MATERIALS SELECTION FOR GAS SENSING. AN HSAB PERSPECTIVE
1
Bogdan-Catalin Serban1, Mihai Brezeanu1, Cornel Cobianu1, Stefan Costea1, Octavian Buiu1, Alisa Stratulat1, Nicolae Varachiu2
Honeywell Romania SRL, Sensors and Wireless Laboratory Bucharest (SWLB), 2Honeywell EMEA Six Sigma 3 George Constantinescu, BOC Tower, Entrance A, 4th Floor, Sector 2, 020339, Bucharest, Romania
[email protected]
Table 1 – List of soft, borderline and hard bases and acids, according to HSAB principle
Abstract This paper introduces the Hard Soft Acid Base (HSAB) concept as a promising tool for the selection of gas sensing layers. Target gas molecule - sensitive layer tandems are discussed and interpreted in the terms of this theory. Sensing layers suitable for carbon dioxide, nitrogen dioxide, sulphur dioxide, and hydrogen sulphide detection are presented and classified according to this concept. For oxygen and mineral acids detection, an indirect HSAB approach is discussed. The paper explains how the HSAB principle can be useful in designing gas sensing layers for different types of sensing structures, such as: surface acoustic waves (SAW), colorimetric, chemoresistive, etc. Keywords: Hard Soft Acid Base (HSAB) Concept, Gas Molecule, Sensor, Sensitive Layers
1. INTRODUCTION The Hard Soft Acid Base (HSAB) concept, introduced by Ralph Pearson in the early 1960s as an attempt to unify organic and inorganic reactions chemistry, explains, among others, chemical reactivity of different complexes, preferences of some chemical species to interact with other species, and reaction mechanisms [1, 2]. This concept operates with Lewis acids and bases, a well-known classification, according to which a molecule capable to donate electrons pair is a base, while a molecule capable to accept it is an acid. Pearson classified Lewis acids and Lewis bases as soft, hard and borderline. Soft acids and bases exhibit large ionic radius, high polarizability, and low oxidation state. By contrast, hard species tend to have small atomic/ionic radius, low polarizability and high oxidation state [3, 4]. Borderline species have an intermediate character between hard and soft species. Examples of hard, soft and borderline acids and bases are given in Table 1 [5]. According to this concept, the species classified as described above and presented in the table react preferentially with species of similar hardness or softness. In other words, a hard acid prefers to interact to hard bases, 978-1-4799-3917-6/14/$31.00 © 2014 IEEE
Soft
Borderline
Hard
Bases
H2S, C2H4, RSH, R2S, CO, CN-, RCN, H2-, R3P, C6H6, RS-, I-
C6H5NH2, C5H5N, C3H4N2 N2, Br-, N3-
ROH, RO-, HO-, R2O, N2H4, R-NH2, H2O, CO32-, F-, Cl-
Acids
Cd2+, Cu+, Ag+, carbenes, I2, Hg2+
SO2, Bi3+, Ni2+, 2+ Zn ,C6H5+, Pb2+, Cu2+
CO2, SO3, H+, Li+,Mg2+, Al3+, Co3+ Ga3+, Ti4+, La3+, In3+, Zr4+
soft acids prefer to interact to soft bases, while a borderline acid tends to bond to borderline bases. The interaction between soft acids and soft bases is mostly covalent, whereas the interaction between hard acids and hard bases is predominantly ionic. Despite its limitations [6], the HSAB concept has proven to be a useful investigation tool in many fields of chemistry such as: ¾ Medicinal chemistry, toxicology: The HSAB concept and the subsequent classifications have been successfully applied to chemical-induced toxicity in biological systems. Chelation therapy (the use of cysteine, N-acetyl cysteine, 2,3dimercaptopropane-1-sulphonate (DMPS), DMPS + cysteine or DMPS + N-acetyl cysteine in the case of cadmium intoxication) and a lot of other medical treatments, biological interactions and toxicological observations are in agreement with the HSAB theory [7-10]; ¾ Quantum dot functionalization and design of quantum dot solar cell: The selection of ligands with appropriate anchors for surface modification of quantum dots can be optimized by employing the HSAB concept [11-20]; 21
