Ammonia sensing using a fibre optic long period grating with a porous nanostructured coating formed from silica nanospheres

Ammonia Sensing Using a Fibre Optic Long Period Grating with a Porous Nanostructured Coating Formed from Silica Nanospheres. S. Korposha,b, W. Battya, S. Kodairab, S-W. Leeb, S.W. Jamesc, S.M. Toplissc and R.P. Tatamc a Cranfield University at Kitakyushu, bGraduate School of Environmental Engineering, The University of Kitakyushu, Kitakyushu, Japan. c School of Engineering, Cranfield University, Cranfield, MK43 0AL, UK. ABSTRACT The transmission spectrum of a fibre optic long period grating (LPG) coated with a porous multilayer coating of thickness of order 400 nm formed from silica nanospheres is shown to exhibit a strong sensitivity to the infusion of a functional, chemically sensitive material into the coating. Subsequently, the transmission spectrum of the LPG shows sensitivity to changes in the properties of the functional material when exposed to a particular chemical species in an aqueous solution. The operation of such a device as an ammonia sensor is demonstrated, exhibiting 1 ppm sensitivity. The sensing mechanisms are discussed. Keywords Fibre optic long period grating, ammonia sensing, silica nanoparticles

1. INTRODUCTION The deposition of functional coatings of thickness of order 100 nm onto fibre optic long period gratings (LPGs) has been shown to offer promise for the development of chemical sensors. Changes in the refractive index and/or thickness of functional coatings of materials of refractive index higher than that of silica have been shown to have a significant influence on the transmission spectrum of the LPG 1 . In recent years there have been a number of theoretical2,3,4 and experimental5,6 studies of the physical mechanisms behind this effect, with reports of methods to enhance the sensitivity7, and demonstrations of the operation of such devices as chemical sensors8. The original work in this area focussed on the layer by layer deposition of coatings from nanoscale building blocks using electrostatic self assembly9 and the Langmuir Blodgett technique 10. More recently there has been interest in the use of larger nanospheres of materials such as SnO2 to create porous coatings that have been used to monitor humidity11 . In this paper we extend this concept to investigate the possibility of developing an ammonia sensor, by infusing a dye into a porous film formed by the self-assembly of silica nanospheres.

2. LONG PERIOD GRATINGS An LPG is a core-cladding mode coupling device formed by a periodic modulation of the refractive index of the core of the optical fibre. The period of an LPG typically lies in the range of 100 μm – 1mm, and it acts to couple light from the core mode to a discrete set of symmetric cladding modes at wavelengths governed by the phase matching condition12:

λ ( x ) = (n core − n clad ( x ) )Λ

(1)

where λ(x) represents the wavelength at which coupling occurs to the LP0x mode, ncore is the effective refractive index of the mode propagating in the core of the fibre, nclad(x) is the effective index of the LP0x cladding mode and Λ is the period of the grating. The cladding modes suffer high attenuation, with the result that the transmission spectrum of an LPG contains a series of resonant loss bands. The dependence of the phase matching condition upon the effective refractive index of the cladding mode results in the central wavelengths of the resonance bands exhibiting sensitivity to the refractive index of the surrounding material, including a coating. The deposition of a nanostructured coating onto an LPG is characterised by a thickness at which the central wavelength of the resonance bands become highly sensitive to the coating’s properties. The coating itself can act as a waveguide and the sensitivity of the LPG to the coating’s properties is maximized when the optical thickness of the coating is such that a mode of the coating waveguide is phase matched to one of the lower order cladding modes. This thickness is typically of order 100nm. In Fourth European Workshop on Optical Fibre Sensors, edited by José Luís Santos, Brian Culshaw, José Miguel López-Higuera, William N. MacPherson, Proc. of SPIE Vol. 7653, 76531D © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.866326 Proc. of SPIE Vol. 7653 76531D-1

this “mode transition region”, the cladding modes undergo a reorganization process and the effective refractive indices of the cladding modes exhibit their peak sensitivity to the refractive index and thickness of the coating, causing the observed changes in resonance wavelengths. For coupling to higher order cladding modes, the phase matching condition of equation (1) has turning points, such that it is possible to couple to the same cladding mode at two different wavelengths13. If the period of the LPG is such that the phase matching turning point is accessed, then, as the surrounding refractive index increases, a broad attenuation band develops, with a fixed central wavelength, which subsequently splits into two resonance bands that shift in opposite directions as the refractive index is increased further. It has been shown that an LPG operating at the phase matching turning point offers optimum sensitivity to environmental parameters13. By careful choice of the period and optical thickness of the coating, it is possible to ensure that the phase matching turning point and the mode transition region coincide, offering optimum sensitivity of the LPG to changes in the optical properties of the coating7.

