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Abstract
This paper introduces a surface plasmon resonance (SPR) sensor using tapered silica fiber and photopolymer coating for enhanced refractive index (RI) detection. Tapering the silica fiber to a diameter of 10 µm ensures the evanescent wave leaks into a 1.8-µm thick photopolymer film, which increases the average waveguide RI and broadens the RI detection range accordingly. A 50-nm thick single-side gold film is coated on the photopolymer film, exciting SPR and causing less light transmission loss than a double-side gold film. The method avoids the complex microfabrication processes of conventional polymer optical fiber SPR sensors, while the waveguide RI can be controlled by altering the curing time of the photopolymer during fabrication. The sensor has an overall sensitivity of 3686.25 nm/RIU, enabling RI detection of 1.333 − 1.493. Moreover, the sensor has an ultrahigh sensitivity of 6422.9 nm/RIU in the RI range of 1.423 − 1.493. The temperature response is about 1.43 nm/°C at 20 − 50 °C, which has little impact on RI detection. Finally, we demonstrate that the sensor can grade the severity of hepatic steatosis by measuring the RIs of cytoplasm/triglyceride emulsions with superior sensing performance.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Refractive index (RI) is a fundamental nature of optical material. The detection of RI is required in water quality analysis, concentration measurement, food safety control, and industrial production. Methods using Fabry-Perot interferometer [1], Mach-Zehnder interferometer [2], polarization-maintaining fiber [3], fiber Bragg grating [4], and long-period grating [5] have been proposed to perform RI detection. However, these methods suffer from low RI detection sensitivity or narrow range, failing to meet various applications’ requirements.
Fiber surface plasmon resonance (SPR) sensors can tackle the problems of conventional RI detection methods [6], exhibiting a wide detection range, high sensitivity, and short response time. Light transmission in optical fibers relies on total internal reflection (TIR), generating an evanescent wave at the interface between the fiber waveguide and the metal film. The evanescent wave propagating perpendicular to the interface can penetrate the metal film, exciting surface plasmon wave (SPW). When the light wavelength falls into a specific wavelength range, the satisfied resonance condition will result in SPR. The energy transfer from the evanescent wave to the SPW can cause strong absorption of light and a dip in the transmission spectrum. The SPR sensor can detect analyte RI because the variation of RI will alter the resonance condition and shift the resonance dip [7].
The dependence on TIR means that the waveguide RI sets the upper limit of a fiber SPR sensor's RI detection range [8 − 10]. For the widely used silica optical fiber, the upper limit of RI detection is only about 1.46. Thus, SPR sensors based on polymer optical fiber (POF) with a higher RI than silica optical fiber have been demonstrated to broaden the RI detection range. Recently, D-shaped [11], U-bent [12], and tapered [13] structures have been applied to POF-SPR sensors to improve detection performances. However, poor thermal stability makes it challenging to microfabricate POF [14,15]. Furthermore, the RI of POF cannot be adjusted to suit various application scenarios [16,17].
This paper establishes a tapered fiber SPR sensor for enhanced RI detection based on photopolymer coating. As shown in Fig. 1(a), we fabricate the sensor by coating a tapered silica optical fiber with a photopolymer film and a single-side gold film. Tapering the silica optical fiber ensures the evanescent wave leaks into the photopolymer film, which increases the average waveguide RI and broadens the RI detection range (to 1.333 − 1.493) accordingly. The single-side gold film can excite SPR, causing less light transmission loss than a double-side gold film. The method avoids the complex microfabrication processes of conventional POF-SPR sensors, while the waveguide RI is controllable by altering the curing time of the photopolymer during fabrication. Due to the sensor's wider RI detection range compared to conventional SPR sensors, it can grade hepatic steatosis by detecting a higher RI (1.38 − 1.47) of cytoplasm/triglyceride emulsion than normal liver tissue (see Fig. 1(b)). The principle of hepatic steatosis grading will be elaborated on later.
Fig. 1. Tapered fiber SPR sensor based on photopolymer coating. (a) Schematic diagram of the sensor. Regions A, B, and C are the un-tapered region, taper transition region, and taper waist, respectively. This figure is not to scale. (b) The application of hepatic steatosis grading.
