Surface plasmon resonance detection for capillary electrophoresis separations

Electric field-enhanced backscatter interferometry detection for capillary electrophoresis

Backscatter interferometry (BSI) is a refractive index (RI) detection method that is easily integrated with capillary electrophoresis (CE) and is capable of detecting species ranging from inorganic ions to proteins without additional labels or contrast agents. The BSI signal changes linearly with the square of the separation voltage which has been used to quantify sample injection, but has not been explored as a potential signal enhancement mechanism in CE. Here we develop a mathematical model that predicts a signal enhancement at high field strengths, where the BSI signal is dominated by the voltage dependent mechanism. This is confirmed in both simulation and experiment, which show that the analyte peak area grows linearly with separation voltage at high field strengths. This effect can be exploited by adjusting the background electrolyte (BGE) to increase the conductivity difference between the BGE and analyte zones, which is shown to improve BSI performance. We also show that this approach has utility in small bore capillaries where larger separation fields can be applied before excess Joule heating degrades the separation. Unlike other optical detection methods that generally degrade as the optical pathlength is reduced, the BSI signal-to-noise can improve in small bore capillaries as the larger separation fields enhance the signal.

The development of microfluidic-based western blotting: Technical advances and future perspectives

Over the past two decades significant technical advancement in the field of western blotting has been made possible through the utilization of microfluidic technologies. In this review we provide a critical overview of these advancements, highlighting the advantages and disadvantages of each approach. Particular attention is paid to the development of now commercially available systems, including those for single cell analysis. This review also discusses more recent developments, including algorithms for automation and/or improved quantitation, the utilization of different materials/chemistries, use of projection electrophoresis, and the development of triBlots. Finally, the review includes commentary on future advances in the field based on current developments, and the potential of these systems for use as point-of-care devices in healthcare.

Silicon Photonic Microring Resonator Arrays as a Universal Detector for Capillary Electrophoresis

Electrophoretic separations conventionally rely on chromogenic, fluorogenic, or redox properties for analyte detection that, in many instances, involve chemical modification of samples prior to analysis. For analytes natively lacking chemical signatures, refractive index-based measurements are appealing as a method to detect these molecules without pretreatment. Microring resonators are a type of whispering gallery mode sensor capable of detecting bulk changes in refractive index. Here, we demonstrate the use of silicon photonic microring resonator arrays as a postcolumn detector for capillary electrophoresis. In this approach, we establish the universal detection capabilities of microrings through calibration with analytes lacking unique spectral signatures. Separations of small molecule mixtures are demonstrated using capillary zone electrophoresis. For these separations, the microring resonators maintain a linear response over several orders of magnitude in concentration for three candidate small molecules. Successful separation of three sugars with direct detection is also demonstrated. We further present the successful separation and detection of three model proteins, exemplifying the promise of microring resonators arrays as a biocompatible detector for capillary electrophoresis. Additionally, the spatially offset, array-based nature of the sensing platform enables real-time analysis of analyte mobility and performance characterization-a combination that is not typically provided using single-point detectors.

Wavelength Modulated Back-Scatter Interferometry for Universal, On-Column Refractive Index Detection in Picoliter Volumes

