Ratiometric Oxygen Sensor Research Articles

Ratiometric oxygen sensors of cyclometalated iridium(III) with enhanced quantum yields and variable dynamic ranges.

Four luminescent ratiometric oxygen sensors, pairing phosphorescent cyclometalated iridium with coumarin or BODIPY fluorophores, are presented here. These compounds realize three key improvements over our previous designs, namely higher phosphorescence quantum yields, the ability to access intermediate dynamic ranges better suited for typical atmospheric O2 levels, and the possibility of using visible excitation instead of UV. These ratiometric sensors are accessed via very simple, 1-step syntheses involving direct reaction of the chloro-bridged cyclometalated iridium dimer with the pyridyl-substituted fluorophore. They have phosphorescent quantum yields up to 29% with short to intermediate phosphoresence lifetimes ranging from 1.7 to 5.3 μs in three of the sensors, with the fourth having a long lifetime of 440 μs that is very responsive to oxygen. In one case, visible excitation of 430 nm is used to provide dual emission instead of UV excitation.

Ratiometric Oxygen Sensing with H-NOX Protein Conjugates.

Ratiometric sensors are self-referencing constructs that are functional in cells and tissues, and the read-out is independent of sensor concentration. One strategy for ratiometric sensing is to utilize two-color emission, where one component possesses analyte-dependent emission and the other is independent of analyte concentration, serving as an internal standard. In this way, the intensity ratio of the two components is a quantitative measure of the analyte. In this study, protein-based ratiometric oxygen sensors are prepared using the heme nitric oxide/oxygen-binding protein (H-NOX) from the thermophilic bacterium Caldanaerobacter subterraneus. The native heme cofactor is replaced with a Pd(II) or Pt(II) porphyrin as the oxygen-responsive phosphor. Mutagenesis is performed to incorporate a cysteine residue on the protein surface for thiol/maleimide coupling of the oxygen-insensitive dye, which serves as a Förster resonance energy transfer (FRET) donor for the porphyrin. While both Pd(II)- and Pt(II)-based sensors are responsive over biologically relevant ranges, the Pd sensor exhibits greater sensitivity at lower oxygen concentrations. Together, these sensors represent a new class of protein-based ratiometric oxygen sensors, and the modular platform allows the oxygen sensitivity to be tailored for a specific application. This proof-of-principle study has identified the key considerations and optimal methodologies to develop and subsequently refine protein-based ratiometric oxygen sensors.

Cyclometalated iridium-coumarin ratiometric oxygen sensors: improved signal resolution and tunable dynamic ranges.

In this work we introduce a new series of ratiometric oxygen sensors based on phosphorescent cyclometalated iridium centers partnered with organic coumarin fluorophores. Three different cyclometalating ligands and two different pyridyl-containing coumarin types were used to prepare six target complexes with tunable excited-state energies. Three of the complexes display dual emission, with fluorescence arising from the coumarin ligand, and phosphorescence from either the cyclometalated iridium center or the coumarin. These dual-emitting complexes function as ratiometric oxygen sensors, with the phosphorescence quenched under O2 while fluorescence is unaffected. The use of blue-fluorescent coumarins results in good signal resolution between fluorescence and phosphorescence. Moreover, the sensitivity and dynamic range, measured with Stern–Volmer analysis, can be tuned two orders of magnitude by virtue of our ability to synthetically control the triplet excited-state ordering. The complex with cyclometalated iridium 3MLCT phosphorescence operates under hyperoxic conditions, whereas the two complexes with coumarin-centered phosphorescence are sensitive to very low levels of O2 and function as hypoxic sensors.

Ratiometric Sensor Based on PtOEP-C6/Poly (St-TFEMA) Film for Automatic Dissolved Oxygen Content Detection.

