Ultrasensitive optical imaging with lanthanide lumiphores

Lanthanide-dye hybrid luminophores for advanced NIR-II bioimaging

In vivo luminescence imaging in the second near-infrared window (NIR-II, 1000-2000 nm) is a potent technique for observing deep-tissue life activities, leveraging reduced light scattering, minimized autofluorescence, and moderate absorption attenuation to substantially enhance image contrast. Pushing the frontiers of NIR-II luminescence imaging forward, moving from static to dynamic event visualization, monochromatic to multicolor images, and fundamental research to clinical applications, necessitates the development of novel luminophores featuring bright emission, extendable wavelength, and optimal biocompatibility. Recently, lanthanide-dye hybrid luminophores (LDHLs) are gaining increasing attention for their wavelength extensibility, molecular size, narrowband emission, mega stokes shift, long lifetime, and high photostability. In this review, we will summarize the recent advances of NIR-II LDHLs and their applications in imaging and analysis of living mammals, and discuss future challenges in designing new LDHLs for deep-tissue imaging.

Exchangeable Self-Assembled Lanthanide Antennas for PLIM Microscopy

Lanthanides have unique photoluminescence (PL) emission properties, including very long PL lifetimes. This makes them ideal for biological imaging applications, especially using PL lifetime imaging microscopy (PLIM). PLIM is an inherently multidimensional technique with exceptional advantages for quantitative biological imaging. Unfortunately, due to the required prolonged acquisitions times, photobleaching of lanthanide PL emission currently constitutes one of the main drawbacks of PLIM. In this study, we report a small aqueous-soluble, lanthanide antenna, 8-methoxy-2-oxo-1,2,4,5-tetrahydrocyclopenta[de]quinoline-3-phosphonic acid, PAnt, specifically designed to dynamically interact with lanthanide ions, serving as exchangeable dye aimed at mitigating photobleaching in PLIM microscopy in cellulo. Thus, self-assembled lanthanide complexes that may be photobleached during image acquisition are continuously replenished by intact lanthanide antennas from a large reservoir. Remarkably, our self-assembled lanthanide complex clearly demonstrated a significant reduction of PL photobleaching when compared to well-established lanthanide cryptates, used for bioimaging. This concept of exchangeable lanthanide antennas opens new possibilities for quantitative PLIM bioimaging.

A dye-quenched/sensitized switching upconversion nanoprobe for high-contrast mapping of the pH-related tumor microenvironment

Nanoprobes based on lanthanide-doped upconversion nanoparticles (UCNPs) exhibit promising potential in bioimaging and biosensing due to their unique optical properties. However, conventional UCNP nanoprobes based on the dye quenching effect are still limited in biosensing due to their low upconversion efficiency. The advent of dye-sensitized upconversion has resulted in nanoprobes with significantly enhanced efficiency; however, these still suffer from a high initial emissive background. In view of this, herein, we have constructed a dye-quenched/sensitized switching upconversion nanoprobe for high-contrast imaging of the pH-related tumor microenvironment. Under normal conditions, the luminescence of the nanoprobe at 540 nm (UCL540) was significantly quenched by the employed dye. Upon being triggered by an acid, the dye would switch to its fluorescent form to sensitize the luminescence of UCNPs, affording a significant enhancement of UCL540. The switching from dye-quenched UCL to dye-sensitized UCL jointly enables the detection of a high signal-to-background ratio (high up to 50), allowing for high-contrast mapping of the tumor specific acidic microenvironment in vivo. We believe that this nanoplatform holds considerable promise for acid-related sensing.

