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Fabrication and measurement of optical waveguide sensor based on localized surface plasmon resonance

Fabrication and measurement of optical waveguide sensor based on localized surface plasmon resonance We propose a novel localized surface plasmon resonance (LSPR) sensor system based on polymer material. The pro‑ posed LSPR system consists of the incident medium with low‑loss polymer waveguide and the chemically immobi‑ lized plasmonic nanoparticles for on‑ chip LSPR sensing. Because of low coupling efficiency of conventional methods, usually intricate test equipment such as dark field microscopes equipped with cooled charge ‑ coupled device detec‑ tors is required to perform nanoparticles LSPR sensing. Using a polymer optical waveguide instead of conventional free‑space excitation techniques (e.g., using an objective lens) in the LSPR sensing system offers miniaturization, low cost, and potable sensing capability. The integration of this hybrid plasmonic‑photonic sensor with optical system based on fiber optic is measured to refractive index with a sensitivity of 3.10/RIU. Keywords: Localized surface plasmon resonance, Optical waveguide, Au nanoparticle, Optical fiber, Miniaturization can efficiently combine the light with nanoparticles. A Introduction simple optical measurement set-up is also constructed The oscillations of free electrons in noble metal nanopar - by applying the optical fiber, and the proposed sensor ticles when the light is incident is called localized surface is used to LSPR signal measurement according to the plasmon (LSP), and the phenomenon that the scattered refractive index changes of the surrounding medium. signals in incident lights by nanoparticles are ampli- The LSPR based OWG sensor is expected to the applica - fied with LSP is referred to localized surface plasmon tion as biosensor with advantages in miniaturization, low resonance (LSPR) [1, 2]. One of the most remarkable cost, and portability in the future. effects of LSPR is that the intensity of LSPR is sensitive to changes of the refractive indices around noble metal Materials and methods nanoparticles [3, 4]. The advantages of LSPR sensors hav - Design and fabrication of OWG ing this characteristic are real-time analysis, label-free The OWG of stair-shape is designed as shown in Fig.  1a detection, and high sensitivities [5–7]. However, the most because the situation that the lights directly enter the of LSPR sensors have been developed to method that the detector from the light source without the pass of OWG incident lights are focused to nanoparticles in free-space can cause the noise in the observation of LSPR signal. by the objective lens, which limits the coupling efficiency, Considering the fact that light transmission loss of 0.1 dB miniaturization, cost-effectiveness, and accessibility per 1 mm occurs in the OWG using SU-8 (2075, Micro- [8–12]. chem, Newton, MA, USA), the distance between both In this study, we propose a novel LSPR based optical optical fiber holders is fabricated as short as possible waveguide (OWG) sensor which is fabricated by SU-8 2 mm [13]. If the stair angle (θ) of OWG becomes large, photoresist (PR) through relatively simple process that the light transmission efficiency is decreased due to the increases of the bending angle and the effective length *Correspondence: parkjae@dankook.ac.kr; skilee@dankook.ac.kr of OWG [14, 15]. On the other hand, when θ becomes Department of Electronics and Electrical Engineering, Dankook University, Yongin 16890, Republic of Korea smaller, the light from optical fiber connected to the © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Kim et al. Micro and Nano Syst Lett (2019) 7:7 Page 2 of 5 SU-8 also has good mechanical and thermal stabil- ity, and is suitable for biosensing applications due to its biocompatibility and chemically stable characteristic [20, 21]. The SU-8 was spin-coated (ACE-200, Dong ah, Seoul, Republic of Korea) on 4  in. glass wafer (Boro33, Fiber holder DS semicon, Anyang-si, Republic of Korea) at 1700  rpm 160 µm for 40  s. At this time, the soft bake was sequentially 145 µm OWG performed at 65  °C for 20  min and at 95  °C for 55  min, and the exposure was continued for 60  s at intensity of 25  mW/cm . The post exposure bake was carried out at 65  °C and 95  °C for 10  min and 30  min, respectively. 