applied
sciences
Review
Ammonia Gas Sensors: Comparison of Solid-State
and Optical Methods
Zbigniew Bielecki 1, *, Tadeusz Stacewicz 2 , Janusz Smulko 3
1
2
3
*
and Jacek Wojtas 1
Institute of Optoelectronics, Military University of Technology, 00-908 Warsaw, Poland;
jacek.wojtas@wat.edu.pl
Institute of Experimental Physics, Faculty of Physics, University of Warsaw, 02-093 Warsaw, Poland;
tadeusz.stacewicz@fuw.edu.pl
Faculty of Electronics, Telecommunications and Informatics, Gdansk University of Technology,
80-233 Gdansk, Poland; janusz.smulko@pg.edu.pl
Correspondence: zbigniew.bielecki@wat.edu.pl
Received: 19 June 2020; Accepted: 24 July 2020; Published: 25 July 2020
Abstract: High precision and fast measurement of gas concentrations is important for both
understanding and monitoring various phenomena, from industrial and environmental to medical
and scientific applications. This article deals with the recent progress in ammonia detection using
in-situ solid-state and optical methods. Due to the continuous progress in material engineering
and optoelectronic technologies, these methods are among the most perceptive because of their
advantages in a specific application. We present the basics of each technique, their performance limits,
and the possibility of further development. The practical implementations of representative examples
are described in detail. Finally, we present a performance comparison of selected practical application,
accumulating data reported over the preceding decade, and conclude from this comparison.
Keywords: ammonia detection; NH3 ; MOX sensors; polymer sensors; laser absorption spectroscopy;
CRDS; CEAS; MUPASS; PAS
1. Introduction
Ammonia is a highly toxic chemical substance, common in biological processes, and applied in
technical installation processes (cooling systems, chemical industry, and motor vehicles). The American
Conference of Industrial Hygienists has set a limit to ammonia concentration in air of 25 ppm for
long-term exposure (8 h) and 35 ppm for short-term ones (15 min) [1]. In medicine, the concentration
of ammonia in the breath between 2500 and 5000 ppb is directly related to organ dysfunction and
diabetes [2]. Therefore, the design of novel techniques and sensors which allow accurate and fast
in-situ detection of trace ammonia concentration is highly desirable. Such sensors should satisfy the
specific requirements: high sensitivity, enhanced selectivity, short response time, reversibility, high
reliability, low energy consumption, low cost, safety, broad range of measurement at various operation
temperatures, etc.
The issue of ammonia detection by gas sensors has attracted many researchers. A recent review
paper about ammonia sensing focused on chemical mechanisms of gas sensing by solid-state or
electrochemical sensors, with limited details about optical methods [3]. In our studies, we focus on
optical methods, developed in our research teams. Moreover, another way of solid-state sensors
modulation by UV irradiation was proposed and discussed. This method was advanced in the research
teams preparing this review. These problems have not been considered in such a way in other papers
about gas sensing.
Very advanced approaches like gas chromatography–mass spectrometry (GC-MS) and selective
ion flow tube–mass spectrometry (SIFT-MS) are accurate for NH3 measurement, but their use is
Appl. Sci. 2020, 10, 5111; doi:10.3390/app10155111
www.mdpi.com/journal/applsci
Very advanced approaches like gas chromatography–mass spectrometry (GC-MS) and selective
ion flow tube–mass spectrometry (SIFT-MS) are accurate for NH3 measurement, but their use is
complicated and require qualified staff, laborious samples preparation procedures, and time
consuming measurements, as well as not-compact and expensive instruments which are difficult to
maintain.
Many
applications require faster and easier tools. Therefore, solid and optical detection
Appl.
Sci. 2020,
10, 5111
2 of 17
methods are rapidly developing (Figure 1). In this paper, much attention was paid on the optical
sensors.
complicated
andlike
require
qualified that
staff,we
laborious
samples
and
time consuming
We would
to underline
preselected
thepreparation
considered procedures,
ammonia gas
sensors
and there
measurements,
as
well
as
not-compact
and
expensive
instruments
which
are
difficult
to maintain.
are other promising technologies which can be effectively used for ammonia sensing because
of low
Many
applications
require
faster
and
easier
tools.
Therefore,
solid
and
optical
detection
methods
are
production costs or measurement methods. Good examples are biosensors utilizing bacteria cultures
rapidly
developing
(Figure
1).
In
this
paper,
much
attention
was
paid
on
the
optical
sensors.
or nanotechnology [4–8].
Figure 1. Ammonia detection technology.
We wouldAmmonia
like to underline
2. Solid-State
Sensorsthat we preselected the considered ammonia gas sensors and there
are other promising technologies which can be effectively used for ammonia sensing because of low
Solid-state ammonia sensors can be divided into two groups considered in our paper: metal
production costs or measurement methods. Good examples are biosensors utilizing bacteria cultures
oxide-based sensors and conducting polymer sensors [3].
or nanotechnology [4–8].
Metal oxide-based sensors (MOX) belong to the most investigated groups. Their main features
offer
simplicity,
comprehensive
2.
Solid-State
Ammonia
Sensors detection action, miniature dimensions, flexibility in fabrication,
long life expectancy, low cost, and serviceableness for alarm warning applications. MOX sensors
Solid-state
ammonia
sensors
be divided
into two
groups
considered
in ourofpaper:
metal
change
DC (static)
resistance
whencan
exposed
to ambient
gas or
humidity.
Any change
MOX sensor
oxide-based
sensors
conducting
resistance can
triggerand
an alarm
whenpolymer
toxic gassensors
appears[3].
in an ambient atmosphere.
Metal
oxide-based
sensors
(MOX)
belong
to the
most investigated
groups. Their
main
features
There is a variety of sensitive materials and
methods
of their preparation
for NH
3 detection.
offer
comprehensive
detection
action, miniature dimensions, flexibility in fabrication, long
Metalsimplicity,
oxides, like
SnO2, ZnO, WO
3, TiO2, and MoO3, are most widely utilized for this purpose. They
life
expectancy,
low
cost,
and
serviceableness
for alarm
applications. MOX
sensors
change
DC
are classified into n-type and p-type [3]. Usually,
the warning
n-type semiconducting
metal
oxides
are used
(static)
resistance
when
exposed
to
ambient
gas
or
humidity.
Any
change
of
MOX
sensor
resistance
for gas sensors due to their higher sensitivity [9]. The sensor comprises of grains of various diameter
can
anactive
alarmsurface
when toxic
appears
in anenable
ambient
atmosphere.
withtrigger
a large
area.gas
Smaller
grains
better
sensing properties due to the larger
There
is
a
variety
of
sensitive
materials
and
methods
of
their
preparation
3 detection.
ratio of active surface to the sensor volume. Atoms of oxygen are
bound tofor
theNH
grains.
When Metal
these
oxides,
like
SnO
,
ZnO,
WO
,
TiO
,
and
MoO
,
are
most
widely
utilized
for
this
purpose.
They
2
3
2
3
atoms are displaced by other species present in ambient atmosphere, a potential barrier between are
the
classified
into n-type
p-type
[3]. (static)
Usually,
the n-type
semiconducting
metal
oxides changes.
are used for gas
grains changes.
As a and
result,
the DC
resistance
between
the sensor’s
terminals
sensors
due to their
higher
sensitivity
The
sensor comprises
grains
of various
diameter
Although
the metal
oxide
sensors[9].
have
attractive
properties,oftheir
main
disadvantage
is awith
low
aselectivity
large active
surface
area.
Smaller
grains
enable
better
sensing
properties
due
to
the
larger
ratio
of
in detecting of one particular component in a gas mixture [10]. Moreover, MOX sensors
active
surface
to
the
sensor
volume.
Atoms
of
oxygen
are
bound
to
the
grains.
When
these
atoms
operate at elevated temperatures (up to a few hundreds of Celsius degrees), which requires
are
displaced
by other
species present
ambientof
atmosphere,
a potential
barrier
between the grains
additional
energy
for heating.
Carefulinselection
ingredients,
accelerating
adsorption–desorption
changes.
a result,
the DCproperties,
(static) resistance
between
sensor’stemperature
terminals changes.
processesAs
due
to catalytic
helps to
lower the
operating
and to enhance gas
Although
the
metal
oxide
sensors
have
attractive
properties,
their
main disadvantage
a low
selectivity, pointing at selected gases. The sensor exhibits limited specificity
for NH3 andiscan
be
selectivity
in
detecting
of
one
particular
component
in
a
gas
mixture
[10].
Moreover,
MOX
sensors
affected by acetone, ethanol, hydrogen, methane, nitric oxide, and nitrogen dioxide [11]. Artificial
operate
at elevatedconductance
temperaturesscanning
(up to a few
hundreds of Celsius
which
neural networks,
at periodically
varied degrees),
temperature,
asrequires
well asadditional
principle
energy
for
heating.
