Short Fluorescence Lifetime Research Articles

Synthesis, Optical Spectroscopy, and Laser and Biomedical Imaging Application Potential of 2,4,6-Triphenylpyrylium Tetrachloroferrate and Its Derivatives.

Synthesis, optical spectroscopic properties, two-photon (TP) absorption-induced fluorescence, and laser and bioimaging application potentials of 2,4,6-triphenylpyrylium tetrachloroferrate (1),4-(4-methoxyphenyl)-2,6-diphenylpyrylium tetrachloroferrate (2), 2,6-bis(4-methoxyphenyl)-4-phenylpyrylium tetrachloroferrate (3), and 2,4,6-tris(4-methoxyphenyl)pyrylium tetrachloroferrate (4) are presented. The synthesis involves the conversion of pyrylium tosylates to pyrylium chlorides, followed by transformation into 1-4 on heating to reflux with FeCl3 in acetonitrile. They are characterized using 1H and 13C NMR spectra in CD3OD, and FTIR and Raman spectroscopic techniques. The salts dissolve in organic solvents and water (pH = 7 to 3) even at high concentrations (10-3 M). These solutions absorb light strongly from 500-300 nm. Solutions of 1, 3, and 4 fluoresce with high quantum yield in the 500-700 nm spectral range. Salts 1 and 4 exhibit fluorescence lifetime shortening, line width narrowing, and free-running laser action under intense pulsed laser excitation. Toxicity and cell imaging studies using human cancer cell lines reveal that salts 1 and 3 function as cellular fluorophores in vitro and have no adverse effects on cellular viability at nanomolar ranges. Furthermore, acetonitrile and methanol solutions of salts 1, 3, and 4 exhibit strong two-photon absorption-induced fluorescence, opening potential applications in biomedical imaging and microscopy.

Physical, optical and spectral properties of Sm3+ and Eu3+ ions doped zinc boro-phosphate glass

This study focuses on the synthesis of Sm³⁺ and Eu³⁺ rare earth ions doped zinc boro-phosphate glasses and their physical, optical and spectral characterization. Photoluminescence was carried out through both steady-state and time-domain luminescence measurements. The glasses were prepared via the melt quenching technique, and they exhibit impressive mechanical strength and transparency. Judd-Ofelt analysis of the near-infrared absorption spectra revealed parameters Ω₂, Ω₄, and Ω₆, indicating low covalency and reduced site asymmetry for the rare earth ions compared to those in silicate-based glasses. When excited with violet light, the Sm³⁺-doped samples emitted bright orange luminescence, resulting from radiative relaxation from the ⁴G₅/₂ state to lower energy levels. The calculated branching ratios correspond well with the observed emission peak ratios. However, increasing the Sm³⁺ ion concentration resulted in reduced luminescence intensity and shorter fluorescence lifetimes, due to cross-relaxation effects. In the Sm³⁺/Eu³⁺ co-doped glasses, energy transfer from Sm³⁺ to Eu³⁺ ions was detected upon selective excitation of Sm³⁺ ions. This energy transfer efficiency was found to increase with higher concentrations of Eu³⁺ ions.

Lead-free Ce-doped perovskite scintillators with high figure of merit

Lead halide perovskite scintillators have recently received extensive research attention owing to their short fluorescence lifetimes, low detection limits, and ease of fabrication compared to traditional scintillators. The nontoxic cerium-doped lead-free perovskites with intrinsically efficient and short lifetime d–f transitions are a prospective replacement for the toxic Pb2+. Here, we demonstrated Ce-doped cesium lanthanide chloride perovskites (Cs3LnCl6, Ln = Gd, Y, Lu) synthesized through a facile solution method for the first time. These perovskites exhibit blue-violet emission, which arises from Ce 5d → 4f transitions. Among three types of Cs3LnCl6 perovskites, Ce:Cs3LuCl6 exhibited high photoluminescence quantum yield (PLQY) of 82% and a short excited-state lifetime of approximately 34 ns. When utilized as X-ray scintillators, Ce:Cs3LuCl6 crystals display a high light yield of 8120 photons per MeV and a low detection limit of 36.8 nGy air s−1. Importantly, the figure of merit (FoM), representing the ratio of light yield to decay time, reaches 239, which is the highest reported value for lead-free perovskite scintillators up to now. Additionally, the fabrication of perovskite/PMMA films was undertaken for practical demonstrations in X-ray imaging, resulting in the attainment of a resolution of up to 8.38 lp/mm. We anticipate that this work will inspire the utilization of Ce-doped Cs3LnCl6 perovskites in ultrafast scintillation applications such as high-energy physics, nuclear reaction monitoring, and dynamic X-ray imaging.

