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We present a novel multilayer-coated surface plasmon resonance sensor for dual refractive index range measurements based on a capillary structure. The sensing elements include an internally coated Ag layer and an externally coated bilayer of Au with an overlayer of thin indium tin oxide (ITO). The internal Ag layer was sensitive to higher refractive index (RI) medium while the external Au/ITO layer was sensitive to lower refractive index medium. We evaluated the sensor performance by measuring RI changes in two channels, RI sensitivities were −1951 nm/RIU and 2496 nm/RIU, respectively. This compact, low-cost large RI detection range SPR sensor offers the possibility for wider RI detection range and highly sensitive SPR studies in industry and chemical sensing.
© 2017 Optical Society of America
1. Introduction
Over the past decades, surface plasmon resonance (SPR) technologies have been widely applied to label-free detection, quantitative analysis in physics, chemistry and biochemistry [1–4]. Propagating at the metal/dielectric interface, surface plasmons are extremely sensitive to the changes in the refractive index (RI) of the dielectric [5]. Its high sensitivity to RI variations of the dielectric adjacent to a metal shows great potential in sensing detection [6]. The prism-based SPR sensor were shown to have extremely high sensitivity, but present a number of disadvantages like costly integration, generally bulky instrumentation, and relying on a design difficult to mass produce [7]. Due to its miniaturization, low cost, high sensitivity, and remote sensing capabilities, fiber-optic SPR sensor has become increasingly attractive [8–10]. Theoretically analysis of fiber-based SPR sensors with central holes have been reported [11]. In this configuration, the sensed medium is located inside the holes of the fiber [12–14]. However, the micron size features complicate the experimental realization of this structure. Capillaries have been widely used in many applications [15,16], owing to its simple structure and the suitable size of its hollow core. Therefore, the deposition of the metal layer on the inner wall of the capillary is attractive experimentally for capillary-based SPR sensors.
Fiber optic SPR sensors [17–20] with multi-channel structure may be an effective method for multiplexed RI detection [21,22]. Dual-channel SPR sensors have been developed by obtaining two sensing channels in a single fiber while the reference channel was applied to compensate the changes of RI or temperature [23]. Recently, indium tin oxide (ITO) has been reported to be a better substitute of noble metals for surface plasmon [24], and it has been widely used for multi-channel sensing [25]. However, these multi-channel SPR sensors are in a single fiber by using two coatings of different metal layers, which are sensitive to the same RI range detection. The limited RI range of these sensors affects the applications in some fields, such as the petrochemical industry [26] and chemical industry [27,28].
In this paper, a multilayer-coated and dual-channel SPR sensor based on a capillary structure is presented. The RI sensitivity of each channel has been evaluated numerically. These two sensing channels were realized by coating different metal layers and high index material overlayer. More specifically, the internal channel was coated with a Ag layer and the external channel was a bilayer of Au and ITO. For the external channel, the resonant wavelength had a red shift when RI changed from 1.3253 to 1.3726. With the internal Ag layer, the RI sensitivity of the external channel could be increased from 2215 nm/RIU to 2496 nm/RIU. It has been found that the RI sensitivity of the internal channel is −1951 nm/RIU by varying RI of the liquid medium from 1.5255 to 1.5781. This capillary based SPR sensor has good response in a wide RI range and the RI sensitivities are comparable to the reported fiber optic SPR sensor [29]. The presented sensor has potential applications in petroleum and chemical industry where high RI and low RI liquids are often employed at the same time.
