Fan-Ching Chien

Optical waveguide biosensors constructed with subwavelength gratings

The reflection resonance spectrum of a subwavelength diffraction-grating-coupled waveguide is used to analyze biomolecular interactions in real time. 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 extrinsic labeling of the biomolecules. A theoretical analysis of the subwavelength 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 approximately 10(-5) refractive-index units. The biosensor is successfully applied to study of the dynamic response of an antibody interaction with protein G adsorbed on the sensing surface.

Angular-interrogation attenuated total reflection metrology system for plasmonic sensors.

We develop an angular-interrogation attenuated total reflection (ATR) metrology system for three different plasmonic sensors, namely, a conventional surface plasmon resonance (SPR) device, a coupled-waveguide SPR device, and a nanoparticle-enhanced SPR device. The proposed metrology system is capable of measuring the reflectivity spectra of the transverse magnetic mode and the transverse electric mode simultaneously. Through the optimal control of the fabrication process and use of sophisticated system instrumentation, the experimental results confirm that the developed ATR system is capable of measuring the resonant angle with an angular accuracy of 10(-4) deg.

Nonlinear structured-illumination enhanced temporal focusing multiphoton excitation microscopy with a digital micromirror device

In this study, the light diffraction of temporal focusing multiphoton excitation microscopy (TFMPEM) and the excitation patterning of nonlinear structured-illumination microscopy (NSIM) can be simultaneously and accurately implemented via a single high-resolution digital micromirror device. The lateral and axial spatial resolutions of the TFMPEM are remarkably improved through the second-order NSIM and projected structured light, respectively. The experimental results demonstrate that the lateral and axial resolutions are enhanced from 397 nm to 168 nm (2.4-fold) and from 2.33 μm to 1.22 μm (1.9-fold), respectively, in full width at the half maximum. Furthermore, a three-dimensionally rendered image of a cytoskeleton cell featuring ~25 nm microtubules is improved, with other microtubules at a distance near the lateral resolution of 168 nm also able to be distinguished.

Dynamic particle tracking via temporal focusing multiphoton microscopy with astigmatism imaging

A three-dimensional (3D) single fluorescent particle tracking strategy based on temporal focusing multiphoton excitation microscopy (TFMPEM) combined with astigmatism imaging is proposed for delivering nanoscale-level axial information that reveals 3D trajectories of single fluorospheres in the axially-resolved multiphoton excitation volume without z-axis scanning. Whereas other scanning spatial focusing multiphoton excitation schemes induce optical trapping interference, temporal focusing multiphoton excitation produces widefield illumination with minimum optical trapping force on the fluorospheres. Currently, the lateral and axial positioning resolutions of the dynamic particle tracking approach are about 14 nm and 21 nm in standard deviation, respectively. Furthermore, the motion behavior and diffusion coefficients of fluorospheres in glycerol solutions with different concentrations are dynamically measured at a frame rate up to 100 Hz. This TFMPEM with astigmatism imaging holds great promise for exploring dynamic molecular behavior deep inside biotissues via its superior penetration, reduced trapping effect, fast frame rate, and nanoscale-level positioning.

Surface plasmon resonance biosensors with subwavelength grating waveguide

In this study, a surface plasmon resonance (SPR) biosensor with sub-wavelength grating waveguide for the real-time analysis of biomolecular interactions is developed. The conventional SPR has diffractive grating structure to increase the wave vector for exciting the surface plasmons and then detects biomolecular interactions in high order diffraction light. Using this approach has some disadvantages such as the intensity of high order diffraction light is dimmer to be difficult to measure and the measured reflectivity spectrum is too broadened. The proposed SPR biosensor uses a normally incident white light with the help of subwavelength grating structure and provides a sharper reflectivity spectrum according to waveguide interference both to avoid disadvantages of the conventional SPR biosensor with a grating coupler. When the diffraction grating waveguide structure and the condition of SPR are destroyed by external factors such as slight refractive index changes of the buffer or molecule adsorption on the grating surface, the optical path and momentum of the light coupled through the gold grating into the waveguide are changed and a resonance wavelength shift is induced as a result. By detecting this resonance wavelength shift, the SPR 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. The proposed biosensing metrology system becomes more simply and convenient for real-time biomolecular interaction analysis.