Controllable morphology, excellent stability and non-equivalent doping-induced optical properties of Mn4+-activated K2NaScF6 red phosphor for high-quality warm WLED

Currently, Mn4+ activated fluoride red phosphor has been widely studied to improve the optical performance of white light-emitting diode (WLED) in the field of high-efficiency lighting and wide color gamut backlight display. In this paper, a series of red-emitting K2NaScF6:Mn4+ phosphors are synthesized by a simple co-precipitation method with a non-equivalent substitution of Mn4+ for Sc3+ at room temperature. The crystal structure, morphology and elemental composition of K2NaScF6:Mn4+ phosphors prepared using different reaction conditions are systematically investigated. Interestingly, the K2NaScF6:Mn4+ phosphors exhibit narrowband red emission with low correlated color temperature (CCT), high color purity and short fluorescence lifetime. Compared to equivalent doping, the mechanism of short fluorescence lifetime caused by non-equivalent doping has also been systematically discussed. The optimal doping concentration is determined by setting a series of Mn4+ doping concentrations and the concentration quenching mechanism is analyzed. The crystal field strength (Dq), Racah parameters (B and C) and nephelauxetic ratio (β1) are calculated in detail to evaluate the crystal field environment of Mn4+ in K2NaScF6. The good water resistance of the as-prepared sample K2NaScF6:Mn4+ is verified by comparing with the commercially available phosphor K2SiF6:Mn4+. Moreover, the thermal stability is also analyzed by calculating the activation energy (Ea), chromaticity shift (ΔE) and chromaticity coordinate variation (CCV). Importantly, the as-packaged warm WLED (InGaN chip + YAG:Ce3+ + K2NaScF6:Mn4+) has excellent optical properties (CCT = 4082 K, color rendering index (CRI, Ra = 91.2) and luminous efficiency (LE) = 78.86 lm/W), and it exhibits good output stability at high drive currents. In summary, our work demonstrates that K2NaScF6:Mn4+ red phosphors possess a great potential application in warm WLED for the indoor lighting.

Ln-MOFs with in-situ exsolved perovskite as ratiometric fluorescent for H2O and Cu2+: Spacial charge transfer and detection mechanism

Herein, a ratiometric fluorescent sensor based on CPB⊆Pb/Eu-MOF heterojunction has been designed for rapid, sensitive and visual detection of H2O and Cu2+ in extraction solution. The energy transfer from Pb2+ to Eu3+ in Pb/Eu-MOF has enhanced the luminescent emission of Eu3+, and the CPB⊆Pb/Eu-MOF n-n heterojunction with S-scheme charge transfer can stabilize CPB to activate the detecting for H2O and Cu2+. In-situ XPS and XRD have confirmed the phase transformation from cubic CPB to CsPb2Br5 majorly contributes to color switch when being introduced by H2O. Moreover, the dynamic quenching and electron transfer are verified by the adsorption energy Br-Cu bonding according to DFT calculation and the shortened fluorescence lifetime. Typically, 0.2CPB⊆Pb/Eu-MOF composite exhibits a highly sensitive and selective ratiometric fluorescent sensing. The proper linear ranges from 10.80 % to 23.10 % and a low detection limit of 0.26 % for H2O, while 20–180 μM, R2 = 0.996, and 8.09 μM for Cu2+. More importantly, the paper-based sheet sensor has been manufactured practically in the real floral teas and different solvents for the visual detection of H2O and Cu2+ by naked eyes.