2mm SU-8 Glass Finally, the sample was developed in AZ 1500 thinner (AZ electronic materials, Seoul, Republic of Korea) dur- ing 90  min to complete the OWG fabrication. Figure  1b is micrograph when the optical fibers (FG105LCA, Thor - labs, Newton, NJ, USA) are fixed to the fabricated fiber holder. The optical fiber was immobilized to the fiber Fiber holder using ultraviolet curable resin (NOA61, Norland, holder Optical fiber Cranbury, NJ, USA). Figure  2 shows the field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, OWG Ibaraki, Japan) images of OWG and optical fiber holder. The width and height of fabricated OWG were measured Glass to be 162.4 µm and 126.3 µm, respectively. The width of Fig. 1 Schematic diagram of the OWG and the microscope image. a the fiber holder was also 144.5 µm. Design values and b fabricated SU‑8 OWG are indicated Fabrication of OWG sensor based on LSPR The method to immobilize the Au nanoparticles on source can be directly incident on the opposite optical OWG using SU-8 consists of four steps. In the first step, fiber, which makes it difficult to observe the LSPR signal. the oxygen plasma (CUTE, Femto science, Yongin-si, In order to improve the light transmission efficiency of Republic of Korea) was treated on OWG with 60  W OWG, the θ is selected to 30° within a range in which the power and 0.8  m Torr during 50  s. The oxygen plasma effective length of OWG is shortened and the light is not treatment removes the organic compound and func- directly transmitted between the optical fibers. The width tionalizes the surface of OWG with hydroxyl group [22, of OWG is planned to be 160  µm, considering that the 23]. Next, the sample was reacted with 1% TEOS (98%, coupled optical fiber diameter which includes core and Sigma aldrich, St. Louis, MO, USA) solution in isopro- cladding is 125  µm and the spread of light emitted from panol (99.5%, Daejung, Siheung-si, Republic of Korea) the optical fiber. Finally, the width of optical fiber holder for 3 h, which acts as the cladding in OWG and controls is decided to be 145 µm to allow enough area for the opti- the Au nanoparticles density on the surface [24]. In the cal fiber. A gap of about 10 µm is set on both sides of the third step, 1% 3-(ethoxydimethylsilyl)-propylamine (97%, optical fiber. Sigma aldrich) solution based on isopropanol was coated The OWG was simply fabricated through a single on the OWG for 2 h to form the self-assembled monolay- lithography process (MDA-400M, Midas system, Dae- ers which have the amine groups with positive charges to jeon, Republic of Korea) of SU-8. The excellent optical immobilize the Au nanoparticles on the OWG surface. transparency beyond 400  nm and the low transmission Finally, the Au nanoparticles which are prepared at about loss of SU-8 make it a preferred material for the fabri- 47.8 nm diameter were attached on OWG during 6 h. The cation of various optical components and systems such Au nanoparticles were synthesized through the Turk- as OWGs, splitters, directional couplers, and gratings evich method by reducing the Au(III) chloride trihydrate [16–18]. SU-8 has a high refractive index of 1.59 at the (99.9%, Sigma aldrich) using sodium citrate dehydrate wavelength of 640  nm used in the measurement, which (99%, Daejung) [25, 26]. Since the Au ions are reduced can increase the light guiding efficiency due to the large by citrate, the negative charges are present on the surface contrast of the refractive indices between SU-8 and of the Au nanoparticles, which enables the electrostatic organic-based tetraethyl orthosilicate (TEOS) used as bonding with the positive charges of amine groups on the the cladding [19]. The refractive index of TEOS is 1.38. OWG [27, 28]. The Au nanoparticles are immobilized on Kim et al. Micro and Nano Syst Lett (2019) 7:7 Page 3 of 5 the side surface as well as the top surface of OWG. How- ever, the