Structural Effect of Thioureas on the Detection of Chemical Warfare

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Structural Effect of Thioureas on the Detection of Chemical Warfare Agent Simulants Seonggyun Ha, Minhe Lee, Hyun Ook Seo, Sun Gu Song, Kyung-su Kim, Chan Heum Park, Il Hee Kim, Young Dok Kim, and Changsik Song ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00256 • Publication Date (Web): 04 Aug 2017 Downloaded from http://pubs.acs.org on August 6, 2017

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Structural Effect of Thioureas on the Detection of Chemical Warfare Agent Simulants Seonggyun Ha,a Minhe Lee,a Hyun Ook Seo,b Sun Gu Song,a Kyung-su Kim,a , Chan Heum Park,a Il Hee Kim,a Young Dok Kima and Changsik Song*a a

Department of Chemistry, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do 16419, Republic of Korea. bDepartment of Chemistry and Energy Engineering, Sangmyung University, 20-6, Hongjimun 2-gil, Jongno-gu, Seoul 03015, Republic of Korea. * E-mail: [email protected]. KEYWORDS thiourea; nerve agent; structure–property; quartz crystal microbalance; NMR titration.

ABSTRACT: The ability to rapidly detect, identify and monitor chemical warfare agents (CWAs) is imperative for both military and civilian defense. Since most CWAs and their simulants have an organophosphonate group, which is a hydrogen (H)-bond acceptor, many H-bond donors have been developed to effectively bind to the organophosphonate group. Although thioureas have been actively studied as an organocatalyst, they are relatively less investigated in CWA detection. In addition, there is a lack of studies on the structure-property relationship for gas phase detection. In this study, we synthesized various thioureas of different chemical structures, and tested them for sensing dimethylmethylphosphonate (DMMP), a CWA simulant. Molecular interaction between DMMP and thiourea was measured by 1H-NMR titration and supported by density functional theory (DFT) calculations. Strong H-bond donor ability of thiourea may cause self-aggregation, and CH- interaction can play an important role in the DMMP detection. Gas-phase adsorption of DMMP was also measured using a quartz crystal microbalance (QCM) and analyzed using the simple Langmuir isotherm, showing the importance of structure-induced morphology of thioureas on the surface.

In military and civilian defense, rapidly detecting, identifying and monitoring chemical warfare agents (CWAs) have received increasing attention. In general, CWAs can be classified in four types: choking, nerve, blood and blister agents.1 Among them, the detection of nerve agents such as Sarin, Soman, Tubun and VX has been challenging because they are colorless and odorless, but highly toxic even at very low concentration. Structurally, most nerve agents are organophosphonates, the chemical moieties of which (-P=O- and -P-O-) are a good hydrogen-bond-acceptor or Lewis base. Therefore, Lewis acidic molecules have been utilized for sensing nerve gas agents (or their simulants) through hydrogen (H) bond; for example, hexafluoroisopropyl alcohol, a good H-bond-donor, has been incorporated into various sensing materials.2-3 Thiourea has also been used to detect nerve gas agents (G and V-Series) through H bond of the thioureic N–H protons and phosphonate oxygen.4 In fact, thioureas have been the most commonly used as a H-bond donor in organocatalysis. In 1998, Jacobsen and coworkers reported a thiourea catalyst for the asymmetric hydrocyanation of imines,5-6 and since then thiourea organocatalysts have been widely studied for the enhancement of various organic transformations and their reactivity and selectivity.7-11 In addition, the H-bond donor ability of thioureas has been frequently utilized in anion bindings and sensing, including fluoride, cyanide and carboxylates.12-13 Although thiourea and its derivatives have been actively studied as organocatalysts and anion receptors, they have been less-frequently