Blue-green emitting ZnS0.75O0.25:Ce3+,x%Tb3+ phosphor with tunable fluorescence lifetime

A series of ZnS0.75O0.25:0.1%Ce3+,x%Tb3+ phosphors were prepared by high temperature solid state reaction method. These phosphors exhibited two mixed phases consisting of hexagonal phase ZnS and hexagonal phase ZnO with the average particle size of 13.83 μm and emitted blue-green light. The luminescence mechanism consisted of Zn vacancy defects, the 5d1 → 2F5/2 radiative transitions of Ce3+, the 5D4 → 7F5 and 5D4 → 7F6 radiative transition of Tb3+ induced by the energy transfer of Ce3+ → Tb3+. An equation for the variation of the fluorescence lifetime of ZnS0.75O0.25:0.1%Ce3+,x%Tb3+ phosphors with concentration of Tb3+ fraction was obtained by exponential fitting. The short fluorescence lifetime could be tuned within the range of 113 μs to 550 μs with the increase of Tb3+ concentration, and the color was tunable from blue to blue-green, which is of important application in the field of high-energy particle detection.

Rapid, on-site quantitative determination of mycotoxins in grains using a multiple time-resolved fluorescent microsphere immunochromatographic test strip

The label probe plays a crucial role in enhancing the sensitivity of lateral flow immunoassays. However, conventional fluorescent microspheres (FMs) have limitations due to their short fluorescence lifetime, susceptibility to background fluorescence interference, and inability to facilitate multi-component detection. In this study, carboxylate-modified Eu(III)-chelate-doped polystyrene nanobeads were employed as label probes to construct a multiple time-resolved fluorescent microsphere-based immunochromatographic test strip (TRFM-ICTS). This novel TRFM-ICTS facilitated rapid on-site quantitative detection of three mycotoxins in grains: Aflatoxin B1 (AFB1), Zearalenone (ZEN), and Deoxynivalenol (DON). The limit of detection (LOD) for AFB1, ZEN, and DON were found to be 0.03 ng/g, 0.11 ng/g, and 0.81 ng/g, respectively. Furthermore, the TRFM-ICTS demonstrated a wide detection range for AFB1 (0.05–8.1 ng/g), ZEN (0.125–25 ng/g), and DON (1.0–234 ng/g), while maintaining excellent selectivity. Notably, the test strip exhibited remarkable stability, retaining its detection capability even after storage at 4 °C for over one year. Importantly, the detection of these mycotoxins relied solely on simple manual operations, and with a portable reader, on-site detection could be accomplished within 20 min. This TRFM-ICTS presents a promising solution for sensitive on-site mycotoxin detection, suitable for practical application in various settings due to its sensitivity, accuracy, simplicity, and portability.

Fluorescence lifetime imaging and phasor analysis of intracellular porphyrinic photosensitizers applied with different polymeric formulations

The fluorescence lifetime of a porphyrinic photosensitizer (PS) is an important parameter to assess the aggregation state of the PS even in complex biological environments. Aggregation-induced quenching of the PS can significantly reduce the yield of singlet oxygen generation and thus its efficiency as a medical drug in photodynamic therapy (PDT) of diseased tissues. Hydrophobicity and the tendency to form aggregates pose challenges on the development of efficient PSs and often require carrier systems. A systematic study was performed to probe the impact of PS structure and encapsulation into polymeric carriers on the fluorescence lifetime in solution and in the intracellular environment. Five different porphyrinic PSs including chlorin e6 (Ce6) derivatives and tetrakis(m-hydroxyphenyl)-porphyrin and -chlorin were studied in free form and combined with polyvinylpyrrolidone (PVP) or micelles composed of triblock-copolymers or Cremophor. Following incubation of HeLa cells with these systems, fluorescence lifetime imaging combined with phasor analysis and image segmentation was applied to study the lifetime distribution in the intracellular surrounding. The data suggest that for free PSs, the structure-dependent cell uptake pathways determine their state and emission lifetimes. PS localization in the plasma membrane yielded mostly monomers with long fluorescence lifetimes whereas the endocytic pathway with subsequent lysosomal deposition adds a short-lived component for hydrophilic anionic PSs. Prolonged incubation times led to increasing contributions from short-lived components that derive from aggregates mainly localized in the cytoplasm. Encapsulation of PSs into polymeric carriers led to monomerization and mostly fluorescence emission decays with long fluorescence lifetimes in solution. However, the efficiency depended on the binding strength that was most pronounced for PVP. In the cellular environment, PVP was able to maintain monomeric long-lived species over prolonged incubation times. This was most pronounced for Ce6 derivatives with a logP value around 4.5. Micellar encapsulation led to faster release of the PSs resulting in multiple components with long and short fluorescence lifetimes. The hydrophilic hardly aggregating PS exhibited a mostly stable invariant lifetime distribution over time with both carriers. The presented data are expected to contribute to optimized PDT treatment protocols and improved PS-carrier design for preventing intracellular fluorescence quenching. In conclusion, amphiphilic and concurrent hydrophobic PSs with high membrane affinity as well as strong binding to the carrier have best prospects to maintain their photophysical properties in vivo and serve thus as efficient photodynamic diagnosis and PDT drugs.

