Enhanced Goos-Hänchen Shift of SPR Sensor with TMDCs and Doped PANI/Chitosan Composites for Heavy Metal Ions Detection in Aquatic Environment

A novel surface plasmon resonance (SPR) sensor with transition metal dichalcogenides (TMDCs) and polyaniline (PANI)/chitosan composite for detection of heavy metal ions in an aquatic environment is proposed and analyzed. A novel Goos-Hänchen (GH) shift sensing scheme based on TMDCs and PANI/chitosan structure is proposed. The theory shows that the GH shift can be significantly enhanced in the SPR structure silver (Ag), TMDCs, and PANI/chitosan heterostructures. The refractive index of Cu2+ ion is 1.3516, and the maximum GH shift of the hybrid structure of Ag-MoSe2-PANI/chitosan is−2067 λ at resonance angle 69.19° with MoSe2 4 layers and PANI/chitosan 123 nm. When different Cu2+ ion concentrations are added into the sample layer, the refractive index of the sample and GH shift of the SPR sensor will change. The maximum sensitivity of 2.425 × 106 λ/RIU is obtained by Ag-WSe2-PANI/chitosan structure, which is 463.42 times higher than the traditional SPR Ag film and 112.84 times higher than Ag-PANI/chitosan structure. Therefore, the configuration with GH shift provides a new development direction for the detection of heavy metal ions in an aquatic environment.


Introduction
Heavy metal is a kind of toxic chemical substance, which can cause chronic poisoning when accumulated in the human body to a certain extent, which is a hot research field [1][2][3]. Especially in water environment pollution, heavy metal ions such as mercury (Hg), lead (Pb), manganese (Mn), cadmium (Cd), and chromium (Cr) are becoming an important problem in chemical, physical and health sciences, because these toxic elements tend to accumulate in the food chain and then affect all organisms in a specific biological system [4][5][6][7]. Hg poisoning in an aquatic environment often exists in the form of Hg vapor, which is highly diffusible and fat soluble. Hg in the blood is oxidized into Hg ions and gradually accumulates in the brain tissue. After reaching a certain amount, the brain tissue will be damaged, and the other part will be transferred to the kidney to destroy the human kidney function [8,9]. Cr will destroy the function of the upper respiratory tract after it invades the human body through the respiratory tract. The long-term effect will cause emphysema, bronchiectasis, pulmonary sclerosis, and lung cancer [10]. When Cr enters the human body through the digestive tract, it can cause mouth erosion, nausea, vomiting, diarrhea, and ulcer [11]. Cd can block the metabolism of bone, resulting in osteoporosis, atrophy, deformation, and other symptoms [12]. Other heavy metals, including Mn and Pb, affect the central nervous system, bones, kidneys, liver, and brain [13][14][15].
Surface plasmon resonance (SPR) sensor is widely used in biochemical analysis [16], health monitoring [17], environmental detection [18], and so on, because it can sense the change when the refractive index of the medium to be measured changes slightly. Using SPR sensors, a new type of heavy metal ion detection sensor was constructed, including Hg 2+ , Pb 2+ , Mn 2+ , Cd 2+ , and Cr 2+ , and applied to the analysis of heavy metal ions in the aquatic environment. Compared with the traditional methods, this method has the advantages of high sensitivity, real-time, simple operation, label-free detection and interference, and rapid response [19][20][21]. The most traditional method to excite SPR is the Kretschmann structure, including prism-metaldielectric [22], which has low sensitivity and poor comprehensive performance. To improve the performance of SPR sensor, researchers have been trying to make greater progress by covering the active layer [23], optimizing the metallic transducer layer [24,25], and changing the refractive index and the type of prism [26][27][28], using carved films instead of continuous metallic films [29,30], SPR based on grating [31] and nanoparticles [32,33], and other methods. Another important performance evaluation of the SPR sensor is Goos-Hänchen (GH) shift, which refers to the lateral spatial displacement on the incident plane when the electromagnetic wave packet is reflected from the surface [34]. GH shift has been widely concerned since its discovery and has become a hot spot of academic research. A lot of work has been done in theory [35][36][37] and experiment [38,39], which can be applied to optical measurement [40], biomedicine [41], and chemical sensor [42]. The stationary phase method was originally used to solve the problem of integral steepest descent algorithm in fluid mechanics. However, Artmann uses the stable phase method to calculate the GH shift. However, it is only limited to the case of TM and TE polarized beam total reflection, and the GH shift of beam total reflection under any polarization state cannot be calculated [43]. To enhance the GH shift, the method is to excite surface plasmon polariton (SPP). SPP is the electron coupling of electromagnetic waves along the metal surface, and electrons do collective oscillation on the metal/dielectric interface. Its energy propagates along the metal surface, and the energy perpendicular to the metal surface is exponentially attenuated. The excitation of SPP will be accompanied by the decrease of the reflectivity on the reflection angle/wavelength curve [44]. However, the GH shift excited by traditional SPP (Prism-Au/Ag) is small, which cannot meet the actual needs. Therefore, it is necessary to add other materials to further increase the GH shift.
