Second-Generation Phosgene and Diphosgene Detection Tube

chemosensors

Article Second-Generation Phosgene and Diphosgene Detection Tube

Vladimír Pitschmann 1,2, Lukáš Matˇejovský 3,* , Jiˇrí Zeman 4 , David Vetchý 4, Michal Dymák 5, Martin Lobotka 1, Sylvie Pavloková 4 and ZdenˇekMoravec 6

1 Oritest spol. s.r.o., Cerˇcanskˇ á 640/30, 140 00 Prague, Czech Republic; [email protected] (V.P.); [email protected] (M.L.) 2 Faculty of Biomedical Engineering, Czech Technical University in Prague, Sítná 3105, 272 01 Kladno, Czech Republic 3 Faculty of Environmental Technology, University of Chemistry and Technology, Technická 5, 166 28 Prague, Czech Republic 4 Department of Pharmaceutical Technology, Faculty of Pharmacy, Masaryk University, Palackého tˇr.1946/1, 612 00 Brno, Czech Republic; [email protected] (J.Z.); [email protected] (D.V.); [email protected] (S.P.) 5 National Institute for Nuclear, Chemical and Biological Protection, Kamenná 71, 262 31 Milín, Czech Republic; [email protected] 6 Department of Chemistry, Faculty of Science, Masaryk University, Kotláˇrská 2, 611 37 Brno, Czech Republic; [email protected] * Correspondence: [email protected]

Received: 1 October 2020; Accepted: 29 October 2020; Published: 29 October 2020

Abstract: We have developed a second-generation detection tube for colorimetric and fluorescence detection of phosgene and diphosgene in air. The tube is packed with pellets made of a mixture of microcrystalline cellulose and magnesium aluminum metasilicate treated with a suitable monoterpene (camphor, menthol) to increase porosity and specific surface area. We impregnated the pellets with a specific o-phenylenediamine-pyronin (PY-OPD) based reagent. The detector with this novel indication charge enables phosgene or diphosgene to be selectively and sensitively detected at concentrations lower than as would those posing acute health risk. Owing to the analytical colorimetric and, at the same time, fluorescence signal, the detector is very robust while featuring good sensitivity and variability. The colorimetric limits of detection were 0.3 mg/m3 (tristimulus colorimeter), resp. 5 mg/m3 (with the naked eye), fluorescence detection limits of 0.3 mg/m3 (with the naked eye), all at an air sample volume of 1 dm3. The response was practically immediate, acid vapors and gases, or diethyl chlorophosphate as a simulant of nerve warfare chemical agents, were disruptive.

Keywords: detection tubes; chemosensor; visual and ﬂuorescence detection; phosgene; composite pellets

1. Introduction Phosgene (carbonyl dichloride) and diphosgene (trichloromethyl chloroformate) are toxic compounds that damage the alveolar air/capillary air–blood barrier in the lungs. The change in the permeability of this barrier due to the action of the compounds gives rise to typical pulmonary edema, and the reduced resistance against secondary infection supports the development of bronchopneumonia. 3 Inhalation toxicity of phosgene and diphosgene (LCt50) is 3200 mg min/m , the maximum vapor 3 · 3 concentration (volatility) is 4,300,000 mg/m for phosgene (7.6 ◦C) and 45,000 mg/m (20 ◦C) for diphosgene [1]. Phosgene is an important industrial chemical in the organic synthesis domain. Both phosgene and diphosgene were one of the main types of chemical warfare agents used (CWA)

Chemosensors 2020, 8, 107; doi:10.3390/chemosensors8040107 www.mdpi.com/journal/chemosensors Chemosensors 2020, 8, 107 2 of 10 in the deployment of major chemical weapons, mainly during World War I. Currently, phosgene is listed on Schedule 3 of the Chemical Weapons Convention. Hazards may arise from phosgene and, partly, diphosgene leaks during chemical industrial accidents and from civil wars and terrorist attacks in which the compounds are used as efficient chemical weapons. Great attention is constantly paid to analytical methods for phosgene and diphosgene. Apart from technically advanced physical and physico-chemical detectors and analysers, attention is paid to rapid, simple, and inexpensive detection methods for phosgene/diphosgene in air based on color reactions using detection papers, strips, chalks, and detection tubes [2]. The traditional procedures and methods are based on a nucleophilic substitution reaction (acylation) of phosgene/diphosgene with analytical reagents such as p-dimethylaminobenzaldehyde [3–5], 4,40-bis(dimethylamino)benzophenone [6,7], 4-(p-nitrobenzyl)pyridine [8,9], or anabasine [10]. A number of novel reagents providing colored reaction products that exhibit visible, often predominating fluorescence were developed during the past decade. Examples include chemosensors based on pyronine, rhodamine, iminocoumarin, BODIPY, and other organic compounds [11–25]. Their research is only at the laboratory stage, commercial evaluation is pending. Detection tubes are among the most widespread simple tools to detect phosgene/diphosgene. They have different designs (the indication charge can be completed with a detection solution in an ampoule) and different sensitivity and selectivity levels, and they also have different resistance to the weather conditions and to ageing. Existing detection tubes are almost solely based on the reaction with 4-(p-nitrobenzyl)pyridine and N-phenylbenzylamine or with p-dimethylaminobenzaldehyde in combination with aniline derivatives [10]. Given the existence of new colorimetric and fluorescence reagents (chemosensors), it is not surprising that efforts are made to examine the feasibility of using them in detection tubes. Such devices can be referred to as ‘second generation detection tubes’. The present work is devoted to the development of a detection tube with a chemosensor based on o-phenylenediamine-pyronine [12] in combination with a specifically prepared composite carrier. Such a device could be used for the orientational detection of phosgene/diphosgene in the armed forces and rescue services. We previously used this chemosensor for the preparation of a detection paper for use on liquid CWA [26].

