Sensitivity-Enhanced Fiber Plasmonic Sensor Utilizing Molybdenum

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Sensitivity-Enhanced Fiber Plasmonic Sensor Utilizing Molybdenum Disulfide Nanosheets yaxin zhang, Yaofei Chen, Fan Yang, Shiqi Hu, Yunhan Luo, Jiangli Dong, Wenguo Zhu, Huihui Lu, Heyuan Guan, Yongchun Zhong, Jianhui Yu, Jun Zhang, and Zhen Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00107 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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The Journal of Physical Chemistry

Sensitivity-Enhanced

Fiber

Plasmonic

Sensor

Utilizing Molybdenum Disulfide Nanosheets Yaxin Zhang,1,3,5 Yaofei Chen,1,3,5 Fan Yang,3 Shiqi Hu,1,3 Yunhan Luo,1,2,3* Jiangli Dong,1,2,3 Wenguo Zhu,1,3 Huihui Lu,2,3 Heyuan Guan,3 Yongchun Zhong,3 Jianhui Yu,1,2,3 Jun Zhang,1,3,4 and Zhe Chen3,4

1Guangdong

Provincial Key Laboratory of Optical Fiber Sensing and Communications,

Jinan University, Guangzhou 510632, China

2Key

Laboratory of Optoelectronic Information and Sensing Technologies of Guangdong

Higher Education Institutes, Jinan University, Guangzhou 510632, China

3Department

of Materials Science and Engineering, Jinan University, Guangzhou

510632, China

4Key

Laboratory of Visible Light Communications of Guangzhou, Jinan University,

Guangzhou 510632, China

ACS Paragon Plus Environment

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5These

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authors contribute equally to this work

*Corresponding author: [email protected]

ABSTRACT: Molybdenum disulfide (MoS2), as a representative layered transition metal dichalcogenide (TMDC) material, possesses great potential applications in highlysensitive detection. In this paper, a sensitivity-enhanced surface plasmon resonance (SPR) fiber sensor modified with an overlayer of MoS2 nanosheets is proposed and demonstrated. The sensitivity, which is related to the thickness of the MoS2 overlayer, can be tailored by the number of the deposition cycles. Benefiting from the large surface area, high refractive index, and unique optoelectronic properties, coating MoS2 nanosheet overlayer on the gold film can efficiently improve the sensitivity. The highest sensitivity of 2153.9 nm/RIU was experimentally achieved by depositing the MoS2 nanosheets for two cycles, and it shows a sensitivity enhancement of 37.3%, compared to the case without MoS2 overlayer. Moreover, it is found by experiments that the further deposition of MoS2 causes the decrease of sensitivity, indicating the existence of an optimized thickness for


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MoS2 overlayer. The experimental results are interpreted and verified by numerical simulations companied with theoretical analysis.

1. INTRODUCTION

Optical fiber sensors have attracted considerable attention because of the advantages of high sensitivity, compact size, etc.1-2 Combined with the surface plasmon resonance (SPR) technology, which occurs at a particular wavelength for a given incident angle when the phase matching between the incident light and the surface plasmon wave (SPW) is satisfied,3 the sensitivity of fiber sensor can be further significantly enhanced, endowing itself with the capability in sensing chemical and biological substances.4-5 Moreover, the fiber-based SPR sensors own many advantages over the conventional prism-coupled SPR sensors, such as ease of alignment, online measurement, miniature size, and remote sensing.6-7 In recent years, the side-polished fiber (SPF) or D-shaped fiber, have attracted great interest in configuring an SPR sensor, 8-9 because their flat surface in the polished region greatly facilitates the deposition of metal film with high quality, and provides an ideal platform for functionalization to sensing biochemical substances.10-12


