Cell Membrane-Specific Fluorescent Probe Featuring Dual and Aggregation-Induced Emissions

A novel AIE-based mitochondria-targeting fluorescent probe for monitoring of the fluctuation of endogenous hypochlorous acid in ferroptosis models

Ferroptosis is a way of cell death mainly due to the imbalance between the production and degradation of lipid reactive oxygen species, which is closely associated with various diseases. Endogenous hypochlorous acid (HOCl) mainly produced in mitochondria is regarded as an important signal molecule of ferroptosis. Therefore, monitoring the fluctuation of endogenous HOCl is beneficial to better understand and treat ferroptosis-related diseases. Inspired by the promising aggregation-induced emission (AIE) properties of tetraphenylethene (TPE), herein, we rationally constructed a novel AIE-based fluorescent probe, namely QTrPEP, for HOCl with nice mitochondria-targeting ability and high sensitivity and selectivity. Probe QTrPEP consisted of phenylborate ester and the AIE fluorophore of quinoline-conjugated triphenylethylene (QTrPE). HOCl can brighten the strong fluorescence through a specific HOCl-triggered cleavage of the phenylborate ester bond and release of QTrPE, which has been demonstrated by MS, HPLC, and DLS experiments. In addition, combining QTrPE-doped test strips with a smartphone-based measurement demonstrated the excellent performance of the probe to sense HOCl. The obtained favorable optical properties and negligible cytotoxicity allowed the use of this probe for tracking of HOCl in three different cells. In particular, this work represents the first AIE-based mitochondria-targeting fluorescent probe for monitoring the fluctuation of HOCl in ferroptosis.

Dynamic insights into mitochondrial function: Monitoring viscosity and SO2 levels in living cells

Mitochondria, central organelles pivotal for eukaryotic cell function, extend their influence beyond ATP production, encompassing roles in apoptosis, calcium signaling, and biosynthesis. Recent studies spotlight two emerging determinants of mitochondrial functionality: intramitochondrial viscosity and sulfur dioxide (SO2) levels. While optimal mitochondrial viscosity governs molecular diffusion and vital processes like oxidative phosphorylation, aberrations are linked with neurodegenerative conditions, diabetes, and cancer. Similarly, SO2, a gaseous signaling molecule, modulates energy pathways and oxidative stress responses; however, imbalances lead to cytotoxic sulfite and bisulfite accumulation, triggering disorders such as cancer and cardiovascular anomalies. Our research focused on development of a dual-channel fluorescent probe, applying electron-withdrawing acceptors within a coumarin dye matrix, facilitating monitoring of mitochondrial viscosity and SO2 in live cells. This probe distinguishes fluorescence peaks at 650 nm and 558 nm, allowing ratiometric quantification of SO2 without interference from other sulfur species. Moreover, it enables near-infrared viscosity determination, particularly within mitochondria. The investigation employed theoretical calculations utilizing Density Functional Theory (DFT) methods to ascertain molecular geometries and calculate rotational energies. Notably, the indolium segment of the probe exhibited the lowest rotational energy, quantified at 7.38 kcals/mol. The probe featured heightened mitochondrial viscosity dynamics when contained within HeLa cells subjected to agents like nystatin, monensin, and bacterial lipopolysaccharide (LPS). Overall, our innovative methodology elucidates intricate mitochondrial factors, presenting transformative insights into cellular energetics, redox homeostasis, and therapeutic avenues for mitochondrial-related disorders.

Thermal accelerated urease-driven hyaluronan-targeted melanin nano-missile for bio-radar detection and chemodrug-free phototherapy

Polymer-based nanomotors are attracting increasing interest in the biomedical field due to their microscopic size and kinematic properties which support overcoming biological barriers, completing cellular uptake and targeted blasting in limited spaces. However, their applications are limited by the complex viscous physiological environment and lack of sufficient biocompatibility. This manuscript firstly reports a natural melanin nano-missile of MNP@HA-EDA@Urease@AIE PS (MHUA) based on photothermally accelerated urease-driven to achieve chemodrug-free phototherapy. Compared to conventional nano-missiles that only provide driving force, this photothermally accelerated urease-driven nanomotor is independent of chemodrug to maximise biocompatibility, and achieve ideal therapeutic effect through targeted PTT/PDT. In particular, the thermal effect can not only boost the catalytic activity of urease but also achieve ideally anti-tumor effect. In addition, guided by and AIE PS, the nanomotor can generate 1O2 to achieve PDT and be traced in real time serving as an effective fluorescent bio-radar for intracellular self-reporting during cancer treatment. Finally, the targeting ability of MUHA is provided by hyaluronan. Taken together, this MHUA platform provides a simple and effective strategy for target/fluorescence radar detective-guided PTT/PDT combination, and achieves good therapeutic results without chemodrug under thermal accelerated strategy, providing a new idea for the construction of chemodrug-free nanomotor-therapy system.

