Nanobioanalytical luminescence: Förster-type energy transfer methods

Fluorescent Probes and Quenchers in Studies of Protein Folding and Protein-Lipid Interactions

Fluorescence spectroscopy provides numerous methodological tools for structural and functional studies of biological macromolecules and their complexes. All fluorescence-based approaches require either existence of an intrinsic probe or an introduction of an extrinsic one. Moreover, studies of complex systems often require an additional introduction of a specific quencher molecule acting in combination with a fluorophore to provide structural or thermodynamic information. Here, we review the fundamentals and summarize the latest progress in applications of different classes of fluorescent probes and their specific quenchers, aimed at studies of protein folding and protein-membrane interactions. Specifically, we discuss various environment-sensitive dyes, FRET probes, probes for short-distance measurements, and several probe-quencher pairs for studies of membrane penetration of proteins and peptides. The goals of this review are: (a) to familiarize the readership with the general concept that complex biological systems often require both a probe and a quencher to decipher mechanistic details of functioning and (b) to provide example of the immediate applications of the described methods.

Protein Recognition Based on Temperature-Stimulated Multiparameter Response Virtual Array Sensing Strategy

In the field of array sensing, researchers are committed to miniaturizing array sensing systems while ensuring the acquisition of multiple sensing information. Here, a new strategy called "stimulus responsive array sensing" was presented to obtain virtual multiple sensing without constructing multiple physical sensing units. Based on bioluminescence resonance energy transfer, where luciferase acts as the donor and temperature stimulus response polymers act as the receptors, by using only one sensing unit to output multiple stimulus responsive sensing signals in temperature dimension, an equivalent array sensing could be achieved. This strategy can distinguish and quantify a variety of proteins. More importantly, glucose responsive monomers were doped in polymers; thus, more virtual sensing units can be further increased to obtain more sensing signals, greatly increasing the accuracy of protein recognition, and it can also be used to differentiate several compositions of protein under different glucose concentrations in urine caused by different renal diseases. The results show the potential of the "stimulus responsive array sensing" for analyzing molecular compositions in complex biological systems and show a new tack in array sensing.

Dyads with tunable near-infrared donor-acceptor excited-state energy gaps: molecular design and Förster analysis for ultrafast energy transfer

Bacteriochlorophylls, nature's near-infrared absorbers, play an essential role in energy transfer in photosynthetic antennas and reaction centers. To probe energy-transfer processes akin to those in photosynthetic systems, nine synthetic bacteriochlorin-bacteriochlorin dyads have been prepared wherein the constituent pigments are joined at the meso-positions by a phenylethyne linker. The phenylethyne linker is an unsymmetric auxochrome, which differentially shifts the excited-state energies of the phenyl- or ethynyl-attached bacteriochlorin constituents in the dyad. Molecular designs utilized known effects of macrocycle substituents to engineer bacteriochlorins with S₀ → S₁ (Q_y) transitions spanning 725-788 nm. The design-predicted donor-acceptor excited-state energy gaps in the dyads agree well with those obtained from time dependent density functional theory calculations and with the measured range of 197-1089 cm^-1. Similar trends with donor-acceptor excited-state energy gaps are found for (1) the measured ultrafast energy-transfer rates of (0.3-1.7 ps)^-1, (2) the spectral overlap integral (J) in Förster energy-transfer theory, and (3) donor-acceptor electronic mixing manifested in the natural transition orbitals for the S₀ → S₁ transition. Subtle outcomes include the near orthogonal orientation of the π-planes of the bacteriochlorin macrocycles, and the substituent-induced shift in transition-dipole moment from the typical coincidence with the NH-NH axis; the two features together afforded the Förster orientation term κ² ranging from 0.55-1.53 across the nine dyads, a value supportive of efficient excited-state energy transfer. The molecular design and collective insights on the dyads are valuable for studies relevant to artificial photosynthesis and other processes requiring ultrafast energy transfer.

Hybrid drug nanocrystals

Nanocrystals show promise to deliver poorly water-soluble drugs to yield systemic exposure. However, our knowledge regarding the in vivo fate of nanocrystals is in its infancy, as nanocrystallization is simply viewed as an approach to enhance the dissolution of drug crystals. The dying crystal phenomenon inspired the development of hybrid nanocrystals by physically embedding fluorophores into the crystal lattice. This approach achieved concurrent therapy and bioimaging and is well-established to study pharmacokinetics and nanocrystal dissolution in vivo. Nanocrystals also offer the advantage of long-term durability in the body for interacting with biological tissues and cells. This review introduces the hybrid nanocrystal technique, including the theoretical concepts, preparation, and applications. We also discuss the latest development in self-discriminative hybrid nanocrystals utilizing environment-responsive probes. This review will stimulate further development and application of nanocrystal-based drug delivery systems for theranostic strategies.