Wavelength-modulated back scatter interferometry (M-BSI) is shown to improve the detection metrics for refractive index (RI) sensing in microseparations. In M-BSI, the output of a tunable diode laser is focused into the detection zone of a separation channel as the excitation wavelength is rapidly modulated. This spatially modulates the observed interference pattern, which is measured in the backscattered direction. Phase-sensitive detection using a split photodiode detector aligned on one fringe of the interference pattern is used to monitor RI changes as analytes are separated. Using sucrose standards, we report a detection limit of 700 μg/L in a 75 μm i.d. capillary at the 3σ level, corresponding to a detection volume of 90 pL. To validate the approach for electrophoretic separations, Na⁺ and Li⁺ were separated and detected with M-BSI and indirect-UV absorbance on the same capillary. A 4 mg/L NaCl and LiCl mixture leads to comparable separation efficiencies in the two detection schemes, with better signal-to-noise in the M-BSI detection, but less baseline stability. The latter arises in part from Joule heating, which influences RI measurements through the thermo-optic properties of the run buffer. To reduce this effect, a 25 μm i.d. capillary combined with active temperature control was used to detect the separation of sucrose, glucose, and lactose with M-BSI. The lack of suitable UV chromophores makes these analytes challenging to detect directly in ultrasmall volumes. Using a 55 mM NaOH run buffer, M-BSI is shown to detect the separation of a mixture of 174 mg/L sucrose, 97 mg/L glucose, and 172 mg/L lactose in a 15 pL detection volume. The universal on-column detection in ultrasmall volumes adds new capabilities for microanalysis platforms, while potentially reducing the footprint and costs of these systems.

Recent Advances in Microscale Western Blotting

Western blotting is a ubiquitous tool used extensively in the clinical and research settings to identify proteins and characterize their levels. It has rapidly become a mainstay in research laboratories due to its specificity, low cost, and ease of use. The specificity arises from the orthogonal processes used to identify proteins. Samples are first separated based on size and then probed with antibodies specific for the protein of interest. This confirmatory approach helps avoid pitfalls associated with antibody cross-reactivity and specificity issues. While the technique has evolved since its inception, the last decade has witnessed a paradigm shift in Western blotting technology. The introduction of capillary and microfluidic platforms has significantly decreased time and sample requirements while enabling high-throughput capabilities. These advances have enabled Western analysis down to the single cell level in highly parallel formats, opening vast new opportunities for studying cellular heterogeneity. Recent innovations in microscale Western blotting are surveyed, and the potential for enhancing detection using advances in label-free biosensing is briefly discussed.

Rapid immunoglobulin M-based dengue diagnostic test using surface plasmon resonance biosensor

Surface plasmon resonance (SPR) is a medical diagnosis technique with high sensitivity and specificity. In this research, a new method based on SPR is proposed for rapid, 10-minute detection of the anti-dengue virus in human serum samples. This novel technique, known as rapid immunoglobulin M (IgM)-based dengue diagnostic test, can be utilized quickly and easily at the point of care. Four dengue virus serotypes were used as ligands on a biochip. According to the results, a serum volume of only 1 μl from a dengue patient (as a minimized volume) is required to indicate SPR angle variation to determine the ratio of each dengue serotype in samples with 83-93% sensitivity and 100% specificity.

Facile fabrication of an interface for online coupling of microchip CE to surface plasmon resonance

Background: The aim was to develop a simple route to coupling microchip CE (MCE) to surface plasmon resonance (SPR). MCE is a microfluidic technology that utilizes microfabrication techniques to connect interacting fluid reservoirs. Its advantages include rapid analysis (typically seconds), easy integration of multiple analytical steps and parallel operation. SPR detects changes in refractive index within a short distance from the surface of a thin metal film as variations in light intensity reflected from the back of the film and, thus, does not require labeling. There is a great demand for developing hyphenated techniques like MCE–SPR that are fast, sensitive and inexpensive to analyze biological materials. Materials & Methods: The separation channel and flow cell exist as overlapping regions constructed during the microchip production and buffer solution was delivered mechanically. Such a design has successfully isolated the electrical field inherent in the MCE from the SPR detector. Consequently, the potential interference to the SPR signal (or modulation of the density of surface plasmons at the gold chip) is circumvented. Results: The limits of detection for bovine serum albumin and sodium fluorescein were determined to be 7.5 µM and 3.1 mM, respectively. Conclusion: The technique described, herein, has been successfully applied in the separation of two species. The method offers the advantages of a near zero connection dead volume, electrical shielding from the separation voltage and minimization of the mass transfer effect.

