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Enhanced sensitivity of bimetallic optical fiber SPR sensor based on MoS2 nanosheets
Optics and Lasers in Engineering 128 (2020) 105997
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Enhanced sensitivity of bimetallic optical fiber SPR sensor based on MoS2 nanosheets Qi Wang a,b,∗, Xi Jiang a,1, Li-Ye Niu a,1, Xiao-Chen Fan a a b
College of Information Science and Engineering, Northeastern University, Shenyang 110819, China State Key Laboratory of Synthetical Automation for Process Industries (Northeastern University), Shenyang 110819, China
a r t i c l e
i n f o
Keywords: Optical fiber sensor Surface plasmon resonance Molybdenum disulfide nanosheets Sensitivity enhancement
a b s t r a c t In this paper, we have fabricated and characterized a bimetallic optical fiber surface plasmon resonance (SPR) sensor, followed by molybdenum disulfide (MoS2 ) nanosheets have been coated on the surface of metal film. The performance of the sensor is discussed in terms of the sensitivity and the figure of merit (FOM) theoretically and experimentally. The effect of different thickness of Au/Ag film on the performance of the sensor was studied. The results indicate that the FOM of sensor with silver film is much higher than that with gold film, although gold film is conducive to the sensitivity improvement. Two types of sensors are fabricated, which are the sensor with silver layer and the sensor with bimetallic layers of silver and gold. The sensitivity of the sensor with silver film and the sensor with bimetallic layers are 2141 nm/RIU and 2487 nm/RIU, respectively; and the FOM values are 19.44 RIU−1 and 18.47 RIU−1 , respectively. Then, it is theoretically confirmed that MoS2 has the potential to promote the sensitivity of SPR sensor, and the sensitivity is tuned by the number of layers of MoS2 nanosheets. MoS2 nanosheets successfully bridged the gold film with the help of chemical bonds. The sensitivity of the sensor with MoS2 nanosheets is 3061 nm/RIU, and its FOM value is 23.29 RIU−1 .
1. Introduction In recent decades, the SPR sensor has made great progress in structure research and application. At present, according to the SPR sensor structure classification, there are mainly prism SPR sensors [1], integrated waveguide SPR sensors [2], grating SPR sensors [3] and optical fiber SPR sensors [4–6]. SPR is the phenomenon that the electromagnetic wave irradiates the metal surface and interacts with the free electrons on the metal surface to cause the collective oscillation of electrons in the metal [7]. Fig. 1 shows a general SPR device in which the incident p-polarized light (TM mode) excites the longitudinal collective oscillation of free electrons on the metal surface and generates the surface plasma wave (SPW) on the gold film surface. When the wave vector of the incident P-polarized light matches the wave vector of the SPW: √ 𝜀𝑚 · 𝜀𝑑 𝜔 𝑘𝑠𝑝 = (1) 𝑐 𝜀𝑚 + 𝜀𝑑 𝑘𝑥 =
𝜔√ 𝜔 𝜀0 sin 𝜃0 = 𝑐 𝑐
√
𝜀𝑚 · 𝜀𝑑 𝜀𝑚 + 𝜀𝑑
(2)
Where, kx is the wave vector of incident light along the X-axis, and ksp is the wave vector of the SPW. When the two values are equal, sur∗
1
face plasmon resonance is excited. When the external refractive index perturbation occurs, the value of ksp changes, and surface plasmon resonance occurs at another resonance wavelength. Therefore, the change of external refractive index can be sensitively sense via the shift of resonance wavelength or resonance angel. In 1992, R.C. Jorgenson first made SPR sensor with optical fiber [8], which pushed the optical fiber sensing technology to a new height. Optical fiber sensor has been widely used for its small size, nondestructive testing, anti electromagnetic interference, real-time detection and other advantages [9]. In 1983, Lidberg et al. first introduced the optical fiber SPR technology into the field of biosensors and pioneered the SPR biosensor [10]. After more than ten years of development, SPR sensors have been wildly used in environmental protection, food safety, biomedical and other fields [11–13]. In the field of biomolecule detection, the concentration of biomolecule is usually small, and the sample also contains a variety of other molecules, which requires the sensor to have high sensitivity and specificity. With the improvement of SPR sensor sensitivity, SPR technology will play a greater role in biomedical and disease diagnosis. Therefore, improving the sensitivity of the sensor has been the focus of SPR sensor technology research. At present, the research with regard to enhanced sensitivity of SPR sensor is mainly divided into two aspects. On the one hand, the
Corresponding author. E-mail address: [email protected] (Q. Wang). These authors contributed equally for this work.
https://doi.org/10.1016/j.optlaseng.2019.105997 Received 2 July 2019; Received in revised form 13 November 2019; Accepted 23 December 2019 0143-8166/© 2019 Elsevier Ltd. All rights reserved.
Q. Wang, X. Jiang and L.-Y. Niu et al.
Fig. 1. The model of SPR device.
sensitivity is improved by optimizing the structure of the sensor; on the other hand, different materials are coated into the surface of the metal film to improve the sensitivity. In terms of structural optimization, most of the research mainly includes the improvement of the structure of optical fiber SPR sensors, the types of metal films, and the optimization of the thickness of metal films. Various novel SPR sensors have been designed, such as bimetallic SPR sensors [14], D-type optical fiber SPR sensors [15], long-range SPR sensors [16], and optical fiber taper sensors [17]. Recent studies have shown that many materials, such as oxides, graphene, transition sulfides, and black phosphorus, can enhance the sensitivity of the sensors due to their enhanced SPR signal characteristics. Satyendra K M et al. proposed an optical fiber SPR sensor based on Cu/ZnO [18]. It is demonstrated that the sensitivity of the sensor is higher than that of the sensor uncoated ZnO. Wei et al. introduce graphene into the optical fiber SPR sensor [19], and a single layer of graphene sheet is coated on the surface of the SPR sensor. It is proved that the sensitivity of the graphene-based optical fiber SPR sensor is more than twice that of the conventional gold film optical fiber SPR sensor. Transition metal dichalcogenide (TMDC), similar to graphene, is also a 2D nanomaterial. MoS2 and WS2 , belong to TMDC, have been widely used in the sensor field. The semiconductor MoS2 is a promising material for electronic and optical applications. Due to its high charge mobility, MoS2 has been applied to field effect transistors (FET). Compared with graphene, MoS2 and WS2 have larger band gap and higher optical absorption efficiency, and they also have high specific surface area and good biocompatibility. These make it possible to introduce transition metal dichalcogenide into SPR sensors. Akhilesh Kumar Mishra and his group theoretically investigated the optical fiber SPR sensor based on metal film/graphene/MoS2 structure [20]. It is shown through theoretical analysis that the introduction to MoS2 can better enhance the sensitivity of the sensor, and the structure improves sensitivity to 6000 nm/RIU. Siddharth Kaushik et al. designed MoS2
Optics and Lasers in Engineering 128 (2020) 105997
nanosheets functionalized optical fiber SPR immunosensor, functionalized MoS2 nanosheets as a bridge to connect fiber and antibodies [21]. MoS2 not only enhances the sensitivity of SPR sensors, but also can be used to immobilize biorecognition molecules (antibodies, DNA, enzymes), which further broaden the application of MoS2 in biosensor. Hao Wang et al. coated multilayer tungsten dioxide (WS2 ) nanosheets on the surface of prism SPR sensor [22]. It was experimental confirmed that coating WS2 nanosheets on the outside of the gold film can significantly improve the sensitivity of the sensor, and the maximum sensitivity is 2459.3 nm/RIU. The sensitivity of the sensor is also tuned by the thickness of the WS2 nanosheets on the surface of sensor. The refractive index of molybdenum disulfide is a complex number, and the real part is significantly larger than the imaginary part, so the enhanced sensitivity characteristics of molybdenum disulfide are superior to other materials. Molybdenum disulfide has been proved to be sensitive to some gas specificity, so molybdenum disulfide will have advantages in gas sensing. Some materials can only enhance the sensitivity in a specific refractive index range. Black phosphorus has been proved to have strong enhancement sensitivity to the external medium with refractive index between 1.00–1.20. Although many studies have proved that transition metal sulfide can improve the sensitivity of the sensor, most of the conclusions are based on the theoretical model of numerical analysis, lack of experimental research. In this work, we prove theoretically and experimentally that the MoS2 can enhance the sensitivity of bimetal layer fiber SPR sensor. Two kinds of metal thickness of bimetal layer SPR sensor are optimized theoretically, and the optimum thickness ratio is determined. The experimental results are in accordance with the theoretical simulation, which shows that the designed sensor is reasonable. 2. Theoretical model analysis 2.1. Establishment of numerical model The MoS2 coated silver/gold bimetal optical fiber SPR sensor model is presented in Fig. 2. In order to fabricate the sensing probe, plastic clad silica optical fiber with core diameter of 600 um and numerical aperture of 0.37 is adopted. First, 10 mm length cladding at one end of the optical fiber is removed. A layer of silver film is assembled on the bare core surface. Then a layer of gold film is added to the silver film. Finally, a layer of MoS2 is coated on the surface of the silver film. Light is incident from the fiber at one end and propagates in the fiber. When the light is entirely reflected in the sensing area, the evanescent wave immersed in the surface of the metal film can excite the surface plasmon wave. Because the energy of incident light is absorbed, there is a resonance absorption peak in the reflection spectrum. When the external refractive index of the sensing region changes from n0 to 𝑛0 + Δ𝑛, the position of resonance wavelength shifts. It is well known that gold and silver are the most suitable plasmonic materials for exciting SPR due to their unique dispersion properties, and the dielectric constants of gold and silver are modeled according to the Drude dispersion model [23], as shown in Eq. (3). ε(λ) = 𝜀𝑟 + 𝑖∗𝜀𝑖 = 1 −
( 2
𝜆2 𝜆𝑐
) 𝜆𝑝 𝜆𝑐 + 𝑖∗𝜆
(3)
Fig. 2. The MoS2 coated gold/silver bimetal optical fiber SPR sensor model.
