Sensitivity Improvement of Bimetallic Layer-Based SPR Biosensor Using ZnO and Black Phosphorus

In this paper, a brand-novel surface plasmon resonance (SPR) biosensor structure built by bimetallic layers (Ag, Al2O3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Al}}_{{2}} {\text{O}}_{{3}}$$\end{document}), zinc oxide (ZnO), and black phosphorus (BP) is proposed. The incident light wave wavelength is 633 nm. The angular sensitivity (S), detection accuracy (DA), the figure of merit (FoM), and electric field strength of the proposed construction and other constructions are numerically investigated and compared. To optimize the sensor’s performance, the influence of the thickness of each layer on the proposed construction’s performance was simulated. The proposed construction utilizes the large surface-to-volume ratio of ZnO along with the high biomolecule adsorption of BP for sensing performance improvement. It is found that the proposed sensing structure’s highest sensitivity of 400 °/RIU, that is 294.75% higher than the conventional silver-based sensor. This sensing structure might provide ideas for the construction of precision detection suited SPR sensors.


Introduction
Researchers have been paying more and more attention to surface plasmon resonance (SPR) technology recently because of its high sensitivity, low cost, simple of prepare, real-time, and label-free monitoring [1,2].SPR biosensors have practical applications in several fields due to their unique advantages [3,4], including medical diagnosis [5], virus detection [6], environmental monitoring [7], food safety [8], temperature and pressure detection [9], and liquid measurement [10], among others.Metal materials including gold (Au), silver (Ag), copper (Cu), aluminum (Al), and platinum (Pt) are used as inspire precious metals for SPR responses [11,12].Gold has good optical properties, high oxidation, corrosion resistance, and chemical stability [13], so the conventional gold-based SPR biosensor exhibits better sensitivity compared to conventional SPR sensors with other metals.However, an experiment revealed that the characteristic curve of the SPR biosensor with Au has a large full-width half-maximum (FWHM) rendering the accuracy to be inferior.The SPR sensors with Cu or Pt can achieve good detection accuracy, but their sensitivity is substantially low.The performance parameters of silver-based sensors are excellent, and they also have lower manufacturing costs [1].However, conventional SPR sensors typically have poor performances.The sensitivity of SPR sensor can be improved by adding an appropriate metal to the conventional SPR structure [14].Khaled Aliqab et al. designed a biosensor for Pseudomonas bacteria detection based on a double silver level structure, and the maximum sensitivity of this sensor was 370.8°/RIU [15].Using the Ag-Au bimetallic layer, Awadhesh Kumar et al. created a surface plasma resonance biosensor for the precise detection of ssDNA [16].They achieved a maximal angular sensitivity of 203°/RIU.Nur et al. suggested that the sensing configuration composed of Al 2 O 3 over Au layer with transition metal WS 2 subsequently enhanced the sensitivity as high as 227.5 • ∕RIU [17].Maneesh Kumar Singh et al. reported a novel bio- molecule sensor based on nickel-on-copper where the bimetal combination significantly enhances sensitivity [18].In addition, varieties of bimetallic layer configurations were also suggested to improve the sensor's sensitivity.However, the use of bimetallic material layers often leads to a drop in detection accuracy, which requires further research by researchers.
To decrease the detection accuracy loss associated with increased sensitivity, ZnO can be used as a bonding material due to its large oxygen vacancies on the surface supporting a larger surface-to-volume ratio.[19].ZnO has a strong light absorption ability that can further enhance the phenomenon of SPR.Bhishma Karki and her team used the bi-silver layer and ZnO layer to increase sensing performance [1].In addition to acting as a binder layer, ZnO can help SPR sensors reduce the oxidation issue by the active metal layer and improve the sensors' stability in an open environment.ZnO also can blend well with other materials [20].Rajeev Kumar et al. doped Ga in the ZnO layer of an SPR sensor and got a sensitivity of 264.59 • ∕RIU [21].Adding ZnO to the SPR sensor obtains better per- formance, but there is still considerable potential for improvement.As the demand for industrial processes continues to grow, requiring sensors with more excellent performance.
The efficiency of the sensing medium absorption can also cause differences in sensing performance [22].Plating twodimensional (2D) nanomaterial layers on the sensor chip surface increases the sensing medium adsorption [4,23].Black phosphorus (BP) has appeared as the most desirable 2D nanomaterial for photonic sensors [24].Between graphene and gold layers or between antimonene and gold layers, black phosphorus is anisotropic.Alka Verma et al. demonstrate an extremely sensitive sensor structure employing the feature of black phosphorus and achieved 206.26 • ∕RIU [25].BP has a better capacity to adsorb biomolecules compare to graphene and MoS2 [26].Yesudasu Vasimalla et al. reported an SPR biosensor based on the silica-barium titanate framework and BP to achieve 370 • ∕RIU sensitivity [27].Their team also dis- covered that bismuth ferrate-bromide-BP structure increased performance more effectively than bismuth ferrate-bromidegraphene structure [28].Abdulkarem H. M. Almawgani's team proposed an SPR sensor using black phosphorus as a bimolecular recognition element with a sensitivity of 257.3°/RIU [29].Though the 2D nanomaterial can improve the sensor's angular sensitivity, it is challenging to maintain its accuracy and figure of merit.
In this work, we proposed and simulated a novel SPR sensor structure constructed by ZnO, Ag -Al 2 O 3 bimetallic layer, and BP.To improve the sensor's effectiveness while reducing accuracy loss, ZnO was applied to the structure.The layer of black phosphorus material increases the sensor's angular sensitivity.A numerical analysis of the sensor's performance with a 633 nm incident light source.The thicknesses of ZnO, Ag,Al 2 O 3 , and BP were optimized to achieve the best sensing performance.

