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On-line monitoring of transient radicals and oligomers: o-Phenylenediamine electrooxidation mechanism study by mass spectrometry
Microchemical Journal xxx (xxxx) xxxx
Contents lists available at ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
On-line monitoring of transient radicals and oligomers: o-Phenylenediamine electrooxidation mechanism study by mass spectrometry Jie Jianga,b, GuangMing Zhanga,b, Ling Lic, Hong Zhanga,b, Na Lia,b, YingYing Wangd, Jing Hea,b, ⁎ FengJiao Maoa,b, Kai Yua,b, a
School of Marine Science and Technology, Harbin Institute of Technology at WeiHai, WeiHai, ShanDong 264209, PR China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, Heilongjiang 150040, PR China c Biological and Chemical Engineering Department, WeiHai Vocational College, WeiHai, ShanDong 264210, PR China d Department of Optoelectronic Science, Harbin Institute of Technology at WeiHai, WeiHai, ShanDong 264209, PR China b
ARTICLE INFO
ABSTRACT
Keywords: o-Phenylenediamine On-line monitoring Electrochemistry-mass spectrometry Dehydrogenation Oligomers
Mechanistic insight into the electrooxidation of o-phenylenediamine (oPD) has been achieved by on-line electrochemistry-mass spectrometry (EC-MS). A serious of oxidation products include radical cation as well as soluble oligomers (i.e. dimers, trimers, tetramers, pentamers and hexamers in different oxidation states) were successfully detected and further identified with high-resolution and tandem MS methods. The oligomer itself can undergo dehydrogenation reaction and the products would not participate in the oPD chain propagation. Based on the solid evidences, a new mechanistic pathway for poly(o-phenylenediamine) was formulated. Moreover, the electrooxidation was dependence on the solution pH as well as oxidation voltages. The favored pH environment for oPD polymerization was at around 7.0, whereas only the dimer could be formed in strong acidic (pH ≤ 2.6) and alkaline (pH ≥ 10.6) solution. A high oxidation voltage can accelerate the chain propagation speed, but by-products has generated when the voltage increased to 8.0 V.
1. Introduction Due to the inherent of high conductivity, permselectivity, and electrochromic property, poly(o-phenylenediamine) (PoPD) was intensively studied and widely applied for electrocatalysis, sensors and biosensors, electrochromic displays and supercapacitors [1–6]. Many of these researches have been devoted to explore the mechanistic pathway of PoPD, which is by characterizing the molecular structures of the oxidation products [7,8]. The most preferred techniques include Fourier Transform Infrared Spectrometer, H-nuclear magnetic resonance (FTIR, 1H NMR), UV–vis spectroscopy [9–13]. However, both FTIR and UV–vis are invaluable for determining the specific functional groups and/or conjugate structures of the samples that formed on the electrode, making them useless in providing information on the solution side; the low sensitivity and selectivity of NMR is also considered not suitable for determine the complex chain propagation steps of the polymers. Hence, the discussion about the oligomeric species generated during the oPD oxidation process is still on-going. Based on the advantages of label-free, high sensitivity and selectivity, and superior composition identification power [14–17], mass spectrometry (MS) has been attempted to identify the oPD oligomers such as ⁎
dimers, trimers and tetramers [18–20]. But in these researches the MS analysis was established after the oPD oxidizing reaction. In other words, the off-line route may have missed to capture the short-lived intermediates such as monomer radical cation and oligomer radicals. The absence of real-time data on the whole oxidation procedure would lead to more discussions on the oPD oligomer's formation. It is thus necessary to develop an efficiency interface between the reacting solution and MS inlet, which can monitor the fleeting chain propagation process real-time. In the present work, an innovative electrochemistry-mass spectrometry (EC-MS) technique has been applied for studying the electrooxidation of oPD [21,22]. Various conditions include pH and oxidation voltages effected on reaction progress were investigated. Both highresolution MS and tandem MS (MSn) were used to characterize the soluble oligomers of oPD. By evaluating the MS data, a detailed chain propagation steps for electrooxidation of oPD was proposed. 2. Experiment 2.1. Materials and regents Acetonitrile was HPLC-grade and purchased from Sigma-Aldrich
Corresponding author at: Harbin Institute of Technology at WeiHai, WenHua XiLu 2, WeiHai 264209, PR China. E-mail address: [email protected] (K. Yu).
https://doi.org/10.1016/j.microc.2019.104390 Received 22 August 2019; Received in revised form 2 November 2019; Accepted 2 November 2019 0026-265X/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Jie Jiang, et al., Microchemical Journal, https://doi.org/10.1016/j.microc.2019.104390
