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Ensuring high catalytic activity and durability at low iridium (Ir)usage is still a big challenge for the development of electrocatalysts toward oxygen evolution reaction (OER) in proton exchange membrane water electrolysis (PEMWE). Here, a rapid liquid-reduction combined with surface galvanic replacement strategy is reported to synthesize the sub 2 nm high-entropy alloy (HEA) nanoparticles featured with Ir-rich IrRuNiMo medium-entropy oxide shell (Ir-MEO) and a IrRuCoNiMo HEA core (HEA@Ir-MEO). Advanced spectroscopies reveal that the Ir-rich MEO shell inhibits the severe structural evolution of transition metals upon the OER, thus guaranteeing the structural stability. In situ differential electrochemical mass spectrometry, activation energy analysis and theoretical calculations unveil that the OER on HEA@Ir-MEO follows an adsorbate evolution mechanism pathway, where the energy barrier of rate-determining step is substantially lowered. The optimized catalyst delivers the excellent performance (1.85 V/3.0 A cm-2@80 °C), long-term stability (>500 h@1.0 Acm-2), and low energy consumption (3.98 kWh Nm-3 H2 @1.0 A cm-2) in PEMWE with low Ir usage of ≈0.4 mg cm-2, realizing the dramatical reduction of hydrogen (H2) production cost to 0.88 dollar per kg (H2).

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High-entropy alloys nanoparticles (HEAs NPs) have gained considerable attention due to their extensive compositional tunability and intriguing catalytic properties. However, the synthesis of highly dispersed ultrasmall HEAs NPs remains a formidable challenge due to their inherent thermodynamic instability. In this study, highly dispersed ultrasmall (ca. 2 nm) PtCuGaFeCo HEAs NPs are synthesized using a one-pot solution-based method at 160 °C and atmospheric pressure. The PtCuGaFeCo NPs exhibit good catalytic activity for the oxygen reduction reaction (ORR). The half-wave potential relative to the reversible hydrogen electrode (RHE) reaches 0.88 V, and the mass activity and specific activity are approximately six times and four times higher than that of the commercial Pt/C catalyst. Based on X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analyses, the surface strain and optimized coordination environments of PtCuGaFeCo have led to high ORR activities in acidic media. Moreover, the ultrasmall size also plays an important role in enhancing catalytic performance. The work presents a facile and viable synthesis strategy for preparing the ultrasmall HEAs NPs, offering great potential in energy and electrocatalysis applications through entropy engineering.

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Silver ultrasmall nanoparticles (Ag UNPs) (size < 5 nm) were used as biosensing probes to analyze the efflux kinetics contributing to multidrug resistance (MDR) in single live triple-negative breast cancer (TNBC) cells by using dark-field optical microscopy to follow their size-dependent localized surface plasmon resonance. TNBC cells lack expression of estrogen (ER-), progesterone (PR-), and human epidermal growth factor 2 (HER2-) receptors and are more likely to acquire resistance to anticancer drugs due to their ability to transport harmful substances outside the cell. The TNBC cells displayed greater nuclear and cytoplasmic efflux, resulting in less toxicity of Ag UNPs in a concentration-independent manner. In contrast, more Ag UNPs and an increase in cytotoxic effects were observed in the receptor-positive breast cancer cells that have receptors for ER+, PR+, and HER2+ and are known to better respond to anticancer therapies. Ag UNPs accumulated in receptor-positive breast cancer cells in a time-and concentration-dependent mode and caused decreased cellular growth, whereas the TNBC cells due to the efflux were able to continue to grow. The TNBC cells demonstrated a marked increase in survival due to their ability to have MDR determined by efflux of Ag UNPs outside the nucleus and the cytoplasm of the cells. Further evaluation of the nuclear efflux kinetics of TNBC cells with Ag UNPs as biosensing probes is critical to gain a better understanding of MDR and potential for enhancement of cancer drug delivery.