Recent developments in instrumentation for capillary electrophoresis and microchip-capillary electrophoresis

Over the last years, there has been an explosion in the number of developments and applications of CE and microchip-CE. In part, this growth has been the direct consequence of recent developments in instrumentation associated with CE. This review, which is focused on the contributions published in the last 5 years, is intended to complement the articles presented in this special issue dedicated to instrumentation and to provide an overview of the general trends and some of the most remarkable developments published in the areas of high-voltage power supplies, detectors, auxiliary components, and compact systems. It also includes a few examples of alternative uses of and modifications to traditional CE instruments.

Fabrication and characterization of a sialoside-based carbohydrate microarray biointerface for protein binding analysis with surface plasmon resonance imaging

Monitoring multiple biological interactions in a multiplexed array format has numerous advantages. However, converting well-developed surface chemistry for spectroscopic measurements to array-based high-throughput screening is not a trivial process and often proves to be the bottleneck in method development. This paper reports the fabrication and characterization of a new carbohydrate microarray with synthetic sialosides for surface plasmon resonance imaging (SPRi) analysis of lectin-carbohydrate interactions. Contact printing of functional sialosides on neutravidin-coated surfaces was carried out and the properties of the resulting elements were characterized by fluorescence microscopy and atomic force microscopy (AFM). Sambucus nigra agglutinin (SNA) was deposited on four different carbohydrate functionalized surfaces and differential binding was analyzed to reveal affinity variation as a function of headgroup sialic acid structures and linking bonds. SPRi studies indicated that this immobilization method could result in high quality arrays with RSD < 5% from array element to array element, superior to the conventional covalent linkage used for protein cholera toxin (CT) in a comparison experiment, which yields nonuniform array elements with RSD > 15%. Multiplexed detection of SNA/biotinylated sialoside interactions on arrays up to 400 elements has been performed with good data correlation, demonstrating the effectiveness of the biotin-neutravidin-based biointerface to control probe orientation for reproducible and efficient protein binding to take place. Additionally, the regeneration of the array surface was demonstrated with a glycine stripping buffer, rendering this interface reusable. This in-depth study of array surface chemistry offers useful insight into experimental conditions that can be optimized for better performance, allowing many different protein-based biointeractions to be monitored in a similar manner.

Capillary electrophoresis-based immunoassays: principles and quantitative applications

The use of CE as a tool to conduct immunoassays has been an area of increasing interest over the last decade. This approach combines the efficiency, small sample requirements, and relatively high speed of CE with the selectivity of antibodies as binding agents. This review examines the various assay formats and detection modes that have been reported for these assays, along with some representative applications. Most CE immunoassays in the past have employed homogeneous methods in which the sample and reagents are allowed to react in solution. These homogeneous methods have been conducted as both competitive binding immunoassays and as noncompetitive binding immunoassays. Fluorescent labels are most commonly used for detection in these assays, but enzyme labels have also been utilized for such work. Some additional work has been performed in CE immunoassays with heterogeneous methods in which either antibodies or an analog of the analyte is immobilized to a solid support. These heterogeneous methods can be used for the selective isolation of analytes prior to their separation by CE or to remove a given species from a sample/reagent mixture prior to analysis by CE. These CE immunoassays can be used with a variety of detection modes, such as fluorescence, UV/Vis absorbance, chemiluminescence, electrochemical measurements, MS, and surface plasmon resonance.

GRAVI: Robotized Microfluidics for Fast and Automated Immunoassays in Low Volume

GRAVI, presented here in its automated version, is a new bench-top sized immunoassay platform combining the advantages of microfluidics with those of simplified robotics. Characterized by dramatically reduced time to result (< 10 min) and significantly decreased sample/reagent consumption, the cost-efficient biosensor instrumentation allows performing multimenu analysis with minimal laboratory infrastructure.

