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An optical dosimeter for the selective detection of gaseous phosgene with ultra-low detection limit. Alejandro P. Vargas, Francisco Gámez, Javier Roales, Tania Lopes-Costa, and Jose M. Pedrosa ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00507 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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An optical dosimeter for the selective detection of gaseous phosgene with ultra-low detection limit. Alejandro P. Vargas, Francisco Gámez*, Javier Roales, Tânia Lopes-Costa and José M. Pedrosa* Department of Physical, Chemical and Natural Systems, Universidad Pablo de Olavide, ES-41013 Seville, Spain. KEYWORDS: Colorimetric sensor, Phosgene, Chemical sensor, Nanocrystalline TiO2 film, Screen printing, Disposable sensor

Supporting Information Placeholder phenylamine and 4-(dimethylamino)benzaldehyde) infiltrated in paper strips or glass tubes represent a simple method whose efficiency has not been sufficiently exploited.26 Current advances in optical technologies permit the coupling of this methodology to very affordable and sensitive optical devices. This along with recent advances in nanostructured thin film deposition techniques allows for the fabrication of integrated phosgene detectors. In this work we have exploited these approaches by infiltrating Harrison´s reagent into an inorganic and transparent nanocrystalline matrix prepared from anatase nanoparticles in the form of screen printed films. The high porosity of this matrix allows the rapid diffusion of gas molecules and its transparency enables its use for optical purposes, providing an ideal substrate for the fabrication of colorimetric detectors. The temporal response (absorbance changes) of this simple approach to very low phosgene concentrations results in an ultrasensitive sensor that can be easily monitored in continuous mode during the exposure by an optical fiber spectrophotometer or an inexpensive LEDCCD based device. The experimental details of the proposed procedure as long as the scanning electron microscopy characterization of the screen printed films (Figure S1) can be found in the Supporting Information (SI).

ABSTRACT: We present here a cheap, fast and highly selective dosimeter for the colorimetric detection of gaseous phosgene with an ultralow detection limit. The disposable device is based on the Harrison´s reagent supported into a porous nanocrystalline TiO2 matrix film. We exposed the films to phosgene streams while the absorbance was monitored by an optic fiber in a gas chamber. The pronounced spectral changes were unaffected by humidity and oxygen and permitted us to use the response rate at 464 nm as a very stable calibration signal for quantitative analysis purposes. The use of a specific sensing reaction guaranteed a very high selectivity of the device even against saturated vapors of primary interferences like halide gases and other oxidizing and volatile agents. With this simple method, whose response is compatible with affordable and efficient miniature LED-photodiode devices, we reach an ultralow limit of detection well-below the ppm level.

Amply used as chemical weapon during WW1,1 carbonyl dichloride or phosgene (COCl2) is widely employed nowadays in the chemical industry as a reagent in halogenations and acylation reactions, commonly used in the syntheses of dyes,2 pesticides3 and herbicides.4 It provokes critical lung edema at cumulative exposures of about 150 ppm⋅min and owns a median lethal dose (LD505) value of 500 ppm⋅min,6 easy-to-reach values due to its very high vapor pressure (1.6 atm at 20 ºC). Heavier than air and difficult to evacuate in case of accidental escape, a rapid detection of phosgene at low concentrations is necessary in support of both public and industrial threats. Specifically, the permissible exposure limit is established in 0.1 ppm averaged over a work shift of up to eight hours7 and hence, sensitive, cheap and easy-toautomate selective devices are required for occupational safety purposes. Prototypical gas chromatography methods have been proposed with the drawbacks of being expensive and exhibiting a low portability that does not allow for in-situ detection.8 These pitfalls can be overcome by employing optosensors based on specific chemical events. For instance, sensors of phosgene in the liquid phase based on phenomena as heterogeneous as Förster energy transfer between fluorophores,9 gold nanoparticles aggregation,10 dye-based colorimetric/fluorescence-based methods11–21 or by inducing iodine Raman peaks after phosgene oxidation of an alkali iodide22 are profusely appearing in the literature. However, the detection of phosgene in the gas phase for most of these procedures is only shown as a proof of concept even for those papers where this point is treated explicitly. For instance, we can find the works of Davydova et al.23,24 or Vijri et al.25 based on conductimetric measurements, that employed nanocrystalline diamond layers and modified polyaniline nanofibers respectively. Alternatively, classical dry colorimetric approaches based on specific chemicals like Harrison´s reagent (an equimolar mixture of di-

