Aggregation-Driven Controllable Plasmonic Transition of Silica-Coated Gold Nanoparticles with Temperature-Dependent Polymer-Nanoparticle Interactions for Potential Applications in Optoelectronic Devices

Effect of solvent chain length on the colloidal behavior of alkylamine-protected silver nanoparticles in alkanes: Insights from solvent relaxation NMR

HYPOTHESIS

The dispersion of apolar-ligand-protected nanoparticles (NPs) in alkanes is a complex process diverging from the "like dissolves like" principle, making its prediction beyond the capability of the Hansen solubility parameter (HSP) sphere method. This necessitates experimental investigation at the molecular level to understand dispersion behavior, particularly the role of solvent-ligand interactions.

EXPERIMENTS

Solvent relaxation NMR was applied for the first time to investigate solvent-ligand interactions in the dispersion/agglomeration of hexadecylamine-protected silver nanoparticles (C16-Ag NPs) in alkanes. The dispersibilities in different alkanes were determined from the localized surface plasmon resonance (LSPR) and compared with those predicted from the HSPs.

FINDINGS

The colloidal behavior of C16-Ag NPs in alkanes was notably affected by the length of the solvent chain. LSPR analysis demonstrated that while C16-Ag NPs remained dispersed in pentane, hexane, and octane, they exhibited agglomeration in decane, dodecane, and tetradecane, contradicting the HSP theory predictions. Solvent relaxation NMR revealed that this unexpected agglomeration stems from the strong bonding of longer-chain solvents to surface C16 ligands, leading to significant interaction. In contrast, shorter-chain solvents exhibited weaker bonding, promoting better dispersion. These findings emphasize the importance of solvent choice in NP applications and offer valuable insights into ligand-shell dynamics, furthering the development of NP technologies.

Uniform trehalose nanogels for glucagon stabilization

Glucagon is a peptide hormone that acts via receptor-mediated signaling predominantly in the liver to raise glucose levels by hepatic glycogen breakdown or conversion of noncarbohydrate, 3 carbon precursors to glucose by gluconeogenesis. Glucagon is administered to reverse severe hypoglycemia, a clinical complication associated with type 1 diabetes. However, due to low stability and solubility at neutral pH, there are limitations in the current formulations of glucagon. Trehalose methacrylate-based nanoparticles were utilized as the stabilizing and solubilizing moiety in the system reported herein. Glucagon was site-selectively modified to contain a cysteine at amino acid number 24 to covalently attach to the methacrylate-based polymer containing pyridyl disulfide side chains. PEG2000 dithiol was employed as the crosslinker to form uniform nanoparticles. Glucagon nanogels were monitored in Dulbecco's phosphate-buffered saline (DPBS) pH 7.4 at various temperatures to determine its long-term stability in solution. Glucagon nanogels were stable up to at least 5 months by size uniformity when stored at -20 °C and 4 °C, up to 5 days at 25 °C, and less than 12 hours at 37 °C. When glucagon stability was studied by either HPLC or thioflavin T assays, the glucagon was intact for at least 5 months at -20 °C and 4 °C within the nanoparticles at -20 °C and 4 °C and up to 2 days at 25 °C. Additionally, the glucagon nanogels were studied for toxicity and efficacy using various assays in vitro. The findings indicate that the nanogels were nontoxic to fibroblast cells and nonhemolytic to red blood cells. The glucagon in the nanogels was as active as glucagon alone. These results demonstrate the utility of trehalose nanogels towards a glucagon formulation with improved stability and solubility in aqueous solutions, particularly useful for storage at cold temperatures.

