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Ammonia Gas Sensors: Comparison of Solid-State and Optical Methods

Ammonia Gas Sensors: Comparison of Solid-State and Optical Methods

applied sciences

Review Gas Sensors: Comparison of -State and Optical Methods

Zbigniew Bielecki 1,*, Tadeusz Stacewicz 2 , Janusz Smulko 3 and Jacek Wojtas 1 1 Institute of Optoelectronics, Military University of Technology, 00-908 Warsaw, Poland; [email protected] 2 Institute of Experimental , Faculty of Physics, University of Warsaw, 02-093 Warsaw, Poland; [email protected] 3 Faculty of Electronics, Telecommunications and Informatics, Gdansk University of Technology, 80-233 Gdansk, Poland; [email protected] * Correspondence: [email protected]

 Received: 19 June 2020; Accepted: 24 July 2020; Published: 25 July 2020 

Abstract: High precision and fast measurement of gas 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 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; absorption ; CRDS; CEAS; MUPASS; PAS

1. Introduction Ammonia is a highly toxic , 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 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 , 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 – spectrometry (GC-MS) and selective flow tube– (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 Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 17

Very advanced approaches like –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 Appl.maintain. Sci. 2020 Many, 10, 5111 applications require faster and easier tools. Therefore, solid and optical detection2 of 17 methods are rapidly developing (Figure 1). In this paper, much attention was paid on the optical sensors. complicatedWe would and like require to underline qualified that staff ,we laborious preselected samples the preparationconsidered procedures,ammonia gas and sensors time consuming and there measurements,are other promising as well technologies as not-compact which andcan be expensive effectively instruments used for ammonia which are sensing difficult because to maintain. of low Manyproduction applications costs or requiremeasurement faster and methods. easier tools.Good Therefore,examples are solid and optical util detectionizing bacteria methods cultures are rapidlyor nanotechnology developing [4–8]. (Figure 1). In this paper, much attention was paid on the optical sensors.

Figure 1. Ammonia detection technology.

2. Solid-StateWe would Ammonia like to underline Sensors that 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 2.offer Solid-State simplicity, Ammonia comprehensive Sensors detection action, miniature dimensions, flexibility in fabrication, long expectancy, low cost, and serviceableness for alarm warning applications. MOX sensors changeSolid-state DC (static) ammonia resistance sensors when canexposed be divided to ambient into twogas or groups humidity. considered Any change in our of paper: MOX sensor metal oxide-basedresistance can sensors trigger and an alarm conducting when polymertoxic gas sensors 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 and methods of their preparation for NH3 detection. offer simplicity, comprehensive detection action, miniature dimensions, flexibility in fabrication, long Metal oxides, like SnO2, ZnO, WO3, TiO2, and MoO3, are most widely utilized for this purpose. They lifeare expectancy,classified into low n cost,-type and and serviceableness p-type [3]. Usually, for alarm the warningn-type semiconducting applications. MOX metal sensors oxides change are used DC (static)for gas resistancesensors due when to their exposed higher to sensitivity ambient gas [9]. or The humidity. sensor comprises Any change of grains of MOX of sensorvarious resistance diameter canwith trigger a large an active alarm surface when toxic area. gas Smaller appears grains in an en ambientable better atmosphere. sensing properties due to the larger ratio Thereof active is a varietysurface ofto sensitivethe sensor materials volume. and methods of of their are preparation bound to for the NH grains.3 detection. When Metalthese oxides,atoms are like displaced SnO2, ZnO, by other WO3 ,species TiO2, andpresent MoO in3 ,ambient are most atmosphere, widely utilized a potential for this barrier purpose. between They arethe classifiedgrains changes. into n-type As a andresult,p-type the DC [3]. (static) Usually, resistance the n-type between semiconducting the sensor’s metal terminals oxides changes. are used for gas sensorsAlthough due to theirthe metal higher oxide sensitivity sensors [9have]. The attractive sensor comprisesproperties, of their grains main of variousdisadvantage diameter is a with low aselectivity large active in detecting surface area. of one Smaller particular grains compon enableent better in a sensing gas mixture properties [10]. dueMoreover, to the largerMOX sensors ratio of activeoperate surface at elevated to the sensortemperatures volume. (up Atoms to a offew oxygen hundreds are bound of Celsius to the grains.degrees), When which these requires atoms areadditional displaced energy by other for heating. species presentCareful in selection ambient of atmosphere, ingredients, a potentialaccelerating barrier adsorption–desorption between the grains changes.processes Asdue a result,to catalytic the DC properties, (static) resistance helps to between lower theoperating sensor’s terminals changes. and to enhance gas Although the metal oxide sensors have attractive properties, their main disadvantage is a low selectivity, pointing at selected gases. The sensor exhibits limited specificity for NH3 and can be selectivityaffected by in , detecting , of one , particular , component nitric in a oxide, gas mixture and [10]. Moreover, dioxide [11]. MOX Artificial sensors operateneural networks, at elevated conductance temperatures scanning (up to a fewat periodic hundredsally of Celsiusvaried degrees),temperature, which as requireswell as additionalprinciple energycomponent for heating. or support Careful vector selection machine of ingredients,analysis help accelerating to develop adsorption–desorptionselective sensor systems, processes but only due to tosome catalytic extend properties, [12,13]. The helps algorithms to lower operating support temperatureto determine and even to enhanceparticular gas components selectivity, pointingof the gas at selectedmixtures. gases. The average The sensor detection exhibits limit limited of these specificity sensors for ranges NH3 andfrom can 25 beppb aff toected 100 byppm acetone, (Table ethanol, 1). The hydrogen,selectivity can methane, be further nitric improved oxide, and by nitrogen noble metals dioxide doping [11]. [9,10] Artificial or selecting neural networks, appropriate conductance 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 of these sensors ranges from 25 ppb to 100 ppm (Table1). 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- 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 H2S, 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 functionalized with organic ligands). These materials can be modulated by UV- irradiation (e.g., applying low-cost UV LEDs of different emitted ) [18]. UV light generates 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., , 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 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, , 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 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., C4H8O, CH3OH, C2H3N, CHCl3)[23]. Moreover, the gave repeatable results for different samples of electronic devices. We expect that such sensors should detect NH3 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 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 Table1.