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Liquid exfoliation can be considered as a viable approach for the scalable production of 2D materials due to its various benefits, although the polydispersity in the obtained nanosheet size hinders their straightforward incorporation. Size-separation can help alleviate these concerns, however a correlation between nanosheet size and property needs to be established to bring about size-specific applicability. Herein, size-selected aqueous nanosheet dispersions have been obtained via centrifugation-based protocols, and their chemical activity in the spontaneous reduction of chloroplatinic acid is investigated. Growth of ultrasmall Pt nanoparticles was achieved on nanosheet surfaces without a need for reducing agents, and stark differences in the nanoparticle coverage were observed as a function of nanosheet size. Defects in the nanosheets were probed via Raman spectroscopy, and correlated to the observed size-activity. Additionally, the effect of reaction temperature during synthesis was investigated. The electrochemical activity of the ultrasmall Pt nanoparticle decorated MoS2 nanosheets was evaluated for the hydrogen evolution reaction, and enhancement in performance was observed with nanosheet size, and nanoparticle decoration density. These findings shine light on the significance of nanosheet size in controlling spontaneous reduction reactions, and provide a deeper insight to intrinsic properties of liquid exfoliated nanosheets.

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The fast renal clearance of hydrophilic small molecular anticancer drugs and ultrasmall nanoparticles (NPs) results in the low utilization rate and certain side effects, thus improving the tumor targeting is highly desired but faces great challenges. A novel and general ß-cyclodextrin (CD) aggregation-induced assembly strategy to fabricate doxorubicin (DOX) and CD-coated NPs (such as Au) co-encapsulated pH-responsive nanocomposites (NCs) is proposed. By adding DOX×HCl and reducing pH in a reversed microemulsion system, hydrophilic CD-coated AuNPs rapidly assemble into large NCs. Then in situ polymerization of dopamine and sequentially coordinating with Cu2+ on the surface of NCs provide extra weak acid responsiveness, chemodynamic therapy (CDT), and improved biocompatibility as well as stability. The subsequent tumor microenvironment responsive dissociation notably improves their passive tumor targeting, bioavailability, imaging, and therapeutic capabilities, as well as facilitates their internalization by tumor cells and metabolic clearance, thereby reducing side effects. The combination of polymerized dopamine and assembled AuNPs reinforces photothermal capability, thus further boosting CDT through thermally amplifying Cu-catalyzed Fenton-like reaction. Both in vitro and in vivo studies confirm the desirable outcomes of these NCs as photoacoustic imaging guided trimodal (thermally enhanced CDT, photothermal therapy, and chemotherapy) synergistic tumor treatment agents with minimal systemic toxicity.

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Liquid-liquid phase separation (LLPS) of biopolymers to form condensates is a widespread phenomenon in living cells. Agents that target or alter condensation can help uncover elusive physiological and pathological mechanisms. Owing to their unique material properties and modes of interaction with biomolecules, nanoparticles represent attractive condensate-targeting agents. Our work focused on elucidating the interaction between ultrasmall gold nanoparticles (usGNPs) and diverse types of condensates of tau, a representative phase-separating protein associated with neurodegenerative disorders. usGNPs attract considerable interest in the biomedical community due to unique features, including emergent optical properties and good cell penetration. We explored the interaction of usGNPs with reconstituted self-condensates of tau, two-component tau/polyanion and three-component tau/RNA/alpha-synuclein coacervates. The usGNPs were found to concentrate into condensed liquid droplets, consistent with the formation of dynamic client (nanoparticle) - scaffold (tau) interactions, and were observable thanks to their intrinsic luminescence. Furthermore, usGNPs were capable to promote LLPS of a protein domain which is unable to phase separate on its own. Our study demonstrates the ability of usGNPs to interact with and illuminate protein condensates. We anticipate that nanoparticles will have broad applicability as nanotracers to interrogate phase separation, and as nanoactuators controlling the formation and dissolution of condensates.

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The biological properties of spherical nucleic acids (SNAs) are largely independent of nanoparticle core identity but significantly affected by oligonucleotide surface density. Additionally, the payload-to-carrier (i.e., DNA-to-nanoparticle) mass ratio of SNAs is inversely proportional to core size. While SNAs with many core types and sizes have been developed, all in vivo analyses of SNA behavior have been limited to cores >10 nm in diameter. However, "ultrasmall" nanoparticle constructs (<10 nm diameter) can exhibit increased payload-to-carrier ratios, reduced liver accumulation, renal clearance, and enhanced tumor infiltration. Therefore, we hypothesized that SNAs with ultrasmall cores exhibit SNA-like properties, but with in vivo behavior akin to traditional ultrasmall nanoparticles. To investigate, we compared the behavior of SNAs with 1.4-nm Au102 nanocluster cores (AuNC-SNAs) and SNAs with 10-nm gold nanoparticle cores (AuNP-SNAs). Significantly, AuNC-SNAs possess SNA-like properties (e.g., high cellular uptake, low cytotoxicity) but show distinct in vivo behavior. When intravenously injected in mice, AuNC-SNAs display prolonged blood circulation, lower liver accumulation, and higher tumor accumulation than AuNP-SNAs. Thus, SNA-like properties persist at the sub-10-nm length scale and oligonucleotide arrangement and surface density are responsible for the biological properties of SNAs. This work has implications for the design of new nanocarriers for therapeutic applications.

