Two-photon excitation fluorometric measurement of homogeneous microparticle immunoassay for C-reactive protein

Microbial identification from faces and urine in one step by two-photon excitation assay technique

Two-photon excitation fluorometry (TPX) is a separation-free bioaffinity assay technique which enables accurate diagnostic testing in microvolumes. The technology is currently commercially applied in an automated mariPOC® test system for rapid phenotypic multi-microbe detection of pathogen antigens. The first TPX applications for diagnostics were intended for respiratory infection testing from nasopharyngeal and oropharyngeal samples. Feces and urine are more complex sample matrices and contain substances that may interfere with immunoassay binding or fluorescence detection. Our objective was to study the suitability of these complex matrices in the TPX technique. As expected, feces and urine elevated fluorescence levels but the methodology has the unique property of compensating for matrix effects. Compensation allows reliable separation of specific fluorescence from the fluorescence caused by the matrix. The studied clinical samples did not contain immunoassay inhibitors. The results suggest that the methodology is robust and may provide reliable testing of feces and urine samples with high accuracy.

High-throughput screening of colonization samples for methicillin-resistant Staphylococcus aureus

BACKGROUND

We present here the first application of 2-photon excited fluorescence detection (TPX) technology for the direct screening of clinical colonization samples for methicillin-resistant Staphylococcus aureus (MRSA).

METHODS

A total of 125 samples from 14 patients with previously identified MRSA carriage and 16 controls from low-prevalence settings were examined.

RESULTS

The results were compared to those obtained by both standard phenotypic and molecular methods. In identifying MRSA carriers, i.e. persons with at least 1 MRSA positive colonization sample by standard methods, the sensitivity of the TPX technique was 100%, the specificity 78%, the positive predictive value 75%, and the negative predictive value 100%. The TPX assay sensitivity per colonization sample was 89%, the specificity 93%, the positive predictive value 84%, and the negative predictive value 95%. The median time for a true-positive test result was 3 h and 26 min; negative test results are available after 13 h. The assay capacity was 48 samples per test run.

CONCLUSIONS

The TPX MRSA technique could provide early preliminary results for clinicians, while simultaneously functioning as a selective enrichment step for further conventional testing. Costs and workload associated with hospital infection control can be reduced using this high-throughput, point-of-care compatible methodology.

Nonspecific particle-based method with two-photon excitation detection for sensitive protein quantification and cell counting

A novel easy-to-use homogeneous method utilizing two-photon excitation (TPX) for quantification of proteins or counting of eukaryotic cells in solution has been developed. This highly sensitive technique is based on the adsorption competition between the sample and fluorescently labeled protein to micrometer-sized carboxylate modified polystyrene particles and detection of two-photon excited fluorescence. The adsorption of the labeled protein to the particles was detected as a distinct fluorescence on individual microparticles. Analyte protein or eukaryotic cells interacted with particle surface and reduced the adsorption of labeled protein to the particles resulting in a decrease of the fluorescence. The optimizations of assay conditions were performed separately for protein quantification and cell counting, and the principle of the method was confirmed with the fluorescence microscopy imaging. The protein quantification assay allowed the determination of picogram quantities (1.2 μg/L) of protein, and the cell counting assay allowed three cells in the sample with an average variation of approximately 10% in the signal. The protein assay sensitivity was more than 500-fold improved from the common most sensitive commercial methods. Moreover, the dynamic range of the assay was broad, approximately 4 orders of magnitude. The cell assay has sensitivity comparable to the most sensitive commercial method. The developed method tolerates interfering agents such as neutral detergents found in cell lysate samples even at high concentrations. The method is experimentally fairly simple and allows the expansion for the use of the TPX technology.

