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Abstract
Various optical components employed in biomedical applications have been fabricated using spider silk because of its superior properties, such as elasticity, tensile strength, biodegradability, and biocompatibility. In this study, a highly sensitive fiber optic sugar sensor is fabricated using metal-nanolayer-coated spider silk. The spider silk, which is directly collected from Nephila pilipes, a giant wood spider, is naturally a protein-based biopolymer with great flexibility, low attenuation, and easy functionalization. The surface of the spider silk-based fiber is coated with a metal nano-layer by using the glancing angle deposition technique. This fiber optic sugar sensor is based on the principle of the change in the refractive indices of sugar solutions. The attained experimental results show that the proposed sugar sensor is highly sensitive in the detection of fructose, sucrose, and glucose concentrations. This work may provide a new way to realize precise and sensitive online sugar measurements for point-of-care diagnostics.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The measurement of sugar levels in humans or animals is important for a better standard of living [1]. Owing to this, medical blood testing is widely performed to gain perspective on the overall health of an individual. This is imperative since diabetes is a major health problem that is growing at an accelerated rate in every corner of the world. So much so that global healthcare expenditure for diabetes is expected to reach $490 billion by 2030 [2]. Nowadays, medical blood testing can be done in the comforts of home. It is suggested for the patients with type 1 diabetes to monitor their blood glucose levels with a sensor two hours before and after each meal. The glucometer or glucose sensor is widely used for glucose control. It is proven to exhibit excellent sensitivity and precise measurements in a short time [3]. However, traditional glucose sensors are invasive, and the punctures performed during monitoring cause discomforts and carry risks of infections [4]. In addition, since the single-use test strip is only reactive with the lancet, the tool used to puncture the skin to obtain a blood sample, once, it is not cost-efficient. Moreover, these test strips have expiration dates, and any measurement done after this date may result in erroneous information. Hence, a simple yet practical technique is required in the next generation for simultaneously measuring sugar concentration and distinguishing the type of sugar, such as glucose, fructose, and sucrose.
Recent efforts on fiber optic sensors have made substantial progress for the detection and measurement of various physical variables including strain [5], pressure [6], voltage [7], vibration [8], and glucose concentration [9–12]. Some follow-up methods for glucose concentration measurements obtain higher sensitivity by various types of fiber optics such as fiber ball [13], S-shaped fiber [14], U-shaped fiber [15], photonic crystal fiber [16,17], and fiber Bragg gratings [18,19]. Moreover, these fiber optic sensors are not disposable and can be developed with physical modifications. The physical phenomenon of the surface plasmon resonance (SPR) is commonly utilized for real-time glucose detection [20–24]. The SPR induces a collective oscillation of conduction band electrons generated by the interaction of lightwave and metal layer. The intensity and wavelength of the SPR are highly dependent on the size and shape of metal layer. In comparison to shaped optical fibers, SPR-based fiber optic sensors have short electromagnetic field decay length and less influence in temperature fluctuation. It also has a simple design and a high sensitivity for sugar interaction. To enhance the biocompatibility of fiber optic sensors, native spider silk has attracted increasing attention due to its unique mechanical and optical properties [25–27]. The advantages of the spider silk-based fibers include great elasticity, large tensile strength, and high optical transmission. These significant properties of spider silk-based fibers make various biological applications involving optical guiding, imaging, and sensing [28–31]. Hence, the fabrication of spider silk-based optical fibers with cost-effectiveness, high sensitivity, and compactness needs further demonstration for detecting sugar concentration.
