Towards the fluorogenic detection of peroxide explosives through host–guest chemistry

Two dansyl-modified β-cyclodextrin derivatives (1 and 2) have been synthesized as host–guest sensory systems for the direct fluorescent detection of the peroxide explosives diacetone diperoxide (DADP) and triacetone triperoxide (TATP) in aqueous media. The sensing is based on the displacement of the dansyl moiety from the cavity of the cyclodextrin by the peroxide guest resulting in a decrease of the intensity of the fluorescence of the dye. Both systems showed similar fluorescent responses and were more sensitive towards TATP than DADP.


Synthesis of the peroxide explosives TATP
In a 50 mL round bottom flask equipped with a magnetic stirring bar, 1 ml of acetone (13.5 mmol) was dissolved in 1.16 mL of hydrogen peroxide (32% v/v, 13.5 mmol). The reaction mixture was cooled to 0 ° C and 0.23 mL of conc HNO3 was slowly added. The resulting solution was stirred at room temperature for 24 h. The resultant precipitate was isolated by filtration, washed with deionized H2O and dried, yielding white crystals (0.52 g, 52%). The 1 H-NMR spectrum showed also the presence of a small amount of DADP (< 5 %) 1 H NMR (300 MHz, CDCl3) δ 1.46 (s, 18H).

DADP 1
Inside a 50 mL round bottom flask equipped with a magnetic stirrer 0.5 g (2.25 mmol) of TATP were dissolved in 10 mL of dry dichloromethane and a catalytic amount of p-toluenesulfonic acid was added. The reaction mixture was stirred at room temperature for one week. Then the solvent was removed under a continuous flow of argon. The solid was redissolved in CH2Cl2 and the mixture was washed with cold H2O to remove the p-toluenesulfonic acid. Once isolated the solvent again removed with a continuous stream of argon to give a white solid (132 mg, 39% yield).
The 1 H-NMR spectrum showed also the presence of a small amount of TATP (DADP/TATP 95: 5)

Synthesis of the sensors
Mono-6-deoxy-6-tosyl-β-cyclodextrin (3). 2 In a 250 mL round bottom flask provided with a magnetic stirrer β-CD (7.0 g, 6.2 mmol) was dissolved in 75 mL of H2O. To the resulting whitish dispersion NaOH (2.5 g, 63 mmol) was added. The dispersion became clear gradually. After 0.5 h, 1-(p-toluenesulfonyl)imidazole (1.5 g, 6.8 mmol) was added and the reaction mixture was stirred at room temperature for 1 h. After this time the mixture was acidified with conc hydrochloric acid until pH = 5 and allowed to precipitate in the refrigerator overnight. The obtained solid was filtered under vacuum and washed with hot water, cold water and acetone respectively. Finally it was dried under vacuum to yield 2.4 g (30% yield) of a white powder. Mono-6-deoxy-6-azido-β-cyclodextrin (4). 3 In a 250 mL round bottom flask equipped with a magnetic stirring bar were dissolved 0.813 g of 3 (0.63 mmol) and 0.205 g of NaN3 (3.15 mmol) in 150 mL of H2O and the mixture was left overnight at 86 °C. The reaction mixture was concentrated to half volume. Then 9.5 mL of 1,1,2,2-tetrachloroethane were added and the mixture was stirred for 10 min. Once the complex was formed it was separated by centrifugation (144 rpm, 10 min). The precipitate was filtered under vacuum. To remove 1,1,2,2-tetrachloroethane the mixture was dissolved in water and heated to 50 ºC for 1 h. 1,1,2,2-tetrachloroethane was removed by pipette and the resulting solution was solution was dried in vacuo to yield 4 (0.4 g, 0.44% yield).