¾ Design of polymers – quantum dots hybrid interface: A covalent bond between the polymer back-bond (modified with suitable pendant groups, in accordance with the HSAB principle) and quantum dots avoids the segregation of the phase, a traditional drawback in the design of these types of nanocomposites [21]; ¾ Adsorbtion phenomena: The adsorbtion of different types of cations onto activated carbons was analyzed in terms of the HSAB concept [22]; ¾ Titania functionalization: Ti4+ is a surface defect in TiO2 that introduces specific energy levels in the TiO2 electronic density of states. According to the HSAB theory, Ti4+ cations are hard acids. As a consequence, the carboxylate group (COO-), which is a hard base, could be useful for appropriate functionalization [23]; ¾ Analytical chemistry: The HSAB theory was proven to be an useful tool for predicting favorable equilibrium in reactions involving cations of transition metals [24]; ¾ Corrosion: Design of corrosion inhibitors can be performed by using the HSAB theory [25]; ¾ Surface and adhesion phenomena: The HSAB principle was used in the design and optimization of interactions at the interface between metals and polymers [26]; ¾ Computational chemistry: Theoretical study of reactivity in different types of compounds, such as isatoic anhydride and some derivatives was performed based on the HSAB principle [27]. In this paper, we propose and demonstrate the use of Pearson’s Hard-Soft Acid-Base concept as valuable new tool to coherently explain the selection principle of several gas sensing layers, employed in different types of sensing structures. Novel gas sensing layers, selected and designed in accordance with the HSAB principle, are also presented and discussed.
releases 150 - 200 g of CO2 per mile. The current level of CO2 concentration in open air is in the range of 380 - 440 ppm, depending on the season, location and vicinity with the large urban sites. It is estimated that the increase of CO2 background concentration level to about 550 ppm in the air will increase the average temperature of the planet by a few degrees Celsius. In parallel with intensive research activity to replace the traditional technologies of energy generation with green, non-polluting solutions like solar energy, there is a strong effort involved in developing green coal technologies where the CO2 released from coal burning processes is captured and sequestrated. This implies real-time monitoring of CO2 emissions from transportation pipeline network and storage reservoirs, which are mainly located under the sea. This is the foundation for a significant need for low cost-low power CO2 sensors, able to work in harsh environments. The currently available CO2 sensors are either expensive or high power consuming, or both. Therefore, in the last years, extensive efforts have been devoted to finding or conceiving new carbon dioxide sensitive coatings with high sensitivity and fast response, suitable for room and high temperature detection. From the HSAB concept point of view, carbon dioxide is classified as hard acid. Therefore, in accordance with the HSAB rule, hard bases compounds (or different moieties within these compounds) are potential candidates for CO2 detection. Indeed, inspection of the literature data reveals that the most significant categories of sensitive layer currently employed for CO2 detection are in agreement with this principle: ¾ Polymers sensitive to CO2, such as polyethyleneimine (PEI) [28], poly (3aminopropyltrimethoxysilane octadecyltriethoxysilane copolymer (PA-POS), poly (3-aminopropyl-trimethoxysilane propyltrimethoxysilane copolymer) (PAPPS) [29], polypropyleneimine, aminoalkylpolydimethylsiloxane, polystyrenebound ethylenediamine (PS-EDA), are hard bases, due to the amino groups which are present on their polymeric back-bone; ¾ Small organic molecules sensitive to CO2, such as 7, 10, dioxa-3,4 diaza- 1,5,12, 16hexadecatetrol, diamino-p-menthane, triethanolamine, tri-n-octylamine, 1naphtylmethylamine, benzylamine, pyrene amine, antracene amine, dipropylamine, N,N,-bis (2-
2. THE HSAB CONCEPT IN CARBON DIOXIDE SENSING 2.1 Material selection and sensing results for CO2 sensing using the HSAB concept Global efforts on preserving the health of entire planet and eco-systems are focused on a few key directions like global warming and environment air quality. It is generally agreed that the highest threat for the life on Earth is posed by the greenhouse effect caused by huge CO2 released emissions of electrical power plants, refineries and transportations industries. For example, a car 22
