Coordination-based gold nanoparticle layers

Gold nanoparticle (NP) mono- and multilayers were constructed on gold surfaces using coordination chemistry. Hydrophilic Au NPs (6.4 nm average core diameter), capped with a monolayer of 6-mercaptohexanol, were modified by partial substitution of bishydroxamic acid disulfide ligand molecules into their capping layer. A monolayer of the ligand-modified Au NPs was assembled via coordination with Zr4+ ions onto a semitransparent Au substrate (15 nm Au, evaporated on silanized glass and annealed) precoated with a self-assembled monolayer of the bishydroxamate disulfide ligand. Layer-by-layer construction of NP multilayers was achieved by alternate binding of Zr4+ ions and ligand-modified NPs onto the first NP layer. Characterization by atomic force microscopy (AFM), ellipsometry, wettability, transmission UV-vis spectroscopy, and cross-sectional transmission electron microscopy showed regular growth of NP layers, with a similar NP density in successive layers and gradually increased roughness. The use of coordination chemistry enables convenient step-by-step assembly of different ligand-possessing components to obtain elaborate structures. This is demonstrated by introducing nanometer-scale vertical spacing between a NP layer and the gold surface, using a coordination-based organic multilayer. Electrical characterization of the NP films was carried out using conductive AFM, emphasizing the barrier properties of the organic spacer multilayer. The results exhibit the potential of coordination self-assembly in achieving highly controlled composite nanostructures comprising molecules, NPs, and other ligand-derivatized components.     

Size limitations for the formation of ordered striped nanoparticles

A combination of immiscible molecules in the ligand shell of a gold nanoparticle (NP) has been shown to phase separate into a rippled structure; this phase separation can be used to direct the assembly of the NPs into chains. Here we demonstrate that only NPs within a certain size range can form chains, and we conclude that the rippled morphology of the ligand shell also exists only within that given size range. We corroborate this result with simulations of the ligand arrangement on NPs of various sizes.     

Gold nanoparticles stabilized by thioether dendrimers

Ligand-stabilized gold nanoparticles (Au NPs) are promising materials for nanotechnology with applications in electronics, catalysis, and sensors. These applications depend on the ability to synthesize stable and monodisperse NPs. Herein, the design and synthesis of two series of dendritic thioether ligands and their ability to stabilize Au NPs is presented. The dendrimers have 1,3,5-trisubstituted benzene branching units bridged by either meta-xylene or ethylene moieties. A comparison between the two ligands shows how both size control and the stability of the NPs are influenced by the nature of the ligand-NP wrapping interaction. The meta-xylene-bridged ligands provided NPs with a narrow size distribution centered around a diameter of 1.2 nm, whereas the NPs formed with ethylene-bridged dendrimers lack long-term stability with NP aggregation detected by UV/Vis spectroscopy and transmission electron microscopy. The bulkier tert-butyl-functionalized meta-xylene bridges form larger ligand shells that inhibit further growth of the NPs and thus provide a simple route to stable and monodisperse Au NPs that may find use as functional components in nanoelectronic devices.     

Effect of interacting nanoparticles on the ordered morphology of block copolymer/nanoparticle mixtures

We investigated the effect of hard additives, that is, magnetic nanoparticles (NPs) and metal NPs, on the ordered morphology of block copolymers by varying the NP concentration. To characterize the structural changes of a block copolymer associated with different NP loadings, small-angle X-ray scattering and transmission electron microscopy were performed. Monodisperse maghemite (γ-Fe₂O₃) NPs (7 nm in diameter) and silver (Ag) NPs (6 nm in diameter) with surfaces modified with oleic acids were synthesized, and a cylinder-forming poly(styrene-block-isoprene) diblock copolymer was used as a structure-directing matrix for the NPs. As the NP concentration increased, domains of NP aggregates were observed for both magnetic and metal NPs. In the case of mixtures of cylinder-forming poly(styrene-block-isoprene) and Ag NPs with weak particle–particle interactions, random aggregates of Ag NPs were observed, and the ordered morphology of the block copolymer lost its long-range order with an increase in the NP concentration. However, regular, latticelike aggregates obtained with γ-Fe₂O₃ NPs, because of the strong interparticle interactions, induced an intriguing morphological transformation from hexagonal cylinders to body-centered-cubic spheres via undulated cylinders, whereas the neat block copolymer did not show such a morphological transition over a wide range of temperatures. The interplay between magnetic NPs and the block copolymer was also tested with magnetic NPs of different sizes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3571–3579, 2006     

