Biyan Chen

Influence of silver grain size, roughness, and profile on the extraordinary fluorescence enhancement capabilities of grating coupled surface plasmon resonance

Since the development of fluoroimmunoassays, researchers have sought a method of substantially enhancing fluorescence intensity to extend the limits of detection to new levels of sensitivity. Surface plasmon resonance (SPR) and metal enhanced fluorescence has long been a topic of research and has led to the development of prism- and grating-based SPR systems. However, with the wide coupling range and ease of exciting SPR on plasmonic gratings with a simple microscope objective, they have tremendous potential for revolutionizing the fields of plasmonics, fluorescence, and sensors. In an effort to better understand the influence of grating profile and metal film properties on the extraordinary fluorescence enhancement capabilities of plasmonic gratings, a novel microcontact printing process and different metal deposition techniques were used to fabricate silver gratings with varying grain diameters, roughnesses, heights, and duty cycles using thermal evaporation and RF sputtering. The resulting plasmonic gratings exhibited fluorescence enhancements up to 116× that of dye-coated glass slides using an epifluorescence microscope, much higher than more expensive prism-based SPR systems. This silver grating represents an extraordinary opportunity to quickly and easily enhance fluorescence and widen the detection limits of common fluorescence based assays with little to no equipment modification.

Single-molecule Detection in Nanogap-embedded Plasmonic Gratings

We introduce nanogap-embedded silver plasmonic gratings for single-molecule (SM) visualization using an epifluorescence microscope. This silver plasmonic platform was fabricated by a cost-effective nano-imprint lithography technique, using an HD DVD template. DNA/ RNA duplex molecules tagged with Cy3/Cy5 fluorophores were immobilized on SiO2-capped silver gratings. Light was coupled to the gratings at particular wavelengths and incident angles to form surface plasmons. The SM fluorescence intensity of the fluorophores at the nanogaps showed approximately a 100-fold mean enhancement with respect to the fluorophores observed on quartz slides using an epifluorescence microscope. This high level of enhancement was due to the concentration of surface plasmons at the nanogaps. When nanogaps imaged with epifluorescence mode were compared to quartz imaged using total internal reflection fluorescence (TIRF) microscopy, more than a 30-fold mean enhancement was obtained. Due to the SM fluorescence enhancement of plasmonic gratings and the correspondingly high emission intensity, the required laser power can be reduced, resulting in a prolonged detection time prior to photobleaching. This simple platform was able to perform SM studies with a low-cost epifluorescence apparatus, instead of the more expensive TIRF or confocal microscopes, which would enable SM analysis to take place in most scientific laboratories.

In Situ Characterization of Photothermal Nanoenergetic Combustion on a Plasmonic Microchip

Plasmonic gratings facilitate a robust in situ diagnostic platform for photothermal combustion of nanoenergetic composite thin films using an optical microscope and a high-speed camera. Aluminum nanoparticles (Al NPs) embedded in a fluoropolymer oxidizer are cast onto a plasmonic grating microchip and ignited using a low-power laser. The plasmonic grating enhances both spatial resolution and sufficient photothermal coupling to combust small Al NP clusters, initiating localized flames as small as 600 nm in size. Two-color pyrometry obtained from a high-speed color camera indicates an average flame temperature of 3900 K. Scattering measurements using polarized light microscopy enabled precise identification of individual Al NPs over a large field of view, leading to 3D reconstruction of combustion events.

Surface-plasmon-enhanced Raman and photoluminescence of few-layers and bulk MoS 2 on silver grating

We report the observation of surface-plasmon-enhanced Raman and photoluminescence of few-layers and bulk MoS 2 on silver grating compared with SiO 2 substrate, suggesting an indirect to direct bandgap transition for MoS 2 .

Highly sensitive plasmonic grating platform for the detection of a wide range of infectious diseases

We report the cost-effective fabrication of a plasmonic grating for improved light coupling in a fluorescence-based sensor platform by a simple micro-contact printing technique. The fluorescence of Rhodamine 6G (R6G) film on gratings was enhanced by up to 239-fold with respect to glass using a fluorescence microplate reader. The silver gratings are made suitable for use in biological buffers by a protective alumina layer. The platform has been optimized to detect Interferon-gamma (IFNδ), a commonly used biomarker for M. tuberculosis infection and other autoimmune diseases, with an immunofluorescence assay. The platform demonstrates 500 fg/ml limit of detection (LOD) while commercial enzyme-linked immunosorbent assays (ELISA) report 2 pg/mL LOD. The extraordinary fluorescence enhancement was compared with simulation result to predict and support the experimental analysis. These results can be used to expand the platform for numerous fast, robust, sensitive biosensors.

Fluorescence-based temperature sensor for in-situ imaging local temperature of aluminum nanoparticles on plasmonic gratings

We developed a novel plasmonic grating platform as a fluorescence-based temperature sensor for in-situ imaging of localized temperature and dynamic mapping of temperature in nanoscale due to photothermal heating of aluminum nanoparticles. Al/polymer nanoenergetics films with temperature sensitive dyes (Rhodamine 6G) were prepared and calibrated to obtain temperature-dependent dye fluorescence intensity. A tunable laser heating setup was developed for microscope imaging. We monitored the in-situ laser heating on Al/polymer/R6G systems by microscope and then constructed the spatial thermal mapping over time. Based on the fluorescence microscopic imaging, we can monitor Al nanoparticles movement and morphology changes for Al/polymer/Rhodamine 6G systems induced by laser heating. Al nanoparticles play an important role in laser heating due to the plasmonic, interband and intraband absorption characteristics of Al nanoparticles. Plasmonic grating platforms can not only significantly enhance the photothermal heating of Al compared with glass platforms, but also act as superlensing for sub-diffraction limited imaging of nanoparticles.

Anti-corrosive films on silver plasmonic gratings for fluorescence imaging of single molecules and cancer cells

Silver plasmonic gratings with a thin corrosion protection film enable enhanced fluorescence-based detection, including single molecule, over a much wider fluorescent dye concentration range, 100 μM-1fM, and deeper field penetration than traditional sensor substrates.

Single Molecule Oscillations of an RNA/DNA Duplex in a Plasmonic Nanocavity

We report the visualization of single molecule dynamics in epifluorescence mode through extraordinary plasmonic enhancement provided by silver grating with embedded nanocavities. Cy3/Cy5-labeled DNA/RNA hybrid duplexes were affixed to SiO 2-capped silver gratings produced by soft lithography process. Tracking single-molecule fluorescence revealed damped sub-1 Hz periodic Cy3 intensity fluctuations with strong dependence on the bulk MgCl 2 concentration. Extreme concentration of electric field at the nanocavity edge induces plasmonic heating, which sets up convection deep within the nanocavity. Local fluctuations in Mg 2+ ion concentration promote a bent or unbent duplex conformational state, respectively, by varying degrees of negative charge screening along the duplex backbone. These oscillations continue until the duplex conformational state stabilizes or the dyes bleach. This unique molecular behavior in the nanocavity could be used to study duplex complementarity, structural polymorphisms, and protein-nucleic acid interactions at the single molecule level.