Adsorption of Catalytic Nanoparticles onto Polymer Substrates for Controlled Deposition of Microcapsule Metal Shells

Easy reversible clustering of gold nanoparticles via pH-Induced assembly of PVP-b-PAA copolymer

The growing demand of novel hybrid organic/inorganic systems with exciting properties has contributed to an increasing need for simplifying production strategies. Here, we report a simple method to obtain controlled three-dimensional hybrid architectures, in particular hybrid supracolloids (hSC), formed by gold nanoparticles and a double hydrophilic block copolymer, specifically the poly(acrylic acid)-block-poly(N-vinyl-2-pyrrolidone) (PAA-b-PVP), directly in aqueous medium. The ubiquitous pH-sensitive poly(acrylic acid) (PAA) block initiates the assembly through pH changes, while the poly(N-vinyl-2-pyrrolidone) block assures the close affinity with the AuNPs. We demonstrate that the formation of hybrid supracolloids (hSC) is the result of the synergetic behavior of the two specific polymeric blocks. Additionally, the entire process shows spontaneous and fast switchability. The nanostructured copolymer behaves like a highly swollen hydrogel and displays a disordered internal structure. The driving force for the association of the copolymer chains is induced by the synergetic effects of the decrease in solubility of the poly(acrylic acid) block and the formation of inter and intra chains hydrogen bonds. These were demonstrated by using small angle X-ray scattering (SAXS), quartz crystal microbalance with dissipation monitoring (QCM-D) and scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM-EDX). In turn, the AuNPs are randomly spread all over the polymeric matrix, as demonstrated by field emission gun - scanning electron microscopy (FEG-SEM). A correlation analysis reveals the hSC density depends mostly on the initial concentration of AuNPs. These results can inspire the fabrication of more complex structures with multicomponent composition.

Alginate Hydrogel Beads with a Leakproof Gold Shell for Ultrasound-Triggered Release

Background/Objectives: Focused ultrasound has advantages as an external stimulus for drug delivery as it is non-invasive, has high precision and can penetrate deep into tissues. Here, we report a gold-plated alginate (ALG) hydrogel system that retains highly water-soluble small-molecule fluorescein for sharp off/on release after ultrasound exposure. Methods: The ALG is crosslinked into beads with calcium chloride and layered with a polycation to adjust the surface charge for the adsorption of catalytic platinum nanoparticles (Pt NPs). The coated bead is subject to electroless plating, forming a gold shell. Ultrasound is applied to the gold-plated ALG beads and the release of fluorescein with or without ultrasound stimulation is quantified. Results: Polyethylenimine (PEI), not poly-L-lysine (PLL), is able to facilitate Pt NP adsorption. Gold shell thickness is proportional to the duration of electroless plating and can be controlled. Gold-plated ALG beads are impermeable to the fluorescein cargo and have nearly zero leakage. Exposure to focused ultrasound initiated the release of fluorescein with full release achieved after 72 h. Conclusions: The gold-plated ALG hydrogel is a new material platform that can retain highly water-soluble molecules with a sharp off/on release initiated by focused ultrasound.

Synthesis and Catalytic Properties of New Polymeric Monometallic Composites Based on Copolymers of Polypropylene Glycol Maleate Phthalate with Acrylic Acid

Metal-polymer composites based on copolymers of polypropylene glycol maleate phthalate with acrylic acid and metallic nickel and silver were synthesized for the first time. The objects obtained were characterized by infrared (IR) and Raman spectroscopies, thermogravimetry, a scanning electron microscope with energy dispersive spectroscopy, and atomic emission spectrometry. The catalytic activity of new metal-polymer composites that exhibited a rather high efficiency in the reactions of electrocatalytic hydrogenation of pyridine was studied. It is shown that nanoparticles of metals are evenly distributed in the volume of the polymer matrix; more than 80% of nanoparticles are in the range from 25 to 40 nm and have spherical and rhombic shapes. The reusability of the obtained composites is shown.