Iridium-based Polymeric Multifunctional Imaging Tools for Biochemistry.

Herein, we report the development of a macromolecular multifunctional imaging tool for biological investigations, which is comprised of an N-(2-hydroxypropyl)methacrylamide backbone, iridium-based luminescent probe, glutamate carboxypeptidase II (GCPII) targeting ligand, and biotin affinity tag. The iridium luminophore is a tris-cyclometalated complex based on [Ir(ppy)3] with one of its 2-phenylpyridine ligands functionalized to allow conjugation. Synthesized macromolecular probes differed in the structure of the polymer and content of the iridium complex. The applicability of the developed imaging tools has been tested in flow cytometry (FACS) based assay, laser confocal microscopy, and fluorescence lifetime imaging microscopy (FLIM). The FACS analysis has shown that the targeted iBodies containing the iridium luminophore exhibit selective labelling of GCPII expressing cells. This observation was also confirmed in the imaging experiments with laser confocal microscopy. The FLIM experiment has shown that the iBodies with the iridium label exhibit a lifetime greater than 100 ns, which distinguishes them from typically used systems labelled with organic fluorophores exhibiting short fluorescence lifetimes. The results of this investigation indicate that the system exhibits interesting properties, which supports the development of additional biological tools utilizing the key components (iridium complexes, iBody concept), primarily focusing on the longer lifetime of the iridium emitter.

Pyrazoloquinoline-based fluorescent sensor for the detection of Pb2+, Zn2+ and the realization of an OR-type optical logic gate

A novel derivative of 1H-pyrazolo[3,4-b]quinoline (PQ2OH) has been synthesized and investigated by means of fluorescence and UV–vis absorption spectroscopy. The emission properties of the studied compound are characterized by low fluorescence quantum yields and short fluorescence lifetimes owing to the photoinduced electron transfer process, which is particularly efficient in highly polar solvents. We have employed this process in the optical detection of bivalent cations like Zn2+, Cd2+, Mg2+, Co2+, Ni2+, Ca2+, Pb2+, and Cu2+. The compound exhibits good selectivity towards Pb2+ and Zn2+ with limits of detection of 1.07 × 10−6 M and 1.06 × 10−6 M, respectively. The optical response is based on the recovery of native green fluorescence from pyrazoloquinoline. The sensor is able to detect Pb2+ in river water solutions with good sensitivity and can mimic an OR-type optical logic gate in cooperation with Pb2+ and Zn2+.

Metabolic imaging of human embryos is predictive of ploidy status but is not associated with clinical pregnancy outcomes: a pilot trial.