Careful
selection
of
ingredients,
accelerating
adsorption–desorption
processes
due
component or support vector machine analysis help to develop selective sensor systems,
but only
to
to
catalytic
properties,
helps
to
lower
operating
temperature
and
to
enhance
gas
selectivity,
pointing
at
some extend [12,13]. The algorithms support to determine even particular components of the gas
selected
gases.
The
sensor
exhibits
limited
specificity
for
NH
and
can
be
affected
by
acetone,
ethanol,
3 from 25 ppb to 100 ppm (Table 1). The
mixtures. The average detection limit of these sensors ranges
hydrogen,
methane,
nitricimproved
oxide, and
dioxide
[11]. [9,10]
Artificial
neural networks,
conductance
selectivity can
be further
bynitrogen
noble metals
doping
or selecting
appropriate
operating
scanning at periodically varied temperature, as well as principle component or support vector machine
analysis help to develop selective sensor systems, but only to some extend [12,13]. The algorithms
support to determine even particular components of the gas mixtures. The average detection limit of
these sensors ranges from 25 ppb to 100 ppm (Table 1). The selectivity can be further improved by
noble metals doping [9,10] or selecting appropriate operating temperature [14]. Unfortunately, even
these actions give limited results and require additional advances.
Appl. Sci. 2020, 10, 5111
3 of 17
Another approach improving gas sensitivity and selectivity by MOX sensors utilize low-frequency
noise (flicker noise) which can be more sensitive to the ambient atmosphere than DC resistance
only [15,16]. This idea was proposed more than two decades ago and is still developed. DC resistance
gives a single value due to a change of potential barrier at the presence of ambient gases. At the same
time, the potential barrier fluctuates slowly, and these fluctuations are observed as flicker noise of the
resistance fluctuations. Therefore, low-frequency noise measured as a power spectral density can be
more informative than the value of DC resistance only. We confirmed experimentally that a single
MOX sensor could detect two toxic gases, NH3 and H2 S, by applying low-cost measurement set-up
and flicker noise measurements [17]. MOX sensors are made of porous materials, which generate
quite intense flicker noise components. Low-frequency noise is observed up to a few kHz at least and,
therefore, can be measured using common electronic circuits (e.g., low-noise operational amplifiers
and A/D converters sampling the signals up to tens of kHz only).
Some materials used for MOX sensors are photocatalytic (e.g., WO3 , TiO2 , Au nanoparticles
functionalized with organic ligands). These materials can be modulated by UV-light irradiation
(e.g., applying low-cost UV LEDs of different emitted wavelengths) [18]. UV light generates ions
O2 − , which are weakly bound to the surface of the grains and therefore enhance gas sensitivity. We
confirmed experimentally that this modulation improves sensitivity at low gas concentrations and can
be utilized for the detection of selected organic vapors (e.g., formaldehyde, NO2 [18]).
The MOX sensors exhibit a temporal drift and ageing of their sensitivity. This detrimental effect can
be reduced by algorithms of signal processing or by measuring the derivative parameters (e.g., change
of DC resistance only) at relatively short time intervals [19,20]. Such effects are induced by the structure
and technology of the MOX sensors, which comprise of grains of different size and morphology. Some
ambient gases can be stably adsorbed by the grains and induce the ageing and drift of sensitivity.
These effects can be reduced by pulse heating or intense UV irradiation, used for sensor cleansing [21].
Improvement in gas sensing stability can be reached by applying the materials of very repeatable
structures. Two-dimensional materials (MoS2 , WS2 , phosphorene, graphene, carbon nanotubes) are
of high interest for gas sensing due to their unique properties (high ratio of the sensing area to
volume) and repeatable morphology (e.g., graphene layers) [22]. The sensors provide an opportunity
to detect even a single gas molecule adsorbed by the active surface. An electronic device, using
single-layer graphene for the gate of the field-effect transistor, can detect low gas concentrations and
give repeatable results. Recent experimental studies confirmed that 1/f noise in such device has a
component, called Lorentzian, of the frequency characteristic for the ambient gas (e.g., C4 H8 O, CH3 OH,
C2 H3 N, CHCl3 ) [23]. Moreover, the experiments gave repeatable results for different samples of
electronic devices. We expect that such sensors should detect NH3 molecules at very low concentrations
in a similar way as the organic gases mentioned above. It was experimentally confirmed that the
oxidized graphene sensor is suitable for NH3 detection [24].
These impressive results were achieved for the structures, which are very repeatable and can
be spoiled only by some limited impurities situated on the two-dimensional layers. Unfortunately,
their production is expensive and requires specialized equipment. We may expect that low-cost MOX
gas sensors might be produced using a mixture of graphene flakes decorated with the nanoparticles
of the selected material for gas sensing. These sensors might be less sensitive than the presented
electronic device. However, the morphology of repeatable two-dimensional structures should enhance
their sensing properties in conjunction with very low costs and simplified technology of production.
Further, gas sensing improvement can be achieved by UV-light irradiation because graphene is a
photocatalytic material.
Flexible gas sensing materials are of great interest for emerging applications in wearable electronic
devices for portable health monitoring applications. Detection of NH3 is one of the hot topics in
this area because ammonia may cause severe harm to the human body and is the most common air
contaminant emitted from various sources (e.g., at the decomposition of protein products). The gas
sensing materials should be transparent, mechanically durable, and operate at room temperature.
Appl. Sci. 2020, 10, 5111
4 of 17
New structures of low-cost ammonia gas sensors were proposed recently [25–28]. These structures
comprise different materials, including two-dimensional reduced graphene oxide [26], one-dimensional
nanostructures (nanowires) [27], or polymers [25,28]. The sensors are chemo-resistive and, therefore,
can be used in wearable or handheld portable applications. Moreover, their resistance can be monitored
by utilizing triboelectric charging in self-powered wearable applications.
The second group of solid-state ammonia sensors consists of conducting polymer sensors.
They represent important class of functional organic materials for next-generation sensors. Their features
of high surface area, small dimensions, and unique properties have been used for various sensor
constructions. Many remarkable examples have been reported over the past decade. The enhanced
sensitivity of conducting polymer nanomaterials toward various chemical/biological species and
external stimuli made them ideal candidates for incorporation into the design of the sensors. However,
the selectivity and stability can be further improved.
Advances in nanotechnology allow the fabrication of various conducting polymer nanomaterials [29].
Among the conducting polymers, polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), and poly
(butyl acrylate) (PBuA) or poly (vinylidene fluoride) (PVDF) are the most frequently used in the ammonia
sensor as the active layer [30–32]. Gas sensors with conducting polymer are based on amperometric,
conductometric, colorimetric, gravimetric, and potentiometric measuring techniques. However, their
selectivity and stability should be also improved.
Some recently reported solid-state ammonia sensors are summarized in Table 1.
Table 1. Parameters of some solid-state ammonia sensors.
Sensor
Material
Detection
Limit
Response
Time
Recovery Time
Operation
Temperature
Reference
Metal Oxide
SnO2 /In2 O3
SnO2 /Pd/RGO
SnO
TiO2 /GO/PANI
SnO2 (type
GGS10331)
100 ppb
2 ppm
5 ppm
100 ppm
~0.1 min
7 min
<2 min
~0.5 min
10 s
50 min
30 s
17 s
RT
RT
RT
RT
[33]
[34]
[35]
[36]
5 ppm
<1 min
~few minutes
300–500 ◦ C
[37]
RT
RT
RT
RT
RT
[38]
[39]
[40]
[41]
[42]
Conducting Polymer
PANI/SWNT
PANI-TiO2 -gold
PANI/TiO2
PANI/Cu
PANI/graphene
50 ppb
10 ppm
25 ppb
1 ppm
1 ppm
~few minutes
<1.5 min
~0.1 min
<1 min
~few hours
160 s
23 s
Where: GO—graphene oxide, RGO—reduced graphene oxide, RT—room temperature, PANI—polyaniline,
PPy—polypyrrole, SWNT—single-walled carbon nanotubes.
Colorimetric sensors utilize optical measurements of the sample (porous matrix) interacting with
the ambient gas. Therefore, we present these gas sensors in the group of solid-state sensors. They are
used for the measurement of NH3 in blood, urine, and wastewater. Colorimetric gas sensors are
based on the change in color of a chemochromic reagent incorporated in a porous matrix such as
porphyrin-based or pH indicator-based films. To detect this change in color, three basic components are
needed: a light source, the chemochromic substance, and the light sensor. Liu et al. [43] has developed
a solid-state, portable, and automated device capable to measure total ammonia amount in liquids,
including the biological samples (e.g., urine). The idea of operation of the colorimetric sensor is shown
in Figure 2. A horizontal gas flow channel passing through the sensing chamber, a red LED light
source, and four photodiodes (a sensing-reference pair and a sensing-reference backup pair) were
applied. The target gas is exposed to the sensor which then exhibits a color change proportional to the
NH3 concentration. The photodiodes convert the color change to electronic signals. Such a sensor is of
Appl. Sci. 2020, 10, 5111
5 of 17
Appl. Sci. 2020, 10, x FOR PEER REVIEW
5 of 17
high sensitivity,
short signals.
responseSuch
time,aand
fast reversibility
for NH3 gas
concentrations
ranging
change
to electronic
sensor
is of high sensitivity,
short
response time,
andfrom
fast
2
ppm
to
1000
ppm.
reversibility for NH3 gas concentrations ranging from 2 ppm to 1000 ppm.
