Sensitivity enhancement of a bimetallic surface plasmon resonance biosensor
محورهای موضوعی : فصلنامه نانوساختارهای اپتوالکترونیکی
Hassan Zahmatkeshan
1
,
Mohammad Javad Karimi
2
,
Mojtaba Sadeghi
3
,
Zahra Adelpour
4
1 - Department of Electrical Engineering, Shi.C., Islamic Azad University, Shiraz, Iran
2 - Department of Physics, Shiraz University of Technology, Shiraz, Iran
3 - Department of Electrical Engineering, Shi.C., Islamic Azad University, Shiraz, Iran
4 - Department of Electrical Engineering, Shi.C., Islamic Azad University, Shiraz, Iran
کلید واژه: Biosensor, Sensitivity, Surface plasmon resonance,
چکیده مقاله :
In this study, a plasmonic biosensor with a kreschmann configuration is evaluated by changing the sensing medium's refractive index from 1.330 to 1.335, which includes BK7, gold/silver, silicon, nickel, hexagonal boron nitride, black phosphorus/transition-metal dichalcogenides and sensing medium layers. The sensitivity, figure of merit, quality factor and detection accuracy are the biosensor performance characteristics and are checked at the 633 nm wavelength. The effects of gold and silver layers on the transition-metal dichalcogenides and black phosphorus layers are investigated separately and their performance parameters have been calculated numerically. Since the highest sensitivity is related to the Ag metal with the BP layer, the minimum reflectance and sensitivity as a function of the thickness and number of layers for this structure are examined. The sensitivity of the proposed biosensor (504 deg.RIU-1) is approximately 1.5 times higher than the highest sensitivity reported in comparable studies.
In this study, a plasmonic biosensor with a kreschmann configuration is evaluated by changing the sensing medium's refractive index from 1.330 to 1.335, which includes BK7, gold/silver, silicon, nickel, hexagonal boron nitride, black phosphorus/transition-metal dichalcogenides and sensing medium layers. The sensitivity, figure of merit, quality factor and detection accuracy are the biosensor performance characteristics and are checked at the 633 nm wavelength. The effects of gold and silver layers on the transition-metal dichalcogenides and black phosphorus layers are investigated separately and their performance parameters have been calculated numerically. Since the highest sensitivity is related to the Ag metal with the BP layer, the minimum reflectance and sensitivity as a function of the thickness and number of layers for this structure are examined. The sensitivity of the proposed biosensor (504 deg.RIU-1) is approximately 1.5 times higher than the highest sensitivity reported in comparable studies.
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Islamic Azad University
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Volume 10, Issue 2, Summer 2025, Page 71-83 |
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Research Paper | |
Sensitivity enhancement of a bimetallic surface plasmon resonance biosensor
Hassan Zahmatkeshan1, Mohammad Javad Karimi *2, Mojtaba Sadeghi1, Zahra Adelpour1 1 Department of Electrical Engineering, Shi.C., Islamic Azad University, Shiraz, Iran 2 Department of Physics, Shiraz University of Technology, Shiraz, Iran
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Revised: 2025.08.11 Accepted: 2025.08.14 Published: 2025.08.25
| Abstract: In this study, a plasmonic biosensor with a kreschmann configuration is evaluated by changing the sensing medium's refractive index from 1.330 to 1.335, which includes BK7, gold/silver, silicon, nickel, hexagonal boron nitride, black phosphorus/transition-metal dichalcogenides and sensing medium layers. The sensitivity, figure of merit, quality factor and detection accuracy are the biosensor performance characteristics and are checked at the 633 nm wavelength. The effects of gold and silver layers on the transition-metal dichalcogenides and black phosphorus layers are investigated separately and their performance parameters have been calculated numerically. Since the highest sensitivity is related to the Ag metal with the BP layer, the minimum reflectance and sensitivity as a function of the thickness and number of layers for this structure are examined. The sensitivity of the proposed biosensor (504 deg.RIU-1) is approximately 1.5 times higher than the highest sensitivity reported in comparable studies. |
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Keywords: Biosensor, Sensitivity, Surface plasmon resonance | |
1. INTRODUCTION
Plasmonics plays a significant role in the design of biosensors [1] and photovoltaic cells [2]. Surface plasmon resonance (SPR) biosensors can detect biomaterials, making them valuable for medical diagnostics [3] and food safety [4] applications. These biosensors can identify analytes with minimal changes in their refractive index (RI) [5], achieving high accuracy [6] and sensitivity [7] in real-time [8]. Additionally, they are cost-effective [9] and do not require professional personnel for operation [10].
