B. A. DeGraff

Applications of luminescent transition platinum group metal complexes to sensor technology and molecular probes

Abstract Luminescent transition metal complexes are currently revolutionizing many areas of photochemistry and photophysics. In particular, they are proving useful as molecular probes and sensors. We discuss the design consideration in producing useful sensors and probes. As we show, complexes are amenable to rational design. Applications of inorganic complexes to a variety of sensor technologies are discussed. In addition, problem areas such as sensor–support interactions are covered.

Oxygen sensors based on luminescence quenching of metal complexes: osmium complexes suitable for laser diode excitation.

Oxygen quenching of a series of Os(II) complexes with α-diimine ligands has been studied in a predominantly poly(dimethylsiloxane) (PDMS) polymer and in Gp-163 (an acrylate modified PDMS). Unlike previous Ru(II) complexes used as oxygen sensors, the Os complexes can be excited by readily available, high-intensity, low-cost, red diode lasers at 635, 650, and 670 nm. Variations in the polymer properties have been made in order to delineate the structural features important for satisfactory use of supports for oxygen sensors. A key factor is matching the hydrophobicity of the sensor and support for optimal compatibility and minimizing the size of low oxygen diffusion domains.

Energy degradation pathways and binding site environment of micelle bound ruthenium(II) photosensitizers.

A series of ..cap alpha..-diimine Ru(II) sensitizers were studied in aqueous, alcohol, and sodium lauryl sulfate (NaLS) micellar solutions. The emission efficiency, lifetime, and spectra change dramatically on micellization. From the temperature dependence of the excited-state lifetime and luminescence quantum efficiencies, coupled with spectral fitting, they interpret these changes and elucidate the environment of the micellized sensitizer. The increased efficiencies and lifetimes on micellization arise from decreased rates of deactivation via the photoactive d-d state and by a decrease in other intramolecular nonradiative paths. Radiationless decay theory permits semiquantitative calculation of nonradiative rate constants. A model describing the binding site and local solvent environment for the sensitizers is proposed. Implications of the results for solar energy conversion schemes are described.

Comparison of Methods for Rapid Evaluation of Lifetimes of Exponential Decays

For evaluating exponential luminescence decays, there are a variety of computational rapid integral methods based on the areas of the decay under different binned intervals. Using both Monte Carlo methods and experimental photon counting data, we compare the standard rapid lifetime determination method (SRLD), optimized rapid lifetime determination methods (ORLD), maximum likelihood estimator method (MLE), and the phase plane method (PPM). The different techniques are compared with respect to precision, accuracy, sensitivity to binning range, and the effect of baseline interference. The MLE provides the best overall precision, but requires 10 bins and is sensitive to very small uncorrected baselines. The ORLD provides nearly as good precision using only two bins and is much more immune to uncompensated baselines. The PPM requires more bins than the MLE and has systematic errors, but is largely resistant to baseline issues. Therefore, depending on the data acquisition method and the number of bins that can be readily employed, the ORLD and MLE are the preferred methods for reasonable signal-to-noise ratios.

Oxygen Sensors Based on Luminescence Quenching: Interactions of Tris(4,7-diphenyl-1,10- phenanthroline)ruthenium(II) Chloride and Pyrene with Polymer Supports

Oxygen quenching of [Ru(Ph2phen)3]Cl2 (Ph2phen = 4,7-diphenyl-1,10-phenanthroline) and pyrene has been studied in a series of polymer networks of Gp-163 (a methacryloxy functional polydimethylsiloxane) co-polymerized with one of several co-monomers: styrene, trimethylsilyl-methylmethacrylate (T3642), vinyl-tris(2-methoxy-ethoxy)silane, or vinyl-tris(trimethylsiloxy)silane. Sensor performance was studied as a function of the polymer composition in order to delineate the important features for satisfactory O2 sensor supports. Quenching behavior was examined as a function of polymer structure, including amount and type of co-monomer. This work shows that the earlier two-domain model is too simplistic. The relative affinities of the different domains for the [Ru(Ph2phen)3]Cl2 and pyrene and the efficacy of the domains for O2 quenching are important; however, subtle changes in microstructure within domains can also strongly affect behavior. In particular, T3642 exhibits excellent structural and good quenching properties with [Ru(Ph2phen)3]Cl2.