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Ultrasmall nanoparticles (NPs) are a promising platform for the diagnosis and therapy of cancer, but the particles in sizes as small as several nanometers have an ability to translocate across biological barriers, which may bring unpredictable health risks. Therefore, it is essential to develop workable cell-based tools that can deliver ultrasmall NPs to the tumor in a safer manner. Here, this work uses macrophages as a shuttle to deliver sub-5 nm PEGylated gold (Au) NPs to tumors actively or passively, while reducing the accumulation of Au NPs in the brain. This work demonstrates that sub-5 nm Au NPs can be rapidly exocytosed from live macrophages, reaching 45.6% within 24 h, resulting in a labile Au NP-macrophage system that may release free Au NPs into the blood circulation in vivo. To overcome this shortcoming, two straightforward methods are used to engineer macrophages to obtain "half-dead" and "dead" macrophages. Although the efficiency of engineered macrophages for delivering sub-5 nm Au NPs to tumors is 2.2-3.8% lower than that of free Au NPs via the passive enhanced permeability and retention effect, this safe-by-design approach can dramatically reduce the accumulation of Au NPs in the brain by more than one order of magnitude. These promising approaches offer an opportunity to expand the immune cell- or stem cell-mediated delivery of ultrasmall NPs for the diagnosis and therapy of diseases in a safer way in the future.

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Supported bimetallic nanoparticles (NPs) with ultrasmall sizes and homogeneous alloying are attractive for catalysis. However, facile synthesis of this type of material remains very challenging. Here, the aerosol drying impregnation method for rapid, scalable, and general synthesis of silica-supported bimetallic NPs is proposed. The method relies on aerosol spray drying to promote the mixing and dispersing of binary metal precursors on SiO2 . It is capable of controlling the composition and size of bimetallic NPs and avoids the use of expensive metal complex salts and complicated experiment procedures. Twelve permutations combining a noble metal (Pd, Ru, and Pt) and a base one (Fe, Co, Ni, and Cu) with ultrasmall sizes (1.4-2.2 nm in average size), uniform dispersion, and good alloying are synthesized. Interesting activity and selectivity trends in catalytic semihydrogenation of phenylacetylene over the supported Pd-based NPs can be observed. The silica-supported PdNi NPs deliver both high activity and styrene selectivity. Spectroscopic and density functional theory calculation results reveal the improved chemoselectivity originated from the suitably down-shifted d-band center of the PdNi NPs inducing an increased energy barrier for overhydrogenation and a weakened styrene adsorption.

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Understanding the interactions between nanoparticles (NPs) and proteins is crucial for the successful application of NPs in biological contexts. Protein adsorption is dependent on particle size, and protein binding to ultrasmall (1-3 nm) NPs is considered to be generally weak. However, most studies have involved structured biomacromolecules, while the interactions of ultrasmall NPs with intrinsically disordered proteins (IDPs) have remained elusive. IDPs are abundant in eukaryotes and found to associate with NPs intracellularly. As a model system, we focused on ultrasmall gold nanoparticles (usGNPs) and tau, a cytosolic IDP associated with Alzheimer's disease. Using site-resolved NMR, steady-state fluorescence, calorimetry, and circular dichroism, we reveal that tau and usGNPs form stable multimolecular assemblies, representing a new type of nano-bio interaction. Specifically, the observed interaction hot spots explain the influence of usGNPs on tau conformational transitions, with implications for the intracellular targeting of aberrant IDP aggregation.

