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Critical for metabolism, oxygen plays an essential role in maintaining the structure and function of neurons. Oxygen sensing is important in common neurological disorders such as strokes, seizures, or neonatal hypoxic–ischemic injuries, which result from an imbalance between metabolic demand and oxygen supply. Phosphorescence quenching by oxygen provides a non-invasive optical method to measure oxygen levels within cells and tissues. Difluoroboron β-diketonates are a family of luminophores with high quantum yields and tunable fluorescence and phosphorescence when embedded in certain rigid matrices such as poly (lactic acid) (PLA). Boron nanoparticles (BNPs) can be fabricated from dye-PLA materials for oxygen mapping in a variety of biological milieu. These dual-emissive nanoparticles have oxygen-insensitive fluorescence, oxygen-sensitive phosphorescence, and rigid matrix all in one, enabling real-time ratiometric oxygen sensing at micron-level spatial and millisecond-level temporal resolution. In this study, BNPs are applied in mouse brain slices to investigate oxygen distributions and neuronal activity. The optical properties and physical stability of BNPs in a biologically relevant buffer were stable. Primary neuronal cultures were labeled by BNPs and the mitochondria membrane probe MitoTracker Red FM. BNPs were taken up by neuronal cell bodies, at dendrites, and at synapses, and the localization of BNPs was consistent with that of MitoTracker Red FM. The brain slices were stained with the BNPs, and the BNPs did not significantly affect the electrophysiological properties of neurons. Oxygen maps were generated in living brain slices where oxygen is found to be mostly consumed by mitochondria near synapses. Finally, the BNPs exhibited excellent response when the conditions varied from normoxic to hypoxic and when the neuronal activity was increased by increasing K+ concentration. This work demonstrates the capability of BNPs as a non-invasive tool in oxygen sensing and could provide fundamental insight into neuronal mechanisms and excitability research.

Emissive β-diketones (bdks) and difluoroboron complexes (BF2bdks) exhibit multi-stimuli responsive luminescence, including solvatochromism, viscochromism, aggregation induced emission, thermal and mechanochromic luminescence, halochromism and pH sensing. In this study, a series of six-membered heterocycle-substituted (piperidine, morpholine, 1-methyl piperazine) bdk ligands and boron complexes were synthesized, and their luminescent properties were investigated. All the compounds exhibited red-shifted emission in more polar solvents due to intramolecular charge transfer as well as higher emission intensity in more viscous environments. In response to solubility changes in water/tetrahydrofuran mixtures, while the piperazine bdk ligand showed aggregation caused quenching, the piperidine and morpholine bdks displayed enhanced emission upon aggregation. In the solid state, all ligands exhibited mechanochromism. More dramatic halochromism was observed for the piperidine boron dye spin cast film. In solution, for the boron dyes under varying pH values (1-13), different protonated and deprotonated forms were analyzed according to the measured emission spectra. Graphical abstract Multi-stimuli responsive luminescent properties were investigated for the six-membered heterocycle-substituted β-diketone ligands and difluoroboron complexes.

Significance All organisms sense and respond to their environments. One way bacteria interact with their surroundings is by dynamically extending and retracting filamentous appendages from their surface called pili. While pili are critical for many functions, such as attachment, motility, and DNA uptake, the factors that regulate their dynamic activity are poorly understood. Here, we describe how an environmental signal induces a signaling pathway to promote the retraction of mannose-sensitive hemagglutinin pili in Vibrio cholerae . The retraction of these pili promotes the detachment of V. cholerae from a surface and may provide a means by which V. cholerae can respond to changes in its environment.

Bacteria use surface appendages called type IV pili to perform diverse activities including DNA uptake, twitching motility, and attachment to surfaces. The dynamic extension and retraction of pili are often required for these activities, but the stimuli that regulate these dynamics remain poorly characterized. To address this question, we study the bacterial pathogen

Shear-induced crystallization of dyes in the amorphous state is an effective strategy for generating higher energy emission after mechanical perturbation—a rare phenomenon in mechanoresponsive mate...

Luminescent β-diketones (bdks) and difluoroboron coordinated complexes (BF₂bdks) exhibit many environment-sensitive properties, such as solvatochromism, viscochromism, aggregation-induced emission (AIE), and thermal and mechanochromic luminescence (ML). In a previous study, an azepane-substituted bdk ligand (L1) and boron dye (D1) showed noteworthy luminescence properties but low quantum yields (Φ, L1: 0.26; D1: 0.02) due to free intramolecular bond rotation and twisted intramolecular charge transfer state formation with associated nonradiative decay. Thus, in order to improve the quantum yields, an azepane-substituted bdk ligand (L2) and boron complex (D2) with restricted C–C bond rotation were synthesized, and various luminescence properties were investigated. Restricting bond rotation blue-shifted absorptions and emissions, increased lifetimes, and greatly improved quantum yields (Φ, L2: 0.47; D2: 0.83). Excited state density functional theory calculations displayed twisted geometries for L1 and D1 but more planar geometries for L2 and D2. All compounds showed red-shifted emissions in more polar solvents. For viscochromism, L1 and D1 exhibited higher emission intensity in more viscous media. However, L2 and D2 did not show dramatic viscochromism, substantiating the relation between viscosity sensitivity and intramolecular bond twisting. Additionally, while both ligands showed quenched emission upon aggregation, the dyes exhibited AIE regardless of bond restriction. Thermal and ML studies showed a more dramatic emission shift for L2 than L1 between thermally annealed and melt-quenched states. In summary, the quantum yields of the azepane-substituted bdk ligand and boron dye were successfully improved by restricting the intramolecular C–C bond rotation, making various luminescence properties more promising for environment-sensitive applications.

