| Rhodamine dyes have lots of advantages, such as good stability, wide wavelength range and high fluorescence quantum yield, which endow it many applications in bimolecular labeling, cell tracing, heavy metal detection and membrane materials. The aggregation behavior of rhodamine dyes can be affected by many factors including the structure of dyes themselves, the polarity of solvent, and the structure, type, interaction of surrounding environment. As one kind of xanthene dyes, Rhodamine B(RB) contains different molecular forms in versatile environment. It can easily aggregate to form H-dimer in polar solvents(like water), which exhibits no fluorescence and is one of the factors for fluorescence quenching; while in poor polar solvents(alcohol) or in the absorpt states, J-dimer dominates and it has a better fluorescence property. The aggregation behavior of RB in inorganic gel films, and the adsorption behavior of inorganic gel films, polymer films and nano-composite films fabricated by inorganic sols and polymers were studied in this paper. The main content was briefly described as below:(1) During the sol-gel transition process of alumina sol and boehmite sol, the sol particles aggregated and formed a giant inorganic network. The aggregation behavior of RB changed with the proceeding of sol's condensation degree or the transition of solvents composition. Sols with variant condensation degree were placed between two glass slides or dip-coated to prepare thin films. The UV-vis spectrometer and fluorospectrophotometer were applied to test the as-prepared samples, which was helpful for the analysis of the aggregation behavior in two different sol systems.In the bulk or film state of alumina sol with high content of solid mater, more complicated amorphous structure is in favor of storage of water, which further advocates the formation of H-dimers. With the proceeding of dip-coating process, sol particles will precipitate to form porous structure, thus supplys the room for the storage of RB and solvent molecules. As the solvent in the holes evaporates gradually, RB molecules adsorb onto the wall of the holes and form J-dimers. The aggregation behavior of RB in bulk state of the two sols is similar. However, RB only adopts J-dimer in boehmite gel films, the reason lies in the regular arrangement structure and relatively huge specific surface area of boehmite gel films. Due to the good fluorescence property of J-dimer, fluorescence quenching phenomena does not happen even at high concentration, but improves the luminescence properties of materials. RB doped films were exposed to high humidity and isopropanol environments, respectively. We can conclud from the comparison that alumina sol is better in gas sensing while boehmite is an ideal candidate for preparation of luminescence or absorption materials.(2) In order to make it clear the fabrication method and materials influence on RB aggregation behavior in the film, four kind of sols(alumina, boehmite, polysilicate, mullite sol) were prepared, and the dropwise sols were transferred on to the round glass or silicon substrates by static spin-coating method. It demonstrates that the structure of films, interactions and solvents cooperatively affect the aggregation behavior of RB and their corresponding sensing properties.In the bulk state of alumina, boehmite and mullite sols, RB molecules are surrounded by solvents, only a small parts of RB can absorb onto the sol particles, the main factor influence the aggregation behavior of RB lies in solvents composition. Compared with dip-coated films, spin-coated films are thicker and more water molecules can be stored in the holes of films, thus RB in alumina, boehmite and mullite sol can form H-dimer. With the condensation of sols, the sol-gel transition proceeds and the amount of absorbed RB on the walls of nano-pores also increases, if there is non-hydrogen bonding between RB molecules and spin-coated films, RB molecules in the film can form J-dimer; while for hydrogen bonded RB molecule in the polysilicate sol spin-coated film, it adopts monomer form in the film. Though polysilicate and boehmite spin-coated films with small nano-pores are more stable, the humidity and isopropanol sensing is not obvious. While alumina and mullite sol spin-coated films, with larger nano-pore size, have a good humidity sensing property. But it still has a limitation because the structure of films is prone to destruction by waters. Furthermore, these two spin-coated films can be used for isopropanol sensing. In contrast with spin-coated systems, dip-coated films are better in preparing high luminescence performance of inorganic gel materials, while spin-coated films are more suitable for fabricating gas sensing inorganic gel materials.