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Sens Biosensing Res. Author manuscript; available in PMC 2016 September 01. Published in final edited form as: Sens Biosensing Res. 2015 September 1; 5: 37–41. doi:10.1016/j.sbsr.2015.06.003.
MEISENHEIMER COMPLEX BETWEEN 2,4,6-TRINITROTOLUENE AND 3-AMINOPROPYLTRIETHOXYSILANE AND ITS USE FOR A PAPER-BASED SENSOR Shantelle Hughes, Samuel S. R. Dasary, Salma Begum, Nya Williams, and Hongtao Yu Department of Chemistry and Biochemistry, Jackson State University, Jackson, MS 39217
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Abstract 2,4,6-Trinitrotoluene (TNT) forms a red-colored Meisenheimer complex with 3aminopropyltrenthoxysilane (APTES) both in solution and on solid phase. The TNT-APTES complex is unique since it forms yellow-colored complexes with 2,4,6-trinitrophenol and 4nitrophenol, and no complex with 2,4-dinitrotoluene. The absorption spectrum of TNT-APTES has two absorption bands at 530 and 650 nm, while APTES complexes with 2,4,6-trinitrophenol and 4-nitrophenol have absorption maxima at around 420 nm, and no absorption change for 2,4dinitrotoluene. The TNT-APTES complex facilitates the exchange of the TNT-CH3 proton/ deuteron with solvent molecules. The red color of TNT-APTES is immediately visible at 1 µM of TNT.
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Keywords TNT detection; nitroaromatics; Meisenheimer Complex
Introduction Fast, sensitive and selective detection of 2,4,6-trinitrotoluene (TNT), an explosive, is imperative because of national security and environmental health concerns. The industries, military and terrorists have often used TNT as one of the explosive materials[1]. Because of the excessive use of TNT in war and structure demolition, it is contaminating the soil and groundwater, which causes adverse effects for human health[2–6]. It is well-known that TNT is toxic and carcinogenic and causes anemia, abnormal liver function, and cataracts[1–7].
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Recently, numerous techniques have been developed for the detection of TNT, such as colorimetry[8–14], electrochemical approaches[15–18], luminescence[15, 19–36], Raman spectroscopy[17, 19, 26, 37–39], analytical paper devises[8, 10, 12, 14, 17, 40], and others.[33–36, 41–44]. Most of these techniques exhibit good sensitivity and selectivity. Some
Correspondence should be sent to: Hongtao Yu, Department of Chemistry and Biochemistry, Jackson State University, Jackson, MS 39217; Phone: 601-979-2171; [email protected]. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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require complicated instrumentation, but others use simple paper-based sensors[8, 10, 12, 14, 17, 40]. Several reports took advantage of the Meisenheimer complex formation between TNT and an amine[11, 15, 32, 38, 45–47]. Here we report the binding study using absorption spectroscopy and NMR between 3-aminopropyltriethoxysilane (APTES) and TNT, as well as three TNT-like molecules, 2,4,6-trinitrophenol (TNP), 2,6dinitrotoluene (DNT), and 4-nitrophenol (NP), to understand the selective APTES-TNT complex formation over other nitroaromatics. We further used an APTES-coated filter paper for selective detection of TNT.
Materials and Methods UV-Vis absorption experiments
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Stock solutions of TNT, TNP, NP, and DNT were prepared in toluene (5 mM). To a 2.00 mL solution of 10 mM APTES in a 1 cm quartz cuvette, solutions of TNT or other nitroaromatics (10 – 225 µL) were added to achieve concentrations of 25 – 500 µM. UV-Vis absorption spectra were recorded. NMR Spectroscopy—Samples of 750 µL of 5 mM TNT, APTES, and TNT - APTES complex in d6 - Acetone were recorded with the Varian 500 MHz NMR. The TNT - APTES complex samples were in 1:1, 1:2, and 1:3 mole ratios. Preparation of APTES-coated glass microfiber filter paper Whatman glass microfiber filter paper (GF/F, 24 mm, 1825-024) was soaked in 50% APTES solution for four hours. The paper was dried overnight under nitrogen atmosphere.
