Charged Gold Nanoparticles Research Articles

Photoinduced Electron Transfer between Chlorophyll a and Gold Nanoparticles

Excited-state interactions between chlorophyll a (Chla) and gold nanoparticles have been studied. The emission intensity of Chla is quenched by gold nanoparticles. The dominant process for this quenching has been attributed to the process of photoinduced electron transfer from excited Chla to gold nanoparticles, although because of a small overlap between fluorescence of Chla and absorption of gold nanoparticles, the energy-transfer process cannot be ruled out. Photoinduced electron-transfer mechanism is supported by the electrochemical modulation of fluorescence of Chla. In absence of an applied bias, Chla cast on gold film, as a result of electron transfer, exhibits a very weak fluorescence. However, upon negatively charging the gold nanocore by external bias, an increase in fluorescence intensity is observed. The negatively charged gold nanoparticles create a barrier and suppress the electron-transfer process from excited Chla to gold nanoparticles, resulting in an increase in radiative process. Nanosecond laser flash experiments of Chla in the presence of gold nanoparticles and fullerene (C60) have demonstrated that Au nanoparticles, besides accepting electrons, can also mediate or shuttle electrons to another acceptor. Taking advantage of these properties of gold nanoparticles, a photoelectrochemical cell based on Chla and gold nanoparticles is constructed. A superior performance of this cell compared to that without the gold film is due to the beneficial role of gold nanoparticles in accepting and shuttling the photogenerated electrons in Chla to the collecting electrode, leading to an enhancement in charge separation efficiency.

Functionalized multiwall carbon nanotube/gold nanoparticle composites.

Multiwall carbon nanotubes (MWCNTs) were chemically oxidized in a mixture of sulfuric acid and nitric acid (3:1) while being ultrasonicated. The effect of oxidative ultrasonication at room temperature on development of functional groups on the carbon nanotubes was investigated. The dispersability and the carboxylic acid group concentration of functionalized MWCNTs (fMWNTs) varied with reaction time. The concentration of carboxylic acid groups on fMWNTs increased from 4 x 10(-4) mol/g of fMWNTs to 1.1 x 10(-3) mol/g by doubling the treatment period from 4 to 8 h. The colloidal stability of aqueous fMWCNTs dispersions was enhanced through elongated oxidation. fMWCNTs that were reacted longer than 4 h did not precipitate in aqueous media for at least 24 h. The layer-by-layer self-assembly of polyelectrolytes on fMWCNTs was characterized by zeta potential measurements. The zeta potential of fMWCNTs changed from negative charge to positive charge when cationic polyelectrolytes were self-assembled on their surface. With addition of anionic polyelectrolytes, cationic polyelectrolyte coated fMWCNTs showed the expected charge reversal as expected for multilayer self-assembly. Complex formation of positively charged gold nanoparticles and negatively charged fMWCNTs was achieved with and without polyelectrolyte coatings by electrostatic interaction. The complex formation was characterized by high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy. The here found complex formation of positively charged colloidal gold and defect sites on fMWNTs indicates the location of functional groups on carbon nanotubes. It is suggested that positively charged colloids such as gold nanoparticles could be used for detection of defect sites on carbon nanotubes.

DNA sequence detection using selective fluorescence quenching of tagged oligonucleotide probes by gold nanoparticles.

Simple, fast, economical, and sensitive detection of specific DNA sequences is crucial to pathogen detection and biomedical research. We have designed a novel fluorescent assay for DNA hybridization based on the electrostatic properties of DNA. We exploit the ability to create conditions where single-stranded DNA adsorbs on negatively charged gold nanoparticles while double-stranded DNA does not. Dye-tagged probe sequences have their fluorescence efficiently quenched when they are mixed with gold nanoparticles unless they hybridize with components of the analyte. Subfemtomole amounts of untagged target are detected in minutes using commercially available materials. Target sequences in complex mixtures of DNA and single-base mismatches in DNA sequences are easily detected.

Label-free colorimetric detection of specific sequences in genomic DNA amplified by the polymerase chain reaction.

We document the surprising result that single-stranded DNA adsorbs on negatively charged gold nanoparticles (Au-nps) with a rate that depends on sequence length and temperature. After ss-DNA adsorbs on Au-nps, we find that the particles are stabilized against salt-induced aggregation. These observations can be rationalized on the basis of electrostatics and form the basis for a colorimetric assay to identify specific sequences and single nucleotide polymorphisms on polymerase chain reaction (PCR)-amplified DNA. The assay is label-free, requires no covalent modification of the DNA or Au-np surfaces, and takes on the sensitivity of PCR. Most important, binding of target and probe takes place in solution where hybridization occurs in less than 1 min. As an example, we test PCR-amplified genomic DNA from clinical samples for single nucleotide polymorphisms (SNPs) associated with a fatal arrhythmia known as long QT syndrome.

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.

Light-induced inhibition of chymotrypsin using photocleavable monolayers on gold nanoparticles

Positively charged gold nanoparticles featuring photocleavable units within their surrounding monolayer are switched from non-interacting species to inhibitors of chymotrypsin through UV irradiation.

