Ultrasensitive Biosensor with Hyperbolic Metamaterials Composed of Silver and Zinc Oxide

nanomaterials

Article Ultrasensitive Biosensor with Hyperbolic Metamaterials Composed of Silver and Zinc Oxide

Shuhan Chen 1,* , Shiqi Hu 2, Yichen Wu 1, Dingnan Deng 1, Yunhan Luo 2,* and Zhe Chen 2

1 School of Physics and Electronic Engineering, Jiaying University, Meizhou 514015, China; [email protected] (Y.W.); [email protected] (D.D.) 2 Key Laboratory of Optoelectronic Information and Sensing Technologies of Guangdong Higher Education Institutes, College of Science and Engineering, Jinan University, Guangzhou 510632, China; [email protected] (S.H.); [email protected] (Z.C.) * Correspondence: [email protected] (S.C.); [email protected] (Y.L.)

Abstract: We propose a hyperbolic metamaterial-based surface plasmon resonance (HMM-SPR) sensor by composing a few pairs of alternating silver (Ag) and zinc oxide (ZnO) layers. Aiming to achieve the best design for the sensor, the dependence of the sensitivity on the incidence angle, the thickness of the alternating layer and the metal filling fraction are explored comprehensively. We find that the proposed HMM-SPR sensor achieves an average sensitivity of 34,800 nm per refractive index unit (RIU) and a figure of merit (FOM) of 470.7 RIU−1 in the refractive index ranging from 1.33 to 1.34. Both the sensitivity (S) and the FOM show great enhancement when compared to the conventional silver-based SPR sensor (Ag-SPR). The underlying physical reason for the higher

performance is analyzed by numerical simulation using the ﬁnite element method. The higher sensitivity could be attributed to the enhanced electric ﬁeld amplitude and the increased penetration

Citation: Chen, S.; Hu, S.; Wu, Y.; depth, which respectively increase the interaction strength and the sensing volume. The proposed Deng, D.; Luo, Y.; Chen, Z. HMM-SPR sensor with greatly improved sensitivity and an improved figure of merit is expected to Ultrasensitive Biosensor with find application in biochemical sensing due to the higher resolution. Hyperbolic Metamaterials Composed of Silver and Zinc Oxide. Nanomaterials Keywords: surface plasmon resonance; hyperbolic metamaterials; silver and zinc oxide; sensitivity; 2021, 11, 2220. https://doi.org/ figure of merit 10.3390/nano11092220

Academic Editor: Andrey B. Evlyukhin 1. Introduction Surface Plasma Resonance (SPR) sensors have become a promising method in the Received: 28 July 2021 field of food safety, drug screening and biological sensors over the last two decades [1–3]. Accepted: 25 August 2021 Published: 28 August 2021 To excite SPR, two configurations have usually been employed, namely Kretschmann configuration and grating coupling configuration [4]. In the Kreschmann configuration,

Publisher’s Note: MDPI stays neutral spectral scan and angular scan interrogation are usually employed to measure the reflected with regard to jurisdictional claims in signal [5]. Although the angular scan interrogation is possible to achieve higher signal-to- published maps and institutional affil- noise, the spectral scan has the advantages of low cost, easy fabrication and a compact fiber iations. sensor [6]. Currently, various materials are added to the structure of traditional prism-coupled SPR biosensors to achieve ultra-high detection sensitivity, ultra-high detection accuracy and low detection threshold. For example, metallic nanoparticles and complex nanostructures have been used to enhance the sensitivity of SPR sensors [7,8]. Nanomaterials, such as TiO Copyright: © 2021 by the authors. 2 and graphene, have been deposited to modify the metal for exciting long-range surface Licensee MDPI, Basel, Switzerland. This article is an open access article plasmon [9,10]. distributed under the terms and Recently, hyperbolic metamaterials (HMM), which have been demonstrated to en- conditions of the Creative Commons hance the performance of SPR sensors distinctly, are one of the most concerned metamate- Attribution (CC BY) license (https:// rials with the real part of the permittivity components having opposite signs [11–13]. For creativecommons.org/licenses/by/ instance, the gold/Al2O3 multilayered structure was fabricated as HMM-based sensors to 4.0/).