Appl. Sci. 2020, 10, x FOR PEER REVIEW
5 of 17
change to electronic signals. Such a sensor is of high sensitivity, short response time, and fast
reversibility for NH3 gas concentrations ranging from 2 ppm to 1000 ppm.
Figure
Figure 2.
2. The
The schematic
schematic of
of the
the colorimetric
colorimetric optoelectronic
optoelectronic ammonia
ammonia sensor
sensor [43].
[43].
Optical Methods
Methods
3. Optical
Figure 2. The schematic of the colorimetric optoelectronic ammonia sensor [43].
Most of the optical approaches to ammonia detection are based on the effects of absorption and
3. Optical Methods
luminescence, more rarely on the refraction
refraction or
or the
the light
light reflection.
reflection.
Most
of
the
optical
approaches
to
ammonia
detection
are
on theineffects
of absorption
and The
an optical
opticalabsorption
absorptiontechnique,
technique,thethe
measured
is contained
in sensor
the
sensor
chamber.
In an
measured
gasgas
is based
contained
the
chamber.
luminescence,
more
rarely
on
the
refraction
or
the
light
reflection.
The
radiation
passing
through
the chamber
canabsorbed
be absorbed
by molecules.
gas molecules.
Measuring
the
radiation
passing
through
the chamber
can be
by gas
Measuring
the light
In an optical
absorption
technique, the
measured
gas is contained
in
thespecies
sensor absorbing
chamber. The
light
absorption
at
the
specific
wavelength
(λ),
with
possibly
no
other
gas
in
this
absorption at the specific wavelength (λ), with possibly no other gas species absorbing in
radiation passing through the chamber can be absorbed by gas molecules. Measuring the light
spectral range, the concentration (N) of ammonia can be
be determined.
determined. The concentration estimation
absorption at the specific wavelength (λ), with possibly no other gas species absorbing in this
follows the Beer–Lambert absorption law
spectral range, the concentration (N) of ammonia can be determined. The concentration estimation
follows the Beer–Lambert absorption law
I0
α𝛼(λ𝜆)L𝐿 = 𝜎
σ(λ
NL = 𝑙𝑛
ln
𝜆 )𝑁𝐿
𝐼0I
,
𝛼(𝜆)𝐿 = 𝜎(𝜆)𝑁𝐿 = 𝑙𝑛 ( ),
(1)(1)
(1)
𝐼
where α(λ)
α(λ) is the absorption coefficient defined as the logarithmic
logarithmic ratio of the
the incident
incident (I
(I00) and the
where α(λ)
is the
absorption
coefficient
defined
as
the
logarithmic
ratio
of
the
incident
(I
0) denotes
transmitted
(I) light
intensities,
L
denotes
the
light
path
length
in
the
chamber,
and
σ(λ)
intensities, denotes the light path length
σ(λ) and the the
transmitted
(I)
light
intensities,
L
denotes
the
light
path
length
in
the
chamber,
and
σ(λ)
denotes
the nm,
Ammonia exhibits
exhibits UV
UV and
and IR
IR absorption
absorption bands
bands around
around 135
135
nm, 155
155
nm,
absorption cross section. Ammonia
nm,
absorption
cross
section.
Ammonia
exhibits
UV
and
IR
absorption
bands
around
135
nm,
155
nm,
195 nm, 1.5 µm,
µm, 2 µm,
µm, 3 µm,
The cross-section
cross-section (σ),
(σ),
µm, 4 µm,
µm, 6 µm,
µm, around 11 µm,
µm, and 16 µm
µm (Figure 3). The
195 nm, 1.5 µ m, 2 µ m, 3 µ m, 4 µ m, 6 µ m, around 11 µ m, and 16 µ m (Figure 3). The cross-section (σ),
defining the system sensitivity, is the highest in the UV region. It is about 10-times larger than at the
defining the system−18
sensitivity,
is the highest
in the UV region. It is about 10-times larger than at the
5 -times
5-times
11 µm
cm2 2 ) 2andeven
even1010
largerthan
thanininother
otherNIR
NIRranges
ranges[44].
[44].
µm region
region (e.g.,
(e.g., ~10
~10−18 cm
larger
−18 ) and
5
11 µ m region (e.g., ~10
cm ) and even 10 -times larger than in other NIR ranges [44].
(a)
(b)
(b)
Figure 3.
3. Absorption
Absorption(a)
cross-section
temperatures
in UV
[45,46]
(a). (a).
Absorption
Figure
cross-sectionofofammonia
ammoniaforfordifferent
different
temperatures
in UV
[45,46]
Absorption
cross-section
of
selected
molecules
existing
in
standard
conditions
(1
atm,
288.2
K)
calculated
on
the basis
cross-section
of
selected
molecules
existing
in
standard
conditions
(1
atm,
288.2
K)
calculated
on the
Figure 3. Absorption cross-section of ammonia for different temperatures in UV [45,46] (a). Absorption
of theofHITRAN
database
in IR range
(b).
basis
the
HITRAN
database
in
IR
range
(b).
cross-section of selected molecules existing in standard conditions (1 atm, 288.2 K) calculated on the basis
of the HITRAN
database ininIRthe
range
(b). NIR range, there is a problem with interferences by water vapor
Unfortunately,
UV and
and N2O or NO2 molecules, which occur at high concentrations in air. Their absorption bands might
Unfortunately, in the UV and NIR range, there is a problem with interferences by water vapor
and N2O or NO2 molecules, which occur at high concentrations in air. Their absorption bands might
Appl. Sci. 2020, 10, 5111
6 of 17
Unfortunately, in the UV and NIR range, there is a problem with interferences by water vapor
6 of 17
of 17
and N2 O or NO2 molecules, which occur at high concentrations in air. Their absorption bands 6might
overlap
with
spectra disturbing
disturbing the
the measurements.
Therefore, the
the selection
selection of
of proper
overlap with
with NH
NH333 spectra
spectra
measurements. Therefore,
Therefore,
proper spectral
spectral
overlap
NH
disturbing the
measurements.
the selection
of proper
spectral
range
is
crucial
for
successful
optical
detection.
range
is
crucial
for
successful
optical
detection.
range is crucial for successful optical detection.
Nondispersive
sensing
belongs
to simplest
approaches
of NH3 of
detection.
Figure 4
Nondispersive infrared
infrared(NDIR)
(NDIR)
sensing
belongs
to simplest
simplest
approaches
NH33 detection.
detection.
Nondispersive
infrared
(NDIR)
sensing
belongs
to
approaches
of NH
shows
diagram ofdiagram
such a gas
sensor.
It usually
of a consists
broadband
(cheaper
and
Figure a44schematic
shows aa schematic
schematic
of such
such aa gas
gas
sensor.consists
It usually
usually
of aasource
broadband
source
Figure
shows
diagram of
sensor.
It
consists of
broadband
source
smaller
black
body
emitters
or
IR
LEDs),
absorption
cell,
optical
filters,
and
detectors.
Radiation
from
(cheaper and
and smaller
smaller black
black body
body emitters
emitters or
or IR
IR LEDs),
LEDs), absorption
absorption cell,
cell, optical
optical filters,
filters, and
and detectors.
detectors.
(cheaper
the
broadband
source
passes through
thepasses
chamber
and two
filters.
The and
first filter
covers The
the absorption
Radiation
from
the
broadband
source
through
the
chamber
two
filters.
first filter
filter
Radiation from the broadband source passes through the chamber and two filters. The first
band
of
the
target
gas
(named
active
channel),
while
the
other
covers
a
non-absorbed
spectral
range
covers the
the absorption
absorption band
band of
of the
the target
target gas
gas (named
(named active
active channel),
channel), while
while the
the other
other covers
covers aa
covers
(the
reference channel).
Transmission
bands of
the filtersTransmission
should not overlap
with
absorption
bandsnot
of
non-absorbed
spectral
range
(the
reference
channel).
bands
of
the
filters
should
non-absorbed spectral range (the reference channel). Transmission bands of the filters should not
the
otherwith
gasesabsorption
present in bands
the chamber.
Thus, gases
absorption
in the
active
channelThus,
is proportional
overlap
of the
the other
other
present
in the
the
chamber.
absorptiontoin
inNH
the3
overlap
with absorption
bands of
gases present
in
chamber.
Thus, absorption
the
concentration.
The
light
transmitted
through
the
reference
channel
is
not
attenuated.