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Optics and Lasers in Engineering 128 (2020) 105997
Fig. 3. The SPR curves of the bimetal probes with different thickness ratios (the external refractive index is 1.330).
Table 1 Dispersion coefficients of metals. Dispersion coefficients (𝜇m)
𝜆p 𝜆c
Metal Ag
Au
0.14541 17.614
0.16826 8.9342
Where, 𝜆c represents the collision wavelength, and 𝜆p represents the wavelength corresponding to the plasma frequency. The values of 𝜆c , 𝜆p for gold and silver are given in Table 1. The relationship between the refractive index of the core and the wavelength is obtained by the Sellmeier relation [24]: √ 𝑎3 𝜆2 𝑎1 𝜆2 𝑎2 𝜆2 𝑛1 (𝜆) = 1 + + + (4) 2 2 2 2 2 𝜆 − 𝑏1 𝜆 − 𝑏2 𝜆 − 𝑏3 2 Where a and b are the Sellmeier coefficients, and their values are given as 𝑎1 = 0.6961663, 𝑎2 = 0.4079426, 𝑎3 = 0.8974794
(5)
𝑏1 = 0.0684043, 𝑏2 = 0.1162414, 𝑏3 = 9.896161
(6)
The dispersion relation of MoS2 is obtained in Ref [25], and the polynomial equations of the dispersion relation of MoS2 is given in Ref [25]. There are many numerical models for studying the transmission characteristics of multilayered dielectric film, and the most popular one is the transfer matrix method [26]. A large number of studies have shown that the prediction of the transmission matrix agrees well with the experimental measurements. Therefore, the numerical model analysis of the SPR sensor with multilayer dielectric film uses the transfer matrix method. The schematic diagram of N-layer structure is shown in support information Fig. S1. Here, ɛk is the dielectric constant of the √ k-th film with dk thickness, and its refractive index nk = 𝜀𝑘 . 2.3. Analysis of numerical model Based on the transfer matrix method of multi-layer structure, the influence of different dielectric layers on the performance of the SPR
sensor can be investigated thoroughly, which provides a theoretical basis for the fabrication of excellent SPR sensor in the future. In this work, we propose a reflective optical fiber SPR sensor coated with MoS2 on the outer surface of silver/gold bimetallic layer. It is well known that surface plasmon resonance (SPR) phenomena excited by different kinds of metal films are different, which benefits from their different dispersion characteristics. Additionally, the change of the thickness of the metal film also affects the excitation of surface plasmon resonance (SPR). Previously, it was revealed that the most appropriate thickness of the metal film was about 50 nm [27]. Therefore, the total thickness of the silver/gold bimetallic layer of the SPR probe proposed in this paper is set at 50 nm. In order to study the influence of the thickness ratio of gold and silver on the sensing performance of bimetal layer sensor, molybdenum disulfide was not added to the surface of bimetal layer probe for theoretical analysis. The thickness of the silver film and the gold film are set at 50 nm/0 nm, 40 nm/10 nm, 30 nm/20 nm, 20 nm/30 nm, 10 nm/40 nm and 0 nm/50 nm, respectively. The reflectivity is calculated from the transmission matrix. When the external refractive index is 1.330, the SPR curves of the bimetal probes with different thickness ratios are shown in Fig. 3. With the decrease of the thickness of the silver film (the increase of the thickness of the gold film), the resonance wavelength shifts red and the resonance absorption peak widens. From Fig. 4(a), it can be seen that with the decrease of silver film thickness (the increase of gold film thickness), the sensitivity of the sensor increases, and FWHM also increases. The FOM is considered a reasonable parameter to evaluate the sensor performance, it is given as S/FWHM. As shown in Fig. 4(b), as the gold film increases (the thickness of the silver film decreases), the FOM becomes smaller. Therefore, the performance of the silver film is superior to that of the gold film based on the numerical model analysis. However, the silver film in the experimental environment has poor stability and is easily oxidized. The SPR sensor of the bimetal layer is proposed, and a layer of thin gold film is deposited on the surface of the silver film. The sensitivity of the sensor is also improved, in addition, the gold film isolates the contact between the silver film and the air, thereby improving the stability of the sensor. Therefore, the thicknesses of the silver film and the gold film were determined to be the most suitable thickness ratios of 40 nm and 10 nm, respectively, and
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Optics and Lasers in Engineering 128 (2020) 105997
Fig. 4. The variation of parameters of bimetallic probe with the thickness of metal film. (a) Variation of sensitivity and FWHM with the thickness of metal film. (b) Variation of the FOM with the thickness of metal film.
Fig. 5. The structure of the MoS2 nanosheets.
the sensor sensitivity and FOM were 3131.0 nm/RIU and 86.38 RIU−1 , respectively. MoS2 , as a two-dimensional nano material, is widely used in the manufacture of transistors, photodetectors and optical sensors due to its excellent properties. Fig. 5 shows the ultra-thin MoS2 layer, which
is stacked by S-Mo-S. The thickness of monolayer MoS2 nanosheets is 0.65 nm. Such a small thickness produces a new set of optical and electronic properties, such as larger band gaps in single-layer MoS2 than in bulk MoS2 . The weak van der Waals force between the layers of MoS2 determines that MoS2 can easily be peeled into nanosheets. In addition, MoS2 has good biocompatibility, so MoS2 nanosheets have great potential in the field of biosensing. On the other hand, as a high dielectric constant material, MoS2 is coated on the surface of the metal film, which can enhance the electric field strength of the surface of the metal film. As we know, the electric field strength of the surface of the metal film, the enhancement could directly enhance the sensitivity of the SPR sensor. The research work on the influence of MoS2 on the optical fiber SPR sensor with bimetal layer was carried out by means of the transmission matrix. The gold film surface of the bimetal SPR sensor was coated with a layer of MoS2 nanosheets having a thickness of L = 1 × 0.65 nm, and the thicknesses of the silver film and the gold film were set to 40 nm and 10 nm, respectively. Fig. 6 shows the SPR curve and the resonance wavelength versus refractive index for the probe coated with MoS2 nanosheets. Compared with the probe with bimetallic layer, the resonance wavelength of the optical fiber SPR sensor coated with MoS2 nanosheets shows a red shift Fig. 6. The SPR reflection spectrum of the bimetallic probe with singer layer of MoS2 , the inset of the figure shows the relationship between the resonance wavelength and the refractive index.