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−1 achieve a sensitivity of 30,000 nm/RIU and an FOM of 509 RIU [14]. An Ag/TiO2 HMM- based fiber plasmonic sensor was fabricated to achieve a sensitivity of 9000 nm/RIU and −1 −1 anachieve FOM a ofsensitivity 230.8 RIU of 30,000, which nm/RIU has theand advantagean FOM of 509 of miniaturizationRIU [14]. An Ag/TiO and2 integration HMM- [15]. Amongbased fiber a lot plasmonic of semiconductors, sensor was fabricated ZnO as to a wideachieve band a sensitivity gap semiconductor of 9000 nm/RIU was and found to havean FOM high of 230.8 thermal RIU and−1, which chemical has the stability, advantage and of mini thusaturization it is an ideal and materialintegration to [15]. protect the metallicAmong a layer lot of from semiconductors, oxidation [ 16ZnO]. as a wide band gap semiconductor was found to haveIn high this thermal study, and an HMM-SPR chemical stability, sensor and composed thus it is of an alternating ideal material Ag/ZnO to protect layers the is investi- metallic layer from oxidation [16]. gated. The ZnO layer is used to protect the Ag layer from oxidation. The HMM structure In this study, an HMM-SPR sensor composed of alternating Ag/ZnO layers is inves- composedtigated. The of ZnO Ag/ZnO layer is used multi-layers to protect is th demonstratede Ag layer from tooxidation. greatly The improve HMM thestructure sensitivity of thecomposed SPR sensor. of Ag/ZnO The effectsmulti-layers of the is incidencedemonstrated angle, to greatly alternating improve layer the thicknesssensitivity andof metal fillingthe SPR factor sensor. on The the effects sensitivity of the of incidence the sensor angle, are alternating studied using layer thethickness transfer and matrix metal method (TMM)filling factor [17]. on The the underlying sensitivity of physical the sensor reason are studied for the using higher the transfer performance matrix method is analyzed by using(TMM) finite [17]. elementThe underlying method physical [18]. The reason improvement for the higher is attributedperformance to is the analyzed enhanced by electric fieldusing amplitude finite element and method the increased [18]. The penetrationimprovement depth,is attributed which to respectivelythe enhanced increaseelectric the in- teractionfield amplitude strength and and the increa the sensingsed penetration volume. depth, With which optimized respectively parameters, increase the the average in- S of 34,800teraction nm/RIU strength and and FOMthe sensing of 470.7 volume. RIU− 1Wiareth achievedoptimized inparameters, the range the of 1.33average to 1.34S of RIU. 34,800 nm/RIU and FOM of 470.7 RIU−1 are achieved in the range of 1.33 to 1.34 RIU. 2. Model and Theory 2. Model and Theory In the proposed structure, the SPR sensor based on HMM composed of Ag/ZnO is shownIn inthe Figure proposed1. The structure, Kretschmann the SPR sensor configuration based on is HMM used composed to excite theof Ag/ZnO surface is plasmon shown in Figure 1. The Kretschmann configuration is used to excite the surface plasmon wave, where a BK7 prism is used as the coupling element. The Ag/ZnO bilayer in the wave, where a BK7 prism is used as the coupling element. The Ag/ZnO bilayer in the structurestructure has has aa thickness thickness approximate approximate 30 nm, 30 nm,which which is much is muchsmaller smaller than the than exciting the exciting wavelengthwavelength (500–2000 (500–2000 nm). nm). Therefore, Therefore, the thealternating alternating Ag/ZnO Ag/ZnO layers layerscan be assumed can be assumed to to bebe a uniform medium. medium.

FigureFigure 1. SchematicSchematic diagram diagram of ofthe the proposed proposed SPR SPR sensor sensor with withhyperbolic hyperbolic metamaterial metamaterial composed composed of of Ag/ZnO. Ag/ZnO.

