Fluorogenic Dimers as Bright Switchable Probes for Enhanced Super-Resolution Imaging of Cell Membranes

Super-resolution fluorescence imaging based on single-molecule localization microscopy (SMLM) enables visualizing cellular structures with nanometric precision. However, its spatial and temporal resolution largely relies on the brightness of ON/OFF switchable fluorescent dyes. Moreover, in cell plasma membranes, the single-molecule localization is hampered by the fast lateral diffusion of membrane probes. Here, to address these two fundamental problems, we propose a concept of ON/OFF switchable probes for SMLM (points accumulation for imaging in nanoscale topography, PAINT) based on fluorogenic dimers of bright cyanine dyes. In these probes, the two cyanine units connected with a linker were modified at their extremities with low-affinity membrane anchors. Being self-quenched in water due to intramolecular dye H-aggregation, they displayed light up on reversible binding to lipid membranes. The charged group in the linker further decreased the probe affinity to the lipid membranes, thus accelerating its dynamic reversible ON/OFF switching. The concept was validated on cyanines 3 and 5. SMLM of live cells revealed that the new probes provided higher brightness and ∼10-fold slower diffusion at the cell surface, compared to reference probes Nile Red and DiD, which boosted axial localization precision >3-fold down to 31 nm. The new probe allowed unprecedented observation of nanoscale fibrous protrusions on plasma membranes of live cells with 40 s time resolution, revealing their fast dynamics. Thus, going beyond the brightness limit of single switchable dyes by cooperative dequenching in fluorogenic dimers and slowing down probe diffusion in biomembranes open the route to significant enhancement of super-resolution fluorescence microscopy of live cells.

Pathogenic variants of sphingomyelin synthase SMS2 disrupt lipid landscapes in the secretory pathway

Sphingomyelin is a dominant sphingolipid in mammalian cells. Its production in the trans-Golgi traps cholesterol synthesized in the ER to promote formation of a sphingomyelin/sterol gradient along the secretory pathway. This gradient marks a fundamental transition in physical membrane properties that help specify organelle identify and function. We previously identified mutations in sphingomyelin synthase SMS2 that cause osteoporosis and skeletal dysplasia. Here, we show that SMS2 variants linked to the most severe bone phenotypes retain full enzymatic activity but fail to leave the ER owing to a defective autonomous ER export signal. Cells harboring pathogenic SMS2 variants accumulate sphingomyelin in the ER and display a disrupted transbilayer sphingomyelin asymmetry. These aberrant sphingomyelin distributions also occur in patient-derived fibroblasts and are accompanied by imbalances in cholesterol organization, glycerophospholipid profiles, and lipid order in the secretory pathway. We postulate that pathogenic SMS2 variants undermine the capacity of osteogenic cells to uphold nonrandom lipid distributions that are critical for their bone forming activity.

In Vivo Imaging Evaluation of Fluorescence Intensity at Tail Emission of Near-Infrared-I (NIR-I) Fluorophores in a Porcine Model

Over the last decade fluorescence-guided surgery has been primarily focused on the NIR-I window. However, the NIR-I window has constraints, such as limited penetration and scattering. Consequently, exploring the performance of NIR-I dyes at longer wavelengths (i.e., the NIR-II window) is crucial to expanding its application. Two fluorophores were used in three pigs to identify the mean fluorescence intensity (MFI) using two commercially available NIR-I and NIR-II cameras. The near-infrared coating of equipment (NICE) was used to identify endoluminal surgical catheters and indocyanine green (ICG) for common bile duct (CBD) characterization. The NIR-II window evaluation showed an MFI of 0.4 arbitrary units (a.u.) ± 0.106 a.u. in small bowel NICE-coated catheters and an MFI of 0.09 a.u. ± 0.039 a.u. in gastric ones. In CBD characterization, the ICG MFI was 0.12 a.u. ± 0.027 a.u., 0.18 a.u. ± 0.100 a.u., and 0.22 a.u. ± 0.041 a.u. at 5, 35, and 65 min, respectively. This in vivo imaging evaluation of NIR-I dyes confirms its application in the NIR-II domain. To the best of our knowledge, this is the first study assessing the MIF of NICE in the NIR-II window using a commercially available system. Further comparative trials are necessary to determine the superiority of NIR-II imaging systems.

