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Highly sensitive detection of biological substances using microfluidic enhanced Fabry-Perot etalon-based optical biosensors
Sensors & Actuators: B. Chemical 277 (2018) 62–68
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Highly sensitive detection of biological substances using microfluidic enhanced Fabry-Perot etalon-based optical biosensors Kyung Eun Youa,b,1, Nezam Uddinb,1, Tae Hyun Kimc, Qi Hua Fand, Hyeun Joong Yoona,b,
T ⁎
a
Department of Electrical and Biomedical Engineering, University of Nevada Reno, Reno, NV 89557, USA Department of Electrical Engineering and Computer Science, South Dakota State University, Brookings, SD 57007, USA c Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA d Department of Electrical and Computer Engineering, Michigan State University, East Lansing, MI 48824, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Fabry-Perot etalon Optical biosensors Refractive index Microfluidic device
A microfluidic based optical biosensor is introduced to detect concentrations of biochemical substances in solution using refractive index measurement with high sensitivity and accuracy. The sensor consists of a liquid channel forming a Fabry-Perot cavity between two semitransparent Ag/SiO2 reflective surfaces. Light is transmitted through the cavity to construct interference peaks in the transmission spectra which depend on the refractive index of the test samples in the channel. The refractive index of glucose, potassium chloride, and sodium chloride solutions is measured in different concentrations. Continuous change in refractive index is resolved by observing the peak wavelength shift in the transmitted spectrum. The sensor is characterized using the contact angle measurer, surface profilometer, and spectrophotometer. The proposed Fabry-Perot etalon biosensor shows real time linear responses as well as high accuracy and sensitivity of 10−3 refractive index per percent of glucose, 1.4 × 10−3 and 1.8 × 10-3 refractive index per percent of KCl and NaCl solution, respectively.
1. Introduction The demand for detecting biological and chemical substances in liquid is gradually increasing in various fields, especially in biomedical and healthcare applications [1–4]. Among many analytical techniques, detection methods based on refractive index have many advantages as they are sensitive to the sample concentration/molecular interactions and accurate even with femtoliter to nanoliter volume [5,6]. Many important parameters can be derived, which can be applied to the fields of biology, chemistry, and environment [7–11]. For example, glucose concentrations could be determined from the refractive index of blood samples for the management of diabetes [12]. Over the past years, several types of sensors to measure the refractive indices of liquids have been reported. The majority of liquid sensors are based on optical fibers due to their simple device structure and compact size [13–21]. However, the limitations of these sensors include relatively low sensitivity (grating based fiber) [22,23], weak mechanical strength and durability (tapered fiber sensors and core mismatch sensors) [24,25], and expensive microfabrication procedure (FBG-based sensor) [26–29]. On the other hand, surface plasmon resonance (SPR) sensors [30–32] have
been demonstrated with high accuracy and real-time response, but are costly and sensitive to the environment temperature. A polymer nanostructured Fabry–Perot interferometer with microfluidic network has also been developed [33]. Although it increased the optical interaction with the active layer up to 1–5 μm in depth, which was not possible by SPR, the device structure was complex with a nanopore template, a gold layer and a PDMS layer. In addition, a biochemical sensor based on photonic crystal cavities has been introduced, which generates a proportional shift of resonant wavelength based on the change in concentration of a target molecule [34]. However, the highly sensitive photonic crystal sensors suffer from expensive nanofabrication processes [35,36]. Therefore, Fabry-Perot etalon with microfluidic chamber can be an effective alternative of the above-mentioned techniques for the detection of biological substances in liquid. Fabry-Perot etalons are attractive platforms for label-free bio-sensing as they increase the sensitivity in terms of phase shift or absorption by creating effective interference between two reflective surfaces [11,37–41]. In this paper, a microfluidic based optical biosensor integrated with a Fabry-Perot etalon structure was developed. The Ag layer was used to enhance the signal intensity which provided real time
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Corresponding author at: Department of Electrical and Biomedical Engineering, University of Nevada Reno, 1664 N. Virginia Street, Reno, NV, 89557, USA. E-mail address: [email protected] (H.J. Yoon). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.snb.2018.08.146 Received 20 March 2018; Received in revised form 22 August 2018; Accepted 28 August 2018 Available online 01 September 2018 0925-4005/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic of the principle of the Fabry-Perot etalon (a) Basic structure, (b) Calculation of the reflected wave.
response with high sensitivity and resolution of the liquid compositions. The microfluidic structure eliminated the need of auxiliary pressure to facilitate sample loading to the sensory area [42] and increased the light-matter interactions enabling accurate detection of biological substances with only a small volume [12,43]. Shift of peak wavelength in the transmitted spectrum was measured to calculate the refractive index of the fluid in the microfluidic chamber. The proposed sensor has the features of fast response, mechanical strength, high sensitivity, accuracy, real-time response, easy fabrication and less complex optical set-up to detect biological substances using optical properties. The detection of NaCl, KCl and glucose in liquid has been shown to be useful considering the fact that the diabetic patients need to maintain the level of the key electrolytes (NaCl and KCl) per kg of body weight [44]. 2. Device principle A Fabry-Perot etalon consists of two parallel reflecting surfaces separated by a distance d in which multiple light beams interfere. Due to the highly reflecting plane surfaces facing each other, an infinite number of parallel beams transmit from the semitransparent surfaces (Fig. 1). When these infinite waves are superimposed with decreasing amplitude, the rays form a sharp constructive interference. The equation of free spectral range (FSR) is used to find the gap between the two reflective surfaces from refractive index of a known medium.
c FSR = Δʋ = ʋm + 1− ʋm = 2ndcosθ
Fig. 2. Device structure for the proposed modified microfluidic Fabry-Perot etalon (a) 3D view, (b) Cross-sectional view.
stainless steel syringe needle was used as a tool electrode. The applied voltage was 50 V and the current was 50 mA. Ag thin films (15 nm) and SiO2 thin films (5 nm) were deposited by RF sputtering system. Two Ag layers were used in the proposed Fabry-Perot etalon structure because of their high reflectance (> 90%) and relatively low absorption coefficient compared to other metals (Al, Cu, Au, Cr) in the visible region from 400 to 750 nm [46]. The Ag layer reflected the light repeatedly within the channel, and as the two reflective layers were facing each other in parallel, constructive interference occurred. Light was also able to pass through these semitransparent layers. The transmitted light from the second Ag layer acted as an output spectrum of the sensor and the refractive index was subsequently measured. SiO2 was used to protect the Ag layer from oxidation as well as increase the hydrophilicity of the surface so that the liquid sample can easily flow through the channel. Finally, the positions of the inlet and outlet ports of the upper glass slide were aligned to lower glass substrate. The inlet and outlet tubes were connected to the upper glass slide by using epoxy.
(1)
where ν is the frequency where the maximum transmission occurs, m is the integer which determines the order of transmission peaks, d is the distance between the two glass substrates,θ is the transmission angle, and nis the refractive index of the medium. From the transmission spectrum of the Fabry-Perot etalon cavity filled with liquid, the refractive index of the analyte in the cavity can be extracted using the Eq. (1) with a known d. 3. Materials and methods 3.1. Device fabrication Fig. 2 shows the layer by layer structure with the microfluidic chamber of the proposed Fabry-Perot etalon sensor. Two glass slides were prepared for the top and bottom substrates of the sensor. The glass slides were cleaned by subsequent ultrasonication in acetone, IPA and DI water, followed by Ag and SiO2 deposition and patterning as described in our previous work [45]. The bottom glass substrate has a cavity which was fabricated by photolithography and buffered oxide etch (BOE). The inlet and outlet ports were fabricated by electrical discharge machining (EDM). The glass slide was submerged in the electrolytic solution of NaOH. A tungsten carbide rod (ASTM B777-07, 0.3125″ diameter, 12″ length) was used as a counter electrode and a
3.2. Experimental procedure Fig. 3 shows the experimental setup for measuring the transmittance of the Fabry-perot etalon sensor. Prior to injecting the solutions, all sensors were calibrated to obtain the exact thickness of the cavity by measuring the transmission spectrum with an air gap using equation (1) [45]. The glucose, KCl and NaCl solutions were prepared in different concentrations of 5%, 10%, 15%, 20%, and 25%. The solutions were injected in the microfluidic chamber using the syringe pump. The 63
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the refractive index of the liquid. The light source was a combination of regulated lamps wavelength ranging from 380 to 1050 nm. 4. Results and discussion 4.1. Effect of Ag/SiO2 in the transmittance of the Fabry-Perot etalon sensor The transmission through the glass slide was around 94% (Fig. 4). There was no adsorption in the near ultraviolet wavelength as the band gap of glass is large. The transmittance was reduced when the light passed through the Ag coated glass slide. The complex interaction process between photons and atoms of metal as well as the dielectric layer caused the refraction, reflection and transmission. The optical refraction and absorption occurred due to the interaction of photons with the electron of Ag layer. Although the SiO2 layer was assumed to be transparent, the transmittance was reduced due to adsorption in the visible range. The energy of photon was absorbed for high index dielectric materials (SiO2) at longer wavelengths. The light induced some oscillation on free electrons of Ag when the incident light reached the metal surface. A cloud of negative charge consisting of weakly bound electrons of the metal atoms would be created due to the interaction of photons with the free electrons. These electrons vibrate with the same frequency of light and thus create a reflected light. Some photons with different energy would not interact with the loosely bound electrons and cause transmission with a loss of energy (absorption). The transmission and the reflection path depend on the type of interaction which governs the refractive index. SiO2 has very low contribution in the refractive index value change. In glass or SiO2, the electrons are firmly bound and can only oscillate around their normal position. This movement influences the propagation of light so that its wave velocity is reduced, while there is only a small loss of energy. Overall, the interaction of photons with the free electrons of the Ag layer, bound electrons of SiO2 and liquid causes the change in refractive index value. The interaction is highly dependent on the wavelength of light which affects the refractive index value. As we used very narrow wavelength range for our measurement, the wavelength effect on refractive index measurement can be neglected.
