Sensitivity Enhancement in Surface Plasmon Resonance ... - MDPI

0 downloads 0 Views 2MB Size Report
Jun 27, 2018 - School of Mechanical Engineering and Electronic Information, China ... [email protected] (P.H.); [email protected] (J.P.); ... Center of Fiber Technology, School of Electrical and Electronic Engineering, Nanyang Technological ... When light reflects at a SPR angle, free electrons on the metal ...
sensors Article

Sensitivity Enhancement in Surface Plasmon Resonance Biochemical Sensor Based on Transition Metal Dichalcogenides/Graphene Heterostructure Xiang Zhao 1 , Tianye Huang 1, * Yiheng Wu 1 and Zhuo Cheng 1 1

2

*

ID

, Perry Shum Ping 2 , Xu Wu 1 , Pan Huang 1 , Jianxing Pan 1 ,

School of Mechanical Engineering and Electronic Information, China University of Geosciences (Wuhan), Wuhan 430074, China; [email protected] (X.Z.); [email protected] (X.W.); [email protected] (P.H.); [email protected] (J.P.); [email protected] (Y.W.); [email protected] (Z.C.) Center of Fiber Technology, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore; [email protected] Correspondence: [email protected] or [email protected]

Received: 14 May 2018; Accepted: 21 June 2018; Published: 27 June 2018

 

Abstract: In this work, a surface plasmon resonance (SPR) biosensor based on two-dimensional transition metal dichalcogenides (TMDCs) is proposed to improve the biosensor’s sensitivity. In this sensor, different kinds of two-dimensional TMDCs are coated on both surfaces of metal film. By optimizing the structural parameters, the angular sensitivity can reach as high as 315.5 Deg/RIU with 7-layers WS2 and 36 nm Al thin film, which is 3.3 times of the conventional structure based on single Al thin film. We also obtain maximum phase sensitivity (3.85 × 106 Deg/RIU) with bilayer WS2 and 35 nm Al thin film. The phase sensitivity can be further improved by employing Ag and removing air layer. The proposed configuration is of great potential for biochemical sensing. Keywords: surface plasmon resonance; transition metal dichalcogenides; angular sensitivity; differential phase

1. Introduction Surface plasmon resonance (SPR) is an optical phenomenon which occurs at the metal-dielectric surface. When light reflects at a SPR angle, free electrons on the metal surface can resonate and absorb light energy, consequently leading to a drastic attenuation of reflected light [1,2]. The SPR condition is sensitive to the environment variations and can be utilized as sensors. The biological molecules interactions in the sensing medium are detected by observing the refractive index changes of the sensor region. Due to advantages such as convenient detection, high sensitivity, real-time measurement, SPR sensors have been used to detect and analyze various biological molecules, such as proteins, nucleic acids and viruses, and have a broad prospect in practical applications [3–6]. Sensitivity is one of the most important aspects for biological sensing in particular, and how to enhance the sensitivity becomes a research hotspot for SPR biosensors. Recently, 2D materials such as graphene and transition metal dichalcogenides (TMDCs) have are well-known for their use in constructing SPR sensors due to their unique electrical and optical properties [7]. This is because firstly, the high real part of the dielectric constant allows them to help metal absorb light energy [8]. Secondly, some features such as high surface to volume ratio and tunable biocompatibility can help the biosensor obtain sensitivity enhancements [9]. Finally, when coating these materials on the metal film, they can also protect the metal from oxidation as protective layers [10,11]. Based on these advantages, various 2D-material-assisted SPR sensors are proposed and investigated. Sensors 2018, 18, 2056; doi:10.3390/s18072056

