A side-chain engineering strategy for constructing fluorescent dyes with direct and ultrafast self-delivery to living cells

Labeling selectivity of lipid droplets fluorescent probes: Twisted intramolecular charge transfer (TICT) vs intramolecular charge transfer (ICT)

Fluorescence imaging technology is a versatile and essential tool in the field of biomedical research. To obtain excellent imaging results, the precise labeling of fluorescent probes is an important prerequisite. Nevertheless, the labeling selectivity of most fluorescent probes is not satisfactory, new design concepts are desperately needed. In this context, two isomeric lipid droplets (LDs) fluorescent probes Lipi-Cz-1 and Lipi-Cz-2 have been sophisticatedly developed with TICT and ICT-emitting characteristic, respectively. The more environmentally sensitive TICT-emitting Lipi-Cz-1 exhibits a significantly enhanced labeling selectivity in LDs imaging compared to the ICT-emitting Lipi-Cz-2, sufficiently illustrating the effectiveness of TICT-emitting characteristic in improving labeling selectivity. Additionally, Lipi-Cz-1 displays high photostability and biocompatibility. These advantages enable Lipi-Cz-1 to be finely applied in multimode fluorescence imaging, e.g. time-lapse 3D confocal imaging to monitor changes of the number and size of LDs during starvation, two-photon 3D imaging to compare the variations of LDs in various liver tissues, and STED super-resolution imaging to visualize the nanoscale LDs with the resolution of 65 nm. Overall, these imaging findings validate the effectiveness of the new strategy for improving the labeling selectivity.

Bacterial lipase-responsive polydopamine nanoparticles for detection and synergistic therapy of wound biofilms infection

Wound biofilms represent an elusive conundrum in contemporary treatment and diagnostic options, accredited to their escalating antibiotic resistance and interference in chronic wound healing processes. Here, we developed mesoporous polydopamine (mPDA) nanoparticles, and grafted with rhodamine B (Rb) as biofilm lipase responsive detection probe, followed by π - π stacking mediated ciprofloxacin (CIP) loading to create mP-Rb@CIP nanoparticles. mPDA NPs with a melanin structure could quench fluorescence emissions of Rb. Once encountering biofilm in vivo, the ester bond in Rb and mPDA is hydrolyzed by elevated lipase concentrations, triggering the liberation of Rb and restore fluorescence emissions to achieve real-time imaging of biofilm-infected wounds. Afterwards, the 808 nm near-infrared (NIR) illumination initiates a spatiotemporal controlled antibacterial photothermal therapy (PTT), boosting its effectiveness through photothermal-triggered CIP release for synergistic biofilm eradication. The mP-Rb@CIP platform exhibits dual diagnostic and therapeutic functions, efficaciously treating biofilm-infected wounds in vivo and in vitro. Particularly, the mP-Rb@CIP/NIR procedure expedites wound-healing by alleviating oxidative stress, modulating inflammatory mediators, boosting collagen synthesis, and promoting angiogenesis. Taken together, the theranostic nanosystem strategy holds significant potential for addressing wound biofilm-associated infections.

A small-molecule probe to decipher stress-induced ER microenvironments and ER-Golgi communication

Cellular stress is a crucial factor in regulating and maintaining both organismal and microenvironmental homeostasis. It induces a response that also affects the micropolarity of specific cellular compartments, which is essential for early disease diagnosis. In this contribution, we present a quantitative study of micropolarity changes inside the endoplasmic reticulum (ER) during the G1/S and G2/M phases, using a biocompatible small-molecule fluorophore called ER-Oct. This probe is selectively driven to the ER by its hydrophobicity, and it has the fastest diffusion properties among a series of analogous probes. We found that induced ER stress caused cell cycle arrests leading to an increase in ER micropolarity which is well supported by lambda scanning experiments and fluorescence lifetime imaging microscopy (FLIM) as well. ER-Oct is a versatile staining agent that could effectively stain the ER in various living/fixed mammalian cells, isolated ER, Caenorhabditis elegans, and mice tissues. Furthermore, we used this probe to visualize a well-known biological event, ER to Golgi transport, by live-cell fluorescence microscopy. Our exhaustive investigation of micropolarity using ER-staining dye provides a new way to study ER stress, which could provide a deeper understanding of proteostasis in model systems and even in fixed patient samples.

