Optical Fiber Biomedical Sensor Based on Surface Plasmon Resonance

Optical fiber biomedical sensor based on surface plasmon resonance for measuring and sensing the concentration and the refractive index of sugar in blood serum is designed and implemented during this work. Performance properties such as signal to noise ratio (SNR), sensitivity, resolution and the figure of merit were evaluated for the fabricated sensor. It was found that the sensitivity of the optical fiber-based SPR sensor with 40 nm thick and 10 mm long Au metal film of the exposed sensing region is 7.5μm/RIU, SNR is 0.697, figure of merit is 87.2 and resolution is 0.00026. The sort of optical fiber utilized in this work is plastic optical fiber with a core diameter of 980 μm, a cladding of 20μm, and a numerical aperture of 0.51.


Introduction
Several developments within the field of sensing have been achieved in many areas of science, including biomedicine, biology, chemistry, biochemistry and physics. These developments have provided solutions to various problems associated with the sensors. The difficulties which can face the sensors are ISSN: 0067-2904 linked with the cost, the amount of the sample to be sensitive, the sensor size, and its performance properties such as the signal to noise ratio, sensitivity, and resolution etc. [1][2][3]. The optical fiber-based Surface Plasmon Resonance (SPR) detects numerous points of interest; for example, improved and flexible optical structure, advancement of remote detecting, ceaseless examination, and the in situ observation of superior prism-based SPR detection [4][5][6][7]. Optical excitation of surface plasmon waves (SPW's) has been used for several decades to build up various sensors. The SPR method was used for the first time in 1982 for gas sensing [8]. SPR-based optical sensors are employed for different applications in life sciences, environmental safety, electrochemistry and biomedical diagnostics [9,10] . Surface plasmon is the oscillations of electron density whenever the associated transverse magnetic (TM) polarized waves are controlled parallel to the interface between the dielectric and metal. At the point when the momentum and energy of the incident light are equal to that of the surface plasmon wave, SPR takes place in the metal-dielectric interface. The energy is conveyed by photons transfer to the electrons in a metal to accomplish an obvious dip into the light intensity [11]. The resonance condition relies upon the dielectric constant of each the dielectric and the metal, as well as the wavelength of light and the incident angle. Each sharp dip is shown within the spectrum of the producing signal at the resonance wavelength (spectral interrogation) or the resonance angle (angular interrogation). The wavelength or the angle about which the resonant excitations of surface plasmon take place is extremely delicate to changes in the refractive index of the dielectric adjacent to the metal. Therefore, these changes in the refractive index of the sensing medium can be identified by estimating the resonance angle or the resonance wavelength [10,12,13]. To excite the surface plasmons, optical elements such as optical fiber, high refractive index prism, and diffraction grating are used [14,15]. In 1993, the utilization of optical fibers for surface plasmon resonance sensing was first presented [16]. In this work, an optical fiber sensor based on surface plasmon resonance was designed and implemented for sensing and measuring the refractive index and concentration of sugar in blood serum.

2-Surface Plasmon Resonance
The resonant oscillation of the conduction electrons at the interface between the dielectric and metal, stimulated by an electromagnetic wave, is known as SPR. SPR can be excited by the photons or electrons. When the propagation constant of an incident light is equal to that of the collective oscillation of surface electrons in the metal, their momenta can be matched, and hence resonance occurs. Thus, SPR is highly sensitive to variations in the refractive index (RI) [17].

3-Performance properties
Performance characteristics to be studied are sensitivity, signal to noise ratio, figure of merit, and resolution. In the case of spectral interrogation, sensitivity can be defined as the change in resonance wavelength per unit change in refractive index of the sensing medium, and it can be written as [18]: = ……………. (1) Signal to noise ratio (SNR) and figure of merit (FOM) are inversely proportional to the width of SPR spectral curve, and can be written as [18] : The resolution of the sensor can be defined as the minimum of change in refractive index detectable by the sensor, and is given as [15]:

The experimental work and devices
The experimental setup for measuring the transferred light spectrum consists of the light source (halogen lamp), plastic optical fiber from Thorlabs, the optical spectrum analyzer (OSA) from Thorlabs, and blood serum. The scheme illustrating the components of the SPR based optical fiber biomedical sensor setup is shown in Figure-1).

Optical Sensor Systems
An optical-grade plastic optical fiber with a core diameter of 980µm a numerical aperture of 0.51, without a jacket, and a small part (10mm) of optical fiber in the middle, was embedded in a resin block, then the polishing process was performed. The unclad part was cleaned with distilled water and deposited with about 40nm thickness of gold metal using ION _COATER (COXEM Company, Korea).

Preparation of Solutions with different refractive indices
The sensitive region of the sensor was covered in sucrose /water solutions with various concentrations and, thereby, different refractive indices. We measured the refractive indices of the solutions using an Abbe refractometer. Figure-3 demonstrates the linear relationship between the refractive index values and the solution concentrations.

Results and Discussion
In this work, several parameters including fiber core diameter =980µm, fiber optic numerical aperture (NA) =0.51, gold layer thickness (d) =40nm, sensing length (L) =10mm and various values (1.346, 1.359, 1.382 and 1.39) of refractive index from sucrose /water solutions were found.
The spectra are obtained by recording the transition curves (T) of light through optical fiber. T is calculated from the ratio of the intensity (I) measured in the presence of a sample (sensing medium) and the intensity of the optical signal ) that is evaluated in the absence of the sample.
The transmission (T) is a function of the wavelength in nm. The T-wavelength curve is called SPR curve and, at a special wavelength, named resonance wavelength. A sharp dip happens within T because the energy of incident light transfer into the electrons of the metal and thus reduces the reflected light intensity. The location of this dip depends on the refractive index (n) of the sensing medium. As the refractive index of the sensing medium increases, the resonance wavelength increases because of the sharp dip shifting to the red wavelength, as shown in Figure-4. Figure-5 shows the SPR response curve of the fabricated sensor with a gold layer at various refractive index values of blood serum (sensing medium). It is clear that the resonance wavelength shifts from 495nm to 531nm for the gold metal, as the refractive index of the blood serum varies from 1.3392 to 1.3464. The width and dip position of each (SPR) response curve to the sensor is changed with each sample having a different refractive index. Also, the magnitude of shifting of the dip position increase as the refractive index increases. These variations make the performance parameters, which depend on the SPR curve width, the value of the shifting and the position of dip, change with the changing of resonance wavelength and refractive index of the sensing medium. This occurs because they depend upon the change of the resonance wavelength, the change of refractive index, and the width of the spectral curve. Table-1 shows the experimental performance parameters of the sensor with gold. Table-2 demonstrates the values of the refractive index and concentration of sugar for each sample of blood serum at different resonance wavelengths. The resonance wavelengths were determined from SPR curves for various samples of blood serum (Figure-5), while concentration and refractive index values were calculated from slope equations, as shown in Figures-(3, 4), respectively.

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The concentration of the samples increased as the refractive index increased and the resonance wavelengths increased. This occurs because of the sharp dip shifting to the high wavelength (red wavelength).

Conclusions
This paper presents the usage of the plastic optical fiber as a sensor of sugar concentration in the serum of human blood as a biomedical sample. SPR response curves for different samples of blood serum were recorded and exhibited a dip in the position of resonance. A change in the value of the resonance wavelength occurs for each change in the refractive index, and hence for each change in the concentration of sugar in the blood serum. The sensitivity of the plastic optical fiber-based SPR sensor with 40nm thick Au metal film of the exposed sensing region was 7.5 µm/RIU and the signal to noise ratio was 0.697.