www.mdpi.com/journal/sensors

Sensors 2018, 18, 2056

2 of 10

Graphene has been proposed for the enhancement of the sensitivity [12]. Zeng et al. presented a highly sensitive SPR biosensor based on graphene-MoS2 hybrid nanostructures to enhance its Sensors 2018, 18, x FOR PEER REVIEW 2 of 10 sensitivity [13]. Air layer and graphene sheet for sensitivity enhancement was analyzed in [14]. Other TMDCs like WS with silicon the sensitivity [15]. 2 , MoSe 2 and 2 are combined investigated. Graphene has beenWSe proposed for the enhancement of to theenhance sensitivity [12]. Zeng et al. Wu et al.presented proposeda ahighly SPR biochemical sensor with heterostructures of few-layer BP and 2D materials sensitive SPR biosensor based on graphene-MoS2 hybrid nanostructures to (graphene/MoS /WS ) [16].and According the for previous work, it is found was thatanalyzed the sensor enhance 2its sensitivity [13]. Air 2layer grapheneto sheet sensitivity enhancement 2 /MoSe 2 /WSe in [14]. Other TMDCs like WS MoSe 2 and WSe2and are combined with silicon to To enhance theenhance sensitivitythe performances are highly related to2, the structures functional materials. further [15].both Wu of et these al. proposed a SPR should biochemical sensor with heterostructures of few-layer BP and 2D sensitivity, two aspects be properly optimized. materials (graphene/MoS 2/WS2/MoSe2/WSe2) [16]. According to the previous work, it is found that In this paper, SPR sensor constructed by TMDCs/metal/TMDCs/graphene heterostructure highly related to the structures and functional materials. To further is used the forsensor both performances angular and are phase sensitivity enhancement. By coating different 2D TMDCs enhance the sensitivity, both of these two aspects should be properly optimized. (MoS2 /MoSe2 /WS2 /WSe2 ) at both sides of the metal, the sensitivity of the proposed sensor can be In this paper, SPR sensor constructed by TMDCs/metal/TMDCs/graphene heterostructure is improved by the enhancement of light-material interaction. Angular sensitivity as high as 315.5 Deg/RIU used for both angular and phase sensitivity enhancement. By coating different 2D TMDCs which is(MoS nearly 3 times that of conventional configurations can be obtained. Furthermore, the proposed 2/MoSe2/WS2/WSe2) at both sides of the metal, the sensitivity of the proposed sensor can be SPR configuration suitable for phase detection as well and a Angular pronounced phaseassensitivity up to improved byisthe enhancement of light-material interaction. sensitivity high as 315.5 6 Deg/RIU is predicted. 3.85 × 10 Deg/RIU which is nearly 3 times that of conventional configurations can be obtained. Furthermore, the proposed SPR configuration is suitable for phase detection as well and a pronounced phase 6 Deg/RIU is Model 2. Sensor Configuration and sensitivity up to 3.85 × 10Theoretical predicted.

The schematic diagram of the proposed SPR biosensor is shown in Figure 1a, the configuration 2. Sensor Configuration and Theoretical Model contains seven layers and the operation wavelength is 633 nm which is popular for SPR schematic diagram the refractive proposed SPR biosensor shownacts in Figure the configuration applicationsThe [13,14,17]. BK7 glass of with index of np =is1.5151 as the1a, coupling prism [18]. contains seven layers and the operation wavelength is 633 nm which is popular for SPR applications The refractive index of the air layer is fixed at 1 with thickness of 35 nm. The metal employed in this [13,14,17]. BK7 glass with refractive index of np = 1.5151 acts as the coupling prism [18]. The refractive configuration is Al with dielectric constant of −34.2574 + 0.9108i [19]. Various TMDCs, represented by index of the air layer is fixed at 1 with thickness of 35 nm. The metal employed in this configuration MX2 , are both sides of Al of thin film, the thickness refractive index of TMDCs at 633 nm is coated Al withatdielectric constant −34.2574 + 0.9108i [19]. and Various TMDCs, represented by MX 2, are are shown in Table 1 [20,21]. The graphene layer is coated on the MX /Al/MX hybrid structure coated at both sides of Al thin film, the thickness and refractive index 2of TMDCs 2at 633 nm are shown as the biomolecular andlayer the refractive index graphene given structure as [22]: as the in Table 1recognition [20,21]. Theelement graphene is coated on theofMX 2/Al/MX2 is hybrid biomolecular recognition element and the refractive index of graphene is given as [22]:

iC1 λ iC λ nG = 3.0 +3 1

nG = 3.0 +

3

(1) (1)

where λ is the wavelength and C1 = 5.446 µm−1 . The thickness of the monolayer graphene is 0.34 nm. −1. The thickness of the monolayer graphene is 0.34 nm. where λ is theof wavelength and C1 = 5.446 The refractive index the sensing medium is μm given as ns = 1.33 + ∆n, where ∆n is the index change of The refractive index of the sensing medium is given as ns = 1.33 + ∆n, where ∆n is the index change of the sensing medium. the sensing medium.

Figure 1. Schematic diagramof of the the SPR for (a) angle-sensitivity enhancement and (b) phaseFigure 1. Schematic diagram SPRbiosensor biosensor for (a) angle-sensitivity enhancement and sensitivity enhancement. (b) phase-sensitivity enhancement.

Sensors 2018, 18, 2056

3 of 10

Table 1. The thickness, refractive index and optical constants of different MX2 at λ = 633 nm. Type of TMDC

Thickness of Monolayer (nm)

