Colorimetric Materials for Fire Gas
Katrin Schmitt 1, *, Karina R. Tarantik 1ID , Carolin Pannek 1and Jürgen Wöllenstein 1,2
Fraunhofer Institute for Physical Measurement Techniques IPM, Heidenhofstr. 8, 79110 Freiburg, Germany;
firstname.lastname@example.org (K.R.T.); email@example.com (C.P.);
2Department of Microsystems Engineering—IMTEK, Laboratory for Gas Sensors, University of Freiburg,
Georges-Köhler-Allee 102, 79110 Freiburg, Germany
*Correspondence: firstname.lastname@example.org; Tel.: +49-761-8857-316
Received: 28 February 2018; Accepted: 27 March 2018; Published: 29 March 2018
The damage caused by outbreaks of ﬁre continues to be enormous despite ongoing
improvements in ﬁre detection and ﬁghting. Therefore, the detection of ﬁres at the earliest possible
stage is essential. The latest developments in ﬁre detection devices include the addition of carbon
monoxide (CO) or temperature sensors into the widespread smoke detectors, but also alternative
solutions are searched for. Advantageous is the direct detection of the most relevant ﬁre gases CO
and nitrogen dioxide (NO
), because they are produced very early in a developing ﬁre. A sensitive,
selective, and low-cost method to detect these gases is the use of colorimetric materials combined with
a compact optical readout. In this review, we take account of recent developments in this research
ﬁeld and provide a comprehensive overview on suitable materials for CO and NO
detection in ﬁre
gas sensing and ﬁrst steps towards novel ﬁre gas detectors.
Keywords: colorimetric; sensor; ﬁre gas; carbon monoxide; nitrogen dioxide
In the case of ﬁre, every second counts to prevent the loss of life. In addition, for each additional
minute of response time, property damage increases by 2%. Widespread, state of the art sensors are
smoke detectors which are often not adequate to alarm inhabitants quickly enough. Also heat detectors
measuring the ambient air temperature are too slow to react. With both technologies, nonsmoking ﬁres,
e.g., pure ethanol ﬁres, cannot be detected. A viable alternative is the use of systems relying on gas
detection rather than particle detection. In particular, gas sensors provide a faster and more accurate
ﬁre detecting mechanism since gases are produced prior to aerosols in a developing ﬁre (Figure 1).
Fire gas detectors, if available, are mainly installed in large automatic ﬁre detection systems, and
contain electrochemical cells for carbon monoxide (CO). Often these CO-detecting cells are added
to smoke detectors [
]. Research efforts have focused on metal oxide-based (MOX) gas sensors for
this application for many years [
]. Fire gas detectors containing MOX sensors are commercially
available only for some niche applications such as caravanning, motor boats etc., because they suffer
from high power consumption and a lack of selectivity. Also suspended gate ﬁeld effect transistors
(SGFETs) were proposed for ﬁre gas detection in literature, but have not been marketed until now [
The detection of gases based on chemical reactions developing a visible color change was patented
already in 1919 in the USA as a new type of gas detector [
]. Their main intention was to detect CO
in small quantities by making use of the color change of iodine in the presence of CO. With the
introduction of the so-called Dräger tubes
in 1937, this principle has begun to ﬁnd widespread use in
ﬁre gas detection. These single-use glass tubes, containing the reactive colorimetric material, are broken
Chemosensors 2018,6, 14; doi:10.3390/chemosensors6020014 www.mdpi.com/journal/chemosensors
Chemosensors 2018,6, 14 2 of 15
at one end upon use and permit a quantitative determination of the target gas. The concentration is
read on a scale by the user.
Scheme of the damage caused by ﬁre outbreak occurring over time related to the ﬁre indicators
gas, aerosol, and heat/radiation.
Advances in the control of nanoparticles and synthesis of novel indicator molecules in combination
with optical and micro-structuring methods now allow a conceptually new type of low-cost,
ultra-low-power consuming ﬁre gas detectors based on a colorimetric read-out. No other gas sensing
principle for the speciﬁc detection of the two main ﬁre indicator gases CO and nitrogen dioxide
) can achieve energy self-sufﬁcient, battery-supported operation for up to 10 years. In the
search for suitable colorimetric materials, further requirements for ﬁre detectors regarding sensitivity
and selectivity must be considered: based on the data available for typical test ﬁres (burning wood,
ethanol, polyurethane, n-heptane), relative humidities vary between 25% and 40%. In addition to CO
a signiﬁcant increase of CO and NO
concentrations develop in all test ﬁres. For CO, the background
concentration (no ﬁre) is 1–3 ppm, this value increases to 5–100 ppm depending on the type of ﬁre.
