1. Introduction

Surface plasmon resonance (SPR) technology has been increasingly employed for biomolecular interactions analysis and chemical and biological analysis with the advantages of high sensitivity, real-time online detection and label-free [1–4]. Compared with the traditional prism-based SPR sensor, the fiber-based SPR sensor whose coupling device is optical fiber possesses the advantages of small size, less samples, easy to be integrated and applied to telemeter. Fiber SPR sensor is a natural extension of the miniaturization of the SPR sensor. However, there are two bottlenecks about the fiber SPR sensor of low sensitivity and the difficulty in the multichannel detection cannot be solved effectively.

In the practical application, SPR sensor is indispensable to the multi-channel detection. Firstly, the multi-channel detection can realize the simultaneous detection of the mixture in the same area. With the diversity of the kinds and characteristics of the parameter to be measured, for example, it is necessary to monitor the blood glucose, cholesterol, hemoglobin, urea and all kinds of other useful indicator in the blood at the same time. The conventional single channel SPR sensor has been gradually unable to adapt to the needs of practical detection, multi-channel SPR sensor is urgently needed to be developed. It is becoming the focus of attention to study a novel multi-channel SPR sensor which has the advantages of high-integration and high-throughput. Secondly, it is easy for the different channels to test the same substance simultaneously which will eliminate the interference of background refractive index and the influence of the temperature. The response of the SPR sensor is caused by a variety of factors which depends on the fundamental of the SPR sensor. For example, in the biomedical field, except the change of the sensing film dielectric constant caused by the combination of target molecule and SPR probe which leads to the change of the resonance angle or resonance wavelength (specific response), there also have the additional responses caused by the change of the concentration and temperature, besides that the interaction between the other molecule and the sensing film in the sample solution also leads to the change of the sensing film dielectric constant, these inevitable error changes also lead to the change of the resonance angle or resonance wavelength(non-specific response). Non-specific response has a serious impact on the measuring accuracy of the SPR sensor. In order to solve this problem, it can distinguish the specific response from the non-specific response by proposing the multiple channels. In order to realize SPR multi-channel detection, the prism-based SPR sensor which developed maturely employs the surface of the sensing metal film to divide the grid, and employs the different sensing zones to realize the multi-channel sensing which is the application of the time-multiplexed technology [5]. Recently, there have been some reports about the multi-channel study of the fiber-based SPR sensor [6–12]. However, it is difficult to divide the sensing area into different regions for fiber-based SPR sensor. In this paper, we employ the different cores of the microstructured fiber to fabricate the different sensing channels which solves the multi-channel detection problem of the fiber-based SPR sensor effectively. Meanwhile, the diameter of the seven-core fiber we used in this paper is only 125μm which can insert into the tiny area easily.

The traditional fiber SPR sensor is always based on the multimode fiber. However, it is difficult to control the transmission angle of the multimode fiber. In order to meet the Kretschmann structure and bring the incident angle up to the resonance angle, we generally take the approach of full core injection which covered a certain angle rang, and the resonance curve we take finally is the combination of several resonance curves which are stimulated by several incident angles. The combination of several incident angles cause the broadening of the resonance curve which will impact the sensitivity and accuracy of the SPR sensor based on the multimode fiber. Compared with the multimode fiber, the single mode fiber has the less mode which causes the concentration of the incident angles, even a single incident angle, which compress the width of the resonance curve. Therefore, Wei [13–15] employs the single mode fiber to fabricate the SPR sensor and improve the sensitivity. The proposed three-channel SPR sensor in this paper employs the single mode enhancing sensitivity technique to increase the sensitivity. Based on the further research of the mutual restriction relationship between the refractive index of the solution to be measured, grinding angle, sensitivity and detection range, we propose a novel segmented detection technology.

By employing the fiber end grinding and polishing technology, this three-channel SPR sensor uses circularly symmetric distributed multi cores to fabricate three groups of cone angle structure with different grinding angle. According to the refractive index range of the solution, the testing channel is chose by adjusting the wavelength range and sensitivity of the sensor. These three groups of cone angle structure have the different incident angles which provide three sensing channels. Through the experiment and simulation results, we find that: (1) as the increasing of the grinding angle, the wavelength range of the sensing probe is red shifted and the sensitivity is increasing with the same refractive index of the solution, (2) as the increasing of the grinding angle, the refractive index range of the solution is reducing and the detectable refractive index tends to low refractive index range, (3) with the same grinding angle, the higher is the refractive index of the solution to be measured, the higher is the sensitivity and the wavelength range is red shifted.