Precise determination of the dielectric constant and thickness of a nanolayer by use of surface plasmon resonance sensing and multiexperiment linear data analysis.

Surface plasmon resonance (SPR) sensing and an enhanced data analysis technique are used to obtain precise predictions of the dielectric constant and thickness of a nanolayer. In the proposed approach, a modified analytical method is used to obtain initial estimates of the dielectric constants and thicknesses of the metal film and a nanolayer on the sensing surface of a SPR sensor. A multiexperiment data analysis approach based on a two-solvent SPR method is then employed to improve the initial estimates by suppressing the noise in the measurement data. The proposed two-stage approach is employed to determine the dielectric constant and thickness of a molecular imprinting polymer nanolayer. It is found that the results are in good agreement with those obtained with an ellipsometer and a high-resolution scanning electron microscope.

Multi-experiment linear data analysis for ATR biosensors

The biosensors based on surface plasmon resonance (SPR) are often used as tools for directly detecting the kinetic interaction of unlabelled biological molecules at surface in real time. With the measured SPR reflection spectrum, we can detect a shift in the location and quantity of the reflection spectrum minimum and the half width at half maximum due to the change in the thickness or the refractive index of a thin dielectric film layer. The interested parameters of analyte layer or monolayer, like the molecular size and concentration, can be determined either with analytical approaches or linear data analysis approaches. Depends on the number of parameters need to be resolved, we may need either multiple spectra (two color method) or only one sensing spectrum under the assumption that the other film parameter is given for multiple parameters case. Although it is possible to estimate multiple parameters from only one sensing spectrum by linear estimation techniques, it suffers from not only the shortcoming for larger variance in the estimates from those techniques than that of multiple spectra method but also the difficulty for choosing the appropriate initial value in the estimation process. In this paper, we propose a modified analytic approach to attain suitable initial parameters that close enough to the exact value. Furthermore, we incorporated multi-experiment method into linear estimation algorithms to determine the optimal estimated parameters with smaller variability of the estimated parameters. In that manner, it would be benefit to reject the colored noise accidentally results from experiment process. The experimental data with the multi-experiment linear data analysis demonstrates that it has ability to sense slightly index change in consequence of argon gas flow through the nitrogen.

Plasmon-enhanced optical waveguide biosensors constructed with sub-wavelength gold grating

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.

An investigation into the influence of secondary structures for DNA hybridization using surface plasmon resonance and surface-enhanced Raman scattering

This study utilizes a surface plasmon resonance (SPR) biosensing to investigate the influence of secondary structures on the DNA hybridization and a surface-enhanced Raman scattering (SERS) spectrum to yield analytical data regarding the structure of the oligonucleotides. It is found that the SPR angular shifts associated with the three pairs of 60mer oligonucleotides with prominent secondary structures are lower than those observed for the two pairs of oligonucleotides with no obvious secondary structures. It is also determined that increasing the DNA hybridization temperature from 35 oC to 45 oC reduces secondary structure effects. On the hybridization with mixture target oligonucleotides, the SPR results demonstrate that secondary structures interfere significantly. Although the kinetics of biomolecular interaction analysis is performed by using SPR sensor, the structural information of the oligonucleotides can not observed directly. The SERS spectrum provides the structural information of the oligonucleotides with silver colloidal nanoparticles adapted as a Raman active substrate. Also, the detection limit of the DNA Raman signal has been successfully improved to reach sub-micro molarity of DNA concentration.

A theoretical and experimental investigation into the enhancement of near electro-magnetic field via plasmonic effects

In this study, we use the finite-difference time-domain (FDTD) method and an attenuated-total-reflection (ATR) fluorescent optical microscope to investigate into the enhancement of near electro-magnetic (EM) field via plasmonic effects. In order to enhance the near EM field on the sensing surface, a metallic particle layer is added under the Kretschmann configuration of the conventional surface plasmon resonance sensor based on the ATR method. The affiliation by the simulation and experimental results can help us to understand the mechanisms of surface plasmons and particle plasmons on the sensor surface, and the effects of the EM field enhancement are classified as the surface plasmon effect, particle plasmon effect, interparticle coupling effect, and gap mode effect. By analyzing and comparing the results based on the FDTD method and the ATR fluorescent microscope, we can understand more about the plasmonic effects in order to deign a novel ultra-high resolution plasmonic biosensor.