Therefore,
it
active channel
channel is
is proportional
proportional to
to NH
NH33 concentration.
concentration. The
The light
light transmitted
transmitted through
through the
the reference
reference
active
is
used
to
compensate
instabilities
of
the
light
source.
The
sensitivity
of
NDIR
is
influenced
by
the
channel is
is not
not attenuated.
attenuated. Therefore,
Therefore, it
it is
is used
used to
to compensate
compensate instabilities
instabilities of
of the
the light
light source.
source. The
The
channel
intensity
of
the
source,
the
optical
waveguide
and
detector
parameters.
The
disadvantage
of
this
sensitivity of
of NDIR
NDIR is
is influenced
influenced by
by the
the intensity
intensity of
of the
the source,
source, the
the optical
optical waveguide
waveguide and
and detector
detector
sensitivity
technique
is
the
low
precision
of
the
detecting
small
signal
changes
at
an
eventual
large
background,
parameters. The
The disadvantage
disadvantage of
of this
this technique
technique is
is the
the low
low precision
precision of
of the
the detecting
detecting small
small signal
signal
parameters.
which
results
in
low selectivity
and a high which
detection
limit.
changes
at
an
eventual
large
background,
results
in
low
selectivity
and
a
high
detection
limit.
changes at an eventual large background, which results in low selectivity and a high detection limit.
Appl. Sci. 2020, 10, x FOR PEER REVIEW
Appl. Sci. 2020, 10, x FOR PEER REVIEW
Figure
4.
Design
and
principle
of
operation
of
the
nondispersive
infrared
(NDIR)
sensor.
Figure 4.
4. Design
Design and
and principle
principle of
of operation
operation of
of the
the nondispersive
nondispersive infrared
infrared (NDIR)
(NDIR) sensor.
sensor.
Figure
There are
are various
For example,
performance of
of NDIR
NDIR sensors.
sensors. For
There
various methods
methods to
to improve
improve the
the performance
performance
of
NDIR
sensors.
example, Max-IR
Max-IR
Labs
utilizes
the
NDIR
technique
together
with
fiber-optic
evanescent
wave
spectroscopy
(FEWS)
for
utilizes
the
NDIR
technique
together
with
fiber-optic
evanescent
wave
spectroscopy
(FEWS)
Labs utilizes the NDIR technique together with fiber-optic evanescent wave spectroscopy (FEWS)
ammonia
detection
[47].
The
IR
radiation
is
transmitted
through
a
silver-halide
(AgClxBr1-x)
optical
for
ammonia
detection
[47].
The
IR
radiation
is
transmitted
through
a
silver-halide
(AgCl
xBr1-x)
for ammonia detection [47]. The IR radiation is transmitted through a silver-halide (AgClxBr1-x)
waveguide
withoutwithout
claddingcladding
and the detection
performed
by meansby
of means
the evanescent
field (Figure
5).
optical waveguide
waveguide
without
cladding
and the
the detection
detection
performed
by
means
of the
the evanescent
evanescent
field
optical
and
performed
of
field
−1
−1a. Such
The
maximum
of
the
peak
due
to
ammonia
absorption
was
observed
at
1450
cm
.
Such
sensor
(Figure
5).
The
maximum
of
the
peak
due
to
ammonia
absorption
was
observed
at
1450
cm
−1
(Figure 5). The maximum of the peak due to ammonia absorption was observed at 1450 cm . Such aa
sensor allows
allows
NH33 detection
detection
beyond
ppm limit.
limit.
allows
NH3 detection
beyondbeyond
a 1 ppm
sensor
NH
aa 11limit.
ppm
Figure 5.
5. Diagram
Diagram of
of the
the sensor
sensor based on
on NDIR and
and fiber-optic evanescent
evanescent wave
wave spectroscopy
spectroscopy
Figure
Figure
5. Diagram
of the
sensor based
based onNDIR
NDIR andfiber-optic
fiber-optic evanescent
wave spectroscopy
(FEWS) principles.
principles.
(FEWS)
(FEWS) principles.
Laser spectroscopy
spectroscopy is
is the
the best
best choice
choice for
for trace
trace gas
gas analysis
analysis among
among optical
optical approaches.
approaches. It
It is
is
Laser
choice
for
trace
gas
analysis
among
optical
approaches.
characterized
by
high
sensitivity
and
selectivity.
Ammonia
can
be
detected
using
tunable
laser
characterized by high sensitivity and selectivity.
Ammonia can
can be
be detected
detected using tunable laser
characterized
selectivity. Ammonia
absorption
spectroscopy
(TLAS),
multi-pass
optical
cell
(MUPASS),
cavity
ring down
down spectroscopy
spectroscopy
absorption spectroscopy (TLAS), multi-pass optical cell (MUPASS), cavity ring
(CRDS),
cavity-enhanced
absorption
spectroscopy
(CEAS),
and
photoacoustic
(PAS)
approach.
(CRDS), cavity-enhanced absorption spectroscopy (CEAS), and photoacoustic (PAS) approach.
TLAS is
is aa very
very interesting
interesting technique
technique for
for ammonia
ammonia concentration
concentration measuring
measuring using
using tunable
tunable diode
diode
TLAS
lasers.
The
advantage
of
TLAS
over
other
techniques
consists
in
its
ability
to
achieve
low
detection
lasers. The advantage of TLAS over other techniques consists in its ability to achieve low detection
limits by
by using
using optical
optical path
path extension
extension techniques
techniques and
and improving
improving the
the signal-to-noise
signal-to-noise ratio
ratio (SNR).
(SNR).
limits
Appl. Sci. 2020, 10, 5111
7 of 17
absorption spectroscopy (TLAS), multi-pass optical cell (MUPASS), cavity ring down spectroscopy
(CRDS), cavity-enhanced absorption spectroscopy (CEAS), and photoacoustic (PAS) approach.
TLAS is a very interesting technique for ammonia concentration measuring using tunable diode
lasers. The advantage of TLAS over other techniques consists in its ability to achieve low detection
Appl. Sci. 2020, 10, x FOR PEER REVIEW
7 of 17
limits by using optical path extension techniques and improving the signal-to-noise ratio (SNR).
A single-pass TLAS system, operating in direct absorption measurement mode, consists of a
A single-pass TLAS system, operating in direct absorption measurement mode, consists of a
tunable laser, transmitting optics, sample cell, receiving optics, photodetector, and a signal detection
tunable laser, transmitting optics, sample cell, receiving optics, photodetector, and a signal detection
circuit (Figure 6). The optics matches the laser and photodetector to the gas inside the sample cell.
circuit (Figure 6). The optics matches the laser and photodetector to the gas inside the sample cell. A
A thermoelectric controller (TEC) is used to set the laser-operating temperature to a value where the
thermoelectric controller (TEC) is used to set the laser-operating temperature to a value where the
desired wavelength can be reached due to injection current tuning. The injection current is usually
desired wavelength can be reached due to injection current tuning. The injection current is usually
scanned periodically with a ramp signal, which leads to laser wavelength scanning. The amplitude
scanned periodically with a ramp signal, which leads to laser wavelength scanning. The amplitude
of the scan should cover the absorption transition of interest. Laser radiation passes through the
of the scan should cover the absorption transition of interest. Laser radiation passes through the
absorption cell and the transmitted signal is measured using the photodetector. In absence of the
absorption cell and the transmitted signal is measured using the photodetector. In absence of the
absorption, the detector signal represents laser power changes vs. the current. When absorption occurs,
absorption, the detector signal represents laser power changes vs. the current. When absorption
a dip in transmission is observed. Ratio of the signals registered at the line center corresponding to the
occurs, a dip in transmission is observed. Ratio of the signals registered at the line center
case without absorption (I ) and with absorption (I1 ) can be used to calculate the gas concentration
corresponding to the case 0without absorption (I0) and
with absorption (I1) can be used to calculate
according to the formula (1).
the gas concentration according to the formula (1).
6. Block
diagram
and principle
of operation
of the typical
laser absorption
FigureFigure
6. Block
diagram
and principle
of operation
of the typical
tunabletunable
laser absorption
spectroscopy
spectroscopy
(TLAS)
sensor.
(TLAS) sensor.
Based on TLAS, different components
O, CO22,, and
and HCl
HCl can
can be
components such
such as
as NH
NH33, CO,
CO, O
O22, CH
CH44 , H22O,
TLAS has
has been
been employed
employed for
for various
various applications,
applications,
detected with high selectivity and sensitivity. TLAS
including industrial process monitoring and its control, environmental monitoring, combustion and
flow analyses, trace species measurements,
measurements, and so on. However,
However, the sensitivity
sensitivity of this technique is
−2
−3
−1
−1 (Equation
–10−3 cm
cm
(Equation
(1)).
usually limited to the absorption coefficient value of ~10−2–10
(1)).
