Investigations of energy migration in an organic dendrimer macromolecule for sensory signal amplification

Boosting Excited-State Energy Transfer by Anchoring Dipole Orientation in Binary Thermally Activated Delayed Fluorescence/J-Aggregate Assemblies

Förster resonance energy transfer (FRET) has been widely applied in fluorescence imaging, sensing and so on, while developing useful strategy of boosting FRET efficiency becomes a key issue that limits the application. Except optimizing spectral properties, promoting orientation factor (κ2) has been well discussed but rarely utilized for boosting FRET. Herein, we constructed binary nano-assembling of two thermally activated delayed fluorescence (TADF) emitters (2CzPN and DMAC-DPS) with J-type aggregate of cyanine dye (C8S4) as doping films by taking advantage of their electrostatic interactions. Time-resolved spectroscopic measurements indicated that 2CzPN/Cy-J films exhibit an order of magnitude higher kFRET than DMAC-DPS/Cy-J films. Further quantitative analysing on kFRET and kDET indicated higher orientation factor (κ2) in 2CzPN/Cy-J films play a key role for achieving fast kFRET, which was subsequently confirmed by anisotropic measurements. Corresponding DFT/TDDFT calculation revealed strong "two-point" electrostatic anchoring in 2CzPN/Cy-J films that is responsible for highly orientated transitions. We provide a new strategy for boosting FRET in nano-assemblies, which might be inspired for designing FRET-based devices of sensing, imaging and information encryption.

Real-time fluorescence quenching-based detection of nitro-containing explosive vapours: what are the key processes?

The detection of explosives continues to be a pressing global challenge with many potential technologies being pursued by the scientific research community. Luminescence-based detection of explosive vapours with an organic semiconductor has attracted much interest because of its potential for detectors that have high sensitivity, compact form factor, simple operation and low-cost. Despite the abundance of literature on novel sensor materials systems there are relatively few mechanistic studies targeted towards vapour-based sensing. In this Perspective, we will review the progress that has been made in understanding the processes that control the real-time luminescence quenching of thin films by analyte vapours. These are the non-radiative quenching process by which the sensor exciton decays, the analyte-sensor intermolecular binding interaction, and the diffusion process for the analyte vapours in the film. We comment on the contributions of each of these processes towards the sensing response and, in particular, the relative roles of analyte diffusion and exciton diffusion. While the latter has been historically judged to be one of, if not the primary, causes for the high sensitivity of many conjugated polymers to nitrated vapours, recent evidence suggests that long exciton diffusion lengths are unnecessary. The implications of these results on the development of sensor materials for real-time detection are discussed.

Cooperative porphyrin-quadrupolar based triad for combined two-photon induced fluorescence and singlet oxygen generation

The design and synthesis of a cooperative multichromophoric triad system which combines the large two-photon absorption properties of fluorene-cored bis-donor quadropolar dyes and the remarkable sensitizing properties of the porphyrin subunit (i.e. high intersystem crossing and ability to produce singlet oxygen by energy transfer to oxygen from its triplet excited state) is described. After irradiation the energy can be transferred from the quadrupolar chromophores to the porphyrin with an estimated 80% efficiency via a FRET process. Moreover both the two-photon absorption properties of the quadrupolar subunits and the sensitizing and fluorescence properties of the porphyrin are retained indicating that deleterious competing processes (such as photo-induced electron transfer) are prevented in such molecular architectures thanks to the implemented design. As a result, the two-photon absorption induced singlet oxygen generation efficiency of the triad in the NIR region is found to be enhanced by an order of magnitude as compared to the porphyin subunit. Potential applications of these porphyrin-based multichromophoric systems for photodynamic therapy based upon two-photon excitation in the NIR region might be possible since it overcomes the low two-photon absorption response of porphyrin while fully retaining their remarkable photosensitizing properties.

