Simultaneous monitoring of the fluorescence and refractive index by surface plasmon coupled emission: A proof-of-concept study

Simultaneous acquisition of fluorescence property and refractive index using a single surface plasmon coupled emission (SPCE) measurement has been achieved, thus achieving synchronicity in real time. The SPCE sensor was employed for monitoring the adsorption of volatile organic compounds (VOCs) by dye-encapsulated metal-organic frameworks (Dye@MOFs). Refractive index can reveal surface molecular adsorption and the fluorescence with information on refractive index can provide a comprehensive analysis of the adsorption events of VOCs on the interface. Meantime, the signal intensity can be amplified by combining the responses caused by changes in refractive index and the fluorescence property in parallel. This all-in-one method opens up a route to monitoring multiple processes simultaneously occurring on the interface.     

Attomolar sensitivity in bioassays based on surface plasmon fluorescence spectroscopy

With the aid of a dextran matrix, the metal-induced fluorescence loss of bound fluorophores can be greatly reduced, and the distance dependence of the fluorescence yield could be largely convoluted. This is optimized for the limit of detection assessment of surface plasmon fluorescence spectroscopy. The model system was designed as a direct assay with mouse IgG covalently immobilized to the carboxymethyl dextran matrix of a CM5 sensor chip from Biacore. Time-resolved ultratrace detection of fluorophore (Alexa-Fluor 647)-labeled rabbit anti-mouse antibody down to 500 aM (10-18 M) was accomplished, corresponding to a binding rate of approximately 10 molecules mm-2 min-1.     

Ultrafast dephasing time of localized surface plasmon polariton resonance and the involved damping mechanisms in colloidal gold nanoparticles

In this contribution, for the first time precise in situ measurements of the ultrafast dephasing time T₂ of localized surface plasmon polariton resonances in colloidal gold nanoparticles with the objective to identify the involved damping mechanisms are presented. T₂ is an essential parameter that does not only allow one to determine the field enhancement factor that is of great importance for many applications of nanoparticles, but also reflects the role of different dephasing mechanisms. The most essential result is the observation of a chemical interface damping which causes a dramatic shortening of the dephasing time. While T_2∞ = 9.4 fs can be obtained from the bulk dielectric function, the value shrinks to 3.7 fs if the nanoparticles are in aqueous solution.     

应用表面等离激元提高有机发光器件效率的研究进展

表面等离激元（SPP）存在于金属和介质界面,是光场和金属表面自由电子相互作用而产生的电子集体振荡行为.一方面,由于在金属纳米颗粒表面会形成局域的SPP震荡（LSP）,可以调控金属表面附近分子的发光性质,因此,很多研究者尝试在有机电致发光器件（OLED）中引入金属纳米颗粒,利用LSP改善OLED器件性能;另一方面,在传统发光器件中,由于金属表面等离激元的波矢量和自由光波的波矢量不匹配,无法辐射成自由光波,最终只能以热能的形式耗散掉.通过改变金属表面形貌,如附加光栅结构等方法,使得SPP的能量能够耦合成自由光,从而提高发光器件的外量子效率.利用SPP来提高有机发光器件的效率,已经引起广泛的关注,本文着重综述以下两个方面的工作：一是采用金属纳米颗粒的LSP提高荧光分子辐射跃迁的几率,从而提升发光器件的内量子效率;二是利用有序或无序光栅结构使得SPP与自由光的波矢匹配来提高器件的耦合出光,从而提升外量子效率.     

A novel C-shaped, gold nanoparticle coated, embedded polymer waveguide for localized surface plasmon resonance based detection

In this study, a novel embedded optical waveguide based sensor which utilizes localized surface plasmon resonance of gold nanoparticles coated on a C-shaped polymer waveguide is being reported. The sensor, as designed, can be used as an analysis chip for detection of minor variations in the refractive index of its microenvironment, which makes it suitable for wide scale use as an affinity biosensor. The C-shaped waveguide coupled with microfluidic channel was fabricated by single step patterning of SU8 on an oxidized silicon wafer. The absorbance due to the localized surface plasmon resonance (LSPR) of SU8 waveguide bound gold nano particle (GNP) was found to be linear with refractive index changes between 1.33 and 1.37. A GNP coated C-bent waveguide of 200 μ width with a bend radius of 1 mm gave rise to a sensitivity of ~5 ΔA/RIU at 530 nm as compared to the ~2.5 ΔA/RIU (refractive index units) of the same dimension bare C-bend SU8 waveguide. The resolution of the sensor probe was ~2 × 10(-4) RIU.     

