Curcumin-derivatives as fluorescence-electrochemical dual probe for ultrasensitive detections of picric acid in aqueous media

We are reporting the two curcumin derivatives, ferrocenyl curcumin (Fc-cur) and 4-nitro-benzylidene curcumin (NBC), as a probe through dual modalities, i.e., fluorescence and electrochemical methods, for the detection of nitro-analytes, such as picric acid (PA). The probes exhibited aggregation-induced enhanced emission (AIEE), and the addition of picric acid (PA) demonstrated good and specific fluorimetric identification of PA in the aggregated state. By using density functional theory (DFT), the mechanism of picric acid's (PA) interactions with the probes was further investigated. DFT studies shows evidence of charge transfer from curcumin derivatives probe to picric acid resulting into the formation of an adduct. The reduction of trinitrophenol (PA) to 2, 4, 6-trinitrosophenol was investigated utilizing a probe-modified glassy carbon electrode (GCE) with a good detection limit of 9.63 ± 0.001 pM and 41.01 ± 0.002 pM, respectively, for Fc-cur@GCE and NBC@GCE, taking into account the redox behavior of the probe. The applicability of the designed sensor has been utilized for real-time application in the estimation of picric acid in several water samples collected from the different source.

Dual-Channel Imine-Amine Photoisomerization in a Benzoimidazole and Benzothiazole Coupled System: Photophysics and Applications

A molecule, namely 2-(1H-benzo[d]imidazol-2-yl)-6-(benzo[d]thiazol-2-yl)-4-bromophenol (BIBTB), having a two-way proton transfer unit of thiazole and imidazole moieties was synthesized and characterized by NMR, electrospray ionization mass spectrometry (ESI-MS), and single-crystal diffraction studies. Steady state and time-resolved spectral studies of BIBTB support excited state intramolecular proton transfer (ESIPT), causing imine-amine tautomerization through a two-way 6-membered H-bonded ring, where the N atoms of benzothiazole and the benzoimidazole unit are involved as proton acceptor sites. Interestingly, in a nonpolar and moderately polar solvent, photoisomerization in BIBTB is found to be favored toward the thiazole ring, whereas in a highly polar solvent, it is favored toward the imidazole ring. A spectral comparison of BIBTB with judicially designed molecules 2-(benzo[d]thiazol-2-yl)-4-bromophenol (HBT) and 2-(1H-benzo[d]imidazol-2-yl)-4-bromophenol (BIB) supports these inferences. Theoretical calculation using the Density Functional Theory (DFT) at CAM-B3LYP/6-311+G(d,p) level supports the existence of two low-energy 6-membered hydrogen-bonded planar conformers in the ground state in the gas phase and in solvents of different dielectrics. The potential energy curves (PECs) calculated along the proton transfer (PT) coordinate are found to have a high energy barrier in the ground state and to be barrierless or have a low energy barrier in the excited state for both the forms. The calculated vertical excitation and the emission energy from the relaxed excited and PT states show good correlation with the experimental spectral data. Aggregation of BIBTB in water with red shifted emission was established from X-ray single-crystal structure analysis, solid state emission, and Dynamic Light Scattering (DLS) measurement. The molecule BIBTB also acts as a fluorescence probe for sensing the explosive picric acid in the subnano scale and can be used to determine the proportion of water in dimethyl sulfoxide (DMSO) solvent.

D-π-A-π-D-Configured Imidazole-Tethered Benzothiadiazole-Based Sensor for the Ratiometric Discrimination of Picric Acid: Applications in Latent Fingerprint Imaging

A tetraphenyl imidazole-appended benzothiadiazole-based fluorogenic probe (4,7-bis(4-(1,4,5-triphenyl-1H-imidazol-2-yl)phenyl)benzo[c][1,2,5]thiadiazole (BIPT)) has been successfully synthesized and characterized by NMR and high-resolution mass spectrometry (HRMS) spectral analyses. A low limit of detection (LOD) can be achieved to detect picric acid (PA; 7.89 nM). When benzothiadiazole acceptors are incorporated in the D-A-D probe, it can produce a large Stokes shift (206 nm) as a result. Fascinatingly, the fluorescence signals of BIPT were ratiometrically induced by the interaction with PA and exhibited an apparent emission shift from pink to green. The detection process of BIPT is triggered by an intermolecular charge transfer process, as the charge transfer takes place from the electron-rich imidazole to the electron-deficient PA. Moreover, fluorescence detection of PA has been employed in paper strips. Advantageously, sensor BIPT can potentially be applied to contact mode and real-time detection of PA in environmental water samples. Additionally, the BIPT sensor has been successfully employed for latent fingerprint imaging. The study provides clear insights into the rational design of chemosensors for sensing and real-time applications.