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Detection of TNT on APTES-coated glass microfiber filter paper Solution (1.0 µL) of TNT in toluene with concentrations of 1 – 2000 µM were spotted onto the APTES-coated glass microfiber filter paper in order to see whether there is a color change. To determine the selectivity of the paper sensor, solutions (1 µL of 5 mM) of TNT, DNT, TNP, and NP were spotted onto the filter paper. Measurement of color intensity
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Upon addition of TNT to the APTES-coated glass microfiber filter paper, a red color spot appeared immediately. This color lasted for over a week in a desicator under vacuum. The red colored images were immediately scanned with an EPSON scanner (Perfection Photo 2450). The intensity of the images were analyzed with the Adobe Photoshop software (version CS 5.1), which took an average of intensity of 31×31 pixels of the colored areas (average of three values on the same spot, one near the center and 2 closer to the edge). A common background of the paper was subtracted.
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Results and Discussion UV-Vis spectroscopy study of the interaction of APTES with TNT and TNT-like molecules Figure 1 shows the absorption spectrum of TNT which has an absorption peak at 230 nm and a low intensity peak near 340 nm. There is no absorption in the 400–800 nm range (Figure 1 A). Once TNT is titrated into excess APTES (10 mM) with incremental concentrations of 0, 25, 50, 100, 150, 200, 250, 300, 350, 400, 450, and 500 µM, two bands at 530 nm and 650 nm are formed and increase as the TNT concentration increases (Figure 1 B). The color of the TNT-APTES complex in solution is red immediately after they were mixed. The inset of Figure 1B shows the increase of absorbance at 650 nm as the TNT concentration increases.
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The same titration experiments were also carried out for DNT, NP, and TNP with APTES to see if they also form a complex as TNT does. Figure 2 (left) shows the absorption spectra of 100 µM of pure DNT, NP and TNP in toluene, and Figure 2 (right) shows the spectra of these nitroaromatics when titrated into APTES. DNT and NP do not have any absorption at >400 nm, and TNP has a low shoulder at 400 – 450 nm. When adding DNT into APTES solution in toluene, no absorption peak in the 400 – 800 nm range appears. This indicates that DNT does not form an observable complex with APTES in toluene. When NP or TNP is added to APTES solution in toluene, a new absorption band at 425 nm appears for NP and a new absorption band at 440 nm appears for TNP. Both complexes are yellow in color as shown in Figure 5. NMR study of the APTES-TNT complex
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The structure of the TNT-APTES complex is studied by NMR spectroscopy in acetone-d6 (Figure 3). APTES has five signals (Figure 3A): the methyl and methylene groups of the ethoxy appear at 1.17 and 3.81 ppm, respectively. The three methylene groups of the 3aminopropyl group appear at 3.16 (CH2 connected to N), 1.65 (middle CH2), and 0.61 ppm (CH2 connected to Si), respectively. The amino hydrogens are not seen due to H-D exchange with the solvent (acetone-d6). TNT has two signals (Figure 3B), the two aromatic hydrogens appear at 9.0 ppm and the methyl hydrogens appear at 2.7 ppm. Addition of APTES to TNT in a 1:1, 2:1 and 3:1 (APTES : TNT) mole ratios, no new signals appear, nor significant shifts for any of the signals (spectra not shown). However, the relative signal intensity of the methyl hydrogens (2.7 ppm) versus the aromatic hydrogens (9.0 ppm) of TNT is much lower than that in pure TNT (Figure 3C). Allowing the acetone-d6 solution of the APTES-TNT mixture to stay overnight, it causes the TNT methyl signal to completely disappear. This means that the methyl hydrogens have been exchanged by solvent deuterons. Without the presence of APTES, the TNT methyl signal does not disappear after overnight stay in acetone-d6 (data not shown). This means that the TNT-APTES Meisenheimer complex formation causes the hydrogens of the TNT methyl group to be exchanged with deuterons from the acetone-d6. Although no new NMR signal is observed for the APTESTNT complex, the H-D exchange indicates the complex formation. This also implicates that the TNT-APTES Meisenheimer complex is in fast equilibrium with the non-complexed molecules. We believe that this is the first report of the H-D exchange of the TNT-methyl group caused by the TNT-Meisenheimer complex formation.
