Contrasting impact of coordination polyhedra and site symmetry on the electronic energy levels in nine-coordinated Eu(III) and Sm(III) crystals structures determined from single crystal luminescence spectra.

Lanthanide luminescence is characterised by "forbidden" 4f-4f transitions and a complicated electronic structure. Our understanding of trivalent lanthanide(III) ion luminescence is centered on Eu3+ because absorbing and emitting transitions in Eu3+ occur from a single electronic energy level. In Sm3+ both absorbing and emitting multiplets have a larger multiplicity. A band arising in transitions from the first emitting state multiplet to the ground state multiplet will have nine lines for a Sm3+ complex. In this study, high-resolution emission and excitation spectra were used to determine the electronic energy levels for the lowest multiplet and first emitting multiplet in four Sm3+ compounds with either tricapped trigonal prismatic TTP or capped square antiprismatic cSAP coordination polyhedra but different site symmetry. This was achieved by the use of Boltzmann distribution population analysis and experimentally determined transition probabilities from emission and excitation spectra. Using this analysis it was possible to show the effect of changing three oxygen atoms with three nitrogen atoms in the donor set for two compounds with the same coordination polyhedra and site symmetry. This work celebrates the 40th anniversary of Kirby and Richardson's first report of [Eu(ODA)3]3- luminescence.

Mapping the distribution of electronic states within the 5D4 and 7F6 levels of Tb3+ complexes with optical spectroscopy.

The Tb(III) ion has the most intense luminescence of the trivalent lanthanide(III) ions. In contrast to Eu(III), where the two levels only include a single state, the high number of electronic states in the ground (7F6) and emitting (5D4) levels makes detailed interpretations of the electronic structure-the crystal field-difficult. Here, luminescence emission and excitation spectra of Tb(III) complexes with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA, [Tb(DOTA)(H2O)]-), ethylenediaminetetraacetic acid (EDTA, [Tb(EDTA)(H2O)3]-) and diethylenetriaminepentaacetic acid (DTPA, [Tb(DTPA)(H2O)]2-) as well as the Tb(III) aqua ion ([Tb(H2O)9]3+) were recorded at room temperature and in frozen solution. Using these data the electronic structure of the 5D4 multiplets of Tb(III) was mapped by considering the transitions to the singly degenerate 7F0 state. A detailed spectroscopic investigation was performed and it was found that the 5D4 multiplet could accurately be described as a single band for [Tb(H2O)9]3+, [Tb(DOTA)(H2O)]- and [Tb(EDTA)(H2O)3]-. In contrast, for [Tb(DTPA)(H2O)]2- two bands were needed. These results demonstrated the ability of describing the electronic structure of the emitting 5D4 multiplet using emission spectra. This offers an avenue for investigating the relationship between molecular structure and luminescent properties in detailed photophysical studies of Tb(III) ion complexes.

Crystal structures of two SmIII complexes with dipicolinate [DPA]2- ligands: comparison of luminescent properties of products obtained at different pH values.

The formation of the two title compounds, Na3[Sm(DPA)3]·14H2O tris-odium tris-(pyridine-2,6-di-carboxyl-ato-κ3 O 2,N,O 6)samarate(III) tetra-deca-hydrate, Na3[Sm(C7H3NO4)3]·14H2O, and catena-poly[[[di-aqua-(6-carb-oxy-pyridine-2-carb-oxyl-ato-κ3 O 2,N,O 6)samarium(III)]-µ-pyridine-2,6-di-carboxyl-ato-κ4 O 2,N,O 6:O 2] tetra-hydrate], {[Sm(C7H3NO4)(C7H4NO4)(H2O)2]·4H2O}n, depends on the pH value adjusted with NaOH solution. In both crystal structures, the coordination spheres of the SmIII cations were found to be best described by a tricapped trigonal prism (TTP), with a more regular O6N3 donor set for Na3[Sm(DPA)3]·14H2O than that of O7N2 for [Sm(DPA)(HDPA)(H2O)2]·4H2O. The supra-molecular features of both crystal structures are dominated by O-H⋯O hydrogen bonds between water mol-ecules and the O atoms of the dipicolinato ligands. Samples were made from solutions at pH = 2, pH = 5, pH = 7, and pH = 10, and the crystals present in each sample were ground to a powder. The powder samples were analyzed with powder X-ray diffraction and luminescence spectroscopy. The splitting of the bands in the luminescence spectra recorded on powders at 77 K was observed to vary with the pH.

