Preparation and microscopic visualization of multicolor luminescent immunophosphors

Proteomic Analysis of Biomaterial Surfaces after Contacting with Body Fluids by MALDI-ToF Mass Spectroscopy

We developed a method to identify proteins adsorbed on solid surfaces from a solution containing a complex mixture of proteins by using Matrix-Assisted Laser Desorption/Ionization-Time of Flight mass (MALDI-ToF mass) spectroscopy. In the method, we performed all procedures of peptide mass fingerprint method including denaturation, reduction, alkylation, digestion, and spotting of matrix on substrates. The method enabled us to avoid artifacts of pipetting that could induce changes in the composition. We also developed an algorithm to identify the adsorbed proteins. In this work, we demonstrate the identification of proteins adsorbed on self-assembled monolayers (SAMs). Our results show that the composition of proteins on the SAMs critically depends on the terminal groups of the molecules constituting the SAMs, indicating that the competitive adsorption of protein molecules is largely affected by protein-surface interaction. The method introduced here can provide vital information to clarify the mechanism underlying the responses of cells and tissues to biomaterials.

Time-gated orthogonal scanning automated microscopy (OSAM) for high-speed cell detection and analysis

We report a new development of orthogonal scanning automated microscopy (OSAM) incorporating time-gated detection to locate rare-event organisms regardless of autofluorescent background. The necessity of using long-lifetime (hundreds of microseconds) luminescent biolabels for time-gated detection implies long integration (dwell) time, resulting in slow scan speed. However, here we achieve high scan speed using a new 2-step orthogonal scanning strategy to realise on-the-fly time-gated detection and precise location of 1-μm lanthanide-doped microspheres with signal-to-background ratio of 8.9. This enables analysis of a 15 mm × 15 mm slide area in only 3.3 minutes. We demonstrate that detection of only a few hundred photoelectrons within 100 μs is sufficient to distinguish a target event in a prototype system using ultraviolet LED excitation. Cytometric analysis of lanthanide labelled Giardia cysts achieved a signal-to-background ratio of two orders of magnitude. Results suggest that time-gated OSAM represents a new opportunity for high-throughput background-free biosensing applications.

Time-resolved microscopy for imaging lanthanide luminescence in living cells

Time-resolved luminescence (TRL) microscopy can image signals from lanthanide coordination complexes or other probes with long emission lifetimes, thereby eliminating short-lifetime (<100 ns) autofluorescence background from biological specimens. However, lanthanide complexes emit far fewer photons per unit time than conventional fluorescent probes, making it difficult to rapidly acquire high quality images at probe concentrations that are relevant to live cell experiments. This article describes the development and characterization of a TRL microscope that employs a light-emitting diode (LED, λ(em) = 365 nm) for pulsed epi-illumination and an intensified charge-coupled device (ICCD) camera for gated, widefield detection. Europium chelate-impregnated microspheres were used to evaluate instrument performance in terms of short-lifetime fluorescence background rejection, photon collection efficiency, image contrast, and signal-to-noise ratio (SNR). About 200 nm microspheres were imaged within the time resolution limit of the ICCD (66.7 ms) with complete autofluorescence suppression. About 40 nm microspheres containing ~400 chelate molecules were detected within ~1-s acquisition times. A luminescent terbium complex, Lumi4-Tb®, was introduced into the cytoplasm of cultured cells at an estimated concentration of 300 nM by the method of osmotic lysis of pinocytic vesicles. Time-resolved images of the living, terbium complex-loaded cells were acquired within acquisition times as short as 333 ms, and the effects of increased exposure time and frame summing on image contrast and SNR were evaluated. The performance analyses show that TRL microscopy is sufficiently sensitive and precise to allow high-resolution, quantitative imaging of lanthanide luminescence in living cells under physiologically relevant experimental conditions.

