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Porous silicon Bloch surface and sub-surface wave structure for simultaneous detection of small and large molecules.

Rodriguez GA, Lonai JD, Mernaugh RL, Weiss SM - Nanoscale Res Lett (2014)

Bottom Line: The BSW/BSSW structure consists of a periodic stack of high and low refractive index PSi layers and a reduced optical thickness surface layer that gives rise to a BSW with an evanescent tail that extends above the surface to enable the detection of large surface-bound molecules.The step and gradient BSW/BSSW sensors are designed to maximize both resonance reflectance intensity and sensitivity to large molecules.Size-selective detection of large molecules including latex nanospheres and the M13KO7 bacteriophage as well as small chemical linker molecules is reported.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37235, USA.

ABSTRACT
A porous silicon (PSi) Bloch surface wave (BSW) and Bloch sub-surface wave (BSSW) composite biosensor is designed and used for the size-selective detection of both small and large molecules. The BSW/BSSW structure consists of a periodic stack of high and low refractive index PSi layers and a reduced optical thickness surface layer that gives rise to a BSW with an evanescent tail that extends above the surface to enable the detection of large surface-bound molecules. Small molecules were detected in the sensor by the BSSW, which is a large electric field intensity spatially localized to a desired region of the Bragg mirror and is generated by the implementation of a step or gradient refractive index profile within the Bragg mirror. The step and gradient BSW/BSSW sensors are designed to maximize both resonance reflectance intensity and sensitivity to large molecules. Size-selective detection of large molecules including latex nanospheres and the M13KO7 bacteriophage as well as small chemical linker molecules is reported.

No MeSH data available.


Simulated and experimental reflectance spectra of optimized (a) step and (b) gradient index PSi BSW/BSSW sensor in air. The resonance at the lowest angle for each sensor corresponds to the BSW mode while the other resonances are BSSW modes. Simulations show good agreement with experiment, with small error derived from nonlinear refractive index changes within the PSi multilayer.
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Figure 3: Simulated and experimental reflectance spectra of optimized (a) step and (b) gradient index PSi BSW/BSSW sensor in air. The resonance at the lowest angle for each sensor corresponds to the BSW mode while the other resonances are BSSW modes. Simulations show good agreement with experiment, with small error derived from nonlinear refractive index changes within the PSi multilayer.

Mentions: A resonance condition is distinctly excited when the effective index of a BSW or BSSW mode is matched by the coupling conditions of either a prism or diffraction grating. Prism coupling is compatible with existing surface plasmon resonance biosensing instrumentation. Grating coupling allows for more compact devices, which could be used for point of care diagnostics with microfluidics integration [21]. The BSW mode is confined by the band gap created by the Bragg mirror and by total internal reflection near the surface. Similarly, by reducing the optical thickness of one or more layers within the multilayer through the introduction of a step or gradient refractive index profile, BSSW modes with different effective indices can be supported within the multilayer. The implementation of a single step to break the periodicity of the Bragg mirror refractive index profile shifts the band edge of the Bragg mirror and gives rise to a single BSSW mode confined within the corresponding layer with reduced optical thickness. By creating a gradient refractive index profile, a varying band edge within the multilayer allows a distinct mode within each H layer until prohibited by coupling losses or a band gap is no longer sustained [8]. Although a large sensitivity is important in biosensor design, a sharp and distinct resonance will enhance the minimum detectable shift for an improved detection limit. Therefore, in the design of the step and gradient profile structures, a tradeoff between sensitivity of the resonance position to small changes in refractive index and the resonance intensity was considered. A very small step or gradient refractive index change leads to a very large BSSW sensitivity. However, similar to a WG, the resonance intensity and mode confinement are reduced with a small refractive index contrast between H and L layers due to the reduced mirror strength of the multilayer. For very large refractive index changes within the multilayer, field confinement is increased, resulting in a sharp and distinct resonance; however, BSSW sensitivity decreases as a result of decreased surface area for molecular capture [22]. Figure 3 shows both the simulated (RCWA) and experimental angle-resolved reflectance spectra of an optimized grating-coupled step and gradient index BSW/BSSW sensor. In Figure 3a, the BSW resonance is located at approximately 21° and the single step BSSW mode is located at approximately 25°. In Figure 3b, the BSW mode is located at approximately 15° and the remaining peaks correspond to the different BSSW orders created by the gradient index profile. The different resonance angles are a result of the different refractive index step and gradient depth profiles used in the optimization. Good agreement is observed between the simulations and experiment. Minor deviations are likely a result of a nonlinear refractive index gradient or step caused by the KOH etch [8]. Both the step and gradient BSW/BSSW designs are suitable for size-selective sensing applications. However, the step index sensor has a higher detection sensitivity due to the single well-confined BSSW resonance, as shown in the field profile in Figure 1b, while the gradient index sensor with multiple BSSW modes spatially distributed within several high index layers of the multilayer allows for the determination of the depth of infiltration of molecules within the multilayer.