A ratiometric oxygen sensor based on a platinum octaethylporphyrin (PtOEP)–coumarin 6 (C6)/poly (styrene-trifluoroethyl methacrylate) (poly (St-TFEMA)) film was developed for automatic dissolved oxygen (DO) detection. The oxygen-sensing film according to the dynamic quenching mechanism was prepared by embedding platinum octaethylporphyrin (PtOEP) and coumarin 6 (C6) in poly (styrene-trifluoroethyl methacrylate) (poly (St-TFEMA)). The optical parameter (OP) was defined as the ratio of the oxygen-insensitive fluorescence from C6 to the oxygen-sensitive phosphorescence from PtOEP. A calibration equation expressing the correlation between the OP values and DO content described by a linear function was obtained. A program based on the Labview software was developed for monitoring the real-time DO content automatically. The influence of the excitation intensity and fluctuation on the OP values and the direct luminescence signal (integration areas) was compared, verifying the strong anti-interference ability of the sensor. The detection limit of the sensor was determined to be 0.10 (1) mg/L. The switching response time and recovery time of the sensor were 0.4 and 1.3 s, respectively. Finally, the oxygen sensor was applied to the investigation of the kinetic process of the DO content variation, which revealed an exponential relationship with time.

Stabilization of a gadolinium(III)-porphyrin in aqueous solution for oxygen sensing

Gd(III)-porphyrins are promising phosphorescent materials that are widely available and inexpensive compared to the most widely used phosphorescent materials (e.g., Pt(II)- and Pd(II)-porphyrins). However, Gd(III)-porphyrins are generally unstable in aqueous solutions, which limits their application in the biomedical field. Here, we report a method for stabilizing gadolinium-hematoporphyrin monomethyl ether (Gd-HMME), a Gd(III)-porphyrin, in aqueous solutions using the tripodal (cyclopentadienyl)tris(dimethylphosphito)cobaltate(I) anion, (LOMe–). When not complexed with LOMe–, the luminescence and UV–visible spectra of Gd-HMME in an aqueous solution show the luminescence and absorption characteristics of HMME after several hours of storage. However, after binding with LOMe–, the luminescence and absorption spectra of Gd-HMME-LOMe show no obvious change even after several days of storage in the aqueous solution. Gd-HMME-LOMe can be stored in aqueous solutions for longer than one month, which is sufficient for practical use. The phosphorescence intensity of Gd-HMME-LOMe decreased with increasing oxygen concentration, while its fluorescence intensity remained unchanged, indicating that Gd-HMME-LOMe can be used as a ratiometric oxygen sensor. In the investigated oxygen concentration range, the Stern–Volmer plot exhibits an appropriate linear, with a response factor greater than 10, which is sufficiently high for practical oxygen sensing. Thus, a Gd-porphyrin complex with ratiometric oxygen-sensing ability, which is stable in aqueous solution, was prepared in this study.

White-Light Emission and Tunable Luminescence Colors of Polyimide Copolymers Based on FRET and Room-Temperature Phosphorescence.

Thermally stable copolyimide (CoPI) films exhibiting high optical transparency and room-temperature phosphorescence (RTP) were prepared by copolymerizing fluorescent dianhydride and brominated phosphorescent dianhydride with an alicyclic diamine. The CoPI films underwent a 5 wt % degradation at a temperature higher than 349 °C and exhibited dual fluorescent and phosphorescent emissions owing to their efficient Förster resonance energy transfer from the fluorescent to phosphorescent dianhydride moieties in the main chains, followed by an intersystem crossing from the singlet to triplet state of the latter moiety atoms. The CoPIs displayed bright RTP under a vacuum with various colors produced when adjusting the copolymerization ratio. CoPI with 5 mol % phosphorescent moiety (CoPI-05) emitted white light with high optical transparency owing to the suppression of the PI chain aggregation that causes a yellowish coloration. The copolymerization of fluorescent and phosphorescent PI moieties can control the photoluminescent properties of PI films and is applicable to color-tunable solid-state emitters, ratiometric oxygen sensors, and solar-spectrum converters.

Color-Tunable Thermally Activated Delayed Fluorescence in Oxadiazole-Based Acrylic Copolymers: Photophysical Properties and Applications in Ratiometric Oxygen Sensing.