Based on above description and discussion, we propose and demonstrate a novel segmented detection method. These probes of the three-channel SPR sensor are grinded to three different angles, we employ the sensing probe with small grinding angle to test the solution with high refractive index, and the sensing probe with large grinding angle to test the solution with low refractive index. This segmented detection SPR sensor has the following two advantages:

Firstly, reducing the requirement for bandwidth of light source and spectrometer (the light source and spectrometer with ultra wide bandwidth are always expensive). We employ the rational design and choice of the grinding angle to realize the highly sensitive detection with the wide refractive index range in the working bandwidth of 250nm which means the movements range of the resonant dips.

Secondly, increasing the detection range and also ensuring ultra high sensitivity. Due to the constraint relationship between the detection range and sensitivity, it is difficult for the single detection channel to ensure the wide detection and high sensitivity at the same time. In order to solve this problem, we propose the segmented detection method, employing the sensing probe with small grinding angle to test the solution with high refractive index, and the sensing probe with large grinding angle to test the solution with low refractive index (the sensing probe with small grinding angle has a wide detection range with a low sensitivity, and the sensing probe with large grinding angle has a narrow detection range with a high sensitivity). As we known, this is the first time that the average sensitivity of the fiber-based SPR sensor exceeds 6000nm/RIU, and the average sensitivity of this segmented detection SPR sensor based on the seven-core fiber reaches 7387.10 nm/RIU. Meanwhile, the refractive index detection range of the solution to be measured is 1.333-1.395, and the working bandwidth is 250nm which means the movements range of the resonant dips.

2. Fabrication and structure of the segmented detection SPR sensor

The fiber probe tip is fabricated by using the fiber grinding and polishing method. The white light is launched into the seven cores of the fiber at the same time, and it helps us to employ the CCD of the grinding and polishing machine to locate the fiber. Turn the seven-core fiber axially, when the straight line of the two symmetric cores is perpendicular to the abrasive disk, press down the seven-core fiber to where the angle between the rotational axis of the fiber and vertical direction is $\alpha$, begin to grind the fiber. When the grinding depth fulfills the requirement, lift up the fiber, rotate the fiber by 180° axially, press down the seven-core fiber again, when the angle between the rotational axis of the fiber and vertical direction is $\alpha$, begin to grind the fiber. Then, the single frequency laser is launched into one side of the core which has been finished grinding, and monitoring the light intensity in the other side of the core, when the light intensity reaches its maximum, stop grinding. The seven-core fiber(SM-7C1500(6.1/125)) is shown in the Fig. 1(b). The diameter of the cladding is 125μm, and the core diameter is 8μm. We named each core with a serial number, the 0# core is in the center of the fiber, and the 1#~6# cores distribute in an equilateral hexagon shape, and the side length is 37.5μm. In order to realize the Kretchmann prism configuration, we grind the core 1# and 4# to be 10°(channel I), and the core 2# and 5# to be 12.5° (channel II), the core 3# and 6# to be 15° (channel III) which are shown in the Fig. 1(d). Then the light is in from the core 1#, 3# and5#, out from the core 2#, 4# and 6#. The resonance angle of the channel I is 80°, the resonance angle of the channel II is 77.5°, and the resonance angle of the channel III is 75°. The finished seven-core fiber tip is shown in the Fig. 1(a) and the fiber tip with gold film coating is shown in the Fig. 1(c).

 

Fig. 1 (a) Profile image of the seven-core fiber with serial numbers (0~6#); (b) profile image of the seven-core fiber; (c) image of the sensor probe tip with the gold film coating; (d) sketch diagram of the sensor probe grinding angles(10°, 12.5° and 15°), where SGF means sensing gold film with the thickness of 50nm, RGF means reflecting gold film with the thickness of 300nm.