The effective effective medium medium theory theory (EMT) (EMT) is used is used to evaluate to evaluate the parallel the parallel and perpendicu- and perpendicular lar component of equivalent dielectric tensors of the HMM [19] with the following formu- component of equivalent dielectric tensors of the HMM [19] with the following formulas: las: dAgεAg + dZnOεZnO (1) εx = (1) dAg + dZnO (2) /dAg +d/ZnO ε = (2) z + where and denote the thickness,dAg/ε Agwhiled ZnO /andεZnO denote the dielectric permittivity of the Ag and ZnO layer, respectively. The is obtained according to Ref. where dAg and dZnO denote the thickness, while εAg and εZnO denote the dielectric permit- [20]. The is obtained according to Ref. [21], using the following formula: tivity of the Ag and ZnO layer, respectively. The εAg is obtained according to Ref. [20]. The εZnO is obtained according to Ref. [21], using the following formula:

0.87968λ2 n2 = 2.81418 + − 0.00711λ2 (3) λ2 − 0.30422 Nanomaterials 2021, 11, 2220 3 of 9

where λ denotes the wavelength. The refractive index of the sensing medium is set as ns = 1.33. The refractive index of BK7 prism (np) is obtained according to the formula as follows:

Nanomaterials 2021, 11, x FOR PEER REVIEW 2 2 3 of2 9 2 1.03961212λ 0.231792344λ 1.01046945λ np − 1= + + (4) λ2 − 0.006000698672 λ2 − 0.02001791442 λ2 − 103.5606532

To solve the proposed multilayer0.87968 systems, the transfer matrix method is employed 2.81418 0.00711 (3) due to its simplicity and flexibility. For the 0.3042N-layer model in this study, the propagation characteristicwhere denotes is describedthe wavelength. by transfer The refractive matrix Sindexm [22 of], i.e.,the sensing medium is set as 1.33. The refractive index of BK7 prism () is obtained according to the formula as N ! follows: S11 S12 S = = ∏ I(m−1)m Lm ·IN(N+1) (5) .S21 S22 . . 1= m=1 (4) . . . whereTo thesolve interface the proposed matrix multilayer for each interfacesystems, the in thetransfer structure matrix is method defined is byemployed due to its simplicity and flexibility. For the N-layer model in this study, the propagation characteristic is described by transfer matrix 1Sm [22],1 i.e., rmn Imn = (6) tmn rmn 1 S= =∏ ∙ (5) where tmn and rmn are the reflection and transmission coefficients at the interface mn, whichwhere the can interface be obtained matrix by for the each Fresnel interface formula. in the structure For the TM-polarized is defined by mode, we can get the amplitude of reflection coefficient rm and1 transmission1 coefficient tm at the mth layer: (6) 1 2nm−1 cos θm−1 where and are the reflectiont = and transmission coefficients at the interface mn, (7) m n θ + n θ which can be obtained by the Fresnel formula.m−1 cos Form the TM-polarizedm cos m−1 mode, we can get the amplitude of reflection coefficient and transmission coefficient at the mth layer: nm−1 cos θm − nm cos θm−1 rm = (8) 2cos+ nm−1 cos θm m cos θ m−1 (7) cos cos where θ is the incident angle. The layer matrix through layer m is described by cos cos (8) −i cose ∅m cos0 Lm = i (9) where θ is the incident angle. The layer matrix through0 e layer∅m m is described by ∅ 0 (9) where ∅m is the layer phase thickness as the wave0 traverses∅ the layer m. where ∅ is the layer phase thickness as the wave traverses the layer m. 3. Results and Discussion 3. Results and Discussion The effective permittivity calculated with EMT is plotted in Figure2. The real part of the permittivityThe effective withpermittivityx and zcalculatedcomponents with showEMT is opposite plotted in signs Figure for 2.λ The> 500 real nm, part whichof indi- catesthe permittivity that the alternating with x and Ag/ZnOz components multilayered show opposite structure signs displays for λ > 500 hyperbolic nm, which dispersion inin dicates that the alternating Ag/ZnO multilayered structure displays hyperbolic dispersion this region. The modes in HMM cannot be directly excited due to the momentum mismatch. in this region. The modes in HMM cannot be directly excited due to the momentum mis- Therefore, a BK7 prism is used to couple the spatial light into the HMM structure and match. Therefore, a BK7 prism is used to couple the spatial light into the HMM structure realizeand realize the the match match of of the the two two wave-vectors wave-vectors [13]. [13].