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Optics and Lasers in Engineering 128 (2020) 105997
Fig. 7. (a) The SPR curves of MoS2 with different layers, (b) Variation of sensitivity and FWHM with the layers of MoS2 nanosheets.
trend, and the SPR curve becomes wider. The sensitivity and FWHM of the sensor are 3485.5 nm/RIU and 48.05 nm, respectively. MoS2 with different layers is deposited on the outer surface of the gold film. The SPR curves of MoS2 with different layers are shown in Fig. 7(a). With the increase of layers, the SPR curves show a trend of redshift and become wider. When the number of layers of MoS2 nanosheets reaches three layers, the sensitivity of the sensor decreases sharply and the SPR curve begins to distort. Fig. 7(b) shows the change trend of sensor sensitivity and FWHM with the increase of layers of MoS2 nanosheets. The sensitivity and FWHM increase with the increase of layers of MoS2 nanosheets. The maximum sensitivity of the sensor is 4420.4 nm/RIU when the number of layers of MoS2 nanosheets reaches two layers. 3. Experimental section 3.1. Fabrication of bimetallic SPR sensor The fabrication of the bimetal fiber SPR sensor is divided into three steps: (a) pretreatment of the fiber optic probe, (b) assembly of the silver film, and (c) deposition of a layer of gold film. As shown in Fig. 1, the plastic clad silica optical fiber of 80 mm length (Core diameter = 600 um, numerical aperture = 0.37) is selected to make a reflective SPR sensor. A 10 mm cladding at one end of the probe was removed for use as a sensing area. Since the two end faces of the fiber are not flat, it is necessary to repeatedly polish them with sandpaper of different roughness until they become smooth. The bare core is shown in Fig. 8(a). Before assembling the silver film, it is necessary to repeatedly
clean the probe with ethanol and deionized water to remove impurities attached to the surface. The silver film is assembled on the surface of the core by a chemical method, and a layer of silver film is deposited on the surface of the exposed core by silver mirror reaction of silver ammonia solution and glucose solution [28]. The detailed procedure has been described in Ref [28] and the thickness of the silver film can be adjusted by the concentration of the two solutions, so that it is ensured that a layer of silver film having a thickness of about 40 nm can be obtained on the cylinder surface of the core. Fig. 8(b) shows the probe assembled with a layer of silver film. Because the silver film is easy to oxidize, a layer of thin gold film should be coated on the surface of the silver film quickly to prevent contact between silver film and air. The gold film is deposited by magnetron sputtering instrument (JS-1600), which can control the thickness of the gold film by controlling the discharge time and current [29]. The current of 8 mA and the discharge time of 1 min 30 s ensure that the thickness of the gold film deposited on the surface of the silver film is about 10 nm. The probe with a gold film deposited on the surface of the silver film is shown in Fig. 8(c). So far, the fabrication of bimetallic SPR reflective optical fiber sensor has been accomplished. The electron microscope image of the bimetal film of the sensor is shown in Fig. 8(d). 3.2. Preparation of MoS2 –COOH nanocomposite MoS2 powder of 60 mg was dissolved in 30 ml deionized water. MoS2 was evenly dispersed in deionized water by magnetic stirrer. The sodium hydroxide(NaOH) of 1.2 g and the chloroacetic acid(MCA) of 2 g were added to MoS2 dispersion. The mixed solution was placed in an ultra-
Fig. 8. Images of three different types of probes. (a) The fiber’s core without cladding. (b) The sensor assembled with a layer of silver film. (c) The sensor with a gold film deposited on the surface of the silver film. (d) The electron microscope image of the bimetal film of the sensor.
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Optics and Lasers in Engineering 128 (2020) 105997
sonicator for continuous sonication for 3 h. Under ultrasonic treatment, there are quite a few vacancies on the surface of MoS2 [30]. These vacancies are quickly occupied by chlorine atoms in chloroacetic acid, which leads to the modification of the carboxyl group to the surface of MoS2 . Compared with MoS2 , the dispersibility of carboxyl-modified MoS2 is greatly improved. Finally, the MoS2 –COOH dispersion was successfully prepared. 3.3. Fabrication of MoS2 –COOH modified SPR sensor It is difficult to deposit MoS2 –COOH sensitive material on the surface of gold film by dip coating method because of the poor adsorption of gold film. Therefore, a chemical modification method is chosen to replace the traditional dip coating method, which is to modify a specific functional group on the surface of gold film. 4-Aminothiophenol was selected as a bridge between gold film and MoS2 –COOH. The thiol of 4-Aminothiophenol interacted with gold film to form Au–S bond, thus installing amino-group on the surface of gold film. The detailed steps are shown in Fig. S2(a), (b) and (c) in the support information, the 4Aminothiophenol powder used ethanol diluted to 10 mmol, the surface of the gold film of the probe is washed several times with ethanol and deionized water, and then the sensor is placed in a drying oven to be dried. Finally, the gold film of the sensor is completely immersed in the 4-Aminothiophenol ethanol solution for 24 h at room temperature. The sensor for modifying the amino group is immersed in the MoS2 –COOH dispersion for 5 h, which ensures that the reaction of the amino group with the carboxyl group produces a strong chemical bond to firmly bond the MoS2 to the surface of the gold film, followed by thorough rinsing with deionized water to remove unbonded MoS2 –COOH. The electron micrograph of the gold film surface of the sensor and the electron micrograph of the molybdenum disulfide nano film modified on the gold film surface of the sensor are shown in Fig. S3 (a) and (b) in the supporting information, respectively. The physical precipitation method is used to coat the second layer and the third layer of MoS2 nano sheet on the surface of bimetal layer optical fiber probe. The physical precipitation of MoS2 film is to completely cover the sensor area of the probe with the dispersion, put it into the vacuum drying oven, under the effect of temperature, the dispersant in the dispersion is evaporated, thus the second layer of MoS2 nano film is deposited on the surface of the gold film. Repeat the above steps to deposit the third layer of film. The fabrication of the sensing area is completed, and the refractive index sensing experiment of the sensor is carried out. 3.4. Experimental setup The schematic diagram of the refractive index measurement system is shown in Fig. 9. A tungsten halogen lamp source (HL-2000-LL) with a wavelength range of 360–2400 nm was selected as the light source of the measurement system, and the emitted light was transmitted to the SPR probe through the Y-coupler. The reflected light is transmitted along the other end of the Y-coupler to the spectrometer (Maya 2000pro), and the computer connected to the spectrometer records the reflectance spectrum in real time. When the fiber probe is immersed in a sample solution of different refractive index, the condition of the surface plasmon resonance is changed due to the change of the external refractive index of the probe surface, which directly causes the position of the resonance wavelength to drift. The corresponding reflection spectrum is displayed by means of a spectrum analysis software, and a series of SPR curves corresponding to different refractive indexes are obtained. 4. Results and discussion Due to the instability of the silver film, the performance of the silver film probe is tested immediately after the silver film is assembled on the surface of the fiber core. The refractive index range of a series of sample
Fig. 9. The schematic of the refractive index measurement system.
