To demonstrate the enhancement in sensitivity performance, the plots between the reflectance of the structure and incidence angle at distinct refractive indexes (1.33 and 1.335) of the sensing medium are shown in Fig. 2. For the first Fig. 2 (a), it indicates the traditional SPR sensor setup, with the absence of both layers of PtSe2 and BP (\(P=0, B=0\)). Our study demonstrated that the reflectance exhibits a sharp dip at a specific angle range due to the stimulation of SPR. This event shows that the stimulating SPR absorbs the incident light in our biosensor setup. Further, with molecules' cooperation, the refractive index of the sensing medium changes with this setup, the change in resonance angle (\(\varDelta \theta =0.576ᵒ\) and sensitivity is calculated (\(S=115.2 ^\circ /\text{R}\text{I}\text{U})\) as in equation 9. Figure 2(b) indicates a single BP layer (\(B=1\)) and no PtSe2 (\(P=0\)); the other parameters were kept constant as in the previous condition. The values for \(\varDelta \theta =0.59ᵒ\) and \(\text{S}= 118 ^\circ /\text{R}\text{I}\text{U}\) are calculated. It is clearly shown that with a single BP layer, the sensitivity increases when compared with Figure 2(a) without BP. The next case is shown by Figure 2 (c) gives the sensitivity and change in resonance angle (\(\varDelta \theta =0.632ᵒ and S=126.4 ^\circ /\text{R}\text{I}\text{U})\) with the presence of PtSe2 layer and absence of BP layer.
Further, the presence of both layers of PtSe2 and BP (\(\text{P}, \text{B}=1\)) is shown in Figure 2(d). With the modification in the reflectance dip, the sensitivity increases to \(0.65ᵒ\) and the change in resonance angle comes out to be 130\(^\circ /\text{R}\text{I}\text{U}\).We can see that the alteration in the resonance angle of the biosensor in our construction is higher than the typical SPR structure due to the addition of the PtSe2 layer and the BP layer. Only a coupling prism and a metal (Ag) layer make up the typical SPR structure. As a result, we may conclude that combining PtSe2 and a BP layer to our SPR sensor improves its sensitivity significantly compared to a standard construction [Figure 2(a)]. The above analysis easily concludes that the sensing layer's refractive index can regulate the biosensor's sensitivity.
With Fig. 3. we also plot the dependence of the biosensor's sensitivity on the sensing layer’s refractive index to understand better the augmentation of the biosensor's sensitivity by PtSe2 and BP layers. The refractive index of the sensing range is taken from 1.33 to 1.38, with 0.005 variations in between. With the increase in refractive index, the sensitivity enhances significantly, for \(P=0,G=0\) the sensitivity varies from \(115.2^\circ /RIU\) to \(153.8^\circ /RIU\) then for \(P=0,B=1\) it varies to \(156.2 ^\circ /RIU\) from \(117.4^\circ /RIU\). In the next case, when \(P=0,B=1\)the sensitivity, further reaches \(180.2 ^\circ /RIU\)maximum. Lastly, for \(P=B=1\) the highest value of sensitivity as achieved as \(187.4^\circ /RIU\). This analysis signifies that after introducing PtSe2 and BP layer to the structure, the sensitivity increases from \(153.8^\circ /RIU\) to \(187.4^\circ /RIU\).
Further, we expand this study to a greater extent by seeing the impact of further addition of PtSe2 and BP layer on the sensitivity of the proposed biosensor. Firstly, after adding the BP layer with constant PtSe2 layer(P=1), its impact is observed on the sensitivity, as Fig. 4(a). Increasing the number of BP layers can improve reflectance. Furthermore, when the number of BP layers increases, a shift in reflectance dip is observed for a larger incidence angle, implying that the biosensor's sensitivity increases. From Fig. 4 (b), the impact of adding PtSe2 layer on biosensor sensitivity is observed keeping one BP layer. It can be seen from the above plots [Fig. 4 (a) and (b)] that the SPR curve dip gets wider with increasing the number of BP and PtSe2 layers. It signifies that the measurement is complicated at a near resonance angle, influencing biosensors' exactness. The prior discussion gives us the concept that the sensitivity increases to a greater extent by increasing the number of layers of BP and PtSe2. The absorption of biomolecules is higher for BP as compared to PtSe2.With the increase of BP layers, its effect on the reflectance is observed, which is greater than the other layer’s case. The upcoming Fig. 5 (a) gives the sensitivity as a function of a number of BP layers. With the increase in the number of BP layers (1 to 9), the maximum sensitivity is found to be \(275.2^\circ /RIU\) with eight BP layers (\(B=8\)). After eight-layer the sensitivity decreases. This decrement in value is because the utilization rate of the light wave decreases with enhancing the number of BP layers.
