Nanoassembled Thin-Film Gas Sensor II . An Intrinsic Highly Sensitive Fibre Optic Sensor for Ammonia Detection

A layer-by-layer approach was used for the deposition of coatings with a nanometre thickness onto a multimode optical fi bre, from which the cladding had been removed, with the aim of realizing a fi bre optic ammonia sensor. The fi lm was self-assembled with alternate layers of tetrakis-(4-sulfophenyl) porphine (TSPP) and poly(diallyldimethyla mmonium chloride) (PDDA). The exposure of the assembled fi lm to ammonia induced optical changes in the transmission spectrum of the coated optical fi bre, refl ecting the characteristic absorption bands (Soret and Q bands) of the assembled TSPP compound. The fi bre optic sensor showed a linear sensitivity to ammonia in the concentration range of 0.1–20 ppm and the response and recovery times were less than 3 min with a limit of detection of 0.9 ppm at 706 nm.


Introduction
The detection of chemical compounds is very important for monitoring outdoor and indoor environments (e.g., air and soil pollutions and sick building syndrome), (1) diseases (e.g., allergies and cancer), (2) and dangerous substances (e.g., drugs, hidden bombs, and landmines). (3)Sensitive, reliable and cheap sensors for application in different areas of human activities are still required.A sensor for the detection of ammonia gas is desired but has not yet been effectively achieved in the environmental, automotive, chemical and medical areas. (4)A number of different sensor techniques can be employed for the detection of chemical species through the measurement of electrical, (5) amperometric, (6) optical (7) and other physical (5)(6)(7)(8)(9) parameters.
The combination of optical fi bre devices with chemically sensitive coatings offers a universal platform for the development of highly sensitive and selective gas sensors. (10)(13)(14) Different immobilization procedures based on covalent and noncovalent bonds could be applied to the deposition of sensitive elements onto optical fi bres. (15,16)he electrostatic layer-by-layer (LbL) method is a useful tool for the preparation of molecularly assembled fi lms with a good adhesion to the surface. (17,18)The potential of this deposition technique is still expanding because of its versatility and possibility for the fabrication of ordered multilayers including organic, inorganic and biological materials.
Over the past few decades, porphyrins and their derivatives have been considered as sensitive elements for optical sensors (19)(20)(21)(22) owing to their specifi c optical properties in the UV-Vis region.The absorption (or fl uorescence) spectra are dependent on their chemical structures induced by metallation, protonation, and aggregation, which make porphyrins useful sensors of their environmental surroundings. (23)The optical spectrum of porphyrins in solid state is altered compared with that in solution, owing to the presence of strong -interactions. (24)The high extinction coeffi cient (> 200,000 cm −1 M −1 ) makes porphyrin particularly attractive for the development of optical sensors.
In this article, we describe the use of the LbL method for the deposition of a porphyrin thin fi lm onto a multimode silica core/plastic clad optical fi bre with the aim of developing an evanescent wave-type fi bre-optic gas sensor.A short section of the plastic cladding was replaced with a functional coating of alternate PDDA and TSPP layers.The measurement principle of the device is based on the ammonia-induced optical change in the transmission spectrum of the coated optical fi bre.As light travels along the core of the optical fi bre, a small portion of energy penetrates the cladding in the form of an evanescent wave, the intensity of which decays exponentially with the distance from the interface between the cladding and the surrounding environment.The penetration depth (d p ) of the evanescent wave is described by (10) , (1)   where λ is the wavelength of light in free space, n c is the refractive index of the cladding and n eff is the effective refractive index of the mode guided by the optical fi bre.The deposition of a functional coating layer onto the optical fi bre leads to the chemically induced modulation in the transmission spectrum and provides quantitative and qualitative information on the chemical species under examination.The employment of the proposed fi bre optic sensor based on the intrinsic evanescent wave has an additional advantage of offering cheap and compact devices, which is due to the combination of light-emitting diode (LED) and photodetector components.Moreover, the sensitivity of the device can be improved by varying the length of the sensing area and thus the process of fi lm deposition will be less time-consuming.