Heuristics from Modeling of Spectral Overlap in Förster Resonance Energy Transfer (FRET)

Among the photophysical parameters that underpin Förster resonance energy transfer (FRET), perhaps the least explored is the spectral overlap term ( J). While by definition J increases linearly with acceptor molar absorption coefficient (ε_(A) in M^-1 cm^-1), is proportional to wavelength (λ⁴), and depends on the degree of overlap of the donor fluorescence and acceptor absorption spectra, the question arose as to the value of J for the case of perfect spectral overlap versus that for representative fluorophores with incomplete spectral overlap. Here, Gaussian distributions of absorption and fluorescent spectra have been modeled that encompass varying degrees of overlap, full-width-at-half-maximum (fwhm), and Stokes shift. For ε_(A) = 10⁵ M^-1 cm^-1 and perfect overlap, the J value (in M^-1 cm^-1 nm⁴) ranges from 1.15 × 10¹⁴ (200 nm) to 7.07 × 10¹⁶ (1000 nm), is almost linear with λ⁴ (average of λ_abs and λ_flu), and is nearly independent of fwhm. For visible-region fluorophores with perfectly overlapped Gaussian spectra, the resulting value of J ( J_G-0) is ∼0.71 ε_(A)λ⁴ (M^-1 cm^-1 nm⁴). The experimental J values for homotransfer, as occurs in light-harvesting antennas, were calculated with spectra from a static database of 60 representative compounds (12 groups, 5 compounds each) and found to range from 4.2 × 10¹⁰ ( o-xylene) to 5.3 × 10¹⁶ M^-1 cm^-1 nm⁴ (a naphthalocyanine). The degree of overlap, defined by the ratio of the experimental J to the model J_G-0 for perfectly overlapped spectra, ranges from ∼0.5% (coumarin 151) to 77% (bacteriochlorophyll a). The results provide insights into how a variety of factors affect the resulting J values. The high degree of spectral overlap for (bacterio)chlorophylls prompts brief conjecture concerning the relevance of energy transfer to the question "why chlorophyll".

Lyophilization of chemiluminescent substrate reagents for high-sensitive microchannel-based lateral flow assay (MLFA) in point-of-care (POC) diagnostic system

A new method towards successful lyophilization and reconstitution of chemiluminescent substrate while restoring the substrate functionality is reported in this work.

More Than a Light Switch: Engineering Unconventional Fluorescent Configurations for Biological Sensing

Fluorescence is a powerful and sensitive tool in biological detection, used widely for cellular imaging and in vitro molecular diagnostics. Over time, three prominent conventions have emerged in the design of fluorescent biosensors: a sensor is ideally specific for its target, only one fluorescence signal turns on or off in response to the target, and each target requires its own sensor and signal combination. These are conventions but not requirements, and sensors that break with one or more of these conventions can offer new capabilities and advantages. Here, we review "unconventional" fluorescent sensor configurations based on fluorescent dyes, proteins, and nanomaterials such as quantum dots and metal nanoclusters. These configurations include multifluorophore Förster resonance energy transfer (FRET) networks, temporal multiplexing, photonic logic, and cross-reactive arrays or "noses". The more complex but carefully engineered biorecognition and fluorescence signaling modalities in unconventional designs are richer in information, afford greater multiplexing capacity, and are potentially better suited to the analysis of complex biological samples, interactions, processes, and diseases. We conclude with a short perspective on the future of unconventional fluorescent sensors and encourage researchers to imagine sensing beyond the metaphorical light bulb and light switch combination.

The emerging use of bioluminescence in medical research

Bioluminescence is the light produced by a living organism and is commonly emitted by sea life with Ca²⁺-regulated photoproteins being the most responsible for bioluminescence emission. Marine coelenterates provide important functions involved in essential purposes such as defense, feeding, and breeding. In this review, the main characteristics of marine photoproteins including aequorin, clytin, obelin, berovin, pholasin and symplectin from different marine organisms will be discussed. We will focused on the recent use of recombinant photoproteins in different biomedical research fields including the measurement of Ca²⁺ in different intracellular compartments of animal cells, as labels in the design and development of binding assays. This review will also outline how bioluminescent photoproteins have been used in a plethora of analytical methods including ultra-sensitive assays and in vivo imaging of cellular processes. Due to their unique properties including elective intracellular distribution, wide dynamic range, high signal-to-noise ratio and low Ca²⁺-buffering effect, recombinant photoproteins represent a promising future analytical tool in several in vitro and in vivo experiments.