Unique fluorescent probe for the recognition of late apoptosis via translocation from plasma membrane to nucleus

Cell apoptosis is a crucial physiological process playing central roles in key biological and pathological activities. However, the current fluorescent probes for the detection of late apoptosis were "off-on" probes, which were facilely interfered by false positive signals caused by inhomogeneous staining and other factors. Herein, a unique fluorescent probe (NPn) discriminating late apoptosis from early apoptosis and heathy status with two different sets of fluorescent signals have been prepared, to overcome the possible false positive signals. NPn was designed impermeable to biomembranes and simultaneously with high affinity to DNA/RNA, which localized on the plasma membranes of living and early apoptotic cells, while relocated to the nucleus in late apoptotic cells. The hydrophilic amine unit and small ion radius were responsive for its membrane impermeability, which was confirmed with two control molecules without amine group. Using the probe, we have successfully evaluated the cell apoptosis induced by ultraviolet irradiation, rotenone, colchicine, and paclitaxel, demonstrating its potential application in biological researches.

Real-Time Monitoring of Adsorption-Induced Scrolling of Colloidal Inorganic Nanosheets

Inorganic nanotubes have attracted much attention due to their unique physicochemical properties. Nanotubes can be prepared by scrolling exfoliated nanosheets under ambient conditions. However, how the nanosheet scrolled in its colloidal state has not been experimentally visualized. In this paper, we directly observed the scrolling process of nanosheets upon adsorption of organic cations. Exfoliated flat nanosheets of niobate and clay in aqueous colloids were found to scroll by adding organic cations, such as exfoliation reagents, to the colloids. Employment of cationic stilbazolium dye enabled in situ observation of the dye adsorption and scrolling by optical microscopy based on changes in color and morphology of the nanosheets. The scrolling was promoted for nanosheets adsorbed with a stilbazolium dye with a longer alkyl chain, suggesting that the interaction between the hydrophobic parts of the dye cations is the driving force of the scrolling. This finding should encourage research on the formation of nanotubes from nanosheets and also provides important guidelines for the selection of appropriate exfoliation reagents when exfoliating nanosheets from layered crystals.

Visualized Enzyme-Activated Fluorescence Probe for Accurately Detecting β-Gal in Living Cells and BALB/c Nude Mice

Colorectal cancer was one of the major malignant tumors threatening human health and β-Gal was recognized as a principal biomarker for primary colorectal cancer. Thus, designing specific and efficient quantitative detection methods for measuring β-Gal enzyme activity was of great clinical test significance. Herein, an ultrasensitive detection method based on Turn-on fluorescence probe (CS-βGal) was reported for visualizing the detection of exogenous and endogenous β-galactosidase enzyme activity. The test method possessed a series of excellent performances, such as a significant fluorescence enhancement (about 11.3-fold), high selectivity as well as superior sensitivity. Furthermore, under the optimal experimental conditions, a relatively low limit of detection down to 0.024 U/mL was achieved for fluorescence titration experiment. It was thanks to the better biocompatibility and low cytotoxicity, CS-βGal had been triumphantly employed to visual detect endogenous and exogenous β-Gal concentration variations in living cells with noteworthy anti-interference performance. More biologically significant was the fact that the application of CS-βGal in BALB/c nude mice was also achieved successfully for monitoring endogenous β-Gal enzyme activity.

MOF@Au NPs/aptamer fluorescent probe for the selective and sensitive detection of thiamethoxam

The luminescence performance of fluorescent reagents plays a crucial role in fluorescence analysis. Therefore, in this study, a novel bi-ligand Zn-based metal-organic framework, Au nanoparticle (NP) fluorescent material was synthesized using a hydrothermal method with Zn as the metal source. Simultaneously, a DNA aptamer was introduced as a molecular recognition element to develop a Zn-based MOF@Au NPs/DNA aptamer fluorescent probe for the ultra-trace detection of thiamethoxam residues in agricultural products. The probe captured different concentrations of the target molecule, thiamethoxam, through the DNA aptamer, causing a conformational change in the DNA aptamer and bursting the fluorescence of the probe, therefore establishing a fluorometric method for thiamethoxam detection. This method is highly sensitive due to the excellent luminescence properties of the Zn-based MOF@Au NPs, and the DNA aptamer can specifically recognize thiamethoxam, offering high selectivity. The linear range of the method was 2.5-6000 × 10-11  mol L-1 , with a detection limit of 8.33 × 10-12  mol L-1 . This method was applied to the determination of actual samples, such as bananas, and the spiked recovery rate was found to be in the range 84.05-109.07%. Overall, the proposed probe has high sensitivity, high selectivity, and easy operation for the detection of thiamethoxam residues in actual samples.

