bears the distinct advantage of acting simultaneously as separator and detector.
Similarly, simultaneous separation and detection of bovine serum albumin (BSA)
was demonstrated using porous silicon prepared through an asymmetric electrochemical etching process, resulting in a gradient of porosities [103]. By measuring
changes in effective optical thickness of porous silicon samples treated with buffered (pH 4) solutions of BSA, penetration of the BSA into the pores could be
identified. Changes in EOT were observed only at regions of the substrate where
the pores were large enough to admit BSA into the porous structure; the dimensions of these pores were confirmed by AFM and SEM measurements. No penetration was measured for BSA solutions at higher pH (7.2), attributed to the
electrostatic repulsion between the negatively charged protein and the inherently
negative oxide-covered porous silicon. Thus, pH-gating may be used in conjunction
with porosity to contain or exclude proteins and other biomolecules from porous
silicon. Related research has further investigated time-resolved detection of other
gaseous organics [104] as well as interferometric sensing of endocrine disruptors
[105].
Besides examining changes in optical reflectivity, detection of chemicals with
porous silicon has also been investigated by quenching of its visible photoluminescence. Content et al. discovered irreversible quenching of the porous silicon
photoluminescence by nitroaromatics [106]. The efficiency of quenching was established to depend upon the vapor pressures of the analytes with nitrobenzene
extinguishing photoluminescence more completely than either dinitrotoluene
(DNT) or trinitrotoluene (TNT). When comparing approximately equal concentrations of analytes, however, exposure to DNT and TNT showed greater diminution in the silicon photoluminescence. These losses were attributed largely to
chemical oxidation of the porous silicon surface. Detection limits of 2 ppb for
DNT and 1 ppb for TNT could be established, compared to 400 ppm for benzene.
Selective detection of DNT and TNT was accomplished by examining the photoluminescence quenching inflicted by the NO 2 [107] produced from catalytic oxidation. The relatively low concentrations of DNT and TNT required for detection,
upon oxidation, produced levels of NO 2 that were below the sensing limit; hence,
only trace photoluminescence reduction was measured from these catalyzed vapors. To contrast, the greater concentration required for nitrobenzene produced
higher levels of NO 2 that quenched photoluminescence nearly equally to the unoxidized organic vapor. This result offers discrimination between these species
through the comparison of their catalyzed and uncatalyzed quenching efficiencies. Chan et al. have developed a sensor that exploits changes in the photoluminescence of porous silicon multilayers upon adsorption of Gram-negative bacteria [108, 109]. Exposure of a porous microcavity resonator functionalized with a
bacterial binding agent to Escherichia coli resulted in a photoluminescence redshift
of several nanometers (Figure 16.15). Other Gram-negative bacterial strains induced similar shifts; however, exposure to Gram-positive species did not affect the
photoluminescence spectra. The change in refractive index of the silicon microcavity upon binding of the bacteria was proposed to induce photoluminescence
shifts in a similar fashion to changes measured for optical reflectance. Photolumi16 Recent Developments in the Chemistry and Chemical Applications of Porous Silicon
536
Similarly, simultaneous separation and detection of bovine serum albumin (BSA)
was demonstrated using porous silicon prepared through an asymmetric electrochemical etching process, resulting in a gradient of porosities [103]. By measuring
changes in effective optical thickness of porous silicon samples treated with buffered (pH 4) solutions of BSA, penetration of the BSA into the pores could be
identified. Changes in EOT were observed only at regions of the substrate where
the pores were large enough to admit BSA into the porous structure; the dimensions of these pores were confirmed by AFM and SEM measurements. No penetration was measured for BSA solutions at higher pH (7.2), attributed to the
electrostatic repulsion between the negatively charged protein and the inherently
negative oxide-covered porous silicon. Thus, pH-gating may be used in conjunction
with porosity to contain or exclude proteins and other biomolecules from porous
silicon. Related research has further investigated time-resolved detection of other
gaseous organics [104] as well as interferometric sensing of endocrine disruptors
[105].
Besides examining changes in optical reflectivity, detection of chemicals with
porous silicon has also been investigated by quenching of its visible photoluminescence. Content et al. discovered irreversible quenching of the porous silicon
photoluminescence by nitroaromatics [106]. The efficiency of quenching was established to depend upon the vapor pressures of the analytes with nitrobenzene
extinguishing photoluminescence more completely than either dinitrotoluene
(DNT) or trinitrotoluene (TNT). When comparing approximately equal concentrations of analytes, however, exposure to DNT and TNT showed greater diminution in the silicon photoluminescence. These losses were attributed largely to
chemical oxidation of the porous silicon surface. Detection limits of 2 ppb for
DNT and 1 ppb for TNT could be established, compared to 400 ppm for benzene.
Selective detection of DNT and TNT was accomplished by examining the photoluminescence quenching inflicted by the NO 2 [107] produced from catalytic oxidation. The relatively low concentrations of DNT and TNT required for detection,
upon oxidation, produced levels of NO 2 that were below the sensing limit; hence,
only trace photoluminescence reduction was measured from these catalyzed vapors. To contrast, the greater concentration required for nitrobenzene produced
higher levels of NO 2 that quenched photoluminescence nearly equally to the unoxidized organic vapor. This result offers discrimination between these species
through the comparison of their catalyzed and uncatalyzed quenching efficiencies. Chan et al. have developed a sensor that exploits changes in the photoluminescence of porous silicon multilayers upon adsorption of Gram-negative bacteria [108, 109]. Exposure of a porous microcavity resonator functionalized with a
bacterial binding agent to Escherichia coli resulted in a photoluminescence redshift
of several nanometers (Figure 16.15). Other Gram-negative bacterial strains induced similar shifts; however, exposure to Gram-positive species did not affect the
photoluminescence spectra. The change in refractive index of the silicon microcavity upon binding of the bacteria was proposed to induce photoluminescence
shifts in a similar fashion to changes measured for optical reflectance. Photolumi16 Recent Developments in the Chemistry and Chemical Applications of Porous Silicon
536