Aggregation-Induced Emission of Quinoline Based Fluorescent and Colorimetric Sensors for Rapid Detection of Fe3+ and 4-Nitrophenol in Aqueous Medium

New quinoline based fluorescent sensors 4 and 5 were rationally synthesized that exhibited excellent aggregation induced emission (AIE) in an aqueous medium. High fluorescence emission of sensors was accompanied by a noticeable redshift in their absorption and emission spectra that corresponds to the formation of J-aggregates. An AIE feature of sensors 4 and 5 was used for selective detection of Fe³⁺ and 4-NP in an aqueous medium that is attributed to the involvement of intermolecular charge transfer (ICT). The interaction mechanism of sensors with Fe³⁺ and 4-NP was investigated through ¹H NMR titration, Jobs plots, dynamic light scattering (DLS), and DFT analysis. The fluorescence quenching response of sensors 4 and 5 displayed distinguished linear behavior with the concentrations of Fe³⁺ and limits of detection (LOD) were calculated to be 15 and 10 nM, respectively. Further, LOD of sensors 4 and 5 for 4-NP (7.3 and 4.1 nM, respectively) was very low compared to previously reported sensors. Moreover, sensors' coated test strips were fabricated for solid-supported detection of Fe³⁺ and 4-NP. Sensors were successfully applied for the detection and quantification of Fe³⁺ and 4-NP in real water samples. Additionally, sensors were used for the determination of trace amounts of Fe³⁺ in the human serum sample.

Fluorescein based fluorescent and colorimetric sensors for sensitive detection of TNP explosive in aqueous medium: Application of logic gate

Rapid detection of 2,4,6-trinitrophenol (TNP) in real samples has recently attained considerable attention from the perspective of national security, human health, and environmental safety. In this context, cost-effective and convenient detection of TNP explosive was accomplished through two new fluorescein based sensors F2 and F3. Sensors displayed effective fluorescence quenching response towards TNP in the aqueous medium. Highly sensitive fluorescence detection of TNP explosive (detection limit, 0.73 (F2) and 1.7 nM (F3)) was governed by ground-state charge transfer complex formation, facilitated by favorable H-bonding between sensor and TNP explosive. Fluorescence quenching mechanism for the detection of TNP explosive was investigated through UV-Visible absorption, dynamic light scattering (DLS), density functional theory (DFT) calculations, the Benesi-Hildebrand, and Job's plots. Advantageously, sensors displayed selective and immediate colorimetric recognition of TNP explosive. Importantly, sensors exhibited quick response time towards TNP even in the presence of potential interferences that make them highly suitable for practical applications. Sensors were successfully applied for fluorescent and colorimetric detection of TNP explosive in industrial water samples and fabrication of logic gates. Further, convenient contact mode and instant surface sensing of TNP explosive were achieved through the fabrication of fluorescent strips and explosive responsive test kits.

Fluorenone-Based Fluorescent and Colorimetric Sensors for Selective Detection of I- Ions: Applications in HeLa Cell Imaging and Logic Gate

Fluorenone-based fluorescent and colorimetric sensors 1 and 2 have been developed that displayed selective detection of iodide ions in the presence of interferences. Sensors displayed the fluorescence emission enhancement response toward I^- with detection limits of 8.0 and 11.0 nM, respectively, which is accomplished through inhibition of intramolecular charge transfer and C=N isomerization. Excellent sensitivity and unique fluorescence enhancement response of sensors toward I^- make them superior because most of the previously reported iodide sensors are based on the fluorescence quenching mechanism and are less sensitive. The sensing potential of sensors toward I^- ions was investigated through ¹H NMR titration, dynamic light scattering, Job's plots, and density functional theory analysis. Further, sensors displayed reversible behavior by the alternate addition of I^- and Cu²⁺ ions that substantiate their role as recyclable sensors for the on-site detection of I^- ions. Advantageously, fluorescence enhancement response of sensors was favorably used for fluorescence imaging of I^- in live HeLa cells and the design of the logic gate. These sensors were successfully applied in diversified applications such as the preparation of sensors' coated paper strips and the determination of I^- ions in blood serum, food, and real water samples.