3. COATING DEPOSITION Silica nanospheres, with diameters in the range 40–50 nm, were deposited onto the surface of the fibre using the electrostatic self assembly process, as described here. Firstly, the region of optical fibre containing the LPG was rinsed with deionised water and immersed into 1 wt% ethanolic KOH for 20 minutes, leading to a negatively charged surface. The optical fibre was then immersed into a solution containing a positively charged polymer poly(diallyldimethylammonia chloride) (PDDA) for 20 minutes, so that a monolayer of PPDA was deposited onto the surface of the fibre. Subsequently, the fibre was immersed in distilled water and dried by flushing with nitrogen gas. The fibre was then immersed into a solution containing the negatively charged silica nanosphere solution for 20 minutes, such that a monolayer of nanospheres was deposited onto the PDDA layer. The fibre was again rinsed in distilled water and dried. The multilayer film was built up by sequential dipping in the PDDA and silica nanosphere solutions. When the required film thickness had been achieved, the coated fibre was immersed in a solution of a functional dye, tetrakis-(4-sulfophenyl) porphine (TSPP), for 20 minutes, which provides the sensor with its sensitivity to ammonia. Due to the electronegative sulfonic groups present in the TSPP compound, an electrostatic interaction occurs between negatively charged TSPP and the positively charged PDDA compound in the PDD/SiO2 film. After immersion into solution, the fibre was rinsed in distilled water, in order to remove physically adsorbed TSPP, and dried by flushing with nitrogen gas. Porphyrins are tetrapyrrole pigments that occur widely in nature and play an important role in many biological systems14. The optical spectrum of solid state porphyrin is modified compared to that of porphyrin in solution, due to the presence of strong π−π interactions. Interactions with some chemical species can produce further optical spectral changes, thus creating the possibility that they can be applied to optical sensor systems. For instance, exposure of the TSPP, which has sulfonic functional groups, to ammonia leads to the modification of the absorption spectrum.

4. EXPERIMENT AND RESULTS A film of thickness approximately 450 nm, corresponding to 10 cycles of the (PDDA/SiO2) deposition process, was deposited onto an LPG of period 100 μm and length 40 mm, fabricated in a single mode fibre of cut-off wavelength 650 nm. The period was selected to allow the sensor to operate at the phase matching turning point. Figure 1 (a) shows the transmission spectrum recorded with the coated LPG immersed in water, with the coating layers increasing from 0 to 10. In air, the presence of the coating has no measurable effect on the wavelength and extinction of the resonance features of the LPG transmission spectrum. In water, the resonance feature corresponding to coupling to the LP0,20 cladding mode exhibits a small wavelength blueshift of 8.5 nm. The development of the resonance band corresponding to coupling to the LP0,21 mode, which is operating at the phase matching turning point, is also visible. However, because of the low refractive index (RI=1.2) of the porous silica coating, and the coating thickness, this resonance feature has a low extinction ratio. The evolution of the transmission spectrum of the coated LPG when immersed in the TSPP solution is shown in the grey scale plot shown in figure 1(b), where a transmission of 100% is represented by white and 0% by black. Initially, there is no band in the 800 nm wavelength range. As the TSPP infuses into the film, the RI of the film increases (from 1.2 to ca. 1.54, measured using ellipsometry) and the phase matching condition for coupling to LP0,21 is satisfied. A broad single attenuation band is seen to develop rapidly (within 60 sec) and then split in two, similar to the response of dual resonant Long Period Gratings to the increase in reference [7] for increasing coating

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thickness7. The time needed to complete the binding between TSPP and PDDA moieties is less than 600 sec (Figure 8b). The observed response indicates a large increase in the optical thickness of the coating, which is result of the increase in the refractive index of the coating as the TSPP infuses into the porous structure and adsorbs to the silica nanospheres. 110

Transmission / %

100 90

1

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70 60 50

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(a) (b) Figure 1 (a): Transmission spectra of the 100 μm period LPG before and after deposition of the multilayer silica nanosphere coating of thickness approximately 450 nm recorded with the LPG immersed into the silica colloids solution. Arrows indicate the increase of the layers number from 1 to 10. The insert shows the response of the LP0.20 cladding mode resonance (b) The evolution of the transmission spectrum of the coated LPG when immersed in the TSPP solution. The grey scale represents the measured transmission, with white corresponding to 100%, and black to 0%. The abrupt change in the transmission spectrum after 70 s was caused by immersion of the LPG in the TSPP solution.

