Nak-Hyeon Kim

Correlation analysis between plasmon field distribution and sensitivity enhancement in reflection- and transmission-type localized surface plasmon resonance biosensors

We examine the correlation between the plasmon field distribution and the sensitivity enhancement for both reflection- and transmission-type localized surface plasmon resonance (LSPR) biosensors with surface-relief gold nanogratings. In our calculation, the near-field characteristics are obtained from the finite-difference time-domain method and compared with the refractive index sensitivity as a unit target sample moves along the sensor surface. The numerical results show that the highest enhancement of sensitivity is found at the lower grating corners where an interplay between the target sample and the locally enhanced field can occur efficiently. This study suggests that, by localizing biomolecular interactions to the highly enhanced field, we can achieve a significantly improved LSPR detection with high sensitivity and a great linearity in a wide dynamic range.

Numerical study on an application of subwavelength dielectric gratings for high-sensitivity plasmonic detection

Although subwavelength dielectric gratings can be employed to achieve a high sensitivity of the surface plasmon resonance (SPR) biosensor, the plasmonic interpretation verifying the resulting sensitivity improvement remains unclear. The aim of this study is to elucidate the effects of the gratings geometric parameters on the amplification of SPR responses and to understand the physical mechanisms associated with the enhancement. Our numerical results show that the proposed SPR substrate with a dielectric grating can provide a better sensitivity due to the combined effects of surface reaction area and field distribution at the binding region. An influence of adhesion layer on the sensor performance is also discussed. The obtained results will be promising in high-sensitivity plasmonic biosensing applications.

Plasmonic wavelength splitter based on a large-area dielectric grating and white light illumination

An optical process by which transmission wavelengths can be divided selectively by changing a resonance condition of surface plasmons (SPs) is demonstrated. When white light is incident to an SP resonance substrate with a dielectric grating, SP waves are excited at resonance and transmitted into the air via diffraction by a large-area grating pattern fabricated by nanoimprint lithography. While only a limited range of certain wavelengths is allowed to transmit, the peak transmission wavelength can be tuned continuously in the visible band. We also show that multiple wavelengths are transmitted into different directions simultaneously by using a wedge-shaped white light.

Enhancement of field–analyte interaction at metallic nanogap arrays for sensitive localized surface plasmon resonance detection

We investigated the near-field enhancement of a localized surface plasmon resonance (LSPR) structure based on gold nanograting pairs with a nanosized gap. The results calculated by finite-difference time-domain and rigorous coupled-wave analysis methods presented that the nanogap enclosed by two neighboring nanogratings produced significant confinement and enhancement of electromagnetic fields and allowed a sensitive detection in sensing of surface binding events. Gold gratings with a narrow gap distance less than 10 nm showed enhanced refractive index sensitivity due to the intensified optical field at the nanogap, outperforming the LSPR structure with noninteracting nanogratings. Also, we analyzed the effectiveness of using an overlap integral (OI) between analyte and local plasmon field to estimate the detection sensitivity. We found a strong correlation of field-analyte OI with far-field sensor sensitivity.

How to avoid a negative shift in reflection-type surface plasmon resonance biosensors with metallic nanostructures

We experimentally demonstrate that introduction of a dielectric film can prevent the surface plasmon resonance (SPR) curve from being shifted to a smaller angle, called negative shift, which occurs unpredictably when metallic nanostructures deposited on a metal film are exposed to an adsorption of binding analytes. From parylene coating experiments, we find that the proposed reflection-type SPR system with a low refractive index MgF2 film and gold nanorods can provide an enhanced sensitivity by more than 6 times as well as a reliable positive shift. It is due to the fact that use of a dielectric film can contribute to the compensation of an anomalous dispersion relation and the prevention of a destructive interaction of propagating surface plasmons with multiple localized plasmon modes. Our approach is intended to show the feasibility and extend the applicability of the proposed SPR system to diverse biomolecular reactions.

Plasmonic metal–dielectric–metal stack structure with subwavelength metallic gratings for improving sensor sensitivity and signal quality

In this study, we investigated the performance improvement of a localized surface plasmon resonance (LSPR) biosensor by incorporating a metal-dielectric-metal (MDM) stack structure and subwavelength metallic nanograting. The numerical results showed that the LSPR substrate with a MDM stack can provide not only a better sensitivity by more than five times but also a notably improved signal quality. While the gold nanogratings on a gold film inevitably lead to a broad and shallow reflectance curve, the presence of a MDM stack can prevent propagating surface plasmons from interference by locally enhanced fields excited at the gold nanogratings, finally resulting in a strong and deep absorption band at resonance. Therefore, the proposed LSPR structure could potentially open a new possibility of enhanced detection for monitoring biomolecular interactions of very low molecular weights.

Optical determination of thick graphene layer number based on surface plasmon resonance

Abstract. An optical method of measuring the number of layers in a graphene sample is formulated and compared with the conventional surface plasmon resonance (SPR) detection scheme, the latter being appropriate only for a very few graphene layers. Numerical results based on transfer-matrix method support that an alternative method, wherein the SPR substrate includes a dielectric overlayer, is feasible over a wide range of graphene layer numbers. While the multilayer graphene may lead to a broad and shallow SPR curve owing to the nonzero imaginary part in its relative permittivity, the dielectric overlayer makes the resonant surface plasmons less affected by graphene, resulting in a strong and deep absorption band at resonance. Linear regression analysis shows that the measurable graphene layer number can be as high as 50.

Improved biomolecular detection based on a plasmonic nanoporous gold film fabricated by oblique angle deposition.

We demonstrated an enhanced surface plasmon resonance (SPR) detection by incorporating a nanoporous gold film on a thin gold substrate. Nanoscale control of thickness and roughness of the nanoporous layer was successfully accomplished by oblique angle deposition. In biosensing experiments, the results obtained by biotin-streptavidin interaction showed that SPR samples with a nanoporous gold layer provided a notable sensitivity improvement compared to a conventional bare gold film, which is attributed to an excitation of local plasmon field and an increased surface reaction area. Imaging sensitivity enhancement factor was employed to estimate an overall sensor performance of the fabricated samples and an optimal SPR structure was determined. Our approach is intended to show the feasibility and extend the applicability of the nanoporous gold film-mediated SPR biosensor to diverse biomolecular binding events.

In Vitro Biocompatibility Test of Multi-layered Plasmonic Substrates with Flint Glasses and Adhesion Films

Since in vitro neural recording and imaging applications based on a surface plasmon resonance (SPR) technique have expanded dramatically in recent years, cytotoxicity assessment to ensure the biosafety and biocompatibility for those applications is crucial. Here, we report the cytotoxicity of the SPR substrate incorporating a flint glass whose refractive index is larger than that of a conventional crown glass. A high refractive index glass substrate is essential in neural signal detection due to the advantages such as high sensitivity and wide dynamic range. From experimental data using primary hippocampal neurons, it is found that a lead-based flint glass is not appropriate as a neural recording template although the neuron cells are not directly attached to the toxic glass. We also demonstrate that the adhesion layer between the glass substrate and the gold film plays an important role in achieving the substrate stability and the cell viability.