2. Materials and Methods

2.1. Chemicals Materials used for the preparation of carriers included menthol (Dr. Kulich Pharma, Hradec Králové, Czech Republic), camphor (Dr. Kulich Pharma, Hradec Králové, Czech Republic), magnesium aluminum metasilicate (Neusilin® US2, Fuji Chemical Industry, Toyoma, Japan), microcrystalline cellulose (Avicel® PH-101, FMC Biopolymer, Wallingstown, Ireland), microcrystalline cellulose and sodium carboxymethylcellulose (Avicel® RC-581, FMC Biopolymer, Brussels, Belgium), o-phenylenediamine-pyronin (PY-OPD, synthesized by the University of Chemistry and Technology, Prague, Czech Republic; the spectral data corresponded to the literature), chloroform (Merck, Darmstadt, Germany), and puriﬁed water (Ph. Eur. 10). The tests were made with calibrated 1% phosgene (Linde Gas, Prague, Czech Republic), diluted as required with air in 1 dm3 Tedlar bags (SKC, Eighty Four, PA, USA). Diphosgene (Merck, Darmstadt, Germany) was used for the gas chamber test.

2.2. Apparatus Instrumentation used for the preparation of carriers included a Stephan UMC 5 electronic mixer (A. Stephan & Söhne, Hameln, Germany), a PharmTex 35T single-screw extruder and spheronizer (Wyss & Probst Engineering, Ettlingen, Germany) and an AS200 Basic sieve shaker (Retsch, Haan, Germany). A MIRA3 scanning electron microscope (SEM) (Tescan Orsay Holding, Brno, Czech Republic) was used to obtain SEM images of the pellets. The samples were coated with a 20 nm gold layer by Chemosensors 2020, 8, 107 3 of 10 Chemosensors 2020, 8, 107 3 of 10

layer by metal sputtering in argon atmosphere on a Q150R ES Rotatory‐Pumped Sputter Coater / metal sputtering in argon atmosphere on a Q150R ES Rotatory-Pumped Sputter Coater/Carbon Coater Carbon Coater (Quorum Technologies, Laughton, UK). A secondary electron detector was used, and (Quorum Technologies, Laughton, UK). A secondary electron detector was used, and accelerating accelerating voltage of 3 kV was applied for the SEM measurement itself. The specific surface area of voltage of 3 kV was applied for the SEM measurement itself. The speciﬁc surface area of the samples the samples was measured with the BET method on a Quantachrome Autosorb iQ3 gas porismeter was measured with the BET method on a Quantachrome Autosorb iQ3 gas porismeter (Quantachrome (Quantachrome Instruments, Boynton Beach, FL, USA). The samples were degassed at 80 °C, which Instruments, Boynton Beach, FL, USA). The samples were degassed at 80 ◦C, which was reduced to was reduced to 30 °C after attaining the pressure increase rate of 20 mtorr/min, and the samples were 30 ◦C after attaining the pressure increase rate of 20 mtorr/min, and the samples were kept under kept under vacuum until the measurement. vacuum until the measurement. The performance of the detection tubes was tested in a 0.5 m3 gas chamber equipped with a The performance of the detection tubes was tested in a 0.5 m3 gas chamber equipped with a thermostat and a fan. The samples were taken with a manual aspiration system, stroke volume 100 ± thermostat and a fan. The samples were taken with a manual aspiration system, stroke volume 5 cm3 (Kavalier, Votice, Czech Republic). 100 5 cm3 (Kavalier, Votice, Czech Republic). ±An LMG 173 tristimulus colorimeter (Dr. Lange, Düsseldorf, Germany) was used for the An LMG 173 tristimulus colorimeter (Dr. Lange, Düsseldorf, Germany) was used for the objective objective measurement of the indication charge color intensity. A 366 nm radiation wavelength was measurement of the indication charge color intensity. A 366 nm radiation wavelength was obtained obtained from an L92 UV lamp (Leuchtturm, Geesthacht, Germany). from an L92 UV lamp (Leuchtturm, Geesthacht, Germany).