Fig. 10. The SPR reflection spectrum of the sensor probe with Ag film, the inset of the figure shows the relationship between the resonance wavelength and the refractive index.
refractive index solutions used in the experiment is 1.3318 to 1.3701. The refractive index range of the solution obtained by diluting ethanol with deionized water is 1.3301–1.3614. 1.3701 the solution with refractive index is glycerin aqueous solution. Fig. 10 shows the SPR curve of the probe assembled with a layer of silver film, and the illustration shows the linear fitting curve of resonance wavelength and refractive index. As we have seen, the position of the SPR curve shifts toward the long wavelength position as the refractive index increases. The wavelength corresponding to the trough of each SPR curve is recorded, which is called the resonant wavelength. Sensitivity of the sensor can be calculated by 𝛿𝜆res /𝛿ns , where 𝛿ns and 𝛿𝜆res represent the increment of the refractive index and the offset of the corresponding resonant wavelength respectively. As can be seen from the inset of Fig. 10, the sensitivity of SPR probe with the silver film is 2140.42 nm/RIU. It can be seen from Fig. 11 that the sensitivity of the SPR probe of the bimetal layer is significantly better than that of the silver film SPR probe, and the sensitivity is 2486.91 nm/RIU. Moreover, it is worth noting that the width of the SPR curve of the bimetal probe is also larger than that of the silver film probe. These experimental phenomena are consistent with the results of the numerical model in the previous paper. The refractive index measurement experiments of SPR probe based on MoS2 nanosheets were carried out using the same series of refractive index solutions. The experimental results of the refractive index of the sensing area coated with one layer and two layers of MoS2 are shown in Fig. 12. The sensitivity of the probe coated with one layer and two
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Optics and Lasers in Engineering 128 (2020) 105997
the electric field overlap integral of the electric field. 𝛿𝜆 ≈
Fig. 11. The SPR reflection spectrum of the sensor probe with gold and silver bimetal layer, the inset of the figure shows the relationship between the resonance wavelength and the refractive index.
layers of MoS2 was increased to 2544.86 nm/RIU and 2671.87 nm/RIU respectively, but the FWHM of the SPR curve were larger than that of the bimetal probe. The sensitivity of the probe coated with MoS2 is attributed to the enhancement of the electric field on the surface of the gold film. It is well known that the sensitivity of the SPR sensor is directly related to the electric field on the surface of the gold film. In most cases, the increase in sensitivity is related to the enhancement of the electromagnetic field overlap integral around the analyte [31]. According to ref [31], the shift of the resonant wavelength is proportional to
⃗∗ ⃗ 𝑘𝑖 ∫𝑉int 𝐸𝑖 𝛿𝜀𝐸𝑓 𝑑𝑟 2 ∫ 𝐸⃗ ∗ 𝛿𝜀𝐸⃗ 𝑑𝑟 𝑖 𝑉 𝑖
(7)
where Ef is the electric field after the external refractive index changes, and 𝛿𝜀 represents the shift of the resonance wavelength due to the change of the dielectric constant of the analyte from 𝜀 to 𝜀 + 𝛿𝜀. In the above experiments, the sensors coated with one layer and two layers of MoS2 were analyzed. The following experiments explores the sensor coated with three layers of MoS2 , and the results are shown in Fig. 13. The repeatability experiment of the sensor coated with three layers of MoS2 was carried out. The two fitting curves in Fig. 14 are obtained by performing two measurements on the probe under the same experimental conditions. As shown in the figure, for the sample solutions of the same refractive index, the positions of the resonant wavelengths measured twice are almost the same, and the sensitivity of the sensors is 3061.84 nm/RIU and 3072.54 nm/RIU, respectively, which confirms the superior repeatability of the probe. Although the width of the SPR curve of the probe coated with the MoS2 nanosheets is larger than that of the bimetal probe, the sensitivity of the probe increases much more than the FWHM, thus causing the value of the FOM of the probe to become larger. As shown in Table 2, the sensitivity and the FOM of the three probes were compared. In the above experiments, we studied the application of one, two and three layers of MoS2 on the sensing area. It can be seen from the experiment that the refractive index sensitivity of the sensor increases with the increase of the number of layers of the sensor coated with MoS2 . However, the increase of the number of layers results in the broadening and deformation of the spectrum of the sensor. The broadening and de-
Fig. 12. The SPR reflection spectrum of the sensor probe coated MoS2 nanosheets. (a) and (b) are the experimental data of one layer of MoS2 ; (c) and (d) are the experimental data of two layers of MoS2 .
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Fig. 13. (a) The SPR reflection spectrum of the sensor probe coated three layers of MoS2 nanosheets, (b) The relationship between the resonance wavelength and the refractive index. Tabel 2 Performance comparison of SPR sensors with various sensitive layer. Type of sensor
Sensitivity (nm/RIU)
FOM(RIU−1 )
SPR sensor with Au film SPR sensor with bimetallic film Bimetallic SPR sensor with three layers MoS2
2141 2487 3061
19.44 18.47 23.29
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Key R&D Program of China under Grant SQ2019YFC170311, the Fundamental Research Funds for the Central Universities under Grant N180402023 and N172002001, and the National Natural Science Foundation of China under Grant 51607028. Supplementary materials Fig. 14. The fitted curve of the resonant wavelength and the corresponding refractive index, the first measurement (red), the second measurement (black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
formation of the resonance spectrum are enhanced by the coated MoS2 nanosheets, and the more layers are coated, the stronger the scattering is, the more serious the spectral deformation is. 5. Conclusions In summary, the paper proposed a bimetallic optical fiber SPR sensor based on MoS2 nanosheets, the sensitivity of the sensor has been significantly improved. The gold silver bimetal layer SPR sensor not only improves the sensitivity of the sensor, but also improves the stability of the sensor. The introduction of MoS2 further strengthens the sensitivity of the sensor. Due to the unique advantages of SPR sensor in biological detection, using MoS2 with excellent characteristics as sensor material, the proposed sensor will show great potential in the field of biosensor. In addition, MoS2 is also an excellent gas sensitive material, and the designed molybdenum disulfide enhanced SPR sensor can also be used for gas detection.
Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.optlaseng.2019.105997. References [1] Kretschmann E, Raether H. Radiative decay of non-radiative surface plasmons excited by light [J]. Z Naturforschung A 1968;23(12):2135–6. [2] Pang H, Likamwa PL, Cho HJ. Development of planar waveguide based integrated optic SPR (surface plasmon resonance) sensor array[c]. IEEE Conference on Lasers & Electro-optics; 2007. [3] Caucheteur C, Shevchenko Y, Shao LY, et al. High resolution interrogation of tilted fiber grating SPR sensors from polarization properties measurement[J]. Opt Express 2011;19(2):1656. [4] Lewis E, Yang M, Huang Q, et al. Novel optical fiber SPR temperature sensor based on MMF-PCF-MMF structure and gold-PDMS film[J]. Opt Express 2018;26(2):1910–17. [5] Huizhen Y, Wei J, Shuwen C, et al. Fiber-optic surface plasmon resonance glucose sensor enhanced with phenylboronic acid modified au nanoparticles[j]. Biosens Bioelectron 2018;117 937-643. [6] Mishra AK, Mishra SK, Gupta BD. SPR based fiber optic sensor for refractive index sensing with enhanced detection accuracy and figure of merit in visible region[J]. Opt Commun 2015;344:86–91. [7] Wang Q, Zhao W-M. A comprehensive review of lossy mode resonance based fiber optic sensors[j]. Opt Lasers Eng 2018;100:47–60. [8] Jorgenson RC, Yee SS. A fiber-optic chemical sensor based on surface plasmon resonance[J]. Sens Actuators B 1993;12(3):213–20. [9] Wang Q, Jing J-Y, Wang B-T. Highly sensitive SPR biosensor based on graphene oxide and staphylococcal protein a co-modified TFBG for human IgG detection[j]. IEEE Trans Instrum Meas 2019;68(9):3350–7.
Q. Wang, X. Jiang and L.-Y. Niu et al. [10] Liedberg B, Nylander C, Lunstro MI. Surface plasmon resonance for gas detection and biosensing[J]. Sens Actuators 1983;4:299–304. [11] Jang HS, Park KN, Kang CD, et al. Optical fiber SPR biosensor with sandwich assay for the detection of prostate specific antigen[J]. Opt Commun 2009;282(14):2827–30. [12] Yuan Y, Yang X, Gong D, et al. Investigation for terminal reflection optical fiber SPR glucose sensor and glucose sensitive membrane with immobilized GODs[J]. Opt Express 2017;25(4):3884–98. [13] Wang Q, Zhao W-M. Optical methods of antibiotic residues detections: a comprehensive review[J]. Sens Actuators B 2018;269:238–56. [14] Tabassum R, Gupta BD. SPR based fiber-optic sensor with enhanced electric field intensity and figure of merit using different single and bimetallic configurations[J]. Opt Commun 2016;367:23–34. [15] Q. Wang, J.-Y. Jing, X.-Z. Wang, L.-Y. Niu, W.-M. Zhao. A D-shaped fiber long-range surface plasmon resonance sensor with high quality factor and temperature selfcompensation[j]. IEEE Trans Instrum Meas, doi:10.1109/TIM.2019.2920187. [16] Jing J-Y, Wang Q, Zhao W-M, Wang B-T. Long-range surface plasmon resonance and its sensing applications: a review[J]. Opt Lasers Eng 2019;112:103–18. [17] Srivastava SK, Gupta BD. A multitapered fiber-optic SPR sensor with enhanced sensitivity[j]. IEEE Photon Technol Lett 2011;23(13):923–5. [18] Singh S, Mishra SK, Gupta BD. Sensitivity enhancement of surface plasmon resonance based fiber optic refractive index sensor using an additional layer of zinc oxide[J]. Proceeding SPIE 8794, fifth european workshop on optical fibre sensors; 2013. 87941F20 May. [19] Wei W, Nong J, Zhu Y, et al. Graphene/Au-enhanced plastic clad silica fiber optic surface plasmon resonance sensor[j]. Plasmonics 2018;13(2):483–91. [20] Mishra AK, Mishra SK, Verma RK. Graphene and beyond graphene MoS2 : a new window in surface-plasmon-resonance-based fiber optic sensing[j]. J Phys Chem C 2016 acs.jpcc.5b08955.
