Shuo-Hui Cao

Surface Plasmon–Coupled Emission: What Can Directional Fluorescence Bring to the Analytical Sciences?

Surface plasmon-coupled emission (SPCE) arose from the integration of fluorescence and plasmonics, two rapidly expanding research fields. SPCE is revealing novel phenomena and has potential applications in bioanalysis, medical diagnostics, drug discovery, and genomics. In SPCE, excited fluorophores couple with surface plasmons on a continuous thin metal film; plasmophores radiate into a higher-refractive index medium with a narrow angular distribution. Because of the directional emission, the sensitivity of this technique can be greatly improved with high collection efficiency. This review describes the unique features of SPCE. In particular, we focus on recent advances in SPCE-based analytical platforms and their applications in DNA sensing and the detection of other biomolecules and chemicals.

Electric Field Assisted Surface Plasmon-Coupled Directional Emission: An Active Strategy on Enhancing Sensitivity for DNA Sensing and Efficient Discrimination of Single Base Mutation

We have demonstrated the proof-of-principle of electric field assisted surface plasmon-coupled directional emission (E-SPCDE). The combination of SPCDE and electric field control produced a significant synergistic effect to amplify the right signal and suppress the wrong signal intelligently in an active strategy. A novel hairpin structured DNA biosensor based on the quenching and enhancing of fluorescence in SPCDE has been designed. With modulation of the fluorescence coupling efficiency, a high discrimination ratio up to more than 20-fold has been achieved by enhancing the signal of match and suppressing that of mismatch. E-SPCDE has shown a successful application in DNA sensing, eliminating false positives and false negatives in the detection. E-SPCDE should provide an opportunity to create a new generation of miniaturized high-performance sensing platforms especially in chip-based microarrays and to make the manipulation of the nanometer-scale processes more accessible and detectable.

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.

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.

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 charge modulated aptasensor in a single glass conical nanopore

In this work, we have proposed a label-free nanopore-based biosensing strategy for protein detection by performing the DNA-protein interaction inside a single glass conical nanopore. A lysozyme binding aptamer (LBA) was used to functionalize the walls of glass nanopore via siloxane chemistry and negatively charged recognition sites were thus generated. The covalent modification procedures and their recognition towards lysozyme of the single conical nanopore were characterized via ionic current passing through the nanopore membrane, which was measured by recording the current-voltage (I-V) curves in 1mM KCl electrolyte at pH=7.4. With the occurring of recognition event, the negatively charged wall was partially neutralized by the positively charged lysozyme molecules, leading to a sensitive change of the surface charge-dependent current-voltage (I-V) characteristics. Our results not only demonstrate excellent selectivity and sensitivity towards the target protein, but also suggest a route to extend this nanopore-based sensing strategy to the biosensing platform designs of a wide range of proteins based on a charge modulation.

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.

Rapid fluorescence spectroscopic screening method for the sensitive detection of thiabendazole in red wine

In this work, a rapid, sensitive, reliable and cost-effective fluorescence spectroscopic method has been established for the detection of thiabendazole (TBZ) in red wine, based on second-derivative constant-wavelength synchronous fluorescence spectroscopy (DCWSFS) coupled with an ultrasound-assisted liquid–liquid extraction technique. The constant wavelength difference (Δλ) was set at 50 nm between the excitation and emission monochromators. Compared with UV-Vis absorption spectroscopy and conventional fluorescence spectroscopy, this method has the advantages of improving spectral resolution and eliminating the interference of spectral background from red wine. This method avoids the requirement for complicated purification and pretreatment procedures. The detection of TBZ is free from the interference of co-existence pesticides, demonstrating the high selectivity of this method. Satisfactory recoveries of the spiked red wine samples were obtained, ranging from 85.9% to 102.8%. The calibration curve showed good linearity in a range from 0.05 to 1.0 μg mL−1 for TBZ. The detection limit (S/N = 3) was 7.2 ng mL−1 and the relative standard deviation (n = 5) value was 3.9%. The results obtained by this method for analyzing TBZ in red wine samples correlated well with those obtained by the HPLC method. This method could be an attractive alternative for the rapid screening of TBZ in large amounts of samples.