Near-infrared Absorption and Emission Probes with Optimal Connection Bridges for Live Monitoring of NAD(P)H Dynamics in Living Systems

Two NAD(P)H-biosensing probes consisting of 1,3,3-trimethyl-3H-indolium and 3-quinolinium acceptors, linked by thiophene, A, and 3,4-ethylenedioxythiophene, B, bridges are detailed. We synthesized probes C and D, replacing the thiophene connection in probe A with phenyl and 2,1,3-benzothiadiazole units, respectively. Probe E was prepared by substituting probe A's 3-quinolinium unit with a 1-methylquinoxalin-1-ium unit. Solutions are non-fluorescent but in the presence of NADH, exhibit near-infrared fluorescence at 742.1 nm and 727.2 nm for probes A and B, respectively, and generate absorbance signals at 690.6 nm and 685.9 nm. In contrast, probes C and D displayed pronounced interference from NADH fluorescence at 450 nm, whereas probe E exhibited minimal fluorescence alterations in response to NAD(P)H. Pre-treatment of A549 cells with glucose in the presence of probe A led to a significant increase in fluorescence intensity. Additionally, subjecting probe A to lactate and pyruvate molecules resulted in opposite changes in NAD(P)H levels, with lactate causing a substantial increase in fluorescence intensity, conversely, pyruvate resulted in a sharp decrease. Treatment of A549 cells with varying concentrations of the drugs cisplatin, gemcitabine, and camptothecin (5, 10, and 20 μM) led to a concentration-dependent increase in intracellular fluorescence intensity, signifying a rise in NAD(P)H levels. Finally, fruit fly larvae were treated with different concentrations of NADH and cisplatin illustrating applicability to live organisms. The results demonstrated a direct correlation between fluorescence intensity and the concentration of NADH and cisplatin, respectively, further confirming the efficacy of probe A in sensing changes in NAD(P)H levels within a whole organism.

Detecting Disruption of HER2 Membrane Protein Organization in Cell Membranes with Nanoscale Precision

The spatiotemporal organization of proteins within the cell membrane can affect numerous biological functions, including cell signaling, communication, and transportation. Deviations from normal spatial arrangements have been observed in various diseases, and a better understanding of this process is a key stepping stone to advancing development of clinical interventions. However, given the nanometer length scales involved, detecting these subtle changes has primarily relied on complex super-resolution and single-molecule imaging methods. In this work, we demonstrate an alternative fluorescent imaging strategy for detecting protein organization based on a material that exhibits a unique photophysical behavior known as aggregation-induced emission (AIE). Organic AIE molecules have an increase in emission signal when they are in close proximity, and the molecular motion is restricted. This property simultaneously addresses the high background noise and low detection signal that limit conventional widefield fluorescent imaging. To demonstrate the potential of this approach, the fluorescent molecule sensor is conjugated to a human epidermal growth factor receptor 2 (HER2)-specific antibody and used to investigate the spatiotemporal behavior of HER2 clustering in the membrane of HER2-overexpressing breast cancer cells. Notably, the disruption of HER2 clusters in response to an FDA-approved monoclonal antibody therapeutic (Trastuzumab) is successfully detected using a simple widefield fluorescent microscope. While the sensor demonstrated here is optimized for sensing HER2 clustering, it is an easily adaptable platform. Moreover, given the compatibility with widefield imaging, the system has the potential to be used with high-throughput imaging techniques, accelerating investigations into membrane protein spatiotemporal organization.

