Study on the Sensing Coating of the Optical Fibre CO2 Sensor

Abstract

Optical fibre carbon dioxide (CO2) sensors are reported in this article. The principle of operation of the sensors relies on the absorption of light transmitted through the fibre by a silica gel coating containing active dyes, including methyl red, thymol blue and phenol red. Stability of the sensor has been investigated for the first time for an absorption based CO2 optical fiber sensor. Influence of the silica gel coating thickness on the sensitivity and respone time has also been studied. The impact of temperature and humidity on the sensor performance has been examined too. Response times of reported sensors are very short and reach 2–3 s, whereas the sensitivity of the sensor ranges from 3 to 10 for different coating thicknesses. Reported parameters make the sensor suitable for indoor and industrial use.

Full text

Article
Study on the Sensing Coating of the Optical Fibre
CO2Sensor
Karol Wysoki´nski 1, *, Marek Napierała 1,2, Tomasz Sta ´nczyk 3,†, Stanisław Lipi ´nski 1, † and
Tomasz Nasiłowski 1
Received: 31 August 2015; Accepted: 11 December 2015; Published: 17 December 2015
Academic Editor: Michael Tiemann
1InPhoTech, 17 Słomi ´nskiego St31, 00-195 Warszawa, Poland; mnapierala@inphotech.pl (M.N.);
slipinski@inphotech.pl (S.L.); tnasilowski@inphotech.pl (T.N.)
2IPT Safety, Ceramiczna St 8A, 20-150 Lublin, Poland
3Polish Centre for Photonics and Fibre Optics, Rogo´znica 312, 36-060 Głogów Małopolski, Poland;
tstanczyk@pcfs.org.pl
*Correspondence: kwysokinski@inphotech.pl; Tel.: +48-533-779-177; Fax: +48-22-304-6450
† These authors contributed equally to this work.
Abstract: Optical fibre carbon dioxide (CO2) sensors are reported in this article. The principle of
operation of the sensors relies on the absorption of light transmitted through the fibre by a silica
gel coating containing active dyes, including methyl red, thymol blue and phenol red. Stability
of the sensor has been investigated for the first time for an absorption based CO2optical fiber
sensor. Influence of the silica gel coating thickness on the sensitivity and response time has also been
studied. The impact of temperature and humidity on the sensor performance has been examined
too. Response times of reported sensors are very short and reach 2–3 s, whereas the sensitivity of the
sensor ranges from 3 to 10 for different coating thicknesses. Reported parameters make the sensor
suitable for indoor and industrial use.
Keywords: optical fiber sensors; CO2sensors; gas sensors; chemical sensors; sol-gel coatings; silica
gels; indicator dyes; absorption-based sensors
1. Introduction
1.1. An Overview of Current Carbon Dioxide Sensors Applications
Carbon dioxide is an important gas in ventilation, agriculture and many industrial branches.
The possibility of determining the concentration of CO2enables one to control various processes and
improve safety standards. The increase of CO2level is a major concern in a majority of buildings
where it may affect the living comfort and in extreme scenarios, can even cause health issues.
Agriculture and greenhouses are other fields where an appropriate level of CO2content in the
atmosphere is crucial. Miscellaneous industries also require the monitoring of CO2concentration
with fermentation plants and food storehouses being the most well-known ones. The control of
carbon dioxide level is also important in underground mining, especially in coal mines, in which
an increase of CO2concentration may be an indicator of fire or ventilation malfunction.
In the latter application safety is of major importance. Even a small damage of an electronic
device may end in a formation of spark which can lead to explosion. Therefore, all the devices
brought underground need to be validated as anti-sparking ones. Optical remote sensing seems to
be an attractive way of achieving this goal, since no electric current is required, and thus they are
intrinsically explosion safe. Hence, the optical fibres are a promising candidate for a gas monitoring
in harsh environments.
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Sensors 2015,15, 31888–31903
1.2. Commercially Available CO2Sensors
Currently, there are several techniques available on the market, which are used for CO2
detection [1–3]. The most widespread carbon dioxide sensors are based on non-dispersive infrared
(NDIR) detection and do not utilize optical fibres. It is noteworthy that while the measurement
itself is very quick, the diffusion of gas through the protecting membrane is much slower. The
installation of membrane is omitted in sensors dedicated to applications, where dust and other
contaminants are not expected to be present. Capnographs do not utilize such membranes and
thus provide a very short response time. Another type of commercially available CO2sensors are
semiconductor devices, which exhibit different resistance, capacitance or impedance at different
carbon dioxide concentrations [2–4]. They are very cheap, however, they may suffer from
cross-sensitivity to other gases and base signal shifting. Optical fibre fluorescent sensors are another
emerging technology already present on the market, which will be discussed more thoroughly in the
subsequent subsection.
1.3. Optical Fibre Gas Sensors
There are various ways of utilizing the optical fibres in various systems for chemical monitoring.
They can be used for sample illumination and signal reception or they can be sensors per se by using
special active fibre coatings, which are sensitive to the analysed substance.
The most straightforward method of chemical analysis is a measurement of the absorption. It is
especially useful for measuring small concentrations of substances e.g., CO [5,6] or CH4[7]. Optical
fibres can be used to deliver light to the measurement chamber and subsequently to the detectors.
High sensitivity of the method can be further improved by the use of dielectric mirrors in cavity
enhanced spectroscopy [8,9] The absorption of chemicals can be measured by using hollow-core
fibres, where the light is guided in the air inside the fibre [10].
The measurement of the substance concentration can also be performed by investigating the
evanescent wave absorption, e.g., by microstructured optical fibres [10], D-shaped optical fibres [11]
or Plastic Clad Silica (PCS) fibres. Optical fibres may also be deformed e.g., by tapering to achieve
this goal [12]. Such tapers can be made by using either drawing at high temperature [13] or etching
in hydrogen fluoride solution [14].
Refractive index may also be used for an analysis, since it may change in some polymers and
silica gels when in contact with a certain chemical. This fact is widely used in humidity sensing by
employing Fabry-Perot nanocavities at the tips of optical fibres [15,16]. Refractive index change can
also be employed in the coated fibre tapers [17]. The chemical may be adsorbed on the surface of
optical fibre, which facilitates the detection thereof [18]. There is also a class of compounds, which
change their volume when exposed to an analyte e.g., water [19,20].
Fluorescence found many applications in sensing with optical fibres [21]. Usually, a PCS fibre
or optical fibre taper or optical fibre tip is coated with a matrix material with an active substance
incorporated in it. The possible matrix materials are usually chosen from polymer and sol-gel groups.
Measurement of the intensity of the emission peak can be used for determination of concentration of
the analysed substance [22,23]. However, such a measurement procedure requires the detection of
a narrow band of light, which can be achieved by using optical spectrum analysers, but it is also
very expensive. Alternatively, an optical filter can be used for detection, which, however can be
costly, too. Apart from the intensity-based fluorescent sensing, decay time may also be measured in
order to determine the analyte concentration, which is often called a lifetime-based method [22–24].
Lifetime-based sensors are currently commercially available [23]. Both luminescent methods are
widely used for sensing of many parameters such as pH and the concentration of CO2or O2[25].
In a similar way, colour changing dyes can also be used for sensing purposes [23,25,26]. PCS
fibres, fibre tapers, fibre tips or other structures can be coated with a polymer or silica gel containing
an organic indicator dye [27]. The analyte reacts with the indicator inside the sensing layer, which
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changes the colour of the latter. Each form of the dye has got different absorption spectrum. Sensors
based on such solutions have much simpler construction than fluorescent-based sensors.
Another method of chemical sensing is based on the inscription of a long period fibre grating
(LPFG) in the optical fibre. LPFGs are sensitive to many parameters like temperature, strain, refractive
index of surrounding media and other factors [28,29]. Chemical sensors employing LPFGs must be
coated with an analyte-sensitive substance [28,29].
