Time-resolved MIET measurements of blood platelet spreading and adhesion

Human blood platelets are non-nucleated fragments of megakaryocytes and of high importance for early hemostasis. To form a blood clot, platelets adhere to the blood vessel wall, spread and attract other platelets. Despite the importance for biomedicine, the exact mechanism of platelet spreading and adhesion to surfaces remains elusive. Here, we employ metal-induced energy transfer (MIET) imaging with a leaflet-specific fluorescent membrane probe to quantitatively determine, with nanometer resolution and in a time-resolved manner, the height profile of the basal and the apical platelet membrane above a rigid substrate during platelet spreading. We observe areas, where the platelet membrane approaches the substrate particularly closely and these areas are stable on a time scale of minutes. Time-resolved MIET measurements reveal distinct behaviors of the outermost rim and the central part of the platelets, respectively. Our findings quantify platelet adhesion and spreading and improve our understanding of early steps in blood clotting. Furthermore, the results of this study demonstrate the potential of MIET for simultaneous imaging of two close-by membranes and thus three-dimensional reconstruction of the cell shape.

Simultaneous computer-assisted assessment of mucosal and serosal perfusion in a model of segmental colonic ischemia

BACKGROUND

Fluorescence-based enhanced reality (FLER) enables the quantification of fluorescence signal dynamics, which can be superimposed onto real-time laparoscopic images by using a virtual perfusion cartogram. The current practice of perfusion assessment relies on visualizing the bowel serosa. The aim of this experimental study was to quantify potential differences in mucosal and serosal perfusion levels in an ischemic colon segment.

METHODS

An ischemic colon segment was created in 12 pigs. Simultaneous quantitative mucosal and serosal fluorescence imaging was obtained via intravenous indocyanine green injection (0.2 mg/kg), using two near-infrared camera systems, and computer-assisted FLER analysis. Lactate levels were measured in capillary blood of the colonic wall at seven regions of interest (ROIs) as determined with FLER perfusion cartography: the ischemic zone (I), the proximal and distal vascularized areas (PV, DV), and the 50% perfusion threshold proximally and distally at the mucosal and serosal side (P50M, P50S, D50M, D50S).

RESULTS

The mean ischemic zone as measured (mm) for the mucosal side was significantly larger than the serosal one (56.3 ± 21.3 vs. 40.8 ± 14.9, p = 0.001) with significantly lower lactate values at the mucosal ROIs. There was a significant weak inverse correlation between lactate and slope values for the defined ROIs (r = - 0.2452, p = 0.0246).

CONCLUSIONS

Mucosal ischemic zones were larger than serosal zones. These results suggest that an assessment of bowel perfusion from the serosal side only can underestimate the extent of ischemia. Further studies are required to predict the optimal resection margin and anastomotic site.

Stealth and Bright Monomolecular Fluorescent Organic Nanoparticles Based on Folded Amphiphilic Polymer

Fluorescent nanoparticles (NPs), owing to their superior brightness, are an attractive alternative to organic dyes. However, their cellular applications remain limited because of their large size, poor homogeneity, and nonspecific interactions in biological media. Herein, we propose a concept of monomolecular fluorescent organic nanoparticles of high brightness and very small size (10-14 nm) built of a single amphiphilic polymer bearing specially designed fluorescent dyes. We found that high PEGylation of poly(maleic anhydride-alt-1-octadecene (PMAO) favors a single-chain polymer folding into monomolecular stealth NPs with highly reduced nonspecific interactions with proteins and live cells. To ensure high stability of our NPs, the fluorophores (BODIPYs) are covalently linked to the polymer through an optimized linker. Among tested linkers of different lengths and polarity, a short medium-polar linker favoring location of the dyes at NPs interface ensures good fluorescence quantum yield and small particle size. The fluorescence brightness of these NPs has been dramatically enhanced by increasing the bulkiness of the BODIPY dyes that prevents their H-aggregation, reaching 2500000 M^-1 cm^-1 (extinction coefficient × quantum yield). Fluorescence microscopy revealed that the single-particle brightness of these NPs is ∼5-fold higher than that of QDot-585 using the same excitation wavelength (532 nm). Finally, when microinjected inside cells, these small and stealth NPs (10 nm diameter) distribute more evenly than 20 nm QDots inside the cytosol, showing similar spreading as a fluorescent protein. Thus, the developed monomolecular NPs, owing to their small size and stealth properties, are artificial analogues of fluorescent proteins, surpassing the latter >50-fold in terms of brightness.

