Dynamic Nuclear Polarization of Selectively 29Si-Enriched Core@shell Silica Nanoparticles

²⁹Si silica nanoparticles (SiO₂ NPs) are promising magnetic resonance imaging (MRI) probes that possess advantageous properties for in vivo applications, including suitable biocompatibility, tailorable properties, and high water dispersibility. Dynamic nuclear polarization (DNP) is used to enhance ²⁹Si MR signals via enhanced nuclear spin alignment; to date, there has been limited success employing DNP for SiO₂ NPs due to the lack of endogenous electronic defects that are required for the process. To create opportunities for SiO₂-based ²⁹Si MRI probes, we synthesized variously featured SiO₂ NPs with selective ²⁹Si isotope enrichment on homogeneous and core@shell structures (shell thickness: 10 nm, core size: 40 nm), and identified the critical factors for optimal DNP signal enhancement as well as the effective hyperpolarization depth when using an exogenous radical. Based on the synthetic design, this critical factor is the proportion of ²⁹Si in the shell layer regardless of core enrichment. Furthermore, the effective depth of hyperpolarization is less than 10 nm between the surface and core, which demonstrates an approximately 40% elongated diffusion length for the shell-enriched NPs compared to the natural abundance NPs. This improved regulation of surface properties facilitates the development of isotopically enriched SiO₂ NPs as hyperpolarized contrast agents for in vivo MRI.

Nonlinear charge regulation for the deposition of silica nanoparticles on polystyrene spherical surfaces

HYPOTHESIS

We describe the deposition behavior of monodispersed silica nanoparticles on polystyrene spherical particles by using modified pairwise DLVO (Derjaguin-Landau-Verwey-Overbeek) interaction force profiles at pH values between two and twelve. Our modified model contains a new nonlinear charge regulation parameter that considers redistribution of ions, which allows us to realistically express the electrical double layer (EDL) interaction forces.

EXPERIMENTS

Silanol-terminated silica nanoparticles (7.6 ± 0.4 nm), l-lysine-covered silica nanoparticles (7.8 ± 0.4 nm), and polyallylamine hydrochloride-covered polystyrene (PAH/PS) particles (348 ± 1 nm) were synthesized. Then, each type of silica nanoparticle was deposited on the PAH/PS particles at a range of pH values.

FINDINGS

Our new regulation parameter describes the realistic redistribution of charges governed by pH, total salt concentration, ionic strength of solution, and separation distance of particles. We find that this regulation parameter can be roughly approximated from the absolute values of theoretically calculated surface charge density and potential distributions, as well as experimentally measured ζ-potentials. Morphological analysis using electron microscopy of the experimental systems shows that the modified pairwise DLVO interaction forces exceptionally describe the deposition behavior of the silica nanoparticles physically adsorbed on the PAH/PS particle substrates.

Control of Phase Morphology of Binary Polymer Grafted Nanoparticle Blend Films via Direct Immersion Annealing

While the phase separation of binary mixtures of chemically different polymer-grafted nanoparticles (PGNPs) is observed to superficially resemble conventional polymer blends, the presence of a "soft" polymer-grafted layer on the inorganic core of these nanoparticles qualitatively alters the phase separation kinetics of these "nanoblends" from the typical pattern of behavior seen in polymer blends and other simple fluids. We investigate this system using a direct immersion annealing method (DIA) that allows for a facile tuning of the PGNPs phase boundary, phase separation kinetics, and the ultimate scale of phase separation after a sufficient "aging" time. In particular, by switching the DIA solvent composition from a selective one (which increases the interaction parameter according to Timmerman's rule) to an overall good solvent for both PGNP components, we can achieve rapid switchability between phase-separated and homogeneous states. Despite a relatively low and non-classical power-law coarsening exponent, the overall phase separation process is completed on a time scale on the order of a few minutes. Moreover, the roughness of the PGNP blend film saturates at a scale that is proportional to the in-plane phase separation pattern scale, as observed in previous blend and block copolymer film studies. The relatively low magnitude of the coarsening exponent n is attributed to a suppression of hydrodynamic interactions between the PGNPs. The DIA method provides a significant opportunity to control the phase separation morphology of PGNP blends by solution processing, and this method is expected to be quite useful in creating advanced materials.