Visualization of Cellular Membranes in 2D and 3D Conditions Using a New Fluorescent Dithienothiophene S,S-Dioxide Derivative

Cellular membranes play a key role in cell communication with the extracellular environment and neighboring cells. Any changes, including their composition, packing, physicochemical properties and formation of membrane protrusions may affect cells feature. Despite its great importance, tracking membrane changes in living cells is still a challenge. For investigation of processes related to tissue regeneration and cancer metastasis, such as the induction of epithelial-mesenchymal transition, increased cell motility, and blebbing, the possibility to conduct prolonged observation of membrane changes is beneficial, albeit difficult. A particular challenge is conducting this type of research under detachment conditions. In the current manuscript, a new dithienothiophene S,S-dioxide (DTTDO) derivative is presented as an effective dye for staining the membranes of living cells. The synthetic procedures, physicochemical properties, and biological activity of the new compound are presented herein. In addition to the labeling of the membranes in a monolayer culture, its usefulness for visualization of membranes under detachment conditions is also demonstrated. Obtained data have proven that a new DTTDO derivative may be used to stain membranes in various types of experimental procedures, from traditional 2D cell cultures to unanchored conditions. Moreover, due to the specific optical properties, the background signal is reduced and, thus, observation may be performed without washing.

Red-Emitting Dithienothiophene S,S-Dioxide Dyes for Cellular Membrane Staining

A series of dithienothiophene S,S-dioxide (DTTDO) dyes was designed, synthesized, and investigated for their suitability in fluorescent cell imaging. Synthetized (D-π-A-π-D)-type DTTDO derivatives have molecule lengths close to the thickness of the phospholipid membrane, and they contain on both ends two positively charged or neutral polar groups to increase their solubility in water and to ensure simultaneous interaction with polar groups of the inner and outer part of the cellular membrane. DTTDO derivatives exhibit absorbance and emission maxima in the 517-538 nm and 622-694 nm range, respectively, and a large Stokes shift up to 174 nm. Fluorescence microscopy experiments revealed that these compounds selectively intercalate into cell membranes. Moreover, a cytotoxicity assay conducted on a model human live cells indicates low toxicity of these compounds at the concentrations required for effective staining. With suitable optical properties, low cytotoxicity, and high selectivity against cellular structures, DTTDO derivatives are proven to be attractive dyes for fluorescence-based bioimaging.

Rationally designed near-infrared AIEgens photosensitizer for cell membrane-targeted photo-driven theranostics

The complex environment of solid tumors and the migration of cancer cells are important obstacles to the cure of tumors through conventional therapy. Developing secure and efficient photosensitizers (PSs) is the crux to the application of photodynamic therapy (PDT) in the noninvasive clinical treatment of tumors. Herein, a series of PSs (DCTPys) with the same skeleton structure was designed and prepared. The unique molecular structure of DCTPys endows them with aggregation-induced emission (AIE) property and efficient reactive oxygen species (ROS) generation ability. Interestingly, due to their hydrophilic and lipophilic nature, DCTPys have fine staining and visual identification performance for the plasma membrane. In addition (e.g., MeDCTPy-OH), ROS is produced by MeDCTPy-OH under white light irradiation, which could destroy the completeness of cell membranes and cause cell necrosis. Importantly, morphology imaging of the cell membrane using MeDCTPy-OH enables real-time tracking of cancer cell ablation. This allowed cell necrosis and PDT effects to be observed under mild conditions. We conclude that DCTPys are potential cell membrane-selective PSs for PDT, and it is worth systematically exploring the phototherapeutic effect of these PSs on tumors in vivo.

Dual-Modality Molecular Imaging of Tumor via Quantum Dots-Liposome-Microbubble Complexes

Molecular imaging has demonstrated promise for evaluating the expression levels of biomarkers for the early prediction of tumor progression and metastasis. However, most of the commonly used molecular imaging modalities are relatively single and have difficulties imaging complex biological processes. Here, we fabricated αvβ3-integrin-targeted quantum-dots-loaded liposome-microbubble (iRGD-QDLM) complexes that combined ultrasound imaging with optical imaging. The resulting iRGD-QDLM has excellent binding capability to 4T1 breast cancer cells. Ultrasound molecular imaging of 4T1 tumors demonstrated that significantly enhanced ultrasound molecular signals could be observed in comparison with non-targeted QDLM. Importantly, our study also suggested that iRGD-QDL on the surface of microbubbles could be delivered into a tumor by ultrasound-mediated microbubble destruction and adhered to αvβ3 integrin on breast cancer cells, achieving transvascular fluorescent imaging. Our study provides a novel approach to dual-modality molecular imaging of αvβ3 integrin in the tumor tissue.

