Preparation of Langmuir-Blodgett Films from Quinoxalines Exhibiting Aggregation-Induced Emission and Their Acidochromism

The development of aggregation-induced emission (AIE)-exhibiting compounds heavily relies on our evolving comprehension of their behavior at interfaces, an understanding that still remains notably limited. In this study, we explored the preparation of two-dimensional (2D) sensing films from 2,3-diphenylquinoxaline-based diazapolyoxa- and polyazamacrocycles displaying AIE via the Langmuir-Blodgett (LB) technique. This systematic investigation highlights the key role of the heteroatom-containing tether of 2,3-diphenylquinoxalines in the successful fabrication of Langmuir layers at the air-water interface and the transfer of AIE-emitting supramolecular aggregates onto solid supports. Using both diazapolyoxa- and polyazamacrocycles, we prepared AIE-exhibiting monolayer films containing emissive supramolecular aggregates on silica, mica, and quartz glass and characterized them using ultraviolet-visible (UV-vis) and photoluminescence (PL) spectroscopies, atomic force microscopy (AFM) imaging, and fluorescence microscopy. We also obtained multilayer AIE-emitting films through the LB technique, albeit with increased complexity. Remarkably, by employing the smallest macrocycle N2C3Q, we successfully prepared LB films suitable for the visual detection of acidic vapors. This sensing material, which contains a much lesser amount of organic dye compared with traditional drop-cast films, can be regenerated and utilized for real-life sample analysis, such as monitoring the presence of ammonia in the air and the freshness of meat.

Quinoxaline-based azamacrocycles: synthesis, AIE behavior and acidochromism

The development of luminescent molecular materials has advanced rapidly in recent decades, primarily driven by the synthesis of novel emissive compounds and a deeper understanding of excited-state mechanisms. Herein, we report a streamlined synthetic approach to light-emitting diazapolyoxa- and polyazamacrocycles N2CnOxQ and NyCnQ (n = 3-10; x = 2, 3; y = 2-5), incorporating a 2,3-diphenylquinoxaline residue (DPQ). This synthetic strategy based on macrocyclization through Pd-catalyzed amination reaction yields the target macrocycles in good or high yields (46-92%), enabling precise control over their structural parameters. A key role of the PhPF-tBu ligand belonging to the JosiPhos series in this macrocyclization was elucidated through DFT computation. This macrocyclization reaction eliminates the need for complex protecting-deprotecting procedures of secondary amine groups, offering a convenient and scalable method for the preparation of target compounds. Moreover, it boasts a potentially broad substrate scope, making it promising for structure-properties studies within photophysics, sensor development, and material synthesis. Photophysical properties of representative macrocycles were investigated, employing spectroscopic techniques and DFT computation. It was demonstrated that DPQ-containing macrocycles display aggregation-induced emission in a DCM-hexane solvent mixture despite the presence of flexible tethers within their structures. Single-crystal X-ray diffraction analysis of a representative compound N2C8O3Q allowed us to gain deeper insight into its molecular structure and AIE behaviour. The emissive aggregates of the N2C10O3Q macrocycle were immobilized on filter paper yielding AIE-exhibiting test strips for measuring acidity in vapors and in aqueous media.

A Multifunctional Chemical Probe for the Measurement of Local Micropolarity and Microviscosity in Mitochondria

The measurement of physicochemical parameters in living cells can provide information on individual cellular organelles, helping us to understand subcellular function in health and disease. While organelle-specific chemical probes have allowed qualitative evaluation of microenvironmental variations, the simultaneous quantification of mitochondrial local microviscosity (η_m ) and micropolarity (ϵ_m ), along with concurrent structural variations, has remained an unmet need. Herein, we describe a new multifunctional mitochondrial probe (MMP) for simultaneous monitoring of η_m and ϵ_m by fluorescence lifetime and emission intensity recordings, respectively. The MMP enables highly precise measurements of η_m and ϵ_m in the presence of a variety of agents perturbing cellular function, and the observed changes can also be correlated with alterations in mitochondrial network morphology and motility. This strategy represents a promising tool for the analysis of subtle changes in organellar structure.