Optics and Lasers in Engineering 128 (2020) 105997 [21] Kaushik S, Tiwari UK, Pal SS, et al. Rapid detection of Escherichia coli using fiber optic surface plasmon resonance immunosensor based on biofunctionalized molybdenum disulfide (MoS2 ) nanosheets[J]. Biosens Bioelectron 2019;126:501– 509. [22] Wang H, Zhang H, Dong JL, et al. Sensitivity-enhanced surface plasmon resonance sensor utilizing a tungsten disulfide (WS2 ) nanosheets overlayer[J]. Photonics Res 2018;6(06):7–13. [23] Sharma A. On the performance of surface plasmon resonance based fiber optic sensor with different bimetallic nanoparticle alloy combinations[J]. J Phys D Appl Phys 2008;41(5):55106–7. [24] Verma RK, Gupta BD. Surface plasmon resonance based fiber optic sensor for the IR region using a conducting metal oxide film[J]. J Opt Soc Am A 2010;27(4):846– 851. [25] Castellanos-Gomez A, et al. Optical identification of atomically thin dichalcogenide crystals[J]. Appl Phys Lett 2010;96(21):213116. [26] Paliwal N, John J. Lossy mode resonance (LMR) based fiber optic sensors: a review[j]. IEEE Sens J 2015;15(10):5361–71. [27] Mitsushio M, Miyashita K, Higo M. Sensor properties and surface characterization of the metal-deposited SPR optical fiber sensors with Au, Ag, Cu, and Al[j]. Sens Actuators A 2006;125(2):296–303. [28] Zhao Y, Tong R-j, Xia Feng, Peng Yun. Current status of optical fiber biosensor based on surface plasmon resonance. Biosens Bioelectron 2019;142:111505. [29] Jiang X, Wang Q. Refractive index sensitivity enhancement of optical fiber SPR sensor utilizing layer of MWCNT/PtNPs composite[J]. Opt Fiber Technol 2019;51:118–24. [30] Presolski S, Pumera M. Covalent functionalization of MoS2 [j]. Mater Today 2016;19(3):140–5. [31] Shalabney A, Abdulhalim I. Sensitivity-enhancement methods for surface plasmon sensors[J]. Laser Photon Rev 2011;5(4):571–606.
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Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Enhanced sensitivity of bimetallic optical fiber SPR sensor based on MoS2 nanosheets Qi Wang a,b,∗, Xi Jiang a,1, Li-Ye Niu a,1, Xiao-Chen Fan a a b
College of Information Science and Engineering, Northeastern University, Shenyang 110819, China State Key Laboratory of Synthetical Automation for Process Industries (Northeastern University), Shenyang 110819, China
a r t i c l e
i n f o
Keywords: Optical fiber sensor Surface plasmon resonance Molybdenum disulfide nanosheets Sensitivity enhancement
a b s t r a c t In this paper, we have fabricated and characterized a bimetallic optical fiber surface plasmon resonance (SPR) sensor, followed by molybdenum disulfide (MoS2 ) nanosheets have been coated on the surface of metal film. The performance of the sensor is discussed in terms of the sensitivity and the figure of merit (FOM) theoretically and experimentally. The effect of different thickness of Au/Ag film on the performance of the sensor was studied. The results indicate that the FOM of sensor with silver film is much higher than that with gold film, although gold film is conducive to the sensitivity improvement. Two types of sensors are fabricated, which are the sensor with silver layer and the sensor with bimetallic layers of silver and gold. The sensitivity of the sensor with silver film and the sensor with bimetallic layers are 2141 nm/RIU and 2487 nm/RIU, respectively; and the FOM values are 19.44 RIU−1 and 18.47 RIU−1 , respectively. Then, it is theoretically confirmed that MoS2 has the potential to promote the sensitivity of SPR sensor, and the sensitivity is tuned by the number of layers of MoS2 nanosheets. MoS2 nanosheets successfully bridged the gold film with the help of chemical bonds. The sensitivity of the sensor with MoS2 nanosheets is 3061 nm/RIU, and its FOM value is 23.29 RIU−1 .
1. Introduction In recent decades, the SPR sensor has made great progress in structure research and application. At present, according to the SPR sensor structure classification, there are mainly prism SPR sensors [1], integrated waveguide SPR sensors [2], grating SPR sensors [3] and optical fiber SPR sensors [4–6]. SPR is the phenomenon that the electromagnetic wave irradiates the metal surface and interacts with the free electrons on the metal surface to cause the collective oscillation of electrons in the metal [7]. Fig. 1 shows a general SPR device in which the incident p-polarized light (TM mode) excites the longitudinal collective oscillation of free electrons on the metal surface and generates the surface plasma wave (SPW) on the gold film surface. When the wave vector of the incident P-polarized light matches the wave vector of the SPW: √ 𝜀𝑚 · 𝜀𝑑 𝜔 𝑘𝑠𝑝 = (1) 𝑐 𝜀𝑚 + 𝜀𝑑 𝑘𝑥 =
𝜔√ 𝜔 𝜀0 sin 𝜃0 = 𝑐 𝑐
√
𝜀𝑚 · 𝜀𝑑 𝜀𝑚 + 𝜀𝑑
(2)
Where, kx is the wave vector of incident light along the X-axis, and ksp is the wave vector of the SPW. When the two values are equal, sur∗
1
face plasmon resonance is excited. When the external refractive index perturbation occurs, the value of ksp changes, and surface plasmon resonance occurs at another resonance wavelength. Therefore, the change of external refractive index can be sensitively sense via the shift of resonance wavelength or resonance angel. In 1992, R.C. Jorgenson first made SPR sensor with optical fiber [8], which pushed the optical fiber sensing technology to a new height. Optical fiber sensor has been widely used for its small size, nondestructive testing, anti electromagnetic interference, real-time detection and other advantages [9]. In 1983, Lidberg et al. first introduced the optical fiber SPR technology into the field of biosensors and pioneered the SPR biosensor [10]. After more than ten years of development, SPR sensors have been wildly used in environmental protection, food safety, biomedical and other fields [11–13]. In the field of biomolecule detection, the concentration of biomolecule is usually small, and the sample also contains a variety of other molecules, which requires the sensor to have high sensitivity and specificity. With the improvement of SPR sensor sensitivity, SPR technology will play a greater role in biomedical and disease diagnosis. Therefore, improving the sensitivity of the sensor has been the focus of SPR sensor technology research. At present, the research with regard to enhanced sensitivity of SPR sensor is mainly divided into two aspects. On the one hand, the
Corresponding author. E-mail address: [email protected] (Q. Wang). These authors contributed equally for this work.
https://doi.org/10.1016/j.optlaseng.2019.105997 Received 2 July 2019; Received in revised form 13 November 2019; Accepted 23 December 2019 0143-8166/© 2019 Elsevier Ltd. All rights reserved.