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Unique APTES-TNT complex
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We believe that the following complexes are formed between the selected nitroaromatics with APTES (Figure 4). It is known that TNT forms a Meisenheimer complex with an amino group[6, 37, 39, 41, 42]. The three electron-withdrawing nitro groups in TNT enables the toluene ring to be electron-poor enough that TNT forms the red-colored Meisenheimer complex with APTES with two absorption bands at 530 nm and 650 nm. The formation of the Meisenheimer complex in TNT requires the loss of a proton from the TNT-methyl group to the nitrogen of the APTES as illustrated in Figure 4. Therefore, in the back reaction, the hydrogen from the methyl is exchanged with the hydrogen (or deuteron) from the nitrogen of APTES. Repetition of this procedure resulted in nearly complete exchange of the TNT CH3 to CD3. This process is slow and takes overnight to complete. Since there are no new signals appear in both 1H and 13C NMR (not shown) for the TNT-APTES complex, we believe that the Meisenheimer complex is not stable in solution and is in fast equilibrium with individual TNT and APTES molecules. On the contrary, DNT does not form a colored complex with APTES. This is likely due to its one less nitro group than in TNT. Due to one-less electron-withdrawing nitro group in DNT, its benzene ring is not electron-poor enough to form the Meisenheimer complex.
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For the phenolic TNP and NP, a different complex is formed which is yellow with absorption maxima at 400–450 nm. We believe that these complexes are the result of the acid-base reaction between the amine of APTES and the phenolic group to form phenolates (Figure 4), which is also shown by Wanko et al for nitrophenolates[48]. In summary, APTES forms distinct complexes with TNT, different from those with DNT, NP, and TNP. The redcolored APTES-TNT complex can be utilized for both selective and sensitive detection of TNT. APTES-based paper sensor for TNT It is known that a primary amine can form a Meisenheimer complex with the electrondeficient TNT[15, 45, 47]. APTES was chosen because it may have a tight interaction with a fiber glass filter paper on one hand, and forms a Meisenheimer complex with TNT on the other[11, 14, 41]. Soaking the filter paper in 50% APTES for 4 hours produces the paper sensor. The APTES-coated paper must be dried and stored under nitrogen atmosphere since APTES is not stable under oxygen atmosphere due to potential air-oxidation of the amino group.
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TNT solutions in toluene (1 µL) at concentrations of 1, 5, 10, 25, 50, 100, 250, 500, 1000, and 2000 µM were spotted on the APTES-coated paper. As can be seen in Figure 5 (Left), a red-colored spot is formed immediately for each of the spot of TNT. The color intensity increases as the concentration of TNT increases. Based on what can be observed on the filter paper, a red color can be seen with TNT concentration as low as 1 µM. In Figure 5 (Right), the other three nitroaromatics (5 mM) were also spotted along with TNT. While DNT does not produce any color, both phenolic nitroaromatics (TNP and NP) produced a strong yellow color. This shows that this sensor is selective for TNT. To quantify the color intensity of the TNT-APTES complex on the filter paper, these color spots were Sens Biosensing Res. Author manuscript; available in PMC 2016 September 01.