Microchemical Journal xxx (xxxx) xxxx
J. Jiang, et al.
(Darmstadt, Germany). o-Phenylenediamine (99%) and acetic acid were purchased form Sigma-Aldrich (Shanghai, China). Ammonium acetate was from Ruijin Co. (Tianjin, China). Ultrapure water was produced with Milli-Q water purification system (Milford, MA). Polyethylene terephthalate-indium tin oxide sheet (PET-ITO) conductive film was purchased from Kaiwei Co. (Zhuhai, China, 081,035) and cut into squares (Length × Width × Thickness = 18 × 18 × 0.15 mm). 1.0 mM oPD solution was prepared in acetonitrile/water (v:v = 4:1) with 0.1 mM ammonium acetate solution as the electrolyte salt. In parallel experiments, the pH of the solution was adjusted with acetic acid or ammonium to the value of pH 2.6, 3.7, 7.0, 10.6. All the solutions were prepared freshly before to use. 2.2. Electrochemistry-Mass spectrometry (EC-MS) Electrooxidation of oPD was performed by potentiostatic method and monitored by on-line electrochemistry-mass spectrometry (EC-MS) technique. As depicted in Fig. 1, the EC platform is consisted of a conventional two-electrode unit, a platinum plate (Length × Width = 5 × 5 mm) was used as the working electrode (WE) and mounted (not touch) on one corner of the PET-ITO conductive film. This film was functioned as the counter electrode (CE) and the corner named sample corner due to the sample solution (approx. 40 μL) would be loaded on this place. The distance from the tip of the sample corner to the MS inlet is 5 mm. Comparing with traditional electrochemistry methods, two high voltages of 4800 V and 4800 + ΔE V respectively exerted on the CE and WE electrodes. And 4800 V is the spray voltage for the reacting solution and ΔE is assigned to the oxidation voltage of oPD. Each experiment was done at least three times for reproducibility check. For the mass analysis, LTQ/Orbitrap mass spectrometer (Thermo Fisher Scientific, Waltham, MA) was settled to the Full-scan positive ion mode and the mass-to-charge (m/z) range was adjusted to m/z 50–1000. All the data were acquired and proposed by default Xcalibur package (Version 2.0.7 Thermo Fisher Scientific, US). Other basic parameters including tube lens voltages, 110 V; capillary voltage, 50 V; capillary temperature, 275 °C and ion maximum injection time, 10 ms. Default parameters are performed by tandem mass spectrometry experiments.
Fig. 2. EC-MS spectra of electrooxidation of oPD obtained at oxidation voltage of (a) ΔE = 0 V and (b) ΔE = 6.0 V. The data was obtained by low-resolution MS.
monomer [19]. Other peaks observed on this pattern are most likely the contaminants and will not be discussed further. In parallel experiment, the oxidation voltage (ΔE) was increased to 6.0 V and therefore, the total voltage on the WE was 4800 + 6.0 V and on the CE was 4800 V. Compared with the spectrum in Fig. 2a, the newborn peaks appearing on the pattern of Fig. 2b are attributed to a series of oPD oxidation products, including dimer (m/z 211), trimer (m/z 319), tetramer (m/z 425), pentamer (m/z 529) and hexamer (m/z 635) as well as their fragments at single charge protonated state [19]. Notice that the pentamer and hexamer are the first time detected by the EC-MS; the quite low relative intensities of them is probably due to the high solvent insolubility with the increase of molecular chain. Based on tandem MS study, these fragments are due to NH3 loss from their parent ions, i.e. dimer, trimer, tetramer and pentamer. For double check, the high-resolution MS was applied and the accurate m/z values of oPD oligomers as well as their fragments are summarized in Table S1 (see Supplementary data). Observing the inset windows in Fig. 2b, there are cluster peaks beside the oligomeric products. For example, the peaks m/z 315, 317 and 321 around the trimer m/z 319. The ion at m/z 321 is hastily to be seen as the isotope peak of ion m/z 319, but the MS2 spectra of the two ions are obviously different (see Fig. S1, Supplementary data). And the relative intensity of fragmentation peak m/z 304 (its parent ion is m/z 321) is higher than that of fragmentation peak m/z 302 (its parent ion is m/z 319) during reaction time from 1.5 to 5.5 min (see Fig. S2, Supplementary data), evidencing that ion m/z 321 should be assigned to one oxidation product of oPD. Therefore, ion m/z 321 was proposed to
3. Results and discussion 3.1. Electrooxidation of o-phenylenediamine monitored by EC-MS Electrooxidation of 1.0 mM o-Phenylenediamine (oPD) studied by EC-MS was started in acetonitrile/water (v:v = 4:1) solution with 0.1 mM ammonium acetate, and the solution pH was adjusted to 7.0. Since the response time of low-resolution MS is much shorter compared to the high-resolution MS, the reaction was first monitored with lowresolution MS for capturing short-lived intermediates. Fig. 2a shows the low-resolution MS spectrum when the sample solution sprayed out at the voltages of 4800+0 V (WE) and 4800 V (CE). The base peak appeared at m/z 109 is attributed to the protonated form of oPD monomer [M + H]+. A minor peak at m/z 92 is due to the NH3 loss from oPD
Fig. 1. Schematic of EC-MS setup for investing the electrooxidation of oPD. 2
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Fig. 3. Electrooxidation mechanism of oPD proposed by on-line EC-MS.