Photon upconversion in homogeneous fluorescence-based bioanalytical assays

Upconverting phosphors (UCPs) are very attractive reporters for fluorescence resonance energy transfer (FRET)-based bioanalytical assays. The large anti-Stokes shift and capability to convert near-infrared to visible light via sequential absorption of multiple photons enable complete elimination of autofluorescence, which commonly impairs the performance of fluorescence-based assays. UCPs are ideal donors for FRET, because their very narrow-banded emission allows measurement of the sensitized acceptor emission, in principle, without any crosstalk from the donor emission at a wavelength just tens of nanometers from the emission peak of the donor. In addition, acceptor dyes emitting at visible wavelengths are essentially not excited by near-infrared, which further emphasizes the unique potential of upconversion FRET (UC-FRET). These characteristics result in favorable assay performance using detection instrumentation based on epifluorometer configuration and laser diode excitation. Although UC-FRET is a recently emerged technology, it has already been applied in both immunoassays and nucleic acid hybridization assays. The technology is also compatible with optically difficult biological samples, such as whole blood. Significant advances in assay performance are expected using upconverting lanthanide-doped nanocrystals, which are currently under extensive research. UC-FRET, similarly to other fluorescence techniques based on resonance energy transfer, is strongly distance dependent and may have limited applicability, for example in sandwich-type assays for large biomolecules, such as viruses. In this article, we summarize the essentials of UC-FRET, describe its current applications, and outline the expectations for its future potential.

Solid-state time-gated luminescence microscope with ultraviolet light-emitting diode excitation and electron-multiplying charge-coupled device detection

Many naturally occurring materials are autofluorescent, a property that can reduce the discriminative ability of fluorescence methods, sometimes to the point where they cannot be usefully applied. Shifting from the spectral to the temporal domain, it is possible to discriminate fluorophores on the basis of their fluorescence decay lifetime. Luminophores with sufficiently long lifetimes can be discriminated from short-lived autofluorescence using time-gated luminescence (TGL). This technique relies upon the application of a brief excitation pulse followed by a resolving period to permit short-lived autofluorescence to decay, after which detection is enabled to capture persistent emission. In our studies, a high-power UV LED was mounted in the filter capsule of an Olympus BX51 microscope to serve as the excitation source. The microscope was fitted with an Andor DV885 electron-multiplying CCD (EM-CCD) camera with the trigger input synchronized to UV LED operation. Giardia lamblia cysts labeled with the europium chelate BHHST were analyzed against an autofluorescent background with the TGL microscope. The EM-CCD camera captured useful TGL images in real time with a single exposure cycle. With 4x frame averaging, images acquired in TGL mode showed a 30-fold improvement in SNR compared with conventional fluorescence microscopy.

Increasing the luminescence of lanthanide complexes

This review compares the chemical and physical properties of lanthanide ion complexes and of other narrow-emitting species that can be used as labels for cytometry. A series of luminescent lanthanide ion macrocyclic complexes, Quantum Dyes, which do not release or exchange their central lanthanide ion, do accept energy transfer from ligands, and are capable of covalent binding to macromolecules, including proteins and nucleic acids, is described and their properties are discussed. Two methods are described for increasing the luminescence intensity of lanthanide ion complexes, which intrinsically is not as high as that of standard fluorophores or quantum dots. One method consists of adding a complex of a second lanthanide ion in a micellar solution (columinescence); the other method produces dry preparations by evaporation of a homogeneous solution containing an added complex of a second lanthanide ion or an excess of an unbound antenna ligand. Both methods involve the Resonance Energy Transfer Enhanced Luminescence, RETEL, effect as the mechanism for the luminescence enhancement.

Photochemical Characterization of Up-Converting Inorganic Lanthanide Phosphors as Potential Labels

We have characterized commercially available up-converting inorganic lanthanide phosphors for their rare earth composition and photoluminescence properties under infrared laser diode excitation. These up-converting phosphors, in contrast to proprietary materials reported earlier, are readily available to be utilized as particulate reporters in various ligand binding assays after grinding to submicron particle size. The laser power density required at 980 nm to generate anti-Stokes photoluminescence from these particulate reporters is significantly lower than required for two-photon excitation. The narrow photoluminescence emission bands at 520-550 nm and at 650-670 nm are at shorter wavelengths and thus totally discriminated from autofluorescence and scattered excitation light even without temporal resolution. Transparent solution of colloidal bead-milled up-converting phosphor nanoparticles provides intense green emission visible to the human eye under illumination by an infrared laser pointer. In this article, we show that the unique photoluminescence properties of the up-converting phosphors and the inexpensive measurement configuration, which is adequate for their sensitive detection, render the up-conversion an attractive alternative to the ultraviolet-excited time-resolved fluorescence of down-converting lanthanide compounds widely employed in biomedical research and diagnostics.