Sensitive detection of cardiac biomarker using ZnS nanoparticles as novel signal transducers

C-reactive protein (CRP), a 115 kDa pentameric protein, is one of the important cardiac biomarkers that are indicative of coronary heart events. Sensitive detection of CRP in human serum is critical for the diagnosis of coronary heart disease. This work presents a sensitive sandwich immunoassay for the detection of CRP in human serum using zinc sulfide (ZnS) nanoparticles as novel fluorescence signal transducers. In this assay, monoclonal anti-CRP antibodies are used to capture CRP in human serum, and then the captured CRPs are incubated with biotinylated monoclonal anti-CRP and Neutravidin coated ZnS nanoparticle to form sandwich immunocomplexes. Quantification of CRP occurs when zinc ions released from ZnS nanoparticle labels are mixed with zinc-ion sensitive fluorescence indicator Fluozin-3 for fluorescence generation. The developed assay presents a detection limit around 10 pM and a detection range with more than two orders of magnitude.

Confocal detection of planar homogeneous and heterogeneous immunosorbent assays

Optically sectioned detection of fluorescence immunoassays using a confocal microscope enables the creation of both homo- and heterogeneous planar format assays. We report a set assays requiring optically sectioned detection using a model system and analysis procedures for separating signals of a surface layer from an overlying solution. A model sandwich assay with human immunoglobulin G as the target antigen is created on a glass substrate. The prepared surfaces are exposed to antigen and a FITC-labeled secondary antibody. The resulting preparations are either read directly to provide a homogeneous assay or after wash steps, giving a heterogeneous assay. The simplicity of the object shapes arising from the planar format makes the decomposition of analyte signals from the thin film bound to the surface and overlayer straightforward. Measured response functions of the thin film and overlayer fit well to the Cauchy-Lorentz and cumulative Cauchy-Lorentz functions, respectively, enabling the film and overlayer to be separated. Under the conditions used, the detection limits for the homogeneous and heterogeneous forms of the assay are 2.2 and 5.5 ng/ml, respectively. Planar format, confocally read fluorescence assays enable wash-free detection of antigens and should be applicable to a wide range of assays involving surface-bound species.

Methicillin-resistant Staphylococcus aureus screening by online immunometric monitoring of bacterial growth under selective pressure

Rapid, high-throughput screening tools are needed to contain the spread of hospital-acquired methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) strains. Most techniques used in current clinical practice still require time-consuming culture for primary isolation of the microbe. We present a new phenotypic assay for MRSA screening. The technique employs a two-photon excited fluorescence (TPX) detection technology with S. aureus-specific antibodies that allows the online monitoring of bacterial growth in a single separation-free process. Different progressions of fluorescence signals are recorded for methicillin-susceptible and -resistant strains when the growth of S. aureus is monitored in the presence of cefoxitin. The performance of the new technique was evaluated with 20 MRSA strains, 6 methicillin-susceptible S. aureus strains, and 7 coagulase-negative staphylococcal strains and two different monoclonal S. aureus-specific antibodies. When either of these antibodies was used, the sensitivity and the specificity of the TPX assay were 100%. All strains were correctly classified within 8 to 12 h, and up to 70 samples were simultaneously analyzed on a single 96-well microtiter plate. As a phenotypic method, the TPX assay is suited for screening purposes. The final definition of methicillin resistance in any S. aureus strain should be based on the presence of the mecA gene. The main benefit afforded by the initial use of the TPX methodology lies in its low cost and applicability to high-throughput analysis.

Point-of-care testing of proteins

Point-of-care testing (POCT) is a fast developing area in clinical diagnostics that is considered to be one of the main driving forces for the future in vitro diagnostic market. POCT means decentralized testing at the site of patient care. The most important POCT devices are handheld blood glucose sensors. In some of these sensors, after the application of less than 1 microl whole blood, the results are displayed in less than 10 s. For protein determination, the most commonly used devices are based on lateral flow technology. Although these devices are convenient to use, the results are often only qualitative or semiquantitative. The review will illuminate some of the current methods employed in POCT for proteins and will discuss the outlook for techniques (e.g., electrochemical immunosensors) that could have a great impact on future POCT of proteins.