Low-rate smartphone videoscopy for microsecond luminescence lifetime imaging with machine learning

Time-resolved techniques have been widely used in time-gated and luminescence lifetime imaging. However, traditional time-resolved systems require expensive lab equipment such as high-speed excitation sources and detectors or complicated mechanical choppers to achieve high repetition rates. Here, we present a cost-effective and miniaturized smartphone lifetime imaging system integrated with a pulsed ultraviolet (UV) light-emitting diode (LED) for 2D luminescence lifetime imaging using a videoscopy-based virtual chopper (V-chopper) mechanism combined with machine learning. The V-chopper method generates a series of time-delayed images between excitation pulses and smartphone gating so that the luminescence lifetime can be measured at each pixel using a relatively low acquisition frame rate (e.g. 30 frames per second [fps]) without the need for excitation synchronization. Europium (Eu) complex dyes with different luminescent lifetimes ranging from microseconds to seconds were used to demonstrate and evaluate the principle of V-chopper on a 3D-printed smartphone microscopy platform. A convolutional neural network (CNN) model was developed to automatically distinguish the gated images in different decay cycles with an accuracy of >99.5%. The current smartphone V-chopper system can detect lifetime down to ∼75 µs utilizing the default phase shift between the smartphone video rate and excitation pulses and in principle can detect much shorter lifetimes by accurately programming the time delay. This V-chopper methodology has eliminated the need for the expensive and complicated instruments used in traditional time-resolved detection and can greatly expand the applications of time-resolved lifetime technologies.

A pillar[5]arene noncovalent assembly boosts a full-color lanthanide supramolecular light switch

A photo-responsive full-color lanthanide supramolecular switch was constructed from a synthetic 2,6-pyridine dicarboxylic acid (DPA)-modified pillar[5]arene (H) complexing with lanthanide ion (Ln3+ = Tb3+ and Eu3+) and dicationic diarylethene derivative (G1) through a noncovalent supramolecular assembly. Benefiting from the strong complexation between DPA and Ln3+ with a 3 : 1 stoichiometric ratio, the supramolecular complex H/Ln3+ presented an emerging lanthanide emission in the aqueous and organic phase. Subsequently, a network supramolecular polymer was formed by H/Ln3+ further encapsulating dicationic G1via the hydrophobic cavity of pillar[5]arene, which greatly contributed to the increased emission intensity and lifetime, and also resulted in the formation of a lanthanide supramolecular light switch. Moreover, full-color luminescence, especially white light emission, was achieved in aqueous (CIE: 0.31, 0.32) and dichloromethane (CIE: 0.31, 0.33) solutions by the adjustment of different ratios of Tb3+ and Eu3+. Notably, the photo-reversible luminescence properties of the assembly were tuned via alternant UV/vis light irradiation due to the conformation-dependent photochromic energy transfer between the lanthanide and the open/closed-ring of diarylethene. Ultimately, the prepared lanthanide supramolecular switch was successfully applied to anti-counterfeiting through the use of intelligent multicolored writing inks, and presents new opportunities for the design of advanced stimuli-responsive on-demand color tuning with lanthanide luminescent materials.

Sensitization of lanthanide complexes through direct spin-forbidden singlet → triplet excitation

Ln^III complexes may display luminescence within the ultraviolet-visible-near-infrared spectral window and although they render bright emissions mainly due to the classical singlet-triplet-state-assisted ligand sensitization, which would be the photophysical parameters if they could be excited through direct spin-forbidden singlet → triplet transitions? Herein, we report the sensitization of Ln complexes through spin-forbidden S₀ → T transitions in a series of homobimetallic Eu^III, Tb^III, Er^III, and Yb^III complexes with halogen-substituted benzoate ligands. As halogens and Ln^III atomic numbers increase, intense singlet → triplet absorption/excitation bands and relative quantum yields up to 18% were achieved due to an increased spin-orbit coupling effect. Moreover, the near-UV-shifted excitation may enable application in luminescent solar concentrators where Yb^III near-infrared luminescence matches the maximum efficiency of the crystalline Si photovoltaic cell. Therefore, the spin-relaxed excitation channel provides new opportunities to improve the Ln^III complex luminescence and potential within the energy conversion field.

