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Development of a fluorosiloxane polymer-coated optical fibre sensor for detection of organic volatile compounds
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Sensors and Actuators B 132 (2008) 280–289
Development of a fluorosiloxane polymer-coated optical fibre sensor for detection of organic volatile compounds Lurdes I.B. Silva a,b,∗ , Teresa A.P. Rocha-Santos b , A.C. Duarte a a
b
CESAM & Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal Instituto Piaget, Campus Acad´emico de Viseu, Estrada do Alto do Gaio, Lordosa, 3515-776 Viseu, Portugal Received 29 March 2007; received in revised form 7 January 2008; accepted 18 January 2008 Available online 1 February 2008
Abstract A compact optical fibre sensor coated with a fluorosiloxane polymer has been shown to be suitable for monitoring of some different classes of organic volatile compounds (VOCs), namely some chlorinated hydrocarbons, aromatic hydrocarbons, aliphatic hydrocarbons, acetate and alcohols. The sensing component consists of an optical fibre, with the top end surface coated with poly[methyl(3,3,3-trifluoropropyl)siloxane] by dip-coating technique. Variations of the light power guided through the fibre are detected as the organic vapour is sorbed in the thin polymeric film. The experimental set-up is further constituted by an optical source to generate the interrogating signal and a photodiode to measure the intensity modulated signal. In this work some operational conditions such as, temperature of cure of the polymeric material, injection cell temperature, carrier gas flow rate, laser working wavelengths and frequencies were studied in order to achieve higher sensitivity and accuracy concerning sensor system performance. High detection capability for volatile organic compounds, good sensibility, reversibility, reproducibility and linearity were analytical features checked for this sensor system. However, the main advantage of the developed sensor is its very short analytical time (response time and desorption time), found to be less than 20 s. Compounds well known as potentially dangerous for human health and environment, such as toluene and benzene showed changes on the reflected optical power up to 26.0 and 4.0 dB, respectively. © 2008 Elsevier B.V. All rights reserved. Keywords: Fluorosiloxane polymers; Optical fibre sensor; Gas sensor; VOCs sensor
1. Introduction Volatile organic compounds (VOCs) such as benzene, toluene, chloroform, carbon tetrachloride and methanol are consistently higher indoors than outdoors [1]. Human exposure to these chemicals can happen through inhalation, ingestion and skin contact/absorption. Even at low concentration, VOCs can be hazardous since some of these chemicals besides producing short- and long-term adverse health effects, also show carcinogenic, neurotoxic, genotoxic and mutagenic potential. For example, benzene is classified by US EPA as human carcinogenic compound group A [2] and the US Occupational Safety and Health Administration recommend very low exposure limits to benzene, in air of workplaces (1 ppm during an 8-h workday, 40-h workweek) [3]. For short-term exposure, the limit for air-
∗
Corresponding author at: Universidade de Aveiro, Departamento de Quimica, 3810-193 Aveiro, Portugal. Tel.: +351 913431087. E-mail address: [email protected] (L.I.B. Silva). 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.01.039
borne benzene is 5 ppm for 15 min [3]. Therefore the monitoring of VOCs, such as benzene, mainly in confined working areas, is of utmost importance for assuring an indoor air quality (IAQ) acceptable for a healthy environment. Optical fibre sensors have great potential for detection of volatile organic compounds (VOCs). These sensors show many analytical advantages, such as absence of electromagnetic interferences, low weight and small size, which make them suitable for performing remote detection, continuous measurements and in situ monitoring of chemical parameters in industrial processes [4–6]. The sensing principle is based on fact that the energy of the light guided through the optical fibre depends upon the difference between the refractive index of the core and the cladding of the optical fibre. When, the analyte absorbs into the specific porous silica cladding, its refractive index changes, thus leading to a variation of the light power [7]. These variations are proportional to the amount of analyte absorbed on the sensitive surface. Fluorosiloxane polymers constitute an interesting class of polymers for coating the optical fibre sensor because of
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their useful and versatile properties: high thermostability, hydrophobicity, lypophobicity, hydrolitic stability, high chemical inertness and low dielectric constant [8]. Besides their chemical and thermal stability the polymers also have a low glass temperature, which is an interesting property when sorption and diffusion phenomena are concerned [9]. Hence they have many applications in electronics and microelectronic industries, protective coatings, biotechnology, aeronautics [10] and optical sensing [11]. Abdelghani and Jaffrezic-Renault [12], developed a surface plasmon resonance (SPR) fibre sensor employing fluorosiloxane polymers as sensitive component for detection of vapours of chlorinated hydrocarbon and aromatic compounds. This class of polymers have been used as sensing component in many opto-chemical sensors applied to different environmental matrices, such as, on surface acoustic wave sensors development (SAW) for volatile organic compounds detection [13], NIR-evanescent wave absorbance sensors [14] for monitoring non-polar organic compounds in aqueous medium and optical fibre sensors with fixed incident angle of light power for gas and chemical vapour detection [7]. This work aims at the development of an optical fibre sensor coated with a thin polymeric film of poly[methyl(3,3,3trifluoropropyl)siloxane] suitable for monitoring some VOCs with competitive analytical advantages for analytical proposes, such as compact instrumentation, analytical response with both high accuracy and sensitivity besides short analytical time. The influence of the following operational conditions on the sensor system was evaluated: polymeric film temperature cure, injection cell temperature, carrier gas flow rate, laser working wavelengths and frequencies. After establishing the adequate operational conditions, the sensor was tested with different amounts of separate organic vapours, that is, benzene and toluene. The repeatability and reproducibility of the sensor was also estimated, and finally its analytical response was monitored for different classes of VOCs namely, aromatic hydrocarbons (benzene and toluene), chlorinated hydrocarbons (chloroform and carbon tetrachloride), aliphatic hydrocarbons (pentane, hexane and cyclohexane), acetates (ethyl acetate) and some alcohols (butanol, propanol, ethanol and methanol). 2. Materials and experimental 2.1. Optical fibre preparation The optical fibre used for this work was a single-mode optical fibre pigtail (core and cladding diameters of 9 and 125 m, respectively). A directional coupler 50:50 was utilized. A small length of the fibre in Port 3 was mechanically uncladded and than immersed in dichloromethane to remove the protective cladding, without lead micro-particles on fibre surface as checked by optical microscopy on the surface of the optical fibre. The uncladded section was then cleaved on a length of 13.0 mm with a Cleaver V6 (from Future Instrument) precision fibre cleaver. Finally, a thin polymeric film was deposited on the cleaved optical fibre end by dip-coating technique.