Development of a rapid assay methodology for antimicrobial susceptibility testing of Staphylococcus aureus

Development of a new phenotypic technique for rapid antimicrobial susceptibility testing (AST) of methicillin-resistant Staphylococcus aureus is presented. The new technique combines bacterial culturing and specific immunometric detection in a single separation-free process. The technique uses dry chemistry reagents and the recently developed two-photon excitation detection technology, which allows online detection of bacterium-specific growth. The performance of the new technique was evaluated by monitoring the growth of S. aureus reference strains and determining their susceptibility to oxacillin. In the direct analysis of clinical specimens, method specificity and tolerance to interferences caused by other bacteria present in the sample are pivotal. Other bacteria can compete with the bacteria of interest for nutrients, for example. Specificity and tolerance were studied against Staphylococcus epidermidis reference strains. The results suggest that the new technique could allow rapid AST directly from clinical samples within 6 to 8 h. Such a rapid and simple testing methodology would be a valuable tool in clinical microbiology because it would shorten the turnaround times of microbiologic analyses. Advantages of the new approach in relation to conventional methods are discussed.

Rapid method for detection of influenza a and B virus antigens by use of a two-photon excitation assay technique and dry-chemistry reagents

New separation-free assay methods for the rapid detection of influenza A and B virus antigens are presented. The methods employ dry-chemistry reagents and the recently developed two-photon excitation (TPX) fluorescence detection technology. According to the assay scheme, virus antigens are sandwiched by capture antibody onto polymer microspheres and fluorescently labeled antibody conjugate. Consequently, fluorescent immunocomplexes are formed on the surface of microspheres in proportion to the concentration of the analyte in the sample. The fluorescence signal from individual microspheres is measured, separation free, by means of two-photon excited fluorescence detection. In order to demonstrate the applicability of the new assay technique for virus antigen detection, methods for influenza A and B viruses were constructed. The assay method for influenza A virus applied a molecular fluorescent label, whereas the method for influenza B virus required a nanoparticle fluorescent reporter to reach sufficient clinical sensitivity. The new methods utilize a dry-chemistry approach, where all assay-specific reagents are dispensed into assay wells already in the manufacturing process of the test kits. The performance of the assay methods was tested with nasopharyngeal specimens using a time-resolved fluoroimmunoassay as a reference method. The results suggest that the new technique enables the rapid detection of influenza virus antigens with sensitivity and specificity comparable to that of the reference method. The dose-response curves showed linear responses with slopes equal to unity and dynamic assay ranges of 3 orders of magnitude. Applicability of the novel TPX technique for rapid multianalyte testing of respiratory infections is discussed.

Robust estimation of bioaffinity assay fluorescence signals

In this paper, the challenging problem of robust mean-signal estimation of a single-step microparticle bioaffinity assay is investigated. For this purpose, a density estimation-based robust algorithm (DER) was developed. The DER algorithm was comparatively evaluated with four other parameter estimation methods (mean value, median filtering, least square estimation, Welsch robust m-estimator). Two important questions were raised and investigated: 1) Which of the five methods can robustly estimate the mean bioaffinity signal? and 2) How many microparticles need to be measured in order to obtain an accurate estimate of the mean signal value? To answer the questions, bootstrap and coefficient of variation (CV) analyses were performed. In the CV analysis, the DER algorithm gave the best results: The CV ranged from 0.8% to 4.9% when the number of microparticles used for the mean signal estimation varied from 800 to 30. In the bootstrap analysis of the standard error, the DER algorithm had the smallest variance. As a conclusion, it can be underlined that: 1) of all methods tested, the DER algorithm gave the most consistent and reproducible results according to the bootstrap and CV analysis; 2) using the DER algorithm accurate estimates could be calculated based on 80-100 particles, corresponding to a typical assay measurement time of 1 min; and 3) the investigated bioaffinity signals contained a large number of outliers (observations that severely deviate from the majority of data) and therefore robust techniques were necessary for the mean signal estimation tasks.