Multiplexed Biosensing and Bioimaging Using Lanthanide-Based Time-Gated Förster Resonance Energy Transfer

The necessity to scrutinize more and more biological molecules and interactions both in solution and on the cellular level has led to an increasing demand for sensitive and specific multiplexed diagnostic analysis. Photoluminescence (PL) detection is ideally suited for multiplexed biosensing and bioimaging because it is rapid and sensitive and there is an almost unlimited choice of fluorophores that provide a large versatility of photophysical properties, including PL intensities, spectra, and lifetimes.The most frequently used technique to detect multiple parameters from a single sample is spectral (or color) multiplexing with different fluorophores, such as organic dyes, fluorescent proteins, quantum dots, or lanthanide nanoparticles and complexes. In conventional PL biosensing approaches, each fluorophore requires a distinct detection channel and excitation wavelength. This drawback can be overcome by Förster resonance energy transfer (FRET) from lanthanide donors to other fluorophore acceptors. The lanthanides' multiple and spectrally narrow emission bands over a broad spectral range can overlap with several different acceptors at once, thereby allowing FRET from one donor to multiple acceptors. The lanthanides' extremely long PL lifetimes provide two important features. First, time-gated (TG) detection allows for efficient suppression of background fluorescence from the biological environment or directly excited acceptors. Second, temporal multiplexing, for which the PL lifetimes are adjusted by the interaction with the FRET acceptor, can be used to determine specific biomolecules and/or their conformation via distinct PL decays. The high signal-to-background ratios, reproducible and precise ratiometric and homogeneous (washing-free) sensing formats, and higher-order multiplexing capabilities of lanthanide-based TG-FRET have resulted in significant advances in the analysis of biomolecular recognition. Applications range from fundamental analysis of biomolecular interactions and conformations to high-throughput and point-of-care in vitro diagnostics and DNA sequencing to advanced optical encoding, using both liquid and solid samples and in situ, in vitro, and in vivo detection with high sensitivity and selectivity.In this Account, we discuss recent advances in lanthanide-based TG-FRET for the development and application of advanced immunoassays, nucleic acid sensing, and fluorescence imaging. In addition to the different spectral and temporal multiplexing approaches, we highlight the importance of the careful design and combination of different biological, organic, and inorganic molecules and nanomaterials for an adjustable FRET donor-acceptor distance that determines the ultimate performance of the diagnostic assays and conformational sensors in their physiological environment. We conclude by sharing our vision on how progress in the development of new sensing concepts, material combinations, and instrumentation can further advance TG-FRET multiplexing and accelerate its translation into routine clinical practice and the investigation of challenging biological systems.

Bright and stable luminescent probes for target engagement profiling in live cells

The pace of progress in biomedical research directly depends on techniques that enable the quantitative interrogation of interactions between proteins and other biopolymers, or with their small-molecule ligands. Time-resolved Förster resonance energy transfer (TR-FRET) assay platforms offer high sensitivity and specificity. However, the paucity of accessible and biocompatible luminescent lanthanide complexes, which are essential reagents for TR-FRET-based approaches, and their poor cellular permeability have limited broader adaptation of TR-FRET beyond homogeneous and extracellular assay applications. Here, we report the development of CoraFluors, a new class of macrotricyclic terbium complexes, which are synthetically readily accessible, stable in biological media and exhibit photophysical and physicochemical properties that are desirable for biological studies. We validate the performance of CoraFluors in cell-free systems, identify cell-permeable analogs and demonstrate their utility in the quantitative domain-selective characterization of Keap1 ligands, as well as in isoform-selective target engagement profiling of HDAC1 inhibitors in live cells.

Recent Advances in Cellular and Molecular Bioengineering for Building and Translation of Biological Systems

In January of 2020, the Biomedical Engineering Society (BMES)- Cellular and Molecular Bioengineering (CMBE) conference was held in Puerto Rico and themed “Vision 2020: Emerging Technologies to Elucidate the Rule of Life.” The annual BME-CMBE conference gathered worldwide leaders and discussed successes and challenges in engineering biological systems and their translation. The goal of this report is to present the research frontiers in this field and provide perspectives on successful engineering and translation towards the clinic. We hope that this report serves as a constructive guide in shaping the future of research and translation of engineered biological systems.

