Surface-plasmon-coupled emission of quantum dots

We studied surface plasmon-coupled emission (SPCE) of semiconductor quantum dots (QDs). These QDs are water-soluble ZnS-capped CdSe nanoparticles stabilized using lysine cross-linked mercaptoundecanoic acid. The QDs were spin-coated from 0.75% PVA solution on a glass slide covered with 50 nm of silver and a 5-nm protective SiO(2) layer. Excited QDs induced surface plasmons in a thin silver layer. Surface plasmons emitted a hollow cone of radiation into an attached hemispherical glass prism at a narrow angle of 48.5 degrees. This directional radiation (SPCE) preserves the spectral properties of QD emission and is highly p-polarized irrespective of the excitation polarization. The SPCE spectrum depends on the observation angle because of the intrinsic dispersive properties of SPCE phenomenon. The remarkable photostability can make QDs superior to organic fluorophores when long exposure to the intense excitation is needed. The nanosize QDs also introduce a roughness near the metal layer, which results in a many-fold increase of the coupling of the incident light to the surface plasmons. This scattered incident illumination transformed into directional, polarized radiation can be used simultaneously with SPCE to develop devices based on both quantum dot emission and light scattered from surface plasmons on a rough surface.     

Multicolor directional surface plasmon-coupled chemiluminescence

In reports over the past several years, we have demonstrated the efficient collection of optically excited fluorophore emission by its coupling to surface plasmons on thin metallic films, where the coupled luminescence was highly directional and polarized. This phenomenon is referred to as surface plasmon-coupled emission (SPCE). In this current study, we have extended this technique to include chemiluminescing species and subsequentially now report the observation of surface plasmon-coupled chemiluminescence (SPCC), where the luminescence from chemically induced electronic excited states couples to surface plasmons in thin continuous metal films. The SPCC is highly directional and predominantly p-polarized, strongly suggesting that the emission is from surface plasmons instead of the luminophores themselves. This indicates that surface plasmons can be directly excited from chemically induced electronic excited states and excludes the possibility that the plasmons are created by incident excitation light. This phenomenon has been observed for a variety of chemiluminescent species in the visible spectrum, ranging from blue to red, and also on a variety of metals, namely, aluminum, silver, and gold. Our findings suggest new chemiluminescence sensing strategies on the basis of localized, directional, and polarized chemiluminescence detection, especially given the wealth of assays that currently employ chemiluminescence-based detection.     

Demonstration of a surface plasmon-coupled emission (SPCE)-based immunoassay in the absence of a spacer layer

The technique of surface plasmon-coupled emission (SPCE) involves the coupling of light which is emitted from a fluorophore into the surface plasmon of an adjacent thin metal film, giving rise to highly directional emission. We have combined the advantages of SPCE with the high light collection efficiency of supercritical angle fluorescence by carrying out an immunoassay on a paraboloid array biochip in the absence of the conventional SPCE spacer layer normally used to minimize metal quenching of the fluorescence. In this work, we have successfully demonstrated an SPCE-based assay by utilizing the protein assay layer as the spacer layer. A novel 3 × 3 injection molded polymer biochip with paraboloid elements was used. The paraboloid elements served to enhance the light collection efficiency while the top surface was coated with a gold layer to use excitation of surface plasmons and detection of SPCE emission. Theoretical modeling of the gold-protein layer structure showed that the surface plasmon resonance angles were located in the detection range of the paraboloid biochip. The polarization dependence of SPCE emission was also demonstrated. Finally, a human IgG sandwich immunoassay was carried out which exhibited a limit of detection of ~10 ng/ml using 3σ. The results demonstrate the potential of the SPCE-based paraboloid array biochip as a novel platform for high-throughput analysis of biomolecular interactions.     

基于铁薄膜的表面等离子体耦合定向荧光研究

本文报道了以纳米铁膜为基底的SPCDF检测方法. 以荧光素和卟啉为例, 探讨了SPCDF信号的特性, 在固定角度下检测了荧光素-卟啉双荧光团体系的SPCDF信号, 并应用于全血样品的分析.     

First Observation of Surface Plasmon-Coupled Chemiluminescence (SPCC)

In this letter, we report the first observation of surface plasmon-coupled chemiluminescence (SPCC), where the luminescence from chemically induced electronic excited states couples to surface plasmons in a thin continuous silver film. The SPCC is highly directional and predominantly p-polarized, strongly suggesting that the emission is from surface plasmons instead of the luminophores directly themselves. This indicates that surface plasmons can be directly excited from chemically induced excited states. With a wealth of assays that employ chemiluminescence based detection currently in use, then our findings suggest new chemiluminescence sensing strategies based on localized, directional and polarized chemiluminescence detection.     