the firstly formed trimer with polyaniline like structure [9] which subsequently underwent dehydrogenation (oxidation process) to generate the ions m/z 315, 317 and 319. This indicates that the oPD polymerization was accompanied with the oligomer dehydrogenation and these dehydrogenation products may reduce the chain propagation efficiency. Moreover, comparing the relative intensities between the
dehydrogenation products and their fragments, e.g., m/z 211 → m/z 194, m/z 213 → m/z 196, m/z 215 → m/z 198, one can realize that the final dehydrogenation product exhibited the lowest fragment peak intensity. This may due to the strong conjugation inside the molecule, making it difficult to undergo intramolecular oxidation to form a complete oxidation state and thus to lose an NH3 group. 3
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dimer m/z 213 (RT = 0.5 min) and the trimer m/z 319 (RT = 1.5 min), indicating the dehydrogenation process m/z 213 → m/z 211 lagged behind the trimer formation m/z 213 → m/z 319. In addition, the ratio of theoretical peak intensity between ion m/z 211 and its isotope m/z 212 should be 7:1, while on the pattern at RT = 8.5 min in Fig. S3b the calculated value is 2:1. Thus, ion m/z 212 is considered to one oxidation production of oPD instead of isotope attributed to ion m/z 211. Such conclusion can also be referred by ion m/z 214. The relative abundance ratio between peak m/z 213 and peak m/z 214 obtained in Fig. S3b (the pattern at RT = 8.5 min) is 5:3 compared to 7:1 in theory. In previous study by Losito et al. [19], the product m/z 214 was defined as one oxygenated species. Herein, based on elemental formula that proposed by high-resolution MS (see Table S1, Supplementary data), ions m/z 212 and 214 should be the dehydrogenation products of m/z 215 as well. Hence, the commonly accepted 2e−, 2H+ oxidation step of oPD oligomer is, to be more precise, consisted of 1e−, 1H+ oxidation procedure. To the best of our knowledge, this is the first direct confirmation of such behavior occurred during oPD electrooxidation. Moreover, because the relative intensity of dimer m/z 211 kept increasing and exceeded dimer m/z 213 from 8.5 min, dimer m/z 211 is considered not to contribute to the trimer formation. Based on the results and other previous studies [7,18,19,23], a proposed pathway for electrooxidation of oPD is illustrated in Fig. 3. Obviously, the oPD chain propagation was accompanied by oligomer dehydrogenation reaction, (i) the initial stage was oPD monomer oxidized to radical cation (m/z 108), (ii) followed by two radical cations coupling to give dimer m/z 215, (iii) it underwent two 1e−, 1H+ oxidation (dehydrogenation process) steps to produce dimer m/z 213, (iv) most of the dimer m/z 213 combined a radical cation to generate trimer m/z 321 and the rest underwent 2e−, 2H+ oxidation to form dimer m/z 211. By repeating the oxidation of small oligomer and following addition of radical cations, a larger oPD oligomers formed subsequently.
Fig. 4. EC-MS spectra of oPD electrooxidized at pH values of 2.6, 3.7, 7.0 and 10.6. 1.0 mM oPD was dissolved in acetonitrile/water (v:v = 4:1) with 0.1 mM ammonium acetate. Reaction time = 5.0 min and oxidation voltage = 5.0 V. The data was obtained by low-resolution MS.
With the advantage of effective interface between the reacting solution and MS inlet, it is allowing to capture the short-lived intermediates [21]. Herein, the ion at m/z 108 (see the inset window of Fig. 2b) is identified to the radical cation of oPD. It offered solid evidence for this reaction, which was triggered from oxidizing the oPD monomer by losing of one electron. An overoxidized product of oPD monomer can be found at m/z 107 and its postulated formation pathway [23] was depicted in Scheme S1 (see Supplementary data). Li et al. reported that this ion was a semi-quinone radical of oPD which would contribute to form large oligomer [23]. However, the relative intensity of peak m/z 107 was maintained during the entire reaction process observed by EC-MS, suggesting that this side-reaction product did not participate the chain growth of oPD.
3.3. Effects of pH and oxidation voltages For investigating the effects of pH on electrooxidation of oPD, various solutions have been prepared. Fig. 4 shows the MS spectra of oPD oxidized under the same reaction time (5.0 min) and oxidation voltage (5.0 V) but in different pH environments, i.e. pH 2.6, 3.7, 7.0 and 10.6. Comparing the distribution observed for the oxidation products among the four MS spectra, one can conclude that the reaction progress is significantly influenced by the solution pH. Except the oPD monomer, only dimer exists on the spectra of pH 2.6 and 10.6. This may be related to an increase in the proportion of forming non-ionized oPD monomer, which cannot be oxidized to participate in the polymerization. Trimer and tetramer were both detected when the reaction carried out at pH 3.7 and 7.0. However, the relative intensities of the two oligomers on the spectrum of pH 3.7 are lower than them in pH 7.0. This is supported by the mechanism described in Fig. 3, H+ are generated during oligomer formation as well as the dehydrogenation steps, leading to the reaction more favored to perform in neutral environment. Therefore, the oxidation products of oPD are pH dependence and a neutral or weak acidic environment is recommended for the chain growth. When oPD was electrooxidized with different voltages (2.0, 4.0, 6.0 and 8.0 V), the relative intensities of monomer and tetramer m/z 425 versus reaction time (from 0.5 to 4.0 min) were obtained and shown in Fig. S4a and b, respectively. It should be pointed out that many byproducts which cannot be defined at present has been generated at 8.0 V, thus the spectrum is not shown here. As expected, the rate of monomer consumption and oligomer formation is greatly dependence on the oxidizing voltage when ΔE ≤ 6.0 V, that is the higher voltage applied, the faster chain propagation achieved. The ratios of relative intensity between the fully and semi-oxidized states oligomers including dimers (m/z 211 vs. m/z 213) and trimers (m/z 317 vs. m/z 319) were recorded at reaction time of 7 min. Observing from Fig. S4c
3.2. Real-time monitoring of oPD chain propagation Because the electrooxidation of oPD was performed in a droplet by means of the EC setup, it is capable to monitoring the electrooxidation progress of oPD in real time. Fig. S3 shows the MS patterns at different reaction times; the patterns were plotted based on the data from Fig. 2. It should be pointed out that the relative intensities of these oligomer peaks in MS spectra might not strictly reflect the real production of oligomers, but the data remained internally consistent and proved to be extremely useful for evaluating the relative changes of oligomers during the reaction. As can be seen in Fig. S3a, the relative intensity of oPD monomer dramatically decreased as the reaction proceeding, meanwhile, increase at the abundances of oxidation products and their fragments were observed. Specifically, the base peak of the MS spectrum shifted from oPD monomer m/z 109 to the dimer m/z 213 at the reaction time (RT) of 0.5 min. From 1.5 min the peak attributed to the trimer could be found at m/z 319 and its intensity kept growing in the following 6.0 min until it became the most abundant oligomer of the spectrum (RT = 6.5 min). The tetramer peak m/z 425 appeared since 1.5 min, however, the gently raising of relative intensity implies that the chain growing rate was declined from the tetramer, probably due to the rest amount of oPD monomers was too low to maintain the reaction efficiently. Therefore, the continuous changes on the signal intensity of oxidation products clearly reflects the electro-oxidation mechanism of oPD. In particular, the fully oxidized dimer m/z 211 was observed (RT = 2.5 min) (see Fig. S3b) after the appearance of semi-oxidized 4
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and d, the ratios is almost linear increased as the voltages, suggesting that the dehydrogenation reaction can be accelerated under a high voltage. Compared with the values of 317/319 in a narrow range (see yaxis in Fig. S4d), a remarkable difference in the ratios of 211/213 is observed (see y-axis in Fig. S4c), implying that the 1e−, 1H+ oxidation would be restricted in the larger oligomer. Therefore, the efficient oxidizing potentials for oPD chain propagation are 5.0–6.0 V.