Fig. 3. Experimental setup for liquid detection with the proposed Fabry-Perot Etalon sensor.
Fig. 4. Transmittance versus wavelength spectra of glass slide, glass coated with Ag film, and glass slide coated with Ag and SiO2.
syringe pump enabled the sample solution replacement without changing the position of the sensor in which light was pointed. After the chamber was completely filled with the liquid, the transmitted spectrum was measured by a spectrometer. The Filmetrics F-20 software was used to make the detected spectrum visible for analysis [45]. The peak wavelength from the transmittance spectrum was noted to find out
4.2. Simulation using the matrix method The transmittances from the liquid having different concentrations were calculated using the admittance method where each layer is
Table 1 Parameter for Simulation of transmittance spectrum of proposed enhanced Fabry-Perot etalon sensor using admittance method. Layer
Material
Index of refraction
Extinction Co-efficient
Medium 1 2 3
Glass Ag SiO2 Air 5% Glucose solution 10% Glucose solution 15% Glucose solution 20% Glucose solution 25% Glucose solution 5% NaCl solution 10% NaCl solution 15% NaCl solution 20% NaCl solution 25% NaCl solution 5% KCl solution 10% KCl solution 15% KCl solution 20% KCl solution 25% KCl solution SiO2 Ag Glass
1.51674 0.05890 1.45842 1.0003 1.3380 1.3430 1.3480 1.3530 1.3580 1.3418 1.3505 1.3594 1.3684 1.3778 1.33931 1.34643 1.35335 1.35992 1.3676 1.45842 0.05890 1.51674
0.00000 3.65540 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 3.65540 0.00000
4 5 Substrate
64
Physical thickness
15 nm 5 nm 3 nm
5nm 15nm
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Fig. 5. Simulation results of transmission spectra of the Fabry-Perot etalons with air and solution-filled channel (a) Glucose solution, (b) KCl solution, (c) NaCl solution.
Fig. 6. Transmission spectra of the Fabry-Perot etalons with air gap and solution-filled gap from experimental result (a) Glucose solution, (b) KCl solution, (c) NaCl solution.
Perot etalon for air, glucose, KCl and NaCl solutions of different concentrations. The maximum transmission of the sensor occurred when the phase differences between two consecutive rays of the transmitted light were π, 2π, 3π, ..., and Nπ . Here, the phase differences were inversely related to the wavelength and proportional to the refractive index and the thickness of the cavity. Since the thickness of the cavity was fixed, the wavelength of maximum transmission depended on the value of refractive index for a value of phase difference. The transmission spectrum of air was calculated for comparison. The real-time shift of transmission peaks to the right direction was clearly visible with the increase in solution concentration from 5% to 25%. The values corresponded to the refractive index changes of solutions from 1.338 to 1.358, 1.33931 to 1.3676 and 1.3418 to 1.3778 for glucose, KCl and NaCl, respectively.
represented by a characteristic 2 × 2 matrix [47–49]. The characteristic matrix includes effects of the thickness and the refractive index of each film as well as the dependence on the wavelength of light source. The transmittance can be calculated from the overall characteristic matrix considering the effect of all layers. The reference refractive indices of glucose, NaCl and KCl solutions were adapted from previous works [50,51]. The specific value of refractive index was chosen at ∼589.00 nm wavelength with the incident angle of zero degree. The glucose solution mainly absorbs the light in the infrared region [52]. The extinction coefficient was set to zero and the dispersion effect was not taken into consideration as the change of refractive index value would be negligible with wavelength in the visible range. The dispersion and absorption effects by Ag and SiO2 were not considered as the operating wavelength range was too small. The thicknesses of Ag and SiO2 were assumed to be 15 nm and 5 nm, respectively. Table 1 is the summary of parameter used for simulation. Fig. 5 shows the transmittance spectra of the microfluidic Fabry65
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the absorbing medium. The wavelength shifted towards right with increasing the concentration of the liquid which in turn increased the refractive index value. The higher the refractive index value, the lower the FSR value which would result in wider wavelength range. The thickness of the cavity and refractive indices of the NaCl and KCl solutions were determined in a similar way as shown in Fig. 6(b) and (c). The spectrum of glucose was narrower than those of KCl and NaCl while the spectrum of NaCl was wider than that of KCl. This was due to the fact that the refractive index of glucose was lower compared to KCl and NaCl. The light needs less optical path to pass through the glucose solution due to lower density than NaCl and KCl of the same concentrated solution. Table 2 summarizes the peak positions from the transmittance spectra of the Fabry-Perot etalon, gap thickness and refractive index for different liquid solutions. The separation of peak position increased with increasing wavelength for all liquid solution. The thickness of the cavity was found from the two peak wavelengths of air spectrum. As the thickness of the cavity was not changed; the refractive index could be extracted using two peak wavelengths from the transmittance spectrum of the liquid solution. With increasing the gap thickness, transmittance spectrum contains more peak wavelengths in the same wavelength range. Fig. 7 shows the linear response of refractive index with various concentrations of glucose, KCl and NaCl solutions. As the refractive index depended on the amount of analyte present in the liquid, the glucose level could be found from the refractive index value likewise the other liquid solutions. From the linear response, the unknown liquid concentration could be extracted. The sensitivity of glucose measurement was reflected by the slope of the linear fitting, being ∼10−3 refractive index per % of glucose. The sensitivities for KCl and NaCl detection were 1.4 × 10−3 and 1.8 × 10-3 refractive index per percent of liquid concentration, respectively. This sensitivity could be translated into wavelength difference using equation (1). The refractive index was inversely proportional to temperature at a rate of ∼ 0.00014 /o C. This
Table 2 Summary of the peak positions in the transmission spectra with air gap and solution-filled gap, calculated gap thicknesses, and refractive indices. Peak 1 (nm)
Peak 2 (nm)
Gap thick. (μm)
Peak 1 (nm)
Peak 2 (nm)
n
5% Glucose 10% Glucose 15% Glucose 20% Glucose 25% Glucose
591.17 591.17 591.17 591.17 591.17
617.79 617.79 617.79 617.79 617.79
6.86 6.86 6.86 6.86 6.86
588.99 589.74 590.2 590.77 591.18
608.51 609.24 609.65 610.18 610.55
1.3383 1.3429 1.3484 1.3536 1.3582
5% KCl 10% KCl 15% KCl 20% KCl 25% KCl
581.80 581.80 581.80 581.80 581.80
607.09 607.09 607.09 607.09 607.09
6.98 6.98 6.98 6.98 6.98
581.98 583.2 583.9 584.65 585.29
600.67 601.87 602.51 603.21 603.8
1.3392 1.3462 1.3536 1.3605 1.3670
5% NaCl 10% NaCl 15% NaCl 20% NaCl 25% NaCl
589.71 589.71 589.71 589.71 589.71
615.59 615.59 615.59 615.59 615.59
7.01 7.01 7.01 7.01 7.01
595.96 596.99 597.95 599.14 600.05
615.45 616.41 617.31 618.44 619.28
1.3416 1.3510 1.3592 1.3687 1.3776
4.3. Measured device performance The transmittance was measured each time after the liquid of different concentration was injected into the channel. The light source pointed to the same spot of the sensor during the measurement. The spectrum shifted according to the concentration of the liquid. Fig. 6 (a) shows the transmission spectra of the sensor with an air gap and glucose solutions of 5% to 25%. Two consecutive interference peaks were found at 591.17 nm and 617.79 nm for air. With the 5% glucose, two interference peaks were found at 588.99 nm and 608.51 nm, from which the refractive index was deduced to be 1.338. Both the intensity and half power band width reduced with increasing wavelengths in the same spectrum of the liquid, which was attributed to
Fig. 7. Refractive index vs. (a) KCl concentration (%), (b) NaCl concentration (%), (c) Glucose concentration (%). Rhombus: measured values. Continuous line: fitting result. 66
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temperature effect was attributed mainly to decrease in the density of the solution with increasing temperature. The limits of concentration difference that could be determined by the Fabry-Perot sensor were 0.01% glucose, 0.00769% KCl and 0.00555% NaCl solutions, respectively, which was comparable with the other approaches [53,54]. The resolution of the concentration was found from the sensitivity and resolution of the refractive index which was 10-5 refractive index unit. However, the current resolution of the sensor was only limited by the equipment which could further be improved with a higher resolution spectrophotometer in determining the peak wavelength. The maximum resolution of spectrophotometer used in the experiment in determining the peak wavelength is 0.1 Å or 0.01 nm. As discussed before, the sensor measured the positions of the interference peaks. Therefore, the signalto-noise ratio (SNR) depended on the resolution of the spectrophotometer. The SNR was found to be ∼54.69 dB which was already shown in our previous work [45]. Although the sensor produced promising results, limitations remain before it could be readily applied for clinical applications. The current version of our sensor was examined for detecting certain substances in single component solutions. However, biological specimens such as whole blood are complex and modifications to enable selective detection of a desired substance from compound solutions are highly required. Future approaches to improve the specificity of our sensor may include methods immobilizing target molecules with antibodies or enzymes in the active layer using surface functionalization (for example, glucose in the blood using glucose oxidase). Also, combining with microfluidic devices which allow onchip sample preparation such as molecular separation, concentration, and purification, prior to optical detection would potentially improve the precision and expand the sensor functionality.