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scanned with a scanner and quantified by Adobe Photoshop. After subtracting a common background, the thus obtained intensity of the spots is plotted versus the concentration of TNT (Figure 6). The color intensity of the APTES-TNT complex increases with the TNT concentration and levels off at about 2.5 mM of TNT. Triplicate of experiments were conducted to obtain the average of the color intensity. In conclusion, TNT forms a red-colored Meisenheimer complex with APTES on filter paper or in solution with two absorption bands at 530 nm and 650 nm. For the three other TNTlike molecules, DNT does not form a colored complex, while TNP and NP form yellowcolored acid-base type complexes. The difference in the colored complexes formed between APTES and nitroaromatics can be utilized to selectively detect TNT using the APTES coated paper with the lowest detectable concentration of 1 µM TNT.
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Acknowledgments This research was supported in part by NSF through the Partnership for Research and Education in Materials grant (DMR-0611539). Core research facilities were supported by grants from the NSF (CHE-0840450) and NIH (NCRR 8G12MD007581).
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Figure 1.
(A: Top) UV-Vis absorbance spectrum of TNT in toluene (100 µM); (B: Bottom) UVV-is absorbance spectral change when TNT (0, 25, 50, 100, 150, 200, 250, 300, 350, 400, 500 µM) is titrated into APTES toluene solution (10 mM) (Inset: Absorbance change at 650 nm with the increase of TNT concentration).
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Figure 2.
Absorption spectra of DNT (top), TNP (middle), and NP (bottom) in toluene (left) and when added to 10 mM APTES (right) in toluene.
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(A) 1H-NMR of APTES; (B) 1H-NMR of TNT; (C) APTES and TNT mixture (3:1 mole ratio) in acetone-d6.
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Figure 4.
Complex formation between a nitroaromatic compound and the primary amine of the APTES (R-NH2).
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Figure 5.
(A) APTES coated fiber glass filter paper spotted with TNT at concentrations of 1, 5, 10, 25, 50, 100, 250, 500, 1000, 2000 µM. (B) Spotting with 5 mM of TNT (red spot) and other nitroaromatics (TNP: 2,4,6-trinitrophenol, DNT: 2,4-dinitrotoluene, and NP: 4-nitrophenol).
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Color intensity of the TNT-APTES complex on the filter paper versus the concentration of TNT.
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Sens Biosensing Res. Author manuscript; available in PMC 2016 September 01. Published in final edited form as: Sens Biosensing Res. 2015 September 1; 5: 37–41. doi:10.1016/j.sbsr.2015.06.003.
MEISENHEIMER COMPLEX BETWEEN 2,4,6-TRINITROTOLUENE AND 3-AMINOPROPYLTRIETHOXYSILANE AND ITS USE FOR A PAPER-BASED SENSOR Shantelle Hughes, Samuel S. R. Dasary, Salma Begum, Nya Williams, and Hongtao Yu Department of Chemistry and Biochemistry, Jackson State University, Jackson, MS 39217
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Abstract 2,4,6-Trinitrotoluene (TNT) forms a red-colored Meisenheimer complex with 3aminopropyltrenthoxysilane (APTES) both in solution and on solid phase. The TNT-APTES complex is unique since it forms yellow-colored complexes with 2,4,6-trinitrophenol and 4nitrophenol, and no complex with 2,4-dinitrotoluene. The absorption spectrum of TNT-APTES has two absorption bands at 530 and 650 nm, while APTES complexes with 2,4,6-trinitrophenol and 4-nitrophenol have absorption maxima at around 420 nm, and no absorption change for 2,4dinitrotoluene. The TNT-APTES complex facilitates the exchange of the TNT-CH3 proton/ deuteron with solvent molecules. The red color of TNT-APTES is immediately visible at 1 µM of TNT.
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Keywords TNT detection; nitroaromatics; Meisenheimer Complex
Introduction Fast, sensitive and selective detection of 2,4,6-trinitrotoluene (TNT), an explosive, is imperative because of national security and environmental health concerns. The industries, military and terrorists have often used TNT as one of the explosive materials[1]. Because of the excessive use of TNT in war and structure demolition, it is contaminating the soil and groundwater, which causes adverse effects for human health[2–6]. It is well-known that TNT is toxic and carcinogenic and causes anemia, abnormal liver function, and cataracts[1–7].