The ultra-thin atomic structure and huge specific surface area endow the two-dimensional (2D) material with excellent properties. In addition to its excellent physical properties, 2D materials have atomically flat surfaces. Scientists can also stack two-dimensional materials with different properties like Lego building blocks and form heterostructures under the action of van der Waals forces between layers, called van der Waals heterostructures. The 2D material with graphene, transition metal dichalcogenides (TMDCs) (WS 2 /MoS 2 /WSe 2 / MoSe 2 ) and h-BN, as the most common two-dimensional materials, have highly symmetrical crystal structure and lattice orientation and exhibit isotropic physical properties, which have great application advantages in the performance balance of traditional large-area uniform devices [45]. Graphene is the only naturally occurring 2D material and the thinnest material known. Based on stable structure and ultra-high conductivity, graphene may become a key material for advanced research of next-generation transistors [46]. The electric field control characteristics of graphene come from its single atomic layer thickness, and its novel physical effects are ultimately derived from its linear Dirac cone band structure that can be compared to relativistic Dirac fermions. The TMDCs are increasingly used to enhance the performance of SPR sensor due to its high  specific surface area, high dielectric constant, and adjustable bandgap [47,48]. For the sensitivity, Zhao et al. proposed the novel SPR structure with TMDCs coated on both surfaces of the metal film, which the maximum angular sensitivity is 315.5°/RIU and phase sensitivity is 3.85 × 10 6 Deg/RIU [49]. A new structure of SPR sensor based on graphene-MoS 2 hybrid structure was designed by Zeng et al. [50]. Compared with SPR sensor without 2D materials coating, the novel SPR sensor shows the sensitivity improvement factor of more than 500 times. The sensitivity improved SPR sensor based on silicon and TMDCs was designed by Ouyang et al. [51]. The minimum reflectivity and sensitivity of SPR curves in wavelength of 600 ~ 1024 nm were systematically studied by Fresnel equation and transfer matrix method (TMM). Compared with the traditional sensing scheme using Au film, all silicon TMDCs enhanced sensing model shows better performance. The highest sensitivity of 155.68°/RIU can be achieved by prism-Au (35 nm)-silicon (7 nm)-WS 2 (monolayer) hybrid structure at 600 nm excitation wavelength. A SPR sensor with Au-WS 2 structure with enhanced sensitivity is demonstrated by Wang et al. [52]. The results of simulation analysis showed that the maximum sensitivity of 2459.3 nm/RIU was obtained by coating WS 2 monolayer. For GH shift, the potential effects of four different TMDCs monolayers on the spatial and angular GH shift of the beam reflection are investigated theoretically [53]. The GH shift of TMDCs, and direct bandgap semiconductor surfaces is studied by Das and Pradhan [54]. Han et al. theoretically proved the 801.7 λ of GH shift of multi-layer heterostructures [55]. Therefore, 2D material TMDCs can improve the GH shift of SPR sensors. Polyaniline (PANI) is a kind of conductive polymer with good selectivity to some metal ions due to its various redox reactions, excellent electronic properties, and high thermal stability [56]. Graphene oxide/silica@PANI composite was used to adsorb metal ions in a water environment, which proved that the composite material was a promising adsorbent containing metal  [57]. PANI/ZnO nanocomposite is an efficient photocatalytic material, which can degrade water pollutants and adsorb heavy metal ions [58]. A selective solid adsorbent for benzophenone-4 in a water environment based on PANI was developed by Ayadi et al. [59]. In various functional films for the detection of heavy metal ions, chitosan and heavy metal ions have a good chelating effect. Heavy metal ions can automatically combine with the amino group of chitosan, resulting in the change of surrounding refractive index on the fiber surface [60]. Ding et al. reported SPR sensor coated with Au and chitosan/PAA, and the sensitivity is 0.1184 nm/μM in the range of 0.2-50 μM [61]. Chen et al. proposed an SPR sensor with chitosan and high sensitivity of 0.5586 nm/μm to detect Hg 2+ [62]. A single Au thin film SPR sensor based on chitosan is reported for the detection of iron ions [16]. In addition, both PANI and chitosan can be combined with heavy metal ions in water, which can achieve a larger range and high sensitivity of accurate detection.