Fluorescence quantification for surface plasmon excitation

Surface plasmon resonance and surface plasmon fluorescence spectroscopy in combination have the potential to distinguish multicomponent surface processes. However, surface intensity variations from resonance angle shifts lead to a nonlinear response in the fluorescence intensity. We report a method to account for surface intensity variations using the experimentally measured relationship between fluorescence and reflectivity. We apply this method to monitor protease adsorption and proteolytic substrate degradation simultaneously. Multilayer protein substrates are prepared for these degradation studies using a layer-by-layer technique.     

DNA-DNA interaction on dendron-functionalized sol-gel silica films followed with surface plasmon fluorescence spectroscopy

Since we observed that dendron-assembled surface provided high single nucleotide polymorphism discrimination efficiency for DNA microarrays, and that the binding yield for streptavidin increased when biotin was immobilized on top of it, the nanoscale-controlled surface is examined for surface plasmon field-enhanced fluorescence spectroscopy (or SPFS). Firstly, a silica film was coated onto a gold substrate using the sol-gel technique, followed by the covalent immobilization of a layer of second-generation dendrons with a DNA catcher strand at their apex. The thickness of the inorganic interlayer (d=33 nm) was effectively suppressing fluorescence quenching. Thus, the kinetics and affinity characteristics of DNA hybridization could be investigated very sensitively by SPFS. The kinetic rate constants found for DNA hybridization on the dendron-modified surface were larger than those reported for a streptavidin-modified surface by one order of magnitude, except for dissociation rate constant for a single mismatched case. In addition, we observed that the DNA on the cone-shaped linker maintained its capability to capture DNA target strands even after extended storage at ambient conditions.     

Photocurrent enhancement by surface plasmon resonance of silver nanoparticles in highly porous dye-sensitized solar cells

Localized surface plasmon resonance (LSPR) by silver nanoparticles that are photochemically incorporated into an electrode-supported TiO(2) nanoparticulate framework enhances the extinction of a subsequently adsorbed dye (the ruthenium-containing molecule, N719). The enhancement arises from both an increase in the dye's effective absorption cross section and a modest increase in the framework surface area. Deployment of the silver-modified assembly as a photoanode in dye-sensitized solar cells leads to light-to-electrical energy conversion with an overall efficiency of 8.9%. This represents a 25% improvement over the performance of otherwise identical solar cells lacking corrosion-protected silver nanoparticles. As one would expect based on increased dye loading and electromagnetic field enhanced (LSPR-enhanced) absorption, the improvement is manifested chiefly as an increase in photocurrent density ascribable to improved light harvesting.     

Plasmonic surface enhancement of dual fluorescence and phosphorescence emission from organic semiconductors: effect of exchange gap and spin-orbit coupling

Dual singlet-triplet fluorescence-phosphorescence emitting compounds demonstrate that plasmonic surface enhancement is controlled solely by the underlying oscillator strength of a transition: metal-free compounds with weak spin-orbit coupling show no enhancement in phosphorescence efficiency even though fluorescence is amplified.     

Organic vapour sensing using localized surface plasmon resonance spectrum of metallic nanoparticles self assemble monolayer

The response of localized surface plasmon resonance (LSPR) spectra of gold and silver nanoparticles, and gold nanoshells to organic vapors was investigated. The surface area of nanomaterials was sufficiently high for quantitative adsorption of volatile organic compounds (VOCs). Surface adsorption and condensation of VOCs caused the environmental refractive index to increase from n = 1.00 in pure air to as high as n = 1.29 in near saturated toluene vapor. The extinction and wavelength shift of the LSPR spectra were very sensitive to changes in the surface refractive index of the nanoparticles. Responses of the LSPR band were measured with a real-time UV-vis spectrometer equipped with a CCD array detector. The response of silver nanoparticles to organic vapors was most sensitive in changes in extinction, while gold nanoshells exhibited red-shifts in wavelength (∼250 nm/RIU) when exposed to organic vapors. The LSPR spectral shifts primarily were determined by the volatility and refractive indices of the organic species. The T₉₀ response time of the VOC-LSPR spectrum was less than 3 s and the response was completely reversible and reproducible.     