nanoparticles are not attached between OWG and optical fiber in order to prevent the decrease of cou - Glass pling efficiency. When Au nanoparticles are immobilized on the gap between OWG and fiber, the incident light Fiber Fiber from optical fiber will be reflected back to the fiber by the holder Au nanoparticles. The fabrication process of OWG sen - holder sor based on LSPR and the FE-SEM photography at the surface of proposed sensor were displayed in Fig.  3. On the OWG surface, the Au nanoparticles were mostly pre- sent as monomer without excessive aggregation. Results and discussion Optical measurement set‑up The optical measurement set-up for the record of LSPR signal is simply composed as shown in Fig.  4. The light from the laser (Iflex-2000, Qioptiq, Hamble-le-rice, UK) is transmitted to the OWG sensor through the optical Fiber Fiber fiber, which reacts with the Au nanoparticles on the sen - OWG holder holder sor surface. Then, the reacted lights are again collected through the optical fiber to the photodetector (PDA36A, Thorlabs). The collected signals by the photodetector are sent to a computer by the data acquisition (USB-6210, National instruments, Austin, TX, USA) and observed in real-time. In this set-up, the optical fibers are positioned as close to OWG as possible [29]. Refractive index measurement using the OWG sensor based on LSPR Fig. 2 The FE‑SEM photograph of fabricated OWG It is evaluated that the OWG sensor generates the changed LSPR intensities according to the increase of OH OH O Si O Si O OH OH OH O O OH OH OH SU-8 Oxygen plasma 1% (v/v)tetraethyl orthosilicate treatment for 50 sec treatment for 3 hour NH - Au - NH NH 2 2 NH NH NH - 2 2 2 0* 0* SiO SiO 1% (v/v) 3-(ethoxydimethylsilyl)- Immobilization of Au propylaminetreatment for 2 hour nanoparticles for 6 hour Fig. 3 The immobilization process of Au nanoparticles on OWG and the FE‑SEM image of attached nanoparticles on SU‑8 Kim et al. Micro and Nano Syst Lett (2019) 7:7 Page 4 of 5 Optical fiber Photodetector Sample Laser Computer Incident light LSPR signal Input optical fiber Optical waveguide Output optical fiber Fig. 4 The optical measurement set‑up and the schematic diagram of lights pass in OWG refractive index around Au nanoparticles. When six refractive index solutions with a spacing of 0.01 from 1.33 to 1.38 were sequentially supplied to the sensor, the LSPR intensities were gradually increased in Fig.  5a. The refractive index sensitivity which is defined by the normalized intensity change per refractive index unit (RIU) and the coefficient of determination (R ) indicat- 1.38 ing the degree of correlation between the two variables are shown in Fig. 5b [30, 31]. The sensitivity of proposed 7800 1.37 1.36 sensor was 3.10/RIU and the R was 0.99, which mean the high linearity between the refractive index and the 1.35 output of OWG sensor. The normalized intensity is used to calculate the sensitivity because the LSPR intensity 1.34 depends on external factors such as the laser power, the 1.33 gain of photodetector, and the alignment between light source and fiber [32]. The normalized intensity is defined as the relative signal based on the measured intensity at 020406080100 120140 160180 1.33 [33]. Time (s) Conclusion Normalized intensity 1.16 Linear fit of normalized intensity The conventional methods that the light exposes into a 1.14 free space during LSPR measurement have been prob- 1.12 lematic such as the insufficient optical coupling efficiency 1.10 and the requirement of expert skill for the alignment of 1.08 light. To solve these problems, we applied the OWG that 1.06 do not expose the light to the air and suggested using it Sensitivity 1.04 as an LSPR sensor. The low-loss OWG was fabricated on 3.10 /RIU 1.02 glass wafer by SU-8 using a single lithography process 2 R = 0.99 1.00 and the OWG sensor based on LSPR is completed by y = 3.10x-3.13 0.98 immobilizing the Au nanoparticles on OWG. In addition, 1.33 1.34 1.35 1.36 1.37 1.38 it was possible to construct the compact optical system Refractive index unit by using optical fiber, which enables the simple trans - mission and collection