The described methods of chemical detection can be used for monitoring different gases. Not
all of them, however, are suitable for each substance. Humidity levels have been measured by
detecting a change of absorption, refractive index or reflectance in interferometers, LPFGs and other
structures [28–31].
Ammonia sensing has also been thoroughly explored. Sensors based on optical fibres usually
utilize the fact that ammonia is a basic gas and it increases the pH [32,33], but other solutions have
also been reported [17].
Fluorescence has been employed for the detection of oxygen [25,34,35]. Optical fibre O2sensors
utilize special noble metal complexes, which change their fluorescence yield when exposed to
different oxygen concentrations.
Optical fibre sensors of CO2utilize the acidity of this gas. Many publications on CO2sensors are
focused on fluorescence [25,35,36]. Few substances, like pyranine (also known as HPTS) or fluorescein
exhibit dependence between pH and fluorescence yield. Even more solutions have been reported for
optical sensors not utilizing optical fibres, however, such results can be easily reproduced in the fibre
optic field [24,37,38].
2. Proposed CO2Sensor
The aim of the authors was to develop a low-cost, fast responsive optical fibre CO2sensor.
Despite the availability of a great variety of reported fluorescent CO2sensors, the authors recognized
that those solutions are too expensive due to the necessity of filtering the wavelengths. The
absorption-based solution with indicator dye doped coating [27] seems to allow one to conduct gas
sensing in a much simpler way. On the other hand, the reported sensors [27] suffer from long response
times, a low sensitivity to CO2and a high cross-sensitivity to humidity.
Indicator substances usually operate only if they are dissolved in a matrix material. The
most frequently encountered ones are polymers and silica gels. Manifold polymers can be used
for this purpose [19,24,29,36,37,39–41]. Silica gels are made during a controlled hydrolysis of
alkoxysilanes and their derivatives. The most frequently used substrates for silica gel preparation
are tetraethoxysilane [42,43], triethoxymethylsilane [27], triethoxy-n-octylsilane [44] and other
derivatives. A comparison of both polymers and silica gels is presented in Table 1. Due to a high
porosity, a possibility of adjusting the parameters of the coating and other advantages, the authors
have chosen silica gels for the preparation of gas sensors. Other, inorganic matrix materials like ZnO
have also been reported [45], however, they provide less sensing possibilities.
In the reported solution, a fragment of PCS optical fibre acrylic coating is removed and then
it is recoated with silica gel containing pH sensitive dye. Indicator dyes, which can be used for
this purpose include e.g., methyl red, phenol red, phenolphthalein, thymol blue. These substances
change colour when exposed to environments with different pH levels. For example, methyl red
changes from yellow to red when pH decreases [46], phenol red changes from fuchsia to yellow [47]
and thymol blue changes from blue to yellow [48]. The ranges at which a colour changes occur are
quite narrow: 4.8–6.0 for methyl red, 6.4–8.0 for phenol red and 8.0–9.6 for thymol blue [49]. The silica
gel layer after solvent evaporation becomes porous, which facilitates the interaction between the dyes
and carbon dioxide. Some indicators, like thymol blue, do not work properly in a form of powder
or after annealing in silica gel [27]. The interaction between single molecules is responsible for this
phenomenon. Molecules need to be separated to be acceptors or donors of a proton.
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Table 1. Comparison of the most important properties of polymers and silica gels.
Polymers Silica Gels
Porosity varied: low to moderate high
Transparency varied: moderate to high varied: low to high
Surface quality smooth pores
Mechanical behaviour elastic varied: moderately elastic to brittle
Cost of single deposition process low low
Cost of series coating process low moderate (periodic gelling process)
Easiness of deposition easy easy
Easiness of the solution preparation easy moderately difficult
Coatings with different thickness possible to deposit possible to deposit
Solubility of organic dyes limited to those soluble in a
solvent dedicated to the polymer
limited to those soluble
in water and alcohols
Leakage of organic dyes possible unless copolymerized or
immobilized otherwise possible unless immobilized
Carbon dioxide can react with water according to the following equations:
CO2`2H2OÑHCO-
3`H3O+(1)
HCO-
3`H2OÑCO2-
3`H3O+(2)
The formed hydronium ion may subsequently react with the indicator dye, which finally leads
to a colour change. The presented mechanism requires water to be present at the reaction side.
Therefore, sensors utilizing such principle of operation should not be annealed.
3. Experimental Section
Carbon dioxide sensors were prepared according to the procedure described below. A 12 cm
fragment of a plastic clad silica optical fibre was stripped of its acrylic coating by immersing it for
60 s in dichloromethane and subsequent manual taking off the softened polymer. Afterwards we have
coated the fibre with silica gel doped with indicator dye and then we have left it for curing for 24 h.
Silica gels were prepared from triethoxymethylsilane (TEMS) according to the procedure described
in [27]. All the substrates, solvent and a liquid detergent were placed in a plastic vial. The reaction
mixture was stirred for 6 min until a transparent liquid has been obtained. Then a stripped fragment
of PCS fibre has been immersed in the prepared solution. It is possible to control the thickness of the
silica gel layer by managing the fundamental solution parameters [50]. Nevertheless, since available
thickness values for single dip coating process are low, in this work thicker layers have been obtained
by repeating the immersion process several times. Dyes used for the sensors preparation included
thymol blue, phenol red, methyl red and bromothymol blue. The analysed dyes were chosen due to
their pH change range close to neutral, which corresponds to changes induced by CO2gas, which
decreases the pH of water from neutral to weakly acidic. After the curing time, the fibre was attached
to a PMMA slide. One end of the fibre was connected to the light source and the other one was put
inside a detector. For temperature tests a Peltier module was placed under the slide to control the
temperature of the sensor. All the other tests were performed at a constant temperature.
The light source used for phenol red and methyl red samples was a 520 nm pigtailed laser
(Thorlabs, Newton, NJ, USA). Thymol blue and bromothymol blue sensors were illuminated with
650 nm laser, which was FIS visual fault locator. The mentioned wavelengths were chosen due to
big differences between acidic and basic spectra of the dyes. Every pH sensitive dye has at least two
forms specific for certain type of environments (e.g., acidic and basic). Each form of the dye has a
different absorption spectrum. For certain substances it is possible to choose the wavelength range,
within which, the difference of absorption is high. This is evident in the analysed dyes [46–49]. That
provides a possibility of working at a single wavelength instead of working with a broader spectrum
of light. One just needs to choose the wavelength at which there is a big difference between the spectra
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of different forms of a dye. A GL55 series photoresistor (Senba Optoelectronic, Shenzhen, China) was
used for the detection of light.
The composition of an atmosphere inside the chamber was regulated by dosing pure CO2from a
gas pressure bottle. For reducing the concentration of CO2, the chamber was purged with a fresh air.
As a result, the concentration of carbon dioxide decreases slowly, which makes it possible to easily
record the sensor readings at different concentrations. Response time measurements were performed
by a rapid filling the chamber with maximum CO2gas flow and then a subsequent rapid purging
by compressed air. The actual concentration of carbon dioxide inside the chamber was monitored by
two commercial CO2sensors based on NDIR and electrochemical principles of operation. Humidity
was monitored by an electronic meter. Temperature was measured with a pyrometer to ensure that
the actual temperature of the sensor was being reported.
The measurements were carried out in a dedicated gas chamber, which is depicted in Figure 1.
Sensors 2015, 15, page–page
5
of different forms of a dye. A GL55 series photoresistor (Senba Optoelectronic, Shenzhen, China) was
used for the detection of light.