used in the field of CWA sensing, and their structure–property relationship has not been investigated in detail for gas phase detection. Kumar and coworkers studied chromogenic and fluorogenic detection with thioureas for tabun mimic.14-15 Hammond and coworkers used thiourea functionality for enhancing sensitivity toward organophosphonates in carbon nanotube-based flexible chemoresistive sensors,16 but only one type of thiourea (N-3,5-bis(trifluoromethyl)phenyl) was utilized. Gale and coworkers reported the structure–property relationship and compared the binding ability to organophosphonates by controlling the negative charges on the N-aryl group of thioureas.17 However, their study was limited to N-aryl substituents in order to change the donor ability of thioureic N–H, and all the measurements were performed in solution only. In this study, we designed and synthesized thiourea molecules, systematically varying N-substitutions, and investigated molecular characteristics essential for sensing of toxic organophosphonates. Using dimethylmethylphosphonate (DMMP), a nerve gas simulant, the synthesized thioureas were tested thoroughly using a quartz-crystal microbalance (QCM), 1H-NMR spectroscopy, and density-functional theory (DFT) calculations. In the binding of thiourea and DMMP, the N-benzyl group was found to play an important role, although the acidity or H-bond donor ability of N-benzyl thiourea is much less than N-aryl or other electron-withdrawing-group derivatives. The NMR and DFT study suggested that the limited donor ability of N-benzyl group successfully reduced the self-aggregation of thiourea

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molecules, and the additional CH-π interaction strengthened the binding of thiourea and DMMP. Our present work regarding the relationship between the molecular structures of thioureas and the sensing property for nerve gas agents would be helpful in the development of efficient receptor molecules for toxic organophosphonates including CWAs. Experimental Materials and Instruments. All the chemicals were purchased from Sigma-Aldrich, TCI, and Alfa Aesar and used without further purification. TU 1,18 5,18 6,18 7,19 8,19 and 919 were synthesized following published procedures. 1H- and 13C- NMR spectra were recorded using a Bruker 500 MHz spectrometer. The chemical shifts were reported in ppm (δ) with TMS as an internal standard, and the coupling constants (J) are expressed in Hz. High-resolution mass spectra (HRMS) were obtained on a Bruker Daltonics APEX II 3 Tesla FT-ICR-MS. The optimized structures and energy changes at different DMMP binding mode of thiourea were calculated at the B3LYP/d3* level on Gaussian 09.20 Quartz Crystal Microbalance (QCM) Test. The QCM flow cell kits were purchased from BAS Inc. AT-cut 8-MHz quartz crystals with 4-mm diameter Au electrodes on the both sides were purchased from Qrins. Before measuring QCM, the synthesized receptors (i.e., TUs) were coated on the surface of quartz crystal electrodes by the drop-coating method. TUs were dissolved in DMF solvent (2 mg/mL) and 4 L of the resulting solution was dropped on the gold electrodes of QCM quartz using a micro-syringe. Subsequently, the receptor-coated quartz crystals were vacuum-dried and then oven-dried at 80 oC for 1 h to obtain stable film. The coated quartz crystals were tested with DMMP, a surrogate for the warfare agent Sarin. The concentrations of DMMP vapor were controlled from 12 to 120 ppm by a mass flow controller (MFC) using pure nitrogen as the carrier gas. The liquid DMMP was placed into the bubbler chamber, and the nitrogen carrier gas was continuously passed through the bubbler system. The carrier gas was by-passed using an individual flow controller, which was then combined with the DMMP flow. Both flow rates were regulated to obtain a fixed total flow rate, which was set to 340 sccm (Figure S2, the Supporting Information). The concentration of DMMP was calculated by referring to the reference.21 The flow cell kits were designed so that as the diluted DMMP gas passes through the thiourea-coated quartz which can adsorb DMMP vapor gas in the enclosed system to eliminate the other disturbances (Figure 1a). The adsorbed DMMP vapor on thiourea receptor was removed by purging pure nitrogen gas. The measured frequency shift was converted into a mass changes by the following equation: ΔF = -Cf · Δm where ΔF is the observed frequency shift in Hz, C f is the sensitivity factor for the quartz (1.45 MHz g-1 cm2 for 8-MHz ATcut quartz crystal at room temperature), and Δm is the mass change in g/cm2. Synthesis of 1-butyl-3-ethylthiourea (2). Tetrahydrofuran (THF) was dried over sodium metal and distilled. Ethanamine