RESULTS AND DISCUSSION The UV-Vis spectrum of Harrison’s reagent and its precursors (diphenylamine and 4-(dimethylamino)benzaldehyde) in ethanol solution are shown in Figure 1a. The resulting spectrum is not simply the addition of the individual spectra and therefore some reaction between the two components must be taking place as will be discussed below. Once infiltrated, the sensing films were exposed to low and moderate concentrations of phosgene gas and a new band appeared nearly instantly in their UV-Vis spectra centered at ∼464 nm, increasing its intensity with the concentration of the gas and exposure time. This effect is shown in Figure 1b where the nearly instantaneous spectral changes ∆A=A-ATiO2 (i.e. the absorbance of the film corrected with the absorbance of the TiO2 matrix) are shown for a phosgene concentration of 8 ppm. It is foreseen that this band constitutes a colorimetric test for the presence of phosgene in a gas stream and the signal dependence with both time and phosgene concentration will be treated in detail. For much higher phosgene concentration, an additional band appears at 650 nm (see Figure S2) that can be monitored in parallel to be used as a taggant for punctual accidental emissions. This new band can be related to the formation of HCl during the sensing reaction as evidence by the appearance of the same band when we exposed to highly concentrated vapor of HCl (data not shown). In order to shed light into the chemical reaction pathway of Harrison´s reagent and phosgene originally proposed by Ryan et al.27 and shown in Figure 1c, the infiltrated and exposed films were analyzed by laser desorption ionization (LDI) and matrix

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Figure 1. (a) UV-Vis spectrum of Harrison’s reagent and its precursors (diphenylamine and 4(dimethylamino)benzaldehyde) in ethanol solution. (b) Instant response (below two seconds) of our colorimetric detector to 8 ppm of phosgene. (c) Chemical reaction pathway of Harrison’s reagent and phosgene proposed by Ryan et al.27 Adapted with permission from Ryan, T.; Seddon, E.; Seddon, K.; Ryan, C. Phosgene and Related Carbonyl Halides. In Topics in Inorganic and General Chemistry; Elsevier Science, 1996; pp xi–xv. Copyright 1996 Elsevier. assisted laser desorption ionization (MALDI) mass spectrometry all cases confirm the high selectivity of the dosimeter. For further (MS) (see Figure S3). Briefly, it was found that a molecule with clarifying these effects, the temporal evolution of the signal upon m/z=301, consistent with a partial aldehyde-amine condensation, exposure to the main interferents and phosgene at the same gas is already present in Harrison’s reagent before exposure and concentration (0.75 ppm) is depicted in Figure S4. It is worth would explain the non-additive spectrum in Figure 1a. This colornoting that even the exceptional case of a false positive in their imetric reaction has been used as a selective probe for the detecpresence for long exposure times could be considered advantation of diphenylamine upon acid catalyzation.28 Upon exposure, geous given their toxicity. Anyhow, the use of prefilters containanother molecule with nearly the same nominal mass appears, ing sodium iodide and sodium thiosulphate prevents interference which corresponds to the product in the mechanism detailed by by halide gases to a large extent. Importantly, no influence of the Ryan et al.27 (molecule 3 in Figure 1c) and the peak intensity of air humidity or oxygen was observed when the exposure is perthe condensation products becomes comparatively negligible. formed under non-condensing humid N2 or synthetic air (see Additional details of the mass spectrometry analysis can be found Figure S5). Finally, regarding the stability of the device, it was in the SI section. checked that the response remained within a range of ±10 % of the initial calibration signal averaged over triplicate independent In order to study the interference by other volatile compounds samples (see below) for nearly 49 days when stored in an ensuch as halide gases (the main interferents considering that acylatclosed space below 10 ºC (see Figure S6). However, storage for ing agents share many chemical properties with phosgene) and more than 4-5 days at room temperature and open atmosphere typical solvents, we measured the maximum absorbance intensity leads to a small loss of material due to evaporation that would for different vapors against the signal of an atmosphere with 1200 require controlled regeneration and/or additional protection30 of ppm of phosgene after an exposure of 10 seconds (Figure 2). 29 the film when used in those conditions. Given the elevated volatility of the potential interferents, the phosgene concentration was deliberately high to be equal to that of hydrochloric acid but substantially lower than those corresponding to the rest of volatile compounds, evaporated until atmosphere saturation. Despite this, the exposure to phosgene led to the greatest changes. A minimal change was produced by hydrochloric acid, benzoyl chloride and formic acid. The elevated concentration of formic acid (92 times that of phosgene) in the saturated vapor and the little interaction with the films indicates that its potential interference in real-world applications might be negligible. Hydrochloric acid and benzoyl chloride exposures led to changes that were 70 and 7.5 times smaller than those caused by phosgene, being the concentration of benzoyl chloride six times higher than the other two compounds. Hence, a slight interference may be produced by these halides, although their concentrations in the worst scenarios are likely to be much lower than in our experiments. The quantification of this aspect has been conFigure 2. Effects of various saturated volatile compounds on the ducted by evaluating the selectivity index KI/A, defined as the absorbance at 464 nm of the infiltrated Harrison’s reagent. Phossignal-to-concentration ratio for the interferent gas divided by that gene is taken as a reference with a concentration of 1200 ppm, of the target analyte (phosgene). The values of KI/A for each interand the corresponding relative concentrations of the interfering ferent are included in Figure 2. The very low values obtained in

