Abstract
Biological detection based on surface plasmon resonances (SPRs) on metallic Ga-doped zinc oxide (ZnO: Ga) film surfaces is introduced as one of the interesting functionalities of ZnO. SPRs on ZnO: Ga films (ZnO-SPRs) have attracted much attention as alternative plasmonic materials in the infrared (IR) range. This chapter focuses on the structure and optical properties of ZnO-SPR with different layer structure from experimental and theoretical approaches. First, the plasmonic properties of single ZnO: Ga films excited by Kretschmann-type SPRs were investigated. Second, an insulator–metal–insulator structure with a ZnO: Ga film applied as a metal layer is introduced. Finally, hybrid layer structures with the capping of thin dielectric layers to ZnO-SPR (dielectric-assisted ZnO-SPR) were fabricated to enhance SPR properties in the IR range. The biological sensing on ZnO-SPR is experimentally demonstrated by measuring biological interactions. This work provides new insights for fabricating biological sensing platforms on ZnO materials.
Keywords
- oxide semiconductor
- surface plasmon
- near-infrared
- biological detection
- zinc oxide
- heterointerface
1. Introduction
The impurity dopants in zinc oxide (ZnO) produce interesting optical, electrical and magnetic functionalities. For example, photoluminescence behaviors are observed by doping with rare-earth and transition-metal atoms [1, 2, 3, 4]. Ferromagnetic properties are found by the incorporation of transition-metal atoms into the host [5, 6, 7]. Above all, the donor dopants, such as Ga and Al ions, lead to the metallic conductivity of ZnO which has been widely used as transparent electrodes [8, 9]. The metallic properties of ZnO are also prospective candidates for alternatively plasmonic materials in the infrared (IR) range [10, 11, 12, 13]. Unlike classical plasmonic materials such as Ag and Au metals, plasmonic resonances can be controlled by changing electron density in ZnO. Additionally, the electronic structure of ZnO is composed of 4
It is known that ZnO is one of the interesting materials in biological sensing.
Optical and electrical techniques detect biomolecular interactions on ZnO film surfaces. The piezoelectric responses of ZnO have been applied to biosensors based on surface acoustic waves [18]. Electrochemical impedance is needed to use transparent electrodes based on ZnO [19]. These interesting properties of ZnO as biocompatible materials have generated much attention as solid oxide substrates for highly sensitive biosensing platforms. Our group has investigated layer samples of ZnO to improve the optical technology associated with SPRs. In particular, layer samples could readily be employed in biological sensing based on propagation-type SPRs. The benefits of layer samples with large surface areas render film platforms more attractive for industrial applications. Recently, SP waves, which are excited by traverse magnetic polarization, have been produced in ZnO-based optical fibers as biochemical sensing probes [20, 21]. Surface-enhanced infrared absorptions have also been confirmed on ZnO film surfaces using a prism-based SPR method [22, 23]. These previous studies indicate the potential of ZnO-based SPR (ZnO-SPR) biosensors and have motivated the study of biological sensing.
In this chapter, we discuss the structural and optical properties of ZnO-SPR from experimental and theoretical approaches. We first outline single layers’ SPR properties based on ZnO: Ga, which is referred to as a single sample. The single sample is excited by Kretschmann-type SPRs using attenuated total reflection (ATR) optics. Second, the utilization of an insulator–metal–insulator (IMI) structure to a ZnO-SPR (IMI sample) is introduced in relation to the long-range SP mode. Finally, we fabricate hybrid layer structures with the capping of thin dielectric layers to ZnO-SPR (dielectric-assisted ZnO-SPR; hybrid sample) to enhance SPR properties in the IR range. Schematic pictures of single, IMI and hybrid samples are shown in Figure 1. Each sample’s detection sensitivity is discussed from the viewpoint of electric-field distribution (
2. SPRs properties of single ZnO layer surfaces
This section deals with the correlation between SPR phenomena and evanescent fields on metallic ZnO: Ga film surfaces. The fabrication of ZnO: Ga film is introduced as follows [13]. ZnO: Ga films with 5% Ga content were grown on BK-7 glass substrates at 260°C using the pulsed laser ablation (PLD) method. ArF excimer laser pulses (193 nm, 3 Hz, and 1 J/cm2) were irradiated on a ZnO: Ga ceramic target in an O2 atmosphere of 10−4 Pa. The film thickness (
Figure 2(a) shows SPR reflectance spectra of a 174 nm-thick film as a function of incident angle (
The SPR performance was evaluated by examining the bulk sensitivity using a change in the refractive index of glucose-water mixed solution. Figure 3(a) shows the SPR reflectance spectra at
where Δ
This section introduced the SPR responses of single ZnO: Ga film surfaces in the IR range. We could clearly observe the SPR spectra and their field distributions. The bulk sensitivity was evaluated using the glucose-water solution to estimate sensing performance. However, the bulk sensitivity requires improvement for real-time monitoring of biological interactions.