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2.2. Reagents Commercial polymer, poly[methyl(3,3,3-trifluoropropyl) siloxane] (PMTFPS) with a refractive index (real and imaginary values) of 1.383 and 0.01i, available from Aldrich (481645) was dissolved in 0.1% of dichloromethane (Lab Scan Analytical Sciences). All organic volatile compounds (benzene, toluene, chloroform, carbon tetrachloride, pentane, hexane, cyclohexane, ethyl acetate, butanol, propanol, ethanol and methanol) used were analytical grade. 2.3. Polymeric film deposition and cure PMTFPS was deposited as a thin polymeric film on the optical fibre surface, resulting in the sensitive component of the optical fibre sensor system. This sensitive layer was achieved by coating the optical fibre end with a solution of PMTFPS at 0.1% in dichloromethane. The optical fibre was then cured at a given temperature for 1 h. After the whole coating procedure, each fibre experienced a final cure treatment which lasted for 24 h in order to produce a hard porous polymeric film. Finally the fibre was inserted on a continuous stream of nitrogen and left to stabilize for another 24 h. It should be highlighted that the polymer cladding must exhibit particular optical and chemical properties, in order to obtain higher sensitivity and accuracy on the analytical sensor performance. In this work it was evaluated the influence of three different temperatures (20.0, 50.0 and 70.0 ◦ C) for the polymeric material deposition and cure, since polymeric film porosity and uniformity are crucial factors in the analytical sensor response [7]. 2.4. Configuration of the optical fibre sensor for VOCs detection A schematic diagram of the sensor configuration is shown in Fig. 1. The light beam was emitted by a laser diode (1 mW, λ = 1550 nm, λ = 1310 nm) from Oz Optics, that was attached to Port 1 of the directional coupler. The photodetector used, also from Oz Optics, was attached to Port 2. The optical power emitted by the laser (coming from Port 1) is splitted into Ports 3 and 4. Port 3, which was coated with a polymeric film, was placed inside a 6.20 cm long glass tube (analytical tube) in form of T. A constant flow of nitrogen (10.0 mL per min) was maintained through the analytical tube to enhance the interaction between the sensor head (the sensitive component of the optical device) and the organic vapour. Port 4 was cleaved and introduced inside a glass tube with index matching liquid tightly; there is no reflection at this Port 4. The power light guided through Port 3 is reflected at the fibre/polymer interface. Exposure of the fluorosiloxane polymer to organic volatile compounds vapours inside de analytical tube causes a change in its refractive index leading to a variation of the reflected light power. The intensity of the light power variation is then measured by an optical detector.
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Fig. 1. Schematic configuration of the experimental layout (I – injection cell, R – flowmeter, T – glass tube, AO – optical coupler, L – laser diode, F – photodetector, FO – optical fibre, G – index matching liquid, H – tape heater and PC – computer).
In all experiments, VOCs were injected with a gastight micro-syringe (Hamilton) at the top of a glass cell (injection cell). The temperature at the injection cell was controlled by a coiled tape heater (Cole Parmer). The organic compounds were injected as liquids, and after vapourization the organic vapours were carried by a continuous stream of reagent grade nitrogen “Paxair”, controlled with a flowmeter (Sigma) to the sensor tube.
3. Results and discussion 3.1. Influence of the polymeric film cure temperature In Fig. 2, the two top graphs on the left hand side show the sensor behaviour with no carrier gas flow and with a constant flow of carrier gas, respectively. In both cases, the analytical signal was monitored without any VOCs injection. The optical
Fig. 2. Sensor response for any VOCs injection (with no carrier gas flow and with a constant flow of carrier gas) and after injection of 870 g of benzene at three different temperatures of polymeric material cure.
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signal is very stable and no significant optical power changes were detected in the absence of VOCs injections, with or without carrier gas. Fig. 2 also shows the sensor response when 870 g of benzene were injected and the cure of the polymeric material was performed at three different temperatures (20.0, 50.0 and 70.0 ◦ C). In this study the cell injection temperature was maintained at 75.0 ◦ C, it was used a continuous stream of nitrogen around 15 mL min−1 and the experiments were performed at 1550 nm setting the laser mode operating frequency at continuous waveform. At 20.0 ◦ C the optical power change is not enough for analytical purposes. However, it can be observed that the analytical performance and sensitivity of the sensor with a polymeric material cured at 70.0 ◦ C is higher than the analytical response for the sensor with the polymeric material cured at 50.0 ◦ C. The optical power decreases after 12 s of benzene injection, with a maximum decrease after 3 s, the recovering time was found to be 4 s, at 70.0 ◦ C. The thickness of the sensitive film (cured at 70.0 ◦ C) was estimated as about 20 nm by Rutherford backscattering spectrometry (RBS). Since the film thickness largely affects the response time of the sensor, the thickness control of the film becomes a very important tool regarding analytical performance of the sensor system. The nanofilm obtained is in good agreement with the analytical response time of 12 s obtained in Fig. 2 for benzene. 3.2. Influence of the temperature of the injection cell and carrier gas flow rate The optical sensor system response was firstly evaluated for three different injection cell temperatures: 25.0, 50.0 and 75.0 ◦ C. Fig. 3 shows the results obtained when 860 g of
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toluene were injected and the injection cell temperature was maintained at each of the above referred temperatures. It can be concluded that the decrease of the optical power becomes considerably higher when the injection cell temperature increases. The variation of the reflected optical power at 75.0 ◦ C was 27.00 dB. This value was significantly higher than the optical power amplitude change obtained for the others injection cell temperatures. Additionally, and observing the profile of the sensor response, it is possible to conclude that the analytical response of the developed sensor is very well defined and shorter at 75.0 ◦ C (the response time was achieved after 15 s of toluene injection and the recovering time was found to be 7 s). It becomes also apparent that optical signal changes occurs only when vapour molecules are present, and temperatures lower than 25.0 ◦ C are not sufficiently high to bring the compounds under study into the vapour phase. These results prove the suitability of the optical sensor for the determination of organic vapours. Furthermore the sensor can also used to check the availability of organics in confined spaces by modulating the injection temperatures in increasing steps and observing the sensor response. As also shown in Fig. 3, the effect of the injection cell temperature on the sensor performance were also assessed at temperatures higher than 75.0 ◦ C, namely for 100.0 and 120.0 ◦ C. The analytical response (reflected optical power) decreased for a temperature of 100.0 ◦ C and decreased even slightly further for 120.0 ◦ C which may be due to the fact that increasing the injection cell temperature promote a faster analyte vapourization reducing the time of contact and the concentration distribution on the sensor surface of the analyte vapour which consequently affect the sensor response sensitivity.
Fig. 3. Sensor response when 860 g of toluene were injected at different injection cell temperatures (25.0, 50.0, 75.0, 100.0 and 120.0 ◦ C).
