Seyoung Moon

Target-Localized Nanograting-Based Surface Plasmon Resonance Detection toward Label-free Molecular Biosensing

We explore the sensitivity enhancement of label-free detection based on localized surface plasmon resonance using surface-relief nanograting structures. A nanograting structure was modeled, so that target molecular interactions are localized in hot spots of the near fields. The nanograting structure was optimized numerically for the highest enhancement of sensitivity with hybridization between complementary strands of DNA as the model target interaction. Experimentally, angled evaporation was performed to fabricate the target-localized nanograting samples. Measured data confirm the numerical results that sensitivity enhancement by an order of magnitude may be feasible on a per-unit-volume basis through target localization.

Localized surface plasmon resonance detection of layered biointeractions on metallic subwavelength nanogratings

Enhanced detection of multiple targets such as self-assembled monolayer (SAM) formation, DNA hybridization, and ethanol ambient changes was explored using localized surface plasmon resonance (LSPR) excited by metallic surface nanogratings. The sensitivity enhancement depends on the target as well as the nanostructure with a maximum at 242% over a conventional structure when detecting an 11-mercaptoundecanoic acid SAM with an LSPR structure of 200 nm period. The measured enhancement shows smaller target-dependent variance when detecting various layered biointeractions, while structure-dependent variance was much larger. The result suggests the feasibility of the efficient detection of multiple biointeractions at enhanced sensitivity and extends the applicability of a nanostructured LSPR biosensor for diverse biomolecular events.

Surface-enhanced plasmon resonance detection of nanoparticle-conjugated DNA hybridization.

We have investigated surface-enhanced plasmon resonance detection of DNA hybridization. Surface enhancement was based on the excitation of localized surface plasmon using subwavelength nanogratings, at a 300 nm period, coated with 24-mer ssDNA oligonucleotide, while optical signatures of DNA were amplified at the same time by gold nanoparticles conjugated with complementary ssDNA strands. When using nanoparticles of different sizes, maximum sensitivity enhancement, of more than 18 times, was obtained with nanoparticles of 20 nm diameter. This enhancement is mainly due to nanoparticle-associated signal amplification. Additional surface enhancement boosted the detection sensitivity by 57%. We have also confirmed the sensitivity enhancement to be linearly related to nanoparticle volume.

Grating-based surface plasmon resonance detection of core-shell nanoparticle mediated DNA hybridization

In this report, we have investigated enhanced surface plasmon resonance (SPR) detection of DNA hybridization using gold core - silica shell nanoparticles in localized plasmonic fields. The plasmonic fields were localized by periodic linear gratings. Experimental results measured for hybridization of 24-mer single-stranded DNA oligomers suggest that core-shell nanoparticles (CSNPs) on gratings of 400 nm period provide enhanced optical signatures by 36 times over conventional thin film-based SPR detection. CSNP-mediated DNA hybridization produced 3 times larger angular shift compared to gold nanoparticles of the same core size. We have also analyzed the effect of structural variation. The enhancement using CSNPs was associated with increased surface area and index contrast that is combined by improved plasmon coupling with localized fields on gratings. The combined approach for conjugated measurement of a biomolecular interaction on grating structures is expected to lower the limit of detection to the order of a few tens of fg/mm(2).

Fitting-based determination of an effective medium of a metallic periodic structure and application to photonic crystals

Effective permittivities of a metallic periodic structure for which the second-order effective-medium theory does not yield correct results were obtained by numerically fitting to rigorous-coupled-wave analysis (RCWA). The calculated effective medium showed good agreement with RCWA and minimal deviation in the long-wavelength limit with variation in angle of incidence, grating depth, superstrate, and fill factor. In terms of the standard deviation, the effective medium was least affected by the change in grating depths. The calculated effective permittivities were applied to a three-dimensional metallic photonic-crystal structure and produced a photonic bandgap that is consistent with published experimental data.

