A Field-Enhancement Optical Fiber SPR Sensor Using Graphene, Molybdenum Disulfide, and Zinc Oxide

Graphene, molybdenum disulfide (MoS2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}), and zinc oxide (ZnO) are proposed here to enhance the evanescent field of an optical fiber surface plasmon resonance (SPR) sensor. Gold and silver are the plasmonic materials, and the fiber core material is made of polymethylmethacrylate (PMMA). A Fresnel equations-based analysis is used, and the obtained results pointed out higher values of sensitivity, figure of merit, and FWHM (full-width at half maximum) when compared to conventional SPR sensors. In despite of silver-only based SPR sensor has a better performance, oxidation occurs, and the sensor’s lifetime is reduced. The addition of graphene layers leads to sensitivity values 50%\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$50\%$$\end{document} higher than the conventional sensor. On the other hand, the MoS2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}-based sensor improves the interaction of the sensor with the bio-recognition molecules, which is attractive for biomedical applications. When ZnO was added to the silver-based sensor, a highest sensitivity, 4740.9 nm/RIU, was obtained. Graphene-based silver SPR exhibit the highest FOM values.


Introduction
Surface plasmon resonance (SPR) is a well-known physical effect [1][2][3][4][5], which was firstly used for gas detection [6].Since then, it has been a good analytical tool for monitoring biomolecular interactions in various applications, revealing high sensitivity and selectivity, fast response time, and non-labeled detection capability [7,8].Typically, there are two interrogation modes in an SPR biosensor: AIM (angular interrogation mode), where the wavelength of the incident light is fixed, and the angle of incidence is varied; WIM (wavelength interrogation mode), where the incident angle is kept constant, depending on and varying with the wavelength.The resonance phenomenon occurs when the input photons couple to surface plasmons at the substrate metal film interface [4,8,9], which is identified by a minimum in the reflected light.In visible light source applications, gold (Au) and silver (Ag) are often used metals, and they are simply called plasmonic metals [10][11][12][13][14][15][16][17].
Although SPR sensors are widely used for various applications, challenges appear at the sensor design for the detection of analytes with ultra-low concentration or small molecules (molecular weight less than 500 Da) [18][19][20].Some arrangements attempt to struggle with it through the addition of nanomaterials to the SPR structure, with a high refractive index, or large surface area.Graphene, molybdenum disulphide (MoS 2 ), and zinc oxide are proposed nanomaterials deposited on the metallic layer, leading to an increase of the evanescent field and the useful life of the sensor [1].
Graphene consists of a flat sheet of sp 2 carbon atoms with an exceptionally high crystal and electronics quality features high charge carrier mobility, which increases the sensitivity of the [2] sensor.It is worth to mention the good biocompatibility [21][22][23], making it a promising material in the field of biosensors [24,25].A graphene-based SPR biosensor obtained a 25% increase in sensitivity when coating the gold layer with 10 graphene sheets [26].Considering a graphenebased bimetallic SPR sensor, the sensitivity was increased by 22% [27].
2D transition metal dichalcogenides (TMDs) have also attracted interest in recent years [28][29][30].They are materials formed by three atomic structures (X -Y -X), where the Y plane is formed by the transition metal (W, Ti, Mo, etc.) [31].Molybdenum disulfide (MoS 2 ) is a representative of TMDs, and it has specific optical properties that draw attention to its application in SPR sensors.A single layer of MoS 2 has an electronic bandgap of 1.8 eV and a rate optical absorption of 5% [32,33].MoS 2 also increases the interaction of antibodies with the surface of the sensor, increasing its sensitivity [4][5][6].Chen et al. achieved a 30% improvement in the SPR sensor with the addition of two layers of MoS 2 compared to the conventional sensor [34].
Zinc oxide (ZnO) has been recently used in gold, silver, and copper-based sensors, and an increase in sensitivity was observed [8].Furthermore, ZnO has high thermal and chemical stability, good mechanical strength, excitation binding energy, and optical gain [35].With all these properties, it also helps to protect unstable metallic materials (Au, Ag, and Cu), against oxidation and degradation, increasing the useful life of the SPR sensor.
The use of plastic optical fibers or polymer optical fibers (POFs) in SPR sensor applications is already well established in the literature [36][37][38].From a mechanical point of view, POFs have high flexibility in bending, large breakage strain, and high impact resistance [39].From a biological point of view, POFs are more biofriendly and can bind to chemical and biological receptors more efficiently than glass fiber optics [40].POFs have some advantages over glass optical fibers, which are easy, flexible, ventilated, and better accommodation of light from the light source to the fiber, but they are still not used at very long distances due to their relatively high attenuation [41].Among the POFs, the most used ones are based on a PMMA (Poly(methyl methAcrylate)) core because they are readily available in the market, have low cost, and present low loss transmission in the visible region [40,41].
Here, a computational analysis of an optical fiber-based SPR sensor, operating in wavelength interrogation mode (WIM), using gold and silver as the plasmonic metals, and adding graphene, MoS 2 , and ZnO is proposed.An easy-tohandle step-index multimode polymeric optical fiber with a total diameter of 2 mm.This fiber is easier to manipulate and has greater resistance compared to silica optical fibers.Fresnel-based equation modeling was used to evaluate SPR sensor performance parameters, i.e., figure of merit (FOM), sensitivity, and full width at half maximum (FWHM).Furthermore, the simulation results of the three proposed arrangements are presented and discussed.