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ACS Sensors gases is denoted at the top of each bar. The inset chart shows the selectivity index for each one as defined in the text.

Finally, we analyzed the sensitivity variations as a function of the response time t for calibration purposes. For a given c in the range 0-26 ppm and at short and moderate times, the signal follows a linear relation of the form S=B⋅t, where B=d(∆A464A800)/dt. The increment of the signal slope B with c is apparent and hence, from the above-results, we have calibrated the response rate by fitting B to a fourth order polynomial in c (R2=0.99995) with coefficients {1.50⋅10-4, 7.11⋅10-6, 8.82⋅10-7, 4.59⋅10-8}. The resulting fit is plotted in Figure 4a. The complex shape of this figure reflects the overall kinetic path where the phosgene adsorption and diffusion, reaction and secondary reactions are involved. In fact, although the relation between B and the concentration of phosgene is linear in the diluted regime, the linearity suddenly does not hold, probably because the reaction stops being diffusion-controlled. At higher phosgene concentrations, the commented parasite reactions lead to changes in the spectral shape that precludes us to use these data in the calibration curve. The evaluation of the phosgene concentration can be then extracted by numerically solving the above polynomial expression. We have tested our calibration method by measuring the kinetic response of a commercial chloroform stabilized with amylene, which is known to decompose forming phosgene even in cool conditions and the absence of air and light.31 The absorbance rate of the device under a saturated atmosphere of this reagent was used as input in the calibration polynomial. The positive and real root of the above-expression leads to a phosgene concentration of 1.27±0.02 ppm. This confirms the decomposition of the tested chloroform, which was stored under regular laboratory conditions in its original bottle and, although containing amylene as stabilizer, turned partially into phosgene. As it is apparent, the limit of detection (LOD) and quantification (LOQ) depends on the combination of exposure time and phosgene concentration, i.e. the higher the concentration of phosgene, the lower is the time required to detect or quantify the phosgene. Hence, we have also extracted the time required to reach the limit of detection (tLOD) and the limit of quantification (tLOQ) defined by the usual convention signal-to-noise (S/N) ratio equals 3 or 10 respectively. The results are plotted in Figure 4b. It can be observed that although the time required to reach the LOD is about 15 min for the lowest detected concentration (26 ppb) and the LOQ is 16.5 min, they decrease exponentially with the amount of phosgene. So, averaging over 1 min, the LOD is about 1 ppm, a value half than immediately dangerous to life or health limit. At this exposure time, the LOQ is about 2.3 ppm, while for shorter exposure times, it differs significantly from the corresponding LOD as shown in Figure 3c. These values are much lower than those obtained with classical methods using Harrison’s reagent20 and other recently reported colorimetric methods11–19,21, and at least comparable to some commercial tubes with LODs ranging 0.0220 ppm for the same accumulation time.32,33

In Figure 3a, the kinetics of the film response upon different phosgene concentrations in the ppb range at room temperature is presented. In that figure the signal S is evaluated as ∆A464-A800. The absorbance at 800 nm was subtracted from that at 464 nm to account for the detector drift. As expected, the response is higher the higher the phosgene concentration c, but the saturation limit is slowly reached in all cases even in this low concentration regime. It seems timely to mention the ability of the detector to respond to low concentrations, with lower limits in the ppb region as compared to other recently published results.9–12,14–25 For the sake of visualization, Figure 3b plots the spectrum of the sensing films after an exposition time of 103 s for some of the phosgene concentrations used here. Moreover, a visual proof of these changes is shown in Figure 3c where the sensing films have been photographed after exposure to the corresponding phosgene concentration at the same time. As can be seen, the ability of these sensing films to be integrated into very affordable LED-CCD based devices is evident.