Solvatochromism, acidochromism and aggregation-induced emission of propeller-shaped spiroborates

Propeller-shaped pyridyl-enolato-catecholate/-salicyl spiroborates (Sborepy1–6) were synthesized.

Self-Assembly of Fluorescent Organic Nanoparticles for Iron(III) Sensing and Cellular Imaging

Fluorescent organic nanoparticles have attracted increasing attentions for chemical or biological sensing and imaging due to their low-toxicity, facile fabrication and surface functionalization. In this work, we report novel fluorescent organic nanoparticles via facile self-assembly method in aqueous solution. First, the designed water-soluble fluorophore shows a weak and negligible intrinsic fluorescence in water. Upon binding with adenosine-5'-triphosphate (ATP), fluorescent nanoparticles were formed immediately with strongly enhanced fluorescence. These fluorescent nanoparticles exhibit high sensitivity and selectivity toward Fe(3+) sensing with detection limit of 0.1 nM. In addition, after incubation with HeLa cells, the fluorophore shows excellent imaging performance by interaction with entogenous ATP in cells. Finally, this fluorescent system is also demonstrated to be capable of Fe(3+) sensing via fluorescence quenching in cellular environment.

Self-Assembly of Fluorescent Hybrid Core-Shell Nanoparticles and Their Application

In this work, a fluorescent hybrid core-shell nanoparticle was prepared by coating a functional polymer shell onto silver nanoparticles via a facile one-pot method. The biomolecule poly-L-lysine (PLL) was chosen as the polymer shell and assembled onto the silver core via the amine-reactive cross-linker, 3,3'-dithiobis(sulfosuccinimidylpropionate). The fluorescent anticancer drug, doxorubicin, was incorporated into the PLL shell through the same linkage. As the cross-linker possesses a thiol-cleavable disulfide bond, disassembly of the PLL shell was observed in the presence of glutathione, leading to controllable doxorubicin release. The silver core there provided an easily modified surface to facilitate the shell coating and ensures the efficient separation of as-prepared nanoparticles from their reaction mixture through centrifugation. Cell assays show that the prepared hybrid fluorescent nanoparticles can internalize into cells possessing excellent biocompatibility prior to the release of doxorubicin, terminating cancer cells efficiently as the doxorubicin is released at the intracellular glutathione level. Such properties are important for designing smart containers for target drug delivery and cellular imaging.

Fluorescence resonance energy transfer in a binary organic nanoparticle system and its application

Fluorescent organic nanoparticles have a much better photostability than molecule-based probes. Here, we report a simple strategy to detect chemicals and biomolecules by a binary nanoparticle system based on fluorescence resonance energy transfer (FRET). Poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO, energy donor) and poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV, energy acceptor) are utilized to prepare the binary nanoparticle system through a reprecipitation method. Since the FRET process is strongly distance-dependent, a change in the interparticle distance between the two kinds of nanoparticles after introduction of analytes will alter the FRET efficiency. The response of the binary nanoparticle system to cationic polyelectrolytes was investigated by monitoring the FRET efficiency from PFO to MEH-PPV nanoparticles and the fluorescence color of the nanoparticle solutions. Furthermore, the cationic polyelectrolyte pretreated binary nanoparticle system can be used to detect DNA by desorption of nanoparticles from the polyelectrolyte's chains and the detection concentration can go down to 10(-14) M. Thus, the binary nanoparticle system shows great promise for applications in chemical sensing or biosensing.

Nanoparticles made of π-conjugated compounds targeted for chemical and biological applications

This feature article summarizes the recent applications of nanoparticles made of π-conjugated compounds in bio/chemo-sensing, disease therapy, and photoacoustic imaging.