Wavelength
modulation
spectroscopy
(WMS)
is
a
useful
technique
providing
improvement,
Wavelength modulation spectroscopy (WMS) is a useful techniqueSNR
providing
SNR
which is used in
high is
sensitivity
applications
[48].
Small modulation
(with
frequency (with
f ≈ 1–20
kHz) is
improvement,
which
used in high
sensitivity
applications
[48]. Small
modulation
frequency
to kHz)
the laser
currentto
scan,
provided
in awhich
similarisway
as in TLAS
The as
transmitted
fadded
≈ 1–20
is added
thewhich
laser is
current
scan,
provided
in a(Figure
similar7).way
in TLAS
signal,
detected
by
the
photodiode,
is
fed
to
a
lock-in
amplifier.
As
the
laser
is
scanned
across
an
(Figure 7). The transmitted signal, detected by the photodiode, is fed to a lock-in amplifier. As the
absorption
line profile,
theabsorption
transmitted
on-absorption
signal changes
at the frequency
2f, while
laser
is scanned
across an
line
profile, the transmitted
on-absorption
signalof
changes
at the
transmittedofoff-absorption
signal changes
at the frequency
f (Figure
frequency
2f, while the transmitted
off-absorption
signalofchanges
at 8).
the frequency of f (Figure 8).
Appl. Sci. 2020, 10, 5111
Appl. Sci. 2020, 10, x FOR PEER REVIEW
8 of 17
8 of 17
Figure
Theprinciple
principleofofoperation
operation of
of the
and
wavelength
the sensor
Figure
7. 7.The
sensor using
usingaacombination
combinationofofTLAS
TLAS
and
wavelength
modulation
spectroscopy
(WMS)
techniques.
modulation spectroscopy (WMS) techniques.
diode lasers.
lasers.
Figure 8. Idea of first and second harmonic detection with tunable diode
Application of high
frequency minimizes
minimizes 1/f
1/f noise. Therefore, setting the lock-in band
high frequency
band around
around
second harmonic ensures that the sensor
suitable for
for high
high sensitivity
sensitivity applications
applications
sensor becomes
becomes more
more suitable
compared to
Typical sensitivity
sensitivity limits the absorption
the standard
standard TLAS
TLAS approach.
approach. Typical
to the
absorption coefficient
coefficient
−4
−1 . Better limits, about 10
−5 –10
−7 cm
−1 , can be obtained
−4
−1
−5
−7
−1
achievable with WMS and is
about
10
cm
about
10
cm
.
Better
limits,
about
10
–10
cm
,
can
be
obtained
for
is
for
balanced
detection-based
WMS
[49].
This
method
involvesananelectrical
electricalcircuit
circuitthat
that subtracts
subtracts the
balanced
detection-based
WMS
[49].
This
method
involves
photocurrents
one of them registers the reference laser intensity while the other
photocurrents of two
two detectors:
detectors: one
measures
passing
through
the absorption
cell. That
opportunity
to reject common
measures the
thesignal
signal
passing
through
the absorption
cell. provides
That provides
opportunity
to reject
mode
laser
noise.
common
mode
laser noise.
Small absorptions,
absorptions, which
which occur
occur due
due to low densities of molecules in the samples or due to weak
line strengths,
strengths, are
are usually
usually compensated
compensated by
by extending
extending the
the optical
optical path
path with multi-pass cells (White or
Herriott) or cavity-enhanced
They enable
enable the
the optical path to be
cavity-enhanced methods. They
be extended.
extended. If instead of a
single-pass chamber we apply the multi-pass cell (Figure 9), the sensitivity
sensitivity of the sensor
sensor would
would be
be
improved by the
of the
the optical
optical path
path lengthening.
lengthening. A key parameter of multi-pass cells is the
the factor
factor of
ratio of path
path length
length to
to volume.
volume. Moreover, this
this system
system provides an
an opportunity
opportunity for
for the
the simultaneous
simultaneous
application
of
the
WMS
approach
and
2f
detection.
application of the WMS approach and 2f
Appl.
FOR PEER REVIEW
Appl. Sci.
Sci. 2020,
2020, 10,
10, x5111
Appl. Sci. 2020, 10, x FOR PEER REVIEW
99of
of 17
17
9 of 17
Figure 9. Scheme of a multi-pass experimental system for multi-pass optical cell (MUPASS)–WMS
Figure
Schemeofof
a multi-pass
experimental
system
for multi-pass
cell (MUPASS)–
Figure9.9. Scheme
a multi-pass
experimental
system
for multi-pass
opticaloptical
cell (MUPASS)–WMS
spectroscopy.
WMS spectroscopy.
spectroscopy.
Unlike conventional configurations, which involve at least a pair of mirrors separated by
Unlike
conventional
which
involve
least
a pair
mirrors
separated
by exactly
Unlike
conventional
configurations,
which
involve
at least
a of
pair
of mirrors
separated
by
exactly
defined
distances,configurations,
circular
multi-pass
cells
haveatbeen
developed
(Figure
10) [50].
Thanks
to
defined
distances,
circular
multi-pass
cells
have
been
developed
(Figure
10)
[50].
Thanks
to
the
single
exactly
defined
multi-pass
cells have
been expansion.
developed Minimizing
(Figure 10) [50].
Thanks
the
single
piece,distances,
the cell iscircular
especially
robust against
thermal
the cell
size to
is
piece,
the
especially
against
thermal
expansion.
cell size is the
desired
the
single
piece,
the cell isrobust
especially
robust
against
thermalMinimizing
expansion.the
Minimizing
cell for
sizethe
is
desired
forcell
theisdevelopment
of fast
and
portable
gas sensors.
development
fast and portable
gas
sensors.
desired
for theofdevelopment
of fast
and
portable gas sensors.
Figure 10. The circular multi-pass—small volume cell.
Figure 10. The circular multi-pass—small volume cell.
Figure 10. The
circularexploits
multi-pass—small
volume
cell.
Cavity ring-down spectroscopy
(CRDS)
the sample
cells in
the form of optical cavities
Cavity ring-down
spectroscopy
(CRDS)
exploits theAsample
cells scheme
in the form
of optical
cavities
(resonators)
built with mirrors
of very
high reflectivity.
simplified
of such
experiment
is
Cavity ring-down
spectroscopy
(CRDS)
exploits theAsample
cells scheme
in the form
of optical
cavities
(resonators)
built
with
mirrors
of
very
high
reflectivity.
simplified
of
such
experiment
is
presented in Figure 11. Laser pulse injected into the cavity through one of the mirrors is then reflected
(resonators)
built
with11.mirrors
of
veryinjected
high reflectivity.
A simplified
scheme
such
experiment
is
presented
Figure
pulse
the cavity
through
one ofofthe
mirrors
is then
many timesinamong
them.Laser
Its wavelength
must into
be tuned
to the NH
3 spectral line in order to measure
presentedmany
in Figure
Laser
pulse
injected intomust
the be
cavity
through
one3 spectral
of the mirrors
is then
reflected
times 11.
among
them.
Its wavelength
tuned
to the NH
line in order
to
the absorption coefficient of ammonia contained inside. The light transmitted through the exit mirror is
reflected
many
times among
them. Its
wavelength
must be inside.
tuned to
thelight
NH3transmitted
spectral linethrough
in orderthe
to
measure
the
absorption
coefficient
of
ammonia
contained
The
monitored by a detector. Analysis of the output signal by the acquisition system provides opportunity
measure
the
absorption
coefficient
of
ammonia
contained
inside.
The
light
transmitted
through
the
exit mirror is monitored by a detector. Analysis of the output signal by the acquisition system
to determine the Q-factor of the cavity which is limited due to diffraction and mirror losses as well as
exit mirror
is monitored
by a detector.
Analysis
of the
output
by the
system
provides
opportunity
to determine
the Q-factor
of the
cavity
whichsignal
is limited
dueacquisition
to diffraction
and
due to the light absorption or scattering inside the cell. As far as the Q-factor is inversely proportional
provides
opportunity
determine
the Q-factor
of the
cavity which
is limited
to diffraction
and
mirror
losses
as well astodue
to the light
absorption
or scattering
inside
the cell.due
As far
as the Q-factor
to the signal decay time, its value is found due to analysis of the photoreceiver signal by the acquisition
mirror
losses proportional
as well as duetoto the light
absorption
or scattering
the cell.
as the Q-factor
is
inversely
signal
decay time,
its valueinside
is found
dueAstofaranalysis
of the
system. The majority of the approaches exploit a two-step procedure consisting of the decay time
is inversely proportional
the signalsystem.
decay The
time,majority
its value
is found
due to
analysis
of the
photoreceiver
signal by thetoacquisition
of the
approaches
exploit
a two-step
measurement: first, when the cavity is empty (τ0 ), and then, when the cavity is filled with the tested
photoreceiver
signal
by
the
acquisition
system.