Figure 2. 2. TheThe effective effective permittivity permittivity of Ag/ZnO of Ag/ZnO HMM HMM calculated calculated by EMT. by EMT.

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The TMM is used to calculate the reflectionreflection spectraspectra ofof thethe HMM-SPRHMM-SPR andand traditionaltraditional Ag-SPR, and their S, fullfull widthwidth atat halfhalf maximummaximum (FWHM),(FWHM), andand thethe FOMFOM andand depthdepth ofof resonance dipdip (DRD)(DRD) areare furtherfurther calculated calculated [ 23[23].]. For For the the Ag-SPR Ag-SPR sensor, sensor, an an Ag Ag film film with with a thicknessa thickness of of 50 50 nm nm is is used used to to replace replace the the Ag/ZnO Ag/ZnO multi-layersmulti-layers inin thethe structure,structure, shownshown asas Figure1 1.. ForFor thethe HMM-SPRHMM-SPR sensor,sensor, the the pair pair thickness thickness of of Ag/ZnO Ag/ZnO isis d == 30 nm,nm, andand thethe metal fillingfilling fractionfraction isis ff = 0.5. The The pair pair number is set to be 3 based on our early simulation for optimization. With the surrounding surrounding refractive refractive index index (SRI) (SRI) increasing increasing from from 1.330 1.330 to 1.335, to 1.335, the thereflec- re- flectiontion spectra spectra for the for Ag-SPR the Ag-SPR sensor sensor under under different different incidence incidence angles angles are shown areshown in Figure in Figure3a,c, while3a,c ,the while reflection the reflection spectra for spectra the HMM-SPR for the HMM-SPR sensor at sensordifferent at incidence different incidenceangles are anglesshown arein Figure shown 3b,d, in Figure respectively.3b,d, respectively. When the SRI When equals the 1.33, SRI equalsthe resonant 1.33, the wavelengths resonant wavelengths are 633, 658, 709, 824, and 1242 nm corresponding to the incidence angles of are◦ 633,◦ 658,◦ 709,◦ 824, and◦ 1242 nm corresponding to the incidence angles of 85°, 80°, 75°, 8570°,, 80and, 7565°., 70It can, and be 65 observed. It can bethat observed the reso thatnant the wavelength resonant wavelengthis red-shifted is red-shiftedas the inci- asdence the incidenceangle is increased. angle is increased.

Figure 3. The dependence of reflectance for Ag-SPR and HMM-SPR for SRI = 1.330 ( a,b), 1.335 (c,d) with(c,d) differentwith different incident incident angles. angles.

In orderorder toto furtherfurther investigate investigate the the mechanism mechanism of of performance performance improvement, improvement, the the finite fi- elementnite element method method (FEM) (FEM) is used is used to calculate to calculate the the optical optical field field intensity intensity with with the the distance distance of theof the device device for for both both sensors sensors under under the the incidence incidence angle angle of of 65 65°.◦. In In the the calculations, calculations, thethe toptop and bottom ofof thethe computationalcomputational domaindomain areare setset asas periodicperiodic boundaryboundary conditions.conditions. TheThe input powerpower ofof portport inin thethe leftleft sideside isis setset asas 1 1 W/m. W/m. AdaptiveAdaptive meshingmeshing withwith aa maximummaximum element size of 30 nm has been used.used. Figure4 4 shows shows the the distributions distributions of of the the normalized normalized optical optical electric electric field field |E|/|E |E|/|E00|| along with the device distance forfor HMM-SPRHMM-SPR andand Ag-SPRAg-SPR sensors.sensors. It cancan bebe seenseen thatthat thethe electric fieldfield amplitude and the increased penetrationpenetration depthdepth forfor thethe HMM-SPRHMM-SPR sensorsensor areare both improved distinctly comparedcompared withwith Ag-SPRAg-SPR sensors.sensors. TheThe higherhigher sensitivitysensitivity couldcould bebe attributed to the enhanced electric fieldfield amplitude and the increasedincreased penetrationpenetration depth, which respectivelyrespectively increaseincrease thethe interactioninteraction strengthstrength andand thethe sensingsensing volume.volume.

