Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications.

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

Morphology and Size of Bacterial Colonies Control Anoxic Microenvironment Formation in Porous Media.

Bacterial metabolisms using electron acceptors other than oxygen (e.g., methanogenesis and fermentation) largely contribute to element cycling and natural contaminant attenuation/mobilization, even in well-oxygenated porous environments, such as shallow aquifers. This paradox is commonly explained by the occurrence of small-scale anoxic microenvironments generated by the coupling of bacterial respiration and dissolved oxygen (O2) transport by pore water. Such microenvironments allow facultative anaerobic bacteria to proliferate in oxic environments. Microenvironment dynamics are still poorly understood due to the challenge of directly observing biomass and O2 distributions at the microscale within an opaque sediment matrix. To overcome these limitations, we integrated a microfluidic device with transparent O2 planar optical sensors to measure the temporal behavior of dissolved O2 concentrations and biomass distributions with time-lapse videomicroscopy. Our results reveal that bacterial colony morphology, which is highly variable in flowing porous systems, controls the formation of anoxic microenvironments. We rationalize our observations through a colony-scale Damköhler number comparing dissolved O2 diffusion and a bacterial O2 uptake rate. Our Damköhler number enables us to predict the pore space fraction occupied by anoxic microenvironments in our system for a given bacterial organization.

Vapochromism and Magnetochemical Switching of a Nickel(II) Paddlewheel Complex by Reversible NH3 Uptake and Release.

Reaction of [NiCl2 (PnH)4 ] (1) (PnH=6-tert-butyl-pyridazine-3-thione) with NiCl2 affords the binuclear paddlewheel (PW) complex [Ni2 (Pn)4 ] (2). Diamagnetic complex 2 is the first example of a PW complex capable of reversibly binding and releasing NH3 . The NH3 ligand in [Ni2 (Pn)4 (NH3 )] (2⋅NH3 ) enforces major spectroscopic and magnetic susceptibility changes, thus displaying vapochromic properties (λmax (2)=532 nm, λmax (2⋅NH3 )=518 nm) and magnetochemical switching (2: S=0; 2⋅NH3 : S=1). Upon repeated adsorption/desorption cycles of NH3 the PW core remains intact. Compound 2 can be embedded into thin polyurethane films (2P ) under retention of its sensing abilities. Therefore, 2 qualifies as reversible optical probe for ammonia. The magnetochemical switching of 2 and 2⋅NH3 was studied in detail by SQUID measurements showing that in 2⋅NH3 , solely the Ni atom coordinated the NH3 molecule is responsible for the paramagnetic behavior.

Everlasting rhodamine dyes and true deciding factors in their STED microscopy performance.

The authors took an independent and closer look at the family of red-emitting rhodamine dyes known for a decade due to their excellent performance in STED microscopy. After the family was further extended, the true grounds of this performance became clear. Small-molecule protective agents and/or auxiliary groups were attached at two different sites of the dye's scaffold. Thus, a rhodamine core, which is already quite photostable as it is, and an intramolecular stabilizer - a 4-nitrobenzyl or a 4-nitrobenzylthio group were combined to give potentially "everlasting dyes". The fluorescence quantum yields (Φf) and the fluorescence lifetimes (τ) of the modified dyes were thoroughly measured with comparison to those of the parent dyes. The correlation of their STED performance with photostability and fluorescence color stability under illumination in water were explored. Unexpectedly, the anaerobic GSDIM (GOC) buffer proved unhelpful with respect to STED performance. It was demonstrated that, even dyes with a Φf of only 14-17% allow STED imaging with a sufficient photon budget and good signal-to-noise ratio. For the dyes with photostabilizing groups (PSG) the Φf values are 4-5 times lower than in the reference dyes, and lifetimes τ are also strongly reduced. Noteworthy are very high fluorescence color stability and constant or even increasing fluorescence signal under photobleaching in bulk aqueous solutions, which suggests a sacrificing role of the 4-nitrobenzyl-containing moieties. Straightforward and improved recipes for "last-minute" modifications and preparations of "self-healing" red-emitting fluorescent tags are described.

Rhenium-Imido Corroles.