Styrylpyridinium Derivatives for Fluorescent Cell Imaging

A set of styrylpyridinium (SP) compounds was synthesised in order to study their spectroscopic and cell labelling properties. The compounds comprised different electron donating parts (julolidine, p-dimethylaminophenyl, p-methoxyphenyl, 3,4,5-trimethoxyphenyl), conjugated linkers (vinyl, divinyl), and an electron-withdrawing N-alkylpyridinium part. Geminal or bis-compounds incorporating two styrylpyridinium (bis-SP) moieties at the 1,3-trimethylene unit were synthesised. Compounds comprising a divinyl linker and powerful electron-donating julolidine donor parts possessed intensive fluorescence in the near-infrared region (maximum at ~760 nm). The compounds had rather high cytotoxicity towards the cancerous cell lines HT-1080 and MH-22A; at the same time, basal cytotoxicity towards the NIH3T3 fibroblast cell line ranged from toxic to harmful. SP compound 6e had IC50 values of 1.0 ± 0.03 µg/mL to the cell line HT-1080 and 0.4 µg/mL to MH-22A; however, the basal toxicity LD50 was 477 mg/kg (harmful). The compounds showed large Stokes' shifts, including 195 nm for 6a,b, 240 nm for 6e, and 325 and 352 nm for 6d and 6c, respectively. The highest photoluminescence quantum yield (PLQY) values were observed for 6a,b, which were 15.1 and 12.2%, respectively. The PLQY values for the SP derivatives 6d,e (those with a julolidinyl moiety) were 0.5 and 0.7%, respectively. Cell staining with compound 6e revealed a strong fluorescent signal localised in the cell cytoplasm, whereas the cell nuclei were not stained. SP compound 6e possessed self-assembling properties and formed liposomes with an average diameter of 118 nm. The obtained novel data on near-infrared fluorescent probes could be useful for the development of biocompatible dyes for biomedical applications.

Rational Design of MMP-Independent Near-Infrared Fluorescent Probes for Accurately Monitoring Mitochondrial Viscosity

Mitochondrial viscosity affects metabolite diffusion and mitochondrial metabolism and is associated with many diseases. However, the accuracy of mitochondria-targeting fluorescent probes in measuring viscosity is unsatisfactory because these probes can diffuse from mitochondria during mitophagy with a decreased mitochondrial membrane potential (MMP). To avoid this problem, by incorporating different alkyl side chains into dihydroxanthene fluorophores (denoted as DHX), we developed six near-infrared (NIR) probes for the accurate detection of mitochondrial viscosity, and the sensitivity to viscosity and the mitochondrial targeting and anchoring capability of these probes increased by increasing the alkyl chain length. Among them, DHX-V-C₁₂ had a highly selective response to viscosity variations with minimum interference from polarity, pH, and other biologically relevant species. Furthermore, DHX-V-C₁₂ was used to monitor the mitochondrial viscosity changes of HeLa cells treated by ionophores (nystatin, monensin) or under starvation conditions. We hope that this mitochondrial targeting and anchoring strategy based on increasing the alkyl chain length will be a general strategy for the accurate detection of mitochondrial analytes, enabling the accurate study of mitochondrial functions.

