Separation of Semiconducting from Metallic Carbon Nanotubes by Selective Functionalization with Azomethine Ylides

A mild and efficient method for the functionalization of SWNTs by cycloaddition of azomethine ylides derived from trialkylamine-N-oxides is described. Selective reaction of semiconducting carbon nanotubes was achieved by preorganizing the starting N-oxides on the nanotube surface prior to generating the reactive ylides. Separation of met-SWNTs from functionalized sem-SWNTs was successfully accomplished by inducing solubilization of sem-SWNTs in the presence of lignoceric acid.

A gold nanoparticle based approach for screening triplex DNA binders

Nanoparticle assemblies interconnected with DNA triple helixes can be used to colorimetrically screen for triplex DNA binding molecules and simultaneously determine their relative binding affinities based on melting temperatures. Nanoparticles assemble only when DNA triple helixes form between DNA from two different particles and a third strand of free DNA. In addition, the triple helix structure is unstable at room temperature and only forms in the presence of triplex DNA binding molecules which stabilize the triple helix. The resulting melting transition of the nanoparticle assembly is much sharper and at a significantly higher Tm than the analogous triplex structure without nanoparticles. Upon nanoparticle assembly, a concomitant red-to-blue color change occurs. The assembly process and color change do not occur in the presence of duplex DNA binders and therefore provide a significantly better screening process for triplex DNA binding molecules compared to standard methods.

Sequence Dependence of Charge Transport Properties of DNA

The electrical conduction through three short oligomers (26 base pairs, 8 nm long) with differing numbers of GC base pairs was measured. One strand is poly(A)-poly(T), which is entirely devoid of GC base pairs. Of the two additional strands, one contains 8 and the other 14 GC base pairs. The oligomers were adsorbed on a gold substrate on one side and to a gold nanoparticle on the other side. Conducting atomic force microscope was used for obtaining the current versus voltage curves. We found that in all cases the DNA behaves as a wide band-gap semiconductor, with width depending on the number of GC base pairs. As this number increases, the band-gap narrows. For applied voltages exceeding the band-gap, the current density rises dramatically. The rise becomes sharper with increasing number of GC base pairs, reaching more than 1 nA/nm2 for the oligomer containing 14 GC pairs.

Imaging atomic structure in metal nanoparticles using high-resolution cryo-TEM

It has been shown, by imaging gold (200) planes, that it is possible to achieve better than 0.20-nm structural resolution in cryo-transmission electron microscopy (cryo-TEM). This has been done using commercially available cryo equipment and using a 300-kV field emission gun (FEG) TEM. The images of 15-nm gold particles embedded in amorphous frozen water clearly show the (111) planes (separated by 0.235 nm) in gold. Fourier transform demonstrates the presence of (200) planes in the image, proving a resolution of better than 0.20 nm. The experimental results are supported by image simulations using the multislice method. These simulations suggest that it should be possible to achieve the same resolution even in smaller particles and particles of lighter elements. The crucial experimental problem to overcome is keeping the thickness of the amorphous film low and to work at low electron dose conditions.

One-Step Synthesis of Monodisperse Silver Nanoparticles beneath Vitamin E Langmuir Monolayers

The monodisperse silver nanoparticles were synthesized by one-step reduction of silver ions in the alkaline subphase beneath vitamin E (VE) Langmuir monolayers. The monolayers and silver nanocomposite LB films were characterized by surface pressure-area (pi-A) isotherms, transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-vis), selected area electron diffraction (SAED), Fourier transform infrared transmission spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS), respectively. The results showed that the limiting area/VE molecule on different subphases varied. The phenolic groups in the VE molecules were converted to a quinone structure, and the silver ions were mainly reduced to ellipsoidal and spherical nanoparticles. The arrangement of the nanoparticles changed from sparseness to compactness with reaction time. The electron diffraction pattern indicated that the silver nanoparticles were face-centered cubic (fcc) polycrystalline. Silver nanocomposite LB films with excellent quality could be formed on different substrates, indicating that the transfer ratio of monolayer containing silver nanoparticles is close to unity. The dynamic process of reduction of silver ions by VE LB films was also studied through monitoring the conductivity of an Ag2SO4 alkaline solution.

Silver-particle-based surface-enhanced Raman scattering spectroscopy for biomolecular sensing and recognition

In this study, we demonstrate that 2-microm-sized Ag (microAg) powders can be used as a core material for constructing molecular sensing/recognition units operating via surface-enhanced Raman scattering (SERS). This is possible because microAg powders are very efficient substrates for both the infrared and Raman-spectroscopic characterization of molecular adsorbates prepared in a similar manner on silver surfaces; we can obtain an infrared spectrum of organic molecules adsorbed on microAg particles with a very high signal-to-noise ratio by diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), and the Raman spectrum of organic monolayers on powdered silver is an SERS spectrum. The agglomeration of microAg particles in a highly concentrated buffer solution could be prevented by the layer-by-layer deposition of cationic and anionic polyelectrolytes such as poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). In fact, prior to depositing PAA and PAH, 4-aminobenzenethiol (4-ABT) was assembled on the surfaces of the microAg particles as SERS markers. Because of the presence of amine groups of 4-ABT, PAA could be readily deposited on the microAg particles. On the other hand, the outermost PAA layer could also be derivatized with biotin-derivatized poly(L-lysine). The nonspecific interaction of poly(L-lysine) with proteins could be suppressed by grafting poly(ethylene glycol) into the biotin-derivatized poly(L-lysine) molecules. On the basis of the nature of the SERS peaks of 4-ABT, it was confirmed that these biotinylated microAg powders were effective in selectively recognizing the streptavidin arrays. Because a number of different molecules can be used as SERS-marker molecules, such as probable 4-ABT, commercially available microAg powders must be a prospective material in molecular sensing/recognition, particularly via SERS.