Redesigning Solvatochromic Probe Laurdan for Imaging Lipid Order Selectively in Cell Plasma Membranes

Imaging of biological membranes by environmentally sensitive solvatochromic probes, such as Laurdan, provides information about the organization of lipids, their ordering, and their uneven distribution. To address a key drawback of Laurdan linked to its rapid internalization and subsequent labeling of internal membranes, we redesigned it by introducing a membrane anchor group based on negatively charged sulfonate and dodecyl chain. The obtained probe, Pro12A, stains exclusively the outer leaflet of lipid bilayers of liposomes, as evidenced by leaflet-specific fluorescence quenching with a viologen derivative, and shows higher fluorescence brightness than Laurdan. Pro12A also exhibits stronger spectral change between liquid-ordered and liquid-disordered phases in model membranes and distinguishes better lipid domains in giant plasma membrane vesicles (GPMVs) than Laurdan. In live cells, it stains exclusively the cell plasma membranes, in contrast to Laurdan and its carboxylate analogue C-Laurdan. Owing to its outer leaflet binding, Pro12A is much more sensitive to cholesterol extraction than Laurdan, which is redistributed within both plasma membrane leaflets and intracellular membranes. Finally, its operating range in the blue spectral region ensures the absence of crosstalk with a number of orange/red fluorescent proteins and dyes. Thus, Pro12A will enable accurate multicolor imaging of lipid organization of cell plasma membranes in the presence of fluorescently tagged proteins of interest, which will open new opportunities in biomembrane research.

Ultrabright Fluorescent Polymeric Nanoparticles with a Stealth Pluronic Shell for Live Tracking in the Mouse Brain

Visualizing single organic nanoparticles (NPs) in vivo remains a challenge, which could greatly improve our understanding of the bottlenecks in the field of nanomedicine. To achieve high single-particle fluorescence brightness, we loaded polymer poly(methyl methacrylate)-sulfonate (PMMA-SO₃H) NPs with octadecyl rhodamine B together with a bulky hydrophobic counterion (perfluorinated tetraphenylborate) as a fluorophore insulator to prevent aggregation-caused quenching. To create NPs with stealth properties, we used the amphiphilic block copolymers pluronic F-127 and F-68. Fluorescence correlation spectroscopy and Förster resonance energy transfer (FRET) revealed that pluronics remained at the NP surface after dialysis (at one amphiphile per 5.5 nm²) and prevented NPs from nonspecific interactions with serum proteins and surfactants. In primary cultured neurons, pluronics stabilized the NPs, preventing their prompt aggregation and binding to neurons. By increasing dye loading to 20 wt % and optimizing particle size, we obtained 74 nm NPs showing 150-fold higher single-particle brightness with two-photon excitation than commercial Nile Red-loaded FluoSpheres of 39 nm hydrodynamic diameter. The obtained ultrabright pluronic-coated NPs enabled direct single-particle tracking in vessels of mice brains by two-photon intravital microscopy for at least 1 h, whereas noncoated NPs were rapidly eliminated from the circulation. Following brain injury or neuroinflammation, which can open the blood-brain barrier, extravasation of NPs was successfully monitored. Moreover, we demonstrated tracking of individual NPs from meningeal vessels until their uptake by meningeal macrophages. Thus, single NPs can be tracked in animals in real time in vivo in different brain compartments and their dynamics visualized with subcellular resolution.