The composition of an atmosphere inside the chamber was regulated by dosing pure CO
2
from
a gas pressure bottle. For reducing the concentration of CO
2
, the chamber was purged with a fresh
air. As a result, the concentration of carbon dioxide decreases slowly, which makes it possible to
easily record the sensor readings at different concentrations. Response time measurements were
performed by a rapid filling the chamber with maximum CO
2
gas flow and then a subsequent rapid
purging by compressed air. The actual concentration of carbon dioxide inside the chamber was
monitored by two commercial CO
2
sensors based on NDIR and electrochemical principles of
operation. Humidity was monitored by an electronic meter. Temperature was measured with a
pyrometer to ensure that the actual temperature of the sensor was being reported.
The measurements were carried out in a dedicated gas chamber, which is depicted in Figure 1.
Figure 1. Measurement set up used for CO
2
sensing. The red fragment represents the optical fibre sensor.
4. Results and Discussion
4.1. Analysed Indicator Dyes
Several substances acting as CO
2
indicators have been examined. Out of the four tested
dyes—thymol blue, phenol red, methyl red and bromothymol blue—except for bromothymol blue
three were responsive to carbon dioxide concentration changes in air. Bromothymol blue has already
been reported as an indicator in pH and gas sensors [32,42], and the lack of activity reported by the
authors can be attributed either to the interaction between the dye and a sol gel basic catalyst or to
the change of pH sensitivity range due to immobilization of the dye. All the other dyes yielded CO
2
responsive sensors. Sensors incorporating thymol blue and phenol red have already been reported in
the literature, but their sensitivities were much lower [27]. An optical fibre CO
2
sensor utilizing
methyl red has not been reported yet. Making such a sensor was even claimed to be impossible due
to a too low pH colour change range of this dye [27]. However, due to the decomposition of alkyl-
substituted ammonia catalyst upon drying, which can take several days, the pH of the sensing layer
decreases. Therefore, a sensor incorporating thymol blue, which initially has a blue active layer, turns
green and subsequently turns yellow. This colour change is also associated with the decrease of the
sensitivity to CO
2
. Eventually, after 5–10 days when the catalyst decomposes inside the active layer,
the sensor ceases its operation and is no longer working. The authors have encountered a similar
issue with phenol red. Due to the lower range of pH inducing colour change it retained little of its
sensitivity after a few weeks. The opposite problem was observed for methyl red. This dye has a
pH range of colour change within the weakly acidic region. Therefore, initially it was not responsive
to carbon dioxide, but after decomposition of the catalyst, pH of the sensing layer decreased and then
it was able to detect CO
2
within a full range of concentrations. This is why it is possible to prepare
CO
2
sensor with methyl red dye. This is also the first time, when the stability issue for an absorption
based carbon dioxide optical fibre sensor is reported and a method of circumventing thereof
is proposed.
Figure 1. Measurement set up used for CO2sensing. The red fragment represents the optical
fibre sensor.
4. Results and Discussion
4.1. Analysed Indicator Dyes
Several substances acting as CO2indicators have been examined. Out of the four tested
dyes—thymol blue, phenol red, methyl red and bromothymol blue—except for bromothymol blue
three were responsive to carbon dioxide concentration changes in air. Bromothymol blue has already
been reported as an indicator in pH and gas sensors [32,42], and the lack of activity reported by the
authors can be attributed either to the interaction between the dye and a sol gel basic catalyst or to
the change of pH sensitivity range due to immobilization of the dye. All the other dyes yielded CO2
responsive sensors. Sensors incorporating thymol blue and phenol red have already been reported
in the literature, but their sensitivities were much lower [27]. An optical fibre CO2sensor utilizing
methyl red has not been reported yet. Making such a sensor was even claimed to be impossible
due to a too low pH colour change range of this dye [27]. However, due to the decomposition of
alkyl-substituted ammonia catalyst upon drying, which can take several days, the pH of the sensing
layer decreases. Therefore, a sensor incorporating thymol blue, which initially has a blue active layer,
turns green and subsequently turns yellow. This colour change is also associated with the decrease
of the sensitivity to CO2. Eventually, after 5–10 days when the catalyst decomposes inside the active
layer, the sensor ceases its operation and is no longer working. The authors have encountered a
similar issue with phenol red. Due to the lower range of pH inducing colour change it retained little
of its sensitivity after a few weeks. The opposite problem was observed for methyl red. This dye has a
pH range of colour change within the weakly acidic region. Therefore, initially it was not responsive
to carbon dioxide, but after decomposition of the catalyst, pH of the sensing layer decreased and then
it was able to detect CO2within a full range of concentrations. This is why it is possible to prepare CO2
sensor with methyl red dye. This is also the first time, when the stability issue for an absorption based
carbon dioxide optical fibre sensor is reported and a method of circumventing thereof is proposed.
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Sensors 2015, 15, page–page
6
Figure 2. Dependence between the intensity of light transmitted through an optical fibre sensor
containing thymol blue as an indicator dye and the CO
2
concentration.
Figure 2 shows the dependence between the CO
2
concentration and the intensity of the 650 nm
laser light transmitted by the optical fibre sensor comprising thymol blue. The series of experiments
have been performed 24 h after preparation of the sensor. Response of the sensor is logarithmic as a
function of carbon dioxide concentration. Dependence between the intensity of transmitted light and
CO
2
concentration can be described by an equation: I = 2.48ln[CO
2
] + 7.9, where [CO
2
] represents the
percentage of CO
2
in air. R
2
value is equal to 0.99. Character of the dependence between CO
2
concentration and the transmitted light intensity is a result of a way, in which colour of the dye
changes. As the pH of the layer gradually changes, the absorption spectrum of the layer also changes
gradually. The ratio of the intensity of light transmitted at 100% CO
2
concentration and the intensity
of light transmitted at 0.1% CO
2
concentration equals 8.3, which exceeds the value of 1.7 previously
reported for this dye [27]. An increase of the intensity of transmitted light occurs along with a colour
change of an active layer from dark blue to bright yellow, which is consistent with a literature
spectroscopic data [48]. The response characteristics of the sensor have been deteriorating after
performing the experiments. After approximately two weeks it did not respond to CO
2
any more.
The other examined sensor was based on phenol red dye. In the beginning it worked within a
full range of CO
2
concentration. The response characteristics tended to deteriorate after a few days.
The results obtained two weeks after sensor preparation are depicted in Figure 3. The sensor was
illuminated with a 520 nm laser light.
The response of the sensor is linear within a low CO
2
concentration range. The dependence
between CO
2
concentration and the intensity of light transmitted by the fibre is equal to:
I = 1.50[CO
2
] + 2.03, where [CO
2
] represents the percentage of carbon dioxide. R
2
value is equal to
0.97. Sensor detects the gas only until the concentration reaches 0.65%. The ratio of maximum and
minimum signal intensity equals 1.36, which is much less than for thymol blue. However, if only a
0.65% limit is taken into account, than the signal ratio is equal to 3.1 for thymol blue, which makes
the difference smaller. During the operation of the sensor, the active layer changed its colour from
red to pale yellow, which stands in accordance with spectroscopic data available in the literature [47].
The limit of 0.65% of CO
2
concentration is a result of a relatively narrow pH range for which the
colour of phenol red changes. As the basic catalyst decomposes, the pH of an active layer decreases,
thus the available CO
2
detection range narrows down. Taking into account the given analytical
expression for the response of phenol red to carbon dioxide and the experimental uncertainty
(±0.05 a.u.) one can expect that the lower limit of detection should not be higher than 0.03% of CO
2
,
which is suitable for indoor use.
Figure 2. Dependence between the intensity of light transmitted through an optical fibre sensor
containing thymol blue as an indicator dye and the CO2concentration.