In order to detect the heavy metal ions in an aquatic environment, we propose the SPR sensor of Ag-TMDCs-PANI/chitosan hybrid structures to increase the GH shift. By optimizing the number of layers of TMDCs and thickness of PANI/chitosan, comprehensive performance can be improved. The SPR sensor can be used not only in the detection of heavy metal ions in water but also in the detection of diseases and gas. The numerical simulation is calculated by MATLAB software. The simulation conditions and parameter settings have been mentioned in this paper.

Design Configuration and Theoretical Method
We build a multilayer SPR sensing Kretschmann structure including Ag-TMDCs (WS 2 /MoS 2 /WSe 2 /MoSe 2 )-PANI/chitosan as Fig. 1. For the SPR sensing structure, we use He-Ne laser as excitation light with 632.8 nm wavelength and p-polarized incident light. Because our proposed structure is to obtain GH shift, we need a prism made of high refractive index (RI) glass as the incident layer of the light source [48]. The TM polarized light is incident from one side of the prism and then reaches its bottom. It is completely reflected from the other side and thereafter collected and analyzed by the photoelectric detector. We choose SF11 (n 1 = 1.7786 at λ = 632.8 nm) as a prism for the first layer of the SPR sensor [63] combined with a second layer of BK7 glass (n 2 = 1.5151 at λ = 632.8 nm) [49]. The Ag film used as the third layer has a RI of n3 = 0.135 + 3.985i at a wavelength of λ = 632.8 nm [50]. The following fourth layer composed of TMDCs is listed in Table 1 including the thickness of monolayer and RI [49,64]. The final fifth layer is made up of PANI/chitosan with a RI of n5 = 1.803 + 0.318i [65]. In this paper, we take Cu 2+ in heavy metal ions as the sample of analysis. We note that the refractive index of Cu 2+ change corresponding to the wavelength of SPR sensor [61].
From the Ding et al. [61], the linear-fitting curve and experimental data of SPR wavelength and refractive index before the sensor functionalized are depicted in Fig. 2. Figure 2 confirms that surface plasmon waves (SPW) are successfully excited because the increase of RI leads to the shift of the resonance wavelength to a longer wavelength. Therefore, the RI is calculated to be n 6 = 1.3516 at the wavelength of 632.8 nm according to above fitting equation. If the number of layers N is less than 6, it is reasonable to consider individual TMDCs as a non-interacting monolayer [66].
In this paper, the TMM and Fresnel equation are used to calculate the SPR curve and analyze the reflectivity, phase, and GH shift [21]. The reflectivity (R p ) and phase (ψ p ) can be written as follows [67]: The GH shift can be expressed as follows by using the fixed phase method [44]:

Results and Discussions
The reflectivity, phase, and GH shift of traditional prism-BK7-Ag-heavy metal ion structure with n 6 = 1.3516 are shown in Fig. 3. In Fig. 3a, the reflectivity (red line) and phase (blue line) with incident angle are plotted by the Ag film with prism and BK7. When the thickness of Ag is 45 nm, the reflectivity is 0.068 a.u at 53.93°, and the phase   Fig. 3b, the highest GH shift is 50.61 λ. Therefore, when the traditional SPR sensor is optimum thickness, the highest GH shift is obtained. The reflectivity, phase, and GH shift of the Ag-PANI/ chitosan structure of the SPR sensor are shown in Fig. 4. When the thickness of PANI/chitosan is 100 nm, the reflectivity is 0.0018 a.u, and the GH shift is 15.82 λ at 68.13°. The increase of PANI/chitosan thickness, the maximum GH shift is 210.7 λ at 116 nm, and the reflectivity is 9.3776 × 10 −6 a.u at 69.09°. When the thickness of PANI/chitosan is 120 nm, the phase curve changes from "decrement" to "increment," while the corresponding GH shift is changed from positive to negative, and the GH shift is−101.2 λ at 69.29°. Then, with the increase of PANI/ chitosan thickness, its reflectivity increases, and the GH shift decreases. Therefore, for the thickness of Ag (20 nm) and PANI/chitosan (116 nm), the maximum GH shift value 210.7 λ is obtained.
