In situ and sensitive monitoring of configuration-switching involved dynamic adsorption by surface plasmon-coupled directional enhanced Raman scattering.

Surface adsorption studies play a crucial role in numerous fields from surface catalysis to molecular separation. However, investigation on adsorption mechanisms has been restricted to limited analytes and approaches, which calls for an in situ and sensitive surface analysis technique capable of revealing the mechanisms as well as discriminating different adsorbates and their geometry at different adsorption stages. In this study, we employed surface plasmon-coupled directional enhanced Raman scattering (SPCR), a novel technique developed by coupling surface plasmon-coupled emission with SERS, to study conformation-switching involved dynamic adsorption with background suppression and improved sensitivity (nearly 30-fold). We obtained the isotherms for a conformation-changing Raman model analyte, malachite green. An S-type Langmuir model was fitted from the time-resolved SPCR signals sensitively and without any interference from the bulk solution. The reorientation of the analyte from a predominantly parallel configuration to a perpendicular one was captured by the dramatic increase in the intensity ratios of the adsorption-related peaks to the adsorption-unrelated peak. We believe that this new sensitive and selective SPCR technique will be a promising tool for surface adsorption kinetics analysis.

Metallic Nanofilm Enhanced Fluorescence Cell Imaging: A Study of Distance-Dependent Intensity and Lifetime by Optical Sectioning Microscopy.

Simple, stable, easily-fabricated smooth metallic nanofilm can improve the imaging intensity and imaging contrast. However, its application in micrometer-scale cells has not been popularized due to the lack of full understanding of their related fluorescence properties. In this study, fluorescence enhancement of cell imaging on smooth Au nanofilm was investigated over a micrometer-scale range via employment of the optical sectioning method available with a laser scanning confocal fluorescence microscope. The fluorescence enhancement reduced with the distance away from the surface of metallic nanofilm, and this distance dependence was determined by the factors of numerical aperture, dye-substrate distance, and emission wavelength. In addition, distance-dependent fluorescence lifetime images of cells were also measured to study the interaction between fluorophores and metallic film. The enhancement effect of Au nanofilm on fluorescence cell imaging can be induced not only by the standing wave formed by the reflected light and exciting light but also by the interaction between fluorophore and surface plasmons on the metallic nanofilm. Our study on smooth metallic nanofilm should pave the way for utilizing its uniform fluorescence enhancement characteristic for biological imaging.

Excitation-Emission Synchronization-Mediated Directional Fluorescence: Insight into Plasmon-Coupled Emission at Vibrational Resolution.

Light-matter interactions have always been a fundamentally significant topic that has attracted much attention. It is important to reveal a fluorophore-plasmon interaction on the nanoscale. However, as a powerful investigative tool, fluorescence spectroscopy still suffers from a limited spectral resolution and the susceptibility to interfering substances. In this work, excitation-emission synchronization-mediated surface-plasmon-coupled emission (EES-SPCE) is proposed to break the bottleneck. By actively screening the energy transitions for observation, an improved spectral resolution has been achieved, which is advantageous to the investigation of the fluorophore-plasmon interactions under different coupling modes. The spectral information related to the plasmonic interactions through tuning vibrational energy levels is clearly distinguished at directional emission angles. EES-SPCE is demonstrated to selectively and efficiently extract the coupled emission from the vibrational resolution, which would open up the opportunity to improve the capability of spectral feature identification and signal collection for practical applications of plasmonic fluorescence spectroscopy.

Variable-Angle Nanoplasmonic Fluorescence Microscopy: An Axially Resolved Method for Tracking the Endocytic Pathway.

The study of endocytosis, which encompasses diverse mechanisms in biology, requires the utilization of high axial resolution to monitor molecular behavior on both the cell surface and interior of the cell. We have designed a novel axially resolved fluorescence microscopic technique, termed variable-angle nanoplasmonic fluorescence microscopy. The proof-of-principle of this approach is achieved by selectively following the events in the vicinity of a cell membrane or in a cell. We use a 30 nm Au-coated semitransparent coverslip as the nanoplasmonic chip to achieve both surface plasmon resonance excitation and critical angle excitation by tuning the incident angles. This approach leads to improved axial resolution compared to total internal reflection fluorescence microscopy, which is a common imaging technique in cell biology. It offers a unique opportunity to semiquantitatively determine fluorophore axial distributions in the cell. Observing the epidermal growth factor receptor-mediated endocytosis in Caski cells clearly demonstrates the potential application of this new method for cell biology studies.

The synergistic enhancement of silver nanocubes and graphene oxide on surface plasmon-coupled emission.