Metal-shell nanocapsules for the delivery of cancer drugs

Cytotoxic drugs tend to have substantial side effects on healthy tissues leading to systemic toxicity, limited tolerated doses and reduced drug efficacy. A prominent research area focuses on encapsulating cytotoxic drugs for targeted delivery to cancer tissues. However, existing carriers suffer from low drug loading levels and high drug leaching both when circulating systemically and when accumulating in non-target organs. These challenges mean that only few encapsulation technologies for delivery of cytotoxic drugs have been adopted for clinical use. Recently, we have demonstrated efficient manufacture of impermeable metal-shell/liquid core microcapsules that permit localised delivery by triggering release with ultrasound. This method has the potential to improve on existing methods for localised drug delivery because it:We demonstrate here the further miniaturization of both the emulsion droplet template and the thickness of the surrounding metal shell to the nanoscale in an attempt to take advantage of the EPR effect and the excretion of nanoparticles by the hepatobiliary system.

Metal Microcapsules Prepared via Electroless Plating at Liquid-Liquid Interface

We report the facile preparation of metal microcapsules via the formation of a Pickering emulsion, stabilized by catalytic palladium nanoparticles (PdNPs), and subsequent electroless plating at an oil-water interface induced by the adsorbed PdNPs. Metal microcapsules with smooth metal shells are formed by simply agitating the mixture of the plating solution and 1,2-dichloroethane containing poly(vinylpyrrolidone)-coated PdNPs, without external heating or electrical sources as energy inputs. We found that the metal microcapsules had thin copper metal shells with a relatively smooth surface and metallic luster. The metal shells were composed of more than 99 atom % copper in the form of Cu and Cu₂O. We believe that this simple metal microcapsule preparation method may be beneficial to produce novel functional microcapsules with metal shells.

Geometrical Changes in the Hemes of Bacterial Surface c-Type Cytochromes Reveal Flexibility in Their Binding Affinity with Minerals

Microbial extracellular electron transport occurs via the physical and electrical association of outer-membrane c-type cytochromes (OM c-Cyts) with extracellular solid surfaces. However, studies investigating the characteristics of cytochrome binding with solid materials have been limited to the use of purified units of OM c-Cyts dissolved in solution, rather than OM c-Cyts in intact cells, because of the lack of a methodology that specifically allows for the monitoring of OM c-Cyts in whole-cells. Here, we utilized circular dichroism (CD) spectroscopy to examine the molecular mechanisms and binding characteristics of the interaction between MtrC, a unit of OM c-Cyts, in whole Shewanella oneidensis MR-1 cells and hematite nanoparticles. The addition of hematite nanoparticles significantly decreased the intensity of the Soret CD peaks, indicating geometrical changes in the hemes in MtrC associated with their physical contact with hematite. The binding affinity of MtrC estimated using CD spectra changed predominantly depending upon the redox state of MtrC and the concentration of the hematite nanoparticles. In contrast, purified MtrC demonstrated a constant binding affinity following a Langmuir isotherm, with a standard Gibbs free energy of -43 kJ mol^-1, suggesting that the flexibility in the binding affinity of MtrC with hematite was specific in membrane-bound protein complex conditions. Overall, these findings suggest that the binding affinity as well as the heme geometry of OM c-Cyts are flexibly modulated in the membrane complex associated with microbe-mineral interactions.

Encapsulation of Emulsion Droplets with Metal Shells for Subsequent Remote, Triggered Release

A two-step method to encapsulate an oil core with an impermeable shell has been developed. A thin metallic shell is deposited on the surface of emulsion droplets stabilized by metal nanoparticles. This thin shell is shown to prevent diffusion of the oil from within the core of the metal-shell microcapsules when placed in a continuous phase that fully dissolves the oil. The stabilizing nanoparticles are sterically stabilized by poly(vinyl pyrrolidone) chains and are here used as a catalyst/nucleation site at the oil-water interface to grow a secondary metal shell on the emulsion droplets via an electroless deposition process. This method provides the simplest scalable route yet to synthesize impermeable microcapsules with the added benefit that the final structure allows for drastically improving the overall volume of the encapsulated core to, in this case, >99% of the total volume. This method also allows for very good control over the microcapsule properties, and here we demonstrate our ability to tailor the final microcapsule density, capsule diameter, and secondary metal film thickness. Importantly, we also demonstrate that such impermeable microcapsule metal shells can be remotely fractured using ultrasound-based devices that are commensurate with technologies currently used in medical applications, which demonstrate the possibility to adapt these microcapsules for the delivery of cytotoxic drugs.