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Fig. 4 shows the results obtained in the study of the sensor response at 75.0 ◦ C, with different carrier gas flow rates, namely 5.0, 7.7, 10.6, 14.8, 18.6, 24.3 and 30.0 mL min−1 , for 860 g of toluene and 435 g of benzene. The profile of optical power versus carrier nitrogen flow in both toluene and benzene analysis has the same pattern: increases until a maximum and then decreases. Such results can be explained considering that the analytical signal generation depends on the diffusion of the vapour molecules onto the porous silica film, modifying its refractive index and consequently the reflected light power. Therefore, as the gas flow rates increase, the time of interaction between the analyte and the sensitive film become shorter, causing the carry over of the organic vapour molecules by the nitrogen stream, and no proper analytical signal is observed at very high flow rates. However the profile shows a maximum optical power value suggesting that very low flow rates can also lead to very low height analytical signals. In fact, as carrier gas flow rates decreases the diffusion of molecules from the porous sensitive film becomes more difficult and the analytical peaks show a broadening effect associated to lower peak height. As show in Fig. 4b, the highest optical power variation, meaning the difference between the optical value before injection and the lower value obtained after injection, was achieved for a range of carrier gas flow rates between around 10 and 20 mL min−1 . Fig. 4b also shows that high value for optical power change
occur at a nitrogen flow rate of 14.8 mL min−1 with a very low standard deviation for the five injections performed and it is therefore advisable to use a gas flow of around 15 mL min−1 for the best working conditions in toluene. The optimum performance in benzene analysis was attained for a range of nitrogen flow rates between around 10 and 20 mL min−1 as in the case of toluene. However, the highest analytical signal for the five injections, in each flow rate, occurred at 10.6 mL min−1 , where it also occurred the lowest standard deviation. In this case, it would advisable to work at a gas flow of about 11 mL min−1 which is not significantly for away form the best working conditions advised for toluene. As a final remark, and concerning this experiment it is important to highlight that in this sensor, the analyte molecules are adsorbed onto macromolecular chains of a polymer surface, and therefore the control of the velocity and the contact time of the organic vapour phase onto the polymer film (sensitive component of the optical device) become the limiting and the determinant factors on sensitivity of the analytical response. 3.3. Measurements at different laser working wavelengths and frequencies The sensor response was also monitored at different wavelengths on infrared spectral region and at different frequencies of
Fig. 4. Sensor response when 860 g of toluene and 435 g of benzene were injected at various carrier gas flow rates: (a) example of sensor response profile obtained for different carrier gas flow rates and (b) optical power variation obtained for the five injections at each carrier gas flow rates evaluated.
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Fig. 5. Sensor response after injection of 430 g of toluene (a) and 435 g of benzene (b) at 1550 and 1310 nm.
laser operation. The working wavelengths of 1310 and 1550 nm were chosen in order to assess the potential utilization of multiplexing in an available telecommunications network. For this particular study the polymeric film was cured at 70.0 ◦ C, the injector temperature was kept at 75.0 ◦ C and the carrier gas flow maintained at 15 mL min−1 . Fig. 5 shows the sensor response or the decrease of the reflected optical power when 430 g of toluene and 435 g of benzene were injected at two different wavelength by setting the laser mode operating frequency at continuous waveform. As observed in Fig. 5, an increase in wavelengths from 1310 to 1550 nm causes an increase in the reflected optical power for both organic compounds under study, namely toluene and benzene. As already suggested by Bari´ain et al. [5] for the response of an optical fibre sensor to organic compounds at three different working wavelengths an increase in wavelength can causes an increase on the refractive index of the sensitive polymeric film thus leading to an increase in the analytical signal. The higher optical power decrease obtained for toluene compared to benzene could be explained taking into account their boiling temperature values namely, 110.6 ◦ C for toluene and 80.1 ◦ C for benzene. This pattern of increasing analytical signal with increasing boiling temperatures of the analyte was also observed by Abdelmalek et al. [15] during a study of an optical fibre polymer based sensor for aromatic compounds. The sensor response was also evaluated for different modes of laser operation using continuous waveform (CW) and locked
frequencies (270 Hz, 1 and 2 KHz), in order to achieve an analytical signal both as stable and as high as possible. The CW mode means that the laser is continuously pumped and continuously emits light. Otherwise in the locked mode there is an ultrashort pulse generation of the laser beam. The results displayed in Fig. 6 for toluene and benzene, show that for both aromatic hydrocarbons the mean value (for five injections) of the reflected optical power is slightly higher, approximately 1 dB, at continuous waveform output than at any other frequency mode laser operation. Furthermore, at CW mode the obtained standard deviation is lower (0.09 and 0.06 for toluene and benzene, respectively) than at the mode locked frequencies. This could be due to instabilities (pulse unstable spacing, unstable energy or noise background) of the modes locked laser operation. Taking into account all the results obtained with this set of experiments work, one can conclude that a trade off between sensitivity and recovery time was achieved by working at 75.0 ◦ C for the injection cell temperature, using a sensitive film cured at 70.0 ◦ C, 1550 nm as working wavelength and utilizing a CW laser mode operation. 3.4. Figures of merit of the analytical sensor 3.4.1. Potential for linear calibration Fig. 7 shows a set of profiles of optical power changes obtained with increasing injection amounts of benzene (87, 174,
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Fig. 6. Sensor response after injection of toluene (860 g) and benzene (870 g) at different frequencies of data acquisition: (a) typical analytical response and (b) mean of the optical power variation obtained for the five injections and standard deviation at each frequency mode.
Fig. 7. Sensor response for different amounts of benzene (B) in a range between 87 and 2784 g and toluene (T) in a range between 86 and 514 g.
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348, 696, 1392 and 2784 g) and toluene (86, 171, 257, 343, 429 and 519 g). The analytical response time and recovery time of the sensor system are very short in the order of a few seconds, which makes this sensor system a very analytically efficient and competitive alternative for monitoring VOCs. A variation of 0.82 and 2.42 dB on the reflected optical power was obtained when 87 and 86 g of benzene and toluene, respectively were injected, which highlights the high detection capability of the experimental device. A proportional increase on optical power variation, with the increase of the different amounts of benzene and toluene allows concluding about the linearity of the sensor response, as shown in Fig. 8. The estimated of the intercept calibration was not considered to be significantly different of zero at 95% significance and a calibration model y = mx was also used instead of y = mx + b for both benzene and toluene. An estimative of the limit of detection (as three times the residual standard deviation) was found to be 8.30 g which shows the potential of this sensor system for measuring trace amounts of toluene. The developed sensor system showed a higher sensitivity for toluene detection than for benzene, as measured the slope value of the calibration (0.029 and 0.0092 for toluene and benzene, respectively). Moreover, the estimated limit of detection achieved for toluene analysis was lower than the obtained for benzene, specifically 36.0 g. 3.4.2. Repeatability and reproducibility of the sensor system Five sequential injections of 435 g of benzene and 430 g of toluene were used in order to test the repeatability of the sensor response. The time between each injection was around 2 min. Fig. 9 shows the results obtained on this experiment, suggesting that the analytical response of the sensor system is very repetitive, on both benzene and toluene analytical signals. Additionally, and taking into account the obtained results displayed in Fig. 9, it is also possible to infer that the organic vapour absorption and desorption are completely reversible, because when the organic volatile molecules are carried away from the sensitive device, the optical power signal returns to its initial value.
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Fig. 9. Sensor response when a series of toluene (430 g) and benzene (435 g) were injected around 2 min.
√ Using r = 1.96 2s, where s is the standard deviation of sequential experiments, as an estimate of the repeatability [16], then for benzene and toluene they are 0.0002 and 0.0001 dB, respectively. Table 1 shows the optical power variation obtained with injections of different quantities of toluene and benzene with the same sensor, on consecutive weeks. Using SIGMASTAT 3.0 [17], ANOVA was applied to all data on benzene and toluene and it was showed that there is not a statistically significant difference between weeks (α = 0.05). The difference in the mean values among the different weeks is not great enough to exclude the possibility that the difference is just due to random variability after allowing for the effects of differences in concentrations, on both benzene and toluene analysis. 3.4.3. Potential for detection of individual VOCs Figs. 10–12, shows the sensor response when some volatile organic compounds, namely chlorinated hydrocarbons (Fig. 10: chloroform and carbon tetrachloride), aliphatic hydrocarbons Table 1 Optical power changes obtained in consecutive weeks, with injections of different toluene and benzene amounts, for the same sensor dB
Fig. 8. Calibration curves obtained with sensor system for injections of benzene and toluene in a range between 87 and 2784 g and 86 and 514 g, respectively.
