1.0 Introduction -Key Components and Smartphone Integration of MOSPR Sensors
The shift in the optical angle of incidence is the operational principle of SPR sensors, usually leading to the detection of the investigating structures. Optical radiation for localized SPR where the incident light whose wavelength is larger than the structure. The propagation SPR for which the wavelength of the incident light is smaller than the structure is the basis of the SPR. SPR is determined as ‘The Surface Plasmon Resonance. The interaction of magnetic fields is the main principle upon which the sensors are based. With magnetic material’s optical properties in the SPR structure. This MOSPR is a recent field of investigation of biosensing which is in the experimental stage although its vigorous development creates various potentialities for the latest applications. MOSPR requires a device that can only be processed under a magnetic field. The report has been focused on the other designing and development of magnetic optic surface Plasmon resonance (MOSPR) biosensors.
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The basic principle of the MOSPR sensor is a little different from the SPR sensor. The SPR sensor of an optical system conventionally is based on the “Kretchmann configuration”. When an incident light under the condition passes through an angle width with a sensor chip prism, a total reflection of a wave of “p-polarized incident light” is generated, which then confronts the dielectric medium (Rizal et al. 2019). In the MOSPR response, there is a simultaneous excitation of the “transversal magneto-optic Kerr effect (TMOKE)” and SPP happens due to the magnetic field’s presence.
If the vectors of the wavelength of light in both the surface Plasmon wave (ksp) and evanescent wave (kev) are coincident, the following equation is satisfied.
“kev = kp sinθ = (ω/C) (ε n2/(ε + n2))1/2 = ksp”
Here, kev and ksp are wave vectors, θ is the incident angle for light, n, and ε, are constants for the dielectric, and C and ω have been determined as light’s speed and angular frequency (Zhu et al. 2022). The principle is presented.
According to Rizal et al. (2018), the sensitivity of the MOSPR can be defined as
Where ΔRp(A) and ΔRp(B) represent the reflectivity changes because of the H field modulation for both mediums A and B respectively at the incident angle θm (Rizal et al. 2018). The normalization of the difference of the reflectivity changes between both the mediums is achieved by using the maximal value of the first derivative of (ΔRp(A)θ/Δθ(A) which is denoted as ΔRp(A)m.
In the MOSPR response, there is a simultaneous excitation of the ‘transversal magneto-optic Kerr effect (TMOKE)’ and SPP happens due to the presence of an external magnetic field.
2.2 Components of MOSPR-based biomolecule device designing
The MOSPR sensor consists of various layers such as “ferromagnetic layers, nanoparticles, multilayers, alloys and constitutionally graded alloys for the magnetic layer”. MP multilayers are composed of noble ferromagnetic metals such as silver or gold and iron or copper. The very thin dielectric layers can be applied as a protective layer on the surface of the sensor because of their improved sensitivity (Rizal, 2020). More specifically, graphene layers are the most effective regarding this improved sensitivity. In order to obtain selectivity, the carbon surface of these layers is beneficial for the functionalization of the surface [referred to Appendix 3].
There are several components required for MOSPR-based biomolecule device design. The MOSPR setup contains a similar gold layer for an SPR-based device. But, here it is combined with a multilayer of thin magnetic film. A delicate trade between magnetic layer composition and film thickness is essential. A sandwiched cobalt between layers of Au/Co/Au trilayer is required for MOSPR. This trilayer acts as a refraction layer. This Au/Co trilayer is based on a glass substrate. This layer with Au had a thickness of approx. 2 nm and varying co-layer of Co along with a width not less than 1.8 nm were put in the 2 nm buffer glass layer (Wang et al. 2021). This multilayer system is comprised of 20 repeats of Co/Au. The main components of MOSPR are Au (111), Co (111), Au (220), and Au (222). Diffractometric is also required. Cu-Kα radiation of 0.154 nm was deployed for X-ray diffraction and X-ray reflection [Refer to the appendix. 1].
