Light-induced flocculation of gold nanoparticles

Gold particles have been formed in water-in-oil microemulsions doped with a photodestructible surfactant. UV light-induced nanoparticle flocculation has been achieved after photolysis of the photosurfactant, leading to a reduction in the steric stabilization provided by the surfactant layer.     

Hematite spindles with optical functionalities: growth of gold nanoshells and assembly of gold nanorods

The layer-by-layer (LBL) assembly method, combined with the seeded growth technique, have been used to deposit gold shells on the surface of hematite (alpha-Fe(2)O(3)) spindles. While the LBL method yields dense coatings of preformed Au nanoparticles, when AuCl(-)(4) ions are further reduced by a mild reducing agent, thicker, rough nanostructured shells can be grown. The deposition process was monitored by TEM and UV-visible spectroscopy, demonstrating a gradual change in the optical features of the colloids as the surface is more densely covered. The particles so-prepared can find useful applications in cancer therapy and as SERS substrates. Additionally, we show that Au nanorods can be assembled on hematite spindles, providing a flexible way to tune the optical properties of the resulting composite colloids.     

Controlled release of plasmid DNA from gold nanorods induced by pulsed near-infrared light

Pulsed near-infrared laser irradiation induced release of plasmid DNA immobilized on gold nanorods without structural degradation, by selective excitation of longitudinal plasmon oscillation.     

Hemoprotein bioconjugates of gold and silver nanoparticles and gold nanorods: structure-function correlations

Bioconjugates of the hemoproteins, myoglobin, and hemoglobin have been synthesized by their adsorption on spherical gold and silver nanoparticles and gold nanorods. The adsorption of hemoproteins on the nanoparticle surface was confirmed by their molecular ion signatures in matrix assisted laser desorption ionization mass spectrometry and specific Raman features of the prosthetic heme b units. High-resolution transmission electron microscopy (HRTEM) and UV-visible spectroscopy showed that the particles retain their morphology and show aggregation only in the case of silver. The binding of azide ion to the Fe(III) center of the prosthetic heme b moiety caused a red shift of the Soret band, both in the case of the bioconjugates and in free hemoproteins. This was further confirmed by the characteristic signature at 2050 cm-1 in the Fourier-transform infrared spectra, which corresponds to the asymmetric stretching of the Fe(III) bound azide. The retention of the chemical behavior of the prosthetic heme group after adsorption on the nanoparticle is interesting due to its implications in nanoparticle supported enzyme catalysis. The absence of morphology changes after the reaction of bioconjugates with azide ion observed in HRTEM studies implies the stability of nanoparticles under the reaction conditions. All these studies indicate the retention of protein structure after adsorption on the nanoparticle surface.     

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.     

Light-induced aggregation of colloidal gold nanoparticles capped by thymine derivatives

The colloid stability of thymine-coated gold nanoparticles under light irradiation as a function of particle size, surface charge, and exposure time was investigated in alkaline, aqueous solutions as well as in a 0.5 vol % of DMF in H(2)O mixture. With increasing exposure to light irradiation at 280 nm, more and more particles coagulated. Light-induced aggregation of colloidal gold nanoparticles was attributed to reorientation of thymine terminal groups tethered on gold particle surfaces. A smaller particle size and negatively charged surface reduced the rate of photodimerization or even inhibited the photoreaction. UV-vis and FTIR spectroscopy confirmed the photodimerization of terminal thymine molecules under 280 nm light irradiation. The reaction kinetics of thymine photodimerization appears to be a combination of first-order reactions, each having different rates, reflecting the inhomogeneity and high curvature of the gold nanoparticle surfaces.     

Light-induced modulation of self-assembly on spiropyran-capped gold nanoparticles: a potential system for the controlled release of amino acid derivatives

A photoswitchable double-shell structure on Au nanoparticles, consisting of photochromic spiropyran as the first shell, which regulates the assembly and release of an outer shell of amino acid derivatives upon irradiation, is being reported for the first time. The light-regulated changes in the topographic properties of spiropyran-capped Au nanoparticles (i.e., interconversion between the zwitterionic and neutral forms) are exploited for the assembly and release of amino acid-based therapeutic agents such as l-DOPA.     

