Fluorescent sensors for nitroaromatic compounds based on monolayer assembly of polycyclic aromatics

An aggregation-induced emission-active bis-heteroleptic ruthenium(ii) complex of thiophenyl substituted phenanthroline for the selective “turn-off” detection of picric acid

A bis-heteroleptic Ru(ii) polypyridine complex-based AIEgen has been developed for the selective detection of nitroaromatic explosive picric acid in aqueous media.

An AIE-Active Ultrathin Polymeric Self-Assembled Monolayer Sensor for Trace Volatile Explosive Detection

This work has prepared polymeric self-assembled monolayer (SAM) sensors for the detection of trace volatile nitroaromatic compound (NAC) explosives by fluorescence quenching. A typical aggregation-induced emission (AIE) luminogen 1,1,2,2-tetraphenylethene (TPE) polymerizes into PTPE to increase the fluorescence intensity in the SAMs, and the phosphoric acid acts as the anchor group to form stable covalent bonds with the Al₂ O₃ substrate. This design takes advantage of the high sensitivity and good stability of SAMs, and high fluorescence intensity, and "wire effect" of the conjugated polymers. The polymeric SAM sensors are prepared on the Al₂ O₃ silicon wafer and testing paper. Both of them show good response speed, reversibility, selectivity, and sensitivity. The detection limits down to 0.07, 0.35, and 4.11 ppm for TNT, DNB, and NB, respectively, are achieved on the inorganic testing paper. Furthermore, due to the higher fluorescence intensity by interlacing and overlapping of fibers, the detection of the paper can be distinguished by naked eyes even with a low-power handheld UV lamp, which provides an experimental basis for the development of cheap and easy trace NAC explosive sensors.

Design of an AIE-Active Flexible Self-Assembled Monolayer Probe for Trace Nitroaromatic Compound Explosive Detection

In this work, a series of molecules TPE-PA-n (n = 3-11) were designed with classic aggregation-induced emission (AIE) 1,1,2,2-tetraphenylethene (TPE) for self-assembled monolayers (SAMs), which are applied for the detection of trace nitroaromatic compound (NAC) explosives. Phosphoric acid that acts as an anchor is used to connect with TPE through alkyl chains of various lengths. It is found that the alkyl chains play a role in pulling TPE luminogens to aggregate for light emission, which can affect the fluorescence and sensing performance of the SAMs. Ulteriorly, a model is built to explore the influence of the alkyl chain length on the device performance, which is determined by the three effects of the alkyl chain: flexibility, the coupling effect, and the odd-even effect. By comparison, the functional molecules with the chain length of 8 were finally selected and further applied for NAC sensors. By means of fluorescence spectra, the SAM sensor was proved to have good stability, reversibility, selectivity, and sensitivity, and its detection limits for trinitrotoluene, dinitrotoluene, and nitrobenzene were 1.2, 6.0, and 35.7 ppm, respectively. This work provides new ideas for the design and preparation of flexible sensors for trace NAC detection with high performance, low cost, and easy operation.

A Formaldehyde Sensor Based on Self-Assembled Monolayers of Oxidized Thiophene Derivatives

High-performance formaldehyde sensors play an important role in air quality assessment. Herein, a self-assembled monolayer (SAM) sensor for trace formaldehyde (FA) is fabricated based on the fluorescence enhancement of oxidized thiophene derivatives. In the primary SAM molecules, the functional backbone trithiophene (3T) links to the anchor through an n-propyl group. The anchor with an active Si-Cl bond can form a covalent bond with the SiO₂ substrate by solution incubation, which ensures good stability against organic solvents and high sensitivity via monolayer structures. With the alkyl chain's leading, a dense 3T SAM can be obtained on SiO₂. Upon exposure to UV light in the presence of oxygen, 3T can be oxidized into a nonfluorescent but coordination-active product with abundant carbonyl groups, which can be doped with FA and induce a blueshifted fluorescence. With this mechanism, we proposed an SAM-based FA sensor by detecting the enhancement of the blueshifted fluorescence. Reliable reversibility, selectivity, stability, and detection limit lower than 1 ppm are achieved in this system. The work provides an experimental basis for developing a cheap, efficient, and flexible sensor for trace FA detection.