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We employ a fiber-end grinding and polishing method to fabricate the fiber probe. We grind the SCF to be the designed angle and depth by using a fiber grinding machine. We grind the fiber with a 8000-grit grinded paper for 2 hours, and then polish it with a 12000-grit polished paper to make sure the grinded plane completely flat. Figure 2 supplies the grinding step. Similarly to the method in [13], we employ a two-step method to plate the gold film on the fiber tip, with 50nm gold films on the fiber inclined surfaces and 300nm gold film on the fiber end surface. The film thickness measuring method is also same as the method in [13].

 

Fig. 2 Profile images of the seven-core fiber with the (a)No.1#; (b) No.4#; (c) No.6#; (d) No.3#; (e) No.5#; (f) No.2# core being grinded off recorded by the fiber grinding machine. The serial number is referenced from the Fig. 1.(a).

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The sensing experimental setup is shown in the Fig. 3. A super continuum light source (SuperK compact, NKT Photonics) whose spectrum wide is 450~2400nm, is launched into the 1# core of the SCF by using a lens transform system as in the Fig. 3(b), where we place the collimating lens in front of two single mode fibers while placing the microscope objective lens (the magnification of 25) in front of the SCF. The light power is launching into 1# core by adjusting the distance and position of the lens in the transform system), and the reflected beam is received from the SMF by an optical spectrum analyzer (AQ6373, Yokokawa) from the 4# core. When we test the channel II, from the 2# core to 5# core, we rotate the ST(swivel table) in the lens transform system by ~60°; and when we test the channel III, from the 3# core to 6# core, we just need to rotate the ST by another ~60°. The Glycerine aqueous solutions are injected into the microfluidic chip which is used for simulating the blood-vessels by using the micro-injection pump, and the waste solution is transported into the waste reservoir. The Glycerine-aqueous solution index is measured and calibrated by the Abbe refractometer (GDA-2S, Gold).

 

Fig. 3 (a) The experiment setup sketch diagram of the three channel fiber SPR sensor; (b) sketch diagram of the beam lens transform system, where SMF means single mode fiber, SCF means seven-core fiber, ST means swivel table.

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3. Properties analysis of segmented detection SPR sensor

In order to study the properties of the segmented detection SPR sensor based on seven-core fiber, we employ the MATLAB software to calculate the SPR reflection attenuation spectrum according to the method in [16], the simulated conditions are: the fiber core refractive index is 1.467, the thickness of the gold film is 50nm, and the dielectric constant of the gold film is obtained from [17].The simulation results are shown in Fig. 4. With the refractive index range of 1.333~1.385, when the grinding angle is 10°(the SPR incidence angle is 80°) as in the Fig. 4(a), the dynamic range of the resonance wavelength is 626~786nm with a total of 160nm, and the average sensitivity is 3083nm/RIU. When the grinding angle is 12.5°(the SPR incidence angle is 77.5°) which is shown in the Fig. 4(b), the dynamic range of the resonance wavelength is 646~862nm with a total of 216nm, and the average sensitivity is 4152nm/RIU. And when the grinding angle is 15°(the SPR incidence angle is 75°) which is shown in the Fig. 4(c), the dynamic range of the resonance wavelength is 680~1019nm with a total of 339nm, and the average sensitivity is 6523nm/RIU. Therefore, we employ the SCF to fabricate three sensing probes with three different grinding angle, when we employ the different sensing probe to test the solution with the same refractive index, the resonance wavelength is different. Figure 4(d) shows the relationship between the refractive index of the solution to be measured and the resonance wavelength of the different sensing probe respectively, and shows the calculation results of the average sensitivity of the different sensing probe with the refractive index range of 1.333~1.385 and the instantaneous sensitivity in different sensing probe with the different refractive index. It can be seen from the Fig. 4(d), the larger is the grinding angle, the longer is the resonance wavelength. The larger is the refractive index, the larger is the resonance wavelength, and the larger is the sensitivity (the slope of the curves in the figure equals to the instantaneous sensitivity). Here the simulation parameter are as follows, the refractive index of the fiber core is 1.467, the thickness of the gold film is 50nm, and the dielectric constant of the gold film is the same as [16].

 

Fig. 4 Simulated and calculated results of the SPR spectrum with the SPR resonance angle of (a) 80°, (b) 77.5° and (c) 75°, (d) relation between the fiber grinding angle and the testing dynamic range. Where SIM means the simulation results, S means the instantaneous sensitivity, AS means the average sensitivity.