Self-Assembly Mitochondria-Targeting Donor-Acceptor Type Theranostic Nanosphere Activates ROS Storm for Multimodal Cancer Therapy

The rational design of cancer theranostics with natural diagnostic information and therapeutic behavior has been considered to be a big challenge, since common theranostics from photothermal and photodynamic therapy need to be activated with external stimuli of photoirradiation to enable the chemotherapeutic effects. In this contribution, we have designed and synthesized a series of simple theranostic agents, TPA-N-n (n = 4, 8, 12), which could accumulate at the tumor site over 48 h and indicate superior antiproliferative performance in vivo. TPA-N-n was constructed with electron donor triphenylamine-acceptor benzothiadiazole-mitochondria-targeting moiety pyridinium. Complex TPA-N-8 indicated the best cytotoxicity to cancerous HeLa cells, with an IC₅₀ value of 4.3 μM, and could self-assemble to a nanosphere with a size of 161.2 nm in the DMSO/PBS solution. It is worth noting that TPA-N-8 could accumulate in the mitochondria and produce major ROS species O₂•^- and OH• as well as small amounts of ¹O₂ without photoirradiation. Oxidative DNA damage is initiated due to the imbalance of intracellular redox homeostasis from the significant ROS storm. Multimodal synergistic therapy for HeLa cells was activated, as the PINK1-mediated mitophagy from the damaged mitochondria and DNA damage responsive (DDR) induced necroptosis and autophagy. This work not only provided a successful D-A type theranostic agent with superior anticancer performance from multimodal synergistic therapy but also further demonstrated the high efficacy of a mitochondria-targeting strategy for cancer treatment.

Antibacterial Theranostic Agents with Negligible Living Cell Invasiveness: AIE-Active Cationic Amphiphiles Regulated by Alkyl Chain Engineering

To address the threat of bacterial infection in the following post-antibiotic era, developing effective antibacterial approaches is of utmost urgency. Theranostic medicine integrating diagnosis and therapy is a promising protocol to fight against pathogenic bacteria. But numerous reported antibacterial theranostic materials are disclosed to be trapped in the excessive invasiveness to living mammal cells, leading to false positives and possible biosafety risks. Herein, a series of cationic pyridinium-substituted phosphindole oxide derivatives featuring aggregation-induced emission are designed, and alkyl chain engineering is conducted to finely tune their hydrophobicity and investigate their bioaffinity preference for living mammal cells and pathogenic bacteria. Most importantly, an efficient theranostic agent (PyBu-PIO) is acquired that is free from living cell invasiveness with negligible cytotoxicity and yet holds a good affinity for Gram-positive bacteria, including drug-resistant strains, with a superior inactivating effect. Externally applying PyBu-PIO onto Gram-positive bacteria-infected skin wounds can achieve creditable imaging effects and successfully accelerate the healing processes with reliable biosafety. This work proposes living cell invasiveness as a criterion for antibacterial theranostic materials and provides important enlightenment for the design of antibacterial theranostic materials.

Fluorescence labelled XT5 modified nano-capsules enable highly sensitive myeloma cells detection

Accurate diagnosis of cancer cells in early stages plays an important role in reliable therapeutic strategies. In this study, we aimed to develop fluorescence-conjugated polymer carrying nanocapsules (NCs) which is highly selective for myeloma cancer cells. To gain specific targeting properties, NCs, XT5 molecules (a benzamide derivative) which shows high affinity properties against protease-activated receptor-1 (PAR1), that overexpressed in myeloma cancer cells, was used. For this purpose, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[carboxy(polyethylene glycol)-2000]-carboxylic acid (DSPE-PEG₂₀₀₀-COOH) molecules, as a main encapsulation material, was conjugated to XT5 molecules due to esterification reaction using N,N'-dicyclohexylcarbodiimide as a coupling agent. The synthesized DSPE-PEG₂₀₀₀-COO-XT5 was characterized by using FT-IR and¹H NMR spectroscopies and results indicated that XT5 molecules were successfully conjugated to DSPE-PEG₂₀₀₀-COOH. Poly(fluorene-alt-benzothiadiazole) (PFBT) conjugated polymer (CP) was encapsulated with DSPE-PEG₂₀₀₀-COO-XT5 due to dissolving in tetrahydrofuran and ultra-sonication in an aqueous solution, respectively. The morphological properties, UV-vis absorbance, and emission properties of obtainedCPencapsulatedDSPE-PEG₂₀₀₀-COO-XT5(CPDP-XT5) NCs was determined by utilizing scanning electron microscopy, UV-vis spectroscopy, and fluorescent spectroscopy, respectively. Cytotoxicity properties of CPDP-XT5 was evaluated by performing MTT assay on RPMI 8226 myeloma cell lines. Cell viability results confirmed that XT5 molecules were successfully conjugated to DSPE-PEG₂₀₀₀-COOH. Specific targeting properties of CPDP-XT5 NCs and XT5-free NCs (CPDP NCs) were investigated on RPMI 8226 myeloma cell lines by utilizing fluorescent microscopy and results indicated that CPDP-XT5 NCs shows significantly high affinity in comparison to CPDP NCs against the cells. Homology modeling and molecular docking properties of XT5 molecules were evaluated and simulation results confirmed our results.