1,3-Di-naphthalimide Conjugate of Calix[4]arene as a Sensitive and Selective Sensor for Trinitrophenol and This Turns Reversible when Hybridized with Carrageenan as Beads

A fluorescent naphthalimide conjugate of calix[4]arene (L₁ ) has been synthesized and characterized. The selective and efficient detection of trinitrophenol (TNP) by L₁ among nine other different nitroaromatic compounds was demonstrated using absorption and fluorescence spectroscopy. The minimum detection limit is 29 nM, which is the lowest reported so far by any conjugate of calixarene toward TNP. The fluorescence quenching is associated with a high Stern-Volmer constant of 3.3 ± 0.4 × 10⁵ M^-1. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) data revealed a network structure with pores having a weighted average size of 0.66 ± 0.08 μm for L₁ . When incubated with TNP, the pores were filled with fibril structures, as supported by both SEM and TEM data. In order to demonstrate the real time applications, the L₁ has been coated onto a Whatman filter paper and the imprint of TNP contaminated thumb has been detected upon physical contact. The ¹HNMR titration and the studies carried out using the control molecule support the necessity of both the naphthalimide moiety and the calixarene platform for sensing. In order to mend L₁ as a reversible sensor for TNP, the same is incorporated into carrageenan beads (L₁ @C_b ) and the reversible sensing has been shown for three cycles by reusing the same material upon recovery followed by washing it. The solid-state detection of TNP has also been demonstrated using the lyophilized L₁ @C_b bead powder. The fluorescence intensity of L₁ was quenched upon addition of solid TNP to the lyophilized bead powder of L₁ @C_b as studied by fluorescence microscopy. The computational studies show that one of the arms of the calixarene takes a bent conformation, and the 1:1 TNP complex of L₁ is stabilized by exhibiting differential extents of hydrogen bonding interactions with the two arms owing to their conformational difference. The result of such complexation was already felt through the shifts observed in the experimentally measured ¹HNMR spectra.

Active Self-Assembled Monolayer Sensors for Trace Explosive Detection

Trace explosives can be detected with the help of a portable device using a flexible active self-assembled monolayer (SAM). 9,10-diphenyl anthracene with aggregation-induced emission enhancement (AIEE) properties is selected as the fluorophore. Phosphoric acid as the anchor group is linked to the fluorophore through an alkyl chain and able to self-assemble into a dense monolayer on the HfO₂ adhesion layer on a flexible substrate. The dense SAMs show high fluorescence intensity, which can be quenched by nitroaromatic compounds (NACs), and have advantages of high response rate, sensitivity, reversibility, and selectivity.

An adjustable dual-emission fluorescent metal-organic framework: Effective detection of multiple metal ions, nitro-based molecules and DMA

A novel multifunctional Pb(II)-based MOF, [Pb_1.5(DBPT)]₂·(DMA)₃(H₂O)₄ (1), with excellent chemical stability, was successfully assembled by connecting {Pb₂O₁₀} unit with a multi-topic polycarboxylate ligand of 3-(3,5-dicarboxylphenyl)-5-(4-carboxylphenyl)-1-H-1,2,4-triazole (H₃DBPT). It exhibits dual fluorescence emissions at 380 nm (λ_ex = 280 nm) and 540 nm (λ_ex = 380 nm), respectively. Through the adjustable dual fluorescence emissions, it could act as a turn-off and turn-on switch for detecting N,N-dimethylacetamide (DMA) molecule. Moreover, Fe³⁺ ions exert luminescence quenching role on compound 1 at both excitation lengths in water, among which the quenching at λ_ex = 280 nm is of high sensitivity (K_SV = 1.2 × 10⁵), and the quenching at λ_ex = 380 nm is of wide-range. The sensing for metal ions of In³⁺, Zr⁴⁺, and Al³⁺ is also effective at λ_ex = 280 nm, with the K_SV constants of 1.6 × 10⁵, 1.6 × 10⁵, and 4.3 × 10⁴, respectively. More importantly, a series of nitroaromatic compounds (TNP, 2,4,6-trinitrophenol; 4-NA, 4-nitroaniline; NB, nitrobenzene) and nitro-based drugs (MNZ, metronidazole; DMZ, dimetridazole) could be detected at both excitation lengths, demonstrating the advantage of broad range response of fluorescence sensing. Thanks to the excellent chemical stability and unusual dual emission luminescence properties for chemical detection of various metal ions, nitro-based molecules and DMA solvent, the Pb-based MOF reported in this work is, therefore, a very promising multi-response sensor.