Surface‐Plasmon‐Coupled Emission of Rhodamine 110 in a Silica Nanolayer

The first observation of strong directional surface‐plasmon‐coupled emission (SPCE) of Rhodamine 110 in silica nanofilms deposited on silver nanolayers is reported. The preparation of the material is described in detail. The intensity of SPCE exceeds 10 times that of free space fluorescence and total linear light polarization in the SPCE ring is observed. A new experimental setup and an original data collection method is presented. Our material completely preserves its fluorescence properties for at least eight months.     

Analysis of immunoarrays using a gold grating-based dual mode surface plasmon-coupled emission (SPCE) sensor chip

We have developed a novel dual mode immunoassay platform that combines the advantages of real-time, label free measurement of surface plasmon resonance (SPR) and the highly directional surface plasmon-coupled emission (SPCE) using a gold grating-based sensor chip. Since only fluorophore-labeled analyte molecules that are close to the metal surface of the sensor chip will couple to the surface plasmon, SPCE detection is highly surface-specific leading to background suppression and increased sensitivity. Theoretical calculations were done to find SPR and SPCE angles for a sensor chip optimized for Alexa Fluor 647. We have confirmed the SPR and SPCE responses on the dual mode sensor chip using Alexa Fluor 647 labeled anti-mouse IgG. Signal fluctuation of the dual mode sensor chip reader was below 1.2% and 0.8% for SPR and SPCE, respectively. The SPR response in this configuration showed a minimum detection level of 1 μg ml(-1), and the SPCE response showed a minimum detection level of 1 ng ml(-1) for the same sample. A range of human IgG concentrations in human serum was also analyzed with the dual mode sensor chip. The SPCE measurement is more sensitive than the SPR real-time measurement, and substantially extends the dynamic range of the assay platform, as well as enabling independent measurements of co-localized analytes on the same sensor chip region of interest. Since this assay platform is capable of measuring more than 1000 spatially encoded regions of interest on a 1 cm(2) sensor chip, it has the potential for high-content analyses of biological samples with both research and clinical applications.     

Red blood cells do not attenuate the SPCE fluorescence in surface assays

We describe the positive effect of surface plasmon-coupled fluorescence emission (SPCE) on the detection of a signal from a surface immunoassay in highly absorbing or/and scattering samples. A model immunoassay using fluorescently labeled anti-rabbit antibodies that bind to rabbit immunoglobulin on a silver surface was performed, and the signal was detected in the presence of various highly absorbing and/or scattering solutions or suspensions, such as hemoglobin solution, plastic beads, and red blood cells. The results showed that a highly absorbing solution consisting of small molecules (dye, hemoglobin) attenuates the SPCE signal approximately 2-3-fold. In contrast, suspensions with the same absorption containing large particles (large beads, red blood cell suspension) attenuate the SPCE signal only slightly, approximately 5-10%. Also, a suspension of large undyed, highly scattering beads does not reduce the SPCE signal. The effects on the immunoassay signal of the sample background absorption and scattering, the size of the background particles, and the geometry of the experimental set-up are discussed. We believe that SPCE is a promising technique in the development of biosensors utilized for surface-based assays, as well as any assays performed directly in highly absorbing and/or scattering solutions without washing or separation procedures. Figure Red blood cells (unlike hemoglobin) do not attenuate the SPCE fluorescence in surface assays.     

Surface chemistry studies of (CdSe)ZnS quantum dots at the air-water interface

Trioctylphosphine oxide- (TOPO-) capped (CdSe)ZnS quantum dots (QDs) were prepared through a stepwise synthesis. The surface chemistry behavior of the QDs at the air-water interface was carefully examined by various physical measurements. The surface pressure-area isotherm of the Langmuir film of the QDs gave an average diameter of 4.4 nm, which matched very well with the value determined by transmission electron microscopy (TEM) measurements if the thickness of the TOPO cap was counted. The stability of the Langmuir film of the QDs was tested by two different methods, compression/decompression cycling and kinetic measurements, both of which indicated that TOPO-capped (CdSe)ZnS QDs can form stable Langmuir films at the air-water interface. Epifluorescence microscopy revealed the two-dimensional aggregation of the QDs in Langmuir films during the early stage of the compression process. However, at high surface pressures, the Langmuir film of QDs was more homogeneous and was capable of being deposited on a hydrophobic quartz slide by the Langmuir-Blodgett (LB) film technique. Photoluminescence (PL) spectroscopy was utilized to characterize the LB films. The PL intensity of the LB film of QDs at the first emission maximum was found to increase linearly with increasing number of layers deposited onto the hydrophobic quartz slide, which implied a homogeneous deposition of the Langmuir film of QDs at surface pressures greater than 20 mN.m(-1).     

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.     