Optical spectroscopy as a tool for studying the solution chemistry of neodymium(III).

In nature, the elements of the inorganic part of the periodic table are found in three forms: metals, ions in salts & minerals, and ions in solution. The ions may be coordinated to simple or complicated ligands. They may form purely electrostatic or partially covalent bonds. A common trend is that the more covalent bonds an element form, the more we know of its physicochemical properties. The rare earths form purely electrostatic bonds, thus, our understanding of the solution chemistry of these elements is limited-yet important. Most rare earth elements used today pass through hydrometallurgical processes that rely on the solution chemistry of these elements, even through the critical applications are in alloys and functional materials. Through developments in optical spectroscopy, total X-ray scattering, and quantum chemical methods we are posed to remedy this situation: we are ready to create predictive structure-property relationships in the field of lanthanide solution chemistry. The scope of this review is to summarise the state-of-the-art for neodymium(III), to go through the structure-property relationships that are in use. In the form of NdFeB magnets, neodymium plays a crucial role in green energy production and electric propulsion. As a 4f3 ion in solution it is also one of the simpler rare earth ions, and the Nd(III) ion has characteristic optical properties that can be exploited as a handle in physicochemical studies. Here, we start with a critical review of the current concepts used to relate structure and electronic energy levels. We follow with our suggested approach of using the methodology from molecular photophysics to relate optical properties and structure, and conclude with selected literature examples.

Electronic Structure of Neodymium(III) and Europium(III) Resolved in Solution Using High-Resolution Optical Spectroscopy and Population Analysis.

Solution chemistry of the lanthanide(III) ions is unexplored and relevant: extraction and recycling processes exclusively operate in solution, MRI is a solution-phase method, and bioassays are done in solution. However, the molecular structure of the lanthanide(III) ions in solution is poorly described, especially for the near-IR (NIR)-emitting lanthanides, as these are difficult to investigate using optical tools, which has limited the availability of experimental data. Here we report a custom-built spectrometer dedicated to investigation of lanthanide(III) luminescence in the NIR region. Absorption, luminescence excitation, and luminescence spectra of five complexes of europium(III) and neodymium(III) were acquired. The obtained spectra display high spectral resolution and high signal-to-noise ratios. Using the high-quality data, a method for determining the electronic structure for the thermal ground states and emitting states is proposed. It combines Boltzmann distributions with population analysis and uses the experimentally determined relative transition probabilities from both excitation and emission data. The method was tested on the five europium(III) complexes and was used to resolve the electronic structures of the ground state and the emitting state of neodymium(III) in five different solution complexes. This is the first step toward correlating optical spectra with chemical structure in solution for NIR-emitting lanthanide complexes.

Chelating chloride using binuclear lanthanide complexes in water.

Halide recognition by supramolecular receptors and coordination complexes in water is a long-standing challenge. In this work, we report chloride binding in water and in competing media by pre-organised binuclear kinetically inert lanthanide complexes, bridged by flexible -(CH2)2- and -(CH2)3- spacers, forming [Ln2(DO3A)2C-2] and [Ln2(DO3A)2C-3], respectively. These hydrophilic, neutral lanthanide coordination complexes are shown to bind chloride with apparent association constants of up to 105 M-1 in water and in buffered systems. Hydroxide bridging was observed in these complexes at basic pH, which was proven to be overcome by chloride. Thus, these lanthanide complexes show promise towards chloride recognition in biology and beyond. The results described here have clearly identified a new area of anion coordination chemistry that is ripe for detailed exploration.

Effect of buffers and pH in antenna sensitized Eu(III) luminescence.

The photophysics of a europium(III) complex of 1,4,7,10-tetraazacycododecane-1,4,7-triacetic acid-10-(2-methylene)-1-azathioxanthone was investigated in three buffer systems and at three pH values. The buffers-phosphate buffered saline (PBS), 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), and universal buffer (UB)-had no effect on the europium luminescence, but a lower overall emission intensity was determined in HEPES. It was found that this was due to quenching of the 1-azathioxanthone first excited singlet state by HEPES. The effect of pH on the photophysics of the complex was found to be minimal, and protonation of the pyridine nitrogen was found to be irrelevant. Even so, pH was shown to change the intensity ratio between 1-azathioxanthone fluorescence and europium luminescence. It was concluded that the full photophysics of a potential molecular probe should be investigated to achieve the best possible results in any application.

Tb3+ Photophysics: Mapping Excited State Dynamics of [Tb(H2O)9]3+ Using Molecular Photophysics.