Figure 2shows the dependence between the CO2concentration and the intensity of the 650 nm
laser light transmitted by the optical fibre sensor comprising thymol blue. The series of experiments
have been performed 24 h after preparation of the sensor. Response of the sensor is logarithmic as a
function of carbon dioxide concentration. Dependence between the intensity of transmitted light and
CO2concentration can be described by an equation: I= 2.48ln[CO2] + 7.9, where [CO2] represents
the percentage of CO2in air. R2value is equal to 0.99. Character of the dependence between CO2
concentration and the transmitted light intensity is a result of a way, in which colour of the dye
changes. As the pH of the layer gradually changes, the absorption spectrum of the layer also changes
gradually. The ratio of the intensity of light transmitted at 100% CO2concentration and the intensity
of light transmitted at 0.1% CO2concentration equals 8.3, which exceeds the value of 1.7 previously
reported for this dye [27]. An increase of the intensity of transmitted light occurs along with a
colour change of an active layer from dark blue to bright yellow, which is consistent with a literature
spectroscopic data [48]. The response characteristics of the sensor have been deteriorating after
performing the experiments. After approximately two weeks it did not respond to CO2any more.
The other examined sensor was based on phenol red dye. In the beginning it worked within a
full range of CO2concentration. The response characteristics tended to deteriorate after a few days.
The results obtained two weeks after sensor preparation are depicted in Figure 3. The sensor was
illuminated with a 520 nm laser light.
The response of the sensor is linear within a low CO2concentration range. The dependence
between CO2concentration and the intensity of light transmitted by the fibre is equal to:
I= 1.50[CO2] + 2.03, where [CO2] represents the percentage of carbon dioxide. R2value is equal to
0.97. Sensor detects the gas only until the concentration reaches 0.65%. The ratio of maximum and
minimum signal intensity equals 1.36, which is much less than for thymol blue. However, if only a
0.65% limit is taken into account, than the signal ratio is equal to 3.1 for thymol blue, which makes the
difference smaller. During the operation of the sensor, the active layer changed its colour from red to
pale yellow, which stands in accordance with spectroscopic data available in the literature [47]. The
limit of 0.65% of CO2concentration is a result of a relatively narrow pH range for which the colour of
phenol red changes. As the basic catalyst decomposes, the pH of an active layer decreases, thus the
available CO2detection range narrows down. Taking into account the given analytical expression for
the response of phenol red to carbon dioxide and the experimental uncertainty (˘0.05 a.u.) one can
expect that the lower limit of detection should not be higher than 0.03% of CO2, which is suitable for
indoor use.
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Sensors 2015, 15, page–page
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Figure 3. Dependence between the intensity of light transmitted through an optical fibre sensor
containing phenol red as an indicator dye and the CO2 concentration.
The sensor containing methyl red has been operating in a stable fashion for a second week after
preparation. It was tested with a laser light source working at 520 nm. Figure 4 shows the response
characteristics of the sensor.
Signal of this sensor decreased when CO2 concentration increased, which is an opposite situation
in comparison to the previous sensors. Dependence between light intensity and concentration is in
this case logarithmic, similarly to the thymol blue sensor. It can be described by an equation:
I = −0.63ln[CO2] + 4.58. R2 value is equal to 0.96. The quotient of the maximum and minimum intensity
of the signal is equal to 10. When sensor is exposed to CO2, the layer changes its colour from yellow
to red, which is consistent with the literature data [46]. The lowest CO2 concentration, which was
tested was 0.08%, but concentrations down to 0.01%–0.02% should also be possible to measure, since
it was reported for similar solutions [27].
Figure 4. Dependence between the intensity of light transmitted through an optical fibre sensor
containing methyl red as an indicator dye and the CO2 concentration.
4.2. Influence of the Dye Concentration
Since the light goes through the active layer deposited on an optical fibre, the concentration of
an indicator dye is crucial to the response characteristics of the sensor. It is reasonable, that the more
dye is in the sensor, the stronger response should be reported. However, other factors may also affect
the output signal of a sensor, which is presented in Figure 5. Figure 5 and all the other plots presented
henceforth show the results obtained for methyl red-based sensors.
Sensitivity in Figure 5 refers to a ratio of the transmitted signal intensity at 0.1% of CO2 to the
signal intensity at 100% of CO2. This term is present in subsequent figures and is defined in the
same way.
Figure 3. Dependence between the intensity of light transmitted through an optical fibre sensor
containing phenol red as an indicator dye and the CO2concentration.
The sensor containing methyl red has been operating in a stable fashion for a second week after
preparation. It was tested with a laser light source working at 520 nm. Figure 4shows the response
characteristics of the sensor.
Signal of this sensor decreased when CO2concentration increased, which is an opposite situation
in comparison to the previous sensors. Dependence between light intensity and concentration is
in this case logarithmic, similarly to the thymol blue sensor. It can be described by an equation:
I=´0.63ln[CO2] + 4.58. R2value is equal to 0.96. The quotient of the maximum and minimum
intensity of the signal is equal to 10. When sensor is exposed to CO2, the layer changes its colour
from yellow to red, which is consistent with the literature data [46]. The lowest CO2concentration,
which was tested was 0.08%, but concentrations down to 0.01%–0.02% should also be possible to
measure, since it was reported for similar solutions [27].
Sensors 2015, 15, page–page
7
Figure 3. Dependence between the intensity of light transmitted through an optical fibre sensor
containing phenol red as an indicator dye and the CO2 concentration.
The sensor containing methyl red has been operating in a stable fashion for a second week after
preparation. It was tested with a laser light source working at 520 nm. Figure 4 shows the response
characteristics of the sensor.
Signal of this sensor decreased when CO2 concentration increased, which is an opposite situation
in comparison to the previous sensors. Dependence between light intensity and concentration is in
this case logarithmic, similarly to the thymol blue sensor. It can be described by an equation:
I = −0.63ln[CO2] + 4.58. R2 value is equal to 0.96. The quotient of the maximum and minimum intensity
of the signal is equal to 10. When sensor is exposed to CO2, the layer changes its colour from yellow
to red, which is consistent with the literature data [46]. The lowest CO2 concentration, which was
tested was 0.08%, but concentrations down to 0.01%–0.02% should also be possible to measure, since
it was reported for similar solutions [27].
Figure 4. Dependence between the intensity of light transmitted through an optical fibre sensor
containing methyl red as an indicator dye and the CO2 concentration.
4.2. Influence of the Dye Concentration
Since the light goes through the active layer deposited on an optical fibre, the concentration of
an indicator dye is crucial to the response characteristics of the sensor. It is reasonable, that the more
dye is in the sensor, the stronger response should be reported. However, other factors may also affect
the output signal of a sensor, which is presented in Figure 5. Figure 5 and all the other plots presented
henceforth show the results obtained for methyl red-based sensors.
Sensitivity in Figure 5 refers to a ratio of the transmitted signal intensity at 0.1% of CO2 to the
signal intensity at 100% of CO2. This term is present in subsequent figures and is defined in the
same way.
Figure 4. Dependence between the intensity of light transmitted through an optical fibre sensor
containing methyl red as an indicator dye and the CO2concentration.
4.2. Influence of the Dye Concentration
Since the light goes through the active layer deposited on an optical fibre, the concentration of
an indicator dye is crucial to the response characteristics of the sensor. It is reasonable, that the more
dye is in the sensor, the stronger response should be reported. However, other factors may also affect
the output signal of a sensor, which is presented in Figure 5. Figure 5and all the other plots presented
henceforth show the results obtained for methyl red-based sensors.
Sensitivity in Figure 5refers to a ratio of the transmitted signal intensity at 0.1% of CO2to the
signal intensity at 100% of CO2. This term is present in subsequent figures and is defined in the
same way.