Table 1. Parameters of some solid-state ammonia sensors.

Sensor Detection Response Operation Recovery Time Reference Material Limit Time Temperature Metal Oxide

SnO2/In2O3 100 ppb ~0.1 min 10 s RT [33] SnO2/Pd/RGO 2 ppm 7 min 50 min RT [34] SnO 5 ppm <2 min 30 s RT [35] TiO2/GO/PANI 100 ppm ~0.5 min 17 s RT [36] SnO (type 2 5 ppm <1 min ~few minutes 300–500 C[37] GGS10331) ◦ Conducting Polymer PANI/SWNT 50 ppb ~few minutes ~few hours RT [38] PANI-TiO2-gold 10 ppm - - RT [39] PANI/TiO2 25 ppb <1.5 min - RT [40] PANI/Cu 1 ppm ~0.1 min 160 s RT [41] PANI/graphene 1 ppm <1 min 23 s RT [42] 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 ) 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 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 Figure2. A horizontal gas flow channel passing through the sensing chamber, a red LED light source, and four (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 changehigh sensitivity, to electronic short signals. response Such time, a andsensor fast reversibilityis of high sensitivity, for NH3 gas short concentrations response time, ranging 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 2. TheThe schematic schematic of the colorimetric op optoelectronictoelectronic ammonia sensor [43]. [43].

3. Optical Methods 3. Optical MethodsFigure 2. The schematic of the colorimetric optoelectronic ammonia sensor [43]. Most of the optical approaches to ammonia detecti detectionon are based on the effects effects of absorption and 3. Optical Methods luminescence, more rarely onon thethe refractionrefraction oror thethe lightlight reflection.reflection. In an anMost optical optical of the absorption absorption optical approaches technique, technique, to ammoniathe the measur measured detectioned gas gasare is basedcontained is contained on the in effects the in sensor theof absorption sensor chamber. chamber. and The Theradiation radiationluminescence, passing passing throughmore throughrarely the on chamber the refraction chamber can orbe can the absorbed belight absorbed reflection. by gasby molecules. gas molecules. Measuring Measuring the light the In an optical absorption technique, the measured gas is contained in the sensor chamber. The absorptionlight absorption at the at specific the specific wavelength (λ), (λwith), with possibly possibly no no other other gas gas species species absorbing absorbing in in this passing through the chamber can be absorbed by gas molecules. Measuring the light spectral range, the concentration (N) of ammoniaammonia cancan bebe determined.determined. The concentrationconcentration estimation absorption at the specific wavelength (λ), with possibly no other gas species absorbing in this follows the Beer–Lambert absorption law followsspectral the Beer–Lambert range, the concentration absorption (N law) of ammonia can be determined. The concentration estimation follows the Beer–Lambert absorption law   I0 α𝛼𝜆𝐿(λ)L = σ(λ 𝜎𝜆𝑁𝐿)NL = ln 𝑙𝑛 , (1)(1) 퐼I 훼(휆)퐿 = 휎(휆)푁퐿 = 푙푛 ( 0), 퐼 (1) where α(λ) is thethe absorptionabsorption coecoefficientfficient defineddefined as thethe logarithmiclogarithmic ratio ofof thethe incidentincident ((II00) and the transmittedwhere α(( I)λ) light) is the intensities, absorption L coefficient denotes the defined light as path the logarithmiclength in the ratio chamber, of the incident and σ ((λI0)) denotesand the the absorptiontransmitted cross (section. I) light intensities, Ammonia Ammonia L denotesexhibits exhibits the UV UV light and and path IR IR lengthabsorption absorption in the chamber,bands bands around around and σ( λ135 135) denotes nm, nm, 155 155the nm, 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 µµm,m, 4 µµm,m, 6 µµm,m, around 11 µµm,m, and 16 µµmm (Figure3 3).). TheThe cross-sectioncross-section ((σσ),), 195 nm, 1.5 µm, 2 µm, 3 µm, 4 µm, 6 µm, around 11 µm, and 16 µm (Figure 3). The cross-section (σ), definingdefining the system sensitivity, is the highest in the UV region. It is about 10-times larger than at the defining the system sensitivity, is the highest in the UV region. It is about 10-times larger than at the 11 µm region (e.g., ~10−1818 cm2 2) and even 105 5-times larger than in other NIR ranges [44]. 11 µm11 region µm region (e.g., (e.g., ~10 −~10 cm−18 cm) and2) and even even 10 10-times5-times larger larger than inin other other NIR NIR ranges ranges [44 [44].].