hydroxiethyl) ethylenediamine are hard bases, due to the presence of aliphatic amino groups [30-33]; ¾ Inorganic molecules, like sodium carbonate [33] and trans-[carbonyl hydroxyl bis(triphenylphosphine) rhodium(I)] [34] are also hard bases; Based on the HSAB principle, our group has developed several novel CO2 sensing layers. Among these, one can enumerate sensing layers based on polyallylamine (PAA), polyallylamineamino carbon nanotubes (PAA - aCNTs) matrix nanocomposite, polyethyleneimine-amino CNTs matrix nanocomposite (PEI – aCNTs) [35 - 42]. All these types of CO2 sensing layers are hard bases. They were successfully demonstrated using quartz based Surface Acoustic Waves (SAW) devices. The interaction of CO2 with selected polymers having primary and secondary aliphatic amino groups is ionic and reversible, yielding to carbamates. A common example is the reaction of CO2 with polyallylamine (PAA), shown in Fig. 1. CH2
constant for the whole range of CO2 concentration levels investigated. A good linearity of the sensitivity of the SAW sensor for the CO2 concentration in the range of interest can be observed; the measured sensitivity, 30 Hz / 100 ppm of CO2, given by the slope of the linear dependence (Fig. 3), is superior to those reported by other groups after such a long ageing period.
Fig. 2 – Measured frequency response of a SAW sensor coated either with PAA or with the PAA – amino CNTs composite when exposed to different CO2 concentrations levels in an artificial air atmosphere (20% O2, 80% N2)
CH CH2 n
CH2
CH
+ CO2
CH
CH2
CH2 n
NH2
CH2 n +
NH2
NH2
C OO
CH2
CH NH
CH
CH2
CH2 n C O
-
+
CH2 n NH3
O
Fig. 1 – Reaction of PAA with CO2 leading to carbamates Fig. 3 – Experimentally measured frequency shift exhibited by a SAW sensor coated either with PAA or with the PAA – amino CNTs composite
The experimental results of SAW-based sensors employing PAA and amino carbon nanotubes (aCNTs) as sensing layers are presented in Fig. 2, where a SAW delay line is located in the feed-back loop of an oscillator. In an artificial air atmosphere (20% O2, 80% N2), the CO2 concentration was varied between 1200 ppm and 300 ppm. A frequency shift of 1.5 kHz was observed when the PAA-based SAW sensor was exposed to a 1200 ppm CO2 concentration pulse. To further understand the sensor response, the frequency shift as a function of the CO2 concentration is plotted in Fig. 3. Prior to this measurement, the sensor was aged for 89 days at room temperature. A quasilinear increase of the frequency shift with the CO2 concentration indicates that the sensitivity is
As in the case of simple PAA sensing layer, the frequency shift versus CO2 concentration curve corresponding to the PAA-amino-CNTs sensing layer exhibits a linear trend, as depicted in Fig. 3. At the same time, the CO2 sensitivity was also experimentally measured for a SAW device employing polyethyleneimine-amino CNTs matrix nanocomposite (PEI – aCNTs) as sensing layer (Fig. 4). A frequency shift of 700 Hz is obtained when the SAW-based sensor is exposed to 2500 ppm of CO2. In conclusion, if one compares the frequency shifts at 2500 ppm of CO2 for the three sensing 23
layers discussed above (Fig. 5), better sensitivity is obtained when employing simple polymers (PAA) rather than matrix nanocomposites (PAA - aCNTs and PEI - aCNTs, respectively). A possible explanation for this phenomenon is related to the type of amino groups at the surface of the carbon nanotubes. These amino groups are borderline bases and their preference for CO2 molecules is lower in comparison with that exhibited by the aliphatic amino groups which exist in PEI and PAA. For this reason, both PEI – aCNTs and PAA aCNTs matrix nanocomposites exhibit lower sensitivity than simple polymers. At the same time, self-assembled monolayers (SAMs) for CO2 detection by means of a silicon resonant nanosensor were designed in accordance with the HSAB theory [44]. Such a sensing monolayer comprises molecules that have one terminal group attached to Si and the other responsible for CO2 recognition and detection. Such sensitive groups are primary and secondary amines, which are hard bases, therefore suitable for CO2 detection, according to the HSAB principle.