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One can notice that initially an increase of the methyl red concentration improves the sensor
sensitivity. As the concentration rises, the dependence becomes less inclined and eventually goes
down. This phenomenon can be explained by a limited solubility of methyl red in silica gel. The
response characteristics get to the point in the dye concentration domain, when no further response
increase is possible. If one would increase the amount of the dye in the active layer, the excess
of this substance would form a very fine powder during the drying. This was observed as a red,
opaque appearance of an active layer on optical fibre in contrast to yellow, transparent coatings for
low concentrations. What is more, dye in a powdered form may behave in a different way than
dissolved powder. As one can see in Figure 5, after reaching the maximum, the dependence not only
decreases, but even goes below sensor response value equal to 1. This means that after reaching
this point, the sensor works the opposite way i.e., during the increase of the CO2concentration, the
intensity of transmitted light increases. The most reasonable explanation for this behaviour is that an
acidic form of methyl red is more soluble in silica gel matrix than its basic form. Such a phenomenon
may be peculiar to methyl red, since other dyes simply reach the maximum response at a certain
concentration [27].
Sensors 2015, 15, page–page
8
One can notice that initially an increase of the methyl red concentration improves the sensor
sensitivity. As the concentration rises, the dependence becomes less inclined and eventually goes
down. This phenomenon can be explained by a limited solubility of methyl red in silica gel. The
response characteristics get to the point in the dye concentration domain, when no further response
increase is possible. If one would increase the amount of the dye in the active layer, the excess of this
substance would form a very fine powder during the drying. This was observed as a red, opaque
appearance of an active layer on optical fibre in contrast to yellow, transparent coatings for low
concentrations. What is more, dye in a powdered form may behave in a different way than dissolved
powder. As one can see in Figure 5, after reaching the maximum, the dependence not only decreases,
but even goes below sensor response value equal to 1. This means that after reaching this point, the
sensor works the opposite way i.e., during the increase of the CO
2 concentration, the intensity of
transmitted light increases. The most reasonable explanation for this behaviour is that an acidic form
of methyl red is more soluble in silica gel matrix than its basic form. Such a phenomenon may be
peculiar to methyl red, since other dyes simply reach the maximum response at a certain
concentration [27].
Figure 5. Dependence between the sensitivity and the concentration of methyl red in the sensing layer.
Each sample has the same geometrical parameters and the same layer thickness.
4.3. Influence of the Active Layer Thickness on the Response of the Sensor
One can expect that similarly to the concentration, the thickness of an active layer should also
play an important role in the sensor operation. It is possible to check the influence of thickness on the
sensor signal by preparing samples with different thickness of a sensing layer. This can be achieved
by deposition of different number of layers on an optical fibre. The results of such an experiment are
depicted in Figure 6. The plot shows that the response of the sensor (defined the same way like in the
previous subsection) rises when thickness of a sensing layer rises.
Figure 6. The influence of a sensing layer thickness on the sensitivity. Measurements have been
performed on scanning electron microscope.
Figure 5. Dependence between the sensitivity and the concentration of methyl red in the sensing layer.
Each sample has the same geometrical parameters and the same layer thickness.
4.3. Influence of the Active Layer Thickness on the Response of the Sensor
One can expect that similarly to the concentration, the thickness of an active layer should also
play an important role in the sensor operation. It is possible to check the influence of thickness on the
sensor signal by preparing samples with different thickness of a sensing layer. This can be achieved
by deposition of different number of layers on an optical fibre. The results of such an experiment are
depicted in Figure 6. The plot shows that the response of the sensor (defined the same way like in the
previous subsection) rises when thickness of a sensing layer rises.
Sensors 2015, 15, page–page
8
One can notice that initially an increase of the methyl red concentration improves the sensor
sensitivity. As the concentration rises, the dependence becomes less inclined and eventually goes
down. This phenomenon can be explained by a limited solubility of methyl red in silica gel. The
response characteristics get to the point in the dye concentration domain, when no further response
increase is possible. If one would increase the amount of the dye in the active layer, the excess of this
substance would form a very fine powder during the drying. This was observed as a red, opaque
appearance of an active layer on optical fibre in contrast to yellow, transparent coatings for low
concentrations. What is more, dye in a powdered form may behave in a different way than dissolved
powder. As one can see in Figure 5, after reaching the maximum, the dependence not only decreases,
but even goes below sensor response value equal to 1. This means that after reaching this point, the
sensor works the opposite way i.e., during the increase of the CO
2 concentration, the intensity of
transmitted light increases. The most reasonable explanation for this behaviour is that an acidic form
of methyl red is more soluble in silica gel matrix than its basic form. Such a phenomenon may be
peculiar to methyl red, since other dyes simply reach the maximum response at a certain
concentration [27].
Figure 5. Dependence between the sensitivity and the concentration of methyl red in the sensing layer.
Each sample has the same geometrical parameters and the same layer thickness.
4.3. Influence of the Active Layer Thickness on the Response of the Sensor
One can expect that similarly to the concentration, the thickness of an active layer should also
play an important role in the sensor operation. It is possible to check the influence of thickness on the
sensor signal by preparing samples with different thickness of a sensing layer. This can be achieved
by deposition of different number of layers on an optical fibre. The results of such an experiment are
depicted in Figure 6. The plot shows that the response of the sensor (defined the same way like in the
previous subsection) rises when thickness of a sensing layer rises.
Figure 6. The influence of a sensing layer thickness on the sensitivity. Measurements have been
performed on scanning electron microscope.
Figure 6. The influence of a sensing layer thickness on the sensitivity. Measurements have been
performed on scanning electron microscope.
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4.4. Repeatability of the Sensor Response
Sensor aptitude for work should be tested not only during a short test, but also during a longer
experiment. Figure 7presents a plot of the intensity of light transmitted through the optical fibre
sensor during the experiment. The sensor was consecutively exposed to 100% CO2and fresh air. This
results in a number of dips in a sensor operation time plot.
Sensors 2015, 15, page–page
9
4.4. Repeatability of the Sensor Response
Sensor aptitude for work should be tested not only during a short test, but also during a longer
experiment. Figure 7 presents a plot of the intensity of light transmitted through the optical fibre
sensor during the experiment. The sensor was consecutively exposed to 100% CO2 and fresh air. This
results in a number of dips in a sensor operation time plot.
Figure 7. Intensity of the transmitted light for a sensor consecutively exposed to CO2 and fresh air
with CO2 content below 0.1%.
One can observe that the sensor works in a stable, repeatable way. The sequential CO2 exposures
do not affect the base level of the intensity neither for low nor for high CO2 concentrations.
Fluctuations present in Figure 7 are a result of a non-uniform gas distribution during those
fast-paced tests.
4.5. Response Time of the Sensor
The most important parameter of the sensor, which affects the response time, is the thickness of
the sensing layer. The thicker the layer, the more time is needed for CO2 to diffuse through it. Figure
8 presents response time plots for four different layer thicknesses.
(a) (b)
(c) (d)
Figure 8. Response time characteristics for four different sensing layer thicknesses, 0.75 μm, 3 μm,
3.5 μm and 9 μm ((a), (b), (c), (d) respectively).
Figure 7. Intensity of the transmitted light for a sensor consecutively exposed to CO2and fresh air
with CO2content below 0.1%.
One can observe that the sensor works in a stable, repeatable way. The sequential CO2
exposures do not affect the base level of the intensity neither for low nor for high CO2concentrations.
Fluctuations present in Figure 7are a result of a non-uniform gas distribution during those
fast-paced tests.
4.5. Response Time of the Sensor
The most important parameter of the sensor, which affects the response time, is the thickness of
the sensing layer. The thicker the layer, the more time is needed for CO2to diffuse through it. Figure 8
presents response time plots for four different layer thicknesses.
Sensors 2015, 15, page–page
9
4.4. Repeatability of the Sensor Response
Sensor aptitude for work should be tested not only during a short test, but also during a longer
experiment. Figure 7 presents a plot of the intensity of light transmitted through the optical fibre
sensor during the experiment. The sensor was consecutively exposed to 100% CO2 and fresh air. This
results in a number of dips in a sensor operation time plot.
