Sensitivity Enhancement of a Surface Plasmon Resonance Sensor with Platinum Diselenide

The extraordinary optoelectronic properties of platinum diselenide (PtSe2), whose structure is similar to graphene and phosphorene, has attracted great attention in new rapidly developed two-dimensional (2D) materials beyond the other 2D material family members. We have investigated the surface plasmon resonance (SPR) sensors through PtSe2 with the transfer matrix method. The simulation results show that the anticipated PtSe2 biochemical sensors have the ability to detect analytic. It is evident that only the sensitivities of Ag or Au film biochemical sensors were observed at 118°/RIU (refractive index unit) and 130°/RIU, whereas the sensitivities of the PtSe2-based biochemical sensors reached as high as 162°/RIU (Ag film) and 165°/RIU (Au film). The diverse biosensor sensitivities with PtSe2 suggest that this kind of 2D material can adapt SPR sensor properties.


Introduction
We have investigated biochemical sensors based on the surface plasmon resonance technique, which has been applied in extraordinary biological and chemical applications, because of their biocompatibility, excellent sensitivity, and accurate detection for pharmaceutics, for instance, in medical diagnostics and enzyme detection [1][2][3][4]. Surface plasmon resonance (SPR) refers to the resonance excitation of the surface plasmon polaritons (SPPs) at the interface of the negative (metal) and positive (dielectrics) constant materials, which is considered as best suited for untagging sensing and real-time monitoring [5,6].
One of the most effective ways for improving the efficiency of sensing devices is to select a material, such as graphene, to optimize sensing functions [7][8][9][10]. The significant properties of transition metal dichalcogenide (TMDC) materials, such as their absorption rate (~5%), which is higher compared to a graphene monolayer (2.3%), entirely different large tunable band gap than the zero band gap of graphene, and large biosense work function in comparison to graphene, are increasingly becoming preferred in biosensing applications. The outstanding electrical, optical, and chemical properties of TMDCs are converting them as promising candidates and accessory materials in comparison to graphene for the future generation of electronic and optic facilities [11][12][13]. TMDC-based SPR sensors are recommended for refractive index sensing and exhibit highly improved sensitivity [14,15].
A series of ultrasensitive photonic crystal fiber based surface plasmon resonance fiber sensors have been made by Md. Saiful Islam et al. proposed polarization-sensitive sensors that showed a relatively high wavelength sensitivity of 25,000 nm/RIU and a high detection limit [16][17][18]. Wu et al. obtained the highest sensitivity of 134.6 • /RIU by using a graphene layer for the SPR biosensor to enhance the sensitivity caused by the light absorbed [15,19]. L. Wu

Calculation Models and Methods
Z. Lou and other authors provided their synthesized nanoparticle−organic clusters (NOCs) as signal amplification reagents permitting an SPR signal four times higher than that of the sandwich format [36][37][38]. Here, our design presents a proposed structure having a high-sensitivity sensor separately containing PtSe 2 , as shown in Figure 1. In these SPR structures, the Ag or Au film is installed on top of the BK7 coupling prism, and the PtSe 2 layer is the biomolecular recognition element coated on the Ag/Au film surface. The size of the thickness of the PtSe 2 monolayer is 0.375 nm [27]. We have used BK7 glass as a coupling prism. As the first-rank metal for SPP, the Au film or Ag film is chosen at 50 nm. The BK7 glass refractive index can be calculated using the following equation [39]: The refractive index of Ag or Au can be expressed from the Drude-Lorentz model [40]: where λ c and λ p represent the collision and plasma wavelengths. The values of λ c and λ p for Ag are 1.7614 × 10 −5 m and 1.4541 × 10 −7 m, whereas for Au they are 8.9342 × 10 −6 m and 1.6826 × 10 −7 m, respectively. In order to further enhance the sensibility of the designed biochemical sensors, we have covered the metal Ag or Au film surface with PtSe 2 to impede the metal being oxidized. Experimental modulating is used to get the refractive indices as a variable quantity of PtSe 2 , which is composed of real and imaginary parts [41]. The real and imaginary parts of the complex dielectric function of PtSe 2 for a wavelength at 633 nm are exhibited in Figure 2. The expression n s = 1.33 + ∆n is presented as the refractive index in the sensitive medium, and ∆n represents refractive index change in sensitive medium as a result of biological action or a chemical reaction.
