Photonic crystal fiber interferometric pH sensor based on polyvinyl alcohol/polyacrylic acid hydrogel coating Pengbing Hu,1,2 Xinyong Dong,1 Wei Chang Wong,2 Li Han Chen,2 Kai Ni,1 and Chi Chiu Chan2,* 1
Institute of Optoelectronic Technology, China Jiliang University, Hangzhou 310018, China
2
Division of Bioengineering, School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637457, Singapore *Corresponding author:
[email protected] Received 26 November 2014; revised 5 February 2015; accepted 20 February 2015; posted 25 February 2015 (Doc. ID 228620); published 24 March 2015
We present a simple photonic crystal fiber interferometer (PCFI) that operates in reflection mode for pH measurement. The sensor is made by coating polyvinyl alcohol/polyacrylic acid (PVA/PAA) hydrogel onto the surface of the PCFI, constructed by splicing a stub of PCF at the distal end of a single-mode fiber with its free end airhole collapsed. The experimental results demonstrate a high average sensitivity of 0.9 nm/ pH unit for the 11 wt.% PVA/PAA coated sensor in the pH range from 2.5 to 6.5. The sensor also displays high repeatability and stability and low cross-sensitivity to temperature. Fast, reversible rise and fall times of 12 s and 18 s, respectively, are achieved for the sensor time response. © 2015 Optical Society of America OCIS codes: (060.2340) Fiber optics components; (060.2370) Fiber optics sensors; (060.5295) Photonic crystal fibers. http://dx.doi.org/10.1364/AO.54.002647
1. Introduction
The measurement and control of a sample’s pH is very important in scientific research, industrial and agricultural production, civil infrastructure [1–3], and so on. Electrochemistry and colorimetry have been most commonly used in pH measurement. In recent years, fiber-optic pH sensors have been extensively studied and widely used due to their numerous advantages such as compact size, high sensitivity, good biocompatibility, good resistance to chemical corrosion, insusceptibility to electromagnetic interference, and so on. So far several types of fiber-optic pH sensors have been proposed and demonstrated. According to different sensing mechanisms, they can be mainly classified as pH indicator-based and swelling material-based 1559-128X/15/102647-06$15.00/0 © 2015 Optical Society of America
sensors. The former combine an evanescent wave with excitation of the pH indicator. The employed indicators are commonly fluorescent materials [4–7]. However, the fabricated sensors are extremely influenced by light intensity fluctuations and ambient temperature. For this reason, another type of fiber-optic pH sensor is increasingly attracting research interests. Its operation principle is nearly based on variations in the refractive index (RI) of the hydrogel or polyelectrolyte film [8–13], which is immobilized onto the surface of the fiber. These sensors are robust, compatible with optical communication systems, and easily fabricated. Among many reported sensors, fiber-optic interferometric pH sensors [9,10] seem to be more competitive as compared to evanescent field sensors and in-fiber grating sensors [11,12], for their high sensitivity measurement. And recently, photonic crystal fiber (PCF), a new class of optical fiber without rare earth doping, has been increasingly explored in the design and fabrication of optical fiber interferometers 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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[14–19]. The use of PCF has led to the reduction of the impact of temperature on RI measurements of the sensor. In our previous study, we have proposed a miniature pH optical sensor based on a waist-enlarged bitaper and mode excitation. The sensor showed comparable sensing performances, such as sensitivity and time response. However, the synthetic process of tantalum pentoxide coating on the sensor’s tip by chemical conversion method was time-consuming and should be under precise control [20]. In this paper, we propose and demonstrate a new reflective PCF interferometer (PCFI) combined with a layer of polyvinyl alcohol/polyacrylic acid (PVA/ PAA) hydrogel, a pH responsive cross-linked polymeric material, for pH measurement. Two collapsed regions are specially introduced in the PCFI so as to increase the visibility/excitation ratio of the interference fringes, thus improving the measurement accuracy. The reflection operation mode is utilized for the purpose of greater convenience in metrology applications. 2. Sensor Design and Operation Principle