Formation of spherical nanostructures by the controlled aggregation of gold colloids

Stable suspensions of size-uniform spherical assemblies of 5-8 nm gold colloids in toluene are readily obtained by cross linking the colloidal particles using alkanedithiols within a defined range of gold-dithiol molar ratios. The assemblies are very stable and remain suspended in toluene for several months without significant aggregation. These porous gold spheres can be further organized into hierarchically assembled relatively linear chains by the addition of ethanol.

Preparation and encapsulation of highly fluorescent conjugated polymer nanoparticles

A facile method has been developed to prepare aqueous dispersions of encapsulated conjugated polymer nanoparticles exhibiting high fluorescence brightness. Salient features of the nanoparticles include their small diameter and spherical morphology. Encapsulation of the nanoparticles with a silica shell reduces the rate of photooxidation and allows facile attachment of functional groups for subsequent bioconjugation and nanoparticle assembly. Functionalization of the nanoparticle with amine groups followed by the addition of Au nanoparticles resulted in the formation of nanoparticle assemblies, as evidenced by the efficient quenching of the conjugated polymer fluorescence by the Au nanoparticles.

Nanoparticle targeting at cells

Gold nanoparticles have been used for analytical and biomedical purposes for many years. In fact, the labeling of targeting molecules with nanoparticles has revolutionized the visualization of cellular or tissue components by electron microscopy. We report in this study the derivatization of tiopronin-protected nanoparticles with ethylenediamine and poly(ethylene glycol) bis(3-aminopropyl) terminated and their functionalization with the GRGDSP peptide sequence by a straightforward and economical methodology. The particles were subsequently tested in vitro with a human fibroblast cell line to determine the biocompatibility, and the cell-particle interactions, using fluorescence and scanning electron microscopies. The results indicate that tiopronin gold nanoparticles aggregate due to culture medium proteins, whereas the tiopronin gold nanoparticles derivatized with ethylenediamine induce endocytosis, and the same nanoparticles derivatized with poly(ethylene glycol) derivative promote particle-cell adhesion.

Thiol-specific and nonspecific interactions between DNA and gold nanoparticles

The contribution of nonspecific interactions to the overall interactions of thiol-ssDNA and dsDNA macromolecules with gold nanoparticles was investigated. A systematic investigation utilizing dynamic light scattering and cryogenic transmission electron microscopy has been performed to directly measure and visualize the changes in particle size and appearance during functionalization of gold nanoparticles with thiol-ssDNA and nonthiolated dsDNA. The results show that both thiol-ssDNA and dsDNA do stabilize gold nanoparticle dispersions, but possible nonspecific interactions between the hydrophobic DNA bases and the gold surface promote interparticle interactions and cause aggregation within rather a short period of time. We also discuss the adsorption mechanisms of dsDNA and thiol-ssDNA to gold particles.

Colorimetric detection of immunoglobulin G by use of functionalized gold nanoparticles on polyethylenimine film

A method based on use of functionalized gold nanoparticles on polyethylenimine film has been developed for colorimetric detection of immunoglobulin G (IgG). The immunogold nanoparticles were immobilized on quartz slides by recognition between antibody and antigen, with the antigen chemically adsorbed on the polyethylenimine film. By measurement of the UV-visible spectra of the immobilized immunogold, detection of h-IgG was achieved. The detection limit for h-IgG by use of this method can be as low as 0.01 microg mL(-1). This method is quite promising for numerous applications in immunoassay.

Adsorption of Cyanine Dyes on Gold Nanoparticles and Formation of J-Aggregates in the Nanoparticle Assembly

This paper describes the results of an investigation of the interparticle interactions and reactivities in the assembly of gold nanoparticles mediated by cyanine dyes. The combination of the positively charged indolenine cyanine dyes and the negatively charged gold nanoparticles is shown to form a J-aggregate bridged assembly of nanoparticles, in addition to hydrophobic interparticle and electrostatic dye-particle interactions. Such interparticle interactions and reactivities are studied by probing the absorption of J-aggregates and fluorescence from the dyes and the surface plasmon resonance absorption from the nanoparticles. The J-aggregation of the dyes adsorbed on the nanoparticles is shown to play an important role in the assembly of nanoparticles. The spectral evolution of the J-band of the dyes and the surface plasmon resonance band of the nanoparticles was found to be sensitive to the nature of the charge and the structure of the dyes. The fluorescence quenching for the dyes was shown to be quantitatively related to the surface coverage of the dyes on the nanocrystal surfaces. These findings have provided important information for assessing a two-step process involving a rapid adsorption of the dyes on the nanoparticles and a subsequent assembly of the nanoparticles involving a combination of interparticle J-aggregation and hydrophobic interactions of the adsorbed dyes. The results are discussed in terms of the structural effects of the dyes, and the interparticle molecular interactions and reactivities, which provide important physical and chemical insights into the design of dye-nanoparticle structured functional nanomaterials.

