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Colorimetric Materials for Fire Gas Detection—A Review


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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 (NO2), 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 NO2 detection in fire gas sensing and first steps towards novel fire gas detectors.
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Colorimetric Materials for Fire Gas
Detection—A Review
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; (K.R.T.); (C.P.); (J.W.)
2Department of Microsystems Engineering—IMTEK, Laboratory for Gas Sensors, University of Freiburg,
Georges-Köhler-Allee 102, 79110 Freiburg, Germany
*Correspondence:; 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
1. Introduction
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
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.
Figure 1.
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:
Figure 2.
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 [11]:
CO + PdCl2+ H2OPd + 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
95 ±5%.
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]
to [Fe(II)EDTA]
. The latter compound triggers ligand exchange with L = 2,2
-dipyridyl or
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 [
]. Still
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
sensor [
] 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+3H2O2 Mo3+ + 3 CO2+6H+(4)
This reaction proceeds only very slowly, therefore palladium salts are used as catalyst:
Pd2+ +CO+H2OPd0+ 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
(5,10,15,20-tetraphenylporphyrin) zinc
Upon binding of CO or NO2, red-shifted absorption occurs. Details on stability or reversibility are not
yet known [24].
Octamethyltetrabenzoporphyrin iron
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 [25].
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 [26].
Iron(II) phthalocyanine
Binds CO and NO2. It is thermally unstable (loss of amino functionality) [27].
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 [28].
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 [32].
Figure 3.
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·(A)23-F CH3CH3CO2H
4·(A)24-OCH3; 3,5-CH3CH3CH3CO2H
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
for NO
determination [
], 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
with NO
, which are detected by the color reaction. Nitrite reacts in acidic medium (acetic acid) to the
nitrosyl cation:
+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.
Figure 4.
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 [38].
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
sensing [
]. TMPD was found to be highly selective towards NO
with no
cross-sensitivities to other relevant gases [44,45].
Chemosensors 2018,6, 14 9 of 15
Figure 6.
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 [39].
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
(5,10,15,20-tetraphenylporphyrin) zinc
Upon binding of CO or NO2, red-shifted absorption occurs. Details on stability or reversibility are
not yet known [24].
aquacyanocobalt(III)-cobyrinate derivative
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 [46].
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
of NO
]: 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
after NO
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 [47].
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.
4. Conclusions
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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... It is worth noticing that there are several methods available for gas detection exhibiting selectivity and sensitivity, including gas chromatography (GC), mass spectrometry (MS) and optical chemical sensing [144][145][146][147][148][149][150][151][152]. Nonetheless, they are based on robust and costly equipment, which are hardly implemented at remote locations, and therefore, do not allow for on-site monitoring. ...
... Most studied OTFTs are selective to NO 2 . The detection limits are higher than that achieved with colorimetric sensors, but comparable to quartz-enhanced PAS (QEPAS) and H-type longitudinal resonant photoacoustic cells [147,176,177]. It is, however, less performant than Faraday rotation spectroscopy (FRS), a costly and sophisticated technique which uses the magnetic circular birefringence (MCB) effect [177]. ...
... These devices are p-type OTFTs operating at the maximum voltage of −15 to −40 V and exhibiting high selectivity towards H 2 S and CO. OTFTs outperform MOx chemosensors and show comparable performances to SAW sensors, colorimetric sensors and fluorescent probes coupled to optical detectors, QEPAS, micro-electro-mechanical systems (MEMS)-based sensors and PTR-QMS [147][148][149][150]152,179]. ...
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... The early detection of NO 2 has also shown to be a great asset for fire detection and firefighting improvement [5]. Several colorimetric materials were shown to be quite efficient for the detection of CO and NO 2 gases produced in an early fire [6]. Given this scenario, the detection of toxic gases plays an increasingly important role in keeping our environment unpolluted and ourselves safe due to their dangerous effects on both the ecosystem and human health. ...
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An efficient strategy to develop porous materials with potential for NO2 sensing was based in the preparation of a metal-organic framework (MOF), UiO-66(Hf), modified with a very small amount of meso-tetrakis(4-carboxyphenyl) N-methylpyrrolidine-fused chlorin (TCPC), TCPC@MOF. Chlorin’s incorporation into the UiO-66(Hf) framework was verified by several characterization methods and revealed that the as-synthesized TCPC@MOF brings together the chemical stability of UiO-66(Hf) and the photophysical properties of the pyrrolidine-fused chlorin which is about five times more emissive than the porphyrin counterpart. TCPC@MOF was further incorporated into polydimethylsiloxane (PDMS) and the resulting TCPC@MOF@PDMS film was tested in NO2 gas sensing. It showed notable sensitivity as well as a fast response in the range between 0.5 and 500 ppm where an emission intensity quenching is observed up to 96% for 500 ppm. This is a rare example of a chlorin-derivative used for gas-sensing applications through emission changes, and an unusual case of this type of optical-sensing composites of NO2.