[3] X. Liu, Z. Hui, X. Yang, An electrochemical sensor for dopamine based on poly(ophenylenediamine) functionalized with electrochemically reduced graphene oxide, RSC Adv. 4 (8) (2013) 3706–3712. [4] J. Fang, X. Zhang, X. Miao, Y. Liu, S. Chen, Y. Chen, J. Cheng, W. Wang, Y. Zhang, A phenylenediamine-mediated organic electrolyte for high performance graphenehydrogel based supercapacitors, Electrochim. Acta 273 (2018) 495–501. [5] S.L. Mu, The electrocatalytic oxidative polymerization of o-phenylenediamine by reduced graphene oxide and properties of poly(o-phenylenediamine), Electrochim. Acta 56 (11) (2011) 3764–3772. [6] P. Muthirulan, N. Rajendran, Poly(o-phenylenediamine) coatings on mild steel: electrosynthesis, characterization and its corrosion protection ability in acid medium, Surf. Coat. Technol. 206 (8–9) (2012) 2072–2078. [7] M.A.D. Valle, F.R. Díaz, M.E. Bodini, G. Alfonso, G.M. Soto, E.D. Borrego, Electrosynthesis and characterization of o-phenylenediamine oligomers, Polym. Int. 54 (3) (2005) 526–532. [8] D.H. Jang, Y.S. Yoo, S.M. Oh, Electropolymerization mechanism for poly (o-phenylenediamine)(PPD) and its electrocatalytic behavior for O2 reduction, Bull. Korean Chem. Soc. 16 (5) (1995) 392–397. [9] S. Bilal, A.A. Shah, R. Holze, Spectroelectrochemistry of poly(o-phenylenediamine): polyaniline-like segments in the polymer structure, Electrochim. Acta 56 (9) (2011) 3353–3358. [10] R.H. Sestrem, D.C. Ferreira, R. Landers, M.L.A. Temperini, G.M. do Nascimento, Synthesis and spectroscopic characterization of polymer and oligomers of orthophenylenediamine, Eur. Polym. J. 46 (3) (2010) 484–493. [11] K. Ogura, M. Kokura, J. Yano, H. Shiigi, Spectroscopic and scanning tunneling microscopic characterization of virgin and recast films of electrochemically prepared poly(o-Phenylenediamine), Electrochim. Acta 40 (17) (1995) 2707–2714. [12] K.A. Thomas, W.B. Euler, An electrochemical, spectroscopic, and theoretical study of poly(2,3-diaminophenazine), J. Electroanal. Chem. 501 (1) (2001) 235–240. [13] S. Samanta, P. Roy, P. Kar, Influence of pH of the reaction medium on the structure and property of conducting poly (o-Phenylenediamine), Mater. Today 2 (4–5) (2015) 1301–1308. [14] G. Hambitzer, J. Heitbaum, Electrochemical thermospray mass spectrometry, Anal. Chem. 58 (6) (1986) 1067–1070. [15] T.A. Brown, H. Chen, R.N. Zare, Identification of fleeting electrochemical reaction intermediates using desorption electrospray ionization mass spectrometry, J. Am. Chem. Soc. 137 (23) (2015) 7274–7277. [16] S. Jahn, H. Faber, R. Zazzeroni, U. Karst, Electrochemistry/mass spectrometry as a tool in the investigation of the potent skin sensitizer p-phenylenediamine and its reactivity toward nucleophiles, Rapid Commun. Mass Spectrom. 26 (12) (2012) 1453–1464. [17] H. Oberacher, F. Pitterl, R. Erb, S. Plattner, Mass spectrometric methods for monitoring redox processes in electrochemical cells, Mass Spectrom. Rev. 34 (1) (2015) 64–92. [18] I. Losito, N. Cioffi, M.P. Vitale, F. Palmisano, Characterization of soluble oligomers produced by electrochemical oxidation of o-phenylenediamine by electrospray ionization sequential mass spectrometry, Rapid Commun. Mass Spectrom. 17 (11) (2003) 1169–1179. [19] I. Losito, F. Palmisano, P.G. Zambonin, o-phenylenediamine electropolymerization by cyclic voltammetry combined with electrospray ionization-ion trap mass spectrometry, Anal. Chem. 75 (19) (2003) 4988–4995. [20] C. Hempen, S. van Leeuwen, H. Luftmann, U. Karst, Liquid chromatographic/mass spectrometric investigation on the reaction products in the peroxidase-catalyzed oxidation of o-phenylenediamine by hydrogen peroxide, Anal. Bioanal. Chem. 382 (1) (2005) 234–238. [21] K. Yu, H. Zhang, J. He, R.N. Zare, Y. Wang, L. Li, N. Li, D. Zhang, J. Jiang, In situ mass spectrometric screening and studying of the fleeting chain propagation of aniline, Anal. Chem. 90 (12) (2018) 7154–7157. [22] J. Jiang, D. Zhao, H. Zhang, J. He, N. Li, In situ analysis and real-time monitoring of the decomposition of the 2nd grubbs catalyst in CH3CN by droplet spray ionization tandem mass spectrometry, Anal. Methods 9 (28) (2017) 4201–4206. [23] D.J. Li, X.W. Li, Y.X. Xie, X.Q. Cai, G.L. Zou, Identification of intermediate and product from methemoglobin-catalyzed oxidation of o-phenylenediamine in twophase aqueous-organic system, Biochem. (Moscow) 70 (1) (2005) 92–99.