The sensing medium's (SM) RI changes leads to the resonance angle shifts [11]. In SPR sensors that utilize multilayer structures, the Kretschmann [12] and Otto [13] configurations are commonly employed. A thin air gap separates the metal layer from the prism in the Otto configuration, while in the Kretschmann geometry, the prism and the metal layer are in direct contact. Due to the coupling of transverse magnetic (TM) waves or p-polarized incident light with the free electrons on the metal surface, surface plasmons are excited at the metal-dielectric interface [13,14]. Silver (Ag) [15], gold (Au) [16], copper (Cu) [17], aluminum (Al) [18], and nickel (Ni) [19] are recognized as materials with plasmonic properties that can enhance surface plasmon signals in sensors [20].
Conventional sensors sensitivity, comprising a prism, metal layer, and SM [21], achieve a peak reported value of 116 deg RIU-1 [22], which is due to the weak adhesion of the metal layer to the SM [23] resulting in very low sensitivity [24]. Recently, to enhance the sensitivity of these sensors, two-dimensional (2D) nanomaterials such as black phosphorus (BP) [25], graphen (Gr) [5], Mxene [22], transition-metal dichalcogenides (TMDCs) [26], and hexagonal boron nitride (h-BN) [27] are used between the metal layer and the SM. Also, using an antireflection layer like MgF2 enhances light absorption in the biosensor, increasing sensitivity [28]. The general form of TMDCs is MX2 where M represents a metal of transition such as Tungsten (W) and Molybdenum (Mo), and X shows materials of chalcogen like Sulphur (S) and Selenium (Se) [29]. These materials are molybdenum disulfide (MoS2), tungsten disulfide (WS2) molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2).
The use of 2D nanomaterials in SPR sensors, biosensors, and solar cells has grown recently. For instance, Panda et al. designed a plasmonic biosensor for malaria pathogen detection, utilizing a CaF2 prism, titanium oxide, Ag, platinum diselenide, WS2, and SM layers, achieving a maximum sensitivity of 240.10 deg. RIU-1 [30]. Similarly, Kumar et al. analyzed a biosensor for carcinoembryonic antigen (CEA) detection, composed of a prism of BK7, titanium, Ag, MoS2, Gr, and SM layers, reporting a peak sensitivity of 144.72 deg. RIU-1 [31]. Daher et al. investigated a BAK1 prism-based structure with Ag, bismuth ferrite, BP, and SM layers, yielding a 358 deg. RIU-1 sensitivity [32]. Furthermore, Kalpana et al. presented an SPR sensor for colorectal detection, incorporating a CaF2 prism, Ag, MXene, h-BN, BP, and SM layers, and achieving a 315 deg. RIU-1 sensitivity [27].
This study investigates a biosensor of SPR with a prism of BK7 and layers of Ag, Si, Ni, h-BN, BP, and SM. We also explore substituting Au for Ag and TMDCs for BP. Ag offers a cost-effective alternative to Au with comparable accuracy [33]. A narrower full width at half maximum (FWHM) in the reflectance curve shows superior sensor performance. Incorporating a high refractive index Si layer (BK7/Ag/Si/SM) enhances sensitivity compared to the conventional BK7/Ag/SM sensor [11]. The h-BN layer's high-temperature stability and chemical resistance make it suitable for SPR sensors. BP, with its 0.53 nm thickness and desirable bio-recognition element (BRE) properties [27], effectively captures biomolecules [34] due to its high surface-to-volume ratio [35], bandgap of tunable [24], and low thermal conductivity [36], which further increases sensitivity.
The biosensor's performance, comprising sensitivity (S), detection accuracy (DA), quality factor (QF), and figure of merit (FOM), was evaluated using attenuated total reflection (ATR) at the visible 633 nm wavelength (TM-polarized Helium-Neon laser).