Label-Free Detection of Unbound Bilirubin and Nitrophenol Explosives in Water by a Mechanosynthesized Dual Functional Zinc Complex: Recognition of Picric Acid in Various Common Organic Media

High levels of unconjugated bilirubin (UB) in serum lead to asymptomatic and neonatal jaundice and brain dysfunctions. Herein, we have reported the detection of UB at as low as 1 μM in an aqueous alkaline medium using a Zn(II) complex. The specificity of the complex has been validated by the HPLC in the concentration window 6-90 μM, which is rare. The sensory response of the probe at physiological pH against nitro explosives developed it as an instant-acting fluorosensor for picric acid (PA) and 2,4-dinitrophenol (2,4-DNP). Spectroscopic titration provided a binding constant of 4×105  M-1 with PA. The naked eye detection was found to be 15 μM. The solid-state photoluminescent nature of the complex enabled it for PA sensing in the solid phase. Interestingly, the probe remained fluorescent in various volatile and non-volatile organic solvents. As a result, it can also detect PA and 2,4-DNP in a wide range of common organic media. NMR studies revealed the coordination of PA, 2,4-DNP, and UB to the Zn(II) center of the probe, which is responsible for the observed quenching of the probe with the analytes.

A visual and reversible nanoprobe for rapid and on-site determination of hexavalent chromium and lysine based on dual-emission carbon quantum dots coupled with smartphone

A straightforward, largely instrument-free, smartphone-based analytical strategy for hexavalent chromium and lysine (Lys) on-site detection via exploitation of dual-emission carbon quantum dots (DECQDs) has been demonstrated. DECQDs show dual-emission peaks at 439 and 630 nm with the excitation at 375 nm. As a dual-mode detection probe, the fluorescence and ultraviolet adsorption spectra of DECQDs vary with hexavalent chromium concentrations. Most importantly, Lys can restore the fluorescence of the hexavalent chromium added DECQD nanoprobe and change the color of the probe under natural light. At the same time, based on the participation of smartphones, the prepared DECQD probes favor the establishment of visual smart sensors that can also be used for the in-situ detection of targets. The on-site quantitative analysis exhibited a linear range of 5.3-320 μM with a detection limit of 1.6 μM towards Cr(VI) and the differentiation of Lys variation from 1 to 75 mM with a detection limit of 0.3 mM. The probe has been applied for the first time to enable vision-based colorimetric in complex samples such as water, milk and egg. The recoveries of Cr(VI) and Lys in real samples were between 90 and 104%, and the relative standard deviation (RSD) was as low as 0.4%. This work offers new perspectives for fundamental understanding and new design of functional luminescent materials that are applicable for food-safety and rapid and intelligent inspection. A straightforward, large instrument-free, smartphone-based analytical strategy with dual-emission carbon quantum dots was developed for hexavalent chromium and Lys on-site detection via fluorescent and colorimetric twofold readout measure.

Near-infrared probes for luminescence lifetime imaging

Biomedical luminescence imaging in the near-infrared (NIR, 700-1700 nm) region has shown great potential in visualizing biological processes and pathological conditions at cellular and animal levels, owing to the reduced tissue absorption and scattering compared to light in the visible (400-700 nm) region. To overcome the background interference and signal attenuation during intensity-based luminescence imaging, lifetime imaging has demonstrated a reliable imaging modality complementary to intensity measurement. Several selective or environment-responsive probes have been successfully developed for luminescence lifetime imaging and multiplex detection. This review summarizes recent advances in the application of luminescence lifetime imaging at cellular and animal levels in NIR-I and NIR-II regions. Finally, the challenges and further directions of luminescence lifetime imaging are also discussed.

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.

Recent Advancements for the Recognization of Nitroaromatic Explosives Using Calixarene Based Fluorescent Probes

In this era, explosives are easily available compared to the early days. Thus, more effective detection of explosives has become the main concern of homeland security. In the past decades, a large number of sensing materials have been developed for the detection of explosives in solid, vapor, and solution states through fluorescence methods. In recent years, great efforts have been devoted to developing new fluorescent materials with various sensing mechanisms for detecting explosives in order to achieve super-sensitivity, ultra-selectivity, as well as fast response time. Modified calixarenes have high potentials to detect nitroaromatic compounds (NACs) due to their favorable structural properties. It summarizes the detection of NACs by the modified calixarene system formed by the complex. Various methodologies responsible for complex formation and binding mechanisms (PET, FRET, EE, etc.) are the centerpiece of this review. Finally, conclusions and future outlook are presented and discussed.