Smartphone-assisted detection of nucleic acids by light-harvesting FRET-based nanoprobe

Point-of-care assays for optical detection of biomolecular markers attract growing attention, because of their capacity to provide rapid and inexpensive diagnostics of cancer and infectious diseases. Here, we designed a nanoprobe compatible with a smartphone RGB camera for detection of nucleic acids. It is based on light-harvesting polymeric nanoparticles (NPs) encapsulating green fluorescent donor dyes that undergo efficient Förster Resonance Energy Transfer (FRET) to red fluorescent acceptor hybridized at the particle surface. Green-emitting NPs are based on rhodamine 110 and 6G dyes paired with bulky hydrophobic counterions, which prevent dye self-quenching and ensure efficient energy transfer. Their surface is functionalized with a capture DNA sequence for cancer marker survivin, hybridized with a short oligonucleotide bearing FRET acceptor ATTO647N. Obtained 40-nm poly(methyl methacrylate)-based NP probe, encapsulating octadecyl rhodamine 6G dyes with tetrakis(perfluoro-tert-butoxy)aluminate counterions (~6000 dyes per NP), and bearing 65 acceptors, shows efficient FRET with >20% quantum yield and a signal amplification (antenna effect) of 25. It exhibits ratiometric response to the target DNA by FRET acceptor displacement and enables DNA detection in solution by fluorescence spectroscopy (limit of detection 3 pM) and on surfaces at the single-particle level using two-color fluorescence microscopy. Using a smartphone RGB camera, the nanoprobe response can be readily detected at 10 pM target in true color and in red-to-green ratio images. Thus, our FRET-based nanoparticle biosensor enables detection of nucleic acid targets using a smartphone coupled to an appropriate optical setup, opening the way to simple and inexpensive point-of-care assays.

Ratiometric Nanoparticle Probe Based on FRET-Amplified Phosphorescence for Oxygen Sensing with Minimal Phototoxicity

Luminescent oxygen probes enable direct imaging of hypoxic conditions in cells and tissues, which are associated with a variety of diseases, including cancer. Here, a nanoparticle probe that addresses key challenges in the field is developed, it: i) strongly amplifies room temperature phosphorescence of encapsulated oxygen-sensitive dyes; ii) provides ratiometric response to oxygen; and iii) solves the fundamental problem of phototoxicity of phosphorescent sensors. The nanoprobe is based on 40 nm polymeric nanoparticles, encapsulating ≈2000 blue-emitting cyanine dyes with fluorinated tetraphenylborate counterions, which are as bright as 70 quantum dots (QD525). It functions as a light-harvesting nanoantenna that undergoes efficient Förster resonance energy transfer to ≈20 phosphorescent oxygen-sensitive platinum octaethylporphyrin (PtOEP) acceptor dyes. The obtained nanoprobe emits stable blue fluorescence and oxygen-sensitive red phosphorescence, providing ratiometric response to dissolved oxygen. The light harvesting leads to ≈60-fold phosphorescence amplification and makes the single nanoprobe particle as bright as ≈1200 PtOEP dyes. This high brightness enables oxygen detection at a single-particle level and in cells at ultra-low nanoprobe concentration with no sign of phototoxicity, in contrast to PtOEP dye. The developed nanoprobe is successfully applied to the imaging of a microfluidics-generated oxygen gradient in cancer cells. It constitutes a promising tool for bioimaging of hypoxia.

Probing biotin receptors in cancer cells with rationally designed fluorogenic squaraine dimers

Fluorogenic probes enable imaging biomolecular targets with high sensitivity and maximal signal-to-background ratio under non-wash conditions. Here, we focus on the molecular design of biotinylated dimeric squaraines that undergo aggregation-caused quenching in aqueous media through intramolecular H-type dimerization, but turn on their fluorescence in apolar environment due to target-mediated disaggregation. Our structure-property study revealed that depending on the linkers used to connect the squaraine dyes, different aggregation patterns could be obtained (intramolecular dimerization versus intermolecular aggregation) leading to different probing efficiencies. Using a relatively short l-lysine linker we developed a bright fluorogenic probe, Sq2B, displaying only intramolecular dimerization-caused quenching properties in aqueous media. The latter was successfully applied for imaging biotin receptors, in particular sodium-dependent multivitamin transporter (SMVT), which are overexpressed at the surface of cancer cells. Competitive displacement with SMVT-targets, such as biotin, lipoic acid or sodium pantothenate, showed Sq2B targeting ability to SMVT. This fluorogenic probe for biotin receptors could distinguish cancer cells (HeLa and KB) from model non-cancer cell lines (NIH/3T3 and HEK293T). The obtained results provide guidelines for development of new dimerization-based fluorogenic probes and propose bright tools for imaging biotin receptors, which is particularly important for specific detection of cancer cells.