Assembly of Fluorescent Polymer Nanoparticles Using Different Microfluidic Mixers

Nanoprecipitation is a facile and efficient approach to the assembly of loaded polymer nanoparticles (NPs) for applications in bioimaging and targeted drug delivery. Their successful use in clinics requires reproducible and scalable synthesis, for which microfluidics appears as an attractive technique. However, in the case of nanoprecipitation, particle formation depends strongly on mixing. Here, we compare 5 different types of microfluidic mixers with respect to the formation and properties of poly(d-l-lactide-co-glycolide) (PLGA) and poly(methyl methacrylate) NPs loaded with a fluorescent dye salt: a cross-shaped mixer, a multilamination mixer, a split and recombine mixer, two herringbone mixers, and two impact jet mixers. Size and fluorescence properties of the NPs obtained with these mixers are evaluated. All mixers, except the cross-shaped one, yield NPs at least as small and fluorescent as those obtained manually. Notably in the case of impact jet mixers operated at high flow speeds, the size of the NPs could be strongly reduced from >50 nm down to <20 nm. Surprisingly, the fluorescence quantum yield of NPs obtained with these mixers also depends strongly on the flow speed, increasing, in the case of PLGA, from 30 to >70%. These results show the importance of precisely controlling the assembly conditions for loaded polymer NPs. The present work further provides guidance for choosing the optimal microfluidic setup for production of nanomaterials for biomedical applications.

Erratum: Long-term STED imaging of membrane packing and dynamics by exchangeable polarity-sensitive dyes

[This corrects the article DOI: 10.1016/j.bpr.2021.100023.].

Rational Design of Self-Quenched Rhodamine Dimers as Fluorogenic Aptamer Probes for Live-Cell RNA Imaging

With the growing interest in the understanding of the importance of RNAs in health and disease, detection of RNAs in living cells is of high importance. Fluorogenic dyes that light up specifically selected RNA aptamers constitute an attractive direction in the design of RNA imaging probes. In this work, based on our recently proposed concept of a fluorogenic dimer, we aim to develop a robust molecular tool for intracellular RNA imaging. We rationally designed a fluorogenic self-quenched dimer (orange Gemini, o-Gemini) based on rhodamine and evaluated its capacity to light up its cognate aptamer o-Coral in solution and live cells. We found that the removal of biotin from the dimer slightly improved the fluorogenic response without losing the affinity to the cognate aptamer (o-Coral). On the other hand, replacing sulforhodamine with a carboxyrhodamine produced drastic improvement of the affinity and the turn-on response to o-Coral and, thus, a better limit of detection. In live cells expressing o-Coral-tagged RNAs, the carboxyrhodamine analogue of o-Gemini without a biotin unit displayed a higher signal as well as faster internalization into the cells. We suppose that less hydrophilic carboxyrhodamine compared to sulforhodamine can more readily penetrate through the cell plasma membrane and, together with its higher affinity to o-Coral, provide the observed improvement in the imaging experiments. The promiscuity of the o-Coral RNA aptamer to the fluorogenic dimer allowed us to tune a fluorogen chemical structure and thus drastically improve the fluorescence response of the probe to o-Coral-tagged RNAs.

Imaging and Measuring Vesicular Acidification with a Plasma Membrane-Targeted Ratiometric pH Probe

Tracking the pH variation of intracellular vesicles throughout the endocytosis pathway is of prior importance to better assess the cell trafficking and metabolism of cells. Small molecular fluorescent pH probes are valuable tools in bioimaging but are generally not targeted to intracellular vesicles or are directly targeted to acidic lysosomes, thus not allowing the dynamic observation of the vesicular acidification. Herein, we designed Mem-pH, a fluorogenic ratiometric pH probe based on chromenoquinoline with appealing photophysical properties, which targets the plasma membrane (PM) of cells and further accumulates in the intracellular vesicles by endocytosis. The exposition of Mem-pH toward the vesicle's lumen allowed to monitor the acidification of the vesicles throughout the endocytic pathway and enabled the measurement of their pH via ratiometric imaging.