Long-lived lanthanide emission via a pH-sensitive and switchable LRET complex

Lanthanide-based luminescence resonance energy transfer (LRET) can be used as a tool to enhance lanthanide emission for time-resolved cellular imaging applications. By shortening lanthanide emission lifetimes whilst providing an alternative radiative pathway to the formally forbidden, weak lanthanide-only emission, the photon flux of such systems is increased. With this aim in mind, we investigated energy transfer in differently spaced donor–acceptor terbium–rhodamine pairs with the LRET “on” (low pH) and LRET “off” (high pH). Results informed the design, preparation and characterisation of a compound containing terbium, a spectrally-matched pH-responsive fluorophore and a receptor-targeting group. By combining these elements, we observed switchable LRET, where the targeting group sensitises lanthanide emission, resulting in an energy transfer to the rhodamine dye with an efficiency of E = 0.53. This strategy can be used to increase lanthanide emission rates for brighter optical probes.

A pH-sensitive luminescence resonance energy transfer (LRET) was explored as a method to increase photon flux in a terbium-rhodamine-receptor targeting group construct. At low pH, long-lived dye emission and shorter terbium lifetimes were observed.

Luminescence approaches for the rapid detection of disease-related receptor proteins using transition metal-based probes

Protein biomarkers, particularly abnormally expressed receptor proteins, have been proved to be one of the crucial biomarkers for the rapid assessment, diagnosis, prognosis and prediction of specific human diseases. Transition metal based strategies in particular possess delightful strengths in the in-field and real-time visualization of receptor proteins owing to their unique photophysical properties. In this review, we highlight recent advances in the development of detection methods for receptor protein biomarkers using transition metal based approaches, particularly those employing transition metal complexes. We first discuss the strengths and weaknesses of various strategies used for protein biomarker monitoring in live cells. We then describe the principles of the various sensing platforms and their application for receptor protein detection. Finally, we discuss the challenges and future inspirations in this specific field.

Lanthanide-based resonance energy transfer biosensors for live-cell applications

Lanthanide-based, Förster resonance energy transfer (LRET) biosensors enable sensitive, time-gated luminescence (TGL) imaging or multiwell plate analysis of protein-protein interactions (PPIs) in living mammalian cells. LRET biosensors are polypeptides that consist of an alpha-helical linker sequence sandwiched between a lanthanide complex-binding domain and a fluorescent protein (FP) with two interacting domains residing at each terminus. Interaction between the terminal affinity domains brings the lanthanide complex and FP in close proximity such that lanthanide-to-FP, LRET-sensitized emission is increased. A recent proof-of-concept study examined model biosensors that incorporated the affinity partners FKBP12 and the rapamycin-binding domain of m-Tor (FRB) as well as p53 (1-92) and HDM2 (1-128). The sensors contained an Escherichia coli dihydrofolate reductase (eDHFR) domain that binds with high selectivity and affinity to Tb(III) complexes coupled to the ligand trimethoprim (TMP). When cell lines that stably expressed the sensors were treated with TMP-Tb(III), TGL microscopy revealed dramatic differences (>500%) in donor- or acceptor-denominated, Tb(III)-to-GFP LRET ratios between open (unbound) and closed (bound) states of the biosensors. Much larger signal changes (>2500%) and Z'-factors of 0.5 or more were observed when cells were grown in 96-well or 384-well plates and analyzed using a TGL plate reader. In this chapter, we elaborate on the design and performance of LRET biosensors and provide detailed protocols to guide their use for live-cell microscopic imaging studies and high-throughput library screening.

In Situ Rolling Circle Amplification Förster Resonance Energy Transfer (RCA-FRET) for Washing-Free Real-Time Single-Protein Imaging

