Objective Surface plasmon resonance (SPR) sensors based on Kretschmann prism are bulky and need to be equipped with mechanical movable parts, which is not conducive to the miniaturization and remote sensing of devices. Optical-fiber-based SPR technology has been widely used in food safety, environmental monitoring, and other fields owing to its label-free simple pretreatment, fast analysis, high sensitivity, and other excellent characteristics. However, most SPR sensors do not consider sample consumption in the process of sensor monitoring. The exposed optical fiber is fragile, which easily affects the sensor's stability. Highly sensitive optical fibers, such as tapered and D-shaped fibers, are vulnerable and unstable. In addition to the abovementioned problems, the sensitivity of existing SPR fiber sensors needs further improvement. As a high-throughput microscale analysis device, the microfluidic chip system has shown great potential in remote monitoring, biological detection, and other fields in recent years. In this paper, a microfluidic chip based on SPR fiber sensor is designed and used to measure the concentration of a solution. This chip has the advantages of small size, compact structure, and low sample consumption; it also reduces the fiber damage and effectively improves the sensing stability by embedding the sensing fiber into Polydimethylsiloxane (PDMS) substrate. Photonic crystal fiber (PCF) is a new type of optical fiber. PCF comprises a single dielectric, in which air holes are closely arranged in the two-dimensional direction but unchanged in the axial structure to form microstructure cladding. Moreover, it can stimulate a more substantial SPR effect for its unique light-guiding characteristics.
Methods In this paper, PDMS—which has good chemical inertia and good biocompatibility—is used as the main material to fabricate the microfluidic chip. After cooling and forming in 3D mold, PDMS is stably bonded with glass sheet to form the main structure of the chip. The microchip contains a microfluidic channel (diameter: 200nm), whose length is the same as that of the chip. A sensing structure of multimode fiber-photonic crystal fiber-multimode fiber (MMF-PCF-MMF), which is coated with 60-nm gold film on the surface, is embedded in the channel to stimulate SPR effect. Then, mixed solutions of glucose solid sample and deionized water in a certain proportion with refractive index of 1.338--1.425 are used as samples to be tested. The experiment is performed at room temperature (25 °C). The glucose solution in the syringe is injected into the microfluidic chip using a syringe pump at a constant speed; the glucose solution flows through the optical fiber sensing part. When the concentration of the solution injected into the microfluidic chip is changed, the light wave meeting the resonance conditions excites the gold film to produce surface plasmon resonance and the resonant valley is generated on the transmitted spectrum. The position of the resonant valley shifts in the samples with different refractive indexes. By recording the resonance spectra of a series of samples, the sensitivity of the sensor with respect to the change in refractive index can be calibrated.
Results and Discussions When the refractive index of the liquid injected into the microfluidic chip increases from 1.338 to 1.425, the resonant wavelength of SPR spectrum shifts to longer wavelength because of the SPR resonance effect. The relationship between the refractive index and resonant wavelength of the liquid to be measured is extracted, and the sensitivity curve can be obtained by taking the resonant wavelength at n=1.338 as the starting point. The increasing speed of the sensitivity curve is related to the chip sensitivity. The experimental results show that the refractive index sensitivity of PCF SPR sensors can reach up to 8240.6nm/RIU, in which RIU is refractive index unit, thereby showing that these have good high-sensitivity characteristics and meet the application requirements of high sensitivity.
Conclusions In this paper, a novel microfluidic chip embedded with SPR sensor is designed and manufactured by combining the optical fiber structure, SPR effect, and microfluidic system. The volume of chip is approximately 3.5cm×1cm×5cm, which is much smaller than that of the traditional measurement instrument and is conducive to the integration of sensor. After testing the MMF-PCF-MMF structure based on SPR effect, the sensor sensitivity obtained is high (up to 8240.6nm/RIU) in a wide refractive index measurement range of 1.338--1.425. The proposed structure has good refractive sensitivity, small size, acid resistance, and corrosion resistance; these characteristics enable a broader application prospect in the field of biochemistry.