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(0.6 mmol) and 1-isothiocyanatebuthane (0.6 mmol) were dissolved in anhydrous THF (0.6 mL) under a nitrogen atmosphere. The reaction mixture was stirred at room temperature for 3 h and then extracted with dichloromethane. After that the solution was dried over anhydrous Na2SO4 and concentrated using a rotary evaporator. The crude product was purified by column chromatography on silica gel (dichloromethane/hexane gradient 1:3) afforded the product (86 mg, yield: 90%). 1H NMR (500 MHz, CDCl3):  5.79 (br s, 2H, NH), 3.45 (br s, 2H, CH2), 3.39 (br s, 2H, CH2), 1.57 (tt, J = 7.5 Hz, 2H, CH2), 1.37 (tq, J = 7.5 Hz, 2H, CH2), 1.22 (t, J = 7.5 Hz, 3H, CH3), 0.92 (t, J = 7.5 Hz, 3H, CH3) ppm. 13C NMR (125 Hz, CDCl3): 181.34, 44.09, 39.13, 31.04, 20.09, 14.29, 13.75 ppm. HRMS (ESI) m/z: [M]+ Calculated for C7H16N2S 160.1034; found 160.1007. Synthesis of 1-cyclohexyl-3-ethylthiourea (3). Compound 3 was obtained following the synthetic procedure of 2. Yield: (105 mg, yield: 94%). 1H NMR (500 MHz, CDCl3):  5.66 (br s, 1H, NH), 5.57 (br s, 1H, NH), 3.87 (br s, 1H, CH), 3.42 (br s, 2H, CH2), 2.03 (m, 2H, CH2), 1.73 (m, J = 4.0 Hz, 2H, CH2), 1.63 (m, 2H, CH2), 1.37 (m, 2H, CH2), 1.23 (t, J = 7.5 Hz, 3H, CH3), 1.20 (m, 2H, CH2) ppm. 13C NMR (125 Hz, CDCl3): 180.16, 53.05, 38.99, 32.95, 25.42, 24.74, 14.20 ppm. HRMS (ESI) m/z: [M]+ calculated for C9H18N2S 186.1191; found: 186.1161. Synthesis of 1-benzyl-3-ethylthiourea (4): Compound 4 was obtained following the synthetic procedure of 2. (107 mg, Yield: 92%). 1H NMR (500 MHz, CDCl3):  7.30 (m, 5H, Ar-H), 6.33 (br s, 1H, NH), 6.08 (br s, 1H, NH), 4.63 (s, 2H, CH2), 3.38 (s, 2H, CH2), 1.14 (t, J = 7.5, 3H, CH3) ppm. 13C NMR (125 Hz, CDCl3): 181.55, 137.15, 128.88, 127.87, 127.63, 48.51, 39.22, 14.19 ppm. HRMS (ESI) m/z: [M]+ calculated for C10H14N2S: 194.0878; found: 194.0876. Result and Discussion. The H-bond donor ability of thiourea (TU) is well known to be changed depending on the side groups22 and we anticipated different sensitivities toward vapors of organophosphonates (i.e., DMMP). To figure out the structural effect, several TUs designed (1–9), and easily synthesized from the corresponding amine and isothiocyanate (Scheme 1). Firstly, TUs 1–4 were designed and synthesized to control the H-bond donor ability through phenyl, 1° alkyl (n-butyl), 2° alkyl (cyclohexyl), and Nbenzyl groups. Among them, N-phenyl-substituted 1 would have stronger H bond than the others (N-alkyl groups) because of the electronic effect.23 However, the N-benzyl side group of 4 furnished interesting results (see below). TUs 5 and 6 were designed in order to alter the electron density of the N-benzyl group with electron-withdrawing (fluoro) and donating (methoxy), respectively, substituents. Lastly, TUs 7–9 were designed and synthesized to enhance the binding ability by keeping one group of TU with a N-benzyl group, and changing the other to N-benzyl, N-phenyl and N-cyclohexyl group. All the TUs were prepared in good to excellent yields and well characterized by 1 H-, 13C- NMR, and mass spectroscopy.

