A new fiber-optic pH sensor based on a thin-core fiber modal interferometer with electrostatic self-assembled nanocoating is presented. After inserting a segment of thin-core fiber into a standard single-mode fiber, high-order cladding modes are excited and interfere with the core mode to form an in-fiber modal interferometer. The side surface of the sensor is then deposited with poly(allylamine hydrochloride) and poly(acrylic acid) nanocoating by electrostatic self-assembly technique. A fast and linear response is obtained in either acid or alkali solution (in the pH range 2.5 to 10) with resolution of 0.013 pH unit.
© 2009 OSA
Optical biochemical sensors are under rapid development because of their important applications in environmental monitoring and clinical analysis. Among various types of biochemical sensors, the pH sensor is one of the most essential sensors because of the importance of pH control or measurement in numerous fields, e.g., biology, clinical medicine, ecology, etc. Although the titration method has been used for long time, the development of miniaturized optical pH sensor is still a very active research topic. In recent years, fiber-optic pH sensors have attracted increasing interests due to their many advantages, such as small size, remote sensing capability, and the safety for in vivo measurement.
Several types of fiber-optic pH sensors were proposed and demonstrated [1–15]. One type of fiber-optic pH sensor is based on pH indicator immobilized in the matrix materials. The pH indicators that have been proposed include acidochrome dye , fluorescein acrylamide , thymol blue , ethyl violet dye ,neutral red , eosin , the mixture of dipicrylamine and victoria blue , or the mixture of cresol red, bromophenol blue and chlorophenol red [8-10]. The optical properties of the indicators, including absorbance [1, 3–5,7–10], fluorescent intensities [2, 6], and fluorescent lifetime , can be employed for the determination of the pH of the liquid under test. The other type of fiber-optic pH sensor is based on the morphology of nanostructured film, usually known as swelling. Since the refractive index (RI) of a swellable film, e.g. hydrogel  or polyelectrolyte-deposited nanostructured material [13–15], depends on the pH, one can design pH sensors through the measurement of external RI. Since most of the pH indicators work in UV-visible range, while most of fiber-optic components work in near-infrared range, a fiber-optic pH sensor based on swellable film is competitive because of the compatibility with fiber-optic sensor networks and the robustness of the sensor [13–15]. The absorption or fluorescence-based pH sensors have some inherent drawbacks since they are influenced by light intensity fluctuations, temperature, and concentration of indicator. The avoidance of the indicator can solve well the deterioration issues induced by bleaching or leaching of the indicator to make a long-term fiber-optic sensor for on-line pH monitoring applications.
In order to realize a reliable and accurate measurement of the RI of the nanostructured film for pH sensing, several pH sensing schemes based on the resonant optical responses have been recently proposed and demonstrated [13–15]. Corres et al. presented a long-period grating (LPG) pH sensor, in which nanocoating was deposited on the side-surface of the LPG as the reacting functional film and the shift of the resonant wavelength of the LPG is utilized for the determination of pH-induced RI changes. Goicoechea et al. demonstrated a Fabry-Perot (FP) nanocavity pH sensor, in which the nanostructured film was prepared on the fiber-end surface and the white-light interferometry is employed for the measurement of pH induced swelling of the film. The electrostatic self-assembly (ESA) deposition technique was used for the film preparation in both of the above-mentioned works because the ESA has high controllability and repeatability on organic or inorganic nanocoating preparation.
In this paper, we propose a new fiber-optic pH sensor based on fiber modal interferometer (FMI). The fiber device has a short segment of thin-core fiber (TCF) inserted into a standard single-mode fiber (SMF). After nanocoating the FMI with ESA deposition method, the FMI can be used for pH sensing. The basic structure and even the spectrum of this sensor are very similar to those of an LPG sensor. However, the present sensor has not only a slightly higher sensitivity to external RI, but also much lower sensitivity to temperature. Moreover, it has the advantages of the robustness (because of the avoidance of grating degradation issue) and the lower fabrication cost.
2. Thin-core fiber modal interferometer
Figure 1 shows the schematic configuration of the thin-core fiber modal interferometer (TCFMI) we propose. It is different from an LPG, and high-order fiber cladding modes are excited not by resonant scattering, but by mode mismatch at the starting point of the TCFMI. The excited cladding modes will interfere with the core mode and form some local maxima or minima because of the constructively or deconstructively interference, respectively. If one pays attention to a local minimum at the ending point, which results a dip in the transmission spectrum, the relative phase displacement of the interfering two modes can be described asEq. (1) as
The TCFMI sensor was fabricated by using a commercial thin-core optical fiber (Nufern 460-HP). The core diameter of the fiber is ~3.0 μm, and the cutoff wavelength is ~450 nm. Figure 2(a) shows the spectrum responses of the fabricated TCFMI with 2 cm long TCF when the external RI changes. After immersing into the glycerol/deionized-water solution, the transmission dip of the TCFMI shifts to longer wavelength with the sensitivity of 138 nm/R.I.U. The samples under test are glycerol solutions with concentration (volume %) of 0, 0.40, 2.45, 5.06, 7.84, 12.91, 19.82, 37.3, and 62.3%, whose RIs are 1.333, 1.3336, 1.3365, 1.34, 1.3436, 1.3496, 1.3572, 1.373 and 1.3897, respectively. Comparing with the previously reported LPG sensors [14–17], one can see that their sensitivities to external RI are comparable to each other, but the transmission dip of the TCFMI shift to the opposite direction. This is because the cladding mode in the TCF satisfies as one can see from Eq. (2.a).