Difluoroboron β-diketonate materials are used for biomedical oxygen-sensing applications. These fluorophores show both fluorescence (F) and phosphorescence (P) when the dye is embedded in a polymer matrix. The fluorescence is insensitive to oxygen quenching, while the phosphorescence changes intensity depending on the oxygen concentration. This enables oxygen quantification by ratiometric methods with good spatial and temporal resolution. Here, we produce ratiometric-capable oxygen sensors without the need of a heavy atom. This is achieved by substituting methoxyl groups at the meta positions of an aromatic ring, dramatically influencing the electronic transitions of the fluorophore. Altering the aromatic group on the opposing ring tailors the absorption wavelength, luminescence colors, and relative fluorescence to phosphorescence ratio (F/P), which are important for oxygen sensing. The phosphorescence lifetimes are also hundreds of milliseconds long, achieving ultra-sensitivity to oxygen. This study pres...

Emissive β-diketones (bdks) and difluoroboron complexes (BF2 bdks) show multi-stimuli responsive luminescence in both solution and the solid state. A series of bdk ligands and boron coordinated dyes were synthesized with different cyclic amine substituents in the 4-position to explore ring size effects on various luminescent properties, including solvatochromism, viscochromism, aggregation-induced emission (AIE), mechanochromic luminescence (ML) and halochromism. Red-shifted absorption and emission were observed in CH2 Cl2 for both bdk ligands and boron dyes with increasing substituent ring size. The compounds displayed bathochromic emission in more polar solvents, and higher fluorescence intensity in more viscous media. The AIE compounds exhibited enhanced emission when aggregated. For solid-state properties, a large emission wavelength shift was shown for the piperidine substituted bdk after melt quenching on weighing paper. Large blue-shifted emissions were observed in all the boron dye spin cast films after trifluoroacetic acid vapor annealing, and the original emissions were partially recovered after triethylamine vapor treatment.

Significance: Decreasing the oxygen consumption rate (OCR) of tumor cells is a powerful method for ameliorating tumor hypoxia. However, quantifying the change in OCR is challenging in complex experimental systems. Aim: We present a method for quantifying the OCR of two tumor cell lines using oxygen-sensitive dual-emissive boron nanoparticles (BNPs). We hypothesize that our BNP results are equivalent to the standard Seahorse assay. Approach: We quantified the spectral emissions of the BNP and accounted for external oxygen diffusion to quantify OCR over 24 h. The BNP-computed OCR of two breast cancer cell lines, E0771 and 4T07, were compared with their respective Seahorse assays. Both cell lines were also irradiated to quantify radiation-induced changes in the OCR. Results: Using a Bland–Altman analysis, our BNPs OCR was equivalent to the standard Seahorse assay. Moreover, in an additional experiment in which we irradiated the cells at their 50% survival fraction, the BNPs were sensitive enough to quantify 24% reduction in OCR after irradiation. Conclusions: Our results conclude that the BNPs are a viable alternative to the Seahorse assay for quantifying the OCR in cells. The Bland–Altman analysis showed that these two methods result in equivalent OCR measurements. Future studies will extend the OCR measurements to complex systems including 3D cultures and in vivo models, in which OCR measurements cannot currently be made.

OBJECTIVE: Oxygen is essential to wound healing; therefore, accurate monitoring can guide clinical decisions. Clinical wound assessment is often subjective, and tools to monitor wound oxygen are typically expensive, indirect, and highly variable. This study demonstrates the utility of a novel, low-cost oxygen-sensing thin film for serial assessment of wound oxygenation. DESIGN: Dual-layer films were fabricated with boron oxygen-sensing nanoparticles (BNPs) impregnated into a chitosan-polycaprolactone (CS-PCL) layer for direct wound bed contact with a relatively oxygen impermeable calcium alginate surface layer. The BNPs are a dual emissive difluoroboron β-diketonate dye incorporated into poly(lactic acid) nanoparticles (BF(2)nbm(I)PLA). Under UV excitation, the BNPs emit fluorescence based on concentration and oxygen sensitive phosphorescence. The fluorescence/phosphorescence (F/P) ratio is directly proportional to oxygen concentration. METHODS: A series of in vitro oxygen challenges and in vivo murine and porcine wound healing models were used to validate the utility of the film in sensing wound oxygenation. MAIN RESULTS: In vitro testing demonstrated the oxygen-sensing capability of the BNP film and ability to shield ambient oxygen to isolate wound oxygen. In vivo testing demonstrated the ability of the film to accurately monitor relative oxygen changes in a murine wound over time, measuring a 22% F/P increase during acute healing. CONCLUSIONS: This study presents a low-cost, noninvasive, direct, and serial oxygen mapping technology to detect spatial differences in wound oxygenation. Clinical use of the films has the potential to monitor wound healing trajectories and guide wound care decisions.

