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Optimization for Gold Nanostructure-based Surface Plasmon Biosensors Using a Micro-genetic Algorithm Po-Han Fu, Shu-Cheng Lo, Po-Cheng Tsai, Kuang-Li Lee, and Pei-Kuen Wei ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00136 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 8, 2018
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ACS Photonics
Optimization for Gold Nanostructure-based Surface Plasmon Biosensors Using a Micro-genetic Algorithm Po-Han Fu,1 Shu-Cheng Lo,1 Po-Cheng Tsai,1 Kuang-Li Lee,1 and Pei-Kuen Wei1,2,3* 1
Research Center for Applied Sciences, Academia Sinica, 128, Section 2, Academia Road,
Taipei 11529, Taiwan 2
Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224,
Taiwan 3
Institute of Biophotonics, National Yang-Ming University, Taipei 11221, Taiwan
* Corresponding Author: Pei-Kuen Wei, E-mail:
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KEYWORDS: biosensor, genetic algorithm, nanostructure, optimization, surface plasmon.
ABSTRACT: We present a design method and fabrication technique for enhancing detection sensitivity of gold nanostructure-based surface plasmon resonance (SPR) biosensors. In this case, the nanostructure geometry is arbitrary and the optimal design parameters are determined using a micro-genetic algorithm integrated with three-dimensional finite-difference time-domain (3DFDTD) electromagnetic simulations. The corresponding device is fabricated using the hot embossing nanoimprint lithography. Two experimental configurations are used to evaluate the device performance, one is the atomic layer deposition and the other is the protein-protein interaction. The performances of the optimal SPR biosensors are superior to the ones with conventional simple structures under both configurations. The improvements in terms of the fitness function are respectively up to 179% and 128% for the 3D-FDTD simulation and the experimental results. The calculated optical fields indicate that the improvement of the fitness function is resulted from the enhancement of localized SPR at the edges of gold nanostructures.
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INTRODUCTION Over the decades, surface plasmon resonance (SPR) sensing technique has drawn intensive research interests for various applications including medical diagnostics, environmental monitoring, and food safety owing to the feature of real-time and label-free detections with high sensitivity1-3. One of the most common method using a prism for SPR wave excitation is known as the Kretschmann configuration1, 4, where the propagation constants of the obliquely incident light and the SPR wave should be matched. In addition to the Kretschmann configuration, periodic metallic nanostructures provide an additional lattice momentum for phase matching so that the SPR wave can be generated by normal incidence. Periodic nanostructure-based SPR sensor has a small detection volume and no need of a prism. Owing to the facility for the experimental setups, periodic metallic nanostructures have been utilized for bio-detections5-17. The SPR on the periodic metallic surface is known as the Bloch-wave surface plasmon polarization (BW-SPP). For normal incidence, the dispersion relation of the BW-SPP for onedimensional metallic grating is governed by1
2π
λSPR
ε mn2 εm + n
2
=
2π m P
(1)
,where λSPR is the corresponding resonant wavelength, εm and n are respectively the metal permittivity and effective surface refractive index, m is the diffraction order, and P is the grating periodicity. Note that the biomolecular thickness is usually much smaller than the evanescent length of the surface plasmon field. As shown in Figure 1, the effective index change ∆neff can be approximated by
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∆neff = ( na − ns )[1 − exp(
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−2t )] ld
(2) , where na is the adsorbate biolayer (adlayer) refractive index, ns is the refractive index of the bulk solution, t is the thickness of the adlayer, and ld is the decay length of evanescent field (where the amplitude drops to 1/e). When the biomolecular thickness and refractive indices of the solution and adlayer are known, the SPR sensitivity is determined by resonant quality and the evanescent length. The usual evanescent length of propagating SPR is around 200–300 nm. The length is shortened to tens of nanometer when SPR is localized in a small region. The localized SPR (LSPR) can be formed by metallic nanoparticles, holes or sharp edges. However, The LSPR exhibits a much wider resonant linewidth than the BW-SPP. There is a trade-off between the evanescent length and resonant quality. In this paper, we propose increased LSPR effects in periodic gold-capped-nanostructures by introducing nanometer irregularity in the nanostructures. Such configuration reduces the effective evanescent length. The nanostructures are segmented into several discrete sections where the width and position of every section is arbitrary and the geometry is eventually determined using the genetic algorithms (GAs). GAs are the optimization techniques based on the principle of natural selection. GAs are highly effective in handling the problem with a high dimensional variable search space and the solution is not easy to be predicted based on the knowledge of basic disciplines (i.e., electromagnetics)18-19. In addition, GAs exhibit the flexibility that the fitness function for evaluation can be customized for different design principles. In recent years, GAs have been widely used for the optimization of photonic devices19-26, which are shown to be cost-effective in terms of time. However, three-dimensional
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(3D) simulations are required in this case since the geometry of a SPR biosensor is not infinitive in all x, y, and z axes, which may be time-consuming if a classical simple GA (s-GA) with a large number of individuals (e.g., >100) is used. To address this issue, a micro-GA (µ-GA) is an alternative that is suitable for a problem involving 3D simulations. The µ-GAs use a small population of only five individuals that allows for the evaluation of the entire generation by parallel computing with four CPUs. In addition, the µ-GAs reduce the total elapse time to obtain the optimal solution compared with s-GAs owing to the reduction of the number of total function evaluations27. We demonstrate the design and fabrication of gold nanostructure-based SPR biosensors. The optimal design parameter is determined using a µ-GA integrated with 3D finitedifference time-domain (FDTD) simulations and the resultant device is fabricated using the hot embossing nanoimprint lithography28. The result shows that the performance of a SPR sensor is improved if more degrees of freedom are enabled. The improvement of the fitness function is up to 179% and 128% for the optimal design compared with the conventional design for the FDTD simulation and the experiments, respectively. RESULTS AND DISCUSSION Capped Gold Nanostructure-based SPR Biosensors. Figure 2a shows the general view of a gold nanostructure-based SPR biosensor in one period. Light is normally incident in the ydirection from the bottom of cyclic olefin polymer (COP) substrate and the generated SPR wave propagates in the x-direction. The incident electric field Ex is oriented in the x-direction, which is typically defined as TM polarization and the plasmon field Ey is in the y-direction. The x-z crosssectional view of the capped gold nanostructure for −
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