Metallocorroles involving 5d transition metals are currently of interest as near-IR phosphors and as photosensitizers for oxygen sensing and photodynamic therapy. Their syntheses, however, are often bedeviled by capricious and low-yielding protocols. Against this backdrop, we describe rhenium-imido corroles, a new class of 5d metallocorroles, synthesized simply and in respectable (∼30%) yields via the interaction of a free-base corrole, Re2(CO)10, K2CO3, and aniline in 1,2,4-trichlorobenzene at ∼190 °C in a sealed vial under strict anaerobic conditions. The generality of the method was shown by the synthesis of six derivatives, including those derived from meso-tris(pentafluorophenyl)corrole, H3[TPFPC], and five different meso-tris(p-X-phenyl)corroles, H3[TpXPC], where X = CF3, F, H, CH3, OCH3. Single-crystal X-ray structures obtained for two of the complexes, Re[TpFPC](NPh) and Re[TpCF3PC](NPh), revealed relatively unstrained equatorial Re-N distances of ∼2.00 Å, a ∼ 0.7-Å displacement of the Re from the mean plane of the corrole nitrogens, and an Re-Nimido distance of ∼1.72 Å. Details of the corrole skeletal bond distances, diamagnetic 1H NMR spectra, relatively substituent-independent Soret maxima, and electrochemical HOMO-LUMO gaps of ∼2.2 V all indicated an innocent corrole macrocycle. Surprisingly, unlike several other classes of 5d metallocorroles, the Re-imido complexes proved nonemissive in solution at room temperature and also failed to sensitize singlet oxygen formation, indicating rapid radiationless deactivation of the triplet state, presumably via the rapidly rotating axial phenyl group. By analogy with other metal-oxo and -imido corroles, we remain hopeful that the Re-imido group will prove amenable to further elaboration and thereby contribute to the development of a somewhat challenging area of coordination chemistry.

Fast Oxygen Scavenging of Macroporous Poly(Norbornadiene) Prepared by Ring-Opening Metathesis Polymerization.

Emulsion templated norbornadiene is cured via ring-opening metathesis polymerization yielding macroporous poly(norbornadiene) foams of 76% porosity exhibiting appealing stiffness combined with considerable ductility. The foams are readily oxidized in the presence of air at room temperature exhibiting an oxygen uptake capacity of more than 300 mg O2 g-1 foam. In closed volumes of air, a final oxygen level of a maximum of 0.0005 vol%, that is, 5 ppm(v) can be achieved after several hours at room temperature. The synergism of the porous morphology and the chemical nature of the polymer allows for the first example of an organic oxygen scavenger material with properties distinctly surpassing the state-of-the art in the field.

Use of an oxygen planar optode to assess the effect of high velocity microsprays on oxygen penetration in a human dental biofilms in-vitro.

BACKGROUND: Dental plaque biofilms are the causative agents of caries, gingivitis and periodontitis. Both mechanical and chemical strategies are used in routine oral hygiene strategies to reduce plaque build-up. If allowed to mature biofilms can create anoxic microenvironments leading to communities which harbor pathogenic Gram-negative anaerobes. When subjected to high velocity fluid jets and sprays biofilms can be fluidized which disrupts the biofilm structure and allows the more efficient delivery of antimicrobial agents. METHODS: To investigate how such jets may disrupt anoxic niches in the biofilm, we used planar optodes to measure the dissolved oxygen (DO) concentration at the base of in-vitro biofilms grown from human saliva and dental plaque. These biofilms were subject to "shooting" treatments with a commercial high velocity microspray (HVM) device. RESULTS: HVM treatment resulted in removal of much of the biofilm and a concurrent rapid shift from anoxic to oxic conditions at the base of the surrounding biofilm. We also assessed the impact of HVM treatment on the microbial community by tracking 7 target species by qPCR. There was a general reduction in copy numbers of the universal 16S RNA by approximately 95%, and changes of individual species in the target region ranged from approximately 1 to 4 log reductions. CONCLUSION: We concluded that high velocity microsprays removed a sufficient amount of biofilm to disrupt the anoxic region at the biofilm-surface interface.

Correction to: Use of an oxygen planar optode to assess the effect of high velocity microsprays on oxygen penetration in a human dental biofilms in-vitro.

An amendment to this paper has been published and can be accessed via the original article.

Background-Free Fluorescence-Decay-Time Sensing and Imaging of pH with Highly Photostable Diazaoxotriangulenium Dyes.