2. Materials and methods
Figure 1 shows the operational principle of this multilayer-coated capillary based SPR sensor. The capillary based sensing region with two channels was connected to multi-mode fibers (fused-silica fiber with 770 μm core diameter and 800μm cladding diameter, and the numerical aperture was 0.37). Figure 1(b) and (c) depicts the multilayer-coated capillary and the cross sections of the sensor, respectively. A piece of fused-silica capillary (TSG530/660, Polymicro Inc.) with 530 μm and 660 μm inner and outer tubing diameter was selected to implement the SPR-based sensor. A segment of 20 mm length was cleaved and coated with 30 nm Ag layer on its inner surface by using an improved liquid phase deposition method [30]. Then, the Au/ITO layer was fabricated by removing polyimide jacket from the middle of the fused-silica capillary coating the surface with 45 nm Au layer [31] and 10 nm ITO layer using a magnetron sputtering coater (K575XD, E. M. Technologies Ltd). The 10 nm ITO layer was added to adjust the position of resonance wavelength for the outer channel, the deposition ITO layer led to a red shift, and avoided crosstalk between the channels and enhanced the sensitivity [25]. The 7 mm Au/ITO-coated region outside and the 20 mm Ag-coated inside the capillary were the sensing regions which created two independent SPR sensing channels. The light beam from a halogen lamp was launched into the capillary via the multi-mode fiber. Then, the light beam propagated in the capillary and illuminated the mirror Ag layer and Au/ITO layer where surface plasmons wave can be excited, and was guided in total internal reflection on the inner and outer surfaces of the capillary while passing through the sensor. Finally, the resonant wavelengths were measured with a spectrophotometer (Ocean Optics, HR4000). To evaluate the sensing performances of two channels, the sensor was subjected to different RI solutions in the internal and the external channels.
Fig. 1 Design of the capillary based SPR sensor. (a) Schematic diagram of the capillary based multilayer-coated SPR sensor. (b) Structure of the capillary sensing region. (c) Cross section of the sensing region.
In brief, the silver mirror reaction was an optimum selection as a fast chemical liquid phase deposition method to coat the tube inner surface, and the thickness and smoothness of silver layer can be precisely controlled by tuning the deposition time, reaction temperature and flow rate of solution to meet the SPR sensing requirement. Before deposition, the inner wall was sensitized with a SnCl2 solution (0.01 g/ml SnCl2, 5% HCl) for about 20 s. The Sn2+ ions that remained on the glass surface shortened the plating time, and improved the adhesion between the glass surface and the silver particles reduced afterwards. In our experiment, 0.02 g/ml silver nitrate in an alkaline solution (0.03 g/ml NaOH) and 0.01 g/ml glucose solution were used as the plating and reducing agent, respectively. Then, the solution containing the reagents was passed through the glass capillary. A smooth silver surface was obtained in 9 seconds deposition time, at 20 °C and using a 0.05 mL/s flow rate. The thickness of the Ag layer was calculated at 30 nm, which had been described detailedly in our recently published paper [32]. For the dual-channel sensing, the position of the wavelength for 30 nm Ag-coated inner channel was also helpful to reduce the crosstalk between two channels [33].
3. Experimental results and discussion
The light trail for both the capillary based SPR sensor with and without Ag layer coated inside the capillary in the sensing regions are presented in Fig. 2. The light propagated forward in the tubing wall of the capillary based on total internal reflection. Figure 2(a) shows how the light propagates in the Au/ITO layer-based SPR sensor, and Fig. 2(b) shows that the light propagates in the sensor by adding the Ag layer inside the capillary. By comparing Figs. 2(a) and 2(b), it can be easily seen that the light loss of the Ag/Au/ITO layer-based SPR sensor was lower than that of the Au/ITO layer-based SPR sensor of the same length for light traveling, because the Ag layer inside the capillary can reduce the refracted light availability. The surface plasmon polaritons were excited by the evanescent waves from total reflection and the dip of the resonant spectrum was determined by the absorption of the evanescent waves in the sensing region. This meant that more light could be absorbed by the Ag/Au/ITO layer-based SPR sensor than that of the Au/ITO layer-based SPR sensor for the same length.
Fig. 2 Light propagation in the tubing wall of the capillary. (a) Light propagation without Ag layer coated inside the capillary. (b) Light propagation with Ag layer coated inside the capillary.