A julolidine-chalcone-based fluorescent probe for detection of Al3+ in real water sample and cell imaging

A fluorescent probe 1 based on julolidine-chalcone derivative, which can specifically recognize aluminum ion with high selectivity and anti-interference, was developed. Probe 1 has good fluorescence stability and can detect Al³⁺ with turn-on fluorescence in a wide pH range of 4.0-9.0. The probe has good repeatability for the detection of Al³⁺ and fluorescence turn-on and off can be repeated with the alternate Al³⁺ and EDTA. The sensing mechanism is speculated that Al³⁺ will coordinate with hydroxyl oxygen and carbonyl oxygen on the probe through in situ ¹H NMR and HRMS combing with Job's plot. The probe can also detect Al³⁺ in actual water samples and applied to monitor Al³⁺ in biological system.

Antipermeability Strategy to Achieve Extremely High Specificity and Ultralong Imaging of Diverse Cell Membranes Based on Restriction-Induced Emission of AIEgens

Long-term in situ cell membrane-targeted bioimaging is of great significance for studying specific biological processes and functions, but currently developed membrane probes are rarely simultaneously used to image the plasma membrane of animal and plant cells, and these probes lack sufficiently high long-term targeting ability. Herein, we proposed an antipermeability strategy to achieve highly specific and long-term imaging of plasma membranes of both human and plant cells using the steric hindrance effect and restriction-induced emission of AIE-active probes based on an updated membrane model. A certain degree of rigidity of plasma membrane containing a large ratio of rigid cholesterol molecules in the updated membrane model provides a promising opportunity to design antipermeable probes by introducing a rigid steric hindrance group in the probe. The designed antipermeable probes can anchor inside plasma membrane for a long term relying on the combination of the steric hindrance effect and the electrostatic and hydrophobic interactions between the probe and the membrane, as well as light up the membrane via the restriction-induced emission mechanism. The excellent performance in imaging completeness and specificity for both human cells and plant cells clearly shows that these designed probes possess outstanding antipermeability to achieve long-term specific imaging of membrane. These probes also show some advanced features such as ultrafast staining, wash-free merit, favorable biocompatibility, good photostability, and effective resistance to viscosity and pH alteration. This work also provides a valuable design principle for membrane probes of plant cells that the designed probes require a suitable molecular size favoring the penetration of small pores of cell walls.

Dual-Emission GFP Chromophore-Based Derivative for Imaging and Discriminating Aβ Oligomers and Aggregates

β-Amyloid deposition is one of the main pathological features of Alzheimer's disease (AD). The development of fluorescent probes targeting specific β-amyloid species has recently become an attractive strategy to achieve the early diagnosis of AD. In this work, a dual-channel fluorescent protein chromophore derivative C17 was rationally designed and synthesized for the detection and discrimination of Aβ₄₂ aggregates and oligomers. C17 exhibits a specific turn-on emission peak for Aβ₄₂ oligomers at ∼470 nm (peak A) and a peak at ∼600 nm (peak B) for both Aβ₄₂ oligomers and Aβ₄₂ aggregates. Taking advantage of the dual emission of the probe, the dynamic aggregation process of the Aβ₄₂ peptide was monitored in solution. Moreover, double staining of brain sections from transgenic AD mice revealed that peak A of C17 preferentially detected Aβ₄₂ oligomers, whereas peak B was more sensitive to Aβ₄₂ aggregates. The fact that probe C17 can be used for dissecting these two Aβ₄₂ species makes C17 a comprehensive tool for β-amyloid aggregation studies in AD research.

