Kuan Soo Shin

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.

Surface-Enhanced Raman Scattering of 4-Aminobenzenethiol in Ag Sol: Relative Intensity of a1- and b2-Type Bands Invariant against Aggregation of Ag Nanoparticles

4-Aminobenzenthiol (4-ABT) is an unusual molecule, showing variable surface-enhanced Raman scattering (SERS) spectra depending upon measurement conditions. In an effort to reduce ambiguity and add clarity, we have thus conducted an ultraviolet-visible (UV-vis) extinction measurement, along with Raman scattering measurement, after adding 4-ABT into aqueous Ag sol. Upon the addition of 4-ABT, the surface plasmon absorption band of Ag at 410 nm gradually diminished and, concomitantly, a weak and broad band developed at longer wavelengths, obviously because of the aggregation of Ag nanoparticles. At the same time, the Raman scattering peaks of 4-ABT varied in intensity as the Ag particles proceeded to form aggregates. A close examination revealed that the peak intensity of the ring 7a band of 4-ABT, a typical a(1) vibrational mode, could be correlated with the UV-vis extinction of the Ag sol measured at the excitation laser wavelength. In a separate Raman measurement conducted using sedimented Ag colloidal particles, 4-ABT was found not to be subjected to any surface-induced photoreaction, implying that all of the observable Raman peaks were, in fact, solely due to 4-ABT on Ag. The intensities of the b(2)-type bands, such as the ring 3, 9b, and 19b modes of 4-ABT, were then analyzed and found to be invariant with respect to the 7a band, irrespective of the extent of Ag aggregation as far as at a fixed excitation wavelength. The intensity ratio of the b(2)-type/7a bands would then reflect the extent of the chemical enhancement that was involved in the SERS of 4-ABT in aggregated Ag sol.

Silver-Coated Silica Beads Applicable as Core Materials of Dual-Tagging Sensors Operating via SERS and MEF

We have developed dual-tagging sensors, operating via both surface-enhanced Raman scattering (SERS) and metal-enhanced fluorescence (MEF), composed of silver-coated silica beads onto which were deposited SERS markers and dye-grafted polyelectrolytes, for multiplex immunoassays. Initially, a very simple electroless-plating method was applied to prepare Ag-coated silica beads. The Raman markers were then assembled onto the Ag-coated silica beads, after which they were brought to stabilization by the layer-by-layer deposition of anionic and cationic polyelectrolytes including a dye-grafted polyelectrolyte. In the final stage, the dual-tagging sensors were assembled onto them with specific antibodies (antihuman-IgG or antirabbit-IgG) to detect target antigens (human-IgG or rabbit-IgG). The MEF signal was used as an immediate indicator of molecular recognition, while the SERS signals were subsequently used as the signature of specific molecular interactions. For this reason, these materials should find wide application, especially in the areas of biological sensing and recognition that rely heavily on optical and spectroscopic properties.

Adsorption and aggregation characteristics of silver nanoparticles onto a poly(4-vinylpyridine) film: a comparison with gold nanoparticles

The adsorption and aggregation processes of Ag nanoparticles versus Au nanoparticles onto a poly(4-vinylpyridine) (P4VP) surface has been investigated by means of quartz crystal microbalance (QCM), atomic force microscopy (AFM), and Raman scattering spectroscopy. Both the QCM and AFM data indicated that the citrate-reduced Ag and Au nanoparticles are adsorbed onto P4VP, forming only approximately 30% and approximately 17% of surface coverage, respectively, even after 6 h of adsorption in solution. The P4VP film was too thin to observe its normal Raman spectrum, but the Raman peaks of P4VP could be detected upon the adsorption of Ag (or Au) nanoparticles onto the film, due to the surface-enhanced Raman scattering (SERS) effect associated with the localized surface plasmon of Ag (or Au) nanoparticles. When in contact with the solution of Ag (or Au) nanoparticles, the SERS peaks of P4VP thus increased linearly as a function of time, in a manner similar to that shown by the QCM and AFM data. In the interim, however, as the sol solution was drained, the SERS signal of P4VP was intensified about twice probably due to the aggregation of nanoparticles. Eventually, the SERS signal derived from the Ag nanoparticles became two times stronger than that from the same number of Au nanoparticles, at least under the 632.8 nm excitation, suggesting that Ag nanoparticles must be more advantageous than Au nanoparticles in elucidating by SERS the physicochemical characteristics of organic/polymeric surfaces and suggesting their likely advantages in metallic staining in immunoassays.

Silanization of Ag-Deposited Magnetite Particles: An Efficient Route to Fabricate Magnetic Nanoparticle-Based Raman Barcode Materials

Silica-coated Ag nanostructures usable as magnetic nanoparticle-based Raman barcode materials were developed. Initially, 283 nm sized spherical magnetite particles composed of 13 nm sized superparamagnetic Fe(3)O(4) nanoparticles were synthesized, and silver deposition was conducted using butylamine as the reductant of AgNO(3) in ethanol. The Ag-deposited Fe(3)O(4) (Fe(3)O(4)@Ag) particles are found to be efficient surface-enhanced Raman scattering (SERS) substrates with the enhancement factor at 632.8 nm excitation to be about 3 x 10(6). After SERS markers such as benzenethiol, 4-mercaptotoluene, 4-aminobenzenethiol, and 4-nitrobenzenethiol were adsorbed onto the silver surface, poly(allylamine hydrochloride) (PAH) was coated onto them using the layer-by-layer deposition method such that a subsequent base-catalyzed silanization could readily form a 60 nm thick silica shell around the PAH layer by a biomimetic process. The cross-linked silica shells effectively prevented the SERS-marker molecules from being liberated from the surface of the Fe(3)O(4)@Ag particles. Although the gram magnetization decreased nearly to one-half of the initial value because of coating with silver and silica, the remaining magnetization was nonetheless strong enough for the silica-coated Fe(3)O(4)@Ag particles to be used as barcode materials operating via SERS.