Figure 7. Intensity of the transmitted light for a sensor consecutively exposed to CO2 and fresh air
with CO2 content below 0.1%.
One can observe that the sensor works in a stable, repeatable way. The sequential CO2 exposures
do not affect the base level of the intensity neither for low nor for high CO2 concentrations.
Fluctuations present in Figure 7 are a result of a non-uniform gas distribution during those
fast-paced tests.
4.5. Response Time of the Sensor
The most important parameter of the sensor, which affects the response time, is the thickness of
the sensing layer. The thicker the layer, the more time is needed for CO2 to diffuse through it. Figure
8 presents response time plots for four different layer thicknesses.
(a) (b)
(c) (d)
Figure 8. Response time characteristics for four different sensing layer thicknesses, 0.75 μm, 3 μm,
3.5 μm and 9 μm ((a), (b), (c), (d) respectively).
Figure 8. Response time characteristics for four different sensing layer thicknesses, 0.75 µm, 3 µm,
3.5 µm and 9 µm ((a), (b), (c), (d) respectively).
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The plots presented in Figure 8follow the rule mentioned above. What is more, one can notice
that the time needed for the sensor's response to 100% CO2is much lower than the time needed to
get back to the base reading after the end of the CO2pulse. The difference increases significantly
when the thickness increases. The response time for the exposure to CO2changes slightly over the
whole range of thickness. Figure 9shows the dependence between the response time and the sensing
layer thickness.
The lowest reported response times are very short. When switching from low to high carbon
dioxide concentration, the response time is equal to 2 s for 0.75 µm thick silica layer. For the same
thickness, the response time during switching from high to low CO2concentration is equal to 3 s.
These values are much lower than for other similar sensors reported in the literature [27,30]. Such
short response times make it possible to use such sensor for on-line CO2monitoring. Apart from the
low response times, thin layers exhibit satisfactory sensitivity (see Figure 6) equal to 3. Therefore thin
layers are expected to be an optimum choice for versatile applications.
Sensors 2015, 15, page–page
10
The plots presented in Figure 8 follow the rule mentioned above. What is more, one can notice
that the time needed for the sensor's response to 100% CO2 is much lower than the time needed to get
back to the base reading after the end of the CO2 pulse. The difference increases significantly when
the thickness increases. The response time for the exposure to CO2 changes slightly over the whole
range of thickness. Figure 9 shows the dependence between the response time and the sensing layer
thickness.
The lowest reported response times are very short. When switching from low to high carbon
dioxide concentration, the response time is equal to 2 s for 0.75 μm thick silica layer. For the same
thickness, the response time during switching from high to low CO2 concentration is equal to 3 s.
These values are much lower than for other similar sensors reported in the literature [27,30]. Such
short response times make it possible to use such sensor for on-line CO2 monitoring. Apart from the
low response times, thin layers exhibit satisfactory sensitivity (see Figure 6) equal to 3. Therefore thin
layers are expected to be an optimum choice for versatile applications.
(a) (b)
Figure 9. Dependence between the sensing layer thickness and (a) response time during exposure to
100% CO2 (b) response time during exposure to air after CO2 impulse.
4.6. Influence of Temperature
Temperature is a factor which can substantially change during a measurement. Therefore, its
impact on the sensor's response should be thoroughly inspected. The temperature of the sensor was
controlled by Peltier module and it has been measured by a pyrometer at the centre of the optical
fibre sensor. The intensity of light transmitted through the optical fibre increases when the
temperature increases as it is shown in the Figure 10.
The dependence between the intensity of transmitted light and the temperature is weak below
25 °C. Above this temperature, the intensity rises significantly until reaching 55 °C–60 °C range. The
intensity at the highest point (55 °C in Figure 10) is 3.85 times higher than at 20 °C. Therefore it is
important to measure or control simultaneously the temperature while using such optical
fibre sensor.
Figure 10. Dependence between the light intensity transmitted by an optical fibre sensor containing
methyl red and the temperature. Measurement performed at 0.1% of CO2 in air.
Figure 9. Dependence between the sensing layer thickness and (a) response time during exposure to
100% CO2(b) response time during exposure to air after CO2impulse.
4.6. Influence of Temperature
Temperature is a factor which can substantially change during a measurement. Therefore, its
impact on the sensor's response should be thoroughly inspected. The temperature of the sensor was
controlled by Peltier module and it has been measured by a pyrometer at the centre of the optical fibre
sensor. The intensity of light transmitted through the optical fibre increases when the temperature
increases as it is shown in the Figure 10.
The dependence between the intensity of transmitted light and the temperature is weak below
25 ˝C. Above this temperature, the intensity rises significantly until reaching 55 ˝C–60 ˝C range.
The intensity at the highest point (55 ˝C in Figure 10) is 3.85 times higher than at 20 ˝C. Therefore it is
important to measure or control simultaneously the temperature while using such optical fibre sensor.
Sensors 2015, 15, page–page
10
The plots presented in Figure 8 follow the rule mentioned above. What is more, one can notice
that the time needed for the sensor's response to 100% CO2 is much lower than the time needed to get
back to the base reading after the end of the CO2 pulse. The difference increases significantly when
the thickness increases. The response time for the exposure to CO2 changes slightly over the whole
range of thickness. Figure 9 shows the dependence between the response time and the sensing layer
thickness.
The lowest reported response times are very short. When switching from low to high carbon
dioxide concentration, the response time is equal to 2 s for 0.75 μm thick silica layer. For the same
thickness, the response time during switching from high to low CO2 concentration is equal to 3 s.
These values are much lower than for other similar sensors reported in the literature [27,30]. Such
short response times make it possible to use such sensor for on-line CO2 monitoring. Apart from the
low response times, thin layers exhibit satisfactory sensitivity (see Figure 6) equal to 3. Therefore thin
layers are expected to be an optimum choice for versatile applications.
(a) (b)
Figure 9. Dependence between the sensing layer thickness and (a) response time during exposure to
100% CO2 (b) response time during exposure to air after CO2 impulse.
4.6. Influence of Temperature
Temperature is a factor which can substantially change during a measurement. Therefore, its
impact on the sensor's response should be thoroughly inspected. The temperature of the sensor was
controlled by Peltier module and it has been measured by a pyrometer at the centre of the optical
fibre sensor. The intensity of light transmitted through the optical fibre increases when the
temperature increases as it is shown in the Figure 10.
The dependence between the intensity of transmitted light and the temperature is weak below
25 °C. Above this temperature, the intensity rises significantly until reaching 55 °C–60 °C range. The
intensity at the highest point (55 °C in Figure 10) is 3.85 times higher than at 20 °C. Therefore it is
important to measure or control simultaneously the temperature while using such optical
fibre sensor.
Figure 10. Dependence between the light intensity transmitted by an optical fibre sensor containing
methyl red and the temperature. Measurement performed at 0.1% of CO2 in air.
Figure 10. Dependence between the light intensity transmitted by an optical fibre sensor containing
methyl red and the temperature. Measurement performed at 0.1% of CO2in air.
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The phenomenon described above can be a result of an increased solubility of the indicator dye.
Another factor which could also influence the operation of the sensor could be an increased affinity to
CO2of the dye or silica gel.The alternative explanation for the sensor’s behaviour could be a change
of the active layer refractive index. A decrease of the refractive index would increase the amount of
light in the silica core.
A series of thermal experiments has been performed in order to determine the temperature at
which the drop in transmission occurs. It has been noted that the intensity increases until 55 ˝C. Then,
between 55 ˝C and 60 ˝C it does not rise any more. At 60 ˘2˝C it drops abruptly and decreases by
58%. Further increase of the temperature makes the transmission even weaker (see Figure 10). The
decrease of the intensity is linear in time, which is presented in Figure 11.