Tracking cell movements is an important aspect of many biological studies. Reagents for cell tracking must not alter the biological state of the cell and must be bright enough to be visualized above background autofluorescence, a particular concern when imaging in tissue. Currently there are few reagents compatible with standard UV excitation filter sets (e.g. DAPI) that fulfill those requirements, despite the development of many dyes optimized for violet excitation (405 nm). A family of boron-based fluorescent dyes, difluoroboron β-diketonates, has previously served as bio-imaging reagents with UV excitation, offering high quantum yields and wide excitation peaks. In this study, we investigated the use of one such dye as a potential cell tracking reagent. A library of difluoroboron dibenzoylmethane (BF(2)dbm) conjugates were synthesized with biocompatible polymers including: poly(L-lactic acid) (PLLA), poly(ε-caprolactone) (PCL), and block copolymers with poly(ethylene glycol) (PEG). Dye-polymer conjugates were fabricated into nanoparticles, which were stable for a week at 37 °C in water and cell culture media, but quickly aggregated in saline. Nanoparticles were used to label primary splenocytes; phagocytic cell types were more effectively labelled. Labelling with nanoparticles did not affect cellular viability, nor basic immune responses. Labelled cells were more easily distinguished when imaged on a live tissue background than those labelled with a commercially available UV-excitable cytoplasmic labelling reagent. The high efficiency in terms of both fluorescence and cellular labelling may allow these nanoparticles to act as a short-term cell labelling strategy while wide excitation peaks offer utility across imaging and analysis platforms.

Owing to their facile synthesis and desirable optical properties, β-diketones (bdks) are attractive scaffolds for the production of new mechanochromic luminescent (ML) materials. However, ML-active bdk dyes that can access full color luminescent states (i.e., red, green, and blue) have yet to be realized. To synthesize such a set of bdk emitters, two strategies were used to red-shift the emission: substitution with varying donor and acceptor groups; and coordination of BF2 to the β-diketonate moiety. Additionally, 3,4,5-trimethoxy-substituted phenyl rings were used to disrupt the molecular packing to promote ML and enable aggregation-induced emission (AIE). Here, a series of trimethoxy-substituted bdks were prepared via Claisen condensation, each para-substituted with either F, I, or CN groups. Their boronated complexes were synthesized via reaction with boron trifluoride diethyl etherate. Optimized geometries generated from density functional theory calculations indicated that the dye para-substituted me...

The optical properties of meta-alkoxy-substituted difluoroboron dibenzoylmethane dyes were investigated in solution and in the solid state. Meta-alkoxy substitution induced strong intramolecular charge transfer (ICT) from the oxygen-donating substituent to the halide and boron acceptors in the excited state, as compared to the π-π* transition that is observed with para-alkoxy substitution. The optical properties of para- and meta-substituted alkoxy boron dyes were evaluated by calculations, in dilute solution, and in solid-state films. When embedded in amorphous matrixes (e.g., PLA, PMMA, PS, cholesterol), all dyes showed fluorescence (F) and phosphorescence (P) emission. In this report, we show that meta-substitution resulted in enhanced solvatochromism and an increased phosphorescence-to-fluorescence ratio in solid-state films compared to analogous para-substituted samples. With enhanced phosphorescence intensity via the heavy-atom effect, iodo-substituted dyes were further studied in PLA-PEG nanoparticles. Oxygen calibrations revealed stronger phosphorescence and a greater oxygen-sensing range for the meta- versus para-alkoxy-substituted dyes, features that are important for oxygen-sensing materials design.

Difluoroboron β-diketonate (BF2bdk) compounds exhibit solid-state switchable luminescence under excitation by UV light. This property is usually manifested as a blue-shift in emission when dye films are thermally annealed followed by a red-shift in response to mechanical shear (i.e. mechanochromic luminescence). Here we report thiophene and furan heterocycle-substituted dyes bearing short methoxy and long dodecyloxy chain substituents. Optical properties of the dyes were investigated in solution, pristine powders, and films on paper and glass. The structural and thermal properties of the dyes were also investigated by powder x-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. The methoxy-substituted dyes exhibited neither thermal nor mechano-responsive behavior, however, addition of a longer C12 alkoxyl chain substituent resulted in stimuli-responsive behavior. The furan and thiophene dodecyloxy-substituted dyes both exhibit high-contrast reversible luminescence switching between crystalline, blue-shifted and amorphous red-shifted emissive states. For the furan dye, gentle heating produced a green emissive form while melting followed by rapid cooling produced an orange emissive form. The thiophene dye, on the other hand, exhibited blue-shifted emission when annealed below its melting temperature and red-shifted emission when smeared with a cotton swab (mechanochromic luminescence). These transformations for both dyes were found to be completely reversible, making them potential candidates for applications requiring reusable, functional materials.

The mechanochromic luminescent (ML) properties of boron coordinated β-diketones (BF2bdks) have been widely studied, however the stimuli responsive properties of uncoordinated β-diketones (bdks) are less known. While bdk dyes show promising properties, including high contrast ML, rapid room temperature self-erasure, solvatochromism, and aggregation induced emission (AIE), previously reported dyes exhibit emission over a narrow range of wavelengths (∼420–500 nm). To tune luminescence over a broader color range, dimethylamino (DMA) substituted β-diketones were synthesized with a series of electron donating and withdrawing substituents. Solvatochromism, aggregation induced emission (AIE), ML, and thermochromism were investigated. Most dyes showed positive solvatochromic shifts and were responsive to mechanical and thermal stimuli. The solid-state emission wavelengths correlated with the electron withdrawing strength of the para substituent and ranged from blue to orange (488–578 nm). Structural and thermal characterization was performed by powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) respectively. These data indicate that stimuli responsive properties are the result of a crystalline to amorphous phase transition. Additionally, the aggregation induced emission (AIE) properties of iodo and cyano substituted dyes were measured in THF/H2O solutions. Both showed intense emission resulting from aggregation. The sensitivity of these dyes toward matrix polarity was further investigated by fabricating polystyrene films with increasing concentrations of dye and camphoric anhydride, a non-emissive polar dopant. A red-shift in emission was observed in these films which demonstrated that matrix effects can be used to tune the emission of DMA-substituted diketones.

