI-MS is considered a ‘‘soft’’ ablation technique in that the energy
transfer is indirect and relatively gentle, substantially reducing the extent of ion
degradation and simplifying the interpretation of the mass spectra [121–126].
Furthermore, its sensitivity to femtomolar and attomolar levels of analytes and its
tolerance for mixtures and biological contaminants afford the characterization of
minimal sample sizes of only modest purity. Characterization of masses to nearly
200,000 Da in size is possible; however, spectrometry of low-mass analytes is generally not feasible with this method owing to concurrent ablation of the organic
matrix with the cocrystallized analyte. The resultant mass spectrum may exhibit
substantial complexity in the low-mass region due to interference of matrix fragment signals with those of the ionized analyte. To fully optimize this spectrometric
method for specimens of masses less than 1000 Da, the implementation of an
inert and nonablative matrix is imperative.
16 Recent Developments in the Chemistry and Chemical Applications of Porous Silicon
540