Nanostructure-based surface plasmon resonance (SPR) sensors are capable of sensitive, real-time, label-free, and multiplexed detection for chemical and biomedical applications. Recently, the studies of nanostructure-based aluminum sensors have attracted a large attention. However, the intrinsic properties of aluminum metal, having a large imaginary part of the dielectric function and a longer electromagnetic field decay length, limit nanostructure’s surface sensing capability. To improve the surface sensitivity, a nearly guided wave SPR sensor has been proposed, which enables the surface plasmons to spread along the dielectric layer and increases the interaction volume. Here we proposed the combination between Fano resonances in capped nanoslits and a thin nanodielectric top layer to develop highly sensitive nanostructure-based aluminum sensors. We studied the effects of an Al<sub>2</sub>O<sub>3</sub> protection layer on the optical properties, bulk and surface (wavelength and intensity) sensitivities of capped aluminum nanoslits. We found the top layer can enhance the sensitivities of the Wood’s anomaly-dominant resonance or asymmetric Fano resonance in capped aluminum nanoslits. The maximum improvement can be reached by a factor of 16. The maximum wavelength and intensity sensitivities are 6.8 nm/nm and 150 %/nm, respectively. With 1.71 % intensity change (3 times of noise level), the limit of detection of Al<sub>2</sub>O<sub>3</sub> film thickness was 0.018 nm. We attributed the enhanced surface sensitivity for capped aluminum nanoslits to a reduced evanescent length and sharp slope of the asymmetric profile caused by the capped oxide layer and Fano coupling. The protein-protein interaction experiments verified the high sensitivity of the Al<sub>2</sub>O<sub>3</sub>-aliminum capped nanoslits.
We performed experimental measurements and theoretical simulation based on an efficient half-space Green’s function method to investigate the diffraction patterns of light scattered from the biological structure on 1D reflection grating made of metal and polymer. The 1D grating provides higher-order reflected light, which can boost the image signal for off-specular reflection. This can facilitate the micro-ellipsometry imaging experiment when an incident angle of light is at a large angle, while the detection camera is placed at the upright position. The micro-ellipsometry images for s- and p-polarized reflectance and their phase difference (Rs, Rp, and Δ) was taken by a modified Optrel MULTISKOP system with rotating compensator configuration for various angles of incidence and wavelengths ranging from 450nm to 750nm. By using an 80X objective lens, the pixel size for our image is around 164nm. We can further increase the magnification and the numerical aperture by using a substrate collocated with a homemade acrylic resin lens, and the pixel size can be reduced to 50 nm. Based on the above, we study the optical properties of metallic/dielectric nanostructures and nearby biological systems including bacteria, and cancer cells via an imaging micro-ellipsometer combined with detailed theoretical modeling. By using specular and off-specular micro-ellipsometry imaging, we can achieve sufficient sensitivity to collect signals from a small area (around 10μm X 10μm and obtain a 3D image mapping of the morphology and dielectric properties of the biological system of interest.
Development of imaging, sensing, and characterization of cells at Research Center for Applied Sciences (RCAS) of Academia Sinica in Taiwan is progressing rapidly. The research on advanced lattice light sheet microscopy for temporal visualization of cells in three dimensions at sub-cellular resolution shows novel imaging results. Label-free observation on filopodial dynamics provides a convenient assay on cancer cell motility. The newly-developed software enables us to track the movement of two types of particles through different channels and reconstruct the co-localized tracks. Surface plasmon resonance (SPR) for detecting urinary microRNA for diagnosis of acute kidney injury demonstrates excellent sensitivity. A fully automated and integrated portable reader was constructed as a home-based surveillance system for post-operation hepatocellular carcinoma. New microfluidic cell culture devices for fast and accurate characterizations prove various diagnosis capabilities.
An optical label free and high sensitivity plasmonic biosensor using nanoimprint metallic binary grating is presented
based on the phase information of the ellipsometry signal. Plasmonic binary grating was prepared by using soft nanoimprinting
technique which significantly reduce the fabrication cost and can be realized for a transition from a
laboratory-scale method to full-scale technology. The bulk sensitivity measurement from this 1D binary metallic grating
gives a value of refractive index resolution of 1.06×10<sup>-7</sup> RIU. Such a highly sensitive plasmonic biochip was used to
investigate the adsorption of bio-molecules on the nanostructure surface in dynamic mode by monitoring the change in
polarization state or phase of reflected light in the ellipsometry measurement as a sensing signal.