Difluoroboron β-diketonate-polylactides are versatile oxygen-sensing materials. These materials have both fluorescence (F) and oxygen-sensitive, room-temperature phosphorescence (RTP). The fluorescence, being insensitive to oxygen, can act as an internal standard to the changing phosphorescence, and ratiometric sensing of oxygen can be achieved with these simple, single-component materials. To expand the range of colors for this family of fluorophores, a series of thienyl-phenyl-substituted dyes were synthesized with initiator sites for the ring-opening polymerization of lactide. Heavy atoms (Br and I) were added to the dye to modulate the phosphorescence intensity. These halide-substituted thiophene dyes readily aggregated in the poly(lactic acid) (PLA) matrix, generating two fluorescence peaks in air, one for monomer emission and another for aggregate emission. When the dye was dilute in PLA, as a blend, the iodo-thienyl derivative showed impressive singlet-triplet splitting, with blue fluorescence (440 nm) and orange phosphorescence (585 nm), the largest gap recorded for a boron β-diketonate dye. Nanoparticles fabricated from a mixture of PLA and dye-PLA conjugated polymer benefited from the large singlet-triplet splitting to yield oxygen sensitivity at levels between 0 and 21 %, which can be utilized in biological oxygen-sensing applications.

Though the effects of arene substituents on difluoroboron β-diketonate (BF2bdk) mechanochromic luminescence (ML) are known, substitution at the α carbon of the dioxaborine ring has yet to be carefully investigated. Substitution at the α position can force these dyes out of a planar conformation, causing materials to take on unique aggregation-induced enhanced emission (AIEE) character. Furthermore, if ML arises from the formation of H-aggregates upon smearing, as postulated, substitution at the α position could hinder this process and influence ML. To test this hypothesis, luminescent BF2bdk derivatives containing substituents at the α position were synthesized and their optical and morphological properties were characterized by fluorescence spectroscopy, UV/vis spectroscopy, atomic force microscopy (AFM), differential scanning calorimetry (DSC), and density functional theory (DFT) calculations. Specifically, a difluoroboron dibenzoylethane derivative with a methyl group at the α position exhibits bright ...

Difluoroboron β-diketonate compounds exhibit solid-state luminescence phenomena. Among these are reversible mechanochromic luminescence (ML), aggregation induced emission (AIE), and mechanochromic luminescence quenching (MLQ). These properties can be tuned by alterations to the molecular structure. Dyes with varying halide substituents exhibit tunable ML, MLQ, and solid-state emission with high quantum yields. A series of difluoroboron dibenzoylmethane (BF2dbm) dyes with iodide and alkoxyl substituents (BF2dbm(I)OR) were synthesized where R = CH3 (C1), C5H11 (C5), C6H13 (C6), C12H25 (C12), and C18H37 (C18)). The 4-iodo parent compound BF2dbm(I) (H) was made for comparison. By keeping the heavy atom static, the dependence of ML properties on alkyl chain length was probed. The hydrogen derivative is only weakly emissive as a solid and exhibited minimal mechanoresponsive behavior. In contrast, alkoxy dyes exhibited tunable ML and MLQ properties depending on the length of the alkyl chain. Longer chain dyes co...

Summary The presence or absence of oxygen in the environment is a strong effector of cellular metabolism and physiology. Like many eukaryotes and some bacteria, Bacillus subtilis primarily utilizes oxygen during respiration to generate ATP. Despite the importance of oxygen for B. subtilis survival, we know little about how populations adapt to shifts in oxygen availability. Here, we find that when oxygen was depleted from stationary phase B. subtilis cultures, ∼90% of cells died while the remaining cells maintained colony-forming ability. We discover that production of the antimicrobial surfactin confers two oxygen-related fitness benefits: it increases aerobic growth yield by increasing oxygen diffusion, and it maintains viability during oxygen depletion by depolarizing the membrane. Strains unable to produce surfactin exhibited an ∼50-fold reduction in viability after oxygen depletion. Surfactin treatment of these cells led to membrane depolarization and reduced ATP production. Chemical and genetic perturbations that alter oxygen consumption or redox state support a model in which surfactin-mediated membrane depolarization maintains viability through slower oxygen consumption and/or a shift to a more reduced metabolic profile. These findings highlight the importance of membrane potential in regulating cell physiology and growth, and demonstrate that antimicrobials that depolarize cell membranes can benefit cells when the terminal electron acceptor in respiration is limiting. This foundational knowledge has deep implications for environmental microbiology, clinical anti-bacterial therapy, and industrial biotechnology.

Capable of mediating efficient transfection and protein production without eliciting innate immune responses, chemically modified mRNA holds great potential to produce paracrine factors at a physiologically beneficial level, in a spatiotemporally controlled manner, and with low toxicity. Although highly promising in cardiovascular medicine and wound healing, effects of this emerging therapeutic on the microvasculature and its bioactivity in disease settings remain poorly understood. Here, we longitudinally and comprehensively characterize microvascular responses to AZD8601, a modified mRNA encoding vascular endothelial growth factor A (VEGF-A), in vivo. Using multi-parametric photoacoustic microscopy, we show that intradermal injection of AZD8601 formulated in a biocompatible vehicle results in pronounced, sustained and dose-dependent vasodilation, blood flow upregulation, and neovessel formation, in striking contrast to those induced by recombinant human VEGF-A protein, a non-translatable variant of AZD8601, and citrate/saline vehicle. Moreover, we evaluate the bioactivity of AZD8601 in a mouse model of diabetic wound healing in vivo. Using a boron nanoparticle-based tissue oxygen sensor, we show that sequential dosing of AZD8601 improves vascularization and tissue oxygenation of the wound bed, leading to accelerated re-epithelialization during the early phase of diabetic wound healing.