Coupled to a robotic liquid handler, the system dispenses samples and reagents from conventional plates or tubes into microchannels of a microchip (GRAVI- Chip), in which assays are processed and results readout. As in conventional 96-well microtiter plates, the microchannels have a standard spacing of 9 mm to facilitate automation.

With solely gravity and capillary force-driven fluidics within the microchannels, liquids are free to flow while magnetic beads, functionalized with the antibody of choice, are trapped nearby incorporated electrodes by virtue of a magnet array. Following assay performance and electrochemical signal detection in the parallel microchannels, chips are regenerated by magnet release and rinsing of beads out from the microchannels.

Applicability of the presented immunoassay platform, delivering 100 results per hour, is exemplified here with results from the validation of an immunoglobulin assay for antibody quantification in mammalian cell cultures. Adapted to run on the GRAVI platform, this competitive assay covers a dynamic range of two orders of magnitude.

Surface plasmon resonance study of protein-carbohydrate interactions using biotinylated sialosides

Lectins are carbohydrate binding proteins found in plants, animals, and microorganisms. They serve as important models for understanding protein-carbohydrate interactions at the molecular level. We report here the fabrication of a novel sensing interface of biotinylated sialosides to probe lectin-carbohydrate interactions using surface plasmon resonance spectroscopy (SPR). The attachment of carbohydrates to the surface using biotin-NeutrAvidin interactions and the implementation of an inert hydrophilic hexaethylene glycol spacer (HEG) between the biotin and the carbohydrate result in a well-defined interface, enabling desired orientational flexibility and enhanced access of binding partners. The specificity and sensitivity of lectin binding were characterized using Sambucus nigra agglutinin (SNA) and other lectins including Maackia amurensis lectin (MAL), concanavalin A (Con A), and wheat germ agglutinin (WGA). The results indicate that alpha2,6-linked sialosides exhibit high binding affinity to SNA, while alteration in sialyl linkage and terminal sialic acid structure compromises the affinity by a varied degree. Quantitative analysis yields an equilibrium dissociation constant (KD) of 777 +/- 93 nM for SNA binding to Neu5Ac alpha2,6-LHEB. Transient SPR kinetics confirms the K D value from the equilibrium binding studies. A linear relationship was obtained in the 10-100 microg/mL range with limit of detection of approximately 50 nM. Weak interactions with MAL, Con A, and WGA were also quantified. The control experiment with bovine serum albumin indicates that nonspecific interaction on this surface is insignificant over the concentration range studied. Multiple experiments can be performed on the same substrate using a glycine stripping buffer, which selectively regenerates the surface without damaging the sialoside or the biotin-NeutrAvidin interface. This surface design retains a high degree of native affinity for the carbohydrate motifs, allowing distinction of sialyl linkages and investigation pertaining to the effect of functional group on binding efficiency. It could be easily modified to identify and quantify binding patterns of any low-affinity biologically relevant systems, opening new avenues for probing carbohydrate-protein interactions in real time.

Postcolumn renewal of sensor surfaces for high-performance liquid chromatography-surface plasmon resonance detection

The combination of high-performance liquid chromatography (HPLC) with surface plasmon resonance (SPR) for continuous separation and label-free detection of protein samples is described. The detection was realized by electrostatic adsorption of proteins bearing positive and negative charges onto chemically modified SPR sensors in two separate SPR channels. One SPR channel is coated with carboxymethylated dextran which facilitates the detection of positively charged proteins, whereas the other, devoted to the monitoring of negatively charged proteins, is covered with ethylenediamine molecules attached onto a dextran surface. Renewal of the sensor surface in the channels can be accomplished by introducing regeneration solutions through two six-port valves. The coupled technique (HPLC-SPR) was assessed for its analytical figures of merit and applied to the quantification of lysozyme in human milk samples. Unlike the SPR detection of bulk solution refractive index changes during chromatographic peak elutions, the highest sensitivity of SPR is retained in this work since the measurement is performed at the SPR sensor surface where the evanescent field is the strongest. The renewable SPR detection of continuous separations is reproducible and versatile and does not require the separated proteins to contain chromophores or to be prelabeled with a tag (e.g., a redox-active or fluorescent molecule). Such generality makes SPR complementary to other types of chromatographic detectors.