of light. The proposed sensor was Fig. 5 The measurement result. a LSPR intensity changes according to increase of refractive indices. b The amount of increased intensity applied as a refractive index sensor and it was confirmed per refractive index unit. The intensities are normalized as reference that the OWG sensor based on LSPR exhibits a very lin- to the signal at 1.33 ear response with the change of refractive index. Based Normalized intensity Intensity (mV) Kim et al. Micro and Nano Syst Lett (2019) 7:7 Page 5 of 5 14. Marcuse D (1978) Length optimization of an S‑shaped transition between on these results, if a number of OWGs are integrated into offset optical waveguides. Appl Opt 17:763–768 a single chip, it is expected that the various target mol- 15. Remouche M, Georges F, Meyrueis P (2012) Flexible optical waveguide ecules can be multi measured in real-time as the biosen- bent loss attenuation effects analysis and modeling application to an intrinsic optical fiber temperature sensor. Optic Photon J 2:1–7 sor application. 16. Mogensen KB, El‑Ali J, Wolff A, Kutter JP (2003) Integration of polymer waveguides for optical detection in microfabricated chemical analysis Acknowledgements systems. Appl Opt 42:4072–4079 Not applicable. 17. Liu H, Huang Y, Jiang H (2016) Artificial eye for scotopic vision with bioinspired all‑ optical photosensitivity enhancer. Proc Natl Acad Sci USA Authors’ contributions 113:3982–3985 SKL and JHP devised the idea and supervised the project. SKL, JHP, and HMK 18. Parida OP, Bhat N (2009) Characterization of optical properties of SU‑8 discussed the design and experimental setup. HMK performed the experi‑ and fabrication of optical components. In: International conference on ment and drafted the manuscript. All authors edited the manuscript. All optics photonics. pp 4–7 authors read and approved the final manuscript. 19. Lin CH, Lee GB, Chen SH, Chang GL (2003) Micro capillary electrophoresis chips integrated with buried SU‑8/SOG optical waveguides for bio ‑ Funding analytical applications. Sens Actuators A Phys 107:125–131 This work was supported by the National Research Foundation of Korea (NRF) 20. Abgrall P, Conedera V, Camon H, Gue AM, Nguyen NT (2007) SU‑8 as a grant funded by the Korea government (MSIT ) (No. NRF‑2018R1A2B6001361). structural material for labs‑ on‑ chips and microelectromechanical sys‑ tems. Electrophoresis 28:4539–4551 Availability of data and materials 21. Borreman A, Musa S, Kok AAM, Diemeer MB, Driessen A (2002) Fabrication The datasets supporting the conclusions of this article are included within the of polymeric multimode waveguides and devices in SU‑8 photoresist article. using selective polymerization. In: Proceedings of the symposium IEEE/ LEOS Benelux Chapter 83–86 Competing interests 22. Eddings MA, Johnson MA, Gale BK (2008) Determining the optimal The authors declare that they have no competing interests. PDMS‑PDMS bonding technique for microfluidic devices. J Micromech Microeng 18:067001 Received: 23 April 2019 Accepted: 7 June 2019 23. Hook DA, Olhausen JA, Krim J, Dugger MT (2010) Evaluation of oxygen plasma and UV ozone methods for cleaning of occluded areas in MEMS devices. J Microelectromech Syst 19:1292–1298 24. Ruano‑Lopez JM, Aguirregabiria M, Tijero M, Arroyo MT, Elizalde J, Berganzo J, Aranburu I, Blanco FJ, Mayora K (2006) A new SU‑8 process to References integrate buried waveguides and sealed microchannels for a lab‑ on‑a‑ 1. Langer J, Novikov SM, Liz‑Marzan LM (2015) Sensing using plasmonic chip. Sens Actuators B Chem 144:542–551 nanostructures and nanoparticles. Nanotechnology 26:322001 25. Schulz F, Homolka T, Bastús NG, Puntes V, Weller H, Vossmeyer T (2014) 2. Manzano M, Vizzini P, Jia K, Adam PM, Ionescu RE (2016) Development Little adjustments significantly improve the Turkevich synthesis of gold of localized surface plasmon resonance biosensors for the detection of nanoparticles. Langmuir 30:10779–10784 Brettanomyces bruxellensis in wine. Sens Actuators B Chem 223:295–300 26. Kim HM, Park JH, Jeong DH, Lee HY, Lee SK (2018) Real‑time detection 3. Dahlin AB, Tegenfeldt JO, Höök F (2006) Improving the instrumental of prostate‑specific antigens using a highly reliable fiber ‑ optic localized resolution of sensors based on localized surface plasmon resonance. Anal surface plasmon resonance sensor combined with micro fluidic channel. Chem 78:4416–4426 Sens Actuators B Chem 273:891–898 4. Kim HM, Nam KT, Lee SK, Park JH (2018) Fabrication and measurement of 27. Ji X, Song X, Li J, Bai Y, Yang W, Peng X (2007) Size control of gold microtip‑array‑based LSPR sensor using bundle fiber. Sens Actuatiors A nanocrystals in citrate reduction: the third role of citrate. J Am Chem Soc Phys 271:146–152 129:13939–13948 5. Srivastava SK, Verma RK, Gupta BD (2009) Theoretical modeling of a local‑ 28. Kim HM, Jeong DH, Lee HY, Park JH, Lee SK (2019) Improved stability of ized surface plasmon resonance based intensity modulated fiber optic gold nanoparticles on the optical fiber and their application to refractive refractive index sensor. Appl Opt 48:3796–3802 index sensor based on localized surface plasmon resonance. Opt Laser 6. Jeong HH, Erdene N, Park JH, Jeong DH, Lee HY, Lee SK (2013) Real‑time Technol 114:171–178 label‑free immunoassay of interferon‑ gamma and prostate‑specific 29. Xiao L, Jin W, Demokan MS, Ho HL, Tam HY, Ju J, Yu J (2006) Photopolymer antigen using a fiber ‑ optic localized surface plasmon resonance sensor. microtips for efficient light coupling between single ‑mode fibers and Biosens Bioelectron 39:346–351 photonic crystal fibers. Opt Lett 31:1791–1793 7. Xie L, Yan X, Du Y (2014) An aptamer based wall‑less LSPR array chip 30. Chen J, Shi S, Su R, Qi W, Huang R, Wang M, Wang L, He Z (2015) Optimi‑ for label‑free and high throughput detection of biomolecules. Biosens zation and application of reflective LSPR optical fiber biosensors based Bioelectron 53:58–64 on silver nanoparticles. Sensors 15:12205–12217 8. McFarland AD, Van Duyne RP (2003) Single silver nanoparticles as real‑ 31. Mohammadi SZ, Beitollahi H, Tajik S (2018) Nonenzymatic coated screen‑ time optical sensors with zeptomole sensitivity. Nano Lett 3:1057–1062 printed electrode for electrochemical determination of acetylcholine. 9. Xiang G, Zhang N, Zhou X (2010) Localized surface plasmon resonance Micro Nano Syst Lett 6:9 biosensing with large area of gold nanoholes fabricated by nanosphere 32. Sanders M, Lin Y, Wei J, Bono T, Lindquist RG (2014) An enhanced LSPR lithography. Nanoscale Res Lett 5:818–822 fiber ‑ optic nanoprobe for ultrasensitive detection of protein biomarkers. 10. SadAbadi H, Badilescu S, Packirisamy M, Wüthrich R (2013) Integration of Biosens Bioelectron 61:95–101 gold nanoparticles in PDMS microfluidics for lab ‑ on‑a‑ chip plasmonic 33. Kim HM, Uh M, Jeong DH, Lee HY, Park JH, Lee SK (2019) Localized surface biosensing of growth hormones. Biosens Bioelectron 44:77–84 plasmon resonance biosensor using nanopatterned gold particles on the 11. Park JH, Byun JY, Mun H, Shim WB, Shin YB, Li T, Kim MG (2014) A regener‑ surface of an optical fiber. Sens Actuators B Chem 280:183–191 atable, label‑free, localized surface plasmon resonance (LSPR) aptasensor for the detection of ochratoxin A. Biosens Bioelectron 59:321–327 12. Devi RV, Doble M, Verma RS (2015) Nanomaterials for early detection of Publisher’s Note cancer biomarker with special emphasis on gold nanoparticles in immu‑ Springer Nature remains neutral with regard to jurisdictional claims in pub‑ noassays/sensors. Biosens Bioelectron 68:688–698 lished maps and institutional affiliations. 13. Sum TC, Bettiol AA, Van Kan JA, Watt F, Pun EYB, Tung KK (2003) Proton beam writing of low‑loss polymer optical waveguides. Appl Phys Lett 83:1707–1709 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Micro and Nano Systems Letters Springer Journals

Fabrication and measurement of optical waveguide sensor based on localized surface plasmon resonance

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Springer Journals
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Copyright © 2019 by The Author(s)
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Engineering; Circuits and Systems; Electrical Engineering; Mechanical Engineering; Nanotechnology; Applied and Technical Physics