A reusable fluorescent sensor from electrosynthesized water-soluble oligo(1-pyrenesulfonic acid) for effective detection of Fe3+

Electrosynthesized oligo(1-pyrenesulfonic acid) displays high selectivity, low detection limit and outstanding reversibility in the detection of Fe³⁺ in aqueous solution.

Ormosil-coated conjugated polymers for the detection of explosives in aqueous environments

A fluorescence-based sensor for detecting explosives, based on a conjugated polymer coated with an ormosil layer, has been developed for use in aqueous environments. The conjugated polymer Super Yellow was spin-coated onto glass substrates prior to a further spin-coating of an MTEOS/TFP-TMOS-based ormosil film, giving an inexpensive, solution-based barrier material for ruggedization of the polymer to an aqueous environment. The sensors showed good sensitivity to 2,4-DNT in the aqueous phase at micromolar and millimolar concentrations, and also showed good recovery of fluorescence when the explosive was removed.

Selective Sensing of Metal Ions and Nitro Explosives by Efficient Switching of Excimer-to-Monomer Emission of an Amphiphilic Pyrene Derivative

An amphiphilic pyrene derivative exhibiting unusually stable excimer emission due to strong aggregation is presented. The aggregated system served as an intelligent sensor for metal ions and nitro explosives in aqueous media. The excimer displayed excellent selectivity toward Cu²⁺ among the tested cations. The observation was interpreted on the basis of chelation of metal ions involving the hydroxyl and amino groups of two molecules, leading to the ligand-to-metal charge-transfer (CT) process. The excimer was further applied for the cell imaging of Cu²⁺ ions. Also, while treating the excimer with various nitro explosives, it displayed efficient 2,4,6-trinitrophenol sensing, corroborating mainly the CT process from pyrene to the analyte due to intercalation of the analyte within pyrene.

Detection of nitroaromatic explosives using π-electron rich luminescent polymeric nanocomposites

Novel electron rich luminescent nanocomposite polymers for sensing nitroaromatic explosives by PET and FRET phenomenon.

Fluorescent diphenylfluorene-pyrenyl copolymer with dibenzothiophene-S,S-dioxide and adamantane units for explosive vapor detection

This proposed strategy of incorporating SO units can be used as a promising approach to the development of fluorescent conjugated sensing materials.

Multifunctional Benzothiadiazole-Based Small Molecules Displaying Solvatochromism and Sensing Properties toward Nitroarenes, Anions, and Cations

Aryl or heteroaryl 5-substituted imidazo-benzothiadiazole derivatives were synthesized and shown to display remarkable solvatofluorochromism and selectively sense mercury(II) cations, acetate anions, and nitroaromatic derivatives, with discrimination between p-nitrophenol and picric acid. These novel sensors are of importance these days, as the detection of explosives is a high priority in issues of national security and environmental protection. To determine the ion binding properties of the sensors, their absorption and fluorescence emission spectra upon binding different cations and anions were compared. Significant shifts in the spectra were only observed for mercury(II) and acetate. The binding of these two ions was further studied using (1)H NMR. The binding properties of different nitroaromatic compounds were also determined, and the results showed the importance of the presence of a phenol group in the guest molecule. Specifically, the two sensors were shown to discriminate between p-nitrophenol and picric acid. Finally, the mechanism of fluorescence quenching upon addition of nitrophenols was determined by computational methods.