Refractive Index

Dielectric Constant

MoS2 MoSe2 WS2 WSe2

0.65 0.70 0.80 0.70

5.0805 + 1.1723i 4.6226 + 1.0063i 4.8937 + 0.3124i 4.5501 + 0.4332i

24.4368 + 11.9121i 20.3560 + 9.3039i 23.8511 + 3.0578i 20.5156 + 3.9423i

In this paper, for the SPR curve calculation and sensing performance analysis, the transfer matrix method (TMM) is employed [23]. In the proposed structure, the thickness, the refractive index, and the dielectric constant of each layer are defined as dk , nk and εk , respectively. The incident angle corresponding to the minimum reflectance is called resonance angle and the angular sensitivity is calculated by probing the spectral shifts of the resonance angle [3] and defined as SA = ∆θ res /∆n [24], where ∆θ res represents the change of resonance angle. Furthermore, we also discuss phase sensitivity which is defined as Sp = ∆ϕ/∆n [13], where ∆ϕ is the differential phase changes corresponding to ∆n. 3. Results and Discussions In order to obtain the optimal angular sensitivity for the proposed configuration, we firstly calculate the angular sensitivity with various number of MX2 layers and Al thickness. It should be noted that in this calculation, the MX2 layers at both sides are changed simultaneously. As shown in Figure 2a–d, when the refractive index of sensing medium changes from 1.330 to 1.335 (∆n = 0.005), the SPR curves shows three important features enumerated below. (1)

(2) (3)

When the thickness of Al thin film is fixed, the sensitivity increases with more MX2 layers mainly due to the enhanced light energy absorption. However, it will decrease rapidly when the number of MX2 layers exceeds the optimal number which is defined as the number of MX2 layers with the highest sensitivity. The optimal numbers of MX2 layers will increase when the thickness of Al increases. With the same thickness of Al thin film, the enhancement effect offered by different kinds of MX2 are not the same.

It is known that the improvement effect caused by TMDCs is related to their dielectric constants. As illustrated in Table 1, MoS2 has a larger real part of the dielectric constant than others, which means its absorption ability is stronger [15]. Nevertheless, the ability to absorb light is not the only crucial factor to affect sensitivity; electron energy loss related to the imaginary part of the dielectric constant can lead to a counteraction [13]. Comparing to other TMDCs, the WS2 layers have much lower energy loss because of their small imaginary part of the dielectric constant. According to the conditions above, the optimized parameters and the corresponding angular sensitivity for different TMDCs are summarized in Table 2. The highest sensitivities can achieve 214.8 Deg/RIU, 210.1 Deg/RIU, 315.5 Deg/RIU and 286.3 Deg/RIU for the structures containing 3-layer MoS2 with 22 nm Al film, 4-layer MoSe2 with 24 nm Al film, 7-layer WS2 with 36 nm Al film and 7-layer WSe2 with 30 nm Al film, respectively. The corresponding SPR curves are shown in Figure 2e–h.

Sensors 2018, 18, 2056 Sensors 2018, 18, x FOR PEER REVIEW

4 of 10 4 of 10

Figure 2. (a–d) Variation of the angular sensitivity as a function of the number of MX2 layers N with Figure 2. (a–d) Variation of the angular sensitivity as a function of the number of MX2 layers N with various thickness of Al thin film dAl and (e–h) the reflection spectra with different TMDCs materials various thickness of Al thin film dAl and (e–h) the reflection spectra with different TMDCs materials under optimal structures. under optimal structures.

Sensors 2018, x FOR PEER REVIEW Sensors 2018, 18, 18, 2056

5 10 of 10 5 of

Table 2. Optimized values of thickness of gold and the number of MoS2 layers with corresponding Table 2. Optimized of thickness of gold and the number of MoS2 layers with corresponding change in angularvalues sensitivity. change in angular sensitivity.

Type of TMDC MoS2 Type of TMDC MoSe2 MoS2 2 WS MoSe2 2 WSe WS 2 WSe2

Optimal Thickness of Al (nm) 22 Optimal Thickness of Al (nm) 24 22 36 24 30 36

Optimal Number of TMDC Layers Angular Sensitivity (Δn = 0.005) 3 214.8 Deg/RIU Optimal Number of TMDC Layers Angular Sensitivity (∆n = 0.005) 4 210.1 Deg/RIU 3 214.8 Deg/RIU 7 315.5 Deg/RIU 4 210.1 Deg/RIU 286.3 Deg/RIU 7 7 315.5 Deg/RIU

