Simple biosensor with high selectivity and sensitivity: thiol-specific biomolecular probing and intracellular imaging by AIE fluorogen on a TLC plate through a thiol-ene click mechanism

Aggregation-Induced Emission of Tetraphenylethene-Conjugated Phenanthrene Derivatives and Their Bio-Imaging Applications

In this study, a series of rationally designed emissive phenanthrene derivatives were synthesized and their aggregation-induced emission (AIE) properties in tetrahydrofuran (THF)/water mixtures were investigated. Two tetraphenylethene (TPE) segments were conjugated to both ends of the phenanthrene core at the para-positions and meta-positions, resulting in pTPEP and mTPEP derivatives, respectively. While the TPE-conjugated phenanthrene derivatives did not show any emission when dissolved in pure THF, they showed strong sky-blue emissions in water-THF mixtures, which is attributed to the restriction of intramolecular motions of TPE segments by aggregation. Furthermore, silica nanoparticles loaded with these AIE-active compounds were prepared and proved to be promising intracellular imaging agents.

A thiol probe for measuring unfolded protein load and proteostasis in cells

When proteostasis becomes unbalanced, unfolded proteins can accumulate and aggregate. Here we report that the dye, tetraphenylethene maleimide (TPE-MI) can be used to measure cellular unfolded protein load. TPE-MI fluorescence is activated upon labelling free cysteine thiols, normally buried in the core of globular proteins that are exposed upon unfolding. Crucially TPE-MI does not become fluorescent when conjugated to soluble glutathione. We find that TPE-MI fluorescence is enhanced upon reaction with cellular proteomes under conditions promoting accumulation of unfolded proteins. TPE-MI reactivity can be used to track which proteins expose more cysteine residues under stress through proteomic analysis. We show that TPE-MI can report imbalances in proteostasis in induced pluripotent stem cell models of Huntington disease, as well as cells transfected with mutant Huntington exon 1 before the formation of visible aggregates. TPE-MI also detects protein damage following dihydroartemisinin treatment of the malaria parasites Plasmodium falciparum. TPE-MI therefore holds promise as a tool to probe proteostasis mechanisms in disease.

Proteostasis is maintained through a number of molecular mechanisms, some of which function to protect the folded state of proteins. Here the authors demonstrate the use of TPE-MI in a fluorigenic dye assay for the quantitation of unfolded proteins that can be used to assess proteostasis on a cellular or proteome scale.

A reusable and naked-eye molecular probe with aggregation-induced emission (AIE) characteristics for hydrazine detection

We report a reusable fluorogenic probe for naked-eye sensing of hydrazine in solution and in the gaseous phase.

pH-Regulated Reversible Transition Between Polyion Complexes (PIC) and Hydrogen-Bonding Complexes (HBC) with Tunable Aggregation-Induced Emission

The mimicking of biological supramolecular interactions and their mutual transitions to fabricate intelligent artificial systems has been of increasing interest. Herein, we report the fabrication of supramolecular micellar nanoparticles consisting of quaternized poly(ethylene oxide)-b-poly(2-dimethylaminoethyl methacrylate) (PEO-b-PQDMA) and tetrakis(4-carboxylmethoxyphenyl)ethene (TPE-4COOH), which was capable of reversible transition between polyion complexes (PIC) and hydrogen bonding complexes (HBC) with tunable aggregation-induced emission (AIE) mediated by solution pH. At pH 8, TPE-4COOH chromophores can be directly dissolved in aqueous milieu without evident fluorescence emission. However, upon mixing with PEO-b-PQDMA, polyion complexes were formed by taking advantage of electrostatic interaction between carboxylate anions and quaternary ammonium cations and the most compact PIC micelles were achieved at the isoelectric point (i.e., [QDMA(+)]/[COO(-)] = 1), as confirmed by dynamic light scattering (DLS) measurement. Simultaneously, fluorescence spectroscopy revealed an evident emission turn-on and the maximum fluorescence intensity was observed near the isoelectric point due to the restriction of intramolecular rotation of TPE moieties within the PIC cores. The kinetic study supported a micelle fusion/fission mechanism on the formation of PIC micelles at varying charge ratios, exhibiting a quick time constant (τ1) relating to the formation of quasi-equilibrium micelles and a slow time constant (τ2) corresponding to the formation of final equilibrium micelles. Upon deceasing the pH of PIC micelles from 8 to 2 at the [QDMA(+)]/[COO(-)] molar ratio of 1, TPE-4COOH chromophores became gradually protonated and hydrophobic. The size of micellar nanoparticles underwent a remarkable decrease, whereas the fluorescence intensity exhibited a further increase by approximately 7.35-fold, presumably because of the formation of HBC micelles comprising cationic PQDMA coronas and PEO/TPE-4COOH hydrogen-bonded cores, an inverted micellar structures compared to initial PIC micelles. Moreover, the pH-mediated schizophrenic micellar transition from PIC to HBC with tunable AIE characteristic was reversible.