FRET-based nanobiosensor for detection of scopolamine in hairy root extraction of Atropa belladonna

A simple, sensitive, selective, and rapid optical nanobiosensor based on FRET was designed to detect tropane alkaloids as anti-cholinergic agents in natural and transgenic hairy roots extracts of Atropa belladonna. To achieve that, conjugation of tioglycolyic acid capped cadmium telluride quantum Dots, M₂ muscarinic receptor (Cd/Te QDs-M₂R) and conjugation of scopolamine-rhodamine123 (Sc-Rho123) were performed. More specifically, proportional amounts of M₂ muscarinic receptor and quantum dots (QDs) were conjugated while scopolamine (as a tropane alkaloid) and rhodamine123 were also combined and these moieties functioned as donor and acceptor pairs, respectively. The system response was linear over the range of 0.01-4µmolL^-1 of scopolamine hydrochloride concentration with a detection limit of 0.001µmolL^-1. The developed nanobiosensor was successfully used for in vitro recognition of scopolamine as an anti-cholinergic agent in the investigated plant extracts. In addition, Agrobacterium rhizogenesis mediated gene transfer technique was employed to generate hairy roots and to enhance the production of tropane alkaloids in the studied medicinal plant.

Improving the luminescence properties of aequorin by conjugating to CdSe/ZnS quantum dot nanoparticles: Red shift and slowing decay rate

Changing the properties of photoprotein aequorin such as the wavelength emission and decay half-life by using bioluminescence resonance energy transfer (BRET) phenomenon is the main aim in this paper. BRET system was set up with CdSe/ZnS quantum dot nanoparticles as an acceptor molecule and photoprotein as an energy donor molecule. Quantum dots are semiconductor nanoparticles with very interesting optical properties, including broad excitation spectra, narrow and the symmetric band width emission spectra, tunable by their sizes, compositions, negligible photo-bleaching and good chemical and photo-stability. In this QD-BRET system, aequorin is conjugated to the carboxyl groups on quantum dot surface by EDC/NHS chemistry as cross linker. Bioluminescence energy generates by aequorin upon adding Ca(2+) and transfers to the quantum dots in a radiationless manner and emits at a longer wavelength. The determined bioluminescent parameters for this method included aequorin activity, emission spectra and decay half-life time. In fact, this spectrum tuning strategy resulted in a change in bioluminescent properties of photoprotein, therefore, the maximum emission wavelength shifted from 455 to 540nm and the decay time increased from 3.76 to 12.11s. Nowadays, photoproteins with different characteristics are capable of being employed as a reporter in multi-analyte detections and in vivo imaging.

Enabling Aequorin for Biotechnology Applications Through Genetic Engineering

In recent years, luminescent proteins have been studied for their potential application in a variety of detection systems. Bioluminescent proteins, which do not require an external excitation source, are especially well-suited as reporters in analytical detection. The photoprotein aequorin is a bioluminescent protein that can be engineered for use as a molecular reporter under a wide range of conditions while maintaining its sensitivity. Herein, the characteristics of aequorin as well as the engineering and production of aequorin variants and their impact on signal detection in biological systems are presented. The structural features and activity of aequorin, its benefits as a label for sensing and applications in highly sensitive detection, as well as in gaining insight into biological processes are discussed. Among those, focus has been placed on the highly sensitive calcium detection in vivo, in vitro DNA and small molecule sensing, and development of in vivo imaging technologies. Graphical Abstract.

A homogenous fluorescence quenching based assay for specific and sensitive detection of influenza virus A hemagglutinin antigen

