New luminescent oxygen-sensing and temperature-sensing materials based on gadolinium(III) and europium(III) complexes embedded in an acridone–polystyrene conjugate

Terpyridyl-Imidazole Based Ligand Coordinated to Ln(Hexafluoroacetyl acetonate)3 Core: Synthesis, Structural Characterization, Luminescence Properties, and Thermosensing Behaviors in Solution and PMMA Film

A new array of ternary lanthanide complexes of the form [Ln(hfa)3(tpy-HImzphen)] have been synthesized and thoroughly characterized wherein Ln = LaIII (1), EuIII (2), SmIII (3), and TbIII (4); hfa = hexafluoroacetylacetonate; and tpy-HImzphen = 2-(4-[2,2':6',2″]terpyridin-4'-yl-phenyl)-1H-phenanthro[9,10-d]imidazole. Incorporation of tpy-HImzphen onto the Ln-hfa moiety induced a bathochromic shift of the absorption window of the complexes into the visible region. Extensive investigations of the luminescence characteristics have been conducted both at RT and at 77 K to understand the deactivation pathways of the complexes. Both steady-state and time-resolved emission spectral behaviors indicate four distinctive behaviors upon incorporation of tpy-HImzphen onto the lanthanide core, viz., a huge red-shift of the ligand-centered peak for LaIII; almost complete energy transfer for EuIII; very little energy transfer for SmIII, while reverse energy transfer in the case of TbIII. In addition, the EuIII-complex exhibits its excellence in luminescence thermometry in the solution state as well as in poly(methyl methacrylate) (PMMA) thin films. The thermosensitive luminescence response in solution was further utilized to mimic set-reset flip-flop logic operation. A plausible energy transfer scheme has been devised to explain dissimilar luminescence behaviors in the complexes. The role of LMCT was also considered for the observed thermosensing property of the Eu(III) complex.

Elucidating Bacterial Spore Dynamics through Lanthanide-Enhanced Live Imaging

Identifying and distinguishing dormant and active bacterial spores are vital for biosecurity, food safety, and space exploration. Yet, there is a lack of simple, quick, and nondestructive methods to achieve this. The common Schaeffer-Fulton method is both sample-destructive and requires significant operator involvement. In this study, we employed lanthanide-beta-diketonate complexes to directly observe both dormant and germinated single spores. Staining is instantaneous and requires minimal sample processing. The complex stains areas outside the core of dormant spores, leaving the core hollow and nonfluorescent. However, upon germination, the complex enters the core, making it brightly fluorescent. This difference was noted in five bacterial species including Bacillus, Clostridium, and Clostridioides. Various lanthanides and beta-diketonates can be mixed to form a range of spore-visualizing complexes. Due to their low toxicity, these complexes allow for live imaging of single germinating spores. We demonstrate low-cost imaging using a USB microscope as well as imaging of spores in milk matrices. This method provides a valuable tool for studying bacterial spores.

Composites Based on Polylactide Doped with Amorphous Europium(III) Complex as Perspective Thermosensitive Luminescent Materials

This work reports fabrication of polylactide (PLA) films doped with various additives of an amorphous Eu(III) complex. We study the temperature behavior of the luminescence intensity and lifetime of the PLA-Eu(III) composites in the range of 298–353 K and investigate the mechanism of luminescence temperature quenching. The peak relative sensitivity of the films reaches 20.1 %×K−1 and exceeds the respective characteristics of all known lanthanide-containing thermosensors designed for the range of physiological temperatures. The produced films can be potential novel materials for luminescent thermosensors.

Temperature-Sensitive Chameleon Luminescent Films Based on PMMA Doped with Europium(III) and Terbium(III) Anisometric Complexes

The spin-coating technique was used to produce composite films consisting of PMMA polymer doped with anisometric complexes of Eu(III) and Tb(III). It was found that an increase in the content of Tb3+ complexes intensifies emission of both ions due to the intermolecular energy transfer from the Tb(III) complex to the Eu(III) complex, which results in the increase in the relative luminescence quantum yield of Eu(III) ion by 36%. The temperature sensitivity of the film luminescence intensity and lifetime in the range of 296–363 K was investigated. The maximum relative sensitivity of the films reaches 5.44% × K−1 and exceeds that of all known lanthanide-containing thermal sensors designed for measuring physiological temperatures. In combination with changing luminescence color, such a sensitivity makes these films promising colorimetric thermal sensors for in situ temperature measurements.