The proposed pH sensor without coating consists of a stub of PCF fusion spliced to a single mode fiber (SMF), as illustrated in Fig. 1. Fabrication of the PCFI was undertaken by using the electric arc discharge of a fusion splicer. It has two collapsed regions, which are located at the two ends of the PCF. The length of the collapsed region can be controlled by the arc power and duration [21]. The first collapsed region broadens the propagating light and partially excites the core mode to cladding modes, and the region’s length determines its insertion loss and excitation property, which will in turn influence the interferometric fringes of the fabricated PCFI. The second collapsed region is introduced to prevent solutions from entering into the voids of the PCF. Besides, a reflective silver layer is deposited on its end to improve its reflectivity. When the input core mode propagates into the first collapsed region, its mode field diameter is broadened, causing excitation of the core and cladding modes. After travelling a distance of L (the length of the PCF), they will recombine at the second collapsed region before reflecting back to the first collapsed
region for the second interference. This gives rise to the interference between the two modes, forming a Mach–Zehnder interferometer (MZI). Thus, the proposed reflective PCFI is actually a combination of two cascaded identical fiber MZIs, which has led to an improved visibility/excitation ratio of the interference fringes [14]. Based on previous studies [22], the normalized transmission function of the PCFI can be expressed as follows: 2 2π p · Δn · L ; (1) Tλ Rm r1 r2 2 r1 · r2 cos λ where Rm is the silver layer reflectivity; λ is the operation wavelength; r1 and r2 are the coupling coefficients of the core and cladding modes of the MZIs, respectively; and Δn is the RI difference between the core and cladding modes of the MZIs. Because Δn varies with the ambient RI, Tλ will change in wavelength when the ambient RI changes, resulting in shifting of the spectrum. As a consequence, the RI can be determined by demodulating the wavelength shift. The free spectrum range (FSR) and the extinction ratio (ER) of the interferogram for the pexpPCFI, 2 ∕Δn · L and 10 log r pressed as λ r2 4 ∕ 1 p 4 r1 − r2 , respectively, are theoretically double of those of the PCFI without collapsing the second region. Owing to the enhanced FSR and ER, highprecision measurement with a broad measuring range can be realized for the PCFI-based sensor. Moreover, the RI sensitivity of the PCFI remains unchanged for its unchanged total length before and after the second collapsing, as ascertained by many reported literatures [14,22]. To construct a pH sensor, the pH sensitive PVA/ PAA film is deposited onto the surface of the PCFI. When the pH sensor is immersed in liquids with varying pH values, the film will swell and shrink with variations according to the change of pH value. The RI of the film will be changed with the swelling and deswelling of the film. Hence, the pH value can be monitored in real time based on the interferometric spectrum shift of the sensor. 3. Fabrication and Experimental Results A. Sensor Fabrication
Fig. 1. Schematic diagram and micrograph of the proposed reflective PCFI. 2648
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We fabricated two PCFIs of the lengths L of 10 and 35 mm. The cross-sectional view of the used PCF and the micrograph of the PCFI are shown in the inset of Fig. 1. It can be seen from the micrograph that the first collapsed region is about 117 μm and the second collapsed region about 87 μm. The second collapsed region is made to be short, because strong arc power will make the PCF’s end rounded and thus raise the insertion loss [15]. The two collapsed regions were manually introduced by a high precision fusion splicer (Sumitomo Electronic Type 39) with arc powers and durations of 50 steps and 0.60 s, and 45 steps and 0.50 s. Figure 2 shows the reflection
Fig. 2. Reflection spectra of the PCFIs with lengths of (a) 10 mm and (b) 35 mm during fabrication.
spectra of the PCFIs with lengths of 10 and 35 mm during fabrication. Interferometric fringes are observed over the spectral range of 1530–1600 nm. As expected, the ERs of the PCFIs with collapsed ends are improved. For example, the ER of the 10 mm long PCFI with collapsed ends, 21 dB, is much larger than that of the PCFI without collapsed ends, 7 dB. This improvement in ER may be due to different coupling coefficients for collapsed regions [17]. The FSRs of the PCFIs with collapsed ends are nearly extended twice in comparison with those of PCFIs without collapsed ends, which is in good agreement with our theoretical analysis. The interference patterns in Fig. 2 were Fourier transformed to acquire the spatial frequency spectra of the interference patterns. As shown in Fig. 3, there is one dominant spatial frequency and several comparably moderate spatial frequencies for each pattern except zero frequency which represents the DC components in the pattern. This implies that the interference fringes of the PCFI are actually formed by the core mode and several cladding modes. Above all, with respect to the dominant single spatial frequency, the spatial frequency increases with the
Fig. 3. Spatial frequency spectra of the PCFIs with lengths 10 and 35 mm with and without the collapsed end.