[5] [6] [7] [8] [9]
[10] [11]
[12] [13] [14]
[15]
[16]
[17] [18]
[19]
[20]
5. Conclusions
[21]
A microfluidic enhanced Fabry-Perot etalon liquid sensor with two semi-transparent Ag/SiO2 surfaces is highly accurate for the refractive index measurement of glucose, KCl and NaCl solutions. The refractive index was linearly proportional to the liquid concentration with a satisfactory R2 value. Therefore, the concentration of certain substance can be correctly estimated from the linear response showing the potential as a tool for disease detection and management. Conclusively, the proposed sensor enables accurate and sensitive detection of specific biological and chemical substances in liquid with simple optical set-up. These features of the sensor are expected to surmount the limitations of other sensors including commercial glucometers.
[22]
[23]
[24]
[25] [26]
[27]
Acknowledgments
[28]
This work was partly supported by the NASA Grant Award 80NSSC18M0022 and the National Science Foundation Awards # 1700785 and # 1700787. Also acknowledged is the support from Sanford Health–South Dakota State University Collaborative Research Seed Grant Program (3×6660, 2S6661).
[29]
[30]
Conflict of interests
[31]
The authors declare no competing financial interests. [32]
References [1] T. Wei, Y. Han, Y. Li, H.-L. Tsai, H. Xiao, Temperature-insensitive miniaturized fiber inline Fabry-Perot interferometer for highly sensitive refractive index measurement, Opt. Express 16 (8) (2008) 5764–5769. [2] M.F. Ferreira, G. Statkiewicz-Barabach, D. Kowal, P. Mergo, W. Urbanczyk, O. Frazão, Fabry-Perot cavity based on polymer FBG as refractive index sensor, Opt. Commun. 394 (2017) 37–40. [3] T. Takahashi, T. Hizawa, M. Nobuo, M. Taki, K. Sawada, K. Takahashi, Surface stress sensor based on MEMS Fabry–Perot interferometer with high wavelength selectivity for label-free biosensing, J. Micromech. Microeng. 28 (5) (2018) 054002. [4] L. Li, L. Xia, Z. Xie, D. Liu, Thinned fiber based Mach-Zehnder interferometer for
[33]
[34]
[35] [36]
67
measurements of liquid level and refractive index, OFS2012 22nd International Conference on Optical Fiber sensors, Int. Soc. Opt. Photonics (2012) 84217K. X. Fan, I.M. White, S.I. Shopova, H. Zhu, J.D. Suter, Y. Sun, Sensitive optical biosensors for unlabeled targets: a review, Anal. Chim. Acta 620 (1) (2008) 8–26. M. Xhoxhi, A. Dudia, A. Ymeti, Interferometric evanescent wave biosensor principles and parameters, IOSR-JAP 7 (6) (2015) 84–96. Y.-J. Rao, Recent progress in applications of in-fibre Bragg grating sensors, Opt. Lasers Eng. 31 (4) (1999) 297–324. A.D. Kersey, M.A. Davis, H.J. Patrick, M. LeBlanc, K. Koo, C. Askins, M. Putnam, E.J. Friebele, Fiber grating sensors, J. Light. Technol. 15 (8) (1997) 1442–1463. M. Deng, C.-P. Tang, T. Zhu, Y.-J. Rao, L.-C. Xu, M. Han, Refractive index measurement using photonic crystal fiber-based Fabry-Perot interferometer, Appl. Opt. 49 (9) (2010) 1593–1598. W. Liang, Y. Huang, Y. Xu, R.K. Lee, A. Yariv, Highly sensitive fiber Bragg grating refractive index sensors, Appl. Phys. Lett. 86 (15) (2005) 151122. Z.L. Ran, Y.J. Rao, W.J. Liu, X. Liao, K.S. Chiang, Laser-micromachined Fabry-Perot optical fiber tip sensor for high-resolution temperature-independent measurement of refractive index, Opt. Express 16 (3) (2008) 2252–2263. P. Domachuk, I. Littler, M. Cronin-Golomb, B. Eggleton, Compact resonant integrated microfluidic refractometer, Appl. Phys. Lett. 88 (9) (2006) 093513. M. Han, F. Guo, Y. Lu, Optical fiber refractometer based on cladding-mode Bragg grating, Opt. Lett. 35 (3) (2010) 399–401. T. Guo, H.-Y. Tam, P.A. Krug, J. Albert, Reflective tilted fiber Bragg grating refractometer based on strong cladding to core recoupling, Opt. Express 17 (7) (2009) 5736–5742. O. Frazão, T. Martynkien, J. Baptista, J. Santos, W. Urbanczyk, J. Wojcik, Optical refractometer based on a birefringent Bragg grating written in an H-shaped fiber, Opt. Lett. 34 (1) (2009) 76–78. T. Allsop, R. Reeves, D.J. Webb, I. Bennion, R. Neal, A high sensitivity refractometer based upon a long period grating Mach–Zehnder interferometer, Rev. Sci. Instrum. 73 (4) (2002) 1702–1705. P. Wang, Y. Semenova, Q. Wu, G. Farrell, Y. Ti, J. Zheng, Macrobending singlemode fiber-based refractometer, Appl. Opt. 48 (31) (2009) 6044–6049. H. Liang, H. Miranto, N. Granqvist, J.W. Sadowski, T. Viitala, B. Wang, M. Yliperttula, Surface plasmon resonance instrument as a refractometer for liquids and ultrathin films, Sens. Actuators B Chem. 149 (1) (2010) 212–220. O. Frazão, P. Caldas, J. Santos, P. Marques, C. Turck, D. Lougnot, O. Soppera, FabryPerot refractometer based on an end-of-fiber polymer tip, Opt. Lett. 34 (16) (2009) 2474–2476. C.-H. Chen, T.-C. Tsao, J.-L. Tang, W.-T. Wu, A multi-D-shaped optical fiber for refractive index sensing, Sensors 10 (5) (2010) 4794–4804. Q. Wang, G. Farrell, All-fiber multimode-interference-based refractometer sensor: proposal and design, Opt. Lett. 31 (3) (2006) 317–319. G. Laffont, P. Ferdinand, Tilted short-period fibre-Bragg-grating-induced coupling to cladding modes for accurate refractometry, Meas. Sci. Technol. 12 (7) (2001) 765. L.-Y. Shao, A.P. Zhang, W.-S. Liu, H.-Y. Fu, S. He, Optical refractive-index sensor based on dual fiber-Bragg gratings interposed with a multimode-fiber taper, IEEE Photonics Technol. Lett. 19 (1) (2007) 30–32. J. Villatoro, D. Monzón-Hernández, D. Talavera, High resolution refractive index sensing with cladded multimode tapered optical fibre, Electron. Lett. 40 (2) (2004) 106–107. K.Q. Kieu, M. Mansuripur, Biconical fiber taper sensors, Ieee Photonics Technol. Lett. 18 (21) (2006) 2239–2241. S.J. Mihailov, C.W. Smelser, D. Grobnic, R.B. Walker, P. Lu, H. Ding, J. Unruh, Bragg gratings written in all-SiO2 and Ge-doped core fibers with 800-nm femtosecond radiation and a phase mask, J. Light. Technol. 22 (1) (2004) 94. K. Zhou, Y. Lai, X. Chen, K. Sugden, L. Zhang, I. Bennion, A refractometer based on a micro-slot in a fiber Bragg grating formed by chemically assisted femtosecond laser processing, Opt. Express 15 (24) (2007) 15848–15853. D. Grobnic, C. Smelser, S. Mihailov, R. Walker, P. Lu, Fiber Bragg gratings with suppressed cladding modes made in SMF-28 with a femtosecond IR laser and a phase mask, IEEE Photonics Technol. Lett. 16 (8) (2004) 1864–1866. J. Thomas, E. Wikszak, T. Clausnitzer, U. Fuchs, U. Zeitner, S. Nolte, A. Tünnermann, Inscription of fiber Bragg gratings with femtosecond pulses using a phase mask scanning technique, Appl. Phys. A 86 (2) (2007) 153–157. F.