1st week
2nd week
3rd week
4th week
Toluene (g) 85.74 171.48 257.22 342.97 428.71 514.45
2.42 4.86 7.31 9.72 12.23 14.85
2.42 4.85 7.30 9.72 12.24 14.85
2.43 4.86 7.31 9.72 12.25 14.86
2.42 4.86 7.32 9.72 12.23 14.84
Benzene (g) 87.02 174.04 348.08 696.16 1392.32 2784.64
0.82 1.71 3.36 6.45 12.70 25.80
0.81 1.72 3.37 6.44 12.70 25.81
0.82 1.71 3.35 6.45 12.71 25.8
0.83 1.71 3.36 6.43 12.70 25.80
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high potential for VOCs monitoring not only to the laboratorial scale but also for confined industrial atmospheres. 4. Conclusions
Fig. 10. Sensor response for injection of chlorinated hydrocarbons, namely chloroform (2972 g) and carbon tetrachloride (3178 g).
(Fig. 11: pentane, hexane, cyclohexane) and an acetate (ethyl acetate) and alcohols (methanol, ethanol, propanol and butanol) were injected with the same experimental conditions as in the case of benzene and toluene. The developed sensor show high analytical potential for different classes of VOCs monitoring as it could be observed in Figs. 10–12. As a final remark, and observing Fig. 12, changes of 24 dB in optical power reflected were detected, more specifically in the determination of methanol, which is a good indicator of the potential application of this sensor system. Additionally no variations in optical signal were observed after 2 months, of continuous operation, which suggest a good analytical stability for the sensor device. Furthermore, this analytical device showed
An optical fibre sensor for VOCs detection based on a fluorosiloxane polymer coating has been developed for monitoring of benzene, toluene, chloroform, carbon tetrachloride, cyclohexane, pentane, hexane, ethyl acetate, butanol, ethanol, propanol and methanol that were used as examples of VOCs of interest. The poly[methyl(3,3,3-trifluoropropyl)siloxane] polymer was found to be suitable for sensor coating applications, concerning VOCs detection. The results show that the developed sensor system exhibit higher analytical performance at 1550 nm, at continuous waveform laser frequency mode operation and no higher than 80 ◦ C operational temperature is required. The main advantage of this newly developed sensor is the very short analytical time (response time and recovery time), approximately 20 s. Furthermore, the sensor system showed high linearity, repeatability, reproducibility and stability of the analytical signal. The sensitivity for toluene was higher than for benzene that could be detected at an estimated detection limit of 8.30 g. For alcohols, changes of up to 24 dB in the reflected optical power were detected, indicating the suitability of this analytical device for alcohols monitoring. The reduced dimensions, experimental design simplicity, low cost and maintenance make the sensor system very suitable for air quality monitoring and control. Additionally the easiness of the assembling process of the optical devices, and the large variety of materials, which can be deposited as sensitive membrane, confer high potential to the experimental device for in situ monitoring of organic volatile vapours in industrial environments and in indoor atmospheres.
Fig. 11. Sensor response for injections of two different classes of VOCs, namely some aliphatic hydrocarbons (3219 g of pentane, 1260 g of hexane and 1550 g of cyclohexane) and an acetate (1802 g of ethyl acetate).
Fig. 12. Sensor response for some alcohols namely methanol (1580 g), ethanol (1573 g), propanol (1573 g) and butanol (1606 g).
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Acknowledgements This work has been developed under the scope of the FCT (Portugal) funded research project POCTI/CTA/44899/02: “Development of a new Optical Fibre Chemical Sensor for in situ monitoring of VOCs (VOCSENSOR)” and a Ph.D. grant (SFRH/BD/17288/2004) awarded to Lurdes I.B. Silva is also gratefully acknowledged. References [1] EPA, Indoor Air Quality, Sources of Indoor Air Pollution – Organic Gases (Volatile Organic Compounds – VOCs), updated on October 22, 2006. http://www.epa.gov/iaq/voc.html. [2] EPA, Technology Transfer Network, Air Toxics Website, Benzene, updated on October 22, 2006. http://www.epa.gov/ttnatw01/hlthef/benzene.html. [3] NIOSH Pocket Guide to Chemical Hazards, NIOSH, Publication No. 2005-151, updated on October 22, 2006. http://www.cdc.gov/niosh/npg/ npgd0049.html. [4] M. Campbell, Sensor Systems for Environmental Monitoring, vol. 1, Blackie Academic & Professional, London, 1997, pp. 3, 4. [5] C. Bari´ain, I. Mat´ıas, I. Romeo, J. Garrido, M. Laguna, Behavioral experimental studies of a material towards development of optical fibre organic compounds sensor, Sens. Actuators B 76 (2001) 25–31. [6] C. Elos´ua, C. Bari´ain, I.R. Mat´ıas, F.J. Arregui, A. Luquin, M. Laguna, Volatile alcoholic compounds fibre optic nanosensor, Sens. Actuators B 115 (2006) 444–449. [7] A. Abdelghani, J.M. Chovelon, N. Jaffrezic-Renault, M. Lacroix, H. Gagnaire, C. Veillas, B. Berkova, M. Chomat, V. Matejec, Optical fibre sensor coated with porous silica layers for gas and chemical vapour detection, Sens. Actuators B 44 (1997) 495–498. [8] J.S. Bergstr¨om, L.B. Hilbert Jr., A constitutive model for predicting the large deformation thermomechanical behaviour of fluoropolymers, Mech. Mater. 37 (2005) 899–913. [9] C. Demathieu, M.M. Chehimi, J.-F. Lipskier, Inverse gas chromatographic characterization of functionalized polysiloxanes. Relevance to sensors technology, Sens. Actuators B 62 (2000) 1–7. [10] B. Ameduri, B. Boutevin, Update on fluoroelastomers: from perfluoroelastomers to fluorosilicones and fluorophosphazenes, J. Fluorine Chem. 126 (2005) 221–229.
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Biographies Lurdes Silva graduated in Chemistry and Environmental Sciences at Instituto Superior de Estudos Interculturais e Transdisciplinares of Instituto Piaget (Viseu, Portugal) in 2003. She worked at Instituto Piaget as Assistant Lecturer for 3 years and she is now working both at Department of Chemistry at the University of Aveiro and Instituto Piaget towards her Ph.D. degree in Chemistry. Her research interests include the development of chemical fibre optical sensors. Teresa Rocha-Santos graduated in Analytical Chemistry in 1996 and obtained a Ph.D. in Chemistry in 2000 at the University of Aveiro (Portugal). She is Assistant Professor of Environmental Chemistry at Instituto Piaget (Portugal) since 1999 and her main research interest is Analytical and Environmental Chemistry. Armando da Costa Duarte graduated in Chemical Engineering at University of Oporto (Portugal) and obtained a Ph.D. in Public Health Engineering at University of Newcastle-upon-Tyne (England) in 1981. He is Professor of Chemistry at University of Aveiro (Portugal) since 1995 and his main research interest is Analytical and Environmental Chemistry (http://www.cesam-ua.com/aduarte).