Selective Heterogeneous Nucleation and Growth of Size-Controlled Metal Nanoparticles on Carbon Nanotubes in Solution

We present a novel approach to the in situ deposition of size-controlled platinum nanoparticles on the exterior walls of carbon nanotubes (CNTs). The reduction of metal ions in ethylene glycol (EG), by the addition of a salt such as sodium dodecyl sulfate (SDS), p-CH3C6H4SO3Na, LiCF3SO3, or LiClO4, results in high dispersions and high loadings of platinum nanoparticles on CNTs without aggregation. We have performed controlled experiments to elucidate the mechanism. By exploiting the salt effect, our method effectively depresses homogeneous nucleation, leading to selective heterogeneous metal nucleation and growth, even on unmodified CNTs. In the 2.3-9.6 nm size range, the size of platinum nanoparticles, at 50% loading, can be controlled by changing the concentration of metal ions, the reaction temperature, the reducing reagent or the means by which reactive solutions are added. Our method provides a flexible route towards the preparation of novel one-dimensional hybrid materials, for which a number of promising applications in a variety of fields can be envisioned.

Comparative analysis of the structure of sterically stabilized ferrofluids on polar carriers by small-angle neutron scattering

Results of experiments on small-angle neutron scattering from ferrofluids on polar carriers (pentanol, water, methyl-ethyl-ketone), with double-layer sterical stabilization of magnetic nanoparticles, are reported. Several types of spatial structural organization are observed. The structure of highly stable pentanol-based samples is similar to that of stable ferrofluids based on organic non-polar carriers (e.g., benzene) with mono-layer covered magnetic nanoparticles. At the same time, the effect of the interparticle interaction on the scattering is stronger in polar ferrofluids because of the structural difference in the surfactant shell. The structure of the studied methyl-ethyl-ketone- and water-based ferrofluids essentially different from the previous case. The formation of large (>100 nm in size) elongated or fractal aggregates, respectively, is detected even in the absence of external magnetic field, which corresponds to weaker stability of these types of ferrofluids. The structure of the fractal aggregates in water-based ferrofluids does not depend on the particle concentration, but it is sensitive to temperature. A temperature increase results in a decrease in their fractal dimension reflecting destruction of the aggregates. In addition, in water-based ferrofluids these aggregates consist of small (radius approximately 10 nm) and temperature-stable primary aggregates.

Synthesis of AgcoreAushellBimetallic Nanoparticles for Immunoassay Based on Surface-Enhanced Raman Spectroscopy

Layered core-shell bimetallic silver-gold nanoparticles were prepared by coating Au layers over Ag seeds by a seed-growth method. The composition of Ag100-xAux particles can vary from x=0 to 30. TEM and SEM images clearly show that the bimetallic nanoparticles are of core-shell structure with some pinholes on the surface. Strong surface-enhanced Raman (SER) signals of thiophenol and p-aminothiophenol have been obtained with these colloids. It was found that the SERS activity of aggregated colloids critically depends on the molar ratio of Ag to Au. With the increase of the Au molar fraction, the SERS activity enhances first and then weakens, with the maximal intensity being 10 times stronger than that of Ag colloids. The AgcoreAushell nanoparticles were then labeled with monoclonal antibodies and SERS probes and used for immunoassay analysis. In the proposed system, antibodies immobilized on a solid substrate can interact with the corresponding antigens to form a composite substrate, which can capture reporter-labeled AgcoreAushell nanoparticles modified with the same antibodies. The immunoreaction between the antibodies and antigens was demonstrated by the detection of characteristic Raman bands of the probe molecules. AgcoreAushell bimetallic nanoparticles, as a new SERS active and biocompatible substrate, will be expected to improve the detection sensitivity of immunoassay.

Raman and surface enhanced Raman microscopy of microstructured polyethylenimine/DNA multilayers

We analyze microstructured multilayer films of poly(ethyleneimine) (PEI) and DNA by employing Raman and surface enhanced Raman spectroscopy (SERS). The microstructuring of the samples allows a simultaneous measurement of signal and reference in a single analytic process. Silver nanoparticles are implemented in the microstructured multilayers for SERS measurements. The recorded SERS spectra of PEI/DNA are dominated by the Raman bands of the DNA bases which show a larger mean enhancement than bands belonging to DNA backbone vibrations. Our results show that the combination of SERS and microstructured multilayer films provides an adapted way to characterize the polyelectrolytes as well as to measure the enhancement factor and the distance dependence for the SERS active silver nanoparticles. Furthermore, microstructured polyelectrolyte films containing SERS active nanoparticles are used for sensing molecules.