Covalent Organic Polymers and Frameworks for Fluorescence-Based Sensors

Following the advancements and diversification in synthetic strategies for porous covalent materials in the literature, the materials science community started to investigate the performance of covalent organic polymers (COPs) and covalent organic frameworks (COFs) in applications that require large surface areas for interaction with other molecules, chemical stability, and insolubility. Sensorics is an area where COPs and COFs have demonstrated immense potential and achieved high levels of sensitivity and selectivity on account of their tunable structures. In this review, we focus on those covalent polymeric systems that use fluorescence spectroscopy as a method of detection. After briefly reviewing the physical basis of fluorescence-based sensors, we delve into various kinds of analytes that have been explored with COPs and COFs, namely, heavy metal ions, explosives, biological molecules, amines, pH, volatile organic compounds and solvents, iodine, enantiomers, gases, and anions. Throughout this work, we discuss the mechanisms involved in each sensing application and aim to quantify the potency of the discussed sensors by providing limits of detection and quenching constants when available. This review concludes with a summary of the surveyed literature and raises a few concerns that should be addressed in the future development of COP and COF fluorescence-based sensors.

Facile Microwave-Assisted Synthesis of Functionalized Carbon Nitride Quantum Dots as Fluorescence Probe for Fast and Highly Selective Detection of 2,4,6-Trinitrophenol

Functionalized carbon nitride quantum dots (CNQDs) are fabricated by moderate carbonization of L-tartaric acid and urea in oil acid media by a facile microwave-assisted solvothermal method. The obtained CNQDs are monodispersed with a narrow size distribution (average size of 3.5 nm), and exhibit excellent selectivity and sensitivity of fluorescence quenching for 2,4,6-trinitrophenol (TNP) with a quenching efficiency coefficient Ksv of 4.75 × 104 M-1. This sensing system exhibits a fast response time within 1 min and a wide linear response range from 0.1 to 15 μM. The limit of detection is as low as 87 nM, which is comparable or lower than the other probes. The application of the developed probe to the detection of TNP in spiked water samples yields satisfactory results. The mechanism of fluorescence quenching is also discussed. Graphical Abstract An optical sensor based on functionalized carbon nitride quantum dots (CNQDs) were fabricated from L-tartaric acid and urea by a facile one-pot microwave-assisted solvothermal method, and were effectively utilized to the detection of 2,4,6-trinitrophenol (TNP) based on fluorescence (FL) quenching.

Aggregation-directed High Fidelity Sensing of Picric Acid by a Perylenediimide-based Luminogen

Widespread use of picric acid (PA) in chemical industries and deadly explosives poses dreadful impact on all living creatures as well as the natural environment and has raised global concerns that necessitate the development of fast and efficient sensing platforms. To address this issue, herein, we report a perylenediimide-peptide conjugate, PDI-1, for detection of PA in methanol. The probe displays typical aggregation caused quenching (ACQ) behaviour and exhibits a fluorescence "turn-off" sensory response towards PA which is unaffected by the presence of other interfering nitroaromatic compounds. The sensing mechanism involves PA induced aggregation of the probe into higher order tape like structures which leads to quenching of emission. The probe possesses a low detection limit of 5.6 nM or 1.28 ppb and a significantly high Stern-Volmer constant of 6.87×10⁴  M^-1 . It also exhibits conducting properties in the presence of PA vapours and thus represents a prospective candidate for vapour phase detection of PA. This is, to the best of our knowledge, the first example of a perylenediimide based probe that demonstrates extremely specific, selective and sensitive detection of PA and thus grasps the potential for application in practical scenarios.

A composite nanosensing array with two response channels for trinitrobenzoic acid optical test

In this paper, the authors reported a composite platform for the optical sensing of 2,4,6-trinitrobenzoic acid (TBA). This composite sensing platform consisted of luminescent rare earth MOF and a rhodamine-derived sensing probe. Its composite structure was confirmed by means of XRD, IR, TGA and photophysical measurement. It was found that there were two emission components in this sensing platform, which were rhodamine emission component and Eu(III) emission component, respectively. The rhodamine emission was enhanced with increasing TBA concentration, while the Eu(III) emission component was quenched by increasing TBA concentration, showing self-calibrated sensing signals for ratiometric fluorescent sensing. In addition, rhodamine absorption in visible region was increased with increasing TBA concentration, showing colorimetric sensing. The above mentioned two sensing channels both showed high selectivity and linear response towards TBA with LOD of 2.4 μM. By comparing emission lifetime under various TBA concentrations, the sensing mechanism was convinced as the combination of rhodamine emission turn on effect initiated by TBA -released protons and Eu(III) emission turn off effect caused by an ET (energy transfer) procedure from EuBTC to TBA. This sensing platform showed naked eye sensing and two sensing channels with linear response and high selectivity.