Figure 1: Components of MOSPR-based biomolecule device designing
Apart from these components, a proper source of light, prism and detector is also required for the development of the MOSPR sensor (Rizal et al. 2022). The use of iron instead of copper can also be beneficial theoretically, although the effectiveness of the copper based multilayer is better than the iron based multilayer in the MOSPR sensor.
2.3 Understanding of different components of the MOSPR sensor
MOSPR sensor is made up of different components. One of the components of MOSPR, Au or gold, does not have any magnetic property, hence it is categorized as a non-magnetic (NM) metal. According to Drude’s conduction of electron theory in metal, the effects of magneto-optical (MO) are very small for gold (Rizal et al. 2022). The gold plate layer acts as a refractive medium for light rays. After the light rays confront the gold plate medium, those are refracted back towards the Au/Co dielectric medium and the light rays passed through it. These refracted rays are then picked up by the magnetic device sensor. As a result, a picture of the structure that is being used to examine by the x-rays on an x-ray plate. This is important for medical purposes, this principle is used to carry out MRI, CT scans, and many more [Refer to the appendix. 2].
According to Drude’s theory, Co, Ni and Fe are the “3D transition ferromagnetic metals (FM)” whose alloys as well as multilayers are the most appropriate for the study of the MOSPR because of their magnificent properties of MO and optical such as electromagnetic coupling and improved permittivity. These can be operated with the help of H fields and optical radiation (Qin et al. 2022). Although having high MO effects, these FMs cannot be utilized as the sole metallic components; rather they can be used by implanting these with NM components or oxides in the Core-shell or several-layered structural forms in order to increase the effects of MO. After obtaining the SPR in MOSPR, a huge strengthening of “transverse MO Kerr effects” is created.
For the improvement of the optical modulation and MO respectively, various dielectric and FM layers can be used (Rizal and Belotelov, 2019). The theoretical demonstration explained that only at a specific optimum condition of a design, the coupling between “incident optical radiation” and the marked “surface plasmon polaritons mode” can occur by the synergetic action which is created between MO and optical radiation-induced fields.
2.4 Integration of different components of the MOSPR sensor
The gold-plated layer acts as an x-ray diffraction and X-ray reflection (XRR) medium. After the incident light confronted the gold plate, it reflected on the Au/Co trilayer. The x-ray diffraction (XRD) layers are used for the identification of many unknown nanostructure materials in the field of geology, material science, environmental science, biology, engineering purposes, and many others. The XRD is useful in the determination of the quality of nanostructure multilayer films, and crystal structure, to identify any mismatch between the substrate and individual layers due to strains.
Figure 2: Integration of different components of the MOSPR sensor
The XRR is used for the determination of bilayer thickness, to measure the density of the film, and to determine surface roughness (Petrov et al. 2020). When an incident light passes through a prism to a sensor chip within an angle within the total reflection condition, a wave of p-polarized incident light is generated, which then confronts the dielectric medium. Thus, the device of MOSPR is important for determining the structures of materials, and integration of the components of biomedical devices which requires a magnetic field.
Modulation of MOSPR is performed by utilizing the “magneto-optic Kerr effect (MOKE)”. The MOSPR uses magneto-plasmonic sensors of Co/Au with three-fold sensitivity to increase effectiveness as compared to SPR. The incident angle of the MOSPR sensor can improve the sensitivity level (Fan et al. 2020). In order to retain the sensitivity, the oxidation of Co is prevented by the Co film. The magnetic field induces transverse MOKE (TMOKE) when it is perpendicular to the incidence plane. This influences the reflected light’s polarization plane’s rotation. In the case of Co film, its thickness is important in the context of sensitivity as the thinner Co film can weaken the TMOKE’s action. Even the thicker Co film can be capable of reducing the fugitive field’s intensity in order to weaken the excitation of SPW (Chai et al. 2021). Apart from that, the optimized incident angle of the MOSPR sensor can improve the sensing.
