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Abstract
In this manuscript, an immense sensitive single-core double-slotted square-shaped SPR biosensor externally coated with gold nano films is designed and critically analyzed. The sensor shows a wide range of sensing capacity between 1.41 to 1.49 RIU. Two circular slots are constructed to hold more analytes inside the slots. The sensor shows extremely sensitive behavior with the highest sensitivity of 36000nm/RIU with the maximum amplitude sensitivity of 1380 RIU−1. The maximum resolution of the sensor is 9×10−5 RIU. The sensor parameters are optimized in COMSOL Multiphysics software by observing fabrication tolerance using percentage variations of parameters. The experimental data of the proposed sensor also shows good polynomial fitting and a high figure of merit (FOM). Because of the newer structure, high sensitivity, large detection range, high stability and high resolution the proposed sensor can be a tough candidate in the realm of bio-sensing and biochemical applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In today's technology-based life, biosensors have become a fundamental element to detect biological samples very sharply. In this circumstance, surface plasmon resonance (SPR) has become the most promising technique to detect biological [1,2] and biochemical [3–6] samples. SPR based sensors are broadly used for virus detection [7,8], glucose sensing [9,10], blood group detection [11], antigen-antibody interaction [11], gas sensing [11], water testing [12], food quality measurements [13,14], environment testing and monitoring [15], bio imaging [16], cancer cell detection [17], telemedicine [11], biomedical [18] and biochemical applications etc. [11].
SPR is a mechanism that occurs at metal surfaces when the light beams penetrate from the fiber core to cladding and hit the metal surface releasing free electrons [19]. When the frequencies of the incident light beams and the released electrons coincide with one another, the surface plasmon wave (SPW) is generated [20]. By using this optical characteristic, biosensors detect the biological samples [21].
The theoretical background of SPR was first introduced in 1950 and in 1957 the surface plasmon (SP) was introduced by Rithie et al [22]. Later in 1968 by Kretschmann and Otto introduced that the optical excitation SPW has been basically two major ways like Attenuated Total Reflection (ATR) in prism coupler based structures and diffraction at diffraction grating [23,24]. Then everyone tried to apply this technique of SPR. So in 1983 based on an attenuated total reflection method, the application of SPW had been introduced by Nylander and Liedberg [25]. Prism coupler-based structures are in bulky size and large mechanical setup process [26]. Cosmic expansion had been created after the fundamental idea of Photonic crystal fiber (PCF) given by Yeh et al. in 1978 [27] and invented 2D PCF by P. Russel et al. in 1992 [28].
PCF has updated techniques over conventional optical fiber where air holes act as cladding regions with lower and core material as fused silica for higher density medium and also used the plasmonic material for creating SPR phenomenon [29]. PCF provides the facilities independent of design structure, lightweight and microstructured in size and, highly sensing performance with broad detection range [30]. Since the light is desolated on the fiber core then created the evanescent pole polarized light that hit the plasmonic material and created Surface Plasmon Wave (SPW) [20]. This generation of SPW is dependent on the surrounding medium refractive index (RI) with the function of wavelength [31]. For particular RI at constant frequency maximum energy of core to surface plasmon polariton mode (SPP) is transferred and it is called resonance condition [32]. To interrogate this wavelength for particular RI we detect the unknown biological sample with very fast and smart response by using this PCF based SPR biosensor [33].
From the development of PCF based SPR biosensors, we found two types of sensing approaches [34]. Those are the internal and external sensing responses. Internal sensing approach or it is called nanowire [35–37] where the analyte layer and plasmonic material layer are placed internally. Another sensing approach is external where the sensing layer is placed externally [30]. For improving the sensing performance different geometric structures of PCF have been found like hexagonal [20,31,32], circular lattice [33,34], spiral lattice [38–40], D-shape [41–47 ,44–49], micro channel [44–56], slotted PCF [55–58] and various alphabetic shape (U shape, V shape, H shape and X shape etc.) [59,60]. The sensing performance is greatly affected by the use of plasmonic material. Various plasmonic materials have been introduced like gold, silver, copper, aluminum, indium tin oxide (ITO) [46], Titanium oxide (TiO2) [40] and silicon nitride (Si3N4) [48]. Among them silver shows more sharp and narrow resonance peaks but has the oxidation problem so another metal layer like TiO2 [40], Si3N4 [48] is used to remove the oxidation problem but those bimetal structures create fabrication complexity. So gold is the most promising plasmonic material because of its chemical and temperature stability.