Synthesis and Characterization of Alkoxysilane-Bearing Photoreversible Cinnamic Side Groups: A Promising Building-Block for the Design of Multifunctional Silica Nanoparticles

The present study reports on the synthesis of a new alkoxysilane-bearing light-responsive cinnamyl group and its application as a surface functionalization agent for the development of SiO₂ nanoparticles (NPs) with photoreversible tails. In detail, cinnamic acid (CINN) was activated with N-hydroxysuccinimide (NHS) to obtain the corresponding NHS-ester (CINN-NHS). Subsequently, the amine group of 3-aminopropyltriethoxysilane (APTES) was acylated with CINN-NHS leading to the generation of a novel organosilane, CINN-APTES, which was then exploited for decorating SiO₂ NPs. The covalent bond to the silica surface was confirmed by solid state NMR, whereas thermogravimetric analysis unveiled a functionalization degree much higher compared to that achieved by a conventional double-step post-grafting procedure. In light of these intriguing results, the strategy was successfully extended to naturally occurring sepiolite fibers, widely employed as fillers in technological applications. Finally, a preliminary proof of concept of the photoreversibility of the obtained SiO₂@CINN-APTES system has been carried out through UV diffuse reflectance. The overall outcomes prove the consistency and the versatility of the methodological protocol adopted, which appears promising for the design of hybrid NPs to be employed as building blocks for photoresponsive materials with the ability to change their molecular structure and subsequent properties when exposed to different light stimuli.

Formation of kinetically trapped small clusters of PEGylated gold nanoparticles revealed by the combination of small-angle X-ray scattering and visible light spectroscopy

Gold nanoparticles coated with polyethylene glycol (PEG) are able to form clusters due to the collapse of the surface-grafted polymer chains when the temperature and ion concentration of the aqueous medium are increased. The chain collapse reduces the steric repulsion, leading to particle aggregation. In this work, we combine small angle X-ray scattering (SAXS) and visible light spectroscopy to elucidate the structure of the developing clusters. The structure derived from the SAXS measurements reveals a decrease in interparticle distance and drastic narrowing of its distribution in the cluster, indicating restricted particle mobility and displacement within the cluster. Surprisingly, instead of forming a large crystalline phase, the evolving clusters are composed of about a dozen particles. The experimental optical extinction spectra measured during cluster formation can be very well reproduced by optical simulations based on the SAXS-derived structural data.

Kinetic and Mechanistic Studies of the Formation of Silver Nanoparticles by Nicotinamide as a Reducing Agent

Here, in the present study, silver nanoparticles (SNPs) in the size range 6-10 nm have been synthesized by a chemical reduction method using nicotinamide (NTA), an anti-inflammatory agent, and cetyltrimethylammonium bromide (CTAB), a good stabilizing agent, to preparing the nanoparticles in the 6-10 nm size range. Kinetic studies on the formation of SNPs have been performed spectrophotometrically at 410 nm (strong plasmon band) in aqueous medium as a function of [AgNO₃], [NTA], [NaOH], and [CTAB]. The plot of ln(A _∞ - A _t ) versus time exhibited a straight line and the pseudo-first-order rate constants of different variables were calculated from its slope. On the basis of experimental findings, a plausible mechanism was proposed for the formation of SNPs colloid. From the mechanism, it is proved that the reduction of silver ions proceeded through the formation of silver oxide in colloidal form by their reaction with hydroxide ions and NTA after performing their function and readily undergo hydrolysis to form nicotinic acid as a hydrolysis product with the release of ammonia gas. The preliminary characterization of the SNPs was carried out by using a UV-visible spectrophotometer. The detailed characterization of SNPs was also carried out using other experimental techniques such as Fourier transform infrared spectroscopy (FTIR), field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and powder X-ray diffraction (PXRD). SNPs show a remarkable catalytic activity of up to 90% for the reduction of the cationic dye methylene blue.

Gold, Silver and Iron Oxide Nanoparticles: Synthesis and Bionanoconjugation Strategies Aimed at Electrochemical Applications

Nanomaterials have revolutionized the sensing and biosensing fields, with the development of more sensitive and selective devices for multiple applications. Gold, silver and iron oxide nanoparticles have played a particularly major role in this development. In this review, we provide a general overview of the synthesis and characteristics of gold, silver and iron oxide nanoparticles, along with the main strategies for their surface functionalization with ligands and biomolecules. Finally, different architectures suitable for electrochemical applications are reviewed, as well as their main fabrication procedures. We conclude with some considerations from the authors' perspective regarding the promising use of these materials and the challenges to be faced in the near future.