Figure 3: (a) θspp MO modulation by an alternating magnetic field. The inset defines the graph of MO modulation factor (Δθ). (b) Schematic plot of the production of the TMOKE effect’s enhancement during the satisfying SPP excitation condition. (c) Schematic response of MOSPR and SPR sensors to the similar fluctuation in the outer refractive index (Δn).
The amplification of the effects of MO of the MP multilayer depends on the satisfactory condition of SPP excitation. The biosensor of MOSPR monitors the fluctuations of the amplified ΔRpp/Rpp which is influenced by the SPP’s excitation. In MOSPR, the electromagnet is positioned in a crosswise configuration along with an MP transducer which is the proper substitute of the Au layer (Suzuki et al. 2021). The large MO activity and angular and sharp resonance of the MP transducer enhance the MOSPR’s sensor feature. In the 3-layered structure Au/Cr/Co multilayer, the intervening chromium layer is the necessary element for the enhancement of the transducer’s stability. This is because of the deficiency of adhesion between the other two layers, viz. Au and Co, which results in a decrease in the MP transducer’s sensitivity.
There is a high possibility of increasing the sensitivity of the MP transducer by substituting the layer of Co with the layer of Fe as the FM. Iron’s optical absorption and MO activity are similar to copper; its magnetic field is lower than the latter for which the adhesion of the Fe with the gold layer is better than with the copper (Sharma et al. 2020). The thickness plays a significant role in this case. The ideal thickness for the gold layer in the multilayer is 32 nm and 30 nm, along with the 5 nm and 3 nm of iron, respectively. The main reason behind this difference is the prime excitation condition of the SPP for the specific thickness of each layer in the multilayer. For this case, the reflectance value is reduced at the incident angle in order to maximize the application of biosensing.
The thickness of the iron layer helps in the determination of the width of the angular curve of the Rpp. For this reason, a higher amount of iron induces a greater “optical absorption” in the regions of multilayer that broadens the angular curve of Rpp and ΔRpp/Rpp respectively (Chai et al. 2021). Additionally, iron can operate the multilayer’s MO activity, which can amplify the Δθ. Hence, the thickness of the iron layer in the multilayer can have an effect on ferromagnetic film absorption as well as on the MO activity.
Although the theoretical explanation reveals that the improvement in the sensitivity using AU/Fe/Au trilayer is somewhat lower because of the roughness of the interface, which decreases the optimal thickness of iron effectively and leads to reduced MO activity as compared to expected theoretical activity of Fe (Xiao et al. 2022). Additionally, this trilayer provides high chemical stability as well as the resistance to the chemical oxidation. The latest MP transducers’ development is assisted with the sharper interfaces like epitaxial structures or the utilization of the ferromagnetic material Fe/Co alloys. In case of using Fe/Co alloys, Fe can supply stability and reduce the deficiency in Au/Co system’s resistance (Li et al. 2019). Apart from that, the external medium interface of the MP transducer is gold, which is fully compatible with the biofunctionalization protocols of renowned SPR.
The use of X-ray, specifically “high angle X-ray diffraction (XRD)” or the low angle X-ray reflection (XRR) is used in the MOSPR sensor for the characterization of the foreign nanostructured materials of environmental science, biology, material science, geology and engineering (Rizal, 2020). The utilization of XRD is confined to the determination of the crystal structure, the nanostructure microlayer film’s quality and “lattice mismatch” of individual layers and substrates because of stress or strain and dislocation density. On the other hand, the utilization of XRR is confined to the calculation of the thickness of the bilayer and the layer.