The sensitivity of this device to ammonia in water was characterised by sequential immersion of the coated LPG into ammonia solutions with different concentrations (0. 1, 1, 10 ppm). In order to record the base line, the coated LPG fibre was immersed several times into a 150 μL of distilled water. After each deposition the fibre was dried using N2. The response of the spectrum is shown in Figure 2(a). The dynamic response of the sensor was assessed by immersing the LPG into an ammonia solution of volume 150 μL and of concentration 1 ppm, followed by drying and immersion into an ammonia solution of concentration 10 ppm. The transmission at the central wavelength of the resonance band, 800nm, was monitored. The results are shown in Figure 2(b), where “air”,, “H2O” and “NH3” regions correspond to the transmission recorded at 800 nm after drying the LPG and immersing it into water and ammonia solutions, respectively. 2200

3500

air 2000

2500

LP0,21 air

2000 NH3-10 ppm

1500

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H2 O

1000 600

650

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time / sec

wavelength / nm

(a)

(b)

Figure 2 (a): Response of the spectrum to immersion into water and into ammonia solutions of different concentrations: “H2O”, LPG immersed into water; “air”, LPG in air after drying with N2 gas; NH3 1 ppm, LPG immersed into ammonia solution of concentration of 1 ppm; NH3 10 ppm, LPG immersed into ammonia solution of concentration of 10 ppm. (b): Dynamic response to the varying of the ammonia concentration recorded at 800 nm: “H2O”, LPG immersed into water; “air”, LPG in air after drying with N2 gas; NH3 1 ppm, LPG immersed into ammonia solution of concentration of 1 ppm; NH3 10 ppm, LPG immersed into ammonia solution of concentration of 10 ppm.

After several immersions into water, the recorded spectrum was stable, demonstrating the stability of the coated device in an aqueous environment (the H2O region indicated in Figure 2(a)). After immersion into an ammonia

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solution (1 ppm) a decrease in the extinction ratio of the transmission at the central wavelength of the resonance band at 800 nm corresponding to coupling to LP0,21 was seen. After immersion into 10 ppm a further decrease in extinction was seen, reaching a steady state within 100 seconds, as shown in Figure 2 (b). The resonance feature corresponding to coupling to the LP0,20 cladding mode exhibited a small red shift of 0.5 nm for 1ppm ammonia concentration and 1.5 nm for 10 ppm ammonia concentration. The refractive index change in the coating is believed to be caused by two processes that occur after exposure to the ammonia solution; Firstly, ammonia or hydroxide ion that is generated by the acid base equilibrium reaction of ammonia in water leads to the deprotonation from the pyrolle ring of TSPP and hence it weakens the interaction between TSPP molecules, causing a change in the absorption spectrum of the TSPP. Secondly, in the presence of ammonium ions that are a by-product of the deprotonation of TSPP and the acid base equilibrium reaction of ammonia in water, the electrostatic interaction between TSPP and PDDA can be disturbed, causing desorption of TSPP from the PDDA/SiO2 film and consequently a decrease in the refractive index of the film .It was observed that, if the sensor was repeatedly exposed to a certain concentration of ammonia following a washing and drying cycle, the response was not reproducible, in that on each subsequent exposure to ammonia, the extinction of the band was further reduced each time. The extinction of the band would still change in time and the effect saturate, in the way indicated in figure 2(b). However, after a number of repeated exposures, with the number being dependent on the concentration of the ammonia solution to which the device was exposed, the sensitivity was exhausted, and exposure to the ammonia solution would produce a spectrum equivalent to that obtained after deposition of the porous film, but before TSPP infusion, as shown in figure 1(a). These observations indicate that the second process is dominant in the sensing mechanism. It was found that the TSPP compound could be infused into the porous film again by immersion of the LPG into a solution of TSPP.

SUMMARY In summary, the response of the transmission spectrum of an LPG of period 100μm to the deposition of a multilayer film of silica nanospheres and the subsequent infusion of a porphyrin into the porous coating has been characterised. The operation of this device as a ammonia sensor with a minimum detection level of 1ppm and a response time of approximately 100s has been demonstrated. It was found that repeated exposure to ammonia solution incurred a reduction in sensitivity and a change in the baseline that would correspond to the egress of TSPP from the film.

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