Effect of Polystyrene Microsphere Surface to Fluorescence Lifetime Under Two-Photon Excitation

Molecular assays such as immunoassays are often performed using solid carriers and fluorescent labels. In such an assay format a question can be raised on how much the fluorescence of the label is influenced by the bio-affinity binding events and the solid carrier surface. Since changes in fluorescence intensity as labels bind to surfaces are notoriously difficult to quantify other approaches are preferred. A good indicator, independent of the fluorescence intensity of the label, is the fluorescence lifetime of the marker fluorophore. Changes in fluorescence lifetime reliably indicate the presence of dynamic quenching, energy transfer or other de-excitation processes. A microsphere based assay system is studied under two-photon excitation. Changes in fluorescence lifetime are studied as labeled protein conjugates bind on microsphere surfaces--both direct on the surface and with a few nanometer distance from the surface. Fluorescence signal is measured from individual polystyrene microspheres and the fluorescence lifetime histogram is simultaneously recorded. The results indicate that self-quenching and quenching by the polystyrene surface are both present in such a system. However, the effect of the surface can be avoided by increasing the distance between the surface and the label. Typical distances achieved by a standard sandwich type of assay, are already sufficient to overcome the surface induced quenching in fluorescence detection.

Calibration of bioaffinity assays using kinetic data

Bioaffinity assays are usually calibrated by using a set of standard measurements fitted to a simple empirical model. In this paper, a new calibration approach based on mechanistic model of reaction kinetics is presented. When the calibration assay is known in terms of reaction mechanism, incubation time, initial concentration, and rate constants, one can back-calculate concentrations of unknown samples measured in a nonequilibrium time point. This paper describes a calculation method of unknown sample concentrations based on kinetically measured single calibration assay point. The theoretical results are verified by two common in-vitro diagnostic assays.

A novel separation-free assay technique for serum antibodies using antibody bridging assay principle and two-photon excitation fluorometry

A new technique for separation-free detection of antigen-specific antibodies is presented. The new technique employs antibody bridging assay principle and the recently developed ArcDia TPX fluorescence detection technology. According to the assay scheme, antibody molecules from the sample bind with one arm to an antigen on polymer microspheres and with the other arm to a fluorescently labeled secondary antigen reagent. Consequently, fluorescent immunocomplexes are formed on the surface of microspheres in proportion to the concentration of the analyte in the sample. The fluorescence signal from individual microspheres is measured by means of two-photon excited fluorescence detection. In order to demonstrate the applicability of the new assay technique, an assay for anti-adenovirus antibodies was constructed. The function of the assay method was tested both with monoclonal anti-adenovirus antibody preparation (standard analyte), and with positive serum samples. Standard class-specific ELISA was used as a reference method. The new assay method provides comparable sensitivity and precision, and wider dynamic range for IgG antibodies than the ELISA method. The standard curve showed linear response (R(2)=0.999) with a dynamic range of three orders of magnitude, detection limit (mean+3S.D.) of 8 pM, and intra-assay signal precision of 5%. Applicability of the new method for clinical serodiagnostics is discussed.

A lab-on-a-chip compatible bioaffinity assay method for human alpha-fetoprotein

A new lab-on-a-chip compatible binding assay platform is introduced. The platform combines dry-chemistry bioaffinity reagents and the recently introduced ArcDia TPX binding assay technique. The technique employs polymer microspheres as a solid phase reaction carrier, fluorescently labeled antibody conjugates, and detection of fluorescence emission from the surface of individual microspheres by two-photon excitation fluorescence. Signal response of the technique is independent of the reaction volume, thus the technique is particularly well suited for detection of bioaffinity reactions from miniature volumes. Performance of the new assay platform is studied by means of an immunometric assay of human alpha-fetoprotein (hAFP) in 384-plate format, and the results are compared to those of a corresponding wet-chemistry assay method. The results show that the ArcDia TPX detection technique can be combined with dry-chemistry reagents without compromises in assay performance. The microchip field has so far been characterized with a lack of microchip-compatible detection platforms which would allow cost-effective microchip design and sensitive bioaffinity detection. The presented detection technique is expected to provide a solution for this shortage.