Sensors 2015, 15, page–page
11
The phenomenon described above can be a result of an increased solubility of the indicator dye.
Another factor which could also influence the operation of the sensor could be an increased affinity
to CO2 of the dye or silica gel.The alternative explanation for the sensor’s behaviour could be a change
of the active layer refractive index. A decrease of the refractive index would increase the amount of
light in the silica core.
A series of thermal experiments has been performed in order to determine the temperature at
which the drop in transmission occurs. It has been noted that the intensity increases until 55 °C. Then,
between 55 °C and 60 °C it does not rise any more. At 60 ± 2 °C it drops abruptly and decreases by
58%. Further increase of the temperature makes the transmission even weaker (see Figure 10). The
decrease of the intensity is linear in time, which is presented in Figure 11.
Figure 11. Dependence between the intensity of light transmitted through the sensor and the time
after reaching 60 °C. Measurement performed at 0.1% of CO2 in air.
It is important to highlight that before reaching the temperature of 60 °C, the sensor was
operational. After reaching 60 °C it did not respond to CO2 any more. The time needed for the whole
change is approximately equal to 100 s at 0.1% of CO2. The reason for such an abrupt transition is
probably related to the nature of methyl red indicator dye, since such behaviour has not been
reported for other silica gel-based optical fibre sensors [35]. Therefore, the most probable explanation
for the analysed phenomenon is that methyl red in silica gel at 60 °C losses its ability to bind hydrogen
ion. This can be caused either by a decrease of humidity in the silica gel and subsequent precipitation
of the dye or by molecular properties of methyl red. The decay time of 100 s is probably needed to
achieve the threshold temperature for the whole optical fibre sensing region.
The described process was repeatable. However, the optical fibre regained its ability to sense
carbon dioxide after cooling to temperatures lower than 60 °C. For instance, the sensor resumed
operation after cooling to 43 °C, which occurred approximately 4 min after the start of cooling from
60 °C. This speaks in favour of the humidity-based hypothesis described in the previous paragraph.
It is important to notice that the drop of the transmission is non-differentiable as a function of
temperature. This could indicate that a high order phase transition may occur, since the imaginary
part of the dielectric constant (responsible for absorption) is non-differentiable.
4.7. Cross-Sensitivity to Other Gases
Equations (1) and (2) indicate that the processes employing CO2 inside the sensing layer require
water to be present at the reaction site. Thereby, humidity may also affect the operation of the sensor.
We expected that the humidity dependence should also be relatively low, similar to the solution
reported in [30]. The results are shown in Figure 12.
When increasing the relative humidity from 36% to 84%, the intensity of light increases only by
13%. It is consistent with the results reported for methylene blue [30]. Measured range of relative
humidity is typical for majority of applications. It is necessary to highlight the fact, that the response
of the sensor to humidity can also be dependent on temperature.
Figure 11. Dependence between the intensity of light transmitted through the sensor and the time
after reaching 60 ˝C. Measurement performed at 0.1% of CO2in air.
It is important to highlight that before reaching the temperature of 60 ˝C, the sensor was
operational. After reaching 60 ˝C it did not respond to CO2any more. The time needed for the whole
change is approximately equal to 100 s at 0.1% of CO2. The reason for such an abrupt transition is
probably related to the nature of methyl red indicator dye, since such behaviour has not been reported
for other silica gel-based optical fibre sensors [35]. Therefore, the most probable explanation for the
analysed phenomenon is that methyl red in silica gel at 60 ˝C losses its ability to bind hydrogen ion.
This can be caused either by a decrease of humidity in the silica gel and subsequent precipitation
of the dye or by molecular properties of methyl red. The decay time of 100 s is probably needed to
achieve the threshold temperature for the whole optical fibre sensing region.
The described process was repeatable. However, the optical fibre regained its ability to sense
carbon dioxide after cooling to temperatures lower than 60 ˝C. For instance, the sensor resumed
operation after cooling to 43 ˝C, which occurred approximately 4 min after the start of cooling from
60 ˝C. This speaks in favour of the humidity-based hypothesis described in the previous paragraph.
It is important to notice that the drop of the transmission is non-differentiable as a function of
temperature. This could indicate that a high order phase transition may occur, since the imaginary
part of the dielectric constant (responsible for absorption) is non-differentiable.
4.7. Cross-Sensitivity to Other Gases
Equations (1) and (2) indicate that the processes employing CO2inside the sensing layer require
water to be present at the reaction site. Thereby, humidity may also affect the operation of the sensor.
We expected that the humidity dependence should also be relatively low, similar to the solution
reported in [30]. The results are shown in Figure 12.
When increasing the relative humidity from 36% to 84%, the intensity of light increases only by
13%. It is consistent with the results reported for methylene blue [30]. Measured range of relative
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humidity is typical for majority of applications. It is necessary to highlight the fact, that the response
of the sensor to humidity can also be dependent on temperature.
Although the cross-sensitivity to water vapour is low, the sensor responds strongly to liquid
water and dew. For instance, the transmitted intensity strongly increases when the sensing part of
the fibre is immersed in water. Therefore during the operation the sensor should be protected against
water and other liquids.
Carbon dioxide is not the only gas which can influence the pH of a sensing layer. There are
several other gases, e.g., NO2and SO2, which also react with water and produce H3O+ions. Similarly,
NH3is a basic gas, which increases pH of the sensing layer. All the mentioned gases may influence
the readings of the reported sensor. However, they are usually present in the atmosphere at low
levels, which may be too low to affect the sensor operation.
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12
Although the cross-sensitivity to water vapour is low, the sensor responds strongly to liquid
water and dew. For instance, the transmitted intensity strongly increases when the sensing part of
the fibre is immersed in water. Therefore during the operation the sensor should be protected against
water and other liquids.
Carbon dioxide is not the only gas which can influence the pH of a sensing layer. There are
several other gases, e.g., NO2 and SO2, which also react with water and produce H3O+ ions. Similarly,
NH3 is a basic gas, which increases pH of the sensing layer. All the mentioned gases may influence
the readings of the reported sensor. However, they are usually present in the atmosphere at low
levels, which may be too low to affect the sensor operation.
Figure 12. Dependence between relative humidity and the intensity of transmitted light. The
measurements were carried out at 18 °C with a sensor containing methyl red.
It is possible to design and prepare more selective sensors for solely detecting gases more acidic
than carbon dioxide or more basic than this gas. For instance, a sensor similar to the reported ones,
but also containing a buffer, which would keep a pH approximately around 4 would be less affected
by typical fluctuations of CO2 and it would mostly respond to more acidic gases. However, a dye
which changes its colour within lower pH values should be used for this purpose instead of methyl
red or phenol red. In a similar way ammonia can also be detected. Other solutions like the use of
certain polymers and not using a matrix material at all [32] have already been reported and also may
help solving the problem of cross-sensitivity.
Thus a sensor matrix incorporating: (a) the reported CO2 sensor; (b) sensor of strongly acidic
gases; (c) sensor of basic gases; (d) humidity sensor and (e) temperature sensor should be
self-consistent and should provide the option of multigas sensing.
4.8. Final Recommended Solution
The authors have tested several sensors with different indicator dyes, different response times
and different sensitivities. Thymol blue, due to its instability, was found not to be useful for further
work. Phenol red yielded a sensor with 0.65% maximum limit of detection, which is suitable for
ventilation monitoring. Methyl red provided a sensor which could operate at low and high CO2,
concentrations, and therefore it is recommended dye for versatile applications. Taking into account
the examined silica gel thickness, thin layers (approximately 0.75 μm) are suitable for the majority of
applications, since the sensitivity reaches a value of 3 and response times are very low (2–3 s). To the
best of our knowledge, these response times for thin silica gel layers are the lowest ones reported for
an absorption-based optical fibre CO2 gas sensor. Similarly, the reported sensitivity for a thick silica
gel coating is the highest for such a sensor type.