Because of rising fabrication difficulty for internal sensing approaches, slotted external sensing approaches PCF based SPR sensor has become most popular in recent times. Akter et al. proposed a biosensor in visible to near infrared wavelength which is dual open channel that shows the maximum wavelength (WS) is 5000 nm/RIU with amplitude sensitivity (AS) only 396 RIU−1 [54]. Z. Yang et al. introduced a newer structure that is a concave square shape PCF based SPR sensor that is very low loss and shows maximum WS 10700 nm/RIU [61]. H. Han et al. designed an alphabetic H-shaped PCF based SPR biosensor with a large detection range that showed maximum WS 25900 nm/RIU [59]. Sakib et al. demonstrated a novel approach circularly slotted PCF based SPR sensor that showed maximum WS 16000 nm/RIU with AS 780 RIU−1 [57]. H. Sarker et al. expressed a newly structured like rectangular shape air holes slotted PCF based SPR sensor that showed a maximum WS 22000 nm/RIU with AS 1782 RIU−1 [58]. M. T. Rahman et al. propounded a micro channel circular slotted gold coated PCF based SPR sensor that showed maximum WS 25000 nm/RIU and AS 1897 per RIU [55].
Because of the above mentioned proposed sensor having two or more slots structured so its arising fabrication difficulty. In these circumstances, we propose a simple dual slot single core, a wide range highly sensitive sensor that shows maximum WS of 36000 nm per RIU with the maximum amplitude sensitivity of 1380 RIU−1. The proposed sensor also shows good cure fitting characteristics of resonance wavelength and high figure of merit (FOM). For high sensitivity, large detection range, high stability and high resolution the proposed sensor can be a promising candidate for the purpose of bio-sensing application.
2. Structural description of the proposed sensor with theoretical background
Figure 1(a) represents the cross sectional transverse view of our proposed square-structured PCF based SPR sensor.
Fig. 1. (a) Schematic cross sectional view of the proposed square structure PCF based SPF sensor with Λ= 2$\mathrm{\mu}$ m, d1 = 0.25µm, d2 = 0.50µm, d3 = 0.60µm and tg = 30nm, (b) Stacked preform of the proposed sensor and (c) 3D view of the proposed PCF.
Figure 1(b) represents the stacked preform of the proposed sensor and Fig. 1(c) represents the 3D view of the proposed PCF. This sensor consists of inner hexagonal ring of air holes and the center air hole is deleted to form a fiber core and confine the light in the core. The hexagonal shaped ring is made of six air holes. The distance between air holes 1, 2, 3 and 4 in figure is same and the distance of the air holes 5, 6, 7, 8, 9, 10, 11 and 12 is same from the core.
This proposed structure is called square structure because the air hole arrangement is like a square structure sensor. Square structure is created by dividing the structure into four parts and each part is similar. Those are the air hole no. 1, 5, 6 and inner one hexagonal ring air hole diameter d2, the air hole no. 2, 7, 8 and inner one hexagonal ring air hole diameter d2, air hole no. 3, 9, 10 and inner one hexagonal ring air hole diameter d3 and finally the air hole no. 11, 12, 4 and inner air hole diameter d3.
Two air holes in the first hexagonal ring are omitted to create a more evanescent field. Two air holes are kept small in this ring in the opposite direction of the y axis near the slot to reduce the distance between the plasmonic material and the core. For this reason, evanescent fields can easily strike the metal and produce more free electrons. We used two slots with a gold layer in the cladding because it has no oxidation problem and it is chemically inactive. Slotted design is more beneficial because it needs only a small amount of gold rather than full external coating. The first hexagonal air hole ring is formed by constructing the air holes apart 45 degrees from each other and second one are formed by constructing the air holes apart from 45 degrees from each other except the ring which is used as a slot.