30

7

286.3 Deg/RIU

It should be noted that the intensity of reflection light is very weak at resonance. In order to effectively shift,of itreflection is necessary increase the power or adopt It shouldmeasure be notedthe thatresonance the intensity light to is very weak at incident resonance. In order to detectorsmeasure with high sensitivity.shift, For itexample, assuming 10 mW incidentpower light and 0.1 deg angular effectively the resonance is necessary to increase the incident or adopt detectors resolution [25], theFor reflected power at the SPR dipincident is estimated be0.1 6.5deg μWangular and theresolution reflected power with high sensitivity. example, assuming 10 mW light to and [25], and its nearest measurable neighbor is 0.32 μW, which can be thevariation reflectedbetween power atthe theresonance SPR dip isangle estimated to be 6.5 µW and the reflected power variation between detected byand commercially-available photodiodes a visible wavelength [26]. Also note by that, theeasily resonance angle its nearest measurable neighbor isat0.32 µW, which can be easily detected in our calculation, the sensing medium is set to be homogeneous in order to make fair comparison commercially-available photodiodes at a visible wavelength [26]. Also note that, in our calculation, previously works [12–16]. In fact, when the biosensors for detecting with size thewith sensing medium is set to be homogeneous in SPR order to make are fairused comparison with cells previously of several microns [27], a homogeneous sensing medium layer is reasonable. To further investigate works [12–16]. In fact, when the SPR biosensors are used for detecting cells with size of several the surface the index change caused byisthe sensing target is applied with finite microns [27], asensitivity, homogeneous sensing medium layer reasonable. To further investigate the thickness. surface Under the condition for by WSthe 2, the sensitivities different sensing layer thickness sensitivity, theoptimal index change caused sensing target iswith applied with finite thickness. Under theare showncondition in Figurefor 3.WS It is2 ,shown that the surface sensitivity increases with thicker sensinginlayer. optimal the sensitivities with different sensing layer thickness are shown FigureEven 3. with 10 nm thesensitivity surface sensitivity can be still assensing high aslayer. 37 Deg/RIU. It is shown thatthickness, the surface increases with thicker Even with 10 nm thickness, the surface sensitivity can be still as high as 37 Deg/RIU.

Figure 3. The change in surface sensitivity as the function of the thickness of biomolecules on Figure 3. The change in surface sensitivity as the function of the thickness of biomolecules on the the surface. surface.

ToTo further illustrate thethe contribution of of MX sensitivity, wewe have drawn thethe variation of of thethe 2 on further illustrate contribution MX 2 on sensitivity, have drawn variation reflectance with incident Deg to to90 90Deg Degfor for different number of layers reflectance with incidentangle anglevarying varyingfrom from 60 Deg thethe different number of layers when when the thickness Al thin is fixed 30 and nm and ns = 1.3300 in Figure It is indicated the thickness of Alofthin film film is fixed at 30atnm ns = 1.3300 in Figure 4a–d.4a–d. It is indicated that the that the minimum reflectivity, representing the capability of light energy absorption [15], approaches minimum reflectivity, representing the capability of light energy absorption [15], approaches zero zero firstly when thenumber numberofofMX MX22 layers layers increase, increase, which means energy firstly when the means the thecontribution contributionofoflight light energy absorption exceeds thethe electron energy loss. Meanwhile, thethe full width at at half maximum (FWHM) absorption exceeds electron energy loss. Meanwhile, full width half maximum (FWHM) becomes broader caused byby electron energy loss of of MX [13]. With thethe further increase of of thethe becomes broader caused electron energy loss MX 2 layers [13]. With further increase 2 layers MX layers, the FWHM keeps getting broader and the minimum reflectivity begins to diverge from MX 2 2 layers, the FWHM keeps getting broader and the minimum reflectivity begins to diverge from zero. This is because the the SPRSPR process mustmust satisfy the energy conservation T+R+ = 1, where T, R and zero. This is because process satisfy the energy conservation TA +R +A = 1, where T, R A denote the transmission, reflection and absorption, respectively. UnderUnder SPR condition, since the and A denote the transmission, reflection and absorption, respectively. SPR condition, since total reflection is fulfilled, the transmission T is close 0. When number TMDCs theinternal total internal reflection is fulfilled, the transmission T to is close to 0.the When the of number of layers TMDCs is insufficient, the absorbed energy is not able to promote SPRaexcitation. Therefore, layers is insufficient, the light absorbed light energy is not able atostrong promote strong SPR excitation. increasing theincreasing TMDCs layers enhance the can lightenhance absorption, in higher sensitivity. this Therefore, the can TMDCs layers the resulting light absorption, resulting inIn higher

sensitivity. In this condition, the absorption A is enhanced while the reflection R is reduced. However,

Sensors 2018, 18, 2056

Sensors 2018, 18, x FOR PEER REVIEW

6 of 10

6 of 10

condition, the absorption A is enhanced while the reflection R is reduced. However, the absorption the absorption will to bethe saturated to the electron energy loss when further adding enhancement will enhancement be saturated due electrondue energy loss when further adding TMDCs layers. TMDCs layers. In this condition, the absorption A is degraded while the reflection R is increased. In this condition, the absorption A is degraded while the reflection R is increased.