A mechanical stability enhanced luminescence lanthanide MOF test strip encapsulated with polymer net for detecting picric acid and macrodantin

The improper use of organic explosives and antibiotics have brought serious threats to the public health and the environmental safety, exploiting cost-effective and handy luminescent sensors with water stability and high selectivity in monitoring and detecting these hazardous substances are of utmost importance. Herein, we developed a simple yet powerful luminescent test strip sensor in a facile way. As for fabricating this test strip, the filter paper used for filtering lanthanide MOF (Ln-MOF) of [Tb(HIP)(H₂O)₅]·(H₂O)·(HIP)_1/2 (Tb-HIP, where HIP is 5-hydroxyisophthalate) powders was firstly recycled, and encapsulated with poly(methyl methacrylate) (PMMA) polymer net. The as-fabricated Tb-HIP test strips exhibit enhanced mechanical stability than the un-encapsulated ones, and show characteristic green emission of Tb³⁺. These test strips can behave as promising highly selective luminescent probes for picric acid (PA) and macrodantin (MDT) even existence of relevant potentially competing analytes. The detection limit for PA is 0.26 μM, and for MDT is 0.21 μM. In addition, the sensors can be successfully applied to detect PA in the river water samples as well as MDT in serum samples with satisfactory results. More importantly, the Tb-HIP test strips are highly efficient, recyclable luminescence sensors to detect PA and MDT.

p-Pyridine BODIPY-based fluorescence probe for highly sensitive and selective detection of picric acid

A novel fluorescence probe p-PBP for PA was synthesized based on a basic N atom as the electronic donor. The probe could detect PA over TNT, CE, PETN, RDX, HMX, NB, NT, DNT, NP, DNP, and common inorganic explosive ions (K⁺, Ba²⁺, NH₄⁺, NO₃^-, ClO₃^-, and ClO₄^-), and common ions (Na⁺, Ca²⁺, and Mg²⁺) with high selectivity. The fluorescence quenching was attributed to the photo-induced electron transfer (PET) processes from the excited state of p-PBP to the ground state PA. The detection limit of probe p-PBP for PA was as low as 13.06 nmol/L, which is far lower than the concentration stipulated by the Environmental quality standards for surface water. The response time was less than 30 s. Hence, the fluorescence probe p-PBP was successfully developed to detect the concentration level of PA in real samples, which would provide a novel quantitative analysis method of PA in forensic science.

Squaraine Dyes: Molecular Design for Different Applications and Remaining Challenges

Squaraine dyes are a class of organic dyes with strong and narrow absorption bands in the near-infrared. Despite high molar absorptivities and fluorescence quantum yields, these dyes have been less explored than other dye scaffolds due to their susceptibility to nucleophilic attack. Recent strategies in probe design including encapsulation, conjugation to biomolecules, and new synthetic modifications have seen squaraine dyes emerging into the forefront of biomedical imaging and other applications. Herein, we provide a concise overview of (1) the synthesis of symmetrical and unsymmetrical squaraine dyes, (2) the relationship between structure and photophysical properties of squaraine dyes, and (3) current applications of squaraine dyes in the literature. Given the recent successes at overcoming the limitations of squaraine dyes, they show high potential in biological imaging, in photodynamic and photothermal therapies, and as molecular sensors.