Dipyrrylmetheneboron difluorides as labels in two-photon excited fluorometry. Part II--Nucleic acid hybridization assays

Five two-photon excitable dipyrrylmetheneboron difluoride labels (dipyrrylmethene-BF(2) labels) with fluorescence emission maximum between 530 and 590 nm, and a frequently used rhodamine label, TAMRA, were conjugated to aminomodified oligonucleotides. The performance of the labeled oligonucleotides was studied in a separation-free nucleic acid hybridization assay using ArcDia TPX bioaffinity assay technology. The results show that oligonucleotide conjugates of dipyrrylmethene-BF(2) labels provide higher two-photon excited fluorescence yield and better assay sensitivity than corresponding TAMRA conjugate. The effect of conjugation on photophysical properties of the labels and performance of the labeled oligonucleotides in separation-free hybridization assay is discussed.

Plasmonic technology: novel approach to ultrasensitive immunoassays

At the Center for Fluorescence Spectroscopy, we have taken advantage of the favorable properties of surface plasmon-coupled emission (SPCE) to improve fluorescence-based immunoassays. SPCE occurs when excited fluorophores near conducting metallic structures efficiently couple to surface plasmons. These surface plasmons, appearing as free electron oscillations in the metallic layer, produce electromagnetic radiation that preserves the spectral properties of fluorophores but is highly polarized and directional. SPCE immunoassays provide several advantages over other fluorescence-based methods. This review explains new approaches to fluorescence immunoassays, including our own use of SPCE for simultaneous detection of more than one fluorescent marker and performance of immunoassays in the presence of an optically dense medium, such as whole blood.

Point-of-care immunotesting: Approaching the analytical performance of central laboratory methods

The use of point-of-care (POC) immunoassays has increased significantly and the menu of analytes continues to expand. Most of the rapid immunoassays are currently based on simple manual assay devices such as the immunochromatographic, agglutination, and immunofiltration assays. Although automated readers have recently been introduced at an increasing pace, the major benefit of these genuinely hand-portable assay devices is that they do not usually necessitate instrumentation but can be performed anywhere. Significant advances in assay and detection technologies have, however, recently facilitated the introduction of truly quantitative, sophisticated immunoassay methods to POC settings as well, with the analytical performance characteristics approaching those of conventional laboratory assays. Furthermore, innovative assay technologies such as those based on immunosensors have been introduced to POC testing (POCT) without ever being employed in clinical laboratories. However, further simplification of the assay procedures and analyzers is still feasible, and strong efforts are directed towards the development of miniaturized and simple, yet sensitive and quantitative, novel assay technologies to keep up with the increasing expectations set on future POC immunotesting.

Dipyrrylmetheneboron Difluorides as Labels in Two-Photon Excited Fluorometry. Part I-Immunometric Assays

Seven different two-photon excitable dipyrrylmetheneboron difluoride labels (dipyrrylmethene-BF(2) labels) and a frequently used TAMRA label were conjugated to mouse IgG against alpha-fetoprotein in variable substitution degrees. Altogether 40 IgG conjugates were prepared, and studied with respect to one-photon absorption and emission properties, and two-photon fluorescence efficiency using 1064 nm laser as illumination source. Performance of the IgG conjugates as tracers in a separation-free immunometric assay of alpha-fetoprotein was evaluated using two-photon excitation assay technology, ArcDia TPX. The results show that the dipyrrylmethene-BF(2) labels provide subpicomolar sensitivity, which is an order of magnitude better than that of TAMRA label. The effect of chromophore structure and substitution degree of IgG-label conjugates on the assay performance is discussed.