3. IMI structures for sensing performance
The evanescent depth of SPR plays an essential role in determining the sensing performance of plasmonic devices [26]. Above all, an IMI structure can control the evanescent field by changing the thickness of the metal layer [27]. This IMI structure is applied to slightly asymmetric dielectric media [28]. The slightly asymmetric IMI structure can provide benefits for aqueous sensing solutions. To date, no attention has been paid to the SPR property based on a ZnO-related IMI structure. The employment of IMI geometry to SPR excitations on ZnO: Ga films remains unclear. In this section, we introduce asymmetric IMI structures consisting of water, ZnO: Ga, and a cytop polymer. In particular, water-based IMI structures are one of the interesting sensing platforms in SPR applications. The benefits of SPR are confirmed from the bulk sensitivity based on index changes. We discuss the sensing performance of ZnO-SPR based on the proof-of-concept of an IMI structure.
The fabrication of IMI samples was performed as follows. The refractive index of a cytop polymer is kept close to water, which can excite SPRs using IMI structures with ZnO: Ga films. A Cytop polymer (perfluropolymer) film (1.8 mm-thickness) was deposited on BK-7 glass substrates using a polymer content of 9% in fluoride solvent by a spin coating method (2500 rpm for 50s). The coated polymer films were annealed at 220°C for 2 h in the air to evaporate the solvent. ZnO: Ga films were fabricated on polymer/glass substrates using PLD at room temperature. ArF excimer laser pulses (193 nm, 5 Hz, and 1 J/cm2) were focused on ZnO: Ga targets in an O2 flow of 10−4 Pa [29].
The IMI sample SPR reflectance spectra are shown in Figure 4(a). The peak positions gradually moved within the 4000 to 6000 cm−1 range with an increasing incident angle from 60.5o to 64o in 2o increments [29]. The SPR spectra were observed even at a film thickness of 22 nm. The SPR peak dependence on the incident angle of light for the IMI sample was higher than that for the single ZnO film. In addition, the IMI sample showed narrower spectral features than those of the single ZnO films. These behaviors were also confirmed by theoretical SPR spectra (Figure 4(b)) [29].
The SP mode of an IMI structure is separated into two types of plasmon branches comprising the short-range and long-range SP modes. The dispersion curves of both SP modes can be described using the Maxwell relations in a planar structure [30]:
where
Figure 4(d) shows a depth profile of a mean-square evanescent field at 4500 cm−1 of the
Figure 5(a) shows the SPR reflectance spectra (
In this section, we investigated the SPR properties and sensing performance of the IMI sample. The SPR spectra of the IMI sample displayed narrower features than those of the single film. This result provided the extended evanescent field depth and the long propagation distance. Consequently, the sensitivity of the IMI sample was markedly enhanced compared to that of the single film.
4. Dielectric-assisted ZnO-SPR
In Sections 2 and 3, we introduced the SPR properties of the single film and IMI sample. The single film showed broad SPR resonances and weak evanescent fields. The use of an IMI structure increased the evanescent field because of the long-range SP mode. However, the IMI sample produced a large sensing volume on the sample surface due to the long penetration depth, where it is not easy to detect small changes in the refractive index near the sample surfaces. Therefore, there is a need to find new strategies for circumventing the structural limitations of ZnO-SPRs in terms of penetration depth and evanescent field. Consequently, we propose a new structural concept based on dielectric-assisted SPRs to overcome some difficulties associated with ZnO-SPRs for real-time monitoring of biological interactions.