Organosilica Cages Target Hepatic Sinusoidal Endothelial Cells Avoiding Macrophage Filtering

Over the last years, advancements in the use of nanoparticles for biomedical applications have clearly showcased their potential for the preparation of improved imaging and drug-delivery systems. However, compared to the vast number of currently studied nanoparticles for such applications, only a few successfully translate into clinical practice. A common "barrier" that prevents nanoparticles from efficiently delivering their payload to the target site after administration is related to liver filtering, mainly due to nanoparticle uptake by macrophages. This work reports the physicochemical and biological investigation of disulfide-bridged organosilica nanoparticles with cage-like morphology, OSCs, assessing in detail their bioaccumulation in vivo. The fate of intravenously injected 20 nm OSCs was investigated in both healthy and tumor-bearing mice. Interestingly, OSCs exclusively colocalize with hepatic sinusoidal endothelial cells (LSECs) while avoiding Kupffer-cell uptake (less than 6%) under both physiological and pathological conditions. Our findings suggest that organosilica nanocages hold the potential to be used as nanotools for LSECs modulation, potentially impacting key biological processes such as tumor cell extravasation and hepatic immunity to invading metastatic cells or a tolerogenic state in intrahepatic immune cells in autoimmune diseases.

Synthesis of size-controlled and highly monodispersed silica nanoparticles using a short alkyl-chain fluorinated surfactant

Highly monodispersed silica nanoparticles (SiNPs) were synthesised using a fluorinated surfactant, HOCH2CH(CF3)CO2H, and its efficiency was compared with efficiencies of five other surfactants. The size of the SiNPs (∼50-200 nm) was controlled by controlling the surfactant amount. The short alkyl-chain fluoro surfactant was found to be more efficient at producing monodispersed SiNPs than its long alkyl-chain fluoro or non-fluorinated surfactant counterparts.

Surface functionalization-dependent localization and affinity of SiO2 nanoparticles within the biofilm EPS matrix

The contribution of the biofilm extracellular polymeric substance (EPS) matrix to reduced antimicrobial susceptibility in biofilms is widely recognised. As such, the direct targeting of the EPS matrix is a promising biofilm control strategy that allows for the disruption of the matrix, thereby allowing a subsequent increase in susceptibility to antimicrobial agents. To this end, surface-functionalized nanoparticles (NPs) have received considerable attention. However, the fundamental understanding of the interactions occurring between engineered NPs and the biofilm EPS matrix has not yet been fully elucidated. An insight into the underlying mechanisms involved when a NP interacts with the EPS matrix will aid in the design of more efficient NPs for biofilm control. Here we demonstrate the use of highly specific fluorescent probes in confocal laser scanning microscopy (CLSM) to illustrate the distribution of EPS macromolecules within the biofilm. Thereafter, a three-dimensional (3D) colocalization analysis was used to assess the affinity of differently functionalized silica NPs (SiNPs) and EPS macromolecules from Pseudomonas fluorescens biofilms. Results show that both the charge and surface functional groups of SiNPs dramatically affected the extent to which SiNPs interacted and localized with EPS macromolecules, including proteins, polysaccharides and DNA. Hypotheses are also presented about the possible physicochemical interactions which may be dominant in EPS matrix-NP interactions. This research not only develops an innovative CLSM-based methodology for elucidating biofilm-nanoparticle interactions but also provides a platform on which to build more efficient NP systems for biofilm control.