A near-infrared fluorogenic dimer enables background-free imaging of endogenous GPCRs in living mice

Fluorescent probes are commonly used in studying G protein-coupled receptors in living cells; however their application to the whole animal receptor imaging is still challenging. To address this problem, we report the design and the synthesis of the first near-infrared emitting fluorogenic dimer with environment-sensitive folding. Due to the formation of non-fluorescent H-aggregates in an aqueous medium, the near-infrared fluorogenic dimer displays a strong turn-on response (up to 140-fold) in an apolar environment and exceptional brightness: 56% quantum yield and ≈444 000 M⁻¹ cm⁻¹ extinction coefficient. Grafted on a ligand of the oxytocin receptor, it allows the unprecedented background-free and target-specific imaging of the naturally expressed receptor in living mice.

A near-infrared emitting fluorogenic dimer with environment-sensitive folding and exceptional brightness enables background-free and target-specific imaging of the naturally expressed oxytocin GPCR in living mice.

Lipid-core/polymer-shell hybrid nanoparticles: synthesis and characterization by fluorescence labeling and electrophoresis

Among the lipid nanoparticles, lipid polymer hybrid nanoparticles (HNPs) composed of an oily core and a polymeric shell display interesting features as efficient drug carriers due to the high loading capability of the oil phase and the stability and surface functionalization of the polymer shell. Herein, we formulated lipid-core/polymer-shell hybrid nanoparticles (HNPs) using a simple nanoprecipitation method involving Vitamin E Acetate (VEA) as the oily core and a tailor-made amphiphilic polymer as a wrapping shell. The fluorescence labeling of the oil, using a newly developed green fluorogenic BODIPY tracker, and of the polymer using a covalent attachment of a red emitting rhodamine was done to assess the formation, the composition and the stability of these new hybrid nanoparticles using dual color electrophoresis gel analysis. This technique, combined to conventional DLS and electronic microscopy analysis, allowed us to quickly determine that 20 wt% of the polymer was an optimal ratio for obtaining stable HNPs by nanoprecipiation. Finally, we showed that using different polymeric shells, various HNPs can be obtained and finely discriminated using a combined approach of electrophoresis and two-color labeling.

Molecular Tuning of Styryl Dyes Leads to Versatile and Efficient Plasma Membrane Probes for Cell and Tissue Imaging

The plasma membrane (PM) plays a major role in many biological processes; therefore, its proper fluorescence staining is required in bioimaging. Among the commercially available PM probes, styryl dye FM1-43 is one of the most widely used. In this work, we demonstrated that fine chemical modifications of FM1-43 can dramatically improve the PM staining. The newly developed probes, SP-468 and SQ-535, were found to display enhanced photophysical properties (reduced cross-talk, higher brightness, improved photostability) and, unlike FM1-43, provided excellent and immediate PM staining in 5 different mammalian cell types including neurons (primary culture and tissue imaging). Taking advantage of these features, we successfully used SP-468 in STED super resolution neuronal imaging. Additionally, we showed that the new probes displayed differences in their internalization pathways compared to their parent FM1-43. Finally, we showed that the new probes kept the ability to stain the PM of plant cells. Overall, this work presents new useful probes for PM imaging in cells and tissues and provides insights on the molecular design of new PM targeting molecules.