To study the spectral transmission characteristics of the Au/ITO layer channel (outer channel), the resonant wavelength versus RIs were measured. The RI responses of outer channel for the two structures in Fig. 2 were tested during the injection of several samples with different RIs from 1.3253 to 1.3726. The samples we used in the outer channel were prepared with sodium chloride at different concentrations in deionized water, and RIs were measured using an Abbe refractometer (WAY-2S). Experimental transmission spectra of outer channel with different metal layer structures are shown in Figs. 3(a) and (b). It can be seen that the resonant wavelength had red shift when the RI changed from 1.3253 to 1.3726. In Fig. 3 (c), we compared the transmission spectra of outer channel with different metal layer structures for a RI of 1.3253. It can be clearly seen that the dip became more intense (14.8% for Ag/Au/ITO structure and 7.9% for Au/ITO structure) and the line width narrowed (67 nm for Ag/Au/ITO structure and 81 nm for Au/ITO structure) with the addition of the Ag layer in the tubing wall of the capillary, which is better than previously reported in the literature [34]. Figure 3(d) shows the good linearity of the refractive index calibration curves for two structures. The RI sensitivity for Ag/Au/ITO structure was 2496 nm/RIU and thus, higher than the RI sensitivity for Au/ITO structure at 2215 nm/RIU. The Ag layer also serves to increase light transmission inside the tubing wall. The phenomena in Figs. 3(c) and (d) are attributed that the existence of the inner wall Ag layer enhances the transmission efficiency inside the tubing wall of the capillary resulting in deep depth, narrow width and high RI sensitivity.
Fig. 3 Spectral characteristics of the capillary based sensor. (a) Transmission spectra of the Au/ITO layer-based SPR sensor. (b) Transmission spectra of the Ag/Au/ITO layer-based SPR sensor. (c) Transmission spectra of Au/ITO layer-based and Ag/Au/ITO layer-based SPR sensors with the RI of 1.3253. (d) Variation of resonant wavelength as a function of different RI solutions.
To validate the inner sensing channel, we fabricated and tested the capillary based SPR sensor with Ag-coating on the inner wall of the capillary. The light propagated in the liquid medium with sensing length of 20 mm and the thickness of Ag layer was 30 nm (Fig. 4(a)). In these conditions, the surface plasmon waves could be excited on the interface between the silver layer and the capillary wall when appropriate light transmitted in the liquid core of the capillary. The RI of the liquid core had to be higher than that of the capillary wall to satisfy the condition of total reflection [35]. Therefore, RI of the liquid medium in the inner channel had to be higher than 1.46. The optical path in the capillary tube and the high RI liquid medium were shown in Fig. 4(b). The tubing wall of the capillary, Au/ITO layer and the low RI liquid medium (RI lower than 1.46) form the outer sensing channel. The light propagated forward in the tubing wall of the capillary based on total internal reflection. The inner channel assigned as high RI liquid medium (RI higher than 1.46) measurement channel, was fabricated with Ag layer. The transmission mode for the inner channel was same with we shown in Fig. 4(a). Therefore, both the channels of the designed sensor support a wider range of the RI sensing than the normal fiber-optical SPR sensor [36].
Fig. 4 Light propagation inside the capillary-based sensor. (a) Light propagation in single internal channel. (b) Light propagation in both the internal and the external channels.
In order to satisfy the high RI necessary for sensing in the inner channel, the liquid media were mixtures of polymethylphenylsiloxane fluid and kerosene with different volume ratios. The RI was thus varied from 1.5255 to 1.5781, once again verified with the Abbe refractometer. The spectral response to different RIs was shown to blue shift with increasing RI (Fig. 5(a)). The ray transmission model was calculated to compare the performance of this sensor with experimental data. Figure 5(b) presents the dip wavelength shift versus RIs. The experimental dip wavelengths decreased linearly as RI increased from 1.5251 to 1.5734, which is consistent with its theoretical calculation. We can obtain the experimental RI sensitivity of −1951 nm/RIU, which was consistent with the simulated RI sensitivity of −2254 nm/RIU.