Carbazole- and/or triphenylamine-based D–π–D multiarylamino dyes: synthesis, characterization and photophysical properties

One-photon and two-photon fluorescence quenching by benzoyl peroxide of D–π–D multiarylamino dyes was investigated.

Extended cavity pyrene-based iptycenes for the turn-off fluorescence detection of RDX and common nitroaromatic explosives

Extended cavity pyrene-based iptycenes were synthesized for the turn-off fluorescence detection of RDX and common nitro-aromatic explosives.

Fluorescence based explosive detection: from mechanisms to sensory materials

The detection of explosives is one of the current pressing concerns in global security. In the past few decades, a large number of emissive sensing materials have been developed for the detection of explosives in vapor, solution, and solid states through fluorescence methods. In recent years, great efforts have been devoted to develop new fluorescent materials with various sensing mechanisms for detecting explosives in order to achieve super-sensitivity, ultra-selectivity, as well as fast response time. This review article starts with a brief introduction on various sensing mechanisms for fluorescence based explosive detection, and then summarizes in an exhaustive and systematic way the state-of-the-art of fluorescent materials for explosive detection with a focus on the research in the recent 5 years. A wide range of fluorescent materials, such as conjugated polymers, small fluorophores, supramolecular systems, bio-inspired materials and aggregation induced emission-active materials, and their sensing performance and sensing mechanism are the centerpiece of this review. Finally, conclusions and future outlook are presented and discussed.

Two-photon fluorescence spectroscopy and imaging of 4-dimethylaminonaphthalimide peptide and protein conjugates

We report detailed photophysical studies on the two-photon fluorescence processes of the solvatochromic fluorophore 4-DMN as a conjugate of the calmodulin (CaM) and the associated CaM-binding peptide M13. Strong two-photon fluorescence enhancement has been observed which is associated with calcium binding. It is found that the two-photon absorption cross-section is strongly dependent on the local environment surrounding the 4-DMN fluorophore in the CaM conjugates, providing sensitivity between sites of fluorophore attachment. Utilizing time-resolved measurements, the emission dynamics of 4-DMN under various environmental (solvent) conditions are analyzed. In addition, anisotropy measurements reveal that the 4-DMN-S38C-CaM system has restricted rotation in the calcium-bound calmodulin. To establish the utility for cellular imaging, two-photon fluorescence microscopy studies were also carried out with the 4-DMN-modified M13 peptide in cells. Together, these studies provide strong evidence that 4-DMN is a useful probe in two-photon imaging, with advantageous properties for cellular experiments.

Dynamics of fluorescence depolarisation in star-shaped oligofluorene-truxene molecules

Star-shaped molecules are of growing interest as organic optoelectronic materials. Here a detailed study of their photophysics using fluorescence depolarisation is reported. Fluorescence depolarisation dynamics are studied in branched oligofluorene-truxene molecules with a truxene core and well-defined three-fold symmetry, and are compared with linear fluorene oligomers. An initial anisotropy value of 0.4 is observed which shows a two-exponential decay with time constants of 500 fs and 3-8 ps in addition to a long-lived component. The femtosecond component is attributed to exciton localisation on one branch of the molecule and its amplitude reduces when the excitation is tuned to the low energy tail of the absorption spectrum. The picosecond component shows a weak dependence on the excitation wavelength and is similar to the calculated rate of the resonant energy transfer of the localised exciton between the branches. These assignments are supported by density-functional theory calculations which show a disorder-induced splitting of the two degenerate excited states. Exciton localisation is much slower than previously reported in other branched molecules which suggests that efficient light-harvesting systems can be designed using oligofluorenes and truxenes as building blocks.