The
majority
of
the
approaches
exploit
a
two-step
procedure consisting of the decay time measurement: first, when the cavity is empty (𝜏 ), and then,
gas (τA ). These decay times depend on the mirror reflectivity, resonator length, and extinction factor
procedure
consisting
of with
the decay
time gas
measurement:
first, when
cavityonisthe
empty
(𝜏 ),
and then,
when
the cavity
is filled
the tested
(𝜏 ). These decay
timesthe
depend
mirror
reflectivity,
(absorption and scattering of light in the cavity) [51]. Comparing both decay times, the absorber
when the cavity
is filled
with the tested
gas(absorption
(𝜏 ). These decay
times depend
on theinmirror
reflectivity,
resonator
length,
and extinction
factor
and scattering
of light
the cavity)
[51].
concentration can be found.
!
resonator
length,
and
extinction
factor
(absorption
and
scattering
of
light
in
the
cavity)
[51].
Comparing both decay times, the absorber concentration
can
1
1
1 be found.
N
=
−
(2)
Comparing both decay times, the absorber concentration
be found.
cσ(λ) τA τcan
0
,
where c is the light speed and σ(λ) is the 𝑁
absorption cross-section.
𝑁
,
where c is the light speed and σ(λ) is the absorption cross-section.
where c is the light speed and σ(λ) is the absorption cross-section.
(2)
(2)
Appl. Sci. 2020, 10, 5111
10 of 17
Appl. Sci. 2020, 10, x FOR PEER REVIEW
10 of 17
Figure 11.
11. Idea
ring down
down spectroscopy
spectroscopy (CRDS).
(CRDS).
Figure
Idea of
of the
the cavity
cavity ring
There are various
various approaches to CRDS
CRDS with
with pulsed
pulsed lasers
lasers or
or with
with continuously
continuously operating
operating AM
−9
−9
−1 can be
ones. Using
absorption coefficients
coefficients αα== σN
σN<< 10
10 cm
cm−1
modulated ones.
Using these techniques, the absorption
observed. The
The detection
detectionlimit
limitisismainly
mainlyrelated
relatedtotothe
theresonator
resonator
quality
(determined
), but
observed.
quality
(determined
byby
τ0 ),𝜏 but
alsoalso
by
by precision
the precision
of decay
time measurement.
The advantage
of CRDS
over otherspectroscopy
absorption
the
of decay
time measurement.
The advantage
of CRDS over
other absorption
spectroscopyconsists
approaches
consists
not only
in dominant
superiority
results
approaches
not only
in dominant
sensitivity.
Theirsensitivity.
superiorityTheir
also results
from also
minimized
from minimized
impacts
of light
source intensity
fluctuations
or changes
detectoron
sensitivity
changes results.
on the
impacts
of light source
intensity
fluctuations
or detector
sensitivity
the measurement
measurement
results.
This method
is extremely sensitive but requires that the laser frequency is precisely matched to
This method
sensitive
but requires
the laser
frequency
is preciselyand
matched
to
the cavity
mode. is
Inextremely
this way, it
is possible
to obtainthat
a high
Q-factor
of the resonator
efficient
the
cavity
mode.
In
this
way,
it
is
possible
to
obtain
a
high
Q-factor
of
the
resonator
and
efficient
storage of optical radiation. On the other hand, small mechanical instabilities cause changes in the
storagemode
of optical
radiation.
On the other
hand,
small
mechanicalwhich
instabilities
causedisadvantage
changes in the
cavity
frequency
and significant
output
signal
fluctuations,
is the main
of
cavity
mode[52].
frequency
significant output
fluctuations,
which is the
mainordisadvantage
of
this
method
It can and
be minimalized
by use signal
of cavity
length stabilization
[53,54],
by application
this
method
can be
minimalized
use of
cavity length stabilization
[53,54], or by(CEAS)
application
of
the
cavity [52].
with It
dense
mode
structure,by
called
cavity-enhanced
absorption spectroscopy
[55].
of the
dense arrangement
mode structure,
absorption
spectroscopy
(CEAS)
In cavity
CEAS, with
the off-axis
of thecalled
laser cavity-enhanced
and cavity is applied.
Similarly,
to the conventional
[55]. system, the light is repeatedly reflected by the mirrors. However, the reflected beams, which
CRDS
In CEAS,
the off-axis
arrangement
of the laser
cavity is The
applied.
to the
correspond
to different
trips, are
spatially separated
insideand
the resonator.
use ofSimilarly,
highly reflecting
conventional
CRDS
system,
the light
repeatedly
reflected
by result,
the mirrors.
the structure
reflected
mirrors
provides
huge
extension
of theiseffective
optical
path. As
either aHowever,
dense mode
beams,
which occurs,
correspond
to mode
different
trips, are
spatially
separated
inside
theway,
resonator.
The use of
of
low finesse
or the
structure
does
not establish
at all.
In this
sharp resonances
highly
reflecting
mirrors so
provides
hugeisextension
the effective
optical path.
As result,
of
the cavity
are avoided,
the system
much lessof
sensitive
to mechanical
instabilities.
Theeither
CEASa
−1 [56].
dense mode
of low
finesse
occurs,
mode
structure
does not about
establish
at all.
this
sensors
attainstructure
the detection
limit
of about
10−9 or
cmthe
Detailed
information
CRDS
andIn
CEAS
way, sharpisresonances
arepapers
avoided,
so the system is much less sensitive to mechanical
techniques
presented of
in the
the cavity
authors’
[57–59].
instabilities.
sensors
attain
the detection
limit of spectroscopy
about 10−9 cm−1(PAS)
[56]. Detailed
information
Among The
the CEAS
so-called
in-situ
sensors,
photoacoustic
belongs to
the most
about CRDS
CEAS
techniques
is presented
in the
authors’
papers
[57–59].
popular
one. and
In PAS,
conversion
of laser
light energy
into
an acoustic
wave
is applied. The gas sample,
Among
the so-called
in-situ sensors,
photoacoustic
belongswith
to the
most
placed
in photoacoustic
chamber,
is irradiated
by optical spectroscopy
radiation, AM(PAS)
modulated
acoustic
popular one.
Inlaser
PAS,wavelength
conversion isofmatched
laser light
energy
into anline
acoustic
wave is of
applied.
The
frequency.
The
to the
absorption
of a molecule
interest.
If gas
the
sample,
placed
in
photoacoustic
chamber,
is
irradiated
by
optical
radiation,
AM
modulated
with
absorber is present in the cell, a portion of optical radiation is converted onto heat energy. Then, a local
acoustic
frequency.
laser wavelength
is matched
to the
absorption
lineresulting
of a molecule
of interest.
and
periodic
growthThe
of temperature
and pressure
occurs
(Figure
12). The
acoustic
wave is
If the absorber
is present
in the cell,
portion
of optical
radiationplaced
is converted
onto heatThe
energy.
detected
at modulation
frequency
by a very
sensitive
microphone
in the chamber.
PAS
Then, awhich
local is
and
periodic growth
of temperature
and pressure
occurs
signal,
proportional
to the absorber
concentration
(N), is given
by(Figure 12). The resulting
acoustic wave is detected at modulation frequency by a very sensitive microphone placed in the
Q absorber concentration
Q
chamber. The PAS signal, which
proportional
(N), is given by
)∝
A(T, λis
Po Nα(T, λ)Lto the
η = Po Nα(T, λ)
η
(3)
fV
fA
𝜆 ∝𝑃
𝐴 𝑇,
𝑇, 𝜆
𝐿 𝜂 factor
𝑃 𝑁𝛼
𝑇, 𝜆 resonant
𝜂,
where Po denotes average laser
power,
Q 𝑁𝛼
is the
quality
of the
cell, f is the frequency(3)of
modulation, V is the gas volume, A is the cross-sectional area, and η is the system efficiency factor (e.g.,
where Po denotes
average
laser
power, Q is the quality factor of the resonant cell, f is the
microphone
efficiency
and loss
factors).
frequency of modulation, V is the gas volume, A is the cross-sectional area, and η is the
system efficiency factor (e.g., microphone efficiency and loss factors).
Appl. Sci. 2020, 10, 5111
11 of 17
Appl. Sci. 2020, 10, x FOR PEER REVIEW
11 of 17
Figure 12. Idea of photoacoustic spectroscopy.
spectroscopy.
The microphone
signal
is recorded
usingusing
a low-noise
preamplifier
and a lock-in
microphoneoutput
output
signal
is recorded
a low-noise
preamplifier
and voltmeter.
a lock-in
The
strongest
is achieved
the modulation
frequency
is matched
to the
of the
voltmeter.
Theeffect
strongest
effectwhen
is achieved
when the
modulation
frequency
is resonance
matched to
photoacoustic
chamber.
In orderchamber.
to increase
signal,
various the
constructions
of photoacoustic
cells
resonance of the
photoacoustic
Inthe
order
to increase
signal, various
constructions
of
(acoustic
resonators
with higher
Q-factor)
are higher
applied.
photoacoustic
cells (acoustic
resonators
with
Q-factor) are applied.
