Aggregation and adhesion of gold nanoparticles in phosphate buffered saline

In applications in medicine and more specifically drug delivery, the dispersion stability of nanoparticles plays a significant role on their final performances. In this study, with the use of two laser technologies, dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA), we report a simple method to estimate the stability of nanoparticles dispersed in phosphate buffered saline (PBS). Stability has two features: (1) self-aggregation as the particles tend to stick to each other; (2) disappearance of particles as they adhere to surrounding substrate surfaces such as glass, metal, or polymer. By investigating the effects of sonication treatment and surface modification by five types of surfactants, including nonylphenol ethoxylate (NP9), polyvinyl pyrrolidone (PVP), human serum albumin (HSA), sodium dodecyl sulfate (SDS) and citrate ions on the dispersion stability, the varying self-aggregation and adhesion of gold nanoparticles dispersed in PBS are demonstrated. The results showed that PVP effectively prevented aggregation, while HSA exhibited the best performance in avoiding the adhesion of gold nanoparticle in PBS onto glass and metal. The simple principle of this method makes it a high potential to be applied to other nanoparticles, including virus particles, used in dispersing and processing.     

Peptide-induced patterning of gold nanoparticle thin films

In this work, the use of patterned proteins and peptides for the deposition of gold nanoparticles on several substrates with different surface chemistries is presented. The patterned biomolecule on the surface acts as a catalyst to precipitate gold nanoparticles from a precursor solution of HAuCl₄ onto the substrate. The peptide patterning on the surfaces was accomplished by physical adsorption or covalent attachment. It was shown that by using covalent attachment with a linker molecule, the influence of the surface properties from the different substrates on the biomolecule adsorption and subsequent nanoparticle deposition could be avoided. By adjusting the reaction conditions such as pH or HAuCl₄ concentration, the sizes and morphologies of deposited gold nanoparticle agglomerates could be controlled. Two biomolecules were used for this experiment, 3XFLAG peptide and bovine serum albumin (BSA). A micro-transfer molding technique was used to pattern the peptides on the substrates, in which a pre-patterned poly(dimethylsiloxane) (PDMS) mold was used to deposit a lift-off pattern of polypropylmethacrylate (PPMA) on the various substrates. The proteins were either physically adsorbed or covalently attached to the substrates, and an aqueous HAuCl₄ solution was applied on the substrates with the protein micropatterns, causing the precipitation of gold nanoparticles onto the patterns. SEM, AFM, and Electron Beam Induced Current (EBIC) were used for characterization.     

Bimetallic Nanoparticles with Exotic Facet Structures via Iodide‐Assisted Reduction of Palladium

Iodide is arguably the most challenging halide to control as a shape‐directing additive in metal nanoparticle synthesis and the addition of iodide during bimetallic nanoparticle growth often leads to inhomogeneously stellated products. Through judicious control of low micromolar concentrations of iodide ions in solution in a seed‐mediated approach, alloyed gold–palladium tetradecapod nanoparticles have been synthesized with a mixture of both well‐defined convex and concave surfaces. Notably, these particles are uniform and symmetrical, and this unusual combination of convex and concave features in a single nanostructure is not simply an artifact of intersecting spikes, as would be the case with stellated particles. Further, an important new role for iodide in catalyzing the reduction of palladium ions is identified, particularly at the edge sites of the growing gold nanoparticles. This differs from the commonly accepted theory that iodide slows metal ion reduction, and thus opens up promising new routes to the synthesis of other bimetallic nanoparticles with exotic shapes and surface structures.     

Self-assembled and fluorescence enhancement of semiconductor nanoparticles induced by surfactant adsorption

When semiconductor colloidal CdS nanoparticles and nonylphenol are mixed together in dimethyl sulfoxide at room temperature, a self-assembling process is induced. In the course, the size tunable properties of CdS nanoparticles are amplified. A blue shift in the emission spectrum and a strong photoluminescence enhancement are observed without significant change in the absorption features of the colloidal nanoparticles. These results are attributed to the adsorption of nonylphenol onto the nanoparticles surface and to the association process of the surfactant molecules. The surfactant adsorption process provides a nanoparticle surface passivation and induces an associative phase that enlarges the photoluminescence stability. This strategy opens the possibility to improve simultaneously physicochemical and photoluminescence properties of nanocrystals in solution as well as to control their deposition on two-dimensional surfaces.     