The enhancement of surface plasmon-coupled emission (SPCE) by the synergistic effect of silver nanocubes (AgNCs) and graphene oxide (GO) on gold film has been observed with the enhancement factor over 30. The enhancement mechanisms were investigated through simulating the electromagnetic (EM) field patterns of near field and testing different concentration of AgNCs and thickness of dye layer. The enhancement was mainly triggered by the high electromagnetic field of AgNCs, the interaction between localized surface plasmons (LSP) and propagating surface plasmons (PSP) and the assistance of GO. This synergistic enhancement strategy provides a simple way to increase SPCE signal and enable develop a new fluorescence-based detection system.

Surface Plasmon Coupled Fluorescence-Enhanced Interfacial "Molecular Beacon" To Probe Biorecognition Switching: An Efficient, Versatile, and Facile Signaling Biochip.

Integrating probes and a substrate together, a fluorescence-enhanced interfacial "molecular beacon" (FEIMB) is demonstrated, based on directional surface plasmon coupled emission. Through this simple yet efficient interfacial modulation engineering to create an interfacial quencher (graphene oxide)-enhancer (gold nanofilm) pair, the quenching-to-enhancing region of FEIMB can be actively tuned. Therefore, it provides a spatial match between signal transduction and interface-mediated biorecognition switching. Via combination of strong quenching and efficient plasmonic coupling, a synergistically amplified signal-to-background ratio of >1000-fold has been achieved. FEIMBs have been employed in protein and DNA detection, creating a high-performance and universal chip-based plasmon-mediated fluorescence sensing platform.

In Situ Monitoring of Fluorescent Polymer Brushes by Angle-Scanning Based Surface Plasmon Coupled Emission.

Fluorescent polymers have attracted interest in many fields such as sensing, diagnostics, imaging, and organic electronic devices. Real-time techniques to monitor and understand the polymerization process are important for obtaining controllable fluorescence polymers. We present a new technique to in situ monitor the growth process of fluorescent polymer brushes by using angle-scanning based surface plasmon coupled emission (AS-SPCE) approach during electrochemically mediated atom-transfer radical polymerization. The polymer thickness was determined by modeling the location of SPCE emission angle(s) with theoretical calculation. The advantages of unique angle distribution patterns, thickness dependence and effective background rejection of AS-SPCE guarantee the success in the real-time investigation for controllable fabrication of fluorescent polymers.

Optical modulator based on propagating surface plasmon coupled fluorescent thin film: proof-of-concept studies.

We demonstrate that the propagating surface plasmon coupled fluorescent thin film can be utilized as a fluorescence modulator to mimic multiple representative Boolean logic operations. Surface plasmon mediated fluorescence presents characteristic properties including directional and polarized emission, which hold the feasibility in creating a universal optical modulator. In this work, through constructing the thin layer with the specific thickness, surface plasmon mediated fluorescence can be modulated with an ON-OFF ratio by more than 5-fold, under a series of coupling configurations.

High performance dual-mode surface plasmon coupled emission imaging apparatus integrating Kretschmann and reverse Kretschmann configurations for flexible measurements.

A Kretschmann (KR) and reverse Kretschmann (RK) dual-mode surface plasmon coupled emission (SPCE) imaging apparatus based on prism coupling was built up. Highly directional and polarized fluorescence images for both RK and KR configurations were obtained. Besides, surface plasmon field-enhanced fluorescence and free space imaging can also be measured conveniently from this apparatus. Combining the high sensitivity of KR mode and the simplicity of RK mode, the multifunctional imaging system is flexible to provide different configurations for imaging applications. Compared to the free space imaging, SPCE imaging provides enhanced fluorescence, especially large enhancement up to about 50 fold in KR configuration. Additionally, the degree of evanescent field enhancement effect was easily estimated experimentally using the apparatus to compare the different imaging configurations. We believed that the dual-mode SPCE imaging apparatus will be useful in fundamental study of plasmon-controlled fluorescence and be a powerful tool for optical imaging, especially for microarray and biological applications.

Surface Plasmon-Coupled Directional Enhanced Raman Scattering by Means of the Reverse Kretschmann Configuration.

Surface-enhanced Raman scattering (SERS) is a unique analytical technique that provides fingerprint spectra, yet facing the obstacle of low collection efficiency. In this study, we demonstrated a simple approach to measure surface plasmon-coupled directional enhanced Raman scattering by means of the reverse Kretschmann configuration (RK-SPCR). Highly directional and p-polarized Raman scattering of 4-aminothiophenol (4-ATP) was observed on a nanoparticle-on-film substrate at 46° through the prism coupler with a sharp angle distribution (full width at half-maximum of ∼3.3°). Because of the improved collection efficiency, the Raman scattering signal was enhanced 30-fold over the conventional SERS mode; this was consistent with finite-difference time-domain simulations. The effect of nanoparticles on the coupling efficiency of propagated surface plasmons was investigated. Possessing straightforward implementation and directional enhancement of Raman scattering, RK-SPCR is anticipated to simplify SERS instruments and to be broadly applicable to biochemical assays.