Poly (9-(2-diallylaminoethyl)adenine HCl-co-sulfur dioxide) deposited on silica nanoparticles constructs hierarchically ordered nanocapsules: curcumin conjugated nanocapsules as a novel strategy to amplify guanine selectivity among nucleobases

Poly (9-(2-diallylaminoethyl)adenine HCl-co-sulfur dioxide) (Poly A) deposited on silica nanoparticles self-assembles to form hierarchically ordered nanocapsules. These nanocapsules can be conjugated with curcumin. The curcumin-conjugated nanocapsules are found to be spherical in size and their size ranges between 200 and 600 nm. We found that curcumin conjugated with silica nanoparticles marginally shows a selectivity (∼20%) for guanine over adenine, cytosine, thymine and uracil, but this selectivity is extraordinarily amplified to more than 500% in curcumin-conjugated nanocapsules prepared from the above procedure. FT-IR spectra along with lifetime measurements suggest that specific interaction between adenine moieties of Poly A nanocapsules and thymine/uracil does not affect the fluorescence of poly A nanocapsules. Thus, the sensitivity and selectivity for guanine estimation is due to hydrophobic interactions, which are assisted by the low water solubility of guanine as compared to the other nucleobases. The present method illustrates a wider linear dynamic range in the higher concentration range as compared to the reported methods. Finally, the degradation study proves that stability of curcumin is improved dramatically in such nanocapsules demonstrating that nanotechnology could be a viable method to improve selectivity of specific analyte and robustness of probe molecule during fluorescence based bio-sensing.

Fluorescent organic nanoparticles with enhanced fluorescence by self-aggregation and their application to cellular imaging

We report a simple strategy to prepare organic biocompatible fluorescent nanoparticles with enhanced fluorescence. A significant fluorescence enhancement was realized by designing a fluorescent small molecule, 4,4'-(2,7-bis[4-{1,2,2-triphenylvinyl}phenyl]-9H-fluorene-9,9-diyl)bis(N,N,N-trimethylbutan-1-aminum)bromide (TPEFN), with aggregation-induced emission enhancement (AIEE) effect. Nanoparticles of TPEFN can be formed through molecular self-aggregation by gradually increasing the water fraction in TPEFN mixed solution (methanol/water). Fluorescence enhancement by about 120-fold was observed after nanoparticle formation. By addition of the biomolecule adenosine triphosphate (ATP), larger nanoparticles of TPEFN are formed and further fluorescence enhancement can be achieved, yielding a total fluorescence enhancement of 420-fold compared with the TPEFN molecular solution. Both of these nanoparticles show very good biocompatibility. Ultrabright spots present in the confocal laser scanning microscopy image again proved the formation of nanoparticles. Positively charged side chains of TPEFN endow these nanoparticles cationic surfaces. The size of the prepared TPEFN nanoparticles and their cationic surface allow them to be rapidly internalized into cells. Cell viability assays prove that the TPEFN nanoparticles have high biocompatibility. These organic fluorescent nanoparticles show great promise for applications in cellular imaging or biotechnology.

Conjugated oligomer-based fluorescent nanoparticles as functional nanocarriers for nucleic acids delivery

Oligonucleotides such as siRNA and plasmid DNA (pDNA) have great potential for gene therapies. Multifunctional, environment-resistant carriers with imaging capabilities are required to track the assembly and disassembly of oligonucleotides, monitor the delivery processes, and develop new delivery systems. Conjugated polymers and oligomers can potentially be used as novel materials for functional nanocarriers with both delivery and imaging abilities. In this work, a novel π-conjugated oligomer 4,7-(9,9'-bis(6-adenine hexyl)fluorenyl)-2,1,3-benzothiadiazole (OFBT-A) modified with nucleotide adenine (A) groups in its side chains is synthesized and characterized. Fluorescent nanoparticles based on the π-conjugated oligomers OFBT-A are developed as novel functional nanocarriers for oligonucleotides. Single-stranded DNA (ssDNA) TR-T5 labeled with Texas Red (TR) fluorescent dye is selected as a model payload oligonucleotide. The capture abilities and stability of OFBT-A are investigated by monitoring the fluorescence resonance energy transfer (FRET) efficiency between the OFBT-A nanoparticles and TR labels in solution. The OFBT-A/TR-T5 composites are stable in solution at high ionic strengths (0-500 mM) and have a wide working pH range, from 3.0 to 9.5. The in vitro profile demonstrates that the release of the TR-DNA is induced by the ssDNA A43, which has a high charge density. The release process is monitored by measuring the changes in FRET efficiency and fluorescence color for the OFBT-A/TR-T5 composites. Using this carrier, the uptake of TR-DNA by A549 lung cancer cells is observed. Both the OFBT-A nanoparticles and the OFBT-A/TR-T5 composites show high cytocompatibility. We anticipate that these novel functional nanocarriers will provide a safe strategy for monitoring the gene delivery process.