A comparative STM study of Ru nanoparticles deposited on HOPG by mass-selected gas aggregation versus thermal evaporation

Scanning tunneling microscopy was used to compare the morphologies of Ru nanoparticles deposited onto highly-oriented graphite surfaces using two different physical vapour deposition methods; (1) pre-formed mass-selected Ru nanoparticles with diameters between 2 nm and 15 nm were soft-landed onto HOPG surfaces using a gas-aggregation source and (2) nanoparticles were formed by e-beam evaporation of Ru films onto HOPG. The particles generated by the gas-aggregation source are round in shape with evidence of facets resolved on the larger particles. Annealing these nanoparticles when they are supported on unsputtered HOPG resulted in the sintering of smaller nanoparticles, while larger particles remained immobile. Nanoparticles deposited onto sputtered HOPG surfaces were found to be stable against sintering when annealed. The size and shape of nanoparticles deposited by e-beam evaporation depend to a large extent on the state of the graphite support and the temperature. Ru deposition onto unsputtered HOPG is characterised by bimodal growth with large flat particles formed on the substrate terraces and smaller diameter particles aligned along the substrate steps. Evaporation onto sputtered HOPG results in the formation of 2 nm round particles with a narrow size distribution. Finally, thermal deposition onto both sputtered and unsputtered HOPG at 660 °C results in larger particles showing a flat Ru(0 0 0 1) top facet.     

Self-assembling of metal nanoparticles on patterned semiconductor surfaces (Au/GaAs)

The fabrication of selectively deposited arrays of metal nanoparticles on textured semiconductor surfaces is reported. Gold nanoparticles were grown on previously textured surfaces using photostimulated chemical deposition from an aqueous solution of AuCl₃ salt. Surfaces with random and periodic microreliefs were used as templates of sites for nanoparticle deposition. Dendrite-like and quasi-grating-like microreliefs were produced by anisotropic etching of GaAs (100) substrates. Periodic reliefs (diffraction gratings and bi-gratings) were fabricated by the holographic photochemical etching of the same substrates. Our results from AFM, SEM and EDX show that gold predominantly locates on the tops of the microreliefs. Since the surface relief strongly affects the topology of metal deposition, the use of microprofiling of semiconductor surfaces allows designing nanostructure deposition.     

Nanoparticle Formation Mechanism in CVD Reactor with Ionization of Source Vapor

A new chemical vapor deposition (CVD) method, called ionization CVD, was developed, to produce non-agglomerated nanoparticles in which reactant gases are charged. A sonic-jet corona discharger was used as an ionizer in the developed nanoparticle generator. For a tetraethylorthosilicate (TEOS)/O₂ chemical system, SiO₂ nanoparticles were prepared. All particles formed by the ionization CVD were charged unipolarly. SEM micrographs of particles showed that the repulsive Coulombic force between charged particles reduces their coagulation rate and produces non-agglomerated nanoparticles that have a relatively high number concentration and small size. An external field was used to collect the charged particles onto Si wafers. These collected samples indicated that the deposition of charged particles could be controlled by the external electric field. Particle concentration measurement with a condensation nucleus counter at various TEOS concentrations suggested the particle formation mechanism in the ionization CVD was an ion-induced nucleation.     

Novel method for immobilization of enzymes to magnetic nanoparticles

The value of coupling biological molecules such as enzymes to solid materials has long been recognized. To date, protein immobilization onto such surfaces often involves covalent coupling, encapsulation, or non-specific adsorption techniques. Here we demonstrate the feasibility of specifically attaching a haloalkane dehalogenase enzyme to silica-coated or uncoated iron oxide superparamagnetic nanoparticles using affinity peptides. The enzyme was cloned from Xanthobacter autotrophicus strain GJ10 into Escherichia coli to produce fusion proteins containing dehalogenase sequences with C-terminal polypeptide repeats that have specific affinity for either silica or iron oxide. The fusion proteins serve dual functions, allowing for specific inorganic surface binding and for enzymatic activity. The degree of fusion protein adsorption to nanoparticle surfaces was found to exceed that of enzymes that had not been activated with affinity sequences, particularly for iron-oxide nanoparticles. The ability to specifically adsorb cloned affinity-tagged dehalogenase was further demonstrated by selectively adsorbing dehalogenase fusion proteins containing an iron-oxide affinity tripeptide directly from cell lysate. The retention of enzymatic activity was found to be dependent upon the surface chemistry of the nanoparticles. An increase in activity was observed after adsorption of fusion proteins onto the surface of nanoparticles modified by treatment with hydrophilic polyethylene glycol or 3-glycidoxypropyltrimethoxysilane molecules. As a result of this work, it is possible to tag an active enzyme with specific peptides that bind to inorganic nanoparticle surfaces. Because the conjugates self assemble, the novel surface-specific conjugate formation procedure is highly efficient and easily scalable for use in large-scale applications.     