In this study, we propose a low-cost spider silk-based metal-dielectric-coated optical fiber for the detection of sugar solutions with different concentrations. This silk-based fiber optic sensor is based on the changes in the refractive index of the sugar solution. The silk fiber is directly collected from Nephila Pilipes, a local species of giant wood spider. To develop the optic sensor configuration, photocurable resin is applied onto the silk’s surface by an automatic dispenser. The fiber’s surface is then coated with metal nano-layer by the glancing angle deposition technique [32]. The performance of the proposed spider silk-based fiber optic sensor is tested by sensing sets of glucose, fructose, and sucrose solutions with known sugar concentrations that range from 10% to 50%. This encompasses the complete range of sugar concentration found in human blood. The experimental results prove that the silk-based fiber optic sensor is cable of measuring the refractive indices as well as the concentrations of sugar solutions. Moreover, the response time of this fiber optic sugar sensor is 0.1 ms. This makes it feasible for this device to become useful for a multitude of applications in clinical detection and diagnosis.
2. Fiber optic sensor fabrication
A particularly attractive feature of spider silks is that they are composed of the protein fibres. The dragline silk, secreted by the major ampullate glands of spider, comprises of amino acids glycine and alanine. It is the silk usually used as the main building material for the web because of its high tensile strength. Figure 1(a) shows the setup of the manufacturing process of spider silk-based optical fiber. In this research, native dragline silk is obtained from the major ampullate glands of a giant wood spider, Nephila Pilipes. The diameter of the collected spider silk is 10 µm. Then, an automatic dispenser coats the silk’s surface with photocurable resin (Heart-bond TG-1288), which is biocompatible and biodegradable material [33,34]. Because the inner diameter (0.65 mm) of the needle is much larger than the silk’s diameter, the photocurable resin dispensed from the needle envelopes the silk fiber. Photocurable resin is then continuously released through the length of the silk fiber surface, which results in a smooth photocurable resin clad surface. In this coating process, the needle tip is placed near the surface of the spider silk. Figure 1(b) shows a uniformly spun fiber reeled from the spider spigot under ultraviolet exposure at a constant speed of 5 mm/s. The solidified dielectric silk-based optical fiber is attainable because of the spider silk’s excellent wet-rebuilding ability. This automatic reeling process results in a spider silk-based optical fiber with a smooth surface, an even circular cross-section along its length, and homogenous material properties (see Fig. 1(c)). Since the refractive indices of the silk-core and resin-cladding layers are 1.55 and 1.48 [35], optical guiding can be realized due to the principle of total internal reflection. After reeling collection, the spider silk-based optical fiber is measured to have a diameter of 100-µm. A sharp scalpel is used to cut the silk-based optical fiber in a length of 25-mm, and this is then placed on a specialized holder. To make the fiber optic sensor very specific for the sugar detection [12], the sensing surface of the fiber is functionalized with biocompatible gold nano-layer [36]. Figure 1(d) shows the sputter deposition process for coating metal nano-layer on the silk-based fiber surface. Since the silk-based fiber is rotated with respect to the sputtering gun by rotary mechanism, the metal nano-layer is uniformly deposited on the fiber surface. Figure 1(e) shows the scanning electron microscope (SEM) image of the actual spider silk-based metal-dielectric-coated optical fiber. It can be observed that the spider silk-based metal-dielectric-coated optical fiber fabricated by this process is of high diameter uniformity. A commercial focused ion beam system (Helios NanoLab 600i) is used to verify the thickness of gold nano-layer. Figure 1(f) shows that the thickness of the gold nano-layer is about 50 nm. The laminated structures on the gold nano-layer are used to protect the gold nano-layer during the cutting process. Because the dielectric fiber core is a soft material, the cross-section of silk-based metal-dielectric fiber has a little deformation due to milling. The spider silk-based metal-dielectric-coated optical fiber is promising for plasmonic applications.
Fig. 1. (a) Manufacturing process of spider silk-based optical fiber. (b) Electric reeling system with ultraviolet exposure. (c) Microphotograph of spider silk-based optical fiber. (d) Sputter deposition for coating spider silk-based metal-dielectric optical fiber. (e) SEM image of spider silk-based metal-dielectric optical fiber. (f) SEM cross-section image of spider silk-based metal-dielectric optical fiber.