Measuring the Accessible Surface Area within the Nanoparticle Corona Using Molecular Probe Adsorption

The corona phase-the adsorbed layer of polymer, surfactant, or stabilizer molecules around a nanoparticle-is typically utilized to disperse nanoparticles into a solution or solid phase. However, this phase also controls molecular access to the nanoparticle surface, a property important for catalytic activity and sensor applications. Unfortunately, few methods can directly probe the structure of this corona phase, which is subcategorized as either a hard, immobile corona or a soft, transient corona in exchange with components in the bulk solution. In this work, we introduce a molecular probe adsorption (MPA) method for measuring the accessible nanoparticle surface area using a titration of a quenchable fluorescent molecule. For example, riboflavin is utilized to measure the surface area of gold nanoparticle standards, as well as corona phases on dispersed single-walled carbon nanotubes and graphene sheets. A material balance on the titration yields certain surface coverage parameters, including the ratio of the surface area to dissociation constant of the fluorophore, q/KD, as well as KD itself. Uncertainty, precision, and the correlation of these parameters across different experimental systems, preparations, and modalities are all discussed. Using MPA across a series of corona phases, we find that the Gibbs free energy of probe binding scales inversely with the cube root of surface area, q. In this way, MPA is the only technique to date capable of discerning critical structure-property relationships for such nanoparticle surface phases. Hence, MPA is a rapid quantitative technique that should prove useful for elucidating corona structure for nanoparticles across different systems.

Foldable and Extremely Scratch-Resistant Hard Coating Materials from Molecular Necklace-like Cross-Linkers

A flexible hard coating material displaying extreme scratch resistance and foldable flexibility was developed via the design of an organic-inorganic hybrid coating material employing an alkoxysilyl-functionalized polyrotaxane cross-linker (PRX_Si1). PRX_Si1 has a molecular necklace-like structure that can form organic-inorganic cross-linking points and provide large molecular movements. It was postulated that the scratch resistance and flexibility could be simultaneously increased because of the hybrid cross-linking points and dynamic molecular movements. To confirm this hypothesis, the crystalline structure and mechanical properties of the PRX_Si1-based hard coating material were analyzed via transmission electron microscopy, small-angle X-ray diffraction, tensile, pencil hardness, and scratch tests. Finally, the PRX_Si1-based hard coating material could form homogeneously dispersed nanoscale siloxane crystalline domains, and the strain at the break point was 3 times higher than that of a commercial hard coating material, resulting in no defect formation even after 5000 folding test runs. Moreover, the material displayed extremely high pencil hardness (9H) and scratch resistance.

Polyethyleneimine-assisted one-pot synthesis of quasi-fractal plasmonic gold nanocomposites as a photothermal theranostic agent

Gold nanoparticles have been thoroughly used in designing thermal ablative therapies and in photoacoustic imaging in cancer treatment owing to their unique and tunable plasmonic properties. While the plasmonic properties highly depend on the size and structure, controllable aggregation of gold nanoparticles can trigger a plasmonic coupling of adjacent electronic clouds, henceforth leading to an increase of light absorption within the near-infrared (NIR) window. Polymer-engraftment of gold nanoparticles has been investigated to achieve the plasmonic coupling phenomenon, but complex chemical steps are often needed to accomplish a biomedically relevant product. An appealing and controllable manner of achieving polymer-based plasmon coupling is a template-assisted Au+3 reduction that ensures in situ gold reduction and coalescence. Among the polymers exploited as reducing agents are polyethyleneimines (PEI). In this study, we addressed the PEI-assisted synthesis of gold nanoparticles and their further aggregation to obtain fractal NIR-absorbent plasmonic nanoaggregates for photothermal therapy and photoacoustic imaging of colorectal cancer. PEI-assisted Au+3 reduction was followed up by UV-visible light absorption, small-angle X-ray scattering (SAXS), and photo-thermal conversion. The reaction kinetics, stability, and the photothermal plasmonic properties of the as-synthesized nanocomposites tightly depended on the PEI : Au ratio. We defined a PEI-Au ratio range (2.5-5) for the one-pot synthesis of gold nanoparticles that self-arrange into fractal nanoaggregates with demonstrated photo-thermal therapeutic and imaging efficiency both in vitro and in vivo in a colorectal carcinoma (CRC) animal model.