The study of optical transitions in lanthanide(III) ions has evolved separately from molecular photophysics, but the framework still applies to these forbidden transitions. In this study, a detailed photophysical characterization of the [Tb(H2O)9]3+ aqua ion was performed. The luminescence quantum yield (Φlum), excited state lifetime (τobs), radiative (kr ≡ A) and nonradiative (knr) rate constants, and oscillator strength (f) were determined for Tb(CF3SO3)3 in H2O/D2O mixtures in order to map the radiative and nonradiative transition probabilities. It was shown that the intense luminescence observed from Tb3+ compared to other Ln3+ ions is not due to a higher transition probability of emission but rather due to a lack of quenching, quantified by quenching to O-H oscillators in the aqua ion of kq(OH) = 2090 s-1 for terbium and kq(OH) = 8840 s-1 for europium. In addition, the Horrocks method of determining inner-sphere solvent molecules has been revisited, and it was concluded that the Tb3+ is 9-coordinated in aqueous solution.

Electronic Energy Levels and Optical Transitions in Samarium(III) Solvates.

Lanthanide luminescence fascinates with a complicated electronic structure and "forbidden" transitions. By studying the photophysics of lanthanide(III) solvates, a close to ideal average coordination geometry can be used to map both electronic energy levels and transition probabilities. Some lanthanide(III) ions are simpler to study than others, and samarium(III) belongs to the more difficult ones. The 4f5 system has numerous absorption and emission lines in the visible and infrared parts of the spectrum and in this work, the energy levels giving rise to these transitions were mapped, the transition probability between them was calculated, and it was shown that the electronic structures of the samarium(III) solvates in DMSO, MeOH, and water are different.

Invisible strings. The first single crystal of the cTSAP form of [Eu(DOTA)(H2O)]- has an electronic structure similar to one of the reported cSAP forms.

The coordination geometry of lanthanide(III) ions is extremely sensitive to perturbation from the surrounding environment. Changes in the crystal field can be observed as spectral variations in the emission spectra of luminescent lanthanide(III) ions. Europium(III) ions are commonly used to correlate luminescence properties to the crystal field. In solution, kinetically inert complexes as [Ln(DOTA)(H2O)]- can fluctuate and give rise to different diastereomers (H4DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). The [Ln(DOTA)(H2O)]- complex adopts a capped square antiprism (cSAP) geometry but can rotate into a capped twisted square antiprism (cTSAP). The time scale of the solution dynamics in [Ln(DOTA)(H2O)]- is shorter than that of luminescence emission, thus, structural averages are observed in the emission spectra. For the first time, we were able to crystallise both forms of the [Eu(DOTA)(H2O)]- diastereomers. The single crystal structure was combined with single crystal luminescence spectroscopy to reveal the electronic structure of Eu(III) in each form of the complex. The coordination geometry of the crystallized cSAP and cTSAP forms of the complex was compared to ideal coordination polyhedra using a continuous symmetry measure in the AlignIt code. The diastereomers of [Eu(DOTA)(H2O)]- all demonstrate very little deviation from ideal geometries yet nearly identical electronic properties were observed from the two different forms.

Long story short: donor set symmetry in [Eu(DOTA)(H2O)]- crystals determines the electronic structure.

Lanthanide complexes of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid DOTA have been studied in great detail due to their use as MRI contrast agents. Since the first report from Desreux in 1980, the Ln[DOTA]- complexes of gadolinium(III) in particular have been thoroughly investigated. The forms of the nine-coordinated [Ln(DOTA)(H2O)]- complexes are well known, and the ligand backbone has been used extensively to create functional MRI contrast agents, luminescent probes, and as a model system for studying the properties of lanthanide(III) ions. In solution, the photophysical properties have been mapped, but as the structures are not known, direct structure-property relationships have not been created. Here, the electronic properties of two Eu[DOTA] compounds (1 and 2) and a Eu[DOTA]-like compound (3) were studied using single-crystal luminescence spectroscopy. The donor set in the three compounds is identical (4N 4O 1O), and using the symmetry deviation value σideal it was shown that the coordination geometry is close to identical. Nevertheless, the electronic properties evaluated using the luminescence spectrum were found to differ significantly between the three compounds. The magnitude of the crystal field splitting was found not to scale with the symmetry of the coordination geometry. It was concluded that the donor set dictates the splitting, yet the structure-property relationships governing the electronic properties of europium(III) ions still elude us.

A high-sensitivity rapid acquisition spectrometer for lanthanide(III) luminescence.