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Furthermore, we realize the reflection-type TDM technology in fiber which can solve the problem of multi-channel measuring, and also study the increase in sensitivity of the SPR sensor. Figure 5(a) provides the testing results of the reflected attenuating spectrum of channel I (1# core~4# core) in the Glycerine-aqueous solution with index of 1.333, 1.345, 1.355, 1.365 1.375 and 1.385 successively. From the testing results, the dips generated from the channel I move from 625nm to 761nm with a total of 136nm, and the average sensitivity is 2615nm/RIU. With the same testing method, we get the testing results of channel II(2# core~5# core), the dips generated from the channel II move from 654nm to 820nm with a total of 166nm, and the average sensitivity is 3192nm/RIU which is shown in the Fig. 5(b). Figure 5(c) provides the testing results of channel III(3# core~6# core), the dips generated from the channel III move from 682nm to 928nm with a total of 246nm, and the average sensitivity is 4731nm/RIU. Figure 5(d) shows the relationship between the refractive index of the solution to be measured and the resonance wavelength of the different sensing probe respectively, and shows the calculation results of the average sensitivity of the different sensing probe and the instantaneous sensitivity in different sensing probe with the different refractive index. The simulated results and the testing results have a good consistence.

 

Fig. 5 (a) Testing results of the SPR spectrum with channel I working; (b) testing results of the SPR spectrum with channel II working; (c) testing results of the SPR spectrum with channel III working; (d) relation between the refractive index and the resonance wavelength. Where TES means the testing results, S means the instantaneous sensitivity, AS means the average sensitivity.

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4. Results and discussion

After the experiment and theoretical analysis above, we get the conclusions and corollary as follows:

Conclusion(1), for the solution with the same refractive index, the larger is the grinding angle, the longer is the resonance wavelength, and the higher is the sensitivity. For example, when we test the solution with the refractive index of 1.385, the grinding angle of the sensing probe increases from 10° to 15°, the testing resonance wavelength moves from 761nm to 928nm (the simulation resonance wavelength moves from 786.4nm to 1018.9nm), the testing sensitivity increases from 3802nm/RIU to 7264nm/RIU (the simulation sensitivity increases from 4737nm/RIU to 10514nm/RIU).

Conclusion(2), for the same grinding angle of the sensing probe, the higher is the refractive index of the solution, the higher is the instantaneous sensitivity. For example, when we use the channel III with the grinding angle of 15° to test the solution, as the refractive index of the solution increases from 1.333 to 1.385, the testing resonance wavelength increases from 682nm to 928nm (the simulation resonance wavelength increase from 679.7nm to 1018.9nm), and the testing sensitivity increases from 1953nm/RIU to 7264nm/RIU (the simulation sensitivity increases from 1563nm/RIU to 10514nm/RIU).

Inference (3), with the dynamic range of 500~1000nm, the larger is the grinding angle, the narrower is the refractive index range of the solution to be measured, and the detection range tends to the low refractive index range (the simulation data in Table 1 confirms this corollary).

Table 1. The Simulated and calculated results of the resonance wavelength with different refractive index of the solution to be measured

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Inference (4), the segmented detection technology provides a method to test the different refractive index range of the solution through the different sensing channel which breaks off the restrictive relations between the refractive index range of the solution to be measured and the sensitivity of the SPR sensor.

Based on the above conclusions and corollary, we propose a segmented detection method, and we employ the sensing probe with small grinding angle to test the solution with high refractive index, and the sensing probe with large grinding angle to test the solution with low refractive index. In order to increase the sensitivity of the SPR sensor as much as possible, in the premise of ensuring the bandwidth of the light source and spectrometer is 500nm-1000nm (the narrower is the bandwidth, the lower is the demand of the light source and spectrometer), we try to choose the sensing channel with larger grinding angle. The simulating resonance wavelength of the different refractive index of the solution to be measured with different grinding angle of the sensing channel are shown in Table 1, where the average sensitivity is as high as 8995.2nm/RIU, the refractive index detection range is as wide as 1.333-1.395, the dynamic range is as narrow as 626.1-994.8, and the number of channels is 3. Figure 6 provide the relation between the refractive index and the instantaneous sensitivity with different sensing channel.

 

Fig. 6 For the different grinding angle, the relation between the refractive index and the instantaneous sensitivity.