Furthermore, the homogenous quality of the spider silk-based optical fiber with metal coating can be confirmed by performing tensile tests [37]. Figure 2(a) shows the tensile testing machine (Hung-Ta HT-2402) used to test the strength and elasticity of the fabricated spider silk-based fiber. The stationary base is adjusted to accommodate the length of the silk-based fiber, and the load cell is operated to apply tension to the silk-based fiber. As shown in Fig. 2(b), the spider silk-based optical fiber is placed in between the two holding grips of the machine and is subjected to a controlled tension until failure. During the testing process, an extensimeter automatically records the change in gauge length. Figure 2(c) depicts the stress-strain curve of the fabricated spider silk-based optical fiber at room temperature. There are two areas in the stress-strain curve that refer to the different behavior and mechanical properties of materials. The first region is called the linear elastic region, and it is guided by the general Hooke’s law which states that stress applied is proportional to the strain formed. The second region, referred to as the strain hardening or the inelastic region, is characterized by a generally increasing stress as the fiber elongates, and eventually, ends up with a fracture. Figure 2(d) depicts the Young’s modulus calculations derived from the linear elastic region of the stress-strain curve for the silk-based optical fiber. The results indicate that the Young’s modulus decreases as the time progressed. Furthermore, the elongation-at-break is found to be over 48.8% elongation. From this tensile test, it is revealed that the proposed spider silk-based optical fiber exhibits superior mechanical properties than general silica optical fiber. This gives the silk-based optical fiber a vantage point to perform a better role in more stringent conditions.
Fig. 2. (a) Tensile testing machine. (b) Tensile testing for spider silk-based optical fiber. (c) Stress-strain curve and (d) Young’s modulus measurements of the fabricated spider silk-based optical fiber.
3. Experimental principles and measurements
Since a previous study has demonstrated spider silk to be an efficient tool for optical propagation even when immersed in physiological liquid [25], the spider silk-based metal-dielectric-coated optical fiber can be used as a tough, biocompatible device for medical applications dealing with biological media. Figure 3(a) shows the schematic of the experimental arrangement for sensing different sugar concentrations. The insert in Fig. 3(a) indicates the coupling of the spider silk-based metal-dielectric fiber and two general silica multimode optical fibers. The spider silk-based metal-dielectric-coated fiber optic sensor is placed on a specialized holder, and is immersed in a sugar solution. A peristaltic pump (EYELA MP-3000) with infusion tubes is used to control the flow rate of the sugar solution through the fiber optic sensor. A beam of halogen light is coupled with one end of the spider silk-based metal-dielectric-coated fiber optic sensor using a general optical fiber with the alignment of a micro-stage. The efficiency of light coupling is measured to be at 92%. Another general optical fiber is coupled with the other end of the silk-based fiber optic sensor to act as an optical receiver. This also guides the transmitted light wave into a miniature spectrometer (OtO SW2560) that is interfaced with a computer. The spectrometer has a wavelength detection range of 900 nm – 1700 nm, the resolution of 4.5 nm, and the integration time of 50 µs. An optical microscope is employed to capture the optical field intensity from the top view of the silk-based fiber optic sensor.
Fig. 3. (a) Experimental arrangement of the spider silk-based metal-dielectric fiber optic sensor. (b) Experimental and (c) simulation results of the spider silk-based metal-dielectric fiber optic sensor at incident wavelengths of 380 nm – 1700nm.
Figure 3(b) shows the captured image of lightwave propagation along the spider silk-based optical fiber during the experiment. According to physical optics, an optical fiber is capable of efficiently guiding lightwaves only when the refractive index of an optical fiber is higher than the surrounding medium. Here, the refractive index of the spider silk-based optical fiber is high enough to guide and deliver lightwaves in the sugar solution. The white lightwave is propagating inside the spider silk-based optical fiber with little light scattering from the fiber surface. Transmission loss is largely dependent on the fiber properties and surface scatter. The experimental image shows that the spider silk-based optical fiber is fabricated without surface damage which is the fundamental requirement for low loss of transmission. For theoretical observation, the finite-difference time-domain (FDTD) simulation is utilized to demonstrate the propagation performance of the spider silk-based optical fiber [38]. The mesh grid in the simulation is set to 10 nm and a perfectly matched layer boundary is implemented to ensure calculation accuracy. Figure 3(c) shows the simulated electric field patterns of the spider silk-based optical fiber. It is noticeable that the smooth surface of the spider silk-based optical fiber is the key to guide lightwave with low scattering loss. Based on the results above, high refractive index and low transmission loss allow the spider silk-based optical fiber to deliver lightwave for sensing sugar concentration.
