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Single‐molecule localization microscopy has proved promising to unravel the dynamics and molecular architecture of thin biological samples down to nanoscales. For applications in complex, thick biological tissues shifting single‐particle emission wavelengths to the shortwave infrared (SWIR also called NIR II) region between 900 to 2100 nm, where biological tissues are more transparent is key. To date, mainly single‐walled carbon nanotubes (SWCNTs) enable such applications, but they are inherently 1D objects. Here, 0D ultra‐small luminescent gold nanoclusters (AuNCs, <3 nm) and ≈25 nm AuNC‐loaded‐polymeric particles that can be detected at the single‐particle level in the SWIR are presented. Thanks to high brightness and excellent photostability, it is shown that the dynamics of the spherical polymeric particles can be followed at the single‐particle level in solution at video rates for minutes. We compared single particle tracking of AuNC‐loaded‐polymeric particles with that of SWCNT diffusing in agarose gels demonstrating the specificity and complementarity of diffusion properties of these SWIR‐emitting nano‐objects when exploring a complex environment. This extends the library of photostable SWIR emitting nanomaterials to 0D nano‐objects of variable size for single‐molecule localization microscopy in the second biological window, opening unprecedented possibilities for mapping the structure and dynamics of complex biological systems. [ABSTRACT FROM AUTHOR]

Photoactive transition‐metal complexes are luminophores combining high photostability and long luminescence lifetimes. However, reduced optical performance in aqueous solutions has limited their use in biological systems. Herein, the physicochemical and photophysical properties and bioimaging compatibility of Re diimine complexes and near‐infrared (NIR) emitting Cy5 dyes coencapsulated in polymer nanoparticles (NPs) are investigated. By varying the polymers, NPs with sizes from 20 to 70 nm and encapsulating ≤ 40 wt.% of Re complexes, i.e., ≈11 000 Re complexes per NP, are obtained. The photoluminescence (PL) quantum yields of the Re complexes increase eightfold to ≈50% upon encapsulation (vs 6–7% in acetonitrile), resulting in PL brightness up to 108 m−1 cm−1 and PL lifetimes of 3–4 µs. Coencapsulation of Cy5 yields very bright NIR emission upon Re complex excitation. Very close Re‐to‐Cy5 donor–acceptor distances down to ≤2 nm and FRET efficiencies over 90% are confirmed by PL lifetime measurements. The Re‐Cy5 NPs enter mammalian cells for high‐contrast PL imaging in both visible and NIR. This detailed characterization provides a better understanding of the photophysical properties of the transition‐metal‐dye FRET NPs and presents a vital step toward the efficient design of a new class of bright luminescent NP probes. [ABSTRACT FROM AUTHOR]

Brightness is a fundamental property of fluorescent nanomaterials reflecting their capacity to absorb and emit light. In sensing materials, brightness is crucial for high-sensitivity (bio)molecular detection, while in optical bioimaging it ensures high spatial and temporal resolution. Fluorescent organic nanoparticles (NPs) are particularly attractive because of their superior brightness compared to organic dyes. With the ever-growing diversity of organic nanomaterials, it is important to establish universal principles for measuring and estimating their brightness. This tutorial review provides definitions of brightness and describes the major approaches to its analysis based on ensemble and single-particle techniques. We present the current chemical approaches to fight Aggregation-Caused Quenching (ACQ) of fluorophores, which is a major challenge in the design of bright organic nanomaterials. The main classes of fluorescent organic NPs are described, including conjugated polymer NPs, aggregation-induced emission NPs, and NPs based on neutral and ionic dyes. Their brightness and other properties are systematically compared. Some brightest examples of bulk solid-state emissive organic materials are also mentioned. Finally, we analyse the importance of brightness and other particle properties in biological applications, such as bioimaging and biosensing. This tutorial will provide guidelines for chemists on the design of fluorescent organic NPs with improved performance and help them to estimate and compare the brightness of new nanomaterials with literature reports. Moreover, it will help biologists to select appropriate materials for sensing and imaging applications. [ABSTRACT FROM AUTHOR]