FRET detection of proteins using fluorescently doped electrospun nanofibers and pattern recognition

This paper reports the fabrication of solid-state nanofiber sensor arrays and their use for detection of multiple proteins using principal component analysis (PCA). Four cationic and anionic fluorescently embedded nanofibers are generated by an electrospinning method, yielding unique patterns of fluorescence change upon interaction with protein samples. Five metal and nonmetal containing proteins, i.e., hemoglobin, myoglobin, cytochrome c, BSA, and avidin, have been investigated; and the results show that distinct fluorescent patterns can be formed upon the addition of protein samples to the array of solid nanofiber substrates, allowing their unambiguous identification. The nanofiber films are highly repeatable with a batch-to-batch variation of approximately 5% and demonstrated outstanding reusability with less than a 15% loss of fluorescence intensity signal after 5 regenerations of test cycles. For a more practical visualization, a cluster map was generated using PCA of the change-in-fluorescence (ΔI) composite patterns, demonstrating the potential of the method for diagnostic applications.

Energy transfer from silica core-surfactant shell nanoparticles to hosted molecular fluorophores

Very monodisperse water-soluble silica core-surfactant shell nanoparticles (SCSS NPs) doped with a rhodamine B derivative were prepared using micelles of F127 as nanoreactors for the hydrolysis and condensation of the silica precursor tetraethoxysilane (TEOS). The functionalization of the rhodamines with a triethoxysilane group allowed the covalent binding of the fluorophores to the silica core: no leaking of the dye was observed when the NPs were purified either by ultrafiltration (UF) or dialysis. The diameter of the core (d(c) = 10 ± 1 nm) was determined by TEM and subtracted from the hydrodynamic diameter, measured by DLS, (d(H) = 24 nm, PdI = 0.1) to calculate the shell thickness (∼7 nm). The presence of a single population of NPs with a radius compatible with the one measured by DLS after UF was confirmed by AF4-MALS-RI measurements. The concentration of the NPs was measured by MALS-RI. This allowed us to determine the average number of rhodamine molecules per NP (10). The ability of the NPs to host hydrophobic species as cyanines in the SS was confirmed by fluorescence anisotropy measurements. Steady-state and time-resolved fluorescence measurements allowed us to observe the occurrence of a very efficient Förster resonance energy transfer process from the covalently linked rhodamines to the hosted cyanines. In particular, the analysis of the TCSPC data and steady-state measurements revealed that the adsorption of a single cyanine molecule causes an almost complete quenching of the fluorescence of the NP. Thanks to these observations, it was possible to easily determine the concentration of the NPs by fluorescence titration experiments. Results are in good agreement with the concentration values obtained by MALS-RI. Finally, the hosted cyanine molecule could be extracted with (±)-2-octanol, demonstrating the reversibility of the adsorption process.

Nanofibers doped with dendritic fluorophores for protein detection

We report a solid-state, nanofiber-based optical sensor for detecting proteins with an anionic fluorescent dendrimer (AFD). The AFD was encapsulated in cellulose acetate (CA) electrospun nanofibers, which were deacetylated to cellulose to generate secondary porous structures that are desirable for enhancing molecular interactions, and thus better signaling. The protein sensing properties of the fibers were characterized by monitoring the fluorescence response of cytochrome c (cyt c), hemoglobin (Hgb), and bovine serum albumin (BSA) as a function of concentration. Effective quenching was observed for the metalloproteins, cyt c and Hgb. The effect was primarily due to energy transfer of the imbedded fluorescent dendrimers to the protein, as both proteins contain heme portions. Electron transfer, caused through the electrostatic effects in the binding of the anionic dendrimer to the positive patches of globular proteins, could be responsible as well. BSA, on the other hand, triggered a "turn-on" response in fluorescence, suggesting the negatively charged BSA reduces the pi-pi stacking of the partially dispersed, negatively charged dendritic fluorophores through repulsion forces, which results in an increase in fluorescence. Stern-Volmer constants (K(sv)) of the electrospun fibers were found to be 3.4 x 10(5) and 1.7 x 10(6) M(-1) for cyt c and Hgb, respectively. The reusability of the nanofibers is excellent: the nanofibers demonstrated less than 15% change of fluorescence intensity signal in a 5-cycle test.