Quantifying the effects of anesthesia on intracellular oxygen via low-cost portable microscopy using dual-emissive nanoparticles

Intracellular oxygenation is an important parameter for numerous biological studies. While there are a variety of methods available for acquiring in vivo measurements of oxygenation in animal models, most are dependent on indirect oxygen measurements, restraints, or anesthetization. A portable microscope system using a Raspberry Pi computer and Pi Camera was developed for attaching to murine dorsal window chambers. Dual-emissive boron nanoparticles were used as an oxygen-sensing probe while mice were imaged in awake and anesthetized states. The portable microscope system avoids altered in vivo measurements due to anesthesia or restraints while enabling increased continual acquisition durations.

Labelling primary immune cells using bright blue fluorescent nanoparticles

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 (BF2dbm) 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.

Dual-emissive, oxygen-sensing boron nanoparticles quantify oxygen consumption rate in breast cancer cells

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.

Biosurfactant-Mediated Membrane Depolarization Maintains Viability during Oxygen Depletion in Bacillus subtilis

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.

Meta-Alkoxy-Substituted Difluoroboron Dibenzoylmethane Complexes as Environment-Sensitive Materials

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.

Multi-stimuli responsive luminescent azepane-substituted β-diketones and difluoroboron complexes

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 TFA vapor was applied, and reversible fluorescence switching was achieved using the acid/base pair.

Luminescent Difluoroboron β-Diketonate PLA-PEG Nanoparticle

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 postpolymerization 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.

Oxygen Sensing Difluoroboron β-Diketonate Polylactide Materials with Tunable Dynamic Ranges for Wound Imaging

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.

Blue Thermally Activated Delayed Fluorescence from a Biphenyl Difluoroboron β-Diketonate

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.

Difluoroboron β-Diketonate Materials with Long-Lived Phosphorescence Enable Lifetime Based Oxygen Imaging with a Portable Cost Effective Camera

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.

Thienyl Difluoroboron β-Diketonates in Solution and Polylactide Media

Difluoroboron β-diketonates have impressive optical properties in both solution and the solid state. In particular, both fluorescence and room-temperature phosphorescence are present when the dyes are confined to a rigid matrix, such as poly(lactic acid) (PLA). To expand current knowledge and color range capabilities of this unique type of multi-emitting chromophore, a series of thienyl-substituted BF₂bdk complexes have been synthesized. The photophysical properties were investigated in methylene chloride solution and in the solid state as dye/PLA blends. By varying donor ability, i.e. methyl, phenyl, methoxyl, and thienyl substituents, and by changing the dye loading in the PLA media (0.1-10% dye loading) red-shifted emission was achieved, important for biological imaging applications. In dilute CH₂Cl₂ solution, complexes exhibited absorptions ranging from 350 - 420 nm, solid-state fluorescence in PLA ranging from 390 - 500 nm, and oxygen sensitive phosphorescence ranging from 540 - 585 nm in PLA blends. Promising candidates as dye/PLA blends serve as models for dyepolymer conjugates for application as biological oxygen nanoprobes.

Tailoring Oxygen Sensitivity with Halide Substitution in Difluoroboron Dibenzoylmethane Polylactide Materials

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.

Oxygen Sensing Difluoroboron Dinaphthoylmethane Polylactide

Dual emissive luminescence properties of solid-state difluoroboron β-diketonate-poly(lactic acid) (BF2bdk-PLA) materials have been utilized as biological oxygen sensors. Dyes with red-shifted absorption and emission are important for multiplexing and in vivo imaging, thus hydroxyl-functionalized dinaphthoylmethane initiators and dye-PLA conjugates BF2dnm(X)PLA (X = H, Br, I) with extended conjugation were synthesized. The luminescent materials show red-shifted absorbance (~435 nm) and fluorescence tunability by molecular weight. Fluorescence colors range from yellow (~530 nm) in 10 - 12 kDa polymers to green (~490 nm) in 20 - 30 kDa polymers. Room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) are present under a nitrogen atmosphere. For the iodine-substituted derivative, BF2dnm(I)PLA, clearly distinguishable fluorescence (green) and phosphorescence (orange) peaks are present, making it ideal for ratiometric oxygen-sensing and imaging. Bromide and hydrogen analogues with weaker relative phosphorescence intensities and longer phosphorescence lifetimes can be used as highly sensitive, concentration independent, lifetime-based oxygen sensors or for gated emission detection. BF2dnm(I)PLA nanoparticles were taken up by T41 mouse mammary cells and successfully demonstrated differences in vitro ratiometric measurement of oxygen.

Dual-Emissive Difluoroboron Naphthyl-Phenyl β-Diketonate Polylactide Materials: Effects of Heavy Atom Placement and Polymer Molecular Weight

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.