is present in the environment in a background concentration of about 10 ppb and increases to
40 ppb to 3 ppm in the case of ﬁre. Therefore, suitable colorimetric materials must yield detectable
color reactions in the following concentration ranges:
•CO: 5–100 ppm
•NO2: 0.4–3 ppm
Furthermore, the colorimetric materials must be stable for a long period of time (several years)
when used for ﬁre gas detectors, especially regarding thermal stresses, i.e., in the case of a ﬁre, the
color reaction must still take place. Also, the response time of a sensor based on the colorimetric
principle must lie in the range of a few seconds to alarm the inhabitants quickly enough. These factors,
i.e., response time, stability and reproducibility of such sensors are crucially inﬂuenced by the sensor
conﬁguration and the matrix the colorimetric material is embedded in. Especially the matrices are the
topic of ongoing research due to their decisive role in a sensor intended to be commercialized. In the
following, we review currently known or available colorimetric materials suitable for the detection of
CO and NO2as relevant ﬁre gases.
Chemosensors 2018,6, 14 3 of 15
2. Materials for Carbon Monoxide Detection
The earliest description, dated back to 1928, of the colorimetric determination of CO in air or
dissolved in blood can be found in [
]. The colorimetric material used there was iodine pentoxide
(I2O5), which reacts with CO irreversibly as follows:
I2O5+ 5 CO →I2+ 5 CO2(1)
Since the liberated iodine is very easy to detect as a reaction product, this reaction is still used
today in the Dräger tubes
. In comparison, Figure 2shows the extensive technical equipment used for
the same purpose scarcely one hundred years ago:
Experimental setup for the determination of CO in air using I
. Reprinted with permission from [
Martinek and his colleagues already reported a detection limit of 10 ppm in 1928, which roughly
corresponds to the detection limit of the Dräger tubes
. About a decade later, the reaction of palladium(II)
chloride (PdCl2) with CO was described for the first time :
CO + PdCl2+ H2O→Pd + CO2+ 2 HCl (2)
In this method, the concentration of unreacted PdCl
is subsequently determined using potassium
iodide and the CO concentration is calculated therefrom.
PdCl2+ 2 KI →PdI2+ 2 KCl (3)
However, the potassium iodide solution is relatively unstable, but can be replaced by other
color indicators. Initially, the color reaction was determined using standard solutions of known
concentrations. Later, this comparative method was replaced by spectroscopy [
]. At about the same
time, a palladium-ammonium molybdate compound was proposed and patented for the determination
of CO [
]. In this case, pure palladium is dissolved in sulfuric acid to produce palladium sulfate as
catalyst. Ammonium molybdate hexahydrate ((NH
O) is dissolved in water and added in
about 30-fold excess to the palladium sulfate. The carrier or matrix used was silica gel. With this method,
a yellow solution turns blue in the presence of CO (cf. reaction Scheme 1). A sensor based on this reaction
was patented in 1984 by Herskovitz and colleagues [
]. In 1964, another method was published [
CO reacts in a basic solution with the silver salt of p-sulfaminobenzoic acid. The absorption of the silver
colloid solution is determined spectrometrically and is proportional to the CO concentration. With this
Chemosensors 2018,6, 14 4 of 15
method it was possible to measure CO concentrations between 5 and 180 ppm with an accuracy of
Lambert and colleagues [
] also used palladium chloride to determine CO. The actual colorimetric
determination, however, was accomplished by the reduction of the iron complex [Fe(III)EDTA]
. The latter compound triggers ligand exchange with L = 2,2
1,10-phenathroline to form the stable [FeL
compound, the color of which can be determined
spectrophotometrically in the blue/green wavelength range. Sodium molybdate was added as a catalyst.
In further experiments they used, in addition to the palladium chloride compound, iodate and Leuco
Crystal Violet (4,4
-methylidynetris(N,N-dimethylaniline) which turns violet in the presence of
CO, both as a function of the reaction time the CO concentration. Instead of Leuco Crystal Violet,
a promazine hydrochloride complex, also in combination with palladium chloride, was described for
colorimetric determination a few years later [
]. Another publication reports the use of pyronine-G,
which is converted to benzene (also with PdCl
and iodate as base). The absorption of benzene is
proportional to the CO concentration in a range of 20–400 ppm with a detection limit of 1 ppm [
in 1977, Shuler and Schrauzer patented a method for measuring reducing gases [
]. Here, a reversible
reaction for CO determination was described for the ﬁrst time: palladium sulfate serves as a catalyst
for the intrinsically slow reaction of CO with ammonium molybdate, which changes from yellow to
blue, and the reverse reaction with atmospheric oxygen to the ground state (Scheme 1). For the reverse
reaction, a metal salt is needed (e.g., copper, iron, nickel chloride, sulfate, etc.).
