Rashmi Sriram

Silicon photonic crystal nanocavity-coupled waveguides for error-corrected optical biosensing.

A photonic crystal (PhC) waveguide based optical biosensor capable of label-free and error-corrected sensing was investigated in this study. The detection principle of the biosensor involved shifts in the resonant mode wavelength of nanocavities coupled to the silicon PhC waveguide due to changes in ambient refractive index. The optical characteristics of the nanocavity structure were predicted by FDTD theoretical methods. The device was fabricated using standard nanolithography and reactive-ion-etching techniques. Experimental results showed that the structure had a refractive index sensitivity of 10(-2) RIU. The biosensing capability of the nanocavity sensor was tested by detecting human IgG molecules. The device sensitivity was found to be 2.3±0.24×10(5) nm/M with an achievable lowest detection limit of 1.5 fg for human IgG molecules. Additionally, experimental results demonstrated that the PhC devices were specific in IgG detection and provided concentration-dependent responses consistent with Langmuir behavior. The PhC devices manifest outstanding potential as microscale label-free error-correcting sensors, and may have future utility as ultrasensitive multiplex devices.

Comparative study of solution–phase and vapor–phase deposition of aminosilanes on silicon dioxide surfaces

The uniformity of aminosilane layers typically used for the modification of hydroxyl bearing surfaces such as silicon dioxide is critical for a wide variety of applications, including biosensors. However, in spite of many studies that have been undertaken on surface silanization, there remains a paucity of easy-to-implement deposition methods reproducibly yielding smooth aminosilane monolayers. In this study, solution- and vapor-phase deposition methods for three aminoalkoxysilanes differing in the number of reactive groups (3-aminopropyl triethoxysilane (APTES), 3-aminopropyl methyl diethoxysilane (APMDES) and 3-aminopropyl dimethyl ethoxysilane (APDMES)) were assessed with the aim of identifying methods that yield highly uniform and reproducible silane layers that are resistant to minor procedural variations. Silane film quality was characterized based on measured thickness, hydrophilicity and surface roughness. Additionally, hydrolytic stability of the films was assessed via these thickness and contact angle values following desorption in water. We found that two simple solution-phase methods, an aqueous deposition of APTES and a toluene based deposition of APDMES, yielded high quality silane layers that exhibit comparable characteristics to those deposited via vapor-phase methods.

Validation of Arrayed Imaging Reflectometry Biosensor Response for Protein-Antibody Interactions: Cross-Correlation of Theory, Experiment, and Complementary Techniques

One of the critical steps in the development of an analytical technique is to confirm that its experimental response correlates with predictions derived from the theoretical framework on which it is based. This validates the technique quantitatively and, in the case of a biosensor, facilitates a correlation of the sensors output signal to the concentration of the analyte being tested. Herein we report studies demonstrating that the quantitative response of arrayed imaging reflectometry (AIR), a highly sensitive label-free biosensing method, is a predictable function of the probe and analyte properties. We first incorporated a standard one-site Langmuir binding model describing probe-analyte interactions at the surface into the theoretical model for thickness-dependent reflectance in AIR. This established a hypothetical correlation between the analyte concentration and the AIR response. Spectroscopic ellipsometry, surface plasmon resonance, and AIR were then used to validate this model for two biomedically important proteins, fibroblast growth factor-2 and vascular endothelial growth factor. While our studies demonstrated that the 1:1 one-site Langmuir model accurately described the observed response of macrospot AIR arrays, either a two-site Langmuir model or a Sips isotherm better described the behavior of AIR microarrays. These studies confirmed the quantitative performance of AIR across a range of probe-analyte affinities. Furthermore, the methodology developed here can be extended to other label-free biosensing platforms, thus facilitating a more accurate and quantitative interpretation of the sensor response.

Microcavities in photonic crystal waveguides for biosensor applications

In this study, resonant microcavities in photonic crystal (PhC) waveguides are investigated for biosensing applications. The device architecture consists of a PhC waveguide with a defect line for guiding the transmission of light. Resonant microcavities created by changing the radius of a hole adjacent to the defect line are coupled to the PhC waveguide. Detection is based on shifts in the resonance wavelength observed in the transmission spectra. The PhC waveguide device is fabricated on silicon-on-insulator (SOI) wafers using electron beam lithography and reactive-ion etching (RIE). Receptor molecules are attached to the defects in the device by standard amino-silane and glutaraldehyde crosslinking chemistry. Preliminary results demonstrate successful detection of human IgG molecules as the target at large concentration levels of 500 μg/ml. Such PhC waveguide devices are advantageous for medical diagnostics and biosecurity applications as they allow rapid, label-free, and sensitive detection of multiple analytes in a single platform.

Two dimensional photonic crystal biosensors as a platform for label-free sensing of biomolecules

Resonant optical microcavites of two-dimensional photonic crystals (2D PhC) are responsive to refractive index changes in the immediate vicinity and thus provide a label-free platform for sensing biological molecules. Because their active sensing volume is ~ 1 μm3, exceptionally sensitive detection of biomolecules is, in principle, achievable from complex biological samples. Previously, we have demonstrated detection of human-IgG protein and virus-like particles by measuring changes in the optical transmission spectrum from the 2D PhC after it has been treated with analyte and dried. However, this drying step restricts practical utility of the platform especially in the case of clinical diagnostics wherein multiple samples need to be tested in short duration. In our progress toward this, we have demonstrated successful integration of microfluidic channels with the 2D PhC device and we further characterized the temperature and bulk refractive index sensitivity of the device.

Towards an optical concentrator for nanoparticles

Photonic crystal (PC) biosensing platforms have the potential to achieve single-pathogen detection using nanoscale optical resonant cavities. Real-time sample analysis requires the PC sensor to be interfaced with a fluidic environment, but current practical fluidic structures typically have dimensions much larger than the PC sensing cavities. To enhance sensing probability, an on-chip optofluidic structure is being developed to concentrate target material within a narrow sensing region of the microfluidic channel. The device relies on fluid drag forces to propel material along the microfluidic channel. Dielectric material is guided transversely within the microfluidic channel by optical gradient forces due to the evanescent field surrounding a ridge waveguide within the channel. Results of computational modeling are presented.

Single-particle detection of virus simulants under microfluidic flow with two-dimensional photonic crystals (Conference Presentation)

Because of their compatibility with standard CMOS fabrication, small footprint, and exceptional sensitivity, Two-Dimensional Photonic Crystals (2D PhCs) have been posited as attractive components for the development of real-time integrated photonic virus sensors. While detection of single virus-sized particles by 2D PhCs has been demonstrated, specific recognition of a virus simulant under conditions relevant to sensor use (including aqueous solution and microfluidic flow) has remained an unsolved challenge. This talk will describe the design and testing of a W1 waveguide-coupled 2D PhC in the context of addressing that challenge.