Fluorene-Based Fluorometric and Colorimetric Conjugated Polymers for Sensitive Detection of 2,4,6-Trinitrophenol Explosive in Aqueous Medium

Nitroaromatic explosives are a class of compounds that are responsible for various health hazards and terrorist outrages. Among these, sensitive detection of 2,4,6-trinitrophenol (TNP) explosive has always been highly desirable considering public health and national security. In this regard, three fluorene-based conjugated polymers (CP 1, CP 2, and CP 3) were synthesized through the Suzuki-Miyaura coupling reaction and were found to be highly sensitive for fluorescence detection of TNP with detection limits of 3.2, 5.7, and 6.1 pM, respectively. Excellent selectivity of CPs toward TNP was attributed to their unique π-π interactions based on fluorescence studies and density functional theory (DFT) calculations. The high sensitivity of CPs to TNP was attributed to the static quenching mechanism based on the photoinduced electron transfer process and was evaluated by fluorescence, UV-visible absorption, dynamic light scattering, Job's plots, the Benesi-Hildebrand plots, and DFT calculations. CPs were also used for colorimetric and real-water sample analysis for the detection of TNP explosive. Meanwhile, sensor-coated test strips were fabricated for on-site detection of TNP, which makes them convenient solid-supported sensors.

Water-soluble ionic liquid as a fluorescent probe towards distinct binding and detection of 2,4,6-trinitrotoluene and 2,4,6-trinitrophenol in aqueous medium

Owing to the escalating threat of criminal activities and pollution aroused by 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitrophenol (TNP), development of a proficient sensor for the detection of these explosives is highly demanded. Herein, a water-soluble ionic liquid-tagged fluorescent probe, 1-ethyl-3-(3-formyl-4-hydroxybenzyl)-1H-benzimidazol-3-ium chloride (EB-IL) has been designed and synthesized for the detection of TNT and TNP in 100% aqueous medium. The EB-IL fluorescent probe displayed strong cyan-blue fluorescence at 500 nm which gets quenched upon the addition of TNT/TNP over other concomitant nitro-compounds. The distinct binding response of EB-IL towards TNT could be due to the formation of hydrogen bonding between the acidic proton of benzimidazolium (C2-H) and nitro group of TNT. Meanwhile, the selective binding of TNP with EB-IL could be due to the exchange of counter Cl^- anion of EB-IL with picrate anion. The fluorescence quenching of EB-IL by TNT could be attributed to the resonance energy transfer (RET) and that of TNP is ascribed to the anion-exchange process. The developed sensor is extremely selective and sensitive towards TNT and TNP with high quenching constants of 1.94 × 10⁵ M^-1 and 2.32 × 10⁶ M^-1 and shows a lower detection limit of 159 nM and 282 nM, respectively.

A naphthylamide based fluorescent probe for detection of Al3+, Fe3+, and CN- with high sensitivity and selectivity

A naphthylamide based fluorescent chemosensor, N,N'-(1,2-phenylene)bis(1-hydroxy-2-naphthamide) (H₄L), for detection of Fe³⁺ and Al³⁺ cations as well as CN^- anion is reported. This compound has been synthesized by a novel and facile synthetic method with high yield and characterized by FT-IR, ¹H NMR, elemental analysis, and UV-Vis spectroscopy. It could detect Fe³⁺ and Al³⁺ ions in different media with different excitation and emission wavelengths. In DMSO solution, H₄L showed selective ON-OFF quenching of its 451 nm emission in the presence of Fe³⁺. On the other hand, in DMF solution, H₄L exhibited selective OFF-ON fluorescence upon the addition of Al³⁺, the intensity at 429 nm increases drastically by 24-fold. Also, among the anions, the probe can selectively distinguish CN^- by deprotonation of OH and NH groups, as proved by ¹H NMR titration. TD-DFT calculation supports the UV-Vis and fluorescence measurements of the chemosensor.