Reduced scattering and autofluorescence in the second near‐infrared window (NIR‐II, 1000–1700 nm) hold the promise of high‐resolution photoluminescence (PL) in vivo imaging at depths well beyond the µm scale. Yet, contrast agents with bright emission in this spectral region remain limited. Ultrasmall gold nanoclusters (AuNCs) are of particular interest for NIR‐II PL imaging. However, they exhibit limited brightness. Here, high amounts of NIR‐II emitting AuNCs are encapsulated in small polymer nanoparticles (NPs) made of poly(ethyl methacrylates). Using nanoprecipitation, loadings of up to 50 wt% can be reached, corresponding to over 10 000 AuNCs per 60 nm NP. PL quantum yields up to 1.3% and NP brightnesses up to 3.8 × 106 M−1 cm−1 are accomplished. Increasing inter‐AuNC interactions at higher loading result in a bathochromic shift of the PL emission. According to time‐resolved PL measurements, this can be attributed to distinct inter‐AuNC energy transfer pathways between the core and surface‐centered PL modes. Evaluation of the nanohybrids in tissue‐mimicking models for in vivo blood vessel imaging shows that the ultrabright and red‐shifted NIR‐II PL results in significantly improved detection sensitivity and resolution, which makes AuNC‐loaded polymer NPs highly promising candidates for advanced in vivo imaging. [ABSTRACT FROM AUTHOR]

The performance of fluorescence immunostaining is physically limited by the brightness of organic dyes, whereas fluorescence labeling with multiple dyes per antibody can lead to dye self‐quenching. The present work reports a methodology of antibody labeling by biotinylated zwitterionic dye‐loaded polymeric nanoparticles (NPs). A rationally designed hydrophobic polymer, poly(ethyl methacrylate) bearing charged, zwitterionic and biotin groups (PEMA‐ZI‐biotin), enables preparation of small (14 nm) and bright fluorescent biotinylated NPs loaded with large quantities of cationic rhodamine dye with bulky hydrophobic counterion (fluorinated tetraphenylborate). The biotin exposure at the particle surface is confirmed by Förster resonance energy transfer with dye‐streptavidin conjugate. Single‐particle microscopy validates specific binding to biotinylated surfaces, with particle brightness 21‐fold higher than quantum dot‐585 (QD‐585) at 550 nm excitation. The nanoimmunostaining method, which couples biotinylated antibody (cetuximab) with bright biotinylated zwitterionic NPs through streptavidin, significantly improves fluorescence imaging of target epidermal growth factor receptors (EGFR) on the cell surface compared to a dye‐based labeling. Importantly, cetuximab labeled with PEMA‐ZI‐biotin NPs can differentiate cells with distinct expression levels of EGFR cancer marker. The developed nanoprobes can greatly amplify the signal from labeled antibodies, and thus become a useful tool in the high‐sensitivity detection of disease biomarkers. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Efficient and safe delivery of nanoparticles (NPs) into the cytosol of living cells constitutes a major methodological challenge in bio‐nanotechnology. Electroporation allows direct transfer of NPs into the cytosol by forming transient pores in the cell membrane, but it is criticized for invasiveness, and the applicable particle sizes are not well defined. Here, in order to establish principles for efficient delivery of NPs into the cytosol with minimal cytotoxicity, the influence of the size of NPs on their electroporation and intracellular behavior is investigated. For this study, fluorescent dye‐loaded polymer NPs with core sizes between 10 and 40 nm are prepared. Optimizing the electroporation protocol allows minimizing contributions of endocytosis and to study directly the effect of NP size on electroporation. NPs of <20 nm hydrodynamic size are efficiently delivered into the cytosol, whereas this is not the case for NPs of >30 nm. Moreover, only particles of core size <15 nm diffuse freely throughout the cytosol. While electroporation at excessive electric fields induces cytotoxicity, the use of small NPs <20 nm allows efficient delivery at mild electroporation conditions. These results give clear methodological and design guidelines for the safe delivery of NPs for intracellular applications. [ABSTRACT FROM AUTHOR]

Controlling the emission of bright luminescent nanoparticles by a single molecular recognition event remains a challenge in the design of ultrasensitive probes for biomolecules. Herein, we developed 20‐nm light‐harvesting nanoantenna particles, built of a tailor‐made hydrophobic charged polymer poly(ethyl methacrylate‐co‐methacrylic acid), encapsulating circa 1000 strongly coupled and highly emissive rhodamine dyes with their bulky counterion. Being 87‐fold brighter than quantum dots QDots 605 in single‐particle microscopy (with 550‐nm excitation), these DNA‐functionalized nanoparticles exhibit over 50 % total FRET efficiency to a single hybridized FRET acceptor, a highly photostable dye (ATTO665), leading to circa 250‐fold signal amplification. The obtained FRET nanoprobes enable single‐molecule detection of short DNA and RNA sequences, encoding a cancer marker (survivin), and imaging single hybridization events by an epi‐fluorescence microscope with ultralow excitation irradiance close to that of ambient sunlight. [ABSTRACT FROM AUTHOR]