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Through theoretical analysis and reasonable design, we choose the sensing channel with the grinding angle of 13° which has the refractive index range of 1.375-1.395 as the sensing channel I, the sensing channel with the grinding angle of 16° which has the refractive index range of 1.355-1.375 as the sensing channel II, and the sensing channel with the grinding angle of 19° which has the refractive index range of 1.333-1.355 as the sensing channel III. The segmented detection is realized by these three different sensing channels, and the detection range is 1.333-1.395, the average sensitivity reaches 8995.2nm/RIU.

According to the segmented detection method and the sensing channel we choose, the testing results are shown in Fig. 7. Figure 7(a) provides the testing results of the reflected attenuating spectrum of channel I with the refractive index range of 1.375-1.395, the dips generated from the channel I move from 791nm to 941nm, and the average sensitivity is 7500nm/RIU. With the same testing method, we get the testing results of channel II with the refractive index range of 1.333-1.375, the dips generated from the channel II move from 791nm to 935nm, and the average sensitivity is 8700nm/RIU which is shown in the Fig. 7(b). Figure 7(c) provides the testing results of channel III with the refractive index range of 1.333-1.355, the dips generated from the channel III move from 778nm to 942nm, and the average sensitivity is 7455nm/RIU. Figure 7(d) provides the comparisons between simulations and experiments, and the testing results are consistent with the simulated results.

 

Fig. 7 (a) Testing results of the SPR spectrum with channel I working; (b) testing results of the SPR spectrum with channel II working; (c) testing results of the SPR spectrum with channel III working; (d) relation between the refractive index and the resonance wavelength. Here TES means the testing results, SIM means the simulating results, S means the instantaneous sensitivity, AS means the average sensitivity.

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This segmented detection method proposed in this paper has the following two advantages:

Firstly, reducing the requirement for bandwidth of light source and spectrometer (the light source and spectrometer with ultra wide bandwidth are always expensive). We employ the segmented detection method to realize the highly testing average sensitive of 7387.10nm/RIU (simulating average sensitive of 8995.2nm/RIU) with the wide refractive index range of 1.333-1.395 in the bandwidth range of 750nm-1000nm.

Secondly, increasing the detection range and also ensuring ultra high sensitivity. Due to the constraint relationship between the detection range and sensitivity, it is difficult for the single detection channel to ensure the wide detection and high sensitivity at the same time. In order to solve this problem, we propose the segmented detection method, employing the sensing probe with small grinding angle to test the solution with high refractive index, and the sensing probe with large grinding angle to test the solution with low refractive index (the sensing probe with small grinding angle has a wide detection with a low sensitivity, and the sensing probe with big grinding angle has a narrow detection with a high sensitivity).

5. Conclusion

In this paper, we propose and demonstrate a novel segmented detection SPR sensor, which employs the seven-core fiber to realize the reflective three-channel fiber SPR sensor based on TDM technology in single fiber end face at the first time, and also solves two bottlenecks about the fiber SPR sensor of low sensitivity and the difficulty in the multichannel detection. The proposed sensor employs reflection-type time division multiplexing (TDM) technology to realize the three-channel SPR sensing which has the advantages of mixtures detection in the same sensing zone and it can compensate the interferences caused by the temperature variation, nonspecific binding and physical absorption and others. This SPR sensor also has the advantages of online monitoring by inserting into the blood vessel because of its small size.

Furthermore, this paper have deeply researched the relationship between the refractive index of the solution to be measured, the grinding angle, the sensitivity and the detection range. In this paper, we propose a novel segmented detection method which employs the circularly symmetric distributed six cores of the seven-core fiber to couple light into and out to realize the three channels SPR sensing and testing, through the reasonable design and choice, it is easy to employ the different sensing channel to test the solution with different refractive index range. And this segmented detection method reduces the requirement for bandwidth of light source and spectrometer which lays the foundation of using the light source and spectrometer with narrow bandwidth and high stability in SPR experimental study. This is the first time that employing the fiber-based SPR sensor to realize the wide detection with the refractive index range of 1.333~1.395 and the bandwidth of 250nm, the average sensitivity and the maximum sensitivity of the sensor reach 7387.1nm/RIU and 8502.5nm/RIU respectively, the testing results are consistent with the simulated results, and has a high application value.