4. Conclusions By means of EC-MS investigations, the fleeting and complex chain propagation steps during electrooxidation of oPD were monitored in real time. The oxidation products include monomer radical cation as well as soluble oligomers (dimers, trimers, tetramers, pentamers and hexamers in different oxidation states) have been successfully detected and identified. The oligomer itself also underwent 1e−, 1H+ dehydrogenation reaction and some of the products would not participate in the oPD chain propagation. The favored pH environment for oPD oligomers formation was at around 7.0, whereas only the dimer could be formed in strong acidic (pH ≤ 2.6) or basic (pH ≥ 10.6) solution due to oPD monomer became non-ionized state. The most efficiency oxidation potential is determined to 5.0–6.0 V, however, side-reaction would occurr when the voltage rose to 8.0 V. These solid evidences can account for the various properties of PoPD synthesized under different experimental conditions. Declaration of Competing Interest There is no conflict of interest to declare. Acknowledgements This research is supported by the National Nature Science Foundation of China (Grant No. 21804027), Shandong Provincial Natural Science Foundation, China (Grant No. ZR2018PB016), and the Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 2019071). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.microc.2019.104390. References [1] J.H. Wang, H. Li, Y.H. Cai, D.J. Wang, L. Bian, F.Q. Dong, H.L. Yu, Y. He, Direct blue light-induced autocatalytic oxidation of o-Phenylenediamine for highly sensitive visual detection of triaminotrinitrobenzene, Anal. Chem. 91 (9) (2019) 6155–6161. [2] H.C. Xia, X.H. Xu, Q.H. Song, BODIPY-Based fluorescent sensor for the recognization of phosgene in solutions and in gas phase, Anal. Chem. 89 (7) (2017) 4192–4197.
5
Contents lists available at ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
On-line monitoring of transient radicals and oligomers: o-Phenylenediamine electrooxidation mechanism study by mass spectrometry Jie Jianga,b, GuangMing Zhanga,b, Ling Lic, Hong Zhanga,b, Na Lia,b, YingYing Wangd, Jing Hea,b, ⁎ FengJiao Maoa,b, Kai Yua,b, a
School of Marine Science and Technology, Harbin Institute of Technology at WeiHai, WeiHai, ShanDong 264209, PR China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, Heilongjiang 150040, PR China c Biological and Chemical Engineering Department, WeiHai Vocational College, WeiHai, ShanDong 264210, PR China d Department of Optoelectronic Science, Harbin Institute of Technology at WeiHai, WeiHai, ShanDong 264209, PR China b
ARTICLE INFO
ABSTRACT
Keywords: o-Phenylenediamine On-line monitoring Electrochemistry-mass spectrometry Dehydrogenation Oligomers
Mechanistic insight into the electrooxidation of o-phenylenediamine (oPD) has been achieved by on-line electrochemistry-mass spectrometry (EC-MS). A serious of oxidation products include radical cation as well as soluble oligomers (i.e. dimers, trimers, tetramers, pentamers and hexamers in different oxidation states) were successfully detected and further identified with high-resolution and tandem MS methods. The oligomer itself can undergo dehydrogenation reaction and the products would not participate in the oPD chain propagation. Based on the solid evidences, a new mechanistic pathway for poly(o-phenylenediamine) was formulated. Moreover, the electrooxidation was dependence on the solution pH as well as oxidation voltages. The favored pH environment for oPD polymerization was at around 7.0, whereas only the dimer could be formed in strong acidic (pH ≤ 2.6) and alkaline (pH ≥ 10.6) solution. A high oxidation voltage can accelerate the chain propagation speed, but by-products has generated when the voltage increased to 8.0 V.