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Considering the dismal survival rate, novel therapeutic strategies are warranted to improve the outcome of pancreatic ductal adenocarcinoma (PDAC). Combining nanotechnology for delivery of chemotherapeutics-preferably radiosensitizing agents-is a promising approach to enhance the therapeutic efficacy of chemoradiation. We assessed the effect of biodegradable ultrasmall-in-nano architectures (NAs) containing gold ultra-small nanoparticles (USNPs) enclosed in silica shells loaded with cisplatin prodrug (NAs-cisPt) combined with ionizing radiation (IR). The cytotoxic effects and DNA damage induction were evaluated in PDAC cell lines (MIA PaCa2, SUIT2-028) and primary culture (PDAC3) in vitro and in the chorioallantoic membrane (CAM) in ovo model. Unlike NAs, NAs-cisPt affected the cell viability in MIA PaCa2 and SUIT2-028 cells. Furthermore, NAs-cisPt showed increased γH2AX expression up to 24 h post-IR and reduced ß-globin amplifications resulting in apoptosis induction at DNA and protein levels. Similarly, combined treatment of NAs-cisPt + IR in PDAC3 and SUIT2-028 CAM models showed enhanced DNA damage and apoptosis leading to tumor growth delay. Our results demonstrate an increased cytotoxic effect of NAs-cisPt, particularly through its release of the cisplatin prodrug. As cisplatin is a well-known radiosensitizer, administration of cisplatin prodrug in a controlled fashion through encapsulation is a promising new treatment approach which merits further investigation in combination with other radiosensitizing agents.

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BACKGROUND: Iron oxide nanoparticles have been approved by food and drug administration for clinical application as magnetic resonance imaging (MRI) and are considered to be a biocompatible material. Large iron oxide nanoparticles are usually used as transversal (T2) contrast agents to exhibit dark contrast in MRI. In contrast, ultrasmall iron oxide nanoparticles (USPIONs) (several nanometers) showed remarkable advantage in longitudinal (T1)-weighted MRI due to the brighten effect. The study of the toxicity mainly focuses on particles with size of tens to hundreds of nanometers, while little is known about the toxicity of USPIONs. RESULTS: We fabricated Fe3O4 nanoparticles with diameters of 2.3, 4.2, and 9.3 nm and evaluated their toxicity in mice by intravenous injection. The results indicate that ultrasmall iron oxide nanoparticles with small size (2.3 and 4.2 nm) were highly toxic and were lethal at a dosage of 100 mg/kg. In contrast, no obvious toxicity was observed for iron oxide nanoparticles with size of 9.3 nm. The toxicity of small nanoparticles (2.3 and 4.2 nm) could be reduced when the total dose was split into 4 doses with each interval for 5 min. To study the toxicology, we synthesized different-sized SiO2 and gold nanoparticles. No significant toxicity was observed for ultrasmall SiO2 and gold nanoparticles in the mice. Hence, the toxicity of the ultrasmall Fe3O4 nanoparticles should be attributed to both the iron element and size. In the in vitro experiments, all the ultrasmall nanoparticles (< 5 nm) of Fe3O4, SiO2, and gold induced the generation of the reactive oxygen species (ROS) efficiently, while no obvious ROS was observed in larger nanoparticles groups. However, the ·OH was only detected in Fe3O4 group instead of SiO2 and gold groups. After intravenous injection, significantly elevated ·OH level was observed in heart, serum, and multiple organs. Among these organs, heart showed highest ·OH level due to the high distribution of ultrasmall Fe3O4 nanoparticles, leading to the acute cardiac failure and death. CONCLUSION: Ultrasmall Fe3O4 nanoparticles (2.3 and 4.2 nm) showed high toxicity in vivo due to the distinctive capability in inducing the generation of ·OH in multiple organs, especially in heart. The toxicity was related to both the iron element and size. These findings provide novel insight into the toxicology of ultrasmall Fe3O4 nanoparticles, and also highlight the need of comprehensive evaluation for their clinic application.

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During recent decades, ultrasmall inorganic nanoparticles have attracted considerable interest due to their favorable biodistribution, pharmacokinetics and theranostic properties. In particular, AGuIX nanoparticles made of polysiloxane and gadolinium chelates were successfully translated to the clinics. In an aqueous medium, these nanoparticles are in dynamic equilibrium with polysiloxane fragments due to the hydrolysis of Si-O-Si bonds. Thanks to high-performance liquid chromatography coupled with electrospray ionization mass spectrometry, all these fragments were separated and identified.