Diketopyrrolopyrrole-based ratiometric fluorescent probe for the sensitive and selective detection of cysteine over homocysteine and glutathione in living cells

A new fluorescent probe (DPP-AC) based on diketopyrrolopyrrole with an acrylate group was designed and synthesized for the sensitive and selective detection of biological thiols.

Silicon effect of dendritic polyphenyl derivatives: enhancement of aggregation-induced emission and emission color adjustment

Polyphenyl compounds could not emit strong fluorescence and adjust the emission colors. The “silicon effect” appeared to resolve this problem.

An AIE-active fluorescence turn-on bioprobe mediated by hydrogen-bonding interaction for highly sensitive detection of hydrogen peroxide and glucose

An AIE-active “turn-on” bioprobe is designed for hydrogen peroxide detection based on an imine-functionalized tetraphenylethene derivative.

Insight into the strong aggregation-induced emission of low-conjugated racemic C6-unsubstituted tetrahydropyrimidines through crystal-structure-property relationship of polymorphs

Racemic C6-unsubstituted tetrahydropyrimidines (THPs) are a series of fluorophores with a strong aggregation-induced emission (AIE) effect. However, they do not possess the structural features of conventional AIE compounds. In order to understand their AIE mechanism, here, the influences of the molecular packing mode and the conformation on the optical properties of THPs were investigated using seven crystalline polymorphs of three THPs (1-3). The racemic THPs 1-3 have low-conjugated and highly flexible molecular structures, and hence show practically no emission in different organic solvents. However, the fluorescence quantum yields of their polymorphs are up to 93%, and the maximum excitation (λex) and emission (λem) wavelengths of the polymorphs are long at 409 and 484 nm, respectively. Single-crystal structures and theoretical calculation of the HOMOs and LUMOs based on the molecular conformations of these polymorphs indicate that the polymorphs with the shortest λex and λem values possess a RS-packing mode (R- and S-enantiomers self-assemble as paired anti-parallel lines) and a more twisted conformation without through-space conjugation between the dicarboxylates, but the polymorphs with longer λex and λem values adopt a RR/SS-packing mode (R- and S-enantiomers self-assemble as unpaired zigzag lines) and a less twisted conformation with through-space conjugation between the dicarboxylates. The molecular conformations of 1-3 in all these polymorphs are stereo and more twisted than those in solution. Although 1-3 are poorly conjugated, the radiative rate constants (kr) of their polymorphs are as large as conventional fluorophores (0.41-1.03 × 108 s-1) because of improved electronic conjugation by both through-bond and through-space interactions. Based on the obtained results, it can be deduced that the strong AIE arises not only from the restriction of intramolecular motion but also from enhanced electronic coupling and radiatively-favored inter-crossed local excitation (LE) and intramolecular charge transfer (ICT) excitation states. The abnormal molecular structures, easily-controllable self-assembly of the R- and S-enantiomers, and the strong AIE effect make THPs very useful fluorophores for applications and theoretical research.

A turn-on fluorogenic Zn(ii) chemoprobe based on a terpyridine derivative with aggregation-induced emission (AIE) effects through nanofiber aggregation into spherical aggregates

The self-assembly of the terpyridine-based ligand exhibited strong emission in the presence of Zn²⁺ due to the formation of coordination bonds between the terpyridine moieties and the Zn²⁺ by the aggregation-induced emission effect.

Biosensing by luminogens with aggregation-induced emission characteristics

This tutorial review outlines the concept of aggregation-induced emission and its utility in biosensing applications.