Strategies for label-free optical detection

A large number of methods using direct detection with label-free systems are known. They compete with the well-introduced fluorescence-based methods. However, recent applications take advantage of label-free detection in protein-protein interactions, high-throughput screening, and high-content screening. These new applications require new strategies for biosensors. It becomes more and more obvious that neither the transduction principle nor the recognition elements for the biomolecular interaction process alone determine the quality of the biosensor. Accordingly, the biosensor system has to be considered as a whole. This chapter focuses on strategies to optimize the detection platform and the biomolecular recognition layer. It concentrates on direct detection methods, with special focus on optical transduction. Since even this restriction still leaves a large number of methods, only microrefractometric and microreflectometric methods using planar transducers have been selected for a detailed description and a listing of applications. However, since many review articles on the physical principles exist, the description is kept short. Other methods are just mentioned in brief and for comparison. The outlook and the applications demonstrate the future perspectives of direct optical detection in bioanalytics.

Potential of label-free detection in high-content-screening applications

The classical approach of high-content screening (HCS) is based on multiplexed, functional cell-based screening and combines several analytical technologies that have been used before separately to achieve a better level of automation (scale-up) and higher throughput. New HCS methods will help to overcome the bottlenecks, e.g. in the present development chain for lead structures for the pharmaceutical industry or during the identification and validation process of new biomarkers. In addition, there is a strong need in analytical and bioanalytical chemistry for functional high-content assays which can be provided by different hyphenated techniques. This review discusses the potential of a label-free optical biosensor based on reflectometric interference spectroscopy (RIfS) as a bridging technology for different HCS approaches. Technical requirements of RIfS are critically assessed by means of selected applications and compared to the performance characteristics of surface plasmon resonance (SPR) which is currently the leading technology in the area of label-free optical biosensors.

Integrated Label-Free Protein Detection and Separation in Real Time Using Confined Surface Plasmon Resonance Imaging

We demonstrate a label-free protein detection and separation technology for real-time monitoring of proteins in micro/nanofluidic channels, confined surface plasmon resonance imaging (confined-SPRi). This was achieved by fabricating ultrathin fluidic channels (500 nm high, 500 microm wide) directly on top of a specialized SPRi sensor surface. In this way, SPRi is uniquely used to detect proteins deep into the fluidic channel while maintaining high lateral accuracy of separated products. The channel fluid and proteins were driven electrokinetically under an external electric field. For this to occur, the metallic SPR sensor (46 nm of Au on 2 nm of Cr) was segmented into an array of squares (each 200 microm x 200 microm in size and spaced 8 microm apart) and coated with 30 nm of CYTOP polymer. In this work, we track label-free protein separation in real time through a simple cross-junction fluidic device with an 8-mm separation channel length under 30 V/cm electric field strength.

Integrated Refractive Index Optical Ring Resonator Detector for Capillary Electrophoresis