1. Introduction Due to the inherent of high conductivity, permselectivity, and electrochromic property, poly(o-phenylenediamine) (PoPD) was intensively studied and widely applied for electrocatalysis, sensors and biosensors, electrochromic displays and supercapacitors [1–6]. Many of these researches have been devoted to explore the mechanistic pathway of PoPD, which is by characterizing the molecular structures of the oxidation products [7,8]. The most preferred techniques include Fourier Transform Infrared Spectrometer, H-nuclear magnetic resonance (FTIR, 1H NMR), UV–vis spectroscopy [9–13]. However, both FTIR and UV–vis are invaluable for determining the specific functional groups and/or conjugate structures of the samples that formed on the electrode, making them useless in providing information on the solution side; the low sensitivity and selectivity of NMR is also considered not suitable for determine the complex chain propagation steps of the polymers. Hence, the discussion about the oligomeric species generated during the oPD oxidation process is still on-going. Based on the advantages of label-free, high sensitivity and selectivity, and superior composition identification power [14–17], mass spectrometry (MS) has been attempted to identify the oPD oligomers such as ⁎
dimers, trimers and tetramers [18–20]. But in these researches the MS analysis was established after the oPD oxidizing reaction. In other words, the off-line route may have missed to capture the short-lived intermediates such as monomer radical cation and oligomer radicals. The absence of real-time data on the whole oxidation procedure would lead to more discussions on the oPD oligomer's formation. It is thus necessary to develop an efficiency interface between the reacting solution and MS inlet, which can monitor the fleeting chain propagation process real-time. In the present work, an innovative electrochemistry-mass spectrometry (EC-MS) technique has been applied for studying the electrooxidation of oPD [21,22]. Various conditions include pH and oxidation voltages effected on reaction progress were investigated. Both highresolution MS and tandem MS (MSn) were used to characterize the soluble oligomers of oPD. By evaluating the MS data, a detailed chain propagation steps for electrooxidation of oPD was proposed. 2. Experiment 2.1. Materials and regents Acetonitrile was HPLC-grade and purchased from Sigma-Aldrich
Corresponding author at: Harbin Institute of Technology at WeiHai, WenHua XiLu 2, WeiHai 264209, PR China. E-mail address: [email protected] (K. Yu).
https://doi.org/10.1016/j.microc.2019.104390 Received 22 August 2019; Received in revised form 2 November 2019; Accepted 2 November 2019 0026-265X/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Jie Jiang, et al., Microchemical Journal, https://doi.org/10.1016/j.microc.2019.104390
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(Darmstadt, Germany). o-Phenylenediamine (99%) and acetic acid were purchased form Sigma-Aldrich (Shanghai, China). Ammonium acetate was from Ruijin Co. (Tianjin, China). Ultrapure water was produced with Milli-Q water purification system (Milford, MA). Polyethylene terephthalate-indium tin oxide sheet (PET-ITO) conductive film was purchased from Kaiwei Co. (Zhuhai, China, 081,035) and cut into squares (Length × Width × Thickness = 18 × 18 × 0.15 mm). 1.0 mM oPD solution was prepared in acetonitrile/water (v:v = 4:1) with 0.1 mM ammonium acetate solution as the electrolyte salt. In parallel experiments, the pH of the solution was adjusted with acetic acid or ammonium to the value of pH 2.6, 3.7, 7.0, 10.6. All the solutions were prepared freshly before to use. 2.2. Electrochemistry-Mass spectrometry (EC-MS) Electrooxidation of oPD was performed by potentiostatic method and monitored by on-line electrochemistry-mass spectrometry (EC-MS) technique. As depicted in Fig. 1, the EC platform is consisted of a conventional two-electrode unit, a platinum plate (Length × Width = 5 × 5 mm) was used as the working electrode (WE) and mounted (not touch) on one corner of the PET-ITO conductive film. This film was functioned as the counter electrode (CE) and the corner named sample corner due to the sample solution (approx. 40 μL) would be loaded on this place. The distance from the tip of the sample corner to the MS inlet is 5 mm. Comparing with traditional electrochemistry methods, two high voltages of 4800 V and 4800 + ΔE V respectively exerted on the CE and WE electrodes. And 4800 V is the spray voltage for the reacting solution and ΔE is assigned to the oxidation voltage of oPD. Each experiment was done at least three times for reproducibility check. For the mass analysis, LTQ/Orbitrap mass spectrometer (Thermo Fisher Scientific, Waltham, MA) was settled to the Full-scan positive ion mode and the mass-to-charge (m/z) range was adjusted to m/z 50–1000. All the data were acquired and proposed by default Xcalibur package (Version 2.0.7 Thermo Fisher Scientific, US). Other basic parameters including tube lens voltages, 110 V; capillary voltage, 50 V; capillary temperature, 275 °C and ion maximum injection time, 10 ms. Default parameters are performed by tandem mass spectrometry experiments.
Fig. 2. EC-MS spectra of electrooxidation of oPD obtained at oxidation voltage of (a) ΔE = 0 V and (b) ΔE = 6.0 V. The data was obtained by low-resolution MS.