Adsorption of surface functionalized silica nanoparticles onto mineral surfaces and decane/water interface

The adsorption of silica nanoparticles onto representative mineral surfaces and at the decane/water interface was studied. The effects of particle size (the mean diameters from 5 to 75?nm), concentration and surface type on the adsorption were studied in detail. Silica nanoparticles with four different surfaces [unmodified, surface modified with anionic (sulfonate), cationic (quaternary ammonium (quat)) or nonionic (polyethylene glycol (PEG)) surfactant] were used. The zeta potential of these silica nanoparticles ranges from ?79.8 to 15.3?mV. The shape of silica particles examined by a Hitachi-S5500 scanning transmission electron microscope (STEM) is quite spherical. The adsorption of all the nanoparticles (unmodified or surface modified) on quartz and calcite surfaces was found to be insignificant. We used interfacial tension (IFT) measurements to investigate the adsorption of silica nanoparticles at the decane/water interface. Unmodified nanoparticles or surface modified ones with sulfonate or quat do not significantly affect the IFT of the decane/water interface. It also does not appear that the particle size or concentration influences the IFT. However, the presence of PEG as a surface modifying material significantly reduces the IFT. The PEG surface modifier alone in an aqueous solution, without the nanoparticles, yields the same IFT reduction for an equivalent PEG concentration as that used for modifying the surface of nanoparticles. Contact angle measurements of a decane droplet on quartz or calcite plate immersed in water (or aqueous nanoparticle dispersion) showed a slight change in the contact angle in the presence of the studied nanoparticles. The results of contact angle measurements are in good agreement with experiments of adsorption of nanoparticles on mineral surfaces or decane/water interface. This study brings new insights into the understanding and modeling of the adsorption of surface-modified silica nanoparticles onto mineral surfaces and water/decane interface.     

Ag2O nanoparticle clusters coated with porous gelatin-g-PMMA copolymer

Herein a special nanoparticle cluster coated with a porous copolymer is designed and prepared. At first, Ag₂O nanoparticles (secondary particles) were fabricated in gelatin solution by a facile chemical approach. Then these nanoparticles were entrapped in a copolymerization system containing gelatin, methyl methacrylate (MMA), an initiator, and using water as a solvent. The nanoparticle clusters coated with porous gelatin-g-PMMA copolymer (Ag₂O/gelatin-g-PMMA) were prepared by grafting methyl methacrylate onto gelatin, followed by coating solidification. One significant feature for our approach is that every Ag₂O aggregated cluster has been coated with porous gelatin-g-PMMA copolymer film in a unique way, and the Ag₂O nanoparticle could penetrate and escape from the coating freely in water by ultrasonication. As a result, this study provides a new approach to prepare monodispersed nanoparticles by ordered porous copolymers with controlled releasing.     

Wide-field optical sizing of single nanoparticles with 10 nm accuracy

There is an increasing demand for new technologies to rapidly measure individual nanoparticles in situ for applications,including early-stage diagnosis of human diseases and environmental monitoring. Here, we demonstrate a label-free wide-field optical microscopy capable of sizing dispersed non-luminescent dielectric nanoparticles(with diameters down to 22 nm) with 10 nm accuracy. This technique utilizes enhanced nanoparticle-perturbed scattering by surface plasmons created on a gold film.In the meantime, an azimuthal rotation illumination module is installed on this microscope and a differential image processing technique is carried out. The relationship between the scattering intensity and the particle size was experimentally measured with good consistency with the theoretical prediction. The capability of precise measurement of the size of dispersed nanoparticles within a larger field of view in a label-free, non-invasive and quantitative manner may find broad applications involving single nanoparticle chemistry and physics.     

Carbon nanotubes as nanoparticles collector

This communication reports on a new method for the collection of nanoparticles using carbon nanotubes (CNT) as collecting surfaces, by which the problem of agglomeration of nanoparticles can be circumvented. CNT (10–50 nm in diameter, 1–10 μm in length) were grown by thermal CVD at 923 K in a 7 v/v% C₂H₂ in N₂ mixture on electroless nickel-plated copper transmission electron microscopy (TEM) grids and Monel coupons. These samples were then placed downstream of an arc plasma reactor to collect individual copper nanoparticles (5–30 nm in diameter). It was observed that the Cu nanoparticles preferentially adhere onto CNT and that the macro-particles (diameter >1 μm), a usual co-product obtained with metal nanoparticles in the arc plasma synthesis, are not collected. Cu–Ni nanoparticles, a catalyst for CNT growth, were deposited on CNT to grow multibranched CNT. CNT-embedded thin films were produced by re-melting the deposited nanoparticles.     