When sufficiently thick dielectric layers are placed on Ag and Au-metal layer surfaces, waveguide modes are supported in addition to the conventional SP mode [32, 33]. However, the sensing performance of a waveguide is comparable to that of a classical SPR. However, the Ag and Au-based SPR devices with thin dielectric layers (e.g., Al2O3 and SiO2) show SP waves along the thin dielectric layer surfaces with a thickness insufficient to support a waveguide mode. The SP waves of dielectric-assisted SPRs sufficiently penetrate the analyte region, unlike the waveguide mode. Therefore, the introduction of thin dielectric layers on top of Ag and Au metallic films has been reported to improve the sensing performance in the visible range [34].
In this section, we report on the capping of thin dielectric layers to a ZnO-SPR sensing platform to enhance the detection sensitivity in the IR range. Here, we define dielectric-assisted ZnO-SPR devices as “hybrid samples”. The insertion of dielectric layers changes the
The fabrication of hybrid samples was conducted as follows. Cytop polymer films (2.2 μm thickness) were deposited on BK-7 glass substrates. Metallic ZnO: Ga films with a thickness of 22 nm were fabricated on the polymer-coated substrates using a PLD method at room temperature (RT). Fabrication conditions of the polymer and ZnO: Ga films were the same as those of the IMI sample. In this study, we selected Ga2O3 as a dielectric layer. 190 m-thick Ga2O3 dielectric layers were deposited over ZnO: Ga layers on the polymer/glass substrates at RT in an O2 flow of 10−4 Pa using the PLD method [35].
Figure 6(a) shows a hybrid sample’s experimental SPR reflectance spectra with a Ga2O3 thickness of 190 nm [35]. The peak positions moved within the 4000–5500 cm−1range as the incident angle increased in 2.5o increments. The dependence of the SPR peak on the incident angle of light for the hybrid sample was higher than that for the IMI sample. The hybrid sample exhibited narrower spectral features than those of the IMI sample. These SPR behaviors were further reproduced by theoretical SPR spectra (Figure 6(b)) [35]. The difference in SPR response between the hybrid and IMI samples is attributed to the propagation length. An experimental
Figure 7(a) shows the SPR reflectance spectra of the hybrid sample [35]. The dip peaks shifted to lower wavenumbers with increasing glucose concentration. A wavenumber shift of 19 cm−1 at 1 g/dL was observed from the SPR reflectance spectra taken at
This relation is used to quantify the general performance of a biosensor [36]. This normalization allows for comparison with other sensing platforms. The hybrid sample provided a higher FoM value (51.0 RIU−1) than did the IMI sample (36.1 RIU−1). The enhanced FoM was attributed to the narrowing of the spectral linewidth resulting from the insertion of the Ga2O3 layer.
5. Real-time monitoring of biological interactions
We evaluated the SPR sensing performance of the hybrid sample using the biological interactions between biotin-PEG-DPPE and streptavidin. PEG and DPPE indicate poly(ethylene glycol) and 1, 2 dipalmitoyl-
The Ga2O3 layer surface of the hybrid sample was chemically modified using a SAM of
Biological interactions were conducted as follows [35]. The hybrid and IMI samples were washed with ethanol several times and then dried using nitrogen gas. Both samples were first exposed to a phosphate-buffered saline (PBS) solution with pH = 7.4 [PBS (1) process], until a stable SPR signal was obtained. Next, both samples were exposed to a solution of biotin-PEG-DPPE (100 μg/mL in PBS) for 35 min, followed by washing with PBS for 5 min [PBS (2) process]. After the sample surfaces were treated with biotin-PEG-DPPE, bovine serum albumin (BSA) was introduced to confirm non-specific protein adsorption to the sample surfaces [BSA process], followed by another PBS wash [PBS (3) process]. PBS solutions with different streptavidin concentrations (0–10 μg/dl in PBS) were then introduced [Streptavidin process]. The streptavidin concentrations were used to monitor the specific biological bonding of biotin and avidin [38].
Figure 9(a) shows the SPR reflectance spectra measured after each process [35]. The SPR peak position shifted to small wavenumbers after the surfaces were treated with biotin-PEG-DPPE and streptavidin. The SPR peak shift (Δ
The present detection limit of the hybrid samples was approximately 1 μg/ml (15 nM) due to the background noises. Industrial applications are expected to detect bio-molecule concentration at the ng/dl (~ pM) level. The spectral features of the present ZnO-SPRs were markedly influenced by the interface roughness, relating to the sensing activity for biomolecular detection. Recently, high-sensitive SPR detection at the pM levels of small biomolecules (e.g. biotin) using nanophotonic devices such as cavity structures and metamaterials with layer structures have been reported [39, 40, 41]. Recently, biological sensing based on ZnO-related SPR and metamaterials are reported by some papers [42, 43, 44]. Therefore, monitoring biological interactions at the pM level could be realized by enhancing the structural and crystalline quality of ZnO-SPR sensors. The detection sensitivity of ZnO-SPR is expected to improve the employment of new nano-plasmonic structures such as cavities and metamaterials.