Nanosized Organo-Silica Particles with "Built-In" Surface-Initiated Atom Transfer Radical Polymerization Capability as a Platform for Brush Particle Synthesis

A facile synthetic method was developed to prepare sub-5 nm organo-silica (oSiO₂) nanoparticles through the self-condensation of atom transfer radical polymerization (ATRP)-initiator-containing silica precursors. The obtained oSiO₂ nanoparticles were characterized by a combination of nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), dynamic light scattering (DLS), and small-angle neutron scattering (SANS). The accessibility of the surface-Br initiating sites was evaluated by the polymerization of poly(methyl methacrylate) (PMMA) ligands from the surface of the oSiO₂ nanoparticles using surface-initiated atom transfer radical polymerization (SI-ATRP). The ultrasmall size, tunable composition, and ease of surface modification may render these organo-silica nanoparticle systems with built-in SI-ATRP capability an interesting alternative to conventional silica nanoparticles for functional material design.

Quantifying the level of nanoparticle uptake in mammalian cells using flow cytometry

Reliable quantification of nanoparticle uptake in mammalian cells is essential to study the effects of nanoparticles in the fields of medicine and environmental science. Most conventional quantification methods, such as electron microscopy or confocal imaging, are laborious and semi-quantitative and therefore not readily applicable to routine analyses. Here, we developed assays to quantify fluorescently labelled nanoparticle uptake in mammalian cells using a flow cytometer. The first approach was to measure the percentage of nanoparticle-containing cells based on a cutoff fluorescence intensity as set from a histogram of control cells, which is a quick and easy way to relatively compare nanoparticle uptake in the same set of experiments. The second approach was to measure the calibrated fluorescence intensity of the nanoparticle-treated cells in molecules of equivalent soluble fluorophore (MESF) values using calibration beads, which allows for comparisons between different sets of experiments. We successfully applied the developed assays to more readily measure fluorescence-labelled silica nanoparticle uptake in A549 lung carcinoma cells in a quantitative rather than semi-quantitative manner. We further tested the assays with nine different types of mammalian cells and investigated the correlation between cell type/size and nanoparticle uptake.

A high throughput method to investigate nanoparticle entrapment efficiencies in biofilms

The commercial use of nanoparticles has increased in recent years due to their unique characteristics, including high surface area, modifiable shape and surface charge and size-dependent properties. Consequently, a greater number of nanomaterials are now being released into the environment and inevitably interact with the natural ecosystem. Bacterial biofilms have the potential to capture and retain nanoparticles, however the factors determining the specific nanoparticle entrapment efficiencies of biofilms are not yet fully understood. Based on fluorescent intensity measurements we developed a simple and straightforward method that allowed the entrapment of different silica nanoparticles by two Pseudomonas strains to be quantified. It was determined that, regardless of nanoparticle size or surface functionalisation, Pseudomonas putida biofilms showed enhanced entrapment efficiencies compared to Pseudomonas fluorescens biofilms. It was also noted that both biofilms showed a higher entrapment capacity towards positively charged NPs. The method developed has the potential to be utilized for high throughput biofilm screening studies in order to develop a new understating of the relationship between nanoparticle characteristics and its uptake by bacterial biofilms.

Classification, Synthesis, and Application of Luminescent Silica Nanoparticles: a Review

Luminescent materials are of worldwide interest because of their unique optical properties. Silica, which is transparent to light, is an ideal matrix for luminescent materials. Luminescent silica nanoparticles (LSNs) have broad applications because of their enhanced chemical and thermal stability. Silica spheres of various sizes could be synthesized by different methods to satisfy specific requirements. Diverse luminescent dyes have potential for different applications. Subject to many factors such as quenchers, their performance was not quite satisfying. This review thus discusses the development of LSNs including their classification, synthesis, and application. It is the highlight that how silica improves the properties of luminescent dye and what role silica plays in the system. Further, their applications in biology, display, and sensors are also described.