Influenza pandemics cause millions of deaths worldwide. Effective surveillance is required to prevent their spread and facilitate the development of appropriate vaccines. In this study, we report the fabrication of a homogenous fluorescence-quenching-based assay for specific and sensitive detection of influenza virus surface antigen hemagglutinins (HAs). The core of the assay is composed of two nanoprobes namely the glycan-conjugated highly luminescent quantum dots (Gly-QDs), and the HA-specific antibody-modified gold nanoparticle (Ab-Au NPs). When exposed to strain-specific HA, a binding event between the HA and the two nanoprobes takes place, resulting in the formation of a sandwich complex which subsequently brings the two nanoprobes closer together. This causes a decrease in QDs fluorescence intensity due to a non-radiative energy transfer from QDs to Au NPs. A resulting correlation between the targets HA concentrations and fluorescence changes can be observed. Furthermore, by utilizing the specific interaction between HA and glycan with sialic acid residues, the assay is able to distinguish HAs originated from viral subtypes H1 (human) and H5 (avian). The detection limits in solution are found to be low nanomolar and picomolar level for sensing H1-HA and H5-HA, respectively. Slight increase in assay sensitivity was found in terms of detection limit while exposing the assay in the HA spiked in human sera solution. We believe that the developed assay could serve as a feasible and sensitive diagnostic tool for influenza virus detection and discrimination, with further improvement on the architectures.

A theoretical analysis of the potential role of π-π stacking interactions in the photoprotolytic cycle of firefly luciferin

Firefly oxyluciferin is a photoacid that presents a pH-sensitive fluorescence, which results from pH-dependent changes on the conformation of self-aggregated π-π stacking complexes. Luciferin is a derivative of oxyluciferin with very similar fluorescence and photoacidic properties. This similarity indicates that luciferin is also expected to be able to form π-π stacking complexes, but no pH-sensitive fluorescence is found for this compound. Here, a theoretical approach is used to rationalize this finding. We have found that luciferin only forms π-π stacking complexes in the ground state at acidic pH. At basic pH and in the excited state, luciferin is present as a dianion. This species is not able to self-aggregate, owing to repulsive electrostatic interactions. Thus, this emissive species is not subject to π-π stacking interactions; this explains its pH-insensitive fluorescence.

Red-shifted emission from 1,2-dioxetane-based chemiluminescent reactions

Commercial chemiluminescent reagents emit across a broad portion of the electromagnetic spectrum (400-500 nm). A challenge to the use of chemiluminescence to monitor biological processes is the presence of interfering substances in the biological optical window. In the present study, longer wavelength emitting fluorophores (the organic dyes Alexa 568 and Alexa 647), and a semiconductor nanoparticle (QDOT800) were used to red-shift the emission from commercially available 1,2-dioxetane-based chemiluminescent substrate reactions. By adding non-conjugated fluorescent emitters into chemiluminescent reaction mixtures, an emission peak occurred at the predicted wavelength of the fluorescent emitter. The excitation and emission from QDOT800 was preserved in the presence of a 100 µm-thick glass barrier separating it from the chemiluminescent reaction components. The maximum tissue phantom penetration by QDOT800 emission was 8.5 mm; in comparison, the native chemiluminescent emission at 500 nm was unable to penetrate the thinnest tissue phantom of 2.5 mm. The described method for red-shifted emissions from chemiluminescent reactions does not require direct interaction between the chemiluminescent reaction and the fluorescent emitters. This suggests that the mechanism of chemiluminescent excitation of fluorophores and QDOT800 is not exclusive to chemiluminescence resonance energy transfer or sensitized chemiluminescence, but rather by broad energization from the native chemiluminescent emission.

Application of enzyme bioluminescence for medical diagnostics

Nowadays luciferases are effectively used as analytical instruments in a great variety of research fields. Of special interest are the studies dealing with elaboration of novel analytical systems for the purposes of medical diagnostics. The ever-expanding spectrum of clinically important analytes accounts for the increasing demand for new techniques for their detection. In this chapter we have made an attempt to summarize the results on applications of luciferases as reporters in binding assays including immunoassay, nucleic acid hybridization assay, and so on. The data over the last 15 years have been analyzed and clearly show that luciferase-based assays, due to extremely high sensitivity, low cost, and the lack of need for skilled personnel, hold much promise for clinical diagnostics.

Point-of-care platforms

Point-of-care applications are gaining increasing interest in clinical diagnostics and emergency applications. Biosensors are used to monitor the biomolecular interaction process between a disease biomarker and a recognition element such as a reagent. Essential are the quality and selectivity of the recognition elements and assay types used to improve sensitivity and to avoid nonspecific interactions. In addition, quality measures are influenced by the detection principle and the evaluation strategies. For these reasons, this review provides a survey and validation of recognition elements, assays, and various types of detection methods for point-of-care testing (POCT) platforms. Common applications of clinical parameters are discussed and considered. In this ever-changing field, a snapshot of current applications is needed. We provide such a snapshot by way of a table including literature citations and also discuss these applications in more detail throughout.