Controlling chemical reactivity at solid-solution interfaces by means of hydrophobic magnetic nanoparticles

Hydrophobic magnetic nanoparticles are employed to reversibly regulate the hydrophobic/hydrophilic properties of surfaces and to control the electrochemistry and bioelectochemistry at chemically modified electrodes. Selective bioelectrocatalytic transformations at relay-functionalized electrodes are accomplished by the magnetic attractions of the hydrophobic magnetic nanoparticles with coadsorbed hydrophobic redox relays to the electrode. The selective activation of one of two biocatalysts solubilized in the aqueous electrolyte solution in the absence or presence of hydrophobic magnetic nanoparticles results in the specific activation of bioelectrocatalytic processes. The magnetic attraction and retraction of hydrophobic magnetic nanoparticles to and from semiconductor nanoparticle (CdS)-functionalized electrodes enable the control of the photocurrent directions at the electrode from cathodic to anodic directions, respectively. The magnetic attraction of the hydrophobic magnetic nanoparticles to the surfaces is also employed to control biorecognition and biocatalytic transformations at solid supports. The magnetic attraction and retraction of the hydrophobic magnetic nanoparticles to and from the surfaces allow the blockage and activation of DNA hybridization, polymerization, and enzymatic digestion, respectively.

Bioconjugation of Ln3+-doped LaF3 nanoparticles to avidin

The binding of Eu3+-doped LaF3 nanoparticles with biotin moieties at the surface of the stabilizing ligand layer to avidin, immobilized on cross-linked aragose beads, is described. The biotin moieties were attached to the nanoparticles by reaction of an activated ester with the amino groups on the surface of the nanoparticles resulting from the 2-aminoethyl phosphate ligands that were coordinated to the surface through the phosphate end. This strategy of employing the reactions of amines with activated esters provides a general platform to modify the surface of the 2-aminophosphate stabilized Ln3+-doped LaF3 nanoparticles with biologically relevant groups. Significant suppression of nonspecific binding to the avidin modified aragose beads has been realized by the incorporation of poly(ethylene glycol) units via the same reaction of a primary amine with an activated ester. The particle size distribution of the functionalized nanoparticles was within 10-50 nm, with a quantum yield of 19% in H2O for the LaF3 nanoparticles codoped with Ce3+ and Tb3+. A discreet, 4 unit poly(ethylene glycol) spaced heterobifunctional cross-linker, functionalized with biotin and N-hydroxysuccinimide at opposite termini, was covalently linked to the 2-aminoethyl phosphate ligand via the N-hydroxysuccinimide activated ester, making an amide bond, imparting biological activity to the particle. Modification of the remaining unreacted amino groups of the stabilizing ligands was done with Me(OCH2CH2)3CH2CH2(C=O)-NHS (NHS = N-hydroxysuccinimide).

Simple Electrochemical Method for Deposition and Voltammetric Inspection of Silver Particles at the Liquid−Liquid Interface of a Thin-Film Electrode

A novel experimental methodology for depositing and voltammetric study of Ag nanoparticles at the water-nitrobenzene (W-NB) interface is proposed by means of thin-film electrodes. The electrode assembly consists of a graphite electrode modified with a thin NB film containing decamethylferrocene (DMFC) as a redox probe. In contact with an aqueous electrolyte containing Ag(+) ions, a heterogeneous electron-transfer reaction between DMFC((NB)) and Ag(+)((W)) takes place to form DMFC(+)((NB)) and Ag deposit at the W-NB interface. Based on this interfacial reaction, two different deposition strategies have been applied. In the uncontrolled potential deposition protocol, the electrode is immersed into an AgNO(3) aqueous solution for a certain period under open circuit conditions. Following the deposition step, the Ag-modified thin-film electrode is transferred into an aqueous electrolyte free of Ag(+) ions and voltammetrically inspected. In the second protocol the deposition was carried out under controlled potential conditions, i.e., in an aqueous electrolyte solution containing Ag(+) ions by permanent cycling of the electrode potential. In this procedure, DMFC((NB)) is electrochemically regenerated at the electrode surface, hence enabling continuation and voltammetric control of the Ag deposition. Hence, the overall electrochemical process can be regarded as an electrochemical reduction of Ag(+)((W)) at the W-NB interface, where the redox couple DMFC(+)/DMFC acts as a mediator for shuttling electrons from the electrode to the W-NB interface. Ag-particles deposited at the W-NB interface affect the ion transfer across the interface, which provides the basis for voltammetric inspection of the metal deposit at the liquid-liquid interface with thin-film electrodes. Voltammetric properties of thin-film electrodes are particularly sensitive to the deposition procedure, reflecting differences in the properties of the Ag deposit. Moreover, this methodology is particularly suited to inspect catalytic activities of metal particles deposited at the liquid-liquid interface toward heterogeneous electron-transfer reactions occurring at the at the liquid-liquid interface.

Laser-Assisted Synthesis of Au−Ag Alloy Nanoparticles in Solution

By using laser-induced heating, we prepared Au-Ag nanoalloys via three different procedures: (i) mixture of Au nanoparticles and Ag(+) ions irradiated by a 532 nm laser, (ii) mixture of Au and Ag nanoparticles irradiated by a 532 nm laser, and (iii) mixture of Au and Ag nanoparticles irradiated by a 355 nm laser. Procedure i is advantageous for the production of spherical alloy nanoparticles; in procedures ii and iii, nanoalloys with a sintered structure have been obtained. The morphology of the obtained nanoalloys depends not only on the laser wavelength but also on the concentration of nanoparticles in the initial mixture. When the total concentration of Ag and Au nanoparticles in the mixture is increased, large-scale interlinked networks have been observed upon laser irradiation. It is expected that this selective heating strategy can be extended to prepare other bi- or multi-metallic nanoalloys.