Synthesis of highly stable cyanine-dye-doped silica nanoparticle for biological applications

Cyanine dyes are widely used in biological labeling and imaging because of their narrow near infrared emission, good brightness and high flexibility in functionalization, which not only enables multiplex analysis and multi-color imaging, but also greatly reduces autofluorescence from biological matter and increases signal-to-noise ratio. Unfortunately, their poor chemical- and photo-stability strongly limits their applications. The incorporation of cyanine dyes in silica nanoparticles provides a solution to the problem. On one hand, the incorporation of cyanine dyes in silica matrix can enhance their chemical- and photo-stability and increase brightness of the nanomaterials. On the other hand, silica matrix provides an optimized condition to host the dye, which helps to maintain their fluorescent properties during application. In addition, the well-established silane technique provides numerous functionalities for diverse applications. However, commercially available cyanine dyes are not very stable at high alkaline conditions, which will gradually lose their fluorescence over time. Our results showed that cyanine dyes are very vulnerable in the reverse micelle system, in which they will lose their fluorescence in less than half an hour. The existence of surfactant could greatly promote degradation of cyanine dyes. Fluorescent silica nanoparticles cannot be obtained at the high alkaline condition with the existence of surfactant. In contrast, the cyanine dyes are relatively stable in Stöber media. Owing to the fast formation of silica particles in Stöber media, the exposure time of cyanine dye in alkaline solution was greatly reduced, and highly fluorescent particles with good morphology and size distribution could be obtained via Stöber approach. However, the increasing water content in the Stöber could reduce the stability of cyanine dyes, which should be avoided. This research here provides a clear guidance on how to successfully synthesize cyanine dye-doped silica nanoparticles with good morphology, size distribution, stability and brightness.

Ultra-small dye-doped silica nanoparticles via modified sol-gel technique

In modern biosensing and imaging, fluorescence-based methods constitute the most diffused approach to achieve optimal detection of analytes, both in solution and on the single-particle level. Despite the huge progresses made in recent decades in the development of plasmonic biosensors and label-free sensing techniques, fluorescent molecules remain the most commonly used contrast agents to date for commercial imaging and detection methods. However, they exhibit low stability, can be difficult to functionalise, and often result in a low signal-to-noise ratio. Thus, embedding fluorescent probes into robust and bio-compatible materials, such as silica nanoparticles, can substantially enhance the detection limit and dramatically increase the sensitivity. In this work, ultra-small fluorescent silica nanoparticles (NPs) for optical biosensing applications were doped with a fluorescent dye, using simple water-based sol-gel approaches based on the classical Stöber procedure. By systematically modulating reaction parameters, controllable size tuning of particle diameters as low as 10 nm was achieved. Particles morphology and optical response were evaluated showing a possible single-molecule behaviour, without employing microemulsion methods to achieve similar results. Graphical abstractWe report a simple, cheap, reliable protocol for the synthesis and systematic tuning of ultra-small (< 10 nm) dye-doped luminescent silica nanoparticles.

Optical Characteristics and Tumor Imaging Capabilities of Near Infrared Dyes in Free and Nano-Encapsulated Formulations Comprised of Viral Capsids

Near infrared (NIR) fluorescent molecules and nanosized structures can serve as potential optical probes for image-guided removal of small tumor nodules (≲ 1 mm diameter). Although indocyanine green (ICG) remains as the only FDA-approved NIR dye, other organic dyes are under extensive development for enhanced imaging capabilities. One such dye is BrCy106-NHS where bromine is substituted for aromatic structures in cyanine dyes. Herein, we investigate the absorption and fluorescence characteristics of ICG and BrCy106-NHS, and quantitatively assess their tumor imaging capabilities in free (non-encapsulated) and a nano-encapsulated form that utilizes the capsid protein (CP) from genome-depleted plant-infecting brome mosaic virus as the encapsulating shell. We refer to these nanoconstructs as optical viral ghosts (OVGs). For example, when fabricated at CP to dye concentration ratio of 200, value of the spectrally integrated fluorescence emission for BrCy106-NHS-doped OVGs is ∼60 times higher than that of ICG-doped OVGs. Our analysis of homogenized mice intraperitoneal tumors indicate that the averaged total fluorescence emission associated with the use of BrCy106-NHS-doped can be at least about 44 times greater than that of ICG-doped OVGs. Our results suggest that OVGs containing BrCy106-NHS may potentially serve as effective optical probes for tumor imaging.