Browsing by Author "Lee, Seunghyun"

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    A single molecule immunoassay by localized surface plasmon resonance
    (IOP Publishing, 2010) Mayer, Kathryn M.; Hao, Feng; Lee, Seunghyun; Nordlander, Peter; Hafner, Jason H.
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    Interfacial Chemistry in Nanophotonics
    (2012) Lee, Seunghyun; Hafner, Jason H.
    Nanophotonics, especially plasmonics is a kind of very active research area, which deals with the interaction behavior between electromagnetic radiation and metallic nanostructures. It has attracted enormous attention over recent decades due to its great potential of ripple effects on electronics, energy, environmental, and medical industries as well as scientific interests. In particular, noble metal nanoparticles exhibit localized surface plasmon resonance (LSPR), which is the collective oscillating excitation of the free electrons on the surface of metal nanoparticles when light is incident on the particle. The LSPR extinction peak is very sensitive to the dielectric environment near the particle surface and can be tailored by the particle's sizes and shapes. These properties allow LSPR-active substrate using plasmonic gold nanoparticles to be a great transducer for biosensing with real-time and label-free measurement. In addition, the plasmonic gold nanoparticles such as gold nanorod and bipyramid are prepared by the seed-mediated and surfactant-directed method based on the cetyltrimethylammonium bromide (CTAB), which has a great influence on the synthesis. In the growth mechanism, it is believed that CTAB interacts with different facet and defects on the growing nanoparticles to produce different rate of gold ion reduction onto the nanoparticles to generate anisotropic growth. Therefore, CTAB layer is greatly interesting because the modification of nanoparticles surface chemistry is essential to biological targeting, film formation, and assembly of complex structures. Surface enhanced Raman spectroscopy (SERS) of gold nanorods in CTAB solution has been used to analyze a surfactant structural transition based on the distance dependent electromagnetic enhancement. As the surfactant concentration in the gold nanorod solution was reduced, a structural transition in the surfactant layer between 2 mM and 5 mM CTAB solution was observed through a sudden increase in the signal from the alkane chains. A structural transition in the CTAB layer that stabilizes gold nanorods was identified by comparing the intensities of different bands within the CTAB molecule. Therefore, the surface manipulation and analysis of the nanostructures and their interface with controlled environment provide important insight into their structural function and interpretation, and many opportunities for biomedical applications.
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    Manipulation of gold nanorod surfaces with self-assembled monolayers
    (2008) Lee, Seunghyun; Hafner, Jason H.
    Gold nanorods exhibit spectral peaks due to localized surface plasmon resonances (LSPR) which are sensitive to the optical properties of their environment. Accordingly, molecular binding to the surface of gold nanorods generates a shift in the spectral peak. In addition, self-assembled monolayers (SAMs) with well-defined and controllable molecular architectures on gold nanorod surfaces enable us to create a gold nanorod biosensor based on LSPR. We have demonstrated a simple immunoassay based on LSPR of gold nanorods. Here we report on the optimization of SAMs on gold nanorods for sensing applications, and on the kinetics of their formation, etching, and molecular displacement on gold nanorod surfaces.
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    Monodispersed mesoscopic star-shaped gold particles via silver-ion-assisted multi-directional growth for highly sensitive SERS-active substrates
    (Springer Nature, 2024) Kim, Sumin; Yoo, Sunghoon; Nam, Dong Hwan; Kim, Hayoung; Hafner, Jason H.; Lee, Seunghyun
    Surface-enhanced Raman scattering (SERS) exploits localized surface plasmon resonances in metallic nanostructures to significantly amplify Raman signals and perform ultrasensitive analyses. A critical factor for SERS-based analysis systems is the formation of numerous electromagnetic hot spots within the nanostructures, which represent regions with highly concentrated fields emerging from excited localized surface plasmons. These intense hotspot fields can amplify the Raman signal by several orders of magnitude, facilitating analyte detection at extremely low concentrations and highly sensitive molecular identification at the single-nanoparticle level. In this study, mesoscopic star-shaped gold particles (gold mesostars) were synthesized using a three-step seed-mediated growth approach coupled with the addition of silver ions. Our study confirms the successful synthesis of gold mesostars with numerous sharp tips via the multi-directional growth effect induced by the underpotential deposition of silver adatoms (AgUPD) onto the gold surfaces. The AgUPD process affects the nanocrystal growth kinetics of the noble metal and its morphological evolution, thereby leading to intricate nanostructures with high-index facets and protruding tips or branches. Mesoscopic gold particles with a distinctive star-like morphology featuring multiple sharp projections from the central core were synthesized by exploiting this phenomenon. Sharp tips of the gold mesostars facilitate intense localized electromagnetic fields, which result in strong SERS enhancements at the single-particle level. Electromagnetic fields can be further enhanced by interparticle hot spots in addition to the intraparticle local field enhancements when arranged in multilayered arrays on substrates, rendering these arrays as highly efficient SERS-active substrates with improved sensitivity. Evaluation using Raman-tagged analytes revealed a higher SERS signal intensity compared to that of individual mesostars because of interparticle hot spots enhancements. These substrates enabled analyte detection at a concentration of 10− 9 M, demonstrating their remarkable sensitivity for trace analysis applications.