Polymer-based emitters are a promising route to the production of low-cost, scalable solution-processable luminescent materials. Here we describe a series of acrylic oxadiazole-based donor-acceptor monomers with tunable emission from blue to orange, with quantum yields as high as 96%. By introducing structural constraints that limit donor-acceptor orbital overlap, thermally activated delayed fluorescence (TADF) was observed in these materials. Polymerization by Cu(0) reversible deactivation radical polymerization (RDRP) gave high-molecular-weight copolymers (Mn > 20 kDa) with dispersities ranging from 1.10 to 1.45, using a room-temperature procedure with Cu wire as a catalyst. One of these materials, which had phenothiazine as donor moiety, exhibited conformationally dependent dual emission, giving a mixture of prompt fluorescence and delayed fluorescence peaks, whose relative ratios varied based on the amount of O2 present during measurement. We demonstrate that this material can combine prompt and delayed fluorescence to act as a single-component, all-organic, ratiometric oxygen sensor without external calibrant. Application to ratiometric oxygen sensing is demonstrated both using a polymer thin film and via incorporation of this material into water-soluble polymer dots (Pdots), with a ratiometric response to O2 throughout the range of partial pressures relevant to biological environments.

An amine linker group modulates luminescent properties in a Rhenium(I) tricarbonyl complex. How can it be applied for ratiometric oxygen sensing?

The UV–Vis absorption spectrum of the rhenium(I) tricarbonyl complex [(NH2-phen)Re(CO)3Br] (ReNN-NH2Br, a deep-orange solid) in solution shows three bands located at 300, 370 and 445 nm. TD-DFT calculations and spectroscopic data indicate that the higher energy band corresponds to a ligand centered transition (LC), while the two lower energy bands have been ascribed to have major ILCT and MLCT character. Excitation at 370 nm leads to two emission bands centered at around 490 nm and 590 nm, which have different nature, 1ILCT and 3MLCT, respectively. Both show a low luminescent quantum yield, especially the lowest energy band. The emission quantum yields in argon-saturated solutions are largely increased, specially for the band at 590 nm. Our results demonstrate that adding the amino group has a non-innocent effect over the luminescent properties of the complex when is compared with those of [(phen)Re(CO)3Br] (ReNNBr). The capacity of these complexes to act as singlet oxygen sensitizers and their application as oxygen sensors were explored. Both complexes, ReNNBr and ReNN-NH2Br, were incorporated in silsesquioxane matrices and tested as ratiometric oxygen sensors using the intrinsic emission of the matrix as an emission internal reference signal. The SSO1-ReNN-NH2Br films was the most sensitive material for this application.

Benzo[ghi]perylene & coronene as ratiometric reversible optical oxygen nano-sensors

The development of novel single molecular probe based optical ratiometric oxygen sensor with simple fabrication, high sensitivity, and linear response is highly desirable. Herein, benzo[ghi]perylene (BzP) and coronene (Cron) were explored as dual-emissive ratiometric oxygen nano-sensors that displayed linear plots at a low probe concentration (6 μM). The sensor fabrication was simple, and the probes exhibited efficient fluorescence quenching of monomer emission by molecular oxygen. However, the excimer emission of probe remained unaffected. Thus a ratiometric method is developed based on the dual emitting luminophore. The method can be used to estimate dissolved oxygen in liquids and 5–100% of oxygen concentration in gaseous mixtures.

Cyclometalated iridium-BODIPY ratiometric O2 sensors.

Here we introduce a new class of ratiometric O2 sensors for hypoxic environments. Two-component structures composed of phosphorescent cyclometalated Ir(iii) complexes and the well-known organic fluorophore BODIPY have been prepared by the 1 : 1 reaction of bis-cyclometalated iridium synthons with pyridyl-substituted BODIPY compounds. Two different cyclometalating ligands are used, which determine the relative energies of the iridium-centered and BODIPY-centered excited states, and the nature of the linker between iridium and BODIPY also has a small influence on the photoluminescence. Some of the conjugates exhibit dual emission, with significant phosphorescence from the iridium site and fluorescence from the BODIPY, and thus function as ratiometric oxygen sensors. Oxygen quenching experiments demonstrate that as O2 is added the phosphorescence is quenched while the fluorescence is unaffected, with dynamic ranges that are well suited for hypoxic sensing (pO2 < 160 mmHg).

