Triplet Shelving in Fluorescein and Its Derivatives Provides Delayed, Background-Free Fluorescence Detection

Fluorescence-informed photoacoustic discrimination of multiple chromophores by lifetime mapping optically gated responses

Synchronously Amplified Photoacoustic Image Recovery (SAPhIRe) offers improved background suppression using non-linear properties of modulatable contrast agents. Using SAPhIRe, multiple contrast agents in the same absorption window can be detected independently based on their unique triplet-state lifetimes. Here, we have demonstrated the unmixing of rose bengal and eosin Y signals from solution based on triplet-state lifetime mapping using both fluorescence and photoacoustics. Varying the pump-probe delay enables resolution and recovery of fast-decaying rose bengal and of slowly decaying eosin Y modulated photoacoustic signals, resulting from optically gated triplet state residence. Distinct images were reconstructed within tissue-mimicking phantom using the fitting coefficients of triplet-state lifetimes. Fluorescence was used to screen for modulation prior to photoacoustic imaging. The results suggest that lifetime unmixing can be utilized to simultaneously detect multiple pathologies with overlapping spectra using photoacoustic imaging.

Sequential Two-Photon Delayed Fluorescence Anisotropy for Macromolecular Size Determination

Time-resolved fluorescence anisotropy (FA) uses the fluorophore depolarization rate to report on rotational diffusion, conformation changes, and intermolecular interactions in solution. Although FA is a rapid, sensitive, and nondestructive tool for biomolecular interaction studies, the short (∼ns) fluorescence lifetime of typical dyes largely prevents the application of FA on larger macromolecular species and complexes. By using triplet shelving and recovery of optical excitation, we introduce optically activated delayed fluorescence anisotropy (OADFA) measurements using sequential two-photon excitation, effectively stretching fluorescence anisotropy measurement times from the nanosecond scale to hundreds of microseconds. We demonstrate this scheme for measuring slow depolarization processes of large macromolecular complexes, derive a quantitative rate model, and perform Monte Carlo simulations to describe the depolarization process of OADFA at the molecular level. This setup has great potential to enable future biomacromolecular and colloidal studies.

Probing emission of a DNA-stabilized silver nanocluster from the sub-nanosecond to millisecond timescale in a single measurement

A method for measuring emission over a range of sub-nanosecond to millisecond timescales is presented and demonstrated for a DNA-stabilized silver nanocluster (DNA-AgNC) displaying dual emission. This approach allows one to disentangle the temporal evolution of the two spectrally overlapping signals and to determine both the nano- and microsecond decay times of the two emission components, together with the time they take to reach the steady-state equilibrium. Addition of a second near-infrared laser, synchronized with a fixed delay, enables simultaneous characterization of optically activated delayed fluorescence (OADF). For this particular DNA-AgNC, we demonstrate that the microsecond decay times of the luminescent state and the OADF-responsible state are similar, indicating that the OADF process starts from the luminescent state.

Spectroscopy study of dimerization of fluorone dyes in AOT reverse micelles

The dimerization processes and its thermodynamic parameters of fluoronic dyes (fluorescein (F), eosin (E), erythrosine (ER), bengal rose (BR)) in reverse micelles of AOT with different hydrodynamic radius R_h are studied. The dimerization constants and its efficiency (the degree of dimerization of dye molecules (1-X)) were determined from the experimental data. It is found that an increase in the intercombination conversion due to the heavy atom effect leads to an increase of the value of (1-X). At the same time, the heavy atom effect affects the dye dimerization process for all the studied values of R_h. The linear dependence of (1-X) on R_h is observed. The slope of this dependence is affected by both the mass of the internal heavy atom and the charge of the anionic forms of dyes. It was found that there is a different structure of dye dimers for different R_h for all the studied systems - different angles α between the molecules in the dimer. A linear dependence of α on R_h is observed. At the same time, the growth gradients α(R_h) practically do not differ for F, E, and BR and they are of the greatest importance for the studied water-micellar solutions of dyes. The growth of α from R_h is insignificant for ER. The thermodynamic parameters (such as Gibbs potential ΔG, enthalpy (ΔH and entropy ΔS) were calculated from the experimentally measured dependences of the dimerization constant on the temperature. ΔH < 0 and ΔS < 0 in micellar solutions of the studied dyes at all values of R_h, that indicates that the dimerization reaction in the studied systems is controlled by enthalpy. The obtained linear relationship between TΔS and ΔH indicates the existence of enthalpy-entropy compensation in the dimeric reactions of the molecules of the studied dyes. The linear correlation observed between the values of TΔS and ΔH allowed us to establish that the higher the molecular weight of the halogen in the dye molecule, the more effectively an increase in ΔH contributes to the dimeric stability.