Dynamic covalent chemistry in live cells for organelle targeting and enhanced photodynamic action

Organelle-specific targeting enables increasing the therapeutic index of drugs and localizing probes for better visualization of cellular processes. Current targeting strategies require conjugation of a molecule of interest with organelle-targeting ligands. Here, we propose a concept of dynamic covalent targeting of organelles where the molecule is conjugated with its ligand directly inside live cells through a dynamic covalent bond. For this purpose, we prepared a series of organelle-targeting ligands with a hydrazide residue for reacting with dyes and drugs bearing a ketone group. We show that dynamic hydrazone bond can be formed between these hydrazide ligands and a ketone-functionalized Nile Red dye (NRK) in situ in model lipid membranes or nanoemulsion droplets. Fluorescence imaging in live cells reveals that the targeting hydrazide ligands can induce preferential localization of NRK dye and an anti-cancer drug doxorubicin in plasma membranes, mitochondria and lipid droplets. Thus, with help of the dynamic covalent targeting, it becomes possible to direct a given bioactive molecule to any desired organelle inside the cell without its initial functionalization by the targeting ligand. Localizing the same NRK dye in different organelles by the hydrazide ligands is found to affect drastically its photodynamic activity, with the most pronounced phototoxic effects in mitochondria and plasma membranes. The capacity of this approach to tune biological activity of molecules can improve efficacy of drugs and help to understand better their intracellular mechanisms.

Amplified Fluorescence in Situ Hybridization by Small and Bright Dye-Loaded Polymeric Nanoparticles

Detection and imaging of RNA at the single-cell level is of utmost importance for fundamental research and clinical diagnostics. Current techniques of RNA analysis, including fluorescence in situ hybridization (FISH), are long, complex, and expensive. Here, we report a methodology of amplified FISH (AmpliFISH) that enables simpler and faster RNA imaging using small and ultrabright dye-loaded polymeric nanoparticles (NPs) functionalized with DNA. We found that the small size of NPs (below 20 nm) was essential for their access to the intracellular mRNA targets in fixed permeabilized cells. Moreover, proper selection of the polymer matrix of DNA-NPs minimized nonspecific intracellular interactions. Optimized DNA-NPs enabled sequence-specific imaging of different mRNA targets (survivin, actin, and polyA tails), using a simple 1 h staining protocol. Encapsulation of cyanine and rhodamine dyes with bulky counterions yielded green-, red-, and far-red-emitting NPs that were 2-100-fold brighter than corresponding quantum dots. These NPs enabled multiplexed detection of three mRNA targets simultaneously, showing distinctive mRNA expression profiles in three cancer cell lines. Image analysis confirmed the single-particle nature of the intracellular signal, suggesting single-molecule sensitivity of the method. AmpliFISH was found to be semiquantitative, correlating with RT-qPCR. In comparison with the commercial locked nucleic acid (LNA)-based FISH technique, AmpliFISH provides 8-200-fold stronger signal (dependent on the NP color) and requires only three steps vs ∼20 steps together with a much shorter time. Thus, combination of bright fluorescent polymeric NPs with FISH yields a fast and sensitive single-cell transcriptomic analysis method for RNA research and clinical diagnostics.