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Meanwhile, lots of concern has been paid on the two-dimensional transition metal dichalcogenides (TMDCs),12 because they have shown great potential applications in catalysts, nano-lubricants, microelectronic devices, medical, optoelectronic devices, etc. Among the family of TMDCs, MoS2 becomes the most popular one and attracted the most interests,13-14 which is based on the fact that the MoS2 material is easily available in the form of a mineral, molybdenite, but other TMDCs semiconductors are expected to have qualitatively similar properties.15 When MoS2 is thinned to a two-dimensional layer structure, its excellent properties in optoelectronic semiconductors emerge, and provide a way to improve the performance of sensors.16-18 Maharana et. al theoretically studied the effect of graphene on the electric field enhancement of SPR, revealing the mechanism in the performance enhancement by 2D materials.19 Wang et. al explored the effect of WS2 on the sensitivity of prism-coupled SPR sensor.20 Mishra et. al systematically studied the graphene/MoS2 hybrid structure on the performance enhancement of fiber-based SPR sensor.21 In a word, it has been widely demonstrated that two-dimensional materials show great potentials in improving the sensitivity of SPR sensors.22-27 However, most of the reported works were based on the prism-coupled SPR configuration, and the sensors


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were modified with the mono-layer or few layers of two-dimensional materials, which needs complex process in the fabrication and modification.

In this work, MoS2 nanosheets are introduced into the SPR fiber sensor by the physical deposition method to improve the sensing performance. The influence of the MoS2 overlayers thickness on the refractive index (RI) sensitivity is studied and discussed in detail. The thicknesses of MoS2 overlayer can be tailored by repeating different cycles of the drop-and-dry process. Experimental results show that with the increase of the MoS2 overlayer thickness, the SPR sensitivity increases first and then decreases, which is also discussed and explained theoratically and numerically.

2. EXPERIMENTS


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Figure 1. (a) Measured profile of the polished fiber. (b) Cross section view of the polished fiber obtained by an SEM. (c) (d) Schematic diagrams of the proposed sensor and its cross section.

The proposed SPR sensor was constructed on an SPF, which was fabricated by sidepolishing a multi-mode fiber (MMF, MM-S105/125-22A, NUFERN) using a homemade wheel polishing system. The MMF, with the core and cladding diameters of 105 and 125 μm, was coarsely polished for 5 minutes and then finely polished for about 120 minutes to achieve a relatively-smooth surface. The profile of the polished section was characterized by an optical microscope (Zeiss Axio Scope A1). The lengths of the polished and flat regions are ~ 13 and ~ 6 mm, respectively, as shown in Figure 1(a). The


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cross section of the SPF was characterized by a scanning electron microscopy (SEM, ULTRA 55 FESEM-ZEISS) and shown in Figure 1(b), by which the residual thickness can be estimated as ~ 70 μm. In the subsequent metal deposition process, a thin layer (~ 5 nm) of chromium and a gold film (~ 50 nm) were successively deposited onto the polished side by a vacuum coating machine (E6080, Sichuan Sisheng Vacuum Equipment Co. Ltd., China), wherein the chromium layer was applied to enhance the adhesion force between the fiber and the gold film. Choosing gold as the plasmonic material is based on the fact that gold is more chemically-inert to solution and solutes typically used in biochemical contexts, compared to the other metal such as silver and aluminum. For typical SPR sensors, the optimal thickness of gold film is ~50 nm, because the resonance dip will become shallower or broader, if the thickness is above or below the optimal one.28

Figures 1(c) and 1(d) show the schematic diagrams of the proposed sensor and its cross section, respectively. The deposition of MoS2 overlayer on the gold film was realized by dropping and drying the MoS2 alcohol suspension (Nanjing XFNANO Materials Tech Co., Ltd) on the surface of gold film. The concentration of the suspension is 1mg/ml, and


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the suspended MoS2 nanosheets have the diameter ranging from 0.05-1μm and the number of layers from 1 to 10. Before deposition, the MoS2 suspension was decanted into centrifuge tube and treated by ultrasonication for at least 30 minutes to avoid agglomeration. Then, a certain volume of suspension (~ 300 μl) was directly dropped onto the gold film surface, and the sensor was kept at room temperature for at least 12 hours until the alcohol totally evaporates. In order to study influence of the thickness of the MoS2 overlayer on the sensing performance, the same deposition process described above was repeated for more cycles to tailor the thickness of the overlayer.

