Signal enhancement of surface plasmon-coupled directional emission by a conical mirror

A simple strategy for increasing the collection efficiency of surface plasmon-coupled emission (SPCE) is demonstrated. SPCE is a near-field phenomenon occurring when excited fluorophores are in close proximity to a subwavelength metal film. The energy of the fluorophores induces surface plasmons that radiate the coupled energy at highly specific angles. In an attempt to maximize the collected emission, a conical mirror was placed around the coupling prism. The result was a nearly 500 fold enhancement over the free space signal as detected from a single point from a poly(vinyl alcohol) layer doped with ruthenium. Coupling this large enhancement with LED excitation could lead to the development of inexpensive, handheld fluorescent devices with high sensitivity.     

Theory and simulation of surface plasmon-coupled directional emission from fluorophores at planar structures

A theoretical approach to surface plasmon-coupled emission (SPCE) from planar structures is developed. It is used for simulations. The results are compared to experimental findings. The match is almost perfect concerning emission angles. Power relations and decay times are reproduced qualitatively. The theory is based on Fresnel plane wave refraction at planar multilayered structures and the Weyl identity for expressing the dipolar radiation in terms of plane waves. One-dimensional integrals, used for the numerical computations, are derived for the fields, powers, and decay enhancements. This theoretical approach is shown to be well suited for design of SPCE setups and for prediction and explanation of experimental results. It also shows promise for refinement and optimization of SPCE, concerning enhancement of weak fluorophores, and usage of decay times.     

DNA hybridization using surface plasmon-coupled emission

We describe a new approach to measuring DNA hybridization using surface plasmon-coupled emission (SPCE). Excited fluorophores are known to couple with surface oscillations of electrons in thin metal films, typically 50 nm thick silver on a glass prism. These surface plasmons then radiate into the glass at a sharply defined angle determined by the emission wavelength and the optical properties of the glass and metal. This radiation has the same spectral profile as the emission spectrum of the fluorophores. We studied the emission due to Cy3-labeled DNA oligomers bound to complementary unlabeled oligomers which were themselves bound to the metal surface. Hybridization resulted in SPCE due to Cy3-DNA into the prism. Directional SPCE was observed whether the sample was illuminated from the sample side or through the glass substrate at the surface plasmon angle for the excitation wavelength. A large fraction of the total potential emission is coupled to the surface plasmons resulting in improved sensitivity. When illuminated through the prism at the surface plasmon angle, the sensitivity is increased due to the enhanced intensity of the resonance evanescent field. It is known that SPCE depends on proximity to the silver surface. As a result, changes in emission intensity are observed due to fluorophore localization even if hybridization does not affect the quantum yield of the fluorophore. The use of SPCE resulted in suppression of interfering emission from a noncomplementary Cy5-DNA oligomers due to weaker coupling of the more distant fluorophores with the surface plasmons. We expect SPCE to have numerous applications to nucleic acid analysis and for the measurement of bioaffinity reactions.     

Ultraviolet surface plasmon-coupled emission using thin aluminum films

Surface plasmon-coupled emission (SPCE) is the directional radiation of light into a substrate due to excited fluorophores above a thin metal film. To date, SPCE has only been observed with visible wavelengths using silver or gold films. We now show that SPCE can be observed in the ultraviolet region of the spectrum using thin (20 nm) aluminum films. We observed directional emission in a quartz substrate from the DNA base analogue 2-aminopurine (2-AP). The SPCE radiation occurs within a narrow angle at 59 degrees from the normal to the hemicylindrical prism. The excitation conditions precluded the creation of surface plasmons by the incident light. The directional emission at 59 degrees is almost completely p-polarized, consistent with its origin from surface plasmons due to coupling of excited 2-AP with the aluminum. The emission spectra and lifetimes of the SPCE are those characteristic of 2-AP. Different emission wavelengths radiate at slightly different angles on the prism providing intrinsic spectral resolution from the aluminum film. These results indicate that SPCE can be used with numerous UV-absorbing fluorophores, suggesting biochemical applications with simultaneous surface plasmon resonance and SPCE binding assays.     