Characterization of Room-Temperature Ionic Liquids to Study the Electrochemical Activity of Nitro Compounds

Over the past few years, room-temperature ionic liquid (RTIL) has evolved as an important solvent-cum-electrolyte because of its high thermal stability and excellent electrochemical activity. Due to these unique properties, RTILs have been used as a solvent/electrolyte/mediator in many applications. There are many RTILs, which possess good conductivity as well as an optimal electrochemical window, thus enabling their application as a transducer for electrochemical sensors. Nitroaromatics are a class of organic compounds with significant industrial applications; however, due to their excess use, detection is a major concern. The electrochemical performance of a glassy carbon electrode modified with three different RTILs, [EMIM][BF4], [BMIM][BF4] and [EMIM][TF2N], has been evaluated for the sensing of two different nitroaromatic analytes: 2,6-dinitrotoluene (2,6 DNT) and ethylnitrobenzene (ENB). Three RTILs have been chosen such that they have either a common anion or cation amongst them. The sensory response has been measured using square wave voltammetry (SQWV). We found the transducing ability of [EMIM][BF4] to be superior compared to the other two RTILs. A low limit of detection (LOD) of 1 ppm has been achieved with a 95% confidence interval for both the analytes. The efficacy of varying the cationic and anionic species of RTIL to obtain a perfect combination has been thoroughly investigated in this work, which shows a novel selection process of RTILs for specific applications. Moreover, the results obtained from testing with a glassy carbon electrode (GCE) have been replicated using a miniaturized sensor platform that can be deployed easily for on-site sensing applications.

Selective sensing of 2,4,6-trinitrophenol (TNP) in aqueous media with "aggregation-induced emission enhancement" (AIEE)-active iridium(iii) complexes

A series of new phosphorescent cyclometalated iridium(iii) complexes which possess aggregation-induced emission enhancement (AIEE) detect 2,4,6-trinitrophenol (TNP) selectively with high quenching constants in aqueous media. The sensing mechanism was systematically investigated by mass spectrometry, ¹H and ¹⁹F NMR spectroscopy. X-ray crystal structure analysis reveals an O-HO interaction between TNP and the ancillary ligand which explains the high selectivity for TNP compared to other nitro-aromatics.

New fluorescent through-space conjugated polymers: synthesis, optical properties and explosive detection

New through-space conjugated polymers based on a tetraphenylethene foldamer are explored and utilized in explosive detection in aqueous media.

Simple Bisthiocarbonohydrazone as a Sensitive, Selective, Colorimetric, and Ratiometric Fluorescent Chemosensor for Picric Acids

A bisthiocarbonohydrazone-based chemosensor molecule (R1) containing a tetrahydro-8-hydroxyquinolizine-9-carboxaldehyde moiety has been synthesized and characterized as a new ratiometric fluorescent probe for picric acid (PA). The ratiometric probe R1 is a highly selective and sensitive colorimetric chemosensor for PA. The association between the chemosensor and PA and the ratiometric performance enabled by the key role of excited state intramolecular proton transfer in the detection process are demonstrated. Selectivity experiments proved that R1 has excellent selectivity to PA over other nitroaromatic chemicals. Importantly, the ratiometric probe exhibited a noteworthy change in both colorimetric and emission color, and this key feature enables R1 to be employed for detection of PA by simple visual inspection in silica-gel-coated thin-layer chromatography plates. Probe R1 has been shown to detect PA up to 3.2 nM at pH 7.4. Microstructural features of R1 and its PA complex have been measured by a field emission scanning electron microscope, and it clearly proves that their morphological features differ dramatically both in shape and size. Density function theory and time-dependent density function theory calculations were performed to establish the sensing mechanism and the electronic properties of probe R1. Furthermore, we have demonstrated the utility of probe R1 for the detection of PA in live Vero cells for ratiometric fluorescence imaging.