APTS and rGO co-functionalized pyrenated fluorescent nanonets for representative vapor phase nitroaromatic explosive detection

For the first time, flexible PVP/pyrene/APTS/rGO fluorescent nanonets were designed and synthesized via a one-step electrospinning method to detect representative subsaturated nitroaromatic explosive vapor. The functional fluorescent nanonets, which were highly stable in air, showed an 81% quenching efficiency towards TNT vapor (∼10 ppb) with an exposure time of 540 s at room temperature. The nice performance of the nanonets was ascribed to the synergistic effects induced by the specific adsorption properties of APTS, the fast charge transfer properties and the effective π-π interaction with pyrene and TNT of rGO. Compared to the analogues of TNT, the PVP/pyrene/APTS/rGO nanonets showed notable selectivity towards TNT and DNT vapors. The explored functionalization method opens up brand new insight into sensitive and selective detection of vapor phase nitroaromatic explosives.

Synthesis, optical properties and explosive sensing performances of a series of novel π-conjugated aromatic end-capped oligothiophenes

Four novel terthiophene (3T) derivatives, have been synthesized by employing Grignard coupling reaction via end-capping of naphthyl (NA) or pyrenyl (Py) unit to the one or two ends of 3T. It has been shown that both increasing electron donating strength and extending conjugation are effective approaches to improve the photochemical stability of the oligothiophene. Fluorescence studies demonstrated that the emission of the 3T derivatives is sensitive to the presence of some important nitro-containing explosives in their ethanol solution, in particular, 2,4,6-trinitrophenol (PA) and 3,5-dinitro-2,6-bispicrylamino pyridine (PYX). As an example, the detection limits of 4 to PA and PYX were determined to be 6.21 × 10(-7)mol/L and 8.95 × 10(-7)mol/L, respectively. Based on the discovery, a colorimetric detection method has been developed. The sensitive and selective response of the modified 3T to the explosives have been tentatively attributed to the adsorptive affinity of the compounds to the explosives, and to the higher probability of the electron transfer from the electron-rich 3T derivatives to the electron-poor nitro-containing explosives. No doubt, present study broadens the family of fluorophores which may be employed for the development of fluorescent sensors.

Fabrication of a novel cholic acid modified OPE-based fluorescent film and its sensing performances to inorganic acids in acetone

A self-assembled monolayer (SAM)-based fluorescent film was designed and prepared by chemical immobilization of a novel oligo(p-phenylene- ethynylene) (OPE) with cholic acid moieties at the ends of its side chains (Film 1). As a control, a similar film, Film 2, of which OPE brings no side chains, was also prepared. The structures of the films were characterized by contact angle, XPS, ATR-IR and fluorescence measurements. Fluorescence studies revealed that the emission of Film 1 is sensitive to the presence of trace amount of some inorganic acids in acetone, such as HCl, H(2)SO(4), HNO(3), and H(3)PO(4), etc., whereas the acids as studied showed little effect on the emission of Film 2. The difference in the sensing performances of the two films have been rationalized by considering presence or absence of a possible cavity, a substructure appearing above the OPE adlayer which is something like a dimer of cholic acid (CholA) formed at specific environment.

Fluorescent film sensors based on SAMs of pyrene derivatives for detecting nitroaromatics in aqueous solutions

The detection of nitroaromatics in aqueous solutions by a novel pyrene-functionalized film has been investigated in the present study. The pyrene moieties were attached on the glass surface via a long flexible spacer based on self-assembled monolayer technique. Steady-state fluorescence measurements revealed that these surface-attached pyrene moieties exhibited both monomer and excimer emission. Nitroaromatics such as 2,4,6-trinitrotoluene, 2,4-dinitrotoluene, and 2,4,6-trinitrophenol (picric acid) were found to efficiently quench the fluorescence emission of this film. The quenching results demonstrated that the excimer emission of these surface-confined pyrene moieties is more sensitive to the presence of nitroaromatics than the monomer emission. The quenching mechanism was examined through fluorescence lifetime measurement and it revealed that the quenching is static in nature and may be caused by electron transfer from the polycyclic aromatics to the nitroaromatics. Furthermore, the response of the film to nitroaromatics is fast and reversible, and the obtained film shows promising potentials in detecting explosives in aqueous environment.

