Plasmon-enhanced two-photon excitation fluorescence of rhodamine 6G and an Eu-diketonate complex by a picosecond diode laser.

Two-photon excited fluorescence (TPEF) of rhodamine 6G (Rh6G) and tris(dibenzoylmethane) mono(5-aminophenanthroline) europium (Eu-TDPA) was measured using a pulsed diode laser head (<45 mW, 975 nm, 90 ps pulse width, 40 MHz). Fluorophores were cast on a glass slide modified with triangular silver nanoprisms. A photon-counting photomultiplier detected the TPEF of Rh6G on a glass substrate (1361 Hz) and on the nanoprisms (6322 Hz). On the other hand, Eu-TDPA did not exhibit TPEF on a glass substrate. TPEF was only observed when the extinction of the nanoprisms on the substrates was larger than 0.1. The nanoprisms enhanced the TPEF of these two fluorophores up to the detectable level using a low-power laser diode.

Spectroscopic analysis of two distinct equilibrium states for the exchange reaction of sodium cholate and oligo-DNA on single-walled carbon nanotubes.

Understanding the quantitative analysis of the transition adsorption structures of molecules on single-walled carbon nanotubes (SWNTs) is of importance from the point of view of both fundamental science and applications of nanotubes. Absorption spectroscopy reveals that two different equilibrium states are existent for the exchange reaction of sodium cholate (SC) and oligo-DNA (single-stranded 20-mer cytosine) on SWNTs. This is derived from the transitions of the adsorption structures of different chirality-types of SWNTs and SC/DNA at certain SC concentrations below the critical micelle concentration of SC.

Multimode resonances in silver nanocuboids.

A rich variety of dipolar and higher order plasmon resonances have been predicted for nanoscale cubes and parallopipeds of silver, in contrast to the simple dipolar modes found on silver nanospheres or nanorods. However, in general, these multimode resonances are not readily detected in experimental colloidal ensembles, due primarily to the usual variation of size and shape of the particles obscuring or blending the individual extinction peaks. Recently, methods have been found to prepare silver parallopipeds with unprecedented shape control by nucleating the silver onto a tightly controlled suspension of gold nanorods (Okuno, Y.; Nishioka, K.; Kiya, A.; Nakashima, N.; Ishibashi, A.; Niidome, Y. Uniform and Controllable Preparation of Au-Ag Core-Shell Nanorods Using Anisotropic Silver Shell Formation on Gold Nanorods. Nanoscale 2010, 2, 1489-1493). The optical extinction spectra of suspensions of such monodisperse particles are found to contain multiple extinction peaks, which we show here to be due to the multimode resonances predicted by theoretical studies. Control of the radius of the nanoparticle edges is found to be an effective way to turn some of these modes on or off. These nanoparticles provide a flexible platform for the excitation, manipulation, and exploration of higher order plasmon resonances.

Anionic gold ions desorbed from gold nanorods and nanospheres.

The desorption and ionization of gold anions and cations from gold nanorods and nanospheres were investigated using a matrix-assisted laser desorption/ionization mass spectrometry instrument. The signal intensities of the gold anions from nanorods with a volume of 4.8 × 103 nm3 were about 28 times larger than the signal intensities of gold anions from nanospheres of similar volume. Gold cations did not show a remarkable difference depending on the nanoparticles' shapes. The unique desorption behavior of gold anions suggests their potential as reporter ions in mass spectrometry.

Rational concept to recognize/extract single-walled carbon nanotubes with a specific chirality.

