Jason C. Sanchez

Efficient blue-emitting silafluorene–fluorene-conjugated copolymers: selective turn-off/turn-on detection of explosives

The synthesis and spectroscopic characterization of a series of new blue-emitting silafluorene–fluorene copolymers is described. The polymers are synthesized using kinetically controlled hydrosilylation copolymerization of 1,1-dihydridosilafluorene with a series of 9-substituted 2,7-diethynylfluorenes. The polymers contain a trans-only framework with molecular weights in the range of 13 000–20 000, as determined by gel permeation chromatography (GPC) using polystyrene standards, and by 1H NMR spectroscopy using dimethylphenylsilane as an end-capping marker group. The three stereoregular polymers synthesized include a 9,9-dihydridofluorene (PSF1), a 9,9-dimethyl-9H-fluorene (PSF2), and a 9,9′-spirobifluorene (PSF3) comonomer with the frameworks. These fluorenyl units are conjugated through the silicon center of the silafluorene moiety by bridging vinylene groups. Quantum yields of fluorescence range from 20 to 100% with PSF3 having the highest quantum efficiency. Polymers PSF1-3 emit in the blue region of the spectrum (∼475 nm), showing good color purity with little change in luminescence properties between the solution and solid-state phases. The polymers were tested for explosives detection properties by a fluorescence-quenching mechanism. Targeted explosives include laboratory prepared TNT, DNT, picric acid, RDX, HMX, PETN, TNG, and Tetryl, as well as production line PETN and C-4. All three polymers exhibit detection of explosive particulates with limits as low as 1 pg cm−2 for Tetryl. Polymer PSF1 simultaneously acts as a selective fluorescence “turn-on” sensor for nitrate ester explosives when irradiated with UV light. In the presence of nitrate ester-based explosives such as PETN, PSF1 initially exhibits fluorescence quenching, but continued exposure to UV-light (302 nm), promotes a photochemical reaction forming a luminescent green fluorenone copolymer. This is the first example of a single material acting as both a turn-off and turn-on selective fluorescent sensor for an explosive material.

Visual detection of trace nitroaromatic explosive residue using photoluminescent metallole-containing polymers.

ABSTRACT: The detection of trace explosives is important for forensic, military, and homeland security applications. Detection of widely used nitroaromatic explosives (trinitrotoluene [TNT], 2,4‐dinitrotoluene [DNT], picric acid [PA]) was carried out using photoluminescent metallole‐containing polymers. The method of detection is through the quenching of fluorescence of thin films of the polymer, prepared by spray coating organic solutions of the polymer, by the explosive analyte. Visual quenching of luminescence (λem≈400–510 nm) in the presence of the explosive is seen immediately upon illumination with near‐UV light (λex=360 nm). Detection limits were observed to be as low as 5 ng for TNT, 20 ng for DNT, and 5 ng for PA. In addition, experiments with normal production line explosives and their components show that this technology is also able to detect composition B, Pyrodex®, and nitromethane. This method offers a convenient and sensitive method of detection of trace nitroaromatic explosive residue.

Polymerization of a boronate-functionalized fluorophore by double transesterification: applications to fluorescence detection of hydrogen peroxide vapor

The double transesterification polymerization of 3′,6′-bis(pinacolatoboron)fluoran and pentaerythritol is reported. A model dimeric compound was synthesized to demonstrate the effectiveness of bis-diols to undergo a double transesterification, which is driven by formation of the energetically favored six-membered di-ester ring from a monomer containing a five-membered di-ester ring. This synthetic procedure provides a new route to boronate based polymers, avoiding unstable boronic acid monomers. Formation of poly-3′,6′-bis(1,3,2-dioxaborinane)fluoran, with a molecular weight of 10 000, is complete after 48 h at 50 °C. The thermodynamic stability of the six-membered boronic ester rings present in the polymer backbone also improves the stability of the polymer and its resistance to oxidation under ambient and UV light conditions. A surface detection method for the analysis of H2O2 vapor by a fluorescence turn-on response was explored. The fluorescent response results from oxidative deprotection of the boronate functionalities forming green luminescent fluorescein. Detection limits as low as 3 ppb were observed for H2O2 over an 8 h period. Detection of H2O2 in liquids can also be carried out through spot tests at concentrations as low as 1 ppm after 5 min. This new vapor-phase sensor for H2O2 provides a robust, low-cost alternative to current technology for potential applications as a self-integrating sensor for the detection of H2O2 as well as the direct monitoring of H2O2 levels in areas such as cargo shipments, chemical facilities, and pulp bleaching.

Selective detection of trace nitroaromatic, nitramine, and nitrate ester explosive residues using a three-step fluorimetric sensing process: a tandem turn-off, turn-on sensor

Abstract:  Detection of trace quantities of explosive residues plays a key role in military, civilian, and counter‐terrorism applications. To advance explosives sensor technology, current methods will need to become cheaper and portable while maintaining sensitivity and selectivity. The detection of common explosives including trinitrotoluene (TNT), cyclotrimethylenetrinitramine, cyclotetramethylene‐tetranitramine, pentaerythritol tetranitrate, 2,4,6‐trinitrophenyl‐N‐methylnitramine, and trinitroglycerin may be carried out using a three‐step process combining “turn‐off” and “turn‐on” fluorimetric sensing. This process first detects nitroaromatic explosives by their quenching of green luminescence of polymetalloles (λem ≈ 400–510 nm). The second step places down a thin film of 2,3‐diaminonaphthalene (DAN) while “erasing” the polymetallole luminescence. The final step completes the reaction of the nitramines and/or nitrate esters with DAN resulting in the formation of a blue luminescent traizole complex (λem = 450 nm) providing a “turn‐on” response for nitramine and nitrate ester‐based explosives. Detection limits as low as 2 ng are observed. Solid‐state detection of production line explosives demonstrates the applicability of this method to real world situations. This method offers a sensitive and selective detection process for a diverse group of the most common high explosives used in military and terrorist applications today.