Uncontrolled release of encapsulated drugs and contrast agents from biodegradable polymer nanoparticles (NPs) is a central problem in drug delivery and bioimaging. In particular, it concerns polymeric NPs prepared by nanoprecipitation, where this release (so-called burst release) can be very significant, leading to side effects and/or bioimaging artifacts. Here, we systematically studied the effect of the chemical structure of cargo molecules, BODIPY dye derivatives, on their capacity to be loaded into ∼50 nm PLGA NPs without leakage in biological media. Absorption and fluorescence spectroscopy suggested that all the dyes, except the most polar BODIPY derivative, formed blended structures with polymer NPs. Fluorescence correlation spectroscopy of dye-loaded NPs in the presence of serum proteins revealed that only the most hydrophobic BODIPY dyes, bearing one octadecyl chain or two octyl chains, remain inside the NPs, while all other derivatives are released into the serum medium. The time-lapse absorption and fluorescence studies confirmed this result, suggesting the release kinetics for the leaky NPs on the time scale of hours. Fluorescence microscopy of living cells incubated with BODIPY-loaded NPs showed that most of them exhibit strong dye leakage observed as a homogeneous distribution of fluorescence all over the cytoplasm. Importantly, NPs loaded with the most hydrophobic dyes exhibited high stability showing a dotted pattern in the perinuclear region, typical for endosomes and lysosomes. Our results highlight the significance of the cargo hydrophobicity for efficient encapsulation inside polymeric NPs prepared by nanoprecipitation, which enables designing stable cargo-loaded nanomaterials for bioimaging and drug delivery. [ABSTRACT FROM AUTHOR]

Dye‐loaded polymer nanoparticles (NPs) emerge as a powerful tool for bioimaging applications, owing to their exceptional brightness and controlled small size. However, aggregation‐caused quenching (ACQ) and leakage of dyes at high loading remain important challenges of these nanomaterials. The use of bulky hydrophobic counterions has been recently proposed as an effective approach to minimize ACQ and dye leakage, but the role of counterion structure is still poorly understood. Here, a systematic study based on ten counterions, ranging from small hydrophilic perchlorate up to large hydrophobic tetraphenylborate derivatives, reveals how counterion nature can control encapsulation and emission of a cationic dye (rhodamine B octadecyl ester) in NPs prepared by nanoprecipitation of a biodegradable polymer, poly‐lactide‐co‐glycolide (PLGA). We found that increase in counterion hydrophobicity enhances dye encapsulation efficiency and prevents dye adsorption at the particle surface. Cellular imaging studies revealed that ≥95 % encapsulation efficiency, achieved with most hydrophobic counterions (fluorinated tetraphenylborates), is absolutely required because non‐encapsulated dye species at the surface of NPs are the origin of dye leakage and strong fluorescence background in cells. The size of counterions is found to be essential to prevent ACQ, where the largest species, serving as effective spacer between dyes, provide the highest fluorescence quantum yield. Moreover, we found that the most hydrophobic counterions favor dye–dye coupling inside NPs, leading to ON/OFF fluorescence switching of single particles. By contrast, less hydrophobic counterions tend to disperse dyes in the polymer matrix favoring stable emission of NPs. The obtained structure‐property relationships validate the counterion‐based approach as a mature concept to fight ACQ and dye leakage in the development of advanced polymeric nanomaterials with controlled optical properties. Counterion‐enhanced encapsulation and emission: A systematic study based on ten counterions revealed that their size and hydrophobicity is essential to prevent aggregation‐caused quenching and ensure efficient encapsulation of a cationic rhodamine dye in polymeric nanoparticles. Here, the use of counterions is established as mature concept for preparation of bright and leakage‐free fluorescent nanomaterials for bioimaging applications. [ABSTRACT FROM AUTHOR]

The key challenge in the field of fluorescent nanoparticles (NPs) for biological applications is to achieve superior brightness for sizes equivalent to single proteins (3-7 nm). We propose a concept of shell-cross-linked fluorescent micelles, in which PEGylated cyanine 3 and 5 bis-azides form a covalently attached corona on micelles of amphiphilic calixarene bearing four alkyne groups. The fluorescence quantum yield of the obtained monodisperse NPs, with a size of 7 nm, is a function of viscosity and reached up to 15 % in glycerol. In the on-state they are circa 2-fold brighter than quantum dots (QD-585), which makes them the smallest PEGylated organic NPs of this high brightness. FRET between cyanine 3 and 5 cross-linkers at the surface of NPs suggests their integrity in physiological media, organic solvents, and living cells, in which the NPs rapidly internalize, showing excellent imaging contrast. Calixarene micelles with a cyanine corona constitute a new platform for the development of protein-sized ultrabright fluorescent NPs. [ABSTRACT FROM AUTHOR]

