Introduction

Chikungunya is an RNA virus belonging to the Togaviridae family’s alphavirus genus. Chikungunya is a Kimakonde word that means “to twist.” In 1952, CHIKV was found for the first time in the United Republic of Tanzania. It was later found in other places in Africa and Asia. In 1967, Thailand and India were the first countries to report urban outbreaks. Since 2004, there have been more and bigger CHIKV outbreaks. CHIKV has been found in over 110 countries in Asia, Africa, Europe, and the Americas. The reverse transcriptase-polymerase chain reaction may be used to identify the Chikungunya virus in early blood samples (RT–PCR). Other assays can reveal chikungunya immune response. After a week of infection, these tests look for viral antibodies. Antibody levels are apparent within a week after sickness starts and can last 2 months [1].

During 2005–2006, there were more than two hundred thousand reported chikungunya cases on Réunion Island in the Indian Ocean, with a case-fatality rate of 1/1000. In 2006, India experienced a major chikungunya outbreak, with more than one million cases reported in 151 districts and ten states [2]. In Ahmedabad, with a population of roughly four million, there were approximately 60,000 suspected cases of chikungunya. During this outbreak, the attack rate (the percentage of a population affected by the disease) reached 45% in some areas. To determine the impact of the pandemic on Ahmedabad, death rates in 2006 were compared to the average number of deaths in the same months during the preceding four years. The analysis found 2944 additional fatalities during the chikungunya pandemic (August–November 2006). Chikungunya has been present in India since 1973, when the attack rate was 37.5%. However, no deaths directly related to chikungunya have been documented in India [3], unlike the Réunion Island outbreak. The most common vectors for transmitting the disease in these areas were Aedes albopictus on Réunion Island and Aedes aegypti in India.

Surface plasmon resonance (SPR) sensors are particular, accurate, and quantifiable, and do not require molecular labeling (label-free). Any target, from tiny molecules like methane and carbon dioxide to microorganisms, may be identified. SPR is an affinity/immunoassay specialist. It employs a planar waveguide to create an evanescent wave, where light is internally reflected. Gold metallization of the waveguide surface interacts with electromagnetic waves at the metal–liquid contact. Light coupling into the metal layer and reduction of reflection intensity are proportional to refractive index variances in the fluid near the metal surface. Continuous monitoring of a refractive index change owing to macromolecular surface binding (e.g., an immunoreaction) is possible. It thus allows immediate, real-time tracking of surface affinity responses. SPR biosensors can detect even minute shifts in the refractive index in areas where biomolecules bind with receptors mounted on an optical transducer’s surface [4, 5]. Prism-based SPR sensors are popular nowadays. A photonic crystal-based biosensor has also been proposed for chikungunya detection. The maximum sensitivity 3.43 × 103 nm/RIU has been reported here [6]. The application of SPR-based biosensors for the detection of pathogens in water has been examined by several researchers [7,8,9].

Platinum diselenide (PtSe2) is a new transition metal dichalcogenide (TMD) family member with exceptional electrical and optical properties. For instance, high carrier mobility electrical qualities have been used in solar cells, and other properties, such as a variable band gap and strong optical absorption, have also been observed [10,11,12,13]. 2D TMDs have the potential to be used in biosensor technology because of their outstanding biocompatibility [12, 14] with the analyte.

This work developed a new SPR sensor structure using a PtSe2 heterostructure together with silicon/ Ag/ and a comprehensive examination of the traditional structure was performed. For sensor analysis, angular interrogation, a method based on attenuated total reflection (ATR), is considered here. The sensitivity (S), detection accuracy (DA), the figure of merit (FOM), quality factor (QF), and other features of the sensor are investigated here in this work and are discussed in the following sections.

Structure of Proposed SPR Sensor

A schematic model of a biosensor based on the Kretschmann configuration is shown in Fig. 1. This proposed SPR biosensor is a five-layer system consisting of a BK7 prism, a silver (Ag) layer, a silicon (Si) layer, and a platinum diselenide (PtSe2) monolayer and sensing medium. P-polarized light with an operating wavelength of 633 nm is directed at the prism/metal interface beyond the critical angle. A photodetector can be used to identify and examine the reflected light.

Fig. 1
figure 1

Schematic diagram of the proposed sensor

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The proposed SPR design parameters are summarized in Table 1.