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Development of a fluorosiloxane polymer-coated optical fibre sensor for detection of organic volatile compounds Lurdes I.B. Silva a,b,∗ , Teresa A.P. Rocha-Santos b , A.C. Duarte a a
b
CESAM & Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal Instituto Piaget, Campus Acad´emico de Viseu, Estrada do Alto do Gaio, Lordosa, 3515-776 Viseu, Portugal Received 29 March 2007; received in revised form 7 January 2008; accepted 18 January 2008 Available online 1 February 2008
Abstract A compact optical fibre sensor coated with a fluorosiloxane polymer has been shown to be suitable for monitoring of some different classes of organic volatile compounds (VOCs), namely some chlorinated hydrocarbons, aromatic hydrocarbons, aliphatic hydrocarbons, acetate and alcohols. The sensing component consists of an optical fibre, with the top end surface coated with poly[methyl(3,3,3-trifluoropropyl)siloxane] by dip-coating technique. Variations of the light power guided through the fibre are detected as the organic vapour is sorbed in the thin polymeric film. The experimental set-up is further constituted by an optical source to generate the interrogating signal and a photodiode to measure the intensity modulated signal. In this work some operational conditions such as, temperature of cure of the polymeric material, injection cell temperature, carrier gas flow rate, laser working wavelengths and frequencies were studied in order to achieve higher sensitivity and accuracy concerning sensor system performance. High detection capability for volatile organic compounds, good sensibility, reversibility, reproducibility and linearity were analytical features checked for this sensor system. However, the main advantage of the developed sensor is its very short analytical time (response time and desorption time), found to be less than 20 s. Compounds well known as potentially dangerous for human health and environment, such as toluene and benzene showed changes on the reflected optical power up to 26.0 and 4.0 dB, respectively. © 2008 Elsevier B.V. All rights reserved. Keywords: Fluorosiloxane polymers; Optical fibre sensor; Gas sensor; VOCs sensor
1. Introduction Volatile organic compounds (VOCs) such as benzene, toluene, chloroform, carbon tetrachloride and methanol are consistently higher indoors than outdoors [1]. Human exposure to these chemicals can happen through inhalation, ingestion and skin contact/absorption. Even at low concentration, VOCs can be hazardous since some of these chemicals besides producing short- and long-term adverse health effects, also show carcinogenic, neurotoxic, genotoxic and mutagenic potential. For example, benzene is classified by US EPA as human carcinogenic compound group A [2] and the US Occupational Safety and Health Administration recommend very low exposure limits to benzene, in air of workplaces (1 ppm during an 8-h workday, 40-h workweek) [3]. For short-term exposure, the limit for air-
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Corresponding author at: Universidade de Aveiro, Departamento de Quimica, 3810-193 Aveiro, Portugal. Tel.: +351 913431087. E-mail address: [email protected] (L.I.B. Silva). 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.01.039
borne benzene is 5 ppm for 15 min [3]. Therefore the monitoring of VOCs, such as benzene, mainly in confined working areas, is of utmost importance for assuring an indoor air quality (IAQ) acceptable for a healthy environment. Optical fibre sensors have great potential for detection of volatile organic compounds (VOCs). These sensors show many analytical advantages, such as absence of electromagnetic interferences, low weight and small size, which make them suitable for performing remote detection, continuous measurements and in situ monitoring of chemical parameters in industrial processes [4–6]. The sensing principle is based on fact that the energy of the light guided through the optical fibre depends upon the difference between the refractive index of the core and the cladding of the optical fibre. When, the analyte absorbs into the specific porous silica cladding, its refractive index changes, thus leading to a variation of the light power [7]. These variations are proportional to the amount of analyte absorbed on the sensitive surface. Fluorosiloxane polymers constitute an interesting class of polymers for coating the optical fibre sensor because of
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their useful and versatile properties: high thermostability, hydrophobicity, lypophobicity, hydrolitic stability, high chemical inertness and low dielectric constant [8]. Besides their chemical and thermal stability the polymers also have a low glass temperature, which is an interesting property when sorption and diffusion phenomena are concerned [9]. Hence they have many applications in electronics and microelectronic industries, protective coatings, biotechnology, aeronautics [10] and optical sensing [11]. Abdelghani and Jaffrezic-Renault [12], developed a surface plasmon resonance (SPR) fibre sensor employing fluorosiloxane polymers as sensitive component for detection of vapours of chlorinated hydrocarbon and aromatic compounds. This class of polymers have been used as sensing component in many opto-chemical sensors applied to different environmental matrices, such as, on surface acoustic wave sensors development (SAW) for volatile organic compounds detection [13], NIR-evanescent wave absorbance sensors [14] for monitoring non-polar organic compounds in aqueous medium and optical fibre sensors with fixed incident angle of light power for gas and chemical vapour detection [7]. This work aims at the development of an optical fibre sensor coated with a thin polymeric film of poly[methyl(3,3,3trifluoropropyl)siloxane] suitable for monitoring some VOCs with competitive analytical advantages for analytical proposes, such as compact instrumentation, analytical response with both high accuracy and sensitivity besides short analytical time. The influence of the following operational conditions on the sensor system was evaluated: polymeric film temperature cure, injection cell temperature, carrier gas flow rate, laser working wavelengths and frequencies. After establishing the adequate operational conditions, the sensor was tested with different amounts of separate organic vapours, that is, benzene and toluene. The repeatability and reproducibility of the sensor was also estimated, and finally its analytical response was monitored for different classes of VOCs namely, aromatic hydrocarbons (benzene and toluene), chlorinated hydrocarbons (chloroform and carbon tetrachloride), aliphatic hydrocarbons (pentane, hexane and cyclohexane), acetates (ethyl acetate) and some alcohols (butanol, propanol, ethanol and methanol). 2. Materials and experimental 2.1. Optical fibre preparation The optical fibre used for this work was a single-mode optical fibre pigtail (core and cladding diameters of 9 and 125 m, respectively). A directional coupler 50:50 was utilized. A small length of the fibre in Port 3 was mechanically uncladded and than immersed in dichloromethane to remove the protective cladding, without lead micro-particles on fibre surface as checked by optical microscopy on the surface of the optical fibre. The uncladded section was then cleaved on a length of 13.0 mm with a Cleaver V6 (from Future Instrument) precision fibre cleaver. Finally, a thin polymeric film was deposited on the cleaved optical fibre end by dip-coating technique.