Difluoroboron β-diketonate (BF2bdk) compounds show environment-sensitive optical properties in solution, aggregation-induced emission (AIE) and multi-stimuli responsive fluorescence switching in the solid state. Here, a series of 4-azepane-substituted β-diketone (bdk) ligands (L-H, L-OMe, L-Br) and their corresponding difluoroboron dyes (D-H, D-OMe, D-Br) were synthesized, and various responsive fluorescence properties of the compounds were studied, including solvatochromism, viscochromism, AIE, mechanochromic luminescence (ML) and halochromism. Compared to the β-diketones, the boron complexes exhibited higher extinction coefficients but lower quantum yields, and red-shifted absorption and emission in CH2Cl2. Computational studies showed that intramolecular charge transfer (ICT) dominated rather than π–π* transitions in all the compounds regardless of boron coordination. In solution, all the bdk ligands and boron dyes showed red-shifted emission in more polar solvents and increased fluorescence intensity in more viscous media. Upon aggregation, the emission of the β-diketones was quenched, however, the boronated dyes showed increased emission, indicative of AIE. Solid-state emission properties, ML and halochromism, were investigated on spin cast films. For ML, smearing caused a bathochromic emission shift for L-Br, and powder X-ray diffraction (XRD) patterns showed that the “as spun” and thermally annealed states were more crystalline and the smeared state was amorphous. No obvious ML emission shift was observed for L-H or L-OMe, and the boronated dyes were not mechano-active. Trifluoroacetic acid (TFA) and triethylamine (TEA) vapors were used to study halochromism. Large hypsochromic emission shifts were observed for all the compounds after exposure to TFA vapor, and reversible fluorescence switching was achieved using the acid/base pair.

Structure–mechanical property studies confirm that the extent of mechanochromic luminescence in crystalline organic fluorophores positively correlates with the extent of plasticity., The structure and mechanical properties of crystalline materials of three boron difluoride dibenzoylmethane (BF2dbm) derivatives were investigated to examine the correlation, if any, among mechanochromic luminescence (ML) behaviour, solid-state structure, and the mechanical behaviour of single crystals. Qualitative mechanical deformation tests show that the crystals of BF2dbm(tBu)2 can be bent permanently, whereas those of BF2dbm(OMe)2 exhibit an inhomogeneous shearing mode of deformation, and finally BF2dbmOMe crystals are brittle. Quantitative mechanical analysis by nano­indentation on the major facets of the crystals shows that BF2dbm(tBu)2 is soft and compliant with low values of elastic modulus, E, and hardness, H, confirming its superior suceptibility for plastic deformation, which is attributed to the presence of a multitude of slip systems in the crystal structure. In contrast, both BF2dbm(OMe)2 and BF2dbmOMe are considerably stiffer and harder with comparable E and H, which are rationalized through analysis of the structural attributes such as the intermolecular interactions, slip systems and their relative orientation with respect to the indentation direction. As expected from the qualitative mechanical behaviour, prominent ML was observed in BF2dbm(tBu)2, whereas BF2dbm(OMe)2 exhibits only a moderate ML and BF2dbmOMe shows no detectable ML, all examined under identical conditions. These results confirm that the extent of ML in crystalline organic solid-state fluorophore materials can be correlated positively with the extent of plasticity (low recovery). In turn, they offer opportunities to design new and improved efficient ML materials using crystal engineering principles.

The dual-emissive properties of solid-state difluoroboron β-diketonate-poly(lactic acid) (BF2bdkPLA) materials have been utilized for biological oxygen sensing. In this work, BF2dbm(X)PLA materials were synthesized, where X = H, F, Cl, Br, and I. The effects of changing the halide substituent and PLA polymer chain length on the optical properties in dilute CH2Cl2 solutions and solid-state polymer films were studied. These luminescent materials show fluorescence, phosphorescence, and lifetime tunability on the basis of molecular weight, as well as lifetime modulation via the halide substituent. Short BF2dbm(Br)PLA (6.0 kDa) and both short and long BF2dbm(I)PLA polymers (6.0 or 20.3 kDa) have fluorescence and intense phosphorescence ideal for ratiometric oxygen sensing. The lighter halide-dye polymers with hydrogen, fluorine, and chlorine substitution have longer phosphorescence lifetimes and can be utilized as ultrasensitive oxygen sensors. Photostability was also analyzed for the polymer films.

Surface modification of nanoparticles and biosensors is a dynamic, expanding area of research for targeted delivery in vivo. For more efficient delivery, surfaces are PEGylated to impart stealth properties, long circulation and enable enhanced permeability and retention (EPR) in tumor tissues. Previously, BF2dbm(I)PLA was proven to be a good oxygen nanosensor material for tumor hypoxia imaging in vivo, though particles were applied directly to the tumor and surrounding region. Further surface modification is needed for this dual-emissive oxygen sensitive material for effective intravenous (IV) administration and passive and active delivery to tumors. In this paper an efficient synthesis of a new dual-emissive material BF2dbm(I)PLA-mPEG is presented and in vitro stability studies were conducted. It is found that fabricated nanoparticles are stable for 24 weeks as a suspension, while after 25 weeks the nanoparticles swell and both dye and polymer degradation escalates. Preliminary studies show BF2dbm(I)PLA-mPEG nanoparticle accumulation in a window chamber mammary tumor 24 hours after IV injection into mice (C57Bl/6 strain) enabling tumor oxygen imaging.