6. Conclusion
This chapter reported on the structural and optical properties of ZnO-SPR with different layer structures (single, IMI, and hybrid samples) from experimental and theoretical approaches. First, the SPR properties and sensing performance of the single sample were investigated. Second, the IMI sample was introduced in ZnO-SPR devices to enhance sensing activity. Finally, the hybrid sample with the capping of thin dielectric layers to ZnO-SPR was developed to monitor biological interactions between biotin and streptavidin. This work highlights new insights for fabricating biological sensing platforms on ZnO materials.
References
- 1.
Jadwisienczak WM, Lozykoqski HJ, Xu A, Patel B. Visible emission from ZnO doped with rare-earth ions. Journal of Electronic Materials. 2002; 31 :776-684. DOI: 10.1007/s11664-002-0235-z - 2.
Badalawa W, Matsui H, Osone T, Hasuike N, Harima H, Tabata H. Correlation between structural and luminescent properties in Eu3+-doped ZnO epitaxial layers. Journal of Applied Physics. 2011; 109 :053502. DOI: 10.1063/1.3549633 - 3.
Panda J, Sasmal I, Nath TK. Magnetic and optical properties of Mn-doped ZnO vertically aligned nanorods synthesized by hydrothermal technique. AIP Advances. 2016; 6 :035118. DOI: 10.1063/1.4944837 - 4.
Babikier M, Wang D, Wang J, Li Q , Sun J, Yan Y, et al. Cu-doped ZnO nanorod arrays: The effects of copper precursor and concentration. Nanoscale Research Letters. 2014; 9 :199. DOI: 10.1186/1556-276X-9-199 - 5.
Neal JR, Behan AJ, Ibrahim RM, Blythe HJ, Ziese M, Fox AM, et al. Room-temperature magneto-optics of ferromagnetic transition-metal-doped ZnO thin films. Physical Review Letters. 2006; 96 :197208. DOI: 10.1103/PhysRevLett.96.197208 - 6.
Matsui H, Tabata H. Lattice, band and spin engineering on Zn1- x Cox O. Journal of Applied Physics. 2013;113 :183525 - 7.
Ali N, Singh B, Khan ZA, Vijava AR, Tarafder K, Ghosh S. Origin of ferromagnetism in Cu-doped ZnO. Scientific Reports. 2019; 9 :2461. DOI: 10.1038/s41598-019-39660-x - 8.
Jin ZC, Hamberg I, Granqvist CG. Optical properties of sputter-deposited ZnO: Al thin films. Journal of Applied Physics. 1988; 64 :5117. DOI: 10.1063/1.342419 - 9.
Yamamoto N, Makino H, Osone N, Ujihara A, Ito T, Hokari H, et al. Structural, electrical and optical properties of highly transparent conductive ZnO films. Thin Solid Films. 2012; 520 :4131. DOI: 10.3169/itej.66.555 - 10.
Buonsanti R, Llordes A, Aloni S, Helms BA, Milliron DJ. Tunable infrared absorption and visible transparency of colloidal aluminum-doped zinc oxide nanocrystals. Nano Letters. 2011; 11 :4706. DOI: 10.1021/nl203030f.nnichsen - 11.
Sachet E, Losego MD, Guske J, Franzen S, Maria JP. Mid-infrared surface plasmon resonance in znc oxide semiconductor thin films. Applied Physics Letters. 2013; 102 :051111. DOI: 10.1063/1.4791700 - 12.
Naik GJ, Kim J, Boltasseva A. Oxides and nitrides as alternative plasmonic materials in the optical range. Optical Materials Express. 2011; 1 (6):1090-1099. DOI: 10.1364/OME.1.001090 - 13.
Badalawa W, Matsui H, Ikehata A, Tabata H. Surface plasmon modes guided by ZnO: Ga layers bounded by different dielectrics. Applied Physics Letters. 2011; 99 :011913. DOI: 10.1063/1.3608313 - 14.