Fluorescence signal enhancement via isothermal nucleic acid amplification is an important approach for sensitive imaging of intra- or extracellular nucleic acid or protein biomarkers. Rolling circle amplification (RCA) is frequently applied for fluorescence in situ imaging but faces limitations concerning multiplexing, dynamic range, and the required multiple washing steps before imaging. Here, we show that Förster resonance energy transfer (FRET) between fluorescent dyes and between lanthanide (Ln) complexes and dyes that hybridize to β-actin-specific RCA products in HaCaT cells can afford washing-free imaging of single β-actin proteins. Proximity-dependent FRET could be monitored directly after or during (real-time monitoring) dye or Ln DNA probe incubation and could efficiently distinguish between photoluminescence from β-actin-specific RCA and DNA probes freely diffusing in solution or nonspecifically attached to cells. Moreover, time-gated FRET imaging with the Ln-dye FRET pairs efficiently suppressed sample autofluorescence and improved the signal-to-background ratio. Our results present an important proof of concept of RCA-FRET imaging with a strong potential to advance in situ RCA toward easier sample preparation, higher-order multiplexing, autofluorescence-free detection, and increased dynamic range by real-time monitoring of in situ RCA.

Single-Chain Lanthanide Luminescence Biosensors for Cell-Based Imaging and Screening of Protein-Protein Interactions

Lanthanide-based, Förster resonance energy transfer (LRET) biosensors enabled sensitive, time-gated luminescence (TGL) imaging or multiwell plate analysis of protein-protein interactions (PPIs) in living cells. We prepared stable cell lines that expressed polypeptides composed of an alpha helical linker flanked by a Tb(III) complex-binding domain, GFP, and two interacting domains at each terminus. The PPIs examined included those between FKBP12 and the rapamycin-binding domain of m-Tor (FRB) and between p53 (1–92) and HDM2 (1–128). TGL microscopy revealed dramatic differences (>500%) in donor- or acceptor-denominated, Tb(III)-to-GFP LRET ratios between open (unbound) and closed (bound) states of the biosensors. We observed much larger signal changes (>2,500%) and Z′-factors of 0.5 or more when we grew cells in 96- or 384-well plates and analyzed PPI changes using a TGL plate reader. The modular design and exceptional dynamic range of lanthanide-based LRET biosensors will facilitate versatile imaging and cell-based screening of PPIs.

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Non-invasive, microscopic imaging or screening of protein-protein interactions

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Intracellular assembly of sensor polypeptides with luminescent Tb(III) complexes

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High dynamic range with time-gated detection of Tb(III)-to-GFP sensitized emission

The microbial threat: Can rare earths help?

Despite an ever increasing demand for reliable and cheap methods in the detection and quantification of microbes, surprisingly few investigations have explored or utilized the luminescence of rare earths in the microbial context, neither in conventional, that is, plating and microscopic imaging techniques, nor in advanced methods like fluorescence flow cytometry. We have thus investigated the potential of some rare earth complexes and hybrid materials for microbiological analysis. We found fairly simple procedures for internal staining (dyes inside the bacterial cell) and external staining (dyes on the cell surface). The present paper is predominantly relying on microscopic imaging and luminescence spectroscopies (excitation, emission, decay times), but also evaluates model rare earth microspheres to estimate an eventual rare earth based stain for a fast and sensitive bacteria enumeration with luminescence flow cytometry.

Lanthanide-Based Optical Probes of Biological Systems

The unique photophysical properties of lanthanides, such as europium, terbium, and ytterbium, make them versatile molecular probes of biological systems. In particular, their long-lived photoluminescence, narrow bandwidth emissions, and large Stokes shifts enable experiments that are infeasible with organic fluorophores and fluorescent proteins. The ability of these metal ions to undergo luminescence resonance energy transfer, and photon upconversion further expands the capabilities of lanthanide probes. In this review, we describe recent advances in the design of lanthanide luminophores and their application in biological research. We also summarize the latest detection systems that have been developed to fully exploit the optical properties of lanthanide luminophores. We conclude with a discussion of remaining challenges and new frontiers in lanthanide technologies. The unprecedented levels of sensitivity and multiplexing afforded by rare-earth elements illustrate how chemistry can enable new approaches in biology.