2. Materials and methods
2.1 Preparation of the photopolymer
The amine co-initiator, sensitizer dye, and multifunctional acrylate monomer of the photopolymer are methyldiethanolamine (C5H13NO2), eosin Y (C20H6Br4Na2O5), and pentaerythritol triacrylate (C14H18O7), respectively [18]. 8 g methyldiethanolamine, 0.5 g eosin Y, and 91.5 g pentaerythritol triacrylate are uniformly mixed at room temperature to obtain the photopolymer glue. Excess bubbles in the photopolymer glue are removed. An Abbe refractometer (GDA-2S, Gold) is applied to confirm that the RI of the prepared photopolymer glue is 1.48. When a 532-nm laser irradiates the eosin Y, its triplet state will react with the methyldiethanolamine to initiate the polymerization of the pentaerythritol triacrylate, curing the photopolymer glue. The RI of the cured photopolymer is 1.52, measured by a gem refractometer (RHG-181, Azzota).
2.2 Fabrication of the sensor
Figure 1(a) shows the structure of the sensor. A silica single-mode fiber (SMF) without coating is tapered by a fiber tapering working station (IPCS-5000-SMT, Idealphotonics) to form the backbone of the sensor. The sensor comprises the un-tapered (region A) and tapered region. The tapered region includes the taper transition region (region B) and the taper waist (region C). The diameter of the tapered fiber waist in region C is controlled by adjusting the stretching speed and length of the fiber tapering working station. The tapered fiber waist's RI is 1.465. As the diameter of the tapered fiber waist in region C decreases, light cannot be confined in the fiber core and instead leaks to surrounding areas.
Then, we immerse region C in prepared photopolymer glue and couple the light from a 532-nm laser into the tapered SMF using a 4× microscope objective. The evanescent field of the laser can cure the photopolymer glue surrounding region C. The laser illumination time determines the thickness of coated photopolymer film in region C. After coating the photopolymer film, we remove region C from the glue and rinse region C with deionized water to wipe off excess photopolymer droplets. The photopolymer film is dried for two hours at room temperature. Finally, we fix region C in the vacuum chamber of a magnetron sputter (JS-1600, HTCY) to coat a 50-nm thick single-side gold film on the surface of the photopolymer film.
As shown in the top left corner of Fig. 1(a), we simulate the light field of a cross-section in region C. The wavelength of the light source is set at 500-1200 nm. Since the photopolymer film RI is 1.52, higher than the fiber core RI, most light propagates in the photopolymer film instead of the fiber core. The light intensity in the photopolymer film adjacent to the single-side gold film is low because of excited SPR. What's more, the light intensity on the surface of the gold film is higher than that in the adhered photopolymer film, which also confirms that SPR has been excited.
2.3 Experimental setup
Figure 2 shows the experimental setup for the RI detection using the tapered fiber SPR sensors. We place a sensor in a polydimethylsiloxane (PDMS) microfluidics chip. We use a syringe pump (LSP01-1A, Longer Pump) to inject prepared sucrose solutions with different RIs (1.333 − 1.493) into the PDMS chip. The RIs of sucrose solutions are measured by the Abbe refractometer (GDA-2S, Gold) before injection. After each RI detection test, the sucrose solution will be drained into a waste bucket. A supercontinuum white light laser (SuperK COMPACT, NKT Photonics) with a 450–2400 nm spectral range is connected to the sensor. The transmitted laser is detected by an optical spectrum analyzer (OSA, AQ6373, Yokogawa), whose wavelength range is 350–1200 nm.
3. Results and discussion
In this section, we will investigate the influences of tapered fiber waist diameter and photopolymer film thickness on the sensors’ performance, demonstrate the sensors’ resistance to temperature variation, and establish the sensors’ capability of hepatic steatosis severity grading.
3.1 Influence of tapered fiber waist diameter without photopolymer film
We first simulate the influence of tapered fiber waist diameter on SPR sensing performance. In these simulations, the tapered fiber's RI is set at 1.465. A 50-nm thick single-side gold film is directly coated on the surface of the tapered fiber waist. The wavelength of the testing light source is set at 500 − 1200 nm and the RI range of the testing solutions is 1.333 − 1.413. As shown in Fig. 3, tapered fiber waist diameters of 7, 10, and 15 µm are tested.