2.2 All-Differential CO2 Sensing Using the HSAB Approach Most of the CO2 sensors employing polymers with amino groups experience cross sensitivity with water, which is as major drawback. Based on the HSAB approach, a novel carbon dioxide differential sensing scheme (Fig. 6) using a SAW sensor was developed by our group [45 - 47]. Unlike the traditional differential sensing systems, which are based on an appropriately functionalized sensing layer in the sensing loop and on an uncoated surface in the reference loop, the new “all-differential” CO2 sensing concept provides a better response subtraction between the two paths.
Fig. 6 - SAW based chemical differential sensing system A1 and A2 - RF oscillator SAW delay line
In the scheme depicted in Fig. 6, A1 is an RF oscillator with SAW delay line and sensing layer (6) in the positive feed-back loop oscillating on frequency f1, while A2 is an RF oscillator with SAW delay line and reference, non-sensing layer (7) oscillating on frequency f2. 1 and 2 are input Interdigital Transducers (IDTs) and, respectively, output of SAW delay line 1, while 3 and 4 are inputs and output IDTs for SAW delay line 2. The tandem polyallylamine (PAA) (as sensing layer) - PAA hydrochloride (as reference layer) was chosen to demonstrate this new concept. Both polymers exhibit quasi-similar sensing properties towards humidity. According to the HSAB rule, PAA is a hard base, due to the presence of amino groups onto its backbone, and, thus, can have a strong interaction with CO2, which is a hard acid. At the same time, in PAA hydrochloride, due to the reaction of PAA with the hydrochloric acid, the ability of the polymer to interact with CO2 is suppressed (the electrons pair is blocked), while the
Fig. 4 Frequency variation when the PEI composite sensor is exposed to 2500 ppm CO2 concentration in an artificial air atmosphere (20% O2, 80% N2)
Fig. 5 - Frequency shifts exhibited by SAW devices employing different sensing layers when exposed to 2500 ppm of CO2
24
sensitivity towards humidity remains the same as for PAA. Experimental results show that, after exposing the sensor to a temperature variation in the range 20°C – 60°C, the response frequencies of the sensing oscillator and of the reference oscillator have similar (high) variations with temperature (Fig. 7), while the all-differential SAW structure response (f1–f2) is almost equal to zero. At the same time, the measured response of the “all-differential” SAW sensor when varying relative humidity in the range 15% - 50% (Fig. 8) clearly demonstrates the cancellation of the humidity effect in the final sensor response due to the presence of the reference layer having the same humidity behavior as the sensing layer.
3. HSAB CONCEPT DIOXIDE SENSING
IN
NITROGEN
Due to the highly harmful potential of toxic gases on human health, the maximum legally allowed emission limits in polluting industries, (automotive, aerospace, power plants, refineries), were gradually decreased in recent years. In the case of nitrogen dioxide (NO2) emissions, the National Ambient Air Quality Standards (NAAQS) impose the permissible annual average of NO2 in ambient air to be no higher than 53 ppb, while for short term exposures limits (STEL) such values should not exceed 1 ppm in 15 minutes. Unfortunately, the gas sensing accuracy of the currently available electrochemical and low cost NO2 sensors is in the range 1-2 ppm, which makes them unsuitable for air quality monitoring applications. At the same time, if one considers the huge applicability potential of the sensors in the mobile phones market, the state-of-the-art solutions for NO2 detection are both bulky and expensive. For this reason, in the last decade, great effort was dedicated to finding or conceiving new NO2 sensitive coatings with high sensitivity, low-power consumption, and fast response. From the HSAB concept point of view, nitrogen dioxide is classified as a soft acid. Therefore, compounds (or different moieties within this compounds) that are soft bases are potential candidates for NO2 detection. Literature review shows, indeed, that soft bases such as graphene [48 - 51], carbon nanotubes [52], graphene-CNTs matrix nanocomposite [53], and graphite [54] were used as NO2 sensing layer in different types of sensing structures. Recently, by using the same HSAB criterion, our group has designed functionalized monolayers of silicon meant for NO2 detection by means of a silicon resonant nanosensor [55]. The key sensing layer moiety is based on CNTs, as described in Fig. 9.