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Recently, numerous techniques have been developed for the detection of TNT, such as colorimetry[8–14], electrochemical approaches[15–18], luminescence[15, 19–36], Raman spectroscopy[17, 19, 26, 37–39], analytical paper devises[8, 10, 12, 14, 17, 40], and others.[33–36, 41–44]. Most of these techniques exhibit good sensitivity and selectivity. Some
Correspondence should be sent to: Hongtao Yu, Department of Chemistry and Biochemistry, Jackson State University, Jackson, MS 39217; Phone: 601-979-2171; [email protected]. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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require complicated instrumentation, but others use simple paper-based sensors[8, 10, 12, 14, 17, 40]. Several reports took advantage of the Meisenheimer complex formation between TNT and an amine[11, 15, 32, 38, 45–47]. Here we report the binding study using absorption spectroscopy and NMR between 3-aminopropyltriethoxysilane (APTES) and TNT, as well as three TNT-like molecules, 2,4,6-trinitrophenol (TNP), 2,6dinitrotoluene (DNT), and 4-nitrophenol (NP), to understand the selective APTES-TNT complex formation over other nitroaromatics. We further used an APTES-coated filter paper for selective detection of TNT.
Materials and Methods UV-Vis absorption experiments
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Stock solutions of TNT, TNP, NP, and DNT were prepared in toluene (5 mM). To a 2.00 mL solution of 10 mM APTES in a 1 cm quartz cuvette, solutions of TNT or other nitroaromatics (10 – 225 µL) were added to achieve concentrations of 25 – 500 µM. UV-Vis absorption spectra were recorded. NMR Spectroscopy—Samples of 750 µL of 5 mM TNT, APTES, and TNT - APTES complex in d6 - Acetone were recorded with the Varian 500 MHz NMR. The TNT - APTES complex samples were in 1:1, 1:2, and 1:3 mole ratios. Preparation of APTES-coated glass microfiber filter paper Whatman glass microfiber filter paper (GF/F, 24 mm, 1825-024) was soaked in 50% APTES solution for four hours. The paper was dried overnight under nitrogen atmosphere.
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Detection of TNT on APTES-coated glass microfiber filter paper Solution (1.0 µL) of TNT in toluene with concentrations of 1 – 2000 µM were spotted onto the APTES-coated glass microfiber filter paper in order to see whether there is a color change. To determine the selectivity of the paper sensor, solutions (1 µL of 5 mM) of TNT, DNT, TNP, and NP were spotted onto the filter paper. Measurement of color intensity
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Upon addition of TNT to the APTES-coated glass microfiber filter paper, a red color spot appeared immediately. This color lasted for over a week in a desicator under vacuum. The red colored images were immediately scanned with an EPSON scanner (Perfection Photo 2450). The intensity of the images were analyzed with the Adobe Photoshop software (version CS 5.1), which took an average of intensity of 31×31 pixels of the colored areas (average of three values on the same spot, one near the center and 2 closer to the edge). A common background of the paper was subtracted.
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Results and Discussion UV-Vis spectroscopy study of the interaction of APTES with TNT and TNT-like molecules Figure 1 shows the absorption spectrum of TNT which has an absorption peak at 230 nm and a low intensity peak near 340 nm. There is no absorption in the 400–800 nm range (Figure 1 A). Once TNT is titrated into excess APTES (10 mM) with incremental concentrations of 0, 25, 50, 100, 150, 200, 250, 300, 350, 400, 450, and 500 µM, two bands at 530 nm and 650 nm are formed and increase as the TNT concentration increases (Figure 1 B). The color of the TNT-APTES complex in solution is red immediately after they were mixed. The inset of Figure 1B shows the increase of absorbance at 650 nm as the TNT concentration increases.