Aqueous-organic phase transfer of gold nanoparticles and gold nanorods using an ionic liquid

The water-immiscible ionic liquid, [C4MIM][PF6], is a solvent medium that allows complete transfer of gold nanoparticles from an aqueous phase into an organic phase. Both spherical and rod-shaped gold nanoparticles are efficiently transferred from an aqueous solution into the organic phase without requiring the use of thiols. The sizes and shapes of the gold nanoparticles were preserved during the phase-transfer process when a surfactant was added to the ionic liquid. This process offers a simple approach for obtaining solutions of differently sized and shaped gold nanoparticles in ionic liquids.     

DNA bimodified gold nanoparticles

We report a general approach to bimodify gold nanoparticles (GNPs) with two different DNA strands via DNA template reaction. Two thioctic acid modified DNA strands, one at 5' end and one at 3' end, were attached to GNPs through bivalent thiol-gold bond. By sequence design, assemblies of 5 nm GNPs chains, 10 nm GNPs chains and alternative arrangement of 5 and 10 nm GNPs could be achieved. Gel electrophoresis, transmission electron microscope (TEM), UV-vis spectra were used to characterize the assemblies. It is believed that this new kind of bimodified GNPs with two different DNA strands at different ends would enrich the toolbox of DNA-GNP conjugates and provide diverse selectivity for further assembly.     

On-demand electrochemical release of DNA from gold surfaces

Electrochemical quartz crystal microbalance (EQCM) is used to probe the electrochemically triggered release of nucleic acids from gold surfaces to solutions of physiological pH. The immobilization of nonthiolated DNA onto the gold surface is followed by an electrostatic desorption at -1.0 V (vs. Ag/AgCl). Steady-state frequency signals, corresponding to the removal of 261- and 644-ng/cm2 single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA), respectively, were attained within 75 and 330 s. As expected for electrostatic repulsion, the amount released can be manipulated by tuning the potential. The small nonsteady-state frequency signals observed at lower potentials indicate promise for a sustained DNA release. Applicability to gold ultramicroelectrodes (12.5-microm radius) is demonstrated in connection with voltammetric blocking experiments. We expect that such on-demand electrochemical release would be a useful addition to the arsenal of nonviral gene delivery routes.     

Formation and plasmonic response of self-assembled layers of colloidal gold nanorods and branched gold nanoparticles

The plasmonic properties of self-assembled layers of rod- and branched-shaped gold nanoparticles were investigated using optical techniques. Nanoparticles were synthesized by a surfactant-guided, seed-mediated growth method. The layers were obtained by gradual assembly of nanoparticles at the interface between a polar and a nonpolar solvent and were transferred to a glass slide. Polarization and angle-dependent extinction measurements showed that the layers made of gold nanorods were governed by an effective medium response. The response of the layers made by branched gold particles was characterized by random light scattering. Microscopic mapping of the spatial mode structure demonstrates a uniform optical response of the nanoparticle layers down to a submicrometer length scale.     

Evolution of ultrafine gold seed nanoparticles with temperature and time and synthesis of plasmonic nanoshells

Features of the evolution of ultrafine gold nanoparticles synthesized by the Duff method with temperature and time have been studied in relation to their use as seeds for the formation of plasmonic nanoshells. A quantitative relation has been revealed between the duration of preheating of such particles at a preset temperature and their size. The obtained relation indicates that Au nanoparticles grow mainly via the Ostwald ripening mechanism. Using anisotropic composite FeOOH/Ag particles as an example, it has been shown that the obtained information may be used to substantially decrease the duration of the synthesis of metal nanoshells on diverse cores.     

Remotely triggered liposome release by near-infrared light absorption via hollow gold nanoshells

An elusive goal for systemic drug delivery is to provide both spatial and temporal control of drug release. Liposomes have been evaluated as drug delivery vehicles for decades, but their clinical significance has been limited by slow release or poor availability of the encapsulated drug. Here we show that near-complete liposome release can be initiated within seconds by irradiating hollow gold nanoshells (HGNs) with a near-infrared (NIR) pulsed laser. Our findings reveal that different coupling methods such as having the HGNs tethered to, encapsulated within, or suspended freely outside the liposomes, all triggered liposome release but with different levels of efficiency. For the underlying content release mechanism, our experiments suggest that the microbubble formation and collapse due to the rapid temperature increase of the HGN is responsible for liposome disruption, as evidenced by the formation of solid gold particles after the NIR irradiation and the coincidence of a laser power threshold for both triggered release and pressure fluctuations in the solution associated with cavitation. These effects are similar to those induced by ultrasound and our approach is conceptually analogous to the use of optically triggered nano-"sonicators" deep inside the body for drug delivery. We expect HGNs can be coupled with any nanocarriers to promote spatially and temporally controlled drug release. In addition, the capability of external HGNs to permeabilize lipid membranes can facilitate the cellular uptake of macromolecules including proteins and DNA and allow for promising applications in gene therapy.     

