Luminescent Terbium Complexes Research Articles

Examination of the spectroscopic characteristics of bright green emitting, octa coordinated luminescent terbium (III) complexes

Mononuclear heteroleptic green emissive Tb(III) complexes were synthesized utilizing 1,3-diketone and N-based bidentate auxiliary ligands. The structural and photoluminescence characteristics were analyzed employing various spectroscopic and analytical techniques. Band gap values obtained from optical and cyclovoltammetric studies were found in good alignment with each other, illustrating their semiconducting behavior and thereby their potential applications in fabrication of photocells and photovoltaic. The photoluminescent spectra of T1 – T4 complexes exhibited distinct peaks at about 491, 547, 583 and 622 nm. The green emission in these complexes can be attributed to the emission peak observed at approximately 547 nm, which arises from the 5D4→7F5 transition. The thermal investigation revealed their high thermal stability, which further indicates their potential utilization in display devices.

Luminescent Tb(III) complexes with Lewis bases for displays: Synthesis and spectral investigation

Octa-coordinated luminescent terbium (III) complexes were synthesized with fluorinated 1,3-diketone [4,4,4-trifluoro-1-(2-furyl)-1,3-butanedione (TFFB)] and nitrogen donor auxiliary moieties. The structural investigation of ternary complexes was done via elemental analysis, FT-IR, 1H NMR, electrochemical, absorption and photoluminescence analyses. Absorption patterns evinced ligand based broad bands. The luminescent nature of complexes proposed the good sensitization of Tb(III) cation by different ligands. The emission patterns of compounds demonstrated sharp emission peaks which are assigned to Tb3+ ion. Most intense peak present at 546 nm credited to 5D4 → 7F5 transition is accountable for green emissive nature of Tb(III) complexes. Outcomes of colorimetric study also support the PL emission results. The unique luminescent and conducting characteristics of prepared materials affirmed their usefulness in practical display applications.

A Dendrimer-Based Time-Gated Concentric FRET Configuration for Multiplexed Sensing

Förster resonance energy transfer (FRET) is widely used for the development of biological probes and sensors. In this context, the norm for multiplexed detection is deployment of multiple probes, each a discrete donor-acceptor pair. Concentric FRET (cFRET) probes enable multiplexed sensing with a single vector but, to date, have only been developed around semiconductor quantum dots, which may limit the scope of biological applications for such probes. Here, we demonstrate that dendrimers labeled with a luminescent terbium complex (Tb) are a viable and advantageous alternative platform for cFRET probes. Polyamidoamine dendrimers were functionalized with Tb, biotin, NeutrAvidin, and three types of dye-labeled oligonucleotide probes to establish a network of competitive and sequential Tb-to-dye and dye-to-dye FRET pathways. These probes were characterized physically and photophysically, and a time-gated multiplexed assay for DNA targets was demonstrated. The time-gating offered by the Tb allowed the rejection of background autofluorescence from serum. More broadly, this dendrimer-based architecture shows that cFRET is a general concept and is an important step toward a new generation of probes for biological sensing.

Photoluminescence performance of green light emitting terbium (III) complexes with β-hydroxy ketone and nitrogen donor ancillary ligands.

An efficient and cost-effective technique, solution precipitation approach is adopted to synthesize five bright green luminescent terbium (III) complexes by employing the main β-hydroxy ketone ligand, 2-hydroxy-4-ethoxyacetophenone, and ancillary ligands like bathophenanthroline, 5,6-dimethyl-1,10-phenanthroline, 1,10-phenanthroline, and 2,2-bipyridyl. The elemental compositions and binding mode of ligand to terbium (III) ion can be validated by using energy dispersive X-ray analysis, elemental analysis, Fourier transform infrared, and proton nuclear magnetic resonance spectroscopy. The complexes are thermally stable up to 158°C and possess the cubic shaped particles as confirmed by thermogravimetric analysis and scanning electron microscopic study, respectively. The band-gap energy (3.02-2.92 eV) of complexes is reckoned through diffuse reflectance spectra, which tailors them as potential candidates in the field of military radars. The photoluminescence studies unveil that the complexes exhibit the bright green luminescence corresponding to 5 D4 → 7 F5 transition of Tb3+ ion (548 nm) under the excitation wavelength of 395 or 397 nm. The Commission International de I'Eclairage chromaticity coordinates (x, y) and color purity substantiates the green emission of complexes. The energy transfer mechanism elucidates that the main ligand and ancillary ligands sensitize Tb3+ ion, which in turn enhances the luminescence efficiency of the emissive layer of white organic light emitting diodes. The results reveal that the complexes are considered as good contenders in the field of display devices and laser technology. Lastly, in vitro antimicrobial and antioxidant activity proclaim the potent antimicrobial and antioxidant actions of complexes via tube dilution and 2, 2-diphenyl-1-picrylhydrazyl assays, respectively.