A calorimetric biosensor and its application for detecting a cancer cell with optical imaging

This paper reports a biochemical sensor for a label-free detection of cancer cell. A microcalorimeter has monitored the signal created from generated heat due to biochemical reaction between cancer cell and antibody in real time. HER2/neu receptor which is a remarkable biomarker of several cancers, especially breast cancer, was detected by using antibody, in this case HERCEPTIN®. In this study, the microcalorimeter was fabricated by using MEMS (Micro Electro Mechanical Systems)-technology. For higher sensitivity of microcalorimeter, it is important to reduce a parasitic heat transfer from a thermopile to a silicon substrate which has higher thermal conductivity (k = 148 W/m·K). Hot junctions of the proposed microcalorimeter are released from a silicon substrate by bulk micromachining to reduce loss of generated heat by reactions between biomolecules. NIH3T6.7 cell line with overexpressed HER2/neu receptor and FITC conjugated HERCEPTIN® were used. Each volume of biomolecules was 10 µl. HER2/neu receptor was detected by measuring output voltage change of the microcalorimeter about 7.92V/mole. For confirming a reliability of data, the measured data was compared with fluorescent data.

Monte Carlo study of coherent diffuse photon transport in a homogeneous turbid medium: a degree-of-coherence based approach

We employ a Monte Carlo (MC) algorithm to investigate the decoherence of diffuse photons in turbid media. For the MC simulation of coherent photons, the degree of coherence, defined as a random variable for a photon packet, is associated with a decoherence function that depends on the scattering angle and is updated as a photon interacts with a medium via scattering. Using a slab model, the effects of medium scattering properties were studied, which reveals that a linear random variable model for the degree of coherence is in better agreement with experimental results than a sinusoidal model and that decoherence is quick for the initial few scattering events followed by a slow and gradual decrease of coherence.

Investigation of an effective medium theory for metallic periodic structures: a fitting-based approach

While the effective medium theory (EMT) has been useful to explain optical characteristics of a dielectric periodic structure analytically, it has failed to describe metallic structures correctly. In this paper, a fitting-based approach is introduced to applying an effective medium theory to structures that include metallic material. The effective indices of a metallic medium were first obtained by numerically fitting to reflectance characteristics calculated with rigorous coupled wave analysis (RCWA). Searching for an effective medium has been performed through binary searches rather than a time-consuming simulated-annealing algorithm. The calculated effective medium showed results that are in good agreement with RCWA. The deviation was minimal in the long-wavelength limit when angles of incidence, grating depths, or refractive indices of a superstrate are varied. In particular, TE polarization showed more robust features against the variations while TM polarization was more sensitive to the modeling parameters. In terms of the standard deviation, the calculated effective medium was the least affected by the change of grating depths. The applicability of the fitting-based approach was investigated by applying it to a three-dimensional metallic photonic crystal. Simulation results based on the fitting-based EMT perfectly reproduced broad photonic bandgap as observed in published experimental data. Also, the fitting-based approach provided valid results in the wider wavelength range than a traditional EMT.

Gold @ silica core-shell nanoparticle for enhanced surface plasmon resonance detection of DNA hybridization in combination with gold nanowire gratings

Metallic nanoparticles have drawn much interest due to their distinct plasmonic characteristics especially in imaging and sensing applications. Surface plasmon resonance (SPR) based biosensors have evolved in many ways, among which sensitivity enhancement towards molecular sensing capability came up with strategies to overcome the hard limit of the intrinsic sensitivity of gold thin film. Recently adoption of signal contrast materials has proven successful in biochemical sensing applications. This study employs gold-SiO2 core-shell nanoparticles (CSNPs) as a strong SPR signal contrast agents. To reveal the underlying physics for the contrast mechanism, the particle characteristics were analytically evaluated in terms of light interaction coefficients. We experimentally demonstrate the effect of the CSNPs by applying them to acquire enhanced signal in DNA hybridization sensing scheme. We also applied gold nanowire grating structure on conventional gold thin film to further amplify the intrinsic sensitivity, where localized surface plasmon and locally amplified evanescent fields take parts. The results suggest that CSNPs and the grating structure cooperatively enhance the sensitivity and the role of nanowire gratings was analyzed with numerical methods to allow optimum sensitivity enhancement in terms of fill factor variations. The effects of field localization, amplification and enlarged signature of CSNPs are also discussed.

Detection of DNA hybridization with LSPR induced by surface relief nanostructure and particle plasmon

DNA hybridization can be measured with enhanced sensitivity based on localized surface plasmon (LSP) induced by surface nanowire structure. Changes made to the structure result in higher plasmon momentum, which can be coupled to a particle plasmon induced by gold nanoparticles to which DNA molecules are adsorbed. With the insight gained from near-field pattern via calculation, target localization effect is also experimentally shown. We expect that orders of magnitude can be improved in terms of sensitivity if one is to combine the effect of particle-to-LSP coupling and target localization scheme.