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Abstract

We propose a novel localized surface plasmon resonance (LSPR) sensor system based on polymer material. The pro‑ posed LSPR system consists of the incident medium with low‑loss polymer waveguide and the chemically immobi‑ lized plasmonic nanoparticles for on‑ chip LSPR sensing. Because of low coupling efficiency of conventional methods, usually intricate test equipment such as dark field microscopes equipped with cooled charge ‑ coupled device detec‑ tors is required to perform nanoparticles LSPR sensing. Using a polymer optical waveguide instead of conventional free‑space excitation techniques (e.g., using an objective lens) in the LSPR sensing system offers miniaturization, low cost, and potable sensing capability. The integration of this hybrid plasmonic‑photonic sensor with optical system based on fiber optic is measured to refractive index with a sensitivity of 3.10/RIU. Keywords: Localized surface plasmon resonance, Optical waveguide, Au nanoparticle, Optical fiber, Miniaturization can efficiently combine the light with nanoparticles. A Introduction simple optical measurement set-up is also constructed The oscillations of free electrons in noble metal nanopar - by applying the optical fiber, and the proposed sensor ticles when the light is incident is called localized surface is used to LSPR signal measurement according to the plasmon (LSP), and the phenomenon that the scattered refractive index changes of the surrounding medium. signals in incident lights by nanoparticles are ampli- The LSPR based OWG sensor is expected to the applica - fied with LSP is referred to localized surface plasmon tion as biosensor with advantages in miniaturization, low resonance (LSPR) [1, 2]. One of the most remarkable cost, and portability in the future. effects of LSPR is that the intensity of LSPR is sensitive to changes of the refractive indices around noble metal Materials and methods nanoparticles [3, 4]. The advantages of LSPR sensors hav - Design and fabrication of OWG ing this characteristic are real-time analysis, label-free The OWG of stair-shape is designed as shown in Fig.  1a detection, and high sensitivities [5–7]. However, the most because the situation that the lights directly enter the of LSPR sensors have been developed to method that the detector from the light source without the pass of OWG incident lights are focused to nanoparticles in free-space can cause the noise in the observation of LSPR signal. by the objective lens, which limits the coupling efficiency, Considering the fact that light transmission loss of 0.1 dB miniaturization, cost-effectiveness, and accessibility per 1 mm occurs in the OWG using SU-8 (2075, Micro- [8–12]. chem, Newton, MA, USA), the distance between both In this study, we propose a novel LSPR based optical optical fiber holders is fabricated as short as possible waveguide (OWG) sensor which is fabricated by SU-8 2 mm [13]. If the stair angle (θ) of OWG becomes large, photoresist (PR) through relatively simple process that the light transmission efficiency is decreased due to the increases of the bending angle and the effective length *Correspondence: parkjae@dankook.ac.kr; skilee@dankook.ac.kr of OWG [14, 15]. On the other hand, when θ becomes Department of Electronics and Electrical Engineering, Dankook University, Yongin 16890, Republic of Korea smaller, the light from optical fiber connected to the © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Kim et al. Micro and Nano Syst Lett (2019) 7:7 Page 2 of 5 SU-8 also has good mechanical and thermal stabil- ity, and is suitable for biosensing applications due to its biocompatibility and chemically stable characteristic [20, 21]. The SU-8 was spin-coated (ACE-200, Dong ah, Seoul, Republic of Korea) on 4  in. glass wafer (Boro33, Fiber holder DS semicon, Anyang-si, Republic of Korea) at 1700  rpm 160 µm for 40  s. At this time, the soft bake was sequentially 145 µm OWG performed at 65  °C for 20  min and at 95  °C for 55  min, and the exposure was continued for 60  s at intensity of 25  mW/cm . The post exposure bake was carried out at 65  °C and 95  °C for 10  min and 30  min, respectively. 