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

SummaryLanthanide-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.

Synthesis, NMR and optical features of intense green color terbium(III) complexes

The paper reports the three highly green luminescent terbium(III) complexes functionalized with 1,1,1,5,5,6,6,6-octafluorohexane-2,4-dione (L) as main ligand, 1,10-phenanthroline (phen) and2,2′-bipyridyl (bipy) as ancillary ligands. These complexes were characterized by means of IR, 1H-NMR, elemental analysis, UV–vis, TG-DTG and photoluminescence (PL) spectroscopy. In 1H-NMR spectra, large highfield and lowfield shifts are noticed in these paramagnetic complexes. The proton resonances of phen and bipy have been shifted to highfield, while, the resonance of methine proton in main ligand has been shifted to lowfield. In this way, all the proton signals are resolved as revealed in NMR spectra. The photoluminescent features clearly indicate that the substitution of solvent molecules by ancillary ligand in the coordination sphere enhance the emission intensity and decay time of the complexes. Furthermore, the intramolecularenergy transfer mechanism shows that an efficient transfer of energy from triplet level of ligands to the emitting level of terbium(III) ion takes place in the complexes. In this manner, main ligand and ancillary ligands sensitize the terbium(III) ion yielding the intense and characteristic green luminescence in the complexes. The CIE color coordinates of Tb(L)3bipy complex exactly matches with standard pure green color (x = 0.21, y = 0.71) of NTSC (National Television Standard Committee) which makes them pure green promising component for potencial application as luminescent materials. The beauty of complexes is that these acts as NMR shift reagent as well as these are of great interest in the field of emitting material and display devices.

Terbium(III) complexes sensitized with β-diketone and ancillary ligands: Synthesis, elucidation of photoluminescence properties and mechanism

A β-diketone 1-(2-hydroxyphenyl)-3-phenylpropane-1,3-dione (HHPPP) has been used to synthesize binary and ternary terbium(III) complexes of the form Tb(HPPP)L [where L = H2O (C1), 1,10-phenanthroline (C2), 2,2′-bipyridyl (C3), 2,2′-biquinoline (C4) and neocuproine (C5)]. These complexes are characterized by various techniques including IR, 1H-NMR, elemental analyses, powder XRD, TG–DTG, UV–visible and photoluminescene spectroscopy. The emission spectra of complexes exhibit characteristics bands of terbium(III) ion with 5D4 → 7F5 (545 nm) as the most prominent band. The photophysical properties clearly reveal that the substitution of solvent molecules by anciliary ligands essentially increases the luminescence intensity and decay time of ternary complexes, suggesting the sensitization of Tb(III) ion by both HPPP and anciliary ligands. The sensitization process in complexes is also delineated by proposed energy transfer mechanism. The color coordinates of the complexes fall under green region of Commission Internationale de L’Eclairage diagram. These results demonstrate that luminescent terbium(III) complexes could be a deserving component for organic light emitting diodes and display devices.

Luminescent Alkyne-Bearing Terbium(III) Complexes and Their Application to Bioorthogonal Protein Labeling.

Two new bifunctional macrocyclic chelate ligands that form luminescent terbium(III) complexes featuring an alkyne group for conjugation to (bio)molecules via the Cu(I)-catalyzed "click" reaction were synthesized. Upon ligation, the complexes exhibit a significant luminescent enhancement when excited at the λ(max) of the "clicked" products. To demonstrate the utility of the complexes for luminescent labeling, they were conjugated in vitro to E. coli aspartate/glutamate-binding protein incorporating a genetically encoded p-azido-L-phenylalanine or p-(azidomethyl)-L-phenylalanine residue. The complexes may prove useful for time-gated assay applications.

Total protein concentration quantification using nanobeads with a new highly luminescent terbium(iii) complex

Using 40 nm large polyglycidyl methacrylate (PGMA) nanobeads loaded with a new Tb(iii) complex, it is possible to determine total protein concentration (TPC) down to 50 ng mL−1in just 10 minutes.

Time-Resolved Nucleic Acid Hybridization Beacons Utilizing Unimolecular and Toehold-Mediated Strand Displacement Designs.