The Microenvironment in Immobilized Enzymes: Methods of Characterization and Its Role in Determining Enzyme Performance

The liquid milieu in which enzymes operate when they are immobilized in solid materials can be quite different from the milieu in bulk solution. Important differences are in the substrate and product concentration but also in pH and ionic strength. The internal milieu for immobilized enzymes is affected by the chemical properties of the solid material and by the interplay of reaction and diffusion. Enzyme performance is influenced by the internal milieu in terms of catalytic rate (“activity”) and stability. Elucidation, through direct measurement of differences in the internal as compared to the bulk milieu is, therefore, fundamentally important in the mechanistic characterization of immobilized enzymes. The deepened understanding thus acquired is critical for the rational development of immobilized enzyme preparations with optimized properties. Herein we review approaches by opto-chemical sensing to determine the internal milieu of enzymes immobilized in porous particles. We describe analytical principles applied to immobilized enzymes and focus on the determination of pH and the O₂ concentration. We show measurements of pH and [O₂] with spatiotemporal resolution, using in operando analysis for immobilized preparations of industrially important enzymes. The effect of concentration gradients between solid particle and liquid bulk on enzyme performance is made evident and quantified. Besides its use in enzyme characterization, the method can be applied to the development of process control strategies.

Life sensors: current advances in oxygen sensing by lanthanide complexes

Oxygen is essential for life. Inspired by the importance of oxygen, we present this critical and current review on lanthanide-based oxygen sensors.

Fluorescence resonance energy transfer aptasensor between nanoceria and graphene quantum dots for the determination of ochratoxin A

In the present work, colloidal cerium oxide nanoparticles (nanoceria) and graphene quantum dots (GQDs) were firstly synthesized by sol-gel method and pyrolysis respectively, which all have a uniform nano-size and significant fluorescence emission. Due to the fluorescence emission spectrum of nanoceria overlapped the absorption spectrum of GQDs, fluorescence resonance energy transfer (FRET) between nanoceria and GQDs could occur effectively by the electrostatic interaction. Based on it, a sensitive ratiometric fluorescence aptasensor for the determination of ochratoxin A (OTA), a small molecular mycotoxin produced by Aspergillus and Penicillium strains, has been successfully constructed. In which, probe DNA1@nanoceria and DNA2@GQD were designed to complementary with OTA aptamer, both could adsorb each other, leading to the occur of FRET. After adding of OTA aptamer and then introducing of OTA, the FRET would be interrupted/recovered due to the specific affinity of OTA and its aptamer, the fluorescence recovery value would increase with the addition of OTA. Under the optimal experimental conditions (pH 7, m_{GQD/nanoceria} 2, c_aptamer 100 nM, incubation time 30 min), the constructed ratiometric fluorescence aptasensor exhibited a satisfying linear range (0.01-20 ng mL^-1), low limit of detection (2.5 pg mL^-1) and good selectivity towards OTA, and has been successfully applied for the analysis of real sample peanuts with good accuracy of the recoveries ranged from 90 to 110%.

Mixed-Lanthanide Porous Coordination Polymers Showing Range-Tunable Ratiometric Luminescence for O2 Sensing

Luminescent porous coordination polymers (PCPs) are emerging as attractive oxygen-sensing materials, but they are mostly based on single-wavelength luminometry. Here, we report a special mixed-lanthanide strategy for self-referenced ratiometric oxygen sensing. A series of isostructural, pure-lanthanide, or mixed-lanthanide PCPs, MCF-53(Tb/Eu_x), were synthesized by solvothermal reactions. Single-crystal X-ray diffraction revealed that MCF-53(Tb/Eu_x) is composed of complicated two-dimensional coordination networks, which interdigitate to form a three-dimensional supramolecular structure retaining one-dimensional ultra-micropores. MCF-53(Tb/Eu_x) can undergo multiple single-crystal to single-crystal structural transformations upon desorption/adsorption of coordinative and lattice guest molecules, and the lanthanide metal ions are partially exposed on the pore surface at the guest-free state. Tb(III) ions are not luminescent and only act as separators between Eu(III) ions, and the Tb(III)/Eu(III) mixing ratio can tune the relative emission intensities, luminescence lifetimes of the Eu(III) phosphorescence, and the ligand fluorescence, giving rise to not only ratiometric photoluminescence oxygen sensing but also tunable emission-color-changing ranges.