A label free, non-destructive and high sensitivity biosensor with sub-nanometer thickness resolution is presented. We
investigate various sequences of DNA attached on gold nanoparticles (AuNPs) on top of a layer of self assembling
molecules. A strategy to amplify localized surface plasmon resonance (LSPR) response is made by sandwiching DNA
sequences between two AuNPs. We monitor the induced changes in polarization state or phase of reflected light from the
surface as a function of the photon energy as a sensor signal by using ellipsometry and compare that with theoretical
The morphologic changes of living cells under drug interactions were studied by using 80-nm gold nanoparticles and dark-field optical section microscopy. The gold nanoparticles were coated with poly (L-lysine), which attached to the membranes of various cells by way of electrostatic attractive force. A three-dimensional (3-D) morphological image was obtained by measuring the peak scattering intensities of gold nanoparticles at different focal planes. An algorithm for the reconstruction of 3-D cell morphology was presented. With the measured nanoparticle images and calculations, we show morphologic changes of lung cancer cells under the interaction of cytochalasin D drug at different times.
We present a planar evanescent wave (PEW) technique combined with phase contrast optical microscopy to study the interactions between cells and gold nanoparticles (AuNPs). The PEW method employs a dual-fiber-line guide to couple light into a thin glass slide. It produces a uniform and long evanescent wave near the glass surface, as verified by the optical near-field measurement. High-contrast AuNP images are obtained by the PEW illumination. At the same time, cells are observed only by using the phase contrast microscopy. The nanoparticles and cell images indicated that unmodified AuNPs had no interactions with cells, possibly due to the negative surface charges on both cells and nanoparticles. The electrostatic concept was further verified by coating AuNPs with positively charged poly (L-lysine). DNA aptamers for surface mucin glycoprotein were coated on AuNPs to demonstrate the application for single nanoparticle tracking.
Three kinds of periodic gold nanostructures: nanoslit array, nanohole array and nanogrid, were fabricated and compared.
These nanostructures were made on a 130nm-thick gold film with the same 600nm-period. Each array size was
150μm x 150μm in square. The transmission spectra show 630nm peak in air and about 830nm peak in water environment.
These peaks are verified by the resonances of surface plasmonic waves on the outside surface. The wavelength
sensitivities of the surface plasmon resonance in aqueous condition are tested. They are 590.9nm, 556nm and 514nm per
refractive index change for the nanoslit, nanogrid and nanohole structures, respectively. The higher sensitivity of nanoslit
array is attributed to the extraordinary transmission of transverse magnetic wave in the nanoslit gap.
Chip-based biosensor arrays for label-free and high-throughput detection were fabricated and tested. The sensor array was composed of a 150-nm-thick, 50-nm-gap, and 600-nm-period gold nanoslits. Each array size was 100 µm×100 µm. A transverse-magnetic polarized wave in these metallic nanostructures generated resonant surface plasmons at a wavelength of about 800 nm in a water environment. Using the resonant wavelength shift in the nanoslit array, we achieved detection sensitivity up to 668 nm per refractive index unit, about 1.7 times larger than that reported on an array of nanoholes. An antigen–antibody interaction experiment in an aqueous environment verified the sensitivity in a surface binding event.
This study develops a coupled waveguide-surface plasmon resonance (CWSPR) biosensor with a sub-wavelength grating structure for the real-time analysis of biomolecular interactions. In the proposed optical metrology system, normally incident white light is coupled into the waveguide layer through the sub-wavelength grating structure thereby enhancing the wave vector which excites the surface plasmons on the metal sensing surface. The proposed CWSPR biosensor not only retains the same sensing sensitivity as that of a conventional surface plasmon resonance device, but also yields a sharper dip in the reflectivity spectrum and therefore provides an improved measurement precision. Moreover, the metrology setup overcomes the limitations of the conventional Kretschmann attenuated total reflection approach and is less sensitive to slight variations in the angle of the incident light. The experimental results confirm that the current CWSPR biosensor provides a straightforward yet powerful technique for real-time biomolecular interaction analysis.