Counterion-insulated near-infrared dyes in biodegradable polymer nanoparticles for in vivo imaging

Polymeric nanoparticles (NPs) are highly attractive for biomedical applications due to their potential biodegradability and capacity to encapsulate different loads, notably drugs and contrast agents. For in vivo optical bioimaging, NPs should operate in the near-infrared region (NIR) and exhibit stealth properties. In the present work, we applied the approach of ionic dye insulation with bulky hydrophobic counterions for encapsulation of near-infrared cyanine dyes (Cy5.5 and Cy7 bearing two octadecyl chains) into biodegradable polymer (PLGA) NPs. We found that at high dye loading (20-50 mM with respect to the polymer), the bulkiest fluorinated tetraphenylborate counterion minimized best the aggregation-caused quenching and improved fluorescence quantum yields of both NIR dyes, especially of Cy5.5. In addition, bulky counterions also enabled formation of small 40 nm polymeric NPs in contrast to smaller counterions. To provide them stealth properties, we prepared 40 nm dye-loaded PEGylated NPs through nanoprecipitation of synthetic PLGA-PEG block copolymer with the dye/counterion salt. The obtained NIR NPs loaded with Cy5.5 dye salt allowed in vivo imaging of wild-type mice with a good contrast after IV injection. Compared to the bare PLGA NPs, PLGA-PEG NPs exhibited significantly slower accumulation in the liver. Biodistribution studies confirmed the preferential accumulation in the liver, although PLGA and PLGA-PEG NPs could also be distributed in other organs, with the following tendency: liver > spleen > lungs > kidney > heart > testis > brain. Overall, the present work validated the counterion approach for encapsulation of NIR cyanine dyes into biodegradable polymer NPs bearing covalently attached PEG shell. Thus, we propose a simple and robust methodology for preparation of NIR fluorescent biodegradable polymer NPs, which could further improve the existing optical imaging for biomedical applications.

Dynamic tracing using ultra-bright labeling and multi-photon microscopy identifies endothelial uptake of poloxamer 188 coated poly(lactic-co-glycolic acid) nano-carriers in vivo

The potential of poly(lactic-co-glycolic acid) (PLGA) to design nanoparticles (NPs) and target the central nervous system remains to be exploited. In the current study we designed fluorescent 70-nm PLGA NPs, loaded with bulky fluorophores, thereby making them significantly brighter than quantum dots in single-particle fluorescence measurements. The high brightness of NPs enabled their visualization by intravital real-time 2-photon microscopy. Subsequently, we found that PLGA NPs coated with pluronic F-68 circulated in the blood substantially longer than uncoated NPs and were taken up by cerebro-vascular endothelial cells. Additionally, confocal microscopy revealed that coated PLGA NPs were present in late endothelial endosomes of cerebral vessels within 1 h after systemic injection and were more readily taken up by endothelial cells in peripheral organs. The combination of ultra-bright NPs and in vivo imaging may thus represent a promising approach to reduce the gap between development and clinical application of nanoparticle-based drug carriers.

Long-term STED imaging of membrane packing and dynamics by exchangeable polarity-sensitive dyes

Understanding the plasma membrane nanoscale organization and dynamics in living cells requires microscopy techniques with high spatial and temporal resolution that permit for long acquisition times and allow for the quantification of membrane biophysical properties, such as lipid ordering. Among the most popular super-resolution techniques, stimulated emission depletion (STED) microscopy offers one of the highest temporal resolutions, ultimately defined by the scanning speed. However, monitoring live processes using STED microscopy is significantly limited by photobleaching, which recently has been circumvented by exchangeable membrane dyes that only temporarily reside in the membrane. Here, we show that NR4A, a polarity-sensitive exchangeable plasma membrane probe based on Nile red, permits the super-resolved quantification of membrane biophysical parameters in real time with high temporal and spatial resolution as well as long acquisition times. The potential of this polarity-sensitive exchangeable dye is showcased by live-cell real-time three-dimensional STED recordings of bleb formation and lipid exchange during membrane fusion as well as by STED-fluorescence correlation spectroscopy experiments for the simultaneous quantification of membrane dynamics and lipid packing that correlate in model and live-cell membranes.