Fluorescence imaging of the activity of glucose oxidase using a hydrogen-peroxide-sensitive europium probe

A method for optical imaging of the activity of glucose oxidase (GOx) using a fluorescent europium(III) tetracycline probe for hydrogen peroxide is presented. A decay time in the microsecond range and the large Stokes shift of 210 nm of the probe facilitate intensity-based, time-resolved, and decay-time-based imaging of glucose oxidase. Four methods for imaging the activity of GOx were compared, and rapid lifetime determination imaging was found to be the best in giving a linear range from 0.32 to 2.7 m Unit/mL. The detection limit is 0.32 m Unit/mL (1.7 ng mL(-1)) which is similar to that of the time-resolved (gated) imaging using a microtiterplate reader. Fluorescent imaging of the activity of GOx is considered to be a useful tool for GOx-based immunoassays with potential for high-throughput screening, immobilization studies, and biosensor array technologies.

Hydrophilic labeling reagents of dipyrrylmethene-BF2 dyes for two-photon excited fluorometry: syntheses and photophysical characterization

Recently introduced bioaffinity assay technology, ArcDia TPX, is based on two-photon excited fluorescence (TPE) and it enables separation-free ultra-sensitive immunoassays from microvolumes. Here we present syntheses of novel two-photon excitable fluorescent labeling reagents which have been specially designed to be used as label molecules in the ArcDia TPX assay technique. The labeling reagents are based on dipyrrylmetheneboron difluoride (dipyrrylmethene-BF2) chromophore, which have been substituted with aryl, heteroaryl or arylalkenyl chemical groups to extend the pi-electron conjugation. These substitutions results in a series of dipyrrylmethene-BF2 fluorophores with different photophysical properties. Dipyrrylmethene-BF2 fluorophores have been further substituted with a dipeptide linker unit and finally activated as succinimidyl esters to enable specific coupling with primary amino groups. The dipeptide linker serves as a spacer arm between the label and a target, and enhances the solubility of the label in aqueous solutions. Study of the chemical and photophysical performance of the new labeling reagents is described. The new labeling reagents exhibit high fluorescence quantum yields, and molar absorption coefficients. The results show that the new labels with the hydrophilic dipeptide linker unit provide large two-photon excitation cross-sections, high fluorescence quantum efficiency and good solubility in aqueous solutions. The results suggest that the novel dipyrrylmethene-BF2 labels are highly applicable to bioaffinity assays based on two-photon excitation of fluorescence.

Fluorescent nanoparticles as labels for immunometric assay of C-reactive protein using two-photon excitation assay technology

We describe the use of fluorophore-doped nanoparticles as reporters in a recently developed ArcDia TPX bioaffinity assay technique. The ArcDia TPX technique is based on the use of polymer microspheres as solid-phase reaction carrier, fluorescent bioaffinity reagents, and detection of two-photon excited fluorescence. This new assay technique enables multiplexed, separation-free bioaffinity assays from microvolumes with high sensitivity. As a model analyte we chose C-reactive protein (CRP). The assay of CRP was optimized for assessment of CRP baseline levels using a nanoparticulate fluorescent reporter, 75 nm in diameter, and the assay performance was compared to that of CRP assay based on a molecular reporter of the same fluorophore core. The results show that using fluorescent nanoparticles as the reporter provides two orders of magnitude better sensitivity (87 fM) than using the molecular label, while no difference between precision profiles of the different assay types was found. The new assay method was applied for assessment of baseline levels of CRP in sera of apparently healthy individuals.

Nanoparticle labels in immunosensing using optical detection methods

Efforts to improve the performance of immunoassays and immunosensors by incorporating different kinds of nanostructures have gained considerable momentum over the last decade. Apart from liposomes, which will not be discussed here, most groups focus on artificial, particulate marker systems, both organic and inorganic. The underlying detection procedures may be based either on electro-magnetical or optical techniques. This review will be confined to the latter only, comprising nanoparticle applications generating signals as diverse as static and time-resolved luminescence, one- and two-photon absorption, Raman and Rayleigh scattering as well as surface plasmon resonance and others. In general, all endeavors cited are geared to achieve one or more of the following goals: lowering of detection limits (if possible, down to single-molecule level), parallel integration of multiple signals (multiplexing), signal amplification by several orders of magnitude and prevention of photobleaching effects with concomitant maintenance of antigen binding specificity and sensitivity. Inorganic nanoparticle labels based on noble metals, semiconductor quantum dots and nanoshells appear to be the most versatile systems for these bioanalytical applications of nanophotonics.