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Scheme 1. Synthesis of thioureas.a The TU-coated quartz crystal was dried under vacuum (1 h), and further oven-dried at 80 oC for 1 h, resulting in a stable film. As shown in Figure 1b, the frequency shifts were monitored under a flow of nitrogen and DMMP, and their concentrations were controlled using mass flow controllers. When the coated TU on the quartz crystal adsorbs DMMP vapor, the resonance frequency shifts to lower values and are proportional to the adsorbed mass of DMMP. The adsorbed DMMP molecules could be removed by purging with pure nitrogen. All the TUs were measured under the same conditions. The QCM analysis revealed that the TU’s interaction with a DMMP vapor was significantly affected by the molecular structure of TU, especially the presence of an N-benzyl moiety. Figure 1c compares the degree of frequency shifts (ΔF) of all the TUs at the DMMP concentration of 60 ppm. In the case of TUs 1–4, one group of TU was fixed with an ethyl group and the other was varied as N-phenyl (1), N-n-butyl (2), N-cyclohexyl (3), and N-benzyl (4) groups. Interestingly, TU 4 with an Nbenzyl group resulted in very high ΔF (3910 Hz) when compared to TU 1 (25.2 Hz), 2 (167 Hz), and 3 (209 Hz). Remarkably, TU 1, the N-phenyl group of which renders it stronger in H-bond donor strength, appeared less reactive toward DMMP than TU 4. We suspect that not only the H-bond donor strength, but also other secondary interaction might involve in the case of the N-benzyl group.

aReagents

and conditions: amine derivative (0.1 M), isothiocyanate derivative (1 equiv), THF, rt, 1-24 h, yield: 90-94%.

The interactability of the synthesized TUs with the simulant (DMMP) was investigated by QCM measurements (Figure 1). QCM can detect very small mass changes in real-time through frequency shifts of a quartz-crystal resonator. The resonance frequency because of the piezoelectric effect of a quartz crystal shifts by adsorption and desorption of an analyte, and such shift can be easily monitored by electrodes. 24 Before the measurements, the synthesized TUs were coated on the surface of a quartz crystal (8 MHz, AT-cut, Figure 1a) by a drop-coating method. Drop coating was chosen since a thin film was produced uniformly and easily. A 4-L solution of a TU in DMF (2 mg/mL) was dropped on the crystal using a micro-syringe.

(a) N2 IN DMMP IN

N2 OUT DMMP OUT

① DMMP blowing

② N2 blowing

+1

Receptor Coated Quartz

QCM Flow Cell Kit : TU

(b)

: DMMP

: TU-DMMP Complex

(c) 12 ppm

-600 : DMMP blowing

-1000

: N2 blowing : Delta F

-1200 0

(d)

500 1000 1500 2000 2500

Time (s)

4000

4 (Fmax = 100)

25

F (Hz / nmol)

-400

F (Hz) @ DMMP 60 ppm

-200

-800

[TU-DMMP]

120 ppm

0

F (Hz)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3000 2000 1000 0 0

1

2

3

4

5

6

7

8

9 10

TU

20

8 (Fmax = 71)

15 10

2 (Fmax = 0.22) 1 (Fmax = N/A)

5 0 0

20

40

60

80 100 120 140

DMMP concentration (ppm)

Figure 1. (a) Schematic diagram of the QCM measurement set-up and the Langmuir isotherm model for adsorption and desorption of DMMP on thiourea (θ: fractional occupancy, Keq: equilibrium constant, and pDMMP: partial pressure of DMMP). (b) The representative QCM response curve with TU 4 on increasing concentrations of DMMP from 12 to 120 ppm. (c) The QCM frequency shifts of TU receptors (1-9) against the DMMP concentration of 60 ppm. (d) Saturation curves of the QCM frequency shifts depending on the DMMP concentrations with TUs 1, 2, 4, and 8. The fitted values of ΔFmax, which is proportional to the total number of available sites [S]0, were also presented.