2mm SU-8 Glass Finally, the sample was developed in AZ 1500 thinner (AZ electronic materials, Seoul, Republic of Korea) dur- ing 90  min to complete the OWG fabrication. Figure  1b is micrograph when the optical fibers (FG105LCA, Thor - labs, Newton, NJ, USA) are fixed to the fabricated fiber holder. The optical fiber was immobilized to the fiber Fiber holder using ultraviolet curable resin (NOA61, Norland, holder Optical fiber Cranbury, NJ, USA). Figure  2 shows the field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, OWG Ibaraki, Japan) images of OWG and optical fiber holder. The width and height of fabricated OWG were measured Glass to be 162.4 µm and 126.3 µm, respectively. The width of Fig. 1 Schematic diagram of the OWG and the microscope image. a the fiber holder was also 144.5 µm. Design values and b fabricated SU‑8 OWG are indicated Fabrication of OWG sensor based on LSPR The method to immobilize the Au nanoparticles on source can be directly incident on the opposite optical OWG using SU-8 consists of four steps. In the first step, fiber, which makes it difficult to observe the LSPR signal. the oxygen plasma (CUTE, Femto science, Yongin-si, In order to improve the light transmission efficiency of Republic of Korea) was treated on OWG with 60  W OWG, the θ is selected to 30° within a range in which the power and 0.8  m Torr during 50  s. The oxygen plasma effective length of OWG is shortened and the light is not treatment removes the organic compound and func- directly transmitted between the optical fibers. The width tionalizes the surface of OWG with hydroxyl group [22, of OWG is planned to be 160  µm, considering that the 23]. Next, the sample was reacted with 1% TEOS (98%, coupled optical fiber diameter which includes core and Sigma aldrich, St. Louis, MO, USA) solution in isopro- cladding is 125  µm and the spread of light emitted from panol (99.5%, Daejung, Siheung-si, Republic of Korea) the optical fiber. Finally, the width of optical fiber holder for 3 h, which acts as the cladding in OWG and controls is decided to be 145 µm to allow enough area for the opti- the Au nanoparticles density on the surface [24]. In the cal fiber. A gap of about 10 µm is set on both sides of the third step, 1% 3-(ethoxydimethylsilyl)-propylamine (97%, optical fiber. Sigma aldrich) solution based on isopropanol was coated The OWG was simply fabricated through a single on the OWG for 2 h to form the self-assembled monolay- lithography process (MDA-400M, Midas system, Dae- ers which have the amine groups with positive charges to jeon, Republic of Korea) of SU-8. The excellent optical immobilize the Au nanoparticles on the OWG surface. transparency beyond 400  nm and the low transmission Finally, the Au nanoparticles which are prepared at about loss of SU-8 make it a preferred material for the fabri- 47.8 nm diameter were attached on OWG during 6 h. The cation of various optical components and systems such Au nanoparticles were synthesized through the Turk- as OWGs, splitters, directional couplers, and gratings evich method by reducing the Au(III) chloride trihydrate [16–18]. SU-8 has a high refractive index of 1.59 at the (99.9%, Sigma aldrich) using sodium citrate dehydrate wavelength of 640  nm used in the measurement, which (99%, Daejung) [25, 26]. Since the Au ions are reduced can increase the light guiding efficiency due to the large by citrate, the negative charges are present on the surface contrast of the refractive indices between SU-8 and of the Au nanoparticles, which enables the electrostatic organic-based tetraethyl orthosilicate (TEOS) used as bonding with the positive charges of amine groups on the the cladding [19]. The refractive index of TEOS is 1.38. OWG [27, 28]. The Au nanoparticles are immobilized on Kim et al. Micro and Nano Syst Lett (2019) 7:7 Page 3 of 5 the side surface as well as the top surface of OWG. How- ever, the nanoparticles are not attached between OWG and optical fiber in order to prevent the decrease of cou - Glass pling efficiency. When Au