2.3.2.3. CarrierCarrier PreparationPreparation

® ® AA powder powder mixture mixture of of 44 44 g g of of Avicel Avicel® PH-101 PH‐101 microcrystalline microcrystalline cellulose, cellulose, 11 11 g g of of Avicel Avicel® RC-581RC‐581 microcrystallinemicrocrystalline cellulose cellulose with with sodium sodium carboxymethylcellulose, carboxymethylcellulose, 25 g of magnesium25 g of magnesium alumino metasilicate alumino ® (Neusilinmetasilicate® US2) (Neusilin and 20 US2) g of mentholand 20 g (batch of menthol M20.0) (batch or camphor M20.0) (batch or camphor C20.0) was(batch homogenized C20.0) was inhomogenized a mixer at 1500in a rpmmixer for at 5 1500 min andrpm subsequentlyfor 5 min and wetted subsequently with puriﬁed wetted water with (mixturepurified/ waterwater ratio(mixture/water 1:1.15) at a ratio ﬂow rate1:1.15) of 100at a mL flow/min. rate The of pellets100 mL/min. were prepared The pellets from were the moistenedprepared from mass bythe extrusionmoistened/spheronization mass by extrusion/spheronization on an axial single-screw on an extruder axial single/spheronizer‐screw extruder/spheronizer at 60 rpm through at a grid 60 rpm of 1.25through mm mesh. a grid The of 1.25 extrudate mm mesh. was then The spheronizedextrudate was at 1000then rpmspheronized for 20 min. at 1000 The product rpm for was 20 min. dried The in product was dried in a hot air oven at 60 °C for 24 h. The reference pellet batch (N) contained neither a hot air oven at 60 ◦C for 24 h. The reference pellet batch (N) contained neither menthol nor camphor. Thementhol physical nor properties camphor. ofThe the physical pellets (bulkproperties density, of the tapped pellets density, (bulk Hausner density, ratio,tapped hardness, density, friability, Hausner pycnometricratio, hardness, density, friability, interparticular pycnometric and intraparticular density, interparticular porosity, and and sphericity) intraparticular were determined porosity, and by conventionalsphericity) were methods determined [27]. by conventional methods [27].

2.4.2.4. CarrierCarrier (Pellet)(Pellet) ImpregnationImpregnation and and Detection Detection Tube Tube Preparation Preparation AA 100100 gg batchbatch of 0.8–1.25 mm mm pellets pellets was was impregnated impregnated with with 50 50 mL mL of of 1% 1% PY PY-OPD‐OPD solution solution in inchloroform. chloroform. The The impregnated impregnated pellets pellets were were allowed allowed to air to dry air dryto complete to complete solvent solvent evaporation. evaporation. Dry Dryand and non non-agglomerated‐agglomerated carriers, carriers, light light-yellow‐yellow in in color, color, were were stored stored in in a a hermetically hermetically sealed containercontainer protectedprotected from from sunlight. sunlight. TheThe detectiondetection tubestubes werewere preparedprepared asas follows: follows: TheThe chargecharge waswas pouredpoured intointo aa 55 mmmm diameter diameter glassglass tube tube to to obtain obtain a a 10 10 mm mm long long layer. layer. This This was was stabilized stabilized by by using using polyethylene polyethylene (PE) (PE) elements elements to to enableenable air air to to ﬂow flow through through the the system. system. The The tubes tubes were were hermetically hermetically sealed sealed (the total(the lengthtotal length of 100 of mm). 100 Themm). construction The construction of the detectionof the detection tube and tube its and use its are use shown are shown in Figure in 1Figure. 1.

Figure 1. Detection tube design (1—indication charge; 2—PE element) and its use. Figure 1. Detection tube design (1—indication charge; 2—PE element) and its use. 2.5. Detection Tube Testing 2.5. DetectionWe selected Tube testing Testing in a gas chamber in which a speciﬁc volume of diphosgene solution in hexane had beenWe evaporated,selected testing as the in basica gas method chamber of detectionin which tubea specific performance volume testing.of diphosgene The detection solution tube in hexane had been evaporated, as the basic method of detection tube performance testing. The

Chemosensors 2020, 8, 107 4 of 10 response to phosgene/diphosgene was evaluated based on (i) VIS color intensity assessed with the naked eye and measured on a tristimulus colorimeter; and (ii) intensity of ﬂuorescence excited under a UV lamp (assessed with the naked eye). The stability of charge was tested on detection tubes exposed to elevated temperatures in a dryer for a prolonged period of time. The maximum temperature applied was 60 ◦C (selected with regard to the material of structural elements—polyethylene), the maximum heat burden duration was 2 months. The ageing process of the charge (evaluated with the naked eye and on the tristimulus colorimeter) and its response to phosgene/diphosgene in air were evaluated periodically. The color changes were evaluated based on the experience gained by the printing industry, where the human eye is capable of noting ∆E values measured with a tristimulus colorimeter as follows: ∆E < 0.2 imperceptible; ∆E = 0.2–0.5 very weakly; ∆E = 0.5–1.5 weakly; ∆E = 1.5–3.0 clearly, ∆E = 3.0–6.0 very clearly, ∆E = 6.0–12.0 strongly, ∆E > 12.0 very strongly.