Does fluorescence lifetime imaging microscopy (FLIM)-based metabolic imaging assessment of human blastocysts prior to frozen transfer correlate with pregnancy outcomes? FLIM failed to distinguish consistent patterns in mitochondrial metabolism between blastocysts leading to pregnancy compared to those that did not. FLIM measurements provide quantitative information on NAD(P)H and flavin adenine dinucleotide (FAD+) concentrations. The metabolism of embryos has long been linked to their viability, suggesting the potential utility of metabolic measurements to aid in selection. This was a pilot trial enrolling 121 IVF couples who consented to have their frozen blastocyst measured using non-invasive metabolic imaging. After being warmed, 105 couples' good-quality blastocysts underwent a 6-min scan in a controlled temperature and gas environment. FLIM-assessed blastocysts were then transferred without any intervention in management. Eight metabolic parameters were obtained from each blastocyst (4 for NAD(P)H and 4 for FAD): short and long fluorescence lifetime, fluorescence intensity, and fraction of the molecule engaged with enzyme. The redox ratio (intensity of NAD(P)H)/(intensity of FAD) was also calculated. FLIM data were combined with known metadata and analyzed to quantify the ability of metabolic imaging to differentiate embryos that resulted in pregnancy from embryos that did not. De-identified discarded aneuploid human embryos (n = 158) were also measured to quantify correlations with ploidy status and other factors. Statistical comparisons were performed using logistic regression and receiver operating characteristic (ROC) curves with 5-fold cross-validation averaged over 100 repeats with random sampling. AUC values were used to quantify the ability to distinguish between classes. No metabolic imaging parameters showed significant differences between good-quality blastocysts resulting in pregnancy versus those that did not. A logistic regression using metabolic data and metadata produced an ROC AUC of 0.58. In contrast, robust AUCs were obtained when classifying other factors such as comparison of Day 5 (n = 64) versus Day 6 (n = 41) blastocysts (AUC = 0.78), inner cell mass versus trophectoderm (n = 105: AUC = 0.88) and aneuploid (n = 158) versus euploid and positive pregnancy embryos (n = 108) (AUC = 0.82). The study protocol did not select which embryo to transfer and the cohort of 105 included blastocysts were all high quality. The study was also limited in number of participants and study sites. Increased power and performing the trial in more sites may have provided a stronger conclusion regarding the merits of the use of FLIM clinically. FLIM failed to distinguish consistent patterns in mitochondrial metabolism between good-quality blastocysts leading to pregnancy compared to those that did not. Blastocyst ploidy status was, however, highly distinguishable. In addition, embryo regions and embryo day were consistently revealed by FLIM. While metabolic imaging detects mitochondrial metabolic features in human blastocysts, this pilot trial indicates it does not have the potential to serve as an effective embryo viability detection tool. This may be because mitochondrial metabolism plays an alternative role post-implantation. This study was sponsored by Optiva Fertility, Inc. Boston IVF contributed to the clinical site and services. Becker Hickl, GmbH, provided the FLIM system on loan. T.S. was the founder and held stock in Optiva Fertility, Inc., and D.S. and E.S. had options with Optiva Fertility, Inc., during this study. The study was approved by WCG Connexus IRB (Study Number 1298156).

Non-equivalent Mn4+ doping in mixed-anion host of K3Na(MoO2F4)2·H2O achieving short fluorescence lifetime and intense zero phonon line

The parity-selection rule for the Mn4+ d → d intra-configurational transition becomes relaxed after placing it into Mo6+-based distorted octahedra, which induced an oxyfluoride phosphor with fast decay and intense zero phonon line emission.

Impacts of heteroatom substitution on the excited state dynamics of π-extended helicenes.

Benzo-annulated aza[9]helicene ([9]AH) and thia[9]helicene ([9]TH) were prepared as novel π-extended heterohelicenes. [9]TH showed a quite short fluorescence lifetime of ∼0.3 ns and intense phosphorescence at low temperature that were attributed to its larger spin-orbit coupling and faster intersystem crossing between pseudo-degenerate S1/2 and triplet states.

Host-sensitized borate phosphors ZnGdB5O10:Mn2+/Dy3+/Sm3+ .

Photoluminescence energy transfer is a good strategy to enhance the efficiency or tuning of emission colors. A phosphor host containing Gd3+ may facilitate the host-sensitization effect and transfer the so-absorbed photon energy to other activators. Zn1-xMnxGdB5O10 (0.005 ≤ x ≤ 0.07), ZnGd1-yDyyB5O10 (0.01 ≤ y ≤ 0.09), and ZnGd1-zSmzB5O10 (0.01 ≤ z ≤ 0.09) were synthesized via the traditional high-temperature solid-state method. A powder X-ray diffraction technique was employed to confirm the phase purity and successful doping. In all phosphors, by monitoring the characteristic emission of Mn2+, Dy3+ and Sm3+, the excitation spectra of all were found to contain the typical absorption belonging to Gd3+; in addition, the largely shortened fluorescence lifetimes of Gd3+ after Mn2+, Dy3+ or Sm3+ doping strongly proved the existence of the host-sensitization effect. Along with Gd3+-sensitization, Mn2+ doped at the Zn2+ site emits a close-to-ideal red light. Dy3+ and Sm3+ doped at the Gd3+ site emit close to white and orange light, respectively. The calculated internal quantum efficiency is 25.5% for Zn0.995Mn0.005GdB5O10, 17.0% for ZnGd0.97Dy0.03B5O10 and 17.4% for ZnGd0.97Sm0.03B5O10. The high thermal stability of the photoluminescent emission for Mn2+, Dy3+, and Sm3+ can be demonstrated through in situ high-temperature experiments, which suggest possible enhanced energy transfer efficiency at high temperatures.