Selective detection of aqueous nitrite ions by surface-enhanced Raman scattering of 4-aminobenzenethiol on Au.

In this work, we have devised a selective nitrite-ion detection method based on the surface-enhanced Raman scattering (SERS) of 4-aminobenzenethiol (4-ABT) on Au. This is possible because, firstly, SERS is a very surface-sensitive technique with monolayer detection capability, and secondly, the amine group of 4-ABT reacts readily with nitrites in acidic media, forming a diazonium group, which can subsequently form an azo bond by reacting with a variety of benzene derivatives. From the peak intensity of the diazonium group, the presence of nitrite ions above 20 μM can be identified readily. From the peak intensity of the azo moiety alone, it is even possible to detect nitrite ions at concentrations as low as 5 μM, without interference from other anions. This work clearly illustrates the usefulness of SERS in environmental science research.

Shock tube study of the rate coefficient of H + O2 → OH + O

Abstract The H + O 2 → OH + O reaction was reexamined by monitoring OH radical concentrations using cw UV laser absorption spectroscopy in H 2 -O 2 -Ar test gas heated by 1850-3550 K shock waves. The rate coefficient expression (1.0 ± 0.1) × 10 14 × exp(−7690 ± 250 K/T) cm 3 mol −1 s −1 was derived. Fitting the present data together with those of Shin and Michael [J. Chem. Phys. 95 (1991) 262] gave k 4 = (7.6 ± 0.7) × 10 13 exp(−7065 ± 140 K/ T ) cm 3 mol −1 s −1 for the temperature range 1100–3550 K. The reverse reaction rate coefficient was found to be (9.3 ± 0.8) × 10 12 cm 3 mol −1 s −1 independent of temperature from 1850 to 3550 K.

Poly(ethylenimine)-stabilized silver nanoparticles assembled into 2-dimensional arrays at water-toluene interface.

A one-pot, size-controlled preparation of amine-functionalized silver nanoparticles is possible using poly(ethylenimine) (PEI) as a reducing and a stabilizing agent simultaneously. The PEI-stabilized Ag nanoparticles thus prepared in aqueous phase can further be assembled into 2-dimensional (2-D) arrays at the water-toluene interface by heating. By the addition of benzenethiol (BT) into the toluene, a more robust 2-D film is formed, not only at the interface but also on the inner surface of the sampling bottle. The latter Ag film can also be formed, through brief contact with the mixture, on glass slides and even on dielectric beads and cotton fabrics. These Ag-coated films are highly surface-enhanced Raman scattering (SERS) active and also exhibit a very intense SERS spectrum of 4-aminobenzenethiol (4-ABT) or 4-nitrobenzenethiol (4-NBT) via a place-exchange reaction that takes place between BT and 4-ABT or 4-NBT. In addition, the Ag-coated fabrics would be used as antibacterial gauzes for the treatment of burned skin and also as antistatic mats for the ready dissipation of electrical energy buildup. The present electroless deposition method of Ag is thus expected to serve as a technique in high demand in various fields.

Rhodamine B isothiocyanate-modified Ag nanoaggregates on dielectric beads: A novel surface-enhanced Raman scattering and fluorescent imaging material

Rhodamine B isothiocyanate (RhBITC) is a prototype dye molecule that is widely used as a fluorescent tag in a variety of biological applications. We report in this work that once RhBITC is adsorbed onto Ag on silica or polystyrene beads, it exhibits not only a strong surface-enhanced Raman scattering (SERS) signal but also a measurable amount of fluorescence. The RhBITC-modified Ag-deposited silica or polystyrene beads disperse well in ethanol, and they are also readily coated in water with polyelectrolytes for their further derivatization with biological molecules of interest that can bind to target molecules. The application prospects of these materials are thus expected to be very high especially in the areas of biological sensing and recognition that rely heavily on optical and spectroscopic properties. For instance, on the basis of the nature of the SERS peaks of RhBITC, those Ag-deposited silica or polystyrene beads were confirmed, after attaching biotin groups over RhBITC, to selectively recognize streptavidin molecules down to concentrations of 10(-13)M based on a signal-to-noise ratio of 3. The biotin-streptavidin interaction was also confirmed from the photoluminescence of RhBITC.

Novel fabrication and catalytic application of poly(ethylenimine)-stabilized gold–silver alloy nanoparticles

Novel synthesis of amine-stabilized Au–Ag alloy nanoparticles with controlled composition has been devised using poly(ethylenimine) (PEI) as a reducing and a stabilizing agent simultaneously. The composition of Au–Ag alloy nanoparticles was readily controlled by varying the initial relative amount of HAuCl4 and AgNO3. Due to the presence of abundant amine functional groups in PEI, which could act as the dissolving ligand for AgCl, the precipitation problem of Ag+ in the presence of Cl− from the gold salt was avoided. On this basis, the relatively high concentrations of HAuCl4 and AgNO3 salts were used for the fabrication of Au–Ag alloy nanoparticles. The PEI thus plays triple roles in this study that include the co-reducing agents for HAuCl4 and AgNO3, the stabilizing agents for Au–Ag alloy nanoparticles, and even the dissolving agents for AgCl. As a novel material for use in catalysis, the Au–Ag alloy nanoparticles including pure Au and Ag samples were exploited as catalysts for the reduction of 4-nitrophenol in the presence of NaBH4. As the Au content was increased in the Au–Ag alloy nanoparticles, the rate constant of the reduction was exponentially increased from pure Ag to pure Au.