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Magnetic nanoparticles are central to the development of efficient hyperthermia treatments, magnetic drug carriers, and multimodal contrast agents. While the magnetic properties of small crystalline iron oxide nanoparticles are well understood, the superparamagnetic size limit constitutes a significant barrier for further size reduction. Iron (oxy)hydroxide phases, albeit very common in the natural world, are far less studied, generally due to their poor crystallinity. Templating ultrasmall nanoparticles on substrates such as graphene is a promising method to prevent aggregation, typically an issue for both material characterization and applications. We generate ultrasmall nanoparticles, directly on the carbon framework by the reaction of a graphenide potassium solution, charged graphene flakes, with iron(II) salts. After mild water oxidation, the obtained composite material consists of ultrasmall potassium ferrite nanoparticles bound to the graphene nanoflakes. Magnetic properties as evidenced by magnetometry and X-ray magnetic circular dichroism, with open magnetic hysteresis loops near room temperature, are widely different from classical ultrasmall superparamagnetic iron oxide nanoparticles. The large value obtained for the effective magnetic anisotropy energy density Keff accounts for the presence of magnetic ordering at rather high temperatures. The synthesis of ultrasmall potassium ferrite nanoparticles under such mild conditions is remarkable given the harsh conditions used for the classical syntheses of bulk potassium ferrites. Moreover, the potassium incorporation in the crystal lattice occurs in the presence of potassium cations under mild conditions. A transfer of this method to related reactions would be of great interest, which underlines the synthetic value of this study. These findings also give another view on the previously reported electrocatalytic properties of these nanocomposite materials, especially for the sought-after oxygen reduction/evolution reaction. Finally, their longitudinal and transverse proton NMR relaxivities when dispersed in water were assessed at 37 °C under a magnetic field of 1.41 T, allowing potential applications in biological imaging.

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High-performance photothermal theranostics is urgently desired for cancer therapy because of their good controllability and noninvasive features. The relatively low photothermal conversion efficiency is still at the drawbacks because of the absence of efficient extraneous carriers. Herein, a carrier-free nanomedicine is developed to in vivo self-deliver organic photothermal agents for efficient cancer phototheranostics. By a facile self-assembly strategy, the near-infrared (NIR)-absorbing conjugated oligomer IDIC-4F is fabricated into a carrier-free nanoparticle (DCF-P), showing ultrasmall size of nearly 4.0 nm with a nearly 100% of drug loading capacity. Notably, DCF-P achieves a superhigh photothermal conversion efficiency of 80.5% that is far greater than that of IDIC-4F-loaded nanomicelle DCF-M (57.3%). With the guidance of NIR fluorescence and photoacoustic dual-imaging, it is verified that DCF-P could well achieve tumor-preferential accumulation and retention at 4 h postinjection, and meanwhile shows highly efficient in vivo tumor elimination with good biosafety. This study thus contributes a novel concept for designing ultrasmall nanoparticle characteristics of preferential accumulation in tumors, and also provides a strategy for creating high-performance carrier-free nanomedicine via highly ordered molecular stacking.

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The knowledge of the fate of metal-containing nanoparticles in biological media in aqueous media is of utmost importance for the future use of these promising theranostic agents for clinical applications. A methodology based on the combination of TDA-ICP-MS and CE-ICP-MS was applied to study the degradation pathway of AGuIX, a phase 2 clinical ultrasmall gadolinium-containing nanoparticle. Nanoparticle size measurements and gadolinium speciation performed in different media (phosphate buffer, urine and serum) demonstrated an accelerated dissolution of AGuIX in serum, without any release of free gadolinium for each medium.

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Ultrasmall metal-organic frameworks (MOFs) may generate unique properties to expand the scope of applications. However, the synthesis is still a great challenge. Herein, we propose a strategy to synthesize ultrasmall MOFs by high gravity technology. With the aid of tremendous intensification of molecular mixing and mass transfer in high-gravity field, six typical MOFs were obtained instantaneously in a continuous way. These samples are monodispersed with sub-5 nm in size, smaller than the previously reported values and even close to the length of one crystal unit cell. As a proof-of-concept, catalytic activity for Knoevenagel reaction can be significantly enhanced using ultrasmall ZIF-8. Conversion time of benzaldehyde was decreased by 94 % or 75 % compared to those using conventional or hierarchically porous ZIF-8. More importantly, this approach is readily scalable with the highest space-time yield for nano-MOFs, which may promote the convenient synthesis and practical applications of ultrasmall MOFs in large-scale.