Optical Properties of Material Layers
Figure 1 illustrates the proposed SPR biosensor configuration using Ag -Al 2 O 3 bimetallic layer.The overall structure is designed based on the Kretschmann structure, and a 633 nm light source is used as the excitation light source.Six layers made up the entire construction.For the connection prism, the BK7 prism is chosen, and the ZnO layer is bonded to the BK7 prism as an absent layer.A bimetallic structure is made of Ag and Al 2 O 3 is present on the ZnO layer's upper side, while Black P is present above the bimetallic structure.The BK7 prism's refractive index has been determined by Eq. ( 1) [13]: The Ag layer's thickness has been adjusted between 35 and 50 nm.Its refractive index is calculated employing the Drude-Lorentz model [16]: where p and c respectively represent for the plasma wavelength and collision wavelength: The thickness of Al 2 O 3 layer is set at 10 to 20 nm.Al 2 O 3 's refractive index changes depending on the incident light's wavelength, and its refractive index is 1.7659 when the wavelength is 633 nm [14].
ZnO has a strong light-absorbing capacity and its refractive index is [6]: (1) A single-layer black phosphorus is 0.54 nm thick.The quantity of layers is indicated by the constant B. We can calculate the refractive index of black phosphorus using Eq. ( 6) [18]: Lastly, the detecting medium layer's refractive index is n s = 1.330 + Δn s and Δn s = 0.007 is the refractive index change caused by biomolecular concentration.

Performance Parameters
Three parameters were applied to assess the performance of the proposed SPR biosensor: sensitivity (S), detection accuracy (DA), and the figure of merit (FoM).The ratio between the shift in resonance angle ( Δ SPR ) with the variation of the medium's refractive index ( Δn S ) is used to determine the sensitivity.The sensitivity is represented by Eq. ( 7) [11]: It is essential to decrease the reflectance curve's FWHM to keep the detection accuracy.Equation ( 8) is used to describe the detection accuracy [12]: The merit factor (FoM) is described as the ratio of sensitivity (S) to FWHM [30]:

Experimental Feasibility Analysis
In this paper, the proposed sensor structure is simulated and numerically analyzed by COMSOL Multiphysics software.The proposed sensor structure can be fabricated by existing technology.The possible implementation methods are as follows: the BK7 coupling prism needs several cleaning sessions with acetone vapor, methanol, and deionized water solution [13].ZnO can be grown on the glass substrate by using the spray-pyrolysis technique [31].The silver layer is deposited by the thermal evaporator system using physical vapor deposition (PVD).The atomic layer deposition method (ALD) can be used to deposit the Al 2 O 3 layer, which needs to be performed below 200 °C [32,33].The ALD method can achieve precise thickness control.The BP nanolayer can be fabricated by chemical vapor deposition (CVD) and then transferred to the Al 2 O 3 layer [24].Finally, the sensor chip is transferred to a BK7 coupling prism.The fabricated sensor (6) n BP = 3.5 + i0.01 structure is placed on a goniometer rotating base.The sensor chip is irradiated with a 633 nm laser source and the position of the laser source is fixed.The angle of the incident light is changed by rotating the goniometer.Making test samples of different refractive indices and using the sensor chip to detect different samples in rotation.The reflected light is detected by a photodetector, and the electrical signal from the photodetector is sent to a lock-in-amplifier for measuring the reflected intensity at different incident angles [34][35][36].