Fluorescent organic nanoparticles based on branched small molecule: preparation and ion detection in lithium-ion battery

Fluorescent organic nanoparticles (FONs) as a new class of nanomaterials can provide more advantages than molecule based probes. However, their applications in specific metal ion detection have rarely been exploited. We design and synthesize a branched small-molecule compound with triazole as a core and benzothiadiazole derivative as branches. By a facile reprecipitation method, nanoparticles (NPs) of this compound can be prepared in aqueous solutions, which can show high selectivity and sensitivity to Fe(III) ions based on fluorescence quenching. In addition, the fluorescence intensity of these NPs is resistant to pH changes in solutions. Such characters of this kind of NPs can be utilized in Fe(3+) impurity detection in a promising cathode material (LiFePO4) for lithium ion batteries. When exposed to Fe(3+), both the triazole and benzothiadiazole groups contribute to the fluorescence quenching of NPs, but the former one plays a more important role in Fe(3+) impurity detection. The sensing mechanism has also been investigated which indicates that a Fe-organic complex formation may be responsible for such sensing behavior. Our findings demonstrate that specific metal ion detection can be realized by FONs and have extended the application field of FONs for chemical sensing in aqueous solutions.

Preparation of high-quality organic semiconductor nanoparticle films by solvent-evaporation-induced self-assembly

Organic semiconductor nanoparticles are expected to be used in organic optical and electronic devices due to their unique optical and electrical properties. However, no method has been reported for the preparation of high-quality organic nanoparticle films without remaining additives and being capable of dealing with binary nanoparticle blends. We developed a simple approach to fabricate high-quality organic semiconductor nanoparticle films from their aqueous solutions by solvent-evaporation-induced self-assembly. Only volatile solvents are employed in the nanoparticle solutions, so the self-assembled nanoparticle films are free of additives. Moreover, this method is also suitable for fabricating thin films containing binary nanoparticles. Therefore, it paves the way for potential applications of organic semiconductor nanoparticles in nanoscale optical and electronic devices.

Aggregation-induced emission enhancement in organic ion pairs

We present here a new example of aggregation-induced emission enhancement (AIEE), which involves an original mechanism based on the formation of organic ion pairs. The phenol 4-hydroxy-7-nitrobenzoxadiazole (NBDOH) is dissociated in water at pH 5.0 to give the corresponding phenolate, which is poorly fluorescent in this medium. We bring evidence that fluorescence quenching is due to an interaction with water molecules. In the presence of a relatively bulky ammonium salt, specifically tetrabutylammonium bromide (TBAB), NBDOH forms a hydrophobic salt, TBA(+)NBDO(-). This has no influence on the fluorescence of the anion as long as the salt is dissolved. However, the salt readily crystallizes in the medium and transition to the solid state is accompanied by a strong increase in fluorescence intensity. This effect can be explained by two reasons. The anions are protected from water molecules, and above all, the presence of the bulky cations prevents parallel-stacking of the anions, thus leading to an original molecular arrangement that is favorable to fluorescence. As the nature of the organic cation may be easily changed, the versatility of the system is very interesting for the design of new organic micro- and nanoparticles that must be fluorescent in the solid state, possibly in an aqueous environment.