Materials
Tetrakis-(4-sulfophenyl) porphine (TSPP) and poly(diallyldimethylammonium chloride) (PDDA, Mw: 200,000-350,000, 20 wt% in H 2 O) were purchased from Tokyo Kasei, Japan (Scheme 1).Deionized pure water (18.3MΩ•cm) was obtained by reverse osmosis followed by ion exchange and fi ltration (Nanopure Diamond, Barnstead, Japan).An HCS200 multimode silica core/plastic cladding optical fi bre (OF) with core and cladding diameters of 200 and 400 μm, respectively, was purchased from Ocean Optics (USA).Standard ammonia gas of 100 ppm in dry air was purchased in a cylinder from Japan Air Gases Corp.All of these chemicals were of analytical grade and used without further purifi cation.

Optical fi bre preparation
The electrostatic layer-by-layer adsorption method was employed for the deposition of a porphyrin thin fi lm onto a multimode optical fi bre (OF).A schematic illustration of this method using PDDA and TSPP is shown in Fig. 1(a).A multimode optical fi bre from which the plastic cladding has been removed over an area 1 cm in length was rinsed in ethanol and distilled water prior to fi lm deposition.The plastic cladding could be easily burned off from the fi bre using a burner fl ame (temperature < 500°C, the property of the silica core is not changed within this temperature range).One end of the optical fi bre was connected to a deuterium-halogen light source (DH-2000-Ball, Mikropack) and the other end was connected to a spectrometer (S1024DW, Ocean Optics).The stripped section of the optical fi bre was fi xed within a special deposition cell for fi lm preparation, as shown in Fig. 1(b).
Before assembly, the previously stripped section of the optical fi bre was cleaned with concentrated sulfuric acid (96%), rinsed several times with deionized water, and treated with 1 wt% ethanolic KOH (ethanol/water = 3:2, v/v) for about 10 min with sonication to functionalize the surface of the silica core with a OH group.The fi bre core was then rinsed with deionized water, and dried by fl ushing with dry nitrogen gas.The fi lm is denoted (PDDA + /TSPP − ) x , where x = 5 and indicates the number of adsorption cycles.The fi lm was prepared by the alternate deposition of PDDA (5 mg mL −1 in water) and Scheme 1. Structural models of the polycation (PDDA) and porphyrin (TSPP) compounds used in this study: (17) SS, side length of square; DS, diagonal length of square.TSPP (1 mM in water) (where one cycle is considered to be a combined PDDA + /TSPP − bilayer) by introducing a coating solution (150 μL) into the deposition cell with intermediate processes of water washing and drying by fl ushing with nitrogen gas being undertaken between the application of layers.In every case, the outermost surface of the alternate fi lm was TSPP.The assembly process was monitored using an S1024DW spectrometer (Ocean Optics).The absorbance was determined by taking the logarithm of the ratio of the transmission spectrum of the coated fi bre, T(λ), to the transmission spectrum measured prior to fi lm deposition, T 0 (λ), .
The assembly process was characterised and the thickness of the fi lm was measured using a quartz crystal microbalance technique, as described in our previous work. (25)