Blue-shifted aggregation-induced emission of siloles by simple structural modification and their application as nitro explosive chemosensors

By substituting methyl groups at ortho positions of peripheral tetraphenyl rings on the silacyclopentadiene ring, intramolecular rotations were successfully controlled and the photophysical properties were varied.

Bigger and Brighter Fluorenes: Facile π-Expansion, Brilliant Emission and Sensing of Nitroaromatics

π-Expanded butterfly-like 2D fluorenes and 3D spirobifluorenes 1-5 were synthesized via a DDQ-mediated oxidative cyclization strategy with a high regioselectivity. Through structural modification via π-expansion, it was possible to achieve near-ultraviolet absorption, bright-blue emission, very high near-unity fluorescence quantum yields in solution as well as in film states, and deep-lying HOMO energy levels with excellent thermal stabilities. Furthermore, these electron-rich compounds displayed a notable behavior towards sensing of nitroaromatic explosives, such as picric acid, up to a detection limit of 0.2 ppb.

Supercoiled fibres of self-sorted donor-acceptor stacks: a turn-off/turn-on platform for sensing volatile aromatic compounds

To ensure the comfortable survival of living organisms, detection of different life threatening volatile organic compounds (VOCs) such as biological metabolites and carcinogenic molecules is of prime importance. Herein, we report the use of supercoiled supramolecular polymeric fibres of self-sorted donor-acceptor molecules as "turn-off/turn-on" fluorescent sensors for the detection of carcinogenic VOCs. For this purpose, a C3-symmetrical donor molecule based on oligo(p-phenylenevinylene), C3OPV, and a perylene bisimide based acceptor molecule, C3PBI, have been synthesized. When these two molecules were mixed together in toluene, in contrast to the usual charge transfer (CT) stacking, supramolecular fibres of self-sorted stacks were formed at the molecular level, primarily driven by their distinct self-assembly pathways. However, CT interaction at the macroscopic level allows these fibres to bundle together to form supercoiled ropes. An interfacial photoinduced electron transfer (PET) process from the donor to the acceptor fibres leads to an initial fluorescence quenching, which could be modulated by exposure to strong donor or acceptor type VOCs to regenerate the respective fluorescence of the individual molecular stacks. Thus, strong donors could regenerate the green fluorescence of C3OPV stacks and strong acceptors could reactivate the red fluorescence of C3PBI stacks. These supercoiled supramolecular ropes of self-sorted donor-acceptor stacks provide a simple tool for the detection of donor- or acceptor-type VOCs of biological relevance, using a "turn-off/turn-on" fluorescence mechanism as demonstrated with o-toluidine, which has been reported as a lung cancer marker.

“Not quenched” aggregates of a triphenylene derivative for the sensitive detection of trinitrotoluene in aqueous medium

“Not quenched” porous aggregates of triphenylene derivative 4 have been utilized for the detection of TNT in solution, solid and vapour phases with detection limits of 22.7 attograms cm⁻².

AIE luminogen-functionalised mesoporous nanomaterials for efficient detection of volatile gases

Mesoporous silica nanoparticles functionalised with aggregation-induced emission (AIE) luminogen via a carbon-nitrogen double bond are fabricated into films by a dip-coating method. The as-made films can serve as efficient fluorescent sensors for the naked-eye detection of volatile acid gases by colour and emission changes, as well as for the detection of 2,4-dinitrotoluene vapours by fluorescence quenching.