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Figure 4. The distribution of normalized optical electric field |E|/|E0| along with the device dis- 0 Figuretance.Figure 4.4. TheThe distributiondistribution of of normalized normalized optical optical electric electric field field |E|/|E |E|/|E0|| along along with with the the device device distance. distance. To quantitativelyquantitatively analyzeanalyze thethe sensingsensing performance,performance, thethe S,S, FWHM,FWHM, DRDDRD andand FOMFOM areare plottedTo inquantitatively FigureFigure5 5.. TheThe analyze SS increasesincreases the sensing whenwhen thetheperformance, incidenceincidence angleanglethe S, decreases.FWHM,decreases. DRD AtAt anandan incidenceincidence FOM are angleplotted of in 6565° Figure◦ with the5. The SRI S region increases ranging when from the incidence 1.330 to 1.335, angle the decreases. average AtS of an the incidence HMM- SPRangle and of Ag-SPR 65° with sensors sensorsthe SRI is isregion 27,800 27,800 ranging and and 12,600 12,600 from nm/RIU, nm/RIU, 1.330 to and 1.335, and the the theaverage average average FOM FOM S ofis 600 isthe 600 HMM-and and 26 − 26RIUSPR RIU− 1and, respectively.1 ,Ag-SPR respectively. sensors The The HMM-SPRis HMM-SPR27,800 and sensor 12,600 sensor shows nm/RIU, shows a asensitivity and sensitivity the average enhancement enhancement FOM is 600of of 120.6% and 26 overRIU− the1, respectively. Ag-SPR sensor. The HMM-SPR sensor shows a sensitivity enhancement of 120.6% over the Ag-SPR sensor.

Figure 5. ((aa)) S, S, (b (b) )FWHM, FWHM, (c ()c DRD) DRD and and (d ()d FOM) FOM distributions distributions with with the theincidence incidence angle angle for the for the HMM-SPRFigure 5. (a and) S, ( Ag-SPRb) FWHM, sensors, (c) DRD respectively.respectively. and (d) FOM distributions with the incidence angle for the HMM-SPR and Ag-SPR sensors, respectively.

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Figure 6a,c and Figure 6e,g show the dependence of reflection spectrums on thick- Figure6 a,c6a,c and and Figure Figure6e,g 6e,g show show the the dependence dependence of reflection of reflection spectrums spectrums on thickness on thickness of the alternating layer (d) and the metal filling fraction (f) for HMM-SPR with SRI = ofness the of alternating the alternating layer layer (d) and (d) the and metal the metal filling filling fraction fraction (f ) for ( HMM-SPRf) for HMM-SPR with SRI with = 1.330SRI = 1.330 and 1.335 respectively. The resonance wavelengths of the sensors are all shown to and1.330 1.335 and respectively.1.335 respectively. The resonance The resonance wavelengths wavelengths of the sensors of the sensors are all shown are all to shown be blue- to be blue-shifted with an increase in f. The influence of d and f on the S and FWHM of HMM- shiftedbe blue-shifted with an with increase an increase in f. The in influence f. The influence of d and of fd onand the f on S andthe S FWHM and FWHM of HMM-SPR of HMM- SPR are shown in Figure 7. The S increases with decreasing f and increases with increasing areSPR shown are shown in Figure in Figure7. The 7. SThe increases S increases with with decreasing decreasingf and f and increases increases with with increasing increasingd. d. However, the FWHM is shown to increase with decreasing f and decrease with increas- However,d. However, the the FWHM FWHM is shownis shown to increaseto increase with with decreasing decreasingf and f and decrease decrease with with increasing increasing d. By comparing the reflection spectrum in Figure 6 and S distribution in Figure 7, it ding. By d. comparing By comparing the reflectionthe reflection spectrum spectrum in Figure in Figure6 and 6 Sand distribution S distribution in Figure in Figure7, it can 7, it can be seen that the higher S for HMM-SPR can be attributed to the higher effective re- becan seen be seen that thethat higher the higher S for HMM-SPRS for HMM-SPR can be ca attributedn be attributed to the higherto the higher effective effective refractive refractive index for SPR at a longer resonant wavelength. indexfractive for index SPR atfor a SPR longer at a resonant longer resonant wavelength. wavelength.