Pure surface plasmon resonance enhancement of the first hyperpolarizability of gold core-silver shell nanoparticles

We report the optical second harmonic (SH) response from gold core-silver shell nanoparticles supported at a liquid-liquid interface in the spectral region where the second harmonic generation (SHG) frequency is resonant with the surface plasmon (SP) resonance excitation of the nanoparticles. We compare these results with that obtained by classical linear optical absorption spectroscopy and show that the nonlinear optical response is dominated by the SP resonance enhancement with negligible contributions from the interband transitions. As a result, the SH spectrum exhibits two clear SP resonance bands attributed to the two SP resonances of the composite nanostructure formed by the gold core-silver shell nanoparticles. Absolute values of the hyperpolarizabilities are measured by hyper Rayleigh scattering (HRS) and compared that of pure gold nanoparticles. The hyperpolarizability measured at a harmonic energy of 3.0 eV, enhanced through excitation of the high energy SP resonance of the nanoparticle, increases with the silver content whereas the hyperpolarizability measured at a harmonic energy of 2.4 eV, enhanced through the excitation of the low energy SP resonance of the nanoparticle, decreases because of the shift of this resonance away from the harmonic frequency. The hyperpolarizability determined by HRS and the square root of the SHG intensities, scaling with the nanoparticle hyperpolarizability, have similar trends with respect to the silver content indicative of closely related adsorption properties yielding similar surface concentrations at the liquid-liquid interface.     

Monolithically integrated dye-doped PDMS long-pass filters for disposable on-chip fluorescence detection

We report the fabrication of high quality monolithically integrated optical long-pass filters, for use in disposable diagnostic microchips. The filters were prepared by incorporating dye molecules directly into the microfluidic chip substrate, thereby providing a fully integrated solution that removes the usual need for discrete optical filters. In brief, lysochrome dyes were added to a poly(dimethylsiloxane) (PDMS) monomer prior to moulding of the microchip from a structured SU-8 master. Optimum results were obtained using 1 mm layers of PDMS doped with 1200 microg mL(-1) Sudan II, which resulted in less than 0.01% transmittance below 500 nm (OD 4), >80% above 570 nm, and negligible autofluorescence. These spectral characteristics compare favourably with commercially available Schott-glass long-pass filters, indicating that high quality optical filters can be straightforwardly integrated into the form of PDMS microfluidic chips. The filters were found to be robust in use, showing only slight degradation after extended illumination and negligible dye leaching after prolonged exposure to aqueous solutions. The provision of low cost high quality integrated filters represents a key step towards the development of high-sensitivity disposable microfluidic devices for point-of-care diagnostics.     

In vivo fluorescence imaging using two excitation and/or emission wavelengths for image contrast enhancement

Steady-state fluorescence imaging can be used in conjunction with selective exogenous or endogenous fluorescent compounds for the diagnosis of skin lesions, for example cancer. Depending on the excitation and emission properties of the fluorescent compound used, various excitation and/or emission wavelengths can be chosen in order to allow fluorescence imaging. Unwanted background signals such as autofluorescence and scattering can decrease the image quality and, hence, the diagnosis potential of this imaging method. We have used an inexpensive dual excitation and/or emission wavelength approach in order to suppress the unwanted background signal and allow contrast enhanced fluorescence imaging. One excitation and/or emission wavelength is at the corresponding maximum of the fluorescent compound, while the second is at a nearby excitation/emission minimum. The first image contains the emission from the fluorescent compound used combined with the signal from the unwanted background. The second image provides an image of just the unwanted background signal. The difference of both images taken, thus gives a contrast enhanced image of the skin lesion. The method relies on the assumption that the background signal does not change significantly due to the small changes in wavelength for excitation or emission. Image ratio methods have already been applied towards diagnosis of basal cell carcinomas after administration of aminolevulinic acid-induced protoporphyin IX. In this study, we describe in vivo measurements in mice where the second image, usually the background signal only, contains new unwanted image data. This simple method can successfully resolve the desired image, thus demonstrating the versatility of the image processing procedure.     

Estimation of dielectric function of biotin-capped gold nanoparticles via signal enhancement on surface plasmon resonance

Biotin-capped gold nanoparticles assembled on flat gold with volume fraction f are studied by surface plasmon resonance (SPR) spectroscopy and atomic force microscopy (AFM) in order to estimate the dielectric function of the gold nanoparticles based on the Maxwell-Garnett (MG) theory. The complex dielectric function (epsilon',epsilon') of the spherical nanoparticles at three representative wavelengths in the vis-near-IR region, i.e., lambda = 543, 632.8, and 1152 nm, is estimated for a surface homogeneously covered with nanoparticles in order to discuss the wavelength dependence of the dielectric function. The SPR response of a surface covered with particles in 2D aggregates is also analyzed. The experimental SPR curve of the particle aggregates deviates from the theoretical predictions, suggesting dipole interactions between particles.     