monomer [19]. Other peaks observed on this pattern are most likely the contaminants and will not be discussed further. In parallel experiment, the oxidation voltage (ΔE) was increased to 6.0 V and therefore, the total voltage on the WE was 4800 + 6.0 V and on the CE was 4800 V. Compared with the spectrum in Fig. 2a, the newborn peaks appearing on the pattern of Fig. 2b are attributed to a series of oPD oxidation products, including dimer (m/z 211), trimer (m/z 319), tetramer (m/z 425), pentamer (m/z 529) and hexamer (m/z 635) as well as their fragments at single charge protonated state [19]. Notice that the pentamer and hexamer are the first time detected by the EC-MS; the quite low relative intensities of them is probably due to the high solvent insolubility with the increase of molecular chain. Based on tandem MS study, these fragments are due to NH3 loss from their parent ions, i.e. dimer, trimer, tetramer and pentamer. For double check, the high-resolution MS was applied and the accurate m/z values of oPD oligomers as well as their fragments are summarized in Table S1 (see Supplementary data). Observing the inset windows in Fig. 2b, there are cluster peaks beside the oligomeric products. For example, the peaks m/z 315, 317 and 321 around the trimer m/z 319. The ion at m/z 321 is hastily to be seen as the isotope peak of ion m/z 319, but the MS2 spectra of the two ions are obviously different (see Fig. S1, Supplementary data). And the relative intensity of fragmentation peak m/z 304 (its parent ion is m/z 321) is higher than that of fragmentation peak m/z 302 (its parent ion is m/z 319) during reaction time from 1.5 to 5.5 min (see Fig. S2, Supplementary data), evidencing that ion m/z 321 should be assigned to one oxidation product of oPD. Therefore, ion m/z 321 was proposed to
3. Results and discussion 3.1. Electrooxidation of o-phenylenediamine monitored by EC-MS Electrooxidation of 1.0 mM o-Phenylenediamine (oPD) studied by EC-MS was started in acetonitrile/water (v:v = 4:1) solution with 0.1 mM ammonium acetate, and the solution pH was adjusted to 7.0. Since the response time of low-resolution MS is much shorter compared to the high-resolution MS, the reaction was first monitored with lowresolution MS for capturing short-lived intermediates. Fig. 2a shows the low-resolution MS spectrum when the sample solution sprayed out at the voltages of 4800+0 V (WE) and 4800 V (CE). The base peak appeared at m/z 109 is attributed to the protonated form of oPD monomer [M + H]+. A minor peak at m/z 92 is due to the NH3 loss from oPD
Fig. 1. Schematic of EC-MS setup for investing the electrooxidation of oPD. 2
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Fig. 3. Electrooxidation mechanism of oPD proposed by on-line EC-MS.
the firstly formed trimer with polyaniline like structure [9] which subsequently underwent dehydrogenation (oxidation process) to generate the ions m/z 315, 317 and 319. This indicates that the oPD polymerization was accompanied with the oligomer dehydrogenation and these dehydrogenation products may reduce the chain propagation efficiency. Moreover, comparing the relative intensities between the
dehydrogenation products and their fragments, e.g., m/z 211 → m/z 194, m/z 213 → m/z 196, m/z 215 → m/z 198, one can realize that the final dehydrogenation product exhibited the lowest fragment peak intensity. This may due to the strong conjugation inside the molecule, making it difficult to undergo intramolecular oxidation to form a complete oxidation state and thus to lose an NH3 group. 3
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dimer m/z 213 (RT = 0.5 min) and the trimer m/z 319 (RT = 1.5 min), indicating the dehydrogenation process m/z 213 → m/z 211 lagged behind the trimer formation m/z 213 → m/z 319. In addition, the ratio of theoretical peak intensity between ion m/z 211 and its isotope m/z 212 should be 7:1, while on the pattern at RT = 8.5 min in Fig. S3b the calculated value is 2:1. Thus, ion m/z 212 is considered to one oxidation production of oPD instead of isotope attributed to ion m/z 211. Such conclusion can also be referred by ion m/z 214. The relative abundance ratio between peak m/z 213 and peak m/z 214 obtained in Fig. S3b (the pattern at RT = 8.5 min) is 5:3 compared to 7:1 in theory. In previous study by Losito et al. [19], the product m/z 214 was defined as one oxygenated species. Herein, based on elemental formula that proposed by high-resolution MS (see Table S1, Supplementary data), ions m/z 212 and 214 should be the dehydrogenation products of m/z 215 as well. Hence, the commonly accepted 2e−, 2H+ oxidation step of oPD oligomer is, to be more precise, consisted of 1e−, 1H+ oxidation procedure. To the best of our knowledge, this is the first direct confirmation of such behavior occurred during oPD electrooxidation. Moreover, because the relative intensity of dimer m/z 211 kept increasing and exceeded dimer m/z 213 from 8.5 min, dimer m/z 211 is considered not to contribute to the trimer formation. Based on the results and other previous studies [7,18,19,23], a proposed pathway for electrooxidation of oPD is illustrated in Fig. 3. Obviously, the oPD chain propagation was accompanied by oligomer dehydrogenation reaction, (i) the initial stage was oPD monomer oxidized to radical cation (m/z 108), (ii) followed by two radical cations coupling to give dimer m/z 215, (iii) it underwent two 1e−, 1H+ oxidation (dehydrogenation process) steps to produce dimer m/z 213, (iv) most of the dimer m/z 213 combined a radical cation to generate trimer m/z 321 and the rest underwent 2e−, 2H+ oxidation to form dimer m/z 211. By repeating the oxidation of small oligomer and following addition of radical cations, a larger oPD oligomers formed subsequently.
Fig. 4. EC-MS spectra of oPD electrooxidized at pH values of 2.6, 3.7, 7.0 and 10.6. 1.0 mM oPD was dissolved in acetonitrile/water (v:v = 4:1) with 0.1 mM ammonium acetate. Reaction time = 5.0 min and oxidation voltage = 5.0 V. The data was obtained by low-resolution MS.
With the advantage of effective interface between the reacting solution and MS inlet, it is allowing to capture the short-lived intermediates [21]. Herein, the ion at m/z 108 (see the inset window of Fig. 2b) is identified to the radical cation of oPD. It offered solid evidence for this reaction, which was triggered from oxidizing the oPD monomer by losing of one electron. An overoxidized product of oPD monomer can be found at m/z 107 and its postulated formation pathway [23] was depicted in Scheme S1 (see Supplementary data). Li et al. reported that this ion was a semi-quinone radical of oPD which would contribute to form large oligomer [23]. However, the relative intensity of peak m/z 107 was maintained during the entire reaction process observed by EC-MS, suggesting that this side-reaction product did not participate the chain growth of oPD.