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The same titration experiments were also carried out for DNT, NP, and TNP with APTES to see if they also form a complex as TNT does. Figure 2 (left) shows the absorption spectra of 100 µM of pure DNT, NP and TNP in toluene, and Figure 2 (right) shows the spectra of these nitroaromatics when titrated into APTES. DNT and NP do not have any absorption at >400 nm, and TNP has a low shoulder at 400 – 450 nm. When adding DNT into APTES solution in toluene, no absorption peak in the 400 – 800 nm range appears. This indicates that DNT does not form an observable complex with APTES in toluene. When NP or TNP is added to APTES solution in toluene, a new absorption band at 425 nm appears for NP and a new absorption band at 440 nm appears for TNP. Both complexes are yellow in color as shown in Figure 5. NMR study of the APTES-TNT complex
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The structure of the TNT-APTES complex is studied by NMR spectroscopy in acetone-d6 (Figure 3). APTES has five signals (Figure 3A): the methyl and methylene groups of the ethoxy appear at 1.17 and 3.81 ppm, respectively. The three methylene groups of the 3aminopropyl group appear at 3.16 (CH2 connected to N), 1.65 (middle CH2), and 0.61 ppm (CH2 connected to Si), respectively. The amino hydrogens are not seen due to H-D exchange with the solvent (acetone-d6). TNT has two signals (Figure 3B), the two aromatic hydrogens appear at 9.0 ppm and the methyl hydrogens appear at 2.7 ppm. Addition of APTES to TNT in a 1:1, 2:1 and 3:1 (APTES : TNT) mole ratios, no new signals appear, nor significant shifts for any of the signals (spectra not shown). However, the relative signal intensity of the methyl hydrogens (2.7 ppm) versus the aromatic hydrogens (9.0 ppm) of TNT is much lower than that in pure TNT (Figure 3C). Allowing the acetone-d6 solution of the APTES-TNT mixture to stay overnight, it causes the TNT methyl signal to completely disappear. This means that the methyl hydrogens have been exchanged by solvent deuterons. Without the presence of APTES, the TNT methyl signal does not disappear after overnight stay in acetone-d6 (data not shown). This means that the TNT-APTES Meisenheimer complex formation causes the hydrogens of the TNT methyl group to be exchanged with deuterons from the acetone-d6. Although no new NMR signal is observed for the APTESTNT complex, the H-D exchange indicates the complex formation. This also implicates that the TNT-APTES Meisenheimer complex is in fast equilibrium with the non-complexed molecules. We believe that this is the first report of the H-D exchange of the TNT-methyl group caused by the TNT-Meisenheimer complex formation.
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Unique APTES-TNT complex
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We believe that the following complexes are formed between the selected nitroaromatics with APTES (Figure 4). It is known that TNT forms a Meisenheimer complex with an amino group[6, 37, 39, 41, 42]. The three electron-withdrawing nitro groups in TNT enables the toluene ring to be electron-poor enough that TNT forms the red-colored Meisenheimer complex with APTES with two absorption bands at 530 nm and 650 nm. The formation of the Meisenheimer complex in TNT requires the loss of a proton from the TNT-methyl group to the nitrogen of the APTES as illustrated in Figure 4. Therefore, in the back reaction, the hydrogen from the methyl is exchanged with the hydrogen (or deuteron) from the nitrogen of APTES. Repetition of this procedure resulted in nearly complete exchange of the TNT CH3 to CD3. This process is slow and takes overnight to complete. Since there are no new signals appear in both 1H and 13C NMR (not shown) for the TNT-APTES complex, we believe that the Meisenheimer complex is not stable in solution and is in fast equilibrium with individual TNT and APTES molecules. On the contrary, DNT does not form a colored complex with APTES. This is likely due to its one less nitro group than in TNT. Due to one-less electron-withdrawing nitro group in DNT, its benzene ring is not electron-poor enough to form the Meisenheimer complex.