Zwitterionic Stealth Dye-Loaded Polymer Nanoparticles for Intracellular Imaging

Intracellular applications of fluorescent nanoparticles (NPs) as probes and labels are currently limited by significant molecular crowding and the high level of complexity encountered inside living cells. The solution is to develop very small, bright, and noninteracting (stealth) NPs. Combining these properties requires implementing the stealth behavior through the thinnest possible hydrophilic shell. Here, we propose a one-step process for preparing ultrasmall and bright stealth NPs based on a zwitterionic (ZI) methacrylate-based copolymer. Dye-loaded polymer NPs are assembled through nanoprecipitation of the copolymer together with the salt of a rhodamine B derivative and a bulky hydrophobic counterion to achieve high particle brightness. We found that 10 mol % ZI groups in the polymer yield NPs of less than 15 nm that are stable in physiological salt conditions and practically resistant to protein adsorption, as suggested by fluorescence correlation spectroscopy. The combination of the very small size with the nonfouling nature of these particles enables spreading of ZI polymer NPs in the whole cytosol after their microinjection into living cells. In addition, single-particle tracking showed up to four times faster diffusion of ZI NPs in the cytosol compared to PEGylated NPs. The obtained dye-loaded ZI polymer NPs open the route to intracellular single-particle tracking and biosensing applications.

Near infrared fluorogenic probe as a prodrug model for evaluating cargo release by nanoemulsions

We developed Pro-HD, a NIR fluorogenic prodrug model. We evaluated its efficient cell delivery using biocompatible nanoemulsions and its hydrolysis into the fluorescent HD drug model once delivered in cancer cells.

Switchable Solvatochromic Probes for Live-Cell Super-resolution Imaging of Plasma Membrane Organization

Visualization of the nanoscale organization of cell membranes remains challenging because of the lack of appropriate fluorescent probes. Herein, we introduce a new design concept for super-resolution microscopy probes that combines specific membrane targeting, on/off switching, and environment sensing functions. A functionalization strategy for solvatochromic dye Nile Red that improves its photostability is presented. The dye is grafted to a newly developed membrane-targeting moiety composed of a sulfonate group and an alkyl chain of varied lengths. While the long-chain probe with strong membrane binding, NR12A, is suitable for conventional microscopy, the short-chain probe NR4A, owing to the reversible binding, enables first nanoscale cartography of the lipid order exclusively at the surface of live cells. The latter probe reveals the presence of nanoscopic protrusions and invaginations of lower lipid order in plasma membranes, suggesting a subtle connection between membrane morphology and lipid organization.

Agonist-induced membrane nanodomain clustering drives GLP-1 receptor responses in pancreatic beta cells

The glucagon-like peptide-1 receptor (GLP-1R), a key pharmacological target in type 2 diabetes (T2D) and obesity, undergoes rapid endocytosis after stimulation by endogenous and therapeutic agonists. We have previously highlighted the relevance of this process in fine-tuning GLP-1R responses in pancreatic beta cells to control insulin secretion. In the present study, we demonstrate an important role for the translocation of active GLP-1Rs into liquid-ordered plasma membrane nanodomains, which act as hotspots for optimal coordination of intracellular signaling and clathrin-mediated endocytosis. This process is dynamically regulated by agonist binding through palmitoylation of the GLP-1R at its carboxyl-terminal tail. Biased GLP-1R agonists and small molecule allosteric modulation both influence GLP-1R palmitoylation, clustering, nanodomain signaling, and internalization. Downstream effects on insulin secretion from pancreatic beta cells indicate that these processes are relevant to GLP-1R physiological actions and might be therapeutically targetable.

Nanodomain segregation and clustering of the glucagon-like peptide-1 receptor, a key target for type 2 diabetes therapy, is regulated by agonist binding, leading to compartmentalization of downstream signaling and clathrin-dependent internalization and impacting pancreatic beta cell responses.