Fresnel Equations Model
The design of the proposed optical fiber SPR sensor uses the multilayer Fresnel-based equations [42], where the reflectivity can be calculated.This model assumes that all layers are non-magnetic, uniform, and isotropic [42].The resultant of the matrix or the individual transference matrix can be defined as [43]: where M k represents the propagation from the k layer to the k + 1 layer, and q k represents the optical admittance as a function of the polarization, since the surface plasmons are excited by a p-polarized wave.Fresnel equations calculate the reflection coefficient for a p-polarized incident wave, defined by [43]: The reflectance ( R p ) of a multilayer system for a p-polarized light is [44]: For a sensor operating in the wavelength interrogation mode (WIM), the sensitivity relates to the resonant wavelength variation, res , and the RI variation, n s , of the sur- rounding medium, n s .It is called refractometric sensitivity, being given by [44]: Figure of merit (FOM) relates two parameters, sensitivity, and FWHM.It provides information about the resonance wavelength shift and the sensor accuracy.It is defined by [45]:

Graphene
Recent studies have evidenced graphene as a good alternative in SPR sensors due to its electrical characteristics.The dielectric properties of graphene can be adjusted by chemical or electrostatic changes in its charge density, which (4) S n = res n s . ( makes it possible for SPR devices to be activated at certain frequencies [9].
The number of layers deposited on the sensor affects the sensor's performance.A graphene layer is approximately 0.34 nm.The complex refractive index of graphene is calculated by the following equation [46]: where is the wavelength and C is approximately 5.446 m −1 , which was found from the measurement of graphene opacity by Nair et al. [47].
Meshginqalam et al. proposed an SPR biosensor based on a graphene layer, showing that the deposition on the surface of the thin gold film increases the sensitivity by 23% [48].Ryu et al. obtained an increase in sensitivity of 13% adding layers of graphene to the sensor [49].
The effect of graphene layers involving thin films of gold and silver has been analyzed [10].Besides the sensitivity improvement, the sensor's lifetime increases by avoiding oxidation.In some manufactured graphene-based sensor configurations, the increase in sensitivity is around two times compared of a conventional one.Different methods are available for graphene synthesis and sensor deposition [11].Moreover, a graphene-based gold optical fiber sensor was recently proposed, where its sensitivity was two times higher than the conventional one, revealing a fast response and good reuse [3].

Molybdenun Disulfide
When a molybdenum crystal thickness is reduced to a monolayer (0.65 nm), a set of physical and optical properties is built up.The bandgap increases from 1.2 to 1.8 eV, due to quantum confinement effects [12].Furthermore, the monolayer has an optical absorption of approximately 5% , being ( 6) able to manufacture ultra-sensitive photodetectors.In comparison, the optical absortpion of graphene is approximately 2.3% [13].Therefore, molybdenum disulfide can be used in SPR sensors to improve light absorption to provide sufficient excitation energy for charge transfer.In [7], a sensor based on MoS 2 nanosheets was manufactured, where a technique of liquid exfoliation, assisted by ultrasound, was used for manufacturing.This technique can also be used for graphene synthesis.For the deposition process, an immersion coating-based technique (dip coating) was employed.Another sensor used hybrid structures based on graphene and MoS 2 , where the sensitivity was increased when MoS 2 layers were added [14].
Recently, SPR sensors based on MoS 2 nanosheets were applied for the detection of E. coli, increasing the contact area of the sensor, consequently improving sensitivity as more antigens will bind to [7] antibodies.Chen et al. deposited molybdenum nanosheets for two cycles on a conventional SPR sensor, obtained an increase of more than 30% in the sensitivity of the sensor [21].