There are three different air holes whose diameter is denoted d1, d2 and d3 respectively where d1 = 0.25µm, d2 = 0.50µm and d3 = 0.60µm. The distance between center to center of the air hole is called pitch and represented as Λ. The optimum value of the pitch is found out $2\mathrm{\mu} m$ by varying the pitch with other structural parameters remains constant. The distance between core and the small air hole diameter d1 in the x-axis which helps to create birefringence is 1.75Λ. The thickness of the gold layer, analyte layer and PML layer is denoted by tg, t1 and t2 respectively where tg= 30nm, t1=$0.8\mathrm{\mu} m$ and t2=$1\mathrm{\mu} m$. The radius of the analyte and PML layer 2.56Λ and 3.06Λ where the PML layer is used to absorb surface radiation. We set a boundary condition to remove the back scattering. These all parameters are collected by performing numerical analysis and simulation using COMSOL Multiphysics Software.
There are different types of PCF fabrication methods such as stack and draw, extrusion, sol–gel casting, injection modeling and drilling etc. Among them, the stack and draw method is a very easy and common method which is very fast, clean and flexible [62]. Figure 1(b) represents the stacked preform structure of the proposed sensor. Here solid rod is used for the structure of the PCF. The capillary is demonstrated the air holes of the sensor structure. The thin wall capillary is denoted the air holes diameter of d3 and the thick wall capillary is represented the smaller air holes of d1.
Fused Silica is used as the background material because it has low absorption loss and high mechanical strength. The RI of fused silica is characterized by the Sellmeire’s Equation [44]:
For coating the gold layer CVD (Chemical Vapor Deposition) technique is used to decrease roughness [3]. The dielectric constant of the plasmonic material Gold (Au) is obtained from the Drude-Lorentz π model [48]:
The most important parameter confinement loss is analyzed the sensor performance such as amplitude sensitivity, wavelength sensitivity etc. So measuring confinement loss is the first step to analyze sensor performance. Loss is obtained from the equation as follows [39]:
Here, Im(neff) is the imaginary part of the RI, ko is the number of wavelengths where $\; ko = \frac{{2\pi }}{\lambda }$ and $\lambda $ is the wavelength in free space at which analysis has to be performed. Wavelength sensitivity of the sensor is obtained using wavelength interrogation method which is expressed by [53]:
where Δ${\lambda _{peak}}$ is the difference in the wavelength of two nearest peak shifts and $\mathrm{\Delta }{n_a}$ is the two nearest analyte RI variations.Amplitude sensitivity of the sensor is obtained using amplitude interrogation method which is expressed by [63]:
Here, $\partial \alpha \; ({\lambda ,{n_a}\; } )$ is the difference between two nearer spectral losses and $\partial ({\lambda ,{n_a}} )$ represents the loss of the corresponding analyte.
The FOM is one of the most important parameters which represented the better detection range of a sensor. FOM is calculated from the equation as follows [57]:
Here, $FWHM$ is the Full Width at Half Maxima of the loss characteristic curve and sensitivity is the wavelength sensitivity which is calculated by Eq. (4) for individual analyte.
3. Simulation results and performance anlaysis
3.1 Dispersion relation characteristics
The basic operating principle of PCF based SPR sensor is the interaction between evanescent field and the free electron is emitted from the plasmonic metal. The evanescent field induces when an electromagnetic wave propagates through the core and when an evanescent field pounded the plasmonic surface, it induced SPW. The propagation of SP waves that propagate parallel to the interface travels through the plasmonic metal surface. When the fundamental mode of core guided and the mode of SPP is matched by their frequency, a keen confinement loss peak occurred and transferred the maximum energy [64].
Any amount of change of analyte RI caused a significant change in loss curve. The principle is used to detect any unknown analyte by either the shifting of peak or the loss variation. During the analysis of this SPR sensor we have used a y-polarized mode that creates the powerful coupling with the surface electrons in the plasmonic layer.
Figure 2(a) has shown x polarization core guided mode, Fig. 2(b) and Fig. 2(c) are shown y polarization core guided mode and SPP mode respectively. The effective RI(neff) of y polarized core guided mode, the loss spectra of core guided y polarized mode and (neff) of SPP mode are shown in Fig. 2(d) with the refractive index na 1.47 and metal layer 30nm. The solid gray line is SPP mode that intersects the solid orange line at the wavelength of 1.83$\mathrm{\mu} m$. The intersecting phenomenon is also known as phase matching condition and 1.83$\mathrm{\mu} m$ is considered as resonance wavelength. At this point, we observe a sharp peak on the curve and this indicates the highest confinement loss for that curve. Due to the asymmetric design, this sensor is birefringence. Because of the birefringence, the sensitivity is better than any other symmetric biosensor.