Structural and optical properties of novel surfactant-coated Yb@TiO<Subscript>2</Subscript> nanoparticles

In this paper a novel hybrid approach to synthesise composite nanoparticles is presented. It is based on the laser ablation of a bulk target (Yb) immersed in a reversed micellar solution which contains nanoparticles of a different host material (TiO₂ nanoparticles) previously synthesised by chemical method. This approach thus exploits the advantages of the chemical synthesis through reversed micellar solution (size control, nanoparticle stabilisation), and of the laser ablation (“clean” synthesis, no side reactions). Central role is played by the microscopic processes controlling the deposition of the ablated Yb atoms onto the surface of TiO₂ nanoparticles which actually behave as nucleation seeds. The structural features of the resulting Yb@TiO₂ composite nanoparticles have been studied by Transmission Electron Microscopy, whereas their peculiar optical properties have been explored by UV–Vis spectroscopy and steady-state fluorescence. Results consistently show the formation of Yb and TiO₂ glued nanodomains to form nearly spherical and non-interacting nanoparticles with enhanced photophysical properties.     

Optimal sample preparation for nanoparticle metrology (statistical size measurements) using atomic force microscopy

Optimal deposition procedures are determined for nanoparticle size characterization by atomic force microscopy (AFM). Accurate nanoparticle size distribution analysis with AFM requires non-agglomerated nanoparticles on a flat substrate. The deposition of polystyrene (100 nm), silica (300 and 100 nm), gold (100 nm), and CdSe quantum dot (2–5 nm) nanoparticles by spin coating was optimized for size distribution measurements by AFM. Factors influencing deposition include spin speed, concentration, solvent, and pH. A comparison using spin coating, static evaporation, and a new fluid cell deposition method for depositing nanoparticles is also made. The fluid cell allows for a more uniform and higher density deposition of nanoparticles on a substrate at laminar flow rates, making nanoparticle size analysis via AFM more efficient and also offers the potential for nanoparticle analysis in liquid environments.     

Nanoparticle Probes for Quantifying Supramolecular Determinants of Biosurface Affinity

Interactions between macromolecular systems and biosurfaces are complicated by the complexity of these multivalent interactions and challenges in quantifying affinities. In this study, a library of gold nanoparticles (AuNPs) with different functional head groups as multivalent probes to quantify biosurface affinity, using hair as a model targeted substrate, is used. The adhesion of the AuNPs is quantified by inductively coupled plasma mass spectrometry. Using this method it is demonstrated that multiple supramolecular forces affect affinity. As expected, electrostatic interaction is a strong driving force for adhesion of the nanoparticle tags onto hair in aqueous solution, evidenced by a much higher level of gold adsorption for cationic AuNPs compared to anionic or neutral AuNPs. Functionalized cationic AuNPs are synthesized with systematically varied terminal groups and are screened for deposition onto hair. AuNP adhesion onto hair in water generally decreases as a function of increasing hydrophobicity; however, electron‐rich aromatic rings provide significantly enhanced attachment. Although the intact, healthy hair cuticle is considered negatively charged and hydrophobic, the findings indicate that hydrophobic interactions are not as critical to deposition of AuNPs onto hair as the electrostatic component from the presence and accessibility of the cationic moieties, which are the greatest drivers for deposition onto hair.     

Sonochemical preparation of composite nanoparticles of Au/gamma-Fe2O3 and magnetic separation of glutathione

We prepared Au/gamma-Fe2O3 composite nanoparticles by sonochemically reducing Au(III) ions employing no stabilizer in the aqueous solution to form stable Au nanoparticles and allowing them to attach onto the surface of gamma-Fe2O3 particles with an average size of 21 nm. Size of the formed Au nanoparticle depended on the initial concentration of Au(III) ions. The number of the Au nanoparticles, supported on each gamma-Fe2O3 particle was controlled by changing the relative amounts of Au(III) ions and gamma-Fe2O3 particles. The composite nanoparticles exhibited a high affinity with glutathione, a tripeptide with mercapto group so that separation and manipulation of glutathione in aqueous solutions could be performed by application of external magnetic field. Because the surfaces of the Au nanoparticles were not shielded by any stabilizers, or naked, sonochemically prepared Au/gamma-Fe2O3 composite nanoparticles seemed to show stronger affinity to the glutathione than those by the radiochemical method.     