Q. Wang, X. Jiang and L.-Y. Niu et al.
Fig. 1. The model of SPR device.
sensitivity is improved by optimizing the structure of the sensor; on the other hand, different materials are coated into the surface of the metal film to improve the sensitivity. In terms of structural optimization, most of the research mainly includes the improvement of the structure of optical fiber SPR sensors, the types of metal films, and the optimization of the thickness of metal films. Various novel SPR sensors have been designed, such as bimetallic SPR sensors [14], D-type optical fiber SPR sensors [15], long-range SPR sensors [16], and optical fiber taper sensors [17]. Recent studies have shown that many materials, such as oxides, graphene, transition sulfides, and black phosphorus, can enhance the sensitivity of the sensors due to their enhanced SPR signal characteristics. Satyendra K M et al. proposed an optical fiber SPR sensor based on Cu/ZnO [18]. It is demonstrated that the sensitivity of the sensor is higher than that of the sensor uncoated ZnO. Wei et al. introduce graphene into the optical fiber SPR sensor [19], and a single layer of graphene sheet is coated on the surface of the SPR sensor. It is proved that the sensitivity of the graphene-based optical fiber SPR sensor is more than twice that of the conventional gold film optical fiber SPR sensor. Transition metal dichalcogenide (TMDC), similar to graphene, is also a 2D nanomaterial. MoS2 and WS2 , belong to TMDC, have been widely used in the sensor field. The semiconductor MoS2 is a promising material for electronic and optical applications. Due to its high charge mobility, MoS2 has been applied to field effect transistors (FET). Compared with graphene, MoS2 and WS2 have larger band gap and higher optical absorption efficiency, and they also have high specific surface area and good biocompatibility. These make it possible to introduce transition metal dichalcogenide into SPR sensors. Akhilesh Kumar Mishra and his group theoretically investigated the optical fiber SPR sensor based on metal film/graphene/MoS2 structure [20]. It is shown through theoretical analysis that the introduction to MoS2 can better enhance the sensitivity of the sensor, and the structure improves sensitivity to 6000 nm/RIU. Siddharth Kaushik et al. designed MoS2
Optics and Lasers in Engineering 128 (2020) 105997
nanosheets functionalized optical fiber SPR immunosensor, functionalized MoS2 nanosheets as a bridge to connect fiber and antibodies [21]. MoS2 not only enhances the sensitivity of SPR sensors, but also can be used to immobilize biorecognition molecules (antibodies, DNA, enzymes), which further broaden the application of MoS2 in biosensor. Hao Wang et al. coated multilayer tungsten dioxide (WS2 ) nanosheets on the surface of prism SPR sensor [22]. It was experimental confirmed that coating WS2 nanosheets on the outside of the gold film can significantly improve the sensitivity of the sensor, and the maximum sensitivity is 2459.3 nm/RIU. The sensitivity of the sensor is also tuned by the thickness of the WS2 nanosheets on the surface of sensor. The refractive index of molybdenum disulfide is a complex number, and the real part is significantly larger than the imaginary part, so the enhanced sensitivity characteristics of molybdenum disulfide are superior to other materials. Molybdenum disulfide has been proved to be sensitive to some gas specificity, so molybdenum disulfide will have advantages in gas sensing. Some materials can only enhance the sensitivity in a specific refractive index range. Black phosphorus has been proved to have strong enhancement sensitivity to the external medium with refractive index between 1.00–1.20. Although many studies have proved that transition metal sulfide can improve the sensitivity of the sensor, most of the conclusions are based on the theoretical model of numerical analysis, lack of experimental research. In this work, we prove theoretically and experimentally that the MoS2 can enhance the sensitivity of bimetal layer fiber SPR sensor. Two kinds of metal thickness of bimetal layer SPR sensor are optimized theoretically, and the optimum thickness ratio is determined. The experimental results are in accordance with the theoretical simulation, which shows that the designed sensor is reasonable. 2. Theoretical model analysis 2.1. Establishment of numerical model The MoS2 coated silver/gold bimetal optical fiber SPR sensor model is presented in Fig. 2. In order to fabricate the sensing probe, plastic clad silica optical fiber with core diameter of 600 um and numerical aperture of 0.37 is adopted. First, 10 mm length cladding at one end of the optical fiber is removed. A layer of silver film is assembled on the bare core surface. Then a layer of gold film is added to the silver film. Finally, a layer of MoS2 is coated on the surface of the silver film. Light is incident from the fiber at one end and propagates in the fiber. When the light is entirely reflected in the sensing area, the evanescent wave immersed in the surface of the metal film can excite the surface plasmon wave. Because the energy of incident light is absorbed, there is a resonance absorption peak in the reflection spectrum. When the external refractive index of the sensing region changes from n0 to 𝑛0 + Δ𝑛, the position of resonance wavelength shifts. It is well known that gold and silver are the most suitable plasmonic materials for exciting SPR due to their unique dispersion properties, and the dielectric constants of gold and silver are modeled according to the Drude dispersion model [23], as shown in Eq. (3). ε(λ) = 𝜀𝑟 + 𝑖∗𝜀𝑖 = 1 −
( 2
𝜆2 𝜆𝑐
) 𝜆𝑝 𝜆𝑐 + 𝑖∗𝜆
(3)
Fig. 2. The MoS2 coated gold/silver bimetal optical fiber SPR sensor model.
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Fig. 3. The SPR curves of the bimetal probes with different thickness ratios (the external refractive index is 1.330).
Table 1 Dispersion coefficients of metals. Dispersion coefficients (𝜇m)
𝜆p 𝜆c
Metal Ag
Au
0.14541 17.614
0.16826 8.9342
Where, 𝜆c represents the collision wavelength, and 𝜆p represents the wavelength corresponding to the plasma frequency. The values of 𝜆c , 𝜆p for gold and silver are given in Table 1. The relationship between the refractive index of the core and the wavelength is obtained by the Sellmeier relation [24]: √ 𝑎3 𝜆2 𝑎1 𝜆2 𝑎2 𝜆2 𝑛1 (𝜆) = 1 + + + (4) 2 2 2 2 2 𝜆 − 𝑏1 𝜆 − 𝑏2 𝜆 − 𝑏3 2 Where a and b are the Sellmeier coefficients, and their values are given as 𝑎1 = 0.6961663, 𝑎2 = 0.4079426, 𝑎3 = 0.8974794
(5)
𝑏1 = 0.0684043, 𝑏2 = 0.1162414, 𝑏3 = 9.896161
(6)
The dispersion relation of MoS2 is obtained in Ref [25], and the polynomial equations of the dispersion relation of MoS2 is given in Ref [25]. There are many numerical models for studying the transmission characteristics of multilayered dielectric film, and the most popular one is the transfer matrix method [26]. A large number of studies have shown that the prediction of the transmission matrix agrees well with the experimental measurements. Therefore, the numerical model analysis of the SPR sensor with multilayer dielectric film uses the transfer matrix method. The schematic diagram of N-layer structure is shown in support information Fig. S1. Here, ɛk is the dielectric constant of the √ k-th film with dk thickness, and its refractive index nk = 𝜀𝑘 . 2.3. Analysis of numerical model Based on the transfer matrix method of multi-layer structure, the influence of different dielectric layers on the performance of the SPR
sensor can be investigated thoroughly, which provides a theoretical basis for the fabrication of excellent SPR sensor in the future. In this work, we propose a reflective optical fiber SPR sensor coated with MoS2 on the outer surface of silver/gold bimetallic layer. It is well known that surface plasmon resonance (SPR) phenomena excited by different kinds of metal films are different, which benefits from their different dispersion characteristics. Additionally, the change of the thickness of the metal film also affects the excitation of surface plasmon resonance (SPR). Previously, it was revealed that the most appropriate thickness of the metal film was about 50 nm [27]. Therefore, the total thickness of the silver/gold bimetallic layer of the SPR probe proposed in this paper is set at 50 nm. In order to study the influence of the thickness ratio of gold and silver on the sensing performance of bimetal layer sensor, molybdenum disulfide was not added to the surface of bimetal layer probe for theoretical analysis. The thickness of the silver film and the gold film are set at 50 nm/0 nm, 40 nm/10 nm, 30 nm/20 nm, 20 nm/30 nm, 10 nm/40 nm and 0 nm/50 nm, respectively. The reflectivity is calculated from the transmission matrix. When the external refractive index is 1.330, the SPR curves of the bimetal probes with different thickness ratios are shown in Fig. 3. With the decrease of the thickness of the silver film (the increase of the thickness of the gold film), the resonance wavelength shifts red and the resonance absorption peak widens. From Fig. 4(a), it can be seen that with the decrease of silver film thickness (the increase of gold film thickness), the sensitivity of the sensor increases, and FWHM also increases. The FOM is considered a reasonable parameter to evaluate the sensor performance, it is given as S/FWHM. As shown in Fig. 4(b), as the gold film increases (the thickness of the silver film decreases), the FOM becomes smaller. Therefore, the performance of the silver film is superior to that of the gold film based on the numerical model analysis. However, the silver film in the experimental environment has poor stability and is easily oxidized. The SPR sensor of the bimetal layer is proposed, and a layer of thin gold film is deposited on the surface of the silver film. The sensitivity of the sensor is also improved, in addition, the gold film isolates the contact between the silver film and the air, thereby improving the stability of the sensor. Therefore, the thicknesses of the silver film and the gold film were determined to be the most suitable thickness ratios of 40 nm and 10 nm, respectively, and
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Fig. 4. The variation of parameters of bimetallic probe with the thickness of metal film. (a) Variation of sensitivity and FWHM with the thickness of metal film. (b) Variation of the FOM with the thickness of metal film.