2. Design and modeling
Figure 1 depicts a biosensor of SPR with seven layers designed in a Kretschmann geometry.
Fig. 1. The proposed SPR biosensor
|
The coupling of light to the surface plasmon polaritons (SPPs) occurs through the BK7 prism, and its RI is given by [37,38].
where 
is the incident light wavelength, and 
= 1.03961212, 
= 0.231792344, 
= 1.0104694, 
= 0.00600069867, 
= 0.0200179144, and 
= 103.560653.
The RI of the metal layers is presented by the Drude‒Lorentz model as follows [39]:
The parameters of wavelength of plasma (
) and wavelength of collision (
) for Ag, Au and Ni are listed in Table 1.
Table1. 
and 
for Ag, Au and Ni at the wavelength of 633 nm
|
| Metal | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1.7614 | 1.4541 | Ag | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
8.9342 | 1.6826 | Au | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
2.8409 | 2.5381 | Ni |
Monolayer thickness (nm) | RI (λ=633 nm) | 2D nanomaterials |
1 | 1.65 | h-BN |
0.53 | 3.5+0.01i | BP |
0.65 | 5.0805+1.1723i | MoS2 |
0.70 | 4.6226+1.0063i | MoSe2 |
0.80 | 4.8937+0.3124i | WS2 |
0.70 | 4.5501+0.4332i | WSe2 |
The intensity of reflected light is calculated using the transfer matrix method (TMM), as given by [37],
In the above equations, 
, 
, 
, and 
are the incident angle, RI of the prism, permittivity and thickness of the k-th layer, respectively.
The amplitude reflection coefficient is 
:
And the reflectance coefficient:
Biosensor performance characteristics are 
, 
, DA, QF, and the FOM. The sensitivity () is given by,
where 
is the shift in the resonance angle, and 
is the RI change of the SM, respectively. The resonance angle (
) is the angle at which the dip appears in the reflectance spectrum. Additionally, the reflectance at the bottom of the dip, denoted as 
, represents the minimum value of the reflectance. 
, 
, and the 
are defined as follows:
3. RESULTS AND DISCUSSION
We studied an SPR biosensor comprising a prism of BK7, Ag/Au, Si, Ni, h-BN, BP/TMDCs, and SM layers. For these ten structures (two metals (Ag and Au) and five 2D material (BP, MoS2, MoSe2, Ws2, WSe2)), the effects of layer thickness (
) and the number of layers (L) on the performance characteristics of the biosensor are analyzed. The three-layer structure consisting of Si, Ni, and h-BN is present in all ten examined configurations.
Table 3 lists the optimal layer thicknesses for maximum biosensor sensitivity. For example, row one of the Table denotes that the optimal thicknesses and the number of layer for the Au/ Si/Ni/ h-BN/ BP structure are 16 nm/ 5 nm/ 39 nm/ 1 L/ 2L, respectively. The comparison of the rows in the table indicates that structures containing silver exhibit greater sensitivity than those that include gold. Additionally, structures incorporating BP show higher sensitivity compared to TMDC structures. Among the TMDCs, the structures made of Tungsten provides greater sensitivity for the sensor.
Table 3: The performance parameters (S, FOM, QF, DA & Rmin) of the designed structures with Ag/Au (column 1) and 2D nanomaterials of BP/TMDCs (column 3).