Results and Discussion
In this paper, the ZnO, Al 2 O 3 , and BP are added to the con- ventional silver-based structure.Firstly, the performance of four sensor structures-the standard structure, BK7-Ag-Al 2 O 3 -sensing medium, BK7-ZnO-Ag-Al 2 O 3 -sensing medium, and BK7-ZnO-Ag-Al 2 O 3 -BP-sensing medium has been examined as the refractive index of the sensing medium changes by 0.007 (from 1.330 to 0.007), and their performances have been compared.Next, with the goal to optimize the sensor's performance parameters, we study and evaluate the thickness optimization of the BK7-ZnO-Ag-Al 2 O 3 -BP- sensing medium construction.

Comparative Study of Four SPR Sensor Models
Figure 2 displays the characteristic curves for the four SPR sensor configurations to the detecting medium's refractive index variation as 0.007.Figure 2 a depicts the reflectivity curve of the typical silver-based construction.The resonance angle shift for traditional silver-based construction is 0.95°, and the angular sensitivity is 135.71 • ∕RIU .An efficient method to increase the sensor's angular sensitivity lies in the deposition of high refractive index metal material on the stimulated noble metal layer [37].The bimetallic layer structure in Fig. 2b is created by covering the traditional silverbased structure with an Al 2 O 3 layer.The bimetallic layer sensor's resonance angular shift is 0.55° better than the traditional silver-based construction, and the angular sensitivity is increased to 214.29 • ∕RIU .According to Fig. 2c, structure 3's resonant angular shift is 1.75° and its angular sensitivity is 250 • ∕RIU .As a high RI conductive metal oxide with exceptional crystal quality and structural homogeneity, ZnO can increase the sensitivity of SPR sensors.Besides, ZnO can be employed as an adhesive layer to increase the sensor's precision thanks to its excellent surface-to-volume ratio [38].In the visible region, BP exhibits considerable optical anisotropy and light matter interactions [26,39].The two-dimensional nanomaterial BP was used to increase the biomolecule adsorption capacity.In Fig. 2d, structure 4 has a maximum reflection angle offset as high as 2.8°.Structure 4's sensitivity is 400 • ∕RIU , which is 294.75% higher than the traditional silver-based structure.The suggested sensor's reflected inclination offset is significantly better than the other three structures.The results show that structure 4(BK7-ZnO-Ag-Al 2 O 3 -BP-sensing medium) exhibits the largest S among the four structures mentioned above.
The precise performance metrics for the four sensor architectures are displayed in Fig. 3.The conventional structure has the lowest sensitivity 135.71°/RIU and its minimum reflectance is also higher.Figure 3b shows that the DA of the characteristic curve is found to decrease for all structures.Increasing the sensitivity inevitably brings a decrease in accuracy and quality factor [40]. Structure 2 shows the sensitivity (214.29 • ∕RIU ) with better FoM ( 57.60RIU −1 ).And the corresponding DA and FoM of structure 3 are obtained as 250°/RIU and 66.30 RIU −1 respectively.Although structure 4(BK7-ZnO-Ag-Al 2 O 3 -BP-sensing medium) has a larger FWHM, which makes the accuracy (DA) lower than the other structures.The optimum sensitivity(400 • ∕RIU ) and FoM(79.29 RIU −1 ) are displayed for structure 4. Therefore, the proposed sensor structure can achieve the purpose of improving performance.

The Thickness Optimization of the Ag-Al 2 O 3 Bimetallic Layer
It is discovered that the thickness of bimetallic layer (Ag-Al 2 O 3 ) greatly impacts the sensor's function.To improve the detection effect on the sensing medium, we analyzed and simulated the impacts of Ag 50 nm,

The Thickness Optimization of the ZnO Layer
The change of performance in accordance with the ZnO layer thickness is offered.Figure 6a and b demonstrate that with ZnO configuration (5 to 25 nm), the resonance angle shift increases with the ZnO layer rising to 20 nm, then it starts to decline.The largest resonance angle change occurs when the ZnO layer is 20 nm thick, which is 2.8°.Hence, out of the BK7-ZnO-Ag-Al 2 O 3 -BP-sensing medium construc- tion optimized for the thickness of ZnO layer, the 25 nm ZnO layer shows better DA, smaller S and FoM of the reflection curve.As shown in Fig. 6, the highest sensitivity of Figure 7 shows the DA and FoM at ZnO thicknesses of 5 nm, 10 nm, 15 nm, 20 nm, and 25 nm, respectively.In Fig. 7, the optimum performance parameters for the proposed sensing configuration are maximum FoM of 79.2864 RIU −1 and DA is 0.1982 for the 20 nm ZnO layer.The result indicates that the higher S and FoM can be obtained by applying a 20 nm ZnO layer.