Ionic aggregation-induced emission dye with bulky counterions for preparation of bright near-infrared polymeric nanoparticles

Dyes exhibiting aggregation-induced emission (AIE) are attractive building blocks for the preparation of bright fluorescent nanomaterials. AIEgens are especially efficient in pure dye aggregates, whereas they are much less explored as dopants in NPs built of hydrophobic polymers. Here, we describe an approach that combines cationic AIEgens with bulky hydrophobic counterions (fluorinated tetraphenylborates) that enables preparation of small and bright AIEgen-loaded polymeric NPs. To this end, we synthesised a cationic tetraphenylethylene (TPE) derivative and studied its salts with counterions of different sizes and hydrophobicities. In organic solvent/water mixtures, all these salts exhibited typical AIE behaviour, whereas only salts with bulky hydrophobic counterions exhibited strongly red-shifted emission in the near-infrared (NIR) region. Encapsulation of these salts into poly(methyl methacrylate-co-methacrylic acid) (PMMA-MA) NPs revealed that bulky counterions ensure (i) formation of small (∼50 nm) AIEgen-loaded polymeric NPs; (ii) good fluorescence quantum yield (up to 30%); and (iii) NIR emission reaching 700 nm. By contrast, AIEgens with small inorganic anions (perchlorate and hexafluorophosphate) blended with PMMA-MA produced large aggregates with emission in the far-red region. Single-particle microscopy revealed that our 50 nm AIEgen-loaded PMMA-MA NPs were 6-fold brighter than the NIR emitting quantum dots (QD705). These NPs feature low cytotoxicity and compatibility with live cell imaging, in contrast to large aggregates of AIEgens with small inorganic counterions that failed to internalize into the cells. The present work shows that combination of cationic AIEgens with bulky counterions opens new routes for the preparation of bright polymer-based nanomaterials.

S-Palmitoylation of junctophilin-2 is critical for its role in tethering the sarcoplasmic reticulum to the plasma membrane

Junctophilins (JPH1-JPH4) are expressed in excitable and nonexcitable cells, where they tether endoplasmic/sarcoplasmic reticulum (ER/SR) and plasma membranes (PM). These ER/SR-PM junctions bring Ca-release channels in the ER/SR and Ca as well as Ca-activated K channels in the PM to within 10-25 nm. Such proximity is critical for excitation-contraction coupling in muscles, Ca modulation of excitability in neurons, and Ca homeostasis in nonexcitable cells. JPHs are anchored in the ER/SR through the C-terminal transmembrane domain (TMD). Their N-terminal Membrane-Occupation-Recognition-Nexus (MORN) motifs can bind phospholipids. Whether MORN motifs alone are sufficient to stabilize JPH-PM binding is not clear. We investigate whether S-palmitoylation of cysteine (Cys), a critical mechanism controlling peripheral protein binding to PM, occurs in JPHs. We focus on JPH2 that has four Cys residues: three flanking the MORN motifs and one in the TMD. Using palmitate-alkyne labeling, Cu(I)-catalyzed alkyne-azide cycloaddition reaction with azide-conjugated biotin, immunoblotting, proximity-ligation-amplification, and various imaging techniques, we show that JPH2 is S-palmitoylatable, and palmitoylation is essential for its ER/SR-PM tether function. Palmitoylated JPH2 binds to lipid-raft domains in PM, whereas palmitoylation of TMD-located Cys stabilizes JPH2's anchor in the ER/SR membrane. Binding to lipid-raft domains protects JPH2 from depalmitoylation. Unpalmitoylated JPH2 is largely excluded from lipid rafts and loses the ability to form stable ER/SR-PM junctions. In adult ventricular myocytes, native JPH2 is S-palmitoylatable, and palmitoylated JPH2 forms distinct PM puncta. Sequence alignment reveals that the palmitoylatable Cys residues in JPH2 are conserved in other JPHs, suggesting that palmitoylation may also enhance ER/SR-PM tethering by these proteins.