Novel fluorescent diazaoxatriangulenium (DAOTA) pH indicators for lifetime-based self-referenced pH sensing are reported. The DAOTA dyes were decorated with phenolic-receptor groups inducing fluorescence quenching via a photoinduced-electron-transfer mechanism. Electron-withdrawing chlorine substituents ensure response in the most relevant pH range (apparent p Ka' values of ∼5 and 7.5 for the p, p-dichlorophenol- and p-chlorophenol-substituted dyes, respectively). The dyes feature long fluorescence lifetimes (17-20 ns), high quantum yields (∼60%), and high photostabilities. Planar optodes are prepared upon immobilization of the dyes into polyurethane hydrogel D4. Apart from the response in the fluorescence intensity, the optodes show pH-dependent lifetime behavior, which makes them suitable for studying 2D pH distributions with the help of fluorescence-lifetime-imaging techniques. The lifetime response is particularly pronounced for the sensors with high dye concentrations (0.5-1 wt % with respect to the polymer) and is attributed to the efficient homo-FRET mechanism.

Luminescence Lifetime Imaging of Chemical Sensors-A Comparison between Time-Domain and Frequency-Domain Based Camera Systems.

Luminescence lifetime based imaging is still the most reliable method for generating chemical images using chemical sensor technology. However, only few commercial systems are available that enable imaging lifetimes within the relevant nanosecond to microsecond range. In this technical note we compare the performance of an older time-domain (TD) based camera system with a frequency-domain (FD) based camera system regarding their measuring characteristics and applicability for O2 and pH imaging in environmental samples and with different indicator dye systems emitting in the visible and near-infrared part of the spectrum. We conclude that the newly introduced FD imaging system delivers comparable if not better results than its predecessor, now enabling robust and simple chemical imaging based on FD luminescence lifetime measurements.

Ambient-temperature near-IR phosphorescence and potential applications of rhenium-oxo corroles.

Presented herein is a first photophysical investigation of a series of rhenium(v)-oxo tris(p-X-phenyl)corroles, Re[TpXPC](O), where X = CF3, F, H, CH3, and OCH3. The complexes all exhibit near-IR phosphorescence in anoxic toluene at room temperature, with emission maxima at 780 ± 10 nm, phosphorescence decay times of 56-74 µs and quantum yields of 1.07-1.52%, the highest reported to date for phosphorescent corrole derivatives. We have also demonstrated that the dyes may serve as indicators in optical oxygen sensors and as sensitizers in triplet-triplet-annihilation-based upconversion. Although the ReO corroles are not superior to benchmark benzoporphyrin complexes that are currently used in these applications, they may prove useful in certain areas such as optical multianalyte sensing owing to improved flexibility in the available spectral properties of the dyes. The high thermal and photochemical stability of the complexes also bodes well for their deployment as new, phosphorescent sensitizers in photodynamic therapy.

Near-infrared fluorescent aza-BODIPY dyes for sensing and imaging of pH from the neutral to highly alkaline range.

New aza-BODIPY pH indicators with spectral properties modulated solely by photoinduced electron transfer (PET) are presented. The pH sensitive hydroxyl group is located in the meta-position of a phenyl substituent with respect to the aza-BODIPY core, which eliminates the conjugation to the chromophore. The new dyes show reversible "on"-"off" fluorescence response upon deprotonation of the receptor but no changes in the absorption spectrum, which is in contrast to state-of-the-art indicators of the aza-BODIPY family. This eliminates potential changes in the efficiency of the inner filter effect and Förster resonance energy transfer (FRET) and makes the new dyes suitable acceptors in light harvesting systems used for ratiometric pH imaging. The introduction of electron-withdrawing or electron-donating groups into the receptor results in a set of indicators suitable for measurements from physiological (pH 7) to very alkaline (pH 13) conditions. The new sensors are particularly promising for monitoring of pH changes in concrete, as was recently shown elsewhere.

DABCOnium: An Efficient and High-Voltage Stable Singlet Oxygen Quencher for Metal-O2 Cells.