Interfacial protein-protein associations

While traditional models of protein adsorption focus primarily on direct protein-surface interactions, recent findings suggest that protein-protein interactions may play a central role. Using high-throughput intermolecular resonance energy transfer (RET) tracking, we directly observed dynamic, protein-protein associations of bovine serum albumin on polyethylene glycol modified surfaces. The associations were heterogeneous and reversible, and associating molecules resided on the surface for longer times. The appearance of three distinct RET states suggested a spatially heterogeneous surface - with areas of high protein density (i.e., strongly interacting clusters) coexisting with mobile monomers. Distinct association states exhibited characteristic behavior, i.e., partial-RET (monomer-monomer) associations were shorter-lived than complete-RET (protein-cluster) associations. While the fractional surface area covered by regions with high protein density (i.e., clusters) increased with increasing concentration, the distribution of contact times between monomers and clusters was independent of solution concentration, suggesting that associations were a local phenomenon, and independent of the global surface coverage.

Quantum dots for fluorescent biosensing and bio-imaging applications

Quantum dots (QDs) have been facilitating the development of sensitive fluorescence biosensors over the past two decades due to their high quantum yield, narrow and tunable emission spectrum and good photostability. The new emerging QDs with improved biocompatibility further promote their biological applications. In this review, we first briefly introduce the prevalently used QDs and their preparation and bioconjugation approaches. Then we summarize QDs-based fluorescent biosensing for proteins and nucleic acids, and QDs-based applications in cellular and in vivo targeting and imaging. Last but not the least, we envision the potential QDs-based applications in future perspectives.

Highly adaptable and sensitive protease assay based on fluorescence resonance energy transfer

Proteases are widely used in analytical sciences and play a central role in several widespread diseases. Thus, there is an immense need for highly adaptable and sensitive assays for the detection and monitoring of various proteolytic enzymes. We established a simple protease fluorescence resonance energy transfer (pro-FRET) assay for the determination of protease activities, which could in principle be adapted for the detection of all proteases. As proof of principle, we demonstrated the potential of our method using trypsin and enteropeptidase in complex biological mixtures. Briefly, the assay is based on the cleavage of a FRET peptide substrate, which results in a dramatic increase of the donor fluorescence. The assay was highly sensitive and fast for both proteases. The detection limits for trypsin and enteropeptidase in Escherichia coli lysate were 100 and 10 amol, respectively. The improved sensitivity for enteropeptidase was due to the application of an enzyme cascade, which leads to signal amplification. The pro-FRET assay is highly specific as even high concentrations of other proteases did not result in significant background signals. In conclusion, this sensitive and simple assay can be performed in complex biological mixtures and can be easily adapted to act as a versatile tool for the sensitive detection of proteases.

Identification of an in vitro interaction between an insect immune suppressor protein (CrV2) and G alpha proteins

The protein CrV2 is encoded by a polydnavirus integrated into the genome of the endoparasitoid Cotesia rubecula (Hymenoptera:Braconidae:Microgastrinae) and is expressed in host larvae with other gene products of the polydnavirus to allow successful development of the parasitoid. CrV2 expression has previously been associated with immune suppression, although the molecular basis for this was not known. Here, we have used time-resolved Förster resonance energy transfer (TR-FRET) to demonstrate high affinity binding of CrV2 to Gα subunits (but not the Gβγ dimer) of heterotrimeric G-proteins. Signals up to 5-fold above background were generated, and an apparent dissociation constant of 6.2 nm was calculated. Protease treatment abolished the TR-FRET signal, and the presence of unlabeled CrV2 or Gα proteins also reduced the TR-FRET signal. The activation state of the Gα subunit was altered with aluminum fluoride, and this decreased the affinity of the interaction with CrV2. It was also demonstrated that CrV2 preferentially bound to Drosophila Gα(o) compared with rat Gα(i1). In addition, three CrV2 homologs were detected in sequences derived from polydnaviruses from Cotesia plutellae and Cotesia congregata (including the immune-related early expressed transcript, EP2). These data suggest a potential mode-of-action of immune suppressors not previously reported, which in addition to furthering our understanding of insect immunity may have practical benefits such as facilitating development of novel controls for pest insect species.

