Girija Gaur

Photonic crystal microring resonator for label-free biosensing

A label-free optical biosensor based on a one-dimensional photonic crystal microring resonator with enhanced light-matter interaction is demonstrated. More than a 2-fold improvement in volumetric and surface sensing sensitivity is achieved compared to conventional microring sensors. The experimental bulk detection sensitivity is ~248nm/RIU and label-free detection of DNA and proteins is reported at the nanomolar scale. With a minimum feature size greater than 100nm, the photonic crystal microring resonator biosensor can be fabricated with the same standard lithographic techniques used to mass fabricate conventional microring resonators.

Comparative Kinetic Analysis of Closed-Ended and Open-Ended Porous Sensors

Efficient mass transport through porous networks is essential for achieving rapid response times in sensing applications utilizing porous materials. In this work, we show that open-ended porous membranes can overcome diffusion challenges experienced by closed-ended porous materials in a microfluidic environment. A theoretical model including both transport and reaction kinetics is employed to study the influence of flow velocity, bulk analyte concentration, analyte diffusivity, and adsorption rate on the performance of open-ended and closed-ended porous sensors integrated with flow cells. The analysis shows that open-ended pores enable analyte flow through the pores and greatly reduce the response time and analyte consumption for detecting large molecules with slow diffusivities compared with closed-ended pores for which analytes largely flow over the pores. Experimental confirmation of the results was carried out with open- and closed-ended porous silicon (PSi) microcavities fabricated in flow-through and flow-over sensor configurations, respectively. The adsorption behavior of small analytes onto the inner surfaces of closed-ended and open-ended PSi membrane microcavities was similar. However, for large analytes, PSi membranes in a flow-through scheme showed significant improvement in response times due to more efficient convective transport of analytes. The experimental results and theoretical analysis provide quantitative estimates of the benefits offered by open-ended porous membranes for different analyte systems.

Influence of interfacial oxide on the optical properties of single layer CdTe/CdS quantum dots in porous silicon scaffolds

Using a combination of continuous wave and time-resolved spectroscopy, we study the effects of interfacial conditions on the radiative lifetimes and photoluminescence intensities of colloidal CdTe/CdS quantum dots (QDs) embedded in a three-dimensional nanostructured silicon (NSi) matrix. The NSi matrix was thermally oxidized under different conditions to change the interfacial oxide thickness. QDs embedded in a NSi matrix with ~0.5 nm of interfacial oxide exhibited reduced photoluminescence intensity and nearly five times shorter radiative lifetimes (~16 ns) compared to QDs immobilized within completely oxidized, nanostructured silica (NSiO2) frameworks (~78 ns). Optical absorption by the sub-nm oxidized NSi matrix partially lowers QD emission intensities while non-radiative carrier recombination and phonon assisted transitions influenced by defect sites within the oxide and NSi are believed to be the primary factors limiting the QD exciton lifetimes in the heterostructures.Using a combination of continuous wave and time-resolved spectroscopy, we study the effects of interfacial conditions on the radiative lifetimes and photoluminescence intensities of sub-monolayer colloidal CdTe/CdS quantum dots (QDs) embedded in a three-dimensional porous silicon (PSi) scaffold. The PSi matrix was thermally oxidized under different conditions to change the interfacial oxide thickness. QDs embedded in a PSi matrix with ∼0.4 nm of interfacial oxide exhibited reduced photoluminescence intensity and nearly five times shorter radiative lifetimes (∼16 ns) compared to QDs immobilized within completely oxidized, porous silica (PSiO2) frameworks (∼78 ns). The exponential dependence of QD lifetime on interfacial oxide thickness in the PSi scaffolds suggests charge transfer plays an important role in the exciton dynamics.