We developed a novel miniaturized and multiplexed, on-capillary, refractive index (RI) detector using liquid core optical ring resonators (LCORRs) for future development of capillary electrophoresis (CE) devices. The LCORR employs a glass capillary with a diameter of approximately 100 mum and a wall thickness of a few micrometers. The circular cross section of the capillary forms a ring resonator along which the light circulates in the form of the whispering gallery modes (WGMs). The WGM has an evanescent field extending into the capillary core and responds to the RI change due to the analyte conducted in the capillary, thus permitting label-free measurement. The resonating nature of the WGM enables repetitive light-analyte interaction, significantly enhancing the LCORR sensitivity. This LCORR architecture achieves dual use of the capillary as a sensor head and a CE fluidic channel, allowing for integrated, multiplexed, and noninvasive on-capillary detection at any location along the capillary. In this work, we used electro-osmotic flow and glycerol as a model system to demonstrate the fluid transport capability of the LCORRs. In addition, we performed flow speed measurement on the LCORR to demonstrate its flow analysis capability. Finally, using the LCORR's label-free sensing mechanism, we accurately deduced the analyte concentration in real time at a given point on the capillary. A sensitivity of 20 nm/RIU (refractive index units) was observed, leading to an RI detection limit of 10-6 RIU. The LCORR marries photonic technology with microfluidics and enables rapid on-capillary sample analysis and flow profile monitoring. The investigation in this regard will open a door to novel high-throughput CE devices and lab-on-a-chip sensors in the future.

Quantitative multispectral biosensing and 1D imaging using quasi-3D plasmonic crystals

We developed a class of quasi-3D plasmonic crystal that consists of multilayered, regular arrays of subwavelength metal nanostructures. The complex, highly sensitive structure of the optical transmission spectra of these crystals makes them especially well suited for sensing applications. Coupled with quantitative electrodynamics modeling of their optical response, they enable full multiwavelength spectroscopic detection of molecular binding events with sensitivities that correspond to small fractions of a monolayer. The high degree of spatial uniformity of the crystals, formed by a soft nanoimprint technique, provides the ability to image binding events over large areas with micrometer spatial resolution. These features, together with compact form factors, low-cost fabrication procedures, simple readout apparatus, and ability for direct integration into microfluidic networks and arrays, suggest promise for these devices in label-free bioanalytical detection systems.

In situ sensing of metal ion adsorption to a thiolated surface using surface plasmon resonance spectroscopy

The kinetics of the adsorption of metal ions onto a thiolated surface and the selective and quantitative sensing of metal ions were explored using surface plasmon resonance (SPR) spectroscopy. The target metal ion was an aqueous solution of Pt2+ and a thin-gold-film-coated glass substrate was modified with 1,6-hexanedithiol (HDT) as a selective sensing layer. SPR spectroscopy was used to examine the kinetics of metal ion adsorption by means of the change in SPR angle. The selectivity of the thiolated surface for Pt2+ over other divalent metal ions such as Cu2+, Ni2+, and Cd2+ was evident by the time-resolved SPR measurement. SPR angle shift, deltatheta(SPR), was found to increase logarithmically with increasing concentration of Pt2+ in the range of 1.0 x 10(-5)-1.0 mM. The rate of Pt2+ adsorption on HDT observed at both 0.1 and 1 mM Pt2+ accelerates until the surface coverage reaches approximately 17%, after which the adsorption profile follows Langmuirian behavior with the surface coverage. The experimental data indicated that heavy metal ions were adsorbed to the hydrophobic thiolated surface by a cooperative mechanism. A mixed self-assembled monolayer (SAM) composed of HDT and 11-mercaptoundecanoic acid was used to reduce the hydrophobicity of the thiol-functionalized surface. The addition of hydrophilic groups to the surface enhanced the rate of adsorption of Pt2+ onto the surface. The findings show that the adsorption of metal ions is strongly dependent upon the hydrophilicity/hydrophobicity of the surface and that the technique represents an easy method for analyzing the adsorption of metal ions to a functionalized surface by combining SPR spectroscopy with a SAM modification.

Unconventional detection methods for microfluidic devices

The direction of modern analytical techniques is to push for lower detection limits, improved selectivity and sensitivity, faster analysis time, higher throughput, and more inexpensive analysis systems with ever-decreasing sample volumes. These very ambitious goals are exacerbated by the need to reduce the overall size of the device and the instrumentation - the quest for functional micrototal analysis systems epitomizes this. Microfluidic devices fabricated in glass, and more recently, in a variety of polymers, brings us a step closer to being able to achieve these stringent goals and to realize the economical fabrication of sophisticated instrumentation. However, this places a significant burden on the detection systems associated with microchip-based analysis systems. There is a need for a universal detector that can efficiently detect sample analytes in real time and with minimal sample manipulation steps, such as lengthy labeling protocols. This review highlights the advances in uncommon or less frequently used detection methods associated with microfluidic devices. As a result, the three most common methods - LIF, electrochemical, and mass spectrometric techniques - are omitted in order to focus on the more esoteric detection methods reported in the literature over the last 2 years.

