Hyang Bong Lee

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.

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.

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.

Silver salts of aromatic thiols applicable as core materials of molecular sensors operating via SERS and fluorescence

We have developed dual-tagging sensors, operating via both Raman and fluorescence spectroscopy, composed of silver aromatic thiolates (AgSRs) modified with fluorescent organic dye for multiplex immunoassays. Owing to the photo-induced production of SERS-active Ag nanoparticles, AgSRs exhibit the surface-enhanced Raman scattering (SERS) spectra of corresponding thiols. The fluorescence dye-modified AgSRs were accordingly fabricated using dye-grafted polyelectrolytes during layer-by-layer deposition of cationic and anionic polyelectrolytes onto AgSRs. In the final stage, the tagging sensors assembled with either specific biotin group or specific antibodies (anti-h-IgG or anti-r-IgG) were employed to detect either streptavidin molecules or target antigens (h-IgG or r-IgG), respectively. Since numerous AgSRs can be used as the core materials, multiple bioassays are expected to be accomplishable using the present methodology. The fluorescence signal may be used as an immediate indicator of molecular recognition, while the SERS signals can be used subsequently as the signature of specific molecular interactions.

Silver-coated dye-embedded silica beads: a core material of dual tagging sensors based on fluorescence and Raman scattering.

We have developed a new type of dual-tag sensor for immunoassays, operating via both fluorescence and surface-enhanced Raman scattering (SERS). A one-shot fluorescence image over the whole specimen allows us to save considerable time because any unnecessary time-consuming SERS measurements can be avoided from the signature of the fluorescence. Dye-embedded silica beads are prepared initially, and then SERS-active silver is coated onto them via a very simple electroless-plating method. The Raman markers are subsequently assembled onto the Ag-coated silica beads, after which they are stabilized by silanization via a biomimetic process in which a poly(allylamine hydrochloride) layer formed on the Raman markers by a layer-by-layer deposition method acting as a scaffold for guiding silicification. In the final stage, specific antibodies are attached to the silica surface in order to detect target antigens. The fluorescence signal of the embedded dye can be used as a fast readout system of molecular recognition, whereas the SERS signals are subsequently used as the signature of specific molecular interactions. In this way, the antibody-grafted particles were found to recognize antigens down to 1 × 10(-10) g mL(-1) solely by the SERS peaks of the Raman markers.

Novel fabrication of au nanoparticle films on planar and curved surfaces of glass and fiber materials.

We have discovered that poly(ethylenimine)-capped Au nanoparticles can be fabricated into a two-dimensional film by adding toluene into the colloidal solution. The Au nanoparticle film layer creeps up the glass wall of the vial after adding benzenethiol into the toluene phase, thus allowing for a robust Au film to form, not only on the planar and curved surfaces of glasses but also on the inside surfaces of capillaries. First, by depositing onto planar glasses, we demonstrate that the Au films are highly surface-enhanced-Raman-scattering (SERS)-active. Second, by depositing Au nanoparticles onto the inside surface of a glass capillary, we demonstrate that benzenethiol molecules present on Au nanoparticles can be easily desorbed by borohydride, maintaining the initial SERS activity. The Au-coated capillary would then be used, for instance, to investigate the relative adsorption strength of diverse organics in situ by SERS. Third, we demonstrate that Au nanoparticle films can be formed even on the surfaces of dielectric beads and flexible fabrics.

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 1.0 × 10−7 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.