Localized surface plasmon resonance biosensor using nanopatterned gold particles on the surface of an optical fiber
Introduction
A biosensor composed of the basic components of a signal converter, a receptor and output equipment can be used to detect a specific biomolecule [1,2]. There are a number of different biosensing systems based on various methods, such as electrochemistry [3,4], ellipsometry [5], and interferometry [6]. Among various biosensing systems, the localized surface plasmon resonance (LSPR) biosensor is probably one of the most popular label-free, real-time biosensing methods in the research field [[7], [8], [9], [10], [11]].
LSPR is a phenomenon of collective oscillations by electrons in noble metal nanoparticles excited by external light [12]. This phenomenon can be applied as sensors because it is highly responsive to changes in the refractive index around metal nanoparticles [13]. In particular, the LSPR sensors with advantages such as quick diagnosis, simple optical set-up, and high sensitivity have applicability to biomarker analysis [14,15], food safety [16], environmental monitoring [17]. Fiber-optic localized surface plasmon resonance (FO LSPR) is based on LSPR with a fiber-optic platform and has advantages in terms of its miniaturization, remote sensing, and lossless signal delivery [[18], [19], [20]]. In previous research, FO LSPR sensors are generally fabricated by immobilizing metal nanoparticles on the surface of an optical fiber using metal colloids [[21], [22], [23], [24]]. However, in this method, it is inevitable that the reproducibility and durability of the sensor deteriorate because the adhesion force between the nanoparticles and substrate is weak. Moreover, the nanoparticles are aggregated and removed on the fiber surface due to the surface tension of the sensor surface during a measurement [[25], [26], [27]]. To obtain a better performance of the FO LSPR sensor, we propose the fabrication of FO LSPR sensor by using focused ion beam (FIB) nanopatterning technology. This process allows nano-scale control of the dimensions of the metallic structures fabricated on the fiber end-face, enabling us to develop a plasmon resonance based sensor with control over the position of the nanoparticles. Moreover, FIB allows these sensors to be developed in a reproducible manner with relative ease [28].
We also fabricated a sensor using a multi-mode fiber, unlike previous research on nanopatterning that mostly use single-mode fibers [[29], [30], [31]]. In the case of fabricating the nanostructures on a cleaved end-face of a single-mode fiber, the fabrication process of the sensor is quick and easy because the nanopatterning region is small. However, the intensity of the LSPR signal is weak and the stability of the signal according to external environment changes is low. The focus of this paper is to develop and fine-tune the fabrication procedures for FIB milling of gold nanostructures on the end-face of multi-mode optical fibers.
Three strategies are followed for the fabrication process of an LSPR biosensor with nanopatterned gold particles on the multi-mode fiber. First, a lift-off process is applied to prevent degradation of the optical properties which is caused by the surface roughness of the optical fiber with FIB milling. In order to enhance the fabrication efficiency of the sensor, i.e., to reduce of the total processing time and to increase of the cost-to-benefit ratio, the nanopattern is partially formed on the core of the optical fiber. Then the gold film remaining on the core is typically milled by the FIB process, which makes it difficult to observe the LSPR signal of the gold nanoparticles. Second, a (3-mercaptopropyl)trimethoxysilane (MPTMS) adhesion layer is used between the gold film and optical fiber to observe a clear LSPR signal because the generally used metal adhesion layers, such as Ti and Cr, exacerbate the plasmon damping due to chemical interface damping [32]. Finally, the control of the beam current is carried out for decreasing the process time without degradation of the nanopatterning resolution. The total process time is decreased by using a high beam current, while the resolution of the patterning is reduced when the same area is patterned by the FIB technique. A large number of nanostructures are needed when using the multi-mode fiber because it has a relatively large core area and background signal compared to a single-mode fiber, leading to an increase in the total process time and cost.
We describe the development of the FIB fabrication procedures to form a metallic nanodisk array with a 70 nm diameter and 30 nm thickness. To verify the sensing characteristics of the fabricated FO LSPR sensor, LSPR spectra for the various refractive indices are measured, and sensitivity is calculated using the spectra. The improvement in the fabrication reproducibility is also assessed by comparing the batch-to-batch variations. By comparing the stability between single-mode and multi-mode fibers in the measurement system, the high stability and good optical properties of the multi-mode fiber are validated. Finally, a prostate-specific antigen (PSA) immunoassay is performed using the fabricated FO LSPR sensors. Our measured results verify that the fabricated FO LSPR sensors have a high reproducibility, durability, good optical properties, and linearity in the measurement system. Furthermore, the PSA immunoassay results verify the feasibility of using the fabricated sensor as a biosensor.
Section snippets
Materials and methods
FIB milling is employed to fabricate a nanodisk array from gold film deposited on the cleaved end-face of a multi-mode optical fiber. The nanoparticles are made only on a portion of the multi-mode optical fiber core, where the gold film is left in the unpatterned core region because of the time and cost-effectiveness in sensor fabrication. The signal reflected from the remaining gold film hides the LSPR signal of the nanoparticles. Therefore, as shown in Fig. 1(a), the gold film left on the
Fabrication result
The FE-SEM images of the fabricated FO LSPR sensor using the FIB etching technique on the optical fiber end-face, after completion of overall fabrication process, are shown in Fig. 5. After the lift-off process in step 8 of Fig. 2, only the gold nanodisk array remains on the surface of the fiber-optic based sensor, which is confirmed by the FE-SEM image (Fig. 5(a)). Fig. 5(b) shows a close up view of the gold nanodisk array and schematic diagram of the fabricated gold nanodisks. The measured
Conclusions
We have developed a nanopatterned FO LSPR biosensor based on the multi-mode optical fiber for quantitatively detecting specific biomolecules. The nanopatterned gold particles can prevent the degradations of the sensing characteristics caused by the weak binding force between the gold nanoparticle and optical fiber surface, as well as by the surface tension on the optical fiber. In addition, the nanopatterning permits the precise adjustment of not only the shape and size of the nanostructures,
Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2018R1A2B6001361).
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