Luminescence properties of ZnSxO1-x:Ce3+ phosphors with tunable short fluorescence lifetime

Fluorescence lifetime of phosphors is a critical index in the field of high energy physics and astrophysical detection. A series of ZnSxO1-x:0.05Ce3+ phosphors with tunable short fluorescence lifetime were prepared by performing high temperature solid state reaction method. The phosphors exhibited two mixed phases consisting of the hexagonal phase ZnO and the hexagonal phase ZnS. They are spherical and the average particle size is 2.24 μm. As the component content of the ZnS in ZnSxO1-x:0.05Ce3+ phosphors varies, the emission wavelength can be tuned from 448 nm to 495 nm, the short fluorescence lifetime can be tuned within the range of 6 μs–200 μs. By performing exponential fitting, we obtained the equation for the variation of fluorescence lifetime of ZnSxO1-x:0.05Ce3+ phosphors with ZnS fraction.

Fluorescence enhancement induced by supramolecular J-aggregation of pyrene dye containing a flexible rotation group

A pyrene dye bearing a flexibly rotated hydrophobic group (3,4,5-tri(dodecyloxy)benzoyl-) was synthesized and fully characterized. In cooling processes for the dye monomer in CHCl3/CH3OH, the monomer was transformed into the J-aggregates which were characterized to possess a typically nanowired shape. A thermodynamic hysteresis of the molar fraction of the aggregates was observed explicitly between the cooling and heating processes. A pronounced self-absorption was involved in the aggregate solutions of the dye at high concentrations. Clearly, the J-aggregation conferred a few intriguing attributes such as a red-shift of maximum absorption peak, a higher quantum yield of absolute fluorescence and a shorter fluorescence lifetime than the monomer. Collectively, these attributes brought about a phenomenon of aggregation-induced emission enhancement (AIEE). In addition, the HOMO and LUMO energies were calculated by DFT for both the monomeric and aggregated states. Furthermore, the stacking parameters in the J-aggregates were evaluated according to the molecular exciton theory. It can be concluded that the J-aggregation enhanced substantially the emission of the dye. Undoubtedly, the strategy for the development of the AIEE molecules containing a flexible rotation group via J-aggregation as revealed in this work will be very useful in the future design of new organic light-emitting dyes.

Binding-induced thermal stabilization of mosR and ndhA G-quadruplex comprising genes by emodin leads to downregulation and growth inhibition in Mtb: Potential as anti-tuberculosis drug

Non-canonical G-quadruplex (G4) DNAs in genome of living organisms are involved inregulation of several cellular activities. Understanding of G-quadruplex interaction with anthraquinones is significant, in the continuing search for an effective therapeutic target against tuberculosis disease causing Mycobacterium tuberculosis virulent strains. Emodin interacts with G4 DNA forming genes, mosR and ndhA, that are essential in ATP synthesis and hypoxic growth of bacterial strains inside host cell. The observed hypochromism, red shift ∼ 6–11 nm, fluorescence quenching, shortening of fluorescence lifetime and appearance of negative induced circular dichroism band during interaction suggest binding of emodin to grooves/loops and end stacking with guanine quartets. The thermodynamically favorable reaction, involving non-intercalative binding mechanism in two distinct modes, is enthalpy driven with affinity constant ∼ 6 × 104 M−1. Inhibition of bacterial growth, Taq polymerase enzyme and significant downregulation of mosR and ndhA genes (2–4 orders) by emodin may be attributed to binding-induced thermal stabilization, ΔTm ∼ 23 and 14 °C in ndhA and mosR G4 DNA complexes, respectively. The studies demonstrate G4 DNA as promising pharmacological targets for developing effective anti-tuberculosis treatments and therapeutic potential of emodin in targeting pathogen persistence genes, particularly in view of emerging multi drug resistant deadly strains.