Then, the TMDCs are added into Ag-PANI/chitosan structure to form prism-BK7-Ag-PANI/chitosan-TMDCswater hybrid SPR sensor, and the thickness of Ag film is 20 nm. Firstly, WS 2 in TMDCs is added to the hybrid structure, and its reflectivity, phase, and GH shift of Ag-WS 2 -PANI/chitosan structure of SPR sensor are shown as Fig. 5. For the WS 2 (monolayer) and the PANI/chitosan (121 nm), the highest GH shift is 267.2λ with an incident angle of 69.24°. For the WS 2 (bilayer), the maximum GH shift is 889.3λ with 69.34° incident angle and 126 nm PANI/chitosan. Then, in the case of 3-layer WS 2 and 130 nm PANI/ chitosan, the GH shift reaches 678.6λ with an incident angle of 69.36°. Subsequently, the GH shift is 728.5 λ at 69.34° by WS 2 4 layers and PANI/chitosan 134 nm. Finally, the GH shift is 826.3 λ at 69.30° with WS 2 5 layers and PANI/ chitosan 138 nm. Therefore, the highest GH shift is 889.3 λ with WS 2 bilayer.
The MoS 2 is added to the Ag-PANI/chitosan structure of SPR sensor as shown in Fig. 6a. When the thickness of MoS 2 increases from monolayer to 3 layers, the phase of MoS 2 is always "Lorentz" type, and the GH shift is always negative. The highest GH shift 682.5 λ at 69.59° is obtained by MoS 2 bilayer and PANI/chitosan 130 nm. In the MoS 2 4 layers, the GH shift is 619.8 λ at 69.66°. Finally, when the MoS 2 is 5 layers, reflectivity is 6.6616 × 10 −7 a.u, and GH shift is−739.9 λ at 69.66°. Subsequently, the GH shift of Ag-MoSe 2 -PANI/chitosan structure of SPR sensor with incident angle is shown as Fig. 6b. In the MoSe 2 monolayer, the reflectivity is 9.2197 × 10 −8 a.u, and optimal GH shift is−2067 λ at 69.40° with PANI/chitosan 123 nm. When the MoSe 2 is bilayer and the thickness of PANI/chitosan is 128 nm, the GH shift is 1451 λ at 69.56°. With the increase of the thickness of MoSe 2 and PANI/chitosan, the GH shift is 755.7 λ at 69.58°,−553.8 λ at 69.77° and−1055 λ at 69.80°, respectively. Hence, the highest GH shift is−2067 λ by monolayer MoSe 2 . Finally, when the WSe 2 is added to the Ag-PANI/chitosan structure, the GH shift of SPR sensor with respect to incident angle is shown as Fig. 6c. For the WSe 2 (monolayer) and PANI/chitosan (121 nm), the GH shift is−766.1 λ at 69.30°. When the thickness of WSe 2 is Fig. 7 Linear fit of wavelength shift and Cu. 2+ ion concentrations from 0.2 to 50 μM and 50 to 500 μM [61] bilayer, the reflectivity is 9.5112 × 10 −8 a.u at 69.41°, and the maximum GH shift 2046 λ. When the thickness of WSe 2 is from 3 to 5 layers, the GH shifts are−927.9 λ at 69.50°, 1197 λ at 69.53°, and−708.1 λ at 69.56°, respectively.
Values of minimum reflectivity, highest GH shift (λ) and resonance angle with optimal layers of TMDCs and the thickness of PANI/chitosan are listed in Table 2. We know that among the four type TMDCs, MoSe 2 has the best performance, which obtains the best GH shift and minimum reflectivity, followed by WSe 2 , WS 2 , and MoS 2 .
Moreover, with the decrement of the maximum GH shift, the reflectivity is increasing, so it can be demonstrated that the smaller the reflectivity, the larger the GH shift. With the gradual increase of the number of TMDCs layers, the bandwidth of the reflection curve becomes wider rapidly, which is due to the energy loss of the TMDCs layer related to the imaginary part of the dielectric function. However, the imaginary part of the dielectric function of the Ag film is smaller than that of the TMDCs film, resulting in a large loss of electron energy [68]. The relationship between concentration and refractive index of Cu 2+ in aquatic environment was analyzed. From Ding et al. [61], the linear fit of wavelength shift and Cu 2+ ion concentrations from 0.2 to 50 μM and 50 to 500 μM is shown as Fig. 7. In the range of 0.2 ~ 50 μM, there is a huge change in wavelength shift (y = 0.1184x + 0.6994), and the highest wavelength shift is about 6.6194 nm. In the range of 50 ~ 500 μM, the change in wavelength shift is y = 0.0117x + 6.1186, and the maximum wavelength shift 11.9686 nm is obtained for 500 μM. Therefore, for the ∆n 6 = 0.002, the wavelength shift is 4.3524 nm and the Cu 2+ ion concentration of 30.8530 μM. For the ∆n 6 = 0.0005, the wavelength shift is 1.04 nm and the Cu 2+ ion concentration of 2.877 μM.