Single-walled carbon nanotubes (SWNTs) have remarkable and unique electronic, mechanical, and thermal properties, which are closely related to their chiralities; thus, the chirality-selective recognition/extraction of the SWNTs is one of the central issues in nanotube science. However, any rational materials design enabling one to efficiently extract/solubilize pure SWNT with a desired chirality has yet not been demonstrated. Herein we report that certain chiral polyfluorene copolymers can well-recognize SWNTs with a certain chirality preferentially, leading to solubilization of specific chiral SWNTs. The chiral copolymers were prepared by the Ni(0)-catalyzed Yamamoto coupling reaction of 2,7-dibromo-9,9-di-n-decylfluorene and 2,7-dibromo-9,9-bis[(S)-(+)-2-methylbutyl]fluorene comonomers. The selectivity of the SWNT chirality was mainly determined by the relative fraction of the achiral and chiral side groups. By a molecular mechanics simulation, the cooperative interaction between the fluorene moiety, alkyl side chain, and graphene wall were responsible for the recognition/dissolution ability of SWNT chirality. This is a first example describing the rational design and synthesis of novel fluorene-based copolymers toward the recognition/extraction of targeted (n, m) chirality of the SWNTs.

Supramolecular hybrid of gold nanoparticles and semiconducting single-walled carbon nanotubes wrapped by a porphyrin-fluorene copolymer.

We describe the design, synthesis, and characterization of a supramolecular hybrid of gold nanometals and semiconducting single-walled carbon nanotubes (SWNTs) wrapped by a porphyrin-fluorene copolymer (1), as well as fabrication of a thin-film transistor (TFT) device using the hybrid. Photoluminescence mapping revealed that the copolymer selectively dissolved SWNTs with chirality indices of (8,6), (8,7), (9,7), (7,6), and (7,5); dissolution of (8,6), and (8,7) SWNTs was especially efficient. The solubilized SWNTs were connected to gold nanoparticles (AuNPs) via a coordination bond to prepare a supramolecular hybrid composed of AuNPs/copolymer 1-wrapped SWNTs, which were studied by atomic force and scanning and transmission electron microscopies. A fabricated TFT device using the semiconducting SWNTs/copolymer 1 shows evident p-type transport with an On/Off ratio of ~10(5). The transport properties of the TFT changed after coordination of the AuNPs with the SWNTs/copolymer 1.

Supramolecular hybrid of metal nanoparticles and semiconducting single-walled carbon nanotubes wrapped by a fluorene-carbazole copolymer.

The first approach for the preparation of metal nanoparticle/semiconducting single-walled carbon nanotube (SWNT) hybrids with specified chirality is described. For this purpose, a copolymer of a fluorene derivative with two long-chain alkyl substituents and a carbazole derivative carrying a thiol group was used. The copolymer was found to selectively dissolve (7,6)- and (8,7)SWNTs, as determined by UV/Vis/NIR absorption and Raman spectroscopy and 2D photoluminescence mapping. Gold and silver nanoparticles with diameters of about 3.8 and about 3.2 nm, respectively, were readily attached along the SWNTs by means of coordination bonds between the nanoparticles and the thiol moieties on the copolymer, as revealed by atomic force and electron microscopy studies. The study provides a novel way to design and fabricate metal nanoparticle/semiconducting SWNT hybrids with specific nanotube chirality.

Controlled-release system mediated by a retro Diels-Alder reaction induced by the photothermal effect of gold nanorods.

Controlled-release systems that respond to external stimuli have received great interest for use in medical treatments such as for drug delivery to specific sites. Gold nanorods have an absorption band at the near-infrared region and convert the absorbed light energy into heat, which is known as a "photothermal effect". Therefore, gold nanorods are expected to act not only as an on-demand thermal converter for photothermal therapy but also as a controller of a drug-release system capable of responding to the near-infrared light irradiation. In this study, to construct a controlled-release system that responds to near-infrared light irradiation, we modified gold nanorods with polyethylene glycol (PEG) through Diels-Alder cycloadducts. When the modified gold nanorods were irradiated by near-infrared light, the PEG chains were released from the gold nanorods because of the retro Diels-Alder reaction induced by the photothermal effect. As a result of the PEG release, the gold nanorods formed aggregates. This type of controlled-release system coupled with the aggregate formation of the gold nanorods triggered by near-infrared light could be expanded to applications of gold nanorods in medical fields such as drug and photothermal therapy.