Solvatochromic Near-Infrared Probe for Polarity Mapping of Biomembranes and Lipid Droplets in Cells under Stress

Can polarity-sensitive fluorescent dyes monitor the response of live cells to fundamental stress conditions, such as deprivation from nutrition and oxidative stress? To address this question, we developed a push-pull dioxaborine probe (DXB-NIR) for biomembranes and lipid droplets featuring strong solvatochromism in the far-red to near-infrared region, high fluorescence brightness, photostability, and two-photon absorption cross section, reaching 13800 GM at 930 nm. In model membranes, DXB-NIR exhibits unprecedented 80 nm shift between liquid ordered and disordered membrane phases, allowing robust imaging of separated membrane microdomains. Two-color imaging of live cells with DXB-NIR enables polarity mapping in plasma membranes, endoplasmic reticulum, and lipid droplets, which reveals that starvation and oxidative stress produce an increase in the local polarity, and this change is different for each of the studied cell compartments. Thus, by pushing the limits of existing solvatochromic dyes, we introduce a concept of polarity mapping for monitoring the response of cells to stress.

Optimizing the Fluorescence Properties of Nanoemulsions for Single Particle Tracking in Live Cells

Nanoemulsions (NEs) are biocompatible lipid nanoparticles composed of an oily core stabilized by a surfactant shell. It is acknowledged that the surface decoration with poly(ethylene glycol), through the use of nonionic surfactants, confers high stealth in biological medium with reduced nonspecific interactions. Tracking individual NE by fluorescence microscopy techniques would lead to a better understanding of their behavior in cells and thus require the development of bright single particles with enhanced photostability. However, the understanding of the relationship between the physicochemical properties and chemical composition of the NEs, on the one hand, and its fluorescence properties of encapsulated dyes, on the other hand, remains limited. Herein, we synthesized three new dioxaborine barbituryl styryl (DBS) dyes that displayed high molar extinction coefficients (up to 120 000 M^-1 cm^-1) with relatively low quantum yields in solvents and impressive fluorescence enhancement when dissolved in viscous oils (up to 0.98). The reported screening of nine different oils allowed disclosing a range of efficient "oil/dye" couples and understanding the main parameters that lead to the brightest NEs. We determine vitamin E acetate/DBS-C₈ as the representative most efficient couple, combining high dye loading capabilities and low aggregation-induced quenching, leading to <50 nm ultrabright NEs (with brightness as high as 30 × 10⁶ M^-1 cm^-1) with negligible dye leakage in biological media. Beyond a comprehensive optical and physicochemical characterization of fluorescent NEs, cellular two-photon excitation imaging was performed with polymer-coated cell penetrating NEs. Thanks to their impressive brightness and photostability, NEs displaying different charge surfaces were microinjected in HeLa cells and were individually tracked in the cytosol to study their relative velocity.

Spontaneous nano-emulsification with tailor-made amphiphilic polymers and related monomers

In general, nano-emulsions are submicron droplets composed of liquid oil phase dispersed in liquid aqueous bulk phase. They are stable and very powerful systems when it regards the encapsulation of lipophilic compounds and their dispersion in aqueous medium. On the other hand, when the properties of the nano-emulsions aim to be modified, e.g. for changing their surface properties, decorating the droplets with targeting ligands, or modifying the surface charge, the dynamic liquid / liquid interfaces make it relatively challenging. In this study, we have explored the development of nano-emulsions which were not anymore stabilized with a classical low-molecular weight surfactant, but instead, with an amphiphilic polymer based on poly(maleic anhydride-alt-1-octadecene) (PMAO) and Jeffamine®, a hydrophilic amino-terminated PPG/PEG copolymer. Using a polymer as stabilizer is a potential solution for the nano-emulsion functionalization, ensuring the droplet stabilization as well as being a platform for the droplet decoration with ligands (for instance after addition of function groups in the terminations of the chains). The main idea of the present work was to understand if the spontaneous emulsification –commonly performed with nonionic surfactants– can be transposed with amphiphilic polymers, and a secondary objective was to identify the main parameters impacting on the process. PMAO was modified with two different Jeffamine®, additionally different oils and different formulation conditions were evaluated. As a control, the parent monomer, octadecyl succinic anhydride (OSA) was also modified and studied in the similar way as that of polymer. The generated nano-emulsions were mainly studied by dynamic light scattering and electron microscopy, that allows discriminating the crucial parameters in the spontaneous process, originally conducted with polymers as only stabilizer.