Time-resolved luminescence energy transfer immunobinding study using a ruthenium-ligand complex as a donor label

Luminescent core-shell imprinted nanoparticles engineered for targeted Förster resonance energy transfer-based sensing

Red-luminescent 200 nm silica nanoparticles have been designed and prepared as a versatile platform for developing FRET (Förster resonance energy transfer) biomimetic assays. Ru(phen)₃²⁺ dye molecules embedded off-center in the silica core provide the long-lived donor emission, and a near-infrared labeled analyte serves as fluorescent acceptor (the measured R₀ of this D-A pair is 4.3 nm). A thin surface-grafted molecularly imprinted polymer (MIP) shell intervenes as selective enrofloxacin-binding element. These nanoparticles have been tested for photochemical detection of enrofloxacin by using a competitive scheme that can be readily performed in MeCN-HEPES (pH 7.5) 7:3 (v/v) mixtures and allows for the antibiotic detection in the μM range (LOD = 2 μM) without optimization of the assay. Given the well-known difficulties of coupling the target-binding-to-MIP and the transducing events, the novel photochemical approach tuned up here will be valuable in future developments of MIP-based assays and optosensors that capitalize also on the advantages of nanomaterials for (bio)analysis.

A Resonance Energy Transfer Immunoassay Based on a Thiol-Reactive Ruthenium Donor Dye and a Longwave-Emitting Acceptor

A novel immunoassay is described that applies a thiol-reactive ruthenium metal-ligand complex as the donor dye in a luminescence energy transfer (LET) detection scheme. Unlike amine-reactive labels, the LET with a thiol label allows improved specificity and better reproducibility of labelling positions on proteins, because the number of reactive thiol groups of proteins is distinctly smaller. This helps to reduce the risk of over-labelling and self-quenching of the fluorophore. The synthesis of the thiol label was significantly improved, resulting in almost quantitative yields of pure product. The absorption and emission maxima of the ruthenium donor dye are at 460 nm and 600 nm, respectively, and a Stokes' shift of 140 nm warrants distinct separation of excitation and emission wavelengths even in turbid samples. A cyanine dye with an absorption maximum at 642 nm was chosen as the acceptor label because it has good overlap with the emission spectrum of the donor label. The emission of the acceptor peaks at 660 nm, thus further increasing the Stokes' shift (to an overall 200 nm). The quantification of anti-HSA with the LET immunoassay is possible with this new approach at concentrations as low as 220 pmol L(-1).

Luminescent metal-ligand complexes as probes of macromolecular interactions and biopolymer dynamics

The knowledge of microsecond dynamics is important for an understanding of the mechanism and function of biological systems. Fluorescent techniques are well established in biophysical studies, but their applicability to probe microsecond timescale processes is limited. Luminescent metal-ligand complexes (MLCs) have created interest mainly due to their unique luminescent properties, such as the exceptionally long decay times and large fundamental anisotropy values, allowing examination of microsecond dynamics by fluorescence methods. MLC properties also greatly simplify instrumentation requirements and enable the use of light emitting diode excitation for time-resolved measurements. Recent literature illustrates how MLC labels take full advantage of well developed fluorescence techniques and how those methods can be extended to timescales not easily accessible with nanosecond probes. MLCs are now commercially available as reactive labels which give researchers access to methods that previously required more complex approaches. The present paper gives an overview of the applications of MLC probes to studies of molecular dynamics and interactions of proteins, membranes and nucleic acids.

Screening Scheme Based on Measurement of Fluorescence Lifetime in the Nanosecond Domain

The authors demonstrate that the fluorescence lifetime of certain fluorescent labels is a useful parameter to detect affinity binding between biotin and streptavidin, as well as between biotinylated bovine serum albumin and streptavidin. The assay is performed in a microplate format, and lifetimes are determined using dye laser-induced fluorescence. Four fluorescent labels are presented that undergo a significant change in their lifetime upon affinity binding. The scheme, referred to as the fluorescence lifetime affinity assay, has several attractive features in that it requires single labeling only, represents a homogeneous assay, allows each of the 2 binding partners to be labeled, and is compatible with the standard microwell formats used in high-throughput screening.

Multiparameter Imaging for the Analysis of Intracellular Signaling

In biological experimentation and especially in drug discovery there is a trend towards more complex test systems. Cell-based assays are replacing conventional binding or enzyme assays more and more. This development is strongly driven by novel fluorescent probes that give insight into cellular processes. Target proteins are studied in their natural environment; this gives much more realistic test results, especially with respect to enzyme location and kinetics. However, in the complex environment of cells, many parameters contribute to the performance of the protein of interest. Therefore, it would be desirable to monitor simultaneously as many of the relevant cellular processes as possible. Here, we discuss the possibilities and limitations provided by multiparameter monitoring of cellular events with fluorescent probes. Some novel examples of the use of fluorescent probes and multiparameter imaging are shown.