Functionalization of amorphous SiO₂ and 6H-SiC(0001) surfaces with benzo[ghi]perylene-1,2-dicarboxylic anhydride via an APTES linker

The successful covalent functionalization of quartz and n-type 6H-SiC with organosilanes and benzo[ghi]perylene-1,2-dicarboxylic dye is demonstrated. In particular, wet-chemically processed self-assembled layers of aminopropyltriethoxysilane (APTES) and benzo[ghi]perylene-1,2-dicarboxylic anhydride are investigated. The structural and chemical properties of these layers are studied by contact angle measurements, attenuated total reflection infrared (ATR-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The optical properties are measured by confocal microscopy. The wetting angles observed for the organic layers are α = 68° for the APTES-functionalized surface, while angles of α = 85° and 78° are determined for dye-functionalized quartz and 6H-SiC surfaces, respectively. However, not all amino groups of the APTES-functionalized surfaces react to bind dye molecules. Further dye functionalization is not uniform throughout the surface, showing different island sizes of the dye and including different chemical environments. The quartz surface exhibits a higher packing density of dyes than the 6H-SiC surface. The fluorescence lifetimes of the surface-attached dye show double exponential decays of about 1.4 and 4.2 ns, largely independent of the substrates.

Optical chemosensors and reagents to detect explosives

This critical review is focused on examples reported from 1947 to 2010 related to the design of chromo-fluorogenic chemosensors and reagents for explosives (141 references).

Fluorescent film sensor for copper ion based on an assembled monolayer of pyrene moieties

We previously reported the construction of a family of fluorescent film sensors for organic copper salts by covalently coupling polycyclic aromatic hydrocarbons on epoxy-terminated self-assembled monolayers on glass plate surfaces. Here we investigate the sensing properties and mechanism of covalently coupling pyrene on a glass plate surface via a long flexible "Y" type spacer. X-ray photoelectron spectroscopy (XPS) and fluorescence spectra measurements demonstrate the covalent attachment of pyrene in our adlayer. Compared with those results obtained in the previous studies, this new film sensor did not show highly selectivity for organic copper salts, which can be attributed to the introduction of sulfonyl groups connecting the pyrene moieties and the spacers. The presence of sulfonyl units made the microenvironments of pyrene relatively hydrophilic and thus showed less screening effect for inorganic ions. The specificity and reversibility of the film sensor toward Cu (II) made it attractive for sensing applications.

Photochemical stabilization of terthiophene and its utilization as a new sensing element in the fabrication of monolayer-chemistry-based fluorescent sensing films

To improve the photochemical stability of α-terthiophene (3T) in air, we purposely introduced a naphthalene unit into its conjugated backbone, resulting in a fluorescent compound, 5-(1-naphthyl)-2,2':5',2''-terthiophene (NA-3T). The compound was further employed as a sensing element for the fabrication of a monolayer-chemistry based fluorescent sensing film. It was demonstrated that the fabricated film is highly sensitive and selective to the presence of picric acid (PA). The detection limit was found to be 3.2 × 10(-7) mol/L. The high sensitivity of the film to PA has been attributed to the specific binding of the film to the analyte because of proton transfer from PA to the amino group in the spacer, which is in accordance with the static nature of the quenching as revealed by fluorescence lifetime measurements. Further experiments demonstrated that the sensing process is fully reversible and free of interference from common organic solvents, acids and bases, etc. In addition, the film is stable, at least, within half a year provided it is properly preserved. More importantly, the present work makes it possible to use oligothiophenes as a new class of sensing elements, which may combine the advantages of conjugated polymers or oligomers and those of fluorescent compounds of low-molecular masses. This effort enlarges, definitely, the applications of oligothiophenes and the space for creating monolayer-chemistry based fluorescent sensing films.