Fig. 5. The structure of the MoS2 nanosheets.
the sensor sensitivity and FOM were 3131.0 nm/RIU and 86.38 RIU−1 , respectively. MoS2 , as a two-dimensional nano material, is widely used in the manufacture of transistors, photodetectors and optical sensors due to its excellent properties. Fig. 5 shows the ultra-thin MoS2 layer, which
is stacked by S-Mo-S. The thickness of monolayer MoS2 nanosheets is 0.65 nm. Such a small thickness produces a new set of optical and electronic properties, such as larger band gaps in single-layer MoS2 than in bulk MoS2 . The weak van der Waals force between the layers of MoS2 determines that MoS2 can easily be peeled into nanosheets. In addition, MoS2 has good biocompatibility, so MoS2 nanosheets have great potential in the field of biosensing. On the other hand, as a high dielectric constant material, MoS2 is coated on the surface of the metal film, which can enhance the electric field strength of the surface of the metal film. As we know, the electric field strength of the surface of the metal film, the enhancement could directly enhance the sensitivity of the SPR sensor. The research work on the influence of MoS2 on the optical fiber SPR sensor with bimetal layer was carried out by means of the transmission matrix. The gold film surface of the bimetal SPR sensor was coated with a layer of MoS2 nanosheets having a thickness of L = 1 × 0.65 nm, and the thicknesses of the silver film and the gold film were set to 40 nm and 10 nm, respectively. Fig. 6 shows the SPR curve and the resonance wavelength versus refractive index for the probe coated with MoS2 nanosheets. Compared with the probe with bimetallic layer, the resonance wavelength of the optical fiber SPR sensor coated with MoS2 nanosheets shows a red shift Fig. 6. The SPR reflection spectrum of the bimetallic probe with singer layer of MoS2 , the inset of the figure shows the relationship between the resonance wavelength and the refractive index.
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Fig. 7. (a) The SPR curves of MoS2 with different layers, (b) Variation of sensitivity and FWHM with the layers of MoS2 nanosheets.
trend, and the SPR curve becomes wider. The sensitivity and FWHM of the sensor are 3485.5 nm/RIU and 48.05 nm, respectively. MoS2 with different layers is deposited on the outer surface of the gold film. The SPR curves of MoS2 with different layers are shown in Fig. 7(a). With the increase of layers, the SPR curves show a trend of redshift and become wider. When the number of layers of MoS2 nanosheets reaches three layers, the sensitivity of the sensor decreases sharply and the SPR curve begins to distort. Fig. 7(b) shows the change trend of sensor sensitivity and FWHM with the increase of layers of MoS2 nanosheets. The sensitivity and FWHM increase with the increase of layers of MoS2 nanosheets. The maximum sensitivity of the sensor is 4420.4 nm/RIU when the number of layers of MoS2 nanosheets reaches two layers. 3. Experimental section 3.1. Fabrication of bimetallic SPR sensor The fabrication of the bimetal fiber SPR sensor is divided into three steps: (a) pretreatment of the fiber optic probe, (b) assembly of the silver film, and (c) deposition of a layer of gold film. As shown in Fig. 1, the plastic clad silica optical fiber of 80 mm length (Core diameter = 600 um, numerical aperture = 0.37) is selected to make a reflective SPR sensor. A 10 mm cladding at one end of the probe was removed for use as a sensing area. Since the two end faces of the fiber are not flat, it is necessary to repeatedly polish them with sandpaper of different roughness until they become smooth. The bare core is shown in Fig. 8(a). Before assembling the silver film, it is necessary to repeatedly
clean the probe with ethanol and deionized water to remove impurities attached to the surface. The silver film is assembled on the surface of the core by a chemical method, and a layer of silver film is deposited on the surface of the exposed core by silver mirror reaction of silver ammonia solution and glucose solution [28]. The detailed procedure has been described in Ref [28] and the thickness of the silver film can be adjusted by the concentration of the two solutions, so that it is ensured that a layer of silver film having a thickness of about 40 nm can be obtained on the cylinder surface of the core. Fig. 8(b) shows the probe assembled with a layer of silver film. Because the silver film is easy to oxidize, a layer of thin gold film should be coated on the surface of the silver film quickly to prevent contact between silver film and air. The gold film is deposited by magnetron sputtering instrument (JS-1600), which can control the thickness of the gold film by controlling the discharge time and current [29]. The current of 8 mA and the discharge time of 1 min 30 s ensure that the thickness of the gold film deposited on the surface of the silver film is about 10 nm. The probe with a gold film deposited on the surface of the silver film is shown in Fig. 8(c). So far, the fabrication of bimetallic SPR reflective optical fiber sensor has been accomplished. The electron microscope image of the bimetal film of the sensor is shown in Fig. 8(d). 3.2. Preparation of MoS2 –COOH nanocomposite MoS2 powder of 60 mg was dissolved in 30 ml deionized water. MoS2 was evenly dispersed in deionized water by magnetic stirrer. The sodium hydroxide(NaOH) of 1.2 g and the chloroacetic acid(MCA) of 2 g were added to MoS2 dispersion. The mixed solution was placed in an ultra-
Fig. 8. Images of three different types of probes. (a) The fiber’s core without cladding. (b) The sensor assembled with a layer of silver film. (c) The sensor with a gold film deposited on the surface of the silver film. (d) The electron microscope image of the bimetal film of the sensor.