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Next, the electronic effect of the N-benzyl group using electronwithdrawing fluorine (5) and electron-donating methoxy (6) groups was examined. However, we did not observe any tendency according to the electronic density of the N-benzyl group; neither TU 5 (1130 Hz) nor TU 6 (2770 Hz) was more reactive than TU 4. Notably both TUs 5 and 6 with N-benzyl derivatives resulted in higher ΔF than TUs 1–3 without a N-benzyl group. Finally, we investigated TUs 7–9, in which one part of TU was fixed with a N-benzyl group and the other side was varied to Nbenzyl (7), N-phenyl (8) and N-cyclohexyl (9) groups. As expected, both TUs 8 and 9 showed high reactivity (F = 3580 Hz and 3030 Hz, respectively) owing to the N-benzyl group effect. Most interestingly, however, TU 7 showed the lowest reactivity (17.9 Hz) among the TUs tested, probably attributed to the increased crystallinity arising from its symmetric structure. In short, the inclusion of a N-benzyl group may increase the reactivity toward DMMP vapor, despite the relatively low donor strength of N-benzyl-substituted thiourea. We have compared our results with the recently published work25-28 concerning the reactivity of receptors with DMMP using QCM, showing that the (mean) sensitivity and adsorbed amount of DMMP on our thiourea receptor (TU 4) was very comparable to other materials from references under the same DMMP concentration of 60 ppm (Table S2, the Supporting Information). The adsorption behaviors of DMMP on TUs were further analyzed by the simple Langmuir isotherm model and the N-benzyl group of TUs appeared to significantly increase the total number of available adsorption sites. As shown in Figure 1d, the resonance frequency shifts of the TU-coated quartz crystal are plotted according to the partial pressures of DMMP vapor. The saturation curves were then fitted using a simple Langmuir isotherm, providing information on the fractional occupancy () and the total number of available sites ([S] 0). When the results were compared for TUs 1, 2, 4 and 8, the fractional occupancy did not show a large difference at a specific DMMP concentration (Table S1, the Supporting Information). However, the total number of available sites [S]0, which can be evaluated by Fmax, showed large variations; with a benzyl group, the total sites increased 300 to 400 times more per mole of the TU molecule (Fmax values of TU 2 = 0.22, TU 4 = 100, and TU 8 = 71). The reason of such an increase in the presence of N-benzyl group is not clear at the moment. However, it should be noted that the molecular structure of TU is very important, and its donor strength is not a determining factor. In order to investigate any morphological effect, we obtained the scanning electron microscopy (SEM) images of TU films (Figure 2). When the TUs were coated on the quartz using a drop-coating method, the surface morphologies were very different depending on their chemical structures. Benzyl groupsubstituted TUs 4 and 8 exhibited rather roughened, particulate morphology, which may serve as binding sites available for DMMP vapor. In contrast, without the benzyl group, TU 1 and 2 tended to show rather crystalline morphology, most of which cannot be easily accessed by DMMP vapor. The morphology analysis indicates that the morphology of TU receptors may play a crucial role in DMMP sensing, which may be the reason why benzyl substituted TUs 4 and 8 showed higher reactivities toward DMMP in QCM measurements.

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(a) TU-1

(b) TU-2

1 m

(c) TU-4

1 m

(d) TU-8

1 m

1 m

Figure 2. FE-SEM images for the drop-coated samples: TUs 1 (a), 2 (b), 4 (c) and 8 (d).