Many difluoroboron β-diketonate (BF2bdk) compounds display mechanochromic luminescence (ML) and substituents can influence their optical properties. For example, the difluoroboron iododibenzoylmethane derivative, BF2dbm(I)OC12H25, exhibits mechanochromic luminescence quenching (MLQ). When annealed films of this dye are smeared under air, the perturbed region becomes dim under UV light. In this study, effects of differing halide substituents on ML were investigated. A series of dyes of the form BF2dbm(X)OC12H25, where X = H, F, Cl, Br, and I, were synthesized. The luminescence properties of these dyes were studied in CH2Cl2 solution, as films on weighing paper, spin-cast films on glass substrates, and as bulk powders. All of the dyes exhibit ML and MLQ. Solid-state emissions from these dyes are also sensitive to annealing temperature. The F, Cl, and Br dyes required higher annealing temperatures than the H and I analogues to achieve ordered emissive states. Spin-cast films of the dyes on glass were studied using atomic force microscopy (AFM) and X-ray diffraction (XRD), revealing a change from an amorphous material in the as-spun state to an ordered, crystalline material after annealing, with the F and Cl dyes showing a propensity to form large lamellar crystallites. Powders were also investigated using XRD and showed changes in diffraction patterns corresponding to thermal annealing and grinding. Finally, halogen substituents greatly diminished the ability of the dye films to recover their ordered emissive states after smearing under ambient conditions. These dyes could be useful for applications in which a more permanent inscription is desirable.

Reversible mechanochromic luminescence (ML) is known for difluoroboron β-diketonates (BF2bdks). Here, we report ML for the methoxy-substituted dinaphthoylmethane (dnmOMe) ligand even without coordination to boron. In addition, dnmOMe shows solvatochromism and aggregation-induced emission (AIE). Diketone thin films showed high contrast emission when smeared, and rapid recovery of the pre-smeared state at room temperature.

Difluoroboron beta-diketonates (BF(2)bdks) show both fluorescence (F) and room-temperature phosphorescence (RTP) when confined to a rigid matrix, such as poly(lactic acid). These materials have been utilized as optical oxygen sensors (e.g., in tumors, wounds, and cells). Spectral features include charge transfer (CT) from the major aromatic donor to the dioxaborine acceptor. A series of naphthyl-phenyl dyes (BF(2)nbm) (1-6) were prepared to test heavy-atom placement effects. The BF(2)nbm dye (1) was substituted with Br on naphthyl (2), phenyl (3), or both rings (4) to tailor the fluorescence/phosphorescence ratio and RTP lifetimeimportant features for designing O-2 sensing dyes by means of the heavy atom effect. Computational studies identify the naphthyl ring as the major donor. Thus, Br substitution on the naphthyl ring produced greater effects on the optical properties, such as increased RTP intensity and decreased RTP lifetime compared to phenyl substitution. However, for electron-donating piperidyl-phenyl dyes (5), the phenyl aromatic is the major donor. As a result, Br substitution on the naphthyl ring (6) did not alter the optical properties significantly. Experimental data and computational modeling show the importance of Br position. The S-1 and T-1 states are described by two singly occupied MOs (SOMOs). When both of these SOMOs have substantial amplitude on the heavy atom, passage from S-1 to T-1 and emission from T-1 to S-0 are both favored. This shortens the excited-state lifetimes and enhances phosphorescence.

Optical properties of biphenyl difluoroboron β-diketonates were studied in poly(lactic acid) (PLA) blends. Increased conjugation lowered the emission energy, decreased the singlet–triplet energy gap and yielded blue thermally activated delayed fluorescence (TADF). The properties of these biphenyl dyes may inform organic light emitting diode (OLED) and bioimaging agent design.

Mechanochromic luminescent (ML) materials, which show a change in emission due to an applied mechanical stimulus, are useful components in a variety of applications, including organic light-emitting diodes, force sensors, optical memory storage, and next-generation lighting materials. While there are many different ML active derivatives, few show room temperature self-erasing. Thin films of the methoxy substituted β-diketone, gbmOMe, initially exhibited blue (428 nm) emission; however, green (478 nm) emission was observed after smearing. The mechanically generated smeared state recovered so rapidly that characterization of its emission was difficult at room temperature using traditional luminescence techniques. Thus, a new complementary metal oxide semiconductor camera imaging method was developed and used to calculate the decay time of the mechanically generated smeared state (i.e., smeared-state decay; τSM) for gbmOMe thin films. Additionally, this method was used to evaluate substrate and film thicknes...

Luminescent difluoroboron β-diketonate poly(lactic acid) (BF2bdkPLA) materials serve as biological imaging agents. In this study, dye structures were modified to achieve emission colors that span the visible region with potential for multiplexing applications. Four dyes with varying π−conjugation (phenyl, naphthyl) and donor groups (-OMe, -NMe2) were coupled to PLLA-PEG block copolymers (~11 kDa) by a post-polymerization Mitsunobu reaction. The resulting dye-polymer conjugates were fabricated as nanoparticles (~55 nm diameter) to produce nanomaterials with a range of emission colors (420–640 nm). For increased stability, dye-PLLA-PEG conjugates were also blended with dye-free PDLA-PEG to form stereocomplex nanoparticles of smaller size (~45 nm diameter). The decreased dye loading in the stereoblocks blue-shifted the emission, generating a broader range of fluorescence colors (410–620 nm). Tumor accumulation was confirmed in a murine model through biodistribution studies with a red emitting dimethyl amino-substituted dye-polymer analogue. The synthesis, optical properties, oxygen-sensing capabilities, and stability of these block copolymer nanoparticles are presented.