Myoglobin immunoassay utilizing directional surface plasmon-coupled emission

We described an immunoassay for the cardiac marker myoglobin on a thin silver mirror surface using surface plasmon-coupled emission (SPCE). SPCE occurs for fluorophores in proximity (within approximately 200 nm) of a thin metal film (in our case, silver) and results in a highly directional radiation through a glass substrate at a well-defined angle from the normal axis. We used the effect of SPCE to develop a myoglobin immunoassay on the silver mirror surface deposited on a glass substrate. Binding of the labeled anti-myoglobin antibodies led to the enhanced fluorescence emission at a specific angle of 72 degrees . The directional and enhanced directional fluorescence emission enables detection of myoglobin over a wide range of concentrations from subnormal to the elevated level of this cardiac marker. Utilizing SPCE allowed us also to demonstrate significant background suppression (from serum or whole blood) in the myoglobin immunoassay. We expect SPCE to become a powerful technique for performing immunoassays for many biomarkers in surface-bound assays.

New separation-free assay technique for SNPs using two-photon excitation fluorometry

A new separation-free method for detection of single nucleotide polymorphisms (SNPs) is described. The method is based on the single base extension principle, fluorescently labeled dideoxy nucleotides and two-photon fluorescence excitation technology, known as ArcDia trade mark TPX technology. In this assay technique, template-directed single base extension is carried out for primers which have been immobilized on polymer microparticles. Depending on the sequence of the template DNA, the primers are extended either with a labeled or with a non-labeled nucleotide. The genotype of the sample is determined on the basis of two-photon excited fluorescence of individual microparticles. The effect of various assay condition parameters on the performance of the assay method is studied. The performance of the new assay method is demonstrated by genotyping the SNPs of human individuals using double-stranded PCR amplicons as samples. The results show that the new SNP assay method provides sensitivity and reliability comparable to the state-of-the-art SNaPshot trade mark assay method. Applicability of the new method in routine laboratory use is discussed with respect to alternative assay techniques.

Detection methods of microsphere based single-step bioaffinity and in vitro diagnostics assays

Microspheres provide a solid phase substrate for bioaffinity binding similar to the walls of traditional test tubes and the wells of microtiter plates. The coated microsphere concentrates analyte molecules in the reaction volume on its surface. When the bioaffinity binding reaction has reached an equilibrium, the local concentration of the analyte in close proximity of the microsphere is orders of magnitude higher than the concentration of the analyte in the total reaction volume. The preparation and quality control of microspheres coated with bioactive material is less costly and labour intensive when compared to test tube or microwell plate coating procedures. In addition, the cost for logistics and transportation of microsphere reagents is lower than that of coated tubes or plates. Moreover, microspheres can be easily used in miniaturised assay formats and several different detection schemes can be employed in the measurement of microsphere-based assays. Several different types of microspheres are commercially available. The microspheres can be manufactured in different sizes from many materials, such as polystyrene, acrylate, and glass. The surface of the microspheres can be activated to enable covalent binding of biomolecules. Further, the microspheres may contain internal fluorochrome or magnetic material, for identification or separation purposes. In this paper we review different assay formats for single-step measurement of bioaffinity assays employing microspheres. The term single-step is used to describe assays where all reagents and the sample are mixed, incubated and measured without separate washing steps.

Reaction kinetics of a two-photon excitation microparticle based immunoassay--from modelling to practice

Real time observation of reaction kinetics is one of the key features of the newly developed microparticle based two-photon excitation fluorescence immunoassay system (TPX). By observing binding reactions at the surface of individual microparticles during the incubation of an assay, the binding constants of an assay become apparent. This paper describes the use of the new system in quantifying the reaction parameters of human thyroid stimulating hormone (hTSH) assay. A mechanistic reaction model for the assay is presented. The reaction model is further shown to precisely predict the behaviour of the assay kinetics over a wide range of analyte concentrations.