Fig. 3. (a), (b), and (c) are simulational results of SPR spectra with 7-, 10-, and 15-µm tapered fiber waist diameters, respectively. The photopolymer film is not coated. (d) The relationships between the sensitivities and the tapered fiber waist diameters.
We then conduct experiments to verify the simulational results. In these experiments, the tapered fiber's RI is 1.465. A 50-nm thick single-side gold film is directly coated on the surface of the tapered fiber waist. The experimental setup is shown in Fig. 2. The wavelength range is 500 − 1200 nm and the RI range is 1.333 − 1.413. As shown in Fig. 4, tapered fiber waist diameters of 7, 10, and 15 µm are tested.
Fig. 4. (a), (b), and (c) are experimental results of SPR spectra with 7-, 10-, and 15-µm tapered fiber waist diameters, respectively. The photopolymer film is not coated. (d) The relationships between the sensitivities and the tapered fiber waist diameters.
The simulational and experimental results are consistent, showing that the sensor can achieve higher RI sensitivity when the diameter of the tapered fiber waist is small. During the taper drawing process, the core and cladding diameters of the SMF decrease proportionally. When the tapered fiber waist diameter is large, most of the light is confined to the core, with only a small part of evanescent waves penetrating the cladding to reach the metal-dielectric interface. As a result, a tapered fiber waist with a larger diameter establishes lower RI sensitivity. Although high RI sensitivity is preferred, the tapered fiber waist with a 7-µm diameter shows a low production rate and a tendency to fracture. Due to the tradeoff between RI sensitivity and fabrication quality, the sensor's tapered fiber waist diameter is chosen as 10 µm.
3.2 Influence of photopolymer film thickness
As mentioned above, the 532-nm laser illumination time determines the thickness of coated photopolymer film. Figure 5 shows the increase in photopolymer film thickness with increasing curing time. Ultimately, the coating thickness tends to saturate with increasing curing time, mainly determined by laser intensity and photopolymer characteristics. We also measure the transmission loss of different coating thicknesses using a 633-nm single-frequency laser. Figure 5 shows the increase in transmission loss with increasing curing time. Thus, properly adjusting the coating thickness to avoid weak detection signals is essential.
We first simulate the influence of photopolymer film thickness on SPR sensing performance. In these simulations, the tapered fiber's RI and waist diameter are set at 1.465 and 10 µm, respectively. A 50-nm thick single-side gold film is coated on the surface of the photopolymer film. The wavelength of the testing light source is set at 500 − 1200 nm and the RI range of the testing solutions is 1.423 − 1.493. As shown in Fig. 6, photopolymer film thicknesses of 1.8, 2.6, 3.2, and 3.65 µm are tested.
Fig. 6. (a), (b), (c), and (d) are simulation results of SPR spectra with 1.8-, 2.6-, 3.2-, and 3.65-µm photopolymer film thickness, respectively. The diameter of the tapered fiber waist is 10 µm.
We then conduct experiments to verify the simulational results. In these experiments, the tapered fiber's RI is 1.465. A 50-nm thick single-side gold film is coated on the surface of the photopolymer film. The experimental setup is shown in Fig. 2. The wavelength range is 500-1200 nm and the RI range is 1.423 − 1.493. As shown in Fig. 7, photopolymer film thicknesses of 1.8, 2.6, 3.2, and 3.65 µm are tested.
Fig. 7. (a), (b), (c), and (d) are experimental results of SPR spectra with 1.8-, 2.6-, 3.2-, and 3.65-µm photopolymer film thickness, respectively. The diameter of the tapered fiber waist is 10 µm.
Figure 8 shows the simulational and experimental relationships between resonance wavelengths and RIs of different photopolymer film thicknesses. Figures 6–8 proves that the simulational and experimental results are consistent. The simulational RI sensitivities of 1.8-, 2.6-, 3.2-, and 3.65-µm thick photopolymer films are 6868.57, 6657.14, 5738.57, and 5220.10 nm/RIU, respectively. The experimental RI sensitivities of 1.8-, 2.6-, 3.2-, and 3.65-µm thick photopolymer films are 6422.86, 6092.29, 5723.71, and 4852.29 nm/RIU, respectively. The sensor has lower RI sensitivity when the photopolymer film is thicker because increased light transmission loss in a thick photopolymer film will weaken the SPR phenomenon. The 3.65-µm thick photopolymer film has the flattest spectra compared with other thicknesses, confirming severe light loss in thick films. A photopolymer film thinner than 1.8 µm may achieve better sensitivity. However, a thickness lower than 1.8 µm has a low production rate because of surface roughness. Due to the tradeoff between RI sensitivity and fabrication quality, the sensor's photopolymer film thickness is chosen as 1.8 µm.