Nucleic acid hybridization probes are sought after for numerous assay and imaging applications. These probes are often limited by the properties of fluorescent dyes, prompting the development of new probes where dyes are paired with novel or nontraditional luminescent materials. Luminescent terbium complexes are an example of such a material, and these complexes offer several unique spectroscopic advantages. Here, we demonstrate two nonstem-loop designs for light-up nucleic acid hybridization beacons that utilize time-resolved Förster resonance energy transfer (TR-FRET) between a luminescent Lumi4-Tb cryptate (Tb) donor and a fluorescent reporter dye, where time-resolved emission from the dye provides an analytical signal. Both designs are based on probe oligonucleotides that are labeled at their opposite termini with Tb and a fluorescent reporter dye. In one design, a probe is partially blocked with a quencher dye-labeled oligonucleotide, and target hybridization is signaled through toehold-mediated strand displacement and loss of a competitive FRET pathway. In the other design, the intrinsic folding properties of an unblocked probe are utilized in combination with a temporal mechanism for signaling target hybridization. This temporal mechanism is based on a recently elucidated "sweet spot" for TR-FRET measurements and exploits distance control over FRET efficiencies to shift the Tb lifetime within or outside the time-gated detection window for measurements. Both the blocked and unblocked beacons offer nanomolar (femtomole) detection limits, response times on the order of minutes, multiplexing through the use of different reporter dyes, and detection in complex matrices such as serum and blood. The blocked beacons offer better mismatch selectivity, whereas the unblocked beacons are simpler in design. The temporal mechanism of signaling utilized with the unblocked beacons also plays a significant role with the blocked beacons and represents a new and effective strategy for developing FRET probes for bioassays.

A time-resolved luminescent competitive assay to detect L-selectin using aptamers as recognition elements

L-selectin is a protein with potential importance for numerous diseases and clinical disorders. In this paper, we present a new aptamer-based luminescent assay developed to detect L-selectin. The sensing system working principle is based on Förster Resonance Energy Transfer (FRET) from a donor terbium complex (TbC) to an acceptor cyanine dye (Cy5). In the present approach, the biotinylated aptamer is combined with Cy5-labelled streptavidin (Cy5-Strep) to yield an aptamer-based acceptor construct (Apta-Cy5-Strep), while L-selectin is conjugated using luminescent TbC. Upon aptamer binding to the TbC-labelled L-selectin (L-selectin-TbC), permanent donor-acceptor proximity is established which allows for radiationless energy transfer to occur. However, when unlabelled L-selectin is added, it competes with the L-selectin-TbC and the FRET signal decreases as the L-selectin concentration increases. FRET from the TbC to Cy5 was observed with time-gated time-resolved luminescence spectroscopy. A significant change in the corrected luminescence signal was observed in the dynamic range of 10–500 ng/mL L-selectin, the concentration range relevant for accelerated cognitive decline of Alzheimer's disease, with a limit of detection (LOD) equal to 10 ng/mL. The aptasensor-based assay is homogeneous and can be realized within one hour. Therefore, this method has the potential to become an alternative to tedious heterogeneous analytical methods, e.g. based on enzyme-linked immunosorbent assay (ELISA).

Time-Gated DNA Photonic Wires with Förster Resonance Energy Transfer Cascades Initiated by a Luminescent Terbium Donor

Functional DNA nanotechnology is a rapidly growing area of research with many prospective photonic applications, including roles as wires and switches, logic operators, and smart biological probes and delivery vectors. Photonic wire constructs are one such example and comprise a Forster resonance energy transfer (FRET) cascade between fluorescent dyes arranged periodically along a DNA scaffold. To date, the majority of research on photonic wires has focused on setting new benchmarks for efficient energy transfer over more steps and across longer distances, using almost exclusively organic fluorescent dyes and strictly DNA structures. Here, we expand the range of materials utilized with DNA photonic wires by demonstrating the use of a luminescent terbium complex (Tb) as an initial donor for a four-step FRET cascade along a ∼15 nm long DNA/locked nucleic acid (LNA) photonic wire. The inclusion of LNA nucleotides increases the thermal stability of the photonic wires while the Tb affords time-gated emission m...

Detecting free hemoglobin in blood plasma and serum with luminescent terbium complexes.