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2.2. Reagents Commercial polymer, poly[methyl(3,3,3-trifluoropropyl) siloxane] (PMTFPS) with a refractive index (real and imaginary values) of 1.383 and 0.01i, available from Aldrich (481645) was dissolved in 0.1% of dichloromethane (Lab Scan Analytical Sciences). All organic volatile compounds (benzene, toluene, chloroform, carbon tetrachloride, pentane, hexane, cyclohexane, ethyl acetate, butanol, propanol, ethanol and methanol) used were analytical grade. 2.3. Polymeric film deposition and cure PMTFPS was deposited as a thin polymeric film on the optical fibre surface, resulting in the sensitive component of the optical fibre sensor system. This sensitive layer was achieved by coating the optical fibre end with a solution of PMTFPS at 0.1% in dichloromethane. The optical fibre was then cured at a given temperature for 1 h. After the whole coating procedure, each fibre experienced a final cure treatment which lasted for 24 h in order to produce a hard porous polymeric film. Finally the fibre was inserted on a continuous stream of nitrogen and left to stabilize for another 24 h. It should be highlighted that the polymer cladding must exhibit particular optical and chemical properties, in order to obtain higher sensitivity and accuracy on the analytical sensor performance. In this work it was evaluated the influence of three different temperatures (20.0, 50.0 and 70.0 ◦ C) for the polymeric material deposition and cure, since polymeric film porosity and uniformity are crucial factors in the analytical sensor response [7]. 2.4. Configuration of the optical fibre sensor for VOCs detection A schematic diagram of the sensor configuration is shown in Fig. 1. The light beam was emitted by a laser diode (1 mW, λ = 1550 nm, λ = 1310 nm) from Oz Optics, that was attached to Port 1 of the directional coupler. The photodetector used, also from Oz Optics, was attached to Port 2. The optical power emitted by the laser (coming from Port 1) is splitted into Ports 3 and 4. Port 3, which was coated with a polymeric film, was placed inside a 6.20 cm long glass tube (analytical tube) in form of T. A constant flow of nitrogen (10.0 mL per min) was maintained through the analytical tube to enhance the interaction between the sensor head (the sensitive component of the optical device) and the organic vapour. Port 4 was cleaved and introduced inside a glass tube with index matching liquid tightly; there is no reflection at this Port 4. The power light guided through Port 3 is reflected at the fibre/polymer interface. Exposure of the fluorosiloxane polymer to organic volatile compounds vapours inside de analytical tube causes a change in its refractive index leading to a variation of the reflected light power. The intensity of the light power variation is then measured by an optical detector.
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Fig. 1. Schematic configuration of the experimental layout (I – injection cell, R – flowmeter, T – glass tube, AO – optical coupler, L – laser diode, F – photodetector, FO – optical fibre, G – index matching liquid, H – tape heater and PC – computer).
In all experiments, VOCs were injected with a gastight micro-syringe (Hamilton) at the top of a glass cell (injection cell). The temperature at the injection cell was controlled by a coiled tape heater (Cole Parmer). The organic compounds were injected as liquids, and after vapourization the organic vapours were carried by a continuous stream of reagent grade nitrogen “Paxair”, controlled with a flowmeter (Sigma) to the sensor tube.
3. Results and discussion 3.1. Influence of the polymeric film cure temperature In Fig. 2, the two top graphs on the left hand side show the sensor behaviour with no carrier gas flow and with a constant flow of carrier gas, respectively. In both cases, the analytical signal was monitored without any VOCs injection. The optical
Fig. 2. Sensor response for any VOCs injection (with no carrier gas flow and with a constant flow of carrier gas) and after injection of 870 g of benzene at three different temperatures of polymeric material cure.
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signal is very stable and no significant optical power changes were detected in the absence of VOCs injections, with or without carrier gas. Fig. 2 also shows the sensor response when 870 g of benzene were injected and the cure of the polymeric material was performed at three different temperatures (20.0, 50.0 and 70.0 ◦ C). In this study the cell injection temperature was maintained at 75.0 ◦ C, it was used a continuous stream of nitrogen around 15 mL min−1 and the experiments were performed at 1550 nm setting the laser mode operating frequency at continuous waveform. At 20.0 ◦ C the optical power change is not enough for analytical purposes. However, it can be observed that the analytical performance and sensitivity of the sensor with a polymeric material cured at 70.0 ◦ C is higher than the analytical response for the sensor with the polymeric material cured at 50.0 ◦ C. The optical power decreases after 12 s of benzene injection, with a maximum decrease after 3 s, the recovering time was found to be 4 s, at 70.0 ◦ C. The thickness of the sensitive film (cured at 70.0 ◦ C) was estimated as about 20 nm by Rutherford backscattering spectrometry (RBS). Since the film thickness largely affects the response time of the sensor, the thickness control of the film becomes a very important tool regarding analytical performance of the sensor system. The nanofilm obtained is in good agreement with the analytical response time of 12 s obtained in Fig. 2 for benzene. 3.2. Influence of the temperature of the injection cell and carrier gas flow rate The optical sensor system response was firstly evaluated for three different injection cell temperatures: 25.0, 50.0 and 75.0 ◦ C. Fig. 3 shows the results obtained when 860 g of
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toluene were injected and the injection cell temperature was maintained at each of the above referred temperatures. It can be concluded that the decrease of the optical power becomes considerably higher when the injection cell temperature increases. The variation of the reflected optical power at 75.0 ◦ C was 27.00 dB. This value was significantly higher than the optical power amplitude change obtained for the others injection cell temperatures. Additionally, and observing the profile of the sensor response, it is possible to conclude that the analytical response of the developed sensor is very well defined and shorter at 75.0 ◦ C (the response time was achieved after 15 s of toluene injection and the recovering time was found to be 7 s). It becomes also apparent that optical signal changes occurs only when vapour molecules are present, and temperatures lower than 25.0 ◦ C are not sufficiently high to bring the compounds under study into the vapour phase. These results prove the suitability of the optical sensor for the determination of organic vapours. Furthermore the sensor can also used to check the availability of organics in confined spaces by modulating the injection temperatures in increasing steps and observing the sensor response. As also shown in Fig. 3, the effect of the injection cell temperature on the sensor performance were also assessed at temperatures higher than 75.0 ◦ C, namely for 100.0 and 120.0 ◦ C. The analytical response (reflected optical power) decreased for a temperature of 100.0 ◦ C and decreased even slightly further for 120.0 ◦ C which may be due to the fact that increasing the injection cell temperature promote a faster analyte vapourization reducing the time of contact and the concentration distribution on the sensor surface of the analyte vapour which consequently affect the sensor response sensitivity.
Fig. 3. Sensor response when 860 g of toluene were injected at different injection cell temperatures (25.0, 50.0, 75.0, 100.0 and 120.0 ◦ C).