Versatile scheme for the step-by-step assembly of nanoparticle multilayers

A versatile scheme for the preparation of nanoparticle (NP) multilayers is presented. The method is based on the step-by-step assembly of NPs and bishydroxamate disulfide ligand molecules by means of metal-organic coordination using easily synthesized tetraoctylammonium bromide (TOAB)-stabilized gold NPs. The assembly of NP multilayers was carried out via a Zr(IV)-coordinated sandwich arrangement of the hydroxamate ligands on Au and glass surfaces. The latter were precoated with electrolessly deposited Au clusters to enable binding of the first NP layer. The new method avoids the need to perform elaborate colloid reactions to prepare the NP building blocks. Au NP monolayer and multilayer films prepared in this manner were characterized by UV-vis spectroscopy, atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (TEM), showing a regular growth of NP layers. The use of coordination chemistry as the binding motif between repeat layers allows for the convenient assembly of hybrid nanostructures comprising molecular and NP components. This was demonstrated by the construction of Au NP multilayers with controlled spacing from the surface or between two NP layers. Drying the samples during or after the construction process induces NP aggregation and changes in the film morphology and optical properties.     

Faraday rotation enhancement of gold coated Fe2O3 nanoparticles: comparison of experiment and theory

Understanding plasmonic enhancement of nanoscale magnetic materials is important to evaluate their potential for application. In this study, the Faraday rotation (FR) enhancement of gold coated Fe(2)O(3) nanoparticles (NP) is investigated experimentally and theoretically. The experiment shows that the Faraday rotation of a Fe(2)O(3) NP solution changes from approximately 3 rad/Tm to 10 rad/Tm as 5 nm gold shell is coated on a 9.7 nm Fe(2)O(3) core at 632 nm. The results also show how the volume fraction normalized Faraday rotation varies with the gold shell thickness. From the comparison of experiment and calculated Faraday rotation based on the Maxwell-Garnett theory, it is concluded that the enhancement and shell dependence of Faraday rotation of Fe(2)O(3) NPs is a result of the shifting plasmon resonance of the composite NP. In addition, the clustering of the NPs induces a different phase lag on the Faraday signal, which suggests that the collective response of the magnetic NP aggregates needs to be considered even in solution. From the Faraday phase lag, the estimated time of the full alignment of the magnetic spins of bare (cluster size 160 nm) and gold coated NPs (cluster size 90 nm) are found to be 0.65 and 0.17 μs. The calculation includes a simple theoretical approach based on the Bruggeman theory to account for the aggregation and its effect on the Faraday rotation. The Bruggeman model provides a qualitatively better agreement with the experimentally observed Faraday rotation and points out the importance of making a connection between component properties and the average "effective" optical behavior of the Faraday medium containing magnetic nanoparticles.     

Liquid crystal-gold nanoparticle composite-modified indium tin oxide (ITO) substrates and their electrochemical characterisation

A simple efficient strategy for the simultaneous synthesis and anchoring of liquid crystal (LC)-stabilised gold nanoparticles (NPs) on indium tin oxide (ITO) substrate is described. A monolayer of 3-mercaptopropyltrimethoxy silane (MPS) compound was formed on ITO and quality of the monolayer was assessed using electrochemical techniques namely cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Gold NP preparation was carried out on this monolayer-modified substrate (and on bare ITO), in a single-step reaction, simply by drop-casting a solution containing an appropriate amount of chloroauric acid and a LC compound possessing a terminal amino group, on the MPS monolayer-modified substrate and heating (70degree) for 2-3 min.. The LC compound served as a reducing agent as well as a capping ligand. LC-capped NPs were chemically anchored onto the ITO substrate through bonding to thiol moiety of the MPS. The CV and EIS analysis of the MPS monolayer showed a complete blocking behaviour for the electron transfer across the electrode/electrolyte interface confirming the formation of a high-quality dense compact monolayer. On the other hand, upon immobilisation of LC-gold NP composite on self-assembled monolayer-modified ITO substrates, both CV and impedance studies showed a small current indicating the gold NP-mediated electron transfer, thus confirming the successful immobilisation of NPs.     