Controlled-release system of single-stranded DNA triggered by the photothermal effect of gold nanorods and its in vivo application.

Gold nanorods have strong absorption bands in the near-infrared region, in which light penetrates deeply into tissues. The absorbed light energy is converted into heat by gold nanorods, the so-called 'photothermal effect'. Hence, gold nanorods are expected to act not only as on-demand thermal converters for photothermal therapy but also as controllers of a drug-release system responding to irradiation by near-infrared light. To achieve a controlled-release system that can be triggered by light irradiation, double-stranded DNA (dsDNA) was modified on gold nanorods. When the dsDNA-modified gold nanorods were irradiated by near-infrared light, the single-stranded DNA (ssDNA) was released from gold nanorods due to the photothermal effect. The amount of released ssDNA was dependent upon the power and exposure time of light irradiation. Release of ssDNA was also observed in tumors grown on mice after light irradiation. Such a controlled-release system of oligonucleotide triggered by the photothermal effect could expand the applications of gold nanorods that have unique optical characteristics in medicinal fields.

Ultrafast spectroscopy and coherent acoustic phonons of Au-Ag core-shell nanorods.

We performed the first investigations of coherent acoustic phonons in Au-Ag core-shell nanorods, which were compared with the results of parental Au nanorods. Both breathing and extensional modes were observed in Au-Ag core-shell nanorods with ~11 nm Ag shell while only extensional modes were detected in other core-shell nanorods with 4-7 nm Ag shell. Young's modulus estimated from the oscillation period of extensional modes was found to be larger for Au-Ag core-shell nanorods with ~4 nm Ag shell, as compared with that of Au nanorods. The value of Young's modulus decreases with the increase of the Ag shell thickness and finally becomes smaller than that of Au nanorods. This phenomenon is interpreted in terms of the surface effects and the existence of grain boundaries in the lattice structure of Ag shell.

Gold-Silver and Gold-Palladium Alloy Nanoparticles as Mass-Probes for Immunosensing.

Silver or palladium shelled gold nanoparticles were fused into alloy nanoparticles by pulsed-laser irradiation. The alloy nanoparticles could carry antibodies on their surfaces without affecting their immune functionalities and interact selectively with antigens on a blotting membrane. Silver or palladium ions desorbed from the alloy nanoparticles as reporter ions upon the UV laser irradiation in a mass spectrometer.

Strong micro-dielectric environment effect on the band gaps of (n,m)single-walled carbon nanotubes.

The electronic states of carbon nanotubes are one of the most fundamental properties of the nanotubes. We now describe the finding that the band gaps of (n,m)SWNTs are strongly affected by the change in microdielectric environments around the isolated nanotubes. In situ photoluminescence (PL) spectroelectrochemistry of the films containing 15 isolated (n,m)single-walled carbon nanotubes (SWNTs) cast on ITO electrodes in organic solvents including DMSO, acetonitirile, DMF, THF, and chloroform was completed and then the oxidation and reduction potentials, and band gaps (ΔE(electr)) of the (n,m)SWNTs in the solvents were determined. We have discovered that the ΔE(electr) of the (n,m)SWNTs become greater as the solvent dielectric constants decreased, which is in sharp contrast to the optical band gaps (ΔE(opt)) that show virtually no solvent dependence. Such a strong solvent dependence of the electrochemical band gaps is due to the difference in the solvation energy of the charged SWNTs produced during the electrochemical processes. The ΔE(electr) of both mod types of the SWNTs, mod = 1 and mod = 2, linearly increased versus the reciprocal of the tube diameter, which agrees with the theory. Moreover, the states of the π-electrons in the SWNTs were evaluated from the dependence of the band gaps on the diameter of the SWNTs. Furthermore, the states of the π-electrons on the sidewalls of the SWNTs were evaluated using the γ(0) values, a parameter representing the measure of the stability or the degree of delocalization of π-electrons in the sidewall of the SWNTs, and revealed that the γ(0) values of the mod = 1 and mod = 2 SWNTs increased with a decrease in the dielectric constants of the solvents in the range of 38-79. This study has enabled us to understand the essential electronic properties of the carbon nanotubes.