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Fig. 4 shows the results obtained in the study of the sensor response at 75.0 ◦ C, with different carrier gas flow rates, namely 5.0, 7.7, 10.6, 14.8, 18.6, 24.3 and 30.0 mL min−1 , for 860 g of toluene and 435 g of benzene. The profile of optical power versus carrier nitrogen flow in both toluene and benzene analysis has the same pattern: increases until a maximum and then decreases. Such results can be explained considering that the analytical signal generation depends on the diffusion of the vapour molecules onto the porous silica film, modifying its refractive index and consequently the reflected light power. Therefore, as the gas flow rates increase, the time of interaction between the analyte and the sensitive film become shorter, causing the carry over of the organic vapour molecules by the nitrogen stream, and no proper analytical signal is observed at very high flow rates. However the profile shows a maximum optical power value suggesting that very low flow rates can also lead to very low height analytical signals. In fact, as carrier gas flow rates decreases the diffusion of molecules from the porous sensitive film becomes more difficult and the analytical peaks show a broadening effect associated to lower peak height. As show in Fig. 4b, the highest optical power variation, meaning the difference between the optical value before injection and the lower value obtained after injection, was achieved for a range of carrier gas flow rates between around 10 and 20 mL min−1 . Fig. 4b also shows that high value for optical power change
occur at a nitrogen flow rate of 14.8 mL min−1 with a very low standard deviation for the five injections performed and it is therefore advisable to use a gas flow of around 15 mL min−1 for the best working conditions in toluene. The optimum performance in benzene analysis was attained for a range of nitrogen flow rates between around 10 and 20 mL min−1 as in the case of toluene. However, the highest analytical signal for the five injections, in each flow rate, occurred at 10.6 mL min−1 , where it also occurred the lowest standard deviation. In this case, it would advisable to work at a gas flow of about 11 mL min−1 which is not significantly for away form the best working conditions advised for toluene. As a final remark, and concerning this experiment it is important to highlight that in this sensor, the analyte molecules are adsorbed onto macromolecular chains of a polymer surface, and therefore the control of the velocity and the contact time of the organic vapour phase onto the polymer film (sensitive component of the optical device) become the limiting and the determinant factors on sensitivity of the analytical response. 3.3. Measurements at different laser working wavelengths and frequencies The sensor response was also monitored at different wavelengths on infrared spectral region and at different frequencies of
Fig. 4. Sensor response when 860 g of toluene and 435 g of benzene were injected at various carrier gas flow rates: (a) example of sensor response profile obtained for different carrier gas flow rates and (b) optical power variation obtained for the five injections at each carrier gas flow rates evaluated.
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Fig. 5. Sensor response after injection of 430 g of toluene (a) and 435 g of benzene (b) at 1550 and 1310 nm.
laser operation. The working wavelengths of 1310 and 1550 nm were chosen in order to assess the potential utilization of multiplexing in an available telecommunications network. For this particular study the polymeric film was cured at 70.0 ◦ C, the injector temperature was kept at 75.0 ◦ C and the carrier gas flow maintained at 15 mL min−1 . Fig. 5 shows the sensor response or the decrease of the reflected optical power when 430 g of toluene and 435 g of benzene were injected at two different wavelength by setting the laser mode operating frequency at continuous waveform. As observed in Fig. 5, an increase in wavelengths from 1310 to 1550 nm causes an increase in the reflected optical power for both organic compounds under study, namely toluene and benzene. As already suggested by Bari´ain et al. [5] for the response of an optical fibre sensor to organic compounds at three different working wavelengths an increase in wavelength can causes an increase on the refractive index of the sensitive polymeric film thus leading to an increase in the analytical signal. The higher optical power decrease obtained for toluene compared to benzene could be explained taking into account their boiling temperature values namely, 110.6 ◦ C for toluene and 80.1 ◦ C for benzene. This pattern of increasing analytical signal with increasing boiling temperatures of the analyte was also observed by Abdelmalek et al. [15] during a study of an optical fibre polymer based sensor for aromatic compounds. The sensor response was also evaluated for different modes of laser operation using continuous waveform (CW) and locked
frequencies (270 Hz, 1 and 2 KHz), in order to achieve an analytical signal both as stable and as high as possible. The CW mode means that the laser is continuously pumped and continuously emits light. Otherwise in the locked mode there is an ultrashort pulse generation of the laser beam. The results displayed in Fig. 6 for toluene and benzene, show that for both aromatic hydrocarbons the mean value (for five injections) of the reflected optical power is slightly higher, approximately 1 dB, at continuous waveform output than at any other frequency mode laser operation. Furthermore, at CW mode the obtained standard deviation is lower (0.09 and 0.06 for toluene and benzene, respectively) than at the mode locked frequencies. This could be due to instabilities (pulse unstable spacing, unstable energy or noise background) of the modes locked laser operation. Taking into account all the results obtained with this set of experiments work, one can conclude that a trade off between sensitivity and recovery time was achieved by working at 75.0 ◦ C for the injection cell temperature, using a sensitive film cured at 70.0 ◦ C, 1550 nm as working wavelength and utilizing a CW laser mode operation. 3.4. Figures of merit of the analytical sensor 3.4.1. Potential for linear calibration Fig. 7 shows a set of profiles of optical power changes obtained with increasing injection amounts of benzene (87, 174,
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Fig. 6. Sensor response after injection of toluene (860 g) and benzene (870 g) at different frequencies of data acquisition: (a) typical analytical response and (b) mean of the optical power variation obtained for the five injections and standard deviation at each frequency mode.
Fig. 7. Sensor response for different amounts of benzene (B) in a range between 87 and 2784 g and toluene (T) in a range between 86 and 514 g.
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348, 696, 1392 and 2784 g) and toluene (86, 171, 257, 343, 429 and 519 g). The analytical response time and recovery time of the sensor system are very short in the order of a few seconds, which makes this sensor system a very analytically efficient and competitive alternative for monitoring VOCs. A variation of 0.82 and 2.42 dB on the reflected optical power was obtained when 87 and 86 g of benzene and toluene, respectively were injected, which highlights the high detection capability of the experimental device. A proportional increase on optical power variation, with the increase of the different amounts of benzene and toluene allows concluding about the linearity of the sensor response, as shown in Fig. 8. The estimated of the intercept calibration was not considered to be significantly different of zero at 95% significance and a calibration model y = mx was also used instead of y = mx + b for both benzene and toluene. An estimative of the limit of detection (as three times the residual standard deviation) was found to be 8.30 g which shows the potential of this sensor system for measuring trace amounts of toluene. The developed sensor system showed a higher sensitivity for toluene detection than for benzene, as measured the slope value of the calibration (0.029 and 0.0092 for toluene and benzene, respectively). Moreover, the estimated limit of detection achieved for toluene analysis was lower than the obtained for benzene, specifically 36.0 g. 3.4.2. Repeatability and reproducibility of the sensor system Five sequential injections of 435 g of benzene and 430 g of toluene were used in order to test the repeatability of the sensor response. The time between each injection was around 2 min. Fig. 9 shows the results obtained on this experiment, suggesting that the analytical response of the sensor system is very repetitive, on both benzene and toluene analytical signals. Additionally, and taking into account the obtained results displayed in Fig. 9, it is also possible to infer that the organic vapour absorption and desorption are completely reversible, because when the organic volatile molecules are carried away from the sensitive device, the optical power signal returns to its initial value.
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Fig. 9. Sensor response when a series of toluene (430 g) and benzene (435 g) were injected around 2 min.
√ Using r = 1.96 2s, where s is the standard deviation of sequential experiments, as an estimate of the repeatability [16], then for benzene and toluene they are 0.0002 and 0.0001 dB, respectively. Table 1 shows the optical power variation obtained with injections of different quantities of toluene and benzene with the same sensor, on consecutive weeks. Using SIGMASTAT 3.0 [17], ANOVA was applied to all data on benzene and toluene and it was showed that there is not a statistically significant difference between weeks (α = 0.05). The difference in the mean values among the different weeks is not great enough to exclude the possibility that the difference is just due to random variability after allowing for the effects of differences in concentrations, on both benzene and toluene analysis. 3.4.3. Potential for detection of individual VOCs Figs. 10–12, shows the sensor response when some volatile organic compounds, namely chlorinated hydrocarbons (Fig. 10: chloroform and carbon tetrachloride), aliphatic hydrocarbons Table 1 Optical power changes obtained in consecutive weeks, with injections of different toluene and benzene amounts, for the same sensor dB
Fig. 8. Calibration curves obtained with sensor system for injections of benzene and toluene in a range between 87 and 2784 g and 86 and 514 g, respectively.
