Acridone and acridinium constructs with red-shifted emission.

Acridinium 9-carboxylic acid derivatives have been extensively used as chemiluminescent labels in diagnostic assays. Triggering acridinium with basic hydrogen peroxide produces a highly strained dioxetanone intermediate, which converts into an acridone in an electronically excited state and emits light at 420-440 nm. Here, we introduce a novel acridinium-fluorescein construct emitting at 530 nm, in which fluorescein is covalently attached to the acridinium N-10 nitrogen via a propyl sulfonamide linker. To characterize the spectral properties of the acridinium-fluorescein chemiluminophores, we synthesized the analogous acridone-fluorescein constructs. Both acridinium and acridone were linked to either 5- or 6-carboxyfluorescein and independently synthesized as individual structural isomers. Using fluorescent acridone-fluorophore tandems, we investigated and optimized the diluent composition to prevent dye aggregation. As monomolecular species, the acridone isomers demonstrated similar absorption, excitation, and emission spectra, as well as the expected fluorescence lifetimes and molecular brightness. Chemical triggering of acridinium-fluorescein tandems, as well as direct excitation of their acridone-fluorescein analogs, resulted in a nearly complete energy transfer from acridone to fluorescein. Acridone-based dyes can be studied with steady-state spectroscopy. Thus, they will serve as useful tools for structure and solvent optimizations, as well as for studying chemiluminescent energy transfer mechanisms in related acridinium-fluorophore tandems. Direct investigations of the light-emitting molecules generated in the acridinium chemiluminescent reaction empower further development of chemiluminescent labels with red-shifted emission. As illustrated by the two-color HIV model immunoassay, such labels can find immediate applications for multicolor detection in clinical diagnostic assays.

A rainbow of acridinium chemiluminescence.

Multicolor chemiluminescent acridinium derivatives were synthesized by attaching various common fluorophores to the N10 -acridinium position through a piperazine linker. Triggering of each acridinium derivative using alkaline hydrogen peroxide resulted in a chemiluminescence spectrum dominated by a strong emission (>95%) from the attached fluorophore. The highly quenched emission from the triggered acridinium, acting as a donor, points to a highly efficient intramolecular energy transfer in acridinium-based chemiluminophore-fluorophore tandems. A variable, and in many cases minimal, spectral overlap between the donor emission and the acceptor absorption may indicate that in such tandems the energy transfer follows the Dexter electron exchange mechanism. Moreover, fluorophores affixed through the acridinium 9-position produce a typical acridinium emission profile, demonstrating the need for close distances and favorable intramolecular orientation of the donor and acceptor moieties for the energy transfer to occur. A family of red-shifted chemiluminescent labels, all sharing a uniform triggering method, will find immediate application in multicolor ligand-receptor assays. Along with the multiplexing capabilities, the red-shifted chemiluminescent detection offers a higher tolerance to green-colored biological interferences and will therefore benefit many screening and diagnostic clinical tests.

Rhodamine-Derived Fluorescent Dye with Inherent Blinking Behavior for Super-Resolution Imaging.

Super-resolution microscopy enables imaging of structures smaller than the diffraction limit. Single-molecule localization microscopy methods, such as photoactivation localization microscopy and stochastic optical reconstruction microscopy, reconstruct images by plotting the centroids of fluorescent point sources from a series of frames in which only a few molecules are fluorescing at a time. These approaches require simpler instrumentation than methods that depend on structured illumination and thus are becoming widespread. The functionalized rhodamine derivative reported in this paper spontaneously converts between a bright and dark state due to pH-dependent cyclization. At pH 7, less than 0.5% of the dye molecules are fluorescent at any given time. Blinking occurs on time scales of seconds to minutes and can therefore be used for single-molecule localization microscopy without sample treatment or activation. The dye is bright and straightforward to use, and it is easy to synthesize and functionalize. Thus, it has potential to become a new and powerful addition to the toolset for super-resolution imaging.

The photostability of the commonly used biotin-4-fluorescein probe.

Biotin-4-fluorescein (B4F) is a commonly used fluorescent probe for studying biotin-(strept)avidin interactions. During a characterization study of an anti-biotin antibody, using B4F as the probe, we noticed a discrepancy in the expected and experimentally determined number of biotin binding sites. Analytical testing showed that the biotin moiety in the probe undergoes a photosensitized oxidation to produce a mixture of biotin sulfoxides which has the potential to impact the quantitation of binding sites using this fluorescent probe.