Assembly of Nanoparticles using Surface‐Grafted Orthogonal Polymer Gradients

AbstractSummary: We report on preparing poly(2‐(dimethylamino)ethyl methacrylate) (PDMAEMA) gradient substrate, wherein molecular weight (MW) and grafting density (σ) of the surface‐anchored PDMAEMA chains vary continuously in two orthogonal directions. Such a specimen is used to control the assembly of charged gold nanoparticles. Increasing MW and σ of the grafted PDMAEMA cause an enhanced binding of the nanoparticles to PDMAEMA, thus leading to an orthogonal number density gradient of surface‐bound gold nanoparticles.magnified image

Size-Dependent Phase Transfer of Gold Nanoparticles from Water into Toluene by Tetraoctylammonium Cations: A Wholly Electrostatic Interaction

In this study, it is demonstrated that the tetraoctylammonium cation can be used directly as a phase-transfer reagent of negatively charged water-based gold nanoparticles. The transference is size-dependent and is based on a wholly electrostatic interaction.

Time-Dependent Complexation of Cysteine-Capped Gold Nanoparticles with Octadecylamine Langmuir Monolayers at the Air−Water Interface

In this paper, we present time-dependent studies on the complexation of cysteine-capped gold nanoparticles with octadecylamine (ODA) Langmuir monolayers. The cysteine molecules bound to the colloidal gold surface via thiolate linkages impart a net negative charge to the particles due to deprotonated carboxylic acid groups. Strong attractive electrostatic interaction between the negatively charged gold nanoparticles and positively charged ODA monolayer drives the complexation process. The extent of complexation of the gold nanoparticles and subsequent Langmuir−Blodgett (LB) film formation is a function of charge on the particles/monolayer. The charge on the nanoparticles/monolayer may be controlled by simple variation of the subphase pH. At pH 9, the carboxylic acid groups on the particles are highly ionized leading to strong electrostatic attraction with the protonated ODA monolayer, while at pH 12, the ODA monolayer is deprotonated leading to a reduction in the electrostatic interaction. The nanoparticle...

Oxygen chemisorption on Au nanoparticles

Oxygen molecules do not adsorb on flat gold surfaces at room temperature, but recent experimental results have demonstrated adsorption of oxygen on small, negatively charged gold nanoparticles. Here we report first-principles density functional calculations of oxygen adsorption on neutral and charged Au n nanoparticles. We find that: (i) Oxygen molecules chemisorb on Au n clusters, with typical binding energies of 0.5–1.5 eV; the binding energy is larger for negatively charged clusters. (ii) In the most stable configurations, the two oxygen atoms are spatially separated and form a linear O–Au–O bridge. The results establish a correlation between the experimental reactivity of gold clusters and the oxygen binding energy.

Surface functionalisation for the self-assembly of nanoparticle/polymer multilayer films

The use of organosilanes as surface functionalising materials has been investigated as a precursor to the adsorption of ligand stabilised gold nanoparticles and the build-up of nanoparticle/polymer multilayer films. The purpose of surface functionalisation here is to produce a uniform surface with the maximum positive charge possible to enable the efficient adsorption of negatively charged gold nanoparticles. It is generally acknowledged that the characteristics of the first layer are important in determining the quality of the subsequent multilayer film and hence careful attention has been paid to its optimisation. Three aminosilanes have been investigated together with various methods for their deposition. The degree of nanoparticle adsorption in the resulting films was characterised using atomic force microscopy and X-ray photoelectron spectroscopy. The surface potential of the aminosilane films was also measured to provide information regarding the surface charge density. Our results show a strong correlation between the nanoparticle density and the initial surface charge density. Films of 3-aminopropyltriethoxysilane adsorbed from toluene yielded the highest level of nanoparticle adsorption.

Use of gold nanoparticles to enhance capillary electrophoresis.

We describe here the use of gold nanoparticles to manipulate the selectivity between solutes in capillary electrophoresis. Two different gold-based nanoparticles were added to the run buffer. In one case, the nanoparticles were stabilized with citrate ions, but in another study, the gold nanoparticles were capped with mercaptopropionate ions (thiol-stablized). Citrate-stabilized gold nanoparticles were used in conjunction with capillaries treated with poly(diallyldimethylammonium chloride) (PDADMAC). The positively charged PDADMAC layer on the capillary walls adsorbs the negatively charged gold nanoparticles. The model solutes that were used to study the effect of the presence of the citrate-stabilized gold nanoparticles are structural isomers of aromatic acids and bases. The presence of the PDADMAC layer and the PDADMAC plus the gold nanoparticles changes both the electroosmotic mobility and the observed mobility of the solutes. These changes in the mobilities influence the observed selectivities and the separations of the system. Thiol-stabilized gold nanoparticles were used without PDADMAC in the capillary. The model solutes studied in this part are various aromatic amines. In this case as well, the presence of the gold nanoparticles modifies the electroosmotic mobility and the observed mobility of the solutes. These changes in the mobilities are manifested in selectivity alterations. The largest change in the selectivities occurs at low concentrations of the gold nanoparticles in the run buffer. The presence of nanoparticles improves the precision of the analysis and increases the separation efficiency.