Fig. 7 – Frequency variation with temperature for the sensing oscillator, the reference oscillator and the all-differential SAW sensor. The reference and sensing oscillator signals vary by nearly 75 kHz, while the all-differential sensor signal varies by 1.7 kHz
CNT
O C
Fig. 8 - Frequency variation with relative humidity (RH) of the sensing oscillator, of the reference oscillator and of the all-differential SAW sensor. The reference and sensing oscillator signals vary by nearly 2.5 kHz, while the all-differential sensor signal varies by 0.2 kHz
O C
O C
O C
O C
NH
NH
NH
NH
NH
O
O
O
O
O
Si
Si
Si
Si
Si
Fig. 9 – Chemical structure of a functionalized silicon nanoresonator sensitive to NO2 designed by Honeywell [55]
25
4. THE HSAB CONCEPT IN SULPHUR DIOXIDE SENSING
O
x
y x
N
Sulfur dioxide (SO2) is one of the most dangerous air pollutants emitted in atmosphere by volcanoes, during the combustion of fossil fuels such as coal and petroleum in power plants, or during the oxidation of organic compounds. The further oxidation of SO2 leads to acid rains having devastating effects on the eco-systems. The SO2 toxicity is very high; therefore its maximum legally allowed level was steadily decreased in the last years. Thus, starting from an initial Threshold Limit Value (TLV) of 2 ppm in air, the current NAAQS requirements for the quality of the ambient air are that the SO2 annual arithmetic average to be smaller than 30 ppb. These regulations against SO2 emissions need a new generation of SO2 sensors able to detect such small concentration values. Such sensors should have low power consumption, cost and size, allowing them to be used in large area distributed wireless sensor networks. As it was already mentioned, a borderline base tends to interact with a borderline acid. According to Pearson’s theory, sulfur dioxide is a borderline acid and has an affinity for borderline bases. Among the borderline bases, one can enumerate aromatic amines, pyridine, azide, bromide, and nitrite ions. Recently, in good agreement with the HSAB theory, our group has proposed pyridine moietybased polymers as sensitive layer for SO2 detection: poly (2-vinylpyridine) (Fig. 10), poly (4vinylpyridine), poly (4-vinylpyridine-cobutylmethacrylate) (Fig. 11), poly (2-vinylpirydineco-styrene) [56]. The sensing structure employed is silicon nano-electromechanical systems (NEMS), where a vibrating functionalized silicon nano-beam changes its resonance frequency as a function of the SO2 concentration in the ambient air. The novelties of this approach come from the HSAB theory-based chemical functionalization of the silicon surface with existing polymeric ultra thin layers and from the use of the reference sensing ultra thin layers (e.g. polystyrene), having the same physical properties like the sensing layer, but no sensing capabilities.