Zinc Oxide
Current interests in zinc oxide have allowed the implementation of optoelectronic devices based on this material.It is considered one of the most important functional oxides among the conductive metal oxides, having a bandgap of 3.37 eV, and an excellent biocompatibility.It is used for various applications such as field emitters, solar cells, UV lasers, LED, and sensors [50].
R. Tabassum et al. [51] reported an experimental study using a copper and zinc oxide optical fiber sensor to detect hydrogen sulfide gas.According to the article, the refractive index of zinc oxide changed with the concentration of the gas, thus making the evanescent wavelength change.Zinc oxide also protected the thin copper film from oxidation and increased the evanescent wave field, optimizing the sensor's sensitivity.

Materials and Methods
Figure 1 illustrates the proposed SPR sensor based on the Kretschmann-Raether structure, which is organized in 4 layers.The first one is the optical substrate, where the light couples, hits the metallic thin film, and its incoming photons ( k x ) inter- act with the surface plasmons ( k sp ).Over the metallic layer is the field-enhancement layer that interacts with the sample (analyte) [4], circulating by a flow cell structure.The chosen analyte was an aqueous solution, having refractive indices  varying around the refractive index of water with increments of 0.002.
Figure 2 illustrates the manufactured sensor.Figure 2C shows the fiber core (PMMA), the metal layer (Au or Ag), and the optimizer metal layer (graphene, MoS 2 , or ZnO), which were used in the simulations.The optical fiber (Fig. 2A) is CK-80, which is a commercial index multimode with a polymeric core (Eska Optical Fiber Division, Mitsubishi Rayon).Its total diameter is 2.00 mm being uncladded by a chemical method [52].Figure 2B depicts the uncladded optical fiber, i.e., with exposed core.The sensor region (Fig. 2B) has a length of 0.5 cm (L), a chosen value based on the simulations.