Using modified β-galactosidase covalently linked to cross-linked polyelectrolyte multilayers (PEM), catalytically active materials have been designed. Their enzymatic activity can be modulated, partially in a reversible way, simply by stretching. This strategy, based on enzyme conformational changes, constitutes a new tool for the development of biocatalytic mechano-responsive materials. [ABSTRACT FROM AUTHOR]

Novel biochemically active compact polyelectrolyte complexes (CoPECs) are obtained through a simple coprecipitation and compaction procedure. As shown for the system composed of poly(acrylic acid) (PAA) and poly(allylamine) (PAH) as polyelectrolytes and alkaline phosphatase (ALP) as enzyme, the enzyme can be firmly immobilized into these materials. The ALP not only remains active in these materials, but the matrix also enhances the specific activity of the enzyme while protecting it from deactivation at higher temperature. The presence of the matrix allows fine control and substantial enhancement of reaction rates by varying the salt concentration of the contacting solution or temperature. The excellent reusability, together with the ease of co-immobilizing other useful components, such as magnetic particles, allowing facile handling of the CoPECs, makes these materials interesting candidates for variable scaffolds for the immobilization of enzymes for small- and large-scale enzyme-catalyzed processes. [ABSTRACT FROM AUTHOR]

ABSTRACT Sulfonation of narrow polydispersity polystyrene, PS, standards remains the method of choice for generating polystyrene sulfonate, PSS, samples with defined composition. Although a variety of sulfonation techniques have been described, relatively little is reported on the material obtained, which is used for so many studies on the fundamental behavior of polyelectrolytes. Here, we show that powdered polystyrene treated with concentrated sulfuric acid (96%) at 90 °C without catalyst yields fully sulfonated PSS. Extensive characterization with 1H and 13C NMR as well as size exclusion chromatography coupled with static and dynamic light scattering shows no evidence of sulfone crosslinking or chain degradation under the conditions used. Though mono-sulfonated as soon as it dissolves in the acid, the PSS contains about 6% meta substitution. Sulfonation kinetics for this heterogeneous reaction depend strongly on particle size, sulfuric acid content and temperature. For preparing perdeuterated PSS from the corresponding PS it is essential to employ D2SO4, as about half of the aromatic units undergo H/D exchange during sulfonation. The remaining ortho H/D may be exchanged with extended exposure to the concentrated sulfuric acid, but the meta site is deactivated. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, /-mbering [ABSTRACT FROM AUTHOR]

Durable compact polyelectrolyte complexes (CoPECs) with controlled porosity and mechanical properties are prepared by ultracentrifugation. Because the starting materials, poly(allylamine hydrochloride) (PAH) and poly(acrylic acid sodium salt) (PAA), are weak acids/bases, both composition and morphology are controlled by solution pH. In addition, the nonequilibrium nature of polyelectrolyte complexation can be exploited to provide a range of compositions and porosities under the influence of polyelectrolyte addition order and speed, and concentration. Confocal microscopy shows these 'saloplastic' materials to be highly porous, where pore formation is attributed to a combination of deswelling of the polyelectrolyte matrix and expansion of small inhomogenities by osmotic pressure. The porosity (15-70%) and the pore size (<5 μm to >70 μm) of these materials can be tuned by adjusting the PAA to PAH ratio, the salt concentration, and the pH. The modulus of these CoPECs depends on the ratio of the two polyelectrolytes, with stoichiometric complexes being the stiffest due to optimized charge pairing, which correlates with maximized crosslinking density. Mechanical properties, pore sizes, and pore density of these materials make them well suited to three dimensional supports for tissue engineering applications. [ABSTRACT FROM AUTHOR]