Staged Surface Patterning and Self‐Assembly of Nanoparticles Functionalized with End‐Grafted Block Copolymer Ligands

Using two orthogonal external stimuli, programmable staged surface patterning and self‐assembly of inorganic nanoparticles (NPs) was achieved. For gold NPs capped with end‐grafted poly(styrene‐block‐(4‐vinylbenzoic acid)), P(St‐block‐4VBA), block copolymer ligands, surface‐pinned micelles (patches) formed from NP‐adjacent PSt blocks under reduced solvency conditions (Stimulus 1); solvated NP‐remote P(4VBA) blocks stabilized the NPs against aggregation. Subsequent self‐assembly of patchy NPs was triggered by crosslinking the P(4VBA) blocks with copper(II) ions (Stimulus 2). Block copolymer ligand design has a strong effect on NP self‐assembly. Small, well‐defined clusters assembled from NPs functionalized with ligands with a short P(4VBA) block, while NPs tethered with ligands with a long P(4VBA) block formed large irregularly shaped assemblies. This approach is promising for high‐yield fabrication of colloidal molecules and their assemblies with structural and functional complexity.     

Nanoscale-enhanced Ru(bpy)3(2+) electrochemiluminescence labels and related aptamer-based biosensing system

A unique multilabeling at a single-site protocol of the Ru(bpy)(3)(2+) electrochemiluminescence (ECL) system is proposed. Nanoparticles (NPs) were used as assembly substrates to enrich ECL co-reactants of Ru(bpy)(3)(2+) to construct nanoscale-enhanced ECL labels. Two different kinds of NP substrates [including semiconductor NPs (CdTe) and noble metal NPs (gold)] capped with 2-(dimethylamino)ethanethiol (DMAET) [a tertiary amine derivative which is believed to be one of the most efficient of co-reactants of the Ru(bpy)(3)(2+) system] were synthesized through a simple one-pot synthesis method in aqueous media. Although both CdTe and gold NPs realized the enrichment of ECL co-reactants, they presented entirely different ECL performances as nanoscale ECL co-reactants of Ru(bpy)(3)(2+). The different effects of these two NPs on the ECL of Ru(bpy)(3)(2+) were studied. DMAET-capped CdTe NPs showed enormous signal amplification of Ru(bpy)(3)(2+) ECL, whereas DMAET-capped gold NPs showed a slight quenching effect of the ECL signal. DMAET-capped CdTe NPs can be considered to be excellent nanoscale ECL labels of the Ru(bpy)(3)(2+) system, as even a NP solution sample of 10(-18) M was still detectable after an electrostatic self-assembly concentration process. DMAET-capped CdTe NPs were further applied in the construction of aptamer-based biosensing system for proteins and encouraging results were obtained.     

Fabrication of dendritic gold nanoparticles by use of an ionic polymer template

A facile method for the fabrication of dendritic gold nanoparticles (NPs) by use of an ionic polymer template has been developed. In situ generation of an imidazolium-based (cationic) polymer, poly[1-methyl-3-(4-vinylbenzyl)imidazolium], with AuCl4- counteranions is achieved by addition of HAuCl4 into a solution containing poly[1-methyl-3-(4-vinylbenzyl)imidazolium chloride]. Subsequent reduction with NaBH4 in water or in a mixture of ethanol and water affords various NPs depending on the conditions, including large dendritic gold NPs that have been analyzed by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and selected area electron diffraction (SAED). The structures of the dendritic gold NPs were found to depend on the ethanol concentration. Scanning electron microscopy (SEM) images of the ionic polymer reveal that the solvent used to deposit the polymer strongly influences its structure and may be correlated to the structure of the resulting NPs.     

Monofunctionalized Gold Nanoparticles Stabilized by a Single Dendrimer Form Dumbbell Structures upon Homocoupling

The assembly of dumbbell structures as organic-inorganic hybrid materials is presented. Gold nanoparticles (NPs) with a mean diameter of 1.3 nm were synthesized in very good yields using a stabilizing dendrimer based on benzylic thioether subunits. The extended dendritic ligand covers the NP surface and contains a peripheral protected acetylene, providing coated and monofunctionalized NPs. These NPs themselves can be considered as large molecules, and thus, applying a wet-chemical deprotection/oxidative acetylene coupling protocol exclusively provides dimers of NPs interlinked by a diethynyl bridge. The concept not only enables access to novel organic/inorganic hybrid architectures but also promises new approaches in labeling technology.     

On the stabilization of gold nanoparticles over silica-based magnetic supports modified with organosilanes

The immobilization of gold nanoparticles (Au NPs) on silica is made possible by the functionalization of the silica surfaces with organosilanes. Au NPs could only be stabilized and firmly attached to silica-support surfaces that were previously modified with amino groups. Au NPs could not be stabilized on bare silica surfaces and most of the NPs were then found in the solution. The metal-support interactions before and after the Au NP formation, observed by X-ray absorption fine structure spectroscopy (XAFS), indicate a stronger interaction of gold(III) ions with amino-modified silica surfaces than with the silanol groups in bare silica. An amino-modified, silica-based, magnetic support was used to prepare an active Au NP catalyst for the chemoselective oxidation of alcohols, a reaction of great interest for the fine chemical industry.     

