Ki-Seok Jeon

Nanogap-engineerable Raman-active nanodumbbells for single-molecule detection

Surface-enhanced Raman scattering (SERS)-based signal amplification and detection methods using plasmonic nanostructures have been widely investigated for imaging and sensing applications. However, SERS-based molecule detection strategies have not been practically useful because there is no straightforward method to synthesize and characterize highly sensitive SERS-active nanostructures with sufficiently high yield and efficiency, which results in an extremely low cross-section area in Raman sensing. Here, we report a high-yield synthetic method for SERS-active gold-silver core-shell nanodumbbells, where the gap between two nanoparticles and the Raman-dye position and environment can be engineered on the nanoscale. Atomic-force-microscope-correlated nano-Raman measurements of individual dumbbell structures demonstrate that Raman signals can be repeatedly detected from single-DNA-tethered nanodumbbells. These programmed nanostructure fabrication and single-DNA detection strategies open avenues for the high-yield synthesis of optically active smart nanoparticles and structurally reproducible nanostructure-based single-molecule detection and bioassays.

Highly uniform and reproducible surface-enhanced Raman scattering from DNA-tailorable nanoparticles with 1-nm interior gap

An ideal surface-enhanced Raman scattering (SERS) nanostructure for sensing and imaging applications should induce a high signal enhancement, generate a reproducible and uniform response, and should be easy to synthesize. Many SERS-active nanostructures have been investigated, but they suffer from poor reproducibility of the SERS-active sites, and the wide distribution of their enhancement factor values results in an unquantifiable SERS signal. Here, we show that DNA on gold nanoparticles facilitates the formation of well-defined gold nanobridged nanogap particles (Au-NNP) that generate a highly stable and reproducible SERS signal. The uniform and hollow gap (∼1 nm) between the gold core and gold shell can be precisely loaded with a quantifiable amount of Raman dyes. SERS signals generated by Au-NNPs showed a linear dependence on probe concentration (R(2) > 0.98) and were sensitive down to 10 fM concentrations. Single-particle nano-Raman mapping analysis revealed that >90% of Au-NNPs had enhancement factors greater than 1.0 × 10(8), which is sufficient for single-molecule detection, and the values were narrowly distributed between 1.0 × 10(8) and 5.0 × 10(9).

Tuning and Maximizing the Single-Molecule Surface-Enhanced Raman Scattering from DNA-Tethered Nanodumbbells

We extensively study the relationships between single-molecule surface-enhanced Raman scattering (SMSERS) intensity, enhancement factor (EF) distribution over many particles, interparticle distance, particle size/shape/composition and excitation laser wavelength using the single-particle AFM-correlated Raman measurement method and theoretical calculations. Two different single-DNA-tethered Au-Ag core-shell nanodumbbell (GSND) designs with an engineerable nanogap were used in this study: the GSND-I with various interparticle nanogaps from ∼4.8 nm to <1 nm or with no gap and the GSND-II with the fixed interparticle gap size and varying particle size from a 23-30 nm pair to a 50-60 nm pair. From the GSND-I, we learned that synthesizing a <1 nm gap is a key to obtain strong SMSERS signals with a narrow EF value distribution. Importantly, in the case of the GSND-I with <1 nm interparticle gap, an EF value of as high as 5.9 × 10(13) (average value = 1.8 × 10(13)) was obtained and the EF values of analyzed particles were narrowly distributed between 1.9 × 10(12) and 5.9 × 10(13). In the case of the GSND-II probes, a combination of >50 nm Au cores and 514.5 nm laser wavelength that matches well with Ag shell generated stronger SMSERS signals with a more narrow EF distribution than <50 nm Au cores with 514.5 nm laser or the GSND-II structures with 632.8 nm laser. Our results show the usefulness and flexibility of these GSND structures in studying and obtaining SMSERS structures with a narrow distribution of high EF values and that the GSNDs with < 1 nm are promising SERS probes with highly sensitive and quantitative detection capability when optimally designed.

Silencing of Metallic Single-Walled Carbon Nanotubes via Spontaneous Hydrosilylation†

Noncovalent approaches havebeen considered appropriate to softly modify the surfaceproperties of carbon nanotubes sufficient for chemical andbiological applications. However, the importance of thedevelopment of covalent bond-forming chemical reactionshas recently been refocused on, since the strategically drivencovalent bonds are known to promptly alter the electricalproperties of carbon nanotubes to secure a high population ofsemiconducting carbon nanotubes from mixtures.

A facile, one-pot synthesis of ultra-long nanoparticle-chained polyaniline wires

Ultra-long nanoparticle-chained polyaniline wires (NPWs) including one-dimensional (1D) Au nanoparticle (NP) chains were synthesized by a one-pot assembly process in water interfaced with a high-density aniline solution. The NPWs exhibited polarization-dependent surface-enhanced Raman scattering along the long axis, which was explained by the field enhancement effect by the embedded Au NP chains. Furthermore, the NPW-based devices exhibited a bistability in their electrical transport properties, which was attributed to charge trapping by the embedded Au NPs. This method allows us to mass-produce NPWs, which should have potential for various optoelectronic applications.

A study on the optoelectronic properties of CuIn1-xGaxSe2 grain boundaries by electrostatic force microscopy

We investigated the electric charge distribution in CuIn1-x GaxSe2 films, with particular emphasis on grain boundaries. Hall measurements, electron beam-induced current and optical beam-induced current measurements are commonly used for the characterization of solar cells, but they do not provide the resolution necessary for the investigation of individual grain boundaries. Therefore, we used an electrostatic force microscopy (EFM) capable of probing the electric charge distribution and the potential gradient of sample surface. EFM experiments were performed at 300 K with a Dimension trade 3100 scanning probe microscope (Digital Instruments). We suggest that grain boundaries should be electron-accumulated area and the inner grain area be the hole-accumulated area. The potential variations between the grain boundaries and inner grain area were estimated to be 60~180 meV