... Vol:. (1234567890) (Schmitt et al. 2018) can also be used to determine CO in the blood, or atmospheric air samples. Another promising method for the detection of CO can be realized using nanocages of B 12 N 12 , B 12 P 12 , and Zn 12 O 12 (Beheshtian et al. 2011;Hadipour et al. 2015;Hojatkashani 2015;Soltani and Javan 2015). ...
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The interactions between carbon monoxide (CO) gas and boron nitride (B12N12) nanocage, in the absence and presence of an external electric field, were investigated using two DFT functionals (B3LYP and B97D), and 6-31G(d) basis set. An external electric field (EF) with its intensity ranging from − 0.514 to + 0.514 V/Å was applied to the B12N12 nanocage, CO molecule, and B12N12–CO complex. Negative EF gradually increased the B64 and B66 bond lengths, while positive EF decreased them in the B12N12 nanocage. Additionally, we obtained a parabolic relationship between band gap (Egap) and the external EF on the B12N12 cluster, with a maximum EF = 0 V/Å, for the B3LYP and B97D functionals. Negative EF enhanced the adsorption energy, while positive EF inhibited the adsorption energy for two functionals of the CO molecule on the B12N12 cluster. Moreover, increase in the EF resulted in a decrease in the dipole moment, Mulliken charge, and band gap energy, and an increase in the sensitivity for B3LYP (19.115 up to 46.738%) and B97D (30.097 up to 61.523%), from − 0.514 to + 0.514 V/Å. Our computational results demonstrated the capability of B12N12 as a sensor for potential applications in the detection of CO under an external electric field. Graphical abstract
We propose a novel sensing film for nitric oxide (NO) gas detection using graphene ink and NNN'N'-tetramethyl-p-phenylenediamine (TMPD). This sensing film can be fabricated by a low-temperature annealing process at about 80°C, which is suitable for flexible gas sensors. The proposed sensing film was deposited on alumina and polyethylene terephthalate (PET) substrates, and resistive NO gas sensors were fabricated. These sensors were able to detect NO gas concentrations from 2 ppm to 10 ppm, and had sufficient NO gas selectivity, regardless of the substrate. The high-temperature annealing process of the sensing film increased the sensitivity to ammonia (NH3) gas, and deteriorated NO gas selectivity.
High-sensitivity and multi-function are of paramount importance in the development of sensors. To this end, a new type of Tm³⁺, Yb³⁺ co-doped BiVO4 inorganic porous nanofibers is proposed, which realizes high sensitivity response to NO2 and methylbenzene, and provides real-time working temperature. The gas sensor can detect the operating temperature by the intensity ratio of the relative intensity bands of Tm³⁺ emission, which associated with two thermally coupled levels, i.e., ³F2,3 → ³H6/³H4 → ³H6 (690/796 nm) intensity ratio, and the maximum relative sensitivity is as high as 0.0237 K⁻¹. While the temperature detection is realized, the addition of rare earth ions also greatly improves the gas sensing performance. More interestingly, the sensing material BVO-T1Y8 porous nanofibers have excellent selectivity and long-term stability, respectively to 3 ppm NO2 at 200 °C and 100 ppm methylbenzene at 340 °C, with the highest response reaching 5.7 and 4.2. In general, the high gas sensitivity and multi-selectivity nanofibers with real-time temperature monitoring performance provides a new attempt for the application of multi-functional sensing materials in processing industrial waste gas.
Humidity interferes most gas sensors, especially colorimetric sensors. The conventional approaches to minimize the humidity interference in colorimetric gas sensing require using extra components, causing unwanted analytes loss, or limiting the choices of sensing probes to only hydrophobic ones. To explore the possibility of minimizing the humidity interference in a hydrophilic colorimetric sensing system, we have developed a hydrogel-incorporated approach to buffer the humidity influence on the colorimetric gas sensing. The hydrogel-incorporated colorimetric sensors show not only high humidity tolerance but also the improved analytical performance. The accuracy and reliability of the hydrogel-incorporated colorimetric sensors have also been validated in field tests. This hydrogel-incorporated approach will open up an avenue to implement hydrophilic recipes into colorimetric gas sensors and extend the application of colorimetric sensors to humid gases detection.
