Kristi A. Kneas

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.

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.

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.

Conventional, confocal and two-photon fluorescence microscopy investigations of polymer-supported oxygen sensors.

Luminescence‐based, polymer‐supported oxygen sensors, particularly those based on platinum group complexes, continue to be of analytical importance. Commercial applications range from the macroscopic (e.g. aerodynamic investigations in wind tunnels, monitoring of oxygen concentration during fermentation, and measurement of biological oxygen demand) to the microscopic (e.g. imaging of oxygen in blood, tissue, cells and other biological samples). Problems hindering the design of improved oxygen sensors include non‐linear Stern–Volmer calibration plots and the multi‐exponentiality of measured lifetime decays, both of which are attributed primarily to heterogeneity of the sensor molecule in the polymer support matrix. Conventional, confocal and two‐photon fluorescence microscopy have proven to be invaluable tools with which the microscale heterogeneity and response of luminescence‐based oxygen sensors can be investigated and compared to the macroscopic response. Results obtained for three ruthenium(II) α‐diimine complexes in polydimethylsiloxane polymer supports indicate the presence of unquenched microcrystals within the polymer matrix that probably degrade oxygen quenching sensitivity and linearity of the Stern–Volmer quenching plot. Two‐photon fluorescence microscopy proved most useful for imaging microcrystals within sensor films, and conventional microscopy allowed direct comparison between microscopic and macroscopic sensor response. The implications of the results in the rational design and mass production of luminescence‐based oxygen sensors are significant.

Comparison of conventional, confocal, and two-photon microscopy for detection of microcrystals within luminescence-based oxygen sensor films

Luminescence-based oxygen sensors, particularly those based on platinum-group complexes are of growing analytical importance. Commercial applications include aerodynamic studies of cars and aircraft in wind tunnels, monitoring of oxygen concentration during fermentation processes and in bioreactors, measurement of biological oxygen demand, and fluorescence detection and imaging of oxygen in blood, tissue, cells and other biological samples. Significant problems in the design and manufacture of polymer-supported, luminescence-based oxygen sensors include the observed non-linearity of the Stern-Volmer calibration plot and the multiexponentiality of measured lifetime decays, both of which are attributed primarily to heterogeneity of the sensor molecule within the polymer matrix. It will be shown that conventional, confocal, and two-photon fluorescence microscopy are invaluable tools with which microcrystals of the sensor molecule can be detected within sensor films. The design of the imaging systems, the measurement methods, and the results will be compared for the three approaches. As a result of the reduction in blur intensity and the minimization of photobleaching, two-photon microscopy provided the easiest and most effective method of microcrystal detection. The implications of the results in the rational design and mass production of luminescence-based oxygen sensors is discussed.

Greening up Auto Part Manufacturing: A Collaboration between Academia and Industry.

Historically, manufacture of automotive electronic components and screen-printing of automotive instrument clusters at DENSO Manufacturing Tennessee, Inc. required washing of equipment such as screens, stencils, and jigs with sizable quantities of volatile organic compounds and hazardous air pollutants. Collaborative efforts between the Maryville College Department of Chemistry and DENSO resulted in a reduction in the use of such solvents, and DENSO remains in compliance with the EPA’s requirements. Individual projects were initiated during an analytical chemistry course when students met with DENSO associates to discuss pressing research problems. During the semester, students designed and performed preliminary experiments and drafted a research proposal that the instructor submitted to DENSO. Funded work was completed under the supervision of the instructor during the summer, and results and recommendations were included in a final report to DENSO. The nature of the collaboration is discussed, as are the results and positive outcomes of the projects.