Gel Formed by Self-Assembly of a Urea-Modified Monopyrrolotetrathiafulvalene Derivative Displays Multi-Stimuli Responsiveness and Absorption of Rhodamine B

A new monopyrrolotetrathiafulvalene-based derivative containing a urea group was designed, synthesized and thoroughly characterized. It proved to be a non-gelator in a single solvent even when heated or sonicated. However, it could self-assemble in a CHCl₃ (CH₂ Cl₂ )/n-hexane mixture to form a thermo-responsive supramolecular organogel. SEM, FT-IR spectroscopy, UV/Vis absorption spectroscopy, and SAXS revealed that in the organogel system, the gelators self-assembled into supramolecular networks with a J-type aggregation mode under the joint effect of π-π stacking, intermolecular hydrogen-bonding, and van der Waals forces. Interestingly, the gel phase was shown to undergo reversible gel-sol transformation induced by Fe³⁺ -Vitamin C (Vc), trifluoroacetic acid-triethylamine (TFA-TEA) and picric acid (PA)-NaOH. In particular, in the presence of picric acid, the experimental results proposed that charge transfer occurred from the electron-donor gelator to the electron-acceptor picric acid due to the possibility of complex formation. Furthermore, the formed organogel could behave as the matrix for encapsulating cationic fluorescent dye from wastewater, and the adsorption efficiency was directly proportional to the concentration of the gelator.

A mitochondria-targeting ratiometric fluorescent probe for the detection of sulfur dioxide in living cells

A mitochondria-targeting ratiometric fluorescent probe was developed for the detection of sulfur dioxide in living cells.

Stepwise elucidation of fluorescence based sensing mechanisms considering picric acid as a model analyte

The precise study of fluorescence-based sensing mechanisms and a step-by-step design experiment for the elucidation of the mechanism of sensing for newly designed sensing systems can be ascertained using the presented tutorial review.

Phenanthroimidazole derivatives as a chemosensor for picric acid: a first realistic approach

A series of phenanthroimidazole (PI) derivatives (M1–M3): 2-phenyl-1H-phenanthro [9,10-d]imidazole (M1), 2-anthryl-1H-phenanthro[9,10-d]imidazole (M2), and 2-pyrenyl-1H-phenanthro[9,10-d]imidazole (M3) were synthesized and characterized using various spectroscopic techniques.

CdS QDs/N-methylpolypyrrole hybrids as fluorescent probe for ultrasensitive and selective detection of picric acid

Inorganic-organic hybrids are an advanced class of luminescent materials showing great promise for fabrication of highly sensitive and selective optical sensors. In the present study, a novel CdS quantum dots/N-methylpolypyrrole (CdS QDs/NMPPY) hybrid was synthesized via the direct polymerization of NMPPY on L-cysteine capped CdS QD aggregates. A number of characterization techniques including FTIR, DLS, FESEM, UV-vis, and fluorescence spectroscopies were used to study the chemical composition, morphology and optical properties of the resultant QDs/polymer hybrid. The as-synthesized CdS QDs/NMPPY hybrid shows a bright emission at 459 nm under excitation at 367 nm in water. Also the results show the role of sodium dodecyl benzenesulfonate (SDBS) to control the mechanism of synthesis and spectroscopic of the prepared CdS/NMPPY hybrid. Moreover, in this work was reported the direct hybridization procedure without other modification such as ligand exchange and coating. We demonstrated that the hybridization of CdS QDs with NMPPY polymer leads to a significant change in fluorescence sensing properties toward nitroaromatic compounds. Further studies unveiled that the emission of CdS QDs/NMPPY hybrid is strongly and selectively quenched by picric acid molecule with a large Stern-Volmer constant of 843,900 M^-1 and an excellent detection limit of 4.6 × 10^-7 M. The changes in the UV-vis spectra of picric acid solutions in the presence and absence of CdS QDs/NMPPY hybrid displayed that the fluorescence quenching occurs through a static quenching mechanism. Finally, the proposed CdS QDs/NMPPY sensor was successfully utilized to determine the amount of picric acid in real water samples.

Selective Detection of Trinitrophenol by Amphiphilic Dimethylaminopyridine-Appended Zn(II)phthalocyanines at the Near-Infrared Region

Novel amphiphilic Zn(II)phthalocyanines (ZnPcs) peripherally substituted with four and eight dimethylaminopyridinium units (ZnPc1 and ZnPc2) were synthesized by cyclotetramerization of the corresponding phthalonitriles. The effect of aggregation and photophysical (fluorescence quantum yields and lifetimes) and photochemical (singlet oxygen generation and photodegradation under light irradiation) properties was investigated. The chemosensing ability of ZnPcs toward explosive nitroaromatic compounds was explored in aqueous medium. This study demonstrates that ZnPc1 and ZnPc2 show fluorescence quenching behavior upon interaction with different nitro analytes and show unprecedented selectivity toward 2,4,6-trinitrophenol with a limit of detection (LOD) of 0.7-1.1 ppm with a high quenching rate constant (K _sv) of 1.6-2.02 × 10⁵. The near-infrared (NIR) fluorescence in thin films was quenched efficiently because of the photoinduced electron-transfer process through strong intermolecular π-π and electrostatic interactions. The sensing process is highly reversible and free from the interference of other commonly encountered nitro analytes. Further, experiments were performed to demonstrate the use of ZnPcs as efficient heterogeneous photocatalysts in the reduction of nitro explosives. The smart dual performance of multicharged ZnPcs in aqueous media quantifies them as attractive candidates in developing sensor materials at the NIR region and to possibly convert the toxic explosives into useful scaffolds. These results provide an interesting perspective toward elaboration of stable fluorescent systems for the selective sensing behavior of nitro explosives and their facile heterogeneous catalytic behavior in the reduction reactions.