Highly efficient ratiometric extracellular oxygen sensors through physical incorporation of a conjugated polymer and PtTFPP in graft copolymers

An amphiphilic graft copolymer (P3) composed of poly(oligo (ethylene glycol) methacrylate) (POEGMA) as a hydrophilic segment and a polystyrene (PS) as a hydrophobic chain was prepared. P3 was used to incorporate a highly efficient hydrophobic oxygen probe platinum(II)-5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)-porphyrin (PtTFPP) in its micelles to enable the application of PtTFPP in aqueous solution for biosensing. For achieving ratiometric oxygen sensing, a hydrophobic conjugated polymer (CP) was used as a fluorescence resonance energy transfer (FRET) donor for PtTFPP - the FRET acceptor. Results showed that diameters of these nano-oxygen sensors ranging from 50 to 60 nm and FRET efficiency as high as 98% could be achieved. Further, FRET contributed to the brightness of PtTFPP in micelles. High quantum yields of PtTFPP in non-FRET micelles and FRET micelles under nitrogen were achieved to be 0.109 and 0.231, respectively. The sensors were found to be able to measure oxygen concentrations from 0.082 to 40.9 mg/L at room temperature with linear Stern-Volmer constants. These sensors were also demonstrated to be suitable for monitoring extracellular oxygen consumption of E. Coli and HeLa cells. This study illustrated a feasible approach to achieve highly efficient ratiometric oxygen sensors without specific structure modification of the efficient PtTFPP probe, however using an amphiphilic graft copolymer approach.

Preparation and application of ratiometric polystyrene-based microspheres as oxygen sensors

Mono-dispersed polystyrene-based microspheres with diameters about 1 μm encapsulating rhodamine moieties as oxygen insensitive internal reference probes and platinum octaethylporphyrin units as oxygen sensitive probes were synthesized as new ratiometric oxygen sensors (Rhod-PtOEP-PS). The dual luminophors of rhodamines and platinum porphyrin moieties exhibited emissions maxima at 585 nm and 644 nm, respectively. The microspheres showed good oxygen sensing properties in different oxygen partial pressures (pO2) and dissolved oxygen (DO) concentrations. It was found the oxygen probes and reference probes in microspheres showed higher photo-stability than their corresponding free fluorophores in solution. The microspheres also showed good sensitivity for air-pressure and cellular oxygen in cell culture medium. These microspheres were used to detect DOs in a few kinds of liquids including some daily drinks and it was found the measured errors were within positive/negative 11% as compared with the measured results using traditional oxygen electrodes.

Platinum(II) Ring-Fused Chlorins as Near-Infrared Emitting Oxygen Sensors and Photodynamic Agents.

Novel near-infrared luminescent compounds based on platinum(II) 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused chlorins are described. These compounds have high photostability and display light emission, in particular simultaneous fluorescence and phosphorescence emission in solution at room temperature, in the biologically relevant 700-850 nm red and near-infrared (NIR) spectral region, making them excellent materials for biological imaging. The simultaneous presence of fluorescence and phosphorescence emission at room temperature, with the phosphorescence strongly quenched by oxygen whereas fluorescence remains unaffected, allows these compounds to be used as ratiometric oxygen sensors in chemical and biological media. Both steady-state (fluorescence vs phosphorescence intensities) and dynamic (dependence of phosphorescence lifetimes upon oxygen concentration) luminescence approaches can be used. Photocytotoxicity studies against human melanocytic melanoma cells (A375) indicate that these compounds display potential as photosensitizers in photodynamic therapy.

Nanoscale LnMOF-functionalized nonwoven fibers protected by a polydimethylsiloxane coating layer as a highly sensitive ratiometric oxygen sensor

Nanoscale SUMOF-6-Eu functionalized nonwoven polypropylene films protected by a polydimethylsiloxane (PDMS) coating layer have been fabricated and used for ratiometric oxygen sensing.

Designing a Microfluidic Device with Integrated Ratiometric Oxygen Sensors for the Long-Term Control and Monitoring of Chronic and Cyclic Hypoxia.