Probing Variations of Reduction Activity at the Plasma Membrane Using a Targeted Ratiometric FRET Probe

Plasma membrane (PM) is the turntable of various reactions that regulate essential functionalities of cells. Among these reactions, the thiol disulfide exchange (TDE) reaction plays an important role in cellular processes. We herein designed a selective probe, called membrane reduction probe (MRP), that is able to report TDE activity at the PM. MRP is based on a green emitting BODIPY PM probe connected to rhodamine through a disulfide bond. MRP is fluorogenic as it is turned off in aqueous media due to aggregation-caused quenching, and once inserted in the PM, it displays a bright red signal due to an efficient fluorescence energy resonance transfer (FRET) between the BODIPY donor and the rhodamine acceptor. In the PM model, the MRP can undergo TDE reaction with external reductive agents as well as with thiolated lipids embedded in the bilayer. Upon TDE reaction, the FRET is turned off and a bright green signal appears allowing a ratiometric readout of this reaction. In cells, the MRP quickly labeled the PM and was able to probe variations of TDE activity using ratiometric imaging. With this tool in hand, we were able to monitor variations of TDE activity at the PM under stress conditions, and we showed that cancer cell lines presented a reduced TDE activity at the PM compared to noncancer cells.

Confronting molecular rotors and self-quenched dimers as fluorogenic BODIPY systems to probe biotin receptors in cancer cells

Probing receptors at the cell surface to monitor their expression level can be performed with fluorogenic dyes. Biotin receptors (BRs) are particularly interesting as they are overexpressed in cancer cells. Herein, to image BRs, we adapted and systematically compared two fluorogenic systems based on BODIPYs: a molecular rotor and a self-quenched dimer that light up in response to high viscosity and low polarity of the membrane, respectively. The fluorogenic dimer proved to be more efficient than the rotor and allowed BRs to be imaged in cancer cells, which can effectively be discriminated from non-cancer cells.

Bulky Barbiturates as Non-Toxic Ionic Dye Insulators for Enhanced Emission in Polymeric Nanoparticles

Bulky hydrophobic counterions (weakly coordinating anions) can insulate ionic dyes against aggregation-caused quenching (ACQ) and enable preparation of highly fluorescent dye-loaded nanoparticles (NPs) for bioimaging, biosensing and light harvesting. Here, we introduce a family of hydrophobic anions based on fluorinated C-acyl barbiturates with delocalized negative charge and bulky non-polar groups. Similarly to fluorinated tetraphenylborates, these barbiturates prevent ACQ of cationic dye alkyl rhodamine B inside polymer NPs made of biodegradable poly(lactic-co-glycolic acid) (PLGA). Their efficiency to prevent ACQ increases for analogues with higher acidity and bulkiness. Their structure controls dye-dye communication, yielding bright NPs with on/off switching or stable emission. They enhance dye encapsulation inside NPs, allowing intracellular imaging without dye leakage. Compared to fluorinated tetraphenylborates known as cytotoxic transmembrane ion transporters, the barbiturates display a significantly lower cytotoxicity. These chemically available and versatile barbiturate derivatives are promising counterion scaffolds for preparation of bright non-toxic fluorescent nanomaterials.

Emerging Solvatochromic Push-Pull Dyes for Monitoring the Lipid Order of Biomembranes in Live Cells

Solvatochromic dyes have emerged as a new class of fluorescent probes in the field of lipid membranes due to their ability to identify the lipid organization of biomembranes in live cells by changing the color of their fluorescence. This type of solvatochromic function is useful for studying the heterogeneous features of biomembranes caused by the uneven distribution of lipids and cholesterols in live cells and related cellular processes. Therefore, a variety of advanced solvatochromic dyes have been rapidly developed over the last decade. To provide an overview of the works recently developed solvatochromic dyes have enabled, we herein present some solvatochromic dyes, with a particular focus on those based on pyrene and Nile red. As these dyes exhibit preferable photophysical properties in terms of fluorescence microscopy applications and unique distribution/localization in cellular compartments, some have already found applications in cell biological and biophysical studies. The goal of this review is to provide information to researchers who have never used solvatochromic dyes or who have not discovered applications of such dyes in biological studies.