Differential interaction of magnetic nanoparticles with tumor cells and peripheral blood cells

PURPOSE

The separation of tumor cells from healthy cells is a vital problem in oncology and hematology, especially from peripheral blood. Magnetic assisted cell sorting (MACS) is a possibility to fulfill these needs.

METHODS

Tumor cell lines and leukocytes from peripheral blood were incubated with carboxymethyl dextran-coated magnetic nanoparticles under various conditions and separated by MACS.

RESULTS

We studied the interaction of magnetic nanoparticles devoid of antibodies with healthy and tumor cells. The magnetic nanoparticles interact with tumor cells and leukocytes and are located predominantly within the cell cytoplasm. Incubation of cell culture cells with magnetic nanoparticles led to a labeling of these cells without reduced biological properties for at least 14 days. The interaction of the magnetic nanoparticles with cells depends on several factors. The ionic strength (osmolality) of the solvent plays an important role. We could show that an increase in osmolality led to a dramatic reduction of labeled leukocytes. Tumor cells, however, are mildly affected. This could be detected not only in pure cultures of tumor cells or leukocytes but also in mixed cell populations.

CONCLUSION

This observation gives us the opportunity to selectively label and separate tumor cells but not leukocytes from the peripheral blood.

Fabrication of Au-DNA-Au nanostructure with new-type DNA-Au conjugate

Nano devices fabricated from DNA and nanoparticles found many important applications in both biotechnology and nanotechnology. Conventionally, DNA was linked to gold surfaces through a linker. In this study, DNAs were directly attached to gold surfaces through S-Au bonds by modifying 5'-end of DNA with a mercapto group. The pH of the reaction mixture played an important role in DNA-gold conjugate fabrication. The DNA-Au conjugates were successfully obtained in pure forms and, with the use of this new type conjugate, Au-DNA-Au nanostructure was prepared. The Au-DNA-Au nanostructure was characterized by AFM.

Phosphonioalkylthiosulfate zwitterions—new masked thiol ligands for the formation of cationic functionalised gold nanoparticles

We report the synthesis and structural characterisation of a new family of stable phosphonioalkylthiosulfate zwitterions, R3P+ (CH2)nS2O3- (R = Ph or Bu, n = 3,4,6, 8 or 10) which behave as cationic masked thiolate ligands with applications in the functionalisation of gold nanoparticles, having potential as new diagnostic biorecognition systems. The ligands were prepared by treatment of omega-bromoalkylphosphonium salts with sodium thiosulfate. The crystal and molecular structures of the zwitterions (R = Ph, n = 3) and (R = Bu, n = 3) were determined. A series of phosphonioalkanethiolate-capped gold nanoparticles dispersed in water was prepared by borohydride reduction of potassium tetrachloroaurate in the presence of the zwitterions in a dichloromethane-water system. UV-visible spectroscopy and scanning transmission electron-microscopy indicated that capped nanoparticles of ca. 5 nm diameter were present.

Gold nanostructures: engineering their plasmonic properties for biomedical applications

The surface plasmon resonance peaks of gold nanostructures can be tuned from the visible to the near infrared region by controlling the shape and structure (solid vs. hollow). In this tutorial review we highlight this concept by comparing four typical examples: nanospheres, nanorods, nanoshells, and nanocages. A combination of this optical tunability with the inertness of gold makes gold nanostructures well suited for various biomedical applications.

Application of the Nanogold-4,4′-bis(methanethiol)biphenyl Modified Gold Electrode to the Determination of Tyrosinase-Catechol Reaction Kinetics in Acetonitrile

The reactivity of tyrosinase adsorbed on nanogold bound with 4,4'-bis(methanethiol)biphenyl monolayer self-assembled on a gold disk with catechol in a dipolar aprotic solvent, acetonitrile (AN), was studied by cyclic voltammetric and amperometric methods. Tyrosinase exhibited characteristics of a Michaelis-Menten kinetic mechanism. The tyrosinase attached to the nanogold continued to react with substrates in AN even when the water content was lower than 0.01 w/w%. The apparent Michaelis-Menten constant K(m) of tyrosinase for catechol is 5.5 +/- 0.4 mM (n = 5).

SERS signals at the anti Stokes side of the excitation laser in extremely high local optical fields of silver and gold nanoclusters

Surface-enhanced anti-Stokes Raman scattering from pumped excited vibrational levels and surface-enhanced hyper Raman scattering show a quadratic dependence on the excitation intensity and are discussed as incoherent two-photon excited Raman processes performed in strongly enhanced local optical fields of silver- or gold nanoclusters, where both effects can experience very similar electromagnetic enhancement conditions.

SERRS labelled beads for multiplex detection

Beads labelled using surface enhanced resonance Raman scattering (SERRS) are highly sensitive and specific tags, with potential applications in biological assays, including molecular diagnostics. The beads consist of a nucleus containing dye labelled silver-nanoparticle aggregates surrounded by a polymer core. The nuclei generate strong SERRS signals. To illustrate the coding advantage created by the sharp, molecularly specific SERRS signals, four specially designed SERRS dyes have been used as labels and three of these have been combined in a multiplex analysis. These dyes use specific groups such as benzotriazole and 8-hydroxyquinoline to improve binding to the surface of the silver particles. The aggregation state of the particles is held constant by the polymer core, this nucleus also contains many dye labels, yielding a very high Raman scattering intensity for each bead. To functionalise these beads for use in biological assays an outer polymer shell can be added, which allows the attachment of oligonucleotide probes. Oligonucleotide modified beads can then be used for detection of specific oligonucleotide targets. The specificity of SERRS will allow for the detection of multiple targets within a single assay.