Embedded Ceria Nanoparticles in Crosslinked PVA Electrospun Nanofibers as Optical Sensors for Radicals

This work presents a new nanocomposite of cerium oxide (ceria) nanoparticles embedded in electrospun PVA nanofibers for optical sensing of radicals in solutions. Our ceria nanoparticles are synthesized to have O-vacancies which are the receptors for the radicals extracted from peroxide in water solution. Ceria nanoparticles are embedded insitu in PVA solution and then formed as nanofibers using an electrospinning technique. The formed nanocomposite emits visible fluorescent emissions under 430 nm excitation, due to the active ceria nanoparticles with fluorescent Ce(3+) ionization states. When the formed nanocomposite is in contact with peroxide solution, the fluorescence emission intensity peak has been found to be reduced with increasing concentration of peroxide or the corresponding radicals through a fluorescence quenching mechanism. The fluorescence intensity peak is found to be reduced to more than 30% of its original value at a peroxide weight concentration up to 27%. This work could be helpful in further applications of radicals sensing using a solid mat through biomedical and environmental monitoring applications.

Exceptional Oxygen Sensing Properties of New Blue Light-Excitable Highly Luminescent Europium(III) and Gadolinium(III) Complexes

New europium(III) and gadolinium(III) complexes bearing 8-hydroxyphenalenone antenna combine efficient absorption in the blue part of the spectrum and strong emission in polymers at room temperature. The Eu(III) complexes show characteristic red luminescence whereas the Gd(III) dyes are strongly phosphorescent. The luminescence quantum yields are about 20% for the Eu(III) complexes and 50% for the Gd(III) dyes. In contrast to most state-of-the-art Eu(III) complexes the new dyes are quenched very efficiently by molecular oxygen. The luminescence decay times of the Gd(III) complexes exceed 1 ms which ensures exceptional sensitivity even in polymers of moderate oxygen permeability. These sensors are particularly suitable for trace oxygen sensing and may be good substitutes for Pd(II) porphyrins. The photophysical and sensing properties can be tuned by varying the nature of the fourth ligand. The narrow-band emission of the Eu(III) allows efficient elimination of the background light and autofluorescence and is also very attractive for use e.g. in multi-analyte sensors. The highly photostable indicators incorporated in nanoparticles are promising for imaging applications. Due to the straightforward preparation and low cost of starting materials the new dyes represent a promising alternative to the state-of-the-art oxygen indicators particularly for such applications as e.g. food packaging.

Optical methods for sensing and imaging oxygen: materials, spectroscopies and applications

We review the current state of optical methods for sensing oxygen. These have become powerful alternatives to electrochemical detection and in the process of replacing the Clark electrode in many fields. The article (with 694 references) is divided into main sections on direct spectroscopic sensing of oxygen, on absorptiometric and luminescent probes, on polymeric matrices and supports, on additives and related materials, on spectroscopic schemes for read-out and imaging, and on sensing formats (such as waveguide sensing, sensor arrays, multiple sensors and nanosensors). We finally discuss future trends and applications and summarize the properties of the most often used indicator probes and polymers. The ESI† (with 385 references) gives a selection of specific applications of such sensors in medicine, biology, marine and geosciences, intracellular sensing, aerodynamics, industry and biotechnology, among others.

Supramolecular networks of 10-(2-hydroxyethyl)acridin-9(10H)-one and 10-(2-chloroethyl)acridin-9(10H)-one

Both 10-(2-hydroxyethyl)acridin-9(10H)-one, C15H13NO2, and 10-(2-chloroethyl)acridin-9(10H)-one, C15H12ClNO, have monoclinic (P21/c) symmetry and supramolecular three-dimensional networks. But the differences in the intermolecular interactions displayed by the hydroxy group and the chlorine substituent lead to stronger intermolecular π-stacking interactions and hydrogen bonding, and hence a significantly higher melting point for the former.

Shine a light on immobilized enzymes: real-time sensing in solid supported biocatalysts

Enzyme immobilization on solid supports has been key to biotransformation development. Although technologies for immobilization have largely reached maturity, the resulting biocatalysts are not well understood mechanistically. One limitation is that their internal environment is usually inferred from external data. Therefore, biological consequences of the immobilization remain masked by physical effects of mass transfer, obstructing further development. Work reviewed herein shows that opto-chemical sensing performed directly within the solid support enables the biocatalyst's internal environment to be uncovered quantitatively and in real time. Non-invasive methods of intraparticle pH and O2 determination are presented, and their use as process analytical tools for development of heterogeneous biocatalysts is described. Method diversification to other analytes remains a challenging task for the future.

Indicators for optical oxygen sensors

Continuous monitoring of oxygen concentration is of great importance in many different areas of research which range from medical applications to food packaging. In the last three decades, significant progress has been made in the field of optical sensing technology and this review will highlight the one inherent to the development of oxygen indicators. The first section outlines the bioanalytical fields in which optical oxygen sensors have been applied. The second section gives the reader a comprehensive summary of the existing oxygen indicators with a critical highlight on their photophysical and sensing properties. Altogether, this review is meant to give the potential user a guide to select the most suitable oxygen indicator for the particular application of interest.