Hypoxia correlates with reduced radiosensitivity and therapy response. Imaging anesthisized mice using fluorescent nanoparticles causes problems in modeling hypoxia. Therefore, a mobile imaging apparatus was developed to image un-anesthetized mice.

Oxygenation in tissue scaffolds continues to be a limiting factor in regenerative medicine despite efforts to induce neovascularization or to use oxygen-generating materials. Unfortunately, many established methods to measure oxygen concentration, such as using electrodes, require mechanical disturbance of the tissue structure. To address the need for scaffold-based oxygen concentration monitoring, a single-component, self-referenced oxygen sensor was made into nanofibers. Electrospinning process parameters were tuned to produce a biomaterial scaffold with specific morphological features. The ratio of an oxygen sensitive phosphorescence signal to an oxygen insensitive fluorescence signal was calculated at each image pixel to determine an oxygenation value. A single component boron dye-polymer conjugate was chosen for additional investigation due to improved resistance to degradation in aqueous media compared to a boron dye polymer blend. Standardization curves show that in fully supplemented media, the fibers are responsive to dissolved oxygen concentrations less than 15 ppm. Spatial (millimeters) and temporal (minutes) ratiometric gradients were observed in vitro radiating outward from the center of a dense adherent cell grouping on scaffolds. Sensor activation in ischemia and cell transplant models in vivo show oxygenation decreases on the scale of minutes. The nanofiber construct offers a robust approach to biomaterial scaffold oxygen sensing.

Luminescent materials are important for imaging and sensing. Aromatic difluoroboron β-diketonate complexes (BF2bdks) are classic fluorescent molecules that have been explored as photochemical reagents, two-photon dyes, and oxygen sensors. A series of BF2bdks with naphthyl and phenyl groups was synthesized, and photophysical properties were investigated in both methylene chloride and poly(lactic acid) (PLA). Polymer molecular weight and dye attachment site along with bromide heavy atom placement were varied to tune optical properties of dye–PLA materials. Systems without heavy atoms have long phosphorescence lifetimes, which is useful for lifetime-based oxygen sensing. Bromine substitution on the naphthyl ring resulted in intense, clearly distinguishable fluorescence and phosphorescence peaks important for ratiometric oxygen sensing and imaging.

Aromatic difluoroboron β-diketonate complexes (BF2bdks) are classic fluorescent molecules that have been explored as photochemical reagents, two-photon dyes, and oxygen sensors. To gain a better understanding of their emissive properties in both solution and polymer matrices, BF2bdks with varying aromatic groups were synthesized and their photophysical properties were investigated in both methylene chloride and poly(lactic acid) (PLA). Absorption spectra showed systematic variations that are well correlated with structural features, including the size of the aryl substituent and the presence of a para electron-donating methoxy substituent. Computational modeling of the absorption spectra with the TD-B3LYP/6-311+G(d)//B3LYP/6-31G(d) formulation of density functional theory and a polarizable continuum model of dichloromethane solvent shows that all systems show intense π-π* one-electron excitations, usually from one of the highest occupied molecular orbitals (HOMO - k, k = 0, 1, 2) to the lowest unoccupied molecular orbital (LUMO). Emission properties are sensitive to the dye structure and medium. Based on spectroscopic and lifetime studies, BF2bdks exhibit comparable fluorescence properties in both solutions and polymers when the diketonate group is functionalized with smaller aromatic ring systems such as benzene. For BF2bdks with larger arene ring systems, such as anthracene, emission from a strong intramolecular charge-transfer (ICT) state was also noted in both solution and in PLA. There are differences in relative intensities of peaks arising from π-π* and ICT excitations depending upon dye loading in PLA. Substituent effects were also observed. Electron-donating methoxyl groups on the aromatic rings lead to enhanced fluorescence quantum yields. For certain dyes, phosphorescence is detected at low temperature or under a nitrogen atmosphere in PLA matrices.

Lifetime-based oxygen imaging is useful in many biological applications but instrumentation can be stationary, expensive, and complex. Herein, we present a portable, cost effective, simple alternative with high spatiotemporal resolution that uses a complementary metal oxide silicon (CMOS) camera to measure oxygen sensitive lifetimes on the millisecond scale. We demonstrate its compatibility with difluoroboron β-diketonate poly(lactic acid) (BF2bdkPLA) polymers which are nontoxic and exhibit long-lived oxygen sensitive phosphorescence. Spatially resolved lifetimes of four BF2bdkPLA variants are measured using nonlinear least squares (NLS) and rapid lifetime determination (RLD) both of which are shown to be accurate and precise. Real-time imaging in a dynamic environment is demonstrated by determining lifetime pixel-wise. The setup costs less than $5000, easily fits into a backpack, and can operate on battery power alone. This versatility combined with the inherent utility of lifetime measurements make this system a useful tool for a wide variety of oxygen sensing applications. This study serves as an important foundation for the development of dual mode real time lifetime plus ratiometric imaging with bright, long lifetime difluoroboron β-diketonate probes.