Ultratrace Detection of Nitroaromatics: Picric Acid Responsive Aggregation/Disaggregation of Self-Assembled p-Terphenylbenzimidazolium-Based Molecular Baskets

1-(p-Terphenyl)-benzimidazolium (TRIPOD-TP) molecules undergo self-assembly to form rodlike structures in aqueous medium, as shown by field-emission scanning electron microscopy, transmission electron microscopy, and dynamic light scattering studies. Upon gradual addition of picric acid (PA), these aggregates undergo an aggregation/disaggregation process to complex morphological structures (10(-12)-10(-10) M PA) and spherical aggregates (10(-9)-10(-8) M PA). These spherical aggregates undergo further dissolution to well-dispersed spheres between 10(-7)-10(-6) M PA. During fluorescence studies, these aggregates demonstrate superamplified fluorescence quenching (>97%) in the presence of 10(-5) to 0.2 equiv of the probe concentration, an unprecedented process with PA. The lowest detection limits by solution of TRIPOD-TP are 5 × 10(-13) PA, 50 × 10(-12) M 2,4-dinitrophenol, 200 × 10(-12) M 2,4,6-trinitrotoluene, and 1 nM 1-chloro-2,4-dinitrobenzene. Paper strips dipped in the solution of TRIPOD-TP demonstrate quantitative fluorescence quenching between 10(-17) and 10(-6) M PA using front-surface steady state studies and can measure as low as 2.29 × 10(-20) g/cm(2) PA.

Fluorescent aggregates of AIEE active triphenylene derivatives for the sensitive detection of picric acid

Fluorescent aggregates of triphenylene derivatives 3 and 5 having aggregation induced emission enhancement (AIEE) characteristics have been developed which selectively detect picric acid.

Aggregates of a hydrazono-sulfonamide adduct as picric acid sensors

A sensitive and selective sensor for picric acid was developed using aggregates of a novel hydrazono-sulfonamide adduct.

π-Electron rich small molecule sensors for the recognition of nitroaromatics

In this review article we provide an overview of the recent developments made in small molecule-based turn-off fluorescent sensors for nitroaromatic explosives with special focus on organic and H-bonded supramolecular sensors.

Redox-switchable copper(I) metallogel: a metal-organic material for selective and naked-eye sensing of picric acid

Thiourea (TU), a commercially available laboratory chemical, has been discovered to introduce metallogelation when reacted with copper(II) chloride in aqueous medium. The chemistry involves the reduction of Cu(II) to Cu(I) with concomitant oxidation of thiourea to dithiobisformamidinium dichloride. The gel formation is triggered through metal-ligand complexation, i.e., Cu(I)-TU coordination and extensive hydrogen bonding interactions involving thiourea, the disulfide product, water, and chloride ions. Entangled network morphology of the gel selectively develops in water, maybe for its superior hydrogen-bonding ability, as accounted from Kamlet-Taft solvent parameters. Complete and systematic chemical analyses demonstrate the importance of both Cu(I) and chloride ions as the key ingredients in the metal-organic coordination gel framework. The gel is highly fluorescent. Again, exclusive presence of Cu(I) metal centers in the gel structure makes the gel redox-responsive and therefore it shows reversible gel-sol phase transition. However, the reversibility does not cause any morphological change in the gel phase. The gel practically exhibits its multiresponsive nature and therefore the influences of different probable interfering parameters (pH, selective metal ions and anions, selective complexing agents, etc.) have been studied mechanistically and the results might be promising for different applications. Finally, the gel material shows a highly selective visual response to a commonly used nitroexplosive, picric acid among a set of 19 congeners and the preferred selectivity has been mechanistically interpreted with density functional theory-based calculations.

N-Alkyl derivative of 1,9-pyrazoloanthrone as a sensor for picric acid

TheN-alkyl derivative of 1,9-pyrazoloanthrone has been synthesized, characterized and evaluated as a potent sensor for picric acid.

A nitroaromatic fluorescence sensor from a novel tripyrenyl truxene

A new fluorescent sensor for 2-nitrophenol and picric acid is successfully synthesized from truxene and ethynyl pyrene.