Active accumulation of gold nanorods in tumor in response to near-infrared laser irradiation.

Gold nanorods, rod-shaped gold nanoparticles, have strong absorbance in the near-infrared region, and the absorbed light energy can be converted to heat, the so-called photothermal effect. The gold nanorods were coated with thermoresponsive polymers, which have different phase transition temperatures that were controlled by adding comonomers, N,N-dimethylacrylamide (DMAA) or acrylamide (AAm) to N-isopropylacrylamide (NIPAM). The phase transition temperatures of poly(NIPAM-DMAA) and poly(NIPAM-AAm)-coated gold nanorods were 38 and 41 °C, respectively, while polyNIPAM-coated gold nanorods showed phase transition at 34 °C. Irradiation of the coated gold nanorods using the near-infrared laser induced a decrease in their sizes due to a phase transition of the polymer layers. Poly(NIPAM-AAm)-coated gold nanorods stably circulated in the blood flow without a phase transition after intravenous injection. Irradiation of near-infrared light at a tumor after the injection resulted in the gold specifically accumulating in the tumor. This novel accumulation technique which combines a thermoresponsive polymer and the photothermal effect of the gold nanorods should be a powerful tool for targeted delivery in response to light irradiation.

Controlled release of PEG chain from gold nanorods: targeted delivery to tumor.

Gold nanorods exhibit strong absorbance of light in the near infrared region, which penetrates deeply into tissues. Since the absorbed light energy is converted into heat, gold nanorods are expected to act as a contrast agent for in vivo bioimaging and as a thermal converter for photothermal therapy. To construct a gold nanorod targeted delivery system for tumor a peptide substrate for urokinase-type plasminogen activator (uPA), expressed specifically on malignant tumors, was inserted between the PEG chain and the surface of the gold nanorods. In other words, we constructed PEG-peptide-modified gold nanorods. After mixing the gold nanorods with uPA, the PEG chain was released from the surface of the gold and subsequently nanorod aggregation took place. The formation of the aggregation was monitored as a decrease in light absorption at 900 nm. Tumor homogenate induced a significant decrease in this absorption. Larger amount of the PEG-peptide-modified gold nanorods bound to cells expressing uPA in vitro compared with control gold nanorods, which had scrambled sequence of the peptide. The PEG-peptide-modified gold nanorods showed higher accumulation in tumor than the control after they were injected intravenously into tumor-bearing mice, however, the density of the peptide on the surface of the gold nanorods was a key factor of their biodistributions. This targeted delivery system, which responds to uPA activity, is expected to be a powerful tool for tumor bioimaging and photothermal tumor therapy.

Efficient solubilization of single-walled carbon nanotubes using tea solutions.

Here we describe the finding that oolong tea and black tea act as excellent carbon nanotube solubilizers to produce individually dissolved single-walled carbon nanotubes (SWNTs), which have been revealed using visible-near IR absorption, photoluminescence and Raman spectroscopy as well as AFM study. The oolong tea solution has a tendency to individually dissolve metallic SWNTs that can be detected by 514 nm-excition, and in solution, the (11,3)SWNTs enriched and (11,0)SWNTs that are contained in as-produced SWNTs were very few. Black tea solution shows similar tendency, while the separation performance was weaker compared to that of the oolong tea. We also used epigallocatechin gallate to solubilize the SWNTs and shed light on the mechanism of the SWNT dissolution. Tea contains numerous components with antioxidant activities; therefore biological and biomedical applications using the present soluble nanotubes might be of interest.

[Theragnostic approaches using gold nanorods and near infrared light].