(a) (b)

FigureFigure 3.3. AbsorptionAbsorption( cross-sectiona cross-section) of of ammonia ammonia for for different different temperatures temperatures in UV in UV [45,46](b [45) , 46(a).]( Absorptiona). Absorption Figurecross-sectioncross-section 3. Absorption of of selected selectedcross-section molecules molecules of existingammonia existing in standard for in standarddifferent conditions conditionstemperatures (1 atm, (1 288.2 atm, in K)UV 288.2 calculated [45,46] K) calculated (ona). theAbsorption basis on the of the HITRAN database in IR range (b). cross-sectionbasis of of the selected HITRAN molecules database existing in IR range in standard (b). conditions (1 atm, 288.2 K) calculated on the basis of the HITRANUnfortunately, database in in IR the range UV and(b). NIR range, there is a problem with interferences by vapor 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 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 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 17 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 17 and N2O or NO2 molecules, which occur at high concentrations in air. Their absorption bands might overlap with NH3 spectra disturbing the measurements. Therefore, Therefore, the selection of proper spectral overlap with NH3 spectra disturbing the measurements. Therefore, the selection of proper spectral range is crucial for successful optical detection. range is crucial for successful optical detection. Nondispersive infraredinfrared (NDIR) (NDIR) sensing sensing belongs belongs to simplest to simplest approaches approaches of NH 3 ofdetection. NH3 detection. Figure4 Nondispersive (NDIR) sensing belongs to simplest approaches of NH3 detection. shows a schematic diagram of such a gas sensor. It usually consists of a broadband source (cheaper and Figure 4 shows a schematic diagram of such a gas sensor. It usually consists of a broadband source smaller emitters or IR LEDs), absorption , optical filters, and detectors. Radiation from (cheaper and smaller black body emitters or IR LEDs), absorption cell, optical filters, and detectors. the broadband source passes through the chamber and two filters. The first filter covers the absorption Radiation from the broadband source passes through the chamber and two filters. The first filter band of the target gas (named active channel), while the other covers a non-absorbed spectral range covers the of the target gas (named active channel), while the other covers a (the reference channel). Transmission bands of the filters should not overlap with absorption bands of non-absorbed spectral range (the reference channel). Transmission bands of the filters should not the other gases present in the chamber. Thus, absorption in the active channel is proportional to NH overlap with absorption bands of the other gases present in the chamber. Thus, absorption in the3 concentration.active channel Theis proportional light transmitted to NH through3 concentration. the reference The light channel transmitted is not attenuated. through the Therefore, reference it active channel is proportional to NH3 concentration. The light transmitted through the reference is used to compensate instabilities of the light source. The sensitivity of NDIR is influenced by the channel is not attenuated. Therefore, it is used to compensate instabilities of the light source. The intensity of the source, the optical waveguide and detector parameters. The disadvantage of this sensitivity of NDIR is influenced by the intensity of the source, the optical waveguide and detector technique is the low precision of the detecting small signal changes at an eventual large background, parameters. The disadvantage of this technique is the low precision of the detecting small signal which 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.

Figure 4. DesignDesign and and principle principle of of operation operation of of th thee nondispersive infrared (NDIR) sensor.

There are various methods to improve the performanceperformance of NDIRNDIR sensors.sensors. For example, Max-IR Labs utilizesutilizes thethe NDIRNDIR technique technique together together with with fiber-optic fiber-optic evanescent evanescent wave wave spectroscopy spectroscopy (FEWS) (FEWS) for ammoniafor ammonia detection detection [47]. [47]. The IRThe radiation IR radiation is transmitted is transmitted through through a silver-halide a silver-halide (AgClxBr1-x) (AgCl opticalxBr1-x) for ammonia detection [47]. The IR radiation is transmitted through a silver-halide (AgClxBr1-x) waveguideoptical waveguide without without cladding cladding and the detectionand the detect performedion performed by means by of means the evanescent of the evanescent field (Figure field5). 1 −1 The(Figure maximum 5). The maximum of the peak of due the topeak ammonia due to ammonia absorption absorption was observed was observed at 1450 cm at −1450. Such cm−1 a. Such sensor a allowssensor allows NH detection NH3 detection beyond beyond a 1 ppm a 1 limit. ppm limit. sensor allows3 NH3 detection beyond a 1 ppm limit.

Figure 5. Diagram of the sensor based on NDIR and fiber-optic evanescent wave spectroscopy FigureFigure 5. 5.Diagram Diagram of of thethe sensorsensor based on on NDIR NDIR and and fiber-optic fiber-optic evanescent evanescent wave wave spectroscopy spectroscopy (FEWS) principles. (FEWS) principles. (FEWS) principles.

Laser spectroscopy is the bestbest choicechoice forfor tracetrace gasgas analysisanalysis amongamong opticaloptical approaches.approaches. It is characterized by high sensitivity and selectivity.selectivity. Ammonia Ammonia can can be be detected using (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 . The advantage of TLAS over other techniques consists in its ability to achieve low detection limits by using optical path extension techniques and improving the signal-to-noise ratio (SNR). 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 , sample cell, receiving optics, photodetector, and a signal detection tunable laser, transmitting optics, sample cell, receiving optics, photodetector, and a signal detection circuit (Figure6). 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 (I0) and with absorption (I1) can be used to calculate the gas concentration corresponding to the case without absorption (I0) and with absorption (I1) can be used to calculate according to the formula (1). the gas concentration according to the formula (1).

FigureFigure 6. Block 6. Block diagram diagram and principle and principle of operation of operation of the typicalof the typical tunable tunable laser absorption laser absorption spectroscopy (TLAS) sensor. spectroscopy (TLAS) sensor.