Sönnichsen C, Franzl F, von Plessen G, Feldmann J. Plasmon resonances in large noble-metal clusters. New Journal of Physics. 2002; 4 :93. DOI: 10.1088/1367-2630/4/1/393 - 15.
Kalusniak S, Sadofev S, Hennberger F. ZnO as a tunable metal: New types of surface plasmon polaritons. Physical Review Letters. 2014; 112 :137401. DOI: 10.1103/PhysRevLett.112.137401 - 16.
Kim J, Dutta A, Memarzadeh B, Kildishev AV, Mosallaei H, Boltasseva A. Mint: Zinc oxide based plasmonic multilayer resonator: Localized and gap surface plasmon in the infrared. ACS Photonics. 2015; 2 :1224. DOI: 10.1021/acsphotonics.5b00318 - 17.
Nader N, Vangala S, Hendrickson JR, Leedy KD, Look DC, Guo J, et al. Investigation of plasmon resonance tunneling through subwavelength hole arrays conductive ZnO films. Journal of Applied Physics. 2015; 118 :173106. DOI: 10.1063/1.4934875 - 18.
Cao X, Cao S, Guo H, Li T, Jie Y, Wang N, et al. Piezotronic effect enhanced label-free detection of DNA using a schottky-contacted ZnO nanowire biosensor. ACS Nano. 2016; 10 :8038-8044. DOI: 10.1021/acsnano.6b04121 - 19.
Wang C, Huang N, Zhuang H, Jiang X. Enhanced performance of nanocrystalline ZnO DNA biosensors via introducing electrochemical covalent biolinkers. ACS Applied Materials & Interfaces. 2015; 7 :7605-7612. DOI: 10.1021/acsmi.5b00040 - 20.
Wang Y, Dong J, Luo Y, Tang J, Lu H, Yu J, et al. Indium tin oxide coated two-mode fiber for enhanced SPR sensor on near-infrared region. IEEE Photonics Journal. 2017; 9 :4801309. DOI: 10.1109/JPHOT.2017.2757513 - 21.
Minn K, Anopchenko A, Yang J, Lee HWH. Excitation of epsilon-near-zero resonance in ultra-thin indium tin oxide shell embedded nanostructured optical fiber. Scientific Reports. 2018; 8 :2342. DOI: 10.1038/s41598-018-19633-2 - 22.
Martínez J, Ródenas A, Aguiló M, Fernadez T, Solis J, Diaz F. Mid-infrared surface plasmon resonance polariton chemical sensing on fiber-coupled ITO coated glass. Optics Letters. 2016; 41 :2493-2496. DOI: 10.1364/OL.41.002493 - 23.
Kalusniak S, Sadofev S, Hennberger F. Resonant interaction of molecular vibrations and surface plasmon polaritons: The weak coupling regime. Physical Review B. 2014; 90 :125423. DOI: 10.1103/PhysRevB.90.125423 - 24.
Matsui H, Ikehata A, Tabata H. Surface plasmon sensors on ZnO: Ga layer surfaces: Electric field distributions and absorption-sensitivity enhancements. Applied Physics Letters. 2015; 106 :011905. DOI: 10.1063/1.4905211 - 25.
Ikehata A, Itoh T, Ozaki Y. Surface plasmon resonance near-infrared spectroscopy. Analytical Chemistry. 2004; 76 :6461-6469. DOI: 10.1021/ac049003a - 26.
Yang F, Sambles JR, Bradberry GW. Long-range surface modes supported by thin films. Physical Review B. 1991; 44 :5855. DOI: 10.1103/PhysRevB.44.5855 - 27.
Dionne JA, Swealtlock LA, Atwater HA, Polman P. Planar metal plasmon waveguides: Frequency-dependent dispersion, propagation, localization, and loss beyond the free electron model. Physical Review B. 2005; 72 :075405. DOI: 10.1103/PhysRevB.72.075405 - 28.
Wendier L, Haupt R. Long-range surface plasmon - polariton in asymmetric layer structures. Journal of Applied Physics. 1986; 59 :3289. DOI: 10.1063/1.336884 - 29.