When we add Cu 2+ ion concentrations of 30.8530 μM into the sample layer, the ∆n 6 = 0.002, and the GH shift will change greatly. The Cu 2+ ion concentrations of is C = 2.877 μM and ∆n 6 = 0.0005. Therefore, by monitoring the change of GH shift (∆GH), the structure can be used as a high sensitivity sensor, and its sensitivity (S P ) is defined as follows [44]: Figure 8 illustrates the S p of six distinct structures including traditional SPR Ag film, Ag-PANI/chitosan structure, Ag-WS 2 (bilayer)-PANI/chitosan structure, Ag-MoS 2 (5 layers)-PANI/ chitosan structure, Ag-MoSe 2 (monolayer)-PANI/chitosan (6) S P = ΔGH Δn 6 structure, and Ag-WSe 2 (bilayer)-PANI/chitosan structure. Table 3 summarizes the value of ∆GH, Cu 2+ concentration, ∆n 6 and highest sensitivity with optimal layers of TMDCs, and the thickness of Ag and PANI/chitosan. Therefore, the Ag-WSe 2 (bilayer)-PANI/chitosan structure contributes to obtain the maximum sensitivity of 2.426 × 10 6 λ/RIU which is 463.42 times higher than that of the traditional SPR Ag film and 112.84 times higher than that of the Ag-PANI/chitosan structure. With the increase of Cu 2+ concentration, the GH shift and sensitivity are increasing. Due to the increase of Cu 2+ concentration, the reaction substance is concentrated in PANI/ chitosan, resulting in an increase in refractive index, which ultimately affects the reflectivity and phase. The performance of GH shift based on SPR sensor is summarized as Table 4. In Tang et al. [69], the GH shift of 12.5 λ is obtained by traditional Au thin film. In Das and Pradhan [54], when MoS 2 of 2D material and air are added to the SPR biosensor, the GH shift is improved to 40.5 λ. We can find that 2D material and air can improve the GH shift of SPR sensor. Therefore, compared with MoS 2 , graphene improves the performance of SPR sensor more significantly. However, when graphene and MoS 2 were added to the Au film of SPR sensor, the GH shift increased to 235.8 λ for reference [44], and the highest sensitivity is obtained as 5.545 × 10 5 λ/RIU. In Han et al. [55], when the ITO and MoSe 2 replace MoS 2 , the GH shift increases to 801.7 λ, and the maximum sensitivity is 8.02 × 10 5 λ/RIU. In this work, we used TMDCs and PANI/chitosan to improve the GH shift and sensitivity of SPR sensors. For the Ag-MoSe 2 -PANI/chitosan, the maximum of GH shift is−2067 λ and sensitivity is 1.608 × 10 6 λ/RIU. For the Ag-WSe 2 -PANI/ chitosan, the highest of GH shift and sensitivity are 2046 λ and 2.426 × 10 6 λ/RIU, respectively. Based on the analysis, we can see that our novel SPR sensor has improved the GH shift and sensitivity significantly.

Conclusion
The GH shift of Kretschmann configuration combined with SPR based 2D nanomaterials is studied. According to the theoretical analysis, TMDCs can improve GH shift and sensitivity.  When the MoSe 2 is monolayer and the thickness of PANI/ chitosan is 123 nm with n 6 = 1.3516, the maximum GH shift of−2067 λ is obtained. The highest GH shift increased more than 41.21 times compared with the traditional SPR Ag film structure. With the increase of TMDCs layers, the positive and negative GH shift appears in the Ag-TMDCs-PANI/chitosan hybrid structure. When Cu 2+ ion concentrations of 30.8530 μM or 2.877 μM added into the sample layer, the GH shift will change greatly, and optimum sensitivity is 2.426 × 10 6 λ/RIU by Ag-WSe 2 -PANI/chitosan structure, and it shows that the proposed structure can be used in the field of high sensitivity sensor. Such configuration with GH shift can be used in various chemical, biomedical and optical sensing fields.

Data Availability
The datasets generated during and/or analyzed during the current study are not publicly available due to [REASON(S) WHY DATA ARE NOT PUBLIC] but are available from the corresponding author on reasonable request.

Competing Interests
The authors declare no competing interests.