Table 1 Layer materials with thicknesses and refractive index for proposed work
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The subsequent sections of the document are arranged as follows: Section “Methodology and Considerations” contains theoretical modeling considerations, and mathematical formulas needed to analyze sensor performance. Section “Results and Discussion” of this work describes the results and analysis, while Section “Conclusion” includes the conclusion and discussions.

Methodology and Considerations

At the interface of the metal-dielectric boundary, electrons are stimulated and form a cloud of electrons, known as surface plasmon, that are oscillating at a certain frequency. Attenuated total reflection (ATR) occurs when light travels through a prism with a thin metal film (typically Au or Ag) at angles greater than the critical angle (AIR). Under AIR circumstances, the photon permeates the metal and passes its energy to oscillating electrons, causing SPR. This happens when the incidence wave vector (Ki) matches the surface plasmon wave vector (Kp) [19]. Light alone cannot excite the plasmon. Therefore, it gets extra acceleration from the prism to achieve resonance. When photon energy is transmitted to oscillating electrons, resonance and a drop in reflected light intensity result. The resonance angle or SPR angle is the largest dip in intensity. On the other side, an evanescent wave will arise and disintegrate exponentially. Evanescent waves detect changes in metal’s refractive index (within 200 nm). Binding of the analyte with the sensing medium causes a change in the refractive index on the other side, affecting electron oscillation. This change in plasmon speed modifies the incident light vector needed for SPR, shifting the resonance angle. Monitoring SPR dip angle over time provides a sensor graph of reaction versus time, visualizing binding. Finally, it is worth noting that SPR devices have emerged as promising contenders for the fast detection of chemical and biological components in domains like environmental monitoring. Its appealing characteristics and functionalities suggest it might be important in fostering public health and safeguarding environments.

The suggested biosensor is envisaged to comprise five layers consisting Fig. 1. In this, the sensing medium blood components such as platelets and plasmas must be utilized for the identification of the chikungunya virus. The glass prism refractive index was found based on the equation [17, 20] as follows:

$${RI}_{prism}={(1+\frac{{A}_{1}{\lambda }^{2}}{{\lambda }^{2}-{B}_{1}}+\frac{{A}_{2}{\lambda }^{2}}{{\lambda }^{2}-{B}_{2}}+\frac{{A}_{3}{\lambda }^{2}}{{\lambda }^{2}-{B}_{3}})}^{1/2}$$
(1)

where \({A}_{i}\) and \({B}_{i}\) are the coefficients which are given as \({A}_{1}\) = 0.971247817, \({A}_{2}\) = 0.216901417, \({A}_{3}\) = 0.904651666, \({B}_{1}\) = 0.00472301995, \({B}_{2}\) = 0.0153575612, and \({B}_{3}\)= 168.68133. Here, “λ” is the wavelength of the incident wave (in nm). The wavelength-dependent refractive index of the Silver layer is expressed by the Drude-Lorentz model [21].

$${RI}_{silver}={(1-\frac{{\lambda }_{cw}{\lambda }^{2}}{{\lambda }_{pw}^{2}({\lambda }_{cw}+i\lambda )})}^{1/2}$$
(2)

where \({\lambda }_{cw}\), collision wavelength = 1.7614 × 10−5 m, and \({\lambda }_{pw}\), plasma wavelength = 1.4541 × 10−7 m of Ag. The graphene refractive index can be calculated by [19, 22]:

$${RI}_{g}={(3+\lambda \frac{K}{3}i)}^{1/2}$$
(3)

where K is a constant equal to 5.446 \(\upmu {\mathrm{m}}^{-1}\). The refractive index of Silicon was given by the following relation [20, 23]:

$${RI}_{s}=1+\frac{10.668493{\lambda }^{2}}{{\lambda }^{2}-{(0.301516485)}^{2}}+\frac{0.00304347{\lambda }^{2}}{{\lambda }^{2}-{(1.13475115)}^{2}}+\frac{1.54133408{\lambda }^{2}}{{\lambda }^{2}-{(1104.0)}^{2}}$$
(4)

For the excitation of SP waves, the SP and TM incident light propagation constants must be the same, which may be expressed mathematically as.