Matsui H, Ikehata A, Tabata H. Asymmetric plasmon structures on ZnO: Ga for high sensitivity in the infrared range. Applied Physics Letters. 2016; 109 :191601. DOI: 10.1063/1/4966598 - 30.
Economou EM. Surface plasmons in thin films. Physical Review B. 1969; 182 :539. DOI: 10.1104/PhysRev.182.539 - 31.
Dastmalchi B, Tassin P, Koschny T, Soukoulis CN. A new perspective on plasmonics: Confinement and propagaton length of surface plasmons for different materials and geometries. Advanced Optical Materials. 2015; 3 :177. DOI: 10.1002/adom.201500446 - 32.
Hayashi S, Nesterenko DV, Rahmouni A, Sekkat Z. Observation of Fano line shapes arising from coupling between surface plasmon polariton and waveguide modes. Applied Physics Letters. 2016; 108 :051101. DOI: 10.1063/1.4940984 - 33.
Abbas A, Linman MJ, Cheng Q. Sensitivity comparison of surface plasmon resonance and plasmon-waveguide resonance biosensors. Sensors and Actuators A. 2011; 156 :169-175. DOI: 10.1016/j.snb.2011.04.008 - 34.
Lahav A, Shalabaney A, Abdulhalim I. Surface plasmon sensor with enhanced sensitivity using top nano dielectric layer. Journal of Nanophotonics. 2009; 3 :031501. DOI: 10.1117/1.3079803 - 35.
Kuranaga Y, Matsui H, Ikehata Am Shimoda Y, Noiri M, Ho YL, Delaunay JJ, et al. Enhancing detection sensitivity of ZnO-based infrared plasmonic sensors using capped dielectric Ga2O3 layers for real-time monitoring of biological interactions. ACS Applied Bio Materials. 2020; 3 :6331-6342. DOI: 10.1021/acsabm.0c00792 - 36.
Otte MA, Sepúlveda B, Ni W, Juste JP, Liz-Marzán LM, Lechuga LM. Identification of the optimal spectra region for plasmonic and nanoplasmonic sensing. ACS Nano. 2010; 4 :349-357. DOI: 10.1021/nn901024e - 37.
Li F, Shishkin E, Mastro MA, Uite JK, Eddy CR, Edgar JH, et al. Photopolymerization of self-assembled monolayers of Diacetylentic Alkylphophonic acids on group-III nitride substrates. Langmuir. 2010; 26 :10725-10730. DOI: 10.1021/la100273q - 38.
Seto H, Yamashita C, Kamba S, Kondo T, Hasegawa M, Matsumoto M, et al. Biotinylation of silicon and nickel surfaces and detection of streptavidin as biosensor. Langmuir. 2013; 29 :9457-9463. DOI: 10.1021/la401068n - 39.
Sreekanth KV, Sreejith S, Mishra A, Chen X, Sun H, Lim CT, et al. Biosensing with the singular phase of an ultrathin metal-dielectric nanophotonic cavity. Nature Communications. 2018; 9 :369. DOI: 10.1038/s41467-018-02860-6 - 40.
Shinohara S, Tanaka D, Okamoto K, Tamada K. Colorimetric plasmon sensors with multilayered metallic nanoparticle sheets. Physical Chemistry Chemical Physics. 2015; 17 :186060-118612. DOI: 10.1039/C5CP02564H - 41.
Sreekanth KV, Sreejith S, Alapan Y, Sitti M, Lim CT, Singh R. Microfluidics integrated lithography-free nanophotonic biosensors for the detection of small molecules. Advanced Optical Materials. 2019; 7 :1801313. DOI: 10.1003/adom.201801313 - 42.
Guo S, Wu X, Li Z, Tong K. High-sensitivity biosensor-based enhanced SPR by ZnO/MoS2 nanowires array layer with graphene oxide nanosheet. International Journal of Optics. 2020; 2020 :7342737. DOI: 10.1155/2020/342737 - 43.
Xu H, Song Y, Zhu P, Zhao W, Liu T, Wang Q , et al. Alcohol sensor based on surface plasmon resonance of ZnO nanoflowers/Au structure. Materials. 2022; 15 :189. DOI: 10.3390/ma15010189 - 44.
Mei GS, Menon PS, Hegde G. ZnO for performance enhancement of surface plasmon biosensor: A review. Materials Research Express. 2020; 7 :012003. DOI: 10.1088/2053-1591/ab66a7