A novel coumarin-based fluorescence chemosensor for Al3+ and its application in cell imaging

As an efficient turn-on fluorescent chemosensor for Al³⁺, a new coumarin derivative (CND) has been designed and synthesized by the condensation of 8-formyl-7-hydroxycoumarin with niacin hydrazide. The spectroscopic studies revealed that the sensor CND exhibited a remarkable fluorescence enhancement towards Al³⁺ with high selectivity and sensitivity in EtOH-HEPES (95:5, v/v, pH = 7.40), which was attributed to the photoinduced electron transfer (PET) and CN isomerization mechanism. Fluorescence titration calculations data showed that the detection limit and the association constants of CND for Al³⁺ were found to be 2.51 × 10^-7 M and 9.64 × 10⁴ M^-1, respectively. The results of experiments, including Job's plot, ¹H NMR titration and ESI-MS, revealed that the stoichiometric binding between CND and Al³⁺ was 1:1. The investigations of the pH dependency of CND for Al³⁺ detection, and the cell imaging suggested the sensor CND could be promisingly applied for the recognition of Al³⁺ in biological cells.

Role of Aromatic Moiety in the Probe Property toward Picric Acid: Synthesis, Crystal Structure, Spectroscopy, Microscopy, and Computational Modeling of a Knoevenagel Condensation Product of d-Glucose

Molecular probes for picric acid (PA) in both solution and solid states are important owing to their wide usage in industry. This paper deals with the design and development of a glucosyl conjugate of pyrene (L 1 ) along with control molecular systems, possessing anthracenyl (L 2 ), naphtyl (L 3 ), and phenyl (L 4 ) moieties, via Knoevenagel condensation of 2,4-pentanedione with d-glucose. The selectivity of L 1 toward PA has been demonstrated on the basis of fluorescence and absorption spectroscopy, and the species of recognition by electrospray ionization mass spectrometry. The role of the aromatic group in the selective receptor property has been addressed among L 1 , L 2 , L 3 , and L 4 . The structural features of the {L 1 + PA} complex were established by density functional theory computations. L 1 was demonstrated to detect PA in solid state selectively over other nitroaromatic compounds (NACs). To study the utility of L 1 in film, cellulose paper strips coated with L 1 were used and demonstrated the selective detection of PA. The observed microstructural features of L 1 and its complex {L 1 + PA} differ distinctly in both atomic force microscopy and scanning electron microscopy, all in the support of the complex formation. Thus, L 1 was demonstrated as a sensitive, selective, and inexpensive probe for PA over several NACs by visual, spectral, and microscopy methods.

Multiple Emitting Amphiphilic Conjugated Polythiophenes-Coated CdTe QDs for Picogram Detection of Trinitrophenol Explosive and Application Using Chitosan Film and Paper-Based Sensor Coupled with Smartphone

Novel multiple emitting amphiphilic conjugated polythiophene-coated CdTe quantum dots for picogram level determination of the 2,4,6-trinitrophenol (TNP) explosive are developed. Four biocompatible sensors, cationic polythiophene nanohybrids (CPTQDs), nonionic polythiophene nanohybrids (NPTQDs), anionic polythiophene nanohybrids (APTQDs), and thiophene copolymer nanohybrids (TCPQDs), are designed using an in situ polymerization method, which shows highly enhanced fluorescence intensity and quantum yield (up to 78%). All sensors are investigated for nitroexplosive detection to provide a remarkable fluorescence quenching for TNP and the quenching efficiency reached 96% in the case of TCPQDs. The fluorescence of the sensors are quenched by TNP through inner filter effect, electrostatic, π-π, and hydrogen bonding interactions. Under optimal conditions, the detection limits of CPTQDs, NPTQDs, APTQDs, and TCPQDs are 2.56, 7.23, 4.12, and 0.56 × 10-9 m, respectively, within 60 s. More importantly, portable, cost effective, and simple to use paper strips and chitosan film are successfully applied to visually detect as little as 2.29 pg of TNP. The possibility of utilizing a smartphone with a color-scanning APP in the determination of TNP is also established. Moreover, the practical application of the developed sensors for TNP detection in tap and river water samples is described with satisfactory recoveries of 98.02-107.50%.