1200 1000

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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404.3

λexc=514.5nm

800 600

373.3

400 200 0 340

360

380

400

-1

420

Raman shift (cm ) Figure 2. Raman spectrum of the MoS2 layers on the SPR sensor.


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Raman spectrum of the deposited layer was measured by using a Raman spectrometer (RENISHAW, UK).20 As shown in Figure 2, the spectrum features with two peaks, namely E2g1 and A1g, which are located at 373.3 and 404.3 cm-1, respectively.The in-plane E2g1 mode results from the opposite vibration of two S atoms with respect to the Mo atom while the A1g mode is associated with the out-of-plane vibration only from S atoms in opposite directions.29 The Raman spectrum indicates that MoS2 nanosheets are successfully deposited onto the surface of the SPR sensor.


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Figure 3. SEM images of (a) the MoS2 on the surface of fiber, and (b)-(d) the side views of the sensors with deposition cycles of 1-3, respectively. Note that the scale bar in (d) is diffrent with that in (b) and (c).

Figure 4. AFM figures for the sensors’ surfaces deposited with MoS2 nanosheets for 0~3 cycles, respectively.

An SEM was used to obtain the morphology of the MoS2 nanosheets deposited on the gold surface, and the images are presented in Figure 3. From the top view of the sensor show in Figure 3(a), the MoS2 nanosheets evenly distribute on the surface, proving the successful deposition of the MoS2 nanosheets again. From the side views of the sensors shown in Figures 3(b)-3(d), it can be found that the deposited nanosheets stack together to form a layer of film on the substrate, and the deposited layers show a porous morphology. Moreover, the thickness increases with the number of deposition cycles: the 1, 2, and 3 deposition cycles result in the thickness of 171.2, 257.9, and 339.4 nm,


10

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respectively. We also measured the AFM figures for the sensors deposited with MoS2 nanosheets for different cycles. As we can see from the measured results, shown in Figure 4, both of the thickness and occupation ratio of MoS2 overlayer increase with deposition cycles, which is consistent with the SEM results shown by Figure 3. Besides, we can also see that the surface of sensor becomes rougher after the deposition of MoS2 nanosheets.

Figure 5. Schematic diagram of experimental setup.

The experimental setup, shown in Figure 5, for the characterization is composed of a tungsten halogen light source (AvaLight-HAL-Mini), the MoS2-SPR sensor, a fiber optic spectrometer (AvaSpec-ULS2048XL), and computer. The two ends of the sensor are


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connected to the light source and spectrometer, respectively. The recorded data from the spectrometer are transferred to the computer for the further process. When a light emitted from the light source couples into the fiber and propagates to the polished region, the Ppolarized component at a certain wavelength (Transverse Magnetic wave) will be significantly attenuated due to the resonance with the surface plasmon wave. As a result, the transmittance spectrum will feature an absorption band at the resonance wavelength, which is extremely sensitive to the surrounding RI (SRI). In turn, the SRI can be demodulated by monitoring the resonance wavelength.

To characterize the performance of the sensor to RI sensing, five solutions with different concentrations were prepared and their refractive indices were characterized by an Abbe refractometer (Edmund NT52-975, Edmund Optics Co. Ltd.). The RIs of the five solutions are measured as 1.332, 1.338, 1.345, 1.351, and 1359, respectively. For measurements, a ~300 μl volume of solution was first dropped on and covers the sensing area, and then the corresponding transmission spectrum was recorded. After each record of the data, the solution covered on the sensor was removed away for the test of the next solution.


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3. RESULTS

Figure 6. (a)Transmission spectra of the SPR fiber sensor without modification. (b) Resonance wavelength as a function of RI and the linear fitting result.

First, we tested the control sample, namely a conventional SPR fiber sensor without modification, in terms of the RI sensing characteristics. According to the experimental procedure, the transmission spectra of the sensor covered by the solutions with different RIs were measured successively, and the corresponding results are shown in Figure 6(a). We can see that as the RI increases, the resonance wavelength shifts to the longer wavelength, and the total shift is 42.399 nm when the RI is changed from 1.332 to 1.359


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RIU. The resonance wavelength depending on RI is presented in Figure 6(b). Linear fitting slope shows the control sensor has a sensitivity of 1568.5 nm/RIU.