Based on TLAS, didifferentfferent componentscomponents suchsuch asas NHNH33,, CO,CO, OO22, CHCH44,,H H2O,O, CO 22,, and and HCl HCl can can be detected with high selectivity and sensitivity. TLAS TLAS has has been been employed for for various applications, including industrial process monitoring and its control, environmental monitoring, combustion and flowflow analyses, trace specie speciess measurements, and so on. However, However, the sensitivity of this technique is 2 3 1 usually limited to the absorption coecoefficientfficient value of ~10−2–10–10−3− cmcm−1 −(Equation(Equation (1)). (1)). Wavelength modulationmodulation spectroscopy spectroscopy (WMS) (WMS) is a useful is a technique useful providingtechnique SNRproviding improvement, SNR improvement,which is usedin which high is sensitivity used in high applications sensitivity [48 a].pplications Small modulation [48]. Small (with modulation frequency (withf 1–20 frequency kHz) is ≈ fadded ≈ 1–20 to kHz) the laser is added current to scan, the which laser iscurrent provided scan, in awhich similar is way provided as in TLAS in a (Figure similar7). way The transmittedas in TLAS (Figuresignal, detected7). The transmitted by the , signal, detected is fed to by a the lock-in photodiode, amplifier. is Asfed theto a laser lock-in is scanned amplifier. across As the an laserabsorption is scanned line profile,across an the absorption transmitted line on-absorption profile, the transmitted signal changes on-absorption at the frequency signal ofchanges 2f, while at the frequencytransmitted of o 2fff-absorption, while the transmitted signal changes off-absorption at the frequency signal of changesf (Figure at8 the). frequency of f (Figure 8). Appl. Sci. 2020, 10, 5111 8 of 17 Appl. Sci. 2020,, 10,, xx FORFOR PEERPEER REVIEWREVIEW 8 of 17

FigureFigure 7. 7.The The principle principle of of operation operation ofof the sensor us usinging a a combination combination of of TLAS TLAS and and wavelength wavelength modulation spectroscopy (WMS)modulation techniques. spectroscopy (WMS) techniques.

Figure 8. Idea of first first and second harmonic detection with tunable diode lasers.

Application of highhigh frequencyfrequency minimizesminimizes 11//ff noise. Therefore, setting the lock-in band around second harmonicharmonic ensures that thethe sensorsensor becomesbecomes moremore suitablesuitable forfor highhigh sensitivitysensitivity applicationsapplications compared toto thethe standardstandard TLASTLAS approach.approach. Typica Typicall sensitivity limits the absorption coecoefficientfficient 4 1 5 7 1 achievable with WMS and isis aboutabout 1010−−4 cmcm−−1. .Better Better limits, limits, about about 10 10−5−–10–10−7 cm− cm−1, can− , canbe obtained be obtained for forbalanced balanced detection-based detection-based WMS WMS [49]. [49 This]. This method method invo involveslves an an electrical electrical circuit circuit that that subtracts subtracts thethe photocurrentsphotocurrents of twotwo detectors:detectors: one of them regist registersers the reference laser intensity while the other measures thethe signal signal passing passing through through the absorptionthe absorption cell. Thatcell. providesThat provides opportunity opportunity to reject commonto reject modecommon laser mode noise. laser noise. Small absorptions, which occur due to low densities of molecules in the samples or due to weak lineline strengths,strengths, areare usuallyusually compensatedcompensated byby extendingextending thethe opticaloptical pathpath withwith multi-passmulti-pass cellscells (White or Herriott) or cavity-enhancedcavity-enhanced methods. They They enable th thee optical path to bebe extended. If instead of a single-pass chamber we apply the multi-pass cell (Figure 99),), thethe sensitivitysensitivity ofof thethe sensorsensor wouldwould bebe improvedimproved byby thethe factorfactor ofof thethe opticaloptical pathpath lengthening.lengthening. A key parameterparameter of multi-passmulti-pass cells isis thethe ratioratio ofof pathpath lengthlength toto volume.volume. Moreover, thisthis systemsystem providesprovides anan opportunityopportunity forfor thethe simultaneoussimultaneous application ofof thethe WMSWMS approachapproach andand 2f2f detection. Appl. Sci. 20202020,, 10,, x 5111 FOR PEER REVIEW 99 of of 17 Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 17

Figure 9. Scheme of a multi-pass experimental system for multi-pass optical cell (MUPASS)–WMS FigureFigure 9.9. SchemeScheme of of a multi-pass a multi-pass experimental experimentalspectroscopy. system system for multi-pass for multi-pass optical optical cell (MUPASS)–WMS cell (MUPASS)– WMS spectroscopy. spectroscopy. Unlike conventional configurations, which involve at least a pair of mirrors separated by exactlyUnlike defined conventionalconventional distances, configurations, circularconfigurations, multi-pass which which cell involve sinvolve have at been leastat leastdeveloped a pair a ofpair mirrors (Figure of mirrors separated 10) [50]. separated Thanks by exactly byto theexactlydefined single defined distances, piece, distances, the circular cell is multi-passcircular especially multi-pass cellsrobust have agai cell beennsts have developedthermal been expansion.developed (Figure 10 Minimizing(Figure)[50]. Thanks 10) [50]. the to cellThanks the size single tois desiredthepiece, single the for cellpiece, the is development especially the cell is robust especially of fast against and robust portable thermal agai expansion.gasnst sensors.thermal Minimizing expansion. the Minimizing cell size is the desired cell forsize the is desireddevelopment for the of development fast and portable of fast gas and sensors. portable gas sensors.