Use of surface plasmon-coupled emission for enhancing light transmission through Top-Emitting Organic Light Emitting Diodes

In this paper, we perform surface plasmon-coupled emission studies on Rhodamine 6G molecules embedded in a corrugated structure of a thin film composed of fluorinated silica particles, and a binding medium. Our results show enhancements of photoluminescence due to surface corrugation. By varying the size of the fluorinated silica nanoparticles we were able to control the surface correlation length scale of the corrugated surface structure. It was found that the coupling efficiency of the directional light emission is strongly correlated to the surface morphology, particularly the surface correlation length, of the corrugated dielectric structure. This substantial enhancement of signal could potentially be utilized in Organic Light Emitting Diode devices to enhance the light emission and transmission through a thin silver layer which can also serve as the cathode in Top-Emitting Organic Light Emitting Diodes.     

Waveguide-modulated surface plasmon-coupled emission of Nile blue in poly(vinyl alcohol) thin films

Surface plasmon-coupled emission (SPCE) phenomenon is the coupling of excited fluorophores near a silver film with surface plasmons, resulting in directional emission into the underlying glass substrates. We report a complex coupling of Nile Blue fluorophore with 50 nm silver mirror, resulting in emission at several angles in the glass substrate, with either s or p polarization. This complex pattern of directional and polarized emission appears to be due to optical waveguide effects occurring when the sample thickness becomes comparable to the emission wavelength. We expect waveguide-modulated SPCE to have applications to biophysics and sensing.     

New immunoassay platform utilizing yeast surface display and direct cell counting

In this study, we report a new immunoassay platform using yeast cell surface display. This method holds promise for very low limit of detection (LOD) and is suitable for 2-Plex antibody recognition. Instead of adopting a conventional enzyme linked immunosorbent assay (ELISA) protocol by detecting the enzymatic activities or other physicochemical properties of the labeled analytes, this approach determines the quantity of an antibody analyte by directly counting the amount of "modified" yeast cells bound with antibody on the cell surface. c-myc and hemagglutinin (HA) tags were employed as an epitope model to demonstrate our approach. This yeast surface display based cell counting immunoassay (abbreviated as YSD-CCI) for anti-c-myc has a detection limit of 0.2 ng/mL, which is about 80 times higher than that of a conventional yeast ELISA under a similar condition. Moreover, the YSD-CCI's capability for 2-Plex antibody detection was demonstrated by simultaneous detection of anti-c-myc and anti-HA using engineered yeast cells expressing intracellular enhanced green fluorescent protein (EGFP) and mCherry, respectively. This proof-of-concept study paves the way for a new ultrasensitive multiplexed immunoassay method for diagnostic applications.     

Sequential injection immunoassay utilizing immunomagnetic beads.

A novel sequential injection immunoassay (SIIA) method is described which utilizes immunomagnetic beads to investigate short-time antibody binding. The method is versatile and flexible and may therefore be adapted to many different applications. Initial results for a competitive assay are also presented. The immunomagnetic bead reactor is created within the flowing stream by retaining immunomagnetic beads with an electromagnet to form an open tube reactor. Thus, the spent beads may be discharged after each analysis. This eliminates the problems of instability of reaction surfaces and eliminates the need for additional time traditionally required for regeneration of the solid-reacting phase in order to not only save time and increase sampling frequency but also to provide each individual sampling cycle with a fresh, uniform portion of beads. The spent beads are collected off line and may be regenerated later. Short-time binding kinetic studies demonstrate linear initial binding under 1 min, which then begins to reach saturation in approximately 10 min. Competitive binding assays of monoclonal mouse IgG (MRC OX-19) to polyclonal sheep anti-mouse IgG immobilized to the immunomagnetic beads show reproducible linear displacement in 30-120-s reactions. Fluorescence detection is utilized with a detection limit of 155 ng/mL, and since the reaction time is typically 2 min or shorter, the sampling frequency is 30 samples/h.     

Plasmonic antennas for directional sorting of fluorescence emission

Spontaneous emission of fluorescent molecules or quantum dots is radiated along all directions when emitters are diluted in a liquid solution, which severely limits the amount of collected light. Besides, the emission direction does not carry any useful information and cannot be used to sort different molecules. To go beyond these limits, optical antennas have been recently introduced as conceptual tools to control the radiation properties for nanoemitters fixed on a substrate. Despite intense recent research, controlling the luminescence directivity remains a challenge for emitters with random positions and orientations, which is a key for several biomolecular screening applications. Here, we present full directional control of the fluorescence emission from molecules in water solution by an optical antenna made of a nanoaperture surrounded by a periodic set of shallow grooves in a gold film. For each emission wavelength, the fluorescence beam can be directed along a specific direction with a given angular width, hereby realizing a micrometer-size dispersive antenna. We demonstrate the fluorescence beaming results from an interference phenomenon and provide physical optics guidelines to control the fluorescence directivity by tuning the groove-nanoaperture distance. This photon-sorting capability provides a new approach for high-sensitivity screening of molecular species in solution.     