The improvement of
of this
this technique,
technique,which
whichallows
allowstotoachieve
achievevery
very
high
detection
sensitivity,
high
detection
sensitivity,
is
is
the
quartz-enhanced
photoacoustic
spectroscopy
(QEPAS).
The
basic
principle
of
operation
is
the quartz-enhanced photoacoustic spectroscopy (QEPAS). The basic principle of
similar, but
but here
here the
the laser
laser beam
beam is
is focused
focused between
between the
the U-shaped
U-shaped prongs of aa resonant
resonant piezo-quartz
piezo-quartz
fork [60]. This
This type of acoustic transducer is sensitive to signals generated by asymmetric
asymmetric prongs
oscillations
acoustic
wave,
induced
by the
laser radiation.
ExternalExternal
sources
oscillations caused
causedbyby
acoustic
wave,
induced
byAM-modulated
the AM-modulated
laser radiation.
of
interference
do not provide
signals
because
they
cause symmetrical
oscillations. oscillations.
Similarly to
sources
of interference
do notoutput
provide
output
signals
because
they cause symmetrical
the
conventional
PAS system, PAS
in QEPAS
set-ups,
the measurements
are performedare
with
a wavelength
Similarly
to the conventional
system,
in QEPAS
set-ups, the measurements
performed
with
modulation
technique
using
the 2f detection
[61].
withCombined
the high Q-factor
the quartz
fork,
a wavelength
modulation
technique
using the
2f Combined
detection [61].
with theofhigh
Q-factor
of
it
gives
the fork,
opportunity
build
ultra-compact
sensors
that reach detection
limits
comparable
that
the
quartz
it givestothe
opportunity
to build
ultra-compact
sensors that
reach
detectiontolimits
of
CEAS. In case
of of
ammonia,
corresponds
to a few
ppb or eventosub-ppb
level.
comparable
to that
CEAS. Init case
of ammonia,
it corresponds
a few ppb
or even sub-ppb level.
Recently reported
reported parameters
parameters of
of some
some optical
optical ammonia
ammonia sensors
sensors are
are given
given in
in Table
Table2.2.
Table
Table 2.
2. Parameters of some optical ammonia
ammonia sensors.
sensors.
Sensing
Sensing
Methods
Methods
7 ppb
Radiation
Radiation
Source
Source
Deuterium
Deuterium
lamp
lamp
QCL
CRDS-cw
MUPASS-WMS
71.3
ppb
ppb
QCL
QCL-DFB
CRDS-cw
CRDS
(open-path)
0.74ppb
ppb
1.3
QCL-DFB
(cw-EC)
6.2 µm
10.33
µm
QCL-DFB
(pulsed)
QCL
EC-QCL
967.35 cm−1
NDIR
NDIR
MUPASS-WMS
(open-path)
CEAS
CRDSPAS
QEPAS
Detection
Detection
Limit
Limit
1 ppm
1 ppm
15 ppb
0.74
0.7 ppb
ppb
<10 ppb
QCL
(cw-EC)
QCL
Wavelength
Wavelength
Filter Filter
200–225 nm
200–225
−1
1103.44 cmnm
1103.44
cm−1
10.33 µm
6.2 µm
10.36
µm
10.6 µm
Other Parameter
Other ParameterReference
Reference
λcenter = 205 nm
[44]
λcenter
= 205
FWHM
= 10
nm nm
Eff.
path
FWHM = 10 nm [62]
length—76.45 m
Eff. Rpath
length—76.45
= 0.9995,
[63]
L = 50 cm
m
R = 0.9998,
R = 0.9995,
L = 50 cm,
[64]
L =Torr
50 cm
p = 115
L = 53 cm,
R = 0.9998,
[65]
ti = 5–10 ns
50 cm,
p=L
220=Torr
[66]
Ammonia
p = 115 Torr
detection in
[67]
L =breath
53 cm,
exhaled
[44]
[62]
[63]
[64]
(cw-EC)
QCL-DFB
967.35 cm−1
[65]
CEAS
15 ppb
(pulsed)
ti = 5–10
ns spectroscopy,
QCL—quantum cascade laser, PAS—photoacoustic
spectroscopy, CRDS—cavity
ring down
QEPAS—quartz-enhanced
photoacousticEC-QCL
spectroscopy, WMS—wavelength
PAS
0.7 ppb
10.36 µm modulation
p =spectroscopy,
220 Torr R—reflectivity
[66]
of mirrors, L—optical cavity length, EC—external cavity, p—pressure. Other methods of gas detection were
QCL
Ammonia detection in
described in our earlier
QEPAS
<10studies
ppb [68,69].
10.6 µm
[67]
(cw-EC)
exhaled breath
QCL—quantum cascade laser, PAS—photoacoustic spectroscopy, CRDS—cavity ring down
Among
many applications, the ammonia sensors for medical purpose are an important part
spectroscopy, QEPAS—quartz-enhanced photoacoustic spectroscopy, WMS—wavelength
of such detectors. The normal concentration of ammonia in the breath of a healthy man is in the
modulation spectroscopy, R—reflectivity of mirrors, L—optical cavity length, EC—external cavity,
range of 0.25–2.9 ppm [70]. An excessive concentration might suggest renal failure, Helicobacter Pyroli,
p—pressure. Other methods of gas detection were described in our earlier studies [68,69].
Appl. Sci. 2020, 10, x FOR PEER REVIEW
12 of 17
Among many applications, the ammonia sensors for medical purpose are an important part of
such
detectors. The normal concentration of ammonia in the breath of a healthy man is in the 12
range
Appl. Sci. 2020, 10, 5111
of 17
of 0.25–2.9 ppm [70]. An excessive concentration might suggest renal failure, Helicobacter Pyroli,
diabetes, and oral cavity disease [71,72]. The main problem in the construction of ammonia sensors
diabetes,
oral cavity
[71,72].
mainsensitivity
problem inbut
the in
construction
of ammonia
for
for
breathand
analysis
doesdisease
not consist
in The
extreme
high selectivity.
More sensors
than 3000
breath
analysis
does
not
consist
in
extreme
sensitivity
but
in
high
selectivity.
More
than
3000
various
various constituents were already detected in exhaled air [73]. Their absorption spectra might
constituents
were already
detected
in exhaled
air and
[73].disturb
Their absorption
spectra might
the
overlap
the spectral
fingerprint
of the
ammonia
the measurement.
Wateroverlap
vapor and
spectraldioxide
fingerprint
of the
ammonia
and disturb
measurement.
Water
andtocarbon
carbon
are the
main
interferences
since the
their
concentrations
can vapor
reach up
5% in dioxide
breath,
are the
main
interferences
since theirdensity
concentrations
can
reach of
upmagnitude.
to 5% in breath,
and monoxide
thus it exceeds
and
thus
it exceeds
the ammonia
by many
orders
Carbon
and
the
ammonia
density
by
many
orders
of
magnitude.
Carbon
monoxide
and
methane
should
also
be
methane should also be taken into account.
takenAs
intoit account.
was mentioned above, the highest sensitivities of ammonia detection using laser
As
it
was
mentionedcan
above,
the highest
sensitivities
ammonia
detection
using
laser
absorption
absorption spectroscopy
be achieved
in UV
and in a of
range
of 10–11
µm, due
to the
largest
values
spectroscopy
can
be
achieved
in
UV
and
in
a
range
of
10–11
µm,
due
to
the
largest
values
of the
of the absorption cross-sections. However, the detection at the wavelengths used
in
absorption
cross-sections.
However,
the
detection
at
the
wavelengths
used
in
telecommunication
telecommunication 1.4–1.6 µm is more convenient because of a large variety of relatively cheap laser
1.4–1.6
is more
convenient
because of a large
variety of relatively
cheap laser
sources,
optical
sources,µm
optical
elements,
and photodetectors.
The measurements
of ammonia
around
1.51 µm
with
elements,
and
photodetectors.
The
measurements
of
ammonia
around
1.51
µm
with
the
detection
the detection limit of 4 ppm was already demonstrated using the CEAS technique [74]. We found
limitsuitable
of 4 ppm
was already
using the
technique
We found
that1.5270409
suitable
that
wavelengths
fordemonstrated
ammonia detection
areCEAS
also the
lines at [74].
1.5270005
µm and
wavelengths
detection are
also wavelength
the lines at 1.5270005
µm and 1.5270409
(Figure 13).
µm (Figure for
13).ammonia
The single-mode
laser
was controlled
with the µm
HighPrecision
The
single-mode
laser
wavelength
was
controlled
with
the
HighPrecision
lambdameter
(model
lambdameter (model WS6) ensuring precision in a short time (1 min) of 0.0005 nm. The WS6)
NH3
ensuring
precision
in
a
short
time
(1
min)
of
0.0005
nm.
