Yu-Hua Weng

Label-free aptasensor based on ultrathin-linker-mediated hot-spot assembly to induce strong directional fluorescence.

We have demonstrated the proof-of-concept of a label-free biosensor based on emission induced by an extreme hot-spot plasmonic assembly. In this work, an ultrathin linking layer composed of cationic polymers and aptamers was fabricated to mediate the assembly of a silver nanoparticles (AgNPs)-dyes-gold film with a strongly coupled architecture through sensing a target protein. Generation of directional surface plasmon coupled emission (SPCE) was thus stimulated as a means of reporting biorecognition. Both the biomolecules and the nanoparticles were totally free of labeling, thereby ensuring the activity of biomolecules and allowing the use of freshly prepared metallic nanoparticles with large dimensions. This sensor smartly prevents the plasmonic assembly in the absence of targets, thus maintaining no signal through quenching fluorophores loaded onto a gold film. In the presence of targets, the ultrathin layer is activated to link NPs-film junctions. The small gap of the junction (no greater than 2 nm) and the large diameter of the nanoparticles (~100 nm) ensure that ultrastrong coupling is achieved to generate intense SPCE. A >500-fold enhancement of the signal was observed in the biosensing. This strategy provides a simple, reliable, and effective way to apply plasmonic nanostructures in the development of biosensing.

Directional surface plasmon-coupled emission of CdTe quantum dots and its application in Hg(II) sensing

We investigated the surface plasmon-coupled emission (SPCE) of CdTe quantum dots (QDs) and developed an SPCE-based quenchometric sensor for Hg(II) ion sensing. CdTe QDs directly synthesized in aqueous solution were attached to a 50 nm-thick Au film through layer-by-layer assembly. The directional emission of the CdTe QDs on the prism side from the surface plasmon coupling was completely p-polarized and observed at a fixed angle of 48.5°, which was consistent with theoretical calculations. An SPCE-based quenchometric sensor for Hg(II) ion sensing was established based on the quenching effect of Hg(II) ions on the fluorescence emission of CdTe QDs. As expected, the SPCE-based sensor enlarged the response range and was more sensitive than that based on free-space detection as a result of the high light-collection efficiency of SPCE. QDs with excellent properties combined with SPCE technology have great potential in detecting analytes at low concentration levels.

Prism‐Based Surface Plasmon Coupled Emission Imaging

A prism-based surface plasmon coupled emission (SPCE) imaging apparatus with a reverse Kretschmann (RK) configuration was developed and applied to dye-doped polymer films. Highly polarized, directional and enhanced fluorescence images were obtained. The angular distribution of the SPCE images was in accordance with the validated theoretical calculation performed using Fresnel equation. Prism-based SPCE imaging combined with microarray technology appears to be a promising platform for rapid and high-throughput analysis, especially for high-density arrays. We believe that prism-based SPCE imaging has potential applications in biochemical research.

Plasmon-mediated fluorescence with distance independence: From model to a biosensing application

In this article, plasmon-mediated fluorescence biosensing is reported to be distance independent through a full-coupling strategy that effectively activates the entire plasmon coupling region. This concept is demonstrated through collecting the directional surface plasmon-coupled emission (SPCE) signal from fluorescent silica nanoparticles with a size that matches the entire coupling region. Based on this design, the spatial distribution of the fluorophores is confined by the dimension of the nanoparticle. Therefore, these encapsulated fluorophores occupy the maximum coupling dominant region and optimally utilize the coupling effect. Being different from the conventional plasmon-mediated fluorescence, the enhanced fluorescence response becomes nearly independent of distance changes on a wide dynamic range from 0nm to 30nm between the fluorescent nanoparticles and metal structure. Full-coupling SPCE appropriately enlarges the distribution of fluorophores, ensuring that the coupling dominant region is filled with enough fluorophores at varying distances to create a stable and detectable signal. This scale of distances is well suited for many biorecognition events. Full-coupling SPCE solves signal deviation challenges originating from the susceptible and unpredictable orientation and conformation of biomolecules on the nanoscale. Immunoassays and DNA detection are shown with high and reliable signals, demonstrating the advantages of distance-independent full coupling. Without the need of a complicated and rigorous architecture for precise distance control, full-coupling SPCE offers great promise for a general platform of chip-based biosensing and bioanalysis.

Surface plasmon coupled emission in micrometer-scale cells: a leap from interface to bulk targets.

Surface plasmon coupled emission (SPCE) technique has attracted increasing attention in biomolecular interaction analysis and cell imaging because of its high sensitivity, low detection volume and low fluorescence background. Typically, the working range of SPCE is limited at nanometers to an interface. For micrometer-scale samples, new SPCE properties are expected because of complex coupling modes. In this work, cells with different subregions labeled were studied using a SPCE spectroscopy system. Angular and p-polarized emission was observed for cell membrane, cytoplasm, and nucleus labeled with DiI, Nile Red, and propidium iodide, respectively. The SPCE signals were always partially p-polarized, and the maximum emission angle did not shift, regardless of variations in emission wavelength, fluorophore distribution and stained layer thickness. Additionally, increased polarization and a broader angle distribution were also observed with an increase in sample thickness. We also investigated the impact of metallic substrates on the SPCE properties of cells. Compared with Au and Ni substrates, Al substrates presented better polarization and angle distribution. Moreover, the real-time detection of the cell labeling process was achieved by monitoring SPCE intensity. These findings expand SPCE from a surface technique to a 3D method for investigating bulk targets beyond the nanoscale interfaces, providing a basis to apply this technique to study cell membrane fluidity and biomolecule interactions inside the cell and to distinguish between cell subregions.

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.

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.

Directional Fluorescence Based on Surface Plasmon-Coupling

Fluorescence technology, including the optical sensing and microscopic imaging, has been playing important roles in biology research and medical diagnosis. However it still remains a great challenge to meet the increasing needs of sensitivity and applicability. Surface plasmon-coupled emission (SPCE) is a novel technique that can significantly improve the ability of fluorescence technology. In SPCE, the excited fluorophores will couple with surface plasmons on a continuous thin metal film, which in turn radiate into the higher refractive index media with a narrow angular distribution. Attributed to the direction emission, the sensitivity can be highly improved with the high collection efficiency. This review will summarize the unique features of SPCE that are important in analytical researches, in particular, with a focus on the recent advancements in the strategies for improving the SPCE performance. The optical imaging based on SPCE and some examples of the analytical applications of SPCE are also highlighted. Recent achievements in SPCE suggest that it could provide new technical platforms with widespread potential applications in various areas, such as nucleic acid, protein and other biochemical sensing.