Coherent, directional, laserlike emission from random gain media

We present experimental evidence for coherent, directional emission from a random gain medium above the threshold for laserlike emission. In particular, we report the first observation (to our knowledge) of a coherent angular backscattering peak above this threshold. A plausible explanation of our findings follows from the assumption that superfluorescent emission occurs in the gain medium above threshold.     

基于高鲁棒性超声超表面的声束定向发射

基于超薄平面型超声超表面构建了超声束定向发射器件,利用有限元仿真验证了声束定向发射的高效性和采用不同单元结构配置时定向发射效果的高鲁棒性。所设计的超表面器件由结构单元周期性排列而成,在保持结构周期性不变的条件下,尽管结构单元几何形状呈随机分布、或者结构中存在几何缺陷,但仍可有效地保持声束定向发射的良好性能。基于超声超表面的超声束定向发射器件具有结构设计简单、定向准确性高、灵活性强、适用范围广等优点,在工业无损检测、水下探测、医学超声工程等领域具有广泛的应用前景。     

Lab-on-a-bubble surface enhanced Raman indirect immunoassay for cholera

We describe a novel sandwich assay based on surface enhanced Raman scattering (SERS) comprised of buoyant silica microspheres coated with antibodies against the β subunit of the cholera toxin (CT), and gold nanoparticles tagged with a Raman reporter, shelled with silica and coated with antibodies against the β subunit of the CT. Together these components couple to form a sandwich which, after incubation, floats on the surface of the sample. The buoyant silica microparticle/nanoparticle reporter combination has been coined a lab on a bubble (LoB). LoB materials may provide a platform for rapid detection of antigen in solution and offers advantages over lateral flow or magnetic pull-down assays. The Raman reporter provides a unique and intense signal to indicate a positive analysis. Our limit of detection for the β subunit of the CT in a buffer based system is 1100 ng.     

Compact magnetic antennas for directional excitation of surface plasmons

Plasmonics is considered as one of the most promising candidates for implementing the next generation of ultrafast and ultracompact photonic circuits. Considerable effort has been made to scale down individual plasmonic components into the nanometer regime. However, a compact plasmonic source that can efficiently generate surface plasmon polaritons (SPPs) and deliver SPPs to the region of interest is yet to be realized. Here, bridging the optical antenna theory and the recently developed concept of metamaterials, we demonstrate a subwavelength, highly efficient plasmonic source for directional generation of SPPs. The designed device consists of two nanomagnetic resonators with detuned resonant frequencies. At the operating wavelength, incident photons can be efficiently channeled into SPP waves modulated by the electric field polarization. By tailoring the relative phase at resonance and the separation between the two nanoresonators, SPPs can be steered to predominantly propagate along one specific direction. This novel magnetic nanoantenna paves a new way to manipulate photons in the near-field, and also could be useful for SPP-based nonlinear applications, active modulations, and wireless optical communications.     

Myoglobin adsorption onto poly(glycidyl methacrylate) microbeads with surface functionalized iminodiacetic acid

In this study, poly(glycidyl methacrylate) [PGMA] microbeads with surface modified iminodiacetic acid (IDA) were used for myoglobin (Mb) adsorption from buffer solutions at different pHs and ionic strengths in a packed-bed column. Attenuated Total Reflectance Fourier Transformed Infrared (ATR-FTIR) spectroscopy and scanning electron microscopy (SEM) measurements before and after the adsorption process confirmed the structural stability of adsorbed Mb. The effects of initial concentration, flow-rate, pH and ionic strength on the adsorption were investigated. The results showed that the maximum amount of protein was adsorbed at a pH 7.0, which is the protein isoelectric point. The adsorption is rationalized in terms of local electrostatic forces acting between the protein and the IDA modified PGMA surface as well as hydrophobic interactions close to the protein isoelectric point, whereas at low pH the global changes give rise to protein–protein repulsion and at high pH protein-support material repulsion.     