The
NH
absorption
coefficient
(at
2
ppm)
−6
−1
3
absorption coefficient (at 2 ppm) reaches here about 3.5∙10 cm , and experimental techniques like
−6 cm−1 , and experimental techniques like MUPASS–WMS can be effectively
reaches here about
3.5·10
MUPASS–WMS
can
be effectively
used. In this range, carbon dioxide and water vapor interference
used.
In eight
this range,
is
about
times carbon
weaker.dioxide and water vapor interference is about eight times weaker.
Figure 13.
13. Absorption
Absorption spectra
spectra of
of ammonia,
ammonia, H
H22O,
Figure
O, and
and CO
CO22 around
around 1.527
1.527 µm
µm [75].
[75].
Our ammonia sensor used the MUPASS–WMS approach (Figure 9). A single-mode diode laser
was applied
applied as aa light
light source.
source. Output signals were integrated over
(Toptica, model DL100, 20 mW) was
for NH
NH33
1 min. Results of sensor investigation are presented in Figure 14. It provides proper results for
concentrations
concentrations of up to about 1 ppm. For lower values, the deviation from a linear characteristic is
observed. This is
is probably
probablyaaresult
resultofofpoor
poor
regulation
of reference
concentration
inside
the sensor
regulation
of reference
concentration
inside
the sensor
due
due
to3NH
3 deposit
onwalls
the walls
the system,
causes
a systematic
error
of measured
data.
In
to NH
deposit
on the
of theofsystem,
whichwhich
causes
a systematic
error of
measured
data. In
order
order
to the
avoid
the concentration
measurement
disturbed
by NH3 molecules
on (and
the walls
to avoid
concentration
measurement
disturbed
by NH3 molecules
adsorbedadsorbed
on the walls
then
(and
then desorbed),
was
kept at a temperature
desorbed),
the sensor the
wassensor
kept at
a temperature
of 50 ◦ C. of 50 °C.
Nevertheless,
lines,
wewe
achieved
a good
immunity
of the
Nevertheless, due
due to
tothe
theproper
properchoice
choiceofofspectral
spectral
lines,
achieved
a good
immunity
of
detection
against
H2OHand
CO
2
at
the
concentrations
which
might
occur
in
breath
(Figure
13).
The
the detection
against
O
and
CO
at
the
concentrations
which
might
occur
in
breath
(Figure
13).
2
2
detection
limitlimit
of 1 of
ppm
was was
better
thanthan
in other
experiments
preformed
at a at
spectral
range
of 1.5
The
detection
1 ppm
better
in other
experiments
preformed
a spectral
range
of
1.5 µm [74]. Our multi-pass sensor is suitable for rough monitoring of ammonia in the exhaled air and
for detection of morbid states (>1.5 ppm).
Appl. Sci. 2020, 10, x FOR PEER REVIEW
13 of 17
µm [74].
Our10,multi-pass
Appl.
Sci. 2020,
5111
sensor is suitable for rough monitoring of ammonia in the exhaled air
and
13 of
17
for detection of morbid states (>1.5 ppm).
Figure 14.
14. Results
with MUPASS–WMS
MUPASS–WMS system
system [75].
[75].
Figure
Results of
of ammonia
ammonia measurement
measurement with
4.
Conclusions
4. Conclusions
Based on the analysis,
analysis, it can
can be
be concluded
concluded that
that the
the further
further development
development of
of in-situ
in-situ ammonia
ammonia
detection
technologies
will
focus
on
the
improvement
of
sophisticated
laboratory
equipment,
e.g.,e.g.,
the
detection technologies will focus on the improvement of sophisticated laboratory equipment,
mass
spectrometers
and lasers.
One can
the development
of sensors
dedicated
to a specific
the mass
spectrometers
and lasers.
Oneexpect
can expect
the development
of sensors
dedicated
to a
application.
This
article
focuses
on
the
second
group
of
the
sensors
in
which
two
technologies
with
high
specific application. This article focuses on the second group of the sensors in which two
potential
for application
development
have been distinguished:
solid-state
sensors and
technologies
with highand
potential
for application
and development
have ammonia
been distinguished:
laser
absorption
spectroscopy.
and operational
parametersMetrological
of such sensors
largely
depend
solid-state
ammonia
sensorsMetrological
and laser absorption
spectroscopy.
and
operational
on
the
application.
Basically,
they
must
be
characterized
by
fast
operation,
the
largest
measuring
parameters of such sensors largely depend on the application. Basically, they must be characterized
range
and
the highest
measurement
precision,range
high reliability,
selectivity,
as well as small
dimensions,
by fast
operation,
the
largest measuring
and the highest
measurement
precision,
high
and
low cost.
Sensors as
belonging
the dimensions,
former groupand
canlow
detect
theSensors
ammonia
with a concentration
reliability,
selectivity,
well as to
small
cost.
belonging
to the former
of
several
within
tens of seconds.
advantage
in very
smalltens
dimensions
andTheir
very
group
canppm
detect
the ammonia
with a Their
concentration
of consists
several ppm
within
of seconds.
low
cost. Materials
and nanotechnology
arevery
key technologies
for the development
this
advantage
consistsengineering
in very small
dimensions and
low cost. Materials
engineeringof and
sensor
group.
We
underlined
that
recent
advances
in
materials
and
measurement
methods
enhance
nanotechnology are key technologies for the development of this sensor group. We underlined that
gas
detection
by such
low-costand
gasmeasurement
sensors in portable
applications.
We detection
are conscious
that low-cost
our research
recent
advances
in materials
methods
enhance gas
by such
gas
presents
a limited
of NH3We
gasare
sensors.
Thisthat
issueour
is aresearch
hot topicpresents
in the gas
sensingnumber
area, and
sensors in
portablenumber
applications.
conscious
a limited
of
other
attractive
sensors
were
developed
recently
(e.g.,
capacitive
sensors
[76],
colorimetric
sensing
NH3 gas sensors. This issue is a hot topic in the gas sensing area, and other attractive sensors were
materials
[77],
or advanced
MOX sensors
[78]).[76],
The latter
group ofsensing
sensorsmaterials
can detect[77],
up toorfour
orders
developed
recently
(e.g., capacitive
sensors
colorimetric
advanced
of
lower
ammonia
fractions
a second.
However,
their costs
much
higher,
mainly
due to the
MOX
sensors
[78]).inThe
latter of
group
of sensors
can detect
up toare
four
orders
of lower
ammonia
in
lasers
and
high-quality
optical
components.
The
further
improvement
of
these
sensors
is
much
more
fractions of a second. However, their costs are much higher, mainly due to the lasers and
dependent
the development
of lasers
than improvement
photodetectorsof
[80],
because
they are
alreadymore
at a
high-qualityonoptical
components.
The [79]
further
these
sensors
is much
very
high level.
It mainly
concerns
progress
lasers
for mid-infrared
sinceare
onealready
can expect
dependent
on the
development
of alasers
[79] in
than
photodetectors
[80],radiation,
because they
at a
the
highest
sensitivity
and
selectivity
in
this
spectral
range.
However,
miniaturization,
reliability,
and
very high level. It mainly concerns a progress in lasers for mid-infrared radiation, since one can
reduction
production
costs areand
still selectivity
a current challenge
for all optical
components.
It should
expect theofhighest
sensitivity
in this spectral
range.sensor
However,
miniaturization,
be
also
mentioned
that
MOX
sensors
can
utilize
optical
methods
in
some
sense
by
applying
UV
to
reliability, and reduction of production costs are still a current challenge for all optical light
sensor
improve
the sensitivity
selectivity
of gas
detection.
Therefore,
types
of gas
sensorsinstart
to
components.
It should and
be also
mentioned
that
MOX sensors
can both
utilize
optical
methods
some
interlace
and optical
sensors
technologies
to reduceand
costsselectivity
and popularize
applications.
We
sense by MOX
applying
UV light
to improve
the sensitivity
of gastheir
detection.
Therefore,
believe
thatofdifferent
typesstart
of gas
sensors find
complementary
areas technologies
of new applications.
both types
gas sensors
to interlace
MOX
and optical sensors
to reduce costs and
popularize their applications. We believe that different types of gas sensors find complementary
Author Contributions: Z.B.: conceptualization, writing—original draft, visualization. T.S.: writing—review
areas
of new
applications.
&
editing,
investigation.
J.S.: writing—review & editing, investigation. J.W.: formal analysis, investigation,
writing—review & editing. All authors have read and agreed to the published version of the manuscript.
Author Contributions: Z.B.: conceptualization, writing—original draft, visualization. T.S.: writing—review &
Funding: The publication was supported by The Polish National Centre for Research and Development as part of
editing,
investigation.
J.S.: writing—review & editing, investigation. J.W.: formal analysis, investigation,
the “Sense”
project, ID 347510.
writing—review & editing.
Conflicts of Interest: The authors declare no conflict of interest.
Appl. Sci. 2020, 10, 5111
14 of 17
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