Tailored nanoporous gold for ultrahigh fluorescence enhancement

We report molecular fluorescence enhancement of free-standing nanoporous gold in which the nanoporosity can be arbitrarily tailored by the combination of dealloying and electroless gold plating. The nanoporous gold fabricated by this facile method possesses unique porous structures with large gold ligaments and very small pores, and exhibits significant improvements in surface enhanced fluorescence as well as structure rigidity. It demonstrates that the confluence effect of improved quantum yield and excitation of fluorophores is responsible for the large fluorescence enhancement due to the near-field enhancement of nanoporous gold, which arises from the strong electromagnetic coupling between neighboring ligaments and the weakening of plasmon damping of the large ligaments because of the small pore size and large ligament size, respectively.     

Fourier transform spectroscopy of surface polariton bragg reflection in gratings

Periodic gratings on metal surface have been used for surface polariton decay length measurements for 5–15m wavelength by means of a commercial Fourier-transform spectrometer. The effect of different grating parameters as well as that of the spectrometer light beam configuration on transmission spectra has been investigated. Gaps in the transmission spectrum due to Bragg reflection have been studied in details.     

Liquid-core waveguide TCSPC sensor for high-accuracy fluorescence lifetime analysis

Liquid-core waveguide (LCW) has many advantages such as the elimination of optical artifacts typically exhibited in systems employing lenses and filters. However, due to the effect of temporal dispersion, LCWs are typically employed in steady-state fluorescence detection microsystems rather than in fluorescence lifetime measurement (FLM) systems. In this paper, we present a compact liquid-core waveguide time-correlated single-photon counting (LCW-TCSPC) sensor for FLM. The propagation of excitation within the LCW is analyzed both analytically and in simulations, with results in agreement with experimental characterization. Results reveal an optimal region within the LCW for highly accurate FLM. The proposed prototype achieves excellent excitation rejection and low temporal dispersion as a result of optimization of the propagation length of the excitation within the LCW. The prototype achieves a detection limit of 5 nM for Coumarin 6 in dimethyl sulfoxide with < 3% lifetime error. The techniques proposed for analyzing the LCW for TCSPC based FLM and prototype demonstration pave the way for developing high-performance fluorescence lifetime measurement for microfluidics and point-of-care applications.

A compact liquid-core waveguide time-correlated single-photon counting (LCW-TCSPC) sensor for fluorescence lifetime measurement (FLM) is presented. Results reveal an optimal propagation length region within the LCW for highly accurate FLM. The prototype achieves a detection limit of 5 nM for Coumarin 6 in dimethyl sulfoxide with < 3% lifetime error.

     

Fiber-optic surface plasmon resonance for vapor phase analyses

Fiber-optic sensors based on surface plasmon resonance (SPR) for direct refractive index (RI) measurements of samples with the RI between 1.00 and 1.30 are described. Most applications of SPR sensors are designed to function near the refractive index of water (1.3330 RI). The RI changes of aqueous solution (RI, ca. 1.34) can easily be monitored by silica-fiber (RI, 1.4601 at 550 nm) based SPR sensor. With regard to gas species detection, the fiber-optic SPR sensor must be modified for sensitivity to changes in refractive index near 1.0008 (i.e., RI of air). However, the silica waveguide has a prohibitively high RI for unmodified monitoring of the RI changes of gas. The silica-fiber based SPR probe design presented here is based upon the modification of the probe geometry to the ability to tune the SPR coupling wavelength/angle pair. In this study, the tapered silica-based fiber SPR sensors are shown to directly determine the RI changes of gas species and the density change of dry air.     

Antigen-antibody interactions: binding studies with fluorescence and surface plasmon resonance exemplified by acid-traseolide as antigen

The interaction between the musk fragrance acid-traseolide and monoclonal antibodies (mAB) generated against this odorant has been investigated with two different techniques. Fluorescence spectroscopy was used to study the quenching of tryptophan fluorescence of the antibody upon binding acid-traseolide. This spectroscopic approach is based on measurements under equilibrium conditions. The second technique exploited the surface plasmon resonance (SPR) phenomenon. The acid-traseolide was immobilized in the surface matrix and upon presenting mAB changes in SPR were recorded in real time during the association reaction. The SPR approach can be considered as a kinetic method. Although having a different origin, both methods lead to comparable equilibrium dissociation constants (Kd). However, the results obtained with fluorescence spectroscopy were more accurate and reproducible. Not only the association of acid-traseolide with antibody was evaluated, also Fab fragment and peptide (H3-peptide) mimicking the heavy chain CDR3 of this antibody were included in this study. The Kd-values, determined by both methods, increase in the order mAB < Fab < H3-peptide because of diminishing recognition.