Porous silicon Bloch surface and sub-surface wave structure for simultaneous detection of small and large molecules.

Rodriguez GA, Lonai JD, Mernaugh RL, Weiss SM - Nanoscale Res Lett (2014)

Simulated and experimental reflectance spectra of optimized (a) step and (b) gradient index PSi BSW/BSSW sensor in air. The resonance at the lowest angle for each sensor corresponds to the BSW mode while the other resonances are BSSW modes. Simulations show good agreement with experiment, with small error derived from nonlinear refractive index changes within the PSi multilayer.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4128542&req=5

Figure 3: Simulated and experimental reflectance spectra of optimized (a) step and (b) gradient index PSi BSW/BSSW sensor in air. The resonance at the lowest angle for each sensor corresponds to the BSW mode while the other resonances are BSSW modes. Simulations show good agreement with experiment, with small error derived from nonlinear refractive index changes within the PSi multilayer.
Mentions: A resonance condition is distinctly excited when the effective index of a BSW or BSSW mode is matched by the coupling conditions of either a prism or diffraction grating. Prism coupling is compatible with existing surface plasmon resonance biosensing instrumentation. Grating coupling allows for more compact devices, which could be used for point of care diagnostics with microfluidics integration [21]. The BSW mode is confined by the band gap created by the Bragg mirror and by total internal reflection near the surface. Similarly, by reducing the optical thickness of one or more layers within the multilayer through the introduction of a step or gradient refractive index profile, BSSW modes with different effective indices can be supported within the multilayer. The implementation of a single step to break the periodicity of the Bragg mirror refractive index profile shifts the band edge of the Bragg mirror and gives rise to a single BSSW mode confined within the corresponding layer with reduced optical thickness. By creating a gradient refractive index profile, a varying band edge within the multilayer allows a distinct mode within each H layer until prohibited by coupling losses or a band gap is no longer sustained [8]. Although a large sensitivity is important in biosensor design, a sharp and distinct resonance will enhance the minimum detectable shift for an improved detection limit. Therefore, in the design of the step and gradient profile structures, a tradeoff between sensitivity of the resonance position to small changes in refractive index and the resonance intensity was considered. A very small step or gradient refractive index change leads to a very large BSSW sensitivity. However, similar to a WG, the resonance intensity and mode confinement are reduced with a small refractive index contrast between H and L layers due to the reduced mirror strength of the multilayer. For very large refractive index changes within the multilayer, field confinement is increased, resulting in a sharp and distinct resonance; however, BSSW sensitivity decreases as a result of decreased surface area for molecular capture [22]. Figure 3 shows both the simulated (RCWA) and experimental angle-resolved reflectance spectra of an optimized grating-coupled step and gradient index BSW/BSSW sensor. In Figure 3a, the BSW resonance is located at approximately 21° and the single step BSSW mode is located at approximately 25°. In Figure 3b, the BSW mode is located at approximately 15° and the remaining peaks correspond to the different BSSW orders created by the gradient index profile. The different resonance angles are a result of the different refractive index step and gradient depth profiles used in the optimization. Good agreement is observed between the simulations and experiment. Minor deviations are likely a result of a nonlinear refractive index gradient or step caused by the KOH etch [8]. Both the step and gradient BSW/BSSW designs are suitable for size-selective sensing applications. However, the step index sensor has a higher detection sensitivity due to the single well-confined BSSW resonance, as shown in the field profile in Figure 1b, while the gradient index sensor with multiple BSSW modes spatially distributed within several high index layers of the multilayer allows for the determination of the depth of infiltration of molecules within the multilayer.

Bottom Line: The BSW/BSSW structure consists of a periodic stack of high and low refractive index PSi layers and a reduced optical thickness surface layer that gives rise to a BSW with an evanescent tail that extends above the surface to enable the detection of large surface-bound molecules.The step and gradient BSW/BSSW sensors are designed to maximize both resonance reflectance intensity and sensitivity to large molecules.Size-selective detection of large molecules including latex nanospheres and the M13KO7 bacteriophage as well as small chemical linker molecules is reported.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37235, USA.

ABSTRACT
A porous silicon (PSi) Bloch surface wave (BSW) and Bloch sub-surface wave (BSSW) composite biosensor is designed and used for the size-selective detection of both small and large molecules. The BSW/BSSW structure consists of a periodic stack of high and low refractive index PSi layers and a reduced optical thickness surface layer that gives rise to a BSW with an evanescent tail that extends above the surface to enable the detection of large surface-bound molecules. Small molecules were detected in the sensor by the BSSW, which is a large electric field intensity spatially localized to a desired region of the Bragg mirror and is generated by the implementation of a step or gradient refractive index profile within the Bragg mirror. The step and gradient BSW/BSSW sensors are designed to maximize both resonance reflectance intensity and sensitivity to large molecules. Size-selective detection of large molecules including latex nanospheres and the M13KO7 bacteriophage as well as small chemical linker molecules is reported.

No MeSH data available.