Paul Hartmann

Effects of polymer matrices on calibration functions of luminescent oxygen sensors based on porphyrin ketone complexes.

The design of luminescent oxygen sensors is guided by optimizing sensitivity and/or the form of the calibration function. Both qualities are governed by the molecular processes of luminescence quenching. To evaluate the influence of matrix effects, we prepared membranes based on oxygen-sensitive phosphorescent complexes of porphyrin ketones dissolved in plasticizer-free poly(vinyl chloride) (PVC) and polystyrene (PS). In a PVC matrix, both platinum(II) and palladium(II) octaethylporphyrin ketones exhibited perfectly linear Stern-Volmer intensity plots and almost single-exponential excited state decays. In a PS matrix, the sensitivity of palladium(II) octaethylporphyrin ketone was among the highest reported to date. Yet, slightly nonlinear Stern-Volmer plots and nonexponential decays illustrate the significance of matrix effects of PS. Addition of plasticizers to PVC-based sensors allowed tuning of the oxygen sensitivity in a wide range, while the Stern-Volmer plots became pronouncedly nonlinear. For the plasticizer bis(2-ethylhexyl) adipate, the decay profile was single-exponential in the absence but nonexponential in the presence of oxygen, which is attributed to a distribution of quenching rate constants.

Oxygen flux fluorescence lifetime imaging

We describe a device capable of imaging distributions of surface PO2 as well as oxygen flux (e.g., into the human skin). Oxygen is monitored by a phase-angle-sensitive imaging technique using the effect of luminescence lifetime quenching of ruthenium complexes incorporated into a polymer layer system. The dyes are excited by modulated radiation of blue light-emitting diodes. The phase-shifted luminescence is detected by an intensified CCD camera. Several modulation techniques have been comparatively investigated. The obtained images correspond pixel-by-pixel to the local luminescence lifetimes, which are a direct measure for the distribution of surface PO2, or alternatively for the oxygen flux through a permeable diffusion barrier. For an illuminated sensor area of 10 cm2 and a spatial resolution of 1 mm at the sensor layer, the oxygen resolution is better than ΔPO2 = 0.3 torr in the absence of oxygen, and better than ΔPO2 = 4 torr at PO2= 160 torr.

Response characteristics of luminescent oxygen sensors

Abstract Nonlinea Stern-Volmer quenching characteristics are a common feature of oxygen sensors containing Ru(II) complexes immobilized in solid-state matrices. We discuss the origins of this behaviour and address the implications of the molecular processes on sensor performance with the help of two different sensor systems: (1) For tris(2,2′-bipyridyl)Ru(II) dichloride (Rubpy) adsorbed on silica-gel the influence of both the absorption isotherm and heterogeneity of the indicators environment significantly contributes to the intensity and lifeime Stern-Volmer behaviour. A modified method to determine an average liftime from parameters received by a multi-exponential decay analysis is employed to show that static quenching is absent in this system. (2) For tris(4,7′-diphenyl-1,10′-phenanthroline)Ru(II) diperchlorate (RuPh 2 phen) dissolved in polystyrene a two-component approach is shown to be adequate for the description of the intensity and mean lifetime Stern-Volmer behaviour, as well as the excited state decay. The origin of this behaviour is discussed in consideration of lifetime distributions present in disordered media. The pre-exponentially weighted mean lifetime is used to show that static quenching is also absent in this system. The importance of the thermal history of the sample is noted.

Photobleaching of a ruthenium complex in polymers used for oxygen optodes and its inhibition by singlet oxygen quenchers

We investigated the effects of photobleaching on the photophysical properties of tris(4,7-diphenyl-1,10-phenanthrolin) Ru(II) in thermoplastic polymers. Illumination of the sensors by superbright LEDs resulted in a decrease of the emission intensity and decay time parameters. Absorption and matrix-assisted laser desorption/ionization (MALDI) spectra revealed the emergence of photoproducts. Photobleaching effects increased with increasing oxygen concentration. Singlet molecular oxygen, a product of the oxygen quenching process, is strongly involved in photoeffects on both the intensity and the decay time. The ability of photobleaching inhibition using singlet oxygen scavengers is illustrated.