installed on top of the BK7 coupling prism, and the PtSe2 layer is the biomolecular recognition element coated on the Ag/Au film surface. The size of the thickness of the PtSe2 monolayer is 0.375 nm [27]. We have used BK7 glass as a coupling prism. As the first-rank metal for SPP, the Au film or Ag film is chosen at 50 nm. The BK7 glass refractive index can be calculated using the following equation [39]: ( ) The refractive index of Ag or Au can be expressed from the Drude-Lorentz model [40]: where λc and λp represent the collision and plasma wavelengths. The values of λc and λp for Ag are 1.7614 × 10 −5 m and 1.4541 × 10 −7 m, whereas for Au they are 8.9342 × 10 −6 m and 1.6826 × 10 −7 m, respectively. In order to further enhance the sensibility of the designed biochemical sensors, we have covered the metal Ag or Au film surface with PtSe2 to impede the metal being oxidized. Experimental modulating is used to get the refractive indices as a variable quantity of PtSe2, which is composed of real and imaginary parts [41]. The real and imaginary parts of the complex dielectric function of PtSe2 for a wavelength at 633 nm are exhibited in Figure 2. The expression ns = 1.33 + Δn is presented as the refractive index in the sensitive medium, and Δn represents refractive index change in sensitive medium as a result of biological action or a chemical reaction.   The transfer matrix method has been used [42] to investigate the reflectivity of the incident TM-polarized light of the N-layer model, as the matrix method is accurate and without approximations. MATLAB was used to compute the analogous SPR modulation. In these PtSe2 biosensors, all layers are piled in the direction of the vertical BK7 glass coupling prism, and each layer is named by the refractive index (nk), thickness (dk), and dielectric constant (εk), respectively. The resonance angle is the minimum reflectance corresponding to the incident angle. The tangential field of the first boundary Z = Z1 = 0 is correlated with the tangential field of the final boundary Z = ZN−1: where V and U represent the components of magnetic and electric fields at the limiting surface. M and k present the characteristic matrix of the composite architecture and the kth layer in the N-layered model, respectively. P-polarized light is made up of: sin cos The transfer matrix method has been used [42] to investigate the reflectivity of the incident TM-polarized light of the N-layer model, as the matrix method is accurate and without approximations. MATLAB was used to compute the analogous SPR modulation. In these PtSe 2 biosensors, all layers are piled in the direction of the vertical BK7 glass coupling prism, and each layer is named by the refractive index (n k ), thickness (d k ), and dielectric constant (ε k ), respectively. The resonance angle is the minimum reflectance corresponding to the incident angle. The tangential field of the first boundary Z = Z 1 = 0 is correlated with the tangential field of the final boundary Z = Z N−1 : where V and U represent the components of magnetic and electric fields at the limiting surface. M and k present the characteristic matrix of the composite architecture and the kth layer in the N-layered model, respectively. P-polarized light is made up of: After mathematical simplifications, we get the p-polarized light N-layer complex reflection coefficient r p , and the corresponding amplitude reflection coefficient (R p ) might be obtained by the square of r p : The transformation of sensing medium in refractive index (∆n) can initiate the alteration of the resonance angle (∆θ), and the sensitivity can be indicated as SR1 = ∆θ/∆n [43].
The detection accuracy (DA) is the ratio in the reflectance curve of shift in resonance angle (∆θ res ) to the full width at half-maximum (FWHM): The figure of merit (FOM) is the ratio in the reflectance curve of sensitivity (S) to the FWHM [44]:

Results and Discussion
Because of the low refractive index of BK7 glass, it was chosen as the coupling prism in these expected biochemical sensors. For the conventional SPR, the Kretschmann geometry biochemical sensor mostly offers a simplex metallic layer to survey SPP, as illustrated in Figure 3. The values of the sensitivity of the conventional Ag or Au metal were 118 • /RIU and 130 • /RIU, as shown in Figure 3a,c, respectively, by using BK7 glass prism. However, this value is not satisfactory for biochemical sensor sensitivity [45,46]. In this paper, we have developed SPR biochemical sensors by using 2D TMDC PtSe 2 materials to enhance the sensitivities of the sensors.