We also examined the effect of relative humidity and other interference gases on DMMP sensing. Firstly, we tested the selectivity of TUs for DMMP toward other gas molecules. Under the similar conditions for DMMP sensing, vapors of acetone, acetonitrile, ethanol, methanol, hexane and toluene were introduced to the QCM flow cells equipped with a TU 4-coated quartz crystal. The concentration-normalized frequency changes indicated that TU 4 adsorbed at least 30 times more DMMP than other gas molecules tested, which suggested that TU 4 is highly selective toward organophosphonates (Figure S5, the Supporting Information). Secondly, we measured the change of TU’s sensing ability according to the relative humidity (Figure S6, the Supporting Information). We implemented additionally the H2O-bubbling system using a mass flow controller (MFC), and the relative humidity was set to 7, 20, 30.5, and 45%. The total flow was fixed at 55 sccm and the concentration of DMMP was adjusted to 98 ppm. Interestingly, the results indicated that the frequency change (F) under humidity tended to increase from the dry condition as the relative humidity increased up to 30.5% (from 109 up to 149 Hz), presumably due to the formation of DMMP clusters with H2O molecules.29 However, it should be noted here that in the case of the relative humidity of 30.5%, the frequency change up on adsorption/desorption appeared unstable. In addition, no absorption of DMMP was observed in QCM over the relative humidity of 45%, which showed the interference of humidity arising from the competitive binding of water molecules on TUs. The 1H-NMR titration experiment revealed that the benzyl group of TUs strengthened the intermolecular interactions toward an organophosphonate (Figure 3). It has been known that the chemical shift of thiourea N-H changes according to the degree of H bond with an acceptor (i.e., DMMP), and its association constant (Ka) can be obtained by non-linear regression analysis (Figure 3a and Supporting Information).30 The 1H-NMR spectra of TU 1, 2, 4 or 8 in chloroform-d (~100 mM) indicate that the chemical shift of the N–H protons shifted downfield with increasing amount of DMMP (up to ~35 equivalents), confirming the formation of the TU-DMMP complex through H bonding (Figure 3b and Supporting Information). In Figure 3c, the amount of N–H peak shifts was plotted against the equivalents of DMMP toward each TU. Interestingly, TUs with a Nbenzyl group (4 and 8) showed the large shifts compared to TU 2 of alkyl groups and TU 1 of a N-phenyl group (4 > 8 > 2 > 1).

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Table 1. Association constants (Ka) and Gibbs free energies (Ga) of TU-DMMP interactions from 1H-NMR titrations. TU

Ka (M-1)

Ga (kJ/mol)

1

5.3

-4.15

2

2.3

-2.06

3

2.1

-1.89

4

0.7

0.61

2.1 Å 2.9 Å

H-bonding (NH∙∙∙O)

The non-linear regression analysis furnished the Ka values at 25 oC (Table 1 and Figures S5-8, the Supporting Information); 5.3 for the largest TU 4 and 0.7 for the smallest TU 1, corresponding to the ΔG in the range -4.15 ~ 0.61 kJ/mol and suggesting relatively weak TU-DMMP interactions. The electronwithdrawing N-phenyl group of TU 1 should increase the acidity of N–H, thus increase the H-bond donor strength. However, the incorporation of the N-phenyl group seems to apparently weaken the H-bond strength (4 vs 8 and 2 vs 1). This (apparent) weakening of donor strength with a N-phenyl group toward DMMP is attributed to the self-aggregation of thiourea itself. We reasoned that the increased donor strength because of the phenyl group actually promoted TU–TU self-aggregation, which in turn decreased the TU–DMMP interaction. To support this hypothesis, the TU’s chemical shift changes were measured by varying its concentration in chloroform-d (Figure S11, the Supporting Information). Indeed, the presence of a N-phenyl group induced self-aggregation to a greater extent; the degree of peak shifts (self-aggregation) appeared in the order of TU 8 > 1 > 4 > 2, corresponding to the H-bond donor strength. Thus, we may conclude that the incorporation of a phenyl group renders TU more acidic or increase the H bonding, and also promotes more self-aggregation. Although the decreased interaction of TUs 1 and 8 with DMMP than TUs 2 and 4, respectively, can be explained by adverse effect of phenyl, it is still not clear why N-benzyl-substituted TU 4 is better than unsubstituted TU 2 (or TU 8 better than TU 1). (a)

TU

DMMP

TU-DMMP

(c)