Hemolysis, the rupturing of red blood cells, can result from numerous medical conditions (in vivo) or occur after collecting blood specimen or extracting plasma and serum out of whole blood (in vitro). In clinical laboratory practice, hemolysis can be a serious problem due to its potential to bias detection of various analytes or biomarkers. Here we present the first "mix-and-measure" method to assess the degree of hemolysis in biosamples using luminescence spectroscopy. Luminescent terbium complexes (LTC) were studied in the presence of free hemoglobin (Hb) as indicators for hemolysis in TRIS-buffer, and in fresh human plasma with absorption, excitation and emission measurements. Our findings indicate dynamic as well as resonance energy transfer (FRET) between the LTC and the porphyrin ligand of hemoglobin. This transfer leads to a decrease in luminescence intensity and decay time even at nanomolar hemoglobin concentrations either in buffer or plasma. Luminescent terbium complexes are very sensitive to free hemoglobin in buffer and blood plasma. Due to the instant change in luminescence properties of the LTC in presence of Hb it is possible to access the concentration of hemoglobin via spectroscopic methods without incubation time or further treatment of the sample thus enabling a rapid and sensitive detection of hemolysis in clinical diagnostics.

A macrocyclic tetraamine bearing four phenol groups: a new class of heptadentate ligands to provide an oxygen-sensitive luminescent Tb(III) complex with an extendable phenol pendant arm.

This paper presents a 1,4,7,10-teraazacyclododecane-based tetrakis-phenol as a protonated ligand precursor and its oxygen-sensitive luminescent terbium(III) complex with an extendable phenol pendant arm (Φ = 0.91 under N2, Φ = 0.031 under air), in which the potentially N4O4-octadentate ligand unprecedentedly coordinates to the Tb(3+) ion in a N4O3-heptadentate fashion.

Electrochemical signal response for vitamin B1 using terbium luminescent nanoscale building blocks as optical sensors

A luminescent terbium complex was covalently modified onto anisotropic gold nanostructures (nanoparticles and nanorods) respectively. Two novel emissive gold nanoprobes were successfully developed for selective and rapid determination of vitamin B1 in water. More significantly, electrochemical sensing performances were investigated based on green emissive gold nanoparticles and nanorods which were fabricated by anchoring gold nano-sensors on the surface of glassy carbon electrodes. Both the fluorescence spectra and cyclic voltammetry curves exhibited selective signal changes in the presence of vitamin B1 in comparison with vitamin B2, B3, B4, B5 and B6. It is considered to be the first example that different dimensional fluorescent nano-probes were selected and applied as dual opto-electrochemical sensors for recognizing the same analyte.

Terbium-containing graphene oxide and its opto-electrochemical response for hypochlorite in water

A luminescent terbium complex was incorporated into graphene oxide and the material was used for monitoring hypochlorite in pure water (detection limit 0.04μM). The evidence from the cyclic voltammetry curves revealed that the signal changes of the probe were dependent on the presence of hypochlorite. The emission spectra displayed luminescence quenching effects upon the addition of hypochlorite.

Specific recognition of DNA depurination by a luminescent terbium(iii) complex

Recognition of DNA depurination is of great importance for early cancer detection. Current analytical methods for this purpose are usually complicated. Luminescent lanthanide complexes possess some fascinating optical properties that have shown potential applications in biomedical research. In this study, a novel terbium(III) complex (TbL) has been demonstrated to be capable of recognizing purine nucleobases in DNA as a selective time-resolved luminescence probe. TbL consists of a luminescent terbium(III) center and two nitroimidazole groups linked by a linear polydentate ligand. The luminescence of TbL is enhanced remarkably upon reaction with oligonucleotides or natural DNA containing purine bases in aqueous solution, while it is quenched dramatically as depurination occurs to DNA. Mechanistic studies using circular dichroism and fluorescence spectroscopies reveal that the luminescence enhancement results from the preferential intercalation between the nitroimidazole moieties of TbL and the purine bases of DNA, which regulate the electron withdrawing effect of nitro groups via hydrogen bonds and thereby affect the energy transfer from the ligand to the metal center of the probe. This mechanism is also supported by molecular dynamics simulation results for the reaction. The distinct luminescence responses of TbL in the presence and absence of purine bases in DNA make it a sensitive probe for DNA depurination in physiological conditions.