Luminescent gold nanoclusters for bioimaging applications

Luminescent nanomaterials have emerged as attractive candidates for sensing, catalysis and bioimaging applications in recent years. For practical use in bioimaging, nanomaterials with high photoluminescence, quantum yield, photostability and large Stokes shifts are needed. While offering high photoluminescence and quantum yield, semiconductor quantum dots suffer from toxicity and are susceptible to oxidation. In this context, atomically precise gold nanoclusters protected by thiol monolayers have emerged as a new class of luminescent nanomaterials. Low toxicity, bioavailability, photostability as well as tunable size, composition, and optoelectronic properties make them suitable for bioimaging and biosensing applications. In this review, an overview of the sensing of pathogens, and of in vitro and in vivo bioimaging using luminescent gold nanoclusters along with the limitations with selected examples are discussed.

EuNPs-mAb fluorescent probe based immunochromatographic strip for rapid and sensitive detection of porcine epidemic diarrhea virus

Porcine epidemic diarrhea (PED), induced by porcine epidemic diarrhea virus (PEDV) causes acute diarrhea, vomiting, dehydration and high mortality in neonatal piglets, resulting in significant economic losses in the pig industries. In this study, an immunochromatographic assay (ICA) based on a EuNPs-mAb fluorescent probe was developed and optimized for rapid detection of PEDV. The limit of detection (LOD) of the ICA was 0.218 μg/mL (2.725 × 10³ TCID₅₀/mL) and its linear detection range was 0.03125-8 μg/mL (3.91 × 10²-10⁵ TCID₅₀/mL). The ICA was also validated for the detection of PEDV in swine stool samples. 60 swine stool samples from southern China were analyzed by the ICA and RT-PCR, and the results showed that the coincidence rate of the ICA to RT-PCR was 86.67%, which was significantly higher than that of AuNPs based ICA. The ICA is sensitive and specific and can achieve on-site rapid detection of swine stool samples. Therefore, the ICA has a great potential for PED diagnosis and prevention.

Time-resolved microfluidic flow cytometer for decoding luminescence lifetimes in the microsecond region

Time-resolved luminescence detection using long-lived probes with lifetimes in the microsecond region have shown great potential in ultrasensitive and multiplexed bioanalysis. In flow cytometry, however, the long lifetime poses a significant challenge to measure wherein the detection window is often too short to determine the decay characteristics. Here we report a time-resolved microfluidic flow cytometer (tr-mFCM) incorporating an acoustic-focusing chip, which allows slowing down of the flow while providing the same detection conditions for every target, achieving accurate lifetime measurement free of autofluorescence interference. Through configuration of the flow velocity and detection aperture with respect to the time-gating sequence, a multi-cycle luminescence decay profile is captured for every event under maximum excitation and detection efficiency. A custom fitting algorithm is then developed to resolve europium-stained polymer microspheres as well as leukemia cells against abundant fluorescent particles, achieving counting efficiency approaching 100% and lifetime CVs (coefficient of variation) around 2-6%. We further demonstrate lifetime-multiplexed detection of prostate and bladder cancer cells stained with different europium probes. Our acoustic-focusing tr-mFCM offers a practical technique for rapid screening of biofluidic samples containing multiple cell types, especially in resource-limited environments such as regional and/or underdeveloped areas as well as for point-of-care applications.

High pressure induced local ordering and tunable luminescence of La2Hf2O7:Eu3+ nanoparticles

This work highlights the high-pressure induced site swapping and improved ordering of Eu³⁺ in La₂Hf₂O₇:Eu³⁺ nanocrystals which leads to red-orange-yellow tunable emission at low-moderate-high pressure regime and enhanced correlated color temperature.

UV-Vis-NIR absorption spectra of lanthanide oxides and fluorides

Absorption spectra of inorganic lanthanide fluorides and oxides.

The human body at cellular resolution: the NIH Human Biomolecular Atlas Program

Transformative technologies are enabling the construction of three-dimensional maps of tissues with unprecedented spatial and molecular resolution. Over the next seven years, the NIH Common Fund Human Biomolecular Atlas Program (HuBMAP) intends to develop a widely accessible framework for comprehensively mapping the human body at single-cell resolution by supporting technology development, data acquisition, and detailed spatial mapping. HuBMAP will integrate its efforts with other funding agencies, programs, consortia, and the biomedical research community at large towards the shared vision of a comprehensive, accessible three-dimensional molecular and cellular atlas of the human body, in health and under various disease conditions.