-C. Chien, C.-Y. Lin, J.-N. Yih, K.-L. Lee, C.-W. Chang, P.-K. Wei, C.-C. Sun, S.J. Chen, Coupled waveguide–surface plasmon resonance biosensor with subwavelength grating, Biosens. Bioelectron. 22 (11) (2007) 2737–2742. A. Hassani, M. Skorobogatiy, Design of the microstructured optical fiber-based surface plasmon resonance sensors with enhanced microfluidics, Opt. Express 14 (24) (2006) 11616–11621. J.-G. Huang, C.-L. Lee, H.-M. Lin, T.-L. Chuang, W.-S. Wang, R.-H. Juang, C.H. Wang, C.K. Lee, S.-M. Lin, C.-W. Lin, A miniaturized germanium-doped silicon dioxide-based surface plasmon resonance waveguide sensor for immunoassay detection, Biosens. Bioelectron. 22 (4) (2006) 519–525. T. Zhang, P. Pathak, S. Karandikar, R. Giorno, L. Que, A polymer nanostructured Fabry–Perot interferometer based biosensor, Biosens. Bioelectron. 30 (1) (2011) 128–132. Ya-nan Zhang, Yong Zhao, Tianmin Zhou, Qilu Wu, Applications and developments of on-chip biochemical sensors based on optofluidic photonic crystal cavities, Lab Chip 18 (1) (2018) 57–74. S. Mandal, D. Erickson, Nanoscale optofluidic sensor arrays, Opt. Express 16 (3) (2008) 1623–1631. N. Skivesen, A. Têtu, M. Kristensen, J. Kjems, L.H. Frandsen, P.I. Borel, Photoniccrystal waveguide biosensor, Opt. Express 15 (6) (2007) 3169–3176.
Sensors & Actuators: B. Chemical 277 (2018) 62–68
K.E. You et al.
ethanol-water mixtures using spectroscopic refractometry near the critical angle, Appl. Opt. 54 (28) (2015) 8453–8458. [54] A. Crespi, Y. Gu, B. Ngamsom, H.J. Hoekstra, C. Dongre, M. Pollnau, R. Ramponi, H.H. van den Vlekkert, P. Watts, G. Cerullo, Three-dimensional Mach-Zehnder interferometer in a microfluidic chip for spatially-resolved label-free detection, Lab Chip 10 (9) (2010) 1167–1173.
[37] M.E. Caldwell, E.M. Yeatman, Surface-plasmon spatial light modulators based on liquid crystal, Appl. Opt. 31 (20) (1992) 3880–3891. [38] D. Majchrowicz, M. Hirsch, P. Wierzba, M. Bechelany, R. Viter, M. Jędrzejewska‑Szczerska, Application of thin ZnO ALD layers in fiber-optic fabrypérot sensing interferometers, Sensors 16 (3) (2016) 416. [39] V. Bhatia, K.A. Murphy, R.O. Claus, M.E. Jones, J.L. Grace, T.A. Tran, J.A. Greene, Optical fibre based absolute extrinsic Fabry-Perot interferometric sensing system, Meas. Sci. Technol. 7 (1) (1996) 58. [40] T. Dohi, K. Matsumoto, I. Shimoyama, The optical blood test device with the micro fabry-perrot interferometer, micro electro mechanical systems, 17th IEEE International Conference on.(MEMS) (2004) 403–406. [41] T. Zhang, Z. Gong, R. Giorno, L. Que, A nanostructured fabry-perot interferometer, Opt. Express 18 (19) (2010) 20282–20288. [42] J. Tian, Y. Lu, Q. Zhang, M. Han, Microfluidic refractive index sensor based on an all-silica in-line Fabry–Perot interferometer fabricated with microstructured fibers, Opt. Express 21 (5) (2013) 6633–6639. [43] Ana C. Fernandes, V. Krist, Ulrich Gernaey, Krühne, Connecting worlds–a view on microfluidics for a wider application, Biotechnol. Adv. (2018). [44] A.R. Gosmanov, E.O. Gosmanova, E. Dillard-Cannon, Management of adult diabetic ketoacidosis, Diabetes Metab. Syndr. Obes. Targets Ther. 7 (2014) 255. [45] N. Uddin, M. Shrestha, B. Zheng, H.-J. Yoon, X. Wang, Q.H. Fan, Liquid sensors based on enhanced fabry-perot etalons, IEEE Sens. J. 17 (22) (2017) 7348–7354. [46] H. Ennaceri, A. Benyoussef, A. Ennaoui, A. Khaldoun, Optical properties of front and second surface silver-based and molybdenum-based mirrors, Int. J. Eng. Technol. 8 (6) (2016). [47] S.A. Furman, A. Tikhonravov, Basics of Optics of Multilayer Systems, Atlantica Séguier Frontieres, 1992. [48] A. Macleod, Optical Thin Films, Handbook of Thin Film Deposition, Third Edition, Elsevier, 2012, pp. 271–311. [49] K.-O. Peng, R. Marcel, Derivatives of transmittance and reflectance for an absorbing multilayer stack, Appl. Opt. 24 (4) (1985) 501–503. [50] C.-Y. Tan, Y.-X. Huang, Dependence of refractive index on concentration and temperature in electrolyte solution, polar solution, nonpolar solution, and protein solution, J. Chem. Eng. Data 60 (10) (2015) 2827–2833. [51] D.R. Yoder-Short, Interferometric measurement of glucose by refractive index determination, U.S. Patent No. 5,168,325, 1992. [52] K. Singh, G. Sandhu, B. Lark, S. Sud, Molar extinction coefficients of some carbohydrates in aqueous solutions, Pramana 58 (3) (2002) 521–528. [53] H. Sobral, M. Peña-Gomar, Determination of the refractive index of glucose-
Kyung Eun You received the B.Eng. in Biomedical Engineering from the University of Melbourne, Australia in 2009 and M.S in Medical Science from Yonsei University, Korea in 2013. She is currently a Ph.D. student in the Department of Electrical and Biomedical Engineering at the University of Nevada Reno. Her research interest includes circulating tumor cells, cancer diagnosis and biosensors. Nezam Uddin was born in Dhaka, Bangladesh, in 1990. He received the B.Sc. in Electrical and Electronic Engineering (EEE) from Khulna University of Engineering & Technology (KUET) in 2012 and M.S. degrees in Electrical Engineering and Computer Science from South Dakota State University (SDSU) in 2017. His research interests include micro/nano photonics, bio-entities interaction of light with nanostructures, fiber-optic sensing/actuation devices and systems. Tae Hyun Kim received his Ph.D. degree in the Electrical Engineering and Computer Science from the University of Michigan in 2017. Now he is a postdoc at California Institute of Technology. His research interest includes microfluidics, 3D printing and circulating tumor cells. Qi Hua Fan received his Ph.D. in applied physics from the University of Aveiro in 1999. He is currently an Associate Professor at Michigan State University. Dr. Fan’s research interest includes plasma sources for large-area coatings and plasma processing of nanostructured materials for energy harvesting, energy storage, and electro-optical devices. Hyeun Joong Yoon received the B.S., M.S., and Ph.D. degrees in Electrical Engineering from Ajou University, Korea, in 1999, 2001, and 2007, respectively. He was a postdoctoral research fellow in the Department of Chemical Engineering at the University of Michigan between 2010 to 2015. He is currently an assistant professor in the Department of Electrical and Biomedical Engineering at the University of Nevada Reno. His current research interest includes MEMS, biosensors, circulating tumor cells, and microfluidics.