1st week
2nd week
3rd week
4th week
Toluene (g) 85.74 171.48 257.22 342.97 428.71 514.45
2.42 4.86 7.31 9.72 12.23 14.85
2.42 4.85 7.30 9.72 12.24 14.85
2.43 4.86 7.31 9.72 12.25 14.86
2.42 4.86 7.32 9.72 12.23 14.84
Benzene (g) 87.02 174.04 348.08 696.16 1392.32 2784.64
0.82 1.71 3.36 6.45 12.70 25.80
0.81 1.72 3.37 6.44 12.70 25.81
0.82 1.71 3.35 6.45 12.71 25.8
0.83 1.71 3.36 6.43 12.70 25.80
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high potential for VOCs monitoring not only to the laboratorial scale but also for confined industrial atmospheres. 4. Conclusions
Fig. 10. Sensor response for injection of chlorinated hydrocarbons, namely chloroform (2972 g) and carbon tetrachloride (3178 g).
(Fig. 11: pentane, hexane, cyclohexane) and an acetate (ethyl acetate) and alcohols (methanol, ethanol, propanol and butanol) were injected with the same experimental conditions as in the case of benzene and toluene. The developed sensor show high analytical potential for different classes of VOCs monitoring as it could be observed in Figs. 10–12. As a final remark, and observing Fig. 12, changes of 24 dB in optical power reflected were detected, more specifically in the determination of methanol, which is a good indicator of the potential application of this sensor system. Additionally no variations in optical signal were observed after 2 months, of continuous operation, which suggest a good analytical stability for the sensor device. Furthermore, this analytical device showed
An optical fibre sensor for VOCs detection based on a fluorosiloxane polymer coating has been developed for monitoring of benzene, toluene, chloroform, carbon tetrachloride, cyclohexane, pentane, hexane, ethyl acetate, butanol, ethanol, propanol and methanol that were used as examples of VOCs of interest. The poly[methyl(3,3,3-trifluoropropyl)siloxane] polymer was found to be suitable for sensor coating applications, concerning VOCs detection. The results show that the developed sensor system exhibit higher analytical performance at 1550 nm, at continuous waveform laser frequency mode operation and no higher than 80 ◦ C operational temperature is required. The main advantage of this newly developed sensor is the very short analytical time (response time and recovery time), approximately 20 s. Furthermore, the sensor system showed high linearity, repeatability, reproducibility and stability of the analytical signal. The sensitivity for toluene was higher than for benzene that could be detected at an estimated detection limit of 8.30 g. For alcohols, changes of up to 24 dB in the reflected optical power were detected, indicating the suitability of this analytical device for alcohols monitoring. The reduced dimensions, experimental design simplicity, low cost and maintenance make the sensor system very suitable for air quality monitoring and control. Additionally the easiness of the assembling process of the optical devices, and the large variety of materials, which can be deposited as sensitive membrane, confer high potential to the experimental device for in situ monitoring of organic volatile vapours in industrial environments and in indoor atmospheres.
Fig. 11. Sensor response for injections of two different classes of VOCs, namely some aliphatic hydrocarbons (3219 g of pentane, 1260 g of hexane and 1550 g of cyclohexane) and an acetate (1802 g of ethyl acetate).
Fig. 12. Sensor response for some alcohols namely methanol (1580 g), ethanol (1573 g), propanol (1573 g) and butanol (1606 g).
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Acknowledgements This work has been developed under the scope of the FCT (Portugal) funded research project POCTI/CTA/44899/02: “Development of a new Optical Fibre Chemical Sensor for in situ monitoring of VOCs (VOCSENSOR)” and a Ph.D. grant (SFRH/BD/17288/2004) awarded to Lurdes I.B. Silva is also gratefully acknowledged. References [1] EPA, Indoor Air Quality, Sources of Indoor Air Pollution – Organic Gases (Volatile Organic Compounds – VOCs), updated on October 22, 2006. http://www.epa.gov/iaq/voc.html. [2] EPA, Technology Transfer Network, Air Toxics Website, Benzene, updated on October 22, 2006. http://www.epa.gov/ttnatw01/hlthef/benzene.html. [3] NIOSH Pocket Guide to Chemical Hazards, NIOSH, Publication No. 2005-151, updated on October 22, 2006. http://www.cdc.gov/niosh/npg/ npgd0049.html. [4] M. Campbell, Sensor Systems for Environmental Monitoring, vol. 1, Blackie Academic & Professional, London, 1997, pp. 3, 4. [5] C. Bari´ain, I. Mat´ıas, I. Romeo, J. Garrido, M. Laguna, Behavioral experimental studies of a material towards development of optical fibre organic compounds sensor, Sens. Actuators B 76 (2001) 25–31. [6] C. Elos´ua, C. Bari´ain, I.R. Mat´ıas, F.J. Arregui, A. Luquin, M. Laguna, Volatile alcoholic compounds fibre optic nanosensor, Sens. Actuators B 115 (2006) 444–449. [7] A. Abdelghani, J.M. Chovelon, N. Jaffrezic-Renault, M. Lacroix, H. Gagnaire, C. Veillas, B. Berkova, M. Chomat, V. Matejec, Optical fibre sensor coated with porous silica layers for gas and chemical vapour detection, Sens. Actuators B 44 (1997) 495–498. [8] J.S. Bergstr¨om, L.B. Hilbert Jr., A constitutive model for predicting the large deformation thermomechanical behaviour of fluoropolymers, Mech. Mater. 37 (2005) 899–913. [9] C. Demathieu, M.M. Chehimi, J.-F. Lipskier, Inverse gas chromatographic characterization of functionalized polysiloxanes. Relevance to sensors technology, Sens. Actuators B 62 (2000) 1–7. [10] B. Ameduri, B. Boutevin, Update on fluoroelastomers: from perfluoroelastomers to fluorosilicones and fluorophosphazenes, J. Fluorine Chem. 126 (2005) 221–229.
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Biographies Lurdes Silva graduated in Chemistry and Environmental Sciences at Instituto Superior de Estudos Interculturais e Transdisciplinares of Instituto Piaget (Viseu, Portugal) in 2003. She worked at Instituto Piaget as Assistant Lecturer for 3 years and she is now working both at Department of Chemistry at the University of Aveiro and Instituto Piaget towards her Ph.D. degree in Chemistry. Her research interests include the development of chemical fibre optical sensors. Teresa Rocha-Santos graduated in Analytical Chemistry in 1996 and obtained a Ph.D. in Chemistry in 2000 at the University of Aveiro (Portugal). She is Assistant Professor of Environmental Chemistry at Instituto Piaget (Portugal) since 1999 and her main research interest is Analytical and Environmental Chemistry. Armando da Costa Duarte graduated in Chemical Engineering at University of Oporto (Portugal) and obtained a Ph.D. in Public Health Engineering at University of Newcastle-upon-Tyne (England) in 1981. He is Professor of Chemistry at University of Aveiro (Portugal) since 1995 and his main research interest is Analytical and Environmental Chemistry (http://www.cesam-ua.com/aduarte).