Ewa Miller

Investigation of drying silica gel by fluorescence methods

Abstract Results of steady-state and time-resolved fluorescence studies for a model drying process of selected silane gels are discussed in the paper. Gels which contained a fluorescence probe in the form of pyrene, at pH 4.1 and 5.9 were dried in the oxygen-free atmosphere, using a vacuum system. After subsequent stages of the drying process the spectra of fluorescence excitation and emission as well as fluorescence decay curves were recorded using the time-resolution techniques. On this basis micropolarity of the environment of pyrene encapsulated in gel pores and changes in the excimer intensity of a probe during the process were determined. A kinetic model of drying for the tested gels and the value of a drying rate constant were specified on the basis of the determination of the number of moles of water and ethanol evaporated from the gel. The analysis of lifetime distributions of particular pyrene forms led to the conclusions concerning changes in the fluorophore environment during the gel–xerogel transition connected with the evaporation of solvents from gel pores and next with a decrease of the pore sizes.

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.

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 pHs growth. Quenching mechanism of 3-amino phenylboronic acid fluorescence by quantum dots needs further experiments to be fully explained.

Stability of pyrene-1-carboxylic acid and other pyrene derivatives immobilized on silane gels

The main aim of this research was to select a silane gel with appropriate composition containing fluorophore immobilized in it, in view of application of this carrier in the construction of a fluorescence optical sensor. Results of stationary fluorescence studies on gelation of silane gel with different composition, obtained as a result of acidic or basic hydrolysis using the fluorophores: pyrene-1-carboxylic acid and 1,3-bis(1-pyrenyl)propane, are presented. The form of probe during sol-gel transition and in the formed gel is discussed. The presence of a protonated form of PyCOOH(2)(+) was observed in the gel obtained by acidic hydrolysis and of PyCOO(-) ions and neutral form of PyCOOH in the gel obtained with the use of basic hydrolysis. The aim of the research was also to estimate suitability of the silane gel, which contained a selected immobilized fluorophore, for application in the optical sensor. This criterion includes determination of the kinetics of fluorophore elution from the gel as well as estimation of the interactions of fluorophore immobilized in the gel with microenvironment. Therefore, to carry out this part of the research, the silane gel was formed into a thin layer deposited on a glass surface. Next, the kinetics of elution of pyrene-1-carboxylic acid, 1,3-bis(1-pyrenyl)propane as well as pyrene and aminopyrene from this layers by water and ethanol was investigated. Elution rate constants were determined with the use of the first-order kinetics. Three stages of the elution with different duration were distinguished: I 0-4 min, II 4 min-1h and III, the slowest one-above 1h. Stability of the probes immobilized in these gels during storage in water or ethanol for several days was also determined.

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.

Fluorescence studies of a silane gel to be applied as a carrier in optical sensors

Results of fluorescence studies on the degree of pyrene and aminopyrene elution from a silane gel by water and ethanol are discussed in the paper. The studies were carried out for gels obtained on glass plates that differed in composition. Silane monomers: TEOS—tetraethylorthosilane, APTES—3-aminopropyltriethoxysilane, GPTMS—3-glycidoxypropylmethoxysilane, PDMS—polydimethylsilane, TMAOH—tetraethylammonium hydroxide, water and ethanol were used. A fluorophore was trapped in the gel either physically or using covalent bonds with the amino group of a carrier and glutaraldehyde. Emission spectra of the probe steady-state fluorescence in the gel were recorded on subsequent days of storage of plates with the gel in fresh solvent. The kinetics of aminopyrene decay in the sol and silane gel was also recorded. On the basis of results obtained, the degree of elution of fluorophore from the gel was determined in view of the application of the silane gel as a carrier in the optical sensor and the nature of the probe environment.