Synthesis and Properties of Dendronized Chitosan

Summary: Chitosan films and microspheres were prepared and their surfaces were functionalized with first generation dendritic molecules. The films were modified by Weisocyanate dendron, while Behera's and bis Behera's amine dendrons were used to modify the microspheres. Prior to dendronization films were prepared by blending chitosan with 18% of polyvinyl pyrrolidone (PVP), and casting the resulting mixture. The degree of dendronization reached was 28%. The microspheres were prepared by coacervation/precipitation, after which the surfaces were activated with either epychlorohydrine (ECH) or 1,4-butanediol diglycidyl ether (BDGE). The oxirane groups were utilized to form covalent bonds between chitosan and dendrons. The degree of dendronization yielded with Behera's amine was 60% for both activating agents. When bis Behera's amine was used, the dendronization reached values of 15 and 21% when ECH or BDGE were used, respectively. The dendronized products were characterized through spectroscopic and microscopic studies and by determination of swelling indexes. Only one of the surfaces was dendronized in every film, which therefore presented a hydrophobic and a hydrophilic surface. Since these films maintain the properties of chitosan, they offer interesting potential as dressings for exuding wounds. The different surfaces make the microspheres potentially applicable as carriers for delivery and controlled release of drugs.     

Charge-driven flocculation of poly(L-lysine)-gold nanoparticle assemblies leading to hollow microspheres

An unusual aggregation phenomenon that involves positively charged poly(L-lysine) (PLL) and negatively charged gold nanoparticles (Au NPs) is reported. Discrete, submicrometer-sized spherical aggregates are found to form immediately upon combining a PLL solution with gold sol (diameter approximately 14 nm). These PLL-Au NP assemblies grow in size with time, according to light scattering experiments, which indicates a dynamic flocculation process. Water-filled, silica hollow microspheres (outer diameter approximately microns) are obtained upon the addition of negatively charged SiO2 NPs (diameter approximately 13 nm) to a suspension of the PLL-Au NP assemblies, around which the SiO2 NPs form a shell. Structural analysis through confocal microscopy indicates the PLL (tagged with a fluorescent dye) is located in the interior of the hollow sphere, and mostly within the silica shell wall. The hollow spheres are theorized to form through flocculation, in which the charge-driven aggregation of Au NPs by PLL provides the critical first step in the two-step synthesis process ("flocculation assembly"). The SiO2 shell can be removed and re-formed by decreasing and increasing the suspension pH about the point-of-zero charge of SiO2, respectively.     

Combined Blip and Staircase Response of Ascorbic Acid‐Stabilized Copper Single Nanoparticle Collision by Electrocatalytic Glucose Oxidation

The current response of the collision of ascorbic acid‐stabilized copper (Cu) single nanoparticles (NPs) on a gold (Au) ultramicroelectrode (UME) surface was observed by using an electrocatalytic amplification method. Here, the glucose oxidation electrocatalyzed by oxidized Cu NPs was used as the indicating reaction. In this system, the NP collision signals were obtained simultaneously by both direct particle electrolysis and electrocatalytic amplification. For example, when the applied potential was high enough for Cu NP oxidation, a blip response combined with a staircase response was observed as a current signal. The blip part in the single Cu NP collision signal indicates the self‐oxidation of a Cu NP, and the staircase part indicates the steady‐state electrocatalytic reaction by oxidized Cu NP.     

Gold nanoparticle self-assembly in two-component lipid Langmuir monolayers

Self-assembly processes are considered to be fundamental factors in supramolecular chemistry. Langmuir monolayers of surfactants or lipids have been shown to constitute effective 2D "templates" for self-assembled nanoparticles and colloids. Here we show that alkyl-coated gold nanoparticles (Au NPs) adopt distinct configurations when incorporated within Langmuir monolayers comprising two lipid components at different mole ratios. Thermodynamic and microscopy analyses reveal that the organization of the Au NP aggregates is governed by both lipid components. In particular, we show that the configurations of the NP assemblies were significantly affected by the extent of molecular interactions between the two lipid components within the monolayer and the monolayer phases formed by each individual lipid. This study demonstrates that multicomponent Langmuir monolayers significantly modulate the self-assembly properties of embedded Au NPs and that parameters such as the monolayer composition, surface pressure, and temperature significantly affect the 2D nanoparticle organization.     