N-(but-3-yn-1-yl)-5-(dimethylamino)naphthalene-1-sulfonamide (5)
1-Amino-3-butyne (0.2 mL, 2.4 mmol) and triethylamine (0.33 mL, 2.4 mmol) were dissolved in 10 mL of dry DCM in a 50 mL round bottom flask equipped with magnetic stirrer. A solution of dansyl chloride (0.54 g, 2 mmol) in dry CH2Cl2 (10 mL) was added through a compensated addition funnel. The reaction mixture was left for 3 h in an ice-water bath under argon and then 3 days with vigorous stirring at room temperature. After this time the reaction mixture was washed with 35 ml of 1M NaOH and then twice with 70 mL of deionized water. It was dried with MgSO4 and the solvent was removed under vacuum. The crude product was purified by column chromatography using silica gel as stationary phase and as eluent a mixture of CH2Cl2: MeOH 95: 5 to obtain 5 (380 mg, 63% yield). In a 50 mL round bottom flask equipped with magnetic bar stirring were dissolved 0.25 mL (1.8 mmol) of 4-pentyne-1-amine and 0.25 ml (1.8 mmol) of triethylamine in 10 mL dry DCM. To this solution was added, through a compensated addition funnel, 0.404 g (1.5 mmol) of dansyl chloride dissolved in 10 mL dry CH2Cl2. The reaction mixture was left for 3 h at 0 °C in an icewater bath under argon and then 3 days with vigorous stirring at room temperature. After this time the reaction mixture was washed with 35 mL of 1M NaOH and then twice with 70 mL of deionized water. It was dried with MgSO4 and the solvent removed under vacuum. Product purification was performed by column chromatography using silica gel as stationary phase and as eluent a mixture of CH2Cl2: MeOH 95: 5 to obtain 357.6 g (75% yield). Finally, 182 mg (0.92mmol) of sodium ascorbate dissolved in 5 mL of H2O were added. The mixture was allowed to react for two days. After this time, the solvent mixture was removed. Once dry, it was dissolved in 30 mL of 8% aqueous NH3 to remove the copper ion. After three days the mixture was subjected to column chromatography (MeCN:H2O 7:3) and the fractions containing the product were combined. The solvent was removed and the crude was treated with ethyl acetate to remove excess fluorophore and finally centrifuged. The solid was washed with ethyl acetate and dried in vacuum to yield 1 (0.114 g, 17% yield).

Sensor 2
In a 50 mL round bottom flask of two-necked 94.83 mg (0.3 mmol) of 6 and 250 mg (0.2 mmol) of 4 were dissolved in 20 mL of DMSO. Once cyclodextrin was dissolved 59.8 mg (0.3 mmol) copper acetate was added by dragging it with 5 mL of DMSO up to a total reaction volume of 25 mL. Finally, 118.86 mg (0.6 mmol) of sodium ascorbate dissolved in 5 mL of H2O were added. The mixture was allowed to react for two days. After this time the solvent mixture was removed. Once dry, it was dissolved in 30 mL of a solution of NH3 8% and after three days a Cu complex was formed. After this time was performed a column with MeCN:H2O 7:3 and the fractions containing the product were combined, the solvent was removed and then redissolved in ethyl acetate to remove excess fluorophore observed in the 1H-NMR and finally centrifuged. The solid is washed with ethyl acetate and dried in vacuum obtaining 68 mg (23% yield).

Spectroscopic studies
All the measurements were carried out with a initial concentration of the sensor (1 or 2) of·10 -6 M in H2O:MeOH (95:5) solution. The measuring range was 400 nm to 700 nm, the excitation wavelength was 340 nm and the slits of excitation/ emission were 5/5 respectively. Both peroxide explosives were added dissolved in MeOH (5 x 10 -4 M).
In a typical titration experiment, 2.5 mL of sensor 1 (10 -6 M in H2O/MeOH, 95:5) was placed inside a 1 cm path length quartz cuvette. Then 5 µL of TATP (5 x 10 -4 M in MeOH) were added to the sensor, the mixture was heated at 40 ºC for 10 min and the fluorescence emission was recorded. The titration was performed by successive additions of 5 µL aliquots of the peroxide explosive following the same protocol. The total increase in the volume of the mixture was less than 3 %. In the preliminary experiments at room temperature, the mixture of sensor and peroxide explosive were kept 1 min at room temperature before measuring the fluorescence.