3. Results and Discussion

3.1. Properties of the Carrier Silica gel as a traditional carrier (treated with sodium carbonate) enabled a very sensitive charge with PY-OPD to be prepared, however, it was not adequately stable and failed to exhibit fluorescence, especially at low concentrations. This stimulated us to develop a specific carrier eliminating the above shortcomings of silica gel. Our laboratories have been studying the preparation and use of pellets with different compositions for a number of years. For example, we have developed composite pellets containing microcrystalline cellulose and MgO and proposed their use as carriers for 4-(p-nitrobenzyl)pyridine and N-phenylbenzylamine in phosgene/diphosgene detection tubes [9]. Other past experiments were aimed at increasing the pellet’s sorption capacity by adding magnesium aluminum metasilicate; such carriers were then used in cholinesterase detection tubes serving to identify nerve chemical warfare agents [28,29]. As another method to attain still better sorption properties, the pellet mixture (microcrystalline cellulose and magnesium aluminum metasilicate) was treated with menthol or camphor. Comparative studies gave evidence that, unlike the previous types, such pellets are well suited to the preparation of indication charge for phosgene/diphosgene with the PY-OPD chemosensor, with a colorimetric and, at the same time, fluorescence indication effect. The best properties were observed with pellets treated with the highest fraction (20%) of menthol or camphor (M20.0, C20.0). Such pellets had a much (approximately two-fold) larger specific surface area than the untreated pellets (N), and this was not appreciably affected by impregnation with PY-OPD. We observed an increased interparticular porosity and for M20.0, also increased intraparticular porosity. Selected physical and structural properties of carriers are shown in Table1 and Figure2.

Table 1. Physical properties of pellets before and after impregnation with PY-OPD.

Property N N/PY-OPD M20.0 M20.0/PY-OPD C20.0 C20.0/PY-OPD Bulk density, g/cm3 0.777 0.003 0.758 0.006 0.500 0.005 0.502 0.001 0.583 0.004 0.588 0.000 ± ± ± ± ± ± Tapped density, g/cm3 0.796 0.004 0.790 0.007 0.509 0.001 0.520 0.002 0.596 0.004 0.624 0.002 ± ± ± ± ± ± Hausner ratio 1.024 0.000 1.042 0.005 1.019 0.008 1.036 0.005 1.024 0.000 1.060 0.004 ± ± ± ± ± ± Hardness, N 11.99 1.57 12.28 1.59 3.64 0.48 3.58 0.55 6.99 1.12 7.03 1.11 ± ± ± ± ± ± Friability, % 0.14 0.03 0.13 0.03 0.19 0.04 0.18 0.04 0.17 0.04 0.17 0.05 ± ± ± ± ± ± Pycnometric pellet density, g/cm3 1.6467 0.0065 1.6459 0.0082 1.6227 0.008 1.6159 0.0019 1.6596 0.0029 1.6528 0.0015 ± ± ± ± ± ± Interparticular porosity, % 52.80 0.21 53.97 0.35 69.19 0.31 68.95 0.09 64.90 0.24 64.41 0.00 ± ± ± ± ± ± Intraparticular porosity, % 2.76 0.38 2.81 0.48 4.02 0.05 4.42 0.11 2.29 0.17 2.69 0.09 ± ± ± ± ± ± Sphericity 0.982 0.010 0.977 0.017 0.986 0.010 0.980 0.018 0.982 0.012 0.980 0.014 ± ± ± ± ± ± Speciﬁc surface area, m2/g 65.90 0.84 63.73 0.19 121.14 5.32 127.72 0.55 113.44 3.44 119.94 6.94 ± ± ± ± ± ±

3.2. Color Characteristics and Analytical Data The detection was based on the specific cyclization reaction of phosgene/diphosgene with the chemosensor PY-OPD (Figure3) occurring on a specific composite carrier. This analytical reaction provided a violet product exhibiting clearly visible red fluorescence under a UV lamp. This fluorescence is characteristic of charges with this carrier at analytical concentrations (roughly up to 100 mg/m3). No fluorescence was observed if the treatment with menthol or camphor was omitted. The analytical Chemosensors 2020, 8, 107 5 of 10

Chemosensors 2020, 8, 107 5 of 10 signal was emitted almost immediately uniformly along the whole length of the indication charge and its intensity depended on the phosgene/diphosgene concentration in air. Chemosensors 2020, 8, 107 5 of 10

Figure 2. SEM images of pellets: (A)—reference pellet (N) with no camphor or menthol; (B)— reference pellet, impregnated with PY‐OPD; (C)—M20.0; (D)—M20.0 impregnated with PY‐OPD; (E)—C20.0; (F)—C20.0 impregnated with PY‐OPD; 1—whole pellet, 2—surface detail; 3—cross‐ section detail.