Colorimetric sensing technologies have been widely used for both quantitative detection of specific analyte and recognition of a large set of analytes in gas phase, ranging from environmental chemicals to biomarkers in breath. However, the accuracy and reliability of the colorimetric gas sensors are threatened by the humidity interference in different application scenarios. Though substantial progress has been made toward new colorimetric sensors development, unless the humidity interference is well addressed, the colorimetric sensors cannot be deployed for real-world applications. Although there are comprehensive and insightful review articles about the colorimetric gas sensors, they have focused more on the progress in new sensing materials, new sensing systems, and new applications. There is a need for reviewing the works that have been done to solve the humidity issue, a challenge that the colorimetric gas sensors commonly face. In this review paper, we analyzed the mechanisms of the humidity interference and discussed the approaches that have been reported to mitigate the humidity interference in colorimetric sensing of environmental gases and breath biomarkers. Finally, the future perspectives of colorimetric sensing technologies are also discussed.
We report a new type of self-powered gas sensors based on the combination of a colorimetric film with hierarchical micro/nano-structures and organic photovoltaic cells. The transmittance of the colorimetric film with micro/nano-structures coated with N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD) changes by reacting with NO2 gas and it is measured as a current output of the photovoltaic cell. For this purpose, materials for the organic photovoltaic cells were carefully chosen to match the working wavelength of the TMPD. Micropost array and nanowires increase the surface area for the gas reaction and thus improves the transmittance changes by NO2 gas (6.7 % change for plain film vs. 27.7 % change for the film with hierarchical micro/nano-structures to 20 ppm NO2). Accordingly, the colorimetric device with the hierarchical structures showed a response of ΔI/I0=0.27 to 20 ppm NO2, which is a 71 % improvement compared to those of plain sensing film. Furthermore, it showed a high selectivity against other gases such as H2S and CO with almost negligible responses. Since the current output change of photovoltaic cell is utilized as a sensor signal, no extra electrical power is required for the operation of gas sensors. We also integrated the sensor device with an electrical module and demonstrated a self-powered gas alarm system.
Interest in mobile chemical sensors is on the rise, but significant challenges have restricted widespread adoption into commercial devices. To be useful these sensors need to have a predictable response, easy calibration, and be integrable with existing technology, preferably fitting on a single chip. With respect to integration, the CMOS imager makes an attractive template for an optoelectronic sensing platform. Demand for smartphones with cameras has driven down the price and size of CMOS imagers over the past decade. The low cost and accessibility of these powerful tools motivated us to print chemical sensing elements directly on the surface of the photodiode array. These printed colorimetric microdroplets are composed of a nonvolatile solvent so they remain in a uniform and homogeneous solution phase, an ideal medium for chemical interactions and optical measurements. By imaging microdroplets on the CMOS imager surface we eliminated the need for lenses, dramatically scaling down the size of the sensing platform to a single chip. We believe the technique is generalizable to many colorimetric formulations, and as an example we detected gaseous ammonia with Cu(II). Limits of detection as low as 27 ppb and sensor-to-sensor variation of less than 10% across multiple printed arrays demonstrated the high sensitivity and repeatability of this approach. Sensors generated this way could share a single calibration, greatly reducing the complexity of incorporating chemical sensors into mobile devices. Additional testing showed the sensor can be reused and has good selectivity; sensitivity and dynamic range can be tuned by controlling droplet size.
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The detection of the toxic gas carbon monoxide (CO) in the low ppm range is required in different applications. We present a study of the reactivity of different gasochromic rhodium complexes towards the toxic gas carbon monoxide (CO). Therefore, the binuclear rhodium complexes with different ligands were prepared and their influence regarding reaction velocity and sensitivity towards CO was investigated. The most promising rhodium complex was embedded into a polymer with which glass substrates were coated. The reactivity towards CO of these layers was also investigated.
In this work, the determination of the products of interaction of NO 2 molecules with copper phthalocyanine films was performed using IR and Raman spectroscopy. It was stated that on contact with π electron network of phthalocyanine the oxidizing NO2 gas causes the transfer of an electron from the phthalocyanine ring to NO2 molecule. The formation of [CuPc]⁺ and NO2⁻ charged chemical forms was confirmed experimentally by Raman and IR spectroscopies, respectively. In order to determine the suggested structure of charge transfer complex the bands in the Raman spectra of CuPc film which are more sensitive to NO2 were assigned by means of measuring the isotopic shifts of corresponding modes upon ¹⁵N-substitution of β-CuPc. Besides, the correlation between sensor response, oxidative properties and structural features of CuPc film structure was demonstrated.