Scheme 1. Reaction scheme for the reversible detection of CO.
In a similar fashion, Goswami and colleagues patented in 1994 a speciﬁc and reversible CO
] based on molybdenum or tungsten salts with palladium salts as catalyst and metal salts for
the reverse reaction. In general, the following redox reaction takes place:
2 Mo6+ +3CO+3H2O→2 Mo3+ + 3 CO2+6H+(4)
This reaction proceeds only very slowly, therefore palladium salts are used as catalyst:
Pd2+ +CO+H2O→Pd0+ CO2+2H+
3 Pd0+ 2 Mo6+ →3 Pd2+ + 2 Mo3+ (5)
For the reverse reaction the metal salts, e.g., iron compounds, are used:
Mo3+ + 3 Fe3+ →Mo6+ + 3 Fe2+ (6)
The atmospheric oxygen is sufﬁcient for this reverse reaction, and the molybdenum compound
regains its initial state and can react again. Another, relatively simple method was presented by Pal
and colleagues in 1987 [
]. They used silver nitrate (AgNO
) as an indicator in a gelatine solution and
were thus able to determine CO in the concentration range of 2–100 ppm, with a standard deviation of
2.6% at 10 ppm CO.
Metalloporphyrins and metallophthalocyanines can also be used for the detection of CO.
The reactions of these metal compounds with CO are reversible. Table 1presents a compilation
of currently known complexes:
Chemosensors 2018,6, 14 5 of 15
Table 1. Metal complexes for the detection of CO.
Complex Description and Characteristics
Upon binding of CO or NO2, red-shifted absorption occurs. Details on stability or reversibility are not
yet known .
This compound is stable in the solid state and binds CO. In solution, a color change from green to red
occurs. The compound is only stable in solution when pyridine is added as a stabilizer .
Chemosensors 2018,6, 14 6 of 15
Table 1. Cont.
Complex Description and Characteristics
Ruthenium porphyrin derivative
The CO-complexed molecule binds another CO reversibly and stronger than other porphyrins. There is
no information about a color change .
Binds CO and NO2. It is thermally unstable (loss of amino functionality) .
Iron pincer complex
Extremely selective reaction with CO; no cross-sensitivities to NO or SO2are known. Complete
regeneration is accomplished by heating at 100 ◦C for 5 min. No detailed information on gas
sensitivities is available yet .
Chemosensors 2018,6, 14 7 of 15
Another promising material for CO detection is based on binuclear rhodium complexes.
They show a fast and selective reaction towards CO and are completely reversible [
]. The general
reaction scheme of the rhodium complexes [Rh
with CO is shown
in Figure 3. Table 2gives the deﬁnition of possible ligands. An investigation of different rhodium
complexes, obtained by varying the residues of the phosphines, regarding their reaction velocity
towards CO exposure is published in .
General two-step reaction scheme of the binuclear rhodium complex with the formula
. The color of the complex turns from violet to orange-yellow.
Cf. Table 2for the deﬁnition of the ligands.
Table 2. Ligands of the rhodium compounds 1·(A)2–5·(A)2.
Compound X= R= A=
3. Materials for Nitrogen Dioxide Detection
Colorimetric materials for NO
can roughly be divided into three types: the ﬁrst is essentially
based on the N-(1-naphthyl)-ethylenediamine (NED) proposed by Saltzman in 1954 for the ﬁrst time
], the second of Jacobs and Hochheiser 1958 [
], and the third, on metal
complexes. In the following the three classes are brieﬂy described:
3.1. Saltzman Method
The Saltzman method originally uses a mixture of sulfanilic acid, NED, and acetic acid. Thus,
a stable staining could be achieved upon contact of the solution with NO
with a detection limit
of a few ppb. The solution had a low cross-sensitivity to ozone, for other gases it was negligible.
The Saltzman method is one of the most sensitive colorimetric determination methods for NO
has the advantage that all reagents are mixed in a solution and the color reaction develops over time.
However, only a moderate long-term stability of the solution is also reported. However, this does not
have to apply to an indicator layer in a suitable matrix, e.g., a polymer.