Fig. 8. (a) The simulation relationships between resonance wavelengths and RIs of different photopolymer film thicknesses. (b) The experimental relationships between resonance wavelengths and RIs of different photopolymer film thicknesses.
3.3 Influence of tapered fiber waist diameter with photopolymer film
Although we have confirmed that a 10-µm tapered fiber waist diameter is optimal for the SPR sensor, we investigate the influence of other diameters on SPR sensing performance when coating photopolymer film. In simulations, the tapered fiber's RI and photopolymer film thickness are set at 1.465 and 1.8 µm, respectively. A 50-nm thick single-side gold film is coated on the surface of the photopolymer film. The wavelength of the testing light source is set at 500 − 1200 nm and the RI range of the testing solutions is 1.423 − 1.493. As shown in Figs. 9(a) and 9(b), tapered fiber waist diameters of 7 and 15 µm are tested. We then conduct experiments to verify the simulational results. In these experiments, the tapered fiber's RI is 1.465. A 50-nm thick single-side gold film is coated on the surface of the photopolymer film. The experimental setup is shown in Fig. 2. The wavelength range is 500 − 1200 nm and the RI range is 1.423 − 1.493. As shown in Fig. 9(c) and 9(d), photopolymer film thicknesses of 7 and 15 µm are tested.
Fig. 9. (a) and (b) are simulational results of the SPR spectra with 7- and 15-µm tapered fiber waist diameters, respectively. The thickness of the photopolymer film is 1.8 µm. (c) and (d) are experimental results of the SPR spectra with 7- and 15-µm tapered fiber waist diameters, respectively. The thickness of the photopolymer film is 1.8 µm.
In the simulations, the average RI sensitivities of the fiber tapers with 7- and 15-µm diameters are 8581.43 and 5884.29 nm/RIU, respectively. The resonance wavelength of the fiber taper with a 7-µm diameter at 1.493 RI exceeds the detection limit (i.e., > 1200 nm). These results are consistent with those of Fig. 3, in which photopolymer films are not coated on the fiber tapers. However, during the experiments, we observe that the RI sensitivity of the 7-µm diameter is lower than that of the 10-µm diameter when comparing Fig. 9(c) with Fig. 7(a). The average RI sensitivities of 7-, 10-, and 15-µm diameters are 5347.14, 6422.9, and 3920.43 nm/RIU, respectively. The possible reason is that the strong evanescent field increases the surface roughness of photopolymer film during curing, weakening the SPR phenomenon.
3.4 Sensing performance of optimal parameters
As mentioned above, the sensor's optimal tapered fiber waist diameter and photopolymer film thickness are 10 and 1.8 µm, respectively. We have demonstrated the optimal sensor's performance in the RI range of 1.423 − 1.493 in Fig. 7(a). The resonance wavelength shifts from 675.5 nm to 1125.1 nm with rising RI. As shown in Fig. 10(a), we establish the experimental results of SPR spectra in the RI range of 1.333 − 1.413. The experimental setup is shown in Fig. 2. The resonance wavelength shifts from 535.3 nm to 628.6 nm with rising RI. As shown in Fig. 10(b), the average RI sensitivity in the 1.333 − 1.413 and 1.423 − 1.493 are 1332.9 and 6422.9 nm/RIU, respectively. The overall RI sensitivity is 3686.25 nm/RIU. Table 1 compares the performance of the sensor discussed in this work and those presented in previously published papers. Our sensor provides a much wider RI measurement range with very high sensitivity.
Fig. 10. (a) Experimental results of SPR spectra in the RI range of 1.333 − 1.413. (b) The relationships between the resonance wavelengths and RIs.