Nanoparticle-containing structures as a substrate for surface-enhanced Raman scattering

Metallic nanostructures were prepared through the alternate immersion of derivatized glass slides in solutions of gold nanoparticles (NPs) and a propanedithiol linker molecule. Nanostructures consisting of 1-17 depositions of gold NPs were synthesized, and these substrates were characterized using UV-vis spectroscopy and atomic force microscopy. Subsequently, the surface-enhanced Raman scattering (SERS) of oxazine 720 was obtained at two excitation wavelengths (632 and 785 nm) from all substrates. Maximum SERS enhancement was observed for 9 and 13 NP depositions for 632 and 785 nm excitations, respectively. The difference in the number of NP depositions required for maximum enhancement is attributed to different wavelengths which can excite distinct aggregate structures within the metallic substrate. Therefore, these NP-containing structures can be "tuned" to yield maximum SERS enhancement for the excitation source being used by varying the number of NP depositions.     

Synthesis of copper nanoparticles stabilized by polyoxyethylenesorbitan monooleate

We studied the influence of synthesis parameters and the composition of the reaction mixture on the size and morphology of copper nanoparticles (NPs). Use of a surfactant (polyoxyethylenesorbitan monooleate) is promising for confining NP growth and stabilizing NPs. Concentration ranges of existence were determined for copper NP dispersions stable to aggregation and sedimentation. Scanning electron microscopy and dynamic light scattering were used to show that: the NP size varied from 10 to 65 nm, the average diameter was 25–35 nm, and the shape was spherical. The sizes of copper NP aggregates were determined.     

Molecularly imprinted aptamers of gold nanoparticles for the enzymatic inhibition and detection of thrombin

We prepared thrombin-binding aptamer-conjugated gold nanoparticles (TBA-Au NPs) through a molecularly imprinted (MP) approach, which provide highly efficient inhibition activity toward the polymerization of fibrinogen. Au NPs (diameter, 13 nm), 15-mer thrombin-binding aptamer (TBA(15)) with different thymidine linkers, and 29-mer thrombin-binding aptamer (TBA(29)) with different thymidine linkers (Tn) in the presence of thrombin (Thr) as a template were used to prepare MP-Thr-TBA(15)/TBA(29)-Tn-Au NPs. Thrombin molecules were then removed from Au NPs surfaces by treating with 100 mM Tris-NaOH (pH ca. 13.0) to form MP-TBA(15)/TBA(29)-Tn-Au NPs. The length of the thymidine linkers and TBA density on Au NPs surfaces have strong impact on the orientation, flexibility, and stability of MP-TBA(15)/TBA(29)-Tn-Au NPs, leading to their stronger binding strength with thrombin. MP-TBA(15)/TBA(29)-T(15)-Au NPs (ca. 42 TBA(15) and 42 TBA(29) molecules per Au NP; 15-mer thymidine on aptamer terminal) provided the highest binding affinity toward thrombin with a dissociation constant of 5.2 × 10(-11) M. As a result, they had 8 times higher anticoagulant (inhibitory) potency relative to TBA(15)/TBA(29)-T(15)-Au NPs (prepared in the absence of thrombin). We further conducted thrombin clotting time (TCT) measurements in plasma samples and found that MP-TBA(15)/TBA(29)-T(15)-Au NPs had greater anticoagulation activity relative to four commercial drugs (heparin, argatroban, hirudin, and warfarin). In addition, we demonstrated that thrombin induced the formation of aggregates from MP-TBA(15)-T(15)-Au NPs and MP-TBA(29)-T(15)-Au NPs, thereby allowing the colorimetric detection of thrombin at the nanomolar level in serum samples. Our result demonstrates that our simple molecularly imprinted approach can be applied for preparing various functional nanomaterials to control enzyme activity and targeting important proteins.     

Helicoidal Patterning of Nanorods with Polymer Ligands

Chiral packing of ligands on the surface of nanoparticles (NPs) is of fundamental and practical importance, as it determines how NPs interact with each other and with the molecular world. Herein, for gold nanorods (NRs) capped with end‐grafted nonchiral polymer ligands, we show a new mechanism of chiral surface patterning. Under poor solvency conditions, a smooth polymer layer segregates into helicoidally organized surface‐pinned micelles (patches). The helicoidal morphology is dictated by the polymer grafting density and the ratio of the polymer ligand length to nanorod radius. Outside this specific parameter space, a range of polymer surface structures was observed, including random, shish‐kebab, and hybrid patches, as well as a smooth polymer layer. We characterize polymer surface morphology by theoretical and experimental state diagrams. The helicoidally organized polymer patches on the NR surface can be used as a template for the helicoidal organization of other NPs, masked synthesis on the NR surface, as well as the exploration of new NP self‐assembly modes.