Multiplexed Tracking of Protease Activity Using a Single Color of Quantum Dot Vector and a Time-Gated Förster Resonance Energy Transfer Relay

Semiconductor quantum dots (QDs) are attractive probes for optical sensing and imaging due to their unique photophysical attributes and nanoscale size. In particular, the development of assays and biosensors based on QDs and Förster resonance energy transfer (FRET) continues to be a prominent focus of research. Here, we demonstrate the application of QDs as simultaneous donors and acceptors in a time-gated FRET relay for the multiplexed detection of protease activity. In contrast to the current state-of-the-art, which uses multiple colors of QDs, multiplexing was achieved using only a single color of QD. The other constituents of the FRET relay, a luminescent terbium complex and fluorescent dye, were assembled to QDs via peptides that were selected as substrates for the model proteases trypsin and chymotrypsin. Loss of prompt FRET between the QD and dye signaled the activity of chymotrypsin; loss of time-gated FRET between the terbium and QD signaled the activity of trypsin. We applied the FRET relay in a series of quantitative, real-time kinetic assays of increasing biochemical complexity, including multiplexed sensing, measuring inhibition in a multiplexed format, and tracking the proteolytic activation of an inactive pro-protease to its active form in a coupled, multienzyme system. These capabilities were derived from a ratiometric analysis of the two FRET pathways in the relay and permitted extraction of initial reaction rates, enzyme specificity constants, and apparent inhibition constants. This work adds to the growing body of research on multifunctional nanoparticles and introduces multiplexed sensing as a novel capability for a single nanoparticle vector. Furthermore, the ability to track both enzymes within a coupled biological system using one vector represents a significant advancement for nanoparticle-based biosensing. Prospective applications in biochemical research, applied diagnostics, and drug discovery are discussed.

Quantum Dots as Simultaneous Acceptors and Donors in Time-Gated Förster Resonance Energy Transfer Relays: Characterization and Biosensing

The unique photophysical properties of semiconductor quantum dot (QD) bioconjugates offer many advantages for active sensing, imaging, and optical diagnostics. In particular, QDs have been widely adopted as either donors or acceptors in Förster resonance energy transfer (FRET)-based assays and biosensors. Here, we expand their utility by demonstrating that QDs can function in a simultaneous role as acceptors and donors within time-gated FRET relays. To achieve this configuration, the QD was used as a central nanoplatform and coassembled with peptides or oligonucleotides that were labeled with either a long lifetime luminescent terbium(III) complex (Tb) or a fluorescent dye, Alexa Fluor 647 (A647). Within the FRET relay, the QD served as a critical intermediary where (1) an excited-state Tb donor transferred energy to the ground-state QD following a suitable microsecond delay and (2) the QD subsequently transferred that energy to an A647 acceptor. A detailed photophysical analysis was undertaken for each step of the FRET relay. The assembly of increasing ratios of Tb/QD was found to linearly increase the magnitude of the FRET-sensitized time-gated QD photoluminescence intensity. Importantly, the Tb was found to sensitize the subsequent QD-A647 donor-acceptor FRET pair without significantly affecting the intrinsic energy transfer efficiency within the second step in the relay. The utility of incorporating QDs into this type of time-gated energy transfer configuration was demonstrated in prototypical bioassays for monitoring protease activity and nucleic acid hybridization; the latter included a dual target format where each orthogonal FRET step transduced a separate binding event. Potential benefits of this time-gated FRET approach include: eliminating background fluorescence, accessing two approximately independent FRET mechanisms in a single QD-bioconjugate, and multiplexed biosensing based on spectrotemporal resolution of QD-FRET without requiring multiple colors of QD.

Terbium to Quantum Dot FRET Bioconjugates for Clinical Diagnostics: Influence of Human Plasma on Optical and Assembly Properties

Förster resonance energy transfer (FRET) from luminescent terbium complexes (LTC) as donors to semiconductor quantum dots (QDs) as acceptors allows extraordinary large FRET efficiencies due to the long Förster distances afforded. Moreover, time-gated detection permits an efficient suppression of autofluorescent background leading to sub-picomolar detection limits even within multiplexed detection formats. These characteristics make FRET-systems with LTC and QDs excellent candidates for clinical diagnostics. So far, such proofs of principle for highly sensitive multiplexed biosensing have only been performed under optimized buffer conditions and interactions between real-life clinical media such as human serum or plasma and LTC-QD-FRET-systems have not yet been taken into account. Here we present an extensive spectroscopic analysis of absorption, excitation and emission spectra along with the luminescence decay times of both the single components as well as the assembled FRET-systems in TRIS-buffer, TRIS-buffer with 2% bovine serum albumin, and fresh human plasma. Moreover, we evaluated homogeneous LTC-QD FRET assays in QD conjugates assembled with either the well-known, specific biotin-streptavidin biological interaction or, alternatively, the metal-affinity coordination of histidine to zinc. In the case of conjugates assembled with biotin-streptavidin no significant interference with the optical and binding properties occurs whereas the histidine-zinc system appears to be affected by human plasma.