Theory and practice in optical pH sensing

Abstract Optical pH sensors (pH optodes) are based upon pH-dependent changes of the optical properties of thin proton-permeable layers. Most research on pH optodes has been carried out on pH optodes for use in the physiological pH range. However, in recent years, the working range has been extended to other pH ranges as well. The basic technology, performance characteristics and design parameters of specific pH optodes are illustrated. Practical problems associated with instrumental difficulties as well as physical and chemical interferences such as background signals, accuracy and drift, time response, system calibration and durability are discussed.

FLIM of luminescent oxygen sensors: clinical applications and results

Abstract Oxygen supply is a critical parameter for the vitality of human skin. With the development of fluorescent lifetime imaging (FLIM) of oxygen optodes it became possible to measure the distribution of transcutaneous oxygen partial pressure of, or the oxygen flux into the skin, which is related to the cutaneous microcirculation. Aims of this study were: testing the sensor under clinical conditions, standardisation of handling of sensor membranes, their application to the skin, the documentation and visualisation of p O 2 changes after provocation tests (intracutaneous application of histamine and recall antigens). Clinical measurements are performed easily and within a short time. The sensors do not irritate the skin of the patients. The patients can move around between the measurements. For the first time, localized changes of p O 2 were quantified and visualised two-dimensionally for example in histamine-induced hyperaemic areas as well as in recall-antigen-induced inflammatory reactions. This new technique enables us to obtain clinically relevant data about the oxygen supply of the skin quickly. This is of significant clinical relevance in determining the level of amputation in patients with peripheral arterial occlusion.

Specific solvent effects of linear alcohols on the emission spectrum and the excited state decay of tris(2,2′-bipyridyl) ruthenium(I)

Abstract This study aims at a quantitative extraction of specific solvent effects of hydroxylic solvents on the non-radiative decay of tris(2,2′-bipyridyl) ruthenium(II). For this purpose the emission spectra, quantum yields and excited state lifetimes of the dye were measured in a series of monovalent linear alcohols. Separation of the non-radiative decay via the energy gap was achieved by evaluating the temperature dependence of quantum yields to account for temperature-activated transitions. The parameters determining the shapes of the emission spectra were calculated by a modified Franck-Condon analysis including the anharmonic Morse potential, and correlated with the non-radiative rate with the help of the energy gap law. The specific effects of hydroxylic solvents were finally obtained by comparison with the well-known behavior in non-hydroxylic solvents, and interpreted with the help of the energy gap law theory.

Static and dynamic quenching of luminescent species in polymer media.

A method developed for quantitative determination of static and dynamic contributions to luminescence quenching is applied to Ru(II) complexes in polymer matrices (silica gel and polystyrene), quenched by oxygen. This method is based on both intensity and lifetime quenching experiments. The curvature of intensity Stern-Volmer plots is related to the results.

Fluorescence Lifetime Imaging of the Skin PO2 Supply

The quality of the local blood microcirculation and the cutaneous “respiration” are important factors in the diagnosis of circulatory disturbances, which may arise as a consequence of a number of clinical diseases. Some of the more important are smokers leg, burns, ulcerations, or tumors of the skin.

Specific solvent effects of hydroxylic solvents on the emission properties of ruthenium(ii)tris(2,2′-bipyridyl) chloride

The excited state of Ru(II)[bpy]32+ dissolved in hydroxylic solvents is subject to specific solvent effects, which were hitherto not understood on a quantitative basis. We determined the solvent effects of linear monovalent alcohols on the energy gap law of internal conversion with the help of lifetime and intensity measurements. An on-line method for measurement of the temperature dependence of quantum efficiencies was introduced. A modified Franck-Condon analysis of emission spectra by taking into account the shape of a Morse potential of the involved states was applied to compute excited-state energies.