Design of Novel, Water Soluble and Highly Luminescent Europium Labels with Potential to Enhance Immunoassay Sensitivities

Overview on the Development of Alkaline-Phosphatase-Linked Optical Immunoassays

The drive to achieve ultrasensitive target detection with exceptional efficiency and accuracy requires the advancement of immunoassays. Optical immunoassays have demonstrated significant potential in clinical diagnosis, food safety, environmental protection, and other fields. Through the innovative and feasible combination of enzyme catalysis and optical immunoassays, notable progress has been made in enhancing analytical performances. Among the kinds of reporter enzymes, alkaline phosphatase (ALP) stands out due to its high catalytic activity, elevated turnover number, and broad substrate specificity, rendering it an excellent candidate for the development of various immunoassays. This review provides a systematic evaluation of the advancements in optical immunoassays by employing ALP as the signal label, encompassing fluorescence, colorimetry, chemiluminescence, and surface-enhanced Raman scattering. Particular emphasis is placed on the fundamental signal amplification strategies employed in ALP-linked immunoassays. Furthermore, this work briefly discusses the proposed solutions and challenges that need to be addressed to further enhance the performances of ALP-linked immunoassays.

Design of novel tripyridinophane-based Eu(III) complexes as efficient luminescent labels for bioassay applications

In this work, the development of highly luminescent europium(III) complexes in water solution is reported, including their syntheses, analyses of their photophysical properties and applications in bioassays. Three Eu(III) complexes are derived from new ligands based on a tripyridinophane platform. There are four distinct sections in the structure of these ligands: an 18-membered polyaminocarboxylic macrocycle to bind efficiently lanthanide ions in aqueous solutions, three chromophoric subunits (4-(phenylethynyl)pyridine moieties) to effectively sensitize the emission of the metal, two peripheral moieties to solubilise the complex in aqueous media (sulfonate, sulfobetaine or glucose groups) and a free NH₂ group available for grafting or bioconjugation. In our synthetic procedure, a pivotal macrocyclic platform is obtained with a high yield in the crucial macrocyclization step due to a metal template ion effect (74% yield). In Tris aqueous buffer (pH 7.4), the Eu(III) complexes show a maximum excitation wavelength at 320 nm, a suitable overall quantum yield (14%), a relatively long lifetime (0.80 ms) and a one-photon brightness in the range of 10 000 M^-1 cm^-1. Importantly, these photophysical properties are retained at dilute concentrations, even in the presence of a very large excess of potentially competing species, such as EDTA or Mg²⁺ ions. Furthermore, we report the bioconjugation of a Eu(III) complex labelled by an N-hydroxysuccinimide ester reactive group with an antibody (anti-glutathione-S-transferase) and the successful application of the corresponding antibody conjugate in the detection of GST-biotin in a fluoroimmunoassay. These new complexes provide a solution for high sensitivity in Homogeneous Time-Resolved Fluorescence (HTRF®) bioassays.

Photoluminescence-based immunochemical methods for determination of C-reactive protein and procalcitonin

Modern, sensitive, rapid, and selective analytical methods for the detection of inflammatory markers are a crucial tool for the assessment of inflammation state, efficacy of medical intervention, and the prediction of future diseases. Their development requires understanding of current state for point-of-care testing of inflammatory markers and identification of their crucial drawbacks. This review summarizes the progress in the application of luminescent labels for immunoassays. The luminescent labels became more popular in the latest decade due to their high sensitivity, selectivity, and robustness. This review presents a constructive analysis of different luminescent labels such as fluorescent organic dyes, quantum dots, long-lived emissive nanoparticles, and up-converting nanocrystals, as well as a range of the strategies for inflammatory markers determination. The advantages and disadvantages of all classes of luminescent labels are demonstrated, and the strategies of labels modification for their improvement are discussed. The current approaches for the creation of luminescent probes and robust assays are also highlighted.