Table 1. Comparison of different fiber SPR sensors for RI measurement
3.5 Temperature response
We investigate the optimal sensor's response to temperature. The sensor is still placed in a PDMS microfluidics chip and immersed in deionized water. The PDMS microfluidics chip is placed in a temperature control box with stable humidity. We set the temperature at a certain value and record SPR spectra after 5 minutes. As the temperature changes at 20 − 50 °C, the RI of the photopolymer varies accordingly, leading to shifts of resonance wavelengths. As shown in Fig. 11, the simulational and experimental results are consistent. As the temperature increases, the resonance spectra shift to longer wavelengths. The reason is that the rising temperature will decrease the photopolymer RI, equivalent to an analyte RI increase. In the experiments, the resonance wavelength shifts from 536.3 nm to 578.2 nm with rising temperature. The temperature response is about 1.43 nm/°C, which has little impact on RI detection.
Fig. 11. (a) Simulational temperature response of the sensor at 20 − 50 °C. (b) The simulational relationships between resonance wavelengths and temperatures. (c) Experimental temperature response of the sensor at 20 − 50 °C. (d) The experimental relationships between resonance wavelengths and temperatures.
3.6 Hepatic steatosis severity grading
Finally, we demonstrate the sensor's capability of hepatic steatosis severity grading. Hepatic steatosis is characterized by triglyceride (TG) accumulation within the hepatocytes without inflammation or injury [19]. Although there are no or few symptoms of hepatic steatosis, it may progress into cirrhosis of the liver. Thus, preventive diagnosis is essential. The severity of hepatic steatosis is graded based on the percentage of TG within the hepatocytes. The TG volume percentages of grades 1 (mild), 2 (moderate), and 3 (severe) are 5%−33%, 33%−66%, and >66%, respectively [20] (see Fig. 12(a)). It is known that the RI of a normal human hepatocyte and TG are around 1.38 [21] and 1.47 [22], respectively. RI is linearly dependent on the TG volume percentages. Thus, the RIs of grades 1, 2, and 3 are 1.39 − 1.41, 1.41 − 1.44, and 1.44 − 1.47, respectively.
Fig. 12. (a) Histological features of grade 1, 2, and 3 hepatic steatoses (hematoxylin and eosin stain) [20]. These figures are reproduced under the terms of Creative Commons Attribution 4.0. (b) SPR spectra of hepatic steatosis severity grading.
The sensor in this work can grade the severity of hepatic steatosis with high sensitivity by measuring the RIs of cytoplasm/TG emulsions. Due to experimental constraints, we utilize sucrose solutions with an RI of 1.38 to imitate the cytoplasm while mixing sucrose solutions with TG to form emulsions. As a result, 10% (grade 1), 50% (grade 2), and 90% (grade 3) TG emulsions are prepared. As shown in Fig. 12(b), the resonance wavelengths of 10%, 50%, and 90% TG emulsions are 608.9, 680.4, and 810.2 nm, respectively. According to Fig. 10(b), resonance wavelengths of 608.9, 680.4, and 810.2 nm correspond to RI of 1.388, 1.425, and 1.461, respectively. Thus, the presented sensor is capable of hepatic steatosis grading.
4. Conclusion
In conclusion, the tapered silica fiber SPR sensor using photopolymer coating developed in this study shows high sensitivity and a wide detection range, making it suitable for various applications. We prove that tapered fiber waist diameter and photopolymer film can affect the sensing performance. The optimal tapered fiber waist diameter and photopolymer film thickness are 10 and 1.8 µm, respectively. An optimized sensor has an overall sensitivity of 3686.25 nm/RIU, allowing it to detect RIs ranging from 1.333 to 1.493. The temperature response of the sensor is about 1.43 nm/°C at 20 − 50 °C, which has little impact on RI detection. Additionally, we demonstrate that the sensor can grade the severity of hepatic steatosis because of its superior sensing performance.
Funding
National Natural Science Foundation of China (61975039, 62175046, 62205086); China Postdoctoral Science Foundation (2022M720940); Natural Science Foundation of Heilongjiang Province (YQ2020F011); 111 Project (B13015); National Defense Basic Scientific Research Program of China (JCKYS2023604SSJS003); Fundamental Research Funds of Harbin Engineering University (3072023CFJ2501).
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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