Control of oxygen over cell cultures in vitro is a topic of considerable interest, as chronic and cyclic hypoxia can alter cell behaviour. Both static and transient hypoxic levels have been found to affect tumour cell behaviour; it is potentially valuable to include these effects in early, in vitro stages of drug screening. A barrier to their inclusion is that rates of transient hypoxia can be a few cycles/hour, which is difficult to reproduce in traditional in vitro cell culture environments due to long diffusion distances from control gases to the cells. We use a gas-permeable three-layer microfluidic device to achieve spatial and temporal oxygen control with biologically-relevant switching times. We measure the oxygen profiles with integrated, ratiometric optical oxygen sensors, demonstrate sensor and system stability over multi-day experiments, and characterize a pre-bleaching process to improve sensor stability. We show, with both finite-element modelling and experimental data, excellent control over the oxygen levels by the device, independent of fluid flow rate and oxygenation for the operating flow regime. We measure equilibration times of approximately 10 min, generate complex, time-varying oxygen profiles, and study the effects of oxygenated media flow rates on the measured oxygen levels. This device could form a useful tool for future long-term studies of cell behaviour under hypoxia.

Luminescence ratiometric oxygen sensor based on gadolinium labeled porphyrin and filter paper

A fluorescence–phosphorescence ratiometric oxygen sensing system based on gadolinium labeled hematoporphyrin monomethyl ether (Gd-HMME) and filter paper was developed. A compact and simple measurement system was constructed utilizing fiber optoelectronic sensing device. Optical parameter (OP) was defined as the ratio of filter paper background fluorescence to oxygen sensitive Gd-HMME phosphorescence. We demonstrate that the OP is independent of excitation intensity in this ratiometric method. The measured results of oxygen concentration were calibrated and displayed by a Labview-based software. The fluctuation is about 0.1% for a wide range of oxygen concentration above 10% and much less than 0.1% for oxygen concentrations lower than 3%. A detection limit of 0.01% was achieved. The response time was determined to be about 4s. These results indicate that the system we have developed can be used for oxygen detection in the whole range and is especially accurate for very low oxygen concentrations.

Four new lead(ii)–iridium(iii) heterobimetallic coordination frameworks: synthesis, structures, luminescence and oxygen-sensing properties

Crystalline lead(ii)–iridium(iii) heterobimetallic coordination frameworks were prepared and characterized, and one was mixed with C480 to form a ratiometric oxygen sensor for the determination of oxygen in real gas.

Virtual issue: molecular sensors.