Tripyridinophane Platform Containing Three Acetate Pendant Arms: An Attractive Structural Entry for the Development of Neutral Eu(III) and Tb(III) Complexes in Aqueous Solution

We report a detailed characterization of Eu³⁺ and Tb³⁺ complexes derived from a tripyridinophane macrocycle bearing three acetate side arms (H₃tpptac). Tpptac^3- displays an overall basicity (∑ log K_i^H) of 24.5, provides the formation of mononuclear ML species, and shows a good binding affinity for Ln³⁺ (log K_LnL = 17.5-18.7). These complexes are also thermodynamically stable at physiological pH (pEu = 18.6, pTb = 18.0). It should be noted that the pGd value of Gd-tpptac (18.4) is only slightly lower than that of commercially available MRI contrast agents such as Gd-dota (pGd = 19.2). Moreover, a very good selectivity for these ions over the endogenous cations (log K_CuL = 14.4, log K_ZnL = 12.9, and log K_CaL = 9.3) is observed. The X-ray structure of the terbium complex shows the metal coordinated by the nine N₆O₃ donor set of the ligand and one inner-sphere water molecule. DFT calculations result in two Eu-tpptac structures with similar bond energies (ΔE = 0.145 eV): one structure in which the water is coordinated to the metal ion and one structure in which the water molecule is farther away from the ion, bound to the ligand with an OH-π bond. By detailed luminescence experiments, we demonstrate that the europium complex in aqueous solution presents a hydration equilibrium between nine-coordinate, dehydrated [Eu-tpptac]⁰ and ten-coordinate, monohydrated [Eu-tpptac(H₂O)]⁰ species. A similar trend is observed for the terbium complex. Despite the presence of this hydration equilibrium, the H₃tpptac ligand sensitizes Eu³⁺ and Tb³⁺ luminescence efficiently in buffered water at physiological pH. Particularly, the terbium complex displays a long excited-state lifetime of 2.24 ms and an overall quantum yield of 33% with a brightness of 3600 M^-1 cm^-1. Such features of Ln³⁺ complexes of H₃tpptac indicate that this platform appears to be particularly appealing for the further development of luminescent lanthanide labels.

π-Expanded Thioxanthones - Engineering the Triplet Level of Thioxanthone Sensitizers for Lanthanide-Based Luminescent Probes with Visible Excitation

Bright lanthanide based probes for optical bioimaging must rely on the antenna principle, where the lanthanide-centred excited state is formed by a complex sensitization process. Efficient sensitization of lanthanide-centred emission occurs via triplet states centred on the sensitizing chromophore. Here, the triplet state of thioxanthone chromophores is modulated by extending the π-system. Three thioxanthone chromophores-thioxanthone, benzo[c]thioxanthone, and naphtho[2,3-c]thioxanthone were synthesised and characterised. The triplet state energies and lifetimes is found to change as expected, and two dyes are found to be suitable sensitizers for europium(iii) luminescence. Reactive derivatives of thioxanthone and benzo[c]thioxanthone were prepared and coupled to a 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) lanthanide binding pocket. The photophysics and the performance in optical bioimaging of the resulting europium(iii) complexes were investigated. It is concluded that while the energetics favour efficient sensitization, the solution structure does not. While it was found that the complexes are too lipophilic to be efficient luminescent probes for optical bioimaging, we successfully demonstrated bioimaging using europium(iii) luminescence following 405 nm excitation.

Polyanionic Polydentate Europium Complexes as Ultrabright One- or Two-photon Bioprobes

A family of europium (III) complexes based on a polydentate ligand functionalized by charge-transfer antennae presents remarkable one- and two-photon photophysical proper-ties in water or buffer. A detailed analysis of their emission properties suggests that the wrapping of the ligand around the central rare-earth ion results in an overall Cs symmetry in agreement with the theoretical simulation and that about 65-70 % of the emission intensity is concentrated in the hypersensitive 5 D0 →7 F2 transition at 615 nm. Their brightness is excellent, in the range of the best lanthanide bioprobes making them very attractive for bio-imaging experiments.

Shining light on the antenna chromophore in lanthanide based dyes

Lanthanide based dyes and assays exploit the antenna effect, where a sensitiser-chromophore is used as a light harvesting antenna and subsequent excited state energy transfer populates the emitting lanthanide centred excited state. A rudimentary understanding of the design criteria for designing efficient dyes and assays based on the antenna effect is in place. By preparing kinetically inert lanthanide complexes based on the DO3A scaffold, we are able to study the excited state energy transfer from a 7-methoxy-coumarin antenna chromophore to europium(iii) and terbium(iii) centred excited states. By contrasting the photophysical properties of complexes of metal centres with and without accessible excited states, we are able to separate the contributions from the heavy atom effect, photoinduced electron transfer quenching, excited state energy transfer and molecular conformations. Furthermore, by studying the photophysical properties of the antenna chromophore, we can directly monitor the solution structure and are able to conclude that excited state energy transfer from the chromophore singlet state to the lanthanide centre does occur.