The addition of the 2D TMDC PtSe 2 material between the metallic (Au or Ag) film and sensing medium in the structure of biosensors increased the sensitivities. The results clearly suggest a large improvement in the sensitivities when the PtSe 2 monolayer covers the previous structure, as shown in Figure 3b,d. For the Ag film or Au film, we have obtained sensitivities equal to 162 • /RIU and 165 • /RIU at the layers accumulating 16 and 12 layers, respectively.
In future experiments, 2D PtSe 2 nanosheets can be prepared by the liquid-phase exfoliation (LPE) method, and then the nanosheets will be coated on the Ag or Au thin film (~50 nm) with the BK7 substrate by using the spin coating method [47]. The SPR sensors can be constructed by connecting the PtSe 2 -Ag/Au-BK7 multilayer films and the BK7 prism with the refractive index matching solution. Based on these biochemical sensor results, the 2D TMDC PtSe 2 material might increase sensitivities. The reflectivity of PtSe 2 layers changed with the variation in the incident angle, which is shown in Figure 4a,c, illustrating that sensitivities varied with different layers of PtSe 2 -based biochemical sensors from n s = 1.33 to n s = 1.37 to the sensing medium refractive index. The reflectivity changes with the incident angle are shown in Figure 4b Figure 5a presents distributions of the electric field for the proposed SPR sensors for PtSe 2 material on Ag or Au film, respectively. It has been investigated that coating PtSe 2 on the Ag film might enhance the electric field at the sensor/sensing medium interface, where PtSe 2 gets the maximum electric field strength, since PtSe 2 appears in the middle and is located at the last single layer of PtSe 2 .

Results and Discussion
Because of the low refractive index of BK7 glass, it was chosen as the coupling prism in these expected biochemical sensors. For the conventional SPR, the Kretschmann geometry biochemical sensor mostly offers a simplex metallic layer to survey SPP, as illustrated in Figure 3. The values of the sensitivity of the conventional Ag or Au metal were 118°/RIU and 130°/RIU, as shown in Figure 3a,c, respectively, by using BK7 glass prism. However, this value is not satisfactory for biochemical sensor sensitivity [45,46]. In this paper, we have developed SPR biochemical sensors by using 2D TMDC PtSe2 materials to enhance the sensitivities of the sensors. The addition of the 2D TMDC PtSe2 material between the metallic (Au or Ag) film and sensing medium in the structure of biosensors increased the sensitivities. The results clearly suggest a large improvement in the sensitivities when the PtSe2 monolayer covers the previous structure, as shown in Figure 3b,d. For the Ag film or Au film, we have obtained sensitivities equal to 162°/RIU and 165°/RIU at the layers accumulating 16 and 12 layers, respectively.
In future experiments, 2D PtSe2 nanosheets can be prepared by the liquid-phase exfoliation (LPE) method, and then the nanosheets will be coated on the Ag or Au thin film (~50 nm) with the BK7 substrate by using the spin coating method [47]. The SPR sensors can be constructed by connecting the PtSe2-Ag/Au-BK7 multilayer films and the BK7 prism with the refractive index matching solution.  Based on these biochemical sensor results, the 2D TMDC PtSe2 material might increase sensitivities. The reflectivity of PtSe2 layers changed with the variation in the incident angle, which is shown in Figure 4a Figure 5a presents distributions of the electric field for the proposed SPR sensors for PtSe2 material on Ag or Au film, respectively. It has been investigated that coating PtSe2 on the Ag film might enhance the electric field at the sensor/sensing medium interface, where PtSe2 gets the maximum electric field strength, since PtSe2 appears in the middle and is located at the last single layer of PtSe2.