Figure 4. Variation of the reflectance respect to the number of (a) , (c)2, MoSe Figure 4. Variation of the reflectance respect to different the different number ofMoS (a) MoS 2, WS (b) 2WS (c) MoSe 2 2 , (b) 2 andand (d) (d) WSe with thethe thickness of Al thinthin filmfilm is fixed at 30 andand the the refractive index of of 2 layers with thickness of Al is fixed at nm 30 nm refractive index WSe 2 layers sensing medium ∆n ∆n = 1.3300. sensing medium = 1.3300.

sensitivity of conventional structure based single thin film is not high enough since TheThe sensitivity of conventional structure based on on single Al Al thin film is not high enough since the metallic layer cannot absorb enough light energy to excite a strong SPR. However, TMDCs has the metallic layer cannot absorb enough light energy to excite a strong SPR. However, TMDCs a real part benefits SPR SPR excitation excitation[15]. [15].Figure Figure5 shows 5 shows haslarger a larger real partofofdielectric dielectricconstant, constant, which which benefits thethe configuration and reflectance curves for the conventional SPR sensors and the proposed one with configuration and reflectance curves for the conventional SPR sensors and the proposed one with optimal parameters. Prominent sensitivity improvement up to 3.3 times can be observed in the WS optimal parameters. Prominent sensitivity improvement up to 3.3 times can be observed in the 2configuration. Figure 6 plots the electric field distributions in these two structures. There is WSassisted 2 -assisted configuration. Figure 6 plots the electric field distributions in these two structures. a stronger field enhancement in theinWS structure compared to the one,one, which There is a stronger field enhancement the2-assisted WS2 -assisted structure compared toconventional the conventional further verifies the positive contribution provided by thebyTMDCs. In addition to sensitivity, figure of which further verifies the positive contribution provided the TMDCs. In addition to sensitivity, merit (FOM) is also one of the important aspects that affects the sensing performance. According figure of merit (FOM) is also one of the important aspects that affects the sensing performance. to our calculation, the structure the highestthe FOM. ThisFOM. is because According to our calculation, thewithout structureTMDCs withoutdemonstrates TMDCs demonstrates highest This isthe energy induced by the TMDCs will broaden FWHM. As weAs know, this phenomenon is also because theloss energy loss induced by the TMDCs will broaden FWHM. we know, this phenomenon reported in other works of TMDCs-based SPR sensors [12–16]. Therefore, introducing TMDCs is also reported in other works of TMDCs-based SPR sensors [12–16]. Therefore, introducing TMDCsinto intothe theSPR SPRsensor sensorwill willcontribute contributetotosensitivity sensitivityenhancement enhancementbut butnot notFOM FOMenhancement. enhancement.

Sensors 2018, 18, 2056 Sensors 2018, 18, x FOR PEER REVIEW Sensors 2018, 18, x FOR PEER REVIEW

7 of 10 7 of 10 7 of 10

Figure5.5. Variation Variation of of the thereflectance reflectancewith withrespect respecttotothe theincident incidentangle anglefor for(a) (a) theconventional conventional Figure Figure 5. Variation of the reflectance with respect to the incident angle for the (a) the conventional biochemicalsensor sensorbased basedon on singleAl Al film,and and(b) (b)the the proposedconfiguration configurationbased basedon on optimal biochemical biochemical sensor based single on single film, Al film, and (b) proposed the proposed configuration based optimal on optimal angle sensitivity. angle sensitivity. angle sensitivity.

Figure Figure6.6.The Thefield fielddistributions distributionsalong alongthe thedirection directionperpendicular perpendiculartotothe theprism prismfor forthe theproposed proposed Figure 6. The field distributions along the direction perpendicular to the prism for the proposed configuration with seven WS layers and the conventional configuration without WS layers. 2 2 layers and the conventional configuration without WS 2 2 layers. configuration with seven WS configuration with seven WS2 layers and the conventional configuration without WS2 layers.

Besides angular sensitivity, thethe differential phase change between p-polarized and s-polarized Besides angular sensitivity, the differential phase change between p-polarized and s-polarized Besides angular sensitivity, differential phase change between p-polarized and s-polarized reflective wave is another approach to detect the analyte [28–30]. The variation of the phase sensitivity reflective wave is another approach to detect the analyte [28–30]. The variation of the phase sensitivity reflective wave is another approach to detect the analyte [28–30]. The variation of the phase sensitivity with respect the different number MX 2and and thickness areare showed Figure 7a–d. Inthe thethe structure with respect totothe number ofofMX thickness are showed ininFigure 7a–d. In structure with respect to different the different number of 2MX 2 and thickness showed in Figure 7a–d. In structure 5 Deg/RIU for bilayer MoS2 5 showing in Figure 1a, we can obtain the highest sensitivity of 1.12 × 10 showing in Figure 1a, we the highest sensitivity of 1.12 ×of101.12 Deg/RIU for bilayer 2 with forMoS bilayer MoS2 showing in Figure 1a,can weobtain can obtain the highest sensitivity × 105 Deg/RIU 5 Deg/RIU for monolayer MoSe2 with 35 nm Al film, 1.375 × 105 Deg/RIU 5 Deg/RIU with 30 nm Al film, 2.02 × 10 5 5 30 nm Al film, 2.02 × 10 for monolayer MoSe with 35 nm Al film, 1.37 × 10 Deg/RIU for 2 MoSe2 with 35 nm Al film, 1.37 × 10 Deg/RIU with 30 nm Al film, 2.02 × 10 Deg/RIU for monolayer 5 5 Deg/RIU forfor 3-layer WSWS 235 with 35 nmnm Al film and × 10 bilayer 2 with 35 35 nmnm Al Al film, 5 Deg/RIU 3-layer WS Al 35 film and 4.56 × and 104.56 for bilayerfor WSe with WSe 35WSe nm Al film, respectively. 2 with 2bilayer 3-layer 2 nm with Al film 4.56 × 10Deg/RIU for 2 with film, respectively. Comparing with angular sensitivity, the optimal number of MX 2 layers for phase Comparing with angular sensitivity, optimal number ofthe MXoptimal phase sensitivity is less.for phase 2 layers for respectively. Comparing with the angular sensitivity, number of MX2 layers sensitivity is less. sensitivity is less.