The next plot [Fig. 5 (b)] gives the impact of the number of PtSe2 layers on biosensor sensitivity. The plot is not following the same trend as in Fig. 5 (a); it decreases after the second layer. The peak value of sensitivity is found to be \(215^\circ /RIU\) at \(P=2\). So, with these discussions, we conclude that the optimization of the BP layer is done followed by the optimization of the PtSe2 layer, and the maximum sensitivity we are getting is \(275.2^\circ /RIU\) (for \(P=1, B=8\)). Using constant angular interrogation, various plots have been drawn for conclusive evidence. Multiple Y-axis parameters (FWHM and minimum reflectivity) with a single X-axis showing sensitivity is plotted for variable BP layers [\(0 to 8\)] with mono (\(P=1\)) layer of PtSe2[Fig. 6 (a)] at RI of sensing layer 1.38. The maximum and minimum value of sensitivity attained was \(180.2 ^\circ /RIU\) and \(275.2^\circ /RIU\) with values of minimum reflectance and FWHM varying from 0.07129 to 0.23796 and 4.73\(^\circ\) to 6.38\(^\circ\) respectively. Fig. 6 (b) shows sensitivity's functional relationship between DA and FoM. The number of layers taken is the same as in the previous case. The highest values obtained with this plot for both parameters [DA and FoM] were 0.21 to 0.16 and 38.1 to 43.1, respectively. It depicts that the electric field distribution at the greatest sensitivity is accurate and sharper, while vice versa in the lowest sensitivity value. With next Fig. 6 (c), the plots for DA and FoM are plotted for biosensor sensitivity. These parameter calculations took constant BP layer (\(B=1\)) and varying PtSe2 layers (0 to 2).
The maximum sensitivity, FoM, and DA were calculated as \(215^\circ /RIU\),36.1, and 0.17. Similarly, with Fig. 6 (d), the maximum values of FWHM and minimum reflectance (\({R}_{min}\)) values come out to be \(5.95^\circ\) and 0.32291.
Figure 7 depicts the relation of electric field distribution inside the sensor with the distance of the prism interface. At the Ag-TiO2 boundary, the electric field intensity increases, further increasing at the TiO2-BP boundary. After the last boundary of BP-sensing media, its value decreases exponentially. It was observed that when light absorption is greater, field intensity enhances. It demonstrates that higher light absorption causes considerable SPW excitation. Figure 7 (a-c) shows the electric field distribution plots for the three thickness cases, D2 of PtSe2, 2 nm,3.3 nm, and 4 nm. With the upcoming Table 2, a comparison is made with the similar previous reported works, and an analysis is made with the help of performance parameters like sensitivity, DA, and FoM. This paper clearly shows that the achieved sensitivity and FoM are higher than earlier works.
With the upcoming Table 2, a comparison is made with the similar previous reported works, and an analysis is made with the help of performance parameters like sensitivity, DA, and FoM. This paper clearly shows that the achieved sensitivity and FoM are higher than earlier works.
Table 2
Comparative survey of present work with earlier reported works
Wavelength
|
Design Configuration
|
Sensitivity
|
DA
|
FoM
|
Reference
|
632.8 nm
|
BK7 + (ZnO, Ag, Au, Graphene)
|
76
|
-
|
13.79
|
[35]
|
632.8 nm
|
BK7+(Au & WSe2)
|
179.32
|
0.17
|
-
|
[36]
|
633 nm
|
BK7 + (Ag + BlueP + MoS2)
|
230.66
|
-
|
34.58
|
[37]
|
633 nm
|
BK7 + (Au, Al2O3, WS2)
|
227.25
|
1.1123
|
28.26
|
[38]
|
633 nm
|
BK7 + (Au + MoS2 + Au + graphene)
|
182
|
-
|
-
|
[39]
|
633 nm
|
BK7 + (Ag/Au, PtSe2)
|
165
|
0.1412
|
14.12
|
[12]
|
633 nm
|
BK7 + (Ag, PtSe2 & BP)
|
275.2
|
0.16
|
43.1
|
Present work
|