Difluoroboron β–diketonate poly(lactic acid) materials exhibit both fluorescence (F) and oxygen sensitive room-temperature phosphorescence (RTP). Introduction of halide heavy atoms (Br and I) is an effective strategy to control the oxygen sensitivity in these materials. A series of naphthyl-phenyl (nbm) dye derivatives with hydrogen, bromide and iodide substituents were prepared for comparison. As nanoparticles, the hydrogen derivative was hypersensitive to oxygen (0–0.3%), while the bromide analogue was suited for hypoxia detection (0–3% O2). The iodo derivative, BF2nbm(I)PLA, showed excellent F to RTP peak separation and an 0–100% oxygen sensitivity range unprecedented for metal-free RTP emitting materials. Due to the dual emission and unconventionally long RTP lifetimes of these O2 sensing materials, a portable, cost-effective camera was used to quantify oxygen levels via lifetime and red/green/blue (RGB) ratiometry. The hypersensitive H dye was well matched to lifetime detection, simultaneous lifetime and ratiometric imaging was possible for the bromide analogue, whereas the iodide material, with intense RTP emission and a shorter lifetime, was suited for RGB ratiometry. To demonstrate the prospects of this camera/material design combination for bioimaging, iodide boron dye-PLA nanoparticles were applied to a murine wound model to detect oxygen levels. Surprisingly, wound oxygen imaging was achieved without covering (i.e. without isolating from ambient conditions, air). Additionally, would healing was monitored via wound size reduction and associated oxygen recovery, from hypoxic to normoxic. These single-component materials provide a simple tunable platform for biological oxygen sensing that can be deployed to spatially resolve oxygen in a variety of environments.

In solid-state mechanochromic luminescence (ML) materials, it remains a challenge to establish the origin of fluorescence color changes upon mechanical action and to determine why only some fluorophores exhibit ML behavior. The study of mechanical properties by nanoindentation, followed by ML experiments on green- and cyan-emitting polymorphs of difluoroboron avobenzone reveals that upon smearing, the plastically deformable cyan form shows a prominent color change to yellow, while in the harder green form the redshifted emission is barely detectable. Crystal structure analysis reveals the presence of slip planes in the softer cyan form that can facilitate the formation of recoverable and low energy defects in the structure. Hence, the cyan form exhibits prominent and reversible ML behavior. This suggests a potential design strategy for efficient ML materials.

A simple phosphoroscope with no moving parts is described. In one scan the total luminescence, the long-lived phosphorescence, and the short-lived fluorescence can be determined. A 50% duty cycle excitation from a diode laser is used to excite the sample, and from the digitized waveform the phosphorescence is extracted from the off period, the total emission from the full cycle, and the fluorescence from the on period corrected for the phosphorescence contribution. The performance of the system is demonstrated using room-temperature phosphorescence of organic dyes in boric acid glasses, a multi-emissive boron-polymer dye, and a europium chelate.

The polymeric metal complex iron tris(dibenzoylmethane-polylactide), Fe(dbmPLA)3, was synthesized from d,l-lactide and Fe(dbmOH)3, a hydroxyl functionalized metalloinitiator. Reactions are fast relative to those with the dbmOH ligand initiator even when no Sn(oct)2 or DMAP catalyst is added, indicating that iron serves as both a dbm protecting group and a catalyst for lactide ring-opening polymerization (ROP). Kinetics experiments show good molecular weight control to ∼60−70% conversion (e.g., PDIs < 1.1; no evidence of transesterification via 1H NMR spectroscopic analysis; continued chain growth when Fe(dbmPLA)3 is heated with additional lactide monomer). For comparison, Fe(bdk)3 (bdk = β-diketonate) complexes were tested as lactide ROP catalysts using dbmOH and benzyl alcohol as the initiators. The red-orange Fe(dbmPLA)3 polymers were analyzed by GPC, 1H NMR and UV-vis spectroscopy. Demetalation with dilute HCl yields dbmPLA macroligands for chelation to other metals. The polymerization mechanism using ...

In this study, nanoparticles based on difluoroboron dibenzoylmethane-poly(lactic acid) (BF(2)dbmPLA) are prepared. Polylactic acid or polylactide is a commonly used degradable polymer, while the boron dye possesses a large extinction coefficient, high emission quantum yield, two-photon absorption, and sensitivity to the surrounding environment. BF(2)dbmPLA exhibits molecular-weight-dependent emission properties and can be formulated as stable nanoparticles, suggesting that its unique optical properties may be useful in multiple contexts for probing intracellular environments. Here we show that BF(2)dbmPLA nanoparticles are internalized into cultured HeLa cells by endocytosis, and that within the cellular milieu, they retain their fluorescence properties. BF(2)dbmPLA nanoparticles are photostable, resisting laser-induced photobleaching under conditions that destroy the fluorescence of a common photostable probe, LysoTracker Blue. Their endocytosis is also lipid-raft-dependent, as evidenced by their significant colocalization with cholera toxin B subunit in membrane compartments after uptake and their sensitivity of uptake to methyl-beta-cyclodextrin. Additionally, BF(2)dbmPLA nanoparticle endocytosis utilizes microtubules and actin filaments. Internalized BF(2)dbmPLA nanoparticles do not accumulate in acidic late endosomes and lysosomes but within a perinuclear nonlysosomal compartment. These findings demonstrate the feasibility of using novel BF(2)dbmPLA nanoparticles exhibiting diverse emission properties for in situ, live cell imaging and suggest that their endogenous uptake occurs through a lipid-raft-dependent endocytosis mechanism.