A malonitrile-functionalized metal-organic framework for hydrogen sulfide detection and selective amino acid molecular recognition

A novel porous polymeric fluorescence probe, MN-ZIF-90, has been designed and synthesized for quantitative hydrogen sulfide (H₂S) fluorescent detection and highly selective amino acid recognition. This distinct crystalline structure, derived from rational design and malonitrile functionalization, can trigger significant enhancement of its fluorescent intensity when exposed to H₂S or cysteine molecules. Indeed this new metal-organic framework (MOF) structure shows high selectivity of biothiols over other amino acids and exhibits favorable stability. Moreover, in vitro viability assays on HeLa cells show low cytotoxicity of MN-ZIF-90 and its imaging contrast efficiency is further demonstrated by fluorescence microscopy studies. This facile yet powerful strategy also offers great potential of using open-framework materials (i.e. MOFs) as the novel platform for sensing and other biological applications.

A selective glutathione probe based on AIE fluorogen and its application in enzymatic activity assay

In this work, we design and synthesize a malonitrile-functionalized TPE derivative (TPE-DCV), which can react with thiol group through thiol-ene click reaction, leading to the fluorescence change of the system. Combined with the unique AIE property, TPE-DCV can selectively detect glutathione (GSH) but not cysteine or homocysteine. As the cleavage of GSSG with the aid of glutathione reductase produces GSH, which turns on the fluorescence of TPE-DCV, the ensemble of TPE-DCV and GSSG can thus serve as a label-free sensor for enzymatic activity assay of glutathione reductase. We also apply TPE-DCV for the detection of intracellular GSH in living cells.

A sensitivity tuneable tetraphenylethene-based fluorescent probe for directly indicating the concentration of hydrogen sulfide

A selective tetraphenylethene-based fluorescent H₂S probe was designed and synthesized, which exhibits tuneable sensitivity and could directly indicate the concentration of H₂S.

Construction of a fluorescence turn-on probe for highly discriminating detection of cysteine

We developed a cysteine specific probe by utilizing the remarkable difference in reactivity toward discriminating cysteine from homocysteine and glutathione. This probe was also successfully used for detection of Cys in living cells and monitoring cystathionine γ-lyase activityin vitro.

Synergizing nucleic acid aptamers with 1-dimensional nanostructures as label-free field-effect transistor biosensors

Since the introduction by Gold et al. in 1990, nucleic acid aptamers had evolved to become a true contender in biosensors for protein and cell detections. Aptamers are short strands of synthetically designed DNA or RNA oligonucleotides that can be self-assembled into unique 3-dimensional structures and can bind to different proteins, cells or even small molecules at a high level of specificity and affinity. In recent years, there had been many reports in literature in using aptamers in place of conventional antibodies as capture biomolecules on the surface. This is mainly due to the better thermal stability properties and ease in production. Consequently, also these characteristics allowed the aptamers to find use in field effect transistors (FETs) based upon 1D nanostructured (1D-NS) as label-free biosensing. In terms of designing label-free platforms for biosensors applications, 1D-NS FET had been an attractive option due to reported high sensitivities toward protein targets arising from the large surface area for detection as well as to their label-free nature. Since the first aptamer-based 1D-NS FET biosensor had surfaced in 2005, there had been many more improvements in the overall design and sensitivity in recent years. In this review, the latest developments in synergizing these two interesting areas of research (aptamers and 1D-NS FET) will be discussed for a range of different nanowire types as well as for the detection results.

Thiol reactive probes and chemosensors

Thiols are important molecules in the environment and in biological processes. Cysteine (Cys), homocysteine (Hcy), glutathione (GSH) and hydrogen sulfide (H₂S) play critical roles in a variety of physiological and pathological processes. The selective detection of thiols using reaction-based probes and sensors is very important in basic research and in disease diagnosis. This review focuses on the design of fluorescent and colorimetric probes and sensors for thiol detection. Thiol detection methods include probes and labeling agents based on nucleophilic addition and substitution, Michael addition, disulfide bond or Se-N bond cleavage, metal-sulfur interactions and more. Probes for H₂S are based on nucleophilic cyclization, reduction and metal sulfide formation. Thiol probe and chemosensor design strategies and mechanism of action are discussed in this review.

A seminaphthofluorescein-based fluorescent chemodosimeter for the highly selective detection of cysteine

A fluorescent chemodosimeter for cysteine detection was developed based on a tandem conjugate addition and intramolecular cyclization reaction. The method exhibited an excellent selectivity for cysteine over other biothiols such as homocysteine and glutathione.