Detecting luminescence beyond 750-800 nm becomes problematic as most conventional detectors are less sensitive in this range, and as simple corrections stops being accurate. Lanthanide luminescence occurs in narrow bands across the spectrum from 350-2000 nm. The most emissive lanthanide(III) ions have bands from 450 nm to 850 nm, some with additional bands in the NIR. Investigating NIR bands are hard, but the difficulties already start at 700 nm. In general, the photon flux from lanthanide(III) emitters is not great, and the bands beyond 700 nm are very weak, we therefore decided to build a spectrometer based on cameras for microscopy with single-photon detection capabilities. This was found to allieviate all limitations and to allow for fast and efficient recording of luminescence spectra in the range from 450 to 950 nm. The spectrometer characteristics were investigated and the performance was benchmarked against two commercial spectrometers. We conclude that this spectrometer is ideal for investigating lanthanide luminescence, and all other emitters with emission in the target range.

A europium(III)-based nanooptode for bicarbonate sensing - a multicomponent approach to sensor materials.

Lanthanide luminescence contains detailed chemical information and can be used to report on several chemical analytes. This has been exploited through elaborate synthesis of responsive lanthanide complexes. Here, we report on a less elaborate approach and assemble four different nanooptodes. Europium(III) is used to sense the bicarbonate concentration. The signal from the optode was enhanced 100 times using antenna chromophore and the response was modulated by the addition of lipophilic cations.

Delicate, a study of the structural changes in ten-coordinated La(III), Ce(III), Pr(III), Nd(III), Sm(III) and Eu(III) sulfates.

We recently presented a new method that allows for a direct structural comparison of coordination complexes. The main difference from other methods is that our AlignIt approach uses common scaling and orientation of the complexes when computing the symmetry deviation, σideal, values. Here, six apparently isostructural lanthanide(III) sulfates K6[(Ln)2(SO4)6]/K5Na[(Ln)2(SO4)6] with ten-coordinated lanthanide(III) sites (Ln(O)10) were prepared, and single-crystal structures were determined and compared using the symmetry deviation (σideal) values. The six structures were shown to fall into two groups: Pr(III) and Eu(III) are identical (σideal = 0.04) bar in size. With a maximum σideal of 0.07, the same was found to be true for La(III), Ce(III), Nd(III), Sm(III) structures. The two groups are shown to be significantly different (σideal > 2.7), yet the coordination geometries of all six are best described as bicapped square antiprisms (σideal = 1.15-1.68). The structures differ in more than symmetry as the smaller lanthanides are shown to crystallize with one sodium and five potassium ions, while the larger lanthanides crystallize with six potassium ions. This does not change the structure and we postulate that the structural variation is due to delicate size matching between alkali and lanthanide ions. For the first six lanthanides, this structure is very robust, which is confirmed as seven doped systems readily crystallized. These doped systems were prepared in order to use europium(III) luminescence as a structural probe. Unsurprisingly, only the doped Eu-La and Eu-Ce systems were found to be luminescent. Between these two and all the europium(III) systems, the intricate structural differences were shown to be enough to change both crystal field splitting and luminescence lifetime. We conclude that even in simple dilution experiments, where luminescent lanthanide(III) ions are introduced in 'innocent' hosts materials, the structure can act as a modulator for the observed properties.

We are never ever getting (back to) ideal symmetry: structure and luminescence in a ten-coordinated europium(III) sulfate crystal.

Our theoretical treatment of electronic structures in coordination complexes often rests on assumptions of symmetry. Experiments rarely provide fully symmetric systems to study. In solutions, fluctuations in solvation, variations in conformations, and even changes in constitution occur and complicate the picture. In crystals, lattice distortion, energy transfer, and phonon quenching play a role, but we are able to identify distinct symmetries. Yet the question remains: How is the real symmetry in a crystal compared to ideal symmetries?

A Nanobody-on-Quantum Dot Displacement Assay for Rapid and Sensitive Quantification of the Epidermal Growth Factor Receptor (EGFR).

Biosensing approaches that combine small, engineered antibodies (nanobodies) with nanoparticles are often complicated. Here, we show that nanobodies with different C-terminal tags can be efficiently attached to a range of the most widely used biocompatible semiconductor quantum dots (QDs). Direct implementation into simplified assay formats was demonstrated by designing a rapid and wash-free mix-and-measure immunoassay for the epidermal growth factor receptor (EGFR). Terbium complex (Tb)-labeled hexahistidine-tagged nanobodies were specifically displaced from QD surfaces via EGFR-nanobody binding, leading to an EGFR concentration-dependent decrease of the Tb-to-QD Förster resonance energy transfer (FRET) signal. The detection limit of 80±20 pM (16±4 ng mL-1 ) was 3-fold lower than the clinical cut-off concentration for soluble EGFR and up to 10-fold lower compared to conventional sandwich FRET assays that required a pair of different nanobodies.