Temporally and spectrally resolved imaging microscopy of lanthanide chelates

The combination of temporal and spectral resolution in fluorescence microscopy based on long-lived luminescent labels offers a dramatic increase in contrast and probe selectivity due to the suppression of scattered light and short-lived autofluorescence. We describe various configurations of a fluorescence microscope integrating spectral and microsecond temporal resolution with conventional digital imaging based on CCD cameras. The high-power, broad spectral distribution and microsecond time resolution provided by microsecond xenon flashlamps offers increased luminosity with recently developed fluorophores with lifetimes in the submicrosecond to microsecond range. On the detection side, a gated microchannel plate intensifier provides the required time resolution and amplification of the signal. Spectral resolution is achieved with a dual grating stigmatic spectrograph and has been applied to the analysis of luminescent markers of cytochemical specimens in situ and of very small volume elements in microchambers. The additional introduction of polarization optics enables the determination of emission polarization; this parameter reflects molecular orientation and rotational mobility and, consequently, the nature of the microenvironment. The dual spectral and temporal resolution modes of acquisition complemented by a posteriori image analysis gated on the spatial, spectral, and temporal dimensions lead to a very flexible and versatile tool. We have used a newly developed lanthanide chelate, Eu-DTPA-cs124, to demonstrate these capabilities. Such compounds are good labels for time-resolved imaging microscopy and for the estimation of molecular proximity in the microscope by fluorescence (luminescence) resonance energy transfer and of molecular rotation via fluorescence depolarization. We describe the spectral distribution, polarization states, and excited-state lifetimes of the lanthanide chelate crystals imaged in the microscope.

Microspectroscopic imaging of nodulation factor-binding sites on living Vicia sativa roots using a novel bioactive fluorescent nodulation factor

A novel bioactive fluorescent nodulation (Nod) factor, NodRlv-IV(BODIPY FL-C16), has been synthesized by attaching a BODIPY FL-C16 acyl chain to the primary amino group of chitotetraose deacetylated at the nonreducing terminus by recombinant NodB. The binding of the fluorescent Nod factor to root systems of Vicia sativa was investigated with fluorescence spectral imaging microscopy (FSPIM) and fluorescence ratio imaging microscopy (FRIM). Spatially resolved fluorescence spectra of living and labeled Vicia sativa root systems were measured by FSPIM. Strong autofluorescence, inherent to many plant systems when excited at 488 nm, was corrected for by utilizing the difference in fluorescence emission spectra of the autofluorescence and NodRlv-IV(BODIPY FL-C16). A methodology is presented to break down the in situ fluorescence emission spectra into spatially resolved autofluorescence and BODIPY FL fluorescence spectra. Furthermore, an FRIM method was developed for correcting autofluorescence in fluorescence micrographs for this system. After autofluorescence correction it was shown that NodRlv-IV(BODIPY FL-C16) was concentrated in the root hairs, but was also bound to other parts of the root surface.

Robert Feulgen Prize Lecture 1995. Electronic light microscopy: present capabilities and future prospects

Electronic light microscopy involves the combination of microscopic techniques with electronic imaging and digital image processing, resulting in dramatic improvements in image quality and ease of quantitative analysis. In this review, after a brief definition of digital images and a discussion of the sampling requirements for the accurate digital recording of optical images, I discuss the three most important imaging modalities in electronic light microscopy--video-enhanced contrast microscopy, digital fluorescence microscopy and confocal scanning microscopy--considering their capabilities, their applications, and recent developments that will increase their potential. Video-enhanced contrast microscopy permits the clear visualisation and real-time dynamic recording of minute objects such as microtubules, vesicles and colloidal gold particles, an order of magnitude smaller than the resolution limit of the light microscope. It has revolutionised the study of cellular motility, and permits the quantitative tracking of organelles and gold-labelled membrane bound proteins. In combination with the technique of optical trapping (optical tweezers), it permits exquisitely sensitive force and distance measurements to be made on motor proteins. Digital fluorescence microscopy enables low-light-level imaging of fluorescently labelled specimens. Recent progress has involved improvements in cameras, fluorescent probes and fluorescent filter sets, particularly multiple bandpass dichroic mirrors, and developments in multiparameter imaging, which is becoming particularly important for in situ hybridisation studies and automated image cytometry, fluorescence ratio imaging, and time-resolved fluorescence. As software improves and small computers become more powerful, computational techniques for out-of-focus blur deconvolution and image restoration are becoming increasingly important. Confocal microscopy permits convenient, high-resolution, non-invasive, blur-free optical sectioning and 3D image acquisition, but suffers from a number of limitations. I discuss advances in confocal techniques that address the problems of temporal resolution, spherical and chromatic aberration, wavelength flexibility and cross-talk between fluorescent channels, and describe new optics to enhance axial resolution and the use of two-photon excitation to reduce photobleaching. Finally, I consider the desirability of establishing a digital image database, the BioImage database, which would permit the archival storage of, and public Internet access to, multidimensional image data from all forms of biological microscopy. Submission of images to the BioImage database would be made in coordination with the scientific publication of research results based upon these data.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrasensitive bioanalytical assays using time-resolved fluorescence detection