3.2. Color Characteristics and Analytical Data The detection was based on the specific cyclization reaction of phosgene/diphosgene with the chemosensor PY‐OPD (Figure 3) occurring on a specific composite carrier. This analytical reaction provided a violet product exhibiting clearly visible red fluorescence under a UV lamp. This fluorescence is characteristic of charges with this carrier at analytical concentrations (roughly up to Figure3 2. SEM images of pellets: (A)—reference pellet (N) with no camphor or menthol; (B)— 100 mg/mFigure). 2. NoSEM fluorescence images of pellets: was (observedA)—reference if the pellet treatment (N) with with no camphor menthol or menthol;or camphor (B)—reference was omitted. reference pellet, impregnated with PY‐OPD; (C)—M20.0; (D)—M20.0 impregnated with PY‐OPD; The analyticalpellet, impregnated signal was with emitted PY-OPD; almost (C)—M20.0; immediately (D)—M20.0 uniformly impregnated along with the PY-OPD; whole ( Elength)—C20.0; of the (E)—C20.0; (F)—C20.0 impregnated with PY‐OPD; 1—whole pellet, 2—surface detail; 3—cross‐ indication(F)—C20.0 charge impregnated and its intensity with PY-OPD; depended 1—whole on the pellet, phosgene/diphosgene 2—surface detail; 3—cross-section concentration detail. in air. section detail.

3.2. Color Characteristics and Analytical Data The detection was based on the specific cyclization reaction of phosgene/diphosgene with the chemosensor PY‐OPD (Figure 3) occurring on a specific composite carrier. This analytical reaction provided a violet product exhibiting clearly visible red fluorescence under a UV lamp. This fluorescence is characteristic of charges with this carrier at analytical concentrations (roughly up to 100 mg/m3). No fluorescence was observed if the treatment with menthol or camphor was omitted. The analytical signal was emitted almost immediately uniformly along the whole length of the FigureFigure 3. 3. PhosgenePhosgene reaction reaction with with PY PY-OPD‐OPD and detection tube indication charge. indication charge and its intensity depended on the phosgene/diphosgene concentration in air. The sensitivity was veriﬁed and the phosgene/diphosgene detection limits (limit of detection, LOD) were assessed with the naked eye and measured instrumentally. This was made based on the color/visible ﬂuorescence dependence on the analyte concentration within the range of 0–100 mg/m3,

Figure 3. Phosgene reaction with PY‐OPD and detection tube indication charge.

ChemosensorsChemosensors 2020 2020, ,8 8, ,107 107 66 of of 10 10

TheThe sensitivity sensitivity waswas verifiedverified andand thethe phosgene/diphosgenephosgene/diphosgene detection limitslimits (limit(limit ofof detection,detection, LOD) were assessed with the naked eye and measured instrumentally. This was made based on the ChemosensorsLOD) were2020 assessed, 8, 107 with the naked eye and measured instrumentally. This was made based on6 the of 10 color/visiblecolor/visible fluorescence fluorescence dependence dependence onon thethe analyte concentration withinwithin thethe rangerange of of 0–100 0–100 mg/m mg/m3,3 , asas shown shown in in Figure Figure 4.4. TheThe colorcolor intensityintensity increaseincrease (ΔE) measured onon thethe tristimulustristimulus colorimetercolorimeter is is asplottedplotted shown in in inFigure Figure Figure 5. 5. 4 . The color intensity increase ( ∆E) measured on the tristimulus colorimeter is plottedFromFrom in Figure the the obtainedobtained5. data,data, itit cancan bebe deduceddeduced that the sensitivity ofof detectiondetection (k(k == Δ ΔE/c)E/c) ofof thethe tristimulustristimulusFrom the byby obtained thethe colorimetercolorimeter data, it cancorrespondedcorresponded be deduced toto that the the change sensitivity of Δ ΔEE of == detection 0.30.3 withwith (k increasingincreasing= ∆E/c) of unitunit the tristimulusconcentrationconcentration by theof of the colorimeterthe pollutant pollutant corresponded (1(1 mg/mmg/m33).). InIn to lineline the with change this, of ∆theE =LOD0.3 with withwith increasing thethe tristimulustristimulus unit concentration colorimeter colorimeter ofisis 0.3 the 0.3 mg/m pollutant mg/m3 3and and (1 the mgthe /corresponding mcorresponding3). In line with levellevel this, forfor the thethe LOD naked with eye the is tristimulus 5 mg/m33 for colorimeterfor aa samplesample is 11 0.3 dmdm mg3 3volume. volume./m3 and theTheThe corresponding LOD LOD could could be be level improved improved for the byby naked increasingincreasing eye is 5thethe mg sample/m3 for volume—e.g., a sample 1 dm atat3 volume. airair samplingsampling The LOD ofof 33 dm coulddm3;3 ;the the be improvedcolorimetriccolorimetric by increasingLODLOD waswas the aroundaround sample 0.10.1 volume—e.g., mg/mmg/m33 (tristimulus(tristimulus at air sampling colorimeter), of 3 dm 3resp.;resp. the colorimetric << 22 mg/mmg/m3 LOD 3 (to(to wasthethe aroundnakednaked eye). eye). 0.1 mg /m3 (tristimulus colorimeter), resp. < 2 mg/m3 (to the naked eye). AsAs demonstrated demonstrated in in Figure Figure4 4, ,4, the thethe detection detectiondetection sensitivity sensitivity under under the thethe UV UVUV lamp lamplamp was was higher higher than than than under under under naturalnatural daylight.daylight. daylight. ItIt It isis is estimated estimated estimated that thatthat the thethe LOD LODLOD for forfor ﬂuorescence fluorescence observed observed with withwith the thethe naked nakednaked eye eye eye was was was 10-fold 10 10‐‐ lower,foldfold lower, lower, thereby thereby thereby reaching reaching reaching the LOD thethe LODLOD of the ofof tristimulus thethe tristimulustristimulus colorimeter colorimeter (0.3 (0.3 mg(0.3/ mmg/mmg/m3). We33).). We alsoWe alsoalso examined examined examined the feasibilitythethe feasibility feasibility of objective of of objective objective ﬂuorescence fluorescencefluorescence intensity intensityintensity measurement measurement (Horiba (Horiba Fluorolog FluorologFluorolog 3 Extreme, 33 Extreme,Extreme, Kyoto, Kyoto, Kyoto, Japan) theJapan)Japan) data, the the however, data, data, however, however, was not reproducible.waswas notnot reproducible.reproducible. The borosilicate The borosilicate glass tube glass material tubetube was materialmaterial especially waswas especially disturbing.especially Thedisturbing.disturbing. use of a The quartzThe use use glass ofof aa or quartzquartz plastic glassglass tubing oror plasticplastic instead tubingtubing might instead be a solution might to bebe this aa solutionsolution problem. toto This thisthis will problem.problem. be the objectiveThisThis will will be of be ourthe the objective future objective research. of of ourour futurefuture research.research.