Fig. 2. EM field distribution in (a) x-polarized core mode (b) y-polarized core mode (c) y-polarized SPP mode (d) dispersion relation between SPP mode and y-polarized core guided mode at RI = 1.47 and tg = 30nm.
3.2 Analyte variation loss spectra
All the performance measurement parameters can be calculated by computing the confinement loss, such as amplitude sensitivity, wavelength sensitivity and sensor resolution. Apropos, confinement loss could be the first step for examining the performance of any plasmonic sensor. Confinement loss can be calculated from Eq. (3). The confinement loss spectra for the variation of analyte RI from 1.41 to 1.49 are illustrated in Fig. 3 where the red shift is occurring. Due to the increment of analyte RI from 1.41 to 1.42, 1.42 to 1.43, 1.43 to 1.44, 1.44 to 1.45, 1.45 to 1.46, 1.46 to 1.47, 1.47 to 1.48 and 1.48 to 1.49 resonance wavelength shifts from 1.39µm to 1.4µm, 1.40µm to 1.43µm, 1.43µm to 1.47µm, 1.53µm to 1.64µm, 1.64µm to 1.83µm, 1.83µm to 2.1µm and 2.1µm to 2.46µm respectively. According to Fig. 3 the lowest confinement loss 2.36 dB/cm at 1.39µm and the highest peak 161.25 dB/cm at 2.46µm are certainly observed.
Fig. 3. (a) Analyte RI varying loss characteristic curve from 1.41 to 1.45 and (b) loss characteristic curve from 1.41 to 1.49.
As increasing the analyte refractive index, the corresponding loss peak wavelength shifts are 10nm for 1.41 to 1.42, 30nm for 1.42 to 1.43, 40nm for 1.43 to 1.44, 60nm for 1.44 to 1.45, 110nm for 1.45 to 1.46, 190nm for 1.46 to 1.47, 280nm for 1.47 to 1.48 and 360nm for 1.48 to 1.49 respectively. Besides the theoretical wavelength sensitivity are 1000, 3000, 4000, 6000, 11000, 19000, 28000, 36000 nm/RIU. Therefore 13500nm/RIU the average wavelength sensitivity can be achieved in the range of RI between 1.41 and 1.49.
Amplitude sensitivity of this particular sensor is illustrated in Fig. 4 with analyte RI of 1.41 to 1.49 the theoretical amplitude sensitivity of 32, 48, 74, 138, 332, 799, 1380, 390 RIU−1 respectively. Another important parameter is sensor resolution, which indicates the slightest change of RI of the sensor can be detected. The smallest value of sensor resolution indicates the sensor has the capability for small change of analyte’s RI. The proposed sensor has maximum sensor resolution 9.09×10−9 RIU, which suggests the capability of detecting 10−6 scaled smallest change in analyte’s RI.
Fig. 4. (a) Amplitude sensitivity curve with $\wedge=z\mu\textrm{m}$ and tg = 30nm, (b) Intensity curve for analyte RI 1.41 to 1.48.
3.3 Performance analysis based on gold layer thickness variation
One of the most important concerns is to select plasmonic metal that can provide free electrons for making strong coupling between core and SPP mode to strengthen the sensor performance and the metal layer thickness which can provide better sensitivity as well.
There are some plasmonic materials that have been utilized to increase sensor performance such as silver (Ag), aluminum (Al) and gold (Au). But some of these materials (silver and aluminum) have several drawbacks that contribute to decreased sensor performance. For example, silver (Ag) and aluminum (Al) have oxidation problems [38], which might compensate by using an extra layer of graphene or Ti2O5 [65]. But an additional layer could emerge damping loss [66]. In this case, gold has no oxidation problem and due to no additional layer damping loss may be avoided. Therefore the gold is used as a plasmonic layer for sensors [67–70].