A coupled waveguide-surface plasmon resonance (CWSPR) biosensor constructed with sub-wavelength grating structure is developed and used to analyze biomolecular interaction in real time. The normal incident white light is coupled into the waveguide layer through the sub-wavelength grating, and hence it has an enhanced wave vector to excite the localized surface plasmons on the metal grating surface. The CWSPR biosensor with the surface plasmon resonance (SPR) mode and the waveguide mode not only retains the same sensing sensitivity as that of a conventional SPR device, but also yields sharper dips in the reflectivity spectrum and therefore provides an improved measurement precision. Moreover, without the limitation of a conventional attenuated total reflection coupler and with the help of
normal incidence, the system is more flexible and feasible for protein microarray and imaging applications.
We propose a fiber-optic biosensor based on the generation of local surface plasmons (LSR) near a nano fiber tip. The nano-optical fiber biosensor was made by shaping the fiber to form a taper with a tip size less than 100nm. Gold nanoparticles with 12nm diameter were immobilized to the tapered fiber's tip by modifying tip surface with NH<sub>2</sub> groups. Most of light in the fiber tip is confined by the total internal reflection. It generates substantial evanescent wave near the tip and effectively excites LSR on the nanogolds. The evanescent wave excitation results in very low background light and high signal to noise ratio. Using this nanofiber sensor, we have achieved a sensitivity of reflective index unit, ~4800 (% RIU<sup>-1</sup>) in the intensity measurement. Furthermore, the LSR is excited only near the tip region. It takes advantage of ultra small detection area. Only micro-liter sample solution is needed for the detection.
We present a sensitive nano-optical fiber biosensor made by shaping a fiber to form a taper with a tip size under 100 nm. A 40-nm-thick layer of gold is coated around the tapered fiber and a surface plasmon wave is excited near the tip to achieve a sensitivity of the reflective index unit of ~4000 (% RIU–1) in the intensity measurement. A 3-D coded finite-difference time-domain approach verifies the excitation of the surface plasmon wave and the differences among its intensities in media of various refractive indices. The nanotip fiber sensor has the merits of a low background light and an ultrasmall detection region. Only a microliter of sample solution is required for detection.
In this paper, the reflection resonance spectrum of a sub-wavelength diffraction grating-coupled waveguide is used to analyze biomolecular interactions in real time. When the diffraction grating waveguide structure is destroyed by external factors such as slight refractive index changes of the buffer or molecule adsorption on the grating surface, the optical path of the light coupled through the grating into the waveguide is changed and a resonance wavelength shift is induced as a result. By detecting this resonance wavelength shift, the optical waveguide biosensor provides the ability to identify the kinetics of the biomolecular interaction on an on-line basis without the need for the extrinsic labeling of the biomolecules. A theoretical analysis of the sub-wavelength optical waveguide biosensor is performed. A biosensor with a narrow reflection resonance spectrum, and hence an enhanced detection resolution, is then designed and fabricated. Currently, the detection limit of the optical waveguide sensor is found to be approximately 10<sup>-5</sup> refractive index units. The developed biosensor is successfully applied to study the kinetics of an antibody interaction with protein G adsorbed on the sensing surface.
We present a new method to improve the brightness of tapered fiber probes for near-field scanning optical microscope. The new probes are fabricated by adding high refractive index materials onto the pulled tapered fiber tips before coated with metal. With a tip size of 100 nm, the far-field optical power of the new tapered probe which has 25 nm thickness of zinc sulfide on tip end is about 5 times larger than the same sized traditional fiber probe.
We have used the near-field scanning optical microscope (NSOM) to study the inhomogeneity as well as initiate photochemical processes in conjugated polymer films. A simple transmission-mode NSOM is constructed for these studies. A low noise, large area Si photo-detector is mounted directly between the PZT scanning stage and the sample. This method provides a simple way to covert the commercial AFM/STM scanning stage to a near-field optical microscope.