An Easily Available Ratiometric Reaction-Based AIE Probe for Carbon Monoxide Light-up Imaging

AIE-based nanoaggregate tracker: high-fidelity visualization of lysosomal movement and drug-escaping processes

High-fidelity imaging and long-term visualization of lysosomes are crucial for their functional evaluation, related disease detection and active drug screening. However, commercial aggregation-caused quenching probes are not conducive to precise lysosomal imaging because of their inherent drawbacks, like easy diffusion, short emission and small Stokes shift, let alone their long-term tracing due to rapid photobleaching. Herein we report a novel aggregation-induced emission (AIE)-based TCM-PI nanoaggregate tracker for direct visualization of lysosomes based on the building block of tricyano-methylene-pyridine (TCM), wherein introduced piperazine (PI) groups behave as targeting units to lysosomes upon protonation, and the self-assembled nanostructure contributes to fast endocytosis for enhanced targeting ability as well as extended retention time for long-term imaging. The piperazine-stabilized TCM-PI nanoaggregate shifts the emission maximum to 677 nm in an aqueous environment, and falls within the desirable NIR region with a large Stokes shift of 162 nm, thereby greatly reducing biological fluorescent background interference. In contrast with the commercially available LysoTracker Red, the essential AIE characteristic of high photostability can guarantee three-dimensional high-fidelity tracing with low photobleaching, and little diffusion from lysosomes, and especially overcome the AIE bottleneck to target specificity. Consequently, the AIE-based nanoaggregate tracker successfully achieves the high-fidelity and long-term tracing of lysosomal movement and even monitors the drug-escaping process from lysosomes to cell nuclei, which provides a potential tool to benefit drug screening.

Fluorogenic Enzyme-Triggered Domino Reactions Producing Quinoxalin-2(1H)-one-based Heterocycles

A simple and effective biocompatible domino reaction triggered by a model protease and leading to the formation of strongly fluorescent quinoxalin-2(1H)-one N-heterocycles is described. Some positive attributes including versatility and the ability to provide outstanding fluorescence "OFF-ON" responses were revealed by this work. They open the way for practical applications of this novel type of "covalent-assembly"-based fluorescent probe in the fields of sensing and bioimaging.

Sequencing of Small DNA Fragments with Aggregated-Induced-Emission Molecule-Labeled Nucleotides

Sequencing by synthesis is a significant method for high-throughput DNA sequencing. Herein, we synthesized terminal aggregated-induced-emission luminogen (AIEgen) labeled nucleotides (dNTPs-HCAP) that could serve as substrates for some polymerases and applied them into the sequencing of small DNA fragments. In the process of DNA amplification, ratiometric AIEgens are released from dNTPs-HCAP and aggregate through the effects of phosphatase, which results in changes in the ratiometric fluorescent signals. With the AIEgen-labeled nucleotides, we accomplished the sequencing of small DNA fragments through double changes in fluorescence. In addition, we achieved the differentiation of single nucleotide polymorphisms through rolling circle amplification reactions without the addition of signal probes, which is fast and cost-effective. The introduction of ratiometric AIEgens into DNA synthesis makes the detection of DNA sequences more efficient and accurate. Therefore, the development of AIEgen-labeled nucleotides is meaningful for the study of DNA sequencing methods.

CO Sense and Release Flavonols: Progress toward the Development of an Analyte Replacement PhotoCORM for Use in Living Cells

Carbon monoxide (CO) is a signaling molecule in humans. Prior research suggests that therapeutic levels of CO can have beneficial effects in treating a variety of physiological and pathological conditions. To facilitate understanding of the role of CO in biology, molecules that enable fluorescence detection of CO in living systems have emerged as an important class of chemical tools. A key unmet challenge in this field is the development of fluorescent analyte replacement probes that replenish the CO that is consumed during detection. Herein, we report the first examples of CO sense and release molecules that involve combining a common CO-sensing motif with a light-triggered CO-releasing flavonol scaffold. A notable advantage of the flavonol-based CO sense and release motif is that it is trackable via fluorescence in both its pre- and postsensing (pre-CO release) forms. In vitro studies revealed that the PdCl₂ and Ru(II)-containing CORM-2 used in the CO sensing step can result in metal coordination to the flavonol, which minimizes the subsequent CO release reactivity. However, CO detection followed by CO release is demonstrated in living cells, indicating that a cellular environment mitigates the flavonol/metal interactions.