$$(\omega /c){({\varepsilon }_{prism})}^{1/2}\;\mathrm{sin}\;{\alpha }_{in}= (\omega /c){({\varepsilon }_{Ag}{\varepsilon }_{d }/({\varepsilon }_{Ag}{+ \varepsilon }_{d }))}^{1/2}$$
(5)

where \({\varepsilon }_{prism},\;\omega,\; {\varepsilon }_{Ag},\;{\alpha }_{in},\; \text{and }{\varepsilon }_{d }\) are the permittivity of the prism, the frequency of the incident wave, the permittivity of the metal, the angle of incidence, and the dielectric permittivity, respectively. Along the z-axis, the various levels of the SPR structure are stacked. The dielectric constant and thickness of each layer are \({\varepsilon }_{K}\; and\; {d}_{K}\) respectively. The tangential field components at the first interface (z = 0) may be expressed mathematically in terms of those at the final interface (z = \({z}_{N-1}\)) as

$$\left[\begin{array}{c}{EF}_{1}\\ {MF}_{1}\end{array}\right]={FL}_{T}\left[\begin{array}{c}{EF}_{N-1}\\ {MF}_{N-1}\end{array}\right]$$
(6)

where \({EF}_{1}\) and \({EF}_{N-1}\) are the first and the last boundaries of tangential electric fields, respectively. \({MF}_{1}\) and \({MF}_{N-1}\) are the first and the last boundaries of tangential magnetic fields, respectively. \({FL}_{T}\) is the transfer matrix that connects the first to the last boundary field. For the jth layer, the characteristic matrix (Cj) is expressed by [24]:

$${C}_{j}=\left[\begin{array}{c}\begin{array}{cc}\mathrm{cos}\;{\phi }_{j}& -i\frac{\mathrm{sin}\;{\phi }_{j}}{{P}_{j}}\\ -{\mathit{iP}}_{j}\mathrm{sin}\;{\phi }_{j}& \mathrm{cos}\;{\phi }_{j}\end{array}\end{array}\right]$$
(7)

\({\phi }_{j}\) is the phase shift of the light wave due to propagation through the jth layer. Its value is

$${\phi }_{j}=\frac{2\pi }{\lambda }{d}_{j}{({\varepsilon }_{j}-{({n}_{1}\;\text{sin}\;{\alpha }_{1})}^{2})}^{1/2}$$
(8)

where \({n}_{1}\) and \({\alpha }_{1}\) are the refractive index and the incident angle of the prism. For TM waves, \({P}_{j}\) is given by:

$${P}_{j}={\left({\varepsilon }_{j}-{\left({n}_{1}\;\text{sin}\;{\alpha }_{1}\right)}^{2}\right)}^\frac{1}{2}/{\varepsilon }_{j}$$
(9)

The system transfer matrix (\({C}_{T}\)) is given as the product of the individual matrices of the layers.

$${C}_{T}={C}_{Ag}{C}_{S}{C}_{G}=\left[\begin{array}{c}\begin{array}{cc}{C}_{11}& {C}_{12}\\ {C}_{21}& {C}_{22}\end{array}\end{array}\right]$$
(10)

The reflection coefficient (\({K}_{r}\)) of the structure is given in terms of the system transfer matrix \({C}_{ij}\) as [25]

$${K}_{r}=\frac{\left({C}_{11}+{C}_{11}{P}_{N}\right){P}_{1}-({C}_{21}+{C}_{22}{P}_{N})}{\left({C}_{11}+{C}_{11}{P}_{N}\right){P}_{1}+({C}_{21}+{C}_{22}{P}_{N})}$$
(11)

The reflection intensity (R) of the proposed system is expressed as [26]

$$R=r.{r}^{*}={\left|r\right|}^{2}$$
(12)

Sensor sensing capability of biosensor computed by this parameter sensitivity (S) [27]. Mathematically sensitivity is expressed as \(\mathrm{S}=\frac{{\Delta\uptheta }_{\mathrm{SPR}}}{{\mathrm{\delta n}}_{\mathrm{s}}}\); its unit is \({\mathrm{DegRIU}}^{-1}\). The notations \({\Delta\uptheta }_{\mathrm{SPR}}\) and \({\mathrm{\delta n}}_{\mathrm{s}}\) signify the variation in SPR angle at resonance and change in the RI of the sensing medium.

FWHM is the SPR curve width at half of the reflectance, and expressed as: FWHM = \({\uptheta }_{\mathrm{b}}- {\uptheta }_{\mathrm{a}}\), unit is in Deg [28]. The detection accuracy (DA) parameter calculates the sensor’s exactness. Numerically expressed by [29]: \(\mathrm{DA}={FWHM}^{-1}\) (expressed in \({\mathrm{Deg}}^{-1}\)). Another parameter that calculates the quality of the sensor is the figure of merit (FOM) or quality factor (Q). Mathematically expressed by [30]: \(FOM=S . DA\), its unit is RIU−1.