Group 12 metal monoselenocarboxylates: synthesis, characterization, structure and their transformation to metal selenide (MSe; M = Zn, Cd, Hg) nanoparticles

Reactions of [MCl2(tmeda)] with potassium salts of monoselenocarboxylic acids gave complexes of the general formula [M(SeCOR)2(tmeda)] (M = Zn, Cd; R = Ph, Tol; Tol = C6H4-p-CH3; tmeda = Me2NCH2CH2NMe2). The analogous mercury complexes were unstable at room temperature and afforded HgSe nanoparticles during the course of reaction. All the complexes were characterized by elemental analysis, IR, UV-vis, NMR (1H, 13C, 77Se, 113Cd) data. The X-ray structural analysis of [Cd(SeCOPh)2(tmeda)] revealed that the complex is a discrete monomer having an approximate tetrahedral coordination environment around the central metal atom with monodentate (Se-bonded) selenocarboxylates. Thermal behavior of these complexes was studied by TG analysis. Pyrolysis in a furnace or in HDA (hexadecylamine) gave MSe nanoparticles, which were characterized by XRD, EDAX, SEM and absorption spectroscopy.

Fluorescent core–shell silica nanoparticles: towards “Lab on a Particle” architectures for nanobiotechnology

Novel nanoscale fluorescent materials are integral to the progress of emergent fields such as nanobiotechnology and facilitate new research in a variety of contexts. Sol-gel derived silica is an excellent host material for creating fluorescent nanoparticles by the inclusion of covalently-bound organic dyes. Significant enhancements in the brightness and stability of organic dye emission can be achieved for silica-based core-shell nanoparticle architectures at length scales down to tens of nanometers with narrow size distributions. This tutorial review will highlight these findings and describe the evolution of the fluorescent core-shell silica nanoparticle concept towards integration of multiple functionalities including mesoporosity, metal nanoshells and quantitative chemical sensing. These developments point towards the development of "lab on a particle" architectures with promising prospects for nanobiotechnology, drug development and beyond.

Nanoparticle Conjugation Increases Protein Partitioning in Aqueous Two-Phase Systems

We describe the effect of bioconjugation to colloidal Au nanoparticles on protein partitioning in poly(ethylene glycol) (PEG)/dextran aqueous two-phase systems (ATPS). Horseradish peroxidase (HRP) was conjugated to colloidal Au nanoparticles by direct adsorption. Although HRP alone had very little phase preference, HRP/Au nanoparticle conjugates typically partitioned to the PEG-rich phase, up to a factor of 150:1 for conjugates of 15-nm colloidal Au. Other protein/Au nanoparticle conjugates exhibited partitioning of greater than 2000:1 to the dextran-rich phase, as compared with approximately 5:1 for the free protein. The degree of partitioning was dependent on polymer concentration and molecular weight, nanoparticle diameter, and in some instances, nanoparticle concentration in the ATPS. The substantial improvements in protein partitioning achievable by conjugation to Au nanoparticles appear to result largely from increased surface area of the conjugates and require neither chemical modification of the proteins or polymers with affinity ligands, increased polymer concentrations, nor addition of high concentrations of salts. Adsorption to colloidal particles thus provides an attractive route for increased partitioning of enzymes and other proteins in ATPS. Furthermore, these results point to ATPS partitioning as a powerful means of purification for biomolecule/nanoparticle conjugates, which are increasingly used in diagnostics and materials applications.

Gold nanoparticle localization at the core surface by using thermosensitive core-shell particles as a template

We report novel thermosensitive hybrid core-shell particles via in situ gold nanoparticle formation using thermosensitive core-shell particles as a template. This method for the in situ synthesis of gold nanoparticles with microgel interiors offers the advantage of eliminating or significantly reducing particle aggregation. In addition, by using thermosensitive microgel structures in which the shell has thermosensitive and gel properties in water--whereas the core itself is a water-insoluble polymer--we were able to synthesize the gold nanoparticles only at the surface of the core, which had reactive sites to bind metal ions. After the gold nanoparticles were synthesized, electroless gold plating was carried out to control the thickness of the gold nanoshells. The dispersions of the obtained hybrid particles were characterized by dynamic light scattering and UV-vis absorption spectroscopy, and the dried particles were also observed by electron microscopy. Adaptation of the technique shown here will create a number of applications as optical, electronic, and biomedical functional materials.