Robust Dual Optical Sensor for pH and Dissolved Oxygen.

As part of moving our optical pH and dissolved oxygen (DO) optical chemosensors toward industrial applications, we decided to explore a many-sensors-in-one principle. It was tested if physical segregation of the optical sensor components in a single sensor polymer could remove cross-talk and quenching. It was found that a design concept with an oxygen-responsive dye in polymer nanoparticles and a pH-responsive dye in an organically modified siloxane polymer resulted in a robust pH/O2 dual optical sensor. Individually, the O2-sensitive nanoparticles, a known component for optical DO sensing, and the pH sensor are operational. Thus, it was decided to test if nanoparticles enclosed within the pH-sensitive responsive sol-gel (i) could work together if segregated and (ii) could operate with a single intensity signal that is without a reference signal; developments within industrial optical sensor technology indicate that this should be feasible. The prototype optode produced in this work was shown to have a negligible drift over 60 h, bulk diffusion-limited DO response, and independent response to pH and O2. On the individual optode, pH calibration was found to show the expected sigmoidal shape and pKa, while the complexity of the calibration function for the DO signal was significant. While the engineering of the sensor device, optics, and hardware are not robust enough to attempt generic sensor calibration, it was decided to demonstrate the design concept in simple fermentation experiments. We conclude that the dual sensor design with the physical segregation of components is viable.

Revisiting the assignment of innocent and non-innocent counterions in lanthanide(III) solution chemistry.

Lanthanides are found in critical applications from display technology to renewable energy. Often, these rare earth elements are used as alloys or functional materials, yet access to them is through solution processes. In aqueous solutions, the rare earths are found predominantly as trivalent ions and charge balance dictates that counterions are present. The fast ligand exchange and lack of directional bonding in lanthanide complexes have led to questions regarding the speciation of Ln3+ solvates in the presence of various counterions and the distinction between innocent = non-coordinating and non-innocent = coordinating counterions. There is limited agreement as to which group counterions belong to, which led to this report. By using Eu3+ luminescence, it was possible to clearly distinguish between coordinating and non-coordinating ions. To interpret the results, it was required to bridge the descriptions of ion pairing and coordination. The data-in the form of Eu3+ luminescence spectra and luminescence lifetimes from solutions with varying concentrations of acetate, chloride, nitrate, sulfate, perchlorate and triflate-was contrasted to those obtained with ethylenediaminetetraacetic acid (EDTA4-), which allowed for the distinction between three Ln3+-anion interaction types. It was possible to conclude which counterions are truly innocent (e.g. ClO4- and OTf-) and which clearly coordinate (e.g. NO3- and AcO-). Finally, a considerable amount of data from systems studied under similar conditions allowed the minimum perturbation arising from the inner sphere or outer sphere coordination in Eu3+ complexes to be identified.

Rapid and Wash-Free Time-Gated FRET Histamine Assays Using Antibodies and Aptamers.

Histamine (HA) is an indicator of food freshness and quality. However, high concentrations of HA can cause food poisoning. Simple, rapid, sensitive, and specific quantification can enable efficient screening of HA in food and beverages. However, conventional assays are complicated and time-consuming, as they require multiple incubation, washing, and separation steps. Here, we demonstrate that time-gated Förster resonance energy transfer (TG-FRET) between terbium (Tb) complexes and organic dyes can be implemented in both immunosensors and aptasensors for simple HA quantification using a rapid, single-step, mix-and-measure assay format. Both biosensors could quantify HA at concentrations relevant in food poisoning with limits of detection of 0.19 µg/mL and 0.03 µg/mL, respectively. Excellent specificity was documented against the structurally similar food components tryptamine and l-histidine. Direct applicability of the TG-FRET assays was demonstrated by quantifying HA in spiked fish and wine samples with both excellent concentration recovery and agreement with conventional multistep enzyme-linked immunosorbent assays (ELISAs). Our results show that the simplicity and rapidity of TG-FRET assays do not compromise sensitivity, specificity, and reliability, and both immunosensors and aptasensors have a strong potential for their implementation in advanced food safety screening.