Figure 10. The circular multi-pass—small volume cell. Figure 10. The circular multi-pass—small volume cell. Cavity ring-down spectroscopyFigure 10. The (CRDS) circular exploits multi-pass—small the sample volume cells incell. the form of optical cavities (resonators)Cavity ring-down built with mirrorsspectroscopy of very (CRDS) high reflectivity.exploits the Asample simplified cells schemein the form of such of optical cavities is Cavity ring-down spectroscopy (CRDS) exploits the sample cells in the form of optical cavities (resonators)presented in built Figure with 11. Lasermirrors pulse of very injected high into reflectivity. the cavity A through simplified one scheme of the mirrors of such is experiment then reflected is presented(resonators) in built Figure with 11. mirrors Laser ofpulse very injected high reflectivity. into the cavityA simplified through scheme one of of the such mirrors experiment is then is many times among them. Its wavelength must be tuned to the NH3 in order to measure presented in Figure 11. Laser pulse injected into the cavity through one3 of the mirrors is then reflectedthe absorption many coe timesfficient among of ammonia them. Its contained wavelength inside. must The be lighttuned transmitted to the NH through spectral the line exit in mirrororder to is reflected many times among them. Its wavelength must be tuned to the NH3 spectral line in order to measuremonitored the by absorption a detector. coefficient Analysis of of the ammonia output signal contained by the inside. acquisition The light system transmitted provides through opportunity the exitmeasure mirror the is absorption monitored coefficient by a detector. of ammonia Analysis contained of the outputinside. Thesignal light by transmitted the acquisition through system the to determine the Q-factor of the cavity which is limited due to diffraction and mirror losses as well as providesexit mirror opportunity is monitored to determine by a detector. the Q-factor Analysis of theof thecavity output which signal is limited by the due acquisition to diffraction system and due to the light absorption or scattering inside the cell. As far as the Q-factor is inversely proportional mirrorprovides losses opportunity as well as to due determine to the light the Q-factorabsorption of theor scattering cavity which inside is limitedthe cell. due As farto diffractionas the Q-factor and to the signal decay time, its value is found due to analysis of the photoreceiver signal by the acquisition ismirror inversely losses proportionalas well as due to to the lightsignal absorption decay time, or scattering its value inside is found the cell. due As to far analysis as the Q-factor of the system. The majority of the approaches exploit a two-step procedure consisting of the decay time photoreceiveris inversely proportional signal by the to acquisition the signal system. decay Thtime,e majority its value of theis foundapproaches due toexploit analysis a two-step of the measurement: first, when the cavity is empty (τ0), and then, when the cavity is filled with the tested procedurephotoreceiver consisting signal byof thethe decayacquisition time measursystem.ement: The majority first, when of thethe approachescavity is empty exploit (𝜏 ),a andtwo-step then, gas (τA). These decay times depend on the mirror reflectivity, resonator length, and factor whenprocedure the cavity consisting is filled of withthe decay the tested time gasmeasur (𝜏 ).ement: These decayfirst, when times the depend cavity on is the empty mirror (𝜏 ),reflectivity, and then, (absorption and scattering of light in the cavity) [51]. Comparing both decay times, the absorber resonatorwhen the cavitylength, is filledand extinctionwith the tested factor gas (absorptio (𝜏). Thesen decayand scattering times depend of light on the in mirror the cavity) reflectivity, [51]. concentration can be found. ! Comparingresonator length, both decay and times,extinction the absorber factor (absorptio concentration1 n1 and can1 scattering be found. of light in the cavity) [51]. N = (2) Comparing both decay times, the absorber concentrationcσ(λ) τA − τcan0 be found. 𝑁 , (2) where c is the light and σ(λ) is the absorption cross-section. 𝑁 , (2) 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. Appl. Sci. 2020, 10, 5111 10 of 17 Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 17

Figure 11. Idea of the cavity ring down spectroscopy (CRDS).