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Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Highly sensitive detection of biological substances using microfluidic enhanced Fabry-Perot etalon-based optical biosensors Kyung Eun Youa,b,1, Nezam Uddinb,1, Tae Hyun Kimc, Qi Hua Fand, Hyeun Joong Yoona,b,
T ⁎
a
Department of Electrical and Biomedical Engineering, University of Nevada Reno, Reno, NV 89557, USA Department of Electrical Engineering and Computer Science, South Dakota State University, Brookings, SD 57007, USA c Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA d Department of Electrical and Computer Engineering, Michigan State University, East Lansing, MI 48824, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Fabry-Perot etalon Optical biosensors Refractive index Microfluidic device
A microfluidic based optical biosensor is introduced to detect concentrations of biochemical substances in solution using refractive index measurement with high sensitivity and accuracy. The sensor consists of a liquid channel forming a Fabry-Perot cavity between two semitransparent Ag/SiO2 reflective surfaces. Light is transmitted through the cavity to construct interference peaks in the transmission spectra which depend on the refractive index of the test samples in the channel. The refractive index of glucose, potassium chloride, and sodium chloride solutions is measured in different concentrations. Continuous change in refractive index is resolved by observing the peak wavelength shift in the transmitted spectrum. The sensor is characterized using the contact angle measurer, surface profilometer, and spectrophotometer. The proposed Fabry-Perot etalon biosensor shows real time linear responses as well as high accuracy and sensitivity of 10−3 refractive index per percent of glucose, 1.4 × 10−3 and 1.8 × 10-3 refractive index per percent of KCl and NaCl solution, respectively.
1. Introduction The demand for detecting biological and chemical substances in liquid is gradually increasing in various fields, especially in biomedical and healthcare applications [1–4]. Among many analytical techniques, detection methods based on refractive index have many advantages as they are sensitive to the sample concentration/molecular interactions and accurate even with femtoliter to nanoliter volume [5,6]. Many important parameters can be derived, which can be applied to the fields of biology, chemistry, and environment [7–11]. For example, glucose concentrations could be determined from the refractive index of blood samples for the management of diabetes [12]. Over the past years, several types of sensors to measure the refractive indices of liquids have been reported. The majority of liquid sensors are based on optical fibers due to their simple device structure and compact size [13–21]. However, the limitations of these sensors include relatively low sensitivity (grating based fiber) [22,23], weak mechanical strength and durability (tapered fiber sensors and core mismatch sensors) [24,25], and expensive microfabrication procedure (FBG-based sensor) [26–29]. On the other hand, surface plasmon resonance (SPR) sensors [30–32] have
been demonstrated with high accuracy and real-time response, but are costly and sensitive to the environment temperature. A polymer nanostructured Fabry–Perot interferometer with microfluidic network has also been developed [33]. Although it increased the optical interaction with the active layer up to 1–5 μm in depth, which was not possible by SPR, the device structure was complex with a nanopore template, a gold layer and a PDMS layer. In addition, a biochemical sensor based on photonic crystal cavities has been introduced, which generates a proportional shift of resonant wavelength based on the change in concentration of a target molecule [34]. However, the highly sensitive photonic crystal sensors suffer from expensive nanofabrication processes [35,36]. Therefore, Fabry-Perot etalon with microfluidic chamber can be an effective alternative of the above-mentioned techniques for the detection of biological substances in liquid. Fabry-Perot etalons are attractive platforms for label-free bio-sensing as they increase the sensitivity in terms of phase shift or absorption by creating effective interference between two reflective surfaces [11,37–41]. In this paper, a microfluidic based optical biosensor integrated with a Fabry-Perot etalon structure was developed. The Ag layer was used to enhance the signal intensity which provided real time
⁎
Corresponding author at: Department of Electrical and Biomedical Engineering, University of Nevada Reno, 1664 N. Virginia Street, Reno, NV, 89557, USA. E-mail address: [email protected] (H.J. Yoon). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.snb.2018.08.146 Received 20 March 2018; Received in revised form 22 August 2018; Accepted 28 August 2018 Available online 01 September 2018 0925-4005/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic of the principle of the Fabry-Perot etalon (a) Basic structure, (b) Calculation of the reflected wave.
response with high sensitivity and resolution of the liquid compositions. The microfluidic structure eliminated the need of auxiliary pressure to facilitate sample loading to the sensory area [42] and increased the light-matter interactions enabling accurate detection of biological substances with only a small volume [12,43]. Shift of peak wavelength in the transmitted spectrum was measured to calculate the refractive index of the fluid in the microfluidic chamber. The proposed sensor has the features of fast response, mechanical strength, high sensitivity, accuracy, real-time response, easy fabrication and less complex optical set-up to detect biological substances using optical properties. The detection of NaCl, KCl and glucose in liquid has been shown to be useful considering the fact that the diabetic patients need to maintain the level of the key electrolytes (NaCl and KCl) per kg of body weight [44]. 2. Device principle A Fabry-Perot etalon consists of two parallel reflecting surfaces separated by a distance d in which multiple light beams interfere. Due to the highly reflecting plane surfaces facing each other, an infinite number of parallel beams transmit from the semitransparent surfaces (Fig. 1). When these infinite waves are superimposed with decreasing amplitude, the rays form a sharp constructive interference. The equation of free spectral range (FSR) is used to find the gap between the two reflective surfaces from refractive index of a known medium.
c FSR = Δʋ = ʋm + 1− ʋm = 2ndcosθ
Fig. 2. Device structure for the proposed modified microfluidic Fabry-Perot etalon (a) 3D view, (b) Cross-sectional view.
stainless steel syringe needle was used as a tool electrode. The applied voltage was 50 V and the current was 50 mA. Ag thin films (15 nm) and SiO2 thin films (5 nm) were deposited by RF sputtering system. Two Ag layers were used in the proposed Fabry-Perot etalon structure because of their high reflectance (> 90%) and relatively low absorption coefficient compared to other metals (Al, Cu, Au, Cr) in the visible region from 400 to 750 nm [46]. The Ag layer reflected the light repeatedly within the channel, and as the two reflective layers were facing each other in parallel, constructive interference occurred. Light was also able to pass through these semitransparent layers. The transmitted light from the second Ag layer acted as an output spectrum of the sensor and the refractive index was subsequently measured. SiO2 was used to protect the Ag layer from oxidation as well as increase the hydrophilicity of the surface so that the liquid sample can easily flow through the channel. Finally, the positions of the inlet and outlet ports of the upper glass slide were aligned to lower glass substrate. The inlet and outlet tubes were connected to the upper glass slide by using epoxy.
(1)
where ν is the frequency where the maximum transmission occurs, m is the integer which determines the order of transmission peaks, d is the distance between the two glass substrates,θ is the transmission angle, and nis the refractive index of the medium. From the transmission spectrum of the Fabry-Perot etalon cavity filled with liquid, the refractive index of the analyte in the cavity can be extracted using the Eq. (1) with a known d. 3. Materials and methods 3.1. Device fabrication Fig. 2 shows the layer by layer structure with the microfluidic chamber of the proposed Fabry-Perot etalon sensor. Two glass slides were prepared for the top and bottom substrates of the sensor. The glass slides were cleaned by subsequent ultrasonication in acetone, IPA and DI water, followed by Ag and SiO2 deposition and patterning as described in our previous work [45]. The bottom glass substrate has a cavity which was fabricated by photolithography and buffered oxide etch (BOE). The inlet and outlet ports were fabricated by electrical discharge machining (EDM). The glass slide was submerged in the electrolytic solution of NaOH. A tungsten carbide rod (ASTM B777-07, 0.3125″ diameter, 12″ length) was used as a counter electrode and a
3.2. Experimental procedure Fig. 3 shows the experimental setup for measuring the transmittance of the Fabry-perot etalon sensor. Prior to injecting the solutions, all sensors were calibrated to obtain the exact thickness of the cavity by measuring the transmission spectrum with an air gap using equation (1) [45]. The glucose, KCl and NaCl solutions were prepared in different concentrations of 5%, 10%, 15%, 20%, and 25%. The solutions were injected in the microfluidic chamber using the syringe pump. The 63
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the refractive index of the liquid. The light source was a combination of regulated lamps wavelength ranging from 380 to 1050 nm. 4. Results and discussion 4.1. Effect of Ag/SiO2 in the transmittance of the Fabry-Perot etalon sensor The transmission through the glass slide was around 94% (Fig. 4). There was no adsorption in the near ultraviolet wavelength as the band gap of glass is large. The transmittance was reduced when the light passed through the Ag coated glass slide. The complex interaction process between photons and atoms of metal as well as the dielectric layer caused the refraction, reflection and transmission. The optical refraction and absorption occurred due to the interaction of photons with the electron of Ag layer. Although the SiO2 layer was assumed to be transparent, the transmittance was reduced due to adsorption in the visible range. The energy of photon was absorbed for high index dielectric materials (SiO2) at longer wavelengths. The light induced some oscillation on free electrons of Ag when the incident light reached the metal surface. A cloud of negative charge consisting of weakly bound electrons of the metal atoms would be created due to the interaction of photons with the free electrons. These electrons vibrate with the same frequency of light and thus create a reflected light. Some photons with different energy would not interact with the loosely bound electrons and cause transmission with a loss of energy (absorption). The transmission and the reflection path depend on the type of interaction which governs the refractive index. SiO2 has very low contribution in the refractive index value change. In glass or SiO2, the electrons are firmly bound and can only oscillate around their normal position. This movement influences the propagation of light so that its wave velocity is reduced, while there is only a small loss of energy. Overall, the interaction of photons with the free electrons of the Ag layer, bound electrons of SiO2 and liquid causes the change in refractive index value. The interaction is highly dependent on the wavelength of light which affects the refractive index value. As we used very narrow wavelength range for our measurement, the wavelength effect on refractive index measurement can be neglected.