Fig. 5 Normalized intensity transmission spectra of silver-coated capillary SPR sensor with different RIs of liquid media. (a) Experimental. (b) Linear relationship between the resonant wavelength of the silver-coated capillary SPR sensor and RI of the liquid medium.
To investigate the RI response of two different channels, the multilayer-coated capillary SPR sensor was tested experimentally (Fig. 6). For the outer channel, the transmission spectra of the sensor was measured with different concentrations of sodium chloride solution corresponding to various RIs from 1.3253 to 1.3726. In that experiment, the inner channel was filled with a fixed high RI of 1.5781. The two SPR dips associated with the inner and outer channels are well separated for the whole RI range, while the SPR response associated to the inner channel nearly did not change (The standard deviation for inner channel was 2.06 nm) with the variation of the outer channel refractive index. The resonant wavelength of the outer channel red shifted in the 700 to 850 nm region when RI changed from 1.3253 to 1.3726. A similar experiment was carried for the sensitivity of the inner channel (Fig. 6(b)). In this experiment, the RI solution of the outer channel was constant at 1.3726 throughout the experiment, while the inner channel was subjected to RI changes from 1.5255 to 1.5781. The spectral dips of the inner channel showed a shift to shorter wavelengths as the RI of the internal sample increased, which had small effect on the plasmon resonance of the outer channel. The standard deviation for outer channel was 4.2 nm. We also demonstrate that the smaller RI gap between two channels, the larger measurement error be suffered for the detection. Lastly, both resonances were tracked simultaneously (Fig. 6(c)). The sensor still had two obvious dips when we changed the RIs of two channels at the same time.
Fig. 6 Experimental spectral characteristics of the capillary based sensor. (a) Transmission spectra of channel one with different high RI of liquid media. (b) Transmission spectra of the inner channel with different low RI of liquid media. (c) Transmission spectra of double channels with various RI of liquid media. (d) Experimental linear fitting curves of two channels. The black points were obtained from Fig. 6(a) and (b), and the red points were obtained from Fig. 6(c).
The calibration curves for two dips are shown in Fig. 6(d). The black points were obtained from Fig. 6(a) and (b), the calibration curves were calculated a constant RI for the other channel. We obtained a RI sensitivity of 2513 nm/RIU for the outer channel and −1937 nm/RIU for the inner channel, which is consistent with the previously measure RI sensitivity of 2496 nm/RIU for the outer channel and −1951 nm/RIU for the inner channel. It also can be observed that the wavelength of the two dips and RI of sample satisfied good linear relationship. In addition, the response measured was not affected with the simultaneous measurement of both channels. To demonstrate that, the red points in Fig. 6(d) were the resonant wavelengths when we tracked two channels simultaneously and thus, even if we changed the RIs of both the channels at the same time, the results were still accurate using the calibration curves we calculated. These experimental results showed that both two channels with low crosstalk resulted in a sensor with a wider RI measurement range than a single channel. The dual-channel sensor has potential application for chemical industry such as multi-refractive index chemical solutions detection [27,28] (2, 4-dichlortoluene, n (C7H6Cl12) = 1.55 and water, n = 1.33) at same time and oil well (oil, n>1.5 and gas, n<1.4 detection at same time) detection.
4. Conclusion
We proposed and demonstrated a potentially cost-effective multilayer-coated SPR capillary sensor. The inner Ag-coated channel detected the high RI liquid medium, whose resonant wavelength blue shifted when RI increased from 1.5255 to 1.5781. The outer channel coated with Au/ITO layers was sensitive to lower RI liquid medium detection. Its resonant wavelength red shifted as RI increased from 1.3253 to 1.3726, which is separated from the internal channel. Benefiting from the wider RI detection range, it demonstrated that this SPR sensor can potentially be used for industry or biochemical sensing environments with large RI measurement.
Funding
National Natural Science Foundation of China (NSFC) (Grant Nos. 61520106013, 11474043, and 61137005)
Acknowledgments
The authors would like to thank financial supports from the National Natural Science Foundation of China (NSFC) and Chinese government scholarship.
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