Biomolecular interaction analysis under electrophoretic flow conditions

Combining the advantages of electrophoresis with the advantages of biomolecular interaction analysis (BIA) enables the biospecific detection of separated molecules; for example it permits differentiation between a complementary single-stranded DNA and a single nucleotide polymorphism. In order to integrate these two techniques, it is necessary to investigate whether it is possible to detect a biomolecular interaction under electrophoretic flow conditions. To this end a novel detection system was developed for electrophoresis that utilizes a label-free and time-resolved detection technique: reflectometric interference spectroscopy (RIfS). The biological functions of important analytes were investigated using this system. Although RIfS can be used as a postcolumn detector, it is also possible to use it to detect relevant substances under electrophoretic flow conditions. DNA-LNA, biotin-streptavidin and protein-protein interactions were detected using this coupled electrophoresis-RIfS set-up.

Denaturation and renaturation of self-assembled yeast iso-1-cytochrome c on Au

We have made surface plasmon resonance (SPR) measurements of yeast iso-1-cytochrome c (Cyt c) on a gold surface. Angle-resolved SPR curves are recorded as a function of urea concentration before and after self-assembly of the Cyt c. Exposure to a urea solution causes denaturation of Cyt c, which shifts the minimum in the SPR curve to a larger angle and decreases the signal amplitude. The Gibbs free energy change for denaturation of the protein on Au is calculated from the change of the SPR signal amplitude with urea concentration. We find that (1) Cyt c can be reversibly denatured and renatured, depending on the urea concentration, and (2) the Gibbs free energy change for denaturation of Cyt c on Au surface in water, DeltaG degrees (water), is 1.5 kcal/mol, which is approximately 4 times less than that in bulk solution.

Survey of the year 2003 commercial optical biosensor literature

In the year 2003 there was a 17% increase in the number of publications citing work performed using optical biosensor technology compared with the previous year. We collated the 962 total papers for 2003, identified the geographical regions where the work was performed, highlighted the instrument types on which it was carried out, and segregated the papers by biological system. In this overview, we spotlight 13 papers that should be on everyone's 'must read' list for 2003 and provide examples of how to identify and interpret high-quality biosensor data. Although we still find that the literature is replete with poorly performed experiments, over-interpreted results and a general lack of understanding of data analysis, we are optimistic that these shortcomings will be addressed as biosensor technology continues to mature.

Surface Plasmon-Coupled Emission with Gold Films

In a recent report we demonstrated efficient collection of emission by coupling to surface plasmons on a thin silver film, resulting in a directional signal in the glass substrate. We call the phenomenon surface plasmon coupled emission (SPCE). In the present report we examined sulforhodamine 101 (S101) in thin polymer films on 50 nm thick gold films on glass. We observed efficient SPCE through thin gold films. This result was surprising because metallic gold is typically an efficient quencher of fluorescence. The energy effectively coupled through the gold film into the glass at a sharply defined angle, but somewhat less sharp than for a comparable silver film. About 50% of the total emission appeared as SPCE, irrespective of direct excitation or excitation via the plasmon resonance evanescent wave. The emission was p-polarized with different wavelengths appearing at different angles. The lifetime of S101 was mostly unaffected by the gold film. These results indicate that SPCE occurs over long distances, larger than for quenching by energy transfer to the gold. We conclude highly efficient detection devices can be constructed by using fluorophores on gold-coated surfaces.