Fig. 3. Experimental setup for liquid detection with the proposed Fabry-Perot Etalon sensor.
Fig. 4. Transmittance versus wavelength spectra of glass slide, glass coated with Ag film, and glass slide coated with Ag and SiO2.
syringe pump enabled the sample solution replacement without changing the position of the sensor in which light was pointed. After the chamber was completely filled with the liquid, the transmitted spectrum was measured by a spectrometer. The Filmetrics F-20 software was used to make the detected spectrum visible for analysis [45]. The peak wavelength from the transmittance spectrum was noted to find out
4.2. Simulation using the matrix method The transmittances from the liquid having different concentrations were calculated using the admittance method where each layer is
Table 1 Parameter for Simulation of transmittance spectrum of proposed enhanced Fabry-Perot etalon sensor using admittance method. Layer
Material
Index of refraction
Extinction Co-efficient
Medium 1 2 3
Glass Ag SiO2 Air 5% Glucose solution 10% Glucose solution 15% Glucose solution 20% Glucose solution 25% Glucose solution 5% NaCl solution 10% NaCl solution 15% NaCl solution 20% NaCl solution 25% NaCl solution 5% KCl solution 10% KCl solution 15% KCl solution 20% KCl solution 25% KCl solution SiO2 Ag Glass
1.51674 0.05890 1.45842 1.0003 1.3380 1.3430 1.3480 1.3530 1.3580 1.3418 1.3505 1.3594 1.3684 1.3778 1.33931 1.34643 1.35335 1.35992 1.3676 1.45842 0.05890 1.51674
0.00000 3.65540 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 3.65540 0.00000
4 5 Substrate
64
Physical thickness
15 nm 5 nm 3 nm
5nm 15nm
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Fig. 5. Simulation results of transmission spectra of the Fabry-Perot etalons with air and solution-filled channel (a) Glucose solution, (b) KCl solution, (c) NaCl solution.
Fig. 6. Transmission spectra of the Fabry-Perot etalons with air gap and solution-filled gap from experimental result (a) Glucose solution, (b) KCl solution, (c) NaCl solution.
Perot etalon for air, glucose, KCl and NaCl solutions of different concentrations. The maximum transmission of the sensor occurred when the phase differences between two consecutive rays of the transmitted light were π, 2π, 3π, ..., and Nπ . Here, the phase differences were inversely related to the wavelength and proportional to the refractive index and the thickness of the cavity. Since the thickness of the cavity was fixed, the wavelength of maximum transmission depended on the value of refractive index for a value of phase difference. The transmission spectrum of air was calculated for comparison. The real-time shift of transmission peaks to the right direction was clearly visible with the increase in solution concentration from 5% to 25%. The values corresponded to the refractive index changes of solutions from 1.338 to 1.358, 1.33931 to 1.3676 and 1.3418 to 1.3778 for glucose, KCl and NaCl, respectively.
represented by a characteristic 2 × 2 matrix [47–49]. The characteristic matrix includes effects of the thickness and the refractive index of each film as well as the dependence on the wavelength of light source. The transmittance can be calculated from the overall characteristic matrix considering the effect of all layers. The reference refractive indices of glucose, NaCl and KCl solutions were adapted from previous works [50,51]. The specific value of refractive index was chosen at ∼589.00 nm wavelength with the incident angle of zero degree. The glucose solution mainly absorbs the light in the infrared region [52]. The extinction coefficient was set to zero and the dispersion effect was not taken into consideration as the change of refractive index value would be negligible with wavelength in the visible range. The dispersion and absorption effects by Ag and SiO2 were not considered as the operating wavelength range was too small. The thicknesses of Ag and SiO2 were assumed to be 15 nm and 5 nm, respectively. Table 1 is the summary of parameter used for simulation. Fig. 5 shows the transmittance spectra of the microfluidic Fabry65
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the absorbing medium. The wavelength shifted towards right with increasing the concentration of the liquid which in turn increased the refractive index value. The higher the refractive index value, the lower the FSR value which would result in wider wavelength range. The thickness of the cavity and refractive indices of the NaCl and KCl solutions were determined in a similar way as shown in Fig. 6(b) and (c). The spectrum of glucose was narrower than those of KCl and NaCl while the spectrum of NaCl was wider than that of KCl. This was due to the fact that the refractive index of glucose was lower compared to KCl and NaCl. The light needs less optical path to pass through the glucose solution due to lower density than NaCl and KCl of the same concentrated solution. Table 2 summarizes the peak positions from the transmittance spectra of the Fabry-Perot etalon, gap thickness and refractive index for different liquid solutions. The separation of peak position increased with increasing wavelength for all liquid solution. The thickness of the cavity was found from the two peak wavelengths of air spectrum. As the thickness of the cavity was not changed; the refractive index could be extracted using two peak wavelengths from the transmittance spectrum of the liquid solution. With increasing the gap thickness, transmittance spectrum contains more peak wavelengths in the same wavelength range. Fig. 7 shows the linear response of refractive index with various concentrations of glucose, KCl and NaCl solutions. As the refractive index depended on the amount of analyte present in the liquid, the glucose level could be found from the refractive index value likewise the other liquid solutions. From the linear response, the unknown liquid concentration could be extracted. The sensitivity of glucose measurement was reflected by the slope of the linear fitting, being ∼10−3 refractive index per % of glucose. The sensitivities for KCl and NaCl detection were 1.4 × 10−3 and 1.8 × 10-3 refractive index per percent of liquid concentration, respectively. This sensitivity could be translated into wavelength difference using equation (1). The refractive index was inversely proportional to temperature at a rate of ∼ 0.00014 /o C. This
Table 2 Summary of the peak positions in the transmission spectra with air gap and solution-filled gap, calculated gap thicknesses, and refractive indices. Peak 1 (nm)
Peak 2 (nm)
Gap thick. (μm)
Peak 1 (nm)
Peak 2 (nm)
n
5% Glucose 10% Glucose 15% Glucose 20% Glucose 25% Glucose
591.17 591.17 591.17 591.17 591.17
617.79 617.79 617.79 617.79 617.79
6.86 6.86 6.86 6.86 6.86
588.99 589.74 590.2 590.77 591.18
608.51 609.24 609.65 610.18 610.55
1.3383 1.3429 1.3484 1.3536 1.3582
5% KCl 10% KCl 15% KCl 20% KCl 25% KCl
581.80 581.80 581.80 581.80 581.80
607.09 607.09 607.09 607.09 607.09
6.98 6.98 6.98 6.98 6.98
581.98 583.2 583.9 584.65 585.29
600.67 601.87 602.51 603.21 603.8
1.3392 1.3462 1.3536 1.3605 1.3670
5% NaCl 10% NaCl 15% NaCl 20% NaCl 25% NaCl
589.71 589.71 589.71 589.71 589.71
615.59 615.59 615.59 615.59 615.59
7.01 7.01 7.01 7.01 7.01
595.96 596.99 597.95 599.14 600.05
615.45 616.41 617.31 618.44 619.28
1.3416 1.3510 1.3592 1.3687 1.3776
4.3. Measured device performance The transmittance was measured each time after the liquid of different concentration was injected into the channel. The light source pointed to the same spot of the sensor during the measurement. The spectrum shifted according to the concentration of the liquid. Fig. 6 (a) shows the transmission spectra of the sensor with an air gap and glucose solutions of 5% to 25%. Two consecutive interference peaks were found at 591.17 nm and 617.79 nm for air. With the 5% glucose, two interference peaks were found at 588.99 nm and 608.51 nm, from which the refractive index was deduced to be 1.338. Both the intensity and half power band width reduced with increasing wavelengths in the same spectrum of the liquid, which was attributed to
Fig. 7. Refractive index vs. (a) KCl concentration (%), (b) NaCl concentration (%), (c) Glucose concentration (%). Rhombus: measured values. Continuous line: fitting result. 66
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temperature effect was attributed mainly to decrease in the density of the solution with increasing temperature. The limits of concentration difference that could be determined by the Fabry-Perot sensor were 0.01% glucose, 0.00769% KCl and 0.00555% NaCl solutions, respectively, which was comparable with the other approaches [53,54]. The resolution of the concentration was found from the sensitivity and resolution of the refractive index which was 10-5 refractive index unit. However, the current resolution of the sensor was only limited by the equipment which could further be improved with a higher resolution spectrophotometer in determining the peak wavelength. The maximum resolution of spectrophotometer used in the experiment in determining the peak wavelength is 0.1 Å or 0.01 nm. As discussed before, the sensor measured the positions of the interference peaks. Therefore, the signalto-noise ratio (SNR) depended on the resolution of the spectrophotometer. The SNR was found to be ∼54.69 dB which was already shown in our previous work [45]. Although the sensor produced promising results, limitations remain before it could be readily applied for clinical applications. The current version of our sensor was examined for detecting certain substances in single component solutions. However, biological specimens such as whole blood are complex and modifications to enable selective detection of a desired substance from compound solutions are highly required. Future approaches to improve the specificity of our sensor may include methods immobilizing target molecules with antibodies or enzymes in the active layer using surface functionalization (for example, glucose in the blood using glucose oxidase). Also, combining with microfluidic devices which allow onchip sample preparation such as molecular separation, concentration, and purification, prior to optical detection would potentially improve the precision and expand the sensor functionality.