When compared to commercially available NDIR sensors, the reported sensor has much faster
response time (2–3 s instead of 60 s [1]) and can be used for measuring higher CO2 concentrations (up
to 100% instead of 0.5%–1% for NDIR). On the other hand, the resolution of NDIR devices within the
low concentration range is much higher. The reported sensor has a faster response than commercially
Figure 12. Dependence between relative humidity and the intensity of transmitted light. The
measurements were carried out at 18 ˝C with a sensor containing methyl red.
It is possible to design and prepare more selective sensors for solely detecting gases more acidic
than carbon dioxide or more basic than this gas. For instance, a sensor similar to the reported ones,
but also containing a buffer, which would keep a pH approximately around 4 would be less affected
by typical fluctuations of CO2and it would mostly respond to more acidic gases. However, a dye
which changes its colour within lower pH values should be used for this purpose instead of methyl
red or phenol red. In a similar way ammonia can also be detected. Other solutions like the use of
certain polymers and not using a matrix material at all [32] have already been reported and also may
help solving the problem of cross-sensitivity.
Thus a sensor matrix incorporating: (a) the reported CO2sensor; (b) sensor of strongly
acidic gases; (c) sensor of basic gases; (d) humidity sensor and (e) temperature sensor should be
self-consistent and should provide the option of multigas sensing.
4.8. Final Recommended Solution
The authors have tested several sensors with different indicator dyes, different response times
and different sensitivities. Thymol blue, due to its instability, was found not to be useful for further
work. Phenol red yielded a sensor with 0.65% maximum limit of detection, which is suitable for
ventilation monitoring. Methyl red provided a sensor which could operate at low and high CO2,
concentrations, and therefore it is recommended dye for versatile applications. Taking into account
the examined silica gel thickness, thin layers (approximately 0.75 µm) are suitable for the majority of
applications, since the sensitivity reaches a value of 3 and response times are very low (2–3 s). To the
best of our knowledge, these response times for thin silica gel layers are the lowest ones reported for
an absorption-based optical fibre CO2gas sensor. Similarly, the reported sensitivity for a thick silica
gel coating is the highest for such a sensor type.
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When compared to commercially available NDIR sensors, the reported sensor has much faster
response time (2–3 s instead of 60 s [1]) and can be used for measuring higher CO2concentrations
(up to 100% instead of 0.5%–1% for NDIR). On the other hand, the resolution of NDIR devices
within the low concentration range is much higher. The reported sensor has a faster response than
commercially available electrochemical sensors and it has similar resolution (measurement range
from 0.035% to 1% or 5% or from 0.2% to 95%, response time of several minutes [2]).
4.9. Self-Referencing Arrangement
The described sensor can possibly find application in manifold areas. We propose a simple
self-referencing arrangement of the sensing system, in which light from the source passes through
an optical fibre coupler and then it passes independently through the sensor and the reference arm.
The arrangement is shown in Figure 13.
Sensors 2015, 15, page–page
13
available electrochemical sensors and it has similar resolution (measurement range from 0.035% to
1% or 5% or from 0.2% to 95%, response time of several minutes [2]).
4.9. Self-Referencing Arrangement
The described sensor can possibly find application in manifold areas. We propose a simple
self-referencing arrangement of the sensing system, in which light from the source passes through an
optical fibre coupler and then it passes independently through the sensor and the reference arm. The
arrangement is shown in Figure 13.
Figure 13. Self-referencing arrangement of the sensor. Lines represent optical fibres.
In such a sensing system the concentration of CO2 would be determined from the intensity ratio
of both signals passing through the sensor and the reference arm. What is more, the described sensor
arrangement enables one to omit the problem of the light source power instability.
5. Conclusions
Broad range CO2 optical fibre sensors have been presented. The reported sensors are made by
deposition of an active silica gel coating onto a plastic clad silica optical fibre. Considering the tested
indicator dyes, methyl red was found to be the most utilitarian one, since it operates over a whole
range of CO2 concentrations. It is noteworthy that a methyl red CO2 optical fibre sensor has not been
reported before and it was even claimed to be impossible to manufacture one. The stability of these
absorption-based carbon dioxide optical fibre sensors has been examined for the first time. A method
of obtaining a stable sensor has been proposed and experimentally verified with good results. What
is more, the phenol red-containing sensor also operated stably within the low range of carbon dioxide
concentrations. This may destine phenol red-based sensors to indoor use and methyl red ones to
industrial applications. It has been noted that an increase in the active layer thickness increases the
sensitivity of the sensor, but it also substantially increases the response time. Humidity was found to
have a weak influence on the response of the sensor. Reported sensors exhibit high sensitivities
reaching I0.1%/I100% of 10 for the 9 μm thick layers. For an active layer thickness of 0.75 μm the response
time can be as low as 2 s for switching from low to high CO2 concentration and 3 s for the opposite
process. The reported sensor thus exhibits much lower response time and higher sensitivity than
other absorption-based solutions presented elsewhere in the literature.
Author Contributions: Tomasz Nasiłowski conceived of the project and Marek Napierała coordinated it. Karol
Wysokiński designed the experiments and performed measurements. Tomasz Stańczyk and Stanisław Lipiński
designed and prepared the measurement chamber and prepared the light sources. The manuscript was written
by Karol Wysokiński and was revised by Marek Napierała and Tomasz Nasiłowski.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Yasuda, T.; Yonemura, S.; Tani, A. Comparison of the Characteristics of Small Commercial NDIR CO2
Sensor Models and Development of a Portable CO2 Measurement Device. Sensors 2012, 12, 3641–3655.
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Figure 13. Self-referencing arrangement of the sensor. Lines represent optical fibres.
In such a sensing system the concentration of CO2would be determined from the intensity ratio
of both signals passing through the sensor and the reference arm. What is more, the described sensor
arrangement enables one to omit the problem of the light source power instability.
5. Conclusions
Broad range CO2optical fibre sensors have been presented. The reported sensors are made by
deposition of an active silica gel coating onto a plastic clad silica optical fibre. Considering the tested
indicator dyes, methyl red was found to be the most utilitarian one, since it operates over a whole
range of CO2concentrations. It is noteworthy that a methyl red CO2optical fibre sensor has not been
reported before and it was even claimed to be impossible to manufacture one. The stability of these
absorption-based carbon dioxide optical fibre sensors has been examined for the first time. A method
of obtaining a stable sensor has been proposed and experimentally verified with good results. What is
more, the phenol red-containing sensor also operated stably within the low range of carbon dioxide
concentrations. This may destine phenol red-based sensors to indoor use and methyl red ones to
industrial applications. It has been noted that an increase in the active layer thickness increases the
sensitivity of the sensor, but it also substantially increases the response time. Humidity was found
to have a weak influence on the response of the sensor. Reported sensors exhibit high sensitivities
reaching I0.1%/I100% of 10 for the 9 µm thick layers. For an active layer thickness of 0.75 µm the
response time can be as low as 2 s for switching from low to high CO2concentration and 3 s for the
opposite process. The reported sensor thus exhibits much lower response time and higher sensitivity
than other absorption-based solutions presented elsewhere in the literature.
Author Contributions: Tomasz Nasiłowski conceived of the project and Marek Napierała coordinated
it. Karol Wysoki ´nski designed the experiments and performed measurements. Tomasz Sta´nczyk and
Stanisław Lipi´nski designed and prepared the measurement chamber and prepared the light sources. The
manuscript was written by Karol Wysoki´nski and was revised by Marek Napierała and Tomasz Nasiłowski.
Conflicts of Interest: The authors declare no conflict of interest.
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© 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open
access article distributed under the terms and conditions of the Creative Commons by
Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
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