A Bio-Bar-Code Assay Based upon Dithiothreitol-Induced Oligonucleotide Release

The recently developed bio-bar-code assay for the PCR-less detection of protein and nucleic acid targets has been shown to be extraordinarily sensitive, exhibiting low attomolar sensitivity for protein targets and high zeptomolar sensitivity for nucleic acid targets. In the case of DNA detection, the original assay relies on three distinct oligonucleotide strands on a single nanoparticle for target identification and signal amplification. Herein, we report the development of a new nanoparticle probe that can be used in the bio-bar-code assay, which requires only one thiolated oligonucleotide strand. This new assay relies on the ability to liberate the adsorbed thiolated oligonucleotides from the gold nanoparticle surface with dithiothreitol (DTT), which simplifies the assay and increases its quantitative capabilities. The utility of this new DTT-based system is demonstrated by detecting a mock mRNA target using both fluorescent and scanometric assay readouts. When the scanometric readout is used, the sensitivity of the assay is 7 aM and quantification can be accomplished over the low-attomolar to the mid-femtomolar concentration range.

Nanometer-scale modification and welding of silicon and metallic nanowires with a high-intensity electron beam

We demonstrate that a high-intensity electron beam can be applied to create holes, gaps, and other patterns of atomic and nanometer dimensions on a single nanowire, to weld individual nanowires to form metal-metal or metal-semiconductor junctions, and to remove the oxide shell from a crystalline nanowire. In single-crystalline Si nanowires, the beam induces instant local vaporization and local amorphization. In metallic Au, Ag, Cu, and Sn nanowires, the beam induces rapid local surface melting and enhanced surface diffusion, in addition to local vaporization. These studies open up a novel approach for patterning and connecting nanomaterials in devices and circuits at the nanometer scale.

Nanoparticle-DNA conjugates bearing a specific number of short DNA strands by enzymatic manipulation of nanoparticle-bound DNA

Self-assembling of metallic nanoparticles to form well-defined nanostructured structures is a field that has been receiving considerable research interest in recent years. In this field, DNA is a commonly used linker molecule to direct the assembly of the nanoscale building blocks because of its unique recognition capabilities, mechanical rigidity, and physicochemical stability. This study reported our novel approach to generate gold nanoparticle-DNA conjugates bearing specially designed DNA linker molecules that can be used as building blocks to construct nanoassemblies with precisely controlled structure or as nanoprobes for quantitative DNA sequence detection analysis. In our approach, gold nanoparticle-DNA conjugates bearing a specific number of long double-stranded DNA strands were prepared by gel electrophoresis. A restriction endonuclease enzyme was then used to manipulate the length of the nanoparticle-bound DNA. This enzymatic cleavage was confirmed by gel electrophoresis, and digestion efficiency of 90% or more was achieved. With this approach, nanoparticle conjugates bearing a specific number of strands of short DNA with less than 20-base can be achieved.

Sensitivity of Metal Nanoparticle Surface Plasmon Resonance to the Dielectric Environment

Electrodynamic simulations of gold nanoparticle spectra were used to investigate the sensitivity of localized surface plasmon band position to the refractive index, n, of the medium for nanoparticles of various shapes and nanoshells of various structures. Among single-component nanoparticles less than 130 nm in size, sensitivities of dipole resonance positions to bulk refractive index are found to depend only upon the wavelength of the resonance and the dielectric properties of the metal and the medium. Among particle plasmons that peak in the frequency range where the real part of the metal dielectric function varies linearly with wavelength and the imaginary part is small and slowly varying, the sensitivity of the peak wavelength, lambda, to refractive index, n, is found to be a linearly increasing function of lambda, regardless of the structural features of the particle that determine lambda. Quasistatic theory is used to derive an analytical expression for the refractive index sensitivity of small particle plasmon peaks. Through this analysis, the dependence of sensitivity on band position is found to be determined by the wavelength dependence of the real part, epsilon', of the particle dielectric function, and the sensitivity results are found to extend to all particles with resonance conditions of the form, epsilon' = -2chin(2), where chi is a function of geometric parameters and other constants. The sensitivity results observed using accurate computational methods for dipolar plasmon bands of gold nanodisks, nanorods, and hollow nanoshells extend, therefore, to particles of other shapes (such as hexagonal and chopped tetrahedral), composed of other metals, and to higher-order modes. The bulk refractive index sensitivity yielded by the theory serves as an upper bound to sensitivities of nanoparticles on dielectric substrates and sensitivities of nanoparticles to local refractive index changes, such as those associated with biomolecule sensing.

Photon-by-Photon Determination of Emission Bursts from Diffusing Single Chromophores

One of the difficulties in diffusion-type single-molecule experiments is the determination of signal amid photon-counting noise. A commonly used approach is to further average the noisy time trace by binning, followed by placing a threshold to discriminate signal from background. The choice of smoothing parameters and the placement of the threshold may impact on the efficiency with which the information-rich region can be harvested, among other potential complications. Here we introduce a procedure that operates on the data sequence photon by photon, thereby relieving the incertitude in choosing binning-thresholding parameters. We characterize this procedure by detecting the two-photon emission bursts from diffusing single gold nanoparticles. The results support our burst-finding procedure as a reliable and efficient way of detecting and harvesting photon bursts from diffusing experiments.