In detail, the reaction proceeds as follows: sulfanilic acid and NED form nitrite ions in contact
, which are detected by the color reaction. Nitrite reacts in acidic medium (acetic acid) to the
−+2H+→NO++ H2O. (7)
The nitrosyl cation causes sulfanilic acid and NED to form an azo dye (red-violet). In this
case, ﬁrst a diazotization reaction takes place, in which a diazonium ion is formed; this is followed
Chemosensors 2018,6, 14 8 of 15
by azo coupling (in the para-position to the NH
group of 1-naphthylamine). Figure 4shows the
Partial reactions in the Saltzman method: the nitrosyl cation causes sulfanilic acid and NED
to form an azo dye (red-violet). In this case, the ﬁrst step is the diazotization reaction, in which a
diazonium ion is formed; this is followed by the azo coupling.
Thomas and colleagues further developed the method and implemented a calibration because
the color change reaction is not linear [
]. Further modiﬁcations of the Saltzman method can
be found in [
]. Huygen [
] compared various azo dyes, including N,N-(1-naphthyl,acetyl)
ethylenediamine-toluene monosulfate (ANEDA), N-(1-naphthyl)ethylenediamine dihydrochloride
(NEDA), 1,8-diaminonaphthalene, 1-naphthylamine, and dimethylaniline. The ﬁrst three yielded the
best results. Nash [
] additionally added basic guaiacol solution to the reagents and buffered with
glycerol. This resulted in a higher stability of the solution.
Smith patented a variation of the Saltzman method in 1972, in which a kind of paste was developed
by addition of e.g., MgSO
and glycerol, and this paste was further processed it into a pellet. As a
result, the stability of the reagents could be further improved; moreover, this method allowed the
measurement directly in air without the need for any equipment by means of a small sensor .
About 20 years ago, Tanaka and colleagues intensiﬁed the work on colorimetric NO
. Instead of NED, they used N,N-dimethyl-1-naphthylamine (NA) and, through the use
of porous glass, also increased the active surface, which further increased the sensitivity of the sensors
(<100 ppb). Figure 5shows the structural formulas of the reagents used, Figure 6shows an example of
the absorption spectra of the reagents when exposed to NO2.
Figure 5. Structural formulas of (a) sulfanilamide (SA) and (b)N,N-dimethyl-1-naphthylamine (NA).
The investigation of further redox indicators from the group of aromatic amines is still ongoing:
Alexy et al. used N,N
-diphenyl-1,4-phenylenediamine (DPPD), o-dianisidine, N,N
(DPB) and N,N,N
-tetramethyl-1,4-phenylenediamine (TMPD) immobilized in a gas-permeable
polymeric layers for NO
]. TMPD was found to be highly selective towards NO
cross-sensitivities to other relevant gases [44,45].
Chemosensors 2018,6, 14 9 of 15
Absorption spectra of porous glass slides with Saltzman reagents, after exposure to 600 ppb
for 8 h (ﬂow rate 1 L/min): with sulfanilamide (SA) (I), N,N-dimethyl-1-naphthylamine (ND) (II),
SA and ND (4:1) (III), and SA and ND (8:5) (IV). Reprinted with permission from .
3.2. Jacobs and Hochheiser Method
In the Jacobs and Hochheiser method from 1958, 0.1 m NaOH is used as the absorption medium
and the sulfanilamide is dissolved in phosphoric acid. The indicator, as in the Saltzman method,
is NED. The colorimetric reaction is readout spectrophotometrically. This reaction was used in an
automatic sequence sampler to monitor NO
emissions in urban areas on a 24 h basis. The reaction
is also sensitive to sulfur dioxide, but when oxidized to sulfate by adding hydrogen peroxide, this
does not interfere with the reaction. The limit of detection of the Jacobs and Hochheiser method is in
the ppb range. Later, sodium arsenite was also used as the absorption medium, this being referred to
as a modiﬁed Jacobs and Hochheiser method.
3.3. Metal Complexes
Similar to the detection of CO, also NO
can be measured using different types of metal complexes.
Table 3gives an overview of the metal complexes used for NO
detection that can be found in literature.
Chemosensors 2018,6, 14 10 of 15
Table 3. Metal complexes for the detection of NO2.
Complex Description and Characteristics
Upon binding of CO or NO2, red-shifted absorption occurs. Details on stability or reversibility are
not yet known .
Reacts very selectively to NO2but is not completely reversible. Captures a concentration range of
25–800 ppb within a few seconds. Was intended to be commercialized in a ﬁre gas detector, but the
compound turned out to be unstable from approx. 50 ◦C .
Chemosensors 2018,6, 14 11 of 15
Table 3. Cont.
