Self-assembly of poly(ethylenimine)-capped Au nanoparticles at a toluene-water interface for efficient surface-enhanced raman scattering.

Branched poly(ethylenimine) (PEI)-capped Au nanoparticles are prepared at room temperature using PEI as the reductant of hydrogen tetrachloroaurate (HAuCl4). The size of Au nanoparticles, ranging from 10 to 70 nm, is readily controlled by varying the relative amount of PEI used initially versus HAuCl4. The PEI-capped Au nanoparticles are further demonstrated to be assembled into a large area of 2-D aggregates at a toluene-water interface either by heating the mixture or by adding benzenethiol to the toluene phase at room temperature. Both films are quite homogeneous, but Au nanoparticles appear to be more closely packed in the film assembled via the mediation of benzenethiol. The optical property of the PEI-capped Au films is controlled by the amount of benzenethiol added to the toluene phase. The obtained large area of PEI-capped Au film exhibits strong SERS activity of benzenethiol and also exhibits a very intense SERS spectrum of 4-nitrobenzenethiol via a place-exchange reaction that takes place between benzenethiol and 4-nitrobenzenethiol. Because the proposed method is cost-effective and is suitable for the mass production of diverse Au films irrespective of the shapes of the underlying substrates, it is expected to play a significant role in the development of optical nanotechnology especially for surface plasmon-based analytical devices.

Easy deposition of Ag onto polystyrene beads for developing surface-enhanced-Raman-scattering-based molecular sensors.

We describe a very simple electroless plating method that can be used to prepare Ag-coated polystyrene beads. Robust Ag nanostructures are reproducibly fabricated by soaking polystyrene beads in ethanolic solutions of AgNO(3) and butylamine. When the molar ratio of butylamine to AgNO(3) is far below 1.0, distinct nanosized Ag particles are formed on the polystyrene beads, but by increasing the amount of butylamine, network-like Ag nanostructures are formed that possess very broad UV/vis absorption characteristics extending from the near-UV to near-infrared regions. In conformity with the UV/vis absorption characteristics, the Ag-deposited polystyrene beads were highly efficient surface-enhanced Raman scattering (SERS) substrates, with an enhancement factor estimated using 4-aminobenzenethiol (4-ABT) as a model adsorbate to be larger than 1.1x10(6). On the basis of the nature of the SERS peaks of 4-ABT, those Ag-deposited polystyrene beads were confirmed, after attaching biotin groups over 4-ABT, to selectively recognize streptavidin molecules down to concentrations of 10(-11) g mL(-1) (i.e., approximately 0.2 pM). Since a number of different molecules can be used as SERS-marker molecules (such as 4-ABT), multiple bioassays are readily accomplished via SERS after attaching appropriate host or guest molecules onto them.

A practical procedure for producing silver nanocoated fabric and its antibacterial evaluation for biomedical applications.

A novel and universal procedure has been developed for producing nanosized stable silver particles on cotton fabrics in a simple and cost-effective manner with complete control of the silver loading level on the fabrics; the antibacterial effect of Ag-nanocoated fabrics on various bacteria was evaluated by growth inhibition; for biomedical applications, skin irritation tests on guinea pigs were performed and no side effects were observed.

A facile deposition of silver onto the inner surface of a glass capillary tube for micro-surface-enhanced Raman scattering measurements.

Silver can be deposited very efficiently onto glass substrates using only ethanolic solutions of AgNO3 and butylamine. This paper reports that the inner surface of a glass capillary can also be coated evenly with silver by shaking it after soaking in ethanolic solutions of AgNO3 and butylamine; the silver deposited outside the capillary can be easily wiped off with cotton wool before drying. The grain size of the silver deposited onto the inner surface can be readily controlled within the range from 20 to 100 nm by varying the relative molar ratio of butylamine and AgNO3 used as reactants. Due to its nanoaggregated structure, the Ag coated capillary is a very efficient surface-enhanced Raman scattering (SERS) active substrate, particularly usable in the microanalysis of chemicals; the detection limit of adenine is as low as 1.0x10(-7) M based on a signal-to-noise (S/N) ratio of 3. Since the proposed method is cost-effective and is suitable for the mass production of Ag coated capillaries, we fully expect it to play a significant role in the development of SERS based microchip analyzers and even in the fabrication of Ag coated hollow glass waveguides.

microAg particle-based molecular sensing/recognition via surface-enhanced Raman spectroscopy.