Smart designs in chemosensing! The articles of this virtual issue focus on molecular sensors, also known as chemosensors or probes, and highlight current research in the area of chemosensing and how wise designs can lead to selective sensing of target analytes. Articles show diverse approaches in probe design based on different sensing principles and the detection of a wide range of analytes. A molecular sensor, also known as a chemosensor or probe, is a molecular or supramolecular-based system that is able to transform probe–analyte interactions into a signal which allows analyte sensing. The most common output signals are usually optical or electrochemical changes. In particular, optical responses have become very popular due to the possibility of using low-cost, widely available instrumentation and even the possibility of detecting target species with the naked eye. The design of these molecular sensors and probes has advanced and gained importance in the last years due to the progress made in the knowledge on the preparation of host–guest complexes and on studies on how this interaction can be used for the design of selective molecular probes. This is a very interdisciplinary filed that in many cases involves organic or inorganic synthesis, photo-physical studies, and analytical applications. In this context, the design of new molecular sensors to detect anions, cations, and neutral species has gained primary significance recently due to the importance to detect target analytes in environmental and biological samples. Molecular sensors for metal cations were developed more than two decades ago, whereas probes for anions and neutral species have been investigated more recently. Articles already accepted show diverse approaches for the design of chemical probes that are based on different sensing principles and display sensing features to a wide range of analytes such as anions, cations, neutral molecules, and gases. Molina et al. (DOI: 10.1002/open.201402022) prepared aryl or heteroaryl 5-substituted imidazo-benzothiadiazole derivatives and found an interesting selective dual-channel sensing of mercury(II) cations and acetate anions. Moreover the probes also display sensing features in the presence of nitroaromatic compounds with a selective response to the explosive picric acid. In a close research approach to the design of sensing probes for target chemical substances that could be used as weapons, Martínez-Máñez, Costero, and co-workers (DOI: 10.1002/open.201402014) designed a molecular sensor for the selective chromogenic recognition of diisopropylfluorophosphate (DFP), a Sarin and Soman mimic, in 99:1 (v/v) water/ acetonitrile and in the gas phase. Anion sensing probes were studied in the works of Ito et al. and Yam and co-workers. Ito and Shinoda (DOI: 10.1002/open.201402049) prepared macrotricyclic octadentate cyclen dimer ligands that were able to afford cage-shaped dinuclear Na+ complexes. The authors found an interesting selectivity for dicarboxylates of different length. For instance, N-Boc-Asp dianion was strongly coordinated inside the cavity of the receptor whereas N-Boc-Glu dianion, which was one carbon longer, did not. On the other hand Yam et al. (DOI: 10.1002/open.201402019) designed probes using alkynylplatinum(II) terpyridine complexes with an amide-based receptor moiety and studied their sensing features in the presence of different spherical and non-spherical anions. As an example of molecular sensors for cations Lodeiro and co-workers (DOI: 10.1002/open.201402020) prepared a probe based in a bipyridyl unit that was covalently anchored to a rhodamine B fluorophore and studied the interaction of this molecular sensor with Cu2+, Zn2+, Cd2+, Hg+, and Hg2+. Different sensing responses were found to mimic certain logic operations. Finally, two works from Liz-Marzán and Pérez-Prieto described the use of inorganic supports for the design of probes. Liz-Marzán and co-workers (DOI: 10.1002/open.201402009) developed a simple procedure to prepare three-dimensional surface-enhanced Raman scattering (SERS) detection substrates and used these structures for the detection of malachite green in water and of 1-naphthalenethiol from the gas phase. Moreover Pérez-Prieto and co-workers (DOI: 10.1002/open.201402021) used CdSe/ZnS nanoparticles that were decorated with pyrene units as a sensing platform. The authors showed that the probe could be applied as a ratiometric oxygen sensor in organic solvents. In summary, this virtual issue in ChemistryOpen on “Molecular Sensors” includes nice examples in this highly interdisciplinary research field. Overall, the contributions beautifully illustrate current research in the area of chemosensing and how wise designs can lead to selective sensing of target analytes. Prof. Ramón Martínez-Máñez Guest Editor Co-chair of the Editorial Advisory Board ChemistryOpen

Pyrene-Capped CdSe@ZnS Nanoparticles as Sensitive Flexible Oxygen Sensors in Non-Aqueous Media.

A flexible, highly sensitive sensor of oxygen in non-aqueous solvents is described. It consists of CdSe/ZnS nanoparticles decorated with a considerable number of pyrene units, thus making the formation of the pyrene excimer possible. The emission of the pyrene excimer and that of the nanoparticle are suitably separated from each other and also from the excitation wavelength. This sensor can be applied as a ratiometric oxygen sensor by using the linear response of the pyrene excimer lifetime combined with the linear response of the nanoparticle excited state lifetime. This nanohybrid has been assayed in seven media with different dielectric constants and viscosities over the whole oxygen concentration range. In addition, the sensor versatility provides an easy way for monitoring oxygen diffusion through systems.

Luminescence and photosensitivity of gadolinium labeled hematoporphyrin monomethyl ether

Photodynamic therapy for deep-lying lesions needs an appropriate imaging modality, precise evaluation of tissue oxygen and an effective photosensitizer. Gadolinium based metalloporphyrins Gd(III)-HMME is proposed in this study as a potential multifunctional theranostic agent, as photosensitizer, ratiometric oxygen sensor and MRI contrast agent. The time resolved spectroscopy revealed the luminescence peak of Gd(III)-HMME at 710 and 779 nm with a lifetime of 64 μs in oxygen-free methanol to be phosphorescent. This phosphorescence is strongly dependent on dissolved oxygen concentration. Its intensity in oxygen saturated methanol solution is 21% of that in deoxygenated solution. The singlet oxygen quantum yields ΦΔ of HMME and Gd(III)-HMME in air saturated methanol solution were determined to be 0.79 and 0.40 respectively using comparative spectra method. These phenomena indicate that the oxygen sensibility and production of singlet oxygen of Gd(III)-HMME can fulfill the requirement of PDT treatment.