Optical DNA detection based on gold nanorods aggregation

Sequence-specific DNA detection is important in various biomedical applications such as gene expression profiling, disease diagnosis and treatment, drug discovery and forensic analysis. Herein, the localized surface plasmon resonance properties of unmodified gold nanorods (GNRs) in 1 mM cetyltrimethyl-ammonium bromide solution were used for sensing DNA sequences, with good simplicity and sensitivity. The intensity of typical plasmon resonance absorption bands of the GNRs decreased with increasing cDNA concentration. The detection of a 30-mer single-stranded oligonucleotide as a model reached a detection limit of about 0.1 pM. This study will be significant for as-prepared GNRs for future application in biological systems.     

DNA aptamer folding on gold nanoparticles: from colloid chemistry to biosensors

We have investigated the effect of the folding of DNA aptamers on the colloidal stability of gold nanoparticles (AuNPs) to which an aptamer is tethered. On the basis of the studies of two different aptamers (adenosine aptamer and K+ aptamer), we discovered a unique colloidal stabilization effect associated with aptamer folding: AuNPs to which folded aptamer structures are attached are more stable toward salt-induced aggregation than those tethered to unfolded aptamers. This colloidal stabilization effect is more significant when a DNA spacer was incorporated between AuNP and the aptamer or when lower aptamer surface graft densities were used. The conformation that aptamers adopt on the surface appears to be a key factor that determines the relative stability of different AuNPs. Dynamic light scattering experiments revealed that the sizes of AuNPs modified with folded aptamers were larger than those of AuNPs modified with unfolded (but largely collapsed) aptamers in salt solution. From both the electrostatic and steric stabilization points of view, the folded aptamers that are more extended from the surface have a higher stabilization effect on AuNP than the unfolded aptamers. On the basis of this unique phenomenon, colorimetric biosensors have been developed for the detection of adenosine, K+, adenosine deaminase, and its inhibitors. Moreover, distinct AuNP aggregation and redispersion stages can be readily operated by controlling aptamer folding and unfolding states with the addition of adenosine and adenosine deaminase.     

Photochemical and analytical applications of gold nanoparticles and nanorods utilizing surface plasmon resonance

In recent years, plasmonics has emerged as a promising tool in the fields of analytical chemistry and biochemistry. In particular, surface plasmon resonance at the surfaces of gold nanostructures has led to the development of widespread interest in gold nanoparticles. In this review, we describe some of the recent progress in the manufacture and use of gold nanoparticles, with particular emphasis on gold nanorods. Furthermore, the spectroscopic and photochemical applications of gold nanospheres and nanorods are described.     

Strong metal-support interactions between gold nanoparticles and ZnO nanorods in CO oxidation

The catalytic performances of supported gold nanoparticles depend critically on the nature of support. Here, we report the first evidence of strong metal-support interactions (SMSI) between gold nanoparticles and ZnO nanorods based on results of structural and spectroscopic characterization. The catalyst shows encapsulation of gold nanoparticles by ZnO and the electron transfer between gold and the support. Detailed characterizations of the interaction between Au nanoparticles and ZnO were done with transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and FTIR study of adsorbed CO. The significance of the SMSI effect is further investigated by probing the efficiency of CO oxidation over the Au/ZnO-nanorod. In contrast to the classical reductive SMSI in the TiO(2) supported group VIII metals which appears after high temperature reduction in H(2) with electron transfer from the support to metals, the oxidative SMSI in Au/ZnO-nanorod system gives oxygen-induced burial and electron transfer from gold to support. In CO oxidation, we found that the oxidative SMSI state is associated with positively charged gold nanoparticles with strong effect on its catalytic activity before and after encapsulation. The oxidative SMSI can be reversed by hydrogen treatment to induce AuZn alloy formation, de-encapsulation, and electron transfer from support to Au. Our discovery of the SMSI effects in Au/ZnO nanorods gives new understandings of the interaction between gold and support and provides new way to control the interaction between gold and the support as well as catalytic activity.     

Fluorescence properties of gold nanorods and their application for DNA biosensing

This communication reveals new and unique optical properties with respect to enhanced fluorescence of gold nanorods as they elongate; a novel strategy for DNA hybridization studies based on monitoring the fluorescence intensity of gold nanorods has been demonstrated.