Admicellar polymerization and characterization of thin poly(2,2,2-trifluoroethyl acrylate) film on aluminum alloys for in-crevice corrosion control

Corrosion control of aluminum alloys in the aerospace industry has been of great interest in recent years, especially the aging of certain fleets in the United States Air Force. A thin film of poly(2,2,2-trifluoroethyl acrylate) (PTFEA) has been deposited on aluminum alloy coupons by admicellar polymerization for the purpose of in situ control of corrosion in narrow gaps. Polymerization conditions were chosen based on contact angle measurements, and the final product film was characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. Surface characterization studies have shown that the polymeric film is approximately 10 nm thick with nonuniform deposition at this scale. The modified surface is highly hydrophobic and able to delay salt solution uptake (3.5 wt % NaCl) for a period of up to 6 h in crevice corrosion tests. PTFEA films reduced the corroded area to 20% compared to 65% for a bare aluminum control and to 33% for poly(methyl methacrylate) (PMMA) film in a 24 h crevice test. PTFEA film exhibits better corrosion protection than PMMA film because it has higher hydrophobicity than a PMMA-modified surface and comparable properties as a corrosion barrier.     

Feasibility of Using Bimetallic Plasmonic Nanostructures to Enhance the Intrinsic Emission of Biomolecules

Detection of the intrinsic fluorescence from proteins is important in bio-assays because it can potentially eliminate the labeling of external fluorophores to proteins. This is advantageous because using external fluorescent labels to tag biomolecules requires chemical modification and additional incubation and washing steps which can potentially perturb the native functionality of the biomolecules. Hence the external labeling steps add expense and complexity to bio-assays. In this paper, we investigate for the first time the feasibility of using bimetallic nanostructures made of silver (Ag) and aluminum (Al) to implement the metal enhanced fluorescence (MEF) phenomenon for enhancing the intrinsic emission of biomolecules in the ultra-violet (UV) spectral region. Fluorescence intensities and lifetimes of a tryptophan analogue N-acetyl-L-tryptophanamide (NATA) and a tyrosine analogue N-acetyl-L-tyrosinamide (NATA-tyr) were measured. Increase in fluorescence intensities of upto 10-fold and concurrent decrease in lifetimes for the amino acids were recorded in the presence of the bimetallic nanostructures when compared to quartz controls. We performed a model protein assay involving biotinylated bovine serum albumin (bt-BSA) and streptavidin on the bimetallic nanostructured substrate to investigate the distance dependent effects on the extent of MEF from the bimetallic nanostructures and found a maximum enhancement of over 15-fold for two layers of bt-BSA and streptavidin. We also used finite difference time domain (FDTD) calculations to explore how bimetallic nanostructures interact with plane waves and excited state fluorophores in the UV region and demonstrate that the bimetallic substrates are an effective platform for enhancing the intrinsic emission of proteins and other biomolecules.     

Photochemical precipitation of copper particles on the quartz surface modified with polybutoxytitanium

Results of photochemical precipitation of copper nanoparticles on the surface of polybutoxytitanium film applied to the surface of quartz sublayer and modified by the UV irradiation with the excitation wavelength 254 nm are presented. Photocatalytic activity of polybutoxytitanium film modified by the UV light was confirmed. It was established that photoprecipitated copper nanoparticles form a uniform stable coating consisting of tightly packed particles of 50 nm diameter and 8 nm height.     

Preparation of conductive,flexible and transparent films by in situ deposition of polypyrrole nanoparticles on polyethylene terephthalate

In this study polypyrrole (PPy) nanoparticles were deposited as a thin film on the modified surface of polyethyleneterephthalate (PET) by in situ chemical polymerization in the presence of sodium dodecylsulfate (SDS), sodium dodecylbenzenesulfonate (DBSNa) and mixture of them as the surfactant. The surface of PET was modified by KOH before deposition and was investigated for conductivity and adhesion of PPy nanoparticles to PET. Resulting conductive flexible films were characterized by UV–Vis spectroscopy, fieldemission scanning electron microscopy, contact angle measurements and four-point-probe technique for conductivity. Direct morphological observation (FESEM) and electrical measurements indicated that the morphology, conductivity and the nature of deposited PPy films depend on surfactant, surface modification of PET and monomer concentration. In optimized process condition, uniform conductive films of PPy were obtained with good adhesion to PET.     