Luminescence-Based Oxygen Sensors

The area of oxygen sensors encompasses a broad range of sensing techniques, devices, and applications. The latter range from monitoring combustion mixtures to use in fish farming.1 The majority of the techniques and devices involve transduction of the changes in oxygen concentration or pressure to changes in an electrical parameter such as voltage, current, or resistance. This sensing approach is older and has more working devices on the market. A more recent approach involves transduction via changes in luminescence characteristics of both organic and inorganic dyes. This approach has attracted considerable interest for applications such as pressure sensitive paints (PSP) and for situations amenable to use of fiber optic technology

Fluorescence Microscopy Study of Heterogeneity in Polymer-supported Luminescence-based Oxygen Sensors

Despite the great potential of fluorescence microscopy, its application to date has largely been in the study of biological specimens. It will be shown that conventional fluorescence microscopy provides an invaluable tool with which to study the photophysics of polymer-supported luminescence-based oxygen sensors. The design of the imaging system, the measurement methods, and the data analysis used in the investigation of sensor systems are described. Fluorescence microscopic images of sensor films in which microheterogeneous regions exhibiting enhanced luminescence intensity and poorer oxygen quenching relative to the bulk response are shown. This is the first direct evidence that sensor molecules in various domains of the polymer support can exhibit different oxygen quenching properties. It will be shown that µ- and nano-crystallization of the sensor molecule are the probable source of both the observed heterogeneous microscopic responses and the microscopic and macroscopic nonlinear Stern-Volmer plots. The implications of these results in the rational design of luminescence-based oxygen sensors are discussed.

Luminescence-Based Oxygen Sensors: ReL(CO)3Cl and ReL(CO)3CN Complexes on Copolymer Supports

A new class of luminescent rhenium complexes has been tested as oxygen sensors based on luminescent quenching. ReL(CO)3Cl and ReL(CO)3CN (L = 2,2′-bipyridine or 1,10-phenanthroline and substituted analogues) have several features that seem to indicate suitability as oxygen sensors. These include simple synthesis, long excited-state lifetimes, and high luminescence quantum yields. Intensity and lifetime oxygen quenching measurements were used to study the complexes in various polymer supports including homopolymers of PDMS (polydimethylsiloxane), a methacryloxy containing PDMS (Gp-163), and trimethylsilylmethylmethacrylate (T3642), and copolymers containing Gp-163 and T3642. In contrast to previous studies utilizing [Ru(4,7-diphenyl-1,10-phenanthroline)3]2+ as an oxygen sensor, quenching of the Re complexes proved much more sensitive to the polymer support. With suitable supports, the rhenium chloro complexes demonstrated significant quenching; but the cyano complexes, in spite of being robust in solution, exhibited severe photochemical instability in polymers. The potential of this class of complexes as oxygen sensors and as molecular probes as well as the ramifications in the design of new and different types of sensors is discussed.

Oxygen Sensors Based on a Quenching of Tris-(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) in Fluorinated Polymers

Luminescence quenching of Ru(II) complexes by oxygen has proved a powerful method of quantitative oxygen analysis. It has become clear that the polymer support for the sensor molecule plays a pivotal role in the sensor performance. The current work is devoted to understanding how the physical and photophysical properties of a sensor respond to changes in polymer composition. An oxygen quenching study was conducted on [Ru(Ph2phen)3]Cl2(Ph2phen=4,7-diphenyl-1,10-phenanthroline), in copolymer supports consisting of GP-163 (a polydimethylsiloxane (PDMS) with varying amounts of pendant acrylate groups) combined with a number of alkyl methacrylates with long chain alkyl or fluorinated alkyl esters. Increasing the chain length or the degree of fluorination on the hydrocarbon chains enhances performance. However, there is an optimal chain length for the fluorinated hydrocarbons for sensitivity, linearity, and physical properties. Too long a chain yields reduced quenching sensitivity and yields cloudy polymers. All systems showed some degree of heterogeneity as indicated by nonlinear Stern-Volmer quenching plots, but their intensity quenching data could be successfully fit with a two-site model.

Self-Referencing Intensity Measurements Based on Square-Wave Gated Phase-Modulation Fluorimetry

An adaptation of square-wave gated phase-modulation (GPM) fluorimetry allows for self-referenced intensity measurements without the complexity of dual excitation or dual emission wavelengths. This AC technique utilizes square-wave excitation, gated detection, a reference emitter, and a sensor molecule. The theory and experimental data demonstrating the effectiveness and advantages of the adapted GPM scheme are presented. One component must have an extremely short lifetime relative to the other. Both components are affected identically by changes in intensity of the excitation source, but the sensor intensity also depends on the concentration of the analyte. The fluctuations of the excitation source and any optical transmission changes are eliminated by ratioing the sensor emission to the reference emission. As the concentration of the analyte changes, the corresponding sensor intensity changes can be quantified through several schemes including digitization of the signal and digital integration or AC methods. To measure pH, digital methods are used with Na3[Tb(dpa)3] (dpa = 2,6-pyridinedicarboxylic acid) as the long-lived reference molecule and fluorescein as the short-lived sensor molecule. Measurements from the adapted GPM scheme are directly compared to conventional ratiometric measurements. Good agreement between the data collection methods is demonstrated through the apparent pKa. For the adapted GPM measurements, conventional measurements, and a global fit the apparent pKa values agree within less than 2%. A key element of the adapted GPM method is its insensitivity to fluctuations in the source intensity. For a roughly 8-fold change in the excitation intensity, the signal ratio changes by less than 3%.