We demonstrate enhanced detection sensitivity of a slow light Mach-Zehnder interferometer (MZI) sensor by incorporating multi-hole defects (MHDs). Slow light MZI biosensors with a one-dimensional photonic crystal in one arm have been previously shown to improve the performance of traditional MZI sensors based on the increased lightmatter interaction that takes place in the photonic crystal region of the structure. Introducing MHDs in the photonic crystal region increases the available surface area for molecular attachment and further increases the enhanced lightmatter interaction capability of slow light MZIs. The MHDs allow analyte to interact with a greater fraction of the guided wave in the MZI. For a slow light MHD MZI sensor with a 16 μm long sensing arm, a bulk sensitivity of 151,000 rad/RIU-cm is demonstrated experimentally, which is approximately two-fold higher than our previously reported slow light MZI sensors and thirteen-fold higher than traditional MZI biosensors with millimeter length sensing regions. For the label-free detection of nucleic acids, the slow light MZI with MHDs also exhibits a two-fold sensitivity improvement in experiment compared to the slow light MZI without MHDs. Because the detection sensitivity of slow light MHD MZIs scales with the length of the sensing arm, the tradeoff between detection limit and device size can be appropriately mitigated for different applications. All experimental results presented in this work are in good agreement with finite difference-time domain-calculations. Overall, the slow light MZI biosensors with MHDs are a promising platform for highly sensitive and multiplexed lab-on-chip systems.
The formation of resonant photonic structures in porous silicon leverages the benefit of high surface area for improved molecular capture that is characteristic of porous materials with the advantage of high detection sensitivity that is a feature of resonant optical devices. This review provides an overview of the biosensing capabilities of a variety of resonant porous silicon photonic structures including microcavities, Bloch surface waves, ring resonators, and annular Bragg resonators. Detection sensitivities > 1000 nm/RIU are achieved for small molecule detection. The challenge of detecting molecules that approach and exceed the pore diameter is also addressed.
A colorimetric biosensing system based on a porous silicon (PSi) rugate filter is demonstrated. Using an imaged-based technique that monitors RGB intensity, a spectral shift less than 0.25nm can be reliably detected. The porous silicon rugate filter demonstrates a sensitivity of 310 nm/RIU, which corresponds to a detection limit near 7×10<sup>-4</sup> RIU. In this work, an external light source and camera are employed for proof-of-concept demonstration. By utilizing a smartphone camera LED and smartphone camera as the light source and detector, respectively, this system could serve as an effective, low-cost, point-of-care diagnostic tool.
Porous silicon structures have been demonstrated as effective biosensors due to their large surface area, size-selective
filtering capabilities, and tunable optical properties. However, porous silicon surfaces are highly susceptible to oxidation
and corrosion in aqueous environments and solutions containing negative charges. In DNA sensing applications, porous
silicon corrosion can mask the DNA binding signal as the typical increase in refractive index that results from a
hybridization event can be countered by the decrease in refractive index due to corrosion of the porous silicon matrix.
Such signal ambiguity should be eliminated in practical devices. In this work, we carefully examined the influence of
charge density and surface passivation on the corrosion process in porous silicon waveguides in order to control this
process in porous silicon based biosensors. Both increased DNA probe density and increased target DNA concentration
enhance the corrosion process, leading to an overall blueshift of the waveguide resonance. While native porous silicon
structures degrade upon prolonged exposure to solutions containing negative charges, porous silicon waveguides that are
sufficiently passivated to prevent oxidation/corrosion in aqueous solution exhibit a saturation effect in the corrosion
process, which increases the reliability of the sensor. For practical implementation of porous silicon DNA sensors, the
negative charges from DNA must be mitigated. We show that a redshift of the porous silicon waveguide resonance
results from either replacing the DNA target with neutral charge PNA or introducing Mg2+ ions to shield the negative
charges of DNA.