Diffusion-facilitated direct determination of intrinsic parameters for rapid photoinduced bimolecular electron-transfer reactions in nonpolar solvents.

Bimolecular fluorescence-quenching reactions involving electron-transfer between electronically excited 5,10,15,20-tetraphenyl-21H,23H-porphine (TPP*) and 1,4-benzoquinone (BQ) or 1,4-naphthoquinone (NQ) were investigated using a set of alkane solvents that enabled the rapid reaction kinetics to be probed over a wide viscosity range, while minimizing changes in other relevant solvent parameters. Relative diffusion coefficients and reaction distances were recovered directly from analysis of fluorescence decay curves measured on a nanosecond time scale. The electron transfer from TPP* to BQ requires reactant contact, consistent with tightly associated exciplex formation in these nonpolar solvents. In contrast, electron transfer from TPP* to NQ displays a clear distance dependence, indicative of reaction via a much looser noncontact exciplex. This difference is attributed to the greater steric hindrance associated with contact between the TPP*/NQ pair. The diffusion coefficients recovered from fluorescence decay curve analysis are markedly smaller than the corresponding measured bulk relative diffusion coefficients. Classical hydrodynamics theory was found to provide a satisfactory resolution of this apparent discrepancy. The calculated hydrodynamic radii of TPP and NQ correlate very well with the van der Waals values. The hydrodynamic radius obtained for BQ is a factor of 6 times smaller than the van der Waals value, indicative of a possible tight cofacial geometry in the (TPP(+)/BQ(-))* exciplex. The present work demonstrates the utility of a straightforward methodology, based on widely available instrumentation and data analysis, that is broadly applicable for direct determination of kinetic parameter values for a wide variety of rapid bimolecular fluorescence quenching reactions in fluid solution.

Immunochromatographic assay using gold nanoparticles for measuring salivary secretory IgA in dogs as a stress marker.

The concentration of salivary secretory immunoglobulin A (sIgA) is a well-known stress marker for humans. The concentration of salivary sIgA in dogs has also been reported as a useful stress marker. In addition, salivary sIgA in dogs has been used to determine the adaptive ability of dogs for further training. There are conventional procedures based on enzyme-linked immunosorbent assay (ELISA) for measuring salivary sIgA in dogs. However, ELISA requires long assay time, complicated operations and is costly. In the present study, we developed an immunochromatographic assay for measuring salivary sIgA in dogs using a dilution buffer containing a non-ionic surfactant. We determined 2500-fold dilution as the optimum condition for dog saliva using a phosphate buffer (50 mM, pH 7.2) containing non-ionic surfactant (3 wt% Tween 20). The results obtained from the saliva samples of three dogs using immunochromatographic assay were compared with those obtained from ELISA. It was found that the immunochromatographic assay is applicable to judge the change in salivary sIgA in each dog. The immunochromatographic assay for salivary sIgA in dogs is a promising tool, which should soon become commercially available for predicting a dog's psychological condition and estimating adaptive ability for training as guide or police dogs.

Effect of diffusion on the photoinduced reaction between a tetra-anionic porphyrin and methylviologen cation in methanol.

The quenching of the fluorescence decay of electronically excited 5,10,15,20-tetraphenyl- 21H, 23H-porphinetetrasulfonate (TPPS (4-)*) in the presence of methylviologen cations (MV (2+)) was measured at various ionic strengths in methanol. Analysis of the fluorescence decay curves revealed strong evidence for the presence of a second fluorescent species over the entire range of ionic strength used in this work, which is attributed to solvent-separated ion pairs (TPPS (4-)-S-MV (2+)). Transient effects of the fluorescence decays were analyzed, and values for the effective reaction distance, R NH, and the diffusion coefficients, D, were obtained. Diffusion coefficients were independently measured for TPPS (4-) and MV (2+) using the Taylor dispersion method. The values for D obtained by the analysis of the transient effect were found to be smaller than those for the sum of the diffusion coefficients of TPPS (4-) and MV (2+) obtained by the Taylor dispersion method and a possible explanation for this result is given.