Sensors 2018, 18, 2056

8 of 10

Sensors 2018, 18, x FOR PEER REVIEW

8 of 10

FigureFigure 7. Variation of the sensitivity ofMX MX layers with various 7. Variation of angular the angular sensitivityasasa afunction function of of number number of 2 2 layers with various 2 , (b) MoSe 2 , (c) WS 2 , and (d) WSe 2 . thickness of Al thin film, (a) MoS thickness of Al thin film, (a) MoS2 , (b) MoSe2 , (c) WS2 , and (d) WSe2 .

To further improve the phase sensitivity, we propose another SPR biosensor based on Kretschmann

To further improve the phase sensitivity, we propose another SPR biosensor based on Kretschmann configuration, as shown in Figure 1b. In this sensor, Ag is used to replace Al and the air layer is removed. configuration, as shown in Figure In this and sensor, is used replace Alin and the 3. airThe layer is removed. The optimal conditions of Ag1b. thickness MX2Ag layers are to summarized Table best phase The optimal conditions of Ag thickness and MX layers are summarized in Table 3. The 2 with monolayer WS2 and 46 nm Ag film.best phase sensitivity as high as 3.85 × 106 Deg/RIU can be achieved sensitivity as high as 3.85 × 106 Deg/RIU can be achieved with monolayer WS2 and 46 nm Ag film. Table 3. Variation of the angular sensitivity as a function of number of MX2 layers with various

Variation the angular sensitivity as a function of number of MX2 layers with various Table 3. thickness of Alof thin film. thickness of Al thin film. Type of TMDC Optimal Thickness of Al (nm) Optimal Number of TMDC Layers Angular Sensitivity (Δn = 0.005) MoS2 Type of TMDC MoSe 2 WS2 MoS 2 2 WSe MoSe 2 WS2 WSeFor 2

40 Optimal Thickness 40 of Al (nm) 4046 4044

2 Optimal Number of 2 TMDC Layers 21 21

46 44 performances

1 1 reported

6.32 × 105 Deg/RIU Angular1.54 Sensitivity (∆n = 0.005) × 105 Deg/RIU 3.85 × 105 6Deg/RIU Deg/RIU 6.32 × 10 4.57 105 5Deg/RIU Deg/RIU 1.54 ××10

3.85 × 106 Deg/RIU 4.57 105 Deg/RIU 2D-material-assisted ×SPR sensors

comparison, the of previously are summarized in Table 4. Significant enhancements on both angular sensitivity and phase sensitivity can be obtained in the proposed sensors. For comparison, the performances of previously reported 2D-material-assisted SPR sensors are

summarized in Table 4. Significant enhancements on both angular sensitivity and phase sensitivity Table 4. Comparison with the formerly reported 2D-material-assisted SPR sensors. can be obtained in the proposed sensors. 2D Material Metal Angular Sensitivity Phase Sensitivity Graphene Au 134.6 Deg/RIU Table 4. Comparison with the formerly reported 2D-material-assisted SPR Graphene and MoS2 Al 190.4 Deg/RIU 4 Au 8.19 × 10 Deg/RIU Graphene and MoS2 2D Material Metal Sensitivity Phase Sensitivity Au Angular155.7 Deg/RIU WS2 1.20 ×-106 Deg/RIU WSe2 Graphene Au Au 134.6 Deg/RIU BP and TMDCs/graphene Deg/RIU 6.75 ×-103 Deg/RIU Graphene and MoS2 Al Ag 190.4 279.0 Deg/RIU graphene 315.5 WS2 andand Graphene MoS2 Au Al - Deg/RIU 8.19 × 104 Deg/RIU 3.85 ×-106 Deg/RIU WS2 and WS2graphene Au Ag 155.7 Deg/RIU

WSe2 BP and TMDCs/graphene WS2 and graphene WS2 and graphene

Au Ag Al Ag

279.0 Deg/RIU 315.5 Deg/RIU -

1.20 × 106 Deg/RIU 6.75 × 103 Deg/RIU 3.85 × 106 Deg/RIU

References sensors.[12] [13] [14] References [15] [15] [12] [16] [13] This [14]work This [15]work