Porous silicon biosensors using quantum dot signal amplifiers

We demonstrate the use of colloidal quantum dots (QDs) as refractive index signal amplifiers for the dual-mode, optical detection of biotin in streptavidin-functionalized porous silicon (PSi) biosensors. The nanoporous silicon host matrix was first analyzed to determine the relationship between different formation conditions, the resulting pore size distributions, and the efficiency with which different sized target molecules infiltrate and bind to the pore walls. Non-specific detection of glutathione conjugated with QDs was then demonstrated using PSi films with different average pore diameters. The specific detection of small biotin molecules was confirmed when the QD-biotin conjugates resulted in a six-fold increase in the reflectance fringe shift and a distinctive fluorescence spectrum upon exposure to an optimized, streptavidin-functionalized PSi sensor. A biotin detection limit of 0.5 fg/mm2 is achievable.

Effects of x-ray and gamma-ray irradiation on the optical properties of quantum dots immobilized in porous silicon

The effects of X-ray and gamma irradiation on the optical properties of CdTe/CdS quantum dots (QDs) immobilized in a functionalized porous silicon film have been investigated via continuous wave and time-resolved photoluminescence measurements. Carrier lifetimes of the QDs and photoluminescence intensities decrease with increasing exposure dose from 500 krad(SiO2) to 16 Mrad(SiO2).

Resonant photonic structures in porous silicon for biosensing

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.

Influence of Ionizing Radiation and the Role of Thiol Ligands on the Reversible Photodarkening of CdTe/CdS Quantum Dots.

We investigate the influence of high energy photons and thiol ligands on the photophysical properties of sub-monolayer CdTe/CdS quantum dots (QDs) immobilized in porous silica (PSiO2) scaffolds. The highly disperse, uniform distributions of QDs in a three-dimensional PSiO2 framework ensure uniform interaction of not only radiation but also subsequent surface repassivation solutions to all immobilized QDs. The high optical densities of QDs achieved using PSiO2 enable straightforward monitoring of the QD photoluminescence intensities and carrier lifetimes. Irradiation of QDs in PSiO2 by high energy photons, X-rays, and γ-rays leads to dose-dependent QD photodarkening, which is accompanied by accelerated photooxidative effects in ambient environments that give rise to blue-shifts in the peak QD emission wavelength. Irradiation in an oxygen-free environment also leads to QD photodarkening but with no accompanying blue-shift of the QD emission. Significant reversal of QD photodarkening is demonstrated following QD surface repassivation with a solution containing free-thiols, suggesting reformation of a CdS shell, etching of surface oxidized species, and possible reduction of photoionized dark QDs to a neutral, bright state. Permanent lattice displacement damage effects may contribute toward some irreversible γ radiation damage. This work contributes to an improved understanding of the influence of surface ligands on the optical properties of QDs and opens up the possibilities of engineering large area, low-cost, reuseable, and flexible QD-based optical radiation sensors.

Porous silicon devices and applications (Conference Presentation)

Three-dimensional nanoporous silicon (PSi), with inherently large surface areas, tunable pore sizes, film thicknesses, and effective refractive indices, has been utilized as a platform for the detection of biomolecules and high-dose radiation. A brief overview of the fabrication and characterization of the nanoporous framework is presented for novel applications that benefit from such sponge-like, high surface area devices. For many of these applications, it is necessary to ensure that the PSi surfaces are well-passivated and stabilized for subsequent conjugation with linker molecules and for emitters to maintain their emissive properties post-integration with the porous matrix. We present a detailed analysis of the influence that varied levels of interfacial oxide (SiOx) growth has on the optical properties of quantum dots (QDs) immobilized within the PSi thin-films. Reflectance spectroscopy, continuous wave photoluminescence (CWPL) and time-resolved photoluminescence (TRPL) studies provide a comprehensive understanding of the complex QD exciton dynamics at the PSi/SiOx-QD interfaces. The gradual conversion of PSi thin-films into fully-oxidized porous silicon oxide (PSiO2) thin-films is shown to significantly suppress non-radiative recombination pathways of photogenerated QD excitons and achieve almost a five-fold increase in QD exciton lifetimes. This conversion of PSi into PSiO2, a wide bandgap nanoporous material, also circumvents loss of QD emission due to absorption by PSi based devices. Future avenues of research into PSi based devices will be presented based on analyzing the optical scattering response of nanoscale PSi annular rings fabricated over PSi Bragg mirrors via dark field microscopy.