(b)

1.2

a

b

14 eq

1.0

10 eq

0.8

7 eq

3 eq 1.5 eq 0 eq

b 7.00

7.00

ppm (t1)

6.50

6.50

6.00

6.00

a

 (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Chemical shift (ppm)

8 2 1

0.6 0.4 0.2 0.0

5.50

5.50

4

0

5

10

15

20

DMMP equiv

Figure 3. (a) Schematic representation of thiourea and DMMP complexation through H bond, and its equilibrium constant (Ka). (b) Representative 1H-NMR titration of TU 4 using various equivalents of DMMP. (c) Chemical shifts (Δδ) of TU’s N-H in 1H-NMR were plotted against the equivalents of DMMP. The saturation curves were fitted using non-linear regression analysis.

1.9 Å

CH-

Figure 4. Calculated structure between 4 and DMMP by H bonding and CH- interaction.

To gain further insight into the binding mechanism between N-benzyl-substituted TU 4 and DMMP, density functional theory (DFT) calculation was performed using B3LYP functional with the empirical dispersion of Grimme (B3LYP-D3BJ) and d3 basis set employing the Gaussian0920 suite of programs. As shown in Figure 4, the DFT calculation revealed the possible existence of CH- interactions from the benzyl group. TU 4 seems to bind to DMMP mainly through the H bonds of N-Hs and O=P. However, the methyl group of DMMP was pointed to-ward the π-electrons of the N-benzyl moiety, presumably forming CH- bonds.31 Therefore, the N-benzyl group in the TU structures has an advantage for DMMP sensing through extra CH- interaction as well as H bonding. In addition, the donor strength of TU’s N-H with the N-benzyl group seems suitable for reducing the self-aggregation. Conclusion Various thioureas with combinations of N-aryl and N-alkyl functional groups were synthesized, and their sensing properties to DMMP, a CWA simulant, were investigated. The thioureas with a range of H bond strength were subjected to QCM measurements and 1H-NMR titrations in the presence of DMMP, and the resulting data were analyzed by simple Langmuir isotherm and non-linear regression for binding constants. Interestingly, benzyl-substituted TU 4 showed the highest sensitivity to DMMP in the QCM measurement and NMR titration, albeit its H-bond strength was limited compared to N-phenyl derivatives such as 1. The analysis based on the simple Langmuir isotherm suggested that the benzyl group increased the available binding sites ([S]0) for thioureas in the film state. The 1H-NMR titrations and subsequent non-linear regression analyses showed strong H-bond donor strength of phenyl derivatives induced self-aggregation, whereas N-benzyl-substituted 4 interacted most favorably with DMMP. The DFT calculations showed the interesting interaction between the  electrons of the N-benzyl group and the CH of DMMP. We concluded that the H-bonding interaction between thioureas and organophosphonates should be adjusted to avoid self-aggregation, and CH- interaction may play an important role in sensing organophosphonates. Supproting Information Available: The following file are available free of charge (SI_se-2017-00256h.pdf). Contents: Calculation of FMax, equilibrium constant (K) and Go of receptors; linear fitting of receptors from response curve; the schematic diagram of MFC/bubbler system; the response curve of TU-coated QCM sensor; comparison of the mean sensitivity and the adsorbed amount; comparison of QCM tests for various gas molecules; QCM test depending on relative humidity; the

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NMR titrations of TUs with DMMP; 1H NMR and 13C NMR spectra of TUs 1, 2, 4, and 8. Acknowledgements This research was supported by Civil-Military Technology Cooperation Program of Korea funded by the Ministry of Trade, Industry & Energy, and by the Agency for Defense Development through Chemical and Biological Defense Research Center. The authors also thank the Nano Material Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST), Republic of Korea (2012M3A7B4049644).

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ACS Sensors

Table of Contents Graphic

2.1 Å 2.9 Å Receptor Coated Quartz

: TU

: DMMP

1.9 Å

H-bonding (NH∙∙∙O)

: TU-DMMP

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CH-