[15] [16] This work This work

Sensors 2018, 18, 2056

9 of 10

4. Conclusions In this paper, SPR biosensors by using 2D TMDCs are proposed to enhance the sensitivity. In such sensors, the functional materials are coated on both sides of the metal layer and the impacts of material type, layer number, and metal thickness on the sensing performance are investigated and analyzed in detail. The results show that the angular sensitivity and phase sensitivity can reach as high as 315.5 Deg/RIU with 7-layers WS2 and 3.85 × 106 Deg/RIU with 1-layers WS2 , respectively. The proposed configuration can be promising a candidate for high performance biosensing. Author Contributions: Conceptualization was designed by T.H.; methodology, validation, and formal analysis were carried out by X.Z., X.W., P.H., J.P., and Y.W.; investigation and data curation were carried out by T.H., P.S.P., and Z.C.; writing-original draft preparation was redacted by X.Z.; writing-review & editing was performed by T.H., and X.Z. Funding: This work was supported by the National Natural Science Foundation of China (61605179), and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (162301132703 and G1323511794). Conflicts of Interest: The authors declare no conflicts of interest.

References 1. 2. 3.

4. 5.

6. 7. 8. 9. 10. 11.

12. 13.

14. 15.

Otto, A. Excitation of nonradiative surface plasma waves in silver by method of frustrated total reflection. Zeitschrift für Physik a Hadrons and Nuclei 1968, 216, 398–410. [CrossRef] Kretschmann, E.; Raether, H. Notizen: Radiative decay of nonradiative surface plasmons excited by light. Zeitschrift für Naturforschung A 1968, 23, 2135–2136. [CrossRef] Ahn, H.; Song, H.; Choi, J.R.; Kim, K. Localized Surface Plasmon Resonance Sensor Using DoubleMetal-Complex Nanostructures and a Review of Recent Approaches. Sensors 2018, 18, 98. [CrossRef] [PubMed] Zeng, S.; Baillargeat, D.; Ho, H.P.; Yong, K.T. Nanomaterials enhanced surface plasmon resonance for biological and chemical sensing applications. Chem. Soc. Rev. 2014, 43, 3426–3452. [CrossRef] [PubMed] Zeng, S.; Sreekanth, K.V.; Shang, J.; Yu, T.; Chen, C.K.; Yin, F.; Baillargeat, D.; Coquet, P.; Ho, H.P.; Kabashin, A.V.; et al. Graphene–Gold Metasurface Architectures for Ultrasensitive Plasmonic Biosensing. Adv. Mater. 2015, 27, 6163–6169. [CrossRef] [PubMed] Wang, G.; Wang, C.; Yang, R.; Liu, W.; Sun, S. A Sensitive and Stable Surface Plasmon Resonance Sensor Based on Monolayer Protected Silver Film. Sensors 2017, 17, 2777. [CrossRef] [PubMed] Maurya, J.B.; François, A.; Prajapati, Y.K. Two-Dimensional Layered Nanomaterial-Based One-Dimensional Photonic Crystal Refractive Index Sensor. Sensors 2018, 18, 857. [CrossRef] [PubMed] Kooyman, R.P.H. Handbook of surface plasmon resonance. R. Soc. Chem. 2008, 2, 15–34. Zhu, Y.; Murali, S.; Cai, W.; Li, X.; Suk, J.W.; Potts, J.R.; Ruoff, R.S. Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 2010, 22, 3906–3924. [CrossRef] [PubMed] Homola, J. Surface plasmon resonance sensors for detection of chemical and biological species. Chem. Rev. 2008, 108, 462–493. [CrossRef] [PubMed] Zhang, N.; Humbert, G.; Gong, T.; Shum, P.P.; Li, K.; Auguste, J.L.; Wu, Z.; Hu, J.; Feng, L.; Dinh, Q.X.; et al. Side-channel photonic crystal fiber for surface enhanced Raman scattering sensing. Sens. Actuators B Chem. 2016, 233, 195–201. [CrossRef] Verma, R.; Gupta, B.D.; Jha, R. Sensitivity enhancement of a surface plasmon resonance based on biomolecules sensor using graphene and silicon layers. Sens. Actuators B Chem. 2011, 160, 623–631. [CrossRef] Zeng, S.; Hu, S.; Xia, J.; Anderson, T.; Dinh, X.Q.; Meng, X.M.; Coquet, P.; Yong, K.T. Graphene-MoS2 hybrid nanostructures enhanced surface plasmon resonance biosensors. Sens. Actuators B Chem. 2015, 207, 801–810. [CrossRef] Wu, L.; Jia, Y.; Jiang, L.; Guo, J.; Dai, X.; Xiang, Y.; Fan, D. Sensitivity improved SPR biosensor based on the MoS2 /graphene-aluminum hybrid structure. J. Lightwave Technol. 2016, 35, 82–87. [CrossRef] Ouyang, Q.; Zeng, S.; Dinh, X.Q.; Coquet, P.; Yong, K.T. Sensitivity enhancement of MoS2 nanosheet based surface Plasmon resonance biosensor. Procedia Eng. 2016, 140, 134–139. [CrossRef]

Sensors 2018, 18, 2056

16.