Two Water-Soluble and Wash-Free Fluorogenic Probes for Specific Lighting Up Cancer Cell Membranes and Tumors

The construction of microenvironment-sensitive probes with good cell membrane-targetability can reveal the fundamental properties of cell membranes. Herein, two polarity-sensitive probes, termed MEMs were reported for the first time to specifically light up cancer cell membranes. Both probes were designed with tetrahydroquinoxaline coumarin amide as the fluorophore, and quaternary ammonium groups were appended to increase water solubility and target cell membranes. In vitro studies showed that the fluorescence of both probes displayed strong polarity dependence and had a wide linear range to polarity (Δf). MEMs also displayed excellent cell membrane targeting ability and could long-term light up cell membranes with red fluorescence and a wash-free process. More excitingly, MEMs could specifically light up cancer cell membranes, revealing that cancer cells might have lower cell membrane polarity than normal cells. In vivo studies showed that MEMs could also effectively distinguish tumors from normal tissues. Overall, this work has not only developed two polarity-sensitive probes with good cell membrane targetability, but also provided new insights and methods for an in-depth understanding of cancer cells and cancer diagnosis.

A two-photon fluorescence silica nanoparticle-based FRET nanoprobe platform for effective ratiometric bioimaging of intracellular endogenous adenosine triphosphate

Two-photon fluorescence imaging is one of the most attractive imaging techniques for monitoring important biomolecules in the biomedical field due to its advantages of low light scattering, high penetration depth, and suppressed photodamage/phototoxicity under near-infrared excitation. However, in actual biological imaging, organic two-photon fluorescent dyes have disadvantages such as high biological toxicity and their fluorescence efficiency is easily affected by the complex environment in organisms. In this study, a novel nanoprobe platform with two-photon dye-doped silica nanoparticles was developed for FRET-based ratiometric biosensing and bioimaging, with endogenous ATP chosen as the target for detection. The nanoprobe has three components: (1) a two-photon dye-doped silica nanoparticle core, which serves as an energy donor for FRET; (2) amino-modified hairpin primers with carboxy fluorescein as an energy acceptor for FRET; (3) an aptamer acting as a recognition unit to realize the probing function. The nanoprobe showed ratiometric fluorescence responses for ATP detection with high sensitivity and high selectivity in vivo. Moreover, the nanoprobe showed satisfactory ratiometric two-photon fluorescence imaging of endogenous ATP in living cells and tissues (penetration depth of 190 nm). These results indicated that novel two-photon silica nanoparticles can be constructed by doping a two-photon fluorescent dye into silica nanoparticles, and they can effectively solve the disadvantages of two-photon fluorescent dyes. These excellent performances indicate that this novel nanoprobe platform will become a very valuable molecular imaging tool, which can be widely used in the biomedical field for drug screening and disease diagnosis and other related research.

A ratiometric near-infrared fluorescent probe based on a novel reactive cyanine platform for mitochondrial pH detection

A near-infrared reactive cyanine platform (probe A) was prepared by condensation of 9-chloro-1,2,3,4-tetrahydro-10-methyl-acridinium iodide with Fisher's aldehyde. A near-infrared fluorescent probe (probe B) was prepared by modifying a reactive chlorine atom of probe A with tert-butyl(2-aminoethyl)carbamate through a substitution reaction. The deprotection of the Boc group of probe B was achieved under an acidic condition, affording an amine-functionalized cyanine dye (probe C). A near-infrared ratiometric fluorescent probe (probe D) for mitochondrial pH detection was synthesized by conjugating a FRET coumarin donor to a FRET cyanine acceptor (probe C) through an amide bond connection. Probe A shows low fluorescence of 2% due to an electron-withdrawing chlorine atom, while probes B-D display high fluorescence quantum yields of 60%, 32%, and 35% in aqueous solutions containing 10% ethanol, respectively. Probes B-D show strong fluorescence with push-pull molecular structures in neutral and basic pH conditions. However, protonation of the probe's second amine at the 9-position under acidic condition disrupts the push-pull feature of the probes, resulting in fluorescence quenching of the new cyanine fluorophores. The probes can selectively stain mitochondria, while probe D was employed to detect pH changes in HeLa cells and Drosophila melanogaster first-instar larvae.