Figure 4. Dependence of the M20.0/PY‐OPD charge VIS/UV color intensity on the concentration of Figure 4. Dependence of the M20.0/PY M20.0/PY-OPD‐OPD charge VISVIS/UV/UV color intensity on the concentration of diphosgene (the air volume was 1 dm3). diphosgene (the air volume was 1 dm33).

45 45 C20.0 40 C20.0 y = 0.4164x 40 R² = 0.8225y = 0.4164x 35 M20.0 R² = 0.8225 35 M20.0 30 30

E* 25 y = 0.3825x  E* 25 R² = 0.8805y = 0.3825x  20 R² = 0.8805 20 15 15 10 10 5 5 0 0 0 20406080100 0 20406080100Diphosgene concentration (mg/m3)

Diphosgene concentration (mg/m3)

FigureFigure 5. 5.Charge Charge color color intensity intensity (tristimulus (tristimulus colorimetry) colorimetry) in dependence in dependence on concentration on concentration of diphosgene of (theFigurediphosgene air volume5. Charge (the was air color 1volume dm 3intensity). was 1 dm(tristimulus3). colorimetry) in dependence on concentration of diphosgene (the air volume was 1 dm3). At this stage of the research, only the analytical data of the designed detection tube can be compared with the known ‘first generation’ detection tubes. Detection tubes for industrial use Chemosensors 2020, 8, 107 7 of 10 Chemosensors 2020, 8, 107 7 of 10 At this stage of the research, only the analytical data of the designed detection tube can be compared with the known ‘first generation’ detection tubes. Detection tubes for industrial use are aredesigned designed to allow to allow the detection the detection of phosgene of phosgene at the at level the level of hygienic of hygienic limits limits (0.08–0.4 (0.08–0.4 mg/m mg3) or/m close3) or closeconcentrations. concentrations. Therefore, Therefore, they theycontain contain a sufficiently a sufficiently high high indicator indicator layer layer (60–80 (60–80 mm), mm), usually supplementedsupplemented by a protectiveprotective layerlayer againstagainst interferinginterfering influences.influences. They are evaluatedevaluated accordingaccording toto thethe lengthlength ofof thethe colorcolor ofof thethe indicatorindicator layer.layer. The secondsecond group consistsconsists of detectiondetection tubestubes used by thethe armedarmed forcesforces oror rescuerescue servicesservices toto orientorient thethe phosgenephosgene/diphosgene,/diphosgene, which are muchmuch simplersimpler inin construction,construction, contain an indicator layer about 10 mm high and areare usuallyusually partpart ofof specialspecial electricelectric pumpspumps/chemical/chemical semi-automaticsemi‐automatic detectors.detectors. Their detection limitlimit (or(or thethe lowerlower limitlimit of thethe measuring range)range) isis usuallyusually aroundaround 55 mgmg/m/m33 and they are evaluatedevaluated mainly according to the intensityintensity of thethe colorcolor ofof thethe indicatorindicator layerlayer [[2,30].2,30]. This group alsoalso includesincludes thethe ‘second‘second generation’generation’ detectiondetection tubetube designed by us,us, which,which, inin additionaddition toto colorimetriccolorimetric evaluationevaluation (in(in daylight),daylight), alsoalso enablesenables evaluationevaluation based onon fluorescence.fluorescence. The The colorimetric colorimetric detection detection limit limit (with (with the the naked naked eye) eye) meets meets the the requirements requirements of potentialof potential users, users, other other detection detection limits limits (tristimulus (tristimulus colorimetry, colorimetry, fluorescence) fluorescence) significantly significantly exceed exceed them andthem approach and approach the detection the detection limits usual limits for usual calibrated for calibrated detection detection tubes for tubes industrial for industrial use. The proposed use. The deviceproposed is for device single is usefor single only. use only.