There are variousvarious approaches to CRDSCRDS withwith pulsedpulsed laserslasers oror withwith continuouslycontinuously operatingoperating AM −99 −1 modulated ones.ones. Using these techniques, the absorptionabsorption coecoefficientsfficients αα == σσNN< < 1010− cmcm− can be observed. The detectiondetection limitlimit isis mainlymainly related related to to the the resonator resonator quality quality (determined (determined by byτ0 ),𝜏 but), but also also by theby precisionthe precision of decay of decay time measurement. time measurement. The advantage The advantage of CRDS overof CRDS other absorptionover other spectroscopy absorption approachesspectroscopy consists approaches not only consists in dominant not only sensitivity. in dominant Their sensitivity. superiority Their also resultssuperiority from also minimized results impactsfrom minimized of light source impacts intensity of light fluctuations source intensity or detector fluctuations sensitivity or changesdetector on se thensitivity measurement changes results.on the measurementThis method results. is extremely sensitive but requires that the laser frequency is precisely matched to the cavityThis method mode. is In extremely this way, itsensitive is possible but requires to obtain that a high the laser Q-factor frequency of the resonatoris precisely and matched efficient to storagethe cavity of opticalmode. In radiation. this way, On it theis possible other hand, to obtain small a mechanical high Q-factor instabilities of the resonator cause changes and efficient in the cavitystorage mode of optical frequency radiation. and significant On the other output hand, signal small fluctuations, mechanical which instabilities is the maincause disadvantage changes in the of thiscavity method mode [frequency52]. It can and be minimalized significant output by use signal of cavity fluctuations, length stabilization which is the [53 ,main54], or disadvantage by application of ofthis the method cavity [52]. with It dense can be mode minimalized structure, by called use of cavity-enhanced cavity length stabilization absorptionspectroscopy [53,54], or by(CEAS) application [55]. of theIn cavity CEAS, with the o ffdense-axis arrangementmode structure, of the called laser cavity-enhanced and cavity is applied. absorption Similarly, spectroscopy to the conventional (CEAS) CRDS[55]. system, the light is repeatedly reflected by the mirrors. However, the reflected beams, which correspondIn CEAS, to di thefferent off-axis trips, arearrangem spatiallyent separated of the laser inside and the resonator.cavity is Theapplied. useof Similarly, highly reflecting to the mirrorsconventional provides CRDS huge system, extension the li ofght the is e ffrepeatedlyective optical reflected path. Asby result,the mirrors. either aHowever, dense mode the structurereflected ofbeams, low finessewhich occurs,correspond or the to modedifferent structure trips, are does sp notatially establish separated at all. inside In this the way, resonator. sharp resonancesThe use of ofhighly the cavity reflecting are avoided, mirrors soprovides the system huge is extension much less of sensitive the effective to mechanical optical path. instabilities. As result, The either CEAS a 9 1 sensorsdense mode attain structure the detection of low limit finesse of about occurs, 10− orcm the− [ 56mode]. Detailed structure information does not aboutestablish CRDS at all. and In CEAS this techniquesway, sharp isresonances presented of in the the cavity authors’ are papers avoided, [57 –so59 the]. system is much less sensitive to mechanical instabilities.Among The the CEAS so-called sensors in-situ attain sensors, the detection photoacoustic limit of spectroscopyabout 10−9 cm− (PAS)1 [56]. Detailed belongs toinformation the most popularabout CRDS one. and In PAS, CEAS conversion techniques of laseris pres lightented energy in the into authors’ an acoustic papers wave [57–59]. is applied. The gas sample, placedAmong in photoacoustic the so-called chamber, in-situ sensors, is irradiated photoacous by opticaltic spectroscopy radiation, AM (PAS) modulated belongs with to the acoustic most frequency.popular one. The In laser PAS, wavelength conversion isof matched laser light to theenergy absorption into an lineacoustic of a molecule wave is ofapplied. interest. The If gas the absorbersample, placed is present in inphotoacoustic the cell, a portion chamber, of optical is irradiated radiation by is convertedoptical radiation, onto heat AM energy. modulated Then, a localwith andacoustic periodic frequency. growth The of temperaturelaser wavelength and pressure is matched occurs to the (Figure absorption 12). The line resulting of a molecule acoustic of interest. wave is detectedIf the absorber at modulation is present frequency in the cell, by a veryportion sensitive of optical microphone radiation placed is converted in thechamber. onto heat The energy. PAS signal,Then, a which local isand proportional periodic growth to the absorberof temperature concentration and pressure (N), is given occurs by (Figure 12). The resulting acoustic wave is detected at modulation frequency by a very sensitive microphone placed in the Q Q chamber. The PAS signal, whichA(T, λ is) proportionalPoNα(T, λ) Lto theη absorber= PoNα( Tconcentration, λ) η (N), is given by (3) ∝ f V f A 𝐴𝑇, 𝜆 ∝𝑃𝑁𝛼𝑇, 𝜆 𝐿 𝜂𝑃𝑁𝛼𝑇, 𝜆 𝜂, (3) where Po denotes average laser power, Q is the quality factor of the resonant cell, f is the frequency of modulation, V is the gas volume, A is the cross-sectional area, and η is the system efficiency factor (e.g., microphonewhere Po denotes efficiency average and loss laser factors). power, Q is the quality factor of the resonant cell, f is the 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. IdeaIdea of photoacoustic spectroscopy.

The microphonemicrophone output output signal signal is recordedis recorded using us aing low-noise a low-noise preamplifier preamplifier and a lock-in and voltmeter.a lock-in Thevoltmeter. strongest The e ffstrongestect is achieved effect whenis achieved the modulation when the frequency modulation is matched frequency to the is resonancematched to of the photoacousticresonance of the chamber. photoacoustic In order chamber. to increase In theorder signal, to increase various the constructions signal, various of photoacoustic constructions cells of (acousticphotoacoustic resonators cells (acoustic with higher resonators Q-factor) with are higher applied. Q-factor) are applied. The improvement of of this this technique, technique, which which allows allows to to achieve achieve very very high high detection detection sensitivity, sensitivity, is isthe the quartz-enhanced quartz-enhanced photoacoustic photoacoustic spectroscopy spectroscopy (Q (QEPAS).EPAS). The The basic basic principle principle of of operation is similar, butbut herehere thethe laserlaser beambeam isis focusedfocused betweenbetween thethe U-shapedU-shaped prongs ofof aa resonantresonant piezo-quartzpiezo-quartz fork [[60].60]. This This type of acoustic transducer is sensitive to signalssignals generatedgenerated byby asymmetricasymmetric prongsprongs oscillations causedcaused by by acoustic acoustic wave, wave, induced induced by the by AM-modulated the AM-modulated laser radiation. laser radiation. External External sources ofsources interference of interference do not provide do not output provide signals output because signals they because cause symmetricalthey cause symmetrical oscillations. oscillations. Similarly to theSimilarly conventional to the conventional PAS system, PAS in QEPAS system, set-ups, in QEPA theS measurements set-ups, the measurements are performed are with performed a wavelength with modulationa wavelength technique modulation using technique the 2f detection using the [61 2].f Combineddetection [61]. with Combined the high Q-factor with the of high the quartzQ-factor fork, of itthe gives quartz the fork, opportunity it gives tothe build opportunity ultra-compact to build sensors ultra-compact that reach detectionsensors that limits reach comparable detection tolimits that ofcomparable CEAS. In caseto that of of ammonia, CEAS. In it case corresponds of ammonia, to a fewit corresponds ppb or even to sub-ppb a few ppb level. or even sub-ppb level. Recently reported parameters ofof somesome opticaloptical ammoniaammonia sensorssensors areare givengiven inin TableTable2 .2.

Table 2.2. Parameters of some optical ammonia sensors.sensors.