Figure 7. Transmission spectra and the resonance wavelength depending on the surrounding RI for the MoS2-SPR fiber sensors modified by depositing MoS2 nanosheets for (a)(b) 1, (c)(d) 2, and (e)(f) 3 cycles.


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Following the same procedures, the RI sensing characteristics of the sensors modified with MoS2 nanosheets by different deposition cycles were measured, and the corresponding results are shown in Figure 7. Results show that the transmission spectrum keeps shifting to the longer wavelength with the increase of SRI for all the modified sensors, but the shift amounts of the sensors are different. To be specific, the resonance wavelength shift amounts are 49.186, 58.512, and 39.453 nm for the sensors modified by 1 - 3 deposition cycles, respectively. The linear fittings were implemented to obtain the sensitivities, and the sensitivities for the sensors deposited with MoS2 nanosheets for 1, 2, and 3 cycles are 1781.5, 2153.9, and 1396.48 nm/RIU, respectively. Notably, the deposition of MoS2 would cause the transmission to become shallower, which can be attributed to the relatively-high RI in both the real and imaginary part of the MoS2 material.30


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2500

Sensitivity(nm/RIU)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2153.9

2000 1500

1781.5 1568.5

1396.48

1000 500 0 -1

0

1

2

3

4

Number of the deposition cycles Figure 8. Relationship between sensitivity and the number of the deposition cycles of MoS2 nanosheets.

To facilitate the comparison, the sensitivity as a function of the number of the deposition cycles is also plotted in Figure 8. It can be found that the sensitivity increases first and then decreases with the increase of deposition cycles. For the 1 and 2 cycles, the sensitivity shows a remarkable improvement, compared to that without modification. When the sensor is deposited for 2 cycles, the sensitivity reaches the maximum of 2153.9 nm/RIU. However, further deposition will lead a decrease of the sensitivity. The sensitivity abruptly decreases to 1396.48 nm/RIU after the third deposition cycle, and it is even 11% lower than the 1568.5 nm/RIU of the case without modification. Consequently, it can be concluded that the deposition of two-dimensional nanomaterial MoS2 can enhance the


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sensitivity of the SPR sensor. However, when MoS2 overlayer is too thick, the sensitivity becomes lower. In addition, because the resonance dip almost disappears at the fourth deposition cycle, the cases with more cycles (≥4) of depositions are not considered here.

4. DISCUSSION

To understand the experimental results, we construct a phenomenological model to simulate the MoS2-SPR sensor and perform the corresponding analysis. Considering the porous and loose morphology of the overlayer, as shown in Figure 3, an effective dielectric model is employed to simulate the overlayer, which can be expressed by20

neff  f analyte  nanalyte  f MoS  nMoS 2

2

(1)

where neff, nanalyte, and nMoS2 are the effective refractive indices of the overlayer, analyte solution, and MoS2, respectively; fanalyte and fMoS2 are the volume fraction of the analyte and MoS2, respectively, in the region over the gold film within the height of overlayer, and they have the relationship of fanalyte + fMoS2 = 1.


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The finite element method (FEM, COMSOL Multiphysics) was employed for the simulations. In the simulations, the RIs of the fiber core and cladding were set as 1.4457 and 1.4378, respectively; the Drude model was employed for the dispersion relationship of gold film;31 the Cr layer was ignored for simplifying the simulations; the RI of the analyte solution was changed from 1.332 to 1.360. It is worth noting that because of the large core diameter, a large number of modes exist in the fiber core. Among them, the mode that possesses the largest value in the effective index imaginary part, i.e. the largest attenuation, was picked out for the analysis. The transmission T(λ) at the wavelength λ can be calculated by32

T ( )  exp[

4π



imag(n ) L] eff

(2)

where imag(neff) is the imaginary part of the effective index of the picked mode, and L (~6 mm) is the length of the SPR sensor as shown in Figure 1(a).


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Figure 9. The simulated electric field diagram and transmission spectra by COMSOL. (a)(b) AuSPR fiber sensor, (c)(d) Au-MoS2-SPR fiber sensor.