Conference Paper
One approach to realize selective chemical sensors is the integration of colorimetric materials as gas sensitive substances. Color changes, due to the reaction with the target gas, can be detected optically. These kinds of sensors offer various advantages, such as fast response times and the use of simple instrumentation. Being selective, reliable and energy-saving, the integration into different low-power applications such as fire detectors, RFID-labels or energy autarkic systems and sensor networks is possible. Therefore a low-cost and fully reversible sensor system capable to detect CO, NO 2 , NH 3 and C 2 H 4 at low concentrations in air was developed.
The determination of residual palladium chloride following reaction with carbon monoxide microdiffused from 0.5 ml blood has been accomplished by the formation of a highly stable, violet-colored complex with promazine hydrochloride. An aliquot of palladium solution from the center well of the diffusion cell is sufficient for the analysis, and in the concentration range of 1–11 ugm Pd(II)/ml of complex (equivalent to 0–30 vol% CO), the absorbance conforms to Beer's Law. For volumes percent CO the coefficient of variance is 6.9% and the standard deviation is 0.73.
The design, fabrication and validation of an optoelectronic sensor implemented in an easy-to-use portable device for the selective and sensitive detection of CO in air is reported herein. The system is based on the colour changes observed in the binuclear rhodium complex of formula [Rh-2[C6H4)P(C6H5)(2)](2)(O2CCF3)(2)] (CF3CO2H)(2) (1) upon coordinating CO molecules in axial positions. Complex 1 is used supported on cellulose chromatography paper. In this support, colour changes to the naked eye are observed for CO concentrations above 50 ppm. The probe is also implemented in a simple portable optoelectronic device. The cellulose support containing probe 1 in this device is placed inside a small dark chamber, is illuminated with a tricolour LED emitting at 624, 525 and 470 nm, respectively corresponding to red (R), green (G) and blue (B) light, and reflected light is detected by a photodiode. With a transimpedance amplifier, the current generated by the photodiode is transformed into a voltage compatible with the 10-bit analogue-to-digital converter (ADC) port. Colour changes are measured as the distance d between the R, G and B data of the blank (probe without CO) and that for a certain CO concentration. Typical calibration curves are fitted using a bi-exponential equation. This system offers a typical response time of a few minutes (ca. 7 min) and a limit of detection of 11 ppm. The probe in the cellulose supports is also highly reversible. The optoelectronic device is portable (dimensions 14 x 8.5 x 3.5 cm; weighs approximately 270 g) and is powered by AA batteries. In addition, no variations in experimental parameter d upon exposure to CO2, N-2, O-2, Ar, water-saturated air and vapours of chloroform, hexane, ethanol, acetone, methane, toluene or formaldehyde are observed. Besides, colour changes are found for acetonitrile vapour, NO and NO2, but only at high concentrations. For validation purposes, the device was used to determine the CO present in the 4-shed accumulated smoke of two cigarette types after passing smokers' lungs.
Gas sensors based on 2,9,16,23-tetra(2,6-dimethylphenoxy)metallophthalocyanine (TDMP-MPc) films on interlocking silver electrodes were fabricated by solvent casting from chloroform solutions containing 2,9,16,23-tetra(2,6-dimethylphenoxy)copper, cobalt or nickel phthalocyanine (TDMP-CuPc, TDMP-CoPc or TDMP-NiPc). The sensitivity of the gas sensors to the presence of NO2 and the depletion of O2 was investigated. It was confirmed that the gas sensors based on TDMP-CuPc and TDMP-CoPc films could detect 1 ppm NO2 at room temperature and that the current responses to the presence of NO2 and the depletion of O2 were considerably quick. These results indicate that adsorption and desorption of NO2 and O2 on the surfaces of TDMP-MPc films are easily achieved at room temperature.
Coloration reactions between nitrogen dioxide (NO2) and organic compounds, such as diazocoupling reactions, were demonstrated to occur in pores by exposing organically impregnated porous glass chips to NO2. An NO2 concentration of under 1 ppm can be estimated from the absorbance changes after the coloration reactions. Good linear relationships were obtained between the absorbance changes and exposure time at 200 ppb, indicating the possibility of environmental-level analysis by accumulation. Application to an actual analysis showed that this method can give the variations in NO2 concentrations at the environmental level.