Singlet oxygen (1 O2 ) causes a major fraction of the parasitic chemistry during the cycling of non-aqueous alkali metal-O2 batteries and also contributes to interfacial reactivity of transition-metal oxide intercalation compounds. We introduce DABCOnium, the mono alkylated form of 1,4-diazabicyclo[2.2.2]octane (DABCO), as an efficient 1 O2 quencher with an unusually high oxidative stability of ca. 4.2 V vs. Li/Li+ . Previous quenchers are strongly Lewis basic amines with too low oxidative stability. DABCOnium is an ionic liquid, non-volatile, highly soluble in the electrolyte, stable against superoxide and peroxide, and compatible with lithium metal. The electrochemical stability covers the required range for metal-O2 batteries and greatly reduces 1 O2 related parasitic chemistry as demonstrated for the Li-O2 cell.

NIR Phosphorescent Intramolecularly Bridged Benzoporphyrins and Their Application in Oxygen-Compensated Glucose Optode.

A glucose optode measuring the internal oxygen gradient is presented. The multilayer biosensor is composed of (i) analyte-impermeable transparent support, (ii) first oxygen-sensing layer combined with an enzymatic layer, (iii) diffusion barrier, and (iv) second oxygen-sensing layer. To make this design suitable for measurement in subcutaneous tissue, a pair of NIR phosphorescent indicators with very different spectral properties is chosen. Combination of a conventional Pt(II) tetrabenzoporphyrin dye (absorption and emission maxima at 617 and 772 nm, respectively) used in the first layer and a new intramolecularly bridged Pt(II) complex (absorption and emission maxima at 673 and 872 nm, respectively) in the second layer enables efficient separation of both emission signals. This specially designed dye class is accessible via Scholl-reaction from tetraphenyltetrabenzoporphyrin complexes. For the first time, the new optode allows simultaneous glucose and oxygen measurement in a single spot and therefore accurate compensation of oxygen heterogeneities resulting from fluctuations in the tissue. The presented material covers the dynamic ranges from 0 to 150 hPa O2 and from 0 to 360 mg/dL (20 mM) glucose (at 37 °C).

Nanoparticle-based fluoroionophore for analysis of potassium ion dynamics in 3D tissue models and in vivo.

The imaging of real-time fluxes of K+ ions in live cell with high dynamic range (5-150 mM) is of paramount importance for neuroscience and physiology of the gastrointestinal tract, kidney and other tissues. In particular, the research on high-performance deep-red fluorescent nanoparticle-based biosensors is highly anticipated. We found that BODIPY-based FI3 K+-sensitive fluoroionophore encapsulated in cationic polymer RL100 nanoparticles displays unusually strong efficiency in staining of broad spectrum of cell models, such as primary neurons and intestinal organoids. Using comparison of brightness, photostability and fluorescence lifetime imaging microscopy (FLIM) we confirmed that FI3 nanoparticles display distinctively superior intracellular staining compared to the free dye. We evaluated FI3 nanoparticles in real-time live cell imaging and found that it is highly useful for monitoring intra- and extracellular K+ dynamics in cultured neurons. Proof-of-concept in vivo brain imaging confirmed applicability of the biosensor for visualization of epileptic seizures. Collectively, this data makes fluoroionophore FI3 a versatile cross-platform fluorescent biosensor, broadly compatible with diverse experimental models and that crown ether-based polymer nanoparticles can provide a new venue for design of efficient fluorescent probes.

Ultrabright Red-Emitting Photostable Perylene Bisimide Dyes: New Indicators for Ratiometric Sensing of High pH or Carbon Dioxide.

New pH-sensitive perylene bisimide indicator dyes were synthesized and used for fabrication of optical sensors. The highly photostable dyes show absorption/emission bands in the red/near-infrared (NIR) region of the electromagnetic spectrum, high molar absorption coefficients (up to 100 000 m-1 cm-1 ), and fluorescence quantum yields close to unity. The absorption and emission spectra show strong bathochromic shifts upon deprotonation of the imidazole nitrogen atom, which makes the dyes promising as ratiometric fluorescent indicators. Physical entrapment of the indicators into a polyurethane hydrogel enables pH determination at alkaline pH values. It is also shown that a plastic carbon dioxide solid-state sensor can be manufactured by immobilization of the pH indicator in a hydrophilic polymer, along with a quaternary ammonium base. The influences of the plasticizer, different lipophilic bases, and humidity on the sensitivity of the sensor material are systematically investigated. The disubstituted perylene, particularly, features two deprotonation equilibria, enabling sensing over a very broad pCO2 range of 0.5 to 1000 hPa.