Förster resonance energy transfer methods for quantification of protein-protein interactions on microarrays

Methods based on Förster (or fluorescence) resonance energy transfer (FRET) are widely used in various areas of bioanalysis and molecular biology, such as fluorescence microscopy, quantitative real-time polymerase chain reaction (PCR), immunoassays, or enzyme activity assays, just to name a few. In the last years, these techniques were successfully implemented to multiplex biomolecular screening on microarrays. In this review, some fundamental considerations and practical approaches are outlined and it is demonstrated how this very sensitive (and distance-dependent) method can be utilized for microarray-based high-throughput screening (HTS) with a focus on protein microarrays. The advantages and also the demands of this dual-label technique in miniaturized multiplexed formats are discussed with respect to its potential readout modes, such as intensity, dual wavelength, and time-resolved FRET detection.

Bioluminescence and its impact on bioanalysis

There is an increasing need for versatile yet sensitive labels, posed by the demands for low detection in bioanalysis. Bioluminescent proteins have many desirable characteristics, including the ability to be detected at extremely low concentrations; no background interference from autofluorescent compounds present in samples; and compatibility with many miniaturized platforms, such as lab-on-a-chip and lab-on-a-CD systems. Bioluminescent proteins have found a plethora of analytical applications in intracellular monitoring, genetic regulation and detection, immuno- and binding assays, and whole-cell biosensors, among others. As new bioluminescent organisms are discovered and new bioluminescence proteins are characterized, use of these proteins will continue to dramatically improve our understanding of molecular and cellular events, as well as their applications for detection of environmental and biomedical samples.

Lifetime imaging of FRET between red fluorescent proteins

Numerous processes in cells can be traced by using fluorescence resonance energy transfer (FRET) between two fluorescent proteins. The novel FRET pair including the red fluorescent protein TagRFP and kindling fluorescent protein KFP for sensing caspase-3 activity is developed. The lifetime mode of FRET measurements with a nonfluorescent protein KFP as an acceptor is used to minimize crosstalk due to its direct excitation. The red fluorescence is characterized by a better penetrability through the tissues and minimizes the cell autofluorescence signal. The effective transfection and expression of the FRET sensor in eukaryotic cells is shown by FLIM. The induction of apoptosis by camptothecine increases the fluorescence lifetime, which means effective cleavage of the FRET sensor by caspase-3. The instruments for detecting whole-body fluorescent lifetime imaging are described. Experiments on animals show distinct fluorescence lifetimes for the red fluorescent proteins possessing similar spectral properties.

Cell-based assays: fuelling drug discovery

It has been estimated that over a billion dollars in resources can be consumed to obtain clinical approval, and only a few new chemical entities are approved by the US Food and Drug Administration (FDA) each year. Therefore it is of utmost importance to obtain the maximum amount of information about biological activity, toxicological profile, biochemical mechanisms, and off-target interactions of drug-candidate leads in the earliest stages of drug discovery. Cell-based assays, because of their peculiar advantages of predictability, possibility of automation, multiplexing, and miniaturization, seem the most appealing tool for the high demands of the early stages of the drug-discovery process. Nevertheless, cellular screening, relying on different strategies ranging from reporter gene technology to protein fragment complementation assays, still presents a variety of challenges. This review focuses on main advantages and limitations of different cell-based approaches, and future directions and trends in this fascinating field.

Visualization of the Orai1 homodimer and the functional coupling of Orai1-STIM1 by live-cell fluorescence lifetime imaging

The Orai1-STIM1 constructed store-operated Ca2+ channels (SOCs) have been found to exert several essential Ca2+ entry/signaling cascades, e.g., the generation of immune response in T lymphocytes. Although biochemical and novel imaging evidence appear to indicate that Orai1 and STIM1 interact with each other to achieve store-operated Ca2+ entry (SOCE), the detailed mechanism of functional SOCE in situ has yet to be fully understood. In this study, green fluorescence protein (EGFP as donor) targeted to either the N- or C-terminal of Orai1 (wild type or delta1-90+delta267-301 double deletion type) and mOrange (as acceptor) tagged STIM1 were used to comprise a fluorescence resonance energy transfer (FRET) pair within living PC12 cells. The fluorescence lifetime map and histogram/distribution of each single cell, determined by one-photon excitation fluorescence lifetime imaging microscopy (FLIM), was used to visualize FRET and show the Orai1 homodimer and Orai1-STIM1 binding. Both the color-coded lifetime map and the distribution of EGFP-tagged Orai1 significantly changed after the administration of thapsigargin, the SOCE stimulating agent. The FRET efficiency from each experimental set was also calculated and compared using double exponential analysis. In summary, we show the detailed interactions Orai1-Orai1 and Orai1-STIM1 within intact living cells by using the FLIM-FRET technique.