3.3. Effects of pH and oxidation voltages For investigating the effects of pH on electrooxidation of oPD, various solutions have been prepared. Fig. 4 shows the MS spectra of oPD oxidized under the same reaction time (5.0 min) and oxidation voltage (5.0 V) but in different pH environments, i.e. pH 2.6, 3.7, 7.0 and 10.6. Comparing the distribution observed for the oxidation products among the four MS spectra, one can conclude that the reaction progress is significantly influenced by the solution pH. Except the oPD monomer, only dimer exists on the spectra of pH 2.6 and 10.6. This may be related to an increase in the proportion of forming non-ionized oPD monomer, which cannot be oxidized to participate in the polymerization. Trimer and tetramer were both detected when the reaction carried out at pH 3.7 and 7.0. However, the relative intensities of the two oligomers on the spectrum of pH 3.7 are lower than them in pH 7.0. This is supported by the mechanism described in Fig. 3, H+ are generated during oligomer formation as well as the dehydrogenation steps, leading to the reaction more favored to perform in neutral environment. Therefore, the oxidation products of oPD are pH dependence and a neutral or weak acidic environment is recommended for the chain growth. When oPD was electrooxidized with different voltages (2.0, 4.0, 6.0 and 8.0 V), the relative intensities of monomer and tetramer m/z 425 versus reaction time (from 0.5 to 4.0 min) were obtained and shown in Fig. S4a and b, respectively. It should be pointed out that many byproducts which cannot be defined at present has been generated at 8.0 V, thus the spectrum is not shown here. As expected, the rate of monomer consumption and oligomer formation is greatly dependence on the oxidizing voltage when ΔE ≤ 6.0 V, that is the higher voltage applied, the faster chain propagation achieved. The ratios of relative intensity between the fully and semi-oxidized states oligomers including dimers (m/z 211 vs. m/z 213) and trimers (m/z 317 vs. m/z 319) were recorded at reaction time of 7 min. Observing from Fig. S4c
3.2. Real-time monitoring of oPD chain propagation Because the electrooxidation of oPD was performed in a droplet by means of the EC setup, it is capable to monitoring the electrooxidation progress of oPD in real time. Fig. S3 shows the MS patterns at different reaction times; the patterns were plotted based on the data from Fig. 2. It should be pointed out that the relative intensities of these oligomer peaks in MS spectra might not strictly reflect the real production of oligomers, but the data remained internally consistent and proved to be extremely useful for evaluating the relative changes of oligomers during the reaction. As can be seen in Fig. S3a, the relative intensity of oPD monomer dramatically decreased as the reaction proceeding, meanwhile, increase at the abundances of oxidation products and their fragments were observed. Specifically, the base peak of the MS spectrum shifted from oPD monomer m/z 109 to the dimer m/z 213 at the reaction time (RT) of 0.5 min. From 1.5 min the peak attributed to the trimer could be found at m/z 319 and its intensity kept growing in the following 6.0 min until it became the most abundant oligomer of the spectrum (RT = 6.5 min). The tetramer peak m/z 425 appeared since 1.5 min, however, the gently raising of relative intensity implies that the chain growing rate was declined from the tetramer, probably due to the rest amount of oPD monomers was too low to maintain the reaction efficiently. Therefore, the continuous changes on the signal intensity of oxidation products clearly reflects the electro-oxidation mechanism of oPD. In particular, the fully oxidized dimer m/z 211 was observed (RT = 2.5 min) (see Fig. S3b) after the appearance of semi-oxidized 4
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and d, the ratios is almost linear increased as the voltages, suggesting that the dehydrogenation reaction can be accelerated under a high voltage. Compared with the values of 317/319 in a narrow range (see yaxis in Fig. S4d), a remarkable difference in the ratios of 211/213 is observed (see y-axis in Fig. S4c), implying that the 1e−, 1H+ oxidation would be restricted in the larger oligomer. Therefore, the efficient oxidizing potentials for oPD chain propagation are 5.0–6.0 V.
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4. Conclusions By means of EC-MS investigations, the fleeting and complex chain propagation steps during electrooxidation of oPD were monitored in real time. The oxidation products include monomer radical cation as well as soluble oligomers (dimers, trimers, tetramers, pentamers and hexamers in different oxidation states) have been successfully detected and identified. The oligomer itself also underwent 1e−, 1H+ dehydrogenation reaction and some of the products would not participate in the oPD chain propagation. The favored pH environment for oPD oligomers formation was at around 7.0, whereas only the dimer could be formed in strong acidic (pH ≤ 2.6) or basic (pH ≥ 10.6) solution due to oPD monomer became non-ionized state. The most efficiency oxidation potential is determined to 5.0–6.0 V, however, side-reaction would occurr when the voltage rose to 8.0 V. These solid evidences can account for the various properties of PoPD synthesized under different experimental conditions. Declaration of Competing Interest There is no conflict of interest to declare. Acknowledgements This research is supported by the National Nature Science Foundation of China (Grant No. 21804027), Shandong Provincial Natural Science Foundation, China (Grant No. ZR2018PB016), and the Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 2019071). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.microc.2019.104390. References [1] J.H. Wang, H. Li, Y.H. Cai, D.J. Wang, L. Bian, F.Q. Dong, H.L. Yu, Y. He, Direct blue light-induced autocatalytic oxidation of o-Phenylenediamine for highly sensitive visual detection of triaminotrinitrobenzene, Anal. Chem. 91 (9) (2019) 6155–6161. [2] H.C. Xia, X.H. Xu, Q.H. Song, BODIPY-Based fluorescent sensor for the recognization of phosgene in solutions and in gas phase, Anal. Chem. 89 (7) (2017) 4192–4197.
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