Effective transport of aggregated hypericin encapsulated in SBA-15 nanoporous silica particles for photodynamic therapy of cancer cells

Photodynamic therapy (PDT) represents an interesting modality for the elimination of damaged biomaterials and cells. This treatment takes advantage of the photosensitizing properties of molecules that are active only when irradiated with light. In the present work, a dual property of hypericin, a hydrophobic molecule with high performance in photodiagnostics and photodynamic therapy, was exploited. The non-fluorescent and photodynamically inactive form of hypericin aggregates was loaded into the nanopores of SBA-15 silica particles. The synthesized particles were characterized by infrared spectroscopy, thermogravimetry, differential thermal analysis, small-angle X-ray scattering and transmission electron microscopy. Hypericin aggregates were confirmed by absorption spectra typical of aggregated hypericin and by its short fluorescence lifetime. Release of hypericin from the particles was observed toward serum proteins, mimicking physiological conditions. Temperature- and time-dependent uptake of hypericin by cancer cells showed gradual release of hypericin from the particles and active cellular transport by endocytosis. A closer examination of SBA-15-hypericin uptake by fluorescence lifetime imaging showed that aggregated hypericin molecules, characterized by a short fluorescence lifetime (∼4 ns), were still present in the SBA-15 particles upon uptake by cells. However, monomerization of hypericin in cancer cells was observed by extending the hypericin fluorescence lifetime by ∼8 ns, preferentially in lipid compartments and the plasma membrane. This suggests a promising prognosis for delayed biological activity of the entire cargo, which was confirmed by effective PDT in vitro. In summary, this work presents an approach for safe, inactive delivery of hypericin that is activated at the target site in cells and tissues.

Luminescence Enhancement of K2LiAlF6:Mn4+ Phosphors by Zn2+-Mediated Charge Compensation for Fast-Response Backlighting Applications.

Wide-spectrum displays require narrow-band red phosphors activated by Mn4+, but most of them, such as K2SiF6:Mn4+, have long fluorescence decay lifetimes (>7 ms) that hinder their use in fast-response backlights. Interestingly, K2LiAlF6:0.05Mn4+ has a shorter fluorescence lifetime (3.43 ms), but its disadvantage is that its luminescence intensity is relatively weak. So, in this study, the luminescence intensity of K2LiAlF6:0.05Mn4+ is improved by doping with Zn2+. The experimental results show that enhancement of the luminous intensity is as high as 39%, the fluorescence lifetime is only increased by 13% (3.89 ms), and it is still less than 4 ms. Through experiments and differential charge density calculations, it has been revealed that the luminescence intensity improvement is due to an increased crystalline quality during the synthesis process. Specifically, the co-doping of Zn2+ reduces the formation of impurity ions Mn2+ and Mn3+ and the generation of K+ vacancies caused by nonequivalent doping. We demonstrate the advantage of this phosphor over K2SiF6:Mn4+ in terms of response speed by using a camera. It emits only weak red light after the blue chip stops working for 5 ms, indicating its potential application in next-generation fast-response displays.

Photophysical characterization of isothiazologuanosine, a unique isomorphic and isofunctional fluorescent analogue of guanosine

Application of fluorescence techniques to investigate molecular interactions with nucleic acids is complicated by their poor emission, making the substitution of natural nucleobases by fluorescent nucleoside analogues (FNAs) a useful strategy. A breakthrough in fluorescent nucleoside analogues has been the development of thienoguanosine (thG) and isothiazologuanosine (tzG), two isosteric mimics of guanosine (G). Due to its N7 atom needed in Hoogsteen base pairs and enzyme recognition, tzG is also an isofunctional G surrogate. Herein, we integrated fluorescence spectroscopy measurements with quantum mechanical (QM) calculations to characterize the mechanisms underlying tzG photophysics in different solvents. In dioxane and ethyl acetate, tzG existed primarily as a H1 keto-amino tautomer with short fluorescence lifetime (τ ∼ 2 ns) and low quantum yield (ϕ ∼ 0.10). In buffer, the H1 tautomer (ϕ = 0.36, τ = 8.84 ns) coexisted with a weakly emissive H3 keto-amino tautomer. The two tautomers were also observed in methanol, but with a 30% decrease in ϕ and τ values for the major H1 tautomer. QM calculations suggested that the main non-radiative pathway of tzG-H1 involves NS bond loosening and is responsible for the more solvent-sensitive ϕ and τ values compared to thG. This pathway is much more efficient for tzG-H3, for which an additional pathway to a dark nπ* state and a large coupling with triplet states further explain its very low emission. This study lays the ground for rationally using tzG as a sensitive FNA.