QTR-FRET: Efficient background reduction technology in time-resolved förster resonance energy transfer assays

A novel homogeneous assay system QTR-FRET (Quencher modulated Time-Resolved Förster Resonance Energy Transfer) combining quenching resonance energy transfer (QRET) and time-resolved Förster resonance energy transfer (TR-FRET) was developed to reduce background signal in the conventional energy transfer applications. The TR-FRET functionality is often limited by the lanthanide donor background signal leading to the use of low donor concentration. QTR-FRET reduces this background by introducing soluble quencher molecule, and in this work the concept functionality was proven and compared to previously introduced QRET and TR-FRET technologies. Comparison was performed with three different Eu³⁺-chelates exhibiting different luminescent lifetime and stability. The side-by-side comparison of the three signaling systems and Eu³⁺-chelates was demonstrated in a model assay with Eu³⁺-chelate conjugated biotin and streptavidin (SA) or Cy5-SA conjugate. Comparison of the methodologies showed increased signal-to-background ratios when comparing QTR-FRET to TR-FRET, especially at high Eu³⁺-biotin concentrations. Quenching the non-bound Eu³⁺-biotin improved the assay performance, which suggests that an improved assay performance can be attained with the QTR-FRET method. QTR-FRET is expected to be especially useful for Eu³⁺-labeled ligands with low affinity or assays requiring high Eu³⁺-ligand concentration. The QTR-FRET indicated potential for multi-analyte approaches separately utilizing the direct QRET-type Eu³⁺-chelate signal and energy transfer signal readout in a single-well. This potential was hypothesized with Avi-KRAS nucleotide exchange assay as a second biologically relevant model system.

An open-source software analysis package for Microspheres with Ratiometric Barcode Lanthanide Encoding (MRBLEs)

Multiplexed bioassays, in which multiple analytes of interest are probed in parallel within a single small volume, have greatly accelerated the pace of biological discovery. Bead-based multiplexed bioassays have many technical advantages, including near solution-phase kinetics, small sample volume requirements, many within-assay replicates to reduce measurement error, and, for some bead materials, the ability to synthesize analytes directly on beads via solid-phase synthesis. To allow bead-based multiplexing, analytes can be synthesized on spectrally encoded beads with a 1:1 linkage between analyte identity and embedded codes. Bead-bound analyte libraries can then be pooled and incubated with a fluorescently-labeled macromolecule of interest, allowing downstream quantification of interactions between the macromolecule and all analytes simultaneously via imaging alone. Extracting quantitative binding data from these images poses several computational image processing challenges, requiring the ability to identify all beads in each image, quantify bound fluorescent material associated with each bead, and determine their embedded spectral code to reveal analyte identities. Here, we present a novel open-source Python software package (the mrbles analysis package) that provides the necessary tools to: (1) find encoded beads in a bright-field microscopy image; (2) quantify bound fluorescent material associated with bead perimeters; (3) identify embedded ratiometric spectral codes within beads; and (4) return data aggregated by embedded code and for each individual bead. We demonstrate the utility of this package by applying it towards analyzing data generated via multiplexed measurement of calcineurin protein binding to MRBLEs (Microspheres with Ratiometric Barcode Lanthanide Encoding) containing known and mutant binding peptide motifs. We anticipate that this flexible package should be applicable to a wide variety of assays, including simple bead or droplet finding analysis, quantification of binding to non-encoded beads, and analysis of multiplexed assays that use ratiometric, spectrally encoded beads.

Fluorescence lifetime imaging of upper gastrointestinal pH in vivo with a lanthanide based near-infrared τ probe

Lanthanide complex was successfully applied in the design of pH-responsive NIR τ probe for quantitative in vivo imaging.