Due to the alternation of gold layer thickness (30nm, 35nm, 40nm, 45nm) in terms of wavelength and amplitude sensitivity in Fig. 5(a) and Fig. 5(b) respectively is illustrated for analyte RI of 1.47 and 1.48. Figure 6 represents the intensity curve. In wavelength sensitivity Fig. 5(a), the red shift occurred for increasing the thickness, where 30nm of gold layer exhibited sharp peaks with moderately higher loss. The reason is the thin layer that could reduce damping loss [66] and 1215 RIU−1 for 45nm.
Fig. 5. (a) Confinement loss spectra with variation of gold layer thickness and (b) Amplitude sensitivity variation with the change of gold layer thickness for tg = 30nm, 35nm, 40nm, 45nm for analyte RI of 1.47 & 1.48.
Because of the variation of gold layer amplitude sensitivity is obtained 1380 RIU−1 for 30nm, 1351 RIU−1 for 35nm and 1302 RIU−1 for 40nm. The sensor exhibited the highest amplitude sensitivity at gold layer thickness of 30nm. Using 30nm of gold layer provided better performance, which is why 30nm is chosen as optimum.
3.4 Performance analysis based on pitch variation
Figure 7(a) and Fig. 7(b) illustrate the variation of pitch from 1.8µm to 2.1µm with the wavelength $\lambda $ for refractive index 1.47 and 1.48. As the extension of the pitch, the core space is increased that’s why light confinement in the core is reduced and confinement loss gradually increases. A slight red shifting in the wavelength sensitivity curve can be observed with scaling up the pitch.
Fig. 7. (a) Loss spectra with pitch (Λ) variation at 1.8µm to 2.1µm for analyte RI 1.47 to 1.48 (b) Amplitude sensitivity curve due to pitch (Λ) variation where tg = 30nm.
For this biosensor, we have acquired maximum amplitude sensitivity 1379.64 RIU−1 for pitch 2µm whereas for pitch 1.8µm, 1.9µm and 2.1µm amplitude sensitivity is achieved 484.681 RIU−1, 798.698 RIU−1 and 1246.09 RIU−1. Since maximum amplitude sensitivity is achieved for pitch value 2µm as the optimum value to get better performance.
3.5 Performance analysis based on the air hole diameter
The impact of the air hole diameter on the performance of the sensor is examined and discussed concerning WS and AS. Figure 8(a) shows the sense of confinement loss due to the ± 2% to ±5% variations of the small air hole radius from its optimum value of analyte RI of 1.47 and 1.48. The confinement loss decreases with the increment of small air hole diameter because of the amount of leakage from the core to the cladding increases [58]. Though loss peak varies, there is no variation in WS. Figure 8(b) shows the AS of the proposed sensor showing the effect of air hole diameter on AS. It shows that the curve becomes balanced to the increasing diameter of the air hole. Figure 9(a) exhibits the loss spectra for analyte RI 1.47 and 1.48 with the ± 2% to ± 5% variations of the medium radius air hole radius of the optimum value. It’s shows that the confinement loss decreased by 1.8445 dB/cm, 4.4135 dB/cm,1.6397 dB/cm, 6.1156 dB/cm with −2% and −5% value while the loss increases 2.63 dB/cm, 6.85698 dB/cm,0.6514 dB/cm,6.988 dB/cm with +2% and +5% variation of medium diameter from its optimum value of analyte RI of 1.47 and 1.48. With the increase of wavelength, the confinement loss decreases.
Fig. 8. (a) Confinement loss spectra and (b) Amplitude sensitivity variation due to the variation of the air hole ${d_1}\; \; $ ${\pm} 2\%$ and ${\pm} 5\%$ from the optimum value for RI 1.47 and 1.48.
Fig. 9. (a) Confinement loss spectra and (b) Amplitude sensitivity variation due to the variation of the air hole ${d_2}\; \; $ ${\pm} 2\%$ and ${\pm} 5\%$ from the optimum value for RI 1.47 and 1.48.
Figure 9(b) represents the amplitude sensitivities for this variation of medium diameter. Here, the loss peak increased by the increment of the wavelength due to ±5% to ±2% variation. AS increases to 89.72128 RIU−1 and 91.19226 RIU−1 with +2% and +5% variation and reduces to 87.60894 RIU−1 and 85.64928 RIU−1 with −2% and −5% variation of air hole diameter [63].
