In this study, four hybrid sol-gel films which were indicated as Film-A, Film-B, Film- C, and Film-D and containing one of the amino acids: L-tryptophan (Trp) as a model molecule were synthesized with and without TMOS, APTES and MTMOS by using trimethoxysilane terminated poly(propylene glycol) (ICPTS-PPG) to elucidate the release behaviour of stimuli responsive gels upon solvent combination. Moreover, the formation of hybrid sol-gel network was followed by monitoring the fluorescence emission spectra of Trp and found to be completed within 40 minutes. Meanwhile, the swelling experiments in ethanol-water mixture with different compositions were performed. The degree of swelling in feed composition containing 25% water was found to be very low, but the value increased steadily with increasing concentration of ethanol. It was found that hybrid sol-gel has the highest swelling degree in 75% ethanol-25% water mixture. Therefore, solvent uptake experiments were carried out in ethanol-water (75:25, v/v) mixture and revealed that hybrid sol-gel Film-D having MTMOS without TMOS and APTES, exhibited highest absorption of solvent compared with the other hybrid sol-gel films. According to data obtained for solvent uptake behaviours of hybrid sol-gel films, diffusional exponents and rate constants of each films in swelling kinetic were calculated by using equilibrium plots. The results showed that the magnitude of diffusional exponents were in the order of Film-B<Film-A<Film-C<Film-D and order of rate constants were inverse of diffusional exponents. This indicates the pseudo-Fickian diffusion with different swelling rates [74].
Afterwards, Emission spectra of Trp in ethanol-water mixture (75:25,v/v) for each hybrid sol-gel films were obtained to determine the concentration of Trp released by each of the gels. The results revealed that fractional release of Trp at initial of time interval was in the order of Film-B<Film-A<Film-D<Film-C. It was observed that Film-C showed the highest amount of Trp release at swelling equilibrium, followed by Film-B, Film-D and Film-A. Fluorescence spectra of Trp on which both from the surface of dried films and after films immersed in 75% ethanol-25% water mixture for a week were obtained to analyze the complete removal of Trp from hybrid sol-gel films. Emission spectra of Trp showed that removal of Trp from the sol-gel matrix was not completed. These results showed that some amount of Trp was trapped in
sol-gel matrix indicating insufficient swelling ratio of hybrid sol-gel films. After the swelling procedure in ethanol-water mixture (3:1, v/v) was completed for each hybrid sol-gel films, deswelling behaviours of hybrid sol-gels in distilled water were investigated and results showed that the smaller swelling ratio the greater rate of deswelling, and it was found that Film-C and Film-A hybrid sol-gel films exhibited higher Trp release in water solution when compared with other films.
Solvent uptake experiment for hybrid sol-gel films in 75% ethanol-25% 1M trifluoro acetic acid solution mixture with pH of 1.0 revealed that Film-C absorbed much more solvent compared to that of Film-A. This indicates that intramolecular interactions within sol-gel Film-A containing APTES which is able to make hydrogen bonding. But when Trp release of each film compared at low pH values, different behaviour was observed. It means that Film-A exhibited slightly faster Trp release rate than that of Film-C. This shows weak interactions occur between Trp and APTES at low pH values. On the other hand, experimental results related to Trp release in water at neutral pH value revealed that both Film-A and Film-C exhibit low
Trp diffusion because of low swelling degree of gels in water compared to that of
obtained at both lower and higher pH values. But Film-C exhibits higher release of
Trp than that of Film-A in water solution. This shows that Trp is much more trapped
in Film-A compared to Film-C.
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APPENDICES
APPENDIX A.1: Trp Calibration Curves in Different Solvent Mixtures
Figure A.1 : Fluorescence spectra of 10-3M Trp in 75% ethanol-25% water depending on concentrations. λex=290 nm. Inset: emission intensity
at 352 nm depending on time.
The equation of linear calibration (standard) curve was obtained by using the first five data, So the equation is as follows, F(352nm)=0.88+6.37x105[Trp].
Figure A.2 : Fluorescence spectra of Trp in water λex=290 nm. Inset: emission intensity at 356 nm depending on time.
The equation of linear calibration (standard) curve was obtained by using the first five data, So the equation is as follows, F(356nm)=1.35+5.30x105[Trp].
Figure A.3 : Fluorescence spectra of 10-3M Trp (75% ethanol-25% 1M CF3COOH) in 75% ethanol-25% 1M CF3COOH depending on concentrations. λex=290 nm. Inset: emission intensity at 347nm depending on time. The equation of linear calibration (standard) curve was obtained by using the first five data, So the equation is as follows, F(347nm) =0.27+2.09x105[Trp].
APPENDIX A.2: Photos of Hybrid Sol-Gels
Figure A.4 : Photos of hybrid sol-gels; A) after synthesized, B) After deswelled in water, C) Trp-imp gel swollen in ethanol, D) non-imp sol-gel swollen in ethanol.
Figure A.5 : Photos of hybrid sol-gels; 0) Dried Trp-imp gel after synthesized, 1) After swollen in 75% ethanol-25% water mixture at pH of 6.64 2)
swollen in 75% ethanol-25% 0.1M acetic acid mixture at pH of 4.61 3) sol-gel swollen in 75% ethanol-25% 0.1M sodium bicarbonate mixture at pH of 10.75.
CURRICULUM VITAE
Name Surname: Erdal Serin
Place and Date of Birth: Kadıköy 02.03.1983
Address: Altayçeşme Mah. Çam Sk. No:8/13 34843 Maltepe/İstanbul E-Mail: [email protected]
B.Sc.: Metallurgical and Material Engineering, Kocaeli University
PRESENTATIONS ON THE THESIS
Serin E., Güney O*., 2013: Synthesis of Stimuli-responsive Hybrid Sol-Gel Films for Controlled Release of Tryptophan. International Porous and Powder Materials
Symposium and Exhibition (PPM), September 06-09 2013. Cesme Izmir, TURKEY