FigureFigure 6.6. The dependence of of the the reflectance reﬂectance on on dd andand f fforfor HMM-SPR HMM-SPR with with SRI SRI = 1.330 = 1.330 (a,b (a,c–)c and) and Figure 6. The dependence of the reflectance on d and f for HMM-SPR with SRI = 1.330 (a,b,c) and 1.335 (e,f,g), respectively. 1.3351.335 ((ee–,fg,),g), respectively. respectively.

Figure 7. The dependence of S (a) and FWHM (b) on the f and d in the case of SRI changing from Figure 7. The dependence of S (a) and FWHM (b) on the f and d in the case of SRI changing from Figure1.330 to 7. 1.335.The dependence of S (a) and FWHM (b) on the f and d in the case of SRI changing from 1.3301.330 toto 1.335.1.335. Figure 8a shows the reflection spectra of HMM-SPR (d = 30 nm, f = 0.5) and Ag-SPR Figure8 8aa shows shows the the reflection reflection spectra spectra of of HMM-SPR HMM-SPR ( d(d= = 3030 nm,nm,f f == 0.5)0.5) andand Ag-SPRAg-SPR at different SRI, respectively. The depth of the reflection dip for the HMM-SPR sensor is at differentdifferent SRI,SRI, respectively.respectively. TheThe depthdepth ofof thethe reflectionreflection dipdip forfor thethe HMM-SPRHMM-SPR sensorsensor isis significantly larger than that for the Ag-SPR sensor. The resonant wavelength is redshifted significantlysignificantly largerlarger thanthan thatthat forfor thethe Ag-SPR sensor. The resonant wavelengthwavelength isis redshiftedredshifted with the increasing SRI. Figure 8b shows the dependence of the resonant wavelength on withwith thethe increasingincreasing SRI.SRI. FigureFigure8 8bb shows shows the the dependence dependence of of the the resonant resonant wavelength wavelength on on RIU for both sensors. The resonant wavelength is shown to be linear with the RIU. The RIU forfor bothboth sensors.sensors. The resonant wavelength is shown to bebe linearlinear withwith thethe RIU.RIU. TheThe slopes of the fitting lines represent the average S of the sensors. With the SRI ranging from slopesslopes of the the fitting fitting lines lines represent represent the the averag averagee S of S the of the sensors. sensors. With With the SRI the ranging SRI ranging from 1.33 to 1.34, the average S of the HMM-SPR and Ag-SPR sensors is 34,800 nm/RIU and from1.33 to 1.33 1.34, to 1.34,the average the average S of Sthe of HMM-SPR the HMM-SPR and andAg-SPR Ag-SPR sensors sensors is 34,800 is 34,800 nm/RIU nm/RIU and 15,714 nm/RIU, respectively. A sensitivity enhancement up to 121.4% was achieved. The and15,714 15,714 nm/RIU, nm/RIU, respectively. respectively. A sensitivity A sensitivity enhancement enhancement up to up 121.4% to 121.4% was wasachieved. achieved. The FWHM and FOM are shown in Figure 8c,d. The average FOM are 470.7 RIU−1 and 321.3 TheFWHM FWHM and andFOM FOM are shown are shown in Figure in Figure 8c,d.8c,d. The Theaverage average FOM FOM are are470.7 470.7 RIU RIU−1 and−1 321.3and RIU−1 for −HMM-SPR and Ag-SPR, respectively. 321.3RIU−1 RIUfor HMM-SPR1 for HMM-SPR and Ag-SPR, and Ag-SPR, respectively. respectively.

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Figure 8.8. The dependencedependence ofof thethe reﬂectance reflectance on on the the refractive refractive index index for for the the HMM-SPR HMM-SPR (a) ( anda) and Ag-SPR Ag- sensorSPR sensor (b). Comparison(b). Comparison of the of resonant the resonant wavelength wavelength (c). Comparison (c). Comparison of S (ofd). S (d).