17. 18. 19. 20.

21. 22. 23. 24. 25.

26.

27.

28.

29. 30.

10 of 10

Wu, L.; Guo, J.; Wang, Q.; Lu, S.; Dai, X.; Xiang, Y.; Fan, D. Sensitivity enhancement by using few-layer black phosphorus-graphene/TMDCs heterostructure in surface plasmon resonance biochemical sensor. Sens. Actuators B. Chem. 2017, 249, 542–548. [CrossRef] Khageswar, S.; Kumar, M.S.; Kumar, G.P. He-Ne laser (632.8 nm) pre-irradiation gives protection against DNA damage induced by a near-infrared trapping beam. J. Biophotonics 2009, 2, 140–144. Sreekanth, K.V.; Zeng, S.; Yong, K.T.; Yu, T. Sensitivity enhanced biosensor using graphene-based one-dimensional photonic crystal. Sens. Actuators B. Chem. 2013, 182, 424–428. [CrossRef] Jha, R.; Sharma, A.K. Chalcogenide glass prism based SPR sensor with Ag-Au bimetallic nanoparticle alloy in infrared wavelength region. J. Opt. Pure Appl. Opt. 2009, 11, 045502. [CrossRef] Li, Y.; Chernikov, A.; Zhang, X.; Rigosi, A.; Hill, H.M.; van der Zande, A.M.; Chenet, D.A.; Shih, E.M.; Hone, J.; Heinz, T.F. Measurement of the optical dielectric function of monolayer transition-metal dichalcogenides: MoS2 , MoSe2 , WS2 , and WSe2 . Phys. Rev. B 2014, 90, 205–422. [CrossRef] Liu, H.L. Optical properties of monolayer transition metal dichalcogenides probed by spectroscopic ellipsometry. Appl. Phys. Lett. 2014, 105, 201905. [CrossRef] Bruna, M.; Borini, S. Optical constants of graphene layers in the visible range. Appl. Phys. Lett. 2009, 94, 031901. [CrossRef] Wu, L.; Ling, Z.; Jiang, L.; Guo, J.; Dai, X.; Xiang, Y.; Fan, D. Long-Range Surface Plasmon with Graphene for Enhancing the Sensitivity and Detection Accuracy of Biosensor. IEEE Photonics J. 2016, 8, 1–9. [CrossRef] Maharana, P.K.; Jha, R.; Palei, S. Sensitivity enhancement by air mediated graphene multilayer based surface plasmon resonance biosensor for near infrared. Sens. Actuators B Chem. 2014, 190, 494–501. [CrossRef] Sreekanth, K.V.; Alapan, Y.; ElKabbash, M.; Wen, A.M.; Ilker, E.; Hinczewski, M.; Gurkan, U.A.; Steinmetz, N.F.; Strangi, G. Enhancing the Angular Sensitivity of Plasmonic Sensors Using Hyperbolic Metamaterials. Adv. Opt. Mater. 2016, 4, 1767–1772. [CrossRef] [PubMed] Corcoran, B.; Monat, C.; Grillet, C.; Moss, D.J.; Eggleton, B.J.; White, T.P.; O'Faolain, L.; Krauss, T.F. Green light emission in silicon through slow-light enhanced third-harmonic generation in photonic-crystal waveguidese. Nat. Photonics 2009, 3, 206–210. [CrossRef] Song, C.L.; Jin, T.; Yan, R.P.; Qi, W.Z.; Huang, T.Y.; Ding, H.F.; Tan, S.H.; Nguyen, N.T.; Xi, L. Opto-acousto-fluidic microscopy for three-dimensional label-free detection of droplets and cells in microchannels. Lab Chip 2018, 9, 1267–1390. [CrossRef] [PubMed] Zeng, S.; Yu, X.; Law, W.C.; Zhang, Y.; Hu, R.; Dinh, X.Q.; Ho, H.P.; Yong, K.T. Size dependence of Au NP-enhanced surface plasmon resonance based on differential phase measurement. Sens. Actuators B Chem. 2013, 176, 1128–1133. [CrossRef] Raether, H. Surface plasmons on smooth and rough surfaces and on gratings. Springer Tracts Mod. Phys. 1983, 111, 354401–373633. Wong, C.L.; Chua, M.; Mittman, H.; Choo, L.X.; Lim, H.Q.; Olivo, M. A Phase-Intensity Surface Plasmon Resonance Biosensor for Avian Influenza A (H5N1) Detection. Sensors 2017, 17, 2363. [CrossRef] [PubMed] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).