A butterfly-shaped ESIPT molecule with solid-state fluorescence for the detection of latent fingerprints and exogenous and endogenous ONOO- by caging of the phenol donor

The latent fingerprints (LFPs) at the crime scene are unique and stable, which are considered as an important clue in criminal justice and forensic identification. Herein, a butterfly-shaped molecule DPTS with solid fluorescence plus excited-state intramolecular proton transfer (ESIPT) properties was used to develop for enhancing the visualization of the LFPs. Considering the solid fluorescence of DPTS, the color and efficiency of DPTS with a large Stokes shift (216 nm) can be tuned by changing the morphology of its aggregates, and gradually red-shifted (green-yellow-red) with increasing water content. Furthermore, its effectiveness for the detection of LFPs was demonstrated on various different substrates including paper box, tinfoil and weighting paper. The emissive fingerprint of DPTS obtained gave good fluorescence images with high contrast and resolution such as the core, delta, bifurcation, ridge termination, independent ridge and pores. Caging of the phenol donor of DPTS with a sensitive biomarker group provided DPTS-ONOO^-, which had high sensitive with detection limit of 5 nM and the quantification limit of 21 nM toward ONOO^-. Modularly derived DPTS-ONOO^- was synthesized and demonstrated specific fluorescence imaging of exogenous and endogenous peroxynitrite (ONOO^-) in living macrophage cells.

BioAIEgens derived from rosin: how does molecular motion affect their photophysical processes in solid state?

The exploration of artificial luminogens with bright emission has been fully developed with the advancement of synthetic chemistry. However, many of them face problems like weakened emission in the aggregated state as well as poor renewability and sustainability. Therefore, the development of renewable and sustainable luminogens with anti-quenching function in the solid state, as well as to unveil the key factors that influence their luminescence behavior become highly significant. Herein, a new class of natural rosin-derived luminogens with aggregation-induced emission property (AIEgens) have been facilely obtained with good biocompatibility and targeted organelle imaging capability as well as photochromic behavior in the solid state. Mechanistic study indicates that the introduction of the alicyclic moiety helps suppress the excited-state molecular motion to enhance the solid-state emission. The current work fundamentally elucidates the role of alicyclic moiety in luminogen design and practically demonstrates a new source to large-scalely obtain biocompatible AIEgens.

Photostable AIE probes for wash-free, ultrafast, and high-quality plasma membrane staining

Plasma membrane (PM), a fundamental building component of a cell, is responsible for a variety of cell functions and biological processes. However, it is still challenging to acquire its morphology and morphological variation information via an effective approach. Herein, we report a PM imaging study regarding an aggregation-induced emission luminogen (AIEgen) called tetraphenylethylene-naphthalimide+ (TPE-NIM+), which is derived from our previously reported tetraphenylethylene-naphthalimide (TPE-NIM). The designed AIEgen (TPE-NIM+) shows significant characteristics of ultrafast staining, high photostability, wash-free property, and long retention time at the PM, which can structurally be correlated with its positively charged quaternary amine and hydrophobic moiety. TPE-NIM+ is further applied for staining of different cell lines, proving its universal PM imaging capability. Most importantly, we demonstrate that TPE-NIM+ can clearly delineate the contours of densely packed living cells with high cytocompatibility. Therefore, TPE-NIM+ as a PM imaging reagent superior to currently available commercial PM dyes shall find a number of applications in the biological/biomedical fields and even beyond.

Reaction-Based Ratiometric and Colorimetric Chemosensor for Bioimaging of Biosulfite in Live Cells, Zebrafish, and Food Samples

In this work, a reaction-based ratiometric and colorimetric sensor was designed and synthesized for probing bisulfite (HSO₃^-) by coupling coumarin (CM) with barbituric (BA) moiety. Further tests have shown that CM-BA has high selectivity and sensitivity for the recognition of HSO₃^-, which can be applied for the detection of HSO₃^- in environmental and biological systems very effectively. The fluorescence intensity ratios (F₄₆₂/F₅₆₈) exhibited an outstanding HSO₃^--dependent response with ultrafast response time (within 20 s) and a lower detection limit (105 nM). Meanwhile, the color of the CM-BA solution changed from green to colorless during the recognition process, and its fluorescence changed from green to blue. The mechanism of response is confirmed by the density functional theory (DFT) model. In summary, CM-BA has demonstrated low toxicity and good permeability, which can be applied for imaging HSO₃^- in cells and zebrafish safely and effectively. Besides, this novel sensor CM-BA successfully realized the quantification of the concentration of HSO₃^- in paper strips and food samples.