Small-molecule fluorescent probes for imaging gaseous signaling molecules: current progress and future implications

Endogenous gaseous signaling molecules including nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H2S) have been demonstrated to perform significant physiological and pharmacological functions and are associated with various diseases in biological systems. In order to obtain a deeper insight into their roles and mechanisms of action, it is desirable to develop novel techniques for effectively detecting gaseous signaling molecules. Small-molecule fluorescent probes have been proven to be a powerful approach for the detection and imaging of biological messengers by virtue of their non-invasiveness, high selectivity, and real-time in situ detection capability. Based on the intrinsic properties of gaseous signaling molecules, numerous fluorescent probes have been constructed to satisfy various demands. In this perspective, we summarize the recent advances in the field of fluorescent probes for the detection of NO, CO and H2S and illustrate the design strategies and application examples of these probes. Moreover, we also emphasize the challenges and development directions of gasotransmitter-responsive fluorescent probes, hoping to provide a general implication for future research.

A Nuclear-Localized Naphthalimide-Based Fluorescent Light-Up Probe for Selective Detection of Carbon Monoxide in Living Cells

A nuclear-localized fluorescent light-up probe, NucFP-NO₂, was designed and synthesized that can detect CO selectively in an aqueous buffer (pH 7.4, 37 °C) through the CO-mediated transformation of the nitro group into an amino-functionalized moiety. This probe triggered a more than 55-fold "turn-on" fluorescence response to CO without using any metal ions, e.g., Pd, Rh, Fe, etc. The enhanced response is highly selective over a variety of relevant reactive oxygen, nitrogen, and sulfur species and also various biologically important cationic, anionic, and neutral species. The detection limit of this probe for CO is as low as 0.18 μM with a linear range of 0-70 μM. Also, this fluorogenic probe is an efficient candidate for monitoring intracellular CO in living cells (RAW 264.7, A549 cells), and the fluorescence signals predominantly localize in the nuclear region.

Enhancement of the Aggregation-Induced Emission by Hydrogen Bond for Visualizing Hypochlorous Acid in an Inflammation Model and a Hepatocellular Carcinoma Model

As an important reactive oxygen species, hypochlorous acid (HClO) is produced in various physiological processes. The abnormal rise of the HClO level is associated with a large number of inflammatory diseases. In this work, we develop a simple, aqueous-soluble aggregration-induced emission (AIE) probe for sensing HClO with significant aggregation-induced fluorescence (>1000 times). Two probes, CH₃O-TPE-Py⁺-N⁺ (COTN) and OH-TPE-Py⁺-N⁺ (HOTN) (TPE, tetraphenylethylene), are synthesized for sensing HClO by the cleavage of the Py⁺-N⁺ group; the reaction products are CH₃O-TPE-CHO (COT) and OH-TPE-CHO (HOT), respectively. The hydrophobicity of the probes is changed with the increased aggregation-induced emission. During the process, HOTN shows significantly better response than COTN. The slightly different chemical structures of COTN and HOTN result in a significant response to HClO. The theoretical calculation data support the theory that the hydrogen bond contributes to the excellent sensitivity for HClO. On the basis of the good response to HClO in vitro, HOTN is used to image inflammation and hepatocellular carcinoma in vivo because these diseases always produce high HClO levels.

The development of a hemicyanine-based ratiometric CO fluorescent probe with a long emission wavelength and its applications for imaging COin vitroandin vivo

A novel ratiometric fluorescent probe,Hcy-CO, with long-wavelength emission was developed for visualizing CO in living cells and zebrafish.

Deeper insight into protease-sensitive "covalent-assembly" fluorescent probes for practical biosensing applications

We report a rational and systematic study devoted to the structural optimisation of a novel class of protease-sensitive fluorescent probes that we recently reported (S. Debieu and A. Romieu, Org. Biomol. Chem., 2017, 15, 2575-2584), based on the "covalent-assembly" strategy and using the targeted enzyme penicillin G acylase as a model protease to build a fluorescent pyronin dye by triggering a biocompatible domino cyclisation-aromatisation reaction. The aim is to identify ad hoc probe candidate(s) that might combine fast/reliable fluorogenic "turn-on" response, full stability in complex biological media and ability to release a second molecule of interest (drug or second fluorescent reporter), for applications in disease diagnosis and therapy. We base our strategy on screening a set of active methylene compounds (C-nucleophiles) to convert the parent probe to various pyronin caged precursors bearing Michael acceptor moieties of differing reactivities. In vitro stability and fluorescent enzymatic assays combined with HPLC-fluorescence analyses provide data useful for defining the most appropriate structural features for these fluorogenic scaffolds depending on the specifications inherent to biological application (from biosensing to theranostics) for which they will be used.