Results and Discussion
In this work, the step-index multimode plastic optical fiber, reference CK-80, sold by Eska Optical Fiber Division of the company Mitsubishi Rayon is used as a substrate for the biochip (sensing region of the sensor).The fiber core is polymethyl methacrylate (PMMA) with a diameter of 1960 µm.The core is covered by a 40 µm thick fluorinated polymer shell, totaling a fiber diameter of 2 mm.This type of fiber is more malleable than silica fiber and less brittle, making it easier to handle for cutting, peeling, and polishing, without splintering.
Considering the configuration of the SPR biosensor (Fig. 1), the transmitted power SPR curves for gold (a) and silver (b) sensors based on Kretschmann-Raether were presented.A wide range of thicknesses of gold and silver metals were analyzed.For thicknesses less than 40 nm or greater than 65 nm, the SPR effect was small or non-existent, where the dip in the reflectance curve was reduced substantially.In the range of 40 to 65 nm, the best choice considering the sensor performance parameters was the thickness of 50 nm, which was used for gold and silver sensors.
In the computational results generated by the Fresnel equations multilayer, the aqueous medium is considered the reference analyte.To calculate sensitivity, a fixed value of 0.002 is added to the refractive index of water for different wavelengths.In addition, manufacturing errors such as oxidation, for example, were taken into account in the simulation, especially with silver, which possess a high oxidation factor.A layer of silver oxide of 2 nm was considered for simulations over the silver thin film.The deposition of the optimizing layer before the oxide formation is an alternative to reduce or eliminate the silver oxidation.
For a 50 nm gold sensor, a pronounced dip was observed in the SPR curve (Fig. 3).The sensitivity, FWHM, and FOM values of 2220 nm/RIU, 34.78 nm, and 63.83 were reported, respectively.When the 50 nm silver sensor was considered, a lower FWHM was observed (Fig. 3).Its obtained metric Fig. 4 Transmitted power of the gold-and silver-based graphene sensor values, i.e., sensitivity, FWHM, and FOM, are 3450 nm/ RIU, 17.68 nm, and 194.02, respectively.The results of the silver-based sensor are better than the gold-based sensor; however, the lifetime of the silver-based sensor is shorter due to its high oxidation rate.An alternative to reduce this wear is the coating of the silver thin film with materials such as graphene, molybdenum disulfide, and zinc oxide, which, in addition to optimizing its metrics, protect it from the external environment.
Layers of graphene were added to the 50 nm gold and silver sensors, and the transmitted power curves are depicted in Fig. 4. A range of 1 to 20 graphene layers was considered for calculations.In the gold-based graphene sensor, the highest sensitivity was 3540.7 nm/RIU for 18 deposited graphene layers, an increase of 61% .For the same number of graphene layers, the FWHM and FOM values were respectively 36.5 nm and 97.02.In the silver-based graphene sensor, the highest sensitivity was 4550.9 nm/RIU for 12 graphene layers, an increase of 32% , with FWHM and FOM values of 25.18 nm and 180.74, respectively.
As the number of graphene layers increases, the dips redshift linearly for both arrangements.In addition, the metal tends to degrade less, due to the protection of graphene, i.e., increasing the sensor lifetime.The sensitivity optimization is due to the excellent electrical conductivity, large surface area, and high refractive index of the graphene layers.
Afterwards, a range of 1 to 16 layers of MoS 2 has been added to gold-and silver-based sensors.The results are depicted in Fig. 5.As the number of layers of MoS 2 increases, the resonant wavelength tends towards the infrared.In the gold-based MoS 2 sensor, the sensitivity reached 3600.7 nm/RIU for 13 MoS 2 layers, 63% increase over con- ventional gold sensor, while FWHM and FOM were, respectively, 38.45 nm and 93.64.It shows a great improvement compared to the conventional gold sensor.In the silver-based MoS 2 sensor, the best sensitivity was 3990.8 nm/RIU for 14 layers of MoS 2 , 16% improvement over silver sensor.The FWHM and FOM were respectively 34.313 nm and 116.301.
These results show that by adding layers of MoS 2 , the electric field intensity that penetrates the sample solution enhances and also the contact area with the sample increases.Addition of MoS 2 improves the interaction of the sensor with the biorecognition molecules and can be used in the detection of pathologies or even in the analysis of biological concentrations in different samples [34].
When zinc oxide (ZnO) was used, its thickness varied from 10 to 30 nm with a step of 4 nm for the silver sensor, while for the gold one, the thickness varied from 10 to 20 nm, with a step of 2 nm.For a 14 nm gold-based ZnO sensor, the measured maximum sensitivity was 2340 nm/ RIU, which means a small increase of 120 nm/RIU compared to the conventional gold sensor.The measured values For the silver-based ZnO sensor, the improvement in sensitivity was significant, reaching a value of 4740.9 nm/RIU when 26 nm of ZnO were deposited, 37% improvement over conventional silver sensor.The values for FWHM and FOM were, respectively, 81.69 nm and 51.71.
The improvement in the parameters, mainly in the silverbased sensor, after the addition of ZnO can be related to a series of characteristics of the oxides and semiconductors.ZnO does not oxidize easily when in contact with the environment, unlike silver.Zinc oxide has high thermal and chemical stability, being able to be in contact with different samples; it also has a large piezoelectric coefficient and a high optical excitation binding energy, causing it to increase the evanescent field of the sensor, improving its sensitivity and the other performance parameters (Fig. 6).
Table 1 summarizes the obtained metrics for the SPR sensors based on graphene, molybdenum disulfide, zinc oxide, and pure one which is the conventional sensor, without the addition of the optimizing materials.Considering sensitivity, ZnO silver-based sensor shows the highest value, although FWHM is also higher and reduces the overall performance.As FOM (Eq.5) involves sensitivity and FWHM, a graphene silver-based sensor is the best choice for all the proposed arrangements.When SPR curves are compared, the graphenebased ones have also the best morphologies, where the dips  are located on the visible range and both have a reflectivity close to zero (Fig. 4).The lowest FWHM was also obtained for graphene-based silver sensor, which is related to the signal-to-noise ratio (SNR) [57].When FWHM is lower, the highest is SNR, which is appropriate for SPR sensors [57].Table 2 presents the sensitivity results for some sensors already published.Comparing with our proposed silver-or gold-based sensors, using any field-enhancement material (Table 1), our device sensitivity is higher than that shown in Table 1.

Conclusion
With the addition of materials such as graphene, molybdenum disulfide, and zinc oxide, a high increase in the evanescent field of the proposed gold-based and silverbased SPR sensors has been reported.This leads to a higher change in the resonant wavelength, thus increasing the sensitivity, except for the gold-based ZnO sensor.Graphenebased SPR-based sensors show the best performance, and therefore, it is a good alternative to enhance the performance of SPR sensors.We also can conclude that it is possible to use these sensors in the biomedical area, mainly due to the biocompatibility of these materials, which provides a higher improvement of the accuracy and sensitivity.In addition to the optimization of the sensor, the lifetime of silver-based sensors is increased by the natural protection caused by the deposition of graphene, MoS 2 , and ZnO.

Fig. 1
Fig. 1 SPR biosensor multilayer structure based on the Kretschsmann-Raether model, where dielectric constants, wave vectors, and layer thicknesses are indicated

Fig. 2
Fig. 2 Manufactured sensor model, A optical fiber, B optical fiber without the shell in the central region, C sensor manufactured with the metallic layer and the optimizer material

Fig. 3
Fig. 3 Transmitted power of conventional gold and silver sensor

Fig. 5
Fig. 5 Transmitted power of the gold-and silver-based MoS 2 sensor

Fig. 6
Fig. 6 Transmitted power of the gold-and silver-based zinc oxide sensor