Electrochemical Characterization of Polyelectrolyte/Gold Nanoparticle Multilayers Self-Assembled on Gold Electrodes

Polyelectrolyte/gold nanoparticle multilayers composed of poly(l-lysine) (pLys) and mercaptosuccinic acid (MSA) stabilized gold nanoparticles (Au NPs) were built up using the electrostatic layer-by-layer self-assembly technique upon a gold electrode modified with a first layer of MSA. The assemblies were characterized using UV-vis absorption spectroscopy, cyclic and square-wave voltammetry, electrochemical impedance spectroscopy, and atomic force microscopy. Charge transport through the multilayer was studied experimentally as well as theoretically by using two different redox pairs [Fe(CN)(6)](3-/4-) and [Ru(NH(3))(6)](3+/2+). This paper reports a large sensitivity to the charge of the outermost layer for the permeability of these assemblies to the probe ions. With the former redox pair, dramatic changes in the impedance response were obtained for thin multilayers each time a new layer was deposited. In the latter case, the multilayer behaves as a conductor exhibiting a strikingly lower impedance response, the electric current being enhanced as more layers are added for Au NP terminated multilayers. These results are interpreted quite satisfactorily by means of a capillary membrane model that encompasses the wide variety of behaviors observed. It is concluded that nonlinear slow diffusion through defects (pinholes) in the multilayer is the governing mechanism for the [Fe(CN)(6)](3-/4-) species, whereas electron transfer through the Au NPs is the dominant mechanism in the case of the [Ru(NH(3))(6)](3+/2+) pair.

Fourier Transform Raman and Density Functional Theory Studies on the Adsorption Behavior of p-Hydroxybenzoic Acid on Silver Nanoparticles

Two models of p-hydroxybenzoic acid (PHBA) adsorbed on the surfaces of silver nanoparticles were established, each of them corresponding to an experimental configuration. The first model is PHBA adsorbed on one Ag atom through the carboxyl group, and the second one is PHBA adsorbed on two Ag atoms through the carboxyl and hydroxyl groups. The Raman spectra of these two models using DFT-B3PW91 with lanl2dz were calculated; it was found that the calculated Raman frequencies were in good agreement with experimental values. So one can conclude that the simplified models are probably reasonable to describe some surface-enhanced Raman experiments.

Low-Temperature Synthesis of Amorphous Carbon Nanocoils via Acetylene Coupling on Copper Nanocrystal Surfaces at 468 K: A Reaction Mechanism Analysis

A new type of amorphous helical carbon nanofibers has been synthesized using copper nanocatalysts and an acetylene gas source at atmospheric pressure. The nanofibers are grown at 468 K, which is the lowest temperature by ordinary metal-catalyzed thermal chemical vapor deposition of hydrocarbon, and exhibit a symmetric growth mode in the form of twin helices. IR, XRD, Raman, and C/H molar ratio analyses reveal a polymer-like structure with a weak trans-polyacetylene feature. The nanofibers are a mixture of solid polymers and a small amount of carbon. A reaction mechanism has been proposed on the basis of the previous studies of acetylene adsorption, desorption properties, and surface reactions on copper (111), (110), and (001) planes under ultrahigh-vacuum (UHV) conditions as well as the results obtained in our study. The reaction mechanism of acetylene on copper single-crystal surfaces under UHV conditions indeed reflects the reaction mechanism under practical catalytic conditions at atmospheric pressure. The nanofibers grow mainly via acetylene coupling to solid polymers on copper nanocrystal surfaces. Acetylene also couples to yield small amounts of liquid oligomers and gaseous products, and undergoes slight carbon deposition during the fiber growth.

Deposition Method for Preparing SERS-Active Gold Nanoparticle Substrates

Surface-enhanced Raman scattering or SERS, discovered some 20 years ago, has recently become a promising tool for routine biofluid assays in a clinical setting. Many attempts have been made to produce cheap and reproducible SERS-active substrates. In this study, we report on the fabrication of SERS-active substrates through the convective assembly of gold (Au) particles on electrostatically charged glass slides. We show that, by a proper control of the initial particle concentration in an evaporating Au suspension droplet, it is possible to obtain a closely packed colloidal film capable of generating SERS activity. Finally, AFM and SERS measurements of the resulting films reveal comparability in performance with previous silane-immobilized Au colloidal films. The minimum electromagnetic enhancement factor of our films is estimated to be about 2 x 10(4).

Identification of Peptides Using Gold Nanoparticle-Assisted Single-Drop Microextraction Coupled with AP-MALDI Mass Spectrometry

A novel technique, gold nanoparticle-assisted single-drop microextraction (SDME) combined with atmospheric pressure matrix-assisted laser desorption/ionization mass spectrometry (AP-MALDI-MS) for the identification of peptides has been described. The SDME of peptides from aqueous solution was achieved using gold nanoparticles prepared in toluene as the acceptor phase. A simple phenomenon of isoelectric point (pI) of the peptides has been utilized successfully to extract the peptides into a single drop of nanogold in toluene. After extraction, a single-drop nano gold solution was directly spotted onto the target plate with an equal volume of matrix, proportional, variant-cyanohydroxy cinnamic acid ( proportional, variant-CHCA) and analyzed in AP-MALDI-MS. The parameters, such as solvent selection, extraction time, agitation rate, and pH effect, were optimized for the SDME technique. Using this technique, in aqueous solution, the lowest concentration detected for Met- and Leu-enkephalin peptides was 0.2 and 0.17 microM, respectively. In addition, the application of this technique to obtain the signal for the selected peptides in a mass spectrum in the presence of matrix interferences such as 1% Triton X-100 and 6.5 M urea has been showed. The application was extended to identify the peptides spiked into urine.