Two AIEE-active α-cyanostilbene derivatives containing BF2 unit for detecting explosive picric acid in aqueous medium

Two novel α-cyanostilbene derivatives bearing triphenylamine and BF₂ groups are synthesized (named TPE-B and TPE-BN). The fluorescent emissions of compounds TPE-B and TPE-BN are hypochromatically shifted and bathochromically shifted, respectively, with increasing polarity of the solvents, suggesting that the two compounds have characteristic polarity-dependent solvatochromic effects. Furthermore, they show obvious aggregation-induced emission enhancement (AIEE) phenomenon in THF/water mixture solutions. Meanwhile, compounds TPE-B and TPE-BN emit orange and yellow fluorescence in their solid states, respectively. Most significantly, in aqueous medium, compounds TPE-B and TPE-BN can selectively and sensitively detect picric acid (PA) among a number of nitroaromatic compounds, and their limits of detection (LOD) are calculated as 1.26 × 10⁻⁶ M and 1.51 × 10⁻⁶ M, respectively. The recognition mechanism for PA can be attributed to the photo-induced electron transfer (PET) process and this is supported by density functional theory (DFT) calculation. This research provides two novel compounds for the rational design of AIEE-active materials for sensing systems.

Two α-cyanostilbene derivatives bearing triphenylamine and BF₂ groups exhibit AIEE properties and can detect explosive picric acid in aqueous medium.

Aggregation induced emission enhancement (AIEE) of tripodal pyrazole derivatives for sensing of nitroaromatics and vapor phase detection of picric acid

Organosoluble tris-pyrazole compounds aggregate in organic/aqueous solvent mixtures, showing aggregation-induced emission enhancement (AIEE), the latter being quenched by picric acid.

Photo-physical properties of salts of a di-topic imidazole-tethered anthracene derivative in solid and solution

Combined effects of Dexter-quenching and protonation to change the photoluminescence of a di-topic anthracene–amine conjugate by nitrophenols and aromatic carboxylic acids in solid and solution are presented.

“Receptor free” inner filter effect based universal sensors for nitroexplosive picric acid using two polyfluorene derivatives in the solution and solid states

“Receptor free” and “interaction free” detection of nitroexplosive PA at remarkably low limit of detection (LOD) values of 110 nM and 219 nM using two new fluorescent polymers via the inner filter effect mechanism.

Integrated pyrazolo[1,5-a]pyrimidine-hemicyanine system as a colorimetric and fluorometric chemosensor for cyanide recognition in water

A new probe for cyanide detection based on the integrated pyrazolo[1,5-a]pyrimidine-hemicyanine (PpHe) system was synthesized in an efficient and straightforward manner using microwave-assisted heating. Photophysical studies in a 100% aqueous solution demonstrated high cyanide selectivity and detection limits as low as 600 and 86 nmol L^-1 for UV-vis absorption and fluorescence emission, respectively. Both values are well below 1900 nmol L^-1, which is the maximum concentration permitted for drinking water by the World Health Organization (WHO). HRMS analysis and NMR experiments were performed to confirm the mechanism of detection based on blocking the ICT phenomenon via nucleophilic addition of CN^- on the C˭N⁺ bond (iminium salt moiety) of the probe.

Recent Advances of the Polymer Micro/Nanofiber Fluorescence Waveguide

Subwavelength optical micro/nanofibers have several advantages, such as compact optical wave field and large specific surface area, which make them widely used as basic building blocks in the field of micro-nano optical waveguide and photonic devices. Among them, polymer micro/nanofibers are among the first choices for constructing micro-nano photonic components and miniaturized integrated optical paths, as they have good mechanical properties and tunable photonic properties. At the same time, the structures of polymer chains, aggregated structures, and artificial microstructures all have unique effects on photons. These waveguided micro/nanofibers can be made up of not only luminescent conjugated polymers, but also nonluminous matrix polymers doped with luminescent dyes (organic and inorganic luminescent particles, etc.) due to the outstanding compatibility of polymers. This paper summarizes the recent progress of the light-propagated mechanism, novel design, controllable fabrication, optical modulation, high performance, and wide applications of the polymer micro/nanofiber fluorescence waveguide. The focus is on the methods for simplifying the preparation process and modulating the waveguided photon parameters. In addition, developing new polymer materials for optical transmission and improving transmission efficiency is discussed in detail. It is proposed that the multifunctional heterojunctions based on the arrangement and combination of polymer-waveguided micro/nanofibers would be an important trend toward the construction of more novel and complex photonic devices. It is of great significance to study and optimize the optical waveguide and photonic components of polymer micro/nanofibers for the development of intelligent optical chips and miniaturized integrated optical circuits.