Ultrabright Fluorescent Polymeric Nanofibers and Coatings Based on Ionic Dye Insulation with Bulky Counterions

Preparation of bright fluorescent materials based on polymers is hampered by a fundamental problem of aggregation-caused quenching (ACQ) of encapsulated dyes. Here, ultrabright fluorescent polymeric nanofibers and coatings are prepared based on a concept of ionic dye insulation with bulky hydrophobic counterions that overcomes the ACQ problem. It is found that bulky hydrophobic counterion perfluorinated tetraphenylborate can boost >100-fold the fluorescence quantum yields of cationic dye octadecyl rhodamine B at high loading (30 wt %) in biocompatible poly(methyl methacrylate) (PMMA). The concept is applicable to both rhodamine and cyanine dyes, which results in bright fluorescent polymeric materials of four different colors spanning from blue to near-infrared. It allows for preparation of electrospun polymeric nanofibers with >50-fold higher dye loading by mass (30 wt %, >20-fold higher molarity for rhodamine dyes) while preserving good fluorescence quantum yields (31%), which implies drastic improvement in their fluorescence brightness. The counterion-based polymeric materials are also validated as coatings of model medical devices, such as stainless steel fiducials and 3D-printed stents of complex geometry. Spin-coated fluorescent polymeric films loaded with a dye paired with bulky counterions exhibit excellent biocompatibility and low toxicity. Moreover, counterion-modified materials show much better stability against dye leakage in the presence of living cells and a serum-containing medium, compared to materials based on the dye with a small inorganic anion. Overall, by pushing the barriers of ACQ, our counterion approach emerges as a powerful tool to develop ultrabright fluorescent polymeric materials ranging from nano- and macroscale.

µIVC-Useq: a microfluidic-assisted high-throughput functionnal screening in tandem with next generation sequencing and artificial neural network to rapidly characterize RNA molecules

The function of an RNA is intimately linked to its three-dimensional structure. X-ray crystallography or NMR allow the fine structural characterization of small RNA (e.g., aptamers) with a precision down to atomic resolution. Yet, these technics are time consuming, laborious and do not inform on mutational robustness and the extent to which a sequence can be modified without altering RNA function, an important set of information to assist RNA engineering. On another hand, thought powerful, in silico predictions still lack the required accuracy. These limitations can be overcome by using high-throughput microfluidic-assisted functional screening technologies, as they allow exploring large mutant libraries in a rapid and cost-effective manner. Among them, we recently introduced the microfluidic-assisted In Vitro Compartmentalization (µIVC), an efficient screening strategy in which reactions are performed in picoliter droplets at rates of several thousand per second. We later improved µIVC efficiency by using in tandem with high throughput sequencing, thought a laborious bioinformatic step was still required at the end of the process. In the present work, we strongly increased the automation level of the pipeline by implementing an artificial neural network enabling unsupervised bioinformatic analysis. We demonstrate the efficiency of this "µIVC-Useq" technology by rapidly identifying a set of sequences readily accepted by a key domain of the light-up RNA aptamer SRB-2. This work not only shed some new light on the way this aptamer can be engineered, but it also allowed us to easily identify new variants with an up-to 10-fold improved performance.

Enzyme-free amplified detection of cellular microRNA by light-harvesting fluorescent nanoparticle probes

Detection of cellular microRNA biomarkers is an emerging powerful tool in cancer diagnostics. Currently, it requires multistep tedious protocols based on molecular amplification of the RNA target, e.g. RT-qPCR. Here, we developed a one-step enzyme-free method for microRNA detection in cellular extracts based on light-harvesting nanoparticle (nanoantenna) biosensors. They amplify the fluorescence signal by effective Förster resonance energy transfer (FRET) from ultrabright dye-loaded polymeric nanoparticle to a single acceptor and thus convert recognition of one microRNA copy (through nucleic acid strand displacement) into a response of >400 dyes. The developed nanoprobes of 17-19 nm diameter for four microRNAs (miR-21, let-7f, miR-222 and miR-30a) exhibit outstanding brightness (up to 3.8 × 10⁷ M^-1cm^-1) and ratiometric sequence-specific response to microRNA with the limit of detection (LOD) down to 1.3 pM (21 amol), equivalent to 24 RT-qPCR cycles. They enable quantitative detection of the four microRNAs in RNA extracts from five cancerous cell lines (human breast cancer - T47D and MCF7, head and neck cancer - CAL33 and glioblastoma - LNZ308 and U373) and two non-cancerous ones (Hek293 and MCF10A), in agreement with RT-qPCR. The results confirmed that let-7f and especially miR-21 are systematically overexpressed in all studied cancerous cell lines. These nanoparticle biosensors are compatible with low-cost portable fluorometers and small detection volumes (11 amol LOD), opening a route to rapid point-of-care cancer diagnostics.