O
y
N
Fig. 10 – The structure of poly (2-vinylpyridine co-styrene)
Fig. 11 – The structure of poly (4-vinylpyridineco-butylmethacrylate)
5. “INDIRECT HSAB” CONCEPT IN MINERAL AND ORGANIC ACIDS SENSING Mineral and organic acid detection is an extremely important issue related to the safe operation of numerous, highly complex industrial areas, such as: semiconductor manufacturing (where hydrogen chloride and hydrogen fluoride are employed), chemical plants (where hydrogen fluoride is used) and food industry (employing acetic acid, phosphoric acid, propionic acid, butyric acid). One of the most widely used solutions for the detection of mineral and organic acids is a colorimetric sensor [57]. When such a structure is exposed to a target gas, it changes its color in direct proportion with the gas concentration. The monitoring systems read color intensity changes and determine the gas concentration. The formula of the solution used to impregnate the cellulose substrate comprises a source of nitrite ions (sodium nitrite), a diazotization coupling agent (sulfanilic acid), a coupling agent for the diazonium salt (chromotropic acid), a pH indicator (methyl red), a pH buffer material (sodium hydroxide with sodium borate), a stabilizer (sodium bromide) and a humectant (a polyalcohol). All mentioned above are being dissolved in methanol. The sensor shows good sensitivity and fast response time. Building on this structure, our group has followed the HSAB approach in developing a chlorophyll-based sensor for mineral and organic acid detection [58]. Chlorophyll is one of the most studied metallic biomolecules, found in various plants and algae. There are different types of chlorophyll, with different chemical structures: chlorophyll a (molecular formula C55H72O5N4Mg), chlorophyll b (C55H70O6N4Mg), chlorophyll c1 (C35H30O5N4Mg), etc. All these structures contain the Mg2+ cation surrounded by a tetrapyrrolic ring. According to the HSAB theory, the magnesium (II) cation (small, doubly charged) is classified as a hard acid, while nitrogen atoms, present in the 26
tetrapyrrolic ring (due to large electron delocalization), are borderline bases [59]. One of the most important statements of the HSAB theory is that hard acids prefer to react with hard bases, while soft acids prefer to react with soft acids. As a consequence of this principle, the central magnesium in the chlorophyll structure cation can be easily removed from the tetrapyrrolic ring in reaction with different cations, such as Cu (II), Zn (II), a mineral acid like HCl (as described in Fig. 12), or an organic acid like CH3COOH. In the presence of a mineral or an organic acid, chlorophyll a is converted into pheophytin a, while chlorophyll b is converted into pheophytin b. All these chemical reactions are accompanied by color changes. While both chlorophyll a and b are bright green, pheophytin b is green-grey and pheophytin b is olive-green. These color changes can be easily exploited for designing a colorimetric method for mineral acid sensing.
N
N
NH
Mg N H C20H39
O C O
6. THE HSAB APPROACH IN FLUORESCENCE QUENCHING-BASED OXYGEN SENSING In various fields, such as automotive, medical applications (like anesthesia monitoring), and environmental monitoring, it is of high importance to determine the oxygen concentration. Recently, many devices based on the fluorescence quenching of organic molecules were developed in order to determine the concentration of oxygen [60-63]. At the same time, inorganic fillers, such as Al2O3, ZrO2, TiO2 Fe2O3, Cu2O, highly dispersed onto the surface of cellulose and its derivatives, were used in manufacturing fluorescence quenching – based O2 sensors [64]. In order to ensure a robust interface (either ionic or covalent) between the fluorophore and the polymeric support coated with a metal oxide or with bulk metal, the HSAB concept has again proven to be a valuable tool in the rational design of the filler – fluorophore tandem [65]. Organic fluorophores can be functionalized with appropriate moieties which can act as soft, borderline or hard base. For instance, pyrene alkane thiols as the ones depicted in Figs. 13-15 can act, due to their thiols groups, as soft bases [66]. If Cu2O is used as inorganic filler (with Cu+ acting as a soft acid), pyrene thiols or appropriate mercaptides (soft bases) can be used for generating the covalent bond. The Lewis acid (soft, borderline, or hard) is either the bulk metal (e.g., Au) or the cations of the metal oxides (Al3+, Ti4+, Cu+, etc). By using this approach, ionic interaction (i. e., hard acid-hard base tandem) or covalent interaction (e.g., soft acid- soft base tandem) can be designed at interfaces. Pyrene 1-decanoic acid, pyrene 1-butyric acid and pyrene 1-dodecanoic acid are other examples that can act as hard base in a basic medium (such as carboxylate salt). For instance, if Al3+ (a hard acid) is a major species at the surface of the metal oxides (in the case of Al2O3 used as filler), or Ti4+ (in the case of TiO2), pyrene carboxylic acid is converted to the carboxilate form (hard base). The interface is ensured by the ionic interaction between anion and cation.
N
2H+ N
N H
H O
case the sensing reaction speculates the structural HSAB mismatching in the sensing element.