A novel platform self-assembled from squaraine-embedded Zn(ii) complexes for selective monitoring of ATP and its level fluctuation in mitotic cells

Using multiple interactions, a simple self-assembly based on a Zn(ii) coordination compound and squaraine () demonstrated a selective turn-on fluorescence response to ATP in the near infrared (NIR) region. More importantly, the self-assembly has been successfully applied to ATP imaging in the mitochondria of the gastric cancer cell line SGC-7901 and monitoring of level fluctuation of ATP during the mitotic period.

Inner Filter Effect and Resonance Energy Transfer Based Attogram Level Detection of Nitroexplosive Picric Acid Using Dual Emitting Cationic Conjugated Polyfluorene

A novel conjugated cationic polyfluorene (polyelectrolyte) derivative, PFBT, was developed by means of simple and cost-effective oxidative coupling polymerization method. PFBT displayed dual state emission in dimethyl sulfoxide (DMSO) as well as in water, a characteristic phenomenon of polyfluorene homopolymers, and tested for nitroexplosive analytes detection to observe a remarkable fluorescence quenching response for picric acid (PA) in the both solvents. The polymer PFBT demonstrated substantial selectivity and ultrasensitivity toward nitroexplosive PA in both the solvents (DMSO and H₂O) with exceptional quenching constant values of 2.69 × 10⁴ and 2.18 × 10⁵ M^-1 and a ultralow limit of detection of 92.7 nM (21.23 ppb) and 0.19 nM (43.53 ppt) in respective solvents. Furthermore, economical portable test strip devices were prepared for easy and fast on-site PA sensing, which can detect up to 0.22 ag level of PA. PA sensing in vapor phase was also established, that could detect up to 42.6 ppb level of PA vapors. Interestingly, the mechanism of sensing in DMSO solvent was attributed to substantial inner filter effect and photoinduced electron transfer, while in H₂O the sensing occurs via possible resonance energy transfer and photoinduced electron transfer, which is exceptional and not reported earlier for a single probe.

Amorphous Carbon Dots and their Remarkable Ability to Detect 2,4,6-Trinitrophenol

Apparently mundane, amorphous nanostructures of carbon have optical properties which are as exotic as their crystalline counterparts. In this work we demonstrate a simple and inexpensive mechano-chemical method to prepare bulk quantities of self-passivated, amorphous carbon dots. Like the graphene quantum dots, the water soluble, amorphous carbon dots too, exhibit excitation-dependent photoluminescence with very high quantum yield (~40%). The origin and nature of luminescence in these high entropy nanostructures are well understood in terms of the abundant surface traps. The photoluminescence property of these carbon dots is exploited to detect trace amounts of the nitro-aromatic explosive - 2,4,6-trinitrophenol (TNP). The benign nanostructures can selectively detect TNP over a wide range of concentrations (0.5 to 200 µM) simply by visual inspection, with a detection limit of 0.2 µM, and consequently outperform nearly all reported TNP sensor materials.

Photostable and Low-Toxic Yellow-Green Carbon Dots for Highly Selective Detection of Explosive 2,4,6-Trinitrophenol Based on the Dual Electron Transfer Mechanism

Advances in the development of fluorescent carbon dots (CDs) for detecting nitro-explosives have attracted great interest. However, developing long-wavelength luminescence CDs for highly selective determination of 2,4,6-trinitrophenol (TNP) and getting insight into the detection mechanism remain further to be investigated. Here, excitation-independent yellow-green emission CDs with good photostability and low biotoxicity were introduced for detecting TNP selectively. Then, two types of electron transfer (ET) processes including hydrogen-bond interaction-assisted ET and proton transfer-assisted ET are suggested to be responsible for their photophysical behavior. Finally, the visual detection of TNP has been successfully developed by a CD-based indicator paper. The facile, highly sensitive, and selective detection for TNP in both of a solution and a solid phase makes CDs potentially useful in environmental sensor applications.