| DA (deg.-1) | QF (RIU-1) | FOM (RIU-1) | S (deg.RIU-1) | Type and number of layers | Si(nm)/Ni(nm)/h-BN(L) | Type and thickness (nm) |
0.0127 | 0.1655 | 81.44 | 80.40 | 492 | BP: 2 | 5/39/1 | Au: 16 |
0.0149 | 0.1020 | 29.38 | 28.95 | 288 | MoS2: 1 | 5/38/1 | Au: 10 |
0.0092 | 0.1071 | 32.11 | 31.82 | 300 | MoSe2: 1 | 5/39/1 | Au: 10 |
0.0418 | 0.1185 | 43.60 | 41.78 | 368 | WS2: 1 | 5/39/1 | Au: 7 |
0.0002 | 0.1239 | 47.31 | 47.31 | 382 | WSe2: 1 | 5/39/3 | Au: 8 |
0.0021 | 0.1672 | 84.24 | 84.06 | 504 | BP: 2 | 5/36/2 | Ag: 16 |
0.0305 | 0.1041 | 30.18 | 29.26 | 290 | MoS2: 1 | 5/39/1 | Ag: 10 |
0.0091 | 0.1082 | 32.46 | 32.16 | 300 | MoSe2: 1 | 5/39/1 | Ag: 10 |
0.0003 | 0.1279 | 50.64 | 50.62 | 396 | WS2: 1 | 5/39/1 | Ag: 10 |
0.0005 | 0.1237 | 47.49 | 47.46 | 384 | WSe2: 1 | 5/39/3 | Ag: 8 |
Overall, Table 3 indicates that the optimized biosensor is the BK7/Ag/Si/Ni/ h-BN/ BP structure with performance parameters; S (504 deg. RIU-1), FOM (84.06 RIU-1), QF (84.24 RIU-1) and DA (0.1672 deg.-1). The corresponding optimal thicknesses and number of layers are Ag (16 nm), Si (5 nm), Ni (36 nm), h-BN (2 layers), and BP (2 layers). Therefore, we focused on the BK7/Ag/Si/Ni/h-BN/BP/SM structure and examined four layout scenarios; i) Ag/Si, ii) Ag/Si/Ni, iii) Ag/Si/Ni/h-BN and iv) Ag/Si/Ni/h-BN/BP. Figure 2 illustrates the SPR reflectance curve for these configurations. This figure shows that adding layers increases both the depth and angular shift of the reflectance dip, enhancing the sensor's operational parameters. Additionally, Table 4 presents the thickness and number of layers and performance parameters for these configurations. The table demonstrates that adding a Ni layer to the first structure, h-BN to the second, and BP to the third enhances sensitivity. The fourth structure's sensitivity (504 deg RIU-1) is 100% higher than the second structure's (250 deg RIU-1).
.
Fig. 2. The reflectance curve for (a) Ag/Si, (b) Ag/Si/Ni, (c) Ag/Si/Ni/h-BN, (d) Ag/Si/Ni/h-BN/BP structures
.
Table 4. The thickness, number of layers, and performance parameters for (i) Ag/Si, (ii) Ag/Si/Ni, (iii) Ag/Si/Ni/h-BN, (iv) Ag/Si/Ni/h-BN/BP structures
Structure | Ag (nm) | Si (nm) | Ni | h-BN | BP | S deg. RIU-1 | FOM RIU-1 | QF RIU-1 | DA deg.-1 |
|
i | 50 | 5 | - | - | - | 204 | 62.11 | 64.06 | 0.3140 | 0.0304 |
ii | 20 | 5 | 39 | - | - | 250 | 64.14 | 68.31 | 0.2733 | 0.0610 |
iii | 20 | 5 | 39 | 3 L | - | 328 | 71.88 | 72.84 | 0.2221 | 0.0133 |
iv | 16 | 5 | 36 | 2 L | 2 L | 504 | 84.06 | 84.24 | 0.1672 | 0.0021 |
Ref. |
| DA deg.-1 | QF RIU-1 | S deg.RIU-1 | SPR sensor configuration |
[11] | - | 0.1700 | 52 | 305 | BK7/Ag/Si/Franckeite |
[27] | - | 0.1660 | 52.34 | 315 | CaF2/Ag/MXene/h-BN/BP |
[32] | - | - | - | 358 | BAK1/Ag/BiFeO3/BP |
[40] | 0.480 | 0.3072 | 97.07 | 316 | CaF2/Ag/BaTiO3/Ni/MXene |
This work | 0.012 | 0.1655 | 81.44 | 492 | BK7/Au/Si/Ni/h-BN/BP |
This work | 0.002 | 0.1672 | 84.24 | 504 | BK7/Ag/Si/Ni/h-BN/BP |
*Corresponding author: Mohammad Javad Karimi Address: Department of Physics, Shiraz University of Technology, Shiraz, Iran Email: karimi@sutech.ac.ir DOI: https://doi.org/10.71577/jopn.2025.1203950 |
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