Sensing Detection Radiation Sensing Detection Radiation Wavelength Other Parameter Reference Methods Limit Source Wavelength Other Parameter Reference Methods Limit Source λcenter = 205 nm NDIR 1 ppm Deuterium FilterFilter 200–225 nm λcenter = 205 nm [44] NDIR 1 ppm lamp FWHM = 10 nm [44] Eff. path MUPASS-WMS 7 ppblamp QCL 200–2251103.44 cm nm1 FWHM = 10 nm [62] − length—76.45 m CRDS-cw Eff. Rpath= 0.9995, length—76.45 MUPASS-WMS 71.3 ppb ppb QCL-DFB QCL 1103.44 10.33 µ cmm −1 [63] [62] (open-path) L = 50 cmm R = 0.9998, QCL CRDS-cwCRDS 0.74 ppb 6.2 µm L =R50 = cm,0.9995, [64] 1.3 ppb QCL-DFB(cw-EC) 10.33 µm [63] (open-path) p = 115L = Torr 50 cm QCL-DFB 1 L = 53 cm, CEAS 15 ppb 967.35 cm− R = 0.9998, [65] (pulsed)QCL ti = 5–10 ns CRDSPAS 0.74 0.7 ppb ppb EC-QCL 10.366.2 µmµm p = L220 = Torr50 cm, [66] [64] (cw-EC) Ammonia QCL p = 115 Torr QEPAS <10 ppb 10.6 µm detection in [67] QCL-DFB(cw-EC) L = 53 cm, CEAS 15 ppb 967.35 cm−1 exhaled breath [65] QCL—quantum cascade laser, PAS—photoacoustic(pulsed) spectroscopy, CRDS—cavityti = 5–10 ring downns spectroscopy, QEPAS—quartz-enhancedPAS 0.7 ppb photoacoustic EC-QCL spectroscopy, WMS—wavelength 10.36 µm modulation p = spectroscopy,220 Torr R—reflectivity [66] of mirrors, L— length, EC—external cavity, p—pressure. Other methods of gas detection were QCL Ammonia detection in QEPASdescribed in our earlier<10 studies ppb [68,69]. 10.6 µm [67] (cw-EC) exhaled breath AmongQCL—quantum many applications,cascade laser, the PAS—photoacoustic ammonia sensors forspectroscopy, medical purposeCRDS—cavity are an ring important down 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

Appl.such Sci.detectors.2020, 10, 5111The normal concentration of ammonia in the breath of a healthy man is in the 12range 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 fordiabetes, breath and analysis oral cavity doesdisease not consist [71,72 in]. Theextreme main sens problemitivity in but the in construction high selectivity. of ammonia More sensorsthan 3000 for variousbreath analysis constituents does notwere consist already in extreme detected sensitivity in exhaled but inair high [73]. selectivity. Their absorption More than spectra 3000 various might overlapconstituents the spectral were already fingerprint detected of the in exhaled ammoni aira and [73]. disturb Their absorptionthe measurement. spectra mightWater overlapvapor and the carbonspectral dioxide fingerprint are the of the main ammonia interferences and disturb since thethei measurement.r concentrations Water can vaporreach up and to carbon 5% in dioxidebreath, andare thethus main it exceeds interferences the ammonia since their density concentrations by many can orders reach of up magnitude. to 5% in breath, Carbon and monoxide thus it exceeds and methanethe ammonia should density also be by taken many into orders account of magnitude.. and methane should also be takenAs into it account.was mentioned above, the highest sensitivities of ammonia detection using laser absorptionAs it was spectroscopy mentioned can above, be achieved the highest in UV sensitivities and in a ofrange ammonia of 10–11 detection µm, due using to the laser largest absorption values ofspectroscopy the absorption can be achievedcross-sections. in UV andHowever, in a range the ofdetection 10–11 µm, at due the to thewavelengths largest values used of thein telecommunicationabsorption cross-sections. 1.4–1.6 µm However, is more the convenient detection because at the wavelengthsof a large variety used of in relatively telecommunication cheap laser 1.4–1.6sources,µ opticalm is more elements, convenient and photodetectors. because of a large The measurements variety of relatively of ammonia cheap laseraround sources, 1.51 µm optical with theelements, detection and limit photodetectors. of 4 ppm was The already measurements demonstrated of ammonia using the around CEAS 1.51 techniqueµm with [74]. the We detection found thatlimit suitable of 4 ppm wavelengths was already for demonstrated ammonia detection using theare CEASalso the technique lines at [1.527000574]. We found µm and that 1.5270409 suitable wavelengthsµm (Figure for13). ammonia The single-mode detection arelaser also wavelength the lines at 1.5270005was controlledµm and 1.5270409with the µHighPrecisionm (Figure 13). The single-mode laser wavelength was controlled with the HighPrecision lambdameter (model WS6) lambdameter (model WS6) ensuring precision in a short time (1 min) of 0.0005 nm. The NH3 absorptionensuring precision coefficient in a(at short 2 ppm) time reaches (1 min) here of 0.0005 about nm.3.5∙10 The−6 cm NH−1,3 andabsorption experimental coefficient techniques (at 2 ppm) like reaches here about 3.5 10 6 cm 1, and experimental techniques like MUPASS–WMS can be effectively MUPASS–WMS can be· effectively− − used. In this range, and water vapor interference isused. about In eight this range, times carbonweaker. dioxide and water vapor interference is about eight times weaker.

2 2 Figure 13. Absorption spectra of ammonia, H2O, and CO 2 aroundaround 1.527 1.527 µmµm [75]. [75].