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sonicator for continuous sonication for 3 h. Under ultrasonic treatment, there are quite a few vacancies on the surface of MoS2 [30]. These vacancies are quickly occupied by chlorine atoms in chloroacetic acid, which leads to the modification of the carboxyl group to the surface of MoS2 . Compared with MoS2 , the dispersibility of carboxyl-modified MoS2 is greatly improved. Finally, the MoS2 –COOH dispersion was successfully prepared. 3.3. Fabrication of MoS2 –COOH modified SPR sensor It is difficult to deposit MoS2 –COOH sensitive material on the surface of gold film by dip coating method because of the poor adsorption of gold film. Therefore, a chemical modification method is chosen to replace the traditional dip coating method, which is to modify a specific functional group on the surface of gold film. 4-Aminothiophenol was selected as a bridge between gold film and MoS2 –COOH. The thiol of 4-Aminothiophenol interacted with gold film to form Au–S bond, thus installing amino-group on the surface of gold film. The detailed steps are shown in Fig. S2(a), (b) and (c) in the support information, the 4Aminothiophenol powder used ethanol diluted to 10 mmol, the surface of the gold film of the probe is washed several times with ethanol and deionized water, and then the sensor is placed in a drying oven to be dried. Finally, the gold film of the sensor is completely immersed in the 4-Aminothiophenol ethanol solution for 24 h at room temperature. The sensor for modifying the amino group is immersed in the MoS2 –COOH dispersion for 5 h, which ensures that the reaction of the amino group with the carboxyl group produces a strong chemical bond to firmly bond the MoS2 to the surface of the gold film, followed by thorough rinsing with deionized water to remove unbonded MoS2 –COOH. The electron micrograph of the gold film surface of the sensor and the electron micrograph of the molybdenum disulfide nano film modified on the gold film surface of the sensor are shown in Fig. S3 (a) and (b) in the supporting information, respectively. The physical precipitation method is used to coat the second layer and the third layer of MoS2 nano sheet on the surface of bimetal layer optical fiber probe. The physical precipitation of MoS2 film is to completely cover the sensor area of the probe with the dispersion, put it into the vacuum drying oven, under the effect of temperature, the dispersant in the dispersion is evaporated, thus the second layer of MoS2 nano film is deposited on the surface of the gold film. Repeat the above steps to deposit the third layer of film. The fabrication of the sensing area is completed, and the refractive index sensing experiment of the sensor is carried out. 3.4. Experimental setup The schematic diagram of the refractive index measurement system is shown in Fig. 9. A tungsten halogen lamp source (HL-2000-LL) with a wavelength range of 360–2400 nm was selected as the light source of the measurement system, and the emitted light was transmitted to the SPR probe through the Y-coupler. The reflected light is transmitted along the other end of the Y-coupler to the spectrometer (Maya 2000pro), and the computer connected to the spectrometer records the reflectance spectrum in real time. When the fiber probe is immersed in a sample solution of different refractive index, the condition of the surface plasmon resonance is changed due to the change of the external refractive index of the probe surface, which directly causes the position of the resonance wavelength to drift. The corresponding reflection spectrum is displayed by means of a spectrum analysis software, and a series of SPR curves corresponding to different refractive indexes are obtained. 4. Results and discussion Due to the instability of the silver film, the performance of the silver film probe is tested immediately after the silver film is assembled on the surface of the fiber core. The refractive index range of a series of sample
Fig. 9. The schematic of the refractive index measurement system.
Fig. 10. The SPR reflection spectrum of the sensor probe with Ag film, the inset of the figure shows the relationship between the resonance wavelength and the refractive index.
refractive index solutions used in the experiment is 1.3318 to 1.3701. The refractive index range of the solution obtained by diluting ethanol with deionized water is 1.3301–1.3614. 1.3701 the solution with refractive index is glycerin aqueous solution. Fig. 10 shows the SPR curve of the probe assembled with a layer of silver film, and the illustration shows the linear fitting curve of resonance wavelength and refractive index. As we have seen, the position of the SPR curve shifts toward the long wavelength position as the refractive index increases. The wavelength corresponding to the trough of each SPR curve is recorded, which is called the resonant wavelength. Sensitivity of the sensor can be calculated by 𝛿𝜆res /𝛿ns , where 𝛿ns and 𝛿𝜆res represent the increment of the refractive index and the offset of the corresponding resonant wavelength respectively. As can be seen from the inset of Fig. 10, the sensitivity of SPR probe with the silver film is 2140.42 nm/RIU. It can be seen from Fig. 11 that the sensitivity of the SPR probe of the bimetal layer is significantly better than that of the silver film SPR probe, and the sensitivity is 2486.91 nm/RIU. Moreover, it is worth noting that the width of the SPR curve of the bimetal probe is also larger than that of the silver film probe. These experimental phenomena are consistent with the results of the numerical model in the previous paper. The refractive index measurement experiments of SPR probe based on MoS2 nanosheets were carried out using the same series of refractive index solutions. The experimental results of the refractive index of the sensing area coated with one layer and two layers of MoS2 are shown in Fig. 12. The sensitivity of the probe coated with one layer and two
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Optics and Lasers in Engineering 128 (2020) 105997
the electric field overlap integral of the electric field. 𝛿𝜆 ≈
Fig. 11. The SPR reflection spectrum of the sensor probe with gold and silver bimetal layer, the inset of the figure shows the relationship between the resonance wavelength and the refractive index.
layers of MoS2 was increased to 2544.86 nm/RIU and 2671.87 nm/RIU respectively, but the FWHM of the SPR curve were larger than that of the bimetal probe. The sensitivity of the probe coated with MoS2 is attributed to the enhancement of the electric field on the surface of the gold film. It is well known that the sensitivity of the SPR sensor is directly related to the electric field on the surface of the gold film. In most cases, the increase in sensitivity is related to the enhancement of the electromagnetic field overlap integral around the analyte [31]. According to ref [31], the shift of the resonant wavelength is proportional to
⃗∗ ⃗ 𝑘𝑖 ∫𝑉int 𝐸𝑖 𝛿𝜀𝐸𝑓 𝑑𝑟 2 ∫ 𝐸⃗ ∗ 𝛿𝜀𝐸⃗ 𝑑𝑟 𝑖 𝑉 𝑖
(7)
where Ef is the electric field after the external refractive index changes, and 𝛿𝜀 represents the shift of the resonance wavelength due to the change of the dielectric constant of the analyte from 𝜀 to 𝜀 + 𝛿𝜀. In the above experiments, the sensors coated with one layer and two layers of MoS2 were analyzed. The following experiments explores the sensor coated with three layers of MoS2 , and the results are shown in Fig. 13. The repeatability experiment of the sensor coated with three layers of MoS2 was carried out. The two fitting curves in Fig. 14 are obtained by performing two measurements on the probe under the same experimental conditions. As shown in the figure, for the sample solutions of the same refractive index, the positions of the resonant wavelengths measured twice are almost the same, and the sensitivity of the sensors is 3061.84 nm/RIU and 3072.54 nm/RIU, respectively, which confirms the superior repeatability of the probe. Although the width of the SPR curve of the probe coated with the MoS2 nanosheets is larger than that of the bimetal probe, the sensitivity of the probe increases much more than the FWHM, thus causing the value of the FOM of the probe to become larger. As shown in Table 2, the sensitivity and the FOM of the three probes were compared. In the above experiments, we studied the application of one, two and three layers of MoS2 on the sensing area. It can be seen from the experiment that the refractive index sensitivity of the sensor increases with the increase of the number of layers of the sensor coated with MoS2 . However, the increase of the number of layers results in the broadening and deformation of the spectrum of the sensor. The broadening and de-
Fig. 12. The SPR reflection spectrum of the sensor probe coated MoS2 nanosheets. (a) and (b) are the experimental data of one layer of MoS2 ; (c) and (d) are the experimental data of two layers of MoS2 .
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Optics and Lasers in Engineering 128 (2020) 105997
Fig. 13. (a) The SPR reflection spectrum of the sensor probe coated three layers of MoS2 nanosheets, (b) The relationship between the resonance wavelength and the refractive index. Tabel 2 Performance comparison of SPR sensors with various sensitive layer. Type of sensor
Sensitivity (nm/RIU)
FOM(RIU−1 )
SPR sensor with Au film SPR sensor with bimetallic film Bimetallic SPR sensor with three layers MoS2
2141 2487 3061
19.44 18.47 23.29
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Key R&D Program of China under Grant SQ2019YFC170311, the Fundamental Research Funds for the Central Universities under Grant N180402023 and N172002001, and the National Natural Science Foundation of China under Grant 51607028. Supplementary materials Fig. 14. The fitted curve of the resonant wavelength and the corresponding refractive index, the first measurement (red), the second measurement (black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
formation of the resonance spectrum are enhanced by the coated MoS2 nanosheets, and the more layers are coated, the stronger the scattering is, the more serious the spectral deformation is. 5. Conclusions In summary, the paper proposed a bimetallic optical fiber SPR sensor based on MoS2 nanosheets, the sensitivity of the sensor has been significantly improved. The gold silver bimetal layer SPR sensor not only improves the sensitivity of the sensor, but also improves the stability of the sensor. The introduction of MoS2 further strengthens the sensitivity of the sensor. Due to the unique advantages of SPR sensor in biological detection, using MoS2 with excellent characteristics as sensor material, the proposed sensor will show great potential in the field of biosensor. In addition, MoS2 is also an excellent gas sensitive material, and the designed molybdenum disulfide enhanced SPR sensor can also be used for gas detection.
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