Free-Standing Metal Films Prepared via Electroless Plating at Liquid-Liquid Interfaces

We report a simple preparation of free-standing metal films via electroless plating (ELP) at the liquid-liquid (L-L) interface between an aqueous electroless plating solution and an organic solvent. The use of ELP does not require any external energy in the form of heating and stirring. We find that the affinity of the organic solvent for the palladium nanoparticles (PdNPs) as catalysts and the vertical position of the organic and aqueous phases in the biphasic system are important considerations for synthesizing a robust copper film. Specifically, 1,2-dichloroethane which has an appropriate affinity for PdNPs and a higher density than water was found to be a good candidate for use as the organic phase in this system. However, a poor-quality copper film was obtained in the system with 1-hexanol as the organic phase. We also controlled the microscale surface structure of the copper films by using different concentrations of the injected PdNP dispersion. A high density of PdNPs caused smaller regions of metal growth, which contributed to the formation of smoother metal films. Moreover, under the optimal synthesis condition, we confirmed the electrical conductivity of the obtained copper film to be 1.16 × 10^-7 Ωm. We believe that this metal film preparation represents a promising way to produce a range of metal film structures through the use of flexible L-L interfaces as templates.

Particle-Stabilized Fluid-Fluid Interfaces: The Impact of Core Composition on Interfacial Structure

The encapsulation of small molecule drugs in nanomaterials has become an increasingly popular approach to the delivery of therapeutics. The use of emulsions as templates for the synthesis of drug impregnated nanomaterials is an exciting area of research, and a great deal of progress has been made in understanding the interfacial chemistry that is critical to controlling the physicochemical properties of both the encapsulated material and the templated material. For example, control of the interfacial tension between an oil and aqueous phase is a fundamental concern when designing drug delivery vehicles that are stabilized by particulate surfactants at the fluid interface. Particles in general are capable of self-assembly at a fluid interface, with a preference for one or the other of the phases, and much work has focussed on modification of the particle properties to optimize formation and stability of the emulsion. An issue arises however when a model, single oil system is translated into more complex, real-world scenarios, which are often multi-component, with the incorporation of charged active ingredients and other excipients. The result is potentially a huge change in the properties of the dispersed phase which can lead to a failure in the capability of particles to continue to stabilize the interface. In this mini-review, we will focus on two encapsulation strategies based on the selective deposition of particles or proteins on a fluid-fluid interface: virus-like particles and polymer microcapsules formed from particle-stabilized emulsion templates. The similarity between these colloidal systems lies in the fact that particulate entities are used to stabilize fluid cores. We will focus on those studies that have described the effect of subtle changes in core composition on the self-assembly of particles at the fluid-fluid interface and how this influences the resulting capsule structure.

Metal/Carbon Hybrid Nanostructures Produced from Plasma-Enhanced Chemical Vapor Deposition over Nafion-Supported Electrochemically Deposited Cobalt Nanoparticles

In this work, we report development of hybrid nanostructures of metal nanoparticles (NP) and carbon nanostructures with strong potential for catalysis, sensing, and energy applications. First, the etched silicon wafer substrates were passivated for subsequent electrochemical (EC) processing through grafting of nitro phenyl groups using para-nitrobenzene diazonium (PNBT). The X-ray photoelectron spectroscope (XPS) and atomic force microscope (AFM) studies confirmed presence of few layers. Cobalt-based nanoparticles were produced over dip or spin coated Nafion films under different EC reduction conditions, namely CoSO₄ salt concentration (0.1 M, 1 mM), reduction time (5, 20 s), and indirect or direct EC reduction route. Extensive AFM examination revealed NP formation with different attributes (size, distribution) depending on electrochemistry conditions. While relatively large NP with >100 nm size and bimodal distribution were obtained after 20 s EC reduction in H₃BO₃ following Co²⁺ ion uptake, ultrafine NP (<10 nm) could be produced from EC reduction in CoSO₄ and H₃BO₃ mixed solution with some tendency to form oxides. Different carbon nanostructures including few-walled or multiwalled carbon nanotubes (CNT) and carbon nanosheets were grown in a C₂H₂/NH₃ plasma using the plasma-enhanced chemical vapor deposition technique. The devised processing routes enable size controlled synthesis of cobalt nanoparticles and metal/carbon hybrid nanostructures with unique microstructural features.

A two-step synthesis for preparing metal microcapsules with a biodegradable polymer substrate

Novel method of producing metal microcapsules, using nanoparticles as the stabiliser, allows better control of nanoparticle distribution thus optimisation of metal shells.