Chemiluminescence Detection in the Study of Free-Radical Reactions. Part 2. Luminescent Additives That Increase the Chemiluminescence Quantum Yield

The present review examines the use of chemiluminescence detection to evaluate the course of free radical reactions in biological model systems. The application of the method is analyzed by using luminescent additives that enhance the luminescence thanks to a triplet-singlet transfer of the electron excitation energy from radical reaction products and its emission in the form of light with a high quantum yield; these additives are called chemiluminescence enhancers or activators. Examples of these substances are provided; differences between the so-called chemical and physical enhancers are described; coumarin derivatives, as the most promising chemiluminescence enhancers for studying lipid peroxidation, are considered in detail. The main problems related to the use of coumarin derivatives are defined, and possible ways of solving these problems are presented. Intrinsic chemiluminescence and the mechanism of luminescence accompanying biomolecule peroxidation are discussed in the first part of the review.

Optically Modulated and Optically Activated Delayed Fluorescent Proteins through Dark State Engineering

Modulating fluorescent protein emission holds great potential for increasing readout sensitivity for applications in biological imaging and detection. Here, we identify and engineer optically modulated yellow fluorescent proteins (EYFP, originally 10C, but renamed EYFP later, and mVenus) to yield new emitters with distinct modulation profiles and unique, optically gated, delayed fluorescence. The parent YFPs are individually modulatable through secondary illumination, depopulating a long-lived dark state to dynamically increase fluorescence. A single point mutation introduced near the chromophore in each of these YFPs provides access to a second, even longer-lived modulatable dark state, while a different double mutant renders EYFP unmodulatable. The naturally occurring dark state in the parent YFPs yields strong fluorescence modulation upon long-wavelength-induced dark state depopulation, allowing selective detection at the frequency at which the long wavelength secondary laser is intensity modulated. Distinct from photoswitches, however, this near IR secondary coexcitation repumps the emissive S₁ level from the long-lived triplet state, resulting in optically activated delayed fluorescence (OADF). This OADF results from secondary laser-induced, reverse intersystem crossing (RISC), producing additional nanosecond-lived, visible fluorescence that is delayed by many microseconds after the primary excitation has turned off. Mutation of the parent chromophore environment opens an additional modulation pathway that avoids the OADF-producing triplet state, resulting in a second, much longer-lived, modulatable dark state. These Optically Modulated and Optically Activated Delayed Fluorescent Proteins (OMFPs and OADFPs) are thus excellent for background- and reference-free, high sensitivity cellular imaging, but time-gated OADF offers a second modality for true background-free detection. Our combined structural and spectroscopic data not only gives additional mechanistic details for designing optically modulated fluorescent proteins but also provides the opportunity to distinguish similarly emitting OMFPs through OADF and through their unique modulation spectra.

Structure and luminescence of DNA-templated silver clusters

DNA serves as a versatile template for few-atom silver clusters and their organized self-assembly. These clusters possess unique structural and photophysical properties that are programmed into the DNA template sequence, resulting in a rich palette of fluorophores which hold promise as chemical and biomolecular sensors, biolabels, and nanophotonic elements. Here, we review recent advances in the fundamental understanding of DNA-templated silver clusters (Ag N -DNAs), including the role played by the silver-mediated DNA complexes which are synthetic precursors to Ag N -DNAs, structure-property relations of Ag N -DNAs, and the excited state dynamics leading to fluorescence in these clusters. We also summarize the current understanding of how DNA sequence selects the properties of Ag N -DNAs and how sequence can be harnessed for informed design and for ordered multi-cluster assembly. To catalyze future research, we end with a discussion of several opportunities and challenges, both fundamental and applied, for the Ag N -DNA research community. A comprehensive fundamental understanding of this class of metal cluster fluorophores can provide the basis for rational design and for advancement of their applications in fluorescence-based sensing, biosciences, nanophotonics, and catalysis.