The Optimization of the Black Phosphorus Layer
The 2D nanomaterial of BP as a protective layer has been added to the proposed sensing configuration.It is employed to increase the stability and sensitivity of SPR sensor [8]. Figure 8 displays the characteristic curves for the variation of BP layer at the BK7-ZnO-Ag-Al 2 O 3 -BP- sensing structure.According to Fig. 8, the reflection angle shift for the 3 layers BP configuration is 0.25° and − 0.95° (resonance angle shifts to the right) for the 4 layers BP configuration.For the BK7-ZnO-Ag-Al 2 O 3 -BP-sensing structure, the lower performance is obtained at 3 layers BP configuration and 4 layers BP configuration.The sensitivity around 307.14

Electric Field Strength Analysis
The analyte sensing surface's electric field strength and the intensity of the SPR response are closely related; this connection shows the intensity of the SPR response increases with the electric field strength [41,42].Figure 9 shows the electric field intensity distribution for the conventional SPR sensor structure and BK7-ZnO-Ag-Al 2 O 3 -BP-sensing medium configuration at the sensing medium's refractive index of 1.330.According to Fig. 9, the electric field strength is increased at the interface between the Ag layer and the ZnO layer and reached the maximum intensity at the junction between the BP layer and the sensing medium.At this point, the SPR has absorbed the majority of the incident light's energy, minimizing the reflectivity.Figure 9c and d depict the 1D distribution of the electric field strength for the BK7-ZnO-Ag-Al 2 O 3 -BP-sensing medium struc- ture and the conventional silver-based SPR sensor structure, respectively.The maximal electric field intensity increases from 4.57 × 10 5 V∕m to 4.75 × 10 5 V∕m for the conventional silver-based SPR sensor with ZnO, Ag-Al 2 O 3 and BP.Comparing the suggested sensor struc- ture to the traditional silver-based sensor structure, the electric field intensity is improved.Therefore, the addition of ZnO, Ag-Al 2 O 3 bimetallic layer structure, and BP layer can accomplish a better SPR sensing performance.Table 1 compares the proposed configuration performances with other similar SPR biosensors.Although the DA of the proposed SPR configuration is lower than other configurations, the S and FoM of the proposed configuration are better than those, while it detects precisely.With its excellent parametric performance, this new sensor structure is better suited for sensing applications.

Conclusion
This paper proposed a new SPR sensing configuration using the BK7 prism, ZnO, an Ag-Al 2 O 3 bimetallic layer, and BP.The theoretical research has been considered to explain the impact of layer thickness on sensing performance using the BK7-ZnO-Ag-Al 2 O 3 -BP-sensing medium configuration.The ZnO not only enhances the SPR effect but also reduces the loss of accuracy, due to its large surface-to-volume ratio and strong light absorption capacity.The 2D material black phosphorus not only has unique optical properties but also has high molecular adsorption which can increase the adsorption rate of the sensing medium.The addition of BP significantly increases the sensitivity of the proposed SPR sensor.The proposed sensing configuration obtains a maximum angular sensitivity of 400°/RIU, which is correspondingly 294.75%, 186.67%, and 160% greater than the standard silver-based configuration, the BK7-Ag-Al 2 O 3 -sensing configuration, and the BK7-ZnO-Ag-Al 2 O 3 -sensing con- figuration.The proposed SPR sensor configuration has excellent angular sensitivity and sensing capabilities, and it offers great potential for applications in biochemistry and biomedicine.

Fig. 1
Fig. 1 Schematic diagram of the proposed sensing configuration

Fig. 5 Fig. 6 3 400•
Fig. 5 Performance parameters for different Ag and Al 2 O 3 layer thicknesses.a FWHM and D. b Sensitivity and FoM

Fig. 7 Fig. 8
Fig. 7 Performance parameters for different ZnO thicknesses.a FWHM and sensitivity.b DA and FoM

Fig. 9
Fig. 9 Electric field distribution for the n s = 1.330 .a 2D electric field distribution of traditional silver-based construction SPR sensor.b 2D electric field distribution of structure 4 sensor.c 1D electric field dis- •∕RIU is attained for one layer of BP with corresponding DA and FoM as 0.23 and 70.99 RIU −1 , respectively.It can be observed that the highest S (400 • ∕RIU ) and maximum FoM (79.29 RIU −1 ) are attained for 2 layers of BP.Therefore, two layers of BP materials are used in our proposed sensor structure.