Concentration gradient immunoassay. 1. An immunoassay based on interdiffusion and surface binding in a microchannel

We describe a novel microfluidic immunoassay method based on the diffusion of a small-molecule analyte into a parallel-flowing stream containing a cognate antibody. This interdiffusion results in a steady-state gradient of antibody binding site occupancy transverse to convective flow. In contrast to the diffusion immunoassay (Hatch, A.; Kamholz, A. E.; Hawkins, K. R.; Munson, M. S.; Schilling, E. A.; Weigl, B. H.; Yager, P. Nat. Biotechnol. 2001, 19, 461-465.), this antibody occupancy gradient is interrogated by a sensor surface coated with a functional analogue of the analyte. Antibodies with at least one unoccupied binding site may specifically bind to this functionalized surface, leading to a quantifiable change in surface coverage by the antibody. SPR imaging is used to probe the spatial distribution of antibody binding to the surface and, therefore, the outcome of the assay. We show that the pattern of antibody binding to the SPR sensing surface correlates with the concentration of a model analyte (phenytoin) in the sample stream. Using an inexpensive disposable microfluidic device, we demonstrate assays for phenytoin ranging in concentration from 75 to 1000 nM in phosphate buffer. At a total volumetric flow rate of 90 nL/s, the assays are complete within 10 min. Inclusion of an additional flow stream on the side of the antibody stream opposite to that of the sample enables simultaneous calibration of the assay. This assay method is suitable for rapid quantitative detection of low molecular weight analytes for point-of-care diagnostic instrumentation.     

Investigation of cocaine plumes using surface acoustic wave immunoassay sensors

Vapor sensors, aka electronic noses, are becoming an increasingly popular analytical tool for detection and identification of small molecules in the gas phase. In this paper, we present the results of a series of experiments demonstrating real-time vapor phase detection of cocaine molecules. A distinctive response or signature was observed under laboratory conditions in which the cocaine vapors were presented using an INEL vapor generator and under "field" conditions facilitated by the Georgia Bureau of Investigation (GBI) Crime Lab. For these experiments, the sensor component was a two-port resonator on ST-X quartz with a center frequency of approximately 250 MHz. On this cut of quartz, a temperature-compensated surface acoustic wave is generated via an interdigital transducer. Antibenzoylecgonine (anti-BZE) antibodies are attached to the electrodes on the device surface via a protein-A cross linker. We observed a large transient frequency shift accompanied by baseline shift with the anti-BZE coated sensor. After repeated experiments and the use of numerous controls, we believe that we have achieved real time molecular recognition of cocaine molecules.     

Electromagnetic and thermal models of emission of a cooled directional for local endocavity hyperthermia

Possibilities of creation of controlled temperature fields in deep-seated biological tissue with the use of a microwave endocavity applicator with directed emission and surface cooling are explored. Mathematical models are proposed and calculated that make it possible to construct electromagnetic and thermal fields in biotissue depending on the specific thermophysical and ultrasound characteristics of the medium being irradiated, and to reveal situations and effects which should be achieved to solve problems of practical medicine in the field of local microwave hyperthermia of tissues. Kiev Polytechnical Institute, Kiev, Ukraine. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 69, No. 5, pp. 785–789, September–October, 1996.     

Molecular recognition for electronic noses using surface acoustic wave immunoassay sensors

In this paper, we present the results of a series of experiments on vapor phase surface acoustic wave (SAW) sensors using a layer of antibodies as the chemically sensitive film. For these experiments, the sensor component was a ST-quartz resonator with a center frequency of approximately 250 MHz. Anti-FITC antibodies were attached to the electrodes on the device surface via a protein-A crosslinker. SAW resonator devices with various coatings were mounted in TO-8 packages, inserted into a sensor head module and subjected to various fluorescent analyte gases. Numerous controls were performed including the use of coated and uncoated devices along with devices coated with antibodies which were not specific for the target analyte. The SAW immunosensor response was monitored and a baseline frequency shift was observed when the analyte being presented was the antigen for the immobilized antibody. To provide an independent measure of antibody/antigen binding, the devices were removed from the sensor head, washed with a buffer solution to remove any unbound analyte, and then inspected using a confocal laser scanning microscope (CLSM). Since all the analytes being used in these experiments were fluorescent, this afforded us the opportunity to visualize the attachment of the analyte to the antibody film. Given the high resolution of the CLSM, we were able to identify the location of the attachment of the fluorescent analytes relative to the 1.5 /spl mu/m wide electrodes of the SAW device. We believe that these experiments demonstrate that we have achieved real time molecular recognition of these small molecules in the vapor phase.