This article reviews the use of time-resolved fluorimetric detection of lanthanide chelate luminescence as a detection method for ultrasensitive bioanalytical assays. Assay formats and detection methods, and the principle of time-resolved fluorimetric detection, are described. Detection systems, assay formats, reagents, and instrumentation for time-resolved fluorimetric detection are outlined. A review of published and commercially available immunoassays and DNA hybridization assays using time-resolved fluorimetric detection of lanthanide chelate luminescence is given.

Time-resolved detection of lanthanide luminescence for ultrasensitive bioanalytical assays

The principles and practice of the application of time-resolved lanthanide chelate luminescence (or fluorescence) as a detection method for ultrasensitive bioanalytical assays such as immunoassays and nucleic acid hybridization assays are reviewed. The various lanthanide chelate-based detection systems which have been developed for use in heterogeneous and homogeneous assay formats are described, including reagents, assay methods, and instrumentation, along with recent improvements in these methods. Detection systems described include those based on dissociative enhancement of lanthanide ions, direct labeling with luminescent chelates, enzyme-amplified lanthanide luminescence, lanthanide luminescence quenching, and energy transfer.

Fluorochromes with long-lived fluorescence as potential labels for pulsed laser immunocytofluorometry: photophysical characterization of pyrene derivatives

An apparatus for laser-induced time-resolved fluorescence measurements, in conjunction with pyrene derivatives endowed with long-lived excited singlet states, was employed for immunocytofluorometric measurements. N-(1-pyrene)maleimide, 1-pyrenesulfonyl chloride and 1-pyreneisothiocyanate were conjugated with immunoglobulin, antimouse-IgG, and the fluorescence decays of both free and conjugated forms were investigated. Bi-exponential decays were obtained in all cases with time constants of the short-lived component in the range 3-4.7 ns and the long-lived one in the range 20-55 ns. Only the spectral distribution of the two components is essentially affected upon conjugation. The persistence of the long-lived component, well above the lifetime of autofluorescence, and of the antibody specificity, as shown by immunodiffusion tests, upon conjugation indicates that this technique could be advantageously adopted in immunocytofluorometry.

Time-resolved delayed luminescence image microscopy using an europium ion chelate complex

Improvements and extended applications of time-resolved delayed luminescence imaging microscopy (TR-DLIM) in cell biology are described. The emission properties of europium ion complexed to a fluorescent chelating group capable of labeling proteins are exploited to provide high contrast images of biotin labeled ligands through detection of the delayed emission. The streptavidin-based macromolecular complex (SBMC) employs streptavidin cross-linked to thyroglobulin multiply labeled with the europium-fluorescent chelate. The fluorescent chelate is efficiently excited with 340-nm light, after which it sensitizes europium ion emission at 612 nm hundreds of microseconds later. The SBMC complex has a high quantum yield orders of magnitude higher than that of eosin, a commonly used delayed luminescent probe, and can be readily seen by the naked eye, even in specimens double-labeled with prompt fluorescent probes. Unlike triplet-state phosphorescent probes, sensitized europium ion emission is insensitive to photobleaching and quenching by molecular oxygen; these properties have been exploited to obtain delayed luminescence images of living cells in aerated medium thus complementing imaging studies using prompt fluorescent probes. Since TR-DLIM has the unique property of rejecting enormous signals that originate from scattered light, autofluorescence, and prompt fluorescence it has been possible to resolve double emission images of living amoeba cells containing an intensely stained lucifer yellow in pinocytosed vesicles and membrane surface-bound SBMC-labeled biotinylated concanavalin A. Images of fixed cells represented in terms of the time decay of the sensitized emission show the lifetime of the europium ion emission is sensitive to the environment in which it is found. Through the coupling of SBMC to streptavidin,a plethora of biotin-based tracer molecules are available for immunocytochemical studies.