[5] [6] [7] [8] [9]
[10] [11]
[12] [13] [14]
[15]
[16]
[17] [18]
[19]
[20]
5. Conclusions
[21]
A microfluidic enhanced Fabry-Perot etalon liquid sensor with two semi-transparent Ag/SiO2 surfaces is highly accurate for the refractive index measurement of glucose, KCl and NaCl solutions. The refractive index was linearly proportional to the liquid concentration with a satisfactory R2 value. Therefore, the concentration of certain substance can be correctly estimated from the linear response showing the potential as a tool for disease detection and management. Conclusively, the proposed sensor enables accurate and sensitive detection of specific biological and chemical substances in liquid with simple optical set-up. These features of the sensor are expected to surmount the limitations of other sensors including commercial glucometers.
[22]
[23]
[24]
[25] [26]
[27]
Acknowledgments
[28]
This work was partly supported by the NASA Grant Award 80NSSC18M0022 and the National Science Foundation Awards # 1700785 and # 1700787. Also acknowledged is the support from Sanford Health–South Dakota State University Collaborative Research Seed Grant Program (3×6660, 2S6661).
[29]
[30]
Conflict of interests
[31]
The authors declare no competing financial interests. [32]
References [1] T. Wei, Y. Han, Y. Li, H.-L. Tsai, H. Xiao, Temperature-insensitive miniaturized fiber inline Fabry-Perot interferometer for highly sensitive refractive index measurement, Opt. Express 16 (8) (2008) 5764–5769. [2] M.F. Ferreira, G. Statkiewicz-Barabach, D. Kowal, P. Mergo, W. Urbanczyk, O. Frazão, Fabry-Perot cavity based on polymer FBG as refractive index sensor, Opt. Commun. 394 (2017) 37–40. [3] T. Takahashi, T. Hizawa, M. Nobuo, M. Taki, K. Sawada, K. Takahashi, Surface stress sensor based on MEMS Fabry–Perot interferometer with high wavelength selectivity for label-free biosensing, J. Micromech. Microeng. 28 (5) (2018) 054002. [4] L. Li, L. Xia, Z. Xie, D. Liu, Thinned fiber based Mach-Zehnder interferometer for
[33]
[34]
[35] [36]
67
measurements of liquid level and refractive index, OFS2012 22nd International Conference on Optical Fiber sensors, Int. Soc. Opt. Photonics (2012) 84217K. X. Fan, I.M. White, S.I. Shopova, H. Zhu, J.D. Suter, Y. Sun, Sensitive optical biosensors for unlabeled targets: a review, Anal. Chim. Acta 620 (1) (2008) 8–26. M. Xhoxhi, A. Dudia, A. Ymeti, Interferometric evanescent wave biosensor principles and parameters, IOSR-JAP 7 (6) (2015) 84–96. Y.-J. Rao, Recent progress in applications of in-fibre Bragg grating sensors, Opt. Lasers Eng. 31 (4) (1999) 297–324. A.D. Kersey, M.A. Davis, H.J. Patrick, M. LeBlanc, K. Koo, C. Askins, M. Putnam, E.J. Friebele, Fiber grating sensors, J. Light. Technol. 15 (8) (1997) 1442–1463. M. Deng, C.-P. Tang, T. Zhu, Y.-J. Rao, L.-C. Xu, M. Han, Refractive index measurement using photonic crystal fiber-based Fabry-Perot interferometer, Appl. Opt. 49 (9) (2010) 1593–1598. W. Liang, Y. Huang, Y. Xu, R.K. Lee, A. Yariv, Highly sensitive fiber Bragg grating refractive index sensors, Appl. Phys. Lett. 86 (15) (2005) 151122. Z.L. Ran, Y.J. Rao, W.J. Liu, X. Liao, K.S. Chiang, Laser-micromachined Fabry-Perot optical fiber tip sensor for high-resolution temperature-independent measurement of refractive index, Opt. Express 16 (3) (2008) 2252–2263. P. Domachuk, I. Littler, M. Cronin-Golomb, B. Eggleton, Compact resonant integrated microfluidic refractometer, Appl. Phys. Lett. 88 (9) (2006) 093513. M. Han, F. Guo, Y. Lu, Optical fiber refractometer based on cladding-mode Bragg grating, Opt. Lett. 35 (3) (2010) 399–401. T. Guo, H.-Y. Tam, P.A. Krug, J. Albert, Reflective tilted fiber Bragg grating refractometer based on strong cladding to core recoupling, Opt. Express 17 (7) (2009) 5736–5742. O. Frazão, T. Martynkien, J. Baptista, J. Santos, W. Urbanczyk, J. Wojcik, Optical refractometer based on a birefringent Bragg grating written in an H-shaped fiber, Opt. Lett. 34 (1) (2009) 76–78. T. Allsop, R. Reeves, D.J. Webb, I. Bennion, R. Neal, A high sensitivity refractometer based upon a long period grating Mach–Zehnder interferometer, Rev. Sci. Instrum. 73 (4) (2002) 1702–1705. P. Wang, Y. Semenova, Q. Wu, G. Farrell, Y. Ti, J. Zheng, Macrobending singlemode fiber-based refractometer, Appl. Opt. 48 (31) (2009) 6044–6049. H. Liang, H. Miranto, N. Granqvist, J.W. Sadowski, T. Viitala, B. Wang, M. Yliperttula, Surface plasmon resonance instrument as a refractometer for liquids and ultrathin films, Sens. Actuators B Chem. 149 (1) (2010) 212–220. O. Frazão, P. Caldas, J. Santos, P. Marques, C. Turck, D. Lougnot, O. Soppera, FabryPerot refractometer based on an end-of-fiber polymer tip, Opt. Lett. 34 (16) (2009) 2474–2476. C.-H. Chen, T.-C. Tsao, J.-L. Tang, W.-T. Wu, A multi-D-shaped optical fiber for refractive index sensing, Sensors 10 (5) (2010) 4794–4804. Q. Wang, G. Farrell, All-fiber multimode-interference-based refractometer sensor: proposal and design, Opt. Lett. 31 (3) (2006) 317–319. G. Laffont, P. Ferdinand, Tilted short-period fibre-Bragg-grating-induced coupling to cladding modes for accurate refractometry, Meas. Sci. Technol. 12 (7) (2001) 765. L.-Y. Shao, A.P. Zhang, W.-S. Liu, H.-Y. Fu, S. He, Optical refractive-index sensor based on dual fiber-Bragg gratings interposed with a multimode-fiber taper, IEEE Photonics Technol. Lett. 19 (1) (2007) 30–32. J. Villatoro, D. Monzón-Hernández, D. Talavera, High resolution refractive index sensing with cladded multimode tapered optical fibre, Electron. Lett. 40 (2) (2004) 106–107. K.Q. Kieu, M. Mansuripur, Biconical fiber taper sensors, Ieee Photonics Technol. Lett. 18 (21) (2006) 2239–2241. S.J. Mihailov, C.W. Smelser, D. Grobnic, R.B. Walker, P. Lu, H. Ding, J. Unruh, Bragg gratings written in all-SiO2 and Ge-doped core fibers with 800-nm femtosecond radiation and a phase mask, J. Light. Technol. 22 (1) (2004) 94. K. Zhou, Y. Lai, X. Chen, K. Sugden, L. Zhang, I. Bennion, A refractometer based on a micro-slot in a fiber Bragg grating formed by chemically assisted femtosecond laser processing, Opt. Express 15 (24) (2007) 15848–15853. D. Grobnic, C. Smelser, S. Mihailov, R. Walker, P. Lu, Fiber Bragg gratings with suppressed cladding modes made in SMF-28 with a femtosecond IR laser and a phase mask, IEEE Photonics Technol. Lett. 16 (8) (2004) 1864–1866. J. Thomas, E. Wikszak, T. Clausnitzer, U. Fuchs, U. Zeitner, S. Nolte, A. Tünnermann, Inscription of fiber Bragg gratings with femtosecond pulses using a phase mask scanning technique, Appl. Phys. A 86 (2) (2007) 153–157. F.