Figure 10(a) illustrates the loss spectra for analyte RI 1.47 and 1.48 with the ±2% to ±5% variations of the larger air hole diameter of the optimum value. It shows that the CL increased with the increment of wavelength. It increases 7.9 dB/cm and 24.028 dB/cm due to −5% to +5% from its optimum value for analyte RI 1.47 and 1.48 respectively.
Fig. 10. (a) Confinement loss spectra and (b) Amplitude sensitivity variation due to the variation of the air hole ${d_3}\; \; $ ${\pm} 2\%$ and ${\pm} 5\%$ from the optimum value for RI 1.47 and 1.48.
Figure 10(b) illustrates the loss spectra and amplitude sensitivities with ±2% and ±5% variation of for analyte RI of 1.40 and 1.41 which shows that there is none of significant change of amplitude sensitivities when radius varies ±2% to ±5% from its optimum value [63].
3.6 Sensor performance of resonance wavelength curve fitting and Figure of Merit
For calibration simplicity, high linearity or high polynomial fit is required [63] and it has great impact on average sensitivity and resolution [62]. The proposed sensor shows nonlinear characteristics according to Fig. 11 of the resonance wavelength (RW) with the increment of refractive index (RI). Linearity characteristics largely depend on two terms, first one is the R-square value and second one is the RMS (Root Mean Square) value. A sensor will be accurately fitted when R-square value is near to be unity and RMS value is near to be zero. By considering these two terms a second order polynomial fit is suited for this sensor where ${R^2} = 0.9942$. The proposed sensor shows high performance and calibration is simple because of good polynomial fit.The second order polynomial equation is:
where y and x corresponds resonance wavelength (RW) and refractive index (RI) respectively.FOM is one of the most important parameters which convey the ability of high detection range and better FOM can improve the sensor performance. Figure 12 represents the FOM of the stated sensor and we obtained FOM from Eq. (6) which defines that FOM is the ratio of sensitivity and FWHM (Full Width at Half Maxima).
So if the value of FOM is high that means sensor shows high sensitivity and better performance. The maximum FOM within the range of 1.42 to 1.49 of the refractive index is 385 RIU−1 from the figure we can see that with the increasing of refractive index (RI) the FOM curve increased. Because of sensitivity is increased and FWHM (Full Width at Half Maxima) is decreased.
The circumstantial result of the proposed sensor is being put in Table 1.
Table 1. circumstantial results of the sensor-
3.7 Description of practically experimental arrangement
Figure 13 shows the practical sensing setup to detect the unknown analyte. First, light is penetrated through the sensor fiber filled with the unknown analyte. Then come the second phase, optical spectral analyzer (OSA) which detects the sensing area for the unknown analyte. If the analysis shows the loss curve shifts right due to the increase of analyte RI, it is indicated as a red shift. And if shifts are left, it is indicated as a blue shift. The analyzed report is then sent to the computer to plot the loss curve to analyze the performance and sensitivity of the sensor. But this setup is very complex and sophisticated to arrange. So this part is being worked on. We wish to include this setup in our upcoming work.
According to Table 2 we can say that our proposed sensor is a newer structure and shows better results among the previously existing work. Our sensor shows more sensitive performance with large detection range and promising candidate to sensing response with very sharply and smartly.
Table 2. performance comparison with previously designed sensor
4. Conclusion
A highly sensitive single core double slotted plasmonic biosensor is detailed analyzed throughout the overall manuscript. The sensor is externally coated with gold and shows a huge sensitivity of 36000nm/RIU. Also covering a large detection range is 1.41 to 1.49 RIU. The sensor can consume more analytes due to the circular slots. And have a maximum resolution of 9 × 10−5 RIU. Literature study of previously proposed sensors reveals that the sensor can undoubtedly compete with the other existing works in the field. The sensor thus can be used in a great medium to detect bio-molecular analytes in the era of bio-sensing.
Acknowledgments
Alhamdulillah, by the grace of Almighty Allah (SWT), we have done a research paper. We would like to gratefully and sincerely thank Md. Rabiul Hasan and Rifat Ahmmed, Rajshahi University of Engineering and Technology for their constant inspiration, patience, necessary guidance, continuous help, suggestions, technical support and most importantly, their friendly dealing during this research work. They encouraged our not only perform the research work but also to grow as an independent thinker.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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