Porous matrix materials in optical sensing of gaseous oxygen.

The review provides comparison of porous materials that act as a matrix for luminescent oxygen indicators. These include silica-gels, sol-gel materials based on silica and organically modified silica (Ormosils), aerogels, electrospun polymeric nanofibers, metal-organic frameworks, anodized alumina, and various other microstructured sensor matrices. The influence of material structure and composition on the efficiency of oxygen quenching and dynamic response times is compared and the advantages and disadvantages of the materials are summarized to give a guide for design and practical application of sensors with desired sensitivity and response time.

Early non-destructive biofouling detection and spatial distribution: Application of oxygen sensing optodes.

Biofouling is a serious problem in reverse osmosis/nanofiltration (RO/NF) applications, reducing membrane performance. Early detection of biofouling plays an essential role in an adequate anti-biofouling strategy. Presently, fouling of membrane filtration systems is mainly determined by measuring changes in pressure drop, which is not exclusively linked to biofouling. Non-destructive imaging of oxygen concentrations (i) is specific for biological activity of biofilms and (ii) may enable earlier detection of biofilm accumulation than pressure drop. The objective of this study was to test whether transparent luminescent planar O2 optodes, in combination with a simple imaging system, can be used for early non-destructive biofouling detection. This biofouling detection is done by mapping the two-dimensional distribution of O2 concentrations and O2 decrease rates inside a membrane fouling simulator (MFS). Results show that at an early stage, biofouling development was detected by the oxygen sensing optodes while no significant increase in pressure drop was yet observed. Additionally, optodes could detect spatial heterogeneities in biofouling distribution at a micro scale. Biofilm development started mainly at the feed spacer crossings. The spatial and quantitative information on biological activity will lead to better understanding of the biofouling processes, contributing to the development of more effective biofouling control strategies.

pH-sensitive perylene bisimide probes for live cell fluorescence lifetime imaging.

Several new perylene bisimide (PBI) probes comprising oligo-guanidine conjugates and cationic hydrogel nanoparticle structures were designed for sensing intracellular pH in live cell fluorescence lifetime imaging microscopy (FLIM). Using adherent mammalian cells (2D) and neurosphere (3D) cell models, we evaluated their performance by confocal FLIM-TCSPC. The nanoparticle PBI probe showed stable pH calibration and lifetime changes from 4.7 to 3.7 ns between pH 4.4 and 8 attributed to photo-induced electron transfer (PET). The molecular oligo-guanidine probe showed fast cell penetration and bright staining, but its calibration is affected by the microenvironment being unreliable for quantitative FLIM. Thus, nanoparticle structures are preferred for the design of quantitative pH measurement by FLIM. High brightness and photostability, efficient staining of different cell types and positive optical response to acidification in fluorescence intensity and lifetime modalities are the advantages of the nanoparticle PBI probes compared to conventional pH probes such as BCECF (2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein). Other PBI derivatives with stronger PET can be developed for future high-resolution FLIM of intracellular pH.

Ultrabright planar optodes for luminescence life-time based microscopic imaging of O2 dynamics in biofilms.

New transparent optodes for life-time based microscopic imaging of O2 were developed by spin-coating a µm-thin layer of a highly luminescent cyclometalated iridium(III) coumarin complex in polystyrene onto glass cover slips. Compared to similar thin-film O2 optodes based on a ruthenium(II) polypyridyl complex or a platinum(II) porphyrin, the new planar sensors have i) higher brightness allowing for much shorter exposure times and thus higher time resolution, ii) more homogeneous and smaller pixel to pixel variation over the sensor area resulting in less noisy O2 images, and iii) a lower temperature dependency simplifying calibration procedures. We used the new optodes for microscopic imaging of the spatio-temporal O2 dynamics at the base of heterotrophic biofilms in combination with confocal imaging of bacterial biomass and biofilm structure. This allowed us to directly link biomass distribution to O2 distribution under both steady state and non-steady state conditions. We demonstrate that the O2 dynamics in biofilms is governed by a complex interaction between biomass distribution, mass transfer and flow that cannot be directly inferred from structural information on biomass distribution alone.

Red light-excitable oxygen sensing materials based on platinum(II) and palladium(II) benzoporphyrins.

New optical oxygen-sensing materials make use of highly luminescent NIR platinum(II) and palladium(II) complexes with benzoporphyrins. Bulk optodes based on polystyrene and sensing nanobeads based on poly(styrene-block-vinylpyrrolidone) and polysulfone are prepared and characterized. The versatility of the new materials is demonstrated. The features include excellent compatibility with most common excitation sources, high brightness, and suitability for subcutaneous oxygen monitoring.