Introduction

The oscillations of free electrons in noble metal nanoparticles when the light is incident is called localized surface plasmon (LSP), and the phenomenon that the scattered signals in incident lights by nanoparticles are amplified with LSP is referred to localized surface plasmon resonance (LSPR) [1, 2]. One of the most remarkable effects of LSPR is that the intensity of LSPR is sensitive to changes of the refractive indices around noble metal nanoparticles [3, 4]. The advantages of LSPR sensors having this characteristic are real-time analysis, label-free detection, and high sensitivities [5,6,7]. However, the most of LSPR sensors have been developed to method that the incident lights are focused to nanoparticles in free-space by the objective lens, which limits the coupling efficiency, miniaturization, cost-effectiveness, and accessibility [8,9,10,11,12].

In this study, we propose a novel LSPR based optical waveguide (OWG) sensor which is fabricated by SU-8 photoresist (PR) through relatively simple process that can efficiently combine the light with nanoparticles. A simple optical measurement set-up is also constructed by applying the optical fiber, and the proposed sensor is used to LSPR signal measurement according to the refractive index changes of the surrounding medium. The LSPR based OWG sensor is expected to the application as biosensor with advantages in miniaturization, low cost, and portability in the future.

Materials and methods

Design and fabrication of OWG

The OWG of stair-shape is designed as shown in Fig. 1a because the situation that the lights directly enter the detector from the light source without the pass of OWG can cause the noise in the observation of LSPR signal. Considering the fact that light transmission loss of 0.1 dB per 1 mm occurs in the OWG using SU-8 (2075, Microchem, Newton, MA, USA), the distance between both optical fiber holders is fabricated as short as possible 2 mm [13]. If the stair angle (θ) of OWG becomes large, the light transmission efficiency is decreased due to the increases of the bending angle and the effective length of OWG [14, 15]. On the other hand, when θ becomes smaller, the light from optical fiber connected to the source can be directly incident on the opposite optical fiber, which makes it difficult to observe the LSPR signal. In order to improve the light transmission efficiency of OWG, the θ is selected to 30° within a range in which the effective length of OWG is shortened and the light is not directly transmitted between the optical fibers. The width of OWG is planned to be 160 µm, considering that the coupled optical fiber diameter which includes core and cladding is 125 µm and the spread of light emitted from the optical fiber. Finally, the width of optical fiber holder is decided to be 145 µm to allow enough area for the optical fiber. A gap of about 10 µm is set on both sides of the optical fiber.

Fig. 1
figure 1

Schematic diagram of the OWG and the microscope image. a Design values and b fabricated SU-8 OWG are indicated

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The OWG was simply fabricated through a single lithography process (MDA-400M, Midas system, Daejeon, Republic of Korea) of SU-8. The excellent optical transparency beyond 400 nm and the low transmission loss of SU-8 make it a preferred material for the fabrication of various optical components and systems such as OWGs, splitters, directional couplers, and gratings [16,17,18]. SU-8 has a high refractive index of 1.59 at the wavelength of 640 nm used in the measurement, which can increase the light guiding efficiency due to the large contrast of the refractive indices between SU-8 and organic-based tetraethyl orthosilicate (TEOS) used as the cladding [19]. The refractive index of TEOS is 1.38. SU-8 also has good mechanical and thermal stability, 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, DS semicon, Anyang-si, Republic of Korea) at 1700 rpm for 40 s. At this time, the soft bake was sequentially 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/cm2. The post exposure bake was carried out at 65 °C and 95 °C for 10 min and 30 min, respectively. Finally, the sample was developed in AZ 1500 thinner (AZ electronic materials, Seoul, Republic of Korea) during 90 min to complete the OWG fabrication. Figure 1b is micrograph when the optical fibers (FG105LCA, Thorlabs, Newton, NJ, USA) are fixed to the fabricated fiber holder. The optical fiber was immobilized to the fiber holder using ultraviolet curable resin (NOA61, Norland, Cranbury, NJ, USA). Figure 2 shows the field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Ibaraki, Japan) images of OWG and optical fiber holder. The width and height of fabricated OWG were measured to be 162.4 µm and 126.3 µm, respectively. The width of the fiber holder was also 144.5 µm.