Live-cell imaging of the nucleolus and mapping mitochondrial viscosity with a dual function fluorescent probe

Visualization of sub-cellular organelles allows the determination of various cellular processes and the underlying mechanisms. Herein, we report a fluorescent probe, bearing push-pull substituents emitting at 600 nm and its application in cellular imaging. The probe shows dual imaging of mitochondria and nucleoli and maps mitochondrial viscosity in live cells under various physiological variations and show minimum cytotoxicity. Nucleolar staining is confirmed by RNAase digestion.

Nile Red-Based GPCR Ligands as Ultrasensitive Probes of the Local Lipid Microenvironment of the Receptor

The local lipid microenvironment of transmembrane receptors is an essential factor in G protein coupled receptor (GPCR) signaling. However, tools are currently missing for studying endogenously expressed GPCRs in primary cells and tissues. Here, we introduce fluorescent environment-sensitive GPCR ligands for probing the microenvironment of the receptor in living cells using fluorescence microscopy under no-wash conditions. We designed and synthesized antagonist ligands of the oxytocin receptor (OTR) by conjugating a high-affinity nonpeptidic OTR ligand PF-3274167 to the environment-sensitive fluorescent dye Nile Red. The length of the polar PEG spacer between the pharmacophore and the fluorophore was adjusted to lower the nonspecific interactions of the probe while preserving a strong fluorogenic response. We demonstrated that the new probes embed into the lipid bilayer in the vicinity of the receptor and convey information about the local polarity and the lipid order via the wavelength-shifting emission of the Nile Red fluorophore.

Fluorogenic Squaraine Dendrimers for Background-Free Imaging of Integrin Receptors in Cancer Cells

To overcome the limited brightness of existing fluorogenic molecular probes for biomolecular targets, we introduce a concept of fluorogenic dendrimer probe, which undergoes polarity-dependent switching due to intramolecular aggregation-caused quenching of its fluorophores. Based on a rational design of dendrimers with four and eight squaraine dyes, we found that octamer bearing dyes through a sufficiently long PEG(8) linker displays >400-fold fluorescence enhancement from water to non-polar dioxane. High extinction coefficient (≈2,300,000 m^-1  cm^-1 ) resulted from eight squaraine dyes and quantum yield (≈25 %) make this octamer the brightest environment-sensitive fluorogenic molecule reported to date. Its conjugate with cyclic RGD used at low concentration (3 nm) enables integrin-specific fluorescence imaging of cancer cells with high signal-to-background ratio. The developed dendrimer probe is a "golden middle" between molecular probes and nanoparticles, combining small size, turn-on response and high brightness, important for bioimaging.

Fluorescent labeling of biocompatible block copolymers: synthetic strategies and applications in bioimaging

Among biocompatible materials, block copolymers (BCPs) possess several advantages due to the control of their chemistry and the possibility of combining various blocks with defined properties. Consequently, BCPs drew considerable attention as biocompatible materials in the fields of drug delivery, medicine and bioimaging. Fluorescent labeling of BCPs quickly appeared to be a method of choice to image and track these materials in order to better understand the nature of their interactions with biological media. However, incorporating fluorescent markers (FM) into BCPs can appear tricky; we thus intend to help chemists in this endeavor by reviewing recent advances made in the last 10 years. With the choice of the FM being of prior importance, we first reviewed their photophysical properties and functionalities for optimal labeling and imaging. In the second part the different chemical approaches that have been used in the literature to fluorescently label BCPs have been reviewed. We also report and discuss relevant applications of fluorescent BCPs in bioimaging.