(3) Layer-by-layer(Lb L) assembly technique is a versatile approach for constructing two-dimential or three-dimential materials. While the investigation of aggregation and absorption behavior of rhodamine dyes in Lb L system is not enough, the research on the inorganic sol/polymer nano-composite layer-by-layer assembly film as absorbent is even less. RB dyes absorption and release behavior in polyvinylpyrrolidone(PVPON) / polyacrylic acid(PAA) Lb L assembly film were paid attention to. In addition, the adsorption ability of(PVPON/PAA)n film was compared with that of inorganic sol spin-coated film and inorganic sol/polymer spin-coated Lb L nano-composite film.Compared with rhodamine 6G(R6G) and rhodamine 110(R110) dyes, RB shows higher affinity to polymer molecules of(PVPON/PAA)n films, thus it has the best absorption capacities among rhodamine derivatives. The absorption and release behavior of RB in(PVPON/PAA)n Lb L assembly films shows a great dependence on temperature and p H value. When p H is 5.5,(PVPON/PAA)n films begin to disintegrate, so the appropriate investigating p H range is from 0.5 to 4.5. At p H 3.0 to 4.5 and low temperature, the film can absorb RB rapidly and keep relatively higher RB storage amount. But in the region where p H is less than 2.0 or at higher temperature, films will release most of absorbed RB and only a small part of it is left. When p H is above 5.5, the films will disintegrate, and all the absorbed RB molecules will be released. Analyzing the absorption behavior of RB in(PVPON/PAA)n films, we can conclude that only PAA contributes to the absorption capacity. When stained by high concentration of RB solutions, the dyes adsorption capacity of inorganic sol spin-coated films and inorganic sol/polymer spin-coated Lb L nano-composite films both are much lower than that of(PVPON/PAA)n dip-coated Lb L assembly films which stained by lower concentration of RB solutions.(4) The huge specific surface area of boehmite gel nano-particles and the functional group COOH of PAA make them exhibit adsorption behavior of RB molecules, and thus they are utilized to construct the complex film via layer-by-layer assembly technique. As the spin-coated Lb L composite film's adsorption behavior of RB was far from the expectation, the dip-coating method was utilized to make the nano-composite film. In acid condition, boehmite sol can be better to maintain its relatively stable structure as colloid, its surface is rich of hydroxyl groups and presents positive charge. Complex precipitation was observed when boehmite sol was mixed with PAA solution at the same volume ratio, so it has a certain film-forming property. The concentration and p H value of assemble solution both had a direct relationship with the films property. The change of surrounding environment did not affect boehmite sol particle with large diluted time, and the films prepared by which were also more stable. Through the analysis of the FT-IR spectra of(Boehmite/PAA)n nano-composite films prepared at different p H values, we concluded that electrostatic interaction was the main driving force, while hydrogen bonding acted as the associated driving force at low p H values, such as p H 2.0- p H 3.0. At p H 2.0, the film adopted non-linear to linear growth mode. When dipped in basic solution for a certain time, the nano-composite film fabricated at optimized condition would swell and detached from the substrate into fragments. With thermal cross-linking, the stability of the film was improved obviously, which made the film overcome the binding force between the film and substrate when dipped in basic solution, then wholly detached from the substrate to be an integral free-standing film.Boehmite and PAA double adsorption of RB determines the prominent adsorption property of(Boehmite/PAA)n Lb L nano-composite films than that of(PVPON/PAA)n Lb L films. The absorption process of the free-standing film belongs to the pseudo-first-order model and it is a chemical and exothermic absorption process. In addition, it agrees with Freundlich isotherm model. It is also worth to mention that the free-standing nano-composite film can be repeated many times for RB adsorption. The successfully Lb L assembly of Boehmite nano-particle and weak polyelectrolyte PAA, broadens Lb L technique application fields, supplies methods for fabrication functional materials and lays a theoretical foundation for it. |
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