Fig. 2
figure 2

The FE-SEM photograph of fabricated OWG

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Fabrication of OWG sensor based on LSPR

The method to immobilize the Au nanoparticles on OWG using SU-8 consists of four steps. In the first step, the oxygen plasma (CUTE, Femto science, Yongin-si, Republic of Korea) was treated on OWG with 60 W power and 0.8 m Torr during 50 s. The oxygen plasma treatment removes the organic compound and functionalizes the surface of OWG with hydroxyl group [22, 23]. Next, the sample was reacted with 1% TEOS (98%, Sigma aldrich, St. Louis, MO, USA) solution in isopropanol (99.5%, Daejung, Siheung-si, Republic of Korea) for 3 h, which acts as the cladding in OWG and controls the Au nanoparticles density on the surface [24]. In the third step, 1% 3-(ethoxydimethylsilyl)-propylamine (97%, Sigma aldrich) solution based on isopropanol was coated on the OWG for 2 h to form the self-assembled monolayers which have the amine groups with positive charges to immobilize the Au nanoparticles on the OWG surface. Finally, the Au nanoparticles which are prepared at about 47.8 nm diameter were attached on OWG during 6 h. The Au nanoparticles were synthesized through the Turkevich method by reducing the Au(III) chloride trihydrate (99.9%, Sigma aldrich) using sodium citrate dehydrate (99%, Daejung) [25, 26]. Since the Au ions are reduced by citrate, the negative charges are present on the surface of the Au nanoparticles, which enables the electrostatic bonding with the positive charges of amine groups on the OWG [27, 28]. The Au nanoparticles are immobilized on the side surface as well as the top surface of OWG. However, the nanoparticles are not attached between OWG and optical fiber in order to prevent the decrease of coupling efficiency. When Au nanoparticles are immobilized on the gap between OWG and fiber, the incident light from optical fiber will be reflected back to the fiber by the Au nanoparticles. The fabrication process of OWG sensor 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 present as monomer without excessive aggregation.

Fig. 3
figure 3

The immobilization process of Au nanoparticles on OWG and the FE-SEM image of attached nanoparticles on SU-8

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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, which reacts with the Au nanoparticles on the sensor 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].

Fig. 4
figure 4

The optical measurement set-up and the schematic diagram of lights pass in OWG

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Refractive index measurement using the OWG sensor based on LSPR

It is evaluated that the OWG sensor generates the changed LSPR intensities according to the increase of 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 (R2) indicating the degree of correlation between the two variables are shown in Fig. 5b [30, 31]. The sensitivity of proposed sensor was 3.10/RIU and the R2 was 0.99, which mean the high linearity between the refractive index and the output of OWG sensor. The normalized intensity is used to calculate the sensitivity because the LSPR intensity depends on external factors such as the laser power, the 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 1.33 [33].

Fig. 5
figure 5

The measurement result. a LSPR intensity changes according to increase of refractive indices. b The amount of increased intensity per refractive index unit. The intensities are normalized as reference to the signal at 1.33

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Conclusion

The conventional methods that the light exposes into a free space during LSPR measurement have been problematic such as the insufficient optical coupling efficiency and the requirement of expert skill for the alignment of light. To solve these problems, we applied the OWG that do not expose the light to the air and suggested using it as an LSPR sensor. The low-loss OWG was fabricated on glass wafer by SU-8 using a single lithography process and the OWG sensor based on LSPR is completed by immobilizing the Au nanoparticles on OWG. In addition, it was possible to construct the compact optical system by using optical fiber, which enables the simple transmission and collection of light. The proposed sensor was applied as a refractive index sensor and it was confirmed that the OWG sensor based on LSPR exhibits a very linear response with the change of refractive index. Based on these results, if a number of OWGs are integrated into a single chip, it is expected that the various target molecules can be multi measured in real-time as the biosensor application.