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For the phenolic TNP and NP, a different complex is formed which is yellow with absorption maxima at 400–450 nm. We believe that these complexes are the result of the acid-base reaction between the amine of APTES and the phenolic group to form phenolates (Figure 4), which is also shown by Wanko et al for nitrophenolates[48]. In summary, APTES forms distinct complexes with TNT, different from those with DNT, NP, and TNP. The redcolored APTES-TNT complex can be utilized for both selective and sensitive detection of TNT. APTES-based paper sensor for TNT It is known that a primary amine can form a Meisenheimer complex with the electrondeficient TNT[15, 45, 47]. APTES was chosen because it may have a tight interaction with a fiber glass filter paper on one hand, and forms a Meisenheimer complex with TNT on the other[11, 14, 41]. Soaking the filter paper in 50% APTES for 4 hours produces the paper sensor. The APTES-coated paper must be dried and stored under nitrogen atmosphere since APTES is not stable under oxygen atmosphere due to potential air-oxidation of the amino group.
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TNT solutions in toluene (1 µL) at concentrations of 1, 5, 10, 25, 50, 100, 250, 500, 1000, and 2000 µM were spotted on the APTES-coated paper. As can be seen in Figure 5 (Left), a red-colored spot is formed immediately for each of the spot of TNT. The color intensity increases as the concentration of TNT increases. Based on what can be observed on the filter paper, a red color can be seen with TNT concentration as low as 1 µM. In Figure 5 (Right), the other three nitroaromatics (5 mM) were also spotted along with TNT. While DNT does not produce any color, both phenolic nitroaromatics (TNP and NP) produced a strong yellow color. This shows that this sensor is selective for TNT. To quantify the color intensity of the TNT-APTES complex on the filter paper, these color spots were Sens Biosensing Res. Author manuscript; available in PMC 2016 September 01.
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scanned with a scanner and quantified by Adobe Photoshop. After subtracting a common background, the thus obtained intensity of the spots is plotted versus the concentration of TNT (Figure 6). The color intensity of the APTES-TNT complex increases with the TNT concentration and levels off at about 2.5 mM of TNT. Triplicate of experiments were conducted to obtain the average of the color intensity. In conclusion, TNT forms a red-colored Meisenheimer complex with APTES on filter paper or in solution with two absorption bands at 530 nm and 650 nm. For the three other TNTlike molecules, DNT does not form a colored complex, while TNP and NP form yellowcolored acid-base type complexes. The difference in the colored complexes formed between APTES and nitroaromatics can be utilized to selectively detect TNT using the APTES coated paper with the lowest detectable concentration of 1 µM TNT.
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Acknowledgments This research was supported in part by NSF through the Partnership for Research and Education in Materials grant (DMR-0611539). Core research facilities were supported by grants from the NSF (CHE-0840450) and NIH (NCRR 8G12MD007581).
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Figure 1.
(A: Top) UV-Vis absorbance spectrum of TNT in toluene (100 µM); (B: Bottom) UVV-is absorbance spectral change when TNT (0, 25, 50, 100, 150, 200, 250, 300, 350, 400, 500 µM) is titrated into APTES toluene solution (10 mM) (Inset: Absorbance change at 650 nm with the increase of TNT concentration).
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Figure 2.
Absorption spectra of DNT (top), TNP (middle), and NP (bottom) in toluene (left) and when added to 10 mM APTES (right) in toluene.
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(A) 1H-NMR of APTES; (B) 1H-NMR of TNT; (C) APTES and TNT mixture (3:1 mole ratio) in acetone-d6.
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Figure 4.
Complex formation between a nitroaromatic compound and the primary amine of the APTES (R-NH2).
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Figure 5.
(A) APTES coated fiber glass filter paper spotted with TNT at concentrations of 1, 5, 10, 25, 50, 100, 250, 500, 1000, 2000 µM. (B) Spotting with 5 mM of TNT (red spot) and other nitroaromatics (TNP: 2,4,6-trinitrophenol, DNT: 2,4-dinitrotoluene, and NP: 4-nitrophenol).
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Color intensity of the TNT-APTES complex on the filter paper versus the concentration of TNT.
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