Optical measurements
The desired gas concentrations were produced using a two-arm fl ow system, as shown in Fig. 2(a).Dry compressed air and ammonia gas of 100 ppm were passed through two fl owmeters, and the two fl ows were recombined with a fi nal analyte concentration (volume fraction) c in the measurement chamber calculated using, , where z is the mole fraction of ammonia, and L 1 and L 2 are the fl ow rates of dry air and ammonia gas, respectively.L (where L = L 1 + L 2 ) was maintained at 1 L min −1 and ammonia concentration was adjusted by varying L 1 and L 2 .A custom designed sensor chamber made of Tefl on (Fig. 2(b)) was used to estimate the ammonia response.The optical fi bre coated with the functional fi lm was inserted in the chamber and connected to the light source and spectrometer, as shown in Fig. 2(b).
The sensor response at a given analyte concentration was measured every second by recording the transmission spectrum of the fi lm deposited on the optical fi bre.The difference spectrum was plotted by subtracting a spectrum measured at a given analyte concentration from the spectrum recorded in the presence of dry air.The baseline spectrum of each experiment was recorded by passing dry air through the measurement chamber until the signal measured at wavelengths of 350, 470 and 706 nm reached equilibrium.The dynamic sensor response was also measured at the same wavelengths.
The optical fi bre sensor response (SR) was calculated using where I 0 and I describe the light intensities of the PDDA + /TSPP − fi lm in the absence and presence of the analyte gas, respectively, measured at a given wavelength.

Optical spectra of PDDA + /TSPP − alternate layers
The assembly of the PDDA and TSPP layers after each deposition cycle was measured by monitoring the optical change in the transmission spectra of the optical fi bre. Figure 3(a) shows the evolution of the transmission spectrum of the optical fi bre during the deposition of a fi ve-cycle PDDA + /TSPP − thin fi lm.The absorbance spectra were derived from the transmission spectra using eq.( 2).The largest absorbance due to the deposition of the (PDDA + /TSPP − ) bilayer was observed at a wavelength of 420 nm, which corresponds to the Soret band.The absorbance increased in proportion to the number of adsorption cycles (Fig. 3(b)).The absorbance spectra of the (PDDA + /TSPP − ) fi lm are characterized by a double peak in the Soret band occurring at 420 and 480 nm, and by a pronounced peak of the Q band at 706 nm.These spectral characteristics suggest that TSPP molecules exist in the J-aggregate state, in which the absorbance maxima of the Soret and Q bands are redshifted compared with those in the monomeric state. (17,26,27)The aggregation state of TSPP and hence its spectral features are controlled by the protonation/deprotonation of the porphyrin pyrrole ring. (17)Figure 3(c) shows the absorbance change monitored at two Soret bands (420 and 480 nm) and at the Q band (706 nm) versus the number of adsorption cycles.

Optical response to ammonia
Ammonia-induced optical changes in the transmission spectrum of the (PDDA + /TSPP -) 5 fi lm are shown in Fig. 4. As ammonia concentration increases from 0 to 20 ppm, the intensity change occurs at several wavelengths; at 706 nm, intensity increases, whereas at 350 and 470 nm it decreases.Upon exposure of the (PDDA + /TSPP − ) 5 fi lm to ammonia, the largest intensity change was observed at 706 nm.The interaction between ammonia and TSPP molecules leads to the deprotonation from the pyrolle ring and hence affects the interaction between TSPP molecules.Similarly, the largest change in absorbance is observed at 706 nm (Q band), which is attributed to the aggregation structure of TSPP. (26)he dynamic sensor response of the (PDDA + /TSPP -) 5 film to ammonia was monitored at 350, 470 and 706 nm (Fig. 5(a)).As can be seen from the result, the sensor response is fully reversible for low ammonia concentrations (up to 1 ppm).However, at higher concentrations the recovery time of the sensor response takes longer to return to the base line.The base line may be recovered when fl ushed with air for a suffi cient time, as shown in Fig. 5(a).Alternatively, the sensor response can be regenerated by rinsing for a few seconds in distilled water. (25)The calibration curve at each wavelength was plotted from the recorded spectra at given ammonia concentrations.The sensor shows linear responses at all wavelengths for a wide concentration range from 0.1 to 20 ppm and the highest sensitivity was observed at 706 nm (Fig. 5(b)).
Table 2 shows a summary of the sensor parameters, including sensitivity, response and recovery times and limit of detection (LOD) measured at different wavelengths.The response and recovery times (t 90 ) of the sensor to increasing ammonia concentration were within 1.6-2.5 and 1.8-3.2min, respectively (see Fig. 5(a)).The sensitivity of the sensor depends on the wavelength and has different directions; for 350 and 470 nm it is negative and for 706 nm it is positive.The highest sensitivity was measured at 706 nm, corresponding to the optical change of the Q band of TSPP.The current sensor system has a limit of detection (LOD) on the ppm order ranging from 0.9 to 2.6 ppm.The limit of detection was defi ned according to LOD =3σ/m, where σ ≈ 0.31 is the standard deviation, and m is the slope (∆I/∆c) of the calibration curve, where c is the ammonia concentration and I is the measured intensity (mV). (28)The presence of different features Fig. 3. Evolution of the spectrum of a multilayer fi lm of PDDA + /TSPP -that was deposited onto a 200-μm-core-diameter multimode optical fi bre with a stripped silica core of 1 cm: (a) transmission spectra (data as measured); (b) absorbance spectra (derived from the transmission spectra using eq.( 2)); (c) absorbance change due to fi lm deposition monitored at wavelengths of 420 nm (squares), 480 nm (circles), and 706 nm (triangles).