For the characterization of the developed spider silk-based metal-dielectric-coated fiber optic sugar sensor, three sets of solutions with different sugars, namely fructose, sucrose, and glucose, are prepared with varying concentrations. These solutions have sugar concentrations that range from 10%-50% and are varied by 10% increments. Fructose, sucrose, and glucose powders are separately diluted in 40 ml of deionized distilled water to create the sugar solutions. The refractive indices of these three sets of sugar solutions with five different concentrations are measured using a digital refractometer (Dogger C9W-RSD501). Measurements using the spider silk-based metal-dielectric-coated fiber optic sugar sensor are carried out by confining the three sugar solutions of known concentrations at constant room temperature. To demonstrate the time stability and repeatability of the silk-based fiber optic sensor, the experiments are repeated 10 times at 5-minute intervals for each level of concentration of the sugar solutions. The sensing abilities of the silk-based fiber optic sensor are estimated by recording the transmitted intensity spectra in the presence of sugar solutions.
Figure 4 shows the transmission spectra of the silk-based fiber optic sensor in various sugar solutions with five varying concentrations. It can be noted that the transmission peak of around 1300 nm is red-shifted with the increasing the concentration of sugar solutions. The small red-shift of the transmission peak is due to the increase in the real part of refractive index in the sensing layer. It is also observed in Fig. 4(a) that the amplitude of the transmission peak decreases from 50.7% to 38.2% as the fructose concentration increases from 10% to 50%. Whereas, the change in the amplitude of the transmission peak for different glucose concentrations is not obvious in Fig. 4(c). To further evaluate the sensing performance of the proposed sensor, the influence of sugar concentration on the absorption spectrum is also studied. Figure 5 represents the measured absorption spectra for different sugar concentrations of fructose, sucrose, and glucose solutions. The absorption peaks are at 1191 nm, 1195, and 1197 nm for 10% fructose, sucrose, and glucose solutions, respectively. In Fig. 5(c), the distributions of the absorption spectra are almost the same at different glucose concentrations. By comparison of the transmission and absorption spectra, the variation of the transmission peaks is a significant indicator to differentiate various sugar solutions and concentrations.
Fig. 4. Measured transmittance spectra for different sugar concentrations of (a) fructose, (b) sucrose, and (c) glucose solutions.
Fig. 5. Measured absorbance spectra for different sugar concentrations of (a) fructose, (b) sucrose, and (c) glucose solutions.