Mn4+-Doped Magnesium Titanate-A Promising Phosphor for Self-Referenced Optical Temperature Sensing.

Phosphors based on magnesium titanate activated with Mn 4 + ions are of great interest because, when excited with blue light, they display a strong red-emitting luminescence. They are characterized by a luminescence decay which is strongly temperature dependent in the range from -50 ∘ C to 150 ∘ C, making these materials very promising for temperature sensing in the biochemical field. In this work, the optical and thermal properties of the luminescence of Mg 2 TiO 4 are investigated for different Mn 4 + doping concentrations. The potential of this material for temperature sensing is demonstrated by fabricating a fiber optic temperature microsensor and by comparing its performance against a standard resistance thermometer. The response of the fiber optic sensor is exceptionally fast, with a response time below 1 s in the liquid phase and below 1.1 s in the gas phase.

Trace Ammonia Sensors Based on Fluorescent Near-Infrared-Emitting aza-BODIPY Dyes.

Highly sensitive ammonia sensors for environmental monitoring are presented. The sensing materials are based on fluorescent BF2-chelated tetraarylazadipyrromethene dyes (aza-BODIPYs) dyes physically entrapped in polyurethane hydrogels and dispersed in silicone rubber. This layer is covered by a hydrophobic porous Teflon membrane used as an additional proton barrier and light scattering layer. The dual-lifetime referenced (DLR) sensors make use of near-infrared (NIR)-emitting Egyptian blue as a reference material and in combination with optical fibers are read-out via a compact phase-fluorometer. The detectable concentration range can be tuned by the choice of aza-BODIPY dye or/and the hydrogel matrix. The most sensitive sensor has a limit of detection (LOD) of 0.11 µg/L and the upper detectable concentration of 300 µg/L. No cross-sensitivity toward pH is observed. The sensors show remarkable operational stability with no noticeable drift over a period of 2 weeks.

Sodium-Selective Fluoroionophore-Based Optodes for Seawater Salinity Measurement.

A new fluorescent sensor for Na+ is presented. The sensor relies on a Na+ selective fluoroionophore based on a bright red-emitting BODIPY chromophore. The fluorescence of the fluoroionophore is enhanced upon binding of Na+-ions to the highly selective aza-crown ether receptor due to reduction of the photoinduced electron transfer (PET) quenching. Solid state sensing materials were prepared by physically embedding the fluoroionophore into water-swellable biocompatible polymer matrices (polyurethane hydrogels), thus enabling continuous measurements of aqueous samples. Despite the simple design, the sensor showed no leaching of the indicator and featured fast and reversible response. Among different polyurethane hydrogels investigated, the hydrogel D1 featuring the highest water uptake was found to be the most suitable due to the highest dynamics between "off" and "on" states. Due to little or no cross sensitivity to other ions (e.g., Mg2+, Ca2+, K+) and its insensitivity to potential changes in pH, this sensor is promising for use in clinical diagnostics and for biological and marine applications. Fiber-optic sensors based on referenced read-out with a compact phase fluorimeter were prepared. To demonstrate their practical applicability, the sensors were used to determine the salinity in the seawater and brackish water of the Baltic Sea.

Singlet Oxygen during Cycling of the Aprotic Sodium-O2 Battery.

Aprotic sodium-O2 batteries require the reversible formation/dissolution of sodium superoxide (NaO2 ) on cycling. Poor cycle life has been associated with parasitic chemistry caused by the reactivity of electrolyte and electrode with NaO2 , a strong nucleophile and base. Its reactivity can, however, not consistently explain the side reactions and irreversibility. Herein we show that singlet oxygen (1 O2 ) forms at all stages of cycling and that it is a main driver for parasitic chemistry. It was detected in- and ex-situ via a 1 O2 trap that selectively and rapidly forms a stable adduct with 1 O2 . The 1 O2 formation mechanism involves proton-mediated superoxide disproportionation on discharge, rest, and charge below ca. 3.3 V, and direct electrochemical 1 O2 evolution above ca. 3.3 V. Trace water, which is needed for high capacities also drives parasitic chemistry. Controlling the highly reactive singlet oxygen is thus crucial for achieving highly reversible cell operation.