Gold nanoparticles have unique optical properties such as surface-plasmon and photothermal effects. Such properties have resulted in gold nanoparticles having several clinical applications. Gold nanorods (which are rod-shaped gold nanoparticles) show a surface plasmon band in the near-infrared region. They have therefore been proposed as contrast agents for bioimaging, or as heating devices for photothermal therapy. Polyethylene glycol-modified gold nanorods systemically administrated into mice can be detected with integrating sphere, and the stability of the gold nanorods in blood flow evaluated. After intravenous injection of gold nanorods followed by near-infrared laser irradiation, significant tumor damage triggered by the photothermal effect was observed. To deliver gold nanorods to the target tissue, thermosensitive polymer gel-coated gold nanorods were prepared. After intravenous injection of the gel-modified gold nanorods and irradiation of the tumor, a larger amount of gold was detected in the irradiated tumor than in the non-irradiated tumor. This effect is due to the hydrophobic interaction between the cellular membrane or the extracellular matrix and the gel surfaces induced by the photothermal effect. Furthermore, the photothermal effect enhanced the permeability of the stratum corneum of the skin. As a result of treatment of the skin with ovalbumin and gold nanorods followed by near-infrared light irradiation, a significant amount of protein was detected in the skin. The gold nanorods therefore showed several functions as a photothermal nanodevice for bioimaging, thermal therapy, and a drug delivery system.

PNIPAM gel-coated gold nanorods for targeted delivery responding to a near-infrared laser.

Gold nanorods can be used as photothermal converters, permitting near-infrared (NIR) light to be transmitted deep into tissues without causing damage. We prepared hybrid nanorods with a core-shell structure, i.e., a single gold nanorod encapsulated in a poly (N-isopropylacrylamide) nanogel. Hybrid nanorods demonstrated remote, reversible, pulsatile phase transition and in vivo action after irradiation using a NIR laser.

Characterization of silver ions adsorbed on gold nanorods: surface analysis by using surface-assisted laser desorption/ionization time-of-flight mass spectrometry.

Surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-MS) indicated AgBr2-, which adsorbed on gold nanorod surfaces, was a key material to control the anisotropic growth of gold nanorods.

Imaging mass spectrometry of gold nanoparticles in a tissue section as an immunohistochemical staining mass probe.

For analysis of low abundance peptides in a tissue section, immunohistochemical staining through antibody-antigen interaction is a usual technique. The antibody is conjugated with a probe moiety that aids in highly sensitive detection. Gold nanoparticles, which show excellent chemical stability and variation of surface modifications, are expected to act as a sensitive mass probe to desorb gold ions (Au+ , Au2 + , Au3 + ) that are distinguishable from fragment ions from organic molecules. Here, green fluorescent proteins (GFP) in a tissue section of a transgenic zebrafish were detected by the gold mass probe conjugated with antibodies. Due to the efficient ionization and desorption of gold ions, imaging mass spectrometry of Au2 + ions indicated the distribution of gold nanoparticles stained in a tissue section, and the mass signal distribution was consistent with the area where the GFP-expressing cells were distributed. Conventional immunofluorescence techniques showed intense autofluorescence that come from intrinsic fluorophores in the tissue section. In contrast, the gold nanoparticles acted as an immunostaining mass probe that displayed significantly lower background signals.

In vivo monitoring of intravenously injected gold nanorods using near-infrared light.

Gold nanorods showing surface plasmon (SP) bands in the near-IR region are used as bioimaging probes that respond to near-IR light in mice. The SP bands of intravenously injected polyethylene glycol-modified gold nanorods are directly monitored from the mouse abdomen by using a spectrophotometer equipped with an integrating sphere. The absorbance at 900 nm from the gold nanorods immediately increases after injection and reaches a plateau. The injection of phosphatidylcholine-modified gold nanorods also increases the absorbance at 900 nm, but the absorbance decreases single exponentially with a 1.3-min half-life. In vivo spectral changes of gold nanorods depend on the surface characteristics, and can be observed in real time using simple spectroscopic measurements.