To quantitatively estimate the performance of the proposed silk-based fiber optic sensor, a commercial refractometer is used to measure the refractive index of each concentration for the three sugar solutions. Figure 6(a) shows the refractive index as a function of the concentration for the three sets of sugar solutions. It is observed that the refractive index of the sugar solutions is proportional to the sugar concentration. In the same level of concentration, the refractive index of sucrose solution is higher than that of fructose and glucose solutions. For example, the refractive indices of glucose, fructose, and sucrose solution are 1.408, 1.416, and 1.418 at 50% concentration, respectively. The proposed silk-based fiber optic sensor is sensitive to the variation in the refractive indices of sugar solution. The transmission peak of the silk-based fiber optic sensor is strongly dependent on the sugar concentration, which is the essential of this sensor. The sugar concentration can be determined from the slope of the transmission T versus wavelength λ for the peak near 1300 nm. The slope is calculated by finding a secant line to the transmission peak. The transmission peak is decreasing if it goes down from center to right, and the slope is negative. Figure 6(b) shows the calculations for the different sugar concentrations based on the slope -ΔT/Δλ evaluated at λ =1300 nm. The slopes and corresponding standard deviations are calculated by 10 repeated measurements. The error bars shown in Fig. 6(b) indicate the standard deviation of the 10 repeated measurements at each concentration for the three sets of sugar solutions. Table 1 shows the calculated average slopes and the corresponding standard deviations for three sugar solutions. Each of the calculated slopes represents the type and concentration of sugar solution at that particular condition. For example, 10% and 50% concentrations of the glucose solution are measured to obtain the slopes of 0.11 and 0.088 at the transmission peak, respectively. Based on Fig. 6(b), the average slope with low standard deviation indicates that the silk-based fiber optic sensor provides precision measurements to distinguish the transmission features induced by different sugar solutions with varying concentrations. The measurement precision is defined as P = SD / Avg, where SD and Avg are the standard deviation and the average of a set of peak intensity measurements. The measurement precisions for each level of concentration of three sugar solutions are all less than 1% as the time progressed. The experimental results imply that a sugar solution with unknown concentration can be estimated accurately by the slope of the transmission peak at λ =1300 nm. For instance, the measured slopes of 0.035, 0.068, and 0.11 represent the sucrose, fructose, and glucose solutions with 10% concentration, respectively.
Fig. 6. (a) Refractive index and (b) transmittance slope at λ = 1300 nm as a function of concentration for different sugar solutions.
Table 1. Measured average slopes and the corresponding standard deviations for three sugar solutions
The sensing sensitivity can be mathematically defined as S = ΔI / Δn, where Δn is the refractive index difference of different sugar concentrations and ΔI is the intensity difference of the transmission peak in the case of Δn [39]. The intensity of the transmission peak is expressed in counts, and the refractive index of sugar concentration is expressed in refractive index unit (RIU). The sensitivities of the proposed silk-based fiber optic sensor are calculated to be 136123, 98957, and 51013 counts/RIU for fructose, sucrose, and glucose solutions, respectively. This is a relatively high sensitivity compared to that of the currently reported optical fiber sensors [39]. The limit of detection (LOD) is defined as LOD = ΔImin / S, where ΔImin is the minimum detectable change of the peak intensity and S is the sensitivity corresponding to the peak intensity [39]. The LOD of the proposed silk-based fiber optic sensor are calculated to be 2.71×10−5, 3.73×10−5, and 7.25×10−5 RIU for fructose, sucrose, and glucose solutions, respectively. As a result, the sensitivity of the proposed sensor completely encompasses the range of sugar concentrations found in human blood.
4. Conclusion
In the current work, a novel strategy using metal-nanolayer-coated spider silk-based optical fiber is proposed for sugar concentration sensing. Compared to traditional glass-based optical fibers, the spider silk-based optical fiber exhibits notable flexibility and biocompatibility, and thus is more suitable and safer for the measurements in biological environments. The liquid collection capability of wet-rebuilt spider silk is utilized for the formation of the silk-based optical fiber. Furthermore, the glancing angle deposition technique is used to coat metal nano-layer on the fiber surface for enhancing the sensitivity of sugar concentration detection [12,24]. The authentication of the proposed silk-based fiber optic sensor is done by recording the transmission spectra for the three sets of sugar solutions with varying sugar concentrations that the ranges from 10% to 50%. The slopes at the transmission peak can be used to determine the concentrations and types of sugar found in the solutions. The repeatability, reusability, stability, and selectivity of this silk-based fiber optic sensor are demonstrated by repeatedly performing measurements. Optical sensing could still be achieved one year later with the same sensitivity when the proposed silk-based sugar sensor is stored in dry and dust-free conditions at room temperature [29]. With further development, this newly proposed silk-based fiber optic sugar sensor will be useful for implantable medical devices and treatment strategies in biomedical applications.
Funding
Ministry of Science and Technology, Taiwan (108-2221-E-010-012-MY3, 110-2221-E-492-006, 111-2221-E-A49-102-MY2).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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