Time-resolved fluorescence lifetime imaging (FLIM) in the near-infrared region of 900–1700 nm not only allows a deep tissue penetration depth but also offers the unique benefit of the quantitative visualization of molecular events in vivo and is independent of local luminescence intensity and fluorophore concentration. Herein, we report the design of a wide-range pH sensitive molecular probe based on Yb³⁺ porphyrinate. The Yb³⁺ probe shows increasing NIR emission and lifetime with pK_a values of ca. 6.6 from pH 9.0 and 5.0 and also displays an elongated lifetime from ca. 135 to 170 μs at lower pH values (5.0–1.0) due to aggregation and reduced exposure to water at low pH values. Importantly, the probe is able to monitor a wide range of in vivo gastrointestinal pH values in mice models and the potential applications in imaging-guided gastrointestinal diagnostics and therapeutics were revealed. This study shows that lifetime contrast is important for preclinical imaging; lanthanide complexes could be successfully used in the design of stimuli-responsive NIR τ probes for advanced in vivo imaging.

A three-dimensional ratiometric sensing strategy on unimolecular fluorescence-thermally activated delayed fluorescence dual emission

Visualized sensing through fluorescence signals is a powerful method for chemical and physical detection. However, the utilization of fluorescent molecular probes still suffers from lack of precise signal self-calibration in practical use. Here we show that fluorescence and thermally activated delayed fluorescence can be simultaneously produced at the single-molecular level. The thermally activated delayed fluorescence serves as a sensing signal with its wavelength and lifetime both altered correlating to polarity, whereas the fluorescence always remains unchanged as an internal reference. Upon the establishment of a three-dimensional working curve upon the ratiometric wavelength and photoluminescence lifetime vs. polarity, disturbance factors during a relevant sensing process can be largely minimized by such a multiple self-calibration. This strategy was further applied into a precise detection of the microenvironmental polarity variation in complex phospholipid systems, towards providing new insights for convenient and accurate diagnosis of membrane lesions.

New photoluminescent strategies are required to realize high-performance ratiometric sensors for bio-sensing applications. Here, the authors demonstrate a fluorescence-thermally activated delayed fluorescence dual emission strategy for ratiometric sensors with high precision.

The role of ligand to metal charge-transfer states on the luminescence of Europium complexes with 18-membered macrocyclic ligands

The stabilization of a divalent Europium provides an efficient pathway for the quenching of the luminescence in ten-coordinate macrocyclic complexes.

Highly luminescent, biocompatible ytterbium(iii) complexes as near-infrared fluorophores for living cell imaging

We report three synthetic methods to prepare biocompatible Yb³⁺ complexes, which displayed high NIR luminescence with quantum yields up to 13% in aqueous media. This renders β-fluorinated Yb³⁺ porphyrinoids a new class of NIR probes for living cell imaging including time-resolved fluorescence lifetime imaging.

Herein, we report the design and synthesis of biocompatible Yb³⁺ complexes for near-infrared (NIR) living cell imaging. Upon excitation at either the visible (Soret band) or red region (Q band), these β-fluorinated Yb³⁺ complexes display high NIR luminescence (quantum yields up to 23% and 13% in dimethyl sulfoxide and water, respectively) and have higher stabilities and prolonged decay lifetimes (up to 249 μs) compared to the β-non-fluorinated counterparts. This renders the β-fluorinated Yb³⁺ complexes as a new class of biological optical probes in both steady-state imaging and time-resolved fluorescence lifetime imaging (FLIM). NIR confocal fluorescence images showed strong and specific intracellular Yb³⁺ luminescence signals when the biocompatible Yb³⁺ complexes were uptaken into the living cells. Importantly, FLIM measurements showed an intracellular lifetime distribution between 100 and 200 μs, allowing an effective discrimination from cell autofluorescence, and afforded high signal-to-noise ratios as firstly demonstrated in the NIR region. These results demonstrated the prospects of NIR lanthanide complexes as biological probes for NIR steady-state fluorescence and time-resolved fluorescence lifetime imaging.

Thermal annealing effects on La2Hf2O7:Eu3+ nanoparticles: a curious case study of structural evolution and site-specific photo- and radio-luminescence

High temperature annealed La₂Hf₂O₇:Eu³⁺ nanoparticles favor tunneling of Eu³⁺ to symmetric sites showing orange emission, whereas low temperature annealed samples favor red emission.