In this study, we demonstrate that powders of commercially available 2-microm-sized Ag (microAg) 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. The Raman spectrum of organic monolayers on powdered silver is a SERS spectrum. The agglomeration of microAg particles in a highly concentrated buffer solution could be prevented by the deposition of polar molecules like 1,4-phenylenediisocyanide (1,4-PDI), and mixed self-assembled monolayers of 1,4-PDI and N-(+)-biotinyl-6-aminocaproic acid on microAg particles were then confirmed via the SERS of 1,4-PDI to selectively recognize the avidin arrays formed on a separate biotinylated substrate. According to a dose response curve, avidin at >10(-6)g/mL could be easily identified by the present method. In addition, the non-specific adsorption of microAg particles was found to be negligibly small, probably because the Ag particles were too heavy to be retained on organic substrates solely by non-specific interaction.

Facile method to prepare surface-enhanced-Raman-scattering-active Ag nanostructures on silica spheres.

In this paper we describe a very simple electroless plating method used to prepare Ag-coated silica beads. Robust Ag nanostructures can be reproducibly fabricated by soaking silica beads in ethanolic solutions of AgNO(3) and butylamine. When the molar ratio of butylamine to AgNO(3) is far below 1.0, distinct nanosized Ag particles are formed on the silica beads, but by increasing the amount of butylamine, network-like Ag nanostructures are formed that possess very broad UV/vis absorption characteristics extending from near-UV to near-infrared regions. In conformity with the UV/vis absorption characteristics, the Ag-deposited silica beads were highly efficient surface-enhanced Raman scattering (SERS) substrates, with the enhancement factor estimated using benzenethiol as a model adsorbate to be larger than 10(6). Since another silica layer can be readily deposited onto the Ag surface, the Ag-coated silica beads should be invaluable in the development of SERS-based biosensors.

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.

Novel fabrication of Ag thin film on glass for efficient surface-enhanced Raman scattering.

This paper describes a very simple electroless-plating method used to prepare optically tunable nanostructured Ag films. Very stable Ag films can be reproducibly fabricated simply by soaking glass substrates in ethanolic solutions of AgNO3 and butylamine. The grain size of silver can be readily controlled to range from 20 to 150 nm, and these nanostructural features correlated well with their UV/vis absorption characteristics, as well as with their surface-enhanced Raman scattering (SERS) activities. It is also very advantageous that the Ag films prepared exhibit very even SERS activity over an area up to hundreds thousand square-micrometers, and the enhancement factor estimated using benzenethiol as a prototype adsorbate reaches approximately 2 x 10(5). Since the proposed method is cost-effective and is suitable for the mass production of diverse Ag films irrespective of the shapes of the underlying substrates, it is expected to play a significant role in the development of surface plasmon-based analytical devices.

New strategy for ready application of surface-enhanced resonance Raman scattering/surface-enhanced Raman scattering to chemical analysis of organic films on dielectric substrates.

Dropping of appropriately concentrated AgNO3 and NaBH4 solutions, as well as laser-ablated Ag sols, onto organic molecules results in the formation of aggregated Ag nanoparticles that can induce surface-enhanced Raman scattering (SERS) for the molecules. The addition of flocculating agents such as alkali halides can further increase the Raman signals. We demonstrate in this work that Raman spectra can be obtained even for 0.01 monolayers of R6G on Si simply by spreading silver nanoparticles and/or fabricating Ag nanoparticles and nanoaggregates at the gaps and vacant sites of R6G molecules. The application prospect of the present methodology is extremely high, not only because of its simplicity but also because of the fact that the observation of vibrational spectra is one of the most incisive methods for understanding the chemical and physical phenomena on a variety of surfaces.

Gelation of octadecyltrimethoxysilane at the air/water interface: effect of perfluorotetradecanoic acid and divalent cadmium ions.

The effect of an acidic surfactant, perfluorotetradecanoic acid (PFTDA), on the gelation of octadecyltrimethoxysilane (ODTMS) monolayers at the air/water interface was investigated using a Langmuir film balance, attenuated total reflection (ATR) infrared spectroscopy, and atomic force microscopy (AFM). The gelation of ODTMS was greatly accelerated by adding only 2 mol % PFTDA into the monolayer; in this case, the hydrolysis of ODTMS was completed within a few minutes, whereas otherwise it took 18 h. ATR-IR spectroscopy clearly demonstrated that the gelation of a 49:1 ODTMS/PFTDA monolayer proceeded on a pure water subphase mainly via hydrolysis followed by condensation. In the presence of PFTDA, the local acidity at specific sites is presumed to be very high, catalyzing the hydrolysis of ODTMS. The extremely fast rate is attributed not only to the lateral diffusion of PFTDA in the mixed monolayer but also to proton propagation where the proton(s) involved in the initial hydrolysis are transposed rapidly to the nearby methoxy groups for subsequent hydrolysis. The catalytic activity of PFTDA can be neutralized, however, simply by the addition of multivalent metallic ions such as Cd2+ to form saltlike complexes with PFTDA; the rate of 2-D sol-gel processes can thus be easily regulated by a minute amount of PFTDA and/or Cd2+ added into the reaction system.