-C. Chien, C.-Y. Lin, J.-N. Yih, K.-L. Lee, C.-W. Chang, P.-K. Wei, C.-C. Sun, S.J. Chen, Coupled waveguide–surface plasmon resonance biosensor with subwavelength grating, Biosens. Bioelectron. 22 (11) (2007) 2737–2742. A. Hassani, M. Skorobogatiy, Design of the microstructured optical fiber-based surface plasmon resonance sensors with enhanced microfluidics, Opt. Express 14 (24) (2006) 11616–11621. J.-G. Huang, C.-L. Lee, H.-M. Lin, T.-L. Chuang, W.-S. Wang, R.-H. Juang, C.H. Wang, C.K. Lee, S.-M. Lin, C.-W. Lin, A miniaturized germanium-doped silicon dioxide-based surface plasmon resonance waveguide sensor for immunoassay detection, Biosens. Bioelectron. 22 (4) (2006) 519–525. T. Zhang, P. Pathak, S. Karandikar, R. Giorno, L. Que, A polymer nanostructured Fabry–Perot interferometer based biosensor, Biosens. Bioelectron. 30 (1) (2011) 128–132. Ya-nan Zhang, Yong Zhao, Tianmin Zhou, Qilu Wu, Applications and developments of on-chip biochemical sensors based on optofluidic photonic crystal cavities, Lab Chip 18 (1) (2018) 57–74. S. Mandal, D. Erickson, Nanoscale optofluidic sensor arrays, Opt. Express 16 (3) (2008) 1623–1631. N. Skivesen, A. Têtu, M. Kristensen, J. Kjems, L.H. Frandsen, P.I. Borel, Photoniccrystal waveguide biosensor, Opt. Express 15 (6) (2007) 3169–3176.
Sensors & Actuators: B. Chemical 277 (2018) 62–68
K.E. You et al.
ethanol-water mixtures using spectroscopic refractometry near the critical angle, Appl. Opt. 54 (28) (2015) 8453–8458. [54] A. Crespi, Y. Gu, B. Ngamsom, H.J. Hoekstra, C. Dongre, M. Pollnau, R. Ramponi, H.H. van den Vlekkert, P. Watts, G. Cerullo, Three-dimensional Mach-Zehnder interferometer in a microfluidic chip for spatially-resolved label-free detection, Lab Chip 10 (9) (2010) 1167–1173.
[37] M.E. Caldwell, E.M. Yeatman, Surface-plasmon spatial light modulators based on liquid crystal, Appl. Opt. 31 (20) (1992) 3880–3891. [38] D. Majchrowicz, M. Hirsch, P. Wierzba, M. Bechelany, R. Viter, M. Jędrzejewska‑Szczerska, Application of thin ZnO ALD layers in fiber-optic fabrypérot sensing interferometers, Sensors 16 (3) (2016) 416. [39] V. Bhatia, K.A. Murphy, R.O. Claus, M.E. Jones, J.L. Grace, T.A. Tran, J.A. Greene, Optical fibre based absolute extrinsic Fabry-Perot interferometric sensing system, Meas. Sci. Technol. 7 (1) (1996) 58. [40] T. Dohi, K. Matsumoto, I. Shimoyama, The optical blood test device with the micro fabry-perrot interferometer, micro electro mechanical systems, 17th IEEE International Conference on.(MEMS) (2004) 403–406. [41] T. Zhang, Z. Gong, R. Giorno, L. Que, A nanostructured fabry-perot interferometer, Opt. Express 18 (19) (2010) 20282–20288. [42] J. Tian, Y. Lu, Q. Zhang, M. Han, Microfluidic refractive index sensor based on an all-silica in-line Fabry–Perot interferometer fabricated with microstructured fibers, Opt. Express 21 (5) (2013) 6633–6639. [43] Ana C. Fernandes, V. Krist, Ulrich Gernaey, Krühne, Connecting worlds–a view on microfluidics for a wider application, Biotechnol. Adv. (2018). [44] A.R. Gosmanov, E.O. Gosmanova, E. Dillard-Cannon, Management of adult diabetic ketoacidosis, Diabetes Metab. Syndr. Obes. Targets Ther. 7 (2014) 255. [45] N. Uddin, M. Shrestha, B. Zheng, H.-J. Yoon, X. Wang, Q.H. Fan, Liquid sensors based on enhanced fabry-perot etalons, IEEE Sens. J. 17 (22) (2017) 7348–7354. [46] H. Ennaceri, A. Benyoussef, A. Ennaoui, A. Khaldoun, Optical properties of front and second surface silver-based and molybdenum-based mirrors, Int. J. Eng. Technol. 8 (6) (2016). [47] S.A. Furman, A. Tikhonravov, Basics of Optics of Multilayer Systems, Atlantica Séguier Frontieres, 1992. [48] A. Macleod, Optical Thin Films, Handbook of Thin Film Deposition, Third Edition, Elsevier, 2012, pp. 271–311. [49] K.-O. Peng, R. Marcel, Derivatives of transmittance and reflectance for an absorbing multilayer stack, Appl. Opt. 24 (4) (1985) 501–503. [50] C.-Y. Tan, Y.-X. Huang, Dependence of refractive index on concentration and temperature in electrolyte solution, polar solution, nonpolar solution, and protein solution, J. Chem. Eng. Data 60 (10) (2015) 2827–2833. [51] D.R. Yoder-Short, Interferometric measurement of glucose by refractive index determination, U.S. Patent No. 5,168,325, 1992. [52] K. Singh, G. Sandhu, B. Lark, S. Sud, Molar extinction coefficients of some carbohydrates in aqueous solutions, Pramana 58 (3) (2002) 521–528. [53] H. Sobral, M. Peña-Gomar, Determination of the refractive index of glucose-
Kyung Eun You received the B.Eng. in Biomedical Engineering from the University of Melbourne, Australia in 2009 and M.S in Medical Science from Yonsei University, Korea in 2013. She is currently a Ph.D. student in the Department of Electrical and Biomedical Engineering at the University of Nevada Reno. Her research interest includes circulating tumor cells, cancer diagnosis and biosensors. Nezam Uddin was born in Dhaka, Bangladesh, in 1990. He received the B.Sc. in Electrical and Electronic Engineering (EEE) from Khulna University of Engineering & Technology (KUET) in 2012 and M.S. degrees in Electrical Engineering and Computer Science from South Dakota State University (SDSU) in 2017. His research interests include micro/nano photonics, bio-entities interaction of light with nanostructures, fiber-optic sensing/actuation devices and systems. Tae Hyun Kim received his Ph.D. degree in the Electrical Engineering and Computer Science from the University of Michigan in 2017. Now he is a postdoc at California Institute of Technology. His research interest includes microfluidics, 3D printing and circulating tumor cells. Qi Hua Fan received his Ph.D. in applied physics from the University of Aveiro in 1999. He is currently an Associate Professor at Michigan State University. Dr. Fan’s research interest includes plasma sources for large-area coatings and plasma processing of nanostructured materials for energy harvesting, energy storage, and electro-optical devices. Hyeun Joong Yoon received the B.S., M.S., and Ph.D. degrees in Electrical Engineering from Ajou University, Korea, in 1999, 2001, and 2007, respectively. He was a postdoctoral research fellow in the Department of Chemical Engineering at the University of Michigan between 2010 to 2015. He is currently an assistant professor in the Department of Electrical and Biomedical Engineering at the University of Nevada Reno. His current research interest includes MEMS, biosensors, circulating tumor cells, and microfluidics.
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