Our ammonia sensor used the MUPASS–WMS approach (Figure9 9).). AA single-modesingle-mode diodediode laserlaser (Toptica, modelmodel DL100,DL100, 2020 mW)mW) waswas appliedapplied asas aa lightlight source.source. Output signals were integrated over 1 min. Results of sensor investigation are presentedpresented in Figure 1414.. It provides proper results forfor NHNH33 concentrations of up to about 1 ppm. For lower values, the deviation from a linear characteristic is observed. This isis probablyprobably a a result result of of poor poor regulation regulation of referenceof reference concentration concentration inside inside the sensorthe sensor due dueto NH to3 NHdeposit3 deposit on the on walls the walls of the of system, the system, which which causes ca auses systematic a systematic error oferror measured of measured data. Indata. order In orderto avoid to theavoid concentration the concentration measurement measurement disturbed disturbed by NH3 moleculesby NH3 molecules adsorbed adsorbed on the walls on (andthe walls then (anddesorbed), then desorbed), the sensor the was sensor kept atwas a temperature kept at a temperature of 50 ◦C. of 50 °C. Nevertheless, due due to to the the proper proper choice choice of of spectr spectralal lines, lines, we we achieved achieved a good a good immunity immunity of the of detectionthe detection against against H2O H and2O andCO2 COat the2 at concentrations the concentrations which which might might occur occur in breath in breath (Figure (Figure 13). The13). Thedetection detection limit limit of 1 ofppm 1 ppm was wasbetter better than than in other in other experiments experiments preformed preformed at a atspectral a spectral range range of 1.5 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

Appl.µm [74]. Sci. 2020 Our, 10 multi-pass, 5111 sensor is suitable for rough monitoring of ammonia in the exhaled air13 ofand 17 for detection of morbid states (>1.5 ppm).

Figure 14. Results of ammonia measurement with MUPASS–WMS systemsystem [[75].75].

4. Conclusions Based on the analysis,analysis, it cancan bebe concludedconcluded thatthat thethe furtherfurther developmentdevelopment ofof in-situin-situ ammoniaammonia detection technologiestechnologies willwill focus focus on on the the improvement improvement of of sophisticated sophisticated laboratory laboratory equipment, equipment, e.g., e.g., the massthe mass spectrometers and lasers.and lasers. One canOne expect can expect the development the development of sensors of sensors dedicated dedicated to a specific to a application.specific application. This article This focuses article on thefocuses second on group theof second the sensors group in whichof the two sensors technologies in which with hightwo potentialtechnologies for applicationwith high and potential development for applicat have beenion distinguished:and development solid-state have ammoniabeen distinguished: sensors and lasersolid-state absorption ammonia spectroscopy. sensors Metrologicaland laser absorption and operational spectroscopy. parameters Metrological of such sensors and largely operational depend onparameters the application. of such sensors Basically, largely they depend must be on characterized the application. by fastBasically, operation, they themust largest be characterized measuring rangeby fast and operation, the highest the measurement largest measuring precision, rang highe reliability,and the highest selectivity, measurement as well as small precision, dimensions, high andreliability, low cost. selectivity, Sensors as belonging well as tosmall the dimensio former groupns, and can low detect cost. the Sensors ammonia belonging with a concentrationto the former ofgroup several can ppm detect within the ammonia tens of seconds. with a Theirconcentration advantage of consistsseveral ppm in very within small tens dimensions of seconds. and Their very lowadvantage cost. Materials consists engineering in very small and nanotechnologydimensions and are very key technologieslow cost. Materials for the development engineering of and this sensornanotechnology group. We are underlined key technologies that recent for advancesthe developme in materialsnt of this and sensor measurement group. We methods underlined enhance that gasrecent detection advances by suchin materials low-cost and gas measurement sensors in portable methods applications. enhance gas We detection are conscious by such that low-cost our research gas presentssensors in a limitedportable number applications. of NH 3Wegas are sensors. conscious This that issue our is aresearch hot topic presents in the gas a limited sensing number area, and of otherNH3 gas attractive sensors. sensors This issue were is developed a hot topic recently in the ga (e.g.,s sensing capacitive area, sensorsand other [76 attractive], colorimetric sensors sensing were materialsdeveloped [77 recently], or advanced (e.g., capacitive MOX sensors sensors [78]). [76], The lattercolorimetric group ofsensing sensors materials can detect [77], up toor four advanced orders ofMOX lower sensors ammonia [78]). in The fractions latter ofgrou a second.p of sensors However, can detect their costsup to are four much orders higher, of lower mainly ammonia due to the in lasersfractions and of high-quality a second. opticalHowever, components. their costs The are further much improvement higher, mainly of these due sensors to the is muchlasers more and dependenthigh-quality on optical the development components. of lasers The [79further] than improvement photodetectors of [80 these], because sensors they areis much already more at a verydependent high level. on the It mainlydevelopment concerns of alasers progress [79] inthan lasers photodetectors for mid-infrared [80], radiation, because they since are one already can expect at a thevery highest high level. sensitivity It mainly and selectivityconcerns ina progress this spectral in lasers range. for However, mid-infrared miniaturization, radiation, reliability,since one andcan reductionexpect the of highest production sensitivity costs are and still selectivity a current challenge in this spectral for all optical range. sensor However, components. miniaturization, It should bereliability, also mentioned and reduction that MOX of sensors production can utilize costs optical are still methods a current in some challenge sense by for applying all optical UV lightsensor to improvecomponents. the sensitivityIt should andbe also selectivity mentioned of gas that detection. MOX sensors Therefore, can bothutilize types optical of gas methods sensors in start some to interlacesense by MOXapplying and opticalUV light sensors to improve technologies the sensitiv to reduceity and costs selectivity and popularize of gas theirdetection. applications. Therefore, We believeboth types that of di gasfferent sensors types start of gas to interlace sensors find MOX complementary and optical sensors areas technologies of new applications. 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 editing, of new investigation. applications. 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 theediting, “Sense” investigation. project, ID 347510.J.S.: writing—review & editing, investigation. J.W.: formal analysis, investigation, 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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