High-resolution images of oxygen distributions in microheterogeneous samples are obtained by twophoton laser scanning microscopy (2P LSM), using a newly developed dendritic nanoprobe with internally enhanced two-photon absorption (2PA) cross-section. In this probe, energy is harvested by a 2PA antenna, which passes excitation onto a phosphorescent metalloporphyrin via intramolecular energy transfer. The 2P LSM allows sectioning of oxygen gradients with near diffraction-limited resolution, and lifetime-based acquisition eliminates dependence on the local probe concentration. The technique is validated on objects with a priori known oxygen distributions and applied to imaging of pO 2 in cells.

Imaging oxygen in 3D with submicron spatial resolution can be made possible by combining phosphorescence quenching technique with multiphoton laser scanning microscopy. Because Pt and Pd porphyrin-based phosphorescent dyes, traditionally used as phosphors in biological oxygen measurements, exhibit extremely low two-photon absorption (2PA) cross-sections, we designed a nanosensor for oxygen, in which a 2P absorbing antenna is coupled to a metalloporphyrin core via intramolecular energy transfer (ET) with the purpose of amplifying the 2PA induced phosphorescence of the metalloporphyrin. The central component of the device is a polyfunctionalized Pt porphyrin, whose triplet state emission at ambient temperatures is strong, occurs in the near infrared and is sensitive to O 2 . The 2PA chromophores are chosen in such a way that their absorption is maximal in the near infrared (NIR) window of tissue (e.g., 700−900 nm), while their fluorescence is overlapped with the absorption band(s) of the core metalloporphyrin, ensuring an efficient antenna-core resonance ET. The metalloporphyrin-antenna construct is embedded inside the protecting dendritic jacket, which isolates the core from interactions with biological macromolecules, controls diffusion of oxygen and makes the entire sensor water-soluble. Several Pt porphyrin-coumarin based sensors were synthesized and their photophyics studied to evaluate the proposed design.

Fluorescence quenching in solutions of a pendant-functionalized conjugated polymer chemosensor (ttp-PPETE) has been evaluated in the presence of a variety of transition metal cations, including Ni2+, Co2+, Cu2+, Fe2+, and Cr6+. Photophysical analysis of the emission quenching revealed a static quenching mechanism that demonstrated strong positive deviations from predicted linear behavior. The enhanced emission quenching mechanism was found to correlate closely with the relative loading of the cationic analytes on the polymer chemosensor. This behavior has been attributed to rapid energy transfer along the polymer backbone. A modified Stern−Volmer static quenching model has been successfully applied that incorporates an energy-transfer term that takes into account different energy-transfer mechanisms. Both Förster and Dexter energy-transfer enhancements were observed for ttp-PPETE quenching depending upon the identity of the quencher analyte involved. Stern−Volmer constants in all cases were on the order of 105 M-1 for the transition metals reported. Photophysical characterization for ttp-PPETE includes absorbance, emission, and single-photon counting lifetimes in the absence and presence of varying concentrations of the analytes.

The demonstration of a nonlinear optical technique for directly monitoring adsorption of surfactants on the surface of microparticles in colloids is reported. In this approach, dye molecules with strong hyperpolarizability are first adsorbed on the particle surface to give detectable second-harmonic generation. The surfactant is then added to the colloidal solution in competition with dye for adsorption on the surface. The displacement of the dye molecules on the surface results in a decrease of the second-harmonic signal, indicating the adsorption of the surfactant molecules. A continuous flow/titration system in combination with a high-repetition-rate femtosecond laser allows the adsorption to be monitored in real time. This approach was first demonstrated on a methacrylate polymeric surfactant on latex and talc particles in an aqueous solution. The adsorption free energy and surface density of this surfactant on these particles have been determined.