3.3. Selectivity and Interferences The PY-OPDPY‐OPD colorimetry colorimetry/fluorescence/fluorescence chemosensor chemosensor was was initially initially developed developed by Zhouby Zhou X. et X. al. et [12 al.] for[12] thefor detectionthe detection of phosgeneof phosgene and and G-type G‐type nerve nerve chemical chemical warfare warfare agents agents oror theirtheir simulantsimulant diethylchlorophosphate (DCP) (DCP) and and for for discerning discerning one one from from the the other. other. Hence, Hence, assessment assessment of the of effect the eofff ectDCP of DCPon the on proposed the proposed charge charge was was a priority a priority for for us. us. In In normal normal conditions, conditions, PY PY-ODP‐ODP with DCP providesprovides aa yellow yellow product product by by phosphorylation. phosphorylation. However, However, the the detection detection tube’s tube’s composite composite charge charge on on its ownits own was was light light orange orange color, color, and so and its interactionsso its interactions with low with DCP low concentrations DCP concentrations could not could be observed not be withobserved the naked with the eye; naked a deeper eye; orange a deeper color orange only resultedcolor only at resulted high DCP at concentrations high DCP concentrations (tube held above (tube 3 theheld open above vial). the open When vial). viewed When under viewed the under UV lamp, the UV the lamp, charge the exposed charge toexposed DCP vapors to DCP ( vapors250 mg (≥/m250, 3 ≥ samplemg/m3, volumesample volume 1 dm ) provided 1 dm3) provided yellow fluorescenceyellow fluorescence (Figure6 (Figure). 6).

Figure 6.6. Schematic of of the PY PY-OPD‐OPD reaction with DPC and the eeffectﬀect ofof DCPDCP ﬂuorescencefluorescence onon thethe indicationindication charge.charge.

The PY-OPDPY‐OPD reaction reaction with with phosgene phosgene/diphosgene/diphosgene gives gives rise torise HCl to (Figure HCl (Figure3), which 3), can which interfere can withinterfere the detection.with the detection. In fact, this In fact, problem this problem has been has tackled been tackled by Hu, by Y. et Hu, al. Y. [13 et], al. who [13], developed who developed a new chemosensora new chemosensor (rhodamine (rhodamine dye fused dye withfused benzimidazole), with benzimidazole), which which does notdoes generate not generate HCl. HCl. We took We thistook issue this issue into considerationinto consideration when when developing developing the composite the composite charge charge for the for detection the detection tube tube but didbut notdid experiencenot experience any any interfering interfering eﬀ ecteffect of of spontaneous spontaneous HCl HCl formation formation on on the the phosgenephosgene/diphosgene/diphosgene detection. Therefore, Therefore, we we conclude that the composite material composition and propertiesproperties createcreate favorablefavorable reactionreaction conditions.conditions. As As demonstrated demonstrated by by Table Table 22,, interferinginterfering eeffectsﬀects only arose from thethe presencepresence ofof veryvery highhigh concentrationsconcentrations ofof HClHCl (or(or acidacid compoundscompounds inin general).general). Mixtures of diphosgene withwith toluene,toluene, acetone, acetone, hexane, andhexane, other common and solventsother did notcommon interfere either.solvents did not interfere either.

