Wei-Peng Cai

Investigation of surface-enhanced Raman scattering from platinum electrodes using a confocal Raman microscope: dependence of surface roughening pretreatment

Abstract In order to establish an appropriate surface roughening procedure for obtaining high-quality surface Raman spectra from Pt electrodes, various roughening conditions for the SERS from the adsorbed pyridine, thiocynate and hydrogen are assessed in terms of the corresponding surface Raman intensities, enhancement factors and surface homogeneity. The repetitive square-wave oxidation reduction cycle (SWORC), triangular-wave ORC (TWORC) and platinization have been performed in the present study. The enhancement factor ( G ) is calculated based on the confocal feature of a confocal microprobe Raman system, showing one to two orders of amplification of Raman signal for adsorbed pyridine on roughened Pt surfaces. The involvement of charge transfer (CT) enhancement is inferred from the SERS intensity-potential profiles that are dependent on excitation lines. In general, the Pt surfaces with different roughness factors ( R ) can be divided into three categories: (1) the mildly roughened surface with R of 20–30 seems more adequate for the study of SERS mechnism including calculation of G ; (2) the moderately roughened surface with R ranging from 20 to 100, providing homogeneous morphologies, is suitable for investigating surface adsorption and reactions; (3) the highly roughened surface with R ranging from 100 to 300, with non-uniform morphologies, could only be used for investigating species having small Raman cross-sections such as hydrogen adsorption.

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.

Can surface Raman spectroscopy be a general technique for surface science and electrochemistry

Surface-enhanced Raman spectroscopy has almost been restricted to the study of only three noble metals of Au, Ag and Cu for two decades. Recently, a new confocal Raman microscope and special surface pretreatments have allowed the acquisition of high-quality surface Raman spectra of organic and inorganic molecules adsorbed on bare Pt, Ni, Co, Fe, Pd, Rh, Ru and Si electrodes over a wide applied potential range for the first time. The present results demonstrate several advantages of in situ surface Raman spectroscopy that could probably make it a general technique widely used in surface science and electrochemistry.

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.

Extending surface Raman spectroscopic studies to transition metals for practical applications: III. Effects of surface roughening procedure on surface-enhanced Raman spectroscopy from nickel and platinum electrodes

Abstract Surface-enhanced Raman spectroscopy (SERS) has been applied successfully to the in situ study of Ni and Pt electrodes with different surface roughnesses. The appropriate surface roughening procedure is indispensable for obtaining good-quality surface Raman signals from transition metals, with the surface enhancement factor ranging from one to three orders of amplification. The potential-dependent SERS spectra show that methanol is dissociated to CO at the surface, leading to catalytic poisoning of the reaction sites and, more interestingly, the onset potential of the CO oxidation is affected considerably by the surface roughness. In this paper two important capabilities of in situ surface Raman spectroscopy are emphasized: (i) to probe the adsorbate–metal vibration in the low frequency region and (ii) to study highly rough transition metal surfaces with dark color that are widely used for practical electrocatalysis.

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.