The changes of sensitivities for PtSe2 on Ag and Au surface biosensors are plotted in Figure 5b. The changes of sensitivities for PtSe 2 on Ag and Au surface biosensors are plotted in Figure 5b. It can be seen that the values of the highest sensitivities of PtSe 2 occur at 162 • /RIU and 165 • /RIU, respectively. Then, the sensitivity curves decrease with an increasing PtSe 2 layer thickness, causing the gold or silver film coating on the PtSe 2 nanosheet overlayer to affect the evanescent electric field distribution, which, in turn, has an impact on the decay length [47]. Figure 5c shows the electric field distributions of 2.0 nm PtSe 2 on Ag film and Au film biochemical sensors, separately. The refractive indices of the inductive media changed within the range of 1.33 to 1.37, and the change in the electric field of graphene/sensing medium interface has been clearly observed. It was found that the designed PtSe 2 -based biochemical sensors are sensitive when detecting small changes in sensitive media. A major change of the surface wave characteristics might occur as a result of small changes in the refractive index of sensitive medium close to the interface, which may produce change in the electric field. Figure 5d gives the change of FOM for this PtSe 2 biosensor. When the thickness of the 2D PtSe 2 material was from 0 to 7 nm, and the change of width was kept from 1 • to 3 • , we calculated the FOM using Equation (11). From this calculation, we can determine that the FOM decreases with the increase of thickness on both Ag and Au films. Table 1 presents the information about change in thickness of PtSe 2 , layer numbers (L), highest sensitivity (S), minimum resonance angle (θ min ) at n s = 1.33, and resonance angle change (∆θ) from n s = 1.330 to n s = 1.335. The smallest reflectivity (R min ), FWHM, and FOM were at n s = 1.33, and DA was at n s = 1.33 to n s = 1.34 in these PtSe 2 biochemical sensors on Ag or Au films. The thicknesses of Ag and Au were set to 50 nm. We set the range of the incident angle from 0 • to 90 • . The thicknesses of PtSe 2 layers came from reference [41], and layer numbers were calculated from the thickness of the PtSe 2 monolayer, which was 0.375 nm. The reflectivity curve steps forward at a higher angle with an increase in the number of PtSe 2 layers. Once the PtSe 2 layers are augmented to the optimized value, maximum sensitivity can be obtained. For PtSe 2 on Ag and Au, the sensitivities achieved maximum values as the layers accumulated to 16 and 12, respectively. As a result of certain restrictions on the angle range, there was a direct relation of the variation of the resonance angle with the sensitivity (i.e., as the variation of the resonance angle lowered, the sensitivity decreased after exceeding the optimized PtSe 2 layers). In the end, we are also aware that most high-sensitivity sensors are accompanied by an increase in SPR linewidth and a reduction in figure of merit (FOM), and a higher FOM usually means a lower sensitivity [48]. Table 1 also lists the FWHM, DA, and FOM data for these PtSe 2 biosensors. Table 1. Change in thickness of PtSe 2 , layer numbers (L), highest sensitivity (S), minimum resonance angle (θ min ) at n s = 1.33, and resonance angle change (∆θ) from n s = 1.330 to n s = 1.335. The smallest reflectivity (R min ), full width at half-maximum (FWHM), and figure of merit (FOM) is at n s = 1.33, and detection accuracy (DA) is at n s = 1.33 to n s = 1.34 for the PtSe 2 SPR biochemical sensors. When the dielectric refractive index change is ∆n, the reflection angle of the prism with a lower refractive index gave a larger result, which means that when the refractive index of the prism is low, a relatively higher sensitivity can be obtained [20]. Therefore, higher sensitivity can be obtained by using a prism with a low refractive index. The refractive indexes of silicon and BK7 were calculated as 3.420 and 1.515 at λ = 633 nm, respectively [15,49]. Therefore, to improve the sensitivity, BK7 was chosen as the coupling prism in the proposed architecture. Table 2 gives the information about the optimized layers of different architectures, changes in resonance angle (∆θ), highest sensitivity (S), and FOM from another two previous papers in our laboratory.
Comparing with these data of graphene, MoS 2 , black phosphorous heterojunction, and diversification architectures [51], PtSe 2 -based biosensors did not have the highest sensitivity. However, this high sensitivity and simple biosensor structure makes it easy to achieve commercial production.

Conclusions
On the basis of the Kretschmann configuration, we have theoretically investigated a new type of biosensor structure with the insertion of a PtSe 2 layer. In this paper, SPR biochemical sensors utilizing 2D TMDC PtSe 2 have been designed and emulated to improve the sensitivities. Moreover, the various characters of PtSe 2 on different metal substrates have been discussed in detail. In order to obtain a high sensitivity, we used BK7 as the coupling prism in this structure because of its low refractive index. Ag or Au films were covered with PtSe 2 to enhance the sensitivities as well as the protective layers. It was found that they enhanced the sensitivities in the advised biochemical sensors, and the sensitivities of PtSe 2 were 162 • /RIU and 165 • /RIU for Ag and Au substrates at PtSe 2 layers 16 and 12, respectively. We are convinced that these kinds of sensors can have potential applications in environmental monitoring, biological detection, medical diagnosis, chemical examination, and so on.