nanoparticles are immobilized on the gap between OWG and fiber, the incident light Fiber Fiber from optical fiber will be reflected back to the fiber by the holder Au nanoparticles. The fabrication process of OWG sen - holder sor based on LSPR and the FE-SEM photography at the surface of proposed sensor were displayed in Fig.  3. On the OWG surface, the Au nanoparticles were mostly pre- sent as monomer without excessive aggregation. Results and discussion Optical measurement set‑up The optical measurement set-up for the record of LSPR signal is simply composed as shown in Fig.  4. The light from the laser (Iflex-2000, Qioptiq, Hamble-le-rice, UK) is transmitted to the OWG sensor through the optical Fiber Fiber fiber, which reacts with the Au nanoparticles on the sen - OWG holder holder sor surface. Then, the reacted lights are again collected through the optical fiber to the photodetector (PDA36A, Thorlabs). The collected signals by the photodetector are sent to a computer by the data acquisition (USB-6210, National instruments, Austin, TX, USA) and observed in real-time. In this set-up, the optical fibers are positioned as close to OWG as possible [29]. Refractive index measurement using the OWG sensor based on LSPR Fig. 2 The FE‑SEM photograph of fabricated OWG It is evaluated that the OWG sensor generates the changed LSPR intensities according to the increase of OH OH O Si O Si O OH OH OH O O OH OH OH SU-8 Oxygen plasma 1% (v/v)tetraethyl orthosilicate treatment for 50 sec treatment for 3 hour NH - Au - NH NH 2 2 NH NH NH - 2 2 2 0* 0* SiO SiO 1% (v/v) 3-(ethoxydimethylsilyl)- Immobilization of Au propylaminetreatment for 2 hour nanoparticles for 6 hour Fig. 3 The immobilization process of Au nanoparticles on OWG and the FE‑SEM image of attached nanoparticles on SU‑8 Kim et al. Micro and Nano Syst Lett (2019) 7:7 Page 4 of 5 Optical fiber Photodetector Sample Laser Computer Incident light LSPR signal Input optical fiber Optical waveguide Output optical fiber Fig. 4 The optical measurement set‑up and the schematic diagram of lights pass in OWG refractive index around Au nanoparticles. When six refractive index solutions with a spacing of 0.01 from 1.33 to 1.38 were sequentially supplied to the sensor, the LSPR intensities were gradually increased in Fig.  5a. The refractive index sensitivity which is defined by the normalized intensity change per refractive index unit (RIU) and the coefficient of determination (R ) indicat- 1.38 ing the degree of correlation between the two variables are shown in Fig. 5b [30, 31]. The sensitivity of proposed 7800 1.37 1.36 sensor was 3.10/RIU and the R was 0.99, which mean the high linearity between the refractive index and the 1.35 output of OWG sensor. The normalized intensity is used to calculate the sensitivity because the LSPR intensity 1.34 depends on external factors such as the laser power, the 1.33 gain of photodetector, and the alignment between light source and fiber [32]. The normalized intensity is defined as the relative signal based on the measured intensity at 020406080100 120140 160180 1.33 [33]. Time (s) Conclusion Normalized intensity 1.16 Linear fit of normalized intensity The conventional methods that the light exposes into a 1.14 free space during LSPR measurement have been prob- 1.12 lematic such as the insufficient optical coupling efficiency 1.10 and the requirement of expert skill for the alignment of 1.08 light. To solve these problems, we applied the OWG that 1.06 do not expose the light to the air and suggested using it Sensitivity 1.04 as an LSPR sensor. The low-loss OWG was fabricated on 3.10 /RIU 1.02 glass wafer by SU-8 using a single lithography process 2 R = 0.99 1.00 and the OWG sensor based on LSPR is completed by y = 3.10x-3.13 0.98 immobilizing the Au nanoparticles on OWG. In addition, 1.33 1.34 1.35 1.36 1.37 1.38 it was possible to construct the compact optical system Refractive index unit by using optical fiber, which enables the simple trans - mission and collection of light. 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Published: Jun 26, 2019

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