Real-time imaging mitochondrial viscosity dynamic during mitophagy mediated by photodynamic therapy

Photodynamic therapy has been generally developed and approved as a promising theranostic technique in recent years, which requires photosensitizers to bear high efficiency of reactive oxygen species production, precisely targeting ability and excellent biocompatibility. The real-time monitoring the microenvironments such as viscosity dynamic involved in mitophagy mediated by photodynamic therapy is significantly important to understand therapeutic process but barely reported. In this work, a pyridinium-functionalized triphenylamine derivative, (E)-4-(2-(4'-(diphenylamino)-[1,1'-biphenyl]-4-yl)vinyl)-1-methylpyridin-1-ium iodide (Mito-I), was exploited as photosensitizer for mitochondria-targeted photodynamic therapy and as fluorescent probe for imaging the mitochondrial viscosity dynamic during mitophagy simultaneously. The results indicated that the additional phenyl ring in Mito-I was beneficial to promote its efficiency of singlet oxygen production. The excellent capability of targeting mitochondria and singlet oxygen generation allowed Mito-I for the specifically mitochondria-targeted photodynamic therapy. Moreover, Mito-I displayed off-on fluorescence response to viscosity with high selectivity and sensitivity. The observed enhancement in fluorescence intensity of Mito-I revealed the increasingly mitochondrial viscosity during mitophagy mediated by the photodynamic therapy of Mito-I. As a result, this work presents a rare example to realize the mitochondria-targeting photodynamic therapy as well as the real-time monitoring viscosity dynamic during mitophagy, which is of great importance for the basic medical research involved in photodynamic therapy.

Two-Color Visualization of Cholesterol Fluctuation in Plasma Membranes by Spatial Distribution-Controllable Single Fluorescent Probes

Visualizing cholesterol (CL) fluctuation in plasma membranes is a crucially important yet challenging task in cell biology. Here, we proposed a new imaging strategy based on permeability changes of plasma membranes triggered by different CL contents to result in controllable spatial distribution of single fluorescent probes (SF-probes) in subcellular organelles. Three spatial distribution-controllable SF-probes (PMM-Me, PMM-Et, and PMM-Bu) for imaging CL fluctuation in plasma membranes were rationally developed. These SF-probes target plasma membranes and mitochondria at normal CL levels, while they display solely staining in plasma membranes and mitochondria at increased and decreased CL levels, respectively. These polarity-sensitive probes also show distinct emission colors with fluorescence peaks of 575 and 620 nm in plasma membranes and mitochondria, respectively. Thus, the CL fluctuation in plasma membranes can be clearly visualized by means of the spatially distributed and two-color emissive SF-probes.

Organelle-Targeted Photosensitizers for Precision Photodynamic Therapy

Subcellular organelles are the cornerstones of cells, and destroying them will cause cell dysfunction and even death. Therefore, realizing precise organelle targeting of photosensitizers (PSs) can help reduce PS dosage, minimize side effects, avoid drug resistance, and enhance therapeutic efficacy in photodynamic therapy (PDT). Organelle-targeted PSs provide a new paradigm for the construction of the next generation of PSs and may provide implementable strategies for future precision medicine. In this Review, the recent targeting strategies of different organelles and the corresponding design principles of molecular and nanostructured PSs are summarized and discussed. The current challenges and opportunities in organelle-targeted PDT are also presented.