Figure 3. (a) Temporal evolution of the signal (S) upon exposure to phosgene stream of different concentrations. The background signal at 800 nm is subtracted to account for changes in the baseline. (b) Spectral variation of the infiltrated TiO2 films after phosgene exposure (1000 s) at different concentrations. (c) Photographs of the color modification of the sensing films upon exposure to different phosgene concentrations (1000 s of exposure time). Photograph number 1 corresponds to an unexposed film, and it increases according to the color scale included for the sake of visualization.

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The Supporting Information is available free of charge on the ACS Publications website. Experimental details, scanning electron microscopy characterization of the screen printed films, spectrum at high phosgene concentrations, chemical reaction pathway of Harrison´s reagent and phosgene originally proposed by Ryan et al.27, analysis of the infiltrated and exposed films by laser desorption ionization (LDI) and matrix assisted laser desorption ionization (MALDI) mass spectrometry (MS), additional details of the mass spectrometry analysis, temporal evolution of the signal upon exposure to the main interferents and phosgene, exposure of the films under noncondensing humid N2 or synthetic air and temporal evolution of the signal of the films when stored in an enclosed space below 10 ºC (PDF).

Figure 4. (a) Calibration curve of the colorimetric rate response B (see text for details) of the infiltrated films as a function of the phosgene concentration. Symbols stand for experimental data while the dashed line corresponds to the polynomial fit described in the text. Error bars correspond to one standard deviation (±1σ) of triplicate samples. (b) Semi-log plot of the experimental values of LOD (S/N=3) and LOQ (S/N=10) reached for different exposure times. Error bars are calculated from the error propagation of B.

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interests.

To summarize, we have characterized the spectral response of Harrison’s reagent in the presence of phosgene and potential interferents when infiltrated in nanocrystalline TiO2 transparent films. Firstly, the UV-vis spectrum of the reagent does not follow an additivity rule. This fact is indicative of a chemical reaction which is consistent with the formation of a condensation product as checked with mass spectrometry methods, which also confirmed the reaction mechanism between the reagent and phosgene proposed by Ryan.27 Secondly, the appearance of a band at 464 nm in the absorbance spectrum of the film under exposure to gaseous phosgene was employed as a very selective colorimetric probe. Only hydrochloric acid or benzoyl chloride at very high relative concentrations has been proven to slightly interfere with the detection of phosgene. Finally, we have calibrated the response rate against the phosgene concentration. The limit of detection/quantification of the resulting device is superior to those reported recently in the literature and lies well below the ppm limit and it is shown that, averaging over 1 min, the limit of detection is one-half the “dangerous to life or health” limit. It should be remarked that the selectivity and the quantitative capacity of the sensor will not be affected by the interferents within the time frame required for the detection of phosgene concentrations in these stipulated limits. The films have been tested against a decomposing chloroform sample with promising results.

ACKNOWLEDGMENTS Funding from Ministry of Economy and Competitiveness of Spain (MINECO), Projects MAT2014-57652-C2-2-R and PCIN-2015169-C02-02 (under the project M-Era-NET/0005/2014), is gratefully acknowledged. Funding from the Operative Programme FEDER Andalucía (Junta de Andalucía) through project P12 FQM-2310 also contributed to the present research. Thanks to the Mass Spectrometry and Laser Spectroscopy group of the Pablo de Olavide University for its assistance during the mass spectrometry experiments. REFERENCES (1)

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CONCLUSIONS We have built a simple, cheap and disposable device for the colorimetric discrimination of phosgene in gas streams. The spectroscopic response is the result of specific oxidations of a colorimetric probe that has been infiltrated in an inorganic nanocrystalline matrix. We have specified a calibration procedure that allowed for the temporal detection and quantification of phosgene with very high low LOD/LOQs. The colorimetric device is proposed as a readily, easily accessible and selective tool for the analysis of low concentrations of phosgene in real gas samples.

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