Figure 2(b) shows the temperature response of the TCFMI, which is measured by using a temperature-controlled oven. The transmission dip is observed to shift to longer wavelength with increasing of temperature, which is also opposite to the temperature response of normal LPG sensors. The measured temperature sensitivity is 0.012 nm/°C, which is about a twentieth of that of conventional LPG sensor. The lower temperature sensitivity is a desirable merit for an RI sensor because of the suppression of the cross sensitivity to temperature.
3. Fabrication and characterization of TCFMI pH sensor
A nanocoating was deposited on the side surface of TCFMI by solution dipping layer-by-layer ESA technology  for pH sensing. The method is based on the construction of molecular multi-layers by the electrostatic attraction between oppositely charged polyelectrolyte. Poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) were utilized as the cationic and anionic materials in the experiment. All chemicals are purchased from Sigma-Aldrich and used without further purification.
The ESA method is shown schematically on the left-up corner in Fig. 3 . Both PAH and PAA were dissolved in 1M NaCl aqueous solution with concentration of 2 g/L. After cleaned by piranha solution (H2SO4 and H2O2 in concentration of 7:3), the TCFMI was washed with large amount of deionized water and dried by nitrogen gas. Therefore, the fiber surface is negatively charged . The fiber was then immersed into PAH and PAA alternatively with reaction of 4 min to form a monolayer of polymer through electrostatic adsorption. After each monolayer was formed, the fiber was rinsed by deionized water for 1 min to remove the excess molecules. Each polycation/polyanion layer (PAH/PAA) is called a bilayer. After depositing 25 bilayers, the optical fiber was baked in an oven at 60 °C for 10 hours.
The deposited nanocoating was characterized by utilizing scanning electron microscopy (SEM). Figure 3(a) shows the SEM photos of the naked fiber surface, and Fig. 3(b) and Fig. 3(c) show the surface morphology of the nanocoating with different magniscales. Some local defects were observed in the image with small magniscale, as shown in Fig. 3(b). Meanwhile, it also shows that the nanocoating has a bumpy surface with nano-indents. The thickness of the nanocoating measured by the SEM is around 100 nm.
The transmission spectra of the TCFMI before and after the nanocoating deposition were recorded by an optical spectrum analyzer (ANDO, AQ6317) for a comparison. As shown in Fig. 4 , the fabricated TCFMI has a transmission dip at 1565 nm, resulting from a destructive interference as discussed before. The transmission dip shifts to 1568 nm after the deposition of nanocoating. It attributes to the nanocoating induced changes of the effective index of the reacting cladding mode, since the temperature effect is neglectable in our measurement.
The fabricated TCFMI pH sensor was tested with HCl and NaOH aqueous solutions. The sensor was packaged with a shallow aluminum groove for the test. Figure 5 shows the spectral response of the sensor to different buffer solutions. One can see that the transmission dip shifts to shorter wavelength with decreasing of pH value in acidic range, and also shifts to shorter wavelength with increasing of pH value in alkaline range. This is because the ionization state of the PAH/PAA multi-layer is modulated by the pH value through swelling/deswelling and reaches maximum at the neutral pH . The modulation of ionization state induces a change of the density of the polyelectrolyte nanocoating, which results a change of the RI of nanocoating. The sensitivities of the fabricated TCFMI pH sensor are 0.32 nm/pH unit and −0.45 nm/pH unit for acid and alkali solutions, respectively. The sensor can be alternatively used for either acid or alkali solution because the corresponding calibration curves is different in the two cases, which reduces the operating range of the sensor.
The dynamic response of the pH sensor was also measured in the experiment. The optical spectrum analyzer keeps sweeping with period of 8 s. The buffer solutions with pH value of 5 and 6 are alternately added into the testing groove. As shown in Fig. 6 , the rise time (tr) of the sensor is 120 s and the fall time (t f) is 200 s. The variation of dip wavelength for a given pH value was measured as 0.004 nm. Since the wavelength sensitivity of the sensor is 0.3 nm/pH unit, one can deduce the effective resolution of the pH sensor as 0.013 pH unit.
4. Discussion and conclusion
We have presented a new type of fiber-optic pH sensor based on a TCF modal interferometer. The TCFMI has a regular spectrum with a high extinction-ratio transmission dip. Because of the resonant nature with the cladding mode, the transmission dip is highly sensitivity to external RI. Comparing to the well-know LPG-based RI sensor, the TCFMI has a comparable RI sensitivity and much lower temperature-sensitivity. A drawback of the sensor is the relatively high insertion loss, which was measured in the experiment as around 7 dB.
After depositing the side surface with PAH/PAA multi-layers by ESA technique, the TCFMI sensor has been used to measure acid and alkali solutions. Although some defects of nanocoating were observed by SEM imaging, as shown in Fig. 3(b), the sensor showed high performance on the pH measurement. The resolution of pH sensor is as high as 0.013 pH unit, thanks to the highly stable spectrum response of the sensor. It is also because the reacting side-surface of the fiber sensor is relatively large, and thus the requirement of defect control is not as rigorous as that of a fiber-optic pH sensor based on a fiber-end nanocavity.
The TCFMI sensor proposed in this paper has advantages of low-cost, robustness, reversibility, high sensitivity to external RI, and low temperature sensitivity, and is therefore very suitable for use as remote single-point monitoring biosensors or disposable low-cost fiber-optic bioprobes.
This work was supported by Natural Science Foundation of China (Grant No: 60607011 and 20876134) and Key Program of Natural Science Foundation of China (Grant No: 90101024).
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