Complex Description and Characteristics
So far, only results on the reaction of Cu/Co. phthalocyanines on NO2exist, in which the
conductivity is measured (no color change) [
]. The reaction is reversible. A detailed description of
the chemical reaction (IR or Raman spectroscopic studies) upon binding of NO
can be found in [
Chemosensors 2018,6, 14 12 of 15
3.4. Other Methods
In 1984 Bajeva and colleagues presented another simple and sensitive method for the determination
]: guaiacol is used in basic solution both as an absorption medium and a coupling reagent.
With p-nitroaniline, a red dye is formed after the diazotization reaction. Information on the detection
limit cannot be found here. Raman used benzidine in 1991 for the diazotization reaction with orcinol
was absorbed onto sodium arsenite. This reaction causes a yellow color (460 nm), which can
be detected spectroscopically [
]. The absorption according to Lambert-Beer is given as linear in the
concentration range of 0.04–0.48 µg/mL nitrite .
A year later, Kaveeshwar and colleagues [
] published a similar procedure to Bajeva’s: o-nitroaniline
is used as an absorption and diazotization reagent. The red-violet color reaction results from coupling
to 1-amino-2-naphthalene sulfonic acid (ANSA). Kumar and colleagues presented in 1993 a further
modiﬁcation of the above-mentioned methods [
]: they again used sodium arsenite as an absorption
medium and p-nitroaniline and chromotropic acid in acetate solution for the color reaction. The resulting
solution has a maximum absorption at 515 nm and is reported as linear in the concentration range of
g/mL nitrite. This method was also compared with the Saltzman method, and the detection limit
was determined to be 0.5
g/mL. In Pandurangappa et al. also sodium arsenite as an absorption medium
is mentioned [
]. The color reaction is based on the reaction of nitrite with aminophenyl benzimidazole in
an acidic environment and NED as an indicator. The maximum absorption is 555 nm and is described as
linear according to Lambert-Beer in the concentration range 0–10
g in 25 mL solution. Parmar et al. [
used, in addition to sodium arsenite as the absorption medium, aminoacetophenone in acidic solution
and phloroglucinol as an indicator with an absorption maximum at 420 nm. The detectable concentration
range is given as 0.008–0.12 µg/mL.
The present review shows the results of a literature search on currently known or available
colorimetric materials for the detection of CO and NO
; the focus was on their suitability for
implementation in ﬁre gas detectors. In principle, irreversible compounds are available for single-use
sensors for CO such as iodine pentoxide in Dräger tubes
, or reversible indicator molecules for
long-term warning sensors, such as different binuclear rhodium complexes, molybdenum compounds
with catalysts, or different (metallo-)porphyrins or (metallo-)phthalocyanines. Iron pincer complexes
are promising, but their investigation is still basic research. For NO
detection, sensors based on azo
dyes such as NED-based reagents or TMPD, have shown good results, but also metal complexes such
as porphyrins or phthalocyanines. Table 4summarizes the results.
Table 4. Summary of colorimetric compounds suitable for CO and NO2detection in ﬁre gases.
Materials for CO Detection
Compound Reversibility Measuring Range/Limit of Detection (ppm) Cross-Sensitivity
Iodine pentoxide irreversible 5–10
Molybdenum complexes reversible 2–100 n.a.
Metal complexes Partly reversible 5–180 Towards NO2, SO2
Rhodium complexes reversible 10–200
Materials for NO2Detection
Compound Reversibility Measuring Range/Limit of Detection (ppm) Cross-Sensitivity
Organic dyes Partly reversible <0.1 ppm Towards O3
Metal complexes Partly reversible 25–800 ppb Towards CO, SO2
Yet for an implementation in ﬁre gas sensors, not only the gas sensing characteristics of the
colorimetric compound itself is decisive, but also its long-term stability in changing environments,
i.e., temperature and humidity variations. Also, hazardous compounds will probably not come into
use (sodium arsenite, azo dyes). Another crucial inﬂuence on the sensing characteristics has the matrix
in which the colorimetric material is embedded. The matrix must ensure gas permeability and, at the
Chemosensors 2018,6, 14 13 of 15
same time, be thermally stable in a ﬁre and insensitive to variations in the relative humidity in the
environment. Further challenging is the response time of a sensor based on the colorimetric principle,
which is inﬂuenced not only by the material itself, but also by the sensor conﬁguration and the matrix.
Sensor response times of a few seconds can be already achieved [
]. Future research should address
the development of a matrix with the abovementioned characteristics, in combination with low-cost
and compact optical sensors for read-out. This will play a vital role for the implementation of the
colorimetric principle in commercial ﬁre gas detectors.
We acknowledge ﬁnancial support from the German Federal Ministry of Education and
Research (BMBF—FKZ 13N14076).
Conﬂicts of Interest: The authors declare no conﬂict of interest.
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