The effects of oxygen concentration on ultraviolet luminescence of ZnO films by sol–gel technology and annealing

ZnO thin films were successfully deposited on SiO₂/Si substrate using the sol–gel technique and annealed in various annealing atmospheres at 900 °C by rapid thermal annealing (RTA). X-ray diffraction revealed the (002) texture of ZnO thin films. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that the grains of the ZnO thin film were enlarged and its surface was smoothed upon annealing in oxygen. PL measurement revealed two ultraviolet (UV) luminescence bands at 375 and 380 nm. The intensity of the emission peak at 380 nm became stronger as the concentration of oxygen in the annealing atmosphere increased. The X-ray photoelectron spectrum (XPS) demonstrated that a more stoichiometric ZnO thin film was obtained upon annealing in oxygen and more excitons were generated from the radiative recombination carriers consistently. Additionally, the UV intensity increased with the thickness of ZnO thin film.     

银膜表面等离子体耦合辐射特性的仿真分析

表面等离子体耦合辐射(SPCE)是传统表面等离子体共振(SPR)的逆过程:当分子足够靠近金属薄膜表面时( < 200 nm),其受激辐射的能量可以耦合成SPR模式并定向辐射到棱镜中.由于具有场增强特性、高收集效率和优异的表面选择性, SPCE作为一种新的表面分析技术已经在荧光和拉曼光谱领域得到了有效的应用.本文采用光学互易定理简化传统SPCE的计算方法.通过计算,我们得到了SPCE一维和二维辐射功率密度分布,表面选择性,辐射角的波长色散特性,辐射角半峰宽与银膜厚度的关系.仿真结果与已报到的实验结果吻合良好,验证了该方法的有效性.     

Enhanced modulation of magnetic field on surface plasmon coupled emission (SPCE) by magnetic nanoparticles

This article demonstrates the enhancement of magnetic nanoparticles on magnetic field modulation of surface plasmon coupled emission (SPCE), and this method is designed as a biosensor to prove the feasibility of magnetic field modulated SPCE to be employed in the field of biosensing and biodetection.     

Enhanced modulation of magnetic field on surface plasmon coupled emission (SPCE) by magnetic nanoparticles

The obvious enhancement effect of magnetic nanoparticles (MNPs) introduced in Cr/Co/Cr/Au substrate on the pulsed magnetic field-modulated surface plasmon coupled emission (SPCE) was investigated, and the observed enhancement factor was 4 comparing with the magnetic field modulated SPCE without MNPs. This is the new observation for the magnetic field modulated SPCE, and this method was designed as a biosensor, which to our knowledge, is the first application of magnetic field-modulated SPCE in biosensing and detection field. This strategy is a universal approach to increase the fluorescence signal and helps to build the new SPCE based stimulus-response system.     

The impact of ultraviolet light on bacterial adhesion to glass and metal oxide-coated surface

Biofouling of glass and quartz surfaces can be reduced when the surface is coated with photocatalytically active metal oxides, such as TiO2 (anatase form) or SnO2. We measured the attachment of eight strains of bacteria to these two metal oxides (TiO2 and SnO2), and to an uncoated glass (control; designated Si-m) before and after exposure to UV light at wavelengths of 254 nm (UVC) or 340 nm UV (UVA). TiO2-coated surfaces were photocatalytically active at both 254 and 340 nm as evidenced by a decrease in the water contact angle of the surface from 59 degrees +/-2 to <5 degrees. The water contact angle of the SnO2 surface was reduced only at 254 nm, while contact angle of the Si-m glass surface was not altered by light of either wavelength. Bacterial adhesion decreased by 10-50% to photocatalyzed glass surfaces. In all cases, bacteria exposed to the UV light were completely killed due to a combination of exposure to UV light and the photocatalytic activity of the glass surfaces. These results show that UV light irradiation of TiO2-coated surfaces can be an effective method of reducing bacterial adhesion.     

RESOLUTION OF TWO EMISSION SPECTRA FOR TRYPTOPHAN USING FREQUENCY-DOMAIN PHASE-MODULATION SPECTRA

We describe a novel application of frequency-domain fluorometry which allows resolution of the decay times and emission spectra of samples which display multi-exponential decay kinetics. This method does not require any previous knowledge about the decay times or any assumptions about the shape of the emission spectra. We record the wavelength-dependent phase angles and modulations (phase angle and modulation spectra) using a number of light modulation frequencies. The data is analyzed by non-linear least-squares to recover the emission spectra and their associated decay times. Phase and modulation spectra (PM Spec) were used to recover the emission spectra associated with the two decay times of tryptophan at pH = 7 (0.54 and 3.44 ns). The emission spectra of these components are centered at 340 and 355 nm, respectively, with the amplitude of the 0.54 ns component contributing 6% to the total emission. These results are in agreement with previous time-resolved studies by Szabo and Rayner [J. Am. Chem. Soc. 102, 554-563 (1980)]. Control experiments were performed on mixtures of N-acetyl-L-tryptophanamide (NATA) and PPD, which demonstrate our ability to recover the spectra and decay times from two component mixtures. NATA itself displayed a single decay time and only one emission spectrum.