Minhee Kang

Beyond the SERS: Raman Enhancement of Small Molecules Using Nanofluidic Channels with Localized Surface Plasmon Resonance

This work was supported by KRIBB (Korea Research Institute of Bioscience & Biotechnology) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2010–0017693).

Silver nanoislands on cellulose fibers for chromatographic separation and ultrasensitive detection of small molecules

High-throughput small-molecule assays play essential roles in biomedical diagnosis, drug discovery, environmental analysis, and physiological function research. Nanoplasmonics holds a great potential for the label-free detection of small molecules at extremely low concentrations. Here, we report the development of nanoplasmonic paper (NP-paper) for the rapid separation and ultrasensitive detection of mixed small molecules. NP-paper employs nanogap-rich silver nanoislands on cellulose fibers, which were simply fabricated at the wafer level by using low-temperature solid-state dewetting of a thin silver film. The nanoplasmonic detection allows for the scalable quantification and identification of small molecules over broad concentration ranges. Moreover, the combination of chromatographic separation and nanoplasmonic detection allows both the highly sensitive fluorescence detection of mixed small molecules at the attogram level and the label-free detection at the sub-nanogram level based on surface-enhanced Raman scattering. This novel material provides a new diagnostic platform for the high-throughput, low-cost, and label-free screening of mixed small molecules as an alternative to conventional paper chromatography.

Repeated Solid-state Dewetting of Thin Gold Films for Nanogap-rich Plasmonic Nanoislands.

This work reports a facile wafer-level fabrication for nanogap-rich gold nanoislands for highly sensitive surface enhanced Raman scattering (SERS) by repeating solid-state thermal dewetting of thin gold film. The method provides enlarged gold nanoislands with small gap spacing, which increase the number of electromagnetic hotspots and thus enhance the extinction intensity as well as the tunability for plasmon resonance wavelength. The plasmonic nanoislands from repeated dewetting substantially increase SERS enhancement factor over one order-of-magnitude higher than those from a single-step dewetting process and they allow ultrasensitive SERS detection of a neurotransmitter with extremely low Raman activity. This simple method provides many opportunities for engineering plasmonics for ultrasensitive detection and highly efficient photon collection.

A Deformable Nanoplasmonic Membrane Reveals Universal Correlations Between Plasmon Resonance and Surface Enhanced Raman Scattering

A quantitative correlation between plasmon resonance and surface enhanced Raman scattering (SERS) signals is revealed by using a novel active plasmonic method, that is, a deformable nanoplasmonic membrane. A single SERS peak has the maximum gain at the corresponding plasmon resonance wavelength, which has the maximum extinction product of an excitation and the corresponding Raman scattering wavelengths.

Engineering Hot Spots on Plasmonic Nanopillar Arrays for SERS: A Review

Nanopillar arrays have provided unique optical properties due to their multi-dimensional architectures with large surface area. Recently, surface enhanced Raman spectroscopy (SERS) has taken full benefits of nanopillar arrays for highly sensitive chemical and biosensing. This article gives an overview of hot spot engineering on nanopillar arrays for SERS. Nanopillar arrays are very beneficial for providing high density plasmonic nanostructures, which induce the oscillation of free electrons to create highly localized electric fields, i.e., electromagnetic hot spots, for highly intense SERS detection. The diverse methods have successfully demonstrated the nanofabrication of hotspot-rich nanopillar arrays on silicon or glass substrates. Tailoring hot spots enables ultrasensitive detection of biomolecules at low concentrations and even allows single-molecule level detections. This review overviews the nanofabrication methods for nanopillar array construction, the design strategies for electromagnetic hot spot generation on nanopillar arrays, and their SERS applications.

Bioplasmonic Alloyed Nanoislands Using Dewetting of Bilayer Thin Films

Unlike monometallic materials, bimetallic plasmonic materials offer extensive benefits such as broadband tuning capability or high environmental stability. Here we report a broad range tuning of plasmon resonance of alloyed nanoislands by using solid-state dewetting of gold and silver bilayer thin films. Thermal dewetting after successive thermal evaporation of thin metal double-layer films readily forms AuAg-alloyed nanoislands with a precise composition ratio. The complete miscibility of alloyed nanoislands results in programmable tuning of plasmon resonance wavelength in a broadband visible range. Such extraordinary tuning capability opens up a new direction for plasmonic enhancement in biophotonic applications such as surface-enhanced Raman scattering or plasmon-enhanced fluorescence.

Optically Patternable Metamaterial Below Diffraction Limit

We report an optically patternable metamaterial (OPM) for ultraviolet nanolithography below the diffraction limit. The OPM features monolayered silver nanoislands embedded within a photosensitive polymer by using spin-coating of an ultrathin polymer, oblique angle deposition, and solid-state embedment of silver nanoislands. This unique configuration simultaneously exhibits both negative effective permittivity and high image contrast in the ultraviolet range, which enables the surface plasmon excitation for the clear photolithographic definition of minimum feature size of 70 nm (≲ λ/5) beyond the near-field zone. This new metamaterial provides a new class of photoresist for ultraviolet nanolithography below the diffraction limit.

Solid-state alloyed gold-silver nanostructures for plasmon resonance tailoring as highly sensitive SERS substrates

This work reports straightforward approach for AuAg alloyed nanostructures with solid-state dewetting. Successive thin films evaporation and thermal annealing enable fabrication of AuAg alloyed nanostructures with well-defined sizes and shapes. The complete miscibility of Au and Ag leads to programmable tailoring of the plasmon resonance over the entire visible wavelength region by varying corresponding film thickness. Such wide-range tuning capability of AuAg alloyed nanostructures is expected to be excellent candidates for highly sensitive surface-enhanced Raman scattering (SERS) substrates.

A modified Plasmon Ruler equation for quasi-ordered plasmonic nanoislands

This work report the modified Plasmon Ruler equation for quasi-ordered nanoislands using deformable nanoplasmonic membrane that able to modulate interstitial gap between neighboring nanoislands. Depending on interstitial gap, the plasmon resonance wavelength shift was achieved by averaging the plasmon resonance shift of individual nanoislands that reflects the size distribution of nanoislands. This distance dependence of plasmon coupling in quasi-ordered nanoislands provides a useful guideline to expect the plasmon resonance wavelength for surface-enhanced Raman scattering (SERS).

Hierarchically structured LED lens for wide angle and high efficiency illumination

This work presents an effective method for the wide angle and high efficiency illumination with hierarchical structures on lens surfaces, which were fabricated by a photolithography and a capillary force lithography.