First, we simulated the SPR sensor without modification. Figure 9(a) and (b) show the electric field distribution at the resonance wavelength of 520 nm and the transmission spectra for the picked mode. We can see that because of the excitation of SPR, the evanescent field, namely the electric field penetrating into the analyte solution, is significantly enhanced in the vicinity of the gold surface. The resonance wavelength has a shift of 74.6 nm when the RI is changed from 1.332 to 1.360.


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Then, the MoS2 overlayer was included in the simulation, where the overlayer thickness the and the fMoS2 were set as 30 nm and 0.1, respectively, and the dispersion relationship of the MoS2 materials was referred to the published data.30 It should be pointed out that because the measured MoS2 overlayer thickness is the largest one presented in the SEM Figure 3(c), the overlayer thickness used in simulation was obtained by multiplying the measured one by the occupation ratio, which is evaluated from the 3D AFM Figure 4(c). The simulated results are presented in Figures 9(c) and 9(d), from which the resonance wavelength shift of 92.8 nm (larger than the 74.6 nm for the unmodified sensor) can be found, demonstrating that the modification of MoS2 overlayer can increase the RI sensitivity. The enhancement of sensitivity can be explained by the enhancement of evanescent field, which penetrates into the analyte solution, induced by the attachment of MoS2 overlayer as shown by comparing the Figures 9(a) and 9(c), because the sensitivity is positively-related to the integral overlap between the evanescent field and analyte solution.33


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Figure 10. The simulated transmission spectra. (a) Au-SPR fiber sensor, (b) Au-MoS2-SPR fiber sensor.

Using the same parameters employed in COMSOL and the transfer matrix method (TMM),34 we also simulated the transmission spectra (shown in Figure 10) of the sensors before and after the deposition of MoS2 nanosheets. Similar to the results shown in Figure 9, the resonance wavelength shift increases from 86.1 to 120.8 nm due to the introduction of MoS2 overlayer, again demonstrating the sensitivity enhancement resulted from the MoS2 deposition. Moreover, we can see from Figures 9 and 10 that the full width at half maximum (FWHM) shows a significant increase. This arises from the relatively-large imaginary part (0.16~1.92 at 500~800nm) of the refractive index for MoS2 material,30 which causes the high loss to the evanescent field.23


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Figure 11. Schematic diagram of the cross section of the sensor.

It is notable that even though the MoS2 overlayer can enhance the evanescent field, the overlayer thickness Hover (shown in Figure 11) will increase with the number of deposition cycles at the same time, which has been demonstrated by Figure 3. The further increase of Hover would thus decrease the Hana, i.e. the distance of evanescent field penetrating into the analyte solution as shown in Figure 11, resulting in the decrease of the integral overlap between the evanescent field and the analyte solution, which has a positive correlation with the sensitivity.33 Besides, the significant roughness of the sensor’s surface with 3 cycles deposition, as shown in Figure 4(d), will cause a high scattering loss to SPW and the decrease of propagation length. The above illustrations can explain why


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the RI sensitivity tends to decrease, even to be lower than that of un-modified sensor at the deposition cycles of 3.

We also make a comparison between the reported-relevant and our works, which are presented in Table 1. Firstly, in terms of the sensitivity enhancement, the result achieved in this work is higher than most of the published results obtained no matter by simulations or by experiments. Secondly, employing the side-polished fiber instead of a prism as the SPR coupling scheme can provide a small size and more compact SPR sensor. Thirdly, in this work, the MoS2 nanosheets instead of the monolayer or few-layer 2D materials was used for the modification layer, which can significantly reduce the complexity of operation and the cost.