Construction of fluorescent rotors with multiple intramolecular rotation sites for visualization of cellular viscous compartments with elevated fidelity

Viscosity-sensitive fluorescent dyes are widely utilized to image viscous intracellular compartments with high fidelity. However, the sensitivity of many fluorescent rotors needs improvement for bioimaging applications. Herein, we proposed to construct a fluorescent rotor with multiple intramolecular rotation sites to elevate its sensitivity to environmental viscosity. The fabricated fluorescent rotor showed evidently increased sensitivity to viscosity and had the potential to image intracellular viscous compartments with improved fidelity. By decorating the rotor with sidechains of different lengths, we successfully fabricated two fluorescent probes, TAPI-6 and TAPI-16, to visualize the mitochondria and plasma membrane, respectively, with high fidelity. The two probes were also successfully utilized to clearly visualize the mitochondria and plasma membranes in skeletal muscle tissue, cardiac muscle tissue, and liver tissue, demonstrating the potential of the fluorescent rotor for bioimaging applications. We believe that the strategy of increasing the sensitivity to viscosity using multiple rotation sites is valuable for the construction of fluorescent rotors, and the presented fluorescent probes in this work can serve as powerful tools for biological research.

A NIR-I light-responsive superoxide radical generator with cancer cell membrane targeting ability for enhanced imaging-guided photodynamic therapy

Photodynamic therapy (PDT), as an emerging treatment modality, which takes advantage of reactive oxygen species (ROS) generated upon light illumination to ablate tumours, has suffered from a limited treatment depth, strong oxygen dependence and short ROS lifespan. Herein, we developed a highly efficient NIR-I light (808 nm laser) initiated theranostic system based on a fluorescent photosensitizer (EBD-1) with cancer cell membrane targeting ability, which can realize large penetration depth in tissue, generate superoxide radicals (O2 -˙) to relieve the oxygen-dependence, confine the ROS oxidation at the cell membrane, and self-report the cell viability during the PDT process. In vivo experiments demonstrated that EBD-1 under 808 nm light successfully accomplished remarkable cancer ablation. This work will be beneficial for the design of novel photosensitizers for PDT-based theranostic systems.

Lanthanide MOFs for inducing molecular chirality of achiral stilbazolium with strong circularly polarized luminescence and efficient energy transfer for color tuning

We present herein an innovative host–guest method to achieve induced molecular chirality from an achiral stilbazolium dye (DSM). The host–guest system is exquisitely designed by encapsulating the dye molecule in the molecule-sized chiral channel of homochiral lanthanide metal–organic frameworks (P-(+)/M-(−)-TbBTC), in which the P- or M-configuration of the dye is unidirectionally generated via a spatial confinement effect of the MOF and solidified by the dangling water molecules in the channel. Induced chirality of DSM is characterized by solid-state circularly polarized luminescence (CPL) and micro-area polarized emission of DSM@TbTBC, both excited with 514 nm light. A luminescence dissymmetry factor of 10⁻³ is obtained and the photoluminescence quantum yield (PLQY) of the encapsulated DSM in DSM@TbTBC is ∼10%, which is close to the PLQY value of DSM in dilute dichloromethane. Color-tuning from green to red is achieved, owing to efficient energy transfer (up to 56%) from Ln³⁺ to the dye. Therefore, this study for the first time exhibits an elegant host–guest system that shows induced strong CPL emission and enables efficient energy transfer from the host chiral Ln-MOF to the achiral guest DSM with the emission color tuned from green to red.

Homochiral Ln-MOFs are synthesized to encapsulate achiral dyes to induce strong circularly polarized luminescence with a luminescence dissymmetry factor of 10⁻³.

Cancer cell discrimination and dynamic viability monitoring through wash-free bioimaging using AIEgens

Cancer cell discrimination and cellular viability monitoring are closely related to human health. A universal and convenient fluorescence system with a dual function of wide-spectrum cancer cell discrimination and dynamic cellular viability monitoring is desperately needed, and is still extremely challenging. Herein we present a series of aggregation-induced emission luminogens (AIEgens) (denoted as IVP) which can allow accurate discrimination between cancer and normal cells and dynamic monitoring of cellular viability through mitochondria-nucleolus migration. By regulating the lengths and positions of alkyl chains in IVP molecules, we systematically studied the discrimination behavior of these AIEgens between cancer cells and normal cells and further investigated how they can migrate between the mitochondria and nucleolus based on the change of mitochondrial membrane potential (ΔΨ m). Using IVP-02 as a model molecule, wash-free bioimaging, excellent two-photon properties, and low cytotoxicity were demonstrated. This present work proves that these designed IVP AIEgens show great potential for cancer identification and metastasis monitoring, as well as activity evaluation and screening of drugs.