Chemosensors 2020, 8, 107 8 of 10 Chemosensors 2020, 8, 107 8 of 10 Table 2. Interference (C20.0, M20.0), sample volume 1 dm3 (vapor above an open bottle) 3 Table 2. InterferenceChemical (C20.0, M20.0), sampleColo volumer Change/Daylight 1 dm (vapor aboveFluorescence an open bottle). Acetone No No Chemical Color Change/Daylight Fluorescence Carbon disulphide No No NitromethaneAcetone NoNo No No CarbonPyridine disulphide NoNo No No 2‐(Butylamino)ethanethiolNitromethane Slight Noyellowing No No Pyridine No No 2‐Chloroethyl ethyl ether No No 2-(Butylamino)ethanethiol Slight yellowing No 2‐Chloroethyl ethyl sulphide No No 2-Chloroethyl ethyl ether No No 2-ChloroethylN,N‐Dimethylformamide ethyl sulphide Slight Noyellowing No No N,N-DimethylformamideEthylenediamine SlightSlight yellowingyellowing No No N,NEthylenediamine‐Dimethylethanolamine SlightSlight yellowingyellowing No No N,N-DimethylethanolamineHydrogen chloride Deep Slight orange yellowing color Yellow No HydrogenAmmonia chloride DeepGreen orange‐yellow color color YellowNo AmmoniaDCP Green-yellowDeep orange color Yellow No FuelDCP petrol Deep orangeNo color YellowNo FuelNitric petrol acid Red‐brown No color Yellow No Nitric acid Red-brown color Yellow 3.4. Stability (Temperature, Ageing) 3.4. Stability (Temperature, Ageing) The degree of charge resistance to elevated temperatures was assessed from accelerated stability tests.The Figure degree 7a illustrates of charge resistancethe characteristic to elevated patterns temperatures of changes was in assessed the appearance from accelerated (ΔE) of the stability C20.0 tests.and M20.0 Figure charges7a illustrates during the heat characteristic stress at patterns 60 °C for of changes42 days. in The the appearancetwo charges ( ∆exhibitedE) of the C20.0a similar and M20.0increasing charges darkening during trend, heat stress which at did 60 ◦notC for differ 42 days. significantly The two from charges the reference exhibited charge a similar (N). increasing The tube darkeningcharge darkening trend, whichalso correlated did not di withﬀer signiﬁcantlythe response fromto phosgene/diphosgene. the reference charge Still, (N). Thethe two tube charges charge darkeningremained partly also correlated functional with (color the intensity response was to phosgene about 35%/diphosgene. lower) in the Still, 42 the days two of charges heat stress remained at 60 partly°C (Figure functional 7b). (color intensity was about 35% lower) in the 42 days of heat stress at 60 ◦C (Figure7b). No appearance change or phosgene/diphosgene phosgene/diphosgene detection intensity decrease was observed after 4 months of charge storage in normal conditions (at 20 ◦°C).C). Based on our experience we expect the detection tubes with with the the novel novel indication indication charge charge stored stored in in typical typical storage storage conditions conditions (at (at ≤30 30°C)◦ C)to be to ≤ bestable stable for for 1 year 1 year as as a aminimum. minimum. In In this this respect, respect, the the detection detection tubes tubes would would match commercially available products.

16 35 C20.0 y = 0.3409x C20.0 14 M20.0 R² = 0.9094 30 M20.0 12 N 25 y = 0.297x 10

R² = 0.8702 E* E* y = ‐0.4892x + 29.158 

 20 R² = 0.8886 8 15 6 y = 0.2678x 10 4 R² = 0.735 y = ‐0.3294x + 22.151 R² = 0.9394 2 5 (a) (b) 0 0 0 6 12 18 24 30 36 42 0 6 12 18 24 30 36 42 Days of heat stress at 60 °C (h) Days of heat stress at 60 °C (h)

Figure 7. (a(a) )Dependence Dependence of of the the indication indication charge charge appearance appearance (ΔE) (∆ onE) the on duration the duration of heat of heatstress stress at 60 3 at°C, 60 and◦C, ( andb) dependence (b) dependence of the of theindication indication charge charge performance performance (response (response ΔE∆ Eto to 20 20 mg/m mg/m3 of diphosgene) on the duration of heat stress atat 6060 ◦°C.C.

Chemosensors 2020, 8, 107 9 of 10

4. Conclusions The newly designed simple detection tube for the orientation detection of phosgene/diphosgene in air enabling the response to be evaluated by colorimetric and fluorescence measurements seems (based on available sources) to be the first of this type. This result was achieved by combining the known (but only recently published) PY-OPD chemosensor with a new composite carrier, specifically developed for this purpose. This strategy appears to be promising. It enables scientists to test a range of chemosensors (recently published in scientific literature) and design additional non-conventional carriers (with emphasis on an increased sorption capacity and on compatibility with the chemosensor) for use in dangerous gas detection tubes. We believe that continuation of this research may bring about new interesting facts helping organic chemists in their efforts to synthesize an ‘ideal’ chemosensor for phosgene/diphosgene and, perhaps, also other dangerous chemical substances.

Author Contributions: Conceptualization: V.P., L.M., and J.Z.; Methodology: V.P., M.L., L.M., D.V., and J.Z.; Formal analysis: L.M., J.Z., S.P., Z.M., M.D., and L.M.; Validation: V.P., M.L., and L.M.; Writing—manuscript preparation: V.P. and L.M.; Writing—review and editing: V.P., L.M., D.V., and J.Z.; Resources: V.P.; Project supervisor: V.P. All authors have read and agreed to the published version of the manuscript. Funding: The work was funded in the framework of a security research project of the Ministry of Interior of the Czech Republic, no. VI20192022172. Conﬂicts of Interest: The authors declare that they have no competing interest.

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