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The multifunctional biological properties of Ce ions including antioxidant, anti-inflammatory, antibacterial and anti-cancer effects are very encouraging for development of Ce-containing biomaterials with therapeutic properties. Herein, novel Ce3+/Ce4+ ions containing mesoporous bioactive glass ultrasmall nanoparticles (Ce-BGn) were prepared by a facile one-pot ultrasound-assisted sol-gel method. Interestingly, Ce2O3 incorporation exerted a significant influence on the particle size and textural properties of mesoporous BGn (SiO2 - CaO binary glass system). Ce-BGn exhibited ultrasmall nanoparticle size (< 30 nm), mesoporous texture (pore size up to 2.82 nm and pore volume up to 0.191 cm3/g) and large specific surface area ca. 132.9 m2/g. Notably, in situ formation of CeO2 nanospheres (3-6 nm) was detected at the surface and in the amorphous glass matrix of mesoporous Ce-BGn. Importantly, X-ray photoelectron spectroscopy (XPS) revealed the presence of 72.57 % Ce3+ and 27.43 % Ce4+ at the surface of mesoporous Ce-BGn with Ce3+/Ce4+ ratio = 2.66. Furthermore, mesoporous Ce-BGn exhibited high catalase-mimic activity and showed sustained release of Ce (2.5-32 ppm), Ca (85-327 ppm) and Si (54-200 ppm) ions within 4 weeks along with excellent bone-like hydroxyapatite formation. Finally, the in vitro biological behavior of mesoporous Ce-BGn in cell cultures of human skin fibroblasts (HSF) revealed that mesoporous Ce-BGn (with concentrations up to 300 µg/mL) possess good cyto-biocompatibility. Taken together, novel ultrasmall mesoporous Ce-BGn showed remarkable catalase-mimic activity via surface containing Ce3+/Ce4+ ions which can scavenge ROS (Ce3+↔ Ce4+) and decompose H2O2 molecules into H2O and O2. In addition to that, Ce-BGn demonstrated sustained release of bioactive ions (Ce, Ca and Si), excellent bone-like hydroxyapatite formation and good cyto-biocompatibility.

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Fluorescence imaging (FI) in the second near-infrared region (NIR-II, 1000-1700 nm) has attracted great attention for brain tumor imaging due to its deep penetration and high resolution. However, traditional NIR-II organic fluorescent nanoparticles (NPs) are usually hindered by uncontrolled large size (~30-100 nm), marked aggregation-caused quenching (ACQ) effect, and limited blood circulation (~1-3 h), which have great impact on efficient NIR-II FI of deep brain tumors. Herein, starlike polymer brush-based ultrasmall TQFP-10 NPs, with bright NIR-II fluorescence, prolonged blood circulation, and enhanced tumor accumulation, are facilely prepared for efficient orthotopic glioblastoma (GBM) imaging. Compared with traditional method prepared NPs (physically coated TQF@NPs and PEG modified TQF-PEG5K NPs), the ultrasmall (~8 nm) TQFP-10 NPs display a higher NIR-II fluorescence QY (1.9%), which is 2.1- and 3.8-fold higher than TQF@NPs (0.9%) and TQF-PEG5K NPs (0.5%), respectively. In addition, TQFP-10 NPs present a 10.6-fold higher blood circulation half-life (t1/2 = 8.5 h) than that of TQF-PEG5K NPs. Consequently, TQFP-10 NPs exhibit 4.2- and 33-fold higher maximal tumor to normal tissue ratio in subcutaneous and in situ NIR-II FI of GBM, respectively, than TQF@NPs and TQF-PEG5K NPs, attractively realizing GBM imaging. This work provides a general strategy for constructing ultrasmall NIR-II fluorescent NPs with simultaneously improved NIR-II fluorescence and blood circulation for efficient brain tumor imaging.

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We describe here a method to synthesize ultrasmall nanocapsules with a diameter of 6 nm, exhibiting a well-defined core-shell morphology. Remarkably, the nanocapules are synthesized in a miniemulsion process without the need of large amounts of surfactant as commonly used in the microemulsion process. Ultrasmall nanocapsules with an oil core and a silica shell are formed by the concurrent processes of a sol-gel reaction and Ostwald ripening. Using solvents with different water solubilities and alkoxysilanes with different reactivities, we demonstrate that sizes of obtained nanocapsules depend on the ripening rate and alkoxysilane conversion rate. The method can be also used for encapsulating natural oils such as peppermint oil and limonene. This work shows that the Ostwald ripening phenomenon can be employed beneficially for the preparation of very small colloids.