Results and Discussion

This section of the manuscript shows the results and discussions of the proposed simulation study. Figure 2 gives the reflectance spectra for the different scenarios of layers of silicon (Si) and PtSe2 (Pt). The computed parameters such as SPR angle (θ), minimum reflectance (Rmin), sensitivity, FWHM, DA, and FOM for (a) [Si = 0, Pt = 0], (b) [Si = 0, Pt = 1], (c) [Si = 1, Pt = 0], and (d) [Si = 1, Pt = 1] are given in Table 2.

Fig. 2
figure 2

Reflectance plots for different incidence angles for a Si = 0, Pt = 0; b Si = 0, Pt = 1; c Si = 1, Pt = 0; and d Si = 1, Pt = 1

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Table 2 Performance parameters calculated for Silicon and PtSe2 layers
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For the conventional SPR setup, i.e., Prism/Ag/Sensing Medium (Si = 0, Pt = 0), the angular sensitivity was computed to be 169.1 deg/RIU maximum. After that, inserting a mono layer of PtSe2 with no silicon layer (Si = 0, Pt = 1) in between the sensing medium and Ag layer, the highest sensitivity enhanced to 174.4 deg/RIU. Furthermore, for Si = 1, and Pt = 0, the maximum value of angular sensitivity varies to 239.7 deg/RIU. For single silicon and single PtSe2 layer (Si = 1, Pt = 1) (Fig. 2d), the maximum computed value for angular sensitivity attained is 257.3 deg/RIU. So, these results show that after the addition of both Si and Pt layers over the conventional design, the sensitivity is enhanced due to maximum optical light absorption with the presence of silicon and PtSe2.

Silver Layer Optimization

The angular sensitivity gets calculated for three different layer thicknesses of Ag (45 nm, 50 nm, and 55 nm). The RI considered here is 1.33 (infected plasma), 1.35 (normal plasma), 1.38 (infected plasma), and 1.39 (normal plasma). The computed sensitivity value maximum for 55 nm of thickness attained is 269.3 deg/RIU for normal platelets. Therefore, the optimized thickness of the Ag layer 55nm has been considered (Fig. 3).

Fig. 3
figure 3

Variation in angular sensitivity for different Ag thicknesses

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The values of angular sensitivity by altering the number of silicon and PtSe2 layers have been calculated (Table 3). As the number of silicon and PtSe2 layers increases, the absorbance of the optical signal has been increased, and the dip on the resonance is also increased, ultimately increasing the sensitivity of the sensor. The number of silicon and PtSe2 has been increased by more than three layers; the sensitivity decreases. In the three layers of silicon and PtSe2, we attained the maximum sensitivity of 287.3 and 279.9 in deg/RIU for both plasma cases. Figure 4a implies that the sensitivity can be further enhanced by increasing the silicon layers (1 to 3) because of the change in the reflectance dip to a greater incidence angle. We also observed that the same trend is observed by enhancing the PtSe2 layers (PtSe2 = 1 to 3) also with a single silicon layer (Fig. 4b).

Table 3 Sensitivity calculations for the number of silicon layers and PtSe2 layers for platelets and plasma cases
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Fig. 4
figure 4

Sensitivity as a function of the number of a silicon layers and b PtSe2 layers

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Furthermore, the comparison with other SPR-based sensors reported earlier is shown here. Table 4 shows that the proposed study’s sensitivity parameters are more comparable to other studies in the reported literature.

Table 4 Comparison of sensitivity parameter values of current SPR work with latest SPR works
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Conclusion

An RNA-based virus called the chikungunya virus infects mosquitoes which bite humans. The most typical signs of this infection include fever, joint swelling, discomfort, and muscle soreness. With the advantages like label-free sensing, real-time detection, and low cost, SPR sensors are popular sensing techniques. Here we proposed a layered heterostructure architecture of SPR sensors consisting of BK7 prism (coupling)/Ag (metal)/silicon/PtSe2(TMD)/sensing medium (analyte) for the early detection of chikungunya virus. The analysis of the sensor has been carried out using TMM. The thickness of the Ag layer has been optimized. The optimum thickness of Ag layer 55 nm has been taken. Optimizing the layers has been done to enhance the sensor’s sensitivity. For three silicon layers and one PtSe2 layer, the highest sensitivity has been calculated as 287.3 deg/RIU for the case of plasma. The four types of blood cells have been analyzed using the proposed SPR sensor.