Time-resolved flow cytometry for the measurement of lanthanide chelate fluorescence: I. Concept and theoretical evaluation

The concept of a flow cytometer suited for the time-resolved measurement of lanthanide chelate luminescence with a decay time on the order of 10 microseconds to 2 ms is presented and evaluated. The instrument proposed encompasses a continuous-wave laser for fluorescence excitation and an optical switch for the elimination of cellular autofluorescence decaying within 1 ns to 1 microseconds during the luminescence detection period. The slowly decaying fluorescence is to be quantified by a photon-counting system, whereas light scatter and prompt fluorescence parameters are acquired by a conventional detection system. The detection limit of the method, in terms of the smallest detectable number of fluorescing chelates per cell, is examined. It was found to be nearly 30,000 complexes of a europium chelate with a decay time of 1.6 ms and a quantum efficiency of 17%, independent of fast decaying cellular autofluorescence or prompt dye emission intensity. The probability of cells passing through the instrument without being detected while the laser beam is turned off was estimated, and the implications for cell throughput and sorting performance of the instrument were assessed. At typical fluorescence detection intervals of 500 microseconds to 1 ms, cell flow rates of 100-200 particles per second lead to detection probabilities of more than 90% and sorting purities comparable to those found in conventional fluorescence-activated cell sorting.

Time-resolved flow cytometry for the measurement of lanthanide chelate fluorescence: II. Instrument design and experimental results

A time-resolved flow cytometer capable of measuring a luminescence with a decay time in the range of 10 microseconds to 2 ms, typical for some lanthanide chelates, is presented. The instrument permits acquisition of conventional light scatter and prompt fluorescence signals as well as detection of slowly decaying luminescence by a photon counting unit for a selectable time period of 1 microsecond to 1 ms. During photon counting, the laser beam is turned off by an acoustooptic deflector. The design of a flow chamber with an average geometrical light collection efficiency of 35% over a distance of 1.7 mm is presented and analyzed by ray tracing. A pulse processing system featuring digital integration of the conventional signals and a transputer system for the acquisition and the transfer of the measured parameter values to a host computer is described. Instrument function is verified with lyophilized human lymphocytes stained for the CD8 antigen with dye-loaded liposomes. Quantitation of cell-associated europium chelate fluorescence, displaying a decay time of 1.6 ms, is demonstrated. Elimination of fast decaying background emission generated by DNA-associated ethidium bromide is shown. The background generated by instrument components in the time-gated measurement channel is characterized, and measures for its complete elimination are discussed.

Mapping and chromosome analysis: the potential of fluorescence in situ hybridization

Fluorescence in situ hybridization (FISH) is a method widely used for the delineation of chromosomal DNA. FISH is applied in many areas of basic research as well as in clinical cytogenetics. In this review important technical improvements as well as the various applications of this method are summarized. In the first part different labeling and detection procedures are described and the potential of various kinds of probes are discussed. Recent developments in optical instrumentation and digital imaging procedures are outlined in the second part. The following important applications of FISH are discussed: (a) new strategies for high resolution mapping of DNA sequences; (b) detection of chromosomal aberrations in clinical material; (c) techniques allowing the simultaneous detection of numerous probes by multiple color FISH; and (d) the new approach of comparative genomic hybridization, allowing a rapid and comprehensive analysis of chromosomal imbalances in cell populations, which is particularly useful for the cytogenetic analysis of tumor samples.

Use of ferro-electric liquid crystal shutters for time-resolved fluorescence microscopy

A technique is described to modify a standard fluorescence microscope for time-resolved visualization of delayed luminescing substances with decay times from 50 microseconds to several milliseconds. The modification consists of synchronized operation of a mechanical shutter, positioned in an aperture plane in the excitation pathway, simultaneously with a ferro-electric liquid crystal (FLC) shutter on the emission side. Operation of the microscope is through a microprocessor interfaced keypad by which all timing parameters can be adjusted for optimal suppression of fast decaying luminescence. Accuracy of the timing was within 1 microsecond. Prompt fluorescence was suppressed up to 10(6) times, as determined for bright prompt fluorescing microspheres. The use of the FLC shutter resulted in a reduction in emission intensity by a factor of 8 (due to the use of polarizers, the lower transmission of the FLC devices, and IR blocking filters). No significant image degradation due to shutter operations was observed. The modified microscope was successfully used for the visualization of delayed luminescing immunolabels, such as inorganic phosphor particles and lanthanide chelates, as well as naturally phosphorescing materials.