Theoretical and experimental study on the effects of pH and surfactant on the internal charge transfer process in distyrylnaphthalene-based conjugated oligoelectrolytes.

In this study we have investigated the effects of pH and surfactant on the internal charge transfer (ICT) process in the DSNN derivative, DSNN-NMe+3 (4,4'-bis(4'-(N,N-bis(6″-(N,N,N-trimethylammonium)hexyl)amino)-styryl) naphthalene tetraiodide) with the aim to show that environmentally-induced changes in the degree of ICT process determine the spectral response of the DSNN chromophore. Obtained results showed that DSNN chromophore exhibits evident changes in linear optical properties (absorption/emission wavelengths, quantum yield) upon protonation. These changes are a manifestation of the attenuation of the internal charge transfer processes, which accompanies binding of proton to the nitrogen atoms of the dialkylamino groups at the termini of DSNN chromophore. The results obtained in this study clearly demonstrated the sensitivity of the ICT process in DSNN upon protonation, which, together with the affinity of DSNN towards biological and artificial membranes, may open new perspectives for its utility in fluorescence-based sensing. Moreover, the studied compound showed substantial surfactochromic effects in the ionic and non-ionic surfactant solutions, which indicate the formation of various self-organized DSNN-surfactant aggregates. The structure of these aggregates is determined by the type of specific intermolecular interactions between the chromophore and surfactant molecules. The knowledge of the nature of these interactions may be substantial in the future development of DSNN-based sensing platforms with suitable optical properties.

Fluorescence properties of 3-amino phenylboronic acid and its interaction with glucose and ZnS:Cu quantum dots.

Preliminary results of a study of the interaction between 3-amino phenylboronic acid and glucose or ZnS:Cu quantum dots are presented in this paper. ZnS:Cu quantum dots with mercaptopropionic acid as a capping agent were obtained and characterized. Quenching of 3-amino phenylboronic acid fluorescence was studied by steady-state and timeresolved measurements. For fluorescence quenching with glucose the results of steady-state measurements fulfill Stern-Volmer equation. The quenching constants are increasing with growing pH. The decay of fluorescence is monoexponential with lifetime about 8.4 ns, which does not depend on pH and glucose concentration indicating static quenching. The quenching constant can be interpreted as apparent equilibrium constant of estrification of boronic group with diol. Quantum dots are also quenching 3-amino phenylboronic acid fluorescence. Fluorescence lifetime, in this case, is slightly decreasing with increasing concentration of quantum dots. The quenching constants are increasing slightly with pH's growth. Quenching mechanism of 3-amino phenylboronic acid fluorescence by quantum dots needs further experiments to be fully explained.

Quantum dots as a possible oxygen sensor.

Results of studies on optical properties of low toxicity quantum dots (QDs) obtained from copper doped zinc sulfate are discussed in the paper. The effect of copper admixture concentration and solution pH on the fluorescence emission intensity of QDs was investigated. Quenching of QDs fluorescence by oxygen was reported and removal of the oxygen from the environment by two methods was described. In the chemical method oxygen was eliminated by adding sodium sulfite, in the other method oxygen was removed from the solution using nitrogen gas. For elimination of oxygen by purging the solution with nitrogen the increase of fluorescence intensity with decreasing oxygen concentration obeyed Stern-Volmer equation indicating quenching. For the chemical method Stern-Volmer equation was not fulfilled. The fluorescence decays lifetimes were determined and the increase of mean lifetimes at the absence of oxygen support hypothesis that QDs fluorescence is quenched by oxygen.

Thermostability of glucose oxidase in silica gel obtained by sol-gel method and in solution studied by fluorimetric method.

The thermostability of glucose oxidase entrapped in silica gel obtained by sol-gel method was studied by thermostimulated fluorescence of FAD at pH 5 and 7 and compared with that of the native enzyme in the solution and at the presence of ethanol. The unfolding temperatures were found to be lower for the enzyme immobilised in gel as compared with the native enzyme but higher as for the enzyme at the presence of ethanol. In gel, the thermal denaturation of glucose oxidase is independent on pH while in solution the enzyme is more stable at pH 5. The investigation the enzyme in different environment by steady-state fluorescence of FAD and tryptophan, synchronous fluorescence and time-resolved fluorescence of tryptophan indicates that the state of the molecule (tertiary structure and molecular dynamics) is different in gel and in solution. The ethanol produced during gel precursor hydrolysis is not the main factor influencing the thermostability of the enzyme but more important are interactions of the protein with the gel lattice.