Tailored Colloidal Stability and Rheological Properties of Graphene Oxide Liquid Crystals with Polymer-Induced Depletion Attractions

Graphene oxide liquid crystallinity (GO LC) has been widely exploited for high-performance graphene-based applications. In this regard, colloidal stability of GO LC suspension is a crucial requirement, particularly while polymers are often added to the GO LC. Unfortunately, current level of knowledge on how polymers influence the structure and properties of GO LC is not sufficient to systematically guide the development of applications. Here, we investigate the microstructure and rheological properties of GO LC suspensions in the presence of polymer additives with varying molecular weights and concentrations. Similar to conventional colloidal systems, non-negligible polymer-induced interactions are found in GO LC suspensions, which can effectively modulate the interaction among GO platelets and the relevant physical properties. On the basis of extensive small-angle X-ray scattering and rheological measurements, we demonstrate that, contrary to the general perception, polymer-induced depletion attraction can increase the colloidal stability of GO, while also preventing the vitrification of GO LC. In addition, a proper level of polymer additive can reduce the viscosity of GO LC suspensions by orders of magnitude, providing an effective route to GO LC-based solution processing. After all, the colloidal stability and rheological properties of GO can significantly impact the quality of GO. Therefore, we believe that our finding will be of great interest in the field of graphene-based applications, as it presents effective strategies for improving properties.

Plasmonic and colloidal stability behaviours of Au-acrylic core-shell nanoparticles with thin pH-responsive shells

The localised surface plasmon resonance (LSPR) of Au nanoparticles (NPs) as well as its interaction with nearby entities provides a wealth of fundamental and practical information at the nanometre scale. A number of studies have investigated core-shell NPs with Au cores and polymer shells that are temperature-responsive. However, there are very few studies of pH-responsive Au-polymer NP shells. Precipitation polymerisation is a scalable method and here we establish such a method to synthesise pH-responsive Au-poly(methyl methacrylate) copolymer core-shell NPs without the need for pre-functionalisation. The comonomers used were methacrylic acid (MAA) or 2-carboxyethyl acrylate (CEA) and the shells were crosslinked with ethylene glycol dimethacrylate. A series of five core-shell systems with collapsed shell thicknesses less than 30 nm are studied. The shell-thicknesses for the CEA-based core-shell NPs are relatively thin (≤5 nm) compared to related Au-polymer core-shell NPs prepared using precipitation polymerisation. The LSPR properties of the core-shell NPs were dependent on the shell thickness and were successfully simulated using finite difference time domain (FDTD) calculations. Two systems are considered further as exemplars. The MAA-based core-shell system with the thickest shell exhibited enhanced colloidal stability to added electrolyte. The CEA-based core-shell dispersion with the thinnest shells displayed reversible pH-triggered aggregation and was cytocompatible for HeLa cells. Proof-of-concept data are presented that demonstrate intracellular pH reporting.

Enhanced Mechanical Properties of Polymer Nanocomposites Using Dopamine-Modified Polymers at Nanoparticle Surfaces in Very Low Molecular Weight Polymers

While incorporation of nanoparticles in a polymer matrix generally enhances the physical properties, effective control of the nanoparticle/polymer interface is often challenging. Here, we report a dramatic enhancement of the mechanical properties of polymer nanocomposites (PNCs) using a simple physical grafting method. The PNC consists of low molecular weight poly(ethylene glycol) (PEG) and silica nanoparticles whose surfaces are modified with dopamine-modified PEG (DOPA-mPEG) brush polymers. With DOPA-mPEG grafting, the nanoparticle surface can be readily altered, and the shear modulus of the PNC is increased by a factor of 10⁵ at an appropriate surface grafting density. The detailed microstructure and mechanical properties are examined with small-angle X-ray scattering (SAXS) and oscillatory rheometry experiments. The attractive interactions between particles induced by DOPA-mPEG grafting dramatically improve the mechanical properties of PNCs even in an unentangled polymer matrix, which shows a much higher shear modulus than that of a highly entangled polymer matrix.