O OCH3
C20H39
O C O
HN
H O
O OCH3
Fig. 12 – The reaction of chlorophyll-chore a with a mineral acid (HCl) leading to the formation of pheophytin-chore
The sensing layer, based on chlorophyll and a hygroscopic compound, such as cyclodextrines (Į or ȕ or Ȗ or mixture of these ), xylitol or maltitol, is a much simpler and environmental-friendly solution compared to the ones employed in currentlyavailable sensing structures and by other colorimetric sensing structures available on the market. The hygroscopic component ensures the amount of humidity necessary for the in-situ ionization of mineral or organic acids. Thus, the whole formula composition for mineral acid detection comprises only two elements, is environment friendly and is based on a single chemical reaction [58]. The method will be further named “indirect HSAB” due to the fact that, in comparison with other examples where the sensing elements and target molecules are an acid–base tandem (soft – soft, borderline – borderline or hard – hard), in this 27
CH2SH
montmorilonite and dickite). Both isolator polymer and clay have the role to improve the thermal behavior of the sensitive matrix nanocomposite. This sensing approach proposal is in good agreement with the HSAB principle: H2S, which is a soft base, has a strong interaction with cations like Cu+, Ag+, Hg2+, classified as soft acids:
CH2O(CH2)5SH
Fig. 14 - The structure of 5-pyrene-1-yl-methoxypentane-1-thiol CH2O(CH2)8SH
Fig. 13 – The structure of 1-pyrene-1yl-methanethiol
ʹ ݑܥା ܪଶ ܵ ൌ ݑܥଶ ܵ ՝ ʹܪା ʹ݃ܣା ܪଶ ܵ ൌ ݃ܣଶ ܵ ՝ ʹ ܪା 8. CONCLUSIONS DEVELOPMENT
FURTHER
The HSAB rule was discussed as a new promising tool for the selection and rational design of materials for gas sensing. Several target gas molecule-sensitive layer tandems described in literature are discussed and interpreted in the view of this theory. Different types of gas molecules, such as carbon dioxide, nitrogen dioxide, hydrogen sulphide, and sulphur dioxide, and corresponding sensitive layers are classified according to this concept. For oxygen and mineral acids detection, an “indirect HSAB” approach is presented. The paper discusses how the HSAB principle can be useful for the design of gas sensing layers used in different types of sensors: surface acoustic waves (for CO2), colorimetric (for mineral and organic acids), chemoresistive (for H2S), nanocantilever (for CO2, SO2 and NO2), fluorescent quenching–based (for O2). Based on the HSAB approach, a novel CO2 differential sensing scheme using a SAW sensor was presented. Without claiming that this method will exhaustively discriminate between sensitive and non-sensitive layers, it is believed that the HSAB concept is a valuable tool for identifying the most appropriate gas sensing layers. This statement is supported by results reported in literature, by theoretical findings and by experimental measurements performed by our group. As future work, more detailed calculations in terms of electronegativity and local hardness are required for a better evaluation of the sensing layer suitability towards different types of gases.
Fig. 15 - The structure of 8-pyrene-1-yl-methoxy pentane 1-thiol
7. THE HSAB APPROACH HYDROGEN SULPHIDE SENSING
AND
IN
Hydrogen sulphide (H2S) is a flammable, irritating, corrosive, typically bad-smelling, and extremely toxic gas. The toxicity of H2S can be comparable with that of hydrogen cyanide, which is considered a broad-spectrum poison. The detection of the H2S can be performed by means chemoresistive, electrochemical (amperometric or potentiometric), conducting polymers or optical principles [67 - 68]. Conducting polymers-based sensors for H2S detection have received a special attention in the last few years. Polyaniline (PANI) nanofibers composites containing metal salts such as CuCl2 present an enhanced response to H2S when compared to simple PANI. The sensing principle is based on the increase in the conductivity of the film containing PANI and metal salts, due to the reaction between the metal salt and H2S, with formation of hydrochloric acid [69]: ݈ܥݑܥଶ ܪଶ ܵ ൌ ܵݑܥ ʹ݈ܥܪ Based on this sensing concept, our group has proposed two types of matrix nanocomposite for H2S sensing at elevated temperature (150°C 200°C). The first sensing structure comprises three key elements: derivative of emeraldine base, metal salt (Cu (I) or Ag (I) and an isolator polymer (such as polyvinyl acetate, polystyrene or polyvinyl chloride). In the second one, the isolator polymer is replaced by clay (muscovite, kaolinite,
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