Table 1. Comparison of different schemes for sensitivity enhancement of SPR-based sensor. Plamonic interface

Coupling scheme

Measurement range（RIU）

Sensitivity

Ag-Graphene19

Prism

1.330-1.370

91.8 a.u./RIU

22%

Simulation

Au-Graphene35 Cu(Al)-GrapheneSilicon36 Rh-Ag-SiliconGraphene37 Rh-Graphene38

Prism

~1.330

-

25%

Simulation

Fiber Core

1.330-1.332

24000 nm/RIU

-

Simulation

MgF2 Prism

1.0000-1.0005

220 o/RIU

-

Simulation

MgF2 Prism

1.330-1.360

259 o/RIU

-

Simulation

Prism

1.333-1.360

~2715.1 nm/RIU

20.2%

Experiment

Au-Graphene39

Enhanceme Simulation/ Experiment nt


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Graphene/AgNPs40

U-bent Fiber

1.340-1.356

~1198 nm/RIU

50%

Experiment

Au-MoS2 (this work)

Side-polished Fiber

1.332-1.359

~2153.9 nm/RIU

37.3%

Experiment

5. CONCLUSION

In summary, a sensitivity-enhanced SPF-based SPR sensor modified with MoS2 nanosheets is proposed and demonstrated. The MoS2 nanosheets are deposited onto the surface of the gold film by the drop-and-dry method, and the sensitivity that is related to the thickness of the MoS2 nanosheet overlayer can be tailored by depositing MoS2 for different cycles. With the increase of MoS2 nanosheet thickness, the sensitivity increases first and then decreases, for which a detailed explanation is presented. The highest sensitivity of 2153.9 nm/RIU is achieved by the two cycles of deposition, showing an enhancement of 37.3% when compared with the sensor without the modification of MoS2 nanosheets. By means of a simple drop-and-dry process, this work demonstrated an effective strategy to improve the sensor performance. The proposed MoS2 modified SPR fiber sensors could hold considerable potential in biochemical detections by exploiting the


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additional advantages of MoS2, such as large surface area and the abundant surface functional groups.

AUTHOR INFORMATION

Corresponding Author

Yunhan Luo, E-mail: [email protected]

Author Contributions

5These

authors contributed equally.

Acknowledgement

This work is partly supported by National Natural Science Foundation of China (NSFC) (61575084, 61805108, 61705087); Special Research Fund for Central Universities (21618404, 21617332); Science and Technology Projects of Guangdong Province (2017A010101013); Science & Technology Project of Guangzhou (201707010500,


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201807010077, 201704030105, 201605030002); Joint Fund of Pre-research for Equipment, Ministry of Education of China (6141A02022124).

REFERENCES

(1) Santos, D.; Guerreiro, A.; Baptista, J. M. Numerical investigation of a refractive index SPR Dtype optical fiber sensor using COMSOL multiphysics. Photonic Sensors 2013, 3 (1), 61-66. (2) Sharma, A. K.; Gupta, B. On the performance of different bimetallic combinations in surface plasmon resonance based fiber optic sensors. Journal of applied physics 2007, 101 (9), 093111. (3) Zhao, J.; Cao, S.; Liao, C.; Wang, Y.; Wang, G.; Xu, X.; Fu, C.; Xu, G.; Lian, J.; Wang, Y. Surface plasmon resonance refractive sensor based on silver-coated side-polished fiber. Sensors and Actuators B: Chemical 2016, 230, 206-211. (4) Sharma, A. K.; Jha, R.; Gupta, B. Fiber-optic sensors based on surface plasmon resonance: a comprehensive review. IEEE Sensors Journal 2007, 7 (8), 1118-1129. (5) Zeng, S.; Baillargeat, D.; Ho, H.-P.; Yong, K.-T. Nanomaterials enhanced surface plasmon resonance for biological and chemical sensing applications. Chemical Society Reviews 2014, 43 (10), 3426-3452. (6) Mishra, S. K.; Tripathi, S. N.; Choudhary, V.; Gupta, B. D. Surface plasmon resonance-based fiber optic methane gas sensor utilizing graphene-carbon nanotubes-poly (methyl methacrylate) hybrid nanocomposite. Plasmonics 2015, 10 (5), 1147-1157. (7) Kim, S. A.; Kim, S. J.; Moon, H.; Jun, S. B. In vivo optical neural recording using fiber-based surface plasmon resonance. Optics letters 2012, 37 (4), 614-616. (8) Liu, L.-h.; Chen, Z.; Bai, C.-h.; LI, Z. The effect of refractive index of material overlaid side


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