Superspin-glass like behavior of nanoparticle La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> obtained by mechanochemical milling

Single-phase perovskite compound La_0.7Ca_0.3MnO₃ was synthesised by a high-energy ball milling in a single step processing. Structure and morphology characterizations revealed nanoparticle nature of this mixed valent manganite with the average particle diameter of 9 nm. Comprehensive set of magnetic measurements showed that the system can be described as an ensemble of interacting magnetic nanoparticles where each particle possesses high magnetic moment, i.e., superspin. Furthermore, magnetic behavior showed contributions from both superspin-glass (SSG) and superparamagnetic (SP) states, and the prevailing properties depended on the experimental conditions. It was established that SSG state dominated in low magnetic fields up to 500 Oe while in higher applied fields suppression of collective behavior occurred and individual characteristics of nanoparticles prevailed. It was also concluded that the applied method of synthesis produced system with high magnetic anisotropy as well as with the large nanoparticle shell whose thickness amounts 30% of a particle diameter.     

Surface functionalization of silica-coated magnetic nanoparticles for covalent attachment of cholesterol oxidase

A systematic approach towards the fabrication of highly functionalized silica shell magnetic nanoparticles, presently used for enzyme immobilization, is herein fully presented. The synthesis of bare maghemite (γ-Fe₂O₃) nanoparticles was accomplished by thermal co-precipitation of iron ions in ammonia alkaline solution at harsh reaction conditions, respectively. Primary surface engineering of maghemite nanoparticles was successfully performed by the proper deposition of silica onto nanoparticles surface under strictly regulated reaction conditions. Next, the secondary surface functionalization of the particles was achieved by coating the particles with organosilane followed by glutaraldehyde activation in order to enhance protein immobilization. Covalent immobilization of cholesterol oxidase was attempted afterwards. The structural and magnetic properties of magnetic silica nanocomposites were characterized by TEM and vibrating sample magnetometer (VSM) instruments. X-ray diffraction measurements confirmed the spinel structure and average size of uncoated maghemite nanoparticles to be around 20 nm in diameter. SEM-EDS spectra indicated a strong signal for Si, implying the coating procedure of silica onto the particles surface to be successfully accomplished. Fourier transform infrared (FT-IR) spectra analysis confirmed the binding of amino silane molecules onto the surface of the maghemite nanoparticles mediated Si-O-Si chemical bonds. Compared to the free enzyme, the covalently bound cholesterol oxidase retained 50% of its activity. Binding of enzyme onto chemically modified magnetic nanoparticles via glutaraldehyde activation is a promising method for developing biosensing components in biomedicine.     

Room temperature synthesis of an optically and thermally responsive hybrid PNIPAM–gold nanoparticle

Composites of metal nanoparticles and environmentally sensitive polymers are useful as nanoactuators that can be triggered externally using light of a particular wavelength. We demonstrate a synthesis route that is easier than grafting techniques and allows for the in situ formation of individual gold nanoparticles encapsulated by an environmentally sensitive polymer, while also providing a strong interaction between the polymer and the metal particle. We present a one-pot, room-temperature synthesis route for gold metal nanoparticles that uses poly-N-isopropyl acrylamide as the capping and stabilizing agent and ascorbic acid as the reducing agent and achieves size control similar to the most common citric acid synthesis. We show that the composite can be precipitated reversibly by temperature or light using the non-radiative decay and conversion to heat of the surface plasmon resonance of the metal nanoparticle. The precipitation is induced by the collapse of the polymer cocoon surrounding each gold nanoparticle, as can be seen by surface plasmon spectroscopy. The experiments agree with theoretical models for the heat generation in a colloidal suspension that support fast switching with low laser power densities. The synthesized composite is a simple nanosized opto-thermal switch.     

Laser ablation synthesis of monodispersed magnetic alloy nanoparticles

Monodispersed CoPt alloy nanoparticles were synthesized by a pulsed laser ablation (PLA) technique coupled with a low-pressure operating differential mobility analyzer (LP-DMA). The CoPt alloy nanoparticles were generated by laser ablating a solid Co–Pt target. In CoPt alloy nanoparticles synthesized from a target with a Co composition of 75 at%, the nanoparticle surfaces were covered by an oxide layer and exhibited a core-shell structure. In contrast, no shell was observed in particles generated from a target with a Co:Pt ratio of 50:50 at%. According to an EDX analysis, the compositions of the individual nanoparticles were almost the same as that of the target material. Finally, the magnetic hysteresis loops of the CoPt alloy nanoparticles exhibited ferromagnetism.