Table1 1 summarizes summarizes the the results results of of various various SPR SPR sensors sensors concerning concerning the the S S and and FOM FOM fromfrom thethe previousprevious reports,reports, includingincluding thethe HMM-AuHMM-Au grating,grating, HMM-SPR-PrismHMM-SPR-Prism NanorodNanorod HMM-Prism, HMM-SP-FMFHMM-SP-FMF andand Au/AgAu/Ag multilayer-prism SPR sensors. It isis shownshown thatthat thethe Ag/ZnOAg/ZnO HMM-SPRHMM-SPR sensor sensor proposed proposed in in this this paper paper possesses possesses a much a much higher higher S and S FOM. and FOM. Table 1. Comparison of various SPR sensors. Table 1. Comparison of various SPR sensors. Detection Configuration S (nm/RIU) FOM (RIU−1) Reference RangeDetection (RIU) Range Configuration S (nm/RIU) FOM (RIU−1) Reference Ag/ZnO HMM 1.33–1.34(RIU) 34,800 470.7 This paper HMM-AuAg/ZnO GratingHMM 1.3333–1.3336 1.33–1.34 30,000 34800 590 470.7 This [14 paper] NanorodHMM-Au HMM-Prism Grating 1.3333–1.3336- 30,000 30000 330 590 [24 [14]] NanorodHMM HMM-Prism Fiber 1.33–1.40 - 9000 30000 230.8 330 [15 [24]] Au/Ag ~1.3558 4154 56.9 [25] multilayer-PrismHMM Fiber 1.33–1.40 9000 230.8 [15] Au/AgAg/ZnO multilayer- bilayers 1.30–1.37 3161 - [16] ~1.3558 4154 56.9 [25] Prism 4. ConclusionsAg/ZnO bilayers 1.30–1.37 3161 - [16] An ultrasensitive HMM based SPR biosensor composed of alternating Ag/ZnO layers is4. numericallyConclusions investigated using TMM. The underlying physical reason for the higher performanceAn ultrasensitive is analyzed HMM using based FEM. SPR The biosensor higher sensitivity composed for of HMM-Sensoralternating Ag/ZnO is attributed layers tois numerically the enhanced investigated electric field using amplitude TMM. andThe theunderlying increased physical penetration reason depth, for the which higher re- spectivelyperformance increased is analyzed the interactionusing FEM. strength The higher and sensitivity the sensing for volume.HMM-Sensor The effects is attributed of the incidenceto the enhanced angle, alternatingelectric field layer amplitude thickness and and the metal increased filling penetration factor on the depth, performance which re- of thespectively sensor areincreased studied. the The interaction S and FOM strength are demonstrated and the sensing to increase volume. with The a decrease effects inof thethe incidenceincidence angle.angle, alternating The S is demonstrated layer thickness to increase and metal with filling decreasing factor onf at the a certainperformanced and toof increasethe sensor with are increasing studied. Thed at S a and certain FOMf. Withare demonstrated the SRI region to ranging increase from with 1.33 a decrease to 1.34, an in averagethe incidence S of 34,800 angle. nm/RIU The S is anddemonstrated FOM of 470.7 to increase RIU−1 for with the decreasing proposed HMM-SPRf at a certain sensor d and areto increase achieved. with The increasing proposed d biosensorat a certain with f. With greatly the improvedSRI regionS ranging and FOM from is expected1.33 to 1.34, to findan average application S of 34,800 in ultrasensitive nm/RIU and biochemical FOM of 470.7 sensing RIU-1 field. for the proposed HMM-SPR sensor

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Author Contributions: Original draft preparation, S.C.; methodology, S.H.; investigation, D.D. and Y.W.; writing—review and editing, Y.L.; supervision, editing and revision, Z.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Provincial Key Platform and Scientific Research Projects of Universities in Guangdong Province (No. 2019KTSCX167); Guangdong Basic and Applied Ba- sic Research Foundation (No. 2020A1515110283); National Natural Science Foundation of China (62175094). Data Availability Statement: The data presented in this study are available on request from the corresponding author upon reasonable request. Conflicts of Interest: The authors declare no conflict of interest.

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