interferometer length and is nearly reduced by half after the introduction of the collapsed end; the results are well consistent with the preceding analysis and can be ascertained as in [17]. After being cleaned in warm piranha solution (95%–97% sulfuric acid and 30% hydrogen peroxide in a 7:3 ratio), the 10 mm long PCFI was inserted into a tube with an inner diameter of 130 μm to prevent coating on the surface of the PCFI cladding. The PCFI with tubing was then dipped into a Tollen’s reagent [23], and the silver mirror formed on the end of the PCFI within ∼1 min . The silinization process was performed in 3-trimethoxysilyl propyl methacrylate in deionized water to create chemical bonds for the fiber surface adherence to the hydrogel and a silane film linked to the mirror to prevent it from degrading [24]. Then the PCFI was cleaned and dip coated with PVA/PAA hydrogel to construct a compact pH sensor. The PVA/PAA blending solutions of 11 wt.% were prepared by dissolving and diluting the blend (an 8:1 mixture of PVA with PAA) in distilled water under stirring for 1 h at 60°C [14]. The PCFI was then dip coated into the resultant blending solution with a withdrawing speed of 30 mm/min by using a dip coater. After that, the coated PCFI was placed in an oven for drying at 45°C and for crosslinking at 130°C. At last, it was soaked in water to remove any uncross-linked polymer residues. All the reagents used were purchased from Sigma-Aldrich. B. Sensor Performance
The experimental setup is shown in Fig. 4. Light from an amplified spontaneous emission (ASE) source propagated through an optical fiber circulator to the coated fiber sensor. It was reflected back by the sensor and through the circulator again, and finally detected by an optical spectrum analyzer (OSA) with a resolution of 0.02 nm. The sensor was kept and tested in water, where pH was varied by adding hydrohchloric acid (HCl) or sodium hydroxide (NaOH). A commercial pH–thermometer (PB-11, Sartorious Mechatronics) with measuring accuracy of 0.005 and 0.2°C was mounted in proximity to the sensor to provide real-time pH value and temperature readings of the solution. The optical spectra of the PCFI before and after coating with 11 wt.% PVA/PAA hydrogel were recorded for a comparison. As shown in Fig. 5(a), the PCFI after coating has little interference in air mainly
Fig. 4. Experimental setup for the proposed pH sensor. 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
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Fig. 6. pH response of the sensor before and after coating with PVA/PAA hydrogel.
Fig. 5. (a) Optical spectra of the sensor before and after coating with PVA/PAA hydrogel. (b) Optical spectra of the sensor after coating with PVA/PAA hydrogel at varying pH values.
due to its RI being higher than that of the PCF cladding, whereas it has a strong interference with a large extinction ratio (>10 dB) when immersed in water, therefore rendering it capable of pH sensing applications. Obviously, water entering into the film reduces its effective RI lower than that of PCF cladding as most other polymers do [14]. When we increased the pH value from 2.5 to 6.5 at an interval of 0.25, the dip wavelength of the interference pattern was blueshifted gradually, as shown in Fig. 5(b). This is largely because the ionization state of the film is modulated by the pH value, which determines its volume. As the pH value increases, the volume increases and the RI decreases. Figure 6 exhibits the wavelength shifts of the sensor with the comparison of the bared sensor (no coating) in the varying pH values in both ascending and descending orders. Within the pH measurement range, the bared sensor shows no obvious changes of the dip wavelength, therefore eliminating the impact of the RI variation of the aqueous solution induced by the pH value on pH sensing. As for the pH sensor, it responds monotonically with an average measurement sensitivity of 0.9 nm/pH unit, and a small but acceptable degree of hysteresis (