How small should nanoparticles be in order to travel freely through the cytosol similar to proteins? Answering this question remains a challenge, because the majority of nanoparticles are relatively large and their size cannot be finely tuned to match that of proteins. Here, poly(methyl methacrylate) copolymers with varied fraction and type of charged groups (carboxylate, sulfonate, and trimethylammonium) are developed, yielding nanoparticles with controlled sizes from 50 to 7 nm through nanoprecipitation. Loading these nanoparticles with a rhodamine dye/bulky counterion pair at 10wt% makes them highly fluorescent. After their coating with polyethylene glycol groups for preventing non‐specific protein binding and microinjection into living cells, the first systematic study of the size effect on diffusion in the cytosol for solid nanoparticles of the same nature is realized. Single‐particle‐tracking data provide evidence for distinct particle sieving in the cytosol, suggesting that only nanoparticles below a critical size of 23 nm exhibit free diffusion and spreading. These findings show the size limitations imposed by intracellular crowding and compartmentalization, which is critical for applications of nanomaterials in the cytosol. The proposed concept of polymer design opens the route to organic nanoparticles of ultrasmall sizes and high loading for bioimaging and drug‐delivery applications. What is the ideal size of nanoparticles (NPs) for intracellular applications? Here, a series of dye‐loaded polymer NPs with sizes from 50 down to 7 nm is synthesized and microinjected into living cells. Single‐particle tracking of these particles suggests that only NPs smaller than a critical size of 23 nm exhibit free diffusion and spreading in the cytosol. [ABSTRACT FROM AUTHOR]

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. • Fluorescent nanoprobes for microRNA detection in cell extracts are developed. • They amplify signal up to 130-fold by light-harvesting from ~400 FRET donor dyes. • They enable ratiometric analysis of microRNA with limit of detection down to 1.3 pM. • Biosensors for four microRNA are validated in 5 cancer and 2 healthy cell lines. • They are compatible with low-cost portable fluorometers and small detection volumes. [ABSTRACT FROM AUTHOR]

Nucleic acids are important biomarkers in cancer and viral diseases. However, their ultralow concentration in biological/clinical samples makes direct target detection challenging, because it leads to slow hybridization kinetics with the probe and its insufficient signal-to-noise ratio. Therefore, RNA target detection is done by molecular (target) amplification, notably by RT-PCR, which is a tedious multistep method that includes nucleic acid extraction and reverse transcription. Here, a direct method based on ultrabright dye-loaded polymeric nanoparticles in a sandwich-like hybridization assay with magnetic beads is reported. The ultrabright DNA-functionalized nanoparticle, equivalent to ≈10 000 strongly emissive rhodamine dyes, is hybridized with the magnetic bead to the RNA target, providing the signal amplification for the detection. This concept (magneto-fluorescent sandwich) enables high-throughput detection of DNA and RNA sequences of varied lengths from 48 to 1362 nt with the limit of detection down to 0.3 fm using a plate reader (15 zeptomoles), among the best reported for optical sandwich assays. Moreover, it allows semi-quantitative detection of SARS-CoV-2 viral RNA directly in clinical samples without a dedicated RNA extraction step. The developed technology, combining ultrabright nanoparticles with magnetic beads, addresses fundamental challenges in RNA detection; it is expected to accelerate molecular diagnostics of diseases., (© 2024 The Author(s). Small published by Wiley‐VCH GmbH.)

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., (© 2021 Wiley-VCH GmbH.)

Nanoprecipitation is a straightforward yet powerful technique to synthesize polymer nanoparticles loaded with various biologically active compounds or contrast agents. Particle formation in this approach is kinetically controlled, and various assembly parameters have been used to control the size distribution and properties of the formed nanoparticles. Here, the influence of the nature of the polymer on the formation of nanoparticles in nanoprecipitation is studied systematically by varying its hydrophobicity and charge over a broad range. For this, methacrylate copolymers with different types and fractions of hydrophobic, hydrophilic, and charged side groups are synthesized. Nanoprecipitation of these polymers shows that particle size increases with increasing global hydrophobicity of the polymers. At the same time, both hydrophilic and charged groups reduce particle size. In this way, we achieve control over particle size from ∼10 to 200 nm. Furthermore, the effect of the polymer nature on the photophysical properties of nanoparticles loaded with a fluorescent dye, a rhodamine B derivative with a bulky hydrophobic counterion (fluorinated tetraphenylborate), is studied. It is found that the hydrophobic/hydrophilic balance of the polymer modulates to a large extent the spectral properties and fluorescence quantum yield of the dye encapsulated at high concentration, which reflects changes in the dye aggregation within the polymer matrix. Thus, we show how polymer chemistry can tune kinetically controlled formation of nanoparticles and encapsulation of the load. The concepts introduced here should be valuable tools for the design of nanoparticles for imaging and drug-delivery applications.