2.5 Structure design for mobile application assembly
In this process, the use of a “fiber-optic SPR (FO-SPR) structure” is essential. This enhances greatly the flexibility and simplicity of the “optical alignment” along with “light coupling”. If compared with prism-based SPR devices, this enhances greatly the flexibility and simplicity of the light coupling along with optical alignment FO-SPR, which has not required different optical components like slides, optical benches, prisms, and other expansive mechanical parts (Rizal 2020). All the components of optical nature have been connected with the optical fiber, which has been within a smartphone as a lightweight designed case for it, a small structure suitable for a handy smartphone. The control channel (CC), measurement channel (MC), and reference channel (RC) are activated by incoming light illuminated by the phone LED flash entering into the device mechanisms for this specific system. Whereas, the end face emitted lights of the fiber have usually been detected by the phone camera. Silica capillaries act as an element that has been coated along with a “50 nm gold film” (Xi et al. 2022). A smart application reads the frequency of 2 Hz with the intensity of light of the camera. The operation was simplified to ensure reliable operation.
This structure helps to make the MOSPR compatible with the smartphone without any negative effect to the display and the interface of the touch-screen during the process of detection due to the fixation of the components of MOSPR to the backside of the phone case (Fan et al. 2020). This structure can allow the assembly of the sensing components along with the smartphone into an instrument as well as the easy disassembly of these after the evaluation.
The polished end-facing fibres are fixed in the equivalent slot of the backside of the smartphone while the camera with the LED flash of the smartphone is positioned with the “lead-in and lead-out” fibres’ end faces. Since the smartphone’s LED flashlight is a cool source, the red light is collected by applying a cheap plastic lens behind the filter. In order to contain the stray light, the “lead-in and lead-out” fibres are protected with the help of a black rubber tube (Xiao et al. 2022). The “lead-in and lead-out” fibres’ other endpoints are bundled within the connectors of the optical fibre, which makes it simpler to attach them to the sensing components of the flow cell and to assemble and disassemble the sensing components.
While developing the structure of the MOSPR for the smartphone, one issue has been observed. In other SPR systems, high-performance light sources are used, but in the case of integration of SPR systems in the smartphone, its LED flashlight is relatively a low-performance source of light, which raises power instability-related problems (Li et al. 2019). In order to solve this power instability-related problem of the LED flashlight, the addition of a reference channel, the HPOF for the observation of the potency of the LED flashlight has occurred. The adjacent ends of the reference fiber and lead-in fiber are attached to guarantee that the light reaches both fibers in the exact condition. This results in affecting simultaneously the evaluating channel, reference channel, and the control channel in the same way if any variations in the LED flashlight have occurred.
Figure 4: (a) Schematic of MOSPR sensor designed for smartphones. (b) Installed SPR sensor on an Android-based smartphone. (c) 3D illustration of the internal structure of the optomechanical attachment. (d) The camera of the smartphone captures the images of the measurement channel, control channel, and reference channel; then, the images are rapidly processed to obtain the relative intensity. The data points are plotted and displayed on the screen.
The SPR system in the smartphone employs intensity modulation by the application of a thin band filter between the lead-in fibres and the smartphone’s flash that can supply more or less “monochromatic incident light”. For imaging purposes, this light can interact with the sensing components which can be analyzed by the smartphone’s camera (Zhang et al. 2022). In this way, the MOSPR system can be designed for the smartphone which can be appropriate for both real-time and label-free optical detection of different biological interactivities as well as the evaluation of the molecular interactivities’ kinetic parameters.
In order to use the MOSPR sensor in smartphones to treat it like a microscope, a series of lenses along with other “optical components” are necessary. This can also help in visualization of the output in a digitalized form.
3.0 Conclusion
The devices of MOSPR and SPR can be important for biomedical purposes. These systems can be put inside or linked with smartphones. The traditional devices for measuring and then determining the nanostructures in the field of geology, material science, environmental science, biology, engineering purposes, determination of the quality of nanostructure multilayer films, crystal structure, to identify any mismatch between the substrate and many others are generally heavy and cannot be fitted within a smartphone. However, smartphone developers in association with medical practitioners have figured out a way to integrate the systems to create a smart device that can measure different human physiological conditions, which makes use of phone sensors, and a phone camera. But, it is needed to be an improvement in this technology for better results in the future.
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