Yao-Qun Li

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.

A Novel Synchronous Fluorescence Spectroscopic Approach for the Rapid Determination of Three Polycyclic Aromatic Hydrocarbons in Tea with Simple Microwave-Assisted Pretreatment of Sample

Many polycyclic aromatic hydrocarbons (PAHs) are carcinogenic, and some have been reported to be present in tea. People can be exposed to PAHs through tea consumption. Therefore, there is real importance for the determination of PAHs in tea. Because of the complex matrix of tea, it is hard to detect PAHs in tea without cleanup and chromatographic separation procedures. In this research, for the first time, a novel synchronous fluorescence spectroscopic approach coupling nonlinear variable-angle synchronous and matrix-isopotential synchronous scanning modes has been developed for the rapid determination of benzo(a)pyrene (BaP), benzo(k)fluoranthene (BkF), and anthracene (AN) in tea with simple microwave-assisted pretreatment of samples. This novel technique is able to resolve the spectra of the three PAHs well, even with interference from other EPA PAHs. The detection limits for BaP, BkF, and AN in tea were 0.18-0.28, 0.55-0.89, and 0.64-3.58 μg/kg, respectively, depending on various teas, with satisfactory recoveries ranging from 77.1 to 116%. The relative standard deviations achieved for BaP, BkF, and AN were 1.5, 6.6, and 8.5% for green tea; 2.9, 7.4, and 2.1% for oolong tea; and 5.6, 5.4, and 5.8% for black tea, respectively. Our results showed good correlation with those of gas chromatography-mass spectrometry. The approach developed is simple, reliable, and cost-efficient, providing an attractive alternative for the rapid selective screening of PAHs in tea.

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.

A novel approach for simultaneous determination of polycyclic aromatic hydrocarbons by Shpol'skii non-linear variable-angle synchronous fluorescence spectrometry

A new method of combining low-temperature Shpolskii effect with non-linear variable-angle synchronous fluorescence spectrometry (L-NLVASFS) has been proposed to increase spectral resolution. This coupled method was applied successfully to the simultaneous identification and quantification of some polycyclic aromatic hydrocarbons (PAHs) in mixtures, which cannot be determined by non-linear variable-angle synchronous fluorescence spectrometry at room-temperature (R-NLVASFS). The usefulness of this method is demonstrated by the analyses of synthetic mixtures and several real samples of airborne particulates.

Electrical, Optical, and Docking Properties of Conical Nanopores

The diffusion-influenced translocation behavior of individual nanoparticles upon passage through a conical nanopore has been elucidated by using a pressure-reversal, resistive-pulse technique, as reported by Lan and White in this issue of ACS Nano. We outline here some recent progress in conical nanopore analysis, and we present some prospects for future developments. Compared to cylindrical nanopores, the geometric change brought about by tapered nanopores causes a dramatic difference in electrical and optical properties. Such conical nanopores may also be integrated into microfluidic chips to capture cells or nanoparticles, one per nanopore, and then to release them. These advances hold the promise of making conical nanopores useful as highly efficient actuators and sensors.

Rapid simultaneous determination of protoporphyrin IX, uroporphyrin III and coproporphyrin III in human whole blood by non-linear variable-angle synchronous fluorescence technique coupled with partial least squares

Fluorescence spectroscopy provides high sensitivity in quantitative analysis. However, due to spectral interference, it is difficult to determine the individual components of fluorescent multi-component mixtures in such complicated and important body matrices as blood, urine and feces without any pre-separation. In this study, a simple and rapid approach based on non-linear variable-angle synchronous fluorescence spectrometry coupled with partial least squares analysis (NLVASF/PLS) was developed for the simultaneous determination of protoporphyrin IX (PP), uroporphyrin III (UP) and coproporphyrin III (CP). The detection limits were 0.18, 0.29 and 0.24 nmol L(-1) for protoporphyrin IX (PP), uroporphyrin III (UP) and coproporphyrin III (CP), respectively. The individual components of blood porphyrins were quantified, by this method, simultaneously in one scan with only about 30s. The recoveries of this method were above 80% in human whole blood samples. This method provided a potential tool for the determination of porphyrins in whole blood and the differential diagnosis of porphyria, especially for rapid routine screening of large number of samples.

Synchronous Fluorescence Spectroscopy and Its Applications in Clinical Analysis and Food Safety Evaluation

Synchronous fluorescence spectroscopy (SFS) plays an important role in the simultaneous analysis of multicomponent samples due to the remarkable advantages of spectral simplification, light scattering reduction, and selectivity improvement over conventional fluorescence spectroscopy. The selectivity and resolution are further enhanced by the combination of synchronous fluorescence approaches with other techniques, such as derivative technique, low-temperature technique, and chemometrics. In this chapter, the methodologies of various SFS approaches are compendiously introduced. Representative examples of biological, environmental, and clinical applications of these techniques are briefly summarized. An emphasis is placed on the development of rapid and simple synchronous fluorescence-based approaches for the determination of carcinogenic polycyclic aromatic hydrocarbons in foods and the differential analysis of porphyrins in human body samples.

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.