Conclusion
A fi bre optic ammonia sensor based on a PDDA + /TSPP − alternate thin fi lm deposited on the core of a multimode optical fi bre using a layer-by-layer approach is demonstrated.The intensity of the light propagating through the optical fi bre decreases proportionally with the increase in the thickness of the TSPP layers deposited over the optical fi bre.The exposure of the fi ve-cycle PDDA + /TSPP − fi lm to ammonia induces changes in the absorption spectrum via the deprotonation of TSPP, which could be observed in the transmission spectrum of the coated optical fi bre.The highest sensitivity (0.98 mV ppm -1 ) was observed when measured at 706 nm, which corresponds to the Q band of the porphyrin compound; for low-cost-sensor development it is possible to use a simple LED-photodiode system operating at approximately 700 nm.The sensor showed a linear sensitivity to the presence of ammonia with a limit of detection of 0.9 ppm in the concentration range of 0.1-20 ppm and the sensor response and recovery times were less than 4 min.The demonstrated sensor offers an opportunity for the detection of different chemicals by coating an optical fi bre with an appropriate sensitive material.Further work is needed to optimize sensor performance and to study the effects of coating thickness, relative humidity and presence of the other chemical compounds on sensor parameters.

Acknowledgment
This work was supported by MEXT via the Kitakyushu knowledge-based Cluster Project.

Fig. 1 .
Fig. 1.(a) Schematic illustration of the layer-by-layer adsorption of TSPP and PDDA on a multimode optical fi bre and (b) deposition cell used for coating the optical fi bre.

Fig. 2 .
Fig. 2. (a) Apparatus of a two-arm fl ow gas generation system: F1 and F2 are fl owmeters; L i represents the concentration of the gases in the different arms of the system.(b) Schematic illustration of the measurement setup: light source, Ocean optics light source emitting light in the range of wavelengths from 200 to 1,100 nm; spectrometer, Ocean Optics S1024DW spectrometer.

Fig. 4 .
Fig.4.Optical transmission difference spectra of the optical fi bre consisting of a fi ve-cycle PDDA + /TSPP − alternate fi lm for ammonia concentrations ranging from 0-20 ppm.Difference spectra derived from Fig.3(a) were obtained by subtracting a spectrum measured in ammonia atmosphere from a spectrum measured in air.

Fig. 5 .
Fig. 5. (a) Dynamic response of the optical fiber consisting of a five-cycle PDDA + /TSPP − alternate fi lm for ammonia concentrations ranging from 0-20 ppm at 350, 470, and 706 nm.(b) Calibration curves at 350 nm (squares), 470 nm (rhombuses), and 706 nm (circles).Lines show the linear fi tting and are provided only as guides for the eye.